Non-refractory Material Research Articles

Formation of location-controlled crystalline islands using substrate-embedded seeds in excimer-laser crystallization of silicon films

Single-crystal thin-film transistors on nonrefractory materials such as glass can be realized if monocrystalline islands of sufficient sizes can be grown at a predetermined position. By artificially controlling the super-lateral growth phenomenon observed in excimer-laser crystallization, this could be achieved. In this letter, we present such a method in which the silicon filling of a very small indentation fabricated in the substrate will act as a seed for lateral growth of large grains. When the melt is deep in these indentations, lateral growth is preceded by a vertical growth phase during which grains become occluded, so that a high yield of monocrystalline islands is obtained.

Temperature dependence of the growth rate for nanocrystalline diamond films deposited from an Ar/CH4 microwave plasma

We have investigated the effect of substrate temperature on the growth rate and properties of nanocrystalline diamond thin films prepared by microwave plasma-assisted chemical vapor deposition on (100) Si from a 1% methane (CH4) precursor in argon (Ar). In previous work we have shown that the carbon dimer C2 is the dominant growth species for this CH4/Ar system without the addition of molecular hydrogen. In the present work, the apparent activation energy for this growth process from C2 was determined from a standard Arrhenius-type analysis of the growth rate data for substrate temperatures between 500 and 900 °C. The measured value of 5.85±0.438 kcal/mol (0.254±0.019 eV/atom) is shown to be in close agreement with the results of recent modeling studies of the energetics of C2 addition to the diamond (110)–(1×1):H surface. These results have important implications for low-temperature diamond coating of nonrefractory materials such as glasses.

Modelling of short arc re-ignition

A unified mathematical description of phenomena leading to short arc re-ignition is presented. Computational results for a 2 mm long post-arc channel appearing at current zero after the burning of a 900 A arc on silver electrodes are discussed. The analysis concerns both cathode and anode electrode layers, for which computations were run simultaneously. For silver electrodes representing non-refractory materials, the dominant role of the cathode sheath in the re-ignition process has been proved analytically. Some mathematical simplifications, which may be used depending on the process dynamics, are proposed to decrease computation time but whilst still realizing acceptable accuracy.

Refractory precursor components of Semarkona chondrules and the fractionation of refractory elements among chondrites

Chondrules from the Semarkona (LL3.0) chondrite show refractory and common lithophile fractionation trends similar to those observed among the chondrite groups. It appears that chondrules are mixtures of a small number of pre-existing solid components, and we infer that chondrule precursor materials were related to the nebular components involved in the lithophile element fractionations recognized in ordinary chondrites. Compositional trends among the chondrules can be used to deduce the compositions of these components. We use instrumental neutron activation analysis to measure many (~20) of the lithophile elements in 30 chondrules. The amounts of oxidized iron were calculated from other compositional parameters; concentrations of Si were estimated using mass-balance considerations. The data were corrected for the diluting effects of non-lithophile constituents. Plots of lithophile elements versus a reference refractory element such as Al show that there were two major chondrule silicate precursor components: a refractory, olivine-rich, FeO-free one, and a non-refractory, SiO 2-, FeO-rich one. The refractory component probably forms from olivine-enriched condensates formed above the condensation temperature of enstatite. The non-refractory component must have formed from fine-grained materials that were able to equilibrate down to lower nebular temperatures. Chondrite matrix may have had an origin similar to that of the non-refractory material, and constitutes a third lithophile-bearing component that took part in chondrite fractionation processes. The low abundance of refractories and Mg in ordinary and enstatite chondrites was produced by the loss of materials having a higher refractory-element/Mg ratio than that in the refractory component of chondrules.

Ion enhanced film bonding

Two commonly used methods of depositing a thin film in vacuum are evaporation and sputtering. Evaporation has the advantage of rapid deposition in relatively high vacuum for non-refractory materials. Evaporated films are, however, liable to be weakly bonded to the substrate, because of the low arrival energy of the evaporant, unless effects such as chemical bonding occur. Sputtered films generally bond strongly but sputtering is a much slower deposition process than evaporation so that there may be a high density of contaminating inclusions. As charging also affects the sputtering process special techniques such as r.f. sputtering must be employed for insulating targets. A saddle-field ion source has been developed which operates with cold cathodes and without a magnetic field at pressures compatible with evaporation. Evaporated films deposited in the presence of a beam from this source exhibit the bonding properties of sputtered films. The beam contains a high proportion of energetic neutrals so that the method can be applied to both insulating and conducting substrates. The ion source is compact and can be installed generally in existing vacuum equipment. The source geometry can be varied to ensure that the area covered by the beam matches the deposition area.

Ion-enhanced film bonding

Abstract Two commonly used methods of depositing a thin film in vacuum are evaporation and sputtering. Evaporation has the advantage of rapid deposition in relatively high vacuum for non-refractory materials. Evaporated films are, however, liable to be weakly bonded to the substrate, because of the low arrival energy of the evaporant, unless effects such as chemical bonding occur. Sputtered films generally bond strongly but sputtering is a much slower deposition process than evaporation so that there may be a high density of contaminating inclusions. As charging also affects the sputtering process special techniques such as r.f. sputtering must be employed for insulating targets. A saddle-field ion source has been developed which operates with cold cathodes and without a magnetic field at pressures compatible with evaporation. Evaporated films deposited in the presence of a beam from this source exhibit the bonding properties of sputtered films. The beam contains a high proportion of energetic neutrals so that the method can be applied to both insulating and conducting substrates. The ion source is compact and can be installed generally in existing vacuum equipment. The source geometry can be varied to ensure that the area covered by the beam matches the deposition area.

Element correlations and the bulk composition of the Moon

A great number of element correlations have been observed in lunar samples. It is known from theoretical and experimental studies that in the solar nebula the elements condensed in groups according to their condensation temperatures and chemical affinities. One of these groups - the refractory elements - is represented by the early condensates or high temperature condensates (h.t.c.). From element correlations and group relations we estimate the bulk Moon to contain about 50 % of h.t.c.; the other 50%, the non-refractory portion, consists mainly of (Mg, Fe)-silicates and minor phases of about chondritic composition. Recently we have found strong evidence that most of the lunar highland samples represent mechanical mixtures of a differentiated (feldspathic) lunar component and a primary component from the last accretion stage of the Moon. The contribution of the h.t.c. in this primary material is estimated to 21 %. Hence, an inhomogeneous accretion of the Moon is indicated. After the formation of a highly refractory core relatively more and more non-refractory material was added until the Moon reached its final mass. The composition of the primary matter observed in the lunar highlands gives us an important clue to the composition of the non-refractory portion of the Moon and thus leads to a more reliable estimation of the lunar bulk composition.

The properties of high-pressure steady-state discharges in hydrogen

The results of a large number of measurements of total voltage, voltage gradient, current density and excitation temperature over a range of current, pressure, gap length and electrode condition for high-pressure steady-state discharges in hydrogen are given. Tungsten and copper electrodes were used as these are typical refractory and non-refractory materials respectively. The results show that in the absence of metal vapour the cathode and column regions are quite independent, the state of one not affecting that of the other. With increasing current a change from the glow to an arc column occurs at a critical value of the gas temperature. At short gaps the column transition current is modified by heat transfer with the electrodes. With tungsten electrodes, increase in current causes a continuous transition between glow and arc at the cathode which is due to thermionic emission. With copper electrodes the cathode transition is discontinuous, erratic and independent of cathode temperature, being a function rather of the surface condition.