Interstitial helium clustering in molybdenum; effect on surface blistering

Calculated results are reported which indicate the possibility of clustering of interstitial helium in molybdenum. The possible relation of this clustering with surface blistering is discussed.     

He and H sequential implantation induced surface blistering and exfoliation of Si covered with oxide layer

Si wafers with a 220 nm top oxide layer were sequentially implanted at room temperature with 40 keV He and 35 keV H ions at fluence of 5× 10¹⁶/cm² and 1× 10¹⁶/cm², respectively. Techniques of scanning electron microscopy, atomic force microscopy and cross-sectional transmission electron microscopy (XTEM) were used to characterize the thermal evolution of surface damage as well as defect microstructures. Surface blisters as well as the localized exfoliation (～ 0.42 μm in depth) have been observed for samples annealed at temperatures of 500 ℃ and above. XTEM observations reveal a variety of defect microstructures, including cavities, platelets, nanometer or micrometer sized cracks and dislocations. The platelets and cracks are mainly distributed at the depth of about 0.42 μm parallel to the sample surface, which are responsible for the occurrence of the observed surface features. The relations between surface damage and defect microstructures are described in detail.     

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.     

Vacancy defects induced in sintered polished UO2 disks by helium implantation

Vacancy defects have been investigated in sintered polished and annealed uranium oxide disks. Slow positron beam coupled with Doppler broadening spectrometer was used to probe the track region of 1 MeV ³He ions implanted in uranium dioxide (UO₂) disks. The low and high momentum annihilation fractions, S and W, respectively, were measured in the first micrometer near surface region of the disks as a function of positron energy. The S and W values indicate that the 1 MeV He ions induce vacancy defects in the track region of their range. The vacancy defect depth distribution is heterogeneous. The positron trapping at these vacancy defects increases with the depth and with the implantation fluence indicating an increase of the vacancy defect concentration. The nature of the induced vacancy defects does not change with the fluence.     

He and H sequential implantation induced surface blistering and the exfoliation of Si covered with an oxide layer

Si wafers with a 220 nm top oxide layer were sequentially implanted at room temperature with 40 keV He and 35 keV H ions at a fluence of 5×10¹⁶ /cm² and 1×10¹⁶ /cm² , respectively. Techniques of scanning electron microscopy, atomic force microscopy and cross-sectional transmission electron microscopy （XTEM） were used to characterize the thermal evolution of surface damage as well as defect microstructures. Surface blisters as well as the localized exfoliation （～0.42 μm in depth） have been observed for samples annealed at temperatures of 500℃ and above. XTEM observations reveal a variety of defect microstructures, including cavities, platelets, nanometer or micrometer sized cracks and dislocations. The platelets and cracks are mainly distributed at a depth of about 0.42 μm parallel to the sample surface, which are responsible for the occurrence of the observed surface features. The relationship between surface damage and defect microstructures is described in detail.     

Radiation damage in diamonds subjected to helium implantation

Optical spectroscopy and volume “swelling” measurements were used to study radiation damage and graphitization of diamonds implanted with helium ions at temperatures from 77 to 373 K. It is established that the radiation damage decreases as the implantation temperature increases. This effect is explained by radiation-stimulated annealing of defects caused by damaging. It is shown that the result of formation of a graphitized layer is determined not by the implantation dose but by the level of radiation damage. It is found that the lower the implantation temperature, the lower the annealing temperatures required for the formation of a graphitized layer. It is shown that annealing of radiation defects and the formation of a graphitized layer in a diamond occur up to 1600°C.     

Thermal evolution of vacancy defects induced in sintered UO2 disks by helium implantation

A slow positron beam coupled with Doppler broadening (DB) spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO₂ disks implanted with different 1 MeV ³He fluences and annealed in ArH₂ or in vacuum. The S(E) and W(E) behaviors indicate that for fluences in the range from 2 × 10¹⁴ to 2 × 10¹⁶³He cm⁻², the vacancy defects distribution evolves with the annealing temperature in the range from 264 to 700 °C under ArH₂. This evolution is found to be dependent on the ³He fluence implanted in the sintered UO₂ disks. For the lowest fluence of 2 × 10¹⁴³He cm⁻², the S(W) plot with positron energy as the running parameter suggests that only the concentration of vacancy defects decreases when annealing temperature increases. For the highest implantation fluences (from 5 × 10¹⁵ to 2 × 10¹⁶³He cm⁻²) the S(W) plot suggests that the nature of the vacancy defects changes in the annealing temperature range from 260 to 400 °C. Measurements performed in implanted UO₂ disks annealed in vacuum have revealed a partial recovery of the vacancy defects possibly due to their recombination with mobile oxygen interstitials. The role of the hydrogen infusion into the disk is also discussed.     

Investigations of He implantation and subsequent annealing effects in InP

The influence of 70 keV He⁺ ion implantation and subsequent annealing of Cz-indium phosphide (InP) samples has been investigated using a slow positron beam-based Doppler broadening spectrometer. Three samples with ion fluences of 1 × 10¹⁶, 5 × 10¹⁶ and 1 × 10¹⁷ cm⁻² were studied in the as-implanted condition as well as after annealing at 640 °C for times between 5 and 40 min. It was found that the line-shape parameter of the positron-electron annihilation peak in the implanted layer increases after 5 min annealing, then after longer annealing times it starts to decline gradually until it reaches a value close to the value of the as-grown sample. This implies that vacancy-like defects can be created in InP by He implantation followed by short-thermal annealing at T > 600 °C. Comparison of the results with a study where cavities were observed in He-implanted InP has been carried out.     

Effect of implantation temperature on the blistering behavior of hydrogen implanted GaN

GaN epitaxial layers were implanted by 100 keV H⁺ ions at different implantation temperatures (LN₂, RT and 300 °C) with a fluence of 2.5×10¹⁷ cm^?2. The implanted samples were characterized using Nomarski optical microscopy, AFM, XRD, and TEM. Topographical investigations of the implanted surface revealed the formation of surface blistering in the as-implanted samples at 300 °C and after annealing at higher temperature for the implantation at LN₂ and RT. The physical dimensions of the surface blisters/craters were dependent on the implantation temperature. XRD showed the dependence of damage-induced stress on the implantation temperature with higher stress for the implantation at 300 °C. TEM investigations revealed the formation of a damage band in all the cases. The damage band was filled with large area microcracks for the implantation at 300 °C, which were responsible for the as-implanted surface blistering.     

Effect of helium implantation on SiC and graphite

Effects of helium implantation on silicon carbide(SiC) and graphite were studied to reveal the possibility of SiC replacing graphite as plasma facing materials. Pressureless sintered SiC and graphite SMF-800 were implanted with He+ions of 20 ke V and 100 ke V at different temperatures and different fluences. The He+irradiation induced microstructure changes were studied by field-emission scanning electron microscopy(FESEM), atomic force microscopy(AFM), and transmission electron microscopy(TEM).     

Investigation of the high-current arc in helium

The secondary and erosion characteristics of the electric-arc plasmatorch were studied experimentally. Formulas for arc voltage calculation depending on helium flow rate, diameter of the inner cavity of cylindrical copper anode, arc current, and distance from the nozzle to metal melt were derived.     

Laser transfer of diamond nanopowder induced by metal film blistering

Blister-based laser induced forward transfer (BB-LIFT) is a promising technique to produce surface microstructures of various advanced materials including inorganic and organic micro/nanopowders, suspensions and biological micro-objects embedded in life sustaining medium. The transferred material is spread over a thin metal film irradiated from the far side by single laser pulses through a transparent support. Interaction of the laser pulse with the metal–support interface under optimized conditions causes formation of a quickly expanding blister. Fast movement of the free metal surface provides efficient material transfer, which has been investigated for the case of diamond nanopowder and diamond-containing suspension. The unique features of the given technique are universality, simplicity and efficient isolation of the transferred material from the ablation products and laser heating.     

Optical properties of electron irradiation induced defects in InP

The optical properties of deep hole traps H₄ and H₅ in p type and of the deep electron trap E₁₁ in n type InP, introduced by electron irradiation, have been studied using deep level optical spectroscopy. Comparison of the optical threshold with the thermal activation energy of H₅ level shows that it is highly relaxed with a Frank-Condon shift d_Fc = 0.45 eV. The electron level E₁₁ is weakly relaxed and its optical cross section σ ⁰ is well accounted for by transitions to the Γ_6c minimum. The optical absorption σ_p⁰ associated to level H₄ shows two successive onsets at hν = 0.5 and hν = 1.2 eV which can be attributed to hole transitions to the Γ_7–8 and to the L_4–5 valence band extrema, respectively. The deduced Frank-Condon shift, d_Fc = 0.23 eV, agrees with the measured difference of 40 meV between its apparent activation energy E_a and its thermal activation energy E_T.     

Fission isomerism induced by helium ions

Delayed fission events due to isomeric fission have been produced using He³ and He⁴ beams from the MP Tandem Van de Graaff accelerator. The isomeric nuclei which recoil from the target and subsequently fission in flight were detected by a square cone array of polycarbonate foil detectors mounted along the beam direction. From the distribution of fission tracks along the detector foils, half-lives in the nanosecond region were deduced.     

Laser induced excitation transfer in helium glow discharge

Some experimental results on the temporal decay of the fluorescence induced by a resonant laser pulse excitation focused onto a helium gas discharge are presented. In particular, excitation transfer between singlet-singlet and singlet- triplet sub-levels has been studied when 2¹ S → 3¹ P and 2¹ P → 4¹ D transitions of He I are optically pumped.     

Influence of helium implantation of optical fibre faceplates on acid etching

In order to achieve preferential etching of a structured multi-component glass fibreoptic faceplate the surface has been ion implanted to enhance the differences in chemical reactivity of the various glass constituents. While HF dissolution is enhanced at silicate glass components due to ion beam electronic excitation, changes in the glass stability of the heavy metal fibre core due to nuclear collision damage were evident at the end of the ion penetration range after HNO₃ acid attack.     

氦在材料中基于扩散机理的热释放特征

以随机扩散理论为基础,研究该机理下材料中氦的热释放可能呈现的特征,考察了氦的初始深度分布和扩散系数等因素对氦的热释放率的影响;阐明了随机扩散模型下和脱附模型下的氦释放特征之异同;指出对实验观察到的热解析谱的分析应和基底中氦的深度分布的分析以及氦的聚集状态的分析相结合,才能对氦的热释放机理给出正确的理解和判断. 关键词： 随机扩散 热释放 氦