Ceramic spherical body and method for manufacturing same

Abstract

The objective of the present invention is to provide a ceramic spherical body having high hydrophilicity and reduced wear during pulverization.　The present invention is a ceramic spherical body which contains zirconia as a main component, and has a monoclinic crystal percentage of at most 3.0 vol%, a tetragonal crystal percentage of 80 vol% to 95 vol%, a surface roughness Sa of 0.005 μm to 0.015 μm, and a ratio (A / B) of 0.07 to 0.30, where (A) is the peak maximum value at 3,200-3,400 cm-1 derived from OH groups on the surface of the ceramic spherical body, and (B) is the sum of peak maximum values at 450-500 cm−1, 550-600 cm-1, and 750-850 cm−1 derived from Zr-O bonds.

Description

Ceramic sphere and its manufacturing method

[0001] The present invention relates to a ceramic sphere and a method for producing the same.

[0002] Mills such as ball mills, vibration mills, sand mills, and bead mills that use milling media are widely used for finely grinding powders used in electronic materials and dispersing pigments for ink applications. As the milling media such as balls and beads (hereinafter sometimes simply referred to as "media") used in these mills, ceramic sintered bodies containing zirconia as their main component, which have excellent wear resistance and impact resistance, are used.

[0003] As a ceramic sintered body mainly composed of zirconia, ZrO ２ and Y ２ O ３ The composition ratio of Al is limited. ２ O ３ amount and SiO ２ In one example, Patent Document 1 discloses media that are said to have improved durability and wear resistance by controlling the amount of sintered ceramic spherical bodies. These spherical ceramic bodies are polished after sintering to improve surface smoothness and wear resistance.

[0004] Japanese Patent Application Laid-Open No. 2001-316178

[0005] These media are often washed with water before and after use, but ceramic spheres, which generally consist primarily of zirconia, are hydrophobic, so they tend to clump together or trap air during washing, making them difficult to clean. In particular, micro-sized media with a diameter of 0.1 mm or less have a light weight and a large surface area, making the hydrophobic effect more pronounced. As a result, they float in water, and when the supernatant liquid is discarded after washing, some of the media is discarded along with the supernatant liquid, creating a handling issue of loss.

[0006] Furthermore, because these micro-sized media have a large surface area, when they are used for grinding or dispersion, they are subject to a great deal of wear due to collisions between the media and the material being dispersed. In particular, initial wear occurs during the initial stages of use due to unevenness on the media surface. As a result, quality issues have been raised, such as the components of the media contaminating the material being dispersed and deteriorating the product characteristics.

[0007] In view of the above problems, the present invention aims to provide ceramic spherical bodies that are highly hydrophilic and have reduced wear during crushing, and a method for producing the same.

[0008] In order to solve the above problems, the present invention mainly has the following features: (1) A ceramic spherical body mainly composed of zirconia, wherein the proportion of monoclinic crystals is 3.0% by volume or less, the proportion of tetragonal crystals is 80% by volume or more and 95% by volume or less, the surface roughness Sa is 0.005 μm or more and 0.015 μm or less, and the surface roughness Sa is 3200 to 3400 cm 2 derived from OH groups on the surface of the ceramic spherical body. －１ and the peak maximum (A) at 450 to 500 cm due to the Zr—O bond. －１ , 550-600cm －１ and 750-850 cm －１ (2) A ceramic spherical body according to the above (1), in which the ratio (A / B) of the sum (B) of the maximum peak values ​​of the ceramic spherical body and the maximum peak values ​​of the ceramic spherical body is 0.07 or more and 0.30 or less. (3) A ceramic spherical body according to the above (1), in which the proportion of monoclinic crystals is 9.0% by volume or less after a pressure cooker test (PCT) is carried out for 72 hours under conditions of a temperature of 121°C and a humidity of 100%. (4) A ceramic spherical body according to the above (1), in which the proportion of monoclinic crystals is 9.0% by volume or less after a water penetration rate measured by a penetration rate method is 2.0 x 10 －４ g ２ / s or more 80×10 －４ g ２ / s or less. (4) Ceramic spherical bodies according to any one of (1) to (3) above, having an average particle size of 10 μm or more and 150 μm or less. (5) A method for producing ceramic spherical bodies according to any one of (1) to (4) above, comprising a polishing step of polishing the surface of a spherical ceramic mainly composed of zirconia to obtain a polished spherical ceramic, and a heat treatment step of heat treating the polished spherical ceramic at 100°C to 1300°C, in this order, wherein the surface roughness Sa of the polished spherical ceramic is 0.005 μm or more and 0.015 μm or less. (6) A method for producing ceramic spherical bodies according to (5) above, wherein the heat treatment step is carried out in an air atmosphere. (7) A method for producing ceramic spherical bodies according to (5) above, wherein the polishing apparatus used in the polishing step is a bead mill.

[0009] According to the ceramic spherical bodies of the present invention, ceramic spherical bodies having high hydrophilicity and suppressing the amount of wear during grinding can be obtained.

[0010] Schematic diagram showing the chemical bonds on the surface of the ceramic spheres of the present invention, which are mainly composed of zirconia. －１ These are the results of infrared spectroscopy measurements.

[0011] [Ceramic Spherical Body] The ceramic spherical body of the present invention is made of a ceramic sintered body containing zirconia as a main component. Hereinafter, in this specification, the ceramic sintered body as a final product, i.e., the ceramic sintered body as an intermediate body obtained by sintering once or more times in the manufacturing process, other than the grinding media, will be collectively referred to as an "intermediate sintered body." Furthermore, both the ceramic sintered body as a final product and the intermediate sintered body will be collectively referred to simply as a "sintered body."

[0012] The ceramic spherical bodies of the present invention are obtained by forming a ceramic raw material powder (hereinafter simply referred to as "raw material powder") containing zirconia as the main component into a spherical shape. Here, "containing zirconia as the main component" in this specification means that the ratio of zirconia is 90% by weight or more, and it is preferable that the ratio of zirconia is 93% by weight or more of the total components, because particularly high strength can be obtained.

[0013] The content of each component in ceramics can be determined as follows. First, a ceramic sample is crushed using a universal testing machine, and approximately 0.3 g of crushed pieces are placed in a platinum crucible and melted with potassium hydrogen sulfate. This is then dissolved in dilute nitric acid to obtain a constant solution. The content of each metal element is quantified using ICP atomic emission spectroscopy, and the content is then calculated by converting it to its oxide. Hereinafter, the components in the ceramic spherical bodies of the present invention will be referred to either as metal elements or as oxides.

[0014] In addition to the main components described above, the ceramic spherical body of the present invention contains yttria (Y ２ O ３ ), ceria (CeO ２ ), alumina (Al ２ O ３ It is preferable to include, for example, magnesium (MgO), calcia (CaO), etc. These function as stabilizers and can improve the strength and toughness of the ceramic spheres. Among these, it is preferable to include yttria. The yttria content is preferably 4.6 / 95.4 or more and 5.6 / 94.4 or less, and more preferably 4.7 / 95.3 or more and 5.5 / 94.5 or less, in terms of the yttria / zirconia weight ratio in the ceramic spheres. By setting the yttria / zirconia weight ratio to 4.6 / 95.4 or more, the proportion of the monoclinic phase of zirconia during sintering can be suppressed, thereby further reducing the amount of wear when used for grinding. Furthermore, by setting the yttria / zirconia weight ratio to 5.6 / 94.4 or less, the proportion of the cubic phase increases, thereby relatively reducing the proportion of the tetragonal phase, thereby further reducing the amount of wear when used for grinding.

[0015] The ceramic spherical body of the present invention has a thermal conductivity of 3200 to 3400 cm originating from OH groups on the surface of the ceramic spherical body. －１ and the peak maximum (A) at 450 to 500 cm due to the Zr—O bond. －１ Peak maximum value B1, 550-600 cm －１ Peak maximum B2 and 750-850 cm －１ It is important that the ratio (A / B) of the sum of the peak maximum values ​​of B1 and B2 (B = B1 + B2 + B3) of the peak maximum values ​​B1 and B2 is 0.07 or more and 0.30 or less. Conventional ceramic spheres have a low A / B ratio of approximately 0.02, exhibiting hydrophobicity and poor cleanability. Especially when the ceramic spheres are small in diameter, they tend to float in water during cleaning and are discarded together with the supernatant liquid after rinsing. Therefore, by setting the A / B ratio to 0.07 or more, the amount of hydrogen-bonding OH groups on the ceramic sphere surface is sufficient, preventing the floating of small ceramic spheres in water. If the A / B ratio is less than 0.07, the hydrophilicity is insufficient and floating occurs in water. A / B is preferably 0.13 or more. The higher the A / B ratio, the more hydrophilic the ceramic spheres can be imparted, but from a practical standpoint, it is 0.30 or less. A method for increasing the A / B ratio is to heat treat the grinding media after polishing at 100 to 1300°C by the method described later. As shown in Figure 1, this is ２ Adsorbed water 1 (H ２ This is because 2Zr-O) is excited by heat to become Zr-OH according to the following formula, generating hydroxyl groups bonded to the surface: 2Zr-O + HO → 2Zr-OH An OH group (3 in Figure 1) with adsorbed water (1 in Figure 1) attached to a hydroxyl group via a hydrogen bond (2 in Figure 1) is called a hydrogen-bonded OH group, and a Zr-OH group with no adsorbed water attached (4 in Figure 1) is called an isolated Zr-OH group.

[0016] The peak maximum values ​​A and B can be determined by the following method. First, the ceramic spherical body is set in an FT-IR device and purged with nitrogen for 30 minutes, after which diffuse reflectance FT-IR measurement is performed. At this time, an Au vapor-deposited film is used as a reference. Next, the relative diffuse reflectance R of the sample, which is the measurement result, is subjected to Kubelka-Munk transformation ((1-R ２) / 2R), and the 3200-3400 cm band derived from the hydrogen-bonded OH group was observed in the spectrum after Kubelka-Munk conversion. －１ The maximum value of the peak is A, and monoclinic ZrO ２ and tetragonal ZrO ２ 450-500 cm －１ The maximum value of the peak B1, 550 to 600 cm －１ The maximum value of the peak B2 and 750-850 cm －１ The sum B (B=B1+B2+B3) of the maximum values ​​B3 of the peaks can be calculated.

[0017] The ceramic spherical body of the present invention has a water permeation rate of 2.0 × 10 －４ g ２ / s or more 80.0×10 －４ g ２ / s or less. －４ g ２ When the water penetration rate is 40.0 g / s or more, the ceramic spherical bodies have good compatibility with water and have high hydrophilicity, so that floating in water is further suppressed. ２ The higher the water penetration rate, the more hydrophilic the film can be imparted. －４ g ２ The permeation rate can be calculated from the results of measuring the weight of the permeated solvent and the permeation time when ceramic spheres are densely packed in a pipe-shaped column with a mesh on the bottom and immersed in a solvent.

[0018] A method for increasing the water penetration rate is, for example, to heat treat the grinding media after polishing in an air atmosphere at 700° C. to 1300° C. by the method described below.

[0019] The ceramic spheres of the present invention have a tetragonal crystal content of 80% by volume or more and 95% by volume or less. When the tetragonal crystal content is 80% by volume or more, the tetragonal crystals transform into monoclinic crystals (pressure-induced transformation) and expand in volume when stress is applied, suppressing cracking in the media and reducing wear. However, when the tetragonal crystal content is less than 80% by volume, this effect is reduced. The tetragonal crystal content is preferably 88% by volume or more. On the other hand, when the tetragonal crystal content is greater than 95% by volume, deterioration is likely to occur in high-temperature water, and the ceramic spheres are more likely to break or wear when the water temperature rises due to prolonged grinding and dispersion. The tetragonal crystal content is preferably 94% by volume or less.

[0020] The ceramic spherical body of the present invention has a monoclinic crystal content of 3.0% by volume or less. If the monoclinic crystal content is greater than 3.0% by volume, the amount of wear increases. The monoclinic crystal content is preferably 2.0% by volume or less, and more preferably 1.0% by volume or less. The lower the monoclinic crystal content, the better. However, since the monoclinic crystal content tends to increase due to the energy required in the polishing process and the heat during drying, a content of 0.1% by volume or more is practically preferable.

[0021] The ceramic spherical bodies of the present invention preferably have a cubic crystal ratio of 5% to 20% by volume. A cubic crystal ratio of 20% by volume or less increases the relative proportion of tetragonal crystals, further suppressing the increase in wear caused by pressure-induced transformation. Furthermore, a cubic crystal ratio of 5% by volume or more reduces the likelihood of deterioration (phase transition from tetragonal to monoclinic) in high-temperature water, further suppressing breakage or wear of the ceramic spherical bodies when the water temperature rises due to prolonged grinding and dispersion.

[0022] Furthermore, the ceramic spheres of the present invention preferably have a monoclinic fraction of 9.0% by volume or less after a pressure cooker test (PCT) involving continuous 72-hour wet heat treatment at a temperature of 121°C and 100% humidity. Zirconia is known to transform from a tetragonal to a monoclinic phase when immersed in a hydroxyl-containing solvent, such as water or alcohol. When used as a dispersion medium in such a solvent for an extended period of time, the monoclinic fraction in the dispersion medium increases, resulting in a deterioration in wear resistance. This phenomenon can be simulated using an accelerated PCT device. Ceramic spheres with a monoclinic fraction of 9.0% by volume or less after the test are suitable as dispersion media because they are less susceptible to increased wear and cracking or breakage.

[0023] The monoclinic transformation due to hydrothermal degradation tends to increase as the surface area of ​​the ceramic spheres increases, as the probability of contact with water molecules increases. Therefore, it is preferable to finish the ceramic spheres with a smooth surface with as few irregularities as possible during the surface polishing process used to manufacture the ceramic spheres.

[0024] The proportion of each crystal phase in the ceramic spheres can be measured by powder X-ray diffraction.

[0025] The ceramic spherical body of the present invention preferably has a surface roughness Sa (arithmetic mean) of 0.005 μm or more and 0.015 μm or less. If the surface roughness Sa is greater than 0.015 μm, the initial wear amount increases quadratically. The surface roughness Sa is preferably 0.12 μm or less. The smaller the surface roughness Sa, the better, but from a practical standpoint, it is 0.005 μm or more, preferably 0.05 μm or more. The surface roughness Sa can be measured using a laser microscope in accordance with ISO 25178 (measurement method: JIS 0681-6:2014).

[0026] An example of a method for reducing the surface roughness Sa is to polish the surface of the ceramic spherical body using a bead mill polishing device.

[0027] The ceramic spheres of the present invention preferably have an average particle size X of 10 μm or more and 150 μm or less. Having an average particle size X of 150 μm or less significantly improves hydrophilicity and suppresses floating in water. On the other hand, having an average particle size X of 10 μm or more makes it easier to obtain ceramic spheres using currently established manufacturing methods. The average particle size X can be adjusted to the above range by sieve classification, as described below.

[0028] [Method for producing ceramic spherical bodies] The ceramic spherical bodies of the present invention can be produced by various methods. As an example, a production example by a rolling granulation molding method will be described in detail below.

[0029] (Forming step) The raw material powder can first be formed into a spherical shape using a rolling granulation method. The rolling granulation method is a method in which spherical fine particles are formed by alternately adding ceramic raw material powder and a liquid binder containing a binding agent and water into a rotating drum, and then the particles and powder are rotated together to grow, thereby producing a spherical molded body.

[0030] (Drying step) Since the obtained green body contains moisture, if it is subjected to the sintering step described below as it is, the moisture inside the green body may evaporate rapidly, which may cause cracks in the green body. Therefore, before being subjected to the sintering step, the green body is subjected to a drying step in which the moisture inside the green body is gradually reduced using a dryer or the like.

[0031] (Sintering process) The compact thus formed and dried is placed in a sagger (or similar) and fired in a firing furnace to remove the binder and bond the powder particles together, resulting in a sintered ceramic body. In the sintering process, firing at 1350 to 1450°C for 1 to 3 hours is preferred.

[0032] (HIP process) The sintered body that has undergone the sintering process can be used as grinding media after being polished and sintered at a low temperature, as described below. However, in order to further reduce defects in the grinding media, it is preferable to carry out the hot isostatic pressing process, as described below. Below, we will explain the case where the hot isostatic pressing process is carried out after the sintering process. Note that if the hot isostatic pressing process is not carried out, the sintered body after the sintering process described above is not an "intermediate sintered body," but in the following explanation of the case where the hot isostatic pressing process is carried out, it will be described as an "intermediate sintered body."

[0033] As described above, the intermediate sintered body obtained in the sintering step is then preferably subjected to a hot isostatic pressing step (hereinafter referred to as "HIP treatment") in which a high temperature and an isotropic pressure are simultaneously applied to the workpiece. By subjecting the intermediate sintered body to HIP treatment, defects such as voids and cracks remaining inside the intermediate sintered body can be removed without changing the shape.

[0034] The HIP treatment is preferably carried out at a temperature 0°C to 50°C lower than the sintering temperature in the sintering step. If the temperature is lower than this, the ceramic powder such as zirconia may not diffuse sufficiently during the HIP treatment, resulting in defects. On the other hand, if the HIP treatment temperature is higher than the sintering temperature, grain growth of the intermediate sintered body may occur, resulting in a decrease in strength and increased variation in strength. The HIP treatment temperature is more preferably 0°C to 40°C lower than the sintering temperature in the sintering step, and even more preferably 0°C to 30°C lower.

[0035] The pressure for the HIP treatment needs only to be sufficient to remove defects, and treatment can be carried out without problems at a pressure of 100 MPa or more. To achieve a high pressure state, treatment is preferably carried out in an Ar gas atmosphere.

[0036] (Polishing step) In the method for producing ceramic spheres of the present invention, the surface roughness Sa (arithmetic mean) of the polished spherical ceramic obtained by the polishing step is 0.005 μm or more and 0.015 μm or less. If the surface roughness Sa is greater than 0.015 μm, the initial wear amount of the finally obtained ceramic spheres increases quadratically. The smaller the surface roughness Sa, the better, but from a practical standpoint, it is 0.005 μm or more. The surface roughness Sa can be measured using a laser microscope based on ISO 25178 (measurement method: JIS 0681-6:2014).

[0037] As a method for reducing the surface roughness Sa, there is a method for polishing the surface of the spherical ceramics using a bead mill polishing device.

[0038] Methods for polishing the surface of spherical ceramics include barrel polishing and polishing with a bead mill. Among these, polishing with a bead mill is preferred because it allows for the production of ceramic spherical bodies with reduced wear. Polishing with a bead mill allows for polishing with higher energy, resulting in ceramic spherical bodies with smoother surfaces and reduced wear.

[0039] (Heat Treatment Step) The polished ceramic spheres are then subjected to a heat treatment step. This heat treatment step can impart hydrophilicity to the ceramic spheres. This is done by applying a hydrophilic property to the ceramic spheres as shown in FIG. ２ Adsorbed water 1 (H ２ This is because 2Zr-O + HO → 2Zr-OH is excited by heat to form Zr-OH according to the following formula, generating hydroxyl groups bonded to the surface as shown in Figure 1: 2Zr-O + HO → 2Zr-OH. The presence of these isolated Zr-OH groups 4 and hydrogen-bonded OH groups 3 improves the hydrophilicity of the ceramic spheres. Furthermore, by performing this heat treatment process after polishing, the surface condition of the ceramic spheres described above is maintained, making them more likely to exhibit hydrophilicity.

[0040] The temperature of the heat treatment step is preferably 100°C to 1300°C. By setting the temperature of the heat treatment step to 100°C or higher, the effect of imparting hydrophilicity to the ceramic spheres is enhanced. On the other hand, by setting the temperature of the heat treatment step to 1300°C or lower, an increase in surface roughness Sa and an increase in the amount of wear during grinding can be more effectively suppressed. However, at 300 to 600°C, the proportion of monoclinic crystals increases within the temperature range of 100°C to 1300°C, and durability tends to decrease. Therefore, the temperature range is more preferably 100°C to 200°C or 700°C to 1300°C.

[0041] The heat treatment is preferably carried out in the presence of water vapor, more preferably in the air, and more preferably in a high humidity atmosphere.

[0042] Furthermore, it is preferable to classify the sintered body by a classification step. The classification step allows the desired average particle size, minimum particle size, and maximum particle size to be achieved. Examples of classification methods include sieve classification, in which classification is performed using a mesh sieve. Sieve classification may be performed using two sieves stacked on top of each other to separate coarse powder with a relatively large particle size from fine powder with a relatively small particle size in a single operation.

[0043] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0044] [Measurement method] (1) Peak intensity ratio A / B by infrared spectroscopy The sample was packed into a container inside the FT-IR device, purged with nitrogen for 30 minutes, dried in the sample chamber, and then measured by infrared spectroscopy. The measurement conditions were as follows: Device: Varian 7000 (manufactured by Varian) Accessory: Diffuse reflectance measurement (Burns collector) Light source: Globar (SiC) Detector: DTGS Wavenumber resolution: 4 cm －１ Number of measurements: 256 Reference: Au vapor deposition film The measurement results were subjected to Kubelka-Munk conversion to obtain the wavenumber for hydrogen-bonding OH groups of 3300 to 3500 cm －１ The peak maximum (A) in the vicinity of 450 to 500 cm originates from the Zr—O bond. －１ Peak maximum value B1, 550-600 cm －１Peak maximum B2, 750-850 cm －１ The intensity ratio A / B to the sum of the maximum peak intensity B3 (B (= B1 + B2 + B3)) was calculated.

[0045] (2) Ratio of crystalline phases The sample was embedded in resin, cross-sectioned, and mirror-polished to prepare a measurement sample. This was attached to a sample holder and measured using wide-angle X-ray diffraction (micro-area X-ray diffraction). The measurement conditions were as follows: X-ray source: CuK-ray (using a multilayer mirror); Output: 50 kV, 22 mA; Slit system: 100 μmφ pinhole; Measurement range: 2θ = 27° to 33°, 70° to 77°; Accumulation time: 3600 seconds / frame.

[0046] From the measurement results, the content of each zirconia crystal phase was calculated using the following formula: Monoclinic content (%) = [{I ｍ (111) + I ｍ (1-1-1)} / {I ｍ (111) + I ｍ (1-1-1) + I ｔ＋ｃ (111)} × 100 Cubic crystal content (%) = [I ｔ＋ｃ (111) / {I ｍ (111) + I ｍ (1-1-1) + I ｔ＋ｃ (111)}] × [I ｃ (400) / {I ｃ (400) + I ｔ (400) + I ｔ (004)} × 100 Tetragonal content (%) = 100 - monoclinic content - cubic content Here, I indicates the diffraction intensity. The subscripts m, t, and c indicate monoclinic, cubic, and tetragonal crystals, respectively. The numbers in parentheses for the diffraction intensity indicate the Miller indices for each crystal.

[0047] (3) Surface Roughness Sa The surface roughness Sa was measured as follows. Using a laser microscope (Keyence VK-X-150) with a 150x objective lens, the digital zoom was adjusted so that the measurement range was X / 3 (μm) square, where X is the diameter of the ceramic spherical object to be measured, and the arithmetic mean height Sa of 10 spherical objects was measured with a laser. The average value of the 10 spherical objects was calculated and used as the surface roughness Sa.

[0048] (4) Water Permeation Rate The permeation rate was measured by the following method. The ceramic spheres to be measured were densely packed in a column made of an aluminum pipe with an outer diameter of 9 mm, an inner diameter of 7 mm, and a height of 200 mm, with a nylon mesh with an opening of 18 μm attached to the bottom. The column was placed in a vial with an inner diameter of 15 mm and a height of 65 mm containing 15 g of water, so that the bottom of the column was immersed in the water, and the weight of the column alone was measured every 30 seconds. The weight change from the measurement result was taken as the permeation weight of water, and the square of the permeation weight of water (W ２ ) and penetration time (t) (W ２ / t) was calculated by the least squares method and used as the penetration rate.

[0049] (5) Average particle size The particle size was measured using the following method. An image of an aggregate of ceramic spheres was taken with a digital microscope (Keyence VHX-2000) at a magnification of 10 to 200 times. Using image analysis and measurement software (Mitani Shoji Co., Ltd.'s "WinROOF" (registered trademark)), the image for measurement was binarized based on the brightness of the image. The binarized image was separated into circular shapes using the least mean square method, and the diameter of each separated circle was calculated as the diameter of each ceramic sphere. The number average value of the diameters of 1,000 ceramic spheres was taken as the average particle size X.

[0050] (6) Floating Amount of Ceramic Spherical Objects The floating amount of ceramic spherical objects was measured using the following method. 20 ml of water was placed in a disposable cup with an inner diameter of 52 mm and a height of 70 mm, and 10 g of the ceramic spherical objects obtained in the Examples and Comparative Examples were added from above at a rate of 1 g / s. The cup was then stirred for 30 seconds, and the floating beads were scooped up with a spoon, dried, and weighed. The evaluation results were ranked as follows: 0.00 to 0.20 g: "1," 0.21 to 0.40 g: "2," 0.41 to 0.60 g: "3," and 0.61 g or more: "4." The lower the number, the higher the hydrophilicity.

[0051] (7) Abrasion Wear Volume To evaluate the abrasion wear volume of the ceramic spheres obtained in the examples and comparative examples, the abrasion wear volume during abrasion operation using only beads and solvent was measured using the following method. 121 g of ceramic spheres were placed in a microbead-compatible bead mill (PCM-LR, manufactured by Asada Iron Works Co., Ltd.), and 1 L of pure water was circulated at a flow rate of 200 cc / min while stirring at a peripheral speed of 12 m / s. After stirring for 2 hours, the ceramic spheres were removed, dried, and their weights (Z [g]) were measured. The abrasion volume was calculated by subtracting Z g from the original weight (121 g). The evaluation results were ranked as follows: 0.00 to 0.50 g: "1," 0.51 to 1.00 g: "2," 1.01 to 1.50 g: "3," and 1.51 to 2.00 g: "4." The smaller the number, the less abrasion volume.

[0052] (8) Crushing Load The crushing load was measured as an index of the durability of the ceramic spheres against cracking and breakage using the following method. A compressive load was applied to one ceramic sphere at a loading rate of 5.0 gf / sec using a microcompression tester (MCT-510 manufactured by Shimadzu Corporation), and the load value at which it broke was measured. Measurements were conducted on 30 ceramic spheres, and the average value was used. The higher this value, the less risk there is of cracking or breakage in the media that can be used.

[0053] (9) Monoclinic Ratio After Hydrothermal Testing As an indicator of the hydrothermal degradation resistance of ceramic spheres, the monoclinic ratio after an accelerated pressure cooker test (PCT) was measured using the following method. Approximately 10 g of ceramic spheres was filled into a crucible, and the crucible was placed in a PCT apparatus (Hirayama Seisakusho's "HIRAYAMA" PC-242HS). Treatment was then carried out for 72 consecutive hours at a temperature of 121°C and humidity of 100%. After completion of the treatment, the ceramic spheres were removed and naturally dried to remove residual moisture. The monoclinic content was calculated using the same procedure as described in (2) above. A lower monoclinic ratio indicates higher hydrothermal degradation resistance, and thus provides high durability for long-term use as dispersion media.

[0054] (10) Crushing Load after Hydrothermal Testing As an index of the durability of the ceramic spheres after the accelerated hydrothermal degradation test, the crushing load after the hydrothermal test was measured by the method described in (8) above. The higher this value, the more suitable the ceramic spheres are for use as media with a low risk of cracking or breakage due to hydrothermal degradation, even after long-term use.

[0055] Example 1 Yttrium chloride was added to zirconium oxychloride so that the mass ratio of yttria / zirconia in terms of oxides in the resulting ceramic spherical bodies was 4.9 / 95.1, and a raw material powder was prepared by coprecipitation.

[0056] Next, the raw material powder was granulated by a rolling granulation molding method to form a compact having an average particle size X of about 30 μm after sintering.

[0057] The compact thus obtained was dried and then fired at 1400°C for 2 hours to obtain an intermediate sintered body (sintering process). The intermediate sintered body was then subjected to HIP treatment at 1380°C and 120 MPa for 1.5 hours (hot isostatic pressing process). The surface of the obtained sintered body was polished for 10 hours using a wet bead mill polishing machine with an abrasive at a peripheral speed of 14 m / s, followed by sieve classification. The temperature was then increased at 100°C / h in an air atmosphere and heat-treated at 100°C for 1 hour to produce the ceramic spheres shown in Table 1.

[0058] Examples 2 to 4, 6 to 14, Comparative Examples 1 to 6, 11 to 18 Ceramic spheres were prepared in the same manner as in Example 1, except that the heat treatment temperature, yttria / zirconia ratio, and average particle size were changed as shown in Table 1.

[0059] [Example 5] After sieve classification, ceramic spheres were produced in the same manner as in Example 1, except that they were heated at 100°C / h in a tube furnace while flowing nitrogen at 1 atmosphere, and then heat-treated at 1000°C for 1 hour.

[0060] [Comparative Examples 7 and 8] Ceramic spheres were prepared in the same manner as in Example 1, except that they were heat-treated at 100°C for 1 hour and then left to stand in water at 98°C for 48 hours to cause a phase transition of part of the tetragonal crystals of the ceramic spheres to monoclinic crystals.

[0061] Comparative Examples 9 and 10 Ceramic spherical bodies were produced in the same manner as in Example 1, except that in the polishing step, instead of bead mill polishing, a wet barrel polishing machine was used to polish the surface for 6 hours.

[0062] The evaluation results are shown in Tables 2 and 3.

[0063]

[0064]

[0065]

[0066] As shown in Examples 1 to 14, by increasing the A / B ratio to improve hydrophilicity, ceramic spherical bodies that do not easily float in water were obtained. For reference, the 2500 to 4000 cm of Examples 2 and 5 and Comparative Example 1 were obtained. －１ The results of infrared spectroscopy are shown in FIG.

[0067] In Comparative Examples 1, 11, and 15, which were not heat-treated, the small A / B ratio resulted in fewer OH groups on the ceramic sphere surface, and a larger amount of OH groups floated in water. In Comparative Examples 2, 3, 7, 8, 12, 13, 16, and 17, the large A / B ratio resulted in more OH groups on the ceramic sphere surface, and as a result, the amount of OH groups floating in water was smaller, but the higher monoclinic crystal ratio resulted in a larger amount of wear. In Comparative Examples 4, 9, 10, 14, and 18, the large (A / B) ratio and low monoclinic crystal ratio resulted in an increased surface roughness, resulting in a larger amount of wear.

[0068] Furthermore, in Examples 1 to 2, 7 to 8, and 11 to 12, in which the heat treatment temperature was 200°C or less, the monoclinic ratio after the hydrothermal test (accelerated test at a temperature of 121°C, humidity of 100%, and for 72 hours) was also kept to a low range of 9.0% or less. As a result, the crushing load value after the hydrothermal test was generally maintained at the same level as before the test, and these media can be suitably used as media with high reliability for long-term use.

[0069] 1. Adsorbed water 2. Hydrogen bond 3. Hydrogen-bonded OH group 4. Isolated Zr-OH group

Claims

1. A ceramic sphere mainly composed of zirconia, The proportion of monoclinic crystals is 3.0% by volume or less, The proportion of tetragonal crystals is 80% by volume or more and 95% by volume or less, Surface roughness Sa is 0.005 μm or more and 0.015 μm or less, 3200 to 3400 cm originating from OH groups on the surface of the ceramic spheres －１ and the peak maximum (A) at 450 to 500 cm due to the Zr—O bond. －１ , 550-600cm －１ and 750-850 cm －１ a ratio (A / B) of the sum (B) of the maximum peak values ​​of the ceramic spheres (A, B) to the sum (B) of the maximum peak values ​​of the ceramic spheres (A, B) is 0.07 or more and 0.30 or less, and the proportion of monoclinic crystals after a pressure cooker test (PCT) is carried out for 72 hours under conditions of a temperature of 121°C and a humidity of 100%, is 9.0% by volume or less.

2. The water penetration rate by the penetration rate method is 2.0 x 10 －４ g ２ / s or more 80×10 －４ g ２ 2. The ceramic sphere according to claim 1, wherein the pore size is 1 / s or less.

3. 3. The ceramic sphere according to claim 1, wherein the average particle size is 10 μm or more and 150 μm or less.

4. 3. A method for producing the ceramic spherical body according to claim 1, comprising the steps of: polishing the surface of a spherical ceramic whose main component is zirconia to obtain a polished spherical ceramic; and heat treating the polished spherical ceramic at 100°C to 1300°C, in that order; wherein the surface roughness Sa of the polished spherical ceramic is 0.005 μm or more and 0.015 μm or less.

5. The method for producing ceramic spheres according to claim 4, wherein the heat treatment step is carried out in an air atmosphere.

6. 5. The method for producing ceramic spheres according to claim 4, wherein the polishing step involves polishing using a bead mill.