Ultrasound generation with high power and coil only EMAT concepts

Abstract

Electro-magnetic acoustic transducers (EMATs) are intended as non-contact and non-destructive ultrasound transducers for metallic material. The transmitted intensities from EMATS are modest, particularly at notable lift off distances. Some time ago a concept for a "coil only EMAT" was presented, without static magnetic field. In this contribution, such compact "coil only EMATs" with effective areas of 1-5cm(2) were driven to excessive power levels at MHz frequencies, using pulsed power technologies. RF induction currents of 10kA and tens of Megawatts are applied. With increasing power the electroacoustic conversion efficiency also increases. The total effect is of second order or quadratic, therefore non-linear and progressive, and yields strong ultrasound signals up to kW/cm(2) at MHz frequencies in the metal. Even at considerable lift off distances (cm) the ultrasound can be readily detected. Test materials are aluminum, ferromagnetic steel and stainless steel (non-ferromagnetic). Thereby, most metal types are represented. The technique is compared experimentally with other non-contact methods: laser pulse induced ultrasound and spark induced ultrasound, both damaging to the test object's surface. At small lift off distances, the intensity from this EMAT concept clearly outperforms the laser pulses or heavy spark impacts.

Ultrasound irradiation accelerates the lipase-catalyzed synthesis of methyl caffeate in an ionic liquid

“…Using pulse mode of ultrasound operation [38], Fig. 5A and B shows the effects of different power (90, 120, 150, 180 and 210 W) and ultrasound frequency (15,20,25,30 and 35 kHz) on MC yield at different reaction time when the CA concentration was 0.5 g/L, reaction temperature was 75 • C, and Novozym 435 concentration was 60 g/L.…”

Experimental Demonstration and Circuitry for a Very Compact Coil-Only Pulse Echo EMAT

“…Similar rise times and amplitudes have been observed in practice and simulated for the real transducer system, and—the purpose here—transient and strong magnetic fields and Lorentz forces occur close to the windings of the small coil. Although the field topology, the achieved flux density, and the forces between the coil and the target can be computed with a quasi-analytical method [11], a much more rough and simple calculation [10] suffices for this practical work: about 50% of the available electrical energy in the capacitor (here: 0.125 J) can momentarily convert into magnetic energy within the gap between the coil and the target metal. The volume of a 0.5 mm gap with an 8 mm diameter (the diameter of the coil) is close to 25 mm 3 , and then the momentary magnetic energy density E m in the gap becomes about 5 MJ/m 3 .…”

“…Contactless ultrasound excitation in metallic test objects is also possible with a simple compact induction coil alone (Figure 1b), without a permanent magnet [5,6,7,8,9,10,11]. As the transduction effect is proportional to the RF eddy current and the exciting RF induction field itself (“self-biasing”), the RF Lorentz forces follow a quadratic characteristic.…”

“…As the transduction effect is proportional to the RF eddy current and the exciting RF induction field itself (“self-biasing”), the RF Lorentz forces follow a quadratic characteristic. Then the affected acoustic pressure becomes proportional to the square of the induction current, or in more detail, equals the energy density (=magnetic pressure) of the RF induction field at the metal target’s surface [10]. Consequently, because the ultrasound intensity is proportional to the square of the acoustic pressure, the acoustic intensity increases with the fourth power of the RF induction current.…”

“…Beyond certain power levels, the RF flux densities can become higher than the practically available flux density from a permanent magnet, and then the emitted ultrasound intensity can readily outperform conventional EMATs. Intense electrical pulses and magnetic flux densities with amplitudes of more than 10 kV, 10 kA, and about 10 T have been reported, resulting in strong ultrasound intensities in the target up to kilowatt levels [10]. High-voltage capacitors and a spark gap switch provide the microsecond high-power pulses for this simple compact coil transmitter.…”