Characterization of Layers with Metal-Coated Polymer Spheres for use in Ultrasound Transducers
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The properties of materials used in ultrasound transducers affect the properties of the finished transducer, making it important to characterize the acoustic properties of these materials. The study in this thesis is focused on metal-coated polymer spheres, MPS, and characterization of composites with MPS and epoxy for use as bonding and acoustic matching layers in ultrasound transducers. Samples consisting of silicon, glass and a composite of spheres and epoxy were made to determine a method for making a monolayer of the spheres. Samples consisting of PZT, a monolayer of spheres and epoxy, and a load material were also made to see the effect of the sphere layer, with a 1D model being used to identify the acoustic properties of the layer, i.e. characteristic acoustic impedance and the speed of sound. The 1D analytical model was supplemented by 2D simulations for more accurate modeling. Thicker 0-3 composite samples were made and measured to determine the longitudinal speed of sound and the characteristic acoustic impedance in this material, and the values from the samples were compared with 2D simulations and values from the 1D models. The fitting of the 1D Mason model to the impedance spectra from the trial samples gave a characteristic acoustic impedance between 2.7 and 3.1 MRayl and a speed of sound between 2700 m/s and 3200 m/s for the samples with 20 μm spheres. The later PZT samples gave a characteristic acoustic impedance between 2.9 and 3.1 MRayl and a speed of sound between 2400 m/s and 2500 m/s for samples with 40 μm spheres. The lower value of the speed of sound for the 40μm spheres was also indicated by the thicker 0-3 composite samples, where the 20 μm sphere samples had a speed of sound of 2586 ± 50 m/s, while the 40 μm sphere samples had a speed of sound of 2449 ± 31 m/s. FEM simulations for the speed of sound on the other hand indicated that the speed of sound of the 40 μm sphere layer should be higher than for the 20 μm sphere layer. The speed of sound samples with 40 μm spheres did also show more clearly air-bubbles for all samples, which has most likely affected the results, making the measured speed of sound of the 40 μm sphere layers lower than the real value. The FEM simulations of the thermal conductivity showed that 2D simulations can be used as an indicator for the thermal conductivity of a layer. The calculated thermal conductivity decreased slightly with increased sphere diameter, and it also showed a decrease in value with a thin layer of polymer between the sphere and the boundary. For the layers with the same thickness as the boundary, the thermal conductivity was 0.320 ± 0.002 W/(m*K) for the 40 μm sphere layer and 0.325 ± 0.001 W/(m*K) for the 20 μm layer.