Dopant effects on high temperature mechanical properties of zirconium carbide ceramics

Combining theoretical and experimental methods, the effects of 5?vol.-% WC dopant on the microstructure evolution, sintering behaviour and mechanical properties of ZrC ceramics were investigated. WC dopant was found to improve the high temperature elastic modulus and bending strength of ZrC ceramics. Both calculations and experimental results showed that the formation of (Zr, W)C solid solution promote dissolution of the impurity oxygen from the starting powder into the lattice sites, resulting in less oxygen defects in grain boundaries. Internal friction curve can also conform that the ZrC ceramics doped with WC have cleaner grain boundaries, which improved higher elastic modulus and bending strength in WC doped ZrC ceramics at elevated temperature.     

Achieving both high electromechanical properties and temperature stability in textured PMN-PT ceramics

Textured piezoelectric ceramics, such as textured Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) ceramics, have attracted considerable attention from both academia and industry, as they possess crystal-like piezoelectric properties, high composition homogeneity, and low manufacturing cost. However, the main difficulty with the textured piezoelectric ceramics is the presence of BaTiO₃ (BT) templates, which greatly reduces their piezoelectricity and phase transition temperature. Thus, it is highly recommended to fabricate textured piezoelectric ceramics using as few templates as possible. Here, we successfully fabricated high-quality <001>-textured PMN-28PT ceramics (texturing degree of 99%) by using an extremely small amount of BT templates (1 vol.%) with the help of CuO/B₂O₃ sintering aids. The textured PMN-28PT ceramic exhibits 80% piezoelectric coefficient (d₃₃ ∼ 1200 pC/N), 96% electromechanical coefficient (k₃₃ ∼ 88%) and the same temperature stability (T_rt ∼ 100, T_c ∼ 150°C) when compared to its single crystal counterpart. In addition, by using an alternating current electric field poling (AC-poling), the piezoelectric coefficient d₃₃ and dielectric permittivity ε₃₃ of the textured PMN-28PT ceramics were further enhanced around 5–8%. It is believed that the advantages of high electromechanical properties, low cost, and easy mass production of textured PMN-28PT ceramic will make it a promising candidate for advanced electromechanical devices.     

Enhanced high temperature properties of ZTA prestressed ceramics reinforced by cordierite coating

Zirconia toughened alumina (ZTA) has a wide range of applications in high temperature. Though some routes were reported to enhance the mechanical properties at room temperature, the preparation of ZTA with improved high temperature properties was still unknown. In this work, cordierite/ZTA pre-stressed ceramics (marked as Cor-ZTA30) have been fabricated successfully by using prestressed coating reinforcement method. The relationships among the temperature, residual stress and the flexural strength of Cor-ZTA30 have been established based on experimental data. Due to the compressive stresses exists in the surface layer of Cor-ZTA30 hinder the crack extension, the flexural strength of Cor-ZTA30 was greatly enhanced at the temperatures below 1000°C. In addition, the difference in flexural strength between coated and uncoated ZTA ceramics decreased with increasing temperature until 1000°C. It indicated that the residual stress vanished entirely as the temperature above 1000°C, which resulted in the same flexural strength for ZTA and Cor-ZTA30. Therefore, strengthening ZTA ceramics by prestressing is available under the temperatures below 1000°C.     

Preparation of carbon/carbon‐ultra high temperature ceramics composites with ultra high temperature ceramics coating

In order to increase the oxidation resistance of carbon/carbon (C/C) composites at long‐term high temperature, C/C‐Ultra High Temperature Ceramics composites (UHTCs) with a dual‐layer UHTCs oxidation coating was successfully designed and fabricated. The microstructure and ablation resistance were investigated and discussed. After ablation in arc‐heated wind tunnel with temperature being 2200°C for 1000s, the mass ablation rate and linear ablation rate were ?1.9 × 10^?2 mg/cm²s and 2.9 × 10^?5 mm/s, respectively. The formation of thermodynamically compatible oxide scale including ZrO₂ skeleton and SiO₂ or Zr–Si–O glass on the surface were mainly contributed to the excellent ablation resistance of the composite.     

Polymer-derived SiHfN ceramics: From amorphous bulk ceramics with excellent mechanical properties to high temperature resistant ceramic nanocomposites

Within the present work, additive-free amorphous bulk SiHfN ceramics with excellent mechanical properties were prepared by a resource-efficient low-temperature molding method, namely warm-pressing. As densification mechanism viscous flow has been identified based on cross-linking reaction. The critical problems concerning gas evolution and crystallization inducing bloating and cracking are addressed through controlled thermolysis and pressure. The microstructural evolution of the SiHfN ceramics indicates that the incorporation of Hf in perhydropolysilazane not only increases the ceramic yield (97.4 wt%) and crystallization resistance (1300 °C), but also suppresses the transformation from α-Si₃N₄ to β-Si₃N₄ at high temperatures (1700 °C). Especially, HfN/α-Si₃N₄ nanocomposites converted by the SiHfN ceramics at 1500 °C show a slight weight loss of 3.13 wt%, indicating the high temperature resistance of the ceramic nanocomposites. The method proposed in this work opens a new strategy to fabricate additive-free polycrystalline Si₃N₄- and amorphous Si₃N₄-based (nano)composites.     

Promising high temperature thermoelectric properties of dense Ba2Co9O14 ceramics

Layered cobalt oxides have shown high thermoelectric properties. The n = 1 member of the Ba_n+1Co_nO_3n+3(Co₈O₈) family, Ba₂Co₉O₁₄, a new layered cobalt oxides family with Co(II) and Co(III) in the CdI₂ layers, has been synthesized by solid state reaction and sintered as dense ceramics (relative density ∼ 93%) by Spark Plasma Sintering. It presents promising p-type thermoelectric properties at high temperature. The dimensionless figure of merit ZT is 0.032 at 660 K and 0.04 at 1000 K, which is about one quarter to one third of the ZT value of Ca₃Co₄O₉ ceramics.     

Porous alumina-spinel ceramics for high temperature applications

In order to reduce energy costs, high-temperature insulation porous refractory ceramics have been subjected to increasing demands. Among the techniques used to produce these materials (such as the addition of foaming agents and organic compounds), the pore generation via phase transformation presents key aspects, such as easy processing and the absence of toxic volatiles. In this study, this technique was applied to produce porous ceramics by decomposing an aluminum-magnesium hydro-carbonate known as hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O). It was found out that by using this complex compound, a large fraction of pores can be generated and kept at high temperatures (above 1300 °C) due to the in situ formation of spinel-like phases (MgAl₂O₄).     

Thermoelectric properties of Yb-doped La0.1Sr0.9TiO3 ceramics at high temperature

The effect of ytterbium doping on the thermoelectric properties of La_0.1Sr_0.9TiO₃ ceramics has been investigated at the temperature range between 300 K and 1000 K. Samples with different ytterbium concentrations have been synthesized by the conventional solid state reaction technique. An X-ray diffraction pattern suggests that the dominant crystal structure is of perovskite, with a small amount of pyrochlore phase of Yb₂Ti₂O₇. The electrical resistivities of all samples exhibit a minimum in the temperature range between 300 K and 1000 K. The minimum values of electrical resistivity are 1.5 mΩ cm, 2.0 mΩ cm, 3.9 mΩ cm, 7.1 mΩ cm, and 9.0 mΩ cm for x=0.01, 0.03, 0.05, 0.07, and 0.10, respectively. With the increasing ytterbium content, the electrical resistivity enhanced dramatically, the Seebeck coefficients are increased marginally, and the thermal conductivities are reduced moderately. The lowest thermal conductivity of 3.9 W/mK is obtained in sample of La_0.1Sr_0.89Yb_0.01TiO₃, which exhibits maximum figure of merit 0.20 at 963 K.     

自蔓延高温合成法在陶瓷领域应用的发展

前言 自蔓延高温合成技术（Self—propagating High-temperatuer Synthesis）即SHS，它是许多金属和非金属难熔化合物在燃烧合成时依靠自身放热，来合成材料的技术。某些物质在合成时自身可以放热并利用此热量可以使合成反应继续下去的这种现象在19世纪中、后期被一些科学工作者发现，经过一个多世纪的努力，直到20世纪的五、六十年代人们才开始使其在受控的状态下和冶金机械等技术结合起来，发展成为具有普遍意义的制备材料新技术并应用于工业生产。     

Phase content and dielectrical properties of sintered BaSrTiO ceramics prepared by a high temperature hydrothermal technique

Ba_xSr_1−xTiO₃ ceramic powders with varying barium content were prepared by a high temperature hydrothermal technique and sintered at 1300 °C for 1–8 h. Their dielectrical, phase and structural properties were investigated. It was computed from the XRD spectra that the samples with a small amount of strontium, no more than 10% of the initial Ba:Sr share, had a single phase structure with x = 0.77–0.79 and Curie point T_c = 37–53 °C. Samples with a higher initial Sr ratio developed a two-phase structure and two Curie points. T_c(x) dependence showed that all the experimental data followed a linear trend and were close to the values obtained from the conventional solid state technique, while the dielectric constant was almost one order of magnitude smaller.     

Mechanical properties of zirconia-based ceramics asfunctions of temperature

Commercially available ceramics, MgO–ZrO₂, CeO₂–ZrO₂, and an in-house fabricated zirconia-toughened mullite were examined in this study for use as a structural component in diesel engines. The fast fracture strengths of these materials were measured by loading ASTM C-1161-B specimens in four-point flexure at 30 MPa/s and at 20, 200, 400, 600, and 850 °C. The dynamic fatigue or slow crack growth susceptibility was assessed at 20 and 850 °C by combining the fast fracture strengths with strength data obtained by testing the same specimens in four-point flexure at 0.30 and 0.003 MPa/s stressing rates, as specified in the ASTM C 1368 standard. Fracture toughness was measured following the ASTM C-1421 standard and using chevron notch specimens in three-point flexure at room and elevated temperatures. The strength of the zirconia-toughened mullite was invariant to increases in the temperature and decreases in the loading rate, while the MgO–ZrO₂ and CeO₂–ZrO₂ materials exhibited strength degradation as temperatures increased and the loading rates decreased. Temperature was observed to have the greatest influence on facture toughness. As temperatures increased, the fracture toughness values dramatically decreased for all the materials examined in this study. Improvements in the fracture toughness are needed most for these ceramic materials in order to meet the structural requirements and to develop a more durable and reliable diesel engine component.     

Bioactivity and biodegradability of high temperature sintered 58S ceramics

Bioactive glasses are often considered in bone tissue engineering applications where mechanical strength is essential. As such, bioactive glass scaffolds are often sintered to improve mechanical strength. However, sintering can lead to crystallization, which reduces bioactivity and biodegradability. It has generally been considered that amorphous biomaterials exhibit better bioactivity. However, the in-vitro bioactivity and biodegradability of the sintered 58S made from initial amorphous powder and partially crystalline powder with the same chemical compositions (60SiO₂-36CaO-4P₂O₅ (mol%)) have not been compared before.In this study, 58S bioactive glass (fully amorphous) and glass-ceramic (partially crystallized) powders were synthesized using the sol-gel process, followed by heat-treating at 600 °C for 3 h (calcination). The powders were mixed with carboxymethyl cellulose solution as a binder, shaped in a cylindrical mold, dried, and then sintered at 1100 °C for 5 h. The in-vitro bioactivity and biodegradability of the sintered samples were assessed in simulated body fluid (SBF) for times up to 28 days. The specimens were investigated before and after immersion in SBF using X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). The In-vitro bioactivity and biodegradability rate of the sintered 58S produced from the glass ceramic powder were higher than that from fully amorphous powder. This study shows that the initial structure after calcination is important and affects the subsequent crystallization during sintering. Therefore, crystallinity and formation of hydroxyapatite after calcination are important controlling mechanisms that can increase the bioactivity and biodegradability rate of sintered 58S.     

Improving low temperature degradation of 3Y-TZP ceramics via high temperature carburizing

3Y-TZP ceramics are prepared by solid state method and surface carburization process, and the effect of surface carburization on its the low temperature degradation is studied. The conventional sintered samples completely lost its mechanical properties after aging for 15 h, while the failure time of the surface carburized samples are 300 h. In addition, the nuclear growth rate of the surface carburized samples (αd) and the nucleation rate (Nr) is lower than that of sintered samples, αd plays a dominant role in the degradation process at low temperature and is the key factor determining the aging rate. At the same time, it is found that carbon is dissolved in zirconia lattice in the form of electrically neutral atoms, which will not destroy the original charge balance and produce new oxygen vacancies when entering the interstitial site. More importantly, the precipitation rate of Y³⁺ from zirconia lattice is the key factor to determine the low-temperature phase transition of tetragonal-monoclinic(T-M). The treatment method of surface carburization has significantly improved the low-temperature degradation performance of 3Y-TZP ceramics, which provides a basis for the application of zirconia ceramics in low-temperature and humid environment.     

Excellent electrostrictive properties of low temperature sintered PZT ceramics with high concentration LiBiO2 sintering aid

The effect of LiBiO₂ (LBO) additive on the sintering of Pb_0.97La_0.03(Zr_0.53Ti_0.47)_0.9925O₃ (PLZT) ceramics was carefully investigated. 6.0 wt% LBO added PLZT powders could be fully densified to 98% relative density at a temperature as low as 950 °C. It is worthy to notice that there are distinct enhancements in piezoelectric and electrostrictive properties by increasing the soaking time from 2 h to 7 h, which could mainly originate from the improvement of crystallinity and grain size of PLZT ceramics. By controlling the soaking time and concentration of LBO addition, PLZT ceramics sintered at 950 °C could exhibit high curie temperature of 240 °C and very high S₁₁ of 0.22% under 3.0 kV/mm, which is even better than that of traditionally sintered PZT-5, PMN–PZT, and this is very promising for actuators designed in multilayer structure in high temperature environment.     

Electrocaloric properties of high dielectric constant ferroelectric ceramics

In the last two decades, the electrocaloric (EC) effect which is associated to the temperature (θ) dependence of the macroscopic polarization P(E, θ) under electric field E has been spasmodically studied in ferroelectric materials in order to find an alternative to the classical refrigeratory devices using freon. Basically, large electrocaloric temperature variation ΔT originates from electric field-induced phase transition at the Curie temperature, but temperature changes of the sample are difficult to measure and depend on the experimental conditions. In this paper, the electrocaloric effect has been quantified directly and precisely by measuring the thermal energy exchanged under isothermal conditions using a modified Differential Scanning Calorimetry (DSC) apparatus. The DSC technique allowed to compare the EC properties of high-dielectric-constant (ɛ) ceramics in the vicinity of ferroelectric–paraelectric phase transition. The measurements were also simulated starting from polarization versus electric field hysteresis loops for different temperatures. It is shown excellent agreement between simulations and direct DSC measurements, except in a limited temperature range where the hysteresis of the polarization versus temperature is high.     

Mechanical properties of quartz ceramics at their service temperature

Translated from Steklo i Keramika, No. 10, pp. 21–22, October, 1989.     

A multifluid corresponding states principle for the thermodynamic properties of fluid mixtures

A generalised multifluid corresponding states principle is presented for the correlation and prediction of the thermodynamic properties of fluid mixtures. The multifluid model is an extension of earlier work which was based on the known properties of two (nonspherical) reference fluids. The new model is tested for the prediction of the orthobaric densities of LNG mixtures using all the pure components as the reference fluids. Comparisons with the two-fluid corresponding states method are shown and the advantages and disadvantages of the two methods are outlined. For simple nonpolar mixtures, such as LNG, both methods can give predictions which are well within the accuracy of the experimental data using, at most, only one adjustable coefficient to characterize each binary system.     

Properties of BST ceramics prepared by high temperature hydrothermal process

Barium strontium titanate (BST) was produced in a pressure vessel at 220 °C using the hydrothermal technique. Ba and Sr concentrations, temperature, reaction time and Ti concentration were varied to study the effects of processing on the formation of BST. The chemical composition and crystal structure were analysed by X-ray diffractometry. Peak fitting software was used to separate contributions from different BST phases and to calculate the relative volumes of the phases. From the obtained data Ba losses relative to initial Ba:Sr ratio were estimated. It was found that the amount of Ti was critical for BST phase formation. A small amount of Ti, less than 0.19 mol/l, in a Ba rich solution leads to a single-phase structure, as Sr greatly outperforms Ba. An increase of Ti above 0.225 mol/l decreases Ba losses and creates a solid solution of two phases: one is tetragonal, almost Sr-free, and the other is cubic BST. When the initial concentration of Sr is increased above 30% the obtained BST became triple-phased with different Ba:Sr ratios.