Thermogravi metric Study of the Oxidation of ZrB[sub 2] in the Temperature Range of 800° to 1500°C

Abstract

A thermogravimetric system is described for the determination of the total oxygen consumption, sample weight-change, and vaporization rate during the oxidation of ZrBrf. The results show an increasing deviation from para-bolic kinetics with increasing temperature at 250 mm oxygen pressure. The following equation is used to describe the total oxygen consumption in the temperature range of 800 ~ 1500 ~ C () (000) Ano2 moles O2-5.41 X 10-4 exp t 1/2 Jr 2.86 exp t A cm 2 2RT RT Because ZrB2 is oxidized stoichiometrically, this result can be converted to the recession of ZrB2 as follows () (. 000)-25,000 t 1/2-p 2.10 X 101 exp-t 5 (cm)-4.06 X 10-8 exp 2RT RT The total oxygen consumed in the oxidation of ZrB2 has been investigated by Berkowitz (1) over the temperature range of 1150~176 Parabolic oxidation was reported in that study as well as in the study by Kuriakose and Margrave (2), who measured sample weight change over the temperature range of 950 ~ 1250~ In both of these studies, the effect of oxygen partial pressure on the oxidation behavior was investigated at 1056~ Kuriakose and Margrave found the increase in the parabolic rate constant to be directly proportional to the oxygen pressure over the oxygen partial pressure range of 100-740 Tort. Berkowitz substantiated this finding but found no oxygen pressure dependence at 1287~ over the range of 8-40 Torr or at 1557~ over the range of 20-700 Torr. Clougherty et al. (3) have measured recession rates of ZrB2 at temperatures up to 1900~ and reported their data on the basis of a parabolic time dependence. In view of the current technological need for materials with good oxidation resistance at high temperatures , this study was undertaken to obtain additional quantitative data on the oxidation of ZrB2 by independent determinations of sample weight-change, total vaporization, and total oxygen consumption. These measurements allow the evaluation of the amount of ZrO2 and B203 formed. Experiment A common method of studying the kinetics of an oxidation process is by continuously recording weight-change at constant temperature. For many materials this method has proved to be very successful. However , in the study of the oxidation of some alloys and high-temperature refractory materials, care must be used in interpreting weight-change data because of the formation of volatile oxides. Weight-change measurements indicate only the amount of oxide products remaining on the sample. If volatile oxides are formed, then other methods, such as the measurements of total oxygen consumed by means of pressure changes or volume changes, must be used to determine reaction rates. Thermal conductivity measurements of the gas stream before and after reaction have been used successfully (4). Some experimental difficulties are encountered in all of these methods, particularly at very high temperatures. The techniques described here are designed for the thermogravimetric measurement of total oxygen consumption for materials that form a volatile oxide which is condensable at temperatures below the reaction temperature. Slight modifications of the system can be made to measure separately the vaporization rates and sample weight-change. During this study of the oxidation of ZrBf, three methods were used to separate into parts the oxidation processes which occur simultaneously. These methods were: 1. Sample weight-change-conventional weight-change measurements in which only the weight increase due to the oxidation products remaining on the sample are recorded. 2. Oxygen consumption-measurement of the weight increase of the sample plus the weight of any volatile oxides that are condensable within the temperature range of the system. 3. Vaporization-only the weight of the condensable oxides is measured. Measurements of sample weight-change were made with an Ainsworth FV-AU-1 recording microbalance. The capacity of the balance is 10g and the sensitivity is-+3 ~,g. A total weight-change of 100 mg can be recorded. In the conventional weight-change measurements , the gaseous B203 collected on the cooler parts of the furnace and represented an unknown weight loss. Single-crystal sapphire rods were used to suspend the samples, and measurements were made only to 1500~ At 1500~ and above, it was not possible to complete a run because of a reaction which destroyed the sapphire rods. It was observed that the reaction product of ZrBf, probably B203, reacts quite destructively with most materials at high temperatures. Rh, Pt, Ir, A1203, ThOf, ZrO2 were tried without success in attempts ~o 1195) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see