This study has investigated the effect of shelf aging, for up to one year in air, on the properties of gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE). A variety of techniques were ...used to characterize the properties of treated samples. Differential scanning calorimetery (DSC) was used to characterize the morphology. The extent of cross-linking in a polymer network was detected by swelling measurements. The durometer hardness test was used to measure the relative hardness of this material, and changes in density were also measured. Results from all these measurements were combined to explain the changes in the microstructure of the aged, irradiated UHMWPE. This study shows that crystallinity is increased with radiation dose and with aging due to chain scission, which leads to a reduction in the molecular weight of the material. This allows the chains to rearrange to form crystalline regions. Positron annihilation lifetime spectroscopy confirms these conclusions. Fractional free volumes have been deduced from lifetime parameters, which correlate with the data obtained by the other techniques.
A magnetically guided slow positron beam is being constructed at Qatar University and is currently being optimised for regular operation. This is the first positron beam in the Middle East, as well ...as being the first Arabic positron beam. Novel features in the design include a purely magnetic in-line deflector, working in the solenoid guiding field, to eliminate un-moderated positrons and block the direct line of sight to the source. The impact of this all-magnetic transport on the Larmor radius and resultant beam characteristics are studied by SIMION simulations for both ideal and real life magnetic field variations. These results are discussed in light of the coupled effect arising from electrostatic beam extraction.
The influence of 70
keV He
+ ion implantation and subsequent annealing of Cz–indium phosphide (InP) samples has been investigated using a slow positron beam-based Doppler broadening spectrometer. ...Three samples with ion fluences of 1
×
10
16, 5
×
10
16 and 1
×
10
17
cm
−2 were studied in the as-implanted condition as well as after annealing at 640
°C for times between 5 and 40
min. It was found that the line-shape parameter of the positron–electron annihilation peak in the implanted layer increases after 5
min annealing, then after longer annealing times it starts to decline gradually until it reaches a value close to the value of the as-grown sample. This implies that vacancy-like defects can be created in InP by He implantation followed by short-thermal annealing at
T
>
600
°C. Comparison of the results with a study where cavities were observed in He-implanted InP has been carried out.
Observations of vacancy clusters formed in Czochralski (Cz) Si after high energy ion implantation are reported. Vacancy clusters were created by 2
MeV Si ion implantation of 1
×
10
15
ions/cm
2 and ...after annealing between 600 and 650
°C. Doppler broadening measurements using a slow positron beam have been performed on the self-implanted Si samples, both as-implanted and after annealing between 200 and 700
°C for time intervals ranging from 15 to 120
min. No change in the
S parameter was noted after the thermal treatment up to 500
°C. However, the divacancies (V
2) created as a consequence of the implantation were found to start agglomerating at 600
°C, forming vacancy clusters in two distinct layers below the surface; the first layer is up to 0.5
μm and the second layer is up to 2
μm. The
S–
W plots of the data suggest that clusters of the size of hexavacancies (V
6) could be formed in both layers after annealing for up to an hour at 600
°C or half an hour at 650
°C. After annealing for longer times, it is expected that vacancies are a mixture of V
6 and V
2, with V
6 most probably dominating in the first layer. Further annealing for longer times or higher temperatures breaks up the vacancy clusters or anneals them away.
Ultra high molecular weight polyethylene was bombarded with He
+ and Ar
+ ions to fluences ranging from 10
13 to 2×10
16 ions/cm
2. Rutherford backscattering and nuclear reaction analysis were ...applied to study mechanism of oxygen uptake and hydrogen release induced by ion beam bombardment. The influence of ion bombardment on positron annihilation lifetime parameters is also discussed. Hydrogen release was observed with increasing ion dose and was correlated to the ion stopping power. An important effect observed, was the rapid oxidation of samples after exposure to air.
A study of the degree and rate of cross-linking of low-density polyethylene (LDPE) as a result of irradiation by high-energy electron beam has been performed using positron annihilation lifetime ...(PAL) technique. The PAL measurements were carried out at room temperature with a conventional fast–fast coincidence system. The lifetime spectra were analysed into four components using the PATFIT program to extract the positron parameters such as lifetime, mean lifetime, intensities, free volume radius, and fractional free volume. Almost all parameters exhibited differences between the unirradiated and irradiated LDPE. It can be concluded that the effect of irradiation on polyethylene results in intensive network formation, which is intensified as the electron energy is increased. The results manifest another proof of the usefulness of positron techniques in the study of the microstructure of polymers.
The corrosion properties of duplex-treated and nitrided Ti–6Al–4V have been investigated in 0.025, 0.25 and 2.5 M NaCl using electrochemical techniques, Positron Annihilation Spectroscopy (PAS), ...X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS). The electrochemical results showed a three-fold increase in corrosion rate for every ten-fold increase in chloride concentration. The morphology and surface composition studied by XRD showed the presence of TiN and Ti
2N layers. SEM and EDS showed the presence of pitting and the presence of oxide and chloride products. At the lowest concentration of NaCl no aluminum was detected in the surface layers while at the highest concentration no chloride byproducts were detected. PAS indicated that the damage caused by the NaCl changes in nature and becomes shallower as the concentration increases.
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•Theoretical consideration of self-sustainable MECs for H2 evolution.•Key challenges that must be overcome for the real application of self-sustainable MECs.•Efforts us far to achieve ...self-sustainable MECs and enhance H2 production yield.•Future perspectives for self-sustainable MECs for industrial use.
Microbial electrolysis cell (MEC) technology is a promising bioelectrochemical hydrogen production technology that utilizes anodic bio-catalytic oxidation and cathodic reduction processes. MECs require a lower external energy input than water electrolysis; however, as they also require the application of external power sources, this inevitably renders MEC systems a less sustainable option. This issue is the main obstacle hindering the practical application of MECs. Therefore, this review aims to introduce a self-sustainable MEC technology by combining conventional MECs with advanced carbon–neutral technologies, such as solar-, microbial-, osmotic-, and thermoelectric-powers (and their combinations). Moreover, new approaches to overcome the thermodynamic barriers and attain self-sustaining MECs are discussed in detail, thereby providing a working principle, current challenges, and future perspective in the field. This review provides comprehensive insights into reliable hydrogen production as well as the latest trends towards self-sustainable MECs for practical application.
Diagnostic studies have been performed on a 70
μm thick diamond film with a view to its development as a possible field-assisted moderator (FAM) for slow positron emission. The film was coated on one ...side with a 200
nm gold electrode and on the other with a similar thickness of gold in the form of a fine mesh (10
μm lines, 40
μm spaces) in the central part of the diamond surface and a solid edge for electrical contact. A potential difference of up to 300
V could be held across the film with a leakage current of ∼10
nA. Although positron re-emission was observed from the diamond surface, the re-emitted fraction was observed to decrease when a bias of 300
V was applied across the film. This negative result is interpreted in terms of the drifting of thermalised positrons to the gold mesh lines, a conclusion supported by Doppler broadening measurements. The potential for the development of a diamond-film-based positron beam tagger is discussed.
