Effect of extraction methods on kinetic,chemical composition and antibacterial activities of Tunisian Thymus vulgaris. L. essential oil

The present study aims to compare two innovative extraction techniques: microwave-assisted hydrodistillation (MAHD) and solvent-free microwave extraction (SFME) through traditional extraction techniques: hydrodistillation (HD) and steam distillation (SD) for their efficiency in the extraction of the volatile compounds from Tunisian Thymus vulgaris leaves; the kinetic, yield, composition and antibacterial activities of the essential oil were assessed in vitro. Results show that the essential oils extracted by microwaves were quantitatively (yield) similar to those obtained through the conventional methods, but qualitatively, essential oils analysed by gas chromatography–mass spectrometry (GC–MS) presented 17, 11, 11 and 8 compounds obtained through SFME, MAHD, SD and HD, respectively, mostly consisting of carvacrol (89.24–41.17%), followed by γ-terpinene (11.37–1.37%) and para-cymene (27.95–2.05%). The essential oils were screened for antibacterial activity against 5 microorganisms. All essential oils obtained by studied extraction methods showed the same resistance against Gram (?) and Gram (+) bacteria. The SFME method gave the best results: rapid kinetic of extraction (30 min vs. 35 min for MAHD, 120 min for SD, and 180 min for HD), less energy saving and cleanest process.     

N-type negative differential resistance, hysteresis, and oscillations in the current-voltage characteristics of microwave diodes

The dc current-voltage characteristics of microwave diodes are experimentally studied on simultaneous exposure of the diodes to a high-frequency (100 MHz) high-amplitude harmonic signal. In the current-voltage characteristics of the microwave diodes, regions of N-type negative differential resistance, hysteresis, and current oscillations are observed depending on the amplitude and frequency of the high-frequency signal. A comparative analysis of the experimentally observed effects is performed, and possible mechanisms for the effects are discussed.     

Hydrophilicity and surface energy of polyethylene modified by radiation grafting of acrylamide

Summary LDPE films were modified via direct γ-radiation grafting of acrylamide (AAm) from its 30% aqueous solution. Equilibrium swelling of the grafted samples in water, and a water/PAAm molar ratio in the swelled films were determined as a function of PAAm content. Contact angles formed by sessile drops of water or diiodomethane on the starting and modified LDPE were measured with following calculation of a surface energy in these samples. A gradual filling of LDPE surface with PAAm as well as changes in hydrophilic-hydrophobic properties of the composition during the grafting process was discussed. It has been found an essential increase in surface energy of the modified LDPE up to ∼15% of the grafted PAAm content with a further stabilization of the energy value. Received: 12 March 2001/Revised version: 26 June 2001/Accepted: 27 June 2001     

A method for determining the texture scattering angle and the relative remanent magnetization of anisotropic permanent magnets

A Mössbauer-spectroscopy method for determining the texture scattering angle and the relative remanent magnetization of anisotropic permanent magnets is described. The method has been experimentally verified on powder-based permanent magnets made of barium ferrite BaFe₁₂O₁₉.     

Coupled heat and moisture transfer in multi-layer building materials

A dynamic mathematical model for simulating the coupled heat and moisture migration through multi-layer porous building materials was proposed. Vapor content and temperature were chosen as the principal driving potentials. The discretization of the governing equations was done by the finite difference approach. A new experimental set-up was also developed in this study. The evolution of transient temperature and moisture distributions inside specimens were measured. The method for determining the temperature gradient coefficient was also presented. The moisture diffusion coefficient, temperature gradient coefficient, sorption–desorption isotherms were experimentally evaluated for some building materials (sandstone and lime-cement mortar). The model was validated by comparing with the experimental data with good agreement. Another advantage of the method lies in the fact that the required transport properties for predicting the non-isothermal moisture flow only contain the vapor diffusion coefficient and temperature gradient coefficient. They are relatively simple, and can be easily determined.     

Phase Transition of Ag-Enriched Ag2Te

The electrical and thermoelectric properties of Ag₂Te containing excess Ag (0.25 at. %) were studied. At 0.1–0.12 at. % excess Ag, Ag₂Te undergoes an electronic phase transition attributable to the formation of a new Ag sublattice.     

Image diffusion and cross-talk in passive matrix electrochromic displays

A passive matrix electrochromic display (ECD) working in transparent or reflective mode is described. In this paper, we report the main reasons of image diffusion and cross-talk in passive matrix ECD and initial computer simulation results of the 7×7 matrix display. These suggest that the insulation of electrolyte under each pixel and threshold (nonlinearity) of coloration current can in practice completely eliminate the image diffusion and cross-talk. The matrix addressed ECD based on solid polymer electrolyte and two complementary electrodes, (WO₃+0.1TiO₂, TiO₂–0.25CeO₂), in which all pixels were insulated by negative photoresist material was elaborated. An additional protected thin film (0.2Li₂O–0.2CeO₂–0.6SiO₂) deposited on working electrode was used to provide the enhanced threshold effect. The ECD exhibited excellent memory effect and substantially free from image diffusion and cross talk effects.