Part I: Study on the performance of a solid state electrochemical cell using yttria-doped thoria electrolyte. Part II: Calculation of the Fe-Ni phase diagram from 1184K̊ to 300K̊

Author

Sujit Das

Date of Award

8-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

C. R. Brooks

Committee Members

J. E. Spruiell, E. E. Stansbury

Abstract

The main purpose of this investigation was to study the performance of a solid state electrochemical cell using yttria-doped thoria electrolytes, which have been extensively used these days in high temperature thermodynamic research. Attaining equilibrium for these cells was slow and required attention only about twice a day. Advantage was taken of the time available to carry out a computer and graphical calculation of the Fe-Ni phase diagram in the solid region, as the understanding of this system is of interest in understanding the behavior of superalloys. Thus, this is a two-part thesis—Part I dealing with the performance of a solid state electrochemical cell using yttria-doped thoria electrolytes and Part II with the calculation of the Fe-Ni diagram from 1184°K to 300°K.

In the solid state electrochemical cell study, yttria-doped thoria solid electrolytes have been fabricated in our laboratory using our newly developed nitrate process. Their performance was then studied by the electrochemical cell consisting of Cr-Cr₂0₃ and Ta-Ta₂0₅ as electrodes. Both a single chamber and a double chamber experimental arrangements were tried.

Yttria-doped thoria electrolytes of composition Th0₂ + 6.625 mole% Y ₂0 ₃ made from 99.99% purity thorium nitrate and 99.9% purity yttrium nitrate powders by our newly developed nitrate process gave the best properties. Electrolytes were transparent in color and had 99.96% of the theoretical density. Pycnometrically measured densities of these fired pellets have agreed with those calculated for the anion vacancy model. Electrolytes of composition Th0₂ + 9.2 mole% Y₂0₃ made from 99.9% purity thorium and yttrium nitrate powders by our nitrate process and also Th0₂ +5.3 mole% Y₂0₃ electrolytes made by directly mixing respective 99.9% purity oxide powders did not give satisfactory results.

The solid state electrochemical cell using Ta-Ta₂0₅ and Cr-Cr₂0₃ as electrodes, and Th0₂ + 6.625 mole% Y₂0₃ as the electrolyte was studied. The performance of the cell was not satisfactory. Various reasons have been attributed to this failure. One of the important reasons may be the inability to obtain a p₀₂ of the order of 10^-25 atm. in the cell environment (particularly the Cr-Cr₂0₃ inner chamber). This low value of P₀₂ corresponds to the equilibrium pressure of Ta-Ta₂0₅ electrode at 1000°C. The cell emf was found in general to increase with time and was very much dependent on the flow rate of the inert gas. The resistance of the cell was found high, of the order of kilo-ohms. These characteristics may be due to a new cell Ta-Ta₂0₅/YDT/p₀₂ (inner chamber) becoming operative because of the formation of a layer of Cr₂0₃ (caused by inadequate P₀₂, atmosphere). This new cell theoretically has a larger cell emf and a lower resistance than the actual cell in consideration at that time. The emf developed across this new cell will be very much dependent on P₀₂ in the environment of inner chamber.

In the calculation of the Fe-Ni diagram, using computer programs available in this Department, various sources of solution thermochemistry information at high temperatures (solid region) of the Fe-Ni system have been analyzed in order to determine the best fit with the most recent experimental α - γ boundary. Based on the recent pulse calorimetric measurements of C_p and the heat of the order-disorder transformation for VI1 the Ni^Fe alloy, the γ-Ni₃Fe and α-Ni₃Fe boundaries have been determined graphically.

. On the basis of the least negative data of ΔG^M_γ reported to this date, ΔG^XS_γ aG^ has been fitted to a Legendre polynomial; and assuming &alpha’ to behave as an ideal solution, excellent agreement of the calculated α-γ boundary was obtained with the most recent experimental one. The eutectoid transformation (γ → α + Ni₃Fe (ordered)) was found to be at 565°K and 52 at. % Ni, and the maximum solubility of nickel in the b.c.c. phase was near 9 at. %. Due to the absence of any well-established model to take into account the effect of loss of ordering with the increasing temperature on ΔG^F(Ni₃Fe), proper calculations could not be made with the γ-Ni₃Fe boundary. The ordered NiFe could not be taken into consideration in our present phase boundary calculation, as no thermodynamic data are available on NiFe.

Recommended Citation

Das, Sujit, "Part I: Study on the performance of a solid state electrochemical cell using yttria-doped thoria electrolyte. Part II: Calculation of the Fe-Ni phase diagram from 1184K̊ to 300K̊. " Master's Thesis, University of Tennessee, 1982.
https://trace.tennessee.edu/utk_gradthes/14989