Ultrasonic motor using two sector-shaped piezoelectric transducers for sample spinning in high magnetic field

This paper presents the design, fabrication process, and evaluation of an ultrasonic motor for sample spinning in a high magnetic field of solid-state Nuclear Magnetic Resonance (NMR). To decrease effects of the magnetic field on rotation, all motor components are made of materials that have low magnetic permeability. The motor, including the sample casing, is a maximum 31 mm in diameter and 50 mm high. The motor has two sector-shaped piezoelectric transducers. One transducer generates two different vibration modes, longitudinal and flexural, when two sinusoidal voltages are applied to transducers. To confirm that transducers can be driven in a high magnetic field, the effect of the magnetic field on the transducer was evaluated. The motor was driven at a frequency of 329.0 kHz. The maximum rotation speed and starting torque were 1.50 × 10³ rpm and 26 μ Nm when applied voltage was 40 V_p-p. The rotation speed, controlled by a proportional-integral control system, was 1.20 × 10³ rpm in a 7.0-T magnetic field. The motor was also applied to the sample spinning system of a high-resolution NMR spectrometer. We succeeded in obtaining ¹ H-NMR signals of H₂ O. The motor can therefore be used for a sample spinning system in a high magnetic field.