Deuterium implantation in graphite

A study has been made of deuterium ion implantation in polycrystalline graphite over the temperature range 100 K to 450 K. Deuterium depth profiles are obtained using a method based on observing the energy spectra of the tritons and protons from D-D reactions initiated with a 200 keV probing deuteron beam. In marked contrast to the low adsorption for gaseous hydrogen previously reported, graphite is found here to retain large concentrations of implanted deuterium with atomic loadings exceeding unity in the deeper regions of a cold target. The amount of deuterium retained continues to decrease with increasing temperature until at 150°C the retention has fallen to approximately half that at −160°C. No evidence is found for a layer of surface deuterium of the type seen in metals such as copper. Scanning electron micrographs of irradiated areas show no observable surface deformation for doses of up to 2.4 × 10¹⁹ deuterons/cm².     

Deuterium release as D2 and HD after D 2 + implantation in palladium and niobium

Moscow Physics Research Institute. Translated from Atomnaya Énergiya, Vol. 70, No. 2, pp. 94–96, February, 1991.     

Deuterium trapping in deep traps of differently oriented pyrolytic graphite exposed to D2 gas at 1473 K

Due to their importance for tritium inventories in future DT fueled fusion machines, experimental data on H isotope diffusion, absorption and retention in deep traps (E_b ≅ 4.3 eV) of graphites exposed to hydrogen at elevated temperatures have been reviewed. Deuterium retention was studied in edge- and basal-oriented pyrolytic graphite (PG) and polycrystalline RG-Ti-91 damaged by irradiation with 200 keV carbon ions. Deuterium loading was done by soaking in D₂ gas at 1473 K, and the resulting D retention was measured by nuclear reaction analysis. The microstructure was studied by cross-sectional TEM, SEM and microprofilometry. The concentration of strong traps created by irradiation and estimated by the amount of accumulated deuterium was shown to saturate with the damage above ≈1 dpa at about 1000 appm. In non-damaged and damaged graphites deuterium diffuses via porous grain boundaries and along basal planes within crystallites, while its migration through the graphite lattice along the c direction was found to be negligible. Radiation modifications of PG retard deuterium diffusion and decrease the rate of its chemical erosion by a factor of five. The amount of deuterium accumulated in strong traps in graphites is mainly influenced by their macro- and microstructure, while the degree of graphitization seems to be less important. Derivations are made of the susceptibility of damaged graphites, in particular, CFCs to the retention of hydrogen isotopes in deep traps.     

Deuterium retention in tungsten at fluences of up to 10 D/m using D ion beams

Deuterium retention in two types of polycrystalline tungsten (PCW) was studied as a function of incident ion fluence, ion energy, and specimen temperature. (i) D retention at 300 K, as a function of D⁺ fluence, demonstrated a trend to saturation in both the Rembar hot-rolled thin foil and Plansee tungsten plate. At 500 K, new D retention results for the Plansee PCW showed an increasing trend with increasing incident D⁺ fluence without any indication of saturation, in agreement with previous results for Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895]. Even when the incident D⁺ fluence was increased to 8 × 10²⁵ D⁺/m², which is in the fluence range of plasma devices, there was still no sign of saturation. (ii) The temperature dependence results for the Plansee PCW show a decreasing trend in D retention as the temperature is increased from 300 to 500 K. These results differ from previous studies of Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895], but are similar to those seen for single crystal tungsten [M. Poon, A.A. Haasz, J.W. Davis, R.G. Macaulay-Newcombe, J. Nucl. Mater. 313-316 (2003) 199]; an explanation for the different behaviour is suggested. (iii) Varying the D⁺ energy from 100 to 500 eV/D⁺ plays a minor role in the amount of D retained, suggesting that D retention in W depends more on the W structure, incident ion fluence and specimen temperature, rather than on the incident ion energy when the energy is below the threshold for damage formation (∼960 eV for D on W).     

Deuterium permeation behavior for damaged tungsten by ion implantation

The deuterium (D) permeation behaviors for ion-damaged tungsten (W) by 3 keV D₂⁺ and 10 keV C⁺ were studied. The D permeability was obtained for un-damaged W at various temperatures. For both D₂⁺ and C⁺ implanted W, the permeability was clearly reduced. But, for the D₂⁺ implanted W, the permeability was recovered by heating at 1173 K and it was almost consistent with that for un-damaged W. In the case of C⁺ implanted W, the permeability was not recovered even if the sample was heated at 1173 K, indicating that the existence of carbon would prevent the recovery of permeation path in W. In addition, transmission electron microscope (TEM) observation showed the voids were grown by heating at 1173 K and not removed, showing the existence of damages would not largely influence on the hydrogen permeation behavior in W in the present study.     

Deuterium retention in plasma spray tungsten coatings exposed to low-energy, high flux D plasma

Two types of porous plasma spray tungsten coatings deposited onto stainless steel and graphite substrates were exposed to low-energy (76 eV ), high-flux (10²² D/m² s) D plasma to ion fluences of (3-4) × 10²⁶ D/m² at various temperatures. Deuterium retention in the W coatings was examined by thermal desorption spectroscopy and the D(³He,p)⁴He nuclear reaction, allowing determination of the D concentration at depths up to 7 μm. The relatively high D concentration (above 0.1 at.%) at depths of several micrometers observed after D plasma exposure at 340-560 K can be related to accumulation of D₂ molecules in pores, while at temperatures above 600 K deuterium is accumulated mainly in the form of D atoms chemisorbed on the inner pore surfaces. At exposure temperatures above 500 K, the D retention in the plasma spray W coating on graphite substrate increases significantly due to trapping of diffusing D atoms at carbon dangling bonds located at the edge of a graphite crystallite.     

Deuterium retention and release in tungsten co-deposited layers

A systematic study of the influence of the deposition conditions on the deuterium retention in co-deposited tungsten layers formed both by magnetron sputtering and in the PISCES-B linear device has been carried out. Experimental parameters such as the tungsten deposition rate, the incident particle energy and the substrate temperature are shown to affect the level of deuterium retention in the layers. A decreased retention for increased substrate temperature and deposition rates, and an increased retention for increasing incident deuterium particle energy are observed. A scaling equation is proposed to describe the influence of the conditions during the co-deposition process (surface temperature, incident particle energy and deposition flux) on the deuterium retention. In addition, the desorption kinetics of deuterium has been studied by TDS. Two desorption stages at 473-573 K and at 1073 K have been observed.     

Embedding of coated particles in SiC

The disposal of spent HTR fuel elements requires a relatively large volume due to the integration of fuel particles and moderator graphite. This can be reduced by separating the coated particles from the graphite matrix. However, the coated particles cannot be disposed without a suitable embedding or backfill. Silicon carbide has been identified as a potential embedding material with less porosity as is the case with graphite. Therefore, a method has been developed to proof the technical feasibility of embedding coated particles in silicon carbide.     

The evolution of He+ irradiation-induced point defects and helium retention in nuclear graphite

The atomic-level study of point defect evolution in nuclear graphite is essential for a deep understanding of irradiation-induced property changes. The evolution of helium ion irradiation-induced point defects and helium retention in nuclear graphite ETU-10 and ETU-15 were studied by positron annihilation Doppler broadening (PADB) experiments and thermal desorption spectroscopy (TDS) measurements. The graphite samples were implanted with 10¹⁵, 10^16_, and 10¹⁷ cm^?2 of 200 keV He⁺ at operation temperatures below 373 K. Frenkel pairs were created during ion irradiation and they annihilated during annealing. Three stages of interstitial-monovacancy annihilation are suggested. At low temperatures, the initial annihilation would be refined only to the recombination of intimate metastable Frenkel pairs. When temperature increases, the annihilation would expand to a larger extent that isolate interstitials and vacancies annihilate with each other. In the case of high doses irradiation, vacancy clusters form at elevated temperatures. The retention and release of helium is tightly related to the evolution of the defects, especially the vacancies. The small over-pressured He-V clusters (He_nV) are thought to be the most possible form of helium retention under irradiation.     

Deuterium retention in sintered boron carbide exposed to a deuterium plasma

Depth profiles of deuterium up to a depth of 10 μm have been measured using the D(³He,p)⁴He nuclear reaction in a resonance-like technique after exposure of sintered boron carbide, B₄C, at elevated temperatures to a low energy (≈200 eV/D) and high ion flux (≈10²¹ m⁻² s⁻¹) D plasma. The proton yield was measured as a function of incident ³He energy and the D depth profile was obtained by deconvolution of the measured proton yields using the program SIMNRA. D atoms diffuse into the bulk at temperatures above 553 K, and accumulate up to a maximum concentration of about 0.2 at.%. At high fluences (?10²⁴ D/m²), the accumulation in the bulk plays a major role in the D retention. With increasing exposure temperature, the amount of D retained in B₄C increases and exceeds a value of 2 × 10²¹ D/m² at 923 K. The deuterium diffusivity in the sintered boron carbide is estimated to be D = 2.6 × 10⁻⁶exp{−(107 ± 10) kJ mol⁻¹/RT} m² s⁻¹.     

TEM studies of the defects introduced by ion implantation in SiC

We have undertaken a systematic study of the defects formed by ion implantation in SiC for a large variety of experimental conditions. B, N, Al and Ne ions were implanted into 6H–SiC at room temperature RT and at 650°C. Multiple energy implants were carried out in order to obtain “flat” dopant profiles. The samples were annealed from 1100°C to 1750°C for various duration times. Transmission Electron Microscopy (TEM) analysis was carried out on cross-sectional samples using weak beam dark field imaging conditions. All these defects are of interstitial type (clusters or loops). A statistical analysis of digital images was performed to extract the depth-distributions of the defects. The depth-distributions were compared with Monte-Carlo simulations of the ion implantation process. It is shown that when implanted at RT, the defect distributions follow the “damage” profiles i.e., defects appear in regions where atomic displacements occur in the target. In contrast, the defects found after implantation at 650°C always mirror the “range” profile before and after annealing. We show that there is a concentration threshold under which no defect appear. These results are discussed in terms of point defect annihilation, clustering and dopant activation in SiC.     

Structure and strain measurements on SiC formed by carbon ion implantation

Thin buried silicon carbide layers have been formed by high-dose carbon ion implantation into silicon and subsequent annealing. The formation of SiC during implantation and the structure of carbide layers after annealing are investigated by X-ray diffraction measurements using a four-circle goniometer. A detailed stress analysis of the epitaxially aligned 3C---SiC precipitates formed during implantation is presented. The three-dimensional strain and stress tensors are calculated for different doses. With increasing dose, stress relaxation accompanied by a transition from isotropic to anisotropic strain/stress states is observed. The dose dependence of the peak intensities of 3C---SiC present in the as-implanted state is studied. Stress tensors show a further relaxation in the annealed state.     

Monte Carlo simulation of ion implantation in crystalline SiC

We report first the results of a simulation study of ion implantation in crystalline 6H-SiC. Calculations were performed with a Monte Carlo code modified to account for the hexagonal lattice structure of the material. After an approximate determination of empirical parameters of electronic energy loss, performed by comparison of simulated profiles with experimental data as reported in the literature, a detailed study of the effects of beam-target orientation has been made for a few specific cases. Results have been compared with those of similar simulations made in cubic 3C---SiC, where the same model parameters were used, in order to emphasize differences due to the different crystallographic structure and surface orientation of the two phases. Conditions which originate deep channeling tails in the implanted profiles are identified, as well as conditions suitable to obtain the minimum width profile.     

Deuterium retention in tungsten exposed to low-energy, high-flux clean and carbon-seeded deuterium plasmas

Depth profiles of deuterium trapped in tungsten exposed to a low-energy (≈200 eV/D) and high deuterium ion flux (about 1 × 10²¹ D/m² s) in clean (We use the term ‘clean’ in quotation marks having in mind the impossibility to obtain absolutely clean plasma. In our case the conception ‘clean’ D plasma means the plasma without intentionally introduced carbon impurities.) and carbon-seeded D plasmas at an ion fluence of about 2 × 10²⁴ D/m² and various temperatures have been measured up to a depth of 7 μm using the D(³He, p)⁴He nuclear reaction at a ³He energy varied from 0.69 to 4.0 MeV. The deuterium retention in single-crystalline and polycrystalline W increases with the exposure temperature, reaching its maximum value at about 500 K (for ‘clean’ plasma) or about 600 K (for carbon-seeded plasma), and then decreases as the temperature grows further. It is assumed that tungsten carbide formed on the W surface under exposure to the carbon-seeded D plasmas serves as a barrier layer for diffusion and prevents the outward transport of deuterium, thus increasing the D retention in the bulk of tungsten.     

Measurement of erosion yields for a SiC surface on H+, D+ and Ar+ bombardment

Changes in the distribution of silicon solute in a 99.8% pure nickel crystal after 75 keV Ni⁺ ion bombardment at 16°C and 500°C, are reported. The concentration depth profiles were measured by simultaneous sputter-profiling and secondary ion mass spectrometry (SIMS). At 16°C the results are satisfactorily explained by the formation and subsequent migration to sinks of nickel interstitial-silicon complexes. The same mechanisms can be used to explain the results obtained at 500°C but the effect observed could also be due to the migration of vacancy-silicon complexes to sinks and the possibility that both mechanisms occur simultaneously cannot be ruled out.     

2MeV He~+离子注入修复太赫兹发射源

为修复因加载过高外置偏压而被击穿的太赫兹光电导天线的发射晶体,采用离子注入法对光电导间隙区域进行了2 MeV He+离子注入处理.经过1016 ions/cm2 He+璃子处理后,光电导天线的电阻由约800 Ω变为60 MΩ,基本恢复到了击穿前的水平.通过对比辐照前后太赫兹发射源在空气气氛中的时域谱和频域谱,发现离子辐照后太赫兹信号强度有了十分明显的增强,时域光谱峰值信号电流由辐照前的2 nA增加到了辐照后的8 nA,峰值功率增幅超过了一个量级,且发现处理后光电导天线的频谱相对于辐照前有了明显的展宽.     

Formation of Si/SiC multilayers by low-energy ion implantation and thermal annealing

Si/SiC multilayer systems for XUV reflection optics with a periodicity of 10-20 nm were produced by sequential deposition of Si and implantation of 1 keV ions. Only about 3% of the implanted carbon was transferred into the SiC, with a thin, 0.5-1 nm, buried SiC layer being formed. We investigated the effect of thermal annealing on further completion of the carbide layer. For the annealing we used a vacuum furnace, a rapid thermal annealing system in argon atmosphere, and a scanning e-beam, for different temperatures, heating rates, and annealing durations. Annealing to a temperature as low as 600 °C resulted in the formation of a 4.5 nm smooth, amorphous carbide layer in the carbon-implanted region. However, annealing at a higher temperature, 1000 °C, lead to the formation of a rough poly-crystalline carbide layer. The multilayers were characterized by grazing incidence X-ray reflectometry and cross section TEM.     

Chemical sputtering of graphite by H+ ions

Methane production during graphite bombardment by H⁺ ions is observed. The temperature dependence of the graphite sputtering yield is obtained for the temperature range 100–1400°C. A simple model of chemical sputtering of graphite under H⁺ ions bombardment is presented.