Combined effects of phase-shift and power distribution on efficiency of dual-high-frequency sonochemistry

•A single-piezo dual-high-frequency ultrasound reactor was constructed.•Phase-shift between two harmonic frequencies strongly affected efficiency.•Most efficient waveforms were non-sinusoidal with a single maxima/minima per cycle.•Further advances require consistent piezoelectrics with resonant harmonic frequencies. In an effort to increase the efficiency of sonochemical reactors, this study investigates a single-source, dual-high-frequency ultrasound reactor. Experiments were conducted with a variety of piezoelectric crystals and reactor components, and for each reactor design a range of power distributions and phase shifts between the two frequencies were evaluated. Certain dual-frequency configurations produced up to a threefold increase in sonochemical efficiency, while others yeilded no improvement over a single frequency. These results led to two significant findings. First, phase-shift had a strong effect on sonochemical efficiency for both harmonic and non-harmonic frequency combinations. Second, the most efficient dual-harmonic-frequency waveforms had a single peak per half-cycle, rather than two unique peaks. If dual-frequency, single-source ultrasound reactors are to become more efficient they must be able to consistently control the phase angle of and power distribution between harmonic waves to create an optimal waveform.