Corrosion effects of nuclear tracks in polymers

Usually ion tracks are studies ex-situ, i.e. after removal of the ion-irradiated sample from the evacuated irradiation vessel. The exposure to ambient air leads to degradation effects which are frequently neglected and consequently have been studied rarely. They have never been explained consistently. In this work we compile and discuss them in the frame of today's general understanding of ion tracks in polymers. It appears that some pristine polymers such as polyethylene are covered by a surface layer which is badly permeable for gaseous contaminants. Ion irradiation leads to destruction of this protective layer so that oxygen and moisture from the ambient air can easily penetrate through the ion tracks, and eventually even into the surrounding pristine material. Moisture absorption along the tracks gives rise to some faint conductivity. Oxygen uptake adds new trapping centers to the existing ones along the tracks, so that on one hand an increased dopant uptake capability is recorded, and on the other hand oxygen-sensitive mobile dopants are immobilized along the ion tracks. Photooxidation processes contribute to the fading of the optical blackening of irradiated polymers.     

A study of the basic properties of electrochemical track etching

The basic properties of the electrochemical track etching method proposed by Tommasino were studied for PC and PET foils irradiated with fission fragments and/or alpha-particles. Etching was performed in a specially designed double-wall vessel applying electric fields of different strengths and frequencies. The variation in the diameters of the discharge spots produced around the tracks of fission fragments entering PC and PET foils at right angles was systematically studied as a function of the strength and frequency of the electric field, etching time and etchant temperature. For alpha-tracks registered in PC foils the dependence of the discharge spot diameter on particle energy was also determined. It was found that the production of discharge spots started at a threshold field strength depending on the type of particle. The temperature dependence of the growing rate of discharge spots followed the Arrhenius law, but with a reduced activation energy as compared to that obtained for the chemical etching rate of the bulk material.     

Funnel-type etched ion tracks in polymers

It is shown that conical track etching is a much more complicated process than generally assumed. The choice of the corresponding parameters (i.e. the ratios of concentrations and diffusion coefficients of both etchant (e.g. NaOH) and stopping solutions (e.g. HCl) and the etching temperature) determines the ratio of polymer dissolution to etchant penetration. The latter value controls the counterplay of diffusion, etching, ionic conductivity, field emission and capacitive effects, which is decisive for both the final track shapes and their electronic properties. The stages of track evolution during etching under different conditions are outlined in detail. Both transparent conical nanopores and “funnel-type” tracks can be obtained, the latter consisting of a shorter cone and a residual latent track. Depending on the internal structure of that latent track segment, such funnel-type tracks either allow smooth transmission of the rectified currents or they emit unipolar current spikes. Not only the study of electronic properties of single ion tracks, but also of a multitude of tracks makes sense. Depending on the applied parameters, the individual track properties may either just add up, or new effects may be found that emerge from the interaction of the tracks among each other. This is preferentially the case for spike-emitting tracks, where effects such as phase-locked spike synchronization can be found as described by neural network theory.     

SHI induced nano track polymer filters and characterization

Swift heavy ion irradiation produces damage in polymers in the form of latent tracks. Latent tracks can be enlarged by etching it in a suitable etchant and thus nuclear track etch membrane can be formed for gas permeation / purification in particular for hydrogen where the molecular size is very small. By applying suitable and controlled etching conditions well defined tracks can be formed for specific applications of the membranes. After etching gas permeation method is used for characterizing the tracks. In the present work polycarbonate (PC) of various thickness were irradiated with energetic ion beam at Inter University Accelerator Centre (IUAC), New Delhi. Nuclear tracks were modified by etching the PC in 6N NaOH at 60 (±1) °C from both sides for different times to produce track etch membranes. At critical etch time the etched pits from both the sides meet a rapid increase in gas permeation was observed. Permeability of hydrogen and carbon dioxide has been measured in samples etched for different times. The latent tracks produced by SHI irradiation in the track etch membranes show enhancement of free volume of the polymer. Nano filters are separation devices for the mixture of gases, different ions in the solution and isotopes and isobars separations. The polymer thin films with controlled porosity finding it self as best choice. However, the permeability and selectivity of these polymer based membrane filters are very important at the nano scale separation. The Swift Heavy Ion (SHI) induced nuclear track etched polymeric films with controlled etching have been attempted and characterized as nano scale filters.      

Dynamics of ultrashort pulsed laser radiation induced non-thermal ablation of graphite

We report on the dependence of a laser radiation induced ablation process of graphite on the applied pulse duration of ultrashort pulsed laser radiation smaller than 4 ps. The emerging so-called non-thermal ablation process of graphite has been confirmed to be capable to physically separate ultrathin graphitic layers from the surface of pristine graphite bulk crystal. This allows the deposition of ablated graphitic flakes on a substrate in the vicinity of the target. The observed ablation threshold determined at different pulse durations shows a modulation, which we ascribe to lattice motions along the c axis that are theoretically predicted to induce the non-thermal ablation process. In a simple approach, the ablation threshold can be described as a function of the energy penetration depth and the absorption of the applied ultrashort pulsed laser radiation. Based on the analysis of the pulse duration dependence of those two determining factors and the assumption of an invariant ablation process, we are able to reproduce the pulse duration dependence of the ablation threshold. Furthermore, the observed pulse duration dependences confirm the assumption of a fast material specific response of graphite target subsequent to optical excitation within the first 2 ps.     

Coupled chemical reactions in dynamic nanometric confinement: V. The influence of Li+ and F− ions on etching of nuclear tracks in polymers

Etching of continuous nuclear tracks in thin polymer foils from both sides is known to lead to the formation of double-conical nanopores. In this work and related ones we try to find out how this etching kinetics is modified when materials are added which react with each other upon their contact towards some new product that influences the etching. For that purpose we have chosen here Li⁺ and F^? ions as the additions, which react with each other to form LiF precipitations. The coupled etching and precipitation kinetics is recorded by measuring the electrical current that is transmitted through the foils upon application of a low-frequency alternating sinusoidal voltage. Depending on the etchant concentrations, the etching temperature and the time of Li⁺ and F^? addition, different effects are found that range from (a) no alteration of the transmitted current at all, via (b) the emergence of an alternating current with a temperature-dependent amplitude, and (c) the complete vanishing of any transmitted current at all, towards (d) chaotic transmitted current histories with phases with strong current spike emission and (e) rather quiet phases, alternating with each other in a rather unsystematic way. The observed effects are ascribed to (a) the enhanced penetration efficiency of both the Li⁺ and F^? ions through the polymeric bulk and/or latent ion tracks after the removal of the polymer's protective surface layer by the etchant, (b) the high mobility of preferentially the F^? ions within the polymer, (c) the LiF precipitation within the polymer or on its surface upon encounter of Li⁺ and F^? ions, (d) the nanofluidic properties of narrow etched tracks covered with Li⁺ ions on the wall surfaces and F^? ions beyond, and/or (e) the formation of LiF membranes within the etched tracks.     

Different shapes of tracks in phlogopite, biotite and soda lime glass

Etched track opening geometries in Biotite, Phlogopite and soda-lime glass irradiated with swift heavy ions [¹⁹⁷Au(11.64 MeV/n), ¹³⁶Xe(11.56 MeV/n), ⁵⁸Ni(11.56 MeV/n)] at different angle of incidence have been studied using appropriate chemical etching technique. Different geometries (Hexagonal, irregular polygon, triangular in case of Biotite and Phlogopite, and circular & elliptical in case of soda lime glass detector) of heavy ion tracks are reported in the present investigations using optical microscope. The different shapes of these heavy ions track geometries are found to be related with various target-projectile parameters (viz: type of projectiles, energy, stopping power, angle of incidence of the projectile, density of defects and its reactivity with etchant, etching conditions and chemical structure of the detectors). The dependence of different shapes of heavy ion tracks in isotropic and anisotropic medium on the variation of radiation damage densities along the ion trajectories have also been discussed in the present paper.      

GaP台面腐蚀研究

高效GaP绿色发光二极管制造中,器件的隔离和台面制作具有十分重要意义。本文介绍了用于GaP发光器件的隔离和台面制作的碱性铁氰化钾化学腐蚀技术。观察了腐蚀温度,腐蚀时间对腐蚀深度和表面形貌间关系。结果表明:碱性铁氰化钾对GaP台面腐蚀是一种优良的腐蚀剂,具有较高的腐蚀速率(2微米/分),并可获得光滑无凹坑或少凹坑的腐蚀面。对出现的实验现象从机理上作了解释。     

Thickness-dependent effective surface resistances of nearly ferroelectric superconductors

The effective surface resistance of nearly ferroelectric superconducting film in the dielectriclike response is theoretically investigated based on the electrodynamics of the nearly ferroelectric superconductors. We calculate the intrinsic film surface resistance for isolated thin film and the effective surface resistance for a superconductor/dielectric layered structure. It is found that the thickness-dependent surface resistance has two different behaviors separated by a critical film thickness being equal to the London penetration length. That is, a nonresonant dependence is seen when the film thickness is less than the London penetration length, and an anomalously resonant behavior is found when the film thickness is larger than the London penetration length. The nonresonant dependence is similar to that of a cuprate superconductor and it further is characterized by some other critical thicknesses. As for the anomalous resonant region it is seen only in a nearly ferroelectric superconductor.     

Coupled chemical reactions in dynamic nanometric confinement: Ag2O membrane formation during ion track etching

In this study, continuous swift heavy ion tracks in thin polymer foils were etched from both sides to create two conical nanopores opposing each other. Shortly before both cones merged, one of the nanopores was filled with a silver salt solution, whereas etching of the other cone continued. At the moment of track breakthrough, the etchant reacted with the silver salt solution by forming an impermeable and insulating membrane. Continued etching around the thus-created obstacle led to repetitive {etchant – silver salt solution} interactions. The coupling of the two chemical reactions, {etchant – polymer} and {etchant – silver salt solution}, within the confinement of etched tracks, with continuously changing shapes, showed a highly dynamic nature as recorded by measuring both the electrical current and the optical transmission across the foils. At low etching speeds, a central membrane that grew in radius and thickness with time until, at a critical thickness, the membrane became rather impermeable was formed. However, at high etching speeds, the emerging reaction products exhibited a sponge-like consistency, which allowed for their infinite growth. This precipitation was accompanied by a pronounced current spike formation. A simple theoretical model explains, at a minimum, the basic features.     

Radial dose distribution from carbon ion incident on liquid water

We report calculations of the radial dose deposited along carbon-ion tracks in liquid water using different techniques depending on the energy range of secondary electrons. The models are developed in relation with the experimental data on electron penetration lengths. For electrons with energies higher than 45 eV, we use the Katz model. However, the main focus is on the low-energy electrons, which are largely responsible for DNA damage within 10 nm from the tracks. For these electrons, the dose calculation is based on their random walk behaviour. The results of this combined approach are compared to experimental measurements. Contributions to the deposited energy by electrons of different ranges of energy are discussed.     

Coupled chemical reactions in dynamic nanometric confinement: IV. Ion transmission spectrometric analysis of nanofluidic behavior and membrane formation during track etching in polymers

In recent papers, it was shown that coupled chemical-topological reactions (CCRs) with both NaOH etchant and silver salts, performed in thin swift-heavy ion-irradiated polymers under the application of a test voltage across the polymer foils, eventually gave rise to characteristic current/voltage features and Bode plots that were tentatively attributed to the formation of Ag₂O membranes within the etched tracks. The same was also found when replacing the silver ions by lithium ions, and adding fluoride ions to the NaOH etchant, to promote LiF membrane formation. Ion Transmission Spectrometry (ITS) enabled us to reconfirm the existence of these membranes beyond doubt. The membrane thickness was determined to be ～0.2–0.4?µm in the best cases.

ITS also revealed that hitherto membrane formation occurs only in ～1% of all tracks, or even less. The reason for this poor abundance seems to be that the decisive factor for membrane formation, which is the firm anchoring of the emerging solid Ag₂O or LiF reaction products on the etched track walls, was hitherto rarely fulfilled. We attribute this tentatively to the too high test voltage applied for controlling the CCR process that might hinder the product anchoring on the walls by promoting nanofluidic electromigration. Indeed, voltage reduction seems to improve the situation.     

Ionizing particles may create shockwaves

To find a model that describes the gas diffusion on irradiated polymers (Makrofol KG polycarbonate) the diffusion constants have been measured with argon as diffusion gas. The polymers were irradiated with uranium, gold and lead ions of about 10 MeV/u and ion fluences between 1×10¹⁰ and 4×10¹¹ ions/cm². The ion irradiated probes show two quite different dependencies of the diffusion constant on the ion energy loss. These effects are strongly related to the fluence of the irradiation. In case of low ion fluences, the diffusion constant is up to 8 times higher than that of pristine material. In the probes with high ion fluences we observe a decrease of diffusion constant to half the value of the pristine material. To understand the dependence of the diffusion constant on ion fluences we apply a model of compacting. This model describes the compacting ability of shockwaves arising from latent tracks. A track formation model is suggested. When an ion penetrates the foil it creates shockwaves around its path. These shockwaves put compacting forces on earlier created latent tracks in the same foil.     

Optimization of porous silicon preparation technology for SERS applications

A series of porous silicon samples prepared at different etching parameters, namely etchant composition, etching time and current density, was investigated as substrates for surface-enhanced Raman scattering (SERS). Silver nanostructures were deposited on porous silicon by immersion plating method and Rhodamine 6G was used as analyte. The relation between the etching parameters, morphology of porous silicon surface and its SERS efficiency after silver deposition is examined. We show that a high HF content in the etchant allows the formation of a film with close-packed silver nanocrystals, which possess strong surface enhancement properties.     

核微孔过滤器的生产、特性和应用

核微孔过滤器是利用重离子辐照高分子材料后经化学蚀刻而制成的新型过滤介质,它具有真实的孔径和高选择过滤性能．本文讨论了核微孔过滤器的生产原理、方法、特性和应用． The nuclear track microfilter has a true pore size and high selectivity,which are produced by irradiating a variety of polymers such as PC, PET, Kapton and PVDF using heavy ions and subsequent etching with the appropriate etchant. The pore size and porosity of the nuclear trackmicrofilters can be preselected according to what is required, their properties as well as already tested or possible applications are discussed.     

Observations of fission-tracks in zircons by atomic force microscope

The fission-track (FT) method is a dating technique based on the observation of damage (tracks) by spontaneous fission of ²³⁸U left in a mineral. The date is calculated from the track density and the uranium concentration in the mineral. This is possible because the number of tracks is a function of uranium concentration and time since the start of track accumulation. Usually, the number of tracks is counted under an optical microscope after etching (chemical expansion of a track). However, as FT density per unit area rises, it becomes difficult to count the number of tracks. This is due to the fact that FTs overlap one another and are unable to be readily distinguished. This research examines the potential of atomic force microscope (AFM) for FT dating using zircons, which are likely to show higher FT density than other minerals due to their high U concentrations.To obtain an AFM image for a sample prepared for FT dating, removing the static electricity of the sample is essential to avoid an unexpected movement of the cantilever. A grain should be wider than about 30 μm to bring the cantilever on the mineral surface. Polishing with a fine grained compound is very important. There is not much difference in sharpness between images by AC mode (scanning with vibrating cantilever at a constant cycle) and Contact mode (scanning with the cantilever always in close contact with the surface). To confirm how tracks can be identified with the AFM, an AFM image was compared with an image obtained with the optical microscope. When change in the number of tracks and their shapes were observed through stepwise etching, the track expanded as the etching time increased. In addition, the etching rate was slower for large tracks than those for small tracks. This implied that the AFM can be used to observe etching of zircons with different degrees of nuclear fission damage. A track that could not be seen with the optical microscope due to insufficient etching could be observed by AFM methods, indicating the possibility of FT dating with high track densities using AFM after relatively short etching periods.     

Alternative procedure for LR 115 chemical etching and alpha tracks counting

An improved procedure for etching and analysis of alpha tracks induced in LR-115 detectors is proposed with the advantages of simplicity and its relatively low cost. A new type of detector holder was designed to etch and rinse efficiently up to 100 detectors. We develop a simple and reliable methodology with a semiautomatic track count using a Nikon digital camera coupled to a PC and employing software “SCION” freely available on the Internet. Track images are binarized prior the application of software “SCION” so that original track shapes are not distorted, space resolution is improved and track counting has low dependence on focus and illumination level. High discrimination for tracks is achieved when marks and rips perturb the detector surface. An image generator of nuclear tracks is included to study the effect of track overlapping effect on counting.     

A novel method of fabricating porous silicon material: ultrasonically enhanced anodic electrochemical etching

Ultrasonically enhanced anodic electrochemical etching is developed to fabricate luminescent porous silicon (PS) material. The samples prepared by the new etching method exhibit superior characteristics to those prepared by conventional direct current etching. By applying ultrasonically enhanced etching, PS microcavities with much higher quality factors can be fabricated. The improved quality induced by ultrasonic etching can be ascribed to increased rates of escape of hydrogen bubbles and other etched chemical species from the porous silicon pillars' surface. This process will cause the reaction between the etchant and the silicon wafer to proceed more rapidly along the vertical direction in the silicon pores than laterally.     

Secondary ion mass spectrometry measurements of deuterium penetration into silicon by low pressure RF glow discharges

Abstract

Using secondary ion mass spectrometry (SIMS) the penetration of deuterium into Si(100) substrates as a result of exposure to deuterium low pressure rf discharges has been determined as a function of exposure time, thermal contact of the Si wafers to the substrate electrode, substrate doping, and discharge pressure. For undoped (100) single crystal Si exposed without intentional heating to a 25 m torr D₂ plasma for 1 min the deuterium concentration in the near-surface region (0—30 nm) approaches 10²¹ at.cm^?3. It drops off with depth, but is still greater than 10¹⁷ at.cm.^?3 at a silicon depth of 200 nm. The large penetration depth, the observation that lowering the substrate temperature decreases the rate of deuterium uptake, and the dependence of deuterium penetration on the substrate doping type indicate that hydrogen diffusion is of primary importance. The presence of a 50 nm thick oxide layer on the Si substrate during plasma exposure lowers the deuterium near-surface concentration in the Si substrate by about three orders of magnitude, while the presence of 10 nm of thermal oxide reduces the deuterium uptake only insignificantly. Heavily B and As doped polycrystalline Si show less deuterium penetration, while undoped polycrystalline Si shows more deuterium uptake than undoped single crystal Si for the same plasma treatment.     

Diffusion along the Grain Boundaries in Crystals with Dislocations

A generalization of the Fisher model of the grain boundary diffusion is suggested, which takes into account the diffusion along short circuit diffusion paths (i.e., dislocations) in the bulk of crystalline grains. For the B-regime of the grain boundary diffusion, three different penetration modes have been found: at the short times the penetration depth of the element diffusing along the grain boundary is given by the Whipple solution of the Fisher model, but with the pipe diffusion coefficients along the dislocation cores instead of the volume diffusivities; at the intermediate times the penetration depth is a weak function of time, and at the large times the penetration depth again increases with time according to the Whipple solution, however, the rate of this increase is much smaller than in the initial period of time. The applications of the model for diffusion in nanomaterials are discussed.