α-Alumina manufacturing method

Abstract

To provide a method for producing α-alumina that can produce alumina mainly composed of α phase at lower temperatures.SOLUTION: A method for producing α-alumina comprises heat-treating a starting material that comprises an aluminum compound comprising aluminum and chlorine at an atomic ratio of Al:Cl=25:75 to 80:20, in an atmosphere comprising superheated steam. Preferably, the aluminum compound is at least one selected from an aluminium chloride represented by the chemical formula AlCl3 and a basic aluminium chloride represented by the chemical formula [Al2(OH)nCl6-n]m (0<n<6, m≤10). Preferably, the heating treatment comprises a step of holding the starting material at a temperature of 900°C or higher.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing α-alumina by heat treating an aluminum compound. [Background technology]

[0002] There are several types of alumina (aluminum oxide) with different crystallinity and properties, and they are used for a variety of purposes depending on their properties. Among them, α-alumina, whose main crystalline phase is the α phase, is widely used industrially due to its high hardness, chemical stability, and mechanical strength. In recent years, α-alumina has also been expected to be used as a matrix material for ceramic matrix composites (CMCs), which combine ceramic fibers with ceramics.

[0003] Alpha-alumina is primarily composed of the alpha phase, which is stable at high temperatures, and is produced by heating aluminum hydroxide (Al(OH)3) or boehmite (AlO(OH)) through intermediate aluminas such as gamma-alumina, delta-alumina, and theta-alumina. The production of alpha-alumina via intermediate alumina requires heating at a high temperature of approximately 1300°C or higher. Therefore, a method for producing alpha-alumina at lower temperatures is desirable.

[0004] For example, Patent Document 1 describes a method for producing α-alumina in which aluminum hydroxide obtained by the Bayer process or transition alumina obtained by calcining it is crushed and fired at 600 to 1400°C in an atmosphere containing hydrogen chloride gas or an atmosphere containing chlorine gas and water vapor. Non-Patent Document 1 also reports on the effect of water vapor on the phase transition from γ-type or θ-type transition alumina to α-alumina. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-290914 [Non-patent literature]

[0006] [Non-Patent Document 1] Hiroaki YANAGIDA, Goro YAMAGUCHI and Joe KUBOTA, "The Role of Water Vapor in Formation of Alpha Alumina from Transient Alumina," Journal of the Ceramic Industry Association, 1966, Vol. 74, No. 856, pp. 371-378 Summary of the Invention [Problem to be solved by the invention]

[0007] Non-Patent Document 1 describes that water vapor has the effect of lowering the temperature at which the phase transition occurs from γ-alumina or θ-alumina to α-alumina. However, the proportion of the α phase in the α-alumina produced was not investigated, and the heating temperature in the experiment was 1000°C or higher, which is not sufficient for lowering the temperature.

[0008] The α-alumina manufacturing method described in Patent Document 1 aims to produce α-alumina powder with extremely low sodium, iron, and calcium contents and a narrow particle size distribution (paragraph

[0011] ). For this purpose, pulverized aluminum hydroxide, such as that obtained by the Bayer process, is used as the raw material, and calcination is carried out in an atmosphere containing hydrogen chloride gas or an atmosphere containing chlorine gas and water vapor. While Patent Document 1 does not describe in detail the effect of the calcination atmosphere, it is believed that the sodium, iron, and calcium are removed as chlorides by using hydrogen chloride gas or chlorine gas. Because hydrogen chloride gas and chlorine gas are corrosive gases, corrosion resistance must be considered for materials used in calcination furnaces and other equipment. Furthermore, Patent Document 1 lists calcination temperatures of 600 to 1400°C, but the examples only include results from calcination at 1100°C. Thus, there is still room for further research into methods for producing α-alumina at lower temperatures.

[0009] Among CMCs, development of alumina fiber-reinforced alumina composites (Al2O3 / Al2O3) is underway as an oxide-based material used in relatively low-temperature ranges. Alumina fiber-reinforced alumina composites are manufactured by impregnating a woven fabric made of alumina fibers with a solution containing a matrix (alumina) precursor and then sintering the fabric. As mentioned above, the formation of α-alumina requires heating at high temperatures of 1300°C or higher. However, in the manufacture of alumina fiber-reinforced alumina composites, sintering at temperatures around 1150°C is unavoidable due to the heat resistance and cost of the alumina fibers. Because the phase transition to α-alumina involves a relatively large volume shrinkage, a method for forming α-alumina at a lower temperature is desirable, in order to maximize the difference between the phase transition temperature and the sintering temperature and thereby increase the mechanical strength of the matrix.

[0010] The present disclosure has been made in view of the above circumstances, and aims to provide a method for producing α-alumina that can produce alumina having mainly the α phase at a lower temperature. [Means for solving the problem]

[0011] In order to solve the above problems, the method for producing α-alumina disclosed herein is characterized in that a raw material containing an aluminum compound containing aluminum and chlorine in an atomic ratio of Al:Cl=25:75 to 80:20 is heat-treated in an atmosphere containing superheated steam to produce alumina mainly having the α phase. [Effects of the Invention]

[0012] The method for producing α-alumina disclosed herein uses an aluminum compound containing a predetermined proportion of chlorine as a starting material. It is believed that chlorine in the aluminum compound tends to stabilize the γ-phase produced by heating, thereby suppressing crystal growth of the α-phase and making the phase transition from the γ-phase to the α-phase less likely to proceed. In this regard, the method for producing α-alumina disclosed herein involves heat treatment in an atmosphere containing superheated steam. The term "superheated steam" used herein refers to saturated steam (hereinafter simply referred to as "steam") generated by boiling water, which is further heated to a temperature exceeding 100°C. The superheated steam facilitates the desorption of chlorine from the aluminum compound. Therefore, even if the γ-phase initially forms upon heating, the γ-phase is not stabilized due to the desorption of chlorine. The superheated steam also serves to generate α-phase nuclei and promote crystal growth. As a result, even at relatively low heat treatment temperatures of 1000°C or less, the phase transition from the γ-phase to the α-phase proceeds, enabling the production of α-alumina primarily composed of the α-phase. In this way, the method for producing α-alumina disclosed herein incorporates chlorine into the structure of the raw aluminum compound, and utilizes the effect of the chlorine being removed to produce α-alumina at a lower temperature. Therefore, by using the method disclosed herein, for example, in the production of an alumina fiber material-reinforced alumina composite, the difference between the α-alumina production temperature and the sintering temperature can be reduced. This promotes neck growth between the particles that make up the matrix, thereby improving the mechanical strength of the matrix. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 shows the X-ray diffraction patterns of the powders of the examples. [Figure 2] FIG. 1 shows the X-ray diffraction patterns of the powders of the reference examples. [Figure 3] FIG. 1 is a diagram showing the results of thermogravimetry-differential thermal analysis when measured in a mixed gas. [Figure 4] FIG. 10 is a diagram showing the analysis results of the gas evolved during the temperature rise process in the mixed gas. [Figure 5]FIG. 1 is a diagram showing the results of thermogravimetry-differential thermal analysis when measured in air. [Figure 6] FIG. 10 is a diagram showing the analysis results of the gas generated during the temperature rise process in air. [Figure 7] FIG. 1 is a diagram in which the Cl / Al value is plotted against the temperature of heat treatment. [Figure 8] FIG. 2 shows NMR spectra of each powder in the examples. [Figure 9] FIG. 1 shows NMR spectra of each powder of the reference examples. [Figure 10] FIG. 1 is a plot of the peak intensity ratio of 6-coordination versus the temperature of heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0014] The following describes embodiments of the method for producing α-alumina according to the present disclosure. However, the embodiments are not limited to the following embodiments, and various modifications and improvements that can be made by those skilled in the art can be made.

[0015] [Raw materials] The method for producing α-alumina disclosed herein uses a raw material containing an aluminum compound containing aluminum and chlorine in an atomic ratio of Al:Cl=25:75 to 80:20. One type of aluminum compound may be used alone, or two or more types may be used in combination. If the chlorine content is too low, it becomes difficult to produce α-alumina at low temperatures. Furthermore, the solubility in water becomes very low. This makes it difficult to uniformly disperse the aluminum compound in the fiber material when the fiber material is impregnated with a solution containing the aluminum compound, for example, in the production of an alumina fiber-reinforced alumina composite material.

[0016] Suitable aluminum compounds include aluminum chloride, which has the formula AlCl3, aluminum nitrate, which has the formula [Al2(OH) n Cl 6-n ] mExamples include basic aluminum chloride represented by (0 < n < 6, m ≦ 10). The latter basic aluminum chloride is an inorganic polymer, and the basicity varies depending on the value of n in the chemical formula. The basicity is a value calculated by n / 6 × 100 (%). For the reason of suppressing the stabilization of the γ phase and promoting the phase transition to the α phase, a form with a high basicity, that is, a form with a large value of n and less Cl, is desirable. In the chemical formula, a preferred value of n is 4 < n ≦ 5.5.

[0017] The aluminum compound may be a solid such as powder or a liquid in which the aluminum compound is dispersed or dissolved in a solvent, such as a basic aluminum chloride solution. In the case of a liquid, the solvent may be removed in advance to form a solid before the heat treatment. The raw material may contain other components as long as it does not inhibit the formation of the α phase in alumina.

[0018] [Heat treatment] In the method for producing α-alumina of the present disclosure, the above-mentioned raw material is heat-treated in an atmosphere containing superheated steam. As the heat treatment apparatus, a firing furnace capable of controlling the atmosphere may be used. The pressure of the heat treatment may be atmospheric pressure.

[0019] The atmospheric gas used for the heat treatment may consist solely of superheated steam, or may contain other gases. Examples of such gases include oxygen gas, nitrogen gas, helium gas, and argon gas. It is desirable that the atmospheric gas does not contain chlorine-containing gases such as chlorine gas and hydrogen chloride gas. Superheated steam may be generated using tap water, ion-exchanged water, distilled water, ultrapure water, or other water, or hot water obtained by heating such water (hereinafter collectively referred to as "raw water"). The atmosphere containing superheated steam may be generated by supplying pre-prepared superheated steam to the heat treatment apparatus, or by supplying water vapor obtained by boiling raw water to the heat treatment apparatus and heating it to a temperature exceeding 100°C during the heat treatment. When other gases are mixed, the other gas and superheated steam or water vapor may be pre-mixed and then supplied, or the other gas and superheated steam or water vapor may be supplied separately. From the viewpoint of effectively exerting the effects of superheated steam, the atmosphere containing superheated steam preferably contains 20% by volume or more of superheated steam relative to the total volume of the atmospheric gas, and more preferably 50% by volume or more, 80% by volume or more, or 100% by volume.

[0020] The heat treatment involves heating the raw material to a predetermined temperature and maintaining that state for a predetermined period of time. The heating rate during heating is not particularly limited, but can be, for example, 0.5°C / min to 30°C / min. The maintenance temperature can be set appropriately depending on the desired α-phase ratio, the raw material, the composition of the atmospheric gas, and the like, and can be set, for example, in the range of 850°C to 1050°C. A preferred embodiment of the heat treatment includes a maintenance step in which the raw material is maintained at a temperature of 900°C or higher. This embodiment allows for the generation of a large amount of α-phase within a practical maintenance time. The maintenance time can also be set appropriately depending on the heat treatment temperature, the desired α-phase ratio, and the like, and can be set, for example, in the range of 1 hour to 50 hours. A maintenance time of 1 hour to 10 hours is particularly practical. For example, a suitable maintenance step involves maintaining the raw material at a temperature of 950°C to 1000°C for 2 hours or more.

[0021] The heat treatment produces alumina mainly having the α phase. The proportion of the α phase in the produced alumina can be determined, for example, by solid-state NMR (nuclear magnetic resonance) measurement. 27 It can be determined based on the Al NMR spectrum. 27 In the Al NMR spectrum, α-alumina consists of only hexacoordinated Al, while γ-alumina contains both hexacoordinated and tetracoordinated Al. When the amount of pentacoordinated Al increases, the alumina becomes amorphous. Therefore, as shown in the examples below, solid 27 The Al NMR spectrum can be separated into peaks for four-, five-, and six-coordinate systems by peak fitting, and the intensity ratio of the six-coordinate system peak when the integral value of the peak intensities for the three coordinate systems is set to 1 can be regarded as the proportion of the α phase. In this case, the intensity ratio of the six-coordinate system peak is preferably 0.8 or more, 0.9 or more, or 1.0. [Example]

[0022] Next, the present disclosure will be described more specifically with reference to examples.

[0023] <Production of α-alumina> [Ingredient preparation] [Al2(OH) n Cl 6-n ] m A solution containing basic aluminum chloride (4.9≦n≦5.1, m≦10) was heated under reduced pressure using a rotary evaporator to remove the solvent. The resulting powdery basic aluminum chloride (hereinafter referred to as "basic aluminum chloride powder") was used as the raw material.

[0024] [Heat treatment] (1) Example The basic aluminum chloride powder was placed in a tubular furnace and heat-treated in an atmosphere containing superheated steam and oxygen gas. The heat treatment was carried out by raising the temperature of the tubular furnace at a rate of 5°C / min and maintaining the set temperature in the range of 400 to 1000°C for 2 hours. A mixture of steam at a flow rate of 80 mL / min and oxygen gas at a flow rate of 20 mL / min was supplied to the tubular furnace. The volumetric ratio of superheated steam in the atmospheric gas in the tubular furnace was 80% by volume, and the partial pressure of the superheated steam (P H2O ) is 8 x 10 4 It is Pa.

[0025] (2) Reference example For comparison, the atmosphere in the tubular furnace was changed to air, and basic aluminum chloride powder was heat-treated in the same manner as in the example, except that air was supplied to the tubular furnace at a flow rate of 100 mL / min and the upper limit of the set temperature was changed to 1,050°C.

[0026] <Measurement of crystalline phase> The crystalline phase in the heat-treated powders was measured using an X-ray diffractometer (Rigaku Corporation, "RINT2500"). Measurements were performed using CuKα radiation at a tube voltage of 50 kV, a tube current of 300 mA, and a scanning rate of 1° / min. The measurement results are shown in Figures 1 and 2. Figure 1 shows the X-ray diffraction patterns of the powders of the Examples, and Figure 2 shows the X-ray diffraction patterns of the powders of the Reference Examples.

[0027] As shown in Figure 1, in the powder of the example heat-treated in a mixed gas atmosphere of superheated steam and oxygen gas, only the gamma phase (e.g., at 2θ = 46°) was formed when the heat treatment temperature was 800°C or lower. However, the alpha phase (e.g., at 2θ = 25.6°) began to form at 850°C, and the phase transition to the alpha phase progressed rapidly at temperatures above 900°C. Furthermore, when heat-treated at 1000°C, only the alpha phase was formed, i.e., single-phase alpha-alumina was formed. On the other hand, as shown in Figure 2, in the powder of the reference example heat-treated in an air atmosphere, some alpha phase formation was observed at heat treatment temperatures of 400°C and 500°C. However, as the temperature increased, the alpha phase did not grow but the gamma phase formed more, and even at 1000°C, the gamma phase was formed. Furthermore, when heat-treated at 1050°C, only the alpha phase was formed.

[0028] Furthermore, the heat treatment temperature was set to 900°C or 950°C, and the holding time was extended from 2 hours to 10 hours, and then to 50 hours. The crystalline phases of the heat-treated powder were measured using an X-ray diffractometer (same as above). Table 1 shows the main types of crystalline phases that were produced for the heat treatment atmosphere, temperature, and holding time. [Table 1]

[0029] As shown in Table 1, when heat treatment was performed in an atmosphere of a mixture of superheated steam and oxygen gas (H2O / O2), much of the material underwent a phase transition to the α phase after just two hours at 900°C, and after 50 hours it was all α phase. When heat treatment was performed at 950°C, all of the material was α phase after 10 hours. In contrast, when heat treatment was performed in an air atmosphere, the phase transition to the α phase progressed after 10 hours at 950°C, but the γ phase was still present even after 50 hours.

[0030] Thus, even when using the same basic aluminum chloride powder, if the heat treatment is carried out in an air atmosphere, the phase transition to the α-phase is difficult to proceed. However, if it is carried out in an atmosphere containing superheated steam, the phase transition to the α-phase can proceed at a temperature about 100 °C lower compared to the case of carrying out in an air atmosphere. And according to the method for producing α-alumina of the present disclosure, α-alumina can be produced at a temperature about 400 °C lower compared to the conventional sintering temperature of about 1300 °C.

[0031] <TG-DTA (Thermogravimetry-Differential Thermal Analysis) / MS (Mass Spectrometry)> Using a thermogravimetry-differential thermal analyzer (「STA2500」manufactured by Netzsch Japan Co., Ltd.) and a mass spectrometer (「JMS-Q1500GC」manufactured by JEOL Ltd.), the thermal decomposition behavior and simultaneous analysis of the generated gas during the temperature increase process of the raw material basic aluminum chloride powder were carried out. The heating rate was 10 °C / min. The measurement was carried out in a mixed gas of steam equivalent to a flow rate of 100 mL / min and oxygen gas with a flow rate of 100 mL / min (corresponding to the heat treatment of the previous example, the partial pressure (P H2O ) is 5×10 4 Pa), and in air with a flow rate of 200 mL / min (corresponding to the heat treatment of the previous reference example). The measurement results are shown in FIGS. 3 to 6. FIG. 3 shows the thermogravimetry-differential thermal analysis results when measured in the mixed gas, and FIG. 4 shows the analysis results of the generated gas during the temperature increase process in the mixed gas. FIG. 5 shows the thermogravimetry-differential thermal analysis results when measured in air, and FIG. 6 shows the analysis results of the generated gas during the temperature increase process in air. For convenience of explanation, in FIG. 4, the analysis results of the main generated gases H 35 Cl (m / z = 36) and H 37 Cl (m / z = 38) are shown. Similarly, in FIG. 6, the results of the main generated gases H2O (m / z = 18), H 35 Cl (m / z = 36), H 37 Cl (m / z = 38), and Cl2 (m / z = 70) are shown.

[0032] As shown in Fig. 3, when the temperature was raised in the mixed gas, the weight of the basic aluminum chloride powder gradually decreased and became substantially constant at 600 °C or higher. Also, a heat generation peak was observed over the range of 260 to 500 °C. These thermal decomposition behaviors correspond to the analysis results of the generated gas shown in Fig. 4. As shown in Fig. 4, mainly the generation of HCl (hydrogen chloride) gas was confirmed around 260 °C and 350 °C, and almost nothing was generated at 600 °C or higher. Thus, in the mixed gas, Cl in the basic aluminum chloride is mainly desorbed as HCl gas at 260 to 500 °C. In the mixed gas, the basic aluminum chloride is completely decomposed around 600 °C. <0000​​​​​​​​​​​​The raw basic aluminum chloride powder and the heat-treated powder were analyzed using an X-ray photoelectron spectrometer (ULVAC-PHI, Inc., ESCA 1800MC) to determine the Al / Cl ratio. AlKα (1486.6 eV) X-rays were used to acquire Al2p and Cl2p spectra, and the Al / Cl ratio was determined using the analysis software (PHI Multipak v9.4) provided with the analyzer. The heat-treated powders measured were those subjected to the same heat treatment as in the previous example (held in a superheated steam and oxygen gas mixed gas atmosphere for 2 hours) or the reference example (held in an air atmosphere for 2 hours), with 18 different heat treatment atmospheres or temperatures. Figure 7 shows a plot of the Cl / Al ratio versus heat treatment temperature. The Cl / Al ratio for the raw basic aluminum chloride powder was 0.43. As shown in Figure 7, when heat-treated in a mixed gas atmosphere, the Cl / Al ratio approached zero at 600 °C. On the other hand, when heat treatment was performed in an air atmosphere, the Cl / Al value approached 0 at 900°C. Thus, it was confirmed that when heat treatment was performed in an atmosphere containing superheated steam, Cl was desorbed at a lower temperature.

[0036] <Solid state NMR (nuclear magnetic resonance) measurement> Using a nuclear magnetic resonance spectrometer (JEOL Ltd. "JNM-ECA600II"), the solid of the powder after heat treatment was 27 The Al NMR spectrum was measured. The heat-treated powder was filled into a HXMAS probe with a diameter of 3.2 mm, and the measurement was performed at a magnetic field strength of 14.1 T and a rotation speed of 20 kHz. 27 The chemical shift reference for Al NMR was set to the peak of an Al(NO3)3 aqueous solution at 0 ppm. Figure 8 shows the NMR spectrum of each powder in the examples, and Figure 9 shows the NMR spectrum of each powder in the reference example. Note that in Figures 8 and 9, the NMR spectra of the reference α-alumina (α-Al2O3) and γ-alumina (γ-Al2O3) are also shown with dotted lines. As shown in Figures 8 and 9, α-alumina is composed only of hexacoordinated Al, while γ-alumina contains both hexacoordinated and tetracoordinated Al. When the amount of pentacoordinated Al increases, amorphous alumina results.

[0037] As shown in Figure 8, in the example where heat treatment was performed for 2 hours in a mixed gas atmosphere of superheated steam and oxygen gas, peaks were observed for tetra- and hexa-coordinated Al at temperatures below 800°C, indicating the formation of the γ phase. At 900°C, the hexa-coordinated Al peak shifted to the left, indicating the progression of the phase transition to the α phase. At 1000°C, only hexa-coordinated Al, i.e., the α phase, was confirmed. On the other hand, as shown in Figure 9, in the reference example where heat treatment was performed for 2 hours in an air atmosphere, peaks were observed for tetra- and hexa-coordinated Al at temperatures below 1000°C. The hexa-coordinated Al peak shifted slightly to the right at 800°C and 900°C, but remained almost unchanged from that of α-alumina. This indicates that the α phase and the γ phase coexist below 1000°C. Furthermore, it was confirmed that the α phase, consisting of only hexa-coordinated Al, was formed at 1050°C.

[0038] In addition, to determine the ratio of the α phase in the powder after heat treatment, the obtained solid 27 The Al NMR spectrum was separated into peaks for tetracoordination, pentacoordination, and hexacoordination by peak fitting, and the intensity ratio of the hexacoordination peak was calculated when the integral value of the peak intensities for the three coordinations was set to 1. Figure 10 shows a plot of the hexacoordination peak intensity ratio versus heat treatment temperature. As shown in Figure 10, the hexacoordination peak intensity ratio for the powder of the example that was heat-treated for 2 hours in a mixed gas atmosphere of superheated steam and oxygen gas was confirmed to be 0.8 at 900 °C and 1.0 at 1000 °C. From these results, it can be said that approximately 80% of the alumina obtained by heat treatment at 900 °C in the example was α-phase, and all of the alumina obtained by heat treatment at 1000 °C was α-phase. [Industrial Applicability]

[0039] The method for producing α-alumina disclosed herein enables the production of alumina having a predominantly α-phase at lower temperatures, and is useful for, for example, the production of alumina fiber-reinforced alumina composites, the low-temperature synthesis of porous alumina bodies and α-alumina powders, and the like.

Claims

1. A method for producing α-alumina, characterized by heat-treating a raw material containing one or more aluminum compounds selected from basic aluminum chlorides represented by the chemical formula [Al 2 (OH) n Cl 6-n ] m (4<n≦5.5, m≦10) in an atmosphere containing superheated steam to produce alumina mainly having the α phase.

2. 2. The method for producing α-alumina according to claim 1, wherein the heat treatment comprises a holding step of holding the raw material at a temperature of 900° C. or higher.

3. 3. The method for producing α-alumina according to claim 2, wherein the holding step is a step of holding the raw material at a temperature of 950° C. or higher and 1000° C. or lower for 2 hours or longer.

4. 4. The method for producing α-alumina according to claim 1, wherein the atmosphere containing superheated steam contains 20% by volume or more of the superheated steam relative to the total volume of the atmospheric gas.

5. 5. The method for producing α-alumina according to claim 1, wherein the atmosphere containing the superheated steam contains oxygen gas.

6. The alumina having mainly the α phase is a solid state alumina obtained by solid state nuclear magnetic resonance (NMR) measurement. ２７ 6. The method for producing α-alumina according to claim 1, wherein, in an Al NMR spectrum, when the sum of the integrated values ​​of the peak intensities of tetracoordination, pentacoordination, and hexacoordination is taken as 1, the peak intensity ratio of the hexacoordination is 0.8 or more.

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