Method for producing porous silicon

Abstract

To prevent Mg2Si and SiO2 from being present in a produced porous silicon.SOLUTION: A method for producing porous silicon includes: a reduction step of obtaining an intermediate product containing Si and MgO by bringing Mg vapor into contact with a raw material containing SiO2 under reduced pressure and under conditions of Mg vapor pressure lower than the equilibrium pressure in the following reaction formula (1); and a cleaning step of removing MgO from the intermediate product.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for producing porous silicon.

Background Art

[0002] The method for producing porous silicon disclosed in Non-Patent Document 1 includes a reduction step and a washing step. The reduction step is a step of producing an intermediate product containing Si and MgO by reducing SiO2 contained in porous diatomaceous earth using Mg vapor generated by heating metallic Mg. The washing step is a step of obtaining porous silicon by washing the intermediate product with an acid to remove MgO.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the reduction step of the conventional production method, the charged amount of metallic Mg is usually adjusted to an amount that generates Mg vapor exceeding 1 molar equivalent with respect to the number of moles of SiO2 contained in diatomaceous earth. In this case, by the reaction of the generated Si and Mg vapor, a by-product Mg2Si is generated. The generated Mg2Si reacts with an acid in the washing step to become silane gas and then is oxidized to become SiO2, which becomes an impurity contained in the porous silicon. In addition, since silane gas ignites in the air, it is difficult to handle. Therefore, the treatment for generating silane gas should be avoided as much as possible.

[0005] On the one hand, when the charged amount of metallic Mg is set to an amount that generates 1 molar equivalent or less of Mg vapor, SiO2, which is an unreacted substance, remains due to insufficient Mg vapor. Since SiO2 is not removed in the washing step, it becomes an impurity contained in the porous silicon. Although SiO2 can be removed by treatment with hydrofluoric acid, there is a practical situation where treatment with hydrofluoric acid, which is a harmful substance to the human body, should be avoided as much as possible. Therefore, it is preferable to perform the reduction step so as not to generate Mg2Si and SiO2.

[0006] However, as described above, in the case of the conventional manufacturing method, when the charged amount of metallic Mg is increased, Mg2Si is generated, and when the charged amount of metallic Mg is decreased, SiO2 remains. Therefore, it is difficult to perform the reduction step so as not to generate Mg2Si and SiO2 in the conventional manufacturing method.

Means for Solving the Problem

[0007] The method for producing porous silicon that achieves the above object is a method for producing porous silicon, comprising a reduction step of obtaining an intermediate product containing Si and MgO by bringing Mg vapor into contact with a raw material containing SiO2 under reduced pressure and under a Mg vapor pressure equal to or lower than the equilibrium pressure of the following reaction formula (1), and a washing step of removing MgO from the intermediate product.

[0008]

Chemical Formula

[0009] In the method for producing porous silicon, the Mg alloy is preferably at least one selected from Mg2Si, MgCa alloy, MgCu2, MgNi2, and MgSn alloy.

[0010] In the method for producing the porous silicon, it is preferable to perform the reduction step at 700 ° C or lower. In the method for producing the porous silicon, the porous silicon preferably has a pore volume of 0.6 cm or more with a pore diameter of 20 nm or less calculated based on the BJH method. 3 / g or more.

Advantages of the Invention

[0011] According to the production method of the present invention, it is possible to suppress the inclusion of Mg2Si and SiO2 in the porous silicon.

Brief Description of the Drawings

[0012]

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Modes for Carrying Out the Invention

[0013] Hereinafter, an embodiment embodying the present invention will be described. The method for manufacturing the porous silicon of the present embodiment includes a reduction step and a cleaning step described below.

[0014] <Reduction Step> The reduction step is a step of obtaining an intermediate product containing Si and MgO by a reduction reaction in which Mg vapor is brought into contact with a raw material containing SiO2 (hereinafter referred to as Si raw material) to reduce SiO2.

[0015] (Si Raw Material) Examples of the Si raw material include quartz, silica gel, diatomaceous earth, and sand. The Si raw material may contain components other than SiO2. The proportion of SiO2 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.

[0016] The shape and size of the Si raw material are not particularly limited and can be appropriately selected according to the use of the porous silicon to be manufactured. The Si raw material is, for example, granular. The average particle diameter of the granular Si raw material is, for example, 100 μm or less, preferably 20 μm or less, and more preferably 5 μm or less. In this specification, the "average particle diameter" means the median diameter (d 50 ) measured by the laser diffraction scattering method.

[0017] (Pressure Conditions) The first pressure condition of the above reduction reaction is under reduced pressure. Under reduced pressure means under a pressure of less than 1 atmosphere (101325 Pa). Under reduced pressure, for example, it is preferably 100 Pa or less, and more preferably 20 Pa or less. As the pressure in the reduction step decreases, Mg vapor diffuses more easily into the reaction system. As a result, the contact opportunity between the Si raw material and Mg vapor increases, and the reduction reaction in the reduction step proceeds more easily. In addition, since the reduction reaction proceeds more easily, the temperature required to advance the reduction step can be lowered.

[0018] The second pressure condition of the above reduction reaction is the Mg vapor pressure below the equilibrium pressure of the following reaction formula (1).

[0019]

Chem.

[0020] (Method for generating Mg vapor) As long as it is possible to bring Mg vapor into contact with the Si raw material so as to satisfy the above second pressure condition, the method for generating Mg vapor is not particularly limited. Examples of the method for generating Mg vapor include a method of generating Mg vapor by heating a reducing material serving as a Mg source such as a Mg alloy and metallic Mg.

[0021] Among these, a method using a Mg alloy (hereinafter referred to as a low equilibrium pressure Mg alloy) whose equilibrium pressure of the reaction for generating Mg vapor is below the equilibrium pressure of reaction formula (1) is particularly preferable. In the reaction system for generating Mg vapor from the low equilibrium pressure Mg alloy, the Mg vapor pressure does not exceed the equilibrium pressure of reaction formula (1). Therefore, the reaction for generating Mg vapor from the low equilibrium pressure Mg alloy and the reaction for bringing Mg vapor into contact with the Si raw material can be carried out in the same reaction system. In this case, the equipment used in the reduction process can be simplified.

[0022] Examples of low equilibrium pressure Mg alloys include Mg2Si, MgCa alloys, MgCu2, MgNi2, and MgSn alloys. Table 1 below shows the equilibrium pressures at 600 °C, 700 °C, and 800 °C for the reactions that generate Mg vapor from low equilibrium pressure Mg alloys. Note that the numerical values of the Mg vapor pressure in Table 1 are values expressed in common logarithms. The numerical values for Mg2Si in Table 1 are the equilibrium pressures of Reaction (1).

[0023] Among the low equilibrium pressure Mg alloys, those with an equilibrium pressure for the reaction that generates Mg vapor close to the equilibrium pressure of Reaction (1) are preferred.

[0024]

Table 1

[0025] The charging amount of the reducing material serving as a Mg source such as Mg alloy and metallic Mg is an amount that generates Mg vapor exceeding 1 molar equivalent with respect to the number of moles of SiO2 contained in the Si raw material. In other words, the charging amount of the reducing material is an amount equal to or more than the amount capable of reducing all of the SiO2 contained in the Si raw material. For example, the amount of the reducing material that generates 1 molar equivalent of Mg vapor with respect to the number of moles of SiO2 contained in the Si raw material is defined as the reference amount. At this time, the charging amount of the reducing material is 1 times or more 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 charging amount of the reducing material is, for example, 2 times or less the reference amount.

[0026] (Temperature conditions) The temperature of the above reduction reaction is not particularly limited as long as Mg vapor exists at a temperature equal to or lower than the equilibrium pressure of reaction formula (1). The temperature of the above reduction reaction is, for example, 500 °C or higher and 900 °C or lower. Also, when producing porous silicon having pores with a small pore diameter, it is preferable to lower the temperature of the above reduction reaction. By lowering the temperature of the above reduction reaction, the pores with a pore diameter of 20 nm or less in the porous silicon can be increased. In this case, the temperature of the above reduction reaction is, for example, 700 °C or lower, preferably 650 °C or lower, and more preferably 600 °C or lower. Also, by lowering the temperature of the above reduction reaction, porous silicon with a small size of Si crystallites can be produced.

[0027] (Reaction time) The reaction time of the above reduction reaction can be appropriately selected as an appropriate time according to the above pressure conditions and temperature conditions. The reaction time of the above reduction reaction is, for example, 3 hours or more and 24 hours or less.

[0028] (Specific example of the reduction process) With reference to FIG. 2, a specific example of the reduction process will be described. The stainless-steel reaction vessel 10 is a vessel configured to allow the flow of gas inside and outside the vessel. Inside the reaction vessel 10, a raw material dish 11 and a reduction dish 12 are accommodated. The raw material dish 11 is disposed on the legs 11a that stand upright from the bottom of the reaction vessel 10. The raw material dish 11 is a mesh-shaped dish having air permeability. The raw material dish 11 is disposed with a powdery Si raw material A. The reduction dish 12 is disposed below the raw material dish 11 at the bottom of the reaction vessel 10. The reduction dish 12 is disposed with a powdery low equilibrium pressure Mg alloy B. Therefore, in the reaction vessel 10, the Si raw material A and the low equilibrium pressure Mg alloy B are disposed in a non-contact state with each other.

[0029] The reaction vessel 10 containing the Si raw material A and the low equilibrium pressure Mg alloy B is placed into the vacuum furnace 13. Then, while reducing the pressure inside the vacuum furnace 13, the inside of the vacuum furnace 13 is heated to a temperature at which Mg vapor is generated from the low equilibrium pressure Mg alloy B. The Mg vapor generated from the low equilibrium pressure Mg alloy B diffuses into the reaction vessel 10. Then, when the Mg vapor contacts the Si raw material A in the raw material dish 11, SiO2 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 taken out 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.

[0030] <Washing process> The washing process is a process of removing MgO by treating the intermediate product obtained by the reduction process with an acid or the like. By removing MgO from the intermediate product, the target porous silicon is obtained.

[0031] The acid used in the washing process is not particularly limited as long as it can remove MgO. Examples of the acid used in the washing process include hydrochloric acid, nitric acid, and sulfuric acid. The concentration of the acid used in the washing process can be appropriately selected according to the type of the acid.

[0032] The cleaning process optionally includes a further cleaning process of the solid content after cleaning with acid using distilled water, alcohol, or the like. Further, the cleaning process optionally includes a process of drying the solid content after cleaning.

[0033] <Porous silicon> The porous silicon produced by the production method of this embodiment has three-dimensionally connected pores. The pore diameter (diameter) of the pores of the porous silicon is, for example, 1 nm or more and 1000 nm or less. The porous silicon is, for example, a microporous material having a pore diameter of less than 2 nm, a mesoporous material having a pore diameter of 2 to 50 nm, and a macroporous material having a pore diameter of more than 50 nm.

[0034] The production method of this embodiment can adjust the pore size distribution and the Si crystallite size of the porous silicon produced by adjusting the temperature of the reduction reaction in the reduction step. Therefore, the production method of this embodiment is suitable for producing porous silicon having many small pores and porous silicon having a small Si crystallite size.

[0035] An example of porous silicon having many small pores is that the pore volume of pores of 20 nm or less calculated based on the BJH method is 0.5 cm 3 / g or more, preferably the pore volume of pores of 20 nm or less is 0.6 cm 3 / g or more.

[0036] An example of porous silicon having many small pores is that the pore volume of pores of 46 nm or less calculated based on the BJH method is 0.6 cm 3 / g or more, preferably the pore volume of pores of 46 nm or less is 0.7 cm 3 / g or more.

[0037] An example of porous silicon having many small pores is that the value P of the main peak of the pore distribution plot based on the BJH method is 3 nm or more and 20 nm or less, and the pore volume of the above value P + 10 nm or less calculated based on the BJH method is 0.5 cm 3It is above / g. Note that the value P of the main peak of the pore size distribution plot is the pore diameter of the main pores of the porous silicon.

[0038] An example of porous silicon with a small crystallite size of Si has a crystallite size of 100 nm or less, preferably a crystallite size of 70 nm or less. The crystallite size is a numerical value calculated by Scherrer's formula.

[0039] Porous silicon can be used for various applications. Examples of applications of porous silicon include, for example, the negative electrode material of a power storage device such as a lithium-ion secondary battery. In particular, porous silicon with a small crystallite size of Si is suitable as the negative electrode material of a lithium-ion secondary battery because it is advantageous for expansion and contraction during charge and discharge.

[0040] Next, the operation and effects of this embodiment will be described. (1) The method for producing porous silicon includes a reduction step and a washing step. The reduction step is a step of obtaining an intermediate product containing Si and MgO by bringing Mg vapor into contact with a Si raw material under reduced pressure and under conditions of a Mg vapor pressure equal to or lower than the equilibrium pressure of reaction formula (1). The washing step is a step of removing MgO from the intermediate product.

[0041] According to the above configuration, the above reduction reaction is carried out under reduced pressure. In this case, since Mg vapor easily diffuses into the reaction system, the contact opportunity between the Si raw material and Mg vapor increases and the reduction reaction easily proceeds. Therefore, it is possible to suppress the remaining of SiO2 in the intermediate product.

[0042] In addition, according to the above configuration, the reduction reaction for reducing SiO2 contained in the Si raw material is carried out under conditions where the Mg vapor pressure is below the equilibrium pressure of Reaction Formula (1). In this case, the forward reaction in which Mg2Si decomposes into Mg vapor and Si proceeds, and the reverse reaction for generating Mg2Si from Mg vapor and Si does not proceed or hardly proceeds. Therefore, in the reaction system in which Si and Mg vapor generated by the reduction reaction are mixed, the reaction for generating Mg2Si from Si and Mg vapor generated by the reduction reaction is suppressed. Further, even if Mg2Si is generated, the generated Mg2Si is decomposed into Mg vapor and Si.

[0043] Therefore, in the case of the above configuration, even if an amount of Mg vapor equal to or greater than the amount of SiO2 is supplied into the reaction system so that SiO2 does not remain in the intermediate product, the generation of Mg2Si is suppressed. Therefore, a reduction step that does not generate Mg2Si and SiO2 in the intermediate product can be easily performed. As a result of obtaining an intermediate product that does not contain Mg2Si and SiO2, it is possible to suppress the inclusion of Mg2Si and SiO2 in the porous silicon obtained through the washing step.

[0044] (2) The Mg vapor used in the reduction step is Mg vapor generated by heating a Mg alloy disposed in a non-contact state with respect to the Si raw material. The Mg alloy is a low equilibrium pressure Mg alloy in which the equilibrium pressure of the reaction for generating Mg vapor is below the equilibrium pressure of Reaction Formula (1).

[0045] According to the above configuration, in the reaction system in which the Si raw material is disposed, by heating the low equilibrium pressure Mg alloy to generate Mg vapor, the Mg vapor pressure in the reaction system becomes below the equilibrium pressure of Reaction Formula (1). Therefore, the above reduction reaction that satisfies the condition of having a Mg vapor pressure below the equilibrium pressure of Reaction Formula (1) can be easily performed.

[0046] (3) The temperature of the reduction reaction in the reduction step is 700 °C or lower. According to the above configuration, porous silicon having pores with a small pore diameter can be manufactured. Further, according to the above configuration, porous silicon with a small size of Si crystallites can be manufactured.

Example

[0047] <Examples 1 to 4> (Synthesis of Mg2Si) Si powder (30 g) with a particle size of 300 μm or less and metallic Mg powder (53.5 g) with a particle size of 180 μm or less were mixed. The obtained mixture was placed in a stainless steel container with a lid and heated at 600 °C for 6 hours in an Ar atmosphere to obtain powdery Mg2Si.

[0048] (Reduction process) As shown in Fig. 2, the Si raw material was placed on the raw material dish 11 in the stainless steel reaction vessel 10, and Mg2Si was placed on the reduction dish 12 in the reaction vessel 10. The reaction vessel 10 was placed in the vacuum furnace 13 and the vacuum furnace 13 was heated at 600 °C to 800 °C for 6 to 12 hours while evacuating with a rotary pump. 。Plus After the heat treatment, the powdery intermediate product in the raw material dish 11 was recovered.

[0049] As the raw material, quartz powder (2 g) with an average particle size of 0.8 μm was used. The charged amount of the Mg alloy was set to 2 times (5.1 g) or 1.3 times (3.3 g) of the above reference amount.

[0050] As shown in Table 2, in Examples 1 to 4, the charged amount of Mg2Si, the heating temperature, and the heating time were varied. Also, the calculated values of the Mg vapor pressure in the reaction vessel 10 are shown in Table 2. The numerical values of the Mg vapor pressure in Table 2 are the values expressed in common logarithm. As shown in Fig. 1 and Table 2, the Mg vapor pressure in the reaction vessel 10 in Examples 1 to 4 is below the equilibrium pressure of Reaction Formula (1).

[0051] (Washing process) The intermediate product was put into a 15 mass% HCl aqueous solution and stirred for 20 hours or more while maintaining at 0 °C, and then 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 powdery porous silicon.

[0052] <Examples 5 to 6> As the Si raw material, instead of quartz powder, a powder with an average particle size of 6 μm obtained by pulverizing silica gel was used. Except for the point of changing the Si raw material, intermediate products and porous silicon were obtained by performing the same reduction process and washing process as in Examples 1 to 4. The charged amount, heating temperature, and heating time of Mg2Si in the reduction processes of Examples 5 to 6 are as shown in Table 2. Also, the calculated values of the Mg vapor pressure in the reaction vessel 10 are shown in Table 2. As shown in FIG. 1 and Table 2, the Mg vapor pressure in the reaction vessel 10 in Examples 5 to 6 is below the equilibrium pressure of Reaction Formula (1).

[0053] <Comparative Examples 1 to 5> Instead of Mg2Si, powdered metallic Mg was used. Except for the point of using metallic Mg and the point of changing the charged amount of metallic Mg, intermediate products and porous silicon were obtained by performing the same reduction process and washing process as in Examples 1 to 4. The charged amount, heating temperature, and heating time of metallic Mg in the reduction processes of Comparative Examples 1 to 5 are as shown in Table 2. Also, the calculated values of the Mg vapor pressure in the reaction vessel 10 are shown in Table 2. As shown in FIG. 1 and Table 2, the Mg vapor pressure in the reaction vessel 10 in Comparative Examples 1 to 5 is higher than the equilibrium pressure of Reaction Formula (1).

[0054] <Comparative Example 6> In the reduction process, instead of heating the vacuum furnace 13 under reduced pressure, the vacuum furnace 13 was heated at 1 atm in an Ar atmosphere. Except for the point of heating the vacuum furnace 13 at 1 atm in an Ar atmosphere, intermediate products and porous silicon were obtained by performing the same reduction process and washing process as in Examples 1 to 4. The charged amount, heating temperature, and heating time of the Mg alloy in the reduction process of Comparative Example 6 are as shown in Table 2. Also, the calculated values of the Mg vapor pressure in the reaction vessel 10 are shown in Table 2. As shown in FIG. 1 and Table 2, the Mg vapor pressure in the reaction vessel 10 in Comparative Example 6 is below the equilibrium pressure of Reaction Formula (1).

[0055]

Table 2

[0056] From the X-ray diffraction patterns of Example 1 and Example 5 after the reduction process shown in FIGS. 3 and 5, it can be confirmed that the intermediate products of Example 1 and Example 5 contain Si and MgO, and do not contain unreacted SiO2 and by-product Mg2Si. And from the X-ray diffraction patterns of Example 1 and Example 5 after the washing process shown in FIGS. 4 and 6, it can be confirmed that in the porous silicon of Example 1 and Example 5, MgO is removed and it is composed of Si crystals. Although the illustration of the X-ray diffraction pattern is omitted, Examples 2 to 4 also had the same results as Example 1.

[0057] On the other hand, from the X-ray diffraction pattern of Comparative Example 3 after the reduction process shown in FIG. 7, it can be confirmed that the intermediate product of Comparative Example 3 contains Mg2Si in addition to Si and MgO. Although the illustration of the X-ray diffraction pattern is omitted, the intermediate products of Comparative Examples 2, 4, and 5 also contained Mg2Si in the same manner as Comparative Example 3. Also, although the illustration of the X-ray diffraction pattern is omitted, the intermediate product of Comparative Example 1 did not contain Mg2Si, but unreacted SiO2 remained.

[0058] The charged amount of Mg2Si in Examples 1 to 6 is 1.3 times or 2.0 times the reference amount. From the results of the above Examples 1 to 6 and Comparative Examples 1 to 5, it can be seen that by performing the reduction reaction at a Mg vapor pressure below the equilibrium pressure of Reaction Formula (1), while supplying Mg vapor equal to or more than the amount of SiO2, the generation of Mg2Si can be suppressed. Also, since this result was the same regardless of whether quartz or silica gel was used as the Si raw material, it is considered that it does not depend on the form of the Si raw material.

[0059] Also, from the X-ray diffraction pattern after the reduction step of Comparative Example 6 shown in FIG. 8, it can be confirmed that Si is not generated in the intermediate product of Comparative Example 6 in which the reduction reaction was carried out under non-reduced pressure, and unreacted SiO2 remains.

[0060] <Analysis of pores> Using a specific surface area and pore size distribution meter, the nitrogen adsorption / desorption isotherms of the porous silicon particles of each example were measured by the gas adsorption method. Based on the obtained nitrogen adsorption / desorption isotherms, pore distribution plots were created using the BJH (Barret-Joyner-Halenda) method and the BET (Brunauer-Emmett-Teller) method, and the crystallite size of Si constituting the particles, specific surface area (BET value), peak value P, and pore volume for each size were calculated. The results are shown in Tables 3 and 4.

[0061] Representing the examples, the adsorption / desorption isotherms and pore distribution plots of Example 1 are shown in FIGS. 9 and 10, and the adsorption / desorption isotherms and pore distribution plots of Example 2 are shown in FIGS. 11 and 12. Note that the peak value P shown in Table 3 is the value of the main peak of the pore distribution plot.

[0062]

Table 3

[0063]

Table 4

[0064] In Comparative Examples 1 to 6, the size of the crystallites of Si contained in the porous silicon tended to be larger than that in Examples 1 to 6. For the porous silicon in Comparative Examples 1 to 6, the correlation between the temperature of the reduction reaction and the size of the crystallites of Si could not be confirmed. Also, Comparative Examples 1 to 6 tended to have less pore volume of 10 nm or less, 20 nm or less, and 46 nm or less compared to Examples 1 to 6.

Description of Signs

[0065] A…Si raw material B…Low equilibrium pressure Mg alloy 10…Reaction vessel 11…Raw material dish 11a…Leg 12…Reduction dish 13…Vacuum furnace

Claims

**Claim 1** A method for producing porous silicon, comprising: Under reduced pressure and under conditions of Mg vapor pressure below the equilibrium pressure of the following reaction formula (1), by bringing Mg vapor into contact with a raw material containing SiO ２ a reduction step of obtaining an intermediate product containing Si and MgO by bringing Mg vapor into contact with a raw material containing a washing step of removing MgO from the intermediate product, characterized in that it is a method for producing porous silicon. 【Chemical 1】 **Claim 2** The Mg vapor is Mg vapor generated by heating an Mg alloy disposed in a non-contact state with respect to the raw material, The Mg alloy has an equilibrium pressure of the reaction for generating the Mg vapor equal to or lower than the equilibrium pressure of Reaction Formula (1). The method for producing porous silicon according to Claim 1. **Claim 3** The Mg alloy is Mg ２ Si, MgCa alloy, MgCu ２ , MgNi ２ The method for producing porous silicon according to claim 2, which is at least one selected from MgSn alloy. **Claim 4** The method for producing porous silicon according to any one of Claims 1 to 3, wherein the reduction step is carried out at 700 °C or lower. **Claim 5** The amount of pores with a diameter of 20 nm or less calculated based on the BJH method of the porous silicon is 0.6 cm ３ / g or more, and the method for producing porous silicon according to any one of claims 1 to 4.

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