Urea effect on the mechanism of mullite crystallization

Mullite is an excellent structural material due to its physical and mechanical properties. In this study, mullite was obtained by the sol–gel process, using silicic acid, aluminum nitrate, and urea. The urea effect was studied by evaluating samples obtained from urea/Al³⁺ ratio equal to 0, 1, and 3. The kinetic study was conducted using the isoconversional, non-isothermal, Flynn–Wall–Ozawa method. The sample prepared without urea, which is the least homogeneous one, formed spinel and α-alumina at 1150 °C, and Al-poor mullite together with α-alumina, at 1200 °C. The Al-poor mullite crystallization process from this sample showed the lowest yield. The sample prepared with urea/Al³⁺ ratio equal to 1, which has an intermediate behavior, formed spinel at 1100 °C, Al-poor mullite at 1150 °C, and α-alumina together with Al-poor mullite at 1250 °C. However, the sample prepared with urea/Al³⁺ ratio equal to 3, the most homogeneous, formed spinel and Al-rich mullite at 1100 °C. This sample formed Al-poor mullite at 1200 °C with the highest yield. Moreover, the sample synthesized without urea showed a higher porosity and a greater amount of hexacoordinated aluminum at 350 °C. All samples showed the same kinetic model, Šesták and Berggren (SB) for Al-poor mullite crystallization. The samples synthesized with urea crystallized mullite through the same kinetic parameters and constant values of the activation energy, but the sample prepared without urea followed different kinetic parameters and values of activation energy which changed over the course of the crystallization.     

Synthesis and properties of in situ Si3N4-reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Silicon nitride (94.5% α, 5.5% β), BaCO₃, Al₂O₃, and SiO₂ powders were mixed and pressureless sintered to produce a ceramic matrix composite consisting of 30 vol% barium aluminosilicate (BaO·Al₂O₃·2SiO₂ or BAS) matrix reinforced with in situ grown whiskers of β-Si3N4. In situ X-ray studies of the reactions indicated that BaCO₃ decomposes first to yield BaO which reacts with SiO₂ to yield a series of barium silicates which then react with Al₂O₃ between 950 and 1300°C to yield hexacelsian BAS. The sintering times were varied in order to develop a material system that combines the favourable properties of BAS with the high strength of Si₃N₄. In situ high-temperature X-ray studies after composite processing did not reveal any changes in the BAS or Si3N4 up to temperatures of 1300°C. Dilatometry studies of the sintered composite indicated a low-temperature transformation between 230 and 260°C with the temperature of transformation and volume change associated with the hexagonal to orthorhombic transformation decreasing with an increase of sintering time. Room- and high-temperature (1400°C) strengths were evaluated using four-point bend flexural tests. Composites exhibited near theoretical densities and an increase in flexural strength that was primarily dependent on the higher α- to β-Si₃N₄ transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructure and property enhancement of silicon nitride-barium aluminum silicate composites with β-Si3N4 seed addition

Si₃N₄-barium aluminum silicate (BAS) self-reinforced composites have been prepared by pressureless sintering at 1800 °C for 2 h. The β-Si₃N₄ seeds incorporated in the starting α-Si₃N₄ powders encouraged the α- to β-Si₃N₄ phase transformation, and the final bimodal microstructure with large grains, consequently, led to the improvement of the fracture toughness, from 7.74 to 8.34 MPa m^1/2. The almost-complete crystallized BAS benefited the high-temperature mechanical properties. The residual stress, crack deflection, grain bridging, and pullout were considered as the major toughening mechanisms in this composite.     

Evolution of microstructure in flyash-containing porcelain body on heating at different temperatures

15 wt% flyash (a calcined byproduct of thermal power plant) was incorporated in a normal triaxial kaolin-quartz-feldspar system by replacing equivalent amount of quartz. The differences in microstructural evolution on heating the compact mass of both normal and flyash-containing porcelain at different temperatures (1150–1300°C) were examined using scanning electron microscopy (SEM) operating in secondary electron image (SEI) mode. Microstructure of normal porcelain did not show the presence of mullite and quartz grains at 1200°C and the viscosity of silica-rich glass restricted the growth of mullite crystals at 1250°C. Flyash porcelain, on the other hand, shows the presence of primary mullite aggregates in the clay relict and a significant growth of mullite crystals in a low viscosity glassy matrix at 1200°C itself. At 1300°C, both the bodies show a larger region of more elongated (> 1 μm) secondary mullite along with clusters of smaller sized primary mullite (< 1 μm). Small primary mullite crystals in the clay relict can be distinguished from elongated secondary mullite crystals in the feldspar relict in their size. Primary mullite aggregates remain stable also at higher temperatures. XRD studies were carried out for quantitative estimation of quartz, mullite and glass, which supported the SEM observations. An attempt was also made to correlate their mechanical strength with the constituent phases.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Semiconducting ceramics produced using nanocrystalline barium titanate powder

Positive temperature coefficient of resistance ceramics of composition (Ba_0.89Ca_0.08Pb_0.03)TiO₃ + Y₂O₃ + MnO + SiO₂ have been produced using barium titanate powder with an average crystallite size of 125 nm prepared by calcining barium titanyl oxalate at 900°C. The effect of firing temperature on their microstructure and electrical properties has been studied. The results demonstrate that the ceramics possess semiconducting properties starting at a firing temperature of 1205–1215°C. The room-temperature resistivity of the ceramics has a minimum at t _firing ≈ 1245–1250°C. The samples sintered at 1250–1260°C have the largest positive temperature coefficient of resistance. The highest electric strength (360 V/mm at ρ_25°C = 290 Ω cm) is offered by the thermistor materials sintered at 1260°C, which is 60–70°C below the firing temperature of analogous ceramics produced by solid-state reaction.     

Densification and mechanical properties of mullite-SiC nanocomposites synthesized through sol-gel coated precursors

Mullite-SiC nanocomposites are synthesized by introducing surface modified sol-gel mullite coated SiC particles in the matrix and densification and associated microstructural features of such precursor are reported. Nanosize SiC (average size 180 nm) surface was first provided with a mullite precursor coating which was characterized by the X-ray analysis and TEM. An average coating thickness of 120 nm was obtained on the SiC particles. The green compacts obtained by cold isostatic pressing were sintered in the range 1500–1700°C under pressureless sintering in the N₂ atmosphere. The percentage of the theoretical sintered density decreases with increase in SiC content. A maximum sintered density of 97% was achieved for mullite-5 vol.% SiC. The fractograph of the sintered composite showed a highly dense, fine grained microstructure with the SiC particles uniformly distributed along the grains as well as at the grain boundaries inside the mullite. The Vicker’s microhardness of mullite-5 vol.% SiC composite was measured as 1320 kg/mm² under an applied indentation load of 500 g. This value gradually decreased with an increase in SiC content.     

Effect of heat treatment in air on the structure and properties of barium osumilite reinforced with Nicalon fibres

Microstructural studies have been carried out on glass-ceramic matrix composites, consisting of barium osumilite reinforced with Nicalon fibres, which have been subjected to heat treatment in air in the range 600–1100 °C. Parallel studies have involved the measurement of the friction stress between fibre and matrix and the flexural strength of the composite. The matrix was shown to consist of barium osumilite, hexacelsian, mullite and a silica-rich glass, the thermal mismatch of these different phases leading to the development of appreciable strains. Whilst high-temperature treatments caused the formation of voids due to flow of the glassy phase, the major factor controlling the mechanical properties of the composite was the fibre/matrix interface. A change in microstructure, from a weak carbon-rich interface to one where the fibre and matrix were strongly bonded together by a silica layer, was thus reflected in an increase in the interfacial friction stress and a change in fracture behaviour from one showing fibre pull-out and delamination to one with brittle characteristics.     

Mechanical properties and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/mullite composite

An Al₂O₃/5 vol.% mullite composite was synthesized by using reaction sintering of Al₂O₃/0.78 wt.% SiC at 1,600 °C for 2 h in air. The phase analysis of the Al₂O₃/mullite composite was carried out using X-ray diffraction (XRD). There were two kinds of mullite in alumina/mullite composite, namely, 3Al₂O₃·2SiO₂ and Al_5.65Si_0.35O_9.175. The microstructure of the Al₂O₃/mullite composite was investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). The mechanical properties such as Young’s modulus, Poisson’s ratio, hardness, toughness and strength of the Al₂O₃/mullite composite were investigated. The influence of mullite on the composite is discussed.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

The preparation and characterization of SiC-AlN-Fe2Si composites with high porosity from allophane

The reaction products of an allophane heated with carbon at 850–1600 °C in the stream of nitrogen for a given time were characterized by X-ray diffractometry. As a result, it was found that cristobalite and mullite were stable phases at 850–1300 °C, β-Si₃N₄ and α-Al₂O₃ at 1300–1500 °C, and SiC-AlN-Fe₂Si at temperatures higher than 1500 °C. SiC-AlN-Fe₂Si composites with high porosity of about 50% were easily prepared by a heat treatment at a temperature higher than 1500 °C with carbon in a stream of nitrogen. The formation mechanism of the composites is kinetically discussed from a viewpoint of small-pore shrinkage and large-pore expansion by volume diffusion during heating. The resultant microstructure of the composites obtained is also discussed.     

SiCw/BAS复合材料的显微结构及力学性能的研究

本文采用热压烧结法制备出致密的SiC_w增强BAS玻璃陶瓷基复合材料．结果表明,BAS基体晶化后获得以钡长石为主晶相和莫来石为次晶相的复相BAS玻璃陶瓷．晶须的加入对BAS基体有显著的强韧化效果,加入30vol％SiC_w可使材料的室温抗弯强度和断裂韧性分别由基体的156MPa和1．40MPa·m^1/2提高到356MPa和4．06MPa·m^1/2．TEM观察结果表明,晶须／基体界面结合良好,无界面反应物和非晶层的存在．断口形貌和压痕裂纹扩展路径的SEM观察结果表明,复合材料的主要增韧机制为裂纹偏转、晶须的拔出和桥接．     

Fast-sintering of hydrothermally synthesized BaTiO3 powders and their dielectric properties

Hydrothermally synthesized barium titanate (BaTiO₃) powders with a submicrometre particle size have been fast-sintered with a heating rate of ∼ 200 °C min^-1 at various temperatures (1250–1350 °C) for short times (5 and 15 min). The microstructures and dielectric properties of the sintered samples are studied and compared with those sintered conventionally. The sample fast-sintered at 1250 °C for 5 min had the highest dielectric constant value of approximately 3700 with an average grain size of about 1 μm. Both the dielectric constant and the Curie–Weiss temperature are found to be dependent on the grain size of the sintered samples, particularly when the average grain size is less than 5 μm. This has been attributed to the presence of internal stress in the fine-grained BaTiO₃. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile and creep performance of a novel mullite fiber at high temperatures

The novel fiber CeraFib75 with a composition near to pure mullite was analyzed with respect to its potential for high-temperature applications. This mullite fiber free of glass phase was aimed to overcome the strength of commercial oxide fibers at high-temperatures. Tensile tests at room and high temperatures ranging from 900 to 1400 °C and creep tests were performed. Nextel™720, another crystalline mullite-alumina fiber, was tested as a reference. Microstructure and crystal phase analysis of the new fiber revealed mullite grains with traces of γ- and α-alumina in-between; it contains occasionally defects causing a reduced strength at room-temperature. Remarkably, at temperatures beyond 1200 °C, CeraFib75 presented a higher tensile strength than Nextel™720. During tensile tests at 1400 °C, an extended region of inelastic deformation was observed for CeraFib fibers only, which was related to a grain boundary sliding mechanism. Creep rates were of the same order of magnitude for both fibers.     

Effect of finishing rolling temperature on fire resistance and dynamic strain aging behavior of a structural steel

The influence of finishing rolling temperature (FRT) on dynamic strain aging (DSA) behavior and high-temperature resistance of a fire resistant steel microalloyed with Mo and Nb was investigated by means of tensile tests performed at temperatures ranging from 25 to 600 °C and strain rates of 10⁻⁴ to 10⁻¹ s⁻¹. In these steels, DSA manifestations are less intense than those observed for carbon steels and they take place at higher temperatures. The precipitation behavior of the steels was also considered. Hardness of samples heat treated at 100–600 °C displayed a maximum at 400 °C. Samples treated at this temperature and tensile tested at 600 °C did not show a higher yield stress than the untreated specimens. Results obtained indicated that DSA in the fire resistant steel might have a contribution for its fire resistance. The empirical activation energies related to the appearance of serrations on the stress–strain curves and to the maxima on the variation of tensile strength with temperature suggested that the high-temperature strengthening associated with DSA in this steel is the dynamic interaction of interstitial-substitutional solute dipoles and dislocations. The steel with lower FRT is more susceptible to DSA because of its higher amount of carbon in solid solution and showed better results in terms of high-temperature resistance.     

Effects of environment on creep behavior of two oxide/oxide ceramic–matrix composites at 1200 °C

The tensile creep behavior of two oxide/oxide ceramic–matrix composites (CMCs) was investigated at 1200 °C in laboratory air, in steam, and in argon. The composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, have no interface between the fiber and matrix, and rely on the porous matrix for flaw tolerance. The matrix materials were alumina and aluminosilicate. The tensile stress–strain behavior was investigated and the tensile properties were measured at 1200 °C. Tensile creep behavior of both CMCs was examined for creep stresses in the 80–150 MPa range. Creep run-out defined as 100 h at creep stress was achieved in air and in argon for stress levels ≤100 MPa for both composites. The retained strength and modulus of all specimens that achieved run-out were evaluated. The presence of steam accelerated creep rates and reduced creep life of both CMCs. In the case of the composite with the aluminosilicate matrix, no-load exposure in steam at 1200 °C caused severe degradation of tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated. Poor creep performance of both composites in steam is attributed to the degradation of the fibers and densification of the matrix. Results indicate that the aluminosilicate matrix is considerably more susceptible to densification and coarsening of the porosity than the alumina matrix. The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the U.S. Government.     

Effect of Ca/Si ratio on the microstructures and properties of CaO–B2O3–SiO2 glass-ceramics

CaO–B₂O₃–SiO₂ glass-ceramics were synthesized by sol–gel method, and the effect of Ca/Si ratio on the microstructures, electrical properties and mechanical characteristics of this ternary system was investigated. The results showed that the increase of CaO content is favorable for the crystallization of CaO–B₂O₃–SiO₂ system and formation of the desired glass-ceramics. The bending strength of the sintered glass-ceramics increases with CaO content by increasing of crystalline phases. When the Ca/Si ratio increases, the dielectric constant (ε _r) decreases and loss (tanδ) increases gradually. The thermal expansion coefficient (TEC) enhances by increasing CaO contents due to the formation of other crystal phases with large TEC value. The glass-ceramics exhibit low dielectric constant and loss (ε _r < 4.7, tanδ < 5 × 10^-4 at 1 MHz), high resistivity (ρ > 10¹² Ω · cm), as well as excellent mechanical properties (σ ≈ 160 MPa, α ≈ 3.6 × 10⁻⁶/°C).     

Sintering Characteristics of a Powder Injection Molded Ceria-Stabilized Zirconia–Mullite Composite

Powder injection molding (PIM) technology has the potential for economically manufacturing several complex-shaped zirconia–mullite components in mass production. The sintering behavior of a zirconia–mullite composite fabricated by injection molding was analyzed in this paper. The focus of this study is to assess the dependence on properties and microstructure on PIM processing conditions. The sintered density of the samples displayed a strong dependence on sintering temperature. The hardness of the samples followed a similar trend as sintered density. A maximum fracture toughness of 4.1 ± 0.3 MPa · m^1/2 and strength around 450 ± 60 MPa was observed for samples sintered at 1500°C for 4 h. The properties from this study are significantly higher than the values reported in majority of the prior studies where other technologies like uniaxial and cold isostatic pressing were used to fabricate zirconia–mullite composites. The above results support the suitability of PIM as a manufacturing process for complex-shaped zirconia–mullite components with good mechanical properties.     

Temperature-dependent dielectric and microwave absorption properties of silicon carbide fiber-reinforced oxide matrices composite

Simultaneous improvement of mechanical and microwave absorption properties of the composites at high temperatures still undergoes considerable challenges. We have investigated the high-temperature microwave absorbing properties of the silicon carbide fiber-reinforced oxide matrices (SiC_f/mullite–SiO₂) composite on the basis of our previous work. Results indicate that the complex permittivity increases from 8.19 ? j5.09 to 16.39 ? j9.83 at 10 GHz with the temperature rising from 200 to 600 °C. The SiC_f/mullite–SiO₂ composite has relatively high tanδ values indicating superior microwave attenuation ability. The reflection loss (RL) values of the composite increase with rising thickness. It can be noticed that the RL response curves of different thicknesses are basically consistent at 200 and 400 °C. In addition, the RL value of the composite is less than ??5 dB in the whole X band when the thickness is under 2.9 mm and the temperature is below 400 °C. The hybrid oxide matrices of mullite and SiO₂ are beneficial to improve the dielectric properties, especially high-temperature microwave absorption properties of the SiC fiber-reinforced ceramic matrix composite. The superior microwave absorption properties indicate that the SiC_f/mullite–SiO₂ composite is a promising candidate in aircraft engine nozzle and aerodynamic heating parts of aircrafts at high temperatures.     

Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C

The tensile creep behavior of a N610™/LaPO₄/Al₂O₃ composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10⁻⁸ s⁻¹. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10⁻⁶ s⁻¹. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al₂O₃ (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.