Formation of carbon nanofibers via carbon monoxide disproportionation

Experimental results suggesting that carbon nanofibers are formed from amorphous carbon released at several compact active sites are reported. It was shown that the sites in question are catalyst crystal lattice defects formed at the crystallite contact boundaries.     

An algorithm for deducing an effective sieve residue from the rate of powder passage through a sieve

The effect of non-ideal aperture distribution of a sieving surface on the sieve residue has been the object of many experimental studies. Unfortunately, the sieving kinetics of many sieving techniques do not result in a simple probability relationship between the size distribution function of the powder and the sieve aperture distribution function. If one specifies a certain type of sieving kinetics, then one can apply probability theory to the rate of movement of powder through the sieve and the size aperture distribution function of the sieve. Under such specified sieving conditions it can be shown that it is possible to deduce a size characteristic parameter of the powder being sieved, which in this publication is described as the kinetic residue of the sieve. An algorithm for the automatic computation of the kinetic residue from the rate of powder sieving is described. The possible implementation of this algorithm to automate sieve analysis is explored.     

Photodegradation of ethylene–propylene copolymer and ethylene–propylene–ethylidenenorbornene terpolymer

Significant progress has been made in recent years regarding the photooxidation of olefin copolymers, but questions still remain. This paper reviews the progress and probes the photooxidative chemistry of ethylene–propylene (EP) and ethylene/propylene/diene monomer (EPDM) copolymers. Both stabilized and unstabilized polymer plaques were irradiated in a xenon are and the surface chemistry followed using infra-red spectroscopy. Model compounds were used to help elucidate the chemistry caused by unique structural features present in the copolymers. Volatile products evolved during photooxidation were determined giving valuable insight into the degradation chemistry.     

Dislocation Structures in Creep-Deformed Polycrystalline MgO

Secondary creep of polycrystalline MgO with grain sizes of 100 and 190 μm was investigated at 1300° to 1460°C under compressive loads of 2.5 to 5.5 kgf/mm². The dependence of creep rate on load follows a power law with an exponent of 3.2±0.3. The process is thermally activated, with an activation energy of 76 ± 12 kcal/mol. The creep rate is independent of grain size. The dislocation structure was investigated by transmission electron microscopy. The total dislocation density follows the relation, σ= bG √ρ, commonly found for metals. The dislocations form a 3-dimensional network in which many dislocation segments lie in their slip or climb planes. On the basis of this structure, a model is proposed in which glide is the principal cause of deformation but the rate-limiting process, i.e. annealing of the network, is diffusion-controlled. Theoretical estimates and experimental results agree within 1 order of magnitude.     

Leaching of Polyphase Nuclear Waste Ceramics: Microstructural and Phase Characterization

The leaching of complex polyphase nuclear waste ceramics is described in the context of the geochemically established dissolution behavior of the constituent phases. Static leach tests conducted on ceramic monoliths in deionized water, in simulated silicate, and in brine groundwaters, conforming to Materials Characterization Center standards and an accelerated, microscopic leach test, were used to identify the processes. Dissolution and formation of surface passivation layers are discussed in terms of hydrolysis and the adsorption of the metal hydroxocomplexes onto the monolith surface. The factors observed to affect dissolution are pertinent to the leaching of other polyphase nuclear waste ceramics.     

Habit Changes of Sapphire Grown from PbO-PbF2, and MoO3-PbF2 Fluxes

Single crystals of Al₂O₃ (sapphire) were grown from PbO-PbF₂ and MoO₃-PbF₂ fluxes; they varied from flat plates (PbF₂-rich melts) to equidimensional crystals (PbO- or MoO₃-rich melts). The primary growth planes are basal {0001}, first-order rhombohedral {1011}, and second-order rhombohedral {0112}. The habit change is interpreted on the basis of F^- contamination and Pb²⁺ surface adsorption. Possible ion species in the melts and their relative importance on crystal growth from these systems are discussed.     

Lipids of seven cereal grains

Grain samples of representative varieties of barley, corn, oats, rye, sorghum, triticale, and wheat grown commercially in the north central US were analyzed. Chemical constituents of the varieties studied are presented to provide an overview of their characteristics. Lipids of the milled grain samples were solvent extracted, classified by silicic acid column chromatography, and separated by thin layer chromatography. Fatty acid composition of the total lipid was determined by gas liquid chromatography and the fatty acid content was determined by saponification and extraction. Total lipid content of the grains ranged from 2.3% for ‘Polk’ wheat to 6.6% for ‘Chief’ oats. Lipid composition varied considerably. The row crops, corn and sorghum, have a high neutral lipid and low glycolipid content. The small grain varieties have a more balanced distribution among neutral lipids, glycolipids, and phospholipids. Fatty acid composition of the total lipid was similar for all grains. Minor qualitative differences were noted among the lipid classes of the 7 cereals.     

Hertzian Stress Cracks in Beryllia and Glass

The effect of Hertzian stresses resulting from loads acting at the points of contact of beryllia spheres was determined. As-drawn glass and glass ground with various grades of silicon carbide were indented with beryllia spheres, 1 in. in diameter. Also, pairs of these beryllia spheres were pressed together. Circular cracks due to Hertzian stresses were produced; an optical technique was developed for detecting them in a beryllia surface. The outer crack radius was proportional to the cube root of the maximum applied load. Average values obtained for the respective inner crack radii were used to determine average values of the minimum load to fracture these materials. The average minimum load to fracture 1 in. diameter beryllia spheres in air at room temperature under dynamic loading was 235 1b. The crack radius in glass and beryllia was about 20% greater than the radius of the contact surface. This was not significantly affected (in glass) by flaw density. It is considered that the Hertz analysis does not give the correct location and value of the maximum tensile stress when finite displacements of material occur.