Influence of spoliation in poly(2-hydroxy ethyl methacrylate) soft contact lens on its free volume and optical transparency

The calcification in poly(2-hydroxy ethyl methacrylate) contact lens was investigated using positron annihilation spectroscopy (PLS). The two poly(2-hydroxy ethyl methacrylate) (PHEMA) lenses of different companies were calcified employing a simple mechanism of calcification in abiotic aqueous solutions. The calcium deposit was analyzed using energy dispersive X-ray spectroscopy (EDS). Calcified lenses showed decrease in ortho-positronium (o-Ps) lifetime and free volume hole size of the lens material suggesting diffusion of Ca²⁺ into these cavities. The change in optical property viz. refractive index of these calcified lenses were measured and correlated with positron results. To find a better correlation, a series of worn spoilt PHEMA lenses of the same power with mainly calcium deposits, were similarly characterized using PLS and refractive index. These results correlate well with the free volume of the material. For hydrophilic lenses this correlation is reported for the first time.     

Positron lifetime study of the devitrification process in a cordierite glass

The devitrification of the 13.3% MgO-32.0% Al₂O₃-52.7% SiO₂-2.0% BaO ceramic-glass via isochronal heating up to a stable crystalline structure has been studied by the positron annihilation technique. The lifetime spectra showed two components. This devitrification results as a two-stage process. The long-lifetime ₂, attributed to the pick-off annihilation ofortho-Ps states, decreased on crystallization, but it was insensitive to the hexagonal-orthorhombic transformation of the crystalline cordierite. On the contrary, the effective lifetime of positrons annihilating in states that are not Ps, increased. This increase has been attributed to the Al/Si order process inherent to the hexagonal-orthorhombic transformation. Moreover, the Ps fraction in the crystalline phases was higher than in the glassy phase. However the free volume model might qualitatively explain the relationship observed betweenI ₂ and ₂.     

变温条件下HDPE/CB的正电子湮灭特性

对掺杂了25份（phr）导电炭黑（CB）的样品进行了正电子湮灭寿命谱的变温测量，在玻璃化转变温度以上，随着温度的增加，自由体积的大小线性增加，在玻璃化转变温度以下，自由体积的大小基本不变，o-Ps的强度在－40℃下基本不变，从－40℃以上开始增加，但在50℃以上上升有加快的趋势，与PTC材料的PTC现象和导电性能有一定的关联。     

Structural study of polymer hydrogel contact lenses by means of positron annihilation lifetime spectroscopy and UV–vis–NIR methods

A study has been conducted in order to determine presence of free volume gaps in the structure of structure of polymer hydrogel contact lenses made in phosphoryl choline technology and of the degree of defect of its structure. The study was made by means of positron annihilation lifetime spectroscopy. As a result of the conducted measurements, curves were obtained, which described numbers of counts of the acts of annihilation in the time function. The conducted studies revealed existence of three components τ₁, τ₂ and τ₃. The τ₃ component is attributed to the pick-off annihilation of o-Ps orthopositronium trapping by free volume gaps and provides information about geometrical parameters of the volumes. At the same time, the UV–vis–NIR spectrometry examination was conducted on the same samples in the spectral range 200–1,000 nm.     

Electroless deposition catalysed by gold in the photographic gelatin layer studied with positron annihilation

Abstract

The effects of free volume holes in the photographic gelatin in the electroless deposition process was studied by positron annihilation technology in this paper. The results show that, after complete activation, the average volume of the free volume holes in photographic gelatin decreased by about 0.011 nm³ which is equivalent to the volume of a gold atom. On an average basis, every free volume hole was filled by a gold atom which catalysed the electroless copper deposition. The copper grain formed in the electroless copper deposition was not filamentary in shape but a spherical stack. After electroless copper deposition was completed, the average volume of the free volume holes in the photographic gelatin decreased by about 0.020 nm³ which is equivalent to the volume of three copper atoms. Therefore, the deposition of copper in isotropic and spherical form was not hindered by free volume holes in photographic gelatin.     

Investigation of Er-doped Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> transparent ceramics by positron annihilation spectroscopy

0.25 at.% Er-doped Sc₂O₃ transparent ceramics fabricated using the two-step sintering method with different combinations of sintering temperatures were investigated by positron annihilation spectroscopy. Analysis of the broadening of the annihilation photopeak revealed the presence of the same type of defect in all samples. The lack of long lifetimes (τ ≥ 2 ns) suggested no positronium formation or the lack of trapping sites large enough to trap positronium for long enough time for the annihilation to be observed. Analysis of positron annihilation lifetime revealed the presence of a single lifetime that ranged from 208 to 219 ps, depending on the sintering conditions. These results also suggest the absence of a significant presence of vacancy clusters and other larger open-volume defects, and that the dominant open-volume defect corresponds to monovacancies and/or complex defects associated with monovacancies. The bulk lifetime of Er-doped scandia is estimated to be equal or lower than 208 ps.     

Characterization of lattice defects in metallic materials by positron annihilation spectroscopy: A review

Positron is an excellent probe of lattice defects in solids. A thermallized positron delocalized in lattice can be trapped at open volume defects, e.g. vacancies, dislocations, grain boundaries etc. Positron annihilation spectroscopy is a non-destructive technique which enables characterization of open volume lattice defects in solids on the atomic scale. Positron lifetime and Doppler broadening of annihilation photo-peak are the most common observables related to positron annihilation process. Positron lifetime spectroscopy enables to identify defects in solids and to determine their concentrations while coincidence measurement of Doppler broadening provides information about local chemical environment of defects. This article provides a review of the state-of-art of defect characterization in bulk metallic materials by positron annihilation spectroscopy. Advanced analysis of positron annihilation data and recent developments of positron annihilation methodology are described and discussed on examples of defect studies of metallic materials. Future development in the field is proposed as well.     

正电子湮没技术在高分子薄膜研究中的应用

介绍了正电子湮没技术在高分子薄膜研究中的最新进展.常规的正电子湮没技术和慢正电子湮没技术,可以准确地探测高分子薄膜微观缺陷(自由体积)尺寸、分数、浓度、深度分布,该测试技术将在研究各种高分子薄膜的微结构-性能关系、表面效应、界面效应等方面发挥积极的作用.     

Positron lifetime spectroscopy characterization of thermal history effects on polycarbonate

The effect of a short-term anneal aboveT _g on the free volume cavity size and concentration and on the fracture toughness of polycarbonate is examined. The positron annihilation lifetime (PAL) technique is used to measure the change in free volume concentration and cavity size during isothermal relaxation experiments at 10, 20 and 30 ° C. An activation energy of 16.5 kJ mol^–1 is calculated for the relaxation of the annealed polycarbonate, compared to 12.3 kJ mol^–1 for the unannealed material. The fracture toughness and brittle fracture morphology of compact tension specimens are unchanged by the anneal. The similarity in the PAL parameters and physical properties between the unannealed polycarbonate and the material annealed aboveT _g suggests that the short-term anneal does not appreciably alter the structural state of glassy polycarbonate.     

Thermal Relaxation and High Temperature Creep of Zr55Cu30Al10Ni5 Bulk Metallic Glass

The free volume model is applied to isothermal relaxation and hightemperature creep. For this analysis, the time dependent flow behaviourof Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass (BMG) near the glass transition temperature (T _g ) is expressed as a trade off between stress inducedgeneration and diffusion controlled annihilation of free volume. Thestrain rate-stress relation over a wide strain rate-range (10^–7to 10^–2 s^–1) was established for three different temperatures near T _g . It was found that the thermal relaxation behaviour and creep kinetics arecontrolled by the mobility of atoms with an activation energy of 161 kJ/mol.     

Doppler broadening measurements of the electron-positron annihilation radiation in nanocrystalline ZrO<Subscript>2</Subscript>

ZrO₂ powders have been ground by ball mill grinder to achieve the particle size down to 10 nm. Typical defects introduced during ball mill grinding have been studied by positron annihilation lifetime measurement technique and coincidence Doppler broadened positron annihilation radiation spectroscopic technique. Coincidence Doppler broadened positron annihilation spectra for ball mill ground and unground ZrO₂ samples have been analyzed by constructing ratio curve with defects free Al single crystal. Results indicate an increase of cation defects in ZrO₂ samples due to the reduction of particle size by the ball mill grinding process.     

Ultra-fine particles of amorphous TiO2 studied by means of positron annihilation spectroscopy

Microscopic structures and mechanical properties of amorphous TiO2 ultra-fine particles have been studied by positron annihilation spectroscopy. A positron lifetime of about 1.5 ns was found as the longest lifetime component 3 which is attributed to the ortho-positronium annihilation in open space. The lifetime 3 and the corresponding intensity I3 was independent of atmosphere: vacuum and various gasses. Accordingly, a micro-void is contained inside the particle as the open space. An intermediate lifetime 2 was approximately 0.4 ns. The intensity I2 was strongly affected by the atmosphere. No effect was found for the particles, the surface of which was coated by silicon dioxide film. This component is arisen from the positron annihilation on the particle surface and the particles contain the surface cracks. When the volume of particles packed in an ampoule was decreased by 1/4 times by compression, the longest component as well as the intermediate one disappeared and only the shortest-lifetime component 1 of about 0.24 ns remained. To conclude, the particles are so soft that the micro-voids and the surface cracks are collapsed by compression.     

Sintering of CuO investigated by positron lifetime spectroscopy

The sintering process of CuO under isochronal annealing has been investigated by positron lifetime spectroscopy. A long-lived component of 370±20 psec, observed in as-pressed samples and samples annealed atT900 K, has been attributed to continuous channels along three-grain junctions. A short-lived component of about 224 psec is attributed to positron traps at two-grain interfaces. The annihilation parameters exhibit recovery stages which may be correlated with the stages of sintering. A drastic recovery, starting at 975 K, is correlated with the final stage of sintering. The results suggest a lifetime of approximately 147 psec for positrons in the bulk of CuO.     

Non‐destructive evaluation of fatigue damage and fatigue crack initiation in type 316 stainless steel by positron annihilation line‐shape and lifetime analyses

Rotating bending fatigue tests were conducted using type 316 stainless steel. The fatigue tests were periodically terminated, and fatigue damage and fatigue crack initiation were non‐destructively and sequentially evaluated by positron annihilation line‐shape and lifetime analyses. The counter‐jig and anticoincidence methods were used for positron annihilation line‐shape and lifetime analyses, respectively, to enhance the analytical precision. The fatigue crack lengths were monitored by a plastic replication technique, and related to the parameters in both analyses. S‐parameter obtained in the line‐shape analysis increased with increasing fatigue damage, while it was difficult to detect fatigue crack initiation and subsequent small fatigue crack growth. That was because the precision of line‐shape analysis was limited. On the other hand, both fatigue damage and fatigue crack initiation were successfully detected by lifetime analysis. Positron annihilation lifetime also increased with increasing fatigue damage, and lifetime was longer at the notch root with fatigue crack than at the smooth section without crack. It was considered that the precision of lifetime analysis was high enough to detect high dislocation density areas at the fatigue crack tips.     

Effects of plasticizer and strain on the percolation threshold in polyisoprene–carbon nanocomposites: Positron annihilation lifetime spectroscopy and electrical resistance measurements

A study of the effects of plasticizer and stretching strain on the percolation transition in polyisoprene–carbon nanocomposites (PCNC) is reported. The ortho-positronium (oPs) accessible free volume sites are measured by positron annihilation lifetime spectroscopy (PALS) in relaxed and stretched PCNC samples containing different amounts of plasticizer. The lifetime of oPs, τ₃, is related to the size of the free volume, and the intensity, I₃, to the concentration of free volume sites. The number of free volume cavities is found to decrease during stretching regardless of the content of either carbon nanoparticles (CNP) or plasticizer. The free volume cavity size reaches its maximum value in the region of percolation transition. The percolation threshold is determined by measurements of electrical resistance (ERM). Both PALS and ERM show that the percolation threshold shifts to higher concentration of CNP under stretching strain. A shift of the percolation threshold to lower concentration of CNP was observed for addition of plasticizer. It is interesting that addition of CNP increases the mean size of free volume cavities below the percolation threshold and decreases it at CNP concentrations exceeding the percolation threshold. The relative number of free volume cavities represented by I₃ also decreases at CNP concentrations exceeding the percolation threshold. The results are interpreted as filling of cavities above the percolation threshold. ERM during application of cyclic tensile stress revealed fatigue of the tensoresistance effect in samples containing 10 m.p. of CNP with and without added plasticizer.     

Modified C-t technique for determining the generation lifetime profile in MeV He implanted silicon

Depth profiling of carrier lifetime can be a very useful method for investigation of minority charge carrier lifetime modification by ion implantation. A modified pulse C-t method has been developed for this purpose using the MOS structure fabricated on bulk Si prior to ion implantation. This technique enables the determination of lifetime distribution in the near surface region. Verification of the method was done on the devices implanted with MeV He⁺ ions for different doses. The extracted lifetime profiles as well as the advantages and limitations of the method are also discussed.     

Measuring the tensor polarization of positronium

A method for measuring the tensor polarization in a positronium (Ps) beam is proposed, which is based on the determination of the Ps lifetime as a function of the orientation of a homogeneous magnetic field. The dependence of the orthopositronum (o-Ps) lifetime on the angle between the directions of the magnetic field and the tensor polarization of the Ps beam can be determined from the results of measurements from two or more field orientations.     

正电子湮没技术在高聚物中的应用

本文介绍了正电子湮没技术的原理、技术方法 ,在自由体积测试方面的应用 ,以及国内研究进展。     

Electrical characterization of shallow cobalt-silicided junctions

The impact of the thickness of the cobalt-silicide layer on the electrical diode characteristics will be reported, with particular emphasis on the current-voltage (I-V characteristics, the surface and bulk carrier recombination lifetime and the low-frequency noise. It is shown that for thicker silicides the generation-recombination of holes in the top n⁺-layer is enhanced, giving rise to a higher diffusion current, an increase in the noise and a larger surface recombination velocity. It is believed that these parameters are closely linked, as the underlying physical phenomenon is the minority carrier recombination. In addition, it is shown by transmission electron microscopy that the silicide-silicon interface roughness is increased for larger cobalt thickness, which may explain some of the observations.     

Features of positronium migration in a solid containing nanopores, by an example of sorption and annihilation of positrons in vaterite

The features of migration of positrons in the bulk of vaterite (metastable modification of CaCO₃) were studied using positron annihilation lifetime spectroscopy (PALS). Comparison of the lifetime of ortho-positronium determined from the PALS data, which corresponds to the time of its residence in the vaterite nanopores, with the absorption results (BET adsorption isotherm) allowed evaluation of the diffusion coefficient of positronium in the perfect sections of the vaterite crystal lattice, equal to 0.4 × 10^?1 cm² s^?1.