Method for producing porous silicon

Abstract

This method for producing porous silicon includes: a reduction step for obtaining an intermediate product containing Si and MgO by bringing Mg vapor into contact with an Si raw material containing silicon oxide under the conditions where the Mg vapor pressure is equal to or less than the equilibrium pressure of reaction a formula (1) described below; and a washing step for removing MgO from the intermediate product. In the reduction step, metal Mg disposed in a non-contact state with respect to the Si raw material is heated in the presence of getter metal to generate the Mg vapor. The getter metal includes Si.

Description

Manufacturing method for porous silicon

[0001] This disclosure relates to a method for producing porous silicon.

[0002] The method for producing porous silicon disclosed in Non-Patent Document 1 comprises a reduction step and a washing step. In the reduction step, Mg vapor generated by heating metallic Mg is used to remove SiO contained in porous diatomaceous earth. ２ The first step involves reducing the substance to produce an intermediate product containing Si and MgO. The second step involves washing the intermediate product with acid to remove the MgO and obtain porous silicon.

[0003] Nature 2007, 446, 172-175

[0004] When manufacturing porous silicon, it is necessary to appropriately control the pore size. In conventional manufacturing methods, one factor that makes it difficult to control the pore size of porous silicon is the grain growth of Si generated during the reduction process. If Si grain growth progresses excessively, the pores become coarser, making it difficult to form fine pores.

[0005] The method for producing porous silicon according to the present disclosure includes a reduction step to obtain an intermediate product containing Si and MgO by contacting a Si raw material containing silicon oxide with Mg vapor under conditions of an Mg vapor pressure below the equilibrium pressure of the following reaction formula (1), and a washing step to remove MgO from the intermediate product, wherein in the reduction step, Mg vapor is generated by heating metallic Mg, which is placed in a non-contact state with the Si raw material, in the presence of a getter metal, and the getter metal contains Si.

[0006]

[0007] In the reduction step of one embodiment of the method for producing porous silicon, the metal Mg and the getter metal are arranged on the path through which the Mg vapor generated from the metal Mg reaches the Si raw material.

[0008] In the reduction step of one embodiment of the method for manufacturing porous silicon, the metal Mg and the getter metal are placed in a reduction dish with an open top, and the getter metal is placed on top of the metal Mg and covers the metal Mg.

[0009] In one embodiment of the method for producing porous silicon, in the reduction step, an obstacle is placed between the Si raw material and the metal Mg to increase the distance that the Mg vapor generated from the metal Mg travels to the Si raw material.

[0010] In the reduction step of one embodiment of the method for producing porous silicon, the metal Mg and the getter metal are arranged in contact with each other. In the reduction step of one embodiment of the method for producing porous silicon, the amount of the getter metal charged is 0.10 equivalents or more and 2.0 equivalents or less in molar equivalents relative to the amount of the metal Mg charged.

[0011] In the reduction step of one embodiment of the method for producing porous silicon, the amount of metal Mg charged is one to two times the amount that generates one molar equivalent of Mg vapor relative to the number of moles of O (oxygen) in the silicon oxide contained in the Si raw material.

[0012] According to the method for producing porous silicon of the present invention, the progression of crystal grain growth of the generated Si can be suppressed.

[0013] Figure 1 shows the temperature-Mg vapor pressure curves for reaction equations (1) and (2). Figure 2 is an explanatory diagram of the reduction process. Figure 3 is an explanatory diagram of the arrangement of the reaction vessel in the vacuum furnace. Figure 4 is an enlarged view of the reaction vessel shown in Figure 3. Figure 5 is an explanatory diagram of an example of a modified reaction vessel. Figure 6 is an explanatory diagram of the arrangement of metallic Mg and getter metal in the reduction process. Figure 7 is the X-ray diffraction pattern of the intermediate product of Example 1. Figure 8 is an explanatory diagram of the reduction process in the modified example. Figure 9 is an explanatory diagram of the reduction process in the modified example.

[0014] An embodiment of the present invention will be described below. The method for producing porous silicon according to this embodiment comprises the reduction step and the washing step described below.

[0015] <Reduction Process> The reduction process is a process in which an intermediate product containing Si and MgO is obtained by a reduction reaction in which Mg vapor is brought into contact with a Si raw material containing silicon oxide to reduce the silicon oxide. Mg vapor is generated by heating metallic Mg and a getter metal that are placed in a non-contact state with the Si raw material.

[0016] [Si raw material] Si raw material contains silicon dioxide as its main component. Examples of silicon dioxide include SiO₂, SiO₂ ２ Examples include the following. The Si raw material may consist only of silicon dioxide, or it may contain components other than silicon dioxide. The proportion of silicon dioxide in the Si raw material is, for example, 50% by mass or more, preferably 90% by mass or more, and more preferably 99% or more.

[0017] The Si raw material is, for example, in powder form. The average particle size (D50) of the powdered Si raw material is, for example, 5 μm or less, preferably 3 μm or less, and more preferably 1 μm or less. The average particle size (D50) of the powdered Si raw material is, for example, 0.5 μm or more. In this specification, "average particle size (D50)" means the median diameter measured by laser diffraction scattering method.

[0018] The particle size of the Si raw material may be controlled by further grinding the Si raw material powder. For example, the Si raw material powder can be ground using a grinding device such as a bead mill. Alternatively, the Si raw material powder may be ground by adding a dispersion medium and wet grinding using a grinding device such as a bead mill. The grinding process is preferably carried out under conditions in which the lump-like Si raw material is volume-ground. For example, in the first half of the grinding process, the ground particles generated by surface grinding due to frictional or shear force are further volume-ground in the second half of the grinding process under conditions in which impact or compressive force is used. By performing the grinding process under conditions in which volume-ground particles are produced, the aspect ratio of the Si raw material particles can be reduced. By using Si raw material with a small aspect ratio, the aspect ratio of the porous silicon particles obtained in the end can be reduced.

[0019] The Si raw material may be a granule formed by agglomerating Si raw material powder, in other words, a granule in which Si raw material powder is the primary particle. The granule can be obtained, for example, by spray drying a slurry containing silicon oxide powder and a dispersion medium. The dispersion medium is, for example, water, or a mixed solvent of water and a non-aqueous solvent. The slurry may contain other components such as dispersants and binders. Examples of spraying methods in spray drying include spraying methods using a disc type, a pressurized nozzle, a pressurized two-fluid nozzle, a pressurized four-fluid nozzle, etc. The average particle diameter (D50) of the granule is, for example, 3 μm or more, preferably 5 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more. The average particle diameter (D50) of the granule is, for example, 20 μm or less, preferably 15 μm or less, and more preferably 13 μm or less.

[0020] [Metallic Mg] Metallic Mg is a source of Mg vapor that reduces silicon oxide to Si and MgO. Metallic Mg is, for example, in powder form. The average particle size (D50) of powdered metallic Mg is, for example, 2 mm or more, preferably 3 mm or more.

[0021] The shape of metallic Mg is not limited to powder form. The metallic Mg may be in other shapes, such as lumps, rods, or plates. Regardless of its shape, the metallic Mg is preferably too large to pass through a sieve with a mesh size of 2 mm, and more preferably too large to pass through a sieve with a mesh size of 3 mm. In the case of rod-shaped metallic Mg, for example, the diameter is preferably 2 mm or more, and more preferably 3 mm or more. In the case of plate-shaped metallic Mg, for example, the product of the thickness and the minimum planar dimension is 4 mm. ２ Preferably, it is 9 mm or more ２ It is more preferable that the above conditions are met. Furthermore, there is no particular upper limit to the dimensions of the metallic Mg. The longest dimension of the metallic Mg is, for example, less than 15 cm, regardless of its shape.

[0022] The charged amount of metallic Mg is an amount that generates Mg vapor exceeding 1 molar equivalent with respect to the number of moles of O (oxygen) of silicon oxide contained in the Si raw material. In other words, the charged amount of the reducing material is an amount that can reduce all of the silicon oxide contained in the Si raw material. For example, the amount of metallic Mg that generates 1 molar equivalent of Mg vapor with respect to the number of moles of O (oxygen) of silicon oxide contained in the Si raw material is used as a reference amount. At this time, the charged amount of metallic Mg is 1 times or more of the reference amount, preferably 1.1 times or more, more preferably 1.2 times or more, and still more preferably 1.3 times or more. Also, the charged amount of metallic Mg is, for example, 2 times or less of the reference amount.

[0023] [Getter metal] The getter metal is selected from metals that can absorb Mg vapor generated from metallic Mg at a Mg vapor pressure below the equilibrium pressure of the following reaction formula (1). That is, the getter metal is a metal that reacts with Mg to form a Mg alloy at a Mg vapor pressure below the equilibrium pressure of the following reaction formula (1). The getter metal is used to reduce the Mg vapor pressure in the reaction system.

[0024]

[0025] Reaction formula (1) is Mg ２ It shows a reversible decomposition reaction that decomposes Si into Mg vapor and Si. In FIG. 1, a temperature-Mg vapor pressure curve V1 of reaction formula (1) (hereinafter, may be referred to as curve V1) is shown. The numerical value on the vertical axis of FIG. 1 is a numerical value expressed in common logarithm. At a Mg vapor pressure below the equilibrium pressure of reaction formula (1), the reaction that decomposes Mg ２ Si into Mg vapor and Si proceeds, and at a Mg vapor pressure exceeding the equilibrium pressure, the reaction that generates Mg ２ Si from Si and Mg vapor proceeds. Therefore, the getter metal is a metal that absorbs Mg vapor generated from metallic Mg in the presence of Si and Mg vapor so as not to generate Mg ２ Si on the Si raw material (silicon oxide) side to be reduced, and reduces the Mg vapor pressure in the reaction system.

[0026] At a Mg vapor pressure below the equilibrium pressure of the reaction formula (1), examples of the metal for Mg alloying include Si, Sn, Ni, Ca, Cu, and Bi. Among these, Si is a metal that alloyes with Mg in a smaller amount. Therefore, the getter metal contains Si. Thereby, the Mg vapor pressure in the reaction system can be efficiently reduced with a small amount of the getter metal. An example of the getter metal contains Si alone. Further, the getter metal may further contain other metals that are metals other than Si among the above metals for Mg alloying. In this case, the mass ratio of Si in all the getter metals is preferably, for example, 50% by mass or more, 80% by mass or more, or 95% by mass or more.

[0027] The charged amount of the getter metal is set, for example, according to the charged amount of metallic Mg and the temperature conditions of the reduction reaction. When the temperature of the reduction reaction is 500°C or higher and 900°C or lower, the charged amount of the getter metal is, for example, 0.10 equivalent or more and 2.0 equivalents or less. The above charged amount of the getter metal may be 0.25 equivalent or more, 0.30 equivalent or more, or 0.40 equivalent or more. Further, the above charged amount of the getter metal may be 1.0 equivalent or less, 0.75 equivalent or less, or 0.60 equivalent or less. Also, in the above case, the charged amount of the getter metal is, for example, 0.1 equivalent or more and 4.0 equivalents or less in terms of molar equivalent to the number of moles of O (oxygen) of silicon oxide contained in the Si raw material.

[0028] Further, when the getter metal contains Si alone, the charged amount of the getter metal is preferably an amount such that the ratio R defined by the following (Formula 1) is within a specific range. The ratio R indicates the relationship between the amount of metallic Mg and the amount of the getter metal in the reaction system.

[0029] Ratio R = (number of moles of metallic Mg) / (number of moles of getter metal + number of moles of metallic Mg) … (Formula 1) When the temperature of the reduction reaction is 500°C or higher and 900°C or lower, the ratio R is, for example, 0.33 or more and 0.91 or less. The ratio R may be 0.60 or more, or 0.65 or more. Further, the ratio R may be 0.80 or less, 0.75 or less, or 0.70 or less.

[0030] The getter metal is, for example, in powder form. In this case, the average particle diameter (D50) of the getter metal is, for example, 200 μm or less, preferably 150 μm or less. The smaller average particle diameter (D50) of the getter metal, that is, the larger specific surface area of the getter metal, increases the absorption efficiency of Mg vapor. Also, the average particle diameter (D50) of the getter metal is, for example, 10 μm or more.

[0031] [Pressure conditions] The pressure condition for the reduction reaction is a Mg vapor pressure below the equilibrium pressure of the above reaction formula (1). When it is less than the equilibrium pressure of reaction formula (1), the forward reaction that decomposes Mg ２ [[ID=​​​​​​​​​​​​​​​​​​​​This can lead to coarsening of the pores in the resulting porous silicon. Therefore, by lowering the reduction reaction temperature to 800°C or below, it becomes easier to obtain porous silicon with fine pores.

[0033] [Reaction Time] The reaction time for the reduction reaction described above can be appropriately selected according to the pressure and temperature conditions described above. For example, the reaction time for the reduction reaction described above is between 3 hours and 24 hours.

[0034] [Reaction Equations] The reaction equations (1) to (3) of the reactions proceeding in the reaction system of the reduction process are described below. In the following reaction equations, "Mg(s)" represents metallic Mg (solid), "Mg(g)" represents Mg vapor, "Mg(g+s)" represents Mg in an unspecified state (solid or gaseous), and "M" represents the getter metal. Also, x is a value that satisfies 1 ≤ x ≤ 2, and y is a value that satisfies 1 ≤ y ≤ 2.

[0035]

[0036] When the reaction system is heated, Mg vapor is generated from metallic Mg according to reaction equation (2) above. A portion of the generated Mg vapor and a portion of the metallic Mg are absorbed by the getter metal according to reaction equation (3) above to form Mg alloy. ｘ M is produced. Also, based on the above reaction equation (3), Mg ｘ The reverse reaction also proceeds, in which Mg vapor is generated from M. Furthermore, as shown in reaction equation (4) above, the generated Mg vapor reduces silicon oxide, which is the Si raw material, to produce Si and MgO, which constitute the intermediate products.

[0037] Figure 1 also shows the temperature-Mg vapor pressure curve V2 (hereinafter sometimes referred to as curve V2) of the above reaction equation (2). At Mg vapor pressures below curve V2, the reaction of changing from solid to gas proceeds, and at Mg vapor pressures above curve V2, the reaction of changing from gas to solid proceeds. Curve V2 is higher than curve V1 of reaction equation (1) over the entire temperature range. In this embodiment, by carrying out the equilibrium reaction of reaction equation (2) and the equilibrium reaction of reaction equation (3) in the same system, the Mg vapor pressure in the reaction system is reduced to an Mg vapor pressure lower than the equilibrium pressure of reaction equation (2). Note that when the getter metal is Si, reaction equation (3) is the same as reaction equation (1).

[0038] [Specific Examples of the Reduction Process] Refer to Figure 2 to explain specific examples of the reduction process. Hereinafter, "metallic Mg" may be written as "metallic magnesium."

[0039] The stainless steel reaction vessel 10 is a vessel configured to allow the flow of gas between the inside and outside of the vessel. Inside the reaction vessel 10 are a raw material dish 11 and a reduction dish 12. The raw material dish 11 is placed on legs 11a that rise upright from the bottom of the reaction vessel 10. The raw material dish 11 is a mesh-like dish that allows for airflow. Powdered Si raw material A is placed in the raw material dish 11.

[0040] The reaction vessel 10 is not limited to stainless steel; it may be made of a material that does not corrode in reaction with Mg, such as a carbon material. The reaction vessel 10 may also be the furnace chamber of a vacuum furnace. Furthermore, the reaction vessel 10 may have a stirring function to agitate the Si raw material A placed in the raw material dish 11. For example, the reaction vessel 10 may have a function to vibrate part of the vessel or the entire vessel. In this case, the Si raw material A placed in the raw material dish 11 is agitated as the raw material dish 11 vibrates in conjunction with the vibration of the reaction vessel 10.

[0041] An example of a reducing dish 12 is a dish-shaped member with an open top, positioned below the raw material dish 11 at the bottom of the reaction vessel 10. A mixture of powdered metallic magnesium B1 and powdered getter metal B2 is placed in the reducing dish 12. The mixing state of metallic magnesium B1 and getter metal B2 in the reducing dish 12 is not particularly limited; it may be a uniformly mixed state or an unevenly mixed state.

[0042] Within the reaction vessel 10, the metallic magnesium B1 and the getter metal B2 are arranged so as not to be in contact with the Si raw material A. The reaction vessel 10 may also have a stirring function to stir the mixture of metallic magnesium B1 and getter metal B2 placed in the reducing dish 12. The details of the stirring function are the same as the stirring function for the Si raw material A placed in the raw material dish 11.

[0043] Furthermore, the metallic magnesium B1 and the getter metal B2 are not limited to being a mixture. For example, the metallic magnesium B1 and the getter metal B2 may be arranged separately on the reducing dish 12. In this case, the getter metal B2 is positioned between the metallic magnesium B1 and the Si raw material A, that is, on the path from which the Mg vapor generated from metallic magnesium B1 reaches the Si raw material A. In one example, the getter metal B2 is positioned closer to the metallic magnesium B1 on the above path. In other words, the getter metal B2 is positioned on the above path such that the distance between the getter metal B2 and metallic magnesium B1 is shorter than the distance between the getter metal B2 and the Si raw material A. In this case, the vaporized Mg vapor is more likely to come into contact with the getter metal B2. Therefore, the action of reducing the Mg vapor pressure in the reaction system based on the getter metal B2 can be carried out more efficiently.

[0044] Furthermore, it is more preferable that the metallic magnesium B1 and the getter metal B2 are arranged in a state of contact with each other, for example, by being arranged in a mixed state or by having the getter metal B2 placed on top of the metallic magnesium B1. In this case, the metallic magnesium B1 that has been melted by heating can immediately come into contact with the getter metal B2. This makes it possible to further efficiently reduce the Mg vapor pressure in the reaction system based on the getter metal B2.

[0045] Note that the arrangement of the raw material dish 11 and the reduction dish 12 is not limited to the configuration shown in Figure 2. For example, they may be arranged with the raw material dish 11 at the bottom and the reduction dish 12 at the top, or they may be arranged side by side at the same height. In addition, multiple raw material dishes 11 and reduction dishes 12 may be arranged in the same reaction vessel 10. One example is an arrangement in which the raw material dishes 11 and reduction dishes 12 are stacked alternately in the reaction vessel 10 with a certain separation distance between them. Another example is an arrangement in which multiple raw material dishes 11 are stacked on top of one reduction dish 12 in the reaction vessel 10. Furthermore, since the reducing material placed in the reduction dish 12 expands during the reduction reaction, it is preferable to place the reduction dish 12 and the raw material dish 11 at a predetermined distance apart.

[0046] Next, the reaction vessel 10 containing Si raw material A, metallic magnesium B1, and getter metal B2 is placed inside the vacuum furnace 13. Then, the vacuum inside the vacuum furnace 13 is reduced in pressure and heated to a temperature at which the reduction reaction proceeds. This heats the metallic Mg in the presence of the getter metal. Multiple reaction vessels 10 may be arranged side by side inside the vacuum furnace 13.

[0047] For example, as shown in Figures 3 and 4, multiple reaction vessels 20 are stacked and arranged inside the vacuum furnace 13. Each reaction vessel 20 comprises a box-shaped container body 21 that opens upwards and a lid member 22 that closes the opening of the container body 21. Metallic magnesium B1 and getter metal B2 are placed on the bottom surface of the container body 21. Therefore, in this case, the container body 21 corresponds to a reduction dish 12 that is open at the top. In addition, a raw material dish 11 containing Si raw material A is placed above the metallic magnesium B1 and getter metal B2 inside the container body 21. The raw material dish 11 is spaced apart from the metallic magnesium B1 and getter metal B2. By stacking multiple reaction vessels 20 inside the vacuum furnace 13 in this way, it becomes possible to increase the amount of porous silicon that can be synthesized in a single process.

[0048] Furthermore, as shown in Figure 5, multiple raw material dishes 11 containing Si raw material A may be stacked vertically above the metallic magnesium B1 and getter metal B2 within the container body 21 of the reaction vessel 20. In this case, it is possible to achieve a configuration in which multiple raw material dishes 11 are stacked in a single reduction dish 12 (container body 21).

[0049] The Mg vapor generated based on the above reaction equations (2) and (3) is diffused into the reaction vessel 10. When the Mg vapor comes into contact with the Si raw material A in the raw material dish 11, the silicon oxide contained in the Si raw material A is reduced to Si and MgO. After a predetermined reaction time has elapsed, the reaction vessel 10 is removed from the vacuum furnace 13, and the intermediate product containing Si and MgO generated in the raw material dish 11 of the reaction vessel 10 is recovered.

[0050] The mixing state of metallic magnesium B1 and getter metal B2 in the reducing dish 12 is not particularly limited; they may be uniformly mixed or separated.

[0051] Figure 6 shows an example of the arrangement of metallic magnesium B1 and getter metal B2 in the reducing dish 12. In the example shown in Figure 6, metallic magnesium B1 is placed at the bottom of the reducing dish 12. Getter metal B2 is placed on top of metallic magnesium B1 and covers it. A portion of the getter metal B2 may be placed between the metallic magnesium B1 pieces located at the bottom of the reducing dish 12. In this case, the direct diffusion of Mg vapor generated from metallic magnesium B1 into the reaction vessel 10 is suppressed. This makes it possible to efficiently reduce the Mg vapor pressure in the reaction vessel 10.

[0052] In detail, the Mg vapor generated from metallic magnesium B1 upon heating reacts with the getter metal B2 surrounding the metallic magnesium B1, causing the getter metal B2 to become an Mg alloy. Furthermore, when the equilibrium state of reaction equation (3) is reached, Mg vapor is generated from the Mg alloy. The Mg vapor generated from the Mg alloy then diffuses into the reaction vessel 10 and reacts with the Si raw material A in the raw material dish 11.

[0053] In this case, the equilibrium pressure of reaction equation (3) is dominant over the equilibrium pressure of reaction equation (2) in the reaction vessel 10. The equilibrium pressure of reaction equation (3) is lower than the equilibrium pressure of reaction equation (2). Therefore, because the equilibrium pressure of reaction equation (3) is dominant, the Mg vapor pressure in the reaction vessel 10 becomes close to the equilibrium pressure of reaction equation (3) and is lower than the equilibrium pressure of reaction equation (2). Furthermore, when arranged as shown in Figure 6, the Mg vapor pressure in the reaction vessel 10 can be reduced even with a small amount of getter metal B2 added.

[0054] <Cleaning Process> The cleaning process includes a first cleaning process and a second cleaning process. By going through the first and second cleaning processes, porous silicon is obtained.

[0055] The first washing step involves treating the intermediate product obtained in the reduction step with an acid. In the first washing step, a magnesium salt is generated from the MgO contained in the intermediate product, and the generated magnesium salt is washed away. The acid used in the first washing step is not particularly limited; any acid capable of generating a magnesium salt from MgO is acceptable. Examples of acids used in the first washing step include hydrochloric acid, nitric acid, and sulfuric acid. The concentration of the acid used in the first washing step can be appropriately selected depending on the type of acid. As an example, the reaction equation when hydrochloric acid is used is shown below.

[0056] MgO+2HCl→MgCl ２ +H ２ The second washing step is a step in which any magnesium salts, etc., remaining from the first washing step are washed away using a rinsing solution. An example of a rinsing solution is water or an acidic aqueous solution with a pH of 6 or less. The acid contained in the acidic aqueous solution is preferably an acid that can be removed with an alcohol such as ethanol, for example, an organic acid such as acetic acid. An example of an acidic aqueous solution is a 1% acetic acid aqueous solution. If necessary, a further washing treatment using an alcohol such as ethanol may be performed after the second washing step.

[0057] Another example of a rinsing solution is a polar organic solvent such as ethanol, isopropyl alcohol, or acetone. In this case, the second washing step is performed using only the polar organic solvent, without washing with water or an acidic aqueous solution with a pH of 6 or lower.

[0058] <Porous Silicone> Details of the porous silicon produced by the manufacturing method of this embodiment are described below.

[0059] The silicon content of the porous silicon is, for example, 50% by mass or more and 100% by mass or less, preferably 80% by mass or more, and more preferably 90% by mass or more. The above silicon content refers to the mass ratio of silicon atoms contained in the porous silicon.

[0060] The average particle size (D50) of porous silicon is, for example, 1 μm or more and 3 μm or less. Preferably, the average particle size (D50) of porous silicon is 1.5 μm or more. Also, preferably, the average particle size (D50) of porous silicon is 2.5 μm or less.

[0061] Porous silicon has pores that are connected in three dimensions. One example of porous silicon has fine pores of 30 nm or less, and the pore density is 0.1 cm². ３ / g or more 0.8cm ３ It is less than or equal to / g. The pore size is preferably 0.2 cm. ３ It is 0.3 cm or more per g, and more preferably 0.3 cm ３ The amount is 1 / g or more. Furthermore, the pore size is preferably 0.6 cm. ３ It is less than or equal to / g. In this specification, pore size means the number of pores per unit mass calculated based on the BJH (Barret-Joyner-Halenda) method.

[0062] The total pore size of porous silicon is, for example, 0.1 cm. ３ / g or more 0.8cm ３ The amount is less than or equal to / g. The total pore size of porous silicon is preferably 0.2 cm. ３ It is 0.3 cm or more per g, and more preferably 0.3 cm ３ The amount is 1 / g or more. Furthermore, the total pore size of the porous silicon is preferably 0.6 cm. ３ It is less than / g.

[0063] The specific surface area of ​​porous silicon is, for example, 50 m². ２ / g or more 250m ２ It is less than or equal to / g. The specific surface area of ​​the porous silicon is preferably 100 m². ２ It is 1 / g or more. Furthermore, the specific surface area of ​​the porous silicon is preferably 200 m². ２ It is less than or equal to / g. In this specification, the specific surface area of ​​porous silicon is N by the BET (Brunauer-Emmett-Teller) method. ２ This refers to the BET surface area per unit mass measured by adsorption.

[0064] Porous silicon may contain other components besides silicon. Examples of other components include oxygen and magnesium compounds. The proportion of other components is, for example, 10% by mass or less, and preferably 6% by mass or less.

[0065] Porous silicon can be used in a variety of applications. One example of an application for porous silicon is as an electrode material for energy storage devices. Energy storage devices include, for example, secondary batteries such as nickel-metal hydride secondary batteries or lithium-ion secondary batteries. Energy storage devices may also be all-solid-state batteries or semi-solid-state batteries, or electric double-layer capacitors.

[0066] (Function) Next, the function of this embodiment will be described. In the manufacturing method of this embodiment, in the reduction step, the Si raw material is reduced by bringing the Mg vapor generated by heating the metallic Mg and the getter metal into contact with the Si raw material. Specifically, the silicon oxide contained in the Si raw material is reduced to Si and MgO with Mg vapor, thereby producing an intermediate product containing Si and MgO.

[0067] Here, the reduction reaction in which silicon oxide is reduced to Si and MgO by Mg vapor is an exothermic reaction that releases heat. Therefore, as the reduction reaction progresses, the temperature of the intermediate products generated rises. If the temperature of the intermediate products rises excessively, the crystal grain growth of Si produced by the reduction reaction is promoted. As a result, the crystallite size of Si that makes up the final porous silicon becomes larger. The increase in the crystallite size of Si leads to the coarsening of the pores in porous silicon, which is a factor that inhibits the formation of fine pores.

[0068] Therefore, when metallic Mg is used as the Mg source to generate Mg vapor, the Mg vapor pressure in the reaction system rises rapidly due to the high evaporation rate of metallic Mg, causing the reduction reaction to proceed rapidly. This leads to an excessive rise in the temperature of the intermediate product, which in turn promotes the grain growth of the resulting Si crystals.

[0069] Therefore, in the manufacturing method of this embodiment, in the reduction step, metallic Mg is heated in the presence of a getter metal that reacts with metallic Mg or Mg vapor to form an Mg alloy, thereby generating Mg vapor. The getter metal absorbs the Mg vapor generated from metallic Mg and forms an Mg alloy, which suppresses a rapid increase in the Mg vapor pressure in the reaction system. This suppresses the rapid progress of the reduction reaction and also suppresses the excessive temperature rise of the intermediate product due to the heat generated during the reduction reaction. As a result, the progress of Si grain growth caused by excessive heat being applied to the generated Si is suppressed.

[0070] Furthermore, in the manufacturing method of this embodiment, Si is used as the getter metal, which is a metal that alloys with Mg at a Mg vapor pressure below the equilibrium pressure of reaction equation (1) above. This makes it possible to efficiently reduce the Mg vapor pressure in the reaction system to below the equilibrium pressure of reaction equation (1). As a result, Si produced from silicon oxide by the reduction reaction reacts with Mg vapor to form Mg ２ This can suppress the formation of silicon.

[0071] (Effects) Next, the effects of this embodiment will be described. (1) The method for producing porous silicon includes a reduction step to obtain an intermediate product containing Si and MgO by contacting a Si raw material containing silicon oxide with Mg vapor under conditions of an Mg vapor pressure below the equilibrium pressure of the above reaction equation (1), and a washing step to remove MgO from the intermediate product. In the reduction step, Mg vapor is generated by heating metallic Mg, which is placed in a non-contact state with respect to the Si raw material, in the presence of a getter metal. The getter metal contains Si.

[0072] According to the above configuration, grain growth of Si caused by excessive heat being applied to the generated Si during the reduction process can be suppressed. Therefore, the manufacturing method including the above configuration is suitable for producing porous silicon having fine pores, i.e., porous silicon with a large specific surface area. Electrodes using porous silicon with a large specific surface area have small volume changes due to the expansion and contraction of Si during charging and discharging, thus suppressing poor contact and capacity reduction caused by volume changes.

[0073] In addition, since Si, the getter metal, is a metal that readily forms Mg alloys, it can efficiently reduce the Mg vapor pressure in the reaction system with a small amount of material. Furthermore, when Si is used as the getter metal, the Mg vapor pressure in the reaction system during the reduction process becomes the equilibrium pressure of reaction equation (1). Therefore, it is possible to suppress an excessive decrease in the reaction efficiency of the reduction reaction that produces the intermediate product from the Si raw material, which would otherwise occur due to the getter metal lowering the Mg vapor pressure in the reaction system. In other words, the reaction rate of the reduction reaction that produces the intermediate product can be maximized within a range where Si grain growth can be suppressed.

[0074] (2) In the reduction process, the metallic Mg and the getter metal are arranged in the path from the metallic Mg to the Si raw material. With the above configuration, the Mg vapor pressure in the reaction system can be efficiently reduced.

[0075] (3) In the reduction process, the metallic Mg and the getter metal are placed in a reducing dish 12 that is open at the top. The getter metal is placed on top of the metallic Mg and covers it. With the above configuration, the vapor pressure of Mg in the reaction system can be reduced more efficiently.

[0076] (4) In the reduction process, the metallic Mg and the getter metal are arranged in contact with each other. In this case, the metallic magnesium that has been melted by heating can immediately come into contact with the getter metal. This allows the action of lowering the Mg vapor pressure in the reaction system based on the getter metal to proceed more efficiently.

[0077] (Example of modification) This embodiment can be implemented with the following modifications. This embodiment and the following examples of modifications can be combined with each other to the extent that they do not contradict each other technically.

[0078] ○In the reduction process, an obstacle may be placed between the Si raw material and the metallic Mg to increase the distance that the Mg vapor generated from the metallic Mg travels to the Si raw material. The obstacle is made of a material that does not allow the metallic Mg to pass through. When the above obstacle is placed, the distance that the Mg vapor generated from the metallic Mg travels to the Si raw material is increased, which can suppress the rapid progress of the reduction reaction caused by a large amount of Mg vapor reaching the Si raw material at the same time. As a result, the temperature rise during the reduction reaction and the grain growth of Si that accompanies the temperature rise can be suppressed.

[0079] In the example shown in Figure 8, the obstacle wall 14 is positioned at a certain distance above the reducing dish 12 so as to cover the metallic magnesium B1 placed inside the reducing dish 12. In this case, as indicated by the arrows, the Mg vapor generated from the reducing dish 12 diffuses into the reaction vessel 10 by circling around the outside of the obstacle wall 14, and then reaches the Si raw material A which is located above the obstacle wall 14.

[0080] In the example shown in Figure 9, the raw material dish 11, which was a permeable mesh dish in the example shown in Figure 2, is changed to a raw material dish 15 with a non-permeable shape. In this case as well, as indicated by the arrows, the Mg vapor generated from the metallic magnesium B1 in the reduction dish 12 diffuses into the reaction vessel 10 by circling around the outside of the raw material dish 15, and then reaches the Si raw material A placed in the raw material dish 15. In this case, the raw material dish 15 corresponds to the aforementioned obstacle.

[0081] Furthermore, the obstacles used to increase the distance traveled by the Mg vapor generated from metallic Mg to the Si raw material are not limited to the configurations shown in Figures 8 and 9. For example, a partition wall may be provided as the obstacle to divide the reaction vessel 10 into a Si raw material side where the Si raw material is located and a metallic Mg side where metallic Mg is located, and a passage section such as a hole may be provided in the partition wall to allow metallic Mg to pass through in part.

[0082] ○ The getter metal and the Mg alloy of the getter metal after the reduction process may be reused. For example, in the example shown in Figure 2, after the first reduction process, the intermediate composition generated in the raw material dish 11 may be replaced with new Si raw material, and metal Mg may be added to the reduction dish 12, and the second reduction process may be performed. In this case, the getter metal and the Mg alloy of the getter metal remaining in the reduction dish 12 after the first reduction process are reused in the second reduction process.

[0083] (Example 1) [Reduction Process] As shown in Figure 2, Si raw material was placed in the raw material dish 11 inside the stainless steel reaction vessel 10, and metallic Mg and getter metal were placed in the reduction dish 12 inside the reaction vessel 10. The reaction vessel 10 was placed in a vacuum furnace 13, and the vacuum furnace 13 was heated at 700°C for 12 hours while evacuating with a rotary pump. The pressure inside the vacuum furnace 13 at this time was 1 Pa. After the heat treatment, the powdered intermediate product in the raw material dish 11 was recovered.

[0084] For the Si raw material, SiO powder (90 g) with an average particle size (D50) of 0.8 μm was used. For the metallic Mg, an average particle size (D50) of 6 mm was used. The amount of metallic Mg charged was 1.3 times the standard amount (64.5 g). The standard amount is the amount of metallic Mg that generates 1 molar equivalent of Mg vapor relative to the number of moles of O (oxygen) in the SiO contained in the Si raw material.

[0085] For the getter metal, Si with an average particle size (D50) of 30 μm was used. The amount of getter metal used was 37.3 g, which resulted in the above-mentioned ratio R being 0.67. [Washing process] The intermediate product was added to a 15% by mass HCl aqueous solution and stirred for more than 20 hours while being maintained at 0°C, after which the solid content was recovered by filtration. Next, the recovered solid content was washed with distilled water and ethanol, and vacuum dried at 120°C for 12 hours to obtain powdered porous silicon.

[0086] (Comparative Example 1) Porous silicon was obtained by the same method as in Example 1, except that only metallic Mg was placed in the reducing dish 12 in the reaction vessel 10, and no getter metal was placed.

[0087] (Analysis by X-ray diffraction) The intermediate products after the reduction step in Example 1 and Comparative Example 1 were analyzed by X-ray diffraction. The X-ray diffraction pattern of the intermediate product of Example 1 is shown in Figure 7. From the X-ray diffraction pattern shown in Figure 7, it can be seen that the intermediate product of Example 1 contains Si and MgO. In addition, although the illustration of the X-ray diffraction pattern is omitted, when the porous silicon after the washing step in Example 1 was analyzed by X-ray diffraction, it was confirmed that MgO was removed by the disappearance of the MgO peak.

[0088] As shown in Figure 7, the X-ray diffraction pattern of the intermediate product of Example 1 shows a peak indicating Si crystals (indicated by a black circle). A high peak intensity indicates that Si crystal grain growth is progressing more rapidly. For Example 1 and Comparative Example 1, we compared the peak Ps at 2θ = 28.5, which is a representative peak indicating Si crystals. As a result, the peak intensity of peak Ps in Comparative Example 1 was 4500. In contrast, the peak intensity of peak Ps in Example 1 was 3300, which was less than 75% of the peak intensity of peak Ps in Comparative Example 1. From these results, it can be seen that by performing a reduction reaction to reduce the Si raw material with Mg vapor generated from metallic Mg and getter metal, the progression of Si crystal grain growth in the resulting intermediate product can be suppressed.

[0089] (Example 2 and Comparative Examples 2-4) Similar to Example 1, Si raw materials were placed in the raw material dish 11 inside the stainless steel reaction vessel 10, and metallic Mg and one of Si, Sn, Bi, or Ni as a getter metal were placed in the reduction dish 12 inside the reaction vessel 10. The types of getter metals used in each example are shown in Table 1. The amount of metallic Mg charged was 1.3 times the standard amount. The amount of getter metal charged was such that the above-mentioned ratio R was 0.67. The reaction vessel 10 was placed in a vacuum furnace 13, and the vacuum furnace 13 was heated at 700°C for 12 hours while evacuating with a rotary pump. After the heat treatment, the powdered intermediate product in the raw material dish 11 was recovered.

[0090] (Analysis by X-ray diffraction) The intermediate products of Example 2 and Comparative Examples 2-4 were analyzed by X-ray diffraction. The amount of Si contained in the intermediate product was measured from the X-ray diffraction pattern. The reduction rate (amount of Si measured / amount of SiO charged) was then calculated from the measured amount of Si and the amount of SiO used as the Si raw material. The results are shown in Table 1.

[0091]

[0092] As shown in Table 1, the reduction rate in Example 2, which used Si as the getter metal, was approximately 100%, while the reduction rates in Comparative Examples 2 to 4, which used Sn, Bi, or Ni as the getter metal, were 60 to 80%. This means that under the processing conditions of the reduction step where the reduction rate is 100% when the getter metal is Si, unreacted Si raw material remains when the getter metal is Sn, Bi, or Ni. From these results, it can be seen that when Si is used as the getter metal, the reaction efficiency of the reduction step is higher compared to when other metals are used as the getter metal.

[0093] A...Si raw material B1...Metal Mg B2...Getter metal 10...Reaction vessel 11,15...Raw material dish 11a...Legs 12...Reduction dish 13...Vacuum furnace 14...Obstruction wall

Claims

1. A method for producing porous silicon, comprising: a reduction step to obtain an intermediate product containing Si and MgO by contacting a Si raw material containing silicon oxide with Mg vapor under conditions where the Mg vapor pressure is below the equilibrium pressure of the reaction equation (1) below; and a washing step to remove MgO from the intermediate product, wherein in the reduction step, Mg vapor is generated by heating metallic Mg, which is placed in a non-contact state with respect to the Si raw material, in the presence of a getter metal, and the getter metal contains Si.

2. The method for producing porous silicon according to claim 1, wherein in the reduction step, the getter metal is arranged on the path through which the Mg vapor generated from the metal Mg reaches the Si raw material.

3. The method for producing porous silicon according to claim 1 or 2, wherein in the reduction step, the metal Mg and the getter metal are placed in a reducing dish with an open top, and the getter metal is placed on top of the metal Mg and covers the metal Mg.

4. The method for producing porous silicon according to any one of claims 1 to 3, wherein, in the reduction step, an obstacle is placed between the Si raw material and the metal Mg to increase the distance that the Mg vapor generated from the metal Mg travels to the Si raw material.

5. The method for producing porous silicon according to any one of claims 1 to 4, wherein in the reduction step, the metal Mg and the getter metal are arranged in contact with each other.

6. The method for producing porous silicon according to any one of claims 1 to 5, wherein the amount of getter metal charged is 0.10 equivalents or more and 2.0 equivalents or less in molar equivalents relative to the amount of metal Mg charged.

7. The method for producing porous silicon according to any one of claims 1 to 6, wherein the amount of metallic Mg charged is one to two times the amount required to generate one molar equivalent of Mg vapor relative to the number of moles of O (oxygen) in the silicon oxide contained in the Si raw material.

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