Dynamics of Si-H vibrations in an amorphous environment

We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions.     

Application of ultrasound in comminution

This paper deals with a new technology for fine grinding of hard materials, based on a high-compression roller mill with ultrasonic capabilities. The machine was tested by producing fine powders from hard rocks, with and without ultrasonic activation, permitting the beneficial effects of ultrasound to be evaluated. The experimental set-up allows the following operational parameters to be measured: material flow, applied torque, angular velocity of the rollers, stress on the shafts, ultrasonic energy applied and the vibration amplitude and phase behaviour of the transducer roller. It is found that the application of ultrasonic energy diminishes the torque required, the stress over the shafts and the total energy consumed for the same grinding results. In addition, a reduction in the erosion of the grinding surfaces was found. The optimal value of the applied ultrasonic power was determined by measuring the specific rate of breakage, a parameter that refers to the energy consumed for the generation of 1 ton of material, for each size range.     

An ultrasonic transducer for high power applications in gases

In the field of the generation of airborne ultrasound, we have described and analysed in a previous paper¹ a piezoelectric transducer based on a novel flexural vibrating stepped plate. This transducer has now been improved for high power applications, obtaining, with a remarkable increase in efficiency and directivity, sound pressure levels in free field greater than 160 dB.