A method for mass production of silicon nitride nanowires

Abstract

The application discloses a method for preparing silicon nitride nanowires in large quantities, and belongs to the field of ceramic material preparation. The method comprises the following steps: repeatedly folding a carbon paper into a wave shape according to a certain length, mixing silicon powder and silicon dioxide powder, and uniformly scattering the mixed powder into the crevices of the folded wave-shaped carbon paper. The obtained carbon paper is placed in a graphite furnace, nitrogen is filled into the graphite furnace to 0.1 Mpa, and the temperature is raised to 1300-1700 DEG C; after the reaction temperature is reached, the nitrogen in the sintering furnace is pumped out by a mechanical pump to reduce the air pressure in the furnace, and the low-pressure state is maintained for 1-20 min; then nitrogen is filled into the furnace to increase the pressure in the furnace, and the pressure is maintained for 1-20 min; the above pumping and filling processes are repeated until the total holding time reaches 0.5-10 h, and the preparation is finally ended; and a large quantity of silicon nitride nanowires are generated on the surface of the carbon paper. The silicon nitride nanowires prepared by the method have the advantages of simple process, low raw material cost and large output, and are very suitable for direct industrial production.

Description

TECHNICAL FIELD

[0001] The application relates to a method for mass production of silicon nitride nanowires and belongs to the technical field of ceramic material preparation. BACKGROUND

[0002] The silicon nitride nanowire is an important semiconductor compound and ceramic material, has a high length-diameter ratio, good high-temperature resistance, chemical stability, photoelectric properties, mechanical properties and a series of excellent performances, and has a high thermal conductivity of 270 W.m -1 ·K -1 Due to the excellent performances, the silicon nitride nanowire has a very wide application prospect in many fields such as nanocomposite materials, optical nanodevices, aerospace and solar cells. However, the practical application of the silicon nitride nanowire is limited due to the high production cost and uncontrollable production process.

[0003] The application patent 'Preparation method of high-alpha-phase silicon nitride powder and super-long silicon nitride nanowire' (publication number: CN110436934.A) uses ammonia to pretreat silicon powder, and then generates a high-alpha-phase silicon nitride accumulation body soft block under the condition of a nitrogen-hydrogen-argon mixed gas atmosphere through slow heating, segmented heat preservation and gradual reduction of auxiliary argon. A large number of silicon nitride nanowires are covered on the soft block. In the preparation process, the reaction atmosphere needs to be changed for many times, the reaction temperature and the heating rate need to be adjusted, and the experimental process is complex and time-consuming.

[0004] The application patent 'Preparation method of silicon nitride nanowire and silicon nitride nanowire' (publication number: CN107161962.B) uses a binary, ternary or quaternary layered double hydroxide metal hydroxide LDH containing one or several of Fe, Co, Ni, Cu and Mo as a catalyst precursor, uniformly mixes the silicon powder, heats the mixture to 1000-1400 DEG C under the protection of a carrier gas, pretreats the mixture with hydrogen for 10-20 min, performs a nitriding reaction under a gas containing a nitrogen source, performs chemical vapor deposition, and purifies the deposited product to obtain Si3N4 nanowires. The method introduces a metal catalyst, has high purification difficulty, has high production cost and is not suitable for commercial production.

[0005] The invention patent "Preparation method of high-purity alpha-phase silicon nitride nanowire" (publication number: CN112607715.B) stirs and dissolves the catalyst in the carbon nanotube slurry to obtain a carbon nanotube slurry containing the catalyst, puts the carbon nanotube slurry containing the catalyst into a freeze dryer to obtain a freeze-dried carbon nanotube precursor, spreads the reaction silicon source powder on the bottom of a graphite crucible, then places the freeze-dried carbon nanotube precursor on the surface of the reaction silicon source layer, puts the graphite crucible containing the reactants into a tube furnace, cools to room temperature after the reaction is completed, and washes and dries to obtain high-purity alpha-phase silicon nitride nanowire. The reaction preparation process is complex, time-consuming, and requires carbon nanotubes to participate in the reaction, which is too high in preparation cost and small in yield.

[0006] It can be seen that further improvement is needed on the basis of the existing method to further realize the preparation of a large number of silicon nitride nanowires. SUMMARY

[0007] The present application is to improve some problems existing in the preparation process of silicon nitride nanowires at present, and a new method for large-scale preparation of silicon nitride nanowires by carbothermal reduction is proposed. In the present application, the possible reactions of silicon powder and silicon dioxide generating Si3N4 under nitrogen atmosphere include:

[0008] 3SiO2(s)+6C(s)+2N2(g)→Si3N4(s)+6CO(g) (1)

[0009] 3Si(s)+2N2(g)→Si3N4(s) (2)

[0010] SiO2(s)+Si(s)→2SiO(g) (3)

[0011] 3SiO(g)+3C(s)+2N2(g)→Si3N4(s)+CO(g) (4)

[0012] Among them, reaction (1) and reaction (2) are carbon thermal reduction or direct nitriding reactions of solid-phase silicon sources SiO2 and Si with N2 to generate silicon nitride, and reaction (3) and reaction (4) are carbon thermal reduction reactions of gas-phase silicon source SiO with N2 to generate silicon nitride. According to the crystal growth mechanism, one-dimensional nanowires are generally generated in low supersaturation conditions by gas-solid (VS) mechanism or gas-liquid-solid (VLS) mechanism, that is, silicon nitride nanowires are mainly generated by gas-phase silicon source in reaction (4). Therefore, in order to improve the yield of silicon nitride nanowires, reactions (1) and (2) must be inhibited and reactions (3) and (4) must be promoted.

[0013] The present application utilizes the cyclic charging and discharging to form the oscillating N2 pressure to regulate the reaction process: firstly, the nitrogen partial pressure is reduced by pumping, the nitridation process in reactions (1) and (2) is inhibited, the rapid nucleation of silicon nitride is avoided, and the reaction (3) is promoted to generate a large amount of SiO gas phase; then, nitrogen is filled, because the reaction rate of the gas phase silicon source with N2 is faster than that of the solid phase silicon source, the silicon nitride is induced to nucleate in large amounts according to reaction (4) through the gas phase reaction mechanism, and the whole process system is in a negative pressure state, the generated silicon nitride always maintains a low supersaturation, and thus the anisotropic growth of silicon nitride into one-dimensional morphology is promoted by using the surface energy. The cyclic charging and discharging process forms the oscillating N2 pressure in the reaction furnace, so as to promote the generation of a large amount of silicon nitride nanowires.

[0014] In addition, the preparation mechanism of the carbon paper includes: folding the carbon paper at certain intervals, folding into a wave shape, artificially preparing a small angle region, and putting raw material powder into the crevice of the wave-shaped carbon paper according to a certain mass. Under high temperature conditions, the reaction of silicon dioxide and silicon powder produces silicon monoxide gas which hides between the carbon papers, and the relative supersaturation of silicon monoxide on the surface of the carbon paper is too low to assist the nucleation and growth process of silicon nitride, so that the silicon nitride grows preferentially in one-dimensional direction, and a large amount of silicon nitride nanowires are generated on the surface of the carbon paper.

[0015] Specifically, the purpose of the present application is to provide a method for preparing a large amount of silicon nitride nanowires, which comprises the following steps:

[0016] (1) Mixing: uniformly mixing silicon powder and silicon dioxide to obtain a mixed powder.

[0017] (2) Carbon paper preparation: repeatedly folding the carbon paper into a wave shape, and uniformly scattering the mixed powder of step (1) into the crevice of the folded wave-shaped carbon paper.

[0018] (3) Synthesis: placing the carbon paper obtained in step (2) into a graphite furnace, filling nitrogen to 0.1 MPa, and heating to 1300-1700℃; after reaching the reaction temperature, pumping out the nitrogen in the sintering furnace with a mechanical pump to reduce the gas pressure in the furnace, maintaining a low pressure state for 1-20 min, then filling nitrogen to increase the pressure in the furnace, and maintaining for 1-20 min. Repeat the above pumping and charging process until the total holding time reaches 0.5-10 h, then end; finally, a large amount of silicon nitride nanowires are generated on the surface of the carbon paper.

[0019] Further, the average particle size of the silicon powder in step (1) is 50-800 mesh, and the average particle size of the silicon dioxide is 0.1-2 μm.

[0020] Further, the mass ratio of the silicon powder and the silicon dioxide in step (1) is (0.1-5):1, and the two powders are mixed uniformly by ball milling. According to the calculation of the possible reactions (1)-(4) and the fact that a small amount of vapor produced in the experiment will be volatilized and not participate in the reaction, the above mass ratio is finally obtained.

[0021] Further, the thickness of the carbon paper used in step (2) is 0.1-5 mm, preferably 0.2-1 mm.

[0022] Further, the interval length of the folded carbon paper in step (2) is 2-20 cm, preferably 3-5 mm. The carbon paper is the growth substrate of the silicon nitride nanowires, and different interval lengths will result in different amounts of silicon nitride nanowires. If the interval length is too small, the growth substrate is small, the vapor volatilizes more, and the raw materials are wasted seriously. If the interval length is too large, most of the vapor volatilizes and reacts before reaching the highest position of the carbon paper, and a small amount of silicon nitride nanowires will be generated on the surface of the carbon paper with a high height, which also leads to waste of raw materials.

[0023] Further, the opening angle of the folded wave-shaped carbon paper in step (2) is 20-45°, preferably 25-35°.

[0024] Further, the mass of the powder added in each carbon paper gap in step (2) is 0.5-5 g.

[0025] Further, the heating rate of the reaction furnace in step (3) is 5-50℃ / min.

[0026] Further, in step (3), the nitrogen pressure in the reaction furnace forms a shock type cyclic change, and the holding process always maintains a negative pressure. The specific characteristics are that the pressure in the furnace after pumping is 0.001-0.04 Mpa, and the pressure in the furnace after charging is 0.06-0.10 Mpa.

[0027] Further, in step (3), the nitrogen pressure decreases at a rate of 5-50 kPa / min during the pumping process, and the nitrogen pressure increases at a rate of 10-100 kPa / min during the charging process.

[0028] The application also provides the silicon nitride nanowires prepared by the method.

[0029] The innovative idea of the present application is that, without using a catalyst, on the basis of the traditional carbon thermal reduction method for preparing silicon nitride nanowires, the growth process of the silicon nitride nanowires is regulated by using cyclic pumping and gas regulation to adjust the nitrogen partial pressure. During the pumping process, the nitriding process is inhibited, which promotes the generation of gaseous silicon source, and during the inflation process, the silicon nitride nanowires grow in a gas-solid mechanism under the condition of low supersaturation. Moreover, the carbon paper is folded into a wave shape, and a large number of small-angle regions are artificially prepared to assist in regulating the nucleation and growth process of the silicon nitride, so that the silicon nitride nanowires grow preferentially in one dimension and are generated in large quantities on the surface of the carbon paper.

[0030] Compared with the prior art, the present application has the following advantages:

[0031] The main raw materials used in the present application are Si powder and SiO2 powder, which are low in cost, easy to obtain, high in yield, do not use catalysts, simple in process, and easy to realize commercial production.

[0032] The method regulates the nucleation and growth process of silicon nitride by means of oscillation type gas pressure, which is simple in process, greatly improves the yield, and can control the macroscopic size of the silicon nitride nanowires by changing the size of the carbon paper.

[0033] The silicon nitride nanowires prepared by the present application have a high aspect ratio, high thermal conductivity, excellent mechanical properties, and high purity, and have a broad application prospect in the fields of composite materials, heat dissipation, and energy storage. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0035] Figure 1 It is a macroscopic photo of the growth of the silicon nitride nanowires synthesized in Example 1 on the carbon paper.

[0036] Figure 2 It is a macroscopic photo of the silicon nitride nanowires synthesized in Example 1.

[0037] Figure 3 It is a scanning electron microscope (SEM) photo of the silicon nitride nanowires synthesized in Example 1.

[0038] Figure 4 It is an XRD spectrum of the silicon nitride nanowires synthesized in Example 1.

[0039] Figure 5 It is a scanning electron microscope (SEM) photo of the silicon nitride nanowires synthesized in Example 2.

[0040] Figure 6 XRD pattern of the synthesized silicon nitride nanowires in Example 2;

[0041] Figure 7 Macro photograph of the synthesized silicon nitride nanowires in Comparative Example 1;

[0042] Figure 8 Macro photograph of the synthesized silicon nitride nanowires in Comparative Example 2;

[0043] Figure 9 Scanning electron microscope (SEM) photograph of the synthesized silicon nitride nanowires in Comparative Example 3;

[0044] Figure 10 Macro photograph of the synthesized silicon nitride nanowires in Comparative Example 4;

[0045] Figure 11 Macro photograph of the synthesized silicon nitride nanowires in Comparative Example 5;

[0046] Figure 12 Macro photograph of the synthesized silicon nitride nanowires in Comparative Example 6;

[0047] Figure 13 Macro photograph of the synthesized silicon nitride nanowires in Comparative Example 7;

[0048] Figure 14 Process diagram of folding carbon paper. DETAILED DESCRIPTION

[0049] In order to make the inventive purposes, technical solutions and beneficial technical effects of the present application clearer, the present application will be described in detail below in combination with specific embodiments. It should be understood that the embodiments described in the present specification are only for the purpose of explaining the present application, and are not intended to limit the present application.

[0050] Example 1

[0051] 4.5 g of silicon powder with a particle size of 100 mesh and 3 g of silicon dioxide with a particle size of 0.5 μm were mixed uniformly, and a carbon paper with a thickness of 1 mm was folded every 5 cm, corresponding to the interval length l in the formula (1), so that the entire carbon paper had a wavy shape, and the opening angle of the wavy carbon paper was 35°, corresponding to the angle θ in the formula (2). Figure 14 Figure 14 ​The mixed powder is evenly spread in the crevice of the carbon paper, and the mass of the mixed powder in each crevice is 2.5 g. Subsequently, the carbon paper is placed in a graphite furnace, and the temperature is raised to 1500°C in 240 minutes and maintained for 2 hours. After the reaction temperature is reached, the mechanical pump is started to reduce the nitrogen pressure in the furnace to 0.04 MPa at a rate of 25 kPa / min. After the pressure is maintained for 15 minutes, nitrogen is filled into the furnace at a rate of 25 kPa / min to 0.10 MPa, and the pressure is maintained for 15 minutes. The above process is repeated until the total holding time reaches 2 hours. After the reaction is completed, a large amount of silicon nitride nanowires is obtained on the surface of the carbon paper.

[0052] A digital picture of the growth of the silicon nitride nanowires on the carbon paper is shown in Figure 1 , and the black surface of the carbon paper is almost covered by white flocculent products, which indicates that the yield of the silicon nitride nanowires is large. The white flocculent products on the surface of the carbon paper can be easily collected by using a pair of tweezers, and the collected silicon nitride nanowires are shown in Figure 2 . At the same time, there are gray-black mixed powder residues in the crevice, which are solid substances formed by the sintering of the unreacted silicon powder and the silicon dioxide powder at high temperature. Figure 3 It can be seen that the prepared silicon nitride nanowires are uniform in thickness and have a large aspect ratio. XRD analysis is shown in Figure 4 It can be seen that the prepared silicon nitride nanowires have two phases of α-Si3N4 and β-Si3N4, and do not have other impurities, i.e., have a high purity.

[0053] Example 2

[0054] 8 g of silicon powder with a particle size of 150 mesh and 4 g of silicon dioxide with a particle size of 1 μm are uniformly mixed, and a carbon paper with a thickness of 0.2 mm is folded every 4 cm to form a wave-shaped carbon paper with an opening angle of 25°. The mixed powder is evenly spread in the crevice of the carbon paper, and the mass of the mixed powder in each crevice is 4 g. Subsequently, the carbon paper is placed in a graphite furnace, and the temperature is raised to 1600°C in 240 minutes and maintained for 1 hour. After the reaction temperature is reached, the mechanical pump is started to reduce the nitrogen pressure in the furnace to 0.03 MPa at a rate of 50 kPa / min. After the pressure is maintained for 20 minutes, nitrogen is filled into the furnace at a rate of 50 kPa / min to 0.08 MPa, and the pressure is maintained for 20 minutes. The above process is repeated until the total holding time reaches 4 hours. After the reaction is completed, a large amount of silicon nitride nanowires is obtained on the surface of the carbon paper.

[0055] The SEM image of the prepared silicon nitride nanowires is shown in Figure 5 , and the growth of the nanowires is relatively uniform and has a large aspect ratio. XRD analysis is shown in Figure 6It can be seen that the prepared silicon nitride nanowires have two phases, α-Si3N4 and β-Si3N4, and no other impurities, which means they have high purity.

[0056] Comparative Example 1

[0057] It is basically the same as Example 1, except that the nitrogen pressure is kept constant at 0.1 MPa during the heat preservation process.

[0058] Digital images of silicon nitride nanowires grown on carbon paper can be found here. Figure 7 Because the nitrogen gas was kept at a constant pressure, the growth driving force was insufficient compared to Example 1, resulting in a lower yield of silicon nitride nanowires.

[0059] Comparative Example 2

[0060] The reaction was basically the same as in Example 1, except that the low pressure stage was maintained for 60 minutes and the high pressure stage was maintained for 60 minutes, meaning that only one nitrogen pressure oscillation cycle was performed during the 2-hour reaction process.

[0061] Digital images of silicon nitride nanowires grown on carbon paper can be found here. Figure 8 Because only one oscillation cycle was performed, the silicon nitride nanowires grew very thinly, resulting in uneven growth on the carbon paper and making them difficult to peel off.

[0062] Comparative Example 3

[0063] The method is essentially the same as in Example 2, except that during the low-pressure oscillation cycle, nitrogen gas is only introduced up to 0.03 MPa. The silicon nitride nanowires prepared using this method have extremely low yields on carbon paper. SEM images of the silicon nitride nanowires are shown below. Figure 9 The silicon nitride nanowires prepared using this method suffer from low yield and uneven aspect ratio due to insufficient growth driving force caused by the constant low nitrogen partial pressure.

[0064] Comparative Example 4

[0065] It is basically the same as Example 1, except that the opening angle of the wavy carbon paper is 15°.

[0066] Digital images of silicon nitride nanowires grown on carbon paper can be found here. Figure 10 Because the opening angle is too small, the steam is not easy to evaporate, and there is basically no growth of silicon nitride nanowires on the upper layer of the carbon paper, resulting in a small yield.

[0067] Comparative Example 5

[0068] It is basically the same as Example 1, except that the opening angle of the wavy carbon paper is 60°.

[0069] Digital images of silicon nitride nanowires grown on carbon paper can be found here.Figure 11 Because the opening angle is too large, the vapor volatilization does not fully contact the carbon paper for reaction, and the yield of silicon nitride nanowires is small.

[0070] Comparative Example 6

[0071] The same as Example 1, except that the graphite carbon paper without folding is used as the reaction container

[0072] The digital picture of the growth of silicon nitride nanowires on the graphite carbon paper without folding is shown in Figure 12 Because the vapor volatilization is serious, it basically does not react with the carbon paper, and only a thin layer is formed on the surface, the yield is extremely small and difficult to collect.

[0073] Comparative Example 7

[0074] The same as Example 1, except that the carbon paper is folded every 8 cm to present a wavy shape.

[0075] The digital picture of the growth of silicon nitride nanowires on the graphite carbon paper is shown in Figure 13 Because the interval of the carbon paper is too large, the vapor volatilization, most of the vapor volatilization does not reach the highest position of the carbon paper, and cannot react. It can be seen that the carbon paper located above basically has no silicon nitride nanowires generated near the fold.

[0076] The above description is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any skilled person in the art can easily think of various equivalent modifications or replacements within the technical range disclosed by the present application, and these modifications or replacements should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be determined by the protection scope of the claims.

Claims

1. A method for mass production of silicon nitride nanowires, characterized by, The method comprises the following steps: (1) mixing: uniformly mixing silicon powder and silicon dioxide to obtain mixed powder; (2) carbon paper preparation: repeatedly folding the carbon paper into a wave shape, and uniformly spreading the mixed powder of step (1) into the crevices of the folded wave-shaped carbon paper; (3) synthesis: placing the carbon paper obtained in step (2) into a graphite furnace, filling nitrogen to 0.1 Mpa, and heating to 1300-1700℃; after reaching the reaction temperature, the nitrogen in the sintering furnace is pumped out by a mechanical pump to reduce the gas pressure in the furnace, and the low pressure state is maintained for 1-20 min, then nitrogen is filled to increase the pressure in the furnace, and maintained for 1-20 min; The above pumping and filling process is repeated until the total holding time reaches 0.5-10 h, and then the process is ended; in step (3), the nitrogen pressure in the reaction furnace forms an oscillating cyclic change, and the holding process always maintains a negative pressure.

2. The method of claim 1, wherein, The mass ratio of the silicon powder and the silicon dioxide in step (1) is (0.1-5):1, and the two powders are uniformly mixed by ball milling.

3. The method of claim 1, wherein, The thickness of the carbon paper used in step (2) is 0.1-5 mm.

4. The method of claim 3, wherein, The thickness of the carbon paper used in step (2) is 0.2-1 mm.

5. The method of claim 1, wherein, The interval length of the folded carbon paper in step (2) is 2-20 cm.

6. The method of claim 5, wherein, The interval length of the folded carbon paper in step (2) is 3-5 mm.

7. The method of claim 1, wherein, The opening angle of the folded wave-shaped carbon paper in step (2) is 20-45°.

8. The method of claim 7, wherein, The opening angle of the folded wave-shaped carbon paper in step (2) is 25-35°.

9. The method of claim 1, wherein, The mass of the powder added into each crevice of the carbon paper in step (2) is 0.5-5 g.

10. The method of claim 1, wherein, The heating rate of the reaction furnace in step (3) is 5-50℃ / min.

11. The method of claim 1, wherein, The pressure in the furnace after pumping in step (3) is 0.001-0.04 Mpa, and the pressure in the furnace after filling is 0.06-0.10 Mpa.

12. The method of claim 1, wherein, The descending rate of the nitrogen pressure in the pumping process in step (3) is 5-50 kPa / min, and the ascending rate of the nitrogen pressure in the filling process is 10-100 kPa / min.

13. The silicon nitride nanowire prepared by the method of any one of claims 1-12.

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