Thermoelectric measurements of energy deposition during shock-wave consolidation of metal powders of several sizes

The degree of shock energy localization within individual particles and between neighboring particles of different size was explored during shock-wave consolidation of spherical metal powders. The thermoelectric voltage generated by the passage of a shock wave through a copper powder-constantan powder interface was recorded. The sizes of the copper and constantan powders were varied between mean diameters of 40 and 98 µm. Shock-wave pressures of 5 GPa were applied by flyer plate impact, and the resulting voltage versus time signals were collected with a 10 ns time resolution. In order to analyze the signals, a simulation of the thermocouple system was developed to account for the effects of multiple particle interactions and a slightly nonplanar copper-constantan interface. The resulting simulated voltage versus time signals are a good match for the observed signals when the size ratio of the copper and constantan particles is less than a factor of 2, and reveal the preferential deposition of energy in smaller particles at the expense of larger particles within the size range examined. The amount of energy localized near particle surfaces was found to be a majority of all the energy, with a significant minority deposited throughout the particle bulk

Strain modification in coherent Ge and SixGe1–x epitaxial films by ion-assisted molecular beam epitaxy

We have observed large changes in Ge and SixGe1–x layer strain during concurrent molecular beam epitaxial growth and low-energy bombardment. Layers are uniformly strained, coherent with the substrate, and contain no dislocations, suggesting that misfit strain is accommodated by free volume changes associated with injection of ion bombardment induced point defects. The dependence of layer strain on ion energy, ion-atom flux ratio, and temperature is consistent with the presence of a uniform dispersion of point defects at high concentration. Implications for distinguishing ion-surface interactions from ion-bulk interactions are discussed

Etching of High Purity Zinc

A method of etching high purity zinc to reveal various etch figures on {101¯0} planes is presented in this paper. Etch figures are formed by polishing in a dichromic acid solution after the introduction of mercury to the crystal surface. No measurable aging time is required to form etch figures at newly formed dislocation sites when mercury is on the surface prior to deformation. The mercury concentrates at the sites where etch figures form and may be removed by vacuum distillation and chemical polishing before it appreciably affects the purity of the bulk of the crystal

Shock wave initiation of the Ti5Si3 reaction in elemental powders

Elemental powder mixes were subjected to plane wave shock processing which reduced the initial porosity to essentially zero. Two powder mixes in a 5:3 Ti:Si atomic ratio were used: -325 mesh Ti and Si (<45 mu m), and -100 mesh Ti and Si (<150 mu m) with shock pressures up to 7.3 GPa and shock energies up to 671 J/g. Shock pressures were calculated using hugoniot parameters for porous elemental powder mixtures and shock energies were taken to be the work done by the shock (P Delta V/2). Shock energy thresholds for complete reaction of the elemental powders were found which depend upon powder particle size and the initial porosity of the powder. The threshold energy for the larger powder mix was found to be similar to 8O% larger than that for the smaller powder. A decrease in initial porosity from 0.49 to 0.40 caused an increase in threshold shock energy of about 75% for both powders. At shock energies slightly below the threshold energy, evidence for the reaction of solid Ti and liquid Si was observed in small isolated regions. These regions contained spherical micronodules with the composition of TiSi2 in Si. The results are compared to those of previous studies reported in the literature, and mechanisms for reaction initiation and the observed threshold values are proposed

Correlation of shock initiated and thermally initiated chemical reactions in a 1:1 atomic ratio nickel-silicon mixture

Shock initiated chemical reaction experiments have been performed on a 1:1 atomic ratio mixture of 20- to 45-µm nickel and –325 mesh crystalline silicon powders. It has been observed that no detectable or only minor surface reactions occur between the constituents until a thermal energy threshold is reached, above which the reaction goes to completion. The experiments show the energy difference between virtually no and full reaction is on the order of 5 percent. Differential scanning calorimetery (DSC) of statically pressed powders shows an exothermic reaction beginning at a temperature which decreases with decreasing porosity. Powder, shock compressed to just below the threshold energy, starts to react in the DSC at 621 °C while powder statically pressed to 23% porosity starts to react at about 30 °C higher. Tap density powder starts to react at 891 °C. The DSC reaction initiation temperature of the shock compressed but unreacted powder corresponds to a thermal energy in the powder of 382 J/g which agrees well with the thermal energy produced by a shock wave with the threshold energy (between 384 and 396 J/g). (Thermal energies referenced to 20 °C.) A sharp energy threshold and a direct correlation with DSC results indicates that the mean thermal energy determines whether or not the reaction will propagate in the elemental Ni+Si powder mixture rather than local, particle level conditions. From this it may be concluded that the reaction occurs on a time scale greater than the time constant for thermal diffusion into the particle interiors

Dislocations and etch figures in high purity zinc

A method of etching high purity zinc single crystals to reveal various etch figures on {1010} planes is presented in the preceding paper. The procedure involves the introduction of mercury to the crystal surface prior to a chemical polish with dichromic acid. The mercury was found to be concentrated at the etch figures. This paper presents the results of several experiments which support the conclusion that there exists a one-to-one correspondence between etch figures and dislocations. Some observations of slip on (0001) basal planes and {1212} pyramidal planes, and of twinning in zinc are also presented

Orientation Dependence of a Dislocation Etch for Zinc

The dislocation etch for (101-[bar]0] surfaces of zinc reported by Brandt, Adams, and Vreeland have been further explored. Additional surface orientations have been found where dislocation etching takes place. These orientations cover an area located between 3 degrees and 12.2 degrees to the [0001], and the area is symmetric about that axis. Attempts to produce dislocation etching on within 2 degrees of (0001) were generally unsuccessful. This is in contrast to etching of many crystals which takes place only within a few degrees of a low index plane

Several Techniques for One-Dimensional Strain Shock Consolidation of Multiple Samples

We explored three methods of shock wave powder consolidation which retain the one-dimensional nature of a plane shock wave and allow multiple samples to be consolidated. The first technique uses a porous sintered metal cylinder as a shock fixture. The sintered material is chosen to match as closely as possible the solid density and compressibility of the powder to be investigated. The second method does not require a compatible porous material. A cylindrical target cavity is separated into multiple regions by thin sheet metal dividers. The dividers are of the same scale thickness as the powder size to retain a one dimensional condition in most of the compact. The third method may be the most interesting technologically. A powder media of near impedance match to the material under study is selected which resists bonding under the shock conditions to be used. Pressed greens of the material to be consolidated are then embedded in this pressure -transmitting media. The greens are then shocked along with the 'non-stick' media. In our experiment, a discontinuously reinforced metal matrix composite (MMC) was shock consolidated to near net shape by this method. Ti powder was mixed with SiC powder, pressed into a green with corners and radii, and embedded in fine zirconia powder. The shock wave generated by a 304 stainless steel flyer plate accelerated to 1.0 km/ s fully consolidated the MMC without bonding the zirconia. The compact was recovered with well defined corners and flat surfaces

Defect formation and diffusion mechanism in ion-assisted molecular-beam epitaxy

A simple moving boundary diffusion model has been used to characterize defect incorporation kinetics during ion-assisted molecular-beam epitaxy. The model permits analysis of the dependence of the final defect concentration on the growth rate, defect diffusivity, defect production range, and the shape of defect depth distribution. The results indicate a linear dependence of the final defect concentration on the ion-to-atom flux ratio which is in the growth-rate-limited regime of the model. Comparison between the model and the film strains measured by x-ray rocking curve analyses has been made and reveals that the thermal spike energy deposited by the bombarding ions during epitaxial growth has a significant effect on the apparent activation energy of the defect migration. A transition temperature above which the defect migration is thermally activated and below which the defect migration is cascade assisted can be defined. The experimentally observed temperature dependence of the defect concentration can be attributed to cascade-assisted diffusion of the defects. Comparison between the model and the multisite multiply activated migration model for low-energy dopant incorporation has also been made. The results show the similarity between the defect incorporation and dopant incorporation which gives a unified view of both processes