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  • Experimental visualization of microstructure evolution during suspension plasma spraying of thermal barrier coatings
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-18
    Ashish Ganvir, Rosa Filomena Calinas, Nicolaie Markocsan, Nicholas Curry, Shrikant Joshi

    This paper investigates the evolution of microstructure of thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) through a careful experimental study. Understanding the influence of different suspension characteristics such as type of solvent, solid load content and median particle size on the ensuing TBC microstructure, as well as visualizing the early stages of coating build-up leading to formation of a columnar microstructure or otherwise, was of specific interest. Several SPS TBCs with different suspensions were deposited under identical conditions (same substrate, bond coat and plasma spray parameters). The experimental study clearly revealed the important role of suspension characteristics, namely surface tension, density and viscosity, on the final microstructure, with study of its progressive evolution providing invaluable insights. Variations in suspension properties manifest in the form of differences in droplet momentum and trajectory, which are found to be key determinants governing the resulting microstructure (e.g., lamellar/vertically cracked or columnar).

  • Sintering behavior of columnar thermal barrier coatings deposited by axial Suspension Plasma Spraying (SPS)
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-18
    Dapeng Zhou, Jürgen Malzbender, Yoo Jung Sohn, Olivier Guillon, Robert Vaßen

    During the last decade, Suspension Plasma Spraying (SPS) attracted a lot of interest as an alternative process to produce columnar Thermal Barrier Coatings (TBCs). In this study, columnar TBCs were deposited with SPS. After spraying, samples were isothermally annealed at 1373 K for 1 h, 3 h, 10 h and 50 h, respectively. Microstructures and mechanical properties of the ceramic coatings were investigated as a function of annealing time. Annealing resulted in healing of micro-cracks, coarsening of pores, growth of domain size, companied with a decrease of porosity within columns. The change of coating microstructure led to change of mechanical properties. In addition, residual stress in SPS coatings was also investigated. Furthermore, as-sprayed coatings and pre-annealed coatings were subjected to burner rig tests. Short time pre-annealing allowed to enhance thermal cycling lifetime of such SPS coatings. The thermal cycling results were related to microstructure modifications of coatings.

  • High thermal stability of electric field-induced strain in (1−x)(Bi0.5Na0.5)TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 lead-free ferroelectrics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-17
    Li Jin, Wenting Luo, Liang Wang, Ye Tian, Qingyuan Hu, Lei Hou, Lin Zhang, Xu Lu, Hongliang Du, Xiaoyong Wei, Gang Liu, Yan Yan

    In ferroelectric materials high electric field-induced strain (EFIS) with good thermal stability is of much interest from both fundamental research and potential applications. Here we propose a strategy to achieve high thermally stable EFIS based on electrostrictive effect and thermal stability of polarization. According to this strategy, we synthesized (1−x)(Bi0.5Na0.5)TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 (BNT-xBCZT) ferroelectric ceramics in order to tailor the thermal stability of dielectric permittivity, polarization and EFIS. A dielectric platform with a wide temperature region is induced by increasing x from 0.24 to 0.36 gradually. From 30 °C to 150 °C, a variation of 20 % polarization results in a change of 36 % EFIS, suggesting a good thermal stability as expect. Temperature-insensitive electrostrictive coefficient Q33 ranges from 0.0264 m4/C2 to 0.0314 m4/C2. These results not only prove the effectiveness of this strategy, but also suggest that this strategy can be applied to other ferroelectric materials to improve the thermal stability.

  • A colloidal approach to prepare binder and crack-free TiO2 multilayer coatings from particulate suspensions: application in DSSCs
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-17
    Z. Gonzalez, J. Yus, A.J. Sanchez-Herencia, J. Dewalque, L. Manceriu, C. Henrist, B. Ferrari

    Well-compacted and crack-free TiO2 multilayer coatings have been manufactured from a colloidal approach based on the preparation of particulate suspensions for DSSC. The study of the suspension parameters to optimize dispersion and stabilization of the TiO2 nanoparticle in the liquid media as well as a thermal stabilization step between the layers have been defined as two key points in the processing method to obtain interconnected microstructures, free of defects and heterogeneities, that prevent the application of an additional scattering layer or any kind of specific or clean conditions during deposition. The sintering process at low temperature, 450⁰C, has allowed obtaining open microarchitectures avoiding the complete densification and favoring the dye adsorption. A thickness of 12.8 µm resulted in a successful dye loading of 4.52 × 10-10 mol·mm-2 and a photoefficiency of 5.7%, both in the range of the others particulate systems. EIS measurements were also made to study the transfer charge phenomena

  • Preparation and ionic conduction of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte using inorganic Germanium as precursor
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-17
    Zhijian Sun, Lei Liu, Yuxiao Lu, Guangyue Shi, Jiajun Li, Lei Ma, Jie Zhao, Hongli An

    NASION-type Li1.5Al0.5Ge1.5(PO4)3(LAGP) is prepared by a novel sol-gel method using low cost inorganic Germanium (GeO2) as the precursor. The composition and phase transformation during the heating of the LAGP precursors are analyzed using thermogravimetric-differential scanning calorimetry (TG-DSC) and X-ray diffraction (XRD). The structures and morphologies of the LAGP are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the LAGP annealed at 900℃ is partially crystallized and consists of a large number of nanoscale grains surrounded by amorphous regions. The LAGP particles present an irregular morphology with a large size distribution over a range of 0.2 to 1 μm. In addition, ionic conductivities of the prepared LAGP first increase and then decrease with an increase in the sintering temperature and time. A high ionic conductivity (4.18 × 10-4Scm-1) with an activation energy of 0.30 eV are obtained for the LAGP sample sintered at 900 °C for 8 h.

  • Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-17
    Yong-Hyeon Kim, Young-Wook Kim, Kwang-Young Lim, Seoung-Jae Lee

    Two different SiC ceramics with a new additive composition (1.87 wt% Y2O3–Sc2O3–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m1/2, 25.4 ± 1.2 GPa, and 99 Wm-1 K-1 at room temperature, respectively.

  • Assessment of the Performance of Y2SiO5-YSZ/YSZ Double-layered Thermal Barrier Coatings
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-17
    Wei Hao, Qihui Zhang, Chen Xing, Fangwei Guo, Meiyu Yi, Xiaofeng Zhao, Ping Xiao

    Y2SiO5 is a promising material for the thermal barrier coatings due to its low thermal conductivity, high temperature stability and exceptional resistance for molten silicate attack. However, it suffers low fracture toughness and low coefficient of thermal expansion compared with yttria-stabilized zirconia (YSZ). In this study, a composite coating approach, i.e., incorporating YSZ into Y2SiO5 coating, was employed to overcome those limitations. The double-layered Y2SiO5-YSZ/YSZ coatings were fabricated using atomospheric plasma spraying and tested under thermal cycling at 1150 °C. The phase compositions, microstructure, mechanical properties and the failure behavior were evaluated. It was found that the amorphous phase during spraying would crystallize at high temperature accompanied by volume shrinkage, leading to cracks and spallation in the coating. With YSZ addition, the composite coatings exhibited a much longer lifetime than the single phase Y2SiO5 coating due to a lower volume shrinkage and enhanced toughness.

  • Validation of an experimental-numerical approach for the high temperature behaviour of open-cell ceramic foams
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-18
    Christoph Settgast, Yvonne Ranglack-Klemm, Jana Hubalkova, Martin Abendroth, Meinhard Kuna, Horst Biermann

    Open-cell ceramic foams are used as filters in casting processes of molten metal to reduce the amount of non-metallic inclusions. These filters are exposed to high temperatures and loadings, sometimes for a longer time. Therefore, the dimensional stability has to be considered, which suffers from inelastic deformations, in particular creep deformations.A recent work of the authors describes the creep deformation behaviour of such foam structures using an experimental-numerical approach based on bulk material creep tests. Now, the results of this prediction are validated by creep curves of real foam samples. This comparison shows a good accordance for the considered parameters (load, temperature, time, and creep phase).Moreover, the influences of different structural parameters on the elastic foam stiffness and foam creep resistance are investigated. The relative foam density has the greatest impact, followed by uniaxial pore stretching in one direction and strut shape, whereas polydispersity has only a very small effect.

  • A phase field model of pressure-assisted sintering
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-15
    Branislav Dzepina, Daniel Balint, Daniele Dini

    The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.

  • Mixed-ionic and electronic conduction and stability of YSZ-graphene composites
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-16
    Daniel Marinha, Manuel Belmonte

    Composites of 8 mol% Yttria-stabilized Zirconia (YSZ) containing 0, 7, 10 and 14 vol.% of graphene nano-platelets (GNP) were fully densified by Spark Plasma Sintering. The effect of GNP on the electrical performance of the composites was analyzed by impedance spectroscopy as a function of temperature (150 – 800 °C) and oxygen partial pressure (0.21 – 10-20 atm). Results show that below GNP percolation threshold (7.1 vol.%), the electrical behavior is dominated by the matrix oxygen-ion conductivity. Above the threshold, the conductivity is predominantly electronic provided by the GNP network. The total conductivity of composites was used as an indicator of GNP stability in different atmospheres. YSZ/GNP composites remain stable in inert conditions up to 600 °C, and in reducing conditions up to 800 °C, making them good alternatives to perovskite-based materials used for electrochemical applications.

  • Carbon-Free Synthesis and Luminescence Saturation in a Thick YAG:Ce Film for Laser-Driven White Lighting
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-14
    Xu Jian, Baofu Hu, Chao Xu, Jian Wang, Bingguo Liu, Hui Li, Xinliang Wang, Baoli Du, Yuxuan Gong

    Good thermal dispersion is essential for phosphors used in laser-lighting applications. Luminescent films naturally show better thermal dispersion than bulk phosphor because the distance to the supporting substrate is shorter, which facilitates heat dissipation. However, the underlying mechanism for luminescence saturation in films is still unclear. In addition, the synthesis of luminescent films always involves the use of carbon-rich organic materials, which can introduce both pores and carbon residues. Here, we present a facile sol–gel route to synthesize YAG:Ce thick films suitable for laser lighting applications. Inorganic aluminum hydroxides are used as gelling agent, which solves the carbon residue problem. A series of YAG:Ce films of different thicknesses was produced at relatively low temperature (975 °C). The YAG:Ce film shows no luminescence saturation under 4.1 W blue laser excitation. This means this approach represents a strong potential candidate for applications like automotive headlamps and many other devices.

  • Influence of sintering temperature and pressure on the 3C-6H transition of silicon carbide
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-15
    Florimond Delobel, Sébastien Lemonnier, Élodie Barraud, Julien Cambedouzou

    The influence of Spark Plasma Sintering pressure and temperature on the evolution of the phase composition of two sources of β-SiC ceramic were studied based on X-ray diffraction and Raman scattering measurements. A commercial powder and a powder derived from a preceramic polymer precursor were chosen, and the investigation was undertaken in the [1850-2200°C] temperature range and in the [17-127 MPa] pressure range. Analyses revealed that the 3C-6H phase transition depends on sintering pressure, which is shown to stabilize the cubic phase. The stability domains of the cubic phase as a function of sintering P and T were determined for both powdery precursors and revealed that the cubic phase stability is also linked to the nature of powdery precursor and in particular to its crystallinity state. The improvement of cubic phase stability at higher temperature by applying pressure allows sintering β-SiC ceramic with high density.

  • Analysis of Sb-doped ceria: Magnetism, conductivity, dielectric, specific heat and optical properties
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-13
    T. Kolodiazhnyi, T. Charoonsuk, M. Spreitzer, N. Vittayakorn

    We report on magnetism, charge transport, dielectric properties and specific heat of Ce1−xSbxO2 ceramics sintered at 1650 ∘C with a final antimony content of 0 ≤ x ≤ 0.0017. In contrast to other donor dopants, such as Nb, Ta, U and W, charge compensation of antimony in Ce1−xSbxO2 does not involve the formation of the Ce3+ ions as revealed by the magnetic susceptibility data. Therefore, we conclude that antimony is mainly present as Sb3+ ion and acts as an acceptor dopant in Ce1−xSbxO2. This conclusion is also supported by a very low electrical conductivity of the Sb-doped ceria that shows an activation energy Eσ ∼ 0.97 eV. This activation energy is close to that observed in oxygen conducting acceptor-doped ceria and is significantly higher than the typical Eσ ∼ 0.1-0.3 eV values reported for n-type CeO2. Below 10 K, both an anomaly in the dielectric loss and a small specific heat surplus in Sb-doped ceria indicate a low-energy dipolar relaxation probably associated with a local dynamics of the off-centered Sb3+ point defects.

  • Estimating thermal conductivities and elastic moduli of porous ceramics using a new microstructural parameter
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-12
    Kang Guan, Haitao Ren, Qingfeng Zeng, Zhiqiang Feng, Jianqing Wu, Zhenya Lu, Pinggen Rao, Yangfang Cheng, Zhenyuan Gong, Yougen Yu

    Porous ceramics are numerically constructed based on the convexity of the void phase: microstructures with convex pores are representative of isolated or randomly overlapping spherical pores, while particulate materials with non-convex pores are composed of randomly overlapping, partial overlapping or partially sintered solid spheres. Finite element simulations show that, given the porosity, thermal conductivities and elastic moduli for convex porosity are larger than the values for non-convex pores. These conditions are not well described by solely porosity. By contrast, this study proposes a new microstructural parameter, <lp2>/(<ls2>+<lp2>), to estimate thermal conductivities and elastic moduli for both convex and non-convex pores. <ls2> and <lp2> are respectively mean-square solid chord length and mean-square pore chord length of cross-sections, which can be conveniently extracted from SEM images combined with chord length distributions of solid and void.

  • Electrical properties of bismuth ferrites: Bi2Fe4O9 and Bi25FeO39
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-11
    Antonio Perejón, Eva Gil-González, Pedro E. Sánchez-Jiménez, Anthony R. West, Luis A. Pérez-Maqueda

    Bi2Fe4O9 was prepared by solid-state reaction and the electrical properties measured by impedance spectroscopy. After annealing in O2 at 900 °C, Bi2Fe4O9 is an electrically-homogeneous insulator. Its high frequency permittivity is constant (~14.1) over the temperature range 300–400 °C and shows no evidence of incipient ferroelectricity at lower temperatures. On annealing in N2 at 900 °C, the pellets gradually decompose.Bi25FeO39 was prepared by both solid-state reaction and mechanosynthesis. It showed a modest amount of mixed conduction of both oxide ions and holes. Impedance analysis showed a complex response that best fitted an equivalent circuit consisting of a parallel combination of long-range conduction and short range dielectric relaxation elements.The electrical conductivity of both Bi2Fe4O9 and Bi25FeO39 is less than that of BiFeO3 prepared by solid-state reaction, which indicates that any leakage conductivity of BiFeO3 is not due to the possible presence of small amounts of these secondary phases.

  • High-temperature properties and interface evolution of silicon nitride fiber reinforced silica matrix wave-transparent composite materials
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-09
    Xuejin Yang, Bin Li, Duan Li, Changwei Shao, Changrui Zhang

    In order to improve the high-temperature performance of wave-transparent materials especially for the high-speed aircrafts application, filament winding combined with sol-gel method was adopted to the fabrication of unidirectional silicon nitride fiber reinforced silica matrix composites. The mechanical properties and the interface evolution at high temperatures were investigated. The results show that the composite sintered in N2 maintains a flexural strength of 210MPa at up to 1200°C, while its counterpart prepared in air experiences a dramatic reduction to about 73MPa. The degradation is due to the partial oxidation of silicon nitride fibers at the fiber matrix interface. Besides, it is also notable that the bending strength of these two composites undergoes a similar growth from about 160 to 210MPa when tested under 900°C, which can be explained by the release of thermal stress on the silicon nitride fibers.

  • Unified tensile model for unidirectional ceramic matrix composites with degraded fibers and interface
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-07
    C. -P. Yang, F. Jia, B. Wang, T. Huang, G.-Q. Jiao

    In order to overcome the roughness of the previously proposed micromechanical model [Acta Mech. Sin. (2011) 382], an enhanced multiscale analytical model was thus developed based on the rule of mixture, shear-lag theory and statistical approach to forecast the load carrying capacity of the prestressed ceramic matrix composites (CMCs) subjected to high-temperature oxidation. For comprehensive characterization of the mechanical degradation mechanisms, the oxidation induced fiber necking (or embrittlement) and fiber-matrix interface weakening were both taken into account. The suggested model was then applied to 2D-C/SiC composites. The influences of interface friction resistance, interface recession length, fiber necking factor and oxidation duration upon the residual mechanical property were investigated. Parametric analysis demonstrates that the modified formulations are much more reasonable than the previous model. The predicted residual tensile modulus and strength for the 2D-C/SiC composite agree well with the experimental data and furthermore the microscopic damage mechanisms were correlated properly with the macroscopic fracture morphologies.

  • 更新日期:2018-09-09
  • High electric field-induced strain with ultra-low hysteresis and giant electrostrictive coefficient in barium strontium titanate lead-free ferroelectrics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-08
    Li Jin, Wenting Luo, Lei Hou, Ye Tian, Qingyuan Hu, Liang Wang, Lin Zhang, Xu Lu, Hongliang Du, Xiaoyong Wei, Yan Yan, Gang Liu

    Ferroelectrics with high electrostrictive properties are of great interest in fundamental researches and industrial applications. In this work, the phase structural evolution, dielectric properties as well as electrostrictive properties in barium strontium titanate [(Ba1-xSrx)TiO3, BST] lead-free ferroelectrics with x from 0.05 to 0.4 were investigated in details. High electric field-induced strain (~0.2%) at 60 kV/cm are obtained in x = 0.05 and 0.1 compositions. More importantly, almost purely positive strains with ultra-low hysteresis (<8%) determined from bipolar strain-electric field curves are observed in compositions with x from 0.05 to 0.3, suggesting the dominating role of electrostrictive effect. Temperature-insensitive and composition-insensitive longitudinal electrostrictive coefficient Q33 for BST ceramics with giant values from 0.0409 m4/C2 to 0.0479 m4/C2 is identified. These features suggest that BST ceramics not only possess high electric field-induced strain with ultra-low hysteresis and giant Q33, but also are good candidates for potential application in high-precision actuator devices.

  • Ionic conductivity of space charge layers in acceptor doped ceria
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-04
    Dries Van Laethem, Johan Deconinck, Annick Hubin

    The ionic conductivity of acceptor doped ceria is strongly influenced by grain boundaries and interfaces. Most experiments show a decrease in ionic conductivity and an increase in electronic conductivity in these regions. Classical models explain this observation by the formation of space charge layers, that are depleted of mobile ionic charge carriers and enriched in small polarons. However, some experiments demonstrate an increase in ionic conductivity and recent models show that the space charge layers can also be enriched in mobile ionic species. Because of these contradictions, it is still not clear whether nanocrystalline or thin film ceria can offer superior ionic conductivity or not. To aid this debate, we calculate the ionic conductivity of yttrium doped ceria in regions of net charge density using kinetic Monte Carlo simulations. Through an appropriate choice of the charge densities, these calculations allow to demarcate the possible conductivity gains from space charge layers.

  • Development of an instrumented and automated flash sintering setup for enhanced process monitoring and parameter control
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-05
    João Vitor Campos, Isabela Reis Lavagnini, Rafael Vieira de Sousa, Julieta Adriana Ferreira, Eliria Maria de Jesus Agnolon Pallone

    Heterogeneous grain size distributions due to cathodic reduction of samples sintered by flash sintering (FS) are a recurring problem. Herein, we present an experimental setup for FS with refined control of the process parameters, especially the electric current density, to control the power peak that could reduce the grain size heterogeneity of sintered materials. The real-time control of the equipment enabled the development of a Multi-Step Flash Sintering (MSFS) process. This technique involves varying the maximum current density at the onset of the abrupt shrinkage of the sample during the flash phenomenon, smoothing the shrinkage and power density curves, avoiding power peak and allowing its control in real time. Tetragonal stabilised zirconia (3YSZ) was sintered in this equipment by both FS and MSFS. Preliminary results of this study were promising in terms of density control and improvement of grain size homogeneity.

  • Process development for additive manufacturing of functionally graded alumina toughened zirconia components intended for medical implant application
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-05
    Eric Schwarzer, Stefan Holtzhausen, Uwe Scheithauer, Claudia Ortmann, Thomas Oberbach, Tassilo Moritz, Alexander Michaelis

    Additive manufacturing processes become more and more important in almost all areas, also for medical implant application. Medical implants made of ceramics are known for their excellent biological compatibility. For increasing the ingrowth of cells into an implant, a function-optimized structure with a defined porosity seems to be advantageous. Additive manufacturing is suited to realize defined and filigree structures. Lithography-based ceramic manufacturing (LCM), for example, has a high resolution, but still a limited material portfolio. Alumina toughened zirconia is a very interesting material for medical applications, but not commercially available for LCM. In this study, the development of an ATZ suspension usable in the LCM process with a bulk material density higher than 99 % after sintering is presented. Furthermore, printing of lattice test structures for the evaluation of accuracy in dependence of printing dimension is demonstrated.

  • Experimental and Computational Analysis of PSZ 10- and PSZ 20-Derived Si-C-N Ceramics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-03
    Ingo Jürgen Markel, Jürgen Glaser, Martin Steinbrück, Hans Jürgen Seifert

    A Si-C-N ceramic was synthesized by pyrolysis of the polysilazane Ceraset® PSZ 10 and PSZ 20. The preceramic polymer was crosslinked by gradual heating up to 300 °C and pyrolyzed at temperatures from 1000 °C to 1500 °C in flowing Ar- and N2-containing atmospheres, respectively. The influence of the pyrolysis temperature and the nitrogen partial pressure (pN2) on the high-temperature stability, phase reactions and crystallization behavior was investigated by STA, XRD and SEM/EDX. Additionally, gaseous pyrolysis- and reaction products were analyzed by on-line mass spectrometry. The CALPHAD (CALculation of PHAse Diagrams) method was used to calculate pyrolysis products and high-temperature phase stabilities as a function of temperature and pAr/pN2. It is shown that the observed experimental findings are quantitatively confirmed and explained by the modelling.

    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-09-01
    Gregory N. Morscher, Ryan Maxwell

    Acoustic emission, electrical resistance, and surface optical techniques were used to monitor matrix cracking and fiber-breakage during fatigue in tension for [0/90]2s SiC-based laminate composite single-notch specimens. Acoustic emission sensors were positioned in several locations including on the edge of the specimen which enabled location of events through the width and the location of internal tunnel-type cracks. Surface optical techniques, including digital image correlation, enabled the extent of surface crack growth. From these two sets of data a simple circuit could be constructed of the different damaged and undamaged regions in the region of the notch that was in good agreement with the change in electrical resistance, thus establishing a correlation with change in ER and damage development. The unique placement of AE sensors on the edge of the specimen also enabled the capture and location of what are believed to be fiber failure events prior to ultimate failure.

  • A New Family of Cu-doped Lanthanum Silicate Apatites as Electrolyte Materials for SOFCs: Synthesis, Structural and Electrical Properties
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-31
    Tianrang Yang, Hailei Zhao, Mengya Fang, Konrad Świerczek, Jie Wang, Zhihong Du

    La9.67Si6-xCuxO26.5-x (LSC, x = 0, 0.1, 0.3 and 0.5) are synthesized by a citric-nitrate method. Substitution Si with Cu promotes the densification process of silicate apatite. Unit cell parameters and volume increase linearly with Cu content. The Rietveld refinement reveals a much more distorted (Si,Cu)O4 tetrahedra in the oxygen stoichiometric La9.67Si5.5Cu0.5O26 sample. The structural observation from high temperature XRD implies a second-order phase transition in La9.67Si5.5Cu0.5O26. Cu-doping decreases the activation energy of oxygen ion conduction and increases the conductivity of LSC materials in the temperature range of 550-800 °C. La9.67Si5.5Cu0.5O26 shows the conductivity values of 29.3 and 12.3 mS cm-1 at 800 °C and 650 °C, respectively. The oxygen ion transference number of La9.67Si5.5Cu0.5O26 is higher than 0.99. These attractive properties make the La9.67Si5.5Cu0.5O26 a promising oxygen ion conducting electrolyte for applications of solid oxide fuel cells, oxygen sensors, oxygen separation membranes, etc.

  • Flash sintering of ceramics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-31
    Mattia Biesuz, Vincenzo M. Sglavo

    Flash sintering is a novel densification technology for ceramics, which allows a dramatic reduction of processing time and temperature. It represents a promising sintering route to reduce economic, energetic and environmental costs associated to firing. Moreover, it allows to develop peculiar and out-of-equilibrium microstructures.The flash process is complex and unusual, including different simultaneous physical and chemical phenomena and their understanding, explanation and implementation require an interdisciplinary approach from physics, to chemistry and engineering. In spite of the intensive work of several researchers, there is still a wide debate as for the predominant mechanisms responsible for flash sintering process.In the present review, the most significant and appealing mechanisms proposed for explaining the “flash” event are analyzed and discussed, with the aim to point out the level of knowledge reached so far and identify, at least, possible shared theories useful to propose future scientific activities and potential technological implementations.

  • Microstructural study of dielectric breakdown in glass-ceramics insulators
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-30
    V. Fuertes, M.J. Cabrera, J. Seores, D. Muñoz, J.F. Fernández, E. Enríquez

    Novel glass-ceramic materials based on Na and Ca-rich feldspar crystallizations with a hierarchical micro-nanostructure shown the largest dielectric strength, > 57 kV/mm, reported at room temperature in ceramic insulators, due to a large amount of interfaces that favor scattering processes of charge carriers. Dielectric breakdown tests with temperature indicated they withstood up to 200 °C, with dielectric strengths of 30 kV/mm and 44 kV/mm for anorthite and albite-based glass-ceramics, respectively. These values are even larger than the ones obtained at room temperature for most of the current ceramic insulators. Microstructural characterization and micro-Raman spectroscopy carried out after breakdown allow determining the dielectric breakdown mechanisms. Glass phases in the surroundings of the crater because of local melting and fast cooling are identified. These results make feldspar based glass-ceramics suitable for electrical insulator applications at room and high temperature. Moreover, dielectric breakdown mechanism may allow tailoring new high insulating application in the future.

  • Gelcasting of through-pore hydroxyapatite ceramics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-29
    Zhen Wu, Zhengren Zhou, Youliang Hong

    Although the near-net-shape forming ability of gelcasting has been well demonstrated, current attention mainly was focused on manufacturing compact ceramics. In this work we demonstrated that the combination of the gelcasting technique and the ceramic sintering process could prepare the near-net-shape through-pore hydroxyapatite (HAP) ceramics, in which the pore diameter at the range of < 2 μm could be adjusted by changing the content of HAP in green bodies and with the aid of alcohol immersion. Experiments demonstrated that the gels in green bodies and alcohol immersion played important roles in improving the strength of green bodies and decreasing the shrinkage and deformation of green bodies, and thus improving the through-pore and near-net-shape forming of HAP ceramics. Such method reported here possibly extends the gelcasting technique to manufacture other through-pore ceramics for precise filtration, protein separation, or high performance liquid chromatography.

  • Anisotropies in structure and properties of hot-press sintered h-BN-MAS composite ceramics: effects of raw h-BN particle size
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-29
    Bo Niu, Delong Cai, Zhihua Yang, Xiaoming Duan, Yangshan Sun, Hailiang Li, Wenjiu Duan, Dechang Jia, Yu Zhou

    Effects of raw h-BN particle size from 0.5 μm to 11 μm on the phase compositions, texture degree, bending strength, fracture toughness and thermal conductivity of hot-press sintered h-BN-MAS composite ceramics were investigated. Larger h-BN grain can facilitate the nanocrystallization of MAS phase due to the inhibiting crystallization effect of h-BN on α-cordierite. Texture degree of h-BN-MAS composite ceramics increased significantly with increasing raw h-BN particle size, and the 11.0μmBN-MAS composite ceramic shows typical textured structure. The h-BN-MAS composite ceramics show anisotropy in mechanical properties and thermal conductivity, and the anisotropy increased significantly with increasing raw h-BN particle size. The 0.5μmBN-MAS sample shows excellent mechanical properties, and the 10μmBN-MAS sample shows strong anisotropy in thermal conductivity.

  • The Acceleration of Crystal Growth of Gold-Doped Glasses within the System BaO/SrO/ZnO/SiO2
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-28
    Christian Thieme, Michael Kracker, Christian Patzig, Katrin Thieme, Christian Rüssel, Thomas Höche

    Glasses with the base composition 8 BaO ∙ 8 SrO ∙ 34 ZnO ∙ 50 SiO2 were studied with respect to the crystallization of a phase with the composition Ba0.5Sr0.5Zn2Si2O7. Some glasses were doped with gold and/or antimony oxide. All glasses exhibited solely surface crystallization. If the glasses contained both antimony and gold, they showed red coloration after different heat treatments and the crystal growth velocities were enhanced, depending on the chosen temperature, by around a factor of 30. Crystals were proven to be oriented statistically, while in all other glasses of the system under consideration, i.e., in those which did not contain both gold and antimony, an orientation of the crystals with their c-axes perpendicular to the surface was obtained. This effect is explained by a continuous nucleation in front of the growing crystals.

  • Effect of the phase transformation on fracture behaviour of fused silica refractories
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-25
    Yajie Dai, Yucheng Yin, Xiaofeng Xu, Shengli Jin, Yawei Li, Harald Harmuth

    In this paper, the influence of phase transformation on the properties and fracture behaviour of fused silica refractory was investigated. The virgin fused silica refractory is amorphous, and possible failure is attributed to the propagation of a single crack in the structure. Due to the crystallization and phase transformation of low-/high- temperature cristobalite subpolymorphs occurring during the heat treatment, microcracks are formed especially in the matrix and at the grain boundary. This microcracking enables the development of sizable fracture process zone, which is responsible for the increase of specific fracture energy even with the decrease of strength. Therefore, the heat-treated specimens exhibit lower brittleness and higher strain tolerance before failure compared with the virgin fused silica refractory. All of these properties represent a better thermal shock resistance. Furthermore, microcracking causes a characteristic temperature dependence of Young’s Modulus due to phase transformation and partial crack closure at increased temperatures.

  • Enhancing Temperature Stability in Potassium-Sodium Niobate Ceramics through Phase Boundary and Composition Design
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-25
    Xiang Lv, Jiagang Wu, Chunlin Zhao, Dingquan Xiao, Jianguo Zhu, Zehui Zhang, Chenhui Zhang, Xi-xiang Zhang

    Phase boundaries and composition design were explored to achieve both high piezoelectricity and favorable temperature stability in potassium-sodium niobate ceramics, using (1-x)(K,Na)(Nb,Sb)O3-xBi(Na,K)(Zr,Sn,Hf)O3 ceramics. A rhombohedral-tetragonal (R-T) phase boundary was constructed at x=0.035-0.04 by co-doping with Sb5+ and Bi(Na,K)(Zr,Sn,Hf)O3. More importantly, a superior temperature stability was observed in the ceramics with x=0.035, accompanying with a stable unipolar strain at room temperature to 100 °C. The ceramics with x=0.035 also exhibited improved piezoelectric properties (e.g., piezoelectric coefficient d33~465 pC/N and electromechanical coupling factor kp=0.47) and Curie temperature (Tc~240 °C). The Rietveld refinement and in-situ temperature-dependent piezoresponse force microscopy (PFM) results indicated that the enhancement of the piezoelectric properties was caused by the easy domain switching, high tetragonal fraction, and tetragonality, while the improved temperature stability mainly originated from the stable domain structures.

  • Monoclinic-celsian ceramics formation: Through thermal treatment of ion-exchanged 3D printing geopolymer precursor
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-23
    Shuai Fu, Peigang He, Meirong Wang, Meng Wang, Ruifei Wang, Jingkun Yuan, Dechang Jia, Jianye Cui, Kuanlong Wang

    Based on conventional ion exchange method from zeolite materials, we herein report a facile synthetic procedure to prepare monoclinic celsian ceramics through thermal treatment of ion-exchanged 3D printing geopolymer precursors. In this contribution, both ion exchange process and thermal evolution of celsian precursors were systematically investigated. The results proved 3D printing Na- and K- based geopolymer were ideal precursors with low residual Na+ content (0.10 meq/g) and K+ content (0.05 meq/g) and translated into monoclinic celsian after being heated at 1400 °C. With increasing Sr2+ doping concentration from 20% (mole ratio) to 30% (mole ratio), more significant phase transition results (hexagonal → monoclinic) were observed. All 3D printing geopolymer precursors kept fine integrity and stable 3D structure upon ion exchange process and heating, indicating the combination of geopolymer technique and 3D printing opens up a versatile and robust way to yield monoclinic celsian ceramics and related components of complex shapes.

  • Garnet-like Li7-xLa3Zr2-xNbxO12 (x = 0−0.7) solid state electrolytes enhanced by self-consolidation strategy
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-23
    Pengcheng Zhao, Yu Xiang, Yuehua Wen, Meng Li, Xiayu Zhu, Songtong Zhao, Zhaoqing Jin, Hai Ming, Gaoping Cao

    Garnet-like Li7La3Zr2O12 electrolytes with Nb doping are synthesized by self-consolidation method. Different from conventional methods such as cold or hot isostatic pressing, not any pressing assistance is employed throughout the preparation process. Although the preparation process is dramatically simplified, both density and ionic conductivity of the obtained samples are enhanced. Nb-doped content plays a key role in the sintering of the packed precursor powders and in the structure stabilization of the obtained bulk samples. The optimized 0.60 mol Nb-doped sample with relative density of 94%, fine particle boundaries, and solitary cubic structure possesses the maximum total ionic conductivity of 5.22 × 10-4 S cm-1 at 30 °C, which is comparable to the highest reported value of the samples prepared by conventional pressing methods. This work verifies that self-consolidation strategy is effective, reliable, and productive for the preparation of cubic Li7La3Zr2O12 electrolyte, which would significantly facilitate the development of ceramic electrolyte membrane technology.

  • Improved toughness and electromagnetic shielding-effectiveness for graphite-doped SiC ceramics with a net-like structure
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-23
    Mengyong Sun, Yuhang Bai, Mingxing Li, Shangwu Fan, Laifei Cheng

    The graphite-doped SiC ceramics with net-like structure was fabricated via tape casting and pressureless sintering. The ceramics exhibited a step-like fracture mode, which could be attributed to the net-like structure composed of long columnar SiC grains, layered graphite, and the three-modal pore distribution. The formation of warped epitaxial graphene and large size graphite could be attributed to the pyrolysis of organics in the tape casting system. In the net-like structure, the SiC grains provide the high strength, whereas the layered graphite and three-modal pores were used to deflect the cracks and release the stress at the tip, following the crack-tip-shielding mechanism. The sample with a net-like structure exhibited a combination of a variety of extrinsic toughening mechanisms, such as crack deflection, crack bridging, crack branching and delamination, pull-out, and rupture of layered graphite, which led to improved fracture toughness of 7 MPa m1/2, flexural strength of 400 MPa, and (work of fractur) WOF of 3.3 KJ m−2. When increasing the graphite content, the electrical conductivity of the graphite-doped SiC ceramics significantly increased from 7.15 × 10-4 to 216 S/m. The high shielding effectiveness of 34.1 dB was due to the multi-absorption on the various surfaces during the multi-reflection by the net-like structure.

  • Effect of Zeta Potential on Properties of Foamed Colloidal Suspension
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-23
    Wenlong Huo, Xiaoyan Zhang, Ke Gan, Yugu Chen, Jie Xu, Jinlong Yang

    We reveal that zeta potential has a notable effect on properties of foamed suspension, which remains poorly explored. It is demonstrated low zeta potential is beneficial for foam stability, and the experimental results show that absolute value of zeta potential of 30–40 mV is the boundary between stable foams and unstable foams. High zeta potential above 40 mV gives rise to instability of foamed colloidal suspension, owing to strong interparticle repulsion force that prevents particles from forming a closely packed particle network at liquid/air interfaces. Besides, it is found that high zeta potential leads to low foamability of foamed colloidal suspension. It is also demonstrated herein that stable and uniform foams with thin and homogeneous-thickness wall without agglomerates can be prepared at the isoelectric point (IEP) using long-chain surfactant, which generates weak agglomerated particles that can be re-separated to monodisperse particles due to the steric hindrance effect of surfactant.

  • Innovative carbon-bonded filters based on a new environmental-friendly binder system for steel melt filtration
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    C. Himcinschi, C. Biermann, E. Storti, B. Dietrich, G. Wolf, J. Kortus, C.G. Aneziris

    New carbon-bonded alumina filters for steel melt filtration were developed. The carbonaceous matrix was based on a new, environmental friendly binder system based on lactose and tannin. The filter preparation was analogous to the production of conventional foam filters according to the Schwartzwalder process. The processing as well as the rheology of the slurries was investigated. An addition of n-Si increased the carbon yield and the cold crushing strength (CCS) of the samples. Higher values of CCS were obtained after coating of the filters with alumina. The material was characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The applicability of these new filters was assessed in impingement tests with a steel melt, in which three out of four recipes survived the thermal shock.

  • Microstructure and Phase Evolution of Atmospheric Plasma Sprayed Mn-Co-Fe Oxide Protection Layers for Solid Oxide Fuel Cells
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Nikolas Grünwald, Yoo Jung Sohn, Xiaoyan Yin, Norbert H. Menzler, Olivier Guillon, Robert Vaßen

    Dense protective layers are needed to reduce chromium-related degradation in SOFC stacks. In particular, atmospheric plasma sprayed (APS) Mn1,0Co1,9Fe0,1O4 (MCF) coatings demonstrated low degradation rates in stack tests. We show that short-term annealing in air induces crack healing within these coatings. Parallel to this effect, a phase transformation is observed originating from oxidation that proceeds by solid state-diffusion. The present contribution reveals the basic mechanisms of the microstructural and phase changes of coatings in long-term annealing tests of up to 10,000 h at 700 °C. The layer develops differently at the air-facing surface and in the bulk. Due to cation deficiency, oxidation is dominated by cation outward diffusion, leading to a Co-enriched surface layer. The bulk displays a fine distribution of the initial (rock salt) and the final (spinel) phases. Understanding the mechanisms leading to these irreversible changes enables predictions to be made concerning durable protective coatings in SOFCs.

  • Critical roles of the rhombohedral-phase inducers in morphotropic NaNbO3-BaTiO3-ABO3 quasi-ternary lead-free piezoelectric ceramics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    He Qi, Ruzhong Zuo, Jing-feng Li, Longtu Li

    A morphotropic phase boundary (MPB) between rhombohedral (R) and tetragonal (T) phases was identified in a few (0.9-x)NaNbO3-0.1BaTiO3-xABO3 (x = 0-0.05) lead-free systems. Critical roles of R-phase inducers were specially evaluated in terms of phase boundary position, microstructure and piezoelectric responses. The results indicate not only the tolerance factor of the ABO3 additive but also its ferroelectricity and corresponding volume change would influence the formation of phase boundary and further determine dielectric and ferroelectric responses. The piezoelectric coefficient d33 of MPB compositions was compared with theoretically-calculated d33-cal according to d33 = 2Pr·ε33·Q33, demonstrating that the piezoelectric response of these systems should be determined by combined effects of the phase coexistence, nano-scale domains and particularly enhanced dielectric responses. The largest d33 ~305 pC/N, the highest ε33T/εo ~2815 and the lowest Pr ~14.7 μC/cm2 were achieved in the MPB composition with 3.75% SrZrO3. These experimental results provide a valuable reference for designing new NaNbO3-based lead-free piezoelectric materials.

  • The effect of electric fields on grain growth in MgAl2O4 spinel
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Wei Qin, Dereck N.F. Muche, Ricardo H.R. Castro, Klaus van Benthem

    The application of electrostatic fields during processing of oxide ceramic microstructures was previously reported to enhance densification and grain growth. In this study effects of the externally applied electrostatic field strength on grain growth in MgAl2O4 were investigated. Free sintering of green bodies showed accelerated grain growth by about 20% in the presence of an applied nominal field strength of 0.95 kV/cm. In contrast to previous studies, annealing of dense microstructures in the presence of a nominal electric field strength as high as 2.22 kV/cm revealed no additional grain growth. A machine learning algorithm for grain size analysis enabled grain size distributions including up to 30,000 grains. Due to the resulting counting statistics for microstructure analysis, it was discovered that the applied electrostatic fields caused grain growth predominantly during the early stages of sintering, i.e., at lower green body densities, hence suggesting an enhancement of surface diffusion.

  • Phase equilibria and thermodynamic evaluation of BaO-TiO2-YO1.5 system
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Xiaomei Liu, Zhu Li, Jing Wang, Rulin Zhang, Wajid Ali, Shihua Wang, Xionggang Lu, Chonghe Li

    A series of ceramic samples were prepared and characterized at 1400 °C in the BaO-TiO2-YO1.5 system. The obtained experimental results were adopted to the present thermodynamic evaluation to derive a set of thermodynamic database for the BaO-TiO2-YO1.5 system. The database was constructed by the CALPHAD method where the binary parameters from BaO-TiO2 and TiO2-YO1.5 systems were re-optimized, those from the BaO-YO1.5 system were simulated by our previous assessments. Thermodynamics descriptions of all liquid and terminal solid solution phases were treated by the substitutional solution model, the H-BaTiO3 (BaTiO3, hexagonal structure) and C-BaTiO3 (BaTiO3, cubic structure) were described by the compound energy formalism model, and other binary intermediate phases were treated as the stoichiometric compounds. Finally, some thermodynamic diagrams were calculated and compared with the experimental results. Good agreement between present calculations and the experimental results demonstrates that the present thermodynamic database is self-consistent and credible.

  • In situ measurement of temperature and reduction of rutile titania using energy dispersive x-ray diffraction
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Harry Charalambous, Shikhar Krishn Jha, Xin Li Phuah, Han Wang, Haiyan Wang, John Okasinski, Thomas Tsakalakos

    Using in situ energy dispersive x-ray diffraction (EDXRD) the average specimen temperature of TiO2 in the steady state of flash is experimentally determined. Comparison of the microstructure for flash sintering and conventional sintering in addition to the temperature determined from calibration of the unit cell expansion indicates that rapid Joule heating during flash sintering causes densification and grain growth comparable with conventional sintering. An average temperature approximation model is proposed to account for greybody radiation and thermal conduction. The inhomogeneity of the grain growth across the sample length is confirmed to correlate with inhomogeneity in temperature distribution and this suggests a current induced Peltier effect in n-type TiO2.

  • Prolong the durability of La2Zr2O7/YSZ TBCs by decreasing the cracking driving force in ceramic coatings
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Bo Cheng, Zhi-Yuan Wei, Lin Chen, Guan-Jun Yang, Cheng-Xin Li, Chang-Jiu Li

    Service lifetime and thermal insulation performance are both crucial for the application of thermal barrier coatings (TBCs). In this study, layered structure design under equivalent thermal insulation conception is introduced to lower the cracking driving force in TBCs, and with the goal of prolonging TBCs lifetime. Three groups of layered LZO/YSZ TBCs were designed with same thermal insulation of 500 μm YSZ, the LZO layers were deliberately designed with different initial elastic modulus. Virtual crack closure technique (VCCT) calculation result showed that the energy release rates at the crack tips are 28.2, 22, and 18.8 N/m corresponding to the initial elastic modulus of 70, 60, and 50 GPa. After gradient thermal cyclic tests with surface temperature of 1300 °C, TBCs with lowest initial elastic modulus showed the longest lifetime, and more than double of pure YSZ TBCs. This study provides a new option for the improvement of TBCs lifetime.

  • Image Analysis of the Porous Yttria-Stabilized Zirconia (YSZ) Structure for a Lanthanum Ferrite-impregnated Solid Oxide Fuel Cell (SOFC) Electrode
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Chengsheng Ni, Mark Cassidy, John T.S. Irvine

    Image analysis and quantification were performed on porous scaffolds for building SOFC cathodes using the two types of YSZ powders. The two powders (U1 and U2) showed different particle size distribution and sinterability at 1300 °C. AC impedance on symmetrical cells was used to evaluate the performance of the electrode impregnated with 35-wt.% La0.8Sr0.2FeO3. For example, at 700 °C, the electrode from U2 powder shows a polarization resistance (Rp) of 0.21 Ω cm2, and series resistance (Rs) of 8.5 Ω cm2 for an YSZ electrolyte of 2-mm thickness, lower than the electrode from U1 powder (0.25 Ω cm2 for Rp and 10 Ω cm2 for Rs) does. The quantitative study on image of the sintered scaffold indicates that U2 powder is better at producing architecture of high porosity or long triple phase boundary (TPB), which is attributed as the reason for the higher performance of the LSF-impregnated electrode.

  • Preparation, crystal structure and luminescence properties of red-emitting Lu3Al5O12: Mn4+ ceramic phosphor
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-22
    Yunli Zhang, Song Hu, Yingli Liu, Zhengjuan Wang, Wei Ying, Guohong Zhou, Shiwei Wang

    A series of red-emitting Mn4+ doped Lu3Al5O12 (LuAG) ceramic phosphors were successfully prepared by a simple solid-state reaction method in a high-temperature muffle. MgO was co-doped as sintering aids and Mg2+ ions helped to realize the charge balance. The relations between the luminescence properties, crystal structures and the microstructures were well established. Results indicated that MgO promoted the densification of the ceramics as the specimens’ relative densities were up to 99%. Moreover, the substitution of Al3+ with Mg2+ have changed crystal structures and further affected the luminescent properties. Overall, the obtained ceramic phosphors showed strong red-light emission under excitation of ultraviolet and blue light. By optimizing the Mg2+ and Mn4+ concentration, a quantum efficiency (QE) as high as 47.8% can be achieved under the excitation of 460 nm light, indicating that the LuAG: Mn4+ ceramic phosphors are promising candidates for WLEDs applications.

  • Dielectric Relaxation and ac Conductivity in Magnetoelectric YCrO3 Ceramics: A Temperature Dependent Impedance Spectroscopy Analysis
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-20
    Ashish Kumar Mall, Ashish Garg, Rajeev Gupta

    Here, we report on the temperature and frequency dependent electrical conduction and dielectric behavior of YCrO3 ceramics. Dielectric studies reveal a peak in the dielectric constant ∼ 230 K, suggesting presence of spin-charge coupling. Also, an additional broad peak found at ∼ 450 K is reminiscent of a relaxor like behavior for YCrO3, attributed to a diffused phase transition. The nature of dc conductivity is of Arrhenius type and shows an abrupt change in the activation energy at ∼ 230 K and ~ 450 K. The activation energy suggests that the polaronic hopping mechanism stabilizes at low temperature while, at higher temperatures, the process is associated with the diffusion of double ionized oxygen vacancies. However, ac conductivity suggests that the overlapping large polaron tunneling conduction mechanism drives the ac conduction below 300 K and above 300 K, the conduction behavior is consistent with the correlated barrier hopping conduction mechanism.

  • Enhanced pyroelectric and piezoelectric responses in W/Mn-codoped Bi4Ti3O12 Aurivillius ceramics
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-20
    Yanxue Tang, Zong-Yang Shen, Qiuxiang Du, Xiangyong Zhao, Feifei Wang, Xiaomei Qin, Tao Wang, Wangzhou Shi, Dazhi Sun, Zhiyong Zhou, Shujun Zhang

    The pyroelectric and piezoelectric properties of 4 at% Mn-doped Bi4Ti2.9W0.1O12 (BiTW-Mn) Aurivillius ceramic were investigated and compared to Bi4Ti2.9W0.1O12 (BiTW) counterpart, which were fabricated using a conventional solid state reaction method. High resistivities of 4.9 × 1012 and 2.5 × 1011 Ω·cm at 100 °C were obtained in the W-doped and W/Mn-codoped BiT ceramics, respectively. They showed similar activation energies and ionic-p-type mixed conduction mechanisms. Higher pyroelectric coefficients of 57.1 μC/m2K and piezoelectric coefficients of 21 pC/N, as well as much lower dielectric loss of 0.003 were achieved in W/Mn-codoped ceramics. These property changes were mainly induced by M n Ti − V o defect dipoles. The effect of acceptor doping was evidenced by an internal bias field, shown by a horizontal offset in the polarization-field behavior. The improved properties together with high thermal stability indicate that BiTW-Mn may be a promising candidate for pyroelectric and piezoelectric devices at elevated temperatures.

  • Ferroelectric and dielectric behaviors of sol-gel derived perovskite PMN-PT/PZT heterostructures via compositional development: An interface-dependent study
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-20
    Huiting Sui, Huajun Sun, Xiaofang Liu, Dingguo Zhou, Renxin Xu

    Phase-pure perovskite 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3/Pb(Zr0.52Ti0.48)O3 (PMN-PT/PZT) heterostructures were prepared via compositional development using a sol-gel route. Interface-dependent microstructure, insulating, ferroelectric and dielectric performance were investigated. Relatively enhanced ferroelectricity (Pr=21.81 μC/cm2, Ec = 61.88 kV/cm) and dielectricity (εr = 1959, tanδ = 0.0152) were obtained for the heterostructure with the maximum number of interfaces (7 interfaces, namely F-7 type). Presumably, this behavior is due to the reduced leakage current density (10-9–10-8 A/cm2 at ± 400 kV/cm), which arises from columnar grain growth mode with more depletion layers generated between different compositions acting as a potential barrier for the movement of free carriers. A simplified equivalent circuit is used to provide a comprehensive explanation of the enhanced performance mechanism. These results highlight inherent issues in designing structures with specific interfaces and provide a new approach for designing high-performance PMN-PT-based thin film.

  • Mullite-bonded SiC-whisker-reinforced SiC matrix composites: Preparation, characterization, and toughening mechanisms
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-19
    Yang Luo, Shili Zheng, Shuhua Ma, Chunli Liu, Xiaohui Wang
  • Harmonized toughening and strengthening in pressurelessly reactive-sintered Ta0.8Hf0.2C-SiC composite
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-19
    Buhao Zhang, Jie Yin, Yihua Huang, Jian Chen, Xuejian Liu, Zhengren Huang

    Ta0.8Hf0.2C-27 vol%SiC (99.0% in relative density) composite was toughened and strengthened via pressurelessly in-situ reactive sintering process. HfC and β-SiC particles were formed after reaction of HfSi2 and carbon black at 1650 °C. Ta0.8Hf0.2C was obtained from solid solutioning of HfC and commercial TaC. The β-α phase transformation of SiC proceeded below 2200 °C. High aspect ratio, platelet-like α-SiC grains formed and interconnected as interlocking structures. Toughness and flexural strength values of 5.4 ± 1.2 MPa∙m1/2 and 443 ± 22 MPa were measured respectively. The toughening mechanisms by highly directional growth of discontinuous α-SiC grains were crack branching, bridging and deflection behaviors.

  • A new Li-based ceramic of Li4MgSn2O7: synthesis, phase evolution and microwave dielectric properties
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-17
    Ruzhong Zuo, He Qi, Fang Qin, Qianlong Dai

    A pure-phase Li4MgSn2O7 (L4MS) was successfully synthesized through optimizing the calcination condition. Microwave dielectric properties of the L4MS ceramic with the phase evolution were investigated together with its low-temperature sintering. The sample maintains a single L4MS phase as sintered below 1200 °C, such that τf remains a constant value of ~12.4 ppm/°C. Accompanied by the appearance of impurity phases (Li2SnO3)ss and especially (MgO)ss at higher sintering temperatures, excellent microwave dielectric properties of εr = 13.1~13.5, Q × f = 106800~126810 GHz and τf = 0~-4.2 ppm/°C are obtained in samples sintered at 1215~1260 °C for 4 h. Reduction of sintering temperature using LiF sintering aid also helps achieve pure-phase dense L4MS ceramic. The L4MS + x wt.% LiF ceramic exhibits εr~13.7, Qxf~97000 GHz (x≤3) and τf~8-13 ppm/°C sintered at 850 °C for potential LTCC applications, and εr ~13.9, Qxf~146000 GHz and τf ~1.5-6 ppm/°C (x≥4) as sintered 1000 °C, exhibiting large potentials for microwave dielectric candidates.

  • Fabrication of one-step co-fired proton-conducting solid oxide fuel cells with the assistance of microwave sintering
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-18
    Bin Wang, Lei Bi, X.S. Zhao

    A microwave sintering technique is reported for fabricating co-sintered proton-conducting solid oxide fuel cells. With this method, high-quality ceramic electrolyte membranes can be prepared at 1100 °C, thus enabling the fabrication of entire cells in a single step. The microwave sintering method not only enhances electrolyte densification but also improves the cathode/electrolyte interface, which is critical for improving fuel cell performance. The power output of the co-sintered cell prepared under the microwave conditions (up to 449 mW cm-2 at 700 °C) was significantly higher than that of the cell fabricated using the traditional co-sintering method (approximately 292 mW cm-2 at the same temperature). Electrochemical analysis revealed that the enhanced electrolyte density and the improved cathode/electrolyte interface achieved by using the microwave sintering technique decrease both the ohmic resistance and the polarisation resistance of the cell, leading to good fuel cell performance.

  • The effect of SiC addition on photoluminescence of YAG:Ce phosphor for white LED
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-16
    Dusan Bucevac, Vladimir Krstic

    An effective way of improving photoluminescence (PL) of YAG:Ce by addition of small amount of SiC and sintering in air was described. The breakdown of SiC during sintering process in air was employed to provide the presence of SiO2 and CO both of which are known to be beneficial in enhancing the PL of YAG:Ce phosphor. SiC in the form of a fine powder was added to YAG:Ce powder and sintered to densities of >99% of theoretical density. The highest luminescence was measured in sample containing 0.08 wt% SiC. The effect of the formed SiO2 and CO was discussed and their contribution to the emission intensity was assessed. The enhancement of PL intensity is attributed to the formation of vacancies, both on Y sub-lattice and on oxygen sub-lattice and their ability to release the electrons for subsequent reduction of Ce4+ to Ce3+ which plays a role of luminescence activator.

  • Space-resolved study of binder burnout process in dry pressed ZnO ceramics by neutron imaging
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-15
    Lise Donzel, David Mannes, Michael Hagemeister, Eberhard Lehmann, Jan Hovind, Nikolay Kardjilov, Christian Grünzweig

    A novel non-destructive space-resolved method for studying binder burnout is presented. Debinding of dry pressed zinc oxide green bodies was analyzed by neutron imaging. Neutrons in contrast to X-rays allow penetrating samples with large dimensions up to 80 mm and at the same time detecting variations in the organics content as small as 0.1 wt%. We used neutron radiography (2D) and tomography (3D) to investigate with a spatial resolution in the order of 100 µm the distribution of binder during burnout under three different conditions. First a reference case with green bodies placed on a plate in a lab furnace. Second, a configuration with a reduced volume of air with green bodies enclosed in a sagger and third a green body placed on an improved supporting structure. The method is extendable to other particulate materials and green bodies shaped by processes involving organic binder such as casting, extrusion and injection.

  • Thermophysical, environmental, and compatibility properties of nitrate and nitrite containing molten salts for medium temperature CSP applications: A critical review
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-14
    T. Delise, A.C. Tizzoni, M. Ferrara, N. Corsaro, C. D’Ottavi, S. Sau, S. Licoccia

    The production of electric energy from solar radiation is nowadays one of the most investigated and developed "carbon free" technology. A throughout investigation of the ceramics most commonly used as heat transfer fluids and/or heat storage materials for concentrating solar power systems (i.e. alkaline and heart alkaline nitrate/nitrite mixtures) is here reported. The study stems by the need to base materials selection on an accurate and critical knowledge of all their characteristics, including their thermophysical, environmental compatibility, and economic features.At this purpose, a rating criterion have been established, to readily show the advantages and disadvantages of each material, and to highlight which characteristics of the examined materials need to be further investigated and improved. Nitrate/nitrite mixtures have also been compared with other ceramics used for thermal storage, such as solid fillers, liquid metals, other salt mixtures, or phase change materials.

  • Fracture toughness testing of biomedical ceramic-based materials using beams, plates and discs
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-11
    Renan Belli, Michael Wendler, Anselm Petschelt, Tanja Lube, Ulrich Lohbauer

    The testing of fracture toughness becomes problematic when only limited amount of material is available that hinders the production of typical beam specimens to be tested in bending. Here we explore fracture toughness testing methodologies that allow for small discs and plates having surface cracks to be tested in biaxial flexure using the Ball-on-3-balls (B3B) set-up, or sawed notches as in the Compact Tension geometry. The B3B-KIc test has shown to be versatile and account for a very small overestimation of the KIc-value in the order of 0.8–1.25% due to in-plane crack mispositioning, and a maximum of 4% if a worst-case scenario of additional out-of-plane mispositioning is assumed. The geometrical factor in the standard SCF method, derived by Newman and Raju, resulted in an overestimation of ~8% of the KIc-value compared to the new calculation by Strobl et al. for materials with Poisson´s ratio <0.3.

  • Stable and efficient all-inorganic color converter based on phosphor in tellurite glass for next-generation laser-excited white lighting
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-11
    Yang Peng, Yun Mou, Hao Wang, Yong Zhuo, Hong Li, Mingxiang Chen, Xiaobing Luo

    Laser lighting is considered as a next-generation high-power lighting due to its high-brightness, directional emission, and quasi-point source. However, thermally stable color converter is an essential requirement for white laser diodes (LDs). Herein, we proposed a stable and efficient phosphor-in-glass (PiG) in which YAG:Ce3+ and MFG:Mn4+ phosphors were embedded into tellurite glass matrixes. The glass matrixes with low-melting temperature and high refractive index were prepared by designing their composition. The luminescence of YAG:Ce3+ PiGs was adjusted by controlling phosphor thickness. Aiming to compensate for red emission, multi-color PiGs were realized by stacking MFG:Mn4+ layers on YAG:Ce3+ layer. The phosphor crystals are chemically stable and maintain intact in the glass matrix. Furthermore, white LDs were fabricated by combining the PiGs with blue LDs. As the phosphor thickness increases, the chromaticity of white LDs shifts from cool to warm white, and the white LDs exhibit excellent thermal stability under different excitation powers.

  • Development of sapphirine opaque glazes for ceramic tiles
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-11
    Ruqiang Li, Ming Lv, Jingpei Cai, Kang Guan, Fan He, Weixiong Li, Cheng Peng, Pinggen Rao, Jianqing Wu

    The purpose of this work is to produce more excellent opaque wall tile glazes by using sapphirine instead of zirconium silicate as an opacifier. In order to achieve it, the chemical compositions were precisely adjusted in the system of SiO2-Al2O3-MgO-K2O-Na2O-B2O3. The morphological characteristics of the glaze were determined by differential scanning calorimetry (DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). And the optical properties of the glaze were characterized by a spectrophotometer and a gloss meter. The results reveal that well-shaped crystals of sapphirine (Mg2Al4SiO10) with needle-like morphology were formed as only crystal phase. The glaze is provided with better opacity whiteness and gloss compared with commercial zircon-based glass-ceramic glazes, and it is mainly composed of cheap mineral raw materials. Those features make it as an alternative one for improving the properties of conventional opaque ceramic glazes.

  • Creep study on Alumina and Alumina/SWCNT nanocomposites
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-11
    Miguel Castillo-Rodríguez, Antonio Muñoz, Arturo Domínguez-Rodríguez

    Alumina and alumina/SWCNT nanocomposites have been sintered by spark plasma sintering, obtaining relative densities higher than 99%. Microstuctural characterization revealed a grain microstructure in the submicron range, where alumina/SWCNT nanocomposites exhibited a good CNTs dispersion thought the ceramic matrix. Creep experiments performed in both materials showed a similar mechanical behavior, where the addition of CNTs seems to have a negligible effect on the strain rate, in contrast to results reported by other authors. Grain boundary sliding accommodated by lattice diffusion has been identified as the high temperature deformation mechanisms in both samples, alumina and alumina/SWCNT nanocomposites. We have discussed about the role of CNTs influence on the plasticity of these composites.

  • Investigation on the phase stability of cubic perovskite BaCo0.7Fe0.2Nb0.1O3-δ oxygen-permeable membrane
    J. Eur. Ceram. Soc. (IF 3.794) Pub Date : 2018-08-11
    Jian Zhang, Jialu Zhang, Lanqian Li, Chao Zhang, Yuwen Zhang, Xionggang Lu

    The phase stability of the cubic perovskite-type oxide BaCo0.7Fe0.2Nb0.1O3−δ (BCFNO) has been examined by means of X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). And, the timescale on the second phases has been established by using the TOPAS 4.2. Compared with Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCFO), for samples annealed at T = 1023 K for t = 64 d, the rhombohedral and hexagonal phases formed on the surface of cubic perovskite BCFNO surface simultaneously and the amount of them is smaller. As for the rhombohedral phase, it comes out firstly along the grain boundary, and whose amount in equilibrium is about 5%. In contrast to the rhombohedral phase, the hexagonal phase is more likely to form at lower temperature which lead to the microcracks. In brief, obtaining the eligible phase stability is crucial for the industrial application of the oxygen permeation membrane.

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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