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.

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.

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.