Phase transformations in xerogels of mullite composition

Monophasic and diphasic xerogels have been prepared as precursors for mullite (3Al₂O₃-2SiO₂). Monophasic xerogel was synthesized from tetraethyl orthosilicate and aluminium nitrate nanohydrate and the diphasic xerogel from colloidal suspension of silica and boehmite. The chemical and structural evolutions, as a function of thermal treatment in these two types of sol-gel-derived mullite precursor powders, have been characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy (IRS). Monophasic xerogel transforms to an aluminium-silicon spinel from an amorphous structure at 980 ° C. The spinel then changes into mullite on further heating. Diphasic xerogel forms mullite at 1360 ° C. The components of the diphasic powder react independently up to the point of mullite formation. The transformation in the monophasic powder occurs rapidly and yields strongly crystalline mullite with no other phases present. The diphasic powder, however, transforms rather slowly and contains remnants of the starting materials (-Al₂O₃, cristobalite) even after heating at high temperatures for long periods (1600 ° C, 6 h). The diphasic powder could be sintered to high density but not the monophasic powder, in spite of its molecular-level homogeneity.     

Sol-emulsion-gel synthesis of hollow mullite microspheres

Hollow mullite microspheres were obtained from emulsified diphasic sols by an ion extraction method. The surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined microspheres were investigated by using thermogravimetry analysis (TGA), differential thermal analysis (DTA), Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), optical and scanning electron microscopy (SEM) and particle size analysis. TGA indicated the removal of most of the volatiles, i.e. 30.77 wt% up to about 500°C. Crystallization of the Si-Al spinel at 900°–970°C in gel microspheres was confirmed by DTA and XRD. XRD results also showed the formation of orthorhombic mullite at 1200°C. FTIR indicated the sequence of transformations taking place during heat-treatment of gel microspheres at different temperatures. The optical and scanning electron microscopy confirmed the spherical morphology of the gel and calcined particles. Formation of hollow microspheres with a single cavity was identified by SEM. The particle size distribution of the mullite microspheres calcined at 1300°C/1h exhibited a size range of 6–100 m with an average particle size (d ₅₀) of 22.5 m.     

Role of hydrolysis water-alcohol mixture on mullitization of Al2O3-SiO2 monophasic gels

Two kinds of monophasic gels were synthesized from silicon ethyl ester (TEOS) and aluminium nitrate mixture by varying the water-alcohol content of the system. The mullitization reaction paths of the two gels were studied by differential thermal analysis (DTA) and X-ray diffraction techniques. The first kind of monophasic gel exhibited cubic mullite (Si-Al spinel) on the 980 °C exotherm and a further two more broad exotherms at 1150 and 1243 °C. This phenomenon resembles the DTA thermogram of a kaolinitic clay. The later two exotherms have been explained as due to the formation of mullite by two different reaction paths from the two phases appearing in the 980 °C exotherm. Nucleation and crystallization of mullite from amorphous alumino silicate phase takes place at 1150 °C, and transformation of cubic mullite to orthorhombic mullite takes place at 1245 °C. The second kind of monophasic gel exhibited only one exotherm and produced exclusively tetragonal mullite at 980 °C. The results suggest that pure mullite formation is possible when water induced hydrolysis of TEOS is avoided during the gelation process.     

Characterization of mono- and diphasic mullite precursor powders prepared by aqueous routes. 27Al and 29Si MAS-NMR spectroscopy investigations

The structural evolution from amorphous to crystalline mullite, for different 3Al₂O₃ · 2SiO₂ mono- and diphasic precursors, has been investigated by ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The crystallization has also been studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the ²⁹Si NMR spectra. A close agreement is found with the composition deduced from the lattice parameters measured by XRD. For monophasic precursors the amount of hexa-coordinated aluminium atoms decreases when the temperature increases while Al(IV) and Al(V) increase. Al(VI) practically completely disappears just before the crystallization at 980 °C. An alumina-rich mullite 2Al₂O₃ · SiO₂ (21 mullite) is then formed through a strong exotherm. An enthalpy of 75 kJ per mol is determined for the crystallization of the 21 mullite. At higher temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the ²⁹Si NMR spectroscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO₆ octahedra before the crystallization at 980 °C is higher. Spinel crystallizes and continues to become richer in alumina until it reacts with silica to form the stoichiometric 32 mullite at 1260–1275 °C. The nature of the crystallization is related to the local composition of the amorphous alumino-silicate network and to the amount of AlO₆ octahedra on approaching 980 °C.     

Oxidation behaviour of a pressureless sintered AlN-SiC composite

The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y₂O₃. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300–1450°C for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400°C for 100 h, in relationship with the microstructure of the exposed surfaces.The material starts to oxidize notably at temperatures higher than 1300°C. The oxidation kinetics is parabolic in the temperature range 1350–1450°C, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina.The surface modification induced by long term oxidation does not affect mechanical strength until 1200°C, while after oxidation at 1400°C, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation.     

Preparation of MgO-doped mullite by sol-gel method,powder characteristics and sintering

Mullite doped with MgO in quantities ranging from 0.01 to 1.5 wt% were prepared by the sol-gel method. The mullitization temperature decreases with increased MgO dopant content. The XRD patterns of the MgO-doped mullite calcined at temperatures up to 1600° C for durations ranging from 1 to 10 h did not show the presence of any other phase except mullite. The IR spectrum shows a broadening of the Al-O absorbance band at 1175cm^–1 with MgO content, indicating the solid solution of MgO. Sintering temperature decreases with increased MgO dopant content. The microstructure observed consisted of equiaxed grains. The TEM observation of the microstructure showed the presence of glassy pockets at the triple grain junctions. The thermal coefficient of expansion and dielectric constant were not changed up to 0.75wt% MgO dopant concentration. The three-point bend strength observed for 0.3wt% MgO-doped mullite at room temperature was 300 MPa and decreased below 200 MPa at 1400° C.     

Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide

Dense SiC ceramics were obtained by hot pressing of -SiC powders using Al₂O₃-Y₂O₃ and La₂O₃-Y₂O₃ additive systems. The effect of the addition of an amount of ultrafine SiC to commercial silicon carbide powder was evaluated. Sintering behaviour and microstructure depended on type and amount of liquid phase, as densification proceeded via a classical solution-reprecipitation mechanism. A core/rim structure of SiC grains indicated that reprecipitation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorphous silicates. Values of flexural strength up to 750 MPa at RT and up to 550 MPa at 1000 °C were measured. At 1300 °C a strong degradation of strength was attributed to softening of the amorphous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa·m^1/2 and hardness from 21 to 23 GPa.     

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m²/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 m were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.     

Effect of NiO and/or TiO2 mullite formation and microstructure from gels

Polymeric and colloidal gels with a constant molar ratio of (Al+Ni and/or Ti)/Si=3/1 and various (Al/Ni and/or Ti) ratios (up to 21.42 mol% NiO+TiO2) were prepared and used to study the effect of the precursor chemical homogeneity on mullite formation processes and the resulting microstructure. Both kinds of gel precursors were preheated at 750°C for 3 h in order to obtain appropriate gel-derived glasses for further thermal processing. After annealing for several time periods at temperatures between 750 and 1500°C, differences in crystallization pathways were observed. Polymeric gels crystallized Al–Si and NiAl2O4 spinels from the amorphous form at temperatures in the range between 900 and 1000°C, depending on the amount of aluminium substitution. Mullite formation was initiated at temperatures between 1100 and 1200°C, except for the higher substituted 3:2 mullite in which it was produced at 1000°C. In constrast, -Al2O3 and NiAl2O4 spinel were the first crystalline phases identified at 750°C in specimens from colloidal gels, whereas mullite was formed at temperatures higher than 1200°C. In specimens with high substitution, mullite was observed at lower temperatures. Although the sequences of reaction from either kind of gel were rather different, mainly at low temperatures (as could be inferred from the chemical homogeneity attained in both gel-derived glasses), the final set of crystalline phases after long annealing at 1400°C was quite similar. Differences in the microstructure of specimens from either type of gel precursor after annealing at 1400°C concerned the size of mullite particles and the presence of secondary phases in specimens derived from colloidal precursors. © 1998 Kluwer Academic Publishers     

Microstructure and permittivity of sintered BaTiO3: influence of particle surface chemistry in an aqueous medium

The influence of changes in the surface chemistry and surface composition of colloidal BaTiO₃, due to its dissolution and adsorption/precipitation of Ba²⁺ in an aqueous medium, on the microstructure and permittivity of sintered powder compacts was investigated. For BaTiO₃ powder with Ba-deficient (Ti-excess) surface prepared at pH 3, grain growth was enhanced at 1350 °C (above the eutectic) and permittivity was reduced (relative to stoichiometric BaTiO₃ prepared at pH 9) with increasing sintering temperature due to the liquid phase formed at grain boundaries. This same sample showed minimal grain growth and moderate enhancement of sinterability at 1300 °C (below the eutectic) attributed to sliding of the Ti-excess surface phase. BaTiO₃ powder treated at pH 3 and subsequently adjusted to pH 10 results in a core-shell structure with a varying near-surface stoichiometry, and produced abnormal grain growth for the compact sintered at 1350 °C. Permittivity of this sample was significantly reduced at 1350 °C due to the formation of the liquid phase, while exhibiting a similar permittivity to that of the stoichiometric sample when sintered at 1300 °C, despite significant microstructural coarsening. We conclude that changes in the surface-phase Ba/Ti ratio of particulate precursors, due to dissolution, adsorption and precipitation reactions in aqueous media, are as significant in determining the mechanical and electronic properties of the sintered material as are variations in the bulk stoichiometry of BaTiO₃.     

Characterization of the spinel phase in a diphasic mullite gel using dynamic X-ray diffraction

Dynamic X-ray diffraction (DXRD) has been used in an effort to identify the specific phase changes which are responsible for observed thermal events at 980 °C in mullite gel precursors. Specifically, changes in the evolution of the common and strongest diffraction peak (d = 0.139 nm) corresponding to both transient alumina phases and the Al-Si spinel were followed in order to descriminate between these two phases. Results which compare the DXRD results for a diphasic mullite gel and a boehmite gel are presented and suggest that the Al-Si spinel phase forms at 980 °C in diphasic gels along with - and/or -Al₂O₃. These results are corroborated by separate TEM measurements which indicate the presence of both phases in samples quenched from 1000 °C.     

Mullite formation from non-crystalline precursors

X-ray diffraction, differential thermal analysis, and²⁹Si and²⁷Al MAS NMR spectroscopy were used to characterize the formation of mullite from non-crystalline precursors. The precursors were produced by the sol-gel route from organic starting compounds (SGM), and by coprecipitation of inorganic starting materials (CM). There is NMR evidence that both types of mullite precursor consist of Si-O and Al-O tetrahedra, and of Al-O octahedra. Furthermore, fivefold-coordinated Al-O polyhedra, or alternatively strongly distorted Al-O tetrahedra, do occur in the precursors. SGM and CM precursors transform to mullite in multi-step reactions with intermediate formation of-Al₂O₃, and a non-crystalline nearly pure SiO₂ phase. The-Al₂O₃ compound has a highly distorted spinel structure, and may contain some minor amount of Si. The diffusion-controlled decomposition of the precursors to-Al₂O₃ + SiO₂ at low temperature (1000°C) suggests that the precursors are diphasic, consisting of nanometre-sized, long-range-disordered Al₂O₃- and SiO₂-rich domains. The reaction of-Al₂O₃ + SiO₂ above about 1000°C initially produces Al₂O₃-rich mullite. This is explained by a possible nucleation of mullite on the surfaces of-Al₂O₃ crystallites. The gradual transformation of the unstable, low-temperature (1000 to 1400°C) Al₂O₃-rich mullites to the stable high-temperature ( 1400°C) 3/2-type mullites (3Al₂O₃ · 2SiO₂) is essentially controlled by the annealing temperatures, whereas annealing times play a minor role.     

In situ reacted TiB2-reinforced mullite

In situ formation of TiB₂ in mullite matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed and pressureless-sintered samples, in addition to TiB₂, TiC was also found to be dispersed phases in mullite matrix. However, in the case of pressurelesssintered samples, mullite/TiB₂ composite with 98% relative density can be obtained through a preheating step held at 1300 °C for longer than 3 h and then sintering at a temperature above 1600 °C. Hot-pressed composite containing 30 vol% TiB₂ gives a flexural strength of 427 MPa and a fracture toughness of 4.3 MPam^1/2. Pressureless-sintered composite containing 20 vol% TiB₂ gives a flexural strength of 384 MPa and a fracture toughness of 3.87 MPam^1/2.     

Crystallization of a coprecipitated mullite precursor during heat treatment

Powder of mullite composition (3Al₂O₃·2SiO₂) has been made by a coprecipitation method. The evolution of mullite in this precursor powder during heat treatment has been studied using differential thermal analysis, electron microscopy and X-ray diffraction techniques. It is shown that during calcination below 1100°C the coprecipitate develops -Al₂O₃ and perhaps cristobalite crystallites within the basic grains, whose morphology is otherwise invariant with temperature. Mullite forms above 1100°C by reaction of these -Al₂O₃ and SiO₂ crystallites, and the grain morphology changes markedly. Small exothermic events occur at 1000 and 1250 °C. The former is associated with the decomposition of a small content of aluminosilicate or perhaps with the conversion of - to -Al₂O₃, and the latter with mullite formation. For comparison, the behaviour of a polymeric mullite precursor during calcination is also examined. This material showed a large exothermic event at 1000°C which could be associated with the decomposition of the (amorphous) aluminosilicate to crystalline -Al₂O₃ and SiO₂, and a small exothermic event at 1250° C due to mullite formation.     

Effect of pH on 980 °C spinel phase-mullite formation of Al2O3-SiO2 gels

Different reaction paths of mullite formation via sol-gel processing techniques are reviewed. These variations are due to differences in hydrolysis/gelation behaviours of the silica and alumina components used. Variations of pH during processing without altering other variables follow three different routes of mullite formation. In the highly acidic region(pH 1), the gel does not exhibit a 980 °C exotherm but forms -Al₂O₃. Mullite forms at high temperature by diminution of -Al₂O₃ and -cristobalite, respectively. In the pH range of 3–4.5, gels exhibit a 980 °C exotherm and develop only mullite. In the highly alkaline region (pH 14), the gel produces a Si-Al spinel phase at the 980 °C exotherm and mullite formation at the 1330 °C exotherm takes place from the intermediate Si-Al spinel phase.     

Some properties of mullite powders prepared by chemical vapour deposition

The sinterability of mullite (3Al₂O₃·2SiO₂) powder prepared by chemical vapour deposition was examined to improve the conditions for fabricating dense mullite ceramics. The starting powder contained not only mullite, but also a small amount of -Al₂O₃ (Al-Si spinel) and amorphous material. Although the compressed powder was fired at a temperature between 1550 and 1700 °C for 1, 3 and 5 h, the relative densities of the sintered compacts were limited to 90%: (i) due to the creation of pores/microcracks during the solid state reaction (1100–1350 °C), and (ii) due to restriction on the rearrangement of grains because the amount of liquid phase (1550–1700 °C) was insufficient. Calcination of the starting powder was effective for preparation of easily sinterable powder with homogeneous composition. When the compact formed by compressing the calcined powder at 1400 °C for 1 h was fired at 1650 °C for 3 h, the relative density was raised up to 97.2%; moreover, mullite was the only phase detected from the sintered compact. The sintered compact was composed of polyhedral grains with sizes of 1–2 m and elongated grains with long axes of 6 m.     

Preparation and study of YSTZ-Al2O3 nanocomposites

Yttria stabilized zirconia-alumina (YSTZ-Al₂O₃) nanocomposite system with various Al₂O₃ concentrations has been synthesized by sol-gel route. The experimental techniques XRD, DTA, TGA, FT-Raman, FT-IR, SEM, Vickers hardness measurements, density measurements and Impedance spectroscopy were used to characterize the synthesized specimens. DTA result shows two exothermic reactions: one around 760°C and another around 960°C. XRD results confirm that the specimen starts to crystallize on heating above 750°C. Well resolved XRD reflections corresponding to tetragonal (t) ZrO₂ were obtained after the specimens were heated at 1000°C. FT-Raman results confirmed that the crystallites developed above 750°C was t-ZrO₂. It was observed from the XRD and DTA results that the bulk and grain boundary region crystallize independently in two different temperatures with a difference in temperature of about 200°C. The crystallization temperatures increase with Al₂O₃ contents. At 1300°C, the pure YSTZ and 5 and 10 wt % Al₂O₃ added YSTZ specimens underwent structural transformation from tetragonal to monoclinic ZrO₂. But, the tetragonal symmetry remains stable at 1300°C with an addition of 15 wt % Al₂O₃. The system which retain its tetragonal symmetry at its processing temperature (1300°C) gives high hardness and maximum density values. Almost 100% theoretical density value was obtained at 1300°C with an addition of 15 wt % of Al₂O₃.     

Study on oxidation behavior of (Nd,Y)- and (Nd,Yb)-α-Sialon

The oxidation behavior of multi-cation -Sialons containing Nd and Y or Yb has been investigated for the compositions (Nd_0.18Y_0.18)Si_10.38Al_1.62O_0.54N_15.46 and (Nd_0.18Yb_0.18)Si_10.38Al_1.62O_0.54N_15.46 respectively in the temperature range of 1200°C to 1400°C in air. The grains of silicate containing Nd and Y as well as Nd and Yb were observed in preferred orientation on the surface of the materials oxidized at 1200°C or 1300°C for 20 h for (Nd,Y)- and (Nd,Yb)--Sialon respectively. By increase of oxidation temperature from 1300°C to 1400°C, bubble, which was caused by softening of silicate oxidation layer, occurred and glassy phase then appeared obviously. The phases formed on the surfaces of multi-cation -Sialons during the oxidation were also discussed in this paper.     

Low temperature needle like mullite grain formation in sol-gel precursors coated on SiC porous substrates

Dense mullite coating having thickness in the range of 3 to 5 μm was produced from sol-gel mullite precursor coated on SiC porous substrates at heat treatment temperatures as low as 1300 °C. Mullite formed in the coating layer was characterised by X-ray diffraction. The precursors have an average particle size of 170 nm and the mullite formed in the coating in situ has a grain size of 3-5 μm. Mullite grains formed on the SiC have needle like morphology. The mullite formation has been explained on the basis of reaction between the silica-alumina nano precursor and the needle like morphology has been similar to that formed from a liquid phase. The gas permeation analysis shows that there is considerable difference between gas pressure while using SiC substrate before and after coating and hence clearly indicated reduction in pore size. This particular approach is good since usual mullite formation is at high temperature and is difficult to attain small grain size. Further, in situ formed mullite, in this investigation covers the SiC surface protecting the SiC from oxidation at high temperature.     

Densification of LaGaO3 at low sintering temperatures via Fe3+ substitution at Ga3+ site

Attempts to achieve near theoretical density in polycrystalline LaGaO₃ compacts using Fe₂O₃ as a dopant have been made. The B-site substituted lanthanum gallates with iron doping ranging from 15 to 55 mole% FeO_1.5 (Fe₂O₃) were synthesized via solid state reaction using oxide and nitrate precursors and sintered at 1200°C for 24 to 72 h and at 1300°C for 12 to 36 h. Systematic variation in the lattice constants and microstructural aspects was followed as a function of mole fraction of Fe₂O₃. It was found that the parent LaGaO₃ structure could take up as high as 55 mole% of Fe₂O₃ retaining the orthorhombic crystal structure. Compositions containing 35 mole% Fe₂O₃ soaked for 48 h or more at 1200°C and for as low as 12 h at 1300°C resulted in perfectly dense compacts. Bodies containing lower dopant concentrations (x_{Fe 2 O 3} 0.25) were found to have a substantial degree of porosity with good intergranular connectivity, while a significant amount of grain growth and broad grain size distribution was an interesting feature in samples containing mole fractions higher than 0.35.