New eutectic alloys and their heats of transformation
- PDF / 948,209 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 84 Downloads / 181 Views
I.
INTRODUCTION
T H E R E are various ways in which heat storage can reduce costs and make more efficient use of power generating equipment. A thermal storage unit would eliminate the difference between peak and off-peak hours, storing heat when demand is less than the generating capacity and returning it as needed. An efficient way to store heat is to utilize congruent phase transformations. The storage density depends upon the latent heat of transformation, which is the entropy change times the transformation temperature. This entropy change is small for solid-solid transformations, but much larger on melting. The first materials considered for high temperature heat storage were inorganic salts. ~ 5 The problems found in molten salts were associated with their low thermal conductivity, their highly corrosive nature, and their large volume changes during melting. The lithium and fluoride compounds having the best heat storage densities are relatively expensive materials. Metals diminish these problems, and can therefore be strongly competitive even if they have somewhat lower storage densities. The use of metallic systems for heat storage was first analyzed by Birchenall and Telkes.6 pryor 7 calculated the heats of fusion of many binary and ternary eutectics, and the most promising of these were measured by Birchenall and Riechman. 8 Materials were selected in the range of 300 to 1000 ~ on the basis of cost, availability, and storage density. Analysis of the results obtained by Riechman shows that alloys have the best observed heat storage densities if their melting temperatures lie between 700 and 850 K. Better alloys are sought in the melting ranges 900 to 1000 K and 700 K by further alloying. Therefore, it is necessary to determine congruently melting compositions in multicomponent systems which are estimated to melt in the desired temperature ranges, and evaluate the heats of fusion. In ternary systems the phase diagrams often are not known, and few thermal data are available. In some cases the formation of ternary intermetallic compounds is reported. With this lack of information any theoretical apDIANA FARKAS, formerly a Graduate Student, University of Delaware, is now Assistant Professor, Department of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. C.E. BIRCHENALL is Distinguished Professor of Metallurgy, Department of Chemical Engineering, University of Delaware, Newark, DE 19711. Manuscript submitted June 13, 1983. METALLURGICAL TRANSACTIONS A
proach to the problem is a crude approximation. Nevertheless, such an approach can be the useful starting point of a trial-and-error method for experimental determination. An experimental method for finding and characterizing eutectics was needed that would work with no information at all on the system but would be able to use available information to converge more rapidly. The combined results of DTA, electron microprobe, and metallography provide such a trial-and-error method. Any theoretical approximation or e
Data Loading...
