JPH1025158A - Sintered compact for termistor and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a sintered compact for a thermistor having a high stability at a high temperature, especially in >=600 deg.C temperature range, and to provide a method for producing the same. SOLUTION: This five sintered compact for a thermistor having >=95% relative density, is obtained by wet-mixing high purity Y2 O5 powder and Cr2 O3 powder, drying the mixed powder, then calcining the powder at 1300 deg.C for 5hr to obtain YCrO3 single phase powder, wet-crushing the calcined powder, drying the crushed powder, forming a compact having an optical shape, burning the compact under any atmosphere of nitrogen gas, hydrogen gas an inert gas and a mixed gas of them under 10<-20> -10-4 atm oxygen partial pressure at 1500-1700 deg.C, e.g. approximately 1600 deg.C, for approximately 1hr, and then heat- treating the sintered compact at >=600 deg.C, e.g. at 1000 deg.C for >=10hr, especially for 200hr. As a heating furnace for burning, a tungsten heater, etc., are preferred to a carbon heater.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a sintered body for a thermistor (NTC thermistor) having a negative temperature coefficient and a method for producing the same. INDUSTRIAL APPLICABILITY The sintered body for a thermistor of the present invention has high stability at high temperatures, and can be used particularly as a sintered body for a thermistor used at 600 ° C. or higher.

[0002]

2. Description of the Related Art Materials for thermistors used at high temperatures include: (1) Al ₂ O ₃ and Cr ₂ O _3;
A material having a corundum-type crystal structure mainly composed of (for example, JP-A-50-118294), (2) MgAl
Materials mainly composed of compounds having a spinel-type crystal structure such as ₂ O ₄ , MgCr ₂ O ₄ and MgFe ₂ O ₄ (JP-A-49-63995, etc.);

(3) A material mainly composed of a compound having a perovskite type crystal structure having a high melting point and conductivity, for example, a material having a La (Al ₁ -x Cr _x ) O ₃ -based composition (Japanese Unexamined Patent Publication No. 51-108298), a material using LaCrO ₃ in a thin film on an insulating substrate (Japanese Patent Application Laid-Open No. 61-108298).
161701), and LaCrO ₃ and MgA
l ₂ O ₄ and a mixture material (JP 51-95297, JP-JP 51-23691 Publication), etc. are used.

[0004]

However, the NTC thermistor currently used is practically 500 ° C. or 6 ° C.
Most of them are in the use range below 00 ° C. In fact,
Even if it is provided as a high temperature thermistor,
In a region exceeding the above temperature, the conductive characteristics become unstable, and many of them are not suitable for long-time use. In addition, the above (1) ~
Among the thermistor materials of (3), some perovskite compounds have high heat resistance. However, materials having high heat resistance are generally difficult to sinter, and require a sintering aid for densification. However, there is a problem that the heat resistance of the sintered body is reduced by the sintering aid. Occurs. Furthermore, when elements such as lanthanum and neodymium and their oxides remain in the material, they react with moisture in the air to form hydroxides, which may cause the element to collapse.

[0005] YCrO ₃ -based materials have high heat resistance and excellent conductivity, but due to their high heat resistance, the addition of a sintering aid in air is essential for densification. It is. However, the addition of the sintering aid reduces the heat resistance of the material and makes the material characteristics unstable, such as a large change in resistance over time. In addition, the compactness of the sintered body is low,
That is, low density means that the proportion of pores opened on the material surface is high, and the area of the element exposed to the atmosphere is large. In such a device, the specific resistance of the thermistor is greatly affected by the atmosphere, such as a change in oxygen partial pressure or adsorption of gas to the device. In order to reduce the influence of the atmosphere, it is effective to provide a heat-resistant case or the like around the element to prevent the element from being directly exposed to the atmosphere. However, this causes new problems such as a slow response speed.

On the other hand, it is known that sinterability of a compound containing chromium is generally improved when fired in a reducing atmosphere. Further, the YCrO ₃ -based material is also fired in a reducing atmosphere, whereby the densification can be promoted. However, the sintered body thus obtained has a problem in that the valence of the metal forming the carrier such as chromium is easily changed, or the initial specific resistance is easily changed.

An object of the present invention is to provide a sintered body for a thermistor comprising a sintered body containing substantially only a perovskite type crystal phase of YCrO ₃ and a method for producing the same. I do. The sintered body for a thermistor of the present invention has very few impurities, has stable material properties, and is sufficiently densified. for that reason,
The thermistor using the sintered body of the present invention is 500 to 600
Even at a high temperature of 1000 ° C. or more, especially about 1000 ° C., which is much higher than this temperature, it can be practically used without any problem.

[0008]

A sintered body for a thermistor according to the first invention is a sintered body for a thermistor containing only a perovskite-type crystal phase mainly composed of YCrO ₃ as a crystal phase. The relative density is 95% or more, and impurities other than the perovskite-type crystal phase contained in the sintered body are 0.5% by volume or less with respect to 100% by volume of the perovskite-type crystal phase. I do.

The compound constituting the “perovskite-type crystal phase of YCrO ₃ ” includes Y ₂ O ₃ and Cr ₂ O ₃ ,
After mixing so that the ratio of Y to Cr becomes 1: 1, it is obtained by molding and firing. Prior to molding, 1200
It is preferable to calcine at ~ 1500C for several hours. And, in the present invention, despite not using a sintering aid,
A sintered body having a "relative density" of "95% or more" can be obtained. If the relative density of the sintered body is 95% or more, there are substantially no pores opened on the surface of the sintered body, and the influence of the firing atmosphere on the conductive characteristics of the thermistor is greatly reduced.

The above-mentioned "sintered body for thermistor" does not require a sintering aid for firing. Therefore, “impurities” other than the perovskite type crystal phase are “0.5% by volume”.
Below. " And the less the impurities, the higher the heat resistance of the obtained thermistor. Therefore,
In addition to not using a sintering aid, it is preferable to use as pure Y ₂ O ₃ and Cr ₂ O ₃ as possible as raw materials. The impurity is more preferably 0.1% by volume or less, and a thermistor having higher heat resistance can be obtained.

Further, the method for producing a sintered body for a thermistor according to the second invention is characterized in that the crystal phase after firing contains only a perovskite crystal phase of YCrO ₃ and impurities other than the perovskite crystal phase are mixed with the perovskite crystal phase. Phase 1
After mixing and molding the raw material powders so as to be 0.5% by volume or less with respect to 00% by volume, selected from nitrogen gas, hydrogen gas, inert gas, and a mixed gas of two or more of these gases. And firing in an atmosphere having an oxygen partial pressure of 10 ⁻²⁰ to 10 ⁻⁴ atm at a temperature of 1500 ° C. or more, and then heat-treating in air at 900 ° C. or more for 10 hours or more. And

As the above-mentioned "raw material powder", Y ₂ O ₃ and Cr ₂ O ₃ , which are oxides of Y or Cr, are generally used. Good. In addition, as these raw material powders, it is particularly preferable to use high-purity powders having a purity exceeding 99.8%. This further reduces impurities in the sintered body, and improves the heat resistance and the like of the obtained thermistor.

The calcination is performed in an inert atmosphere or a reducing atmosphere as described above. The reducing atmosphere is formed by mixing an inert gas, particularly a nitrogen gas, with a hydrogen gas of about several percent. In addition, firing is performed under the “oxygen partial pressure” in the atmosphere.
As "10 ^-20 to 10 ^-4 atm". If the sintering is performed in an oxidizing atmosphere having a high oxygen partial pressure, such as in the air, the density of the obtained sintered body decreases.

On the other hand, when the oxygen partial pressure in the atmosphere is reduced to 10 by using a graphite heater or a heat insulating material, for example.
If the pressure is lower than ^-20 atm, the volatilization of the Cr component becomes intense, and the compactness of the sintered body also decreases. In addition, the composition of the sintered body fluctuates due to the volatilization and reduction of the Cr component. As described above, when a heater made of a carbon-based material such as graphite is used, problems such as a decrease in denseness and a change in composition occur. Therefore, as in the third invention, it is preferable to heat and sinter with a heater made of a material other than carbon, for example, a material such as tungsten.

If the firing temperature is lower than 1500 ° C., the densification of the sintered body becomes insufficient. Increasing the firing temperature improves sinterability and promotes densification. However, if the sintering temperature exceeds 1700 ° C., the amount of the Cr component volatilized increases, which may cause a reduction in the density and a change in the composition. Therefore, the firing temperature is 1500 to 1700 ° C., and especially 1550 ° C.
The temperature is preferably set to about 1650 ° C. The firing time may be 30 minutes to several hours, particularly about 1 hour.

In the "heat treatment", even in an atmosphere having an oxygen partial pressure other than the atmosphere, oxidation of Cr and the like is promoted, and a heat treatment effect such as stabilization of the quality of the sintered body is exhibited. However, in an atmosphere having an oxygen partial pressure lower than the atmosphere, the heat treatment effect becomes insufficient, and in an atmosphere having an oxygen partial pressure higher than the atmosphere, the amount of Cr volatilized in the form of CrO ₃ increases. Also, preparing a special atmosphere other than the atmosphere increases costs.

Further, when the heat treatment temperature is lower than 900 ° C., the heat treatment effect becomes insufficient. When the heat treatment temperature is higher than the required temperature, there is no particular problem from the viewpoint of the heat treatment effect. However, since the heat treatment is a long-time heat treatment of “10 hours or more”, if the treatment temperature is too high, the Cr component volatilizes from the surface of the sintered body. Also, it seems that the linearity between the reciprocal of the temperature and the logarithm of the specific resistance slightly decreases. The heat treatment temperature is 900 to 1300 ° C., particularly 1
The temperature is preferably from 000 to 1100C, more preferably around 1000C. Further, the heat treatment time is preferably at least 20 hours, particularly about 200 hours. If the heat treatment time is less than 10 hours, a sufficient heat treatment effect cannot be obtained, and the quality of the sintered body deteriorates, for example, a change in resistance becomes large.

The specific resistance and the thermistor constant of the sintered body for a thermistor of the present invention in which no sintering aid is used are almost determined by the firing conditions. However, it is also effective to substitute a part of Y or Cr with another element in order to obtain desired conductive properties. However, in that case, pure YC
Heat resistance or durability may be inferior to rO ₃ . Therefore, it is necessary to select an element according to a use temperature range and a use situation. The element to be replaced with Y or Cr to form a solid solution is a rare earth element other than Y, an alkaline earth metal, a transition metal, or the like.
What is necessary is just to select from solid solution. However, when firing in a reducing atmosphere, the addition of such a metal species is not preferred, since the control of the conductive properties becomes difficult if a metal species other than Cr whose valence is liable to change is included. Also, the use of a highly volatile metal species such as an alkali metal is not preferred.

The sintered body for a thermistor according to the present invention is excellent in heat resistance because the amount of impurities brought from the sintering aid and the like is extremely small. Although the high heat resistance is correlated with the low diffusion coefficient of ions, by performing heat treatment to stabilize the specific resistance, a change in specific resistance due to diffusion of oxygen or the like can be suppressed. . Further, in the firing in a reducing atmosphere, the YCrO ₃ -based material is considered to have undergone liquid phase sintering, and the temperature at which the liquid phase starts to be generated is 1400 from the curve showing the correlation between the sintering temperature and densification. Expected to be around ° C. Therefore, even when used in a reducing atmosphere, there is no particular problem in a temperature range of about 1000 ° C.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described specifically with reference to examples. Y ₂ O ₃ powder (BET specific surface area; 1)
0.8 m ² / g, purity> 99.9%) and Cr ₂ O ₃ powder (BET specific surface area; 2.7 m ² / g, purity 99.8)
%) Was wet-mixed. After drying, Y ₂ O ₃ and Cr ₂ O ₃
Was calcined at 1300 ° C. for 5 hours to obtain a YCrO ₃ single phase powder.

The calcined powder is pulverized by a wet method, dried, formed into a disk having a diameter and thickness of about 10 mm, and subjected to a CIP (cold isostatic pressing) treatment at a pressure of 1.5 t / cm ^2. gave. This compact was fired and heat-treated under the conditions shown in Table 1 to obtain a sintered body. In addition, a tungsten heater furnace was mainly used for firing. For comparison, in the same manner, the firing conditions or heat treatment conditions deviated from the second invention, the examples using a carbon heater furnace as the firing furnace, and the examples using the sintering aid (sample No. 13) were also performed. A compact was produced. The addition and mixing of the sintering aid were performed simultaneously with the wet pulverization of the calcined powder.

[0022]

[Table 1]

Table 1 shows the density of the sintered body as a relative density with respect to the theoretical density of YCrO ₃ (5.751 g / cm ³ ). According to Table 1, when the tungsten heater furnace was used, the sinterability was superior to that of the carbon heater furnace, and the relative density was 95% at the firing temperature of 1600 ° C.
It can be seen that the above dense sintered body was obtained. When a carbon heater furnace is used, the relative density becomes 95% or more when the firing temperature is increased to 1700 ° C.
However, the volatilization of Cr during firing was so severe that a porous portion was formed near the surface of the obtained sintered body.

The obtained sintered body was formed into a disk having a diameter of 7 mm and a thickness of 5 mm by polishing, and then heat-treated in a high-purity alumina crucible under the conditions shown in Table 1. In Table 1, * indicates that the value is out of the range of the first invention, and ** indicates that the value is out of the range of the second invention. Also,
Sample Nos. 11 and 12 using a carbon heater furnace
Is outside the scope of the third invention. The firing time was all one hour, the pressure of the firing atmosphere was normal pressure, and the heat treatment was air atmosphere. The sintering aid added to Sample No. 13 was Mg-Ca
-Al-Si-O-based composition. In this sample No. 13, since the sintering aid was used, the firing temperature was 1450.
Although the temperature was lowered to as low as ° C., a sintered body having a sufficiently high relative density was obtained.

Next, platinum electrodes were formed at both ends (two circular end faces) of the heat-treated disk-shaped specimen, and resistance values in the air and in nitrogen were measured. Measurement is 900
C., 750.degree. C., and 650.degree. C. in this order, and the resistance value after holding the specimen at each temperature for 20 minutes was taken as a measured value. Tables 2 and 3 show the specific resistance and thermistor constant (B constant) calculated from the results. B value is as follows (1)
It was determined by the formula.

Further, based on the above results, the rate of change ΔR ₁ of the resistivity ρ _{N2 in} nitrogen from the resistivity ρ _{AIR in} the atmosphere was calculated using the following equation (2). Further, the rate of change ΔR ₁
The temperature difference [Delta] T ₁ corresponding calculated based on the change in the specific resistance of the atmosphere. The following equation (3) was used for the calculation. Table 4 shows the obtained values of ΔR ₁ and ΔT ₁ . As the value of B _{AIR in} the equation (3), the B value obtained from the specific resistance at 900 ° C. and 650 ° C. was used for convenience. B = ln (ρ _T1 / ρ _T2 ) / (1 / T ₁ −1 / T ₂ ) (1) ΔR ₁ (%) = 100 (ρ _N2 −ρ _AIR ) / ρ _AIR (2) ΔT ₁ (° C.) = B _AIR · T / (ln (ρ _N2 / ρ _AIR ) · T + B _AIR ) −T (3) T, T ₁ , T ₂ : measured temperature ρ _T1 , ρ _T2 : specific resistance at temperature T ₁ , T ₂ B _AIR : B constant in the atmosphere (900 to 650 ° C)

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

According to the results shown in Tables 2 to 4, all the samples showed stable resistance values in the measurement in the atmosphere. However, in the measurement in nitrogen, the resistance values of Sample Nos. 2 and 11 continued to increase even after the holding time of 20 minutes. In these samples, the specific resistance in nitrogen is increased by several tens% or more as compared with that in the atmosphere, and it can be seen that an error of several tens degrees Celsius or more is caused when converted into temperature. The sintered bodies other than Samples Nos. 2 and 11 exhibited stable resistance values even when measured in nitrogen, indicating that the influence of the measurement atmosphere was large for the sintered bodies of Samples Nos. 2 and 11 having low density. Incidentally, even in the sample No. 12 which is considerably denser than the samples No. 2 and 11, the specific resistance in nitrogen is several% to that in the atmosphere.
This is thought to be due to a porous portion formed near the surface of the specimen.

In each of the samples other than the above three samples, the resistance change in nitrogen was equivalent to 2 ° C. or less in terms of temperature, and the higher the measurement temperature, the smaller the change. Also, 750
It can be said that the rate of change above ℃ is almost within the error range. In sample No. 8, in which the heat treatment temperature was high, the crucible made of alumina was colored a little purple, and it appeared that Cr was volatilized during the heat treatment, but there was no problem with the characteristics. Except for sample No. 8, there was no coloring of the crucible during the heat treatment.

Subsequent to the above evaluation, a durability test was performed on each of the samples except for Samples Nos. 2, 11, and 12, which were inferior in the rate of change in specific resistance. The durability test was performed by exposing the test piece to the air at 1100 ° C. for 300 hours, and then measuring the resistance value in the air. The method of measuring the resistance value is the same as described above. The results are shown in Tables 5 and 6. Table 6
ΔR ₂ and ΔT ₂ were determined by the following equations (4) and (5). ΔR ₂ (%) = 100 (ρ _DUR −ρ _AIR ) / ρ _AIR (4) ΔT ₂ (° C.) = B _AIR · T / (ln (ρ _DUR / ρ _AIR ) · T + B _AIR ) −T (5) ρ _DUR : Specific resistance after endurance test (in air)

[0033]

[Table 5]

[0034]

[Table 6]

According to the results shown in Tables 5 and 6, Sample No. 3
In Nos. And 4, the heat treatment was insufficient, so that the resistance value after the durability test was reduced by several tens of percent. This is equivalent to a temperature change of 10 ° C. or more, which is a problem. Sample No. 13 to which the sintering aid was added also showed a large change in resistance, indicating that the presence of impurities made the material characteristics unstable. The specific resistance change rates of the test pieces other than Sample Nos. 3, 4, and 13 were within 10 ° C. in terms of temperature change, and the temperature change tended to be smaller at higher temperatures. In particular, sample No. 1,
In 5, 6, 9 and 10, although the temperature of the durability test was higher than the heat treatment temperature, the specific resistance change rate was small similarly to Sample Nos. 7 and 8. Although the change rate of the specific resistance in Table 6 seems to be a large value at first glance, these samples have a large B constant, and therefore, when converted to a temperature change, the change is small.

[0036]

According to the first invention, the sintered body mainly composed of YCrO ₃ has a relative density as high as 95% or more and has no pores opened to the outside. High stability to the atmosphere. Further, since no sintering aid is used, impurities are extremely small, and components are not easily diffused or moved. As a result, heat resistance and durability are excellent. Further, according to the second invention, the sintered body for a thermistor of the first invention can be easily obtained by firing in a specific atmosphere and then performing heat treatment in the air at a specific temperature and time.

Claims (3)

[Claims] 1. A sintered body for a thermistor containing only a perovskite-type crystal phase mainly composed of YCrO ₃ as a crystal phase, wherein the relative density of the sintered body is 95% or more, and A sintered body for a thermistor, characterized in that the content of impurities other than the perovskite-type crystal phase is 0.5% by volume or less based on 100% by volume of the perovskite-type crystal phase. 2. YCrO ₃ as a crystal phase after firing.
The raw material powder is mixed so as to contain only the perovskite-type crystal phase mainly composed of, and 0.5% by volume or less of impurities other than the perovskite-type crystal phase with respect to 100% by volume of the perovskite-type crystal phase, and then molding. After that, nitrogen gas,
It is selected from hydrogen gas, inert gas, and a mixed gas of two or more of these gases, and has an oxygen partial pressure of 10 ^-20.
Firing at a temperature of 1500 ° C. or more in an atmosphere of 10 to ⁴ atm,
A method for producing a sintered body for a thermistor, comprising heat-treating for at least 0 hour. 3. The method for producing a sintered body for a thermistor according to claim 2, wherein the firing is performed using a heater made of a material other than a carbon-based material.