JPH07335404A - Manufacture of positive temperature coefficient thermistor - Google Patents

Abstract

PURPOSE:To obtain a positive temperature coefficient thermistor having excellent electric characteristics, especially having a large positive resistance temperature coefficient and a high breakdown voltage, to be used as an exothermic substance or switching device. CONSTITUTION:This manufacturing method of a positive temperature coefficient thermistor is characterized by adding an excessive amount, 0.5-3.0mol%, of TiO2 powder against 1mol of a main component, to a calcined substance mainly composed of barium titanate and its solid solution and containing a very small amount of semiconductorizing elements, Si, Mn, and Al added. Besides, the ratio of DA to DB DA/DB is controlled into a range 0.5-1.0, when the average particle diameter of the calcined crushed powder at that time is represented by DA, and that of the excessive TiO2 powder by DB.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a positive temperature coefficient thermistor having a resistance value rapidly increasing at a specific temperature, and more particularly to a method of manufacturing a positive temperature coefficient thermistor having a large resistance temperature coefficient and a large withstand voltage.

[0002]

2. Description of the Related Art Barium titanate is converted into a semiconductor by adding a trace amount of a rare earth element such as Y, La or Ce or a transition metal such as Nb or Ta, and has a positive resistance temperature characteristic (Positive Temperature Coefficient; PTC
It is widely known in the past to exhibit the characteristics. That P
Utilizing the TC characteristics, it has been applied to various applications such as an overcurrent protection element, a temperature control element, a motor starting element, and a heater element.

On the other hand, as a method of manufacturing such a positive temperature coefficient thermistor, the following method is generally used. First, the blended raw materials are mixed using a ball mill, a disper mill or the like, dehydrated and dried with a filter press, a drum dryer or the like, and then these mixed powders are calcined. Next, the calcined powder is pulverized by a ball mill or the like, a binder such as polyvinyl alcohol is added to form a slurry, and the slurry is granulated by a spray dryer or the like to form a desired shape. Furthermore, this molded body is usually placed in the air at 1300 to 1400 ° C.
Main firing is carried out at a high temperature, and electrodes are applied to the obtained sintered body to obtain the final product.

[0004]

One of the characteristics required for such a positive temperature coefficient thermistor is to increase the withstand voltage.

This is (1) application to high voltage circuits,
(2) This is because there are advantages such as miniaturization of the element (when the rated voltage is the same).

In order to achieve this improvement in withstand voltage, it is necessary to make the crystal grain diameter of the ceramics itself as small as possible, and various studies have been conventionally conducted in terms of its composition or process. .

In terms of composition, the grain size can be suppressed by adding a grain growth suppressing element such as Ca, but there is a drawback that the resistance value at room temperature becomes large.

On the other hand, from a process standpoint, it is possible to suppress grain growth by setting the firing temperature as low as possible, but there is a problem that the PTC characteristics, particularly the decrease of the temperature coefficient of resistance becomes remarkable. Occurs.

In addition to such a problem of withstand voltage, when a positive temperature coefficient thermistor is used in a heater-related product for temperature control, if the working voltage is different, the temperature coefficient of resistance decreases, which is desirable. Problems such as not showing the exothermic temperature of occurs.

In view of the above points, it is an object of the present invention to provide a positive temperature coefficient thermistor having a large temperature coefficient of resistance and a high withstand voltage.

[0011]

In order to achieve the above-mentioned object, the present invention provides a main component composed of barium titanate or a solid solution thereof with a rare earth element or an oxide of Nb, Sb or Bi as a semiconducting element. A raw material containing at least one of these, and further added with SiO ₂ , MnO ₂ , and Al ₂ O ₃ is calcined, and when pulverized, TiO ₂ is further added, and then molded and fired, and then an electrode is provided. It is a thing.

[0012]

Next, the operation of the present invention will be described.

It has long been pointed out by Heywang that the mechanism of manifestation of PTC characteristics is due to grain boundaries with potential barriers.

At temperatures above the Curie point, the barrier increases exponentially with increasing temperature. The grain boundary resistance is shown by the following equation.

P = p ₀ exp (φ / kT) (1) where p ₀ , k: constant φ: potential barrier height T: absolute temperature Further, the barrier height φ is φ = e ² Ns. ^{_{2 / 2ε 1 · ε 0 ·}} Nd (2) e: electron charge, Ns: acceptor density Nd: donor concentration, epsilon _1: a dielectric constant epsilon _0: the dielectric constant of a vacuum.

On the other hand, it is known that the crystal system undergoes a phase transition from a tetragonal perovskite structure to a cubic system around the Curie point.

Therefore, to increase the temperature coefficient of resistance in the PTC characteristic means to increase φ in the equation (1), that is, to increase Ns in the equation (2), and also in terms of crystal structure. Are required to suppress the production of substances that inhibit the phase transition around the Curie point and increase the phase transition rate.

Based on such a viewpoint, the present invention has been achieved as a result of earnest research on the composition side and the process side. The operation will be described below.

In the composition of the present invention, Al, Mn, and Si are components that segregate particularly in the grain boundary portion and greatly affect the PTC characteristics. By adding a slight excess of Ti to this, Al,
It is considered that the acceptor concentration is increased by reacting with Mn, and as a result, Ns in the equation (2) is increased and the resistance temperature coefficient is increased. Furthermore, in the manufacturing process, by adding excess Ti after calcination, it is possible to more effectively develop this function selectively only at the grain boundaries. On the other hand, from the crystal structure, PTC produced by a normal manufacturing process
By-products found in the device (Ba ₄ Ti ₁₃ O ₃₀ phase and Ba ₂
Although formation of liquid phase components such as TiSi ₂ O ₈ phase) is already recognized during the calcination, the production method of the present invention suppresses the formation of these by-products and thus the phase transition rate is increased. Conceivable. Further, it is considered that since the generation of the liquid phase component described above is suppressed, the grain growth at the time of sintering is suppressed, and the crystal grain size is made fine, so that the withstand voltage is improved.

As described above in detail, by using the method for manufacturing a PTC thermistor of the present invention, a PTC thermistor having a large PTC characteristic, particularly a large temperature coefficient of resistance and a suppressed crystal grain size can be obtained.

Therefore, since the PTC characteristic has little voltage dependency and the withstand voltage is improved, it can be applied to further miniaturization and higher voltage.

[0022]

EXAMPLES The present invention will be described below with reference to examples.

(Example 1) (Ba _0.8 Pb _0.1 Ca _0.1 )
TiO ₃ + 0.002Y ₂ O ₃ + 0.02SiO ₂ +0.0
Barium carbonate (BaCO ₃ ) as a raw material so as to have a composition of 005 MnO ₂ + 0.01Al ₂ O ₃ ,
Titanium oxide (TiO ₂ ), lead oxide (PbO), calcium carbonate (CaCO ₃ ), yttrium oxide (Y ₂ O ₃ ),
Silicon dioxide (SiO ₂ ), manganese dioxide (MnO ₂ ) and aluminum oxide (Al ₂ O ₃ ) were weighed. next,
These are wet mixed in a ball mill and then dried,
It was calcined at 0 ° C. for 2 hours. Then, excess TiO ₂ was added to the calcined powder, and the mixture was wet pulverized with a ball mill and dried.

Next, 5% of polyvinyl alcohol as a binder was added to this pulverized powder, and the mixture was granulated and press-molded at a pressure of 800 kg / cm ² . This molded product is about 135 in air
Fired at 0 ° C for 1 hour, diameter 20mm, thickness 2.0mm
A disk-shaped sintered body of was obtained. Further, this sintered body was plated with Ni, and then silver paste was applied and baked to form an electrode.

Next, various electrical characteristics of the sample thus obtained are measured. From the resistance-temperature characteristic curve, the room temperature resistance value (R ₂₅ ), the resistance temperature coefficient (α), and the withstand voltage (V _BD ) were evaluated. The results are shown in (Table 1).

[0026]

[Table 1]

Here, the temperature coefficient of resistance was determined according to the following equation. _{[Ln (R 2 / R 1} ) / (T 2 -T 1)] × 100 (% /
However, R ₁ and T ₁ are the resistance value twice that of R ₂₅ and the temperature at that time R ₂ and T ₂ ; the resistance value and the temperature at (T ₁ +10) ° C.

Sample numbers 1 to 3, 6, 9, 11, 12
Is a comparative example of the present invention. Sample number 3, 6, 9
Although the amount of excess TiO ₂ is within the range of the present invention, it was added at the time of mixing the raw materials.
The amount of iO ₂ is outside the range of the present invention.

As is clear from (Table 1), the amount of TiO ₂ added in excess is within the range of the present invention, and those added after calcination have a large temperature coefficient of resistance and a suppressed crystal grain size, resulting in a withstand voltage. Is also found to be high.

On the other hand, in the samples outside the range of the present invention, the temperature coefficient of resistance and the withstand voltage were not improved, and in particular, excess Ti
Regarding the sample numbers 11 and 12 containing a large amount of O ₂ , the room temperature resistance value becomes extremely high.

Example 2 Raw materials are blended so as to have the same composition as in Example 1 and wet mixed in a ball mill. Next, this mixture was dried and then calcined at 1100 ° C. for 2 hours. Further, 1.0 mol% of TiO ₂ was added to this calcined powder, and wet pulverization was performed with a ball mill. At this time, the average particle size of the calcined pulverized powder was D _A , and the average particle size of the excess TiO ₂ powder was D _B.

Here, the crushing time of the sample was adjusted so that the ratio D _A / D _B of D _A and D _B would be the sample numbers 1 to 9 in (Table 2).

[0033]

[Table 2]

Further, this pulverized powder was granulated, molded and fired in the same manner as in (Example 1) to form an electrode. Next, various electrical characteristics of the sample thus manufactured were measured. From the resistance-temperature characteristic, R ₂₅ , α, V
_{The BD} was evaluated. The evaluation results are shown in (Table 2).

As is clear from (Table 2), when the D _A / D _B ratio is within the range of the present invention of 0.5 to 1.0, the increase in the room temperature resistance value is suppressed and the crystal grain size is suppressed. Therefore, the temperature coefficient of resistance and the withstand voltage are improved. But D _A
When the / D _B ratio is out of the range of the present invention, R ₂₅ increases and at the same time, α and V _BD decrease. The reason for this is as follows.

It is generally considered that the solid phase reaction between barium and titanium proceeds by unidirectional ion diffusion from titanium to barium. Therefore, it is considered that the smaller the particle size of titanium, the easier this reaction proceeds. In the present invention, by making the average particle size of the calcined pulverized powder smaller than the average particle size of TiO ₂ with excessive addition, the reaction with barium can be suppressed and the reaction at the grain boundary can be carried out more effectively. ,
Furthermore, since it also has the effect of suppressing the above-mentioned by-products,
It is considered that the temperature coefficient of resistance and the withstand voltage can be further improved. However, if D _A / D _B is less than 0.5, the reaction at the grain boundaries becomes non-uniform, so the effect of the present invention is considered to be reduced.

[0037]

As is apparent from the above description, the present invention comprises barium titanate or its solid solution as a main component,
0.5-3 semiconductive dopant or Nb, Sb, at least one of the oxides of Bi, Al, Mn, to the calcined powder Si is formed by adding, further, the TiO ₂ with respect to the main component 1 mol. The thermistor element is formed by adding 0 mol% and controlling the ratio of the average particle size of the calcined pulverized powder to the excess TiO ₂ to be 0.5 to 1.0. As a result, it is possible to suppress the crystal grain size, promote uniform grain growth, and improve the withstand voltage. Further, the temperature coefficient of resistance can be improved by increasing the acceptor density at the grain boundary.

Therefore, since it can be expected to be applied to a high power circuit and miniaturization of elements, its industrial utility value is great.

Claims (3)

[Claims] 1. A main component comprising barium titanate or a solid solution thereof, and a rare earth element or Nb,
At least one of Sb and Bi oxides, and further S
When the raw material added with iO ₂ , MnO ₂ , and Al ₂ O ₃ is calcined and then crushed, TiO ₂ is further added, followed by molding,
A method for manufacturing a positive temperature coefficient thermistor, which comprises firing and finally applying an electrode. _{2. When} the average particle diameter of TiO ₂ added during pulverization is D _B and the average particle diameter of the calcined raw material is D _A ,
The method for manufacturing a positive temperature coefficient thermistor according to claim 1, wherein D _A / D _B is set to 0.5 to 1.0. 3. The method for producing a positive temperature coefficient thermistor according to claim 1, wherein the amount of TiO ₂ added during milling is 0.5 to 3.0 mol% with respect to 1 mol of the main component.