Thermistor element

A thermistor element in which in a thermistor body having first and second outer electrodes formed on a pair of its end surfaces, a first inner electrode connected to the first outer electrode and a second inner electrode connected to the second outer electrode are so disposed that their ends are opposed a predetermined distance away from each other.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to thermistor elements, and more 
particularly, to a thermistor element which is suitably used as a surface 
mounted type tip component and is subject to small variations in 
resistance value and B-value. 
2. Description of the Prior Art 
One example of NTC (negative temperature coefficient) thermistor elements 
conventionally used as a surface mounted type tip component is shown in 
FIG. 2. An NTC thermistor element 1 has a structure in which electrodes 3 
and 4 are formed on both end surfaces of a single plate type thermistor 
body 2. Used as the thermistor body 2 is a ceramic sintered body obtained 
by cutting to a predetermined size a ceramic wafer obtained by slicing a 
ceramic sintered body block. 
The above described NTC thermistor element 1 has been conventionally 
fabricated in the following manner. More specifically, ceramic powder for 
forming the thermistor body 2 is first calcined, to obtain a calcined raw 
material. A binder is then mixed with the calcined raw material, and a 
mixed raw material obtained is granulated. The granulated raw material is 
formed to a predetermined size, to obtain a formed body. The formed body 
is sintered and is sliced in the direction of thickness, to obtain a 
ceramic wafer having a thickness corresponding to the thickness of the 
element. The ceramic wafer is annealed at temperatures of 900 to 
1100.degree. C. and then, is cut to a predetermined size by a dicing saw 
and is barrel polished and then, outer electrodes 3 and 4 are formed on 
both end surfaces thereof. The outer electrodes 3 and 4 are formed by 
applying conductive pastes such as Ag pastes and baking the same at a 
predetermined temperature for approximately ten minutes. 
The conventional tip type NTC thermistor element obtained in the above 
described manner has not been widely applied at the present time. The 
reason for this is that the variation in resistance value of the NTC 
thermistor element 1 obtained is large, i.e., tens of percent, and the 
variation in B-value is also very large, i.e., approximately 1.0 to 2.0 
percent. Consequently, it is desired that the variation in resistance 
value and the variation in B-value are reduced. 
TABLE 1 
______________________________________ 
RESISTANCE B-VALUE 
R25.degree. C. 
R3CV B.sub.25/50 
B3CV 
CHIP SIZE (k .OMEGA.) 
(%) (k) (%) 
______________________________________ 
3.2 .times. 1.6 .times. 1.0 
15.125 13.2 3502 1.8 
2.0 .times. 1.25 .times. 1.0 
11.536 15.2 3489 1.0 
______________________________________ 
On the other hand, in the NTC thermistor element 1 shown in FIG. 2, the 
resistance value thereof is adjusted by changing the thickness of the 
ceramic body 2. More specifically, when the resistance value of the NTC 
thermistor element 1 deviates from a desired resistance value, the 
resistance value is adjusted by decreasing the thickness of the thermistor 
body 2 and specifically, the thickness of the above described ceramic 
wafer by polishing or changing the thickness to which the ceramic sintered 
body block is sliced to change the thickness of the ceramic wafer. 
Therefore, the variation in thickness is significantly large between 
manufacturing lots, so that NTC thermistor elements are forced to greatly 
vary in thickness although they have the same resistance value. As a 
result, the NTC thermistor element having a small thickness has the 
disadvantage of being, for example, chipped or cracked in the actual use. 
Furthermore, NTC thermistor elements having a series of resistance values 
are respectively constructed using different types of materials. 
Consequently, if an attempt is made to make the thicknesses of the NTC 
thermistor elements having a series of resistance values constant, one 
type of material provides only one type of NTC thermistor element having a 
definite resistance value, so that a large number of types of materials 
are required. In order to avoid this, a series of types of NTC thermistor 
elements each having a definite resistance value are made from one type of 
material by changing the thicknesses of the elements. Therefore, the NTC 
thermistor elements having a series of resistance values greatly vary in 
thickness from 0.5 to 1.3 mm 
Additionally, the conventional NTC thermistor element 1 has the 
disadvantage of easily varying in characteristics with time and having 
insufficient life characteristics because the outer electrodes 3 and 4 are 
exposed to the surfaces. 
Moreover, the variation in resistance value of the conventional NTC 
thermistor element 1 is significantly affected in construction by the 
variation in size of the thermistor body 2 and the variation in size 
between the outer electrodes. Accordingly, significantly high precision is 
required for the NTC thermistor element to take a desired resistance 
value. On the other hand, a PTC (positive temperature coefficient) 
thermistor element has the same disadvantages as those of the NTC 
thermistor element. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a thermistor element which 
is subject to small variations in resistance value and B-value, does not 
easily vary in thickness, and is superior in life characteristics. 
A thermistor element according to the present invention comprises first and 
second outer electrodes formed on opposed outer surfaces of a thermistor 
body and first and second inner electrodes respectively connected to the 
first and second outer electrodes. The first and second inner electrodes 
extend into the thermistor body and are formed in the thermistor body. In 
addition, the first and second inner electrodes are so disposed that their 
respective ends are opposed a predetermined distance away from each other. 
Remaining edges, other than edges respectively connected to the first and 
second outer electrodes, of the first and second inner electrodes are 
preferably disposed in the thermistor body, so that the periphery of the 
inner electrodes is covered tight, thereby to enhance environment 
resistance and therefore, life characteristics. 
Furthermore, the above described thermistor body is more preferably 
constructed using a monolithic type sintered body obtained by laminating a 
plurality of ceramic green sheets, along with first and second inner 
electrode materials, followed by cofiring. 
In the present invention, the first and second inner electrodes extend into 
the thermistor body, so that the resistance value of the thermistor 
element can be adjusted by adjusting the distance between the opposed ends 
of the first and second inner electrodes. Consequently, the resistance 
value of the thermistor element can be adjusted without changing the 
thickness of the element. 
Furthermore, the resistance value of the thermistor element is adjusted by 
changing the distance between the first and second inner electrodes, 
thereby making it possible to adjust the resistance value with high 
precision. 
Additionally, in a structure in which edges, other than edges respectively 
connected to the outer electrodes, of the first and second inner 
electrodes are disposed in the thermistor body, the inner electrodes are 
covered tight, thereby to enhance environment resistance and therefore, 
life characteristics. 
Moreover, when a sintered body obtained by laminating a plurality of 
ceramic green sheets, along with first and second inner electrode 
materials, followed by cofiring is used as a thermistor body, the 
variations in diameter of ceramic particles and distribution of pores can 
be reduced, and the area of the pores in the sintered body can be reduced. 
More specifically, when thin ceramic green sheets are laminated and a 
laminated body obtained is sintered to obtain a sintered body having a 
thickness corresponding to the thickness of the element, the variations in 
diameter of the ceramic particles and distribution of the pores can be 
made smaller and the sintered body obtained becomes denser, as compared 
with a case where a thick sintered body block is sliced to obtain a 
ceramic wafer. Furthermore, in the present invention, the resistance value 
is designed and controlled by adjusting the distance between the inner 
electrodes, thereby to make it possible to decrease the effect of the 
variation in size of the thermistor body and the variation in size between 
the outer electrodes on the resistance value. Consequently, it is possible 
to reduce the variation in resistance value and the variation in B-value 
of the thermistor element obtained. 
According to the present invention, the first and second inner electrodes 
are so disposed that the respective ends are opposed a predetermined 
distance away from each other. Accordingly, the resistance of the 
thermistor element can be adjusted by changing the distance between the 
first and second inner electrodes. Consequently, the resistance value of 
the thermistor element can be varied without changing the thickness of the 
element. Therefore, thermistor elements which differ in resistance value 
can be obtained although they have the same shape, thereby to make it 
possible to provide a thermistor element most suitable for a tip 
component. 
The resistance value of the thermistor element has been conventionally 
adjusted by changing the thickness of the element. Consequently, the 
thermistor elements have conventionally ranged in thickness from 0.5 to 
1.25 mm. Therefore, the thermistor element having a small thickness is 
liable to be cracked when it is mounted on a substrate, for example. In 
addition, the thermistor element may not, in some cases, reach constant 
strength in the bending test. On the other hand, in the thermistor element 
according to the present invention, the thickness thereof can be made 
constant as described above, so that the transverse strength thereof can 
be kept constant, thereby to make it possible to prevent accidents such as 
the above described cracking in the case of mounting. 
Furthermore, the resistance value can be determined by adjusting the 
distance between the first and second inner electrodes, thereby to make it 
easy to design the resistance value and make it possible to easily 
fabricate a thermistor element having a desired resistance value. 
Additionally, the inner electrodes can be formed with high precision by 
printing conductive pastes, and the resistance value is determined by the 
printing precision of the inner electrodes, thereby to make it possible to 
obtain a thermistor element having a desired resistance value with high 
precision. 
Furthermore, thermistor elements which differ in resistance value can be 
obtained by changing the distance between the inner electrodes, thereby to 
make it possible to design the thermistor elements which differ in 
resistance value even by using a small number of types of materials, and 
make it also possible to cut material cost. 
Additionally, if the edge portion, which is not connected to the outer 
electrode, of the first and second inner electrodes is disposed in the 
thermistor body, it is possible to provide a thermistor element having 
high environment resistance and therefore, having superior life 
characteristics. 
Moreover, if the thermistor body provided with the first and second inner 
electrodes is constructed by laminating ceramic green sheets and cofiring 
the ceramic green sheets, along with first and second inner electrode 
materials, the variations in diameter of ceramic particles and 
distribution of pores are smaller and the thermistor body obtained becomes 
denser, as compared with the conventional method of slicing a sintered 
body block to obtain a ceramic wafer. Consequently, it is possible to 
obtain a thermistor element which is subjected to small variations in 
resistance value and B-value. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1, 3 and 4, description is made of a thermistor element 
according to one embodiment of the present invention. In the present 
embodiment, an NTC thermistor element will be described. 
An NTC thermistor element 11 has a structure in which first and second 
outer electrodes 13 and 14 are formed on both end surfaces 12a and 12b of 
a thermistor body 12. A first inner electrode 15 and a second inner 
electrode 16 are formed so as to be in a position at the same height, that 
is, be positioned in the same plane in the thermistor body 12. The first 
inner electrode 15 is connected to the first outer electrode 13, while the 
second inner electrode 16 is connected to the second outer electrode 14. 
In addition, the first and second inner electrodes 15 and 16 are so 
disposed that their ends 15a and 16a are opposed a constant distance away 
from each other. 
Since the first and second inner electrodes 15 and 16 are provided as 
described above, the resistance value of the NTC thermistor element 11 can 
be varied by changing the distance between the ends 15a and 16a of the 
first and second inner electrodes 15 and 16. More specifically, the 
resistance value of the NTC thermistor element 11 can be adjusted by 
changing the distance between the ends 15a and 16a of the first and second 
inner electrodes 15 and 16, so that the resistance value of the NTC 
thermistor element 11 can be adjusted without changing the thickness of 
the element. In the present embodiment, therefore, the resistance value of 
the NTC thermistor element 11 can be designed with the element having 
sufficient strength, thereby to make it possible to prevent such accidents 
that the thermistor body 12 is cracked or chipped at the time of barrel 
polishing and mounting. 
Furthermore, the resistance value of the NTC thermistor element 11 can be 
varied by changing the distance between the ends 15a and 16a of the first 
and second inner electrodes 15 and 16, thereby to make it possible to 
increase the thickness of the element with the resistance value thereof 
being constant, while making it possible to design a series of resistance 
values of the element with the thickness thereof being constant, as 
compared with the conventional example. 
Meanwhile, the thermistor body 12 provided with the first and second inner 
electrodes 15 and 16 is preferably obtained by laminating a plurality of 
ceramic green sheets, along with first and second inner electrode 
materials, followed by cofiring, as in the specific examples of 
experiments as described later. In this case, by using a sintered body 
obtained by laminating a plurality of ceramic green sheets, followed by 
cofiring, the variations in diameter of ceramic particles and distribution 
of pores are smaller and the thermistor element 12 obtained becomes 
denser, as compared with the conventional method of slicing a sintered 
body block to obtain a ceramic wafer. Consequently, it is possible to 
reduce the variations in resistance value and B-value. 
In obtaining the thermistor body 12, however, the plurality of ceramic 
green sheets need not be laminated, along with the first and second inner 
electrode materials. For example, the thermistor body 12 may be 
constructed by affixing two ceramic sintered bodies through the first and 
second inner electrodes 15 and 16. 
Additionally, in the above described embodiment, edges, other than edges 
respectively connected to the outer electrodes 13 and 14, of the first and 
second inner electrodes 15 and 16 are disposed in the thermistor body 12, 
as obvious from FIG. 4. Consequently, the first and second inner 
electrodes 15 and 16 are not exposed to the exterior, thereby to enhance 
environment resistance and therefore, life characteristics. When the 
environment resistance is not particularly required, however, the edges, 
other than the edges connected to the outer electrodes, of the first and 
second inner electrodes 15 and 16 may be exposed to the exterior of the 
thermistor body 12. 
Description is now made of the specific examples of experiments. 
Prepared as a raw material is one obtained by mixing Mn.sub.3 O.sub.4, NiO 
and Co.sub.3 O.sub.4 at a weight ratio of 45:25:30. The raw material is 
calcined at a temperature of 1000.degree. C. for two hours and then, the 
raw material calcined is ground by a pulverizer. 
10 to 20% by weight of polyvinyl alcohol serving as an organic binder, 0.5% 
by weight of glycerine serving as a plasticizer, and 1.0% by weight of a 
polyvinyl type dispersant are added to the calcined raw material ground 
and are mixed for 16 hours. A mixed material obtained is changed into a 
slurry for forming a sheet by passing through a 250-mesh to remove coarse 
grains. The slurry obtained is formed by the Doctor blade process, to 
fabricate a ceramic green sheet having a thickness of 50 .mu.m. This 
ceramic green sheet is cut to a predetermined size, and conductive pastes 
for forming the inner electrodes 15 and 16 shown in FIG. 4 are printed on 
the surface of one ceramic green sheet obtained by the cutting. A 
plurality of ceramic green sheets are laminated above and below the 
ceramic green sheet having the conductive pastes printed thereon, to 
obtain a laminated body having the entire thickness of 1550 .mu.m. The 
laminated body obtained is then pressed in the direction of thickness and 
is cut into a lot of laminated chips in a rectangular plane shape 
measuring 2.4 mm.times.1.5 mm. The laminated chips obtained are sintered 
at a temperature of 1200.degree. C. for two hours and then, are barrel 
polished, to obtain a thermistor body 12 shown in FIG. 1 measuring 
2.0.times.1.25.times.1.0 mm. 
Ag pastes are applied to both end surfaces of the thermistor body 12 
obtained and are baked at a temperature of 850.degree. C. for ten minutes, 
thereby to form first and second outer electrodes 13 and 14 to obtain an 
NTC thermistor element 11. 
Meanwhile, as the above described NTC thermistor element 11, five types of 
NTC thermistor elements are fabricated by printing conductive pastes made 
of a material having a specific resistance .rho..sub.25 =500 .OMEGA.cm 
such that the distance between the ends 15a and 16a of the first and 
second inner electrodes 15 and 16 becomes 0.3 mm, 0.4 mm, 0.5 mm, 1.0 mm 
and 1.4 mm after sintering. 
Measurements are made on the resistance values and the B-values at a 
temperature of 25.degree. C. of the five types of NTC thermistor elements 
measuring 2.0.times.1.25.times.1.0 mm obtained in the above described 
manner. The results, along with the variations in resistance value and 
B-value, are shown in Table 2. In addition, the results of Table 2 are 
shown in a graph of FIG. 5. 
TABLE 2 
______________________________________ 
DISTANCE 
BETWEEN RESISTANCE B-VALUE 
INNER R25.degree. C. 
R3CV B.sub.25/50 
B3CV 
ELECTRODES (k .OMEGA.) 
(%) (k) (%) 
______________________________________ 
0.3 2.762 4.8 3450 0.21 
0.4 3.179 5.2 3452 0.19 
0.5 3.564 5.0 3446 0.20 
1.0 5.337 6.1 3449 0.18 
1.4 6.752 5.3 3457 0.19 
______________________________________ 
As can be seen from the results of Table 2 and FIG. 5, the larger the 
distance between the ends 15a and 16a of the first and second inner 
electrodes 15 and 16 is, the larger the resistance value is. However, the 
B-value is not greatly varied, and the variations in resistance value and 
B-value are small and stable. Consequently, it is found that NTC 
thermistor elements which differ in resistance value are obtained without 
changing the outside diameter by changing the distance between the inner 
electrodes 15 and 16. 
Various NTC thermistor elements are then fabricated by setting the distance 
between the ends 15a and 16a of the first and second inner electrodes 15 
and 16 to 0.3 mm and changing the covering depth of the outer electrodes 
13 and 14 with the side surfaces of the thermistor body 12 (a distance a 
shown in FIG. 1), and the resistance values of the NTC thermistor elements 
are measured. The results are shown in FIG. 6 and Table 3. For comparison, 
the conventional NTC thermistor element 1 measuring 
2.0.times.1.25.times.1.0 mm in Table 1 is prepared, and the change in 
resistance value thereof is examined by changing the covering depth of the 
outer electrodes. The results are also shown in FIG. 6. 
TABLE 3 
______________________________________ 
COVERING RESISTANCE 
DEPTH OF R25.degree. C. 
B3CV 
ELECTRODE (k .OMEGA.) 
(%) 
______________________________________ 
0 2.788 5.2 
0.3 2.762 6.0 
0.4 2.730 5.9 
0.5 2.680 5.4 
______________________________________ 
As can be seen from FIG. 6 and Table 3, in the NTC thermistor element 
according to the present embodiment, even if the covering depth a of the 
outer electrodes is changed, the resistance value thereof hardly varies. 
On the other hand, in the conventional NTC thermistor element, if the 
covering depth a of the outer electrodes is changed, the resistance value 
thereof greatly varies. Consequently, according to the present embodiment, 
it is possible to obtain an NTC thermistor element having a desired 
resistance value without considering the covering depth a of the outer 
electrodes which is a factor of the variation in resistance value. 
The life test is then carried out with respect to the NTC thermistor 
element according to the above described embodiment. As the life test, the 
high temperature leaving test in which the NTC thermistor element is left 
at a temperature of 120.degree. C. and the humidity leaving test in which 
the NTC thermistor element is left for a long time in an environment of 
80.degree. C. and 65 percent relative humidity are adopted, and the rate 
of change in resistance value of the NTC thermistor element is measured, 
to evaluate life characteristics. In addition, life characteristics are 
similarly evaluated with respect to the conventional NTC thermistor 
element shown in FIG. 2. The results are shown in FIG. 7. 
As can be seen from FIG. 7, in the NTC thermistor element according to the 
present embodiment, the rate of change in resistance value is very low, 
and the variation in resistance value is small and stable even if 1000 
hours have elapsed in any one of the high temperature leaving test and the 
humidity leaving test. On the other hand, in the conventional NTC 
thermistor element, the rate of change in resistance value is 
significantly raised with the elapse of time, and the variation in 
resistance value is large. 
NTC thermistor elements of the same size as the above described size are 
then fabricated by altering the conductive material composing the first 
and second inner electrodes 15 and 16 as shown in Table 4 and setting the 
distance between the first and second inner electrodes to 0.3 mm. The 
resistance values and the B-values at a temperature of 25.degree. C. and 
the variations in resistance value and B-value of the respective NTC 
thermistor elements obtained are also shown in Table 4. 
TABLE 4 
______________________________________ 
TYPE OF RESISTANCE B-VALUE 
INNER R25.degree. C. 
R3CV B.sub.25/50 
B3CV 
ELECTRODE (k .OMEGA.) 
(%) (k) (%) 
______________________________________ 
Ag--Pd (7:3) 
3.021 5.3 3449 0.20 
Pt--Au 2.793 6.6 3443 0.18 
Pd 2.961 5.8 3450 0.12 
Pt--Au--Pd 2.833 4.9 3447 0.17 
______________________________________ 
As can be seen from Table 4, it is possible to provide an NTC thermistor 
element which is subject to small variations in resistance value and 
B-value and is superior in reliability, similarly to the results shown in 
Table 2 even if the material composing the inner electrodes is altered. 
Characteristics in a Case Where the Number of Inner Electrodes is Changed 
As shown in FIGS. 8A and 8B, NTC thermistor elements 21 and 31 in which a 
plurality of first and second inner electrodes are respectively formed are 
fabricated using the same material as that of the NTC thermistor element 
shown in FIG. 1. The distance between the first and second inner 
electrodes is set to 0.5 mm. Measurements are made on the resistance 
values at a temperature of 25.degree. C. and the variations in resistance 
value of the NTC thermistor element 11 according to the present embodiment 
shown in FIG. 1 and the respective NTC thermistor elements shown in FIGS. 
8A and 8B. The results are shown in Table 5. 
TABLE 5 
______________________________________ 
RESISTANCE 
NUMBER OF R25.degree. C. 
R3CV 
ELECTRODES (k .OMEGA.) 
(%) 
______________________________________ 
1 4.215 5.1 
2 3.864 5.4 
3 3.402 4.9 
______________________________________ 
As can be seen from Table 5, even if a plurality of inner electrodes are 
formed in the direction of thickness, the variation in resistance value is 
very small. 
Other Modified Examples 
As shown in FIG. 4, the first and second inner electrodes 15 and 16 are 
formed in the same rectangular shape. As shown in FIG. 9, however, the 
first and second inner electrodes 15 and 16 may be respectively formed in 
a shape other than the rectangular shape by forming concave portions 15b 
and 16b. In addition, as shown in FIG. 10, the widths of the first and 
second inner electrodes 15 and 16 may be made different from each other. 
Further, the lengths of the first and second inner electrodes 15 and 16 
may be made different from each other. 
Furthermore, as shown in FIG. 11, one of the inner electrodes 15 may be 
divided into a plurality of electrode parts 151 to 153. 
Additionally, as shown in FIG. 12, the first and second inner electrodes 15 
and 16 may be formed on different planes. 
Although description was made of an NTC thermistor element by way of 
example, the present invention can be also applied to a PTC thermistor 
element. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.