WO2014074930A1

WO2014074930A1 - Very low cost, low-viscosity phosphorus-based liquid glass for heat transfer and thermal energy storage

Info

Publication number: WO2014074930A1
Application number: PCT/US2013/069313
Authority: WO
Inventors: Justin Raade; Benjamin ELKIN; Leo Finkelstein
Original assignee: Halotechnics Inc
Current assignee: Halotechnics Inc
Priority date: 2012-11-08
Filing date: 2013-11-08
Publication date: 2014-05-15
Anticipated expiration: 2015-05-08

Abstract

A system of mixtures of oxides is disclosed. These compositions can have liquidus temperatures less than 400°C and thermal stability limits greater than 1200°C. Such oxide compositions have low viscosity over their entire range of stability, allowing them to be pumped as a heat transfer fluid and thermal storage medium with low parasitic power losses.

Description

VERY LOW COST, LOW-VISCOSITY PHOSPHORUS-BASED LIQUID GLASS FOR HEAT TRANSFER AND THERMAL ENERGY STORAGE

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Appl. No.

61/723,929, filed Nov. 8, 2013, and U.S. Provisional Patent Appl. No. 61/783,989, filed Mar. 14, 2013, the entirety of which applications are incorporated by reference in their entirety.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] The present invention was made with Government support under Award No. DE- AR0000174 awarded by the Advanced Research Projects Agency— Energy (ARPA-E). The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0003] It is imperative that we reduce our usage of fossil fuels to address pressing societal concerns - climate change and environmental degradation, energy security, and price volatility. Solar thermal power, a compelling source of renewable electricity at large scale, represents a possible solution to fossil fuel use. The recently announced SunShot Initiative from the Department of Energy (DOE) sets the aggressive goal of making solar electricity cost competitive with fossil fuels by 2020. With further technological advances, solar thermal power will become cheaper than natural gas or coal and able to provide electricity day and night. However, electricity from solar thermal power currently costs too much to be directly competitive with fossil fuel-based power. Furthermore, although solar thermal plants have the capability of storing heat in order to produce power after sundown, this represents a significant capital cost to plant developers. [0004] The solar thermal power market is an emerging industry with enormous growth potential. A recent report published by Greenpeace and the European Solar Thermal Electricity Association projects an installed capacity over 68,000 MW by 2020, enough to power over 50 million households ["Concentrating solar power outlook 2009," Greenpeace International, SolarPACES, and ESTELA, 2009]. Solar thermal power plants generate electricity by focusing sunlight using mirrors onto a receiver, then passing a fluid through the receiver to collect the heat, and finally using the heated fluid to boil water and drive a steam turbine generator. At the heart of these plants is the heat transfer fluid. It is possible to store large quantities of the heated fluid in an insulated tank during the day, and to discharge this thermal energy after sundown to continue generating power ["Survey of thermal storage for parabolic trough power plants," subcontractor report NREL/SR-550-27925, 2000]. However, this storage represents an additional capital cost to the project developer and must be made cheaper in order to economically provide power from the sun day and night. [0005] In order to achieve large scale commercial deployment and to compete with fossil fuels, there is a crucial need across the solar thermal power industry to lower costs and develop viable thermal storage. To achieve these goals solar technology developers are pushing to increase the operating temperature of their systems, thereby lowering their levelized cost of the electricity and reducing the cost of storage. High temperatures necessitate the adoption of very stable heat transfer fluids and thermal storage materials (synthetic oil and even molten salt would break down). The present invention provides a molten glass material and thermal energy storage system that will enable further advances in the solar power industry.

BRIEF SUMMARY OF THE INVENTION

[0006] In a first aspect, the invention provides an oxide containing phosphorus oxide, in an amount of from about 25 to about 90 mol%. The oxide also contains at least two oxides selected from: lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%.

[0007] In a second aspect, the invention provides a method for storing or transporting thermal energy. The method includes heating a composition with thermal energy, wherein the composition contains an oxide of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 shows the PLK phase diagram, with the 340 °C eutectic highlighted in the shaded oval. [0009] Figure 2 shows (L to R) Monofrax CS-3, CS-5, and M rods after a 200-hr corrosion test at 1100 °C.

[0010] Figure 3 shows viscosity data for glass 3i from three separate trials, measured at 400-1180 °C. [0011] Figure 4 shows heat capacity data obtained for glass 3i.

[0012] Figure 5 shows a differential scanning calorimetry (DSC) trace obtained for glass 5a, containing phosphorus oxide, sodium oxide, calcium oxide, and boron oxide.

[0013] Figure 6a shows a DSC trace obtained for Green glass (6a), containing phosphorus oxide, sodium oxide, calcium oxide, boron oxide, and vanadium oxide. Figure 6b shows thermogravimetric analysis (TGA) data for Green glass.

[0014] Figure 7 shows a DSC trace obtained for glass 7a, containing phosphorus oxide, sodium oxide, calcium oxide, boron oxide, vanadium oxide, and potassium oxide.

[0015] Figure 8 shows a DSC trace obtained for glass 8a, containing phosphorus oxide, sodium oxide, calcium oxide, boron oxide, vanadium oxide, potassium oxide, and zinc oxide. [0016] Figure 9 shows a DSC trace obtained for glass 9, containing phosphorus oxide and sodium oxide.

[0017] Figure 10 shows a DSC trace obtained for glass 10, containing phosphorus oxide, sodium oxide, and calcium oxide. DETAILED DESCRIPTION OF THE INVENTION

I. General

[0018] The present invention provides higher order oxide compositions containing phosphorous oxide and at least two oxides selected from lithium oxide, sodium oxide, potassium oxide, and boron oxide. The compositions can reduce the cost of concentrated solar power (CSP) electricity and thermal storage in two ways: 1) by enabling significantly higher operating temperatures in the plants and more effective sensible heat thermal storage, and 2) by enabling the use of a lower cost thermal storage material. The current state of the art thermal storage system is a two-tank sensible heat system using molten salt as the thermal storage media, a technology deployed in many commercial CSP plants. The high capital cost of this technology— over $100/kWht— represents a significant impediment to the construction and operation of plants with thermal storage. This high cost is primarily due to the inefficient use of the most expensive component of the system, the thermal storage medium. With sensible heat systems, the amount of thermal energy stored is directly proportional to the temperature difference of the storage material. Typical materials have a small temperature difference between the hot tank (at 400 °C) and the cold tank (at 300 °C). The present invention, on the other hand, provides storage materials with surprisingly large temperature differences. The materials of the present invention can store as much heat as conventional systems while using as little as 1/8^Λ, or less, of the storage material. This surprising reduction is further improved by reducing the cost of the storage material itself by 50% or more.

II. Definitions

[0019] As used herein, the term "oxide" refers to a compound having a least one oxygen atom bound to a non-oxygen atom. Alkali metal oxides refer to oxides of sodium, potassium, lithium, rubidium, and cesium. Useful oxides in the present invention include, but are not limited to, phosphorus oxide (P₂O₅), lithium oxide (Li₂0), sodium oxide (Na₂0), potassium oxide (K₂0), boron oxide (B₂0₃), calcium oxide (CaO), bismuth oxide (Bi₂0₃), copper oxide (CuO), vanadium oxide (V₂0₅), lead oxide (Pb₃0₄), zinc oxide (ZnO), and magnesium oxide (MgO). Other oxides can be used in the compositions of the invention.

[0020] "Thermal energy" refers to portion of energy in a system that gives rise to the temperature of the system. A "thermal source" refers to a member of the system from which thermal energy is transferred to other members of the system. Thermal energy can include, for example, radiation of varying wavelengths including, but not limited to, wavelengths in the infrared and visible portions of electromagnetic spectrum. Thermal energy can also include heat in fluids or solids which can be transferred by convection or conduction.

Thermal energy can also be generated by mechanical compression or electrical resistive elements. Thermal energy is transferred between members of the system as heat. "Heating" a composition of the invention refers to raising the temperature of the composition. Heating can be conducted by processes including, but not limited to, convection, conduction, mechanical compression, electrical resistive heating, and irradiation. Examples of thermal sources include, but are not limited to, natural sources such as sunlight and geothermal sources. Examples of thermal sources also include systems such as power plants and components thereof. III. Low-Order Alkali-Phosphate Glasses

[0021] Various low-order alkali-phosphate oxide glass eutectics are known, with liquidus and composition data displayed in Table 1. Alkali oxides of rubidium (Rb₂0) and cesium (Cs₂0) are not considered as they are extremely cost-prohibitive. [0022] Existing data is inconsistent and very limited. For example, liquidus data varies 188 °C for the phosphorus/lithium/potassium oxide system (see, entries la-le) and 35 °C for the phosphorus/sodium/potassium oxide system (see, entries lf-lj), while the

phosphorus/lithium/potassium oxide system has but one data point (see, lk). Data is available for only one four-component system (see, phosphorous/ lithium/sodium/potassium oxide; entry 11), where the 652 °C reported liquidus temperature is unexpectedly high.

Additionally, there is little variation in the molar percentage of phosphorus pentoxide across this phase space. Phosphorus pentoxide molar compositions never exceed 50%, as P₂0₅ is most often introduced as alkali-metaphosphate (X-PO3) batch component salts. Each mole of alkali-metaphosphate salt introduces equimolar quantities of P₂0₅ and alkali oxide, no doubt responsible for P2O5 compositions hovering in the neighborhood of 50% molar (the PLNK system is lower at 33% due to the presence of three alkali oxides rather than two as in other cases).

Table 1. Existing alkali-phosphate glass eutectics^a with composition data.

aSee, Phase Diagrams for Ceramists, American Ceramic Society/NIST, vol. 1-4, 7, 1964- 1989. IV. High-Order Alkali-Phosphate Glasses

[0023] The present invention provides higher-order compositions that include over 50% molar P2O5, leading to several useful alkali-phosphate glasses for heat transfer and storage. The compositions were synthesized using batch components capable of adding P₂0₅ independently of accompanying alkali oxides. Other oxides and thermally stable compounds were then added to the alkali-phosphate system so as to enhance stability and lower viscosity over very broad temperature ranges.

A. Glasses for Heat Transfer and Thermal Energy Storage

[0024] In one aspect, the invention provides glasses containing alkali metal oxides and phosphorus oxide. The oxides of the invention contain phosphorus oxide and at least two oxides selected from lithium oxide, sodium oxide, potassium oxide, and boron oxide. Unless otherwise specified, "phosphorus oxide" refers to P₂0₅, "lithium oxide" refers to Li₂0, "sodium oxide" refers to Na₂0, "potassium oxide" refers to K₂0 , and "boron oxide" refers to B₂O₃. In some embodiments, the present invention provides an oxide containing phosphorus oxide, in an amount of from about 25 to about 90 mol%, as well as at least two oxides selected from: lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%.

[0025] In some embodiments, the present invention provides an oxide consisting essentially of phosphorus oxide and two or more oxides selected from lithium oxide, sodium oxide, potassium oxide, and boron oxide in any of the amounts described herein. In some embodiments, the oxide consists of phosphorus oxide and two or more oxides selected from lithium oxide, sodium oxide, potassium oxide, and boron oxide in any of the amounts described herein. [0026] The oxides of the invention can contain any suitable amount of phosphorus oxide. In general, the oxides contain from about 25 mol% to about 90 mol% phosphorus oxide. The oxides can contain from about 25 to about 75 mol% phosphorus oxide, or from about 50 to about 75 mol% phosphorus oxide, or from about 30 to about 70 mol% phosphorus oxide, or from about 30 to about 60 mol% phosphorus oxide, or from about 25 to about 50 mol% phosphorus oxide, or from about 40 to about 50 mol% phosphorus oxide. The oxides can contain from about 50 to about 90 mol% phosphorus oxide, or about 65 to about 75 mol% phosphorus oxide. The oxides can contain about 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or about 80 mol% phosphorus oxide.

[0027] The oxides of the invention can contain any suitable amount of lithium oxide. In general, the oxides contain from about 0 mol% to about 50 mol% lithium oxide. The oxides can contain, for example, from about 1 to about 50 mol% lithium oxide. The oxides can contain from about 10 to about 50 mol% lithium oxide, or from about 20 to about 50 mol% lithium oxide, or from about 25 to about 50 mol% lithium oxide, or from about 30 to about 50 mol% lithium oxide, or from about 40 to about 50% lithium oxide, or from about 10 to about 40 mol% lithium oxide, or from about 20 to about 30 mol% lithium oxide, or from about 35 to about 45 mol% lithium oxide. The oxides can contain from about 1 to about 25 mol% lithium oxide, or from about 5 to about 15 mol% lithium oxide. The oxides can contain about

I, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, or about 25 mol% lithium oxide.

[0028] The oxides of the invention can contain any suitable amount of sodium oxide. In general, the oxides contain from about 0 mol% to about 75 mol% sodium oxide. The oxides can include, for example, from about 1 to about 75 mol% sodium oxide. The oxides can contain from about 5 to about 75 mol% sodium oxide, or from about 10 to about 75 mol% sodium oxide, or from about 25 to about 75 mol% sodium oxide, or from about 50 to about 75% sodium oxide, or from about 10 to about 60 mol%> sodium oxide, or from about 10 to about 55 mol%> sodium oxide, or from about 20 to about 50 mol%> sodium oxide, or from about 20 to about 45 mol%> sodium oxide, or from about 20 to about 40 mol%> sodium oxide, or from about 30 to about 40 mol% sodium oxide. The oxides can contain from about 1 to about 25 mol%> sodium oxide, or from about 5 to about 15 mol%> sodium oxide. The oxides can contain about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11,

I I .5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, or about 25 mol%> sodium oxide.

[0029] The oxides of the invention can contain any suitable amount of potassium oxide. In general, the oxides contain from about 0%> to about 75%> potassium oxide. The oxides can include, for example, from about 1 to about 75 mol%> potassium oxide. The oxides can contain from about 1 to about 50 mol% potassium oxide, or from about 1 to about 40 mol% potassium oxide, or from about 1 to about 30 mol% potassium oxide, or from about 1 to about 25% potassium oxide, or from about 5 to about 60 mol% potassium oxide, or from about 10 to about 60 mol% potassium oxide, or from about 10 to about 50 mol% potassium oxide, or from about 10 to about 40 mol% potassium oxide, or from about 10 to about 30 mol% potassium oxide, or from about 10 to about 20 mol% potassium oxide, or from about 30 to about 40 mol% potassium oxide. The oxides can contain about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or about 35 mol% potassium oxide.

[0030] The oxides of the invention can contain any suitable amount of boron oxide. In general, the oxides contain from about 0% to about 50 mol% boron oxide. The oxides can include, for example, from about 1 to about 50 mol% boron oxide. The oxides can contain from about 5 to about 45 mol% boron oxide, or from about 5 to about 30 mol% boron oxide, or from about 5 to about 20 mol% boron oxide, or from about 1 to about 25 mol% boron oxide, or from about 1 to about 20 mol% boron oxide, or from about 1 to about 10 mol% boron oxide, or from about 1 to about 5 mol% boron oxide. The oxides can contain about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or about 10 mol% boron oxide.

[0031] In some embodiments, the invention provides an oxide containing: phosphorus oxide, in an amount of from about 25 to about 90 mol%; and at least two oxides selected from: lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%. In some embodiments, the invention provides an oxide containing: phosphorus oxide, in an amount of from about 25 to about 75 mol%; lithium oxide, in an amount of from about 25 to about 50 mol%; and potassium oxide, in an amount of from about 1 to about 25 mol%. In some embodiments, the invention provides oxide containing phosphorus oxide, in an amount of from about 40 to about 50 mol%; lithium oxide, in an amount of from about 35 to about 45 mol%; and potassium oxide, in an amount of from about 10 to about 20 mol%. In some embodiments, the oxide contains phosphorus oxide, in an amount of about 45 mol%; lithium oxide, in an amount of about 39 mol%; and potassium oxide, in an amount of about 16 mol%. In some embodiments, the oxides contain phosphorus oxide, in an amount of from about 25 to about 75 mol%; sodium oxide, in an amount of from about 5 to about 75 mol%; and potassium oxide, in an amount of from about 5 to about 60 mol%. In some embodiments, the oxides contain phosphorus oxide, in an amount of from about 30 to about 70 mol%; sodium oxide, in an amount of from about 10 to about 55 mol%; and potassium oxide, in an amount of from about 10 to about 50 mol%.

[0032] The oxides of the invention can contain two, three, four, five, or more oxides in addition to phosphorus oxide. For example, the oxides can contain phosphorus oxide and at least three oxides selected from lithium oxide, sodium oxide, potassium oxide, and boron oxide. The oxides can contain phosphorus oxide, lithium oxide, sodium oxide, potassium oxide, and boron oxide. The oxides can contain phosphorus and boron oxide, as well as at least one oxide selected from lithium oxide, sodium oxide, and potassium oxide. The oxides contain phosphorus and boron oxide, as well as at least one oxide selected from calcium oxide, bismuth oxide, and copper oxide.

[0033] In some embodiments, the invention provides an oxide containing phosphorus oxide as described above, and further containing at least three oxides selected from: lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%. In some embodiments, the oxide contains lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%.

[0034] In some embodiments, the oxide contains phosphorus oxide, in an amount of from about 25 to about 90 mol%; and boron oxide, in an amount of from about 1 to about 10 mol%. In some such embodiments, the oxide further contains at least one oxide selected from lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; and potassium oxide, in an amount of from about 1 to about 75 mol%. In some such embodiments, the oxide further contains at least one oxide selected from calcium oxide (CaO), in an amount of from about 1 to about 25 mol%; bismuth oxide (Bi₂0₃), in an amount of from about 1 to about 10 mol%; and copper oxide (CuO), in an amount of from about 1 to about 5 mol%.

[0035] The oxides of the present invention can include one or more additional components including, but not limited to, potassium fluoride (KF), calcium fluoride (CaF₂), vanadium oxide (V₂0₅), lead oxide (Ρ¾0₄), zinc oxide (ZnO), and magnesium oxide (MgO). In some embodiments, the oxide contains phosphorus oxide, in an amount of from about 25 to about 90 mol%; boron oxide, in an amount of from about 1 to about 10 mol%; and at least one fluoride selected from calcium fluoride and potassium fluoride, in an amount of from about 1 to about 25 mol%, and calcium fluoride, in an amount of from about 1 to about 25 mol%.

[0036] In some embodiments, the oxide contains: phosphorus oxide, in an amount of from about 50 to about 90 mol%; lithium oxide, in an amount of from about 1 to about 25 mol%; sodium oxide, in an amount of from about 1 to about 25 mol%; potassium oxide, in an amount of from about 1 to about 25 mol%; and boron oxide, in an amount of from about 1 to about 10 mol%.

[0037] In some embodiments, the oxide contains: phosphorus oxide, in an amount of from about 65 to about 75 mol%; lithium oxide, in an amount of from about 5 to about 15 mol%; sodium oxide, in an amount of from about 5 to about 15 mol%; potassium oxide, in an amount of from about 5 to about 15 mol%; and boron oxide, in an amount of from about 1 to about 5 mol%.

[0038] In some embodiments, the oxide contains: phosphorus oxide, in an amount of about 72 mol%; lithium oxide, in an amount of about 8.5 mol%; sodium oxide, in an amount of about 8.5 mol%; potassium oxide, in an amount of about 8.5 mol%; and boron oxide, in an amount of about 2.5 mol%.

[0039] In general, an oxide of the invention can be prepared by weighing specific amounts of suitable starting materials and mechanically grinding or otherwise bringing the starting materials into uniform association with each other. The resulting mixture is then heated in an appropriate container, such as an alumina or porcelain crucible, in a furnace or similar apparatus. Synthesis temperatures can regulated to processes such as foaming, outgassing, and reaction of gaseous components in the batch. Examples of suitable starting materials include, but are not limited to, ammonium dihydrogen phosphate (NH₄H₂P0₄), boric acid (H₃BO₃), sodium carbonate (Na₂C0₃), sodium sulfate (Na₂S0₄), potassium carbonate (K₂C0₃), potassium sulfate (K₂S0₄), potassium fluoride (KF), lithium carbonate (Li₂C0₃), cuprous oxide (Cu₂0), bismuth trioxide (Bi₂0₃), calcium carbonate (CaC0₃), and calcium fluoride (CaF₂).

[0040] Accordingly, some embodiments of the invention provide methods for making an oxide as described herein. The methods include forming a precursor mixture containing a phosphorus oxide precursor and at least two precursors selected from a lithium oxide precursor, a sodium oxide precursor, a potassium oxide precursor, and a boron oxide precursor. The methods include heating the precursor mixture under conditions sufficient to form the oxide. In some embodiments, the phosphorus oxide precursor is ammonium dihydrogen phosphate. In some embodiments, the potassium oxide precursor is potassium carbonate. In some embodiments, the potassium oxide precursor is potassium sulfate. In some embodiments, an oxide prepared using potassium sulfate exhibits a glass melt that is more fluid than a corresponding oxide prepared using potassium carbonate. One of skill in the art will appreciate that the temperatures and heating rates used in the methods for preparing the oxides will depend in part on factors such as the particular precursor materials used and the relative quantities of the precursor materials.

B. Methods for Storing and Transporting Thermal Energy [0041] In another aspect, the present invention provides a method for storing and transporting thermal energy. The method includes heating a composition with thermal energy, wherein the composition includes and oxide containing: phosphorus oxide, in an amount of from about 25 to about 90 mol%, as well as at least two oxides selected from: lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%. Heating the composition with thermal energy causes the temperature of the composition to increase.

[0042] Any of the oxides described above can be useful in the methods of the present invention. In some embodiments, the composition includes an oxide containing: phosphorus oxide, in an amount of from about 25 to about 75 mol%; lithium oxide, in an amount of from about 25 to about 50 mol%; and potassium oxide, in an amount of from about 1 to about 25 mol%. In some embodiments, the composition includes an oxide containing phosphorus oxide, in an amount of from about 40 to about 50 mol%; lithium oxide, in an amount of from about 35 to about 45 mol%; and potassium oxide, in an amount of from about 10 to about 20 mol%. In some embodiments, the composition includes an oxide containing phosphorus oxide, in an amount of about 45 mol%; lithium oxide, in an amount of about 39 mol%; and potassium oxide, in an amount of about 16 mol%. In some embodiments, the compositions includes and oxide containing phosphorus oxide, in an amount of from about 25 to about 75 mol%; sodium oxide, in an amount of from about 5 to about 75 mol%; and potassium oxide, in an amount of from about 5 to about 60 mol%. In some embodiments, the compositions includes an oxide containing phosphorus oxide, in an amount of from about 30 to about 70 mol%; sodium oxide, in an amount of from about 10 to about 55 mol%; and potassium oxide, in an amount of from about 10 to about 50 mol%.

[0043] In some embodiments, the composition includes an oxide containing phosphorus oxide as described above, and further containing at least three oxides selected from: lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%. In some embodiments, the oxide contains lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%.

[0044] In some embodiments, the composition includes an oxide containing phosphorus oxide, in an amount of from about 25 to about 90 mol%, and boron oxide, in an amount of from about 1 to about 10 mol%. In some such embodiments, the oxide in the composition further contains at least one oxide selected from lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; and potassium oxide, in an amount of from about 1 to about 75 mol%. In some such

embodiments, the oxide in the composition further contains at least one oxide selected from calcium oxide, in an amount of from about 1 to about 25 mol%; bismuth oxide, in an amount of from about 1 to about 10 mol%; and copper oxide, in an amount of from about 1 to about 5 mol%. In some embodiments, the composition includes an oxide containing phosphorus oxide, in an amount of from about 25 to about 90 mol%; boron oxide, in an amount of from about 1 to about 10 mol%; and at least one fluoride selected from potassium fluoride, in an amount of from about 1 to about 25 mol%, and calcium fluoride, in an amount of from about 1 to about 25 mol%. [0045] In some embodiments, the composition includes and oxide containing: phosphorus oxide, in an amount of from about 50 to about 90 mol%; lithium oxide, in an amount of from about 1 to about 25 mol%; sodium oxide, in an amount of from about 1 to about 25 mol%; potassium oxide, in an amount of from about 1 to about 25 mol%; and boron oxide, in an amount of from about 1 to about 10 mol%.

[0046] In some embodiments, the composition includes and oxide containing: phosphorus oxide, in an amount of from about 65 to about 75 mol%; lithium oxide, in an amount of from about 5 to about 15 mol%; sodium oxide, in an amount of from about 5 to about 15 mol%; potassium oxide, in an amount of from about 5 to about 15 mol%; and boron oxide, in an amount of from about 1 to about 5 mol%.

[0047] In some embodiments, the composition includes and oxide containing: phosphorus oxide, in an amount of about 72 mol%; lithium oxide, in an amount of about 8.5 mol%;

sodium oxide, in an amount of about 8.5 mol%; potassium oxide, in an amount of about 8.5 mol%; and boron oxide, in an amount of about 2.5 mol%.

[0048] In some embodiments, compositions consisting essentially of any of the oxides described above are used in the methods of the invention. In some embodiments,

compositions consisting of any of the oxides described above are used in the methods of the invention.

[0049] As described above, heating the composition with or otherwise exposing the composition to thermal energy causes the temperature of the composition to increase. The temperature of the composition can increase, for example, by from about 1 °C to about 750 °C. In some embodiments, the temperature of the composition can increase by about 5 °C, 10, 25, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, or by about 700 °C. The temperature of the composition can increase to a temperature of about 30, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700 °C, or greater. Other temperatures or temperature increases can be achieved depending on factors including, but not limited to, the source of the thermal energy and the environment in which the composition is placed. [0050] Any suitable source of thermal energy can be used in the methods of the invention. Useful thermal sources include, but are not limited to, a concentrated solar power plant, a fossil fuel power plant, a nuclear power plant, sunlight, a geothermal source, fuel combustion, and a heat pump. The thermal source can also be waste heat from an industrial process (e.g., metal production or glass production). In some embodiments, the invention provides a method for storing thermal energy as described above, wherein the thermal energy originates from a source selected from the group consisting of sunlight, a concentrated solar power plant, a fossil fuel power plant, fuel combustion, an industrial process, and a heat pump. V. Examples

Example 1 : Phosphorus-lithium-potassium (PLK) reference composition

[0051] The PLK system studied corresponds to le as listed in Table 1, the phase diagram for which is shown Figure 1. This specific composition was of interest due to its reported 340 °C melting point, the lowest melting data reported in any alkali-phosphate glass literature.

[0052] Due to the geometry of this phase space, the alleged 340 °C eutectic composition can be synthesized from a variety of batch components. Arguably the simplest route to synthesis was calculated as 29.4 mol% K₃PO₄ and 70.6 mol% (LiP0₃)₃. A small, 10 g batch was synthesized and observed qualitatively during stages of melting. The PLK glass was melted at 800 °C with little outgassing, forming a clear, vitreous, and fluid liquid. Upon cooling to 500 °C, vitreous glass character was maintained but with steeply increased viscosity. By 425-400 °C, the glass no longer flowed under the influence of gravity.

Although DSC analysis was never conducted with the PLK glass, the 340 °C reported eutectic might realistically correspond to the glass transition temperature rather than a temperature at which the glass might function as a suitable heat transfer and thermal energy storage fluid.

Example 2: Phosphorous-sodium-potassium (PNK) glasses

[0053] Utilizing a high-throughput chemistry rapid screening workflow, compositions of phosphorus pentoxide (via ammonium di-hydrogen phosphate), sodium oxide (via sodium carbonate), and potassium oxide (via potassium carbonate) were screened across broad intervals.

[0054] Various aspects of the screening procedure are described in United States Patent Application Pub. No. 2012/0056125 Al, the entirety of which is incorporated herein by reference. Briefly, components were ground with a mortar and pestle and dehydrated in an oven at elevated temperatures. Glass mixtures were formulated using automated robotic systems for dispensing powdered and liquid materials. The powder dispensing system was the MTM Powdernium from Symyx Technologies (Sunnyvale, California), which measures each component as it is being dispensed and records the final weight with high accuracy. The mixtures were dispensed into a plate containing 24 wells of small ceramic crucibles in a 4x6 array. After dispensing, the plate was placed in a furnace and heated to for a period of time sufficient to ensure complete melting and homogenization of each mixture. After melting the plate was allowed to cool and stored in a desiccator until subsequent testing.

[0055] Overall, 48 samples were synthesized in 250 mg - 500 mg batches and measured via Netzsch DSC Fl 404 Pegasus for glass transition temperature, liquidus temperature, and heat of fusion, where available.

Example 3 : Alkali phosphate glasses

[0056] A number of larger batch compositions (-200 g each) were synthesized for qualitative screening. Overall, thirteen distinct compositions were prepared with the inclusion of twelve different batch components. The experimental range of compositions is summarized in Table 2 as "Experiment No. 3," with detailed composition information recorded in Table 3.

Table 2. Alkali-phosphate glass compositions

[0057] These series of formulations exhibited important characteristics, including:

acceptable thermal stability and fluidity over the temperature range 400 - 1200 °C; low cost; and low toxicity.

[0058] Weight percent conversions from starting batch materials to final oxide materials can be calculated as follows.

[0059] NH₄H₂PO₄ (g) * (141.95 (g/mol) P₂O₅)/(230.06 (g/mol) NH₄H₂P0₄) = P₂0₅ (g) [0060] H₃BO₃ (g) * (61.83 (g/mol) B₂0₃)/(123.66 (g/mol) (H₃B0₃)₂) = B₂0₃ (g) [0061] Na₂C0₃ (g) * (61.98 (g/mol) Na₂O)/(105.99 (g/mol) Na₂C0₃) = Na₂0 (g) [0062] Na₂S0₄ (g) * (61.98 (g/mol) Na₂O)/(142.04 (g/mol) Na₂S0₄) = Na₂0 (g) [0063] K₂C0₃ (g) * (94.2 (g/mol) K₂0)/(138.21 (g/mol) K₂C0₃) = K₂0 (g)

[0064] K₂S0₄ (g) * (94.2 (g/mol) K₂0)/(174.26 (g/mol) K₂S0₄) = K₂0 (g)

[0065] Li₂C0₃ (g) * (29.88 (g/mol) Li₂0)/(73.89 (g/mol) Li₂C0₃) = Li₂0 (g)

[0066] CaC0₃ (g) * (56.07 (g/mo) CaO)/(l 00.09 (g/mol) CaC0₃) = CaO (g)

Table 3a. Experimental summary of large-batch alkali phosphate glass compositions

Table 3b. Experimental summary of large-batch alkali phosphate glass compositions

3m 63.9 2.3 7.6 7.6 7.7 10.9

[0067] The large batch formulations were synthesized in order to characterize the high- phosphate (>50 mol%) glass phase space. For other systems, the phosphate concentrations have hovered around 50% as a result of 1 :1 alkali-phosphate-containing batch processing materials. In order to prepare the high-phosphate glasses, batch materials that facilitated P₂O₅ addition without the inclusion of alkali oxides were evaluated. In early iterations, phosphoric acid (H₃PO₄) proved too hygroscopic to process in open air. After surveying other batch materials, ammonium di-hydrogen phosphate (ADP, or NH₄H₂PO₄) was identified. [0068] ADP adds half a mole of P₂O₅ for every mole of batch component and exhibits low hygroscopic behavior. ADP also adds a considerable weight percentage of ammonia (NH₃) and water (H₂0) to the melt. During heating, considerable outgassing occurs, such that the heating rate must be carefully controlled to prevent excessive foaming and subsequent furnace damage. This challenge aside, ADP's gas content is believed to be beneficial to the melt's overall fluidity. Without wishing to be bound by any particular theory, trapped gaseous molecules are believed to increase the concentration of non-bonding oxygen in the melt, leading to decreased viscosity as a result of less internal molecular resistance to flow.

[0069] While many traditional glass systems use alkali carbonate salts {i.e., X-CO₃) as a source of alkali oxides, the present invention was made with certain alkali sulfates {i.e., X- SO₄). This proved beneficial to the glass' overall stability and fluidity, particularly when potassium sulfate (K₂SO₄) was used to replace potassium carbonate (K₂CO₃) as a batch material. Given high enough synthesis temperatures, both candidate batch components decompose into potassium oxide and a gaseous byproduct (C0₂ and S0₂/S0₃, respectively). However, K₂SO₄ has greater thermal stability than K₂CO₃, resulting in a higher concentration of SO_x gas in the melt. This is believed to contribute to a more fluid glass melt, where the gases create non-bonding oxygen connections that reduce the liquid's resistance to flow. Another benefit of K₂SO₄ is that it is available at a third of the cost of K₂CO₃.

[0070] Each composition in Table 3 was weighed using an analytical balance, mechanically ground via mortar and pestle, and synthesized in alumina or porcelain crucibles in time-and- temperature programmable furnaces capable of reaching 1200 °C. While no DSC thermal analysis (liquidus, glass transition, heat capacity, heat of fusion) was initially conducted with these samples, viscosity and crystallization dynamics were qualitatively observed over all temperatures (400 - 1200 °C) through numerous thermal cycles to assess repeatability.

Example 4: Physical characterization of a novel alkali-phosphate glass.

[0071] The chemical composition of glass 3i resulted in an unexpected combination of favorable physical properties. Of the 62 glass samples surveyed, glass 3i exhibited superior fluid properties, thermal properties, materials compatibility. The glass itself is completely clear, which can be advantageous in a solar receiver designed to utilize infrared radiation.

[0072] Corrosion and materials compatibility tests using the glass 3i were carried out at high temperature. In these tests, -50 g of the glass were melted in covered alumina crucibles while rods of candidate refractory ceramics were suspended in the melt for 200 hours at 1100 °C. After the test, refractory samples were cross-sectioned and corrosion assessed. The first round of testing saw Monofrax CS-3, CS-5, and M samples exhibit a substantial ring of corrosion at the glass-air-ceramic interface (see Figure 2), as is common in the glass processing industry. [0073] Long-term compatibility tests were conducted in graphite crucibles at high temperature, where the beneficial fluid properties of the glass were restored at low (400 °C) temperatures with no corrosion to the graphite crucible. Because graphite (carbon) crucibles are susceptible to burning in air at high temperature, an inert (nitrogen, argon, etc.) cover gas should be used for processing. Graphite showed excellent compatibility with the glass, where traditional alumina-based refractory ceramics were very sensitive to corrosion from the high- phosphate glass composition.

[0074] Due to the nature of heat transfer and thermal energy storage applications, material stability of the glass was studied during repeated thermal cycling. This was tested by rapidly transferring a graphite crucible of the molten glass between furnaces set at 450 °C and 1100 °C. The glass was allowed to equilibrate in each respective furnace for at least one hour, and the procedure was repeated ten times consecutively. After the last cycle, the glass showed no signs of crystallization or devitrification and retained its glassy, fluid character.

[0075] A significant weight percent of glass batch components will be outgassed during melting, where a majority of the gas released will be NH₃. There will also be smaller quantities of C0₂, H₂0, and SO_x released during batch component melting. As a result, slow, controlled heating to 500 °C should be conducted with foaming and outgassing occurring below this temperature. Careful control can prevent violent reaction of the gaseous elements incorporated into the batch. After holding at 500 °C for several hours, the melt can be ramped to 1100 °C for final outgassing. Graphite crucible/liner materials, including an inert N₂ gas purge should be used, because phosphate glasses are particularly sensitive to corrosion and pollution from alumina-based refractory ceramics.

[0076] Viscosity was subsequently measured using a molten glass viscometer (Orton 1600V) at 400 - 1180 °C, with results summarized in Table 4 and shown in Figure 3. The viscosity of the glass over these temperatures is well within the range necessary for use with molten glass pump apparatuses. [0077] The heat capacity of the glass was measured using a Netzsch Fl Pegasus 404 autosampler DSC. Heat capacity data is displayed below in Figure 4.

Table 4. Viscosity data for glass 3i.

[0078] Additional glasses were synthesized with components as summarized in Table 5. Table 5. Multi-component oxide glasses of the invention.

Example 5: Na^-CaO-P^-B^ Glass

[0079] Quaternary oxides containing sodium (I), phosphorus (V), calcium (II), and boron (III) oxides were synthesized as summarized in Experiment No. 5 in Table 5. Na₂0, P₂0₅, and CaO were kept in the same ratio relative to one another, while the levels of boron trioxide were varied over eleven compositions. The best overall composition tested included approximately 5% by weight of boron trioxide, as summarized in Table 6.

[0080] The liquidus temperature (410 °C) for glass 5a was measured via differential scanning calorimetry (DSC), with the data trace depicted in Figure 5. Based upon high temperature furnace tests with glasses of similar composition, the glass is expected to be stable to 1200 °C with minimal weight loss or thermal decomposition for heat transfer and thermal storage applications.

Example 6: Na?Q-CaO-P?0_s-B?C -V?Cs glass (Green glass)

[0081] This is a molten glass oxide composition comprised of sodium (I), phosphorus (V), calcium (II), boron (III), and vanadium (V) oxides. The original Na₂0-CaO-P₂0₅-B₂0₃ components were kept in the same ratio relative to one another, while the levels of vanadium pentoxide were varied over six compositions. The best overall composition tested included approximately 8% by weight of vanadium pentoxide. See, Table 7. The glass is referred to here as Green glass because of the emerald green color it assumes upon quench cooling. Table 7. Composition of glass 6a (Green glass)

[0082] The liquidus temperature (422 °C) for the Green glass was measured via differential scanning calorimetry (DSC), with the data trace depicted in Figure 6a. Additionally, the DSC measured a heat capacity of 1.44 J/g-K. Green glass is stable to 1200 °C, as measured by negligible weight loss in a muffle furnace at this upper temperature limit for several hours. Green glass was also subjected to thermo gravimetric analysis (TGA) to show that there was minimal weight loss at temperatures up to 1000 °C (the maximum operating range of that instrument). See, Figure 6b.

Example 7: Green glass + K?0 [0083] This is a molten glass oxide composition comprised of sodium (I), phosphorus (V), calcium (II), boron (III), vanadium (V), and potassium (I) oxides. The original Green glass components were kept in the same ratio relative to one another, while the levels of potassium oxide were varied to replace sodium oxide over five compositions. The best overall composition tested, glass 7a, included approximately 16% by weight of potassium oxide. See, Table 8:

Table 8. Composition of Green glass + K?Q (glass 7a)

[0084] The liquidus temperature (370 °C) for glass 7a was measured via differential scanning calorimetry (DSC), with the data trace depicted Figure 7. This represents an improvement over the Green glass sample in Example 6, as an earlier melting onset and much lower viscosity than the Green glass were observed. Based upon high temperature furnace tests with glasses of similar composition, glass 7a is expected to be stable to 1200 °C with minimal weight loss or thermal decomposition for heat transfer and thermal storage applications. Example 8: Green glass + K?Q + ZnO

[0085] This is a molten glass oxide composition comprised of sodium (I), phosphorus (V), calcium (II), boron (III), vanadium (V), potassium (I), and zinc (II) oxides. The original glass 7a components were kept in the same ratio relative to one another, while the levels of zinc oxide were varied to replace some calcium oxide over three compositions. The best overall composition tested, glass 8a, included approximately 2% by weight of zinc oxide. See, Table 9:

Table 9. Composition of Green glass + ?0 + ZnO (glass 8a)

[0086] The liquidus temperature (380 °C) for glass 8a was measured via differential scanning calorimetry (DSC), with the data trace depicted in Figure 8. Earlier melting onset and liquidus temperatures were observed, along with much lower viscosity, than the other glasses. Based upon high temperature furnace tests with glasses of similar composition, glass 8a is expected to be stable to 1200 °C with minimal weight loss or thermal decomposition for heat transfer and thermal storage applications.

Example 9: E490 (glass 9)

[0087] E490 is a glass composition of sodium (I) and phosphorus (V) oxides derived from the extensive literature of binary and ternary oxide phase diagrams and eutectics.

[0088] E490 was synthesized at the eutectic region, with mol% 55.9 Na₂0 - 44.1 P₂0₅. This composition's glass transition (386 °C) and liquidus temperatures (580 °C) were measured via differential scanning calorimetry (DSC), with the data trace depicted in Figure 9. Based upon high temperature furnace tests with glasses of similar composition, E490 is expected to be stable to 1200 °C with minimal weight loss or thermal decomposition for heat transfer and thermal storage applications. Example 10: E410 (glass 10)

[0089] E410 is a glass composition of sodium (I), phosphorus (V), and calcium (II) oxides derived from the extensive literature of binary and ternary oxide phase diagrams and eutectics [4]. This was the last composition screened before moving to higher-order oxide

compositions, for which there exists little phase diagram literature.

[0090] E410 was synthesized at the eutectic region, with mol% 49.1 Na₂0 - 44.2 P₂0₅ - 6.7 CaO. E410's glass transition (283 °C) and liquidus temperatures (495 °C) were measured via differential scanning calorimetry (DSC), with the data trace depicted in Figure 10. The addition of calcium oxide to E490 results in E410, with an 85 °C liquidus melting point reduction. Based upon high temperature furnace tests with glasses of similar composition, E410 is expected to be stable to 1200 °C with minimal weight loss or thermal decomposition for heat transfer and thermal storage applications. [0091] Molten glasses were visually observed in a molten state by manual agitation in a crucible. These observations were compared to Brookfield viscosity standards manually agitated in ajar to gain a qualitative understanding as to the glass viscosity at various temperatures. Although still viscous, glasses 9, 10, and 5a flowed at approximately 600 °C. Green glass, glass 6a, flowed nicely at 550 °C, while glasses 7a and 8a flowed at 525 °C and 500 °C, respectively.

[0092] Although the foregoing has been described in some detail by way of illustration and example for purposes of clarity and understanding, one of skill in the art will appreciate that certain changes and modifications can be practiced within the scope of the appended claims. In addition, each reference provided herein is incorporated by reference in its entirety to the same extent as if each reference was individually incorporated by reference.

Claims

WHAT IS CLAIMED IS: 1. An oxide comprising:

phosphorus oxide, in an amount of from about 25 to about 90 mol%; and at least two oxides selected from the group consisting of:

lithium oxide, in an amount of from about 1 to about 50 mol%, sodium oxide, in an amount of from about 1 to about 75 mol%, potassium oxide, in an amount of from about 1 to about 75 mol%, and boron oxide, in an amount of from about 1 to about 50 mol%.

2. The oxide of claim 1, comprising from about 50 to about 90 mol% phosphorus oxide.

3. The oxide of claim 1 , comprising from about 65 to about 75 mol% phosphorus oxide.

4. The oxide of claim 1, comprising:

phosphorus oxide, in an amount of from about 25 to about 75 mol%; lithium oxide, in an amount of from about 25 to about 50 mol%; and potassium oxide, in an amount of from about 1 to about 25 mol%.

5. The oxide of claim 1, comprising:

phosphorus oxide, in an amount of from about 40 to about 50 mol%; lithium oxide, in an amount of from about 35 to about 45 mol%; and potassium oxide, in an amount of from about 10 to about 20 mol%.

6. The oxide of claim 1, comprising:

phosphorus oxide, in an amount of about 45 mol%;

lithium oxide, in an amount of about 39 mol%; and

potassium oxide, in an amount of about 16 mol%.

7. The oxide of claim 1, comprising:

phosphorus oxide, in an amount of from about 25 to about 75 mol%; sodium oxide, in an amount of from about 5 to about 75 mol%; and potassium oxide, in an amount of from about 5 to about 60 mol%.

8. The oxide of claim 1, comprising: phosphorus oxide, in an amount of from about 30 to about 70 mol%;

sodium oxide, in an amount of from about 10 to about 55 mol%; and potassium oxide, in an amount of from about 10 to about 50 mol%.

9. The oxide of claim 1, comprising at least three oxides selected from the group consisting of:

lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; potassium oxide, in an amount of from about 1 to about 75 mol%; and boron oxide, in an amount of from about 1 to about 50 mol%.

10. The oxide of claim 1, comprising:

11. The oxide of claim 1 , comprising:

phosphorus oxide, in an amount of from about 25 to about 90 mol%; and boron oxide, in an amount of from about 1 to about 10 mol%.

12. The oxide of claim 11, further comprising at least one oxide selected from the group consisting of:

lithium oxide, in an amount of from about 1 to about 50 mol%; sodium oxide, in an amount of from about 1 to about 75 mol%; and potassium oxide, in an amount of from about 1 to about 75 mol%.

13. The oxide of claim 11, further comprising at least one oxide selected from the group consisting of:

calcium oxide, in an amount of from about 1 to about 25 mol%; bismuth oxide, in an amount of from about 1 to about 10 mol%; and copper oxide, in an amount of from about 1 to about 5 mol%.

14. The oxide of claim 11, further comprising at least one fluoride selected from the group consisting of:

potassium fluoride, in an amount of from about 1 to about 25 mol%; and calcium fluoride, in an amount of from about 1 to about 25 mol%.

15. The oxide of claim 1, comprising

phosphorus oxide, in an amount of from about 50 to about 90 mol%;

lithium oxide, in an amount of from about 1 to about 25 mol%; sodium oxide, in an amount of from about 1 to about 25 mol%; potassium oxide, in an amount of from about 1 to about 25 mol%; and boron oxide, in an amount of from about 1 to about 10 mol%.

16. The oxide of claim 1, comprising

phosphorus oxide, in an amount of from about 65 to about 75 mol%;

lithium oxide, in an amount of from about 5 to about 15 mol%; sodium oxide, in an amount of from about 5 to about 15 mol%; potassium oxide, in an amount of from about 5 to about 15 mol%; and boron oxide, in an amount of from about 1 to about 5 mol%.

17. The oxide of claim 1, comprising

phosphorus oxide, in an amount of about 72 mol%;

lithium oxide, in an amount of about 8.5 mol%;

sodium oxide, in an amount of about 8.5 mol%;

potassium oxide, in an amount of about 8.5 mol%; and

boron oxide, in an amount of about 2.5 mol%.

18. A method for preparing an oxide of any one of claim 1 to 17, the method comprising:

forming a precursor mixture comprising a phosphorus oxide precursor and at least two precursors selected from the group consisting of a lithium oxide precursor, a sodium oxide precursor, a potassium oxide precursor, and a boron oxide precursor; and

heating the precursor mixture under conditions sufficient to form the oxide.

19. The method of claim 18, wherein the phosphorus oxide precursor comprises ammonium dihydrogen phosphate.

20. The method of claim 18, wherein the potassium oxide precursor comprises potassium sulfate.

21. A method for storing or transporting thermal energy comprising heating a composition with thermal energy, wherein the composition comprises an oxide of any one of claims 1 to 17.

22. The method of claim 21, wherein the composition comprises an oxide of claim 17.

23. The method of claim 21 , wherein the thermal energy originates from a source selected from the group consisting of sunlight, a concentrated solar power plant, a fossil fuel power plant, fuel combustion, an industrial process, and a heat pump.