Hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 Reversible Phase Change Compositions

The invention relates to reversible liquid/solid phase change compositions. More particularly, the invention relates to hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 phase change compositions containing additives to modify the super-cooling properties of the Mg(NO.sub.3).sub.2 /MgCl.sub.2 liquid phase.

Mg(NO.sub.3).sub.2.6H.sub.2 O/MgCl.sub.2.6H.sub.2 O eutectic undergoes a phase transition at about 59.degree. C. (about 138.degree. F.) releasing or alternately absorbing a substantial quantity of latent heat of fusion. As such, the material has advantageous properties for use as an encapsulated heat storage medium for use in conjunction with solar energy systems for space or water heating applications. The material is particularly highlighted for such use by Yoneda and Takanashi "EUTECTIC MIXTURES FOR SOLAR HEAT STORAGE," Solar Energy, No. 21, pp. 61-63, herein fully incorporated by reference. In order to improve the material for such applications, however, there is a need to develop additives or a series of additives to modify the supercooling properties of hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 compositions, and most advantageously, additives which are highly effective in a very small amount.

Hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 means reversible liquid/solid phase change compositions which, exclusive of any additives or impurities, contains in admixture from about 25-45 weight percent Mg(NO.sub.3).sub.2, from about 10-25 weight percent MgCl.sub.2, and from about 40-50 weight percent water (to 100 weight percent). Most preferred is a eutectic mixture having a melt/freeze temperature of about 59.degree. C. (138.degree. F.), and prepared by admixing about 34 weight percent Mg(NO.sub.3).sub.2 and about 19 weight percent MgCl.sub.2 in about 47 weight percent water. The predominate crystal forms in compositions of the invention are the hexahydrates.

"Supercooling" refers to a discrepancy between the temperature at which freezing initiates and the thawing temperature of a given hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 composition, when cooled and heated under quiet conditions.

"Additive(s)" includes in addition to the nucleating additives specified herein, precursors of such additives nondetrimental to the purposes of the invention.

The invention is the discovery of a new class of highly active nucleating additives for hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 compositions. The invention particularly is a composition comprising hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2, and which includes as a nucleating additive to such composition, one or more of the group of MgO, Mg(OH).sub.2, and NaBO.sub.2 added in an amount effective to suppress the supercooling properties of the hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 liquid phase. Sufficient of the additive(s) is present in the preferred practice to suppress supercooling to no greater than about 2.degree. C. taken on an averaging basis over repeated cycles.

The effective amount of additive is determined by testing a given composition over repetitive phase change cycles. A given nucleating additive, if present in an insufficient amount may, for example, exhibit only temporary effectiveness. Such behavior is overcome by adding slightly greater amounts of that given additive.

The compositions hereof are ideally packaged in individual encapsulating means for use in conjunction with solar heating systems. Exemplary of suitable and known encapsulating means for the heat storage compositions hereof, are water impervious foils of plastic/metal laminates. Also, closed cell plastic foams have been suggested in which the heat storage composition may be encapsulated within the cells of the foam structure, as illustrated, for example, in U.S. Pat. No. 4,003,426, incorporated herein by reference. Yet other useful encapsulating means are metal or plastic cans, such as inexpensive aerosol cans; metal or plastic pipe, such as extruded polyethylene pipe; and the like. Further details respecting such encapsulating means are given in Report Nos. ORO/5217-8 and NSF RANN SE C906 FR76 1, both available from the National Technical Information Service, Springfield, Virginia, and fully incorporated herein by reference.

The data of the Table below are generated by preparing hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 samples according to the ratios indicated, and wherein the mixed samples of each specified ratio are placed into a two-ounce glass vial, which is then heated to above 70.degree. C. under agitation to achieve a uniform dispersion or admixture. A glass-encased thermocouple is inserted through the cap of each vial to measure temperature changes at the center of each sample prior to, during and after freezing. The cooling temperature is controlled and maintained within the range of about 20.degree.-25.degree. C. (room temperature). Sample No. 1, below, is a control sample of analytical grade Mg(NO.sub.3).sub.2 /MgCl.sub.2, whereas Samples Nos. 3-8 and 11-14 are comparative examples outside the scope of the invention.

It is understood that the less effective additives of the comparative examples (Sample Nos. 3-8 and 11-15) may be used in conjunction with the highly effective additives of Sample Nos. 2, 10 and 11 to prepare improved Mg(NO.sub.3).sub.2 /MgCl.sub.2 phase change compositions. While the data also illustrate that very little of the highly effective species of additives (Sample Nos. 2, 10 and 11) produce marked benefits (average supercooling less than about 2.degree. C.) such additives, of course, may be incorporated in larger amounts, but preferably would not exceed the amount of about 2.0 weight percent of the weight of the phase change composition.