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  • Phase stability, microstructure, and dielectric properties of quaternary oxides In12Ti10A2BO42 (A: Ga or Al; B: Mg or Zn)
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-07
    Felipe Francisco Castillón‐Barraza; Alejandro Durán; Mario Humberto Farías; Francisco Brown; Guillermo Tiburcio Munive; Fernando Cubillas; Victor Emmanuel Alvarez‐Montaño

    Quaternary oxides In12Ti10Al2MgO42 (ITAM), In12Ti10Al2ZnO42 (ITAZ), In12Ti10Ga2MgO42 (ITGM), and In12Ti10Ga2ZnO42 (ITGZ), were synthesized by the solid‐state reaction method and the dielectric behavior is reported. The samples were submitted to different sintering temperatures (1473‐1773 K) for 24 hours and the phase stability and microstructure were analyzed by X‐ray powder diffraction (XRD) and scanning electron microscopy (SEM). It is found that the phase decomposition occurs above of 1573 K. Microstructure images showed an increase in the grains size as sintering temperature was raised. The dielectric permittivity as a function of temperature and frequency showed acceptable dielectric constant (15‐30) and low dielectric loss (tan δ << 1) values in a wide range of temperature. The band gap obtained by the optical spectrum analysis is about 3.5 eV indicating good dielectric insulating compounds. Furthermore, the electrical conductivity, the activation energy, and the conduction mechanisms are analyzed and discussed in a whole range of temperature. The good dielectric values, ε′ (15‐20) and tan δ (~0.004), and their behavior (almost independent of the frequency and temperature) almost constant within a wide range of temperature make these quaternary oxides interesting in electroceramic applications.

    更新日期:2018-07-18
  • Thermodynamic properties of Si(OH)4(g) based on combined experimental and quantum chemistry data
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    Andrey V. Plyasunov; Alexander S. Zyubin; Tatiana S. Zyubina

    The volatility of silicon‐based ceramics in combustion environments is primarily controlled by the formation of gaseous Si(OH)4. The heat capacities and entropies of this species at 298.15‐2100 K and P = 0.1 MPa have been studied with the B3LYP density functional theory for the 6‐311+G(d,p) basis set in different approximations: a harmonic oscillator, an anharmonic oscillator, and with corrections for hindered rotors. Experimentally based Gibbs energies of Si(OH)4(g) at 424‐1661 K have been employed to evaluate the Gibbs energy of formation, , and the entropy, , of Si(OH)4(g) at = 298.15 K and P = 0.1 MPa. We found that the QC and “experimental” values are very close for the harmonic and anharmonic oscillator approximations, but not for the “hindered rotor” approximation. This conclusion is also supported by calculations of the OH rotational energy for Si(OH)4 molecule, where the potential barrier was found to exceed 12 kJ/mol. Finally, we recommend the thermodynamic properties of Si(OH)4 in the ideal gas state at P = 0.1 MPa over the temperature range of 298‐2100 K.

    更新日期:2018-07-16
  • Low gallium‐content, dysprosium III‐doped, Ge–As–Ga–Se chalcogenide glasses for active mid‐infrared fiber optics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-07
    Zhuoqi Tang; David Furniss; Nigel C. Neate; Trevor M. Benson; Angela B. Seddon

    A systematic investigation is presented, for the first time, of a 1000 ppmw (parts per million, by weight) Dy3+‐doped Ge–As–Ga–Se chalcogenide glass series, with a fixed low Ga content of 1 atomic% (at. %), suitable for active mid‐infrared fiber optics. Seven glasses constitute the series, which have increasing average coordination number from 2.49 to 2.61, in steps of 0.02, with the GeSe2, As2Se3, and Ga2Se3 stoichiometries kept. Glass formation is confirmed using X‐ray diffraction and differential scanning calorimetry. Fourier transform infrared spectroscopy is reported for the series. Parallel plate viscometry enables prediction of fiber‐drawing temperatures and, with differential thermal analysis, determines the potential for fiber fabrication. X‐ray diffraction of samples after parallel‐plate viscometry shows that Ge25As9Ga1Se65 (at. %) alone, in the glass‐series, devitrifies to form the single‐crystalline phase: monoclinic‐GeSe2; scanning electron microscope imaging suggests that this phase is both surface and bulk grown. Overall, the recommended host glass at. % compositions for doping with rare‐earth ions and drawing to active mid‐infrared fiber are: Ge17.5As18Ga1Se63.5, Ge15As21Ga1Se63, and Ge12.5As24Ga1Se62.5.

    更新日期:2018-07-16
  • Thermo‐structural characterization of (As2Se3)100‐x‐(As2Te3)x glasses for infrared optics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-06
    Daniela Brandová; Roman Svoboda

    Differential scanning calorimetry was used to study thermokinetic behavior of (As2Se3)100−x(As2Te3)x infrared glasses (seven compositions along the pseudo‐binary were investigated). Glass‐transition kinetics was described in terms of the Tool‐Narayanaswamy‐Moynihan model and the relaxation motions were interpreted using Raman spectroscopy data. The enthalpy relaxation kinetics was found to be absolutely uninfluenced by changing Se/Te ratio. On the other hand, DSC crystallization behavior changed significantly with increasing As2Te3 content: Te‐rich compositions show marked affinity toward crystallization, whereas in case of the compositions with 0‐34 mol.% As2Te3 crystal growth is significantly suppressed. X‐ray diffraction analysis indicated presence of monoclinic As2Se3, As2Te3, and As2Se(Te)3 phases. The complex crystallization behavior occurring in case of the Te‐rich compositions was described by superposition of two autocatalytic kinetic signals. Based on the information from infrared microscopy the two overlapping crystallization processes can be attributed to the respective formations of spherulitic and needle‐shaped crystallites. Best glass stability was identified in case of the (As2Se3)66(As2Te3)34 composition, which appears to be a potential candidate for optic applications in the far‐infrared region of the spectrum.

    更新日期:2018-07-16
  • Growth of noncongruent LiTaO3 crystal by chemical vapor phase equilibration using niobium‐based two‐phase powder
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-06
    Chao‐Yang Zhang; Xiao‐Fei Yang; Zi‐Bo Zhang; Wing‐Han Wong; Edwin Yue‐Bun Pun; De‐Long Zhang

    Noncongruent nearly stoichiometric/lithium‐deficient LiTaO3 crystal was grown by lithium‐rich/lithium‐deficient chemical vapor‐phase‐equilibration (VPE) technique, and low‐cost two‐phase powder LiNbO3‐Li3NbO4 or LiNbO3‐LiNb3O8 instead of expensive LiTaO3‐Li3TaO4 or LiTaO3‐LiTa3O8 was used in the VPE experiments. The LiTaO3 crystalline phase in the lithium‐rich/lithium‐deficient crystal was confirmed by X‐ray analysis. The lithium‐rich/lithium‐deficient VPE‐induced lithium‐oxide (Li2O) molar content increase/reduction was measured as a function of VPE duration using gravimetric method, and empirical relations between them are presented for both cases. We show that both the lithium‐rich and lithium‐deficient VPE techniques based on the two‐phase powder LiNbO3‐Li3NbO4 or LiNbO3‐LiNb3O8 can be successfully used to adjust the lithium‐oxide content in a LiTaO3 crystal and produce a noncongruent nearly stoichiometric or lithium‐deficient crystal plate with desired lithium‐oxide content.

    更新日期:2018-07-16
  • Investigations on ferromagnetism of Li and Mn codoped LiZnN by first‐principles calculations
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-03
    Yan Cui; Ji Guo Zhu; Hua Long Tao; Shi Min Liu; Yun Zhuo Lv; Ming He; Bo Song; Zhi Hua Zhang

    Electronic structures and magnetic properties of Li and Mn codoped LiZnN systems were investigated by using the first‐principles calculations. Mn‐doped LiZnN systems preferred the antiferromagnetic states, while Mn/Li codoped LiZnN systems were ferromagnetic and more stable than the Mn‐doped LiZnN systems. The magnetic moments mostly arose from Mn‐3d states, which hybridized with N‐2p and Li‐2s states near the Fermi level. The ferromagnetic coupling was suggested to be attributed to the p‐d hybridization by the Mn–N–Mn chain. The results indicated that nonstoichiometry of Li was very essential for the ferromagnetism of Li(Zn,Mn)N system.

    更新日期:2018-07-16
  • Comparison of AC to DC current sources for field‐assisted sintering of aluminum alloy 5083 powder
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 16 July 20
    Frank Kellogg; Michael Kornecki; Selva Vennila Raju; Brandon McWilliams; Raymond Brennan

    The viability of resistively sintering aluminum 5083 powder with an alternating current (AC) was explored under both as‐atomized and cryomilled conditions. Samples were processed under an AC field and a direct current (DC) field. Samples processed by both methods exhibited similar microstructures and densities at different die wall temperatures. For as‐atomized powders, similar densities (99% of theoretical [TD]) were achieved at die wall temperatures of 564°C under DC fields and 525°C under AC fields. For cryomilled 5083, densities up to 90% of theoretical were achieved at 550°C under DC fields, while density values 99% of TD were achieved at 500°C under AC fields. Based on these findings, it has been determined that AC fields can be used as an alternative to SPS for achieving optimal density.

    更新日期:2018-07-16
  • Direct observation of Eu atoms in AlN lattice and the first‐principles simulations
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-29
    Liang‐Jun Yin; Sheng‐Hui Zhang; Hui Wang; Xian Jian; Xin Wang; Xin Xu; Ming‐Zhen Liu; Chang‐Ming Fang

    Rare‐earth metal (Eu)‐doped aluminum nitride has potential application as luminescence materials due to its unusual mechanical and physical properties, as well as high chemical stability. Here, we investigate the energetics, local structure and optical and electronic properties by means of a combination of experimental observations (XDR, SEM, HR‐TEM and XANES) and first‐principles simulations. Present study has revealed that Eu ions are likely to be co‐doped with O in the form of Eu–O pairs. Eu ions or Eu–O pairs favor participation at the surfaces of the AlN crystallites. Our analyses show dependences of the Eu valence and electronic/optical properties on the local chemical composition and structure. The obtained information helps us to realize tuning of the optical properties of the luminescent materials via composition and site occupation modification.

    更新日期:2018-07-16
  • Insights into the ternary eutectic microstructure formed in inter‐colony regions in Al2O3‐ZrO2(Y2O3) system
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-14
    Lian‐sheng Fu; Guo‐qing Chen; Xue‐song Fu; Wen‐long Zhou

    Al2O3/ZrO2(Y2O3) pseudo‐binary eutectic melt may solidify in the dendritic form at the low G/v ratio (thermal gradient G divided by growth rate v) associated with the Y2O3 microsegregation along the monovariant line. Especially, in the system with high Y2O3 content (≥4.5 mol%), an Al2O3/ZrO2/YAG ternary eutectic in‐situ microstructure growing with a “plane front” will appear at the boundaries of dendritic structures. The volume fraction of the ternary eutectic increases with the increase of Y2O3 content. It was found that the amount of ternary eutectic can be well predicted by the Scheil equation (or nonequilibrium lever rule).

    更新日期:2018-07-15
  • Crystallization behavior of iron‐ and boron‐containing nepheline (Na2O●Al2O3●2SiO2) based glasses: Implications on the chemical durability of high‐level nuclear waste glasses
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-13
    Ambar Deshkar; Mostafa Ahmadzadeh; Alex Scrimshire; Edmund Han; Paul A Bingham; Donna Guillen; John McCloy; Ashutosh Goel

    The present study focuses on understanding the relationship between iron redox, composition, and heat‐treatment atmosphere in nepheline‐based model high‐level nuclear waste glasses. Glasses in the Na2O–Al2O3–B2O3–Fe2O3–SiO2 system with varying Al2O3/Fe2O3 and Na2O/Fe2O3 ratios have been synthesized by melt‐quench technique and studied for their crystallization behavior in different heating atmospheres – air, inert (N2) and reducing (96%N2‐4%H2). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass‐ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometer (VSM). While iron coordination in glasses and glass‐ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass‐ceramics produced as a result of heat treatment in air and N2 atmospheres developed a golden/brown colored iron‐rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Further, while this iron‐rich layer was observed in glass‐ceramics with varying Al2O3/Fe2O3 ratio, it was absent from glass‐ceramics with varying Na2O/Fe2O3 ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron‐rich layer on the surface of glass‐ceramics on the chemical durability of high‐level nuclear waste glasses have been discussed.

    更新日期:2018-07-14
  • Thermo‐mechanical properties of mullite ceramics: New data
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-13
    Thomas F. Krenzel; Jürgen Schreuer; Derek Laubner; Michel Cichocki; Hartmut Schneider

    Coefficients of elastic stiffnesses and thermal expansion of hot isostatically pressed, reaction‐ sintered and technical fused mullite ceramics were measured between 100 K and 1673 K in comparison to single crystal mullite employing resonant ultrasound spectroscopy and dilatometry, respectively. Additionally, chemical and phase compositions, and the microstructure of the ceramics were studied using X‐ray diffraction techniques and scanning electron microscopy. Our studies revealed that despite polycrystallinity and slight porosity of up to 1.6% the elastic behavior of the hot isostatically pressed ceramics are near to ideal aggregate elastic properties of mullite single crystal, e. g. their bulk moduli fit within 0.7% to B = 170.0 GPa of single crystal mullite. On the other hand, with B = 155 GPa the reaction sintered mullite behaves significantly softer. The difference can be explained with more tight grain‐to‐grain contacts in hot isostatically pressed ceramics as compared to reaction sintered materials. The thermal expansion of both types of ceramics almost coincides with the corresponding averaged behavior of single crystal mullite. For example, between 573 K and 1273 K the volume expansion coefficients of all these materials are (18.0±0.3)·10‐6K−1. Obviously, the microstructural features are less important for the macroscopic thermal expansion. Due to heterogeneous microstructure and high α‐alumina and zirconia contents the corresponding properties of fused‐mullite refractory deviate strongly from those of the other mullite materials.

    更新日期:2018-07-14
  • Dense Freeze‐cast Li7La3Zr2O12 Solid Electrolytes with Oriented Open Porosity and Contiguous Ceramic Scaffold
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-13
    Lucienne Buannic; Maninpat Naviroj; Sarah M. Miller; Jakub Zagorski; Katherine T. Faber; Anna Llordés

    Freeze casting is used for the first time to prepare solid electrolyte scaffolds with oriented porosity and dense ceramic walls made of Li7La3Zr2O12 (LLZO), one of the most promising candidates for solid state battery electrolytes. Processing parameters ‐ such as solvent solidification rate, solvent type, and ceramic particle size ‐ are investigated, focusing on their influence on porosity and ceramic wall density. Dendrite‐like porosity is obtained when using cyclohexane and dioxane as solvents. Lamellar porosity is observed in aqueous slurries resulting in a structure with the highest apparent porosity and densest ceramic scaffold but weakest mechanical properties due to the lack of interlamellar support. The use of smaller LLZO particle size in the slurries resulted in lower porosity and denser ceramic walls. The intrinsic ionic conductivity of the oriented LLZ ceramic scaffold is unaffected by the freeze casting technique, providing a promising ceramic scaffold for polymer infill in view of designing new types of ceramic‐polymer composites.

    更新日期:2018-07-14
  • Hydration mechanisms of hybrid cements as a function of the way of addition of chemicals
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-13
    Ana Fernández‐Jiménez; Inés Garcia‐Lodeiro; Olga Maltseva; Angel Palomo

    In this study fly ash contents of up to 50 % were used to produce binders with mechanical strength greater than or equal to type CEM I 42.5R cement (reference). The specific aim pursued in this research was to determine whether the procedure used to add the chemical activator (in solid or liquid form) can affect strength development in the cement or the nature of the reaction products formed. Two experimental procedures were deployed: a) the chemical activator was dissolved in the mixing water; b) the chemical activator was ground, as a solid, into the fly ash. The solid state method induced slightly higher mechanical strength than when the activator was dissolved in hydration water. The way the chemical activator was added affected not only early age reaction kinetics, but also the nature and composition of the reaction products, the liquid state favouring the formation of phases AFt and AFm.

    更新日期:2018-07-14
  • Improvement of Reliability and Dielectric Breakdown Strength of Nb‐doped PZT Films via Microstructure Control of Seed
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-13
    Song Won Ko; Wanlin Zhu; Charalampos Fragkiadakis; Trent Borman; Ke Wang; Peter Mardilovich; Susan Trolier‐McKinstry

    A Pb(Zr,Ti)O3 (PZT) seed layer without Pb‐deficient defective areas was developed to improve the dielectric breakdown strength and lifetime of thin film piezoelectric actuators. The proportion of defective area in the seed layers was reduced by adjusting the amount of Pb excess in the solution, combined with implementation of a dense, large‐grained (> 200 nm) Pt bottom electrode. The optimal Pb excess amount in the solution was about 20 at%; seeding was improved when a slightly Ti‐rich composition (relative to the morphotropic phase boundary) was utilized. It was found that the dielectric breakdown strength and lifetime of PZT films improved as the proportion of visible defective area on the PZT seed layer decreased. Dielectric breakdown strength increased from approximately 300 kV/cm to 1 MV/cm. The lifetime, characterized by highly accelerated lifetime testing (HALT), was increased 60 times by reducing the fraction of defective area. The activation energy (Ea) and voltage acceleration factor (N) for failure of devices (e.g. patterned PZT films) were 1.12 ± 0.03 eV and 4.24 ± 0.07 respectively.

    更新日期:2018-07-14
  • Laser‐induced metallization of porous Si3N4 ceramic and its brazing to TiAl alloy
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-13
    Y. X. Zhao; H. Bian; W. Fu; Y. Hu; X. G. Song; D. Liu

    The poor wettability of traditional brazing filler alloys on the surface of ceramics always lead to the formation of defects in the joints and weaken the bonding strength eventually, especially the porous ceramics. Metallization on ceramics is an effective way to improve the wettability. In this work, laser‐induced cladding process was applied to metalize the surface of porous Si3N4 ceramic, and the traditional AgCu eutectic filler alloy can wet on the metalized surface completely. The metalized porous Si3N4 ceramic brazed to TiAl alloy successfully using AgCu filler alloy. The interfacial microstructure and mechanical property of the porous Si3N4/ TiAl alloy brazed joint was significantly improved by the novel laser‐induced metallization process.

    更新日期:2018-07-14
  • Quantitative prediction of the structure and properties of Li2O–Ta2O5–SiO2 glasses via phase diagram approach
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-29
    Linling Tan; John C. Mauro; Jie Peng; Xinlei Yang; Mingying Peng

    Tantalum silicate glasses serve as laser host materials to take advantage of their high refractive index and the ability to tailor their physical properties in the design of high‐performance photonic and photoelectric components. However, successful attainment of feature control in tantalum‐doped materials remains a longstanding problem due to the limited understanding of local structure around the tantalum ions, a problem that lies at the heart of predicting the micro‐ and macroscopic properties of these glasses. Herein, we present a novel approach for predicting the local structural environments in tantalum silicate glass based on a phase diagram approach. The phase relations and glass formation region of Li2O–Ta2O5–SiO2 ternary systems are explored to calculate the structure and additive physical properties of lithium tantalum silicate glasses. These measured and calculated results are in good quantitative agreement, indicating that the phase diagram approach can be applied broadly to Li2O–Ta2O5–SiO2 ternary glass systems. Using the phase diagram approach, the local structure of tantalum can be directly obtained. Each Ta atom is surrounded by six atoms, and its polyhedron, the TaO6 octahedron, bonds through oxygen to Li and Ta. As a network modifier, Ta5+ depolymerizes the silicate glass structure by modulating the local structure of lithium atoms in Li2O–Ta2O5–SiO2 ternary glass system. The compositional dependence of structure in lithium tantalum silicate glasses is quantitatively determined based on the structure of the nearest neighbor congruent compound through the lever rule. These findings offer a precise prediction of tantalum silicate glass properties with quantitative control over local structural environment of the disordered materials.

    更新日期:2018-07-12
  • Significantly improved photoluminescence of the green‐emitting β‐sialon:Eu2+ phosphor via surface coating of TiO2
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-29
    Chenning Zhang; Tetsuo Uchikoshi; Rong‐Jun Xie; Lihong Liu; Yoshio Sakka; Naoto Hirosaki

    The β‐sialon:Eu2+ phosphor particles were successfully coated by TiO2 nanoparticles via the sol‐gel method. The TiO2‐coated β‐sialon:Eu2+ phosphor had a significantly improved photoluminescence (PL) performance under the 365 nm excitation, due to the localized surface plasmon resonance (LSPR) at the interface between the TiO2 coating layer and phosphor surface. The emission intensity of the TiO2‐coated β‐sialon:Eu2+ prepared with the titanium (IV) tetrabutoxide (Ti(OC4H9)4, TTBO):H2O = 1:0.5 volume ratio was dramatically increased by ~24%. When the preparation temperature was 500°C, it was responsible for superior PL intensity by considering the important domination factors of higher anatase content and spherical particle shape of the TiO2 coating layer to the LSPR effect. The coating around the phosphor surface by the TiO2 nanoparticles would be an effective technique to improve the PL efficiency of phosphor for the application in the white light‐emitting diodes (LEDs), by utilizing the LSPR effect of the semiconductor coating layer, instead of conventional metal plasmonic materials.

    更新日期:2018-07-12
  • Phase equilibria, structure, oxygen nonstoichiometry, and thermal expansion of oxides in the 1/2Y2O3–SrO–1/2Fe2O3 system
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-29
    Anastasia S. Urusova; Victoria V. Rudik; Mikhail Yu. Mychinko; Vladimir A. Cherepanov
    更新日期:2018-07-12
  • Effects of CuO on constrained sintering of a polycrystalline TiO2 ceramics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-27
    Cheng‐Fan Lee; Cheng‐Feng Wu; Jau‐Ho Jean

    Effects of CuO on constrained sintering of a polycrystalline TiO2 ceramics have been investigated. The densification temperature of TiO2 is reduced from 1100‐1200°C for pure TiO2 to 900°C with the presence of 0.5‐3 mol% CuO under free sintering. For the samples with 1 mol% CuO, the constrained densification is slowed down, but a high sintered density of >95% at 950°C, which is close to that sintered freely, is still obtained. The above results are caused by the formation of CuO‐rich film at the grain boundaries, which reduces grain‐boundary energy and enhances grain‐boundary migration kinetics of TiO2. To confirm the above findings, molecular dynamics simulation, at which the ratio of grain boundary energy of TiO2 between with and without CuO agrees well with that obtained experimentally, is conducted.

    更新日期:2018-07-12
  • Kinetics of microwave synthesis of AlN by carbothermal‐reduction‐nitridation at low temperature
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-27
    Hideaki Chikami; Jun Fukushima; Yamato Hayashi; Hirotsugu Takizawa

    Aluminum nitride (AlN) was synthesized at 1000‐1400°C from a mixture of alumina and carbon powders using 2.45 GHz microwaves in a N2 atmosphere. High nitridation ratios (>0.90) were obtained in the temperature range 1200‐1400°C. The apparent activation energy of the carbothermal reduction and nitridation (CRN) reaction using 2.45 GHz microwave irradiation was calculated from the nitridation ratio. The value obtained, 79.9 kJ/mol, is 11% of the energy reported for conventional synthesis using α‐Al2O3 as the raw material. This result indicates that 2.45 GHz microwave irradiation could promote the kinetics of the CRN reaction, and AlN could be effectively synthesized at low temperature.

    更新日期:2018-07-12
  • Properties of t‐zirconia prepared by a composite‐assisted nonhydrolytic sol‐gel
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-27
    Christine Warwar Damouny; Christine Khoury; Oz M. Gazit

    High surface area and highly crystalline tetragonal Ni‐Si‐doped ZrO2 samples were prepared using a nonhydrolytic sol‐gel method. The synthesis involved the condensation of zirconium chloride and isopropyl ether in the presence of a chitosan‐based composites containing Ni and Si (Ni@Si‐CS), followed by calcination to remove the CS. The interactions between the zirconium precursor and the Ni@Si‐CS composites were studied and correlated with the effect on morphologies, crystalline structure, phase compositions, and surface area. These parameters were evaluated for different Ni@Si‐CS to ZrO2 mass ratios. It was found that increasing the concentration of Ni@Si‐CS composite led to smaller grains of ZrO2 nanocrystals and an overall material with a higher surface area. The stability of the tetragonal/cubic phases following calcination at 800°C was correlated with the presence of low amounts of Ni2+ in the sublattice of ZrO2 and the presence of the Si‐chitosan, which acted as steric stabilizer.

    更新日期:2018-07-12
  • Fabrication and microstructure characterizations of transparent Er:CaF2 composite ceramic
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-27
    Zuodong Liu; Mengying Jia; Guoqiang Yi; Bingchu Mei; Qiangshan Jing; Peng Liu

    A novel layered transparent Er:CaF2 composite ceramic was proposed in the present study. Er:CaF2 nanoparticles were synthesized by a chemical coprecipitation method. The crystal structures and morphologies of synthesized nanoparticles were performed by X‐ray diffraction (XRD) and field emission scanning electron microscope (FE‐SEM) measurements, respectively. Transparent composite ceramic was fabricated by the combination of multistep dry pressing and hot‐pressed sintering method without any sintering aids or binders. The average grain size of 2% Er‐doped and 5% Er‐doped layers were about 30 and 55 μm, respectively. The thickness of interfacial between two different Er‐doped layers was 150‐200 μm. For a 1.5 mm thickness transparent Er:CaF2 composite ceramic, the optical transmittance reached 44.9% at 500 nm and 53.6% at 1200 nm. The luminescence spectra and thermal conductivities of transparent ceramic specimens were also discussed.

    更新日期:2018-07-12
  • Targeted doping builds a high energy density composite piezoceramics for energy harvesting
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-25
    Xiaole Yu; Yudong Hou; Mupeng Zheng; Jing Yan; Wenxu Jia; Mankang Zhu

    To build piezoceramics with high transduction coefficient (d33 × g33) is the key to improve the power generation capability of piezoelectric energy harvester. Here, a new targeted doping strategy has been proposed to significantly increase the energy density of piezoceramics. Taking the modification of 0.2Pb(Zn1/3Nb2/3)O3‐0.8Pb(Zr0.5Ti0.5)O3 (PZN‐PZT) as an example, dual functions can be achieved by introducing appropriate amount of target‐doped (Zn0.1Ni0.9)TiO3 (ZTN9) based on its pyrolysis characteristics. On the one hand, Ni2+ ions enter the perovskite matrix to replace Zn2+ ions to form equivalent doping; on the other hand, it induces the formation of 0‐3 ZnO/perovskite composite structure, and both of which promote the large increase in d33 × g33 due to the changes in the domain configuration are more conducive to the ferro‐/piezoelectricity. In all compositions, 0.67 mol% ZTN9 added specimen has a maximum value (12 433 × 10−15 m2/N) of the d33 × g33. The cantilever piezoelectric energy harvester fabricated with this material generates up to 4.50 μW/mm3 of power density at 1 g acceleration, which is capable of quickly charging a 47 μF electrolytic capacitor and then lit 135 parallel‐connected commercial blue light‐emitting diodes (LEDs), showing its important application in implementing self‐powered microsensors.

    更新日期:2018-07-12
  • The effect of electrochemical cycling on the strength of LiCoO2
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 11 July 20
    Lin Feng; Xuefeng Lu; Tingting Zhao; Shen Dillon
    更新日期:2018-07-12
  • Improved Dielectric Strength and Energy Storage Density in Ba6‐3xLa8+2xTi18O54 (x = 0.5, 2/3, and 0.75) Ceramics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Jia Yi Yang; Hai Yang Zhou; Xiao Li Zhu; Xiang Ming Chen

    Dielectric strength and energy storage density in Ba6‐3xLa8+2xTi18O54 (x = 0.5, 2/3, and 0.75) ceramics were investigated as functions of composition and microstructure. With increasing x, though the dielectric constant decreased from 113 to 102, the energy storage density increased from 2.3 J/cm3 to 3.2 J/cm3 due to the increased dielectric strength for ceramics prepared by conventional sintering. The energy storage was further improved to 4.2 J/cm3 in ceramics prepared by spark plasma sintering under an electric field of 1058 kV/cm. Both dielectric strength and energy storage density in the present ceramics indicated the strong processing and microstructure dependence. The optimum dielectric strength and energy storage density were achieved in the dense ceramics with fine grains, while both dielectric strength and energy storage density decreased in the ceramics with coarse columnar grains.

    更新日期:2018-07-12
  • Eu3+ doped CsPbBr1.5I1.5 Quantum Dots Glasses: A Strong Competitor among Red Fluorescence Solid Materials
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Rongrong Yuan; Jianming Liu; Huiling Zhang; Zelong Zhang; Guangzhan Shao; Xiaojuan Liang; Weidong Xiang

    CsPbBr1.5I1.5 quantum dots (QDs) glasses are synthesized by traditional melting and thermal treated method, CsPbBr1.5I1.5 QDs glasses show vast potential as red fluorescence component in warm WLED applications due to their moderate emission wavelength as well as good opacity property. However, the quantum yield of QDs glasses is still low, therefore, Eu3+ ions is chosen to introduce into CsPbBr1.5I1.5 QDs, the quantum yield is enhanced to 64.7%. After sequence of testing operations, we find that 6.5%CsPbBr1.5I1.5: 0.28%Eu3+ QDs glasses is a strong competitor among red fluorescence solid materials.

    更新日期:2018-07-12
  • Dual color tuning in Ce3+ doped oxyfluoride ceramic phosphor plate for white LED application
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Alejandro Arredondo; Haggeo Desirena; Ivan Moreno; Ignacio E. Orozco Hinostroza; Elder De la Rosa

    Ceramic phosphor plates of cerium (Ce3+) doped oxyfluoride were fabricated by the solid state reaction method. These phosphors exhibit efficient emission, with the novel feature of color tuning by varying both the doping concentration and excitation wavelength. As the Ce3+ concentration increases, the excitation spectrum broadens by a factor of 1.6, and the excitation peak wavelength shifts from 390 nm to 435 nm, and there is a variation in excitation energy of ~ 10%. Luminescence spectrum of low Ce3+ concentration samples is tuned from blue to green with the change of excitation wavelength. The emission peak exhibits a shift of 58 nm into the red spectral region, varying the Ce3+ concentration from 0.05 to 0.1 mol%; while this shift is only 6 nm when Ce3+ content changes from 0.25 to 1 mol%. Photoluminescence (PL) quantum yield has achieved 76%. The crystal structure was examined using X‐ray diffraction to explain its possible influence on the redshift luminescence. A proof of concept of white LED was constructed using a 450 nm blue LED chip with an oxyfluoride phosphor plate, showing a luminous efficacy (LE) of 64 lm/W with a color rendering index of 74.

    更新日期:2018-07-12
  • Phase evolution of reactive sputtering synthesized holmium silicate coatings
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Wanpeng Hu; Jie Zhang; Hongqiang Nian; Jingyang Wang

    Holmium silicates are promising environmental barrier coatings (EBCs) materials for Si‐based non‐oxide ceramics due to their excellent thermal and mechanical properties. In the present work, the phase evolution of holmium silicate coatings synthesized by reactive sputtering was systematically studied. The monosilicate crystallized in an X1‐Ho2SiO5 phase at 1000 °C and transformed to an X2‐Ho2SiO5 phase at 1200 °C. The phase transition is not problematic for the coating process because the X1‐ and X2‐Ho2SiO5 phases have similar coefficients of thermal expansion (CTEs). For the holmium disilicates, the stoichiometry obviously influenced polymorphic phase evolutions. Relatively Ho2O3‐rich compounds promoted the formation of γ‐Ho2Si2O7 phase at lower temperatures. This study on phase evolution versus temperature and stoichiometry can provide a guideline to optimize the processing and the properties of advanced holmium silicate EBCs.

    更新日期:2018-07-12
  • Silicon Oxycarbide Glasses and Glass‐Ceramics: “All‐Rounder” Materials for Advanced Structural and Functional Applications
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Christina Stabler; Emanuel Ionescu; Magdalena Graczyk‐Zajac; Isabel Gonzalo‐Juan; Ralf Riedel

    Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Youngs moduli, hardness values, glass transition and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior.

    更新日期:2018-07-12
  • Cycling‐ and heating‐induced evolution of piezoelectric and ferroelectric properties of CuO‐doped K0.5Na0.5NbO3 ceramic
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Tao Wang; You Liao; Dongmei Wang; Qiaoji Zheng; Jie Liao; Fengyu Xie; Wenjing Jie; Dunmin Lin

    For accepter‐doped perovskite piezoelectric ceramics, macroscopic properties of the materials (e.g., hardening, fatigue and aging) are closely related to microscopic characteristics (e.g., oxygen vacancies and defect dipoles). In this work, the relationship of macroscopic and microscopic characteristics in CuO‐doped K0.5Na0.5NbO3 ceramic has been studied by subjecting the material to electric field cycling, quenching, heating and consequently aging. The introduction of CuO in K0.5Na0.5NbO3 generates (CuNb”‘ – Vo••)’ and (Vo•• – CuNb”‘ – Vo••)•. The defect dipoles exhibit obviously the pinning effect on ferroelectric domains and thus induce a completely pinched/double P‐E loop and excellent hardening piezoelectricity of high Qm of 2235. With the destruction of short‐range symmetry uniformity between defect dipoles and ferroelectric dipoles induced by electric field cycling, quenching and heating, the ceramic can be depinned and softened. As a result, the depinned ceramic possesses an opened single ferroelectric hysteresis loop and the significantly decreasing Qm. A distinctive aging is observed in the depinned ceramic. This study provides deep insights into the evolution of electrical properties of accepter‐modified alkali niobate perovskite ceramics under electric field cycling, quenching and heating.

    更新日期:2018-07-12
  • Understanding the structural origin of intermediate glasses
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-12
    Siva Priya Jaccani; Siddharth Sundararaman; Liping Huang

    Intermediate glasses show nearly constant elastic moduli with temperature and/or pressure. These glasses would prove useful in designing a‐thermal optical fibers for enhanced telecommunication, fiber sensing applications and in designing glass products for applications where a broad range of thermal and mechanical stimulation is expected. In this study, intermediate glasses belonging to the Na2O‐SiO2, Na2O‐Al2O3‐SiO2 and Na2O‐TiO2‐SiO2 glass systems were identified from in‐situ high‐temperature Brillouin light scattering (BLS) experiments. Glasses important for engineering applications like the international simple glass (ISG) and the less brittle glass (LBG) were also found to exhibit intermediate behaviors. In‐situ Raman spectroscopy was used to investigate their structural evolution from room temperature to temperatures beyond Tg. Raman spectra along with molecular dynamics simulations revealed common structural signatures that intermediate glasses with different compositions possess. Our study showed that the intermediate elastic behaviors come from a delicate balance between the stiffening effect associated with conformation changes in the medium range flexible rings and the softening effect due to the weakening of short range chemical bonds with temperature.

    更新日期:2018-07-12
  • α‐Alumina and spinel react into single phase high‐alumina spinel in < 3 seconds during flash sintering
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    David Kok; Devinder Yadav; Emanuele Sortino; Scott J. McCormack; Kuo‐Pin Tseng; Waltraud M. Kriven; Rishi Raj; Martha L. Mecartney

    In‐situ X‐ray diffraction measurements at the Advanced Photon Source show that α‐Al2O3 and MgAl2O4 react nearly instantaneously and completely, and nearly completely to form single‐phase high‐alumina spinel during voltage‐to‐current type of flash sintering experiments. The initial sample was constituted from powders of α‐Al2O3, MgAl2O4 spinel, and cubic 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) mixed in equal volume fractions, the spinel to alumina molar ratio being 1:1.5. Specimen temperature was measured by thermal expansion of the platinum standard. These measurements correlated well with a black body radiation model, using appropriate values for the emissivity of the constituents. Temperatures of 1600‐1736°C were reached during the flash, which promoted the formation of alumina‐rich spinel. In a second set of experiments the flash was induced in a current rate method where the current flowing through the specimen is controlled and increased at a constant rate. In these experiments we observed the formation of two different compositions of spinel, MgO•3Al2O3 and MgO•1.5Al2O3, which evolved into a single composition of MgO•2.5Al2O3 as the current continued to increase. In summary, flash sintering is an expedient way to create single‐phase, alumina‐rich spinel.

    更新日期:2018-07-12
  • Luminescence effects in reactive powder sintered silica glasses for radiation sensing
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    Ruth E Shaw; Christopher A.G. Kalnins; Nigel Antony Spooner; Carly Whittaker; Stephan Grimm; Kay Schuster; David Ottaway; Jillian Elizabeth Moffatt; Georgios Tsiminis; Heike Ebendorff‐Heidepriem

    Silica glasses doped with rare earth ions are potential materials for optical fibre radiation detection and dosimetry applications. High sensitivity to radiation requires fibres with large cores that can be reliably fabricated using glass made in a novel process from the reactive powder sintering of silica. The luminescence and dosimetric properties of a range of rare earth doped silica materials produced using this novel technique are reported here.

    更新日期:2018-07-12
  • High‐Q and temperature‐stable microwave dielectrics in layer cofired Zn1.01Nb2O6/TiO2/Zn1.01Nb2O6 ceramic architectures
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    Jie Zhang; Yu Luo; Zhenxing Yue; Longtu Li

    A multi‐layer cofired architecture was proposed and demonstrated to achieve high‐Q and temperature‐stable microwave dielectrics in a derived system, Zn1.01Nb2O6‐TiO2. This approach could effectively allow the chemical reactions between Zn1.01Nb2O6 and TiO2 occur at a rather narrow area (~ 12 μm), the interfaces of heterogenous layers, where the diffusion of Zn, Nb and Ti could be observed. Such interfaces could act as the in‐situ “glues” to connect each layer well. The effects of stacking scheme and TiO2 content on the microwave dielectric properties of layered architectures were investigated systematically. The resonant frequency, Q‐factor and electric field distribution were reported using the eigenmode solver of HFSS (High Frequency Structure Simulator). Among the available layer‐architectures, the optimized microwave dielectric characteristic was observed in Zn1.01Nb2O6/TiO2/Zn1.01Nb2O6 stacked with 0.058 mol TiO2 (~1.84 vol%). The τf can be effectively tuned to ~ +0.53 ppm/°C, and importantly a high Q×f value ~ 99 500 GHz together with εr ~ 26.8 was achieved. This design could be beneficial for opening up new ways to develop high‐performance microwave dielectrics based on current material systems, and therefore to meet with the high requirements for 5G wireless communication components and multilayer packing technology.

    更新日期:2018-07-12
  • Textured dense Zinc Oxide layers for active noise cancelling windows
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    Jörg Lüchtenborg; Delf Kober; Alfred P. Weber; Jörg Melcher; Jens Günster

    Dense ZnO films with a strong c‐axis texture have been deposited on TCO‐Glass, Glass and Si wafers, respectively, with a two‐step pressureless wet chemical method using zinc acetate dihydrate as Zn‐precursor. The crystallographic structure of the films has been studied with XRD and SEM. Optical measurements reveal a high transparency of the ZnO films with a thickness of up to 10 μm. This new cost effective route for ZnO film deposition does not require expensive sophisticated equipment and is easily upscaled.

    更新日期:2018-07-12
  • Mn4+‐related photoemission enhancement via energy transfer in La2MgGeO6:Dy3+,Mn4+ phosphor for plant growth LEDs
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    Wei Chen; Linli Shen; Chenyang Shen; Zelong Zhang; Xiaojuan Liang; Weidong Xiang

    A double perovskite‐type substrate of La2MgGeO6 (LMGO) was successfully synthesized via a high‐temperature solid‐state reaction method and was co‐doped with Mn4+ and Dy3+ to form a new deep‐red phosphor (LMGO:Mn4+,Dy3+) for artificial plant growth LEDs. This extraordinary phosphor can exhibit strong far‐red emission with a maximum peak at 708 nm between 650 and 750 nm, which can be ascribed to the 2Eg→2A2g spin‐forbidden transition of Mn4+. The X‐ray diffraction (XRD) patterns and high‐resolution transmission electron microscopy (HRTEM) clarified that the La3+ sites in the host were partly replaced by Dy3+ ions. Moreover, we discovered energy transfers from Dy3+ to Mn4+ by directly observing the significant overlap of the excitation spectrum of Mn4+ and the emission spectrum of Dy3+ as well as the systematic relative decline and growth of the emission bands of Dy3+ and Mn4+, respectively. With the increase in the activator (Mn4+) concentration, the relationship between the luminescence decay time and the energy transfer efficiency of the sensitizer (Dy3+) was studied in detail. Finally, an LED device was fabricated using a 460 nm blue chip, and the as‐obtained far‐red emitting LMGO:Mn4+,Dy3+ phosphors for Wedelia chinensis cultivation. As expected, the as‐fabricated plant growth LED‐treated Wedelia chinensis cultured in the artificial climate box with overhead LEDs demonstrated that after 28 days of irradiation, the average plant growth rate and the total chlorophyll content were better than those of specimens cultured using the commercial R‐B LED lamps, indicating that the as‐prepared phosphor could have a potential application in the agricultural industry.

    更新日期:2018-07-12
  • Iron‐depleted Bi‐YIG having enhanced gyromagnetic properties suitable for LTCC processing
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-10
    Ning Jia; Zhang Huaiwu; Vincent G. Harris

    The traditional method of ferrite synthesis is based upon solid‐state reaction. The advantages of this approach include simplicity, cost effectiveness, and suitability for commercial process scaling. However, there exist shortcomings in that impurities and defects adversely affect magnetic properties. Here, iron‐depleted compositions of bismuth substituted yttrium iron garnet (i.e., Bi:YIG) were studied with emphasis placed on the impact of synthesis processing parameters upon the structure and gyromagnetic properties. It was shown that ferrimagnetic resonance linewidth experienced a minimum of 180.3 Oe (X‐band) when the sample stoichiometry was 4% depleted in iron (x=0.2), a reduction of 29.3% from the parent compound. Concomitantly, iron‐depletion caused a systematic change in lattice parameter that resulted in a dilation of Goodenough‐Kanamori‐Anderson bond angles increasing exchange energy and magnetization. The substitution of Bi for Y on the dodecahedron sites altered the reaction path and decreased the activation energy effectively reducing sintering temperature. In order to establish a functional processing protocol, an ion diffusion model is presented to explain the demonstrated processing, structure, and performance paradigm.

    更新日期:2018-07-12
  • Ferroelectric and magnetoelectric origins of multiferroic SmCrO3
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-27
    Jun Ding; Liwei Wen; Haidong Li; Haoshan Hao; Ying Zhang

    We investigated the magnetic and ferroelectric properties of orthochromite SmCrO3 by using density functional theory simulations. The atom coordinates of both Pbnm and Pna21 were calculated, and Pna21 was found to be the ground state structure. The spontaneous polarization in Pna21 structure is sensitive to delicate structure change which is induced by different antiferromagnetic order, and its direction is along the c axis. The spin structure of Pbnm was analyzed and it was found to not support ferroelectricity. The results revealed the origin of ferroelectricity of multiferroic SmCrO3 and explained the magnetoelectric effect.

    更新日期:2018-07-08
  • Grain size variation in nanocrystalline silicon carbide irradiated at elevated temperatures
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 05 July 20
    Limin Zhang; Weilin Jiang; Wensi Ai; Liang Chen; Chenlong Pan; Tieshan Wang

    This study reports on ion irradiation‐induced grain size variations in nanocrystalline SiC films on Si substrates. The SiC grains with average size ranging from ~2 to 20 nm were embedded in amorphous SiC matrices. Irradiation was performed using 5 MeV Xe23+ ions to 1.15 × 1016 ions/cm2 at 700 K. The irradiated films were characterized using X‐ray diffraction, transmission electron microscopy, and Raman spectroscopy. Significant grain growth is observed for smaller grains that tend to saturate at ~8 nm. In contrast, irradiation of larger grains (~20 nm in size) leads to a decrease in the grain size, which could be associated with the production of lattice disorder within the grains. Homonuclear C‐C bonds in the irradiated amorphous SiC matrix are found to be graphitized. This bonding transformation could limit or inhibit grain growth and contribute to the size saturation. The results from this study may suggest nanocrystalline SiC as a promising candidate structural or cladding material for applications in advanced nuclear reactors.

    更新日期:2018-07-08
  • An optimal spectral model for phosphor‐converted white light‐emitting diodes used in the mesopic vision
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-25
    Zi‐shan Zheng; Le Wang; Hong Zhang; Tian‐liang Zhou; Peng Zheng; Pei Liang

    White light‐emitting diodes (WLEDs) for road lighting are required to have both the high scotopic to photopic ratio (S/P) and color rendering index (CRI). However, there is a trade‐off between S/P and CRI, and WLEDs commonly having the S/P of 1.68‐2.38 usually exhibit a low CRI. In this work, to provide a best solution to the trade‐off problem we proposed an optimal spectral model for the phosphor‐converted WLEDs (pc‐WLEDs) aiming to figure out the optimal phosphor combination. The Monte Carlo Algorithms combined with the Genetic Algorithm were adopted to obtain the optimization of CRI and S/P by varying the spectral power distributions of WLEDs through adjusting the spectral parameters. Considering the spectral requirements of pc‐WLEDs based on the mesopic vision, we chose CaAlSiN3:Eu2+ and Y3(Ga,Al)5O12:Ce3+ as the red‐ and green‐emitting phosphors to prepare WLEDs with both high S/P and CRI, respectively. The simulation based on the optimal spectral model led to an optimal pc‐WLED with a high S/P of 2.0‐2.14, Ra > 80 and correlated color temperature (CCT) of 4000‐5000 K, which matches very well with the experimental results of S/P = 2.064, Ra = 93.9, and CCT = 4981 K for the two‐phosphor converted WLEDs. It implies that the optimal spectral model would be used effectively for the spectral design and phosphor selection for WLEDs.

    更新日期:2018-07-08
  • Microwave sintering of complex shapes: From multiphysics simulation to improvements of process scalability
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 05 July 20
    Charles Manière; Shirley Chan; Eugene A. Olevsky

    The microwave sintering homogeneity of large and complex shape specimens is analyzed. A new approach enabling the fabrication of complex shapes ceramics via 3D printing and microwave sintering is presented. The use of a dental microwave cavity is shown to enable a substantial level of densification of complex shape components while restricting the grain growth. The homogeneity of the processed samples during microwave sintering is studied by an electromagnetic‐thermal‐mechanical simulation. The realistic densification behavior, that phenomenologically takes into account the microwave effect, is included in the modeling framework. The simulation indicates the sharp correlation between the microwave field distribution in the cavity, the temperature profile, and the specimen's shape distortion.

    更新日期:2018-07-08
  • Greatly enhanced photocurrent in inorganic perovskite [KNbO3]0.9[BaNi0.5Nb0.5O3‐σ]0.1 ferroelectric thin‐film solar cell
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-22
    Jian Chen; Weijie Pei; Guang Chen; Qingfeng Zhang; Yinmei Lu; Haitao Huang; Mingkai Li; Yunbin He

    Inorganic perovskite [KNbO3]0.9[BaNi0.5Nb0.5O3‐σ]0.1 (KBNNO) ferroelectric thin films with narrow band gap (1.83 eV) and high room‐temperature remnant polarization (Pr = 0.54 μC/cm2) was grown successfully on the Pt(111)/Ti/SiO2/Si(100) substrates by pulsed laser deposition. Ferroelectric solar cells with a basic structure of ITO/KBNNO/Pt were further prepared based on these thin films, which exhibited obvious external‐poling dependent photovoltaic effects. When the devices were negatively poled, the short‐circuit current and open‐circuit voltage were both significantly higher than those of the devices poled positively. This is attributed to enhanced charge separation under the depolarization field induced by the negative poling, which is superimposed with the built‐in field induced by the Schottky barriers at the interfaces between KBNNO and the two electrodes. When a poling voltage of ‐1 V was applied, the device showed a short‐circuit current as high as 27.3 μA/cm2, which was by two orders of magnitude larger than that of the KBNNO thick‐film (20 μm) devices reported previously. This work may inspire further exploration for lead‐free inorganic perovskite ferroelectric photovoltaic devices.

    更新日期:2018-07-08
  • Investigation of antimicrobial properties and in‐vitro bioactivity of Ce3+‐Sr2+dual‐substituted nano hydroxyapatites
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-11
    Lakshmanaperumal Sundarabharathi; Mahendran Chinnaswamy; Deepalekshmi Ponnamma; Hemalatha Parangusan; Mariam Al Ali Al‐Maadeed
    更新日期:2018-07-08
  • Simple and low‐cost synthetic route for SiBCN ceramic powder from a boron modified cyclotrisilazane
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-06
    T Ganesh Babu; Renjith Devasia

    This study reports a simple and low‐cost synthetic route for preparing SiBCN ceramic powder via pyrolysis of boron modified cyclotrisilazane (BCTS). BCTS resins were synthesized by reacting boric acid with 1, 3, 5‐trimethyl‐1̍, 3̍, 5̍‐trivinylcyclotrisilazane (CTS) in the molar ratio of 1:1, 1:3 and 1:5. The boron modification of CTS resin resulted in optimum properties for preceramic polymers such as solubility in common solvents, processable viscosity (< 20 cps) and high ceramic yield (>80 wt. %). The polymer to ceramic conversion was carried out at 1450 and 1650°C under a nitrogen atmosphere. The study demonstrated that the changes in CTS concentration and pyrolysis temperature significantly affected the evolution of ceramic phases, morphology and elemental composition which were thoroughly investigated through XRD, SEM and HRTEM techniques. The results revealed the formation of β‐SiC, β‐Si3N4 and oxide ceramic phases with BCTS in the molar ratio of 1:1 and 1:3; whereas, β‐SiC, β‐Si3N4 and turbostratic BN(C) ceramic phases were obtained with BCTS in the molar ratio of 1:5.

    更新日期:2018-07-08
  • Facile synthesis and spectroscopic characterization of siliconitride phosphors for white light emitting diodes
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-06
    Chung‐Hsin Lu; Che‐Yuan Yang; Sudipta Som; Subrata Das

    A modified chemical vapor deposition (CVD) technique is used to synthesize the color‐tunable siliconitride Sr2‐1.5x‐yCexEuySi5N8 (x = 0.000‐0.016 and y = 0.000‐0.020) phosphors. In comparison with the conventional solid‐state method, the CVD approach successfully improved the crystallinity, particle size distribution, and photoluminescence through the enhanced gas‐solid reaction. Under blue excitation, Sr1.98Eu0.02Si5N8 exhibited a red emission band at 618 nm. The incorporation of Ce3+ ions increased the emission intensity of Eu2+ ions by approximately 10% owing to the enhanced absorption and dipole‐dipole energy transfer process from Ce3+ to Eu2+ ions. It resulted in a shift of the emission colors from yellow to red region. The external and internal quantum efficiencies of Sr1.906Ce0.06Eu0.004Si5N8 were calculated as 54% and 70%, respectively. The activation energy of thermal stability for Sr1.906Ce0.06Eu0.004Si5N8 was evaluated as 0.31 eV. A white LED with a color rendering index of 80 and a CCT of 4964 K was successfully fabricated with the present phosphors. The current research demonstrated a new series of Sr2Si5N8: Ce3+, Eu2+ phosphors with color‐tunability for fabricating white LEDs with high color‐rendering index.

    更新日期:2018-07-08
  • Fabrication and Properties of Transparent Nd‐doped BaF2 Ceramics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-06
    Jianlin Li; Xianqiang Chen; Lingfei Tang; Yiyu Li; Yiquan Wu

    Nd:BaF2 nanoparticles have been prepared via co‐precipitation and a pumping filtration wash method. The phase composition and morphology of the synthesized nanoparticles were investigated by X‐Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE‐SEM) analyses, respectively. SEM observations revealed the powder's particle size to be approximately 100‐200 nm after calcination at 600 °C for 5h in Ar. Then the transparent Nd:BaF2 ceramics were fabricated by the one‐step vacuum sintering method at a temperature of 1200 °C for 10h. SEM observations of the polished and thermally etched cross sections of the sintered ceramic revealed a highly homogenous microstructure with average grain size of 420μm. Optical property characterization revealed that the transmittance of the ceramic reaches a maximum of ~70% in the infrared wavelength range, and an emission peak located at 1058nm, excited by 808nm light.

    更新日期:2018-07-08
  • Understanding the atomistic origin of hydration effects in single and mixed bulk alkali‐silicate glasses
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-07-06
    Khagendra Baral; Aize Li; Wai‐Yim Ching

    Silica‐water interaction plays an essential role for the mechanical strength and chemical durability of alkali‐doped silicate glasses. A comprehensive study of single and mixed alkali silicate glasses with 30% molar content of Li2O, Na2O and K2O, and half‐half mixture of Li2O‐Na2O, Li2O‐K2O and Na2O‐K2O in hydrated models is carried out using density functional theory methods. Information on atomic geometry, electronic structure, interatomic bonding, partial charge distribution, mechanical and optical properties are obtained and compared. It confirms that water in the solvated and confined bulk models can be either dissociated or remains as H2O molecule depending on the distribution and specific alkali elements. A quantum mechanical metric, the total bond order density is used to unravel the atomistic origin of the internal cohesion and strength of glasses in different environments. In particular, we show that the mechanical strength of bulk alkali silicate glasses is enhanced by hydration with some evidence that mixing of alkali ions tends to degrade the strength of the hydrated glasses. These results are discussed in the context of experimental observations and a few existing simulations using classical molecular dynamics.

    更新日期:2018-07-08
  • Synthesis of Ti0.2Zr0.8B2 solid‐solution nanopowders by molten salt assisted borothermal reduction
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-25
    Da Liu; Yanhui Chu; Siyi Jing; Beilin Ye; Xiya Zhou

    High‐purity Ti0.2Zr0.8B2 solid‐solution nanopowders were successfully synthesized via a molten salt assisted borothermal reduction at 1323‐1373 K using ZrO2, TiO2 and amorphous B as starting materials. The Ti0.2Zr0.8B2 solid‐solution nanopowders synthesized at 1323 K show the largest specific surface area of 12.24 m2/g and the lowest equivalent average particle size of 86 nm. Meanwhile, they exhibit the high compositional uniformity and the good single‐crystal hexagonal structure. This study provides a new method to synthesize the high‐purity solid‐solution nanopowders of the transition‐metal borides.

    更新日期:2018-07-05
  • Oxygen stoichiometry, chemical expansion or contraction, and electrical properties of rutile, TiO2±δ ceramics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-22
    Yun Dang; Anthony R. West

    Rutile, TiO2 is increasingly oxygen‐deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a chemical contraction effect attributed to partial collapse of columns of edge‐sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen‐deficient samples quenched from above 700°C were n‐type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450‐500°C were p‐type and believed to be slightly oxygen‐rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross‐over between n‐ and p‐type behavior.

    更新日期:2018-07-05
  • Influences of rare‐earth oxide additives on the formation and properties of porous Si2N2O ceramic
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-20
    Jiangbo Wen; Hongjie Wang; Lei Fan; Liting Wei; Min Niu; Hongfei Gao; Zhixin Cai

    Here we prepared porous silicon oxynitride (Si2N2O) ceramics by reaction sintering of SiO2 and Si3N4 using five different rare‐earth oxides (RE2O3, RE = Lu, Yb, Y, Sm, and La) as sintering aids. The influences of RE2O3 on the formation, densification, microstructure, and mechanical properties of Si2N2O ceramics have been investigated in detail. The results have indicated that with the increase in RE ionic radius, the formation temperature of Si2N2O decreases, and the densification process could be promoted by RE2O3 with larger RE3+ ionic radius. In addition, microstructures and mechanical properties are highly dependent on the RE2O3 additives. With the increase in RE3+ ionic radius, Si2N2O changes from platelike crystals to elongated crystals. The samples doped with La2O3 and Sm2O3 with elongated crystals exhibit higher flexural strength and higher Vickers hardness.

    更新日期:2018-07-05
  • Polar domain structural evolution under electric field and temperature in the (Bi0.5Na0.5)TiO3‐0.06BaTiO3 piezoceramics
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 03 July 20
    Jinyan Zhao; Nan Zhang; Wei Ren; Gang Niu; David Walker; Pamela A. Thomas; Lingyan Wang; Zuo‐Guang Ye
    更新日期:2018-07-04
  • Design of tailored biodegradable implants: The effect of voltage on electrodeposited calcium phosphate coatings on pure magnesium
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-21
    Miroslava Horynová; Michaela Remešová; Lenka Klakurková; Karel Dvořák; Ivana Ročňáková; Shaokun Yan; Ladislav Čelko; Guang‐Ling Song
    更新日期:2018-07-02
  • Fabrication of Gd2O3‐MgO nanocomposite optical ceramics with varied crystallographic modifications of Gd2O3 constituent
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-20
    Nan Wu; Xiaodong Li; Ji‐Guang Li; Qi Zhu; Xudong Sun

    The fabrication of Gd2O3‐MgO nanocomposite optical ceramics via hot‐pressing using sol‐gel derived cubic‐Gd2O3 and MgO nanopowders was investigated. The precursor powder calcined at 600°C had an average particle size of 12 nm. The effects of hot‐pressing temperature on constituent phases, microstructure, mid‐infrared transmittance, and microhardness were studied. The crystallographic modifications of Gd2O3 phase varied with the increase in sintering temperature from 1250 to 1350°C. The monoclinic‐Gd2O3 phase was retained for the composite sintered at 1350°C and the sample had an average grain size of 90 nm, excellent transmission (80.4%‐84.8%) over 3‐6 μm wavelength range, and enhanced hardness value of 14.1 GPa.

    更新日期:2018-07-02
  • Sintering aid (ZnO) effect on proton transport in BaCe0.35Zr0.5Y0.15O3‐δ and electrode phenomena studied by distribution function of relaxation times
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-19
    Ashok Kumar Baral; Yoed Tsur

    In this work, we have employed an evolutionary programming technique to analyse the impedance spectroscopy (IS) of a ceramic proton conductor BaCe0.35Zr0.5Y0.15O3‐δ (BCZY) to study its proton transport properties as well as the electrode reactions kinetics at the Pt|BCZY interface. The distribution functions of relaxation times (DFRT) obtained from the impedance analysis were used to evaluate the physical parameters such as relaxation time constant, resistance, reactance/capacitance, conductivities, activation energies, grain‐boundary space charge potential Φ(0, T), and electrode polarization resistance. The effect of ZnO sintering aid (2 wt%) on charge‐transfer processes at bulk, grain boundaries and Pt|BCZY interface were thoroughly studied. Φ(0, T) is increased and grain‐boundary conductivity (σgb) is decreased by an order of magnitude due to the presence of Zn at the grain boundaries, whereas bulk properties appear to be unchanged. Variation in σgb with temperature is explained by correlating the grain‐boundary capacitance (Cgb), relaxation time (τgb), space charge effect, and the defect associations (eg, Y‐OH). The anodic functions, for example, hydrogen oxidation kinetics at the electrode (Pt) surface and Pt‐BCZY interface, are explained through the symmetrical cell study and the behavior of relaxation time constants (τ) with temperature.

    更新日期:2018-07-02
  • Thermochemical Model on the Carbothermal Reduction of Oxides During Spark Plasma Sintering of Zirconium Diboride
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-29
    David Pham; J. Houston Dycus; James M. LeBeau; Venkateswara R. Manga; Krishna Muralidharan; Erica L. Corral

    Carbon was used to reduce oxides in spark plasma sintered ZrB2 ultra‐high temperature ceramics. A thermodynamic model was used to evaluate the reducing reactions to remove B2O3 and ZrO2 from the powder. Powder oxygen content was measured and carbon additions of 0.5 wt% and 0.75 wt% were used. A C‐ZrO2 pseudo binary diagram, ZrO2‐B2O3‐C pseudo ternaries and Zr‐C‐O potential phase diagrams were generated to show how the reactions can be related to an open system experiment in the tube furnace. Scanning transmission electron microscopy identified impurity phases composed of amorphous Zr‐B‐O with lamellar BN and a Zr‐C‐O ternary model was calculated under SPS sintering conditions at 1900 °C and 6 Pa to understand how oxides can be retained in the microstructure.

    更新日期:2018-07-01
  • 更新日期:2018-06-30
  • Reversible upconversion switching for Ho/Yb codoped (K,Na)NbO3 ceramics with excellent luminescence readout capability
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-20
    Haiqin Sun; Yao Zhang; Jian Liu; Dengfeng Peng; Qiwei Zhang; Xihong Hao
    更新日期:2018-06-30
  • Molecular structure of CaO–FeOx–SiO2 glassy slags and resultant inorganic polymer binders
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 28 June 20
    Arne Peys; Claire E. White; Daniel Olds; Hubert Rahier; Bart Blanpain; Yiannis Pontikes
    更新日期:2018-06-30
  • Strong magnetoelectric effect in pulse laser deposited [Ba(Zr0.2Ti0.8)O3‐0.5(Ba0.7Ca0.3)TiO3]/CoFe2O4 bilayer thin film
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-19
    Jyoti Rani; Varun K. Kushwaha; Jayant Kolte; Chakkalakal V. Tomy

    Polycrystalline bilayer thin film of multiferroic [Ba(Zr0.2Ti0.8)O3‐0.5(Ba0.7Ca0.3)TiO3]/CoFe2O4([BZT‐0.5BCT]/CFO) has been deposited on Pt/Si (100) substrate using a pulsed laser deposition technique. The dielectric analysis reveals a significant change in the dielectric constant (~39% at a typical frequency of 100 Hz) at room temperature when a magnetic field is applied, in addition to a substantial improvement in the saturation polarization. A low leakage current density (~ 5 × 10−7 A/cm2) and a high magnetoelectric coupling coefficient (αE) both in the transverse (~2.085 V/Oe cm) as well as in the longitudinal (~0.708 V/cm Oe) directions, indicate in‐principle usability of this system for multifunctional device applications in thin film form.

    更新日期:2018-06-30
  • The upconversion luminescence modulation and its enhancement in Er3+‐doped Na0.5Bi0.5TiO3 based on photochromic reaction
    J. Am. Ceram. Soc. (IF 2.956) Pub Date : 2018-06-14
    Kaixuan Li; Laihui Luo; Yuanyuan Zhang; Weiping Li; Yafei Hou

    A large and reversible upconversion (UC) luminescence modulation has been found in the Na0.498Bi0.498TiO3:0.002Er (NBT:0.002Er) based on the photochromic reaction. The dependence of luminescence modulation of the ceramics on the wavelength of irradiation light and sintering temperature was investigated. It was found that the optimized sintering temperature and irradiation wavelength were 1130°C and 405 nm, respectively. The highest ΔRt (defined as: ΔRt = (R0 – Rt)/R0×100(%), where R0 and Rt are the initial emission intensity and that after different irradiation time, respectively) value of 44.9% was obtained for the ceramics sintered at 1130°C after irradiation at 405 nm. More importantly, for the poled ceramics, ΔRt value was promoted to a high value of 75.5%, which was 168% of that of the unpoled ones. The mechanism of luminescence modulation and its enhancement via electric field poling were discussed. This study demonstrated that electric field poling was an effective strategy to enhance the PC reaction in the NBT ceramics.

    更新日期:2018-06-30
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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