Sintered bodies and their manufacturing methods, as well as orthodontic brackets

Abstract

This invention provides a sintered body with excellent mechanical strength and light transmittance, a method for manufacturing the same, and a dental orthodontic bracket. The sintered body is characterized by comprising crystalline particles having cubic and tetragonal crystal domains, and using zirconium oxide with a stabilizing element and lanthanum dissolved in it as a matrix. The content of the stabilizing element is 1 mol% or more and 6 mol% or less. The average moment strength of the sintered body is 1.00 kgf·cm or more, and the linear transmittance of visible light at a wavelength of 600 nm is 35% or more when the sample thickness is 1 mm.

Description

[Technical Field]

[0001] This disclosure relates to a sintered body, a method for manufacturing the same, and a dental orthodontic bracket. [Previous Technology]

[0002] Sintered bodies with zirconium oxide as the main component and light transmittance (hereinafter also referred to as "light-transmitting zirconium oxide sintered bodies") have superior mechanical strength compared to glass or alumina. Therefore, light-transmitting zirconium oxide sintered bodies have been studied as materials for applications that require not only optical properties but also mechanical properties.

[0003] For example, Patent Document 1 discloses a translucent zirconia sintered body suitable as a material for dental materials or external components. This translucent zirconia sintered body is a zirconia sintered body containing 3 mol% (moles) of yttrium oxide.

[0004] Patent Document 2 discloses a translucent zirconia sintered body suitable as a material for dental applications, particularly for orthodontic braces. This translucent zirconia sintered body is a zirconia sintered body containing 8 mol% yttrium oxide.

[0005] Patent document 3 discloses a translucent zirconia sintered body formed by dissolving a stabilizer and lanthanum in zirconia.

[0006] Patent Document 4 discloses a translucent zirconia sintered body that exhibits high mechanical strength without requiring post-sintering treatments such as grinding, and is formed by dissolving a stabilizer and lanthanum in zirconia. [Prior Art Documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2008-050247 [Patent Document 2] Japanese Patent Application Publication No. 2009-269812 [Patent Document 3] Japanese Patent Application Publication No. 2017-105689 [Patent Document 4] Japanese Patent Application Publication No. 2020-001988 [Summary of the Invention]

[0008] [Problem to be Solved by the Invention] The translucent zirconia sintered bodies of Patent Documents 3 and 4 have higher light transmittance than previous translucent zirconia sintered bodies and translucent alumina, and possess mechanical strength suitable for use in orthodontic brackets. However, due to the increasing demands for higher strength and improved aesthetics in orthodontic brackets in recent years, orthodontic brackets incorporating translucent zirconia sintered bodies require even higher mechanical strength and light transmittance.

[0009] The purpose of this disclosure is to provide a sintered body with excellent mechanical strength and light transmittance in a transparent zirconia sintered body with lanthanum dissolved in solid solution and the domain structure of the crystalline particles controlled, a method for manufacturing the same, and at least one of a dental orthodontic bracket.

[0010] [Means for Solving the Problem] The inventors have studied sintered bodies, their manufacturing methods, and orthodontic brackets. They discovered that by controlling the state of the sintered material under specific sintering conditions, sintered bodies and orthodontic brackets possessing both excellent mechanical strength and translucency can be obtained, thus completing this invention.

[0011] That is, the present invention is as described in the claims, and the subject matter of the disclosure is as follows. [1] A sintered body, characterized in that it comprises crystalline particles having cubic and tetragonal crystal domains, and uses zirconium oxide with a stabilizing element and lanthanum dissolved in it as a matrix, wherein the content of the stabilizing element is 1 mol% or more and 6 mol% or less, the average torque strength of the sintered body is 1.00 kgf·cm or more, and the linear transmittance of visible light at a wavelength of 600 nm is 35% or more when the sample thickness is 1 mm. [2] The sintered body as described in [1], wherein the deviation of the torque strength is 0.30 or less. [3] The sintered body as described in [1] or [2], wherein the lanthanum content is 1 mol% or more and 10 mol% or less. [4] The sintered body as described in any one of [1] to [3], wherein the stabilizing element is at least one selected from the group consisting of yttrium, scandium, calcium, magnesium and cerium. [5] The sintered body as described in any one of [1] to [4] comprises at least one of a lanthanide element other than lanthanum or a transition metal. [6] A method for manufacturing a sintered body is a method for manufacturing a sintered body as described in any one of [1] to [5], characterized by comprising: a mixing step, mixing a zirconium oxide raw material, a stabilizing element raw material and a lanthanum raw material to obtain a mixed powder; a forming step, forming the obtained mixed powder to obtain a shaped body; a sintering step, placing the obtained shaped body in an inner container and placing the inner container in an outer container, and sintering at a sintering temperature of 1650°C or higher to obtain a sintered body; and a cooling step, cooling from the sintering temperature to 1000°C at a cooling rate of more than 1°C / min. [7] The method for manufacturing a sintered body as described in [6], wherein the outer container is made of carbon. [8] A dental orthodontic bracket comprising a sintered body as described in any one of [1] to [5]. [9] A dental orthodontic bracket, characterized in that it comprises a sintered body containing crystalline particles having cubic and tetragonal crystal domains, and uses zirconium oxide with a stabilizing element and lanthanum dissolved in it as a matrix, wherein the content of the stabilizing element is 1 mole% or more and 6 mole% or less, the average torque strength of the dental orthodontic bracket is 1.00 kgf·cm or more, and after hot water treatment at 140°C for 72 hours, the linear transmittance of visible light at a wavelength of 600 nm is 10% or more at a sample thickness of 1 mm.

[10] The dental orthodontic bracket as described in [9], wherein the deviation of the torque strength is 0.30 or less. [Effects of the Invention]

[0012] By means of this disclosure, a sintered body with excellent mechanical strength and light transmittance, a method for manufacturing the same, and at least one of an orthodontic bracket can be provided.

Implementation Method

[0014] Hereinafter, the sintered body disclosed herein will be described with an example of an embodiment shown.

[0015] The sintered body of this embodiment is a sintered body that not only contains lanthanum (La) but also has lanthanum dissolved in zirconium oxide (hereinafter also referred to as "lanthanum-solution zirconium oxide sintered body"). By dissolving lanthanum in the solid solution, the microstructure of the crystalline particles of the sintered body becomes fine.

[0016] In the sintered body of this embodiment, the lanthanum is dissolved in zirconium oxide, which can be confirmed by the powder X-ray diffraction (hereinafter also referred to as "XRD (X-Ray Diffractometry)") pattern. The sintered body of this embodiment exhibits a peak of 2θ = 30 ± 2° (hereinafter also referred to as "main peak") in XRD measurements using CuKα rays (λ = 0.15418 nm) as the XRD source. The main peak is a peak that repeats the XRD peak of tetragonal zirconium oxide (2θ = 30.0 ± 2°) and the XRD peak of cubic zirconium oxide (2θ = 29.6 ± 2°), and is the XRD peak with the strongest diffraction intensity in the XRD pattern of the sintered body. The lattice parameter obtained from the main peak is larger than that of the sintered body without lanthanum dissolution, thereby confirming that lanthanum is dissolved in zirconium oxide in the sintered body of this embodiment. For example, in the case of a lanthanum-solution zirconium oxide sinter containing lanthanum and 3 mol% yttrium as a stabilizing element, its lattice constant is larger than that of a sinter containing only 3 mol% yttrium as a stabilizing element, with the remainder being zirconium oxide. This larger lattice constant can be confirmed by the shift of the main peak towards a lower angle in the XRD pattern.

[0017] Furthermore, the sintered body of this embodiment is preferably substantially free of composite oxides containing lanthanum and zirconium, and lanthanum oxides (hereinafter also referred to as "lanthanum oxides, etc."). By not containing lanthanum oxides, the sintered body of this embodiment becomes a sintered body with higher light transmittance. The absence of lanthanum oxides can be confirmed by the absence of XRD peaks in the XRD pattern of the sintered body of this embodiment that correspond to peaks other than those of zirconium oxide. Examples of lanthanum oxides, etc., include La2Zr2O7 and La2O3.

[0018] In this embodiment, the lanthanum content of the sintered body is preferably 1 mol% or more. By containing 2 mol% or more of lanthanum, the domains in the crystal particles tend to become finer. The lanthanum content (mol%) is the molar ratio of lanthanum (calculated as oxide) to the total of zirconium oxide (calculated as oxide), stabilizing elements (calculated as oxide), and lanthanum (La2O3) (calculated as oxide) in the sintered body. For example, when yttrium is included as a stabilizing element, the lanthanum content [mol%] can be calculated in the form of {La2O3[mol] / (Y2O3+La2O3+ZrO2)[mol]}×100.

[0019] To ensure that all lanthanum is dissolved in zirconium oxide, the lanthanum content of the sintered body is preferably 10 mol% or less. Furthermore, the lanthanum content is preferably 1 mol% or more, 2 mol% or more, or 3 mol% or more. Additionally, it is preferably 10 mol% or less, 7 mol% or less, or 6.5 mol% or less. These upper and lower limits can be any combination. Therefore, for example, examples of lanthanum content include: 1 mol% or more and 10 mol% or less, further 1 mol% or more and 7 mol% or less, further 2 mol% or more and 10 mol% or less, further 2 mol% or more and 7 mol% or less, further 2 mol% or more and 6.5 mol% or less, and further 3 mol% or more and 6.5 mol% or less.

[0020] The sintered body of this embodiment preferably does not contain lanthanide rare earth elements other than lanthanum. Examples of lanthanide rare earth elements other than lanthanum include europium (Eu), thorium (Gd), tbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thorium (Tm), ytterbium (Yb), and argonium (Lu). The sintered body of this embodiment preferably does not contain lanthanide rare earth elements other than lanthanum, but considering the measurement error of composition analysis, the content of lanthanide rare earth elements other than lanthanum in the sintered body of this embodiment can be exemplified as 0.6 mol% or less.

[0021] The sintered body of this embodiment contains a stabilizing element. The stabilizing element is dissolved in zirconium oxide. By dissolving lanthanum and the stabilizing element in zirconium oxide, even at low temperatures such as room temperature, the sintered body's crystal grains contain both cubic and tetragonal crystal domains.

[0022] The stabilizing element is an element that has the function of stabilizing zirconium oxide. The stabilizing element is preferably at least one selected from the group consisting of yttrium (Y), scandium (Sc), calcium (Ca), magnesium (Mg) and cerium (Ce), more preferably at least one selected from the group consisting of calcium, magnesium and yttrium, and even more preferably yttrium.

[0023] In order to partially stabilize the zirconium oxide, the content of the stabilizing element in the sintered body of this embodiment is 1 mol% or more and 6 mol% or less. Preferably, the content of the stabilizing element is 1 mol% or more or 2 mol% or more. Alternatively, it is preferably 6 mol% or less, 5 mol% or less, 4.9 mol% or less, or 4 mol% or less. These upper and lower limits can be any combination. Therefore, the content of the stabilizing element is preferably 2 mol% or more, and preferably 5 mol% or less, 4.9 mol% or less, or 4 mol% or less. The content of the stabilizing element is preferably 2 mol% or more, and preferably 5 mol% or less, 4.9 mol% or less, or 4 mol% or less. Furthermore, the content of the stabilizing element (mol%) is the molar ratio of the stabilizing element to the total of zirconium oxide, the stabilizing element converted to oxides, and lanthanum (La2O3) in the sintered body. When yttrium is included as a stabilizing element, the content of the stabilizing element [mol%] can be calculated in the form of {(Y2O3)[mol] / (Y2O3+La2O3+ZrO2)[mol]}×100.

[0024] The sintered body of this embodiment is a so-called zirconia sintered body, which is a sintered body with zirconia as the matrix (main component). Therefore, the total content of stabilizing elements and lanthanum contained in the sintered body of this embodiment only needs to be less than 50 mol%, preferably less than 20 mol% or less or less than 10 mol%, and more preferably more than 1 mol% or more or more than 2 mol%. These upper and lower limits can be any combination. The zirconia content of the sintered body of this embodiment is preferably more than 50 mol%, more than 60 mol%, more than 80 mol%, more than 83 mol%, or more than 90 mol%.

[0025] The sintered body of this embodiment may also contain alumina (Al2O3). By containing alumina, the light transmittance of the sintered body, especially the high-strength sintered body, is easily improved. When the sintered body of this embodiment contains alumina, the alumina content is preferably 100 ppm or more by mass, or 200 ppm or more by mass, and preferably 2000 ppm or less by mass, or 1000 ppm or less by mass. These upper and lower limits can be any combination. Therefore, the alumina content is preferably, for example, 100 ppm or more by mass and 2000 ppm or less by mass, and further preferably 200 ppm or more by mass and 1000 ppm or less by mass. The alumina content (ppm by mass) is the mass ratio of aluminum (Al2O3) converted to oxides to the total mass of zirconium oxide, stabilizing elements converted to oxides, lanthanum (La2O3), and aluminum (Al2O3) converted to oxides in the sintered body. In the case of aluminum oxide and lanthanum, and yttrium as a stabilizing element, the aluminum oxide content [mass ppm] can be calculated in the form of {Al2O3[g] / (Y2O3+La2O3+Al2O3+ZrO2)[g]}×1000000.

[0026] The sintered body of this embodiment has the aforementioned composition, but may also contain unavoidable impurities. Hafnium oxide (HfO2) can be listed as an example of an unavoidable impurity. Furthermore, in the calculation of composition-related values ​​such as theoretical density, the content of stabilizing elements or additives in this embodiment, hafnium oxide (HfO2), as an unavoidable impurity, can be calculated as zirconium oxide (ZrO2).

[0027] As a preferred composition of the sintered body of this embodiment, the following molar composition can be listed: Zirconia: 90 mol% or more and 95 mol% or less; Stabilizing element: 2 mol% or more and 5 mol% or less; Lanthanum: 2 mol% or more and 6.5 mol% or less.

[0028] The following molar composition can be listed as a particularly preferred composition of the sintered body of this embodiment: Zirconia: 92 mol% or more and 94 mol% or less; Stabilizing element: 2 mol% or more and 4 mol% or less; Lanthanum: 3 mol% or more and 5 mol% or less.

[0029] The stabilizing element in the composition is preferably yttrium.

[0030] The sintered body of this embodiment has cubic and tetragonal crystal domains in its crystalline particles. Because the crystalline particles contain cubic and tetragonal crystal domains, not only is the light transmittance high, but the strength is also improved. In this embodiment, a domain refers to at least one of the crystallites or aggregates of crystallites in the crystalline particles, and is a portion formed by the same continuous crystalline structure. Furthermore, a cubic crystal domain refers to a domain with a cubic fluorite-type crystalline structure, and a tetragonal crystal domain refers to a domain with a tetragonal fluorite-type crystalline structure. The presence of cubic and tetragonal crystal domains in the crystalline particles of the sintered body of this embodiment can be confirmed by Rietveld analysis of the XRD pattern. That is, by Rietveld analysis of the XRD pattern, it can be confirmed that the sintered body contains cubic and tetragonal crystals. Furthermore, based on the fact that the crystallite diameters of cubic and tetragonal crystals are smaller than the grain size, calculated using Rietwald analysis, it can be confirmed that cubic and tetragonal crystal domains are contained within the crystalline particles. Moreover, when confirming that the sintered body contains cubic and tetragonal crystal domains within its crystalline particles, it is sufficient to confirm that the average crystallite diameter (described later) is smaller than the average grain size. The sintered body of this embodiment contains crystalline particles having both cubic and tetragonal crystal domains, and preferably is composed of crystalline particles having both cubic and tetragonal crystal domains. Thus, the sintered body of this embodiment has crystalline particles that at least contain domains composed of crystals with different crystal structures. Therefore, the sintered body of this embodiment differs from sintered bodies containing crystalline particles whose crystal structure is solely composed of cubic zirconia and crystalline particles whose crystal structure is solely composed of tetragonal zirconia.

[0031] The sintered body of this embodiment includes the aforementioned domain, and therefore its crystal structure includes a cubic fluorite structure and a tetragonal fluorite structure. Furthermore, the sintered body of this embodiment is preferably substantially free of monoclinic crystals. Here, "substantially free of monoclinic crystals" can be exemplified by XRD peaks in the XRD pattern where no monoclinic crystals were detected.

[0032] The lanthanum concentrations in the cubic and tetragonal crystal regions can be the same, but in the sintered body of this embodiment, the lanthanum concentrations in the cubic and tetragonal crystal regions of the crystalline particles can also be different, and thus the lanthanum concentration in the cubic crystal region can be higher than that in the tetragonal crystal region. In this embodiment, the lanthanum concentration in each region can be observed by compositional analysis under a transmission electron microscope (hereinafter also referred to as "TEM").

[0033] In this embodiment, the sintered body preferably has an average crystallite size (hereinafter also referred to as "average crystallite size") calculated based on the half-width at half maximum (FWHM) of the main peak (hereinafter also referred to as "FWHM"). In this embodiment, the average crystallite size is calculated without distinguishing between cubic and tetragonal crystals. By having an average crystallite size of 250 nm or less, further 200 nm or less, further 150 nm or less, and further 130 nm or less, light transmittance is easily improved. Furthermore, by having an average crystallite size of 100 nm or less, further 60 nm or less, further 50 nm or less, and further 30 nm or less, light scattering is further suppressed. As a result, the light transmittance of the sintered body is further improved.

[0034] The average crystallite diameter is preferably small, but in the sintered body of this embodiment, it can be 2 nm or more, 5 nm or more, 10 nm or more, or 15 nm or more.

[0035] An average crystallite diameter of 255 nm or less can be confirmed by an FWHM of 0.1536° or more in the XRD pattern of the sintered body of this embodiment. Therefore, the sintered body of this embodiment preferably has an FWHM of 0.1536° or more. The larger the FWHM, the smaller the average crystallite diameter. For example, regarding FWHM, it is 0.154° or more when the average crystallite diameter is 250 nm or less, 0.1635° or more when it is 200 nm or less, 0.178° or more when it is 150 nm or less, 0.187° or more when it is 130 nm or less, and 0.25° or more when it is 100 nm or less. FWHM is preferably 0.3° or more, and further 0.4° or more. On the other hand, the higher the crystallinity, the smaller the FWHM of the XRD peak, but the maximum FWHM that can be measured in a normal XRD measurement is only about 40°. The FWHM of the main peak of the sintered body in this embodiment can be categorized as 1° or less, and further as 0.7° or less.

[0036] In this embodiment, the XRD pattern is measured using CuKα rays as the XRD source. The measurement conditions are as follows: Accelerating current / voltage: 40 mA / 40 kV; XRD source: CuKα rays (λ=1.5405 Å); Measurement mode: Step scan; Scanning conditions: 0.04° / second; Measurement range: 2θ=20°~80°; Diverging slit: 0.5 deg; Scattering slit: 0.5 deg; Light receiving slit: 0.3 mm; Detector: Scintillation counter.

[0037] XRD patterns can be determined using a general powder X-ray diffraction apparatus (e.g., Ultima III, manufactured by Rigaku). Additionally, crystalline XRD peaks are detected by determining the 2θ peak apex using general analytical software (e.g., JADE7, manufactured by MID). The following are some analytical conditions for XRD patterns: Fitting conditions: Automatic, background refinement, dispersion-type pseudo-Voigt function (peak shape); Background removal method: Fitting method; Kα2 removal method: Kα1 / Kα2 ratio = 0.497; Smoothing method: B-spline curve smoothing conditions: Quadratic differential method, σ cutoff value = 3, χ threshold value = 1.5.

[0038] Furthermore, the crystallite diameters of the cubic and tetragonal crystals contained in the crystalline particles of the sintered body of this embodiment can be determined by Rietwald analysis of the XRD pattern of the sintered body of this embodiment. That is, by Rietwald analysis, the XRD pattern of the sintered body is separated into XRD peaks originating from cubic crystals and XRD peaks originating from tetragonal crystals. It is only necessary to obtain the half-width of the XRD peaks of each separated crystalline structure, and then calculate the crystallite diameter based on the obtained half-width using the following Scherrer equation.

[0039] D=K×λ / ((β-B)×cosθ) In the above formula, D is the crystallite diameter of each crystal (nm), K is the Scherrer constant (1.0), λ is the wavelength of CuKα (0.15418 nm), β is the half-width (°), B is the device constant (0.1177°), and θ is the diffraction angle of the XRD peak (°). The XRD peaks with the calculated half-width are the XRD peaks of the tetragonal crystal with 2θ=30.0±2° and the XRD peaks of the cubic crystal with 2θ=29.6±2°.

[0040] The average crystal grain size of the sintered body in this embodiment is preferably 20 μm or more, or 30 μm or more. It is also preferably 100 μm or less, 90 μm or less, or 60 μm or less. These upper and lower limits can be any combination. Therefore, the average crystal grain size of the sintered body can be, for example, 20 μm or more and 100 μm or less, 30 μm or more and 90 μm or less, and further, 30 μm or more and 60 μm or less. By having an average crystal grain size within this range, a sintered body with high light transmittance is obtained. In this embodiment, the average crystal grain size can be measured using a planimetric method.

[0041] The sintered body of this embodiment is preferably high in density. The density varies depending on the amount of stabilizing element and lanthanum. The density of the sintered body of this embodiment may be 6.0 g / cm3 or more and 6.2 g / cm3 or less, and further 6.0 g / cm3 or more and 6.12 g / cm3 or less.

[0042] In this embodiment, the sintered body preferably has an arithmetic mean surface roughness (hereinafter also referred to as "Ra") of 20 nm or more, or 25 nm or more. Furthermore, it is preferably 60 nm or less, or 45 nm or less. These upper and lower limits can be any combination. Therefore, the arithmetic mean surface roughness is, for example, 20 nm or more and 60 nm or less, and preferably 25 nm or more and 45 nm or less. When Ra exceeds 60 nm, the mechanical strength, especially the fracture strength, decreases. A smaller Ra is better, but even after polishing or similar treatments, the Ra of the sintered body is around 20 nm.

[0043] In this embodiment, the sintered body preferably has a maximum surface height (hereinafter also referred to as "Rz") of 100 nm or more, or 300 nm or more. Alternatively, it is preferably 1000 nm or less, or 900 nm or less. These upper and lower limits can be any combination. Therefore, the maximum surface height can be, for example, 100 nm or more and 1000 nm or less, and preferably 300 nm or more and 900 nm or less.

[0044] In this embodiment, the sintered body preferably has a root mean square height (hereinafter also referred to as "Rq") of 10 nm or more, or 20 nm or more. Alternatively, it is preferably 100 nm or less, or 50 nm or less. These upper and lower limits can be any combination. Therefore, the root mean square height of the surface can be, for example, 10 nm or more and 100 nm or less, and preferably 20 nm or more and 50 nm or less.

[0045] In this embodiment, Ra, Rz and Rq can be measured by means of the method according to Japanese Industrial Standard (JIS) B 0601.

[0046] Preferably, the sintered body of this embodiment has no grinding marks on its surface. Typically, the surface of a sintered body shortly after sintering (also referred to as the "sintered surface") is rough, and therefore, post-processing such as grinding is performed to smooth the surface. However, grinding marks are generated on the surface of the sintered body during post-processing. Grinding marks are generated along with grinding, and can be, for example, regular stripe patterns. In contrast, the sintered body of this embodiment preferably has practical smoothness in the post-sintering state, in which case grinding marks are absent and Ra, Rz, and Rq are satisfied. Grinding marks can be confirmed by SEM observation of the surface of the sintered body.

[0047] The sintered body of this embodiment has high light transmittance (translucency). The sintered body of this embodiment has a linear transmittance of 35% or more for visible light at a wavelength of 600 nm when the sample thickness is 1 mm. A higher linear transmittance results in a sintered body with higher transparency, which is therefore preferable. The linear transmittance of the sintered body of this embodiment for visible light at a wavelength of 600 nm when the sample thickness is 1 mm is preferably 40% or more, and more preferably 45% or more. Examples of linear transmittance include 50% or less, and further, 48% or less.

[0048] Preferably, the sintered body of this embodiment is aesthetically pleasing even when used as a dental orthodontic bracket (hereinafter also referred to as a "bracket") containing the sintered body of this embodiment, and further when it is installed in the dental row for a long period of time as a bracket, without any sense of disharmony due to its translucency. Hereinafter, the bracket containing the sintered body of this embodiment will be described with reference to the drawings. Furthermore, in the following drawings, for the sake of easy understanding of each structure, the actual structure may sometimes differ from the scale and number of elements in each structure.

[0049] Furthermore, the XYZ coordinate system is appropriately shown as a three-dimensional orthogonal coordinate system in the diagram. In the XYZ coordinate system, the Y-axis direction is set as the depth direction of the groove. The X-axis direction is set as the width direction of the groove. The Z-axis direction is set as the direction orthogonal to both the X-axis and Y-axis directions. In any of the X-axis, Y-axis, and Z-axis directions, the side indicated by the arrow shown in the diagram is designated as the "+" side, and the opposite side is designated as the "-" side.

[0050] Furthermore, in the following description, the positive side (+X side) in the X-axis direction is referred to as the "gingival side," and the negative side (-X side) in the X-axis direction is referred to as the "occlusal side." Furthermore, the positive side (+Z side) in the Z-axis direction is referred to as the "labial / buccal side," and the negative side (-Z side) in the Z-axis direction is referred to as the "dental surface side." Furthermore, in the following description, the shape and structure of each structure of the bracket are not particularly limited, and the shape and structure of all known bracket structures can be used.

[0051] FIG1 is a schematic perspective view showing an example of the shape of an orthodontic bracket. As shown in FIG1, the orthodontic bracket (100) of this embodiment includes a pair of opposing sidewall portions (20) in the gingival-occlusal direction and a groove (30) for maintaining an arched line between the sidewall portions (20) on a base (10) mounted on a crown.

[0052] The sidewall portion (20) has a gingival sidewall portion (20a) and an occlusal sidewall portion (20b). Furthermore, the sidewall portion (20) has a pair of recesses (40) facing each other in the gingival-occlusal direction. The recesses (40) have a gingival recess (40a) and an occlusal recess (40b).

[0053] The groove (30) has a pair of groove sides (31) facing each other in the gingival-occlusal direction and a groove bottom surface (32). The groove sides (31) have a gingival groove side (31a) and an occlusal groove side (31b).

[0054] Figure 2 is a schematic plan view showing an example of the shape of a tooth alignment bracket. Surface (90) is a surface that is orthogonal to the depth direction of the groove and bisects the width of the groove depth. Figure 3 is a schematic cross-sectional view obtained by cutting the tooth alignment bracket of Figure 2 at surface (90).

[0055] As shown in Figure 3, in this embodiment, the so-called inside-outside size (hereinafter also referred to as "I / O") refers to the following distance (d1), that is, the distance (d1) between the bottom surface (32) of the wire groove and the tooth surface side of the base (10) on the line that divides the wire groove (30) into two equal parts on the gingival side and the occlusal side in the cross-sectional view of the orthodontic bracket at the surface that is orthogonal to the wire groove depth direction and divides the wire groove depth width into two equal parts.

[0056] As shown in Figure 3, in this embodiment, the so-called gingival sidewall thickness, in the cross-sectional view of the orthodontic bracket at the surface orthogonal to the depth direction of the groove and bisecting the width of the groove depth, is defined as point A at the corner where the bottom surface (32) of the groove intersects with the side surface (31a) of the gingival groove, point B at the edge of the gingival recess (40a) closest to the occlusal side, and point C at the intersection of a straight line passing through point A that is parallel to the line that bisectes the groove (30) on the gingival and occlusal sides and a perpendicular line passing through point B that is perpendicular to the straight line.

[0057] Figure 4 is an enlarged view showing the case where the corner where the bottom surface of the groove intersects with the gingival side groove has an arc-shaped curved surface portion (R). As shown in Figure 4, the orthodontic bracket (100) in this embodiment sometimes has an arc-shaped curved surface portion (R) at the corner where the bottom surface of the groove (32) intersects with the gingival side groove (31a). In other words, the bottom surface of the groove (32) and the gingival side groove (31a) sometimes have a flat portion (P1) and an arc-shaped curved surface portion (R1). In this case, the corner where the bottom surface of the groove (32) intersects with the gingival side groove (31a), i.e., point A, refers to the boundary point of the flat portion (P1) and the curved portion (R1) on the gingival side groove (31a).

[0058] The aesthetic appeal of the bracket containing the sintered body of this embodiment, derived from its light transmittance, can be evaluated, for example, by converting the linear transmittance of the groove portion after hot water treatment at 140°C for 72 hours to a value obtained by measuring the sample thickness of 1 mm (hereinafter also referred to as "converted transmittance"). Figures 5 and 6 are schematic side views illustrating an example of a method for preparing a test sample for measuring the converted transmittance in an orthodontic bracket. The test sample for measuring the converted transmittance in the bracket is cut out in such a way that it includes the region (Q) of the single-point chain frame shown in Figure 5 or Figure 6.

[0059] The region (Q) shown in Figure 5 includes dimensions (q1), (q2), (q3), and (q4). Dimension (q1) is 0.1 mm or more, preferably 0.3 mm or more, from the bottom surface (32) of the groove towards the tooth surface. It is also preferably 3 mm or less, 2 mm or less, or 1 mm or less. These upper and lower limits can be any combination. Therefore, dimension (q1) has, for example, a size of 0.1 mm or more and 3 mm or less, preferably 0.1 mm or more and 2 mm or less, and more preferably 0.3 mm or more and 1 mm or less, from the bottom surface (32) of the groove towards the tooth surface. Dimension (q2) is preferably 0.1 mm or more, or 0.3 mm or more, from the bottom surface (32) of the groove towards the lip / cheek side. It is also preferably 3 mm or less, 2 mm or less, or 1 mm or less. These upper and lower limits can be any combination. Therefore, dimension (q2) has a size of, for example, 0.1 mm or more and 3 mm or less, preferably 0.1 mm or more and 2 mm or less, and more preferably 0.3 mm or more and 1 mm or less, from the bottom surface (32) of the groove toward the labial / buccal side. Dimension (q3) preferably has a size of 0.1 mm or more, or 0.3 mm or more, from the corner where the bottom surface (32) of the groove intersects with the gingival side surface (31a) of the groove toward the tooth surface. Alternatively, it is preferably 3 mm or less, 2 mm or less, or 1 mm or less. These upper and lower limits can be any combination. Therefore, dimension (q3) has a size of, for example, 0.1 mm or more and 3 mm or less, preferably 0.1 mm or more and 2 mm or less, and more preferably 0.3 mm or more and 1 mm or less, from the corner where the bottom surface (32) of the groove intersects with the gingival side surface (31a) of the groove toward the tooth surface. The dimension (q4) is preferably 0.1 mm or more, or 0.3 mm or more, in the direction towards the occlusal side from the corner where the bottom surface (32) of the groove intersects with the side surface (31b) of the occlusal groove. Alternatively, it is preferably 3 mm or less, 2 mm or less, or 1 mm or less. These upper and lower limits can be any combination. Therefore, the dimension (q4) in the direction towards the occlusal side from the corner where the bottom surface (32) of the groove intersects with the side surface (31b) of the occlusal groove has, for example, a dimension of 0.1 mm or more and 3 mm or less, preferably 0.1 mm or more and 2 mm or less, and more preferably 0.3 mm or more and 1 mm or less. Furthermore, the dimension (not shown) in the depth direction of the groove (30) in region (Q) is not particularly limited; for example, it can be the same as the width of the groove (30) of the orthodontic bracket (100).

[0060] Region (Q) only needs to have the stated dimensions, but from the viewpoint of the accuracy of linear transmittance measurement, in order to prevent incident light from being scattered by a portion of the sidewall portion, as shown in FIG6, region (Q) preferably only has the stated dimensions (q1), i.e., q1 > 0 and q2 = q3 = q4 = 0. For example, the sample containing the region (Q) described above can also be further cut to produce a measurement sample with q1 > 0 and q2 = q3 = q4 = 0. The measurement sample in the transmittance measurement only needs to be set to have a surface roughness Ra ≦ 0.02 μm.

[0061] The linear transmittance in the conversion transmittance measurement can be measured using a general micro-spectrophotometer (e.g., MSV-370, manufactured by Nippon Spectrophotometer Co., Ltd.). Incident light is irradiated onto the surface of the test sample facing the bottom of the groove (hereinafter also referred to as the "adhesion surface") in the conversion transmittance measurement. The aperture position is adjusted so that transmitted light can be obtained from the bottom of the groove, and the measurement is performed. The aperture position refers to the area range through which linear light is transmitted in the conversion transmittance measurement. Figure 7 is an enlarged view showing the area near the aperture position of the sintered body when the conversion transmittance of the orthodontic bracket is measured. As shown in Figure 7, the aperture (200) is located within the bottom surface (32) of the groove of the sintered body (102). The following conditions can be listed as the conditions for measurement. Light source: Halogen lamp; Measurement wavelength: 600 nm; Aperture size: 400 μm × 100 μm (100 μm is the width of the bottom surface of the groove); Light transmission direction: From the bonding surface (grinding and finishing) to the bottom surface of the groove (unfinished); Number of measurement sites: 5.

[0062] In this embodiment, the converted transmittance of the bracket can be obtained by measuring the linear transmittance of the test sample in the converted transmittance measurement and converting it into the linear transmittance when the sample thickness is 1 mm using the following formula (1). For example, the linear transmittance of a sample with a thickness of x mm (e.g., x is 0.1 or more and 3 or less, preferably x is 0.2 or more and 2 or less, more preferably x is 0.3 or more and 1 or less, and even more preferably x is 0.3 or more and 0.5 or less) can be measured, and the linear transmittance of the sample with a thickness of 1 mm can be calculated using formula (1). T1 = (Tx × 0.01)^(1 / x) × 100 [%] ··· (1) T1: Converted transmittance [%] Tx: Measured value of linear transmittance when the sample thickness is x mm [%] x: Measured sample thickness [mm]

[0063] Any sample that has undergone aging treatment can be used for the determination. The aging treatment can be as follows: after setting the surface roughness to Ra≦0.02 μm, pure water and the sample are placed in a stainless steel pressure-resistant container, and then placed in an autoclave and kept at 140°C for 72 hours.

[0064] The preferred transmittance is 10% or higher, 12% or higher, or 14% or higher. Alternatively, it is 25% or lower or 18% or lower.

[0065] Furthermore, in the measurement of the converted transmittance of the bracket containing the sintered body of this embodiment, the bottom surface of the groove refers to the planar portion. For example, if a part of the shape of the bottom surface has an arc-shaped curved portion, the curved portion is not included in the bottom surface of the groove.

[0066] The sintered body of this embodiment has high strength. The torque strength of the orthodontic bracket containing the sintered body of this embodiment may vary depending on the size of the bracket. For example, the torque strength in the bracket mainly depends on the size of the bracket, namely the inner and outer dimensions and the thickness of the gingival sidewall. Specifically, there is a tendency that if the inner and outer dimensions and the thickness of the gingival sidewall increase, the value of the torque strength also increases. Regarding the bracket of this embodiment, when the inner and outer dimensions and the thickness of the gingival sidewall are all 1.0 mm or less, the torque strength is preferably 1.0 kgf·cm or more, and when the inner and outer dimensions and the thickness of the gingival sidewall are all 0.6 mm or less, the torque strength is more preferably 1.0 kgf·cm or more. In addition, when the inner and outer dimensions and the thickness of the gingival sidewall are all 0.4 mm or more, or 0.3 mm or more, the torque strength is preferably 1.0 kgf·cm or more. In this embodiment, the torque strength of the bracket containing the sintered body of this embodiment can be obtained by pressing stainless steel wire into the wire groove and determining the value when the bracket breaks.

[0067] The deviation of the torque strength in the bracket is preferably 0.30 or less, further preferably 0.25 or less, and even further preferably 0.20 or less. Alternatively, it may be 0.01 or more, or 0.05 or more. In this embodiment, the deviation of the torque strength of the bracket including the sintered body of this embodiment is the absolute value of the difference between each measured value obtained by measuring the torque strength of multiple (e.g., 2 or more and 50 or less) brackets and their average value, and refers to the largest value.

[0068] The flexural strength of the sintered body in this embodiment is preferably 500 MPa or more, and further preferably 600 MPa or more. In order to broaden the applicable applications, the flexural strength of the sintered body in this embodiment is preferably 800 MPa or more, and further preferably 1000 MPa or more.

[0069] Preferably, the sintered body of this embodiment has a breaking toughness equal to or greater than that of a transparent zirconia sintered body containing cubic zirconia, such as a sintered body containing 8 mol% yttrium and the remainder containing zirconia. Therefore, the sintered body of this embodiment can be used as a component employing a previously used transparent zirconia sintered body. Examples of breaking toughness for the sintered body of this embodiment include 1.7 MPa·m0.5 or more, further including 1.8 MPa·m0.5 or more, further including 2 MPa·m0.5 or more, further including 2.2 MPa·m0.5 or more, and examples include 2.5 MPa·m0.5 or less, or 3.0 MPa·m0.5 or less.

[0070] In this embodiment, the failure toughness can be determined by either the indentation fracture (IF) method according to JIS R1607 or the single edge precracked beam (SEPB) method, with the value determined by the SEPB method being particularly preferred. Furthermore, the failure toughness determined by the IF method shows a higher value than the failure toughness value determined by the SEPB method.

[0071] As explained above, the sintered body of this embodiment has both excellent mechanical strength and light transmittance, and therefore can be used for known applications of light-transmitting zirconia sintered bodies, such as window materials, decorative components, and exterior components of electronic devices. In particular, it can be better used for orthodontic components and orthodontic brackets that require aesthetics.

[0072] Next, the method for manufacturing the sintered body of this embodiment will be described.

[0073] In this embodiment, the sintered body of this embodiment can be manufactured by the following manufacturing method, characterized by including: a mixing step, in which zirconium oxide raw material, stabilizing element raw material and lanthanum raw material are mixed to obtain a mixed powder; a forming step, in which the obtained mixed powder is formed to obtain a shaped body; a sintering step, in which the obtained shaped body is placed in an inner container and the inner container is placed in an outer container, and sintering is performed at a sintering temperature of 1650°C or higher to obtain a sintered body; and a cooling step, in which the temperature is cooled from the sintering temperature to 1000°C at a cooling rate of more than 1°C / min.

[0074] In the mixing step, the zirconium oxide raw material, the stabilizing element raw material, and the lanthanum raw material are mixed to obtain a mixed powder. The mixing method is arbitrary, and can be either wet mixing or dry mixing, as long as the zirconium oxide raw material, the stabilizing element raw material, and the lanthanum raw material can be uniformly mixed. To further improve the uniformity of the obtained mixed powder, the mixing method is preferably wet mixing, and more preferably wet mixing using at least one of a wet ball mill and a wet stirred mill.

[0075] The zirconium oxide raw material is zirconium oxide or its precursor, such as zirconium oxide powder with a specific surface area of ​​more than 4 m2 / g and less than 20 m2 / g by Brunauer-Emmett-Teller (BET).

[0076] The stabilizing element raw material is a powder containing at least one compound selected from the group consisting of yttrium, scandium, calcium, magnesium and cerium, preferably a powder containing a compound containing yttrium or a precursor thereof.

[0077] Furthermore, the zirconium oxide raw material is preferably zirconium oxide powder containing stabilizing elements (hereinafter also referred to as "zirconia containing stabilizing elements"). This zirconium oxide powder serves as both the zirconium oxide raw material and the stabilizing element raw material. The stabilizing element contained in the zirconium oxide powder is preferably at least one selected from the group consisting of yttrium, scandium, calcium, magnesium, and cerium, more preferably yttrium. The zirconium oxide powder containing stabilizing elements is zirconium oxide powder containing 1 mol% or more and 6 mol% or less of stabilizing elements in terms of oxide content, preferably zirconium oxide powder with a BET specific surface area of ​​4 m² / g or more and 20 m² / g and containing 1 mol% or more and 6 mol% or less of stabilizing elements. The amount of stabilizing element contained in the zirconium oxide powder containing stabilizing elements is preferably 2 mol% or more and 5 mol% or less, and further 2 mol% or more and 4 mol% or less, in terms of oxide content.

[0078] Lanthanum raw materials may include compounds containing lanthanum, and may include at least one selected from the group consisting of lanthanum oxide, lanthanum hydroxide, lanthanum nitrate, lanthanum sulfate, lanthanum chloride, lanthanum carbonate and pyrochlore-type La2Zr2O7, preferably selected from at least any one of the group consisting of lanthanum hydroxide, lanthanum oxide and La2Zr2O7, more preferably at least any one of lanthanum hydroxide and lanthanum oxide, and even more preferably lanthanum hydroxide.

[0079] The mixed powder may also contain alumina raw material. The alumina raw material may include compounds containing aluminum, preferably at least one selected from the group consisting of alumina, aluminum hydroxide, aluminum carbonate, and spinel, and more preferably alumina. As preferred alumina, at least one of α-alumina and γ-alumina may be included, and further, α-alumina may be included.

[0080] The composition of the mixed powder can be any desired proportion, for example, zirconium oxide is 84 mol% or more and 98 mol% or less, stabilizing element is 1 mol% or more and 6 mol% or less, and lanthanum is 1 mol% or more and 10 mol% or less.

[0081] The following molar compositions are preferred for the mixed powder: Zirconia: 90 mol% or more and 95 mol% or less, preferably 92 mol% or more and 94 mol%; Stabilizing element: 2 mol% or more and 5 mol% or less, preferably 2 mol% or more and 4 mol%; Lanthanum: 2 mol% or more and 6.5 mol% or less, preferably 3 mol% or more and 5 mol% or less.

[0082] The stabilizing element in the composition is preferably yttrium.

[0083] In the molding step, the mixed powder is molded to obtain a molded article. The molding method is arbitrary as long as a molded article of the desired shape can be obtained. As a molding method, at least one selected from the group consisting of compression molding, injection molding, sheet molding, extrusion molding and casting molding is included, preferably at least one of compression molding and injection molding.

[0084] The shape of the molded body is arbitrary, for example, it can be a circular plate, a cylindrical shape, a polyhedral shape, or a toothed straightening bracket or a semiconductor manufacturing jig, or other complex shapes, etc., any shape corresponding to the purpose or use. In the sintering step, by placing the molded body in the inner container and placing the inner container in the outer container for sintering, a sintered body with a highly smooth surface can be obtained. The container during sintering is used to avoid the reduction of the smoothness of the sintered body surface caused by direct contact between the sintered material and the ambient medium flow of the sintering furnace, that is, to avoid the increase of the surface roughness of the sintered body caused by the direct exposure of the sintered material to the ambient gas flow introduced from outside the system into the sintering furnace.

[0085] Figure 8 is a schematic diagram showing an example of the configuration of the molded body in the sintering step. The molded body (300) is disposed inside the inner container (301), which is disposed inside the outer container (302). The inner container can be any shape in which the molded body can be disposed, and the outer container can be any shape in which the inner container can be disposed.

[0086] The inner container and the outer container are any containers other than sealed containers, that is, containers that do not block the flow of ambient gas. For example, containers that do not become closed systems relative to the environment in the sintering furnace, without directly exposing the sintered material to the ambient gas flow introduced from outside the system into the sintering furnace. Specific containers include covered containers, such as covered crucibles or covered saggers. Furthermore, in Figure 8, the inner container (301) and the outer container (302) are shown as covered containers. The inner container (301) is shown with a lid on when the sintered material is contained and the inner container (301) is not sealed, and the outer container (302) is shown with a lid on when the inner container (301) is contained and the outer container (302) is not sealed.

[0087] The material of the inner container is at least one of metal oxides and metal nitrides, preferably metal oxides, more preferably at least one selected from the group consisting of alumina, zirconium oxide, mullite, yttrium oxide, spinel, magnesium oxide, silicon nitride and boron nitride, and even more preferably at least one selected from the group consisting of alumina, zirconium oxide, mullite and yttrium oxide, and even more preferably yttrium oxide.

[0088] The material of the outer container is selected from at least one of the group consisting of carbon, metal oxides, and metal nitrides. From the viewpoint of obtaining a sintered body with both excellent mechanical strength and light transmittance, and suppressing deviations in mechanical strength between sintered bodies, the material of the outer container is preferably carbon. Furthermore, when the outer container is made of carbon, it is cheaper than outer containers made of metal oxides or metal nitrides, and has high industrial applicability, thus it is preferred. In the manufacturing method of this embodiment, the container is dualized, so the carbon from the container does not adhere to the surface of the sintered object, making it easy to obtain a sintered body with excellent mechanical strength and light transmittance. Furthermore, the inventors have found that when using a carbon outer container, in particular, deviations in mechanical strength between sintered bodies are suppressed.

[0089] Furthermore, in the case where the sintering step is the two-stage sintering method described later, it is sufficient to place the primary sintered body in the inner container instead of the formed body. That is, in the secondary sintering, it is sufficient to place the primary sintered body in the inner container and then place the inner container in the outer container for sintering.

[0090] In the sintering step, the shaped body obtained in the forming step is sintered at a sintering temperature of 1650°C or higher. It is believed that by sintering at 1650°C or higher, the crystalline structure of the sintered body becomes a high-temperature crystalline structure. By passing the sintered body with the high-temperature crystalline structure through a cooling step, cubic and tetragonal crystal domains are generated in the crystalline structure of the crystalline particles, and a sintered body containing the crystalline structure of the sintered body of this embodiment can be obtained. The sintering temperature is 1650°C or higher, preferably 1700°C or higher, more preferably 1725°C or higher, and even more preferably 1750°C or higher. When using a general-purpose sintering furnace, the sintering temperature may be 2000°C or lower, 1900°C or lower, or 1800°C or lower.

[0091] If sintering is performed at the sintering temperature, the sintering method is arbitrary. For example, at least one selected from the group consisting of atmospheric pressure sintering, pressure sintering and vacuum sintering can be listed, with atmospheric pressure sintering and pressure sintering being preferred.

[0092] As a preferred sintering step, examples include: a sintering method using only atmospheric pressure sintering (hereinafter also referred to as "one-stage sintering method"); or a sintering step including one-stage sintering and two-stage sintering (hereinafter also referred to as "two-stage sintering method"), wherein the one-stage sintering is to obtain a one-stage sintered body by firing the shaped body at a temperature above 1000°C and below 1650°C, and the two-stage sintering is to sinter the one-stage sintered body at a temperature above 1650°C.

[0093] The one-stage sintering method simply involves subjecting the sintering step to pressureless sintering to obtain a sintered body. Pressureless sintering is a method of sintering by heating the material without applying external force during sintering. In this embodiment, the sintered body is produced by pressureless sintering the shaped body obtained in the forming step. The sintering temperature can be 1600°C or higher, preferably 1700°C or higher and 1900°C or lower. The sintering environment can be either an oxidizing environment or a reducing environment. For simplicity, an atmospheric environment is preferred.

[0094] The two-stage sintering method is a method of producing a primary sintered body by first sintering a shaped body and then performing a second sintering on the primary sintered body. The primary sintering is preferably performed at a temperature above 1000°C and below 1650°C. The primary sintering environment is preferably an oxidizing or reducing environment, more preferably an oxidizing environment, and further preferably an atmospheric environment. As a preferred primary sintering method, atmospheric sintering at a temperature above 1000°C, above 1400°C, and below 1650°C, or below 1520°C, is also an example. This results in a finer microstructure in the obtained primary sintered body. Furthermore, pores are less likely to form within the crystalline particles of the primary sintered body.

[0095] Secondary sintering involves sintering the primary sintered body at a temperature of 1650°C or higher, preferably 1700°C or higher, more preferably 1725°C or higher, and even more preferably 1750°C or higher. To obtain a sintered body with high strength, the secondary sintering temperature is below 2000°C, preferably below 1900°C, and more preferably below 1800°C. By setting the secondary sintering temperature below 2000°C, it is less likely to generate coarse crystalline particles.

[0096] In order to obtain a sintered body with higher density, the secondary sintering is preferably hot isostatic pressing (hereinafter also referred to as "HIP (High Temperature Insostatic Pressing)").

[0097] The HIP treatment time (hereinafter also referred to as "HIP time") may be 10 minutes or more or 30 minutes or more and 4 hours or less or 2 hours or less. In order to fully remove the porosity of the sintered body, the HIP time is preferably 10 minutes or more during the HIP treatment.

[0098] The pressure medium (hereinafter also referred to as "pressure medium") used in HIP processing can be exemplified by argon, nitrogen, oxygen, etc., but the simplest is ordinary argon.

[0099] The pressure of the HIP process (hereinafter also referred to as "HIP pressure") is preferably 5 MPa or higher, and more preferably 50 MPa or higher. By using a HIP pressure of 5 MPa or higher, the removal of pores in the sintered body can be further promoted. There is no specific upper limit for the pressure, but when using a conventional HIP device, the HIP pressure is 200 MPa or lower.

[0100] In the cooling step, the temperature is reduced to 1000°C from the secondary sintering temperature at a cooling rate of more than 1°C / min. By setting the secondary sintering temperature to 1650°C or higher and the cooling rate to more than 1°C / min, preferably 5°C / min or higher, and more preferably 8°C / min or higher, a sintered body with cubic and tetragonal crystal domains and high light transmittance can be obtained. When the cooling rate is less than 1°C / min, precipitates or monoclinic crystals will form, and the resulting sintered body will have low light transmittance. In order to obtain a lanthanum solid solution zirconia sintered body with higher light transmittance, the cooling rate from the sintering temperature to 1000°C can be set to a cooling rate of more than 10°C / min, more preferably 15°C / min or higher, and even more preferably 30°C / min or higher, and even more preferably 50°C / min or higher. There is no particular upper limit to the cooling rate, but examples include below 150°C / min or below 100°C / min.

[0101] The manufacturing method of this embodiment may also include an annealing step for heat treatment of the sintered body after the cooling step. By subjecting the sintered body to the annealing step, the light transmittance of the sintered body can be further improved. The annealing step may be performed on the sintered body in an oxidizing environment at a temperature of 900°C or higher and 1200°C or lower, preferably 980°C or higher and 1030°C or lower.

[0102] The sintered body obtained by the method tends to have a smaller deviation in mechanical strength. Specifically, the deviation in torque strength of the sintered body of this embodiment with a bracket shape can be listed as 0.30 or less, further as 0.25 or less, and further as 0.20 or less, and can also be listed as 0.01 or more or 0.05 or more. The deviation in torque strength is preferably within the above range, and more preferably a smaller deviation. Since the occurrence of poor bracket strength can be effectively suppressed, the minimum torque strength of the sintered body is maintained at a high level and the reliability of the sintered body is improved. In addition, production stability is improved by improving yield. [Example]

[0103] Hereinafter, the present invention will be specifically described by way of examples and comparative examples. However, the present invention is not limited to the examples.

[0104] (Determination of average crystal grain size) After planar grinding of the sintered sample, mirror polishing was performed sequentially using diamond abrasive grains of 9 μm, 6 μm and 1 μm. The polished surface was kept at 1400°C for 1 hour, and after thermal etching, SEM observation was performed. Based on the obtained SEM images, the average crystal grain size was determined by planar measurement method.

[0105] (Identification of Crystalline Structure) The crystalline structure of each sintered body sample was identified and confirmed by analyzing the XRD patterns obtained from XRD measurements. The presence or absence of impurity layers was also confirmed. XRD measurements were performed on mirror-polished sintered body samples using a standard powder X-ray diffraction apparatus (Ultima III, manufactured by Rigaku). The XRD measurements were performed using CuKα rays as the X-ray source under the following conditions: Accelerating current / voltage: 40 mA / 40 kV; X-ray source: CuKα rays (λ=1.5405 Å); Measurement mode: Step scan; Scanning conditions: 0.04° / sec; Measurement range: 2θ=20°~80°; Diverging slit: 0.5 deg; Scattering slit: 0.5 deg; Receiving slit: 0.3 mm; Detector: Scintillation counter.

[0106] The XRD pattern was determined using a standard powder X-ray diffraction apparatus (Ultima III, manufactured by Rigaku). Additionally, the crystalline XRD peaks were detected by determining the 2θ of the peak apex using standard analytical software (JADE7, manufactured by MID). The XRD pattern analysis was performed under the following conditions: Fitting conditions: Automatic, background refinement, dispersion-type pseudo-Voigt function (peak shape); Background removal method: Fitting method; Kα2 removal method: Kα1 / Kα2 ratio = 0.497; Smoothing method: B-spline curve smoothing conditions: quadratic differential method, σ cutoff value = 3, χ threshold value = 1.5.

[0107] (Determination of average crystallite diameter) For the XRD pattern obtained using the same determination method as for the identification of the crystalline phase, the average crystallite diameter of the sintered sample was determined using the Scherrer formula. D=K×λ / ((β-B)×cosθ) In the formula, D is the average crystallite diameter (nm), K is the Scherrer constant (1.0), λ is the wavelength of CuKα (0.15418 nm), β is the half-width (°), B is the device constant (0.1177°), and θ is the diffraction angle of the XRD peak (°). The XRD peaks at which the half-width was determined were the XRD peaks of tetragonal crystal with 2θ=30.0±2° and the XRD peaks of cubic crystal with 2θ=29.6±2°. Furthermore, regarding the main peak, the peaks corresponding to the cubic (111) plane of zirconium oxide and the peaks that overlapped with the peaks corresponding to the tetragonal (111) plane were considered as single peaks. In addition, FWHM used "Integral Analysis for Windows" (Version 6.0) manufactured by Rigaku Corporation to obtain the results.

[0108] (Determination of Torque Strength) A sintered body in the shape of a bracket was used as a sample for determining torque strength. The bracket was fixed to a pedestal as a sample, and a stainless steel wire (0.019 × 0.025 inches) was threaded through the groove of the sample for fixation. The surface of the groove of the sample was prepared with HIP treatment. The stainless steel wire was pressed into the groove, and the torque strength at the point of bracket breakage was measured. More than one sample was prepared for each embodiment and comparative example, and the minimum torque strength, the maximum torque strength, the average value of each measured value (i.e., the average torque strength), and the maximum deviation of each measured value from the average value (i.e., the torque strength deviation) were determined.

[0109] (Determination of linear transmittance) In the determination of linear transmittance, for each embodiment and comparative example, a sintered body in the shape of a test piece was used as the sample.

[0110] The linear transmittance of a sintered body (a rectangular plate-shaped body of 30 mm × 25 mm and 1.0 mm thickness, hereinafter referred to as "test piece") was measured according to the method of JIS K321-1. Visible light with a wavelength of 600 nm was irradiated onto the test sample, and the light beam passing through the test sample was detected using an integrating sphere to measure the linear transmittance. The linear transmittance obtained in this way is set as "1 mm thickness transmittance". A general haze meter (device name: haze meter NDH2000, manufactured by NIPPON DENSHOKU) was used for the measurement. Furthermore, before the measurement, both sides of the test piece were surface ground, and then mirror polished with diamond abrasive grains of 9 μm, 6 μm and 1 μm respectively to make the surface roughness Ra less than 0.02 μm.

[0111] (Determination of Converted Transmittance) In the determination of converted transmittance, for each embodiment and comparative example, the sample after aging treatment of the sintered body in the bracket shape was used as the sample for measurement. Aging was carried out by placing pure water and the sample into a stainless steel pressure vessel, then placing it into an autoclave, and maintaining it at 140°C for 72 hours.

[0112] After cutting the bracket in a manner including region Q shown in Figure 5 and removing the cut portion, the cut portion of the bracket was mirror-polished using the same method as the test piece to achieve a surface roughness Ra of less than 0.02 μm. Pure water and the polished bracket were placed in a stainless steel pressure-resistant container, and then placed in an autoclave and kept at 140°C for 72 hours to perform aging treatment, and test samples were prepared.

[0113] A microspectrophotometer (device name: MSV-370, manufactured by Nippon Spectrophotometer Co., Ltd.) was used in the measurement. Incident light was irradiated onto the bonding surface of the test sample. The aperture position was adjusted so that transmitted light could be obtained from the bottom surface of the groove, and the measurement was performed. The linear transmittance obtained in this way was set as the linear transmittance of the support. The measurement was performed under the following conditions: Light source: halogen lamp; Measurement wavelength: 600 nm; Aperture size: 400 μm × 100 μm (100 μm is the width direction of the bottom surface of the groove); Light transmission direction: from the bonding surface (grinding and finishing) to the bottom surface of the groove (unfinished); Number of measurement sites: 5.

[0114] In this embodiment and comparative example, the converted transmittance is the linear transmittance when the sample thickness is 1 mm, that is, it is obtained by converting the sample thickness to the linear transmittance when it is 1 mm using the following formula (1). T1 = (Tx × 0.01)^(1 / x) × 100 [%] ··· (1) T1: Converted transmittance [%] Tx: Linear transmittance of the bracket when the sample thickness is x mm [%] x: Measured sample thickness [mm]

[0115] Example 1: La(OH)3 powder was added to zirconia powder at a mass ratio of 11.7% by mass to 3 mol% yttrium-containing zirconia powder (BET specific surface area of ​​7 m² / g), and dispersed in pure water to prepare a slurry with a solid content of 50% by mass. The obtained slurry was pulverized using a wet ball mill with zirconia balls of 10 mm diameter. The average particle size of the obtained mixed powder was 0.4 μm. The slurry was dried and granulated using a spray dryer to prepare the raw material powder.

[0116] The obtained mixed powder is mixed with an organic binder containing wax, plasticizer and thermoplastic resin, and then injection molded to obtain a rectangular plate-shaped molded body (test piece shape) with a thickness of 30 mm × 25 mm and a thickness of 1.0 mm, and a bracket-shaped molded body with dimensions of I / O: 0.506 mm and gingival sidewall thickness: 0.506 mm.

[0117] The obtained molded body was heated in the atmosphere at 450°C and then calcined in the atmosphere at 1500°C for 2 hours to obtain a primary sintered body. The obtained primary sintered body was placed inside a yttrium oxide inner container. The inner container containing the primary sintered body was placed inside a carbon outer container.

[0118] Under this condition, in a 99.9% argon atmosphere, the primary sintered body was subjected to HIP treatment at a heating rate of 600°C / h, a HIP temperature of 1750°C, a HIP pressure of 150 MPa, and a holding time of 1 hour. After HIP treatment, the body was cooled from the sintering temperature to room temperature to obtain a HIP-treated body. Furthermore, the cooling rate from the HIP temperature to 1000°C was 45°C / min.

[0119] A colorless and translucent sintered body was obtained by heat-treating the obtained HIP-treated body at 1000°C for 1 hour in atmospheric air. The obtained sintered body is a sintered body containing 4.1 mol% lanthanum and 2.9 mol% yttrium zirconium oxide in solid solution, with an average crystal grain size of 45 μm. In addition, according to the XRD pattern shown in Figure 9, the average crystallite diameter is 27 nm. Based on the fact that the average crystallite diameter is smaller than the average crystal grain size, it was confirmed that the obtained sintered body contains tetragonal and cubic crystal domains in the crystal particles. The linear transmittance was measured for the obtained test piece-shaped sintered body, and the torque intensity and linear transmittance were measured for the bracket-shaped sintered body. For the sintered body in the shape of a bracket, a test sample with a thickness q1 of 0.3 mm is taken out by grinding and cutting. After aging treatment, the linear transmittance T0.3 [%] when the sample thickness is 0.3 mm is measured, and the linear transmittance T1 [%] when the sample thickness is set to 1 mm is calculated by formula (1) and used as the conversion transmittance.

[0120] Example 2 Except that the shape of the bracket-shaped molded body was set to I / O: 0.813 mm and the gingival sidewall thickness: 0.813 mm for injection molding, the sintered body of this example was obtained using the same method as in Example 1. The linear transmittance of the obtained test piece-shaped sintered body was measured, and the torque strength and converted transmittance of the bracket-shaped sintered body were measured.

[0121] Comparative Example 1 An alumina-based orthodontic bracket, which serves as a standard orthodontic bracket, was prepared using the following method. High-purity alumina powder (purity 99.99%, BET specific surface area 14 m2 / g) was mixed with an organic binder containing wax, plasticizer, and thermoplastic resin to obtain an alumina composite.

[0122] The obtained alumina composite was injection molded to obtain a rectangular plate-shaped molded body with a thickness of 1.0 mm and a bracket-shaped molded body with dimensions of I / O: 0.508 mm and gingival sidewall thickness: 0.508 mm.

[0123] The obtained molded body was heated at 450°C in the atmosphere and then calcined at 1300°C in the atmosphere for 2 hours to obtain a primary sintered body. The primary sintered body was subjected to HIP treatment in a 99.9% argon atmosphere at a heating rate of 600°C / h, a HIP temperature of 1500°C, a HIP pressure of 150 MPa, and a holding time of 1 hour. After HIP treatment, the body was cooled from the sintering temperature to room temperature to obtain a HIP-treated body, which was used as the sintered body of this comparative example. The sintered body of this comparative example is colorless and translucent. The linear transmittance was measured for the obtained test piece-shaped sintered body, and the torque strength and converted transmittance were measured for the bracket-shaped sintered body. For a sintered body in the shape of a bracket, a test sample with a thickness q1 of 0.3 mm is taken out by grinding and cutting. After measuring the linear transmittance T0.3 [%] when the sample thickness is 0.3 mm, the linear transmittance T1 [%] when the sample thickness is set to 1 mm is calculated by formula (1) and used as the conversion transmittance.

[0124] Comparative Example 2: Except that the shape of the bracket-shaped molded body was set to I / O: 0.814 mm and the gingival sidewall thickness: 0.814 mm for injection molding, the sintered body of this comparative example was obtained using the same method as Comparative Example 1. The linear transmittance of the obtained test piece-shaped sintered body was measured, and the torque strength and converted transmittance of the bracket-shaped sintered body were measured.

[0125] Comparative Example 3 Except that the HIP temperature was set to 1300°C, the sintered body of this comparative example was obtained using the same method as in Comparative Example 1. The linear transmittance was measured for the obtained test piece-shaped sintered body, and the torque strength and converted transmittance were measured for the bracket-shaped sintered body. For the bracket-shaped sintered body, a test sample with a sample thickness q1 of 0.3 mm was taken out by grinding and cutting. After measuring the linear transmittance T0.3 [%] when the sample thickness was 0.3 mm, the linear transmittance T1 [%] when the sample thickness was 1 mm was calculated by Equation (1) and used as the converted transmittance.

[0126] Comparative Example 4: Except that the shape of the bracket-shaped molded body was set to I / O: 0.814 mm and the gingival sidewall thickness: 0.814 mm for injection molding, the sintered body of this comparative example was obtained using the same method as in Comparative Example 3. The linear transmittance of the obtained test piece-shaped sintered body was measured, and the torque strength and converted transmittance of the bracket-shaped sintered body were measured.

[0127] Comparative Example 5: Except that no outer container was used, a lidded zirconia container was used as the inner container, and the shape of the bracket-shaped molded body was set to I / O: 1.016 mm and gingival sidewall thickness: 0.864 mm for injection molding, a sintered body containing 4.1 mol% lanthanum and 2.9 mol% yttrium dissolved in zirconia was obtained using the same method as in Example 1, and this was used as the sintered body of this comparative example. The linear transmittance of the obtained test piece-shaped sintered body was measured, and the torque strength of the bracket-shaped sintered body was measured.

[0128] Comparative Example 6: Except that a yttrium oxide outer container was used instead of a carbon outer container, and the shape of the bracket-shaped molded body was set to I / O: 1.016 mm and gingival sidewall thickness: 0.864 mm for injection molding, a sintered body containing 4.1 mol% lanthanum and 2.9 mol% yttrium dissolved in zirconium was obtained using the same method as in Example 1, and this was used as the sintered body of this comparative example. The linear transmittance of the obtained test piece-shaped sintered body was measured, and the torque strength of the bracket-shaped sintered body was measured.

[0129] Table 1 shows the composition of the sintered body, the size of the tooth alignment bracket and the HIP temperature in the embodiments and comparative examples.

[0130] [Table 1] Composition [mol%] Orthodontic bracket dimensions HIP temperature [℃] ZrO2 Y2O3 La2O3 Al2O3 Internal and external dimensions [mm] Gingival lateral wall thickness [mm] Example 1 93.0 2.9 4.1 0.0 0.506 0.506 1750 Example 2 93.0 2.9 4.1 0.0 0.813 0.813 1750 Comparative Example 1 0.0 0.0 0.0 100.0 0.508 0.508 1500 Comparative Example 2 0.0 0.0 0.0 100.0 0.814 0.814 1500 Comparative Example 3 0.0 0.0 0.0 100.0 0.508 0.508 1300 Comparative Example 4 0.0 0.0 0.0 100.0 0.814 0.814 1300 Comparative Example 5 93.0 2.9 4.1 0.0 1.016 0.864 1750 Comparative Example 6 93.0 2.9 4.1 0.0 1.016 0.864 1750

[0131] Table 2 shows the results of torque strength, linear transmittance, and converted transmittance of the sintered bodies in the Examples and Comparative Examples. In addition, Table 3 shows the calculated value obtained by dividing the torque strength by the inner and outer dimensions or the thickness of the gingival sidewall as the torque strength per unit size of the bracket.

[0132] [Table 2] Torque strength Linear transmittance [%] Transmittance conversion [%] Mean moment strength [kgf·cm] Maximum torque strength [kgf·cm] Minimum moment strength [kgf·cm] Torque strength measurement number N Torque strength deviation test piece Orthodontic bracket Example 1 1.05 1.14 0.98 11 0.05 45 14 Example 2 1.33 1.67 1.09 15 0.16 45 14 Comparative Example 1 0.63 0.76 0.51 12 0.13 11 8.1 Comparative Example 2 1.00 1.13 0.78 14 0.15 11 8.1 Comparative Example 3 0.98 1.21 0.60 14 0.17 13 2.4 Comparative Example 4 1.33 1.82 0.62 5 0.49 13 2.4 Comparative Example 5 0.98 - - 3 - 46 - Comparative Example 6 1.34 - - 1 - 34 -

[0133] [Table 3] The calculated value is obtained by dividing the torque strength by the internal and external dimensions or the thickness of the gingival lateral wall. Average moment strength [kgf·cm] / internal and external dimensions [mm] Average torque strength [kgf·cm] / Gingival lateral wall thickness [mm] Example 1 2.08 2.08 Example 2 1.64 1.64 Comparative Example 1 1.24 1.24 Comparative Example 2 1.23 1.23 Comparative Example 3 1.93 1.93 Comparative Example 4 1.63 1.63 Comparative Example 5 0.96 1.13 Comparative Example 6 1.32 1.55

[0134] Based on the measurement results of the sintered bodies of Examples 1 and 2, it can be confirmed that the sintered body of this embodiment possesses both excellent mechanical strength and permeability, and thus the deviation in mechanical strength is small. On the other hand, it can be confirmed that the sintered bodies of Comparative Examples 1 to 4 generally have poor permeability. In addition, it can be confirmed that the sintered bodies of Comparative Example 5 (which uses a double container without an inner container and an outer container during sintering) and Comparative Example 6 (which does not use a carbon outer container during sintering) do not possess the same excellent mechanical strength and permeability as the examples.

[0135] This application claims priority based on Japanese Patent Application No. 2021-89267, filed on May 27, 2021, and cites all the contents of that Japanese Patent Application. [Simplified Explanation of the Diagram]

[0013] Figure 1 is a schematic perspective view showing an example of the shape of a dental orthodontic bracket. Figure 2 is a schematic plan view showing an example of the shape of a dental orthodontic bracket. Figure 3 is a schematic cross-sectional view obtained by cutting the dental orthodontic bracket of Figure 2 at surface (90). Figure 4 is a diagram showing the case where the corner where the bottom surface of the groove intersects with the side surface of the gingival groove has an arc-shaped curved surface portion (R), and is an enlarged view showing the vicinity of the corner. Figure 5 is a schematic side view showing an example of the method for preparing the test specimen in the conversion transmittance measurement of a dental orthodontic bracket. Figure 6 is a schematic side view showing an example of the method for preparing the test specimen in the conversion transmittance measurement of a dental orthodontic bracket. Figure 7 is an enlarged view showing the vicinity of the aperture position of the sintered body when measuring the conversion transmittance of the dental orthodontic bracket. Figure 8 is a schematic diagram showing an example of the configuration of the formed body in the sintering step. Figure 9 is a diagram showing the result of the X-ray diffraction pattern of Example 1.

Claims

1. A sintered body, characterized in that it comprises crystalline particles having cubic and tetragonal crystal domains, and uses zirconium oxide with a stabilizing element and lanthanum dissolved in it as a matrix, wherein the content of the stabilizing element is 2 mol% or more and 6 mol% or less, the stabilizing element being at least one selected from the group consisting of yttrium, scandium, calcium, magnesium and cerium, the content of the lanthanum contained in the sintered body is 3 mol% or more and 10 mol% or less, the average torque strength of the sintered body is 1.00 kgf·cm or more, and the linear transmittance of visible light at a wavelength of 600 nm is 35% or more when the sample thickness is 1 mm, wherein the average torque strength is obtained by processing the sintered body into the shape of a tooth alignment bracket, fixing the bracket on a base with the inner and outer dimensions and the thickness of the gingival sidewall being 1.0 mm or less, pressing stainless steel wire into the wire groove of the bracket for fixation, and measuring the value at which the bracket breaks.

2. The sintered body as claimed in claim 1, wherein the deviation of the torque strength is less than 0.

30.

3. The sintered body as described in claim 1 or claim 2, comprising at least one of a lanthanide element other than lanthanum or a transition metal.

4. A method for manufacturing a sintered body, characterized by comprising: The process includes a mixing step, in which zirconium oxide raw material, stabilizing element raw material, and lanthanum raw material are mixed to obtain a mixed powder; a forming step, in which the obtained mixed powder is formed to obtain a shaped body; a sintering step, in which the obtained shaped body is placed inside a yttrium oxide inner container and the inner container is placed inside a carbon outer container, and sintered in an argon atmosphere at a sintering temperature of 1650°C or higher and a pressure of 50 MPa or higher for 10 minutes or more and 4 hours or less to obtain a sintered body; and a cooling step, in which the temperature is reduced from the sintering temperature to 1000°C at a cooling rate of more than 1°C / min.

5. The method for manufacturing a sintered body as described in claim 4, wherein the outer container is made of carbon.

6. A dental orthodontic bracket comprising a sintered body as described in any one of claims 1 or 2.

7. A dental orthodontic bracket, characterized in that it comprises a sintered body containing crystalline particles having cubic and tetragonal crystal domains, and using zirconium oxide with a stabilizing element and lanthanum dissolved in it as a matrix, wherein the content of the stabilizing element is 2 mol% or more and 6 mol% or less, and the stabilizing element is at least one selected from the group consisting of yttrium, scandium, calcium, magnesium, and cerium; the total content of the lanthanum contained in the sintered body is 3 mol% or more and 8 mol% or less; the average torque strength of the dental orthodontic bracket is 1.00 kgf·cm or more, and after hot water treatment at 140°C for 72 hours, the linear transmittance of visible light at a wavelength of 600 nm is 10% or more at a sample thickness of 1 mm; the average torque strength is obtained by processing the sintered body into the shape of a dental orthodontic bracket, with both the inner and outer dimensions and the thickness of the gingival sidewall being 1.0 mm. For cases with a diameter of less than mm, the bracket is fixed on the base, and a stainless steel wire is pressed into the wire groove of the bracket for fixation. The value at which the bracket breaks is then measured.

8. The orthodontic bracket as claimed in claim 7, wherein the deviation of torque strength is less than 0.30.

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