A TEM and HREM study of particle formation during barium titanate synthesis in aqueous solution

The formation mechanisms of barium titanate particles from an amorphous TiO2 gel during synthesis in aqueous solution at temperatures between 20 and 80°C have been investigated. It was found that barium ions diffuse into the gel almost immediately, with nanocrystalline BaTiO3 particles being formed after heating to just 40°C. These particles grew to dimensions of about 100 nm as the temperature was increased to 80°C, consuming the remaining TiO2 gel. Some remnants of gel were found on particle surfaces in a sample taken at this temperature but after holding the sol at 80°C for 2 or 4 h, the particle surfaces became “cleaner” and more rounded. It is proposed on the basis of these observations that the BaTiO3 particles were formed by an in-situ transformation of the amorphous TiO2 gel. The mechanism by which (i) the particles were then rounded off and (ii) the final gel fragments were incorporated into the main BaTiO3 particles was, however, less clear

Low temperature hydrothermal synthesis of Ba(Mg1/3Ta2/3)O3 sol-derived powders

Powders of the microwave dielectric material barium magnesium tantalate Ba(Mg1/3Ta2/3)O3 have been produced by hydrothermal synthesis at moderately low temperatures (160 to 350°C). It was found that while it is relatively straightforward to produce the material in the desired perovskite phase at or below 200°C, the powder particles tend to be highly irregular in morphology with extremely small dimensions (of the order of 10 nm) and deficient in magnesium (with some precipitation of the excess magnesium as the hydroxide). The effects of both higher synthesis temperatures and different feedstock preparation were thus investigated with the aim of improving the precipitation of magnesium under hydrothermal conditions in order to produce a more homogeneous, stoichiometric powder and significant progress was made. It was found that when near-stoichiometric particles are formed, they adopt rounded morphologies and exhibit larger particle sizes (around 30–50 nm). These results show that the hydrothermal feedstock and the synthesis temperature used have a profound effect on particle stoichiometry, which in turn affects the growth morphology of the particles

Microstructure and mechanical properties of pressureless sintered alumina-silver composites

The silver toughened alumina ceramics, which were fabricated via a conventional ceramic processing route using commercially available alumina and silver oxide powders as the starting materials, exhibit a higher fracture toughness (6 to 9 MPam0.5) than the monolithic alumina ceramic ( 2.5 to 3.0 MPam0.5). The weak interfacial bonding between the silver inclusions and the alumina matrix is characterised by the occurrence of voids or pores at the interface, the formation of which is due to the high vapour pressure of molten silver at the sintering temperature. The thermal expansion mismatch between the metallic phase and the ceramic matrix generates residual strains in the composite structure. A microstructural study using in-situ TEM observation showed that the thermal strains were accommodated largely by the silver grains, which are much more deformable than the rigid alumina grains. Both the well established dislocation rings and deformation twins were observed to occur in the silver grains entrapped at the grain boundaries and grain junctions of the alumina matrix

A transmission electron microscope study of hydrothermally synthesized yttrium disilicate powders

The crystallization of three hydrothermally synthesized Y2Si2O7 precursor powders at temperatures between 991 and 1038°C has been studied using transmission electron microscopy. Powders prepared in acidic or near-neutral conditions were found to be highly inhomogeneous both chemically and microstructurally, with a wide range of crystalline phases formed. In contrast to this, a powder prepared in alkaline conditions was found to be very homogeneous. Small crystalline nuclei were formed in this powder on heating to 1006°C which grew rapidly at 1038°C to form large single crystal particles. The majority of these had the α-Y2Si2O7 phase but some were of the y-Y2Si2O7 phase. The reasons for the formation of this y-phase are unclear, although it would be expected to transform to α-Y2Si2O7 on further heat treatment at 1200°C. Weaknesses in the current crystal structure data for yttrium disilicate phases are identified and suggestions made for rectifying them

Electrophoretic deposition infiltration of 2-D woven SiC fibre mats with mixed sols of mullite composition

Electrophoretic deposition (EPD) has been used to infiltrate a dual-component mixed sol of mullite composition into woven, electrically conducting SiC (Nicalon) fibre preforms. Both silica and alumina precursors had a near-equiaxed particle shape and their controlled colloidal mixing at an optimised pH (3.1) has lead to their extensive heterocoagulation on a nanometre scale. The mechanisms of particle migration in the EPD cell are not fully clear at this stage. It is suggested that both sol species migrate simultaneously as a ‘composite’ sol particle to the fibre mat serving as one electrode in the cell. Owing to the small particle size of the silica and alumina used, both components were able to infiltrate the spaces within the fibre tows. Good particle packing and a high solids-loading were achieved, producing a firm matrix deposit which adhered to the fibres. The maintenance of the stoichiometric mullite composition in the deposited material was confirmed by XRD analysis of the deposited material heat-treated at 1350 °C for 5h. The high-quality infiltrated fibre mats serve as preforms for the fabrication of mullite matrix composites

Silica glass segregation in 3 wt.% LiF doped hot-pressed Y₂Si₂O₇

Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y2Si2O7 containing rounded glassy Y-doped SiO2 inclusions; excess glassy SiO2-rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO2 excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined

Treatment of Strontium Hexaferrite Powder Synthesized Conventionally to Produce High Coercivity

Strontium hexaferrite powder, produced conventionally from strontium carbonate and iron oxide has been treated in nitrogen and hydrogen atmospheres and then calcined in air. Magnetic measurements after the gas treatment stage indicated a marked decrease of intrinsic coercivity and an increase in saturation magnetisation. During the calcination stage there was a recovery of the magnetic properties. The material now exhibited a remanence and saturation magnetisation close to that of the starting hexaferrite, but a very much higher intrinsic coercivity. The highest coercivity obtained was around 400 kA/m, which is very high for a hexaferrite powder, particularly one synthesized conventionally, without extensive milling. The high coercivity was attributed to a much finer grain size