Ceramic electrode materials, multilayer ceramic capacitors, and methods for manufacturing them.

Abstract

To provide a ceramic electrode material made of barium titanate, a ceramic capacitor using the same, and a method for manufacturing the same. [Solution] The ceramic electrode material of the present invention is made of barium titanate having oxygen vacancies and having a cubic crystal structure in a temperature range of at least 300K to 400K. The method for producing the ceramic electrode material made of oxygen-vacant barium titanate of the present invention includes annealing a barium titanate raw material having a stoichiometric composition in a vacuum in the presence of a Group 2 element metal.

Description

[Technical Field] 【0001】 The present invention relates to ceramic electrode materials, multilayer ceramic capacitors, and methods for manufacturing them. [Background technology] 【0002】 Multilayer ceramic capacitors are used in circuits for noise reduction, power supply voltage smoothing, and filtering, and are essential components in mobile phones, smartphones, and televisions. To achieve miniaturization and higher capacitance of such multilayer ceramic capacitors, there is a need for increased density and thinning of the internal electrodes. 【0003】 In recent years, techniques have been developed to control the electrical resistance by controlling the amount of oxygen in barium titanate used in multilayer ceramic capacitors (see, for example, Non-Patent Documents 1 and 2). 【0004】 According to Non-Patent Literature 1, it is reported that the electrical resistance of barium titanate ceramics is reduced by doping them with oxygen vacancies by heating them to 900-1250°C in a tube furnace and treating them for 20-40 hours in a flow of 5% H2 / 95% N2 or pure H2 (99.999%). However, since barium titanate undergoes a structural transition with increasing temperature, it is difficult to use it as an electrode in this state. 【0005】 Recently, according to Non-Patent Literature 2, it was found that barium titanate single crystals doped with oxygen vacancies by being placed in a tantalum crucible with titanium powder and sealed in a quartz tube not only have low electrical resistance but also do not undergo structural transitions. Furthermore, according to Non-Patent Literature 2, such barium titanate single crystals can exhibit ferroelectricity and are polar metals. 【0006】 Non-Patent Document 3 relates to ferroelectric capacitors and reports that by using a polar metal as an electrode, the thickness of the ferroelectric can be reduced to a thickness exceeding the limit. This suggests that there are new possibilities for polar metals. 【Prior Art Documents】 【Non-Patent Documents】 【0007】 【Non-Patent Document 1】 T. Kolodiazhnyi et al., Phys. Rev. Lett., 104, 147602 (2010) 【Non-Patent Document 2】 X. Yang et al., Quantum Materials volume 8, Article number: 47 (2023) 【Non-Patent Document 3】 D. Puggioni et al., J. Appl. Phys., 124, 174102 (2018) 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0008】 An object of the present invention is to provide a ceramic electrode material made of barium titanate, a ceramic capacitor using the same, and a method for manufacturing them. 【Means for Solving the Problems】 【0009】 The ceramic electrode material according to the present invention is made of barium titanate having oxygen deficiency and having a cubic crystal structure in a temperature range of at least 300 K or more and 400 K or less, thereby solving the above problems. The barium titanate may have a cubic crystal structure in a temperature range of 3 K or more and 700 K or less. The barium titanate may satisfy BaTiO 3-x (0 < x ≦ 0.1). The barium titanate may have an electrical resistivity of 0.01 Ωcm or more and 1 Ωcm or less in a temperature range of 3K or more and 700K or less. The barium titanate may have the characteristics of a polar metal in a temperature range of 3K or more and 700K or less. The lattice constant a of the barium titanate may satisfy 0.398 nm < a < 0.402 nm in a temperature range of 3K or more and 700K or less. The method for producing a ceramic electrode material composed of barium titanate having oxygen deficiency according to the present invention includes annealing a barium titanate raw material having a stoichiometric composition in a vacuum in the presence of a metal of Group 2 element, thereby solving the above problems. The Group 2 element may be at least one element selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The annealing may be performed in a temperature range of 1050 °C or more and 1500 °C or less. The annealing may be performed in a temperature range of 1150 °C or more and 1250 °C or less. The mass ratio of the metal of the Group 2 element to the barium titanate raw material may be in the range of 0.01 or more and 0.5 or less. The mass ratio of the metal of the Group 2 element to the barium titanate raw material may be in the range of 0.02 or more and 0.1 or less. The annealing may be performed at a degree of vacuum of 1×10 -4 Pa or more and 1 Pa or less. The annealing may be performed for a time of 30 minutes or more and 24 hours or less. The method may further include repeating the annealing. The multilayer ceramic capacitor according to the present invention has a multilayer structure in which dielectric layers made of ceramic and electrode layers made of the above ceramic electrode material are alternately laminated, thereby solving the above problems. The dielectric layer may be selected from the group consisting of barium titanate, calcium zirconate, calcium titanate, lithium niobate, potassium niobate, strontium titanate with lanthanum addition, and barium zirconate. The dielectric layer may have a thickness in the range of 1 nm or more and 1 mm or less, and the electrode layer may have a thickness in the range of 1 nm or more and 1 mm or less. The method for manufacturing the multilayer ceramic capacitor according to the present invention includes forming a laminate in which a dielectric sheet containing a dielectric and an internal electrode sheet containing the ceramic electrode material are laminated, and sintering the laminate, thereby solving the above problems. 【Advantages of the Invention】 【0010】 Since the ceramic electrode material of the present invention is made of barium titanate having oxygen deficiency, its electrical resistivity is low and it functions as an electrode material. Furthermore, barium titanate has a cubic crystal structure in the temperature range of at least 300 K to 400 K and does not have a structural phase transition. Therefore, even when used as an electrode of a ceramic capacitor, the electrode will not be destroyed within the operating temperature range. By using the ceramic electrode material of the present invention, a multilayer ceramic capacitor can be provided. 【0011】 The method for manufacturing the ceramic electrode material of the present invention is to anneal the raw material barium titanate in the presence of a Group 2 metal in a vacuum. Therefore, no special technology or equipment is required, which is advantageous for practical application. 【Brief Description of the Drawings】 【0012】 [Figure 1] A diagram showing a flowchart for manufacturing the ceramic electrode material of the present invention [Figure 2] A schematic diagram showing the multilayer ceramic capacitor of the present invention [Figure 3] A diagram showing a flowchart for manufacturing the multilayer ceramic capacitor of the present invention [Figure 4] A diagram showing the experimental apparatus used in the manufacturing [Figure 5] Figure showing the temperature dependence of the electrical resistivity of the samples of Example 4 and Example 5 [Figure 6] Figure showing the temperature dependence of the XRD pattern of the sample of Example 4 [Figure 7] Figure showing the temperature dependence of the lattice constant (a) and the isotropic thermal vibration parameter (Beq) of the sample of Example 4 [Figure 8] Figure showing the temperature dependence of the second harmonic oscillation of the sample of Example 4 【Embodiments for Carrying Out the Invention】 【0013】 Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, the same elements are denoted by the same numbers, and the description thereof will be omitted. In this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value, respectively. 【0014】 (Embodiment 1) In Embodiment 1, the ceramic electrode material of the present invention and its manufacturing method will be described in detail. 【0015】 The ceramic electrode material of the present invention is composed of polycrystalline barium titanate but has oxygen deficiency. Therefore, the electrical resistivity becomes low and it functions as an electrode material. For simplicity, if an appearance of dark blue to black can be visually recognized, it can be determined that it has oxygen deficiency. 【0016】 Furthermore, barium titanate in the ceramic electrode material of the present invention has a cubic crystal structure in a temperature range of at least 300K or more and 400K or less. Conventional barium titanate is known to have a tetragonal crystal structure in a temperature range of 300K or more and 390K or less, and a cubic crystal structure at 392K or more, and undergoes a structural phase transition. However, in the ceramic electrode material of the present invention, barium titanate maintains a cubic crystal structure in a temperature range of at least 300K or more and 400K or less and does not have a structural phase transition. Therefore, even when used as an electrode of a ceramic capacitor, the electrode will not be physically damaged. 【0017】 Furthermore, the inventors of this invention discovered that barium titanate in the ceramic electrode material of the present invention can exhibit ferroelectricity in a temperature range of at least 300K to 400K, and is a polar metal. As mentioned above, single-crystal barium titanate is known as a polar metal, but there are no reports of polycrystalline ceramic barium being a polar metal. As exemplified by oxide high-temperature superconductors and nonlinear optical elements, there are many materials whose properties differ between single crystals and polycrystalline forms, even if they are the same material. Even if the single-crystal barium titanate described in Non-Patent Document 2 is a polar metal, it is not conceivable that polycrystalline ceramic barium titanate would also be a polar metal. 【0018】 In the ceramic electrode material of the present invention, barium titanate preferably has polar metal properties in a temperature range of 200K to 500K, and more preferably in a temperature range of 3K to 700K. This allows the dielectric layer in the multilayer ceramic capacitor described later to be thinned to an atomic layer thickness, which is advantageous for miniaturization. 【0019】 In the ceramic electrode material of the present invention, barium titanate preferably has a cubic crystal structure in a temperature range of 200K to 500K, and more preferably has a cubic crystal structure in a temperature range of 3K to 700K. This makes it possible to provide a stable electrode that does not suffer physical damage even when used over a wide temperature range. 【0020】 Barium titanate preferably belongs to the Pm3￣m space group (￣ is an overbar notation for 3, space group No. 221 in International Tables for Crystallography) in a temperature range of at least 300K to 400K, more preferably in a temperature range of 200K to 700K, still more preferably in a temperature range of 3K to 700K. The lattice constant a (nm) preferably satisfies 0.398nm < a < 0.402, and more preferably satisfies 0.399nm ≦ a ≦ 0.4015. Since the crystal is cubic, the other parameters describing the crystal lattice are a = b = c, α = β = γ = 90°. 【0021】 In the ceramic electrode material of the present invention, barium titanate preferably has an electrical resistivity of 0.01 Ωcm or more and 1 Ωcm or less in a temperature range of at least 300K to 400K. Thereby, it functions as an electrode. Barium titanate more preferably has an electrical resistivity of 0.01 Ωcm or more and 1 Ωcm or less in a temperature range of 200K to 500K, and still more preferably in a temperature range of 3K to 700K. Thereby, a stable electrode that does not physically break even when used in a wide temperature range can be provided. Even more preferably, barium titanate is oxygen-deficient so as to have an electrical resistivity of 0.01 Ωcm or more and 0.2 Ωcm or less in a temperature range of 300K to 400K. 【0022】 In the ceramic electrode material of the present invention, barium titanate is preferably represented by BaTiO 3-x (0 < x ≦ 0.1). Within this range, it is oxygen-deficient, has a low electrical resistivity, and does not have a structural phase transition in the above temperature range. The composition of barium titanate can be analyzed by an electron probe microanalyzer (EPMA) and thermogravimetric analysis (TGA). Barium titanate is more preferably represented by BaTiO 3-x (0 < x ≦ 0.05). Even with a very small amount of oxygen deficiency, it maintains a low electrical resistivity and does not have a structural phase transition within the above range. 【0023】 Next, a method for manufacturing the ceramic electrode material of the present invention will be described. Figure 1 shows a flowchart illustrating the manufacturing process of the ceramic electrode material of the present invention. 【0024】 The method for manufacturing the ceramic electrode material described above according to the present invention comprises the following steps. Step S110: A barium titanate raw material having a stoichiometric composition is annealed in a vacuum in the presence of a Group 2 element metal. The inventors of this invention have discovered that metals of Group 2 elements function as good oxygen getters, and have succeeded in efficiently creating oxygen deficiency. 【0025】 In step S110, the barium titanate raw material having a stoichiometric composition may be commercially available barium titanate or may be synthesized using a solid-phase reaction. In this case, the raw materials should be mixed so that the titanium in the titanium-containing raw material and the barium in the barium-containing raw material are in a molar ratio of 1:1. For simplicity, if the white appearance is visible, it can be determined that the barium titanate raw material has a stoichiometric composition. 【0026】 In step S110, the Group 2 element is preferably at least one element selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements allow for efficient removal of oxygen from the barium titanate raw material. The Group 2 element is more preferably Ca and / or Ba. These metals are readily available and easy to handle. 【0027】 In step S110, the mass ratio of the group 2 element metal to the barium titanate raw material is preferably in the range of 0.01 to 0.5. Within this range, oxygen can be efficiently removed from the barium titanate raw material. More preferably, the mass ratio of the group 2 element metal to the barium titanate raw material is in the range of 0.01 to 0.1, and even more preferably, in the range of 0.01 to 0.05. This allows the group 2 element metal to react with the barium titanate raw material, efficiently removing oxygen while suppressing the formation of the second phase. 【0028】 In step S110, the vacuum level is preferably 1 × 10⁻⁶ -4 The range may be Pa or more and 1 Pa or less, and more preferably 1 × 10 -3 The pressure can be in the range of Pa or more and 0.5 Pa or less. Within this range, oxygen can be efficiently removed from the barium titanate raw material. 【0029】 In step S110, the annealing treatment is preferably carried out in a temperature range of 1050°C to 1500°C. This allows for efficient removal of oxygen from the barium titanate raw material. More preferably, the annealing treatment is carried out in a temperature range of 1100°C to 1300°C, and even more preferably, in a temperature range of 1150°C to 1250°C. This allows for even more efficient removal of oxygen from the barium titanate raw material. 【0030】 In step S110, the annealing treatment is preferably carried out for a period of 30 minutes to 24 hours, more preferably for a period of 1 hour to 10 hours, and even more preferably for a period of 2 hours to 6 hours. This allows for efficient removal of oxygen from the barium titanate raw material. 【0031】 In step S110, the barium titanate raw material and the group 2 element metal are preferably arranged spaced apart from each other. This allows them to react directly and suppress the formation of the second phase. More preferably, the barium titanate raw material and the group 2 element metal are each covered with an alumina crucible or titanium foil, etc. This further suppresses the formation of the second phase. In particular, using titanium foil allows the titanium foil itself to function as a getter, thus promoting the removal of oxygen. 【0032】 Step S110 may be repeated after step S110. This ensures the uniformity of the sample and allows for further removal of oxygen from the barium titanate raw material. The annealing treatment may be repeated preferably one to five times, and more preferably two to four times. 【0033】 (Embodiment 2) Embodiment 2 describes a multilayer ceramic capacitor using the ceramic electrode material of the present invention, and a method for manufacturing the same. 【0034】 Figure 2 is a schematic diagram showing the multilayer ceramic capacitor of the present invention. 【0035】 The multilayer ceramic capacitor 200 of the present invention has a laminated structure in which a dielectric layer 210 made of ceramic and an electrode layer 220 made of ceramic electrode material are alternately stacked. Here, the ceramic electrode material is the same as the ceramic electrode material described in Embodiment 1, so its description is omitted. The multilayer ceramic capacitor 200 may further include external electrodes 230 provided spaced apart on two opposing surfaces of the laminated structure. 【0036】 The dielectric layer 210 is not particularly limited as long as it contains a dielectric, but is preferably selected from the group consisting of barium titanate, calcium zirconate, calcium titanate, lithium niobate, potassium niobate, lanthanum-doped strontium titanate, and barium zirconate. These are materials well known in the art and are commonly used in multilayer ceramic capacitors. In particular, the dielectric layer 210 may be made of barium titanate. As a result, both the dielectric layer 210 and the electrode layer 220 are made of barium titanate, making it possible to provide a monolithic multilayer ceramic capacitor 200. With such a monolithic multilayer ceramic capacitor 200, mismatches in thermal expansion coefficients and interface degradation that tend to occur when combining dissimilar materials can be avoided, material procurement and management can be made more efficient, and manufacturing costs can be reduced. Furthermore, since the dielectric layer and the electrode layer are made of the same material, it is easier to adjust the manufacturing conditions and it is possible to improve the yield. 【0037】 The thickness of the dielectric layer 210 is not particularly limited, but is preferably in the range of 1 nm to 1 mm. Since the ceramic electrode material of the present invention can be applied as the electrode layer 220, the dielectric layer 210 can function even when thinned to the thickness of an atomic layer, taking advantage of the properties of polar metals. The thickness of the dielectric layer 210 is more preferably in the range of 1 nm to 1 μm, and even more preferably in the range of 1 nm to 500 nm. 【0038】 The thickness of the electrode layer 220 is not particularly limited, but is preferably in the range of 1 nm to 1 mm, more preferably 1 nm to 1 μm, and even more preferably in the range of 1 nm to 500 nm. 【0039】 The edges of the electrode layer 220 are in contact with the external electrodes 230, thereby providing alternating electrical conductivity between the electrode layer 220 and the two external electrodes 230. The external electrodes 230 may be made of known materials such as copper (Cu), nickel (Ni), or tin (Sn). 【0040】 There are no particular restrictions on the size of the multilayer ceramic capacitor 200, but as an example, the height in the thickness direction of the dielectric layer 210 and electrode layer 220 may be in the range of 0.1 mm to 1 mm. By adopting the electrode layer 220 of the present invention, the dielectric layer 210 can be made thinner, thereby increasing the capacitance per unit volume. As described above, since the dielectric layer 210 can be made thinner than conventional capacitors, the overall capacitance of the capacitor can be improved by increasing the degree of lamination. As a result, both high performance and miniaturization can be achieved. 【0041】 Figure 3 shows a flowchart illustrating the manufacturing process of the multilayer ceramic capacitor of the present invention. 【0042】 The method for manufacturing the multilayer ceramic capacitor described above in the present invention comprises the following steps. Step S310: Form a laminate by stacking a dielectric sheet containing a dielectric material and an internal electrode sheet containing a ceramic electrode material. Step S320: Sinter the laminate. 【0043】 In step S310, the dielectric can be the dielectric powder described above. In step S310, the ceramic electrode material is the ceramic electrode material powder described in Embodiment 1, so its description is omitted. The average particle size of the powder may be in the range of 1 nm to 300 nm in volume-based median system (d50). Within this range, the thinning described above becomes possible. 【0044】 In step S310, a laminate may be formed by simply stacking dielectric powder and ceramic electrode material powder, or a laminate may be formed by applying a dielectric slurry containing dielectric powder and an electrode slurry containing ceramic electrode material powder. 【0045】 Dielectric slurry is prepared by adding a binder such as polyvinyl butyral (PVB) resin, an organic solvent such as ethanol or toluene, and a plasticizer to dielectric powder and mixing. Similarly, electrode slurry is prepared by adding a binder such as polyvinyl butyral (PVB) resin, an organic solvent such as ethanol or toluene, and a plasticizer to ceramic electrode material powder and mixing. 【0046】 A dielectric slurry is applied to a substrate such as an alumina substrate or a silicon substrate using a die coating method or a doctor blade method to form a dielectric green sheet. Next, an electrode slurry is applied to the dielectric green sheet using a die coating method or a doctor blade method to form an electrode green sheet. This process is repeated to form a laminate. 【0047】 In step S320, there are no particular restrictions as long as sintering is possible, but for example, if the powder is used as a laminate in step S310, sintering may be performed using plasma discharge sintering (SPS). 【0048】 If a slurry is used to form a laminate in step S310, for example, after debinding in a nitrogen atmosphere at a temperature range of 200°C to 250°C, it may be fired in a reducing atmosphere at a temperature range of 1100°C to 1300°C. In this way, a multilayer ceramic capacitor 200 is obtained having a laminated structure in which a dielectric layer 210 made of ceramic and an electrode layer 220 made of ceramic electrode material are alternately laminated. 【0049】 Following step S320, plating may be performed with copper (Cu), nickel (Ni), and tin (Sn) to form the external electrode 230. 【0050】 The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. [Examples] 【0051】 [Synthesis of barium titanate as a raw material] Barium titanate with a stoichiometric composition, which is the raw material, was prepared by a solid-phase reaction method. Specifically, barium carbonate (manufactured by Rare Metallic Co., Ltd., purity 99.99%) and titanium oxide (manufactured by Rare Metallic Co., Ltd., purity 99.99%) were mixed in a 1:1 ratio of barium to titanium, and the mixture was calcined multiple times. The calcined powder was then calcined in an oxygen stream at 1300°C for 48 hours to obtain the raw material powder. The obtained raw material powder was white, and its identity as barium titanate was confirmed using a desktop X-ray diffractometer (Rigaku Corporation, MiniFlex). 【0052】 [Examples 1-12] In Examples 1 to 12, ceramic electrode materials were manufactured by annealing barium titanate raw materials under the conditions shown in Table 1, using the experimental apparatus shown in Figure 4 and the method shown in Figure 1. Specifically, the prepared barium titanate raw materials having a stoichiometric composition were annealed in a vacuum in the presence of calcium or barium as a Group 2 metal element (step S110 in Figure 1). 【0053】 Figure 4 shows the experimental apparatus used in the manufacturing process. 【0054】 As shown in Figure 4, barium titanate raw material 420 and a group 2 element metal 430 were placed in a quartz tube 410. The barium titanate raw material 420 was compacted and placed in an alumina crucible 440. The alumina crucible 440 was placed on titanium foil 450, and the group 2 element metal 430 was placed inside the titanium foil 450 to prevent direct reaction between the barium titanate raw material 420 and the group 2 element metal 430 inside the alumina crucible 440. These contents were placed in an alumina crucible 460, and quartz wool 470 was placed at the bottom of the quartz tube 410 as a cushioning material to prevent damage to the quartz tube 410. 【0055】 Quartz tubes 410 were vacuum-sealed to the vacuum levels shown in Table 1, placed in an electric furnace, and annealed under the conditions shown in Table 1. In Examples 3 to 12, the samples were annealed two or more times to ensure uniformity. The annealed samples are referred to as the samples of Examples 1 to 12, respectively. 【0056】 Samples of Examples 1 to 12 were observed. Subsequently, electrodes were attached to both sides of the samples of Examples 1 to 12, and the temperature dependence of the electrical resistivity was measured by the four-terminal method using an electrical resistivity measuring device (manufactured by Custom Design Co., Ltd., PPMS). The temperature dependence of the lattice constant of the samples of Examples 1 to 12 was measured using a desktop X-ray diffractometer and a synchrotron radiation X-ray diffractometer (BL02B2 beamline of SPring-8). The composition analysis of the samples of Examples 1 to 12 was performed by an electron probe microanalyzer (EPMA) and thermogravimetric analysis (TGA). Using a titanium sapphire regenerative amplifier as an excitation light source (pulse width: 100 fs, repetition frequency: 1 kHz, wavelength: 1200 nm), the nonlinear optics of the samples of Examples 1 to 12 was measured using an optical parametric amplifier. These results are shown in FIGS. 5 to 9 and will be described later. All measurements were carried out under atmospheric pressure. 【0057】 [Example 13] In Example 13, annealing was performed in the same procedure as in Examples 1 to 12, except that a Group 2 element metal was not used. The sample thus obtained was referred to as the sample of Example 13, and the characteristics of the sample of Example 13 were evaluated and measured in the same manner as in Examples 1 to 12. 【0058】 [Table 1] 【0059】 [Table 2] 【0060】 The colors of the samples of Examples 1 to 4 and Examples 5 to 12 changed from white to dark blue by the annealing treatment. From this, it was found that the barium titanate raw material was oxygen-deficient by the annealing treatment. On the other hand, the color of the sample of Example 13 was gray, and the oxygen deficiency was not sufficient. According to the composition analysis, it was confirmed that all the samples of Examples 1 to 12 satisfied BaTiO 3-x (0 < x ≦ 0.05). 【0061】 Figure 5 shows the temperature dependence of the electrical resistivity of the samples in Example 4 and Example 5. 【0062】 The electrical resistivity of the samples in Examples 4 and 5 was 0.3 Ωcm or less in the temperature range below room temperature, and in the temperature range of 0.01 Ωcm to 1 Ωcm in the temperature range of 3 K to 700 K. In particular, as shown in Table 2, in the temperature range of 300 K to 400 K, the electrical resistivity satisfied the range of 0.01 Ωcm to 0.2 Ωcm. This indicates that oxygen-deficient barium titanate has an electrical resistivity equivalent to that of metal and functions as an electrode material. Although not shown in the figures, the electrical resistivity of the samples in Examples 1 to 3 and Examples 6 to 12 was similar to that of Examples 4 and 5. This indicates that metals of Group 2 elements function as getter materials that adsorb oxygen. 【0063】 Furthermore, even in the sample of Example 13, which did not use a Group 2 element metal during the annealing process, the titanium foil functioned as a getter material that adsorbed oxygen, resulting in a decrease in electrical resistivity, although it was not sufficient. 【0064】 Figure 6 shows the temperature dependence of the XRD pattern of the sample in Example 4. Figure 7 shows the lattice constant (a) and isotropic thermal vibration parameter (B) of the sample in Example 4. eq This is a diagram showing the temperature dependence of ). 【0065】 Figure 7(A) shows the temperature dependence of the lattice constant (a) of the barium titanate raw material, indicated by a dashed line. As reported in Non-Patent Literature 1, the barium titanate raw material exhibited a structural phase transition, showing rhombohedral (R), orthorhombic (O), tetragonal (T), and then cubic crystal structures as the temperature changed from low to high. Although not shown, the sample of Example 13, which did not use a Group 2 element metal during the annealing treatment, also exhibited a structural phase transition similar to that indicated by the dashed line. 【0066】 On the one hand, according to FIGS. 6, 7 and Table 2, the sample of Example 4 showed a cubic crystal structure in the temperature range of 3K to 700K, belonged to the space group of Pm3￣m, and had no structural phase transition. Also, the lattice constant a of the sample of Example 4 satisfied the range of 0.399 nm ≤ a ≤ 0.4015 nm in the temperature range of 3K to 700K. 【0067】 Although not shown, the samples of Examples 1 to 3 and the samples of Examples 5 to 12 also similarly had a cubic crystal structure and no structural phase transition at least in the range of 300K or more and 400K or less. Although there are reports on single crystals in Non-Patent Document 2, barium titanate composed of a ceramic that does not have a structural phase transition in the temperature range of 3K or more and 700K or less has not been reported so far. 【0068】 FIG. 7(B) shows a diagram showing the temperature dependence of the isotropic thermal vibration parameter (B eq ). According to FIG. 7(B), the isotropic thermal vibration parameter of each element changed monotonically with the increase in temperature, and no peculiar behavior or discontinuous points were confirmed. This result indicates that the sample of Example 4 maintained a stable cubic crystal structure at least in the temperature range of 3K or more and 700K or less. Furthermore, since the thermal vibrations of each element were appropriately controlled, it can be seen that the stability of the crystal structure is high. 【0069】 The barium titanate of the present invention has oxygen deficiency and low electrical resistivity, so it functions as a ceramic electrode material. Furthermore, since it maintains its crystal structure even at 300K to 400K in the temperature range where it is used as a capacitor, the electrodes will not be physically damaged. Therefore, if the barium titanate of the present invention is used for the internal electrodes of a multilayer ceramic capacitor, a multilayer ceramic capacitor made entirely of ceramics can be provided. 【0070】 FIG. 8 is a diagram showing the temperature dependence of the second harmonic oscillation of the sample of Example 4. 【0071】 As shown in Figure 8, the sample in Example 4 oscillated with the second harmonic from low temperature to room temperature, demonstrating an important indicator of ferroelectricity. Although not shown, the samples in Examples 1 to 3, and in Examples 5 to 12, similarly oscillated with the second harmonic. This fact may at first glance seem to contradict the structural analysis results for a cubic crystal. However, even when a sample exhibits a cubic crystal structure (Pm3-m), the presence of local or dynamic non-centrosymmetrical properties can cause the oscillation of the second harmonic, as shown in Cohen, RE, Nature, 1992, 358, 136-138. Therefore, the oscillation of the second harmonic is not inconsistent with the sample having a cubic crystal structure. 【0072】 The inventors of this application have succeeded in removing more oxygen from the barium titanate raw material by using a metal of a Group 2 element as a getter material. They believe that as a result of removing more than a certain amount of oxygen, they obtained barium titanate that does not undergo a structural phase transition and can exhibit ferroelectric properties. 【0073】 This demonstrates that the oxygen-deficient barium titanate of the present invention is a polar metal that possesses both low metallic electrical resistivity and ferroelectric properties. According to Non-Patent Literature 3, by using a polar metal as the electrode material, the dielectric layer of a multilayer ceramic capacitor can be made as thin as possible, for example, between 1 nm and 140 nm. As a result, the multilayer ceramic capacitor can be miniaturized. [Industrial applicability] 【0074】 As described above, the present invention is made of barium titanate that is oxygen-deficient and has a cubic crystal structure in a temperature range of at least 300K to 400K, and therefore has low electrical resistivity and functions as a ceramic electrode material. The ceramic electrode material of the present invention is applicable to multilayer ceramic capacitors, high-frequency devices, ceramic heaters, gas sensors, electrochemical devices, energy harvesting devices, and the like. [Explanation of Symbols] 【0075】 200 Multilayer Ceramic Capacitors 210 Dielectric layer 220 Internal electrode layer 410 Quartz tube 420 Barium titanate raw material 430 Metals of Group 2 elements 440 Alumina Crucible 450 Titanium Foil 460 Alumina Crucible 470 Quartz Wool

Claims

[Claim 1] A ceramic electrode material made of barium titanate, which has oxygen vacancies and a cubic crystal structure in a temperature range of at least 300K to 400K. [Claim 2] The ceramic electrode material according to claim 1, wherein the barium titanate has a cubic crystal structure in a temperature range of 3 K to 700 K. [Claim 3] The aforementioned barium titanate is BaTiO ３－ｘ A ceramic electrode material according to claim 1 or 2, satisfying (0 < x ≤ 0.1). [Claim 4] The barium titanate has an electrical resistivity of 0.01 Ωcm to 1 Ωcm in a temperature range of 3 K to 700 K, as described in any one of claims 1 to 3. [Claim 5] The barium titanate has polar metal properties in a temperature range of 3K to 700K, as described in any one of claims 1 to 4. [Claim 6] The ceramic electrode material according to any one of claims 1 to 5, wherein the lattice constant a of the barium titanate satisfies 0.398 nm < a < 0.402 nm in a temperature range of 3 K to 700 K. [Claim 7] A method for producing a ceramic electrode material made of barium titanate having an oxygen deficiency, A method comprising annealing a barium titanate raw material having a stoichiometric composition in a vacuum in the presence of a group 2 element metal. [Claim 8] The method according to claim 7, wherein the group 2 element is an element selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). [Claim 9] The method according to claim 7 or 8, wherein the annealing treatment is performed in a temperature range of 1050°C to 1500°C. [Claim 10] The method according to claim 9, wherein the annealing treatment is performed in a temperature range of 1150°C to 1250°C. [Claim 11] The method according to any one of claims 7 to 10, wherein the mass ratio of the group 2 element metal to the barium titanate raw material is in the range of 0.01 or more and 0.5 or less. [Claim 12] The method according to claim 11, wherein the mass ratio of the Group 2 element metal to the barium titanate raw material is in the range of 0.02 or more and 0.1 or less. [Claim 13] The aforementioned annealing treatment is 1 × 10 －４ The method according to any one of claims 7 to 12, wherein the procedure is carried out at a vacuum level of Pa or more and 1 Pa or less. [Claim 14] The method according to any one of claims 7 to 13, wherein the annealing treatment is performed for a period of 30 minutes or more and 24 hours or less. [Claim 15] The method according to any one of claims 7 to 14, further comprising repeating the annealing process. [Claim 16] A multilayer ceramic capacitor comprising a laminated structure in which dielectric layers made of ceramic and electrode layers made of the ceramic electrode material described in any one of claims 1 to 6 are alternately stacked. [Claim 17] The multilayer ceramic capacitor according to claim 16, wherein the dielectric layer is selected from the group consisting of barium titanate, calcium zirconate, calcium titanate, lithium niobate, potassium niobate, lanthanum-doped strontium titanate, and barium zirconate. [Claim 18] The dielectric layer has a thickness in the range of 1 nm to 1 mm. The multilayer ceramic capacitor according to claim 16 or 17, wherein the electrode layer has a thickness in the range of 1 nm to 1 mm. [Claim 19] Forming a laminate by laminating a dielectric sheet containing a dielectric and an internal electrode sheet containing a ceramic electrode material according to any one of claims 1 to 6, The laminate is sintered A method for manufacturing a multilayer ceramic capacitor according to any one of claims 16 to 18, encompassing the above.

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