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Description  |
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Ceramic semiconductor bodies of this type have found wide application as
self-regulating heaters, as current-regulating resistors, and as
current-responsive or temperature-responsive switching devices or the like
and, for many of these purposes, it is desirable that the semiconductor
bodies display low resistivity at room temperature, that the bodies
display a very sharp increase in resistivity over a narrow temperature
range, that the bodies display very high peak resistivity at a selected
temperature level, and that the bodies display consistent resistivity
properties at various applied voltage levels. For achieving wide
application, it is also desirable that the semiconductor bodies retain
their original electrical and thermal properties over a long service life,
that the bodies be adapted for use in a variety of environments, and
particularly that the bodies be adapted for manufacture at low cost with
consistently reproducible electrical and thermal properties.
In this regard, it had been found that semiconductor materials which
display very high peak resistivities at low input voltages could be
obtained by the use of manganese oxide additions to the base ceramic
compositions. However, these manganese-modified materials have had poor
voltage sensitivity properties and have tended to display much lower peak
resistivities at higher applied voltage levels. These modified materials
were also found to have other undesirable characteristics which tended to
deter successful application of the semiconductor materials. It had then
been found that the further use of silicon additions would improve the
voltage sensitivity properties of manganese-modified semiconductor
materials where the materials were sintered in selected atmospheres but it
had not been possible to achieve consistently reproducible materials of
this type having suitably low room temperature resistivity, having high
peak resistivities and good voltage sensitivity characteristics while
using the same convenient and economical techniques which are
conventionally used in semiconductor manufacture.
It is an object of this invention to provide novel, improved low cost
ceramic semiconductor materials; to provide such materials which have low
room temperature resistivity; to provide such semiconductor materials
which display very high peak resistivity; to provide such materials which
have excellent voltage sensitivity properties; and to provide such
materials which are adapted to be manufactured using conventional
manufacturing techniques including air sintering.
Other objects, advantages and details of the novel and improved ceramic
semiconductors of this invention appear in the following detailed
description of preferred embodiments of the invention, the detailed
description referring to the drawings in which:
FIGS. 1-4 are graphs illustrating the electrical and thermal properties of
the ceramic semiconductor materials of this invention.
The base ceramic composition utilized in this invention preferably
comprises barium titanate alone or barium titanate with partial
substitution of other titanates such as calcium titanate, lead titanate or
strontium titanate or with partial substitution of corresponding stannates
and zirconates. Typically a stoichiometric or slightly titanium-rich
composition is used, the composition preferably incorporating an excess of
not more than about 2 mol percent titanium oxide. In preferred embodiments
of this invention, for example, the base ceramic composition comprises
barium titanate alone, a mixture of barium and strontium titanate, a
mixture of barium and calcium titanate, or a mixture of barium, calcium
and strontium titanates, such compositions incorporating an excess of
titantium in the range from 0 to 2 mol percent.
In accordance with this invention, additions are made to these base ceramic
compositions, these additions including silicon oxide, an activator
selected from the group consisting of manganese and ruthenium oxide, and
certain dopants such as selected heavy rare earths of relatively small
ionic radius, the proportions of these additions each being maintained
within precisely controlled limits for achieving the objectives of this
invention. Thus, it has been found that additions of certain light rare
earths such as lanthanum, cerium and samarium of relatively large ionic
radius have tended to produce ceramic semiconductor materials having
excessively high room temperature resistivities when such dopants have
been used with manganese and silicon-modified materials. Accordingly, it
is an important part of this invention that the dopants used for the base
ceramic compositions be limited to yttrium, niobium and the rare earths
heavier than dysprosium. Preferably, for example, the dopants are limited
to yttrium, dysprosium, holmium and niobium. Preferably also, the
proportion of these dopants are maintained at relatively low levels in the
range from about 0.25 to 0.60 mol percent, thereby to obtain ceramic
semiconductor materials having room temperature resistivities on the order
of about 100 ohm-centimeters or less. For example, where the preferred
yttrium dopant is used, the addition to the base ceramic composition is
preferably maintained within the range from 0.35 to 0.60 mol percent;
holmium is preferably used in the range from 0.35 to 0.60 mol percent;
dysprosium is preferably maintained in the range from 0.30 to 0.45 mol
percent; and, where a niobium dopant is used, the dopant addition is
preferably from 0.20 to 0.30 mol percent.
It is found that the silicon oxide addition must be controlled within
narrow limits. Thus, silicon additions to the composition of less than
about 2.0 mol percent tend to produce ceramic semiconductor materials
having excessive voltage sensitivity whereas use of more than about 4.0
mol percent silicon tends to cause deterioration of the peak resistivity
of the semiconductor materials. Preferably then, in accordance with this
invention, the silicon addition to the base ceramic composition is
maintained in the range from 2.0 to 4.0 mol percent to obtain
semiconductor materials having peak resistivities in excess of 10.sup.7
ohm-centimeters at low applied voltages and to maintain peak resistivity
of the materials on the order of about 10.sup.6 ohm-centimeters at applied
voltages of greater than about 600 volts per centimeter.
In accordance with this invention it has also been found that, if
semiconductor materials having desired low room temperature resistivities
on the order of 100 ohm-centimeters or less are to be obtained, much lower
additions of manganese must be used than was previously thought necessary.
Thus, in the preferred embodiments of this invention, the manganese
addition to the base ceramic composition is maintained within the range
from about 0.03 to 0.10 mol percent. Alternately, if desired, the
manganese activator is replaced with ruthenium as an activator, the
addition of ruthenium preferably being maintained in the range from about
0.001 to 0.02 mol percent.
If desired, aluminum additions up to about 0.5 mol percent may also be made
to the base ceramic compositions in conventional manner to regulate
resistivity.
In using the base ceramic compositions with additions thereto controlled
within limits as above-described, it is possible to sinter the
compositions in air in a very economical and convenient manner while still
achieving semiconductor materials of desirable and consistently
reproducible electrical and thermal properties. That is, any conventional
starting materials are used and are mixed and calcined in any conventional
manner. Typically, for example, the starting materials of the base
compositions comprise barium, strontium, calcium and lead carbonates and
titanium dioxide whereas the additions thereto are made with manganese
carbonate and silicon dioxide and with oxides of ruthenium of the selected
rare earths or other dopants, and of aluminum. Where such starting
materials are used, the materials are commonly mixed by ball milling or in
other conventional manner and are then calcined. Alternately where other
starting materials are used, other conventional techniques are used for
mixing and calcining the materials.
In accordance with this invention, the calcined materials in proportions as
provided by this invention are then combined with a small quantity of an
organic binder in conventional manner and are further mixed and sieved and
are pressed to form disc elements of a thickness of about 0.100 inches and
a mass of about 20 grams, also in a conventional way. Finally, in
accordance with this invention, the pressed discs are sintered in air in
any convenient and economical way. Preferably, for example, the pressed
discs are initially heated in air at a temperature on the order of
1200.degree. C., below the temperature at which any liquid phase occurs.
It is believed that this preheating or heat soaking allows orderly grain
growth to occur prior to liquid phase formation and minimizes grain growth
at the peak sintering temperature. Preferably, also the pressed disc
elements are then fired in air at a peak sintering temperature in the
range from 1300.degree. C. to about 1450.degree. C. for from 15 to 30
minutes, the temperature and duration of the peak sintering temperature
being selected with respect to the specific materials used and with
respect to the mass of the disc elements. Preferably, for example, where
the base ceramic composition comprises barium titanate, the pressed disc
elements are fired at a peak temperature of about 1350.degree. C. Where
the base ceramic compositions include 10 or 20 mol percent strontium
titanate, the peak sintering temperatures are preferably 1375.degree. C.
and 1400.degree. C. respectively. Alternately, where the base composition
comprises barium titanate with 10 mol percent of lead titanate, a lower
peak sintering temperature of about 1335.degree. C. is used. Where the
pressed disc elements have a mass of about 20 grams, the elements are
preferably fired at peak temperature for about 15 minutes. The shortest
possible sintering time adequate for completely sintering the discs is
preferred to minimize excessive grain growth in the elements. Thereafter,
firing of the pressed discs in air is preferably continued for about 1 to
4 hours at a lower or anneal temperature on the order of 1225.degree. C.
Finally, layers of electrically conductive materials are applied to the
opposite disc surfaces of the sintered elements to provide ohmic contact
to the elements in a conventional manner.
In this way, the ceramic semiconductor materials of this invention are
provided with desirable properties as above-described. That is, when the
above-noted additions are made to the described base compositions, and
when these compositions are sintered in air as noted above, the resulting
ceramic semiconductor reaction products are characterized by low room
temperature resistivity on the order of 100 ohm-centimeters or less; are
characterized by very high peak resistivities of about 10.sup.7
ohms-centimeters or more; and are characterized by excellent voltage
sensitivity properties, by improved retention of resistivity properties
over a long service life, and by stability in use in a variety of
environments. Most important, these desirable properties are consistently
reproducible in an economical and convenient manner by sintering in air.
For example, in a preferred embodiment of this invention, conventional
starting materials are combined in proportions to produce a base ceramic
composition comprising 90 mol percent barium titanate and 10 mol percent
calcium titanate, the base composition having an excess of 1 mol percent
titanium. To these base composition starting materials are then added
selected starting materials to add 2 mol percent silicon oxide 0.08 mol
percent manganese oxide, 0.40 mol percent holmium oxide and 0.10 mol
percent aluminum oxide. After suitable mixing, calcining, sieving,
addition of organic binder and pressing to form disc elements of about 20
grams mass having a thickness of about 0.100 inches as above-described,
the pressed discs were preheated at a temperature of 1200.degree. C. for 1
hour in air, were fired in air at a peak sintering temperature of
1360.degree. C. for about 15 minutes, and were thereafter maintained in
air at a temperature of 1225.degree. C. for about 3 hours. After
application of ohmic contact layers to opposite surfaces of the sintered
discs, the resulting ceramic semiconductor reaction product displayed
electrical and thermal properties as illustrated in FIG. 1. That is, with
input voltage at a minimum (less than 1 volt per centimeter), the reaction
product displayed a room temperature resistivity on the order of about 100
ohm-centimeters, displayed a very sharp increase in resistivity over a
narrow temperature range, and displayed a peak resistivity on the order of
10.sup.8 ohm-centimeters at a temperature of about 225.degree. C. as
illustrated by curve 12. Similarly, when the same semiconductor element
was tested at an applied voltage level of about 700 volts per centimeter,
the element displayed significant voltage independence by providing a peak
resistivity of better than 10.sup.7 ohm-centimeters as illustrated by
curve 14 in FIG. 1. The semiconductor elements also displayed good
retention of its resistivity properties in use and was stable in various
conventional environments in which such elements are commonly used.
In another preferred embodiment of this invention, the base ceramic
composition comprised barium titanate alone having no excess of titanium.
To this base composition was added 4 mol percent silicon, 0.06 mol percent
manganese, 0.55 mol percent yttrium, and 0.20 mol percent aluminum. After
mixing and calcining and after binder addition and pressing as noted
above, the composition was preheated in air at 1200.degree. C. for 1 hour,
was fired in air at a peak sintering temperature of 1360.degree. C. for 20
minutes, and was then maintained in air at a temperature of 1225.degree.
C. for 90 minutes. After addition of ohmic contacts, the sintered elements
displayed properties as shown in FIG. 2. That is, at minimum applied
voltage levels, the elements displayed room temperature resistivity of 100
ohm-centimeters and peak resistivity at 250.degree. C. of better than
10.sup.7 ohm-centimeters as illustrated by curve 16 in FIG. 2. Similarly,
at an applied voltage level of about 210 volts per centimeter, the
semiconductor elements showed only very slight decrease in peak
resistivity as illustrated by curve 18 in FIG. 2.
In another preferred embodiment of this invention, the base ceramic
composition comprised 90 mol percent barium titanate, 5 mol percent
strontium titanate, and 5 mol percent calcium titanate, without excess of
titanium. To this base composition was then added 4 mol percent silicon,
0.06 mol percent manganese, 0.55 mol percent yttrium, and 0.20 mol percent
aluminum. After preparation of pressed disc elements as above-described,
the elements were preheated in air at 1200.degree. C. for 1 hour, were
fired in air at 1350.degree. C. for 15 minutes, and were maintained in air
at 1225.degree. C. thereafter for 3 hours. As shown by curve 20 in FIG. 3,
these semiconductor elements provided with ohmic contact layers displayed
room temperature resistivity of about 100 ohm-centimeters and peak
resistivity of better than 10.sup.7 ohm-centimeters at 250.degree. C.
under low applied voltage and, as shown by curve 22 in FIG. 3, also
displayed excellent peak resistivity even at applied voltage levels of 685
volts per centimeter.
In another preferred embodiment of this invention, the base ceramic
composition comprised 80 mol percent barium titanate and 20 mol percent
strontium titanate without excess of titanium. To this base composition
was added 20 mol percent silicon, 0.03 mol percent manganese, and 0.35 mol
percent yttrium. After preparation of pressed disc elements as
above-described, the elements were preheated in air at 1200.degree. C. for
1 hour, were fired in air at 1400.degree. C. for 15 minutes, and were
maintained in air at 1225.degree. C. for an additional 2 hours. As shown
by curve 24 in FIG. 4, these semiconductor materials displayed room
temperature resistivity of less than 100 ohm-centimeters and peak
resistivity at 200.degree. C. of better than 10.sup.7 ohm-centimeters at
low applied voltage levels and, as shown by curve 26 in FIG. 4, retained
excellent peak resistivity even at an applied voltage of 600 volts per
centimeter.
These and other preferred examples of the ceramic semiconductor materials
of this invention are set forth in Table I, this table further
illustrating the desirable peak resistivities achieved by showing the
logarithm of the ratio of resistivity maximums and minimums (.sup.R max,
.sup.R min) and illustrating the desirable voltage sensitivity properties
achieved by showing the logarithm of the ratios of peak resistivities
(.sup.R,.sup.R 0) at selected voltage gradients between the stated value
and less than 1 volt/centimeter.
TABLE I
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Air Sintering Volt.
(Temp./Time)
Resistivity Sens.
Pre
Peak
Ann.
(ohm-cm)
Log Log
Volt
Relative Gram Mols (Oxides)TiBaSrCaPbSiDyHoYNbAlMnRu
Min.degree. C
Min.degree. C
Min.degree. C
Tp.Rm.
Peak
##STR1##
##STR2##
Grad V/cm
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1 101
90 10 2 0.4 .10
.08 1200
1360
1225
60
15
180
120
1.times.10.sup.8
5.92 -1.5
700
2 100
100 4 .55 .20
.06 1200
1360
1225
60
20
90
101
5.7.times.10.sup.7
5.88 -0.6
210
3 100
90 5 5 4 .55 .20
.06 1200
1350
1225
60
15
180
100
5.times.10.sup.7
5.71 -1.5
685
4 100
80 20 2 .35 .03 1200
1400
1225
60
15
120
70
1.times.10.sup.7
5.22 -1.0
600
5 100
80 10
10
2 .40 .04 1300
1225
15
120
99
1.times.10.sup.7
5.17 -0.5
510
6 101
90 2 8 4 .45 .20
.07 1200
1350
1225
60
15
120
100
5.times.10.sup.7
5.68 -1.2
1000
7 101
89 3 8 2 .40 .04 1370
1225
30
360
95
1.times.10.sup.8
6.10 -1.0
465
8 102
80 20 2 .35 .03 1200
1400
1225
60
15
120
82
6.5.times.10.sup.7
5.90 -1.2
610
9 102
85 15 2 .40 .06 1180
1360
1227
60
15
360
101
1.6.times.10.sup.9
7.22 -2.0
1000
10 1015
80 20 2 .20
.10
.04 1200
1400
1225
60
40
20
134
1.1.times.10.sup.7
4.93 -0.5
600
11 102
80 20 2 .30 .10 0.01
1200
1400
1225
75
15
65
133
5.times.10.sup.6
4.60 -1.2
610
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It should be understood that although preferred embodiments of the ceramic
semiconductor materials of this invention have been described by way of
illustrating this invention, this invention includes all modifications and
equivalents of the disclosed embodiments falling within the scope of the
appended claims.
* * * * *
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