Liquid thallium self-diffusion measurements
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to determine initial radioactivity levels prior to the measurement. A similar procedure was used with the entire capillary contents followinga period of isothermal diffusion (controlled to within 1~ to determine the final average radioactivity level. Average radioisotope concentrations were determined by gas-flow proportional counting with a 90 to 10 mixlure of argon and methane. Specific activities of the thallium ranged from 11,500 to 35,000 cpm per gram, and total counts recorded ranged from 106 to 2 • 106. These count rate data were subsequentlyused to calculate the radioisotope concentrations needed to evaluate the diffusion coefficients. Diffusion coefficients were evaluated from the solution to the diffusion equation for the modified shear cell p r o b l e m 4 uiz.:
~
-
4(LI+Lz' ~ z - nt=~z z( 1 sEi n 2~ nTLrz 1
-
exp - (n~)2o
[l]
in which
(-C~(t)(L~ + L2) - C s L t - COL2)* (Co - Cs)(L1 + Lz)
2\
*The factor of 2 in this expression was inadvertantly omitted in earlier reports.4,6,7
Liquid Thallium Self-Diffusion Measurements ROGER E. BARRAS, HUGH A. WALLS, AND ANTHONY L. HINES D e n s i t y d a t a a r e a v a i l a b l e for liquid t h a l l i u m f r o m m e a s u r e m e n t s by C r a w l e y 1 and by M a r t i n e z and Walls, 2 and C a h i l l and G r o s s e 3 have e x p e r i m e n t a l l y d e t e r m i n e d v i s c o s i t i e s . C a h i l l and G r o s s e also p r e s e n t e d s e l f diffusion c o e f f i c i e n t s for liquid thallium c a l c u l a t e d f r o m t h e i r v i s c o s i t y data using equations b a s e d on F r e n k e l ' s kine t i c t h e o r y v i a the S t o k e s - E i n s t e i n r e l a t i o n s h i p b e tween v i s c o s i t y and diffusion. H o w e v e r , the continued l a c k of e x p e r i m e n t a l l y d e t e r m i n e d s e l f - d i f f u s i o n data f or t h a l l i u m p r o m p t e d the w o r k r e p o r t e d h e r e , in which diffusion c o e f f i c i e n t s w e r e m e a s u r e d o v e r a t e m p e r a t u r e r a n g e f r o m 318.3 to 450.5~ The modified s h e a r c e l l method of B r o o m e and Walls a w a s a l t e r e d slightly s to p r o v i d e a m e lt in g c h a m b e r and c o n t r o l l e d a t m o s p h e r e mounted above the c a p i l l a r y f i l l ing por t . The s h e a r c e l l s e g m e n t s w e r e f a b r i c a t e d f r o m cold-rolled low-carbon steel, and the capillary diam was 1.59 ram. Capillary lengths are noted with other data given later. Thallium purity was 99.999 wt pct, and the thallium204 isotope used was produced in the University of Texas Nuclear Reactor Laboratory. Thallium rods were cast under controlled atmosphere and then cleaned with sulfuric acid prior to use in the diffusion measurements. A knownquantity of each rod was also dissolved in a controlled volume of sulfuric acid to provide a sample ROGER E. BARRAS is Development Engineer, E. I. du Pont de Nemours and Company, Wilmington, Delaware. HUGH A. WALLSis Professor of Mechanical Engineering, University of Texas at Austin, Austin,
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