The present invention relates to 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, the present invention relates to 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. BF4 ). 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.

Background of the Invention

Phase change materials (PCMs) are materials which have a high latent heat associated with a phase transition and have potential for use in energy storage applications, amongst others.

PCMs with solid to solid phase transitions are of a particular interest due to desirable properties such as low-volume change during transition, easier encapsulation and higher safety at high temperatures than solid to liquid phase transition PCMs.

(a) Phase Change Materials

Phase change materials (PCMs) have a high latent heat therefore large amounts of energy can be stored and released during phase change transitions. During a phase change, the system remains at a constant temperature, hence heat of a specific temperature can be stored or released for an above ambient temperature PCM. Energy is released during a cooling transition and stored during a heating transition.

Phase change materials are categorised as, solid to liquid, liquid to gas and solid to solid phase transitions. However, liquid to gas transitions are not commonly used in Thermal Energy Stores (TES) due to large volume changes.

The physical properties of PCMs can be altered with the addition of nucleators, which can reduce supercooling (cooling below transition temperature with no phase change) or nucleate a preferred phase. A PCMs transition temperature can also be altered with the addition of new salts, sometimes known as eutectics, like the addition of a salt to water, an existing salt or a solution, results in the depression of the systems transition temperature. A eutectic is the composition of the system where all components transition simultaneously at a single transition temperature.

(b) Solid to Liquid Phase Change Materials

The most common form of phase change materials have liquid to solid transitions. Energy is released during freezing and absorbed during melting. During freezing nucleation hopefully occurs spontaneously, initiating crystallisation of the solid phase.

Due to the existence of a liquid phase, the material must be encapsulated to avoid loss of material and ensure safety in applications. Furthermore, as the phase change from a solid to liquid results in a change in density of the materials, this must be accounted for in the encapsulation of these materials.

(c) Solid to Solid Phase Change Materials

Often no visible change is observed during a solid to solid phase transition and low volume change is observed. This is beneficial in their application as PCMs as they are less challenging to encapsulate than solid to liquid PCMs as volume change does not need to be considered as much. Furthermore, as no liquid phase exists, there is no chance of PCM leaking during a phase transition and the safety of their application is improved which is especially important in the application of high temperature PCMs.

Phase change materials (PCMs) traditionally store and release thermal energy by undergoing melt/crystallisation cycles. PCMs can be used in multiple applications. PCMs can be used as: thermal stores (for example, in scenarios that hot water tanks are used), or high heat capacity bricks (clays, or magnetite or feolite or iron oxide containing blocks), and as thermal buffers (for example a PCM will thermally buffer an object that oscillates in temperature above and below the PCM transition temperature).

Potassium tetrafluoroborate (KBF4) is an example of an inorganic salt that undergoes a solid to solid phase transition, sometimes known as a plastic deformation transition, or sometimes known as a polymorphic transition. In comparison to solid to solid transitions present in organic molecules such as pentaerythritol, the reported latent heats of these materials are lower. However, unlike organic materials, these materials do not degrade at higher temperatures (many organics degrade above 200°C), therefore allowing a wider useable temperature range) and are non-combustible.

The polymorphic transition of tetrafluoroborate salts has been of academic interest due to the interesting calorimetric properties. In this regard, we refer to Table 1 below.

Table 1 : Review of some inorganic salts that undergo solid to solid transitions.

There is a known problem in the field of PCMs of obtaining solid to solid phase change materials which can be used in heat batteries and which provide desired temperature ranges for phase changes. Very few of these materials are known to exist and there is a significant need and requirement for such materials for the development of heat batteries.

It is an object of at least one aspect of the present invention to obviate or mitigate at least one more of the aforementioned problems.

It is a further object of at least one aspect of the present invention to provide an improved phase change material that comprises tetrafluoroborate salts which undergo a solid to solid phase transition.

It is an object of at least one aspect of the present invention to provide a phase change material (PCM) which is a solid to solid phase 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.

It is another object of at least one aspect of the present invention to provide a phase change material (PCM) which is 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.

It is another object of at least one aspect of the present invention to provide a phase change material (PCM) which is a solid to solid transition material which provides a high temperature PCM active over a wide temperature range of about 100°C - 200°C or about 135°C - 155°C.

It is another object of at least one aspect of the present invention that tetrafluoroborate salts can be used as solid to solid phase transition PCMs and as solid to liquid PCMs by utilising both transitions. In this scenario the PCM may reach temperatures of >1 ,500°C.

Summary of the Invention

According to a first aspect of the present invention there is provided a phase change material (PCM) comprising:

at least one or a plurality of tetrafluoroborate salts which has a solid to solid (polymorphic) transition;

wherein the PCM has a phase change in the region of about -270°C to about 3,000°C temperature range.

The present invention relates to 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, 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 or a plurality of tetrafluoroborate salts which are capable of having a solid to solid phase transition.

The tetrafluoroborate salts may be capable of at least one, two or more, three or more or a plurality of solid to solid phase transitions. The phase transitions may occur at different temperatures.

The phase change material (PCM) of the present invention may therefore function as a thermal storage material which comprises at least one or a plurality of solid to solid phase change materials (PCMs) wherein the phase change material (PCM) comprises the tetrafluoroborate anion (BF4 ). The tetrafluoroborate anion may be part of an organic salt, inorganic salt and/or metal salt.

The inorganic salt and/or metal salt of the tetrafluoroborate anion (BF4 ) may therefore function and be used as a material that changes phase between two solid phases.

The inorganic salt and/or metal salt of the tetrafluoroborate anion (BF4 ) may therefore be used for thermal storage and/or thermal buffering in, for example, a heat battery.

Other suitable applications of the phase change materials (PCMs) of the present invention include heat transportation and automotive applications.

Furthermore, the phase change materials (PCMs) of the present invention may also be used as barocaloric materials. This therefore permits the tetrafluorobo rates of the present invention to be utilised as barocaloric materials, where the change in solid to solid transition point temperature under pressure may be exploited in, for example, a heat pump type scenario. This can be used for both heating and cooling generation, similar to a vapour compression heat pump.

The tetrafluoroborate salt may comprise at least one anion or a plurality of anions of tetrafluoroborate (e.g. BF4 ).

A preferred tetrafluoroborate salt may be KBF4 or may comprise substantially KBF4.

The phase change material (PCM) may also comprise any one of or combination of the following additives: thermal conductivity improving additives; stabilising additives (e.g. shape stabilising additives) and/or transition point tuning stabilising additives.

In particular embodiments, the phase change material (PCM) of the present invention may comprise:

• One or more tetrafluoro borate salts 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.%; 10 - 90 wt.%; 20 - 90 wt.%; 30 - 90 wt.%; about 100 wt.%; and/or optionally

• One or more thermal conductivity improving additives in the following amounts: 0 - 30 wt.%; 2- 20 wt.%; 5 - 15 wt.%; and/or optionally

• One or more stabilising additives in the following amounts: 0 - 40 wt.%; 0 - 30 wt.%; 0 - 20 wt.%;

3 - 30 wt.%; 5 - 15 wt.%; and/or optionally

• One or more transition point tuning stabilising additives in the following amounts: 0 - 40 wt.%;

0 - 30 wt.%; 0 - 20 wt.%; 3 - 30 wt.%; 5 - 15 wt.%.

By wt.% in the present application means weight percent which is sometimes written as w w e.g. weight percent of the component in the phase change material (PCM).

The thermal conductivity improving additives, stabilising additives and transition point tuning stabilising additives may be optional components in the phase change material (PCM).

The stabilising additives may be shape stabilising additives which may be used to stabilise any shape formed by the PCM.

In particular embodiments, the phase change material (PCM) of the present invention may comprise KBF4 in the following amounts: 10 - 100 wt.%; 20 - 100 wt.%; 30 - 100 wt.%; 40 - 60 wt.%; 50 - 100 wt.%; 10 - 90 wt.%; 20 - 90 wt.%; 50 - 90 wt.%; 60 - 90 wt.%; 70 - 90 wt.%; or about 100 wt.%.

The tetrafluoro bo rate salt may comprise a mixture of tetrafluoro bo rate salts such as KBF4 and NH4BF4. In particular embodiments, the tetrafluoroborate salt may be about a 50:50 mol% molar ratio mixture of KBF4 and NH4BF4. This is a mixture of about one mole of KBF4 with about one mole of NH4BF4.

Alternatively, a mixture of tetrafluoroborate salts comprising KBF4 and NH4BF4 may comprise a molar ratio mixture of: about 10 - 90 mol% of KBF4 and 10 - 90 mol % of NH4BF4; about 20 - 80 mol% of KBF4 and 20 - 80 mol% of NH4BF4; or about 30 - 60 mol% of KBF4 and 30 - 60 mol% of NH4BF4.

By mol% in the present application means the percentage of the total moles that is of a particular component in the phase change material (PCM). Mole percent is equal to the mole fraction for the component multiplied by 100: mol % a = X3 c 100. The sum of the mole percents for each component in the phase change material (PCM) will be equal to 100.

Further particular embodiments may comprise any of the following: about 20 mol% KBF4 and 80 mol% NH4BF4; about 40 mol% KBF4 and 60 mol% NH4BF4; about 50 mol% KBF4 and 50 mol% NH4BF4; about 60 mol% KBF4 and 40 mol% NH4BF4; or about 90 mol% KBF4 and 10 mol% NH4BF4

The present inventors have also found that the tetrafluoroborate salts of the present invention may be used to form phase change materials with a solid to solid phase transition with no requirement for a nucleating agent. This is a significant and surprising finding to the inventors.

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).

The phase change materials (PCMs) of the present invention may be repeatedly thermally cycled with very little or substantially no detrimental effect and no substantial degradation on the phase change material (PCM) itself. For example, the phase change materials (PCMs) may be repeatedly thermally cycled over temperature ranges described in the present invention such as up to: 10 thermal cycles; 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.

CLAIMS

1 . A phase change material (PCM) comprising:

at least one or a plurality of tetrafluoroborate salts which has a solid to solid (polymorphic) transition;

wherein the PCM has a phase change in the region of about -270°C to about 3,000°C temperature range.

2. A phase change material (PCM) according to claim 1 , 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.

3. A phase change material (PCM) according to any preceding claim, wherein the phase change material (PCM) functions as a thermal storage material and the phase change material (PCM) comprises tetrafluoroborate anions (BF4 ) which is part of an inorganic salt and/or metal salt.

4. A phase change material (PCM) according to any preceding claim, wherein the at least one or a plurality of tetrafluoroborate salts is in the form of an inorganic salt and/or metal salt and is used for thermal storage and/or thermal buffering in a heat battery, transportation and/or automotive applications.

5. A phase change material (PCM) according to any preceding claim, wherein the phase change material (PCM) comprises:

• One or more tetrafluoroborate salts 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.%; 10 - 90 wt.%; 20 - 90 wt.%; 30 - 90 wt.%; about 100 wt.%; and/or optionally

• One or more thermal conductivity improving additives in the following amounts: 0 - 30 wt.%; 2-20 wt.%; 5 - 15 wt.%; and/or optionally

• One or more stabilising additives in the following amounts: 0 - 40 wt.%; 0 - 30 wt.%; 0 - 20 wt.%; 3 - 30 wt.%; 5 - 15 wt.%; and/or optionally

• One or more transition point tuning stabilising additives in the following amounts: 0 - 40 wt.%; 0 - 30 wt.%; 0 - 20 wt.%; 3 - 30 wt.%; 5 - 15 wt.%.

6. A phase change material (PCM) according to any preceding claim, wherein the at least one or a plurality of tetrafluoroborate salts is used as a barocaloric materials where the solid to solid transition point temperature capable of being changed under pressure.

7. A phase change material (PCM) according to any preceding claim, wherein the tetrafluoroborate salts is or comprises KBF4 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.%.

8. A phase change material (PCM) according to any preceding claim, wherein the tetrafluoroborate salt may comprises a mixture of tetrafluoroborate salts of KBF4 and NH4BF4 in a ratio of: about 10 - 90 mol% of KBF4 and 10 - 90 mol % of NH4BF4; about 20 - 80 mol% of KBF4 and 20 - 80 mol % of NH4BF4; or about 30 - 60 mol% of KBF4 and 30 - 60 mol % of NH4BF4.

9. A phase change material (PCM) according to any preceding claim, wherein the tetrafluoroborate salt may comprises a mixture of tetrafluoroborate salts of KBF4 and NH4BF4 in a ratio of: about 20 mol% KBF4 and 80 mol% NH4BF4; about 40 mol% KBF4 and 60 mol% NH4BF4; about 50 mol% KBF4 and 50 mol% NH4BF4; about 60 mol% KBF4 and 40 mol% NH4BF4; or about 90 mol% KBF4 and 10 mol% NH4BF4.

10. A phase change material (PCM) according to any preceding claim, wherein the phase change material does not comprise a nucleating agent.

1 1 . A phase change material (PCM) according to any preceding claim, wherein the phase change materials (PCMs) as capable of being repeatedly thermally cycled with very little or substantially no detrimental effect and no substantial degradation on the phase change material (PCM) itself.

12. A phase change material (PCM) according to any preceding claim, wherein the phase change materials (PCMs) as capable of being repeatedly thermally cycled up to: 10 thermal cycles; 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.

13. A phase change material (PCM) according to any preceding claim, wherein the phase change material (PCM) does not comprise any stabilising additive.

14. A phase change material (PCM) according to any preceding claim, wherein the phase change material (PCM) as in a pressed (i.e. compacted) form such as a pressed pellet.

15. A phase change material (PCM) according to any preceding claim, 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)

b. Sodium (Na)

c. Potassium (K)

d. Rubidium (Rb)

e. Caesium (Cs)

f. Magnesium (Mg)

g. Calcium (Ca)

h. Strontium (Sr)

i. Barium (Ba

j. Iron (Fe)

k. Manganese (Mn)

L. Zinc (Zn)

m. Zirconium (Zr)

n. Titanium (Ti)

o. Cobalt (Co)

p. Aluminium Al)

q. Copper (Cu)

r. Nickel (Ni).

16. A phase change material (PCM) according to any preceding claim, 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.

17. A phase change material (PCM) according to any preceding claim, 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: 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.

18. A phase change material (PCM) according to any preceding claim, 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.

19. A phase change material (PCM) according to any preceding claim, 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 100°C - 200°C or about 135°C - 155°C.

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

21. A phase change material (PCM) according to any preceding claim, wherein the phase change materials (PCM) comprise any one of or combination of the following salts: UBF4, NaBF4, KBF4, RbBF4, CSBF4 and NH4BF4.

22. A phase change material (PCM) according to any preceding claim, 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+, AI3+, Ag+,: an inorganic cation, such as NH4+, N02+, NH2-NH3+ (Hydrazinium);

an organic cation, such as 1-Ethyl-3-methylimidazolium; or

other cations that may be found in an ionic liquid.

23. A phase change material (PCM) according to any preceding claim, 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+.

24. A phase change material (PCM) according to any preceding claim, 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

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

26. A phase change material (PCM) according to any preceding claim, 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 (KBF4);

NaBF4;

NH4BF4;

L1BF4;

Sr(BF4)2;

Ca(BF4)2;

NH4H(BF4)2;

(NH4)3H(BF4)4;

Ba(BF4)2;

Cr(BF4)2;

Pb(BF4)2;

Mg(BF4)2;

AgBF4;

RbBF4;

Ba(CI04)2;

CSBF4;

Zn(BF4)2;

Fe(BF4)2;

Fe(BF4)3;

Ni(BF4)2;

Ni(BF4)3;

Mn(BF4)2;

CO(BF4)2; and

Zn(BF4)2.

27. A phase change material (PCM) according to any preceding claim, wherein the tetrafluoroborate salt is a hydrate, or another solvate.

28. A phase change material (PCM) according to any preceding claim, wherein the tetrafluoroborate salt is hydrated tetrafluoroborate salt selected from any one of or combination of the following: magnesium tetrafluoroborate hexahydrate ([Mg(H20)6](BF4)2); iron tetrafluoroborate hexahydrate; the cobalt tetrafluoroborate hexahydrate; and zinc tetrafluoroborate hexahydrate.

29. A phase change material (PCM) according to any preceding claim, 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).

30. A heat battery comprising a phase change material (PCM) according to any of claims 1 to 29.

31 . A heat battery according to claim 30, wherein the phase change material (PCM) functions as a thermal storage medium.

32. A heat battery according to any of claims 30 and 31 , wherein the heat battery comprises heat exchangers and insulation.

33. A heat battery according to any of claims 30 to 32, wherein the heat battery is in the form of the receptacle comprising a cylindrical member with an attachable cap e.g. a screw-on cap.

34. Use of a solid to solid phase change material (PCM) according to any of claims 1 to 29 in transportation, automotive and barocaloric applications.