The effect of ion implantation, including Ar + ion with influences (1 × 10 13 - 10 15 ions/cm 2), on the electrical and optical properties of ultrahigh molecular weight polyethylene (UHMWPE) were investigated with particular emphasis placed on the sensor performance to be used in the field of radiation detection. The obtained results focusing on the effect of the different influences showed a significant change in the electrical conductivity, capacitance and loss tangent. The absorption spectra for UHMWPE samples were recorded and the values of the allowed direct and indirect optical energy gap (E opt) d, (E opt) in of UHMWPE and energies of the localized states for the virgin and implanted samples were calculated. We found that the optical energy gap values decreased as the radiation dose increased. The results can be explained on the basis of the ion beam radiation-induced damage in the linear chains of UHMWPE, with cross-linking generated after implantation. The observed changes in both the optical and the electrical properties suggest that the UHMWPE film may be considered as an effective material to achieve ion-radiation detection at room temperature.
Ion implantation is a technology in which one type of external action leads to various defects in materials. The ion beam irradiation affects the polymer structure by cross-linking and degradation [
The extent and the type of defects depend on the nature of radiation as well as its energy and dose. As a result of irradiation, the chemical bonds are damaged and various types of radicals are created. Chen et al. [
The studied UHMWPE was supplied by Goodfellow Ltd. (Cambridge, UK). The processing characteristics included Mw = 120,000 g/mol, Mw/Mn = 3.4, Tm = 132˚C, Tc = 112˚C and density d = 0.95 g/cm3. The polymer samples used were flat, rectangular wafers with a thickness of 1 mm.
Ion bombardment was carried out in a vacuum at room temperature by means of commercial blazers MPB 202 RP ion implanter at the institute of Electronic Material Technology (ITME), Poland. The beam density was maintained below 0.1 A/cm in order prevent any increases in the sample temperature. The 160 and 300 keV Argon influences ranged from 1 × 1013 to 1 × 1015 ions/cm2, which were originally applied by Turos et al. [
The absorption spectra of the virgin and treated samples were recorded in the wavelength range of 200 - 1100 nm using a lambda 950 Perkin Elmer UV-Vis spectrometer. A perfectly flat piece of the UHMWPE thin sheet was placed vertically in the path of the sample beam, while the reference beam directly reached the detection point. Therefore, the spectral data obtained are absolute values.
The electrical properties of all samples were studied before and after ion implantation. The electrical contacts were made to the samples using silver paste. After this, the impedance, resistance, dielectric loss (tanδ) and capacitance measurements were carried out using a Hewlett Packard 4284A LCR meter, which was operated at 0.8 V over the frequency range of 100 Hz - 1 MHz at room temperature. The AC conductivity was calculated using the relation σ(ω) = t/AR. The dielectric constant was calculated using the relationships of ε = Cp/Co, where Cp is the capacitance measured using the LCR meter; and Co = εoA/t, where εo is the permittivity of vacuum and A and t are the cross-sectional area and thickness of the sample, respectively. The exposed area was 0.083 cm2 and the measurements were performed in air.
The study of the optical absorption and particularly the absorption band edge is a useful tool for providing information about the electronic band structure, localized states, type of optical transition and optical energy gap in polymeric materials [
Optically transparent polymers acquire some color after implantation. The color changes to deep brown or grey with increasing ion influence, while a metallic luster appears at high influences (1 × 1015 cm−2), which is fairly consistent with a previous study [
greater than 1015 ion/cm2. It is clear that for the sharp absorption edges in the curves of the virgin and Ar+-implanted UHMWPE samples 656 - 662 and 482 - 492, there are no significant change in the stability of the polymers up to a influence of 1015 ions cm−2, which shows good recyclability in the irradiation detection when a constant wavelength shifts before and after the detection of the ion beam. This sensor can be useful for the monitoring of ion bombardment and quality control in ion bombardment synthesis in the lab and biomedical industry.
The absorption coefficient α(ω) of the optical absorption near the band edge shows an exponential dependence on the photon energy hω (
α ( ω ) = α 0 exp ( ℏ ω / Δ E ) (1)
where α0 is a constant and ∆E is the width of the band tails of the localized states.
Equation (1) can be written as:
ln α ( ω ) = ln α 0 + ℏ ω Δ E (2)
The variation of the logarithmic of the absorption coefficient (lnα(ω)) with the photon energy after UHMWPE exposure to irradiation is presented in
The values of the optical energy gap for the UHMWPE before and after implantation were estimated using the Mott and Davis’ model [
α ( ω ) ℏ ω = B ( ℏ ω − E o p t ) n (3)
where B is a constant, α is the absorption coefficient and Eopt is the optical energy gap of the system and (n) is the index used to determine the nature of the electronic transitions during the absorption process. According to Equation (3), the direct and indirect transitions of (Eopt)d and (Eopt)in can be obtained by extrapolating the linear portions of the curves, which represents (αħω)2 and (αħω)1/2 compared to the photon energy (ħω) for UHMWPE at different ion fluences. The linear fit for this relation is shown in
as the change in the degree of disorder. From the density of state model, it is known that Eopt decreases with increasing degree of disorder of the amorphous phase [
The implantation-induced disorder of polymers leads to a change in the conductance due to alteration of the electronic structure. Typically, the conductance increases with ion influence due to the carbonization of the polymer. An exception is the high-fluence implantation of metals, which increases the metal volume fraction and also contributes to the charge carrier transport.
The increase in conductivity at a given frequency due to irradiation may be attributed to the scissioning of polymer chains and of the subsequent increase in the free radicals, unsaturation, etc. [
The plot of the dielectric loss tangents (tanδ) versus log frequency for pristine and irradiated UHMWPE samples in the frequency range of 300 Hz - 1 MHz is shown in
A simple optical/ electrical sensor is demonstrated as a device to detect the ions. A study of 160 keV energy Ar+ ion-induced effects in UHMWPE has been
conducted. The measurements of the optical spectra in the range of 360 - 840 nm have been carried out for the unirradiated and irradiated samples and optical constants have been determined. The analysis of the results of the optical study reveals that the absorption coefficient of the polymer increases up to an influence of 1014 ions cm−2, which is probably due to cross-linking without any degradation effects. Furthermore, these results indicate that UHMWPE gets chemically degraded at the highest Ar+ ion influence used, which was 1015 ions cm−2. There is an exponential increase in the conductivity with the log of frequency and the effect is significant after implantation. However, the tan δ and the dielectric constant ε decrease with an increase in ion influence. As the influence increases, the polymer surface becomes hydrogenated amorphous carbon when the influence exceeds 1 × 1015 ions/cm2 because of the degradation of the polymer from the scissioning of bonds. This is also corroborated by optical absorption spectra of UWMHDPE implanted by 160 keV Ar+ ions with different influences.
The author declares no conflict of interest.
El-Muraikhi, M. (2019) Ion Radiation Detection Using Implanted Ultrahigh Molecular Weight Polyethylene Structures (UHMWPE). Materials Sciences and Applications, 10, 12-24. https://doi.org/10.4236/msa.2019.101002