Growth of he bubbles in al during annealing

Abstract

The temperature dependent growth of He bubbles in Al films implanted at room temperature to various He concentrations is investigated by electron energy-loss spectroscopy (EELS) and transmission electron microscopy (TEM). EELS reveals even the weakest changes in He density within the bubbles by measuring the pressure shift of the He 1¹S₀?2¹P₁ transition. This is applied to investigate the mechanisms driving the growth of bubbles in the temperature range 20°C ? T ? 500°C. For T?200°C indication is found that bubbles relax by emission of interstitial dislocation loops. At higher temperatures bubble migration and coalescence under absorption of thermal vacancies is evident. The final state is characterized by large cavities filled with He at low pressure as evidenced by the detection of the atomic He series.     

The density and pressure of helium in bubbles in metals

Abstract

The energy shift of the He 1¹S₀?2¹P₁ transition, ΔE(n), can be used to determine the density, n, of He in bubbles in metals. A self-consistent band structure calculation for solid fcc He yields a linear relationship ΔE=C.n with C _th=22 × 10^?3 eV nm³. Systematic electron energy loss spectroscopy and transmission electron microscopy studies of He bubbles in Al for various He doses and temperatures result in C_exp=(24±8).10^?3 eV nm³ in agreement with theory. The analysis is consistent with the assumption that dislocation loop punching is the dominant bubble growth mechanism during high-dose room temperature implantation. The application to He bubbles in Ni indicates a maximum He density of n=0.2 × 10³ nm^?3 for which He should be solid at room temperature.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Vacancy trapping and helium decoration in Sb ion-implanted tungsten studied by Mössbauer spectroscopy

Mössbauer effect measurements have been performed using sources of¹¹⁹Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·10¹⁶ He/cm² leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.     

Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature (T _M) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold–silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7T _M, respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.     

纳米SrTiO3的光声光谱研究

利用光声光谱技术对不同退火温度的纳米SrTiO₃粉末进行了研究．结果表明，纳米SrTiO₃粉末随退火温度升高，吸收边红移，说明能隙变窄，这与颗粒长大、晶格参数减小有关．在700nm附近出现的宽吸收带是由氧缺位俘获的电子在缺陷附加能级上的跃迁所产生的，其变化趋势强烈地依赖于颗粒尺寸 关键词：     

A description of the laser-induced annealing and diffusion behaviour of implanted silicon crystals

Abstract

It is shown that as a result of the Greenwood-Foreman-Rimmer loop punching mechanism applied for helium bubble growth in nickel implanted with 5 keV He⁺ ions at 273 K, a considerable amount of helium remains outside the bubbles which are visible in a transmission electron microscope (TEM). It is also shown that even when it is assumed that there is an energy barrier with an upper limit equal to the formation energy of a self-interstitial atom, not all implanted helium can be accumulated in the bubbles below the critical dose for blistering.

The experimental observation of bubble growth in a helium pre-implanted nickel specimen during 1 MeV e^? irradiation may demonstrate that indeed a significant amount of helium remains between the bubbles visible in TEM.     

Annealing Behaviour of Helium Bubbles in Titanium Films by Thermal Desorption Spectroscopy and Positron Beam Analysis

Helium-containing Ti Glms are prepared using magnetron sputtering in the helium-argon atmosphere. Isochronal annealing at different temperatures for an hour is employed to reveal the behaviour of helium bubble growth. Ion beam analysis is used to measure the retained helium content. Helium can release largely when annealing above 970K. A thermal helium desorption spectroscopy system is constructed for assessment of the evolution of helium bubbles in the annealed samples by linear heating （OAK/s） from room temperature to 1500K. Also, Doppler broadening measurements of positron annihilation radiation spectrum are performed by using changeable energy positron beam. Bubble coarsening evolves gradually below 680K, migration and coalescence of small bubbles dominates in the range of 68-970 K, and the Ostwald ripening mechanism enlarges the bubbles with a massive release above 970K.     

Thermal desorption spectroscopy of he from Ni at and below saturation

Trapping of helium after implantation at energies of 8 to 150 keV and fluences up to 10¹⁹ He-ions/cm² in nickel at room temperature is studied by measuring the thermal desorption spectra during linear heating up to 1000°C. At several annealing stages the trapped helium is measured by means of the nuclear reaction ³He(d, α)H and the target surface is observed by laser scattering and with the scanning electron microscope.

The thermal desorption spectra depend strongly on the implantation fluence but only slightly on the implantation energy, indicating a similar trapping of He in the lattice for the implantation energies used here, The temperature at which desorption starts decreases with increasing fluence. Above the critical fluence for blistering an additional low temperature (150°C) desorption maximum is found.

The desorption peak at 150°C can be approximated theoretically with a single jump desorption process of first order and a Gaussian distribution of activation energies around 1 eV. The measurements indicate that at higher temperatures (>300°C) helium desorption is partly due to the opening of helium bubbles at the target surface.     

注氦尖晶石的红外光谱研究

利用傅立叶变换红外光谱仪对注He尖晶石样品随退火温变化而引起光吸收性能的变化进行了研究。 发现尖晶石样品在626.4 cm-1附近的吸收峰随注入剂量的增加向小波数方向移动, 而在随后退火过程该吸收峰随退火温度的增加而向大波数方向回复。 该吸收峰的回复行为依赖于注入剂量和退火温度。 认为在626.4 cm-1附近吸收峰随注入剂量和退火温度的这种变化与尖晶石中He的俘获以及释放有关。 The infrared absorption behavior of helium implanted spinel with annealing temperature was studied by Fourier transformed infrared (FTIR) spectroscopy. It was found that the absorbance peak at 626.4 cm-1 shifted to smaller wave numbers with the increase of implantation fluence, while on subsequent annealing the absorbance peak shifted back to larger wave numbers with the increase of annealing temperature. The shift of the peak at 626.4 cm-1 with He implantation/annealing is considered to be related with the trapping and release of helium atoms in lattice sites in the spinel crystal.     

Study of He bubbles in Al by electron energy loss spectroscopy

High energy electron energy loss spectroscopy (ELS) has been used to study He bubbles in Al, which were obtained by irradiation of He ions or α-particles of energy ranging from 500 eV to 8 keV and fluences 1x10²⁰m^?2 and 5x10²⁰m^?2. ELS reveals surface plasmon losses of the Al cavities as well as pressure shifts of the He-resonance lines as large as 1 eV. This is viewed as evidence for the existence of a so-called super-dense He in the bubbles. ELS is therefore a promising tool for obtaining information on the He pressure within the bubbles.     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

Positron and deuteron depth profiling in helium-3-implanted electrum-like alloy

In spite of previous extensive studies, the helium behavior in metals still remains an issue in microelectronics as well as in nuclear technology. A gold-silver solid solution (Au₆₀Ag₄₀: synthetic gold-rich electrum) was chosen as a relevant model to study helium irradiation of heavy metals. After helium-3 ion implantation at an energy ranging from 4.2 to 5.6 MeV, nuclear reaction analysis (NRA) based on the ³He(d,p)⁴He reaction, was performed in order to study the thermal diffusion of helium atoms. At room temperature, NRA data reveal that a single Gaussian can fit the He-distribution, which remains unchanged after annealing at temperatures below 0.45 of the melting point. Slow positron implantation spectroscopy, used to monitor the fluence dependence of induced defects unveils a positron saturation trapping, which occurs for He contents of the order of 50-100 appm, whereas concentrations larger than 500 appm seem to favor an increase in the S-parameter of Doppler broadening. Moreover, at high temperature, NRA results clearly show that helium long range diffusion occurs, though, without following a simple Fick law.     

Influence of helium metastable states on the E-H mode transition in an inductively coupled plasma

A helium (He) inductively coupled plasma (ICP) source combined with two diode laser systems with the wavelengths of 396.5 nm (2¹S–4¹P) and 1082.9 nm (2³S–2³P) was configured to investigate the influence of He metastable states on the E-H mode transition. The population densities of 2¹S and 2³S states were measured by laser absorption spectroscopy as the functions of RF power, He pressure, and the distance from RF antenna. Correlations between the E-H mode transition and the population density ratio of the 2³S metastable state to the 2¹S metastable state as well as the line intensity ratio (LIR) of the 3³D-2³S transition to the 3¹P–2¹S transition were investigated. The E-H mode transition of He ICP was analyzed by the electron impact ionization rate and it was confirmed that the 2³S metastable state plays a key role in the E-H mode transition.     

Evolution,migration and clustering of helium-vacancy complexes in RAFM steel- depth resolved positron annihilation Doppler broadening study

Indian Reduced Activation Ferritic Martensitic steel is implanted with 130 keV helium ions to a fluence of 5 × 10¹⁴ and 1 × 10¹⁶ ions/cm² and investigated using positron annihilation spectroscopy. The samples were characterised by defect sensitive S and W-parameters using depth resolved slow positron beam. A dose dependency is observed in the nucleation and growth of helium bubbles with annealing temperature. An experimental evidence for the migration of smaller helium-vacancy complexes is observed via the variation in thickness/width of irradiated layer with temperature. The S–W plot clearly shows the regions corresponding to defect annealing, bubble nucleation and growth.     

Structure and dynamics of the He 2 * (a 3Σ u + molecular complex in condensed phases of helium

An investigation is made of the absorption spectra of triplet metastable helium molecules in the a ³Σ _u ⁺ state in liquid ⁴He and ³He at various pressures and in dense ³He gas. An analysis of the spectrum corresponding to the a ³Σ _u ⁺ → c ³Σ _g ⁺ transition confirms the conclusion that there is a microscopic bubble surrounding the molecule in liquid helium. A simple approximation is proposed for the wave function of the valence electron of the molecule and the parameters of the bubble are determined for various experimental conditions. The coefficient of molecular recombination in liquid ³He and ⁴He was determined experimentally at various pressures and in dense cold ³He gas. The results show good agreement with the theory of mutual recombination limited by molecular diffusion under conditions of strong van der Waals interaction. It is shown that in the condensed phases of helium the polarization of the molecules under the action of the magnetic field does not lead to suppression of their mutual recombination, and this is confirmed experimentally.     

Helium wetting: Theme and variations on inhomogeneous helium

The surprising discovery, in 1991, that liquid helium does not wet a cesium surface at low temperature has triggered an important activity both theoretical and experimental: helium has become a model system for the study of wetting transitions. After summarizing the main theme of helium wetting, I will focus on more recent studies, such as the structure and excitations of helium interfaces, experiments on the capillary rise, the “surfactant effect” of helium-3 impurities on liquid helium-4 and the “quantum prewetting transition” of pure helium-3. Unexpected consequences on the phase separation of³He?⁴He mixtures in restricted geometry will be drawn.     

The Lamb-shift experiment in Muonic helium

We propose to measure several transition frequencies between the 2S and the 2P states (Lamb shift) in muonic helium ions (μ ⁴He^?+? and μ ³He^?+?) by means of laser spectroscopy, in order to determine the alpha-particle and helion root-mean-square (rms) charge radius. In addition, the fine and hyperfine structure components will be revealed, and the magnetic moment distribution radius will be determined. The contribution of the finite size effect to the Lamb shift (2S???2P energy difference) in μHe^?+? is as high as 20 %. Therefore a measurement of the transition frequencies with a moderate (for laser spectroscopy) precision of 50 ppm (corresponding to 1/20 of the linewidth) will lead to a determination of the nuclear rms charge radii with a relative accuracy of 3 ×10^???4 (equivalent to 0.0005 fm). The limiting factor for the extraction of the radii from the Lamb shift measurements is given by the uncertainty of the nuclear polarizability contribution. Combined with an ongoing experiment at MPQ aiming to measure the 1S???2S transition frequency in the helium ion, the Lamb shift measurement in μHe^?+? will lead to a sensitive test of problematic and challenging bound-state QED terms. This measurement will also help to clarify the discrepancy found in our previous μ _p experiment. Additionally, a precise knowledge of the absolute nuclear radii of the He isotopes and the hyperfine splitting of μ ³He^?+? provide a relevant test of few-nucleon theories.     

Ions at helium interfaces

The boundaries between different phases of condensed helium provide an interesting testing ground for studying ions in a quantum matter matrix. Here we consider the simplest positive and negative ions in helium — snowballs and electron bubbles, respectively — being trapped at the liquid-liquid interface of phaseseparated³He–⁴He mixtures and at the liquid-solid interface of⁴He. A comparison of experimental results for the trapping with predictions of the snowball and the bubble models shows that the models are in qualitative accord with the observations, but disagree in detail. It is suggested to use such studies for refinements of the ion models. In addition multielectron bubbles (=mesoscopic ions) and electrons on helium films are briefly discussed.     

Roles of local He concentration and Si sample orientation on cavity growth in amorphous silicon

(111)- and (100)-oriented Si samples were implanted with Si⁺ ions at 1 MeV to a dose of 1?×?10¹⁶?cm^?2 and with 5?×?10¹⁶ He⁺ cm^?2 at 10?keV or 50?keV and eventually annealed in the 800–1000°C temperature range. Sample characterisation was carried out by cross-section transmission electron microscopy, positron annihilation spectroscopy and nuclear reaction analysis. In addition to the formation of He bubbles at the projected range of He, bubbles were observed after solid-phase epitaxial growth (SPEG) of the embedded amorphous Si layer. The He threshold concentration required to obtain thermally stable bubbles in amorphised Si is between one and four orders of magnitude lower than in c-Si. Since bubble formation and growth take place in the a-Si phase, the interaction with SPEG during annealing was studied by considering (100) and (111) Si. Both the SPEG velocity and the resulting defects play a role on bubble spatial distribution and size, resulting in bigger bubbles in (111) Si with respect to (100) Si.