Production of ultrafine alpha alumina powders and fabrication of fine grained strong ceramics

Hydrous alumina powders, pure, seeded with alpha alumina, containing ammonium nitrate and containing both ammonium nitrate and seeds, were prepared by hydroxide precipitation. Their crystallization and sintering behaviour were investigated and mechanical properties of the ceramics were tested. Pure hydrous alumina transformed to alpha alumina crystals, with a size of ca. 200 nm, at 1200°C, after undergoing the usual metastable phase changes during heat-treatment. The powder needed to be sintered at 1600°C to achieve a high density. The ceramic had an average grain size of ca. 9 m. Seeding lowered the transformation temperature to ca. 1120°C and caused the transformation to begin at ca. 600°C. The material could be sintered at 1500°C and had a grain size of 2 m. The nitrate, predominantly present as ammonium nitrate, lowered the transformation temperature to ca. 1150°C and altered the proportion of the intermediate phases. However, the materials still had to be sintered at 1500°C to achieve >97% density. When both seed particles and nitrate ions were present the material almost completely transformed at 950°C to uniform crystals of alpha alumina with a size <60 nm that sintered to >99% theoretical density at 1450°C. The final ceramic had a uniformly grained (<1.0 m) microstructure and exhibited strength up to 800 M Pa.     

Synthesis and microstructure of boron-doped alumina membranes prepared by the sol–gel method

Pure and boron-doped γ-Al₂O₃ membranes have been synthesized by the sol–gel method. The thermal stability of the unsupported alumina membrane was studied by determining the pore structure (including average pore size, pore volume and BET surface area). The average pore size of the pure alumina membrane increased sharply after sintering at temperatures higher than 1000°C. Addition of 16% boron can considerably stabilize the pore structure of the unsupported alumina membrane. The pore diameter for the B-doped membrane was stabilized within 13 nm after sintering at 1200°C for 5 h. The substantial increase in the pore size for the pure alumina membrane at the sintering temperature of 1000–1200°C was accompanied by the phase transformation from γ-Al₂O₃ to -Al₂O₃. The addition of boron can raise the temperature of this phase transformation significantly and, thus, improves the thermal stability of the membranes.     

硼掺杂的γ-A12O3催化膜的制备及其热稳定性的研究

在勃姆石AlOOH溶胶中引入一定量的H2BO3溶液,经不同温度的热处理,制成不同硼掺杂含量的无支撑的γ-A12O3催化膜。用XRD、BET分别对膜的晶相和膜的微孔结构,包括比表面积、孔径和孔容进行了研究,结果发现：随着硼含量的增加,在低温下,膜的比表面积和孔容都不断增加,而对孔径的影响不大;经1200℃处理后,未掺杂硼的膜的比表面积,孔径和孔容分别为5.4m2/g,49nm和0.063cm3/g,而经掺杂16％摩尔硼的膜的比表面积,孔径和孔容分别为35m2/g,13nm和0.225cm3/g,这说明硼的掺杂对γ-A12O3膜的热稳定性有很好的改善作用。     

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 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.     

Novel aqueous sol–gel preparation and characterization of barium M ferrite, BaFe12O19 fibres

Gel fibres of barium M ferrite, BaFe12O19, were blow spun from an aqueous inorganic sol and calcined at temperatures up to 1200°C. The ceramic fibres were shown by X-ray diffraction to be single phase crystalline M ferrite at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a favourable grain structure of less than 0.1 m at this temperature and maintained a small grain size of less than 4 m even up to 1200°C, an important factor in the magnetic properties of this material.     

Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina

Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 m. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions.     

Characterization of the Pore Structure of Ceramics via Propagation of Light and Infrared Radiation

The pore size distributions of alumina and magnesia ceramics were determined by measuring the directional-hemispherical transmittance and reflectance. These values are highly sensitive to changes of the pore structure. The partially sintered alumina samples were measured at room temperature in a wavelength range from 0.5 to 6 m. The equation of radiative transfer can be solved for absorbing and scattering media by a three-flux solution. With this three-flux solution the scattering coefficients were derived from the measured directional-hemispherical transmittance and reflectance. The scattering coefficients can also be calculated theoretically by the Mie theory, if the pore size distribution is known. Finally, the quantitative pore size distribution was determined by fitting the theoretical scattering coefficients to the experimental scattering coefficients. To check the correctness of the derived pore-size distribution, scanning electron microscopy (SEM) and atomic force microscopy (AFM) pictures of the alumina samples were taken. The pore-size distribution was then derived by counting the pores and determining the diameters D of the spherical pores. Both results agree well and show that the new procedure is a valuable tool to extract structural information during the final sintering state.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

Magnetic Co2Y ferrite, Ba2Co2Fe12O22 fibres produced by a blow spun process

Gel fibres of Co2Y,Ba2Co2Fe12O22, were blow spun from an aqueous inorganic sol and calcined at temperatures of up to 1200°C. The ceramic fibres were shown by X-ray diffraction to form crystalline Co2Y at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a small grain size of 1–3 m across the hexagonal plane and 0.1–0.3 m thickness at 1000°C. This only increased to 3 m in diameter and 1 m thickness even at temperature up to 1200°C. The fibrous nature combined with the improved microstructures could be an important factor in improving the magnetic properties of this material.     

Hydrothermal stability of pure and modified microporous silica membranes

The hydrothermal stability of microporous (0.6 nm) silica membranes prepared by the sol-gel process was studied at 600 and 800 °C in a 50 mol% steam atmosphere. The membranes remained microporous after calcination and hydrothermal treatment at 600 °C for 30 h but a substantial reduction in the specific surface area (48%) accompanied by a 77% decline in the micropore volume was observed. Hydrothermal treatment at 800 °C for 30 h resulted in complete densification of the membranes. The effect of alumina and magnesia on the hydrothermal stability of the membranes was investigated. Both Al₂O₃ and MgO were introduced into the membranes by doping the starting silica sol with controlled amounts of the corresponding nitrate salts. Alumina did not change the pore structure of the silica membranes which retained a large part of their microporosity after hydrothermal treatment at 600 °C compared to pure silica membranes. Doping with magnesia, however, resulted in lower specific surface areas relative to those of pure and alumina-doped silica membranes after drying and calcination. These effects on the stability of the membranes are explained by assuming structural changes in the membranes catalysed by magnesia.     

Effect of microstructure on oxygen permeation in SrCo0.8Fe0.2O3−δ

The effect of microstructure on oxygen permeation in SrCo0.8Fe0.2O3– membranes was investigated using disc samples fabricated under different processing conditions of applied pressure and sintering temperature. The average grain size of the samples was found to remain unchanged as a function of applied pressure, but increased considerably when the sintering temperature was increased from 950 to 1200°C. This change in grain size has a strong effect on the oxygen permeation flux, which increased considerably as the grain size was decreased. The density as well as the microhardness of these samples were also measured and found to change slightly as the processing conditions were changed.     

Thermal stability of aluminas by hydrothermal treatment of an alkoxide-derived gel

Alumina precursors were prepared by hydrothermal treatment of alkoxide-derived alcogels. The crystalline structure of precursor beohmites and their microstructural change during heat treatment were examined and the specific surface area of the alumina precursors after heating was measured. The alumina prepared by hydrothermal treatment at 270 °C retained high specific surface areas at high temperatures; e.g. 35.0, 8.3 and 5.4 m²g^–1 at 1200, 1400 and 1500 °C, respectively. The thermal stability of the aluminas depended on the hydrothermal temperatures. For excellent thermal stability, the following factors are necessary: (1) grain growth of beohmite as an alumina precursor, and a grain size of more than 20 nm for the (1 2 0) plane; (2) a crystallite size for the (2 0 0) plane exceeding that for the (0 0 2) plane; (3) anisotropic growth of the beohmite crystal. In the transition alumina region ( 1200 °C), the thermal stability of the alumina is caused by raising the transformation temperature, resulting from decreasing the number of grain boundaries by beohmite growth. In the -alumina region (> 1200 °C), inhibiting the three-dimensional grain growth achieves thermal stability, resulting from preservation of the anisotropic structure introduced into the beohmite.     

Microstructural evolution in nanocrystalline alumina containing silicate glasses

Pure nanocrystalline -alumina powders were coated with different fractions (5, 10, and 15 vol%) of SiO₂-SrO glass using the sol-gel technique. The isostatically cold pressed powders were pressureless sintered in air for 5 h in the temperature range of 1250°C to 1550°C. The relative densities were ranged between 60 to 90% of the theoretical and were composition dependent. The density was increased with the sintering temperature. In pure alumina, the to phase transformation went to completion by sintering at 1250°C. However, in the glass-coated samples, transition -alumina was mostly retained after sintering at the same temperature. Pure nanocrystalline alumina sintered at 1350°C exhibited vermicular structure with isolated pores. The microstructure of the low glass-containing samples exhibited nanocrystalline to submicron size grains arranged in platelet-shaped clusters. Samples with higher glass contents exhibited also micron-size needle-shape grains of strontium aluminate.     

Influence of annealing on properties and structure of alumina

Alumina bodies were prepared from pure alumina powder (98.9% Al₂O₃ consisting of 82% > 53m). The powder was compacted by hot-pressing at 1200° C, Compacted bodies were annealed at 1300, 1400 and 1500° C. Annealing continued at each maximum temperature for 25, 50 and 100 h. Strong bodies were obtained with maximum bulk density of 2.32 g cm^–3 and minimum apparent porosity of 30.21%. The change in sintering parameters with annealing was correlated with developed structure.     

Preparation of nanocrystalline SrBi2Ta2O9 powders using sucrose-PVA as the polymeric matrix

Nanocrystalline powders of SrBi₂Ta₂O₉ (Strontium Bismuth Tantalate) have been prepared through evaporation of a polymer-based aqueous precursor solution. The precursor solution was obtained by homogeneous dispersion of the water-soluble metal salts (i.e., strontium nitrate, bismuth nitrate and tartarate complex of tantalum) in a polymeric matrix created by an aqueous solution mixture of sucrose and polyvinyl alcohol. Complete evaporation of the precursor solution (at 200°C) resulted in a fluffy, porous, carbonaceous mass, which on calcination at 750°C/2 h yield the single-phase SrBi₂Ta₂O₉ powders with average particle size 35 nm. The compacted powders, after sintering at 1000°C/4 h, show density of 96.8% of its theoretical value and dielectric constant value of 862 with Curie temperature (T _c) at 287°C, when measured at 100 KHz.     

Phosphor-doped BaTiO3: Microstructure development and dielectric properties

The phosphor cation retards BaTiO₃ formation during the first reaction stage, and favours Ba₂TiO₄ formation. As a consequence, the end of the reaction is only accomplished at 1150 °C. The average particle size of the synthesized powder, after attrition milling was <1.0 m. Isopressed bars were sintered from 1200–1400 °C. Dilatometric measurements showed the existence of two sintering mechanisms; these were confirmed by means of isothermal sintering experiments. Microstructural development at low sintering temperature differs from that corresponding to higher ones. Very high permittivity values were measured in the 1325–1350 °C sintered samples. This anomalous dielectric behaviour is associated with the presence of phosphor.     

Sintering temperature, microstructure and resistivity of polycrystalline Sm0.2Ce0.8O1.9 as SOFC's electrolyte

In this work, near-completely soft-agglomerated Sm_0.2Ce_0.8O_1.9 powders have been prepared. The pellets were formed and sintered at various sintering conditions of temperature and time. It was found that the sintering conditions have significant effects on the pellet resistivity. By the measurements with the DC four-probe method, it was found that the overall resistivity of the polycrystalline Sm_0.2Ce_0.8O_1.9 material sintered at 1500°C increases linearly with the reciprocal of the average grain size. The AC impedance spectroscopy has been used to distinguish the grain resistivity and the apparent grain boundary resistivity. The brick layer microstructural model has been used to provide an estimate of the apparent grain boundary resistivity and to relate the electrical properties to the microstructural parameters. By lowering the sintering temperature to 1100–1200°C, the true grain boundary resistivity was nearly two orders lower than that sintered at 1500°C, and thus the overall resistivity decreases to about 31 ohm-cm at 700°C measurement. This makes the Sm_0.2Ce_0.8O_1.9 material capable of working as SOFC's electrolyte at temperatures lower than 700°C.     

Ceramic joining IV. effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 m thick cladding layers of copper on a 125 m thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 m grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.     

Low-temperature sintering of zinc oxide varistors

High-field varistors in the system ZnO-CoO-MnO-Bi₂O₃ were fabricated using powders prepared by a previously developed coprecipitation process. Following calcination, the powders were compacted and densified by conventional pressureless sintering at temperatures below 750° C in air, The effects of sample green density, sintering temperature, and grain-growth inhibitor on densification and microstructure development were investigated. Addition of aluminium at the 125 p.p.m level was used to inhibit grain growth. Samples with densities >0.98 theoretical and grain sizes <1m were fabricated by high-pressure cold-isostatic pressing followed by sintering at 730° C. For comparison, typical commercial varistor devices have grain sizes of about 20 m and switching fields of approximately 2 kV cm^–1 after sintering at 1200 to 1400° C. As a result of the fine grain size, our high-field varistors had switching fields of 45 kV cm^–1 at a current density of 10 A cm^–2. Consistent with earlier work on extremely high-density varistors (>0.98 theoretical) prepared from similar powders, nonlinearity coefficients of about 10 were measured for current densities between 2.5 and 10 A cm^–2.     

Sintering behavior of Ni/Y2O3-ZrO2cermet electrodes of solid oxide fuel cells

The sintering behavior of Ni/Y₂O₃-ZrO₂(YSZ) cermet electrode coating on 3 mol% Y₂O₃-ZrO₂electrolyte was studied under moist and dry hydrogen atmosphere at 1000°C for up to 2000 h. The sintering behavior of Ni/YSZ cermet electrodes was dominated by the agglomeration and grain growth of Ni particles in the cermets, which was critically related to the content of Ni and YSZ phases in the cermet. For pure Ni electrode coating, the sintering was substantial and cross plane cracks and isolated Ni island were formed after sintering at 1000°C for only 250 h. However with the addition of YSZ phase, the sintering of Ni/YSZ cermet anode coatings was significantly reduced. For the cermet composition of Ni (50 vol%)/YSZ (50 vol%), the change in the surface porosity and pore size distribution after sintering at 1000°C for 2000 h was very small. The microstructural stability of the Ni (50 vol%)/YSZ (50 vol%) cermet electrodes was also demonstrated by the performance stability tested under current load of 250 mAcm^–2at 1000°C for over 2500 h.