PHASE CHANGE MATERIALS (PCMS) WITH SOLID TO SOLID TRANSITIONS

Abstract

There is herein described phase change materials (PCMs) comprising at least one or a plurality (e.g. a mixture) of tetrafluoroborate salts that are capable of undergoing a solid to solid phase transition. In particular, there is described phase change materials (PCMs) comprising at least one or a plurality (e.g. a mixture) of tetrafluoroborate salts where there is at least one tetrafluoroborate salt or a plurality of tetrafluoroborate salt which have a solid to solid phase transition. The tetrafluoroborate salt may comprise at least one anion or a plurality of the same or different anions of tetrafluoroborate (e.g. BF.sub.4—). The PCM may have a solid to solid phase change in the region of about −270° C. to about 3,000° C., about −50° C. to about 1,500° C., about 0° C. to about 1,000° C., or about 0° C. to about 500° C. temperature range.

Claims

1.-34. (canceled)

35. A heat battery comprising a thermal storage material in the form of a phase change material (PCM), said PCM comprising: at least one or a plurality of tetrafluoroborate salts which has a solid to solid (polymorphic) transition; wherein the PCM has a solid-to-solid phase change in the region of about −270° C. to about 3,000° C. temperature range; the PCM comprises tetrafluoroborate anions (BF.sub.4.sup.−) which is part of an organic salt, inorganic salt and/or metal salt with the proviso that the PCM comprises no nucleating agent; the PCM comprising tetrafluoroborate anions (BF.sub.4.sup.−) has increased bulk density and is in a pressed (i.e. compacted) or melt cast form; and wherein the PCM is capable of being repeatedly thermally cycled without any significant degradation to the PCM material.

36. A heat battery according to claim 35, wherein the at least one or plurality of tetrafluoroborate salts are capable of at least one, two or more, three or more or a plurality of solid to solid phase transitions which occur at different temperatures.

37. A heat battery according to claim 35, wherein the solid to solid transition point temperature is capable of being changed under pressure.

38. A heat battery according to claim 35, wherein the tetrafluoroborate salts is or comprises KBF.sub.4 in the following amounts: 10-100 wt. %; 20-100 wt. %; 30-100 wt. %; 40-60 wt. %; 50-100 wt. %; 50-90 wt. %; 60-90 wt. %; 70-90 wt. %; about 100 wt. %.

39. A heat battery according to claim 35, wherein the tetrafluoroborate salt comprises a mixture of tetrafluoroborate salts of KBF.sub.4 and NH.sub.4BF.sub.4 in a ratio of: about 10-90 mol % of KBF.sub.4 and 10-90 mol % of NH.sub.4BF.sub.4; about 20-80 mol % of KBF.sub.4 and 20-80 mol % of NH.sub.4BF.sub.4; about 30-60 mol % of KBF.sub.4 and 30-60 mol % of NH.sub.4BF.sub.4; about 20 mol % KBF.sub.4 and 80 mol % NH.sub.4BF.sub.4; about 40 mol % KBF.sub.4 and 60 mol % NH.sub.4BF.sub.4; about 50 mol % KBF.sub.4 and 50 mol % NH.sub.4BF.sub.4; about 60 mol % KBF.sub.4 and 40 mol % NH.sub.4BF.sub.4; or about 90 mol % KBF.sub.4 and 10 mol % NH.sub.4BF.sub.4.

40. A heat battery according to claim 35, wherein the phase change materials (PCMs) is capable of being repeatedly thermally cycled up to: 50 thermal cycles; 70 thermal cycles; 100 thermal cycles; 200 thermal cycles; 500 thermal cycles; 1,000 thermal cycles; 5,000 thermal cycles; and 10,000 thermal cycles.

41. A heat battery according to claim 35, wherein the phase change material (PCM) is in a pressed (i.e. compacted) form such as a pressed pellet.

42. A heat battery according to claim 35, wherein the at least one or the plurality of tetrafluoroborate salts is selected from any one of or any combination of the following tetrafluoroborate salts: a. Lithium (Li) tetrafluoroborate salts; b. Sodium (Na) tetrafluoroborate salts; c. Potassium (K) tetrafluoroborate salts; d. Rubidium (Rb) tetrafluoroborate salts; e. Caesium (Cs) tetrafluoroborate salts; f. Magnesium (Mg) tetrafluoroborate salts; g. Calcium (Ca) tetrafluoroborate salts; h. Strontium (Sr) tetrafluoroborate salts; i. Barium (Ba) tetrafluoroborate salts; j. Iron (Fe) tetrafluoroborate salts; k. Manganese (Mn) tetrafluoroborate salts; l. Zinc (Zn) tetrafluoroborate salts; m. Zirconium (Zr) tetrafluoroborate salts; n. Titanium (Ti) tetrafluoroborate salts; o. Cobalt (Co) tetrafluoroborate salts; P. Aluminium (Al) tetrafluoroborate salts; q. Copper (Cu) tetrafluoroborate salts; r. Nickel (Ni) tetrafluoroborate salts.

43. A heat battery according to claim 35, wherein the PCM has a solid to solid phase change in the region of: about −50° C. to about 1,500° C.; about 0° C. to about 1,000° C.; or about 0° C. to about 500° C. temperature range; about −270° C. to about 3,000° C.; about −50° C. to about 1,500° C.; about −50° C. to about 500° C.; about 0° C. to about 1,000° C.; about 0° C. to about 500° C.; about 0° C. to about 400° C.; about 0° C. to about 300° C.; about 0° C. to about 200° C.; about 0° C. to about 100° C.; about 100° C.-400° C.; about 150° C.-300° C.; 200° C.-300° C.; about 260° C.-290° C.; or about 270° C.-280° C.

44. A heat battery according to claim 35, wherein the phase change material (PCM) comprises a solid to solid transition material which provides a PCM active over a wide temperature range over any of the following temperature range of about 0° C.-50° C. or about 20° C.-30° C.; about 100° C.-200° C. or about 135° C.-155° C.

45. A heat battery according to claim 35, wherein the phase change material (PCM) is air and moisture stable in the atmosphere and will be stable under any desired formed shape.

46. A heat battery according to claim 35, wherein the phase change materials (PCM) comprise any one of or combination of the following salts: LiBF.sub.4, NaBF.sub.4, KBF.sub.4, RbBF.sub.4, CsBF.sub.4 and NH.sub.4BF.sub.4.

47. A heat battery according to claim 35, wherein the phase change materials (PCM) comprise a cation selected from any one of or combination of the following: a metal cation, such as Li+, Na+, K+, Cs+, Rb+, Mg2+, Sr2+, Fe2+, Fe3+, Pt+, Al3+, Ag+: an inorganic cation, such as NH4+, NO2+, NH2-NH3+(Hydrazinium); or an organic cation, such as 1-Ethyl-3-methylimidazolium.

48. A heat battery according to claim 35, wherein the phase change materials (PCM) comprise a cation selected from any one of or combination of the following: Li+, NH4+, Na+, K+, Mg2+, Ca2+.

49. A heat battery according to claim 35, wherein the phase change material (PCM) forms a thermal storage medium which comprises a number of other components and/or additives that act as: a. Thermal conductivity enhancers b. Shape stabilising c. Processing aids.

50. A heat battery according to claim 35, wherein the phase change material (PCM) also comprises a range of other non-tetrafluoroborate salts to alter the transition temperature of the tetrafluoroborate salt.

51. A heat battery according to claim 35, wherein the phase change material (PCM) comprises at least one of or a combination of any of the following non-limiting list of inorganic tetrafluoroborate salts: potassium tetrafluoroborate (KBF.sub.4); NaBF.sub.4; NH.sub.4BF.sub.4; LiBF.sub.4; Sr(BF.sub.4).sub.2; Ca(BF.sub.4).sub.2; NH.sub.4H(BF.sub.4).sub.2; (NH.sub.4).sub.3H(BF.sub.4).sub.4; Ba(BF.sub.4).sub.2; Cr(BF.sub.4).sub.2; Pb(BF.sub.4).sub.2; Mg(BF.sub.4).sub.2; AgBF.sub.4; RbBF.sub.4; CsBF.sub.4; Zn(BF.sub.4).sub.2; Fe(BF.sub.4).sub.2; Fe(BF.sub.4).sub.3; Ni(BF.sub.4).sub.2; Ni(BF.sub.4).sub.3; Mn(BF.sub.4).sub.2; Co(BF.sub.4).sub.2; and Zn(BF.sub.4).sub.2.

52. A heat battery according to claim 35, wherein the tetrafluoroborate salt is a hydrate, or another solvate; or magnesium tetrafluoroborate hexahydrate ([Mg(H2O)6](BF.sub.4)2); iron tetrafluoroborate hexahydrate; the cobalt tetrafluoroborate hexahydrate; and zinc tetrafluoroborate hexahydrate.

53. A heat battery according to claim 35, wherein different tetrafluoroborates salts are mixed together and/or with other components (e.g. sodium chloride) to depress the melting point of the phase change material (PCM).

54. A heat battery according to claim 35, wherein the heat battery comprises heat exchangers and insulation.

Description

DESCRIPTION OF THE FIGURES

[0157] Embodiments of the present invention will now be described, by way of example only, with reference to the following Figures:

[0158] FIG. 1 is a graph showing the thermal cycling of potassium tetrafluoroborate (KBF.sub.4) according to an embodiment of the present invention;

[0159] FIG. 2 is a graph showing the simultaneous thermal analysis of KBF.sub.4 performed from 25° C. to 350° C. according to an embodiment of the present invention;

[0160] FIG. 3 is a graph showing the simultaneous thermal analysis of KBF.sub.4 from 25° C. to 550° C. according to an embodiment of the present invention;

[0161] FIG. 4 is a graph showing first and third thermal cycling of a 50:50 mol % KBF.sub.4—NH.sub.4BF.sub.4 mixture according to an embodiment of the present invention;

[0162] FIG. 5 is a graph showing the phase diagram of a NH.sub.4BF.sub.4— KBF.sub.4 phase change material (PCM) according to an embodiment of the present invention;

[0163] FIG. 6 is the DSC analysis of KBF.sub.4 using apparatus from Mettler Toledo according to an embodiment of the present invention;

[0164] FIG. 7 is the DSC analysis of KBF.sub.4 using TA instruments DSC 2500 according to an embodiment of the present invention;

[0165] FIG. 8 is a representation of calibrated heat capacity measurements carried out using a sapphire standard according to an embodiment of the present invention;

[0166] FIG. 9 is a comparison of thermal conductivity results of melted and pressed KBF.sub.4 vs other inorganic compounds, Na.sub.3PO.sub.4 and borax according to an embodiment of the present invention;

[0167] FIG. 10 is a DSC analysis performed between 75° C. and 350° C. of KBF.sub.4 after 10 thermal cycles between 450° C. and 600° C. using TA Instruments DSC 2500 according to an embodiment of the present invention;

[0168] FIG. 11 is a representation of the thermal performance of an aluminium heat battery containing KBF.sub.4: a) on the top of FIG. 11 this shows both the charging and discharging of the heat battery over one thermal cycle; b) on the bottom of FIG. 11 this shows a more detailed look at the charging following the input and output temperature of the heat exchange fluid, as well as the accumulative energy used during charging according to an embodiment of the present invention;

[0169] FIG. 12 is a representation of thermal cycling over 25 cycles using an aluminium heat exchanger with molten KBF.sub.4 according to an embodiment of the present invention;

[0170] FIG. 13 is a representation of thermal cycling data for KBF.sub.4 and NaBF.sub.4 up to 350° C. and for NH.sub.4BF.sub.4 up to 250° C. according to an embodiment of the present invention;

[0171] FIG. 14 is a representation of powder X-ray diffraction patterns of anhydrous LiBF.sub.4 cycled between 0° C. and 50° C. according to an embodiment of the present invention;

[0172] FIG. 15 is a representation of powder X-ray diffraction patterns for NaBF.sub.4 thermally cycled between 50° C. and 350° C. according to an embodiment of the present invention;

[0173] FIG. 16 is a representation of RbBF.sub.4 salt cycled between 20° C. and 300° C. and powder patterns collected for the transition of the salt according to an embodiment of the present invention;

[0174] FIG. 17 is a representation showing thermal cycling of LiBF.sub.4 and KBF.sub.4 between room temperature and 350° C. containing 25 mol % and 50 mol % LiBF.sub.4 according to an embodiment of the present invention;

[0175] FIG. 18 shows the thermal cycling of 50 mol % LiBF.sub.4 and KBF.sub.4 mixture cycled up to 350° C. according to an embodiment of the present invention;

[0176] FIG. 19 shows the normalised variable temperature powder patterns for LiBF.sub.4 and KBF.sub.4 mixture for, A—low temperature before cycling, B—mid heating transition, C—high temperature phase, D—mid cooling transition and E—low temperature phase after transition according to an embodiment of the present invention;

[0177] FIG. 20 shows the variable temperature powder patterns for LiBF.sub.4 and KBF.sub.4 mixture for, A—low temperature before cycling, B— mid heating transition, C— high temperature phase, D mid cooling transition and E—low temperature phase after transition according to an embodiment of the present invention;

[0178] FIG. 21 shows powder patterns in 5°-25° range comparing KBF.sub.4 simulated data (306° C.) and LiBF.sub.4 (80° C.) data with LiBF.sub.4 and KBF.sub.4 (291° C.) according to an embodiment of the present invention;

[0179] FIG. 22 is a representation of the phase transition on heating to 291° C., also shown in powder pattern top of FIG. 21 according to an embodiment of the present invention;

[0180] FIG. 23 therefore represents thermal cycling of NaBF.sub.4 and KBF.sub.4 mixtures between room temperature and 350° C., containing 25 mol % and 50 mol % LiBF.sub.4 according to an embodiment of the present invention;

[0181] FIG. 24 is a representation of thermal cycling of 50 mol % NaBF.sub.4 and KBF.sub.4 mixture up to 350° C. according to an embodiment of the present invention;

[0182] FIG. 25 is a representation of thermal cycling of 50 mol % mixture of NH.sub.4BF.sub.4 and KBF.sub.4 cycled between 50° C. and 350° C. according to an embodiment of the present invention;

[0183] FIG. 26 is a DSC representation of uncycled 50 mol % NH.sub.4BF.sub.4 and KBF.sub.4 cycled between ambient and 300° C. at a rate of 10° C. min.sup.−1 according to an embodiment of the present invention;

[0184] FIG. 27 is a DSC representation of third cycle of 50 mol % NH.sub.4BF.sub.4 and KBF.sub.4 cycled between ambient and 300° C. at a rate of 2° C. min.sup.−1 according to an embodiment of the present invention;

[0185] FIG. 28 is a representation of powder patterns for the collected high temperature phases for KBF.sub.4, NH.sub.4BF.sub.4 and their mixture according to an embodiment of the present invention;

[0186] FIG. 29 is a comparison of DSC data collected for varying compositions of NH.sub.4BF.sub.4 and KBF.sub.4 mixture according to an embodiment of the present invention; and

[0187] FIG. 30 is a phase diagram constructed using DSC data and thermal cycling data where the 40 and 90 mol % compositions have two data points as two transitions were observed in DSC data according to an embodiment of the present invention;

DETAILED DESCRIPTION

[0188] The present invention relates to phase change materials (PCMs) comprising of the tetrafluoroborate anion where there is a solid to solid phase transition; and wherein the PCM has a phase change in the region of: about −270° C. to about 3,000° C.; about −50° C. to about 1,500° C.; about 0° C. to about 1,000° C.; about 0° C. to about 500° C.; about 100° C. to about 400° C.; about 150° C. to about 300° C.; about 200° C. to about 300° C.; about 260° C. to about 290° C.; or about 270° C. to about 280° C.

[0189] The present invention therefore relates to phase change materials (PCMs) comprising at least one or a plurality (e.g. a mixture) of tetrafluoroborate salts that undergo a solid to solid phase transition.

[0190] In particular, the present invention relates to phase change materials (PCMs) comprising at least one or a plurality (e.g. a mixture or range) of tetrafluoroborate salts where there is at least one tetrafluoroborate salt which has a solid to solid transition.

[0191] The tetrafluoroborate salt may comprise at least one anion or a plurality of anions of tetrafluoroborate (e.g. BF.sub.4.sup.−).

[0192] The PCM may typically have a solid to solid phase change in the region of about −50° C. to about 1,500° C., about 0° C. to about 1,000° C. or about 0° C. to about 500° C. temperature range.

[0193] Alternatively, the present invention provides a phase change material (PCM) which comprises a solid to solid transition material which provides a PCM active over a wide temperature range over any of the following: about −270° C. to about 3,000° C.; about −50° C. to about 1,500° C.; about 0° C. to about 1,000° C.; about 0° C. to about 500° C.; about 100° C. to about 400° C.; about 150° C. to about 300° C.; about 200° C. to about 300° C.; about 260° C. to about 290° C.; or about 270° C. to about 280° C.

[0194] In a further preferred alternative, the present invention provides a phase change material (PCM) which comprises a solid to solid transition material which provides a high temperature PCM active over a wide temperature range of about 0° C.-50° C. or about 20° C.-30° C.

[0195] It has been found that the tetrafluoroborate salts of the present invention have a distinct advantage over other high temperature phase change materials with regards to safety. As the high-temperature phase is a solid, as opposed to a liquid, the hazards involved with accidental spillage or handling are considerably reduced. The tetrafluoroborate salts are also non-flammable, as opposed to organic solid to solid PCMs that have been previously discussed in the literature. A solid high temperature phase should correspond to improved compatibility with a wider range of materials, in comparison to molten salts. The tetrafluoroborate salts therefore found by the inventors of the present application have significant technical advantages in the formation of phase change materials which may be used in heat batteries.

[0196] The present invention centres on the use of the polymorphism in tetrafluoroborate salts where there is at least one solid to solid phase transition and the tetrafluoroborate salt is to be used as a phase change material (PCM). The energy of the thermally driven transition can be utilised as a phase change material for thermal energy storage such as in heat batteries.

[0197] FIG. 1 is a graph showing the thermal recycling of potassium tetrafluoroborate (KBF.sub.4).

[0198] Initial small-scale experiments of potassium tetrafluoroborate (KBF.sub.4) were set up using, for example, about 14 g of potassium tetrafluoroborate.

[0199] The results in FIG. 1 show that KBF.sub.4 cycled reproducibly, showing little to no degradation after a large number of cycles such as about 75 thermal cycles. FIG. 1 shows a comparison between the potassium tetrafluoroborate being thermally cycled 9 and 75 times. There is very little difference and therefore very little degradation of the tetrafluoroborate salts phase change material.

[0200] The results show there is some hysteresis between the transition temperatures on heating and cooling, with the transition upon heating occurring at about 289° C. and upon cooling at about 265° C.

[0201] However, there is no observation of supercooling during any of the 75 cycles—showing that KBF.sub.4 can be used without a nucleating agent. This is an important point and surprising finding to the inventors.

[0202] The present inventors have found that it is possible to use tetrafluoroborate in a range of components such as salts and other related mixtures e.g. potassium tetrafluoroborate, other tetrafluoroborate salts, their mixtures and mixtures with other inorganic salts, without the use of a nucleating agent in a phase change material (PCM). By overcoming the requirement for a nucleating agent provides a number of technical advantages such as a cost-effective and very stable system which can be thermally cycled many times without any significant degradation to the tetrafluoroborate phase change material (PCM).

[0203] As shown in FIG. 1, the results also show that KBF.sub.4 could be used without any stabilising additive, due to little degradation occurring in an open system (exposed to air/atmosphere). This is a significant advantage compared to many other PCMs that are air/moisture sensitive.

[0204] In FIG. 2, there is Simultaneous Thermal Analysis (STA) using a combination of Differential Scanning calorimetry (DSC) and Thermogravimetric Analysis (TGA) of KBF.sub.4.

[0205] FIG. 2 shows that the enthalpy of the phase transition differs compared to the value reported in the literature, giving a latent heat of about 153 J g.sup.−1. Due to the density of KBF.sub.4 this results in a volumetric latent heat of about 384 J cm.sup.−3. This is an excellent value for a PCM which is previously unknown to date.

[0206] The thermal analysis also shows that there is no loss in mass, showing that KBF.sub.4 does not thermally degrade or undergo any significant changes with heating to about 350° C.

[0207] KBF.sub.4 has also been successfully thermally cycled with both stainless steel and aluminium for 75 cycles, showing no signs of degradation—with the STA results obtained from these samples showing no discernible difference from the STA results prior to cycling. Therefore, proving that KBF.sub.4 is compatible with both materials up to about 350° C. These materials which could therefore be made into containers and/or heat exchangers. Samples containing copper and a cupronickel alloy were also thermally cycled, however there were clear signs of degradation of the metal (most likely due to air, not the KBF.sub.4).

[0208] FIG. 3 is a graph showing the Simultaneous Thermal Analysis (STA) of KBF.sub.4 from about 25° C. to about 550° C. when contained in an aluminium DSC pan according to an embodiment of the present invention.

[0209] FIG. 3 shows that a sample of KBF.sub.4 was heated to about 550° C. to see whether the sample would melt or thermally degrade at about 530° C., as both had been cited in the literature. However, a large exothermal peak was observed at about 530° C., accompanied by little to no mass loss, as shown in FIG. 3.

[0210] As the pan used to hold the sample was made from aluminium, it is suspected that the sample had reacted with the pan, likely via a substitution reaction, creating element boron and potassium tetrafluoroaluminate (KAlF.sub.4). This clearly defines a useable temperature range when KBF.sub.4 is being contained with aluminium, limiting to a maximum temperature of about 500° C.

[0211] The inventors have also found that it is possible to tailor the transition temperature of the solid to solid tetrafluoroborate salt PCMs of the present invention. This can be achieved by changing the colligative properties (similar to depressing the melting point of ice by adding salt), resulting in more available temperatures of PCM.

[0212] Work was performed into the effect of mixing solid to solid tetrafluoroborate salt PCM materials. Several tetrafluoroborate salts were investigated using any combinations of the following: KBF.sub.4, NH.sub.4BF.sub.4, LiBF.sub.4, NaBF.sub.4 and RbBF.sub.4. The most interesting results were seen when mixing KBF.sub.4 with NH.sub.4BF.sub.4, as shown in FIG. 4. Initial heating saw two thermal events—equivalent to the transitions of NH.sub.4BF.sub.4 and KBF.sub.4, respectively. However, on cooling only one thermal event was observed, and this remained the case with further thermal cycling. This indicates the formation of a new phase or eutectic.

[0213] To further investigate this appearance of one thermal event, in depth thermal cycling experiments with varying NH.sub.4BF.sub.4 amounts were performed, with accompanying DSC thermal analysis.

[0214] The data, shown in Error! Reference source not found., indicates a eutectic composition present around the 50 mol % composition. However, unlike a traditional eutectic, which would occur at a lower temperature point than the transition temperature of its two composites, this eutectic lies between the two temperature points.

[0215] Thermal Characterisation of KBF.sub.4

[0216] The last reported thermal analysis of potassium tetrafluoroborate was in the 1990's. Therefore, to ensure that the latent heat values were accurate, thermal analysis was performed using DSC.

[0217] FIG. 6 is therefore the DSC analysis of KBF.sub.4 using apparatus from Mettler Toledo.

[0218] The analysis was performed using two different DSCs—one from Mettler Toledo, and another from TA Instruments, to ensure the results were not instrument dependent. The results from MT shown in FIG. 6 Error! Reference source not found. give a latent heat of 109 J g.sup.−1, whereas the TA instrument analysis, shown in FIG. 7, gives a latent heat of 120 J g.sup.−1. Both results show hysteresis of the transition on cooling, which has also been observed at larger scales with temperature v time graphs. This is exaggerated in a DSC due to the small sample mass (5-20 mg scale).

[0219] Calibrated heat capacity measurements were also carried out using a sapphire standard. Using several different heating rates with multiple samples, an average heat capacity was calculated. The result is shown in FIG. 8 which shows calibrated heat capacity measurements of KBF.sub.4 using heating rate 2 K min.sup.−1.

[0220] Reported values for heat capacity are quoted at 1.1 to 1.2 J g.sup.−1 K.sup.−1 between 190° C. to 290° C., and 1.1 to 1.15 J g.sup.−1 K.sup.−1 between 290 and 390° C. Experimental values gained from the calibrated DSC analysis are higher than this, however, with an average Cp of 1.4 J g.sup.−1 K.sup.−1 prior to the phase transition (190-290° C.) and 1.6 J g.sup.−1 K.sup.−1 after the phase transition (290-390° C.). This is a significant result as the larger heat capacity will increase the overall heat storage capacity and therefore is a surprising finding.

[0221] The thermal conductivity of the material was also investigated. The initial test was performed using puck (flat disk) of KBF.sub.4 that had been melted in a glassy carbon crucible. These results, using the C-Therm analyser, seemed low in comparison to other inorganic salts, as shown in FIG. 9.

[0222] FIG. 9 therefore shows a comparison of thermal conductivity results of melted and pressed KBF.sub.4 vs other inorganic compounds, Na.sub.3PO.sub.4 and borax. As shown the pressed (i.e. compacted) KBF.sub.4 has improved thermal conductivity.

[0223] The analysis was repeated, this time using a pellet of pressed KBF.sub.4. These results were more aligned to the expected values, likely due the smoother surface of the pressed pellet which resulted in better contact with the probe and less contact with air. This is an important teaching: a melt cast KBF.sub.4 sample had greater bulk density, but the surface was more irregular and therefore reduced heat transfer. The thermal conductivity of the material is still low, and therefore the addition of either a heat exchanger, or an additive such as graphite, is required to allow for efficient heat extraction from the material.

[0224] Usage of thermally conductivity enhancers, such as graphite, graphene, boron nitride, can often increase the rate of corrosion due to galvanic corrosion, especially with graphite, and these additives have a risk of sedimenting out, due to their higher density. In the solid to solid tetrafluoroborate based PCMs, this is not an issue as the PCM is a solid, not a liquid, and so segregation of the additives cannot occur. Also due to the solid nature of the PCM, corrosion is severely limited and is not detectable, even with graphite.

[0225] A summary of the thermal analysis and the new total calculated energy capacity of KBF.sub.4 are shown below in Table 2. The new energy densities, particularly over the 500° C. temperature range, easily overshadow common, cheap sensible heat storage materials such as clay and concrete and feolite etc.

TABLE-US-00002 TABLE 2 Summary of thermal properties of KBF.sub.4 from experimental results H H ΔH S1 C.sub.P S2 C.sub.P K (Δ 250° C.) (Δ 500° C.) J g.sup.−1 J K.sup.−1 g.sup.−1 J K.sup.−1 g.sup.−1 W m.sup.−1 K.sup.−1 J g.sup.−1 J cm.sup.−3 J g.sup.−1 J cm.sup.−3 Lit. 120 1.15 1.1 N/A 407 1021 695 1744 Expt. 109- 1.4 1.6 0.67 490 1225 865 2162 120

[0226] Compatibility of a PCM with different metals is incredibly important when designing and building a containment vessel, and potentially a heat exchanger, of a heat storage device. During the initial thermal cycling experiment of potassium tetrafluoroborate, metal samples were submerged in KBF.sub.4 and heated between 200° C. and 350° C. for 75 cycles. These included copper and aluminium—metals commonly used as the material for heat exchangers in Heat Batteries—a cupronickel alloy, and the stainless steel (SS316) vials that contained the experiment. Copper shows clear signs of corrosion, however, this may be a result of heating over 200° C. exposed to oxygen, as this is known to form cupric oxide (CuO) which is often flakey in appearance. The cupronickel alloy shows less structural damage, but oxidation to form CuO has still occurred due to the formation of the black layer on the surface of the metal. The sample of aluminium appears to have suffered no visible damage or corrosion after 75 thermal cycles—suggesting its suitability as a containment material. The stainless-steel vials also were unchanged after thermal cycling, therefore would also be a good containment material.

[0227] Applying Heat to KBF.sub.4

[0228] Potassium tetrafluoroborate is reported to thermally degrade at high temperatures (no specific temperature value was found in the prior art, only ‘fire conditions’) and to decompose into hazardous decomposition products—hydrogen fluoride, borane oxides and potassium oxides. A low temperature fire (barely visible flame) burns at around 525° C., which is just below the melting temperature of KBF.sub.4. Melting is the easiest way to increase bulk density from powder, therefore, the stability of KBF.sub.4 was investigated up to temperatures of 600° C. by heating in a glassy carbon crucible. After 10 melting and freezing cycles, a sample was thermally analysed using DSC.

[0229] The results, shown in FIG. 10, show no change in latent heat from the pure, uncycled sample. In conclusion, this assures that no degradation occurs when melting KBF.sub.4, which enables melting as a potential route to increasing bulk density of the material. This also assures the safety of workers working with, and in the vicinity of, the material at high temperatures.

[0230] FIG. 10 therefore shows DSC analysis of KBF.sub.4 after 10 thermal cycles between 450° C. and 600° C. using TA Instruments DSC 2500.

[0231] This further shows the stability and technical advantage of using potassium tetrafluoroborate as a phase change material which had not previously been considered.

[0232] Large Scale Testing

[0233] The thermal analysis of potassium tetrafluoroborate had shown that the total energy density (from latent heat and heat capacity) was in fact greater than the reported values in the literature, and could easily compete with, if not surpass, the performance of materials commercially used for high temperature heat storage in the current market. Materials compatibility had discovered aluminium, used below 500° C., and stainless steel to be suitable containment materials.

[0234] Therefore, a large-scale supplier of KBF.sub.4 was found, and quality tests showed excellent comparability to the laboratory grade KBF.sub.4, with no discernible difference in thermal characteristics or impurities. This then allowed two large scale tests to go ahead: one using a Heat Battery infrastructure, an aluminium finned-tube heat exchanger; the other an Alternative Design that removed the need for an internal heat exchanger.

[0235] Heat Battery

[0236] Potassium tetrafluoroborate as received from the supplier, was a very fine powder. This permitted a 17-litre heat battery could be filled with relative ease, as the pourability of the powder allowed it to flow in and around the fins. Once filled, the heat battery was connected to a Julabo High Temperature Circulator, which proceeded to heat up and pump thermal oil around the system. This set-up allowed several thermal cycles to be recorded.

[0237] Thermocouples had been placed strategically throughout the heat battery, but most importantly in oil flowing in and out of the cell, as well as the internal temperature of the KBF.sub.4 material. The performance of the heat battery during charging and discharging is shown in FIG. 11.

[0238] FIG. 11 is a representation of the thermal performance of an aluminium heat battery containing KBF.sub.4: a) on the top of FIG. 11 this shows both the charging and discharging of the heat battery over one thermal cycle; b) on the bottom of FIG. 11 shows a more detailed look at the charging following the input and output temperature of the heat exchange fluid, as well as the accumulative energy used during charging.

[0239] The plateaux of the phase transition were clearly seen during both charging and discharging. There is only a slight lag between the input, output temperature and the internal temperature of the material, therefore the heat exchanger appears to be effectively dispersing the inputted heat to the material. This shows that a finned-tube heat exchanger can still be effective when used with a powdered material, which will have significant total air gaps.

[0240] The thermal properties of the heat battery were extrapolated, and are shown below in Table 3. The calculated specific heats before and after the phase transition, in particular, are somewhat higher than the values gained from DSC. These results are very promising.

TABLE-US-00003 TABLE 3 Thermal properties of KBF.sub.4 in Al heat battery. Specific Specific Specific Specific Specific Specific Specific Specific Heat, 100° C.- Heat, 150° C.- Heat, 270° C.- Heat, 280° C.- Heat, 300° C.- Heat, 310° C.- Heat, 270° C.- Heat, 280° C.- 150° C. 250° C. 300° C. 290° C. 320° C. 325° C. 300° C. 290° C. (kJ/kgK) (kJ/kgK) (kJ/kgK) (kJ/kgK) (kJ/kgK) (kJ/kgK) (kJ/kgK) (kJ/kgK) 1.89 2.26 6.35 11.19 3.12 3.09 128.31 91.17

[0241] Alternative Design

[0242] The compatibility testing discussed earlier showed that aluminium is unsuitable for use with KBF.sub.4 when heating above its melting point. This therefore eliminates the option to use an aluminium heat exchanger with molten KBF.sub.4. This led to the creation of a new design heat store for KBF.sub.4, as well as other high temperature PCM. This design featured a simple ‘cappable’ pipe which may, for example, be a cylinder with a fixable cap such as a screw-on cap. This would allow the heat store (i.e. the prototype heat store) to be easily scalable, in length and in diameter, which should simplify scale-up to shipping container size. The pipes containing the PCM material would act as the heat exchanger, allowing the heat transfer fluid—whether it be air, high temperature steam, or thermal oil—to flow through and around the pipes, bringing or extracting heat.

[0243] In order to melt KBF.sub.4 and thereby increasing the bulk density, stainless steel was required for containment. Pipes with threaded ends, as well as threaded caps may be used.

[0244] One end of a pipe (5.5×25 cm) was fitted with the cap, which was tightly screwed and tested with water at room temperature to ensure a good seal. The prototype container was filled with 500 g of KBF.sub.4 and placed in a glass liner within a tube furnace. A thermocouple was placed in the centre of the material, held in place by an alumina sheath. Firstly, the prototype was heated to 600° C., to ensure all the KBF.sub.4 would melt. The container was then cycled repeatedly between 200 and 350° C. for 25 cycles.

[0245] The cycling data showed good reproducibility over 25 cycles, as shown in FIG. 12 Error! Reference source not found.

[0246] The plateaux had not differed in length, the only discernible difference was in the gradient of the temperature curve; however, this was due to the temperature range being shortened.

[0247] Pelletisation

[0248] An alternative method to increase the bulk density of tetrafluoroborate salts (e.g. KBF.sub.4) for use as phase change materials is to use pressure to compact the powder into a solid pellet. Improving the bulk density without melting would enable the use of aluminium as a containment material.

[0249] To press powder tetrafluoroborate salts (e.g. KBF.sub.4) any suitable means may be used and, for example, a die set and press may be used. The powder compacted reasonably, producing a hard, completely solid pellet. The pellet was then cycled ten times in a furnace up to 350° C., after which there was clear signs of cracking on the pellet. This is expected due to the volume change between the two phases. The pellet had retained its shape, however, and had not crumbled back to a powder, therefore pelleting is a viable option to increase the bulk density.

[0250] The use of additives to increase the structural rigidity is also possible and within scope of the present invention.

[0251] A range of additives may be used including any one of or combination of the following: fiberglass, carbon fibre and graphite flakes. Other tetrafluoroborates and mixtures may also be used.

[0252] Preparation of Tetrafluoroborates Salt Mixtures

[0253] Tetrafluoroborate salts were sourced from the suppliers, Fluorochem (99% KBF.sub.4, 98% NaBF.sub.4, 96% LiBF.sub.4), Alfa Aesar (98% KBF.sub.4, 97% NHa BF.sub.4, 98% RbBF.sub.4) and Sigma-Aldrich (97% NH.sub.4BF.sub.4). All salts with exception to NH.sub.4BF.sub.4 from Sigma-Aldrich were fine, fluid like powders; NH.sub.4BF.sub.4 was granular and required grinding before use.

[0254] Initial testing was carried out on 1:1 molar mixtures of the salts. Approximately 10 g of each salt mixture was prepared by weighing the appropriate mass of each salt and placed in a glass vial.

[0255] Mixing of the salts was carried out on the Resonant Acoustic Mixer (RAM) which operates by oscillating rapidly with a fixed acceleration, which causes displacement of the powder particles and ensures random mixing of the sample. The acceleration chosen for mixing the fine tetrafluoroborate powders was 80 G, and this was carried out for 15 minutes. Sufficient space was left in the vial to allow for movement of the powder. Grinding samples together using a pestle and mortar was also found to be a successful method in creating a uniform mixture.

[0256] Thermal Cycling

[0257] Thermal cycling of the individual salts and their mixtures was carried out on the Torrey Pines Scientific Inc. Programmable Hot Plate HP60. A 10 g sample of salt or salt mixture was placed in a 20 cm.sup.−3 glass vial and cycled between 20° C. and 350° C. Sample temperature was measured using K-type thermocouples held in place with aluminium foil or stainless steel vial caps and a Pico Technologies TC-08 Thermocouple Data Logger.

[0258] Thermal cycling is carried out at this scale as it allows larger material behaviour to be investigated such as sublimation, corrosion (of glass and metal), discolouration and changes in material consistency.

[0259] As multiple samples can be cycled at once, a large amount of data can be collected which can be fairly compared, as the same conditions have been experienced by all samples. Furthermore, as multiple cycles can be performed, changes in material behaviour can be tracked over time.

[0260] Single Salt Analysis

[0261] It has been found that tetrafluoroborate salts according to the present invention can be mixed to form new materials with different phase change temperatures.

[0262] The tetrafluoroborate salts which have been analysed for use in mixtures are combinations of the following: KBF.sub.4; NaBF.sub.4; NH.sub.4BF.sub.4; LiBF.sub.4 and RbBF.sub.4.

[0263] Thermal Analysis

[0264] To understand the salts thermal behaviour, thermal cycling and DSC analysis was carried out.

[0265] Thermal Cycling

[0266] Thermal cycling of 20 g samples was carried out for KBF.sub.4 and NaBF.sub.4 up to 350° C.

[0267] NH.sub.4BF.sub.4 is known to start to sublime at 220° C. and therefore the sample was cycled to only 250° C. Data is shown in FIG. 13.

[0268] FIG. 14 therefore shows the thermal cycling data for KBF.sub.4 and NaBF.sub.4 up to 350° C. and for NH.sub.4BF.sub.4 up to 250° C.

[0269] As expected, sublimation was observed for the sample during thermal cycling.

[0270] Sharp heating and cooling transitions were observed for KBF.sub.4 at 284° C. and 268° C. respectively, with no change over subsequent cycles.

[0271] Slightly shorter plateaus were observed for NaBF.sub.4 at 247° C. and 216° C. for the heating and cooling transitions. The shortening of the plateaus is most likely consequent of a lower energy transition than for KBF.sub.4.

[0272] The NH.sub.4BF.sub.4 cycle shows clear heating and cooling plateaus at 196° C. and 182° C., respectively. Comparing cooling and heating transition temperatures, lower cooling transition temperatures are observed for all salts, likely due to hysteresis or super-cooling of the sample.

[0273] Thermal Properties Comparison

[0274] Thermal analysis was also carried out using a DSC with heating rate 10 K/min. A summary of the literature latent heat values and DSC values is shown in Table 4.

TABLE-US-00004 TABLE 4 Table comparing the literature and DSC values of stored energy and cooling transition temperature for LiBF.sub.4, NaBF.sub.4, KBF.sub.4, RbBF.sub.4 and NH.sub.4BF.sub.4. LiBF.sub.4 NaBF.sub.4 KBF.sub.4 RbBF.sub.4 NH.sub.4BF.sub.4 Literature 27 222 274 249 200 cooling transition temperature (° C.) DSC cooling 26 205 248 222 182 transition temperature (° C.) Thermal cycling — 216 268 — 182 cooling transition temperature (° C.) Literature — 72.4 117.7 — 84.6 energy released (kJ/kg) DSC energy 7.0 55.3 110.2 70.4 98.5 released (kJ/kg)

[0275] Comparing the literature transition temperature values to the DSC and thermal cycling data, it can be observed that experimental data shows slightly lower temperatures, particularly for the DSC data. This is most likely due to super-cooling of the samples due to low sample volume. By comparing the literature values for energy released it can be observed that they are comparable, with exception to NaBF.sub.4. This was attributed to poor data obtained within the literature text.

[0276] Variable Temperature In-Situ PXRD Studies

[0277] The crystal structures for KBF.sub.4 and NH.sub.4BF.sub.4 are characterised, with both the low temperature and high temperature crystal structures available. However, LiBF.sub.4, NaBF.sub.4 and RbBF.sub.4 have published low temperature crystal structures, but no high temperature crystal structures. Hence, using PXRD data gathered at the Diamond Light Source, the high temperature crystal structures of these salts were determined.

[0278] LiBF.sub.4

[0279] The LiBF.sub.4 structure for the low temperature structure was determined and a solid to solid transition was reported at 27° C. Therefore, LiBF.sub.4 was cycled between 0° C. and 50° C. (FIG. 14 Error! Reference source not found.).

[0280] FIG. 14 is a therefore a representation of thermal cycling for LiBF.sub.4 cycled between 0° C. and 50° C. according to an embodiment of the present invention

[0281] During cycling there was no observable change in crystal structure.

[0282] Furthermore, as the transition observed on the DSC was very low energy (7.0 kJ/kg) in comparison to KBF.sub.4 (110.2) it is likely the energy released does not represent a solid to solid transition but the dehydration of a contaminant LiBF.sub.4 hydrate or the transition of an impurity.

[0283] NaBF.sub.4

[0284] The low temperature crystal structure of NaBF.sub.4 has already been determined.

[0285] FIG. 15 shows powder patterns for NaBF.sub.4 cycled between 50° C. and 350° C.

[0286] RbBF.sub.4

[0287] To obtain high temperature data, the RbBF.sub.4 salt was cycled between 20° C. and 300° C. and powder patterns collected for the transition of the salt.

[0288] FIG. 16 is therefore a representation of the RbBF.sub.4 salt which was cycled between 20° C. and 300° C. and powder patterns collected for the transition of the salt.

[0289] RbBF.sub.4 was confirmed to be isostructural with KBF.sub.4 and NH.sub.4BF.sub.4.

CONCLUSIONS

[0290] As the potassium salt has the highest latent heat, potassium tetrafluoroborate salts have some advantages.

TABLE-US-00005 TABLE 5 Table comparing transition temperatures, energy released, low temperature phase and high temperature phase data for the salts LiBF.sub.4, NaBF.sub.4, KBF.sub.4, RbBF.sub.4 and NH.sub.4BF.sub.4. Percent LiBF.sub.4 NaBF.sub.4 KBF.sub.4 RbBF.sub.4 NH.sub.4BF.sub.4 Transition temperature — 205.1 247.8 221.6 182.2 (cooling) (° C.) Energy released — 55.34 110.19 70.44 98.45 (kJ/kg)

[0291] Salt Mixtures

[0292] A number of tests were conducted on KBF.sub.4 due to the salt's high latent heat in comparison with the other tetrafluoroborate salts.

[0293] LiBF.sub.4, NaBF.sub.4 and NH.sub.4BF.sub.4 were chosen as the composite salts to be mixed with KBF.sub.4 as they are readily available and have varying physical properties such as transition temperature and crystal structure, also since they also have BF.sub.4 groups, it was thought they may contribute to the phase change energy more than a salt without a solid-solid phase change. However, it is also possible to change the solid to solid transition point by adding an additive that does not contain the tetrafluoroborate molecule.

[0294] The selection rule for doing so is: addition of a (or multiple) salts that has a common cation with the parent tetrafluoroborate salt. As a non-limiting set of examples, the following may be used: [0295] addition of NaCl to NaBF.sub.4, [0296] addition of KNO.sub.3 to KBF.sub.4, [0297] addition of SrSO.sub.4 to Sr(BF.sub.4).sub.2.

[0298] This is because it is undesirable to have more than three ions in a system as there then exists an enhanced likelihood of undesired by-products forming.

[0299] Addition of K.sub.3PO.sub.4 to Mg(BF.sub.4).sub.2, could result in formation of Mg.sub.2(PO.sub.4).sub.2 (along with KBF.sub.4, and the two starting compounds). Thus, having both more than or equal to two cations and more than or equal to two anions is undesired.

[0300] It was investigated how these factors affect the success of forming a new solid-solid material, such as LiBF.sub.4 and KBF.sub.4 salt mixture.

[0301] Initial analysis was carried out on 20 g samples of 50 mol % and 25 mol % LiBF.sub.4 mixtures. In the 25 mol % mixture, 25% of the molecules were LiBF.sub.4 and 75% were KBF.sub.4, and in the 50 mol % mixture 50% of the molecules were LiBF.sub.4 and 50% were KBF.sub.4. LiBF.sub.4 was found to have no solid to solid transition outside their tested temperature range, however undergoes a melting transition at 296.5° C.

[0302] Thermal Analysis

[0303] The salt mixtures were cycled on the hotplate, the data collected is shown in FIG. 17. For both compositions, two transitions were observed during heating; 274° C. and 227° C. Both temperatures were lower than the transition temperature for the pure salts as LiBF.sub.4 melts at 296.5° C. and KBF.sub.4 transitions at 283° C. It is likely that the presence of two salts causes mutual depression of their transition temperatures.

[0304] FIG. 17 therefore shows thermal cycling of LiBF.sub.4 and KBF.sub.4 between room temperature and 350° C. containing 25 mol % and 50 mol % LiBF.sub.4.

[0305] However, slight differences in plateau length can be observed between the compositions due to variations in LiBF.sub.4 content. It is therefore most likely that the transition temperature of 227° C. corresponds to the LiBF.sub.4 transition, as a shorter melt plateau is observed for the sample with a lower LiBF.sub.4 content.

[0306] The 50 mol % sample was cycled multiple times to observe if any changes in material behaviour were observe. This is shown in FIG. 18.

[0307] FIG. 18 therefore shows the thermal cycling of 50 mol % LiBF.sub.4 and KBF.sub.4 mixture cycled up to 350° C.

[0308] Between cycles of the 50 mol % mixture, no difference can be observed. As a new transition temperature is expected fora homogenous mixture, it is possible that the salts are behaving separately.

[0309] Variable Temperature In-Situ PXRD Studies

[0310] PXRD was carried out on the 50 mol % mixture of LiBF.sub.4 and KBF.sub.4.

[0311] The powder patterns for the full transition are shown in FIG. 19 which shows the normalised variable temperature powder patterns for LiBF.sub.4 and KBF.sub.4.

[0312] Comparing the peaks in the low temperature patterns A and E at 13.5° and 15.5° (marked with asterisk), a change in peak intensity is observed due to preferred orientation. This is most likely due to the crystallization of the LiBF.sub.4 within the capillary during cooling, removing the random orientation of crystals within the sample. There is also a decrease in peak intensity after cycling as shown in Error! Reference source not found. suggesting there is a degradation or melt of one of the mixture components.

[0313] FIG. 19 shows the normalised variable temperature powder patterns for LiBF.sub.4 and KBF.sub.4 mixture for: A—low temperature before cycling; B—mid heating transition; C—high temperature phase; D—mid cooling transition; and E—low temperature phase after transition.

[0314] FIG. 20 shows the normalised variable temperature powder patterns for LiBF.sub.4 and KBF.sub.4 mixture for: A—low temperature before cycling; B— mid heating transition; C— high temperature phase; D mid cooling transition; and E—low temperature phase after transition.

[0315] FIG. 21 shows powder patterns in 5°-25° range comparing KBF.sub.4 simulated data (306° C.) and LiBF.sub.4 (80° C.) data with LiBF.sub.4 and KBF.sub.4 (291° C.).

[0316] The phase transition was observed on heating to 291° C., shown in powder pattern in FIGS. 21 and 22. However, comparing the high temperature powder pattern with the pure KBF.sub.4 high temperature phase (FIG. 21) intensity changes are observed for the highlighted peaks due to preferred orientation.

[0317] Low intensity peaks at 20.19°, 22.47°, and 23.36° are most likely due to a small amount of LiBF.sub.4 present, however due to temperature differences and consequent shifting, the peaks were unable to be matched precisely. However, as no clear new peaks were observed it is probable the LiBF.sub.4 and KBF.sub.4 salts are only acting as a mixture with no new crystal phase or transition temperature.

[0318] NaBF.sub.4 and KBF.sub.4 Salt Mixture

[0319] Analysis was conducted on 25 mol % and 50 mol % NaBF.sub.4 mixtures with KBF.sub.4 which were mixed on the RAM.

[0320] Thermal Analysis

[0321] The 25 mol % and 50 mol % NaBF.sub.4 mixtures were cycled up to 350° C. as shown in FIG. 24.

[0322] FIG. 23 therefore represents thermal cycling of NaBF.sub.4 and KBF.sub.4 mixtures between room temperature and 350° C., containing 25 mol % and 50 mol % LiBF.sub.4.

[0323] Clear transitions can be observed during heating, with the transition at 238° C. corresponding to NaBF.sub.4 and 277° C. to the KBF.sub.4 single solid to solid transitions. The single sodium salt transition appears diminished in the 25 mol % sample due to lower salt content than the 50 mol % sample.

[0324] During cooling, transitions are much less clear with only slight events observed at 261° C. and 180° C. To investigate if any changes occurred through further cycling, the 50 mol %, which displayed clearer transitions, was cycled multiple times. This is shown in FIG. 25 which is a representation of thermal cycling of 50 mol % NaBF.sub.4 and KBF.sub.4 mixture up to 350° C.

[0325] A change in the transition temperature can be observed between cycles, as a new event occurs at 187° C. The appearance of this new transition is important as it suggests the salts are transitioning simultaneously.

[0326] NHa BF.sub.4 and KBF.sub.4 Salt Mixture

[0327] The mixture of NH.sub.4BF.sub.4 with KBF.sub.4 was also chosen, in contrast to the previous salt mixtures only a 50 mol % was cycled as this composition showed the clearest transitions. A 20 g sample was prepared and mixed on the RAM.

[0328] Thermal Analysis

[0329] The 50 mol % sample was cycled up to 350° C. for multiple cycles to determine whether changes in material behaviour occurred over time. This is shown in FIG. 25.

[0330] FIG. 25 is therefore a representation of thermal cycling of 50 mol % mix of NH.sub.4BF.sub.4 and KBF.sub.4 cycled between 50° C. and 350° C.

[0331] During the first heating cycle, two transitions are observed: 199° C. corresponding to the ammonium salt and 280° C. to the potassium salt.

[0332] However, during the second heating cycle only one transition at 217° C. is observed. Furthermore, the cooling transitions appear to occur over a narrower temperature range for subsequent cycles.

[0333] This change in behaviour suggests the formation of a eutectic mixture as the salts are transitioning simultaneously at a new phase transition temperature. Multiple cycles are therefore needed to form a new phase transition temperature and achieve phase mixing, where the salts act as a homogenous system and transition simultaneously. During cycling it was found that sublimation of the sample occurred which was identified as the ammonium salt; hence the composition of the sample will have changed during cycling.

[0334] Further analysis was carried out on DSC as shown in FIG. 26 and FIG. 27 for the first and third cycle respectively.

[0335] FIG. 26 is a DSC representation of uncycled 50 mol % NH.sub.4BF.sub.4 and KBF.sub.4 cycled between ambient and 300° C. at a rate of 10° C. min.sup.−1.

[0336] FIG. 27 is a DSC representation of third cycle of 50 mol % NH.sub.4BF.sub.4 and KBF.sub.4 cycled between ambient and 300° C. at a rate of 2° C. min.sup.−1.

[0337] Through comparison of the first cycle FIG. 26 and third cycle FIG. 27 it is clear that a new broad endothermic transition has emerged at about 228° C.

[0338] Furthermore, there is a change from a broad multiple exothermic peak transition to a broad single peak. This data supports the vial scale thermal cycling data as the emergence of new peaks is indicative of the formation of a eutectic mixture. Comparing the stored energy of the system to KBF.sub.4 (113 kJ/kg) it can be seen that there is a decrease in stored energy.

[0339] Variable Temperature In-Situ PXRD Studies

[0340] To confirm if the salt mixture had formed a new crystal phase, variable temperature PXRD was carried out. Analysis was carried out on a 50 mol % pre-cycled mixture of NH.sub.4BF.sub.4 and KBF.sub.4 to ensure the material was transitioning at the new observed transition temperature. However, during cycling, NH.sub.4BF.sub.4 sublimated, therefore composition is uncertain. The powder patterns obtained for a full cycle are shown in FIG. 28.

[0341] FIG. 28 is therefore a representation of powder patterns for the collected high temperature phases for KBF.sub.4, NH.sub.4BF.sub.4 and their mixture.

[0342] It is clear that both salts transitioned fully into a new high temperature phase. The low temperature phase before transition has broad and undefined peaks notably in the 15° C.-25° C. range. However, after a heating cycle, the peaks appear to have sharpened.

[0343] From FIG. 28 it can be seen that there appears to be no peak overlap of the individual salt phases and therefore, no evidence of the separate salt phases in the high temperature mixture.

[0344] Phase Diagram Construction

[0345] To determine if a eutectic composition of the NH.sub.4BF.sub.4 and KBF.sub.4 mixture exists, thermal cycling of 15 g samples of 10-90 mol % NH.sub.4BF.sub.4 mixture for 5 cycles. Heating transition temperatures were then used to construct a phase diagram.

[0346] Due to a local minima at around 50 mol % NH.sub.4BF.sub.4 indicating the possible presence of a eutectic composition, therefore more data was collected for 2 mol % increments between 40 and 60 mol % NH.sub.4BF.sub.4, to increase data points in this area. DSC of the pre-cycled mixture was also carried out; data fora range of sample is shown in FIG. 29.

[0347] FIG. 29 is therefore a comparison of DSC data collected for varying compositions of NH.sub.4BF.sub.4 and KBF.sub.4 mixture.

[0348] From the DSC data, it can be seen with mixtures dominant in one salt such as 90 mol % KBF.sub.4, the transitions are sharp corresponding to the transition of the dominant salt. However, for compositions with a higher salt ratio for example 60 mol % KBF.sub.4 a shoulder peak can be observed in both the endothermic and exothermic transitions indicating merging of the peaks for each salt. This indicates the approach to a eutectic composition.

[0349] Using the data collected from DSC and thermal cycling a phase diagram was constructed. This is shown in FIG. 30.

[0350] FIG. 30 is therefore a phase diagram constructed using DSC data and thermal cycling data. The 40 and 90 mol % compositions have two data points as two transitions were observed in DSC data.

[0351] From the phase diagram an overall decrease in transition temperature can be seen for both DSC and thermal cycling data. Suggestion of local minima in thermal cycling data was observed for compositions 50 mol % and 70 mol % and for 80 mol % and possibly 90 mol % in the DSC data.

[0352] However, composition of the mixtures is only approximate as the NH.sub.4BF.sub.4 salt was found to sublimate during cycling.

CONCLUSIONS

[0353] The analysis of the thermal and crystallographic data of the tetrafluoroborate salt mixtures it has clearly shown that tetrafluoroborate salt mixtures have very useful properties due to the solid to solid phase change temperatures.

[0354] Tetrafluoroborate salts, LiBF.sub.4, NaBF.sub.4, KBF.sub.4, RbBF.sub.4 and NH.sub.4BF.sub.4 were successfully characterised through the use of thermal cycling, DSC and variable temperature PXRD. The materials were found to have transition temperatures ranging approximately 182° C.-248° C. with stored energy of 50-110 kJ/kg.

[0355] The NH.sub.4BF.sub.4 and KBF.sub.4 mixture was found to be very successful as a new transition temperature of about 217° C. was observed. Therefore, in order to determine if a eutectic composition exists, phase diagram construction was attempted for this mixture, showing a general trend of decreasing transition temperature with increasing NH.sub.4BF.sub.4 content.

[0356] The identification of solid to solid PCMs is beneficial to PCM applications as they are much easier to implement than solid to liquid PCMs for high temperature applications, benefitting from low expansion during phase change and easier encapsulation. Furthermore, the identification of mixtures offers flexibility in phase change temperatures increasing range of suitable applications for solid-solid materials.

[0357] It will be clear to those of skill in the art, that the above described embodiments of the present invention are merely exemplary and that various modifications and improvements thereto may be made without departing from the scope of the present invention. For example, any suitable range and concentrations of tetrafluoroborate salts and components described above may be used.