Showing papers by "Tunis University published in 1987"

A contactless method for investigating the thermal properties of thin films

Abstract: The photothermal deflection technique has been extended as a contactless method to investigate thermal transport in thin films. A theoretical model is developed which quantitatively describes the transport behavior, and is shown to be in excellent agreement with experimental results. This approach yields the thermal diffusivity directly and in a spatially-resolved manner.

Egg‐laying strategy under natural conditions of Leptopilina boulardi, a hymenopteran parasitoid of Drosophila spp

Abstract: The hypothesis of optimal host species selection predicts that when a parasitoid has the choice between two host species, it will choose the species that gives the best survival chances for its progeny. We confirmed this hypothesis by laboratory experiments with Leptopilina boulardi Barb. et al., a cynipid parasitoid which prefers Drosophila melanogaster Meigen (the host species most suitable for parasitoid survival) above D. simulans Sturt. As far as fitness parameters are concerned, the fertility of L. boulardi is higher with D. melanogaster, the egg laying can be spread out over a long period when this host is relatively scarce. This does not occur with D. simulans in which parasitic oviposition stops soon when this host is not abundant. Investigations of this foraging strategy were done under more complex natural conditions. We found that L. boulardi has a type III functional response with D. melanogaster only; furthermore, it seems that a switching effect may exist with this host. Parasitoid females appear to distribute their eggs more regularly on D. melanogaster, thus avoiding superparasitism. This seems to be independent of the relative frequency of this host. However, superparasitism of D. simulans did occur more frequently when this host was scarce. Resume Strategie de ponte de Leptopilina boulardi (Hymenoptere parasite de drosophiles) dans les conditions naturelles Le concept de reponse optimale d'un parasite vis-a-vis de l'hǒte le plus favorable pour son developpement demeure surtout theorique et n'a pu ětre verifie que dans les conditions de laboratoire. Nous avons montre que Drosophila melanogaster s'avere ětre, par rapport a D. simulans, l'hǒte le plus favorable pour le developpement du cynipide parasite Leptopilina boulardi. Une etude sur le terrain a demontre que ce parasite presente une reponse fonctionnelle densite dependante vis-a-vis de D. melanogaster et non vis-a-vis de D. simulans, avec un effet de bascule. D'autre part, il s'avere que ce parasite exploite beaucoup mieux son hǒte, en evitant le superparasitisme, ceci etant demontre au laboratoire et dans la nature. Enfin, il apparaǐt qu'il est capable d'allonger sa periode de ponte lorsque cet hǒte est rare, ce qui ne se produit pas avec D. simulans.

Electronic structure, superconductivity and magnetism in the Zr-Co system

Abstract: A study of low-temperature specific heat in the Zr-Co crystalline system is presented, the results being analysed with the magnetic properties which have been investigated for ZrCo and ZrCo4. Two regions can be distinguished in the Zr-Co system: a Zr-rich concentration range of compounds which are superconductors and a Co-rich concentration range of compounds which are magnetic. Two slight bumps of the electronic specific heat coefficient gamma are observed. One corresponds to Zr2Co which has a high superconductivity temperature Tcs and a low Debye temperature theta D, and the other corresponds to ZrCo2, whose concentration corresponds nearly to the appearance of ferromagnetism. The ZrCo compound shows an unexplained high gamma value.

Investigation of the structural disorder fluctuations in a a-Si:H films by coupled X-ray diffractometry and photodeflection spectroscopy

Abstract: The full width at half maximum (FWHM) of the first diffraction peak and the width of the optical absorption edge Eo (Urbach edge) have been measured on glow discharge (GD) and sputtered (SP) prepared a-Si:H samples. The coherence length L, derived from the FWHM has been plotted vs Eo and also vs the optical gap Eg. It is shown that L increases when Eo decreases. The values of L are always found larger for GD samples than for SP films. The Si matrix relaxation by H atoms is demonstrated in GD materials through an increase of L with Eg. This is not the case for SP materials.