JP7516872B2 - High-purity fine alumina powder - Google Patents

Description

The present invention relates to high-purity fine alumina powder.

Alumina (α-Al ₂ O ₃ ) sintered bodies have excellent properties such as insulation, heat resistance, wear resistance, and corrosion resistance. Therefore, alumina sintered bodies are used in a wide range of fields, including electronic components, refractories, abrasives, insulators, spark plugs, fillers, and catalyst carriers. Alumina sintered bodies are manufactured by molding and sintering alumina powder as the raw material. To obtain alumina sintered bodies with high properties, the raw material alumina powder is required to have excellent sinterability, i.e., to be fine.

When manufacturing sintered antenna bodies, a method is often used in which alumina powder is made into a slurry and this slurry is wet-processed. For example, to improve moldability during molding, the alumina powder slurry is sprayed and granulated to produce a granulated product, which is then molded. Also, a known method is to cast the alumina powder slurry to obtain large alumina products with complex shapes. There is also a method in which alumina powder slurry is applied and fired to produce a sheet-shaped alumina sintered body.

When carrying out such wet processing, it is important to appropriately control the handleability of the slurry, i.e., the properties such as the slurry viscosity. For example, if the slurry viscosity is excessively high, problems will arise during granulation and molding. Furthermore, if the slurry properties change over time, the properties of the final product will not be stable and may vary. For this reason, technologies have been proposed to control the powder properties of the alumina powder and improve the slurry properties.

For example, Patent Document 1 discloses an alumina powder having a ratio (TBD/LBD) of light bulk density (LBD) to heavy bulk density (TBD) of 1.5 or more, and describes that by using this alumina powder, it is possible to form a thin, uniformly thick alumina-containing coating layer (claims 1 and [0032] of Patent Document 1). Patent Document 1 also describes that if the TBD/LBD value is less than 1.5, the bulk density of the individual secondary particles (agglomerated particles) contained in the alumina powder becomes too high, and in this case, precipitation of the alumina agglomerated particles is likely to occur in an alumina slurry containing the alumina powder, making it difficult to ensure the dispersion stability of the alumina powder ([0038] of Patent Document 1).

International Publication No. 2018/047871

However, the techniques proposed so far have been insufficient in achieving both the sinterability of the alumina powder and the slurry properties. In other words, the finer the alumina powder, the better the sinterability. However, when the alumina powder becomes fine, the viscosity of the slurry containing it rises sharply, making handling such as granulation and molding difficult. In particular, in conventional techniques, the alumina powder is generally pulverized intensively to achieve finer size, but alumina powder pulverized intensively by pulverization tends to deteriorate the slurry properties. Furthermore, such alumina powder has low crystallinity, making it problematic as a raw material for manufacturing high-performance alumina sintered bodies.

For example, Patent Document 1 states that the D50 value of the alumina powder is preferably 0.45 to 0.65 μm, and that if the D50 value is less than 0.45 μm, the particles will aggregate too densely ([0044] of Patent Document 1), which means that there is a limit to how fine the alumina powder can be made, i.e., how well it can be sintered.

The present inventors have conducted extensive research in light of the above-mentioned problems. As a result, they have found that an alumina powder in which the 50% particle size _D50 in the volumetric particle size distribution and the BET specific surface area S _BET satisfy a specific relationship and impurities such as sodium are less than a specific amount can achieve both excellent sinterability and slurry properties. They have also found that this alumina powder is suitable as a raw material for a sintered body, which exhibits excellent dielectric properties in the high frequency range in addition to the sinterability and slurry properties.

The present invention was completed based on these findings, and aims to provide an alumina powder that has excellent slurry and sintering properties, as well as excellent dielectric properties in the high frequency range.

The present invention encompasses the following aspects (1) to (6). In this specification, the expression "to" includes both ends of the expression. In other words, "X to Y" is synonymous with "X or more and Y or less."

(1) A high-purity fine alumina powder in which the 50% particle size (D ₅₀ ) and BET specific surface area (S _BET ) in a volumetric particle size distribution satisfy the relationships represented by the formulas: D ₅₀ ≦0.20 μm and D ₅₀ × S _BET ≦2.0 × 10 ^-6 m ³ /g, and the contents of sodium (Na), silicon (Si), iron (Fe) and calcium (Ca) are each 10 ppm or less.

(2) The alumina powder according to (1) above, which satisfies the relationship expressed by the formula: 1.6×10 ⁻⁶ m ³ /g≦D ₅₀ ×S _BET .

(3) Alumina powder according to (1) or (2) above, in which the full width at half maximum (FWHM) of the (113) diffraction line in an X-ray diffraction profile is 0.240° or less.

(4) The alumina powder according to any one of (1) to (3) above, having a pressed bulk density (GD) of 2.20 g/ ^cm3 or more.

(5) The alumina powder according to any one of claims 1 to 4, wherein the 10% particle size D ₁₀ , 50% particle size D ₅₀ and 90% particle size D ₉₀ in the volume particle size distribution satisfy the relationship represented by the formula: (D ₉₀ -D ₁₀ )/D ₅₀ ≦1.1.

(6) Any of the alumina powders (1) to (5) above, having a degree of gelatinization of 80.0% or more.

The present invention provides an alumina powder that has excellent slurry and sintering properties, as well as excellent dielectric properties in the high frequency range.

FIG. 2 shows a schematic cross-sectional view of a hydrostatic pressure type slurry evaluation device. The principle of slurry evaluation by settling pressure measurement is shown. 1 shows an SEM image of alumina powder. 1 shows an SEM image of alumina powder. 1 shows an SEM image of alumina powder. 1 shows an SEM image of alumina powder. 1 shows a particle size distribution curve of alumina powder. The relationship between the firing temperature (sintering temperature) and the density of the alumina sintered body is shown. 1 shows the XRD pattern of alumina powder. The relationship between the shear rate and viscosity of the slurry is shown. The graph shows the change in slurry settling hydrostatic pressure over time.

A specific embodiment of the present invention (hereinafter, referred to as "the present embodiment") will be described. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

High-purity fine alumina powder The high-purity fine alumina powder of this embodiment (hereinafter sometimes collectively referred to as "alumina powder") has a 50% particle size (D ₅₀ ) and a BET specific surface area (S _BET ) in the volumetric particle size distribution that satisfy the relationship expressed by the formula: D ₅₀ ≦0.20 μm and the formula: D ₅₀ ×S _BET ≦2.0×10 ⁻⁶ m ³ /g. In general, the smaller the particle size of the powder, the larger the specific surface area and the greater the number of contact points with other particles. In addition, as the radius of curvature of the particle becomes smaller, the surface becomes more active and the sintering driving force becomes larger. Therefore, from the viewpoint of obtaining high sinterability, D ₅₀ is limited to 0.20 μm or less.

On the other hand, simply reducing the particle size of the powder can lead to problems with handling and can actually worsen sinterability. In other words, when the powder is made fine by methods such as grinding, chipping occurs in the particles in the powder, resulting in angular particles. If such powder is made into a slurry and subjected to wet processing, the slurry viscosity may become too high and the slurry state may become unstable. Even when dry processing is performed, the fluidity of the powder may deteriorate, making it difficult to increase the packing density, which can lead to poor sinterability.

On the other hand, by reducing the average particle size (D ₅₀ ) and keeping D ₅₀ × S _BET small, it is possible to improve the sinterability while maintaining good handleability. That is, if the particle size is the same, the rounder the particle shape, the smaller the specific surface area (S _BET ). Therefore, by reducing D ₅₀ and keeping D ₅₀ × S _BET low, it is possible to make particles with fewer fracture surfaces and chipping even if the particle size is small. In the alumina powder of this embodiment, the shape of the particles contained therein is rounded and the particle size is uniform. Therefore, when it is made into a slurry, it is possible to suppress excessive increase and instability of the slurry viscosity. In addition, the fluidity and moldability are excellent. As a result of these working synergistically, it is possible to achieve both excellent sinterability and handleability. D ₅₀ may be 0.19 μm or less. Furthermore, D ₅₀ × S _BET may be 1.9 × 10 ⁻⁶ m ³ /g or less. On the other hand, if the particle size is too small, it becomes difficult to suppress the adverse effect of deterioration in handling properties. Therefore, _D50 may be 0.05 μm or more, 0.10 μm or more, or 0.15 μm or more.

In addition, the alumina powder of this embodiment has a content of each of sodium (Na), silicon (Si), iron (Fe) and calcium (Ca) of 10 ppm or less. That is, the Na content is 10 ppm or less, the Si content is 10 ppm or less, the Fe content is 10 ppm or less, and the Ca content is 10 ppm or less. Impurities form grain boundary phases in the sintered body. For example, sodium (Na) and calcium (Ca) form a glass phase together with silicon (Si). Also, calcium (Ca) may react with alumina (Al ₂ O ₃ ) in the main phase to form compounds such as CaO.6Al ₂ O _3. Such grain boundary phases are not preferable because they cause abnormal grain growth. That is, when abnormal grain growth occurs, coarse particles are generated locally, and the homogeneous sintering property as a whole may be impaired. As a result, closed voids may be generated, and the final sintered density may be reduced. Also, some of the impurities may be taken into the main phase, reducing the electrical resistance of the main phase. For example, sodium (Na) is incorporated into the main phase alumina (Al ₂ O ₃ ) to become β-alumina (Na ₂ O·11Al ₂ O ₃ ). β-alumina is a material used as a solid electrolyte in batteries and has high electrical conductivity. If there are a lot of impurities like this, the sinterability and electrical resistance (insulation properties) will deteriorate.

In particular, the alumina powder of this embodiment has a small average particle size ( _D50 ) and a large sintering driving force. Therefore, it is easily adversely affected by impurities on sinterability and electrical resistance. Therefore, the content of each of the impurities (Na, Si, Fe, and Ca) is limited to 10 ppm or less. Each content is preferably 9 ppm or less, and more preferably 8 ppm or less.

The alumina powder of this embodiment may further satisfy the relationship expressed by the formula: 1.6×10 ⁻⁶ m ³ /g≦D ₅₀ ×S _BET . When the shape of the alumina particles is maximally rounded, they become spherical particles. When the D ₅₀ ×S _BET of a perfectly spherical particle is calculated using the density of alumina (3.98 g/cm ³ ), this value becomes 1.51×10 ⁻⁶ m ³ /g. Although methods for producing spherical alumina particles are also known, these methods require expensive raw materials and are complicated in their manufacturing process. As will be described in the examples below, the particles constituting the alumina powder of this embodiment are rounded and have a uniform particle size, but are not perfectly spherical particles. In addition, expensive raw materials are not required and the manufacturing process is simple. Therefore, it is not necessary to lower the D ₅₀ ×S _BET as much as for a perfectly spherical particle, and the manufacturing cost can be kept low. D ₅₀ ×S _BET may be 1.7×10 ⁻⁶ m ³ /g or more, and may be 1.8×10 ⁻⁶ m ³ /g or more.

The alumina powder of this embodiment preferably has a full width at half maximum (FWHM) of the (113) diffraction line in the X-ray diffraction (XRD) profile of 0.240° or less, more preferably 0.230° or less. In general, fine powders are produced by strong crushing (strong grinding). However, in alumina powder crushed by applying a strong crushing force, distortion occurs on the crystal surface of the primary particles. Such distortion of the crystal surface may activate the alumina particle surface and adversely affect the handling characteristics and slurry characteristics. In contrast, the alumina powder of this embodiment is composed of fine primary particles, but has little crystal distortion and excellent crystallinity. The smaller the FWHM, the more preferable, but it is typically 0.200° or more.

The alumina powder of this embodiment preferably has a pressed bulk density (GD) of 2.20 g/cm ³ or more. By making the powder have such a high pressed bulk density, the density of the molded body made from this powder can be increased, and as a result, it is possible to suppress the occurrence of pores (defects) even in the sintered body. The pressed bulk density is the bulk density of the molded body (molded piece) obtained by pressurizing the alumina powder under a pressure of 350 kgf/cm ² and no pressing time (0 minutes). The upper limit of the pressed bulk density is preferably as high as possible, but is typically 2.50 g/cm ³ or less.

The alumina powder preferably has a 10% particle size (D ₁₀ ), a 50% particle size (D ₅₀ ), and a 90% particle size (D ₉₀ ) in the volumetric particle size distribution that satisfy the relationship expressed by the formula: (D ₉₀ -D ₁₀ )/D ₅₀ ≦1.1. If the particle size distribution of the alumina powder is too broad, there is a risk that the sinterability and handling properties will deteriorate. Therefore, an excessively broad particle size distribution is not preferable. It is more preferable that (D ₉₀ -D ₁₀ )/D ₅₀ is 1.1 or less.

The alumina powder of this embodiment preferably has a degree of alpha-conversion of 80.0% or more, more preferably 90.0% or more. If the degree of alpha-conversion is less than 80.0%, when a sintered body is made from the alumina powder, the shrinkage during firing is large and pores are likely to form. This makes it difficult to densify the sintered body, which may result in deterioration of its characteristics. Therefore, the higher the degree of alpha-conversion, the better. The degree of alpha-conversion can be determined by determining the X-ray diffraction pattern of the alumina powder using an X-ray diffraction method, and comparing the X-ray diffraction intensities of the (012) and (116) planes of the diffraction pattern with the diffraction intensities of a standard sample with a degree of alpha-conversion of 100%.

The alumina powder of this embodiment has excellent sinterability due to its fine particle size. In fact, the inventors have confirmed that dense sintered bodies can be produced at significantly lower temperatures than with conventional alumina powders.

The alumina powder of this embodiment is fine yet has excellent slurry properties. In other words, a slurry containing this alumina powder has low viscosity and is stable. In general, as the particle size of the powder becomes smaller, the interaction between the particles becomes stronger, making it difficult to disperse well. Therefore, in order to disperse the powder in the slurry, it is necessary to apply a stronger shear force. Even if the powder is dispersed temporarily, the powder is likely to aggregate or gel in the slurry, making the slurry unstable. In contrast, the alumina powder of this embodiment can be dispersed even with a weak shear force, and the slurry maintains a well-dispersed state for a long time. This is because the particle shape is rounded and the particle size is uniform, so the interaction between the particles is small, which is thought to lead to the low viscosity and stabilization of the slurry.

Furthermore, the alumina powder of this embodiment exhibits excellent dielectric properties in the high frequency range. In other words, by using this alumina powder as a raw material, it is possible to produce an alumina sintered body with small dielectric loss and transmission loss. In fact, the inventors have confirmed that it is possible to obtain an alumina sintered body with small dielectric tangent (tan δ) and transmission loss in the high frequency range of 10 GHz or more.

Slurry The slurry of this embodiment is prepared by mixing the above-mentioned alumina powder, a solvent such as water, and, if necessary, a dispersant. The concentration of the alumina powder in the slurry is not limited, but may be, for example, 10 to 70 mass %, or may be, for example, 30 to 70 mass %. The dispersant is not limited, but may be, for example, a polycarboxylate ammonium dispersant, and the content of the dispersant may be, for example, 1 to 10 mass %.

The slurry of this embodiment has a characteristic that the viscosity is low regardless of the shear rate. For example, when the shear rate is increased and decreased in the range of shear rates from 300 to 1000/sec, the viscosity can be reduced to 6 mPa·sec or less. The slurry is a non-Newtonian fluid, and the shear rate and the shear stress do not show a proportional relationship. Therefore, when evaluating the flow characteristics of the slurry, it is preferable to evaluate it using a flow curve, which is the relationship between the shear rate and the shear stress. One such method is a method using a cone-plate type rotational viscometer (cone-plate type rotational viscometer) specified in JIS Z 8803. In the cone-plate type rotational viscometer, the sample (slurry) is sandwiched between a cone and a plate, and the cone is rotated at a constant speed to measure the rotational torque at that time. The shear stress can be calculated from the rotational torque.

The slurry of this embodiment is also characterized by being stable for a long time. The stability of the slurry can be evaluated by examining the settling state in a gravitational field. One such method is settling hydrostatic pressure measurement using a hydrostatic slurry evaluation device. The measurement principle of settling hydrostatic pressure measurement is explained with reference to Figures 1 and 2. As shown in Figure 1, the hydrostatic slurry evaluation device is composed of a settling tube with an open top, a pressure transmission tube whose tip is inserted into this tube, and a pressure sensor installed at the other end of the pressure transmission tube. The slurry is placed in the settling tube and the hydrostatic pressure at depth H is measured. When all the particles in the slurry are suspended, the hydrostatic pressure is maximum. On the other hand, when all the particles in the slurry pass through depth H and settle, the hydrostatic pressure is minimum. Therefore, the dispersion stability of the particles can be evaluated by determining the hydrostatic pressure (P) as a function of time (t). For example, as shown in Figure 2, a well-dispersed slurry has a small change in hydrostatic pressure even after a long time has passed. On the other hand, when the particles in the slurry aggregate or gel, the hydrostatic pressure decreases. The slurry of this embodiment has a good dispersion state, and the change in hydrostatic pressure is small.

Alumina Sintered Body The alumina sintered body of this embodiment is produced by molding the above-mentioned alumina powder and firing (sintering) the resulting molded body. The molding may be performed by a known method such as press molding, casting, injection molding, or extrusion molding. The molded body may also be subjected to a cold isostatic pressing (CIP) treatment. The firing may also be performed under known conditions. For example, firing may be performed under conditions of 1300 to 1500°C in air, vacuum, or hydrogen atmosphere for 1 to 5 hours.

The alumina sintered body of this embodiment is characterized by exhibiting good dielectric properties in the high frequency range. For example, the relative dielectric constant (εr) at 10 GHz can be set to 8.0 to 12.0, and can also be set to 9.0 to 11.0. The dielectric loss tangent (tan δ) at 10 GHz can be set to 0.20×10 ⁻⁴ or less, and can also be set to 0.15×10 ⁻⁴ or less. The alumina sintered body having such a small relative dielectric constant and dielectric loss tangent in the high frequency range is suitable as an antenna material for the fifth generation mobile communication system (5G), although it is not limited thereto.

To explain this point, the fifth generation mobile communications system (5G) is the successor to the fourth generation (4G) represented by smartphones, and commercial services will start in Japan in the spring of 2020. 5G uses radio waves in the extremely high frequency range, including microwaves below 6 GHz and millimeter waves above 24 GHz. 5G, which uses such high frequencies, has three characteristics: high speed and large capacity, highly reliable and low latency communication, and multiple simultaneous connections.

In 5G, which uses high frequencies, it is important to reduce the transmission loss of antenna materials. That is, radio waves transmitted in wireless communication are converted into heat by the antenna material. The amount of transmission loss (a) that occurs at this time is proportional to the product of the frequency of the radio waves (f), the square root of the relative dielectric constant (εr) of the antenna material, and the dielectric tangent (tan δ) of the antenna material, as shown in the following formula (1). In the following formula (1), K is a proportionality constant.

Since transmission loss (a) is proportional to frequency (f), it is necessary to reduce the loss of the antenna material as the frequency used increases. Polytetrafluoroethylene (PTFE) is known as an antenna material in the frequency range before 4G, but as frequencies increase to 5G, PTFE has large losses and is insufficient as an antenna material. The alumina sintered body of this embodiment can reduce the dielectric tangent (tan δ) and transmission loss in the high frequency range compared to PTFE. Therefore, it is expected to be used as an antenna material for 5G.

The alumina sintered body of this embodiment is not limited to use as a 5G antenna material. It goes without saying that it is useful for applications other than antennas, such as circuit boards, capacitor/resistor boards, IC boards, sensor component boards, multilayer boards, packages, RF windows, and semiconductor manufacturing equipment.

Manufacturing method of alumina powder The manufacturing method of the alumina powder of this embodiment is not limited as long as it satisfies the above-mentioned requirements. However, a suitable manufacturing method includes a step of preparing aluminum hydroxide powder and α-alumina seeds (preparation step), a step of mixing the prepared aluminum hydroxide powder with α-alumina seeds to obtain an aluminum hydroxide mixed raw material containing 10 to 20 mass% of α-alumina seeds (mixing step), a step of subjecting the obtained aluminum hydroxide mixed raw material to mechanochemical treatment using a dry bead mill to obtain amorphous aluminum hydroxide having a crystal water content of 21.0 mass% or less and exhibiting an exothermic peak in a temperature range of 750 to 850 ° C. in differential scanning calorimetry (mechanochemical treatment step), and a step of heat treating the obtained amorphous aluminum hydroxide at a temperature in the range of 900 to 1000 ° C. (heat treatment step).

This manufacturing method is characterized by using a specific amorphous aluminum hydroxide as an intermediate raw material, which is produced by subjecting a mixed raw material of aluminum hydroxide powder and α-alumina seeds to mechanochemical processing. This amorphous aluminum hydroxide has an extremely low crystal transition temperature to α-alumina. Therefore, by using this amorphous aluminum hydroxide as an intermediate raw material, fine, high-quality α-alumina powder with a high degree of α-conversion can be easily obtained. Details of each process are explained below.

<Preparation process>
In the preparation step, aluminum hydroxide powder and α-alumina seeds are prepared. As the aluminum hydroxide powder, gibbsite, Bayerite, etc. can be used. However, gibbsite is preferable in consideration of the manufacturing cost. In addition, the aluminum hydroxide powder may be produced by any method, but it is preferable that it is produced by the Bayer process. From the viewpoint of the flowability and ease of handling of the powder, it is preferable that the aluminum hydroxide powder has an average particle size (D ₅₀ ) of 3 to 50 μm and a BET specific surface area (S _BET ) of 0.2 to 5.0 m ² /g. The general-purpose aluminum hydroxide powder has an average particle size and a BET specific surface area within the above range. In the manufacturing method of this embodiment, it is possible to use a general-purpose aluminum hydroxide powder raw material, and as a result, the advantages of reduced manufacturing costs and simplicity can be maximized.

On the other hand, from the viewpoint of lowering the temperature of the alpha-alumina transition of amorphous aluminum hydroxide, it is preferable that the alpha-alumina seeds have a high degree of alpha-conversion and are fine particles. The degree of alpha-conversion is preferably 90% or more, more preferably 95% or more. The average particle size (D ₅₀ ) is preferably 0.1 to 0.5 μm, and the BET specific surface area (S _BET ) is preferably 5 to 15 m ² /g.

<Mixing step>
In the mixing step, the prepared aluminum hydroxide powder is mixed with α-alumina seeds to obtain an aluminum hydroxide mixed raw material containing 10 to 20 mass% of α-alumina seeds. The mixing method is not particularly limited. By adding α-alumina seeds, it is possible to fully exert the effect of lowering the crystal transition temperature of the obtained amorphous aluminum hydroxide. From the viewpoint of lowering the temperature of the alpha transition, the higher the content of the α-alumina seeds, the more preferable. However, high-quality α-alumina seeds are expensive. In this embodiment, from the viewpoint of the balance between the effect of lowering the temperature of the alpha transition and economic efficiency, the content of the α-alumina seeds is set to 10 to 20 mass%. The content of the α-alumina seeds is preferably 15 to 20 mass%.

The reason for the decrease in crystal transition temperature due to α-alumina seeds is speculated as follows. That is, the phenomenon that the α-alumina transition temperature decreases by adding a small amount of α-alumina as seeds to aluminum hydroxide raw material has been known for a long time. In the present invention, the raw materials aluminum hydroxide (gibbsite, etc.) and α-alumina seeds are first adjusted to a mixed raw material powder, and at the same time, they are pulverized into extremely small particles by mechanochemical treatment. As a result, the amorphous aluminum hydroxide primary particles with an increased specific surface area and the α-alumina seeds are closely aggregated at the particle interface, making the desired interaction more uniform. As a result, it is believed that amorphous aluminum hydroxide can undergo α-transition at a low temperature range where phase transition would not normally occur, without passing through an intermediate alumina phase such as chi-alumina.

<Mechanochemical treatment process>
In the mechanochemical treatment step, the obtained aluminum hydroxide mixed raw material is subjected to mechanochemical treatment using a dry bead mill to produce amorphous aluminum hydroxide. Amorphous aluminum hydroxide is mainly composed of amorphous aluminum hydroxide. In a completely crystalline state, aluminum hydroxide is a compound having a composition such as gibbsite (Al(OH) ₃ ). In amorphous aluminum hydroxide, the crystallinity of aluminum hydroxide disappears or decreases, and some of the crystal water is removed. Therefore, amorphous aluminum hydroxide differs in crystal state and amount of crystal water from aluminum hydroxide in a completely crystalline state. The degree of amorphization can be evaluated by the intensity (CPS) of the crystal peak (002) plane of gibbsite measured by X-ray diffraction and the crystal water content (LOI). In the present invention, amorphous aluminum hydroxide refers to one having an intensity of 350 CPS or less of the crystal peak (002) plane of gibbsite measured by X-ray diffraction and an amount of crystal water of 21.0 mass% or less. In contrast, the amount of water of crystallization in non-amorphous gibbsite is about 34.7% by mass.

The amorphous aluminum hydroxide of this embodiment has a crystal water content (LOI) of 21.0% by mass or less. The amount of crystal water in the amorphous aluminum hydroxide is less than that of aluminum hydroxide in a completely crystalline state. As described later, the amorphous aluminum hydroxide of the present invention can be produced by subjecting a mixed raw material consisting of aluminum hydroxide powder and α-alumina seeds to mechanochemical treatment (amorphization treatment) using a dry ball mill. During the amorphization treatment using a dry bead mill, the primary particles constituting the aluminum hydroxide crystals such as gibbsite are ground down to a fine region, and the encapsulated crystal water is expelled to the outside. It is presumed that the grinding proceeds until the amount of crystal water is reduced, and the primary particles become sufficiently fine, making it possible to sufficiently lower the crystal transition temperature to α-alumina. From the viewpoint of promoting amorphization, the crystal water content is preferably 17.0% by mass or less. The lower limit of the crystal water content is not particularly limited, but may typically be 15.0% by mass or more.

The amorphous aluminum hydroxide of this embodiment also exhibits an exothermic peak in a temperature range of 750 to 850°C in differential scanning calorimetry. This exothermic peak corresponds to the crystal transition to α-alumina (α-aluminization). Ordinary aluminum hydroxide has a crystal transition temperature to α-alumina of 1100 to 1200°C. In contrast, the amorphous aluminum hydroxide of the present invention has an extremely low crystal transition temperature of 750 to 850°C. The amorphous aluminum hydroxide preferably exhibits an exothermic peak in a temperature range of 810 to 830°C.

The BET specific surface area of the amorphous aluminum hydroxide is not particularly limited, but is typically 15 to 50 m ² /g. The BET specific surface area can be measured using an automatic specific surface area measuring device (Flowsorb II 2300 model, manufactured by Micromeritics) in accordance with JIS 1626.

The amorphous aluminum hydroxide preferably contains 0.1% by mass or less of elements other than aluminum (Al), oxygen (O), and hydrogen (H). In particular, the sodium (Na) content is preferably 0.01% by mass or less, and the zirconium (Zr) content is preferably 0.05% by mass or less. In producing the final product, which is an easily sintered ceramic, sodium and zirconium are components that inhibit sintering, and it is desirable to have as little of them as possible. Even if aluminum hydroxide such as gibbsite, which has a high sodium content, is used as a raw material during production of the amorphous aluminum hydroxide of the present invention, the sodium contained in the crystals comes out during the amorphous treatment, so it can be easily washed and removed. In addition, if a raw material with a low sodium content is used, the sodium content of the amorphous aluminum hydroxide can naturally be reduced.

When crystals are continuously milled, the newly formed surface area increases and the number of surface atoms and/or molecules that have lost their bonds increases, causing the disorder of their bonds to extend to the vicinity of the surface layer. As a result, the milled particles become activated. In the case of dry milling, the powder aggregates and the apparent surface area decreases. The activated surfaces of the milled particles adsorb moisture and gases in the air, lowering their chemical potential and making them stable. During this series of reactions, various phase transitions occur. These phenomena and effects are called mechanochemical reactions, and the process that causes such mechanochemical reactions is called mechanochemical processing.

In the manufacturing method of this embodiment, a dry bead mill is used as the mechanochemical treatment. In the dry bead mill treatment, a high crushing shear is applied to the mixed raw materials, which effectively induces a mechanochemical reaction. The dry bead mill is a type of media-agitation type grinder, and is composed of a raw material inlet, a cylindrical container (vessel), a rotating stirring member (agitator) installed in the cylindrical container, and an outlet for the processed powder. In addition, the agitator gap in the vessel is filled with a large number of crushing media (beads). In the dry bead mill, the agitator rotates at high speed during operation to stir the beads. At this time, the raw materials fed from the raw material inlet repeatedly collide with the agitator, beads, and the inner wall of the vessel, and are crushed by impact force, shear force, friction force, etc., and a mechanochemical reaction is induced, and the raw materials are discharged from the outlet as dry processed powder.

The detailed mechanism of the mechanochemical reaction caused by the dry bead milling process is unknown, but it is speculated as follows. During the process, the aluminum hydroxide is continuously subjected to a high-shear dry grinding process. As a result, some of the water of crystallization in the aluminum hydroxide is released. At the same time, the temperature inside the device rises due to friction between the ground particles, beads, and the inner wall of the vessel, and as a result, phase transition phenomena such as hydrothermal reactions and dissolution and reprecipitation occur partially in the ground particles (aluminum hydroxide). In fact, when gibbsite (water of crystallization content 34.7% by mass) is used as the raw aluminum hydroxide powder and the mixed amount of α-alumina seeds is 20% by mass, the water of crystallization content of the raw aluminum hydroxide mixture is 27.0% by mass or more, while the water of crystallization content of the dry-processed powder after the dry bead milling process is reduced to 21.0% by mass or less, and in some cases to 17.0% by mass or less. From this, it can be understood that the water of crystallization is released by the dry bead milling process.

In the dry bead mill, the agitator peripheral speed (rotation speed) is preferably 5.0 to 6.0 m/s, more preferably 5.0 to 5.5 m/s, and the bead filling amount is preferably 60 to 70 volume %, more preferably 60 to 65 volume %. The feed amount is preferably 1.0 to 4.0 kg/h, more preferably 2.0 to 3.0 kg/h. The higher the agitator peripheral speed and bead filling amount, the more effectively the mechanochemical treatment is performed, and the higher the quality of amorphous aluminum hydroxide is obtained. The smaller the feed amount, the longer the residence time of the mixed raw materials, and the more the amorphous aluminum hydroxide is promoted. However, if the agitator peripheral speed and bead filling amount are excessively high, stable continuous operation becomes difficult. If the feed amount is excessively small, problems such as a decrease in yield due to leakage of crushed powder become prominent, and production efficiency decreases. If the agitator peripheral speed, bead loading amount, and feed amount are within the above numerical ranges, high-quality amorphous aluminum hydroxide can be obtained with good productivity.

During mechanochemical treatment, the aluminum hydroxide mixed raw material may be treated once (one-pass treatment) or multiple times (two-pass treatment, three-pass treatment, etc.) with a dry bead mill. As mentioned above, the smaller the feed amount, the more the aluminum hydroxide is made amorphous, but the lower the production efficiency. In this regard, by treating multiple times, even if the feed amount is increased, the same results can be obtained as when treating once with a small feed amount. Therefore, high-quality amorphous aluminum hydroxide can be obtained while maintaining production efficiency. It is preferable to set the feed amount to 2.0 to 3.0 kg/hour and perform two passes. In addition, during mechanochemical treatment, a grinding aid may be added to the aluminum hydroxide mixed raw material as necessary. An example of the grinding aid is ethanol.

When a general dry grinding machine other than a dry bead mill, such as a dry ball mill, is used for processing, the mechanochemical reaction of the processed powder is insufficient, and amorphous aluminum hydroxide cannot be obtained. Furthermore, with such a general dry grinding machine, even if the processing time is extended, there is a limit to the amount of atomization of the ground particles, and the specific surface area cannot be sufficiently increased. When a wet grinding machine is used for processing, it is possible to atomize the ground particles to 1 μm or less, but the mechanochemical reaction is insufficient. As a result, amorphous aluminum hydroxide cannot be obtained. Furthermore, when a wet grinding machine is used, there are problems such as increased contamination (impurities) and poor productivity.

In contrast, in the manufacturing method of this embodiment, the dry bead treatment, which is a dry treatment, can sufficiently induce a mechanochemical reaction in the aluminum hydroxide mixed raw material, and as a result, amorphous aluminum hydroxide with an extremely low crystal transition temperature to α-alumina can be easily obtained. In particular, since the manufacturing method of this embodiment employs mechanochemical treatment using a dry beer mill, amorphous aluminum hydroxide, which is difficult to process, can be obtained inexpensively even though general-purpose aluminum hydroxide is used as the starting raw material. Therefore, there is no need for advanced filtering equipment or large drying equipment required for wet grinding treatment, and productivity is excellent. Furthermore, when a continuous device is used as the dry bead mill, continuous processing is possible.

<Heat treatment process>
In the heat treatment step, the obtained amorphous aluminum hydroxide is heat treated (calcined) at a temperature in the range of 900 to 1100°C. If necessary, the heat-treated powder after the heat treatment may be subjected to a crushing treatment or a washing treatment. Since the amorphous aluminum hydroxide of this embodiment has an extremely low crystal transition temperature to α-alumina, α-alumina powder having a sufficiently high degree of α-conversion can be obtained even by heat treatment at a relatively low temperature of 900 to 1100°C. In addition, since the heat treatment temperature is low, crystal grain growth during the heat treatment can be suppressed. Therefore, alumina powder having fine particles and a high degree of α-conversion can be obtained. The heat treatment temperature is preferably 900 to 1100°C, more preferably 1000 to 1100°C. In this manner, the high-purity fine alumina powder of this embodiment can be produced.

The present invention will be described in more detail using the following examples. However, the present invention is not limited to the following examples.

(1) Synthesis of Alumina Powder Example 1 (Example)
<Preparation process>
Aluminum hydroxide powder and α-alumina seeds were prepared as raw materials. Gibbsite (BHP39 manufactured by Nippon Light Metal Co., Ltd.) manufactured by the Bayer process was used as the aluminum hydroxide powder. High-purity fine alumina (manufactured by Nippon Light Metal Co., Ltd.) was used as the α-alumina seeds. The average particle diameter of the α-alumina seeds (high-purity fine alumina) was 0.18 μm.

<Mixing step>
Next, 10 mass % of α-alumina seeds (high-purity fine alumina) was added to the prepared aluminum hydroxide powder and then mixed to prepare a mixed raw material.

<Mechanochemical treatment process>
The resulting mixed raw material was subjected to mechanochemical treatment using a dry bead mill (SDA-1 manufactured by Ashizawa Phiten Tech Co., Ltd.) to obtain a dry-treated powder. The mechanochemical treatment was carried out using PSZ (partially stabilized zirconia) media beads and a grinding aid (ethanol) under conditions of a bead filling rate of 60% by volume, a peripheral speed of 4.5 to 5.0 m/sec, and a feed rate of 1.0 kg/hr, and the mixed raw material was passed through the dry bead mill once.

<Heat treatment process>
The obtained dry-processed powder was filled into a high-purity alumina crucible (purity 99%). The filled dry-processed powder was then heat-treated in a high-speed heating electric furnace (Superburn, manufactured by Motoyama Corporation) to produce a heat-treated powder (α-alumina powder). The heat treatment was performed under the conditions of a heating rate of 200°C/hour, a maximum temperature of 1070°C, and a holding time of 30 minutes.

<Cleaning process>
Next, in order to remove Na, the obtained heat-treated powder was washed by stirring in pure water of twice the mass ratio, and further washed with water of five times the mass ratio. After that, the washed heat-treated powder was dried in a dryer under the conditions of 110°C x 24 hours.

<Crushing process>
The dried heat-treated powder was pulverized using a ball mill to obtain the alumina powder of Example 1.

Example 2 (Comparative Example)
A commercially available high purity alumina powder (product of another company) was obtained and designated as Example 2.

Example 3 (Comparative)
The developed alumina powder (LS ultrafine particles) was used as Example 3.

Example 4 (Comparative)
A commercially available high purity alumina powder (AHP200 manufactured by Nippon Light Metal Co., Ltd.) was obtained and used as Example 4.

(2) Evaluation of Alumina Powder For Examples 1 to 4, various properties were evaluated according to the procedures shown below.

<SEM Observation>
The alumina powder was observed using a scanning electron microscope (SEM) (S4700 manufactured by Hitachi High-Tech Science Corporation, JSM-7200 manufactured by JEOL Ltd.) at a magnification of 50,000 times.

<Amount of impurities>
The amount of impurities (Si, Fe, Ca) in the alumina powder was measured using an ICP emission spectrometer (SP3100 manufactured by Seiko Instruments Inc.). First, the alumina powder was placed in a pressure decomposition vessel and pressure decomposed in a dryer using sulfuric acid for 10 hours. The pressure decomposition product was then adjusted to a constant volume with water to prepare a sample solution. The sample solution was placed in an analyzer, and the emission intensity at the wavelength of each element was measured. The concentration of each element was then calculated using the calibration curve obtained at the same time.

The amount of impurities (Na) in the alumina powder was measured using an atomic absorption spectrometer (polarized Zeeman atomic absorption spectrometer Z-2000, manufactured by Hitachi High-Technologies Corporation). First, the alumina powder was placed in a pressure decomposition vessel and diluted with water to a fixed amount to prepare a sample solution. The sample vessel was then set in the instrument, and the absorbance at a wavelength of 589.0 nm was measured using an air-acetylene flame. The amount of Na was then calculated using the absorbance of the standard solution, which was measured at the same time.

<Powder properties-BET specific surface area>
The BET specific surface area (S _BET ) of the alumina powder was measured in accordance with JIS 1626 using an automatic specific surface area measuring device (Flowsorb II 2300 model, manufactured by Micromeritics).

<Powder characteristics - particle size>
The particle size of the alumina powder was measured using a laser diffraction/scattering type particle size distribution measuring device (Microtrac MT3300 manufactured by Nikkiso Co., Ltd.). First, the alumina powder was dispersed using a homogenizer (US-600T manufactured by Nihon Seimitsu Seisakusho) under conditions of 600 W, 20 kHz, and 1 minute. The dispersed alumina powder was then introduced into a measuring device, where the particle size was measured. The obtained data was analyzed to determine the cumulative 10% particle size (D ₁₀ ), cumulative 50% particle size (average particle size; D ₅₀ ), and cumulative 90% particle size (D ₉₀ ) in the volumetric particle size distribution.

<Powder properties-pressurized bulk density>
The pressed bulk density (GD) of the alumina powder was measured as follows. First, the alumina powder was placed in a mold and pressed and molded at a pressure of 350 kgf/ ^cm2 . At this time, there was no pressing time (0 min). The mass and dimensions of the obtained molded piece (molded body) were measured, and the bulk density was calculated using these values.

<Sinterability>
The sinterability of the alumina powder was evaluated. First, the alumina powder was filled into a mold and uniaxially pressed at a pressure of 350 kgf/ ^cm2 . The resulting molded body was fired in a high-speed heating electric furnace (Superburn, manufactured by Motoyama Corporation) to produce a sintered body. The firing was performed under the conditions of a heating rate of 200°C/hour, a maximum temperature (sintering temperature) of 1350 to 1600°C, and a holding time of 2 hours. The density (bulk density) of the resulting sintered body was measured by the Archimedes method.

<X-ray diffraction>
The alumina powder was analyzed by powder X-ray diffraction (XRD). The analysis was performed as follows. First, the alumina powder was placed on a special sample plate and gently spread to a size of 20 mm x 20 mm x 0.5 mm to prepare a measurement sample. Next, an X-ray diffraction pattern of the measurement sample was obtained using an X-ray diffractometer. The conditions for X-ray diffraction were as follows:

- Apparatus: Rigaku Corporation RINT (horizontal sample type; Ultima II)
- Radiation source: CuKα radiation - Voltage: 40 kV
- Current: 40mA
- Scan speed: 4°/min - Sample width: 0.05°
- Starting angle: 5°
- End angle: 90°

In the obtained diffraction pattern, the peaks (diffraction lines) of the (012), (104), (113), (116) and (300) planes, which are the crystal peaks of α-alumina, were focused on, and the half-widths (full width at half maximum; FWHM) of these peaks were calculated. The peak intensities of the (012) and (116) planes were also compared with the peak intensities (diffraction intensities) of a standard sample (alpha-conversion degree 100%) to determine the degree of alpha-conversion of the alumina powder.

<Slurry characteristics - viscosity>
A slurry (suspension) of alumina powder was prepared, and its viscosity was evaluated. First, 200 g of alumina powder, 164 g of pure water, and 4 g of polycarboxylate ammonium dispersant (Nopcosperse 5600 manufactured by San Nopco Ltd.) were placed in a pot with a capacity of 1 L together with 600 g of a φ20 media ball. The pot was then rotated at a rotation speed of 72 rpm for 2 hours to mix the contents. This produced a slurry with a concentration of 55% by mass.

The viscosity of the resulting slurry was measured using a precision rotational viscometer (RST-CPS, manufactured by Eiko Seiki Co., Ltd.), which is a cone-plate type rotational viscometer. Specifically, the shear rate was changed from 1/sec to 1000/sec over a period of 60 seconds at 25°C, and the viscosity was measured every second.

<Slurry characteristics - Settling hydrostatic pressure>
The settling hydrostatic pressure of the slurry was measured using a hydrostatic pressure type slurry evaluation device (HYSTAP-3, Japan Hotel Goods Supply Co., Ltd.). The slurry used in the measurement was prepared at the time of viscosity evaluation. The settling velocity was calculated taking into consideration interference settling, in which particles dispersed in a fluid settle while interfering with each other, and a good dispersion line was created based on this settling velocity. The settling velocity was calculated with an alumina particle density of 3.98 g/cm ³ , a water density of 1.00 g/cm ³ , a gravitational acceleration of 9.80665 m/sec ² , and a viscosity of the liquid medium (water) of 0.00089 Pa·sec (25° C.).

<Dielectric properties of sintered body>
A sintered body was made from alumina powder, and its dielectric properties were evaluated. First, the alumina powder was filled into a mold and uniaxially pressed at a pressure of 19.6 MPa. The resulting molded body was then vacuum-packed and subjected to cold isostatic pressing (CIP) treatment at a pressure of 245 MPa for 1 minute. The CIP-treated molded body was fired in a high-speed heating electric furnace (Superburn, manufactured by Motoyama Co., Ltd.) to produce a sintered body. The firing was performed under the conditions of a heating rate of 200°C/hour, a maximum temperature of 1500°C, and a holding time of 2 hours.

The dielectric properties of the obtained sintered body were measured at 1 GHz, 5 GHz, and 10 GHz. The value at 1 GHz was measured at room temperature in an air atmosphere using an impedance analyzer (E4991B, manufactured by Keysight Technologies). On the other hand, the values at 5 GHz and 10 GHz were measured using a microwave PNA network analyzer (N5227A, manufactured by Keysight Technologies) in accordance with JIS 1627 under conditions of air atmosphere, temperature 24°C, and humidity 45%.

(3) Evaluation Results <SEM Observation>
For the alumina powders of Examples 1 to 4, SEM images of the powders are shown in Figures 3 to 6, respectively. The alumina powder of Example 1, which is an embodiment, was fine and had a uniform particle size. The particle shape was rounded. There were few fracture surfaces and no chipping particles (Figure 3). On the other hand, the alumina powder of Example 2, which is a comparative example, was fine but had an angular particle shape. Fracture surfaces and chipping particles were observed, although only slightly (Figure 4). The alumina powders of Examples 3 and 4, which are comparative examples, had variable particle sizes. Many fracture surfaces and chipping particles were observed (Figures 5 and 6).

<Impurity amount and powder characteristics>
The impurity amounts and powder properties of the alumina powders of Examples 1 to 4 are shown in Table 1. In Example 1, which is an embodiment, the contents of all impurities, sodium (Na), silicon (Si), iron (Fe), and calcium (Ca), were low, at 10 ppm or less. On the other hand, in Examples 2 to 4, which are comparative examples, the contents of some impurities exceeded 10 ppm. In particular, in Example 3, the contents of Na, Si, Fe, and Ca were all high, at 100 to 200 ppm.

Example 1 satisfied the conditions of D ₅₀ ≦0.20 μm and D ₅₀ × S _BET ≦2.0×10 ^-6 m ³ /g. On the other hand, D ₅₀ × S _BET exceeded 2.0×10 ^-6 m ³ /g in Examples 2 to 4. In particular, Example 2 had a D ₅₀ equivalent to that of Example 1, but had a large S _BET , and as a result, had a large D ₅₀ × S _BET .

Figure 7 shows the particle size distribution curves for the alumina powders of Examples 1 to 4. In Figure 7, the horizontal axis indicates particle size (particle diameter) and the vertical axis indicates frequency. The alumina powder of Example 1 showed a relatively uniform and sharp particle size distribution centered on a particle diameter of 0.2 μm. The alumina powder of Example 2 showed a sharp particle size distribution centered on a particle diameter of 0.2 μm, but there were particles of several μm in size. Therefore, the particle size distribution was broad overall. The alumina powder of Example 3 showed a broader particle size distribution than Examples 1 and 2. There were also quite a few particles of several μm in size. The alumina powder of Example 4 showed a large median particle diameter of 0.4 to 0.5 μm and a broad particle size distribution.

<Sinterability>
The densities (bulk densities) of the sintered bodies made from the alumina powders of Examples 1 to 4 are shown in FIG. 8. The alumina powders of Examples 1 and 2 were densified even at a relatively low firing temperature, and already showed a density of 3.8 g/ ^cm3 or more at 1350°C. The density became almost constant at 1450°C or higher. On the other hand, the alumina powder of Example 3 was less densified than Examples 1 and 2, and the density at 1350°C was only 3.6 to 3.7 g/ ^cm3 . The alumina powder of Example 4 was the least densified, and the density at 1350°C was as low as less than 3.4 g/ ^cm3 . The density finally became constant at 1550°C or higher.

The results of sinterability correspond to the results of average particle size (D ₅₀ ). That is, the alumina powders of Examples 1 and 2 have excellent sinterability because they are fine with D ₅₀ of 0.18 to 0.19 μm, whereas the alumina powders of Examples 3 and 4 have poor sinterability because they are coarse with D ₅₀ of 0.23 to 0.45 μm.

<X-ray diffraction>
For the alumina powders of Examples 1, 2 and 3, the values of the full width at half maximum (FWHM) of the (012), (104), (113), (116) and (300) diffraction lines are shown in Table 2. The XRD pattern of the alumina powder of Example 1 is shown in FIG.

As shown in Table 2, the alumina powder of Example 1 had smaller peak full width at half maximum (FWHM) for all diffraction lines compared to Examples 2 and 3. Also, as shown in FIG. 9, almost no diffraction lines derived from crystal phases other than α-alumina were observed in the XRD pattern of the alumina powder of Example 1. This shows that the alumina powder of Example 1 has small crystal distortion, is extremely excellent in crystallinity, and contains almost no heterogeneous phases.

<Slurry characteristics - viscosity>
The relationship between shear rate and viscosity for the slurries containing the alumina powders of Examples 1 to 4 is shown in Figure 10. Figure 10 shows both the viscosity when the shear rate is increased (indicated by the right-pointing arrow in the figure for some samples) and the viscosity when the shear rate is decreased (indicated by the left-pointing arrow in the figure).

The slurry of Example 1 had a low viscosity and was almost constant regardless of the shear rate. Moreover, there was almost no difference in viscosity when the shear rate was increased or decreased, and it showed a reversible response to the shear rate. This shows that the slurry of Example 1 had a low and stable viscosity and showed a reversible response to the shear rate. On the other hand, the slurries of Examples 2 and 3 had high viscosity. In particular, the slurry of Example 2 had a large difference in viscosity when the shear rate was increased and decreased, even though the average particle size of the alumina powder contained therein was almost the same as that of Example 1, and it showed an irreversible response to the shear rate. The slurry of Example 4, like Example 1, had a low viscosity and was almost constant regardless of the shear rate.

The following was inferred from the viscosity measurements. That is, the alumina powder contained in the slurry of Example 1 is fine, but the particle shape is round and the particle size is uniform. Therefore, even if the particles come into contact or collide with each other in the slurry, they can be quickly avoided and there is little mutual interference. Therefore, the slurry viscosity is low and stable. On the other hand, the particle size of the alumina powder contained in the slurry of Example 2 is almost the same as that of Example 1, but the particle size distribution including the chipping particles is broad and the particle shape is also angular. Therefore, when the particles come into contact or collide with each other, they interfere with each other and the slurry viscosity becomes unstable. Furthermore, the alumina powder contained in the slurry of Example 4 is coarse, so the viscosity is low.

<Slurry characteristics - Settling hydrostatic pressure>
FIG. 11 shows the change in settling hydrostatic pressure over time for the slurries containing alumina powder in Examples 1 and 2. FIG. 11 also shows the settling hydrostatic pressure line (good dispersion line) of the slurry showing an ideal well-dispersed state. The slurry in Example 1 showed a small change in settling hydrostatic pressure over time, and was close to an ideal well-dispersed state. On the other hand, the slurry in Example 2 showed a large change in settling hydrostatic pressure over time. From this, it was inferred that the slurry in Example 1 maintained a well-dispersed state for a long time, whereas the slurry in Example 2 was partially aggregated.

<Dielectric properties of sintered body>
The dielectric properties (relative dielectric constant εr, dielectric loss tangent tan δ, (εr) ^1/2 ×tan δ) of the alumina sintered bodies of Examples 1, 2 and 4 are shown in Table 3. Table 3 also shows the properties of polytetrafluoroethylene (PTFE).

The alumina sintered body of Example 1 had a smaller dielectric tangent and (εr) ^1/2 ×tan δ in the high frequency range (5 GHz, 10 GHz) than the other samples. This shows that Example 1 is an excellent material with small transmission loss as an antenna material.

In contrast, the dielectric loss tangent and (εr) ^1/2 × tan δ of the sintered body of Example 2 in the high frequency region (5 GHz, 10 GHz) were larger than those of Example 1. In Example 2, the BET specific surface area of the alumina powder was larger than that of Example 1, so the molding density was smaller, which is thought to have affected the generation of pores (defects) in the sintered body and the dielectric properties. In addition, the dielectric loss tangent and (εr) ^1/2 × tan δ of the sintered body of Example 4 were larger than those of Examples 1 and 2. In Example 4, the alumina powder contained a large amount of sodium (Na), which is thought to have adversely affected the density and electrical resistance of the sintered body. On the other hand, although the dielectric loss tangent of PTFE at 1 GHz was relatively small, the dielectric loss tangent at 10 GHz was much larger than that of the alumina sintered body (Examples 1, 2, and 4).

The transmission loss (a) of an antenna material is proportional to (εr) ^1/2 × tan δ, as shown in the following formula (1). Therefore, it is found that the sintered body of Example 1 has the smallest transmission loss in the high frequency region of 10 GHz or more, and is therefore promising as an antenna material.

Claims (5)

the 50% particle size (D ₅₀ ) and the BET specific surface area (S _BET ) in the volumetric particle size distribution satisfy the relationships represented by the formulas: D ₅₀ ≦0.20 μm and D ₅₀ ×S _BET ≦2.0×10 ⁻⁶ m ³ /g, and the contents of sodium (Na), silicon (Si), iron (Fe) and calcium (Ca) are each 10 ppm or less;
A high-purity fine alumina powder , in which the 10% particle size D ₁₀ , 50% particle size D ₅₀ and 90% particle size D ₉₀ in a volumetric particle size distribution satisfy the relationship represented by the formula: (D ₉₀ -D ₁₀ )/D ₅₀ ≦1.1. The alumina powder according to claim 1, which satisfies the relationship expressed by the formula: 1.6×10 ⁻⁶ m ³ /g≦D ₅₀ ×S _BET . The alumina powder according to claim 1 or 2, in which the full width at half maximum (FWHM) of the (113) diffraction line in the X-ray diffraction profile is 0.240° or less. The alumina powder according to any one of claims 1 to 3, having a pressed bulk density (GD) of 2.20 g / cm ³ or more. The alumina powder according to any one of claims 1 to 4 , having a degree of gelatinization of 80.0% or more.

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