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Fine-grained mullite nanofibers derived from the diphasic mullite sol were successfully fabricated by electrospinning and subsequent pyrolysis at 1500°C. Polymethylsiloxane and aluminum tri-sec-butoxide were selected as the silicon and aluminum source to synthesize the diphasic sol. Results show that the weight loss of mullite precursor fibers in our work was about 60 wt.%, which is similar with that of fibers fabricated using the monophasic sol. This low weight loss was mainly attributed to the high ceramic yield of polymethylsiloxane and low introduced polyvinylpyrrolidone content, which ensures the integrity of fiber morphology during the sintering process. Mullite fibers with 216 nm average diameter were fabricated after sintered at 1500°C and the corresponding grain size was only ~100 nm, much smaller than that in mullite fibers derived from monophasic sols. Therefore, it can be predicated that mullite fibers in this work should possess a higher mechanical strength than those derived from monophasic sols when the sintering temperature was higher than 1400°C and therefore was an ideal starting materials for the fabrication of mullite nanofibrous ceramics used as the high-temperature thermal insulation materials. [ABSTRACT FROM AUTHOR]

Silver/amorphous silicon (Ag/a-Si) memristor has attracted extensive attention as a candidate for next-generation nonvolatile memories. The optical constant variations of a-Si layer during switching procedure, which are the key for Ag/a-Si memristor to be used in optical applications, have not been clarified yet. To take a further understanding on the optical feathers of Ag/a-Si memristor, here we report the microstructure, resistivity and dispersion relation of amorphous silicon silver (a-Si 1−x Ag x ) thin films prepared by co-sputtering. Ag nanocrystals are observed in the films with size similar to the Ag filament in memristor, and the obtained resistivities of the films march the typical ON/OFF ratio of memristor. Furthermore, both the film's refraction index and extinction coefficient increase with Ag concentration, suggesting that Ag/a-Si memristor combining with silicon waveguide could be applied in optical switch at 1300 and 1550 nm. [ABSTRACT FROM AUTHOR]

A novel temperature resistance buoyancy material with hollow glass microspheres (HGMs) as the filler and borosilicate glass (BG, similar chemical composition to the HGMs) as the high temperature matrix was prepared by a tert-butyl alcohol based on gelcasting process. The effect of solid loading and sintering temperature on microstructure and mechanical properties were investigated. The results showed that HGMs were bonded together by borosilicate glass and 45 wt.% solid loading and 700 °C were the optimal processing parameters to fabricate the samples with the best mechanical and thermal properties, and the corresponding bulk density, compressive strength, thermal conductivity and Young’s modulus were 0.92 g/cm 3 , 14.46 MPa, 0.15 W/m K and 1.24 GPa, respectively. The exponent m value for the samples fabricated in this work was 1.18, lower than that of open- or closed-cell foams, because the sample possess both the closed and open pores. The experimental values of thermal conductivity from 0.07 to 0.16 W/m K were lower than the prediction with the Hashin–Shtrikman upper bound and almost close to that predicted by the Landauer’s relation. Only modulus value of SSL45 sample which possessed the maximum value was closer to Hashin–Shtrikman upper bound, while the modulus value of other samples agreed well with Ashby–Gibson model applied for a porosity ranging from 65% to 89%. The HGMs/BG composite exhibited a low density, low thermal conductivity and excellent temperature resistance, and can be used as the thermal insulation for undersea pipeline or submarine. [ABSTRACT FROM AUTHOR]

The mechanical behaviors of single mullite fiber and binder/mullite fiber interface were experimentally investigated. The growth of the mullite grains of the fiber with the increasing calcination temperature greatly affected the tensile and compression strengths of the mullite fibers. As the calcination temperature went up from 1000 °C to 1500 °C, the average tensile strength decreased from 0.954 GPa to 0.191 GPa, and the Weibull modulus decreased from 5.1 to 1.8. The compressive strength was 50–60% of the average tensile strength under the same calcination temperature. The bonding strength of the binder/fiber interface increased with the increase of the calcination temperature. The mechanical properties and fracture mechanism of the mullite fiber-based porous ceramics with different binder contents were comprehensively analyzed based on the mechanical behavior of the single mullite fiber and the binder/fiber interface. [ABSTRACT FROM AUTHOR]

Porous alumina ceramics with inter-locked plate-like structure were successfully fabricated by the tert -butyl alcohol-based gel-casting method. The effects of Al/Si mole ratio on the structure and properties of the obtained products were investigated. The formation process of the inter-locked structure was analyzed. The products fabricated with the one-step method, in which the growth of plate-like alumina and the formation of the inter-locked structure were combined together, exhibited a high porosity of 76.21%, a compressive strength of 0.525 MPa and a low thermal conductivity of 0.495 W/mK, while the products fabricated by the two-step method after sintered at 1500 °C using the grinded powders after 700 °C heat-treatment as the starting materials possessed a relatively high compressive strength of 4.80 MPa. The obtained porous alumina ceramics were promising flexible insulant materials for the application in high-temperature field. [ABSTRACT FROM AUTHOR]

A novel mullite fibers-based hierarchical heterostructure were fabricated using a sol–gel method. The porous performs were fabricated by mixing the mullite fibers with the aluminum borate xerogel which is derived from the gelation of Al(OC 4 H 9 ) 3 and boric acid. After calcination, the preforms with different mole ratio of Al/B, transformed into the composites with hierarchical structures of the aluminum borate nanowhiskers/mullite fiber and alumina plate/aluminum borate nanowhiskers/mullite fiber. The formed aluminum borate nanowhiskers showed a uniform distribution on the mullite fibers, and the growth process may follow a self-catalysis mechanism. The fabricated hierarchical mullite fiber-based products possessed low density of 0.448 g/cm 3 , low thermal conductivity of 0.132 W/mK, high specific surface area of 230.7 m 2 /g and excellent elastic property with high compression-resilience of 92.7%, and were promising flexible insulant materials for the application in high-temperature heat sealing field. [ABSTRACT FROM AUTHOR]

Abstract: Silica ceramic foams with controllable microstructure were produced by direct foaming method. The silica foams were prepared through three different systems: individual cationic surfactant stabilized system, individual anionic surfactant stabilized system, and the mixed cationic/anionic surfactant stabilized system. The foaming ability and foam stability of the three systems were investigated and the related mechanisms were discussed. The results showed that the mixed system had both long-term stability and good foaming ability compared with the others. In addition, the role of the anionic surfactant (SDS) on microstructure of the ceramic foams was studied. The results showed that the foam structure changed from closed pores to open pores with the increasing concentration of SDS. [Copyright &y& Elsevier]

Abstract: Porous silica ceramics with relatively high strength and novel bi-modal pore structure can be successfully fabricated via a facile tert-butyl alcohol-based gel-casting process by use of silica extracted from coal fly ash. Based on the evaporating of tert-butyl alcohol (TBA) during the drying process, the microstructure with bi-modal pores was obtained. By setting the solid loading (15wt.%) and changing the sintering temperature from 1050 to 1150°C, the porosity of the porous silica ceramics varied within the range of 70.1% and 72.4%, as the bulk density varied from 0.59 to 0.67g/cm3 and small pore sizes from 3.31 to 2.89nm. The compressive strength of sintered silica ceramics increased from 4.38MPa to 5.82MPa, which was attributed to the 3D-framework structure and the connection of silica particles. [Copyright &y& Elsevier]

High emissivity coating plays a critical role in thermal protective system, which can radiate a large amount of aero-convective heat. Silica sol bonded MoSi 2 -SiC-Al 2 O 3 (S-MSA) coating was proved to be promising for mullite fibrous insulation. However, the bonding mechanisms of the coating at elevated temperatures are not clear. In this work, the S-MSA coatings were heat-treated at temperatures from 600 °C to 1500 °C to reveal the bonding mechanisms at elevated temperatures. The S-MSA coatings go through a relatively stable stage (600 °C–1000 °C), a crystallization stage (1100 °C–1200 °C), and a densification stage (1300 °C–1500 °C) at ever increasing temperatures. Results show that both the contact damage resistance and the bonding strength of the calcined coatings exhibit a decrease followed by an increase at elevated calcination temperatures, with the inflection point at 1200 °C, corresponding to the transition temperature of the bonding mechanisms from 600 °C to 1500 °C. [ABSTRACT FROM AUTHOR]

Mullite nanofibrous aerogels with low density, low thermal conductivity and high service temperature (as high as 1400 °C) was fabricated by the freeze-casting method, and two different kinds of ice crystal growth inhibitors, glycerol as the cryoprotector and agarose as the gel-former, were adopted respectively to influence the growth of ice crystal, thereby adjusting the sample microstructure (including the pore size and the pore distribution). The addition of glycerol could influence the growth of the ice crystal, leading to the pore structure changing from the cellular pores to the spherical pores and the decrease of the large pore size, consequently resulting in the increase of the sample density and compressive strength. The addition of agarose could inhibit the growth of the ice crystal due to the formation of gel network, resulting in the disappearance of the large pores (cellular pores). The sample with high agarose content exhibited a relatively high compressive strength of 252.41 kPa due to high density and uniform pore structure. The research of this work provides an effective method to regulate the properties of nanofibrous aerogel to meet the demand of different applications. [ABSTRACT FROM AUTHOR]

The heat resistant buoyancy materials were fabricated by the molding method using the aluminum dihydric phosphate as the matrix and hollow glass microspheres as the filler, respectively. In order to improve the waterproof performance of the buoyancy, two strategies, adding the B 4 C additives and introducing the silica-based coatings, were adopted. The study showed that the samples with the B 4 C content of 10 wt% possessed the optimal comprehensive performance, such as low density (0.540 g/cm3) and low water absorption (29.6%). The effect of the coating types and coating times on the density, water absorption, compressive strength and specific strength of the buoyancy materials were also studied. After 4 times of coating, the density and the compressive strength of the sample increased to 0.62 g/cm3 and 6.0 MPa, respectively; the water absorption of the samples greatly decreased to 10%. Besides, all samples exhibited significant pseudoductility. The buoyancy materials with the above superior properties can be potentially used in the deep see fields. [ABSTRACT FROM AUTHOR]

Abstract High-emissivity MoSi 2 -SiC-Al 2 O 3 -SiO 2 coating with MoSi 2 and SiC as emittance agents, Al 2 O 3 as base skeleton, and silica sol as both dispersive medium of the coating slurry and binder of the coating, was prepared on mullite fibrous insulations via slurry coating and non-firing process. The non-firing coatings were calcined at high temperatures to reveal their reusability after high temperature service. Microstructure, phase composition, interface bonding strength and impact resistance of the non-firing coating and the calcined coatings were investigated comprehensively. Thermal shock resistance, thermal endurance and infrared radiation property of the coatings were studied. The results show that the non-firing coating was dense and flat with all particles bonded together by silica sol; no clear interface existed between the coating and the substrate due to the penetration of the coating slurry; the whole coating was composed of a porous inner layer of about 300 µm in thickness and a top layer of about 100 µm in thickness. The calcined coatings exhibited ideal microstructure with newly formed high temperature phases. Both interface bonding strength and impact resistance of the calcined coatings increased with elevated calcination temperatures, benefited from the good binding effect of silica sol, the gradient structure and the formation of mullite. The coatings remained intact without peeling and spalling after thermal shock tests, and the main defects in the coating surface after thermal shock tests at 1400 °C and 1500 °C were cracks and holes, respectively. The coatings exhibited better long-term thermal endurance at 1400 °C than at 1500 °C, resulted from the weaker oxidation of emittance agents at lower temperature. The emissivity of the non-firing coating at the wavelength range of 3–5 µm and 8–12 µm was as high as 0.9, benefited from the non-firing process, and the emissivity of the calcined coatings were higher than 0.8. [ABSTRACT FROM AUTHOR]

Abstract Silica sol, one of the most commonly used binders in the fabrication of fibrous porous ceramics, can diffuse as the solvent evaporates during the drying stage, leading to an uneven structure of ceramics. To prevent the diffusion of silica particles and prepare homogeneous mullite fibrous porous ceramics, propylene oxide (PO) was introduced into the fabricating procedures. The effects of solid content of silica sol and the molar ratio of PO /silica phase (PO/SiO 2) on the microstructure and physical properties of mullite fibrous porous ceramics were also investigated. The results demonstrated that PO addition strongly consolidated the mullite fibrous skeleton and helped to obtain a homogenous property, because silica phase had been consolidated in situ around the fibers during the gelation stage. Furthermore, the mullite fibrous ceramics prepared with PO addition showed an even microstructure and a much better compressive strength (1.489 MPa), compared with the ones prepared without PO addition (0.868 MPa). Finally, the solid content of silica sol showed a very strong relevance to the properties of the ceramics: the compressive strength raised from 0.731 MPa to 1.715 MPa with the solid content of silica sols increasing from 9 wt% to 18 wt%; while the molar ratio of PO/SiO 2 had little: the compressive strength was around 1.480 MPa, with the corresponding molar ratios of PO/SiO 2 ranging from 0.43 to 1.72. [ABSTRACT FROM AUTHOR]

Directional ice-templated ceramics have unique lamellar porous channels between ceramic walls which run from bottom to top of the samples. This highly oriented morphology results in the anisotropy in mechanical properties both parallel and perpendicular to freezing direction. In this research, fiber-enhanced ice-templated porous ceramics were fabricated by introducing fibers into alumina slurry for freezing. The trans-lamellar fiber bridges connecting adjacent ceramic walls were formed by adjusting the aspect ratio of fibers and freezing velocity, which greatly enhanced the compressive response in two directions and weakened the anisotropy in mechanical properties of the directional ice-templated ceramics. The results showed that the of fiber bridges increased with the increase of fiber aspect ratio and the longitudinal and transverse mechanical properties increased by 73.5% and 232.6%, respectively. [ABSTRACT FROM AUTHOR]

A type of buoyancy material with excellent temperature resistance was successfully prepared through a molding method, and the hollow glass microspheres (HGMs) and silicon oxide (SiO 2 ) were used as the filler and the matrix, respectively. The effect of sintering temperature on mechanical performance, microstructure, and high-temperature performance of the HGMs/SiO 2 composite were investigated in the study. The HGMs/SiO 2 composite exhibited low density (0.47–0.53 g/cm 3 ), high porosity (66.7–75.6%), low thermal conductivity (0.18–0.27 W/(m·K)), and relatively high compressive strength (3.8–5.2 MPa) properties. The values of Weibull moduli m used to predict the variation degree of compressive strength during the compression process was in the range of 5–15 and its values for the samples sintered at 850 °C was 14.64. Two stages namely elastic stage and fracture stage were shown in the stress-strain curves and in the elastic stage, the stress increased linearly with increasing of the strain until reaching the maximum stress. In the fracture stage, the sample gradually destroyed with further increasing of the strain until complete failure. The compressive strength of the HGMs/SiO 2 composites tested at high-temperature was larger than that of the one tested at room temperature. The HGMs/SiO 2 composite exhibited low density, low thermal conductivity and excellent temperature resistance properties and can be used as the structural material or buoyancy material in the deep sea field in the near future. [ABSTRACT FROM AUTHOR]

Porous mullite fiber/silica composite was successfully fabricated by a TBA-based gel-casting, and the influence of mullite fiber content on the composite microstructure and properties were investigated. The results show that the mullite fiber content played an important role in tailoring the microstructure of the composite, consequently influencing the sample properties. The composite fabricated with high fiber content showed a porous fibrous structure with mullite fibers as the fiber skeleton and silica as the binder, and exhibited a low density, high porosity, excellent elasticity and low thermal conductivity. On the contrary, the sample fabricated with low fiber content possessed a high density and high strength due to its dense microstructure which is similar with that of the fiber reinforced ceramic matrix composite. [ABSTRACT FROM AUTHOR]

Fibrous ceramics with the mullite fibers as the matrix and silicon resin as the binder was fabricated by a molding method. The effect of sintering temperature, especially 1500 °C, on the properties of the products, such as porosity, microstructure, mechanical behavior and fracture mechanism, were investigated. This fibrous ceramics exhibited low densities (0.425–0.441 g/cm 3 ), high porosities (81.6–82.3%), low thermal conductivities (0.083–0.089 W/mK), relatively high compressive strength (1.21–1.58 MPa), and high rebound-resilience ratios (70–85%), which made it a promising high-temperature insulation materials. When the sintering temperature increased above 1300 °C, the fracture behavior of the products was mainly attributed to the fibers break and the binders fracture. As the sintering temperature increased to 1500 °C, the grains in the mullite fibers grew up, and the strength of fibers decreased, finally leading to the degradation of the sample mechanical performance. What's important, the compressive strength of the products sintered at 1500 °C was 1.36 MPa and the elastic strain could reach 3.2%, which means that the products can still be put to use in the high-temperature fields. [ABSTRACT FROM AUTHOR]

In our report, a fibrous ceramics with the mullite fibers as the matrix and SiO 2 –AlPO 4 sol as the high-temperature binder were fabricated by the infiltration method. The effects of sintering temperature on the properties, such as porosity, phase transition, microstructure, compressive strength and fracture mechanism, were investigated. Compared with the fibrous ceramics with the SiO 2 –B 2 O 3 binder, the fibrous ceramics with the SiO 2 –AlPO 4 binder exhibited lower densities (0.560–0.595 g/cm 3 ), lower thermal conductivities (0.157–0.165 W/mK), high strain (7–8%), high rebound-resilience (77–90%) and the same level of compressive strengths (1.1–2.1 MPa). During the cooling stage of the heat treatment process, α→β-cristobalite displacive phase transition in the SiO 2 –AlPO 4 binder was successfully suppressed. The fracture behavior of the ceramics tends to be a layer-by-layer failure, which is quite different from that of the traditional ceramics. The non-brittle fracture mode and the excellent elasticity made this fibrous ceramics promising high-temperature elastic insulation materials. [ABSTRACT FROM AUTHOR]

Abstract: 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 (2.25MPa) and high porosity (83%) indicating it is a potential high-temperature seal material. [Copyright &y& Elsevier]

Abstract: Porous Si2N2O ceramics was synthesized by the tert-butyl alcohol-based gel-casting method at a relatively low temperature using Si3N4 and SiO2 powders as the starting materials and spodumene (LiAl(SiO3)2) as the sintering additive. Porous Si2N2O ceramics with relative high compressive strength (10.80MPa) and high porosity (65.5%) were successfully fabricated. The microstructure of porous Si2N2O after sintering at different sintering regimes was studied. The fibers observed in the pores can increase the compressive strength and the specific surface area of the sample. [Copyright &y& Elsevier]

Abstract: Effects of fly ash cenospheres on the microstructure and properties of silica-based composites were studied in this paper. It was found that the density of the samples decreased as the content of fly ash cenospheres increased, while the strength increased inversely when the content was lower than 50wt%. An interlaced microstructure of lath-like and needle-like mullite crystals was discovered in the composites, which was a beneficial factor for strengthening the silica matrix. Furthermore, the results indicated that fly ash cenospheres can promote the densification of silica as a sintering aid and restrain the crystallization of cristobalite as an inhibitor. After sintered at 1200°C for 2h, the density and flexural strength of silica-based composites with 50wt% fly ash cenospheres were 0.92g/cm3 and 14.5MPa separately, and the strength was as high as that of the silica ceramics without fly ash cenospheres. [Copyright &y& Elsevier]

Abstract: Porous silica ceramics with relatively high mechanical strength can be successfully fabricated via a novel tert-butyl alcohol-based gel-casting method. The microstructure similar to stone arch bridges and the sintering neck can be easily observed due to the evaporation of tert-butyl alcohol (TBA) during the drying process. The porosity of samples with different solid loading sintered at 1150°C varied within the range of 56.4% and 70.1%, as the bulk density from 0.67 to 1.04g/cm3. The compressive strength of sintered silica ceramics increased from 4.32MPa to 8.82MPa, which was attributed to the 3D-framework structure and the connection of silica particles. By setting the solid loading (15wt.%) and changing the sintering temperature from 1050 to 1200°C, the porosity of the sintered samples varied within the range of 51% and 72.4%, as the bulk density remarkably varied from 0.59 to 1.18g/cm3 and pore sizes from 3.31 to 2.89nm. [Copyright &y& Elsevier]

Abstract: Sub-micron porous yttria-stabilized zirconia (ZrO2–5mol% Y2O3, YSZ) ceramics with ultra low density were successfully fabricated by a novel tert-butyl alcohol (TBA)-based gel-casting method. In this work, polymerization of acrylamide was realized in tert-butyl alcohol (TBA)/ZrO2 slurries with solid loading ranging from 10 to 30wt.%. Green bodies with ultra-low density (from 1.18 to 1.65g/cm3) could be dried with very small shrinkage (from 0.36% to 0.10%), and relatively high bending strengths (from 26.26 to 112.92MPa) were achieved. By choosing different initial solid loadings, the green bodies were sintered at different temperatures and subsequently characterized in terms of both microstructure and properties to study effects of solid loading and sintering temperature. Both microstructure and properties exhibited obvious dependence on solid loading. By setting the solid loading (20wt.%) and changing the sintering temperature from 1350 to 1500°C, the porosity of the sintered samples varied within the range of 69.9% and 54.1%, as the linear shrinkage varied from 15.8% to 25.2% and pore sizes from 0.58 to 0.44μm. Due to the neck formation between particles, there were also some smaller pores around 1.7 and 3.0nm. But the most important characteristics of the porous structure were the uniformity and the interconnection of pores with sizes about 0.5μm. [Copyright &y& Elsevier]