Showing papers by "Applied Science Private University published in 1974"

A mechanism for the nitridation of silicon powder compacts

Abstract: A mechanism for the nitridation of silicon powder is proposed, based on an interpretation of the microstructure of partially reacted compacts. It is observed that the reaction does not occur at the solid-state interface between the silicon and the nitride product layer. Both silicon and nitrogen are transported through this layer and the removal of silicon results in the formation of pores in the silicon crystals at the nitride-silicon interface. The nitridation reaction takes place within these pores, which subsequently migrate into the silicon grains, and within the original voidage of the compact.

Measurement of K Ic on Small Specimens Using Critical Crack Tip Opening Displacement

Abstract: A method is described whereby on-load values of crack tip crack opening displacement (COD) can be measured at the midsection of precracked three-point bend specimens by infiltration of silicone rubber. A calibration curverelating midsection COD to clip gage displacement was derived from measurements on the silicone rubber "castings." This calibration curve can be used to calculate midsection COD from on-load clip gage displacement and specimen geometry only, without further infiltration measurements. These values of COD have been shown to be simply related to the stress intensity factor, independent of material, as theoretically predicted. The central region of a Charpy specimen in three-point bend has been found to remain in plane strain until well after general yield. Thus, plane strain values of COD at fracture initiation, (COD) c , can be determined from small specimens. Two initiation detection methods are described whereby (COD) c can be determined. These values of (COD) c can be used to accurately predict K I c values which agree with data obtained on larger, more expensive valid ASTM K I c specimens.

Investigation of turbulent convection under a rotational constraint

Abstract: Turbulent convection for a rotating layer of fluid heated from below is studied in this paper. The boundaries of the fluid layer are taken to be free. The underlying principle, used is the Malkus hypothesis that the flow tends to transport the maximum amount of heat possible, subject to certain constraints. By taking the Prandtl number to be infinite, a linear differential constraint and an integral constraint are used. The variational problem that follows then depends on two dimensionless parameters, the Taylor number T and the Rayleigh number R.Asymptotic analysis for the turbulent regime shows that the flow arranges itself so as to tend to offset the stabilizing effect of the rotational constraint, at least in so far as the heat flux is concerned. The dimensionless heat flux, or the Nusselt number, has in general different dependence on T and R, depending on the particular region in the parameter space. For T [les ] O(R), the flow is essentially non-rotating. For O(R) [les ]T [les ] O(R4/3), the flow will always have finitely many horizontal wavenumbers, though the total number of modes increases as T increases in this region. For O(R4/3) [les ] T [les ] O (R3/2), the Nusselt number has a functional dependence proportional to R3/T2, having essentially infinitely many horizontal modes as both R and T increase indefinitely in this region. The last expression is particularly interesting, as it agrees qualitatively with results in finite-amplitude laminar convection. It is also linearly dependent on the layer thickness, as one might expect from dimensional argument. It is suggested that, in the context of the maximum principle, the result in this region of the parameter space may be applicable as well to the same fluid layer with rigid boundaries through the existence of an Ekman layer that is thinner than the thermal layer.

Bedrock intensity attenuation and site factors from San Fernando earthquake records

Abstract: An attenuation formula was derived for the Arias instrumental intensity on bedrock based, in part, on the source spectrum function obtained by Aki. The constants in the formula were calibrated for the San Fernando earthquake by using eight bedrock spectra derived from surface accelerograms. The calibrated formula was then used to compute the Arias bedrock intensities at most of the sites of the ground-level accelerographs and seismoscopes that recorded the earthquake. Maxima accelerations from accelerograms and spectral accelerations from seismoscope records were then used to compute the Arias intensities at the surface by using an empirical relation obtained by Arias. After the instrument sites were classified into four groups, (1) crystalline rock, (2) sedimentary rock, (3) shallow alluvium, and (4) deep alluvium, surface-to-bedrock intensity ratios were correlated with these site classifications which leads to four surface attenuation curves constrained to have the same slope. From the constant differences between these curves, it is possible to define site factors to be applied to bedrock intensity in order to estimate surface intensity for zoning purposes. Conversely, these factors can be applied to surface intensities for deriving bedrock attenuation curves for other earthquakes in which the geology of the instrument sites is only generally known. The site factors relative to unweathered, unfractured crystalline rock outcrops are 1.80, 3.63, 3.74, and 5.12 for classifications 1 through 4, respectively.

The study of powder compaction mechanisms

Abstract: The mechanism whereby powders may be compressed to form solid objects is not properly understood, largely because, until recently, the process has not been studied by sufficiently powerful techniques. This paper describes a number of techniques developed for a detailed study of powder compaction. Methods for measuring specific surface areas of powders and powder compacts, and hence the areas of contact between the particles, are discussed and reasons for selecting an adsorption method are explained. A gas adsorption apparatus capable of determining very small specific surface areas is described, as is the use of the scanning electron microscope. Both techniques were used to investigate particle fracture during compaction. In addition the study of pore size gives valuable insight into the movement of particle fragments once they have been produced. The use of mercury porosimetry in this connection is described.