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  • Recycling of coal fly ash for fabrication of elongated mullite rod bonded porous SiC ceramic membrane and its application in filtration
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Dulal Das; Kayal Nijhuma; Antonio Marsola Gabriel; Gonçalves Parra Filho Daniel; Daniel de Mello Innocentini Murilo

    Mullite bonded SiC ceramic membranes were synthesized by a facile solid-state reaction process, using SiC, solid waste fly ash as raw materials and MoO3 as catalyst for growth of mullite at 1000 °C. The effect of MoO3 catalyst on mullitization reaction and mullite morphology was investigated. Different pore formers were used to enhance the porosity and to observe its effects on the permeability parameters and filtration characteristics. At room temperature Darcian (k1) and non-Darcian (k2) in both water and air flow were measured and clean water flux was determined. The porous SiC ceramics with addition of 5 wt.% MoO3 exhibited a flexural strength of 38.4 MPa at porosity 36.4 vol% and showed 92 % oil removal efficiency from oily wastewater. This technique, combining low-cost materials and the co-sintering at low temperature, can serve as a cost-effective method for the production of high-performance porous SiC ceramic membrnaes for filtration application.

    更新日期:2020-01-17
  • Revealing the formation mechanism of the skin-core structure in nearly stoichiometric polycrystalline SiC fibers
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Pengren Wang; Yanzi Gou; Hao Wang; Yingde Wang

    The polymer-derived SiC fibers have broad application prospects in the fields of aerospace, nuclear industry and high-tech weapon. Oxygen plays an essential role in adjusting the composition, structure and tensile strength of SiC fibers. Our studies have found that introducing too much oxygen during air curing process will form the skin-core structure in nearly stoichiometric polycrystalline SiC fibers. In order to reveal the formation mechanism of the skin-core structure, gradient oxygen was introduced into the fibers. The morphologies, phase distributions and defects of the fibers were well characterized. By strictly controlling the introduction of oxygen, the polycrystalline product fiber exhibits intragranular fracture behavior and excellent high-temperature resistance. The retention rate of its tensile strength can reach up to 91% and 61% after exposure at 1800 °C for 1 h and 10 h, respectively. The present results give valuable insights into the structural optimization of the nearly stoichiometric polycrystalline SiC fibers.

    更新日期:2020-01-17
  • Tensile creep properties and damage mechanisms of 2D-SiCf/SiC composites reinforced with low-oxygen high-carbon type SiC fiber
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Xi Wang; Zhuolin Song; Zanlin Cheng; Dong Han; Mei Li; Chengyu Zhang

    Tensile creep properties of 2D-SiCf/SiC composites reinforced with low-oxygen high-carbon type SiC fibers were studied in vacuum at 1300°C∼1430°C. The fracture morphology was observed by scanning electron microscopy and the damage of fiber in 2D-SiCf/SiC composites was characterized by nanoindentation. Moreover, the microstructure of the composite was investigated by high-resolution transmission electron microscopy. The results show that rupture time is much shortened and steady-state creep rate increase three orders of magnitude when creep temperature is higher than 1400°C. There are two different creep damage mechanisms due to the decrease of interfacial bonding strength at high temperature. The amorphous SiOxCy phase in the fibers can crystallize into SiC and C and the SiC grain grows in the fiber. The microstructural changes lead to the decrease of fiber strength and degrade the creep properties of the composite above 1400°C.

    更新日期:2020-01-17
  • Thermal behaviour and phases evolution during the sintering of porous inorganic membranes
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Juvenal N.G. Deutou; Van Essa L.S. Kamga; R.C. Kaze; E. Kamseu; Vincenzo M. Sglavo

    Anorthite-based highly porous membranes were successfully produced using calcined oyster shell to enhance the pore network. The calcined oyster shells produce CaO responsible for the crystallisation of gehlenite and anorthite at relatively low temperature. While the crystallisation produced nano and meso size of intergranular pores, vitrification of feldspar is responsible for development of the capillary porosities. The increasing sintering temperature from 1200 °C to 1300 °C implies the increase in average pores radius from 1.2 µm to 14.3 µm due to the formation of spherical pores from vitrification. The combination of different class of porosities in the matrices results in the interconnection with improvement of the permeability of the porous network. Porosity, permeability and chemical stability were improved with 20 wt.% of calcined oyster shell addition allowing the possible development of high strength porous network which is promising for the membranes support and other applications including liquid separation as well as liquid filtration where high pressure is used.

    更新日期:2020-01-17
  • High Temperature Absorbing Coatings with Excellent performance combined Al2O3 and TiC material
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Tengqiang Shao; Hua Ma; Jun Wang; Mingde Feng; Mingbao Yan; Jiafu Wang; Zhaoning Yang; Qian Zhou; Heng Luo; Shaobo Qu

    A series of challenges are impeding the development of high temperature electromagnetic (EM) wave absorbing materials in x band. In this study, to deal with this problem, a coating, (1-x).wtAl2O3∼x.wtTiC, is designed and prepared using plasma spraying technology. Its permittivity increases with temperature and TiC content, which endows it with a good EM impedance at high temperature. The coating possesses excellent EM absorbing performance at 800 ℃. When the x value equals 0.2 and thickness 1.6 mm, the coating exhibits an EAB of 3.45 GHz at 800℃, and a reflection loss lower than -8 dB in whole x band. The XRD result shows that only two phases exist in the coating. The SEM images illustrate that TiC is unevenly dispersed in Al2O3, causing loss of conductivity and interface polarization. The finding not only broadens the application of TiC-based materials but also indicates the promising future of this material system.

    更新日期:2020-01-17
  • Flexible and thermal-stable SiZrOC nanofiber membranes with low thermal conductivity at high-temperature
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Xiaoshan Zhang; Bing Wang; Nan Wu; Cheng Han; Chunzhi Wu; Yingde Wang

    Ceramic nanofibers with excellent thermal stability and low thermal conductivity are highly desired for high-temperature thermal insulation applications. However, the incompatibility of thermal stability and low thermal conductivity at high-temperatures largely limit the practical use of conventional single-phase ceramic nanofibers. Here, we report the preparation of multi-phase SiZrOC nanofiber membranes (NFMs) composed of amorphous SiOC and ZrO2 nanocrystals via electrospinning technique. The fabricated SiZrOC NFMs exhibited excellent high-temperature stability (∼1200 °C in Ar) and low thermal conductivity (∼0.1392 W·m-1 K-1 at 1000 °C in N2). The decreased thermal conductivity is achieved through a synergistic mechanism, that the multi-phase interfaces and the ZrO2 nanocrystals create thermal transfer barriers to reduce the heat transfer, whilst the SiOC phase effectively suppresses radiative heat transfer. This unique combination of amorphous SiOC and ZrO2 nanocrystals provides a novel strategy to prepare high-performance thermal insulation materials, and the obtained SiZrOC NFMs are promising high-temperature thermal insulation materials.

    更新日期:2020-01-17
  • Enhanced electrochemical performance and CO2 tolerance of Ba0.95La0.05Fe0.85Cu0.15O3-δ as Fe-based cathode electrocatalyst for solid oxide fuel cells
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-17
    Wenwu Xia; Qiang Li; Liping Sun; Lihua Huo; Hui Zhao

    The perovskite oxide Ba0.95La0.05Fe0.85Cu0.15O3-δ (BLFC) was evaluated as a Fe-based cathode electrocatalyst for solid oxide fuel cells (SOFCs). The crystal structure, electrocatalytic activity for the oxygen reduction reaction (ORR), and CO2 tolerance property of BLFC cathode are systematically investigated. BLFC is found to form a cubic symmetry perovskite oxide with space group Pm 3¯ m. The introduction of Cu reduces the average binding energy (ABE) value of Ba0.95La0.05FeO3-δ and increases the oxygen vacancy concentration. The polarization resistance (Rp) of the BLFC cathode is 0.091 Ω cm2 and the power density of the single cell with BLFC cathode is 1.05 W cm-2 under an output voltage of 0.53 V at 700 °C. The BLFC cathode shows satisfactory CO2 tolerance and chemical stability in the presence of 10% CO2. Furthermore, the relationship of Rp with oxygen partial pressure indicates that the oxygen surface adsorption-diffusion process is the rate-limiting step of the electrode.

    更新日期:2020-01-17
  • Effects of dielectric thickness on energy storage properties of 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 multilayer ceramic capacitors
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-16
    Hongxian Wang; Peiyao Zhao; Lingling Chen; Xiaohui Wang

    Multilayer Ceramic Capacitors (MLCCs) for energy storage applications require a large discharge energy density and high discharge/charge efficiency. Here, 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 (BTBZNT) powders were synthesized via solid-state reactions, and MLCCs with sub-micron grains were fabricated using a two-step sintering method. The BTBZNT MLCCs, which are well-defined relaxor ferroelectrics, possessed relatively high permittivity, dielectric sublinearity, and negligible hysteresis. An AC breakdown strength (BDS) enhancement from 511 to 1047 kV/cm was obtained as the dielectric thickness (D) decreased from 26 to 5 μm. The strong correlation between BDS and D, described as BDS∝D−0.368, agreed well with the general macroscopic theory of thermal breakdown in insulators. The enhanced BDS afforded the BTBZNT MLCCs (D∼9 μm) with excellent energy storage properties with a maximum discharge energy density of 10.5 J/cm3 and discharge/charge efficiency of 93.7% under a maximum electric field of 1000 kV/cm. The MLCCs also exhibited excellent thermal stability with discharge energy density variation <±5% over a wide temperature range of -50 to 175 °C under an electric field of 400 kV/cm. These remarkable performances make BTBZNT MLCCs promising for energy storage applications.

    更新日期:2020-01-17
  • Multi-step domain switching and polarization fatigue in [110]-oriented 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-16
    Yaming Zhou; Qiang Li; Fangping Zhuo; Chao Xu; Qingfeng Yan; Yiling Zhang; Xiangcheng Chu

    Domain switching in ferroelectrics is at the heart of many functionalities, and the visualization of switching pathway is the key to understand the fundamental properties and to promote the applications of high-performance ferroelectrics. Here, we directly documented the multi-step domain switching in [110]-oriented 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-0.33 P T) single crystals under a cycling electric field with the help of in situ polarized light microscopy. Based on the characteristic domain configuration analysis, we demonstrated that the 180° switching process was consisted of multi-step 60° switching. Such a multi-step 60° switching pathway resulted in a large negative strain and an internal electric field, which contributes to a large polarization fatigue rate under the alternating electric field. Our works may provide a window to study the domain switching process and its effect on polarization fatigue in relaxor-ferroelectric single crystals.

    更新日期:2020-01-17
  • A compromise between piezoelectricity and transparency in KNN-based ceramics: the dual functions of Li2O addition
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-16
    Xiaodan Ren; Zhanhui Peng; Bi Chen; Qiangqiang Shi; Xiaoshuang Qiao; Di Wu; Guorong Li; Li Jin; Zupei Yang; Xiaolian Chao

    Transparent ceramics with good electrical performance have recently drawn broad interest as promising multifunctional materials. Here, we report that a superior transmittance (T = 75% at 2000 nm) and good piezoelectricity (d33 ∼ 150 pC/N) can be simultaneously realized in 0.93K0.5Na0.5NbO3-0.07SrZrO3 (KNN-SZ) ceramics by Li2O regulation. The effect of Li2O regulation has two parts: first, the presence of Li2O facilitates the grain growth of KNN-SZ, considering that it melts at a relatively low temperature as a proper sintering aid; second, the introduced Li+ causes local lattice distortion, resulting in the coexistence of orthogonal and tetragonal (O-T) phases. The enlarged grains reduce the light scattering by grain boundaries for a higher optical transmittance; meanwhile, large grains stand as a prerequisite for the macroscopic domain structure favoured for decent piezoelectricity, which could also be partly caused by the coexistence of O-T phases. We believe that these findings might make KNN-based ceramics a preferable candidate for optoelectronic devices.

    更新日期:2020-01-16
  • Redox-thermal behavior of archaeological ceramics from the North Caucasus (Russia, Bronze/Iron Age)
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-15
    Ki Suk Park; Ulrich Schade; Johannes C. Vrijmoed; Sabine Reinhold; Ralf Milke

    The firing behavior of illite-based archaeological ceramics and corresponding firing conditions were investigated, in order to identify the pyrotechnology of ceramic production at archaeological sites in the North Caucasus (Russia, Bronze and Iron Age). Direct observations of the pyrometamorphic degree in the objects by scanning electron microscope (SEM), X-ray powder diffraction (XRD), Raman and Fourier-transform infrared spectroscopy (FT-IR) revealed the thermally induced localization of the redox state within a single object and its influence on the structural distortion, dehydroxylation and total collapse of illite in the ceramics. Fundamental approaches to illite dehydroxylation kinetics and numerical simulations of oxygen diffusion and heat transfer revealed that the firing temperature and time and thickness of the sample, reactivity between oxygen and ceramic pastes and porosity evolution played a decisive role in the firing behavior of the ceramics during the firing at the sites.

    更新日期:2020-01-15
  • Size and scaling effects in barium titanate. An overview
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-15
    Vincenzo Buscaglia; Clive A. Randall

    Ferroelectric perovskites such as BaTiO3 and Pb(Zr,Ti)O3 are well-suited for a variety of applications including piezoelectric transducers and actuators, multilayer ceramic capacitors, thermistors with positive temperature coefficient, ultrasonic and electro-optical devices. Ferroelectricity arises from the long-range ordering of elemental dipoles which determines the appearance of a macroscopic polarization and a spontaneous lattice strain. The confinement of a ferroelectric system in a small volume produces a perturbation of the polar order because of the high fraction of surface atoms and ferroelectricity vanishes when the size of the material is reduced below a critical dimension. This critical size is of a few nanometres in the case of epitaxial thin films and of 10-20 nm for nanoparticles and nanoceramics. The change in properties with decreasing physical dimensions is usually referred to as size effect. Thin films and ceramics are particularly prone to show size effects. A progressive variation of dielectric, elastic and piezoelectric properties of ferroelectric ceramics is already observed when the grain size is reduced below ≈10 μm, i.e. at a length scale much larger than the critical size. In this case it is more appropriate to refer to scaling effects as they are not related to material confinement. The aim of this contribution is to review the current understanding of size and scaling effects in perovskite ferroelectric ceramics and, in particular, in BaTiO3. After a short survey on the intrinsic limits of ferroelectricity and on the impact of particle/grain size on phase transitions, the role of interfaces such as ferroelectric/ferroelastic domain walls and grain boundaries in scaling of dielectric and piezoelectric properties will be discussed in detail. Multiple mechanisms combine to produce the observed scaling effects and the maximization of the dielectric constant and piezoelectric properties exhibited by BaTiO3 ceramics for an intermediate grain size of ≈1 μm. The broad dispersion of experimental data is determined by spurious effects related to synthesis, processing and variation of Ba/Ti ratio. Furthermore, we will consider these size effects, and other properties in relation to the downsizing the modern multilayer BaTiO3 based capacitors.

    更新日期:2020-01-15
  • Microstructure, ionic conductivity and mechanical properties of tape-cast Li1.5Al0.5Ti1.5P3O12 electrolyte sheets
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-15
    Enkhtsetseg Dashjav; Michael Gellert; Gang Yan; Daniel Grüner; Nico Kaiser; Stefan Spannenberger; Irina Kraleva; Raul Bermejo; Marie-Theres Gerhards; Qianli Ma; Jürgen Malzbender; Bernhard Roling; Frank Tietz; Olivier Guillon

    Free-standing Li1.5Al0.5Ti1.5P3O12 electrolyte sheets with a thickness of 50–150 µm were prepared by tape casting followed by sintering at 850–1000 °C in air. While a sintering temperature of 850 °C was too low to achieve appreciable densification and grain growth, a peak relative density of 95 % was obtained at 920 °C. At higher sintering temperatures, the microstructure changed from a bimodal grain size distribution towards exclusively large grains (> 10 µm), accompanied by a decrease in relative density (down to 86 % at 1000 °C). In contrast, ionic conductivity increased with increasing sintering temperature, from 0.1 mS/cm at 920 °C to 0.3 mS/cm at 1000 °C. Sintering behavior was improved by adding 1.5 % of amorphous silica to the slurry. In this way, almost full densification (99.8 %) and an ionic conductivity of 0.2 mS/cm was achieved at 920 °C. Mechanical characterization was carried out on the almost fully densified material, yielding elastic modulus and hardness values of 109 and 8.7 GPa, respectively. The fracture strength and Weibull modulus were also characterized. The results confirm that densification and reduction of grain size improve the mechanical properties.

    更新日期:2020-01-15
  • The irradiation response of ZrC ceramics under 10 MeV Au3+ ion irradiation at 800 ºC
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-14
    Raul Florez; Miguel L. Crespillo; Xiaoqing He; Tommi A. White; Gregory Hilmas; William Fahrenholtz; Joseph Graham

    The microstructural evolution was characterized for ZrC ceramics irradiated with 10 MeV Au3+ ions at 800 °C. Post-irradiation examination showed that ZrC did not amorphize at doses up to 30 displacement per atoms (dpa). Concurrent oxidation of ZrC was found to occur during ion irradiation. Coarsening of the defective microstructure, as a function of dose, was revealed by transmission electron microscopy analysis. Black dot defects were observed at low doses (0.5 dpa), and tangled dislocation networks were formed at 5 dpa and above. Diffraction analysis showed a change in the defect structure occurred at doses close to ∼2.5 dpa. The evolution of lattice parameter with dose indicated that uptake of adventitious oxygen could occur in specimens irradiated at high doses. Raman spectroscopy analysis indicated an increase in non-stochiometry after irradiaton. This work identified specific relationships between dose and microstructure after irradiation, revealing the mechanisms of damage production in ZrCx ceramics.

    更新日期:2020-01-14
  • Low-loss and temperature-stable negative permittivity in La0.5Sr0.5MnO3 ceramics
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-13
    Zhongyang Wang; Kai Sun; Peitao Xie; Runhua Fan; Yao Liu; Qilin Gu; John Wang

    Materials with negative permittivity need to be used at different temperatures, while the negative permittivity behavior affected by large fluctuations in temperature has seldom been studied. In this work, La0.5Sr0.5MnO3 ceramics were prepared by a sol-gel auto-combustion method and subsequent sintering. The negative permittivity behavior, electrical conductivity and reactance of La0.5Sr0.5MnO3 ceramics were systematically studied at various temperatures. The fluctuation in negative permittivity is less than 2.6% and the dielectric loss (tanδ) is less than 0.2 in the temperature range of 50-600 °C. Based on the key governing properties being achieved, the present work experimentally demonstrates that La0.5Sr0.5MnO3 ceramics, as single-phase oxides, can be used as a feasible alternative metamaterial in a wide temperature range.

    更新日期:2020-01-13
  • Tetrahedrites Synthesized via Scalable Mechanochemical Process and Spark Plasma Sintering
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-13
    Peter Baláž; Emanuel Guilmeau; Nina Daneu; Oleksandr Dobrozhan; Matej Baláž; Michal Hegedus; Tristan Barbier; Marcela Achimovičová; Mária Kaňuchová; Jaroslav Briančin

    In this study, we demonstrate the use of elemental precursors (Cu,Sb,S) to synthesize tetrahedrite Cu12Sb4S13 using an industrial eccentric vibratory mill. Mechanochemical synthesis of tetrahedrite leads to the formation of covellite (CuS), skinnerite (Cu3SbS3) or famatinite (Cu3SbS4) in dependence on milling time. However, the composite product can be modified in favour of prevailing tetrahedrite when Spark Plasma Sintering (SPS) treatment is applied after milling. The as-synthesized and sintered products are composed of polydisperse nanosized particles with dimensions up to 250 nm. The thermoelectric measurements reveal a maximum value of figure-of-merit ZT = 0.67 @ 700 K, as a consequence of a relatively high power factor (1.07 mW m-1 K-2) and a low thermal conductivity (1.12 W m-1 K-1). The obtained zT values for products prepared in an industrial mill are comparable to the ones synthesized by milling in laboratory mills. The synthesis of ternary and quaternary sulphides by a scalable and industrializable milling process represents a prospective route for mass production of thermoelectric materials.

    更新日期:2020-01-13
  • Cellular zirconia ceramics processed by direct emulsification
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-11
    Yu. A. Ivanova; C. Freitas; D.V. Lopes; A.V. Kovalevsky; J.R. Frade

    Highly porous zirconia ceramics for prospective use as separators in alkaline electrolysis cells have been processed by emulsification of paraffin in concentrated zirconia suspensions. Effects of processing parameters on porosity, cell size distribution, and pore interconnectivity are studied through Taguchi statistical design. The porosity of zirconia ceramics has shown a strong correlation with paraffin-to-suspension ratio employed in emulsification. High paraffin-to-suspension ratios combined with high gelatin content in the emulsion are beneficial for creating highly porous cellular materials with interconnected cell sizes. The attained microstructural features allow high percolation of ionic species in these highly porous ceramics after impregnation with aqueous electrolytes, facilitating charge transport within the liquid medium inside the ceramic bodies, as required for lower ohmic losses. This work provides guidelines for developing efficient ceramic separators for future use in alkaline electrolysis cells and other applications requiring thermally and chemically-stable ceramics with high and well-interconnected porosity.

    更新日期:2020-01-13
  • From High-Entropy Ceramics to Compositionally-Complex Ceramics: A Case Study of Fluorite Oxides
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-11
    Andrew J. Wright; Qingyang Wang; Chuying Huang; Andy Nieto; Renkun Chen; Jian Luo

    Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf1/3Zr1/3Ce1/3)1-x(Y1/2X1/2)xO2-δ (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young’s modulus (∼210 GPa) and nanohardness (∼18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.

    更新日期:2020-01-13
  • Relationship Between Fractography, Fractal Analysis and Crack Branching
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-11
    J.J. Mecholsky; D.P. DeLellis; N.A. Mecholsky

    A critical part of failure analysis is to understand the fracture process from initiation through crack propagation. Crack propagation in brittle materials can produce crack branching patterns that are fractal in nature, i.e., the crack branching coefficient (CBC). There is a direct correlation between the CBC and strength, σf: σf∝CBC. This appears to be in conflict with the fractal dimensional increment of the fracture surface, D*, which is independent of strength and related to the fracture toughness of the material, Kc: Kc=Ea01/2D*1/2, where E is the elastic modulus and a0, a characteristic dimension. How can D* be constant in one case and CBC be a variable in another case? This paper demonstrates the relationship between D* and CBC in terms of fractographic parameters. Examples of fractal analysis in analyzing field failures, e.g., that involve comminution, incomplete fractures of components, and potential processing problems will be demonstrated.

    更新日期:2020-01-13
  • Synthesis, densification and characterization of Ag doped ceria nanopowders
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-10
    Branko Matovic; Svetlana Butulija; Zorana Dohcevic-Mitrovic; Tamara Minovic Arsic; Jelena Lukovic; Sneyana Boskovic; Jelena Maletaskic

    Nanosized Ag-doped ceria (Ce1-xAlxO2-▯)powders (0.1 ≤ x ≤ 0.04) were obtained by self-propagating room temperature reaction. The solid solubility of Ag into ceria lattice was the highest reported so far. X-ray diffraction analysis and field emission scanning microscopy results showed that the doped samples are single phase solid solutions with fluorite-type structure and all prepared powders were nanometric in size. The average size of Ce1-xAgxO2-▯ particles lies at about 4 nm. Raman spectra revealed an increase in the amount of oxygen vacancies with the increase of Ag concentration, such as is foreseen. The thermal stability of solid solution was followed by XRD. Microstructure development was studied by scanning electron microscopy. By controlling the processing variables, it was possible to obtain high density samples with homogeneous microstructure at low sintering temperature.

    更新日期:2020-01-11
  • Multi-Phase Induced Ultra-Broad 1100-2100 nm Emission of Ni2+ in Nano-Glass Composites Containing Hybrid ZnGa2O4 and ZnF2 Nanocrystals
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-10
    Yindong Zhang; Bochao Sun; Lu Yang; Xiaosong Lu; Zhigang Gao; Yushi Chu; Shu Guo; Shiyu Sun; Shaoqian Zhang; Li Li; Lu Liu; Xinghua Yang; Jing Ren; Jianzhong Zhang

    A new type of Ni2+-doped dual-phase glass ceramics (GCs) is developed by a simple one-step thermal-induced crystallization process. The GCs thus obtained are embedded simultaneously with hybrid ZnGa2O4 and ZnF2 nanocrystals (NCs). When pumped by a readily available 808 nm laser diode, an ultra-broad near-infrared (NIR) emission in a range of 1100-2100 nm is observed at room temperature. The NIR emission band with a full-width-at-half-maximum (FWHM) of more than 450 nm is comparable to the largest value ever reported in Ni2+-doped GCs, and much broader than those of single-phase GCs embedded with either pure ZnGa2O4 or ZnF2 NCs. The microscopic morphologies of the embedded hybrid NCs, and especially the distribution of Ni2+ in the dual-phase GCs are studied by analytical transmission electron microscopy (TEM). The intriguing photoluminescence properties of Ni2+ are thoroughly investigated by steady-state and time-resolved emission spectra. The GCs demonstrated herein hold promise as broadband solid-state NIR-light sources.

    更新日期:2020-01-11
  • High-strength thermal insulating mullite nanofibrous porous ceramics
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-10
    Ying Zhang; Yongjun Wu; Xukun Yang; Dinghe Li; Xueying Zhang; Xue Dong; Xinghe Yao; Jiachen Liu; Anran Guo

    Mullite fibrous porous ceramics is one of the most commonly used high temperature insulation materials. However, how to improve the strength of the mullite fibrous porous ceramics dramatically under the premise of no sacrificing the low sample density has always been a difficult scientific problem. In this study, the strategy of using mullite nanofibers to replace the mullite micron-fibers was proposed to fabricate the mullite nanofibrous porous ceramics by the gel-casting method. Results show that mullite nanofibrous porous ceramics present a much higher compressive strength (0.837 MPa) than that of mullite micron-fibrous porous ceramics (0.515 MPa) even when the density of the mullite nanofibrous porous ceramics (0.202 g/cm3) is only around three quarters of that of the mullite micron-fibrous porous ceramics (0.266 g/cm3). The obtained materials that present the best combination of mechanical and thermal properties can be regarded as potential high-temperature thermal insulators in various thermal protection systems.

    更新日期:2020-01-11
  • Phosphor-SiO2 composite films suitable for white laser lighting with excellent color rendering
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-10
    Huajun Wu; Zhendong Hao; Guo-Hui Pan; Liangliang Zhang; Hao Wu; Xia Zhang; Ligong Zhang; Jiahua Zhang

    Currently, phosphor composite films draw much attention in white laser lighting. In this work, we developed a novel phosphor-SiO2 film via a mild method, which avoiding the phosphor degradation. Commercial colloidal silica was elected as the source of SiO2 and acted as an inorganic binder, gap filler and a protective coating layer. Composite films comprised of mixed Lu3Al5O12:Ce3+ and CaAlSiN3:Eu2+ phosphors exhibit a uniform dense strucure and strong adhesion to the substrate. When excited by blue diodes laser, the optimal film exhibits excellent thermal stability (it maintains 89.1% of the room-temperature intensity at 200 °C), super resistance against laser irradiation (12.9 W/mm2), a broad emission spectra with a full width at half maximum of 180 nm and a high luminous efficiency (183 lm/W). The color rendering index of the film was improved to 85. These outstanding properties indicate that the derived films are a promising candidate for white laser lighting.

    更新日期:2020-01-11
  • Ultrafast synthesis of entropy-stabilized oxide at room temperature
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-10
    Dianguang Liu; Xinyu Peng; Jinling Liu; Liying Chen; Yong Yang; Linan An

    Five-component entropy-stabilized oxide ceramic (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2O) has been successfully synthesized by reaction assisted flash sintering at room temperature. Compared to the conventional sintering technique, the starting materials have been completely transformed to a single rocksalt phase in just a few seconds. Analysis suggests that the electric field significantly accelerated the rate of phase transition. These results clearly demonstrate the feasibility of employing the reaction assisted flash sintering technique to fabricate entropy-stabilized oxides with new chemistries and attractive properties.

    更新日期:2020-01-11
  • Thermal radiation and cycling properties of (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-09
    Shujuan Dong; Fengning Zhang; Neng Li; Jinyan Zeng; Panpan Liang; Hao Zhang; Huiqi Liao; Jianing Jiang; Longhui Deng; Xueqiang Cao

    La2Ce2O7 with low thermal conductivity as a potential candidate of thermal barrier coatings (TBCs) was co-doped with (Ca, Fe) or (Sr, Mn) in order to further improve its thermal radiation at high temperatures. The microstructure, chemical composition, infrared emission properties (reflection and absorption properties) and thermal cycling lifetime of the coatings were respectively investigated. The results revealed that La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ coatings had defected fluorite structure and their infrared emittances were much higher than that of the parent La2Ce2O7. The superior infrared emission could be ascribed to the enhancement of the intrinsic absorption (electron transition absorption), free-carrier absorption and impurity absorption as well as lattice vibration absorption. However, the thermal cycling lifetime of La2Ce2O7 coatings presented a reduction after the (Ca, Fe) or (Sr, Mn) substitution, primarily due to the decrease in the fracture toughness and the increase in the thermal conductivity.

    更新日期:2020-01-09
  • X-ray excited CsPb(Cl,Br)3 perovskite quantum dots-glass composite with long-lifetime
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-08
    Congyong Wang; Hang Lin; Zhijun Zhang; Zhihua Qiu; Hongyi Yang; Yao Cheng; Ju Xu; Xiaoqiang Xiang; Liqiang Zhang; Yuansheng Wang

    CsPbX3 (X = Cl, Br, I) perovskite quantum dots (QDs) have emerged as a kind of brand-new X-ray scintillatior with high performance. Herein, following the phase transformation from amorphous to crystalline, CsPb(Cl,Br)3 QDs are in-situ precipitated from a borate glass matrix. It is demonstrated for the first time that the perovskite QDs-glass composite yields radioluminescence (RL), showing X-ray excited RL ∼1/18 to that of commercial Bi4Ge3O12 (BGO) single crystal. The color of RL is adjustable, dependent on the anionic species. Evidently, the high power X-ray induced damage in material is recoverable just by re-heating it at glass transition temperature. This work highlights the partial settlement of tough issues in perovskite QDs as scintillators, such as, physical stability, service lifetime, Pb-toxicity, and production scale.

    更新日期:2020-01-09
  • Unveiling crystallization mechanism for controlling nanocrystalline structure in glasses
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-08
    Qilin Ye; Kaibo Weng; Shangsheng Guan; Changgui Lin

    Controlling nanocrystalline structure in glasses renders the exploration of new composite multiphase (glass-ceramic) materials with novel functionalities that determined by the precipitated nanocrystals and residual glassy matrix. Previous microstructural investigation of glass-ceramics focused only on one aspect of nanocrystalline structures, e.g., nano-polycrystalline or single nanocrystalline. The recognition of the microscopic mechanism of nanostructure formation in glasses is absent. Here, we use advanced microscopic techniques to show the formation of different nanocrystalline structures composed of nano-polycrystals and single nanocrystals in 80GeS2·20In2S3 and 72.5GeS2·14.5Sb2S3·13RbCl glasses, respectively. Crystallization mechanism for controlling the nanocrystalline structure in glasses was revealed to depend on whether the glass network former participates in crystallization process. The results may shed light not only on glass crystallization mechanism, but also on the fundamental nature of the network structure of chalcogenide glasses.

    更新日期:2020-01-09
  • Enhanced high temperature microwave absorption of La0.9Sr0.1MnO3/MgAl2O4 composite ceramics based on controllable electrical conductivity
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-08
    Hongyao Jia; Wancheng Zhou; Hanyi Nan; Chunhai Wang; Yuchang Qing; Fa Luo; Dongmei Zhu

    The high temperature microwave absorbing efficiency (HTMAE) of xLa0.9Sr0.1MnO3/(1-x)MgAl2O4 composite ceramics was investigated by studying the crystal structure, electrical conductivity, and permittivity. The crystal structure of La0.9Sr0.1MnO3 and MgAl2O4 were maintained, but the Mn3+ and Al3+ ions were exchanged with each other through doping. The conductivity and permittivity of the composite ceramics increased with the increase of La0.9Sr0.1MnO3 content and test temperature. When x = 0.36, the electrical conductivity in La0.9Sr0.1MnO3 significantly enhanced the microwave polarization of the composite ceramics at high temperature. According to transmission/reflection modelling, the composite ceramics with x = 0.24 showed excellent HTMAE near the optimal thickness of 1.8 mm. Although the optimal thickness of the composite with x = 0.36 was reduced to 1.1 mm, the HTMAE was seriously lessened due to an impedance mismatch. xLa0.9Sr0.1MnO3/(1-x)MgAl2O4 are promising as thin and efficient microwave absorbing materials at high temperatures and the microwave permittivity can be further enhanced by adjusting the conductivity of La0.9Sr0.1MnO3.

    更新日期:2020-01-09
  • Significance of modification of slurry infiltration process for the precursor impregnation and pyrolysis process of SiCf/SiC composites
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-07
    Shiv Singh; Jie Yin; Lun Feng; Dehi Pada Mondal; Daejong Kim; Sea-Hoon Lee

    Several intermediate steps were applied before the precursor infiltration and pyrolysis process to improve the infiltration of SiC slurry for promoting the infiltration of SiC slurry into fiber voids. These steps include sonication, popping, electrophoretic deposition, vacuum infiltration and cold isostatic pressing (CIP). The intermediate processes, especially popping and CIP, had a beneficial effect on green density enhancement and improving the homogeneous infiltration of the slurry into fiber fabrics. The density of the SiCfiber/SiCfiller green body was 2.20 g/cm3, which corresponded to 68% of relative density. The SiCf/SiC composite has a high density of 2.65 g/cm3 after seven PIP cycles.

    更新日期:2020-01-07
  • Variable optical properties of La0.5Sr0.5Co1-xNixO3-δ for high-temperature resistant code application
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-07
    Zhenggang Fang; Wenqing Li; Dongshan Gao; Yaru Ni; Chunhua Lu; Zhongzi Xu

    La0.5Sr0.5Co1-xNixO3-δ (x = 0, 0.1, 0.3, 0.5) ceramics were prepared via tape casting and solid state reaction process. The influence of Ni concentration on the optical properties of La0.5Sr0.5Co1-xNixO3-δ has been investigated. Results showed that the reflectance in the range of 0.3–15 μm decreased with the increment of Ni concentration, thereby causing a change in the color phase parameters and emissivity. Based on the difference in L* values and emissivity, the letters (HOT) and QR codes (NJTECH) were fabricated. The developed letters and QR codes could be identified both at room and high temperatures. Furthermore, the QR codes were read out successfully even underwent heat treatment at 1000 °C. The results in this work demonstrate a new application of La0.5Sr0.5Co1-xNixO3-δ ceramics.

    更新日期:2020-01-07
  • The van de Hulst approximation for light scattering and its use for transmittance predictions in transparent ceramics
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-07
    Soňa Hříbalová; Willi Pabst

    The in-line transmittances of transparent spinel and YAG ceramics with a low volume fraction of monosized spherical pores calculated via the exact Mie solution and the so-called van de Hulst approximation are compared. It is shown that the transmittances calculated via the van de Hulst approximation are in surprisingly good qualitative agreement with the exact Mie solutions. In particular, all relevant trends and features of transmittance-versus-wavelength-and-pore-size plots are correctly predicted, so that the van de Hulst approximation may be considered as a perfect tool for material design and process optimization purposes. It avoids the “unphysical” results that can occur in the Rayleigh and Fraunhofer approximations for pores sizes beyond their validity and is much simpler than the Rayleigh-Gans-Debye approximation. Tables listed in the Appendix to this paper allow the reader reconstruct the exact Mie solution results from the easily calculated van de Hulst approximation, if required.

    更新日期:2020-01-07
  • Facile preparation of reduced GO modified porous ceramics with hierarchical pore structure as a highly efficient and durable sorbent material
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-07
    Yubao Bi; Haijun Zhang; Huifang Wang; Quanli Jia; Shaowei Zhang

    Hydrophobic sorbents play a key role in the remediation processes of oil spills/leakages occurring globally. In present work, reduced graphene oxide modified porous diatomite ceramics with hierarchical pore structures and excellent mechanical strength were facilely fabricated using diatomite powder and graphene oxide as starting materials. The microstructures and absorption properties of reduced graphene oxide modified porous diatomite ceramics were investigated, and the results indicated that reduced graphene oxide entered into the pores of diatomite ceramics, and formed a membrane coating on the wall of the pores, which made the diatomite ceramics a highly efficient absorption for organic solvents, and the capability separating the organic counterpart from the oil/water mixture.

    更新日期:2020-01-07
  • Creep properties of high dense La9.33Si6O26 electrolyte for SOFCs
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-03
    D. Ciria; M. Jiménez-Melendo; V. Aubin; G. Dezanneau

    High density La9.33Si6O26 polycrystals were fabricated by conventional and spark plasma sintering starting from nanopowders synthesized by freeze-drying. The materials exhibit a homogeneous microstructure formed by equiaxed grains with average sizes of 1.1 µm and 0.2 µm-diameter depending on the sintering route. Compressive mechanical tests were performed in air at constant strain rate between 900 and 1300 °C. A gradual brittle-to-ductile transition was found with increasing temperature and/or decreasing strain rate. Grain boundary sliding is the main deformation mechanism in the ductile region, characterized by a stress exponent n = 1 for the conventional sintered (large-grained) material and n = 2 for the spark plasma sintered (fine-grained) material; in both cases, the activation energy for creep was 360 kJ/mol. Effective cation diffusivities have been derived from mechanical data by comparison with appropriate models. The creep properties of lanthanum silicates are reported here for the first time.

    更新日期:2020-01-04
  • Novel Ca doped Sr0.7Bi0.2TiO3 Lead-Free Relaxor Ferroelectrics with High Energy Density and Efficiency
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-03
    Peng Zhao; Bin Tang; Feng Si; Chengtao Yang; Hao Li; Shuren Zhang

    Sr0.7Bi0.2TiO3 with high relaxor behavior and energy storage efficiency (η) is expected to be applied in power energy storage capacitors. However, its energy storage density is limited by the relatively low dielectric breakdown strength (DBS). Herein, Sr0.7Bi0.2CaxTiO3 (SBT-xC, x = 0 ∼ 0.15) was prepared to decrease the average grain size of Sr0.7Bi0.2TiO3. This can effectively eliminate the oxygen vacancy and decrease the electrical conductivity and leakage current, which result in the enhanced DBS. Meanwhile, Ca doping increases the relaxor behavior and dielectric constant. When x = 0.1, the composition exhibits high DBS of 480.2 kV/cm and excellent energy storage properties, such as high energy storage density of 2.1 J/cm3 with high η of 97.6% at 290 kV/cm, considerable thermal stability and great frequency stability. Moreover, SBT-0.1C shows high power density of 50.1 MW/cm3. These results suggest that SBT-0.1C is a potential candidate for high performance dielectric energy storage applications.

    更新日期:2020-01-04
  • Sintering Resistance of Suspension Plasma Sprayed 7YSZ TBC under Isothermal and Cyclic Oxidation
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-03
    Bingjie Xiao; Xiao Huang; Taylor Robertson; Zhaolin Tang; Rick Kearsey

    This study examines sintering resistance of a thermal barrier coating (TBC), composed of a 7YSZ suspension plasma sprayed (SPS) top coat (TC), an air plasma sprayed (APS) NiCoCrAl bond coat (BC), and an INCONEL 625 substrate, under isothermal and cyclic conditions with a peak temperature of 1080 °C for 400, 800, and 1300 hours/cycles. Microstructure, phase composition and microstrain were examined using SEM and XRD. Mechanical properties of fracture toughness, hardness and elastic modulus were obtained using nano-indentation. Samples under cyclic conditions presented faster sintering rate than under isothermal condition due to larger compressive strain and frequent heating and cooling cycles. Faster degradation of mechanical properties due to sintering leads to shorter lifetime of SPS coating under cyclic conditions. Moreover, vertical cracks within SPS coatings reduces compressive stress leading to a greater lifetime as compared to APS coatings exposed to similar conditions.

    更新日期:2020-01-04
  • Effect of ZrO2 crystallization on ion exchange properties in aluminosilicate glass
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-03
    Yunlan Guo; Chao Liu; Jing Wang; Jian Ruan; Xiaoyun Li; Jianjun Han; Jun Xie

    Ion exchange has the potential to improve the mechanical properties of glass ceramics. In this work, ZrO2 nanocrystals embedded transparent glass ceramics were prepared and effect of the crystallization on ion-exchange properties was investigated. The crystallization of ZrO2 did not affect the transmittance and Vickers hardness due to the small nanocrystal size and the low crystallinity, but significantly enhanced the ion exchange depth of layer (DOL). X-ray diffraction, high resolution transmission electron microscope, Raman spectra and nuclear magnetic resonance analysis demonstrated that with the crystallization of ZrO2, the charge compensator (Na+) was released, which promoted the transformation of highly coordinated Al into [AlO4]− tetrahedral units and the formation of Na+ balanced non-bridging oxygens. These changes in structure of glass made the Na+ more mobile and increased the DOL upon the crystallization. Results reported here may be useful for the development of glass-ceramic materials suitable for chemical strengthening.

    更新日期:2020-01-04
  • INFLUENCE OF ALBUMIN AND METHYLCELLULOSE ADDITIVES ON THE MECHANICAL BEHAVIOUR OF MULLITE-BASED CELLULAR GREEN BODIES
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-02
    M.L. Sandoval; A.G. Tomba Martinez; M.A. Camerucci

    In this work, the influence of bovine serum albumin (BSA) and methylcellulose (MC) as processing additives on the mechanical behaviour of the mullite-based cellular green bodies determined by diametral compression was studied. Two different forming routes were used to prepare compacts, which are based on the thermal treatment of aqueous suspensions foamed by mechanical stirring. The cellular microstructure of the green bodies was characterized in detail, by the use of 3D-stereological analysis. Parameters obtained relative to the cells’ features (amount, size, type, morphology, etc.) were related to the forming route and the composition of each system. Stress-strain curves were obtained from load-displacement data, and which, together with those calculated mechanical parameters, were ranalyzed in order to get information about the role of the organic binder system.

    更新日期:2020-01-02
  • Chemical evolution of lead in ancient artifacts -A case study of early Chinese lead-silicate glaze
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-02
    Xuesong Yin; Tang Jiao Huang; Hao Gong

    Early Chinese lead glaze has been recognized as an important artifact due to its multiple significances in history and science. Some efforts were made to explore its composition and degradation, but the chemical mechanisms of glaze transformation and incrustation formation, especially the evolution of lead and the roles of colorants, are still not clearly elucidated. In this work, an amorphous Si-rich structure and a layer of dendritic PbCO3 crystal were detected in the degraded glaze. Such characteristics of the incrustation help to attain insights into the chemistries governing glaze decomposition, re-crystallization of the lead compound and formation of the multi-layered structure. In addition, comparative studies on newly constructed lead-based glazes provide solid evidences for the lead transformation and highlight the crucial role of colorants in the glaze degradation. These fresh findings enable better understandings of the lead-silicate glass material and clarify long-term transformation of lead in burial environments.

    更新日期:2020-01-02
  • Microstructures and mechanical properties of high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C ceramics with the addition of SiC secondary phase
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-02
    Kuan Lu; Ji-Xuan Liu; Xiao-Feng Wei; Weichao Bao; Yue Wu; Fei Li; Fangfang Xu; Guo-Jun Zhang

    The relationships between microstructures and mechanical properties especially strength and toughness of high-entropy carbide based ceramics are reported in this article. Dense (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C (HEC) and its composite containing 20 vol.% SiC (HEC-20SiC) were prepared by spark plasma sintering. The addition of SiC phase enhanced the densification process, resulting in the promotion of the formation of the single-phase high-entropy carbide during sintering. The high-entropy carbide phase demonstrated a fast grain coarsening but SiC particles remarkably inhibited this phenomena. Dense HEC and HEC-20SiC ceramics sintered at 1900 °C exhibits four-point bending strength of 332 ± 24 MPa and 554 ± 73 MPa, and fracture toughness of 4.51 ± 0.61 MPa·m1/2 and 5.24 ± 0.41 MPa·m1/2, respectively. The main toughening mechanism is considered to be crack deflection by the SiC particles.

    更新日期:2020-01-02
  • Temperature dependence of Young’s modulus and damping of partially sintered and dense zirconia ceramics
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2020-01-02
    Eva Gregorová; Vojtěch Nečina; Soňa Hříbalová; Willi Pabst

    Young’s modulus and damping of partially sintered and almost fully dense zirconia ceramics (tetragonal zirconia polycrystals with 3 mol.% yttria), obtained by firing to different temperatures (range 1000–1400°C), have been determined via impulse excitation, and the evolution of Young’s modulus and damping of partially sintered zirconia with temperature has been monitored from room temperature to 1400°C and back to room temperature. The room-temperature Young’s modulus of the partially sintered materials obeys the Pabst-Gregorová exponential prediction, which is relatively unusual for partially sintered materials. With increasing temperature Young’s modulus decreases, until the original firing temperature is exceeded and sintering (densification) continues, resulting in a steep Young’s modulus increase. During heating and cooling the temperature dependence obeys a master curve with a typical inflection point at approximately 200 °C, the temperature where damping (internal friction) exhibits a maximum. The reasons for this characteristic behavior of doped zirconia are recalled.

    更新日期:2020-01-02
  • The effects of deposition conditions on hydrogenation, hardness and elastic modulus of W-C:H coatings
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-31
    František Lofaj; Margita Kabátová; Lenka Kvetková; Jozef Dobrovodský

    The additions of C2H2, CH4 and H2 in hybrid PVD-PECVD of W-C:H coatings deposited using High Power Impulse Magnetron Sputtering (HiPIMS) and High Target Utilization Sputtering (HiTUS) were investigated to determine their effects on the content and chemical composition of the amorphous carbon-based boundary phase and mechanical properties of the coatings. Substantial differences were observed: CH4 always produced higher concentrations of hydrogen and lower concentrations of carbon than C2H2 and HiPIMS resulted in higher contents of amorphous carbon-based boundary phase and higher levels of its hydrogenation than HiTUS. The detrimental effects of higher carbon and hydrogen contents in the boundary phase on hardness and indentation modulus were attributed to the consumption of C-C bonds by C-H during hydrogenation and reduction of cross-linking of the polymeric network in the boundary phase. The HiPIMS W-C:H coatings deposited with acetylene and hydrogen exhibited medium (∼20 GPa) hardness and elastic modulus (200 - 220 GPa) with HIT/EIT > 0.1 suggesting improved toughness and wear resistance. These properties were attributed to the optimum combination of hydrogenation, hybridization and cross linking in the carbon-based boundary phase.

    更新日期:2019-12-31
  • Crystal structure and piezoelectric characteristics of various phases near the triple-point composition in PZ-PT-PNN system
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-31
    Tae-Gon Lee; Sun-Woo Kim; Eun-Ji Kim; Sang Jin Lee; Hyun-Gyu Hwang; Youn-Woo Hong; Jeong Seog Kim; Keun Hwa Chae; Ji-Won Choi; Chong-Yun Kang; Sahn Nahm

    Crystal structures and piezoelectric properties of PbZrO3-PbTiO3-Pb(Ni1/3Nb2/3)O3 ceramics near the triple point composition, particularly characteristics of the pseudocubic phase, were investigated. The pseudocubic phase, which formed near the triple point composition, disappeared with increase in the PbZrO3 content. The pseudocubic phase had the Pm3m cubic structure. The tetragonal-pseudocubic morphotropic phase boundary (MPB) structure was developed during the tetragonal-to-cubic phase transformation. However, the rhombohedral phase directly transformed to the cubic phase because the structure of pseudocubic phase was similar to the rhombohedral structure. The specimens with pseudocubic phase and the specimens near pseudocubic phase exhibited nano-sized domains and small coercive electric fields, revealing their low domain wall energies. These specimens exhibited second-order ferroelectric-to-paraelectric phase transition and low Curie temperatures, confirming their low domain wall energies. The enhanced dielectric and piezoelectric properties of these specimens could be attributed to their low domain wall energies.

    更新日期:2019-12-31
  • Correlating the effect of dopant type (Al, Ga, Ta) on the mechanical and electrical properties of hot-pressed Li-garnet electrolyte
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-31
    Gigap Han; Bryan Kinzer; Regina Garcia-Mendez; Heeman Choe; Jeff Wolfenstine; Jeff Sakamoto

    The effect of cubic-phase stabilizing dopant (Al, Ga, Ta) on the mechanical and electrochemical properties of Li garnet solid electrolyte was studied. Dense Li6.25La3Al0.25Zr2O12, Li6.50La3Ta0.50Zr1.5O12, Li6.25La3Ga0.25Zr2O12 were prepared by conventional solid-state synthesis of powder and densified using hot pressing. Ga-LLZO exhibited the highest fracture stress (∼143 MPa), fracture toughness (∼1.22 MPa m1/2) and total conductivity (∼1 mS/cm) of the three materials; however, the bulk conductivity was about 1.5 mS/cm. We believe that the weak grain-boundaries, as evidenced by a predominately intergranular fracture, correlates with a relatively high grain-boundary impedance, thus reducing the value of the total conductivity by about 30% lower than that for the bulk. Based on the combined mechanical and electrical properties, overall, Li6.25La3Ga0.25Zr2O12 exhibits the most favorable combination of some of the most salient properties of the three dopants. We believe the results of this study will facilitate the commercialization of Li metal batteries using Li-garnet ceramic electrolyte.

    更新日期:2019-12-31
  • Blue photo luminescence from 3mol%Y2O3-doped ZrO2 polycrystals sintered by flash sintering under an alternating current electric field
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-30
    Yudai Yamashita; Tsuyoshi Kurachi; Tomoharu Tokunaga; Hidehiro Yoshida; Takahisa Yamamoto

    We investigated the photoluminescence (PL) properties of 3 mol%Y2O3-ZrO2 (3YSZ) polycrystals. The 3YSZ polycrystals were prepared by an isothermal flash sintering technique in which an alternating electric field was applied at a predetermined furnace temperature. After flash sintering, clear, visibly blue PL at 2.82 eV (440 nm) was confirmed under an ultraviolet light (3.4 eV (365 nm)); however, no visible PL was observed from 3YSZ polycrystals conventionally sintered using the same raw powders. Thus, the emission of blue PL was induced by the electric field applied during sintering. The obtained PL band was related to excitation at 3.87 eV (320 nm), which is possibly assigned to the singly ionized associated oxygen vacancy defects (AOD + centers). Furthermore, the blue emission was stable after high temperature annealing at 1400 °C for 1 h in air. We possibly attribute the emission of this PL to the stabilization of AOD + centers by the electric field loading during flash sintering.

    更新日期:2019-12-30
  • Mechanical properties and microstructure of Yb2SiO5 environmental barrier coatings under isothermal heat treatment
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-28
    Byung-Koog Jang; Nobuo Nagashima; Seongwon Kim; Yoon-Suk Oh; Sung-Min Lee; Hyung-Tae Kim

    Yb2SiO5 (ytterbium monosilicate) top coatings and Si bond coat layer were deposited by air plasma spray method as a protection layer on SiC substrates for environmental barrier coatings (EBCs) application. The Yb2SiO5-coated specimens were subjected to isothermal heat treatment at 1400 °C on air for 0, 1, 10, and 50 h. The Yb2SiO5 phase of the top coat layer reacted with Si from the bonding layer and O2 from atmosphere formed to the Yb2Si2O7 phase upon heat treatment at 1400 °C. The oxygen penetrated into the cracks to form SiO2 phase of thermally grown oxide (TGO) in the bond coat and the interface of specimens during heat treatment. Horizontal cracks were also observed, due to a mismatch of the coefficient of thermal expansion (CTE) between the top coat and bond coat. The isothermal heat treatment improves the hardness and elastic modulus of Yb2SiO5 coatings; however, these properties in the Si bond coat were a little bit decreased.

    更新日期:2019-12-29
  • Grinding of hard and brittle ceramic coatings: Force analysis
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-28
    Simanchal Kar; Sravan Kumar; P.P. Bandyopadhyay; Soumitra Paul

    In this work, a grinding force model has been proposed and a correlation between the analytical model and the experimental grinding forces obtained during finishing of plasma sprayed ceramic coatings has been investigated. Thermally sprayed ceramic coatings have low fracture toughness and develop micro brittle fracture under moderate mechanical loading. In this investigation, grinding of air plasma sprayed alumina, zirconia, and titania coatings were studied theoretically and experimentally. It was observed that the ground surface contained micro-cracks and debris of irregular fragmented chips owing to the micro-brittle fracture. A grinding force model is proposed to incorporate the fracture behavior of the ceramic coating. This was substantiated through experimental values showing low grinding forces.

    更新日期:2019-12-29
  • Bioactive silicon nitride by surface thermal treatment
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-28
    Miroslav Hnatko; Michal Hičák; Martina Labudová; Dagmar Galusková; Jaroslav Sedláček; Zoltán Lenčéš; Pavol Šajgalík

    Silicon nitride-based ceramics with SiO2, CaO and Ca3(PO4)2 as sintering additives, have been prepared in order to study the bioactivity. Dense ceramic bodies were oxidized by an oxy-acetylene flame at approx. 1475 °C for 60 seconds, in order to modify the surface in terms of bioactivity enhancement and the formation of optimal porosity for cell viability. During oxidation two concurrent processes occurred on the ceramic body surface: (i) formation of thin glassy layer with a composition close to that of grain boundary phase in ceramic body, and (ii) partial decomposition of silicon nitride matrix. The latter one resulted in the formation of gases (N2 and SiO), which formed bubbles in the viscous surface glassy phase, resulting in porosity required for cell adhesion (small pores) and tissue ingrowth (large pores). The best bioactivity was obtained for oxy-acetylene flame treated Si3N4 ceramics with Ca3(PO4)2 sintering additive.

    更新日期:2019-12-29
  • Effect of residual excess carbon on the densification of ultra-fine HfC powder
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-27
    Jun-Seop Kim; Seung Jun Lee; Feng Lun; Laura Silvestroni; Diletta Sciti; Sea-Hoon Lee

    The residual carbon content of ultra-fine hafnium carbide (HfC) powder was controlled by the optimization of the synthesis process, and the effect of residual carbon on the densification of HfC powder was analyzed. The amount of residual carbon in the HfC powder could be reduced by the de-agglomeration of HfO2 powder before the carbo-thermal reduction (CTR) process. The average particle size of HfO2 powder decreased from 230 to 130 nm after the de-agglomeration treatment. Ultra-fine (d50: 110 nm) and highly pure (metal basis purity: >99.9% except for Zr) HfC powder was obtained after the CTR at 1600 °C for 1 hr using the C/Hf mixing ratio of 3.3. In contrast, the C/Hf ratio increased to 3.6 without the de-agglomeration treatment, indicating that a large amount of excess carbon was required for the complete reduction of the agglomerated HfO2 particles. HfC ceramics with high relative density (>98%) were obtained after spark plasma sintering at 2000 °C under 80 MPa pressure when using the HfC powder with low excess carbon content. In contrast, the densification did not complete at a higher temperature (2300 °C) and pressure (100 MPa) when the HfC powder contained a large amount of residual carbon. The results clearly indicated that residual carbon suppressed the densification of HfC powder in case the carbide powder had low oxygen content, and the residual carbon content could be controlled by the optimization of the synthesis process. The average grain size and Vickers hardness of the sintered specimen were 6.7(±0.7) µm and 19.6 GPa, respectively.

    更新日期:2019-12-27
  • Ta-based 413 and 211 MAX phase solid solutions with Hf and Nb
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-27
    Matteo Griseri; Bensu Tunca; Shuigen Huang; Martin Dahlqvist; Johanna Rosén; Jun Lu; Per O.Å. Persson; Lucia Popescu; Jozef Vleugels; Konstantina Lambrinou

    New bulk MAX phase-based ceramics were synthesized in the Ta–Hf–Al–C and Ta–Nb–Al–C systems. Specifically, (Ta1-x,Hfx)4AlC3 and (Ta1-x,Nbx)4AlC3 stoichiometries with x = 0.05, 0.1, 0.15, 0.2, 0.25 were targeted by reactive hot pressing of Ta2H, HfH2, NbH0.89, Al and C powder mixtures at 1550 °C in vacuum. The produced ceramics were characterized in terms of phase composition and microstructure by X-ray diffraction, scanning electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. The investigation confirmed the existence of such M-site solid solutions with low solute concentrations, as predicted by first-principles calculations. These calculations also predicted a linear trend in lattice parameter evolution with increasing Hf concentration, in agreement with the experimental results. In order to increase the low phase purity of the produced ceramics, Sn was added to form (Ta1-x,Hfx)4(Al0.5,Sn0.5)C3 and (Ta1-x,Nbx)4(Al0.5,Sn0.5)C3 double solid solutions, thus resulting in a higher content of the 413 MAX phase compounds in the produced ceramics.

    更新日期:2019-12-27
  • Characterization of SiC ceramics with complex porosity by capillary infiltration: Part A - Filling by hexadecane at 20 °C
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-26
    J. Roger; M. Avenel; L. Lapuyade

    The characterization of the porosity of ceramic materials is of prime importance for numerous applications. The work is being presented in two parts: A and B. In this part (Part A), several porous SiC samples exhibiting different sizes and porosity spectra were characterized by capillary infiltration of hexadecane at room temperature. The model materials contain a bimodal pore population generated by cracks and porogens. The monitoring of the weight increase in time for each sample evidence the occurrence of two successive filling stages with specific kinetics, except for the reference samples which contain only one kind of pore. The first kinetic is found clearly larger than the second. A new analytical function was used to identify the transition times and the kinetics of each stage. The various values of the kinetics revealed the occurrence of different infiltration mechanisms. The same model correlates silicon infiltration observations as well (Part B).

    更新日期:2019-12-27
  • Characterization of SiC ceramics with complex porosity by capillary infiltration: Part B – Filling by molten silicon at 1500 °C
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-26
    J. Roger; M. Avenel; L. Lapuyade

    In Part A of this study, infiltrations experiments of porous SiC samples by hexadecane with pore-size distributions comprising small and large pores were realized. Two successive stages were identified during the filling of these samples corresponding to the infiltration of the two types of pores. The experimental data were successfully treated with a new analytical function. In Part B, it was found that this function can also be applied to the analysis of the mass gain during molten silicon infiltration at 1500 °C. Prior to silicon infiltration, it was found that the operating temperature induces a shift of the pore size distributions towards larger values. A dissolution-recrystallisation mechanism can also occur during the infiltration of silicon. During the first stage, liquid silicon fills rapidly larger pores than hexadecane. The kinetics are significantly larger with liquid silicon. Consequently, the durations for the complete filling are very short with molten silicon.

    更新日期:2019-12-27
  • Phase Transition between Sphalerite and Wurtzite in ZnS Optical Ceramic Materials
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-25
    Yiyu Li; Wenxia Tan; Yiquan Wu

    The phase transition behavior of zinc sulfide (ZnS) ceramics consolidated via pressureless and hot press sintering has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) analyses. Two types of ZnS powders with different particle sizes and morphologies were employed to study the influence of microstructural features of starting powders on the ZnS phase transition behaviors. The present work has revealed that during sintering of ZnS ceramics, the phase transition behavior varies based on the starting powder particle size and magnitude of the applied pressure. It has been demonstrated that smaller particle sizes lead to an increased degree of “early” phase transformation from sphalerite to wurtzite at 1000 °C. Additionally, the application of uniaxial pressure during sintering can lead to a reverse phase transition from wurtzite to sphalerite while simultaneously inducing twinning, resulting in improved optical transmittance and mechanical hardness.

    更新日期:2019-12-26
  • Origin of high dielectric performance in fine grain-sized CaCu3Ti4O12 materials
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-24
    He Lin; Wentao Xu; Haitao Zhang; Chen Chen; Youfu Zhou; Zhiguo Yi

    We report on high dielectric constant (8.3 × 103, 104 Hz), low dielectric loss (0.029, 104 Hz) as well as fine grain size (∼840 nm) achieved in pure CaCu3Ti4O12 (CCTO) ceramics through a combination of sol-gel method, spark plasma sintering and annealing process. By adjusting the sintering temperature and annealing conditions, the composition variations, valence states and microstructures of CCTO ceramics are systematically studied, which provide direct clues in understanding the origin of their excellent dielectric response. Through the studies on the dielectric, impedance, modulus and conductivity properties of CCTO ceramics, a modified brick-layer model based on two interfacial polarizations originating from sub-grain boundary and grain boundary barriers is proposed to explain their dielectric behaviors. The high dielectric constant of CCTO ceramics is mainly dominated by the sub-grain contribution; and the reduced dielectric loss is attributed to the decrease of electrical conductivity and relaxation loss.

    更新日期:2019-12-25
  • Active Metal Brazing of Graphite Foam-to-Titanium Joints made with SiC-Coated Foam
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-24
    M. Singh; C.E. Smith; R. Asthana; A.L. Gyekenyesi

    Medium-density, high-conductivity carbon foams were joined to titanium using a two-step process that first exposed foam to SiO vapor at 1450 °C for 30 min. under vacuum followed by vacuum brazing Ti using Cusil-ABA to the sides of prismatic foam pieces along the ‘with-rise’ (WR) or foaming direction and the ‘against rise’ (AR) or transvers direction. Well-bonded joints with braze-infiltrated foam and Ti-rich interfaces formed along WR and AR. The un-bonded foam was stronger along WR (785 kPa) than along AR (277 kPa) as were the joints made using coated and uncoated foams. Foam thickness minimally affected joint strength along WR but along AR, joints with thick foam were 58% stronger. The coating marginally (9%) lowered joint strength along WR but led to a nearly 50% strength drop along AR. The experimental foam is more robust and amenable to coating and joining along foaming direction than transverse to it.

    更新日期:2019-12-25
  • Electric transport properties of rare earth doped YbxCa1-xMnO3 ceramics (part III: Point defect chemistry)
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-24
    Meimanat Rahmani; Christian Pithan; Rainer Waser

    A defect chemical model based on charge neutrality and laws of mass action is proposed to clarify the details of the chemistry of point defects for donor-doped YbxCa1-xMnO3. DC-conductivity measurements were carried out in a wide range of partial pressure of oxygen p(O2) ≈ 10−1 down to 10-19 MPa at 750⁰C for the first time without disintegrating the ceramic sample through reduction. A comparison of the experimental observations and the theoretical defect chemical models clearly shows the possibilities for controlling charge carriers in dependence of partial pressure of oxygen p(O2) and dopant concentration. The origin of a plateau state, of a drastic decrease in conductivity in the intermediate and reduction p(O2) regimes are figured out, respectively. In addition, the kind and concentration of the electronic and ionic majority charge carriers are determined and formulated according to the proposed defect chemical model. Furthermore, phase transitions were studied in a wide range of p(O2) at elevated temperatures

    更新日期:2019-12-25
  • Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-24
    Jinchang Sun; Bo Niu; Fan Zhang; Jinyong Zhang; Liwen Lei; Lin Ren

    Hexagonal boron nitride (h-BN) can reinforce boron carbide (B4C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B4C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B4C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B4C matrix and the growth of B4C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B4C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.

    更新日期:2019-12-25
  • Gradient microstructure development and grain growth inhibition in high-entropy carbide ceramics prepared by reactive spark plasma sintering
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-24
    Xiao-Feng Wei; Yuan Qin; Ji-Xuan Liu; Fei Li; Yong-Cheng Liang; Guo-Jun Zhang

    High-entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the single-phase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of high-entropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.

    更新日期:2019-12-25
  • A coupled phase diagram experimental study and thermodynamic optimization of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems, and prediction of the phase diagrams of the Li2O-CaO-Al2O3-SiO2 system
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-23
    Bikram Konar; In-Ho Jung

    A coupled experimental phase diagram study and thermodynamic modeling of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems was conducted at 1 atm total pressure. Differential scanning calorimetry (DSC) measurements were performed in the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems. In addition, the phase relations in the Li2O-CaO-Al2O3 system were determined by equilibration/quenching experiments at 1643 and 1743 K, and the phases were characterized with X-ray diffraction (XRD) and Electron-probe micro analysis-wavelength dispersive spectroscopy (EPMA-WDS). The absence of ternary compounds or solid solutions was confirmed. Congruent melting of Li2CaSiO4 compound in the Li2O-CaO-SiO2 system was determined at 1350 ± 5 K. Thermodynamic optimization of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems was carried out based on new phase diagram experiments and critically evaluated literature data. The phase diagrams of the quaternary Li2O-CaO-Al2O3-SiO2 system were predicted using the thermodynamic models with optimized model parameters.

    更新日期:2019-12-23
  • Incorporation of BN-coated carbon fibers into ZrB2/SiBCN ceramic composites and their ablation behavior
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-23
    Yang Miao; Fengnian Zhang; Zhihua Yang; Dechang Jia; Yu Zhou

    ZrB2/SiBCN composites containing carbon fibers coated with BN were prepared using combination of sol-gel and spark plasma sintering (SPS) techniques. Thermal shock and ablation behavior of these composites were thoroughly analyzed. Sintered composites contained ZrB2, SiC and BN(C) crystalline phases. Thin BN layer passivated carbon fibers and prevented them from reacting with the matrix. With the addition of carbon fiber, diffusion coefficient decreased, and the thermal expansion coefficient increased in comparison to ZrB2/SiBCN composites without carbon fibers. Ablation tests showed no crack formation after carbon fibers were added to the composites. Ablation center exhibited loose and porous network structure. ZrSiO4 formed from ZrO2 and SiO2 reaction in the central ablation region. Presence of ZrSiO4 prevented samples from further ablation-related damages.

    更新日期:2019-12-23
  • Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness
    J. Eur. Ceram. Soc. (IF 4.029) Pub Date : 2019-12-20
    Xuewu Li; Xiaohui Fan; Na Ni; Xiaofeng Zhao; Chuanwei Li; Ping Xiao

    Continuous alumina fiber-reinforced yttria-stabilized zirconia (YSZ) composites with a LaPO4 fiber coating were fabricated by slurry infiltration and spark plasma sintering (SPS). The LaPO4 coating was deposited on the reinforcement alumina fabrics by a modified sol-gel method. The YSZ slurry with good dispersion and stability was prepared by optimizing the pH value, dispersant addition and ball milling time. The fabricated composite with a high density of ∼ 92% has a good flexural strength of 277 ± 43 MPa, and a superior fracture toughness of 15.93 ± 0.75 MPa·m1/2 exhibiting a non-brittle failure behavior. It was found that the LaPO4 coating reduced the residual stress near the fiber/matrix interface to 131 ± 41 MPa, which was 369 ± 63 MPa in the composite without the fiber coating. The LaPO4 coating renders a weak interphase to improve the composite toughness by activating several toughening mechanisms including crack deflection, fiber debonding and pullout, and delamination behavior.

    更新日期:2019-12-21
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