Use of nanocomposites as permeability reducing admixtures J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-03-23 Luca Valentini; Giorgio Ferrari; Vincenzo Russo; Mateja Štefančič; Vesna Zalar Serjun; Gilberto Artioli
Permeability is one of the fundamental properties of concrete structures as it is strictly related to durability. Mitigation of the degradation processes induced by aggressive solutions can be achieved by controlling water penetration through the pore network. In this study, we test the potential use of nano‐composites as waterproofing agents in concrete. Macroscopic measurements show that the addition of a small amount of nanoparticles effectively reduces the extent of the water permeation front. A combination of experiments, based on X‐ray tomography, mercury intrusion porosimetry and BET nitrogen adsorption, and of numerical simulations, are used to interpret the macroscopic observations. These investigations show that C–S–H precipitation away from cement surfaces, induced by the presence of nanoparticles, leads to a refinement of the pore network. Such a microstructural change of the cement matrix results in a net reduction of the overall concrete permeability.
Microfluidic Fabrication of Ceramic Microspheres with Controlled Morphologies J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-03-23 Shuaishuai Liang; Jiang Li; Xiaomin Li; Jia Man; Janine K Nunes; Haosheng Chen
In this study, the diffusion‐induced external gelation is combined with a microfluidic technique to prepare monodisperse ZrO2 ceramic microspheres. The gelation of sol droplets is traced by fluorescence visualization of the local pH, and it illustrates the effect of the external concentration of triggering agent (tetramethylethylenediamine, TMEDA) on the formation of the gel network, which results in three kinds of deformation of the gel particles. The deformation mechanism mainly lies in the imbalanced Laplace pressure exerted on the gel network during the competition between the gelation and the drying processes. By regulating the concentration of TMEDA, the monodisperse ZrO2 ceramic microspheres with high sphericity can be readily fabricated.
Water vapor thermal treatment effects on spark plasma sintered nanostructured ferritic alloy‐silicon carbide systems J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Kaijie Ning; Kathy Lu
Spark plasma sintered pure silicon carbide (SiC) and nanostructured ferritic alloy‐silicon carbide (NFA‐SiC) systems are investigated in a water vapor containing air atmosphere at elevated temperatures up to 1000°C. Both of them exhibit excellent corrosion resistance with a dense amorphous SiO2 layer as the main oxidation barrier. Crystalline α‐quartz and α‐cristobalite from the oxidation of silicides and SiC, respectively, further benefit the corrosion resistance. For the new NFA‐SiC system, the original graphite and silicide phases can be desirably sustained. The NFA‐SiC materials have promising applications in high temperature moist environments and are especially important for nuclear reactor cladding.
Cationic effect of charge compensation on the sulfide capacity of aluminosilicate slags J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Joon Sung Choi; Youngjoo Park; Sunghee Lee; Dong Joon Min
The effect of CaO on the sulfide capacity of CaO‐Al2O3‐SiO2 slags was studied from the viewpoint of the ionic structure of alumina in slag. The aluminum coordination number was analyzed using 27Al 500‐MHz solid nuclear magnetic resonance spectroscopy and the results were compared with those of the sulfide capacity analysis. The sulfide capacity of slag, in the peralkaline region (), exhibited a linear relationship with respect to basicity () as excess free Ca2+ formed a 4‐coordinated aluminum unit structure ([IV]Al; ) and stabilized the sulfide ions (). However, sulfide capacity in the peraluminous region () exhibited a nonlinear relationship with respect to basicity () owing to the structure of higher‐coordinated aluminum units ([V]Al, [VI]Al; Al3+) and the relative lack of Ca2+. Therefore, the sulfide capacity of high Al2O3‐bearing slags strongly depended on the basicity () and stability of sulfide ions (), which depended on the competitive behavior of Ca2+ owing to the structural changes in Al2O3. The effect of the aluminum coordination number on the sulfide capacity was discussed in detail using an analysis of the slag structure and thermodynamics model.
Transmission electron microscopic and optical spectroscopic studies of Ni2+/Yb3+/Er3+/Tm3+ doped dual‐phase glass‐ceramics J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Zhigang Gao; Xiaosong Lu; Yindong Zhang; Shu Guo; Yinyao Liu; Shiyu Sun; Fei He; Piaoping Yang; Jing Ren; Jun Yang
Ni2+/Yb3+/Er3+/Tm3+ codoped transparent glass‐ceramics (GCs) containing both hexagonal β‐YF3 and spinel‐like γ‐Ga2O3 dual‐phase nanoparticles (NCs) are synthesized by melt‐quenching and subsequent heating procedures. Two techniques of transmission electron microscopy (TEM) nanoanalytics and optical spectroscopy are conjugated to understand the distribution of the rare‐earth ions (REs) and transition metals (TMs) in the nanostructured GCs. It is found that the REs are located predominantly in β‐YF3, whereas the TMs in γ‐Ga2O3 NCs. As a result, energy transfer (ET) between the REs and TMs is considerably suppressed due to the large spatial separation (> 3 nm), but it is enhanced between the REs partitioned in the β‐YF3 NCs. This has important implications for intended and demanding photoluminescence functions. For example, an ultrabroadband near‐infrared (NIR) emission in the wavelength region of 1000‐2000 nm covering the entire telecommunications window is observed for the first time. Meanwhile, intense upconversion (UC) emissions covering the 3 primary colors and locating in the first biological window can be also recorded under excitation by a single pump source at 980 nm.
Structural dependence of crystallization in glasses along the nepheline (NaAlSiO4) ‐ eucryptite (LiAlSiO4) join J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 José Marcial; Joey Kabel; Muad Saleh; Nancy Washton; Yaqoot Shaharyar; Ashutosh Goel; John S. McCloy
Lithium and sodium aluminosilicates are important glass‐forming systems for commercial glass‐ceramics, as well as being important model systems for ion transport in battery studies. In addition, uncontrolled crystallization of LiAlSiO4 (eucryptite) in high‐Li2O compositions, analogous to the more well‐known problem of NaAlSiO4 (nepheline) crystallization, can cause concerns for long‐term chemical durability in nuclear waste glasses. To study the relationships between glass structure and crystallization, nine glasses were synthesized in the LixNa1‐xAlSiO4 series, from x = 0 to x = 1. Raman spectra, nuclear magnetic resonance (NMR) spectroscopy (Li‐7, Na‐23, Al‐27, Si‐29), and X‐ray diffraction were used to study the quenched and heat‐treated glasses. It was found that different LiAlSiO4 and NaAlSiO4 crystal phases crystallize from the glass depending on the Li/Na ratio. Raman and NMR spectra of quenched glasses suggest similar structures regardless of alkali substitution. Li‐7 and Na‐23 NMR spectra of the glass‐ceramics near the endmember compositions show evidence of several differentiable sites distinct from known LixNa1‐xAlSiO4 crystalline phases, suggesting that these measurements can reveal subtle chemical environment differences in mixed‐alkali systems, similar to what has been observed for zeolites.
Oxygen vacancy formation in the SrTiO3 Σ5  twist grain boundary from first‐principles J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Maziar Behtash; Yaqin Wang; Jian Luo; Kesong Yang
The SrTiO3 5  twist grain boundary (GB) is studied using first‐principles density functional theory calculations. Three types of GB structures, SrO/SrO (S/S), SrO/TiO2 (S/T), and TiO2/TiO2 (T/T), are modeled and their relative thermodynamic stabilities are examined. Our calculations show that the S/S and S/T structures can be formed within appropriate synthesis conditions, with the S/S structure thermodynamically favored over the S/T structure within a wide range of chemical potentials, while the T/T structure is unlikely to form. The segregation behavior of oxygen vacancies is also investigated by calculating oxygen vacancy formation energies with respect to the distance from GB plane. In the S/S system, oxygen vacancies tend to segregate to the layer adjacent to the GB layer, while in the S/T system, oxygen vacancies tend to segregate to the GB layer itself. In both S/S and S/T systems, oxygen vacancy formation energy is lower than that in bulk SrTiO3. To clearly show the experimental conditions necessary to promote oxygen vacancy formation in the 2 GB systems, we also generate grain boundary phase diagrams for oxygen vacancy with respect to synthesis temperature and oxygen partial pressure. Our calculations reveal different segregation behaviors and distributions of oxygen vacancies in the S/S and S/T systems, providing a possible avenue for GB engineering.
Grain boundary driven mechanical properties of ZrB2 and ZrC‐ZrB2 nanocomposite: A molecular simulation study J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Md Riaz Kayser; Ashfaq Adnan
In this study, we report the grain boundary driven mechanical behavior of 2 polycrystalline ultra‐high‐temperature ceramics (UHTCs), zirconium diboride (ZrB2) and zirconium carbide (ZrC) with zirconium diboride (ZrC‐ZrB2). These nanocomposites were investigated using large‐scale molecular dynamics simulations. First, the atomistic models of the polycrystalline ZrB2 and ZrC‐ZrB2 nanocomposites were subjected to tensile loading to determine their elastic constants and tensile strengths. It was found that the presence of nanoparticles imparts an insignificant effect on the mechanical properties of ZrB2. It has also been observed that the failure mechanisms of both the ZrB2 and ZrC‐ZrB2 nanocomposite are driven by grain boundary deformation. At any instant during the applied load transfer, local tensile stress distribution data indicate that atomic stress becomes much higher near the grain boundaries compared to other locations. The authors performed additional sets of simulations to obtain tensile and shear properties of grain boundary material. When these properties were compared with the adjacent single crystal and overall polycrystalline material properties, it was found that the shear strength and stiffness of the grain boundary materials are significantly lower than the single crystal or polycrystal ZrB2. It is believed that the overall deformation and failure properties of ZrB2 and its composite are controlled by the properties of grain boundary. Hence, the addition of nanoparticles played an insignificant role on the mechanical properties of ZrB2.
Amorphous Ge‐Sb‐Se thin films fabricated by co‐sputtering: Properties and photosensitivity J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Tomáš Halenkovič; Jan Gutwirth; Petr Němec; Emeline Baudet; Marion Specht; Yann Gueguen; Jean‐Christophe Sangleboeuf; Virginie Nazabal
Amorphous Ge–Sb–Se thin films were fabricated by a rf‐magnetron co‐sputtering technique employing the following cathodes: GeSe2, Sb2Se3, and Ge28Sb12Se60. The influence of the composition, determined by energy‐dispersive X‐ray spectroscopy, on the optical properties was studied. Optical properties were analyzed based on variable angle spectroscopic ellipsometry and UV‐Vis‐NIR spectrophotometry. The results show that the optical bandgap range 1.35‐2.08 eV with corresponding refractive index ranging from 3.33 to 2.36 can be reliably covered. Furthermore, morphological and topographical properties of selenide‐sputtered films studied by scanning electron microscopy and atomic force microscopy showed a good quality of fabricated films. In addition, structure of the films was controlled using Raman scattering spectroscopy. Finally, irreversible photoinduced changes by means of change in optical bandgap energy and refractive index of co‐sputtered films were studied revealing the photobleaching effect in Ge‐rich films when irradiated by near‐bandgap light under Ar atmosphere. The photobleaching effect tends to decrease with increasing antimony content.
Mn doping effects on electric properties of 0.93(Bi0.5Na0.5)TiO3‐0.07Ba(Ti0.945Zr0.055)O3 ceramics J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Xingru Zhang; Guicheng Jiang; Feifei Guo; Danqing Liu; Shantao Zhang; Bin Yang; Wenwu Cao
The validity of Mn element on 0.93(Bi0.5Na0.5)TiO3‐0.07Ba(Ti0.945Zr0.055)O3 ceramics (BNT‐BZT‐xMn) is certified by doping. On account of multiple effects introduced by Mn, the appropriate Mn content facilitates property improvement effectively. Compared with pure BNT‐BZT, d33 of the component x = 0.25 increases about 8% up to 187 pC/N and Qm of the component x = 1 increases about 84% up to 197. Thermally stimulated depolarization currents (TSDC) measurement reveals Mn additive is helpful to pyroelectric properties as well. The Mn‐doped component x = 0.125 exhibits better pyroelectric performance at room temperature. Corresponding pyroelectric coefficient and the figures of merit reach up to 0.061 μC/(cm2 °C), Fi=217 pm/V, Fν = 0.023 m2/C, and Fd = 12.6 μPa−1/2, respectively, even superior to lead‐based ceramics. Similar pyroelectric advantage is also observed in the component x = 0.5 near depolarization temperature Td. Mn doping has slight harmful influence on the ferroelectric‐to‐relaxor transition temperature TF−R, as well as Td, but hardly shows restriction on application. These results confirm Mn doping is an available strategy to improve BNT‐based ceramics. Therefore, Mn‐doped BNT‐BZT ceramics will be excellent candidates in area of high‐power piezoelectric application and pyroelectric detectors.
Modulation of electrostriction and strain response in bismuth sodium titanate‐based ceramics J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-01 Yuxing Zhang; Jie Yin; Chunlin Zhao; Bo Wu; Jiagang Wu
The electrostriction and strain response of lead‐free Bi0.5Na0.5TiO3–BaTiO3 piezoceramics with La and Nb [(Bi0.47Na0.47Ba0.06)1−xLaxTi1−yNbyO3] were modified by optimizing the depolarization temperature. The influences of La and Nb on their phase structure and electrical properties were systematically investigated. All the ceramics exhibited a pseudocubic phase, which is independent of the addition of La and Nb. The strain values increased gradually with the addition of La, and a high strain of ~0.5% (80 kV/cm) was attained without any remnant strain when the compositions had a value of x = 0.015. Instead, the piezoelectric constant d33 dropped down ~20 pC/N due to the shift of the Td (or Tf−r) to room temperature. Interestingly, by establishing the relationship between Td and strain values, it should be feasible to optimize the strain property of Bi0.5Na0.5TiO3 (BNT)‐based ceramics by regulating Td to ambient temperature. In addition, a very high electrostriction coefficient Q33 of ~0.0758 m4 C−2 can be found under high temperatures of 125 and 150°C. We believe that the strain and electrostriction behavior of BNT‐based ceramics can be well modified by the modulation of depolarization temperature.
Synergetic enhancement of mechanical and electrical properties in Ce0.8Sm0.1Nd0.1O2−δ/La10Si6O27 composite electrolytes J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-02 Yajie Yuan; Tianjun Li; Hiroki Nishijima; Wei Pan; Jianghong Gong; Kai Wang; Min Wang; Mengfei Zhang; Xiaohui Zhao
It has been demonstrated that the samarium and neodymium codoped ceria (Ce0.8Sm0.1Nd0.1O2−δ abbreviated SNDC) shows high ionic conductivity at intermediate operating temperature (0.012 S/cm at 500°C. However, the poor mechanical properties limited its applications in solid fuel cells and oxygen sensors. Present research reports an approach to improve the mechanical properties of SNDC by adding another kind of oxide solid electrolyte, the apatite‐type lanthanum silicate (La10Si6O27 abbreviated LSO). The SNDC/LSO composites were prepared by mixing the powders with different proportion, and sintered at 1600°C. Their structure, morphology, mechanical, and electrical properties were characterized. It was found that with the 5wt%‐7wt% LSO addition, both the flexural strength and the fracture toughness were improved. The improvement of flexural strength for the SNDC reached as high as 71%. It is also seen that the ionic conductivity of SNDC was enhanced with the adding of 5wt% LSO. The enhanced mechanical properties and electrical conductivity of SNDC/LSO composites make them a promising candidates for high‐performance SOFCs and oxygen sensors.
Low‐temperature synthesis of uranium monocarbide by a Pechini‐type in situ polymerizable complex method J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-02 Hangxu Guo; Jieru Wang; Jing Bai; Shiwei Cao; Denglei Chen; Yalou Sun; Xiaojie Yin; Wei Tian; Cunmin Tan; Qing Huang; Zhi Qin; Qihuang Deng
Uranium monocarbide (UC) was successfully synthesized by the Pechini‐type in situ polymerizable complex technique (IPC) with the organic matter as the only carbon source. In the aqueous process, a mixture of citric acid (CA) and mannitol with was polymerized to form a spongy‐like organic polymeric precursor without any precipitations. The structural evolution and formation mechanism of the precursor were investigated using XRD, DSC‐TG, SEM (EDX), TEM, and FT‐IR. XRD results demonstrated that UC was obtained with the /mannitol/CA molar ratios of 1.0/0.3/1.0 at a low temperature of 1400°C. SEM and TEM analyses revealed that the UO2 nanoparticles were uniformly distributed in the carbon matrix to form UO2/C nanocomposites, and submicrometer‐sized ellipsoidal UC particles cemented together. FT‐IR showed that a ‐CA chelated structure was firstly obtained, achieving the molecular scale mixing of uranium and C. Then the in situ charring guaranteed the intimate contact of UO2 and C, leading to a low reaction temperature in carbothermal reduction owing to a short diffusion distance.
Investigation of phase composition and microwave dielectric properties of MgO‐Ta2O5 ceramics with ultrahigh Qf value J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-02 Mingzhao Dang; Haishen Ren; Xiaogang Yao; Haiyi Peng; Tianyi Xie; Huixing Lin; Lan Luo
Four MgO‐Ta2O5 ceramics with the MgO/Ta2O5 mole ratio x = 1, 2, 3, and 4 were prepared by traditional solid‐state reaction method, and the influence of x on the phase composition, microstructure, and dielectric properties (the dielectric constant εr, the temperature coefficient of resonant frequency τf and the quality factor Qf) of the materials was investigated using XRD, SEM, etc. The results indicated that the ceramics were composed of two crystalline phases MgTa2O6 and Mg4Ta2O9 in the composition range studied, and that the dielectric properties ln ε, 1/Qf, and τf changed proportionally to the fraction of main crystal phases, which meet perfectly with the mixing model proposed in this study. It is obvious that the proportion of the two crystal phases could be precisely controlled by x, and thereby, the dielectric properties can be conveniently and precisely tailored. Our research provided a new microwave dielectric ceramic with the composition of 2MgO‐Ta2O5, which has an ultrahigh Qf value (211 000 GHz), low dielectric constant εr (19.9), and near zero temperature coefficient of resonant frequency τf (8 ppm/°C).
Generation of hydrogen under visible light irradiation with enhanced photocatalytic activity of Bi2WO6/Cu1.8Se for organic pollutants under Vis‐NIR light reign J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-02 Li‐Na Qiao; Huan‐Chun Wang; Yi‐Dong Luo; Hao‐Min Xu; Jun‐Ping Ding; Shun Lan; Yang Shen; Yuan‐Hua Lin; Ce‐Wen Nan
To make better use of solar light, a new Bi2WO6/Cu1.8Se photocatalyst active to visible and near‐infrared light has been synthesized by a facile hydrothermal method. The composites were characterized by X‐ray diffractometry (XRD), scanning electron microscopy (SEM), UV‐vis diffuse reflectance spectroscopy (DRS), and photoluminescene (PL). The photocatalytic activities of Bi2WO6/Cu1.8Se are evaluated by degrading Congo red solution and hydrogen generation from water. It was found that the molar percentage of Cu1.8Se had great effects on the morphology and photocatalytic property of the Bi2WO6/Cu1.8Se heterojunctions, and the composite with suitable molar amount of Cu1.8Se exhibits much enhanced photocatalytic activity for Congo red degradation under visible and near‐infrared light irradiation and for hydrogen generation under visible light compared to Bi2WO6. The significant improvement photocatalytic activity of the composite could be attributed to its good light absorption, suitable band gap structure, and effective separation of photogenerated electron‐hole pairs of Bi2WO6/Cu1.8Se heterojunction. This work presents an efficient multifunction photocatalyst owning the activity both for water splitting under visible light and for organic contaminants decomposition under visible‐near‐infrared light.
Non‐isothermal crystallization kinetics of Al2O3‐YAG amorphous ceramic coating deposited via plasma spraying J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Jian Rong; Kai Yang; Yin Zhuang; Xinghua Zhong; Huayu Zhao; Jinxing Ni; Shunyan Tao; Liang Wang; Chuanxian Ding
Crystallization kinetics of the newly developed Al2O3‐Y3Al5O12 (YAG) amorphous ceramic coating fabricated by atmospheric plasma spraying (APS) were investigated via differential scanning calorimetry (DSC) under non‐isothermal conditions. The phase compositions and microstructure of the as‐sprayed coating were characterized by X‐ray diffraction (XRD) and Scanning electron microscopy (SEM). The glass transition temperature Tg, the onset temperature of crystallization Tc and the peak temperature of crystallization Tp presented heating rate dependence. The related kinetic parameters of activation energies (Eg, Ec, Ep) and Avrami exponents (n) were quantified using various methods including Kissinger, Augis–Bennett, Ozawa and Matusita–Sakka, etc., to understand the phase transition mechanism and crystallization process in depth. A series of parameters including devitrification interval ΔT, thermal stability (Tc, Ec), nucleation resistance Ec/RTg and fragility index F were quantified in order to evaluate the nucleation mechanism, crystallization behavior and thermal stability of Al2O3‐YAG amorphous ceramic coating. Excellent thermal stability was witnessed in the studied coating. Furthermore, the YAG crystalline phases can be reasonably controlled and independently precipitated from the amorphous matrix via proper annealing.
Preparation and photoluminescence enhancement of Au nanoparticles embedded LaPO4:Eu3+ inverse opals J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Zhuangzhuang Chai; Zhengwen Yang; Anjun Huang; Chengye Yu; Jianbei Qiu; Zhiguo Song
In this work, we present a facile preparation approach of Au nanoparticles embedded LaPO4:Eu3+ inverse opal photonic crystals. In the typical preparation process, the transparent LaPO4:Eu3+ sol including HAuCI4 was infiltrated into the opal templates. After the sintering, the 10‐20 nm Au nanoparticles were formed in the interior of nano‐sized wall of LaPO4:Eu3+ inverse opal and the Au nanoparticles embedded LaPO4:Eu3+ inverse opals were obtained. The luminescence of Au nanoparticles embedded LaPO4:Eu3+ inverse opal was investigated. The emission peaks located at the 593 (5D0→7F1), 618 (5D0→7F2) and 698 nm (5D0→7F4) from Eu3+ ions were observed. The 593, 618, and 698 nm emissions of Au nanoparticles embedded LaPO4:Eu3+ inverse opals were enhanced in contrast to these of LaPO4:Eu3+ inverse opal without the Au nanoparticles, which is from the excitation field enhancement caused by the localized surface plasmon resonance of Au nanoparticles.
Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Tae‐Ho Lee; Sung‐Hoon Cho; Tae‐Gon Lee; Hyo Tae Kim; In‐Kyu You; Sahn Nahm
A CaO‐B2O3‐SiO2 (CBS) glass/40 wt% Al2O3 composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al2O3 and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al2O3 composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.
Preparation of homogeneous mullite‐based fibrous ceramics by starch consolidation J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Wenjie Zang; Tao Jia; Xue Dong; Jiachen Liu; Haiyan Du; Feng Hou; Anran Guo
Silica sol is one of the frequently used binders in high‐temperature resisting fibrous porous ceramics, but in the drying process, it can diffuse with water and influence the uniformity of ceramics. To solve this diffusion problem and fabricate homogeneous fibrous porous ceramics, cationic starch was firstly introduced in mullite fibrous system. The effects of starch content and high‐temperature binder content on microstructure, physical, and mechanical properties were also investigated. The results indicate that starch consolidated mullite fibrous ceramics owned a homogeneous 3D skeleton structure, since the introduced starch can absorb both water and silica particles by gelatinization and ensure the even distribution of binders. Compared with the mullite fibrous ceramics fabricated without starch addition, starch consolidated ceramics owned better microstructure and higher interior compression strength. Furthermore, both the starch and silica sol content had great impact on the microstructure, density, porosity, thermal conductivity, and compressive strength of the fibrous ceramic.
Defect engineering on phase structure and temperature stability of KNN‐based ceramics sintered in different atmospheres J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Zhenyong Cen; Xiaohui Wang; Yu Huan; Yichao Zhen; Wei Feng; Longtu Li
Lead‐free MnO‐doped 0.955K0.5Na0.5NbO3‐0.045Bi0.5Na0.5ZrO3 (Abbreviated as KNN‐0.045BNZ) ceramics have been prepared by the conventional solid‐state sintering method in reducing atmosphere ( = 1 × 10−10 atm) and air. For ceramics sintered in reducing atmosphere, only Mn2+ ions exist in ceramics who preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4, whereas Mn2+ ions substitute for Zr4+ ions in B‐site to form defects () at x > 0.4. For ceramics sintered in air, mixed Mn2+, Mn3+, and Mn4+ ions coexist here. The Mn2+ ions preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4 and then Mn2+ ions substitute for Zr4+ ions in B‐site at x > 0.4. Meanwhile, the Mn3+ ions and Mn4+ ions substitute for Nb5+ ions in B‐site to form defects () at x = 0.2‐0.8. The (, , and ) dipolar defects show a positive dipolar defect contribution (DDC) to the , whereas the dipolar defects () show a negative DDC to the . The dipolar defects ( ‐ and ) can help improve the temperature stability of . The 0.4% MnO‐doped KNN‐0.045BNZ ceramics sintered in reducing atmosphere show excellent piezoelectric constant d33 = 300 pC/N and 0.2% MnO‐doped KNN‐0.045BNZ ceramics sintered in air possess optimal piezoelectric constant d33 = 290 pC/N.
Chemical stability of superhard rhenium diboride at oxygen and moisture ambient environmental conditions prepared by mechanical milling J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Mizraim G. Granados‐Fitch; Juan M. Quintana‐Melgoza; Erick A. Juarez‐Arellano; Miguel Avalos‐Borja
In this study, rhenium diboride (ReB2) was obtained by mechanosynthesis at 640 minutes of milling. The obtained ReB2 was stored at oxygen and moisture ambient environmental conditions to know the chemical stability. The results indicate that ReB2 is totally decomposed at oxygen and moisture ambient environmental conditions. Furthermore, the X‐ray diffraction (XRD) analysis of ReB2 samples after 26 months of storage shows that the final products of degradation are HReO4 (liquid), H3BO3, HBO2, and ReO3. Finally, a schematic diagram of the degradation sequence of ReB2 at oxygen and moisture ambient environmental conditions is proposed and validated with a thermodynamic analysis.
Hierarchical growth of BiOCl on SrO‐Bi2O3‐B2O3 glass‐ceramics for self‐cleaning applications J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Vinay P. Singh; Rahul Vaish
We have grown hierarchical structure of bismuth oxycloride (BiOCl) on SrO‐Bi2O3‐B2O3 (SBBO) transparent glass‐ceramic. SBBO glass‐ceramics were fabricated via conventional melt‐quenching technique while BiOCl was grown by etching the glass via HCl. Enhanced visible light driven photocatalytic activity and increasing hydrophobic feature were observed on BiOCl grown SBBO than as‐quenched SBBO glass‐ceramics. Contact angle analysis showed maximum contact angle of 130.7° on the surface of most BiOCl grown SBBO glass‐ceramic. Furthermore, under visible light illumination water contact angle decreased from 130.7° to 30.8°. Such photo‐induced hydrophilicity and catalytic performance in translucent glass‐ceramics lead self‐cleaning applications.
Eu3+‐doped glass as a color rendering index enhancer in phosphor‐in‐glass J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-06 Ignacio E. Orozco Hinostroza; Haggeo Desirena; Jose Hernandez; Jorge Molina; Ivan Moreno; Elder De la Rosa
A new method for improving color rendering index (CRI) and low correlated color temperature (CCT) in high‐power white‐light‐emitting diodes (WLEDs) is proposed. We used a configuration of phosphor‐in‐glass (PIG) and studied light output changes with the increment in concentration of yellow‐emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. The PIG was coupled on the top of blue‐light‐emitting diodes (LED) chip (465 nm). To compensate the lack of red emission in the phosphor, Eu3+‐doped tellurium glass with different europium content was employed as a red emitter. The suitable contents of YAG:Ce3+ and Eu3+ were 7.5 weight percent (wt%) and 3 mol percent (mol%), respectively. The CRI value went from 72 to 82, whereas the CCT was reduced from 24 933 to 6434 K. The proposed structure can improve CCT as well as CRI of WLEDs just by placing a glass on top.
Synthesis of molybdenum nitrides nanosheets by nitriding 2H‐MoS2 with ammonia J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-08 Guo‐Dong Sun; Guo‐Hua Zhang; Kuo‐Chih Chou
Metal nitrides nanosheets possess remarkable physical and chemical properties such as high electrical conductivities, catalytic properties, energy storage, and conversion efficiency. In this paper, molybdenum nitride (Mo5N6, MoN, and Mo2N) nanosheets were synthesized by nitriding and exfoliating the bulk 2H‐MoS2 via dropping N from ammonia at high temperature. Molybdenum nitride nanosheets with the thickness of dozens of nanometers were prepared successfully under different conditions. It was found that the reaction between MoS2 and NH3 began from about 696°C, and reduction products and reaction mechanisms were strongly dependent on the temperature. When there was MoS2, the generated Mo5N6, MoN, and Mo2N can exist stably at even 820, 1020, and 1120°C, respectively. However, they will decompose progressively after MoS2 was consumed completely: at 820°C, Mo5N6 started to decompose to δ‐MoN; at 1020°C, the phase evolution process of MoN can be described as follows: δ‐MoN→ γ‐Mo2N→ β‐Mo2N→ Mo, while at 1120°C, the β‐Mo2N will transform to Mo.
Improving high‐power properties of PZT ceramics by external DC bias field J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-08 Anushka Bansal; Husain N. Shekhani; Maryam Majzoubi; Eberhard Hennig; Timo Scholehwar; Kenji Uchino
This paper concludes that the deterioration of the mechanical quality factor Qm when operated under high power, can be recovered by externally applying positive DC bias field. Material constants for piezoelectric ceramics are generally characterized under low‐power conditions. However, high‐power properties deviate significantly from the ones measured under low‐power conditions (Qm degrades by a factor of ~2). DC Bias field helps to recover the properties of the ceramic under high‐power conditions. The DC bias field of 200 V/mm exhibits an almost equivalent “opposite” change rate to the vibration velocity of 0.1 m/s. It is also notable that the piezoelectric loss tan θ’ can be decreased most effectively under positive DC bias field (1.9% per 100 V/mm for the hard PZT and 3.1% per 100 V/mm for the soft PZT), in comparison with the elastic or dielectric losses. This report presents a comprehensive analysis on the low‐ and high‐power piezoelectric properties of hard and soft Lead Zirconate Titanates (PZT's) under externally applied DC bias field in the k31 resonance mode (transverse extensional).
Role of oxygen vacancies in long persistent phosphor Ca2Ga2GeO7: Zn2+ J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-08 Junhe Zhou; Zhangwen Long; Qi Wang; Dacheng Zhou; Jianbei Qiu; Xuhui Xu
Acting as the electron trap, oxygen vacancies can strongly influence the long persistent luminescence (LPL) properties were verified in Ca2Ga2GeO7: Zn2+ (CGGZ) phosphor. The existence of oxygen vacancies in this self‐excitation material was confirmed by a simple fluorescence probe method. The introduction of zinc ions promotes the generation of oxygen vacancies, as well as optimizes its LPL properties. Thermoluminescence (TL) analysis revealed that electrons in deep trap will effectively transfer to the shallow one via the bridge of interstitial zinc. Accordingly, a mechanism for PL and LPL in this Gallogermanates was provided.
A multiscale methodology quantifying the sintering temperature‐dependent mechanical properties of oxide matrix composites J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-09 Ru Jiang; Lingwei Yang; Haitao Liu; Wei Tan; Xun Sun; Haifeng Cheng; Weiguo Mao
A novel methodology combining multiscale mechanical testing and finite element modeling is proposed to quantify the sintering temperature‐dependent mechanical properties of oxide matrix composites, like aluminosilicate (AS) fiber reinforced Al2O3 matrix (ASf/Al2O3) composite in this work. The results showed a high‐temperature sensitivity in the modulus/strength of AS fiber and Al2O3 matrix due to their phase transitions at 1200°C, as revealed by instrumented nanoindentation technique. The interfacial strength, as measured by a novel fiber push‐in technique, was also temperature‐dependent. Specially at 1200°C, an interfacial phase reaction was observed, which bonded the interface tightly, as a result, the interfacial shear strength was up to ≈450 MPa. Employing the measured micro‐mechanical parameters of the composite constituents enabled the prediction of deformation mechanism of the composite in microscale, which suggested a dominant role of interface on the ductile/brittle behavior of the composite in tension and shear. Accordingly, the ASf/Al2O3 composite exhibited a ductile‐to‐brittle transition as the sintering temperature increased from 800 to 1200°C, due to the prohibition of interfacial debonding at higher temperatures, in good agreement with numerical predictions. The proposed multiscale methodology provides a powerful tool to study the mechanical properties of oxide matrix composites qualitatively and quantitatively.
La2O2SO4:RE/Yb new phosphors for near infrared to visible and near infrared upconversion luminescence (RE=Ho, Er, Tm) J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-10 Xuejiao Wang; Zhipeng Hu; Qi Zhu; Ji‐Guang Li; Xudong Sun
The 3 new upconversion (UC) phosphors of La2O2SO4:RE/Yb (RE=Ho, Er, and Tm, respectively) were derived via facile dehydration of their layered hydroxide precursors that were hydrothermally synthesized at 100°C. Rietveld XRD refinement found contracting cell dimension with decreasing RE3+ size, confirming the direct crystallization of solid solution. The Er3+ and Ho3+ activators both exhibited simultaneous green and red (dominant) emissions under 978‐nm near‐infrared (NIR) laser excitation (NIR‐Vis UC). Particularly, Tm3+ produced a Gaussian‐shaped pure NIR emission band at ~812 nm via its 3H4 → 3H6 transition (NIR‐NIR UC), which is highly desired for NIR biological application. Analysis of the excitation‐power dependent UC properties manifested a 3‐photon mechanism for the 3 phosphors, and the possible photon reactions leading to UC were illustrated.
The origin of the heterogeneous distribution of bismuth in aluminosilicate laser glasses J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-10 Xiaoman Li; Jiangkun Cao; Mingying Peng
As one kind of novel and burgeoning laser materials, bismuth‐doped silicate glasses have aroused increasing attention for the super broadband near‐infrared (NIR) emission. However, the large optical scattering loss, resulting from optical heterogeneity in glass color and refractive index, limits their further applications in telecommunication system. Thus, it is urgent to uncover the essence of heterogeneity in Bi‐doped silicate glasses and subsequently improve glass optical performance. It will give us some hint to homogenize the glass component and Bi active centers so as to boost the development of Bi‐based glass materials. Here, taking 1 typical Bi‐doped calcium aluminosilicate glass as an example, we revealed the origin of the optical heterogeneities in glass color and refractive index through the NIR emission spectra, electron probe microanalyzer (EPMA) of elements and X‐ray photoelectron spectroscopy (XPS) of Bi 4f5/2, Bi 4f7/2, and Al 2p. The inhomogeneous distribution of Bi and aluminum components is responsible for the heterogeneity in this glass system. In addition, we found that tetrahedral coordinated aluminum favors the existence of Bi NIR centers, consequently resulting in enhanced Bi NIR emissions. Furthermore, based on our results and the role of Al3+ in glass network, we demonstrate the homogenizing of glass component by finely tuning glass composition. This work will enrich the understanding of Bi‐doped laser glass and provide a guideline for the design of component‐derived Bi‐doped silicate glasses and fibers with efficient NIR emission and high optical quality.
New insights into nanoindentation crack initiation in ion‐exchanged sodium aluminosilicate glass J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-12 Xiaoyu Li; Liangbao Jiang; Iman Mohagheghian; John P. Dear; Lei Li; Yue Yan
The effect of ion‐exchange on the fracture behavior and the threshold load is investigated for radial crack initiation resulting from cube‐corner indentation. Both tin and air sides of the sodium aluminosilicate float glass are considered. The threshold load and mechanical properties are experimentally measured by nanoindentation. A qualitative explanation of crack initiation is developed by analyzing the stresses at the indentation site. The ion‐exchanged glasses show a lower threshold load for radial crack initiation with a cube‐corner indenter than the raw glass, and this is due to a higher crack driving stress for ion‐exchanged glasses. However, the compressive stress on the surface of the ion‐exchanged glasses can inhibit the expanding of the radial cracks. The air side always shows higher values for the threshold load than the tin side before and after ion‐exchange, which is in accordance with the calculated crack driving stress results.
Process modeling of the low‐temperature evolution and yield of polycarbosilanes for ceramic matrix composites J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-12 Thomas S. Key; Garth B. Wilks; Triplicane A. Parthasarathy; Derek S. King; Zlatomir D. Apostolov; Michael K. Cinibulk
The volatilization of polycarbosilanes is important to the processing and performance of polymer infiltration and pyrolysis‐based ceramic matrix composites. Low molecular weight (MW) polycarbosilane is often present in preceramic polymers and enhances viscosity for the purpose of composite infiltration. Due to the volatility of low MW chains, a model was developed to semi‐empirically determine the MW distribution and then predict the mass yield and evolution of the MW distribution as a function of temperature and time for StarPCS™ SMP‐10. The enthalpy of vaporization, the temperature dependence of the enthalpy of vaporization, the temperature dependence of the normal boiling point and a representation of the molecular weight distribution were fit using a series of thermogravimetric measurements, involving isothermal holds on a particular batch of SMP‐10. Once calibrated for SMP‐10 in this fashion, the molecular weight distribution of different batches of SMP‐10 could be fit using a thermogravimetric measurement involving a reduced temperature‐time series. The model was then predictive of mass loss over time for temperatures below the onset of curing (>90°C). Understanding this volatilization enables improved SiC yield, reduced processing time and minimizing void/bubble formation.
High‐temperature mechanical behavior of ZrB2‐based composites with micrometer‐ and nano‐sized SiC particles J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-12 Shuqi Guo
Using micrometer‐ and nano‐sized SiC particles as reinforcement phase, two ZrB2‐SiC composites with high strength up to 1600°C were prepared using high‐energy ball milling, followed by hot pressing. The composite microstructure comprised finer equiaxed ZrB2 and SiC grains and intergranular amorphous phase. The temperature dependency of flexure strength related to the initial particle size of SiC. In the case of micrometer‐sized SiC, the high‐temperature strength was improved up to 1500°C compared to room‐temperature strength, but the strength degraded at 1600°C, with strength values of 600‐770 MPa. In the case of nano‐sized SiC, the enhanced high‐temperature strength was observed up to 1600°C, with strength values of 680‐840 MPa.
Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-12 Shengfang Shi; Sunghun Cho; Tomoyo Goto; Tohru Sekino
Al2O3/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al2O3 and TiH2 powders. During sintering, TiH2 decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al2O3 matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al2O3. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm1/2, which was 37% greater than that of monolithic Al2O3. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10−1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al2O3 increased the fracture toughness and improved the conductivity of Al2O3.
Anisotropic domain switching in Pb(Mg1/3Nb2/3)O3‐0.30PbTiO3 single crystals with rhombohedral structure J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-14 Yaming Zhou; Qiang Li; Chao Xu; Fangping Zhuo; Qingfeng Yan; Yiling Zhang; Xiangcheng Chu
Anisotropic domain switching paths in ‐, ‐, and ‐poled Pb(Mg1/3Nb2/3)O3‐0.30PbTiO3 single crystals were studied by in situ polarized light microscopic driven by an antiparallel electric field. Orientation‐dependent electric field induced polarization and strain behaviors were investigated systematically. For ‐oriented crystals, only one‐step 71° switching occurred during the domain switching process, resulting in the appearance of stripe domain walls whose traces on (001) plane were along 45° or 135° with respect to  direction. But for ‐oriented samples, a two‐step 71° switching was observed during 109° switching and the projections of formed twin domain walls on the (011) plane are along 35.3° or 144.7° with respect to  direction. Moreover, a three‐step 71° switching was found during 180° switching in ‐oriented samples. It was demonstrated by the produced domain walls whose projections on the (10) plane are along 35.3°, 90° or 160.6° with respect to  direction. The energetically motivated mechanism based on multistep polarization switching process was also proposed to explain the anisotropic domain switching paths. Our results provided a visualized observation on the ferroelectric domain switching process and also laid the solid foundations for controlling polarization order parameter in ferroelectric single crystals.
Molecular simulations of the structure and thermal transport of high alumina aluminosilicate molten core glass fiber J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-14 Bennett Greenberg; Stephen H. Garofalini
The atomistic structure and phonon transport in aluminosilicate glasses made via an interfacial mixing model of the Molten Core process were studied using molecular dynamics simulations. In the simulations, silica glass was brought in contact with different size alumina crystals (to afford core glasses with 4, 18, 24, 29, and 41 mole% alumina concentrations), followed by a melt‐quench process to enable mixing of the phases. The atomistic structure of the resulting glasses and radius of gyration calculations of resultant Al‐O‐Al connected clusters were evaluated. Variation in the 1‐dimensional thermal transport in each glass was also determined and showed that increased alumina concentration in the glasses resulted in increased transport of thermal energy. Results of the structural analyses showed a double peak in the Al‐Al pair distribution function, with the short‐distance peak indicative of edge‐sharing Al‐O‐Al‐O bonding and a longer distance peak of Al‐O‐Al bonding that is not indicative of edge‐sharing structures. The ratio of the first Al‐Al peak to the second Al‐Al peak varied inversely with the thermal transport behavior. An increased radius of gyration of Al‐O‐Al connectivity occurred with increasing alumina concentration, providing a mechanism for the increased thermal transport. Nanosegregation was also observed. Interconnectivity between Al ions created isolated Al‐O‐Al bonded clusters at low alumina concentrations with lower thermal transport than the high alumina glasses, whereas the latter showed a percolated network of Al‐O‐Al bonds that increased thermal transport.
Finite element analysis of fracture statistics of ceramics: Effects of grain size and pore size distributions J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-14 Shingo Ozaki; Yuya Aoki; Toshio Osada; Kyohei Takeo; Wataru Nakao
A novel numerical simulation method based on finite element analysis (FEA), which can evaluate the fracture probability caused by the characteristics of flaw distribution, is considered an effective tool to facilitate and increase the use of ceramics in components and members. In this study, we propose an FEA methodology to predict the scatter of ceramic strength. Specifically, the data on the microstructure distribution (i.e., relative density, size and aspect ratio of pore, and grain size) are taken as the input values and reflected onto the parameters of a continuum damage model via a fracture mechanical model based on the circumferential circular crack emanating from an oval spherical pore. In addition, we numerically create a Weibull distribution based on multiple FEA results of a three‐point bending test. Its validity is confirmed by a quantitative comparison with the actual test results. The results suggest that the proposed FEA methodology can be applied to the analysis of the fracture probability of ceramics.
Facile synthesis of amorphous Cr2O3/N‐doped carbon nanosheets and its excellent lithium storage property J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-16 Deyin Zhang; Mingli Qin; Min Huang; Haoyang Wu; Baorui Jia; Zhiwei Liu; Tingting Liu; Xuanhui Qu; Peng Cao
The Cr2O3 with high‐energy density and relatively low lithium insertion potential is a promising anode candidate for LIBs. However, the intrinsic poor electroconductivity and side effects like volume expansion of Cr2O3 severely limit its capacity and cyclability at high charge/discharge rates. To address the problem, the amorphous Cr2O3/N‐doped carbon nanosheets (denoted as a‐Cr2O3/NC) have been designed and prepared by an easy one‐step solution combustion synthesis method from a uniform solution of chromium nitrate, glucose, and glycine. The as‐synthesized a‐Cr2O3/NC consist of amorphous Cr2O3 particles and N‐doped carbon sheet, where the amorphous Cr2O3 is evenly encapsulated in the carbon sheet support. An anode prepared from the synthesized a‐Cr2O3/NC demonstrates much higher specific capacity and better cycling performance than the crystalline Cr2O3 anode. Upon extended cycling, the a‐Cr2O3/NC anode exhibits good long‐term stability and its reversible capacity retains as high as 782.4 mAh g−1 after 500 cycles at 1 A g−1. Such good performance stems from its unique structure. The amorphous structure of Cr2O3 can furnish a mass of enterable active sites which can favor the lithium ions insertion/extraction, whereas the sheet‐like N‐doped carbon support can increase the electroconductivity and facilitate the transportation of lithium ions and electrons.
Efficient red‐emitting phosphor of Eu3+‐activated (Na0.5Gd1.5)(TiSb)O7 derived via cation‐substitutions in Gd‐Pyrochlore J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-16 Zhiwen Gao; Lei Zhao; Shihua Liu; Zhaoyi Wu; Ruijin Yu
Rare‐earth (RE) titanate pyrochlore with perovskite‐layered structure is a well‐known engineering material in applied in many field. In this work, a red‐emitting phosphor of Gd2−xNaxTi2−2xSb2xO7:Eu3+ (x = 0‐0.5) was developed via cation substitutions of (Sb5+→Ti4+) and (Na+→Gd3+) in Gd2Ti2O7. The motivation is based on the fact that the introduction of cation‐disorders has been regarded to be an effective approach for improving the luminescent efficiency and thermal stability of RE‐activated materials. All the samples were synthesized via facile solid‐state reaction method. The morphology properties were measured via SEM and EDS measurements. The structural Rietveld refinement was performed to investigate the microstructure in pyrochlore lattices. The luminescence properties of Gd2−xNaxTi2−2xSb2xO7:0.15Eu3+ (x = 0‐0.5) has a strict dependence on the cation substitution levels. The band energy of Gd2Ti2O7 is 2.9 eV with a direct transition nature. The incorporation of Sb5+ and Na+ in the lattices moves the optical absorption to a longer wavelength. The cation disorder results in significant improvements of luminescence intensity, excitation efficiency in the blue region, longer emission lifetime and thermal stability.
Zirconium carbide oxidation: Kinetics and oxygen diffusion through the intermediate layer J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-16 Claudia Gasparrini; Richard J. Chater; Denis Horlait; Luc Vandeperre; William E. Lee
Oxidation of hot‐pressed ZrC was investigated in air in the 1073‐1373 K range. The kinetics were linear at 1073 K, whereas at higher temperature samples initially followed linear kinetics before undergoing rapid oxidation leading to a Maltese cross shape of the oxide. The linear kinetics at 1073 K was governed by inward oxygen diffusion through an intermediate layer of constant thickness between ZrC and ZrO2 which was comprised of amorphous carbon and ZrO2 nanocrystals. Diffusion of oxygen through the intermediate layer was measured to be 9 × 10−10 cm2 s−1 using 18O as a tracer in a double oxidation experiment in 16O/18O. Oxidation at 1073 and 1173 K produced samples made of m‐ZrO2 and either t‐ or c‐ZrO2 with an adherent intermediate layer made of amorphous carbon and ZrO2, whereas oxidation at 1273 and 1373 K produced samples with a voluminous oxide made of m‐ZrO2 showing a gap between ZrC and the oxide. A substoichiometric zirconia layer was found at the gap at 1273 K and no carbon uptake was detected in this layer when compared with the top oxide layer. The loss of the intermediate layer and the slowdown of the linear rate constant (g m−2 s−1) at 1273 K compared to 1173 K was correlated with the preferential oxidation of carbon at the intermediate layer which would leave as CO and/or CO2 leaving a gap between ZrC and substoichiometric zirconia.
Tunable emission color of Li2SrSiO4:Tb3+ due to cross‐relaxation process and optical thermometry investigation J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-16 Xiao Zhou; Sha Jiang; Guotao Xiang; Xiao Tang; Xiaobing Luo; Li Li; Xianju Zhou
A series of Li2SrSiO4:xTb3+ (0.2%, 0.4%, 0.6%, 0.8%, 2%, 4%, and 6%) phosphors were prepared by conventional solid‐state reaction. It was found that this silicate phosphor has a wide excitation band at near‐ultraviolet region (230‐300 nm) due to spin‐allowed 4f 8→4f75d1 transitions of Tb3+ ions, with the exact position dependent on the crystal field of the lattice. The cross‐relaxation process originating from 5D3→5D4 and 7F6→7F0 happened between different Tb3+ ions. It leads to the luminescence color of Li2SrSiO4: xTb3+ tuning from blue to green just by controlling Tb3+ concentrations. Furthermore, concentration quenching mechanism, energy migration type, cross‐relaxation rate and efficiency, are discussed in detail. Finally, optical thermometry properties were investigated via temperature‐dependent emission spectra. The results show that low‐concentration‐doped sample (Li2SrSiO4:0.4%Tb3+) shows remarkable optical thermometry based on fluorescence intensity ratio (FIR) between the blue and green emission of Tb3+ ions, whereas the high‐concentration‐doped sample (Li2SrSiO4:4%Tb3+) demonstrates small emission intensity loss. It illustrates that terbium‐doped silicate phosphor is a multifunctional material with potential application for display field and optical thermometry.
Chemical structure and mechanical properties of soda lime silica glass surfaces treated by thermal poling in inert and reactive ambient gases J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-16 Jiawei Luo; Stephen Bae; Mengxue Yuan; Erik Schneider; Michael T. Lanagan; Carlo G. Pantano; Seong H. Kim
This study employed thermal poling at 200°C as a means to modify the surface mechanical properties of soda lime silica (SLS) glass. SLS float glass panels were allowed to react with molecules constituting ambient air (H2O, O2, N2) while sodium ions were depleted from the surface region through diffusion into the bulk under an anodic potential. A sample poled in inert gas (Ar) was used for comparison. Systematic analyses of the chemical composition, thickness, silicate network, trapped molecular species, and hydrous species in the sodium‐depleted layers revealed correlations between subsurface structural changes and mechanical properties such as hardness, elastic modulus, and fracture toughness. A silica‐like structure was created in the inert gas environment through restructuring of Si–O–Si bonds at 200°C in the Na‐depleted zone; this occurred far below Tg. This silica‐like surface also showed enhancement of hardness comparable to that of pure silica glass. The anodic thermal poling condition was found so reactive that O2 and N2 species can be incorporated into the glass, which also alters the glass structure and mechanical properties. In the case of the anodic surfaces prepared in a humid environment, the glass showed an improved resistance against crack formation, which implies that abundant hydrous species incorporated during thermal poling could be beneficial to improve the toughness.
Ferroelectric properties of Li‐doped BaTiO3 ceramics J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-19 Qiwei Lou; Xue Shi; Xuezheng Ruan; Jiangtao Zeng; Zhenyong Man; Liaoying Zheng; Chul Hong Park; Guorong Li
We prepared 3 kinds of Li+‐doped BaTiO3 ceramics by the solid‐state reaction method: (i) (Ba1−xLix)TiO3−x/2 having A‐site Li+, (ii) Ba(Ti1−xLix)O3−3x/2 having B‐site Li+, and (iii) x/2 Li2CO3+BaTiO3 mixed one, for which we investigated the stable site of Li. The density of all prepared ceramics is above 95%. The results show that the lattice structure, the grain size, and the electric properties of Li+‐doped BaTiO3 ceramics are dependent on Li+ site. According to the increase in Li content, the cell volume of Ba1−xLixTiO3−x/2 decreases, but that of BaTi1−xLixO3−3x/2 increases. That of x/2Li2CO3+BaTiO3 decreases by the small addition of Li, but increases by the large addition of Li. All Li+‐doped ceramics show antiferroelectric‐like double hysteresis loops. The shape of loops and the dielectric properties are also dependent on the Li site. We suggest that the role of oxygen vacancy accompanied by the Li‐doping is important. By comparison with the results of 3 type ceramics, it is concluded that at x/2Li2CO3+BaTiO3 ceramics, the Li+ prefers to favorably substitute Ba2+ at A site for the low concentration of Li but its location was changed to Ti4+ site for the high concentration of Li.
Computational study of impact of composition, density, and temperature on thermal conductivity of amorphous silicon boron nitride J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-19 Atreyi Dasmahapatra; Peter Kroll
We study thermal conductivity (κ) of amorphous silicon boron nitride (a‐SiBN) for different compositions and densities as a function of temperature using density functional theory (DFT) calculations and equilibrium molecular dynamic (MD) simulations. Our library of amorphous structures consists of network models comprising 100‐200 atoms and large‐scale models with up to 57 000 atoms generated using the empirical Marian‐Gastreich two‐body potential. Crystalline structures within the Si3N4‐BN system are considered as well. We use 2 distinct approaches to compute thermal conductivity of a‐SiBN. To estimate κ in the high‐temperature limit we feed Clarke's phenomenological model with elasticity data obtained by DFT calculations. We further perform equilibrium MD simulations and apply the Green‐Kubo method. This approach shows decrease of κ with increasing temperature and provides results at high temperatures that agree with results derived within Clarke's model. We find that κ of a‐SiBN depends on composition and increases as the BN content in the structure increases. The effect is pronounced at low temperature but almost vanishes at high temperature. Furthermore, thermal conductivity depends on density and porosity, with a linear relation between κ and density.
Electrophoretic deposition of titanium nitride coatings J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-19 Majid Kavanlouei; Alireza Akbari
Electrophoretic deposition of the titanium nitride (TiN) coatings from suspensions prepared by dispersion of TiN particles in triethanolamine (TEA) containing butanol medium was studied. Effects of the TiN particles concentration (CTiN) on the weight of the deposited coatings, triethanolamine concentration (CTEA=0.25, 0.5, 0.75, and 1 mL/L) on the Zeta potential of the TiN particles, suspension electrical conductivity and pH, as well as effects of the deposition voltage (Vd=60, 90, and 120 V) and time (td =1, 2, and 3 minutes) on the microstructure and thickness of the deposited coatings were investigated. Variations in deposition current density, effective deposition voltage, electrical resistance, and deposited coating weight versus deposition time were recorded. The morphology of the as‐dried coatings was studied using Scanning Electron Microscope (SEM). The results indicated that by increasing the CTiN the weight of deposits increases linearly up to 40 g/L. For suspensions containing CTiN=40 g/L, the optimum CTEA is obtained to be 0.5 mL/L leading to Zeta potential of 43.25 mV. Uniform and crack‐free as‐dried coatings obtained at Vd and td of 90 V and 2 minutes, respectively.
Optical thermometry of Sm3+ on laser‐induced local heating for precipitation of PbS quantum dots in glasses J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-19 Byoungjin So; Jong Heo
Fluorescence intensity ratios (FIRs) of the 640 nm and 602 nm emissions from Sm3+ were recorded at various temperatures T to identify the temperature increases ΔT associated with laser‐induced local heating of Ag nanoparticles. The FIRs increased as intensities of the excitation beam from a 532‐nm continuous‐wave laser increased. Estimated T of the irradiated region increased to as high as 586°C when at laser irradiation of 1.5 W on the surface containing Ag nanoparticles. Local heating due to the surface plasmon resonance of Ag nanoparticles is a main reason for the ΔT that eventually leads to precipitation of PbS quantum dots in glasses.
Intergranular amorphous films formed by DC electric field in pure zirconia J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-19 Nobuhiro Morisaki; Hidehiro Yoshida; Tetsuro Kobayashi; Tomoharu Tokunaga; Takahisa Yamamoto
It is difficult to obtain pure ZrO2 sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO2 can be consolidated without shattering using flash‐sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High‐resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash‐sintering at 1350°C for 5 minutes. Monoclinic ZrO2 flash‐sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash‐sintering.
Fabrication and properties of in situ reduced graphene oxide‐toughened zirconia composite ceramics J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Zhaoyubo Zeng; Yunzhong Liu; Weiping Chen; Xiaoqiang Li; Qifan Zheng; Kaili Li; Ruiran Guo
Graphene oxide and zirconia powders were mixed using a colloidal coating route. In situ reduced graphene oxide‐toughened zirconia ceramics were prepared by spark plasma sintering. Their microstructure, mechanical properties, and toughening mechanisms were investigated. The results show that graphene oxide can be easily reduced in situ during sintering and that it disperses homogeneously within the zirconia substrate. Compared with the toughness of 3 mol.% yttria‐stabilized zirconia, the fracture toughness of in situ reduced graphene oxide‐toughened zirconia increased by up to 175% (from ~6.07 to ~10.64 MPa·m1/2) at 0.09 wt.% graphene oxide with a small increase in hardness. The improvement is more significant than that of prereduced graphene oxide‐toughened cases, and it is associated with the formation of a C‐O‐Zr bond at the interface in addition to conventional toughening mechanisms.
Effect of optical basicity on the stability of yttria‐stabilized zirconia in contact with molten oxy‐fluoride flux J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Jicheng Guo; Thomas Villalon; Uday Pal; Soumendra Basu
Solid oxide membrane (SOM) electrolysis process can produce high‐purity silicon from SiO2 dissolved in molten oxy‐fluoride flux at elevated temperatures. Yttria‐stabilized zirconia (YSZ), the preferred material for the oxygen‐conducting membrane for this application, is found to degrade over time upon exposure to the silica‐containing molten oxy‐fluoride flux. This YSZ degradation is caused by the acidity of the dissolved silica, especially when the optical basicity of the molten flux is lower than that of the yttria present in the YSZ membrane. To counteract this mismatch, the addition of CaO, a basic oxide, to the flux can adjust the optical basicity of the flux and successfully mitigate the YSZ membrane degradation. The detailed correlation between the rate of YSZ membrane degradation and the optical basicity of the flux is investigated by systematically testing a series of flux compositions. It is found that as the oxide optical basicity in the flux approaches that of the yttria in the YSZ, the degradation of the YSZ membrane is mitigated and essentially vanishes when the flux acidity with respected to the yttria is neutralized. This approach provides a guideline for eliminating membrane degradation during the production of silicon using the SOM electrolysis process.
Large electric‐field‐induced strain and enhanced piezoelectric constant in CuO‐modified BiFeO3‐BaTiO3 ceramics J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Weiwei Gao; Jing Lv; Xiaojie Lou
In this work, we fabricated the (1‐x)BiFeO3‐xBaTiO3+y‰ mol CuO ceramics by the modified thermal quenching technique. The pure perovskite phase was formed and a morphotropic phase boundary (MPB) was observed in the ceramics with x = 0.30‐0.33. The addition of CuO can significantly enhance the density of the BiFeO3‐BaTiO3 material. Importantly, an enhanced piezoelectric constant (d33=165 pC/N), a large electric‐field‐induced strain (∆S = 0.54%: peak to peak strain) and a large piezoelectric actuator constant (d33*=449 pm/V) together with a high Curie temperature (TC) of 503°C were observed in the ceramics with x = 0.30 and y = 5. As a result, the enhanced piezoelectricity and large electric‐field‐induced strain could significantly stimulate further researches in BFO‐based ceramics.
α‐Alumina nanospheres from nano‐dispersed boehmite synthesized by a wet chemical route J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Fotini Petrakli; Michalis Arkas; Athena Tsetsekou
In this work, stable aqueous suspensions of nano‐boehmite were developed through a hybrid wet‐chemical route that uses hyperbranched dendritic poly(ethylene)imine (PEI) as template material for boehmite formation aiming at the development of a deagglomerated α‐alumina nanopowder after calcination. The method involves firstly the interaction between the Al precursor and PEI followed by the hydrolysis and polycondensation reactions. The study was aiming to investigate the effect of solids content and pH during reactions on both the stability of the final suspension and the morphology of the resulting nanocrystals. For this purpose, the suspensions were evaluated through viscosity measurements, zeta potential analyses, FT‐IR, DLS and sedimentation studies, whereas after the proper centrifugation, drying, and calcination steps, the as‐received nanocrystals were evaluated through SEM, TEM and XRD studies. In addition, the boehmite nanopowder was studied using Thermogravimetric and Differential Thermal Analysis, whereas its sinterability was evaluated by dilatometric measurements. The investigation showed that the conditions employed affect greatly both the morphology of nanocrystals as well as the dispersion and the stability of the suspensions. The boehmite suspension with the optimum dispersion and stability can lead, after calcination at 1050°C, to a fine deagglomerated α‐alumina nanopowder with a mean size at about 10 nm.
Role of defect structures in stabilization of ferroelectric phase in tin‐substituted lead zirconate titanate J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Adukkadan Anil; Killimangalath Vani; Viswanathan Kumar
This study reports the influence of B‐site acceptor dopants, manganese and copper, on the sequence of phase transformations in antiferroelectric (AFE) Pb(Zr0.60Sn0.30Ti0.10)O3. The sequence of phase transformations below the Curie point have been examined by dielectric, polarization‐electric field and strain‐electric field studies. The parent compound and B‐site Cu2+‐doped composition exhibit the same sequence, FE ← AFE ← MCC with incommensuration in the multicell cubic (MCC) state, whereas in the case of B‐site Mn3+‐doped system, incommensuration is found to be suppressed and only ferroelectric (FE) phase is observed below the Curie point. The underlying mechanism is related to the nature of defect complexes present in the system through detailed Electron Paramagnetic Resonance studies.
Effect of thermal history on high‐valence chromium ion dissolution in merwinite (3CaO·MgO·2SiO2) J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Masanori Suzuki; Norimasa Umesaki; Toshihiro Okajima; Toshihiro Tanaka
Solubility and local structure of transmission elements in calcium silicate compounds has not been well understood. We investigate the local structure of chromium ions dissolved in merwinite (3CaO·MgO·2SiO2) of a monoclinic crystal structure. The acceptance of doping elements into merwinite has not been reported before. We found that chromium ions are soluble in merwinite in air and that chemical valence of the dissolved Cr ions varies with annealing temperature. The absorption edge in the x‐ray absorption near edge structure (XANES) of Cr‐doped merwinite indicated that octahedrally coordinated Cr3+ ions were mainly formed when annealed at 1673 K in air. A pre‐edge peak was also detected, indicating the existence of tetrahedrally coordinated high‐valence Cr ions. Conversely, through annealing of merwinite at 1123 K in air, tetrahedrally coordinated Cr6+ ions were found to be the main form of chromium. XANES spectra simulated by first‐principle calculations were used to explain the structural features in the observed spectra. We propose the coexistence of Cr3+ ions in octahedral Mg2+ sites and high‐valence Cr ions in tetrahedral Si4+ sites. In addition, a change in the chromium ion oxidation state in tetrahedral coordination sites was suggested by XANES spectroscopy of Cr‐doped merwinite synthesized at 1673 K and reannealed at 1123 K.
Preparation of γ‐aluminum oxynitride phosphor with Eu doping by direct nitridation in ammonia and postannealing J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-20 Li Zhang; Huan Luo; Linan Zhou; Qian Liu; Junfeng Li; Wentao Zhang
γ‐aluminum oxynitride (γ‐AlON) with spinel structure has attracted much attention for structural and functional application. γ‐AlON powders were successfully prepared by direct nitridation method of Al/Al(OH)3 starting mixture in ammonia and then calcined at high temperature. XRD, SEM, TEM, EDX, and photoluminescence were conducted to investigate the detail procedure and the optical properties of AlON: xEu phosphors. Nitrogen was introduced by the nitridation of metallic aluminum, appropriate Al amount and nitridation condition was necessary to obtain phase pure γ‐AlON. The as‐prepared AlON powders exhibited multifaceted grain morphology with fine particle size (1‐5 μm). Eu2O3 activator was reduced and transformed to EuAl12O19 by reaction with alumina, which remained in the product when x > 0.25%. Under 331 nm excitation, AlON: xEu phosphors exhibited emission bands of 475 and 410 nm. 475 nm band reached a plateau at x = 0.25% due to the solubility of Eu2+ ion in AlON, whereas 410 nm band showed a linear increase in intensity with Eu2+ doping amount, which was believed to be the contribution of EuAl12O19. The present approach combination of direct nitridation in ammonia and postcalcination process showed potential application for AlON ceramic and AlON phosphors.
Effect of graphite content on properties of B4C‐W2B5 ceramic composites by in situ reaction of B‐Gr‐WC J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-22 Deng Pan; Shufeng Li; Xin Zhang; Bo Pan; Shengyin Zhou; Yabo Fu; Katsuyoshi Kondoh
Strip‐shaped W2B5 reinforced B4C ceramic composites were prepared via in situ reaction of boron(B)‐graphite(Gr)‐WC system by powder metallurgy (P/M). In order to study the effect of the graphite content on the properties of the as‐fabricated ceramic composites, the powder mixture of B‐Gr‐WC with various amounts of Gr powder were blended and consolidated by spark plasma sintering (SPS). The sintering parameters were shown as following: sintering pressure was set as 30 MPa; The three‐step sintering temperature was 1100‐1550‐1700°C and the duration time was set as 5‐5‐6 minutes, respectively. In situ formed strip‐shaped W2B5 particles were dispersed homogeneously in B4C matrix, which resulted in a remarkable improvement on the fracture toughness and mechanical properties. Appropriate 5vol% residual Gr in the composite shows positive effect on the mechanical properties which achieved an optimal counter‐balance of fracture toughness and hardness, the relative density was 99.8%, the Vickers hardness can reach 30.2 GPa, and the fracture toughness was 11.9 MPa·m1/2 when the sintering temperature was set at 1700°C.
Thermal and optical properties of La2O3–Ga2O3– (Nb2O5 or Ta2O5) ternary glasses J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-22 Kohei Yoshimoto; Atsunobu Masuno; Motoi Ueda; Hiroyuki Inoue; Hiroshi Yamamoto; Tatsunori Kawashima
La2O3–Ga2O3–M2O5 (M = Nb or Ta) ternary glasses were fabricated using an aerodynamic levitation technique, and their glass‐forming regions and thermal and optical properties were investigated. Incorporation of adequate amounts of Nb2O5 and Ta2O5 drastically improved the thermal stabilities of the glasses against crystallization. Optical transmittance measurements revealed that all the glasses were transparent over a wide wavelength range from the ultraviolet to the mid‐infrared. The refractive indices of the glasses increased and the Abbe number decreased upon substituting Ga2O3 with Nb2O5, and the decrease in the Abbe number was significantly suppressed when Ta2O5 was incorporated into the glass. As a result, excellent compatibility between high refractive index and lower wavelength dispersion was realized in La2O3–Ga2O3–Ta2O5 glasses. Analysis based on the single‐oscillator Drude–Voigt model provided more systematical information and revealed that this compatibility was due to an increase in the electron density of the glass.
Defect suppression in CaZrO3‐modified (K, Na)NbO3‐based lead‐free piezoceramic by sintering atmosphere control J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-22 Hao‐Cheng Thong; Qi Li; Mao‐Hua Zhang; Chunlin Zhao; Kevin Xiu Huang; Jing‐Feng Li; Ke Wang
During high‐temperature crystal growth, lattice defects will inevitably form inside piezoelectric materials, which can be a hindrance for performance optimization. Through appropriate atmosphere control during sintering, defect levels inside the piezoelectric material can be regulated. Herein, CaZrO3‐modified (K, Na)NbO3‐based lead‐free piezoelectric ceramics with a nominal composition of 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3‐0.05CaZrO3 are produced by sintering in an oxygen‐rich atmosphere. Compared with an air‐sintered sample, the piezoelectric constant of the oxygen‐sintered sample has greatly improved 15% up to 390 pC/N, which is comparable to commercial lead‐based counterparts. In addition, the planar electromechanical coupling factor kp is enhanced from 0.46 to 0.52. A qualitative model related to defect engineering is proposed to support the experimental observations. Our results indicate the feasibility of purposely optimizing the piezoelectric performance by sintering atmosphere control.
Theoretical investigation of anisotropic mechanical and thermal properties of ABO3 (A=Sr, Ba; B=Ti, Zr, Hf) perovskites J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-26 Yuchen Liu; Bin Liu; Huimin Xiang; Yanchun Zhou; Hongqiang Nian; Hongfei Chen; Guang Yang; Yanfeng Gao
As promising TBC (thermal barrier coating) candidates, perovskite oxides own designable properties for their various options of cations and structural diversity, but limited comprehensions of structure‐property relationship delay their engineering applications. In this work, mechanical/thermal properties of ABO3 (A=Sr, Ba; B=Ti, Zr, Hf) perovskites and their anisotropic nature are predicted employing density functional theory. Their theoretical minimum thermal conductivities range from 1.09 to 1.74 W·m−1·K−1, being lower than Y2O3 partially stabilized ZrO2. Reduced thermal conductivities up to 16% along particular directions are reached after considering thermal conductivity anisotropy. All compounds own high hardness while SrZrO3, SrHfO3, and BaHfO3 possess well damage tolerance. We found that small electronegativity discrepancy leads to big anisotropy of chemical bond, Young's/shear moduli and thermal conductivities, together with good damage tolerance. These results suggest that the next generation TBCs with extra low thermal conductivity should be achieved through combining material design and orientation‐growth tailoring.
Enhanced microwave‐absorbing property of precursor infiltration and pyrolysis derived SiCf/SiC composites at X band: Role of carbon‐rich interphase J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-26 Lingwei Yang; Haitao Liu; Mei Zu
Ceramic matrix composites (CMCs) can be microwave‐absorbent when endowing the composite constituents with proper dielectric properties. In this work, we report a new method to enhance the microwave‐absorbing property of CMCs by in situ fabrication of a carbon‐rich interphase at the fiber/matrix interface. This was achieved in a SiC fiber reinforced SiC matrix (SiCf/SiC) composite fabricated by precursor infiltration and pyrolysis (PIP). We found that as the PIP temperature increased from 800 to 1000°C, the microwave‐absorbing property of the SiCf/SiC composite was significantly enhanced at X band, which also surpassed those of the SiC fiber and monolithic SiC ceramic fabricated at the same temperature. The dominant mechanism was studied by decoupling the effect of individual SiC fibers, SiC matrix, and fiber/matrix interface. The results showed that the SiC fiber and SiC matrix were barely microwave‐absorbent, due to their low dielectric losses. The microwave‐absorbing mechanism was finally ascribed to the fiber/matrix interface, which was carbon‐rich, containing Si and O elements. The interphase showed a conductivity that was superior to that of the fiber and the matrix, and mainly dominated the dielectric property of the overall composite. The results highlight the role of carbon‐rich interphase on the microwave‐absorbing property of CMCs.
Energetics of bulk lutetium‐doped Ce1−xLuxO2−x/2 compounds J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-26 Nimrod Yavo; Geetu Sharma; Giora Kimmel; Igor Lubomirsky; Alexandra Navrotsky; Ori Yeheskel
The dissolution enthalpy, ΔHDS, and the formation enthalpy, ΔHf,ox, of bulk lutetium‐doped cerium oxide (LuDC) were studied at 701°C in molten sodium molybdate. For the composition range of Ce1−XLuXO2−X/2, studied 0 ≤ X ≤ 0.3, the ΔHDS decreases linearly and smoothly with lutetium content according to ΔHDS, kJ/mol = 73.5(1.0)−165.1(5.5)·x). The enthalpy of formation, ΔHf,ox, becomes more exothermic linearly with lutetium content. No anomaly in ΔHf,ox is observed at low Lu2O3 concentration as reported previously for several other rare‐earth‐doped ceria systems, suggesting possible differences in clustering and microstructure, which may also be related to difference in processing conditions.
Volatility diagram of ZrB2‐SiC‐ZrC system and experimental validation J. Am. Ceram. Soc. (IF 2.841) Pub Date : 2018-02-26 Ying Lu; Ji Zou; Fangfang Xu; Guo‐Jun Zhang
A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB2 and SiC, the volatility diagram of a ternary ZrB2‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB2‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed.
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