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Патент USA US2407751

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Patented Sept. 17, 1946
2,407,750
UNITED STATES PATENT OFFICE
2,407,750
TEMPERATURE SENSITIVE RESISTOR
Frederick Gordon Smith, Toronto, Ontario, Can'
ada, assignor to Research Consultants Limited,
Toronto, Ontario, Canada, a corporation of On
tario
No Drawing. ‘ Application February 23, 1944,
Serial No. 523,571
14 Claims. (Cl. 201--76)
2
This invention relates to a family of temper
ature sensitive resistance materials and particu
larly to resistors constructed of such materials
which thereby provide a very high negative tem
perature coe?icient of resistance.
It is a general object of the present invention
to provide improved resistors and resistance ma
written as follows:
group IIA of the periodic table of elements ex
perature in atmosphere thereby combining the
metals in group IIA of the periodic table of
elements, excluding mercury, and. the general
formula expressive of these titanates may be
MO.TiO2
where M is magnesium, zinc, cadmium or berryl
terials having high, uniform and stable negative
lium. Each of these compounds is a true titanate
temperature coe?icients of resistance.
and comprises a single crystalline phase which
More particularly it is an object of the inven 10 for the magnesium compound is orthorhombic
tion to provide temperature sensitive resistors
and for the zinc compound is cubic.
composed of true titanates of certain metals of
The several compounds are prepared by com
group II of the periodic table of elements.
bining one molecule of the monoxide of the base
An important object of the invention consists
metal with one molecule of titanium dioxide
in the provision of resistors of one crystalline 15 (T102) . The constituent oxides in powdered form
phase composed of titanates of the metals in
are carefully mixed and heated to a high tem
eluding mercury.
A further object of the invention consists in
ingredients. The several compounds may be pre
pared by sintering at a temperature below their
the provision of a family of titanate resistors 20 respective melting points or they may be melted.
providing a wide range of resistance coeflicients
A temperature of 1300" C. is sufficient to melt the
which overlap so as to permit the selection of
cadmium titanate; 14000 C. is su?‘lcient to melt
resistance ranges for any desired purpose.
the zinc titanate; 1650" C. is suilicient to melt
An important feature of each of the resistors
the magnesium titanate.
is its extremely wide variation in resistance with
Each of these titanates is a hard, strong, stable
temperature change together with extreme sta
crystalline compound unchanging in oxygen or
bility in air or oxygen up to the melting point
air up to its melting point which in the case
of the material which melting point is above the
of the magnesium compound is approximately
melting points of most metals and of most ma- 1610“ C. The zinc compound melts somewhat
terials in commercial usage whereby the resistors 30 above 1300° C. whereas the cadmium compound
may form the essential element of resistance
begins to decompose slightly above 1000" C.
thermometers or the like.
Each of the compounds can be worked in air
An extremely important feature of each of the
with any kind of combustion torch without de
materials is the fact that it is composed of one
composition. They can also be melted in plati
single crystalline phase so that it can be formed
num crucibles and poured or cast in any desired
into minute resistors without causing any ob
form like metal. Each of the compounds is clear,
servable change in the temperature coe?icient
transparent, and almost colorless. Following any
of resistance, and which permits positive dupli
Working to change the shape or size of a mass
cation of the resistance curve at any time.
of the material it‘ is preferably heat treated to
Other and further objects and features of the
a dull red heat in air or in oxygen after being
invention will be more fully understood by those
brought to its ?nal form to make certain that
skilled in the art upon a consideration of the fol
all of the titanium present is in the tetravalent
lowing speci?cation wherein are disclosed several
form.
exemplary embodiments of the invention with the
The material is non-reactive with platinum
understanding that such changes and modi?ca
even at temperatures above the melting point
tions may be made therein as fall within the
of the material which permits the use of platinum
scope of the appended claims without departing
crucibles and platinum wire for terminals or heat- ‘
from the spirit of the invention.
ing elements for the resistors. An important
The ‘resistance materials of the present inven-' ' feature is “that the thermal expansion of the
tion are in a chemical sense titanates of the 50 serveral resistor materials is so similar to that
2,407,750
4
n
a
coeilicient in terms of the "temperature rise or
of platinum that no cracking takes place when
fall required to respectively decrease or increase
the resistance compound is cast about the ends
the resistance by a factor of two.” This temper
of platinum wires which are to act as terminals.
ature change will be herein termed the “half re
The compositions are cheap to manufacture 5 sistance
temperature.” The following tabulation
both because of the simple process of forming
has been determined by experiment with the sev
them and because of the ready availability at
eral resistors
low prices of the basic materials. Pure titanium
dioxide is readily available at a low price and the
Half resistance temperature
'
same is true of the required oxides of magnesium, 1 0
Critical tem
zinc, cadmium and beryllium.
Below critical ' Above critical
perature
As has been previously mentioned the resistors
temperature
temperature
constructed in accordance with this invention
have extremely high negative temperature co
efficients which makes them available for a num
ber of important uses some of which will later
°C.
MgOfl‘iOL
15 ZnO.Ti 2..
Cd0.TiOz..
_
_
.
°C.
34
30
14
°C.
55
76
94
780
560
350
be explained, but before going into detail as to
the range of resistance with temperature changes
Because of the constant temperature coefficient
it might be well to point out some of the advan
of resistance above or below the critical tempera
tages-of the new resistor materials. There is no
ture, standard scales may be prepared for meas
aging of the material in the sense that with 20 uring or recording devices for temperature which
change in age there is no change in resistance
can be made to read in ohms, when the tempera~
or temperature coe?icient of resistance such as
ture of the resistor is changed to vary its resist
is commonly the case with mixed oxide resistors.
ance, or in degrees of temperature when the re
Resistors constructed of the new materials
sistor is used as a thermometer. It will be ap
25
have extremely low “thermal noise” (variation
parent in either case, that if the same titanate
of total resistance due to variation of contact reresistor species is used, one calibration point only
sistance between the granules of powdered or
is needed to ?x the instrument reading to the cor
compressed resistors) which is a common fault
rect resistance or temperature. With mixed ox
of powdered carbon and mixed oxide resistors.
ide and carbon resistors however, the tempera
Because of their stability in air titanate re 3 0 ture coefficient changes with small changes in
sistors do not require vacuum or gas ?lled en
composition and/or particle sizes, .so that a full
velopes.
recording scale must of necessity be prepared for
Each titanate resistor occurs only in one crys
each and every resistor which is required to per
talline phase and hence can be made in any de
sired small size with no observable change in the
01
form quantitatively in measuring instruments.
A single magnesium titanate resistor may be
used to provide the equivalent of a complete se—
temperature coe?icient of resistance. In this
connection it may be pointed out that resistors
ries of resistances from what is commonly re~
using the new materials have been prepared
garded as an excellent insulator having a specific
complete with platinum leads having a total 40 resistance of 1014 ohms at room temperature to a
weight of only .001 grams. With two phase re
good conductor of 50 ohms speci?c resistance at
sistors, or powdered oxide resistors and carbon
1600” C. This extremely wide range is not shown
types, the temperature coefficient changes with
the physical size, especially as the total mass ap
proaches the size of the individual phases or
grains.
This improvement is extremely impor
by any previously known temperature sensitive
resistors.
A number of uses for the resistors of the pres
ent invention have already been mentioned but
tant because in certain types of calorimeters,
several additional speci?c uses may be pointed
small mass and hence small heat capacity of the
out herein. In a great many ?elds and particu
temperature measuring device is an advantage
larly in radio, ?xed resistors for use at room tem
permitting much superior results in use.
59 perature are desired which have very high actual
Each of the titanate compounds mentioned
resistances of the order of one or more megohms.
herein. has an extremely high speci?c resistance
By varying the physical dimensions of a resistor
at atmospheric temperature and each shows a
unit composed of one of the titanate composi
relatively rapid lowering of speci?c resistance
tions the following ranges may be constructed.
with increase in temperature. The curve of tem—
55
Ohms
perature coeiiicient of resistance of each of these
Magnesium titanate ________________ __ ION-1013
titanates shows a point of in?ection the tempera
Zinc titanate _______________________ __ 1013-4010
ture of which may be termed the “critical” tem
Cadmium titanate _________________ __ 1012-109
perature. This coeflicient is less negative above
the critical temperature than it is below the criti 60 An extremely important feature of such meg
cal temperature but for all temperatures above
ohm resistors is the ability to readily determine
and all temperatures below the critical tempera
the exact resistance without the necessity of ac
ture the coe?icient in each case remains con
curately measuring such extremely high resist
stant.
ances which require very careful work and ex—
Except at the point of in?ection mentioned
65
pensive
equipment. With materials of the con
above the relationship between resistance and
stant temperature coefficient of those of the pres
temperature in any of the compounds may best
ent invention, the resistor may be brought to a
be illustrated by the formula
convenient constant elevated temperature below
its critical temperature and the relatively low re
where R is the resistance in ohms, a is a constant 70 sistance measured by simple standard methods.
Then the resistance at room temperature canbe
dependent on the physical dimensions of the
simply calculated from the known temperature
resistor element, 6 is the base of Naperian log
coe?icient of resistance.
arithms, b is the temperature coef?cient of re
Where variable resistors are required, the re
sistance and T is the absolute temperdture.
It is however, more convenient to express the 75 sistance across the terminals attached to a
2,407,750
5
6
titanate resistor may be made to vary continu
ously by varying continuously the temperature of
the resistor material between the two electrodes.
A simple construction permitting this comprises a
suitable sized portion of the resistance material
with electrodes having their ends coaxially dis
posed and separated by several millimeters and
with a thin platinum wire disposed in the mate
rial at right angles to the direction of the elec
trode wires and between and separate therefrom,
with both the electrodes and the heating wire in
good electrical and thermal contact with the re
sistor material.
With this arrangement, if the
great accuracy at low temperatures is not required
the magnesium titanate can be used to cover the
whole range.
I claim:
1. A negative resistance-temperature coe?icient
resistance material consisting of a titanate of a
metal exclusive of mercury of group IIA of the
periodic table of elements.
2. A negative resistance-temperature coe?icient
resistance material consisting of magnesium
titanate.
3. A negative resistance-temperature coeiiicient
resistance material consisting of zinc titanate.
terminations of the electrodes are as close to
4. A negative resistance-temperature coe?icient
gether as stated above, and therefore close to the 15
resistance material consisting of cadmium ti
heating wire, and if the temperature of the outer
tanate.
part of the resistor is low due to thermal radia
5. The method of making a negative resist
tion then there will be a rapid change in the re
ance-temperature coe?‘icient resistance material
sistance across the electrodes when the tempera
ture of the heating wire is suddenly changed.
This results partially from the closeness of the
ends of the electrodes to the heating wire and
partially to the property of the titanate resistor
of conducting heat more rapidly when hot than
when cold which permits a large thermal gradient
to exist in such material. The thermal inertia of
the resistor element may be made very small by
employing a small element with thin electrodes
and heating wire. The temperature of the heat
comprising mixing ?nely divided titanium dioxide
in molecular proportions with a ?nely divided
monoxide of a metal selected from the group
containing magnesium, zinc and cadmium, and
combining the oxides into a true titanate by heat
ing the mixture to a melting temperature.
6. The method of making a negative resist
ance-temperature coeiiicient resistance material
comprising mixing ?nely divided titanium dioxide
in molecular proportions with a ?nely divided
ing wire is controllable by varying the voltages 30 monoxide of a metal selected from the group
containing magnesium, zinc and cadmium, com
across its terminals by standard methods and by
bining the oxides into a true titanate by heating
calibrating the voltage against the resistance
across the resistor electrodes any desired resist
ance may be obtained by applying a known volt
age across the heating wire.
‘
the mixture to a melting temperature and heat
treating the mixture at a dull red heat to con
vert all titanium present to the tetravalen-t form.
35
7. The method of making a resistor having a
The upper limit of the temperature of the re
negative resistance-temperature coe?icient com
sistor element in the neighborhood of its electrode
prising melting and casting in atmosphere about
terminations is approximately 800° C. and thus
two spaced terminal conductors a bead of a
the available ranges of resistance for the three
titanates resistor species modi?ed as above will 40 titanate of a metal selected from the group con
be as follows:
sisting of magnesium, zinc and cadmium and sub
sequently heat treating the bead until it is a clear,
Ohms
transparent substantially colorless mass.
0°—B00°
0°-560°
560°-800°
0°-350°
MgO.'I'iO2 ________________ __
ZnO.TiO2 _________________ __
ZnO.TiO-2 _________________ __
CdO.TiO2 _________________ __
1016-105
1013-104
105-103
1013-104
negative resistance-temperature coe?icient com
prising melting and casting in atmosphere about
350°-800°
CdO.TiO2 _________________ __ 105 --102
the spaced ends of a pair of platinum wires a bead
8. The method of making a resistor having a
of a single crystalline phase of a metallic titanate
For use in circuits where the heating potential
and treating the bead to reduce all titanium
and that across the resistor must be electrically
therein
to the tetravalent form.
separated various obvious arrangements may be 50
9.
An
electrical resistor formed as a shaped,
resorted to.
self-bonded, coherent, monolithic, single crystal
The use of the titanate resistors for resistance
line phase structure consisting of a chemical
thermometers will be handled in a more or less
compound
having the formula MO.TiO2 where M
conventional manner by measuring the voltage
is magnesium, zinc or cadmium.
drop across the variable resistor in series with 55
10. An electrical resistor consisting of a me
a ?xed resistance or by measuring the current
tallic
titanate whose curve of negative resistance
?owing through the resistor at a constant voltage,
temperature coe?icient contains a de?nite point
etc. Because of the tremendous changes in re
of in?exion at a critical temperature above which
sistance available with relatively small changes
the coei?cient is less negative than below.
in temperature the measuring equipment required
11. An electrical resistor consisting of a me
may be quite elementary and cheap and even if
tallic titanate whose curve of negative resistance
its accuracy is not high nevertheless it will indi
temperature coeii‘icient contains a de?nite point
cate thermal readings to relatively close toler
of in?exion at a critical temperature above and
ances.
below which the coe?icient is constant.
Magnesium titanate is most useful for measur
12. An electric resistance element containing
ing temperatures from about 745 to 1600° C., a
titanium
and magnesium in a single crystalline
range which includes most commercial furnace
phase.
temperatures used in the ceramic, glass and
13. An electric resistance element of mono
metallurgical arts. Zinc titanate is most useful
lithic form and of su?icient size to be used alone
in the temperature range from room tempera
containing titanium and zinc, said element be
ture to 560° C. and cadmium titanate, from room
ing of a single crystalline phase.
temperature to 350° C. It will thus be seen that
14. An electric resistance element containing
the three materials permit temperature measure
titanium and cadmium.
ments ranging from zero to 1600° C. but where
FREDERICK GORDON SMITH.
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