Porous YSZ ceramics with unidirectionally aligned pore channel structure: Lowering thermal conductivity by silica aerogels impregnation

The previous report of this work has demonstrated the fabrication and properties of porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels. As a follow-up study, the present work aims at lowering the thermal conductivity of the porous YSZ ceramics by silica aerogels impregnation. The porous YSZ ceramics were immersed in an about-to-gel silica sol. Both the unidirectionally aligned pore channels and the inter-grain pores by grain stacking in the channel-pore wall of the porous YSZ ceramics were impregnated with the silica sol. After aging and supercritical drying, silica aerogels formed in the macroporous network of the porous YSZ ceramics with unidirectionally aligned pore channels. The influences of silica aerogel impregnation on the microstructure and properties of porous YSZ ceramics with unidirectional aligned pore channels were investigated. The porosity decreased after impregnation with silica aerogels. Both microstructure observation and pore size distribution indicated that both channel-pore size and inter-grain pore-size decreased significantly after impregnation with silica aerogels. Impregnating porous YSZ ceramics with silica aerogels remarkably lowered the room-temperature thermal conductivity and enhanced the compressive strength. The as-fabricated materials are thus suitable for applications in bulk thermal isolators.     

Ultralight and resilient Al2O3 nanotube aerogels with low thermal conductivity

This report describes a novel method to produce fluffy and resilient nanotube aerogels by combining solution blow spinning and Atom Layer Deposition (ALD). Polyvinylpyrrolidone (PVP) sponges obtained by blow spinning are used as templates and are deposited in ALD. After removal of template, semitransparent aerogels whose density can be as low as 0.68 mg/cm³ were obtained. The product is heat‐stable, with the ability to retain original shape and keep elastic after being kept at 900°C for 2 hours. It is also heat‐insulated, with a thermal conductivity of 0.022 W/K?m at room temperature. Additionally, when compressed to 60% of the original height for 100 cycles during in‐situ mechanical test, the sponges recovered to around 80% of the original shape, further indicating excellent mechanically elasticity of the aerogel.     

Constructing secondary-pore structure by in-situ synthesized mullite whiskers to prepare whiskers aerogels with ultralow thermal conductivity

Ultralow thermal conductivity and ultralight mullite fibers/mullite whiskers composite aerogels (MF/MW) with secondary-pore structure have been prepared via vacuum impregnation and high-temperature treatment. The in-situ generation of mullite whiskers during vapour-solid reaction process and the mechanism of improving thermal stability have been discussed in detail. Under catalysis condition at 1200 °C, the zero-dimensional nanoparticles of SiO₂-Al₂O₃ aerogels are guided to in-situ transform into one-dimensional mullite whiskers. The secondary-pore structure formed by the overlapped fibers and whiskers in MF/MW reduces the thermal conductivity [as low as 0.0488 W/m^?1 K^?1 compared with that of MF preform (0.0698 W/m^?1 K^?1)] and exhibits excellent thermal stability after 1400 °C heat treatment (0.0503 W/m^?1 K^?1) due to the macropores are decreased and gaseous heat transfer being further weakened effectively. Moreover, the MF/MW exhibits good mechanical performance with high critical compressive stress of 0.2809 MPa, which is more than 317% higher than that of MF preform (0.0673 MPa) at room temperature.     

Inhibited radiation transmittance and enhanced thermal stability of silica aerogels under very-high temperature

Silica aerogels are applied extensively in thermal insulation due to their extra-low density and thermal conductivity. However, this can be heavily undermined under high temperature, especially beyond 700–800 °C, due to not only a high transparency for radiation heat transfer, but also an instability caused by nanoparticle collapse. The present study demonstrated a solution to this problem by two steps. Firstly, the size effect of nanoparticles is explored by experimentally preparing unconventional samples composed of large-diameter particles (larger than 20 nm). The prepared sample microstructures and radiative characteristics are investigated by SEM apparatus and FTIR measurements, respectively. Additionally, based on the fractal structures reconstructed using the Diffusion-Limited Colloid Aggregation (DLCA) method, the effects of particle size on thermal stability of silica aerogels are theoretically investigated and the light shielding performance is numerically analyzed by the Generalized Multiparticle Mie-solution (GMM) method. The simulation and experimental results indicate that the enhanced size of nanoparticles can improve the radiative inhibition and thermal stability property under high temperature significantly. However, the ideal above should be refined as it brings an increasing contribution of conduction heat transfer. Thus, a novel structure of core-mantle nanoparticle is proposed to retain the excellent resistance in radiation but avoid an increased thermal conduction. The present work may pave a new direction for developing aerogels under extreme temperature conditions, compared with the widely-used opacifier addition.     

二氧化硅气凝胶的制备和表征

以正硅酸四乙酯为硅源，通过采取老化、表面修饰、溶剂置换和分级干燥等一系列抑制二氧化硅气凝胶干燥中出现缩裂的有效措施，以非超临界干燥技术最终获得了大块无裂纹的二氧化硅气凝胶。该气凝胶的孔径较小且分布均匀，比表面积为684m^2／g，孔体积可达1．38cm^3／g，最可几孔径为3．221nm，平均孔径达2．871m。同时在实验和理论分析的基础上总结二氧化硅气凝胶缩裂的主要原因和抑制缩裂的有效措施。     

Thermal conductivity of low density carbon aerogels

Carbon aerogels with densities ranging from 0.182 to 0.052?g/cm³, pore sizes ranging from 88 to 227?nm, and particle diameters ranging from 20 to 13?nm were prepared. Thermal conductivity measurements by laser flash method indicate that the lowest thermal conductivity can be obtained at a density of 0.066?g/cm³, in the temperature range from 100 to 300?°C in air. The lowest thermal conductivity is 0.0263?W/m?K at 200?°C. The characteristic density, at which the lowest thermal conductivity can be obtained, is temperature dependent. At a higher temperature, a higher density carbon aerogel will be more efficient in the reduction of the total conductivity by reducing the radiative conductivity.     

Flexible, low-density polymer crosslinked silica aerogels

Polymerization of a di-isocyanate with the amine-modified surface of a sol-gel derived mesoporous silica network crosslinks the nanoparticles of the silica skeleton, and reinforces the otherwise fragile framework. Systematically adjusting the processing variables affecting density produce aerogels whose macroscopic properties could be controlled, and are attributed to changing nanoscale morphology. Aerogels crosslinked using the smallest amount of silica studied exhibit as much as a 40-fold increase in strength over the corresponding non-crosslinked framework, and are flexible.     

从稻壳中获得二氧化硅气凝胶的研究

研究了稻壳在盐酸中浸泡、煮沸、回流、干燥后,置于600℃燃烧,并保温2 h,制得了一系列二氧化硅气凝胶样品。重量法分析表明这些二氧化硅气凝胶中二氧化硅质量分数为99%以上,并采用了激光粒度仪、比表面测定仪（BET方法）、X射线衍射仪（XRD）、红外光谱仪（FT-IR）和扫描电镜（SEM）等研究了二氧化硅气凝胶样品,同时对使用稻壳能够成功制备二氧化硅气凝胶方法进行了探讨。     

Nanoindentation on hybrid organic/inorganic silica aerogels

The hybrid organic/inorganic silica aerogels experiment a drastic mechanical change into rubber behaviour in relation with the pure inorganic silica aerogel as a brittle material. Aerogels were prepared by sol–gel process and drying by venting off the supercritical ethanol, no degradation of the organic polymer was detected. TEOS (tetraethoxysiloxane) and PDMS (polydimethylsiloxane) were used as inorganic and organic precursors, respectively. Depth sensing nanoindentator was used to study the mechanical properties, which is extremely sensitive to small loads (1 mN) and penetration depths (10 nm). The TEOS inorganic clusters and the polymer crosslinking degree influence the microstructure of the hybrid aerogels. Surface indentations maps reveal the different heterogeneities such as the tough silica matrix, the softness of the elastic polymer chains and the plastic microcracks in pores. The values obtained are compatible with the macroscopic ones resulting from uniaxial compression. Creep tests confirm that the compliance parameter increases with the polymer content and results can be theoretically modeled by the Burger model.     

Preparation of stable carbon nanotube aerogels with high electrical conductivity and porosity

Stable carbon nanotube (CNT) aerogels were produced by forming a three-dimensional assembly of CNTs in solution to create a stable gel using a chemical cross-linker, followed by a CO₂ supercritical drying. Thermal annealing of these aerogels in air can significantly improve their electrical and mechanical properties, and increase their surface area and porosity by re-opening the originally blocked micropores and small mesopores in the as-prepared CNT aerogels. Thermally annealed CNT aerogels are mechanically stable and stiff, highly porous (∼99%), and exhibit excellent electrical conductivity (∼1–2 S/cm) and large specific surface area (∼590–680 m²/g).     

Synthesis of highly flexible silica aerogels by photoacids generation

Highly flexible silica aerogels were fabricated by using 2-(3-(triethoxysilyl)propylcarbamoyloxy) ethyl acrylate (TESEA), methyltriethoxysilane (MTES), and polydimethylsiloxane (PDMS) as co-precursors through photopolymerization and sol–gel process, then followed by ambient pressure drying. Attributing to photolysis of the photoacids generators, the radical and Brönsted acid (HSbF₆) were generated simultaneously under irradiation. The radical initiated the photopolymerization of acrylate which induced the gelation and fixed the shape of the gel quickly, the acid allowed the sol–gel process occurring gradually. Due to the enhancement of the polymer, the strength of aerogels was improved. The aerogels had low density (~?0.098 g/cm³), high porosity (~?94.8%), high flexibility and high elastic recovery. Moreover, the silica aerogels have hydrophobic and oleophilic properties, which endows them with an ability of oil water separation.     

Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying

We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm³ and 520 m²/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.     

Carbon-coated mesoporous silica with hydrophobicity and electrical conductivity

The pore surface of mesoporous silica SBA-15 was coated with 2,3-dihydroxynaphthalene (DN) through a dehydration reaction between the surface silanol groups in SBA-15 and the hydroxyl groups of the DN molecules. By the carbonization of DN in the SBA-15 pores, the pore surface was uniformly covered with an extremely thin carbon layer, which comprised only 1-2 graphene sheets. The resulting carbon-coated SBA-15 still possessed the characteristics of the original SBA-15—large surface area and pore volume, long-range ordered structure, and sharp mesopore size distribution. In addition, the carbon-coated SBA-15 showed marked hydrophobicity and high electrical conductivity, both of which are not intrinsic properties of SBA-15. The appearance of these features can be explained from the almost perfect carbon coating on the pore surface. Newly developed graphene coating technique can donate characteristic carbon properties to mesoporous silica.     

白炭黑/Si-69填充剂对轮胎胶料导热系数的影响

考察了不同用量的白炭黑/双(3-三乙氧基硅丙基)-四硫烷(简称Si-69)对轮胎胶料导热性能的影响.结果表明,当白炭黑用量从10份增至15份时,胶料的导热性能降低;继续增加白炭黑的用量,胶料的导热性能提高;白炭黑/Si-69填充胶料的导热性能优于白炭黑填充胶料.     

用激光导热仪测定炭黑填充橡胶的导热系数

用激光导热仪测定了5种填充不同量炭黑N 220的胶料在30～140℃时的导热系数,分析了导热系数随温度和炭黑填充量变化的关系,发现胶料的导热系数均随着温度的升高呈线性增大趋势,随炭黑填充量的增加也逐渐增大.将导热系数与温度和炭黑填充量进行了关联,得到了线性回归方程式,进而确立了适合于计算不同温度和不同炭黑N 220填充量胶料导热系数的关联方程A=0.133 77 2.008 74×10-4t 0.001 64 X.将用该方程计算的结果与实验值进行比较,60个数据点的平均相对误差仪为0.93%.     

Role of silica nanoparticle in multi‐component epoxy composites for electrical insulation with high thermal conductivity

Multicomponent epoxy micro/nano‐composites containing micro‐alumina, micro‐quartz, and nano‐silica were fabricated to develop electrical insulation materials with high thermal conductivity. Simply changing the ratio between the alumina and quartz microparticles caused a trade‐off relationship between the thermal conductivity and electrical insulation. Increasing the alumina content in the epoxy‐alumina/quartz micro‐composites enhanced the thermal conductivity but deteriorated the dielectric strength. An increase in the thermal conductivity without incurring a loss in the dielectric strength was achieved by incorporating silica nanoparticles in the epoxy micro‐composites. Adding silica nanoparticles to the epoxy micro‐composites was found to be more efficient in improving the thermal conductivity compared to increasing the alumina ratio, especially at low alumina/quartz ratios. This behavior corresponded to theoretical models. Therefore, we provide a useful insight, both practical and theoretical, into the more advanced optimization of designing multicomponent epoxy composites for electrical insulation with high thermal conductivity.     

含油气盆地中热传导与岩石热导率研究

含油气盆地中热传导是最主要的热量传递方式,在大地热流一定的情况下制约热传导的主要因素是热导率的大小。前人对岩石热导率与温度、压力、岩石孔隙度的关系以及岩石热导率的各向异性进行了研究。本文对前人的研究成果进行了综合分析,认为下一步应该加强沉积岩平行层面方向和垂直层面方向热导率差异的研究及热导率与岩石输导性能关系的研究。