FINE COAGULATED PARTICLES OF ULTRAFINE MONOCLINIC ZIRCONIA CRYSTALS ORIENTED IN A FIBER BUNDLE-LIKE FORM AND METHOD OF MANUFACTURING THEM

Abstract: Coagulated particles of ultrafine monoclinic zirconia crystals oriented in fiber bundle-like form, wherein primary particles of ultrafine monoclinic zirconia are oriented to join and extended in the direction of C-axis into fine fibrous form of less than 50 Å diameter and they are further gathered into a bundle-like form having from 300 to 2000 Å of width and from 2000 to 10000 Å of length. Such particles are prepared by hydrothermal processing which is applied at a temperature from 120 to 300°C to an acidic aqueous solution containing a water-soluble zirconium compound in an amount of from 0.1 to 2.0 mol/ℓ as Zr, SO₄²⁻ in an amount of from 0.2 to 2.0 mol/ℓ and a water-soluble compound selected from the group consisting of water-soluble magnesium compounds, water-soluble ammonium compounds and mixtures thereof in an amount of from 0.05 to 1.5 mol/ℓ as Mg²⁺ or NH₄.

Description: BACKGROUND OF THE INVENTION This invention concerns coagulated particles of ultrafine monoclinic zirconia crystals oriented in the form of fiber-bundles and a method for their manufacture. More specifically, it relates to coagulated particles of ultrafine monoclinic zirconia crystals in which primary particles of ultrafine monoclinic zirconia crystals oriented in the direction of their C-axis to join into fibers of less than 5 nm (50Å) diameter are gathered to bundles having a width of from 30 to 200 nm (300 to 2000 Å)and a length of from 200 to 1000 nm (2000 to 10000 Å), as well as a method for their manufacture. Since fine anisotropic particles extended in one direction are easily oriented by the external force or flowing in the molding step, they are applied to various kinds of oriented ceramics. Anisotropic zirconia particles are considered essential for providing orientation in the texture of ceramics using zirconia starting material, partially stabilized zirconia or piezoelectric or ferroelectric materials. The present inventor has already filed patent applications regarding anisotropic zirconia particles intended for such a purpose. One of the patent applications concerns oriented coagulated particles of zirconia type crystals prepared by subjecting an aqueous solution of a zirconium salt or a liquid suspension mixture of the aqueous solution with zirconium hydroxide to a heat treatment under an acidic condition of lower than pH 7 to form fine monoclinic zirconia crystals, then subjecting the fine crystals to a heat treatment in an aqueous strong alkali solution such as of KOH in the presence of zirconium hydroxide at a temperature of higher than 90°C and then preferentially growing the zirconia crystals in a specific direction (refer to Japanese Patent Laid-Open No. 180917/1985). Another of the patent applications concerns a method of preparing a sol of flaky crystalline zirconia by subjecting a water soluble zirconium salt or ultrafine crystalline zirconia obtained through hydrolysis of the salt under heating to a heat treatment in an aqueous solution containing hydrochloric acid or nitric acid at a high concentration of greater than 5N at a temperature from 120 to 250°C for more than 50 hours (refer to Japanese Patent Laid-Open No. 201622/1986). A further patent application concerns a method of preparing flaky zirconia type fine crystals as a layered structure of less than 50 nm (500 Å) thickness containing metal ions mainly composed of Zr and sulfate ions (refer to EP-A-0207 469). The foregoing patent applications disclose anisotropic particles respectively having features different from each other and the present inventor has continued a further study and accomplished this invention based on the finding for the method of manufacturing novel fine coagulated particles of monoclinic ultrafine zirconia crystals oriented in a fiber bundle-like form. SUMMARY OF THE INVENTION It is, accordingly, an object of this invention to provide coagulated particles of ultrafine monoclinic zirconia crystals oriented in the form of fiber-bundles, which are useful as a starting material for oriented ceramics, and a method for the manufacutre of such anisotropic coagulated zirconia particles in the form of fiber-bundles. As a result of extensive studies, the present inventor has found that novel and useful coagulated particles of ultrafine monoclimc zirconia crystals oriented in the form of fiber-bundles having a width of from 30 to 200 nm (300 to 2000 Å) and a length of from 200 to 1000 nm (2000 to 10000 Å) comprising fine fibers having a diameter of less than 5 nm (50 Å) of primary particles of ultrafine monoclinic zirconia joined and extended in the direction of their C-axis are obtained by subjecting an acidic aqueous solution containing a water-soluble zirconium compound in an amount of from 0.1 to 2.0 mol/l as Zr, SO₄ 2- in an amount of from 0.1 to 2.0 mol/l and a water-soluble compound selected from the group consisting of water-soluble magnesium compounds, water-soluble ammonium compounds ansd mixtures thereof in an amount of from 0.05 to 1.5 mol/l as Mg² + or NH4⁺, wherein the SO₄² - /Mg² + or NH4 + molar ratio is 0.1 to 20 to hydrothermal processing at a temperature of from 120° to 300°C. Since the anisotropic coagulated zirconia particles have not only a general aspect as oriented fine particles but also of fiber-bundles they are easily mixed with other ceramic materials and extremely reactive and are excellent as the starting material for orienting the textures of ceramics using zirconia as the starting material, partially stabilized zirconia or piezoelectric or ferroelectric materials. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS These and other objects, as well as features of this invention will become more apparent by reading the preferred embodiments of the invention while referring to the accompanying drawings, in which Figures 1 and 2 are explanatory views illustrating the profile of two typical types of coagulated fine particles of ultrafine monoclinic zirconia crystals oriented into fiber bundle-like form according to this invention. DETAILED DESCRIPTION OF THE INVENTION The fine coagulated zirconia particles according to this invention are characterized in that primary particles of ultrafine monoclinic zirconia crystals are oriented to join and extended in the direction of C-axis into fine fibrous form of less than 5 nm (50 Å) diameter, which are further gathered into a bundle-like form of from 30 to 200 nm (300 to 2000 Å) width and from 200 to 1000 nm (2000 to 10000 Å) length. Figs. 1 and 2 show examples of the coagulated fine zirconia particles having a fiber bundle-like form of this invention. It should be noted that the width means that of the narrowest portion "w" of the bundle-like zirconia particle in this invention. In the bundle-like zirconia particle as shown in Fig. 1, the width of the widest portion "W" is generally in a range of from 30 to 250 nm (300 to 2500 Å). The ratio of the width (the narrowest width) and the length of the bundle-like zirconia particles is generally in a range of from 1:2 to 1:20, particularly from 1:5 to 1:10. The fine coagulated particles of ultrafine monoclinic zirconia crystals oriented in a fiber bundle-like form can be synthesized by applying a hydrothermal processing to an acidic aqueous solution containing a water-soluble zirconium compound, SO 2- / 4 and a water-soluble salt selected from the group consisting of water-soluble magnesium salts, water-soluble ammonium salts and mixtures thereof. The water-soluble zirconium compound usable in this invention can include zirconium oxychloride, zirconium hydroxide, zirconium carbonate, zirconium nitrate and zirconium acetate. The zirconium compound may be used alone or in admixture of two or more. The amount of the zirconium compound should be in a range of from 0.1 to 2 mol/ℓ, preferably from 0.2 to 1 mol/ℓ as Zr. The acidic aqueous solution also contains SO₄²⁻ as an essential component. SO₄²⁻ is usually provided from sulfuric acid. A water-soluble sulfate including magnesium sulfate, ammonium sulfate and the like is also used for providing SO₄²⁻. The amount of SO₄²⁻ should be in a range of from 0.2 to 2 mol/ℓ, preferably from 0.6 to 1.5 mol/ℓ. In this case, the amount of sulfuric acid is less than 3 molar times of Zr because it gives better results. The water-soluble magnesium compound usable in this invention includes magnesium sulfate, magnesium hydroxide, magnesium carbonate and the like. Among them, magnesium sulfate is most preferred because it gives better results. The water-soluble ammonium compound includes ammonium sulfate, ammonium hydroxide, ammonium carbonate and the like. Among them, ammonium sulfate is most preferred. The combined use of the magnesium compound and the ammonium compound is also possible. The amount of the magnesium compound or the ammonium compound must be in a range of from 0.05 to 1.5 mol/ℓ, preferably 0.1 ∼ 1 mol/ℓ as Mg²⁺ or NH₄⁺. In this case, Mg²⁺ or NH₄⁺ should have a concentration so that SO₄²⁻ is from 0.1 to 20 molar times, more particularly from 1 to 10 molar times of Mg²⁺ or NH₄⁺. While the coexistence of SO₄²⁻ and Mg²⁺ or NH₄⁺ is indispensable, if the amount of Mg²⁺ or NH₄⁺ is excessive relative to SO₄²⁻, no bundle-like form can be obtained although monoclinic zirconia can be attained. If the amount of Mg²⁺ or NH₄⁺ is insufficient, incorporation of the layered compound containing zirconia sulfate is increased. The hydrothermal processing is carried out at a temperature of from 120 to 300°C, preferably 140 to 230°C and preferably in a closed container. The treating time may be in a range of 10 hours to 14 days, preferably 1 to 5 days. The fiber bundle-like coagulated zirconia particles obtained in this invention can be separated by a usual method of separating the crystals by way of filtration and centrifugal separation, followed by drying. The thus obtained powder can be used as the starting material for orienting the textures of ceramics using zirconia as the starting material, partially stabilized zirconia or piezoelectric or ferroelectric materials. The present invention will further be explained more specifically referring to examples but the invention is no way limited to the following examples. Example 1 16 g of zirconium oxychloride (ZrOCl₂·8H₂O) of reagent grade was dissolved into water and about 10 ml of concentrated aqueous ammonia were added. The resultant precipitates of hydroxides are filtered out, washed with water for once and then the precipitates were completely dissolved by adding 10 g of 36N sulfuric acid in a state where NH₄OH still remained in the precipitate. The total amount of the liquid was about 200 ml and contained Zr at about 0.25 mol/ℓ concentration, SO₄²⁻ at about 0.5 mol/ℓ concentration and NH₄⁺ at about 0.1 mol/ℓ concentration. The solution was tightly closed in a vessel made of polytetrafluoroethylene, heated to 150°C in an autoclave and applied with heat treatment while being maintained for two days to obtain white precipitates. When applying powder X-ray diffractometry for the separated and dried precipitates, it was found that all of the primary particles of ultrafine monoclinic zirconia crystals which were oriented to join and extended in the direction of C-axis into fine fibrous form of about 5 nm (50 Å) diameter and were coagulated or gathered in a bundle-like form, as shown in Figure 1, having a width of the narrowest portion of about 100 nm (1000 Å) and a length of about 800 nm (8000 Å). Example 2 Zirconyl carbonate of reagent grade and sulfuric acid were mixed at about 1:2 molar ratio to prepare an aqueous solution containing Zr at a concentration of about 1 mol/ℓ. According to the chemical analysis, the ratio of Zr:SO₄²⁻ was 1:1.8. Aqueous ammonia was added to the aqueous solution to prepare five sample aqueous solutions each containing Zr at about 0.5 mol/ℓ, and NH₄⁺ at about 0.1, 0.2, 0.4, 0.6 and 0.8 mol/ℓ respectively. These solutions were tightly enclosed in a vessel made of polytetrafluoroethylene and subjected to a heat treatment at 200°C for 5 days to obtain white precipitates respectively. All of them were coagulated particles of fine monoclinic zirconia crystals having less than 5 nm (50 Å) diameter oriented in the form of a fiber-bundle, in which the coagulated particles were as shown in Figure 2 in the case of the solutions containing NH₄⁺ at 0.1, 0.2 and 0.4 mol/ℓ with a width of about 70 nm (700 Å) and a length of about 500 nm (5000 Å), whereas they were somewhat larger particles as shown in Figure 1 in the case of containing NH₄⁺ at 0.6 and 0.8 mol/ℓ with a width of about 100 nm (1000 Å) and a length of about 750 nm (7500 Å). Example 3 When an aqueous solution prepared by adding Zr(OH)₄, (NH₄)₂SO₄ and H₂SO₄ at 0.25 mol/ℓ, 0.4 mol/ℓ and 0.3 mol/ℓ respectively was subjected to a heat treatment in an autoclave at 200°C for 2 days in the same manner as in Example 2, the products were fine bundle-like coagulated zirconia particles having a width of about 60 nm (600 Å) and a length of about 250 nm (2500 Å) as shown in Figure 2. Example 4 When an aqueous solution prepared by adding Zr(OH)₄, MgSO₄ and H₂SO₄ at a ratio of 0.5 mol/ℓ, 0.4 mol/ℓ and 1.5 mol/ℓ respectively was subjected to a heat treatment in an autoclave at 200°C for 3 hours in the same manner as in Example 3, the products were fine bundle-like coagulated zirconia particles having a width of about 80 nm (800 Å) of width and a length of about 500 nm (5000 Å) as shown in Figure 2. The primary monoclinic zirconia crystals of the bundle-like coagulated zirconia particles of Examples 3 and 4 have diameters of less than 5 nm (50 Å).

Claims: Coagulated particles of ultrafine monoclinic zirconia crystals oriented in the form of fiber-bundles having a width of from 30 to 200 nm (300 to 2000 Å) and a length of from 200 to 1000 nm (2000 to 10000 Å) comprising fine fibers having a diameter of less than 5 nm (50 Å) of primary particles of ultrafine monoclinic zirconia joined and extended in the direction of their C-axis. A method for manufacturing the coagulated particles according to claim 1, characterized by subjecting an acidic aqueous solution containing a water-soluble zirconium compound in an amount of from 0.1 to 2.0 mol/l as Zr, SO₄² - in an amount of from 0.2 to 2.0 mol/l and a water-soluble compound selected from the group consisting of water-soluble magnesium compounds, water-soluble ammonium compounds and mixtures thereof in an amount of from 0.05 to 1.5 mol/l as Mg² + or NH₄ + , wherein the SO₄² - /Mg² + or NH₄ + molar ratio is 0.1 to 20, to hydrothermal processing at a temperature of from 120° to 300°C.

Inventors: KATO, ETSURO
Applicants: NISSAN CHEMICAL INDUSTRIES LTD.
Application year: 1986
Main classification: C01G25
IPC codes: C04B35, C01G25