Gas permeation and thermomechanical properties for macroporous alumina focused on necking size at grain boundaries

The gas permeation and thermomechanical properties of macroporous alumina used as a support substrate for microporous ceramic permselective membranes were investigated. The porosity, pore size, and apparent necking size between grains of macroporous alumina were systematically varied, and the relationships between the porous microstructure and material properties were examined. The grain necking size at alumina grain boundaries was evaluated by microstructural observations. The nitrogen gas permeance of the porous alumina increased with increasing pore size. All the measured thermal and mechanical properties decreased with increasing porosity. The properties of porous alumina samples with extensive grain necking showed higher values even in samples with the largest pore size. The high thermal conductivity of porous alumina with extensive grain necking was due to the low interfacial thermal resistance at grain boundaries. Porous alumina with extensive grain necking had high thermal shock strength due to the higher thermal conductivity. It was demonstrated that a porous structure combining high gas permeability and excellent fracture resistance could be successfully achieved.     

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.     

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.     

Alumina/NiCu Laminate under Thermal Shock up to 1000°C: I, Experimental

The mechanical behavior of an alumina/NiCu laminate under thermal shock loading was investigated. The maximum thermal shock temperature was 1000°C. The laminate architecture was the cause of a basic change in the cracking mechanisms, manifested in a dramatic increase in the mechanical residual strength over that of monolithic alumina. The laminated system was constructed by alternating alumina layers coated with copper films with nickel interlayers and joining them by a combination of liquid-state (brazing) and solid-state (diffusion) bonding. The material system was tested by water quenching square-shaped laminated specimens initially at temperatures of up to 1000°C. Three-point bending tests revealed the mechanical strength before and after thermal shock, and SEM analysis described the damage mechanisms and the extent of debonding at the alumina/NiCu interfaces.     

Influence of a Dispersion of Aluminum Titanate Particles of Controlled Size on the Thermal Shock Resistance of Alumina

The possibility of developing fine-grained (∼0.5–3 μm) and dense (≥0.98ρ_th) alumina (90 vol%)–aluminum titanate (10 vol%) composites with improved thermal shock resistance and maintained strength is investigated. One alumina material and one composite with similar microstructures (porosity and grain-size distribution) were fabricated to investigate the effect of Al₂TiO₅ on thermal shock behavior. The size of the Al₂TiO₅ particles was kept under 2.2 μm to avoid spontaneous microcracking. The mechanical and thermal properties of the materials involved in their response to thermal shock and the results for the evolution of indentation cracks of equal initial crack length with increasing Δ T in samples quenched in glycerine are described. The combination of thermal and mechanical properties—thermal conductivity, thermal expansion coefficient, Young's modulus, and toughness—improve the thermal shock resistance of the alumina–aluminum titanate composite in terms of critical temperature increment (>30%). The suitable structural properties of alumina—hardness and strength—are maintained.     

Fabrication and thermal conductivity of highly porous alumina body from platelets with yeast fungi as a pore forming agent

A highly porous alumina body was fabricated by heating a green clinker body consisting of platelets and yeast fungi as a pore forming agent. Four kinds of alumina platelets were used. When green clinker bodies of platelet aggregates (A11) with 10 and 30 mass% of yeast fungi were heated at 1500 °C for 2 h, their porosities reached 72% and 78%, respectively. In contrast, when the green clinker bodies composed of platelets with an average size of 10 µm and an aspect ratio of 25–30 (SERATH①), and 20 mass% of yeast fungi were heated at 1400 °C for 2 h, the porosity of the resultant porous alumina body was also approximately 72%. However, the room temperature thermal conductivities of the porous alumina bodies with 72% porosity derived from A11 and SERATH① were 0.86 and 0.50 W m⁻¹ K⁻¹, respectively. The decrease in the thermal conductivity of the porous alumina body produced from SERATH① is caused by the long path route for heat transfer.     

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.     

Energy efficient lightweight periclase-magnesium alumina spinel castables containing porous aggregates for the working lining of steel ladles

This study presents a new lightweight periclase-magnesium alumina spinel castable (LPSC) for the working lining of steel ladles using porous periclase-spinel aggregates to replace conventional dense magnesia aggregates. The porous periclase-spinel aggregates were produced by an in-situ decomposition technique resulting in an apparent porosity of 23.3% and a median pore size of 5.66?μm. Scanning electron microscopy revealed a better porous aggregate/matrix interface bonding in the LPSC, which significantly improved its strength and thermal shock resistance. Additionally, the higher amount of micropores of the porous aggregates in the LPSC absorbed more penetrated slag from the matrix, which enhanced the slag resistance. Thus, compared with conventional castables, the LPSC had a lower bulk density of 9.2–10.8% and a lower thermal conductivity of 18.8% (1000?°C) while at the same time a higher strength, thermal shock resistance and slag resistance was achieved.     

密集烤烟房用氧化铝-堇青石换热陶瓷材料的制备

本文以矿物原料制备了氧化铝-堇青石换热陶瓷材料,研究了其密度、抗热震性能和热导率等性能,并将其用于烤烟生产工艺中。研究结果表明,随着温度的提高,样品的热导率也有所提高,烧结收缩率也增大;随着堇青石含量的增加,铝矾土含量的降低,样品热导率先增加后降低,并在堇青石含量为20%,1300℃温度下烧结时达到最大值4.69W/(m·K)。此时样品的密度为2.78g/cm3,抗热震性能良好。     

Enhanced ionic conductivity and thermal shock resistance of MgO stabilized ZrO2 doped with Y2O3

The present work focused on the effect of Y₂O₃ co-doping on the phase composition, microstructure, ionic conductivity and thermal shock resistance of 8 mol% MgO stabilized ZrO₂ (Mg-PSZ) electrolyte ceramics for high temperature applications. The addition of Y₂O₃ could promote the process of monoclinic-to-cubic/tetragonal phase transformation and became the metastable phase at room temperature. Meanwhile, the grain size of Mg-PSZ decreased. It was demonstrated that an appreciable increase in the ionic conductivity and compressive strength occurred on substituting MgO with Y₂O₃ in the Mg-PSZ electrolyte ceramics across the measured temperature range. Moreover, the Y₂O₃ addition could restrain the adverse effect of the cyclic thermal shock on the ionic conductivity and compressive strength of Mg-PSZ. The main reason was that the increase of the amount of monoclinic phase caused by cubic/tetragonal-to-monoclinic phase transformation by the cyclic thermal shock was restrained after the Y₂O₃ addition.     

纳米改性Ti(C,N)基金属陶瓷的力学性能及抗热震性能

真空烧结制备了Ti(C,N)基金属陶瓷,测试了不同金属黏结相成分的纳米TiN改性Ti(C,N)基金属陶瓷的力学性能及抗热震性能.力学实验结果表明:金属相含量越多,材料的强度和断裂韧性越高,而硬度则越低.金属相含量相同时,Ni能提供更高的强度与韧性,而Co能带来更高的硬度.热震试验结果表明:热循环温度较低时,40%TiC-10%TiN-15%WC-14%Mo-20%Ni-1%C.50%TiC-10%TiN-15%WC-4%Mo-10%Ni-10%Co-1%C和50%TiC-10%TiN-15%WC-4%Mo-20%Co-1%C(质量分数,下同)3组试样缺口处裂纹的形成均存在一定的孕育期,随着循环温度的升高,孕育期明显缩短,裂纹的扩展速率加快;与金属相为4%Mo-20%Co的金属陶瓷抗热震性能相比,4%Mo-10%Ni-10%Co的较好,14%Mo-20%Ni的最好.扫描电镜观察表明:微孔洞的连通形成裂纹,裂纹主要沿陶瓷相晶界及金属相扩展.     

Novel camphene/photopolymer solution as pore-forming agent for photocuring-assisted additive manufacturing of porous ceramics

This study proposes camphene/photopolymer solutions as a novel pore-forming agent for the photocuring-assisted additive manufacturing of porous ceramics. Unlike conventional techniques using molten camphene, solid camphene can be directly dissolved in the photocurable monomer hexanediol diacrylate (HDDA) at room temperature, which can then crystallize with a dendrite-like morphology based on phase separation at lower temperatures. This unique approach allows alumina suspensions to solidify at ―2 °C and then effectively be photopolymerized using a digital light processing engine, resulting in camphene-rich crystals surrounded by photopolymerized alumina/HDDA walls. Sintered samples exhibited a highly porous structure, with the pores created after the removal of the camphene-rich crystals. Two different pore sizes were obtained in the lower and upper regions of a single layer, due to a decrease in the solidification rate along the building direction, although their porosities were similar (～ 52 vol%). The porous samples exhibited a compressive strength of ～ 265 MPa.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Synthesis of meso-macro alumina using yeast cells as a bio-template and optimization using a genetic algorithm

Meso-Macro porous alumina was fabricated using yeast cells as a pore-forming agent. Alumina powder synthesis was achieved by a low cost process (recrystallisation of alum).The effect of the pore forming agent on the true porosity, bulk density and thermal conductivity of porous alumina was characterized. The results show that the true porosity increased with the increasi ng addition of yeast cells. The bulk density and thermal conductivity at room temperature decreased with the increasing yeast addition. A genetic algorithm method was used to minimize the thermal conductivity of the macro-porous alumina based on the amount of yeast cells used, the sintering temperature, and the hold time. The genetic algorithm found that the best thermal conductivity achievable was equal to 0.152 Watt/m. °C at 20wt% concentration of yeast, a sintering temperature of 1230°C and 1.5 hours of soaking time. The experimental value was 0.14 Watt/m. °C and the slight variance between these values were postulated to be due to experimental error in the measurements.     

Thermal shock resistance of the porous boron nitride/silicon oxynitride ceramic composites

The thermal shock resistance of the porous boron nitride/silicon oxynitride (BN/Si₂N₂O) ceramic composites were tested by the quenching‐strength method with temperature differences of 600‐1400°C. The residual flexural strength of the composites decreased with increasing temperature difference from 600°C to 900°C. This weakening in flexural strength was attributed to the formation of microcracks in the matrix caused by thermal stress damage. Afterward, as the formation of a dense oxidized layer sealed the surface and hindered further oxidation, the residual flexural strength increased with the further increase of temperature difference from 900°C to 1100°C. Finally, when the temperature differences were above 1100°C, the residual flexural strength gradually decreased with increasing temperature difference, which was attributed to the further oxidation and large thermal stress damage. And the thermal shock resistance of the porous BN/Si₂N₂O ceramic can be improved by the introduction of high contents of sintering aids and h‐BN.     

丝网印刷条件对陶瓷发热片抗热震性的影响

本文探讨了各种丝网印刷条件对氧化铝陶瓷发热片热震性能的影响。陶瓷的热震性是表征材料承受温度骤变的一种能力,是材料的综合机械-热性能。陶瓷发热片的抗热震性基本取决于材料本性,但其独特的工艺决定了丝网印刷的诸多条件,包括印刷线路设计、印刷网版膜厚、印刷方向、印刷刮刀行程、印刷压力等,对陶瓷发热片的热震性也有不可轻视的影响。     

Strength and thermal shock resistance of fiber-bonded Si-Al-C-O and Si-Ti-C-O ceramics

Silicon carbide-based fiber-bonded ceramics, obtained from hot pressing of woven silicon carbide fibers, are a cost-effective alternative to ceramic-matrix composites due to their ease of fabrication, involving few processing steps, and competitive thermomechanical properties. In this work, we studied the high-temperature strength and thermal shock resistance of Si-Al-C-O and Si-Ti-C-O fiber-bonded SiC ceramics obtained from hot pressing of two types of ceramic fibers, by mechanical testing in four-point bending. The bending strength of Si-Al-C-O-based fiber-bonded ceramics at room temperature is ∼250–260 MPa and remains constant with temperature, while the bending strength of Si-Ti-C-O increases slightly from the initial 220 to ∼250 MPa for the highest temperature. Both materials retain up to 90% of their room temperature strength after thermal shocks of 1400°C and show no reduction in elastic moduli. After thermal shock, failure mode is the same as in the case of as-received materials.     

Repeated thermal shock behavior of ZrB2-SiC-graphite composite under prestress

The thermal shock resistance of ZrB₂-SiC-graphite composite under nominal prestress of 0, 20, 30, 40 or 50 MPa after subjected to 10 and 30 cycles of thermal shock was evaluated by measuring the residual flexural strength of the tested specimen. In each test the applied prestress kept constant and in each cycle the specimen center was heated to 2000 °C within 5 s through electrical resistance heating method and cooled down naturally to room temperature. A lot of broken SiO₂ bubbles in the tested specimens were observed with a SEM. For the specimen subjected to 10 cycles of thermal shock, the residual flexural strength does not show big change under different levels of prestress, although the thickness of oxide layer increases at larger prestress, which is presumably attributed to the effect of the oxide layer that heals the cracks and the pores and enhances the strength. For the specimen subjected to 30 cycles of thermal shock, the residual strength decreases, in general, with the increase of prestress level. The thermal shock fatigue under different levels of prestress was also tested, and it was found that the increase of prestress may speed the failure of the specimen, indicating that the level of prestress may fatally affect the failure of the material.     

Microscopic regulation of plant morphological pores on mechanical properties of porous mullite materials

Generally, a multilayer structure is present inside a walnut shell, and the residual structure of the walnut shell is retained after impregnation and firing. When the walnut shell is used as a pore-forming agent, this structure helps in improving the mechanical and thermal insulation properties of the lightweight porous materials. In this study, porous mullite materials (PMMs) with plant morphological structure pores were prepared using a-Al₂O₃ and silica powder as the raw materials with addition of sol-impregnated walnut shell powder (WSP). The influence of sol type and firing temperature on the pore structure of the PMMs was analyzed, which affected the compressive strength and thermal conductivity. The plant morphological porous structure was observed in the samples after sol impregnation. After firing at different temperatures, the porous structure gradually contracted and supported the pores, improving the mechanical properties, while the complex porous structure increased the heat conduction path, thereby improving the insulation performance. Using WSP impregnated with silica-sol and zirconia-sol as pore-forming agents, PMMs with higher compressive strength and relatively low thermal conductivity (TC) were prepared.     

高铝质强化日用瓷的研制

通过引入工业氧化铝及复合熔剂,在接近普通硬质瓷的烧成温度下,生产出抗折强度为140MPa、釉面硬度达7373MPa和高热稳定性的刚玉质强化瓷。