High-purity fine alumina powder

Abstract

A high-purity microparticle alumina powder is provided which has excellent slurry properties and sintering properties, excellent fluidity and formability, and furthermore has excellent dielectric properties in the high-frequency region. In this high-purity microparticle alumina powder, the 50% particle diameter (D50) in the volume particle size distribution and the BET specific surface area (SBET) satisfy the relations represented by the formula D50 ≤ 0.20 μm and the formula D50 × SBET ≤ 2.0 × 10-6m3 / g, and the content of sodium (Na), silicon (Si), iron (Fe) and calcium (Ca) is each less than or equal to 10ppm.

Description

[Technical Field]

[0001] The present invention relates to a high-purity fine alumina powder. [Background technology]

[0002] Alumina (α-Al2O3) sintered compacts have excellent properties such as electrical insulation, heat resistance, wear resistance, and corrosion resistance. Therefore, alumina sintered compacts are used in a wide range of fields, including electronic components, refractories, abrasives, insulators, spark plugs, fillers, and catalyst carriers. Alumina sintered compacts are manufactured from alumina powder, which is then molded and fired. To obtain high-performance alumina sintered compacts, the alumina powder used as the raw material must have excellent sinterability, i.e., be fine.

[0003] When producing an alumina sintered body, a method of forming an alumina powder slurry and then wet-treating the slurry is widely used. For example, to improve moldability during molding, the alumina powder slurry is spray-granulated to form granules, and then these granules are molded. Another known method is to cast the alumina powder slurry to obtain large, complex-shaped alumina products. Another method is to produce a sheet-shaped alumina sintered body by applying and firing an alumina powder slurry.

[0004] When performing such wet processing, it is important to appropriately control the handleability of the slurry, i.e., its viscosity and other properties. For example, excessively high slurry viscosity can cause problems during granulation and molding. Furthermore, if the slurry properties change over time, the properties of the final product may become unstable and vary. For this reason, techniques have been proposed to control the powder properties of alumina powder and improve the slurry properties.

[0005] For example, Patent Document 1 discloses an alumina powder having a ratio (TBD / LBD) of loose bulk density (LBD) to loaded bulk density (TBD) of 1.5 or more, and states that by using this alumina powder, it is possible to form a thin, uniformly thick alumina-containing coating layer (claim 1 and

[0032] of Patent Document 1). Patent Document 1 also states 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). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2018 / 047871 Summary of the Invention [Problem to be solved by the invention]

[0007] However, conventionally proposed techniques have been insufficient in achieving both sinterability and slurry properties of alumina powder. In other words, the sinterability improves to a certain extent when alumina powder is made finer. However, when alumina powder is made finer, the viscosity of the slurry containing the powder increases rapidly, making handling difficult, such as granulation and molding. In particular, conventional techniques generally achieve finer alumina powder by intensive pulverization, but alumina powder made fine by intensive pulverization tends to deteriorate slurry properties. Furthermore, such alumina powder has low crystallinity, making it unsuitable as a raw material for producing high-performance alumina sintered bodies.

[0008] For example, Patent Document 1 describes 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 aggregation between particles becomes too dense (Patent Document 1, paragraph

[0044] ), and there is a limit to the refinement of the alumina powder, i.e., to the improvement of sinterability.

[0009] Conventional alumina powders also have problems with moldability. Specifically, fine alumina powders obtained by intensive pulverization have poor fluidity due to the irregular particle shape. Therefore, even when alumina powders are molded by dry or wet processes, it is difficult to obtain high-density compacts.

[0010] The present inventors have conducted extensive research in light of these conventional problems. 50 and BET specific surface area S BET It has been discovered that an alumina powder that satisfies a specific relationship and has impurities such as sodium not exceeding a specific amount can achieve both excellent sinterability and slurry properties. It has also been discovered that this alumina powder has good fluidity and excellent moldability. It has also been discovered that this alumina powder is suitable as a raw material for sintered bodies, exhibiting excellent dielectric properties in the high frequency range in addition to sinterability, slurry properties, and moldability.

[0011] The present invention was completed based on these findings, and an object of the present invention is to provide a high-purity fine alumina powder that has excellent slurry properties and sintering properties, as well as excellent fluidity and moldability, and further exhibits excellent dielectric properties in the high-frequency range. [Means for solving the problem]

[0012] The present invention encompasses the following aspects (1) to (9). In this specification, the expression "to" includes the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0013] (1) 50% particle size in volume particle size distribution (D 50 ) and BET specific surface area (S BET ) is the formula:D50 ≦0.20 μm, and formula: D 50 ×S BET ≦2.0×10 -6 m 3 / g and having a sodium (Na), silicon (Si), iron (Fe) and calcium (Ca) content of 10 ppm or less.

[0014] (2) 50% particle size in the volumetric particle size distribution (D 50 ) and BET specific surface area (S BET ) is the formula:D 50 ≦0.17 μm, and formula: D 50 ×S BET ≦1.8×10 -6 m 3 The high-purity fine alumina powder of (1) above satisfies the relationship expressed as:

[0015] (3) Formula: 1.55×10 -6 m 3 / g≦D 50 ×S BET The alumina powder according to (1) or (2) above, which satisfies the relationship expressed by:

[0016] (4) The alumina powder according to any one of (1) to (3) 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.

[0017] (5) Pressed bulk density (GD) is 2.20 g / cm 3 The alumina powder according to any one of (1) to (4) above.

[0018] (6) 10% particle size D in volume particle size distribution 10 , 50% particle size D 50 and 90% particle size D 90 However, the formula:(D 90 -D 10 ) / D 50 The alumina powder according to any one of (1) to (5) above, which satisfies the relationship expressed as follows:

[0019] (7) The alumina powder according to any one of (1) to (6) above, having a degree of gelatinization of 80.0% or more.

[0020] (8) A method for producing an alumina powder according to any one of (1) to (7), providing aluminum hydroxide powder and α-alumina seeds; mixing the aluminum hydroxide powder with the α-alumina seeds to obtain an aluminum hydroxide mixed raw material containing 1 to 20 mass % of the α-alumina seeds; a step of subjecting the 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; and a step of heat-treating the amorphous aluminum hydroxide at a temperature in the range of 900 to 1100°C to obtain an alumina powder, The average particle diameter (D) of the prepared α-alumina seeds 50 ) is 0.1 to 0.5 μm.

[0021] (9) Average particle size of prepared α-alumina seeds (D 50 ) is 0.1 to 0.3 μm, and the aluminum hydroxide mixed raw material contains 3 to 5 mass % of α-alumina seeds. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a high-purity fine alumina powder that has excellent slurry properties and sintering properties, as well as excellent fluidity and moldability, and further exhibits excellent dielectric properties in the high frequency range. [Brief explanation of the drawings]

[0023] [Figure 1] 1 shows a cross-sectional schematic diagram of a hydrostatic pressure type slurry evaluation device. [Figure 2] The principle of slurry evaluation by settling pressure measurement is shown below. [Figure 3] An SEM image of alumina powder is shown. [Figure 4] An SEM image of alumina powder is shown. [Figure 5] An SEM image of alumina powder is shown. [Figure 6] An SEM image of alumina powder is shown. [Figure 7] 1 shows the particle size distribution curve of alumina powder. [Figure 8] The relationship between the firing temperature (sintering temperature) and the bulk density of the sintered body is shown. [Figure 9] 1 shows an XRD pattern (X-ray diffraction profile) of alumina powder. [Figure 10] The relationship between the shear rate and viscosity of the slurry is shown. [Figure 11] The graph shows the change in slurry settling hydrostatic pressure over time. [Figure 12] An SEM image of alumina powder is shown. [Figure 13] 1 shows the particle size distribution curve of alumina powder. [Figure 14] The relationship between the firing temperature (sintering temperature) and the bulk density of the sintered body is shown. DETAILED DESCRIPTION OF THE INVENTION

[0024] 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.

[0025] <<1. High-purity fine alumina powder>> The high-purity fine alumina powder of this embodiment (hereinafter sometimes collectively referred to as "alumina powder") has a particle size of 50% in the volume particle size distribution (D 50 ) and BET specific surface area (S BET ) is the formula:D 50 ≦0.20 μm, and formula: D 50 ×S BET ≦2.0×10 -6 m 3 / g. Generally, the smaller the particle size of a powder, the larger the specific surface area, and the greater the number of contact points with other particles. Also, as the radius of curvature of the particles decreases, the surface becomes more active, and the driving force for sintering increases. Therefore, from the perspective of obtaining high sinterability, D 50 is limited to 0.20 μm or less.

[0026] 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 powders are refined by methods such as strong grinding, chipping occurs in the particles, resulting in angular particle shapes. If such powders are slurried 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 powder's fluidity may deteriorate, making it difficult to increase the packing density, which can lead to poor sinterability.

[0027] In contrast, the average particle size (D 50 ) and D 50 ×S BET By keeping the specific surface area (S) small, it is possible to improve the sinterability while maintaining good handling properties. BET ) becomes smaller. Therefore, D 50 By reducing D 50 ×S BET By keeping the particle size low, it is possible to produce particles with few fracture surfaces and chipping even if the particle size is small. The alumina powder of this embodiment contains particles with rounded shapes and uniform particle sizes. Therefore, when slurried, excessive increases in slurry viscosity and instability can be suppressed. In addition, the fluidity and moldability are excellent. These factors work synergistically, making it possible to achieve both excellent sinterability and ease of handling. D 50 may be 0.19 μm or less, 0.18 μm or less, 0.17 μm or less, or 0.16 μm or less. 50 ×S BET is 1.9 x 10 -6m 3 / g or less, 1.8×10 -6 m 3 / g or less, 1.7×10 -6 m 3 / g or less, or 1.6 x 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. Therefore, D 50 may be 0.05 μm or more, may be 0.10 μm or more, or may be 0.15 μm or more.

[0028] The alumina powder of this embodiment preferably has a particle size of 50% in the volume particle size distribution (D 50 ) and BET specific surface area (S BET ) is the formula:D 50 ≦0.17 μm, and formula: D 50 ×S BET ≦1.8×10 -6 m 3 / g. The average particle diameter (D 50 ) and D 50 ×S BET By further limiting the particle size, the moldability and sinterability of the alumina powder can be further improved.

[0029] Furthermore, the alumina powder of this embodiment contains sodium (Na), silicon (Si), iron (Fe), and calcium (Ca) at 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). Calcium (Ca) may react with alumina (Al2O3) in the main phase to form compounds such as CaO·6Al2O3. Such grain boundary phases are undesirable because they cause abnormal grain growth. Abnormal grain growth can result in the generation of locally coarse particles, impairing the overall homogeneity of sintering. As a result, closed voids may form, resulting in a low final sintered density. Furthermore, some of the impurities may be incorporated into the main phase, reducing the electrical resistance of the main phase. For example, sodium (Na) is incorporated into the main phase alumina (Al2O3) to form β-alumina (Na2O·11Al2O3). β-alumina is a material used as a solid electrolyte in batteries and has high electrical conductivity. If there are many impurities like this, the sinterability and electrical resistance (insulation) will deteriorate.

[0030] In particular, the alumina powder of this embodiment has an average particle diameter (D 50 ) and a large sintering driving force. Therefore, impurities are easily adversely affected by impurities on sinterability and electrical resistance. Therefore, the content of each impurity (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. In particular, sodium (Na) reduces the sinterability and insulating properties of alumina powder, so it is desirable to keep its content low. Therefore, it is particularly preferable to keep the sodium (Na) content to 5 ppm or less.

[0031] The alumina powder of this embodiment has the formula: 1.55 × 10 -6 m 3 / g≦D 50 ×S BETWhen the shape of an alumina particle is as round as possible, it becomes a perfect spherical particle. D of a perfect spherical particle 50 ×S BET is the density of alumina (3.98 g / cm 3 ), this value is calculated as 1.51 × 10 -6 m 3 / g. Although methods for producing spherical alumina particles are known, these methods require expensive raw materials and are complicated manufacturing processes. As will be explained in the examples below, the particles constituting the alumina powder of this embodiment are rounded and have a uniform particle size, but are not completely spherical. In addition, expensive raw materials are not required and the manufacturing process is simple. Therefore, D 50 ×S BET D does not need to be as low as that of perfectly spherical particles, and production costs can be kept low. 50 ×S BET is 1.6 x 10 -6 m 3 / g or more, and -6 m 3 / g or more, and -6 m 3 / g or more.

[0032] The alumina powder of this embodiment preferably has a full width at half maximum (FWHM) of the (113) diffraction line in its X-ray diffraction (XRD) profile of 0.240° or less, more preferably 0.230° or less. Generally, fine powders are produced by strong crushing (strong pulverization). However, alumina powder crushed by applying a strong crushing force has distortions on the crystal surfaces of its primary particles. Such distortions on the crystal surfaces may activate the alumina particle surfaces, adversely affecting handling properties and slurry characteristics. In contrast, the alumina powder of this embodiment, although composed of fine primary particles, has little crystal distortion and excellent crystallinity. The smaller the FWHM, the better, but it is typically 0.200° or more.

[0033] The pressed bulk density (GD) of the alumina powder of this embodiment is 2.20 g / cm 3More than 2.30 g / cm is preferable. 3 The above is more preferable. By producing powder with a high pressed bulk density in this way, the density of the compact produced from this powder can be increased, and as a result, it is possible to suppress the occurrence of pores (defects) in the sintered compact. The pressed bulk density of alumina powder is 350 kgf / cm 2 The bulk density of the compact (molded piece) obtained by pressure molding under the condition of no pressure time (0 minutes) at a pressure of 1000 kJ / cm. The upper limit of the pressed bulk density is preferably as high as possible, but is typically 2.50 g / cm. 3 The following is the result.

[0034] The alumina powder is preferably 10% particle size in the volume particle size distribution (D 10 ), 50% particle size (D 50 ) and 90% particle size (D 90 ) is expressed by the formula:(D 90 -D 10 ) / D 50 It is preferable that the relationship expressed by (D)≦1.5 is satisfied. If the particle size distribution of the alumina powder is too broad, there is a risk that the sinterability and handling properties may deteriorate. Therefore, an excessively broad particle size distribution is not preferable. 90 -D 10 ) / D 50 may be 1.3 or less, or may be 1.1 or less.

[0035] The alumina powder of this embodiment preferably has a degree of gelatinization of 80.0% or more, more preferably 90.0% or more. If the degree of gelatinization is less than 80.0%, when a sintered body is produced from the alumina powder, 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 properties. Therefore, the higher the degree of gelatinization, the more preferable it is. The degree of gelatinization can be determined by obtaining the X-ray diffraction profile (XRD pattern) of the alumina powder using X-ray diffraction, and comparing the X-ray diffraction intensities of the (012) and (116) planes in the diffraction pattern with the diffraction intensities of a standard sample with a degree of gelatinization of 100%.

[0036] The alumina powder of this embodiment has excellent sinterability due to its fine particle size. In fact, the inventors have confirmed that a dense sintered body can be produced at a significantly lower temperature than with conventional alumina powders.

[0037] Furthermore, the alumina powder of this embodiment has excellent slurry properties despite its fine size. That is, a slurry containing this alumina powder has low viscosity and is stable. Generally, as the particle size of a powder decreases, the interaction between particles becomes stronger, making it difficult to achieve good dispersion. Therefore, a stronger shear force must be applied to disperse the powder in the slurry. Furthermore, even if the powder is dispersed temporarily, it is prone to agglomeration or gelation in the slurry, making the slurry unstable. In contrast, the alumina powder of this embodiment can be dispersed even with weak shear force, and the slurry maintains a well-dispersed state for a long period of time. This is presumably because the particles have a rounded shape and uniform particle size, which reduces the interaction between particles, leading to a low viscosity and stability of the slurry.

[0038] Furthermore, the alumina powder of this embodiment has good fluidity and therefore excellent moldability. That is, due to its good fluidity, it has high packing properties during dry or wet molding, and as a result, a high-density molded body can be obtained.

[0039] Furthermore, the alumina powder of this embodiment exhibits excellent dielectric properties in the high frequency range. Therefore, by using this alumina powder as a raw material, it is possible to produce an alumina sintered body with low dielectric loss and transmission loss. In fact, the inventors have confirmed that it is possible to obtain an alumina sintered body with low dielectric loss tangent (tanδ) and transmission loss in the high frequency range of 10 GHz or higher.

[0040] Such alumina powder can be widely used in electronic components, refractories, abrasives, insulators, spark plugs, fillers, catalyst supports, etc. Because of its particularly excellent sinterability, it is useful when the alumina powder is dry- or wet-molded and fired to produce a sintered body. It is also useful as a raw material when the alumina powder is used as a filler and kneaded with a polymer compound such as a resin to produce a composite material.

[0041] <<2. Slurry>> The slurry of this embodiment is prepared by mixing the above-described 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, an ammonium polycarboxylate dispersant. The content of the dispersant may be, for example, 1 to 10 mass %.

[0042] The slurry of this embodiment is characterized by its low viscosity regardless of shear rate. For example, the viscosity can be maintained at 6 mPa·sec or less when the shear rate is increased and decreased within the shear rate range of 300 to 1000 / sec. Slurries are non-Newtonian fluids, and shear rate and shear stress do not exhibit a proportional relationship. Therefore, when evaluating the flow characteristics of a slurry, it is preferable to evaluate it using a flow curve, which shows the relationship between shear rate and shear stress. One such method is the use of a cone-plate rotational viscometer (cone-plate rotational viscometer) specified in JIS Z 8803. In a cone-plate rotational viscometer, the sample (slurry) is sandwiched between a cone and a plate, and the cone is rotated at a constant speed, measuring the rotational torque. Shear stress can be calculated from the rotational torque.

[0043] The slurry of this embodiment is also characterized by its long-term stability. The stability of a slurry can be evaluated by examining its settling behavior in a gravitational field. One such method is settling hydrostatic pressure measurement using a hydrostatic slurry evaluation device. The measurement principle for settling hydrostatic pressure measurement is explained using 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 with its tip inserted into the tube, and a pressure sensor attached to the other end of the pressure transmission tube. Slurry is poured into the settling tube, and the hydrostatic pressure at a depth H is measured. When all particles in the slurry are suspended, the hydrostatic pressure is maximized. On the other hand, when all particles in the slurry have settled through depth H, the hydrostatic pressure is minimized. Therefore, particle dispersion stability can be evaluated by calculating the hydrostatic pressure (P) as a function of time (t). For example, as shown in Figure 2, a well-dispersed slurry exhibits little change in hydrostatic pressure even after a long period of time has elapsed. On the other hand, when 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.

[0044] <<3. 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, slip casting, injection molding, or extrusion molding. The molded body may also be subjected to cold isostatic pressing (CIP). 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 a hydrogen atmosphere for 1 to 5 hours.

[0045] 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, or even 9.0 to 11.0. In addition, the dielectric loss tangent (tanδ) at 10 GHz can be set to 0.20×10 -4 It can be less than 0.15 x 10 -4The alumina sintered body having such a small relative permittivity and dielectric loss tangent in the high frequency region is suitable as an antenna material for the fifth generation mobile communication system (5G), although it is not limited thereto.

[0046] To explain this point, the fifth-generation mobile communications system (5G) is the successor to the fourth-generation (4G) system typified by smartphones, and commercial services will begin 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, large-capacity communication, highly reliable and low-latency communication, and multiple simultaneous connections.

[0047] For 5G, which uses high frequencies, it is important to reduce the transmission loss of antenna materials. In other words, radio waves transmitted in wireless communication are converted into heat in the antenna material. The amount of transmission loss (a) that occurs during this process is proportional to the product of the frequency of the radio waves (f), the square root of the relative permittivity (εr) of the antenna material, and the dielectric loss tangent (tanδ) of the antenna material, as shown in equation (1) below. In equation (1), K is a proportionality constant.

[0048]

number

[0049] Since transmission loss (a) is proportional to frequency (f), it is necessary to reduce the loss of the antenna material as the operating frequency increases. Polytetrafluoroethylene (PTFE) is known as an antenna material in the frequency range before 4G, but as the frequency increases to 5G, PTFE has large losses and is insufficient as an antenna material. The alumina sintered body of this embodiment can reduce the dielectric loss tangent (tanδ) and transmission loss in the high-frequency range compared to PTFE. Therefore, it is expected to be used as an antenna material for 5G.

[0050] The alumina sintered body of this embodiment is not limited to use as a 5G antenna material, and is, of course, useful for applications other than antennas, such as circuit boards, capacitor / resistor substrates, IC substrates, sensor component substrates, multilayer substrates, packages, RF windows, and semiconductor manufacturing equipment.

[0051] <<4. Alumina Powder Manufacturing Method>> The alumina powder of this embodiment may be produced by any method as long as it satisfies the above-mentioned requirements. However, a preferred production method includes the steps of: preparing aluminum hydroxide powder and α-alumina seeds (preparation step); mixing the prepared aluminum hydroxide powder with the α-alumina seeds to obtain an aluminum hydroxide mixed raw material containing 1 to 20 mass% of the α-alumina seeds (mixing step); mechanochemically treating the obtained aluminum hydroxide mixed raw material 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 heat-treating the obtained amorphous aluminum hydroxide at a temperature range of 900 to 1100°C to obtain an alumina powder (heat treatment step). The average particle diameter (D 50 ) is 0.1 to 0.5 μm.

[0052] This manufacturing method is characterized by using a specific amorphous aluminum hydroxide as an intermediate raw material, which is produced by mechanochemically treating a mixed raw material of aluminum hydroxide powder and α-alumina seeds. 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. Each step is described in detail below.

[0053] <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 preferred in consideration of production costs. Furthermore, the aluminum hydroxide powder may be produced by any method, but it is preferably produced by the Bayer process. From the viewpoint of powder flowability and ease of handling, the aluminum hydroxide powder should have an average particle diameter (D 50 ) is 3 to 50 μm, and the BET specific surface area (S BET ) is 0.2 to 5.0 m 2 / g. General-purpose aluminum hydroxide powders have an average particle size and a BET specific surface area within the above ranges. In the production method of this embodiment, it is possible to use a general-purpose aluminum hydroxide powder raw material, and as a result, it is possible to make the most of the advantages of reduced production costs and simplicity.

[0054] On the other hand, from the viewpoint of lowering the temperature of the gelatinization transition of amorphous aluminum hydroxide, it is preferable that the α-alumina seeds have a high degree of gelatinization. The degree of gelatinization is preferably 90% or more, more preferably 95% or more. Furthermore, from the viewpoint of reducing the size of the alumina powder after production and improving the moldability and sinterability, the average particle diameter (D 50 ) is limited to 0.1 to 0.5 μm. It is preferable that the α-alumina seeds are fine. 50 The BET specific surface area (S BET ) is 5~15m 2 / g is preferred, and 10 to 15m 2 / g is more preferred.

[0055] <Mixing process> In the mixing step, the prepared aluminum hydroxide powder is mixed with α-alumina seeds to obtain an aluminum hydroxide mixed raw material containing 1 to 20 mass% of α-alumina seeds. The mixing method is not particularly limited. Adding α-alumina seeds can fully exert the effect of lowering the crystal transition temperature of the resulting amorphous aluminum hydroxide. From the viewpoint of lowering the temperature of the α-alumina transition, it is desirable to add a certain amount of α-alumina seeds. However, if the amount of α-alumina seeds is excessively large, the alumina powder produced may become coarse and its sinterability may deteriorate. Furthermore, high-quality α-alumina seeds are expensive. The content of α-alumina seeds is preferably 1 to 15 mass%, more preferably 1 to 10 mass%, even more preferably 2 to 8 mass%, and particularly preferably 3 to 5 mass%. By using a small amount of particularly fine α-alumina seeds, a fine alumina powder with excellent moldability and sinterability can be obtained. Therefore, the average particle diameter (D 50 It is particularly preferable to use α-alumina seeds having a fine diameter of 0.1 to 0.3 μm and to set the content of α-alumina seeds to 3 to 5 mass %.

[0056] The decrease in the crystal transition temperature due to the α-alumina seeds is speculated as follows. That is, the phenomenon of the α-alumina transition temperature being decreased by adding a small amount of α-alumina as seeds to an aluminum hydroxide raw material has long been known. In this embodiment, the raw materials, aluminum hydroxide (such as gibbsite) and α-alumina seeds, are first prepared into a mixed raw material powder, and then simultaneously subjected to mechanochemical treatment to be pulverized to a significantly smaller size. As a result, the amorphous aluminum hydroxide primary particles, which have an increased specific surface area, and the α-alumina seeds closely aggregate at the particle interface, making the desired interaction more uniform. As a result, it is believed that amorphous aluminum hydroxide can undergo α-alumina transition at a low temperature range where phase transition would not normally occur, without passing through an intermediate alumina phase such as chi-alumina.

[0057] <Mechanochemical treatment process> In the mechanochemical treatment step, the resulting aluminum hydroxide mixture is subjected to mechanochemical treatment using a dry bead mill to produce amorphous aluminum hydroxide. Amorphous aluminum hydroxide is primarily composed of amorphous aluminum hydroxide. Aluminum hydroxide, in a completely crystalline state, is a compound having a composition similar to gibbsite (Al(OH)3). Amorphous aluminum hydroxide has lost or reduced aluminum hydroxide crystallinity, and some of the water of crystallization has been lost. Therefore, amorphous aluminum hydroxide differs from completely crystalline aluminum hydroxide in terms of its crystalline state and amount of water of crystallization. The degree of amorphization can be evaluated by the intensity (CPS) of the gibbsite crystal peak (002) plane and the amount of water of crystallization (LOI) measured by X-ray diffraction. In this embodiment, amorphous aluminum hydroxide refers to aluminum hydroxide having an intensity (CPS) of the gibbsite crystal peak (002) plane of 350 CPS or less and an amount of water of crystallization of 21.0 mass% or less, as measured by X-ray diffraction. In contrast, the amount of water of crystallization in non-amorphous gibbsite is about 34.7 mass %.

[0058] The amorphous aluminum hydroxide of this embodiment has a water of crystallization content (LOI) of 21.0% by mass or less. The amount of water of crystallization in amorphous aluminum hydroxide is less than that of aluminum hydroxide in a completely crystalline state. As described below, the amorphous aluminum hydroxide of this embodiment 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 aluminum hydroxide crystals such as gibbsite are ground down to a fine region, and the encapsulated water of crystallization is expelled. It is believed that by continuing the grinding process until the amount of water of crystallization is reduced and the primary particles become sufficiently fine, it becomes possible to sufficiently lower the crystal transition temperature to α-alumina. From the viewpoint of promoting amorphization, the water of crystallization content is preferably 17.0% by mass or less. The lower limit of the water of crystallization content is not particularly limited, but may typically be 15.0% by mass or more.

[0059] Furthermore, the amorphous aluminum hydroxide of this embodiment 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 this embodiment 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.

[0060] The BET specific surface area of ​​the amorphous aluminum hydroxide is not particularly limited, but is typically 15 to 50 m 2 The BET specific surface area can be measured using an automatic specific surface area measuring device (Micromeritics, FlowSorb II2300 model) in accordance with JIS1626.

[0061] 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, sinterable ceramics, sodium and zirconium are components that inhibit sintering, and it is desirable to minimize their content. Even if aluminum hydroxide with a high sodium content, such as gibbsite, is used as a raw material during production of the amorphous aluminum hydroxide of this embodiment, the sodium encapsulated in the crystals is released during the amorphousization treatment, allowing it to be easily washed and removed. Furthermore, using a raw material with a low sodium content naturally allows the sodium content of the amorphous aluminum hydroxide to be reduced.

[0062] When a crystal is continuously milled, the newly formed surface area increases, and the number of surface atoms and / or molecules that have lost their bonds increases, causing disruption of their bonding state near the surface layer. As a result, the milled particles become activated. In addition, in the case of dry milling, the powder aggregates, reducing the apparent surface area. The activated surfaces of the milled particles adsorb moisture and gases from 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 processes that cause these mechanochemical reactions are called mechanochemical processes.

[0063] In this embodiment, the mechanochemical treatment is performed using a dry bead mill. In dry bead milling, a high crushing shear is applied to the mixed raw material, effectively inducing a mechanochemical reaction. A dry bead mill is a type of media-agitation mill, consisting of a raw material inlet, a cylindrical container (vessel), a rotating agitator (agitator) installed inside the cylindrical container, and an outlet for the processed powder. The agitator gap in the vessel is filled with numerous crushing media (beads). During operation, the agitator rotates at high speed to agitate the beads. During this process, the raw material introduced through the raw material inlet repeatedly collides with the agitator, beads, and the vessel inner wall, resulting in crushing due to impact, shear, and friction forces, while inducing a mechanochemical reaction. The resulting dry-processed powder is then discharged from the outlet.

[0064] Although the detailed mechanism of the mechanochemical reaction caused by dry bead milling is unknown, it is speculated as follows. During the process, aluminum hydroxide is continuously subjected to high-shear dry milling. This causes some of the water of crystallization to be desorbed from the aluminum hydroxide. At the same time, friction between the ground particles, beads, and the vessel inner wall raises the temperature inside the device, resulting in phase transitions such as partial hydrothermal reactions and dissolution and reprecipitation within 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 20% by mass of α-alumina seeds is added, the water of crystallization content of the raw aluminum hydroxide mixture is 27.0% by mass or more, whereas the water of crystallization content of the dry-processed powder after dry bead milling is reduced to 21.0% by mass or less, and in some cases to 17.0% by mass or less. This suggests that water of crystallization is desorbed during dry bead milling.

[0065] The dry bead mill has an agitator peripheral speed (rotational speed) of preferably 5.0 to 6.0 m / s, more preferably 5.0 to 5.5 m / s, and a bead filling rate of preferably 60 to 70% by volume, more preferably 60 to 65% by volume. The feed rate 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 rate, the more effective the mechanochemical treatment and the higher the quality of amorphous aluminum hydroxide obtained. The smaller the feed rate, the longer the residence time of the mixed raw materials, which promotes the amorphization of aluminum hydroxide. However, excessively high agitator peripheral speeds and bead filling rates make stable continuous operation difficult. An excessively low feed rate leads to significant problems, such as a decrease in yield due to leakage of pulverized powder, resulting in poor production efficiency. When the agitator peripheral speed, bead filling amount, and feed amount are within the above-mentioned ranges, high-quality amorphous aluminum hydroxide can be obtained with good productivity.

[0066] During mechanochemical treatment, the aluminum hydroxide mixed raw material may be treated once (single-pass treatment) or multiple times (two-pass treatment, three-pass treatment, etc.) with a dry bead mill. As mentioned above, the smaller the feed rate, 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 rate is increased, the same results as when treating once with a small feed rate can be obtained. Therefore, high-quality amorphous aluminum hydroxide can be obtained while maintaining production efficiency. It is preferable to set the feed rate to 2.0 to 3.0 kg / hour and perform two passes. Furthermore, during mechanochemical treatment, a grinding aid may be added to the aluminum hydroxide mixed raw material if necessary. An example of the grinding aid is ethanol.

[0067] When a general dry mill 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 mill, even if the processing time is extended, there is a limit to the amount of atomization of the pulverized particles, and the specific surface area cannot be sufficiently increased. When a wet mill is used for processing, it is possible to atomize the pulverized particles to 1 μm or less, but the mechanochemical reaction is insufficient. Therefore, amorphous aluminum hydroxide cannot be obtained. Furthermore, when a wet mill is used, there are problems such as increased contamination (impurities) and poor productivity.

[0068] In contrast, the production method of this embodiment uses dry bead processing, which is a dry process, to sufficiently promote the mechanochemical reaction of the aluminum hydroxide mixed raw material, thereby easily producing amorphous aluminum hydroxide with a significantly low crystal transition temperature to α-alumina. In particular, the production method of this embodiment employs mechanochemical processing using a dry bead mill, and therefore amorphous aluminum hydroxide, which is difficult to process, can be produced inexpensively even using general-purpose aluminum hydroxide as the starting material. Therefore, the method does not require the sophisticated filtration equipment or large-scale drying equipment required for wet milling, and is therefore highly productive. Furthermore, continuous processing is possible when a continuous dry bead mill is used.

[0069] <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 may be subjected to a crushing treatment or a washing treatment. Because the amorphous aluminum hydroxide of this embodiment has an extremely low crystal transition temperature to α-alumina, α-alumina powder with a sufficiently high degree of gelatinization can be obtained even by heat treatment at a relatively low temperature of 900 to 1100°C. Furthermore, the low heat treatment temperature can suppress grain growth during heat treatment. Therefore, alumina powder with fine particles and a high degree of gelatinization can be obtained. The heat treatment temperature is preferably 900 to 1050°C, more preferably 950 to 1050°C. In this manner, the high-purity fine alumina powder of this embodiment can be produced. [Example]

[0070] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0071] [Experimental Example A] In Experiment A, aluminum hydroxide powder and α-alumina seeds were subjected to dry bead treatment and heat treatment to synthesize alumina powder, and its properties were compared with those of conventional alumina powder.

[0072] (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 α-alumina seeds (high-purity fine alumina) had an average particle diameter (D 50 ) was 0.18 μm.

[0073] <Mixing process> Next, α-alumina seeds (high-purity fine alumina) were added to and mixed with the prepared aluminum hydroxide powder to prepare a mixed raw material. At this time, the content of α-alumina seeds in the mixed raw material was adjusted to 10 mass %.

[0074] <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 dry-processed powder. Mechanochemical treatment was carried out using PSZ (partially stabilized zirconia) media beads and a grinding aid (ethanol) at a bead filling rate of 60% by volume, a peripheral speed of 4.5-5.0 m / s, and a feed rate of 1.0 kg / h. The mixed raw material was passed through the dry bead mill once.

[0075] <Heat treatment process> The dry-processed powder was packed into a high-purity alumina crucible (purity 99%). The packed dry-processed powder was then heat-treated in a high-speed heating electric furnace (Superburn, manufactured by Motoyama Corporation) to produce heat-treated powder (α-alumina powder). The heat treatment was performed at a heating rate of 200°C / h, at a maximum temperature of 1070°C, and for a holding time of 30 minutes.

[0076] <Cleaning process> Next, to remove the Na content, the obtained heat-treated powder was washed by stirring in pure water at a mass ratio of 2 times, and then washed with water at a mass ratio of 5 times, and then dried in a dryer at 110°C for 24 hours.

[0077] <Crushing process> The dried heat-treated powder was crushed using a ball mill. Thus, the alumina powder of Example 1 was obtained.

[0078] [Example 2 (Comparative Example)] Commercially available high-purity alumina powder (product of another company) was obtained and used as Example 2.

[0079] [Example 3 (Comparative Example)] The alumina powder as a developed product (LS ultra-fine particles) was used as Example 3.

[0080] [Example 4 (Comparative Example)] Commercially available high-purity alumina powder (AHP200 manufactured by Nippon Light Metal Co., Ltd.) was obtained and used as Example 4.

[0081] (2) Evaluation of alumina powder For Examples 1 to 4, evaluations of various properties were conducted according to the following procedures.

[0082] <SEM observation> The alumina powder was observed using a scanning electron microscope (SEM). The observation was conducted using a scanning electron microscope (S4700 manufactured by Hitachi High-Technologies Corporation, JSM-7200 manufactured by JEOL Ltd.) under the condition of a magnification of 50,000 times.

[0083] <Amount of impurities> The amounts of impurities (Si, Fe, Ca) in the alumina powder were measured using an ICP emission spectroscopic analyzer (SP3100 manufactured by Seiko Instruments Inc.). First, the alumina powder was placed in a pressure decomposition vessel and pressure-decomposed using sulfuric acid in a dryer for 10 hours. Then, the pressure-decomposed product was made up to a fixed volume with water to prepare a sample solution. The sample solution was set in the analyzer, and the emission intensity at the wavelength of each element was measured. Then, the concentrations of each element were calculated using the calibration curve obtained simultaneously.

[0084] The amount of impurity (Na) in alumina powder was measured using an atomic absorption spectrometer (polarized Zeeman atomic absorption spectrometer Z-2000, Hitachi High-Technologies Corporation). First, the alumina powder was placed in a pressure decomposition vessel and diluted with water to a fixed volume to prepare a sample solution. The sample vessel was then placed 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.

[0085] <Powder properties-BET specific surface area> The BET specific surface area (S BET ) was measured in accordance with JIS1626 using an automatic specific surface area measuring device (Flowsorb II2300 model manufactured by Micromeritics Co., Ltd.).

[0086] <Powder characteristics - particle size> The particle size of the alumina powder was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3300, manufactured by Nikkiso Co., Ltd.). First, the alumina powder was dispersed using a homogenizer (US-600T, manufactured by Japan Precision Manufacturing Co., Ltd.) under conditions of 600 W, 20 kHz, and 1 minute. The dispersed alumina powder was then introduced into the 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 diameter (average particle diameter; D 50 ) and cumulative 90% particle size (D 90 ) was sought.

[0087] <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 at 350 kgf / cm 2 The compact was subjected to pressure molding at a pressure of 1000 kJ / min. At this time, no pressure was applied (0 min). The mass and dimensions of the obtained compact (molded body) were measured, and the bulk density was calculated using these values.

[0088] <Sinterability> The sinterability of the alumina powder was evaluated. First, the alumina powder was filled into a mold and subjected to a pressure of 350 kgf / cm. 2It was uniaxially press-molded at the pressure of . The obtained molded body was fired in a high-speed heating electric furnace (Super Burn manufactured by Motoyama Co., Ltd.) to obtain a sintered body. The firing was carried out 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 obtained sintered body was measured by the Archimedes method.

[0089] <X-ray Diffraction> The alumina powder was analyzed by the powder X-ray diffraction (XRD) method. The analysis was carried out as follows. First, the alumina powder was placed on a dedicated sample plate and gently spread to make a measurement sample with a size of 20 mm × 20 mm × 0.5 mm. Next, using an X-ray diffractometer, the X-ray diffraction pattern of the measurement sample was obtained. The conditions for X-ray diffraction were as follows.

[0090] - Equipment: RINT manufactured by Rigaku Corporation (sample horizontal type; UltimaII) - Radiation source: CuKα ray - Voltage: 40 kV - Current: 40 mA - Scan speed: 4° / min - Sample width: 0.05° - Start angle: 5° - End angle: 90°

[0091] In the obtained diffraction pattern, attention was paid to the peaks (diffraction lines) of the (012), (104), (113), (116), and (300) planes, which are the crystal peaks of α-alumina, and the half-width (full width at half maximum; FWHM) of these peaks was calculated. Also, the degree of α-alumina of the alumina powder was determined by comparing the peak intensities of the (012) and (116) planes with the peak intensities (diffraction intensities) of a standard sample (α-degree 100%).

[0092] <Slurry Property - Viscosity> An alumina powder slurry (suspension) was prepared and its viscosity was evaluated. First, 200 g of alumina powder, 164 g of pure water, and 4 g of a polycarboxylate ammonium dispersant (Nopcosperse 5600, manufactured by San Nopco Ltd.) were placed in a 1 L pot along with 600 g of a φ20 media ball. The pot was then rotated at 72 rpm for 2 hours to mix the contents. This produced a slurry with a concentration of 55% by mass.

[0093] 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-and-plate type rotational viscometer. Specifically, the shear rate was changed from 1 / s to 1000 / s over 60 seconds at 25°C, and the viscosity was measured every second.

[0094] <Slurry characteristics - Settling hydrostatic pressure> The settling hydrostatic pressure of the slurry was measured using a hydrostatic slurry evaluation device (HYSTAP-3, manufactured by Japan Hotel Goods Supply Co., Ltd.). The slurry used for the measurement was prepared during the viscosity evaluation. The settling velocity was calculated taking into account interference settling, where particles dispersed in a fluid settle while interfering with each other, and a good dispersion line was created based on this settling velocity. The alumina particle density was set to 3.98 g / cm. 3 , the density of water is 1.00 g / cm 3 , the acceleration due to gravity is 9.80665 m / s 2 The sedimentation velocity was calculated assuming the viscosity of the liquid medium (water) to be 0.00089 Pa·sec (25°C).

[0095] <Dielectric properties of sintered body> Sintered bodies were fabricated from alumina powder and their 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 compact was then vacuum-packed and subjected to cold isostatic pressing (CIP) at a pressure of 245 MPa for 1 minute. The CIP-treated compact was then fired in a high-speed heating electric furnace (Superburn, manufactured by Motoyama Corporation) to produce a sintered body. Firing was performed at a heating rate of 200°C / hour, at a maximum temperature of 1500°C, and for a holding time of 2 hours.

[0096] For the obtained sintered body, the dielectric properties at 1 GHz, 5 GHz, and 10 GHz were measured. 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) under the conditions of an air atmosphere, a temperature of 24 °C, and a humidity of 45% in accordance with JIS 1627.

[0097] (3) Evaluation results <SEM observation> For the alumina powders of Examples 1 to 4, the SEM images of the powders are shown in FIGS. 3 to 6, respectively. The alumina powder of Example 1, which is an example, was fine and had a uniform particle size. Also, the particle shape was rounded. There were few fracture surfaces and no chipping particles were observed (FIG. 3). On the other hand, the alumina powder of Example 2, which is a comparative example, was fine but had an angular particle shape. Also, although there were few fracture surfaces and chipping particles, they were observed (FIG. 4). The alumina powders of Comparative Examples 3 and 4 had a varying particle size. Also, many fracture surfaces and chipping particles were observed (FIGS. 5 and 6).

[0098] <Impurity amount and powder properties> For the alumina powders of Examples 1 to 4, the impurity amount and powder properties are shown in Table 1. In Example 1, which is an example, the content of any of the impurities of sodium (Na), silicon (Si), iron (Fe), and calcium (Ca) was as low as 10 ppm or less. On the other hand, in Comparative Examples 2 to 4, the content of some impurities exceeded 10 ppm. In particular, in Example 3, the content of any of Na, Si, Fe, and Ca was as high as 100 to 200 ppm.

[0099] Example 1 was D 50 ≤ 0.20 μm or less and D 50 × S BET ≤ 2.0 × 10 -6 m 3 / g and satisfied the condition. On the other hand, for Examples 2 to 4, D 50 × S BET was 2.0 × 10-6 m 3 In particular, Example 2 exceeded the D 50 is equivalent to Example 1, but S BET is large, and as a result, D 50 ×S BET was big.

[0100] [Table 1]

[0101] FIG. 7 shows particle size distribution curves for the alumina powders of Examples 1 to 4. In FIG. 7, the horizontal axis represents particle size (particle diameter) and the vertical axis represents frequency. The alumina powder of Example 1 exhibited a relatively uniform and sharp particle size distribution centered around a particle diameter of 0.2 μm. The alumina powder of Example 2 exhibited a sharp particle size distribution centered around a particle diameter of 0.2 μm, but also contained particles of a few μm in size. As a result, the particle size distribution was broad overall. The alumina powder of Example 3 exhibited a broader particle size distribution than Examples 1 and 2. Furthermore, there were quite a few particles of a few μm in size. The alumina powder of Example 4 exhibited a large median particle diameter of 0.4 to 0.5 μm and a broad particle size distribution.

[0102] <Sinterability> The densities (bulk densities) of the sintered bodies made from the alumina powders of Examples 1 to 4 are shown in Figure 8. The alumina powders of Examples 1 and 2 were densified even at relatively low firing temperatures, and already had a bulk density of 3.8 g / cm at 1350°C. 3 The density was almost constant above 1450°C. On the other hand, the alumina powder of Example 3 was inferior to Examples 1 and 2 in densification, and the density at 1350°C was 3.6 to 3.7 g / cm. 3 The alumina powder of Example 4 was the least densified, with a density of 3.4 g / cm at 1350°C. 3 The density finally became constant above 1550°C.

[0103] The sinterability results are based on the average particle diameter (D 50) corresponds to the results. That is, the alumina powders of Example 1 and Example 2 are fine with D 50 being 0.18 - 0.19 μm, so they have excellent sinterability. In contrast, the alumina powders of Example 3 and Example 4 are considered to have poor sinterability because D 50 is coarse at 0.23 - 0.45 μm.

[0104] <X-ray Diffraction> For the alumina powders of Example 1, Example 2, and Example 3, the values of the full width at half maximum (FWHM) of the diffraction lines of (012), (104), (113), (116), and (300) are shown in Table 2. Also, the XRD pattern of the alumina powder of Example 1 is shown in Figure 9.

[0105] As shown in Table 2, for the alumina powder of Example 1, the peak half-width (FWHM) was smaller than that of Example 2 and Example 3 for all diffraction lines. Also, as shown in Figure 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. From this, it was found that the alumina powder of Example 1 has small crystal strain, extremely excellent crystallinity, and almost no foreign phases.

[0106]

Table 2

[0107] <Slurry Property - Viscosity> Regarding the slurries containing the alumina powders of Example 1 to Example 4, the relationship between the shear rate and the viscosity is shown in Figure 10. In Figure 10, both the viscosity when increasing the shear rate (for some samples, indicated by rightward arrows in the figure) and the viscosity when decreasing the shear rate (indicated by leftward arrows in the figure) are shown.

[0108] The slurry of Example 1 had a low viscosity and remained almost constant regardless of shear rate. Furthermore, there was almost no difference in viscosity between increasing and decreasing shear rate, demonstrating a reversible response to shear rate. This indicates that the slurry of Example 1 had a low and stable viscosity and exhibited a reversible response to shear rate. On the other hand, the slurries of Examples 2 and 3 had high viscosities. In particular, the slurry of Example 2 had a large difference in viscosity between increasing and decreasing shear rate, despite the average particle size of the alumina powder contained therein being almost the same as that of Example 1, demonstrating an irreversible response to shear rate. The slurry of Example 4, like Example 1, had a low viscosity and remained almost constant regardless of shear rate.

[0109] The following was inferred from the viscosity measurements. Specifically, the alumina powder contained in the slurry of Example 1 is fine, but the particle shapes are rounded and the particle diameter is uniform. Therefore, even if particles come into contact or collide with each other in the slurry, they can quickly avoid contact, and there is little mutual interference. Therefore, the slurry viscosity is low and stable. On the other hand, the particle diameter 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 chipping particles is broad and the particle shapes are angular. Therefore, when particles come into contact or collide with each other, they interfere with each other, causing the slurry viscosity to become unstable. Furthermore, the alumina powder contained in the slurry of Example 4 is coarse, so the viscosity is low.

[0110] <Slurry characteristics - Settling hydrostatic pressure> Figure 11 shows the change in settling hydrostatic pressure over time for the slurries containing alumina powder in Examples 1 and 2. Figure 11 also shows the settling hydrostatic pressure line (good dispersion line) of the slurry, which indicates 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 partially aggregated.

[0111] <Dielectric properties of sintered body> The dielectric properties (relative permittivity εr, dielectric loss tangent tanδ, (εr)) of the alumina sintered bodies of Examples 1, 2 and 4 were measured. 1 / 2 × tan δ) are shown in Table 3. Table 3 also shows the properties of polytetrafluoroethylene (PTFE).

[0112] The alumina sintered body of Example 1 has a dielectric loss tangent and a dielectric constant (εr) in the high frequency range (5 GHz, 10 GHz). 1 / 2 × tan δ was smaller than those of the other samples. This indicates that Example 1 is an excellent material with small transmission loss as an antenna material.

[0113] In contrast, the sintered body of Example 2 has a dielectric loss tangent and (εr) in the high frequency range (5 GHz, 10 GHz). 1 / 2 × tanδ was larger than in Example 1. In Example 2, the BET specific surface area of ​​the alumina powder was larger than in Example 1, resulting in a smaller compact density, which is thought to have affected the generation of pores (defects) in the sintered body and the dielectric properties. In addition, the sintered body in Example 4 had a lower dielectric loss tangent and (εr) 1 / 2 × tanδ was 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 had an adverse effect on 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 bodies (Examples 1, 2, and 4).

[0114] The transmission loss (a) of the antenna material is (εr) as shown in the following formula (1): 1 / 2 × tan δ. Therefore, the sintered body of Example 1 has the smallest transmission loss in the high frequency range of 10 GHz or higher, and is therefore promising as an antenna material.

[0115]

number

[0116] [Table 3]

[0117] [Experimental Example B] In Experimental Example B, alumina powders were synthesized by varying the particle size and amount of α-alumina seeds added, and the results were evaluated.

[0118] (1) Synthesis of alumina powder [Example 5 (Example)] In Example 5, the average particle size (D 50 ) is 0.16 μm, and the BET specific surface area (S BET ) is 10.2m 2 / g of α-alumina seeds was used. The content of α-alumina seeds in the mixed raw material was changed to 4 mass %. Except for this, an alumina powder was synthesized in the same manner as in Example 1.

[0119] [Example 6 (Example)] Average particle diameter (D 50 ) is 0.23 μm, and the BET specific surface area (S BET ) is 8.29m 2 / g of α-alumina seeds were used. Except for this, an alumina powder was synthesized in the same manner as in Example 5.

[0120] [Example 7 (Example)] An alumina powder was synthesized in the same manner as in Example 5, except that the content of α-alumina seeds in the mixed raw material was changed to 7 mass %.

[0121] [Example 8 (Example)] An alumina powder was synthesized in the same manner as in Example 6, except that the content of α-alumina seeds in the mixed raw material was changed to 7 mass %.

[0122] [Example 9 (Example)] An alumina powder was synthesized in the same manner as in Example 5, except that the content of α-alumina seeds in the mixed raw material was changed to 10 mass %.

[0123] [Example 10 (Example)] An alumina powder was synthesized in the same manner as in Example 6, except that the content of α-alumina seeds in the mixed raw material was changed to 10 mass %.

[0124] [Example 11 (Example)] An alumina powder was synthesized in the same manner as in Example 5, except that the content of α-alumina seeds in the mixed raw material was changed to 20 mass %.

[0125] [Example 12 (Example)] In Example 12, the average particle size (D 50 ) is 0.19 μm, and the BET specific surface area (S BET ) is 13.1m 2 / g of α-alumina seeds was used. The content of α-alumina seeds in the mixed raw material was changed to 20 mass %. Except for this, an alumina powder was synthesized in the same manner as in Example 5.

[0126] (2) Evaluation of alumina powder The powder properties (BET specific surface area, particle size, and pressed bulk density) of the alumina powders obtained in Examples 5 to 12 were evaluated in the same manner as in Examples 1 to 4. In addition, sintered bodies were produced in the same manner as in Examples 1 to 4, except that the sintering temperature was changed to 1300 to 1450°C, and the sinterability of the alumina powder was evaluated.

[0127] (3) Evaluation results The properties of the alumina powders obtained in Examples 5 to 12 are shown in Table 4 together with the manufacturing conditions. When the amount of α-alumina seeds was the same, the alumina powders produced using α-alumina seeds with smaller particle sizes (Examples 5, 7, and 9) had a larger average particle size (D 50 ) was smaller than that of alumina powders produced using α-alumina seeds with larger particle sizes (Examples 6, 8, and 10). As a result, the pressed bulk density and sintered bulk density were also larger. When the particle size of the α-alumina seeds was the same, the pressed bulk density and sintered bulk density of the alumina powders produced under conditions with a small amount of α-alumina seeds (Examples 5 and 7) were larger than that of the alumina powders produced under conditions with a large amount of α-alumina seeds (Examples 9 and 10).

[0128] 12 shows an SEM image of the alumina powder obtained in Example 7. It was found that the alumina powder of Example 7 had a small primary particle size and was hardly aggregated.

[0129] The particle size distribution curve of the alumina powder obtained in Example 7 is shown in Figure 13, along with the results obtained in Examples 1 and 4. The alumina powders of Examples 1 and 7, which were produced by dry bead milling using α-alumina seeds, were finer and had a more uniform particle size distribution than the alumina powder of conventional Example 4. Furthermore, the alumina powder of Example 7 was even finer than the alumina powder of Example 1, which was produced by using a larger amount of α-alumina seeds with a larger particle size.

[0130] The relationship between the firing temperature (sintering temperature) and the sintered bulk density of the alumina powder obtained in Example 7 is shown in Figure 14, along with the results obtained in Examples 1 and 4. The alumina powders of Examples 1 and 7 had significantly higher sinterability than the alumina powder of Example 4, a conventional example. For example, the firing temperature required to obtain the same sintered bulk density for the alumina powder of Example 7 was approximately 200°C lower than that for the alumina powder of Example 4. Furthermore, the alumina powder of Example 7 had even better sinterability than the alumina powder of Example 1, which was produced using a larger particle size α-alumina seed in a larger amount.

[0131] From the above results, it was found that dry bead milling using α-alumina seeds can produce fine alumina powder with excellent moldability and sinterability. It was also found that using α-alumina seeds with smaller particle size and reducing their amount can produce even finer alumina powder with even better moldability and sinterability.

[0132] [Table 4]

Claims

1. 50% particle size in the volume particle size distribution (D ５０ ) and BET specific surface area (S ＢＥＴ ) is represented by the formula: D ５０ ≦0.20 μm, and formula: 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, and the degree of gelatinization is 80.0% or more.

2. 50% particle size in the volume particle size distribution (D ５０ ) and BET specific surface area (S ＢＥＴ ) is represented by the formula: D ５０ ≦0.17 μm, and formula: D ５０ ×S ＢＥＴ ≦1.8×10 －６ m ３ 2. The high-purity fine alumina powder according to claim 1, which satisfies the relationship expressed as: / g.

3. Formula: 1.55×10 －６ m ３ / g≦D ５０ ×S ＢＥＴ The alumina powder according to claim 1 or 2, which satisfies the relationship expressed by the following formula:

4. 4. The alumina powder according to claim 1, wherein the full width at half maximum (FWHM) of the (113) diffraction line in an X-ray diffraction profile is 0.240° or less.

5. Pressed bulk density (GD) is 2.20 g / cm ３ The alumina powder according to any one of claims 1 to 4.

6. 10% particle size D in volume particle size distribution １０ , 50% particle size D ５０ and 90% particle size D ９０ is represented by the formula: (D ９０ -D １０ ) / D ５０ The alumina powder according to any one of claims 1 to 5, which satisfies the relationship expressed as ≦1.

5.

7. A method for producing an alumina powder according to any one of claims 1 to 6, providing aluminum hydroxide powder and α-alumina seeds; mixing the aluminum hydroxide powder with the α-alumina seeds to obtain an aluminum hydroxide mixed raw material containing 1 to 20 mass % of the α-alumina seeds; a step of subjecting the aluminum hydroxide mixed raw material to mechanochemical treatment using a dry bead mill to obtain amorphous aluminum hydroxide having a water of crystallization 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; and heat treating the amorphous aluminum hydroxide at a temperature in the range of 900 to 1100°C to obtain an alumina powder. The average particle diameter (D ５０ ) is 0.1 to 0.5 μm.

8. The average particle diameter (D ５０ 8. The method according to claim 7, wherein the average particle size is 0.1 to 0.3 μm, and the aluminum hydroxide mixed raw material contains 3 to 5 mass % of α-alumina seeds.

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