Method for producing transparent sintered silica using amorphous silica nanopowder

Abstract

The present invention provides a method for producing a silica molded body in which the pore size is minimized and homogenized, and thus pores can be easily removed, and a heat treatment method for producing a silica sintered body with excellent transparency by maximizing the densification process due to viscous flow in the silica molded body during sintering. Therefore, the present invention has an advantage that transparent quartz glass with improved translucency can be produced by using amorphous silica nanopowder.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a transparent silica sintered body using amorphous silica nanopowder. [Background technology]

[0002] Silica glass is a glass made of a high-purity amorphous phase and has a low thermal expansion coefficient (0.5 × 10 -6 / ℃), excellent chemical durability, and high electrical resistance (1,016 Ω·cm), making it suitable for use in laboratory equipment, insulators, etc. Furthermore, because quartz glass has high transmittance in the ultraviolet and visible light ranges, it is the most widely used material for process components in the semiconductor, display, and solar cell industries, such as lenses, optical materials, photomask substrates, and lens materials for excimer stepper equipment.

[0003] Quartz glass can be broadly divided into fused silica glass and synthetic silica glass depending on the manufacturing method. The biggest difference between the two manufacturing methods is the starting material. Fused silica glass is manufactured by melting natural quartz at high temperatures. Therefore, it contains metal impurities and has low purity, resulting in somewhat poor transparency. It also has the disadvantage of requiring high manufacturing costs because it is melted and processed at temperatures above 1,800°C.

[0004] Synthetic silica glass is manufactured by chemical vapor deposition using silicon tetrachloride (SiCl4) and alkoxide methods. Compared to fused silica glass, synthetic silica glass has the advantage of having better optical properties due to its lower impurity content. However, it has the disadvantage of requiring high investment costs to ensure the corrosion resistance of equipment and human safety during the manufacturing process due to the corrosiveness, volatility, and toxicity of harmful gases generated during the manufacturing process.

[0005] Unlike conventional manufacturing methods, amorphous, transparent quartz can be manufactured using nano-sized circular silica powder that exhibits an amorphous phase through molding and sintering processes. The sintering behavior of amorphous materials differs from that of crystalline materials in that densification occurs due to viscous flow. This sintering behavior promotes densification by removing grain boundaries and pores due to increased particle movement caused by increased fluidity as the temperature increases. During the sintering process, when the amorphous material reaches a certain temperature, crystallization occurs, and particle movement mechanisms such as diffusion are activated. However, the energy required to densify the sintered body is consumed in switching from the amorphous phase to the crystalline phase, and therefore further densification due to viscous flow does not occur. At this time, if pores, grain boundaries, and crystalline phases that could not be removed remain inside the sintered body, they scatter light, resulting in a sudden decrease in the translucency of the sintered body.

[0006] The patents and references mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication was individually and specifically indicated by reference. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention relates to a method for producing a transparent silica sintered body by sintering amorphous silica nanopowder. The object of the present invention is to provide a method for producing a transparent silica sintered body that can solve the problem of a rapid decrease in the translucency of the sintered body due to the presence of unremoved pores, grain boundaries, and crystalline phases inside the silica sintered body, which scatter light. [Means for solving the problem]

[0008] To overcome the conventional problems in producing a transparent silica sintered body by sintering silica, the present invention provides a method for producing a transparent silica sintered body, including optimal molding conditions for minimizing and homogenizing the pore size in an amorphous silica molded body, thereby smoothly removing the pores, and optimal heat treatment conditions for maximizing the densification process due to viscous flow within the molded body. [Effects of the Invention]

[0009] The present invention provides a method for producing a silica molded body in which pore size is minimized and uniform, thereby easily eliminating pores, and a heat treatment method for producing a silica sintered body with excellent transparency by maximizing the densification process due to viscous flow within the silica molded body during sintering. Therefore, the manufacturing method of the present invention has the advantage of being able to produce transparent quartz glass with improved translucency using amorphous silica nanopowder. [Brief explanation of the drawings]

[0010] [Figure 1]

[0023] Figure 1 shows the results of analyzing the amorphous silica nanopowder of the present invention, where panel (a) shows the microstructure of the amorphous silica nanopowder, panel (b) shows the crystallinity of the amorphous silica nanopowder, and panel (c) shows the results of Fourier transform infrared spectroscopy (FT-IR) analysis of the amorphous silica nanopowder. [Figure 2] The results are shown for a transparent silica sintered body of the present invention, which was produced by heat treating it at 1200 to 1350° C. for 1 hour. [Figure 3] The results are shown for a transparent silica sintered body of the present invention, which was produced by heat treating it at 1350°C for 1 hour. [Figure 4]1 shows the results of scanning electron microscopy (SEM) analysis of cross sections of transparent silica sintered bodies of the present invention, where panel (a) shows a sintered body produced by heat treatment at 1200°C, panel (b) shows a sintered body produced by heat treatment at 1250°C, panel (c) shows a sintered body produced by heat treatment at 1275°C, panel (d) shows a sintered body produced by heat treatment at 1300°C, panel (e) shows a sintered body produced by heat treatment at 1325°C, and panel (f) shows a sintered body produced by heat treatment at 1350°C. [Figure 5] 1 shows the results of analyzing the transparent silica sintered body of the present invention by X-ray diffraction (XRD). [Figure 6] The transparent silica sintered body of the present invention was produced by sintering at 1325° C. Panel (a) shows a photograph of the sintered body, and panel (b) shows the surface microstructure. [Figure 7] The results are shown for the transparent silica sintered body of the present invention, which was produced by heat treating it at 1215°C for 12 hours, 18 hours, 24 hours, and 30 hours. [Figure 8] 1 shows the results of scanning electron microscopy (SEM) analysis of cross sections of transparent silica sintered bodies of the present invention, where panel (a) shows a sintered body produced by heat treatment at 1215°C for 12 hours, panel (b) shows a sintered body produced by heat treatment at 1215°C for 18 hours, panel (c) shows a sintered body produced by heat treatment at 1215°C for 24 hours, and panel (d) shows a sintered body produced by heat treatment at 1215°C for 30 hours. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention provides a method for producing a transparent silica sintered body using sintering, including: a first step of preparing silica powder; a second step of compression-molding the silica powder to produce a molded body; and a third step of heat-treating the molded body at a temperature of 1310 to 1340°C at a heating rate of 2°C / min for 50 to 240 minutes, or at a temperature of 1200 to 1230°C at a heating rate of 2°C / min for 20 to 28 hours to produce a transparent silica sintered body.

[0012] The silica powder is characterized by having an amorphous spherical particle shape, and the molded body is manufactured by subjecting the silica powder to a primary molding process using a uniaxial press and then a secondary molding process using a cold isostatic press.

[0013] The transparent silica sintered body produced by the above-mentioned manufacturing method is characterized by a shrinkage of 45.5 to 47.5% after heat treatment of the molded body, and by transmitting 19% to 58% of light in the ultraviolet range (wavelength of 200 to 380 nm) and 84% to 85% of light in the infrared range (2000 to 2400 nm).

[0014] (Mode for carrying out the invention) The present invention provides a method for producing a transparent silica sintered body, comprising the steps of:

[0015] The method comprises the following steps: a first step of preparing silica powder; a second step of compression-molding the silica powder to produce a molded body; and a third step of heat-treating the molded body at a temperature of 1310 to 1340°C at a heating rate of 2°C / min for 50 to 240 minutes, or at a temperature of 1200 to 1230°C at a heating rate of 2°C / min for 20 to 28 hours to produce a transparent silica sintered body.

[0016] The manufacturing method of the present invention provides a transparent silica sintered body with high light transmittance and high transparency. To achieve this, the present invention establishes optimal molding conditions to ensure small and uniform pore sizes within the green body, and optimal sintering conditions to maximize densification due to viscous flow of the green body.

[0017] The silica powder is characterized by having an amorphous spherical particle shape, preferably a spherical amorphous silica powder with an average particle diameter of 10 to 900 nm. The silica powder is manufactured by first molding using uniaxial pressing and then second molding using cold isostatic pressing. Uniaxial pressing refers to a method of compressing powder by applying pressure in a single axial direction in an inelastic mold, while cold isostatic pressing refers to a method of compressing powder by applying isostatic pressure using a liquid in an elastic mold. Cold isostatic pressing has the advantage of maintaining a consistent molding density regardless of size, shape, or position, since it is formed using isostatic pressure. The first molding is a process that mainly forms the shape of the molded body, while the second molding is a process that reduces and homogenizes the pore size within the molded body.

[0018] According to an embodiment of the present invention, the molding density of the molded body obtained by the primary molding by the uniaxial pressing and the secondary molding by the cold isostatic pressing is 1 to 1.2 g / cm 3 For this purpose, it is preferable to carry out molding at a molding pressure of 170 to 230 MPa and for a holding time of 3 to 7 minutes.

[0019] The molded body produced using the primary and secondary moldings is heat-treated at a temperature of 1310 to 1340°C at a temperature increase rate of 2°C / min for 50 to 240 minutes, or at a temperature of 1200 to 1230°C at a temperature increase rate of 2°C / min for 20 to 28 hours to produce a transparent silica sintered body.

[0020] The condition of sintering at a temperature of 1310 to 1340°C has the advantage that a transparent sintered body can be produced by heat treatment for only 50 to 240 minutes. However, if the sintering temperature is lower than 1310°C or higher than 1340°C, the transparency of the sintered body may decrease.

[0021] The sintering temperature of 1200 to 1230°C is a condition in which a transparent sintered body can be produced by carrying out heat treatment for 20 to 28 hours. However, if the sintering temperature is lower than 1200°C or if the heat treatment is carried out at a temperature higher than 1230°C, the transparency of the sintered body may decrease.

[0022] The transparent silica sintered body produced by the production method of the present invention is characterized in that the shrinkage of the shaped body after heat treatment is 45.5 to 47.5%, and the density of the sintered body is 2.15 g / cm 3 It is characterized by an increase in

[0023] The transparent silica sintered body produced by the production method of the present invention is characterized by transmitting 19% to 58% of light in the ultraviolet region (wavelengths of 200 to 380 nm) and 84% to 85% of light in the infrared region (2000 to 2400 nm).

[0024] The present invention will be described in detail below with reference to examples.

[0025] Example

[0026] 1. Experimental Method

[0027] 1) Silica raw material powder

[0028] The silica raw material powder used to produce the molded body of the present invention was purchased from Sukgyung AT Co., Ltd. in Korea (SG-700). The particle size and shape of the silica raw material powder were observed using a Field Emission Scanning Electron Microscope (JSM-7100F, manufactured by JEOL). The crystal phase and bond structure of the starting powder were also analyzed using an X-ray Diffractometer (Smartlab®, manufactured by Rigaku Corporation) and a Fourier transform infrared spectrometer (Spectrum400, manufactured by PerkinElmer).

[0029] 2) Manufacturing of molded bodies

[0030] A disk-shaped primary compact measuring Φ15mm x 2mm was produced by uniaxially pressing 0.5g of silica powder into a 15mm diameter circular mold at a pressure of 100psi for 1 minute. To increase the packing density of the primary green body, cold isostatic pressing was performed at a pressure of 150MPa to 250MPa for 5 or 10 minutes to obtain a secondary compact (final green body). The green densities of the green bodies were measured using a dimensional measurement method and compared.

[0031] 3) Sintering of the green body

[0032] The secondary compact was sintered in an electric furnace under atmospheric conditions. The sintering was carried out at a heating rate of 2°C / min at 1200-1350°C for 1 hour. SEM and XRD analyses were performed on the sintered compacts (specimens) sintered under different temperature conditions to examine the microstructure and the presence or absence of crystallization.

[0033] In addition, we attempted to produce transparent sintered bodies by adjusting the sintering time to 12, 18, 24, and 30 hours at 1215°C, a relatively low temperature at which crystallization does not occur, and analyzed the density, microstructure, and crystalline phase of the obtained specimens.

[0034] The transmittance of the sintered body was analyzed using an ultraviolet-visible spectrophotometer (UV-Vis Spectrometer, Lambda365, manufactured by PerkinElmer) and compared.

[0035] 2. Experimental Results

[0036] As shown in Figure 1, the silica powder used in the present invention exhibited the shape of amorphous spherical particles, and the particle size was confirmed to be approximately 700 nm. Panel (c) of Figure 1 shows the FT-IR spectrum results obtained by analyzing the structure of the powder. As a result of the experiment, no peaks other than those of the silica structure, such as hydroxyl groups, were observed. In the FT-IR spectrum results of Figure 1, -1 The peak at 808 cm represents the bending vibration of the Si-O bond. -1 The peak at 1,103 cm represents the symmetrical stretching vibration of Si-O-Si, which connects oxygen atoms between tetrahedra. -1 The peak indicates the Si-O-Si asymmetric stretching structure of bridging oxygen within the tetrahedron.

[0037] Hydroxyl groups and impurities promote the nucleation of amorphous silica particles and, at the same time, lower the crystallization temperature of amorphous silica to promote the formation of a crystalline phase, so it is preferable that they are absent in order to produce a dense, light-transmitting sample. Therefore, the silica powder used in the present invention is found to be an extremely suitable powder.

[0038] During the cold isostatic pressing process using the silica powder, various green bodies were manufactured by adjusting the forming pressure (MPa) and holding time (min). Table 1 shows the green density (g / cm) of each green body manufactured in this way. 3 ) is shown.

[0039] [Table 1]

[0040] The compact produced by molding at a pressure of 200 MPa for 5 minutes had a molding density of 1.12 g / cm 3 It was confirmed that even if a higher pressure and holding time were applied, a relatively constant compact density value was maintained. Therefore, it was decided that all subsequent compacts would be manufactured by compacting for 5 minutes under a pressure of 200 MPa.

[0041] Figure 2 shows a visual comparison of the degree of translucency of sintered body samples heat-treated at various sintering temperatures. From 1200°C to 1325°C, it can be seen that the translucency of the samples increases as the sintering temperature increases. In particular, the sample heat-treated at 1325°C exhibited the highest translucency.

[0042] However, if the sintering temperature is further increased, it can be visually confirmed that the specimen becomes opaque again. In particular, unlike sintered bodies heat-treated at lower temperatures, the specimen sintered at 1350°C has an opaque outer surface but a transparent interior. Figure 3 shows the difference between the surface and interior of the specimen sintered at 1350°C.

[0043] Figure 4 shows the microstructure of the fracture surface of samples sintered at each temperature. At 1200°C, necks began to form between particles, and pores between circular particles were relatively uniformly distributed. At 1250°C, the contact area between particles increased, enlarging the grain boundary area, and the open pore size became more uniform. At 1275°C, the pores were almost entirely closed, and the grain boundaries and residual pores gradually disappeared due to the active viscous flow of particles. At 1300°C, the grain boundaries were almost completely eliminated due to the viscous flow, and a few circular residual pores were observed due to the continuous diffusion of atoms within the pores. At 1325°C, neither grain boundaries nor pores were observed. At 1350°C, a clear difference was observed between the inside and outside of the specimen. The translucent inner surface showed no pores or grain boundaries, as evident from the microstructure, but crystallization occurred in the outer portion, revealing circular crystals of the cristobalite phase.

[0044] Figure 5 shows the XRD analysis results of specimens according to sintering temperature. From 1200°C to 1250°C, an amorphous state is observed with no specific peaks. However, from 1275°C onwards, a fine cristobalite peak is observed. Even in the specimen heat-treated at 1325°C, which has the highest translucency, a cristobalite crystalline phase is observed. The specimen heat-treated at 1350°C was analyzed separately for its surface and interior. The results showed an amorphous peak in the interior, while a strong cristobalite peak was observed on the surface (outside). The difference between the surface and interior is believed to be the result of nuclei being generated and growing from the surface as heat is transferred from the surface to the interior during heat treatment.

[0045] The relational expression for the sintering model of viscous flow in the Frenkel model is as follows:

[0046]

number

[0047] ρ0 is the initial density, ρ g is the density of the glass, γ is the surface energy of the glass, and η(T) is the viscosity value depending on the temperature.

[0048] The density over time (ρ(t)) is affected by the viscosity value over temperature and the surface energy of the glass, but since the change in surface energy during sintering is not significant, the density ultimately depends on the temperature, which affects the viscosity. This theoretical formula can also be confirmed from the shrinkage of the sintered body, and a comparison of shrinkage rates over temperature is shown in Table 2.

[0049] [Table 2]

[0050] It was confirmed that the higher the sintering temperature (℃), the greater the shrinkage (%), and a particularly large shrinkage was observed between 1275℃ and 1300℃. This means that the growth of grains and the disappearance of pores occur in a very short temperature range.

[0051] In addition to the sintering temperature, the holding time also affects sintering. Figure 6 shows a sintered body produced by holding at 1325°C for 5 hours, which exhibits high translucency. Experimental results showed that, unlike the sintered body produced by holding for 1 hour, an opaque sintered body was obtained. Observation of the microstructure indicated that crystallization occurred, resulting in a decrease in translucency. This indicates that holding time is an important variable in silica crystallization. Given that the temperature difference at which translucency decreases due to crystallization is only 25°C, and that silica readily crystallizes even at temperatures where translucency is exhibited depending on the holding time, controlling the translucency through temperature control was deemed difficult. Therefore, to prevent crystallization, which would cause a decrease in translucency, sintering was performed at a relatively low temperature of 1215°C, where crystallization does not occur, and the holding time was adjusted accordingly. Table 3 shows the density of the sintered body as a function of holding time. Experimental results confirmed that the sintered density increased with increasing holding time.

[0052] [Table 3]

[0053] Figure 7 shows the results of comparing the translucency of sintered specimens as a function of holding time. The experiment showed that the specimens were opaque for up to 12 hours, but began to become translucent from 18 hours onwards, with the highest translucency observed at a holding time of 24 hours. However, it was visually confirmed that the translucency decreased again when the specimens were held for more than 24 hours.

[0054] Table 4 shows the comparison results of the shrinkage rate depending on the holding time. The results in Table 4 confirm that in the viscous flow sintering mechanism, the degree of sintering shrinkage is proportional to the time.

[0055] [Table 4]

[0056] Figure 8 shows the results of observing the microstructure of sintered specimens as the holding time was adjusted. The experiment showed that at a holding time of 12 hours, only necks formed between particles, and no densification occurred, resulting in the formation of grain boundaries and numerous pores. At holding times of 18 and 24 hours, no pores or grain boundaries were observed, but at a holding time of 30 hours, inflected sections began to appear.

[0057] The above results are thought to be an early phenomenon of the onset of crystallization, and to confirm this, XRD analysis was performed according to the holding time. Figure 9 shows the XRD analysis results. As a result of the analysis, it was confirmed that an amorphous phase was observed from 12 to 24 hours, but a peak of cristobalite crystals was observed at 30 hours.

[0058] FIG. 10 and Table 5 show the transmittance measured using a UV-visible spectrometer to determine the degree of light transmittance as judged visually.

[0059] [Table 5]

[0060] The measurement results showed that the specimens sintered for 12 hours had a transmittance of 0% for all wavelength bands, while the specimens sintered for 24 hours showed a transmittance of 19% to 58% in the ultraviolet region (wavelengths of 200 to 380 nm) and 84% to 85% in the infrared region (wavelengths of 2000 to 2400 nm).

[0061] 4. Conclusion

[0062] In this invention, we produced a translucent silica sintered body using amorphous, spherical nanosilica powder with a uniform particle size and no impurities. Translucency was induced by densification by adjusting variables such as the manufacturing conditions of the compact, sintering temperature, and holding time. The powder compact exhibited a sintering mechanism based on viscous flow. As densification progressed, pores and grain boundaries disappeared, resulting in translucency. However, the crystallization of cristobalite, which is sensitive to temperature and holding time, reduced translucency.

[0063] To produce a more stable amorphous sintered body, the holding time was adjusted at 1215°C, a relatively low temperature at which crystallization does not occur, resulting in an amorphous silica sintered body that exhibits translucency. This sintered body exhibits a transmittance of 19% to 58% for light in the ultraviolet range (wavelengths of 200 to 380 nm) and 84% to 85% for light in the infrared range (wavelengths of 2000 to 2400 nm). These results indicate that the material can be used to manufacture ultraviolet-transmitting lenses made from existing quartz glass.

[0064] The specific embodiments described herein are meant to represent preferred implementations or examples of the present invention, and are not intended to limit the scope of the present invention in any way. Modifications and other uses of the present invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the claims. [Industrial Applicability]

[0065] The present invention provides a method for producing a transparent silica sintered body, which can solve the problem that if unremoved pores, grain boundaries, and crystalline phases exist inside the silica sintered body, light is scattered, causing a rapid decrease in the light transmittance of the sintered body. Therefore, it is possible to produce materials for process parts most widely used in the semiconductor, display, and solar cell industries, such as lenses, optical materials, photomask substrates, and lens materials for excimer stepper equipment.

Claims

1. A first step of providing a silica powder; a second step of compressing and molding the silica powder to produce a molded body; a third step of heat-treating the molded body at a temperature of 1310 to 1340°C for 50 to 240 minutes at a temperature increase rate of 2°C / min, or at a temperature of 1200 to 1230°C for 20 to 28 hours at a temperature increase rate of 2°C / min, thereby producing a transparent silica sintered body; A method for producing a transparent silica sintered body using sintering, comprising:

2. 2. The method for producing a transparent silica sintered body using sintering according to claim 1, wherein the silica powder has the shape of amorphous spherical particles.

3. 2. The method for manufacturing a transparent silica sintered body using sintering according to claim 1, wherein the green body is manufactured by performing a primary molding process on the silica powder using uniaxial pressing and then a secondary molding process on the silica powder using cold isostatic pressing.

4. The method for producing a transparent silica sintered body using sintering according to claim 1, wherein the transparent silica sintered body has a shrinkage of 45.5 to 47.5% after heat treatment of the molded body.

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