JP3603116B2 - High-density yttria-stabilized zirconia sintered body and method for producing the same - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-density yttria-stabilized zirconia sintered body and a method for producing the same. The high-density yttria-stabilized zirconia sintered body is useful, for example, as a solid electrolyte of a solid oxide fuel cell. Hereinafter, yttria-stabilized zirconia is described as “YSZ”.
[0002]
[Prior art]
A fuel cell is a fossil fuel / power conversion device expected to have higher efficiency than the existing thermal power generation, and among fuel cells, a device using a solid oxide as an electrolyte (solid oxide fuel cell: Solid Oxide Fuel Cell) (Hereinafter, abbreviated as "SOFC")) has attracted attention as having the highest energy conversion efficiency. As a solid oxide used for the electrolyte of the SOFC, for example, a YSZ sintered body is well known.
[0003]
However, when using a YSZ sintered body as the electrolyte of the SOFC, there is a problem in that when the YSZ sintered body is operated at 1000 ° C. or more for about 1000 hours or more, the power generation efficiency is reduced due to the deterioration of the YSZ sintered body. is there. The following can be considered as a cause of the high-temperature and long-time operation of the SOFC;
(1) cracking of the YSZ sintered body;
(2) that the ionic conductivity of the YSZ sintered body is reduced,
(3) The electrode reaction is suppressed by the progress of sintering of the electrode material, and (4) The formation of a low conductive layer by the progress of the solid phase reaction at the YSZ sintered body / electrode interface.
[0004]
In view of such a current situation, various attempts have been made to solve the above problem. For example, attempts have been made to add Al ₂ O ₃ , MgO, etc. to the composition of the YSZ sintered body in order to increase the mechanical strength.
[0005]
However, it has been reported that the addition of a different element increases the mechanical strength of the YSZ sintered body, but at the same time, lowers the conductivity of the YSZ sintered body (for example, LM Navarro, P.S. Recio, JR Jurado, and P. Duran, J. Mater. Sci., 30, 1949 (1995).). It has also been suggested that the added heterogeneous element may precipitate at the electrolyte grain boundaries as the SOFC is operated at a high temperature and for a long time, resulting in an increase in resistance and a decrease in the performance of the SOFC. In any case, current attempts have not largely solved the problems of the prior art.
[0006]
[Problems to be solved by the invention]
From the above viewpoints, there is a strong demand for the development of a high-density YSZ sintered body that has higher mechanical strength than conventional YSZ sintered bodies and that suppresses a decrease in conductivity over time even under high-temperature conditions. However, such a high-density YSZ sintered body has not yet been developed.
[0007]
Accordingly, an object of the present invention is to provide a high-density YSZ sintered body having particularly high mechanical strength and in which the decrease in conductivity over time is suppressed even under high-temperature conditions. It is another object of the present invention to provide a method for manufacturing a high-density YSZ sintered body.
[0008]
[Means for Solving the Problems]
The present inventor has conducted intensive studies to solve the problems of the prior art, and as a result, found that the following high-density YSZ sintered body and a method for producing the same achieve the above objects, and finally complete the present invention. Reached.
[0009]
The present invention relates to the following high-density YSZ sintered body and a method for producing the same.
[0010]
1. A high-density yttria-stabilized zirconia sintered body;
(1) The bulk density of the sintered body is 95% or more of the theoretical density,
(2) the maximum particle size of the sintered body is 2 μm or less, and the average particle size is 1 μm or less;
(3) When the room-temperature bending strength of the sintered body is 300 MPa or more, and (4) when the sintered body is held at a temperature of 1000 ° C. for 1000 hours, the conductivity σ after the lapse of 1000 hours with respect to the initial conductivity σ ₀ A high-density yttria-stabilized zirconia sintered body, wherein the ratio of ₁₀₀₀ (σ ₁₀₀₀ / σ ₀ ) is 0.68 or more.
[0011]
2. A ZrO ₂ powder (yttria-stabilized zirconia) doped with 1 to 20 mol% of Y ₂ O ₃ is pressurized at a pressure of 10 to 60 MPa, and heated at a temperature of 160 to 170 ° C./min using a DC pulse current of 500 to 1500 A. A method for producing a high-density yttria-stabilized zirconia sintered body, comprising: performing heating at a rate of 1100 to 1300 ° C., followed by discharge plasma sintering for 1 to 30 minutes.
[0012]
3.1 Heating temperature (° C.) selected from 1100 to 1300 ° C. and holding time (hour) for spark plasma sintering selected from the range of 1 to 30 minutes (ie, 1/60 to 1/2 hour) 3. The method for producing a high-density yttria-stabilized zirconia sintered body according to the above item 2, wherein the product (° C. · time) of the product is 250 or less.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
High density YSZ sintered body Hereinafter, the requirements (1) to (4) that the high density YSZ sintered body of the present invention should satisfy will be described in detail.
(1) The bulk density of the sintered body is 95% or more of the theoretical density.
[0014]
If the bulk density of the sintered body is less than 95% of the theoretical density (6 g / cm ³ ), the fracture energy of the sintered body with respect to external stress such as friction and impact becomes small, and the mechanical strength is reduced. In addition, since the porosity increases, for example, when used as a solid electrolyte of an SOFC, mixing of fuel gas and air occurs, and the power generation efficiency is significantly reduced. Therefore, the bulk density of the sintered body needs to be 95% or more of the theoretical density, and particularly preferably 98% or more.
(2) The maximum particle size of the sintered body is 2 μm or less, and the average particle size is 1 μm or less.
[0015]
Generally, as the particle size of the sintered body increases, wear due to grain detachment wear tends to occur, but also in the sintered body of the present invention, when the maximum particle size exceeds 2 μm, the stability of the cubic crystal is reduced. Become inferior. Therefore, for example, when this sintered body is used as a solid electrolyte of an SOFC, when the SOFC is maintained at an operating temperature of 1000 ° C. for 1,000 hours or more, the abundance ratio of cubic crystals decreases, and the abundance ratio of tetragonal crystals having low conductivity is reduced. Increase.
[0016]
Further, considering that the mechanical strength of ceramics is proportional to the reciprocal of the square root of the average particle diameter, and when the average particle diameter exceeds 1 μm, there is almost no difference in mechanical strength, and the average particle diameter is 1 μm. It is necessary that:
[0017]
As described above, the high-density YSZ sintered body of the present invention needs to have a maximum particle diameter of 2 μm or less and an average particle diameter of 1 μm or less. Among them, those having a maximum particle diameter of 1 μm or less and an average particle diameter of 0.5 μm or less are particularly preferred.
[0018]
In the present invention, the particle diameter refers to a diameter calculated by observing a high-density YSZ sintered body with a microscope and approximating the observed particles with a spherical shape. The average particle diameter is an average value of diameters calculated by the above-described spherical approximation.
(3) The room temperature bending strength of the sintered body is 300 MPa or more.
[0019]
Since the high-density YSZ sintered body of the present invention can be used as a high-temperature conductive material such as a solid electrolyte of a SOFC and a heat-resistant electrode, it can withstand thermal stress and thermal shock due to repeated temperature rise and fall and temperature distribution. It is necessary to have sufficient strength. Therefore, the room-temperature bending strength of the sintered body needs to be 300 MPa or more, and particularly preferably 320 MPa or more.
[0020]
In the present invention, the value of the room temperature bending strength is a value obtained by measuring the three-point bending strength of the high-density YSZ sintered body at room temperature. Specifically, according to the standard of JIS R1601, three-point bending was performed using a 3 × 4 mm square test piece whose four sides were finished with a diamond polishing machine of # 1000 under the conditions of a span of 30 mm and a load speed of 0.5 mm / min. It is a value of strength (at room temperature).
(4) In the case of holding for 1000 hours at a temperature of the sintered 1000 ° C., the ratio of the conductivity sigma ₁₀₀₀ after 1000 hours passed to the initial conductivity _{σ 0 (σ 1000 / σ 0} ) is 0.68 or more That.
[0021]
In the present invention, the conductivity is determined by shaving a high-density YSZ sintered body into a square rod shape, baking a platinum electrode and a platinum lead wire for voltage measurement at about 1000 ° C. (S / cm).
[0022]
The σ ₀ of the high-density YSZ sintered body of the present invention is usually 0.1 S / cm. For example, when this sintered body is used as a solid electrolyte of an SOFC, the SOFC is operated at an operating temperature of 1000 ° C. for 1000 hours or more. If the conductivity is less than 0.068 S / cm when held, the power generation efficiency is reduced. Accordingly, high-density YSZ sintered body of the present invention requires that σ ₍₀ σ ₁₀₀₀ / σ) ratio of sigma ₁₀₀₀ for ₀ is 0.68 or more.
[0023]
Method for producing high-density YSZ sintered body Hereinafter, the method for producing a high-density YSZ sintered body of the present invention will be described in detail.
[0024]
The raw material powder is not particularly limited as long as it is a ZrO ₂ powder (YSZ powder) doped with 1 to 20 mol% of Y ₂ O _3, and a known or commercially available product can be used. The doping ratio of Y ₂ O ₃ can be appropriately set from 1 to 20 mol%, but is particularly preferably about ₃ to 10 mol%.
[0025]
The method of preparing the YSZ powder is not particularly limited, and may be any synthesis method selected from a solid-phase reaction method, a hydrolysis method, a sol-gel method, a hydrothermal method, and the like. Although the particle size of the YSZ powder is not particularly limited, it is usually preferable that the particle size is submicron or smaller.
[0026]
In the present invention, the above-mentioned YSZ powder is subjected to spark plasma sintering under specific conditions, thereby sintering while suppressing the particle diameter growth of the sintered body, thereby obtaining a high-density YSZ sintered body. That is, a YSZ sintered body having a higher density than the YSZ powder as the raw material can be obtained.
[0027]
In the present invention, it is preferable to use a discharge plasma sintering machine for discharge plasma sintering of the YSZ powder. Using a sintering method based on ON-OFF pulse conduction such as discharge plasma sintering, discharge sintering, and electric sintering, the YSZ powder is compressed into a green compact, and a pulse current is applied to the green compact, It is preferable to perform compression sintering while controlling the temperature of the YSZ powder by controlling the peak current and the pulse width.
[0028]
The spark plasma sintering machine is not particularly limited as long as it can heat, cool and pressurize the YSZ powder and can apply a voltage sufficient to cause a discharge. That is, YSZ powder is preferably subjected to discharge plasma sintering using a discharge plasma sintering machine equipped with a heating / cooling device, a pressurizing device, a discharge device, and a jig for storing the YSZ powder. Generally, graphite is suitable as a jig. Regarding the spark plasma sintering machine and the operation principle thereof, for example, Japanese Patent No. 3007929 (Japanese Patent Laid-Open Publication No. Hei 10-25170 (published on September 22, 1998)) can be referred to.
[0029]
In the manufacturing process of the high-density YSZ sintered body of the present invention, first, while pressing the YSZ powder at a pressure of 10 to 60 MPa, preferably 10 to 30 MPa, a DC pulse current of 500 to 1500 A, preferably 700 to 1100 A is used. The YSZ powder is heated to 1100 to 1300 ° C., preferably 1300 ° C. at a rate of 160 to 170 ° C./min. Thereafter, discharge plasma sintering is performed for 1 to 30 minutes, preferably 1 to 10 minutes while maintaining the temperature. The period of the pulse current to be used is not particularly limited, and generally can be appropriately selected from a range of 300 Hz to 30 KHz.
[0030]
The method for producing the high-density YSZ sintered body of the present invention is not particularly limited as long as the production method satisfies the above-mentioned conditions, and among them, the heating temperature (° C.) particularly selected from the range of 1100 to 1300 ° C. The value of the product (° C. · hour) with the holding time (hour) related to spark plasma sintering selected from the range of 1 to 30 minutes (ie, 1/60 to 1/2 hour) is 20 or more and 250 or less. Is preferred. When the value of the product is 20 or more and 250 or less, a high-density YSZ sintered body having high mechanical strength and in which the decrease in conductivity over time is suppressed even under high temperature conditions is stably manufactured in a short time. be able to.
[0031]
After the discharge plasma sintering, it is preferable to stop the application of the pulse current and the pressure, cool the sintered body to room temperature, and then take out the high-density YSZ sintered body.
[0032]
When graphite is used as a jig, graphite, which is a component thereof, may be included in the vicinity of the surface of the obtained sintered body. Impurities such as graphite contained in the vicinity of the surface of such a sintered body can be easily removed by polishing or heating the surface of the sintered body. When polishing the surface of the sintered body, it is preferable to use a diamond polishing machine, for example. When the surface of the sintered body is heated, for example, it is preferable to heat it in air at about 1000 to 1300 ° C. for about 1 to 3 hours using a normal electric furnace.
[0033]
【The invention's effect】
According to the high-density YSZ sintered body of the present invention, the mechanical strength is higher than that of the conventional YSZ sintered body, and the conductivity of the YSZ sintered body under high temperature conditions (for example, 1000 hours at 1000 ° C.). Since the specific decrease is suppressed, it can be used for various applications including the solid electrolyte of the SOFC.
[0034]
According to the method for producing a high-density YSZ sintered body according to the present invention, a high-density YSZ sintered body having high mechanical strength and in which a decrease in conductivity over time is suppressed even under a high temperature condition is compared with a conventional method. It is possible to manufacture stably at a lower temperature and in a shorter time in comparison.
[0035]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. However, the present invention is not limited by these descriptions.
[0036]
Example 1
As the raw material powder, commercially available YSZ powder (trade name “TZ-8YS” manufactured by Tosoh Corporation: ZrO ₂ powder doped with 8 mol% of Y ₂ O ₃ , average particle diameter 0.3 μm, hereinafter referred to as “8YSZ”) .) Was used. As the spark plasma sintering machine, (trade name "SPS-515S" manufactured by Izumi Tech Co., Ltd.) was used. As the jig, a cylindrical graphite jig having a diameter of 1.5 cm was used.
[0037]
First, 1 g of 8YSZ powder was uniformly placed in a jig, and the inside of the sintering chamber was evacuated to 7 Pa while applying a pressure of 30 MPa. Next, a DC pulse current of 1000 A was applied to the jig, and the 8YSZ powder was heated to 1300 ° C. at a rate of 160 ° C./min. Further, in this state, a DC pulse current of 900 A was applied, and discharge plasma sintering was performed for one minute. After sintering, the application of current and pressure was stopped, and the sintered body was naturally cooled to room temperature to obtain a high-density YSZ sintered body. The value of the product (° C. · hour) of the heating temperature and the holding time related to spark plasma sintering was 21.6.
[0038]
The obtained high-density YSZ sintered body was found by X-ray diffraction to contain graphite of a jig. The fact that the high-density YSZ sintered body contained the graphite of the jig was evident from the results of energy dispersive X-ray analysis (EDX) that C was recognized in addition to Y, Zr, and O. Therefore, the high-density YSZ sintered body was heat-treated at 1000 ° C. for 2 hours to obtain a high-density YSZ sintered body containing no C (the high-density YSZ sintered body after the heat treatment has a peak of 8 YSZ in X-ray diffraction). Only E was observed, and C was not measured by EDX). Hereinafter, the high-density YSZ sintered body obtained by the above method (that is, the high-density YSZ sintered body obtained by spark plasma sintering at 1300 ° C. for 1 minute) is particularly described as “SPS sintered body”.
[0039]
The characteristics of the obtained SPS sintered body were as follows.
[0040]
[X-ray diffraction pattern]
The obtained SPS sintered body shows an X-ray diffraction pattern of a cubic crystal, and the lattice constant obtained from the peak position is a = 0.51398 (1) nm (0.51398 ± 0.00001 nm (the same applies hereinafter)). ), Which is in good agreement with the reported value a = 0.5137 nm (RP Ingel and D. Lewis III, J. Am. Ceram. Soc., 69, 325 (1986).).
[0041]
[Bulk density]
The bulk density of the obtained SPS sintered body was 5.9 g / cm ^{3 as} shown in FIG. This was 98% of the theoretical density (6 g / cm ³ ).
[0042]
(Crystal structure)
As shown in the SEM photograph (b) of FIG. 2, the crystal structure of the obtained SPS sintered body is composed of submicron particles, the maximum particle size of the crystal is 2 μm or less, and the average particle size is It was 1 μm or less. The values of the maximum particle diameter and the average particle diameter calculated by approximating the particles observed by a microscope to a sphere were a maximum particle diameter of 0.8 μm and an average particle diameter of 0.4 μm.
[0043]
[Three-point bending strength]
The three-point bending strength of the obtained SPS sintered body was 329 MPa as shown in FIG. This value is based on the reported 8YSZ sintered body (about 235 MPa; O. Yamamoto, Y. Takeda, N. Imanishi, T. Kawahara, GQ. Shen, M. Mori, and T. Abe, Proceedings of the The value was higher than the strength of Second International Symposium on Solid Oxide Fuel Cells, pp. 437 (1991).
[0044]
(Change over time in AC conductivity)
When the obtained SPS sintered body is held at a temperature of 1000 ° C. for 1000 hours, the initial AC conductivity σ ₀ is 0.18 S / cm, and the reported initial AC conductivity of 8YSZ at 1000 ° C. Rate (0.1 S / cm; NQ Minh, J. Am. Ceram. Soc., 76, 563 (1993)). FIG. 4 shows the results of the change over time in the AC conductivity. After 500 hours, the decrease in conductivity of the SPS sintered body was reduced, and after 1000 hours, the rate of decrease was considerably suppressed.
[0045]
Comparative Example 1
The same YSZ powder “8YSZ” as in Example 1 was sintered (CS sintering) for 2 hours using a conventional external heating electric furnace to produce a YSZ sintered body. Five types of YSZ sintered bodies were manufactured based on differences in sintering temperatures (1200 ° C., 1300 ° C., 1400 ° C., 1500 ° C., and 1600 ° C.). Hereinafter, the five types of YSZ sintered bodies are particularly described as “CS sintered bodies”.
[0046]
The characteristics of the obtained CS sintered body were as follows.
[0047]
[Bulk density]
The bulk density of the obtained CS sintered body is shown in FIG. For example, the bulk density of a CS sintered body obtained by sintering at 1300 ° C. for 2 hours was 4.1 g / cm ³ (68% of the theoretical density). The relative density was 98% or more when the sintering temperature was 1600 ° C. or more.
[0048]
(Crystal structure)
The crystal structure of the obtained CS sintered body is, for example, sintered at 1600 ° C. for 2 hours, and as shown in the SEM photograph (c) of FIG. 2, the crystal grain size grows to several tens μm or more. The grain interface was a structure that was difficult to observe.
[0049]
[Three-point bending strength]
The three-point bending strength of the obtained CS sintered body was, for example, 205 MPa when sintered at 1600 ° C. for 2 hours. The measurement results of the three-point bending strength are shown in FIG.
[0050]
(Change over time in AC conductivity)
When the obtained CS sintered body was held at a temperature of 1000 ° C. for 1000 hours, the initial AC conductivity σ ₀ was 0.18 S / cm, which was almost the same as the value of Example 1. FIG. 4 shows the results of the change over time in the AC conductivity. The AC conductivity of the CS sintered body decreased at a constant rate over time.
[0051]
Consideration of Examples and Comparative Examples According to FIG. 1 (sintering temperature dependence of bulk density of SPS sintered body and CS sintered body), the manufacturing method of the present invention is more effective than the conventional manufacturing method. It can be said that a high-density YSZ sintered body can be manufactured at a lower temperature and in a shorter time.
[0052]
According to FIG. 2 (crystal structures of the SPS sintered body and the CS sintered body), the manufacturing method of the present invention controls the structure more (that is, suppresses the particle diameter growth) than the conventional manufacturing method. It can be said that a high-density YSZ sintered body can be manufactured.
[0053]
According to FIG. 3 (the sintering temperature dependence of the three-point bending strength of the SPS sintered body and the CS sintered body), the manufacturing method of the present invention has a higher density with higher mechanical strength than the conventional manufacturing method. It can be said that a YSZ sintered body can be manufactured. According to the Hall-Petch equation (H = H ₀ + kL ^−1/2 ; H is the strength, L is the particle size, H ₀ and k are constants), the strength of the material increases with the refinement of the particle size. It is considered that the same increase in strength occurred in the production method described above because the growth of the crystal diameter of the sintered body was suppressed and the density was increased.
[0054]
According to FIG. 4 (time-dependent change in the AC conductivity of the SPS sintered body and the CS sintered body), the value of the initial conductivity at 1000 ° C. is substantially the same between the SPS sintered body and the CS sintered body. However, it can be seen that there is a difference in the rate of decrease in AC conductivity over time. In the case of a CS sintered body, a decrease in AC conductivity is observed at a constant rate over time. However, in the case of the SPS sintered body, the decrease rate of the AC conductivity is considerably suppressed after 500 hours, and exceeds the AC conductivity of the CS sintered body after 1000 hours. As the cause of the decrease in conductivity, formation of a complex of oxygen deficiency and Y ^{3+ in} YSZ in a high-temperature and long-time environment and suppression of oxide ion conductivity due to a long-period arrangement of oxygen deficiency are mentioned (J. Kondoh, S. Kikuchi, Y. Tomii, and Y. Ito, J. Electrochem. Soc., 145, 1536 (1998)), and a clear grain boundary exists in the SPS sintered body as shown in FIG. Therefore, it is considered that the long-period arrangement and the like of oxygen deficiency were suppressed to some extent, and a decrease in conductivity over time was suppressed. Therefore, it can be said that the production method of the present invention can produce a high-density YSZ sintered body in which a decrease in conductivity over time is suppressed even under high-temperature conditions.
[0055]
(Other)
The method for producing a high-density YSZ sintered body of the present invention can be applied to various other electrolyte materials, for example, LaGaO ₃ or the like, and has a lower temperature than a conventional external heat sintering method using an electric furnace or the like. In addition, a high-density sintered body having high strength and little decrease in conductivity can be produced in a short time.
[Brief description of the drawings]
FIG. 1 is a diagram showing the sintering temperature dependence of the bulk density of an SPS sintered body and a CS sintered body.
FIG. 2 is a SEM photograph showing the crystal structures of (a) raw material powder, (b) SPS sintered body (1300 ° C., 1 minute sintering) and CS sintered body (1600 ° C., 2 hour sintering). It is.
FIG. 3 is a diagram showing the sintering temperature dependence of the three-point bending strength of an SPS sintered body and a CS sintered body.
FIG. 4 shows the change over time of the AC conductivity when the SPS sintered body (1300 ° C., sintering for 1 minute) and the CS sintered body (1600 ° C., sintering for 2 hours) are maintained at a temperature of 1000 ° C. FIG.

Claims (2)

A high-density yttria-stabilized zirconia sintered body used as a solid electrolyte of a solid oxide fuel cell ;
(1) The bulk density of the sintered body is 95% or more of the theoretical density,
(2) the maximum particle size of the sintered body is 2 μm or less, and the average particle size is 1 μm or less;
(3) The sintered body has a room temperature bending strength of 300 MPa or more.
(4) In the case of holding for 1000 hours at a temperature of the sintered 1000 ° C., the ratio of the conductivity sigma ₁₀₀₀ after 1000 hours passed to the initial conductivity _{σ 0 (σ 1000 / σ 0} ) is 0.68 or more ,as well as
(5) A high-density yttria-stabilized zirconia sintered body characterized in that the crystal phase of the sintered body is cubic . A ZrO ₂ powder (yttria-stabilized zirconia) doped with 1 to 20 mol% of Y ₂ O ₃ is pressurized at a pressure of 10 to 60 MPa, and heated at a temperature of 160 to 170 ° C./min using a DC pulse current of 500 to 1500 A. This is a method for producing a high-density yttria-stabilized zirconia sintered body that is heated at a rate of 1100 to 1300 ° C. and then subjected to discharge plasma sintering for 1 to 30 minutes. ) And a holding time (hour) related to spark plasma sintering selected from the range of 1 to 30 minutes (that is, 1/60 to 1/2 hour) (° C. · hour) in the range of 20 to 250. A production method, wherein the crystal phase of the sintered body is cubic, and the use of the sintered body is a solid electrolyte of a solid oxide fuel cell .

Images