EP0694930A1

EP0694930A1 - Positive characteristic thermistor

Info

Publication number: EP0694930A1
Application number: EP94912679A
Authority: EP
Inventors: Hiroshi Kabushiki Kaisha Komatsu Sasaki; Hiroshi Kabushiki Kaisha Komatsu Inagaki; Takuji Kabushiki Kaisha Komatsu Okumura; Masatoshi Kabushiki Kaisha Komatsu Tamura; Katsuhiko Kabushiki Kaisha Komatsu Sugisawa
Original assignee: Komatsu Ltd
Current assignee: Komatsu Ltd
Priority date: 1993-04-14
Filing date: 1994-04-14
Publication date: 1996-01-31
Also published as: EP0694930A4; WO1994024680A1; KR960701453A

Abstract

A thermistor having a small specific resistance ρ₂₅ at room temperature, a large α and stable characteristics. The semiconductor which constitutes the main body of the thermistor is a BaTiO₃ perovskite-type compound. The quantity of Ti is smaller than that determined by the stoichiometric ratio, which has been preferably in excess conventionally.

Description

TECHNICAL FIELD

The present invention relates to a positive temperature coefficient thermistor, and in particular to the composition of a thermistor main body.

BACKGROUND ART

Ceramics consisting of a BaTiO₃ based perovskite type of compound generally has characteristics that are useful for electrical purposes, such as dielectric characteristics, piezoelectric characteristics, pyroelectric characteristics, and abnormal resistivity, and is widely used in a variety of electronic devices.
Of these, oxide semiconductors in which 0.1 to 0.3 at% Y, Nd, or the like has been added to BaTiO₃ have a high positive temperature coefficient and are thus referred to as positive temperature coefficient thermistors.
Because such positive temperature coefficient thermistors allow temperature regions having a high positive temperature coefficient to be adjusted with the addition of Sr, Pb, or the like, they have come to be regarded as essential in a wide variety of fields, such as low temperature heaters and circuit elements for degaussing color televisions, starting motors, preventing excess current, and measuring temperatures.
The composition and manufacturing method must be adjusted so that, as shown in Figure 3, the specific resistance at room temperature (hereinafter ρ₂₅), the resistance-temperature coefficient (hereinafter α), and the resistance variation (hereinafter J) are compatible with the intended purpose during actual use.
Examples of known methods include the method in which an element such as Mn that controls the surface level of the grain boundary is added in order to increase the resistance variation J and enhance the resistance-temperature coefficient α (Japanese Patent Publications 41-12146 and 42-3855), the method in which SiO₂ is added to lower the specific resistance ρ₂₅ at room temperature (Japanese Patent Publication 51-19599), and the method in which an excess amount of Ti is used (Japanese Patent Publication 41-21869).
A low specific resistance ρ₂₅ at room temperature, a high resistance-temperature coefficient α, and a high resistance variation J are required, however, when BaTiO₃ based positive temperature coefficient thermistors are actually used, but there is a positive correlation between the ρ₂₅ and α, and despite efforts to obtain a thermistor with a low ρ₂₅ and high α, the value (α/log ρ₂₅) has been limited to around 10.
The aforementioned conventional positive temperature coefficient thermistors suffer from drawbacks in that the material must contain Pb, which has a high vapor pressure, so that when sintered at 1000°C or higher, large amounts of Pb vapor are produced, which is extremely harmful to the environment.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a thermistor having the stable characteristics of a low specific resistance ρ₂₅ at room temperature and a substantial resistance-temperature coefficient α.
A second object of the present invention is to provide a positive temperature coefficient thermistor that consumes low levels of electrical power, that does not produce Pb vapor, and that is highly reliable.
To achieve the first object, the present invention is characterized in that the amount of Ti is lower than the stoichiometric ratio in the composition of the semiconductor constituting the main body of the thermistor, which is a BaTiO₃ based perovskite type of compound, a composition in which an excess amount of Ti has been preferred.
That is, it is characterized by comprising a thermistor main body consisting of a barium titanate-based semiconductor formed in such a way that the following formula is satisfied, and electrodes for providing electricity, which are attached to the thermistor main body.

(Ba_1-x-yS_xM_y) Ti_zO₃ + nA O≦x<1, 0<y<1, 0.99≦z<1, 0≦n<0.002 (formula)

S: at least one element selected from Sr, Sn, Zr, Ca, and Pb
M: at least one element selected from Nb, Ta, Bi, Sb, Y, La, Nd, W, Th, Ce, Sm, Gd, and Dy
A: at least one element selected from Mn, Fe, Cu, Cr, F, Cl, Br, K, and V
As a result of extensive tests in which the composition ratios were changed, the inventors discovered that a favorable composition was one in which the amount of Ti is lower than the stoichiometric amount. The present invention was made taking note of this, which is able to provide a thermistor having the stable characteristics of a low specific resistance ρ₂₅ and a high α by using this composition.
The element S in the composition primarily functions to control the Curie temperature, the element M primarily functions in fashioning a semiconductor of the composition, and the element A is believed to control the surface level of the grain boundary.
To achieve the second object, the present invention is characterized in that the composition of the semiconductor constituting the thermistor main body is composed of the following barium titanate-calcium titanate-based semiconductor.

(Ba_1-x-yCa_xY_y) Ti_(1+z) O₃+pSiO₂ + qMn 0.01≦x≦0.2, 0.002≦y≦0.006, 0.001≦z≦0.010, 0.005≦p≦0.03, 0.0005≦q≦0.0015 (formula)

That is, it is characterized in that titanium is added in excess of the stoichiometric ratio and no Pb is added in a conventional barium titanate-strontium titanate-based semiconductor.
As a result of extensive testing in which the composition was changed, the inventors discovered that a positive temperature coefficient thermistor consuming low levels of electrical power could be obtained by using this composition, and the present invention was undertaken in light of this.
The Ca in the above composition effectively refines the crystal grains and enhances the withstand voltage characteristics. The compositional range is 0.01 ≦ x ≦ 0.2 because the characteristics are not effectively improved when x is less than 0.01, and the specific resistance of the element is increased when x is greater than 0.2, making its use impractical.
Y effectively endows the barium titanate with semi-conductivity. The compositional range must be 0.002 ≦ y ≦ 0.006 to achieve a specific resistance having a low value for practical purposes (10 to 1 kΩ·cm).
An amount of Ti in excess of the stoichiometric composition has the most effect on the consumption of electrical power because the grain diameter of the crystal grains and the resistance-temperature coefficient of the element are greatly affected. Here, the Ti compositional range is 0.001 ≦ z ≦ 0.010 because the resistance-temperature coefficient is diminished when lower than 0.001, whereas the crystal grains grow to abnormal lengths when greater than 0.01, and the element resistance during the application of voltage is lowered as a result of the varistor effect, causing greater consumption of electrical power when current is applied. The range above was established for these reasons.
The SiO₂ is effective in lowering the sintering temperature and in suppressing the growth of the crystal grains to abnormal grain lengths. The range is 0.005 ≦ p ≦ 0.03 because of the inadequate results in suppressing abnormal grain growth when lower than 0.005, whereas the crystal grains also grow to abnormal grain lengths when greater than 0.03.
The Mn effectively increases the resistance-temperature coefficient at or beyond the Curie temperature. Because the addition of the Mn increases the specific resistance, however, the compositional range must be 0.0005 ≦ q ≦ 0.0015 in order to obtain the effect described above in the practical range for the specific resistance (no more than 1 kΩ·cm) of the element.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a first practical example of the present invention;
Figure 2 depicts a relation between resistance-temperature coefficient and specific resistance at room temperature and the compositional ratio of a conventional example of a positive temperature coefficient thermistor and a practical example of the present invention;
Figure 3 depicts abnormal resistivity of a positive temperature coefficient thermistor;
Figure 4 depicts a second practical example of a positive temperature coefficient thermistor of the present invention; and
Figure 5 depicts a circuit for measuring the electrical power consumed by a positive temperature coefficient thermistor in a practical example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The practical examples of the present invention will now be explained in detail using the drawings as reference.

[Practical Example 1]

Figure 1 depicts a practical example of a positive temperature coefficients thermistor in the present invention.
This positive temperature coefficient thermistor is characterized in that the composition of a thermistor main body 1 is composed as shown by the following formula.

(Ba_0.7737Sr_0.2221Y_0.0042) Ti_0.9955O₃ + 0.001Mn (formula)

That is, the positive temperature coefficient thermistor comprises the thermistor main body 1 of the composition described above consisting primarily of barium titanate; first electrode layers 2a and 2b consisting of Ni vapor deposited layers formed on the top and bottom of the main body in such a way that the edges are somewhat short of the outer circumferential edge of the main body; and second electrode layers 3a and 3b consisting primarily of silver and formed as the upper layer on the first electrode layers 2a and 2b so that the edges are flush with those of the first electrode layers.
A process for manufacturing this positive temperature coefficient thermistor is described below.
A TiCl₄ source solution was first diluted with distilled water to increase the volume two-fold, part of the TiCl₄ was hydrated, and a TiCl₄ hydrate with a concentration of 239.6 g per mol Ti was thus compounded. Here, the concentration was quantified by the cupferron analysis method.
238.3g of this TiCl₄ hydrate (Ti: 0.994 mol) was taken, and another 400 g distilled water was added to fashion a TiCl₄ aqueous solution.
1145 g of a 12.7 wt% BaCl₂ aqueous solution (Ba: 0.595 mol), 479.2 g of a 16.5 wt% SrCl₂ aqueous solution (Sr: 0.297 mol), and 104.7 g of a 4.53 wt% YCl₃ aqueous solution (Y: 0.0156 mol) were compounded, and the four types of aqueous solutions were mixed.
The mixed solution was added in the form of drops over 4 hours to 1440 g of a 16.7 wt% H₂C₂O₄ (oxalic acid) aqueous solution (H₂C₂O₄:1.902 mol) maintained at 75°C ± 0.5°C to obtain an oxalate comprising (BaSrY) TiO (C₂O₄)₂·4H₂O in the form of a coprecipitate.
The oxalate was filtered and washed with water, and then prefired for 1 hour at 1150°C. Mn(NO₃)₂ was added to the prefired powder in an amount of 0.1 mol% with respect to the total number of mols of the (BaSrY) TiO₃, followed by 1 hour of wet pulverization in a planet-type ball mill to obtain a slurry.

The granulated powder was then prepared to no more than 60 µm with a spray dryer, molded into pellets with a hydraulic press, and fired for 1 hour at 1410°C to yield a semiconductor ceramics. Electrodes were mounted on the ceramics, and the temperature-resistance characteristics were measured. The results are shown in Table 1.

Table 1

Temperature-resistance characteristics in practical examples and comparative examples.
Example	Ti/(Ba+Sr+Y )	ρ₂₅(Ω·cm)	α(%)	J(Digit)	α/logρ₂₅
Pract. Ex.1	0.9955	13.7	24.2	6.06	21.3
Pract. Ex.2	0.9961	12.5	24.0	6.04	21.9
Pract. Ex.3	0.9964	26.2	29.1	6.02	20.5
Pract. Ex.4	0.9972	18.5	29.3	6.15	23.1
Comp. Ex.A	1.0033	28.1	14.3	6.43	9.9
Comp. Ex.B	0.9879	5.73	7.8	4.15	10.3

[Practical Example 2]

The composition for a thermistor main body 1 was prepared as shown by the following formula.

(Ba_0.7744Sr_0.2217Y_0.0039)Ti_0.9961O₃ + 0.001 Mn (formula)

A process for manufacturing this positive temperature coefficient thermistor is described below.
A TiCl₄ source solution was diluted with distilled water in the same manner as in Practical Example 1, A TiCl₄ hydrate with a concentration of 245.0 g per mol Ti was prepared, 122.1 g of this TiCl₄ hydrate (Ti: 0.499 mol) was taken, and 200 g distilled water was added to prepare a TiCl₄ aqueous solution.
573.3 g of a 12.8 wt% BaCl₂ aqueous solution (Ba: 0.400 mol), 239.7 g of a 16.6 wt% SrCl₂ aqueous solution (Sr: 0.149 mol), and 51.8 g of a 3.50 wt% YCl₃ aqueous solution (Y: 5.98 × 10⁻³ mol) were mixed into the aqueous solution above, and the resulting mixed solution was added in the form of drops over 2.5 hours to 720.3 g of a 16.7 wt% H₂C₂O₄ aqueous solution (H₂C₂O₄: 0.954 mol) maintained at 75°C ± 0.5°C to obtain an oxalate.
Elements were manufactured in the same manner as in Practical Example 1, and the characteristics were measured. The results are shown in Table 1.

[Practical Example 3]

The composition for a thermistor main body 1 was prepared as shown by the following formula.

(Ba_0.7731Sr_0.2228Y_0.0041)Ti_0.9964O₃ + 0.001 Mn (formula)

A thermistor element of this composition was manufactured by the same method as in Practical Example 2, and the characteristics were measured, the results of which are shown in Table 1.

[Practical Example 4]

The composition for a thermistor main body 1 was prepared as shown by the following formula.

(Ba_0.7707Sr_0.2254Y_0.0039)Ti_0.9972O₃ + 0.001 Mn (formula)

A thermistor element of this composition was manufactured by the same method as in Practical Examples 2 and 3. The results are shown in Table 1.

[Comparative Example A]

The composition for a thermistor main body 1 was prepared as shown by the following formula as a comparative example.

(Ba_0.7778Sr_0.2180Y_0.0042)Ti_1.0033O₃ + 0.001 Mn (formula)

This composition was outside the upper limits of the compositional range for Ti.
A process for manufacturing this positive temperature coefficient thermistor is described below.
First, a TiCl₄ hydrate with a concentration of 244.5 g per mol Ti was added to 243.4 g of a TiCl₄ hydrate (Ti: 0.9954 mol), and 400 g distilled water was then added to fashion a TiCl₄ aqueous solution.
The concentrations and amounts of the BaCl₂ aqueous solution and other aqueous solutions mixed into this TiCl₄ aqueous solution were the same as in Practical Example 1. The method for manufacturing the element after the mixed aqueous solutions were added in drops was also the same as in Practical Example 1. Table 1 shows the results of measuring the characteristics of this thermistor element.

[Comparative Example B]

The composition for a thermistor main body 1 was prepared as shown by the following formula as a comparative example.

(Ba_0.7703Sr_0.2249Y_0.0048)Ti_0.9879O₃ + 0.001 Mn (formula)

This composition was outside the lower limits of the compositional range for Ti.
The positive temperature coefficient thermistor was then manufactured in the same manner as in Practical Example 1 except for the use of an H₂C₂O₄ (oxalic acid) aqueous solution maintained at 70°C ± 0.5°C. The results of measuring the characteristics of this thermistor element are shown as Comparative Example B in Table 1.
Table 1 clearly shows that the manufacture of a positive temperature coefficient thermistor having a composition in which the Ti compositional range Ti_z defined in the Claims is 0.99 ≦ z < 1 results in a low ρ₂₅ of 12.5 to 26.5 Ωcm, an α of more than 24.00%, and a value J of around 6 digits. As the results of measuring the relation between the amount of Ti (Ti/Ba+Sr+Y) and α/log ρ₂₅ in Figure 2 show, α/log ρ₂₅ was about 10 in the conventional examples (comparative examples), whereas a value at least twice that value can be obtained in the present invention.
The present invention thus allows a better positive temperature coefficient thermistor with a low ρ₂₅ and a high α to be obtained.

[Practical Example 5]

Figure 4 depicts a practical example of a positive temperature coefficient thermistor in the present invention.

This positive temperature coefficient thermistor is characterized by comprising the following barium titanate-calcium titanate-based semiconductor, wherein the composition of the thermistor main body 1S has the compositional ratio shown in the following table.

(Ba_1-x-yCa_xY_y) Ti_(1+z) O₃+pSiO₂ + qM 0.01≦x≦0.2, 0.002≦y≦0.006, 0.001≦z≦0.010, 0.005≦p≦ 0.03, 0.0005≦q≦0.0015 (formula)

Table 2

Compo sition No.	PbO SrCo₃ (mol)	BaCO₃ (mol)	CaCO₃ (mol)	Y₂O₃ (mol)	TiO₂ (mol)	Mn (NO₃)₂ 6H₂O (mol)	SiO₂ (mol)	Specifi c resistan ce (Ω·cm)	Power consu mption (W)	Items
1	0	89.5	10	0.25	100.1	0.08	1.5	53.6	2.94	Pract Ex.1
2	0	89.5	10	0.25	100.3	0.08	1.5	53.3	2.68	Pract Ex. 2
3	0	89.5	10	0.25	100.5	0.08	1.5	56.0	2.82	Pract Ex. 3
4	0	89.5	10	0.25	100.7	0.08	1.5	55.5	2.84	Pract Ex. 4
5	0	89.5	10	0.25	101.0	0.08	1.5	57.1	2.99	Pract Ex. 5
6	0	89.5	10	0.25	101.5	0.08	1.5	60.4	3.25	Comp. Ex 1
7	0	9.5	10	0.25	100.0	0.08	1.5	63.2	3.10	Comp. Ex 2
8	0	84.5	15	0.25	100.3	0.08	1.5	14.2	2.95	Pract Ex 6
9	0	79.5	20	0.25	100.3	0.08	1.5	330	2.60	Pract Ex 7
10	0	74.5	25	0.25	100.3	0.08	1.5	4.3K	--	Comp. Ex 3
11	5/10	74.5	10	0.25	100.3	0.08	1.5	57.0	2.99	Comp. Ex 4

That is, the positive temperature coefficient thermistor comprises a thermistor main body 1S of the composition described above consisting primarily of barium titanate; first electrode layers 2a and 2b consisting of Ni vapor deposited layers formed on the top and bottom of the main body in such a way that the edges are somewhat short of the outer circumferential edge of the main body; and second electrode layers 3a and 3b consisting primarily of silver and formed as the upper layer on the first electrode layers 2a and 2b so that the edges are flush with those of the first electrode layers.
A method for manufacturing the positive temperature coefficient thermistor is described below.
First, commercially available TiO₂, BaCO₃, CaCO₃, and Y₂O₃ powders with a mean particle diameter of 0.3 to 2 µm were blended and wet mixed in the proportions shown in the table above, then dried, and prefired for 4 hours at a temperature of 1000 to 1200°C. The prefired powder thus obtained was blended, wet mixed, and pulverized with SiO₂ with a mean particle diameter 1 µm and Mn(NO₃)₂·6H₂O aqueous solution. An organic binder was added to the resulting slurry, and the material was granulated by a spray dryer. The granulated powder thus obtained was molded to a density of 2.5 to 3.0 g/cm³ by a hydraulic press and fired for 1 hour at 1350°C in the atmosphere, and a barium titanate-based semiconductor ceramics 200 mm in diameter and 2.5 mm in thickness was thus obtained. Commercially available Ag electrodes were printed and baked onto both sides of the semiconductor ceramics, and the specific resistance and power consumption were measured.
The power consumption was measured using the measuring circuit shown in Figure 5. In this measuring circuit, the positive temperature coefficient thermistor 10 was connected via load resistance 11 to a power source 12, this connection could be turned on and off by a switch 13, the voltage across the both ends of the positive temperature coefficient thermistor 10 was measured by a voltmeter 14 connected in parallel to the positive temperature coefficient thermistor 10, and the current flowing to the positive temperature coefficient thermistor was measured by an ampere meter 15 connected in series with the positive temperature coefficient thermistor 10. In this way, the voltage across the both ends of the positive temperature coefficient thermistor 10 and the current flowing through the positive temperature coefficient thermistor 10 were measured to calculate the power consumption. The power consumption P (W) was determined by the following equation. ${P (W) = V}_{p} (v) × I (A)$
In the table above, Composition Nos. 1 through 5, 8, and 9 indicate the present invention, while Compositions Nos. 6, 7, 10, and 11 indicate comparative examples.
A comparison of Composition Nos. 1 through 5 with 6 and 7 reveals that an excess amount of TiO₂ between 0.001 to 0.01 mol (total amount of TiO₂ 100.1 to 101.0 mol) results in a low power consumption equal to or lower than the power consumption of conventional Composition No. 11 containing Pb (3.0 W). An excess amount of Ti of 0.001 mol or less and 0.01 mol or more results in a power consumption of 3.0 W or more.
When the excess amount of TiO₂ was stabilized at 0.3 mol (total amount of TiO₂ 100.3 mol) and the amount of Ca added was changed (Composition Nos. 2, 8, 9, and 10), the addition of Ca in an amount greater than 20 mol increased the specific resistance, which was impractical. When the Ca was added in an amount of no more than 20 mol, the specific resistance was no more than 1 KΩ·cm, and the power consumption was low at no more than 3.0 W.
With this composition, which contains no Pb, a low power consumption conventionally unobtainable without the addition of Pb can be obtained by optimizing the compositional ranges for Ba, Ca, Ti, Y, Mn, and SiO₂. Since, moreover, it contains no Pb, the drawback of producing Pb vapor during manufacture can be resolved.

INDUSTRIAL APPLICABILITY

As described above, the first of the present inventions allows a positive temperature coefficient thermistor with a low ρ₂₅ and substantial α to be obtained because the amount of Ti in the composition of the BaTiO₃ based perovskite type of compound constituting the main body of the thermistor is lower than the stoichiometric ratio.
The second of the present inventions makes it possible to provide a positive temperature coefficient thermistor that has low power consumption during the application of electricity and that does not produce Pb vapor when fired because the composition of the semiconductor constituting the thermistor main body comprises a barium titanate-calcium titanate-based semiconductor containing no Pb.

Claims

A positive temperature coefficient thermistor comprising:
a thermistor main body consisting of a barium titanate-based semiconductor formed in such a way that the composition satisfies the following formula;

(Ba_1-x-yS_xM_y) Ti_zO₃+nA O≦x<1, 0<y<1, 0.99≦z<1, 0≦n< 0.002 (formula)

S: at least one element selected from Sr, Sn, Zr, Ca, and Pb
M: at least one element selected from Nb, Ta, Bi, Sb, Y, La, Nd, W, Th, Ce, Sm, Gd, and Dy
A: at least one element selected from Mn, Fe, Cu, Cr, F, Cl, Br, K, and V
and electrodes for providing electricity, which are attached to the thermistor main body.
A positive temperature coefficient thermistor comprising:
a thermistor main body consisting of a barium titanate-calcium titanate semiconductor formed in such a way that the composition satisfies the following formula;

(Ba_1-x-yCa_xY_y) Ti_(1+z) O₃+pSiO₂ + qMn 0.01≦x≦0.2, 0.002≦y≦ 0.006, 0.001≦z≦0.010, 0.005≦p≦ 0.03, 0.0005≦q≦0.0015 (formula)

and electrodes for providing electricity, which are attached to the thermistor main body.