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  • High energy storage efficiency and thermal stability of A‐site‐deficient and 110‐textured BaTiO3–BiScO3 thin films
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-20
    Waseem Abbas; Derek Ho; Abhijit Pramanick

    The development of thin film dielectrics having both high energy density and energy conversion efficiency, as well as good thermal stability, is necessary for practical application in high‐temperature power electronics. In addition, there is a demand for the development of new Pb‐free high‐energy density dielectric materials due to environmental concerns. In this regard, thin films of weakly coupled relaxors based on solid solutions of BaTiO3–BiMeO3 have shown good promise, because they exhibit a remarkably large polarization over a wide temperature range. Nevertheless, the performance of Pb‐free thin films has lagged behind that of their Pb‐based counterparts in terms of thermal stability and energy conversion efficiency. Toward this end, most recent studies on BaTiO3–BiMeO3 systems have focused on the optimization of material composition, while relatively less attention has been paid to other aspects such as defect chemistry and crystallographic texture. In this study, we examine the effects of A‐site vacancy and crystallographic texture on the energy storage performance of BaTiO3–BiScO3 thin films synthesized using pulsed laser deposition (PLD). It is shown that a high energy storage density (Wr) of ~28.8 J/cm3 and a high efficiency of η >90% are achieved through a combination of moderate A‐site vacancy concentration and (110) crystallographic texture. Furthermore, Wr remains nearly temperature independent while a high efficiency of η >80% is maintained for temperatures up to 200°C, which constitutes one of the best performances for Pb‐free ferroelectric films for high‐temperature capacitor applications.

    更新日期:2020-01-21
  • DC electrical degradation of YSZ: Voltage‐controlled electrical metallization of a fast ion conducting insulator
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-20
    Ana Alvarez; Yanhao Dong; I‐Wei Chen

    DC electrical degradation as a form of dielectric and resistance breakdown is common in thin‐film devices including resistance‐switching memory. To obtain design data and to probe the degradation mechanism, highly accelerated lifetime tests (HALT) are often conducted at higher temperatures with thicker samples. While the mechanism is well established in semiconducting oxides such as perovskite titanates, it is not in stabilized zirconia and other fast oxygen‐ion conductors that have little electronic conductivity. Here we model the mechanism by an oxygen‐driven, transport‐limited, metal‐insulator transition, which finds support in rich experimental observations—including in situ videos and variable temperature studies—of yttria‐stabilized zirconia. This demonstrates that although (electro) reduction does not appreciably alter oxygen stoichiometry in stabilized zirconia, which is fixed by the dopant concentration, it can still raise the chemical potential of electrons enough to eventually reach the conduction‐band level, thereby triggering the insulator‐to‐metal transition and resistance degradation. These results are contrasted with the findings in semiconducting titanates and resistance memory, and to provide new insight into ceramic processing with extremely rapid heating and cooling such as flash sintering and melt processing.

    更新日期:2020-01-21
  • Dense and homogeneous MOX fuel pellets manufactured using the freeze granulation route
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-20
    Florian La Lumia; Laure Ramond; Cecile Pagnoux; Philippe Coste; Florent Lebreton; Jean‐Robert Sevilla; Guillaume Bernard‐Granger

    MOX fuels (UO2‐PuO2) are used in light water nuclear reactors of several countries and are also potential candidates for the fast neutron reactors. Industrial MOX is currently manufactured by a dry route process, involving steps with fine powders. To reduce dusting, enhance MOX powder flow properties, and decrease manufacturing scrap rate, a new wet route process is investigated: the freeze granulation of concentrated water‐based powder suspensions having optimized rheology and dispersion properties. Highly flowable, dustless, and easy‐to‐press MOX granules have been elaborated. Sintering green compacts made of such granules gives highly dense and defect‐free pellets that have a very homogeneous U‐Pu spatial distribution, thus improving the fuel characteristics. Indeed, MOX fuels devoid of large Pu‐rich aggregates are thought to have a better behavior under irradiation in reactors by limiting/preventing the formation of the typical high burnup structure.

    更新日期:2020-01-21
  • Single‐step densification of nanocrystalline CeO2 by the cold sintering process
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-17
    Thomasina H. Zaengle; Arnaud Ndayishimiye; Kosuke Tsuji; Zhongming Fan; Sun Hwi Bang; Jonathan Perini; Scott T. Misture; Clive A. Randall

    This study reports the successful single‐step cold sintering of nanocrystalline cerium dioxide (CeO2) at temperatures ranging from 250°C to 400°C under 500 MPa, using molten hydroxides flux solvents. CeO2 ceramics obtained were 82 to 91% of the theoretical density. Structural and microstructural investigations of the as‐cold sintered CeO2 ceramics were conducted to further understand this new approach to cold sinter ceramics. Electrical conductivity measured by two‐point AC impedance demonstrated an activation energy for grain conductivity of 0.49 eV, with impedance spectra characteristic of nanoscale CeO2.

    更新日期:2020-01-21
  • Phase diagram of the Li2SO4–Na2SO4 system
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-17
    Pavel P. Fedorov; Vera Y. Proydakova; Sergey V. Kuznetsov; Valery V. Voronov; Alexandr A. Pynenkov; Konstantin N. Nishchev

    The thermal analysis and X‐ray powder diffraction studies of the Li2SO4–Na2SO4 system, including the high‐temperature X‐ray diffraction technique, have elucidated four phases of variable composition: three solid solutions based on the α‐Li2SO4, α‐Na2SO4, and α‐LiNaSO4 high‐temperature polymorphs, and a low‐temperature β‐LiNaSO4 phase. α‐Na2SO4‐Base solid solution disintegrates into two phases via a monotectoid phase transformation. It is quite probable that the monotectoid process is related to the conversion of the second‐order phase transition to the first‐order phase transition.

    更新日期:2020-01-21
  • He Irradiation‐induced Lattice Distortion and Surface Blistering of Gd2Zr2O7 Defect‐fluorite Ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-20
    Zhangyi Huang; Mao Zhou; Zhangkai Cao; Zhe Tang; Yutong Zhang; Junjing Duan; Jianqi Qi; Xiaofeng Guo; Tiecheng Lu; Di Wu

    Gd2Zr2O7 ceramics with different grain sizes ranging from nanoscale to submicron scale (91 nm, 204 nm, and 634 nm) were irradiated at room temperature using 190 keV He ions with doses ranging from 5 × 1016 to 5 × 1017 ions/cm2. We fully characterized the pre‐ and post‐irradiation samples using grazing‐incidence X‐ray diffraction (GIXRD), scanning electron microscope (SEM) and atomic force microscope (AFM) as the grain size and degree of irradiation vary. The results suggested that all three Gd2Zr2O7 samples demonstrate outstanding radiation tolerance to displacement damage by retaining their crystallinity after irradiation at 5 × 1017 ions/cm2. which is equal to 16 displacement per atom (dpa) at peak positions. Although lattice expansion was observed at a He irradiation at 5 × 1016 ions/cm2 and beyond, the lattice remained stable for the nanograin ceramic, while the degree of distortion for the sample with the largest grain size (634nm) continuously increased. Moreover, a delayed He bubble evolution process was seen for the nanograin ceramic, which did not appear for the submicron‐grained sample. Interestingly, the grain size‐dependent surface blistering was also found to be a function of ion fluence. After He irradiation at 5 × 1017 ions/cm2 the AFM RMS (root‐mean‐square) roughness variation for Gd2Zr2O7 ceramics of 91, 204 and 634 nm were 4.8, 7.0 and 11.1 nm, respectively.

    更新日期:2020-01-21
  • Critical role of cationic local stresses on the stabilization of entropy stabilized transition metal oxides
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-20
    Lalith Kumar Bhaskar; Varatharaja Nallathambi; Ravi Kumar

    Entropy stabilized transition metal oxides [(MgNiCoCuZn)O] (ESO) in recent years have received considerable attention owing to its unique functional properties. Solution combustion and solid state syntheses resulted in crystallites varying from 5‐15 nm and 3‐5 μm respectively. Phase stability studies showed that all the systems containing Cu2+ ions in the ESO lattice segregated upon slow cooling in the furnace. It was only when ESO was quenched in air from 1000 ºC the lattice stabilized to a single phase. Experiments concomitant with molecular dynamics (MD) simulations demonstrated that the local stress fields around the cations played a critical role in stabilizing the single phase. The local stress fields are a result of Jahn‐Teller distortion induced by the Cu2+ ions in the lattice. It is clearly established that in the absence of the minimization of the local stress fields around the Cu2+ ions, segregation leading to the formation of a multi‐phase material is imminent for this particular composition.

    更新日期:2020-01-21
  • Indentation Densification of Fused Silica assessed by Raman Spectroscopy and constitutive Finite Element Analysis
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-19
    Sebastian Bruns; Tobias Uesbeck; Sindy Fuhrmann; Mariona Tarragó Aymerich; Lothar Wondraczek; Dominique de Ligny; Karsten Durst

    Inelastic deformation of anomalous glasses manifests in shear flow and densification of the glass network; the deformation behavior during indentation testing is linked strongly to both processes. In this paper, the indentation densification field of fused silica is investigated using depth‐resolved Raman spectroscopy and finite element simulations. Through affecting the size of the indent, the normal load and the Raman laser spot size determine the spatial sampling resolution, leading to a certain degree of structural averaging. For appropriate combinations of normal load (indent size) and laser spot diameter, a maximum densification of 18.4 % was found at the indent center. The indentation behavior was modelled by extended Drucker‐Prager‐Cap (DPC) plasticity, assuming a sigmoidal hardening behavior of fused silica with a densification saturation of 21 %. This procedure significantly improved the reproduction of the experimental densification field, yielding a maximum densification of 18.2 % directly below the indenter tip. The degree of densification was found to be strongly linked to the hydrostatic pressure limit below the indenter in accordance to Johnson’s expanding cavity model (J. Mech. Phys. Solids, 18 (1970) 115). Based on the good overlap between FEA and Raman, an alternative way to extract the empirical correlation factor m, which scales structural densification to Raman spectroscopic observations, is obtained. This approach does not require the use of intensive hydrostatic compaction experiments.

    更新日期:2020-01-21
  • SiOC modified carbon‐bonded carbon fiber composite with SiC nanowires enhanced interfibrous junctions
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-19
    Lei Su; Baoyu Li; Hongjie Wang; Min Niu; Xingyu Fan

    Carbon‐bonded carbon fiber (CBCF) composites are promising lightweight and high efficient thermal insulators to be applied in aerospace area, but their practical applications are usually restricted by the low mechanical performance and poor oxidation resistance. To overcome these drawbacks, many efforts have been made in the fabrication of ceramic coated CBCF composites. However, the densities of these modified composites are usually very high, which would result in the reduction of their thermal insulation performance. Herein, we prepared a CBCF composite with SiC nanowires enhanced interfibrous junctions and SiOC ceramic coated carbon fibers (SiCNWs‐SiOC‐CBCF). Similar to CBCF, the SiCNWs‐SiOC‐CBCF exhibits a low density of 0.35 g/cm3 and an anisotropic and highly porous architecture. The SiCNWs‐SiOC‐CBCF possesses a compressive strength of 3.8 MPa and a compression modulus of 195.7 MPa in the X (or Y) direction, ~ 26.7% and 150% higher than those of CBCF, respectively. It can also suffer from an isothermal treatment in air at 900 °C for 120 min. The combination of these properties makes the SiCNWs‐SiOC‐CBCF a good candidate for thermal insulator to be applied in extreme conditions.

    更新日期:2020-01-21
  • Intrinsic second‐order nonlinearity in chalcogenide glasses containing HgI2
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-19
    Andrey Tverjanovich; Evgenii N. Borisov; Mohammad Kassem; Pascal Masselin; Daniele Fontanari; Eugene Bychkov

    Frequency conversion using nonlinear optical (NLO) crystals is widely used in advanced photonic technologies to produce coherent light in the spectral regions where the available laser sources are missing. Isotropic glasses usually do not show second order nonlinear processes like second harmonic or difference frequency generation (SHG, DFG) except for temporarily induced anisotropy under external stimuli. Here we show that a HgI2‐Ga2S3‐GeS2 homogeneous glass exhibits a strong intrinsic SHG response comparable with that of the well‐known NLO single crystal LiNbO3. The origin of this extremely rare phenomenon seems to be non‐centrosymmetric bent HgI2 molecules embedded in a sulfide glassy host. Taking into account the unique properties of chalcogenide glasses (wide IR transmission, low phonon density, unlimited ability to be modified changing the appropriate glass properties, fiber drawing and thin layer design), the observed phenomenon opens up the possibility of creating fundamentally new devices for mid‐IR photonics.

    更新日期:2020-01-21
  • Thermal History and Its Implications: A Case Study for Ion Exchange
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-19
    Liying Zhang; Xiaoju Guo

    Glasses containing monovalent species can be chemically strengthened by the replacement of smaller ions in the glass with larger external ions in the near glass surface. This type of ion exchange puts glass surface under high compressive stress. Glass mainly fails from tension with the presence of surface flaws. Chemical strengthening can change the stress at the flaw tip from tension to compression and further stop the flaw from propagating. Glass damage resistance is therefore significantly improved. For the same glass composition, glass thermal histories can affect the magnitude and depth of the compressive stress generated during ion exchange. In this study, the impact of thermal history on glass physical properties and ion exchange attributes in one alkali‐containing glass formed by fusion draw process was investigated. Multiple thermal treatments were done to rewrite the glass thermal histories. Glass density, refractive index and ion exchange properties as a function of the thermal treatment were studied. It is concluded that ion exchange related properties change dramatically with the glass thermal history.

    更新日期:2020-01-21
  • A New Identification of the Conducting Phase in Tungsten‐Titanium‐Phosphate Glass‐Ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-19
    Lisa Moore; Indrajit Dutta; Bryan Wheaton; Erika Stapleton; Ronald Parysek; Bruce Aitken

    Glass‐ceramics in the tungsten‐titanium‐phosphate system were first reported in 1992. The glass‐ceramics exhibited electrical resistivity as low as log(ρ) = ‐1.8 Ω.cm, which was attributed to an interconnecting network of tungsten sub‐oxide crystals, WO3‐x. A new identification of the conducting phase as a phosphate tungsten bronze, (PO4)2(WO3)2m, is reported. High temperature x‐ray diffraction and electron microscopy have been used to provide a detailed description of the crystalline phases and their development. As the ceramming temperature was raised, the phase assemblage progressed from cubic tungsten oxide and titanium pyrophosphate to phosphate tungsten bronzes. The m‐values of the bronzes ranged from 2‐7 depending on composition.

    更新日期:2020-01-21
  • Structural analysis of oxyfluoride borate glasses and BaF2 crystallization from phase separation
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-18
    Mauricio Rodríguez Chialanza; José Fabian Schneider; Romina Keuchkerian; Mariano Romero; Ricardo Faccio; Alvaro Olivera; Heinkel Bentos Pereira

    Oxyfluoride glasses and glass‐ceramics are being studied due to the possibility of their utilization in many applications, such as thermoluminescent dosimeters, optical fibers, fluorescent concentrators, and temperature sensors. In order to improve the properties of glasses, they are often crystallized. For this crystallization to be effective, knowledge and control of the structure of the starting material is required. Much work was done on silicate glasses; however, a great interest exists in obtaining these kinds of materials in other matrices such as germanates, phosphates, and borates. Here, we present a structural study of some oxyfluoride borate glasses with different concentrations of PbF2 by means of Nuclear Magnetic Resonance (NMR), Raman, and Fourier Transform Infrared (FTIR) spectroscopies. We also analyzed glass‐crystallization using Differential Scanning Calorimetry (DSC). The crystallization study was complemented with the use of Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED), for the purpose of identifying possible mechanisms of crystallization. Our results indicate, from the structural point of view, that the glasses present groups with one or two BO4 species, such as diborate or pentaborate. The small variation in the BO4 fraction with the increase in PbF2 concentration would be an indication of the formation of F−BO3/2 species. Oxyfluoride borate glass containing BaO as a cation modifier and 30 % PbF2 represents a good starting point for obtaining glass‐ceramics with a narrow size distribution of BaF2 nanocrystals. We also propose that the possible mechanism for oxyfluoride borate glass crystallization started with a separation of phases.

    更新日期:2020-01-21
  • Crystal structure, phonon modes, and bond characteristics of AgPb2B2V3O12 (B = Mg, Zn) microwave ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-16
    Rakhi Madhuri; Subodh Ganesanpotti

    AgPb2B2V3O12 (B = Mg, Zn) ceramics with low sintering temperature were synthesized via the conventional solid‐state reaction route. Rietveld refinements of the X‐ray diffraction patterns confirm cubic symmetry with space group . The number of observed vibrational modes and those predicted by group theoretical calculations also confirm the space group. At the optimum sintering temperature of 750°C/4 hours, AgPb2Mg2V3O12 has a relative permittivity of 23.3 ± 0.2, unloaded quality factor () of 26 900 ± 500 GHz (), and temperature coefficient of resonant frequency of 19.3 ± 1 ppm/°C, while AgPb2Zn2V3O12 has the corresponding values of 26.4 ± 0.2, 28 400 ± 500 GHz () and –18.4 ± 1 ppm/°C at 590°C/4 hours. Microwave dielectric properties of a few reported garnets and Pb2AgB2V3O12 (B = Mg, Zn) ceramics were correlated with their intrinsic characteristics such as the Raman shifts as well as width of A1g Raman bands. Higher quality factor was obtained for lower full width at half‐maxima (FWHMs) values of A1g modes. The increase in B‐site bond valence contributes to high and low |τf| with the substitution of Zn2+ by Mg2+. Furthermore, the high ionic polarizability and unit cell volume with Zn2+substitution contribute to increased relative permittivity.

    更新日期:2020-01-17
  • Highly transparent cerium doped glasses with full‐band UV‐shielding capacity
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-17
    Liming Teng; Yuncheng Jiang; Wenna Zhang; Rongfei Wei; Hai Guo

    With thinning ozone layer, considerable attention has been paid to developing materials that block or reduce ultraviolet (UV) transmission. Here, bulk cerium doped glasses with relatively high transparency were successfully manufactured via the melt quenching method. The varied ratio of Ce3+ and Ce4+ in glasses could be deduced by optical properties in photoluminescence excitation, emission spectra, decay curves and absorption spectra. These glasses exhibit excellent UV‐blocking capacity due to the absorption of Ce3+ and Ce4+. And their outstanding physical and chemical stability have been demonstrated in the condition of high temperature, long‐term UV radiation, strong alkali solution and acetone. Hence, our cerium doped glasses are potentially attractive candidates in UV‐shielding device.

    更新日期:2020-01-17
  • Lattice structure and microwave dielectric properties of La[Al1−x(Mg0.5Ti0.5)x]O3 (x = 0–0.2)‐based ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-16
    Jun Fan; Qing Zhao; Kang Du; Fei Wang; Xiao‐Hong Wang; Wen‐Zhong Lu; Wen Lei

    La[Al1−x(Mg0.5Ti0.5)x]O3 (LAMT, x = 0–0.2) ceramics were synthesized by the conventional solid‐state reaction method and formed a solid solution. The pure solid solutions were recorded by X‐ray diffraction (XRD) in every range. Relative permittivity (εr) and structural stability were greatly affected because the Al3+ site was replaced by [Mg0.5Ti0.5]3+. The total ionic polarizability gradually increased with x, and εr gradually increased. The trend of τf is due to change in structural stability. The variation in Q × f value increased firstly and then decreased due to the change in the symmetric stretching mode of Al/MgTi–O. The optimum microwave dielectric properties of LAMT were obtained at x of 0.1 after sintering at 1650 °C for 5 h, and εr = 24.9, Q × f = 79956 GHz, and τf = ‐33 ppm/°C. The CaTiO3 have a large positive τf (+800 ppm/°C), thus, the τf achieved near zero when CaTiO3 and LAMT (x = 0.1) ceramics were mixed with a certain molar mass, and the optimum microwave dielectric properties of 0.65CaTiO3–0.35LaAl0.9(Mg0.5Ti0.5)0.1O3 were as follows: εr = 44.6, Q × f = 32057 GHz, and τf = +2 ppm/°C.

    更新日期:2020-01-17
  • Controllable synthesis of glass ceramics containing YF3:Eu3+ nanocrystals: well preserved Eu and prolonged lifetime
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-16
    Peili Luo; Ping Huang; Jiancheng Wang; Chengfang Yao; Yuye Zhao; Beiying Zhou; Qi Zheng; Xin Zhang; Wan Jiang; Lianjun Wang

    Monolithic luminescent glass‐ceramic (GC) embedded with fluoride‐based nanocrystals (NCs) has drawn much attention as it is stable and possesses long fluorescent lifetime, while only small partial of optically active ions could enter the fluoride NCs prepared by conventional crystallization process. In this work, YF3:Eu3+ embedded GCs have been controllably synthesized by Spark Plasma Sintering at a relatively low sintering temperature (960 ℃) within 10 minutes. The GC samples show typical sharp reddish‐orange emissions peaking at λ= 590 nm and 620 nm, which can be ascribed to the 5D0→7Fj transitions of Eu3+ located in the tetrahedral coordination sites of the YF3 NCs. Significantly, a small R/O ratio (PL intensity ratio of 5D0→7F2/5D0→7F1) suggests that majority of Eu3+ ions are well preserved in YF3 NCs, which is confirmed by the EDS and TEM results that highly crystallized YF3:Eu3+ NCs are homogeneously dispersed into the silica glass matrix without interfacial reaction. Hence, the lifetime of GC sample is prolonged to 6.8 ms. These results demonstrate that Eu3+ could be well protected and resided in YF3 low phonon crystal by this method to fabricate GC composites with high optical performance.

    更新日期:2020-01-17
  • Enhanced photoluminescence quantum yield of red‐emitting CdTe:Gd3+ QDs for WLEDs applications
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-15
    Zhan‐Chao Wu; Zhen Yang; Xing‐Xing Zhang; Ming‐Xia Jiao; Jie Liu; Li Ma; Xiao‐Jun Wang

    In order to improve the quantum yield of red‐emitting CdTe quantum dots (QDs), CdTe:Gd3+ QDs were synthesized by a facile one‐step aqueous method. The composition, morphology, and photoluminescence property of CdTe:Gd3+ QDs were characterized. The results show that the doping of Gd3+ not only leads to a red‐shift in the emission wavelength but also improves the photoluminescence quantum yield (PL QY) of CdTe QDs up to 85.74%. Doping of Gd element causes the Te dangling bond on the surface of CdTe QDs to be destroyed, thus reducing the nonradiative surface recombination, which is considered to be the reason of the increase in PL QY of CdTe QDs. Finally, high color rendition white light was generated from the CdTe:Gd3+ QDs‐assisted phosphor‐converted white light‐emitting diode (WLED). Under operation of 50 mA forward bias current, the fabricated WLED emitted bright warm white light with a high color rendering index of 86, a low correlated color temperature (CCT) of 4020 K, a suitable Commission Internationale de l’Eclairage color coordinates of (0.3651, 0.3223), and an enhanced luminous efficiency of 68.52 lm/W.

    更新日期:2020-01-16
  • BSE‐IA reveals retardation mechanisms of polymer powders on cement hydration
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-15
    Yu Peng; Qiang Zeng; Shilang Xu; Guorong Zhao; Peiming Wang; Xianping Liu

    The retardation effects of two polymer powders, styrene butadiene rubber and ethylene‐vinyl acetate, on cement hydration and the mechanisms in terms of hydration degree and microstructure characteristics were investigated by the combination of backscattered electron image analysis, scanning electron microscopy, energy dispersive spectrometer, and X‐ray diffraction. Results show that the cement hydration can be significantly retarded by both polymers with the reductions in cement hydration degrees and hydration products thickness. Polymers retard cement hydration by agglomerating the polymer particles on the surfaces of cement grains, forming films in different thickness and bonding the calcium ions in the pore solutions. Consequentially, the formation of C‐S‐H and CH is depressed and the Ca/Si ratios in the hydration product layer are raised. The retardation is enhanced with the increase in polymer content. This work helps understand the retardation effect of polymers on cement hydration in depth, which enables rational development of polymer‐modified cement‐based materials for further applications.

    更新日期:2020-01-16
  • Damage evolution in SiC/SiC unidirectional composites by X‐ray tomography
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-15
    A.M. Hilmas; K.M. Sevener; J.W. Halloran

    Melt infiltrated SiC/SiC CMC unidirectional composite specimens were imaged under load using x‐ray micro tomography techniques in order to visualize the evolution of damage accumulation and to quantify damage mechanisms within the composite such as matrix cracking and fiber breaking. The data obtained from these in situ tensile tests were used in comparison with current models and literature results. 3D tomography images were used to measure the location and spacing of matrix cracking that occurred at increasing stress increments during testing within two unidirectional composite specimens. The number of broken fibers and the location of each fiber break gap that occurred within the volume of both specimens was also quantified. The three‐dimensional locations of fiber breaks were correlated to the location of each matrix crack within the volume of the specimen and it was found that at the stress scanned directly before failure, most of the fiber breaks occur within 100 microns of a matrix crack.

    更新日期:2020-01-16
  • A Novel CMAS Resistant Material Based on Thermodynamic Equilibrium Design: apatite‐type Gd10(SiO4)6O3
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Di Wu; Han Zhang; Xiao Shan; Fan Yang; Fangwei Guo; Xiaofeng Zhao; Ping Xiao; Shengkai Gong

    Calcium‐magnesium‐alumino‐silicates (CMAS) melt attack has been a critical issue for the thermal barrier coatings (TBCs) with ever increasing engine operating temperature. In this study, a novel CMAS resistant material apatite‐type Gd10(SiO4)6O3 is developed for TBCs application based on thermodynamic equilibrium design. The chemical reaction of Gd10(SiO4)6O3 bulk and CMAS melt is investigated at 1300ºC. The CMAS corrosion resistance of Gd10(SiO4)6O3 bulk is evaluated and compared with the well‐studied CMAS resistant material Gd2Zr2O7 (GZO). It is found that Gd10(SiO4)6O3 shows a significantly enhanced CMAS resistance, including lower intrinsic CMAS infiltration rate (~1.09 μm/h1/2) and smaller infiltration upper limit (50‐62 μm) for a 20 mg/cm2 CMAS deposition. More importantly, for Gd10(SiO4)6O3, the CMAS infiltration only alters the composition but does not change the crystal structure or destroy microstructural integrity. The reaction mechanism is elucidated as two stages. 1) surface Gd10(SiO4)6O3 quickly transforms into Ca2Gd8(SiO4)6O2 in suit by inter‐diffusion with CMAS melt and then is thermodynamically stable with CMAS melt, thereby effectively inhibiting the further CMAS infiltration. 2) with the ongoing inter‐diffusion of Gd/Ca, the CMAS infiltrated layer slowly thickens and follows a parabolic law. Meanwhile, the CMAS melt gradually precipitates Ca2Gd8(SiO4)6O2 and CaAl2Si2O8 (anorthite) until melt is exhausted.

    更新日期:2020-01-15
  • Quantification of grain boundary defect chemistry in a mixed proton‐electron conducting oxide composite
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    George L. Burton; Sandrine Ricote; Brendan J. Foran; David R. Diercks; Brian P. Gorman

    Dual phase oxide membranes have shown promising hydrogen permeation fluxes in syngas applications due to their high mixed proton and electron conduction (MPEC). However, the conductivity of grain boundaries can be many orders of magnitude lower than that of the bulk and so limits the total conductivity and hydrogen permeation. In this study, the three‐dimensional nanoscale oxygen and cation distributions around grain and phase boundaries in a BaCe0.8Y0.2O3‐δ ‐ Ce0.8Y0.2O2‐δ (BCY‐YDC) membrane were quantified by atom probe tomography (APT) and related to average grain boundary conductivity measured by electrochemical impedance spectroscopy (EIS). Segregation varied among the general high‐angle grain boundaries analyzed, but no trend from orientation analysis was determined. Correlative APT and electron energy loss spectroscopy (EELS) of one YDC grain boundary revealed composition and cerium valence information, respectively, allowing for the determination of vacancies at the grain boundary. While a specific MPEC membrane is characterized, the results are relevant to proton and electron conduction in a number of technologically important ceramics.

    更新日期:2020-01-15
  • Fabrication and optical properties of Tb3+‐doped NaCaPO4 glass‐ceramics and their radiation detection applications
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Jie Xu; Zhaoyang Chen; Minqiang Gai; Ting Liu; Yanwei Fan; Chengfa He

    Tb3+‐doped 25Na2O‐23CaO‐6P2O5‐44B2O3‐2ZrO2 glass was fabricated by conventional melt‐quenching technique. Glass‐ceramics containing NaCaPO4 crystals were then obtained by heating the as‐prepared glasses. Their optical and luminescence properties were studied by FT‐IR spectroscopy, photoluminescence (PL), absorption spectra, thermoluminescence (TL), and optically stimulated luminescence in continuous wave modality (CW‐OSL). The glasses were composed of [PO4], [BO3], and [BO4] basic structural units. The PL excitation and emission spectra exhibited Tb3+‐related transitions, as well as the strongest excitation and emission wavelengths at 370 and 454 nm, respectively. We further investigated the CW‐OSL properties as a function of dopant concentration and time elapsed after irradiation (signal fading). Results indicated that the CW‐OSL intensity reached a maximum when the Tb4O7 concentration was 0.25 mol%. The fading of the OSL signal showed that the OSL signal of Tb3+‐doped NaCaPO4 glass‐ceramics was approximately 65% in 8 days, after which the intensity remained stable. The TL glow curves had a broad peak feature peaking at 180 ± 5ºC. The samples also exhibited good signal reusability and a broad linear dose‐response range (0.03‐1000 Gy). The excellent luminescent and dosimetric properties of these Tb3+‐doped NaCaPO4 glass‐ceramics indicated their potential applications in radiation dosimetry.

    更新日期:2020-01-14
  • Degradation mechanism of Cr2O3‐Al2O3‐ZrO2 refractories in a coal‐water slurry gasifier: Role of stress cracks
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Junfeng Chen; Junli Xiao; Yu Zhang; Yaowu Wei; Youqi Li; Shaowei Zhang; Nan Li

    Durability and corrosion behavior of refractory lining, a core part of a commercial water‐coal slurry gasifier, largely determine its gas yield rate and carbon conversion rate. In this work, corrosion behavior of high chromite‐containing refractory served for 4200 hours in a water‐coal slurry gasifier was studied, and the role of generated stress cracks on its degradation behavior was elucidated. The results demonstrated that the depth of penetration under the assistance of the cracks was up to 2.60 cm, which was much deeper than that in the case free from stress crack. The sub‐surface, accompanied by cracking, provided a pathway for slag penetration and resulted in more severe corrosion. Furthermore, the isolation layer of (Cr, Fe, Al)2O3 solid solution on the refractory’ surface trapped most of the iron oxides. As a result, a further attack from other main corrosive species was controlled by their diffusion through it.

    更新日期:2020-01-14
  • Fused silica contamination layer removal using magnetic field‐assisted finishing
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Julian Long; Daniel Ross; Erik Tastepe; Mikayla Lamb; Yusuke Funamoto; Daichi Shima; Tomosumi Kamimura; Hitomi Yamaguchi

    Fused silica optics used in lasing systems requires a high laser‐induced damage resistance. Processes typically used to polish fused silica lenses induce subsurface and surface damage that collect ceria abrasive, creating a layer of contamination. The contamination can be a precursor to laser damage during use. A preliminary study showed the feasibility of magnetic field‐assisted finishing (MAF) for polishing fused silica and suggested possible beneficial effects of the MAF‐polished surface on the laser‐induced damage threshold (LIDT). This paper proposes a method to examine the fundamental polishing characteristics of MAF for fused silica. Using the proposed method, this paper explores the material removal characteristics of the MAF process and improves the understanding of the MAF polishing mechanism. The 45% improvement of LIDT shows the efficacy of MAF for removing the contamination layer of fused silica surfaces with minimal changes in the surface roughness.

    更新日期:2020-01-14
  • Densification, strengthening and toughening in hafnium carbide with the addition of silicon carbonitride
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Wei Hao; Na Ni; Yi Guo; Chuanwei Li; Xiaohui Fan; Weiwei Xiao; Xiaofeng Zhao; Ping Xiao
    更新日期:2020-01-14
  • SPS‐assisted synthesis of InGaO3(ZnO)m ceramics, and influence of m on the band gap and the thermal conductivity
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Sébastien Préaud; Céline Byl; François Brisset; David Berardan

    In this study, we report on the use of a two‐stage annealing treatment at 1100°C coupled to reactive Spark Plasma Sintering to reduce the synthesis temperature of InGaO3(ZnO)m (m=1 to 9) dense polycrystalline pellets below 1200°C, in order to suppress the volatilization of ZnO and get a better control of the crystalline quality of the pellets. We show that using this treatment, dense single phase pellets can be prepared with randomly oriented grains. Besides, we evidence a monotonic evolution of the band gap in the series from 3.27 eV in InGaO3(ZnO) to 3.02 eV in InGaO3(ZnO)9, as well as a non‐monotonic evolution of the lattice thermal conductivity that reaches a minimum for InGaO3(ZnO)3, lower than 2 W.m‐1.K‐1 above 350°C. Last, we propose a procedure for the high temperature measurement of the thermal diffusivity of oxides by the laser flash method to avoid possible reactions between the measured material and the graphite spray.

    更新日期:2020-01-14
  • Atomistic Understanding of Surface Wear Process of Sodium Silicate Glass in Dry versus Humid Environments
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Seung Ho Hahn; Hongshen Liu; Seong H. Kim; Adri C.T. van Duin

    Understanding surface reactions of silicate glass under interfacial shear is critical as it can provide physical insights needed for rational design of more durable glasses. Here, we performed reactive molecular dynamics (MD) simulations with ReaxFF potentials to study the mechanochemical wear of sodium silicate glass rubbed with amorphous silica in the absence and presence of interfacial water molecules. The effect of water molecules on the shear‐induced chemical reaction at the sliding interface was investigated. The dependence of wear on the number of interfacial water molecules in ReaxFF‐MD simulations was in reasonable agreement with the experimental data. Confirming this, the ReaxFF‐MD simulation was used to find further details of atomistic reaction dynamics that cannot be obtained from experimental investigations only. The simulation showed that the severe wear in the dry condition is due to the formation of interfacial Sisubstrate‐O‐Sicounter_surface bond that convey the interfacial shear stress to the subsurface and the presence of interfacial water reduces the interfacial bridging bond formation. The leachable sodium ions facilitate surface reactions with water producing hydroxyl groups and their key role in the hydrolysis reaction is discussed.

    更新日期:2020-01-14
  • Self‐reduction of Eu3+ to Eu2+ in europium doped Li2B4O7 glass prepared in air
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Xin‐Yuan Sun; Xian‐Chao Le; Zhuohao Xiao; Xuhai Shi; Wen‐Feng Wang; Zuofu Hu; Qing‐Mei Yang; Rongfei Wei; Hai Guo

    Motivated by the detection of neutrons, europium doped Li2B4O7 glasses enriched with both lithium and boron elements with high cross‐section capture were developed. A highly effective method of realizing the self‐reduction of Eu3+ to Eu2+ ions in europium doped Li2B4O7 glasses prepared by high temperature melt‐quenching technology in air were revealed. The self‐reduction of Eu3+ to Eu2+ ions can be easily achieved by the partial replacement of B2O3 with BN within 2 mol% concentration. And the effect of partially replacing B2O3 with BN on the optical properties of europium doped Li2B4O7 glass are systematically studied by transmittance, photoluminescence and radioluminescence spectra, together with the luminescence decay curves.

    更新日期:2020-01-14
  • Strategy to Design High Performance TiB2 Based Materials: Strengthen Grain Boundaries by Solid Solute Segregation
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-14
    Fu‐Zhi Dai; Huimin Xiang; Yanchun Zhou

    TiB2 exhibits a unique combination of excellent properties that makes it promising candidate for applications in extreme environments, where retention of strength at high temperatures is essential. Tailoring grain boundary properties by segregation is believed a prominent way to design high temperature performance of ceramics. In this work, segregation tendencies of solute elements, including Sc, Y, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, in TiB2 grain boundaries and the strengthening/weakening effects induced by segregations are investigated by first‐principles calculations. The results reveal that small atoms tend to segregate to grain boundary sites with local compression strains, while large atoms prefer grain boundary sites with local expansion strains. Deteriorated grain boundary strength is usually caused by additional expansion strain induced by segregation, while improved grain boundary strength results from either enhanced local bonding induced by segregation of small atoms or increased fracture strain due to segregation of large atoms. Cr and V, especially Cr, exhibit strong segregation tendency and improvement on grain boundary strength, which provides useful guidelines for the design of high performance TiB2 based materials.

    更新日期:2020-01-14
  • Thermosetting polymers in cold sintering: The fabrication of ZnO ‐ Polydimethylsiloxane composites
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-13
    Arnaud Ndayishimiye; Zane A. Grady; Kosuke Tsuji; Ke Wang; Sun Hwi Bang; Clive A. Randall

    This study reports the fabrication of the first ceramics‐thermosetting polymer composites by the cold sintering process, for high ceramic volume fraction (v/v > 95 %). The (1–x) ZnO ‐ x PDMS composites, with 0.00 ≤ x ≤ 0.05, were cold sintered at 250 °C, 320 MPa for 60 min. In situ densification studies conducted with the help of a semi‐automated press revealed that the mechanisms driving the densification of the material changes with the polymer content. Relative densities of (1–x) ZnO ‐ x PDMS composites (0.00 ≤ x ≤ 0.05) were above 90%. Impedance spectroscopy of the composites yields insight into the ceramic‐polymer interfaces within the sintered ZnO bulk and suggests long‐range conduction governed by ZnO‐PDMS interface properties for x = 0.03 and x = 0.05. The study also emphasizes on the complexities and opportunities of such ceramic‐dominated Ceramic‐Polymer Composites.

    更新日期:2020-01-14
  • Interplay of Spin, Lattice, Vibration, and Charge Degrees of Freedom: Magneto‐dielectricity in Ca3Mn2O7
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-13
    Pooja Sahlot; Gaurav Sharma; Vasant Sathe; A.K. Sinha; A.M. Awasthi

    From low‐temperature Synchrotron X‐ray diffraction, a precise thermal characterization of octahedral distortions in single phase Ruddlesden‐Popper Ca3Mn2O7 is performed. Highly sensitive close‐steps temperature dependences of Mn‐O‐Mn bond angles connecting MnO6 octahedra clearly reveal the signature of spin‐ordering in the system. Spin‐lattice coupling is thus revealed via the structural distortions, responsible for the evolution of magnetic state. Further, benchmark temperature anomalies observed in the unit cell volume and its polarization‐measure highlight the interplay between spin, lattice, and charge degrees of freedom. Strong spin‐lattice coupling is supported by the Raman spectroscopy results across the magnetic ordering. Dielectric study on Ca3Mn2O7 features relaxor‐like segmented dynamics below the antiferromagnetic ordering. Dipolar relaxations of different origins are spectrally resolved, exhibiting distinct H‐field alterations which identify their allegiance to different magnetic sub‐phases. Dipole‐relaxation characteristics examined under applied magnetic field and the ensuing magneto‐dielectricity consistently correlate with the concurrent magnetic, structural, and vibrational features.

    更新日期:2020-01-14
  • Tunable color emission in LaScO3:Bi3+, Tb3+, Eu3+ phosphor
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-13
    Dayu Huang; Wei Yi; Dang Peipei; Xiao Xiao; Hongzhou Lian; Jun Lin

    LaScO3:xBi3+,yTb3+,zEu3+ (x = 0‐0.04, y = 0‐0.05, z = 0‐0.05) phosphors were prepared via high temperature solid‐state reaction. Phase identification and crystal structures of the LaScO3:xBi3+,yTb3+,zEu3+ phosphors were investigated by X‐ray diffraction (XRD). Crystal structure of phosphors were analyzed by Rietveld refinement and transmission electron microscopy (TEM). The luminescent performance of this trichromatic phosphors is investigated by diffuse reflection spectra and photoluminescence. The phenomenon of energy transfer from Bi3+ and Tb3+ to Eu3+ in LaScO3:xBi3+,yTb3+,zEu3+ phosphors was investigated. By changing the ratio of x, y and z, trichromatic can be obtained in the LaScO3 host, including red, green and blue emission with peak centered at 613, 544 and 428 nm, respectively. Therefore, two kinds of white light emitting phosphors were obtained, LaScO3:0.02Bi3+,0.05Tb3+,zEu3+ and LaScO3:0.02Bi3+,0.03Eu3+,yTb3+. The energy transfer was characterized by decay times of the LaScO3:xBi3+, yTb3+, zEu3+ phosphors. Moreover, absolute internal QY and CIE chromatic coordinates are shown. The potential optical thermometry application of LaScO3:Bi3+,Eu3+ was based on the temperature sensitivity of the FIR. The maximum Sa and Sr are 0.118 K‐1 (at 473.15 K) and 0.795 % K‐1 (at 448.15 K), respectively. Hence, the LaScO3:Bi3+,Eu3+ phosphor is a good material for optical temperature sensing.

    更新日期:2020-01-14
  • Highly permeable Al2O3 microfiltration membranes with holey interior structure achieved through sacrificial C particles
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-13
    Zhixiao Zhang; Tze Chiang Albert Ng; Qilin Gu; Lei Zhang; Zeming He; Zhiyang Lyu; Xiaorong Zhang; Weimin Wang; How Yong Ng; John Wang

    Alumina (Al2O3) microfiltration membranes with holey interior structure exhibit the combined desirable properties of both superior permeability and good retention performance. These membranes are purposely made to consist of a holey intermediate layer with a thickness in the range 12.9‐15.5 μm and a top filtration layer of very thin thickness in the range 1.8‐2.8 μm, fabricated by cosintering with sacrificial carbon (C) particles. The holey structure in the intermediate layer, which can be well controlled by regulating the amount of C particles added to Al2O3 particles, contains plenty of holes of 1‐5 μm in size, thereby possessing much reduced water flow resistance. The thin top filtration layer, which contains uniformly distributed nanopores of 90‐100 nm in size, functions as the selective layer, thereby ensuring a good retention performance. At the C particle content of 30 wt. %, the Al2O3 membrane with holey interior structure demonstrates a super‐high pure water flux (3358 Lm‐2h‐1), which is almost twice that of the membrane without holey interior structure (1697 Lm‐2h‐1), while the mechanical strength and retention performance are well maintained.

    更新日期:2020-01-13
  • Suppression of nitridation of yttria‐doped zirconia during flash sintering
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-12
    Tsuyoshi Kurachi; Yudai Yamashita; Tomoharu Tokunaga; Hidehiro Yoshida; Takahisa Yamamoto

    The effect of direct current (DC) and alternating current (AC) on nitridation of 3 mol% Y2O3‐doped ZrO2 (3YSZ) after keeping in a flash state for 1 hour was investigated. The inside of the DC‐flashed compact was confirmed to exhibit blacking. Scanning transmission electron microscopy, electron energy loss spectroscopy, and X‐ray diffraction analysis revealed that zirconium nitrides formed in the blackened area. In contrast, a uniformly densified compact without blackening was obtained by AC fields. No zirconium nitrides formed in the compacts exposed to AC fields even when the flash state was maintained for 1 hour. Therefore, AC fields are effective to suppress nitridation of 3YSZ during flash sintering.

    更新日期:2020-01-13
  • A novel efficient RhB absorbent of Mo2N/MoO2 composite nanofibers for wastewater treatment
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-12
    Jing Cheng; Yan Xing; Ziyuan Wang; Xiaohui Zhao; XiQiang Zhong; Wei Pan

    Mo2N/MoO2 composite nanofibers have been prepared via an electrospinning and controlled nitridation process. The composite nanofibers exhibit a highly efficient Rhodamine B (RhB) absorption behavior with a rate constant of 0.153 g min−1 mg−1, which is about 20 times of the commercial‐activated carbon material. Furthermore, the nanofibers show stable absorption activity after recycled by an environmental friendly procedure for four times. The excellent absorption performance of Mo2N/MoO2 composite nanofibers demonstrates a promising application of Mo2N‐related materials as an absorbent for wastewater treatment.

    更新日期:2020-01-13
  • Combining effects of TiO6 octahedron rotations and random electric fields on structural and properties in Na0.5Bi0.5TiO3
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-12
    Kaijun Hao; Wenwei Ge; Zhenan Ren; Xiaojuan Liu; Laihui Luo; Xiaobing Li; Haosu Luo; Dwight Viehland

    The structural and dielectric properties of Na0.5Bi0.5TiO3 (NBT) ceramics and crystals have been investigated and are compared to that of Pb(Zr0.55Ti0.45)O3 (PZT55/45) and Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMNT 72/28) ceramics. X‐ray diffraction (XRD) profiles for (100), (110), (111), (200), (220), and (222) (referred to cubic structure) reveal that the monoclinic structure with Cc space group exists both in the NBT single crystal and ceramics. The diffraction profile obtained with high resolution laboratory XRD for the NBT single crystal can be well described, using Cc model instead of R3c model. The dielectric constant of NBT below Thump shows some similarity to that of PZT45/55 ceramics below 50°C in which oxygen octahedron rotations cause the frequency dispersion of the dielectric constant. The temperature‐dependent dielectric constant for NBT can be deconvolved into two independent processes. The lower temperature process shows a typical relaxor characteristic and follows the Vogel‐Fulcher relationship. The other process at higher temperature shows less frequency‐dependent behavior. Comparing the dielectric constant of NBT with that of PZT55/45 and PMNT72/28 reveals that both oxygen octahedral rotations and random electric fields play an important role in the frequency dispersion of the dielectric constant for NBT relaxor feroelectric.

    更新日期:2020-01-13
  • Formation criterion for binary metal diboride solid solutions established through combinatorial methods
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-12
    Tongqi Wen; Beilin Ye; Honghua Liu; Shanshan Ning; Cai‐Zhuang Wang; Yanhui Chu

    Establishing the formation criterion is urgent for accelerating the discovery and design of solid‐solution materials with desirable properties. The previously reported formation criterion mainly focused on solid‐solution alloys, while the formation criterion was rarely established in solid‐solution ceramics. To solve this problem, herein, we take a class of solid‐solution ceramics, namely binary metal diboride ((MxN1‐x)B2) solid solutions, as a prototype. Through combinatorial methods including high‐throughput molten salt syntheses and high‐throughput first‐principles calculations combined with the machine learning approach, the correlation between influential factors, including atomic size difference (δ), mixing enthalpy at 0 K and 0 Pa (), doping condition (φ), and valence electron concentration (VEC), and the formation ability of (MxN1‐x)B2 solid solutions was first studied systematically, and then their formation criterion was well established. The results showed that the influential degree of the aforementioned four factors on the formation ability of (MxN1‐x)B2 solid solutions could be described as follows: δ > > φ > VEC. In addition, a newly proposed parameter, β, could well reflect the formation ability of (MxN1‐x)B2 solid solutions: when β > 0, the single‐phase (MxN1‐x)B2 solid solutions could be successfully synthesized in our work and vice versa. This study may provide a theoretical guidance in the discovery and design of various solid‐solution ceramics, such as the metal borides, carbides, nitrides, etc, with desirable properties.

    更新日期:2020-01-13
  • Optical temperature sensing and luminescent switching properties in Pr/Er doped (K0.5Na0.5)NbO3 materials
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-13
    Yan Zhu; Xuefeng Li; Zizhong Guo; Haiqin Sun; Qiwei Zhang; Xihong Hao

    For optical temperature sensing materials, the emission and excitation bands are extremely critical to measure the temperature by fluorescence intensity ratio (FIR) technique. Singly Ln‐doped optical temperature sensing materials exhibit very few emission bands, which greatly constraints their practical applications of FIR technique. Here, the fabricated Pr/Er co‐doped (K0.5Na0.5)NbO3 materials exhibited multi‐color (red‐green) and dual‐mode (downshifting/upconversion) luminescence properties. The temperature sensitivity can be effectively tuned by choosing different emission or excitation bands. The optimized optical temperature sensitivity reached up to 0.0094 K‐1, much higher than that of most temperature sensing materials. Besides, the samples also showed excellent luminescence modulation properties based on the photochromic reaction. Under sunlight irradiation, the luminescent switching contrast (ΔRt) of the samples reached more than 60%. These results may provide a guiding role in designing and modulating optical temperature sensing properties for multifunctional materials.

    更新日期:2020-01-13
  • Effects of sizes of additive particles on suspensions, microstructures and electrical properties of ZnO varistors
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-13
    Ying Yan; Xin Ren; Qi Cheng; Xuejun Ruan; Manyu Wang; Wenqi Yu; Zheng Yao

    The effects of the sizes of the additives on the dispersion of the mixed suspensions, the microstructures and the comprehensive electrical properties of the ZnO varistors were investigated. The particle size distribution, the viscosity and the zeta potential of the suspension were characterized to evaluate the effect of the sizes of the additives on the dispersion of the metal oxide particles. The potential gradient, the leakage current, the nonlinear coefficient, the voltage ratio and the aging coefficient were considered to estimate the effect of the sizes of the additives on the performance of the ZnO varistors. The electrical testing results showed that a proper amount of milling of the additive particles could improve the comprehensive electrical properties, while an excessive milling produced the opposite effect. When the milling time of the additive particles reached 30 minutes, the dispersion of ZnO‐additive mixed suspension was the best. The as‐prepared varistors showed the optimal electrical performance with potential gradient of 310 V/mm, leakage current of 1 μA, nonlinear coefficient of 32.7, voltage ratio of 1.69 and aging coefficient of 0.59.

    更新日期:2020-01-13
  • Crystallization induced valence state change of Mn2+ → Mn4+ in LiNaGe4O9 glass ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-10
    Ibrahim Morad; Xiaofeng Liu; Jianrong Qiu

    Tetra‐valent manganese (Mn4+) has been regarded as an efficient non‐rare‐earth red‐light emitting ion, which has stimulated continued search of robust hosts and efficient synthetic methods to stabilize Mn4+ centers with strong photoluminescence. In this work, we demonstrate a facile synthetic method for Mn4+ doped glass‐ceramic (GC) based on crystallization induced oxidation state change in an oxide glass. The parent glass with a formula of LiNaGe4O9 is fabricated by melt‐quenching and crystallization is induced by thermal treatment in air. Oxidation of Mn2+ in glass to Mn4+ in the GC is confirmed by both optical spectroscopy and electron paramagnetic resonance (EPR) measurements. After thermal treatment, the characteristic reddish photoluminescence (PL) of Mn2+ in the glass centered at 611 nm disappears and a strong photoluminescence peak at 660 nm attributed to Mn4+ is observed. The conversion to Mn4+ after crystallization in the examined system may have strong implications for synthesis of Mn4+ doped phosphors which always requires rigorous control of the redox equilibrium during synthesis.

    更新日期:2020-01-13
  • Effects of TaB2 and TiB2 on the grain growth behavior and kinetics of HfB2 ceramics during pressureless sintering
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-10
    Qi‐Qi Zhu; Zhan Zhang; Yan Zhang; Qiu‐Yu Liu; Bang‐Chao Song; Wei‐Ming Guo; Hua‐Tay Lin

    HfB2, Hf0.95Ta0.05B2, and Hf0.95Ti0.05B2 powders were self‐synthesized and the grain sizes were 2.04, 0.36, and 1.15 μm, respectively. The three powders were pressureless sintered from 1700℃ to 2000℃ to study and compare effects of the introduction of TaB2 and TiB2 on the grain growth behavior and kinetics of HfB2. The results revealed that HfB2 showed moderately slow grain growth in the whole process. However, significant grain growth consistently happened in Hf0.95Ti0.05B2 and Hf0.95Ta0.05B2 at 1900℃ or above. Eventually, the grain size of Hf0.95Ti0.05B2 increased to almost the same as HfB2, but Hf0.95Ta0.05B2 still possessed smaller grains due to the finest original powders. The grain growth exponent was determined to be ~3, and the dominant growth rate‐controlling mechanism was volume diffusion. The average activation energy of HfB2, Hf0.95Ta0.05B2, and Hf0.95Ti0.05B2 for grain growth at 1700℃‐2000℃ was 191 ± 34, 678 ± 73, and 321 ± 61 kJ/mol, respectively.

    更新日期:2020-01-11
  • Synthesis of the ternary metal carbide solid‐solution ceramics by polymer‐derived‐ceramic route
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-10
    Honghua Liu; Bin Du; Yanhui Chu

    The polymer‐derived‐ceramic (PDC) route has been widely used to fabricate the transition‐metal carbides (TMCs). Previously reported works focused mainly on the synthesis of the single or binary TMCs, while the synthesis of the ternary or more component TMCs was rarely reported. Herein, a class of the ternary TMCs, namely (Nb1/3Zr1/3Ta1/3)C solid‐solution ceramics, was successfully synthesized via PDC route for the first time. The as‐synthesized ceramics exhibited the particle‐like morphology with an average particle size of ~250 nm and showed a single rock‐salt crystal structure of metal carbides. At the same time, they had high compositional uniformity from nanoscale to microscale. In addition, they possessed low‐oxygen impurity content of 0.79 wt% and moderate‐carbon impurity content of 8.98 wt%. Such work provides a novel route to fabricate the ternary or more component TMCs.

    更新日期:2020-01-11
  • 更新日期:2020-01-11
  • A facile pathway to prepare molybdenum boride powder from molybdenum and boron carbide
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-09
    Yu Wang; He Zhang; Shuqiang Jiao; Kuo‐Chih Chou; Guo‐Hua Zhang

    Molybdenum boride is an ideal hard and wear‐resistant material. In this study, a new method is proposed for preparing molybdenum boride, by which Mo first reacts with B4C to generate the mixture of molybdenum boride and C, and then the product is decarburized by molten Ca to generate CaC2. Pure molybdenum boride could be obtained after acid leaching to remove the by‐product CaC2. According to the experimental and thermodynamic calculation results, it is concluded that the single‐phase MoB could be successfully prepared, while Mo2B, Mo2B5, and MoB4 could not be synthesized by this method. Moreover, it was found that the particle size of finally prepared MoB is determined by particle size of raw Mo powder. The residual carbon content of the product could be decreased to 0.10 wt% after first reaction at 1673 K for 6 hours and then decarburization reaction at 1673 K for 6 hours.

    更新日期:2020-01-09
  • Finite element modeling of resistive surface layers by micro‐contact impedance spectroscopy
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-08
    Richard A. Veazey; Amy S. Gandy; Derek C. Sinclair; Julian S. Dean

    Micro‐contact impedance spectroscopy (MCIS) is potentially a powerful tool for the exploration of resistive surface layers on top of a conductive bulk or substrate material. MCIS employs micro‐contacts in contrast to conventional IS where macroscopic electrodes are used. To extract the conductivity of each region accurately using MCIS requires the data to be corrected for geometry. Using finite element modeling on a system where the resistivity of the surface layer is at least a factor of ten greater than the bulk/substrate, we show how current flows through the two layers using two typical micro‐contact configurations. This allows us to establish if and what is the most accurate and reliable method for extracting conductivity values for both regions. For a top circular micro‐contact and a full bottom counter electrode, the surface layer conductivity (σs) can be accurately extracted using a spreading resistance equation if the thickness is ~10 times the micro‐contact radius; however, bulk conductivity (σb) values can not be accurately determined. If the contact radius is 10 times the thickness of the resistive surface, a geometrical factor using the micro‐contact area provides accurate σs values. In this case, a spreading resistance equation also provides a good approximation for σb. For two top circular micro‐contacts on thin resistive surface layers, the MCIS response from the surface layer is independent of the contact separation; however, the bulk response is dependent on the contact separation and at small separations contact interference occurs. As a consequence, there is not a single ideal experimental setup that works; to obtain accurate σs and σb values the micro‐contact radius, surface layer thickness and the contact separation must all be considered together. Here we provide scenarios where accurate σs and σb values can be obtained that highlight the importance of experimental design and where appropriate equations can be employed for thin and thick resistive surface layers.

    更新日期:2020-01-08
  • Modeling nonisothermal crystallization in a BaO∙2SiO2 glass
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-08
    D. C. Van Hoesen; Xinsheng Xia; Matthew E. McKenzie; K. F. Kelton

    The accuracy of a differential thermal analysis (DTA) technique for predicting the temperature range of significant nucleation is examined in a BaO∙2SiO2 glass by iterative numerical calculations. The numerical model takes account of time‐dependent nucleation, finite particle size, size‐dependent crystal growth rates, and surface crystallization. The calculations were made using the classical and, for the first time, the diffuse interface theories of nucleation. The results of the calculations are in agreement with experimental measurements, demonstrating the validity of the DTA technique. They show that this is independent of the DTA scan rate used and that surface crystallization has a negligible effect for the glass particle sizes studied. A breakdown of the Stokes‐Einstein relation between viscosity and the diffusion coefficient is demonstrated for low temperatures, near the maximum nucleation rate. However, it is shown that accurate values for the diffusion coefficient can be obtained from the induction time for nucleation and the growth velocity in this temperature range.

    更新日期:2020-01-08
  • Scaling behavior of dynamic hysteresis of PMN‐PT relaxor ferroelectric ceramics near the morphotropic phase boundary
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-08
    Xiang Xia; Chunbo Li; Jiangtao Zeng; Liaoying Zheng; Guorong Li

    Due to the feature of domains, ferroelectric materials display hysteresis behavior with respect to the change of the applied electric field. Every ferroelectric material has its own unique hysteresis loop reflecting the information of domain reversal under an electric field. In this work, the scaling behaviors of dynamic hysteresis in relaxor (1−x)Pb(Mg1/3Nb2/3)O2‐xPbTiO3 (PMN‐PT) ceramics with different compositions were studied systemically. Our results showed that the evolution of scaling behavior in PMN‐PT ceramics can be divided into three stages, which is independent of the phase structure of the ceramics and the testing electric field frequency. The relationship between hysteresis area and field amplitude E0 obeys the power law ∝ in the low and high E0 regions, where the reorientations of 180° and non‐180° domain are dominant, respectively. However, the dynamic hysteresis area does not follow the power law in the intermediate E0 regions, which is attributed to the interaction of different domain reversal mechanisms. Furthermore, the hysteresis area decreases gradually with increasing frequency at a certain E0 and the time‐dependent domain reversal process was also discussed.

  • Structure and lithium‐ion mobility in Li1.5M0.5Ge1.5(PO4)3 (M = Ga, Sc, Y) NASICON glass‐ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    Igor d’Anciães Almeida Silva; Adriana M. Nieto‐Muñoz; Ana Candida Martins Rodrigues; Hellmut Eckert

    This work reports structural and lithium‐ion mobility studies in NASICON single‐ or multiple phase Li1+xMxGe2‐x(PO4)3 (M = Ga3+, Sc3+, Y3+) glass‐ceramics using solid‐state NMR techniques, X‐ray powder diffraction, and impedance spectroscopy. X‐ray powder diffraction data show the successful incorporation of Ga3+ and Sc3+ into the Ge4+ octahedral sites of the NASICON structure at the levels of x = 0.5 and 0.4, respectively. The glass‐to‐crystal transition was further characterized by multinuclear NMR and electrical conductivity measurements. Among the studied samples, the gallium‐containing glass‐ceramic presented the highest DC conductivity, 1.1 × 10‐4 S/cm at room temperature, whereas for the Sc‐containing samples, the maximum room temperature conductivity that could be reached was 4.8 × 10‐6 S/cm. No indications of any substitution of Ge4+ by Y3+ could be found.

    更新日期:2020-01-08
  • Co‐doping effects of (Al, Ti, Mg) on the microstructure and electrical behavior of ZnO based ceramics
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    Qianying Sun; Guorong Li; Tian Tian; Jiangtao Zeng; Kunyu Zhao; Laoying Zheng; Maud Barré; Jens Dittmer; Francois Gouttenoire; Anthony Rousseau; Abdel Hadi Kassiba

    Co‐doped ZnO based ceramics by using Al, Ti and Mg ions in different ratios were synthesized with the objective to investigate the doping effects on the crystalline features, microstructure and the electrical behavior. For Al and Ti doping, a coexistence of crystalline phases was shown with a major wurtzite ZnO structure and secondary spinel phases (ZnAl2O4, Zn2TiO4 or ZnaTibAlcOd), while Mg doping did not alter significantly the structural features of the wurtzite ZnO phase. The electrical behavior induced by Al, Ti and Mg co‐doping in different ratios was investigated by using Raman, EPR and 27Al and 67Zn solid state NMR. Al doping induces a high electrical conductivity compared to other doping elements. In particular, shallow donors from Zni‐AlZn defect structures are inferred from the characteristic NMR signal at about 185 ppm; i.e. quite far from the usual oxygen coordinated Al. The Knight shift effect emanating from a highly conducting Al doped ZnO ceramics was considered as the origin of this observation. Oppositely, as Ti doping leads to the formation of secondary spinel phases, EPR analysis shows a high concentration of Ti3+ ions which limit the electrical conductivity. The correlation between the structural features at the local order, the involved defects and the electrical behavior as function of the doping process are discussed.

    更新日期:2020-01-08
  • Solid‐state synthesis of multicomponent equiatomic rare‐earth oxides
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    Matheus Pianassola; Madeline Loveday; Jake W. McMurray; Merry Koschan; Charles L. Melcher; Mariya Zhuravleva

    Phase formation in multicomponent rare‐earth oxides is determined by a combination of composition, sintering atmosphere, and cooling rate. Polycrystalline ceramics comprising various combinations of Ce, Gd, La, Nd, Pr, Sm, and Y oxides in equiatomic proportions were synthesized using solid‐state sintering. The effects of composition, sintering atmosphere, and cooling rate on phase formation were investigated. Single cubic or monoclinic structures were obtained with a slow cooling of 3.3°C/min, confirming that rare‐earth oxides follow a different structure stabilization process than transition metal high‐entropy oxides. In an oxidizing atmosphere, both Ce and Pr induce a cubic structure, while only Ce plays that role in an inert or reducing atmosphere. Samples without Ce or Pr develop a single monoclinic structure. The structures formed at initial synthesis may be converted to a different one, when the ceramics are annealed in an additional atmosphere. Phase evolution of a five‐cation composition was also studied as a function of sintering temperature. The binary oxides used as raw materials completely dissolve into a single cubic structure at 1450°C in air.

    更新日期:2020-01-07
  • Reactions of molten silicate deposits with yttrium monosilicate
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    William D. Summers; David L. Poerschke; Aidan A. Taylor; Andrew R. Ericks; Carlos G. Levi; Frank W. Zok

    The article addresses effects of silicate deposit composition on reactions with yttrium monosilicate (YMS), a candidate environmental barrier coating for aero‐engine components. Computed phase equilibria are used to predict the nature and relative proportions of reaction products and the extent of YMS consumption upon reaction with twelve deposits of varying composition at 1300°C. These predictions are compared with results of a corresponding experimental study on three exemplary deposits. Although the nature and sequence of reaction products formed (typically apatite and yttrium disilicate) depend on the Ca:Si ratio of the deposit, the degree of consumption of YMS at equilibrium is relatively insensitive to deposit composition and is predicted to proceed to a greater extent than that in yttrium disilicate. However, sluggish reaction kinetics associated with the formation of a thin apatite layer above the YMS prevents reactions from reaching their terminal equilibrium states within the experimental times investigated (250 hours). For deposit loadings of 18 mg/cm2 (corresponding to a thickness of about 100 µm), the degree of consumption following 250 hours exposures is only about 10%‐40% of the predicted terminal values, depending on deposit composition.

    更新日期:2020-01-07
  • Enhanced piezoelectric properties of Mn‐modified Bi5Ti3FeO15 for high‐temperature applications
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    Qian Wang; Chun‐Ming Wang

    Bi5Ti3FeO15 (BTF) has recently attracted considerable interest as a typical multiferroic oxide, wherein ferroelectric and magnetic orders coexist. The ferroelectric order of BTF implies its piezoelectricity, because a ferroelectric must be a piezoelectric. However, no extensive studies have been carried out on the piezoelectric properties of BTF. Considering its high ferroelectric‐paraelectric phase transition temperature (Tc ~ 761°C), it is necessary to analyze the piezoelectricity and thermal stabilities of BTF, a promising high‐temperature piezoelectric material. In this study, lightly manganese‐modified BTF polycrystalline oxides are fabricated by substituting manganese ions into Fe3+ sites via the conventional solid‐state reaction method. X‐ray diffraction and Raman spectroscopy analyses reveal that the resultant manganese‐modified BTF has an Aurivillius‐type structure with m = 4, and that the substitutions of Fe by Mn lead to a distortion of BO6. The temperature‐dependent dielectric properties and direct‐current (DC) resistivity measurements indicate that the Mn ions can significantly reduce the dielectric loss tanδ and increase the DC resistivity. The piezoelectricity of BTF is confirmed by piezoelectric constant d33 measurements; it exhibits a piezoelectric constant d33 of 7 pC/N. Remarkably, BTF with 4 mol% of Mn (BTF‐4Mn) exhibits a large d33 of 23 pC/N, three times that of unmodified BTF, whereas the Curie temperature Tc is almost unchanged, ~765°C. The increased piezoelectric performance can be attributed to the crystal lattice distortion, decreased dielectric loss tanδ, and increased DC resistivity. Additionally, BTF‐4Mn exhibits good thermal stabilities of the electromechanical coupling characteristics, which demonstrates that manganese‐modified BTF oxides are promising materials for the use in high‐temperature piezoelectric sensors.

    更新日期:2020-01-07
  • Toward a size scale‐up cold sintering process at reduced uniaxial pressure
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    Sun Hwi Bang; Kosuke Tsuji; Arnaud Ndayishimiye; Sinan Dursun; Joo‐Hwan Seo; Stephen Otieno; Clive A. Randall

    Cold sintering is a low‐temperature powder process methodology that enables the densification of ceramics and ceramic‐based composites at significantly reduced times and temperatures. Although the general notion of required pressure for the cold sintering is in the hundreds MPa, some material systems were reasonably demonstrated to be densified in the pressure below 50 MPa, which allows to increase the sample size up to 25 cm2 using a small tabletop laboratory press. Indeed, the pressure requirement has been a major constraint on promoting its application deployments, but this study is intended to propose a path to alleviate that limitation. Five different ceramic and composite systems (three ZnO‐based composites, Li1.5Al0.5Ge1.5(PO4)3, and zeolite Y) with applications in electronic, structural, and energy storage were investigated as a preliminary example of the size scale‐up process. One of the observed challenges of the scale‐up process was to obtain homogeneous microstructure all over the sample as the transient phase evaporation rate may be different upon the localization. In the case of ZnO, the inhomogeneous pellet translucency may pertain to partial anisotropic grain growth within the same sample.

    更新日期:2020-01-07
  • In‐situ observation of AlN formation from Ni‐Al solution using an electromagnetic levitation technique
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-06
    Masayoshi Adachi; Sonoko Hamaya; Yuji Yamagata; Andrew J. Loach; Justin S. Fada; Laura G. Wilson; Roger H. French; Jennifer L. W. Carter; Hiroyuki Fukuyama

    Aluminum nitride is a promising substrate material for AlGaN‐based UV‐LED. In order to develop a robust growth processing route for AlN single crystals, fundamental studies of solution growth experiments using Ni‐Al alloy melts as a new solution system were performed. Al can be stably kept in solution the Ni‐Al liquid even at high temperature; in addition, the driving force of the AlN formation reaction from solution can be controlled by solution composition and temperature. To investigate AlN crystal growth behavior we developed an in situ observation system using an electromagnetic levitation technique. AlN formation behavior, including nucleation and growth, was quantitatively analyzed by an image processing pipeline. The nucleation rate of AlN decreased with increasing growth temperature and decreasing aluminum composition. In addition, hexagonal c‐axis oriented AlN crystal successfully grew on the levitated Ni‐40 mol%Al droplet reacted at low driving force (1960 K), on the other hand, AlN crystal with dendritic morphology appeared on the sample with higher driving force (Ni‐50 mol%Al, 1960 K). Thus, the nucleation rate and crystal morphology were dominated by the driving force of the AlN formation reaction.

    更新日期:2020-01-07
  • Thermal Properties and Elastic Constants of ζ‐Ta4C3‐x
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-07
    Evan C. Schwind; Gregory E. Hilmas; William G. Fahrenholtz

    Thermal and elastic properties were measured for ceramics that contained as much as 96 wt% zeta phase tantalum carbide (ζ‐Ta4C3‐x). The ceramics were produced from tantalum hydride that was milled to reduce particle size and then blended with carbon. Powders were reaction hot pressed at 1800°C for 2 hours under a flowing He environment, which resulted in ζ‐Ta4C3‐x that was about 99% dense. The main secondary phases present in the reacted ceramic were TaC and Ta2O5. ζ‐Ta4C3‐x had a thermal conductivity of 9.6 W/m•K and an electrical resistivity of 160 ± 4.2 μΩ‐cm, which are lower and higher than those of TaC respectively. The Young's modulus was 379 ± 5 GPa and the hardness was 5.1 ± 0.7 GPa, which are also both lower than TaC. This study is the first to report the thermal properties and elastic moduli of high purity ζ‐Ta4C3‐x.

    更新日期:2020-01-07
  • 更新日期:2020-01-06
  • Highly IR transparent ZnS ceramics sintered by vacuum hot press using hydrothermally produced ZnS nanopowders
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-06
    Boo‐Hyun Choi; Dae‐Su Kim; Ku‐Tak Lee; Bum‐Joo Kim; Jeong‐Su Kang; Sahn Nahm

    Hydrothermally synthesized ZnS nanopowders comprising small and large particles were used to synthesize ZnS ceramics. Small particles (200 nm) existed in the gaps between the large particles (0.7 μm) and assisted the densification of the ZnS ceramics. ZnS ceramics sintered at low temperatures (<1000°C) exhibited small grains with large grain‐boundary areas that provided diffusion paths for carbon ions from the graphite mold, resulting in carbonate absorption bands. ZnS ceramics sintered at high temperatures (≥1000°C) for a long time (≥2.0 hours) exhibited a dense microstructure with very large grains (>500 μm). The ZnS liquid phase, which was formed at approximately 980°C, assisted the densification and grain growth of the ZnS ceramics. A 3.0‐mm‐thick ZnS ceramic sintered at 1000°C for 16 hours showed a high Knoop hardness (321 kgf/mm2) and a high transmittance of 71% in the wavelength range 6.0‐12 μm without carbonate absorption bands.

    更新日期:2020-01-06
  • Modeling of thermo‐viscoelastic material behavior of glass over a wide temperature range in glass compression molding
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-06
    Anh Tuan Vu; Anh Ngoc Vu; Tim Grunwald; Thomas Bergs

    In glass compression molding, most current modeling approaches of temperature‐dependent viscoelastic behavior of glass materials are restricted to thermo‐rheologically simple assumption. This research conducts a detailed study and demonstrates that this assumption, however, is not adequate for glass molding simulations over a wide range of molding temperatures. In this paper, we introduce a new method that eliminates the prerequisite of relaxation functions and shift factors for modeling of the thermo‐viscoelastic material behavior. More specifically, the temperature effect is directly incorporated into each parameter of the mechanical model. The mechanical model parameters are derived from creep displacements using uniaxial compression experiments. Validations of the proposed method are conducted for three different glass categories, including borosilicate, aluminosilicate, and chalcogenide glasses. Excellent agreement between the creep experiments and simulation results is found in all glasses over long pressing time up to 900 seconds and a large temperature range that corresponds to the glass viscosity of log (η) = 9.5 – 6.8 Pas. The method eventually promises an enhancement of the glass molding simulation.

    更新日期:2020-01-06
  • Closed porosity ceramics and glasses
    J. Am. Ceram. Soc. (IF 3.094) Pub Date : 2020-01-02
    Cekdar Vakifahmetoglu; Tugce Semerci; Gian Domenico Soraru

    In the last three decades, considerable effort has been devoted to obtain both open and closed porosity ceramics & glasses in order to benefit from unique combination of properties such as mechanical strength, thermal and chemical stability at low‐relative density. Most of these investigations were directed to the production and the analysis of the properties for open porosity materials, and regrettably quite a few compositions and manufacturing methods were documented for closed porosity ceramics & glasses in the scientific literature so far. This review focuses on the processing strategies, the properties and the applications of closed porosity ceramics & glasses with total porosity higher than 25%. The ones below such level are intentionally left out and the paper is set out to demonstrate the porous components with deliberately generated closed pores/cells. The processing strategies are categorized into five different groups, namely sacrificial templating, high‐temperature bonding of hollow structures, casting, direct foaming, and emulsions. The principles underlying these methods are given, with particular emphasis on the critical issues that affect the pore characteristics, mechanical, thermal and electrical properties of the produced components.

    更新日期:2020-01-04
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