Carbon fibre composites based on polyimide/silica ceramers: aspects of structure-properties relationship

Polyimide/silica ceramers, based on the products of the hydrolysis of tetraethoxysilane (TEOS) and a commercial poly(amic acid) solution, were used to fabricate unidirectional carbon fibre composites, which were subsequently evaluated with respect to thermal and mechanical properties. There is evidence to suggest that the silica component of these ceramers is present as dispersed discrete particles at low silica concentration (i.e. 7 wt%) and as fine interconnected domains trapped within the polyimide matrix at higher silica content (i.e. 14 wt%). The dimensions of the silica domains were in the region of 7–20 nm. Carbon fibre composites produced from ceramer solutions (CF/ceramers) were found to exhibit lower thermal expansion and a greater retention of flexural and interlaminar shear properties at elevated temperature than the corresponding polyimide-matrix composites (CF/polyimide). The properties of CF/ceramers were generally better for systems containing the higher amount of silica and were improved further by lowering the pH value of the precursor ceramer solution. This is believed to have resulted from the enhanced fluidity of the ceramer gel within the pre-impregnated fibres, giving rise to a higher packing density of the fibres and a more homogeneous distribution of fibres. CF/ceramers were also found to exhibit a better thermal oxidative stability at 350°C than the corresponding CF/polyimide, although a substantial amount of porosity developed in the case of ceramers with the higher silica content.     

Feedback Insertion-Deletion Codes

Problems of Information Transmission - A new problem of transmitting information over the adversarial insertion-deletion channel with feedback is introduced. Assume that the encoder transmits $$n$$...     

Microinhomogeneity of deformation and mechanical properties of steel in static and impact tests

The article presents the results of the investigation of the mechanical properties and microinhomogeneity of deformation of steel in dependence on its structure. A dependence of the characteristics of ductility and of impact toughness on the level of the microinhomogeneity of deformation is established. The change of ultimate (tensile) strength _u corresponds to the change of microhardness which serves as indirect characteristic of the strength properties of microvolumes. It is shown that impact toughness is a characteristic sensitive to the manifestation of microinhomogeneity of deformation, and that it changes in dependence on its level more than the static characteristics. The obtained results make it possible to choose the optimal structure of the metal.Translated from Problemy Prochnosti, No. 1, pp. 120–122, January, 1990.     

Hot corrosion ofα-SiC ceramics by V2O5 melt

The corrosion behaviour of-SiC in V₂O₅ melt has been investigated at elevated temperatures. The corrosion products on the surface of the specimen are removed using HF. The morphologies are also examined. From the observations of bubble formation in the scale and the temperature dependence of the corrosion rate, a kinetic mechanism is proposed. Based on the consistency of the plotted data with the proposed equation and high values of surface reaction rate constant, a diffusion controlling process has been developed.     

Vibratory comminution of A12O3 powders

Translated from Poroshkovaya Metallurgiya, No. 8(332), pp. 6–9, August, 1990.     

Thermodynamic efficiency of heat exchangers

Expressions are obtained to determine the exergetic efficiency of heat exchangers _a as a function of such dimensionless parameters as the energetic efficiency of the heat exchangers, the relationship between the thermal equivalents of the thermal carriers, the dimensionless initial temperature of the hot heat carrier ¯T₁=T₁/T_m and the temperature of the environment ¯T₀= T₀/T_m. In the domain of high environment temperature the quantity _a rises with the increase in the value of the parameter ¯T₁ while in the domain of low environment temperature the dependence _a=f(¯T₁) is extremal in nature and reached maximal values for ¯T₁=1.75–2.25.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 6, pp. 935–942, December, 1990.     

Computed tomographic imaging of the coronary arterial tree-use of local tomography

It has been demonstrated previously (see E.L. Ritman and A.A. Bove, in State of the Art in Quantitative Coronary Arteriography, p.67-78, 1986) that coronary artery anatomy can be visualized using high-speed, volume-scanning X-ray CT (computed tomography). In the current study it is demonstrated that local image reconstruction (i.e. the reconstruction kernel is ~2(+) mm long), as distinct from more conventional global image reconstruction (i.e. 200(+) mm kernel length), has the advantage of reducing the need for operator interactive image processing. In addition, the local reconstruction algorithm eliminates the need for recording the X-ray projection data over the full transaxial extent of the thorax because it requires only the X-ray attenuation data over the region of the heart. This latter aspect reduces the dynamic range requirements for the sensors and could reduce total X-ray exposure.