Thermal exposure effects on the long‐term behavior of a mullite fiber at high temperature

The behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high‐temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200°C to 1400°C for 25 hours. Quantitative high‐temperature X‐ray analysis and creep tests at 1200°C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as‐received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200°C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina‐rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400°C did not show further changes when exposed again to 1200°C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.     

Creep in Interlaminar Shear of a Nextel™720/aluminosilicate Composite at 1100°C in Air and in Steam

Creep in interlaminar shear of an oxide–oxide ceramic composite was evaluated at 1100°C in air and in steam. Composite consists of a porous aluminosilicate matrix reinforced with mullite/alumina (Nextel^?720) fibers, has no interface between fibers and matrix, and relies on the porous matrix for flaw tolerance. The interlaminar shear strength was 7.6 MPa. Creep behavior was examined for shear stresses of 2–6 MPa. Creep run‐out of 100 h was not achieved. Larger creep strains and higher creep strain rates were produced in steam. However, steam had a beneficial effect on creep lifetimes. Composite microstructure, damage, and failure mechanisms were investigated.     

Effect of coating deposition temperature on monazite coated fiber

Monazite (LaPO₄) was continuously coated on 3M Nextel^™ 720 fiber tows with an ethanolic precursor using hexadecane for immiscible liquid displacement. Coating deposition temperatures were varied from 900 to 1300°C. Fibers coated at 900°C were heat-treated up to 100 h at 1200°C. Coated fibers were characterized by analytical TEM, and tensile strengths were measured by single filament tensile tests. The monazite precursor was characterized by X-ray, DTA/TGA, and mass spectrometry. Microstructure evolution was complex and may have involved recrystallization of large defective grains into smaller grains and then subsequent growth of these grains, along with coarsening of porosity. After 100 h at 1200°C there was significant roughening of the coating–fiber interface, with facetting of alumina grains in the fiber and some lanthanum segregation to these facetted boundaries. Spheroidization of thin coatings was also observed. Tensile strength of coated fiber decreased with increasing deposition temperature and with time at temperature after deposition. Possible reasons for the strength decrease are discussed.     

Enhancing thermal stability of oxide ceramic matrix composites via matrix doping

To overcome the main limitation of oxide ceramic matrix composites (Ox-CMCs) regarding thermal degradation, the use of matrix doping is analyzed. Minicomposites containing Nextel 610 fibers and alumina matrices with and without MgO doping were produced. The thermal stability of the minicomposites was evaluated considering their microstructure and mechanical behavior before and after thermal exposures to 1300 °C and 1400 °C for 2 h. Before heat treatment, both composite types showed very similar microstructure and tensile strength. After heat treatment, densification, grain growth and strength loss are observed. Furthermore, the MgO dopant from the matrix diffuses into the fibers. As a result, abnormal fiber grain growth is partially suppressed and MgO-doped composites show smaller fiber grains than non-doped composites. This more refined microstructure leads to higher strength retention after the heat treatments. In summary, doping the matrix can increase the overall thermal stability without impairing the room-temperature properties of Ox-CMCs.     

Fiber Strength After Grain Growth in Nextel™ 610 Alumina Fiber

Nextel^? 610 alumina fiber tows were heat‐treated at 1100°C–1500°C for 1 to 100 h in air. Tensile strengths and Weibull moduli were measured for 30 filaments after each heat‐treatment. 3‐D grain size and orientation distributions were described using oblate ellipsoids. The number of grains in a 1 inch gauge length and grains with the largest major and minor ellipsoid‐axes were determined from these distributions. The grain with the largest K_EFF for mixed‐mode fracture was also determined, using the maximum energy release rate criteria from grain‐size and orientation distributions. Grain‐size dependence of tensile strength and Weibull modulus was evaluated. Strength had no obvious dependence on grain size for fibers with average major‐axes smaller than 0.25 μm. For fibers with larger grains, grain‐size dependence may involve flaws originating from clumps of grains, rather than a single grain. Possible relationships between strength and grain‐size and other causes of strength degradation after heat‐treatment are discussed.     

Electrical Conductivity of Mullite Ceramics

The electrical conductivity of a lab‐produced homogeneous mullite ceramic sintered at 1625°C for 10 h with low porosity was measured by impedance spectroscopy in the 0.01 Hz to 1MHz frequency range at temperatures between 300°C and 1400°C in air. The electrical conductivity of the mullite ceramic is low at 300°C (≈0.5 × 10^?9 Scm^?1), typical for a ceramic insulator. Up to ≈ 800°C, the conductivity only slightly increases (≈0.5 × 10^?6 Scm^?1 at 800°C) corresponding to a relatively low activation energy (0.68eV) of the process. Above ≈ 800°C, the temperature‐dependent increase in the electrical conductivity is higher (≈10^?5 Scm^?1 at 1400°C), which goes along with a higher activation energy (1.14 eV). The electrical conductivity of the mullite ceramic and its temperature‐dependence are compared with prior studies. The conductivity of polycrystalline mullite is found to lie in‐between those of the strong insulator α‐alumina and the excellent ion conductor Y‐doped zirconia. The electrical conductivity of the mullite ceramic in the low‐temperature field (< ≈800°C) is approximately one order of magnitude higher than that of the mullite single crystals. This difference is essentially attributed to electronic grain‐boundary conductivity in the polycrystalline ceramic material. The electronic grain‐boundary conductivity may be triggered by defects at grain boundaries. At high temperatures, above ≈ 800°C, and up to 1400°C gradually increasing ionic oxygen conductivity dominates.     

Preparation of mullite ceramics with equiaxial grains from powders synthesized by the sol-gel method

Four different alumina content of mullite ceramics were fabricated by powders synthesized using the sol-gel method. The synthesis process of powders, microstructure evolution, mechanical and optical properties of the mullite ceramics were studied. The XRD results showed that the precursors transformed into aluminosilicate spinel phase at 1000 °C and mullite phase at 1200 °C. Equiaxial grains were easy to form in the alumina-rich mullite ceramics while elongated grains were easy to form in the alumina-poor mullite ceramics. With the increase of alumina content, the grain size of the samples firstly increased and then decreased, the number of elongated grains decreased while equiaxed grains increased. The flexural strength, compression strength, fracture toughness, and Vickers hardness all decreased firstly and then increased. While the infrared transmittance increased firstly and then decreased. The transmittance at 4 μm (thickness of 0.75 mm) of the ceramics containing 66mol% Al₂O₃ reached the highest (72%) when sintered at 1780 °C because of the equiaxial grains.     

Electrospun mullite nanofibers derived from diphasic mullite sol

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.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

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.     

Hot pressing of bimodal alumina powders with magnesium aluminosilicate (MAS) addition

High quality alumina ceramics were fabricated by hot-pressed sintering using bimodal alumina with superfine component as raw material and magnesium aluminosilicate (MAS) glass as sintering aid. Densification behavior, microstructure evolution and mechanical properties of alumina were investigated from 1300 °C to 1450 °C. The bimodal alumina powders were sintered to 99.8% of the theoretical value at 1400 °C and a comparative dense microstructure with a few plate-like abnormal grains was observed. With increase of sintering temperature up to 1450 °C, many fine matrix grains were consumed and quite a few abnormal grains impinged upon each other. For the alumina ceramics hot-pressed from bimodal alumina with 30 wt.% superfine component, optimal mechanical properties were obtained at 1400 °C. The bending strength and fracture toughness were 522 MPa and 5.0 MPa m^1/2, respectively.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Fabrication of unidirectional continuous fiber-reinforced mullite matrix composite with excellent mechanical property

This article reports the fabrication of continuous fiber reinforced mullite matrix composite via layer-by-layer assembly method, involving the coating of mullite fiber with BN coating, followed by compositing the coated fibers with mullite matrix and hot pressing. The influences of the fiber coating, fiber content and sintering temperature on the microstructure and mechanical properties of the composite are investigated. By optimizing the sintering temperature and fiber content, the damage of fiber could be avoided and the microstructure and mechanical properties could be improved. The composite containing 30?vol% fibers coated by BN layer sintered at 1300?°C exhibits 90.9% theoretical density with flexural strength value of 203.2?MPa and fracture toughness value of 4.74?MPa?m^1/2. Fracture behavior is investigated to explain the toughening mechanisms. The layer-by-layer assembly method realizes the achievement of an individual architecture featuring fibers distribution and weak interfaces.     

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%.     

Microstructural evolution under load and high temperature deformation mechanisms of a mullite/alumina fibre

A two-phase mullite alumina fibre, the 3M Nextel 720 fibre, has been studied in tension and creep. The fibre shows the highest creep resistance of all current commercial fine oxide fibres up to 1500 °C. The creep mechanisms involve progressive dissolution of mullite and simultaneous reprecipitation of alumina into elongated oriented grains and grain boundary sliding by a thin alumino-silicate liquid phase. The rate of grain growth in creep at a given temperature is dependant on the applied stress. The combination of sub-micron size mullite crystallites and alumina grains gives rise to a high sensitivity to alkaline contamination. Stress enhanced diffusion of the contaminants from the fibre surface results in crack nucleation, dissolution of mullite, formation of a liquid phase and slow crack growth. From 1200 °C, this process is coupled with a fast α-alumina grain growth at the fibre surface.     

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the ‘use temperature’ of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments were studied. Microstructural stability experiments were conducted in air from 1200 to 1500°C and creep tests at temperatures of 1400 to 1700°C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400°C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior are discussed.     

Preparation of mullite by the reaction sintering of kaolinite and alumina

In the present study, mullite specimens and mullite/alumina composites are prepared by reaction sintering kaolinite and alumina at a temperature above 1000°C. The phase and microstructural evolution of the specimens and their mechanical properties are investigated. Primary mullite appears at a temperature around 1200°C. The alumina particles are inert to the formation of primary mullite. Alumina starts to react with the silica in glassy phase to form secondary mullite above 1300°C. The formation of secondary mullite decreases the amount of glassy phase. Furthermore, the addition of alumina reduces the size of mullite grains and their aspect ratio. The strength and toughness of the resulting mullite increase with the increase of alumina content; however, the mechanical properties of the mullite and mullite/alumina composites are lower than those of alumina for their relatively low density.     

Fabrication of ultralight heat-insulating hollow-fiber mullite fiberboards

Mullite fiberboards have been extensively used in heat-insulating refractory materials. To further improve the thermal insulation properties and reduce the density, we fabricated mullite fiberboards by vacuum filtration using hollow mullite fibers based on a ceiba fiber template. The effects of sintering temperature and type and content of high-temperature adhesives on the density, compressive strength, thermal conductivity, microstructure, and volume stability of mullite fiberboards were investigated. The results showed that the obtained mullite fiberboards have ultralow density of 0.1–0.2 g/cm³, low thermal conductivity of 0.0988–0.1230 W/(m·K) (at 500 °C), and compressive strength of 0.08–0.12 MPa, and they exhibit good volume stability at 1300 °C.     

Creep of Nextel™ 610 Fiber at 1100°C in Air and in Steam

Creep of Nextel^?610 fibers was investigated at 1100°C and 100–500 MPa in air and in steam. The effect of loading rate on fiber tensile strength was also explored. The presence of steam accelerated creep and reduced fiber lifetimes. Loading rate had a considerable effect on tensile strength in steam, but not in air. A linear elastic crack growth model was used to predict the creep lifetimes from the constant loading rate data. The dependence of tensile strength on loading rate and the predictability of creep lifetimes suggest that the failure mechanism in steam was environmentally assisted subcritical crack growth. The creep‐rupture data were analyzed in terms of a Monkman‐Grant (MG) relationship. Monkman‐Grant parameters for creep‐rupture data were the same in steam and air, and predicted creep‐rupture at 1100°C in both environments. A grain‐size increase of about 25% was observed by TEM after 100 h at 1100°C in steam, which was about two times that observed in air.     

Electrospinning preparation and microstructure characterization of homogeneous diphasic mullite ceramic nanofibers

In this work, diphasic mullite (3Al₂O₃·2SiO₂) nanofibers with good homogeneity were prepared by electrospinning method. Aluminum nitrate (AN) and aluminum isopropoxide (AIP) were used as alumina sources, commercial colloidal silica as silica source, and polyvinyl alcohol (PVA) as polymer additive. Precursor nanofibers with continuous and uniform structures were acquired at mass ratio of PVA to precursor sol from 0.06 to 0.09. γ-Al₂O₃ phase was obtained at 878 ^°C and mullite phase formed at 1322 ^°C upon heating of the precursor under air atmosphere. Calcined samples suggested mullite as dominant phase at 1300 ^°C and amorphous SiO₂ could even exist at 1400 ^°C. As-prepared nanofibers possessed continuous structures with subequal average diameters at 900–1200 °C. However, such morphological characteristics were lost at temperatures above 1300 ^°C due to rapid growth of crystal grains. Al and Si elements were uniformly distributed in fibers and mixed at nanoscale, confirming homogeneity and diphasic features of nanofibers.