Papers by Keyword: Helium

Abstract: Synthesis of carbon nanotubes from V-type pyrolysis flame is a kind of novel method. It needs more simple equipments. The V-type pyrolysis flame experimental system is introduced. Carbon source is the carbon monoxide and heat source is from acetylene/air premixed flame. Pentacarbonyl iron is acted as catalyst. The sampling time was 5 minutes. This study aims to find the influence rule of helium in synthesis of carbon nanotubes from the V-type pyrolysis flame. Carbon nanotubes with less impurity and high yield were captured successfully in the V-type pyrolysis flame. The carbon nanotubes were between 10nm and 20nm in diameter and dozens of microns in length. Helium played extremely important role in the flame synthesis of carbon nanotubes. The effect of helium is to dilute the concentration of combustible and reduce the flame temperature. Hydrogen also can keep the catalyst particles remain active and increase the yield of carbon nanotubes. Too high or low mass flow of helium was not appropriate for the formation of carbon nanotubes in V-type pyrolysis flame. The carbon nanotubes with "carbon tumour" will appear when the flow of helium is too high. Too low flow of helium will reduce the number of carbon nanotubes due to lack of cooling and dilution of the protection gas. It was found that the optimal flow of helium was about 0.283 SLM according to the experimental results.

Abstract: Comparative study of magnetic properties of highly oriented pyrolytic graphite irradiated with different particles suggests that ions (H+, He+, C+) with the energy in the range of hundreds keV yield larger values of induced magnetization compared to the protons with the energy of several MeV. These values increase with ion fluence but abruptly decrease when the concentration of interstitial defects becomes so large that the graphite stacking sequence is collapsed.

Abstract: The oxide dispersion strengthened (ODS) ferritic steel and non-ODS reduced-activation ferritic (RAF) steel were irradiated at 773 K by means of a dual-beam ion irradiation technique to a dose of 0.4 dpa with simultaneous helium implantation up to 1000 appm. Microstructural changes were investigated by transmission electron microscopy. The RAF steel showed a preferential formation of cavities at grain boundaries, precipitate interfaces and dislocations. In contrast, the ODS ferritic steel showed a homogeneous and fine distribution of cavities in the matrix. This paper discusses the superior resistance of the ODS ferritic steel against development of cavities in terms of the effects of nano-oxide particles dispersed in the matrix.

Abstract: Standard p-type 12 cm Cz Si wafers were implanted by helium ions. The implanted and nonimplanted samples were subsequently subjected to nitrogen plasma treatment and final vacuum annealing. SEM studies have shown the absence of large-scale defects on the top wafer surface and the presence of a layer revealing contrast with surrounding silicon on the cleavage surface at a depth corresponding to the projected range Rp. Scanning over a crater formed by ion sputtering has exposed no defects to the depth of Rp and beyond. At the same time, at a depth of Rp there is a layer with the morphology (structure) significantly different from the surrounding defect-free areas. The measurements of transverse conductivity have shown that the wafer with the formed nitrogen-contained layer possesses dielectric properties with a breakdown voltage up to 15 V.

Abstract: In this paper, helium-charged nanocrystalline Ti films with different bias voltages were deposited by the He-Ar magnetron co-sputtering method. XRD was used to investigate the microstructure of the He-Ti film. Meanwhile, in order to find out the factors of affecting the growth and size of helium clusters, kinetic Monte Carlo (KMC) simulation was carried out to study the growth of helium cluster, based on the simulation of helium behavior in titanium using molecular dynamics.

Abstract: Metal impurities are known to degrade dramatically the performances of silicon-based devices, even at concentrations as low as 1012 cm-3. A specific process, named proximity gettering, has been optimised by some authors in order to reduce the influence of these impurities [1]. This process consists in the building of a favourable impurity trapping zone in a non-active area of the device, by introducing implantation defects. This paper reports on the application of introducing such gettering sites as an approach to control phonon properties in 4H-SiC epilayer, and increase the thermal conductivity.

Abstract: We report on the application of introducing gettering sites as an approach to control some phonons and charge carrier related properties in 4H-SiC epilayer. Helium implantation (at room temperature or 750°C) was first carried out, followed by a proper annealing and gold diffusion, in order to check the gettering efficiency. Raman measurements showed the presence of the desired defect, introduced by ion implantation at RT. The shift of the Fano interference allowed us to calculate the free carriers’ density in each sample. The lowest value was found for the sample implanted at RT. The gettering sites can act as majority carrier traps and reducers of recombination processes, which can be interesting for devices designed for the detection of radiations.

Abstract: Two experimental loops for operation in research reactor LVR-15 in ÚJV Řež are recently under preparation: High Temperature Helium Loop (HTHL) and SuperCritical Water Loop (SCWL). Pure helium will be used as working medium in HTHL and its main physical parameters are: operating pressure 7MPa, max. temperature in the test section 900°C and flow rate 36kg/h. HTHL will include helium purification system, system for dosage of impurities (e.g. CO2, H2, H2O, O2, N2 etc.) and helium sampling. Helium purification experiments and testing of materials in simulated HTR conditions will take place in HTHL in the future. Main parameters of the SCWL are 25MPa, max. temperature in the test section 600°C, flow rate max. 200kg/h. SCWL will be used for corrosion tests of candidate materials, studies of water radiolysis at supercritical conditions and for testing of water chemistry suitable for operation. Both loops possess an irradiation channel with quite complicated internals design, whose complexity is imposed by current constraints on constructional materials of nuclear experimental devices, which limit the choice and maximum surface temperature of material of construction to 500°C for austenitic stainless steel. The working temperature will thus be attained only in a restricted volume of the test section. The channel internals will be briefly described. The mentioned loops will represent novel experimental devices, whose objective is to gain and extend knowledge on materials and environment performance under the influence of radiation.

Abstract: Low-temperature diffusion of palladium (450–700 oC) from an implanted layer (9.5 MeV, 1⋅1013cm-2) deep into the volume of a high-power P-i-N diode (2.5kV, 150A) is compared with that of a sputtered palladium layer (50nm thick), both under the enhancement by the radiation defects from He2+ implantation (10 MeV, 1⋅1012cm-2). Annealing after the palladium implantation (800oC, 60min.) prior to the He2+ implantation is shown to increase the concentration of palladium related deep levels in the damaged region with a subsequent improvement of diode dynamic parameters. The concentration of palladium in-diffusing from the implanted layer with the dose of 1⋅1013cm-2 does not sufficiently reduce excess carrier lifetime close to the anode junction as is the case of the devices with sputtered palladium that give better dynamic parameters like lower maximal reverse recovery current and recovery losses for the same magnitude of leakage current.

Abstract: The effect of high-energy hydrogen and helium implantation and subsequent annealing on generation of radiation defects and shallow donors in the low-doped oxygen-rich FZ n-type silicon was investigated. Samples were implanted with 7 MeV 4He2+ or 1.8 MeV 1H+ to fluences ranging from 1x109 to 3x1011 cm-2 and 1.4x1010 to 5x1012cm-2, resp., and then isochronally annealed for 30 minutes in the temperature range up to 550°C. Results show that radiation damage produced by helium ions remarkably enhances formation of thermal donors (TDs) when annealing temperature exceeds 375°C, i.e. when the majority of vacancy-related recombination centers anneals out. The excess concentration of TDs is proportional to the helium fluence and peaks at 1.6x1014cm-3 if annealing temperature reaches 475°C. Proton irradiation itself introduces hydrogen donors (HDs) which form a Gaussian peak at the proton end-of-range. Formation and annealing of shallow and deep hydrogen-related levels are strongly influenced by electric field at annealing temperatures below 175°C. If annealing temperature exceeds 350°C, HDs disappear and the excessive shallow doping is caused, as in the case of helium irradiation, by radiation enhanced TDs.