Thermomagnetic analysis finds new uses Thermomagnetic analysis offers a new approach to the study of the progress of chemical reactions. The technique itself is not new. But it has only recently been found that it is a useful tool for unraveling certain reactions involving liquid sodium, Dr. Robert G. Charles, a chemist at Westinghouse Research Laboratories, Pittsburgh, said at a symposium on analytical techniques on the horizon. The symposium was part of the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, held in Cleveland, Ohio. Reactions involving liquid sodium are important because the molten metal is widely used as a coolant in nuclear reactors. The temperature of the liquid sodium in a reactor is about 500° C , and there is always the possibility of corrosion caused by reaction with the reactor's container materials. Patricia Biggs of Westinghouse and Dr. Charles have used thermomagnetic analysis to follow the reaction of nickel oxide and various nickel salts with liquid sodium. And in more recent work Dr. Charles, Westinghouse's Dr. L. N. Yannopoulos, and Mrs. Biggs have studied the reactions of iron carbides with liquid sodium. Interaction. Thermomagnetic analysis involves measuring the force of interaction of a sample with a magnetic field as the temperature is varied. In a typical experiment, the sample is placed in a sealed cell surrounded by a furnace. The furnace is within an inhomogeneous magnetic field generated by an electromagnet. The sample cell is suspended from one arm of an analytical balance which is used to measure the changing force of magnetic interaction as the furnace temperature is programed linearly. In principle, thermomagnetic analysis is applicable to almost any system. In practice, however, it is most easily applied to samples containing one or more paramagnetic or ferromagnetic components. One of the main advantages of the method is its very high sensitivity for ferromagnetic components, Dr. Charles points out. Nickel formation. In reactions carried out with nickel compounds in liquid sodium, the sodium acts as a combination solvent and reactant. In studying the reaction of nickel oxide using thermomagnetic analysis, Dr. Charles and Mrs. Biggs find that a reaction occurs quite rapidly at about 300° C. in the presence of
excess sodium. The reaction is NiO (paramagnetic) + 2 Na (paramagnetic) -» Ni (ferromagnetic) -fN a 2 0 (diamagnetic). It's easy to follow the course of this reaction by the very marked increase in magnetic interaction of the system as ferromagnetic nickel metal is produced. Nickel is easily identified in the sealed system by the Curie point (the temperature above which a ferromagnetic substance becomes paramagnetic) at about 350° C. Plots of apparent weight change (due to changes in the magnetic characteristics of the system) versus time, and of temperature versus time, are obtained automatically with a strip-chart recorder. These plots give certain information: • Production or consumption of paramagnetic or ferromagnetic species over the temperature range studied. • Temperature range over which reaction is rapid for those systems that react. • Conclusive identification of those ferromagnetic species that have accessible Curie points. • Quantitative measure of the amount of magnetic material produced or consumed (in favorable instances).
• Information on the kinetics of the reactions that occur (in favorable cases). Iron carbides. The Westinghouse scientists, interest in iron carbides arose because carbon transport through liquid sodium in nuclear reactor loops is a serious matter. Different steels have different affinities for carbon, and the carbon will transfer from one steel to another with usually undesirable effects on the mechanical properties of steel. Scientists have postulated that sodium carbide (Na 2 C 2 ) is involved somehow in the transport. To get more information in this area the Westinghouse group carried out a series of studies of the reactions between liquid sodium and the iron carbides, Fe 3 C and Fe 2 0 C 9 . Thermomagnetic analysis was an ideal technique to use for the study since the iron carbides and the iron metal are ferromagnetic. Also, the ferromagnetic species can be distinguished from each other by their differing Curie points, even when they are present in a mixture. Thus, it was possible to follow the disappearance of iron carbide, when in contact with liquid alkali metal, and also to follow the formation of iron metal or other ferromagnetic species.
Thermomagnetic technique monitors sodium/Fe2oCg reaction Apparent weight increase (mg.) in magnetic field due to species indicated
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Any study of the reactions of iron carbides at high temperatures is com plicated by their tendency to ther mally decompose, Dr. Charles points out. Earlier studies by others have shown that impurities affect the tem perature range of decomposition. Traces of oxygen and especially sul fur increase the thermal stability of iron carbides. Fe 3 C. In the thermomagnetic stud ies with Fe 3 C, decomposition to iron metal and carbon is rapid above 600° C. When excess liquid sodium metal is present, the same thermo magnetic curve is obtained. This indicates that no direct reaction can occur between Fe 3 C and Na, at least at temperatures below the range in which the pyrolysis to iron metal and carbon is important. Support for a lack of reaction of sodium with Fe 3 C was obtained in a second run under identical condi tions except that heating was stopped at 500° C. and the mixture cooled to room temperature. Both the Curie point and the extent of inter action with the magnetic field at room temperature were unaffected. Fe20^9- I n thermomagnetic stud ies with the carbide Fe 2 oC 9 , the Westinghouse scientists found that the ma terial alone is stable to at least 740° C. In a run with sodium present, iron metal was produced from 550° to 650° C. In another thermomagnetic run of Fe 2 oC 9 with excess sodium, the ex periment was interrupted at each of a series of predetermined and in creasing temperatures by turning off the heating current and allowing the tube to cool rapidly to near room tem perature. Reheating then permitted the Westinghouse group to determine any effect on the Curie point of the starting material. The curves ob tained show that a second ferromag netic material having a Curie point of 212° C , and most probably Fe 3 C, is produced as an intermediate as the Fe 2 0 C 9 is consumed. The Westinghouse workers believe that sulfur present as an impurity stabi lizes the Fe 2 oC 9 but then is removed by reaction with liquid sodium, re sulting in decomposition of Fe 2 0 C 9 to Fe 3 C. The Fe 3 C produced then pyrolyzes in its characteristic temperature range. Thus the role of sodium is again an indirect one. From the series of thermomagnetic curves it's possible to construct a dia gram that shows clearly the appear ance and disappearance of the ferro magnetic species involved in this sys tem as the temperature is raised. The diagram shows the power of the thermomagnetic technique in deriv ing a great deal of information about a complicated system. 52 C&EN MARCH 16, 1970
Cell measures oxygen con tent of gases, molten metals A promising technique for process monitoring of a wide range of oxygen contents in gases and molten metals was described at a symposium on ana lytical techniques on the horizon. In the technique, an electrochemical con centration cell containing a solid oxide electrolyte measures the oxygen con centration, Dr. Roy Littlewood of Steel Co. of Canada, Ltd., Hamilton, Ont., explained at the symposium held during the Pittsburgh Conference. Dr. Littlewood points to two prin cipal advantages of the oxygen mon itoring technique. It applies to a wide range of oxygen concentrations, and it gives constant accuracy at all oxygen levels. The method can be used for oxygen partial pressures of 1 atm. down to at least 1 0 3 0 atm. The constant accuracy of the technique arises from the logarithmic depend ence of Ε M F on concentration. The electrochemical concentration cell contains two metallic electrodes. The electrolyte is a refractory mate rial—a high-valence metal oxide such as zirconium dioxide doped with lime. The electrolyte has properties similar to a semiconductor. It conducts by the migration of oxide anions, 02~, through lattice vacancies in the solid. The cell operates only at high temper atures (about 500° C. or greater), Dr. Littlewood points out. In the cell, a gas with a known oxygen concen tration (usually air) and the gas be ing analyzed are in contact with the two electrodes (usually platinum). Applications. The electrochemical oxygen cell has two types of appli cation—direct monitoring of gases and direct monitoring of molten metals. The metals industry finds two main applications in the monitoring of gases. First, the cell can be used as the guide to the efficiency of a proc ess—for example, combustion of fuel in a furnace. In the second type of application it's possible to determine how suitable a gas is for use in a process, such as in the study of conditions in an anneal ing furnace. Oxygen in the gas can be determined to see whether the gas is fit for use in various types of an nealing. Two kinds of industrial electro chemical oxygen meters have been de veloped for gas monitoring, Dr. Little wood points out. One is a sampling instrument in which the cell and heat ing device are housed in a little black box away from the process line. Man ufacturers of this type include West inghouse Electric Co., Pittsburgh, Pa.,
Thermo-Lab Instruments, Inc., Glenshaw, Pa., and Bailey Meter Co., Wickliffe, Ohio. The second type is an in situ instru ment in which the cell is made as a probe and inserted directly in the hot furnace gases or into cool gases. If used with cool gases, the probe must have its own auxiliary heating device. Makers of this type of probe instru ment are George Kent, Ltd., Luton, England, and Thermo-Lab Instru ments. Molten metals. The second major potential industrial application of the electrochemical oxygen analyzer is monitoring molten metals. The in strument should prove invaluable in the steel industry since it can provide a way to determine oxygen in the steel while the steel is still in the fur nace. Here the electrochemical con centration cell is used in a probe which is placed directly in the mol ten metal. The probe gives a direct measurement of the dissolved oxy gen content of the metal. The electrochemical concentration cell used in the probe contains a plati num electrode and electrolyte made from zirconia doped with lime. The molten steel is the other electrode that is in contact with the electrolyte. Scientists in the U.S. and abroad are working on development of elec trochemical probes for oxygen analy sis in steel, according to Dr. Little wood. Canada, perhaps, is the fur thest along. Leigh Instruments, Ltd., Carleton Place, Ont., in conjunction with Dr. J. K. Pargeter, D. K. Faurs^ chou, and J. C. Pope of Canada's De partment of Energy, Mines, and Re sources has developed a commercial oxygen probe for molten steel analy sis. Marketed in the U.S. by LeighAARCO, Inc., Ruxton, Md., the instru ment, called the Leigh Oxygen Probe, has been on sale since the first of this year. Expendable probes are used with the Leigh system. When the system is in operation with a new probe at tached to the lance, the operator thrusts the probe tip through the slag into the liquid metal. The reading starts automatically and free oxygen content is recorded on the recorder chart in parts per million. At the end of the measurement period (about 15 seconds) signal lights and a horn in dicate that the operation is complete. The Leigh instrument is currently being evaluated by about a dozen U.S. and Canadian steel companies, Dr. Littlewood notes.
