Method for manufacturing high-purity silicon nitride powder having excellent crystallinity by using silicon scraps

Abstract

The present invention provides a method for manufacturing a high-purity silicon nitride powder having excellent crystallinity by using silicon scraps, the method comprising the steps of: pulverizing silicon scraps using balls and media; preparing a silicon powder through the pulverization; and heat-treating the prepared silicon powder in a reducing atmosphere to synthesize silicon nitride, wherein in the step of pulverizing silicon scraps, the silicon scraps are pulverized for 24-96 hours. The present invention can implement a simple and efficient process for large-scale synthesis of a pure and highly crystalline Si3N4 powder mainly having an α-phase, even without using a catalyst.

Description

Method for producing high-purity silicon nitride powder with excellent crystallinity using silicon scrap

[0001] The present invention relates to a method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, and more specifically, to a powder milling and heat treatment method for producing silicon nitride powder by recycling silicon scrap left over after manufacturing silicon wafers from high-purity silicon ingots used in semiconductors, solar cells, etc.

[0002] Silicon nitride (Si3N4) ceramics possess a variety of advantages and are used in numerous fields, including automotive, aerospace, medical, electronics, and solar energy. They have been recognized as promising structural and electronic materials. These advantages include low density, high thermal conductivity, a wide bandgap, high mechanical strength, excellent biocompatibility, and excellent chemical and thermal stability.

[0003] It is widely known that crystalline Si3N4 exists as α-phase and β-phase, and that sintering α-Si3N4 at temperatures exceeding 1500℃ causes a phase transformation, forming a columnar β-Si3N4 structure with improved mechanical strength and fracture toughness. Therefore, securing the yield of α-Si3N4 in powder form is very important in the production of Si3N4 ceramics.

[0004] Since the physical properties of sintered ceramics significantly affect the quality of the powder, such as purity, homogeneity, and particle size, various methods for synthesizing Si3N4 powder have been proposed in the past. These methods include carbothermal reduction, combustion synthesis, direct nitridation, diimide synthesis, mechanical alloying, pyrolysis, sol-gel method, organic-inorganic reaction pathways, and radio-frequency glow-discharge synthesis. Since these methods are fundamentally based on reactions of starting materials in the solid, liquid, or gaseous phases, the quality and properties of the raw materials play a very important role.

[0005] Most Si3N4 powder synthesis methods, including these, are complex and require numerous manufacturing steps, making it difficult to control the microstructure and phase composition of the final product. Furthermore, some methods, such as carbothermal reduction, mechanical alloying, and sol-gel methods, produce low-quality powders with low yields. Furthermore, these methods are often labor-intensive and require expensive equipment, making them uneconomical for large-scale industrial production.

[0006] Meanwhile, Si3N4 powder produced by direct nitridation of silicon exhibits superior properties compared to those produced by other methods, including high purity and controlled particle size and morphology. Furthermore, the direct nitridation method is suitable for industrial mass production. Recently, silicon nitride, which has been increasingly used in high-power devices due to its superior mechanical and dielectric breakdown properties compared to aluminum nitride (AlN), can also be produced by nitriding silicon.

[0007] Typically, silicon nitridation is performed in a nitrogen atmosphere at temperatures ranging from 1200 to 1500°C. Nitriding below the melting point of silicon (~1414°C) without the addition of a catalyst can produce high-content α-Si3N4 powders, which are preferred for industrial applications. However, incomplete nitridation of silicon due to limited long-range nitrogen diffusion results in the presence of unreacted residual silicon as an impurity in the final product. Conversely, nitriding above the melting point can result in the formation of undesirable β-Si3N4.

[0008] Therefore, the main task of the present invention is to achieve complete reaction of silicon and obtain desirable α-Si3N4 which can be achieved through optimized powder processing and nitriding conditions.

[0009] Recently, significant efforts are being focused on recycling the large volumes of silicon waste (scrap) produced in the rapid growth of silicon-based device manufacturing in the semiconductor and electronics industries worldwide. Specifically, high-purity silicon, used in the semiconductor and electronics industries for the production of silicon-based devices, is converted from ingots into finished products, generating a significant amount of silicon scrap. This scrap is either stored for future use or discarded. This discarded silicon scrap contributes to environmental pollution and increases manufacturing costs.

[0010] The recycling and reuse of silicon scrap is important not only for health and environmental safety reasons, but also for cost-effectiveness in industrial applications. Silicon scrap has been effectively used in the production of various advanced ceramic materials, including silicon carbide (SiC) and its composites. Recently, several attempts have been made to replace conventional silicon sources for the production of silicon nitride (Si3N4) powder. For example, Hou et al. reported the production of silicon nitride powder from silicon obtained by cutting polysilicon waste using a diamond wire and using FeCl3, NaCl, and Cu additives as catalysts. Another study produced spherical Si3N4 powder containing more than 90 parts by weight of α-phase by adding 7 parts by weight of a halogen salt to silicon sawdust as a catalyst. However, all of these studies involved technologies that utilized catalysts, which can cause environmental pollution and affect the properties of Si3N4. Therefore, a simple and cost-effective method is needed to synthesize high-quality silicon nitride powder without a catalyst, and direct nitridation of silicon shows promising potential to achieve this goal.

[0011] Direct nitridation of silicon is very difficult to obtain pure α-Si3N4 without β-Si3N4 due to the intensive exothermic reaction between silicon and nitrogen. Furthermore, the formation of α-Si3N4 is influenced by many factors, including oxygen impurities on the silicon powder surface, catalyst, nitriding temperature, heating profile, pretreatment conditions, and particle shape and size distribution of the silicon powder. Among these, the nitriding rate and formation of the α-Si3N4 phase are primarily influenced by the nitriding temperature and particle size of the silicon powder. Previous studies have shown that larger silicon particles have lower levels of surface oxygen impurities but larger macropores that allow greater nitrogen diffusion at the expense of reduced surface area. Conversely, smaller silicon particles provide a higher reaction surface area for the same amount of porosity. However, this leads to smaller micropores that inhibit nitrogen diffusion and also contain more surface oxygen impurities. Consequently, to ensure efficient nitridation, it is desirable to use relatively smaller particles with lower oxygen impurities.

[0012] The present invention presents a simple and efficient process route for the large-scale synthesis of pure, highly crystalline Si3N4 powders, primarily α-phase, utilizing silicon scrap. This is achieved by direct nitridation of micron-sized, homogeneous silicon powders derived from ball milling of coarse raw silicon scrap.

[0013] In addition, the present invention presents a process route for achieving a simple and high yield for synthesizing high-quality Si3N4 powder.

[0014] In addition, the present invention opens up new opportunities for the use of silicon waste as a valuable resource for the synthesis of high-quality Si3N4 powder, thereby transforming the semiconductor industry into an environmentally friendly one and reducing manufacturing costs.

[0015] The present invention is intended to solve the problems of the prior art as described above, and an object of the present invention is to provide a method for producing high-purity silicon nitride powder with excellent crystallinity using silicon scrap, which implements a simple and efficient process for large-scale synthesis of pure and highly crystalline Si3N4 powder having mainly α-phase without using a catalyst.

[0016] This is due to the direct nitriding of micron-sized and homogeneous silicon powder derived from ball milling of coarse raw silicon scrap.

[0017] In addition, another object of the present invention is to provide a method for producing high-purity silicon nitride powder with excellent crystallinity using silicon scrap, in order to present an economical, environmentally friendly, and high-yield process route for synthesizing high-quality Si3N4 powder without causing process complexity, cost problems, and contamination problems caused by using a catalyst.

[0018] In addition, another object of the present invention is to provide a method for producing high-purity silicon nitride powder with excellent crystallinity using silicon scrap, so as to open up new opportunities for the use of silicon waste as a valuable resource for the synthesis of high-quality Si3N4 powder, thereby leading to a reduction in the environmental impact and manufacturing cost of the semiconductor industry.

[0019] The present invention provides a method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, comprising the steps of: crushing silicon scrap using a ball and a medium; producing silicon powder through the crushing; and heat-treating the produced silicon powder in a reducing atmosphere to synthesize silicon nitride, wherein the crushing of the silicon scrap is performed for 24 to 96 hours.

[0020] It is preferable that the above medium is ethanol.

[0021] It is preferable that the above-mentioned silicon scrap to be crushed is produced from a solar grade silicon ingot.

[0022] It is preferable that the above-mentioned crushed silicon scrap have an average particle size of more than 0 and less than 1㎛.

[0023] In the step of synthesizing the above silicon nitride, it is preferable that the heat treatment be performed in a mixed gas atmosphere of 95% N2 and 5% H2.

[0024] In the step of synthesizing the above silicon nitride, the heat treatment is preferably performed at a speed range of 5°C / h to 600°C / h in the range of room temperature to 1350°C, and at a speed range of 5°C / h to 50°C / h in the range of 1350°C or higher.

[0025] After the step of synthesizing the above silicon nitride, the step of cooling the synthesized silicon nitride is further included, and it is preferable that the cooling rate be performed in a speed range of 10°C / h to 400°C / h.

[0026] In the step of synthesizing the above silicon nitride, a reducing atmosphere is created using nitrogen, and it is preferable to supply the nitrogen at a flow rate of 50 ml / min to 600 ml / min.

[0027] In addition, the present invention provides a high-purity silicon nitride powder having excellent crystallinity, manufactured by the above-described method, using silicon scrap characterized in that α-phase silicon nitride accounts for at least 85% by weight.

[0028] The present invention, as described above, can implement a simple and efficient process for large-scale synthesis of pure and highly crystalline Si3N4 powder having mainly α phase without using a catalyst.

[0029] In addition, the present invention can provide an economical, environmentally friendly, and high-yield process route for synthesizing high-quality Si3N4 powder without the process complexity, cost, and contamination problems caused by using a catalyst.

[0030] Furthermore, the present invention opens up new opportunities for the use of silicon waste as a valuable resource for the synthesis of high-quality Si3N4 powder, which may lead to a reduction in the environmental impact and manufacturing costs of the semiconductor industry.

[0031] FIG. 1 is a graph showing the effect of ball milling solvent on reducing the particle size of silicon through particle size analysis using a particle size analyzer (PSA) by milling unprocessed silicon powder using different solvents according to one embodiment of the present invention.

[0032] Figure 2 shows the analysis results of silicon scrap (purity 99.9%) for solar cells, which is a silicon raw material according to one embodiment of the present invention.

[0033] FIG. 3 shows the TEM analysis results of powder heat-treated at 1450°C according to one embodiment of the present invention, confirming that the crystallinity of the particles is excellent.

[0034] FIG. 4 is an XRD analysis result of a powder heat-treated at 1450°C according to one embodiment of the present invention, confirming that a silicon nitride powder having no residual silicon and an α phase fraction of 85% is obtained.

[0035] FIG. 5 shows the results of particle size analysis according to milling time of the synthesized silicon nitride powder according to one embodiment of the present invention, and it was confirmed that the bonding between particles occurring during the heat treatment process was not strong, so that grinding was easy.

[0036] FIG. 6 is a TEM analysis result of a synthesized silicon nitride powder according to one embodiment of the present invention after milling for 48 hours, confirming that excellent crystallinity is maintained even after milling.

[0037] Hereinafter, embodiments of the present invention will be described in more detail with reference to the attached drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various forms. These embodiments are provided solely to ensure complete disclosure of the present invention and to fully inform those skilled in the art of the scope of the invention. The drawings may be partially exaggerated in size to accurately illustrate the embodiments of the present invention.

[0038]

[0039] The present invention manufactures silicon nitride powder by milling silicon scrap and then heat-treating it, and is carried out through two steps: ① milling silicon scrap into a state suitable for nitriding, and ② nitriding silicon powder through heat treatment.

[0040]

[0041] <Overview of the process>

[0042] In the present invention, high-quality silicon scrap produced from solar-grade silicon ingots was supplied by a semiconductor company (99.9% purity, Worldex, Korea) and used as a starting material. The supplied silicon scrap was sieved and ball-milled for several hours using 3-mm diameter, 3-mol% yttria-stabilized zirconia balls. To verify the effectiveness of milling solvents in reducing particle size within a short period of time from an industrial perspective, three different milling media were selected: mineral spirits (a type of industrial gasoline used in paints), hexane, and ethanol. After ball-milling, the silicon powder slurry was dried in a vacuum oven at 110°C for 12 hours and then classified to remove powder agglomerates.

[0043] The resulting dried, optimally sized silicon powder (~1 μm) was used in subsequent direct nitridation experiments to produce silicon nitride powder. Nitriding was performed in an alumina tube furnace at 1450°C without a holding time in a mixed gas atmosphere of 95% N2 and 5% H2. To obtain uniform, fine-sized particles and reduce particle agglomeration formed during nitridation, the produced silicon nitride powder was ball milled using ethanol.

[0044] A small amount of silicon slurry was sampled at regular time intervals during ball milling, and the average particle size (d50) and particle size distribution (PSD) behavior were measured using a laser diffraction particle size analyzer (PSA: Masterizer 2000, Malvern, England).

[0045] The purity and composition of the raw silicon scrap, ball-milled silicon powder, and silicon nitride powder formed after nitriding were determined using an X-ray diffraction apparatus (XRD: D-MAX / 2500-PC, Rigaku, Japan) at room temperature. The quantitative analysis of the α- and β-phases of silicon nitride was evaluated through analysis according to the Rietveld refinement method using Fullprof software. The morphology of all powder samples was observed using a scanning electron microscope (SEM: JSM-6500F, JEOL, Japan), while the fine micron-sized particles were observed using a high-resolution spherical aberration-corrected field emission-transmission electron microscope (TEM: JEM-ARM200F, JEOL, Japan). To investigate the surface properties and chemical composition of the powders, irradiation and core-level X-ray photoelectron spectroscopy (XPS: K-Alpha, Thermo Fisher Scientific, USA) were performed. Finally, the oxygen content of all powder samples was measured at room temperature using a nitrogen / oxygen analyzer (ON736 (S / N:17106), LECO Corporation, USA).

[0046]

[0047] <Example>

[0048] 45.6 g of silicon scrap for solar cells, 456 g of 3Φ 3Y-ZrO2 balls, and 65.1 g of ethanol were placed in a 250 ml Nalgen bottle, milled at 155 rpm for 34 hours, dried in a vacuum oven at 110°C for 12 hours, and then sieved to produce silicon powder. After that, the manufactured silicon powder was placed in an atmosphere of a mixed gas atmosphere of 95% nitrogen and 5% hydrogen, and heat-treated according to the schedule as shown in (a) of Fig. 4. The synthesized silicon nitride powder was milled for 48 hours with the same composition as the silicon scrap milling to obtain D 50: High-purity silicon nitride powder of 0.7㎛, Al: 211.4ppm, O2: 1.32% was manufactured.

[0049]

[0050] Milling of Silicon Scrap

[0051] The milling process is a process of synthesizing silicon into silicon nitride through heat treatment, in which the α phase (low-temperature phase) and the β (high-temperature phase) phase are created. In order to manufacture powder with high sinterability by reducing the proportion of the β phase, which is a difficult-to-sinter crystal phase, it is a step of grinding the silicon powder to a size small enough so that nitriding can be completed at a low temperature. It is necessary to use a milling composition that can suppress oxidation during the milling process.

[0052] Any equipment capable of grinding sub-micron powders, such as ball mills, attrition mills, and planetary mills, can be used to grind silicon scrap to a size suitable for nitriding.

[0053] Figure 1 is a graph showing the results of particle size analysis (PSA analysis) of silicon scrap according to milling time when the type of organic solution used as a dispersion medium during a milling process according to one embodiment of the present invention was varied. As shown, it can be confirmed that the milling characteristics differ depending on the type of dispersion medium, and the type of organic solvent must be used differently depending on the required milling characteristics.

[0054] More specifically, three different milling media, including mineral spirits, hexane, and ethanol, were used to evaluate the effectiveness of ball milling on particle size reduction of silicon scrap. This was done from an industrial perspective to identify an efficient milling solvent capable of achieving particle size reduction in a short period of time. Figure 1 (a) shows the PSD (Particle Size Distribution) for a typical milling time of 72 hours using different solvents, demonstrating unimodal behavior.

[0055] Figure 1(b) shows the change in d50 with varying milling time for three different solvents. As shown, d50 was observed to decrease in all three cases as the milling time increased. For mineral spirits, the particle size decreased from 1.79 to 1.30 ㎛ as the milling time increased from 24 to 120 hours. However, for ethanol, the particle size decreased from 1.18 to 0.65 ㎛ as the milling time increased from 24 to 72 hours, and this size was maintained up to 96 hours of milling. As the milling time further increased to 120 hours, d50 slightly increased to 0.83 ㎛, which is believed to be due to particle agglomeration due to over-milling. Therefore, the milling time using ethanol can be set and performed between 24 and 96 hours, more preferably between 72 and 96 hours.

[0056] Ethanol was found to be the most effective in reducing particle size and producing homogeneous, fine, micron-sized silicon particles. This may be due to the relatively lower density of ethanol compared to mineral spirits. A previous study (PM Gandhi, SK Valluri, M Schoenitz, E Dreizin, Effect of organic liquid process control agents on properties of ball-milled powders, Adv Powder Technol 33 (2022), 103332) observed similar behavior when milling Bi2O3 and CuO materials, concluding that denser milling solvents resulted in larger particle sizes. However, hexane, despite having a relatively lower density than ethanol, resulted in larger particle sizes. This suggests that other properties, such as proton affinity and surface tension, in addition to the physical properties of the solvent may also affect powder milling behavior. Furthermore, hexane is not preferred due to its difficult handling and inherently high flammability.

[0057] Since silicon particles with an average particle size of less than 1 μm are considered optimal for efficient nitriding, the present invention also aimed to utilize a similar particle size. After milling with ethanol for 34 hours, silicon particles with an average particle size of less than 1 μm were produced.

[0058] Figure 2 shows the analysis results of silicon scrap (purity 99.9%) for solar cells, which is a silicon raw material according to one embodiment of the present invention. The morphology, phase structure, and PSD behavior of the unprocessed silicon scrap were investigated. Figure 2 shows the corresponding SEM images (a, b) and XRD and PSA graphs (c, d) of the silicon scrap, respectively.

[0059] SEM images at two different magnifications (Fig. 2(a) and (b)) reveal that the silicon scrap particles are rough and irregular in shape with an average particle size of less than 1 mm. Fig. 2(c) shows the XRD pattern of the unprocessed silicon scrap, which exhibits peaks of silicon where no impurity phases are expected to be present. In addition, the PSA data shown in Fig. 2(d) shows a unimodal particle size distribution with an average particle size (d50) of approximately 1 mm, which is consistent with what was observed in the SEM images.

[0060] To characterize the thickness of the SiO2 layer present on the surface of the ball-milled silicon powder, observation by TEM was performed. Fig. 3 shows the corresponding TEM micrograph of the silicon powder. As can be seen in Fig. 3(a), the high-angle annular dark-field scanning-TEM (HAADF-STEM) image shows nanocrystalline silicon powder with particle sizes mostly less than 0.5 μm. In addition, the high-resolution TEM (HRTEM) image along the axis of the

[0110] region with the inserted Fast-Fourier Transform (FFT) shown in Fig. 3(b) shows a highly crystalline structure with an interplanar distance of 0.313 nm along the (111) plane of silicon. In addition, it can be observed that an amorphous SiO2 layer with a thickness of ~1.9 nm exists on the surface of the silicon.

[0061]

[0062] <Nitridation of Silicon Powder>

[0063] The nitriding step is a step in which silicon powder that has been crushed to an appropriate size is reacted with nitrogen (N2) gas to produce silicon nitride powder. This step allows nitrogen to penetrate into the particles due to defects caused by stress between the silicon and silicon nitride layers of the high-curvature powder, thereby nitriding the silicon powder to the inside.

[0064] The heat treatment performed in the nitriding step must remove the SiO2 layer on the surface and adjust the gas composition and heat treatment schedule to facilitate gas penetration into the narrow defects created in the particles.

[0065] After analyzing the characteristics of the silicon powder ball-milled for 34 hours, the powder was nitrided at 1450°C in an atmosphere with a constant nitrogen flow of 300 ml / min using a graphite boat coated with boron nitride.

[0066] The flow of nitrogen serves to supply nitrogen gas for nitriding the silicon powder, and at the same time, to discharge SiO(g), which is generated during the reaction between the silicon oxide film and hydrogen gas, to the outside before it reacts with the nitrogen gas to form silicon nitride. This is expressed in the following chemical formula.

[0067] SiO2(s)+H2(g) → SiO(g)+H2O(g)

[0068] 3SiO(g)+2N2(g) → Si3N4(s)+3 / 2O2(g)

[0069] Therefore, a low nitrogen flow rate does not supply enough nitrogen gas for nitridation, lowering the nitridation rate. A high nitrogen flow rate increases the external outflow of SiO(g), lowering the yield. Furthermore, a high nitrogen flow rate reduces the α-phase fraction. Therefore, a nitrogen gas flow rate of 50 ml / min to 600 ml / min is recommended.

[0070] Since the nitrogen flow rate is affected by the size of the equipment used for nitriding and the amount of powder, it is desirable to apply a range of nitrogen flow rate of 400 m / h to 5000 m / h based on the linear velocity (gas flow rate / cross-sectional area) value including the cross-sectional area of ​​the equipment into which the silicon scrap powder is loaded.

[0071] The nitriding process schedule is shown in Fig. 4(a). After reaching 1350°C at a heating rate of 200°C / h, it reached 1450°C at a relatively slow heating rate of 25°C / h. In the present invention, there are two heat treatment sections with different heating rates, and each section has a different role. The section from room temperature to 1350°C is the section where the temperature is raised to a temperature where the silicon powder and nitrogen gas can react, and can be freely used within the range permitted by the equipment, but it is advantageous to use a high heating rate to reduce the process time. Therefore, considering the performance in a typical atmosphere, the range of 5°C / h to 600°C / h is preferable.

[0072] On the other hand, the nitridation reaction of silicon that proceeds at 1350℃ or higher is an exothermic reaction, so if the nitridation rate is not appropriately controlled, the temperature of the powder will rise higher than the target temperature due to the reaction heat, which will increase the production rate of the high-temperature β phase. In order to obtain a high α phase fraction, it is necessary to control the heating rate, and a high heating rate increases the temperature of the powder, which reduces the α phase fraction, and increases the energy and time required for pulverization of the powder after heat treatment. On the other hand, a low heating rate does not cause a problem in nitriding, but there is a problem that the process cost increases due to the increased process time. Therefore, the heating rate in the nitriding reaction section is preferably 5℃ / h to 50℃ / h.

[0073] To prevent excessive formation of high-temperature phases, cooling was performed at a rate of 120°C / h without holding time at the highest temperature.

[0074] In the present invention, the cooling process proceeds directly without a holding time at the highest temperature, which is intended to suppress the formation of the high-temperature β phase. However, for sufficient nitridation of silicon powder, a continuous supply of thermal energy is necessary. Therefore, if the cooling rate is fast, the nitriding temperature drops below the nitriding temperature before sufficient nitridation is achieved, thereby lowering the nitridation rate. On the other hand, if the cooling rate is slow, the holding time at high temperatures increases, thereby increasing the proportion of the β phase. Therefore, the cooling rate range is preferably 10°C / h to 400°C / h.

[0075] All nitridation experiments were performed in an alumina tube furnace using a mixed gas of 95% N2-5% H2. The degree of nitridation was confirmed to be approximately 90.1%. According to the XRD data shown in Fig. 4(b), although there is no peak of unreacted silicon as an impurity, the degree of nitridation is less than 100%, which may be due to the weight loss of the final product due to the evaporation of SiO and moisture during nitridation.

[0076] Preliminary experiments showed that the degree of nitridation was much less (<2%) for raw silicon scrap with a particle size of approximately 1 mm. In addition, in another preliminary experiment on silicon powder ball-milled for 34 h, unreacted silicon was observed after nitridation at temperatures lower than 1450 °C. Previous studies have also reported the presence of nitrided silicon or Si2N2O impurity peaks below 1450 °C without adding a catalyst. Therefore, nitridation was performed at 1450 °C to ensure complete nitridation of the silicon powder ball-milled for 34 h. In addition, nitriding temperatures higher than 1450 °C result in relatively more β-Si3N4, which is undesirable. The degree of nitridation and α-phase content are not only determined by the particle size, but also depend on the heating profile characteristics such as heating rate, nitriding temperature, and cooling rate. According to Fig. 4 (b), all XRD peaks were assigned to the α-phase or β-phase of the silicon nitride structure, and no additional impurity peaks including silicon were detected.

[0077] To determine the phase content in the nitride powder, analysis was performed using the Rietveld refinement method. Polycrystalline silicon nitride exists primarily in the α- and β-phase polymorphic forms, and accurate quantitative phase assessment is difficult because some peak positions of the α- and β-phases overlap. Nevertheless, the accurate analysis of the polymorphic form present in silicon nitride can be evaluated using the Rietveld method, and the analysis revealed that α-Si3N4 is the dominant phase, accounting for 85% of the powder composition.

[0078] Although β-Si3N4 has higher mechanical properties than α-Si3N4, it is difficult to sinter, so a molded body manufactured from α-Si3N4 powder, which is relatively easy to sinter, is sintered to manufacture a high-density β-Si3N4 sintered body. Therefore, there is no upper or lower limit for the α phase fraction, but a higher α phase fraction is recognized as a high-quality powder with a lower sintering difficulty. The α phase fraction of a representative commercial silicon nitride powder (Denka, Japan) currently on the market is 91%, and the α phase fraction of the silicon nitride powder manufactured by the manufacturing process of this patent is 85-93%, which can be considered to have achieved a competitive level in the market.

[0079] The most feasible mechanism for the formation of α-Si3N4 is the reaction between SiO(g) and N2(g). At high temperatures, the inner silicon core reacts with the outer SiO2 layer to form a gaseous silicon monoxide. Simultaneously, N2 forms a gaseous silicon monoxide.

[0080] The gas flows over the silicon powder, causing SiO(g) to react with N2 to form silicon nitride and oxygen. The oxygen formed in the reaction is then removed by an excess N2 gas flow. In the present invention, adding H2 to the nitriding atmosphere not only removes impurity oxygen present on the surface of the silicon by reacting with oxygen to form water vapor, but also promotes the nitriding reaction until the nitriding reaction is completed.

[0081] It is true that high-purity nitrogen gas could be used as the sole gas in the reaction atmosphere, but H2 gas promotes the removal of SiO2, and P O2 Reduce P SiO, preventing re-oxidation of Si and ultimately enabling the formation of silicon nitride with a higher α-phase fraction. The addition of 5% H2 gas was carefully chosen for its industrial practicality and cost-effectiveness. The inclusion of a small proportion of H2 still provides the desired benefits for efficient nitriding while being compatible with large-scale production processes.

[0082] FIG. 5 is a result of particle size analysis according to milling time of synthesized silicon nitride powder according to one embodiment of the present invention, in which the particle size distribution of unprocessed silicon scrap gradually decreases in d50 size as milling time passes, and the width of the graph is wider when milling is performed for 48 hours compared to when milling is performed for 24 hours, confirming that the bonding between particles occurring during the heat treatment process is not strong, making it easy to grind.

[0083] Figure 6 is a TEM micrograph of silicon nitride powder after nitriding and milling. The STEM image shown in Figure 6(a) shows nanocrystalline silicon nitride powder with a particle size of ~0.5 μm. The HRTEM image presented in Figure 6(b) shows a highly crystalline structure with an interplanar distance of 0.431 nm, corresponding to the most preferred orientation of silicon nitride (101). In addition, the presence of an amorphous SiO2 layer ~1.8 nm thick can be observed on the surface of the silicon nitride.

[0084] Analysis and Evaluation

[0085] An oxygen analyzer was used to evaluate the oxygen impurities present in all powder samples. The oxygen contents (wt%) of the raw silicon scrap, ball-milled silicon powder, and silicon nitride powder were determined to be 0.23%, 1.08%, and 1.32%, respectively. The increase in oxygen content from 0.23% to 1.08% can be attributed to the increased surface area after the ball milling process. However, a higher oxygen content of 1.32% was observed in the silicon nitride powder after nitriding and milling. This may be due to the combined effects of the readily oxidizable nature of silicon nitride under normal oxygen pressure at room temperature and the increased surface area after milling.

[0086] The synthesized silicon nitride powder had an average particle size, d50 of 5.13 μm, and a BET surface area of ​​5.76 m 2 / g contains aggregated particles. However, after milling for 24 hours, d50 decreases to 1.23㎛ and BET surface area is 6.45m 2 / g. Nevertheless, after milling for 48 hours, d50 decreases to 0.7㎛ and BET surface area decreases to 10m 2 / g. XRD and ICP analyses showed that no unreacted silicon (free Si) was observed, and the Al content was low at 211.4 mg / kg. The oxygen content of the milled silicon nitride powder was determined to be 1.32% by an oxygen analyzer.

[0087] To verify the quality of the synthesized powders, a sinterability test was performed. Gas pressure sintering (GPS) was performed on the raw powders without using any sintering additives, and the results were compared with those of commercial silicon nitride powder (0.7 μm, SN9FWS, Denka, Tokyo, Japan). For this purpose, pellet samples (Φ ~ 15 mm and t ~ 2.5 mm) were prepared by uniaxial pressing at 10 MPa for 1 min and isostatic pressing at room temperature at 200 MPa for 5 min. GPS was performed in a boron nitride (BN) crucible, and a mixed bed of 50 wt% silicon nitride and 50 wt% boron nitride (BN) powders was applied inside the crucible. The GPS was performed in a graphite resistance furnace at 1900°C for 4 h under a N2 gas pressure of 0.9 MPa.

[0088] The results showed that both samples exhibited low relative density values ​​of approximately 62% and 56% for the commercial and synthetic powders, respectively, after sintering. This low density is expected to be due to the covalent bonding and low self-diffusion coefficient of silicon nitride. Furthermore, the low density of 6% for the synthetic powder may be attributed to its relatively larger particle size and higher oxygen content compared to the commercial powder.

[0089] As described above, a novel method for large-scale synthesis of pure, highly crystalline silicon nitride powders, primarily composed of α-phase, using silicon scrap was presented. Since processing parameters play a critical role in obtaining the powder under optimal conditions, each powder was precisely analyzed using various relevant methods at each powder processing stage. The results indicate that ethanol is the most effective in obtaining fine-sized, homogeneous silicon powders from silicon scrap. Furthermore, an optimized nitriding heating schedule must be applied for effective nitriding.

[0090] In conclusion, the present invention successfully achieved the production of high-quality, highly crystalline silicon nitride powder through the direct nitridation of silicon scrap without a catalyst. The results showed that the optimal silicon particle size could be achieved by ball milling using ethanol as a solvent for 34 hours. The direct nitridation of the silicon powder was performed at 1450°C, resulting in the formation of a phase dominated by α-Si3N4, which accounted for 85% of the total powder composition on a wt% basis.

[0091] The present invention represents a novel, cost-effective approach to producing high-quality silicon nitride powder while recycling discarded silicon scrap. This novel approach, which utilizes silicon scrap as a silicon source, can be expanded to produce other silicon-based ceramics and their composites. This invention has significant industrial and environmental implications, as it can reduce the large amounts of silicon waste generated in various industries.

[0092]

[0093] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the specific embodiments described above, and various modifications can be made by a person having ordinary skill in the art to which the invention pertains without departing from the gist of the present invention as claimed in the claims. Furthermore, such modifications should not be understood individually from the technical idea or prospect of the present invention.

Claims

1. A step of crushing silicon scrap using a ball and a medium; A step of manufacturing silicon powder through the above grinding; and A step of synthesizing silicon nitride by heat-treating the manufactured silicon powder in a reducing atmosphere; A method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, characterized in that the silicon scrap is crushed for 24 to 96 hours in the crushing step.

2. In paragraph 1, A method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap characterized in that the above medium is ethanol.

3. In paragraph 1, A method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, characterized in that the silicon scrap to be ground is produced from a solar-grade silicon ingot.

4. In paragraph 1, A method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, characterized in that the above-mentioned pulverized silicon scrap has an average particle size of more than 0 and less than 1 ㎛.

5. In paragraph 1, A method for producing high-purity silicon nitride powder with excellent crystallinity using silicon scrap, characterized in that in the step of synthesizing the above silicon nitride, heat treatment is performed in a mixed gas atmosphere of 95% N2 and 5% H2.

6. In paragraph 1, A method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, characterized in that, in the step of synthesizing the above silicon nitride, the heat treatment is performed at a speed range of 5°C / h to 600°C / h in the range of room temperature to 1350°C, and at a speed range of 5°C / h to 50°C / h in the range of 1350°C or higher.

7. In paragraph 1, A method for producing high-purity silicon nitride powder having excellent crystallinity using silicon scrap, characterized in that after the step of synthesizing the above silicon nitride, the step of cooling the synthesized silicon nitride is further included, and the cooling rate is performed in a speed range of 10°C / h to 400°C / h.

8. In paragraph 1, A method for producing high-purity silicon nitride powder with excellent crystallinity using silicon scrap, characterized in that in the step of synthesizing the above silicon nitride, a reducing atmosphere is created using nitrogen, and the nitrogen is supplied at a flow rate of 50 ml / min to 600 ml / min.

9. A high-purity silicon nitride powder having excellent crystallinity manufactured using silicon scrap manufactured by the method of paragraph 1, characterized in that α-phase silicon nitride accounts for at least 85% by weight.

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