U.S. patent number 3,872,419 [Application Number 05/263,192] was granted by the patent office on 1975-03-18 for electrical elements operable as thermisters, varisters, smoke and moisture detectors, and methods for making the same.
Invention is credited to Alexander J. Groves, Alexander J. Groves, Jr..
United States Patent |
3,872,419 |
Groves , et al. |
March 18, 1975 |
ELECTRICAL ELEMENTS OPERABLE AS THERMISTERS, VARISTERS, SMOKE AND
MOISTURE DETECTORS, AND METHODS FOR MAKING THE SAME
Abstract
Electrical elements comprising a metal substrate, a ceramic
semiconductor applied thereon and an outer conductive coating. The
metal substrate may be a ferrous metal, preferably stainless steel,
and may also be a non-ferrous metal, such as aluminum. A bonding
agent, such as nickel aluminide is applied to ferrous metal
substrates by plasma spraying. The ceramic semiconductor material
is also applied to the metal substrate by plasma spraying, and the
preferred ceramic semiconductor material for use with a ferrous
metal substrate is aluminia titania. The ceramic semiconductor
material is coated with a plasma sprayed conductive layer of
copper-glass frit to which terminals may be easily soldered. In one
configuration, the metal substrate is a long thin rod and the
bonding agent, ceramic semiconductor, and copper-glass frit are
applied thereto to form cylindrical coatings. Smaller elements are
made by cutting the coated rod into short lengths. One terminal is
soldered to the copper-glass frit, and the other terminal is the
central wire. This configuration displays some polarity. A second
configuration comprises grinding away strips of the copper-glass
frit wherein two isolated terminals may be attached thereto. The
resultant electrical element has the characteristics of a
resistance drop in response to either an increase in temperature or
humidity, and may be included in a resistance-sensitive alarm
circuit. It is desirable to coat electrical elements used as
temperature sensors with a water proof coating, so that they do not
respond to changes in humidity.
Inventors: |
Groves; Alexander J.
(Bridgeport, CT), Groves, Jr.; Alexander J. (Fairfield,
CT) |
Family
ID: |
23000777 |
Appl.
No.: |
05/263,192 |
Filed: |
June 15, 1972 |
Current U.S.
Class: |
338/25;
200/61.04; 338/21; 338/22SD; 338/34; 338/308; 340/602; 340/628;
200/61.03; 200/61.06; 338/22R; 338/35; 340/595; 340/604;
427/454 |
Current CPC
Class: |
H01C
17/10 (20130101); H01C 7/006 (20130101) |
Current International
Class: |
H01C
7/00 (20060101); H01C 17/10 (20060101); H01C
17/075 (20060101); H01c 007/00 (); H01c
013/00 () |
Field of
Search: |
;338/20-25,308,34-35
;117/7C,71M,217,215,227,93.1PF,221,230,105.2,7C
;260/61.04,61.03,61.06 ;340/235,237R,237S |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Albritton; C. L.
Attorney, Agent or Firm: Mattern, Ware and Davis
Claims
Having described our invention, that we claim as new and desire to
secure
1. An electrical element comprising:
A) a metal substrate;
B) a layer of nickel aluminide deposited on the metal
substrate;
C) a layer of ceramic semiconductor material comprising aluminum
oxide and titanium oxide deposited on the layer of nickel aluminide
and
D) a layer of conductive material forming an electrical terminal
deposited
2. An electrical element as defined in claim 1 wherein the metal
substrate
3. An electrical element as defined in claim 1 wherein the metal
substrate
4. An electrical element as defined in claim 1 wherein the metal
substrate
5. An electrical element comprising:
A. a stainless steel substrate;
B. a layer of nickel and aluminum applied to the stainless steel
substrate by plasma spraying at temperatures in excess of
10,000.degree. F, wherein the nickel and aluminum combine by means
of a synergistic exothermic chemical reaction to form NiAl and
Ni.sub.3 Al;
C. a layer of ceramic semiconductor material applied to the layer
of nickel and aluminum comprising 75-95 percent aluminum oxide and
5-25 percent titanium oxide; and
D. a layer of conductive material forming an electrical terminal
deposited
6. An electrical element comprising: `A. a metal substrate;
B. a layer of a metallic bonding agent deposited on the metal
substrate;
C. a layer of ceramic semiconductor material deposited on the layer
of metallic bonding agent; and
D. a layer of conductive material forming an electrical terminal
comprising a metal-glass frit deposited on the layer of ceramic
semiconductor
7. An electrical element as defined in claim 6 wherein the
metal-glass frit
8. An electrical element as defined in claim 1 wherein the metal
substrate is a ferrous metal, the bonding agent is nickel
aluminide, and the layer
9. An electrical element as defined in claim 1 wherein the
metal-glass frit
10. An electrical element as defined in claim 1 wherein the metal
substrate
11. An electrical element comprising:
A. a cylindrical metal substrate having a diameter in the range of
0.075 inches to 0.125 inches;
B. a layer of metallic bonding agent deposited on the metal
substrate;
C. a layer of ceramic semiconductor material deposited on the
bonding agent; and
D. a layer of conductive material forming an electrical terminal
deposited
12. An electrical element as defined in claim 11 wherein the layer
of ceramic semiconductor material has a thickness of 0.003 inches
to 0.015
13. An electrical element comprising:
A. a cylindrical ferrous metal substrate;
B. a layer of nickel aluminide bonding agent deposited on the metal
substrate;
C. a layer of ceramic semiconductor material deposited on the layer
of bonding agent; and
D. a layer of conductive material forming an electrical terminal
deposited
14. An electrical element as defined in claim 13 wherein the
cylindrical metal substrate has a diameter in the range of 0.075
inches to 0.125
15. An electrical element as defined in claim 14 wherein the layer
of nickel aluminide bonding agent has a depth in the range of 0.001
inches to
16. An electrical element as defined in claim 10 wherein the
cylindrical metal substrate has a diameter in the range of 0.075
inches to 0.125
17. An electrical element as defined in claim 16 wherein the layer
of aluminum oxide and titanium oxide ceramic semiconductor material
has a
18. An electrical element as defined in claim 17 wherein the layer
of nickel aluminide bonding agent has a depth in the range of 0.001
to 0.010
19. An electrical element comprising:
A. a cylindrical stainless steel substrate having a diameter in the
range of 0.075 inches to 0.125 inches;
B. a mixture of nickel and aluminum applied to the stainless steel
metal substrate by plasma spraying temperatures in excess of
10,000.degree. F. wherein the nickel and aluminum combine by means
of a synergistic exothermic reaction to form a layer of bonding
agent comprising NiAl and Ni.sub.3 Al, said layer having a depth of
0.003 to 0.005 inches;
C. a layer of ceramic semiconductor material comprising a mixture
of 75-95% aluminum oxide and 5-25% titanium oxide applied to the
layer of bonding agent by plasma spraying at temperatures in excess
of 10,000.degree. F, wherein the aluminum oxide and titanium oxide
form a porous semiconductor material, said layer having a depth of
0.005-0.009 inches; and
D. a layer of conductive material comprising copper-glass frit
applied over the ceramic semiconductor material by pasma spraying,
said layer having a
20. An electrical element as defined in claim 19 wherein the length
of the cylindrical metal substrate is between 0.5 and 1 inches, and
wherein the bonding agent, ceramic semiconductor material, and
conductive material are
21. An electrical element as defined in claim 20 wherein the layer
of conductive material is ground away at the ends thereof to expose
two
22. An electrical element as defined in claim 21 wherein the
collars have a
23. An electrical element as defined in claim 19 wherein two
flanking longitudinal strips of the copper-glass frit are ground
away to expose two strips of the ceramic semiconductor material and
to separate the
24. An electrical element as defined in claim 23 wherein two
flanking longitudinal strips of the copper-glass frit are ground
away to expose two strips of the ceramic semiconductor material and
to separate the
25. An electrical element as defined in claim 19 wherein a first
electrical lead is connected to the stainless steel metal substrate
and wherein a
26. An electrical element as defined in claim 25 wherein the
electrical
27. An electrical element as defined in claim 23 wherein a first
electrical lead is connected to one portion of the copper-glass
frit and a second electrical lead is connected to the second
portion of the copper-glass
28. An electrical element comprising a body of ceramic
semiconductor material comprising a mixture of 75-95% aluminum
oxide and 5-25% titanium oxide formed by plasma spraying at
temperatures in excess of 10,000.degree. F., wherein the aluminum
oxide and titanium oxide form a cohesive porous semiconductor
material and at least two separate metallic electrodes each in
electrical contact with a different area of the surface of said
body.
Description
BACKGROUND OF THE INVENTION
This invention relates to electrical elements, and more
particularly to electrical elements useful as thermisters, heat,
humidity and moisture detectors, smoke detectors, and varisters.
The invention also relates to methods of fabricating electrical
elements having the above characteristics.
PRIOR ART
Semiconductor materials sandwiched between two electrodes for the
purpose of fire detection have been proposed in the prior art.
These devices have been designed primarily for detecting fires in
aircraft, and are not responsive to small temperature changes in
the range immediately above room temperature, wherein early fire
detection in the home could be accomplished. Further, these devices
have generally been expensive to fabricate including sealing the
devices from the possible absorption of oxygen, and providing
complicated arrangements to insure conductivity between the
semiconductor material and the electrodes.
Therefore, there has been a long felt need for an inexpensive,
rugged, electrical element useful in detecting temperature changes
in the range slightly above room temperature, wherein the
electrical element can be included in alarm systems for early
detection of fires in homes and other buildings.
There has also been a long felt need for an inexpensive electrical
element responsive to moisture and changes in humidity, wherein
such elements can be included in a variety of alarm systems for
such uses as warning of water in basements, warning of unacceptable
humidity levels in storage areas, or monitoring of humidity for
medical purposes, and for a variety of other uses wherein it is
desirable to monitor relative humidity or detect moisture.
There has also been a long felt need for an electrical element
which is responsive to smoke, wherein early detection of fires can
be accomplished by sensing the smoke from the fires before any
significant increase in room temperature occurs. The electrical
elements which are useful in detecting smoke may also be used in
the outside environment where large increases in the temperature of
the air would not be likely to occur. A potential use for such a
device may lie in the field of pollution monitoring.
OBJECTS OF THE INVENTION
It is an object of this invention to provide an electrical element
which operates as a thermister.
It is another object of this invention to provide an electrical
element which is responsive to changes in humidity.
It is an additional object of the invention to provide an
electrical element which is responsive to moisture.
It is a further object of this invention to provide an electrical
element which is responsive to smoke.
It is another object of this invention to provide electrical
elements having the above characteristics which are small.
It is another object of this invention to provide electric elements
which are rugged and inexpensive.
It is an additional object of the invention to provide a element
which is readily includable in alarm circuits.
It is a still further object of this invention to provide methods
for manufacturing such elements.
SUMMARY OF THE INVENTION
Electrical elements according to the invention herein comprise a
metal substrate having a ceramic semiconductor material applied
thereon by plasma spraying. The preferred metal substrate is
ferrous metal, and a bonding agent comprising nickel aluminide is
applied to the ferrous metal substrate by plasma spraying prior to
plasma spraying the ceramic semiconductor thereon. A conductive
coating of copper-glass frit is plasma sprayed over the ceramic
semiconductor material, and the conductive coating may be separated
into at least two portions each comprising one terminal.
The use of plasma spraying techniques fabricating the electrical
elements results in properties of extreme strength and ruggedness.
The materials are not only joined together by a strong mechanical
bond, but also exhibit very good conductivity between the various
layers despite exposure to moisture, oxygen, and the like. The
electrical elements are preferably fabricated in long rods, and the
substrate is preferably stainless steel rod having a diameter of
0.070 to .125 inches. After applying the various coatings, the
elongated electrical element is cut into short lengths to form a
plurality of small electrical elements each approximately 0.75 to
0.8 inches in length. The electrical elements which are to be used
as temperature sensors have a portion of the copper-glass frit
ground away near the ends of the element to expose a collar whereby
electrical arcing at the ends is minimzed. The electrical elements
which are used for moisture detectors, humidity detectors and smoke
detectors have a greater portion of the copper-glass frit gound
away, the ground away portion preferably taking the form of two
flanking longitudinal strips which also serve to separate the glass
copper frit into two distinct terminals.
The invention accordingly comprises the several steps and the
relation of one or more of such steps with respect to each of the
others, and the article possessing the features, properties, and
the relation of elements, which are exemplified in the following
detailed disclosure, and the scope of the invention will be
indicated in the claims.
DRAWINGS
For a fuller understanding of the nature and objects of the
invention, reference should be had to the following detailed
description taken in connection with the accompanying drawings, in
which:
FIG. 1 is a perspective view of an electrical element according to
the invention;
FIG. 2 is a side elevation view, partially in section, of the
electrical element of FIG. 1;
FIG. 3 is an end view of the electrical element of FIG. 1;
FIG. 4 is a perspective view of the electrical element of FIG. 1
having two longitudinal portions of the surface coating ground
away;
FIG. 5 is an end view of the electrical element of FIG. 4;
FIG. 6 is an end view of an electrical element similar to FIG.
1;
FIG. 7 is a schematic perspective view of apparatus for
manufacturing the electrical element of FIG. 1;
FIG. 8 is a graph of resistance vs. temperature for the electrical
element of FIG. 1;
FIG. 9 is a graph of resistance vs. temperature for the
configuration of the electrical element of FIG. 4;
FIG. 10 is a graph of resistance vs. relative humidity for the
electrical element of FIG. 4;
FIG. 11 is a graph of resistance vs. voltage for the the electrical
element of FIG. 1.
The same reference numbers refer to the same elements throughout
the various drawings.
PREFERRED EMBODIMENT
The electrical elements disclosed herein are sensitive to heat,
smoke, humidity, moisture, and voltage. Thus, they are very useful
as sensors in alarm systems for home and industrial environments.
Therefore, although the electrical elements and the methods for
making the same may be adapted for other uses, the preferred
embodiments discussed below relate to electrical elements and the
methods of fabricating the same wherein such elements are useful as
sensors in home and industrial alarm systems.
The electrical elements generally comprise a metal substrate having
a ceramic semiconductor applied thereon, and having a conductor
deposited on the surface of the ceramic semiconductor for use as a
terminal. The method of manufacturing such elements generally
comprises plasma spraying the ceramic semiconductor material onto
the metal substrate, and plasma spraying the conductive material
onto the ceramic semiconductor. A long electrical element is
fabricated, and the long electrical element is thereafter cut into
smaller electrical elements which are appropriately finished by
grinding away portions of the sprayed materials, and attaching lead
wires.
The metal substrate may be any ferrous metal or some nonferrous
metals, such as aluminum. The preferred metal substrate is a mild
stainless steel rod having a diameter in the range of 0.075 - 0.125
inches, and preferably 0.090 inches. It is convenient to work with
a stainless steel rod several feet in length.
The rod is prepared for plasma spraying by abrading the surface
thereof with 40 mesh aluminide oxide, commercially available as
Metcolite "S" from Metco, Inc., Westbury, Long Island, New York. A
relatively fine mesh abrading material is used as it is desirable
to merely remove contaminates from the surface of the rod and
slightly roughen the surface of the rod without pitting or removing
any substantial portion thereof.
Referring now to FIG. 7, a stainless steel substrate rod 10 is
placed in chunks 11 and 12 of a turning machine 13 for rotation
therein. One of the chucks is preferably spring loaded wherein
expansion of the rod 10 due to heating may be accommodated. The
turning machine 13 is provided with a threaded drive rod 14 and
associated fixture 16 for transversing a plasma spray gun 17 along
the length of the rod 10. A transverse speed of 25 feet per minute
is satisfactory. A suitable plasma spray gun is Metco's model M3B,
and details of plasma spray techniques are described in the Metco
Flame Spray Handbook, Volume III - Plasma Flame Process, published
by Metco, Inc. The preferred plasma for use herein is a mixture of
90 percent Nitrogen (N.sub.2) and 10 percent Hydrogen (H.sub.2),
and the gun is operated at temperatures in the range of
10,000.degree. F to 15,000.degree. F, typically 12,000.degree.
F.
The plasma spray gun 17 is used to first preheat the stainless
steel rod 10 to approximately 250.degree. F.
Ceramic semiconductor materials can be plasma sprayed directly onto
the stainless steel or ferrous metal substrates; however, depending
upon the choice of materials, the strength of the bond between the
ceramic semiconductor and the metal substrate can be enhanced by
use of a bonding agent.
The preferred bonding agent is nickel aluminide, commercially
available from Metco. Inc. and identified as its product M-450. The
bonding agent is a mixture of 95 percent nickel and 5 percent
aluminum, which is applied to the substrate by the plasma spray
gun. When subjected to the high temeratures of the plasma spray
gun, the mixture of nickel and aluminum undergoes a synergistic
exothermic reaction wherein the nickel and aluminum combine into
NiAl and Ni.sub.3 Al. The thickness of the layer of bonding agent
applied to the rod 10 may be in the range of 0.001 to 0.010 inches,
preferably 0.003 to 0.005 inches.
The preferred semiconductor material for fabricating the electrical
elements described herein is alumina titania, which comprises a
mixture by weight of 87 percent Al.sub.2 O.sub.3 and 13 percent
TiO.sub.2, and is avaiable from Metco, Inc. as its product Metco
130. It is advantageous to use the nickel aluminide bonding agent
between the alumina titania semiconductor material and the
stainless steel substrate, or with any other ferrous metal
base.
Other typical semiconductor materials which may be used in
fabricating electrical elements of the type described herein are a
mixture of alumina and barium oxide (Al.sub.2 O.sub.3 + BaO),
barium titanate (BaTiO.sub.2), calcium titanate (CaOTiO.sub.2), and
the like.
The ceramic semiconductor material is plasma sprayed over the
bonding agent. As described above, the plasma used is a mixture of
90 percent nitrogen (N.sub.2) and 10 percent hydrogen (H.sub.2).
The hydrogen, which disassociates into two single hydrogen atoms at
the temperatures achieved in the plasma spray gun, combines with
free oxygen present in or around the surface being coated, and
therefore "wipes away" the oxygen which might otherwise create
unwanted oxides.
The thickness of the layer of ceramic semiconductor material may be
in the range of 0.003 to 0.015 inches, and is preferably 0.007
inches.
Application of aluminum titania onto metal substrates by means of
sintering or baking in furnaces results in a low-porosity coating
which is whitish or pale gray in color. Plasma spraying the alumina
titania onto the cylindrical stainless steel substrate coated with
the bonding agent results in a higher porosity coating which is
very dark gray in color. The high porosity of the coating is
probably an effect of both the plasma spraying process and the
smaller diameter of the surface being coated. In the plasma spray
process, the alumina titania is introduced into a high velocity,
high temperature plasma stream wherein the alumina titania is
melted into droplets of varying sizes. The larger droplets hit the
surface being coated and cool very rapidly, bonding onto
neighboring droplets previously deposited and other droplets being
simultaneously deposited. The smaller droplets of alumina titania
in the gaseous stream may be swept away by the plasma rather than
be deposited on the surface. In addition, spraying material onto a
small diameter rod always results in some shadow spraying,
particularly at the portions of the rods which are tangential to
the spray stream. The previously deposited particles cast "shadows"
leaving areas on which material is not deposited. The rotation of
the rod during spraying achieves a uniform coating, but the coating
is nevertheless more porous because of the shadow effect.
The cause of the dark coloring of the ceramic semiconductor
material is not known; however, it may be the result of some
chemical reaction at the high temperatures achieved in the spray
process, or may be some doping effect from the metal substrate or
the nickel aluminide bonding agent.
A conductive layer is applied over the ceramic semiconductor. This
conductive layer preferably comprises a copper-glass frit
commercially available from Metco as XP 1159, the ratio of copper
to glass being 3 to 1. The copper and glass are ground into a very
fine powder and applied by plasma spraying using the
nitrogen-hydrogen plasma at approximately 12,00.degree. F. It is
believed that the copper-glass frit is deposted as small glass
beads partially coated with copper.
The copper-glass frit is applied to a depth in the range of 0.003 -
0.015 inches, and preferably 0.007 inches. However, almost any
depth of copper-glass frit or any other conductor would suffice as
a conductive layer.
The coated rod 10 is removed from the turning machine 13 after the
plasma spraying process is completed. Thereafter, individual
electrical elements are fabricated from the coated rod 10.
Referring now to FIGS. 1-3, there is shown an electrical element 20
comprising a portion of the coated rod 10. The length of element 20
may be as desired, but it has been found that segments of the rod
having length L in the range of 0.5 to 1 inch in length are useful.
In FIG. 2, element 20 has length L equal to 0.8 inch.
Element 20 is comprised of the mild stainless steel metal substrate
rod 10 having diameter D equal to approximately 0.090 inches. The
rod has been coated with a first layer 21 of nickel aluminide
bonding agent, which layer has the thickness B equal to 0.004
inches. The next layer 22 of the electrical element 20 comprises
the alumina titania ceramic semiconductor. Layer 22 has thickness
E, which is preferably 0.007 inches. The outer most layer 23 is the
copper-glass frit which has the dimension F equal to 0.007 inches.
The various layers were applied by plasma spraying process, as
described above.
Referring now to FIGS. 1 and 2, a portion of the copper frit 23 is
ground to expose a collar 26 of the semiconductor layer 22. The
collar 26 is desirable to insure that no shorts exist between the
copper-glass frit 23 and the rod 10 or bonding agent layer 21. Such
shorts are sometimes introduced by smearing of the metal across the
semicondcutor layer during the cutting operation. The collar may be
very narrow, having the dimension indicated at C of typicallly
0.010 inches.
The electrical element 20 is useful as a fire detector having as an
operating principle a decrease in resistance in response to an
increase in heat. Accordingly, the electrical element 20 is
connected in an alarm circuit 30 by solding a first lead 31 to the
copper frit layer 23 at 32, and connecting a second lead 33 to the
rod 10 at 34.
The alarm circuit 30 is preferably of the type which provides a DC
voltage across the element 20. The leads 31 and 33 may be
relatively long, and the element is placed in a position where air
heated by a fire would first accumulate, such as near a ceiling,
near a furnace, or the like. As the electrical element 20 is also
sensitive to humidity or moisture, it may be desirable to dip the
element in varnish or to otherwise provide a waterproof coating so
that the element will not cause false fire alarms in response to
high humidity or moisture. A waterproof coating is particularly
desirable when the element is installed in a kitchen where there
may be steam from cooking, or in other moist areas.
Referring now to the graph of FIG. 8, there are shown two curves A
and B of resistance vs. temperature. Curve A is the resistance vs.
temperature relationship for element 20 having 50 volts DC
potential applied between the leads 31 and 33, lead 31 being
negative. At 80.degree. F, a relatively high room temperature, the
element 20 has a resistance of 1.9 megaohms; at 100.degree. F, the
resistance of element 20 had decreased to less than 1.2 megaohms;
and at 130.degree. F, the resistance of element 20 was 0.65
megaohms. Thus, the resistance at 130.degree. F was approximately
one-third of the resistance at 80.degree. F, and the total
resistance drop was approximately 1.3 megaohms. This large drop in
resistance is more than adequate to trigger even a relatively
insensitive resistance responsive alarm circuit.
Curve B was derived by reversing the polarity of the two leads 31
and 33, the negative lead being connected to the central rod 10. At
80.degree. F the resistance of element 20 was 1.3 megaohms, and the
resistance declined substantially linearally to 0.54 megaohms at
120.degree. F. Curve B therefore also represents a substantial drop
in resistance upon which an alarm circuit can be triggered.
The polarity, i.e., the difference between curve A and curve B of
FIG. 8, indicates that the element 20 has some of the
characteristics of PN junction semiconductor. The conductive metal
substrate and the nickel aluminide bonding coat have available free
electrons, and is roughly equal to an N type material. It is a
conductor rather than a semiconductor. Also, on application of the
ceramic material by plasma spraying, the innermost layer adjacent
to the metal substrate or the layer of bonding agent may become
greatly contaminated with metal, and therefore form a thin layer of
N-type ceramic material which would most likely be a semiconductor,
although isolation of the material of this layer has not been
accomplished and has therefore not been tested.
Referring now to FIG. 6, an alternative electrical element 20C is
shown. Element 20C comprises an aluminum substrate in the form of a
rod 40, and has a layer 42 of the alumina titania applied thereon
by plasma spraying, and a layer 43 of copper-glass frit applied on
the ceramic also by plasma spraying. No bonding agent was required
in fabricating element 20C. Element 20C displays polarity
characteristics similar to those of element 20a. Again, the portion
of the ceramic material immediately adjacent to the aluminum
substrate may become contaminated with aluminum molecules, and
thereby comprise a thin layer of N type semiconductor material.
The combination of metal oxides plasma sprayed onto the metal
substrate probably take the form of an amorphous conglomeration
which is partly crystalline, but irregular. The fact that this
material conducts indicates that some modification of the metal
oxides is formed, the Al.sub.2 O.sub.3 by itself being an excellent
insulator. It is therefore believed that the ceramic material is an
irregular crystalline structure wherein some of the aluminum or
titanium atoms loosely share electrons in their outer most rings,
and wherein some deficiency of electrons exists so that the ceramic
material has the characteristics of P-type semiconductor.
This analogy to a pure PN junction semiconductor material would
account for the variation in resistance between curve A and curve B
of FIG. 8. Curve A, wherein the central terminal is positive,
represents a reverse conduction voltage, wherein higher resistance
results. Curve B, wherein the negative terminal is applied to the
metal substrate, is the equivalent of the forward conduction
voltage situation, and lower resistance and higher conductivity
result.
The decrease in resistance in response to a rise in temperature is
most likely caused by thermal excitation of the material, wherein a
greater number of electrons are free for conduction. This is a well
known phenomena in semiconductor art. It is a surprising discovery
that this phenomena is also found in the devices disclosed herein,
which are fabricated so quickly and inexpensively, and that the
phenomena occurs at temperatures in the proper range for use as a
fire or heat detector in home or industrial environments.
Referring now to FIGS. 4 and 5, there is shown an alternative
electrical element 20a. This element is fabricated similarly to the
electrical element 20, with the additional step of grinding away
two strips of the copper-glass frit wherein the copper-glass frit
layer 23 is divided into two portions 23a and 23b, and two strips
27 of the semiconductor layer 22 are exposed. A silicon carbide
grinding wheel comprising carbon embedded in compressed paper is
used for the grinding operation, as it has sufficient hardness to
remove the copper-glass frit but insufficient hardness to remove
any of the ceramic semiconductor immediately thereunder. In
addition, the wheel is sufficiently flexible to conform to the
cylindrical surface of the ceramic semiconductor layer so that the
copper-glass frit is removed over the entire width of the wheel
rather than at a line tangential to the wheel. The preferred width
W of the path is 0.04 inches, although the width is by no means
critical.
Element 20a is connected to an alarm circuit 30a by means of lead
36 attached to the portion of the copper-glass frit 23a at 37, and
lead 38 attached to the second portion of the copper-glass frit 23b
at 39. The element 20a is preferably dipped in a waterproofing
solution when the element is to be used as a heat sensor. Varnish
works well for this purpose.
Referring now to FIG. 9, there is shown a graph of resistance vs.
temperature for element 23a. The alarm circuit 30a provides a 100
volt DC potential across the element, as indicated in the sketch
included with FIG. 9. At 80.degree. F, the element 20a has a
resistance of 12 megaohms. The resistance declines in a
substantially linear manner to approximately 7 megaohms at
128.degree. F. Thus, the element 23a displays a 50 percent loss of
resistance in the temperature range immediately above average room
temperatures, and is very useful in detecting fires in the home
environment.
The element 20a may also be used to detect moisture, humidity, or
smoke. Referring now to FIG. 10, a graph of resistance vs. relative
humidity is shown. The element 20a is connected to an alarm circuit
as shown in the sketch in FIG. 10, i.e., one lead is soldered to
portion 23a of the copper-glass frit and the second lead is
soldered to the rod 10. A low voltage of 1 volt DC is provided
across the element by the alarm circuit. At a relative humidity of
40%, the element displayed a resistance of 60 megaohms. The
resistance dropped most rapidly between 60% and 70% relative
humidity, the resistance having a value of approximately 20
megaohms at 70% relative humidity. A more gradual drop in
resistance was displayed between 70 and 90% relative humidity, and
at 90% relataive humidity the element displayed a resistance of
approximately 5 megaohms. In the range slightly above 90% relative
humidity, a sharp drop to about 2 megaohms resulted, although this
departure from a smooth curve may have been the result of
instrument error. At 95% relative humidity, the resistance of the
element had dropped to approximately 1 megaohm, and at 99% relative
humidity the resistance was near zero.
The above graph points out the usefulness of the element as a
humidity sensor. In combination with a properly designed alarm
circuit, the element is sufficiently sensitive to be used as a
breath detector for medical applications. The element can also be
used for applications wherein monitoring of dry storage conditions
is desired, or the like.
The element also operates as an almost indestructible water level
sensor; i.e., dipping the element in water causes a nearly complete
loss of resistance. Repeated the wetting and drying of the element
results in no loss of its other sensing properties.
The element is also sensitive to smoke, having the response of
decreased resistance. No standard for concentration of smoke was
derived, and hence no graph is included herein. However, in tests
the element was sufficiently sensitive to trigger an alarm circuit
in response to a puff of concentrated cigarette smoke applied
directly to the element. It is believed that the smoke sensing
characteristic of the element is an extension of the moisture
sensing properties of the element, in the smoke contains a
substantial amount of water, vapor, acids, and the like.
It is believed that the moisture responsive characteristics of the
element are caused by an increased concentration of moisture in the
porous ceramic material. This increase in concentration may take
the form of water vapor, condensed water droplets, or an actual
infusion of water, and may also be acids, other electrolites, or
the like, in similar forms. It is possible that these molecules,
loosely embraced in the physical space of the porous ceramic
material, share some electrons with the crystal instruction of the
ceramic material, and thereby enhance the conductive capabilities
thereof. It is also possible that the moisture is sufficiently
contaminated to be an electrolite in and of itself, or that it
combines with some of the ceramic semiconductor material to become
an electrolite and increase conduction.
In any event, the moisture is not believed to penetrate the ceramic
material to a very great depth, and consequently the decreased
resistivity due to moisture is primarily a surface effect.
Therefore the configuration of the element shown in FIG. 4 is
preferred for moisture sensing.
The electrical elements 20 and 20A also operate as varisters.
Referring now to FIG. 11, there is shown a graph of resistance vs.
supplied DC voltage for element 20 at room temperature. The graph
of FIG. 11 includes two curves A and B, curve A being derived from
applying voltage with the positive terminal connected to the core,
and curve B being derived by connecting the negative terminal to
the core. The resistance of element 20 with the positive terminal
connected to the central core was approximately 3.9 megaohms at 20
volts, and decreased rapidly to about 2.5 megaohms at 40 volts.
Thereafter, a more gradual decrease in resistance was observed, the
resistance having a value of approximately 1.75 megaohms at 70
volts, and about 1.3 megaohms at 100 volts. Curve B, which was
derived by connecting the negative terminal of the voltage supply
to the center of element 20, resulted in a resistance of 2.4
megaohms at 20 volts, with the resistance decreasing relatively
linearly to a value of approximately 1 megaohm at 70 volts.
Thereafter, the resistance remained relatively constant to 100
volts.
Element 20A may therefore be used as a varister, i.e., the
resistance of the element is dependent on the applied voltage. The
ceramic material apparently operates as a leaky di-electric, and
the polarity is again evident. Therefore, the ceramic material, in
addition to acting like a leaky di-electric, retains some of the
characteristics of a PN junction diode.
The electrical elements disclosed herein may be used as sensors in
a variety of alarm systems having many applications. The size of
the elements may be varied wherein quantitative differences in the
response of the elements can be realized; e.g. a longer element
having the same thicknesses of layers of material has a higher
initial resistance. Similarly, the elements can be used in alarm
circuits providing AC across the terminals, wherein the elements
behave somewhat differently but operate along the same principles
disclosed herein.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently
attained and, since certain changes may be made in carrying out the
above method and in the article set forth without departing from
the scope of the invention, it is intended that all matter
contained in the above description or shown in the accompanying
drawings shall be interpreted as illustrative and not in a limiting
sense.
It is also to be understood that the following claims are intended
to cover all of the generic and specific features of the invention
herein described, and all statements of the scope of the invention
which, as a matter of language, might be said to fall
therebetween.
* * * * *