ZIRCONIA SINTERED BODY OF IMPROVED LIGHT TRANSMITTANCE

Abstract: A zirconia sintered body comprising zirconia (ZrO₂), yttria (Y₂O₃) and titania (TiO₂) [Y₂O₃/ZrO₂ = at least 2/98 by mole and TiO₂/(ZrO₂ + Y₂O₃) = 3/97 to 20/80 by mole] has a high translucence giving improved light transmittance. A zirconia sintered body comprising, in addition to ZrO₂, Y₂O₃ and TiO₂, 0.1 to 3 mole% of an oxide of a rare earth element of the lanthanum group has a fluorescence-radiating property as well as a high translucence.

Description: The present invention relates to process for preparing a zirconia sintered body having an excellent translucence giving improved light transmittance. As the translucent zirconia sintered body, a sintered body of the ZrO₂-Y₂O₃ system and a sintered body of the ZrO₂-CaO system are reported in Journal of the American Ceramic Society, 50 , page 532 (1967) and Journal of Less-Common Metals, 13 , page 530 (1967), respectively. However, the light transmittance of each of these sintered bodies is about 10% and they cannot be regarded as truly translucent materials. It is a primary object of the invention to provide a process for preparing a zirconia sintered body having a high light transmittance, which can be used as an optical material. It is considered that, for imparting a translucence to ceramics, a reduction of the scattering of light by pores by increasing the density of a sintered body, and a reduction of the scattering of light by the grain boundary by increasing the particle size of a sintered body, are effective. We carried out research into ZrO₂-Y₂O₃ sintered bodies and found that if TiO₂ is added to the ZrO₂-Y₂O₃ system, the growth of particles in the sintered body is remarkably promoted. However, a remarkable growth of particles generally results because the speed of growth of the particles is higher than the speed of displacement of the pores, and therefore, pores are left in the particles and it is difficult to obtain a dense sintered body. With a view to overcoming this disadvantage, we investigated the properties of the starting material powder and the sintering conditions, and as a result, found that if TiO₂ is added to the ZrO₂-Y₂O₃ system, a remarkable growth of particles and a complete densification can be attained without any adverse effect. Based on this finding, we succeeded in providing a zirconia sintered body having a high translucence. Japanese Patent Specification A59148002 shows ceramic compositions containing 8 wt% yttrium oxide, 8 wt% titanium oxide, 84 wt% zirconium oxide and 18 wt% yttrium oxide, 30 wt% titanium oxide, 52 wt% zirconium oxide. We have found that translucent zirconia sintered shaped articles comprising zirconia, yttria and titania can be further improved. Thus according to the present invention a process for preparing a translucent zirconia sintered body comprising zirconia (ZrO₂), yttria (Y₂O₃) and titania (TiO₂), the molar ratio of Y₂O₃/ZrO₂ being at least 2/98 preferably at least 6/94, more preferably 6/94 to 9/91, and the molar ratio of TiO₂/(ZrO₂ + Y₂O₃) being in the range of from 3/97 to 20/80, preferably 5/95 to 20/80, is characterised in that a shaped article comprising ZrO₂, Y₂O₃ and TiO₂ in the above-mentioned proportions is sintered and then subjected to a hot isostatic pressing treatment, preferably at a temperature of 1400°C to 1700°C in an argon atmosphere. The article is then preferably oxidised in air or oxygen preferably at a temperature of at least 800°C. The sintering is preferably carried out at a temperature of at least 1,400°C, e.g. 1600°C to 1800°C. For a better understanding of the present invention, various preferred embodiments will be described with reference to the accompanying drawings, in which: Figure 1 is a graph showing the visible ray transmittance in Test Example A; and Fig. 2 is a graph showing the visible ray transmittance of sintered body sample No. 14 in Test Example C. The zirconia sintered body of the ZrO₂-Y₂O₃-TiO₂ system which contains 3 to 20 mole% of TiO₂ is composed of a single phase of the tetragonal system at a Y₂O₃ content of about 2 mole% (based on the sum of ZrO₂ + Y₂O₃) and is composed of a single phase of the cubic system at a Y₂O₃ content higher than 6 mole%. When the Y₂O₃ content is intermediate between the above-mentioned two values, the sintered body is composed of a mixed phase of the tetragonal and cubic systems. Since the sintered body composed of a single phase of the cubic system is optically isotropic, there is no scattering by the grain boundary and an optimal translucence can be obtained. It is not necessary for Y₂O₃ to be included in an amount exceeding 9 mole% based on the sum of ZrO₂ and Y₂O₃. The particle size of the sintered body increases with an increase of the amount of TiO₂ added. For example, where sintering is carried out at 1,700°C for 2 hours, the particle size of the sintered body is 50 to 100 µm at a TiO₂ content of 5 mole% and 100 to 200 µm at a TiO₂ content of 10 mole%. The light transmittance tends to increase with an increase of the particle size. If, however, TiO₂ content exceeds 20 mole%, another compound ZrTiO₄ is formed as the second phase and the light transmittance is drastically degraded. Accordingly, in order to obtain a sufficiently high light transmittance, it is most preferable that the TiO₂ content should be 3 to 20 mole%, preferably 5 to 20 mole%, based on the total of ZrO₂, TiO₂ and Y₂O₃. A zirconia sintered body containing a minute amount of an oxide of a rare earth element of the lanthanum group, known from the background art as coloring agent, in addition to at least 6 mole% of Y₂O₃ (based on the sum of ZrO₂ + Y₂O₃) and 3 to 20 mole% of TiO₂ (based on the sum of ZrO₂ , Y₂O₃ + TiO₂) has a fluorescence-radiating property in addition to translucence. The content of the oxide of the rare earth element of the lanthanum group is preferably 0.1 to 3 mole% based on the total composition, and in order to obtain an optimal fluorescence-radiating property, a content of about 1 mole% is preferred. As the oxide of the rare earth element of the lanthanum group, there can be mentioned, for example, Nd₂O₃ , Eu₂O₃ , and Tb₂O₃. A process for the preparation of the sintered body of the present invention will now be described in detail, by way of example. Preferably, the starting material is a fine powder having a high purity. For example, a composition formed by sufficiently mixing a fine powder of the ZrO₂-Y₂O₃ system, having a particle size not larger than 0.1 µm and synthesized by the wet method, with a TiO₂ fine powder having a particle size not larger than 0.5 µm may be used. A solid solution obtained by sufficiently mixing a powder of the ZrO₂-Y₂O₃ system with a TiO₂ powder, calcining the mixture and pulverizing the calcined composition to an average particle sizes not larger than 0.3 µm, is especially preferred. Moreover, a fine powder of the ZrO₂-Y₂O₃-TiO₂ system having an average particle size not larger than 0.3 µm, which is prepared by the co-precipitation method, and a product having an average particle size not larger than 0.3 µm, which is obtained by mixing a powder of the ZrO₂-Y₂O₃ system with a solution of a titanium alkoxide, drying and calcining the mixture, and pulverizing the calcined composition, are preferably used. Incorporation of the oxide of the rare earth element of the lanthanum group may be accomplished by adding an oxalate or the like of the rare earth element to a powder precursor of the ZrO₂-Y₂O₃-TiO₂ system before calcination and then calcining the mixture. A powder as mentioned above is molded to a predetermined shape by a rubber press moulding method, a slip cast moulding method or the like and the molded body is then sintered. The sintering temperature may be at least 1,400°C, and in order to attain a sufficient growth of the particles, preferably the sintering temperature is 1,600 to 1,800°C. Also, preferably the temperature-elevating rate is not higher than 100°C/hr. The sintering atmosphere may be air, but in order to obtain a higher translucence, preferably oxygen is used. The as-sintered body has a high translucence, but in accordance with the present invention in order to further improve the translucence, the sintered body is subjected to hot isostatic pressing. For the treatment conditions, as mentioned above, preferably argon is used as the pressure medium and the temperature is 1,400 to 1,700°C. The pressure is preferably at least 500 atmospheres. Since the sintered body is reduced by this treatment, the sintered body is black. Accordingly, it is desirable that the original order should be restored by oxidizing this black sintered body in air or oxygen. The treatment temperature is preferably at least 800°C, more preferably at least 1,000°C, but the treatment temperature need not be elevated above 1,200°C. The thus-obtained sintered body has a density higher than 99% of the theoretical value and exhibits a high transmittance to rays of the visible-to-infrared region having a wavelength of 350 to 7,000 nm. Furthermore, the sintered body comprising an oxide of a rare earth element of the lanthanum group emits a fluorescence, for example, a red fluorescence in the case of Eu₂O₃ or a green fluorescence in the case of Tb₂O₃ , under irradiation with an ultraviolet ray. The sintered body of the present invention has an excellent translucence and a high refractive index, and the sintered body of the present invention is advantageous in that a large product or a product having a complicated configuration can be easily obtained. The sintered body containing an oxide of a rare earth element of the lanthanum group can be used as a fluorescence-radiating transparent body. Accordingly, the sintered body of the present invention may be utilized as a light-transmitting material for which a heat resistance and heat insulating property are required, for example, as a material of a window of a furnace, a covering tube of a heat generating member or a protecting tube of a lamp, an infrared ray-transmitting material for an infrared ray lens or a window of an infrared a fluorescent display material, a solid oscillating material or a ultraviolet ray dosemeter, and a decorative material for watches or jewelry. The present invention will now be described with reference to the following examples. Powder Production Examples 1 through 3 A mixed aqueous solution of zirconium oxychloride and yttrium chloride was boiled to effect hydrolysis. The obtained sol was dried, calcined at 900°C and pulverized to obtain a fine powder of zirconia containing Y₂O₃. This powder and a solution of titanium isopropoxide were incorporated in ethanol, wet mixing was carried out, and the mixture was dried under reduced pressure, calcined at 950°C and pulverized to obtain a fine powder of the ZrO₂-Y₂O₃-TiO₂ system. Powder Production Examples 4 through 6 A fine powder of zirconia containing Y₂O₃ , which was obtained in the same manner as described in Powder Production Examples 1 through 3, was wet-mixed with a fine powder of titania in ethanol, and the mixture was dried, calcined at 900°c and pulverized to obtain a fine powder of the ZrO₂-Y₂O₃-TiO₂ system. Powder Production Example 7 A solution of titanium isopropoxide was added to a mixed aqueous solution of zirconium oxychloride and yttrium chloride, and the mixture was heated for 3 days to effect hydrolysis. The formed sol was dried, calcined at 920°C and pulverised to obtain a fine powder of the ZrO₂-Y₂O₃-TiO₂ system. Powder Production Examples 8 through 13 A zirconia power was obtained in the same manner as described in Powder Production Examples 1 through 3. Separately, water was added to a solution of titanium isopropoxide to obtain a hydrolysis product of hydrous titania. The above-mentioned zirconia powder was wet-mixed with the hydrolysis product of the hydrous titania, and the mixture was dried, calcined at 1,000°C for 2 hours and pulverized to obtain a fine powder of the ZrO₂-Y₂O₃-TiO₂ system. Powder Production Examples 14 through 18 Neodymium oxalate was added to a mixture of a zirconia powder and a hydrolysis product of hydrous titania, which mixture was obtained in the same manner as described in Powder Production Examples 8 through 13, and the mixture was wet-pulverized, dried and calcined at 1,000°C for 1 hour. The obtained calcined powder was pulverized to obtain a fine powder of the ZrO₂-Y₂O₃-TiO₂ system in which Nd₂O₃ was solid-dissolved. The compositions and primary particle sizes of the thus-obtained fine powders are shown in Table 1. Sintering Example A Each of the powders obtained in Powder Production Examples 1 through 7 was moulded into a disc by using a mold and a rubber press. The moulded body was placed in a tubular furnace, and the temperature was elevated at a rate of 40°C/hr in an oxygen current, maintained at 1,700°C for 2 hours and then lowered. The thus-obtained sintered body was placed in a hot isostatic press and treated under 1,000 atmospheres at 1,500°C for 30 minutes by using argon as the pressure medium. The treated sintered body was black. Accordingly, the sintered body was placed in the tubular furnace again and was maintained at 1,200°C for 4 hours in an oxygen current. The sintered body was recovered after cooling. Each of the obtained sintered bodies has a very high translucence. When the crystal phase was determined by X-ray diffractometry, it was found that the sintered body obtained from the powder of Powder Production Example 4 was composed of a single phase of the tetragonal system but each of the other sintered bodies was composed of a single phase of the cubic system. Test Example A The light transmittance of some of the sintered bodies obtained in sintering Example A was measured. A sample having a thickness of 0.5 to 1.5 mm and having both surfaces mirror-polished was used for the test. The obtained results are shown in Fig. 1. Reference numerals 1, 2, and 3 in Fig. 1 show the results of the sintered products obtained from the powders of Powder Production Examples 2, 5, and 3, respectively. From Fig. 1, it is seen that in the sintered body of the present invention, the transmittance in the visible ray region was higher than 40% at a thickness of 1 mm. Test Example A' In order to examine the effect of TiO₂ , disc-shaped sintered bodies having a thickness of 1 mm were obtained in the same manner as described in Test Example A by using a TiO₂-containing powder (Powder Production Example 2) and a TiO₂-free powder (zirconia containing 8 mole% of Y₂O₃). The TiO₂-containing sample had a high translucence but the TiO₂-free sample was white. When the transmittance in the visible ray region was measured according to the method described in Test Example A, it was found that the light transmittance of the former sample was about 40% but the light transmittance of the latter sample was about 7%. The textures of the sintered bodies of these two samples were observed by a scanning type electron microscope. It was found that, by the addition of TiO₂ , the particle size was increased to about 200 µm and an extreme growth of particles was attained, and the porosity was remarkably reduced. This proves that the addition of TiO₂ is very important for increasing the translucence. Sintering Example B Each of the powders obtained in Powder Production Examples 8 through 13 was moulded into a plate by using a mold and a rubber press. The moulded body was placed in a tubular furnace, and the temperature was elevated at a rate of 50°C/hr in an oxygen current, maintained at 1,500 to 1,750°C for 2 hours and then lowered. Each of the obtained sintered bodies had a high translucence. Each sintered body was cut into halves and the cut pieces were treated under 1,000 atmospheres at 1,500 to 1,700°C in a hot isostatic press (HIP) device by using argon as the pressure medium. Since all of the treated sintered bodies were black, each treated sintered body was placed in the tubular furnace again and maintained at 1,200°C for 2 hours in an oxygen current. All of the sintered bodies recovered after cooling had a very high translucence. The crystal phase of each of the obtained sintered bodies was a cubic phase. The particle sizes and densities of these sintered bodies are shown in Table 2. Test Example B With respect to the sintered bodies obtained in Sintering Example B, the transmittances to visible and infrared rays were measured. A sample having a thickness of 0.8 mm and having both the surfaces mirror-polishing was used for the test. Each sintered body had a good transmittance to rays of the visible-to-infrared region having a wavelength of 0.35 to 7 µm. As the typical value, a linear transmittance to a ray having a wavelength of 0.6 µm is shown in Table 2. Sintering Example C Sintered bodies were prepared in the same manner as described in Sintering Example B by using the zirconia, powders obtained in Powder Production Examples 14 through 18. The crystal phase of each of the obtained sintered bodies was a cubic phase. The particle sizes and densities of the obtained sintered bodies are shown in Table 3. Test Example C With respect to the sintered bodies obtained in Sintering Example C, the transmittances to visible and infrared rays were measured. A sample having a thickness of 0.8 mm and having both the surfaces mirror-polished was used for the test. Each sample had a good transmittance to rays of the visible-to-infrared region having a wavelength of 0.35 to 7 µm. The visible ray transmittance of sintered body sample No. 14 is shown in Fig. 2. Under irradiation with an ultraviolet ray having a wavelength of 0.26 µm, sintered body samples Nos. 13 and 16 emitted a red fluorescence and sintered body sample No. 15 emitted a green fluorescence.

Claims: A process for preparing a translucent zirconia sintered body comprising zirconia (ZrO₂), yttria (Y₂O₃) and titania (TiO₂), the molar ratio of Y₂O₃/ZrO₂ being at least 2/98 and the molar ratio of TiO₂/(ZrO₂ + Y₂O₃) being in the range of from 3/97 to 20/80, characterised in that a shaped article comprising ZrO₂, Y₂O₃ and TiO₂ in the above-mentioned proportions is sintered, subjected to a hot isostatic pressing treatment, and then oxidized in air or oxygen. A process as claimed in Claim 1, characterised in that sintering is carried out at a temperature of at least 1,400°C. A process as claimed in Claim 1, characterised in that the sintering is carried out at a temperature of 1,600°C to 1,800°C. A process as claimed in any one of Claims 1 to 3 characterised in that the sintered shaped article is subjected to a hot isostatic pressing treatment at a temperature of 1,400°C to 1,700°C in an argon atmosphere. A process as claimed in any one of Claims 1 to 4 in which the oxidation is carried out at a temperature of at least 800°C. A process as claimed in any one of Claims 1 to 5 characterised in that the molar rato of Y₂O₃/ZrO₂ is at least 6/94. A process as claimed in any one of Claims 1 to 6 characterised in that the molar ratio of Y₂O₃/ZrO₂ is in the range of from 6/94 to 9/91 and the molar ratio of TiO₂/(ZrO₂ + Y₂O₃) is in the range of from 5/95 to 20/80.

Inventors: TSUKUMA, KOJI
Applicants: TOSOH CORPORATION
Application year: 1986
Main classification: C04B35
IPC codes: C04B35