Chemistry and Chemical Biology ETDs

Publication Date

5-10-1963

Abstract

Self-diffusion in single-crystal tungsten and diffusion of rhenium tracer in single-crystal tungsten have been measured over the temperature range 2660°C to 3230°C by the direct sectioning technique. The initial radioactive layer of tungsten and rhenium tracers was produced on the diffusion samples by bombardment with 9.0-MeV deutrons. It was shown that the initial radioactive layer satisfactorily approximated the boundary-condition requirements of the one-dimensional diffusion equation. The tracers observed in determining the diffusion coefficients were W185, Re183, and Re184. Diffusion heating of the samples in high vacuum was accomplished by an induction-heating arrangement which included an eddy-current concentrator. Temperatures were measured by optical pyrometry, and the system was calibrated by reference to the melting points of tantalum (2996°C), molybdenum (2620°C), niobium (2468°C), and rhodium (1966°C). Sectioning of the diffusion-heated samples was done with a precision surface grinding machine, and the particles from the layers ground off were radiochemically analyzed. Radiochemical procedures were developed for separating pure fractions of tungsten and rhenium from one another and from the grinding wheel particles collected from each layer. The tungsten fraction was mounted as the 8-hydroxy­quinoline derivative of tungsten for beta counting. The final form of the rhenium fraction for gamma counting was tetraphenylarsonium perrhenate. The temperature dependence of the diffusion coefficients in the two systems, obtained by least-squares analyses of the data, is well represented by the equations

D(W) = (42. 8 ± 4. 8) e -( 153' 100 ± 600)/RTcm2 /sec,

and

D(Re) = (275 ± 110) e -< 162• 800 ± 2•500)/RTcm2/sec.

These data have been compared with the predictions of the current models of the diffusion process. The results are consistent with a ring mechanism as the fundamental step in tungsten self-diffusion. The theory of impurity diffusion in metals has not advanced sufficiently to establish a mechanism for diffusion of rheniun tracer in tungsten. However, the high activation energy indicates that there are no easy diffusion paths for rhenium atoms; and since rhenium atoms have nearly the same charge and size as tungsten atoms, it is reasonable to conjecture that rhenium tracer may diffuse through a tungsten lattice by a similar mechanism. In the process of determining an optimum deuteron energy for activation of the diffusion samples, total cross sections as a function of deuteron energy were measured for the reactions, W184(d,p)W185, W186(d,p)W187, and W184(d,2n) 34-day Re184, by the stacked-foil technique. Cross-section values for the reactions leading to the production of both W185 and W187 increased from about 3 mb at 6 MeV to a fairly broad maximum of 300 mb between 11 and 13 MeV. Cross sections for production of 34-day Re184 range from about 3 to 380 mb for deuterons with energies ranging from 7 to 14 MeV. Deuteron energies employed (14 MeV maximum) were not high enough to indicate the position of the maximum for the W184(d, 2n) 34-day Re184 excitation function. The decay of W185 was observed over a period of about 9 half-lives, and a new half-life value of 75.14 ± 0.63 days, with the uncertainty expressed to 3σ, was obtained. The half-life value obtained for W187 was 23.72 ± 0.06 hours.

Language

English

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Milton Kahn

Second Committee Member

Jere D. Knight

Third Committee Member

Jesse LeRoy Riebsomer

Fourth Committee Member

Guido Herman Daub

Fifth Committee Member

Glenn Arthur Crosby

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