UDC 546.271 + 546.171.5 + 541.123.2

CHEMISTRY

G. A. EGORENKO, D. S. STASINEVICH, I. S. ANTONOV

THERMAL ANALYSIS OF THE SODIUM BOROHYDRIDE—HYDRAZINE SYSTEM

(Presented by Academician I. I. Chernyaev, March 24, 1965)

Sodium borohydride is finding ever-increasing application in various fields of technology and scientific research. Its relation to solvents, however, has been systematically studied only for diglyme (¹), water (²), liquid ammonia (³), pyridine (⁴), and dimethylformamide (⁵). Hydrazine is a good solvent for sodium borohydride. There are reports on the use of solutions of sodium borohydride in hydrazine as a high-calorie fuel (⁶), for the preparation of certain derivatives of borohydrides (⁷), etc. It is known, however, only that a saturated solution of sodium borohydride in hydrazine, containing 20.2 wt.% NaBH₄ at 25° and having a vapor pressure of 20 mm Hg, forms a eutectic at −26° (⁸). From this solution sodium borohydride crystallizes in the form of a solvate, presumably with two molecules of hydrazine. The present work gives the results of a study of phase equilibria in the sodium borohydride—hydrazine system over the temperature range from −120° to 80°.

For the study, sodium borohydride preparations containing 99.5 wt.% of the principal substance were used. The hydrazine samples contained 99.5 wt.% N₂H₄ and melted at 1.5°. Phase equilibria were studied by the differential-thermal method, with both cooling curves and heating curves being recorded. The rate of temperature change was 1–2° per minute. The accuracy of temperature measurement was ±0.5°.

Phase equilibria in the sodium borohydride—hydrazine system were studied in the concentration range from 0 to 85.5 wt.% NaBH₄. The results of the investigations are presented in Fig. 1.

Four compounds were found in the system studied: NaBH₄·2N₂H₄; 2NaBH₄·N₂H₄; NaBH₄·N₂H₄ and NaBH₄·6N₂H₄. The first of these compounds melts congruently at 53°. The eutectic between NaBH₄·2N₂H₄ and hydrazine contains 15 wt.% NaBH₄ and melts at −22°. The eutectic between NaBH₄·2N₂H₄ and 2NaBH₄·N₂H₄ contains 46 wt.% NaBH₄ and melts at 44°. The compound 2NaBH₄·N₂H₄ crystallizes from solutions containing 46–54 wt.% NaBH₄. At 56° it decomposes by a peritectic reaction into a solution containing 54 wt.% NaBH₄ and unsolvated sodium borohydride. The branch of the curve corresponding to the solubility of unsolvated sodium borohydride rises very steeply upward, so that it could not be traced by the method of thermal analysis. Some lowering of the temperature of the eutectic arrests for mixtures far from the eutectic composition is apparently associated with the nonequilibrium character of crystallization, caused by the high viscosity of the solutions at these temperatures.

In addition to the thermal effects corresponding to the phase transitions listed above, a number of endothermic effects were observed on the heating curves and, correspondingly, exothermic effects on the cooling curves; these should be attributed to transformations in the solid phase. In the concentration range from 19 to 30 wt.% NaBH₄ a similar effect is observed at −5°, which we attribute to a polymorphic transformation of the solvate NaBH₄·2N₂H₄.

The absence of this effect in the concentration range from 30 to 54.1 wt.% NaBH₄ is probably due to the fact that the transition from one crystalline modification of NaBH₄·2N₂H₄ to another readily occurs only in the presence of a sufficient amount of liquid phase. Halts on the cooling and heating curves at −83° in the region of crystallization of unsolvated sodium borohydride correspond to the known polymorphic transformation of the latter (9–11). The same halt at −83° is observed in

Fig. 1. Phase diagram of the NaBH₄—N₂H₄ system

the concentration range 55–70.2 wt.% NaBH₄, which is apparently associated with the nonequilibrium course of the peritectic reaction, as a result of which part of the unreacted sodium borohydride remains in the solid phase. The halts on the heating curves at 9° in the concentration interval from 37.2 to 70.2 wt.% NaBH₄ should be attributed to the decomposition of the compound NaBH₄·N₂H₄ in the solid state into NaBH₄·2N₂H₄ and 2NaBH₄·N₂H₄. This interpretation of the indicated effects is based on the results of constructing the Tammann triangle and on the character of the heating curves of mixtures, for which the thermal halts were maximal in the concentration region containing 54.1 wt.% NaBH₄. It should be noted that the halts on the cooling curves corresponding to the formation of NaBH₄·N₂H₄, as well as to the polymorphic transformation of NaBH₄·2N₂H₄, were observed with considerable supercooling. Thermal effects at −43° in the concentration region from 0 to 37.2 wt.% NaBH₄ were observed only on the heating curves. These halts were preceded by an exothermic effect near −71°, associated with the release of excess internal energy of the system and indicating nonequilibrium crystallization processes in this concentration interval. The sodium borohydride content at which the halt at −43° is maximal is equal to 16.5 wt.% NaBH₄, which corresponds to the composition NaBH₄·6N₂H₄. This gives grounds to suppose that the halts at −43° may be attributed to the decomposition of the compound NaBH₄·6N₂H₄. The latter is formed upon crystallization of supercooled solutions at a temperature of about −71°. Above −43° the compound NaBH₄·6N₂H₄ decomposes into solid hydrazine and solid NaBH₄·2N₂H₄.

The strong supercooling of mixtures of the system and the exothermic effect on the heating curves make it possible to assume that, upon cooling, the sy-

the system as a whole is prone to nonequilibrium crystallization; therefore, in constructing the phase diagram of sodium borohydride—hydrazine, we used the temperatures of phase transitions on the heating curves, rather than the cooling curves, as our guide. Evidently, it is precisely because of supercooling phenomena that some differences are observed between our data (the solubility at 25° is 23.5 wt.% NaBH₄, and the eutectic melts at −22°) and the data given in (⁸).

Received
19 III 1965

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