Thermal shock resistance of rare-earth doped in-situ SiAlON reinforced h-BN matrix ceramics under vacuum thermal cycling

In-situ SiAlON reinforced BN-matrix ceramics were prepared by hot pressing sintering, and the effects of different rare earth oxides on the thermal shock resistance of the materials were investigated. The effects of rare earth oxides on the phase composition, microstructure, bending strength, thermal properties and thermal shock resistance of the composites were studied. The results show that the phase composition and bending strength of ceramics with different rare earth oxides had no obvious change. However, the influence on the thermal expansion coefficient of the material was notable. The thermal expansion coefficient of the ceramics with CeO₂ increased by 24.6% compared with Sm₂O₃ in the test temperature range. After 50 cycles of thermal shock at Δt = 1150 °C, the residual strength of ceramics with CeO₂ was down to 157.1 MPa, decreased by 40.6% compared with the one tested in room temperature. And the Sm₂O₃-added ceramics reduced by 34.7%–167.1 MPa after thermal shock. The decrease of the residual strength of ceramics is mainly caused by the internal stress generated by the mismatch between the growth of quartz and SiAlON phase in the matrix and the thermal expansion coefficient of the matrix. However, no macro cracks were observed on the surface of the samples after thermal shock.     

In situ synthesis and thermal shock resistance of mullite used for thermal storage ceramics in solar thermal power generation

Abstract

Mullite ceramic, as one of high performance thermal storage ceramics for solar thermal power generation systems, was in situ fabricated via semidry pressing and pressureless sintering in the air. Andalusite (57–68 wt-%) and calcined bauxite (24–29 wt-%) were used as the raw materials, with kaolin and a tiny of boric acid being added to promote the densification and improve the mechanical properties. The best physical properties and thermal shock resistance were obtained on an optimum A3 sample sintered at 1600°C for 3 h, i.e. a bending strength of 120·44 MPa and 30 cycles thermal shock cycling without cracking (wind cooling from 1000°C to room temperature) with a loss of bending strength of 8·7%.     

Preparation and Performance Study of Mullite/Al2O3 Composite Ceramics for Solar Thermal Transmission Pipeline

For lowering sintering temperature of mullite/Al₂O₃ composite ceramics for solar thermal transmission pipeline, kaolin, potassium feldspar, quartz, and γ‐Al₂O₃ were used as raw materials to in situ synthesize the composite ceramics with pressureless sintering method. Densification, mechanical properties, thermal expansion coefficient, thermal shock resistance, phase composition, and microstructure were investigated. The experiment results demonstrated that the introduction of potassium feldspar and quartz decreased the lowest sintering temperatures greatly to 1300°C. The optimum sample A3 sintered at 1340°C obtained the best performances. The water absorption, apparent porosity, bulk density, bending strength, and thermal expansion coefficient of A3 were 0.04%, 0.12%, 2.71 g/cm³, 94.82 MPa, and 5.83 × 10^?6/°C, respectively. After 30 thermal shock cycles (wind cooling from 1100°C to room temperature), no cracks were observed on the surfaces of the sample, and the bending strength increased by ?7.96%. XRD analysis indicated that the main phases of samples before and after 30 thermal shock cycles were consistently mullite, corundum, and α‐cristobalite, while the content of mullite increased after thermal shock. SEM micrographs illustrated that the mullite grains growth and micro‐cracks appeared after thermal shock endowed the composite ceramics with excellent thermal shock resistance.     

Effects of holding time on structure and the properties of spodumene/mullite composite ceramics

In this paper, spodumene/mullite ceramics with good thermal shock resistance were prepared from spodumene, quartz, talc, and clay when the sintering temperature was 1270℃. In the sintering process, the effect of holding time on densification, mechanical properties, phase transformation, microstructure, and thermal shock resistance of the composite ceramics were investigated. The phase transition and microstructures of the ceramics were identified via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The interaction between holding time and bulk density was studied by response surface methodology. The result show that an appropriate holding time can improve the mechanical properties of spodumene/mullite ceramics. When the holding time was kept 90 min, the spodumene/mullite ceramics with the apparent porosity was .47%, the bulk density was 2.28 g/cm³, and bending strength was 63.46 MPa. Furthermore, since no cracks formed after 20 thermal shock cycles for the composite ceramics with a bending strength decreasing rate of 12.66%, it is revealed that spodumene/mullite ceramics exhibit good thermal shock resistance. Therefore, this study can provide beneficial guidance for both industrial production and energy conservation.     

Investigating the effect of andalusite on mechanical strength and thermal shock resistance of cordierite-spodumene composite ceramics

For increasing working stability of cordierite-spodumene composite ceramics for solar heat transmission pipeline, andalusite was utilized as modified additive to improve mechanical strength and thermal shock resistance of the composite ceramics. The effects of andalusite on densification, mechanical strength, thermal stability, phase composition and microstructure were studied. The experiment results showed that andalusite significantly influenced bending strength and thermal shock resistance of the composite ceramics. Especially, specimen B1 with 5 wt% andalusite sintered at 1400 °C achieved the best performances. The linear shrinkage, water absorption, apparent porosity, bulk density and bending strength were 5.62%, 0.02%, 0.06%, 2.19 g cm^?3 and 104.94 MPa, respectively. After 30 thermal shock cycles (wind cooling from 1100 °C to room temperature), the residual strength of the specimen increased to 110.65 MPa, accompanying with ?5.44% strength loss rate. The XRD and SEM analysis illustrated that mullite grains with short rod-like shape could prevent crack growth of inter-granular fracture to enhance bending strength of the specimens. Furthermore, the generation of β-spodumene grains with low thermal expansion coefficient after thermal shock improved thermal shock resistance of the composite ceramics. It is considered that the cordierite-spodumene composite ceramics with high densification, good mechanical strength and excellent thermal stability can be a potential material for high temperature thermal transmission pipeline in solar thermal power generation.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Influence of a Piezoelectric Secondary Phase on Thermal Shock Resistance of Alumina-Matrix Ceramics

Using monolithic alumina ceramics as the reference, the thermal shock behavior of the hot-pressed alumina matrix ceramics with 3 mol% neodymium titanate was investigated. The thermal shock resistance of the materials was evaluated by water quenching and a subsequent three-point bending test to determine the flexural strength degradation. The hot-pressed composite exhibited a temperature differential of the thermal shock resistance 120°C higher than the monolith, mainly because of significantly improved fracture toughness.     

Preparation and thermal shock resistance of solar thermal absorbing corundum ceramics for concentrated solar power

The absorptivity of solar thermal absorber materials affects the heliothermal conversion efficiency of concentrated solar power systems. The solar absorbing ceramics were prepared by the fixed mixture of bauxit, Fe₂O₃, and TiO₂ with adding CuO in different percentages. The absorptivity and thermal shock resistance with the effect of adding CuO in different percentages were studied. Fe₂O₃ and TiO₂ have excellent optical properties, and CuO decreases the material's band gap to boost the electronic transition and increase the material absorptivity. The results showed that the material is sintered at 1380°C with an excellent absorptivity of 94.00% in the spectrum range of 0.3–2.5 μm, and the bending strength is 132.94 MPa. The bending strength was increased by 21.07% after 30 thermal shock cycles (1000°C-room temperature, air cooling). The liquid phase facilitates the synthesis of hercynite with excellent high temperature properties. The hercynite improves the thermal shock resistance of the material.     

烧结助剂对低温制备碳化硅多孔陶瓷性能的影响

分别采用十二烷基苯磺酸钠、氢氧化钠以及NaA分子筛残渣为烧结助剂,碳粉为造孔剂,干压法成型,在1150℃空气气氛下烧结制备碳化硅多孔陶瓷支撑体。考察了助剂添加量对微结构、平均孔径、孔隙率以及抗热震性等方面的影响;分析了添加助剂的低温烧成机理。研究结果表明：三种添加剂均有助于提高支撑体的气体渗透性、抗弯强度和耐热震性;添加NaA分子筛残渣助烧结剂获得的碳化硅多孔陶瓷各项性能最佳,气体渗透率为1300 m³/(m²·h·kPa),强度可达27 MPa,且抗热震性能良好。     

Preparation and characteristics of porous anorthite ceramics with high porosity and high-temperature strength

High-temperature properties including compressive strength, thermal shock behavior, and thermal conductivity of porous anorthite ceramics with high specific strength were tested and analyzed. The results showed that the prepared materials merit high-temperature compressive strength, thermal stability, and conductivity. With the appropriate fabrication parameters, even though containing 0.33 g/cm³ bulk density and 88.2% porosity, its compressive strength could reach 2.03 MPa at 1000°C, 147% of that at room temperature; the residual strength ratio kept as 114.7% after a thermal shock at 1200°C. The X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) showed that anorthite grains refinement and intergranular voids filling by liquid phase were main factors for the high strength. From room temperature to 1200°C, its thermal conductivity only varied from 0.085 to 0.258 W·(m·K)⁻¹. High porosity, a large number of nanoregions in anorthite grains and amorphous phase in grain boundary were main reasons for low thermal conductivity.     

The microstructures and properties of Fe2O3 and TiO2 co-doped corundum ceramics for solar thermal absorbing materials

Solar thermal absorbing materials are the key components of concentrating solar power. In this study, Fe₂O₃ and TiO₂ co-doped corundum ceramics were prepared by pressureless sintering. The effects of different Fe₂O₃/TiO₂ ratios on the phase composition, microstructure, thermal shock resistance and solar absorptance were investigated via XRD and EPMA testing. The results showed that, with the decrease of Fe₂O₃/TiO₂ ratio, the appropriate amount of FeAlTiO₅ would decompose into ferrite particles, which played a bridging role between the corundum grains making the samples have excellent thermal shock resistance. A6 (90% bauxite, 9.5% Fe₂O₃ and 0.5% TiO₂) sintered at 1460 °C had the optimum comprehensive properties, with a bending strength of 154.80 MPa and an absorptance of 89.20% in the spectral range from 0.3 to 2.5 μm. After 30 thermal shock cycles (1000 °C–25 °C, air-cooled), the bending strength of A6 was 222.05 MPa, and the absorptance was 90.40%, which were 43.44% and 1.35% higher than those before thermal shock, respectively. Therefore, it was suitable as an excellent solar thermal absorbing materials.     

Preparation and thermal stability investigation of Al2O3–mullite–ZrO2–SiC composite ceramics for solar thermal transmission pipelines

To obtain composite ceramics with excellent thermal shock resistance and satisfactory high?temperature service performance for solar thermal transmission pipelines, SiC additive was incorporated into Al₂O₃?mullite?ZrO₂ composite ceramics through a pressureless sintering process. The effect of the SiC additive on thermal shock resistance was studied. Also, the variations in the microstructure and physical properties during thermal cycles at 1300 °C were discussed. The results showed that both thermal shock resistance and thermal cycling performance could be improved by adding 20 wt% SiC. In particular, the sample with 50 wt% Al₂O₃, 35 wt% Coal Series Kaolin (CSK), 15 wt% partially yttria?stabilized zirconia (PSZ), and 20 wt% SiC additional (denoted as sample A2) exhibited the best overall performance after firing at 1600 °C. Furthermore, the bending strength of sample A2 increased to 124.58 MPa, with an increasing rate of 13.63% after 30 thermal shock cycles. The increase in thermal conductivity and the formation of mullite were the factors behind the enhancement of thermal shock resistance. During the thermal cycles, the oxidation of SiC particles was favorable as it increased the microstructure densification and also facilitated the generation of mullite, which endowed the composite ceramics with a self?reinforcing performance.     

Excellent mechanical property of fast hot pressure sintering CaZr4(PO4)6 low thermal expansion ceramics

The low mechanical property of the CaZr₄(PO₄)₆ (CZP) ceramics restrict its application prospects in area of high anti-thermal shock applications. Herein, the fast hot pressure sintering (FHP) was adopted to modify the microstructure and optimize the mechanical property of CZP ceramics for the first time. The as-fabricated CZP ceramics exhibited smaller grain size and lower stomatal rates than that of the normal-pressure sintering ceramics owing to the lower processing temperature and shorter insulation time. Consequently, the CZP ceramics prepared with FHP method demonstrated an unprecedented bending strength of 94.5 MPa, with the thermal expansion coefficient of the CZP ceramics remaining at a low level. This study is important for promoting the application of CZP ceramics in flood anti-thermal shock applications, the fast hot pressure sintering would trigger a new direction of improvement methods of sodium zirconium phosphate ceramics.     

Cyclic thermal shock resistance of h-BN composite ceramics with La2O3–Al2O3–SiO2 addition

The effects of La₂O₃–Al₂O₃–SiO₂ addition on the thermal conductivity, coefficient of thermal expansion (CTE), Young's modulus and cyclic thermal shock resistance of hot-pressed h-BN composite ceramics were investigated. The samples were heated to 1000 °C and then quenched to room temperature with 1–50 cycles, and the residual flexural strength was used to evaluate cyclic thermal shock resistance. h-BN composite ceramics containing 10 vol% La₂O₃–Al₂O₃ and 20 vol% SiO₂ addition exhibited the highest flexural strength, thermal conductivity and relatively low CTE, which were beneficial to the excellent thermal shock resistance. In addition, the viscous amorphous phase of ternary La₂O₃–Al₂O₃–SiO₂ system could accommodate and relax thermal stress contributing to the high thermal shock resistance. Therefore, the residual flexural strength still maintained the value of 234.3 MPa (86.9% of initial strength) after 50 cycles of thermal shock.     

Vibration Assisted Hot‐Press Sintering of AlN Ceramics

AlN ceramics is very difficult to be sintered due to its strong covalent bonds and low self‐diffusion. In this work, a new hot‐press sintering system assisted by vibration was used to densify AlN ceramics. During hot‐pressing process an additional vibration caused by pressure fluctuation is imposed on the sample. The well‐densified AlN ceramics had been obtained at 1750°C under 15 MPa, or at 1800°C under 10 MPa which were much milder than traditional hot‐press conditions and would be beneficial to prolong working life of the graphite mold. The samples made by this method have a bending strength about 320 MPa. After annealing, it shows a thermal conductivity of 217 W/mK.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.     

In situ synthesis and thermal shock resistance of a cordierite-mullite composite for solar thermal storage

Cordierite-mullite composite ceramic was synthesized in situ by semidry pressing and pressureless sintering from andalusite, kaolin, γ-Al₂O₃, talc, potassium feldspar, and albite in air. The effects of composition and sintering temperature on the density, bending strength, thermal shock stability, crystal phases, and microstructure of the specimens were studied. The results show that specimen B2 (the theoretical content of cordierite was 20 wt%) has excellent performance, that is, a bending strength of 104.59 MPa, 30 cycles of thermal shock resistance without cracking, and a loss rate of 13.12%. X-ray diffractometer (XRD) analysis and scanning electron microscope (SEM) micrographs showed that spherical cordierite crystals were grown on the surface of the mullite, therefore, the specimen possessed a superior bending strength and thermal shock resistance, where a great number of granules combined to restrain crack initiation as well as propagation over time during the thermal shock test. The thermal conductivity of specimen B2 was determined to be 3.83 W/(m·K) (36°C), and the sensible heat storage density was 1136 kJ/kg, with the temperature difference (ΔT) ranging from 0 to 800°C. Consequently, the cordierite-mullite composite is a potentially applicable material for solar thermal storage.     

Alumina/NiCu Laminate under Thermal Shock up to 1000°C: II, Prediction of the Laminate Behavior

In this investigation, a multilayered, multimaterial system with strong interface subjected to thermal shock loading was analyzed. The analysis was based on a one-dimensional spatio-temporal finite difference scheme of the temperature field, and the thermal residual stresses and zero misfit stress temperature were considered. Using a failure criterion based on crack initiation, the number of broken layers due to thermal shock and residual mechanical strength at room temperature could be predicted. Furthermore, the room temperature residual strength of the laminate as a function of thermal shock temperature was constructed, demonstrating steplike behavior. Using this model, the mechanical behavior of the alumina/NiCu laminate system subjected to thermal shock loading of up to 1000°C was predicted. The model revealed the superiority of this material system over monolithic ceramics under thermal shock conditions.     

Preparation and thermal shock resistance of anorthite solar thermal energy storage ceramics from magnesium slag

Anorthite solar thermal energy storage ceramics were fabricated from magnesium slag solid waste by pressureless sintering. The effects of CaO/SiO₂ ratio and sintering temperature on the physical, chemical, and thermophysical properties of ceramics were explored. X-ray diffraction results demonstrated that thermal shock process contributed to the formation of anorthite, and increasing CaO/SiO₂ ratio promoted the transformation of anorthite (CAS₂) into melilite (C₂AS). Some micro-cracks were found according to SEM analysis, forming by the mismatch of thermal expansion coefficients among phases. The combined effects of the low thermal expansion coefficient of anorthite and micro-crack toughing endowed the ceramic with good thermal shock resistance. Optimum comprehensive performances were observed in the sample with a CaO/SiO₂ ratio of 0.58 sintered at 1160°C, of which the specific thermal storage capacity was 0.63 J·g^-1·°C^-1(room temperature). The bending strength increased by 0.22% after 30 thermal shock times (room temperature-800°C, wind cooling). Therefore, the anorthite ceramics exhibited great potential for solar thermal energy storage.     

Thermal shock behavior of Ti2AlC from 200°C to 1400°C

The thermal shock behavior of Ti₂AlC synthesized by means of self‐propagating high‐temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti₂AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al₂O₃. Although the flexural strength decreases quickly in the temperature range of 300°C‐500°C, no discontinuous decrease in the retained strength is observed in Ti₂AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti₂AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared with the first quench. Quenching at 300°C and above, voids after the pullout of grains and cracks are present, which however are absent in the un‐quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock.