Document Type

Thesis - University Access Only

Award Date

1996

Degree Name

Master of Science (MS)

Department / School

Electrical Engineering

First Advisor

David W. Galipeau

Abstract

Improved humidity sensors can make drying processes much more efficient, resulting in large energy savings (about 6% of the total U. S. energy use in 1980 was for drying). Accurate and inexpensive humidity sensors are extremely important in the fields of meteorology, climatology, and atmospheric sciences. The introduction of new sensing materials and high-precision sensing techniques in recent years suggests that significant improvements can be made in humidity sensors. Some of the more promising materials for high temperature industrial applications are new high performance polymers that are moisture sensitive, stable, and durable. The introduction of surface acoustic wave (SAW) microsensors, which can measure very small changes in mass with low power consumption, offers a sensing technique with the high sensitivity required for humidity measurements in agriculture, meteorology, and microelectronics. The purpose of this work was to investigate structures and materials with the potential to improve the performance and cost of humidity sensors. The materials studied were polyimide thin films and ceramic thick films. The structures studied were thick- and thin-film capacitive and SAW. The sensors were characterized as a function film/substrate interface characteristics, film surface modification, and selectivity. Comparisons to commercial sensors were also made. The thin-film polymer capacitive type sensors had good linearity and V sensitivity. The performance of the thin-film polyimide sensor was similar to that of the commercial capacitive microsensors and had significantly better performance than the thick film sensors. All thick film sensors were found to have non-linear responses and small responses to RH less than 50%. The polyimide-coated SAW sensors have the potential for the highest sensitivity at low RH, but would probably be more expensive than a capacitive microsensor. Teflon and sputter cleaning were found to create a hydrophobic polyimide surface while KOH surface treatments created a more hydrophobic surface and increased the water uptake response approximately seven times. The photosensitive HFDA polyimide was found to be more sensitive to the interferant gases and also less stable than the non-photosensitive HFDA polyimide. A thick film oscillator circuit was designed and fabricated which oscillated with an uncoated SAW frequency sensor.

Library of Congress Subject Headings

Hygrometers -- Design and construction

Format

application/pdf

Publisher

South Dakota State University

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Rights Statement

In Copyright