JP4894728B2 - Translucent alumina sintered body and method for producing the same - Google Patents

Description

  The present invention relates to a translucent alumina sintered body which is excellent in translucency and strength and suitable for dental use, particularly orthodontic brackets and denture mill blanks.

  Translucent alumina ceramics have long been used as a tube material for high-pressure sodium lamps and the like because of its high strength, translucency, and corrosion resistance. Recently, the use as orthodontic brackets and dental materials such as crowns and bridges for tooth restoration has been expanding.

  When used as a dental material, not only mechanical properties such as strength and toughness but also high aesthetics (translucency) are required, and development of materials having both of these properties is desired.

  For improving the mechanical properties of alumina ceramics, for example, a method of compounding with other materials is known. For example, it has been reported that composite with SiC (Patent Document 1) and composite with zirconia (Patent Document 2) have higher strength and higher toughness than pure alumina. It has also been reported that a high fracture toughness value is obtained by compounding with a β-alumina compound having an anisotropic shape (Patent Document 3).

On the other hand, regarding the improvement of translucency, it is necessary to eliminate pores and impurities (heterogeneous phases) that cause scattering. Conventionally, oxides (MgO, La ₂ O ₃ , Y ₂ O ₃ , ZrO ₂ , HfO ₂ ) that suppress grain growth are added to alumina to suppress the formation of intragranular pores due to abnormal grain growth, and alumina It is known to improve translucency. However, there is a problem that the translucency decreases with the increase in the amount of addition.

The amount of additive that maintains the translucency is 0.1 wt% or less for MgO (Patent Document 4), 0.2 wt% or less for La ₂ O ₃ and Y ₂ O ₃ (Patent Document 5), ZrO ₂ , HfO _2. Are reported to be about 0.07 wt% and 0.12 wt% or less (Patent Document 6), respectively. For example, an alumina sintered body to which 2.5 mol% (about 3 wt%) of ZrO ₂ was added had an extremely low visible light transmittance of 25% at a thickness of 0.5 mm (Patent Document 7).

  Regarding the reduction of scattering caused by different components, even in the conventional translucent alumina sintered body to which MgO or the like is added, a small amount of rare element oxide such as lanthanum oxide is added, and the grain boundary phase (spinel phase) A method for bringing the refractive index close to alumina has been reported (Patent Document 8). However, since the amount of rare element oxide added in this method is extremely small, it has not been intended to improve mechanical properties while maintaining high translucency.

  As described above, the translucency of the alumina sintered body decreases due to the presence of a small amount of heterogeneous phase. Therefore, not only the alumina composite sintered body disclosed in Patent Documents 1 to 3, but also the composite sintering reported so far. None of the bodies showed sufficiently high translucency. Compositing with different materials in ceramics is an extremely effective means of improving mechanical properties, but light scattering due to the heterogeneous interface occurs due to compounding, so a sintered body with high translucency should be produced. Was difficult.

JP-A-64-87552 JP 60-235762 A JP-A-63-134551 US Pat. No. 3,026,210 JP 54-148008 A JP 59-6831 A JP 2001-322866 A JP 2000-219570 A

  For example, MgO, which is a sintering aid (for example, a grain growth inhibitor), is added to a conventional translucent alumina sintered body containing different components (0.1 wt% or less), and abnormalities in sintered grains are observed. Although the thing which suppressed grain growth is known, such a sintered compact consists of a crystal grain with a particle size of about 20-50 micrometers, and translucency and intensity | strength were not enough.

  As a result of diligent investigations on the improvement of translucency and strength of the translucent alumina sintered body, the present inventors made a composite of β-alumina structure type aluminate compound having the same refractive index as alumina and alumina. Thus, an alumina sintered body excellent in translucency and strength was found, and the present invention was completed.

  In the translucent alumina sintered body of the present invention, a β-alumina structure type aluminate compound having a refractive index comparable to that of alumina is compounded in a large amount of 1 to 50% which is a composition range that has not been conventionally used. It is an alumina sintered body with high translucency and high strength.

  The present invention is described in detail below.

  The alumina sintered body of the present invention contains 1 to 50 wt% of a β-alumina structure type aluminate compound (hereinafter referred to as aluminate compound), and has a total light transmittance of 45 to visible light having a sample thickness of 1 mm and a wavelength of 600 nm. % Or more.

  Since the aluminate compound has a crystal structure similar to that of alumina, and has a refractive index equivalent to that of alumina, the translucency of the alumina sintered body is maintained even if such a compound is included. By compounding these compounds with alumina, it is possible to suppress light scattering caused by the heterogeneous interface in the sintered particles.

Examples of the aluminate compound in the present invention include LaAl ₁₁ O ₁₈ , LaMgAl ₁₁ O ₁₉ , YMgAl ₁₁ O ₁₉ , CeMgAl ₁₁ O ₁₉ , CaAl ₁₂ O ₁₉ , SrAl ₁₂ O ₁₉ , BaAl ₁₂ O ₁₉ , Mg ₂ NaAl ₁₅ O _{25, SrMgAl 10 O 17, BaMgAl} 10 O 17, Na 2 MgAl 12 O 17, NaAl 11 O 17, CeAl 11 O 18 can be exemplified.

Of these, LaAl ₁₁ O ₁₈ and LaMgAl ₁₁ O ₁₉ are particularly preferable. The refractive index of LaAl ₁₁ O ₁₈ is 1.780 (Ceramic Association, 82 [12], 631-636 (1974)), and the refractive index of alumina is a-axis 1.768, c-axis 1.760 (Infrared Phys). Technol., 39 [4] 235-249 (1998)). Furthermore, LaAl ₁₁ O ₁₈ has an effect of refining the sintered crystal grains of the alumina phase by being combined with alumina. LaMgAl ₁₁ O ₁₉ is also used because of the similarity in structure.

  The aluminate compound content in the sintered body of the present invention is 1 to 50 wt%, and a preferable range for achieving both translucency and strength is 1 to 10 wt%. When the content is small, the mechanical properties are not sufficiently improved, and when it is more than 50 wt%, the transmittance is lowered.

Conventionally, in reports on composite sintering of alumina compounds such as LaAl ₁₁ O ₁₈ and alumina (for example, Patent Document 3), the purpose is to increase toughness and strength using anisotropic growth of aluminate compounds. Therefore, a sintered body with sufficient translucency could not be obtained because of the presence of a large amount of composite phase and insufficient removal of pores affecting translucency.
In the sintered body of the present invention, the total light transmittance for visible light having a wavelength of 600 nm at a sample thickness of 1 mm is 45% or more, particularly 50% or more, more preferably 60% or more, and 70% or more can be achieved.

  In the sintered body of the present invention, the average sintered crystal grain size of the alumina phase is preferably 10 μm or less, more preferably 5 μm or less. This is because in the sintered body of the present invention, the growth of sintered grains of alumina is suppressed by the composite of aluminate compound and alumina.

  In the alumina sintered body, the bending strength is improved as the sintered crystal grain size is smaller. Therefore, the sintered body of the present invention preferably has a high bending strength of 500 MPa or more, particularly 600 MPa or more, more preferably 700 MPa or more. 800 MPa or more can be achieved.

  Next, the manufacturing method of the sintered compact of this invention is demonstrated.

In the sintered body of the present invention, the aluminate compound and / or its precursor are added to the alumina powder and mixed and dispersed so that the aluminate compound in the alumina powder is 1 to 50 wt%. It is manufactured by further subjecting the material sintered in step 1 to HIP treatment.
Although the alumina powder to be used is not particularly limited, commercially available alumina powder (for example, Daimei Chemicals products, Sumitomo Chemical products, etc.) can be used.

When an aluminate compound is used, it is used by mixing and pulverizing with alumina powder using a ball mill or the like. Further, an oxide constituting aluminate as a precursor may be added to alumina powder and mixed and dispersed. For example, in LaAl ₁₁ O ₁₈ , a mixture of LaAlO _{3 as a} precursor and alumina powder is used.

  Sintering without pressure (referred to as primary sintering) usually involves sintering a powder compact as it is in an atmosphere such as air, oxygen, or vacuum. The simplest method is atmospheric sintering. As the sintering condition, it is preferable to perform the sintering at least at a sintering temperature at which the pores in the sintered body (referred to as the primary sintered body) are closed pores (relative density is 95% or more). This is to avoid a decrease in translucency due to penetration of the high-pressure gas into the sintered body in the HIP process.

  In order to improve the translucency of the finally obtained sintered body after the HIP treatment, it is preferable that the number of pores in the sintered body is as small as possible. For this reason, the size of the sintered crystal grains of the primary sintered body is small. Is preferred. The sintered crystal grain size of the primary sintered body is preferably small and is preferably less than 5 μm. If it is less than 5 μm, the plastic flow of particles during HIP treatment becomes active, and the disappearance of residual pores is further promoted. On the other hand, the sintered particle size is preferably more than 1 μm from the viewpoint of translucency and strength.

In order to reduce the particle size of the primary sintered body, the sintering temperature at which the sintered body becomes closed pores is preferably low. When LaAl ₁₁ O ₁₈ is used as the aluminate compound, primary sintering is preferably performed at 1400 ° C. and with LaMgAl ₁₁ O ₁₉ at 1500 ° C. or less.

  The HIP treatment is performed for the purpose of eliminating residual pores in the sintered body. Although the reduction of residual pores occurs at 1200 ° C. or higher, when the HIP processing temperature is 1400 ° C. or lower, the pores are not sufficiently removed and the translucent property of the sintered body is not sufficient. Moreover, when using the precursor which produces | generates an aluminate compound at the time of sintering, since an aluminate compound produces | generates at 1400 degreeC or more, HIP temperature is 1400 degreeC or more, Especially the process of 1500 degreeC or more is preferable.

  As a pressure medium in HIP, argon gas which is usually used is sufficient. Other gases such as nitrogen and oxygen are also applicable. The pressure is preferably 50 MPa or more, particularly preferably in the range of 100 to 200 MPa.

  The sintered body of the present invention has both high strength and translucency, and is suitable as a material used for dental materials, particularly orthodontic brackets or mil-bun ranks for dentures. Since translucent alumina used as a conventional bracket is obtained by high-temperature sintering, the sintered crystal grains are 20 μm or more and have a bending strength of about 300 to 400 MPa. The sintered body of the present invention has higher strength than the conventional alumina sintered body, and it is possible to design a complicated shape such as downsizing of the bracket and self-ligation.

The translucent alumina sintered body of the present invention has high strength and aesthetic properties based on translucency, and is used for orthodontic brackets, denture mill blanks and the like used in the dental field, for example. It has very suitable characteristics as a material. It can also be used for other applications, such as high-pressure sodium lamp tubes and metal halide lamp tubes.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

  The sintered body characteristics of the present invention were evaluated by the following methods.

(1) Total light transmittance The total light transmittance was measured by a double beam spectrophotometer (manufactured by JASCO Corporation, V-650) and a method based on JIS K7361-1. The measurement sample used was processed into a sintered body thickness of 1 mm and subjected to double-sided mirror polishing so that the surface roughness Ra = 0.02 μm or less. An outline of the measuring apparatus is shown in FIG. The light generated from the light source (deuterium lamp and halogen lamp) was transmitted and scattered through the sample, and the total light transmittance was measured using an integrating sphere. The measurement wavelength was measured in the region of 200 to 800 nm, and the total light transmittance in this case was the total light transmittance at a wavelength of 600 nm in the visible light region.

(2) Three-point bending strength The three-point bending strength is measured using a universal testing machine Autograph DCS-2000 (manufactured by Shimadzu Corporation) based on JIS-R-1601, 4 mm wide, 3 mm thick, and 40 mm long. The body was measured under the conditions of a span length of 30 mm and a crosshead speed of 0.5 mm / min. The average value of five test pieces for each sample.

(3) Sintered crystal grain size The sintered crystal grain size of the sintered body is that the sintered body is mirror-polished and the surface subjected to thermal etching at a temperature 100 ° C. lower than the highest experience temperature of the sintered body for 1 hour is observed by SEM. It was measured by. The SEM measurement was performed using a scanning electron microscope JSM-5400 (manufactured by JEOL). The particle size was measured using commercially available particle size measurement software (CERAPART: manufactured by Science Solution) based on the code method. The coefficient for particle shape is described in J. Am. Ceram. Soc. , 52 [8] 443-6 (1969), 1.56 was used.

(4) X-ray diffraction Identification of the crystal phase of the sintered body was performed by X-ray diffraction (XRD) measurement. For measurement, a powder X-ray diffractometer RINT Ultimate III (manufactured by Rigaku) was used, and the sintered surface of the sintered body was measured under conditions of 40 kV and 40 mA.

(5) Sintering density The density of the sintered compact was calculated | required from the measurement of the weight in water by the Archimedes method.

Example 1
LaAlO ₃ was added as a precursor of aluminate to high-purity alumina powder (purity 99.99% or more, manufactured by Daimei Chemical Industry), ball milled for 12 hours, and dried with a rotary evaporator to obtain a raw material powder. The precursor LaAlO ₃ was synthesized by a solid phase reaction method using high-purity alumina (purity 99.99% or more manufactured by Daimei Chemical Industries) and lanthanum oxide (manufactured by Shin-Etsu Chemical Co., Ltd.).

  The raw material powder was formed into a plate-like molded body of 40 mm × 50 mm and thickness 5 mm by applying a pressure of 50 MPa using a uniaxial press apparatus and a mold, and this was put in a rubber mold and molded at a pressure of 200 MPa using a cold isostatic press apparatus. . Next, it was sintered at 1400 ° C. in the atmosphere for 2 hours to obtain a primary sintered body. Further, the primary sintered body was treated with an HIP apparatus in an argon gas medium at a temperature of 1550 ° C. and a pressure of 150 MPa for 2 hours.

Table 1 shows the results of the sintered body density, average particle diameter, total light transmittance (sample thickness 1 mm, wavelength 600 nm), and strength measurement. As a result of the XRD measurement, the sintered body was composed of α-alumina and a β-alumina structure type aluminate compound (LaAl ₁₁ O ₁₈ ).

Example 2
High purity alumina powder (Taimei Chemicals Ltd. Purity 99.99%) lanthanum nitrate hexahydrate as a precursor of _{LaMgAl 11 O 19 (La (NO} 3) 3 · 6H 2 O) and magnesium nitrate (Mg (NO _{3) 2} · _{6H 2} O) was added and stirred and dissolved in aqueous solvent. The obtained slurry was neutralized with aqueous ammonia, and the precipitate was filtered and dried. The obtained dried product was mixed with a ball mill for 12 hours and dried with a rotary evaporator to obtain a raw material powder.

  Next, a sintered body was prepared in the same manner as in Example 1. The density, average particle diameter, total light transmittance measurement, and strength measurement of the obtained sintered body were performed. The results are shown in Table 2.

As a result of the XRD measurement, the sintered body was composed of α-alumina and β-alumina structure type aluminate compound (LaMgAl ₁₁ O ₁₉ ).

Comparative Example 1
Using the high-purity alumina powder described in Example 1, a sintered body was produced under the same conditions as in Example 1 without using an aluminate compound. Table 3 shows the results of the sintered body density, average particle diameter, total light transmittance (sample thickness 1 mm, wavelength 600 nm), and strength measurement.

  In the alumina sintered body to which no aluminate was added, many coarse abnormally grown particles were observed, and only a sintered body with low bending strength was obtained.

Schematic of total light transmittance measuring device Sintered body structure of the present invention ((a) sample number 1-1, (b) sample number 1-2, (c) sample number 2-1) Total light transmittance of the sintered body of the present invention (sample thickness 1 mm) Comparative Example Sintered Structure (Comparative Example 1 Sample No. 3-1) XRD pattern of the sintered body of the present invention

Claims (7)

1 to 10 wt% of LaAl ₁₁ O ₁₈ or 1 to 50 wt% of LaMgAl ₁₁ O _19, and has a total light transmittance of 45% or more for visible light having a wavelength of 600 nm at a sample thickness of 1 mm. Alumina sintered body. The translucent alumina sintered body according to claim 1, wherein the average sintered crystal grain size of the alumina phase is 10 μm or less. The translucent alumina sintered body according to claim 1 or 2 , which has a bending strength of 500 MPa or more. The translucent alumina sintered body according to any one of claims 1 to 3 , wherein a total light transmittance for visible light having a wavelength of 600 nm at a sample thickness of 1 mm is 50% or more. 1-50 wt% of β-alumina structure type aluminate compound and / or precursor thereof is added to alumina powder, mixed and dispersed, molded, sintered under no pressure, and further subjected to hot isostatic pressing (HIP) treatment The manufacturing method of the translucent alumina sintered compact in any one of Claims 1-4 characterized by the above-mentioned. The dental material which uses the translucent alumina sintered compact in any one of Claims 1-4 . The dental material of claim 6 which is either an orthodontic bracket or a denture mill blank.

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