A method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive.

Abstract

The application discloses a method for preparing high-aspect-ratio high-purity Si3N4 whiskers by adopting Mg as an additive in a one-step method, adopts magnesium powder and alpha-Si3N4 powder as main raw materials, and makes Mg and Si3N4 react in a molten salt system formed by high-temperature melting of metal salt to generate MgSiN2 in situ on the surface of Si3N4 particles as a sintering aid, forms Si3N4@MgSiN2 core-shell structure powder, further heats to make MgSiN2 participate in forming a liquid phase, makes alpha-Si3N4 phase change into a beta phase in the liquid phase, and grows into long rod-shaped whiskers, and MgSiN2 is decomposed into Si3N4 and gaseous Mg and N2 at high temperature, so that the high purity of the whiskers is ensured. The method realizes uniform distribution of the additive, reduces the additive consumption, and meanwhile, the decomposition of MgSiN2 can make the whiskers further disperse, and then the aspect ratio and dispersibility of the Si3N4 whiskers are ensured.

Description

Technical Field

[0001] This invention belongs to the field of silicon nitride ceramic whisker technology, specifically relating to a one-step method for preparing high aspect ratio, high purity Si3N4 whiskers using Mg as an additive. Background Technology

[0002] Whiskers are artificially synthesized one-dimensional single-crystal materials. Due to their small size, highly ordered atomic structure, complete crystal structure, and few internal defects, their performance indicators are close to those of ideal crystals. They are often introduced as reinforcing phases into ceramic-based, metal-based, and polymer-based material matrices to strengthen and toughen them. Silicon nitride whiskers, due to their unique lattice characteristics, possess many excellent properties such as high strength, high modulus, high temperature resistance, high thermal conductivity, low density, low expansion, and good chemical stability, making them an ideal reinforcing phase material for composite material systems.

[0003] A method for preparing β-Si3N4 whiskers by directly heating α-Si3N4 at high temperatures, causing a phase transformation and structural rearrangement, results in long, rod-shaped whiskers. During the reaction, sintering aids are generally added to reduce the required temperature and pressure, and these additives also improve the properties of the sintered whiskers. Currently, metal oxide aids are the most well-developed for sintering β-Si3N4 whiskers, while many non-oxide aids such as hydrides, fluorides, and nitrides have also emerged.

[0004] Magnesium silicon nitride, as a non-oxide sintering aid, can effectively reduce the oxygen content in the liquid phase and hinder the formation of lattice oxygen, thereby effectively improving the thermal conductivity and mechanical properties of silicon nitride. Furthermore, it undergoes a decomposition reaction at high temperatures, decomposing into gas and removing it from the sintering system, thus improving the purity of the product. It also has a certain effect on the dispersion of whiskers, reducing the adhesion of whiskers caused by the residual liquid phase after sintering upon cooling.

[0005] Currently, most methods for preparing silicon nitride whiskers through heat treatment involve mixing silicon nitride powder raw materials with sintering aids using mechanical mixing methods such as ball milling. This traditional mixing method can lead to uneven distribution of the aids, with insufficient aids in some raw materials, affecting the phase transformation of silicon nitride and the radial growth of whiskers. To solve this problem, more aids need to be added, which will affect the yield and purity of the whiskers. Summary of the Invention

[0006] To address the problems of low aspect ratio, poor dispersion, and low purity of whiskers caused by the use of oxide additives in existing technologies, as well as the uneven distribution of additives and raw materials, which leads to a decrease in whisker conversion rate and purity, this invention provides a one-step method for preparing high aspect ratio and high purity Si3N4 whiskers using Mg as an additive. Using Si3N4 powder as raw material and magnesium powder as an additive, a molten salt sintering method is used to prepare MgSiN2-coated Si3N4 core-shell structure powder, achieving uniform distribution of the additive. Further heating and sintering then produces high aspect ratio and high purity silicon nitride whiskers with an aspect ratio of 20-40.

[0007] The technical principle of this invention is as follows:

[0008] 3Mg + Si₃N₄ + N₂ → 3MgSiN₂

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive includes the following steps:

[0011] (1) Mix silicon nitride powder, magnesium powder and metal salt to obtain raw material powder;

[0012] The mass ratio of magnesium powder to silicon nitride is 1:(15-100);

[0013] The mass ratio of magnesium powder, silicon nitride powder and metal salt is 1:(1-3);

[0014] The metal salt is one or more of sodium salt, potassium salt, and magnesium salt; wherein the sodium salt is one or two of sodium chloride, sodium bromide, etc., the potassium salt is one or more of potassium fluoride, potassium chloride, potassium bromide, etc., and the magnesium salt is magnesium chloride, and the mixture is in any proportion.

[0015] The process of mixing silicon nitride powder, magnesium powder and metal salt involves grinding the silicon nitride powder, magnesium powder and alkali metal salt and then mixing them using a ball mill at a speed of 300 rpm-400 rpm for 1-12 hours.

[0016] (2) The raw material powder obtained in the above step is subjected to high-temperature sintering under nitrogen atmosphere. After the sintering is completed, it is cooled to room temperature in the furnace to obtain high aspect ratio high purity Si3N4 whiskers. The aspect ratio of the whiskers is 20-40, and the whisker diameter is uniform and there is no residual amorphous phase.

[0017] The high-temperature sintering treatment described herein is performed using pressureless sintering. The first stage sintering temperature is 1000℃-1400℃, and the sintering holding time is 1h-8h. The second stage sintering temperature is 1400℃-1500℃, and the sintering holding time is 1h-12h. The third stage sintering temperature is 1600℃-1800℃, and the sintering holding time is 1h-8h. The heating rate during the sintering process is 1℃ / min-15℃ / min.

[0018] In this invention:

[0019] Preferably, the mass ratio of magnesium powder to silicon nitride in step (1) is 1:(15-30).

[0020] Preferably, in step (1), the mass ratio of magnesium powder to silicon nitride powder to metal salt is 1:(1.5-2.5).

[0021] Preferably, in step (1), the metal salt is one or more of sodium salt, potassium salt, and magnesium salt; wherein, the sodium salt is one or two of sodium chloride, sodium bromide, etc., the potassium salt is one or more of potassium fluoride, potassium chloride, potassium bromide, etc., and the magnesium salt is magnesium chloride. The salt system is divided into three types, namely mono-, binary, and ternary systems. The types of salts can be selected at will, and the mixing ratio can be arbitrary.

[0022] Preferably, in step (1), the ball milling method is used for mixing, with a ball milling speed of 300 rpm-350 rpm and a time of 4 h-12 h.

[0023] Preferably, the pressureless sintering in step (2) is carried out in a nitrogen atmosphere with a pressure ≥0.1MPa. The high N2 pressure during high-temperature sintering can prevent MgSiN2 from decomposing at high temperatures, allowing it to fully exert its role as an additive.

[0024] Preferably, the sintering temperature in the first stage of step (2) is 1300℃ and the holding time is 2h. Before the first stage of sintering, the salt has melted and the molten salt system is formed. Mg dissolves in the molten salt system to achieve uniform dispersion of Mg around the Si3N4 particles. During the sintering process, Mg reacts with Si3N4 to generate MgSiN2 in situ on the surface of Si3N4. Therefore, the process is carried out in the molten salt system, the distribution of Mg is more uniform, and the generation of MgSiN2 is also more uniform, which can better achieve uniform distribution of the additives.

[0025] Preferably, the sintering temperature in the second stage of step (2) is 1400℃ and the holding time is 4h. The holding temperature in this stage needs to be higher than the boiling point of the metal salt to ensure that the molten salt can be removed from the raw material system by heating after the MgSiN2 generation reaction is completed, so as to avoid its influence on the subsequent Si3N4 phase transformation and whisker growth process.

[0026] Preferably, the sintering temperature in the third stage of step (2) is 1650℃-1750℃, the holding time is 1h-4h, the optimal heat treatment process is 1700℃ for 2h, the heating rate during the sintering process is 5℃ / min-15℃ / min, and after the sintering of the three stages is completed, the sample is cooled to room temperature with the furnace.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1. Compared to conventional processes that use metal oxide sintering aids to sinter silicon nitride whiskers, the final product contains a certain amount of intergranular phase containing metal liquid after cooling, which adheres to the whisker surface, causing whisker adhesion and requiring additional processes to remove it, thus affecting whisker quality. The present invention describes a one-step method for preparing high aspect ratio, high-purity Si3N4 whiskers using Mg as an additive. Due to the presence of the reaction 3MgSiN2(s)→3Mg(g)+Si3N4(s)+N2(g), by controlling the temperature and nitrogen atmosphere pressure, the sintering aid MgSiN2, after exerting its auxiliary effect, undergoes a decomposition reaction at high temperature, removing it from the silicon nitride system. This reduces the amount of residual intergranular phase after cooling, slows down intergranular phase adhesion to whiskers, and the decomposition gaseous products also promote whisker dispersion.

[0029] 2. The present invention describes a one-step method for preparing high aspect ratio and high purity Si3N4 whiskers using Mg as an additive. MgSiN2 is generated in situ on the surface of Si3N4 particles via a molten salt method, resulting in a more uniform distribution of the additive and a more complete exertion of its effect. This indirectly reduces the amount of additive used and improves the yield and purity of the whiskers. Attached Figure Description

[0030] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0031] Figure 1 The images show the XRD patterns of Si3N4 whiskers prepared in one step using Mg as an additive in Example 3 of the present invention and Si3N4 whiskers prepared by ball milling using Y2O3 as a sintering aid in Comparative Example 1.

[0032] Figure 2 These are SEM images of whiskers prepared in Examples 1-5 and Comparative Examples 1-2 of the present invention (where (a)-(e) correspond to Examples 1-5, and (f)-(g) correspond to Comparative Examples 1-2, respectively);

[0033] Figure 3 This is an SEM image of the whiskers obtained in Example 3. Detailed Implementation

[0034] The present invention will be further described in detail below through the following embodiments. It should be understood that the following embodiments are only for illustrating the present invention and are not intended to limit the present invention.

[0035] In this specific implementation:

[0036] The silicon nitride powder has an average particle size of 0.5 μm, a purity of >99.5%, an α phase content of >95 wt%, and an oxygen content of 1.35%.

[0037] The magnesium powder has a purity of ≥99.0 wt% and a particle size of 10-150 μm.

[0038] The potassium chloride has a purity of ≥99.5wt%, and the sodium chloride has a particle size of ≤0.1mm;

[0039] The sodium chloride has a purity of ≥99.5wt% and a particle size of ≤0.1mm.

[0040] This invention employs a one-step method using Mg as an additive to prepare high aspect ratio, high-purity Si3N4 whiskers, such as... Figure 1 As shown in the image.

[0041] Example 1:

[0042] A method for preparing high aspect ratio, high purity Si3N4 whiskers in a one-step process using Mg as an additive, the specific steps of which are as follows:

[0043] Step 1: Weigh 46.875g of silicon nitride particles and 3.125g of magnesium powder, add 50g of sodium chloride particles, and ball mill at 300rpm for 4 hours to obtain mixed raw materials;

[0044] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heat to 1300℃ and hold for 2 hours, then raise the temperature to 1400℃ and hold for 4 hours, and finally raise the temperature to 1700℃ and hold for 2 hours. The heating rate is 10℃ / min. After sintering, cool to room temperature with the furnace.

[0045] The final whisker sample had an average length of 7.810 μm, an average diameter of 0.282 μm, and a calculated average aspect ratio of 28.221.

[0046] Example 2:

[0047] A method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive, the specific steps are as follows:

[0048] Step 1: Weigh 48g of silicon nitride particles and 2g of magnesium powder, add 50g of sodium chloride particles and 50g of potassium chloride particles, and ball mill at 300rpm for 8 hours to obtain mixed raw materials;

[0049] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heat to 1300℃ and hold for 2 hours, then raise the temperature to 1400℃ and hold for 4 hours, and finally raise the temperature to 1750℃ and hold for 2 hours. The heating rate is 10℃ / min. After sintering, cool to room temperature with the furnace.

[0050] The final whisker sample had an average length of 9.050 μm, an average diameter of 0.350 μm, and a calculated average aspect ratio of 26.976.

[0051] Example 3:

[0052] A method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive, the specific steps are as follows:

[0053] Step 1: Weigh 46.875g of silicon nitride particles and 3.125g of magnesium powder, add 50g of sodium chloride particles and 50g of potassium chloride particles, and ball mill at 400rpm for 8 hours to obtain mixed raw materials;

[0054] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heat to 1300℃ and hold for 2 hours, then raise the temperature to 1400℃ and hold for 4 hours, and finally raise the temperature to 1700℃ and hold for 2 hours. The heating rate is 10℃ / min. After sintering, cool to room temperature with the furnace.

[0055] The final whisker sample had an average length of 9.147 μm, an average diameter of 0.265 μm, and a calculated average aspect ratio of 35.223.

[0056] Figure 1 The images show the XRD patterns of Si3N4 whiskers prepared in Example 3 using Mg as an additive in a one-step method and Si3N4 whiskers prepared in Comparative Example 1 using Y2O3 as a sintering aid in a ball milling method.

[0057] Figure 3 This is an SEM image of the whiskers obtained in Example 3.

[0058] Example 4:

[0059] A method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive, the specific steps are as follows:

[0060] Step 1: Weigh 49g of silicon nitride particles and 1g of magnesium powder, add 50g of sodium chloride particles and 50g of potassium chloride particles, and ball mill at 400rpm for 12h to obtain mixed raw materials.

[0061] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heat to 1300℃ and hold for 2 hours, then raise the temperature to 1400℃ and hold for 4 hours, and finally raise the temperature to 1650℃ and hold for 2 hours. The heating rate is 10℃ / min. After sintering, cool the furnace to room temperature.

[0062] The final whisker sample had an average length of 7.518 μm, an average diameter of 0.303 μm, and a calculated average aspect ratio of 24.917.

[0063] Example 5:

[0064] A method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive, the specific steps are as follows:

[0065] Step 1: Weigh 49g of silicon nitride particles and 1g of magnesium powder, add 50g of sodium chloride particles and 50g of potassium chloride particles, and ball mill at 400rpm for 12h to obtain mixed raw materials.

[0066] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heat to 1300℃ and hold for 2 hours, then raise the temperature to 1400℃ and hold for 4 hours, and finally raise the temperature to 1700℃ and hold for 4 hours. The heating rate is 10℃ / min. After sintering, cool to room temperature with the furnace.

[0067] The final whisker sample had an average length of 9.421 μm, an average diameter of 0.379 μm, and a calculated average aspect ratio of 25.440.

[0068] Figure 2 These are SEM images of the whiskers prepared in Examples 1-5 and Comparative Examples 1-2 (where (a)-(e) correspond to Examples 1-5, and (f)-(g) correspond to Comparative Examples 1-2, respectively).

[0069] Comparative Example 1:

[0070] The specific steps are as follows:

[0071] Step 1: Weigh 47g of silicon nitride particles, add 3g of yttrium oxide particles, add 100g of anhydrous ethanol, ball mill for 12h to obtain a mixed slurry, and dry the mixed slurry in a 60℃ oven for 12h to obtain a mixed powder.

[0072] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heating them to 1700℃ at a heating rate of 10℃ / min and holding them at that temperature for 2 hours; after sintering, cool them to room temperature in the furnace.

[0073] The difference between Comparative Example 1 and the Example is that Comparative Example 1 uses Y2O3 as a sintering aid by direct addition, while the aid in the Example is MgSiN2 generated by the in-situ reaction of Mg and Si3N4. It is a non-oxide sintering aid with lower oxygen content in the system and higher viscosity of the liquid phase, which is more conducive to the formation of local liquid phase and promotes the independent growth of single whiskers. In addition, MgSiN2 can be removed from the system after sintering, which improves the purity of the sample.

[0074] The final whisker sample had an average length of 6.871 μm, an average diameter of 0.395 μm, and a calculated average aspect ratio of 17.468.

[0075] Comparative Example 2:

[0076] The specific steps are as follows:

[0077] Step 1: Weigh 47g of silicon nitride particles, add 3g of silicon nitride magnesium particles, add 100g of anhydrous ethanol, ball mill for 12h to obtain a mixed slurry, and dry the mixed slurry in a 60℃ oven for 12h to obtain a mixed powder.

[0078] Step 2: Sinter the obtained raw materials under a nitrogen atmosphere, heating them to 1700℃ at a heating rate of 10℃ / min and holding them at that temperature for 2 hours; after sintering, cool them to room temperature in the furnace.

[0079] The difference between Comparative Example 2 and the Example is that Comparative Example 2 uses the direct addition of MgSiN2 as a sintering aid, while the aid in the Example is MgSiN2 generated by the in-situ reaction of Mg and Si3N4. Compared with the direct addition of the aid, the in-situ generation of the aid can better achieve the uniform distribution of the aid, more fully realize the effect of the aid, indirectly reduce the amount of aid used, and improve the yield and purity of the sample.

[0080] The final whisker sample had an average length of 8.542 μm, an average diameter of 0.457 μm, and a calculated average aspect ratio of 19.122.

[0081] Results and Discussion:

[0082] 1. In this invention, the phase composition of the whisker samples prepared in Example 3 and Comparative Example 1 was tested using an X-ray diffractometer (Shimadzu XRD-6100, Japan). The results are shown in [Figure number missing]. Figure 1 ;

[0083] As can be seen, in the final product of Comparative Example 1 using Y2O3 as a sintering aid, a certain proportion of Y-containing glass phase appears. After fine calculation, the mass ratio of Y-containing glass phase in the sample reaches 13.7wt%, which has an adverse effect on the yield and dispersion of whiskers. In contrast, in the final product of Example 3 using MgSiN2 as a sintering aid, only β-Si3N4 phase is present, with no Mg-containing glass phase residue. The whisker product has higher purity and is easier to disperse.

[0084] 2. In this invention, the microstructure and whisker quality of the whisker samples prepared in the examples and comparative examples were observed using a scanning electron microscope (Hitachi Regulus 8100). The results are shown in [Figure number missing]. Figure 2 ;

[0085] As can be seen, when the mass ratio of magnesium to silicon nitride in the raw materials is 1:(15-100), the generated auxiliary agent is sufficient to meet the requirements of whisker growth. Too much magnesium will reduce the whisker yield and cause unnecessary waste. Too little magnesium will result in insufficient auxiliary agent to support the entire process of phase transformation and whisker growth. A mass ratio of magnesium powder to silicon nitride of 1:(15-30) is better, and a mass ratio of magnesium powder to silicon nitride of 1:15 is the best. In the in-situ growth of MgSiN2, the binary molten salt system has better Mg dispersibility than the mono-component system. When the mass ratio of magnesium powder to silicon nitride powder to salt is 1:2 or higher, Mg has better dissolution and dispersion effects in the molten salt system. Increasing the sintering temperature and extending the holding time will increase the average diameter of the whiskers.

[0086] Analysis of Examples 1-5 and Comparative Example 1 shows that using MgSiN2 as a sintering aid results in a smoother and purer whisker surface, while the sample using Y2O3 as a sintering aid contains many residual glass phase particles, which cause the whiskers to stick together and are not conducive to whisker dispersion.

[0087] Analysis of Examples 1-5 and Comparative Example 2 shows that when MgSiN2 is generated in situ using the molten salt method, the distribution of the additives is more uniform, which can achieve similar whisker growth effects with less additives. Furthermore, the formation of magnesium silicon nitride additives on the surface of silicon nitride particles can directionally generate a liquid phase, promote the directional growth of whiskers, and alleviate the cross-growth of whiskers to a certain extent, making the dispersion of whiskers easier.

[0088] 3. In this invention, Nano measurer software was used to measure the whisker length and diameter in the SEM images. The final average whisker diameter, average length, and average aspect ratio results are shown at the end of each embodiment and comparative example. Comparing the measurement data and SEM images of whisker samples obtained from all experiments, it can be found that:

[0089] ① Data from Examples 1 and 3 show that Example 1 used a mono-element molten salt, while Example 3 used a binary molten salt. The results indicate that using a binary molten salt can provide a more balanced molten salt system for shell growth, and the higher the salt content, the better the shell growth effect and the more conducive it is to whisker development.

[0090] ② Comparison of data from Examples 1-5 reveals that the higher the heat treatment temperature and holding time, the larger the diameter of the grown whiskers, which will affect the aspect ratio of the whiskers to a certain extent. The optimal heat treatment process is to hold at 1700℃ for 2 hours.

[0091] ③ By comparing Examples 3-5, it can be found that the whisker growth effect is optimal when the mass ratio of magnesium to silicon nitride in the raw materials is 1:15;

[0092] ④ By comparing Example 3 with Comparative Examples 1-2, it can be found that the whisker prepared by the preparation process proposed in this invention has a better aspect ratio than the preparation process that directly adds sintering aids.

[0093] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. Various process solutions that are not substantially different from the concept of the present invention are all within the scope of protection of the present invention.

Claims

1. A method for preparing high aspect ratio, high purity Si3N4 whiskers in a one-step process using Mg as an additive, characterized in that: Includes the following steps: (1) Mix silicon nitride powder, magnesium powder and metal salt to obtain raw material powder; The mass ratio of magnesium powder to silicon nitride powder is 1:(15-100); The mass ratio of magnesium powder, silicon nitride powder and metal salt is 1:(1-3); The metal salt is one or more of sodium salt, potassium salt, and magnesium salt; wherein the sodium salt is one or two of sodium chloride and sodium bromide, the potassium salt is one or more of potassium fluoride, potassium chloride, and potassium bromide, and the magnesium salt is magnesium chloride, and the mixture is in any proportion. The process of mixing silicon nitride powder, magnesium powder and metal salt involves grinding the silicon nitride powder, magnesium powder and alkali metal salt and then mixing them using a ball mill at a speed of 300 rpm-400 rpm for 1-12 hours. (2) The raw material powder obtained in the above step is subjected to high-temperature sintering under nitrogen atmosphere. After the sintering is completed, it is cooled to room temperature in the furnace to obtain high aspect ratio high purity Si3N4 whiskers with an aspect ratio of 20-40 and no residual amorphous phase. The high-temperature sintering treatment described herein is performed using pressureless sintering. The first stage sintering temperature is 1000℃-1400℃, and the sintering holding time is 1h-8h. The second stage sintering temperature is 1400℃-1500℃, and the sintering holding time is 1h-12h. The third stage sintering temperature is 1600℃-1800℃, and the sintering holding time is 1h-8h. The heating rate during the sintering process is 1℃ / min-15℃ / min.

2. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: In step (1), the mass ratio of magnesium powder to silicon nitride powder is 1:(15-30).

3. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: In step (1), the mass ratio of magnesium powder to silicon nitride powder to metal salt is 1:(1.5-2.5).

4. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: In step (1), the metal salt is one or more of sodium salt, potassium salt, and magnesium salt; among them, the sodium salt is one or two of sodium chloride and sodium bromide, the potassium salt is one or more of potassium fluoride, potassium chloride, and potassium bromide, and the magnesium salt is magnesium chloride. The salt system is divided into three types, namely mono-, binary, and ternary systems. The types of salts can be selected at will, and the mixing ratio can be arbitrary.

5. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: The mixing process described in step (1) involves ball milling at a speed of 300-350 rpm for 4-12 hours.

6. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: The pressureless sintering described in step (2) is carried out in a nitrogen atmosphere with a pressure ≥0.1 MPa.

7. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: In step (2), the sintering temperature in the first stage is 1300℃ and the holding time is 2h.

8. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: In step (2), the sintering temperature in the second stage is 1400℃ and the holding time is 4h.

9. The method for preparing high aspect ratio, high purity Si3N4 whiskers in one step using Mg as an additive according to claim 1, characterized in that: In step (2), the sintering temperature in the third stage is 1700℃ and the holding time is 2h.

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