Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview

Polydimethylsiloxane thermal degradation Part 1. Kinetic aspects

High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces. This damage mechanism explains the measured, but not previously understood, decrease in the ballistic performance of boron carbide at high impact rates and pressures. The formation of these amorphous bands is also an example of how shock loading can result in the synthesis of novel structures and materials with substantially altered properties.

https://science.sciencemag.org/content/sci/299/5612/1563.full.pdf

Shock-induced localized amorphization in boron carbide.

Rice husk is an abundantly available waste material in all rice producing countries. In certain regions, it is sometimes used as a fuel for parboiling paddy in the rice mills. The partially burnt rice husk in turn contributes to more environmental pollution. There have been efforts not only to overcome this but also to find value addition to these wastes using them as secondary source of materials. Rice husk contains nearly 20% silica, which is present in hydrated amorphous form. On thermal treatment, the silica converts to crystobalite, which is a crystalline form of silica. However, under controlled burning conditions, amorphous silica with high reactivity, ultra fine size and large surface area is produced. This micro silica can be a source for preparing advanced materials like SiC, Si3N4, elemental Si and Mg2Si. Due to the high pozzolanic activity, this rice husk silica also finds application in high strength concrete as a substitute for silica fume. Possibility of using this silica as filler in polymers is also studied. The present paper is an attempt to consolidate and critically analyse the research work carried out so far on the processing, properties and application of rice husk silica in various laboratories and also highlighting some results on the processing and characterization of RHA and reactive silica obtained from it in the authors' laboratory.

Review Processing, properties and applications of reactive silica from rice husk—an overview

Rice husk (RH) has now become a source for a number of silicon compounds, including silicon carbide, silica, silicon nitride, silicon tetrachloride, zeolite, and pure silicon. The applications of such materials derived from rice husks are very comprehensive. The methods of synthesizing these silicon-based materials from RHs and their applications are reviewed in this paper.

Silicon-Based Materials from Rice Husks and Their Applications

A study of the distribution of Si in rice hulls was carried out to aid in understanding the observed formation of SIC whiskers by the thermal decomposition and reaction of these natural materials. Scanning electron microscopy, Auger electron spectroscopy, and energy-dispersive X-ray analysis confirm the presence of a major amount of Si in the inner and outer epidermal regions of the rice hull. The Si is present as a hydrated amorphous form of SiO2 which is evidently bound in the inner layer to organic constituents, placing it adjacent to a source of carbon but interfering with its solubility in HF. The SIC produced by pyrolysis is in the form of ultrafine particles with cubic crystal structure and whiskers exhibiting stacking faults and twins perpendicular to the whisker axis, easily visible by transmission electron microscopy. These whiskers are very similar to ones obtained by chemical vapor deposition with SiO and CO as the primary reactants.

Formation and Structure of Silicon Carbide Whiskers from Rice Hulls

Phase relationships in the SiC-AlN system were determined by analytical electron microscopy of local equilibria among adjacent phases in hot-pressed samples and in diffusion couples. At 2100° to 2300°C, a 4H–2H equilibrium exists, the 4H field extending from ∼2 mol% AlN to an upper limit of 11 to 14 mol% AlN. The wurtzite-type 2H (δ) solid solution extends from an impurity-sensitive lower limit of 17 to 24 mol% AlN up to 100 mol% AlN. Semiquantitative Auger electron spectroscopy and energy dispersive X-ray spectroscopy ultrathin-window detector measurements confirmed the assumption that the solid solutions have the general formula (SiC)1-x(AlN)x and belong to the SiC-AlN pseudobinary system. The existence of a miscibility gap below 1950°C was confirmed, but its limits were not determined accurately. A faulted metastable cubic phase, β′, exists below ∼2000°C and contains up to ∼4 mol% AlN. The transformations into the stable a structures occur through diffusion-controlled stacking rearrangements.

Phase Relationships in the Silicon Carbide-Aluminum Nitride System

High-temperature chemistry of the conversion of siloxanes to silicon carbide

High-density SiC-AIN compositions were fabricated from powder mixtures by hot-pressing in the 1700° to 2300°C temperature range. At 2100°C, a 2H solid solution was found from =35 to 100 wt% AlN. The single-phase solid solution samples had steep composition gradients of >10%/μm within the grains. Lattice parameters closely followed Vegard's law. For compositions with <35% AIN, multiphase assemblages were found. Increasing grain size was observed for increasing firing temperature for SiC and AIN. Grain size of the solid solutions was significantly smaller than for SiC or AIN fired to the same temperature. Microhardness values decreased linearly in the solid solution region with increasing AIN content. Flexural strengths of SiC and AIN decreased with increasing firing temperature and increasing grain size. The strengths of SiC, AIN, and the solid solutions were low for materials hot-pressed at 2100°C.

Avigdor Zangvil

Papers

Formation and Structure of Silicon Carbide Whiskers from Rice Hulls

Phase Relationships in the Silicon Carbide-Aluminum Nitride System

High-temperature chemistry of the conversion of siloxanes to silicon carbide

Composition and Properties of Hot-Pressed SiC-AIN Solid Solutions

Structure and Properties of Sylramic™ Silicon Carbide Fiber—A Polycrystalline, Stoichiometric β‐Sic Composition