The installation of a phase adjuster (PA) has been proposed to improve the cooling effect of loop-tube-type thermoacoustic cooling systems. Installing a PA in a circular tube is equivalent to reducing the tube diameter locally. In this paper, we report our numerical investigation of the effects of the configuration parameters of a PA, i.e., diameter reduction and installation position, on the critical temperature of thermoacoustic gas oscillations by utilizing the transfer matrix method. The PA was modeled considering reflections due to the impedance mismatch at the boundaries between different diameters. The calculation results indicate that the critical temperature significantly decreases when a certain level of diameter reduction at appropriate positions is set, and these results correspond to empirical configurations that were experimentally chosen and verified in past studies.

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Using a hydrothermal method, (K0.88Na0.12)NbO3 films were deposited at 240 °C on (100) cSrRuO3//(100)SrTiO3 substrates. Moreover, without any poling treatment, direct and inverse transverse piezoelectric coefficients, e31,f, near 0 kV/cm were approximately −5.0 C/m2 for the as-deposited film. This value was nearly unchanged following the application of an electric field and poling treatment, suggesting that as-deposited films are almost in a fully self-polarized state without the application of an electric field. As-deposited films with a thickness of up to 22 μm showed constant piezoelectricity without any poling treatment. The films did not crack or peel because of substrates due to the small thermal strain originating from the low deposition temperature. The figures of merit (FOM) for the vibration energy harvester [FOM = e31,f2/(ε0εr)] and sensor [FOM = e31,f/(ε0εr)] were estimated to be good at 32.8 GPa and –5.9 GV/m, respectively, primarily because of the low relative dielectric constant of ∼110. Furthermore, the piezoelectric voltage coefficient g31 [= d31/(ε0εr)] was estimated and demonstrated a high value of 0.073 Vm/N.

Reducing the driving temperature of a thermoacoustic system to effectively utilize the unused low-temperature heat source is important for improving the performance of the system. The driving temperature of the thermoacoustic system was reduced by installing multiple stages of prime movers in series, a heat-to-sound transducer, and a prime mover with an increased cross-sectional area. In this study, the oscillation temperature was investigated both experimentally and by stability analysis in order to verify whether the system is operable, and to determine its operating temperature, when changing the installation position of the two-stage prime mover with increased cross-sectional area.

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