Chemistry and Chemical Biology ETDs

Publication Date

5-19-1962

Abstract

Vaporization of hafnium diboride has been studied in a tungsten effusion cell over the temperature range 2175°K. to 2500°K. A low-melting eutectic at 2520°K. prevented the use of the effusion cell technique at higher temperatures. An ultra-high vacuum apparatus, capable of producing vacuums of the order of 1 x 10-12 atm. or better and within which the final series of vapor pressure measurements was made, was designed and constructed during the course of this study. The effect of a high background pressure in producing species more volatile than the ones under investigation was considered to have been reduced to negligible proportions. Hafnium diboride vaporizes congruentiy at a nonstoichiometric composition of HfB1.955±0.010. This composition is very nearly constant up to and including a temperature of 2730°K. Hafnium diboride samples studied at temperatures above this temperature (in the vicinity of 3000°K.) show a singularly different behavior. The Motzfeldt treatment was considered for the determination of both the vaporization coefficient and the equilibrium pressure. The data were used to obtain a vaporization coefficient of 0.098 ± 0.010 and an equilibrium partial pressure of hafnium over HfB1.955 at 2500°K. of 3.1 x 10-7 atm. ± 10%.Analysis of the data indicated that hafnium diboride vaporizes predomi­nantly according to the equation

HfB1.955(s) = Hf(g) + 1.955 B(g)

over the temperature range investigated. A Third Law standard heat of vaporization, ΔH°0, of 477.8 ± 5.5 kcal./mole and an estimated entropy change at 2000°K., ΔS°2000, equal to 100.3 e. u. have been determined for the reaction above. A value of 142 ± 7 kcal./mole was deduced for the heat of vaporization of hafnium metal from data obtained in this research.

Project Sponsors

Los Alamos Scientific Laboratory, University of New Mexico

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

Jesse LeRoy Riebsomer

Third Committee Member

Guido Herman Daub

Fourth Committee Member

Melvin G. Bowman

Fifth Committee Member

Glenn Arthur Crosby

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