阅读说明：本技术 勃姆石结构体及其制造方法 (Boehmite structure and method for producing same ) 是由 佐藤夏希 栗副直树 泽亮介 吉冈达郎 于 2020-04-17 设计创作，主要内容包括：一种勃姆石结构体(1),其含有多个勃姆石粒子(2),并且相邻的勃姆石粒子(2)结合。而且,勃姆石结构体(1)的气孔率为30％以下。勃姆石结构体的制造方法包含下述工序：通过将水硬性氧化铝和含水溶剂进行混合而得到混合物的工序；和将该混合物在压力为10～600MPa、且温度为50～300℃的条件下进行加压和加热的工序。(A boehmite structure (1) containing a plurality of boehmite particles (2), wherein adjacent boehmite particles (2) are bonded. The boehmite structure (1) has a porosity of 30% or less. The method for producing a boehmite structure comprises the steps of: a step of mixing hydraulic alumina with a water-containing solvent to obtain a mixture; and a step of pressurizing and heating the mixture under a pressure of 10 to 600MPa and a temperature of 50 to 300 ℃.)

1. A boehmite structure comprising a plurality of boehmite particles, wherein the boehmite particles adjacent to each other are bonded to each other, and wherein the porosity is 30% or less.

2. The boehmite structure according to claim 1, having a flexural strength measured according to JIS R1601 of 3MPa or more.

3. The boehmite structure according to claim 1 or 2, wherein a boehmite phase is present in a proportion of 50% by mass or more.

4. The boehmite structure according to any one of claims 1 to 3, having pores, and a size of the pores is 5 μm or less.

5. The boehmite structure according to any one of claims 1 to 4, having an average value of total light transmittance in a wavelength region of 380nm to 700nm at a thickness of 0.5mm to 1mm of 20% or more.

6. The boehmite structure according to claim 5, having pores, and a size of the pores is 1 μm or less.

7. The boehmite structure according to claim 5 or 6, wherein the porosity is 5% or less.

8. The boehmite structure according to any one of claims 1-7, further comprising a substance other than the boehmite particles.

9. A method for producing a boehmite structure, comprising the steps of: a step of mixing hydraulic alumina with a water-containing solvent to obtain a mixture; and

and pressurizing and heating the mixture under a pressure of 10 to 600MPa and at a temperature of 50 to 300 ℃.

10. The method for producing a boehmite structure according to claim 9, wherein said hydraulic alumina has an average particle diameter D₅₀Is 5 μm or less.

Technical Field

The present invention relates to a boehmite structure and a method for producing the same.

Background

Boehmite is an aluminum oxidized hydroxide represented by the compositional formula of AlOOH. Boehmite is insoluble in water and hardly reacts with an acid or an alkali at ordinary temperature, and therefore has high chemical stability and excellent heat resistance because of its dehydration temperature as high as about 500 ℃. The boehmite powder having such characteristics is used for resin additives, catalyst raw materials, abrasives, and the like.

Further, the specific gravity of boehmite is about 3.07. Therefore, it is desired to develop a structure which is light in weight and excellent in chemical stability and heat resistance using boehmite. For example, patent document 1 discloses the following: a porous boehmite molded body can be obtained by subjecting a mixture of aluminum hydroxide, a reaction promoter and water to hydrothermal treatment at 140 ℃ or higher and less than 350 ℃. The porous boehmite molded body has a structure in which flaky or needle-shaped boehmite crystals form a connected crystal structure and are connected to form continuous pores, and further has a porosity of 65% or more and a flexural strength of 400N/cm²The above.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2003-238150

Disclosure of Invention

However, in patent document 1, since hydroxides of sodium and calcium are used as the reaction accelerator, they remain as impurities in the resulting structure. Therefore, the production method of patent document 1 has a problem that it is difficult to obtain a boehmite structure that retains the original characteristics of boehmite. Further, when a structure is to be obtained by sintering boehmite powder at a high temperature, the crystal structure of boehmite is changed to α alumina having a high specific gravity (specific gravity of 3.98), and thus there is a problem that a boehmite structure having a light weight cannot be obtained.

The present invention has been made in view of the problems of the above-described conventional technologies. Accordingly, an object of the present invention is to provide a boehmite structure and a method for producing a boehmite structure, which are light in weight, excellent in chemical stability, and reduced in impurity amount.

In order to solve the above problem, a boehmite structure according to a first aspect of the present invention contains a plurality of boehmite particles, and adjacent boehmite particles are bonded to each other. The boehmite structure has a porosity of 30% or less.

The present invention provides a method for producing a boehmite structure according to a second aspect, comprising the steps of: a step of mixing hydraulic alumina with a water-containing solvent to obtain a mixture; and a step of pressurizing and heating the mixture under a pressure of 10 to 600MPa and a temperature of 50 to 300 ℃.

Drawings

Fig. 1 is a sectional view schematically showing an example of a boehmite structure according to the present embodiment.

FIG. 2 shows the X-ray diffraction pattern of the hydraulic alumina used in example 1, and boehmite (AlOOH) and gibbsite (Al (OH) registered in ICSD₃) A chart of the pattern of (a).

Fig. 3 is a diagram showing a reflected electron image of a site 1 in the test sample 1 of example 1.

Fig. 4 is a diagram showing data obtained by binarizing a reflected electron image at position 1 in the test sample 1 of example 1.

Fig. 5 is a diagram showing a reflected electron image of a site 2 in the test sample 1 of example 1.

Fig. 6 is a diagram showing data obtained by binarizing a reflected electron image at a position 2 in the test sample 1 of example 1.

Fig. 7 is a diagram showing a reflected electron image of a site 3 in the test sample 1 of example 1.

Fig. 8 is a diagram showing data obtained by binarizing a reflected electron image at a position 3 in the test sample 1 of example 1.

Fig. 9 is a graph showing the relationship between stress and stroke when the bending strength of the test sample 1 of example 1 is measured in accordance with JIS R1601.

FIG. 10 is a microphotograph showing the results of observing the cross-sections of test sample 2 obtained by molding a mixture of hydraulic alumina and water at normal temperature without applying pressure, test sample 3 obtained by molding at 180 ℃ and 50MPa, and test sample 4 obtained by molding at 180 ℃ and 200MPa in example 2.

Fig. 11 is a graph showing the X-ray diffraction patterns of test samples 5 and 6 obtained by changing the amount of water added to hydraulic alumina in example 3, and the X-ray diffraction patterns of boehmite and gibbsite registered in ICSD.

Fig. 12 is a graph showing the X-ray diffraction pattern of test sample 7 obtained by firing a boehmite structure at 400 ℃ for 1 hour in example 4, and the X-ray diffraction patterns of gamma alumina, gibbsite, and boehmite registered in ICSD.

FIG. 13(a) is a scanning electron micrograph showing hydraulic alumina used for producing test sample 8 of example 5.

Fig. 13(b) is an enlarged photograph of the scanning electron micrograph of fig. 13 (a).

Fig. 14 is a graph showing the relationship between the total light transmittance and the wavelength in test sample 8 of example 5.

Fig. 15 is a graph showing the X-ray diffraction pattern of test sample 8 of example 5 and the X-ray diffraction pattern of boehmite registered in ICSD.

Fig. 16 is a diagram showing a secondary electron image of a site 1 in the test sample 9 of example 6.

Fig. 17 is a diagram showing data obtained by binarizing the secondary electron image at position 1 in test sample 9 of example 6.

Fig. 18 is a diagram showing a secondary electron image of a site 2 in the test sample 9 of example 6.

Fig. 19 is a diagram showing data obtained by binarizing the secondary electron image at the position 2 in the test sample 9 of example 6.

Fig. 20 is a diagram showing a secondary electron image of a site 3 in the test sample 9 of example 6.

Fig. 21 is a diagram showing data obtained by binarizing the secondary electron image at the position 3 in the test sample 9 of example 6.

Detailed Description

The boehmite structure and the method for producing the boehmite structure according to the present embodiment will be described below with reference to the drawings. In addition, the dimensional ratio of the drawings is exaggerated for convenience of explanation and sometimes differs from the actual ratio.

[ Boehmite structure of the first embodiment ]

As shown in fig. 1, a boehmite structure 1 of the present embodiment contains a plurality of boehmite particles 2. Furthermore, the boehmite particles 2 adjacent to each other are bonded to each other, whereby the boehmite structure 1 in which the boehmite particles 2 are combined is formed. Further, pores 3 are present between adjacent boehmite particles 2.

The boehmite particles 2 may be particles formed of only a boehmite phase, or may be particles formed of a mixed phase of boehmite and alumina or aluminum hydroxide other than boehmite. For example, the boehmite particles 2 may be phases formed from boehmite and phases formed from gibbsite (Al (OH)₃) The formed particles are mixed.

The boehmite particles 2 constituting the boehmite structure 1 have an average particle diameter of preferably 300nm to 50 μm, more preferably 300nm to 30 μm, and particularly preferably 300nm to 20 μm, although not particularly limited. When the average particle diameter of the boehmite particles 2 is within this range, the boehmite particles 2 are strongly bonded to each other, and the strength of the boehmite structure 1 can be improved. When the average particle diameter of the boehmite particles 2 is within this range, the ratio of pores present inside the boehmite structure 1 is 30% or less, as described later, and therefore the strength of the boehmite structure 1 can be improved. In the present specification, unless otherwise specified, the value of the "average particle diameter" is a value calculated as an average value of particle diameters of particles observed in several to several tens of fields of view by using an observation means such as a Scanning Electron Microscope (SEM) or a Transmission Electron Microscope (TEM).

The shape of the boehmite particles 2 is not particularly limited, but may be, for example, a spherical shape. The boehmite particles 2 may be whisker-like (needle-like) particles or scale-like particles. The whisker-like particles or the scale-like particles have higher contact with other particles than the spherical particles, and therefore the strength of the boehmite structure 1 as a whole can be improved.

As described above, the boehmite structure 1 is composed of the particle group of the boehmite particles 2. That is, the boehmite structure 1 is composed of a plurality of boehmite particles 2 mainly comprising boehmite, and the boehmite particles 2 are bonded to each other to form the boehmite structure 1. At this time, the boehmite particles 2 may be in a state of point contact with each other, or may be in a state of surface contact in which particle surfaces of the boehmite particles 2 are in contact with each other.

Here, the adjacent boehmite particles 2 are bonded via at least one of an oxide and a hydroxide of aluminum. That is, the boehmite particles 2 are not bonded by an organic binder formed of an organic compound, and are not bonded by an inorganic binder formed of an inorganic compound other than an oxide or hydroxide of aluminum. As described later, the boehmite structure 1 can be formed by heating a mixture of hydraulic alumina and water while pressurizing the mixture. Hydraulic alumina is a compound produced by heat-treating aluminum hydroxide, and contains rho alumina as a main phase. In this way, since the boehmite structure 1 does not contain impurities derived from the reaction accelerator contained in the molded body of patent document 1, the original characteristics of boehmite can be maintained.

The boehmite structure 1 has at least a boehmite phase formed of boehmite (AlOOH), but may have a crystal phase other than the boehmite phase. The crystal phase other than the boehmite phase contained in the boehmite structure 1 may be aluminum hydroxide (al (oh))₃) Gibbsite phase formed, and alumina (Al)₂O₃) The gamma alumina phase formed. However, the boehmite structure 1 is preferably in the boehmite phaseIs a main body. Since boehmite is lightweight and has high chemical stability and heat resistance as described above, boehmite structure 1 having lightweight and excellent chemical stability and heat resistance can be obtained by mainly using a boehmite phase.

In the boehmite structure 1, the boehmite phase is present in a proportion of preferably 50% by mass or more, more preferably 60% by mass or more, and further preferably 70% by mass or more. By increasing the proportion of the boehmite phase, a boehmite structure 1 that is light in weight and excellent in chemical stability and heat resistance can be obtained. The proportion of the boehmite phase in the boehmite structure 1 can be determined by measuring the X-ray diffraction pattern of the boehmite structure 1 by an X-ray diffraction method and then performing Rietveld (Rietveld) analysis.

As described above, the boehmite structure 1 may have aluminum hydroxide (al (oh))₃) The gibbsite phase formed. However, since aluminum hydroxide has reactivity to an acid and a base, it is preferable to reduce the existing ratio of the gibbsite phase in order to further improve the chemical stability of the boehmite structure 1. In order to reduce the existing ratio of the gibbsite phase, the boehmite structure 1 may be heated to dehydrate the gibbsite phase. That is, by heating the boehmite structure 1, a dehydration reaction occurs, and the crystal structure is transformed from gibbsite to boehmite. This reduces the gibbsite phase and increases the boehmite phase, thereby improving the chemical stability of the boehmite structure 1. The heating condition of the boehmite structure 1 is not particularly limited as long as it is a condition under which the dehydration reaction of the gibbsite phase can occur, but it is preferably, for example, heating to 300 ℃ or higher in the air.

Further, as a method for reducing the existing ratio of the gibbsite phase in the boehmite structure 1, it is also preferable to heat hydraulic alumina as a raw material to reduce the existing ratio of gibbsite in the hydraulic alumina. Specifically, hydraulic alumina in which hydraulic alumina is heated to, for example, 300 ℃ or higher to reduce gibbsite is also preferably used. By using the above-described hydraulic alumina in which gibbsite is reduced as a raw material, the proportion of the gibbsite phase in the boehmite structure 1 can be reduced, and the chemical stability of the boehmite structure 1 can be improved. The boehmite structure 1 may be formed by heating a mixture of hydraulic alumina reduced in gibbsite and water under pressure as described later.

The porosity in the cross section of the boehmite structure 1 is preferably 30% or less. That is, when the cross section of the boehmite structure 1 is observed, the average value of the proportion of pores per unit area is preferably 30% or less. When the porosity is 30% or less, the ratio of boehmite particles 2 bonded to each other increases, and thus the boehmite structure 1 becomes dense and the strength improves. Therefore, the machinability of the boehmite structure 1 can be improved. In addition, when the porosity is 30% or less, since the occurrence of cracks in the boehmite structure 1 starting from the pores 3 can be suppressed, the flexural strength of the boehmite structure 1 can be improved. The porosity of the boehmite structure 1 in the cross section is preferably 20% or less, more preferably 10% or less, and still more preferably 5% or less. Since cracks starting from the pores 3 are suppressed as the porosity in the cross section of the boehmite structure 1 is smaller, the strength of the boehmite structure 1 can be improved.

In the present specification, the porosity can be determined as follows. First, a cross section of the boehmite structure 1 is observed to discriminate the boehmite particles 2 and the pores 3. Then, the unit area and the area of the pores 3 in the unit area are measured, and the ratio of the pores 3 per unit area is determined. After such a ratio of the pores 3 per unit area is obtained at a plurality of locations, an average value of the ratio of the pores 3 per unit area is defined as a porosity. In addition, when the cross section of the boehmite structure 1 is observed, an optical microscope, a Scanning Electron Microscope (SEM), or a Transmission Electron Microscope (TEM) may be used. The unit area and the area of the pores 3 in the unit area can also be measured by binarizing an image observed with a microscope.

The size of the pores 3 present inside the boehmite structure 1 is not particularly limited, but is preferably as small as possible. Since the size of the pores 3 is small, cracks starting from the pores 3 are suppressed, and therefore, the strength of the boehmite structure 1 can be improved, and the machinability of the boehmite structure 1 can be improved. The size of the pores 3 of the boehmite structure 1 is preferably 5 μm or less, more preferably 1 μm or less, and further preferably 100nm or less. The size of the pores 3 present inside the boehmite structure 1 can be determined by observing the cross section of the boehmite structure 1 with a microscope, similarly to the porosity described above.

The boehmite structure 1 may have a structure in which the boehmite particles 2 are bonded to each other and the porosity is 30% or less. Therefore, the boehmite structure 1 is not limited in shape as long as it has the above-described structure. The boehmite structure 1 may be formed into a sheet, film, rectangular, block, rod, or sphere shape, for example. When the boehmite structure 1 is in a sheet or film form, the thickness t is not particularly limited, but may be set to, for example, 50 μm or more. The boehmite structure 1 according to the present embodiment is formed by a pressure heating method as described later. Therefore, the boehmite structure 1 having a large thickness can be easily obtained. The thickness t of the boehmite structure 1 may be set to 1mm or more, or may be set to 1cm or more. The upper limit of the thickness t of the boehmite structure 1 is not particularly limited, but may be set to, for example, 50 cm.

As described above, since the plurality of boehmite particles 2 in the boehmite structure 1 are strongly bonded to each other, it has high mechanical strength. Therefore, the boehmite structure 1 preferably has a flexural strength of 3MPa or more as measured in accordance with JIS R1601 (method for testing the room temperature flexural strength of fine ceramics). The flexural strength of the boehmite structure 1 was measured by a 3-point flexural strength test method according to JIS R1601. When the flexural strength of the boehmite structure 1 is 3MPa or more, the mechanical strength is excellent, and therefore the machinability is improved. Therefore, the boehmite structure 1 can be easily used for, for example, a building member requiring high mechanical strength and workability. The flexural strength of the boehmite structure 1 is more preferably 10MPa or more, and still more preferably 50MPa or more. The upper limit of the flexural strength of the boehmite structure 1 is not particularly limited, but may be set to, for example, 200 MPa.

In the boehmite structure 1, the plurality of boehmite particles 2 are not bonded by an organic binder formed of an organic compound, and are not bonded by an inorganic binder formed of an inorganic compound other than an oxide or hydroxide of aluminum. Therefore, the content ratio of the elements other than aluminum among the metal elements contained in the boehmite structure 1 is preferably 5% by mass or less, more preferably 3% by mass or less, and still more preferably 1% by mass or less. Since impurities such as sodium and calcium are hardly present in the boehmite structure 1, the original characteristics of boehmite can be maintained.

As described above, the boehmite structure 1 according to the present embodiment includes a plurality of boehmite particles 2 and is formed by bonding adjacent boehmite particles 2. Further, the boehmite structure 1 has a porosity of 30% or less. In the boehmite structure 1, the plurality of boehmite particles 2 are bonded without using an organic binder or an inorganic binder formed of an inorganic compound other than an oxide or hydroxide of aluminum. Furthermore, since the boehmite structure 1 mainly contains the boehmite phase, the boehmite structure 1 is light in weight and excellent in chemical stability, and further, the amount of impurities can be reduced. Further, since the boehmite structure 1 has a porosity of 30% or less, the boehmite particles 2 are densely arranged, and the mechanical strength of the boehmite structure 1 is improved. Therefore, the boehmite structure 1 can have high machinability.

As shown in fig. 1, the boehmite structure 1 according to the present embodiment can be a structure in which only the boehmite particles 2 are bonded. However, as described later, since the boehmite structure 1 can be obtained by heating the raw material to 50 to 300 ℃ and pressurizing it, for example, a member having low heat resistance can be added to the boehmite structure 1. That is, the boehmite structure 1 may contain a substance other than the boehmite particles 2. In other words, the boehmite structure 1 may contain a substance formed of a material other than the material constituting the boehmite particles 2. Such a substance may be at least one of an organic substance and an inorganic substance. For example, the boehmite structure 1 may contain resin particles or a pigment in addition to the boehmite particles 2. The boehmite structure 1 is not limited to a member having low heat resistance, and may contain metal particles or particles made of an inorganic compound.

[ Boehmite structure of the second embodiment ]

Next, the boehmite structure according to the second embodiment will be described in detail. Note that the same components as those of the first embodiment are denoted by the same reference numerals, and redundant description thereof is omitted.

The boehmite structure of the present embodiment contains a plurality of boehmite particles 2, and adjacent boehmite particles 2 are bonded, as in the first embodiment. Further, the boehmite structure has a porosity of 30% or less. Therefore, the boehmite structure according to the present embodiment is also a structure that is light in weight, excellent in chemical stability, and reduced in the amount of impurities.

The boehmite structure according to the present embodiment is characterized by having a high density and, as a result, has a visible light transmittance. Specifically, the boehmite structure preferably has an average value of total light transmittance of 10% or more, more preferably 15% or more, and further preferably 20% or more in a wavelength region of 380nm to 700nm at a thickness of 0.5mm to 1 mm. The increase in the density of the boehmite structure suppresses scattering of visible light, and thus can improve light transmittance.

Here, in order to increase the density of the boehmite structure and improve the light transmittance, it is preferable to reduce the pore diameter of the pores 3 and to refine the grain boundaries of the plurality of boehmite particles 2. By reducing the pore diameter of the pores 3 and further refining the grain boundaries of the boehmite particles 2, the interfaces between the boehmite particles 2 and the pores 3 and the interfaces between the boehmite particles 2 adjacent to each other are reduced. As a result, scattering of light at the interface can be suppressed, and thus the light transmittance of the boehmite structure can be improved. Therefore, the boehmite structure preferably has pores 3, and the size of the pores 3 is 1 μm or less. When the size of the pores 3 present in the boehmite structure is 1 μm or less, the interface between the boehmite particles 2 and the pores 3 decreases, and therefore scattering of transmitted light can be reduced, and light transmittance can be improved.

In addition, in order to increase the density of the boehmite structure and improve the light transmittance, it is preferable to increase the density of the boehmite structure. Therefore, the boehmite structure preferably has a porosity of 5% or less. When the porosity of the boehmite structure is 5% or less, the number of pores 3 decreases, and the interface between the boehmite particles 2 and the pores 3 also decreases. Therefore, scattering of the transmitted light at the interface is reduced, and the light transmittance of the boehmite structure can be improved. From the viewpoint of further increasing the density of the boehmite structure, the porosity of the boehmite structure is more preferably 3% or less, and still more preferably 1% or less.

As described above, the boehmite structure of the present embodiment has a light transmittance of visible light in addition to the plurality of boehmite particles 2 bonded to each other. Therefore, a structure having excellent design properties can be obtained by adding, for example, a coloring agent.

[ method for producing boehmite structure according to first embodiment ]

Next, a method for producing the boehmite structure 1 according to the first embodiment will be described. The boehmite structure 1 can be produced by mixing hydraulic alumina and a water-containing solvent, and then heating under pressure. Hydraulic alumina is a hydrate produced by heat-treating aluminum hydroxide, and contains rho alumina. Such hydraulic alumina has a property of binding and solidifying by hydration reaction. Accordingly, by using the pressure-heating method, a hydration reaction of hydraulic aluminas proceeds, hydraulic aluminas are bonded to each other, and at the same time, the crystal structure is changed to boehmite, whereby the boehmite structure 1 can be formed.

Specifically, first, a hydraulic alumina powder is mixed with an aqueous solvent to prepare a mixture. The aqueous solvent is preferably pure water or ion-exchanged water. However, the aqueous solvent may contain an acidic substance or a basic substance in addition to water. The aqueous solvent may contain, for example, an organic solvent (e.g., alcohol) as long as it contains water as a main component.

The amount of the solvent added to the hydraulic alumina is preferably an amount that enables the hydration reaction of the hydraulic alumina to proceed sufficiently. The amount of the solvent added is preferably 20 to 200% by mass, more preferably 50 to 150% by mass, based on the hydraulic alumina.

Then, a mixture of hydraulic alumina and an aqueous solvent is filled into the mold. After filling the mixture into a mold, the mold may be heated as necessary. Then, by applying pressure to the mixture inside the mold, the inside of the mold becomes a high-pressure state. At this time, the hydraulic alumina is highly filled, and the hydraulic alumina particles are bonded to each other to increase the density. Specifically, by adding water to hydraulic alumina, the hydraulic alumina undergoes a hydration reaction, and boehmite and aluminum hydroxide are produced on the surface of the hydraulic alumina particles. Then, the mixture is heated and pressurized inside the mold, whereby the generated boehmite and aluminum hydroxide particles are mutually diffused between the adjacent hydraulic alumina particles, and the hydraulic alumina particles are gradually bonded to each other. Thereafter, dehydration reaction is carried out by heating, and the crystal structure is changed from aluminum hydroxide to boehmite. Further, it is presumed that: the hydration reaction of the hydraulic alumina, the interdiffusion between the hydraulic alumina particles, and the dehydration reaction proceed almost simultaneously.

Then, by taking out the molded body from the inside of the mold, a boehmite structure 1 in which a plurality of boehmite particles 2 are bonded to each other by at least one of an oxide and a hydroxide of aluminum can be obtained.

The heating and pressurizing conditions of the mixture of hydraulic alumina and a water-containing solvent are not particularly limited as long as the reaction between hydraulic alumina and the solvent proceeds. For example, it is preferable that a mixture of hydraulic alumina and a water-containing solvent is heated to 50 to 300 ℃ and pressurized at 10 to 600 MPa. The temperature of the mixture of hydraulic alumina and the aqueous solvent when heated is more preferably 80 to 250 ℃, and still more preferably 100 to 200 ℃. The pressure at which the mixture of hydraulic alumina and the aqueous solvent is pressurized is more preferably 50 to 600MPa, and still more preferably 200 to 600 MPa.

Here, as a method for forming the boehmite structure, a method of pressurizing only boehmite powder may be considered. However, even when boehmite powder is put into a mold and pressurized at normal temperature, it is difficult for boehmite particles to react with each other and it is difficult to firmly bond the particles to each other. Therefore, many pores are present in the obtained green compact, and the mechanical strength is insufficient.

In addition, as a method for producing an inorganic member made of ceramic, a sintering method has been known. The sintering method is a method of obtaining a sintered body by heating an aggregate of solid powder formed of an inorganic substance at a temperature lower than the melting point. Therefore, as a method for forming the boehmite structure, a method may be considered in which only boehmite powder is pressed to form a green compact, and then the green compact is fired at 500 ℃. However, when the green compact is fired at 500 ℃, boehmite undergoes a dehydration reaction and the crystal structure changes from boehmite to γ -alumina. Since gamma alumina has a specific gravity of about 3.98, a lightweight structure cannot be obtained. Further, even when the green compact of the boehmite powder is heated at about 500 ℃, the boehmite particles are hard to sinter to each other, and therefore, the resulting structure has many pores and insufficient mechanical strength.

Further, as a method for forming the boehmite structure, a method may be considered in which only boehmite powder is pressed to form a green compact, and then the green compact is fired at 1400 ℃. When the green body of boehmite powder is fired at 1400 ℃, boehmite powders are sintered to each other to form a structure. However, when the boehmite green compact is fired at 1400 ℃, a dehydration reaction of boehmite proceeds and the crystal structure changes from boehmite to α alumina. Therefore, since pores are generated with dehydration and densification is inhibited, and the specific gravity of α -alumina is about 3.98, a dense, lightweight, and strong structure cannot be obtained. In contrast, according to the production method of the present embodiment, since the mixture obtained by mixing hydraulic alumina and the aqueous solvent is heated and pressurized, a dense, lightweight, and strong structure mainly composed of a boehmite phase can be obtained.

As described above, the method for producing the boehmite structure 1 according to the present embodiment includes the steps of: a step of mixing hydraulic alumina with a water-containing solvent to obtain a mixture; and a step of pressurizing and heating the mixture. The heating and pressurizing conditions of the mixture are preferably set to a temperature of 50 to 300 ℃ and a pressure of 10 to 600 MPa. In the production method of the present embodiment, since the boehmite structure 1 is molded under the low temperature conditions as described above, the obtained structure is mainly composed of the boehmite phase. Therefore, the boehmite structure 1 having a light weight, excellent chemical stability, and a reduced amount of impurities can be obtained by a simple method.

[ method for producing boehmite structure according to second embodiment ]

Next, a method for producing a boehmite structure according to a second embodiment will be described. The boehmite structure according to the second embodiment can be produced by mixing hydraulic alumina with a water-containing solvent and then heating the mixture under pressure, as in the first embodiment. Specifically, the method for producing a boehmite structure comprises the steps of: a step of mixing hydraulic alumina with a water-containing solvent to obtain a mixture, and a step of pressurizing and heating the mixture at a temperature of 50 to 300 ℃ and a pressure of 10 to 600 MPa.

However, in the production method of the present embodiment, the particle size of the hydraulic alumina as a raw material is reduced. Specifically, the average particle diameter D of the hydraulic alumina as the raw material₅₀Is 5 μm or less. By the mean particle diameter D of the hydraulic alumina₅₀When the mixture of hydraulic alumina and a solvent is heated and pressurized, the particles are easily densely packed and easily bonded to each other. As a result, the porosity is reduced, and a boehmite structure with high density can be obtained. Further, the average particle diameter D of hydraulic alumina₅₀Can be determined by a laser diffraction scattering method.

As described above, hydraulic alumina having rho alumina as a main phase can be produced by heat-treating gibbsite (aluminum hydroxide). Specifically, can beAnd heating gibbsite at 300-600 ℃ under normal pressure or reduced pressure. Thus, for example, by adjusting the average particle diameter D₅₀Gibbsite of 5 μm or less is heated at 350 to 600 ℃ under normal pressure or reduced pressure to obtain an average particle diameter D₅₀A hydraulic alumina having a particle size of 5 μm or less. The average particle diameter D of the hydraulic alumina as a raw material₅₀More preferably 3 μm or less, and still more preferably 1 μm or less.

[ Member having Boehmite Structure ]

Next, a member including the boehmite structure 1 will be described. As described above, the boehmite structure 1 can be formed into a sheet shape having a large thickness, and is light in weight and excellent in chemical stability. Further, the boehmite structure 1 has high mechanical strength, and can be cut in the same manner as a general ceramic member and also subjected to surface processing. Therefore, the boehmite structure 1 can be preferably used as a building member. The building member is not particularly limited, and examples thereof include an exterior wall material (siding), a roof material, and the like. Further, as the building member, a material for road use and a material for outer ditches can be cited.

The boehmite structure 1 can also be used for a substrate for a thin film circuit, a substrate for a sensor member, a substrate for a semiconductor process, and a ceramic member of a semiconductor manufacturing apparatus.

Examples

The boehmite structure according to the present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited thereto.

[ example 1]

(preparation of test sample 1)

First, hydraulic alumina BK-112 manufactured by Sumitomo chemical Co., Ltd was prepared as hydraulic alumina. The hydraulic alumina had a central particle diameter of 16 μm. FIG. 2 shows the X-ray diffraction pattern of the hydraulic alumina powder, and boehmite (AlOOH) and gibbsite (Al (OH) registered in ICSD₃) The pattern of (2). As shown in fig. 2, the hydraulic alumina is a mixture of boehmite and gibbsite (aluminum hydroxide). This is achieved byIn addition, although not shown in fig. 2, rho alumina is also included in the hydraulic alumina.

Then, ion-exchanged water was weighed to 80 mass% with respect to the hydraulic alumina, and the hydraulic alumina and the ion-exchanged water were mixed using an agate mortar and pestle to obtain a mixture. Then, the obtained mixture was put into a cylindrical molding die (φ 10) having an internal space. Then, the mixture was heated and pressurized under 400MPa, 180 ℃ and 20 minutes to obtain test sample 1 of this example.

(measurement of porosity)

First, a cross section of a cylindrical test sample 1 was subjected to cross section burnishing (CP working). Next, a reflection electron image was observed at a magnification of 2000 times on the cross section of the test sample 1 using a Scanning Electron Microscope (SEM). A reflected electron image obtained by observing 3 positions (positions 1 to 3) of the cross section of the test sample 1 is shown in fig. 3, 5, and 7. In the observed reflection electron image, the white particles are boehmite particles 2, and the black portions are pores 3.

Next, the air pore portions were clarified by binarizing each of the SEM images of 3 fields. The images obtained by binarizing the reflected electron images of fig. 3, 5 and 7 are shown in fig. 4, 6 and 8, respectively. Then, the area ratio of the pore portion was calculated from the binarized image, and the average value was taken as the porosity. Specifically, according to fig. 4, the area ratio of the air hole portion at the position 1 is 1.5%. According to fig. 6, the area ratio of the air hole portion at the position 2 is 0.9%. According to fig. 8, the area ratio of the air hole portion at the position 3 is 2.0%. Therefore, the porosity of the test sample 1 was 1.5% of the average value of the area ratios of the pore portions at the positions 1 to 3.

(measurement of flexural Strength)

For test sample 1, the flexural strength was measured in accordance with JIS R1601. In the graph of fig. 9, the relationship between the stress of the test sample 1 and the stroke of the tester is shown. As shown in fig. 9, the maximum value of the stress of the test sample 1 was 39.4MPa, and therefore the flexural strength of the test sample 1 was 39.4 MPa.

[ example 2]

Ion-exchanged water was weighed to 80 mass% with respect to the same hydraulic alumina as in example 1, and the hydraulic alumina and the ion-exchanged water were mixed using an agate mortar and pestle, thereby obtaining a mixture.

Next, the obtained mixture was dried at normal temperature without pressurization, thereby obtaining test sample 2 of comparative example. Further, the mixture was charged into a cylindrical molding die (φ 10) having an internal space, and the mixture was heated and pressurized under conditions of 50MPa, 180 ℃ and 30 minutes, thereby obtaining test sample 3 of example. Further, the mixture was charged into a cylindrical molding die (φ 10) having an internal space, and the mixture was heated and pressurized under conditions of 200MPa, 180 ℃ and 30 minutes, thereby obtaining a test sample 4 of example.

Next, after the test samples 2 to 4 were each pulverized, the reflection electron images were observed at magnifications of 100 times, 1000 times, and 10000 times using a Scanning Electron Microscope (SEM). In fig. 10, reflection electron images of 100 times, 1000 times, and 10000 times of test samples 2 to 4 are collectively shown.

As shown in fig. 10, in the test sample 2 molded without applying pressure at room temperature, many large pores 10 having a pore diameter of about 100 μm were formed. In addition, in the test sample 2, many large pores 11 having a pore diameter of about several μm and many nano-pores having a pore diameter of several tens to several hundreds nm were formed. Therefore, it is known that the porosity of the resulting structure is at least more than 30% without pressurizing and heating the mixture of hydraulic alumina and water, and as a result, the strength is insufficient.

On the other hand, in the test sample 3 molded under the conditions of 50MPa and 180 ℃ and the test sample 4 molded under the conditions of 200MPa and 180 ℃, the large pores 10 having a pore diameter of about 100 μm were not found. In the test sample 4, large pores 11 having a pore diameter of about several μm were not observed. Therefore, it is understood that the test samples 3 and 4 formed by the pressure-heating method each have a porosity of 30% or less and have high strength.

[ example 3]

(preparation of test sample 5)

Ion-exchanged water was weighed to 80 mass% with respect to the same hydraulic alumina as in example 1, and the hydraulic alumina and the ion-exchanged water were mixed using an agate mortar and pestle, thereby obtaining a mixture. Then, the obtained mixture was put into a cylindrical molding die (φ 10) having an internal space. Then, the mixture was heated and pressurized at 50MPa, 120 ℃ for 20 minutes to obtain test sample 5 of this example.

(preparation of test sample 6)

Ion-exchanged water was weighed to 20 mass% with respect to the same hydraulic alumina as in example 1, and the hydraulic alumina and the ion-exchanged water were mixed using an agate mortar and pestle, thereby obtaining a mixture. Then, the obtained mixture was put into a cylindrical molding die (φ 10) having an internal space. Then, the mixture was heated and pressurized at 50MPa, 120 ℃ for 20 minutes to obtain test sample 6 of this example.

(X-ray diffraction measurement)

For the test samples 5 and 6 obtained as described above, X-ray diffraction patterns were measured using an X-ray diffraction apparatus. Fig. 11 shows the X-ray diffraction patterns of test samples 5 and 6 and the X-ray diffraction patterns of the hydraulic alumina powder as the raw material. Also shown in figure 11 are the X-ray diffraction patterns of boehmite and gibbsite registered in the ICSD.

As is clear from fig. 11, in test sample 5 in which 80 mass% of ion-exchanged water was added to hydraulic alumina, the proportion of boehmite was increased as compared with test sample 6 in which 20 mass% of ion-exchanged water was added. That is, when a mixture of hydraulic alumina and ion-exchange water is prepared, the crystal structure is changed from aluminum hydroxide to boehmite by increasing the amount of ion-exchange water added and further heating and pressurizing. Furthermore, since boehmite has higher chemical resistance than aluminum hydroxide, a boehmite structure having excellent chemical resistance can be obtained by increasing the proportion of boehmite present.

[ example 4]

(preparation of test sample 7)

Ion-exchanged water was weighed to 80 mass% with respect to the same hydraulic alumina as in example 1, and the hydraulic alumina and the ion-exchanged water were mixed using an agate mortar and pestle, thereby obtaining a mixture. Subsequently, the obtained mixture was heated and pressurized under conditions of 400MPa, 180 ℃ and 20 minutes to obtain a boehmite structure of this example. Further, the boehmite structure of this example was heated at 400 ℃ for 1 hour in the air using an electric furnace, whereby test sample 7 of this example was obtained.

(measurement of X-ray diffraction)

With respect to the test sample 7 obtained as described above, an X-ray diffraction pattern was measured using an X-ray diffraction apparatus. The X-ray diffraction pattern of test sample 7, and the X-ray diffraction patterns of gamma alumina, gibbsite (aluminum hydroxide), and boehmite registered in ICSD are shown in fig. 12. Further, the ratio of each phase was determined by measuring the X-ray diffraction pattern of the boehmite structure before heating at 400 ℃ for 1 hour and performing the rietveld analysis.

The ritnwalder analysis showed that the boehmite phase was 65 mass%, the gibbsite (aluminum hydroxide phase) was 25 mass%, and the γ -alumina phase was 10 mass% in the boehmite structure before heating at 400 ℃ for 1 hour. As described above, since aluminum hydroxide reacts with an acid and a base, when aluminum hydroxide remains in the boehmite structure, chemical resistance may be lowered. However, as shown in fig. 12, when the boehmite structure in which gibbsite exists is heated, a dehydration reaction occurs in the gibbsite phase, and the crystal structure is changed from the gibbsite phase to the boehmite phase. Therefore, it is found that by heating the boehmite structure at a temperature at which the dehydration reaction occurs, the gibbsite phase disappears, and the boehmite structure having excellent chemical resistance can be obtained.

[ example 5]

(preparation of test sample 8)

First, an average particle diameter D is prepared₅₀About 0.8 μm aluminum hydroxide (purity: 99.6% available from Showa Kabushiki Kaisha). Then, the aluminum hydroxide was heated at 350 ℃ for 1 hour using an electric furnace with the temperature increase rate and the cooling rate set at 300 ℃/hour. Thereby, an average particle diameter D was obtained₅₀About 0.8 μm of a hydraulic alumina powder.

Fig. 13 shows a scanning electron micrograph of the obtained hydraulic alumina. As shown in fig. 13, it was confirmed that: the obtained hydraulic alumina hardly becomes coarse from the aluminum hydroxide particles as a raw material, and maintains a fine state. In addition, the obtained hydraulic alumina was measured for its X-ray diffraction pattern using an X-ray diffractometer, and as a result, both a peak derived from boehmite and a peak derived from rho alumina were confirmed.

Then, 0.25g of the above hydraulic alumina was charged into a cylindrical molding die (φ 10) having an internal space. Then, 200. mu.L of ion-exchanged water was added to the inside of the mold for molding, and mixed with a plastic spatula. Then, the mixture containing the ion-exchanged water was heated and pressurized under conditions of 400MPa, 200 ℃ and 30 minutes, thereby obtaining test sample 8 of this example. Further, the test sample 8 of this example had a thickness of about 0.75mm and had a high hardness as a sintered body.

(measurement of Total light transmittance)

The total light transmittance of the test sample 8 was measured by using an ultraviolet-visible near-infrared spectrophotometer UV-2600 manufactured by shimadzu corporation. The measurement results are shown in FIG. 14. As shown in fig. 14, although the light transmittance of the test sample 8 around the wavelength of 380nm was about 15%, the light transmittance tended to increase as the wavelength became longer. Further, the light transmittance is more than 45% at a wavelength of about 700 nm. Accordingly, as is clear from fig. 14, the test sample 8 has an average value of total light transmittance of 20% or more in a wavelength region of 380nm to 700 nm.

(X-ray diffraction measurement)

With respect to the test sample 8 obtained as described above, an X-ray diffraction pattern was measured using an X-ray diffraction apparatus. The X-ray diffraction pattern of test sample 8, and the X-ray diffraction pattern of boehmite registered in ICSD are shown in fig. 15. As is clear from fig. 15, in test sample 8, since a peak of boehmite is observed, it is mainly a structure formed of boehmite.

[ example 6]

(preparation of test sample 9)

Test sample 9 of this example was obtained by the same method as in example 5. In addition, test sample 9 of this example also had a high hardness like a sintered body.

(measurement of porosity)

First, a cross section of the cylindrical test sample 9 was subjected to cross section polishing (CP processing). Next, using a Scanning Electron Microscope (SEM), a secondary electron image was observed at 20000 times of the cross section of the test sample 9. Secondary electron images obtained by observing 3 positions (positions 1 to 3) of the cross section of sample 9 are shown in fig. 16, 18, and 20. In the observed secondary electron image, the gray particles are boehmite particles 2, and the black portions are pores 3.

Next, the pore portions were clarified by filling the pore portions with SEM images of 3 fields and binarizing the filled pore portions. The images after the binarization of the secondary electron images of fig. 16, 18, and 20 are shown in fig. 17, 19, and 21, respectively. Then, the area ratio of the pore portion was calculated from the binarized image, and the average value was taken as the porosity. Specifically, according to fig. 17, the area ratio of the air hole portion at the position 1 is 0.60%. According to fig. 19, the area ratio of the air hole portion at the position 2 is 0.28%. According to fig. 21, the area ratio of the air hole portion at the position 3 is 0.13%. Therefore, the porosity of test sample 9 was 0.34% of the average value of the area ratios of the pore portions at positions 1 to 3.

From examples 5 and 6, it can be seen that the use of the average particle diameter D₅₀A boehmite structure having a total light transmittance of 20% or more in an average value in a wavelength region of 380 to 700nm and a porosity of 5% or less can be obtained by using hydraulic alumina having a particle size of 5 μm or less as a raw material.

The present embodiment has been described above with reference to examples, but the present embodiment is not limited to these descriptions, and various modifications and improvements will be apparent to those skilled in the art.

The entire contents of Japanese patent application No. 2019-94630 (application date: 20/5/2019) and Japanese patent application No. 2019-197102 (application date: 30/10/2019) are incorporated herein by reference.

According to the present disclosure, a boehmite structure that is light in weight, excellent in chemical stability, and further reduced in the amount of impurities, and a method for producing the boehmite structure can be provided.

Description of the symbols

1 boehmite structure

2 boehmite particles

3 air holes