Controlled incorporation of mid-to-high Z transition metals in CVD diamond

We report on a general method to fabricate transition metal related defects in diamond. Controlled incorporation of Mo and W in synthetic CVD diamond was achieved by adding volatile metal precursors to the diamond chemical vapor deposition (CVD) growth process. Effects of deposition temperature, grain structure and precursor exposure on the incorporation efficiency were systematically studied, and doping levels of up to 0.25 at.% have been achieved. The metal atoms are uniformly distributed throughout the diamond grains without any indication of inclusion formation. These results are discussed in context of the kinetically controlled growth process of CVD diamond.     

Collision cascades in pure δ -plutonium

Molecular dynamics simulations of the formation and annealing of large collision cascades in delta-phase plutonium are presented. The defect evolution is followed with time up to 2 ns. Simulations are performed with the MEAM potential at three different temperatures; at 600 K where the pure delta phase is thermodynamically stable; at 300 K where the delta phase can only be maintained in a metastable state with minor additions of gallium or aluminum; and at 180 K where plutonium should transform to the alpha phase. It is found in all three cases that the atomic structure within the cascade evolves through a glass-like state. At 600 K, this structure recovers very slowly; at 300 K it persist up to 2 ns with no discernable trend to recover eventually; and at 180 K the amorphous structure initiated by the collision cascade spreads through the entire crystal and converts it to a glass-like structure.     

Change in flow stress and ductility of δ-phase Pu-Ga alloys due to self-irradiation damage

An internal state variable model for the mechanical behavior of aged Pu-Ga alloys is developed and used to predict the change of the material with accumulated self-irradiation damage or age. The material model incorporates microstructural data such as the primary irradiation-induced defect density from cascades, the density and average size of helium bubbles, the initial dislocation density, and the initial average segment length of the dislocation density as input parameters, and then evaluates the stress-strain response of a representative volume element of the material. Given this response at a single material point, the deformation behavior of tensile specimens is predicted, and it forecasts increased strength, decreased strain hardening, and more strain localization with aging. Although the material point behavior showed some slight strain softening, this strain softening is expected to be masked by statistical variations of different volume elements and by the strain rate sensitivity of the material. Hence, it is not expected to appear in the stress-strain response of macroscopic tensile specimens, and only the increase in flow stress will be measured.     

Diagnostic ophthalmology. Bilateral, bullous retinal detachment

The recognition of the grave risk of overwhelming sepsis in splenectomized individuals has led to the search for alternatives to splenectomy. Conservative management, first tried successfully in children and later extended to adult patients, has become the accepted form of treatment and has also been tried with success in haemophiliac children. The case presented here is important because this is the first report of a successful outcome of non-operative treatment of a ruptured spleen in a haemophiliac adult.     

Quantification of the effects of respiration and parallax on inferior vena caval filter position

PURPOSE: The change in inferior vena caval (IVC) filter position at follow-up relative to the filter position at implantation has been used as a criterion for evaluation and comparison of these devices. Perceived changes in filter position may be due to respiratory movement and/or changes in parallax between the initial and follow-up imaging studies. In this study the authors evaluated and attempted to quantify the effects of respiratory movement and parallax. PATIENTS AND METHODS: After placement of an IVC filter, radiographs of the abdomen were taken at maximum inspiration and maximum expiration in 30 patients. The effect of parallax on apparent filter movement was studied by using a phantom. RESULTS: The average filter movement on inspiration/expiration radiographs (corrected for magnification) was 3.6 mm +/- 2.2. An 8.5-mm maximal change secondary to parallax was seen in the phantom study. CONCLUSION: When follow-up images are obtained, efforts should be made to closely reproduce patient positioning and patient respiration to reduce errors in the interpretation of filter migration.     

Diffusional exchange of isotopes in a metal hydride sphere

A model is developed that simulates exchange of one hydrogen isotope with another hydrogen isotope in a spherical metal hydride particle. This is one of the fundamental physical processes operative during isotope exchange in a bed of spherical metal particles and is thus one of the key components in any comprehensive physics-based model of exchange. A critical aspect that must be considered is that the diffusion of one isotope depends not only on its own concentration gradient, but also on the concentration gradient of the other isotope. This coupling arises because the chemical potential of each isotope depends on the concentrations of all isotopes as well as the presence of vacancies within the metal hydride. This thermodynamic coupling is derived from the Gibbs free energy of mixing and the excess chemical potential due to elastic and electronic interactions. Unknown diffusion coefficients appearing in this formulation of isotope transport are determined by fitting to available experimental data. Example calculations illustrate times required for isotope exchange and the asymmetries observed between forward and reverse exchanges due to differences in isotope stoichiometry and diffusivity.     

Stabilization of Ni(I)(1,4,8,11-tetraazacyclotetradecane)+ in a Sol-Gel Matrix: It's Plausible Use in Catalytic Processes

Monovalent nickel is used as a catalyst in a diversity of important chemical systems due to its properties as a strong reducing agent. However, monovalent nickel is unstable in homogeneous systems; its lifetime is very short, and therefore, its efficiency as a catalyst is quite limited. In this article, we demonstrate the catalytic capabilities of Ni(I)(1,4,8,11-tetraazacyclotetradecane)⁺ as a reducing and catalytic agent by its entrapment in the silica sol gel matrix. The EPR results indicate that the sol-gel matrix plays two roles: it inhibits the common mechanism of Ni(I)L⁺ decomposition in aqueous solutions, and it also facilitates its functionality as a heterogeneous catalyst, by stabilizing it for at least several hours. The activity of Ni(I)L⁺ which is entrapped within the matrix, was studied by reducing 2,2-Bis(bromomethyl)-1,3-Propanediol and I₃⁻. The results indicate that Ni(I)L⁺/Ni(III)L−H²⁺ acts as a good reducing agent. These results have important implications for a variety of essential catalytic reactions by shedding light on a new feature of sol-gel matrices and their use as active species stabilizers.     

Atomic-volume variations of α-Pu alloyed with Al, Ga, and Am from first-principles theory

First-principles methods are employed to calculate the ground-state atomic densities (or volumes) of α-Pu alloyed with Al, Ga, and Am. Three configurations for the alloying atom are considered. (i) It is located at the most open and energetically most favorably site. (ii) It is located at the least open site. (iii) It is randomly distributed within the α-Pu matrix. When alloyed with Al or Ga, α-Pu behaves similarly, it expands considerably for configurations (ii) and (iii), while for (i) only small changes of the density occur. Interestingly, for Am, the alloying effects are quite different from that of Al and Ga. Small expansion is noted for the ordered configurations (i) and (ii), whereas for the disordered (iii), only insignificant changes of the density take place. The bonding character is thus differently influenced in Pu by the addition of Al and Ga on one hand and Am on the other. This is consistent with the view that Al and Ga stabilize the δ over the α phase in Pu by a different mechanism than Am, as has been discussed in recent publications. The U.S. Government’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.