Coating processes – Electrical product produced – Resistor for current control
Patent
1992-02-28
1994-09-20
Owens, Terry J.
Coating processes
Electrical product produced
Resistor for current control
324694, 338 35, 2524081, 252963, 252511, 252512, 524424, 524439, 524495, 524496, B05D 512
Patent
active
053487613
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to the use of a swellable plastic, which additives, such as carbon, for example carbon black, metal dust or the like, are dispersed for improving conductivity, for producing a resistive moisture sensor, as well as a method for producing such a resistive moisture sensor.
Resistive moisture sensors, i.e. moisture sensors which change an electrical resistance as a function of humidity, having metal or semiconductor oxides as moisture-sensitive material are known. The said moisture-sensitive materials are mainly used in the form of sinter bodies, ceramic materials, foils, fired thick film pastes and chemically deposited coatings. Moisture sensors of this type are distinguished in the said embodiments by high sensitivity, i.e. large resistance changes with changes of the humidity, but are stable to only a small extent and have relatively sluggish response properties and particularly long response times. Because the moisture-retaining volume of such sensors is comparatively large, the sluggish response characteristics are understandable.
In addition, metal or semiconductor oxide moisture sensors generally have a strong non-linear resistance-moisture characteristic as well as relatively large resistance, which increases the expenditures in connection with the evaluation electronics. Sensors of the above mentioned types can be found, for example, in German Letters Patent 16 98 096, German Published, Non-Examined Patent Applications DE-OS 27 28 092 and DE-OS 30 24 297, in U.S. Pat. No. 3,453,143 and German Published, Examined Patent Application DE-AS 29 38 434.
SUMMARY OF THE INVENTION
It is the object of the invention to make a resistive moisture sensor available which can be produced in a simple manner and which is distinguished by satisfactory linearity of the resistance-moisture characteristic and by a resistance level particularly suited for evaluation. In accordance with the invention it is proposed in this connection to use a swellable plastic and in which additives, such as carbon, metal dust or the like have been dispersed for improving the conductivity, for producing a resistive moisture sensor. Swellable plastics, such as polyimides, have already been proposed in connection with capacitive moisture sensors. But that swellable plastics with appropriate additives for increasing conductivity are suitable for producing resistive moisture sensors has not been suggested in any manner. The mechanism for the function of such a swellable plastic as a matrix for additives which increase conductivity for producing a resistive moisture sensor has by no means been completely clarified. The resistance-moisture characteristic, surprisingly shown to be relatively linear, is put down to an increase in the relative distance of the additives, which increases conductivity caused by swelling of the swellable plastic because of increasing moisture, so that a positive change in resistance is observed with increasing moisture. In this connection the use of a polymer matrix made of a swellable plastic has the advantage that it is possible to operate with relatively thin layers, because of which the response speed can be considerably increased, while the fact that the basic conductivity is attained by additives, such as carbon, for example carbon black, metal dust or the like, offers the chance of producing resistive moisture sensors with advantageous resistance values evaluation electronics connected downstream, while the resistance value can be set over a wide range. Besides the essentially linear positive resistance characteristics, the high response speed is surprising, which is accompanied by a definite improvement along with particularly simple production of the moisture sensor. Polyimides, copolyimides, aramides, polyamides, polyacrylates, poly-methacrylates, polycarbonates, polysulfones or polyethylene are advantageously used as swellable plastics for the production of such resistive moisture sensors, where in a preferred manner up to 50 weight-% of graphite or 3 to 15
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Mitter Helmut
Rossak Norbert
Scharizer Walter
Sollradl Herbert
Cameron Erma
E + E Elektronik Gesellschaft m.b.H.
Lenzing Aktiengesellschaft
Owens Terry J.
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