A comparison of the microstructure of silicon nitride-silicon carbide composites made with and without deoxidized starting material

Two different types of silicon carbide (SiC) matrix composites, with either 10 wt% or 20 wt% silicon nitride (Si₃N₄) reinforcement, were fabricated to investigate the effect of pretreatment on the resulting composite micro-structure. The first type of composite was prepared from as-received α-SiC and α-Si₃N₄ powders, while the second type was prepared from powder compacts that had been deoxidized to eliminate surface silica on the powder particles. The composites were hot isostatically pressed in tantalum cans at 2373 K for 1h under a pressure of 200 MPa. Density measurements showed that full theoretical density was achieved for the composites prepared from the as-received powders, while much lower densities were obtained for the composites prepared from the deoxidized green compacts. Almost all of the α-SiC transformed into β-SiC, and almost all the α-Si₃N₄ transformed into α-Si₃N₄ in the composites made from the as-received powders, while in the composites made from the deoxidized material the α-SiC remained untransformed and both α-Si₃N₄ and β-Si₃N₄ phases were present in significant quantities. High-resolution transmission electron microscopy and Fresnel fringe imaging were used to identify the grain boundary and interphase boundary structure. Most interfaces were found to be covered with ? 1 nm thick amorphous intergranular films in the composites prepared from as-received powders, whereas most interfaces were found to be free of such amorphous intergranular films in the composites prepared from the deoxidized material. Taken together, the presence of intergranular films at the interfaces and the results from density measurements are consistent with the densification and reverse α → β-SiC transformation taking place in the composites made from as-received powders by a liquid-phase sintering route. An incomplete liquid-phase sintering mechanism is also able to explain the microstructure observed in the composites made from the deoxidized material.     

Simple procedure for the production of carbon-polymer support grids for use in X-ray microanalysis

A simple and rapid procedure for the production of low-cost carbon-polymer support grids is described. These grids are particularly useful for X-ray microanalysis of thin specimens because of their low X-ray background properties. The grids can be prepared for immediate use, which guarantees constant quality.     

Rationale for the application of thin,continuous metal films in high magnification electron microscopy

Various metal films of different thicknesses were deposited on to a particle test specimen and their effects on topographic contrast generation and specimen preservation were determined. Tobacco mosaic virus adsorbed on to thin carbon supports or silicon chips was imaged in TEM or high resolution SE-I SEM at a magnification of 350,000×. Tantalum films of 1–2 nm (average mass) thickness produced best contrasts and prevented volume loss of the particles from electron beam damage. Excessively thick films of 5–10 nm thickness blanketed fine structures and caused severe volume losses. Discontinuous 2 nm thick films of gold or platinum decorated the surfaces, caused a loss in topographic contrasts and induced very high volume losses. Thin continuous metal films were necessary to generate high topographic contrast and to prevent volume loss from beam damage by providing sufficient mechanical stability for small topographic features and increased thermal conductivity of the specimen surface.     

Surface roughness and the use of a peak to background ratio in the X-ray microanalysis of bio-organic bulk specimens

The use of a net peak intensity and of a peak to background (P/B)-ratio of sulphur and chlorine is examined in the X-ray microanalysis of a 2·4% w/w S bulk standard in Spurr's epoxy resin. In calculating the P/B-ratio, the background intensity is calculated for the same energy region as for the net peak. Analyses were carried out on the flat top of the standard and on the slope running down from the top on the side not facing the X-ray detector. The results obtained for the peak to local background ratios from the top and the slope yielded a relatively small mean deviation (11%) while net peak intensities ultimately were reduced to 7% or less of the initial value for the flat top. This indicated that a peak to local background ratio is to be preferred in the quantitative analysis of bulk specimens which have poorly defined local tilt and takeoff angles. A second advantage is the inherent correction for beam current fluctuations.     

A new HPF specimen carrier adapter for the use of high‐pressure freezing with cryoscanning electron microscope: two applications: stearic acid organization in a hydroxypropyl methylcellulose matrix and mice myocardium

Cryogenic transmission electron microscopy of high‐pressure freezing (HPF) samples is a well‐established technique for the analysis of liquid containing specimens. This technique enables observation without removing water or other volatile components. The HPF technique is less used in scanning electron microscopy (SEM) due to the lack of a suitable HPF specimen carrier adapter. The traditional SEM cryotransfer system (PP3000T Quorum Laughton, East Sussex, UK; Alto Gatan, Pleasanton, CA, USA) usually uses nitrogen slush. Unfortunately, and unlike HPF, nitrogen slush produces water crystal artefacts. So, we propose a new HPF specimen carrier adapter for sample transfer from HPF system to cryogenic‐scanning electronic microscope (Cryo‐SEM). The new transfer system is validated using technical two applications, a stearic acid in hydroxypropyl methylcellulose solution and mice myocardium. Preservation of samples is suitable in both cases. Cryo‐SEM examination of HPF samples enables a good correlation between acid stearic liquid concentration and acid stearic occupation surface (only for homogeneous solution). For biological samples as myocardium, cytoplasmic structures of cardiomyocyte are easily recognized with adequate preservation of organelle contacts and inner cell organization. We expect this new HPF specimen carrier adapter would enable more SEM‐studies using HPF.