CN100445199C - Preparation method of silicon nitride nanowire or nanoribbon powder material - Google Patents

Abstract

The invention discloses a method for preparing silicon nitride nanowire and nanobelt powder materials at low temperature. SiCl ₄ , NaN ₃ and metal sodium are used as raw materials, and the proportion of reactants is adjusted and reacted at 400-500°C. Washing, filtering and drying can respectively obtain silicon nitride (Si ₃ N ₄ ) nanowire and nanobelt powder materials. The product is mainly β-Si ₃ N ₄ , containing a small amount of α-Si ₃ N ₄ , and the total yield relative to SiCl ₄ is more than 90%. By-products are NaCl and a small amount of nitrogen. The silicon nitride nanowire has an average diameter of 30nm, a silicon nitride nanometer bandwidth of 40-80nm, a thickness of no more than 20nm, and an average length of about 500nm. The reaction operation of the invention is simple and safe, the product is pure and the yield is high.

Description

Preparation method of silicon nitride nanowire or nanoribbon powder material

Technical field:

The invention relates to a method for preparing a silicon nitride nanowire or nanobelt powder material.

Background technique

Due to the advantages of high specific strength, high specific modulus, high temperature resistance, oxidation resistance, wear resistance and thermal shock resistance, silicon nitride ceramics have broad application prospects as structural materials and functional materials. Especially in the application environment of high temperature, high speed and strong corrosive medium often encountered in modern technology, silicon nitride materials also have many special application values. Although its application range is expanding, its application is largely limited by the disadvantages of high preparation cost, reduced high-temperature performance, and inherent brittleness. The preparation of nano-silicon nitride powder and the subsequent development of fine ceramic products are hot spots in the research and development of countries all over the world. In view of the ceramic products sintered with nano-silicon nitride powder as raw material, controlling the composition and structure of ceramics at the nanostructure level (1-100nm) is conducive to giving full play to the potential performance of Si ₃ N ₄ ceramic materials. Because of its extremely small particle diameter, large specific surface area and chemical activity energy, it can significantly reduce the degree of sintering densification of materials and save energy. The firing shrinkage is consistent, the grains are uniform, and the defects are small. The strength and reliability of the prepared structural parts The toughness is also high, and it can overcome brittleness and has good machinability. Silicon nitride one-dimensional nanowires and silicon nitride nanoribbons have different physical and mechanical properties from ordinary nanoparticles, and have great potential in improving the strength and toughness of ceramic materials.

Native silicon nitride is not found in nature. In the past few decades, for the research on the synthesis of silicon nitride, various preparation methods have been developed. Such as carbothermal reduction, high temperature self-propagating combustion (HSH), chemical vapor deposition (CVD), pyrolysis of organic precursors, etc. In addition, there are 15-110nm Si ₃ N ₄ amorphous particles prepared from the mixed gas of SiH ₂ Cl ₂ and NH ₃ by laser irradiation method _, and α - Si ₃ N ₄ (48-77%) and β-Si ₃ N ₄ (20-39%) powder. In the process of seeking low-cost, high-volume and rapid synthesis of high-quality silicon nitride nanomaterials, a research topic that has attracted much attention is to explore the low-temperature initiation of silicon tetrachloride (SiCl ₄ ) and different nitrogen source materials. Fast reaction to prepare silicon nitride nanomaterials. For example, silicon nitride powder can be obtained by reacting SiCl ₄ with ammonia (NH ₃ ) in a hot wire-flow reactor. Similarly, sodium hexafluorosilicate thermally decomposes to produce silicon tetrafluoride (SiF ₄ ), which reacts with NH ₃ or nitrogen at high temperature to prepare silicon nitride materials containing α and β phases, such as particles, whiskers, fibers, etc. . α-Si ₃ N ₄ nanowires can be prepared by using magnesium nitride (Mg ₃ N ₂ ) as a solid nitrogen source and reacting with SiCl ₄ in a sealed kettle at 600°C. Using sodium azide as a solid nitrogen source, in the case of a slight excess of sodium azide, it can react with SiCl ₄ to prepare dendritic and short rod-shaped silicon nitride α and β mixed powder.

"Journal of the American Ceramic Society" reported the study of growing β-Si ₃ N ₄ crystals in (Si/Al/Mg/Y) melts at 1680 °C, and found that the growth rate of silicon nitride and the crystal aspect ratio were related to this depending on the composition of the melt. "Advanced Materials" reported the use of SiCl ₄ liquid as a growth medium for silicon nitride nanocrystals and nanorods, in which the silicon nitride α and β phase seeds were synthesized on-site by SiCl ₄ and NaN ₃ at about 670 °C. Chinese invention patent application CN1491885 discloses a silicon nitride and silicon carbide one-dimensional nanostructure and its preparation method, which uses metal alloy particles containing 20-80% silicon as a catalyst and provides a silicon source, and uses nitrogen or carbon-containing gas or The solid is the source of nitrogen and carbon, and the Si ₃ N ₄ one-dimensional nanostructure is obtained by nitriding or carbonizing the silicon-containing alloy particles in a tube furnace at high temperature. The temperature range is 1200-1600°C. Another Chinese invention patent application CN1562735 discloses a method for preparing silicon nitride powder materials at low temperature. _SiCl4 is used as silicon source and _NaN3 is used as nitrogen source to carry out chemical exchange reaction in a stainless steel reactor, and the reaction product After cleaning, suction filtration and drying, the chemical reaction formula is: 3SiCl ₄ +12NaN ₃ =Si ₃ N ₄ +12NaCl+16N ₂ .

Invention content:

The purpose of the present invention is to provide a method for preparing silicon nitride nanowire or nanobelt powder material, the reaction operation is simple and safe, the obtained product is pure and the yield is high.

For reaching above-mentioned purpose, the present invention adopts following technical scheme: the preparation method of silicon nitride nanowire or nanobelt powder material of the present invention, with SiCl ₄ as silicon source, with NaN ₃ as nitrogen source, and add metal sodium, React in the reaction kettle for 0.5-6 hours under the conditions of 400-600°C temperature and 5-40MPa self-generated pressure of the system, and the product is washed, separated and dried to obtain a white powder product;

Chemical reaction of the present invention can represent with following reaction equation:

3SiCl ₄ +n NaN ₃ +(12-n)Na=Si ₃ N ₄ +12NaCl+(1.5n-2)N ₂ (1)

Where n satisfies 1.334≤n≤11.998. According to the calculation of the standard free energy and enthalpy of the reaction, the following relationship is obtained:

ΔG°＝-3391-187.52*n kJ/mol (2)

ΔH°＝-3617-21.71*n kJ/mol (3)

Among them, the effect of n satisfying 2≤n≤4 is the best.

By adjusting the ratio of SiCl ₄ , NaN ₃ , and Na within an appropriate range, silicon nitride with different morphologies can be obtained. When the mass ratio of SiCl ₄ , NaN ₃ , and Na added is 1:0.4～0.5:0.36～0.4, silicon nitride nanowires can be prepared; the mass ratio of SiCl ₄ , NaN ₃ , Na added is 1: When the ratio is 0.25-0.38:0.40-0.45, silicon nitride nanobelts can be obtained.

In order to better realize the present invention, an inert gas is introduced into the reactor and sealed before the heating reaction is performed. The inert gas is preferably nitrogen or argon.

As a preferred solution, the reaction temperature is 450-500° C.; the autogenous pressure of the reaction system is about 15-25 MPa; and the reaction time is 0.5-5 hours.

The product was washed with water and ethanol, then centrifuged, and then dried in a vacuum oven at 80° C. for 24 hours.

When SiCl ₄ is mixed with NaN ₃ , a violent reaction can occur above 100°C to form polysilicon, sodium chloride and a small amount of silicon nitride, while producing a large amount of nitrogen. Sodium azide is a low-cost solid nitrogen source, which can be decomposed into metallic sodium and nitrogen gas when heated to 410°C alone. It is an explosive and highly toxic substance, and is often used as solid auxiliary nitrogen in high-temperature self-propagating synthesis of silicon nitride. source and catalyst in order to increase the α-Si ₃ N ₄ content in the product. Reduce the amount of NaN ₃ and replace it with a large amount of metallic sodium. The melting point of metallic sodium is low (97.82°C). It not only acts as a reactant, solvent, and endothermic agent to change the chemical reaction and ease the reaction environment, but also avoids the danger of explosion and improves the safety of the reaction. At the same time, metal sodium is also used as a growth medium for silicon nitride nanocrystals, so that relatively pure nanowires and nanobelts can be prepared. In the present invention, the amount of sodium metal is different, which has an important impact on the reaction and the product. The pressure of the reaction system is different, and the ratio of β-Si ₃ N ₄ and α-Si ₃ N ₄ in the product is different, which also has a great influence on the product morphology. Influence. Experiments have proved that the optimal range of n is 2≤n≤4.

The colorless liquid SiCl used in the present invention is an analytically pure reagent, and the white solid crystal _NaN and _metal Na are chemically pure reagents. Metal Na was removed from kerosene, blotted dry with filter paper, and cut into small pieces. SiCl ₄ and NaN ₃ were used without special treatment. Weigh NaN ₃ and metal Na according to the pre-designed ratio and add them to about 60ml stainless steel reaction kettle, and then add metered SiCl ₄ . The operation was carried out in a glove box filled with dry argon at room temperature. Put the sealed reaction kettle into a well-type crucible furnace, react at a constant temperature at a predetermined temperature (400-600° C.) for 0.5-6 hours, then allow it to cool naturally, open the kettle and take out the reaction product. The product is washed with distilled water, washed with absolute ethanol and centrifuged to remove the by-product sodium chloride, and dried in a vacuum oven at 80°C for 24 hours to obtain a white powder sample, weigh and calculate the yield, and the product is nitrogenized The total yield of silicon relative to the raw material _SiCl4 is above 90%.

For n=2, 3, and 4, ΔG°C are -3766, -3954, and -4141kJ/mol respectively, and ΔH° are -3660, -3682, and -3704kJ/mol respectively, indicating that the above reaction is a spontaneous reaction, and a large amount of reaction is released hot. But in the initial stage of the reaction, the formation and nucleation of silicon nitride are in the liquid metal sodium. This reaction is different from SiCl ₄ which only reacts with NaN ₃ without the participation of metal sodium. In addition to providing a crystallization and growth environment, the liquid metal sodium is also a reactant and a temporary diluent and heat sink, thereby moderating the reaction and facilitating the crystal growth of the product into nanowire and nanoribbon shapes. The present invention also studies the influence of excessive sodium metal on the product morphology, and finds that when the sodium metal is excessive to 3-8 times the reaction dose, it is unfavorable for the formation of silicon nitride nanowires and nanobelts, and the product morphology is mostly nano particles and nanorods. This shows that the growth of nanocrystals is accompanied by a chemical reaction, which may be caused by local overheating of the reaction. For the reaction below 400°C, a part of by-product polysilicon particles will be produced in the reaction, which will bring impurities to the product and complicate the purification. When the temperature is above 400°C, the reaction occurs for more than 1 hour. There is no polysilicon in the product, and the reaction is complete. Compared with SiCl ₄ , the yield of silicon nitride is more than 90%, but the reaction time also has a certain influence on the morphology of the product. The product ratio has a certain relationship with the reaction pressure, because the reaction is carried out in a sealed reactor, and the formation of gas in the reaction system makes the pressure change within a certain range. According to ideal gas estimation, when n=4, the maximum pressure of the preparation reaction does not exceed 220 atmospheres.

The resulting product sample structure is with MSAL-XD2 type X-ray powder diffractometer (40kV, 20mA, λ=1.5406 ) analysis, 2θ is in the range of 10-80°. The product morphology was observed with Technai-10 and JEM-2010HR transmission electron microscopes, the microstructure of micro-domain materials was studied with electron diffraction, and its composition was analyzed with EDX. The samples for electron microscopic observation, electron diffraction and EDX analysis are to disperse the product powder in absolute ethanol by ultrasonic waves, and then drop it on the special carbon film copper grid for TEM for observation. The product sample was also observed by JSM-6330F field emission scanning electron microscope and its composition was analyzed by EDX. The sample was prepared by directly distributing the product powder on the double-sided adhesive and sticking it on the copper platform of the sample and spraying gold for observation. Nicolet-type infrared spectrometer was used for FTIR analysis, and the product powder was prepared by KBr pellets. ESCALAB 250 (Thermao Electron Corp.) X-ray energy spectrometer was used for X-ray energy spectroscopic analysis, the Kα line of aluminum was used as the excitation light source, and carbon (binding energy C1s 285.5eV) was used as the internal standard.

Description of drawings:

Figure 1. X-ray diffraction pattern of silicon nitride nanowires.

Figure 2. FTIR spectrum of silicon nitride nanowires.

Figure 3. Transmission electron micrograph (a) and electron diffraction pattern (b) of silicon nitride nanowires.

Figure 4. Scanning electron micrograph (a) and EDX analysis spectrum (b) of silicon nitride nanowires.

Figure 5. TEM photographs of silicon nitride nanowires prepared under different conditions. (a) 1 hour at 400°C, n=4; (b) 5 hours at 500°C, n=4; (c) 1 hour at 500°C, n=3; (d) 5 hours at 500°C, n=3.

Figure 6. X-ray diffraction spectrum of silicon nitride nanoribbons.

Figure 7. TEM photos (a, b) and EDS spectra of silicon nitride nanobelts.

Figure 8. FTIR spectrum of silicon nitride nanoribbons.

Figure 9. X-ray energy spectra of silicon nitride nanowires (a) and nanoribbons (b).

Figures 1-4 show the characterization results of silicon nitride nanowires obtained by reacting at 450° C. for 5 hours when n=4.

，与JCPDS card# 82-0710相符合(粗线，a＝7.634，c＝2.921

)；其余较弱的衍射峰都对应于α-Si₃N₄(细线，JCPDS card# 83-0700)。没有杂质信号。这说明所制备的产物是结晶氮化硅。根据β-Si₃N₄衍射线较强而α-Si₃N₄较弱的特征，可以判断产物主要物相是β-Si₃N₄。采用有关文献的估计方法，得到α-Si₃N₄/β-Si₃N₄质量比等于0.1598，说明产物中β-Si₃N₄约占86.2％，α-Si₃N₄约占13.8％.Wherein Fig. 1 is the X-ray diffraction pattern of product powder. The indexed 17 diffraction envelopes correspond to the diffraction lines of β- _Si3N4 , from which _the calculated lattice constants are a = 7.590 and c = 2.904

, consistent with JCPDS card# 82-0710 (bold line, a=7.634, c=2.921

); the remaining weaker diffraction peaks all correspond to α-Si ₃ N ₄ (thin line, JCPDS card# 83-0700). No impurity signal. This indicates that the prepared product is crystalline silicon nitride. According to the feature that the β-Si ₃ N ₄ diffraction line is stronger and the α-Si ₃ N ₄ is weaker, it can be judged that the main phase of the product is β-Si ₃ N ₄ . Using the estimation method in relevant literature, the mass ratio of α-Si ₃ N ₄ /β-Si ₃ N ₄ is equal to 0.1598, indicating that β-Si ₃ N ₄ accounts for about 86.2% and α-Si ₃ N ₄ accounts for about 13.8% in the product .

Figure 2 shows the FTIR spectrum of this sample. The figure shows strong absorption at 445.6, 578.2, 858.2, 990.7 and 1047 cm ^-1 , indicating that the product is silicon nitride. Among them, 858.2 and 445.6, 578.2 cm ^-1 correspond to the stretching vibration and deformation vibration of Si-N bonds, respectively. 990.7 ^cm corresponds to the Si-N bond asymmetric stretching vibration. 1047 ^cm corresponds to the stretching vibration of the N-Si-O bond. These absorption peaks are characteristic absorptions of silicon nitride. The existence of Si-O bonds comes from the oxidation and purification of the material surface during hydrolysis, resulting in the formation of a small amount of Si-OH and NH bonds. Therefore, a relatively weak absorption peak corresponding to the NH bond can be seen at the position of 1635cm ^-1 . As for the absorption around 3439 cm ^-1 , which corresponds to the absorption of OH bonds, its main cause is the moisture absorbed by the sample in the air.

Figure 3 shows the transmission electron micrograph and electron diffraction pattern of the sample. As shown in Figure 3a, the silicon nitride nanowires have an average diameter of about 30nm, a relatively long length, and a length/diameter ratio of about 25. The electron diffraction in Figure 3b shows clear spots, indicating that the silicon nitride nanowires are well crystallized. According to calculations, its d-value is consistent with that of β-Si ₃ N ₄ , and is consistent with that of β-Si ₃ N in the above-mentioned X-diffraction. ₄ agree.

Figure 4 shows the silicon nitride nanowire scanning electron microscope photo and EDX analysis spectrum of the sample. As shown in Figure 4a, although the shape and size of the silicon nitride nanowires are consistent with those observed by the transmission electron microscope, they are intertwined into bundles. This is because the nanowires that were ultrasonically dispersed in ethanol were observed by transmission electron microscopy, while the powder samples were directly displayed by scanning electron microscopy. The EDX spectrum in Figure 4b shows strong Si and N signals. By calculation, the molar ratio of Si to N content is 0.812:1, which is close to the theoretical value of 0.75:1, but there is a difference that there is more Si. This may be due to the loss of N on the surface of the material due to oxidation and/or hydrolysis. EDX spectrum analysis also found signals of heterogeneous elements such as gold and a small amount of oxygen, which were caused by gold spraying during sample preparation, oxidation or/and hydrolysis of the sample surface, and double-sided tape used for sample preparation.

The above analysis and characterization indicate that silicon nitride nanowires were prepared under the conditions described above. However, for different preparation conditions, the obtained silicon nitride nanowires have different morphologies. Figure 5 shows several different situations, respectively (a) n=4,400°C for 1 hour, (b)n=4,500°C for 5 hours, (c)n=3,500°C for 1 hour, (d)n=3,500°C5 Hour. It can be seen from the figure that factors such as ratio, temperature and reaction time will affect the morphology characteristics of the product nanowires.

Figures 6-8 show the characterization results of silicon nitride nanoribbons obtained by reacting at 450° C. for 5 hours when n=2. Wherein Fig. 6 is the X-ray diffraction pattern of product powder. The indexed diffraction peaks correspond to the diffraction lines of β-Si ₃ N ₄ , which are consistent with JCPDS card# 82-0710 (thick lines); the rest of the weaker diffraction peaks correspond to α-Si ₃ N ₄ (thin lines, JCPDS card# 83-0700). According to the difference of relative intensity, it shows that the prepared product is mainly β-Si ₃ N ₄ . It is estimated that the mass ratio of α-Si ₃ N ₄ /β-Si ₃ N ₄ is equal to 0.0713, indicating that β-Si ₃ N ₄ accounts for about 93.3% and α-Si ₃ N ₄ accounts for about 6.7% in the product. Figure 7 shows Transmission electron micrograph and EDX analysis spectrum of the sample. As shown in Figure 7a, the silicon nitride nanometer bandwidth is 40-80nm, with an average of about 60nm, a length of more than 500nm, and a thickness estimated to be no more than 20nm. The EDX spectrum in Figure 7b shows strong Si and N signals. The molar ratio of Si to N content is 0.792:1, which is close to the theoretical value of 0.75:1. EDX spectrum analysis also found that copper, a small amount of oxygen, etc. Copper is from the TEM copper grid, and oxygen is from the oxidation or/and hydrolysis of the sample surface. Figure 8 shows the FTIR spectrum of this sample. Its infrared absorption characteristics are similar to those of nanowires, indicating that the product is silicon nitride. A weaker absorption peak corresponding to NH bonds and an absorption peak corresponding to OH bonds can also be seen.

Figure 9 shows the X-ray energy spectrum characterization results of silicon nitride nanowires (reacted at 450°C for 5 hours when n=4) and nanoribbons (reacted at 450°C for 5 hours when n=2). It can be seen from the figure that the X-ray energy spectra of nanowire (a) and nanoribbon (b) samples have strong signals at binding energies of 101.8, 152.8 and 397.3eV, corresponding to Si2p, Si2s and N1s binding energies respectively, It is consistent with that reported in the literature, and the N: Si atomic ratios are 1.147 and 1.355, which is relatively close to the theoretical value of Si ₃ N ₄ 1.333, indicating that the product is silicon nitride. The peak at 285.5eV is the C1s signal of the internal standard carbon, and the peak at 532.5eV corresponds to the O1s of oxygen. The presence of oxygen may come from surface oxidation or/and hydrolysis of the material on the one hand, and surface adsorption on the other hand.

Detailed ways:

Embodiment 1: Preparation of silicon nitride nanowires

Take 3.8 grams of NaN ₃ and 2.69 grams of metallic Na into a stainless steel reactor with a capacity of about 60 ml, and then add 5 ml of liquid SiCl ₄ . _SiCl4 is an analytically pure reagent, white solid crystalline _NaN3 and metal Na are chemically pure reagents. Metal Na was removed from kerosene, blotted dry with filter paper, and cut into small pieces. Operations were carried out in a glove box filled with dry argon at room temperature. Cover and seal the reaction kettle, then put it into a well-type crucible furnace, react at a constant temperature of 450°C for 5 hours, cool to room temperature naturally, open the kettle and take out the reaction product. The product was washed with water, washed with absolute ethanol and centrifuged to remove the by-product sodium chloride, and dried in a vacuum oven at 80° C. for 24 hours to obtain 1.895 g of a white powder product. After structure, composition and morphology analysis, it is proved that the product is pure silicon nitride, of which β-Si ₃ N ₄ accounts for about 86.2%, α-Si ₃ N ₄ accounts for about 13.8%, and the total yield relative to SiCl ₄ is 92.5% %, the shape of the product nanowire is uniform, its average diameter is 30nm, and its average length is about 500nm.

Embodiment 2: Preparation of silicon nitride nanowires

Take 3.25 grams of NaN ₃ and 3.0 grams of metal Na into a stainless steel reactor with a volume of about 60 ml, and then add 5 ml of liquid SiCl ₄ . React at a constant temperature of 450°C for 2 hours, cool down to room temperature naturally, open the kettle and take out the reaction product. All the other operations are the same as above-mentioned example 1, obtain 1.947 gram white powder products. After structure, composition and morphology analysis, it is proved that the product is pure silicon nitride, of which β-Si ₃ N ₄ accounts for about 90.3%, α-Si ₃ N ₄ accounts for about 9.7%, and the total yield relative to SiCl ₄ is 94.4% %, the morphology of the product is dominated by nanowires, with a small amount of uniform nanobelts.

Embodiment 3: Preparation of silicon nitride nanobelts

Take 2.70 grams of NaN ₃ and 3.15 grams of metal Na into a stainless steel reactor with a volume of about 60 ml, and then add 5 ml of liquid SiCl ₄ . React at a constant temperature of 500°C for 3.5 hours, cool down to room temperature naturally, open the kettle and take out the reaction product. All the other operations are the same as above-mentioned example 1, obtain 1.863 gram white powder products. After structure, composition and morphology analysis, it is proved that the product is pure silicon nitride, of which β-Si ₃ N ₄ accounts for about 89.7%, α-Si ₃ N ₄ accounts for about 10.3%, and the total yield relative to SiCl ₄ is 90.6 %, the product is dominated by nanowires, and also contains more nanorods and nanobelts.

Embodiment 4: Preparation of silicon nitride nanoribbons

Take 3.8 grams of NaN ₃ and 6.8 grams of metal Na into a stainless steel reactor with a volume of about 60 ml, and then add 10 ml of liquid SiCl ₄ . _SiCl4 is an analytically pure reagent, white solid crystalline _NaN3 and metal Na are chemically pure reagents. All the other operations are the same as above-mentioned example 1, obtain 3.733 grams of white powder products. After structure, composition and morphology analysis, it is proved that the product is pure silicon nitride, of which β-Si ₃ N ₄ accounts for about 93.3%, α-Si ₃ N ₄ accounts for about 6.7%, and the total yield relative to SiCl ₄ is 91.1% %, the uniform width of the nanobelt is 40-80nm, the length is about 500nm, and the thickness is estimated to be no more than 20nm.

Claims (10)

1. A method for preparing a silicon nitride nanowire or nanobelt powder material, using _SiCl4 as a silicon source, _NaN3 as a nitrogen source, reacting in a reactor, the product is washed, separated and dried to obtain a white powder The product is characterized in that: metal Na is also added to the reaction kettle; the reaction is carried out for 0.5-6 hours at a temperature of 400-600 ° C and a self-generated pressure of 5-40 MPa in the system to obtain silicon nitride nanowires or nanobelts; The chemical equation for the reaction is expressed as: 3SiCl ₄ +n NaN ₃ +(12-n)Na=Si ₃ N ₄ +12NaCl+(1.5n-2)N ₂ Where n satisfies 1.334≤n≤11.998. 2. The preparation method according to claim 1, wherein n satisfies 2≤n≤4. 3. The preparation method according to claim 1, characterized in that: SiCl ₄ , NaN ₃ , and Na are added in a mass ratio of 1:0.4-0.5:0.36-0.4 to prepare silicon nitride nanowires. 4. The preparation method according to claim 1, characterized in that the mass ratio of SiCl ₄ , NaN ₃ , and Na added is 1:0.25-0.38:0.40-0.45 to prepare silicon nitride nanobelts. 5. The preparation method according to claim 1, characterized in that: the reaction kettle is heated and reacted after passing an inert gas into the reaction kettle and sealing it. 6. The preparation method according to claim 5, characterized in that: the inert gas is nitrogen or argon. 7. The preparation method according to claim 1, characterized in that: the reaction temperature is 450-500°C. 8. The preparation method according to claim 1, characterized in that: the autogenous pressure of the reaction system is 15-25 MPa. 9. The preparation method according to claim 1, characterized in that: the reaction time is 0.5-5 hours. 10. The preparation method according to claim 1, characterized in that: the product is washed with water and absolute ethanol, then centrifuged and then dried in a vacuum oven at 80°C for 24 hours.

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