JP2012064840A - Method of manufacturing semiconductor ceramic, semiconductor ceramic, and positive thermistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a semiconductor ceramic, a semiconductor ceramic, and a PTC thermistor, capable of avoiding increase in resistance, for obtaining a semiconductor ceramic with excellent reliability.SOLUTION: Ba compound, Ti compound, rare earth compound, and further Ca compound and/or Sr compound are mixed together and thermally treated to prepare first thermal treatment powder comprising BaTiObased composition. Further, alkaline metal compound containing alkaline metal M (for example, Na, Li), Bi compound, Ba compound, and Ti compound are mixed together and thermally treated to prepare second thermally treated powder represented by a general formula (M,Bi)BaTiO(where, 0.025≤x≤0.15). The first thermally treated powder and the second thermally treated powder are mixed together and molded to produce a molding, which is then sintered to provide a semiconductor ceramic. A component element assembly 1 is formed from the semiconductor ceramic.

Description

  The present invention relates to a method of manufacturing a semiconductor ceramic, a semiconductor ceramic, and a positive temperature coefficient thermistor, and more particularly, a method of manufacturing a semiconductor ceramic having a positive temperature coefficient of resistance (hereinafter referred to as “PTC”), and a method of manufacturing the same. The present invention relates to a semiconductor ceramic produced by using the semiconductor ceramic and a positive temperature coefficient thermistor (hereinafter referred to as “PTC thermistor”) suitable for a heater application using the semiconductor ceramic.

The BaTiO ₃ (barium titanate) -based semiconductor ceramic has PTC characteristics that generate heat when a voltage is applied and the resistance value increases rapidly when the Curie point Tc at which phase transition from tetragonal to cubic is exceeded.

  As described above, the semiconductor ceramic having PTC characteristics has a resistance value that increases when the Curie point Tc is exceeded due to heat generation by voltage application, and current hardly flows, and the temperature decreases. And if temperature falls and resistance value becomes small, an electric current will flow easily again and temperature will rise. Semiconductor ceramics are widely used as a thermistor for a heater or a thermistor for starting a motor because it converges to a constant temperature or current by repeating the above-described process.

By the way, since the PTC thermistor used for a heater use is used at high temperature, it needs to have a high Curie point Tc. For this reason, conventionally, a part of Ba in BaTiO ₃ has been replaced with Pb to raise the Curie point Tc.

  However, since Pb is an environmentally hazardous substance, it is required to realize a lead-free semiconductor ceramic that does not substantially contain Pb in consideration of the environment.

And, a part of Ba in BaTiO ₃ is substituted with an alkali metal and Bi, and thereby, lead-free semiconductor ceramics that have attempted to obtain good PTC characteristics have attracted attention.

  This type of semiconductor ceramic can be manufactured by simultaneously preparing and calcining required ceramic raw materials, followed by forming and firing.

However, since alkali metals and Bi have volatility, if calcining is performed simultaneously at such a high temperature that a BaTiO ₃ -based composition is synthesized, the alkali metals and Bi are volatilized, resulting in a composition shift and the like. There is a risk of degrading the PTC characteristics.

Therefore, in Patent Documents 1 and 2, a step of preparing BaTiO ₃ calcined powder, a step of preparing (Bi, Na) TiO ₃ calcined powder, BaTiO ₃ calcined powder and (Bi, Na) TiO ₃ calcined powder. Have been proposed, and a method for producing a semiconductor ceramic composition comprising a step of forming and sintering a mixed calcined powder.

In Patent Documents 1 and 2, the BaTiO ₃ composition is prepared by calcining at a high temperature, while the (Bi, Na) TiO ₃ composition is calcined at a low temperature so that Na and Bi do not volatilize. By mixing and molding both, volatilization of volatile Na and Bi is suppressed, thereby preventing Bi-Na composition shift and suppressing generation of a heterogeneous phase containing Na, and resistance to room temperature. Promoting the decline.

WO2008 / 050875 WO2008 / 050876

However, in Patent Documents 1 and 2, although BaTiO ₃ calcined powder and (Bi, Na) TiO ₃ calcined powder are mixed and baked, BaTiO ₃ and (Bi, Na) TiO ₃ are lattice constants. Therefore, there is a problem that BaTiO ₃ and (Bi, Na) TiO ₃ cannot be smoothly dissolved in the firing process. That is, the ionic radius of Ba ions is larger than the ionic radius of Na ions and Bi ions, and therefore the lattice constant of BaTiO ₃ is also larger than the lattice constant of (Bi, Na) TiO ₃ , and both of them are smooth in the firing process. This makes it difficult to form a solid solution, which may lead to an increase in resistance and a decrease in reliability.

  The present invention has been made in view of such circumstances, and a semiconductor ceramic manufacturing method capable of avoiding a high resistance and obtaining a highly reliable semiconductor ceramic, and using this manufacturing method An object of the present invention is to provide a manufactured semiconductor ceramic and a PTC thermistor using the semiconductor ceramic.

In order to achieve the above object, the present inventors conducted extensive research. As a result, when a (Na, Bi) TiO ₃ -based composition was produced, a part of (Na, Bi) was 0.025 to 0.15. By substituting Ba in this range and increasing the lattice constant of the crystal lattice in advance, solid solution with the BaTiO ₃ -based composition can be carried out smoothly, thereby avoiding high resistance of the semiconductor ceramic. It was possible to improve the reliability. This principle is considered to be the same for an alkali metal having an ionic radius smaller than that of Na, for example, Li.

The present invention has been made on the basis of such knowledge, and a method for producing a semiconductor ceramic according to the present invention includes a semiconductor ceramic in which a BaTiO ₃ compound is a main component and a part of Ba is substituted with an alkali metal and Bi. A first heat-treated powder preparation step of preparing a first heat-treated powder comprising a BaTiO ₃ -based composition, and at least a Ba compound and a Ti compound, and an alkali metal M An alkali metal compound, a Bi compound, a Ba compound, and a Ti compound are mixed and heat-treated to obtain a general formula (M, Bi) _1-x Ba _x TiO ₃ (where x is 0.025 ≦ x ≦ 0.15). A molded body for producing a molded body by mixing and molding a second heat treated powder production step for producing the second heat treated powder represented, and the first heat treated powder and the second heat treated powder. And manufacturing process is characterized by including a firing step of firing the shaped body.

  In the method for producing a semiconductor ceramic according to the present invention, x is preferably 0.025 to 0.075.

  In the method for producing a semiconductor ceramic of the present invention, it is also preferable to add at least one of a Ca compound and an Sr compound in the first heat treatment powder production step.

  In the method for producing a semiconductor ceramic of the present invention, it is preferable to add a rare earth element in the first heat treatment powder production step.

  Moreover, it is preferable that the manufacturing method of the semiconductor ceramic of this invention contains at least any one of Na and Li.

  The semiconductor ceramic according to the present invention is manufactured by any one of the methods described above.

  The PTC thermistor according to the present invention is a PTC thermistor in which external electrodes are formed at both ends of a component body, wherein the component body is formed of the semiconductor ceramic.

According to the method for producing a semiconductor ceramic of the present invention, a Ba compound and a Ti compound (further, a Ca compound, a Sr compound, a rare earth compound, etc.) are mixed and heat-treated, and the first heat-treated powder composed of a BaTiO ₃ -based composition is obtained. A first heat treatment powder production step to be produced and an alkali metal compound containing an alkali metal M (for example, Na, Li), a Bi compound, a Ba compound, and a Ti compound are mixed and heat-treated, and the general formula (M, Bi) _A second heat-treated powder preparation step for producing a second heat-treated powder represented by _1-x Ba _x TiO ₃ (where x is 0.025 ≦ x ≦ 0.15); Since the second heat-treated powder is mixed and molded to form a molded body for producing a molded body and a firing process for firing the molded body, the lattice constant of the crystal lattice of the second heat-treated powder is determined. Increase the size in advance Is possible. Therefore, the difference in the lattice constant of the crystal lattice from the first heat treatment powder can be reduced, and the first heat treatment powder and the second heat treatment powder can be smoothly dissolved in the firing step. . As a result, it is possible to manufacture a semiconductor ceramic that can avoid an increase in resistance and has improved reliability.

  Further, according to the semiconductor ceramic of the present invention, since it is manufactured by any of the above-described methods, a semiconductor ceramic having a low resistance and good reliability can be obtained.

  Furthermore, according to the PTC thermistor of the present invention, since the component body is made of the above-mentioned semiconductor ceramic, it is possible to obtain a PTC thermistor suitable for heater use such as an on-vehicle heater having low resistance and good reliability. it can.

1 is a perspective view showing an embodiment of a PTC thermistor according to the present invention. It is a figure which shows the relationship between substitution molar ratio x of Ba, specific resistance (rho) _0, and resistance change rate (DELTA) (rho) / (rho) ₀ .

  Next, an embodiment of the present invention will be described in detail.

The method for producing a semiconductor ceramic according to the present invention includes a first heat treatment powder preparation step of mixing a Ba compound and a Ti compound and heat-treating to produce a first heat treatment powder made of a BaTiO ₃ -based composition, and an alkali metal M. An alkali metal compound containing Bi, Bi compound, Ba compound, and Ti compound are mixed and heat-treated to obtain a general formula (M, Bi) _1-x Ba _x TiO ₃ (where x is 0.025 ≦ x ≦ 0. 15) The second heat treatment powder production step for producing the second heat treatment powder represented by 15), the first heat treatment powder and the second heat treatment powder are mixed and molded to form a molded body. A body manufacturing step and a firing step of firing the molded body.

As described in [Background Art], a PTC thermistor suitable for use at a high temperature such as a heater is realized by replacing a part of Ba with Pb to realize a high Curie point Tc. I had to give, in consideration of environmental in recent years, _Ba y obtained by substituting a part of Ba with an alkali metal M and Bi (M, Bi, Ln) 1-y TiO 3 (Ln is a rare earth element) -based semiconductor ceramic Has been developed.

  And this kind of semiconductor ceramic can be manufactured through a series of processes of forming and firing after preparing and firing the required ceramic raw materials at the same time.

However, since the alkali metals M and Bi are volatile, if they are simultaneously calcined at a high temperature (for example, 1000 ° C.) at which BaTiO ₃ is synthesized, the alkali metals M and Bi may be volatilized, resulting in a composition shift. There is.

In order to avoid such a situation, as in Patent Documents 1 and 2, (Ba, Ln) TiO ₃ and (M, Bi) TiO ₃ are respectively synthesized separately at a calcining temperature suitable for each, and thereafter A method in which both are mixed, molded, and fired is conceivable.

However, (Ba, Ln) TiO ₃ and (M, Bi) TiO ₃ have a large difference in lattice constant. Therefore, when both are mixed and fired, it becomes difficult to form a solid solution smoothly, resulting in high resistance. There is a risk of reducing the reliability and reliability.

That is, according to Non-Patent Document 1, the ion radius of Ba ²⁺ is 0.135 nm, whereas the ion radius of alkali metal ion M ⁺ , for example, Na ⁺ is 0.102 nm and Li ⁺ is 0.076 nm. In addition, the ion radius of Bi ³⁺ is 0.103 nm. That is, the ionic radius of Ba ²⁺ is larger than the ionic radii of these alkali metal ions M ⁺ and Bi ³⁺ . Therefore (M, Bi) for the lattice constant of the TiO ₃ is smaller than that (Ba, Ln) the lattice constant of _{TiO 3, (M, Bi)} TiO 3 is not easily dissolved (Ba, Ln) to TiO _3, There is a possibility that the resistance of the PTC thermistor is increased and the reliability is lowered. Further, in the alkali metal M, the ion radius of K ⁺ is 0.138 nm, which is close to the ion radius of Ba ²⁺ , but as described above, the ion radius of Bi ³⁺ is smaller than the ion radius of Ba ²⁺ . (M, Bi) lattice constant of TiO ₃ becomes small as compared with (Ba, Ln) the lattice constant of TiO _3, as described above, dissolved in the (M, Bi) TiO ₃ is (Ba, Ln) TiO ₃ It becomes difficult.

R.D.Shannon, “Acta Cryst.”, A32, 1976, p.751-767

Therefore, in this embodiment, by replacing a part of (M, Bi) in advance with Ba having a large ion radius in the second heat treatment powder production step, the lattice constant of the crystal lattice of the second heat treatment powder is changed. It is getting bigger. As a result, the difference in the lattice constant of the crystal lattice between the first heat-treated powder and the second heat-treated powder can be reduced, and the solid solution of both can be smoothly performed. As a result, the resistance of the PTC thermistor is increased. It can be avoided. This effect also occurs in the case of K ⁺ having an ionic radius approximate to Ba ²⁺ for the reasons described above, but is particularly large in the case of Na ⁺ and Li ^{+ having} a smaller ionic radius than Ba ^2+. .

In addition, in the (M, Bi) TiO ₃ -based material, the alkali metal M that is not dissolved in the crystal particles may segregate at crystal grain boundaries, crystal triple points, and the like, leading to a decrease in reliability. By replacing a part of Bi) with Ba in advance to promote solid solution, the alkali metal M can stably enter the crystal lattice, thereby improving the reliability.

  The substitution molar ratio x of Ba in the second heat-treated powder is 0.025 to 0.15.

  That is, when the substitution molar ratio x of Ba is less than 0.025 mol, since the Ba content is small, the lattice constant of the crystal lattice of the second heat-treated powder cannot be sufficiently increased, and the first The solid solution of the heat treated powder and the second heat treated powder cannot be smoothly performed. On the other hand, when the substitution molar ratio x of Ba exceeds 0.15, the content of Ba becomes excessive and synthesis of the second heat-treated powder becomes insufficient. That is, the second heat-treated powder is calcined at a lower temperature than the first heat-treated powder from the viewpoint of preventing the volatilization of the alkali metals M and Bi. Therefore, if the content of Ba is excessively increased, unreacted Ba that has not been sintered increases, which may lead to an increase in resistance or a decrease in reliability.

  Therefore, in this embodiment, the substitution molar ratio x of Ba is set to 0.025 to 0.15, preferably 0.025 to 0.075.

  In addition, since the volatilization of alkali metals M and Bi increases when the calcination temperature is raised, the PTC characteristics are lowered, which is not preferable.

  Hereinafter, the method for producing the semiconductor ceramic will be described in more detail.

  First, an Ln compound containing a Ba compound, a Ti compound, and a rare earth element Ln is prepared as a ceramic raw material. Then, these ceramic raw materials are weighed and mixed to obtain a mixed powder so that the component composition of the semiconductor ceramic becomes a predetermined ratio. Here, the rare earth element Ln is not particularly limited as long as it acts as a semiconducting agent, and for example, one or more selected from the group of Y, Sm, Nd, Dy, and Gd are used. can do.

Next, water or an organic solvent and a polymer dispersant are added to the mixed powder, and the mixture is sufficiently mixed and pulverized in a ball mill together with a pulverizing medium such as PSZ (partially stabilized zirconia) balls. Then, the organic solvent is dried, and then sized using a mesh with a predetermined mesh. Subsequently, heat treatment for 2 hours at 800 to 1000 ° C., to produce a first heat treatment flour perovskite structure composed of BaTiO ₃ based composition.

  Next, an M compound, a Bi compound, a Ba compound, and a Ti compound containing an alkali metal M are prepared as ceramic raw materials. Then, these ceramic raw materials are weighed and mixed to obtain a mixed powder so that the substitution molar ratio x of Ba becomes 0.025 to 0.15.

Next, water or an organic solvent and a polymer dispersant are added to the mixed powder, and the mixture is sufficiently mixed and pulverized in a ball mill with a pulverization medium such as PSZ (partially stabilized zirconia) balls. Then, the organic solvent is dried, and then sized using a mesh with a predetermined mesh. Subsequently, heat treatment is performed at a temperature at which the alkali metals M and Bi do not volatilize, for example, 700 to 900 ° C. for 2 hours, and the general formula (M, Bi) _1-x Ba _x TiO ₃ (where 0.025 ≦ x ≦ 0 .15) is produced as a second heat-treated powder having a perovskite structure.

  Next, a predetermined amount of these first heat-treated powder and second heat-treated powder are weighed, vinyl acetate, polyvinyl organic binder, dispersant, and pure water are added, and mixed and pulverized sufficiently with a pulverizing medium again in a wet manner. Then, the obtained slurry is dried and sized using a mesh having a predetermined opening. Then, press molding is performed, thereby obtaining a molded body.

  From the viewpoint of improving the PTC characteristics, it is also preferable to add Mn so that it becomes 0.0001 to 0.0020 mol part after firing with respect to the total of 1 mol part of the first and second heat treated powders. That is, since Mn has an action as an acceptor, it forms an acceptor level at the grain boundary, thereby contributing to an increase in the temperature coefficient of resistance and further improving the PTC characteristics. Therefore, it is preferable to add a predetermined amount of Mn.

  The addition form of Mn is not particularly limited, and any manganese compound such as manganese oxide sol or powder or manganese nitrate aqueous solution can be used.

  Next, the molded body is heated at 500 to 600 ° C. in a predetermined atmosphere (for example, in an air atmosphere, a nitrogen atmosphere, or a mixed gas stream) to perform a binder removal treatment, and then in a predetermined atmosphere ( For example, in a nitrogen atmosphere or in a mixed gas stream of a reducing atmosphere), firing is performed for a predetermined time at a temperature at which the semiconductor is formed, for example, a maximum firing temperature of 1250 to 1450 ° C., to obtain a component body 1 that is a sintered body.

  Then, the external electrodes 2a and 2b are formed on both ends of the component body 1 by plating, sputtering, coating baking, or the like, thereby producing a PTC thermistor.

As described above, in this embodiment, the Ba compound and the Ti compound (further, the Ca compound, the Sr compound, the rare earth compound, etc.) are mixed and heat-treated to produce the first heat-treated powder made of the BaTiO ₃ -based composition. 1 heat treatment powder preparation step and an alkali metal compound containing an alkali metal M1 (Na, Li), Bi compound, Ba compound, Ti compound and Ba compound are mixed and heat-treated, and the general formula (M1, Bi) _{1 -x} Ba _x TiO ₃ (where x is 0.025 ≦ x ≦ 0.15), a second heat-treated powder production step for producing the second heat-treated powder, the first heat-treated powder and the second The heat treatment powder is then mixed and molded to form a molded body, and a firing step of firing the molded body. Therefore, the lattice constant of the crystal lattice of the second heat treated powder is increased in advance. Possible to It becomes. That is, the difference in the lattice constant of the crystal lattice from the first heat-treated powder can be reduced, and the first heat-treated powder and the second heat-treated powder can be smoothly dissolved in the firing step. . As a result, high resistance can be avoided and a semiconductor ceramic with improved reliability can be manufactured.

  FIG. 1 is a perspective view schematically showing an embodiment of a PTC thermistor according to the present invention.

  That is, this PTC thermistor includes a component element body 1 and a pair of external electrodes 2a and 2b formed on both main surfaces (both ends) of the component element element 1. It is made of semiconductor ceramic. The external electrodes 2a and 2b are formed of a single layer structure or a multilayer structure made of a conductive material such as Cu, Ni, Al, Cr, Ag, Ni—Cr alloy, Ni—Cu or the like.

  Thus, since the PTC thermistor is formed of the semiconductor ceramic manufactured by the manufacturing method of the present invention, a PTC thermistor having good reliability can be obtained without increasing the resistance.

The present invention is not limited to the above embodiment. For example, in the above-described semiconductor ceramic, it is only necessary that BaTiO ₃ is a main component and a part of Ba is replaced with at least an alkali metal and Bi, and a part of Ba is replaced with Ca or Sr according to required PTC characteristics. It is also preferable. Further, even if inevitable impurities are mixed in the semiconductor ceramic, the characteristics are not affected. For example, there is a possibility that PSZ balls used as a grinding medium at the time of wet mixing and grinding may be mixed by about 0.2 to 0.3% by weight, but this does not affect the characteristics. Similarly, trace amounts of Fe, Si, and Cu of about 10 ppm by weight may be mixed in the ceramic raw material, but this does not affect the characteristics. Further, the semiconductor ceramic of the present invention is lead-free, but as described in the section of [Means for Solving the Problems], it should be substantially free of Pb and does not affect the characteristics. However, it does not exclude even Pb that is inevitably mixed in the range of 10 ppm by weight or less.

  Further, the external shape of the PTC thermistor is not limited, and any shape can be used.

  Next, examples of the present invention will be specifically described.

BaCO ₃ , CaCO ₃ , SrCO ₃ , TiO ₂ , and Y ₂ O ₃ were prepared as ceramic raw materials, and a predetermined amount of these ceramic raw materials were weighed and mixed to obtain a mixed powder.

  Next, ethanol as an organic solvent and a polymer-type dispersant (maleic anhydride and ethylene oxide / propylene oxide derivative) are added to the mixed powder, and mixed and pulverized in a ball mill for 24 hours together with PSZ balls, Thereafter, ethanol was dried, and sized with a mesh having an opening of 300 μm. Then, it heat-processed for 2 hours in the temperature range of 800-1000 degreeC, and produced the 1st heat processing powder.

Next, BaCO ₃ , Na ₂ CO ₃ , Bi ₂ O ₃ , and TiO ₂ are prepared as ceramic raw materials, and these are weighed and mixed so that the composition after heat treatment is as shown in Table 1, and mixed powder is prepared. Obtained.

Next, ethanol as an organic solvent and a polymer-type dispersant (maleic anhydride and ethylene oxide / propylene oxide derivative) are added to the mixed powder, and mixed and pulverized in a ball mill for 24 hours together with PSZ balls, Thereafter, ethanol was dried, and sized with a mesh having an opening of 300 μm. Followed by heat treatment for 2 hours at a temperature range of 700 to 900 ° C., to produce a second heat treatment powder composed of _{(Na, Bi) 1-x} Ba x TiO 3.

Next, the first heat-treated powder and the second heat-treated powder are mixed, a vinyl acetate organic binder, Mn ₃ O ₄ sol, the above dispersant, and pure water are added, and again with a PSZ ball for 16 hours in a ball mill. The mixture was pulverized in a wet manner, and the pulverized slurry was dried and sized using a mesh having an opening of 300 μm to prepare a raw material powder. The Mn ₃ O ₄ sol was weighed and added to the calcined powder so as to be 0.00025 mol part in terms of Mn with respect to 1 mol part of the main component.

  And this raw material powder was shape | molded by applying the pressure of 98 Mpa by uniaxial press, and the molded object was obtained.

Next, the molded body was treated to remove the binder for 2 hours at a temperature of 600 ° C. in the atmosphere, and then fired for 2 hours at a maximum firing temperature of 1400 ° C. in a nitrogen atmosphere having an oxygen concentration of 10,000 ppm by volume. _{1-uvwz {(Na, Bi} ) u Ca v Sr w Y z} to obtain a component element of sample No. 1-9 represented by TiO _3].

  Next, this component body is lapped and then dry-plated to form an external electrode having a three-layer structure of NiCr / NiCu / Ag, whereby sample numbers 1 to 9 having a diameter of 12 mm and a thickness of 2.0 mm. A sample was prepared.

Next, for each sample of sample numbers 1 to 9, a direct current four-terminal method was used, a voltage of 0.1 V was applied at a temperature of 25 ° C., and a specific resistance ρ ₀ was measured.

In addition, an energization test was performed to evaluate reliability. That is, a DC voltage of 13 V was applied to each sample and left for 1000 hours. Then, the resistance change rate ρ ₀ before the test and the resistance change rate ρ ₁ after the test are measured at a temperature of 25 ° C., the difference Δρ (= ρ ₁ −ρ ₀ ) is obtained, and the resistance change rate Δρ / ρ ₀ is obtained. Calculated. In this way, ten current samples were conducted for each sample, the average value of the resistance change rate Δρ / ρ ₀ was calculated, and the reliability was evaluated.

Table 1 shows the composition of the second heat-treated powder, the main component composition of the final composition, the specific resistance ρ ₀ at 25 ° C. (room temperature), and the resistance change rate Δρ / ρ ₀ for each sample of sample numbers 1 to 9. ing.

The main component composition was analyzed by inductively coupled plasma emission spectroscopy (ICP-AES). The specific resistance ρ ₀ was determined to be less than 15 Ω · cm, and the resistance change rate Δρ / ρ ₀ was determined to be less than 10%.

Sample No. 1 was found to have a high specific resistance ρ ₀ of 22.1 Ω · cm and a resistance change rate ρ / ρ _{0 of} 38.2%, which is inferior in reliability. This is because Ba is not contained in the second heat-treated powder, so solid solution does not proceed smoothly with the first heat-treated powder, and it seems that the resistance is increased and the reliability is impaired. .

Sample No. 2 has a part of (Na, Bi) substituted with Ba, but since the substitution molar ratio x of Ba is as small as 0.01, the resistivity ρ ₀ is increased to 17.3 Ω · cm. The resistance change rate ρ / ρ ₀ was found to be inferior in reliability at 14.3%.

On the other hand, in Sample Nos. 8 and 9, since the substitution molar ratio x of Ba was too large, 0.20 and 0.40, respectively, the amount of unreacted Ba increased and the specific resistance ρ ₀ exceeded 15.0 Ω · cm. It was found that the resistance was increased, and the resistance change rate Δρ / ρ ₀ exceeded 10% and the reliability was impaired.

On the other hand, in the sample numbers 3 to 7, since the substitution molar ratio x of Ba is 0.025 to 0.15, which is within the range of the present invention, the specific resistance ρ ₀ is less than 15 Ω · cm and the resistance is increased. It can be avoided, and the resistance change rate Δρ / ρ ₀ is also less than 10%, and the reliability is improved.

FIG. 2 is a graph showing the relationship between the substitution molar ratio x of Ba, the specific resistance ρ _0, and the resistance change rate ρ / ρ _0. The horizontal axis represents the substitution molar ratio x of Ba, and the left vertical axis represents the specific resistance ρ ₀ (Ω Cm) and the vertical axis on the right is the resistance change rate ρ / ρ ₀ (%). In the figure, (a) shows the relationship of substitution molar ratio-specific resistance, and (b) shows the relation of substitution molar ratio-resistance change rate.

As is apparent from FIG. 2, the specific resistance ρ ₀ and the resistance change rate ρ / ρ ₀ have the same variation tendency with respect to the substitution molar ratio x. And it turned out that specific resistance (rho) ₀ and resistance change rate (rho) / (rho) ₀ are especially preferable in the range whose substitution molar ratio x is 0.025-0.075.

  Samples Nos. 11 to 16 were prepared by the same method and procedure as in [Example 1] except that the molar amounts v and w of Ca and Sr in the main component were changed.

Next, for each of the samples Nos. 11 to 16, the specific resistance ρ ₀ and the resistance change rate Δρ / ρ ₀ at a temperature of 25 ° C. (room temperature) were obtained by the same method and procedure as in [Example 1].

Table 2 shows the composition of the second heat-treated powder, the main component composition, the specific resistance ρ ₀ at 25 ° C. (room temperature), and the resistance change rate Δρ / ρ ₀ for each sample of sample numbers 11-16.

As in [Example 1], the main component composition was analyzed by inductively coupled plasma emission spectroscopy (ICP-AES), the specific resistance ρ ₀ was less than 15 Ω · cm, and the resistance change rate Δρ / ρ ₀ Judged less than 10% to be good.

As is apparent from Table 2, the specific resistance ρ ₀ and the resistance change rate Δρ are within the range where the Ca substitution molar ratio v is 0.05 to 0.20 and the Sr substitution molar ratio w is 0.02 to 0.10. / Ρ ₀ has obtained good results, and it has been confirmed that the same effect can be obtained when a part of Ba is substituted with Ca and / or Sr.

In the Ba (Na, Bi) TiO ₃ PTC thermistor, the reliability can be improved without causing an increase in resistance. It is particularly useful for high temperature applications such as in-vehicle heaters.

1 Component body 2a, 2b External electrode

Claims (7)

A method for producing a semiconductor ceramic comprising a BaTiO ₃ -based compound as a main component and a part of Ba substituted with an alkali metal and Bi,
A first heat treatment powder preparation step of mixing at least a Ba compound and a Ti compound and heat-treating to produce a first heat treatment powder made of a BaTiO ₃ -based composition;
An alkali metal compound containing an alkali metal M, a Bi compound, a Ba compound, and a Ti compound are mixed and heat-treated to obtain a general formula (M, Bi) _1-x Ba _x TiO ₃ (where x is 0.025 ≦ x ≦ 0.15) a second heat treatment powder production step for producing a second heat treatment powder represented by:
A molded body preparation step of mixing and molding the first heat treated powder and the second heat treated powder to produce a molded body,
And a firing step of firing the molded body.   2. The method of manufacturing a semiconductor ceramic according to claim 1, wherein x is 0.025 to 0.075.   3. The method of manufacturing a semiconductor ceramic according to claim 1, wherein at least one of a Ca compound and an Sr compound is added in the first heat treatment powder manufacturing step.   The method for producing a semiconductor ceramic according to any one of claims 1 to 3, wherein a rare earth compound is added in the first heat treatment powder production step.   The method for producing a semiconductor ceramic according to any one of claims 1 to 4, wherein the alkali metal includes at least one of Na and Li.   6. A semiconductor ceramic produced by the method according to claim 1. A positive temperature coefficient thermistor in which external electrodes are formed on both ends of a component body,
A positive temperature coefficient thermistor, wherein the component body is formed of the semiconductor ceramic according to claim 6.

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