Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

The activation mechanism of oxalic acid on γ-alumina and the formation of α-alumina

Converting the γ phase into the α phase completely is necessary in the presintering stage of industrial alumina (Al₂O₃), which requires high temperature and energy consumption. To reduce the presintering temperature, γ-Al₂O₃ was activated by oxalic acid. XRD, ²⁷Al-MAS-NMR and TG-DSC were used to characterize the γ - alumina before and after activation, and the phase transformation was studied. The formation temperature of α-Al₂O₃ decreased to 1029 °C for oxalic acid activated γ-Al₂O₃, and the α-fraction was 100% for activated γ-Al₂O₃ at 1300 °C. After oxalic acid activation, the diffraction peak intensity of γ-Al₂O₃ decreased significantly; the results of ²⁷Al-MAS-NMR suggested that octahedral [AlO₆] in γ-Al₂O₃ was easier than tetrahedral [AlO₄] to be attacked by oxalic acid, and the formation of pentavalent [AlO₅] with higher reaction activity, which was in favour of the lowering formation temperature of α-Al₂O₃. The dissolution concentration of Al increased after oxalic acid activation, and the dissolution process was controlled by surface reactions. Oxalic acid mainly attacked the octahedral aluminium in γ-Al₂O₃ and extracted Al as three complexes of [Al(C₂O₄)]⁺, [Al(C₂O₄)₂]^- and [Al(C₂O₄)₃]^3-. Oxalic acid activated γ - Al₂O₃ with a lower phase transformation temperature has broad application prospects in the alumina industry.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Effects of alumina sols on the sintering of α-alumina ceramics

The effects of two kinds of alumina sols on the densification behavior of sub-micron grain sized α-alumina ceramics have been investigated. Composition of the sol-derived gels was investigated by energy dispersive spectra and Fourier transform infrared spectra. Structural evolution of the gels at different temperatures was characterized by a combination of X-ray diffraction and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy. Results showed that the gel containing chlorine and carbon transformed to α-alumina at about 950 °C, significantly lower than the other gel which transformed at about 1050 °C. Density measurements and scanning electron microscopy analyses were used to investigate the sintering of alumina ceramics with or without alumina sols. It was found that the alumina sols had profound effects on the densification of alumina ceramics. The ceramic displayed the best densification behavior when the sol containing chlorine and carbon was added.     

Novel heterogeneous zinc triflate catalysts for the rearrangement of α-pinene oxide

New solid acid catalysts based on silica‐supported zinc triflate have been prepared for use in the rearrangement of α-pinene oxide to campholenic aldehyde. These catalysts exhibit considerable activity and can be recycled without loss of selectivity towards the aldehyde. The selectivity towards the aldehyde can be increased to 80% (at 50% conversion) when the reaction is performed at 25°C using hexagonal mesoporous silica (HMS₂₄) as the catalyst support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanochemical synthesis of zinc ferrite from zinc oxide and α-Fe2O3

Mechanochemical synthesis of zinc ferrite (ZnFe₂O₄) from a powder mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) by room temperature grinding using a planetary ball mill was investigated. The grinding enables us to obtain the amorphous mixture of the starting materials. Most of ZnO reacts with α-Fe₂O₃ to convert into insoluble amorphous zinc and iron compounds within 2h-grinding. Prolonged grinding enhances the crystallization of ZnFe₂O₄ from the amorphous compounds. ZnFe₂O₄ crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.     

Hydrostatic treatment of α-aluminium oxide powders part 1. Microstructure of the particles and sintering

Transmission electron microscopy and X-ray radiography are used to show that pressing of -aluminum oxide powders at pressures up to 2.0 GPa is accompanied by crushing of particles and the appearance of dislocations with a density of about 10¹¹ cm^–2. Sintering of these powders at temperatures up to 1550°C reduces the dislocation density 2–4-fold. When the compacts reach a critical density (about 62%) the shrinkage in sintering of nonsintered powders is shown to be independent of the compact density and the pore size. The increase in the sintering rate with increase in the hydrostatic pressure to 0.6 GPa is shown to be caused mainly by an increase in the compact density and at still higher pressures by the mechanical activation of the powders.Translated from Ogneupory, No. 6, pp. 13–19, June, 1994.     

Phase transformation during the sintering of γ-alumina and the simulated Ni-laden waste sludge

This study investigated the fundamental phase transformation process when incorporating simulated nickel-laden waste sludge by alumina-based ceramic system. To match the required sintering conditions for commercial ceramic products as a development of waste-to-resource strategy, the investigation focused on the nickel incorporation behavior of γ-alumina in moderate to low temperature sintering environments (1250–750 °C) under a 3 h sintering process. X-ray diffraction (XRD) and Rietveld refinement were used to show the quantitative distribution of phases. The results demonstrated that γ-alumina interacted intensively with nickel oxide at sintering temperatures above 1000 °C. Furthermore, the densification effect of the sintering process is also illustrated, as this is crucial for developing the needed mechanical strength for practical applications. The findings in this study reveal that there is good potential for achieving the stabilization of nickel by thermal treatment with a γ-alumina containing precursor via moderate ceramic sintering temperatures.     

A comparison of the tape casting of α- and β-alumina

This work concerns the manufacture of planar cell configurations for molten sodium batteries in energy storage devices such as vehicle batteries and stationary storage cells. Tape casting of beta-alumina electrolyte could provide a low-cost mass production route but intriguingly there are only a few reports of tape casting using beta-alumina directly as the raw powder. We first compared tape casting of α-alumina and beta-alumina using polyvinyl butyral (PVB) in conventional formulations. While it is relatively easy to obtain homogeneous α-alumina tape cast sheet, beta-alumina resulted in adhesion to the substrate and cracking. These problems were shown to be attributable to particle characteristics. When the binder was changed to polymethylmethacrylate (PMMA), tape casting of beta-alumina and three different α-alumina powders was facilitated, producing a formulation that was more tolerant to different powder types. Screening of several commercial dispersants provided two which were effective with PMMA and a conventional MEK/ethanol dual solvent.     

Effects of Sc2O3 sintering aid for the densification and mechanical properties of SiC–ZrB2 composites

This study examined the effects of a Sc₂O₃ sintering aid on the density, microstructure and mechanical properties of SiC–5 vol% ZrB₂ composites prepared by hot-pressing. Microstructural studies showed that the addition of Sc₂O₃ not only caused a decrease in the hot-pressing temperature from 1950 to 1750 °C by liquid phase sintering, but also resulted in the formation of crystalline Sc₄Zr₃O₁₂ at the grain boundaries via a reaction with ZrO₂ on the surface of the ZrB₂ powder. The addition of Sc₂O₃ produced a fine-grained microstructure with a 43% (430→615 MPa) and 20% (3.6→4.3 MPa m^1/2) increase in flexural strength and fracture toughness, respectively, compared to the SiC–ZrB₂ composite without Sc₂O₃.     

Is spark plasma sintering suitable for the densification of continuous carbon fibre - UHTCMCs?

For the first time we show that spark plasma sintering can efficiently replace hot pressing for the densification of UHTCMCs, in the present case ZrB₂/SiC composites reinforced with continuous carbon fibres. To this purpose, the same materials were first produced by hot pressing as baseline samples and then by spark plasma sintering (SPS) to compare microstructure and basic mechanical properties. A special emphasis was given to the study of interfaces, in case of both coated and uncoated carbon fibres.SPS allowed for faster sintering but required an adjustment of the temperature to avoid fibre degradation compared to hot pressing. With similar porosity levels, we observed a slight decrease of flexural strength (300 vs 470 MPa), and an improvement of fracture toughness (15 vs 10 MPa√m) for SPSed samples. SPS was proved to be an effective method for the consolidation of continuous fibre reinforced UHTC composites.     

Atomistic modeling of dopant segregation in α-alumina ceramics: Coverage dependent energy of segregation and nominal dopant solubility

Microstructural control is a key aspect in producing ceramics with tailored properties and is often achieved by using dopants in a rather empirical fashion. Atomic scale simulations could provide much needed insight but the long-standing challenge of linking simulation results on isolated grain boundaries to those measured in real ceramics needs to be resolved. Here a novel Monte-Carlo simulation method based on a microstructural model in combination with energies obtained from atomic scale energy minimization is presented. This approach allows, for the first time, the prediction of the nominal solubility of dopants (Y, La and Mg) in a ceramic purely from theory.Results compare well with segregation/precipitation data as a function of grain size, found in the literature. The method can therefore be used in developing experimental guidelines for the effective use of dopants in ceramic production, thus accelerating the development of novel materials required for innovative applications.     

Effects of doping cations on the densification and microstructure of flash-sintered α-Al2O3

The effects of dopants with different valences on the flash sintering behavior of α-Al₂O₃ are investigated. The results indicate that regardless of their valence and ionic radius, all the tested dopants reduce the onset temperature more effectively compared to undoped Al₂O₃. Interestingly, the relative density and grain size of the flash-sintered samples exhibit an inverse linear relationship. The data for the samples doped with non-trivalent cations fall on a different line than those for the samples doped with trivalent cations and the undoped samples. Dopants have an effect on the flash sintering behavior of Al₂O₃ because flash sintering increases the solubility of the dopants in alumina, creating more point defects, and thereby increasing the electrical conductivity of the material. The mechanisms for point defect generation in trivalent and non-trivalent dopants are different.     

Synthesis and spark plasma sintering of the α-Si3N4 nanopowder

A two-step process (milling and then heat treatment) was used for the preparation of α-Si₃N₄ nanopowder. The influence of the milling time and heat treatment temperature as processing parameters were investigated on the formation of α-Si₃N₄. Silicon nitride ceramic was produced by spark plasma sintering at 1700 °C for 15 min, using MgSiN₂ additive. The optimum sample was produced in a 30 h milling time, heat treatment at 1300 °C, and a 22 °C/min heating rate conditions. X-ray fluorescence analysis showed that the purity of the final product is above 98%. Nanoindentation hardness and Young’s modulus of the SPS-ed sample were measured as 17±2.0 GPa and 290±11.0 GPa, respectively.     

Effect of heat treatment on the creep behavior of α/β SiAlON sintered with multication oxide sintering additives

SiAlONs are important materials for high-temperature applications and creep properties of SiAlONs are largely controlled by the amount and type of sintering additives. It has been established that heat treatment can reduce the amount of amorphous intergranular phase through crystallization. However, there is no study on the creep behavior of heat-treated SiAlON ceramics containing multication sintering additives. Therefore, the aim of the study was to investigate the effect of heat treatment on the creep properties of multication containing (Y-Sm-Ca oxides) α/β-SiAlON ceramics. The heat treatments of the sintered samples were carried out at 1600°C for 2 hours. The creep tests were carried out in the range 1300-1400°C under different loads (50-150 MPa). The existing phases and the microstructures of samples before and after creep were investigated using XRD and SEM techniques. It was found that heat treatment resulted in a better creep performance compared to as-sintered samples. The activation energy and stress exponent for heat-treated SiAlONs were also calculated as 708 ± 45 kJ/mol and 1.4, respectively. Compared to the sintered sample values, the results suggested that the acting creep mechanism of grain-boundary sliding and cavitation was reduced with the heat treatment.     

Influence of stabilizing ions and sintering process on the thermal conductivity of α-SiAlON ceramics

α-SiAlON ceramics with different stabilizing ions of Yb, Dy, Nd, Y, Ca, and binary stabilizing ions of (Yb + Ca) and (Yb + Nd) were prepared by spark plasma sintering at 1600°C and gas pressure sintering at 1800 and 1900°C, and their thermal conductivity was investigated. It was found that α-SiAlON ceramics with larger and heavier stabilizing ions had lower thermal conductivity and the thermal conductivity could be further reduced by using binary stabilizing ions, which can be explained by phonon scattering from point defects. At the same time, the samples prepared at lower sintering temperatures showed smaller grain sizes and lower thermal conductivity. The relationship between the thermal diffusivity of samples and temperature was studied, where the dependence of inverse thermal diffusivity on temperature was better fitted by a quadratic fitting function than the usual linear one over a wide temperature range from 25 to 800°C.     

Study of the deactivation and regeneration of copper chromite on γ-alumina and magnesium chromite on γ-alumina catalysts for fuel combustion

The causes of the deactivation of catalysts for fuel combustion MeCr₂O₄/ -Al₂O₃ (Me = Cu or Mg) have been investigated using a variety of complex physical-chemical methods: IRS, ESDR, XPS, TPD and a pulse microcatalytic method. It has been concluded that the observed deactivation of catalysts during fuel combustion is due to the combined effect of high temperatures and reduction media.     

Synthesis and evaluation of acyl-chain- and galactose-6'-modified analogues of α-GalCer for NKT cell activation

α-GalCer is an immunostimulating glycolipid that binds to CD1d molecules and activates invariant natural killer T (iNKT) cells. Here we report a scaled-up synthesis of α-GalCer analogues with modifications in the acyl side chain and/or at the galactose 6'-position, together with their evaluation in vitro and in vivo. Analogues containing 11-phenylundecanoyl acyl side chains with aromatic substitutions (14, 16-21) and Gal-6'-phenylacetamide-substituted α-GalCer analogues bearing p-nitro- (32), p-tert-butyl (34), or o-, m-, or p-methyl groups (40-42) displayed higher IFN-γ/IL-4 secretion ratios than α-GalCer in vitro. In mice, compound 16, with an 11-(3,4-difluorophenyl)undecanoyl acyl chain, induced significant proliferation of NK and DC cells, which should be beneficial in killing tumors and priming the immune response. These new glycolipids might prove useful as adjuvants or anticancer agents.     

The role of zinc oxide in Cu/ZnO catalysts for methanol synthesis and the water–gas shift reaction

All commercial catalysts for methanol synthesis and for the water–gas shift reaction in the low temperature region contain zinc oxide in addition to the main active component, copper. The varied benefits of zinc oxide are analysed here. The formation of zincian malachite and other copper/zinc hydroxy carbonates is essential in the production of small, stable copper crystallites in the final catalyst. Further, the regular distribution of copper crystallites on the zinc oxide phase ensures long catalyst life. Zinc oxide also increases catalyst life in the water–gas shift process by absorbing sulphur poisons but it is not effective against chloride poisons. In methanol synthesis, zinc oxide (as a base) removes acidic sites on the alumina phase which would otherwise convert methanol to dimethyl ether. Although bulk reduction of zinc oxide to metallic zinc does not take place, reduction to copper–zinc alloy (brass) can occur, sometimes as a surface phase only. A new interpretation of conflicting measurements of adsorbed oxygen on the copper surfaces of methanol synthesis catalysts is based on the formation of Cu–O–Zn sites, in addition to oxygen adsorbed on copper alone. The possible role of zinc oxide as well as copper in the mechanisms of methanol synthesis is still the subject of controversy. It is proposed that, only under conditions of deficiency of adsorbed hydrogen on the copper phase, hydrogen dissociation on zinc oxide, followed by hydrogen spillover to copper, is significant. This revised version was published online in June 2006 with corrections to the Cover Date.