Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Glass formation and phase transformations in plasma prepared Al2O3-SiO2 powders

The structure of Al₂O₃-SiO₂ sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al₂O₃ and partially amorphous in the range 52 to 88 wt % Al₂O₃. The crystalline phase was mullite up to 75 wt % Al₂O₃ but at higher Al₂O₃ contents a metastable solid solution of SiO₂ in -Al₂O₃ was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al₂O₃ end of the Al₂O₃-SiO₂ system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al₂O₃ contents up to 60 wt %. The viscosity change is relatively small as the Al₂O₃ content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al₂O₃ concentrations greater than approximately 80 wt %, -Al₂O₃ is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al₂O₃. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO₂ in -Al₂O₃, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO₂ in solution in metastable Al₂O₃ increases the temperature of transformation to -Al₂O₃ to greater than 1500° C compared with 1230° C for pure Al₂O₃.     

Preparation of SiO2-Al2O3 glass powders by the sol-gel process for dental applications

SiO₂-Al₂O₃ binary glass powders with Al₂O₃ contents up to 50 wt% have been synthesized by the sol-gel process. The starting solution (sol) consisted of 1 mole of tetraethoxysilane, 50 moles of water containing aluminum nitrate, 10 moles of ethyl alcohol and 0.02 moles of HCl catalyst. The sol was converted into gel by heating at 85°C and suction by running water. The sintering process was analysed by DTG thermal analysis. The fired powders were examined by XRD. It was confirmed that the gels were converted into the glass structure without crystallization. The measurements of refractive index and powder size of fired powders revealed that these powders could be used as refractive-index-adjustable fillers for dental composite resins. The other potential usage of the SiO₂-Al₂O₃ powders obtained might be in the production of powders for dental cements.To whom correspondence should be addressed at Osaka University, Faculty of Dentistry, Department of Dental Technology, 1-8 Yamadaoka, Suita, Osaka 565, Japan.     

The effect of additives on the crystallization and sintering of 2MgO-2Al2O3-5SiO2 glass-ceramics

Crystallization and sintering behaviour of three cordierite (2MgO-2Al₂O₃-5SiO₂) glasses containing different amount of additives were investigated and compared by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Archimedes method. The stoichiometric 2MgO-2Al₂O₃-5SiO₂ (MAS) glass and the 2MgO-2Al₂O₃-5SiO₂ glass containing 3 wt% of B₂O₃ and 3 wt% of P₂O₅ (MASBP) showed two exotherms (one for -cordierite formation from a glass and the other for -cordierite formation from the -cordierite phase), whereas the 2MgO-2Al₂O₃-5SiO₂ glass containing 2 wt % of B₂O₃, 2 wt% of P₂O₅, and 2 wt % of TiO₂ (MASBPT) showed only a single exotherm representing -cordierite formation. By using Kissinger, Augis-Bennett, Ozawa, and modified Kissinger methods, the activation energy values for -cordierite formation in the MASBP and MASBPT glasses were determined as 310±6 and 326±13 kJ mol^–1, respectively, whereas that in the MAS glass was determined as 868±5 kJ mol^–1. The MASBPT glass showed the lowest peak temperature value for -cordierite formation (980 °C) amongst the three glasses. Both the MASBP and MASBPT glasses showed excellent sintering behaviour (> 99.7% of theoretical density).     

Effect of liquid-forming additives on the sintering and mechanical properties of Al2O3/3Y-TZP (30 vol.%) composite

Al₂O₃/3Y-TZP (30 vol.%) composite was pressurelessly sintered with addition of TiO₂MnO₂ and/or CaOAl₂O₃SiO₂ glass. It was found that TiO₂MnO₂ addition greatly enhanced the densification of the composite by the formation of a low-viscosity liquid at sintering temperature. In contrast, the high-viscosity liquid formed by CaOAl₂O₃SiO₂ glass improved mechanical properties because of its repressing effect on grain growth. The composite could be obtained at a temperature as low as 1400°C by co-doping with TiO₂MnO₂ and CAS glass. Bending strength of 552±64 MPa and fracture toughness of 6.03±0.22 MPa m^1/2 were obtained with a doping level of 2 wt.% TiO₂MnO₂ and 2 wt.% CAS glass.     

Sintering and crystallisation of a P2O5-added Li2O-ZrO2-SiO2 glass powder system

Sintering and crystallisation of a 11.5 wt % Li₂O, 22.8 wt % ZrO₂, 65.7 wt % SiO₂ glass powder with P₂O₅ added were investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and was completed in a very short temperature interval (T 100°C) in less than 30 min. Crystallisation took place just after completion of densification and was almost completed at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallisation stage. The glass powder compacts first crystallised into lithium metasilicate (Li₂SiO₃) and/or zircon (ZrSiO₄) and tridymite (SiO₂) which transformed and/or grew into lithium disilicate (Li₂Si₂O₅), zircon and tridymite after the crystallisation process was essentially complete, so that, a crystallinity degree between 52.4 ± 2.0 and 68.5 ± 3.2 wt % was obtained. P₂O₅ doping little affected the densification. However, adding P₂O₅ remarkably enhanced the zircon and tridymite crystallisation while delaying the Li₂SiO₃ to Li₂Si₂O₅ transformation. The microstructure is characterised by fine crystals uniformly distributed arbitrarily oriented throughout the residual glass phase.     

Sintering and dielectric properties of 0.88Al2O3-0.12TiO2 microwave ceramics by glass addition

The microwave characteristics and the microstructures of 0.88Al₂O₃-0.12TiO₂ with various amounts of MgO-CaO-SiO₂-Al₂O₃ (MCAS) glass sintered at different temperatures have been investigated. The sintering temperature can be lowered to 1300 °C by the addition of MCAS glass. The densities, dielectric constants (ε_r) and quality values (Q×f) of the MCAS-added 0.88Al₂O₃-0.12TiO₂ ceramics decrease with the increase of MCAS glass content. The temperature coefficients of the resonant frequency (τ_f) are shifted to more negative values as the MCAS content or the sintering temperatures increase. The change of the crystalline phases of Al₂TiO₅ phase and rutile-TiO₂ phase has profound effects on the microwave dielectric properties of the MCAS-added Al₂O₃-TiO₂ ceramics. As sintered at 1250 °C, 0.88Al₂O₃-0.12TiO₂ ceramics with 2 wt.% MCAS glass addition exists a ε_r value of 8.63, a Q×f value of 9578 and a τ_f value of +5 ppm/°C.     

Sintering and crystallization of off-stoichiometric BaO·Al2O3·2SiO2 glasses

Glass-ceramics with off-stoichiometric celsian composition of 50 wt% BaO·2SiO₂ – 50 wt% BaO·Al₂O₃·2SiO₂, (B2S-BA2S) were fabricated and investigated for their sintering and crystallization characteristics. (B2S-BA2S) glass powder showed a melting temperature much lowered compared to that of stoichiometric BaO·Al₂O₃·2SiO₂ (BA2S) glass powder and high sintering ability. (B2S-BA2S) glass powder containing B₂O₃, (B2S-BA2S)B and that containing B₂O₃ and TiO₂, (B2S-BA2S)BT revealed much lowered crystallization peak temperatures, but rather low sintered density. By applying Kissiger analysis to differential thermal analysis (DTA) data activation energy values for crystallization were determined as 265, 195 and 242 kJ/mol, respectively for (B2S-BA2S), (B2S-BA2S)B and (B2S-BA2S)BT glasses. X-ray diffraction (XRD) patterns from all the glass-ceramics crystallized at 1100°C for 4 h revealed formation of crystalline phases of -BaO·2SiO₂, monocelsian and hexacelsian. (B2S-BA2S) glass-ceramics crystallized at 1400°C for 4 h showed formation of -BaO·2SiO₂ and monocelsian phases with only trace of metastable hexacelsian phase.     

Preparation and sintering of narrow-sized Al2O3-TiO2 composite powders

Narrow size distribution Al₂O₃-TiO₂ composite powders containing nominally 10 to 60 mol % TiO₂ were prepared by the stepwise hydrolysis of titanium alkoxide in an Al₂O₃ dispersion. Particle size was controlled by selecting the particle size of the starting Al₂O₃ powder. The TiO₂ content was determined by the amount of alkoxide hydrolysed. Composite powder compacts were prepared by filter casting or centrifugal casting the composite powder dispersions. All compacts had similar shrinkage behaviour during sintering. When fired above 1300° C, the compacts containing less than 50 mol % TiO₂ became Al₂TiO₅-Al₂O₃ composite bodies with high densities, the compact containing 50 mol % TiO₂ became an Al₂TiO₅ body with domain structure, and the compact containing 60 mol % TiO₂ formed a TiO₂-Al₂TiO₅ composite structure. When these composite bodies were annealed below 1300° C, they showed different decomposition behaviour and microstructures.     

Sintering and crystallization of off-stoichiometric SrO·Al2O3·2SiO2 glasses

Glass-ceramics with the celsian-corundum binary join composition of 88.8 wt% SrO·Al₂O₃·2SiO₂ – 11.2 wt% Al₂O₃, (SA2S-A), were fabricated by pressureless sintering and investigated for their sintering and crystallization behaviors. The (SA2S-A) glass powder showed crystallization peak and melting temperatures of 1059 and 1550 °C, respectively and high sintering ability. The (SA2S-A) glass powders containing B₂O₃, (SA2S-A)B and those containing B₂O₃ and TiO₂, (SA2S-A)BT showed lowered crystallization peak temperatures of 1033 and 997 °C, respectively. By applying Kissiger analyses to the DTA data of the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders, the activation energy values for crystallization were determined as 488, 370 and 333 kJ/mol, respectively. The Ozawa analyses on the DTA data gave the Avrami parameter values at 1.2, 1.1 and 1.9, respectively for the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders. The x-ray diffraction (XRD) patterns of the (SA2S-A) glass-ceramics, crystallized at 1100 °C for 4 h, showed formation of both the monocelsian and hexacelsian phases. The (SA2S-A)B and (SA2S-A)BT glass-ceramics crystallized at 1100 °C for 1 h, showed formation of the phase-pure monocelsian and did not show any evidence of the hexacelsian formation prior to the monocelsian formation.     

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.     

Low-temperature sintering characteristics of nano-sized BaNd2Ti5O14 and Bi2O3–B2O3–ZnO–SiO2 glass powders prepared by gas-phase reactions

Nano-sized BaNd₂Ti₅O₁₄ (BNT) powders were prepared by spray pyrolysis from solutions containing ethylenediaminetetraacetic acid and citric acid. Treatment at temperatures ≥900 °C and subsequent milling resulted in nanoparticle powders with orthorhombic crystal structures. The mean particle size of the powder post-treated at 1000 °C was 160 nm. Nano-sized Bi₂O₃–B₂O₃–ZnO–SiO₂ glass powder with 33 nm average particle size was prepared by flame spray pyrolysis and used as a sintering agent for the BNT. BNT pellets sintered at 1100 °C without the glass had porous structures and fine grain sizes. Those similarly sintered with the glass had denser structures and larger grains.     

Low-temperature mechanochemical–thermal synthesis of α-Al2O3 nanocrystals

The great capability of high-energy ball milled basic polyaluminium chloride gel (PACl) which consisted of the monomeric Al³⁺ ions and polymerized Al³⁺ species such as [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ with a Keggin like structure to enhance the phase transformation into α-Al₂O₃ nanocrystals at low temperature was verified. PACl gel 10 min milled and annealed at 400 °C, partially transformed to nanocrystalline α-Al₂O₃ with the mean XRD crystallite size in the range of 15–17 nm, embedded in an amorphous or transition alumina matrix. Further crystal growth up to ∼50 nm and phase pure α-Al₂O₃ powder was obtained when heat-treated at 1000 °C. In contrast to this, the non-milled PACl gel transforms to the transition θ-Al₂O₃ phase at 1000 °C. The evolution of α-Al₂O₃ nanocrystals was studied by XRD, TEM, selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) methods.     

Sintering and crystallization of (SrO·SiO2)-(SrO·Al2O3·2SiO2) glass-ceramics

In the ternary SrO-Al₂O₃-SiO₂ system the pseudobinary join composition of 50 wt % SrO·SiO₂–50 wt % SrO·Al₂O₃·2SiO₂ (SS-SA2S) showed a glass melting temperature of 1500 °C and a crystallization peak temperature of 1100 °C. The (SS-SA2S) glass-ceramic pellets prepared by cold pressing and pressureless sintering, showed very low porosity. The (SS-SA2S) glass-ceramics containing B₂O₃ and those containing B₂O₃ and TiO₂ revealed crystallization peak temperatures of 1000 °C and unexpectedly high porosity. By applying Kissinger analyses to the DTA data the activation energy values for crystallization of the three glass-ceramics were determined to range from 196 to 255 kJ/mol. The Ozawa analyses on the DTA data gave the Avrami parameter values at 3.69 to 3.95. The X-ray diffraction (XRD) patterns from the three glass-ceramics revealed formation of the equilibrium crystalline phases of SrO·SiO₂ and SrO·Al₂O₃·2SiO₂ (monocelsian).     

Compaction and sintering behaviour of glass–alumina composites

The effects of Al₂O₃ additions on the compaction and sintering behaviour of a leadborosilicate glass (LG) have been investigated. LG powder was prepared by melting, fritting and milling a glass of the composition: 77PbO, 10B₂O₃, 10SiO₂, 2Al₂O₃ and 1P₂O₅ (wt.%). The mean particle sizes of the powders were: LG, 6.5 μm and Al₂O₃, 3.3 μm. The compaction behaviour of LG–Al₂O₃ powder mixtures can be represented by a new compaction equation: [(D_g−D₀)/(1−D₀)]=(P/P_f)ⁿ, where D_g is the relative green density, D₀ the relative tap density and n and P_f are material constants. The exponent n decreases from 0.192 to 0.065 as the Al₂O₃ content is increased from 0 to 100 vol.%. The Frenkel equation for isothermal shrinkage has been found to be valid. It is shown that in the glass matrix composites the minimum sintering temperature can be determined by measuring the dilatometric deformation temperature. The presence of Al₂O₃ in excess of 15 vol.% has been found to strongly retard the sintering kinetics. An addition of 45 vol.% Al₂O₃ increases the activation energy for sintering from 67 to 112 kcal mol⁻¹. The presence of Al₂O₃ particles also induced a partial crystallisation in LG matrix.     

Nucleation and crystal growth in a fly ash derived glass

The devitrification behaviour of a fly ash derived glass, examined by differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy (SEM), is reported and discussed. The crystallized phases were identified as mullite (3Al₂O₃·2SiO₂) and anorthite (CaO·Al₂O₃·2SiO₂). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 790°C and the activation energy for crystal growth E=370 kJ mol^–1. The crystal morphology was investigated by SEM and the crystal shape found to be consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.     

Sol–gel processing and phase characterization of Al2O3 and Al2O3/SiC nanocomposite powders

Monolithic Al₂O₃ and Al₂O₃/SiC nanocomposite powders were prepared by sol–gel processing. The process involved the precipitation of Al(NO₃)₃·9H₂O with NH₄OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, γ-, δ-, θ-, to α-Al₂O₃. The ²⁷Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the γ-, δ- and θ-Al₂O₃ formed at increasing calcination temperatures. Complete transformation to octahedral Al (α-Al₂O₃) is marked by the abrupt disappearance of tetrahedral Al. Al₂O₃/SiC nanocomposite powders were prepared by adding α-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the ²⁹Si NMR spectra of the Al₂O₃/SiC powders reveal α-SiC, Al₂O₃·xSiO₂ and SiO₂ phases. Stable α-Al₂O₃ and α-Al₂O₃/SiC nanocomposite powders are formed at 1200 and 1300 °C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al₂O₃ in the nanocomposites by stabilising the Al₂O₃ phases with concomitant oxidation of SiO₂.     

Sintering characteristics and crystallization for sol–gel-derived powders for low-dielectric and low-temperature sintering ceramics

Samples in the MgO–Al₂O₃–SiO₂ system were prepared by the sol–gel technique. The coalescence, sintering characteristics, and crystallization were investigated by X-ray diffraction, thermal analysis, and scanning electron microscopy. The dielectric properties and density were measured with an impedance analyzer. Results demonstrated that the obtained cordierite powders synthesized with the sol–gel method distribute uniformly and its effective size is 474 nm. The glass powder could be sintered at 950 °C, and the polymorphic modification cordierite detected in the sintered sample was only stable hexagonal -phase. The sintering densification process was performed mainly in the temperature range from 800 °C to 930 °C, and follows the viscous floating principle. The dielectric constant of the sintered body is 4.2 and its dielectric loss is lower than 0.001 at high frequency (1.5 GHz).     

Al2O3-Ta2O5-rich glass-ceramic with interconnected pores and its sintering

Substitution of SiO₂ in the ternary sodium borosilicate system with Al₂O₃ plus Ta₂O₅ was found to produce glass which decomposed into microphases and/or crystallized after heat treatment. At least one of the phases present was water soluble. The structure of the material was glassy with the presence of a small crystalline content. Crystalline forms found during powder X-ray diffraction analysis of heat treated, leached and then sintered materials were orthorhombic NaTaO₃ plus AIBO₃, orthorhombic NaTaO₃ and orthorhombic Na₂O · 4Ta₂O₅ plus rhombic 9AI₂O₃ · 2B₂O₃, respectively. The specific surface areas of the leached materials ranged between 96 and 304 m²g^–1, while the mean pore radii of interconnected pores were calculated to be between 2.0 and 8.4 nm. A sintering rate of between 1520 and 1580° C for 5 min were estimated from void volume and bulk density measurements.     

Effects of amorphous silica coatings on the sintering behaviours of SiC whisker-reinforced Al2O3 composites

In this study, pressureless sintering of silicon carbide whisker (SiC_w)-reinforced alumina composites was investigated. SiC whiskers or Al₂O₃ powders were coated with amorphous silica, and sintering behaviour was analysed according to the powder characteristics of the composite. It was found that amorphous silica coatings improved densification as compared with uncoated powders, because the viscous flow allows the release of any tensile stress due to differential shrinkage between the matrix and the silicon carbide whiskers. Mullite occurred when amorphous silica coatings reacted with alumina at 1500 °C, which resisted the viscous sintering of the amorphous silica coatings.