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Water-glass-based pastes show good prospect for high-temperature applications, but they are subject to substantial dry-shrinkage. Silicon and aluminum powders were added to prevent the dry-shrinkage. The influence of them on compressive strength, bonding strength and volume of the pastes was discussed. SEM, XRD, and TG/ DSC were employed to study the influencing mechanisms. The results show that Si and Al effectively prevented the dry-shrinkage; Al improved compressive strength and bonding strength, while Si was bad for bonding strength. When Al dosage was 6%, the pastes presented mild expansion of 1.05%, the compressive strength increased by 8.9%, and the bonding strength by 33.4%. [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]

Mullite fibrous ceramics with different types of binders (silica particles, SiO 2 -B 2 O 3 mixture and silicon particles) were successfully fabricated by a TBA-based gel-casting. The influence of the above three types of binder on the microstructure and properties of the ceramics were investigated. The results show the resource of the binders played an important role in tailoring the inner structure of the ceramics, consequently influencing the product properties. All products showed high porosity, low density and low thermal conductivity, attributed to the special 3D fibrous skeleton structure. The products with silica as binder sintered at 1500 °C exhibited relatively low compressive strength because of the high melting point of the silica particles. On the contrary, the products with silicon and SiO 2 -B 2 O 3 as the binder sintered at 1500 °C showed higher compressive strength owing to the low viscosity of the binders during the sintering process. Compared with the products SiO 2 -B 2 O 3 as the binder, the products with silicon as the binder was more suitable for being used in the high-temperature environment (above 1500 °C), since the silicon could transform into the silica phase with high melting point. Besides, with the increase of the silicon content, the compressive strength and the thermal conductivity of the products increased gradually. [ABSTRACT FROM AUTHOR]

A temperature resistance buoyancy material was fabricated through a tert-butyl alcohol gelcasting process with borosilicate glass (BG) and hollow glass microspheres (HGMs) as the matrix and filler, respectively. The effects of the mass ratio of HGMs to BG and sintering temperature on the microstructure, thermal conductivity, and mechanical properties of the composite were studied. The results show that HGMs were bonded together by the BG, and the sample sintered at 750 °C exhibited a broad pore size distribution, from several microns to more than one hundred microns. The thermal conductivity experimental values of all the samples were less than that of Hashin-Shtrikman upper bound (HS + ) prediction but agreed well with that predicted from effective medium percolation theory. The relationship between compressive strength and relative density was predicted by the Gibson-Ashby model, with the calibration factor phi below 0.7. Young′s modulus values obtained from the experiment were below that of HS + prediction. The modulus values of the four types of samples sintered at 650 °C agreed well with the Pabst model prediction, while the values of the samples sintered at 700 °C and 750 °C were distributed in a zone between Ashby-Gibson model and HS + prediction. [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]

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]

The feature of "curing at low temperature, directly used in extreme conditions" made the adhesion technique more suitable for engineering application. The superalloy was successfully bonded at high temperatures by a multi-components modified, phenix-silicone-based adhesive, through forming various ceramics and intermetallics. The intermetallization process not only modified the thermal expansibility of adhesive, but also improved the compositional continuity and reaction extent around bonding interface. With the heating temperature increasing from 500 °C to 1000 °C, various high-temperature-resistant phases in turn generated in adhesive, and the chemical reactions on interfaces became more and more intense. Eventually, the room temperature shear strength of joints calcined at 700 °C reached the maximum value of 28.3 MPa, and the high temperature strength was still up to 18.4 MPa at 800 °C. Moreover, after 6 thermal cycles, the residual strength for joints calcined at 700 °C almost remained ∼13 MPa. The exclusive adhesive absolutely facilitated the repair and connection of engineering superalloy. Image 1 • The convenient use is "one-time connection at room-temperature, valid to 1000 °C without post-processing". • The CTE of adhesive is matched well with superalloys. • The reaction transition layer greatly guaranteed the bonding performance. [ABSTRACT FROM AUTHOR]

Abstract A modified superalloy-used heat-resistant adhesive, which can cure at room temperature and provide excellent bonding effect in a wide temperature range from RT to 1000 °C, was prepared from silicone resin as matrix, and Ni, Al, B 4 C, low-temperature-melting glass powder as additives. After pre-calcination, the bonding strength of adhesive reached 30.1 MPa after 700°C-calcination and maintained above 15.7 MPa during the whole heating process as the mixture of Ni-based intermetallics, heat-resistant ceramic generated and high viscosity glass generated in the adhesive. Without pre-calcination, the bonding strength still kept above 10.5 MPa during test from RT to 900 °C and decreased to 4.3 MPa at 1000 °C. Graphical abstract Image 1 Highlights • This adhesive provides excellent bonding strength from room temperature to 1000 °C for superalloys joining. • The bonding effect of adhesive at high temperatures is estimated by high-temperature shear test. • The practical use is "one-time joining at room temperature, valid to 1000 °C without post-processing". • This adhesive can be used not only for the manufacture of superalloy components but also for the repair of broken parts. [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 novel heat-resistant phosphate-based adhesive for bonding superalloys was developed from phosphates as matrix, and B 4 C, Si as well as low temperature melting Zn-B-Al-Si-R glass as fillers. It can cure at room temperature (RT) and provide no less than 5 MPa in a wider temperature range of RT-1000 °C. The addition of these fillers boosted the thermal expansion of adhesive, improved the structure of bonding area, and provided reactant sources for heat-resistant components. After calcination at 1000 °C, the bonding strength reached the maximum of 7.8 MPa as more heat-resistant ceramics and high-temperature melting glass generated in adhesive, and deeper chemical reactions happen on interface. This novel adhesive greatly facilitated the repair of superalloys, especially for non-weight-bearing superalloy slices or flakes. [ABSTRACT FROM AUTHOR]

Mullite fibrous ceramics with a two-dimensional planar random structure was prepared via a pressure infiltration method. The in-plane and out-of-plane mechanical properties, thermal expansion coefficients and thermal conductivities were compared. MoSi 2 -borosilicate glass coatings were applied on two adjacent surfaces of the as-prepared mullite fibrous ceramics via an in-situ reaction method. The bonding strength and thermal shock resistance of the two coatings were compared. Results show that higher crosslinking degree and more fiber orientating in the planes resulted in the fact that the in-plane compressive and tensile strength of the ceramics were 4.6 times and 2.1 times higher than the out-of-planes values, respectively. The in-plane CTE (Coefficient of Thermal Expansion), thermal conductivity and permeability were also higher than the out-of-plane ones. The interface bonding strength of Coating B (coated perpendicular to the planes) was higher than that of Coating A (coated parallel to the planes), because of the better mechanical locking at the interface and the higher tensile strength of the substrate. Besides, Coating B presented a better thermal shock resistance because the CTE of the substrate parallel to Coating B was closer to the CTE of the coating, and the substrate was more complaint parallel to Coating B so that could absorb some of the thermal stress by deformation. [ABSTRACT FROM AUTHOR]

Buoyancy material with high temperature resistance was successfully prepared through a tert-butyl alcohol gelcasting process. Borosilicate glass (BG) and hollow glass microspheres (HGMs) with different average diameters were used as the matrix and the filler, respectively. Results show that the compressive strength increased with the decrease of the mean diameter of HGMs, and the sample consisting of the filler of HGMs with a small average diameter exhibited high compressive strength of 25.04 MPa. Young's modulus values of the composite sintered at 700 °C and 750 °C were distributed over in the range of the values predicted from Ashby-Gibson model to Hashin-Shtrikman model and other moduli values agreed well with that of Past model prediction. Weibull moduli ( m ) reflecting the variation degree of the sample compressive strength were in the range of 5–15. The compressive strength of the composites composed of the HGMs with small average particle size tested at 500 °C was as high as 7.94 MPa. [ABSTRACT FROM AUTHOR]

Two infrared radiating coatings with borosilicate glass as binder and MoSi 2 as emittance agent were prepared on mullite fibrous ceramics by an in-situ reaction method and a two-step method, respectively. The in-situ reaction coating was prepared from Si, B 2 O 3 and MoSi 2 , in which borosilicate glass was obtained through the in-situ reaction of Si, B 2 O 3 and O 2 . By contrast, the two-step method included the preparation of borosilicate glass powders by a melt-quench technique and then coating preparation with MoSi 2 and the glass powders. The structure, phase composition, thermal endurance, infrared radiation property and impact resistance of the two coatings were compared. The results showed that the in-situ reaction coating was denser, and much less MoSi 2 therein was oxidized during sintering (11.3%) than that in the two-step coating (67.0%), resulted from the oxygen diffusion barrier formed by molten B 2 O 3 at approximately 450 °C. The in-situ reaction coating possessed better thermal endurance due to its higher MoSi 2 content and denser structure. In addition, the in-situ reaction coating had higher emissivity (0.787) and better impact resistance (0.051 J) than the two-step coating (0.706, 0.038 J). [ABSTRACT FROM AUTHOR]

A modified polymer adhesive characterized by “one-time connection at low-temperature without post-treatment” was developed for joining mullite ceramic. Without pre-calcination, the bonding strength of cured adhesive maintained above 6.5 MPa during the whole heating process and even reached 24.8 MPa at 1300 °C. After pre-calcination at 1300 °C, the adhesive obtained the optimum bonding performance as much more heat-resistant ceramics and high-temperature melting glass generated in adhesive. Its bonding strength kept about 45 MPa during test from RT to 700 °C, while the value decreased to 28.5 MPa at 1300 °C because of the melting of newly-formed glass. With increasing thermal-cycling times at 1300 °C, the value could maintain at about 35.2 MPa. [ABSTRACT FROM AUTHOR]

A new practical phosphate adhesive was prepared based on aluminum and phosphate as matrix and SiO 2 and B 4 C as additives, which can be applied to join oxide and non-oxide ceramics under air and argon atmospheres. After being cured at room temperature, the as-received joints do not need subsequent processing and can keep stable shear strength within the whole heat treatment process. Its bonding effect for mullite under air was better than that for SiC under argon. The shear strength of mullite joints could be up to 14.5 MPa after treatment at 1300 °C, while that of SiC joints was about 9 MPa. [ABSTRACT FROM AUTHOR]