JPS5821803B2 - High temperature thermistor - Google Patents

Description

[Detailed description of the invention] The present invention relates to a high temperature thermistor.

Thermocouples and ceramic thermistors are conventionally known as temperature sensing elements for high temperatures.

Among these, ceramic thermistors are being put into practical use because they are excellent in stability, have a long life, and are inexpensive.

Such materials are described, for example, in JP-A-48-705 and JP-A-48-49.
-29493, 49-63995 and Review S
ci, In5tr, 40(4), 544(
(1969), etc., and it has been put into practical use in some cases and is used for temperature control of thermal reactors for purifying automobile exhaust gas, but it has unstable life characteristics and thermal instability. There's a problem.

The present invention aims to provide a thermistor that is stable in use at high temperatures and is chemically and thermally stable.

In the present invention, the composition constituting the temperature-sensitive resistor is NgO.

It is characterized by containing Cr2O3 and TiO2 as main components.

This composition has excellent thermal properties.

Note that the material containing MgO and Cr2O3 is used as the main component of so-called maguro brick,
It is well known that it is extremely useful and excellent as a material for high-grade refractory bricks.

On the other hand, a material obtained by adding Ti02 to the above two components has characteristics of being more thermally stable and less susceptible to being immersed in acids.

Regarding electrical properties, for example, MgOCr2O3A7
203 Regarding Fe2O3 system, JP-A-49-63995
Of course, the Mg0-TiO system has not been investigated, although it is briefly mentioned in .

The present inventors created a sintered body containing Mg02Cr203 and TiO2 as main components and studied its electrical properties, and found that it has excellent characteristics as a material for a high-temperature thermistor.

Here, the method for producing the above-mentioned fired product will be explained. This fired product can be produced, for example, by a general ceramic method.

That is, each powder of MgO, Cr2O3, and TiO2 is weighed, thoroughly mixed in a ball mill, etc., dried, humidified, granulated, molded into a desired shape, and sintered.

4 It is desirable that the raw materials used at this time have high purity;
9.9% or more is good.

Further, in order to obtain uniformity of the sintered body, it is desirable that the raw material be fine particles, be calcined, or be a coprecipitated raw material.

In addition, the temperature of calcination may be 1100 to 1300°C.

The temperature of the main firing is preferably 1400°C or higher.

A dense sintered body can be created even at a low temperature of about 1000°C using a hot pressing method or the like.

Generally, a composition of MgO and Cr 203 is known as a difficult-to-sinter material, and can be heated at very high temperatures, such as 1600°C or higher.
However, it has a fairly large porosity.

The present inventors have found that by adding TiO2 (approximately 0.05% addition is effective), sintering is accelerated and the sintering is made sufficiently dense even at low temperatures.
It has been discovered that even at 0°, the resistance and B constant can be increased, and the characteristics as a thermistor can be improved.

Note that the composition changes slightly due to evaporation of Cr2O3, so it goes without saying that it is a good idea to make this correction at the time of weighing.

As shown in FIG. 1, a temperature-sensitive resistor 11 made of the obtained sintered body or a cut piece thereof is embedded with lead wires 13 in a base 12 that has sufficiently low electrical conductivity even at high temperatures.
For example, a thermistor element is made by bonding with Ru02 paste or platinum paste and baking.

Here, by thermal spraying directly onto one end surface of the base 12,
.

By forming a composition based on MgO, Cr2O3 and T t 02, a thermistor element can also be manufactured very simply and at low cost.

As shown in FIG.

It has a very simple structure compared to the previous example.

In addition, when the thermistor dice is brought into contact with high-temperature reducing gas, MgO is used as shown in Fig. 2.

Temperature sensitive resistor 21 made of Cr2O3 and T i 02
A protective film is applied by thermally spraying alumina etc. onto the surface.

24 and covering the resistor 21 is useful.

In FIG. 2, 22 is a base body, and 23 is a lead wire.

The present inventors have also found that when the porosity of a composition containing Mg02Cr203 and TiO2 as main components is set to 5% or less, there are no continuous pores and the composition becomes stable.

As mentioned above, the composition having a porosity of 5% or less was stable even when used in an exposed state without being covered with a protective film.

The ratio between each component in a composition containing Mg O, Cr 203 and T t 02 as main components is Mg O: Cr
203 is preferably between 45:55 and 55:45.

Next, regarding the component ratio of T i 02, MgO and C
The ratio of TiO2 to the total amount of r2O3, that is, MgO
It is desirable that TiO2 is included so that the ratio of +Cr203:Tt02 is 45-55.

In the above composition range, as shown in Figure 3, the specific resistance gradually changes from about 100 to 1000 Ω-
The B constant from 0°C to 950°C is between 8,000 and 15,000, and even if there is slight variation in the composition, for example due to evaporation of Cr 203 during thermal spraying, the characteristics will not change much. and its characteristics are stable.

Another feature is that the B constant at 600 to 1000°C is almost constant.

Beyond this composition range, excess MgO and 20g of Cr
This is not preferable because the precipitation of is noticeable and the properties become unstable.

Furthermore, if TiO2 exceeds the above composition range, an unknown phase will be seen even by X-ray diffraction, making the properties unstable, which is not preferable.

Furthermore, when TiO2 is not included, the B constant and resistance are low, so it is not preferable to use at temperatures of 600° C., particularly 900° C. or higher.

As is clear from the above description, the present invention has features such as excellent thermal and chemical stability, simple structure, and long life.

Below, non-limiting examples of the invention will be described.

Example l Mg020 mol% y Cr20325 mol%, T
Each powder raw material was weighed so as to have a composition of 0.255 mol%, mixed in a ball mill for 16 hours, and then
After calcination at 200°C, pulverization, granulation, and molding into pellets, main calcination was performed at 1630°C in air for 2 hours.

The characteristics of the fired body thus obtained are as shown by line 1 in FIG.

Note that the porosity of the fired body was 3%.

Example 2 The ratio of Mg0:Cr203:Ti02 is 29:36:35, 40:40:20, 45:45:
Each raw material was weighed to give four types of molar ratios of 10°47:43:10, and pellets were fired in the same manner as in Example 1.

The fired pellets showed the characteristics shown by lines 2, 3, 4, and 5 in FIG. 3 in the order of the four types of molar ratios.

Example 3 Mg045morph s Cr2Q 345 mol%.

A composition consisting of 10 mol% of TtQ2 was prepared as N2-0210.
% atmosphere, plasma spraying was carried out to produce an element as shown in FIG.

Only the tip of this was annealed at 1000° C. and its properties were measured. As Cr 203 had evaporated a little, it showed properties close to line 6 in FIG. 3.

Example 4 The fired pellets and thermally sprayed compositions obtained in Examples 1 to 3 were heated in air at 1000°C for 500 hours, and for 100 hours.
After leaving it in a nitrogen atmosphere at 0°C for 500 hours, we re-measured its characteristics and found that the re-measured values of resistance and B constant were within ±0.5% of the initial values, making it extremely stable. Ta.

[Brief explanation of the drawing]

FIGS. 1 and 2 are cross-sectional views showing examples of the thermistor for a cylindrical portion according to the present invention, and FIG. 3 is a diagram showing the characteristics of the thermistor. 11.21...Temperature-sensitive resistor, 12,22...
... Base body, 13, 23 ... Lead wire, 24.
...Protective film.

Claims (1)

[Scope of Claims] A high temperature thermistor characterized in that a temperature sensitive resistor is made of a composition whose main components are I MgO, Cr2O3, and T i O2. 2. The high temperature thermistor according to claim 1, wherein the composition has a porosity of 5% or less. 3 The molar ratio of MgO and Cr2O3 is 45:55 to 5
2. The high temperature thermistor according to claim 1, wherein Ti02 is in a range of 5:45 to 55 moles or less to 45 moles of MgO and Cr203 combined. 4. The high-temperature thermistor according to claim 1, characterized in that a temperature-sensitive resistor is provided at one end of the high-temperature insulator in which a conductor is embedded. 5. The high-temperature thermistor according to claim 1, wherein the outer surface of the temperature-sensitive resistor is coated with a protective film.