Microstructure evolution and mechanical behavior of a mullite fiber heat-treated from 1100 to 1300°C

In this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ-Al₂O₃ and δ-Al₂O₃ with amorphous SiO₂ at 1150°C. The stable α-Al₂O₃ formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.     

Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Effect of thermal exposure on the microstructure and strength of an alumina-silica ceramic fiber

The microstructure and mechanical properties of an alumina-silica ceramic fiber after thermal exposure at 1100–1300°C were investigated by X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy, transmission electron microscopy analyses and room temperature tensile strength test. The results showed that the fiber was composed of γ-A1₂O₃ and amorphous SiO₂. A phase reaction of γ-A1₂O₃ and amorphous SiO₂ occurred when thermal exposure temperature exceeded 1150°C, and a new mullite phase formed. The grain size of the newly formed mullite increased with the increase of exposure temperature. Both the phase transition and grain growth of mullite had a significant impact on the mechanical properties of the fiber. Tensile strength of the fiber decreased slightly when thermal exposure temperature was below 1150°C, while the strength retention of the fiber decreased sharply to 65.36% as exposure temperature rose to 1200°C. A higher dispersion of tensile strength was also observed at higher exposure temperatures, as revealed by the Weibull statistical model.     

Scheelite coatings on SiC fiber: Effect of coating temperature and atmosphere

Scheelite coating was deposited on SiC fiber tows from various liquid-phase precursors followed by heat treatments between 900 °C and 1100 °C in different atmospheres. The tensile strength was fully retained for the coated fibers treated at 900 °C in vacuum. Subsequent heat treatment at 1100 °C in Ar had little effect on the fiber strength, which is explained by the excepted good thermal stability between the scheelite coating and SiC fiber. However, larger strength degradation and poor spool ability of coated fibers prepared in Ar/air were found. Assisted oxidation of SiC fiber by calcium salts is suggested to be responsible for the much larger strength degradation of fibers prepared in Ar/air.     

Zirconia-alumina multiphase ceramic fibers with exceptional thermal stability by melt-spinning from solid ceramic precursor

Zirconia-alumina multiphase ceramic fibers with 80 wt% (Z80A20 fiber) and 10 wt% (Z10A90 fiber) proportions of zirconia were prepared via melt-spinning and calcination from solid ceramic precursors synthesized by controllable hydrolysis of metallorganics. The zirconia-alumina multiphase fibers had a diameter of about 10 µm and were evenly distributed with alumina and zirconia grains. The Z80A20 and Z10A90 ceramic fibers had the highest filament tensile strength of 1.78 GPa and 1.87 GPa, respectively, with a peak value of 2.62 GPa and 2.71 GPa. The Z80A20 ceramic fiber has superior thermal stability compared to the Z10A90 ceramic fiber and a higher rate of filament strength retention due to the stability in grain size. After heat treatment at 1100 °C, 1200 °C, and 1300 °C for 1 h respectively, the filament tensile strength retention rate of Z80A20 ceramic fibers was 87 %, 80 %, and 40 %. While Z10A90 ceramic fiber was fragile after being heated at 1300 °C. The results showed that the high zirconia content facilitated the fiber's thermal stability.     

Synthesis of high-performance mullite ceramics based on associated rare-earth kaolin

The objective of this work was to prepare high-purity, high-strength mullite ceramics from low-cost, associated rare-earth kaolin (AREK). A reaction sintering process using calcined AREK and γ-Al₂O₃ powders was used to synthesize high-performance mullite ceramics. Mineralogical, morphological, and chemical characteristics of AREK were given. The effects of associated REEs in kaolin and sintering temperature on the microstructural evolution, phase transformation, and physical properties of mullite were studied. The results showed that the mullite contents were 98.8%, the maximum aspect ratio was 8.22 μm, the relative density was 93.04%, and the micro-Vickers hardness and flexural strength were 10.63 GPa and 184.24 MPa, after sintering at 1500°C for 4 h. For comparison, calcined without rare-earth kaolin was also employed as a raw material to synthesize mullite ceramics, and the mullite content prepared by sintering the two kaolin clays at 1320–1480°C for 4 h was quite similar. However, mullite prepared using AREK forms secondary mullite in the temperature range of 1480–1500°C with a significantly higher mullite content, and therefore, the advantages of preparing mullite based on AREK as the raw material are high purity, low mullitization temperature, and high strength.     

Effect of in-situ formation of columnar mullite and pore structure refinement on high temperature properties of corundum casatbles

The effects of in-situ synthesis columnar mullite and pore structure on the hot modulus of rupture (HMOR), thermal shock resistance and corrosion resistance of corundum castables have been investigated in this paper. When 2% nano silica was added, the pore diameters of castables could be decreased to 15 nm (at 110 °C), 1 μm (1100 °C) and 6 μm (1500 °C), respectively. The corresponding reducing magnitude of pore size is 98.5%, 83.3% and 33.3%. The HMOR of castables fired at 1500 °C increased by 110% to 3.64 MPa. Furthermore, after three thermal shock cycles, the residual strength ratio of castables increased from 5.2% to 15.3%. A large amount of cross-distributed columnar mullite was formed between nano silica and α-Al₂O₃ by the two-dimensional nucleation mechanism, which remarkably enhanced the high temperature properties. The penetration index reduced from 30.86% to 19.88%, suggesting that smaller pore size and higher viscosity had a great influence to the penetration process.     

Strength degradation of alumina fiber: Irreversible phase transition after high-temperature treatment

The mechanical properties of alumina AF17-20 fiber after high-temperature treatment have been evaluated through tensile tests on single fiber and bundle. The tensile test on single fibers shows that the temperature has little effect on the elastic modulus of the fibers, which stables around 140 GPa. The test on bundles minimizes the personal errors thus giving a more reliable value of tensile strength. In general, as temperature increases, both the Weibull modulus and the tensile strength decrease gradually. De-sized fibers have the highest tensile strength, but inherent defects like pores still cause slight dispersion of the strength. Further, the strength maintains about 90% after treating below 1200 °C, and this insignificant decline is caused by the decrease of amorphous SiO₂ and the formation of aluminum silicate. In addition, the severe degradation in strength over 1200 °C is mainly attributed to the appearance and growth of mullite grains, which is only about 60% of the initial value.     

Evolution of the structure and mechanical performance of Cansas-II SiC fibres after thermal treatment

Herein, an in-depth analysis of the effect of heat treatment at temperatures between 900 and 1500 °C under an Ar atmosphere on the structure as well as strength of Cansas-II SiC fibres was presented. The untreated fibres are composed of β-SiC grains, free carbon layers, as well as a small amount of an amorphous SiC_xO_y phase. As the heat-treatment temperature was increased to 1400 °C, a significant growth of the β-SiC grains and free carbon layers occurred along with the decomposition of the SiC_xO_y phase. Moreover, owing to the decomposition of the SiC_xO_y phase, some nanopores formed on the fibre surface upon heating at 1500 °C. The mean strength of the Cansas-II fibres decreased progressively from 2.78 to 1.20 GPa with an increase in the heat-treatment temperature. The degradation of the fibre strength can be attributed to the growth of critical defects, β-SiC grains, as well as the residual tensile stress.     

Influences of quartz and muscovite on the formation of mullite from kaolinite

A kaolin containing muscovite and quartz (K-SZ) and a pure kaolin (K-SX) with the addition of potassium feldspar, K₂SO₄ and quartz, respectively, were used to investigate the influences of muscovite and quartz on the formation of mullite from kaolinite in the temperature range 1000–1500 °C. In K-SZ formation of mullite began at 1100 °C, and in K-SX at 1000 °C. In K-SZ quartz accelerated the formation of cristobalite and restrained the reaction of mullite and silica. Muscovite in K-SZ acted as a fluxing agent for silica and mullite before 1400 °C and accelerated the formation of cristobalite. The FTIR band at 896.8 cm^− 1 was used to monitor the formation of orthorhombic mullite.     

Stabilized tialite–mullite composites with low thermal expansion and high strength for catalytic converters

Aluminium titanate (AT) is a potential candidate material for use in demanding high temperature applications, because it exhibits an excellent thermal shock resistance due to its low thermal expansion coefficient and high refractoriness.However, industrial applications of this material are hindered by two major limitations. Its decomposition to α-Al₂O₃ and TiO₂ between 800 and 1280 °C and its low mechanical strength.The present work aims to stabilize aluminium titanate with the addition of Fe₂O₃. The decomposition of aluminium titanate–iron oxide solid solutions when heated at 1100 °C for up to 1000 h was studied. The effect of iron oxide addition on pure aluminium titanate properties was investigated. Additionally, strengthening of the iron stabilized AT with mullite was considered adding mullite (M), 3Al₂O₃·2SiO₂ to tialite body at various amounts (5–50%, w/w). Properties like four point bending strength, thermal expansion coefficient (TEC), and porosity of the composites, were evaluated. Finally, the effect of mullite on the mechanical properties of AT–mullite composites was investigated.It was found that aluminium titanate (iron oxide stabilized)–mullite composites exhibit very good mechanical strength combined with excellent thermal stability.     

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi₂ and commercial powders of Al₂O₃ and spinel (MgAl₂O₄). Careful oxidation of pulverised waste MoSi₂ rendered a precursor mixture of MoO₃ and amorphous SiO₂, which served as pore forming agent and SiO₂ source, respectively. Evaporation of MoO₃ at ～750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10^?6 K^?1) and elastic modulus (2.27 GPa).     

SiC fiber strength after low pO2 oxidation

Hi‐Nicalon?‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO₂ of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO₂, 1400°C with 100 ppm pO₂ or higher, and 1300°C with 50 ppm pO₂ or higher. When active oxidation did not occur, a glass SiO₂ scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO₂. Fiber strengths were lowest after heat treatment at 1300°C and a pO₂ of 3.7 ppm, 1400°C and a pO₂ of 20 ppm, and 1500°C and a pO₂ of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.     

Recycling of aluminum dross and silica fume wastes for production of mullite-containing ceramics: Powder preparation,sinterability and properties

The improper disposal of industrial wastes causes environmental pollution so their recycling for fabrication of new products became an interesting research issue. In this work, sintered mullite-containing ceramics were prepared from aluminum dross and silica fume (up to 40 wt%) waste materials after sintering up to 1500 °C. Before sintering, the starting waste materials were converted into nano powders by mechanical milling alloying method up to 15 h. The obtained waste nano powders were investigated using different techniques as X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). On the other hand, phase identification by XRD, physical properties determination (bulk density and apparent porosity), microstructure by SEM, mechanical and electrical properties of sintered bodies were investigated. The results revealed that mullite phase was formed in higher amounts with increasing both sintering temperature (1500 °C) and silica fume content. At 1300 °C, amorphous mullite was formed in addition to the alumina phase. It is also noted that the apparent porosity and bulk density were reduced with increasing silica content. However, they exhibited opposite trend when the temperature increased from 1300 into 1500 °C. Moreover, with increasing the mullite content, the microhardness, compressive strength, Younges modulus and electrical conductivity were decreased and reached 10.2 GPa, 216.9 MPa, 119.7 GPa and 4.9 × 10 ^?12 S/m, respectively, for the sample that contained higher amount of mullite, while the fracture toughness was improved and reached to 3.44 MPa m^0.5.     

High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

Evolution of the composition,microstructure and electromagnetic properties of HfOC ceramics with pyrolysis temperature

The evolution of phase composition, microstructure, and dielectric characteristics of HfOC ceramics pyrolyzed at various temperatures was studied in this work. When the pyrolysis temperature increased from 900 to 1500 °C, the composition of HfOC ceramics varies from HfO₂ and amorphous carbon (C_amp) at 900 °C to coexistence of HfO₂, C_amp, and HfC at 1100–1300 °C, and HfC and C_amp at 1500 °C. With the continuous consummation of C_amp, its distribution is transformed from a slice-like structure accumulating around the particles to a shell-like structure wrapping around the particles. The atomic ratios of as-obtained HfOC ceramics are HfO_2.0C_2.8, HfO_1.9C_2.7, HfO_1.0C_1.8, and HfO_0.1C_1.1, respectively, after being pyrolyzed at 900, 1100, 1300, and 1500 °C. As the pyrolysis temperature increases, the average value of the real part increases from 13.5 to 16.5, and the imaginary part rises from 12 to 14. The microwave absorption properties of HfOC ceramics need to be enhanced further in the future work.     

Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels

TiO₂-containig single-phase gels with (Al₂O₃ + TiO₂)/(SiO₂) molar ratios 3/2 and 2/1 were prepared by gelling mixtures of aluminium nitrate, tetraethylorthosilicate and titanium isopropoxide. Gels were fast heated at several temperatures up to 1100 °C. Dried and heated gels were characterized by differential thermal analysis (DTA), magic angle spinning nuclear magnetic resonance (MAS-NMR), X-ray powder diffraction (XRD), and scanning and transmission electron microscopies (SEM and TEM). Coupled DTA and XRD results of gels fast heated at 900 °C showed the crystallisation of two mullites as well as a small amount of alumina-silica spinel. ²⁷Al NMR spectra showed the formation of pentacoordinated aluminium before mullite crystallization. The increase of lattice parameters of single-phase mullites heated at 1100 °C indicated that the amount of TiO₂ incorporated into the mullite structure increased on raising the amount of nominal TiO₂ in both series. SEM and TEM images of heated gels at 1100 °C displayed the formation of well-shaped parallelepiped of titanium-doped mullite nanocrystals with crystalline anisotropy.     

Thermal shock resistance of the porous boron nitride/silicon oxynitride ceramic composites

The thermal shock resistance of the porous boron nitride/silicon oxynitride (BN/Si₂N₂O) ceramic composites were tested by the quenching‐strength method with temperature differences of 600‐1400°C. The residual flexural strength of the composites decreased with increasing temperature difference from 600°C to 900°C. This weakening in flexural strength was attributed to the formation of microcracks in the matrix caused by thermal stress damage. Afterward, as the formation of a dense oxidized layer sealed the surface and hindered further oxidation, the residual flexural strength increased with the further increase of temperature difference from 900°C to 1100°C. Finally, when the temperature differences were above 1100°C, the residual flexural strength gradually decreased with increasing temperature difference, which was attributed to the further oxidation and large thermal stress damage. And the thermal shock resistance of the porous BN/Si₂N₂O ceramic can be improved by the introduction of high contents of sintering aids and h‐BN.     

Fabrication and properties of mullite fiber matrix porous ceramics by a TBA-based gel-casting process

A bird nest-like structure was designed by using the mullite fiber as the matrix and SiO₂ as the high temperature binder. This special material was successfully prepared by a TBA-based gel-casting process. The randomly arranged fiber laps bonded by SiO₂ binder was the most important structure characteristic of this porous material. The effect of sintering temperature on the properties, i.e. porosity, bulk density, linear shrinkage, compressive strength, thermal conductivity and the microstructure was studied. The composite exhibited significant pseudoductility. The fracture mechanism of this composite under compression was discussed. The results indicated that the sintering temperature ranging from 1500 to 1600 °C was suitable for yielding mullite fiber matrix porous ceramics which had a low thermal conductivity (0.19–0.22 W/m K), a relatively high compressive strength (3–13 MPa) and a high resilience (66–70%) for applications in the thermal insulators and high-temperature elastic seal field.     

High pressure densification of nanocrystalline mullite powder

Investigations of the high-pressure sintered nanocrystalline mullite powder are presented. The synthesized mullite powder with crystallite size of 51 nm was densified by using high-pressure “anvil-type with hollows” apparatus at 4 GPa over the temperature range of 1100–1500 °C in 100 °C steps. The phase composition and structural parameters of the densified samples were studied as a function of densification temperature. The XRD analysis revealed the appearance of new phases, such as kyanite and corundum, whose development affected the densities of the sintered samples. High relative densities of the sintered samples were obtained because of the application of high pressure. The needle-like microstructure was developed owing to the anisotropic grain growth of mullite. The elongated mullite grains reached the length of approximately 5 µm at 1400 °C, whereas the grains treated at 1500 °C became thicker preserving the same needle length. The Vickers microhardness of the developed microstructures increased with the increase of temperature up to 1400 °C, while at 1500 °C it was slightly reduced due to the grain coarsening.