Capacitance humidity sensor
First Claim
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1. A humidity sensor element, comprising:
- a thin, flexible film consisting essentially of a dielectric, water-absorbing first polymer having a dielectric constant which varies substantially linearly as a function of relative humidity, said first polymer comprising backbone chains containing heterocyclic units in which one or more atoms in the heterocyclic unit are nitrogen, and one or more carbon atoms in the heterocyclic unit have an oxygen atom double bonded thereto, said polymer being substantially free of hydroxyl groups, said first polymer having the general formula;
space="preserve" listing-type="equation">--(A--B).sub.n -- wherein A is a unit containing a heterocyclic unit including at least one --N--C═
O linkage therein, B is an aromatic, alicyclic, or aliphatic, substituted or unsubstituted hydrocarbon unit, and n is at least about 10; and
a pair of thin, water-permeable, conductive layers disposed on opposite sides of said film and integrally bonded thereto, said conductive layers each consisting essentially of a second polymer having electrically conductive particles distributed therein, which conductive particles are effective to render said layers electrically conductive.
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Abstract
A capacitance humidity sensor according to the invention has a dielectric film core which is in contact with a pair of polymeric conductive layers bonded to opposite faces of the core. The dielectric core is made of a polymeric material having a dielectric constant which varies substantially linearly with humidity, such as a polyimide or polyparabanic acid. The conductive layers are made of a polymeric material having conductive particles, such as carbon particles, dispersed therein. Such conductive layers provide superior performance and corrosion resistance in comparison to the metal films commonly employed in the prior art.
59 Citations
47 Claims
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1. A humidity sensor element, comprising:
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a thin, flexible film consisting essentially of a dielectric, water-absorbing first polymer having a dielectric constant which varies substantially linearly as a function of relative humidity, said first polymer comprising backbone chains containing heterocyclic units in which one or more atoms in the heterocyclic unit are nitrogen, and one or more carbon atoms in the heterocyclic unit have an oxygen atom double bonded thereto, said polymer being substantially free of hydroxyl groups, said first polymer having the general formula;
space="preserve" listing-type="equation">--(A--B).sub.n --wherein A is a unit containing a heterocyclic unit including at least one --N--C═
O linkage therein, B is an aromatic, alicyclic, or aliphatic, substituted or unsubstituted hydrocarbon unit, and n is at least about 10; anda pair of thin, water-permeable, conductive layers disposed on opposite sides of said film and integrally bonded thereto, said conductive layers each consisting essentially of a second polymer having electrically conductive particles distributed therein, which conductive particles are effective to render said layers electrically conductive. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29)
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30. A humidity sensor element, comprising:
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a thin, flexible film consisting essentially of a dielectric, water-absorbing first polymer made of poly(parabanic acid) having a dielectric constant which varies substantially linearly as a function of relative humidity; and a pair of water-permeable, conductive layers disposed on opposite sides of said film and integrally bonded thereto, said conductive layers each consisting essentially of a second polymer having electrically conductive particles distributed therein, which conductive particles are effective to render said layers electrically conductive. - View Dependent Claims (31, 32, 33, 34)
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35. A humidity sensor element, comprising:
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a flexible film having a thickness of 0.001 inch or less consisting essentially of a dielectric, water-absorbing polyimide made of units having the formula;
##STR9## having a dielectric constant which varies substantially linearly as a function of relative humidity within the range of 5 to 95 percent relative humidity at temperatures in the range of 15 to 50°
C., which film has a resistivity of at least about 1015 ohm-cm at 35°
C. and 50% relative humidity;a pair of water-permeable, conductive layers having a thickness of 0.0005 inch or less disposed on opposite sides of said film and integrally bonded thereto, said conductive layers each consisting essentially of about 20 to 90 wt. % of a cross-linked cellulosic polymer and about 10 to 80 wt. % of electrically conductive, chain-forming carbon particles having a particle size of about 1 micron or less, which particles are distributed in said cellulosic polymer so that said layer have a resistivity of 125,000 ohm-cm or less. - View Dependent Claims (36)
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37. A method for making a humidity sensor element, comprising:
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(a) disposing a piece of a thin, flexible film made of a dielectric, water-absorbing first polymer having a dielectric constant which varies as a function of relative humidity on a holder of a screen printing apparatus; (b) applying suction through the holder to the film to temporarily secure the film on the holder; (c) positioning a stencil over the film; (d) screen printing the film through the stencil with a liquid composition comprising an electrically nonconductive second polymer, electrically conductive particles and a carrier liquid; (e) drying the liquid composition under conditions effective to form a water-permeable, first conductive layer on one side of said film and integrally bonded thereto, which conductive layer has a shape corresponding to the stencil; (f) repeating steps (a)-(e) to form a second conductive layer on the face of the film opposite the first conductive layer. - View Dependent Claims (38, 39, 40, 41, 42, 43, 44, 45, 46, 47)
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Specification