Semiconductive ceramic composition and semiconductive ceramic device using the same

Provided is a semiconductive ceramic composition comprising a lanthanum cobalt oxide and having a negative resistance-temperature characteristic, which contains, as the side component, a chromium oxide in an amount of from about 0.005 to 30 mol % in terms of chromium, and also a semiconductive ceramic device comprising the composition. The device is usable for rush current inhibition, for motor start-up retardation and for halogen lamp protection, and is also usable in temperature-compensated crystal oscillators.

FIELD OF THE INVENTION 
The present invention relates to a semiconductive ceramic composition 
having a negative resistance-temperature characteristic and also to a 
semiconductive ceramic device comprising the composition. In particular, 
it relates to a semiconductive ceramic composition which is used to form 
devices to be used for rush-current inhibition, those to be used in 
temperature-compensated crystal oscillators, and others, and also to such 
semiconductive ceramic devices comprising the composition. 
BACKGROUND OF THE INVENTION 
Heretofore known are semiconductive ceramic devices having a negative 
resistance-temperature characteristic (hereinafter referred to as a 
negative characteristic) which are characterized in that they have a high 
resistance value at room temperature and that their resistance value is 
lowered with the elevation of the ambient temperature (such devices are 
hereinafter referred to as NTC devices). 
The NTC devices of that type are used variously, for example, in 
temperature-compensated crystal oscillators or for rush-current 
inhibition, motor start-up retardation or halogen lamp protection. 
For example, temperature-compensated crystal oscillators (hereinafter 
referred to as TCXO) comprising NTC devices are used as frequency sources 
in electronic instruments such as those for communication systems. TCXO is 
grouped into a direct TCXO which comprises a temperature-compensating 
circuit and a crystal oscillator and in which the temperature-compensating 
circuit is directly connected with the crystal oscillator inside the 
oscillation loop, and an indirect TCXO in which the 
temperature-compensating circuit is indirectly connected with the crystal 
oscillator outside the oscillation loop. The direct TCXO comprises at 
least two NTC devices and the oscillation frequency from the crystal 
oscillator is subjected to temperature compensation. In this, one NTC 
device has a low resistance value of about 30.OMEGA. or so at room 
temperature (25.degree. C.) for attaining the intended temperature 
compensation at room temperature or lower, while the other has a high 
resistance value of about 3000.OMEGA. or so at room temperature 
(25.degree. C.) for attaining the intended temperature compensation at 
temperatures higher than room temperature. 
NTC devices for rush-current inhibition are those for absorbing initial 
rush currents in electronic instruments. At the switching instant, 
overcurrents are applied to electronic instruments from a switching power 
source. NTC devices for rush-current inhibition act to prevent the 
overcurrent from breaking the other semiconductive devices such as IC and 
diodes and also halogen lamps, or from shortening the life of such devices 
and halogen lamps. After having been switched on, the NTC device of this 
type absorbs the initial rush current to thereby prevent any overcurrent 
from running through the circuit in an electronic instrument, and 
thereafter this is self-heated and thus has a lowered resistance value at 
the higher temperature. In the self-heated, steady state condition, the 
NTC device then acts to reduce the power consumption. 
NTC devices for motor start-up retardation are those for retarding the 
starting-up time for motors started up, for a predetermined period of 
time. In gear motors which are so constructed that a lubricant oil is fed 
to the gearbox after the start of the motor, the gear is often damaged due 
to the insufficient supply of a lubricant oil to the gear if the gear is 
directly rotated at a high speed immediately after the application of an 
electric current to the motor. In order to prevent the gear from being 
damaged in this case, the starting-up motion of the driving motor is 
retarded for a predetermined period of time by the use of an NTC device. 
On the other hand, in motors for driving lapping machines in which a 
grinder is rotated to polish the surface of a ceramic part, the ceramic 
part is often cracked if the lapping disc is rotated at a high speed just 
after the start of the driving motor. In order to prevent the ceramic part 
from being cracked in this case, the starting-up motion of the driving 
motor is retarded for a predetermined period of time by the use of an NTC 
device. For these, the NTC device acts to lower the voltage applied to the 
terminals of the motor being started up, and thereafter it is self-heated 
and thus has a lowered resistance value at the elevated temperature. At 
steady state, the motor is rotated at a desired speed. 
The conventional semiconductive ceramics with such a negative 
resistance-temperature characteristic that have heretofore been used for 
constructing the NTC devices such as those mentioned above comprise spinel 
oxides of transition metal elements such as manganese, cobalt, nickel, 
copper, etc. 
To attain accurate temperature compensation for the oscillation frequency 
in TCXO, it is desirable that the NTC device therein has a large degree of 
resistance-temperature dependence (hereinafter referred to as "constant 
B"). In general, the spinel oxides of transition metal elements have a 
positive relationship between the specific resistance at room temperature 
and constant B. Therefore, those having a small specific resistance at 
room temperature have a small constant B. 
On the other hand, the spinel oxides of transition metal elements having a 
large specific resistance at room temperature have a large constant B. 
Therefore, laminate structures of NTC devices may have a lowered 
resistance value even though each constitutive NTC device has a high 
specific resistance. In that manner, therefore, it may be possible to 
obtain laminated NTC devices having a large constant B. However, the 
laminated NTC devices are problematic in that their capacitance is 
enlarged, resulting in the accuracy in the temperature-compensating 
circuit comprising the NTC laminate being lowered. 
Where NTC devices are used for rush current inhibition, they must be 
self-heated to achieve the lowered resistance value at elevated 
temperatures. However, the conventional NTC devices comprising spinel 
oxides tend to have a smaller constant B if their specific resistance is 
lowered. Therefore, the conventional NTC devices are problematic in that 
they could not have a sufficiently lowered resistance value at elevated 
temperatures and therefore their power consumption at the steady state 
could not be reduced. 
For example, to satisfactorily reduce the resistance value of tabular NTC 
devices at high temperatures, their surface area may be enlarged or their 
thickness may be reduced. However, the increase in the surface area of NTC 
devices is contradictory to the reduction in their size; and the reduction 
in the thickness of NTC devices may not be acceptable in view of their 
strength. 
In order to overcome these problems, there has been proposed a monolithic 
NTC device comprising a plurality of ceramic layers and a plurality of 
inner electrodes each sandwiched between the adjacent ceramic layers, in 
which are formed a pair of outer electrodes at the sides of the laminate 
of such ceramic layers and inner electrodes. In this, the pair of outer 
electrodes are electrically and alternately connected with the inner 
electrodes. However, the space between the facing inner electrodes is 
narrow. Therefore, the monolithic NTC device is still problematic in that 
if an overcurrent (of several amps or higher) is run therethrough at the 
start of switch-on, it is often broken. 
Another NTC device has been proposed which comprises BaTiO.sub.3 and 20% by 
weight of Li.sub.2 CO.sub.3 added thereto, and which may have a rapidly 
enlarged constant B at the phase transition point (see Japanese Patent 
Publication No. 48-6352). However, since this NTC device has a large 
specific resistance of 10.sup.5 .OMEGA.cm or more at 140.degree. C., it is 
problematic in that its power consumption at the steady state is large. 
An NTC device comprising VO.sub.2 exhibiting a rapidly-varying resistance 
characteristic is characterized in that its specific resistance is lowered 
from 10 .OMEGA.cm to 0.01 .OMEGA.cm at 80.degree. C. Therefore, this may 
be advantageously used for rush-current inhibition or for motor start-up 
retardation. However, this VO.sub.2 -containing NTC device is unstable. In 
addition, since this must be produced by reductive baking followed by 
rapid cooling, its shape is limited to only beads. Moreover, since the 
acceptable current value for this is small, up to several tens mA, the NTC 
device of this type cannot be used in switching power sources or driving 
motors where a large current of several amps is used. 
V. G. Bhide and D. S. Rajoria say that rare earth-transition element oxides 
exhibit a negative resistance-temperature characteristic, having a low 
resistance value at elevated temperatures while having a small constant B 
at room temperature and having a large constant B at high temperatures 
(see Phys. Rev. B6, 3!, 1021, 1972). 
For example, the electric characteristics of devices comprising LaCrO.sub.3 
are disclosed by N. Umeda and T. Awa (see Electronic Ceramics, Vol. 7, No. 
1, 1976, p. 34, FIGS. 4 and 5). In this literature, the devices are known 
to exhibit a negative resistance-temperature characteristic. To use for 
rush-current inhibition, these LaCrO.sub.3 -containing NTC devices may be 
good, having a specific resistance of about 10 .OMEGA.cm or so at room 
temperature. However, having a constant B of smaller than 2000K, these 
LaCrO.sub.3 -containing NTC devices are still problematic in that if their 
resistance value is controlled in order to use them for rush-current 
inhibition, their power consumption at the steady state is too large with 
the result that they are heated too highly and are broken. 
Tolochko, et al. say that the substitution of a part of Co in LaCoO.sub.3 
with Cr is effective for gradually increasing the specific resistance of 
the thus-substituted LaCo/CrO.sub.3 (Izv. Akad. Nauk. SSSR, Neorg. Mater., 
Vol. 23, No. 5, 1987, page 832, FIG. 3 and lines 38 to 43). In this 
report, however, they measured the specific resistance of the materials 
only at 20.degree. C., and they did not clarify the characteristics of the 
materials comprising Cr of less than 5 mol %. 
Given the situation, we, the present inventors have assiduously made 
various experiments for producing various semiconductive ceramic 
compositions and for using them under practical conditions, while 
specifically noting oxides of rare earth elements and Co-type elements, 
especially LaCoO.sub.3. The characteristics of LaCoO.sub.3 -containing NTC 
devices are disclosed by A. H. Wlacov and O. O. Shikerowa in Ka , 32, 9!, 
1990, page 2588, FIG. 2, and page 2587, lines 36 to 42. Thus, it is known 
that LaCoO.sub.3 has a lower resistance value than GdCoO.sub.3. 
However, as compared with the conventional spinel-structured transition 
metal oxides, such oxides of rare earth elements and Co-type elements have 
a small constant B, though having a low resistance value at high 
temperatures, and therefore they have not been put to practical use in the 
art. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a semiconductive ceramic 
composition characterized by a low specific resistance at room temperature 
and by a large constant B at high temperatures, and also to provide a 
semiconductive ceramic device which comprises the composition and which 
can be used for rush-current inhibition, for motor start-up retardation, 
for halogen lamp protection and even in instruments through which large 
currents are run. 
Another object of the present invention is to provide a semiconductive 
ceramic composition having a low specific resistance and a large constant 
B at room temperature while still having a large constant B even at 
temperatures lower than room temperature, and also to provide a 
semiconductive ceramic device usable in temperature-compensated crystal 
oscillators. 
Specifically, the first aspect of the present invention is a semiconductive 
ceramic composition comprising a lanthanum cobalt oxide having a negative 
resistance-temperature characteristic, which contains, as a side 
component, chromium oxide in an amount of from about 0.005 to 30 mol % in 
terms of chromium. 
The second aspect of the invention is a semiconductive ceramic composition 
comprising a lanthanum cobalt oxide having a negative 
resistance-temperature characteristic, which contains, as a side 
component, chromium oxide in an amount of from about 0.1 to 10 mol % in 
terms of chromium. 
The third aspect of the invention is a semiconductive ceramic composition, 
which comprises, as the essential component, a semiconductive ceramic 
component of a lanthanum cobalt oxide having a negative 
resistance-temperature characteristic, and containing, as a side 
component, chromium oxide in an amount of from about 0.1 to 30 mol % in 
terms of chromium for use in a temperature compensating device. 
The fourth aspect of the invention is a semiconductive ceramic composition, 
which comprises, as the essential component, a semiconductive ceramic 
component of a lanthanum cobalt oxide having a negative 
resistance-temperature characteristic, and contains, as a side component, 
chromium oxide in an amount of from about 0.5 to 10 mol % in terms of 
chromium. 
The fifth aspect of the invention is a semiconductive ceramic composition, 
which is used to form a device for rush-current inhibition, a device for 
motor start-up retardation, or a device for halogen lamp protection. 
The sixth aspect of the invention is a semiconductive ceramic composition, 
which is used to form a device in temperature-compensated crystal 
oscillators. 
The seventh aspect of the invention is a semiconductive ceramic device 
comprising a semiconductive ceramic part having a negative 
resistance-temperature characteristic and an electrode as formed on the 
surface of said semiconductive ceramic part, which is characterized in 
that said semiconductive ceramic part having a negative 
resistance-temperature characteristic comprises a lanthanum cobalt oxide 
and contains, as a side component, chromium oxide in an amount of from 
about 0.005 to 30 mol % in terms of chromium. 
The eighth aspect of the invention is a semiconductive ceramic device 
comprising a semiconductive ceramic part, in which said semiconductive 
ceramic part comprises lanthanum cobalt oxide and contains, as the side 
component, chromium oxide in an amount of from about 0.1 to 10 mol % in 
terms of chromium. 
The ninth aspect of the invention is a semiconductive ceramic device 
comprising a semiconductive ceramic part, in which said semiconductive 
ceramic part comprises a lanthanum cobalt oxide and contains, as a side 
component, chromium oxide in an amount of from about 0.1 to 30 mol % in 
terms of chromium. 
The tenth aspect of the invention is a semiconductive ceramic device 
comprising a semiconductive ceramic part, in which said semiconductive 
ceramic part comprises a lanthanum cobalt oxide and contains, as the side 
component, chromium oxide in an amount of from about 0.5 to 10 mol % in 
terms of chromium. 
The eleventh aspect of the invention is a semiconductive ceramic device, 
which is used for rush-current inhibition, for motor start-up retardation, 
or for halogen lamp protection. 
The twelfth aspect of the invention is a semiconductive ceramic device, 
which is used in temperature-compensated crystal oscillators.

DETAILED DESCRIPTION OF THE INVENTION 
The chromium content of the semiconductive ceramic composition of the 
present invention is defined to fall between about 0.005 mol % and 30 mol 
% in terms of chromium. This is because if the chromium content is smaller 
than about 0.005 mol %, the chromium oxide added is not satisfactorily 
effective, resulting in the failure in enlarging the constant B of a 
device made of the composition. If, however, it is larger than about 30 
mol %, not only the constant B of a device made of the composition is 
smaller than that of the devices made of chromium-free compositions or 
conventional compositions having a negative resistance-temperature 
characteristic but also the specific resistance of the former is merely 
the same as that of the latter. 
In particular, the chromium content is preferably within the range between 
about 0.1 mol % and about 10 mol %, since the device comprising the 
composition that has a chromium content falling within that range may have 
a constant B of 4000K or higher at high temperatures and therefore the 
device is the most suitable for the inhibition of initial rush currents. 
The chromium content of the semiconductive ceramic composition for 
temperature compensating devices of the present invention is also defined 
to fall between about 0.1 mol % and 30 mol % in terms of chromium. This is 
because if the chromium content is smaller than about 0.1 mol %, the 
chromium oxide added is not satisfactorily effective, resulting in the 
failure in enlarging the constant B of the device made of the composition. 
If, however, it is larger than about 30 mol %, the specific resistance of 
a device made of the composition is too large. 
In particular, the chromium content is preferably within the range between 
about 0.5 mol % and about 10 mol %, since the variation in the specific 
resistance and the constant B at room temperature of a device may depend 
on its chromium content and can be small thereby resulting in the success 
in stable production of temperature-compensating devices having the most 
desirable resistance-temperature characteristic with which the oscillation 
frequency from crystal oscillators can be well compensated relative to the 
ambient temperature. 
In the composition of the present invention, the molar ratio of lanthanum 
to the sum of cobalt and chromium is preferably from about 0.50/1 to 
0.999/1, and most preferably about 0.90 to 0.99/1. This is because if the 
molar ratio is larger than about 0.999/1, the non-reacted lanthanum oxide 
(La.sub.2 O.sub.3) in the sintered ceramic of the composition reacts with 
the water in the ambient air to cause the ceramic to break and become 
unusable as the intended device. If, however, the molar ratio is smaller 
than about 0.50/1, a device made of the composition has a small constant B 
although having an enlarged specific resistance. 
Now, the present invention is described in more detail with reference to 
the following examples, which, however, are not intended to restrict the 
scope of the invention. 
EXAMPLE 1 
A cobalt compound (CoCO.sub.3, Co.sub.3 O.sub.4 or CoO) and a lanthanum 
compound (La.sub.2 O.sub.3 or La(OH).sub.3) were weighed and ground. Added 
thereto was a chromium compound (Cr.sub.2 O.sub.3 or CrO.sub.3) in such a 
manner that the molar ratio of lanthanum to the sum of cobalt and chromium 
in the resulting mixture was 0.95/1. The mixture was wet-milled in a ball 
mill for 24 hours together with pure water and zirconia balls, then dried, 
and thereafter calcined at from 900.degree. to 1200.degree. C. for 2 
hours. A binder was added to the thus-calcined powder, which was further 
wet-milled in a ball mill for 24 hours together with zirconia balls. Then, 
this was filtered, dried and shaped under pressure into discs, which were 
baked at from 1200.degree. to 1600.degree. C. in air for 2 hours to obtain 
sintered discs. Both surfaces of these discs were coated with a 
silver-palladium alloy paste, and baked at from 900.degree. to 
1400.degree. C. in air for 5 hours, thereby forming outer electrodes on 
these discs. Thus were formed herein semiconductive ceramic device 
samples. 
The specific resistance and the constant B of each sample formed herein 
were measured, and the data thus measured are shown in Table 1. In Table 
1, the samples with the mark "*" are outside the scope of the present 
invention, and the other samples are within the scope of the invention. 
The specific resistance (.rho.) is obtained from the following equation: 
EQU .rho.(T)=R(T).times.S/t 
where R(T) is the resistance value at T.degree. C., S is the surface area 
of the outer electrode, and t is the thickness of the semiconductive 
ceramic device sample. 
The specific resistance of each sample as prepared in Example 1, that is 
obtained from the resistance value thereof at -10.degree. C., 25.degree. 
C. and 140.degree. C., may be represented by the following equations: 
EQU .rho.(-10)=R(-10).times.S/t 
EQU .rho.(25)=R(25).times.S/t 
EQU .rho.(140)=R(140).times.S/t 
The constant B is a constant that indicates the variation in the resistance 
depending on the variation in temperature. This may be defined as follows: 
EQU Constant B(T1, T2)={log .rho.(T2)-log .rho.(T1)}/(1/T2-1/T1) 
where .rho.(T1) and .rho.(T2) are the specific resistance at T1.degree. C. 
and T2.degree. C., respectively. The larger the constant B, the smaller 
the reduction in the resistance value with the elevation of temperature. 
On the basis of the above, the constant B of each sample as prepared in 
Example 1 obtained from the specific resistance thereof at -10.degree. C., 
25.degree. C. and 140.degree. C. is as follows: 
EQU B(-10, 25)={log .rho.(-10)-log .rho.(25)}/{1/(-10+273.15)-1/(25+273.15)} 
EQU B(25, 140)={log .rho.(140)-log .rho.(25)}/{1/(140+273.15)-1/(25+273.15)} 
B (-10, 25) is the constant B within the temperature range between 
-10.degree. C. and +25.degree. C.; and B (25, 140 is the constant B within 
the temperature range between 25.degree. C. and 140.degree. C. 
TABLE 1 
__________________________________________________________________________ 
Chromium .rho. (.OMEGA. .multidot. cm) 
Constant B (K) 
Content 
25 140 -10, 25 
25, 140 
Sample No. 
(mol %) 
A B -10 
A/B C D D/C 
__________________________________________________________________________ 
1* 0 1.9 
0.20 
2.5 
9.5 800 2410 
3.0 
2 0.005 2.5 
0.22 
4.7 
11.4 
1420 
2600 
1.8 
3 0.01 3.6 
0.18 
7.1 
20.0 
1510 
3190 
2.1 
4 0.02 5.0 
0.19 
12.6 
26.3 
2050 
3500 
1.7 
5 0.05 6.7 
0.19 
20.6 
35.3 
2530 
3800 
1.5 
6 0.1 9.5 
0.21 
37.4 
45.2 
3080 
4100 
1.3 
7 0.2 12.6 
0.19 
60.5 
66.3 
3510 
4400 
1.3 
8 0.5 18.5 
0.23 
108.1 
80.4 
3960 
4650 
1.2 
9 1 16.1 
0.23 
101.1 
70.0 
4120 
4530 
1.1 
10 2 14.8 
0.22 
85.1 
67.3 
3730 
4490 
1.2 
11 5 12.9 
0.22 
65.7 
58.6 
3640 
4240 
1.2 
12 10 10.9 
0.21 
51.2 
51.9 
3470 
4020 
1.2 
13 20 12.4 
0.56 
39.3 
22.1 
2580 
3320 
1.3 
14 30 14.0 
1.29 
42.9 
10.9 
2510 
2550 
1.0 
15* 31 15.0 
2.01 
45.5 
7.5 2490 
2150 
0.9 
Conventional 
0 10.0 
0.97 
38.0 
10.3 
3000 
2500 
0.8 
Sample 1 
__________________________________________________________________________ 
As seen in Table 1 above, both the specific resistance and the constant B 
of the samples increase with the increase in the chromium content thereof. 
However, when the chromium content is higher than about 0.5 mol %, the 
specific resistance and the constant B lower; when the chromium content is 
higher than 20 mol %, the specific resistance increases while the constant 
B lowers; and when the chromium content is 31 mol %, the constant B (25, 
140) is smaller than the constant B (-10, 25). 
When the chromium content falls between about 0.005 mol % and 30 mol %, the 
constant B (25, 140) is higher than 2500K. In particular, when the 
chromium content falls between 0.1 mol % and 10.0 mol %, both the constant 
B (-10, 25) and the constant B (25, 140) are high, the former being higher 
than 3000K and the latter being higher than 4000K. 
FIG. 1 is a graph showing the dependence on temperature of the specific 
resistance of semiconductive ceramic device samples, in which the vertical 
axis indicates the specific resistance (.OMEGA.cm) and the horizontal axis 
indicates the temperature (.degree.C.) and in which each curve indicates a 
different in the chromium content in each sample. The full lines indicate 
the samples falling within the scope of the present invention, while the 
dotted lines indicate those falling outside the invention. 
As seen in FIG. 1, the semiconductive ceramic device samples of the present 
invention have a small specific resistance at 25.degree. C. of not higher 
than 20 .OMEGA.cm, and still have a small specific resistance even at high 
temperatures of not higher than 10 .OMEGA.cm. 
When a current of 20 A was applied to the semiconductive ceramic device 
samples as prepared herein, those falling within the scope of the present 
invention were not broken. 
Since the samples of the present invention have a large constant B (25, 
140), they inhibit the initial overcurrent while consuming a reduced power 
amount at steady state. Thus, these are excellent as devices for rush 
current inhibition, for motor start-up retardation and for halogen lamp 
protection. 
Conventional Example 1 
Mn.sub.3 O.sub.4, NiO and Co.sub.3 O.sub.4 were weighed in a ratio by 
weight of 6:3:1, and wet-milled in a ball mill for 5 hours along with pure 
water, a binder and zirconia balls. Then, the thus-milled mixture was 
filtered and dried. Next, in the same manner as in Example 1, the 
resulting dry powder was shaped under compression into discs, which were 
baked at 1200.degree. C. in air for 2 hours to obtain sintered discs. Both 
surfaces of these discs were coated with a silver-palladium alloy paste 
and baked at from 900.degree. to 1100.degree. C. for 5 hours in air, to 
thereby form outer electrodes on the discs. Thus were prepared herein 
semiconductive ceramic device samples. 
The electric characteristics of the sample prepared herein were determined 
in the same manner as in Example 1. Of these, the specific resistance 
(.rho.) and the constant B at the predetermined temperatures are shown in 
Table 1. The resistance-temperature characteristic is shown in FIG. 1. 
As shown in Table 1, the constant B (25, 140) of the semiconductive ceramic 
device sample of Conventional Example 1 is smaller than the constant B 
(-10, 25) thereof. Thus, it is known that the energy consumption of this 
conventional sample is large at steady state. 
Comparing the sample of Conventional Example 1 with the samples of Example 
1 of the present invention having the same degree of specific resistance, 
it is revealed that the samples of Example 1 of the invention have a 
higher constant B (25, 140). In general, a reduction in the specific 
resistance results in the reduction in the constant B. As opposed to this, 
however, it is known that the semiconductive ceramic composition of the 
present invention that is characterized by comprising LaCoO.sub.3 and from 
about 0.005 to 30 mol % of chromium added thereto has a higher constant B 
than the sample of Conventional Example 1. 
EXAMPLE 2 
A powdery lanthanum compound (La.sub.2 O.sub.3 or La(OH).sub.3) and a 
powdery cobalt compound (CoCO.sub.3, Co.sub.3 O.sub.4 or CoO) were weighed 
in a molar ratio of lanthanum to cobalt of 0.95/1, to which was added from 
0.01 to 40 mol % of a chromium compound (Cr.sub.2 O.sub.3 or CrO.sub.3). 
The mixture was wet-milled in a ball mill for 16 hours together with pure 
water and nylon balls, then dried, and thereafter calcined at from 
900.degree. to 1200.degree. C. for 2 hours. The resulting mixture was 
further ground in a jet mill, to which was added 5% by weight of a vinyl 
acetate binder along with pure water. This was again wet-milled, then 
dried and granulated. The resulting granules were shaped under pressure 
into discs, which were baked at from 1200.degree. to 1600.degree. C. in 
air for 2 hours to obtain sintered discs. Both surfaces of these discs 
were screen-printed with a silver-palladium alloy paste, and baked at from 
900.degree. to 1200.degree. C. in air for 5 hours, thereby forming outer 
electrodes on these discs. Thus were formed herein semiconductive ceramic 
device samples. 
The specific resistance and the constant B of each sample formed herein 
were measured in the same manner as in Example 1, and the data thus 
measured are shown in Table 2. In Table 2, the samples with the mark "*" 
did not have the intended characteristics applicable to the use of the 
samples as semiconductive ceramic devices for TCXO. The specific 
resistance was derived from the resistance value at 25.degree. C. 
according to the equation employed in Example 1. 
To obtain the constant B, used herein were the same equations as those in 
Example 1. Thus, of the samples of Example 2, the constant B was derived 
from the specific resistance thereof at -30.degree. C., 25.degree. C., 
50.degree. C. and 140.degree. C. to be as follows: 
EQU B(-30, 25)={log .rho.(-30)-log .rho.(25)}/{1/(-30+273.15)-1/(25+273.15)} 
EQU B(25, 50)={log .rho.(50)-log .rho.(25)}/{1/(50+273.15)-1/(25+273.15)} 
EQU B(25, 140)={log .rho.(140)-log .rho.(25)}/{1/(140+273.15)-1/(25+273.15)} 
B(-30, 25) is the constant B within the temperature range between 
-30.degree. C. and +25.degree. C.; B (25, 50) is the constant B within the 
temperature range between 25.degree. C. and 50.degree. C.; and B (25, 140) 
is the constant B within the temperature range between 25.degree. C. and 
140.degree. C. 
TABLE 2 
______________________________________ 
Specific 
Chromium Resistance, 
Content .rho. (25), 
Constant B (K) 
Sample No. 
(mol %) .OMEGA. .multidot. cm 
-30, 25 
25, 50 
25, 140 
______________________________________ 
1* 0.01 2.0 1160 2200 3150 
2* 0.05 3.9 1380 2510 3200 
3 0.1 9.1 3010 3040 3390 
4 0.5 14.6 3310 3700 4010 
5 1.0 18.7 3760 4080 4430 
6 1.5 18.6 3930 4170 4560 
7 2.0 17.7 3990 4140 4620 
8 2.5 17.2 4040 4200 4620 
9 3.0 16.8 4040 4190 4610 
10 3.5 16.4 4030 4180 4610 
11 4.0 15.9 4040 4130 4600 
12 4.5 15.9 4020 4160 4590 
13 5.0 16.2 3980 4130 4590 
14 6.0 16.1 4010 4130 4570 
15 7.0 17.5 3940 4110 4550 
16 8.0 19.3 3890 4070 4520 
17 9.0 21.0 3840 4060 4500 
18 10.0 23.5 3810 4030 4470 
19 15.0 31.4 3670 3900 4380 
20 20.0 37.6 3480 3760 4190 
21 25.0 42.8 3340 3650 4100 
22 30.0 48.9 3200 3510 4010 
23* 35.0 55.1 3020 3250 3620 
24* 40.0 59.3 2890 3010 3230 
Conventional 
0.0 16.2 2760 2750 2750 
Sample 2 
______________________________________ 
As shown in Table 2 above, the specific resistance of the samples increases 
and the constant B thereof increases to be higher than 3000K with the 
increase in the chromium content of the samples. However, when the 
chromium content is not higher than about 0.05 mol %, the constant B is 
lower than 3000K, and when the chromium content is higher than about 30 
mol %, the specific resistance is above 50 .OMEGA.cm, and both are not 
suitable for temperature compensation. As opposed to these, the samples 
falling within the scope of the present invention have low specific 
resistance. Using these, therefore, the surface area of the electrode of 
the devices having a predetermined resistance value may be reduced and the 
capacitance of the devices may be small. Accordingly, the accuracy of the 
devices of the present invention, when used in temperature-compensating 
circuits for temperature compensation in TCXO, is high. 
With the increase in the constant B (-30, 25), the variation in the 
resistance value, relative to temperature, increases, resulting in that 
the devices in temperature-compensating circuits in TCXO can compensate 
low temperatures falling within a broad range. It is known from Table 2 
that the constant B (25, 50) and the constant B (25, 140) of the samples 
of the present invention are both higher than the constant B (-30, 25) 
thereof. 
Of the samples of the present invention having a chromium content falling 
between about 0.1 mol % and 30mol %, all the constant B (-30, 25), the 
constant B (25, 50) and the constant B (25, 140) are higher than 3000K. In 
particular, of those having a chromium content falling between about 0.5 
mol % and 10.0 mol %, the variation in the resistance-temperature 
characteristic, relative to the chromium content, is stably small. Thus, 
the samples of the present invention having a chromium content of from 
about 0.5 mol % to 10.0 mol % are the most suitable as NTC devices in 
temperature-compensating circuits in TCXO. 
FIG. 2 shows the relationship between the chromium content of the 
semiconductive ceramic device samples prepared in Example 2 and the 
constant B thereof, in which the vertical axis indicates the constant B 
(K) and the horizontal axis indicates the chromium content (mol %). In 
FIG. 2, .circle-solid. (filled circle) indicates the constant B (-30, 25); 
.box-solid. (filled rectangle) indicates the constant B (25, 50), and 
.DELTA. indicates the constant B (25, 140). As in FIG. 2, the samples 
having a chromium content of 0.1 mol % or higher all have a constant B of 
higher than 3000K. 
Conventional Example 2 
A semiconductive ceramic device sample was prepared herein in the same 
manner as in Example 2, except that Mn.sub.3 O.sub.4, NiO and Co.sub.3 
O.sub.4 weighed in a ratio by weight of 6:3:1 were used herein. 
The characteristics of the sample prepared herein were determined in the 
same manner as in Example 2. The data are shown in Table 2. 
As seen in Table 2, the constant B (25, 50) at high temperatures of the 
semiconductive ceramic device sample of Conventional Example 2 is smaller 
than the constant B (-30, 25) thereof at low temperatures. In addition, 
both constants B are smaller than 3000K. 
In the composition of the present invention, the molar ratio of lanthanum 
to the sum of cobalt and chromium is not limited to only 0.95/1 but may be 
within the scope between about 0.50/1 and 0.999/1. If the molar ratio of 
lanthanum to the sum of cobalt and chromium is larger than about 0.999/1, 
non-reacted La.sub.2 O.sub.3 in the sintered ceramic reacts with water in 
the air to cause breakage and prevent use as the intended device. If, 
however, the molar ratio is smaller than about 0.50/1, the sintered 
ceramic has a small constant B although having an enlarged specific 
resistance. If so, its constant B is smaller than the constant B of 
conventional semiconductive ceramic devices, and the device comprising the 
sintered ceramic thus having such a small constant B is not suitable for 
the use to which the present invention is directed. 
If desired, lanthanum in the LaCo oxides for use in the present invention, 
such as those mentioned hereinabove, may be partly or wholly substituted 
with any other rare earth elements and bismuth to give, for example, 
La.sub.0.9 Nd.sub.0.1 CoO.sub.3, La.sub.0.9 Pr.sub.0.1 CoO.sub.3, 
La.sub.0.9 Sm.sub.0.1 CoO.sub.3 or Nd.sub.0.95 CoO.sub.3. 
In the above-mentioned examples, produced were semiconductive ceramic 
discs. However, the semiconductive ceramic device of the present invention 
is not limited to only the shape of such discs but may be in any other 
form of laminated devices, cylindrical devices, square chips, etc. In the 
above-mentioned examples, a silver palladium alloy or platinum was used to 
form the outer electrodes on the semiconductive ceramic devices. However, 
such is not limitative, but any other electrode materials of, for example, 
silver, palladium, nickel, copper, chromium or their alloys may also be 
employed to obtain similar characteristics. 
As has been described in detail hereinabove, there is provided according to 
the present invention, a semiconductive ceramic composition comprising a 
lanthanum cobalt oxide with a chromium oxide added thereto in an amount of 
from about 0.005 to 30 mol % in terms of chromium. The composition can 
have a small specific resistivity at steady state, while having a high 
constant B of higher than 3000K at high temperatures. In particular, a 
composition having a chromium content of from about 0.1 to 10 mol % may 
have a much higher constant B, of higher than 4000K, at high temperatures. 
Since the semiconductive ceramic composition of the present invention 
comprises a rare earth-transition metal oxide, especially a lanthanum 
cobalt oxide, it is characterized in that it has a small specific 
resistance at room temperature while having a higher constant B at high 
temperatures than at low temperatures. 
Since the semiconductive ceramic composition of the present invention 
comprises, as the essential component, a lanthanum cobalt oxide and 
contains, as the side component, a chromium oxide in an amount of from 
about 0.1 to 30 mol % in terms of chromium, it has a small specific 
resistance at the steady state and has a high constant B of higher than 
3000K. In particular, the composition having a chromium content of from 
about 0.5 to 10 mol % may have a high constant B of higher than 3500K at 
high temperatures. 
Having the above-mentioned characteristics, the semiconductive ceramic 
composition of the present invention can be used for forming devices to be 
usable in temperature-compensated crystal oscillators and those usable for 
rush current inhibition, for motor start-up retardation and for halogen 
lamp protection. 
In addition, since the semiconductive ceramic composition of the present 
invention comprises a lanthanum cobalt oxide while containing a chromium 
oxide in an amount of from about 0.005 to 30 mol % in terms of chromium, 
it has a low specific resistance at steady state while having a high B 
constant of higher than 2500K at high temperatures. Thus, being different 
from that of conventional semiconductive ceramic devices, the difference 
in the resistance of the device comprising the composition of the 
invention between the electrification thereof at room temperature and that 
at high temperatures (140.degree. C. or so) is large. 
Moreover, since the semiconductive ceramic device of the present invention 
comprises a rare earth-transition element oxide, especially a lanthanum 
cobalt oxide, it is characterized in that it has a small constant B at 
room temperature while having a large constant B at high temperatures. 
Therefore, the device of the invention consumes a reduced amount of energy 
at steady state, and therefore can be used in instruments through which 
large currents run. 
In addition, since the semiconductive ceramic device of the present 
invention comprises, as the essential component, a lanthanum cobalt oxide 
and contains, as the side component, a chromium oxide in an amount of from 
about 0.1 to 30 mol % in terms of chromium, it is characterized in that it 
has a low specific resistance at room temperature while having a high 
constant B of higher than 3000K. 
Having such improved characteristics, the semiconductive ceramic device of 
the present invention can be used for rush current inhibition, for motor 
start-up retardation and for halogen lamp protection, and can be used in 
temperature-compensated crystal oscillators. Temperature-compensated 
crystal oscillators have been specifically referred to herein, in which 
the device of the present invention is usable. Apart from these, the 
device of the present invention is usable in any other 
temperature-compensating circuits to be employed in other instruments. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.