Method for manufacturing zirconia pre-sintered body and its use

Abstract

[Problem] To realize a zirconia pre-sintered body for rapid firing that exhibits superior translucency compared to slow firing. [Solution] The method for manufacturing a zirconia pre-sintered body disclosed herein includes a generation step of manufacturing a zirconia sol slurry containing zirconia powder, a rare earth dispersion step of dispersing rare earth particles with an average particle diameter of 300 nm or less into the zirconia sol slurry, a collecting step of collecting mixed particles containing zirconia and rare earth particles from the zirconia sol slurry, and a heating step of manufacturing partially stabilized zirconia powder by heating the mixed particles. According to this manufacturing method, a zirconia pre-sintered body in which zirconia and rare earth oxides are uniformly dispersed can be produced. This zirconia pre-sintered body exhibits an unprecedented effect of superior translucency during rapid firing compared to slow firing.

Description

METHOD FOR MANUFACTURING ZIRCONIA PRE-SINTERED BODY AND ITS USE

[0001] The technology disclosed herein relates to a method for manufacturing a zirconia pre-sintered body, as well as methods for manufacturing zirconia pre-sintered bodies and zirconia sintered bodies. The present application claims priority based on Japanese Patent Application No. 2024-228087 (filed on December 25, 2025), the entire contents of which are incorporated herein by reference.

[0002] Zirconia sintered bodies are widely used as biomaterials in dental materials (for example, dentures, dental prosthetics, denture milling blanks, dental orthodontic brackets). It is known that when a small amount of rare earth oxides (such as yttria (Y2O3), ytterbia (Yb2O3)) is solid-solubilized as a stabilizing component in these zirconia sintered bodies (hereinafter simply referred to as "sintered bodies"), their strength, toughness, and aesthetic properties are significantly improved. These zirconia sintered bodies are produced by firing a zirconia pre-sintered body (hereinafter simply referred to as "pre-sintered body") that contains zirconia and rare earth oxides. An example of the manufacturing procedure for this zirconia pre-sintered body is disclosed in Patent Document 1. The manufacturing method described in Patent Document 1 includes a mixing step of mixing zirconia sol and rare earth oxides, a drying step of drying the zirconia sol, and a heating step of heating zirconia and rare earth oxides at a low temperature below the sintering temperature.

[0003] In recent years, the rapid firing of pre-sintered bodies in the production of zirconia sintered bodies has been considered. Specifically, in conventional production of zirconia sintered bodies, a long firing process (slow firing) was performed, maintaining a specified firing temperature for several hours (about 1 to 4 hours). In contrast, if rapid firing can be achieved with a holding time of less than 30 minutes, the production efficiency of zirconia sintered bodies can be significantly improved. An example of this rapid firing is disclosed in Patent Document 2. The manufacturing method described in Patent Document 2 sets the holding time for the firing process to less than 20 minutes. According to this Patent Document 2, it is stated that zirconia sintered bodies with the required translucency as dental prosthetic materials can be produced in a short time.

[0004] Japanese Patent Application Laid-open No. 2023-171832 Japanese Patent Application Laid-open No. 2024-7519

[0005] However, it is known that changing the firing process from slow firing to rapid firing in the production of zirconia sintered bodies leads to a decrease in the translucency of the sintered bodies after production. This trade-off between firing time and translucency has not yet been broken, and improvements are sought. For example, in the examples of Patent Document 2, rapid firing with various firing patterns is performed on several types of zirconia pre-sintered bodies. However, the examples in Patent Document 2 demonstrate excellent translucency among the sintered bodies produced by rapid firing, but they do not achieve translucency that greatly exceeds that of sintered bodies produced by slow firing (reference examples in Patent Document 2).

[0006] The technology disclosed herein has been made to break this trade-off and relates to a technology for zirconia pre-sintered bodies for rapid firing that exhibits superior translucency during rapid firing compared to slow firing.

[0007] The inventor of the present invention has conducted studies to realize a zirconia pre-sintered body for rapid firing that solves the above problems, leading to the following insights. First, it has been known that when slow firing is performed, the translucency improves because bubbles within the sintered body are removed due to prolonged heating. However, in rapid firing, it is difficult to sufficiently remove bubbles within the sintered body due to the short heating time. Therefore, the inventor concluded that to improve translucency during rapid firing, it is necessary to control the refractive index associated with the segregation of rare earth oxides. Specifically, after firing, there exists a gradient in the solid solution content of rare earth oxides within the zirconia sintered body. In regions where the solid solution content of rare earth oxides differs locally, the refractive index of transmitted light changes. In other words, the presence of many regions with locally different solid solution content becomes a scattering factor that decreases the translucency of the sintered body. Based on this insight, the inventor concluded that in rapid firing, where bubble removal is difficult, it is better to improve translucency by uniformly dispersing rare earth oxides throughout the sintered body.

[0008] Here, the rapid firing with a short holding time has the characteristic that the crystal structure is less likely to change before and after firing, allowing for quick sintering of the pre-sintered body. The inventor focused on this point and considered that by using a zirconia pre-sintered body with finely dispersed rare earth oxides as a raw material, the translucency of the resulting zirconia sintered body could be improved. As a result of various experiments, it was discovered that when the degree of dispersion of rare earth oxides in the entire zirconia pre-sintered body exceeds a predetermined value, the translucency during rapid firing is superior to that during slow firing, exhibiting an unprecedented effect. The technology disclosed herein relates to this zirconia pre-sintered body for rapid firing.

[0009] First, the method for manufacturing the zirconia pre-sintered body disclosed herein includes a generation step of generating a zirconia sol slurry containing zirconia powder, a rare earth dispersion step of dispersing rare earth particles having an average particle diameter of 300 nm or less into the zirconia sol slurry, a collecting step of collecting mixed particles containing zirconia particles and rare earth particles from the zirconia sol slurry, a heating step of producing partially stabilized zirconia powder by heating the mixed particles, and a forming and heating of heating the partially stabilized zirconia powder after molding it into a desired shape.

[0010] In the rare earth dispersion step of the manufacturing method disclosed herein, rare earth particles with an average particle diameter of 300 nm or less are dispersed into the zirconia sol slurry. When the mixed particles containing these fine rare earth particles are heated, partially stabilized zirconia powder is obtained in which zirconia and rare earth oxides suitably interdiffuse. By heating this partially stabilized zirconia powder after shaping it into a desired form, a zirconia pre-sintered body for rapid firing can be produced.

[0011] Additionally, another aspect of the technology disclosed herein provides a zirconia pre-sintered body. The zirconia pre-sintered body disclosed herein includes zirconia and rare earth oxides. This zirconia pre-sintered body having a tetragonal crystal structure with a c / a axis length ratio of 1.008 or more. Furthermore, in the disclosed pre-sintered body, the difference (X - Y) between the content X (mol%) of the rare earth oxides in the zirconia pre-sintered body measured by XRF analysis and the solution content Y (mol%) of the rare earth oxides in the main phase measured by XRD analysis is 1 mol% or less.

[0012] The zirconia pre-sintered body with the above structure is characterized by having rare earth oxides uniformly dispersed and few second phases where coarse rare earth oxides are segregated. Specifically, the "rare earth oxide content X (based on XRF analysis) in the zirconia pre-sintered body" refers to the total amount of rare earth oxides present throughout the zirconia pre-sintered body. On the other hand, the "solid solution content Y (based on XRD analysis) of rare earth oxides in the main phase" represents the amount of rare earth oxides dispersed in the main phase. In other words, as the difference X - Y becomes smaller, the amount of the second phase, where rare earth oxides are segregated, decreases, leading to improved dispersion of rare earth oxides throughout the pre-sintered body. Here, the zirconia pre-sintered body with the above structure is characterized by the difference X - Y being 1 mol% or less. This zirconia pre-sintered body can be used as a zirconia pre-sintered body for rapid firing, exhibiting superior translucency during rapid firing compared to slow firing due to the pre-uniform dispersion of zirconia and rare earth oxides.

[0013] Additionally, another aspect of the technology disclosed herein provides a method for manufacturing zirconia sintered bodies. The method for manufacturing zirconia sintered bodies disclosed herein includes a pre-sintered body preparation step of accommodating the zirconia pre-sintered body according to any one of claims 6 to 11 inside a firing furnace, a heating step of raising the internal temperature of the firing furnace to a predetermined firing temperature, a holding step of maintaining the predetermined holding time while keeping the firing temperature, and a cooling step of cooling the internal temperature of the firing furnace to a predetermined cooling temperature. In this method for manufacturing zirconia sintered body, the holding time is 30 minutes or less.

[0014] In the method for manufacturing zirconia sintered bodies with the above structure, rapid firing is performed with a holding time of 30 minutes or less for zirconia pre-sintered bodies where the difference X - Y is 1 mol% or less. As a result, zirconia and rare earth oxides can be uniformly dispersed, allowing for the production of zirconia sintered bodies with excellent translucency in a short time.

[0015] Fig.1 is a flowchart showing an overview of the method for manufacturing zirconia pre-sintered bodies according to the first embodiment.Fig.2 is a flowchart showing an overview of the method for manufacturing zirconia sintered bodies according to the first embodiment.Fig.3 is a flowchart showing an overview of the method for manufacturing zirconia pre-sintered bodies according to the second embodiment.

[0016] Below, several embodiments of the technology disclosed herein will be described. Matters not specifically mentioned in this specification but necessary for the implementation of this technology can be understood based on the technical content taught in this specification and general technical knowledge in the relevant field. The content of the technology disclosed herein can be implemented based on the content disclosed in this specification and the technical knowledge in the relevant field. Furthermore, when a numerical range is described as "A to B (where A and B are arbitrary numbers)" in this specification, it means "A is greater than or equal to and B is less than or equal to," and includes the meanings of "greater than A and less than B," "greater than A and less than or equal to B," and "greater than or equal to A and less than B."

[0017] <First Embodiment> 1. Method for Manufacturing Zirconia Pre-sintered Bodies First, the first embodiment of the method for manufacturing zirconia pre-sintered bodies disclosed herein will be explained. Figure 1 is a flowchart showing an overview of the method for manufacturing zirconia pre-sintered bodies according to the first embodiment. As shown in Figure 1, the method for manufacturing zirconia pre-sintered bodies disclosed herein includes a generation step S10, a rare earth dispersion step S20, a collecting step S30, a heating step S40, and a forming and heating S60. In the first embodiment, the rare earth dispersion step S20 includes a rare earth addition step S22 and a precipitation step S24. Additionally, the manufacturing method according to this embodiment performs a particle size adjustment step S50 between the heating step S40 and the forming and heating S60. Each of these steps will be explained below.

[0018] (1) Generation Step S10 In the generation step S10, a zirconia sol slurry containing zirconia powder is generated. Here, "zirconia powder" refers to a powder material that primarily contains zirconia (ZrO2). "Primarily containing zirconia" means that components other than zirconia are not intentionally included. Therefore, powder materials that contain unavoidable impurities derived from raw materials or manufacturing processes are included in the zirconia powder in this technology. Examples of impurities in the zirconia powder include metal materials such as hafnium, calcium, silicon, aluminum, titanium, and compounds of these metals (typically oxides). Another example of an impurity is metallic zirconium. When the total mass of the components in the zirconia powder is set at 100 mol%, it is preferable for the amount of zirconia to be 95 mol% or more (particularly preferably 98 mol% or more). Zirconia powder with low impurities is particularly suitable for use in the manufacturing method according to this embodiment.

[0019] The average particle diameter of the zirconia powder is preferably 10 nm or more, more preferably 20 nm or more, even more preferably 30 nm or more, and particularly preferably 40 nm or more. The grain boundaries of zirconia particles can become the crystal interfaces of the zirconia sintered body after firing. Therefore, as the average particle diameter of the zirconia powder increases, the interface between the main phase and the second phase in the zirconia sintered body after firing tends to decrease, making it easier to improve translucency. On the other hand, the upper limit of the average particle diameter of the zirconia powder is preferably 150 nm or less, more preferably 125 nm or less, and particularly preferably 100 nm or less. By reducing the average particle diameter of the zirconia powder, it is possible to manufacture zirconia pre-sintered bodies that are easy to sinter even with rapid firing. In this specification, "average particle diameter" refers to the particle diameter corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by a particle size distribution analyzer (manufactured by Horiba, model: LA-960).

[0020] The content of zirconia powder in the zirconia sol slurry is not particularly limited and can be 1 wt% or more, 3 wt% or more, or 5 wt% or more. On the other hand, the upper limit of the content of zirconia powder is preferably 40 wt% or less, more preferably 30 wt% or less, and particularly preferably 20 wt% or less. As the content of zirconia powder decreases, the dispersibility within the slurry tends to improve. Here, the "content of zirconia powder" refers to the mass ratio when the total mass of the zirconia sol slurry is set at 100 wt%.

[0021] The method for generating the zirconia sol slurry is not particularly limited, and conventional known generation methods can be appropriately employed. Examples of such generation methods include hydrothermal synthesis and hydrolysis methods. In the hydrothermal synthesis method, a co-precipitate obtained by mixing zirconium salts and alkalis is heated in the presence of a liquid medium at 100 to 200°C (preferably around 120°C). By conducting the hydrolysis reaction at such temperatures, homogeneous nucleation is promoted, resulting in a sharp sol diameter distribution. In the hydrolysis method, zirconium salts are hydrolyzed by heating them in the presence of a liquid medium. These methods allow for the easy generation of zirconia sol slurries in which zirconia powder is suitably dispersed. The liquid medium for the zirconia sol slurry should not dissolve zirconia but can dissolve the rare earth raw materials described later. Water, for example, can be used as such a liquid medium.

[0022] (2) Rare Earth Dispersion Step S20 In the rare earth dispersion step S20, rare earth particles with an average particle diameter of 300 nm or less are dispersed into the zirconia sol slurry. By dispersing such fine rare earth particles in the slurry, partially stabilized zirconia powder in which zirconia and rare earth oxides suitably interdiffuse can be obtained. The average particle diameter of the rare earth particles is preferably 250 nm or less, more preferably 225 nm or less, and particularly preferably 200 nm or less. As the average particle diameter of the rare earth particles decreases, the distribution of rare earth oxides in the partially stabilized zirconia powder can be further homogenized. On the other hand, if the rare earth particles become too small, there is a risk of aggregation in the slurry. From this perspective, the average particle diameter of the rare earth particles is preferably 1 nm or more, more preferably 5 nm or more, and particularly preferably 10 nm or more.

[0023] The means for dispersing rare earth particles into the zirconia sol slurry is not particularly limited, and various means can be selected as necessary. For example, in the rare earth dispersion step S20 of the first embodiment, a rare earth addition step S22 and a precipitation step S24 are performed. By going through these steps, a slurry with dispersed rare earth particles of 300 nm or less can be easily obtained. The following will provide a detailed explanation.

[0024] (2-a) Rare Earth Addition Step S22 In the rare earth addition step S22, rare earth raw materials are dissolved in the zirconia sol slurry. This allows for the uniform presence of rare earth elements within the zirconia sol slurry. In this specification, "rare earth raw materials" refer to compounds that contain rare earth elements. Considering the performance (strength, toughness, aesthetics, etc.) of the zirconia sintered body after firing, yttrium (Y) and ytterbium (Yb) are preferred as rare earth elements, with yttrium being particularly preferable. Additionally, the rare earth raw materials are required to be soluble materials in the zirconia sol slurry. Therefore, preferred rare earth raw materials include halides such as chlorides, bromides, and iodides of the aforementioned rare earth elements, as well as hydroxides, sulfides, sulfates, and nitrates. Among these, yttrium chloride is particularly preferable when considering the overall performance after firing and solution content in the slurry.

[0025] In the rare earth addition step S22, it is preferable to adjust the amount of rare earth raw material dissolved so that the ratio of rare earth elements to Zr elements meets a predetermined range. For example, when the total amount of Zr elements and rare earth elements in the slurry is set at 100 mol%, the amount of rare earth elements is preferably 3 mol% or more, more preferably 3.2 mol% or more, even more preferably 3.4 mol% or more, and particularly preferably 3.5 mol% or more. This ensures the production of partially stabilized zirconia powder containing sufficient rare earth oxides. On the other hand, the upper limit of the amount of rare earth elements is preferably 6 mol% or less, more preferably 5.8 mol% or less, and particularly preferably 5.6 mol% or less. This prevents rare earth oxides that are not incorporated into the zirconia from contaminating the partially stabilized zirconia powder as impurities.

[0026] In the first embodiment, the pH of the zirconia sol slurry before the addition of rare earth raw materials is adjusted to 4 or less. This helps to suppress the aggregation of zirconia particles in the slurry. By making the zirconia sol slurry acidic, the rare earth raw materials can be appropriately dissolved. The pH of the slurry before the addition of rare earth raw materials is preferably 2 or less, more preferably 1 or less, even more preferably 0.7 or less, and particularly preferably 0.5 or less. As the pH of the slurry decreases, the dispersibility of the zirconia particles tends to improve further. The lower limit of the pH of the slurry is not particularly limited and can be 0.1 or more, 0.2 or more, or 0.3 or more. The means for adjusting the pH of the slurry is not particularly limited. For example, hydrochloric acid or other pH adjusters can be added to lower the pH of the slurry. Additionally, if the pH of the slurry generated by hydrothermal synthesis or hydrolysis is already 4 or less, no adjustment may be necessary.

[0027] (2-b) Precipitation Step S24 In the precipitation step S24, the pH of the zirconia sol slurry is raised to 6 to 9. This allows for the precipitation of rare earth compounds within the slurry. Specifically, the zirconia sol slurry supplied to the precipitation step S24 contains dissolved rare earth elements. By precipitating rare earth compounds from this slurry, fine rare earth particles of 300 nm or less can be uniformly present in the slurry. The rare earth particles that precipitate in this step are typically oxides or hydroxides of the rare earth elements. After precipitation, the rare earth particles are uniformly dispersed in the gaps between the zirconia particles in the slurry.

[0028] In this step, it is preferable to add an alkali to the zirconia sol slurry. This allows for easy raising of the pH of the slurry. Alkalis such as ammonia, sodium hydroxide, and potassium hydroxide can be used. Among these, ammonia is particularly preferable as it prevents the contamination of the partially stabilized zirconia powder with metal elements (such as Na) that could act as impurities after production.

[0029] Additionally, in this step, it is preferable to add the alkali while stirring the zirconia sol slurry. This ensures that rare earth compounds precipitate while the zirconia particles are uniformly dispersed in the slurry, further enhancing the degree of dispersion between the zirconia and rare earth compounds. The specific stirring means is not particularly limited, and any conventional stirring apparatus can be used without restriction. Examples of stirring means include ball mills, mixers, dispersers, and kneaders.

[0030] (3) Collecting Step S30 In the collecting step S30, mixed particles containing zirconia and rare earth compounds are collected from the zirconia sol slurry. As mentioned above, in the manufacturing method according to this embodiment, fine rare earth compounds are uniformly dispersed in the slurry before collecting. This allows for the collecting of mixed particles in which zirconia and rare earth compounds are uniformly dispersed. The specific method for collecting the mixed particles is not particularly limited, and any conventional method for separating powder materials from liquids can be employed without restriction. For example, the mixed particles can be collected by appropriately combining means such as filtration, centrifugation, and drying.

[0031] (4) Heating Step S40 In the heating step S40, the mixed particles obtained in the collecting step S30 are heated to produce partially stabilized zirconia powder. Specifically, if the rare earth compounds within the mixed particles are compounds other than oxides, these rare earth compounds will be oxidized to form rare earth oxides during the initial phase of the heating step S40. In the heating step S40, the mixed particles, which contain zirconia and rare earth oxides uniformly dispersed, are heated. Because the distance between zirconia and rare earth oxides is small in these mixed particles, zirconia and rare earth oxides can suitably interdiffusion. This results in the production of partially stabilized zirconia powder in which zirconia and rare earth oxides are uniformly dispersed. By using this partially stabilized zirconia powder, a zirconia pre-sintered body with uniformly dispersed rare earth oxides can be obtained.

[0032] The heating conditions in this step are not particularly limited, and any conventional heating conditions used in the production of zirconia pre-sintered bodies can be employed without restriction. For example, the heating temperature in the heating step S40 is set to a temperature where interdiffusion occurs between zirconia and rare earth oxides, and zirconia does not sinter (e.g., 900°C to 1200°C). This allows for the appropriate production of partially stabilized zirconia powder. The heating atmosphere in the heating step S40 is also not particularly limited and can be atmospheric, oxidizing, or reducing. The time for the heating step S40 can be, for example, between 1.5 hours and 5 hours, or between 2 hours and 4 hours. Additionally, any conventional heating furnaces (e.g., muffle furnaces, electric furnaces, microwave sintering furnaces, etc.) can be used without restriction in the heating step S40.

[0033] (5) Particle Size Adjustment Step S50 The manufacturing method for the pre-sintered body according to this embodiment also includes a particle size adjustment step S50 for adjusting the particle size of the partially stabilized zirconia powder between the heating step S40 and the forming and heating S60. The particle size adjustment step S50 can employ any conventional particle size adjustment techniques without restriction. For example, the partially stabilized zirconia powder obtained after heating can be crushed and then sieved using a mesh to obtain desired particle sizes.

[0034] By controlling the average particle diameter of the partially stabilized zirconia powder within a specific range in this step, higher quality zirconia pre-sintered bodies can be obtained. Specifically, the average particle diameter of the partially stabilized zirconia powder after particle size adjustment is preferably 300 nm or less, more preferably 250 nm or less, and particularly preferably 200 nm or less. By refining the partially stabilized zirconia powder in this step, it becomes easier to produce a dense zirconia pre-sintered body in the forming and heating S60. However, if the average particle diameter of the partially stabilized zirconia powder is made too small, there is a risk that the crystallinity of the zirconia pre-sintered body after shaping and heating (after pre-sintered body firing) will deteriorate, resulting in a decrease in the c / a axis length ratio. From this perspective, the average particle diameter of the partially stabilized zirconia powder after particle size adjustment is preferably 50 nm or more, more preferably 100 nm or more, and particularly preferably 150 nm or more.

[0035] (6) Forming and Heating Step S60 Next, in the forming and heating step S60, the partially stabilized zirconia powder is shaped into a desired form and then heated (fired) to produce the zirconia pre-sintered body. Specifically, in the forming and heating step S60, the partially stabilized zirconia powder obtained after the particle size adjustment step S50 is first mixed with a binder. This mixture is then shaped into the desired form. By heating (firing) this shaped body, the binder is burned off, and some of the partially stabilized zirconia powder melts and adheres together. This results in the production of the zirconia pre-sintered body. The forming and heating step S60 can employ any conventional methods for forming zirconia sintered bodies without specific limitations. Therefore, detailed conditions (types of binders, amounts of binders, forming methods, heating conditions, etc.) are omitted from this specification. The shapes of the bodies produced in this step are not particularly limited and can include, for example, plate-like, disk-like, rectangular, cubic, cylindrical shapes, etc.

[0036] Additionally, the zirconia sintered body obtained after firing, as described later, may contain alumina. This can suppress grain growth during firing, thereby improving the strength of the sintered body after firing. It is preferable to add the alumina source together with the binder in the forming and heating step S60. Examples of alumina sources include alumina powder, alumina sol, hydrated alumina, aluminum hydroxide, aluminum chloride, aluminum nitrate, and aluminum sulfate. These alumina sources will become alumina and be dispersed within the sintered body during the subsequent firing process. The amount of the alumina source should be adjusted so that the amount of Al element is 0.01 mol% or more (preferably 0.02 mol% or more, particularly preferably 0.04 mol% or more) relative to the total mass of the components of the pre-sintered body (100 mol%). This helps to suitably improve the strength of the sintered body. On the other hand, since alumina is a light scattering factor, there is a risk that it may reduce the translucency of the sintered body. Therefore, the amount of the alumina source should be adjusted so that the amount of Al element is 0.2 mol% or less (preferably 0.15 mol% or less, particularly preferably 0.1 mol% or less) relative to the total mass of the components of the pre-sintered body (100 mol%). Additionally, the zirconia sintered body after firing may contain trace amounts of additives (for example, 0.1 mol% or less) for purposes such as coloring. Examples of these trace additives include iron, nickel, cobalt, manganese, niobium, praseodymium, neodymium, europium, and erbium. It is also preferable to add these trace additives together with the binder in the forming and heating step S60.

[0037] 2. Zirconia Pre-sintered Body The method for manufacturing the zirconia pre-sintered body according to the first embodiment has been described above. According to this manufacturing method, a zirconia pre-sintered body in which zirconia and rare earth oxides are uniformly dispersed can be produced. This zirconia pre-sintered body exhibits an unprecedented effect of superior translucency during rapid firing compared to slow firing. The following describes the properties of the pre-sintered body after production.

[0038] The zirconia pre-sintered body according to this embodiment is a shaped body containing zirconia and rare earth oxides. Since zirconia and rare earth oxides have already been described, overlapping explanations are omitted. In this specification, "zirconia pre-sintered body" refers to a pre-sintered body in which zirconia and rare earth oxides are the main components. Here, "zirconia and rare earth oxides being the main components" means that the combined amount of zirconia and rare earth oxides accounts for 90 mol% or more (more preferably 91 mol% or more, even more preferably 92 mol% or more, particularly preferably 93 mol% or more) of the total mass of the components of the pre-sintered body (100 mol%). The amounts of the components in this pre-sintered body will be measured by XRF analysis described later.

[0039] Furthermore, the amount of zirconia in the total mass of the components of the pre-sintered body (100 mol%) is preferably 85 mol% or more, more preferably 86 mol% or more, even more preferably 87 mol% or more, and particularly preferably 88 mol% or more. As the proportion of zirconia in the pre-sintered body increases, the strength, toughness, and resistance to hydrothermal degradation of the zirconia sintered body after firing tend to improve. On the other hand, considering the need to secure the amounts of other additives (such as rare earth oxides), the amount of zirconia should be 96 mol% or less, preferably 95 mol% or less, more preferably 94 mol% or less, and particularly preferably 93 mol% or less.

[0040] The zirconia pre-sintered body has a tetragonal crystal structure with a c / a axis length ratio of 1.008 or more. This c / a axis length ratio can be measured using X-ray diffraction (XRD) on the surface of the zirconia pre-sintered body. Specifically, the surface of the zirconia pre-sintered body is roughly polished using a 9 μm diamond grinding wheel, followed by mirror polishing with a 1 μm abrasive to expose the measurement section. Next, a commercially available X-ray diffraction analysis device (Malvern Panalytical, model: X'Pert Pro Alpha-1) is used to obtain the X-ray diffraction pattern at the measurement section. The X-ray diffraction pattern is then analyzed using crystallographic analysis software (RIETAN-FP). This allows for the derivation of the c / a axis length ratio of the zirconia pre-sintered body. A tetragonal phase with a c / a axis length ratio of 1.008 or more serves as the main phase of stabilized zirconia (typically yttria-stabilized zirconia).

[0041] The conditions for obtaining the X-ray diffraction pattern are as follows: X-ray Source: Cu Kα I Line Tube Voltage: 45 kV Tube Current: 40 mA Measurement Range: 10° ≦ 2θ ≦ 90° Scan Speed: 1.5° / min Step Width: 0.0131°

[0042] In this specification, "main phase" refers to the crystalline phase that constitutes more than 50% of the zirconia pre-sintered body. In other words, the zirconia pre-sintered body according to this embodiment has a ratio of tetragonal phase with a c / a axis length ratio of 1.008 or more that exceeds 50%. Additionally, the crystalline phases other than the main phase (i.e., those with a total ratio of less than 50%) are referred to as "second phases." The second phase can be one or more crystalline phases. That is, the zirconia pre-sintered body according to this embodiment encompasses pre-sintered bodies with three or more crystalline phases. The ratio of the main phase in the zirconia pre-sintered body according to this embodiment is preferably 72% or more, more preferably 74% or more, even more preferably 75% or more, and particularly preferably 76% or more. As the ratio of the main phase increases, the number of boundaries between the main phase and the second phase tends to decrease, improving the permeability of the zirconia sintered body after firing. The upper limit of the ratio of the main phase is not particularly limited and can be 100%. In other words, the zirconia pre-sintered body according to this embodiment may also consist solely of a single-phase body made up of tetragonal phases with a c / a axis length ratio of 1.008 or more. The composition ratios of the crystalline phases can be measured through the analysis of the aforementioned X-ray diffraction patterns.

[0043] Here, the zirconia pre-sintered body according to this embodiment is characterized by the difference (X - Y) being 1 mol% or less, where X is the rare earth oxide content (mol%) in the zirconia pre-sintered body based on XRF analysis, and Y is the solid solution content (mol%) of rare earth oxides in the main phase based on XRD analysis. As mentioned above, "the rare earth oxide content X (based on XRF analysis) in the zirconia pre-sintered body" indicates the total amount of rare earth oxides present throughout the zirconia pre-sintered body. On the other hand, "the solid solution content Y (based on XRD analysis) of rare earth oxides in the main phase" represents the amount of rare earth oxides dispersed in the main phase. In other words, as the difference (X - Y) becomes smaller, the amount of the second phase, where rare earth oxides are segregated, decreases, leading to improved dispersion of rare earth oxides throughout the pre-sintered body. The zirconia pre-sintered body according to this embodiment is characterized by this difference (X - Y) being 1 mol% or less. This zirconia pre-sintered body can be used as a zirconia pre-sintered body for rapid firing, exhibiting superior translucency during rapid firing compared to slow firing due to the pre-uniform dispersion of zirconia and rare earth oxides.

[0044] The difference (X - Y) is preferably 1 mol% or less, more preferably 0.9 mol% or less, even more preferably 0.8 mol% or less, and particularly preferably 0.7 mol% or less. This allows for the production of a zirconia pre-sintered body that is more suitable for rapid firing. On the other hand, the lower limit of the difference (X - Y) is not particularly restricted and can be 0.1 mol% or more.

[0045] The content X of rare earth oxides in the zirconia pre-sintered body is preferably 3 mol% or more, more preferably 3.2 mol% or more, even more preferably 3.4 mol% or more, and particularly preferably 3.5 mol% or more. As the content X of rare earth oxides in the entire pre-sintered body increases, the translucency of the zirconia sintered body after firing tends to improve. On the other hand, a pre-sintered body with a low content of rare earth oxides tends to exhibit improved mechanical properties (strength, toughness, etc.). From this perspective, the content X of rare earth oxides is preferably 6 mol% or less, more preferably 5.5 mol% or less, and particularly preferably 5 mol% or less. In this specification, the "content X of rare earth oxides" refers to the measurement of the content of rare earth elements using a fluorescent X-ray analyzer (XRF: X-ray Fluorescence), converted to the amount of oxides.

[0046] On the other hand, the solid solution content Y of rare earth oxides in the main phase is preferably 3.2 mol% or more, more preferably 3.3 mol% or more, even more preferably 3.4 mol% or more, and particularly preferably 3.5 mol% or more. As the solid solution content Y of rare earth oxides in the main phase increases, the occurrence of second phases where rare earth oxides are segregated can be suppressed. The upper limit of the solid solution content Y of rare earth oxides in the main phase is not particularly restricted and can be approximately the same as the content X of rare earth oxides in the entire pre-sintered body (X = Y). The specific value of the upper limit of the solid solution content Y in the main phase can be 5 mol% or less (typically 4.9 mol% or less, for example, 4.8 mol% or less). The "solid solution content Y of rare earth oxides in the main phase based on XRD analysis" can be measured according to the following procedure. First, as mentioned above, the c / a axis length ratio of the main phase can be measured by analyzing the X-ray diffraction pattern. By substituting this c / a axis length ratio into a predetermined calculation formula, the "solid solution content Y of rare earth oxides in the main phase" can be calculated. This calculation formula is selected from conventional known formulas depending on the type of rare earth oxides. For example, if the rare earth oxide is yttria (Y2O3), the solid solution content Y of rare earth oxides in the main phase can be calculated using the following formula (1). The "solid solution content Y of rare earth oxides in the main phase" in this specification is not limited to those calculated based on the formula (1) below. For example, if the rare earth oxide is ytterbia (Yb2O3), the "solid solution content Y of rare earth oxides in the main phase" can be calculated based on other known formulas.Formula 1

[0047]

[0048] Additionally, it is preferable that the zirconia pre-sintered body according to this embodiment satisfies the condition that the difference (X - Y) is 1 mol% or less in more than 50% (preferably more than 75%) of the surface area of the zirconia pre-sintered body. For example, when measuring a disk-shaped zirconia pre-sintered body with a diameter of 2 cm (surface area: approximately 3 cm2), measurements can be taken by scanning any measurement location (diameter: 1 cm, area: approximately 0.75 cm2). By changing the measurement locations, measurements can be conducted at 2 to 3 locations, and it is preferable that the difference (X - Y) at each measurement position satisfies the condition of being 1 mol% or less. The zirconia pre-sintered body according to this embodiment is produced by firing mixed particles in which fine rare earth compounds are uniformly dispersed. Therefore, it is easy to confirm that the segregation of rare earth elements is suppressed at any of the multiple measurement locations. In other words, according to the technology disclosed herein, it can be confirmed that the majority of the zirconia pre-sintered body satisfies the condition of the difference (X - Y) being 1 mol% or less, resulting in a crystal structure that, when rapidly fired, yields a zirconia sintered body with excellent translucency overall.

[0049] 3. Method for Manufacturing Zirconia Sintered Bodies Having described the zirconia pre-sintered body according to this embodiment, the method for manufacturing zirconia sintered bodies using this zirconia pre-sintered body will now be explained. Figure 2 is a flowchart illustrating the method for manufacturing zirconia sintered bodies according to this embodiment. As shown in Figure 2, the method for manufacturing zirconia sintered bodies includes a pre-sintered body preparation step S110, a heating step S120, a holding step S130, and a cooling step S140. Each of these steps will be described below.

[0050] (1) Pre-sintered Body Preparation Step S110 In this step, the zirconia pre-sintered body is accommodated inside the firing furnace. As mentioned above, this embodiment uses a zirconia pre-sintered body in which rare earth oxides are uniformly dispersed (in other words, where (X - Y) is 1 mol% or less). The means for preparing the zirconia pre-sintered body is not particularly limited. For example, the manufacturing method described in "1. Method for Manufacturing Zirconia Pre-sintered Bodies" can be implemented, or a pre-manufactured zirconia pre-sintered body can be purchased. In this step, the prepared zirconia pre-sintered body is placed inside the heating furnace. Any conventional heating furnace (e.g., muffle furnace, electric furnace, microwave sintering furnace, etc.) can be used without restriction.

[0051] (2) Heating Step S120 In this step, the interior of the firing furnace is heated to a predetermined firing temperature. The firing temperature is typically set within the range of 1400°C to 1700°C (preferably 1550°C to 1650°C). This allows the pre-sintered body containing zirconia and rare earth oxides to be sufficiently sintered. The heating step S120 may be divided into multiple stages. For example, the heating step S120 in this embodiment includes a first heating step S122 and a second heating step S124, as shown in Figure 2.

[0052] (2-a) First Heating Step S122 In the first heating step S122, the temperature is raised to a first temperature of 1000°C to 1100°C at a heating rate of 150°C / min or more. This first temperature is set to be lower than the temperature at which densification of zirconia occurs. In the first heating step S122, the temperature is raised to the first temperature at a faster rate compared to the second heating step S124 described later. Rapidly heating the low-temperature region below the densification temperature has little adverse effect on the sintering (densification) of zirconia. This allows for sufficient density in the zirconia sintered body after production while further shortening the firing time. The heating rate in the first heating step S122 is preferably 160°C / min or more, more preferably 170°C / min or more, even more preferably 180°C / min or more, and particularly preferably 190°C / min or more. This contributes to improving production efficiency by shortening the time of the first heating step S122. Additionally, when using the zirconia pre-sintered body according to this embodiment, improving the heating rate and shortening the first heating step S122 tends to enhance the translucency of the sintered body after firing. On the other hand, the upper limit of the heating rate in the first heating step S122 is preferably 250°C / min or less, more preferably 240°C / min or less, even more preferably 230°C / min or less, and particularly preferably 220°C / min or less. Slowing down the heating rate in the first heating step S122 reduces the temperature gradient between the interior and surface of the pre-sintered body, making it easier to achieve uniform composition and improve strength.

[0053] (2-b) Second Heating Step S124 Next, in the second heating step S124, the temperature is raised to a second temperature of 1500°C to 1700°C at a heating rate of less than 150°C / min and 30°C / min or more. This second temperature is higher than the temperature at which densification of zirconia occurs. In other words, in the second heating step S124, the temperature is raised to the second temperature at a relatively slow heating rate. This ensures that the density of the zirconia sintered body after production is sufficiently secured. The heating rate in the second heating step S124 is preferably 35°C / min or more, more preferably 40°C / min or more, even more preferably 45°C / min or more, and particularly preferably 50°C / min or more. Similar to the first heating step S122, in this embodiment, improving the heating rate and shortening the firing time tends to enhance the translucency of the sintered body after firing. On the other hand, the upper limit of the heating rate in the second heating step S124 is preferably 130°C / min or less, more preferably 120°C / min or less, even more preferably 110°C / min or less, and particularly preferably 100°C / min or less. This allows for better densification of the zirconia.

[0054] (3) Holding Step S130 In the holding step S130, the firing temperature is maintained for a predetermined holding time. This allows the stabilized zirconia within the pre-sintered body to sinter, resulting in the production of the zirconia sintered body. As mentioned above, using the zirconia pre-sintered body according to this embodiment allows for the production of zirconia sintered bodies with excellent translucency, even when the holding time in this step is shortened to 30 minutes or less. The holding time in the holding step S130 can be 20 minutes or less, 15 minutes or less, or even 10 minutes or less. A more preferable holding time is 7.5 minutes or less, 5 minutes or less, 2.5 minutes or less, and particularly 1 minute or less. This further shortens the production time of the sintered body. The lower limit of the holding time is not particularly restricted and can be 0.5 minutes or more. Additionally, the holding time can be set to 0 minutes. Specifically, the method for manufacturing the zirconia sintered body disclosed herein may also include a scenario where the holding step S130 is not performed after the completion of the second heating step S126, and the cooling step S140 is initiated. Even in such a scenario, a zirconia sintered body with uniformly dispersed rare earth oxides and excellent translucency can be produced.

[0055] (5) Cooling Step S140 In the cooling step S140, the interior of the firing furnace is cooled to a predetermined cooling temperature. This allows for the recovery of the zirconia sintered body after firing. The cooling rate in the cooling step S140 is preferably 50°C / min or more, more preferably 75°C / min or more, and particularly preferably 100°C / min or more. Similar to the first heating step S122 and the second heating step S124, improving the cooling rate and shortening the total firing time tends to enhance the translucency of the sintered body after firing. Furthermore, improving the cooling rate contributes to increased production efficiency. On the other hand, the upper limit of the cooling rate in the cooling step S140 is preferably 450°C / min or less, more preferably 425°C / min or less, even more preferably 400°C / min or less, and particularly preferably 400°C / min or less. This helps to suppress the occurrence of cracks due to rapid cooling.

[0056] The zirconia sintered body obtained after production exhibits very excellent translucency, with total light transmittance of 44.5% or more (preferably 45% or more, more preferably 45.5% or more, and particularly preferably 46% or more), despite being subjected to rapid firing. Therefore, according to the zirconia pre-sintered body of this embodiment, it is possible to produce a sintered body with excellent translucency at high production efficiency.

[0057] As mentioned above, in this embodiment, a raw material (zirconia pre-sintered body) in which zirconia and rare earth oxides are pre-uniformly mixed is used. Therefore, it is preferable to shorten the total firing time (the total time from the first heating step S120 to the holding step S140) to avoid significant changes in the crystal structure during the firing process. This allows for the production of a zirconia sintered body with even better translucency. Specifically, the total firing time in this embodiment is preferably 120 minutes or less, more preferably 60 minutes or less, and particularly preferably 30 minutes or less. On the other hand, when the pre-sintered body can be sufficiently sintered, the lower limit of the total firing time is not particularly restricted. For example, the total firing time can be 10 minutes or more, 12 minutes or more, or 14 minutes or more.

[0058] <Other Embodiments> The first embodiment of the technology disclosed herein has been described above. The aforementioned embodiment illustrates one aspect of the technology disclosed herein and does not limit the technology disclosed herein. In other words, when the technology disclosed herein can achieve the goal of manufacturing a zirconia pre-sintered body for rapid firing that exhibits superior translucency compared to slow firing, various configurations can be appropriately modified from the first embodiment described above.

[0059] 1. Second Embodiment In the first embodiment, the rare earth dispersion step S20 includes a rare earth addition step S22 and a precipitation step S24. In other words, in the first embodiment, rare earth elements dissolved in the slurry are precipitated to achieve a slurry containing fine rare earth particles with an average particle diameter of 300 nm or less. However, the rare earth dispersion step S20 is not limited to the procedures described in the first embodiment, when it can disperse fine rare earth particles in the slurry. For example, Figure 3 shows a flowchart outlining the method for manufacturing a zirconia pre-sintered body according to the second embodiment. As shown in Figure 3, the rare earth dispersion step S20 in the second embodiment includes an addition step S26 and a stirring step S28. The following describes each step. The steps other than the rare earth dispersion step S20 are the same as in the first embodiment, so overlapping explanations are omitted.

[0060] (1) Addition Step S26 In the addition step S26, rare earth particles with an average particle diameter of 300 nm or less are added to the zirconia sol slurry. Specifically, in the first embodiment, fine rare earth particles were generated in the zirconia sol slurry by precipitating rare earth particles. In contrast, the addition step S26 in the second embodiment directly adds rare earth particles controlled to an average particle diameter of 300 nm or less into the zirconia sol slurry. Even with this configuration, it has been confirmed through experiments that a zirconia pre-sintered body suitable for rapid firing can be produced. The rare earth particles used here can include oxides or hydroxides of rare earth elements. Among these, rare earth element oxides are preferable for stable production.

[0061] (2) Stirring Step S28 In the stirring step S28, the zirconia sol slurry is stirred to disperse the rare earth particles. This allows for the uniform presence of fine rare earth particles within the zirconia sol slurry, ensuring the reliable production of a zirconia pre-sintered body suitable for rapid firing. Following this stirring step S28, the aforementioned collecting step S30 and heating step S40 are conducted to produce a zirconia pre-sintered body in which zirconia and rare earth oxides are uniformly dispersed.

[0062] Additionally, in the manufacturing method according to this embodiment, it is preferable to perform the dispersion of rare earth particles while maintaining the pH of the zirconia sol slurry in an acidic state. This suppresses the aggregation of each particle (zirconia particles and rare earth particles) in the slurry, further improving the dispersibility of zirconia and rare earth oxides in the zirconia pre-sintered body after production. The pH of the slurry in this step is preferably 4 or less, more preferably 2 or less, even more preferably 1 or less, and particularly preferably 0.5 or less. As the pH of the slurry decreases, the dispersibility of each particle tends to improve. The lower limit of the pH of the slurry is not particularly restricted and can be 0.1 or more, 0.2 or more, or 0.3 or more. The means for adjusting the pH of the slurry is not particularly limited. For example, a pH adjuster such as hydrochloric acid can be added to lower the pH of the slurry. Additionally, if the pH of the slurry generated by hydrothermal synthesis or hydrolysis is already 4 or less, no adjustment may be necessary.

[0063] The other stirring conditions in the stirring step S28 may vary according to various factors such as the viscosity of the zirconia sol slurry, the amount of rare earth particles added, and the average particle diameter, and therefore do not limit the technology disclosed herein. However, examples of stirring means include ball mills, mixers, dispersers, and kneaders. The rotation speed during stirring is preferably 100 rpm or more, more preferably 200 rpm or more, even more preferably 300 rpm or more, and particularly preferably 400 rpm or more. The upper limit of the rotation speed is not particularly restricted and can be 1000 rpm or less. The stirring time is preferably 10 minutes or more, more preferably 20 minutes or more, even more preferably 30 minutes or more, and particularly preferably 40 minutes or more. The upper limit of the stirring time is also not particularly restricted and can be 180 minutes or less. By considering these conditions, the fine rare earth particles can be more uniformly dispersed within the zirconia sol slurry.

[0064] The following examples related to the technology disclosed herein will be described. However, these examples are not intended to limit the technology disclosed herein to the contents described below.

[0065] <First Test> In this test, 13 types of zirconia pre-sintered bodies were prepared under different manufacturing conditions (Samples 1 to 13). XRF analysis and XRD analysis were conducted on each sample to measure the difference (X - Y) between the content X of rare earth oxides in the zirconia pre-sintered body and the solid solution content Y of rare earth oxides in the main phase.

[0066] 1.Sample Preparation (Sample 1) In Sample 1, a zirconia pre-sintered body was produced through the procedures of dissolving and precipitating rare earth raw materials. Specifically, a hydrothermal synthesis was first conducted on an oxychloride zirconium solution to obtain a zirconia sol slurry. The pH of the slurry after synthesis was measured to be pH = 1.5, so the rare earth raw materials were dissolved without adjusting the pH in this sample. Yttrium chloride was used as the rare earth raw material in this sample. The amount of yttrium chloride added was set so that yttrium constituted 4.27 mol% of the total mass of zirconia and yttria (100 mol%). Next, ammonia was added to the zirconia sol slurry, raising the pH of the slurry to 7. This caused the precipitation of yttrium compounds (such as yttria) from the slurry. The average particle diameter of the yttrium compounds after precipitation was measured to be 70 nm using TEM observation. Next, the mixture containing zirconia and yttria was recovered by subjecting the slurry to a drying treatment at 180°C for 5 hours. The mixed particles were then heated at 1120°C for 4 hours to obtain partially stabilized zirconia powder. This zirconia powder was crushed using a ball mill with zirconia balls (diameter: 1 mm). The crushed powder was then sieved to obtain zirconia powder with an average particle diameter of 150 nm to 200 nm. This zirconia powder was shaped into a disk with a diameter of 2 cm and then heated (fired) at 1100°C for 2 hours to obtain a zirconia pre-sintered body.

[0067] (Samples 2-3) In Samples 2 and 3, zirconia pre-sintered bodies were produced using the same procedures as Sample 1, except for differing amounts of yttria added. The amounts of yttria added and the average particle diameters for each sample are shown in Table 1.

[0068] (Samples 4-12) In Samples 4 to 12, rare earth particles with a predetermined average particle diameter were added to the slurry without going through the procedures of dissolving and precipitating rare earth raw materials. Specifically, in Samples 4 to 12, the pH of the slurry generated using the same procedures as Sample 1 was adjusted to 2 before adding the rare earth raw material powder (yttrium oxide particles). The amounts of yttria added and the average particle diameters for each sample are shown in Table 1. After undergoing the same recovery and firing processes as Sample 1, zirconia pre-sintered bodies were obtained.

[0069] 2. Evaluation Test In this test, the content X of rare earth oxides in the zirconia pre-sintered body and the solid solution content Y of rare earth oxides in the main phase were measured according to the measurement procedures described above. Based on the measurement results, the difference (X - Y) between the content X of rare earth oxides in the zirconia pre-sintered body and the solid solution content Y of rare earth oxides in the main phase was measured. The results are shown in Table 1.

[0070] Additionally, in this test, XRD analyses were performed at three different measurement locations on a single sample (pre-sintered body). The c / a axis length ratios at each measurement location were measured based on Rietveld analysis, and the area with the highest crystal ratio was designated as the main phase. The calculated results are also shown in Table 1.

[0071]

[0072] As shown in Table 1, for Samples 1 to 4, 6 to 9, and 11, the difference (X - Y) between the content X of rare earth oxides in the zirconia pre-sintered body and the solid solution content Y of rare earth oxides in the main phase was 1 mol% or less. As mentioned above, such pre-sintered bodies are understood to have yttria uniformly dispersed. In other words, by dispersing yttria particles with an average particle diameter of 300 nm or less in the slurry and subsequently precipitating yttria after dissolving the yttria source, it was found that the segregation of yttria was suitably suppressed, resulting in the production of zirconia pre-sintered bodies.

[0073] <Second Test> Next, in the second test, zirconia sintered bodies were produced using the pre-sintered bodies obtained in the first test. In this test, nine types of pre-sintered bodies (Samples 1 to 5, 8 to 10, and 12) were combined with seven different firing treatments (firing patterns 1 to 6) to produce 22 types of zirconia sintered bodies (Examples 1 to 22). The total light transmittance of the sintered bodies after production was measured to evaluate their translucency.

[0074] 1.Firing Conditions As mentioned above, in this test, six firing patterns were established. The detailed temperature profiles for each firing pattern are shown in Table 2.

[0075]

[0076] 2. Sample Selection As mentioned above, in this test, nine types of pre-sintered bodies (Samples 1 to 5, 8 to 10, and 12) were combined with seven types of firing treatments (firing patterns 1 to 6) to conduct a total of 22 examples. The specific combinations of pre-sintered bodies and firing treatments are shown in Table 3.

[0077] 3. Evaluation Test (1) Firing Time For each of the examples 1 to 22, the total time from the start of firing to the completion of cooling was calculated. The results are shown in Table 3.

[0078] (2) Measurement of Total light Transmittance In this test, total light transmittance was measured to evaluate the translucency of the zirconia sintered bodies after firing. For measuring total light transmittance, the zirconia sintered bodies (examples 1 to 22) were first processed into disk-shaped test pieces with a thickness of 1 mm. Next, diamond slurry (average particle diameter of 0.5 μm) was used as an abrasive to polish both sides of the test pieces to a mirror finish. The total light transmittance with respect to the D65 light source in the thickness direction was then measured. The measurements were conducted using a haze meter NDH4000 manufactured by Nippon Denko Kogyo. The results are shown in Table 3.

[0079]

[0080] As shown in examples 13 and 14, for Sample 4, where (X - Y) exceeds 1 mol%, the translucency after firing was better in example 13 with slow firing (pattern 3) than in example 14 with rapid firing (pattern 1). This is consistent with conventional knowledge, as it is believed that prolonged slow firing removes bubbles within the sintered body. On the other hand, in other samples, superior translucency was exhibited during rapid firing compared to slow firing (see examples 1 to 12 and 15 to 18). This suggests that for samples where (X - Y) exceeds 1 mol%, translucency improves through a mechanism different from bubble removal. In these samples, rare earth oxides are uniformly distributed throughout the pre-sintered body. Therefore, it is inferred that for samples where (X - Y) exceeds 1 mol%, the crystal structure of the sintered body after firing is homogeneous, leading to improved translucency.

[0081] Having described the technology disclosed herein in detail, it should be noted that these are merely illustrative examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. That is, the technology disclosed herein encompasses the forms described in the following items 1 to 9.

[0082] <Item 1> A method for manufacturing a zirconia pre-sintered body comprising: a generation step of generating a zirconia sol slurry containing zirconia powder; a rare earth dispersion step of dispersing rare earth particles having an average particle diameter of 300 nm or less into the zirconia sol slurry; a collecting step of collecting mixed particles containing zirconia particles and rare earth particles from the zirconia sol slurry; a heating step of producing partially stabilized zirconia powder by heating the mixed particles; and a forming and heating of heating the partially stabilized zirconia powder after molding it into a desired shape.

[0083] <Item 2> The method for manufacturing the zirconia pre-sintered body according to item 1, wherein the rare earth dispersion step comprises: a rare earth addition step of dissolving rare earth raw materials into the zirconia sol slurry having a pH of 4 or less; and a precipitation step of precipitating rare earth compounds by raising the pH of the zirconia sol slurry to 6 to 9.

[0084] <Item 3> The method for manufacturing the zirconia pre-sintered body according to item 2, wherein in the precipitation step, ammonia is added to the zirconia sol slurry to raise the pH of the slurry.

[0085] <Item 4> The method for manufacturing the zirconia pre-sintered body according to item 1, wherein the rare earth dispersion step comprises: an addition step of adding rare earth particles having an average particle diameter of 300 nm or less to the zirconia sol slurry; and a stirring step of stirring the zirconia sol slurry to disperse the rare earth particles.

[0086] <Item 5> The method for manufacturing the zirconia pre-sintered body according to any one of items 1 to 4, wherein a particle size adjustment step of adjusting the particle size of the partially stabilized zirconia powder is conducted between the heating step and the forming and heating.

[0087] <Item 6> The method for manufacturing the zirconia pre-sintered body according to any one of items 1 to 5, wherein the rare earth particles are yttria.

[0088] <Item 7> A zirconia pre-sintered body comprising zirconia and rare earth oxides, wherein the zirconia pre-sintered body having a tetragonal crystal structure with a c / a axis length ratio of 1.008 or more, and the difference (X - Y) between the content X (mol%) of the rare earth oxides in the zirconia pre-sintered body measured by XRF analysis and the solution content Y (mol%) of the rare earth oxides in the main phase measured by XRD analysis is 1 mol% or less.

[0089] <Item 8> The zirconia pre-sintered body according to item 7, wherein the rare earth oxide is yttria.

[0090] <Item 9> The zirconia pre-sintered body according to claim 7 or 8, wherein the content X of the rare earth oxides is 3 mol% or more and 6 mol% or less.

[0091] <Item 10> The zirconia pre-sintered body according to any one of claims 7 to 9, wherein the solution content Y of the rare earth elements in the main phase is 3.2 mol% or more and 5 mol% or less.

[0092] <Item 11> The zirconia pre-sintered body according to any one of claims 7 to 10, wherein the presence ratio of the main phase with respect to the entire zirconia pre-sintered body is 75% or more and 100% or less.

[0093] <Item 12> A method for manufacturing a zirconia sintered body, comprising: a pre-sintered body preparation step of accommodating the zirconia pre-sintered body according to any one of claims 6 to 11 inside a firing furnace; a heating step of raising the internal temperature of the firing furnace to a predetermined firing temperature; a holding step of maintaining the predetermined holding time while keeping the firing temperature; and a cooling step of cooling the internal temperature of the firing furnace to a predetermined cooling temperature, wherein the holding time is 30 minutes or less.

Claims

1. A method for manufacturing a zirconia pre-sintered body comprising: a generation step of generating a zirconia sol slurry containing zirconia powder; a rare earth dispersion step of dispersing rare earth particles having an average particle diameter of 300 nm or less into the zirconia sol slurry; a collecting step of collecting mixed particles containing zirconia particles and rare earth particles from the zirconia sol slurry; a heating step of producing partially stabilized zirconia powder by heating the mixed particles; and a forming and heating of heating the partially stabilized zirconia powder after molding it into a desired shape.

2. The method for manufacturing the zirconia pre-sintered body according to claim 1, wherein the rare earth dispersion step comprises: a rare earth addition step of dissolving rare earth raw materials into the zirconia sol slurry having a pH of 4 or less; and a precipitation step of precipitating rare earth compounds by raising the pH of the zirconia sol slurry to 6 to 9.

3. The method for manufacturing the zirconia pre-sintered body according to claim 2, wherein in the precipitation step, ammonia is added to the zirconia sol slurry to raise the pH of the slurry.

4. The method for manufacturing the zirconia pre-sintered body according to claim 1, wherein the rare earth dispersion step comprises: an addition step of adding rare earth particles having an average particle diameter of 300 nm or less to the zirconia sol slurry; and a stirring step of stirring the zirconia sol slurry to disperse the rare earth particles.

5. The method for manufacturing the zirconia pre-sintered body according to claim 1, wherein a particle size adjustment step of adjusting the particle size of the partially stabilized zirconia powder is conducted between the heating step and the forming and heating.

6. The method for manufacturing the zirconia pre-sintered body according to any one of claims 1 to 5, wherein the rare earth particles are yttria.

7. A zirconia pre-sintered body comprising zirconia and rare earth oxides, wherein the zirconia pre-sintered body having a tetragonal crystal structure with a c / a axis length ratio of 1.008 or more, and the difference (X - Y) between the content X (mol%) of the rare earth oxides in the zirconia pre-sintered body measured by XRF analysis and the solution content Y (mol%) of the rare earth oxides in the main phase measured by XRD analysis is 1 mol% or less.

8. The zirconia pre-sintered body according to claim 7, wherein the rare earth oxide is yttria.

9. The zirconia pre-sintered body according to claim 7, wherein the content X of the rare earth oxides is 3 mol% or more and 6 mol% or less.

10. The zirconia pre-sintered body according to claim 7, wherein the solution content Y of the rare earth elements in the main phase is 3.2 mol% or more and 5 mol% or less.

11. The zirconia pre-sintered body according to claim 7, wherein the presence ratio of the main phase with respect to the entire zirconia pre-sintered body is 75% or more and 100% or less.

12. A method for manufacturing a zirconia sintered body, comprising: a pre-sintered body preparation step of accommodating the zirconia pre-sintered body according to any one of claims 7 to 11 inside a firing furnace; a heating step of raising the internal temperature of the firing furnace to a predetermined firing temperature; a holding step of maintaining the predetermined holding time while keeping the firing temperature; and a cooling step of cooling the internal temperature of the firing furnace to a predetermined cooling temperature, wherein the holding time is 30 minutes or less.

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