Application of photoacoustic and photothermal techniques for heat conduction measurements in a free-standing chemical vapor-deposited diamond film

Heat conduction in a free-standing chemical vapor-deposited polycristalline diamond film has been investigated by means of combined front and rear photoacoustic signal detection techniques and also by means of a “mirage” photothermal beam deflection technique. The results obtained with the different techniques are consistent with a value of α=(5.5±0.4)×10⁻⁴ m² · s⁻¹ for thermal diffusivity, resulting in a value ofκ=(9.8±0.7)×10² W·m⁻¹·K⁻¹ for thermal conductivity when literature values for the density and heat capacity for natural diamond are used.     

Heterodyne detection of tunable FIR sidebands

Continuously tunable far infrared (FIR) laser sidebands have been generated in a Schottky diode by nonlinear mixing the radiation from an optically pumped FIR laser operating between 0.6 and 3 THz with that from a microwave source in the 2-4 GHz range. A very sensitive heterodyne detection is adapted to detect the sidebands generated not only at the sum or difference frequency of the two radiations, but also, for the first time in the FIR region, those related to the mixing of the FIR radiation with the second and third harmonics of the microwaves. The high selectivity of the superheterodyne detection allows easy separation of the various sidebands. The global tunability of this system is 2 × 10 GHz for the more powerful FIR emission lines. To demonstrate the tunability of the device absorption signals of several molecules are presented in the whole range of operation of the system (0.6-3 THz) using either direct or lock-in detection techniques.     

Diffusion and vibrational bottleneck in optically pumped submillimetre lasers

By observing the time dependence of the onset of the submm laser power in the millisecond time scale, just after the switching on of the pump laser, the vibrational bottleneck due to diffusion and vibrational relaxation has been clearly exhibited in CH₃F, D₂CO and HCOOH submm lasers. Experimental results are compared with those deduced from a simplified rate equation model. Both show a diffusion limited regime and a collisional regime.     

Phosphors based on NaZr2(PO4)3-type calcium and strontium phosphates activated with Eu2+ and Sm3+

Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sr_0.5Zr₂(PO₄)₃:Eu²⁺, and Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sm³⁺ orthophosphates prepared through precipitation using sol-gel processes are analogs of NaZr₂(PO₄)₃ (NZP) and crystallize in space group R $\bar 3$ . Their crystallographic parameters determined by X-ray diffraction are consistent with the interatomic distances extracted from EXAFS data. Their luminescence spectra obtained under excitation in the range 300?C400 nm contain emission bands between 425 and 525 nm. Substitution of the larger sized cations Eu²⁺ and Sm³⁺ for Ca²⁺ shifts the emission bands to shorter wavelengths and reduces their width because of the decrease in the effect of the crystal field. Analysis of the spectra indicates that Eu²⁺ occupies two types of crystallographic sites (independent interstitial sites of different sizes and shapes in the NZP framework structure). Codoping with Eu and Sm has ensured luminescence with chromaticity coordinates approaching those of white light: (x = 0.27, y = 0.34).     

Interaction of lamb modes with delaminations in plates coated by highly absorbing materials

A theoretical study of Lamb mode propagation through an absorptive bilayer consisting of a steel plate coated by highly absorptive rubber with a finite delamination dividing these layers is presented. Essentially, in such a bilayer structure, steel-like A0 and S0 modes still propagate, although with moderate absorption. Two different types of delaminations are taken into consideration: slip delaminations in which noncontact boundary conditions are assumed only for shear displacement and stress, and stress-free delaminations in which noncontact boundary conditions are assumed for all mechanical displacements and stresses. The calculations, which are based on a modal decomposition method, show that delaminations in absorptive bilayers result in a considerable change of the normal displacement amplitude at the bilayer surfaces inside the delamination region, and for an incident mode steel-like A0 mode, also in the transmission region. Stress-free and slip delaminations can be distinguished exploiting their different effect on the steel-like A0 and S0 incident modes.     

Screen-printed phosphor coatings for white LED emission

We demonstrate the possibilities offered by the screen-printing technique to create thick layers of rare earth doped phosphors on glass slides for light-emitting diodes (LEDs). Using commercial LEDs, the effect of the layer thickness and phosphor concentration on luminescence emission and chromaticity are investigated. The advantages offered by screen-printed multi-phosphors systems are also revealed through a feasibility study permitting to tune the chromatic coordinate of the global light emission by modifying the phosphors characteristics independently.     

Effect of loading a plate with different liquids on the propagation of lamb-like waves studied by laser ultrasonics

Three experimental laser ultrasonic configurations--line excitation with scanning detection, grating excitation with single point detection, and grating excitation with scanning probe beam--are shown to consistently reveal the modified propagation properties of Lamb waves on a polyethylene terephthalate (PET) film that is in contact with different liquids on its two sides. Theoretical predictions concerning the physical nature of different wave modes in symmetric and asymmetric film loading configurations (i.e., their existence, velocity, damping, and polarization) are confirmed by the experimental results.     

Changes in β-carotene bioaccessibility and concentration during processing of carrot puree

The effect of some process and product factors (addition of olive oil, high pressure homogenisation, subsequent thermal or high pressure pasteurisation) on β-carotene bioaccessibility and isomerisation of carrot puree was investigated. High pressure homogenisation could improve β-carotene bioaccessibility by disrupting cells, but only at a pressure higher than 50 MPa. Softening of the cell walls during a subsequent thermal pasteurisation resulted in a further increase in β-carotene bioaccessibility. Unfortunately, the high temperature also induced formation of some undesirable cis-isomers. As cell walls are probably strengthened by high pressure, high pressure pasteurisation could not positively affect the amount of bioaccessible β-carotene. Moreover, the high pressure process induced β-carotene oxidation. A positive effect of olive oil on β-carotene bioaccessibility could only be noticed in combination with a thermal pasteurisation process when β-carotene was solubilised in the oil droplets. Under high pressure however, oil can be crystallised which hinders the solubilisation of β-carotene.     

Enzyme infusion prior to thermal/high pressure processing of strawberries: Mechanistic insight into firmness evolution

Strawberries were infused with fungal pectinmethylesterase (PME) and calcium chloride, followed by a thermal (70 °C–0.1 MPa), a high pressure (25 °C–550 MPa) or a combined thermal-high pressure (70 °C–550 MPa) process. Macroscopic (firmness) and microscopic characteristics were assessed to evaluate the texture of the fruits. In order to interpret the texture changes, the chemical structure of pectin was investigated. Processing of strawberries caused a decrease in firmness, which was limited by infusion of PME and calcium chloride, although the extent of beneficial effects depended on the type of processing. PME was able to decrease the degree of methoxylation of pectin, which was accompanied by an increased crosslinking of the chains. During high pressure or combined thermal-high pressure processing, the degree of methoxylation of pectin in infused strawberries was even further decreased, probably due to a higher activity of the fungal PME under high pressure. In case of the high pressure process, this was reflected in a very firm texture. However, the combined thermal-high pressure process caused more severe tissue damage, in spite of the advantageous pectin properties.Industrial relevanceDuring high pressure processing of strawberries many nutritional and sensorial characteristics are quite well preserved. Unfortunately, texture of strawberries deteriorates during such processes. This paper provides mechanistic insight into how infusion of fungal pectinmethylesterase and calcium ions in strawberries can preserve the firmness of these fruits during high pressure processing.