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Study of mechanical and electrical properties through positron annihilation spectroscopy for different thickness of
Hamdy F M Mohamed1, Howayda G Taha2, I T Zedan3
1Physics Department, Faculty of Science, Minia University, P.O. Box 61519, Minia, Egypt.
Abstract:
This study explores the nano-structural and dielectric behavior of nitrile butadiene rubber (NBR)/polyvinyl chloride (PVC) composite filled with Gamma 600, a newly developed lead-free radiation shielding filler, for protection against ionizing radiation. The NBR/PVC composite was prepared in an 80:20 wt ratio and processed with standard rubber compounding additives. The filler was incorporated at a concentration of 72.6 phr. The NBR/PVC/Gamma 600 blend was processed into two sheets of different thicknesses (1.95 mm and 3.25 mm) to examine the influence of thickness variation on ion transport characteristics and the behavior of the free volume. Wide angle X-ray Diffraction (WAXD) measurements confirmed the loss of semi-crystalline order in thinner samples, reflecting enhanced radiation interaction at reduced thickness. Yield strength, ultimate tensile strength, Young's modulus, and elongation at failure reflect a clear difference in degree of crystallinity between the two thicknesses of the blend, where the thicker NBR/PVC/Gamma 600 (3.25 mm) had a high degree of crystallinity and showed higher yield and ultimate stress than the thinner sample (1.95 mm). The positron annihilation lifetime (PAL) spectroscopy served as a sensitive probe for nanoscopic free volume, while AC conductivity and dielectric spectroscopy assessed the electrical performance following Jonscher's power law and the correlated barrier hopping model. The results revealed a gradual decline in o-Ps lifetime τ3 and intensity I3 over long-term measurement times, indicating progressive structural densification. The thicker samples showed a slightly enhanced electrical conductivity and a more stable dielectric response. The reduction in free volume size reflected by o-Ps lifetime τ3 confirmed this densification process over the 20 h measurement period. These structural changes, along with conductivity trends, suggest improved stability for use in radiation shielding environments. The dielectric constant and loss factor displayed expected frequency-dependent trends, with thicker samples exhibiting higher conductivity. These results presented the usefulness of the dielectric spectroscopy and positron annihilation spectroscopy for investigating the nanostructure of blends exposed to ionized radiation.
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