Showing papers by "Yasser M. Moustafa published in 2001"

Properties of Na2O–Al2O3–B2O3 glasses

Abstract: The density and transformation temperature of (30−y)Na2O·yAl2O3·70B2O3 glasses show a minimum at Al2O3/Na2O≈1. The molar volume increases with increasing Al2O3 concentration and it seems to be constant for Al2O3/Na2O≥1. There is a slight increase in the thermal expansion coefficient up to Al2O3/Na2O≈0.9 followed by a fast decrease. It is assumed that up to Al2O3/Na2O=1, Na2O associates preferentially with Al2O3 to form AlO4 tetrahedra. Infrared spectra are consistent with this assumption. There is a linear dependence between the electric resistivity and the activation energy. Both of these quantities increase with increasing Al2O3 content. It is concluded that the conduction parameters are determined by the Na+–Na+ distance.

Memory switching of Fe2O3–BaO–V2O5 glasses

Abstract: The electrical switching properties of Fe2O3–BaO–V2O5 glasses obtained from the current–voltage (I–V) characteristics at different temperatures have been investigated. It was found that these glasses exhibit a voltage-controlled negative resistance irreversible switching behavior. It can be deduced that the switching process is of memory type and the results are discussed in terms of the electrothermal model. The effect of the temperature, composition and thickness on the threshold voltage, Vth, has been studied. The Vth is found to decrease with increasing content of V2O5 in the glass, the same as the tendency in crystallization. For glasses containing more than 10 mol% Fe2O3, there is a detectable effect of crystallization on the threshold voltage.

Polaronic conduction in barium borate glasses containing iron oxide

Abstract: The conductivity of xFe2O3·(60−x)BaO·40B2O3 glasses can be described using Mott's models for the small polaron hopping and variable-range hopping processes The change of resistivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic one The activation energy is the predominant parameter that affects the conductivity with composition The calculated values of the conductivity parameters indicate that the conduction mechanism is adiabatic in nature The change in both the density and molar volume was discussed in terms of the structural modifications that take place in the glass matrix upon replacing BaO by Fe2O3

Structure and properties of Li2O-Fe2O3-B2O3 glasses

Abstract: Infrared spectra, density, dc conduction and microhardness of (25-x)Li 2 O.xFe 2 O 3 .75B 2 O 3 and 25 Li 2 O.xFe 2 O 3 . (75-x) B 2 O 3 glasses have been investigated. The infrared spectra suggest that Fe 2 O 3 enters the structure as FeO 6 units up to 2.5 mol% and/or FeO 4 for higher contents. There is a linear increase in density with increasing Fe 2 O 3 content. FeO 4 and FeO 6 units occupy their own space at the expense of interstices and this reduces the free volume in the network. The electric conduction is ionic where Li + ions are the charge carriers. The electric conductivity is determined with activation energy rather than the preexponential factor. The change in activation energy is correlated with the changes in the glass structure. A great increase in the hardness for Fe 2 O 3 ≥2.5 mol% reveals an increase in the strength of glass.