Capacitance Humidity Sensor
- PDF / 509,650 Bytes
- 7 Pages / 414.72 x 648 pts Page_size
- 113 Downloads / 262 Views
ANDREW R.K. RALSTON,* PAUL E. THOMA,** CARL F. KLEIN** AND DENICE D. DENTON* *University of Wisconsin-Madison, Materials Science Program, Department of Electrical and Computer Engineering and Department of Chemistry, 1415 Johnson Drive, Madison, WI 53706 "**JohnsonControls, Inc., Central Research, P.O. Box 591, Milwaukee, WI 53201-0591 ABSTRACT A capacitance relative humidity (RH) sensor is described that has a design, construction, and material composition that result in an inexpensive and robust sensor. This sensor has a multilayer, free-standing film construction. It consists of a humidity sensitive polyimide (PI) dielectric core and conductive layers consisting of carbon filled polysulfone on each side of the polyirnide film to form a capacitor. The polyimide used is a BPDA-ODA type, and replaces a PMDA-ODA type polyimide used in a previous version of this sensor. The BPDA-ODA sensor has a nominal capacitance of 200 pF and a nominal sensitivity of 13% at 100% RH. The characteristics of this humidity sensor are discussed and compared to the characteristics of the PMDA-ODA type sensor. Characteristics considered include the PI film moisture uptake and water vapor transmission, and the sensors' sensitivity to relative humidity, frequency response, and aging at 85'C/85% RH. The dual-state sorption model and free volume calculations are used to demonstrate that observed differences in the film are due to differences in chemical composition between the films. INTRODUCTION Many polymeric film materials have been investigated for use in relative humidity (RH) these are polycellulose,1 poly(methyl methacrylate),2-5 various sensing applications. Among plasma deposited materials, 2 ,6-8 and polyimides. 9 -13 Polyimides (PIs) have many properties that make them desirable in RH sensing applications. As a class, they are thermally stable, tough, and are available as resins and free standing films. They are also easily processable, with PI resins used extensively in integrated circuit manufacturing processes, making these materials compatible with integrated sensor packaging designs. While polyimides are thermally stable, they are subject to hydrolysis in the presence of moisture. The polyimide is formed by a thermally activated polycondensation reaction by a dianhydride and a diamine. The C-N bond in the imide ring (see Fig 1, e.g.) can be broken in the presence of moisture, creating a carboxylic acid group, and altering both the polarizability and the moisture uptake properties of the polyimide. These chemical changes are manifested by changes in the dry capacitance of a capacitive sensor using polyimide as the dielectric, and by a change of the sensitivity of the capacitance-RH characteristic. In this work, we investigate the aging characteristics of sensors fabricated using two commercially available PI films as dielectrics. These films are Kapton® HN, made by Du Pont and UpilexV R, formerly made by Ube Industries. The chemistry of Kapton® HN is pyromellitic dianhydride-oxydianaline (PMDA-ODA), with the structure shown in
Data Loading...
