BACKGROUND

The present disclosure relates to the field of temperature detection and of thermal imaging with temperature detection elements made of resistive pastes.

DISCUSSION OF THE RELATED ART

Sensors adapted to temperature detection and thermal imaging made of printed resistive pastes on a low-cost plastic substrate, possibly flexible, are known. Such sensors are for example used to measure the temperature of an object or to measure a thermal gradient for thermal imaging with a maximum sensitivity and accuracy.

Resistive pastes with a negative temperature coefficient, currently called NTC, are for example based on a metal oxide such as antimony tin oxide of chemical formula Sb2O5SnO2or ATO. Heat-sensitive resistive pastes with a positive temperature coefficient, currently called PTC, are currently based on carbon black. In a PTC resistor, the value of the electric resistance increases as the temperature increases. In a NTC resistor, the value of the electric resistance decreases as the temperature increases.

Further, rather than using a heat-sensitive resistor alone, it is known, to increase the measurement accuracy, to assemble NTC and PTC resistors in a Wheatstone bridge.

SUMMARY

An object of the present application is to improve the sensitivity of resistive paste heat-sensitive resistors.

Another object of the present application, more particularly adapted to the Wheatstone bridge assembly, is to increase the resistance value of resistive paste heat-sensitive resistors.

Thus, an embodiment provides a heat-sensitive resistor of negative or positive temperature coefficient, respectively comprising a resistive element based on antimony tin oxide or a resistive element based on carbon black, containing a polymer having a dielectric constant between 2 and 3, a molar mass between 50,000 and 150,000 g/mol, and a glass transition temperature Tg between 40 and 100° C.

According to an embodiment, the polymer belongs to the family of styrenic polymers or to the family of fluorinated polymers.

According to an embodiment, the resistive element comprises from 5% to 40% by mass of dry extract of the polymer with respect to the total mass of the resistive element.

An embodiment also provides a heat-sensitive resistor of positive temperature coefficient having an adjustable temperature threshold with a resistance which is constant below said threshold and which increases along with temperature above said threshold, comprising, in series, a first resistive track made of a heat-sensitive paste of positive temperature coefficient, PTC, and a second resistive track made of a heat-sensitive paste of negative temperature coefficient, NTC.

An embodiment also provides a method of manufacturing a heat-sensitive resistor of negative temperature coefficient, comprising the steps of:forming a first solution comprising antimony tin oxide or carbon black containing a polymer having a dielectric constant between 2 and 3, a molar mass between 50,000 and 150,000 g/mol, and a glass transition temperature Tg between 40 and 100° C.;forming portions of the first solution on a support; andheating the portions.

According to an embodiment, the step of manufacturing the first solution comprises the steps of:providing a second solution comprising antimony tin oxide or carbon black and a first solvent;providing a third solution comprising the polymer and a second solvent; andmixing the second and third solutions.

According to an embodiment, the third solution is by a mass proportion smaller than 30%, preferably from 10 to 30%, with respect to the total mass of the first solution.

According to an embodiment, the third solution comprises from 15 to 30% by mass, for example, 25%, of the polymer in from 85 to 70% by mass, for example, 75%, of the second solvent.

According to an embodiment, the first solvent is selected from the group comprising cyclopentanone, ethyl acetate, tetrahydrofuran, acetone, 3-hexanone, and 2-pentanone for antimony tin oxide or the group comprising cyclopentanone, dibutyl carbitol, and ethylene glycol diacetate for carbon black.

According to an embodiment, the second solvent has an evaporation temperature in the range from 100 to 170° C.

According to an embodiment, the polymer belongs to the family of styrenic polymers or to the family of fluorinated polymers.

According to an embodiment, the second solvent is selected from the group comprising toluene or butyl acetate for styrenic polymers or perfluorotributylamine (FC43) for fluorinated polymers.

According to an embodiment, the evaporation temperature of the first solvent is higher than the evaporation temperature of the second solvent.

An embodiment also provides a method of manufacturing a heat-sensitive resistor of positive temperature coefficient having an adjustable temperature threshold with a resistance which is constant below said threshold and which increases along with temperature above said threshold, comprising the steps of:forming in series on the support a first portion made of a heat-sensitive paste of positive temperature coefficient, PTC, and a second portion made of a heat-sensitive paste of negative temperature coefficient, NTC, the first and/or the second portion being formed with the first solution; andheating the first and second portions to form first and second resistive tracks.

