Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures

Porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels were prepared by freezing YSZ/tert-butyl alcohol (TBA) slurry under different freezing temperatures of ?30, ?78 and ?196 °C, respectively. After removing the frozen TBA via freeze-drying in vacuum at ?50 °C, the green samples were sintered at 1450 °C for 2 h in air. The results showed that the freezing temperature significantly influenced microstructure and properties of the porous YSZ ceramics. Both microstructure observation and pore size distribution indicated that the pore channel size decreased significantly with decreasing freezing temperature, regardless of microstructure variations in the individual sample. Both porosity and room-temperature thermal conductivity of the porous YSZ ceramics varied under different freezing temperatures. Regardless of microstructure variations in the samples under different freezing temperatures, all samples had unidirectional pore channels with increasing pore channel size along the freezing direction. The fabricated samples had remarkably low thermal conductivities both in directions perpendicular and parallel to the channel direction, thus rendering them suitable for applications in thermal insulations.     

Improved Heat Insulation and Mechanical Properties of Highly Porous YSZ Ceramics After Silica Aerogels Impregnation

To further improve heat insulation and mechanical properties, silica aerogels were impregnated into highly porous yttria‐stabilized zirconia (YSZ) ceramics with well‐distributed pores fabricated by tert‐butyl alcohol ‐based gel‐casting process and pressureless sintering. Pore size distribution, room‐temperature thermal conductivities, and compressive strength of the YSZ ceramics before and after impregnation with silica aerogels were examined and compared, respectively. After impregnating porous YSZ ceramics with silica aerogels, the porosity displayed a little decrease, whereas the pore size significantly decreased by one order of magnitude. Based on this microstructure development, the room‐temperature thermal conductivities were significantly lowered and the compressive strength was also promoted. Therefore, the heat insulation and mechanical properties could be simultaneously improved by impregnating porous ceramics with silica aerogels.     

High porous yttria-stabilized zirconia with aligned pore channels: Morphology directionality influence on heat transfer

High porous yttria stabilized zirconia with unidirectionally aligned channels is used in engineering applications with extremely low thermal conductivity. This property is strongly influenced by microstructure features such as pore volume fraction, pore size distribution, random porous microstructure and pore morphology directionality. Although several models are reported in the available literature, but their analytical formulas are formalised for homogeneous structures or they are based on proportion between solid and fluid phases. These differences from real microstructures cause significant computational errors especially when thermal conductivity changes as the function of the measurement direction (parallel or perpendicular). In this context, the application of an intermingled fractal unit's procedure capable of reproducing porous microstructure as well as predicting thermal conductivity has been proposed. The results are in agreement with experimental ones measured for parallel and perpendicular directions and suggest improving the formalisation of fractal modelling in order to obtain an instrument of microstructure design.     

Porous SiCnw/SiC ceramics with unidirectionally aligned channels produced by freeze-drying and chemical vapor infiltration

The current study introduces a methodology for the fabrication of porous silicon carbide nanowire/silicon carbide (SiC_nw/SiC) ceramics with macroscopic unidirectionally aligned channels and reports on their microstructural and mechanical properties. The material was produced by freezing of a water-based slurry of β-SiC nanowires (SiC_nw) with control of the ice growth direction. Pores were subsequently generated by sublimation of the columnar ice during freeze-drying. Chemical vapor infiltration (CVI) of SiC into the open pore network of the SiC_nw aerogel with unidirectionally aligned channels, resulted in the formation of highly porous SiC_nw/SiC ceramics which exhibited a unique microstructure as identified by scanning electron microscopy. The pore size distribution and the mechanical properties of the as-fabricated porous ceramics were examined by mercury intrusion porosimetry and three-point bending and compression tests, respectively, while phase composition was investigated through X-ray diffraction.     

Preparation of high-strength lightweight alumina with plant-derived pore using corn stalk as pore-forming agent

To improve the mechanical and thermal insulation properties of lightweight alumina, which was prepared by using pore-forming agent from biological sources, the silica sol-infiltrated corn stalk was utilized. Spring back and hygroscopicity of corn stalk powder as well as cold compressive strength, thermal conductivity, microstructure, and pore size distribution of lightweight alumina were characterized. The results indicate that impregnation of silica sol leads to different degrees of decrease in spring back height due to achieving better mesh by silica gel between the corn stalk powders, and then improves the formability, although at the same time the large number of hydrophilic groups results in an increase in hygroscopicity. Furthermore, sol impregnated pore-forming agent optimizes the microstructure of the lightweight alumina pores. Lightweight alumina with a cold compressive strength up to 48.64 MPa was produced, and with the silica sol concentration of 3 wt%, lower thermal conductivity values at all test temperatures were obtained. Hence, the use of corn stalk impregnated with the appropriate concentration of silica sol as pore-forming agent could enhance the mechanical and thermal insulation properties of lightweight alumina for the spheroidization of pore shape, randomization of pore distribution as well as miniaturization of pore size.     

Microstructure and gas permeation performance of porous silicon nitride ceramics with unidirectionally aligned channels

Porous Si₃N₄ ceramics with unidirectionally aligned channels were prepared via freezing ceramics suspension with distinct solid contents under different freezing temperatures. The samples obtained using lower solid content in ceramic suspension at higher freezing temperature exhibit larger Darcian and non-Darcian constants due to their higher open porosity, larger pore size and lower tortuosity. Moreover the investigation on individual contributions of viscous energy losses and inertial energy losses on the total pressure drop during permeation process indicates that with decreasing the solid content or the freezing temperature the viscous energy losses increase but the inertial energy losses decrease for samples owing to the differences in their pore structures. It is worth mentioning that porous Si₃N₄ ceramics with unidirectionally aligned channels exhibit larger Darcian and non-Darcian constants than those with similar pore size distributions and open porosity owing to their lower tortuosity, thus rendering them appropriate for filters and membrane supports.     

Porous alumina ceramics with enhanced mechanical and thermal insulation properties based on sol-treated rice husk

To improve the properties of porous alumina ceramics, which were typically prepared by adding pore-forming agents, rice husk (RH) as pore-forming agent was pretreated with zirconia sol. The effects of sol-treatment on the thermal conductivity and compressive strength of the resultant ceramics were characterized. Furthermore, the pore size distribution, pore shape, microstructure, and phase evolution also were studied. The results showed that the RH pretreatment optimizes the microstructure of the ceramic pores. Moreover, complete morph-genetic RH is clearly observed in the pores, which is established as a key factor in improving the properties of the resultant ceramic. The thermal insulation properties are determined to significantly improve, although the thermal conductivity increases slightly with the increment of zirconia sol concentration from 5 to 10?wt%. Meanwhile, after sintering at 1550?°C, the compressive strength is significantly greater for the specimen prepared with 10?wt% zirconia sol-treated RH (65.56?MPa) than that with untreated RH (43.37?MPa). Hence, it was demonstrated that the use of zirconia sol-pretreated RH as a pore-forming agent could enhance the mechanical and thermal insulation properties of porous alumina ceramics.     

Preparation of homogeneous mullite‐based fibrous ceramics by starch consolidation

Silica sol is one of the frequently used binders in high‐temperature resisting fibrous porous ceramics, but in the drying process, it can diffuse with water and influence the uniformity of ceramics. To solve this diffusion problem and fabricate homogeneous fibrous porous ceramics, cationic starch was firstly introduced in mullite fibrous system. The effects of starch content and high‐temperature binder content on microstructure, physical, and mechanical properties were also investigated. The results indicate that starch consolidated mullite fibrous ceramics owned a homogeneous 3D skeleton structure, since the introduced starch can absorb both water and silica particles by gelatinization and ensure the even distribution of binders. Compared with the mullite fibrous ceramics fabricated without starch addition, starch consolidated ceramics owned better microstructure and higher interior compression strength. Furthermore, both the starch and silica sol content had great impact on the microstructure, density, porosity, thermal conductivity, and compressive strength of the fibrous ceramic.     

Processing and properties of silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity

Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity were prepared by sintering nano-SiC powder-carbon black template compacts at 600–1200 °C for 2 h in air. The microstructure of the silica-bonded porous nano-SiC ceramics consisted of SiC core/silica shell particles, a silica bonding phase, and hierarchical (meso/macro) pores. The porosity and thermal conductivity of the silica-bonded porous nano-SiC ceramics can be controlled in the ranges of 8.5–70.2 % and 0.057–2.575 Wm⁻¹ K⁻¹, respectively, by adjusting both, the sintering temperature and template content. Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity (0.057 Wm⁻¹ K⁻¹) were developed at a very low processing temperature (600 °C). The typical porosity, average pore size, compressive strength, and specific compressive strength of the porous nano-SiC ceramics were ∼70 %, 50 nm, 2.5 MPa, and 2.7 MPa·cm³/g, respectively. The silica-bonded porous nano-SiC ceramics were thermally stable up to 1000 °C in both air and argon atmospheres.     

Synthesis and characterization of a xonotlite fibers–silica aerogel composite by ambient pressure drying

Xonotlite fibers (XFs) reinforced silica aerogel composites were prepared by a sol–gel method under ambient pressure drying. XFs were synthesized through a dynamic hydrothermal route and had a noodle-like structure with length of 5–10 μm and average diameter of 150–200 nm. The microstructure analysis showed that XFs were inlaid in silica aerogel matrix by physical combination which contributed to restrict the volume shrinkage of alcogels and maintain the integrality aerogels during drying process. The physical, naonporous and thermal properties of the as prepared aerogel composites were investigated and discussed in detail. The new aerogel composites possessed porous nanostructure, which exhibited typical properties of 0.126 g/cm³ density, 4.132 cm³/g pore volume, and thermal conductivity of 0.0285 W/(m K). The results indicated that the introduced XFs didn’t significantly alter the porosity, hydrophobicity or thermal conductivity of aerogel matrix. It was also found that the aerogel composites had much more outstanding porosity than that of pure aerogel upon calcinations at 800 °C. This study fabricated XFs–silica aerogel composites and explored a new way for silica aerogels to endure and remain monolithic under ambient pressure drying.     

Effect of sintering temperature on compressive strength of porous yttria-stabilized zirconia ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were fabricated by tert-butyl alcohol (TBA)-based gel-casting method for potential applications in heat-insulation materials. The effect of sintering temperature on compressive strength of porous YSZ ceramics was investigated on the basis of measurements linear shrinkage, porosity and pore size. As the sintering temperature increased from 1350 to 1550 °C, a decrease of porosity from 77 to 65%, a decrease of average pore size from and an increase of linear shrinkage from 15.4 to 31.8% were observed. The compressive strength increased remarkably from 3 to 27 MPa with increasing sintering temperature from 1350 to 1550 °C, which was related to the corresponding change of linear shrinkage, porosity, pore size and microstructure. A remarkable decrease of compressive strength with increasing porosity was observed. The compressive strength decreased also with increasing pore size.     

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.     

纳米孔超级绝热材料气凝胶的制备与热学特性

以正硅酸四乙酯(TEOS)为硅源，通过溶胶–凝胶及超临界干燥过程制备了SiO2气凝胶. 同时，采用相对廉价的多聚硅(E–40)为硅源，以三甲基氯硅烷(TMCS)为表面修饰剂，硅油为干燥介质, 在常压条件下制备了同样具有纳米多孔结构的SiO2气凝胶. 用透射电镜、扫描电镜及孔径分布仪对其结构进行了表征，并用动态热线法对其热学特性进行了测试. 结果表明: 两种方法制备的气凝胶均是典型的纳米孔超级绝热材料，后者热导率略高但成本低许多，所以更具应用推广潜力.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Moisture-Resistant and Mechanically Strong Polyimide-Polymethylsilsesquioxane Hybrid Aerogels with Tunable Microstructure

Moisture-resistant and mechanically strong polyimide (PI)-polymethylsilsesquioxane hybrid aerogels with doubly cross-linked structures are synthesized through sol–gel technology and supercritical CO₂ fluid drying. By using bis(trimethoxysilylpropyl) amine as a cross-linker, the end-capped polyamide acid derived from biphenyl-3,3′,4,4′-tetracarboxylic dianhydride and 4,4′-oxydianiline is cross-linked with a silica network using methyltrimethoxysilane as the silica source precursor. The resultant hybrid aerogels show low density (0.12–0.15 g cm⁻³), low thermal conductivity (0.032–0.049 W m⁻¹ K⁻¹), high hydrophobicity (125–140°) and good thermal stability (above 435 °C) with tunable microstructure. With the increase of silica sol volume, the microstructure of hybrid aerogels transforms from fibrous network to hierarchical microstructure. Aerogels with high content of silica sol exhibit good moisture resistance, high Young's modulus (Max. 19.6 MPa), and high specific modulus (Max. 131 kN m kg⁻¹), which are attributed to their unique hierarchical microstructure with a sheet skeleton. These hybrid aerogels are promising in the fields of thermal insulation, aerospace applications and so on.     

二氧化硅气凝胶的气相热导率模型分析

气凝胶是一种超级隔热材料,具有极低的整体热导率。气凝胶的纳米多孔网络结构极大限制了气体分子热运动,使得气凝胶中的气相热导率低于自由气体的气相热导率。本文介绍并讨论了气凝胶气相热导率的基本理论和模型,考察了孔径尺度和气凝胶固相骨架对气相热导率的影响。结果表明,气凝胶气相热导率随气压和孔径的减小而迅速降低,随气凝胶密度的增大而降低。当压力极低时,气凝胶的气相热导率远低于常压下大空间的静止空气。气凝胶纳米固体网格对气相热导率存在重要影响,在(0.01～100)×10⁵ Pa的压力范围内影响尤其显著。     

Synthesis and physicochemical characterization of silica aerogels by rapid seed growth method

Thanks to the high optical transparency and ultra-low thermal conductivity, silica aerogels are ideal materials for energy-saving windows. However, their preparation is commonly based on either one-step base-catalyzed method, or two-step acid-base catalyzed method, which is difficult to inhibit the aggregation of clusters while keeping the size of clusters as small as possible and thus degrading silica aerogel's properties. Here, a new idea for synthesizing silica aerogels is presented from the viewpoint of controlling the growth and aggregation of silica clusters. A certain amount of Tetramethoxysilane (TMOS) used as seed precursor is firstly added into the mixture of methanol and distilled water for hydrolysis. A certain time later, the additional TMOS and a defined amount of ammonia are added to the obtained sol for promoting the rapid formation of the gel in several minutes. The silica aerogels prepared by this method have higher optical transparency and lower thermal conductivity than those prepared by the other two methods. This approach may also shed substantial light on controlling the microstructure of other materials prepared by sol-gel process.     

Effects of mono-dispersed PMMA micro-balls as pore-forming agent on the properties of porous YSZ ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were successfully fabricated by the dry pressing method with different size (1.8–20 μm) and amount (2–60 vol.%) of mono-dispersed poly methyl methacrylate (PMMA) micro-balls. Different PMMA additions with different size and amount were investigated to achieve optimal thermal and mechanical properties. With increases of the amount of PMMA, the porosity of porous YSZ ceramics ranges from 7.29% to 51.6%, the flexural strength increases firstly and then decreases, and the thermal conductivity decreases continuously. With decreases of the diameter of PMMA micro-balls, the mean pore size and thermal conductivity of porous YSZ ceramics decrease, and the flexural strength of porous YSZ ceramics with same porosity increases firstly and then decreases. The porous YSZ ceramics with a higher porosity (18.44 ± 1.24%), the highest flexural strength (106.88 ± 3.2179 MPa) and low thermal conductivity (1.105 ± 0.15 W/m K) can be obtained when the particle diameter and the amount of PMMA are 5 μm and 20 vol.%, respectively.     

Laser surface modification of porous yttria stabilized zirconia against CMAS degradation

Here, we present a new combined freeze-casting and laser processing method for the design of yttria-stabilized zirconia (YSZ) based thermal-barrier coatings. YSZ ceramics with unidirectionally-aligned pore channels were created using the freeze-casting method. After sintering, top view and cross-sectional scanning electron microscopy (SEM) revealed the structural features of the preform, which exhibits a 74 ± 2% volume fraction of porosity and an average pore channel size of 30 ± 3 μm. The measured thermal conductivity of this porous structure was 0.27 ± 0.02 W/(m K), which is eight times lower than that of reported values for dense YSZ. Though high porosity is beneficial both from a structural and thermal response perspective, the open porosity could potentially be an issue from an application stand-point when evaluating the resistance of materials to calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS attack, which can originate from deposits of molten sand, ash, and dust, is one of the major causes of thermal barrier coating failure. Therefore, the surface of the porous samples was modified using a laser process to create a barrier to CMAS infiltration. SEM micrographs aided in determining the optimum laser parameters required to fully seal the surface using a laser treatment. The performance of the original porous and surface-modified YSZ was compared by conducting CMAS infiltration studies. Laser modification was shown to be a viable technique to significantly reduce CMAS infiltration in porous thermal barrier coatings.     

Carbothermal reduction synthesis of high porosity and low thermal conductivity ZrC-SiC ceramics via an one-step sintering technique

In this work, porous ZrC-SiC ceramics with high porosity and low thermal conductivity were successfully prepared using zircon (ZrSiO₄) and carbon black as material precursors via a facile one-step sintering approach combining in-situ carbothermal reduction reaction (at 1600 °C for 2 h) and partial hot-pressing sintering technique (at 1900 °C for 1 h). Carbon black not only served as a reducing agent, but also performed as a pore-foaming agent for synthesizing porous ZrC-SiC ceramics. The prepared porous ZrC-SiC ceramics with homogeneous microstructure (with grain size in the 50–1000 nm range and pore size in the 0.2–4 µm range) possessed high porosity of 61.37–70.78%, relatively high compressive strength of 1.31–7.48 MPa, and low room temperature thermal conductivity of 1.48–4.90 W·m^?1K^?1. The fabricated porous ZrC-SiC ceramics with higher strength and lower thermal conductivity can be used as a promising light-weight thermal insulation material.