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C. B. MURPHY, General Electric Co., Schenectady,
N. Y.
R. R. WEST, State University of New York, College of Ceramics at Alfred University, Alfted, N. Y.
Differential Thermal Analysis of Vanadium Pentoxide and Silica C o m o s i m OF METALLIC components when residual fuel oils are burned in gas turbines has been attributed to constituents in the resulting ash. Vanadium, one of the compounds which cause the most serious corrosion, is present in crude oils as a porphyrin complex (2. 6 ) , and dry ashing at temperatures between 475' to 525' C. is sufficient to produce an inorganic ash from residual fuel ( 4 ) . In an oxidizing atmosphere, vanadium presumably forms the pentoxide. Silicon in various forms such as silica and ethyl orthosilicate reduce this corro. aion somewhat (7). I n the work described here, differential thermal analysis is applied to vanadium pentoxide, silica, and a mixture of these two substances. Experimental
The apparatus consisted of a fixed vertical support, with platinum foil, wrapped about the thermocouple tubing, to form a sample holder. Samples of about 0.2 gram were used. Calcined alumina, the reference material, was also used for imbedding the temperatureprogramming thermocouple, which was platinum-platinum 10% rhodium. A vertically mounted, platinum - wound furnace was lowered over the assembly, and a heating rate of 12.5' C. per minute was maintained. The materials were cleaned quartz, ground to pass through a 100-mesh sieve, and Fisher purified vanadium pentoxide which was examined spectroscopically. A semiquantitative evaluation (x-ray diffraction) for a mixture of these materials after firing is Fired,
c.
Major
600 700
Quartz Quartz Cristobalite
1550
Minor Quartz
Results
The thermogram for quartz shows only one endotherm, but that for both vanadium pentoxide and its mixture with quartz shows two endothermic peaks. The single peak for quartz represents the conversion of quartz I to quartz 11, and the temperature compares favorably to the 573' C. reported for this phenomenon ( 5 ) . The large peak for vanadium pentoxide at 690' C.
952
is associated with the melting point of this compound, which is reported as 670' C. (5). The first endotherm at 510' C. was unexpected because phase studies ( 3 ) reported only a single modification of vanadium pentoxide. The thermogram for the mixture gave peaks corresponding to the quartz inversion and the melting of vanadium pentoxide. That no reaction occurs between quartz and vanadium pentoxide is not surprising-when the pure material is used, the area under the curve is small, and the low temperature endotherm in vanadium pentoxide did not appear at all in the mixture. For vanadium pentoxide, the 510' C. endotherm was reversible on cooling, and the material changed from a salmon color to yellow. To demonstrate that such a peak is not a spurious result, Stone. with his dynamic gas differential thermal analysis apparatus (Robert L. Stone Go.) showed that prior to melting a peak occurred at about 520' C. and oxygen pressures of both 1 and 3 atm. Thus, within these experimental limits, the reaction is not oxygen-dependent. During cooling, the peak appears again but at 430' C. This can be explained assuming a transition from one form of vanadium pentoxide to another. When heated to a higher temperature (510' to 520' C.) where the rate of transition is fast enough to establish a measurable A T , differential thermal analysis shows that the reaction is thermodynamically possible at 430' C . On cooling, however, the reverse reaction is not thermodynamically possible until 430' C. is reached. Because this phenomenon is readily reversible, the u
The thermogram for quartz shows only one endothermic peak, but those for both vanadium pentoxide and its mixtures with quartz show two A.
Quartz B. Quartz containing 7.7% vanadium pentoxide C. Vanadium pentoxide
INDUSTRIAL AND ENGINEERING CHEMISTRY
changes cannot be detected by quenching techniques. Probably this is tlic reason it was not observed in previous phase studies ( 3 ) . Spectrographic analysis showed only five elements present in trace amounts (less than 0.01 $&)-i.e., copper, manganese, beryllium, silver, and barium. Potassium was not detected, using line 4044.14; thus, t.he possibility of fusion of 4VzOj. K 2 0 previously reported ( 3 ) is eliminated. The quantities of impurities present are insufficient to produce the observed thermal effect. Also, the curve obtained from the Chevenard balance for vanadium pentoxide shows neither gain nor loss of weight; thus, the transition is physical rather than an oxidation or reduction reaction. Acknowledgment
The authors wish to rhank R. L. Stone for his thermograms, and H . J. Keily for his spectrographic analysis. literature Cited (1) Buckland, B. O., personal communication. (2) Dunning, H. N., Raybon, N. A., IND. ENG.CHEM. 48, 951 (1956). (3) Holtzberg, F., Reisman, A , , Berry, M., Berkenblit, M., J . Am. Chem. Sac. 78, 1536 (1956). (4) Milner, 0. I., Glass, J. R., Kirchner, J. P., Yurick, .4. N., Anal. Chem. 24, 1730 i 1 0 5 7 ) (5) Rossini, F. D., others, '.Selected Values of Chemical Thermodynamic Properties," Natl. Bur. Standards, Circ. 500, 1952. (6) Skinner, D. A , , IND.ENG. C H L M . 44, 1159 (1952). \-'I-/-
RECEIVhD for
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.qPl'il 5, 1957
ACCEPTEDJanuary 15, 1959
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