Temperature sensitive device

A heater comprises a substrate having an electrically-insulative ceramic coating and a heater track deposited on the coating and electrically connected to a power supply via ends. The heater track consists of a composite material having predetermined proportions of a metal and a material capable of undergoing a reversible change in volume at a predetermined phase transition temperature. The change in volume changes the proportions of metal to material and thus changes the resistivity of the composite material, so that the heater can be used as a self-regulating thermal cut-out device by limiting its own heat output to the phase transition temperature.

This invention relates to a temperature sensitive device and in particular, 
though not exclusively, to such a device for controlling the power 
supplied to a load, for example a resistive heater, in accordance with a 
predetermined threshold temperature. 
Known temperature sensitive devices of this type generally consist of a 
thermostat or a thermal cut-out device, which disconnects, or at least 
reduces, the power supplied to the heater when a predetermined threshold 
temperature is sensed and reconnects, or increases, the supplied power 
when the temperature falls below the threshold temperature. 
Such devices may consist of a mechanical switch including a 
thermally-expansive member, such as a metal rod or a bimetallic strip, 
which undergoes thermal expansion, when heated, and operates a switch at 
the threshold temperature. 
Alternatively, such devices may consist of a temperature-dependent 
resistor, the output of which is compared with a reference signal 
indicative of the threshold temperature. 
However, these conventional temperature-sensitive devices have relatively 
complex constructions and thus tend to be susceptible to malfunction 
during operation, particularly mechanical devices including moving 
components. 
As an alternative to such mechanical devices, U.K. Pat. No. 1,243,410 
discloses the use of vanadium dioxide, which exhibits an abrupt change in 
electrical conductivity at a predetermined transition temperature and can 
thus be employed as both heater and temperature regulator. 
However, vanadium dioxide can only be used as a thermal cut-out at one 
particular temperature, i.e. at its transition temperature, and even when 
the material is suitably doped, as described in U.K. Pat. No. 1,243,410, 
the range of temperatures within which the doped material can be made to 
exhibit a phase transition may be relatively limited. 
It is therefore an object of the present invention to provide a 
temperature-sensitive device, which, on the one hand, is more reliable 
than known mechanical temperature-sensitive devices, and, on the other 
hand, can be made to operate at a temperature selected from a relatively 
wide range of temperatures. 
According to the present invention there is provide a temperature-sensitive 
device comprising an electrically-conductive composite material consisting 
of predetermined proportions of a metal and a material capable of 
undergoing a reversible phase transition at a predetermined temperature, 
said phase transition consisting of a reversible change in volume of said 
phase transition material, thereby effecting a reversible change in said 
proportions and thus in said electrical conductivity of said composite 
material at said temperature. 
In one embodiment, the composite material is deposited on a substrate in 
the form of a heater track, the heat output of which is reduced by a 
decrease in the electrical conductivity when the temperature, at which the 
phase transition occurs, is reached. When the temperature subsequently 
falls below the phase transition temperature, the phase transition 
material undergoes a reverse phase transition so that the electrical 
conductivity, and thus the heat output, of the heater is returned to its 
original value. 
In this manner, the heater is effectively a self-regulating device, which 
limits its own heat output to a predetermined threshold temperature. 
The material capable of undergoing the reversible phase transition may be 
one of a number of suitable materials, such as a ceramic or a polymer, 
which materials undergo the phase transition over a wide range of 
temperatures.

A heater, shown in FIGS. 1 and 2, comprises a substrate 1, preferably 
formed from a metal, having an electrically-insulative ceramic coating 2 
on one side thereof. A heater track 3, preferably in the form of a thick 
film ink, is deposited, such as by any suitable printing technique, onto 
the coating 2 and is electrically connected to a power supply via ends 4 
and 5. A coating 6, of similar or the same composition as coating 2, may 
also be provided on the side of the substrate 1 remote from the heater 
track 3. 
The heater track 3 is formed from a composite material consisting of 
predetermined proportions of a suitable ceramic material and a metal, 
preferably in the form of a powder. 
As shown by the graph in FIG. 3, when a metal is added to an 
electrically-insulative ceramic material, the electrical resistivity, and 
thus conductivity, of the composite material varies, in dependence on the 
relative proportions by volume of the metal and the ceramic material. 
It can be seen from FIG. 3 that, as the metal content is increased, at a 
critical metal content C by volume, a sudden decrease in resistivity, and 
thus a corresponding increase in conductivity, of the composite material 
occurs, because at this point a complete network of interconnecting metal 
particles exists throughout the material, thereby making it a good 
electrical conductor. 
The ceramic material for the composite material is specifically chosen such 
that it undergoes a reversible phase transition, when heated to a 
particular temperature, which causes a change in volume of the ceramic 
material. 
When, therefore, a composite of the selected ceramic and metal, mixed in 
predetermined proportions by volume at room temperature so that the 
composite is a relatively good electrical conductor, is heated to the 
phase transition temperature, the ceramic expands, thereby causing an 
effective decrease in the volume proportion of metal content. The 
proportions of ceramic and metal at room temperature are determined to 
ensure that the expansion of the ceramic, when heated to the phase 
transition temperature, causes the proportion of metal content to decrease 
to below the critical content C, thereby effecting a sudden increase in 
resistivity, and thus a corresponding decrease in conductivity, of the 
composite at this temperature. 
The value of the critical metal content C is generally between 30% and 40% 
by volume, but this concentration can vary considerably, in dependence on 
the particle size and shape before preparation of the composite material. 
In fact, the composite material may be made electrically conductive with a 
much lower metal content, particularly if a fibrous metal material is 
used. 
By utilising a composite material of this type for the material of the 
heater track 3, a voltage can be applied to the heater until it reaches 
the phase transition temperature, at which the ceramic expands, 
effectively reducing the volume proportion of metal content to below the 
critical value C and thus causing a sudden decrease in electrical 
conductivity of the heater track 3. At this point therefore, the heat 
output of the heater track 3 is significantly reduced and it begins to 
cool. As it cools to below the phase transition temperature, a reverse 
phase transition occurs and the ceramic returns to its original volume, 
effectively increasing again the proportions of the metal content to its 
original value above the critical value and thus causing a sudden return 
of the electrical conductivity to its original relatively high value. 
In this manner, the heater is caused to be temperature-sensitive and 
becomes a self-regulating thermal cut-out device by limiting its own heat 
output to the phase transition temperature of the ceramic of the composite 
material. 
A considerable number of ceramic and other types of materials undergo a 
change in volume at different phase transition temperatures, so that a 
suitable material can be selected to provide the correct threshold 
temperature for a particular application for the thermal cut-out device. 
A specific example of a suitable ceramic material is quartz, which has a 
phase transition temperature of approximately 573.degree. C., at which a 
significant change in volume of the material occurs. Any suitable metal, 
which is stable to at least the phase transition temperature of the 
ceramic, may be utilised. Such a heater track, formed from a composite of 
quartz and a suitable metal to provide a thermal cut-out, may have 
applications, for example, in glass ceramic cooking hobs (not shown), 
wherein it is necessary to limit the operating temperature to prevent 
overheating of the glass ceramic cooktop. 
Other suitable materials include polymers, which undergo a phase transition 
known as the "Glass Transition" between a crystalline and an amorphous 
state, accompanied by a change in volume. The polymer materials can be 
loaded with a conductive metal filler to the critical concentration 
referred to hereinbefore and a change in resistivity of the polymer-metal 
composite material is exhibited at the glass transition temperature, when 
the polymer undergoes a significant change in volume. 
Four specific examples of suitable polymers and their approximate 
transition temperatures are shown below. 
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Polymer Transition Temp. (.degree.C.) 
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Polystyrene 100 
Polybutadiene 200 
Nylon-66 322 
Polyethylene terephthalate 
342 
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The transition temperatures of polymers have been found to be particularly 
sensitive to molecular weight changes, so that the transition temperature 
can be readily changed by variation in the molecular weight, thereby 
increasing further the temperature range over which devices, in accordance 
with the invention, can be made to operate. 
Some polymers, such as polybutadiene, may undergo a substantially 
continuous change in volume with temperature rather than an abrupt change, 
but still exhibit a discontinuity in the rate of volume change at the 
transition temperature. After this temperature, there is a marked increase 
in the rate of change of volume, thereby resulting in a higher resistivity 
increase with temperature in the polymer-metal composite material. 
Rather than using the composite material as a self-regulating heater, it 
may be used merely as a temperature-sensitive device, which forms an 
electrical connection to a separate heater, or other load, the heat output 
of which is required to be limited to the threshold phase transition 
temperature of the ceramic of the composite material. As the load heats 
the composite material to the threshold temperature, expansion of the 
ceramic significantly reduces electrical conduction through the material, 
thereby reducing electrical connection of the load to the voltage supply. 
As the heat output of the load decreases to below the threshold 
temperature, the electrical connection is restored. 
A temperature-sensitive device, in accordance with the present invention, 
may be utilised in many other temperature-sensing applications including 
non-destructable fuses, thermostats and other safety cut-outs and sensors. 
If temperature regulation below the threshold temperature is required, such 
as in a cooking hob, an additional temperature sensor, which responds 
continuously to change in temperature would be needed. 
The present temperature-sensitive device is therefore much simpler in 
construction than known thermal cut-outs and other temperature sensors, as 
well as being more reliable in operation, because it has no moving parts, 
which may be susceptible to malfunction.