Method for producing metal borides

Abstract

This method for producing a metal boride comprises preparing: a first unheated raw material containing a metal source that is a metal, a metal oxide, a metal-containing carbonate or a metal-containing hydroxide, boron oxide or boric acid, silicon dioxide, and metallic sodium; or a second unheated raw material containing the metal source, boron oxide or boric acid, and silicon dioxide. Further, this method for producing a metal boride further comprises: heating the first unheated raw material if the first unheated raw material has been prepared; and heating the second unheated raw material in Na vapor if the second unheated raw material has been prepared.

Description

Technical Field

[0001] The present disclosure relates to a method for producing metal borides.

Background Art

[0002] Metal borides have high hardness, high melting points, excellent electrical conductivity, etc., and are used as various functional materials. For example, TiB2 is used as a superhard material (e.g., for grinding and polishing applications), a food-resistant material, an evaporation boat, etc., ZrB2 is used as a heat-resistant ceramic, a refractory material, a superhard material, etc., WB2 is used as a wear-resistant, food-resistant material, etc., and LaB6 is used as a cathode material, a heat ray shielding material, etc. In addition, there are various engineering ceramics.

[0003] Patent Document 1 discloses a method for producing lanthanum boride. Patent Document 1 discloses adding calcium hydride (CaH2) as a reducing agent to powders of lanthanum oxide (La2O3) and boron oxide (B2O3), and heating to 600 °C or higher in a hydrogen atmosphere to reduce and react lanthanum oxide and boron oxide to produce LaB6.

[0004] Patent Document 2 discloses a method for growing a lanthanum hexaboride single crystal by the floating zone method, wherein a good quality and large-sized lanthanum hexaboride single crystal can be grown by setting the melt zone composition B / La (atomic ratio) to 6 to 60 and a growth rate of 1 to 10 cm / hr.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the method described in Patent Document 1, CaO and unreacted CaH2 remain in the reactant. Therefore, the reactant is left in humid air for about 3 days to allow the CaO and CaH2 to weather, and the calcium is dissolved in a dilute acetic acid aqueous solution to leave only LaB6 as the residue, which is then separated using filter paper. Consequently, it takes time and effort to obtain the target substance, the metal boride. In the method described in Patent Document 2, heating to about 1800°C is required, and the floating zone method is a method of growing single crystals using relatively complex equipment, resulting in high costs. There was a need for an easy and inexpensive method to produce metal borides.

[0007] This disclosure was made to solve the problems described above and aims to provide a method for producing metal borides in an easy and inexpensive way. [Means for solving the problem]

[0008] The method for producing metal borides according to this disclosure includes preparing raw materials before heating, which include a metal source that is a metal, a metal oxide, a metal-containing carbonate, or a metal-containing hydroxide, boron oxide or boric acid, silicon dioxide, and metallic sodium, and heating the raw materials before heating. This method allows for the production of metal borides at a temperature much lower than 1800°C, and also allows for the use of inexpensive materials such as silicon dioxide. In another aspect, the method for producing metal borides according to this disclosure comprises preparing a raw material before heating, which includes a metal source that is a metal, a metal oxide, a metal-containing carbonate, or a metal-containing hydroxide, boron oxide or boric acid, and silicon dioxide, and heating the raw material before heating in Na vapor. With this method, washing with water is possible immediately after heating, and there is no need to remove excess Na. In addition, the crucible can be reused, for example.

[0009] Other features of this disclosure are outlined below. [Effects of the Invention]

[0010] Metal borides can be produced in an easy and inexpensive manner.

Brief Description of Drawings

[0011] [Figure 1] It is an example of a flowchart of a method for producing a metal boride. [Figure 2] It is the result of X-ray diffraction of powders obtained at different heating temperatures. [Figure 3] It is the result of X-ray diffraction of the sample before Na removal and before water washing, and the sample after water washing. [Figure 4] It is a diagram showing the SEM photograph of TaB2 and the result of composition analysis by the EDX method. [Figure 5] It is the result of X-ray diffraction of powders obtained at different heating times. [Figure 6] It is the result of X-ray diffraction of powders obtained from raw materials with different composition ratios of B2O3. [Figure 7] It is the result of X-ray diffraction of powders obtained from raw materials with different composition ratios of Na. [Figure 8] It is the result of X-ray diffraction of the substance obtained by using Ta2O5 in the raw material before heating. [Figure 9] It is the result of X-ray diffraction of the substance obtained by using Ti in the raw material before heating. [Figure 10] It is a diagram showing the SEM photograph of TiB2 and the result of composition analysis by the EDX method. [Figure 11] It is the result of X-ray diffraction of the substance obtained by using Nb in the raw material before heating. [Figure 12] It is the result of X-ray diffraction of the substance obtained by using Zr in the raw material before heating. [Figure 13] It is the SEM photograph of ZrB2. [Figure 14] It is the result of X-ray diffraction of the substance obtained by using ZrO2 in the raw material before heating. [Figure 15] It is the result of X-ray diffraction of the substance obtained by using W in the raw material before heating. [Figure 16] It is the result of X-ray diffraction of the substance obtained by using WO3 in the raw material before heating. [Figure 17]This is the result of X-ray diffraction of the substance obtained using La2O3 for the raw material before heating. [Figure 18] This is a flowchart showing the method for manufacturing a metal boride according to Embodiment 2. [Figure 19] This is the result of X-ray diffraction measurement of Example A. [Figure 20] This is the result of X-ray diffraction measurement of Example B. [Figure 21] This is the result of X-ray diffraction measurement of Example C-1. [Figure 22] This is the result of X-ray diffraction measurement of Example C-2. [Figure 23] This is the result of X-ray diffraction measurement of Example C-3. [Figure 24] This is the result of X-ray diffraction measurement of Example C-3’. [Figure 25] This is the result of X-ray diffraction measurement of Example D-1. [Figure 26] This is the result of X-ray diffraction measurement of Example D-2. [Figure 27] This is a SEM photograph of HfB2. [Figure 28] This is the result of X-ray diffraction measurement of Example E-1. [Figure 29] This is the result of X-ray diffraction measurement of Example E-2. [Figure 30] This is the result of X-ray diffraction measurement of Example E-3. [Figure 31] This is the result of X-ray diffraction measurement of Example F. [Figure 32] This is the result of X-ray diffraction measurement of Example G. [Figure 33] This is the result of X-ray diffraction measurement of Example H. [Figure 34] This is a SEM photograph of LaB6. [Figure 35] This is the result of X-ray diffraction measurement of Example I-1. [Figure 36] This is the result of X-ray diffraction measurement of Example I-2. [Figure 37] This is the result of X-ray diffraction measurement of Example I-3. [Figure 38] This is a SEM photograph of CeB6. [Figure 39] These are the results of the X-ray diffraction measurement for Example I-4. [Figure 40] This is an SEM image of the obtained metal boride. [Figure 41] These are the results of the X-ray diffraction measurement for Example I-5. [Figure 42] This is an SEM image of the obtained metal boride. [Figure 43] These are the results of the X-ray diffraction measurement for Example I-6. [Figure 44] This is an SEM image of the obtained metal boride. [Figure 45] These are the results of the X-ray diffraction measurement for Example I-7. [Figure 46] This is an SEM image of YbB6. [Figure 47] These are the results of the X-ray diffraction measurement for Example I-8. [Figure 48] This is a flowchart showing the method for producing metal borides according to Embodiment 3. [Figure 49] This shows the results of the X-ray diffraction measurement of Example J-0. [Figure 50] This is an SEM image of LaB6. [Figure 51] This shows the results of the X-ray diffraction measurement of Example J-0'. [Figure 52] This shows the results of the X-ray diffraction measurement for Example J-1. [Figure 53] This is an SEM image of LaB6. [Figure 54] This shows the results of the X-ray diffraction measurement for Example J-2. [Figure 55] This shows the results of the X-ray diffraction measurement for Example J-3. [Figure 56] This is the result of the X-ray diffraction measurement of Example J-3'. [Modes for carrying out the invention]

[0012] Embodiment 1. Figure 1 is a flowchart showing a method for producing metal boride according to an embodiment. In step S10, a pre-heating raw material is prepared, which includes a metal or metal oxide, boron oxide, silicon dioxide, and metallic sodium, as the raw material for producing the metal boride. In one example, the metal contained in this metal or metal oxide is a transition metal of groups 3 to 6 of the periodic table. In this case, the metal or metal oxide is provided to be a transition metal of groups 3 to 6 or an oxide of any of them. In another example, the metal contained in the metal or metal oxide is a rare earth element. In this case, the metal or metal oxide is provided to be a rare earth element or an oxide of any of them. In yet another example, the metal contained in the metal or metal oxide is a transition element of groups 4 to 6 of periods 4, 5, and 6 of the periodic table. In this case, the metal or metal oxide is provided to be Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, or an oxide of any of these. In yet another example, the metal or metal oxide is provided to be Ta, Ti, Zr, Nb, W, La, or an oxide of any of these.

[0013] In one example, the boron oxide contained in the raw material before heating is B2O3. In addition to the above-mentioned metal or metal oxide and boron oxide, silicon dioxide (SiO2) and metallic sodium (Na) are provided. The raw material before heating can be provided in various forms. In one example, the raw material before heating is prepared by first placing the metal or metal oxide, boron oxide, and silicon dioxide in a crucible, and then adding block-shaped metallic sodium to the crucible. In another example, the raw material before heating is prepared by first placing a compacted molded body (pellet) containing boron oxide and silicon dioxide, and the metal or metal oxide in a crucible, and then adding metallic sodium to the crucible. In yet another example, the raw material powder containing the metal or metal oxide, boron oxide, silicon dioxide, and metallic sodium is mixed in a mortar and pestle to prepare the raw material before heating. In any of these examples, the raw materials may be mixed at any stage of preparation of the raw material before heating, or the mixing of the raw materials may be omitted. Even without mixing the raw materials, a certain degree of reaction can be obtained by heating because the melting point of Na is low, around 100°C. However, the method of providing the raw materials before heating is not limited to these, and any method can be used.

[0014] Next, the process proceeds to step S12. In step S12, the raw material to be heated in step S10 is heated to, for example, 800°C or higher. In another example, this raw material to be heated to 1100°C or higher. The heating time is, for example, 2 hours or more. In one example, the raw material to be heated in a crucible is heated using a well-known heating method. The material of the crucible is, for example, BN, but other materials may be used.

[0015] This heat treatment produces Na2SiO3 and metal borides. Since sodium silicate (Na2SiO3) is a readily synthesized and stable compound, it is thought that the formation of sodium silicate leads to the reduction of boron oxide, resulting in a product primarily composed of boron. The reaction mechanism can be summarized as follows: B2O3+3SiO2+6Na→3Na2SiO3+2B (Formula 1) In this embodiment, a metal or metal oxide is added to the left side of chemical equation 1, so that B, which is shown on the right side of equation 1, reacts with the metal or metal oxide to produce a metal boride. Furthermore, when a metal oxide is used as a raw material, the oxygen in the metal oxide can serve as a raw material for sodium silicate.

[0016] For example, the expected reaction equation when Ta is used as the metal or metal oxide is as shown in Equation 2 below. Ta+B2O3+3SiO2+6Na→TaB2+3Na2SiO3...(Formula 2) For this reaction equation, the change in free energy ΔG at 1027°C was a large negative value of -581 kJ / mol.

[0017] In another example, the expected reaction equation when Ta2O5 is used as the metal or metal oxide is as shown in Equation 3 below. Ta2O5+2B2O3+11SiO2+22Na→2TaB2+11Na2SiO 3 ··· (Formula 3) For this reaction equation, the change in free energy ΔG at 1027°C was a large negative value of -1912 kJ / mol.

[0018] The reason why the free energy changes in the reaction equations 2 and 3 are large negative values ​​is that sodium silicate (Na2SiO3) is very stable and has a large negative formation energy. In addition to the metals or metal oxides mentioned above, metal borides can also be synthesized using these metals or metal oxides as raw materials.

[0019] Next, the process proceeds to step S14. Step S14 is a step to remove residual Na. In step 14, for example, heating under reduced pressure or ethanol injection is performed. When the substance in the crucible is heated under reduced pressure, Na evaporates or sublimes outside the crucible. When ethanol is injected into the crucible, Na becomes sodium alkoxide, which can be removed by washing with water. Note that the washing process washes away both Na and sodium silicate. Therefore, if ethanol is injected and the process is washed with water, the washing process in step S16 can be omitted. Since step S14 is a step to remove Na, it can be omitted if there is no residual Na or if the amount is negligibly small.

[0020] Next, the process proceeds to step S16. In step S16, a water washing treatment is performed. Since sodium silicate (Na2SiO3) is water-soluble, when the sodium silicate and metal boride synthesized in step S12 are treated with water, the sodium silicate (Na2SiO3) can be washed away with water, and the metal boride can be extracted. Therefore, the theoretical yield of metal boride is 100%. Furthermore, the aqueous solution of sodium silicate is highly viscous and can be used as water glass. Water glass is the sodium salt of metasilicic acid. For example, by reacting water glass with a material that reacts with water glass, a strong solidified material can be produced and used for civil engineering purposes.

[0021] According to the embodiment described above, metal borides can be produced at a relatively low temperature of 800°C or higher. This eliminates the need for expensive manufacturing equipment that can withstand high-temperature processing, and also reduces energy consumption. Furthermore, inexpensive materials such as boron oxide and SiO2 can be used in the reaction. Moreover, as mentioned above, metal borides can be easily obtained by simply preparing raw materials containing boron oxide, a metal or metal oxide, SiO2, and Na, and heating them.

[0022] Example 1 TaB2 was produced using B2O3, SiO2, Na, and Ta as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Ta = 1:3:10:0.25. Ta powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. Four BN crucibles containing this raw material before heating were prepared and heated from room temperature to 800°C, 900°C, 1000°C, and 1100°C over 4 hours, respectively, and then heated at these temperatures for 10 hours. Figure 2 shows the results of X-ray diffraction measurements of the powder obtained after removing Na and washing with water. A peak matching the diffraction peak of TaB2 was observed in the powder heated at 800°C for 10 hours. It was found that heating at 1100°C for 10 hours resulted in the synthesis of TaB2 as the main phase and BN as a by-product.

[0023] Figure 3 shows the results of X-ray diffraction measurements of powder before and after washing with water after heating at 1100°C for 10 hours. The upper part of Figure 3 shows the results of X-ray diffraction measurements of the substance from which Na was removed after heating, and the lower part shows the results of X-ray diffraction measurements of the same substance after washing with water. From the waveform in the upper part of Figure 3, it was found that sodium silicate (Na2SiO3) was formed. Furthermore, from the waveform in the lower part of Figure 3, it was found that the sodium silicate was washed away by the water washing process, and TaB2 could be extracted.

[0024] Figure 4A is an SEM image of TaB2 obtained by heating the raw material described above at 1100°C for 10 hours, removing Na, and washing with water. It was found that TaB2 has small particle sizes of 1 μm or less. The average particle size of the primary particles of TaB2 was 500 nm or less. Figure 4B shows the results of compositional analysis of the TaB2 from Figure 4A by energy-dispersive X-ray spectroscopy (EDX). The ratio of Ta to B is approximately 1:2, confirming that it is almost a stoichiometric composition.

[0025] Example 2 TaB2 was produced using B2O3, SiO2, Na, and Ta as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Ta = 1:3:10:0.25. Ta powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. Four BN crucibles containing this raw material before heating were prepared and heated from room temperature to 1100°C over 4 hours. At this temperature, they were heated for 1 hour, 2 hours, 4 hours, and 10 hours, respectively. Figure 5 shows the results of X-ray diffraction measurements of the powder obtained after removing Na and washing with water. A peak matching the diffraction peak of TaB2 was observed in the powder heated at 1100°C for 2 hours, indicating that TaB2 can be synthesized as the main phase even with such a heating time of 2 hours.

[0026] Example 3 TaB2 was produced using B2O3, SiO2, Na, and Ta as raw materials. The molar ratio of the raw materials before heating was set to B2O3:SiO2:Na:Ta = x:3:10:0.25. Ta powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw materials before heating. Four types of raw materials before heating were prepared, with x values ​​of x=0.25, x=0.5, x=1, and x=2. These four types of raw materials before heating were each placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 6 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. By setting the B2O3 composition ratio x to 0.25, a peak matching the diffraction peak of TaB2 was observed when the raw material with a small amount of B2O3 was heated. It was found that TaB2 can be synthesized even with such a small amount of B2O3. When raw materials with B2O3 composition ratios x of 0.5, 1, and 2 were heated, large diffraction peaks of TaB2 were observed. For example, when raw materials with a B2O3 composition ratio x of 1 were heated, it was found that TaB2 was synthesized as the main phase and BN was produced as a by-product.

[0027] Example 4 TaB2 was produced using B2O3, SiO2, Na, and Ta as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Ta = 1:3:x:0.25. Ta powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw materials before heating. Four types of raw materials before heating were prepared, with x values ​​of x=3, x=6, x=8, and x=10. These four types of raw materials before heating were each placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 7 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that when the amount of Na used in the raw materials before heating is small, TaB2 cannot be synthesized as the main phase by setting x=3. On the other hand, it was found that TaB2 could be synthesized as the main phase by increasing the molar ratio of Na contained in the raw materials before heating by setting x=6, x=8, and x=10. Therefore, it was found that TaB2 can be synthesized when the amount of metallic sodium is 6 times or more the molar amount of boron oxide (B2O3).

[0028] Example 5 TaB2 was produced using B2O3, SiO2, Na, and Ta2O5 as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Ta2O5 = 1:3:10:0.125. Ta2O5 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 8 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that TaB2 can be synthesized by using the metal oxide Ta2O5 as a raw material before heating. Since Ta2O5 is a cheaper material than Ta, the use of Ta2O5 can reduce manufacturing costs. Under these manufacturing conditions, it was found that TaB2 was synthesized as the main phase and BN was produced as a by-product. The small BN peak is thought to be due to a BN crucible. In other examples as well, BN formation is thought to be due to a BN crucible.

[0029] Example 6 TiB2 was produced using B2O3, SiO2, Na, and Ti as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Ti = 1:3:10:0.25. Ti powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 9 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that TiB2 can be synthesized by using Ti as a raw material before heating. Under these manufacturing conditions, it was found that TiB2 is synthesized as the main phase and BN is produced as a by-product. The small BN peak is thought to be due to the BN crucible.

[0030] Figure 10 shows the SEM image and EDX results of the generated TiB2. In the SEM image in Figure 10A, TiB2 single crystal grains of several tens of micrometers in size were observed among the fine particles. Many plate-like TiB2 single crystal grains of about 20-25 micrometers in size were observed. The compositional analysis results by EDX shown in Figure 10B showed that the ratio of Ti to B was approximately 1:2, confirming that the TiB2 single crystal grains had a nearly stoichiometric composition.

[0031] Example 7 NbB2 was produced using B2O3, SiO2, Na, and Nb as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Nb = 1:3:10:0.25. Nb powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 11 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that NbB2 can be synthesized with the main phase.

[0032] Example 8 ZrB2 was produced using B2O3, SiO2, Na, and Zr as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:Zr = 1:3:10:0.25. Zr powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 12 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that ZrB2 was synthesized as the main phase and BN was produced as a by-product. The small BN peak is thought to be due to the BN crucible.

[0033] When the synthesized ZrB2 was observed using SEM imaging, many single crystal grains of ZrB2, several tens of micrometers in size, were seen among the fine particles. Figure 13 is an SEM image of the generated ZrB2. In this SEM image, hexagonal plate-like ZrB2 single crystals approximately 30 micrometers in size were observed.

[0034] The melting point of zirconium boride is around 3000°C, so a heating temperature of that magnitude is required to produce a single crystal of ZrB2 from grains. Even when using a flux designed to lower the melting point, the heating temperature can only be reduced slightly from 3000°C, requiring a much higher temperature than 1100°C. In this example, a single crystal of ZrB2 was synthesized at a heating temperature of 1100°C, which is considerably lower than the temperature required for processing at around 3000°C. Other borides also have high melting points, so producing single crystals from grains requires correspondingly high-temperature processing.

[0035] Example 9 ZrB2 was produced using B2O3, SiO2, Na, and ZrO2 as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:ZrO2 = 1:3:10:0.25. ZrO2 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 14 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that ZrB2 can be synthesized by using ZrO2, a metal oxide, as the raw material before heating. Since ZrO2 is a cheaper material than Zr, the use of ZrO2 can reduce manufacturing costs. Under these manufacturing conditions, it was found that ZrB2 was synthesized as the main phase, and BN and ZrN were produced as by-products. The small BN peak is thought to be due to a BN crucible.

[0036] Example 10 Tungsten boride was produced using B2O3, SiO2, Na, and W as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:W = 1:3:10:0.25. W powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 15 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that tungsten boride of various compositions can be synthesized.

[0037] Example 11 Tungsten boride was produced using B2O3, SiO2, Na, and WO3 as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:WO3 = 1:3:10:0.25. WO3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 16 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that tungsten boride of various compositions can be synthesized. It was also confirmed that the sample before washing with water contained Na2SiO3.

[0038] Example 12 LaB6 was produced using B2O3, SiO2, Na, and La2O3 as raw materials. The molar ratio of the raw materials before heating was B2O3:SiO2:Na:La2O3 = 1:3:10:0.167. La2O3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. The raw material before heating, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 17 shows the results of X-ray diffraction measurement of the sample obtained after removing Na and washing with water. From the results of this X-ray diffraction measurement, it was found that LaB6 was synthesized as the main phase and BN was produced as a by-product.

[0039] Embodiment 2. Figure 18 is a flowchart showing a method for producing metal borides according to Embodiment 2. In step S20, a pre-heating raw material is prepared, which includes a metal source that is a metal, a metal oxide, a metal-containing carbonate, or a metal-containing hydroxide, as the raw material for producing the metal boride, and which includes boron oxide or boric acid, silicon dioxide, and metallic sodium. In one example, the metal source includes at least one of a Group 2 metal or a Group 3-7 transition metal. In another example, the metal source includes at least one of Ca, Ti, V, Cr, Mn, Sr, Zr, Nb, Ba, Hf, Ta, W, or a Group 3 element. The metal oxide used as the metal source can be an oxide of one of the metals exemplified herein. In another example, the metal oxide is one of Ta2O5, ZrO2, WO3, La2O3, TiO2, HfO2, Nb2O5, Cr2O3, Mn3O4, Sc2O3, Y2O3, CeO2, Gd2O3, Dy2O3, Er2O3, Yb2O3, or Lu2O3. In yet another example, the metal contained in the metal or metal oxide is a rare earth element. In this case, the metal or metal oxide is provided as a rare earth element or an oxide of any of them.

[0040] For example, a "metal-containing carbonate" that is a metal source contains at least one of CaCO3, SrCO3, or BaCO3. For example, a "metal-containing hydroxide" that is a metal source is La(OH)3.

[0041] The boron oxide or boric acid contained in the raw materials before heating are, for example, B2O3 and B(OH)3, respectively. As raw materials before heating, in addition to the metal source mentioned above and boron oxide or boric acid, silicon dioxide (SiO2) and metallic sodium (Na) are provided. The raw materials before heating can be provided in various forms. For example, the raw materials before heating are prepared by first placing the metal source, boron oxide or boric acid, and silicon dioxide in a crucible, and then adding block-shaped metallic sodium to the crucible. For another example, the raw materials before heating are prepared by first placing a compacted molded body (pellet) containing boron oxide or boric acid and silicon dioxide, and the metal source in a crucible, and then adding metallic sodium to the crucible. For yet another example, the raw material powder containing the metal source, boron oxide or boric acid, silicon dioxide, and metallic sodium is mixed in a mortar and pestle to prepare the raw materials before heating. In each example, the raw materials may be mixed at any stage of preparation before heating, or the mixing of the raw materials may be omitted. Even if the raw materials are not mixed, the reaction will proceed relatively uniformly upon heating because the melting point of Na is low, around 98°C. The manner in which the raw materials are provided before heating is not limited to these examples, and any method can be used.

[0042] Next, the process proceeds to step S22. In step S22, the raw material to be heated, prepared in step S20, is heated to, for example, 550°C or higher. In another example, this raw material to be heated to 800°C or higher. In yet another example, the raw material to be heated to 1100°C or higher. In yet another example, the raw material to be heated is heated at a low temperature of 550°C to 800°C. In this case, for example, heating equipment such as an electric furnace with a maximum temperature of 800°C or lower can be used, so the cost of heating equipment can be reduced compared to using heating equipment with a maximum temperature of 1200°C or 1000°C, and naturally, energy consumption can also be suppressed. The heating time is, for example, 2 hours or more. In another example, the heating time can be 1 hour or more. In one example, the raw material to be heated, placed in a crucible, is heated using a well-known heating method. The material of the crucible is, for example, BN, but other materials may be used.

[0043] This heat treatment generates Na2SiO3 and metal borides. Here, sodium silicate (Na2SiO3) is a stable compound that is easily formed, so assuming the raw material before heating does not contain a metal source, it is thought that as sodium silicate is formed, boron oxide is reduced to obtain a product in which boron is the main component. Since the raw material before heating contains a metal source, metal borides are mostly formed directly by heating the raw material before heating, but a reaction can also occur in which the product in which boron is the main component reacts with the metal component of the metal source to form metal borides.

[0044] Next, the process proceeds to step S24. Step S24 is a step to remove residual Na, but since it is the same as the Na removal in step 14 described in Embodiment 1, the explanation will be omitted.

[0045] Next, the process proceeds to step S26. Step S26 is a water washing process, but since it is the same as the water washing process in step S16 described in Embodiment 1, the explanation is omitted. Next, the process proceeds to step S28. In step 28, the sample is subjected to acid washing. If products that cannot be removed by water washing remain on the sample, acid washing is performed in step 28 to dissolve and remove them. For example, unwanted products are dissolved and removed with a 2 mol / L hydrochloric acid aqueous solution. In another example, a solute concentration or chemical substance suitable for dissolving and removing unwanted products can be used. An example of such a chemical substance is nitric acid. Note that, for example, if there are no substances to be dissolved and removed by acid washing, or if unwanted products cannot be dissolved and removed by acid washing, step S28 is omitted.

[0046] According to the embodiment described above, metal borides can be produced at a relatively low temperature of 550°C or higher. This eliminates the need for expensive manufacturing equipment that can withstand high-temperature processing, and also reduces energy consumption. Furthermore, inexpensive materials such as boron oxide or boric acid and SiO2 can be used in the reaction. Moreover, as mentioned above, metal borides can be easily obtained by simply preparing raw materials containing a metal source, boron oxide or boric acid, SiO2, and Na, and heating them.

[0047] Example A TaB2 was produced using Ta2O5 as the metal source, with the boron source being boron oxide and the boron source being boric acid. For the first sample, where boron oxide was used as the boron source, B2O3, SiO2, Na, and Ta2O5 were prepared as pre-heating raw materials. The molar ratio of the raw materials was B2O3:SiO2:Na:Ta2O5 = 1:6:20:2.3. A mixed compact of B2O3, SiO2, and Ta2O5 powders was prepared by adding block-shaped Na. The pre-heating raw materials, stored in a BN crucible, were heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. The upper part of Figure 19 shows the results of X-ray diffraction measurements of the powder obtained after removing Na and washing with water. From these X-ray diffraction measurements, it was found that TaB2 was synthesized as the main phase and BN was produced as a by-product. The small BN peak is thought to be due to the BN crucible. In other examples as well, the generation of BN is thought to be due to the BN crucible. For the second sample, where boric acid was used as the boron source, B(OH)3, SiO2, Na, and Ta2O5 were prepared as pre-heating raw materials. The molar ratio was B(OH)3:SiO2:Na:Ta2O5 = 2:6:20:2.3. Block-shaped Na was added to a mixed compact of B(OH)3, SiO2, and Ta2O5 powders to prepare the pre-heating raw material. The pre-heating raw material, stored in a BN crucible, was heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. The lower part of Figure 19 shows the results of X-ray diffraction measurements of the powder obtained after removing Na and washing with water. From these X-ray diffraction measurements, it was found that TaB2 was synthesized as the main phase.

[0048] Example B TaB2 was produced under various temperature conditions, using Ta2O5 as the metal source and boron oxide as the boron source. B2O3, SiO2, Na, and Ta2O5 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Ta2O5 = 1:6:20:2.3. A mixed compact of B2O3, SiO2, and Ta2O5 powders was mixed and compacted, to which block-shaped Na was added to prepare the raw material before heating. Seven such raw materials were prepared and each was placed in a BN crucible and heated under the following temperature conditions. 1. The temperature was raised from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. 2. The temperature was raised from room temperature to 1000°C over 3 hours and 20 minutes, and then heated at this temperature for 10 hours. 3. The temperature was raised from room temperature to 800°C over a period of 2 hours and 40 minutes, and then heated at this temperature for 10 hours. 4. The temperature was raised from room temperature to 600°C over 2 hours, and then heated at this temperature for 100 hours. 5. The temperature was raised from room temperature to 600°C over 2 hours, and then heated at this temperature for 10 hours. 6. The temperature was raised from room temperature to 550°C over a period of 1 hour and 50 minutes, and then heated at this temperature for 100 hours. 7. The temperature was raised from room temperature to 500°C over a period of 1 hour and 40 minutes, and then heated at this temperature for 10 hours. Figure 20 shows the results of X-ray diffraction measurements of powders obtained by removing Na after heating these seven samples and then washing them with water. From these X-ray diffraction measurements, it was found that when the heating temperature was 1100°C, TaB2 could be synthesized as the main phase, and when the heating temperatures were 1000°C and 800°C, TaB2 could be synthesized as the main phase and Ta3B4 as the secondary phase. At 600°C, with a heating time of 10 hours, the crystallinity of the metal boride was low, and broad diffraction peaks of TaB2 and Ta5B6 were observed. Even at the same 600°C, when the heating time was 100 hours, TaB2 could be synthesized as the main phase and Ta5B6 as the secondary phase. It was also found that even when the heating temperature was lowered to 550°C, TaB2 could be synthesized as the main phase and Ta5B6 as the secondary phase. Furthermore, when the heating temperature was 500°C, no diffraction peaks of metal boride were observed.

[0049] Example C Metallic borides were produced using carbonates of Group 2 elements as the metal source. Example C-1 describes the synthesis of a metal boride using CaCO3, a carbonate of a group 2 element. More specifically, B2O3, SiO2, Na, and CaCO3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:CaCO3 = 1:4:10:0.25. A mixed compact of B2O3, SiO2, and CaCO3 powders was mixed and compacted, to which block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 21 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that CaB6 was synthesized as the main phase. Example C-2 describes the synthesis of a metal boride using SrCO3, a carbonate of a group 2 element. More specifically, B2O3, SiO2, Na, and SrCO3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:SrCO3 = 1:4:10:0.25. A mixed compact of B2O3, SiO2, and SrCO3 powders was mixed and compacted, to which block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 22 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that SrB6 was synthesized as the main phase. Example C-3 describes the synthesis of a metal boride using BaCO3, a carbonate of a group 2 element. More specifically, B2O3, SiO2, Na, and BaCO3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:BaCO3 = 1:4:10:0.25. A mixed compact of B2O3, SiO2, and BaCO3 powders was mixed and compacted, to which block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 23 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that BaB6 was synthesized as the main phase. In Examples C-1, C-2, and C-3, an unidentified phase was detected in the powder after water washing. It was found that the X-ray diffraction peaks of this unidentified phase disappeared after acid washing. This is demonstrated in the following example. Example C-3' is an X-ray diffraction measurement of the powder obtained by acid washing the water-washed sample from Example C-3, and the results are shown in Figure 24. Comparing the XRD pattern in Figure 24 with the XRD pattern in Figure 23, it can be seen that the crystalline phase corresponding to the unidentified phase disappeared or decomposed into the amorphous phase due to acid washing.

[0050] Example D Metallic borides were produced using oxides of Group IV elements as the metal source. Example D-1 describes the synthesis of a metal boride using TiO2, an oxide of a group 4 element. More specifically, B2O3, SiO2, Na, and TiO2 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:TiO2 = 1:3:10:0.25. TiO2 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 25 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that TiB2 was synthesized as the main phase. Example D-2 describes the synthesis of a metal boride using HfO2, an oxide of a group 4 element. More specifically, B2O3, SiO2, Na, and HfO2 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:HfO2 = 1:3:10:0.25. HfO2 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 26 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that HfB2 was synthesized as the main phase. Figure 27 is an SEM image of the generated HfB2. In this SEM image, single crystal grains of HfB2, several micrometers in size, were observed among the fine particles. Many prismatic single crystal grains of HfB2, approximately 5 to 10 micrometers in size, were observed.

[0051] Example E Metallic borides were produced using elemental Group 5 elements and their oxides as metal sources. Example E-1 describes the synthesis of a metal boride using elemental metal V, a group 5 element. More specifically, B2O3, SiO2, Na, and V were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:V = 1:3:10:0.25. A mixed compact of B2O3 and SiO2 was mixed with V powder, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 28 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that VB2 was synthesized as the main phase. Example E-2 describes the synthesis of a metal boride using V2O5, an oxide of a group 5 element. More specifically, B2O3, SiO2, Na, and V2O5 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:V2O5 = 1:3:10:0.25. A mixed compact of B2O3 and SiO2 was mixed with V2O5 powder, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 29 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that VB2 was synthesized as the main phase. Example E-3 describes the synthesis of a metal boride using Nb2O5, an oxide of a group 5 element. More specifically, B2O3, SiO2, Na, and Nb2O5 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Nb2O5 = 1:3:10:0.125. Nb2O5 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 30 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that NbB2 was synthesized as the main phase. Furthermore, in addition to NbB2, Nb 0.77 It was also found that B2 is synthesized.

[0052] Example F Metallic borides were produced using oxides of Group 6 elements as metal sources. Example F describes the synthesis of a metal boride using Cr2O3, an oxide of a group 6 element. More specifically, B2O3, SiO2, Na, and Cr2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Cr2O3 = 1:3:10:0.125. A mixed compact of B2O3 and SiO2 was mixed with Cr2O3 powder, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 31 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that CrB2 was synthesized as the main phase. Furthermore, it was found that not only CrB2 but also CrB was synthesized as a metal boride.

[0053] Example G Metallic borides were produced using oxides of Group 7 elements as the metal source. Example G describes the synthesis of a metal boride using Mn3O4, an oxide of a group 7 element. More specifically, B2O3, SiO2, Na, and Mn3O4 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Mn3O4 = 1:3:10:0.125. Mn3O4 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 32 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that MnB was synthesized as the main phase. Furthermore, it was found that not only MnB but also MnB2 and MnB4 were synthesized as metal borides.

[0054] Example H Metallic borides were produced using elemental Group 3 elements as metal sources. Example H involves the synthesis of a metallic boride using metallic La, a group 3 element. More specifically, B2O3, SiO2, Na, and La were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:La = 1:3.5:10:0.333. La pieces were added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 33 shows the results of X-ray diffraction measurement of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that LaB6 was synthesized as the main phase. Figure 34 is an SEM image of the obtained LaB6. It was found that LaB6 has a particle size of several hundred μm.

[0055] Example I Metallic borides were produced using oxides of Group 3 elements as the metal source. Example I-1 describes the synthesis of a metal boride using Sc2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Sc2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Sc2O3 = 1:3.5:10:0.167. A mixed compact of B2O3 and SiO2 was mixed with Sc2O3 powder, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 35 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that ScB2 was synthesized as the main phase.

[0056] Example I-2 describes the synthesis of a metal boride using Y2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Y2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Y2O3 = 1:3.5:10:0.167. Y2O3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 36 shows the results of X-ray diffraction measurement of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that YB6 was synthesized as the main phase.

[0057] Example I-3 describes the synthesis of a metal boride using CeO2, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and CeO2 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:CeO2 = 1:3:10:0.25. CeO2 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 37 shows the results of X-ray diffraction measurement of the powder obtained after removing Na and washing the sample with water after heating. From the results of this X-ray diffraction measurement, it was found that CeB6 was synthesized as the main phase. Figure 38 is an SEM image of the obtained CeB6. It was found that CeB6 has a particle size of about 1 μm to 10 μm.

[0058] Example I-4 describes the synthesis of a metal boride using Gd2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Gd2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Gd2O3 = 1:3.5:10:0.167. Gd2O3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 39 shows the results of X-ray diffraction measurements of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that GdB6 was synthesized as the main phase. Furthermore, it was found that not only GdB6 but also GdB4 was synthesized as a metal boride. Figure 40 is an SEM image of the obtained metal boride. Since GdB6 is cubic and GdB4 is tetragonal, the cubic crystal grains shown in the SEM image are thought to be the main phase, GdB6.

[0059] Example I-5 describes the synthesis of a metal boride using Dy2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Dy2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Dy2O3 = 1:3.5:10:0.167. Dy2O3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 41 shows the results of X-ray diffraction measurement of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that DyB6 was synthesized as the main phase. Furthermore, it was found that not only DyB6 but also DyB4 was synthesized as a metal boride. Figure 42 shows an SEM image of the obtained metal boride. Since DyB6 is cubic and DyB4 is tetragonal, the cubic crystal grains in the SEM image are thought to be the main phase DyB6, and the columnar crystal grains are thought to be DyB4.

[0060] Example I-6 describes the synthesis of a metal boride using Er2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Er2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Er2O3 = 1:3.5:10:0.167. A mixed compact of B2O3 and SiO2 was mixed with Er2O3 powder, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 43 shows the results of X-ray diffraction measurements of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that ErB6 was synthesized as the main phase. Furthermore, it was found that not only ErB6 but also ErB4 was synthesized as a metal boride. Figure 44 is an SEM image of the obtained metal boride. Since ErB6 is cubic and ErB4 is tetragonal, the cubic crystal grains in the SEM image are thought to be the main phase ErB6, and the columnar crystal grains are thought to be ErB4.

[0061] Example I-7 describes the synthesis of a metal boride using Yb2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Yb2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Yb2O3 = 1:3.5:10:0.167. Yb2O3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 45 shows the results of X-ray diffraction measurement of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that YbB6 was synthesized as the main phase. Figure 46 is an SEM image of YbB6. It was found that YbB6 has a particle size ranging from approximately 1 μm to 20 μm.

[0062] Example I-8 describes the synthesis of a metal boride using Lu2O3, an oxide of a group 3 element. More specifically, B2O3, SiO2, Na, and Lu2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:Lu2O3 = 1:3.5:10:0.167. Lu2O3 powder was added to a mixed compact of B2O3 and SiO2, and block-shaped Na was added to prepare the raw material before heating. This raw material before heating was placed in a BN crucible and heated from room temperature to 1100°C over 4 hours, and then heated at this temperature for 10 hours. Figure 47 shows the results of X-ray diffraction measurement of the powder obtained after removing Na from the sample, washing with water, and acid washing. From the results of this X-ray diffraction measurement, it was found that LuB6 was synthesized as a secondary phase. It may be possible to synthesize it as the main phase by adjusting the synthesis conditions.

[0063] The results of Examples H and I suggest that metal borides can be synthesized using elemental Group 3 elements not mentioned above as metal sources, or using oxides of Group 3 elements not mentioned above as metal sources. Furthermore, since the lanthanides, which are 15 elements from atomic number 57 to 71, i.e., from lanthanum to lutetium, have similar chemical properties, it is thought that metal borides can be synthesized using elemental Group 3 elements other than those mentioned above, or their oxides, as metal sources.

[0064] Embodiment 3. Figure 48 is a flowchart showing a method for producing metal borides according to Embodiment 3. In step S30, a raw material for producing metal borides is prepared, which includes a metal source that is a metal, a metal oxide, a metal-containing carbonate, or a metal-containing hydroxide, as well as a raw material before heating that includes boron oxide or boric acid and silicon dioxide. The raw material before heating is the same as in Embodiment 2, except that it does not contain metallic sodium.

[0065] Next, the process proceeds to step S32. In step S32, the raw materials prepared in step S30 are heated to, for example, 800°C or higher. The heating time is, for example, 2 hours or more. In another example, the heating time can be 1 hour or more. The raw materials are heated in Na vapor. In one example, a first crucible containing the raw materials and a second crucible containing Na are placed in the same container, and the inside of the container is heated to 800°C or higher. Heating Na to about 800°C allows the container to be filled with Na vapor at about 0.5 atmospheres (atm). The method of supplying Na vapor is not limited to this example and can be any method. For example, the raw materials and Na can be heated with separate heat sources. In yet another example, it is also possible to supply Na vapor generated outside the container into the container. Through this heating treatment, Na2SiO3 and metal borides are produced. The reaction mechanism is as described in Embodiment 1.

[0066] Next, the process proceeds to step S34. In step S34, after heating the raw materials before heating, the sample is removed from the first crucible without breaking the first crucible. For example, when synthesizing metal borides by heating raw materials before heating that contain metallic sodium, the sample may adhere to the crucible, requiring the crucible to be broken to remove the sample, in which case the crucible cannot be reused. On the other hand, when synthesizing metal borides by heating raw materials before heating that do not contain metallic sodium while supplying sodium vapor, the sample does not adhere to the crucible, allowing the sample to be easily removed from the crucible without breaking the crucible. Therefore, the crucible can be reused, and contamination of the crucible material with the sample can be suppressed, enabling efficient industrial manufacturing.

[0067] Next, the process proceeds to step S36. Step S36 is a process of washing the sample removed from the crucible with water. The washing process is as described in Embodiment 1, for example. For example, the raw material heated in Na vapor before heating contains almost no excess metallic Na, so it can be washed with water without heat treatment or ethanol treatment to remove excess Na. Furthermore, since the washing process is performed after removing the sample from the crucible, there is no need to expose the crucible to water, which is convenient for reusing the crucible. Steps S36 and S34 can be interchanged. That is, the sample may be washed with water before being removed from the crucible.

[0068] According to Embodiment 3 described above, in addition to the effects explained in Embodiments 1 and 2, the step of removing excess Na can be omitted and the crucible can be reused.

[0069] Embodiment 4. Embodiment 4 relates to a method for producing a metal boride according to any of Embodiments 1-3, in particular to the synthesis of LaB6. The metal source is, for example, La, La2O3, or La(OH)3. In one example, heating the raw material before heating involves heating it to a temperature between 600°C and 1000°C to produce metal borides with particle sizes ranging from tens to hundreds of nanometers. These metal borides are, for example, those with a particle size of less than 1 μm. In another example, heating to a temperature of 1000°C or higher produces metal borides with particle sizes greater than 1 μm.

[0070] Example J-0 Example J-0 describes the production of metal borides under various heating conditions using La2O3 as the metal source. More specifically, B2O3, SiO2, Na, and La2O3 were prepared as raw materials before heating. The molar ratio of the raw materials was B2O3:SiO2:Na:La2O3 = 1:3.5:10:0.167. A mixed compact of B2O3, SiO2, and La2O3 powders was mixed and compacted, and block-shaped Na was added to prepare the raw materials before heating. Six of these raw materials were prepared and each was placed in a BN crucible. They were heated from room temperature to 1100°C, 1000°C, 800°C, 700°C, 600°C, and 500°C over 4 hours, and these temperatures were maintained for 10 hours, 2 hours, 10 hours, 10 hours, 10 hours, and 10 hours, respectively. Figure 49 shows the results of X-ray diffraction measurements of the powder obtained after heating the sample, removing Na, and then washing with water and acid. From these X-ray diffraction measurements, it was found that LaB6 can be synthesized as the main phase at heating temperatures of 600°C or higher. On the other hand, LaB6 could not be synthesized at a heating temperature of 500°C. Figure 50 shows SEM images of the obtained LaB6. Figures 50A and 50B are SEM images of LaB6 obtained at high heating temperatures of 1100°C and 1000°C, while Figures 50C and 50D are SEM images of LaB6 obtained at low heating temperatures of 800°C and 700°C. At temperatures above 1000°C, the particle size of LaB6 becomes relatively large. On the other hand, it was found that at temperatures below 800°C, the particle size of LaB6 becomes considerably smaller.

[0071] Example J-0' Example J-0' is the result of investigating the effect of post-heating treatment on the crystalline phase in a sample. In Example J-0', the same raw material as in Example J-0 was placed in a BN crucible and heated from room temperature to 1000°C over 4 hours, and this temperature was maintained for 4 hours. After that, X-ray diffraction measurements were performed on the powder obtained after Na removal, the powder obtained by further washing with water, and the powder obtained by further acid washing of the Na-removed and water-washed powder. Figure 51 shows the results of the X-ray diffraction measurements of these three powders. As shown in the upper part of Figure 51, it was found that in the powder obtained after Na removal, Na2SiO3 and LaB6 were obtained as the main phase and NaLaSiO4 as the secondary phase. As shown in the middle part of Figure 51, it was found that in the powder obtained after washing with water after Na removal, LaB6 was obtained as the main phase and NaLaSiO4 as the secondary phase. As shown in the lower part of Figure 51, it was found that in the powder obtained after washing with water and acid washing after Na removal, LaB6 was obtained as the main phase. Thus, it was found that adding a water washing treatment was more effective in removing impurity crystals than simply performing a Na removal treatment, and that further acid washing treatment could remove even more impurity crystals.

[0072] Example J-1 Example J-1 is an example in which La2O3 or La(OH)3 was used as the metal source, the heating time was short, and sodium vapor was used as the sodium source. More specifically, experiments were conducted on the following three examples. For the first experiment, B2O3, SiO2, Na, and La2O3 were used as raw materials before heating, with a molar ratio of B2O3:SiO2:Na:La2O3 = 1:3.5:10:0.167. A mixed compacted powder of B2O3, SiO2, and La2O3 was prepared by adding block-shaped Na to the raw materials before heating. The raw materials before heating were placed in a BN crucible and heated from room temperature to 1000°C over 4 hours, and then heated at this temperature for 2 hours. The second method used B2O3, SiO2, and La2O3 as raw materials before heating, with a molar ratio of B2O3:SiO2:La2O3 = 1:3.5:0.167. A mixed compacted body of B2O3, SiO2, and La2O3 powders was used as the raw material before heating. The raw materials before heating were placed in a BN crucible and heated from room temperature to 1000°C over 4 hours, and then heated at this temperature in Na vapor for 6 hours. The third method used B2O3, SiO2, Na, and La(OH)3 as raw materials before heating, with a molar ratio of B2O3:SiO2:Na:La(OH)3 = 1:3.5:10:0.29. A mixed compacted body of B2O3, SiO2, and La(OH)3 powders was prepared by adding block-shaped Na to it. The raw materials before heating were placed in a BN crucible and heated from room temperature to 1000°C over 4 hours, and then heated at this temperature for 1 hour. Figure 52 shows the results of X-ray diffraction measurements of powders obtained after heating the sample, removing Na, and then washing with water and acid. The upper part of Figure 52 shows the X-ray diffraction results of the first sample. From this, it was found that when La2O3 was used as the metal source, LaB6 was obtained as the main phase after a heating time of 2 hours. The middle part of Figure 52 shows the X-ray diffraction results of the second sample. From these X-ray diffraction results, it was found that when Na vapor was used as the Na source, LaB6 was obtained as the main phase. The lower part of Figure 52 shows the X-ray diffraction results of the third sample. From these X-ray diffraction results, it was found that when La(OH)3 was used as the metal source, LaB6 was obtained as the main phase after a heating time of 1 hour. Figure 53 shows the SEM image of the obtained LaB6. From Figure 53A, it can be seen that the first sample yielded LaB6 with a particle size of 1 μm or larger. From Figure 53B, it can be seen that the second sample yielded LaB6 with a particle size of 1 μm or smaller. From Figure 53C, it can be seen that the third sample synthesized both LaB6 with a particle size of 1 μm or larger and LaB6 with a particle size of 1 μm or smaller.

[0073] Example J-2 Example J-2 is an example in which the heating time and heating temperature were varied when sodium vapor was supplied as the sodium source. More specifically, experiments were conducted on the following four examples. In the first experiment, B2O3, SiO2, and La2O3 were used as raw materials before heating, with a molar ratio of B2O3:SiO2:La2O3 = 1:3.5:0.167. A mixed compacted powder body of B2O3, SiO2, and La2O3 powder was used as the raw material before heating. The raw materials before heating were placed in a BN crucible and heated from room temperature to 1000°C over 4 hours, and then heated at this temperature in Na vapor for 6 hours. The second sample is basically the same as the first sample, but differs only in that the heating temperature was set to 900°C. The third sample is basically the same as the first sample, but differs only in that the heating temperature was set to 800°C and the heating time to 10 hours. The fourth sample is basically the same as the first sample, but differs only in that the heating temperature was set to 700°C and the heating time to 10 hours. Figure 54 shows the results of X-ray diffraction measurements of powders obtained after heating each sample, removing Na, and then washing with water and acid. Figure 54 shows the X-ray diffraction results for the first, second, third, and fourth samples from top to bottom. From these results, it was found that LaB6 can be synthesized by heating in Na vapor at a temperature of 700°C.

[0074] Example J-3 Example J-3 is an example of investigating the effect of the presence or absence of SiO2 on the synthesis results. More specifically, B2O3, SiO2, Na, and La2O3 were prepared in a molar ratio of 1:X:10:0.167, with two pre-heating raw materials: one with X at 3.5 and another with X at 0. For the pre-heating raw material with X at 3.5, block-shaped Na was added to a compacted mixture of B2O3, SiO2, and La2O3 powders. For the pre-heating raw material with X at 0, block-shaped Na was added to a compacted mixture of B2O3 and La2O3 powders. These samples were each placed in a BN crucible and heated from room temperature to 800°C over 4 hours, and then heated at this temperature for 10 hours. Figure 55 shows the results of X-ray diffraction measurements of the powder obtained after removing Na and washing with water and acid for the sample using SiO2, and the results of X-ray diffraction measurements of the powder obtained after removing Na and washing with water for the sample without SiO2. The results of this X-ray diffraction measurement showed that LaB6 could be synthesized as the main phase even when heating at 800°C in samples containing SiO2 in the raw materials before heating. On the other hand, LaB6 could not be synthesized in samples that did not contain SiO2 in the raw materials before heating.

[0075] Example J-3' Example J-3' is basically the same as Example J-3, but differs in that the heating temperature was set to 1100°C. Figure 56 shows the results of the X-ray diffraction measurement. Even when the heating temperature was set to 1100°C, similar to Example J-3 where the heating temperature was 800°C, LaB6 could be synthesized as the main phase in samples containing SiO2 in the raw materials before heating, but LaB6 could not be synthesized in samples that did not contain SiO2 in the raw materials before heating. Considering the example in Example B where metal borides could be synthesized by heating at 550°C, it is possible that LaB6 can also be synthesized at a low heating temperature of 550°C when La2O3 is used as the metal source. All the embodiments described so far are not dependent on any particular embodiment. That is, the embodiments should not be interpreted as being limited to one embodiment, but rather as being embodiments of multiple embodiments.

[0076] The method for producing metal borides according to Embodiments 1-4 allows for the synthesis of metal borides by heating the raw materials at low heating temperatures, such as 550°C to 600°C or 550°C to 800°C. This is because the free energy change ΔG in the formation reactions of various metal borides expected in this synthesis method is a large negative value even at low heating temperatures (e.g., 527°C). Moreover, according to the inventors' research, the ΔG value at low heating temperatures was a larger negative value compared to the ΔG value at high heating temperatures. In fact, for several examples of metal boride synthesis for which thermodynamic parameter values ​​have been reported, ΔG was calculated from the expected reaction equations, and these are exemplified below.

[0077] TiO2+5SiO2+B2O3+10Na→TiB2+5Na2SiO3...(Formula 4) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -870 kJ / mol and -1141 kJ / mol, respectively.

[0078] HfO2+5SiO2+B2O3+10Na→HfB2+5Na2SiO3...(Formula 5) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -689 kJ / mol and -906 kJ / mol, respectively.

[0079] ZrO2+5SiO2+B2O3+10Na→ZrB2+5Na2SiO3...(Formula 6) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -725 kJ / mol and -993 kJ / mol, respectively.

[0080] V2O5+11SiO2+2B2O3+22Na→2VB2+11Na2SiO3...(Formula 7) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -2352 kJ / mol and -2977 kJ / mol, respectively. For the reaction producing 1 mol of metal boride, the ΔG at 1027°C and 527°C was -1176 kJ / mol and -1489 kJ / mol, respectively.

[0081] Nb2O5+11SiO2+2B2O3+22Na→2NbB2+11Na2SiO3...(Formula 8) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -2139 kJ / mol and -2731 kJ / mol, respectively. For the reaction producing 1 mol of metal boride, the ΔG at 1027°C and 527°C was -1070 kJ / mol and -1366 kJ / mol, respectively.

[0082] Ta2O5+11SiO2+2B2O3+22Na→2TaB2+11Na2SiO3...(Formula 9) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -1912 kJ / mol and -2506 kJ / mol, respectively. For the reaction producing 1 mol of metal boride, the ΔG at 1027°C and 527°C was -956 kJ / mol and -1253 kJ / mol, respectively.

[0083] Cr2O3+9SiO2+2B2O3+18Na→2CrB2+9Na2SiO3...(Equation 10) For this reaction equation, the free energy change ΔG at 927°C and 527°C was a large negative value of -1734 kJ / mol and -1984 kJ / mol, respectively. For the reaction producing 1 mol of metal boride, the ΔG was -867 kJ / mol and -992 kJ / mol at 1027°C and 527°C, respectively.

[0084] Mn3O4+13SiO2+3B2O3+26Na→3MnB2+13Na2SiO3...(Formula 11) For this reaction equation, the free energy change ΔG at 927°C and 527°C was a large negative value of -2536 kJ / mol and -2954 kJ / mol, respectively. The ΔG for the reaction producing 1 mol of metal boride was -845 kJ / mol and -985 kJ / mol at 1027°C and 527°C, respectively.

[0085] 2Mn3O4+17SiO2+3B2O3+34Na→6MnB+17Na2SiO3...(Formula 12) For this reaction equation, the free energy change ΔG at 927°C and 527°C was a large negative value of -3625 kJ / mol and -4160 kJ / mol, respectively. The ΔG for the reaction producing 1 mol of metal boride was -604 kJ / mol and -693 kJ / mol at 1027°C and 527°C, respectively.

[0086] La2O3+21SiO2+6B2O3+42Na→2LaB6+21Na2SiO3...(Formula 13) For this reaction equation, the free energy change ΔG at 1027°C and 527°C was a large negative value of -3033 kJ / mol and -4146 kJ / mol, respectively. For the reaction producing 1 mol of metal boride, the ΔG at 1027°C and 527°C was -1517 kJ / mol and -2073 kJ / mol, respectively. Note that this reaction equation is a hypothetical, considerably simplified representation of the reaction in this synthesis method. In reality, in addition to Na2SiO3, NaLaSiO4 is also produced as a byproduct in the formation of LaB6. Similarly, when ΔG was calculated using a simplified reaction equation for the synthesis of CeB6, large negative values ​​were obtained at both temperatures.

[0087] Using metal oxides as the metal source results in a larger negative value for ΔG compared to using elemental metals. Therefore, although low-temperature synthesis is possible with either metal source, for example, using metal oxides as the metal source can reduce the heating temperature required to synthesize metal borides compared to using elemental metals.

Claims

1. The process involves preparing raw materials before heating, which include a metal source that is a metal, metal oxide, metal-containing carbonate, or metal-containing hydroxide, boron oxide or boric acid, silicon dioxide, and metallic sodium. A method for producing a metal boride, comprising heating the raw material before heating to 550°C or higher.

2. The aforementioned heating process generates metal borides and Na ２ SiO ３ A method for producing a metal boride according to claim 1, wherein the following is produced.

3. The Na ２ SiO ３ A method for producing a metal boride according to claim 2, comprising washing away the metal boride with water to remove it.

4. The method for producing a metal boride according to claim 1, characterized in that the heating is performed for two hours or more.

5. The method for producing a metal boride according to claim 1, wherein the metallic sodium is six times the molar amount of the boron oxide.

6. The method for producing a metal boride according to any one of claims 1 to 5, wherein the metal source comprises a metal of group 2 or a transition metal of groups 3 to 7.

7. A method for producing a metal boride according to any one of claims 1 to 5, wherein the metal source comprises Ca, Ti, V, Cr, Mn, Sr, Zr, Nb, Ba, Hf, Ta, W, or a group 3 element.

8. The aforementioned metal boride is LaB ６ The method for producing a metal boride according to claim 1.

9. The aforementioned metal source is La, La ２ O ３ Or La(OH) ３ The method for producing a metal boride according to claim 1, wherein the heating of the raw material before heating involves heating the raw material before heating to 600°C or higher and less than 1000°C to produce a metal boride with a particle size of several hundred nm or less.

10. The process involves preparing a raw material before heating that includes a metal source which is a metal, metal oxide, metal-containing carbonate, or metal-containing hydroxide, boron oxide or boric acid, and silicon dioxide. A method for producing a metal boride, comprising heating the raw material before heating to 700°C or higher in Na vapor.

11. The method for producing a metal boride according to claim 10, wherein, in the heating of the raw materials before heating, the first crucible containing the raw materials before heating and the second crucible containing Na are stored in a container, and the inside of the container is heated to 700°C or higher.

12. A method for producing a metal boride according to claim 11, further comprising removing a sample from the first crucible without breaking the first crucible after heating the raw materials before heating.

13. A method for producing a metal boride according to claim 10, further comprising washing the raw material before heating with water after heating, without heat treatment or ethanol treatment.

14. A raw material to be prepared before heating, comprising a metal source which is a metal, a metal oxide, a metal-containing carbonate, or a metal-containing hydroxide, boron oxide or boric acid, and silicon dioxide. The process involves heating the raw material before heating in Na vapor, The metal source is La, La ２ O ３ or La(OH) ３ A method for producing a metal boride, in which the metal boride having a particle size of less than 1 μm is produced by heating the raw material before heating to 600 °C or higher and lower than 1000 °C in the heating of the raw material before heating.

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