HYDRATED MAGNESIUM NITRATE REVERSIBLE PHASE CHANGE COMPOSITIONS AND THEIR PREPARATION

Abstract: A reversible liquid solid phase change composition comprising hydrated Mg(NO₃)₂ and one or more of MgO, Mg(OH)₂, MgCO₃, CaO, Ca(OH)₂, CaCO₃, MgSO₄, CaSO₄, CuSO₄, NiSO₄, CoSO₄, ZnSO₄, Sr(OH)₂ SrCO₃, Ba(OH)₂, BaO, BaCO₃, or NaBO₂, added to the composition in an effective amount to suppress supercooling of the Mg(NO₃)₂ liquid phase.

Description: HYDRATED Mg(N03)2 REVERSIBLE PHASE CHANGE COMPOSITIONS The invention relates to reversible liquid/solid phase change compositions. More particularly, the invention relates to hydrated Mg(NO3)2 phase change compositions containing nucleating additives to modify the supercooling properties of the Mg(NO3)2 liquid phase. Mg(NO3)2.6H20 undergoes a phase transition at about 890C (about 192 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 hydronic heating and off peak electrical power heat storage applications. 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(NO3)2 compositions, and most advantageously, additives which are highly effective in a very small amount. Hydrated Mg(N03)2 means reversible liquid/solid phase change compositions which, exclusive of any additives or impurities, contain in admixture from about 54 to 62 weight percent Mg(N03)2, and the balance water (to 100 weight percent). Most preferred is a hydrate having a melt/freeze temperature of about 890C (1920F), and prepared by admixing about 57.8 weight percent Mg(N03)2 in about 42.2 weight percent water. The predominant crystal form in compositions of the invention is the hexahydrate. "Supercooling" refers to a discrepancy between the temperature at which freezing initiates and the thawing temperature of a given hydrated Mg(N03)2 composition, when cooled and heated under quiet conditions. "Additive(s)" includes, in addition to the nucleating additives specified herein, precursors of such nucleating additives which are non-detrimental to the purposes of the invention. The invention is the discovery of a new class of highly active nucleating additives for hydrated Mg(N03)2 compositions. The invention particularly is a composition comprising hydrated Mg(N03)2 which includes, as a nucleating additive, one or more of the following: MgO, Mg(OH)2, MgC03, CaO, Ca(OH)2, CaC03, MgS04, CaSO4, Cut04, NiSO4, COSO4, ZnSO4, Sr(OB) SrCO3, Ba(OH)2, BaO, BaCO3, NaB02. The nucleating additive is added in an amount effective to suppress the supercooling properties of the hydrated Mg(N03)2 liquid phase, and preferably in an amount not exceeding about 5.0 weight percent of the weight of the phase change composition. A sufficient amount of the additive(s) is present in the preferred practice to suppress supercooling to no greater than about 40C, and preferably no greater than 20C, taken on an averaging basis over repeated cycles. The invention comprises a reversible liquid/solid phase change composition comprising hydrated Mg(N03)2 and as a nucleating additive, one or more of MgO, Mg(OH)2, MgC03, CaO, Ca(OH)2, CaC03, MgSO4, CaSO4, CuSO4, NiSO4, CoSO4, ZnSO4, Sr(OH)2, SrCO3, Ba(OH)2, BaO, BaC03, or NaBO2, said nucleating additive being added to the composition to suppress average supercooling of the Mg(N03)2 liquid phase to about 40C, or less. The invention further comprises a method of preparing a hydrated Mg(N03)2 composition of suppressed supercooling properties, comprising admixing uniformly in such composition, one or more of MgO, Mg(OH)2, MgCO3, CaO, Ca(OH)2, CaCO3, MgSO4, CaSO4, CuSO4, NiSO4, CoS04, ZnSO4, Sr(OH)2, SrC03, Ba(OH)2, BaO, BaCO3, or NaB02, to suppress average supercooling of the Mg(N03)2 liquid phase to about 40C, or less. The effective amount of the additive(s) 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 as a heat storage device. Exemplary of suitable and known encapsulating means for the heat storage compositions herein described, 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. Patent No. 4,003,426. Other useful encapsulating means are metal or plastic containers, 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. Example The data of the Table below are generated by preparing hydrated Mg(NO3)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 about 1000C under agitation to achieve a uniform dispersion or admixture. A glassencased 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 20-250C (room temperature). Sample No.21, below, is.a control sample of analytical grade Ng(NO3)2 hydrated with deionized water. TABLE Additive No. of Supercooling Sample Material wt.%Tests Ave. C 1 46.24g Mg(NO3)2 + 33.76g H20 Mg(OH)2 0.1 68 0.2 2 " " MgO 0.1 66 0.1 3 " " MgCO3 0.1 5 0.0 4 " " Ca(OH)2 0.1 5 0.0 5 " " CaO 0.1 5 0.0 6 " " CaCO3 0.1 5 0.0 7 " " MgSO4 0.1 15 0.3 8 " " " 0.5 10 0.0 9 " " CaSO4 0.1 5 0.0 10 " " " 0.5 10 1.0 11 " " CuSO4 0.5 10 0.5 12 " " NiSO4 0.5 10 0.9 13 " " CoSO4 0.5 10 1.1 14 " " ZnSO4 0.5 10 2.8 TABLE (Con'd) Additive No. of Supercooling Sample Material wt.%Tests Ave. C 15 46.24g Mg(NO3)2 + 33.76g H2O Sr(OH)2 0.1 5 0.0 16 " " SrCO3 0.1 5 0.0 17 " " Ba(OH)2 0.1 13 0.0 18 " " BaO 0.1 12 0.0 19 " " BaCO3 0.1 18 0.1 20 " " NaBO2 0.5 10 0.5 21 " " None - 6 11.0 It is understood that nucleating additives other than as specifically taught herein may be used in conjunction with the highly effective additives of Sample Nos. 1-20 to prepare improved Mg(N03)2 phase change compositions. While the data also illustrate that very little of the additives used herein produce marked benefits (average supercooling less than about 40C) such additives, of course, may be incorporated in larger amounts, but should not exceed an amount of about 5.0 weight percent of the weight of the phase change composition. Preferably, the amount of the additive is from about 0.02 to about 2.0 weight percent, and most preferably from about 0.05 to 0.5 weight percent of the weight of the phase change composition.

Claims: 1. A reversible liquid/solid phase change composition. comprising hydrated Mg(N03)2 and as a nucleating additive, one or more of MgO, Mg(OH)2, MgCO3, CaO, Ca(OH)2, CaCO3, MgSO4, CaSO4, CuSO4, Nix04, CoSO4, ZnSO4, Sr(OH)2, SrCO3, Ba(OH)2, BaO, BaCO3, or NaBO2, said nucleating additive being added to the composition to suppress average supercooling of the Mg(N03)2 liquid phase to about 40C, or less 2. The composition of Claim 1 having added thereto one or more of the nucleating additives to suppress average supercooling of the Mg(N03)2 liquid phase to about 20C, or less. 3. The composition of Claim 1 or 2, wherein the hydrated Mg(N03)2 consists of about 57.8 weight percent Mg(N03)2 and the balance is water to 100 weight percent. 4. The composition of Claim 1, 2 or 3 hermetically contained within encapsulating means. 5. The composition of any one of Claims 1 to 4 wherein the nucleating additive is present in an amount not greater than about 5.0 weight percent of the weight of the phase change composition. 6. The composition of Claim 5 wherein the nucleating additive is present in an amount of from about 0.02 to about 2.0 weight percent of the weight of the phase change composition. 7. The composition of Claim 5 wherein the nucleating additive is present in an amount of from about 0.05 to about 0.5 weight percent of the weight of the phase change composition. 8. A method of preparing a hydrated Mg(N03)2 composition of suppressed supercooling properties, comprising admixing uniformly in such composition, one or more of MgO, Mg(OH)2, MgCO3, CaO, Ca(OH)2, CaCO3, MgSO4, CaSO4, CuSO4, NiSO4, CoSO4, ZnSO4, Sr(OH)2, SrCO3, Ba(OH)2, BaO, BaCO3, or NaBO2, to suppress average supercooling of the Mg(N03)2 liquid phase to about 40C, or less. 9. The method of Claim 8 including the step of adding one or more of the nucleating additives to suppress average supercooling of the Mg(N03)2 liquid phase to about 20C, or less. 10. The method of Claim 8 or 9, wherein the hydrated Mg(N03)2 consists of about 57.8 weight percent Mg(N03)2 and the balance is water to 100 weight percent. 11. The method of Claim 8, 9 or 10, including the step of adding the nucleating additive in an amount not greater than about 5.0 weight percent of the weight of the phase change composition. 12. The method of Claim 11, including the step of adding the nucleating additive in an amount of from about 0.02 to 2.0 weight percent of the weight of the phase change composition. 13. The method of Claim 11, including the step of adding the nucleating additive in an amount of from about 0.05 to about 0.5 weight percent of the weight of the phase change composition.

Inventors: LANE, GEORGE ASHEL, ROSSOW, HAROLD EVERETT
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1980
Main classification: C09K5
IPC codes: C09K5