Microwave characterization of multi-walled carbon nanotube arrays

A vertically aligned multi-walled carbon nanotube (VACNT) film has been characterized by rectangular waveguide measurements. The complex scattering parameters (S-parameters) are measured by a vector network analyzer at X-band frequencies. The effective complex permittivity and permeability of the VACNT film have been extracted using the Nicolson-Ross-Weir (NWR) approach. The extracted parameters are verified by full wave simulations (CST Microwave Studio) and very good agreement has been obtained. A systematic error analysis is presented and the errors are within the acceptable range. The performance of VACNT films as an absorber is examined, and comparison with the conventional carbon loaded materials shows that a 90% size reduction is possible whilst maintaining the same absorption level. © 2011 EUROPEAN MICROWAVE ASSOC.

Sound radiation from point-excited structures: Comparison of plate and sphere

Acoustic radiation from a structure can be expressed in terms of modal radiation and modal coefficients. This paper investigates the contributions of these two modal properties to radiation excited by a point force. Sound radiation from two basic structures is considered: a baffled rectangular plate and a closed spherical shell. The plate behaviour is familiar, and governed by the relation between the natural frequency of a mode and its coincidence frequency. For a closed spherical shell, there are either zero or two critical frequencies, depending on the radius and thickness. When there are two the shell radiates well both above and below the two frequencies, and poorly in the frequency range between them. © 2012 Elsevier Ltd. All rights reserved.

Acoustic radiation force of a Bessel beam on a porous sphere.

The possibility of using acoustic Bessel beams to produce an axial pulling force on porous particles is examined in an exact manner. The mathematical model utilizes the appropriate partial-wave expansion method in spherical coordinates, while Biot's model is used to describe the wave motion within the poroelastic medium. Of particular interest here is to examine the feasibility of using Bessel beams for (a) acoustic manipulation of fine porous particles and (b) suppression of particle resonances. To verify the viability of the technique, the radiation force and scattering form-function are calculated for aluminum and silica foams at various porosities. Inspection of the results has shown that acoustic manipulation of low porosity (<0.3) spheres is similar to that of solid elastic spheres, but this behavior significantly changes at higher porosities. Results have also shown a strong correlation between the backscattered form-function and the regions of negative radiation force. It has also been observed that the high-order resonances of the particle can be effectively suppressed by choosing the beam conical angle such that the acoustic contribution from that particular mode vanishes. This investigation may be helpful in the development of acoustic tweezers for manipulation of micro-porous drug delivery carrier and contrast agents.

Influence of micro supersonic jets on micro percussion drilling of 316L stainless steel using nanosecond 355NM laser radiation

The interaction phenomena of nanosecond Q-switched diode-pumped solid state (DPSS) laser using 355nm radiation with 0.2mm thick 316L stainless steel foil was investigated at incident laser fluence range of 19 - 82Jcm-2. The characterization study was performed with and without the use of assist gas by utilizing micro supersonic minimum length nozzles (MLN), specifically designed for air at inlet chamber pressure of 8bar. MLN ranged in throat diameters of 200μm, 300μm, and 500μm respectively. Average etch rate per pulse under the influence of three micro supersonic impinging jets, for both oxygen and air showed the average etch rate was reduced when high-speed gas jets were utilized, compared to that without any gas jets, but significant variation was noticed between different jet sizes. Highest etch rate and quality was achieved with the smallest diameter nozzle, suggesting that micro nozzles can produce a viable process route for micro laser cutting.

Radiation Concentrator Device

A radiation concentrator device 21 for improved efficiency in solar systems by limiting reflected light, comprising a radiation concentrator element 27 comprising a radiation transmissive surface 29, a radiation receiving device and disposed on the incident radiation side of the concentrator element 27 is a recapture element 23 for recapturing at least a portion of radiation lost from the concentrator element 27, where the recapturing element allows transmission of incident light into the concentrator element 27, recaptures escaping radiation from the concentrator element and reflecting the radiation back into the concentrator element. Also disclosed are radiation concentrator device 21 where the recapture element 23 comprises ridged or grooved structures to increase the internal reflection of the radiation. The concentrator element 27 may also be provided with luminescent materials.