Temporary changes in electrical properties of polymer dielectrics due to ionizing radiation

Measurements of conductivity, permittivity, and dissipation factor on polystyrene, low-density polyethylene, poly(ethylene terephthalate), and polytetrafluoroethylene under irradiation with x-rays at exposure rates from 0.004 to 400 r/sec. are presented. The radiation-induced anomalous conductivity as well as the induced dielectric loss are interpreted by Maxwell-Wagner polarization due to radiation imbalance in surface layers of the specimen. The nature of radiation-induced steady-state conductivity is also discussed.     

A new antifriction and sealing material based on radiation-modified polytetrafluoroethylene

The current situation with research into the effect of ionizing radiation exposure on the structure and properties of polytetrafluoroethylene was analyzed. New methods of radiation-induced modification, that improve the performance characteristics of polytetrafluoroethylene, were discussed. The results of research into properties of polytetrafluoroethylene irradiated at temperatures exceeding the melting point of its crystalline phase in media with different compositions were presented. It is shown that, under certain irradiation conditions, an extremely strong increase (up to 10 000-fold) in the wear resistance of polytetrafluoroethylene can be achieved, with the creepage decreased up to 100-fold. A conclusion was made that high-temperature radiation-induced modification can yield new modifications of the polymer, in which the advantages offered by untreated polytetrafluoroethylene are combined with high mechanical and triboengineering characteristics.     

Electron-beam charging of poly(methyl methacrylate)

The charging of bulk poly(methyl methacrylate) by irradiation with electrons of 2 MeV energy at room temperature in vacuum was studied. The experimental data obtained using the split Faraday cup are compared with the results of numerical simulation assuming one-dimensional geometry with allowance for the spatial distribution of dose rate and injected-electron current, nonlinear properties of radiation-induced conductivity in the prebreakdown electric-field region, and the intrinsic conductivity of poly(methyl methacrylate). It was shown that published data on the electric field strength measured by means of the electro-optical Kerr effect in electron-beam charged poly(methyl methacrylate) agree satisfactorily with the calculation results.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 3, 2005, pp. 183–189.Original Russian Text Copyright © 2005 by Sadovnichii, Tuytnev, Milekhin.     

Effect of the degree of radiation-induced cross-linking of polytetrafluoroethylene on its wear resistance

The effect of the dose of the γ-irradiation at temperature exceeding the melting point of the crystalline phase on the degree of radiation-induced cross-linking of polytetrafluoroethylene was examined, with the relative content of CF₃ groups and wear rate under conditions of dry sliding friction over steel chosen as characteristic parameters. The wear rate of radiation-cross-linked polytetrafluoroethylene decreases exponentially with an increase in the content of CF₃ groups and can be evaluated with sufficient accuracy from the IR data.     

Optical and electrical analysis of blue polymethyl methacrylate for high-dose dosimetry

The response to gamma radiation of polymethyl methacrylate (“blue PMMA”) containing a blue dye was investigated, with the aim of providing a high-dose dosimeter based on either spectrophotometry or electrical-conductivity measurements. It is found that the 3-mm thick pieces of blue PMMA can be used for dosimetry in a range of absorbed doses from about 5-50 kGy, for which the changes in optical transmission density (absorbance) at different wavelengths in the visible region (402, 596, 612, and 643 nm) are linear functions of dose. Results also show that irradiation of thin 0.1 mm films of blue PMMA produces two components of radiation-induced conductivity: a transient component which can be used to determine the absorbed dose rate and a steady-state component which registers the total absorbed dose in the range 20–80 kGy as based on a suitable calibration. The effects of post-irradiation storage time, day light, and storage temperature on the radiation-induced visible spectrum were investigated. The storage-temperature effect on post-irradiation conductivity measurement was also evaluated.     

Radiation-induced graft polymerization of acrylic acid onto fluorinated polymers. I. Kinetic study on the grafting onto poly(tetrafluoroethylene-ethylene) copolymer

Direct radiation-induced grafting of aqueous acrylic acid (AAc) onto poly(tetrafluoroethylene-ethylene) (ET) film has been studied. The effect of grafting conditions such as monomer concentration, exposure dose, dose rate, and film thickness on the grafting yield was investigated. The dependence of the grafting rate on monomer concentration was found to be 1.2 order. The dependence of the grafting rate on dose rate was found to be 0.6 order regardless of the film thickness. The relationship between the grafting rate and film thickness gave a negative first-order dependence. The results suggest that the grafting process is mainly controlled by monomer diffusion, and it was concluded that this grafting system proceeded by the front mechanism. The swelling behavior increases linearly with degree of grafting. The electrical conductivity and mechanical properties for the trunk and grafted polymer were investigated at different irradiation doses in air and under vacuum irradiations. © 1992 John Wiley & Sons, Inc.     

Influence of cation on charge recombination in dye-sensitized TiO2 electrodes

The reaction of a dye cation recombining with an electron in TiO(2), in the presence of Li(+), Ca(2+), and TBA(+) cations, was studied with laser-induced transient absorption measurements. The active cations, Li(+) and Ca(2+), shorten the dye cation lifetime on sensitized TiO(2) but not ZnO electrodes. By combining the absorbance measurements of the dye cation with simultaneous measurements of the current transient, the contribution of the recombination reaction to the current is identified. Furthermore, classical porous electrode theory is used to quantify the behavior of the heterogeneous electrode, and in doing so, the processes contributing to photoinduced current are identified as Helmholtz layer charging, porous electrode charging, recombination reactions, and surface diffusion of the active cations. The rate of charge recombination is proportional to the concentration of initially deposited active cations. The effect of water on the recombination rate and the current is also observed.     

Radiation-Induced Injection Conductivity of Polymers

Steady-state currents flowing through planar polymer layers under irradiation with 15–50 keV electrons were studied experimentally and theoretically. The ultimate range of electrons was somewhat below the layer thickness. The Monte Carlo method was used to determine the basic transport characteristics of fast electrons in polymers (maximum range, depth distribution of absorbed dose and forward current). It was shown that significant steady-state currents (1 to 10% of the electron beam current) were observed only if the thickness of blocking (unirradiated) layer did not exceed 5 m. The magnitude of these currents was almost unaffected by the polymer type (polymers with minimum radiation-induced conductivity and polymers with electron or hole conductivity were examined). It was found that conventional theories of conductivity of dielectrics failed to explain the observed experimental data. Additional arguments in favor of the hypothesis of streamer mechanism of injection currents through an unirradiated polymer layer were obtained. It is emphasized that the radiation-induced heating of polymer samples can play an important role in the phenomenon under study, acting as an undesirable technical factor, that strongly distorts obtainable experimental data.     

Effect of dose on radiation-induced conductivity in polymers

Numerical simulation of radiation-induced conductivity in polymers upon long-term irradiation on the basis of the generalized Rose-Fowler-Vaisberg model, which allows for both dipolar carrier transport and generation of radiation traps during irradiation, was performed. The unusual properties of radiation-induced conductivity, such as the appearance of a maximum on current transients, the absence of a steady state, and a substantial difference between these curves for the first and subsequent irradiation, are rationalized in terms of the formation of free radicals, the major feature of radiolysis in the chemical aspect. This interpretation does not require the involvement of degradation or crosslinking processes, unlike other interpretations that appear in the literature. With the use of low-density polyethylene as an example, it was shown that radiation-induced conductivity both upon pulse and continuous irradiation can satisfactorily be described with the unified set of parameters of the generalized Rose-Fowler-Vaisberg model.     

Theoretical analysis of the Rose-Fowler-Vaisberg model

Results of numerical calculations based on the Rose-Fowler-Vaisberg model of radiation-induced conductivity in a case polymer upon long (10⁴ s) irradiation at doses of 5 × 10⁵–10⁷ Gy are reported. Two irradiation modes were considered: (1) preliminary irradiation and irradiation repeated at variable times after the end of the first irradiation and (2) probing the virgin and irradiated polymer with a standard pulse of ionizing radiation. It was shown that the properties of radiation-induced conductivity, such as its overshoot kinetics, a considerable difference between current transients for the initial and the repeated irradiation, extremely long annealing times of dose effects, and the absence of a steady state, are naturally explained in terms of this model (with allowance for the generation of radiation-induced traps as regards the last property). The Rose-Fowler-Vaisberg theory should be considered at present a well-approved semiempirical model of radiation-induced conductivity in polymers.     

Universal technique for studying electron transport in polymers

A universal technique was developed for studying electron transport in polymers, which consists in the combination of the time-of-flight technique in the surface and bulk carrier generation modes with measurement of transient radiation-induced conductivity. Central to the technique in question is the time-of-flight method with homogeneous irradiation. The procedure was implemented on the basis of an ELA-50 electron beam device with variable electron energy (3–50 keV). The practicability of the universal technique was demonstrated using the model polymers polyvinylcarbazole and molecularly doped polycarbonate as examples.     

Secondary Electron Emission from an Organic Layer with a Surface Charge

Using secondary electron emission (SEE) techniques, conditions for the traveling of electrons near a charged surface were studied. A simple analytical expression was found to relate the effective coefficient of secondary electron emission from the charged surface of an organic liquid layer with the primary-electron current. At low currents, the relationship is close to a root law, the pattern of the dependence does not change with the varying conductivity of the liquid, its thickness, and the charge spot area. This finding suggests that the effective secondary electron emission coefficient and, hence, the conditions of electron motion near a surface charge depend on the only parameter, the current density of incident electrons. According to the estimates of the dielectric permittivity of a liquid, its resistivity, and ion mobility, the effective SEE coefficient at low charging currents is formed in the ohmic mode of current flow through the liquid.     

On the use of electrokinetics for unraveling charging and structure of soft planar polymer films

During the past decade, much attention has been devoted to the use of electrokinetic phenomena for addressing both charging mechanism and structure of multi-responsive soft polymeric layers whose thickness may range from few tens of nanometers to several microns. In particular, major progress was achieved in the quantitative reconstruction of streaming current data collected over a wide range of physico-chemical conditions using recent theories for electrohydrodynamics of soft diffuse planar interphases. In this article, we review the basics of the methodology adopted for deciphering the mechanisms governing the charging of electric double layers at soft planar films in connection with their structure that may vary according to pH, salt concentration or temperature depending on the responsive character of the system. It is demonstrated how the combination of streaming current, surface conductivity and swelling measurements allows for a comprehensive understanding of the interrelated protolytic, hydrodynamic, electrostatic and structural properties of polymer layers. We discuss the benefits and limits of the approach on the basis of studies carried out on uncharged, moderately charged and highly charged soft polymeric films supported by hard charged carriers. In a final part, the basic processes governing the peculiar electrokinetic properties of soft planar polyelectrolyte multilayers under lateral flow conditions are described.     

Optimisation and characterisation of biosensors based on polyaniline

With lower limits of detection and increased stability constantly being demanded of biosensor devices, characterisation of the constituent layers that make up the sensor has become unavoidable, since this is inextricably linked with its performance. This work describe the optimisation and characterisation of two aspects of sensor performance: a conductive polymer layer (polyaniline) and the immobilised protein layer. The influence of the thickness of polyaniline films deposited electrochemically onto screen-printed electrode surfaces is described in this work in terms of its influence on a variety of amperometric sensor performance characteristics: time to reach steady state, charging current, catalytic current, background current and signal/background ratios. The influence of polymer film thickness on the conductivity and morphology of finished films is also presented.

An electrostatic method of protein immobilisation is used in this work and scanning electron microscopy in conjunction with gold-labelled antibodies and back-scattered electron detection has enabled the direct visualisation of individual groups of proteins on the sensor surface. Such information can provide an insight into the performance of sensors under influence of increasing protein concentrations.     

聚四氟乙烯强酸性阳离子交换纤维的制备研究

采用共辐射引发将苯乙烯接枝到聚四氟乙烯(PTFE)纤维上,然后磺化制备出强酸性和超强酸性离子交换纤维,接枝率随苯乙烯单体浓度和辐射剂量增加而提高,随辐射剂量率的增加而降低,当接枝率为20%左右时,PTFE-co-St-SO3H离子交换纤维的Hammett酸度函数低于-11.99,呈现出超强酸性。     

Theoretical description of the prompt component of pulsed radiation-induced conductivity in polymers in terms of the ion pair mechanism of radiolysis

The properties of the prompt component of radiation-induced conductivity in polymers was theoretically analyzed on the basis of numerical solution of the modified Smoluchowski equation. The time dependences for the concentration of trapped electrons and the geminate-pair current after δ-pulse irradiation were calculated. In addition, calculation for a pulse of a finite duration was performed. The field dependence of the prompt conductivity was determined: deviations from the Ohmís law begin at field strengths on the order of 10⁷ V/m, the character of this dependence strongly varying with a decrease in the electron thermalization length.     

A Monte Carlo simulation of radiation-induced conductivity of disordered solids

The Monte Carlo method was used to perform a theoretical study of the kinetics of radiation-induced conductivity under pulsed irradiation of a disordered solid with traps having different energy distributions. The kinetics of radiation-induced conductivity in a weak electric field was satisfactorily described by the Rose-Fowler-Vaisberg model. In a strong field, radiation-induced conductivity became a nonlinear field strength function because of distortions of the energy spectrum of trap centers.     

Dynamics of copper electrodeposition onto fibrous carbon electrodes with electrical conductivity varying across their thickness

Distribution of copper in its electrodeposition from a sulfuric acid solution onto fibrous carbon electrodes with electrical conductivity varying across their thickness, the deposition rate of copper, and the current efficiency by copper were studied in relation to the electrolysis duration, electrical conductivity of the electrode, geometric current density, and solution flow rate. The main factors governing the distribution of copper across the electrode thickness and the electrolysis parameters beginning from the process onset till the “clogging” of a part of the electrode by the metal were determined.     

Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force-distance curve measurements

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force-distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN(x) films, metal-insulator-metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.     

Radiation-Induced Conductivity of Polymer Composites Filled by Finely Divided Oxides

The radiation-induced conductivity of polymer composites based on polyisoprene rubber filled with finely divided oxides of various chemical natures was studied in the continuous irradiation mode. It was shown that the radiation conductivity is closely related to the molecular mobility of the rubber. It was found that a change in the character of thermal motion of fragments of rubber macromolecules, which is caused by interaction with the filler, could considerably enhance the radiation-induced conductivity of a polymer composite.