A swelling model for stoichiometric SiC at temperatures below 1000°C under neutron irradiation

A simple phenomenological model for the saturation swelling below 1000°C of neutron-irradiated silicon carbide (SiC) is presented in this paper. Under fast neutron irradiation, SiC is known to undergo volumetric expansion (swelling) which quickly saturates at a fast fluence of approximately 10²⁵ n/m² for irradiation temperatures below 1000°C. A previous model due to Balarin attributes swelling to lattice dilation as a result of single point defects. We show in this paper that the experimentally observed linear temperature dependence of saturation swelling can be explained in terms of the formation and growth of small interstitial clusters, resulting directly from collision cascades initiated by energetic neutrons. These loops grow by absorption of mobile carbon interstitials and their composition is subject to stoichiometry constraints, requiring absorption of slower silicon interstitials. Because of cascade re-solution events, the density of loops decreases sharply with temperature as a result of overlap of cascades with larger size loops at higher temperatures. The average radius of these loops increases with temperature. Volumetric swelling is shown to obey a linear temperature dependence as a consequence of the strong decrease in density and the simultaneous increase in average radius, and to saturate with fluence. The model is shown to be consistent with experimental observations. In the temperature range below 500–600°C, swelling seems to be dominated by single point defects, or defect clusters containing only a few atoms, in accordance with the explanation offered by Balarin.     

Pitting corrosion in CVD SiC at 300 °C in deoxygenated high-purity water

SiC is a candidate for nuclear applications at elevated temperatures but has not been fully studied under typical light-water reactor operating conditions, such as moderate temperatures and high pressures. Coupons of high-purity chemical vapor deposited SiC were exposed to deoxygenated, pressurized water at 573 K and 10 MPa for up to 5400 h. Ceramographic examination of the exposed SiC surfaces revealed both embryonic and large, d > 300 μm, pits on the surface after initial exposure for 4000 h. The pits were characterized using scanning electron microscopy for structure and chemistry analysis. Pit densities were also determined by standard counting methods. The chemical analysis revealed that the pits are associated with the formation of silica and subsequent loss of Si, which is expected due to several suggested reactions between SiC and water. Subsequent exposure under nominally identical water chemistry conditions for an additional 1400 h removed the pits and the samples exhibited general corrosion with measurable loss of Si from the surface.     

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.     

Vacancy defects induced in the track region of 132 MeV C irradiated SiC

Positron annihilation lifetime spectroscopy (PALS) and electron paramagnetic resonance (EPR) have been used in this work to investigate vacancy defects induced in the track region of 132 MeV ¹²C irradiated silicon carbide. Irradiations have been performed at room temperature at a fluence of 2.5 × 10¹⁴ cm⁻² in N-low doped 6H–SiC and 3C–SiC monocrystals. Silicon monovacancies have been detected in both polytypes using EPR. Their charge state and concentration have been determined in the track and cascade region of the C⁺ ions. PALS measurements performed as a function of temperature have shown the presence of V_Si–C divacancies in the track region for both polytypes.     

Calculations of neutron-induced production cross-sections of W up to 20 MeV

Neutron-induced cross-sections for the stable isotopes ^{180,182,183,184,186}W in the energy region up to 20 MeV have been calculated. Calculations were made with the codes CEM03.01 and ALICE/ASH, using the following models: the Dubna version of the intranuclear cascade model for the cascade stage of interaction; the hybrid, the geometry dependent hybrid and the exciton model for the pre-equilibrium component; the Hauser–Feshbach and the Weisskopf–Ewing statistical models for the equilibrium component. Effects of some important model parameters such as level density parameter and pairing correction were investigated. Calculated cross-sections were compared with available experimental data in the literature and with ENDF/B-VI T = 300 K and JENDL-3.3 T = 300 K evaluated data libraries.     

Creep-fatigue behaviour of an AISI stainless steel at 550 °C

In the past, a lot of experimental studies have been devoted to creep-fatigue interactions in austenitic stainless steels. Tests have been carried mainly at temperatures of at least 600 °C and at high applied strains, which are supposed to be the most damaging. The present work is dedicated to mechanical tests, TEM observations and lifetime predictions at 550 °C which corresponds to the real industrial temperature in Liquid Metal Fast Breeder Reactors. It is shown that if pure fatigue test results are close to those performed at 600 °C, some of the creep-fatigue results are different, particularly for small applied strains which correspond once more to the industrial conditions. In the 0.25–0.3% strain amplitude range, the stress is larger with hold time than without whatever is the hold time up to 5 h. The numbers of cycles to failure are greatly reduced and no saturation with the hold time is observed, contrary to higher temperature results. The stress–strain behaviour is discussed considering several high temperature mechanisms such as ageing, recovery and viscoplasticity and using TEM observations and stress partitioning into kinematic, isotropic and thermal stresses. Finally, a simple linear damage accumulation model is applied to the 550 °C and 600 °C tests, using the measured stresses. The stress dependence on hold time can partly explain the observed failure results on fatigue life.     

Orientation dependence of near-threshold damage production by electron irradiation of 4H SiC and diamond and outward migration of defects

Low temperature photoluminescence measurements have been made on samples of 4H SiC and diamond irradiated in different crystallographic directions with electrons having energies close to the atomic displacement thresholds. The defects produced in the 4H SiC are found to show some differences from those predicted by molecular dynamics calculations and possible reasons for the differences are discussed. The discussion refers to results from earlier as well as new experiments on the outward migration of defects during irradiation. The results for the energy dependence of the damage introduced into <1 0 0>, <1 1 0> and <1 1 1> oriented diamond are evaluated and shown to be consistent with theory.     

Simulation of ion beam induced crystallization and amorphization in (0 0 1) silicon

The ion beam induced epitaxial crystallization (IBIEC) and the ion beam induced interfacial amorphization (IBIIA) in (0 0 1) silicon caused by 3 MeV Si⁺ and 3 MeV Au⁺ irradiation at 293 K and 623 K are investigated by using a combination of binary collision MC simulations and MD simulations. The energy and angular distribution of the primary recoils is calculated by TRIM and the subcascades caused by the primary recoils are treated by classical MD simulations using a correspondingly large MD cell with 49152 atoms. The resulting topological interface structure is analyzed and compared with that obtained by thermally activated solid phase epitaxy. The rates of crystallization and amorphization are calculated and compared with experimental data. Especially, their dependence on the nuclear deposited energy is discussed.     

Intergranular damage in AISI 316L(N) austenitic stainless steel at 600 °C: Pre-strain and multiaxial effects

This study is devoted to the effect of a multiaxial stress state and of pre-straining on the creep properties of an austenitic stainless steel. Creep tests on both smooth and notched specimens have been carried out on type 316L(N) steel at 600 °C. In comparison to the annealed state, pre-straining caused a substantial increase in creep lifetime but also a dramatic drop in intergranular damage resistance. The effect of a pre-strain on creep ductility was so strong that compact tension specimens in pre-strained state tested under relaxation conditions cracked, whereas specimens in annealed state were not prone to cracking. A model taking into account both pre-strain and multiaxial effects was developed and identified on the basis of local intergranular micro-cracks measurements on notched specimens. It satisfactorily predicts the results of relaxation crack propagation tests. This model may also provide a useful estimation of the relaxation cracking risk of 316L(N) as a function of pre-strain level and stress triaxiality ratio.     

Investigation of kinetic recovery process in low dose neutron-irradiated nuclear graphite by thermal annealing

To investigate the kinetic recovery process of low dose neutron-irradiated graphite, nuclear-grade isotropic graphite IG-110U and ETP-10 were neutron irradiated using the JMTR up to 1.38 × 10²³ n/m² (E_n > 1 MeV) at ~473 K. In-situ measurement of macroscopic length was conducted during the isothermal and isochronal annealing process from room temperature up to 1673 K. From room temperature to 773 K for IG-110U, and to 1023 K for ETP-10, macroscopic lengths, lattice parameters, and unit cell volumes of both specimens recovered to their pre-irradiation values, and this recovery process subdivided into two stages. The activation energies of macroscopic volume recovery at 523–673 K and 673–773 K were determined to be ~0.22 eV and ~0.57 eV for IG-110U, respectively; ~0.13 eV and ~2.59 eV at 523–923 K and 923–1023 K for ETP-10, respectively. The migration of not only single interstitials but also interstitials dissociated from submicroscopic interstitial groups along basal planes followed by vacancy-interstitial recombination play a dominant role in the first stage. The second stage is suggested to proceed via the motion of carbon groups along basal planes for IG-110U, and migration of single interstitials along the c-axis for ETP-10. During 773 K or 1023 K up to 1673 K, macroscopic length continuously shrank with decreasing shrinking rate, even with a turnaround to swell at 1173 K for IG-110U.     

空气中γ射线辐照聚碳硅烷陶瓷先

将聚碳硅烷(PCS)陶瓷先驱丝在开放空气中用γ射线辐照,然后在惰性气氛中高温热解制备出SiC纤维。研究了吸收剂量对先驱丝的结构、热分解特性以及热解产物微观形貌的影响。结果表明,在辐射作用下,空气中的氧与PCS中的部分Si—H发生氧化反应生成Si—OH,同时辐照产物中还形成了Si—C—Si,Si—O—Si等桥联结构。吸收剂量为0.5 MGy时,PCS先驱丝在热解过程中没有并丝;吸收剂量为3.0 MGy时,PCS先驱丝的凝胶含量和陶瓷产率分别达到75%和85.17%。这说明开放的空气气氛能够有效降低PCS先驱丝的不熔化处理剂量。     

Structure and optical properties of sapphire implanted with boron at room temperature and 1000 °C

Many previous studies of ion-implanted sapphire have used gas-forming light ions or heavier metallic cations. In this study, boron (10¹⁷ cm⁻², 150 keV) was implanted in c-axis crystals at room temperature, 500 and 1000 °C as part of a continuing study of cascade density and “chemical” effects on the structure of sapphire. Rutherford backscattering-ion channeling (RBS-C) of the RT samples indicated little residual disorder in the Al-sublattice to a depth of 50–75 nm but almost random scattering at the depth of peak damage energy deposition. The transmission electron micrographs contain “black-spot” damage features. The residual disorder is much less at all depths for samples implanted at 1000 °C. The TEM photographs show a coarse “black-spot damage” microstructure. The optical absorption at 205 nm is much greater than for samples implanted with C, N, or Fe under similar conditions.     

Ion beam synthesis of SiC by C implantation into SIMOX(1 1 1)

We report the conversion of a 65 nm Si(1 1 1) overlayer of a SIMOX(1 1 1) into 30–45 nm SiC by 40 keV carbon implantation into it. High temperature implantation (600 °C) through a SiO₂ cap, 1250 °C post-implantation annealing under Ar ambient (with 1% of O₂), and etching are the base for the present synthesis. Sequential C implantations (fluence steps of about 5 × 10¹⁶ cm^?2), followed by 1250 °C annealing, has allowed to estimate the minimum C fluence to reach the stoichiometric composition as ～2.3 ×  10¹⁷ cm^?2. Rutherford Backscattering Spectrometry was employed to measure layer composition evolution. A two-sublayers structure is observed in the synthesized SiC, being the superficial one richer in Si. Transmission electron microscopy has shown that a single-step implantation up to the same minimum fluence results in better structural quality. For a much higher C fluence (4 × 10¹⁷ cm^?2), a whole stoichiometric layer is obtained, with reduction of structural quality.     

中子照射法合成新型富勒烯C121

利用反应堆中子照射富勒烯混合物C60合成了新型C121的三个同分异构体。实验结果表明较低的热快中子注量比率、原料照射前适当的氧化处理和较长的中子照射时间有利于C121的形成。此研究首次合成了C121的三种同分异构体并且拓展了核技术的应用领域。     

The microstructure evolution of type 17-4PH stainless steel during long-term aging at 350 °C

The microstructure evolution of 17-4 precipitation hardening (17-4PH) stainless steel during long-term aging at 350 °C was studied mainly by X-ray diffraction and transmission electron microscopy. The results showed that the matrix is lath martensite and the precipitation of nano-metric particles of -Cu phase after the alloy has been subjected to solution and temper treatment. When the alloy is aged at 350 °C to 9 months, some reversed austenite is formed and the -Cu precipitates are coarsening according to ripening process. When the alloy aged from 9 to 12 months, some bulk secondary carbides, M₂₃C₆, are precipitated. With the age time extended to 15 months, an amount of reversed austenite is transformed and the G-phase, a kind of intermetallic compound, precipitation occurs nearby the -Cu precipitates in the matrix at this intermediate temperature.     

碳化硅表面锌的吸附及其热氧化的同步辐射光电子能谱研究

利用同步辐射光电子能谱(Synchrotron radiation photoelectron spectroscopy,SRPES)和X射线光电子能谱(X-rayphotoelectron spectroscopy,XPS)技术,研究了金属Zn在6H-SiC表面的吸附和热氧化以及ZnO/SiC的界面形成过程.研究结果表明,在SiC表面沉积金属Zn的初始阶段,Zn可以夺取SiC衬底表面残留的氧并与之成键.随着Zn覆盖度的增加,表面具有金属特性.在2.0×10-4Pa的氧气氛中180℃温度退火下,覆盖的Zn会被部分氧化形成ZnO,还有部分Zn因其在真空中的低蒸发温度而逸出表面.在同样的氧气氛中600℃温度退火后,覆盖的金属Zn全部被氧化而生成ZnO.在氧气氛中退火时,衬底也会轻度氧化,从而导致在ZnO/SiC界面处存在一层很薄的Si的氧化层.根据得到的光电子能谱的实验结果,计算出ZnO/SiC异质结的价带偏移为1.1 eV.     

Activation cross-sections of deuteron induced nuclear reactions on gold up to 40 MeV

Cross-sections of deuteron induced nuclear reactions on gold were measured up to 40 MeV by using the standard stacked foil irradiation technique and high resolution gamma-ray spectroscopy. Experimental cross-sections and derived integral yields are reported for the ¹⁹⁷Au(d,xn)^{197m,197g,195m,195g}Hg and ¹⁹⁷Au(d,x)^{198g,196m,196g,195,194}Au nuclear reactions. The experimental data are analyzed and compared to literature and predictions of the ALICE-IPPE, EMPIRE and TALYS theoretical model codes. The application of the new cross-sections for accelerator technology, medical radioisotope production, thin layer activation and dose calculation is discussed.     

Effect of aging at 700 °C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds

A detailed knowledge of changes in microstructures and mechanical behaviour that occur in austenitic stainless steels with or without Nb/Ti-stabilized weld during heat treatment is of great interest, since the ductility and toughness of the material may change drastically after long aging times. Two kinds of materials, i.e. AISI 321 base and without Ti-stabilized weld steel and AISI 347 base with Nb-stabilized weld steel, were compared during aging at 700 °C up to 6000 h. Both materials present increased amount of precipitate and decreased impact energy as the aging time increases. The decreased extent of impact energy with aging is almost the same for both base materials. However, it presents differences for 347 and 321 weld samples. The latter shows a more drastic decrease of impact energy than the former due to the different amount of precipitates. 321 weld sample precipitates more numerously than 347 weld sample due to the absence of stabilized Ti/Nb on the former. Large amount of carbides is formed on 321 weld sample immediately after welding. The carbides are transformed to sigma phase, which is mainly responsible for the much more sigma phase precipitation compared with other samples, after high-temperature aging. The fractographs showed, in general, brittle fracture mode in 321 weld impact-fractured specimens after aging at 700 °C for 6000 h. However, other samples show ductile fracture mode in general. Several approaches should be employed to control sigma phase precipitation in weld material. These approaches include: decreasing content of ferrite and M₂₃C₆ carbide in weld and selecting Nb added weld wire during welding.     

Thermo-mechanical characteristics of concrete at elevated temperatures up to 310 °C

Both, the normal strength concretes (NSC) and high strength concretes (HSC) have been used in structures which may be exposed to elevated temperatures. Concretes have also been used in the construction of radiation shielding structures. Data on the behaviour of concrete at high temperature is of prime concern in predicting the constructions and safety of buildings in response to certain accidents or particular service conditions. Prediction of mechanical behaviour, thermo-mechanical deformations and moisture migration in non-uniformly heated concrete is important for safe operation of concrete containment.This paper presents the results of an experimental investigation carried out to predict the behaviour of concrete intended for nuclear applications. For this purpose, normal concrete having compressive strength of 40 MPa was designed using limestone aggregates. Cylindrical specimens (110 mm × 22 mm) were made and subjected to heating-cooling cycles at 110, 210 and 310 °C. Measurements were taken for thermal gradient, mass loss, deformations, residual mechanical properties, thermal conductivity, and porosity. This investigation developed some important data on the properties of concrete exposed to elevated temperatures up to 310 °C. Comparisons and interesting conclusions were drawn about the thermal stability at high temperature and the residual mechanical properties of the tested concrete.     

Cross sections for neutron-induced reactions up to 1.6 GeV for target elements relevant for cosmochemical, geochemical, and technological applications

We determined neutron excitation functions from the respective reaction thresholds up to 1.6 GeV for almost 100 target-product combinations relevant for cosmochemical, geochemical, and technological applications. We started with thick target production rates that have been obtained by irradiating iron and stone spheres with protons having energies between 600 MeV and 1.6 GeV. From the particle spectra of primary protons, secondary protons, and secondary neutrons, and the usually well known cross sections for the proton-induced reactions we calculated the production rates only due to protons. By subtracting these data from the measured total production rates we obtained production rates only due to secondary neutrons. With the modelled neutron spectra, guess functions calculated using nuclear model codes, and sophisticated energy-dependent deconvolution procedures we were able to determine almost 100 neutron excitation functions with their uncertainties. With the thus obtained neutron cross sections we are able to describe the experimental production rates in the thick target experiments, meteorites, the lunar surface, and terrestrial surface samples usually within the uncertainties, i.e., to within 10-15%. The adjusted neutron cross sections (a posteriori) are compared to results from the theoretical nuclear model codes INCL4.5/ABLA07 and TALYS. The TALYS code usually describes the a posteriori data reasonably well, i.e., mostly within a factor of a few. The quality of the INCL4.5 + ABLA07 predictions depends on the reaction type and increases with increasing number of ejectiles, i.e., increasing target-product mass difference. The neutron cross section database, though successful in quantifying production rates in terrestrial and extraterrestrial matter, presents by no means a final step and experiments with quasi-monoenergetic neutrons are needed.