A sol–gel-derived α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> crystal interlayer modified 316L porous stainless steel to support TiO<Subscript>2</Subscript>, SiO<Subscript>2</Subscript>, and TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> hybrid membranes

A homogeneous α-Al₂O₃ crystal membrane was fabricated by the sol–gel technique on 316L porous stainless steel (PSS) substrate with an average pore size of 1.0 μm. The preparation process was optimized by carefully choosing the binder, the concentrations of the casting solutions and the sintering temperatures of the membranes. Compared to methylcellulose and polyethylene glycol 20000, polyvinyl alcohol 1750 was found to be the most effective binder to fabricate a homogeneously structured Al₂O₃ membrane without defects. The concentration to prepare an uniform coverage membrane with a thickness of ~10 μm was 0.032 mol/L. When sintered at 1000 °C, γ-Al₂O₃ membrane with ~3 μm grains was obtained. When sintered at 1200 °C, γ-Al₂O₃ completely transformed into α-Al₂O₃ and the grains grew to ~5 μm. Accordingly, the process was applied to a bigger pore-sized PSS with an average pore size of 1.5 μm to fabricate an α-Al₂O₃ intermediate layer to initially modify its surface. A single α-Al₂O₃ crystal layer with a thickness of ~5 μm and an average pore size of 0.7 μm was achieved. Subsequently, TiO₂, SiO₂, and TiO₂–SiO₂ hybrid membranes were tried on the modified PSS. Defect-free microfiltration membranes with average pore sizes of ~0.3 μm were readily fabricated. The results indicate that the sol–gel method is promising to initially modify the PSS substrates and the sol–gel-derived α-Al₂O₃ crystal layer is an appropriate intermediate layer to modify the PSS and to support smaller grain-sized top membranes.     

Preparation of mullite bonded porous SiC ceramics by an infiltration method

A powder compact of α-SiC and α-Al₂O₃ was infiltrated with a liquid precursor of SiO₂, which on subsequent heat treatment at 1500 °C produced a mullite bonded porous SiC ceramics. Results showed that infiltration rate could be estimated by using weight gain measurements and theoretical analysis. The bond phase was composed of needle-shaped mullite which was observed to be grown from a siliceous melt formed during the process of oxide bonding. The porous SiC ceramics exhibited a density and porosity of 2 g cm⁻³ and 30 vol%, respectively, and also a pore size distribution in a range of 2–15 μm with an average pore size of 5 μm. No appreciable degradation of room temperature flexural strength (51 MPa) was observed at high temperatures (1100 °C).     

Thermo-mechanical and Microstructural Characterization of Geopolymers with <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particle Filler

Geopolymers with different content of α-Al₂O₃ particle filler were prepared. The thermo-mechanical and microstructural characterization of the obtained geopolymers were systematically studied by flexural strength and thermal shrinkage measurements, TG-DTA (thermogravimetry and differential thermal analysis), XRD (X-ray diffractometry), and SEM (scanning electron microscopy). The results show that the addition of α-Al₂O₃ particle filler not only increases the onset crystalline temperature but also reduces the crystalline velocity of the geopolymers. The thermal shrinkage of the geopolymers increases with increasing heat treatment temperatures due to the water loss and densification. The flexural strength of the geopolymers increases with the increase of heat treatment temperatures from RT to 1200 °C, and shows a sharp increase in the range from 600 °C to 800 °C due to crystallization and solidification. The increase in content of α-Al₂O₃ particle filler can clearly reduce the thermal shrinkage and maintain a higher porosity at high temperatures. However, it has no distinct influence on the flexural strength after heat treatment. This is mainly attributed to the higher thermal resistance and strength of α-Al₂O₃.     

Fabrication of asymmetric alumina membranes: I. Effect of SrO addition on thermal stabilization of transition aluminas

The effect of SrO addition on the thermal stabilization of transition aluminas with the aim of producing membrane layers (supported and unsupported) has been investigated. Al₂O₃–x wt.% SrO composite powders (x = 1, 3, 5, 8) were synthesized by co-precipitation of the hydroxides from solutions of AlCl₃ and Sr(NO₃)₂ salts using NH₄OH as a precipitating agent. Optimum SrO dopant concentration regarding the transition aluminas stabilization effect was determined to be 5 wt.% based on XRD analysis. STA analysis showed a 30 °C shift versus higher temperatures in the transformation of final transitional alumina (θ-Al₂O₃) to stable alpha phase due to addition of 5 wt.% SrO. The mechanism of transition aluminas thermal stabilization as a result of SrO addition is thoroughly discussed. Unsupported alumina membranes were prepared by drying boehmite sols at 600, 800, 1000 and 1100 °C. The effect of calcination temperature on surface area, pore size distribution of unsupported membranes containing 5 wt.% SrO has been investigated. The microstructure of unsupported and supported membranes revealed quite different. Smaller grains in the supported layers were attributed to the interaction between support and membrane.     

Effect of sintering temperature and holding time on the properties of 3Y-ZrO2 microfiltration membranes

Sintering behavior of supported and unsupported microfiltration membranes prepared from 3 mol% yttria doped zirconia powder was investigated as a function of temperature and holding time in non-isothermal and isothermal densification. Shrinkage that started at 1000°C showed the highest rate between 1200°C and 1300°C although the rate decreased above 1300°C. The activation energy of sintering was calculated at 735 kJ/mol, assuming the grain boundary diffusion mechanism for mass transport. Mean pore size decreased in unsupported membranes and increased in supported ones as the sintering temperature increased up to 1200°C. Dimensional shrinkage of unsupported membrane slabs showed an increase in shrinkage first in the lateral dimension and then in the thickness as the sintering temperature increased. Pore growth and lower hardness in supported membranes, can be explained due to the restricted lateral shrinkage in the supported membranes. Removal of porosity was pronounced above 1100°C and the density increased linearly as a function of holding time. Microhardness of membranes sintered above 1100°C increased as a function of sintering temperature and was higher in unsupported membranes. Samples sintered above 975°C had a100% tetragonal phase structure. Permeability of supported membranes increased as a function of sintering temperature due to pore growth despite a decrease in porosity.     

Thermal formation of corundum from aluminium hydroxides prepared from various aluminium salts

Aluminium hydroxides have been precipitated from various aluminium salts and the differences in their thermal behaviour have been investigated. Pseudoboehmite derived from the nitrate, sulfate and chloride all form γ-Al₂O₃ at ∼ 400°C but the formation of α-A1₂O₃ at 1200°C occurs more readily in the material derived from the sulfate. This contains a higher concentration of anionic impurities related to differences in the solubility of the original aluminium salts. The sulfate is retained in the gel to higher temperatures at which its eventual decomposition may lead to the formation of a reactive pore structure which facilitates the nucleation of α-A1₂O₃.     

Reaction between ∼0.7- to 1-μm α-Al2O3 particles and SiO2 during high-temperature sintering

Al₂O₃:Cr³⁺ and Al₂O₃:Ti³⁺ particles with an average size of ∼1 and 0.7 μm, respectively, have been prepared through mechanical grinding of bulk crystals. Using purpose-designed accessories made from chromium and titanium, we were able to prevent Fe contamination. We studied the reaction between α-Al₂O₃ and SiO₂ particles during high-temperature sintering. The results demonstrate that, in this system at temperatures above 1300°C, corundum particles dissolve in silica to form mullite. The reaction temperature and rate depend on the particle size composition of the Al₂O₃. The heating rate is shown to influence the dissolution rate of corundum particles. Increasing the heating rate from 7 to 15°C/min shifts the dissolution range of alumina particles from 1500 to 1630°C.     

Effect of sintering parameters on the microstructure and properties of strontium modified PZT ceramics prepared using spray-dried powders

PZT powders of the composition Pb_0.94Sr_0.06 (Zr_0.53Ti_0.47)O₃, prepared by spray drying and calcining techniques, were processed to sintered ceramics by conventional cold pressing and sintering at various temperatures and periods between 1000 to 1250°C for 0.5 to 12h. Sintered ceramics were evaluated for their microstructure and electromechanical properties. Highly dense ceramics having bulk density of the order of 97% of the theoretical value could be obtained after sintering at a considerably lower temperature of 1000°C in comparison to the 1300°C generally required for powders prepared by conventional ceramic processing. However, the increase in sintering temperature of reactive spray-dried powders causes the entrapment of closed pores as a result of exaggerated grain growth and subsequent pore coarsening thereby leading to a decrease in the bulk density of the ceramics. It has been observed that minor variations in the sintering parameters influence the porosity, grain size and electromechanical properties. Values of the dielectric constant, piezoelectric strain coefficient and electromechanic coupling factor increase with the increase in grain size and decrease with the increase in porosity of the sintered ceramic whereas the dielectric dissipation factor decreases with the increase in sintering temperature.     

Effect of powder treatment on the crystallization behaviour and phase evolution of Al2O3–High ZrO2 nanocomposites

Al₂O₃–ZrO₂ composites containing nominally equal volume fraction of Al₂O₃ and ZrO₂ have been synthesized through combined gel-precipitation technique. Subsequently the gels were subjected to three different post gel processing treatments like ultrasonication, ultrasonication followed by water washing and ultrasonication followed by alcohol washing. It was observed that while in unwashed samples crystallization took place at low temperature, crystallization was delayed in the washed gels. The phase transition of ZrO₂ in the calcined gels followed the sequence; amorphous → cubic ZrO₂ → tetragonal ZrO₂ → monoclinic ZrO₂. On the other hand, phase transition in alumina followed the sequence amorphous to γ-Al₂O₃, the transition taking place at 650 °C. No α-Al₂O₃ could be detected even after calcination at 950 °C. However, all the sintered samples had α-Al₂O₃. In spite of high linear shrinkage (19–21%) during sintering, the sintered sample had density of only above 70% for all the four varieties of the powders. However, in spite of the low sintered density of the pellets, 31% tetragonal zirconia could be retained after sintering at 1400 °C and it reduced to about 16% at 1600 °C.     

Effect of mixing procedures and compaction pressures on the powder reaction of CuO-η-Al2O3 and ZnO-η-Al2O3 systems

The rate of reaction of the compacted powders of CuO-η-Al₂O₃ and ZnO-η-Al₂O₃ systems was measured in air at 800 to 900° C and the effect of the mixing procedure (dry and wet mixing) and compaction pressure (0 to 8.3×10⁸ Pa) of reactant oxides on the fraction of reaction completed (α) was investigated. In the reaction of the CuO-η-Al₂O₃ system, the α-values obtained for the sample prepared by wet mixing in ethanol were higher than those for the sample prepared by dry mixing in air and were not influenced by the compaction pressure, whereas in the case of dry mixing they varied with the compaction pressure and had a maximum value at 2.1×10⁸ Pa. On the other hand, in the reaction of the ZnO-η-Al₂O₃ system the α-values for the sample obtained by wet mixing were lower than the values obtained by dry mixing, in contrast to the results in CuO-η-Al₂O₃ system, and the α-values for the samples prepared by both dry and wet mixing were not influenced by the compaction pressure. The effect of mixing procedure and compaction pressure of reactant powders on the α-values was found to be explained on the basis of the aggregate size of CuO and ZnO dispersed in the matrix of η-Al₂O₃ fine powder.     

Electroconductivity of sintered bodies of α-Al2O3-TiN composite prepared by CVD reaction in a fluidized bed

α-Al₂O₃ particles whose primary size is ca. 450 nm are smoothly fluidized by forming agglomerates of ca. 200 μm and are coated with TiN crystallites generated by a chemical vapour deposition (CVD) reaction of TiCl₄ and NH₃ at 973 K. The α-Al₂O₃-TiN composite particles and those obtained by mechanically mixing constituent particles are sintered at 1873 K in a nitrogen atmosphere, and the electroconductivity of sintered bodies is measured at 298 K. The electroconductivity of the CVD-hybridized composites is higher than that of mechanically mixed ones. This shows the effectiveness of fluidized-bed CVD processing in the preparation of composite ceramic particles.     

Proton conduction at intermediate temperature and its application in ammonia synthesis at atmospheric pressure of BaCe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−α</Subscript>

Dense ceramic samples of BaCe_1−x Ca _x O_3−α (x = 0.05, 0.10, 0.15, 0.20) have been synthesized by heat treating the precursor prepared from a microemulsion route under lower calcining and sintering temperatures than traditional high-temperature solid-state reaction. The samples exhibited single phase of orthorhombic perovskite. It was found that the samples were almost pure proton conductors in wet hydrogen at 300–600 °C. Ammonia was synthesized successfully from nitrogen and hydrogen gases at atmospheric pressure using BaCe_0.9Ca_0.1O_3−α as an electrolyte of ammonia synthesis reactor.     

The effect of sintering temperature on the development of grain boundary traps in zinc oxide based varistors

A series of zinc oxide based varistors containing 0.5 wt% Bi₂O₃ and 0.5 wt% Mn₂O₃ was prepared by a conventional mixed oxide route and sintered at temperatures between 950° and 1300°C. All samples showed varistor behaviour, although as the sintering temperature was increased from 950°C to 1300°C, the non-linearity coefficient, α, decreased from 22 to 3. Deep level transient spectroscopy of the varistors showed that the main active electron trap migrated to shallower levels within the bandgap as the sintering temperature increased. At the lowest sintering temperature, where α attained the highest values, a second, shallower trap was also activated.     

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.     

AFM and SEM characterization of iron oxide coated ceramic membranes

Alumina–zirconia–titania (AZT) ceramic membranes coated with iron oxide nanoparticles have been shown to improve water quality by significantly reducing the concentration of disinfection by-product precursors, and in the case of membrane filtration combined with ozonation, to reduce ozonation by-products such as aldehydes, ketones and ketoacids. Commercially available ceramic membranes with a nominal molecular weight cut-off of 5 kilodaltons (kD) were coated 20, 30, 40 or 45 times with sol suspension processed Fe₂O₃ nanoparticles having an average diameter of 4–6 nm. These coated membranes were sintered in air at 900 °C for 30 min. The effects of sintering and coating layer thickness on the microstructure of the ceramic membranes were characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). AFM images show a decreasing roughness after iron oxide coating with an average surface roughness of ∼161 nm for the uncoated and ∼130 nm for the coated membranes. SEM showed that as the coating thickness increased, the microstructure of the coating changed from a fine grained (average grain size of ∼27 nm) morphology at 20 coating layers to a coarse grained (average grain size of ∼66 nm) morphology at 40 coating layers with a corresponding increase in the average pore size from ∼57 nm to ∼120 nm. Optimum water quality was achieved at 40 layers, which corresponds to a surface coating morphology consisting of a uniform, coarse-grained structure with open, nano-sized interconnected pores.     

Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from -to -alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C.     

Voigt modelling of size-strain analysis: Application to α-Al2O3 prepared by combustion technique

A comprehensive analysis of size and strain broadened profile shapes in X-ray diffraction line broadening analysis is presented. Both size and strain broadened profiles were assumed to be Voigtian and the derived microstructural parameters (size and strain) were found to be in close agreement with those calculated from model independent Warren-Averbach method. The method is applied to three different alumina samples viz. micron size α-alumina (α-Al₂O₃) prepared by the combustion of aluminium nitrate and urea mixture, annealed samples and commercial α-Al₂O₃ sample. It is likely from the present analysis that a significant Gaussian size contribution is related to narrow size distribution observed from the analysis. It has been concluded that present Voigtian analysis is more reliable and may largely replace the earlier simplified integral breadth methods of analysis often used in line broadening analysis.     

TEM characterization of iron-oxide-coated ceramic membranes

Commercially available porous alumina–zirconia–titania ceramic (AZTC) membranes having a titania surface coating were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and the Brunauer–Emmett–Teller (BET) method. TEM photomicrographs showed the as-received AZTC membrane to be a multi-layered structure consisting of a porous alumina–zirconia–titania core having ultrafine pore sizes, coated by an additional layer of nanoporous titania. Electron diffraction studies revealed an amorphous surface titania layer while the underlying AZTC membrane was crystalline. The AZTC membranes were coated 20, 30, 40, 45, or 60 times with iron oxide (Fe₂O₃) nanoparticles, after which the membranes were sintered in air at 900 °C for 30 min. TEM revealed a relatively uniform nanoporous Fe₂O₃ coating on the sintered, coated membranes, where the Fe₂O₃ coating thickness increased with increasing number of layers. Electron diffraction patterns showed the Fe₂O₃ coating to be crystalline in nature. This was confirmed by the XRD results showing the structure to be α-Fe₂O₃, while the AZTC membrane was a mixture of the anatase and rutile phase of TiO₂ as well as ZrO₂ and corundum, Al₂O₃. The average pore size of the underlying AZTC membrane increased after the Fe₂O₃-coated membrane was sintered. The nanoporosity in the sintered Fe₂O₃ coating increased until 40 layers, beyond which no significant increases in the average pore size were observed. The iron-oxide-coated membrane improved catalytic properties when used in combination with ozone to treat water. The optimal benefit, in terms of water treatment efficacy, was found at 40 layers of Fe₂O₃.     

Enhancement of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel formation from coprecipitated precursor by powder processing

Although low temperature fast coprecipitation technique has been used to synthesize stoichiometric (MgO-nAl₂O₃, n = 1) MgAl₂O₄ spinel forming precursor, delayed spinellization has always been the concern in this process. In this article, the precursor of this ‘fast technique’ has been used for bulk production by further processing by high speed mixing with solvents and mechanical activation by attrition milling in terms of superior spinellization. At 1000°C, MgAl₂O₄ — γ-Al₂O₃ solid solution and MgO phases are formed (spinel formed by 1000°C is regarded as primary spinel). At higher temperatures, due to large agglomerate size, MgO can not properly interact with the exsolved α-Al₂O₃ from spinel solid solution to form secondary spinel; and consequently spinellization gets affected. Solvent treatment and attrition milling of the coprecipitated precursor disintegrate the larger agglomerates into smaller size (effect is more in attrition). Then MgO comes in proper contact with exsolved alumina, and therefore total spinel formation (primary + secondary) is enhanced. Extent of spinellization, for processed calcined samples where some alumina exists as solid solution with spinel, can be determined from the percentage conversion of MgO. Analysis of the processed powders suggests that the 4 h attrited precursor is most effective in terms of nano size (< 25 nm) stoichiometric spinel crystallite formation at ≤ 1100°C.     

Synthesis and thermal expansion hysteresis of Ca<Subscript>1-x</Subscript>Sr<Subscript>x</Subscript>Zr<Subscript>4</Subscript>P<Subscript>6</Subscript>O<Subscript>24</Subscript>

The low thermal expansion ceramic system, Ca_1-xSr{x}Zr₄P₆O₂₄, for the compositions with x = 0, 0.25, 0.50, 0.75 and 1 was synthesized by solid-state reaction. The sintering characteristics were ascertained by bulk density measurements. The fracture surface microstructure examined by scanning electron microscopy showed the average grain size of 2.47 μm for all the compositions. The thermal expansion data for these ceramic systems over the temperature range 25–800°C is reported. The sinterability of various solid solutions and the hysteresis in dilatometric behaviour are shown to be related to the crystallographic thermal expansion anisotropy. A steady increase in the amount of porosity and critical grain size with increase in x is suggested to explain the observed decrease in the hysteresis.