Irradiation effect on dielectric properties and electrical conductivity of Au/SiO2/n-Si (MOS) structures

The Au/SiO₂/n-Si (MOS) structures were exposed to beta-ray irradiation to a total dose of 30 kGy at room temperature. Irradiation effect on dielectric properties of MOS structures were investigated using capacitance−voltage (C−V) and conductance−voltage (G/ω−V) characteristics. The C−V and G/ω−V measurements carried out in the frequency range from 1 kHz to 10 MHz and at various radiation doses, while the dc voltage was swept from positive bias to negative bias for MOS structures. The dielectric constant (ε′), dielectric loss (ε″), loss factor (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C−V and G/ω−V measurements and plotted as a function of frequency at various radiation doses. A decrease in the ε′ and ε″ were observed when the irradiation dose increased. The decrease in the ε′ and ε″ of irradiated MOS structures in magnitude is explained on the basis of Maxwell−Wagner interfacial polarization. Also, the σ_ac is found to decrease with increasing radiation dose. In addition, the values of the tan δ decrease with increasing radiation dose and give a peak. From the experimental results, it is confirmed that the peak of loss tangent is due to the interaction between majority carriers and interface states which induced by radiation.     

Thermal transport properties of hafnium hydrides and deuterides

The thermal conductivities of δ′-, δ-, δ+ε-, and ε-phase hafnium hydrides and deuterides with various hydrogen isotope concentrations (HfH_x, 1.48 ? x ? 2.03; HfD_x, 1.55 ? x ? 1.94) were evaluated within the temperature range of 290-570 K from the measured thermal diffusivity, calculated specific heat, and density. The thermal conductivities of δ′-, δ-, δ+ε-, and ε-phase HfH_x and HfD_x are independent of the temperature within the range 300-550 K and are in the range 0.15-0.22 W/cm K and 0.17-0.23 W/cm K, respectively; these values are similar to and lower than the observed thermal conductivities of α-phase Hf. The experimental results for the electrical resistivities of δ′-, δ-, δ+ε-, and ε-phase HfH_x and HfD_x and the Lorenz number corresponding to the electronic conduction, obtained from the Wiedemann-Franz rule, indicated that heat conduction due to electron migration significantly influences the thermal conductivity values at high temperatures. On the other hand, heat conduction due to phonon migration significantly affects the isotope effects on the thermal transport properties.     

High gamma dose response of the electrical properties of polyethylene terephthalate thin films

Electrical properties of polyethylene terephthalate (PET), irradiated with gamma rays, have been investigated. The PET films were irradiated with high gamma dose levels in the range from 100 to 2000 kGy. The changes in the DC (σ_DC) and the ac (σ_ac) conductivities, with the dose, have been performed. The effect of gamma irradiation on the dielectric constant (ε′) and loss (ε″) has been determined. Also, the dose dependence of the frequency exponent index (S), the resonance frequency (Fc) and the hopping frequency (ω_P) have been obtained. The obtained results show that increasing gamma dose leads to slight increase in σ_DC, σ_ac and ε′, while no change was observed in ε″ value. Meanwhile, S, Fc and ω_P are inversely proportional to the dose. Accordingly, the study suggests the possibility of using PET films in electronic components (capacitors, resistors, etc.), especially that operate at high gamma dose environments for the frequency independent applications.     

Lattice expansion and evolution of damage buildup in Be-implanted InAs

The lattice expansion in InAs single crystal, due to ion-implantation by 80 keV Be ions with the implantation fluencies ranging from 1 × 10¹² to 2 × 10¹⁶ cm⁻², has been investigated by using high resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM), and Rutherford backscattering spectrometry/channeling (RBS/C). In order to clarify the evolution of damage buildup, the nonlinear maximum perpendicular strain ε_m as a function of the Be fluence was obtained and analyzed. The curve of ε_m vs. Be fluence is subdivided into five regions, each having a different damage accumulation behavior. The involved probable mechanisms of microstructural variation in InAs due to Be implantation of different fluencies are analyzed in detail.     

Properties of FINEMET with Fe partially replaced by Mn

Thin films and foils of Fe_73.5−xSi_13.5B₉Cu₁Nb₃Mn_x, the FINEMET based amorphous and nanocrytalline alloys with high Mn doping (x = 9, 11, 13, 15 at%), were studied by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and with Mössbauer spectroscopy (MS), as-quenched (a-q) and after annealing (a). Mn, partially replacing Fe, causes common crystallisation temperature T_cr for the identified crystal structures, decrease of the lattice constants a₀, c₀, decrease of hyperfine parameters: magnetic field H_hf and isomer shift IS for amorphous phases and in consequence the Curie temperature T_Cu.     

Influence of metallurgical variables on delayed hydride cracking in Zr-Nb pressure tubes

During service, Zr-2.5Nb pressure tubes of nuclear power reactors may be prone to suffer from crack growth by delayed hydride cracking (DHC). For a given hydrogen plus deuterium concentration there is a critical temperature (T_C) below which DHC may occur. In this work, T_C was measured for specimens cut from pressure tubes made in Canada (CANDU) and in Russia (RBMK). Hydrogen was added to the specimens to get concentrations ranging from 24 to 60 wt ppm. It was found that T_C was higher than the corresponding precipitation temperature. The crack propagation velocity (V_P), measured in axial direction, increases from a minimum at T_C to a maximum at a temperature close but higher than the precipitation temperature. At lower temperatures, when hydride precipitates are present in the bulk, V_P follows an Arrhenius law: V_P = A exp(−Q/RT), with an activation energy Q of 66-68 kJ/mol for both tubes. The RBMK material presented lower velocities than CANDU one.     

Structural and magnetic characterization of cobalt implanted GaN films

Cobalt ions were implanted into GaN films with multiple energies between 50 keV and 380 keV with two total fluences, 1.25 × 10¹⁶ and 1.25 × 10¹⁷ cm⁻², followed by annealing at temperatures between 600 and 850 °C. The crystal quality and surface morphology of as-implanted and subsequently annealed films were investigated by X-ray diffraction (XRD) 2θ scans, ω-rocking curve measurements and atomic force microscopy (AFM). The profiles of impurities and defects were analyzed by Rutherford backscattering spectrometry (RBS) in random and channeling configurations. The virgin GaN films have an excellent crystal quantity (χ_min = 1.4%) and in the implanted samples 60% disorder induced by ion implantation was recovered after annealing. The annealed sample become ferromagnetic, with a spontaneous magnetization of 0.1 emu/g and a coercive magnetic field of 100 Oe at 10 K, and the Curie point was found to be higher than room temperature.     

Rapid synthesis of Pb5(VO4)3I, for the immobilisation of iodine radioisotopes, by microwave dielectric heating

Rapid synthesis of Pb₅(VO₄)₃I, a potential immobilisation host for iodine radioisotopes, was achieved in an open container by microwave dielectric heating of a mixture of PbO, PbI₂, and V₂O₅ at a power of 800 W for 180 s (at 2.45 GHz). The resulting ceramic bodies exhibited a zoned microstructure, differentiated by inter-granular porosity and phase assemblage, as a consequence of the inverse temperature gradient characteristic of microwave dielectric heating. Liquid PbI₂ within the interior of microwave processed ceramics assisted formation of Pb₅(VO₄)₃I, and reduced inter-granular porosity. In contrast, the exterior of microwave processed ceramics comprised poorly sintered Pb₅(VO₄)₃I with the presence of minor reagent relics. Quantitative microanalysis, electron diffraction and Rietveld analysis, confirmed the synthesis of stoichiometric Pb₅(VO₄)₃I within precision. The crystal structure of Pb₅(VO₄)₃I was found to adopt space group P6₃/m with a = 10.4429(3) Å and c = 7.4865(2) Å.     

Second crossover energy of insulating materials using stationary electron beam under normal incidence

The purpose of this paper is to give some aspects of charging effects on dielectric materials submitted to continuous electron beam irradiation in a scanning electron microscope (SEM). When the dielectric is irradiated continuously, the so-called total yield approach (TYA) used to predict the sign of the charge appeared on electron irradiated insulators fails because the charge accumulated in the dielectric interferes with the electrons emission processes. Based on previous experimental and theoretical works found in the literature, an analysis of the evolution of the electron yield curves σ = f(E₀) of insulators during irradiation is given. The aim of this work is firstly to determine experimentally the second crossover energy E_2C under continuous electron irradiation (charging conditions) and secondly to demonstrate that the charge balance occurs at this beam energy and not at E₂ the energy deduced from non-charging conditions (pulse primary electron beam experiments) as commonly asserted. It is however possible to apply the TYA by substituting the critical energy E₂ for E_2C. The experimental procedure is based on simultaneous time dependent measurements of surface potential, leakage current and displacement current. The study underlines the difference between the landing energy of primary electrons E_L at the steady state and the second crossover energy, E_2C, for charged samples. Some preliminary results are also obtained concerning the influence of the incident beam density on the energy E_2C. The samples used for this study are PMMA, polycrystalline silicone dioxide (p-SiO₂), polycrystalline alumina (p-Al₂O₃) and soda lime glass (SLG).     

Elastic and plastic properties of βZr at room temperature

Room temperature elastic and plastic properties of a single phase β_Zr have been studied by in-situ neutron diffraction compression testing. The measured macroscopic Young’s modulus is ∼60 GPa and the yield strength is ∼500 MPa. Dislocation slip is the major mode of plastic deformation. An Elasto-Plastic Self-Consistent (EPSC) model was used to interpret the experimental results and was shown to be effective in extracting the single crystal properties from the polycrystalline data. The single crystal elastic constants of the β-phase are determined as: C₁₁ = 145.9 ± 2.6 GPa, C₁₂ = 117.4 ± 2.5 GPa and C₄₄ = 29.8 ± 0.2 GPa. The calculated elastic modulus of 〈1 0 0〉, 〈1 1 0〉, 〈1 1 1〉, 〈2 1 1〉 and 〈3 1 0〉 directions was ∼41.2, 66.2, 82.9, 66.2 and 47.7 GPa, respectively. Pencil glide on the {110}, {112} and {123} planes was used in the EPSC model and gave a good simulation to the early part of the plastic deformation. The average β-phase strain is best represented by the peak average method, while in cases where only a limited number of diffraction peaks are available, the {211} grain family is a good candidate for estimation of the average β-phase strain.     

Comparison of microstructural properties and Charpy impact behaviour between different plates of the Eurofer97 steel and effect of isothermal ageing

The microstructure and fracture properties of the Eurofer97 steel plates of thickness 14 mm and 25 mm were investigated in as-received state and in state after long-term thermal ageing (550 °C/5000 h). Detailed microstructure studies were carried out by means of optical light, electron and quantitative electron microscopy. Mechanical properties were evaluated by means of Charpy impact testing and hardness testing and fracture surfaces were fractographically analysed in macro and microscales. The microstructure of the Eurofer97 consisted of tempered martensite with M₂₃C₆ and MX precipitates. Microstructure of 14 mm plate was more homogenous and fine grained than 25 mm plate. Due to different microstructure the t_DBTT of thicker plate was on +10 °C higher than for 14 mm plate for which reached −60 °C. Slight microstructural changes on the level of subgrain consisting of their partial recrystallization and slight carbide coarsening were observed after applied ageing. The isothermal ageing caused evident shift in t_DBTT about +5 °C, which was most likely caused by recrystallization of subgrains.     

Coherent pair production by energetic gamma rays incident on quasicrystals

We develop a Born-approximation theory of coherent pair production (CPP) of electrons by energetic gamma rays incident on an icosahedral quasicrystal, described by a schematic model (K model) that includes phonon and phason disorder. Our main result is a formula for the cross-section dσ_cpp/dε₊ for CPP, differential with respect to the positron energy ε₊ and of order α² in the fine-structure constant α ≈ 1/137, but which is otherwise exact. We discuss results of numerical calculations of dσ_cpp/dε₊ versus y = ε₊/k for gamma rays of energies k = 20 MeV, 200 MeV, and 3 GeV, incident on icosahedral Al-Mn-Si, described as a special case of the K model (vertex model). This consists in placing an Mn atom at each vertex of the relevant Ammann tiles. Our calculations include CPP of types A and B. Both types exhibit vertical intensity drops at irregularly distributed y-values, many of these drops being so large that they should be observable experimentally. They are analogous to the large intensity drops exhibited by coherent bremsstrahlung in quasicrystals. We predict that CPP drops also occur for realistic models of i-Al-Mn-Si at the same y-values as for the vertex model, but whose magnitudes may differ from those predicted by this model.     

Fatigue cracks in Eurofer 97 steel: Part II. Comparison of small and long fatigue crack growth

The fatigue crack growth rate in the Eurofer 97 steel at room temperature was measured by two different methodologies. Small crack growth data were obtained using cylindrical specimens with a shallow notch and no artificial crack starters. The growth of semicircular cracks of length between 10-2000 μm was followed in symmetrical cycling with constant strain amplitude (R_ε = −1). Long crack data were measured using standard CT specimen and ASTM methodology, i.e. R = 0.1. The growth of cracks having the length in the range of 10-30 mm was measured. It is shown that the crack growth rates of both types of cracks are in a very good agreement if J-integral representation is used and usual assumptions of the crack closure effects are taken into account.     

Measurement of transformation temperatures and specific heat capacity of tungsten added reduced activation ferritic-martensitic steel

The on-heating phase transformation temperatures up to the melting regime and the specific heat capacity of a reduced activation ferritic-martensitic steel (RAFM) with a nominal composition (wt%): 9Cr-0.09C-0.56Mn-0.23V-1W-0.063Ta-0.02N, have been measured using high temperature differential scanning calorimetry. The α-ferrite + carbides → γ-austenite transformation start and finish temperatures, namely Ac₁, and Ac₃, are found to be 1104 and 1144 K, respectively for a typical normalized and tempered microstructure. It is also observed that the martensite start (M_S) and finish (M_f) temperatures are sensitive to the austenitising conditions. Typical M_S and M_f values for the 1273 K normalized and 1033 K tempered samples are of the order 714 and 614 K, respectively. The heat capacity C_P of the RAFM steel has been measured in the temperature range 473-1273 K, for different normalized and tempered samples. In essence, it is found that the C_P of the fully martensitic microstructure is found to be lower than that of its tempered counterpart, and this difference begins to increase in an appreciable manner from about 800 K. The heat capacity of the normalized microstructure is found to vary from 480 to 500 J kg⁻¹ K⁻¹ at 500 K, where as that of the tempered steel is found to be higher by about, 150 J kg⁻¹ K⁻¹.     

Thermoelectric power studies of ferromagnet U2ScB6C3

The thermoelectric power (TEP) of a ferromagnet U₂ScB₆C₃ (T_C = 61 K) has been measured in the temperature range 5-300 K. The TEP is positive over the whole measured temperature range and reaches a relatively large value at room temperature of 29 μV/K. Below 30 K and above 200 K the TEP follows a straight line S(T) ∼AT, with slope of 0.23 and 0.085 μV/K², respectively. The change in the slope can be explained by the electron-phonon interaction renormalization effects or spin-reorientation associated with a change in the electronic structure. Analysing the temperature dependence of the ratio [S(T)/T]/[S_300 K/300] and taking into account the specific heat data, we suggest that spin fluctuations are another important factor in determining the thermoelectric power behaviour of U₂ScB₆C₃.     

Crystal structure and magnetic properties of UFe5Ga7

The new ternary compound UFe₅Ga₇ was prepared by an argon arc-melting followed by annealing. This intermetallic compound belongs to the series UFe_xGa_12−x and crystallizes in a structure related with the ThMn₁₂-type, with a = 8.6309 Å and c = 5.0524 Å. EDS elemental analysis yielded the phase composition UFe_4.9(2)Ga_6.8(1), which is very close to the nominal composition. Dc magnetization measurements revealed ferromagnetic type of magnetic ordering in UFe₅Ga₇. The Curie temperature of ∼439 K was estimated by the temperature dependence of magnetization at a low magnetic field of 0.1 T.     

Precipitate phases of a ferritic/martensitic 9% Cr steel for nuclear power reactors

One of the reasons that ferritic/martenstic steels have been considered as candidate materials for nuclear power reactors is their superior creep resistance at elevated temperature. The creep rupture strength of 9% chromium steel could be improved by a fine dispersion of secondary precipitate phase. The precipitate phases in extra-low carbon 9% chromium steel with tempered conditions were investigated by transmission electron microscope and energy-dispersive X-ray analysis. The steel specimens were normalized and then tempered at different temperatures. Niobium-rich MN nitrides (Nb_0.6V_0.3Cr_0.1)N, and two kinds of vanadium nitrides, (V_0.6Nb_0.2Cr_0.2)N and (V_0.45Nb_0.45Cr_0.1)N having a f.c.c. crystal structure, were identified in the steel specimens tempered at 600-780 °C, and 750 or 780 °C respectively. Hexagonal chromium-rich M₂N precipitate phases with different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the tempered steel specimens. The M₂N phase showed a decrease/an increase in its chromium/vanadium content as the tempering temperature was increased. The influence of precipitates and heat treatment conditions on the high temperature properties of 9% Cr steel was discussed.     

Investigation of high temperature phase stability, thermal properties and evaluation of metallurgical compatibility with 304L stainless steel, of indigenously developed ferroboron alternate shielding material for fast reactor applications

Towards the cause of serving economic power production through fast reactors, it is necessary to bring in functionally more efficient and innovative design options, which also includes exploration of cheaper material alternatives, wherever possible. In this regard, the feasibility of using a commercial grade ferroboron alloy as potential alternate shielding material in the outer subassemblies of future Indian fast reactors has been recently investigated from shielding physics point of view. The present study explores in detail the high temperature thermal stability and the metallurgical compatibility of Fe-15.4B-0.3C-0.89Si-0.17Al-0.006S-0.004P-0.003O (wt.%) alloy with SS 304L material. In addition, the high temperature specific heat and lattice thermal expansion characteristics of this alloy have also been investigated as a part of the present comprehensive characterisation program. The Fe-15 wt.%B alloy is constituted of principally of two boride phases, namely tetragonal Fe₂B and orthorhombic FeB phases, which in addition to boron also contains some amount of C and Si dissolved in solid solution form. This Fe-B alloy undergoes a series of phase transformation as a function of increasing temperature; the major ones among them are the dissolution of Fe₂B-lower boride in the matrix through a eutectic type reaction, which results in the formation of the first traces of liquid at 1500 K/1227 °C. This is then followed by the dissolution of the major FeB boride phase in liquid and the melting process is completed at 1723 K/1450 °C. In a similar manner, the thermal stability studies performed on combined Fe-B + 304L steel reaction couples revealed that a pronounced pre-melting or liquid phase formation occurs at a temperature of 1471 K/1198 °C, which is lower than the melting onset of both Fe-B and SS 304L. It is found that within the limits of experimental uncertainty, this pre-melting phenomenon occurred at the same fixed temperature of 1471 K/1198 °C, irrespective of the mass ratios of Fe-B and 304L steel. Further, it is also found that SS 304L is completely soluble in Fe-B alloy and the fused product upon solidification formed a mixture of complex intermetallic borides, such as (Fe,Cr)(B,C), (Fe,Cr)₂(B,C) and (Fe,Ni)₃B. In the temperature range 823-1073 K (550-800 °C), the SS 304L clad is found to interact strongly with the Fe-B alloy. The diffusion layer thickness or the attack layer depth (x) is found to vary with time (t) up to about 5000 h, according to the empirical rate law, x² = k(T)t. The temperature sensitivity of the rate constant, k(T) is found to obey the Arrhenius law, k(T) = k_o exp(−Q/RT), with Q = 57 kJ mol⁻¹, being the effective activation energy for the overall diffusional interaction of Fe-B and SS 304L. The room temperature specific heat capacity of Fe-B alloy is found to be 538 kJ kg⁻¹ K⁻¹. The C_P values measured over 300-1350 K, is found vary smoothly with temperature according to the expression, C_P/kJ kg⁻¹ K⁻¹ = 0.62094 + 0.00012T + 10685.81T⁻². The lattice thermal expansion of both FeB and Fe₂B phases are found to be anisotropic in that the c-axis expansion is found to be more than that along a and b axes. The room temperature volume thermal expansivity of FeB and Fe₂B phases are found to be of the order of 48 × 10⁻⁶ K⁻¹ and 28 × 10⁻⁶ K⁻¹, respectively. The thermal expansion of FeB is found to be more temperature sensitive than that of Fe₂B.     

A study of fullerite ablation with energetic pulsed electrons

We report the ablation of fullerite films deposited on metallic substrates with 3 keV electron pulses generated in a specially designed pulsed discharge tube. During ablation the fragmented species were detected by emission spectroscopy. The emission spectra of C₂ and C₁^∗ (CII) provide the signatures of C₆₀ fragmentation. The vibrational temperature of the C₂ emitted from the ablated fullerite is ∼12,700 ± 1160 K compared with ∼18,230 ± 1150 K for the graphite sample under similar conditions. The fullerite films were produced by vacuum sublimation on Aluminum, Iron and Copper substrates and characterized by Atomic force microscope, X-ray diffraction; Raman and Fourier transform infrared spectroscopy. The comparisons of electron ablation of fullerite films with that of graphite show the similarities and differences of carbon bonding in the caged structure of C₆₀ with that of the planar graphene sheets of graphite.     

Pulsed laser treatments of polyethylene films

A Nd:Yag pulsed laser, 3 ns pulse width, 150 mJ pulse energy, operating at the second harmonics (532 nm) has been used to irradiate in air polyethylene thin films.The thermal and chemical effects induced by the laser irradiation in the polymer are responsible of the hydrogen and C_xH_y groups emission at long irradiation times. A special study, concerning the welding effect between two different types of polyethylene films, one transparent and the other strong absorbent the laser light, was performed. The welding, at the interface of the two pressed polymers, depends on the optical and micro-structural material properties besides the irradiation time. In particular, polymers with different viscosity, melting temperature and crystalline degree exhibit different mechanical behaviour.Different techniques were employed to investigate on the polymeric welding effects, such as the mass quadrupole spectrometry, the scanning electron microscope, the surface profiler and the mechanical strength measurement.