This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 155.33.16.124 On: Sat, 29 Nov 2014 08:17:22 82 C. A. NEUGEBAUER AND M. B. WEBBeffect. If one prepares a series of increasingly thicker films, which consist of increasingly bigger particles, the "free space"between particles not filled with metal will decrease. This means that for any applied field across the film, the thicker films will actually have a much higher E. since there are fewer particles and therefore a much greater voltage drop across each gap. Equation (A3) would then predict that, at any one temperature, the conductance of the thick films should change more rapidly with applied voltage than the thinner ones, since E. changes more rapidly with voltage. This is indeed always observed, and is illustrated in Fig. 11, where the change of conductance with field is plotted logarithmically against the square root of the applied voltage for nickel films of different thickness at 77°K. Small tantalum carbide objects may be made by heating the objects made from tantalum in dilute hydrocarbon atmospheres at temperatures > 20000K. Carburization proceeds quite rapidly above 2300 0 K by diffusion of carbon initially deposited on the surface into the metal, principally along grain boundaries, to form a thin layer of Ta2C which is quickly converted to the TaC. The latter has a tensile strength of 21-25 kg/mm!, a work function <1>=3.17 ev, an emissivity of 0.40-0.46 between 2400 0 and 3500 o K, rising with increasing temperature, and a resistivity p = 102+0.0178 T (OK) ohm em between 1400° and 3500°K.
The kinetics of the gas phase photochemical bromination of isobutane to yield 2-bromo 2-methylpropane was investigated in the temperature range 40—95°C and the reaction found to proceed through an atom and radical chain mechanism analogous to that observed with a number of other hydrocarbons. The thermal reaction was studied at 112 and 130°C and shown to have the same mechanism except for the mode of production of bromine atoms. Bromination was inhibited by hydrogen bromide, a product of the reaction, and also by oxygen. The dissociation energy of the tertiary C–H bond in isobutane was found to be 89.6 kcal/mole at 0°K, in agreement with the idea that tertiary bonds are considerably weaker than primary bonds. A general review of activation energies of bimolecular reactions involved in bromination is given as well as a recapitulation of values of C–H and C–C bond dissociation energies derived from bromination studies.
The effects of long term exposure to dry and humid environments on carbon‐epoxy and graphite‐epoxy composites have been studied. Filament wound Naval Ordnance Laboratory rings were fabricated in 1965 and were placed in dry, distilled water and sea water for 11 years. Moisture desorption tests were conducted in order to determine the water content of specimens exposed to the water environments, The effect of the history on moisture absorption characteristics was investigated by re‐exposure of partially and completely dried specimens to two different environments: distilled water at 60°C and 98% relative humidity at 60°C. The weight gain was measured and diffusion coefficients were calculated. Horizontal shear tests and flexural tests were performed on “wet” specimens (current properties) and on partially and completely dried specimens (residual properties). The shear strength of the carbon‐epoxy composites is degraded by the exposure while that of the graphite‐epoxy composite is slightly increased. The composite flexural strengths are not degraded by the exposure. These results are discussed in terms of failure modes.
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