Preparation and Thermal Shock Resistance of Mullite Ceramics for High Temperature Solar Thermal Storage

Mullite thermal storage ceramics were prepared by low-cost calcined bauxite and kaolin.The phase composition,microstructure,high temperature resistance and thermophysical properties were characterized by modern testing techniques.The experimental results indicate that sample A3(bauxite/kaolin ratio of 5:5)sintered at 1620℃has the optimum comprehensive properties,with bulk density of 2.83 g·cm^(-3)and bending strength of 155.44 MPa.After 30 thermal shocks(1000℃-room temperature,air cooling),the bending strength of sample A3 increases to 166.15 MPa with an enhancement rate of 6.89%,the corresponding thermal conductivity and specific heat capacity are 3.54 W·(m·K)^(-1)and 1.39 kJ·(kg·K)^(-1)at 800℃,and the thermal storage density is 1096 kJ·kg^(-1)(25-800 mullite ceramics;sintering properties;high-temperature thermal storage;thermal shock resistance).Mullite forms a dense and continuous interlaced network microstructure,which endows the samples high thermal storage density and high bending strength,but the decrease of bauxite/kaolin ratio leads to the decrease of mullite content,which reduces the properties of the samples.

Preparation and Thermal Shock Resistance of Mullite and Corundum Co-bonded SiC Ceramics for Solar Thermal Storage

Mullite and corundum co-bonded SiC-based composite ceramics(SiC-mullite-Al2O3)were prepared by using SiC,calcined bauxite and kaolin via pressureless carbon-buried sintering.The low-cost SiC-based composite ceramics designed in this study are expected to be used as thermal storage materials in solar thermal power generation based on the high density and excellent thermal shock resistance.The influences of calcined bauxite addition and sintering temperature on the microstructures,phase compositions,and physical properties of the samples were investigated.Results demonstrated that the introduction of calcined bauxite containing two bonding phases greatly reduced the lowest sintering temperature to 1400℃.The SiC-mullite Al2O3 composite with 40 wt%calcined bauxite sintered at 1500℃exhibited optimum performance.The density and bending strength were 2.27 g·cm^-3 and 77.05 MPa.The bending strength increased by 24.58%and no cracks were observed after 30 thermal shock cycles,while general clay would reduce the thermal shock resistance of SiC.The SiC-mullite-Al2O3 composites with satisfied performance are expected to be used as thermal storage materials in solar thermal power generation systems.

Thermal shock fatigue behavior of TiC/Al_2O_3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference （ΔT） and number of thermal cycles （Ⅳ） on fatigue crack growth （Δa）. The mechanical properties and thermal fatigue resistance of TiC/Al203 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a ＂true＂ cycling effect （thermal fatigue）. Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.

Effect of Nano-ZrO_2 on Microstructure and Thermal Shock Behaviour of Al_2O_3/SiC Composite Ceramics Used in Solar Thermal Power

The Al2O3-ZrO2（3Y）-SiC composite ceramics used in solar thermal power were prepared by micrometric Al2O3,nano-ZrO2 and SiC powders under the condition of pressureless sintering.The bulk density and bending strength of samples with 10vol% nano-ZrO2 sintered at 1480℃ were 3.222 g/cm3 and 160.4MPa,respectively.The bending strength of samples after 7 times thermal shock tests （quenching from 1000℃ to 25℃ in air medium） is 132.0MPa,loss rate of bending strength is only 17%.The effect of nano-ZrO2 content on the microstructure and performance of Al2O3-ZrO2（3Y）-SiC composite ceramic was investigated.The experimental results show that the bending strength of samples with above 10vol% nano-ZrO2 content has decreased,because the volume expansion resulting from t-ZrO2 to m-ZrO2 phase transformation is excessive;Adding proper nano-ZrO2 would be contributed to improve the thermal shock resistance of the composite ceramics.The Al2O3-ZrO2（3Y）-SiC composite ceramic has promising potential application in solar thermal power.

Thermal Shock Resistance of Nano SiC-BN Composites

Nano SiC - BN composite powders were prepared by dissolving analytically pure H3BO3 and CO（ NH2 ）2 with the mole ratio of 1：2.5 in the absolute alcohol, adding 80% E-SiC with 0. 2 μm average grain size while stirring, firing at 850 ℃ in nitrogen （purity： 99. 99%, pressure： O. 92 -0. 93 MPa） for 15 h. Nano SiC -BN composite specimens were prepared by hot-pressed sintering the nano SiC - BN composite powder in N2 atmosphere with 0. 92 - 0. 93 MPa and at 30 MPa axial pressure for 0. 5 - 1 h at 1 750 - 1 800 ℃. The thermal shock resistance of nano SiC -BN composites was studied by three-point bending, TEM and SEM. The results show that, adding BN can decrease the modulus of elasticity of SiC materials, which improves thermal shock resistance;furthermore, because of the large difference of thermal expansion coefficient between matrix SiC and second phase hexa-BN, thermal mismatch effect results in intercrystalline delamination of h-BN grains and forming many micropores in composite ceramic, which can relax the thermal expansion caused by high tempera- ture effectively, and improve the thermal shock resistance significantly.

Effects of mechanical boundary conditions on thermal shock resistance of ultra-high temperature ceramics

The effects of mechanical boundary conditions, often encountered in thermalstructural engineering, on the thermal shock resistance（TSR） of ultra-high temperature ceramics（UHTCs） are studied by investigating the TSR of a UHTC plate with various types of constraints under the first, second, and third type of thermal boundary conditions. The TSR of UHTCs is strongly dependent on the heat transfer modes and severity of the thermal environments. Constraining the displacement of the lower surface in the thickness direction can significantly decrease the TSR of the UHTC plate, which is subject to the thermal shock at the upper surface. In contrast, the TSR of the UHTC plate with simply supported edges or clamped edges around the lower surface is much better.

Design of Composite Ladle Shroud for Improving Thermal Shock Resistance

A ladle shroud is one of the functional refractories for continuous casting,which undergoes severe thermal shock by molten steel when used without pre-heating.The composite ladle shroud with an insulating liner presents excellent thermal shock resistance.Finite element simulation is an effective method to explore the maximum thermal stress for predicting the thermal shock resistance of ladle shrouds.In this paper,the influence of the lining materials and the structure of ladle shrouds on the thermal stress distribution is systematically researched.The working mechanism of the lining material on the body material is also presented.Lining materials with low thermal expansion,elastic modulus and thermal conductivity are helpful to improve the thermal shock resistance and an optimum lining thickness is suggested.The lining material can both serve as thermal resistance for the body material to buffer the thermal stress,and apply a strain load to the body material by the thermal strain to increase the stress.

Properties and appropriate conditions of stress reduction factor and thermal shock resistance parameters for ceramics

Through introducing the analytical problem of the plate with convection into the solution of the transient heat conduction thermal stress field model of the elastic plate, the stress reduction factor is presented explicitly in its dimensionless form. A new stress reduction factor is introduced for the purpose of comparison. The proper- ties and appropriate conditions of the stress reduction factor, the first and second ther- mal shock resistance （TSR） parameters for the high and low Biot numbers, respectively, and the approximation formulas for the intermediate Blot number-interval are discussed. To investigate the TSR of ceramics more accurately, it is recommended to combine the heat transfer theory with the theory of thermoelasticity or fracture mechanics or use a numerical method. The critical rupture temperature difference and the critical rup- ture dimensionless time can be used to characterize the TSR of ceramics intuitively and legibly.

Mechanical Properties and Thermal Shock Resistance of SrAl_(2)Si_(2)O_(8) Reinforced BN Ceramic Composites

Hexagonal boron nitride(h-BN)ceramics have become exceptional materials for heat-resistant components in hypersonic vehicles,owing to their superior thermal stability and excellent dielectric properties.However,their densification during sintering still poses challenges for researchers,and their mechanical properties are rather unsatisfactory.In this study,SrAl_(2)Si_(2)O_(8)(SAS),with low melting point and high strength,was introduced into the h-BN ceramics to facilitate the sintering and reinforce the strength and toughness.Then,BN-SAS ceramic composites were fabricated via hot press sintering using h-BN,SrCO_(3),Al_(2)O_(3),and SiO_(2) as raw materials,and effects of sintering pressure on their microstructure,mechanical property,and thermal property were investigated.The thermal shock resistance of BN-SAS ceramic composites was evaluated.Results show that phases of as-preparedBN-SAS ceramic composites are h-BN and h-SrAl_(2)Si_(2)O_(8).With the increase of sintering pressure,the composites’densities increase,and the mechanical properties shew a rising trend followed by a slight decline.At a sintering pressure of 20 MPa,their bending strength and fracture toughness are(138±4)MPa and(1.84±0.05)MPa·m^(1/2),respectively.Composites sintered at 10 MPa exhibit a low coefficient of thermal expansion,with an average of 2.96×10^(-6) K^(-1) in the temperature range from 200 to 1200℃.The BN-SAS ceramic composites prepared at 20 MPa display higher thermal conductivity from 12.42 to 28.42 W·m^(-1)·K^(-1) within the temperature range from room temperature to 1000℃.Notably,BN-SAS composites exhibit remarkable thermal shock resistance,with residual bending strength peaking and subsequently declining sharply under a thermal shock temperature difference ranging from 600 to 1400℃.The maximum residual bending strength is recorded at a temperature difference of 800℃,with a residual strength retention rate of 101%.As the thermal shock temperature difference increase,the degree of oxidation on the ceramic surface and cracks due to thermal stress are also increased gradually.

Preparation of micro-arc oxidation coatings on magnesium alloy and its thermal shock resistance property

In the NaAlO2-Na2SiO3 compound system, the ceramic coatings were prepared on magnesium alloy by micro-arc oxidation. The morphology, phase composition, and thermal shock resistance of the ceramic coatings were studied by scanning electron microscope, X-ray diffraction and thermal shock tests, respectively. The results showed that the ceramic coating contains MgO, MgAl2O4, as well as a little amount of MgESiO4. The thickness of the ceramic coatings increases with the current density increasing, when the current density is 12 A·dm^-2, the thermal shock resistance of the produced ceramic coating is the best. The hardness of the ceramic coating is up to 10 GPa or so.

Relationship Between Thermal Shock Behavior and Cutting Performance of a Functionally Gradient Ceramic Tool

Based on the deep understanding of the requirements of cutting conditions on ceramic tools, a design model for functionally gradient ceramic tool materials with symmetrical composition distribution was presented in this paper, according to which an Al 2O 3-TiC functionally gradient ceramic tool material FG-1 was synthesized by powder-laminating and uniaxially hot-pressing technique. The thermal shock resistance of the Al 2O 3-TiC functionally gradient ceramics FG-1 was evaluated by water quenching and subsequent three-point bending tests of flexural strength diminution. Comparisons were made with results from parallel experiments conducted using a homogeneous Al 2O 3-TiC ceramics. Functionally gradient ceramics exhibited higher retained strength under all thermal shock temperature differences compared to homogeneous ceramics, indicating the higher thermal shock resistance. The experimental results were supported by the calculation of transient thermal stress field. The cutting performance of the Al 2O 3-TiC functionally gradient ceramic tool FG-1 was also investigated in rough turning the cylindrical surface of exhaust valve of diesel engine in comparison with that of a common Al 2O 3-TiC ceramic tool LT55. The results indicated that the tool life of FG-1 increased by 50 percent over that of LT55. Tool life of LT55 was mainly controlled by thermal shock cracking which was accompanied by mechanical shock. While tool life of FG-1 was mainly controlled by mechanical fatigue crack extension rather than thermal shock cracking, revealing the less thermal shock susceptibility of functionally gradient ceramics than that of common ceramics.

Effect of MnO2 on Properties of SiC-mullite Composite Ceramics for Solar Sensible Thermal Storage

For improving the properties of SiC-mullite composite ceramics used for solar sensible thermal storage, MnO2 was introduced as sintering additive when preparing. The composite ceramics were synthesized by using SiC, andalusite, a-Al2O3 as the starting materials with non-contact graphite-buried sintering method. Phase composition and microstructure of the composites were investigated by XRD and SEM, and the effect of MnOz on the properties of SiC composites was studied. Results indicated that samples SM1 with 0.2 wt% MnO2 addition achieved the optimum properties： bending strength of 70.96 MPa, heat capacity of 1.02 J.（g.K）-1, thermal conductivity of 9.05 W-（m.K）-1. Proper addition of MnO2 was found to weaken the volume effect of the composites and improve the thermal shock resistance with an increased rate of 27.84% for bending strength after 30 cycles of thermal shock （air cooling from 1 100 ℃ to RT）. Key words： SiC-mullite composite ceramics; MnO2; solar sensible thermal storage; non-contact graphite-buried sintering; thermal shock resistance

Effects of Additive CeO_2 on Thermal Shock Resistance of Plasma-Sprayed Cr_2O_3 Coating

The influence of CeO2 with different content on the thermal shock resistance of plasmasprayed Cr2O3 coating was investigated. The thermal shock failure mechanism of coating was also studied. It is found that the thermal shock failure mechanism of coating is thermal stress fatigue destruction, and the destruction takes place at interface of ceramic and bond coating. The experimental results show that the lifetime of coating fracture and failure increase considerably when 3% CeO2 is added into the plasma-sprayed Cr2O3 coating. The suitable content of CeO2 makes the microcracks exist in network form. The microcrack net can release internal stress in coating, delay the crack forming and expanding and decrease holes in coating. Thus the thermal shock resistance increase largely.

Residual Thermal Stresses of Laser Cladding of Intermetallic-Ceramic Composite Coatings

The stresses in laser cladding of Ni3Al-WC composite coating co and in heat affect zone (HAZ) σh have been induced based on considering the influences of laser processing parameters power P and beam traverse speed v.According to the calculated results, certain limits of P and v are necessary in order to obtain crack free coatings. It agrees well with the experimental results.

Research Progress on Thermal Shock Behavior of Porous Ceramics

Thermal-mechanical coupling effect causes a large stress within porous ceramics at high temperatures,resulting in strength attenuation and product reliability decline.Thermal shock resistance is one of the key factors to characterize the reliability of ceramic materials under thermal-mechanical coupling effect.It is important to study the thermal shock resistance of the porous ceramics to evaluate their service performance and improve their service life.To better evaluate the effect of the thermal shock resistance on properties of the porous ceramics,the evaluation theories,experimental characterization methods and influencing factors about the thermal shock performance of the porous ceramics were reviewed in this paper,and some future research directions were prospected.

Preparation and Characterization of Andalusite Ceramic Used for Solar Thermal Power Generation

High-temperature thermal storage material is one of the critical materials of solar thermal power generation system. Andalusite, kaolin, talc, γ-Al2O3 and partially stabilized zireonia were used as the raw materials, and in-situ synthesis of cordierite was adopted to fabricate thermal storage material for solar thermal power generation via pressureless sintering. The phase compositions, microstructures and thermal shock resistances of the sintered samples were analyzed by XRD, SEM and EDS, and the corresponding mechanical properties were measured. The results show that the major phases of the samples are mullite and zirconium silicate, and the pores distribute uniformly. After being sintered at 1 460℃C, A4 sample exhibits a better mechanical performance and thermal shock resistance, its loss rate of bending strength after 30 cycles thermal shock is 3.04%, the bulk density and bending strength are 2.86 g.cm^-3 and 139.66 MPa, respectively. The better thermal shock resistance of the sample is closely related to the effect of zirconium silicate, such as its uniform distribution, nested growth with mullite, low thermal expansion coefficient, high thermal conductivity, etc. This ceramic can be widely used as one of potential thermal storage materials of solar thermal power generation system.

Preparation and Characterization of Mullite-Silicon Carbide Heat Absorbing Ceramics for Solar Thermal Tower Plant

The in-situ synthesized mullite bonded SiC ceramics for solar thermal tower plant were prepared from Silicon carbide （SIC）, manufactured aluminum hydroxide （Al（OH）3） and Suzhou kaolin via semi-dry pressing and pressureless firing. The results indicate that sample B3 （designed mullite content 15 wt%） fired at 1 400 ℃ exhibited optimal performance with a bending strength of 97.41 MPa. Sample B3 can withstand 30-cycles thermal shock without cracking （wind cooling from 1 100 ℃ to room temperature）, and the bending strength after thermal shock decreased by 17.92%. When the service temperature is 600℃, the thermal diffusivity, specific heat capacity, thermal conductivity and heat capacity are 6.48× 10-2 cm：.s-1, 0.69 kJ·kg-1. K-1, 9.62 W·m-1·K-1 and 977.76 kJ·kg-1, respectively. The XRD and SEM results show that SiC, mullite, or-quartz, and tridymite are connected closely, which gives the material a good bending strength. After 30-time thermal shock cycles, the structure of samples becomes loose. SiC grains are intersectingly arranged with rodshape mullite, exhibiting a favorable thermal shock resistance. The addition of Al（OH）3 and Suzhou kaolin can accelerate the synthesis of mullite, thus to reduce the firing temperature effectively. The volume effect of tfidymite is relatively small, improving the thermal shock resistance of materials. A higher designed muUite content yields a lower loss rate of bending strength. The mullite content should not be more than 15 wt% or else the bending strength would be diminished.

Efficient fabrication of light Cf/SiHfBOC composites with excellent thermal shock resistance and ultra-high-temperature ablation up to 1800℃

In this paper,a high-yield Hf-modified SiHfBOC ceramic precursor was developed,and a high-pressure assisted impregnation pyrolysis method was proposed to achieve the preparation of 3D PyC–Cf/SiHfBOC composites.This high-pressure assisted impregnation method significantly improves impregnation filling effect of the precursor in and between fiber bundles compared to dozens of traditional impregnation cycles.After undergoing just 9 precursor infiltration pyrolysis(PIP)cycles,the composites achieved relative density of approximately 90%and density of 1.64 g/cm^(3).The critical temperature difference of the 3D PyC–Cf/SiHfBOC composites after the shock of room temperature(RT)–1000℃is as high as 650℃,which is twice that of traditional ceramic materials,showing good thermal shock resistance.Under the effect of Hf modification,a dense HfO_(2)–SiO_(2)oxide layer(thickness of 93μm)was formed in situ on the surface of the 3D PyC–Cf/SiHfBOC composites,effectively preventing further erosion of the composite matrix by high-temperature oxidation gas.Even in the ultra-high-temperature oxygen-containing environment at 1800℃,it still exhibits an excellent non-ablative result(with a linear ablation rate of 0.83×10^(−4)mm/s).This work not only enriches the basic research on lightweight ultra-high-temperature ceramic composites converted from Hf ceramic precursors,but also provides strong technical support for their applications in ultra-high-temperature non-ablative thermal protection materials for high-speed aircraft.

偏组成堇青石陶瓷材料结构与性能

堇青石陶瓷具有热膨胀系数低、抗热震性好、耐高温等特点,被广泛应用于高低温急剧变化的环境。如何进一步降低堇青石陶瓷的热膨胀系数,提升其使用性能,仍然是国内外研究的热点,以单层片状结构的苏州高岭土、滑石和γ-Al_(2)O_(3)为原料,设计了不同的配比组成并制备了堇青石低膨胀陶瓷样品。采用XRD、SEM、CTE和EPMA等测试技术对样品的结构和性能进行了表征。探究了偏化学计量不同配比组成对合成堇青石陶瓷的结构与性能的影响。结果表明:偏镁样品(Si/Al为1.34时)合成的是单一的纯相堇青石;样品的热膨胀系数为1.91×10^(-6)℃^(-1) (25℃~800℃),抗折强度为50 MPa,气孔率为20%,体积密度为1.90 g·cm^(-3)。风冷热震循环试验(25℃~1000℃) 30次后抗折强度损失率仅为2.66%。偏镁组成中的MgO促进了高温液相的生成,液相传质推动高温化学反应充分进行,提高了样品中的堇青石含量(达到了99 wt.%),生成的堇青石晶粒定向排列,降低了样品的热膨胀系数,提高了样品的抗热震性。

氟化钇对粉煤灰基堇青石陶瓷合成及性能的影响

本文以粉煤灰、滑石、氧化铝和氧化镁为原料,加入质量分数为0%、0.5%和1.0%的氟化钇,通过高温烧结制备了粉煤灰基堇青石陶瓷样品,研究了氟化钇掺量对样品的物相组成、显微结构及主要性能的影响。结果表明,氟化钇的引入可降低堇青石的生成温度,提高堇青石的物相转化率,显著改善材料的力学性能,同时对热学性能亦有一定积极影响。当氟化钇添加量为0.5%时,样品的综合性能最优。当烧结温度为1300℃时,XRD未见明显杂质峰,样品的体积密度为2.15 g/cm^(3),开孔孔隙率为7.49%,吸水率为3.49%,压缩强度为252.27 MPa,经5次热震试验后压缩强度仍然高至204.65 MPa,比热容为0.770~0.931 J/(g·℃)(25~150℃),热膨胀系数为2.54×10^(-6)℃^(-1)(100~800℃)。与不含添加剂的样品相比,添加0.5%氟化钇样品的体积密度增加了5.2%,开孔孔隙率降低了58.6%,吸水率降低了60.6%,而压缩强度增加了4.6倍,比热容增强而热膨胀系数降低。在粉煤灰基堇青石陶瓷的烧结过程中,氟化钇的引入可使其在较低的温度下形成液相,提高元素迁移率,促进晶粒发育完全且具有细化晶粒的明显效果。氟化钇作为一种有效的烧结助剂,微量的添加即可显著改善粉煤灰基堇青石陶瓷的综合性能。本工作拓宽了高效烧结助剂选择范围,可为高质量陶瓷材料的高温烧结提供一定参考。