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Periodicals - LEBANESE SCIENCE JOURNAL VOL.1, NO.2, 2000
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The effects of oxygen deficiency on the structural, magnetic
and electrical properties in perovskite manganites
N. Abdelmoula, K. Guidara, E. Dhahri, A. Cheikhrouhou and J.
C. Joubert
The effects of oxygen deficiency on structural magnetic and electrical
properties of La0.7Sr0.3MnO3-δ
∅δ where ∅ is a vacancy and 0 ≤
δ ≤ 0.15, has been investigated. We present a new method
of creating vacancies in the oxygen sites achieved at atmospheric
pressure. X-ray diffraction patterns reveal that these materials
have a rhombohedral structure for δ ≤ 0.075 and orthorhombic
when δ ≥ 0.1. The magnetization and resistivity measurements
as a function of temperature show a ferromagnetic metallic behavior
for δ ≤ 0.075 at low temperatures and paramagnetic semiconductor
above the Curie temperature TC. The magnitude of the
saturation magnetization Ms and the ferromagnetic phase
are reduced with increasing δ. A ferromagnetic insulating
state below 115K is observed for δ = 0.1 sample. It seems
to be due to an ordering of the Mn3+ and Mn4+
species on the manganese lattice.Back
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Simultaneous structural and magnetic transitions in La0.6Sr0.4MnO3-δ
δ (0.0 ≤ δ ≤ 0.2)
A.Zouari
Non stoichiometric substituted lanthanum manganite was prepared in
the form of polycrystalline products by solid state reaction method.
The structural characterisation was done by X-ray diffraction. All
samples crystallise in a single phase, showing the characteristic
peaks of the perovskite. A structural phase transition from the rhombohedral
phase to the orthorhombic phase was observed at the rate δ =
0.127 corresponding to a concentration in Mn3+ about of
85%. Magnetization measurements gave ferromagnetic order for 0 ≤
δ ≤ 0.127 and antiferromagnetic order for 0.15 ≤ δ
≤ 0.20. The Curie temperature is very sensitive to the presence
of vacancies. The magnetic interactions between the manganese ions
were calculated; this result confirmed the obtained magnetic properties.Back
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Study of double stator induction machine fed by two independent voltage
source inverters
N. Moubayed, F. Meibody-Tabar,
B. Davat and I. Rasoanarivo
One simple and efficient way to apply a power segmentation in AC machine
PWM-VSI set consists in realising multi-star windings and supplying
each one by its own three phase PWM-VSI. This leads theoretically
to use modular inverters of lower power (current or/and voltage) which
allows higher chopping frequency. With this power segmentation method,
it is possible to supply one or more than one of star windings if
for any reason some of them cannot be supplied. In this paper, one
treats the example of a Double Star Induction Machine (DSIM) supplied
by two PWM-VSI. The model of the machine highlights the eventuality
of appearance of high amplitude current ripples if there is a strong
magnetic coupling between the star windings. The conditions of safely
supplying of DSIM are given and are validated by simulation and experimental
results. One presents, finally, the conception of multi-star windings
with weak magnetic coupling in AC machines.Back
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Simplified approach for modeling base current of bipolar transistors
H. Samet and N. Lakhoua
Analytic resolution of the transport equations of minority carriers
in a highly doped semiconductor region can not be achieved in a rigorous
manner. Rinaldi (1993) has presented a general analytic solution of
the transport equations of minority carriers in the emitter region
of a bipolar transistor in the form of a multiple integral asymptotic
series. Taking this solution into consideration, the minority hole
saturation current of the emitter J0e is studied as a function
of the doping and the width of the emitter region. The calculations
take into account the mobility µp, the lifetime τp
and the Band-Gap Narrowing (BGN) of the minority carriers. An approach
to the problem based on average values of the concentration N, the
mobility µp and the lifetime τp of
the minority carriers is also proposed in this paper. The saturation
current is then expressed in a classic form as in the case of a uniform
doping. This approach allows considerable reduction in computation
time. On the other hand, and in comparison with Rinaldi’s calculation,
the approach by the average values can be considered as accurate (error
≈ 1%) in the following conditions:
1) The emitter width W is less than or equal to 1µm (W ≤
1µm )
2) The recombination velocity at the emitter surface is large (Sp
= 107 cm/s),
3) The emitter surface doping is of the order of 1019 cm-3.
For such doping, this approach can also be considered as sufficient
in the case of small recombination velocity (Sp = 102
cm/s). Indeed, the error is inferior to 10% when the emitter width
is less than 0.7 µm.Back
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Four quadrant multiplier
for analog VLSI neural networks
H. Chible
In this paper a four quadrant analog multiplier is presented. The
multiplier can be used for general purpose but it is suitable for
Analog VLSI implementation of artificial neural networks because of
its small silicon area and its low power consumption. The main feature
of the multiplier is the high value of the weight voltage [ 0:Vdd].
The innovative idea in the multiplier proposed is to obtain a linear
function by substituting quadratic function inside the square root
function.Back
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Localization and characterization of ultra thin gate oxide breakdown
regions
N. Akil, P. LeMinh, J. Holleman, V. E. Houtsma,
and P. H. Woerlee
Nano-scale diodes were formed after intentional gate oxide breakdown
of n+ –polysilicon/oxide/p+ –substrate
MOS capacitors by Fowler-Nordheim constant current injection. The
nano-scale diodes called diode-antifuses are created by the formation
of a small link through the oxide between the n+ –poly
and the p+ –substrate with the properties of a diode.
In this paper we report a simple method to localize the antifuse in
terms to characterize the broken down oxide area. The method is based
on the use of the blue-UV light emission from hot carriers to expose
a deposited layer of photo resist on the top of the structure. After
developing the exposed resist, a hole is observed in the photo resist
layer where the antifuse was formed. This method can be applied on
a ULSI circuit under operation to fix the regions of hot carriers
and characterize them easily, or to identify where output power is
lost by the circuit in terms to improve its design.Back
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Stochastic iterative search heuristics for mapping data to multicomputers
N. Mansour
An empirical comparison of a class of five stochastic iterative
search optimization-based heuristics for mapping data to the processor-nodes
of a multicomputer is presented. In this class, three well-known
stochastic heuristics, a new algorithm that we propose, and a new
implementation of a stochastic search paradigm are included. The
well-known heuristics are: Simulated Annealing, Neural Network,
and Genetic Algorithm. The new heuristic is derived from Physics
and is based on Simulated Tempering. The new implementation is for
a Tabu Search heuristic. The empirical comparison of these five
heuristics involves their solution qualities and performances. As
a result of this comparison, we recommend that:
(i) the Neural Network be used for fast mapping of general applications,
(ii) the Genetic Algorithm be used for applications that are mapped
once and are then repeatedly run with the same mapping.Back
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Analyse numérique des conditions d’émission
sur un écoulement de type jet circulaire en régime
laminaire
H. Ben Aissia, Y. Zaouali, S. El Golli
L’étude proposée est relative à une résolution
numérique des équations de Navier-Stockes régissant
les écoulements de type jet circulaire et isotherme en régime
laminaire. Notre travail a été orienté vers
l’étude de l’influence des conditions d’émission
du gaz à la section d’injection sur les paramètres
de l’écoulement à partir de l’analyse
des résultats numériques concernant la vitesse verticale.
Le code de calcul numérique que nous avons développé
utilise une méthode aux différences finies à
maillage décalé. Les résultats obtenus ne concordent
avec ceux proposés dans la littérature que dans la
zone d’affinité. Par ailleurs les conditions d’émission
sont ignorées dams la zone d’écoulement établi.
La validation expérimentale du code de calcul numérique
par des mesures de la vitesse longitudinale a été
réalisée au moyen d’un vélocimètre
laser à effet Doppler en diffusion avant.Back
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Ostéopétrose: bases cellulaires et moléculaires.
Etude expérimentale et étude clinique
A. Elkak, N.Balmain
L’ostéopétrose résulte d’un défaut
de la résorption osseuse conséquence d’une anomalie
au niveau du développement des ostéoclastes. Dans
ce travail nous décrirons à la fois les différents
aspects de l’ostéopétrose et la biologie des
ostéoclastes. Une quantification par analyse d’image
a été faite pour évaluer l’effet d’un
facteur de croissance le M-CSF sur la différenciation des
ostéoclastes chez un enfant présentant une ostéopétrose.
L’analyse statistique est effectuée avec le programme
STAT 2005 (TITN Alcatel). La longueur de contact des ostéoclastes
différenciés rapportée à la longueur
totale des travées osseuses augmente d’une façon
significative après le traitement par le M-CSF passant de
0.8 ± 1.04 avant traitement à 29.30 ± 8.19
après traitement suggérant d’un effet positif
à court terme par le rHu M-CSF chez l’enfant ostéopétrotique.Back
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Aphids as pests and vectors of virus diseases affecting agricultural
crops in Lebanon and Syria
A. S. Talhouk and K. M. Makkouk
Annual surveys to identify aphid species which attack agricultural
crops as well as wild plant species in Lebanon and Syria were conducted
during the period 1969-1976. Ninety four aphid species belonging
to four families (Lachnidae, Calliterpidae, Thelaxidae, and Aphididae)
were identified, and permanent slide mounts were prepared. The role
of some of the identified aphids, as potential vectors of plant
virus diseases of economic importance to agricultural crops known
to occur in Syria and Lebanon, is highlighted.Back
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Nouvelles données sur Echinogammarus berytensis Alouf.
Essai de biogéographie des amphipodes dulçaquicoles
du Liban
N. Alouf
L’Amphipode d’eau douce Echinogammarus berytensis, espèce
présumée disparue, est redécouverte dans une
nouvelle station du Liban. La répartition d’autres
Amphipodes d’eau douce appartenant aux genres Niphargus, Gammarus
et Echinogammarus et la paléogéographie du site de
Beyrouth sont étudiées.Back
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