Fibrous porous mullite ceramics modified by mullite whiskers for thermal insulation and sound absorption

In order to meet the demand for thermal insulation and sound absorption, fibrous porous mullite ceramics (FPMC) with high porosity and an interconnected pore structure were prepared, followed by a pore structure modification with in situ grown mullite whiskers on the three-dimensional framework of the FPMC. The resultant hierarchical material exhibited superior sound absorption performance in the low-to-medium frequency to most reported sound-absorbing materials, as well as a sufficient compressive strength of 1.26 MPa with low thermal conductivity of 0.117 W·m^?1·K^?1. Moreover, the effects of solid content and mullite whiskers on the microstructure and physical properties of the material were analyzed. The increase of solid content led to increased compressive strength and thermal conductivity and decreased frequency corresponding to the first sound absorption peak. The thermal conductivity and compressive strength of the material increased as the mullite whiskers grew, while the median pore size decreased.     

High-temperature elasticity of fibrous ceramics with a bird's nest structure

New fibrous ceramics with polycrystalline mullite fibers as the matrix and silica–boron sols as the high temperature binder, which was inspired by the bird's nest structure in nature, were synthesized. The most important structure characteristic of this fibrous material is that the silica–boron binder only fixed the fibers at the crossing points rather than filled the pores among the fibers. The elastic behavior was investigated, both at room temperature and elevated temperature. Compared to conventional ceramic matrix composites, the samples show a much higher degree of elasticity because of the bending of the fibers. The rebound resilience decreased slowly with the increase of the temperature, but it still remained 86% of that at ambient temperature at 1000 °C. The sample exhibits good elasticity performance, relatively high strength (2.25 MPa) and high porosity (83%) indicating it is a potential high-temperature seal material.     

Effect of recoating slurry compositions on the microstructure and properties of SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer sponge replica technique, followed by recoating with SiC slurries of two different sintering additives of MgO–Al₂O₃–SiO₂ (Slurry 1) and polycarbosilane (Slurry 2). The sintering temperature of SiC RPCs recoated with Slurry 2 was 1100 °C, which was 200 °C lower than that for one recoated with Slurry 1. The prepared SiC RPCs exhibited homogeneous microstructure and contained pores with different sizes which was entrapped in the strut of SiC RPCs, small pores with diameter lower than 4 μm and large pores with diameter higher than 10 μm. Bending strength of SiC RPCs recoated with Slurry 1 was two times higher than that for the non-recoated samples, which was 1.88 MPa and was a little higher than that for one recoated with slurry 2. At the same time, high thermal shock resistance and high refractoriness were achieved for SiC RPCs recoated with Slurry 2.     

Heat treatment effects on microstructure and properties of CVI SiC/SiC composites with Sylramic™-iBN SiC fibers

Chemical-vapor-infiltrated (CVI) SiC/SiC composites with Sylramic?-iBN SiC fibers and CVI carbon, BN, and a combination of BN/C interface coating were heat treated in 0.1-MPa argon or 6.9-MPa N₂ at temperatures to 1800 °C for exposure times up to 100 hr. The effects of thermal treatment on constituent microstructures, in-plane tensile properties, in-plane and through-the-thickness thermal conductivities, and creep behavior of the composites were investigated. Results indicate that heat treatment affected stoichiometry of the CVI SiC matrix and interface coating microstructure, depending on the interface coating composition and heat treatment conditions. Heat treatment of the composites with CVI BN interface in argon caused some degradation of in-plane properties due to the decrease in interface shear strength, but it improved creep resistance significantly. In-plane tensile property loss in the composites can be avoided by modifying the interface composition and heat treatment conditions.     

Thermal behaviour of geopolymers prepared using class F fly ash and elevated temperature curing

This article reports a study of thermal stability of properties upon firing at 800-1200 °C of geopolymer materials prepared using class F fly ash and Na and K alkaline activators. Compressive strength and shrinkage measurements, XRD, SEM (BEI), TGA and MIP were utilised in these studies. The materials were prepared at water/binder ratios in a range of 0.09-0.35, using compaction pressures up to 10 MPa and curing temperatures 80 and 100 °C. Thermal stability of the studied geopolymer materials was rather low. In the samples prepared using sodium-containing activators rapid deterioration of strength at 800 °C was observed, which was connected to a dramatic increase of the average pore size. Initially amorphous structures were replaced by the crystalline Na-feldspars. In materials prepared using fly ash and potassium silicate compressive strength was significantly increased on heating, deterioration of strength started at 1000 °C. After firing these materials remained amorphous with reduced average pore size and significantly increased compressive strength. Compaction at 1-10 MPa reduced shrinkage on firing in all materials. Geopolymer materials prepared using class F fly ash and alkaline activators showed high shrinkage as well as large changes in compressive strength with increasing fired temperature in the range of 800-1200 °C. Thus the materials were found unsuitable for refractory insulation applications.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

叔丁醇基凝胶注模成型制备氧化铝多孔陶瓷

以微米级Al2O3粉料为原料,叔丁醇为溶剂,采用凝胶注模成型工艺制备了氧化铝多孔陶瓷,并研究了Al2O3浆料的固相体积分数(分别为8%、10%、13%和15%)对1 500℃保温2 h烧后氧化铝多孔陶瓷的气孔率、气孔孔径分布、耐压强度、热导率和显微结构的影响.结果表明:当Al2O3浆料的固相体积分数从8%增加到15%时,氧化铝多孔陶瓷烧结体的总气孔率从71.2%逐渐降低至61.2%,气孔平均孔径从1.0 μm逐渐减小至0.78 μm,耐压强度从16.0 MPa逐渐增大至45.6 MPa,而热导率从1.03 W·(m·K)-1逐渐增大至1.83W·(m·K)-1.     

Investigation of the thermal shrink mechanism,thermal conductivity and compressive resistance of TaTiP3O12 ceramics

Dense TaTiP₃O₁₂ ceramics were synthesized by the solid-state method and spark plasma sintering (SPS) with 6 wt% V₂O₅ as a sintering aid, and their phase, microstructure, thermal conductivity, hardness, compressive strength, and expansion property and mechanism were investigated. Results show that the pure phase can be achieved by the two methods. In particular, the sample prepared by SPS possesses a relative density of 97.62% and a porosity of 3.07%, and has better properties than that prepared by the solid-state method. The SPS sample has a thermal conductivity at room temperature of 2.03 w/(m· °C), a Vickers hardness of 4.34 GPa and a compressive strength of 175.98 MPa, which are 0.95, 1.49 and 1.59 times greater than those of the sample prepared by the solid-state method, respectively. In addition, the TaTiP₃O₁₂ ceramic prepared by SPS exhibits a linear ultralow negative thermal expansion property with a coefficient of thermal expansion of ?0.74 × 10^?6 °C ^?1 (-100–400 °C). The negative thermal expansion in TaTiP₃O₁₂ is induced by the coupling effect of [Ta(Ti)O₆] octahedron and [PO₄] tetrahedron caused by the transverse vibration of bridging oxygen atoms.     

SiC-Whisker-Reinforced Cellular SiO2 Composites

Fabrication of lighweight, cellular ceramic composites by foaming sol–gels is presented. Foams of 20-vol%-SiC-whisker-reinforced SiO₂ can be tailored with relative densities as low as 10%, with either open or closed cell structures. In addition to reducing drying shrinkage and thus gel cracking, whisker reinforcement significantly increases the strength of the composite foams relative to pure silica foams.     

Electrospun fabrication,excellent high-temperature thermal insulation and alkali resistance performance of calcium zirconate fiber

CaZrO₃ (CZO) precursor fibers were prepared by sol-gel method and electrospinning technique from solutions which contained aqueous precursors of calcium and zirconium ions and polyethylene oxide. The crystallization of CZO fibers was a concurrent process with the decomposition of organics. The evolution process was characterized by Fourier transform infrared (FT-IR) and Raman spectra, thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The heat-conducting property and high temperature stability of fibers were characterized by the measurements of thermal conductivity and heating permanent linear change, respectively. The fibers were treated in NaOH solution at 80?°C to characterize the alkali resistance. The results showed that CZO fibers had the lower thermal conductivity than the other reported forms of CZO materials, and they possessed excellent stability up to 1100?°C with thermal shrinkage less than 1.2% and excellent corrosion resistance to alkalis. Hence, CZO fiber could be used as a suitable corrosion resistant refractory material for high-temperature thermal insulation.