The rate of reaction of molecular oxygen with tungsten was measured ffOm 2000° to 3300 0 K at pressures from 0.11 to 11.5 torr. High gas-phase mass transfer coefficients were obtained by impinging a high-velocity gas jet on the sample. A maximum reaction rate was observed at each O2 ~ressure, the tempera~ure. of maximum rate increasing with pressure. Below the temperatures of the maXIma, an apparent activatIOn energy near 23 kcal mole-1 was observed. These results are well described by the empirical equations of Perkins, Price, and Crooks, except that no transition to a regime of higher activation energy at lower temperatures was observed here and the maxima were found at slightly higher rates and te:uperatures than previously reported. Appreciable diffusion effects were found only near the observed maXIma. These data and the 10-4 torr data of Schissel and Trulson were correlated in terms of the two-layer oxygen-atom adsorption model for tungsten oxidation kinetics proposed by the latter authors. Kinetic constants for a slightly modified version of this model were obtained.
The kinetics of oxidation of tungsten by CO2 was studied in a stagnation-flow reactor at CO2 partial pressures from 20 to 337 torr. The study covered the temperature range 2200°—3200°K and included measurements of the effect of CO on the reaction kinetics at CO/CO2 ratios up to 5:1. The rates observed below about 2650°K were essentially independent of the velocity of the impinging gas stream, indicating that the surface kinetics primarily determined the rates up to 2650°K at the lower gas-flow rates used and to higher temperatures at higher flow rates. Where surface kinetics were rate determining, the data could be explained simply by incorporating a temperature-dependent ``sticking coefficient'' for CO2 into the two-layer adatom model previously proposed for the W–O2 reaction. The effect of CO was small and was readily incorporated into the model. Velocity-dependent deviations from the rates predicted by the model were observed at temperatures above 2650°K. These were largely explained by simple mass-transfer theory. At low flow rates and high temperatures it appears that the reactions of desorbed oxygen became important.
Nonmagnetic s o l i d s i n a superparamagnetic f e r r o f l u i d are s u b j e c t e d t o body f o r c e s p r o p o r t i o n a l t o the i n t e n s i t y of magnetization of t h e f l u i d and the g r a d i e n t of t h e magnetic f i e l d . the f o r c e equations, and s i n c e the apparent d e n s i t y can b e much l a r g e r than t h e t r u e d e n s i t y , i t i s p o s s i b l e t o l e v i t a t e o r f l o a t dense o b j e c t s . Mixtures of s o l i d s w i t h a d e n s i t y g r e a t e r than t h e apparent d e n s i t y sink w h i l e lower d e n s i t y s o l i d s f l o a t . An apparent d e n s i t y of the f l u i d can be d e f i n e d from I n p r a c t i c e i t i s d i f f i c u l t t o c r e a t e a uniform g r a d i e n t over a l a r g e volume and " s i n g l e gap" magnetic s e p a r a t o r s r e q u i r e very l a r g e magnets o r have a l i m i t e d throughput.problem, "multiple gap" magnetic g r i d s have been designed. Such g r i d s c o n s i s t of p l a n a r a r r a y s of p a r a l l e l b a r s of a l t e r n a t i n g p o l a r i t y , d r i v e n by permanent magnets. When immersed i n f e r r o f l u i d , magnetic g r i d s c r e a t e nonuniform f i e l d g r a d i e n t s and apparent d e n s i t i e s i n t h e f l u i d . However, both a n a l y s i s and experimental measurements show that t h e g r i d a c t s as a b a r r i e r t o p a r t i c l e s below a c r i t i c a l d e n s i t y , while p e r m i t t i n g more dense p a r t i c l e s To overcome t h a t 339 Copyright 0 1980 by Marcel Dekker, Inc. Downloaded by [Michigan State University] at 23:35 08 February 2015 340 FAY AND QUETS t o f a l l through t h e g r i d . Thus, a magnetic g r i d f i l t e r can b e used a s a h i g h throughput b i n a r y s e p a r a t o r of s o l i d s a c c o r d i n g t o their d e n s i t i e s . Such f i l t e r s can b e cascaded f o r more complex s e p a r a t i o n s . S e v e r a l magnetic g r i d f i l t e r s have been d e s i g n e d , b u i l t , and t e s t e d . Magnetic measurements q u a l i t a t i v e l y a g r e e w i t h t h e t h e o r e t i c a l p r e d i c t i o n s . Experiments w i t h s y n t h e t i c m i x t u r e s have demonstrated t h a t good b i n a r y s e p a r a t i o n s c a n b e made.
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