Henk Schut

New Insights in the Nanostructure and Defect States of Hydrogenated Amorphous Silicon Obtained by Annealing

Temperature annealing is used as a tool to study the validity of network models for the nanostructure of hydrogenated amorphous silicon (a-Si:H) and its relation to defect states. The changes in the size of the dominant open volume deficiencies have been studied using Doppler broadening positron annihilation spectroscopy and Fourier transform infrared spectroscopy. It is shown that the dominant open volume deficiencies for as-deposited films are divacancies, which appear to agglomerate into larger open volume deficiencies up to 400 °C. Above this temperature, the largest open volume deficiencies are suggested to be released at the surface of the sample. Fourier transform photocurrent spectroscopy results indicate a dramatic increase in the density of various subgap defect state distributions during temperature annealing. In addition, at least four defect states have been identified. These findings cannot be directly explained by assuming solely dangling bonds as the dominant defects in a-Si:H. We discuss that a model based on an anisotropic disordered network with volume deficiencies does explain our findings better than the classical model based on a continuous random network with solely an isotropic distribution of coordination defects. The claim is made that next to dangling bonds not fully hydrogen-passivated vacancies are significantly contributing to the dominant defect states in a-Si:H.

The Nature and the Kinetics of Light-Induced Defect Creation in Hydrogenated Amorphous Silicon Films and Solar Cells

The nature and the kinetics of light-induced defect creation in hydrogenated amorphous silicon (a-Si:H) films and solar cells are investigated by means of Doppler broadening positron annihilation spectroscopy, Fourier transform photocurrent spectroscopy, and J-V characterization. There is a strong correlation between the open volume deficiencies in a-Si:H and the Staebler-Wronski effect (SWE). The carrier generation and recombination profiles in the absorber layer are spatially correlated, and the recombination due to defects in the top and bottom parts of the absorber layer is different. Furthermore, the various defect distributions in the bandgap have different defect creation kinetics. It is demonstrated that the SWE defect formation kinetics in a solar cell are very complex and can impossibly be described by one time scaling ~ tβ as is often claimed.

From nanopores to macropores: Fractal morphology of graphite

Abstract We present a comprehensive structural characterization of two different highly pure nuclear graphites that compasses all relevant length scales from nanometers to sub-mm. This has been achieved by combining several experiments and neutron techniques: Small Angle Neutron Scattering (SANS), high-resolution Spin Echo SANS (SESANS) and neutron imaging. In this way it is possible to probe an extraordinary broad range of 6 orders of magnitude in length from microscopic to macroscopic length scales. The results reveal a fractal structure that extends from ∼0.6xa0nm to 0.6xa0mm and has surface and mass fractal dimensions both very close to 2.5, a value found for percolating clusters and fractured ranked surfaces in 3D.

Compression Effects on the Phase Behaviour of Miconazole-Poly (1-Vinylpyrrolidone-Co-Vinyl Acetate) Solid Dispersions—Role of Pressure, Dwell Time, and Preparation Method

Compression of miconazole-poly (1-vinylpyrrolidone-co-vinyl acetate) (PVPVA64) solid dispersions prepared by spray drying and hot-melt extrusion was performed to gain insights into effect of compression pressure, dwell time, and preparation method on compression-dependent phase behavior. The solid dispersions prepared by spray drying were initially phase-separated showing two glass transition temperature (Tg), whereas the extruded samples showed one single Tg indicating better mixing. Compression caused mixing of spray-dried solid dispersions at high compression pressures and especially high dwell times. The extruded systems showed no statistically significant differences. However, physical mixtures made up from extruded samples containing 20% and 40% of active pharmaceutical ingredient underwent mixing upon compression. Coincidence Doppler measurements were performed to quantify the free volume of PVPVA64 which is a major contributor to the free volume in the solid dispersion matrix. A small but significant difference was found between the open free volume of the pure polymer subjected to varied manufacturing processes. Compression-induced plastic deformation and plastic flow enhances molecular mobility leading to mixing of different domains in solid dispersions. Different manufacturing methods may result in products with similar free volume, thereby showing similar molecular mobility.

Migration of Open Volume Deficiencies in Hydrogenated Amorphous Silicon During Annealing

The nanostructure of hydrogenated amorphous silicon (a-Si:H) is studied by means of doppler broadening positron annihilation spectroscopy (DB-PAS) and Fourier transform infrared (FTIR) spectroscopy. The evolution of open volume deficiencies is monitored during annealing, demonstrating that small vacancies and other small vacancy clusters that are initially present in the a-Si:H nanostructure agglomerate into larger vacancy clusters. The migration of open volume deficiencies is less pronounced for a-Si:H deposited at higher hydrogen-to-silane gas flow rate ratio, R. FTIR spectroscopy reveals the presence of a peculiar peak in the refractive index in the infrared—and hence the calculated mass density—which occurs just before H effusion from the films starts. The combined results of DB-PAS and FTIR spectroscopy indicate that a stress buildup caused by the accumulation of H2 in agglomerating vacancies during annealing can explain the sudden mass density increase. At higher temperatures, stress is released with the onset of H effusion. The H effusion consists of a two-stage process involving small open volume deficiencies and nanosized voids, contrasting earlier interpretations. The reduced amount of hydrogen migration and enhanced hydrogen passivation degree are suggested as key factors to the reduced light-induced degradation associated with increased R values.

Positron and thermal desorption studies on He ion implanted nuclear graphite

The positron beam Doppler Broadening (DB) and Thermal Helium Desorption Spectroscopy (THDS) techniques are applied to study the behavior of radiation induced point defects in IG-110 nuclear graphite. The defects are introduced by irradiation at room temperature with 200keV He+ at doses ranging from 1015 to 1017 He/cm2. In the thermal desorption spectroscopy, the release of He+ is observed between 500K and 800K. With increasing He+ implantation dose, the fraction of He+ desorbed decreases from 27% to 3%. In the DB-curves showing the S parameter values versus positron implantation depth, the derived vacancy type defect distribution are in accordance with those obtained by SRIM calculations. Subsequent annealing of the implanted samples in steps of 100K for 5 minutes up to 1200K shows a distinct decrease of the S parameter value to its reference value between 500K and 700K. This temperature interval corresponds to the literature values for single vacancy migration energy of 1-2eV.

Postponing Crack Nucleation in 2024 Aluminium Alloy by Dynamic Precipitation from the Supersaturated State

Recently a novel strategy to improve the fatigue resistance of precipitation hardened aluminium alloys has been proposed, which is based on dynamic precipitation in partially aged material. In this paper, the effect of mean stress and alternative temperature treatments that can enhance the high cycle fatigue resistance through the mentioned mechanism were investigated. The material used is an under-aged 2024 aluminium alloy, which showed superior fatigue life compared to the peak aged condition. The recorded behaviour was observed to be more effective at lower stress amplitudes and lower R values. As the dynamic precipitation may not be able to keep up with the damage evolution, dedicated experiments were conducted to insert periods of controlled healing in a stress free state.

Positron annihilation study of Fe-ion irradiated reactor pressure vessel model alloys

The degradation of reactor pressure vessel steels under irradiation, which results from the hardening and embrittlement caused by a high number density of nanometer scale damage, is of increasingly crucial concern for safe nuclear power plant operation and possible reactor lifetime prolongation. In this paper, the radiation damage in model alloys with increasing chemical complexity (Fe, Fe-Cu, Fe-Cu-Si, Fe-Cu-Ni and Fe-Cu-Ni-Mn) has been studied by Positron Annihilation Doppler Broadening spectroscopy after 1.5 MeV Fe-ion implantation at room temperature or high temperature (290 oC). It is found that the room temperature irradiation generally leads to the formation of vacancy-type defects in the Fe matrix. The high temperature irradiation exhibits an additional annealing effect for the radiation damage. Besides the Cu-rich clusters observed by the positron probe, the results show formation of vacancy-Mn complexes for implantation at low temperatures.

Thermal annealing of C ion irradiation defects in nuclear graphite studied by positron annihilation

In order to investigate the thermal behaviour of radiation induced point defects in nuclear graphite, ETU10 graphite was implanted with 350 keV C+ ion to doses of 1015 and 1016 cm-2. The point defects introduced by the implantation were characterized by Positron Annihilation Doppler Broadening (PADB) and their thermal behaviour was studied during in situ annealing at Delft Variable Energy Positron beam (VEP). The annealing was performed for 5 minutes at temperatures ranging from 300 K (as implanted) to 1500 K in steps of 100 K. For both doses, an annealing stage at around 450 K is observed followed by a second stage around 700 K. For the high dose implantation vacancy complexes are found which are stable up to a temperature around 1400K.

Positron annihilation and tribological studies of nano-embedded Al alloys

Positron annihilation studies of aluminium alloys with nanodispersions of insoluble elements, i.e., In, Sn, Pb and Au were reported. The alloys were obtained using a rapid solidification process. For all alloys, except that with Au, the average diameter of nanoparticles in aluminium matrix was 100 nm, and variance of the size distribution was above 50 nm. Positron annihilation studies reveal the presence of monovacancies or divacancies, which were located at the interface between nanoparticles and the matrix. In the as-cast reference pure aluminium sample as well as the aluminium and gold alloy dislocations were identified as well. The isothermal annealing of the obtained alloys and measurement of the annihilation characteristic, i.e., S-parameter, allow us to determine the activation energy of grain boundary migration, which for the alloys was higher by the factor of four than for the reference sample. The measurements of friction parameters for the alloys confirmed the results reported by the other authors that, the friction coefficient was lower by the factor of about two and the specific wear rate was by the factor of about fifty higher than the reference sample. The present study confirmed the attractive positron affinity of the nanoparticles of In, Sn, Pb and Au compared to aluminium matrix.