JP2020138875A - Method of producing ferroelectric ceramic - Google Patents

Abstract

To produce ferroelectric ceramic made by adding a shell material to a core material and calcining the same, the core material being a tungsten bronze type oxide, Sr2NaNb5O15, the shell material being Ta2O5, and hence having stable high-temperature range characteristics capable of maintaining the fluctuation of its relative dielectric constant within a range of ±15% under a high temperature condition of little higher than 273°C.SOLUTION: The method of producing ferroelectric ceramic G of a core-shell structure made by adding a shell material M to a core material S and calcining the same, in which the core material is a tungsten-bronze type oxide, the tungsten-bronze type oxide is Sr2NaNb5O15, and the shell material is Ta2O5.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method for producing a ferroelectric ceramic used for a capacitor, which is assumed to be used in a harsh environment such as an in-vehicle use or an aerospace use.

Conventionally, as this type of strong dielectric ceramic, as shown in FIGS. 7 and 8, in a core shell structure strong dielectric ceramic in which a shell material is added to a core material and fired, BaTIO ₃ (barium titanate) is used as the core material. ), For example, ZrO ₂ (zirconium dioxide) is used as the shell material, and those produced by adding a small amount of ZrO ₂ to this BaTiO ₃ powder and firing are known (Non-Patent Document 1). ..

That is, by adding a small amount of the above ZrO ₂ to the BaTiO ₃ powder and firing it, the zirconium component diffuses from the periphery of the BaTiO ₃ , and the phase given by the BaTi _1-x Zr _x O ₃ is formed around the BaTiO ₃ particles. Then, as shown in FIG. 7, a ferroelectric ceramic G having a core-shell structure having BaTiO ₃ particles as a core and BaTi _1-x Zr _x O ₃ as a shell is produced.

Here, the chemical composition of the ferroelectric ceramic has a higher variable x in BaTi _1-x Zr _x O ₃ as it approaches the surface, that is, a higher Zr component concentration and a pure BaTIO ₃ as it approaches the center. It will be in a close state.

On the other hand, a strong dielectric ceramic having a core-shell structure having a BaTiO ₃ particle as a core and a BaTi _1-x Zr _x O ₃ as a shell is also known as a relaxer dielectric, and such a relaxer dielectric has. , together show a having a broad peak dielectric constant-temperature characteristics, has characteristics that change continuously peak position in accordance with changes in the chemical composition BaTi _1-x Zr _x O ₃ in variable x, relative dielectric constant Since the maximum temperature in the temperature characteristics is determined by the curry temperature of the core material, the relative permittivity fluctuation is 25 ° C. in the temperature range from a low temperature of about -55 ° C to about 125 ° C, which is slightly higher than the curry temperature of the core material. It is known that stable high-temperature characteristics that can be maintained within ± 15% based on the permittivity can be obtained, and the added oxide is not limited to ZrO ₂ , but various oxides and their combination can be used as described above. Relative permittivity temperature characteristics have been established as the EIA X7R standard and have been commercialized.

After that, with the aim of further expanding the stable high-temperature region characteristics, as shown in FIGS. 7 and 8, a ferroelectric ceramic having a core-shell structure formed by adding a shell material to the core material and firing it is described in Patent Document 1. In addition, we invented a ferroelectric ceramic composed of KSr ₂ Nb ₅ O ₁₅ (potassium niobate), which has a Curie temperature higher than that of BaTIO _3, as the core material, and ZrO ₂ (zirconium dioxide) as the shell material.

That is, by adding a small amount of ZrO ₂ to the KSr ₂ Nb ₅ O ₁₅ powder and firing, the zirconium component diffuses from the periphery of KSr ₂ Nb ₅ O ₁₅ and K _1-x Sr _{2 + x} Nb _5-x Zr. given phase _x O ₁₅ is formed around the _KSr 2 _Nb 5 _{O 15} particles, as shown in FIG. _7, the core KSr ₂ Nb _{5 O 15 particles, K 1-x Sr 2 +} x Nb 5-x Zr x O A strong dielectric ceramics G having a core-shell structure having ₁₅ as a shell will be produced.

Here, in the chemical composition of the ferroelectric ceramic G, the closer to the surface, the higher the variable x in K _1-X Sr _{2 + X} Nb _5-X Zr _x O ₁₅ , that is, the higher the Zr component concentration, and the center. The closer it is, the closer it becomes to pure KSr ₂ Nb ₅ O ₁₅ .

In the ferroelectric ceramics G having a core-shell structure having KSr ₂ Nb ₅ O ₁₅ particles as a core and K _1-X Sr _{2 + X} Nb _5-X Zr _x O ₁₅ as a shell, it is also known as a relaxer dielectric. Such a relaxor dielectric exhibits a relative permittivity temperature characteristic having a wide peak, and peaks according to a change in the variable x in the chemical composition K _1-X Sr _{2 + X} Nb _5-X Zr _x O _15. The position also has the characteristic of continuously changing, and the maximum temperature in the relative permittivity temperature characteristics is determined by the Curie temperature of the core material of 156 ° C. Therefore, the temperature is about -55 ° C, which is slightly higher than the Curie temperature of the core material. A flat temperature characteristic has been obtained in the temperature range up to 195 ° C.

In addition, the core-shell structure has typically meant a non-uniform structure in composition such that one core is wrapped in a shell in a single particle, but in recent years, other than such a structure has been used. For example, a structure in which one shell contains a plurality of cores in a single particle, a structure in which a multi-layer shell wraps one core like an onion, or a rod-shaped or wire-shaped core instead of granular shells. The existence of various core-shell structures such as the one-dimensional structure surrounded by is summarized in Non-Patent Document 2, and the core-shell structure dealt with in the text also includes ceramics having nano-sized composition inhomogeneity of such components. It shall include.

Apart from the above, the relative permittivity temperature characteristics of the tungsten bronze ceramics of Sr ₂ NaNb _5-x Ta _x O ₁₅ having a composition in which a part of Nb in Sr ₂ NaNb ₅ O ₁₅ was replaced with Ta were examined, and the details are not described. It is shown in Patent Document 3.

The raw material oxides SrCO ₃ , Na ₂ CO ₃ , Nb ₂ O ₅ and Ta ₂ O ₅ were weighed to a chemical quantitative ratio, sufficiently mixed using a planetary ball mill, and calcined at 1,150 ° C. for 7 hours. After confirming by X-ray diffraction that the obtained calcined product had no different phases other than the same window in the tungsten bronze single phase, the calcined product was again pulverized using a planetary ball mill. The obtained powder was pressure-molded into a disk shape, put into an electric furnace in an oxygen atmosphere, and calcined at a temperature of 1,250 ° C. to 1,350 ° C. for 8 hours to prepare a ceramic sample.

By X-ray diffraction, the ceramic is a tungsten bronze single phase with good crystallinity, and its lattice constant changes almost linearly depending on the value of x of Sr ₂ NaNb _5-x Ta _x O ₁₅ tungsten bronze. Since it was confirmed that the value of x was determined in a first place, it was shown that the obtained ceramics were uniform ceramics having no nano-sized compositional inhomogeneity such as core-shell structure.

Gold electrodes were formed on both sides of the ceramics by sputtering, connected to an LCR meter, and the relative permittivity at each frequency from 1 kHz to 1 MHz was evaluated in the temperature range of up to 400 ° C from the boiling point of liquid nitrogen. As described in Non-Patent Document 3, a wide relative permittivity peak that appears in a temperature range of approximately -100 ° C to 0 ° C and a wide relative permittivity that also appears in a temperature range of approximately -150 ° C to 300 ° C. A generally flat relative permittivity temperature characteristic consisting of two peaks of rate peak was obtained.

The peak position of the relative permittivity peak has a characteristic of continuously transitioning to the low temperature side as the value of x of the chemical composition Sr ₂ NaNb _5-x Ta _x O ₁₅ increases. For example, the high temperature at the composition x = 0. The highest point of the peak position appearing on the side is at 273 ° C, which is considered to correspond to the Curie temperature of Sr ₂ NaNb ₅ O ₁₅ , and at the composition x = 0.2, the peak position appearing on the low temperature side is approximately −150 ° C. Although it covers a wide temperature range such as some, the relative permittivity fluctuation is still large for each composition ceramic, and the relative permittivity is realized in a temperature range wider than -55 ° C to 195 ° C realized by KSr ₂ Nb ₅ O _15. The rate fluctuation could not be suppressed within ± 15% on the basis of 25 ° C.

Patent No. 6267673

T. Armstrong, et al. "Dielectric Properties of Luxed Barium Titanate Ceramics with Zirconia Additions" Journal of the American Ceramic Society Vol. 73 pp. 700-706 (1990) Matias Acosta, et al. , "Core-Shell Lead-Free Piezoelectric Ceramics: Currant Status and Advanced characterization of the Bi1 / 2Na1 / 2TIO3-SrTIO3 Ceramics" Journal 98 pp. 3405-3422 (2015) Hiroshi Iwai, Sakura Miyajima, Sasuke Ishimaru: "Relative Permittivity Temperature Characteristics of Sr2NaNb5-xTaxO15 Ceramics" 64th JSAP Spring Meeting (2017) 15p-P4-6 Lecture Proposal and Poster F. Lin, et al. "Microstructure Refinement of Sintered Alumina by a Two-Step Sintering Technology" Journal of the American Ceramic Society, Vol. 80 pp. 2269-2277 (1997)

In the case of conventional ferroelectric ceramics produced by adding a small amount of ZrO ₂ to the above KSr ₂ Nb ₅ O ₁₅ powder and firing, the temperature is 195 ° C. or higher as shown in the relative permittivity temperature characteristics shown in FIG. Then, the relative permittivity is remarkably lowered, and such a conventional ferroelectric ceramic is used as a capacitor material for automobiles and aerospace at a temperature wider than the temperature range of -55 ° C to 195 ° C. It has the inconvenience that it may not be possible to meet stricter requirements, which require characteristics in the high temperature range where the fluctuation of the relative permittivity is within ± 15% in the range.

An object of the present invention is to solve such inconvenience, and the invention according to claim 1 is a method for producing a ferroelectric ceramic having a core-shell structure, which is obtained by adding a shell material to a core material and firing it. The core material is a tungsten bronze oxide, the tungsten bronze oxide is Sr ₂ NaNb ₅ O ₁₅ (strontium sodium niobate), and the shell material is Ta ₂ O ₅ (ditantal pentoxide). and characterized in that, and KSr ₂ Nb ₅ O ₁₅ core-shell structure ceramic with the addition of ZrO _2, non one example in Patent Document 1 is shown already in practical use has been ZrO ₂ Others were added barium titanate core shell Similar to the expression of the relative permittivity temperature characteristics of structural ceramics, the relative permittivity temperature characteristics that change depending on the composition x of Sr ₂ NaNb _5-x Ta _x O ₁₅ composition x are summed up in the ceramics in principle. In a wide temperature range from -150 ° C to slightly above 273 ° C, which is considered to correspond to the curry temperature of Sr ₂ NaNb ₅ O ₁₅ , for example, a flat ratio that can suppress the relative permittivity fluctuation within ± 15% based on 25 ° C. This is a method for producing ferroelectric ceramics that can have a permittivity temperature characteristic.

As described above, in the invention according to claim 1, the present invention is a method for producing a ferroelectric ceramic having a core-shell structure in which a shell material is added to a core material and fired, wherein the shell material M is an ion. By solidifying in the core material S by diffusion, a ferroelectric ceramic having a core shell structure in which the core material is a tungsten bronze type oxide can be obtained at low cost, and the core material is a tungsten bronze type oxide Sr ₂ NaNb. _Since it is ₅ O ₁₅ and the shell material is Ta ₂ O ₅ , its function is even under temperature conditions slightly higher than 273 ° C, which is considered to correspond to the Curie temperature of Sr ₂ NaNb ₅ O ₁₅ from −150 ° C. Ferroelectric ceramics with stable high-temperature characteristics that can maintain fluctuations in the relative permittivity within ± 15% can be obtained, and are used as capacitor materials for automobiles and aerospace where high-temperature characteristics are required. Can be used.

It is a manufacturing process drawing of the 1st Embodiment of this invention. It is explanatory forward sectional drawing of the 1st Embodiment example of embodiment of this invention. It is a scanning electron microscope image of the first embodiment of the present invention. It is a relative permittivity temperature characteristic figure of the 1st Embodiment of this invention. It is a scanning electron microscope image of the second embodiment of the present invention. It is a relative permittivity temperature characteristic figure of the 2nd Embodiment of this invention. It is explanatory drawing of the embodiment of the conventional product. It is a relative permittivity temperature characteristic figure of the embodiment of the conventional product.

1 to 6 show examples of embodiments of the present invention, FIGS. 1 to 4 are examples of the first embodiment, and FIGS. 5 and 6 are examples of the second embodiment.

In the first embodiment of FIGS. 1 to 4, a ferroelectric ceramic G having a core-shell structure is produced by adding a small amount of a shell material M to a core material S made of a tungsten bronze oxide and firing it.

In this case, Sr ₂ NaNb ₅ O ₁₅ (strontium sodium niobate) is used as the tungsten bronze type oxide, and Ta ₂ O ₅ (ditantalum pentoxide) is used as the shell material M.

In this case, when producing the ferroelectric ceramics G, as shown in FIG. 1, first, the raw material oxides Na ₂ CO ₃ , SrCO ₃ , and Nb ₂ O ₅ are weighed as raw materials for the tungsten bronze type oxide by a chemical quantitative ratio. Then, these raw materials are mixed for 24 hours using a planetary ball mill and calcined at 1,150 ° C. for 8 hours to prepare a tungsten bronze type oxide as the core material S.

Then, it will be confirmed by the X-ray diffraction method that the calcined powder of the produced tungsten bronze type oxide is Sr ₂ NaNb ₅ O ₁₅ single phase.

Next, Ta ₂ O ₅ as the shell material M was directly added to the calcined powder of the tungsten bronze type oxide as the core material S, and the mixture was sufficiently stirred using, for example, a planetary ball mill, and the obtained mixture was obtained. The powder is pressurized at a pressure of about 196 MPa to form pellets, and the pellets are put into a heating furnace and held at a temperature of 1,300 ° C. for about 1 to 3 minutes in an oxygen atmosphere in the heating furnace. , Cooled to 200 ° C. and fired at the same temperature for 8 hours by a two-step firing method, which causes ion diffusion from Ta ₂ O ₅ , and as shown in FIG. 2, the tungsten bronze type oxide is used as the core. Then, a tungsten ceramic G having a core-shell structure having Sr ₂ NaNb _5-x Ta _x O ₁₅ as a shell will be produced.

As described in Non-Patent Document 4, for example, the two-step firing method can produce dense ceramics even under low firing temperature conditions as compared with the normal firing method, and at the same time, grains produced in the final stage of sintering. It is also known to be able to curb growth.

When the grain growth at the final stage of sintering is remarkable, as shown in Non-Patent Document 1, as a result of promoting the diffusion of components between the core and the shell due to the grain growth, the core-shell structure is lost and uniform chemistry. The ceramic has a composition that loses the relative permittivity temperature characteristics peculiar to the core-shell structure, and in the case of the ceramic, for example, Sr ₂ NaNb _5-x Ta _x O ₁₅ composition, instead of that of the core-shell structure, Sr ₂ NaNb _5-x Ta _x O ₁₅ Relative permittivity temperature characteristics of uniform composition are exhibited, and as described above, the relative permittivity fluctuation is ± 15% based on 25 ° C. in a temperature range wider than -55 ° C. to 195 ° C. realized by KSr ₂ Nb ₅ O _15. Although it cannot be suppressed to the inside, this problem can be easily solved by performing the two-step firing as compared with the normal one-step firing.

The surface of the ferroelectric ceramics G thus produced was a scanning electron microscope image shown in FIG.

Gold electrodes are formed on both sides of the ceramics by sputtering, and the relative permittivity at a frequency of 1 kHz is fixed in a sample table with a heating device in the temperature range from the liquid nitrogen temperature to a maximum of 250 ° C. using a liquid nitrogen meter. The relative permittivity was measured while cooling, and the relative permittivity was measured by fixing the sample on a sample fixing table installed in a heating furnace in the atmosphere in the temperature range from room temperature to a maximum of 450 ° C.

As a result, the horizontal axis shown in FIG. 4 is the temperature (° C.), the vertical axis is the relative permittivity, and the relative permittivity shows a maximum of about 1,578 as shown in the relative permittivity temperature characteristic diagram shown under the condition of a frequency of 1 kHz. The relative permittivity becomes about 1,531 at a temperature of 25 ° C, and after showing a minimum of 1,316 at a temperature of 150 ° C, the value of the relative permittivity stabilizes up to a temperature of about 200 ° C, and from the time when the temperature exceeds 230 ° C. The relative permittivity showed a slight increase again and then showed a downward trend.

Below the freezing point, the relative permittivity increases with increasing temperature, reaching 1,301 at about -70 ° C, which corresponds to a 15% decrease in the relative permittivity of 1,531 at a temperature of 25 ° C, and at a temperature of 252 ° C, the temperature is also 25 ° C. After showing 1,301, which corresponds to a 15% decrease in the relative permittivity of 1,531, the relative permittivity decreases as the temperature rises, while conversely, the relative permittivity of 1,531 at a temperature of 25 ° C. increases by 15%. The corresponding 1,760 did not reach the entire temperature range of -70 ° C to 252 ° C, and as a result, this dielectric has a relative permittivity reference with a relative permittivity variation of + 25 ° C over a wide temperature range of -70 ° C to 252 ° C. It was confirmed that it was within ± 15%.

In this first embodiment, as shown in FIGS. 1 and 2, a method for producing a ferroelectric ceramic G having a core-shell structure, which is obtained by adding a shell material M to a core material S and firing in two steps, in which a shell is used. By solidifying the material M into the core material S by diffusion of ions, a ferroelectric ceramic G having a core shell structure in which the core material S is a tungsten bronze type oxide can be obtained at low cost, and the core material S can be obtained at low cost. Is a tungsten bronze type oxide Sr ₂ NaNb ₅ O ₁₅ , and the shell material M is Ta ₂ O _5. Therefore, the core shell in which the component concentration of the shell material M increases from the central portion to the surface portion of the core material S. As a result of producing the ferroelectric ceramics G having a structure, the tungsten bronze type oxide is Sr ₂ NaNb ₅ O ₁₅ as its function. Therefore, in principle, even in a wide temperature range slightly higher than −150 ° C. to 273 ° C. It is possible to obtain a ferroelectric ceramic G having stable high temperature region characteristics in which the fluctuation of the relative permittivity can be maintained within ± 15%.

In the actually obtained first embodiment, even under the temperature condition from −70 ° C. to 254 ° C., the strength having a stable high temperature region characteristic that the fluctuation with respect to the relative permittivity of 25 ° C. can be maintained within ± 15%. Dielectric ceramics G can be obtained, and as a result, the relative permittivity fluctuates even in a temperature range wider than the temperature range from -55 ° C to 195 ° C realized by the ZrO ₂ added KSr ₂ Nb ₅ O ₁₅ core shell structure. It is possible to obtain a ferroelectric ceramic G having stable high temperature region characteristics that can be maintained within ± 15%, and it can be used as a capacitor material for automobiles and aerospace where high temperature region characteristics are required.

5 and 6 are examples of the second embodiment, and similarly to the first embodiment, as shown in FIGS. 1 and 2, the tungsten bronze type oxide as the core material S is Sr ₂ NaNb ₅ O. _15. The shell material M is Ta ₂ O ₅ , and the shell material M is added to the core material S, put into a heating furnace at 1,250 ° C., and fired in an oxygen atmosphere in the heating furnace for 8 hours. As shown in FIG. 2, the ferroelectric ceramics G having a core-shell structure in which the component concentration of the shell material M increases from the center to the surface of the core material S made of tungsten bronze oxide by the usual one-step firing method. It will be produced.

In the ferroelectric ceramics G thus prepared, the surface thereof was a scanning electron microscope image shown in FIG.

Gold electrodes are formed on both sides of the ceramics by sputtering, and the relative permittivity at a frequency of 1 kHz is fixed in a sample table with a heating device in the temperature range from the liquid nitrogen temperature to a maximum of 250 ° C. using a liquid nitrogen meter. The relative permittivity was measured while cooling, and the relative permittivity was measured by fixing the sample on a sample fixing table installed in a heating furnace in the atmosphere in the temperature range from room temperature to a maximum of 450 ° C.

As a result, the horizontal axis shown in FIG. 6 is the temperature (° C.), the vertical axis is the relative permittivity, and the relative permittivity shows a maximum of about 1,249 as shown in the relative permittivity temperature characteristic diagram shown under the condition of a frequency of 1 kHz. The relative permittivity becomes 1,216 at a temperature of 25 ° C., and after showing a minimum of 1,041 at a temperature of 130 ° C., the value of the relative permittivity does not change stably until the temperature is about 200 ° C., and exceeds the temperature of 220 ° C. From around that time, the relative permittivity rose a little and then showed a downward trend.

Below the freezing point, the relative permittivity rises as the temperature rises, reaching about 1,034, which corresponds to a 15% decrease in the relative permittivity of 1,216 at a temperature of 25 ° C at about -65 ° C, and the same temperature 25 at a temperature of 263 ° C. After showing 1,034, which corresponds to a 15% decrease in the relative permittivity of 1,216 at ° C, the relative permittivity decreases as the temperature rises, while conversely, 15% of the relative permittivity 1,216 at a temperature of 25 ° C. The increase of 1,398 did not reach over the entire temperature range of -65 ° C to 263 ° C, resulting in a relative permittivity variation of +25 in this strong dielectric ceramics G over a wide temperature range of -65 ° C to 263 ° C. It was confirmed that it was within ± 15% based on the relative permittivity at ° C.

In the relative permittivity temperature characteristic diagram of FIG. 6, a discontinuous change in the relative permittivity is observed at about 50 ° C., which means that the sample is fixed on a sample table with a heating device from the liquid nitrogen temperature to 50 ° C. Relative permittivity was measured while cooling with liquid nitrogen, whereas in the temperature range of 50 ° C or higher, the sample was fixed on a sample fixation table installed in a heating furnace and the relative permittivity was measured. It was caused as an error due to a change in the suspended permittivity, but without this error, the fluctuation in the relative permittivity would be rather small, but it would not affect the overall conclusion.

The second embodiment is a method for producing a ferroelectric ceramic G having a core-shell structure, which is obtained by adding a shell material M to the core material S and firing it in one step, as in the first embodiment. Since the core material S is a tungsten bronze type oxide Sr ₂ NaNb ₅ O ₁₅ and the shell material M is Ta ₂ O ₅ , the shell material M is dissolved in the core material S by the diffusion of ions. Ferroelectric ceramics G having a core-shell structure in which the core material S is a tungsten bronze type oxide can be obtained at low cost, and the tungsten bronze type oxide is Sr ₂ NaNb ₅ O ₁₅ as its function. It is possible to obtain a ferroelectric ceramic G having a stable high temperature range characteristic in which the fluctuation of the relative permittivity can be maintained within ± 15% even in a wide temperature range slightly over −150 ° C. to 273 ° C.

As shown in FIG. 6, even in the actually obtained second embodiment, the fluctuation with respect to the relative permittivity of 25 ° C. can be maintained within ± 15% under the temperature conditions from −65 ° C. to 263 ° C. It is possible to obtain a ferroelectric ceramic G having high temperature region characteristics, and it can be used as a capacitor material for automobiles and aerospace where high temperature region characteristics are required. ZrO ₂ addition, which is a conventional example of FIG. It has stable high temperature characteristics that can maintain the fluctuation of the relative permittivity within ± 15% even in a temperature range wider than the temperature range from -55 ° C to 195 ° C realized by the KSr ₂ Nb ₅ O ₁₅ core shell structure. Ferroelectric ceramics G can be obtained, and can be used as a capacitor material for automobiles and aerospace where high temperature region characteristics are required.

The one-step firing method as the heat treatment form adopted in the second embodiment has fewer firing parameters and is easier to operate than the two-step firing method, but the continuous firing temperature is relatively high. Although it has a drawback that grain growth is likely to occur and the core-shell structure is likely to be destroyed, a sufficiently stable relative permittivity temperature characteristic can be obtained as shown in this example by appropriately selecting the firing parameters.

Incidentally, upon the addition of of Ta ₂ O ₅ which has a as the shell material M into a calcined powder of a tungsten bronze-type oxide as the core material S, both in the first embodiment and the second embodiment of the above-described embodiments Ta ₂ Although by the solid phase reaction method from O ₅ powder addition, described below liquid phase method, namely by dissolving tantalum penta propoxide, a soluble compound of other tantalum solvent such as, for example, ethanol or isopropyl alcohol, the solution A calcined powder of Sr ₂ NaNb ₅ O ₁₅ as a tungsten bronze type oxide is put therein, and after applying the above tantalum compound to the surface of the powder particles, this is applied in the air at 600 ° C. or higher for about 10 to 30 minutes. A layer of Ta ₂ O ₅ as a shell material M may be formed on the surface of the particles by heating to.

This powder is pressurized at a pressure of about 196 MPa to form pellets, and the pellets are put into a heating furnace having a furnace temperature of 1,200 ° C to 1,300 ° C as the firing temperature, and in an oxygen atmosphere in the heating furnace 7 It is kept for a long time or fired according to other predetermined heat treatment conditions, which causes diffusion of ions from Ta ₂ O ₅ , and as shown in FIG. 2, the above tungsten bronze type oxide is used as a core, and Sr ₂ NaNb _5-X Ta. A tungsten ceramic G having a core-shell structure having _x O ₁₅ as a shell can be obtained.

The present invention is not limited to the above embodiment, and is designed by appropriately changing the manufacturing conditions of the ferroelectric ceramic G depending on the contents of the core material S and the shell material M.

As described above, the intended purpose can be sufficiently achieved.

S core material G ferroelectric ceramics M shell material

Claims (1)

A method for producing a ferroelectric ceramic having a core-shell structure obtained by adding a shell material to a core material and firing the core material. The core material is a tungsten bronze oxide, and the tungsten bronze oxide is Sr ₂ NaNb ₅ O. _{15. A} method for producing a ferroelectric ceramic, wherein the shell material is Ta ₂ O ₅ .