Novel siox / carbon nanofiber and method for producing same

Abstract

[Problem] The present invention focuses on nanofibers having a composite structure of SiOx and carbon as a material to replace conventional SiC fiber materials, and addresses the problem of developing a method for producing the nanofibers. [Solution] This method for producing SiOx / carbon nanofibers comprises sequentially implementing reactions in which (I) an Si substrate is inserted into a thermal CVD device, and a thin film layer of phosphorus is formed on the Si substrate while a mixed liquid of phosphorus / ethanol is atomized using a nebulizer device in a temperature range of 900-1,000°C, and then, (II) a fusion reaction of the phosphorus layer occurs with Si to form a phosphorus-Si mixed liquid layer, and finally, (III) the surface of the Si-phosphorus fusion liquid layer rapidly reaches a supersaturated state, SiOx / carbon nanofibers are precipitated through an oxidation reaction of Si+O2 → SiOx, the growth thereof continues, and an SiOx layer and an amorphous carbon layer precipitated from ethanol are simultaneously deposited on the outside of an SiOx core.

Description

Novel SiOx / carbon nanofibers and their manufacturing method

[0001] The present invention relates to novel SiOx / carbon nanofibers and methods for producing the same.

[0002] The global ceramic matrix composite (CMC-SiC fiber) and carbon matrix composite (CAMC) market refers to advanced materials made of ceramic or carbon fibers embedded in a ceramic or carbon matrix. These materials are known for their high strength, stiffness, and heat resistance, and are used in a variety of applications, including aerospace, defense, energy, automotive, and industrial.

[0003] Although conventional SiC fibers have many advantages as high-performance materials, they also have some drawbacks. For example, 1. High price: Compared to alloy materials, the manufacturing cost is high and the manufacturing process is complicated, which limits their wide range of applications to some extent. 2. Brittleness: They are prone to breakage in high-temperature and high-pressure environments. 3. High thermal expansion rate: Their thermal expansion rate is significantly different from that of other materials. 4. Although they have high strength, they lack toughness and are prone to cracking at some stress concentration points.

[0004] This is because in existing SiC fiber structures, the Si and C elements are normally strongly bonded and do not react with most chemicals, resulting in high strength but lacking toughness. However, under high temperatures and pressures, they react with oxygen to form oxides, destroying the structure and performance of the fiber material.

[0005] Special Publication No. 2012-511492

[0006] In contrast, nanofibers with a composite structure of SiOx and carbon are an ideal material, possessing high strength, high heat resistance, high impact resistance, insulation, high acid resistance and chemical stability.

[0007] This is because the SiOx and carbon composite nanofibers form Si-O and CO bonds with the Si and C elements through the intervention of O atoms, reducing the formation of oxides and improving the oxidation resistance of the nanofiber material; and the presence of O atoms allows the Si and C elements to form stronger Si-O and CO bonds, strengthening the bond strength and improving the heat resistance of the SiOx / carbon nanofibers.

[0008] Therefore, the present invention focuses on nanofibers with a composite structure of SiOx and carbon as an alternative material to conventional SiC fiber materials, and aims to develop a manufacturing method for such nanofibers.

[0009] In the manufacturing method according to the present invention, a Si substrate is inserted into a thermal CVD apparatus, and a phosphorus / alcohol mixed liquid is atomized by a nebulizer at a temperature range of 950 to 1050°C, preferably 900 to 1000°C, while allowing a reaction to occur on the Si substrate for a certain period of time.

[0010] The phosphorus used in the present invention may be any of phosphorus allotropes such as red phosphorus, white phosphorus, and yellow phosphorus, with red phosphorus being preferred.

[0011] The alcohol used is a lower alcohol such as methanol or ethanol, which can atomize the phosphorus-alcohol mixture liquid using a nebulizer, and is preferably ethanol.

[0012] The reaction time is about 1 hour.

[0013] This reaction first forms a thin layer of phosphorus on the Si substrate, then a fusion reaction with Si occurs, forming a phosphorus-Si mixed liquid layer. Finally, the surface of the Si-phosphorus fused liquid layer quickly becomes supersaturated, and through the oxidation reaction of Si + O2 → SiOx, SiOx / carbon nanofibers precipitate and continue to grow. At the same time, an amorphous carbon layer precipitated from the alcohol also accumulates on the outside of the SiOx core, resulting in a core-shell SiOx / carbon nanofiber.

[0014] SiOx / carbon nanofibers with this core-shell structure have an SiOx core and an amorphous carbon layer in the shell, making them nanofiber materials with a very large specific surface area. They are expected to offer high performance in harsh environments, including high heat resistance, toughness, strength, and elastic modulus. Furthermore, the combined structure of the hard and soft properties of SiOx and C provides a perfect combination of toughness and rigidity, improving the overall mechanical performance of SiOx / carbon nanofibers.

[0015] The applications of nanofibers, taking advantage of their excellent properties, are expanding to a wide range of fields, including as additives or replacements for aircraft gas turbine engines, anti-friction materials (automobile brakes), heat exchangers (high-temperature parts), and innovative materials such as Kevlar® fiber, as shown below.

[0016] 1. Aircraft gas turbine engines SiC fibers and nickel alloys have been used in aircraft gas turbine engines, but SiOx / carbon nanofibers are a high-performance, lightweight material that can replace SiC fibers and nickel alloys.

[0017] 2. Anti-friction material (automobile brakes) SiOx / carbon nanofiber has high temperature stability: It has a high melting point and is much more thermally stable than conventional alloy materials.

[0018] 3. Heat Exchanger (High Temperature Section) The application of SiOx / carbon nanofiber in the high temperature section of the heat exchanger is mainly to manufacture the radiator core material of the high temperature plate heat exchanger, which has excellent performance in high temperature and highly corrosive environments, and can significantly improve the service life and heat exchange efficiency of the heat exchanger.

[0019] 4. Use as a substitute for Kevlar (registered trademark) fiber Kevlar (registered trademark) fiber, a type of aramid fiber, is used in materials such as bulletproof vests, but SiOx / carbon nanofiber is used as a type of high-performance bulletproof material and can be used to manufacture bulletproof vests. High hardness: Its hardness is much higher than conventional bulletproof materials such as resin and glass, effectively preventing penetration by bullets and shrapnel. Lightweight: Compared to conventional bulletproof materials, its specific gravity is lower, so it does not place excessive strain on the body and is lightweight and comfortable to wear. Impact resistance: In addition to hardness and lightness, it also has high impact resistance, allowing it to effectively absorb impact energy and reduce damage to the human body.

[0020] In summary, the present invention provides SiOx / carbon nanofibers that are expected to be used as materials for aircraft gas turbine engines, antifriction materials (automobile brakes), heat exchangers (high-temperature parts), bulletproof vests, etc.

[0021] Schematic diagram of a thermal CVD apparatus equipped with a nebulizer. Diagram of the growth mechanism of core-shell SiOx / carbon nanofibers. SEM image of the synthesized core-shell SiOx / carbon nanofibers. Elemental analysis mapping diagram of the synthesized core-shell SiOx / carbon nanofibers. Raman spectroscopy diagram of the synthesized core-shell SiOx / carbon nanofibers. X-ray photoelectron spectroscopy (XPS) diagram of the synthesized core-shell SiOx / carbon nanofibers. Cross-section diagram of the synthesized core-shell SiOx / carbon nanofibers. Diagram of the synthesis process of core-shell SiOx / carbon nanofibers.

[0022] A method for producing SiOx / carbon nanofibers, characterized in that a Si substrate is cleaned and inserted into a thermal CVD apparatus, and a phosphorus / ethanol mixed liquid is atomized by a nebulizer device at a temperature range of 900-1000°C while the reaction is carried out for about an hour, thereby precipitating and growing core-shell SiOx / carbon nanofibers.

[0023] FIG. 1 is a schematic diagram of an apparatus used in the method of the present invention. The method of the present invention will be described using the apparatus of FIG. 1, but the apparatus used in the method of the present invention is not limited to this. The thermal CVD apparatus has three main parts: an electric furnace (6), a nebulizer device (9), and a nitrogen gas canister (8). In FIG. 1, the electric furnace (6), the nebulizer device (9), and the nitrogen gas canister (8) are arranged in order on the left and right.

[0024] (1) This is a method for synthesizing SiOx / carbon nanofibers using a red phosphorus / ethanol mixture and a Si substrate (1). The Si substrate is first cleaned sequentially with acetone, methanol, and ultrapure water using an ultrasonic cleaning device, and then the remaining ultrapure water on the Si substrate (1) is removed using a nitrogen gun. (2) The red phosphorus used in (1) is Phosphorus Powder (Kojundo Chemical Laboratory, 99.5%). (3) The cleaned Si substrate (1) is placed in the electric furnace (6) of a thermal CVD device, the nitrogen valve (7) is opened, and the nitrogen flow rate is set to 0.1 L / min for approximately 10 minutes. The electric furnace (6) of the thermal CVD device is turned on and heated to approximately 1000°C. After waiting for approximately 10 minutes, the nebulizer device (9) is turned on. (4) The nebulizer device (9) (1.65 MHz, Atom Medical Corp., Model 303) described in (3) is a key device in the thermal CVD apparatus of the present invention. It can atomize the phosphorus / ethanol mixed liquid (3), which is then delivered to the surface of the Si substrate (1) in the thermal CVD apparatus via nitrogen gas for synthesis. (5) The proportions of the red phosphorus / ethanol mixed liquid (3) described in (1) were 10 wt% and 100 ml, respectively. (6) The reaction time was approximately 1 hour. After the experiment, the electric furnace (6) of the thermal CVD apparatus was turned off, and the temperature of the electric furnace (6) was allowed to cool to room temperature before the Si substrate (1) was removed.

[0025] Figure 2 shows a schematic diagram of the growth mechanism of core-shell SiOx / carbon nanofibers. First, a phosphorus coating was applied to a Si substrate (1) using a nebulizer (9). The nitrogen valve (7) of a nitrogen gas canister (8) was turned on, and nitrogen gas flowed from the supply gas inlet (10) to the exhaust port (11). As shown in Figure 2(I), a thin phosphorus layer (2) was formed. As shown in Figure 2(II), a fusion reaction occurred between Si and phosphorus, forming a phosphorus-Si mixed liquid layer (3). Finally, as shown in Figure 2(III), the surface of the Si-phosphorus fusion liquid layer (3) quickly became supersaturated. Through the oxidation reaction of Si + O2 → SiOx, SiOx / carbon nanofibers precipitated and continued to grow. Simultaneously, an amorphous carbon layer (4) precipitated from ethanol also deposited on the outside of the SiOx core (5).

[0026] Figure 3 shows a typical SEM image of nanofibers synthesized at a synthesis temperature of approximately 1000°C, where high-density nanofibers with diameters of 200 to 300 nm are observed.

[0027] Figure 4 shows a typical TEM image and the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping and spectrum analysis. The TEM image in Figure 4(a) shows that the fiber diameter is approximately 300 nm. TEM observation also revealed that these nanofibers have a distinct core (spot (i)). An amorphous carbon structure was also observed outside the core (spot (ii)). Figures 4(b)–(d) show elemental mapping and energy-filtered images of the nanofibers. The fiber components are primarily composed of carbon, oxygen, and silicon atoms. Si and O atoms are primarily present in the core of the nanofiber, while C atoms are primarily present in the shell. EDS characterization revealed the following composition ratios: C: 95.54%, O: 2.11%, Si: 1.40%. The EDS spectrum of the nanofiber (Figure 3(f)) displays three distinct peaks: C, O, and Si. However, part of the C peak is due to the thin C film supporting the TEM grid.

[0028] Figure 5 shows the Raman spectrum obtained using a laser with an excitation wavelength of 532 nm. C shares two common features in the Raman spectrum: a G peak near 1580–1620 cm-1 and a D peak near 1350 cm-1. The D peak of the nanofibers was observed to be higher than the G peak. The D peak is caused by the amorphous structure, while the G peak is caused by the elongation of sp2 carbon atoms within the graphitic structure. The strong D peak in the Raman spectrum suggests an amorphous carbon structure due to distortion, vacancies, and strain in the graphite. This feature is common to basic amorphous carbon nanofibers.

[0029] Figure 6 shows the X-ray photoelectron spectroscopy (XPS) spectrum of the nanofibers. The XPS spectrum (Figure 6(a)) clearly indicates the presence of C, O, and Si in the nanofibers. In Figure 6(b), two C1s peaks are located at 284.4 and 285.6 eV, corresponding to sp2 and sp3 atomic layers, respectively. The Si and C recombination peak is located at 282-283 eV, which indicates that the nanofiber is not SiC. In Figure 6(c), XPS peaks at 102.25 and 103.65 eV are clearly observed, corresponding to a low Si oxide composition (Si3+). As shown in Figure 6(c), the nanofibers exhibit x-dependent XPS peaks in the SiOx / C composite. Therefore, the nanofibers have a SiOx component.

[0030] Figure 7 shows a cross-sectional SEM image of the low-density SiOx / carbon nanofiber, revealing the SiOx core and carbon shell of the SiOx / carbon nanofiber.

[0031] Figure 8 shows SEM images of the synthesis process of SiOx / carbon nanofibers. It is clearly seen that SiOx / carbon nanofibers are grown from the Si substrate at a high temperature of about 1000°C.

[0032] In summary, the present invention provides SiOx / carbon nanofibers that are expected to be used as materials for aircraft gas turbine engines, antifriction materials (automobile brakes), heat exchangers (high-temperature parts), bulletproof vests, and other applications.

[0033] 1………………Si substrate 2………………Thin film layer of phosphorus 3………………Phosphorus-Si mixed liquid layer 4………………Amorphous carbon layer 5………………SiOx core 6………………Electric furnace 7………………Nitrogen valve 8………………Nitrogen gas canister 9………………Nebulizer device 10………………Supply gas port 11………………Exhaust port 1………………Si substrate 2………………Thin film layer of phosphorus 3………………Phosphorus-Si mixed liquid layer 4………………Amorphous carbon layer 5………………SiOx core 6………………Electric furnace 7………………Nitrogen valve 8………………Nitrogen gas

Claims

1. A Si substrate is washed and inserted into a thermal CVD device. A phosphorus / alcohol mixture is atomized by a neptiizer device at a temperature range of 950-1050°C for a certain period of time to produce SiO x / SiO2 characterized by precipitating and growing carbon nanofibers x / Method of manufacturing carbon nanofibers.

2. SiO x Core-shell SiO with an amorphous carbon layer around the core x / Carbon nanofiber.

3. The core-shell SiO 2 according to claim 2, which is used as a material for a gas turbine engine for an aircraft, a brake for an automobile, a heat exchanger in a high-temperature part, or a bulletproof vest. x / Carbon nanofiber.

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