U.S. patent number 3,621,446 [Application Number 04/799,808] was granted by the patent office on 1971-11-16 for thermal relay.
This patent grant is currently assigned to Bell Telephone Laboratories. Invention is credited to George E. Smith, Robert H. Walden.
United States Patent |
3,621,446 |
|
November 16, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
THERMAL RELAY
Abstract
In a thermal relay of the type using a conductive heater layer
to switch a layer of vanadium dioxide between nonconducting and
conducting states, design advantages are attained by using a
conductive substrate separated from the vanadium dioxide by a layer
of insulative material having a thermal conductance per unit area
that is within certain prescribed limits.
Inventors: |
George E. Smith (Murray Hill,
NJ), Robert H. Walden (Scotch Plains, NJ) |
Assignee: |
Bell Telephone Laboratories
(Inc., Murray Hill)
|
Family
ID: |
25176805 |
Appl.
No.: |
04/799,808 |
Filed: |
February 17, 1969 |
Current U.S.
Class: |
338/23 |
Current CPC
Class: |
H01C
7/047 (20130101) |
Current International
Class: |
H01C
7/04 (20060101); H01c 007/04 () |
Field of
Search: |
;338/20,23,24,25 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rodney D. Bennett, Jr.
Assistant Examiner: R. Kinberg
Attorney, Agent or Firm: R. J. Guenther Arthur J.
Torsiglieri
Claims
1. A thermal relay comprising: a layer of resistive material having
a resistivity that varies as a function of temperature and having
contacts at opposite ends thereof; a conductive substrate on one
side of the resistive layer and insulated therefrom by a first
layer of insulative material; a conductive heater layer on the
opposite side of the resistive layer and insulated therefrom by a
second layer of insulative material; the first layer of insulative
material having a thermal conductance per unit area G.sub.A which
is less than approximately 500 watts per
2. The thermal relay of claim 1 wherein: the layer of resistive
material has a negative coefficient of resistivity.
3. The thermal relay of claim 2 wherein: the parameter G.sub.A in
watts per centimeter.sup.2 - degree centigrade
4. The thermal relay of claim 3 wherein: the thermal conductivity K
of the first layer of insulative material is approximately
10.sup.-.sup.1 watts per centimeter - degree centigrade and the
parameter G.sub.A substantially conforms to the relation 20
G.sub.A
5. The thermal relay of claim 3 wherein: the thermal conductivity K
of insulative material is approximately 10.sup.-.sup.2 watts per
centimeter - degree centigrade and the parameter
6. The thermal relay of claim 3 wherein: the thermal conductivity K
of the first layer of insulative material is approximately
10.sup.-.sup.3 watts per centimeter - degree centigrade and the
parameter G.sub.A substantially conforms to the relationship
2.0
7. The thermal relay of claim 3 wherein: the first layer of
insulative material is a silicon compound having
8. The thermal relay of claim 7 wherein the first layer of
insulative
9. The thermal relay of claim 3 wherein: the length of the layer of
resistive material is larger than the combined thickness of the
resistive material layer of insulative material and the second
layer of insulative material.
Description
This invention relates to relays, and more particularly, to relays
that make use of the negative temperature coefficient of
resistivity of material such as vanadium dioxide.
The paper "Thin-Film Switching Elements of VO.sub.2 " by K. van
Steensel, F. van de Burg and C. Kooy, Philips Research Reports,
Volume 22, pages 170-177, 1967, describes a four-terminal relay in
which a conductive heater film is used to switch a layer of
vanadium dioxide between conducting and nonconducting states.
Vanadium dioxide is an example of a material which, because of its
negative temperature coefficient of resistivity, will act as a low
resistivity metal if heated above a threshold temperature, but will
act as a high resistivity semiconductor if maintained at a
temperature below the threshold. The relay structure described by
van Steensel et al. comprises a layer of vanadium dioxide on an
insulative substrate such as glass, an insulative film such as
silicon oxide overlaying the vanadium dioxide, and a conductive
heater film overlaying the silicon oxide layer. By passing current
through the heater layer, the vanadium dioxide is switched from a
nonconducting to a conducting state in which it transmits current
between opposite contacts.
It is of course desirable to optimize the operating parameters of
the van Steensel et al. relay to conform to the systems in which it
is to be used. In particular, it is often desirable to optimize the
power required for switching and the time required for switching.
These two parameters, however, are mutually dependent and one
cannot be changed without changing the other.
It is an object of this invention to provide a thermal relay in
which the power required for switching and the time required for
switching can be substantially independently designed.
This and other objects of the invention are attained in an
illustrative embodiment thereof comprising a thermal relay of the
general type described above. Rather than using an insulative
substrate, however, a thermally conductive substrate is used which
is separated from the vanadium dioxide layer by a thin layer of
insulative material. The thermal conductance of the insulative
layer is maintained within a specified range such that a thermal
path is provided from the heater film through the vanadium dioxide
layer to the conductive substrate. With this provision it can be
shown that the power P required for switching and the time t
required for switching can be optimized independently of each
other. Since any desired switching time t and any desired switching
power P can, within a rather broad range, be attained, the
flexibility and number of uses to which the relay may be put are
substantially increased.
These and other objects, features and advantages of the invention
will be better understood from a consideration of the detailed
description taken in conjunction with the accompanying drawing.
DRAWING DESCRIPTION
FIG. 1 is a sectional view of a thermal relay of the prior art;
and
FIG. 2 is a sectional view of a thermal relay in accordance with an
illustrative embodiment of the invention.
DETAILED DESCRIPTION
Referring now to FIG. 1, there is shown a thermal relay 11 of the
type described in the van Steensel et al. paper comprising a layer
12 of vanadium dioxide (VO.sub.2) having contacts 14 and 15 on
opposite ends and bonded to a substrate 13 of insulative material.
A conductive heater film 17 having contacts 18 and 19 is located in
close proximity to the VO.sub.2 layer but is insulated from it by a
layer 20 of insulative material.
When the VO.sub.2 layer 12 is below a threshold temperature,
typically 68.degree. C., it acts as a semiconductor and displays a
high resistivity to current between contacts 14 and 15. When the
layer 12 is heated above its threshold temperature, due to heat
generated by heater film 17, it acts as a low resistivity metal,
and current flows freely between opposite contacts. Thus the
structure of FIG. 1 constitutes a four-terminal thermal relay: When
current is directed through heater film 17, the temperature of the
VO.sub.2 layer 12 is raised above threshold and current is
transmitted through a circuit including contacts 14 and 15; but,
when current through the heater film 17 is terminated, current
through the VO.sub.2 layer is substantially reduced.
Of course, numerous four-terminal relay devices other than that
shown in FIG. 1 are available to the engineer designing a system,
and the one he chooses will significantly depend on the extent to
which device parameters can be optimized to match the requirements
of the system. Two parameters that characterize the device of FIG.
1 are the power P' required by the heater film 17 to switch the
VO.sub.2 layer between conductive states, and the time t' required
for such switching. It can be shown that the switching power P' of
the FIG. 1 device is given by the equation
P' = (.pi..sup.2 lK'.DELTA.T/2) (1) and the switching time t' is
given by
t' = (l.sup.2 /4.alpha.') (2) where K' is the thermal conductivity
of substrate 13, .alpha.' is the thermal diffusivity of the
substrate, .DELTA.T is the difference in temperature between the
transition temperature and the ambient temperature, and l is half
the lateral dimension of the VO.sub.2 layer 12 as shown in FIG. 1.
Notice that only one geometrical parameter, namely the lateral
dimension l, is contained in the expressions for P' and t'. Hence,
switching power and switching time cannot be independently
determined in the design of the structure, and compromises between
the two are inherent.
In accordance with the invention, this drawback is overcome by the
relay 23 of FIG. 2 which uses a conductive substrate 24 rather than
an insulative substrate. The vanadium dioxide layer 25 is separated
from the conductive substrate by an insulative layer 26. A heater
film 27 is separated from the VO.sub.2 layer by an insulative layer
28. As before, heater current between contacts 30 and 31 is used to
switch the states of the VO.sub.2 layer 25, thereby controlling
current between contacts 32 and 33. Since the substrate 24 is a
good conductor of heat, a thermal path is provided that extends
from heater film 27 to the substrate. This thermal path will
transmit a significant quantity of heat compared to that
transmitted by the contacts if the thermal conductance of
insulative film 26 is within a specified range and if the combined
thickness d of insulative layer 28, VO.sub.2 layer 25 and
insulative layer 26 is smaller than the lateral dimension l. When
this is true, it can be shown that the power P required for
switching is given by
P = (3.pi.l.sup.2 K.DELTA.T/8d) (3) and the switching time t is
given by
t = (d.sup.2 /.alpha.) (4) where K is the thermal conductivity of
layer 26, and .alpha. is the thermal diffusivity of layer 26.
Notice that in this case switching power P is a function of the
lateral dimension l and the thickness dimension d while switching
time t is a function only of the thickness dimension d shown in
FIG. 2. This being true, switching time and switching power can be
independently optimized by independent adjustment of the dimensions
l and d.
Substrate 24 should of course be made of a material having high
thermal conductivity such as copper. The insulative layer 26 may
conveniently be silicon dioxide which has a thermal conductivity of
10.sup.-.sup.2 watts per centimeter-degree centigrade. Setting K
equal to 10.sup.-.sup.2, table I shows various values of the
thickness dimension d and the lateral dimension l that should be
used for designing relays having switching powers of between
10.sup.-.sup.1 and 10.sup.-.sup.3 watts and switching times t of
between 10.sup.-.sup.3 and 10.sup.-.sup.6 seconds, in accordance
with equations (3) and (4):------------------------------------
---------------------------------------TABLE I t (sec.) d, l Hi
Speed Lo Speed
_________________________________________________________________________
_ (.mu.m) 10.sup.-.sup.6 10.sup.-.sup.3 Lo Power P 10.sup.-.sup.3
0.6, 2.4 17, 13 (watts) Hi Power 10.sup.-.sup.1 0.6, 24 17, 130
_________________________________________________________________________
_ The first number in each box refers to the thickness dimension d
to be used and the second number to the lateral dimension l. For
example, if a high speed low power relay is designed to have a
switching time t of 10.sup.-.sup.6 seconds and a switching power P
of 10.sup.-.sup.3 watts, the thickness dimension d should be 0.6
microns and the lateral dimension l should be 2.4 microns.
A useful parameter for characterizing the insulative layer 26 is
the conductance per unit area G.sub.A, which is related to the
thermal conductivity K and thickness b by the equation
G.sub.A = K/b (5) Assuming that K .apprxeq. 10.sup.-.sup.2 watts
per centimeter degree C., table I defines a range of values for
G.sub.A assuming d .apprxeq.b, given by
5.9 G.sub.A 170 watts/cm.sup.2 -.degree.C. (6)
Materials having other values of thermal conductivity K could of
course be used as the insulative film 26, but is is unlikely that a
material having a thermal conductivity of substantially less than
10.sup.-.sup.3 or substantially more than 10.sup.-.sup.1 watts/cm -
.degree.C. would be useful in a thermal relay. Assuming that k is
10.sup.-.sup.3, the values of dimensions d and l are those given in
table II:------------------------
---------------------------------------------------TABLE II t
(sec.) d, l Hi Speed Lo Speed
_________________________________________________________________________
_ (.mu.m) 10.sup.-.sup.6 10.sup.-.sup.3 Lo Power P 10.sup.-.sup.3
0.17, 4.0 5.0, 20 (watts) Hi Power 10.sup.-.sup.1 0.17, 40 5.0, 200
_________________________________________________________________________
_
Table II leads to the following range of values of G.sub.A :
2.0 G.sub.A 60 watts/cm.sup.2 - .degree.C. (7)
Assuming that K is 10.sup.-.sup.1 watts/cm-.degree.C., the range of
values for d and l is that given in table
III.----------------------------
-----------------------------------------------TABLE III t (sec.)
d, l Hi Speed Lo Speed
_________________________________________________________________________
_ (.mu.m) 10.sup.-.sup.6 10.sup.-.sup.3 Lo Power P 10.sup.-.sup.3
2.0, 1.6 60, 30 (watts) Hi Power 10.sup.-.sup.1 2.0, 16 60, 300
_________________________________________________________________________
_
Table III leads to the following range of values for G.sub.A :
20 G.sub.A 500 watts/cm.sup.2 - .degree.C. (8)
From relationships (7) and (8), a general range of values for the
thermal conductance per unit area for any reasonable value of
thermal conductivity may be defined as
2.0 G.sub.A 500 watts/cm.sup.2 - .degree.C. (9)
Extensive mathematical analyses which could have been included for
showing the derivation of the relationships given above have, for
reasons of brevity and clarity, not been included. To simplify
computation, these analyses assume a circular configuration of the
various layers of the structures of both FIGS. 1 and 2. To give a
more general statement of the switching power P of the structure of
FIG. 2 equation (3) may be expressed as
P = CAK.DELTA.T/d (10) where A is the area of VO.sub.2 layer 25 and
C is a constant dependent on device configuration which, in the
case of a circular structure, is 3/8. The constant is nearly equal
to 3/8 for structures that are nearly symmetrical such as square
configurations.
In summary, the criteria defining a thermal relay structure in
which the switching power and switching time are mutually
independent have been established. The ability to adjust switching
power and switching time independently substantially increases the
uses to which relays of this type may be put. For example, the
relay of FIG. 2 may be used as an average power limiter in which a
low power threshold and long switching time are simultaneously
required. Moreover, it can be shown that if both the switching
power P and the time t are sufficiently small, the switching energy
of the relay of FIG. 2 is substantially smaller than that of FIG.
1. The ratio of switching energy e.sub.1 of the relay of FIG. 2 to
the switching energy e.sub.2 of a comparable relay of FIG. 1 can be
shown to be
e.sub.1 /e.sub.2 .apprxeq. d/l (11)
While the relay has been described as using a VO.sub.2 layer, it is
to be understood that any material having a negative temperature
coefficient of resistivity could alternatively be used. Various
other modifications and embodiments may be made by those skilled in
the art without departing from the spirit and scope of the
invention.
* * * * *