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Title: Microfabricated liquid density sensors using polyimide-guided surface acoustic waves
Author: Turton, Andrew Charles
Awarding Body: Durham University
Current Institution: Durham University
Date of Award: 2006
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The simultaneous measurements of liquid density and refractive index on the same liquid sample are desirable. This thesis investigates the development of a micro- fabricated liquid density sensor that can be integrated into existing refractometers. A discussion of density sensing techniques and review of suitable sensors is given, leading to the choice of a Love mode surface acoustic wave (SAW) device. Love modes are formed by focussing the acoustic energy in a thin waveguide layer on a surface acoustic wave device. The horizontal-shear wave motion reduces attenuation in liquid environments, and the high surface energy density theoretically gives the highest sensitivity of all SAW devices. This study follows the development of a Love mode liquid density sensor using a polyimide waveguide layer. The novel use of polyimide offers simple and cheap fabrication, and theoretically gives a very high sensitivity to surface loading due to its low acoustic velocity. Love mode devices were fabricated with different polyimide waveguide thicknesses. The optimum thickness for a compromise between low loss and high sensitivity was 0.90 - 1.0 μm. These devices exhibited a linear shift in frequency with the liquid density-viscosity product for low viscosities. The response was smaller for high viscosities due to non-Newtonian liquid behaviour. Dual delay-line structures with a smooth 'reference' and corrugated 'sense' delay- lines were used to trap the liquid and separate the density from the density-viscosity product. A sensitivity up to 0.13 μgcm(^-3)Hz(^-1) was obtained. This is the highest density sensitivity obtained from an acoustic mode sensor. Experimental results show a zero temperature coefficient of frequency is possible using polyimide waveguides. These are the first Love mode devices that demonstrate temperature independence, highlighting the importance of polyimide as a new waveguide material.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available