M. Mohsen

Application of Positron Annihilation Spectroscopy to Study the Recovery of Commercial Pure Al and Al-0.96 wt.% Si Alloys

Isochronal annealing of quenched commercial pure Al (99.95%) and Al-0.96 wt.% Si was investigated between room temperature (RT) and 550 °C. The annealing of defects was studied using positron lifetime and Doppler broadening (DB) techniques. The retardation of annealing defects in the two alloys is interpreted in terms of precipitation of impurities and Si atoms, as well as defect properties (size and concentration) in the framework of the two state trapping model.

Correlation between free-volume parameters and physical properties of polyethylene-nitrile rubber blend

Positron annihilation lifetime spectroscopy (PALS) was used to study the immiscibility of a polar nitrile rubber (NBR) that had been blended with pure and waste, low- and high-density polyethylene (PE). The effect of the weight percent of the rubber added to the PE was also investigated. It was found that a complicated variation (positive and negative) in both free-volume parameters (τ3 and I3) from the values of the initial polymers forms an immiscible blend. These results are supported by a significant broadening in the free-volume hole size distributions. This has been interpreted in terms of interfacial spaces created between the boundaries of the two phases. Furthermore, a correlation was established between the free-volume parameters (τ3 and I3) and the electrical and mechanical properties of the before mentioned polymer blends as a function of the weight percent of waste PE.

The effect of nickel doping on the structural, defect structural, optical and magnetic properties of zinc oxide nanoparticles

Positron annihilation lifetime (PAL) spectroscopy has been applied in the present work to study the defect structure changes of the Zn 1-x Ni x O (0% ≤ x ≤ 10%). This system has been synthesized in nanosize by coprecipitation method. Nanostructure features of the prepared samples have been investigated by X-Ray diffraction (XRD) and TEM. The XRD data has revealed that the prepared samples are crystalline and belong to space group P6 3 mc. The deduced estimated average crystallite size is varying from 20 nm to 96 nm in agreement with TEM measurements The variation of lattice parameters and internal strain that have been deduced from XRD data, have shown that the solubility limit of NiO in ZnO does not exceed 5% and are correlated with PAL parameters. At 7% and 10% concentrations the XRD spectra has revealed the presence of secondary peaks due to creation of a new NiO phase, which indicates that Ni is no more incorporated in the ZnO structure. This has been also confirmed by the variation of the energy gap E g deduced from the UV absorbance spectra and the variation of FTIR absorption bands. The (M-H) curves have shown that the saturation magnetization M s suggest the presence of ferromagnetism which decreases with increasing Ni content up to 5%. This is associated with an increase in the formation of vacancy clusters and positron trapping rate in the interface region. In addition, the vacancy defects play an important role in mediating the ferromagnetism behavior in agreement with the polaron model.

Correlation of Mechanical Properties with Defect Structure in Commercial Pure Al and Al-Mn Alloys Studied by Positron Annihilation Spectroscopy

The variations during isochronal annealing of cold-rolled commercial pure Al (1050) and Al-Mn (3004) has been investigated between room temperature (RT) and 823 K. The annealing stages have been identified as recovery, partial recrystallization, complete recrystallization and grain growth using positron annihilation lifetime (PAL) spectroscopy, Doppler broadening of annihilation radiation (DBAR) spectroscopy and Vickers microhardness (Hv) measurements. The retardation of complete recovery in Al-Mn alloys is due to the presence of vacancy-Mn complexes. A positive correlation has been found between positron annihilation parameters (τav & Snor) and mechanical properties (Hv) for the two alloys under investigation.

Effect of electric field strength and exposure time on epoxy and high density polyethylene measured by positron annihilation lifetime

Using the positron annihilation lifetime technique, the annihilation parameters have been measured for epoxy and high density polyethylene (HDPE) as a function of AC electric field strength and the exposure time. The lifetime spectra have been resolved into three components, the longest component (I3τ3) is attributed to the pick-off annihilation of o-Ps in the amorphous regions. The intermediate one (I2τ2) is due to the annihilation of free positrons, while the shorter component (I1τ1) stems from self annihilation of p-Ps. In HDPE, the o-Ps parameters τ3 andI3 are measured as a function of electric field strengths in the range from 10 to 100 kV/cm exposed for 24 hours. A decrease inI3 of ∼8% is observed from zero to 50 kV/cm followed by an increase of the same order from 50 to 100 kV/cm. By investigating the effect of the exposure time from 2 to 24 hours at 16 and 50 kV/cm, the effect is confirmed and is attributed to the inhibition of o-Ps formation at lower field strength. In epoxy, the effect or exposure time onI3 at 166 and 133 kV/cm shows a similar behavior as in HDPE. At 133 kV/cmI3 decreases by only 2.5%. On the other hand, the changes in τ3 occur at short exposure times. Again at large times the saturation is obtained. These effects are attributed to the expansion of free volume (increase of τ3) competing at longer exposure times with other phenomena, such as liberation of free radicals, which reduce the o-Ps intensityI3 through the conversion to p-Ps. The reactions between o-Ps and free radicals might also lead to free positrons, which could explain the increase ofI2 and the decrease of τ3 at longer exposure times.