Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-18 Lin-Lin Zhu, Young-Jo Park, Lin Gan, Ha-Neul Kim, Jae-Woong Ko, Hai-Doo Kim
We succeeded in the optimization of highly transparent Y2O3 ceramics with a submicrometer grain size approximately 0.6 μm by hot pressing (1300-1550 °C) and a subsequent HIP (1450 °C) treatment using commercial Y2O3 powders as starting powders and ZrO2 as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400 °C for 3 h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2 mm thick) ever reported of 83.4% and 78.3% at 1100 and 400 nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15 nm in size.
Environmental-Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate (CMAS) Glass: Part I, YAlO3 and γ-Y2Si2O7 J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-17 Laura R. Turcer, Amanda R. Krause, Hector F. Garces, Lin Zhang, Nitin P. Padture
The high-temperature (1500 °C) interactions of two promising dense, polycrystalline EBC ceramics, YAlO3 (YAP) and γ-Y2Si2O7, with a calcia-magnesia-alumino-silicate (CMAS) glass have been explored as part of a model study. Despite the fact that the optical basicities of both the EBC ceramics and the CMAS are similar, they both react with the CMAS. In the case of the Si-free YAlO3, the reaction zone is small and it comprises three regions of reaction-crystallization products, including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12 (YAG(ss)). In contrast, only Y-Ca-Si apatite(ss) forms in the case of Si-containing γ-Y2Si2O7, and the reaction zone is an order-of-magnitude thicker. These CMAS interactions are analyzed in detail, and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 and β-Sc2Si2O7) in the accompanying Part II. This is attributed to the presence of the Y in the YAlO3 and γ-Y2Si2O7 EBC ceramics.
Phase Equilibria in the Zirconia–Yttria/Gadolinia–Silica Systems J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-17 Najeb M. Abdul-Jabbar, David L. Poerschke, Cian Gabbett, Carlos G. Levi
Phase equilibria in ZrO2-YO1.5-SiO2 (ZYS) and ZrO2-GdO1.5-SiO2 (ZGS) were experimentally assessed at 1400 °C and 1600 °C as they can offer insight on reactions between thermal barrier coatings (TBCs) based on ZrO2-YO1.5/GdO1.5 and molten silicate deposits in gas turbine engines. Features shared in both systems include the absence of ternary compounds and no ternary solubility in the binary phases. In ZYS however, a quaternary invariant reaction was observed that eliminates the zircon-disilicate equilibrium at higher temperatures. The results suggest no appreciable difference in the reactions between silica and thermal barrier oxides based on ZrO2-YO1.5 or ZrO2-GdO1.5, or environmental barrier coatings based on the corresponding Y/Gd silicates. The phase diagrams derived from these experiments are part of a broader effort to develop thermodynamic databases that can help guide the design of next-generation TBCs.
Solid-state synthesis of pure Ye’elimite J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-17 Y.EL Khessaimi, Y.EL Hafiane, A. Smith, R. Trauchessec, C. Diliberto, A. Lecomte
Ye’elimite is formed during the production of sulfoaluminate cement. In this article, the orthorhombic ye’elimite formation, the optimal synthesis conditions and the microstructural evolution during synthesis, by solid-state reaction from pure oxide raw materials, is investigated. The phase assemblage was substantially affected by temperature and duration of sintering. Making reference to Rietveld quantitative analysis results, optimal solid-state synthesis conditions of ye’elimite was 1300 °C for 3 hours. During ye’elimite synthesis, significant gas releases were observed at different stages of firing using TGA coupled with mass-spectrometer. The gases are the product of carbonate decomposition, gypsum dehydration and sulfate decomposition from the unreacted anhydrite and the formed ye’elimite. Based on the present work, it emerges that a key strategy for forming ye’elimite with a high purity is to compensate sulfate decomposition by the addition of a slight excess of CaSO4 before repeating the firing cycle at optimal conditions. Finally, the porosity was investigated using Archimedes principle measurements compared to BSE-image analysis. It shows the difficulty to achieve dense sintered ye’elimite because of the high decomposition gas releases during the firing process.
Stage-sensitive microstructural evolution of nanostructured TBCs during thermal exposure J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-15 Guang-Rong Li, Guan-Jun Yang, Cheng-Xin Li, Chang-Jiu Li
Nanostructured thermal barrier coatings (TBCs) often exhibit bimodal structure comprised of both nanozones and lamellar zones, and therefore, their sintering behaviour can be different from that of conventional coatings. In this study, changes in the microstructure and properties of nanostructured TBCs were investigated. The results show that their microstructural evolution is highly time-sensitive during long thermal exposure at 1150 °C. In stage I (0–20 h), changes in mechanical properties were significant. The dominant microstructural change was faster healing of flat pores, whereas the macroscopic structure seemed less affected. In stage II (20–500 h), the changes in properties were much slighter and some large macroscopic voids appeared. In brief, the microscopic healing of pores in lamellar zones leads to a significant change in mechanical properties in stage I, whereas sintering of the nanozones leads to macroscopic voids in stage II.
ON THE YTTRIUM TANTALATE – ZIRCONIA PHASE DIAGRAM J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-14 Mary Gurak, Quentin Flamant, Laetitia Laversenne, David R. Clarke
The phase diagram for the YTaO4-ZrO2 quasi-binary has been determined up to 1600 °C. There are three distinct compositional regimes: an extensive YTaO4 solid solution, an extensive ZrO2 solid solution and a two-phase intermediate region. The addition of ZrO2 to YTaO4 decreases the M-T transition temperature almost linearly from 1426 °C to approximately 450 °C at the solubility limit (~28 mol% ZrO2), and then remains constant until the ZrO2 solid solution phase boundary is reached. Within the intermediate region, there exists an extensive two-phase tetragonal (T+t) phase field above the M-T transformation temperature. The transformation exhibits no hysteresis on heating and cooling but nonetheless there is a distribution with temperature in the mass fraction of the monoclinic and tetragonal phases so no unique transformation temperature can be identified. No other high temperature phases were observed but it is suggested that a higher temperature solid solution phase is likely above 1700 °C, based on the similarity in crystallographic relationship between the two tetragonal solid solution structures.
Strengthening mechanism of twin lamellas in transparent AlON ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-14 Huilu Guo, Jian Zhang, Xiaojian Mao, Fang Zhang, Feng Sun, Feng Chen, Jun Wang, Run Tian, Juan Liu, Shiwei Wang
Twin lamellas, were observed in transparent AlON ceramics in our previous work. In this work, the influence of the twin lamellas on mechanical strength was evaluated and its mechanism was investigated by SEM, EBSD, and HRTEM. Both the crystallographic direction and the space direction of a crack will deflect when it propagates across a twin lamella. The dynamic mechanism is that the dislocation propagation in front of the crack tips is blocked by the twin boundaries. Because the fraction of twin boundaries increases with grain size in transparent AlON ceramics, the strengthening effect becomes more notable for large-sized samples resulting in inverse Hall-Petch relation. It is a potentially useful way to advance the mechanical properties of transparent AlON ceramics by controlling the content of twin lamellas.
3D printing of porcelain by layerwise slurry deposition J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-13 Pedro Lima, Andrea Zocca, Wilson Acchar, Jens Günster
The Layerwise Slurry Deposition is a technology for the deposition of highly packed powder layers. A powder bed is achieved by depositing and drying layers of a ceramic suspension by means of a doctor blade. This deposition technique was combined with the binder jetting technology to develop a novel Additive Manufacturing technology, named LSD-print. The LSD-print was applied to a porcelain ceramic. It is shown that it was possible to produce parts with high definition, good surface finish and at the same time having physical and mechanical properties close to those of traditionally processed porcelain, e.g. by slip casting.This technology shows high future potential for being integrated alongside traditional production of porcelain, as it is easily scalable to large areas while maintaining a good definition. Both the Layerwise Slurry Deposition method and the binder jetting technologies are readily scalable to areas as large as >1 m2.
Structural, spectroscopic and thermal properties of hot-pressed Nd:(Ca0.94Gd0.06)F2.06 transparent ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-13 Guoqiang Yi, Weiwei Li, Jinghong Song, Bingchu Mei, Zhiwei Zhou, Liangbi Su
0.5-5.0 at.% Nd3+ doped (Ca0.94Gd0.06)F2.06 transparent ceramics were fabricated by vacuum hot-pressing sintering. The structural, spectroscopic and thermal properties of Nd:(Ca0.94Gd0.06)F2.06 transparent ceramics, as well as the influence of Nd3+ content on these properties were investigated. The as-fabricated ceramic samples exhibited high transparency and nearly pore-free microstructure. The absorption peaks located at 538 nm, 576 nm, 736 nm, 792 nm and 865 nm were attributed to the transitions from ground state to the excited states of Nd3+ ions, and the absorption coefficients increased linearly with Nd3+ content increasing. The emission band of the sample doped with 1 at.% Nd3+ concentration exhibited the highest emission intensity, while the lifetime decreased sharply with the increase of Nd3+ concentration. In addition, with Nd3+ content increasing from 0.5 to 5.0 at.%, the thermal expansivity coefficients increased slightly, while the thermal conductivity decreased from 4.21 to 2.36 W/m·K at room temperature.
p-type/n-type behaviour and functional properties of KxNa (1-x)NbO3 (0.49 ≤ x ≤ 0.51) sintered in air and N2 J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-13 Fayaz Hussain, Iasmi Sterianou, Amir Khesro, Derek C. Sinclair, Ian. M. Reaney
Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N2 using a conventional mixed oxide route. Oxygen vacancies (VO••) played a major role in the semi-conduction mechanism in low p(O2) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O2) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for air- rather than N2-sintered samples with d33 = 125 pC/N and kp = 0.38 obtained for K0.51Na0.49NbO3.
Transparent Highly Oriented 3C-SiC Bulks by Halide Laser CVD J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-12 Hong Cheng, Meijun Yang, Youfeng Lai, Mingwei Hu, Qizhong Li, Rong Tu, Song Zhang, Mingxu Han, Takashi Goto, Lianmeng Zhang
Transparent and highly oriented 3C-SiC bulks were speedily fabricated at deposition temperature (Tdep) of 1623 K by halide laser chemical vapor deposition (HLCVD). The effect of total pressure (Ptot) on the optical transmittance, preferred orientation, microstructure, deposition rate (Rdep) and micro-hardness were investigated. The maximum Rdep of the transparent 3C-SiC reached 2450 µm/h at Ptot = 10 kPa. With an increase in Ptot, the transmittance of 3C-SiC bulks increased firstly, and then decreased. At Ptot = 10 kPa, 3C-SiC bulk, a highly <111 > -oriented and low density of defects, showed the highest transmittance, greater than 55% in the wavelength range of 800 to 1100 nm. At Ptot = 4 kPa and 20 kPa, 3C-SiC bulks showed much lower transmittance, in which contained poorly oriented grains and numerous defects. The Vickers micro-hardness of 3C-SiC bulks increased with increasing Ptot and showed the highest value of 34.8 GPa at Ptot = 40 kPa.
Comparative study on structure, dielectric, and piezoelectric properties of (Na0.47Bi0.47Ba0.06)0.95A0.05TiO3 (A=Ca2+/Sr2+) ceramics: Effect of radii of A-site cations J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-10 Rui-yi Jing, Xiao-ming Chen, Han-li Lian, Xiao-shuang Qiao, Xiao-jing Shao, Jian-ping Zhou
Lead-free ceramics (Na0.47Bi0.47Ba0.06)TiO3, (Na0.47Bi0.47Ba0.06)0.95Ca0.05TiO3, and (Na0.47Bi0.47Ba0.06)0.95Sr0.05TiO3 (BNBT, BNBT-Ca, and BNBT-Sr, respectively) were obtained via solid-state sintering. Due to the different radii between Ca2+ and Sr2+, the mean radii of the A-site cations in BNBT-Ca, BNBT, and BNBT-Sr are different. Effects of radii of the A-site cations on crystallite structure, microstructure, piezoelectric, and dielectric properties of the ceramics were investigated comparatively. X-ray diffraction and Raman spectroscopy results reveal the lattice distortion due to the doping. With the increase in the mean radii of the A-site cations, lattice parameters, tolerance factor, band-gap energy, average size of grains, dielectric constant, and piezoelectric constant of the ceramics increase. The results imply that structure and electrical properties of BNBT-based ceramics are associated with radii of the A-site cations, which provides a possible route to tune structure and electrical properties of BNBT-based materials by changing radii of the A-site cations.
Quasi-static and Dynamic Response of 3D-printed Alumina J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-10 Matthew DeVries, Ghatu Subhash, Alexander Mcghee, Peter Ifju, Tyrone Jones, James Zheng, Virginia Halls
Mechanical properties and microstructure of 3D-printed alumina processed using pressurized spray deposition have been compared to a commercial sintered alumina. The 3D-printed alumina microstructure was found to be bimodal in nature, with alumina particles agglomerated into large spheres, which resulted in 6.1% porosity. Compared to the sintered alumina, the 3D-printed material exhibited lower quasi-static and dynamic compressive strength, negligible differences in quasi-static and dynamic Vickers hardness, and negligible differences in quasi-static and dynamic fracture toughness. However, while the dynamic fracture surfaces of 3D-printed alumina were smooth and planar, large undulations were observed under quasi-static loading. It is concluded that the pressurized spray deposition 3D-printing technique is a promising method for processing alumina with properties comparable to that produced by traditional techniques, and further improvements may be gained by eliminating porosity.
Environmental-Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate (CMAS) Glass: Part II, β-Yb2Si2O7 and β-Sc2Si2O7 J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-10 Laura R. Turcer, Amanda R. Krause, Hector F. Garces, Lin Zhang, Nitin P. Padture
The high-temperature (1500 °C) interactions of two promising dense, polycrystalline EBC ceramics, β-Yb2Si2O7 and β-Sc2Si2O7, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Unlike YAlO3 and γ-Y2Si2O7 in the accompanying Part I paper, little or no reaction is found between the Y-free EBC ceramics and the CMAS. In the case of β-Yb2Si2O7, a small amount of reaction-crystallization product Yb-Ca-Si apatite solid solution (ss) forms, whereas none is detected in the case of β-Sc2Si2O7. The CMAS glass penetrates into the grain boundaries of both EBC ceramics, and they both suffer from a new type of ‘blister’ cracking damage. This is attributed to the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the CMAS glass. The success of a ‘blistering’-damage-mitigation approach is demonstrated, where 1 vol% CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering. The CMAS-glass phase at the grain boundaries promotes rapid CMAS-glass penetration, thereby eliminating the dilatation-gradient.
Effect of texture on oxidation resistance of Ti3AlC2 J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-09 Ludi Xu, Degui Zhu, Yunlong Liu, Tohru S. Suzuki, Byung-nam Kim, Yoshio Sakka, Salvatore Grasso, Chunfeng Hu
Oxidation resistance of textured Ti3AlC2 ceramics was measured in the temperature range 1273-1573 K. It was found that the oxidation was markedly anisotropic and the samples exhibited a better oxidation resistance when tested along a direction transverse to the c-axis. This behavior was attributed to the rapid diffusion of Al within its basal planes to form a passivating Al2O3 scale and it respected Ellingham diagrams. The scales formed had different compositions depending on the testing direction; this response was clearly resulting from the crystallographic orientation. Even at 1473 K after 20 h exposure, the samples tested in a direction transverse to the c axis showed a reduced weight gain which was 45 times lower than one seen on a basal plane.
Electric field induced phase transition and accompanying giant poling strain in lead-free NaNbO3-BaZrO3 ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-08 Mengxian Dou, Jian Fu, Ruzhong Zuo
A series of phase transitions in (1-x)NaNbO3-xBaZrO3 ((1-x)NN-xBZ) ceramics was observed from antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase and finally into ferroelectric rhombohedral phase with increasing x. An electric field induced irreversible phase transition was found in different compositions, irrespective of their virgin phase structures. Particularly, an antiferroelectric orthorhombic phase is irreversibly transformed into a ferroelectric monoclinic phase within 0.02 ≤ x ≤ 0.05, leading to a giant poling strain of ~0.58%. This is much larger than that observed in ferroelectric orthorhombic (0.06 ≤ x ≤ 0.07) and rhombohedral phases (0.08 ≤ x ≤ 0.11) suffering from an irreversible ferroelectric-ferroelectric (monoclinic) phase transition. The synchrotron x-ray diffraction and the measurement of longitudinal and transverse strains suggest that this irreversible phase transition should involve not only a distinct volume expansion, but also an obvious lattice elongation. The present study demonstrates a unique nature of the composition and field dependent phase stability and an underlying mechanism of giant poling strains in NN-BZ ceramics.
Enhanced thermal shock resistance of low-carbon Al2O3-C refractories with direct CVD synthesis of nano carbon decorated oxides J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-08 Yawei Li, Jiangbo Shan, Ning Liao, Shaobai Sang, Dechang Jia
Novel low carbon Al2O3-C refractories were prepared through adopting chemical vapour deposition (CVD) synthesized nano carbon decorated Al2O3 powder. The phase compositions, microstructures, mechanical properties and thermal shock resistance of Al2O3-C refractories were characterized and evaluated. The results show that the morphologies of nano carbon composites are mainly dominated by the concentration of catalyst. Specifically, the growth of MWCNTs is preferred with a Ni2+ concentration at 0.1 mol/L, while higher concentrations e.g. 0.3 mol/L would stimulate the formation of nano-onion like carbon. With the introduction of nano carbon decorated Al2O3 additives, the residual strength after thermal shock can reach 12.4 MPa, which is much higher than the 2 wt% nano carbon black containing specimens (6.4 MPa). The enhanced thermal shock resistance should be attributed to that the nano onion-like carbon reduces the cohesion between the matrix and the Al2O3 particles and decreases the thermal expansion coefficient.
Large and Reversible In-situ Up-conversion Photoluminescence Modulation based on Photochromism via Electric-field and Thermal Stimulus in Ferroelectrics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-07 Yuanyuan Zhang, Laihui Luo, Kaixuan Li, Weiping Li, Yafei Hou
A large and reversible up-conversion (UC) photoluminescence modulation via both thermal and electric-field stimulus based on the photochromic reactions has been developed in a novel material: Er3+/Yb3+ co-doped K0.5Na0.5NbO3 ferroelectrics. After visible light irradiation, the ceramics exhibit a significant photochromism phenomenon turning from pale yellow to gray. The gray color can be eliminated via heat treatment or partly removed by an electric field stimulation. Upon 390-nm light irradiation for 4 min and thermal treatment at 200 °C for 5 min, the ceramics show a large modulation ratio (96%). In addition, the UC photoluminescence modulation can be in-situ operated by alternating the light irradiation and electric-field, showing an enhancing factor of 120% for the 390-nm light irradiated ceramics via applying an electric-field stimulation (12.5 kV/cm, 1 min). The developed ferroelectrics with large and reversible photoluminescence modulation through thermal stimulus and electric-field have great potential for digital memory and optoelectronic device applications.
Large Energy Storage Density, Low Energy Loss and highly stable (Pb0.97La0.02)(Zr0.66Sn0.23Ti0.11)O3 Antiferroelectric Thin-Film Capacitors J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-07 Zhengjie Lin, Ying Chen, Zhen Liu, Genshui Wang, Denis Rémiens, Xianlin Dong
In this work, high performance (Pb0.97La0.02)(Zr0.66Sn0.23Ti0.11)O3 polycrystalline antiferroelectric thin-film was successfully fabricated on (La0.7Sr0.3)MnO3/Al2O3(0001) substrate via a cost-effectively chemical solution method. A large recoverable energy storage density (Wre) of 46.3 J/cm3 and high efficiency (η) of 84 % were realized simultaneously under an electric field of 4 MV/cm by taking full advantage of the linear dielectric response after the electric field induced antiferroelectric-ferroelectric transition. Moreover, the PLZST thin-film displayed high temperature stability. With increasing temperature from 300 K to 380 K, the Wre decreased only 1.3 %. The film also exhibited good fatigue endurance up to 1 × 105 cycling under an electric field of 2.2 MV/cm. Our work underlines the importance of the interface quality between the film and the substrate and the important role of linear dielectric answer after saturation in the improvement of the energy storage density and efficiency of antiferroelectric materials.
Fabrication and Microstructural Characterization of the Novel Optical Ceramic consisting of α-Al2O3@Amorphous Alumina Nanocomposite Core/Shell Structure J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-05 A. Eftekhari, B. Movahedi, G. Dini, M. Milani
In this study, α-Al2O3@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl3 and the commercial α-Al2O3 nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al2O3 phases after the calcination process and the shell material was completely converted to γ-Al2O3 at 1000°C. However, for the α-Al2O3@amorphous alumina core-shell nanoparticles were completely converted to α-Al2O3 at 1000°C. It can be concluded that α-Al2O3 core particles, as the seed crystalline, help to transforming of γ-Al2O3 phase as the shell material directly without forming transitional phases to α-Al2O3. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al2O3@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ~99.8% and the in-line transmittance value is ~80% within the IR range.
MgO as a non-pyrolyzable pore former in porous membrane supports J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-05 A.B. Haugen, A. Geffroy, A. Kaiser, V. Gil
Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-05 Sonia De Almeida-Didry, Meledje Martin Nomel, Cécile Autret, Christophe Honstettre, Anthony Lucas, François Pacreau, François Gervais
Grain boundaries of CaCu3Ti4O12 (CCTO) materials have been shown to play leading role in colossal permittivity. Core-shell design is an attractive approach to make colossal dielectric capacitors by controlling the grain boundaries. Core-shell grains of CCTO surrounded by Al2O3 shell were synthesized by ultrasonic sol-gel reaction from alumina alkoxide precursor. The influence of alumina shell by comparison with bare CCTO grains was studied. Particularly, microstructure, dielectric and electric effects on sintered ceramics are reported. The average grain size and the density are increased compared to undoped CCTO leading to an improvement of permittivity from 58000 to 81000 at 1 kHz. Furthermore a decrease of dielectric loss is found in a frequency range of 102-103 Hz. Moreover, the activation energy of grain boundaries is increased from 0.55 to 0.73 eV and the electrical properties such as breakdown voltage, non-linear coefficient and resistivity are improved with the aim of making industrial capacitors.
Characteristics of porous mullite developed from clay and AlF3·3H2O J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-03 Mohammed Rashad, M. Balasubramanian
Porous mullite with interlocked needle shape microstructure was developed from China clay and aluminium fluoride trihydrate (AlF3·3H2O). The effects of various parameters like sintering temperature, sintering time and the amount of AlF3·3H2O on the phase evolution, microstructure and porosity have been studied. Quantitative analysis of mullite was carried out using X-ray diffraction combined with Rietveld-RIR method (Internal standard method). Porous mullite ceramics with 62 % open porosity have been prepared at a relatively lower temperature of 1400 °C. The results show that the nucleation of mullite can be achieved from 700 °C onwards using hydrated aluminium fluoride without the formation of intermediate topaz crystals. The high amount of water vapour produced within the system during the in-situ reaction has a crucial role in deciding the reaction mechanism.
Friction and wear behaviour of silicon carbide/graphene composites under isooctane lubrication J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-03 Javier Llorente, Manuel Belmonte
The tribological performance of silicon carbide (SiC)/graphene nanoplatelets (GNPs) composites is analysed under oscillating sliding tests lubricated with isooctane, looking to explore their potential as components for gasoline direct injection (GDI) engines. High graphene filler contents (20 vol.% of GNPs) are required to substantially reduce the friction coefficient of SiC ceramics, attaining decreases on friction up to 30% independently of the applied load. For all materials and testing conditions a mild wear regime is evidenced. SiC/20 vol.% GNPs composite also enhances the wear resistance up to 35% at low load, but the addition of GNPs produces a deleterious effect as the load augments. The tribological behaviour depends on the formation and destabilization of a solid lubricant carbon-based tribofilm and strongly correlates with the mechanical properties of the tested materials.
Fabrication and mechanical properties of h-BN based composites containing dual glass phases J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Juanjuan Chen, Jixin Chen, Xi Zhang, Minmin Hu, Meishuan Li
In this work, h-BN based composites containing amorphous silica and ytterbium silicate glass phases were successfully fabricated by in situ hot pressing process. The powder mixtures of h-BN, Yb2O3, SiO2 and sintering additive (Al2O3) were hot pressed at 1880 °C for 1 h under 30 MPa in Ar atmosphere. In the composites, β-Yb2Si2O7 phase produced from the reaction between Yb2O3 and SiO2 disappears completely and transforms to Yb-Si-Al-O glass when the amount of Al2O3 is ≥1.5 wt.%. The Vickers hardness, flexural strength, and compressive strength reached the maximum values of 2.38±0.08 GPa, 337±22 MPa and 950±34 MPa, respectively, as 1.5 wt.% Al2O3 was added. The strengthening effects were attributed to the fine spherical microstructure of Yb-Si-Al-O glass particles, strong [AlO4] coordination state, and residual compressive stress in glass phases.
Thermodynamic Modeling of the K2O-Al2O3 and K2O-MgO-Al2O3 Systems with Emphasis on β- and βʹʹ-Aluminas J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Dong-Geun Kim, Elmira Moosavi-Khoonsari, In-Ho Jung
A critical evaluation and thermodynamic modeling study including key phase diagram experiments was performed to investigate the K2O-Al2O3 and K2O-MgO-Al2O3 systems. For the first time, potassium β- and βʹʹ-alumina solid solutions were described using the Compound Energy Formalism with accurate cation distributions in their sublattices. From the new experimental results, the stability of potassium βʹʹ-alumina was assured up to 1600 °C. A large discrepancy reported in the literature, the eutectic temperature between KAlO2 and β-alumina in the K2O-Al2O3 system, was resolved. A set of self-consistent Gibbs energy functions for all stable phases in the K2O-MgO-Al2O3 system was obtained. As a result, any phase diagram sections and thermodynamic properties of the K2O-MgO-Al2O3 system can be calculated from the optimized Gibbs energy functions. In particular, the cation distribution in the β- and βʹʹ-alumina solid solutions is calculated depending on the non-stoichiometry of solution and temperature.
Copper-based electrodes for IT-SOFC J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Francesca Zurlo, Alessandro Iannaci, Vincenzo M. Sglavo, Elisabetta Di Bartolomeo
Copper and gadolinium doped ceria (GDC) anode supported fuel cells were co-sintered at relatively low temperature (900 °C) and successfully tested in the intermediate temperature (IT) range. The GDC electrolyte densification was promoted by a compressive strain induced by increasing the anodic thickness and was evaluated by SEM investigation. Instead of more commonly used La0.8Sr0.2Fe0.6Co0.4O3-δ, strontium and copper-doped lanthanum ferrite La0.8Sr0.2Fe0.8Cu0.2O3-δ (LSFCu) mixed with 30 wt% GDC (LSFCu-GDC) was employed as cathodic material. Preliminary tests on Cu-GDC/GDC/LSFCu-GDC single cells showed promising results at temperature as low as 650 °C using hydrogen as fuel.
Crack propagation speed in ceramic during quenching J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Yingfeng Shao, Boyang Liu, Xiaohuan Wang, Long Li, Jiachen Wei, Fan Song
The effects of water quenching temperature and specimen size on the propagation speed of thermal shock crack are investigated in real time by water quenching of translucent ceramic and high-speed imaging. The results show that the crack growth rate increases with the increase of quenching temperature difference or specimen size. Within 100 ms, average crack speed is 20.3 mm/s at a temperature difference of 400 °C in 20 mm wide ceramic and is 11.9 mm/s at a temperature difference of 220 °C in 5 mm wide ceramic, respectively. Compare with specimen size, the influence of quenching temperature difference on the crack propagation speed is larger. The calculations based on meso-damage mechanics have similar results to those of experiments. This paper quantitatively studies the thermal-shock crack growth of ceramic in real time and expands the scientific understanding of thermal shock cracking phenomenon of ceramic.
Fabrication of dense nano-laminated tungsten carbide materials doped with Cr3C2/VC through two-step sintering J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Jialin Sun, Jun Zhao, Xiuying Ni, Feng Gong, Zuoli Li
This work investigates the critical roles of two-step sintering (TSS) and laminated structure on the sintering behavior and mechanical properties of functionally graded WC-TiC-Al2O3 nanostructured composite materials doped with Cr3C2/VC. Results show that excellent mechanical properties are achieved for tailored TSS conditions with a hardness of 27.91±2.3 GPa and a flexural strength of 1423.3±23.5 MPa. The desirable mechanical properties are attributed to the suppressed grain growth without densification deterioration. TSS is more effective in facilitating the favorable dispersion of secondary phase toughening nano-particulates in a WC matrix than conventional sintering (CS). Cr3C2/VC dopant plays an important role in maximizing and shifting the temperature range of the kinetic window for WC-Al2O3 composites. Al2O3 crack deflection, transgranular Al2O3, microcracking, WC crack bridging and plate-like WC crack deflection are the major toughening mechanisms. Residual surface compressive stress induced by the graded structure is also an appreciated contribution to the improvement of mechanical properties.
Evaluation of the pore morphology formation of the Freeze Foaming process by in situ computed tomography J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Matthias Ahlhelm, David Werner, Johanna Maier, Johannes Abel, Thomas Behnisch, Tassilo Moritz, Alexander Michaelis, Maik Gude
The so-called Freeze Foaming method aims at manufacturing ceramic cellular scaffolds for diverse applications. One application is dedicated to potential bone replacement material featuring open, micro and interconnected porosity. However, the main challenges of this foaming method is to achieve a homogeneous pore morphology. In a current project, the authors throw light on the bubble/pore and strut formation of this process by in situ computed tomography. This allows for evaluating varying process parameter’s effects on the growth of the ceramic foam during the foaming process. As first result and basis for CT analysis, a stable and reproducible model suspension was developed which resulted in reproducible foam structures. In dependence of selected process parameters like pressure reduction rate or air content in the ceramic suspension resulting Freeze Foams became adjustable with regard to their pore morphology. Pore size and distribution data as well as the porosity were characterized and evaluated accordingly.
Hard and easy sinterable B4C-TiB2-based composites doped with WC J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Simone Failla, Cesare Melandri, Luca Zoli, Guido Zucca, Diletta Sciti
B4C-TiB2 composites were contaminated with WC to study the effect on densification, microstructure and properties. WC was introduced through a mild or a high energy milling with WC-6wt%Co spheres or directly as sintering aid to 50vol% B4C/50vol%TiB2 mixtures. High energy milling was very effective in improving the densification thanks to the synergistic action of WC impurities, acting as sintering aid, and size reduction of the starting TiB2-B4C powders. As a result, the sintering temperature necessary for full densification decreased to 1860 °C and both strength and hardness benefited from the microstructure refinement, 860±40 MPa and 28.5±1.4 GPa respectively. High energy milling was then adopted for producing 75vol% B4C/25 vol% TiB2 and 25 vol% B4C/ 75vol%TiB2 mixtures. The B4C-rich composition showed the highest hardness, 32.2±1.8 GPa, whilst the TiB2-rich composition showed the highest value of toughness, 5.1±0.1 MPa m0.5.
Hypervariate Constitutive Modeling illustrated via Aleatory Uncertainty in a Foundation Model J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Rebecca Brannon, Katharin Jensen, Debasish Nayak
Even if a ceramic's homogenized properties (such as anisotropically evolving stiffness) truly can be predicted from complete knowledge of sub-continuum morphology (e.g., locations, sizes, shapes, orientations, and roughness of trillions of crystals, dislocations, impurities, pores, inclusions, and/or cracks), the necessary calculations are untenably hypervariate. Non-productive (almost derailing) debates over shortcomings of various first-principles ceramics theories are avoided in this work by discussing numerical coarsening in the context of a pedagogically appealing buckling foundation model that requires only sophomore-level understanding of springs, buckling hinges, dashpots, etc. Bypassing pre-requisites in constitutive modeling, this work aims to help students to understand the difference between damage and plasticity while also gaining experience in Monte-Carlo numerical optimization via scale-bridging that reduces memory and processor burden by orders of magnitude while accurately preserving aleatory (finite-sampling) perturbations that are crucial to accurately predict bifurcations, such as ceramic fragmentation.
Mechanical properties of the solid electrolyte Al-substituted Li7La3Zr2O12 (LLZO) by utilizing micro-pillar indentation splitting test J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 An-Ni Wang, Juliane Franciele Nonemacher, Gang Yan, Martin Finsterbusch, Jürgen Malzbender, Manja Krüger
Garnet structured Al-substituted Li7La3Zr2O12 (Al:LLZO) is a promising candidate as electrolyte in all-solid-state Li-ion batteries due to its chemical stability against Li-metal and high voltage cathode materials. In order to ensure long-term stable operation, electrolyte crack growth induced and/or the volume change of the active material on the cathode side needs to be avoided, requiring in particular knowledge of local and global mechanical properties of the electrolyte material. Micro-pillar splitting test was used for the first time on this material to determine the microscopic fracture toughness of single grains and compare it with conventional Vickers indentation fracture toughness (VIF), which represents macroscopic fracture toughness. Both methods yielded comparative results. In conclusion, the micro-pillar splitting test can be used as an advanced locally resolved characterization method that can open up new experimental directions for characterizing and understanding battery materials and enable a targeted approach for material improvements.
Improved thermal stability of ferro/piezo-electric properties of Mn-doped Pb(In1/2Nb1/2)O3-PbTiO3 ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Huimin Qiao, Chao He, Zujian Wang, Xiuzhi Li, Ying Liu, Xifa Long
Thermal stability of piezo-/ferro-electric properties of ferroelectrics is important for the devices working at elevated temperature. A study on thermal stability of ferroelectrics will be greatly helpful for future applications. In this work, thermal behaviors of electrical properties were studied in Mn-doped Pb(In1/2Nb1/2)O3-PbTiO3 (PINT) ceramics. The ferroelectric hysteresis loops of Mn-doped samples change anomalously with increasing temperature compared with the virgin sample. Remnant polarization of PINT ceramics with high manganese content (x ≥ 0.04) exhibits increase trend as temperature increasing, leading to a negative electrocaloric effect which was reported to be beneficial to improving the cooling efficiency. For Mn-doped PINT ceramics, the reduction rate of coercive field reaches a relatively low value of 3%, indicating outstanding ability of depolarization resistance. PINT ceramics with proper dopant concentration show improved aging resistance and thermal stability of piezoelectric property after annealed. Out-of-plane domain configurations show different features at room temperature and elevated temperatures.
The structural and mechanical characterization of TiC and TiC/Ti thin films grown by DC magnetron sputtering J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-03-02 Zsolt Fogarassy, Nikolett Oláh, Ildikó Cora, Zsolt Endre Horváth, Tamás Csanádi, Attila Sulyok, Katalin Balázsi
The formation of TiC and Ti phases and their influence on their mechanical properties was studied in this work. Thin layers were deposited by DC magnetron sputtering at room temperature in ultrahigh vacuum from Ti and C targets.Cubic TiC phase (c-TiC) was formed from 58 to 86 at.% Ti content. First formation of hexagonal Ti (h-Ti) occurred from 86 at.% Ti content. The c-TiC disappears from 90 at.% Ti content. Films with 86 at.% Ti content the c-TiC structure can transform to h-Ti by sequential stacking faults. Dominance of c-TiC(111) texture with increasing Ti content was observed.The hardness of thin films agree with structural observations. The highest hardness value (∼26 GPa) showed the c-TiC thin film with 67 at% Ti content. The nanohardness values showed decreasing character with increasing Ti content over 70 at.%. The lowest values of nanohardness (∼10 GPa) was observed for thin films with only h-Ti phase.
Porous γ-(Y1-xHox)2Si2O7 thermal insulator with excellent high-temperature strength retention and very low thermal conductivity J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-20 Zhen Wu, Wanpeng Hu, Yixiu Luo, Luchao Sun, Jingyang Wang
High-temperature thermal insulation materials challenge extensive candidates with good mechanical, thermal and chemical reliability at high temperatures. Recently, porous γ-Y2Si2O7 was indicated a promising thermal insulator in harsh environment; however, its strength at 1300 °C reduced to 34% of that at room temperature. In this work, we significantly improved its high-temperature strength by doping Ho. Highly porous γ-(Y1-xHox)2Si2O7 solid solution was fabricated by in-situ foam-gelcasting method. Especially, porous γ-(Y2/3Ho1/3)2Si2O7 demonstrated the optimal high-temperature strength, for instance 65% retention at 1300 °C, as well as high compressive strength (13.9 MPa) and low thermal conductivity (0.186 W/(m K)) at room temperature, at the porosity of 79.3%. Interestingly, porous solid solution sample displayed obviously lower thermal conductivity than the two end pure-phase porous materials. Porous γ-(Y1-xHox)2Si2O7 solid solution is clearly highlighted as a promising high-temperature thermal insulator with outstanding high-temperature strength retention and optimal low thermal conductivity.
Tailored dielectric tunability of alkali niobate-based antiferroelectric/relaxor-ferroelectric composites J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-18 Zhiyong Liu, Huiqing Fan, Jinshan Lu, Yuqing Mao, Yang Zhao
High dielectric tunability, low loss and an appropriate level of dielectric permittivity are basic requirements of ferroelectric materials for tunable microwave devices. In this study, 0.96NaNbO3-0.04CaZrO3/0.88(K0.5Na0.5)NbO3-0.12SrZrO3, antiferroelectric/relaxor-ferroelectric composites were designed to tailor dielectric tunability. By tailoring the microstructure of the composites, a high dielectric tunability of 51.78% with a low loss of 0.015 was achieved at the composition ratio of 15/85. The nonlinear behaviours of the composites were explored by Johnson model; with increasing antiferroelectric phase, the contribution of the polarization to the free energy was increased between the antiferroelectric and relaxor-ferroelectric. Furthermore, the high resistance layer at the grain boundary region greatly inhibited the long-term migration of electrons and defective ions (mainly oxygen vacancies) in the composites. Therefore, the dielectric loss was remarkably decreased, and the excellent tunability was still preserved in the composite ceramics.
Phase stability and thermo-physical properties of ZrO2-CeO2-TiO2 ceramics for thermal barrier coatings J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-16 Jinshuang Wang, Junbin Sun, Qiangshan Jing, Bing Liu, Hao Zhang, Yu Yongsheng, Jieyan Yuan, Shujuan Dong, Xin Zhou, Xueqiang Cao
The properties of ZrO2 co-stabilized by CeO2 and TiO2 ceramic bulks were investigated for potential thermal barrier coating (TBC) applications. Results showed that the (Ce0.15Tix)Zr0.85-xO7 (x=0.05, 0.10, 0.15) compositions with single tetragonal phase were more stable than the traditional 8YSZ at 1573 K. These compositions also showed a large thermal expansion coefficient (TEC) and a high fracture toughness, which were comparable to those of YSZ. However, the phase stability, fracture toughness and sintering resistance of the CeO2-TiO2-ZrO2 system showed a decline tendency with the increase of TiO2 content. The TEC of the ceramic bulks decreased with increase of TiO2 content as well because the crystal energy was enhanced with increasing substitution of Zr4+ by smaller Ti4+. The (Ce0.15Ti0.05)Zr0.8O2 had the best comprehensive properties among the (Ce0.15Tix)Zr0.85-xO2 compositions as well as a low thermal conductivity. Therefore, it can be explored as a TBC candidate material for high-temperature applications.
Thermal Cycling Performance of La2Ce2O7/ 50 vol. % YSZ Composite Thermal Barrier Coating with CMAS Corrosion J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-16 Y.X. Kang, Y. Bai, W. Fan, T. Yuan, Y. Gao, C.G. Bao, B.Q. Li
La2Ce2O7 (LC) is receiving increasing attention due to its lower thermal conductivity, better phase stability and higher sintering resistance than yttria partially stabilized zirconia (YSZ). However, the low fracture toughness and the sudden drop of CTE at approximately 350 °C greatly limit its application. In this study, the LC/50 vol. % YSZ composite TBC was deposited by supersonic atmospheric plasma spraying (SAPS). Compared to YSZ or double layered LC/YSZ coating, the thermal cycling life of LC/50 vol. % YSZ coating with CMAS attack increased by 93 % or 91 %. The latter possessed higher fracture toughness (1.48 ± 0.26 MPa·m1/2) than LC (0.72 ± 0.15 MPa·m1/2) and better CMAS corrosion resistance than YSZ owing to the formation of Ca2(LaxCe1-x)8(SiO4)6O6-4x with <001> orientation perpendicular to the coating surface. The sudden CTE decrease of LC was fully suppressed in LC/50 vol. % YSZ coating due to the change of temperature dependent residual stresses induced by YSZ.
Quantifying Particle Segregation in Sequential Layers fabricated by Additive Manufacturing J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-16 Chang-Jun Bae, Arathi Ramachandran, John W. Halloran
Suspensions are used in Ceramic Stereolithography1(CerSLA) and other material processing methods. We applied Fast Fourier Transforms2 (FFTs) in a novel way to signals generated from ceramic processing methods to address flaws resulting from sedimentation. We compared our analysis with traditional measures of sedimentation to show that this novel quantification of sedimentation is able to distinguish finer, subtler patterns of sedimentation. The explanation and application of FFTs and signal processing provided in this article enables the reader to use FFTs and signal processing in a broader way to further optimise and quantify materials processing methods.
Synthesis of ultrafine nano-polycrystalline cubic boron nitride by direct transformation under ultrahigh pressure J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-15 Yoshio Ichida, Hiroaki Ohfuji, Tetsuo Irifune, Takehiro kunimoto, Yohei Kojima, Toru Shinmei
Using a turbostratic pyrolytic boron nitride as a starting material, we synthesized a variety of ultrahard polycrystalline cubic boron nitride (PcBN) as a function of the heating duration changing from 1 to 60 min under a constant temperature and pressure conditions (1950 °C and 25 GPa) using a multi-anvil apparatus. When the heating duration was less than 13 min, ultrafine nano-polycrystalline cBN (U-NPcBN) with the mean grain size of < 50 nm was produced. Among these U-NPcBNs those synthesized with 11–13 min were found to have a uniform texture composed purely of cBN (i.e. with no wurzite BN residue) and a Knoop hardness of > 53 GPa, which is 20 % higher than that of the hardest conventional binderless PcBN in practical use. Furthermore, the PcBNs synthesized with 18–20 min showed a unique nanocrystalline texture composed of relatively coarse grains dispersed in a fine grained matrix and even higher Knoop hardness (54.5–55.2 GPa).
Residual stress measurements in melt infiltrated SiC/SiC ceramic matrix composites using Raman spectroscopy J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-14 Kaitlin Kollins, Craig Przybyla, Maher S. Amer
Raman spectroscopy was utilized to characterize the chemical composition and residual stresses formed in melt infiltrated SiC/SiC CMCs during processing. Stresses in SiC fibers, in SiC chemical vapor (CVI) infiltrated matrix, in SiC melt infiltrated matrix, and in free silicon were measured for two different plates of CMCs. Stresses in the free silicon averaged around 2 GPa in compression, while stresses in the matrix SiC were 1.45 GPa in tension. The SiC CVI phase had stresses ranging between 0.9 GPa and 1.2 GPa in tension and the SiC fibers experienced stresses of. 05 to 0.7 GPa in tension. These results were validated with the proposed model of the system. While the mismatch in the coefficients of thermal expansion between the constituents contributes to the overall residual stress state, the silicon expansion upon solidification was found to be the major contributor to residual stresses within the composite.
Tailoring cathode composite boosts the performance of proton-conducting SOFCs fabricated by a one-step co-firing method J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-14 Hailu Dai, Eman Husni Da'as, Shahid P. Shafi, Huiqiang Wang, Lei Bi
A strategy of tailoring the ceramic cathode composite is presented to improve the performance of proton-conducting solid oxide fuel cells (SOFCs) prepared by a one-step co-firing process. Comparing to the conventional way of using BaCe0.7Zr0.1Y0.2O3-δ (BCZY) in the composite cathode for BCZY-electrolyte based cells, the replacement of BCZY by BaZr0.8Y0.2O3-δ (BZY) mitigates the reaction between the two ceramic phases in the composite cathode during the co-firing process and also keeps the cathode with sufficient porosity for ample gas diffusion which could assist in adequate cathode reactions. As a result, the BCZY-electrolyte based cell with Sm0.5Sr0.5CoO3-δ (SSC)-BZY composite cathode shows a power output of 300 mW cm-2 at 600 °C, which is the largest ever reported for proton-conducting SOFCs prepared by a one-step co-firing process. The strategy of tailoring the composite cathode offers both small ohmic resistance and polarization resistance, providing a promising way to develop single-step co-fired proton-conducting SOFCs.
Comparison of the Proton Irradiation-Induced Damage in SiCf/SiC with Sc-nitrate and Al2O3-Y2O3 Sintering Additives J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-14 Amit Siddharth Sharma, Pipit Fitriani, Bong-Ki Min, Dong-Hyok Yoon
We report microstructural evaluation in proton-irradiated SiCf/SiC up to a fluence level of 1018 p+/m2 (E = 20 MeV) performed at the Korea Multi-purpose Accelerator Complex (KOMAC). To fabricate the SiCf/SiC, a SiC-based matrix with two different sintering additives, Sc-nitrate and Al2O3-Y2O3, were infiltrated into the Tyranno® SiC preform by electrophoretic deposition and subsequent hot pressing. Scanning (SEM) and high resolution (HREM) electron microscopy was used to probe crack formation, pore generation and surface amorphization upon irradiation. SiC matrix was comparatively more crack-resistant in the SiCf/SiC with Sc-nitrate than that with Al2O3-Y2O3. Subsurface porosity evolved in the form of continuous bands for Al2O3-Y2O3 and discrete pockets for Sc-nitrate additives. Selective leaching of Si from the grain surface leads to the formation of a graded structure and surface amorphization. Irradiation-induced roughness on the fibers facilitates easy debonding at the SiCf-PyCcoating interface but no significant changes in the flexural behavior were observed.
Rapid fabrication and phase transition of Nd and Ce co-doped Gd2Zr2O7 ceramics by SPS J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-14 Lan Wang, Xiaoyan Shu, Facheng Yi, Dadong Shao, Kuibao Zhang, Haibin Zhang, Lu Xirui
A series of Nd and Ce co-doped Gd2-xNdxZr2-yCeyO7 (0.0 ≤ x, y ≤ 2.0) ceramics were rapidly fabricated through spark plasma sintering (SPS) within 3 min. The effects of Nd and Ce contents on the phase composition, lattice parameter, active modes, microtopography and microstructure have been investigated in detail. XRD studies reveal that the compositions corresponding to 0.0 ≤ y ≤ 1.0 show a single phase and beyond 1.0 exhibit multiphase. The lattice parameters increase with elevated Nd and Ce content. The grains are densely packed on each other with cube-like shape, and the elements are almost homogeneously distributed in the compound. This synthetic method provides a simple pathway for the preparation of highly densified single phase ceramic at 1600-1700 ℃ for 3 min under pressure of 80 MPa.
Microstructures and mechanical properties of B4C-TiB2-SiC composites fabricated by ball milling and hot pressing J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-13 Qianglong He, Aiyang Wang, Chun Liu, Weimin Wang, Hao Wang, Zhengyi Fu
B4C-TiB2-SiC composites were fabricated via hot pressing using ball milled B4C, TiB2, and SiC powder mixtures as the starting materials. The impact of ball milling on the densification behaviors, mechanical properties, and microstructures of the ceramic composites were investigated. The results showed that the refinement of the powder mixtures and the removal of the oxide impurities played an important role in the improvement of densification and properties. Moreover, the formation of the liquid phases during the sintering was deemed beneficial for densification. The typical values of relative density, hardness, bending strength, and fracture toughness of the composites reached 99.20%, 32.84 GPa, 858 MPa and 8.21 MPa·m1/2, respectively. Crack deflection, crack bridging, crack branching, and microcracking were considered to be the potential toughening mechanisms in the composites. Furthermore, numerous nano-sized intergranular/intragranular phases and twin structures were observed in the B4C-TiB2-SiC composite.
Combustion synthesis of high-temperature ZrB2-SiC ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-12 I.V. Iatsyuk, Yu.S. Pogozhev, E.A. Levashov, A.V. Novikov, N.A. Kochetov, D.Yu. Kovalev
The work is dedicated to researching into combustion kinetics and mechanism as well as the stages of the chemical transformations during self-propagating high-temperature synthesis of ZrB2-SiC based ceramics. Dependences of the combustion temperature and rate on the initial temperature (T0) have been studied. It has been shown that the stages of the chemical reactions of ZrB2 diboride and SiC carbide formation do not change within the range of T0 = 298–700 К. The effective activation energy of the combustion process amounted to 170–270 kJ/mol, from which it has been concluded that chemical interaction through the melt plays a leading role. The stages of the chemical transformations in the combustion wave have been studied by dynamic X-ray diffraction. First, ZrB2 phase forms from Zr-Si melt saturated with boron, and SiC phase is registered later. The SHS method has successfully been used in order to obtain ZrB2-SiC composite powders and compact ceramics with a silicon carbide content of 25-75 %. The ceramics are characterized by a residual porosity of 1.5 %, hardness up to 25 GPa, the elastic modulus of 318 ± 21 GPa, elastic recovery of 36 % and thermal conductivity of 54.9 W/(m×K) at Troom.
Improved thermal shock resistance of SiCnw/PyC core-shell structure-toughened CVD-SiC coating J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-12 Lei Zhuang, Qian-Gang Fu, Xin Yu
In-situ SiC nanowire (SiCnw)/pyrolytic carbon (PyC) core-shell structures were introduced to improve the thermal shock and mechanical properties of chemical vapor deposition (CVD)-SiC coating on carbon/carbon (C/C) composites. The microstructure, phase composition, thermal shock resistance and mechanical properties of the CVD-SiC coating toughened by SiCnw/PyC core-shell structures were studied. The results show that the introduction of SiCnw/PyC core-shell structures can effectively alleviate the mismatch of coefficient of thermal expansion (CTE) between SiC coating and C/C substrate, thus enhancing the thermal shock resistance of the coating. Furthermore, the increased numbers of interfaces in the SiC coating owing to the addition of core-shell structures are beneficial to the mechanical properties of the coating after thermal shock test.
Multiphase imaging of freezing particle suspensions by confocal microscopy J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-11 Dmytro Dedovets, Sylvain Deville
Ice-templating is a well-established processing route for porous ceramics. Because of the structure/properties relationships, it is essential to better understand and control the solidification microstructures. Ice-templating is based on the segregation and concentration of particles by growing ice crystals. What we understand so far of the process is based on either observations by optical or X-ray imaging techniques, or on the characterization of ice-templated materials. However, in situ observations at particle-scale are still missing. Here we show that confocal microscopy can provide multiphase imaging of ice growth and the segregation and organization of particles. We illustrate the benefits of our approach with the observation of particles and pore ice in the frozen structure, the dynamic evolution of the freeze front morphology, and the impact of PVA addition on the solidification microstructures. These results prove in particular the importance of controlling both the temperature gradient and the growth rate during ice-templating.
Micro extrusion of innovative alumina pastes based on aqueous solvent and eco-friendly binder J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-10 J. Bourret, I. El Younsi, M. Bienia, A. Smith, P.-M. Geffroy, J. Marie, Y. Ono, T. Chartier, V. Pateloup
The aim of this work is to develop the micro-extrusion process with an innovative approach linked to respect for the principles of green chemistry. Alumina pastes are prepared in an aqueous medium with psyllium as a natural binder, which is not derived from the petrochemical industry. The challenge is to obtain a concentrated system which is suitable for the micro-extrusion process and which ensures satisfactory structural and mechanical properties of the final parts. The manufacturing of alumina networks is achieved by using the micro-extrusion shaping process consisting of an XYZ displacement platform and a homemade extrusion head. The nozzle diameter is equal to 400µm. The rheological behavior of the paste is studied as well as its capability to be extruded. The green parts are consolidated by heat treatment at 1600°C. Their microstructural characteristics have been studied. Final density equal to 98% have been obtained.
Protecting the MoSi2 healing particles for Thermal Barrier Coatings using a sol-gel produced Al2O3 coating J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-08 A.L. Carabat, M.J. Meijerink, J.C. Brouwer, E.M. Kelder, J.R. van Ommen, S. van der Zwaag, W.G. Sloof
A successful sol-gel process to encapsulate molybdenum di-silicide MoSi2 particles with a closed and thermally stable Al2O3 layer using aluminium tri-sec-butoxide as a precursor is presented. The processing conditions such as precursor selection and temperature were optimized through analysing the interaction of the MoSi2 particles with the sol. The application of the sol-gel based coating was followed by calcining the coated particles at temperatures between 900 and 1200 °C in Ar. The shell composition and the mechanical stability of the microcapsule were analysed by means of X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. Upon calcining at 1200 °C in Ar, the MoSi2 core remains intact as it is, covered by an α - alumina shell with a thickness of about 0.6 µm. The stability tests proved that the encapsulate particles are about five times more oxidation resistant than the uncoated MoSi2 particles.
Escape from the strength-to-toughness paradox: bulk ceramics through dual composite architectures J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-08 Frédéric Monteverde, Cesare Melandri, Simone Failla, Ryan J. Grohsmeyer, Gregory E. Hilmas, William G. Fahrenholtz
This paper describes an approach to escape from the classic strength-toughness trade-off in bulk ceramics using the dual composite architectural concept. The key questions addressed by the research were: can dual composite architectures be fabricated? -how do dual composite architectures affect the physical properties of ceramics? -do dual composite architectures affect the mechanical behavior of composites? -do dual composite architectures have increased damage tolerance at elevated temperatures? These questions were answered positively. Reinforcing granules dispersed in loose powder mixtures were hot-pressed to obtain fully dense dual composite architectures with target overall composition. Defined sub-composites (labeled granule and matrix) of similar compositions, but differing engineered microstructures, were successfully retained. The brittle-to-ductile transition of MoSi2 above 1400 K enabled the simultaneous increase in both strength and toughness, and values of up to 440 MPa and 11.5 MPa√m, respectively, were experimentally determined at 1773 K.
A novel Cr-doped Al2O3-SiC-ZrC composite coating for ablative protection of C/C-ZrC-SiC composites J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-08 Zheng Peng, Wei Sun, Xiang Xiong, Yonglong Xu, Yuntian Chen
A novel Cr-doped Al2O3-SiC-ZrC coating systemis proposed to further improve the ablation resistance of C/C-ZrC-SiC composites. Our approach combines low pressure plasma spray method with slurry impregnation of Zr-Cr-Si-C to achieve a specially tailored oxide-carbide structure. Results show that the as-prepared coating was dense and crack-free, which effectively promoted the ablation resistance of C/C-ZrC-SiC with the mass and linear ablation rates decreased by 66% and 76% respectively, involving a highly dense protective structure of A1xCr2-xO3-SiO2-ZrO2 formed during ablation. The compact and continuous multi-oxide scale with “ZrO2-rosette” skeleton intricated with A11.96Cr0.04O3 was evidenced in the ablation center.
Yb3+ rare earth structural probe and correlation between morphology and spectroscopic properties in La2Mo2O9: Comparative analysis with mixed cubic La2MoWO9 translucent ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-07 M. Bieza, M. Guzik, E. Tomaszewicz, Y. Guyot, G. Boulon
We present and discuss the structural and spectroscopic properties of micro-crystalline Yb3+-doped La2Mo2O9 molybdates (0-25 mol% of Yb3+ ions) synthesized by the high-temperature solid-state reaction in which Yb3+ optical rare earth ions play the role of a structural probe. The main objective is to apply these materials to transparent optical ceramics. The XRD and SEM analysis reveal a strong structural dependence of the crystal structure on Yb3+ ion concentration. The high-resolution of the 2F7/2 ↔ 2F5/2 0-phonon line of Yb3+ ions in absorption and emission spectroscopy at low temperature allows us to evidence three types of polyhedral around Yb3+ ions. Evident differences for cubic Yb3+ -doped both La2Mo2O9 and mixed La2MoWO9 are observed as a result of the introduction of tungsten ions leading to greater disorder in the lattice. First translucent cubic ceramics have been successfully fabricated and the comparison turns out in favor of Yb3+-doped mixed La2MoWO9.
Sintering behavior and dielectric properties of SiO2–BPO4 glass-fluxed ceramics J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-07 J.J. Bian, Y.R. Xie
(1–x)SiO2–xBPO4 (x = 23–70 wt%) glass-fluxed ceramics have been prepared by a traditional ceramic process. The phase assemblage, sintering, crystallization behavior, microwave dielectric properties and chemical compatibility with Ag/Cu have been studied. The SiO2-rich compositions (x = 23–50 wt%) could be well densified at ~975 °C/2 h, while the BPO4-rich compositions demonstrated poor sinterability and porous microstructure. The SiO2-rich compositions sintered at 975 °C/2 h contained BPO4, low temperature cristobalite and glassy phases. Crystallization of BPO4 occurred at lower temperature than that of SiO2. Good combined microwave dielectric properties with Ɛr = ~5, Q×f = 25,000 GHz and τf value of –7.3 ppm/°C could be obtained when 10 wt% TiO2 was added after sintering at 975 °C/2 h. The x = 23 wt% composition chemically reacted with Ag, but exhibited good chemical compatibility with Cu after sintering at 975 °C/2 h.
Carrier concentration optimization for thermoelectric performance enhancement in n-type Bi2O2Se J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-07 Rui Liu, Jin-le Lan, Xing Tan, Yao-chun Liu, Guang-kun Ren, Chan Liu, Zhi-fang Zhou, Ce-wen Nan, Yuan-hua Lin
The Bi2O2Se-based compounds with an intrinsically low thermal conductivity and relatively high Seebeck coefficient are good candidates for thermoelectric application. However, the low electrical conductivity resulted from carrier concentration of only 1015 cm-3 for pristine material, which is too low for optimized thermoelectrics. As a result, the carrier concentration optimization of Bi2O2Se is important and useful to achieve higher power factor. In this work, the effect of Ge-doping at the Bi site has been investigated systematically, with expectations of carrier concentration optimization. It is found that Ge doping is an efficient method to increase carrier concentration. Due to the largely increased carrier concentration via Ge doping, the room temperature electrical conductivity rises rapidly from 0.03 S/cm in pristine sample to 133 S/cm in x = 0.08 sample. Combined with the intrinsically low thermal conductivity, a maximum ZT value of 0.30 has been achieved at 823 K for Bi1.92Ge0.08O2Se, which is the highest ZT value for Bi2O2Se-based thermoelectric materials.
Hierarchical micro-nanostructured albite-based glass-ceramic for high dielectric strength insulators J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-07 V. Fuertes, M.J. Cabrera, J. Seores, D. Muñoz, J.F. Fernández, E. Enríquez
A novel glass-ceramic material based on albite type Na-rich feldspar has been synthesized by conventional ceramic process. High crystallinity, >94% Vol.% is obtained by fast sintering which allows energy saving processing. Albite is the main crystalline phase and tetragonal SiO2 is a secondary phase. Electrical properties were examined by complex impedance, DC measurements, and dielectric breakdown test. Dielectric characterization shows a non-Debye type dielectric behavior with low dielectric constant, 4.6 at 1 MHz, low dielectric losses, (~10-3 at 1 MHz, and a large dielectric strength, ~60 kV/mm), that it is the largest value reported in ceramic insulators. Those dielectric properties are attained by the low glassy phase content in the samples and their unique micro-nanostructure. All these properties make this novel material a very promising candidate in the market of ceramic electrical insulator, highlighting for high-voltage applications.
Asymmetric temperature distribution during steady stage of flash sintering dense zirconia J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-07 Guangxin Liu, Dianguang Liu, Jinling Liu, Yan Gao, Yiguang Wang
Surface temperature of zirconia is measured during flash sintering using thermal imaging camera. The results reveal that in the steady stage the temperature along the longitude of electric field is uneven: the temperature at the end of gauge section close to the anode is much higher than that close to the cathode; and the highest temperature locates at the three-quarter of the length of gauge section from the cathode. We further demonstrate that the asymmetric temperature distribution is related to the flashing since temperature distribution in the specimen without flashing is symmetric. The possible mechanisms that resulted in the asymmetric distribution are discussed.
Microstructure and anisotropic mechanical properties of freeze dried SrTi0.75Fe0.25O3-δ for oxygen transport membrane substrates J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-06 R. Oliveira Silva, J. Malzbender, F. Schulze-Küppers, S. Baumann, M. Krüger, O. Guillon
Hierarchically porous SrTi0.75Fe0.25O3-δ specimens were produced through a freeze drying procedure, which yielded a channel-like porosity, reaching a value of 32%. Compressive testing was used to determine apparent elastic modulus and fracture stresses in the transverse (out-of-plane) and longitudinal (in-plane) direction, revealing a strong dependence onto pore orientation. The lower mechanical stability in the in-plane direction appears to be associated with bending mode of the pore walls, being a result of a lower resistance to crack initiation. Acoustic emissions recorded during compressive tests indicated continuous damage of the pore walls before complete failure of the specimen, which could be also confirmed by complementary in-situ compressive tests in a scanning electronic microscope.
Transparent Sr0.84Lu0.16F2.16: Yb3+, Er3+ glass ceramics: elaboration, structure, up-conversion properties and applications J. Eur. Ceram. Soc. (IF 3.411) Pub Date : 2018-02-06 Jiangkun Cao, Dengke Xu, Fangfang Hu, Xiaoman Li, Weiping Chen, LiPing Chen, Hai Guo
The development of optical temperature sensors is of fundamental and industrial importance for various applications. Despite the great advance in optical temperature-sensing techniques, challenges remain to search for novel sensing materials with low cost, easy fabrication and high sensitivity. Here, transparent glass ceramics (GC) embedded with cubic Sr0.84Lu0.16F2.16:Yb3+/Er3+ nano-crystals were prepared via thermal annealing on the parent glass. The optical and structural properties were investigated. The enhanced emission intensity, obvious Stark splitting and prolonged lifetimes of Er3+ confirm the enrichment of Er3+ ions into formed Sr0.84Lu0.16F2.16 nano-crystals. The temperature sensing performance of Yb3+/Er3+ ions in Sr0.84Lu0.16F2.16GC were investigated based on up-conversion intensity ratio (FIR) from thermally coupled emitting states of Er3+. High energy difference (ΔE Δ E = 839 cm-1) and high absolute sensitivity (27.4 × 10-4 K-1 at 606 K) are obtained. Our results reveal Sr0.84Lu0.16F2.16GC are excellent host for rare earth ions doping and potential candidate for optical thermometry.
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