Xenon-ion irradiation of Co/Si bilayers: Effects of interface structure and ion energy

Heavy-ion irradiation of ferromagnetic thin layers changes their micromagnetic and microstructural properties, due to the production of defects, relaxation or build-up of stress, or changes of grain size. When the ion range exceeds the layer thickness, ion mixing processes take place, leading to the formation of silicide phases. The present study deals with Co(30 or 55 nm)/Si bilayers irradiated at room temperature with 100- or 200 keV Xe ions to fluences of up to 15 × 10¹⁵/cm². The Si(1 0 0) wafers were either crystalline or pre-amorphized by 1 keV Ar⁺ implantation. Rutherford backscattering spectroscopy, in-plane magneto-optical Kerr effect, and X-ray diffraction served to analyse the samples before and after irradiation. The results will be compared with those obtained for other heavy-ions for Co/Si bilayers and in similar studies on Fe/Si bilayers.     

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10¹⁵ to 2 × 10¹⁶ ions/cm². The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr-Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10¹⁶ ions/cm². For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10¹⁵ ions/cm². The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10¹⁶ ions/cm². The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.     

Behavior of crystalline silicon under huge electronic excitations: A transient thermal spike description

Recent experimental works devoted to the phenomena of mixing observed at metallic multilayers Ni/Si irradiated by swift heavy ions irradiations make it necessary to revisit the insensibility of crystalline Si under huge electronic excitations. Knowing that Ni is an insensitive material, such observed mixing would exist only if Si is a sensitive material. In order to extend the study of swift heavy ion effects to semiconductor materials, the experimental results obtained in bulk silicon have been analyzed within the framework of the inelastic thermal spike model. Provided the quenching of a boiling (or vapor) phase is taken as the criterion of amorphization, the calculations with an electron-phonon coupling constant g(300 K) = 1.8 × 10¹² W/cm³/K and an electronic diffusivity D_e(300 K) = 80 cm²/s nicely reproduce the size of observed amorphous tracks as well as the electronic energy loss threshold value for their creation, assuming that they result from the quenching of the appearance of a boiling phase along the ion path. Using these parameters for Si in the case of a Ni/Si multilayer, the mixing observed experimentally can be well simulated by the inelastic thermal spike model extended to multilayers, assuming that this occurs in the molten phase created at the Ni interface by energy transfer from Si.     

Ion-beam irradiation effects on reactively sputtered CrN thin films

The present study deals with CrN/Si bilayers irradiated at room temperature (RT) with 120 keV Ar ions. The CrN layers were deposited by d.c. reactive sputtering on Si(1 0 0) wafers, at different nitrogen partial pressures (2 × 10⁻⁴, 3.5 × 10⁻⁴ and 5 × 10⁻⁴ mbar), to a total thickness of 240-280 nm. The substrates were held at room temperature (RT) or 150 °C during deposition. After deposition the CrN/Si bilayers were irradiated up to fluences of 1 × 10¹⁵ and 1 × 10¹⁶ ions/cm². Structural characterization was performed with Rutherford backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (XTEM) and grazing angle X-ray diffraction (XRD). For the highest nitrogen pressure (5 × 10⁻⁴ mbar) a pure stoichiometric CrN phase was achieved. The results showed that Ar ion irradiation resulted in the variation of the lattice constants, micro-strain and mean grain size of the CrN layers. The observed microstructural changes are due to the formation of the high density damage region in the CrN thin film structure.     

Sputtering and surface state evolution of Bi under oblique incidence of 120 keV Ar ions

The sputtering and surface state evolution of Bi/Si targets under oblique incidence of 120 keV Ar⁺ ions have been investigated over the range of incidence angles 0° ? θ_i ? 60°. Increasing erosion of irradiated samples (whose surface thickness reduced by ∼3% at normal incidence up to ∼8% at θ = 60°) and their surface smoothing with reducing grain sizing were pointed out using Rutherford backscattering (RBS), atomic force (AFM) and X-ray diffraction (XRD) techniques. Measured sputtering yield data versus θ_i with fixed ion fluence to ∼1.5 × 10¹⁵ cm⁻² are well described by Yamamura et al. semi-empirical formula and Monte Carlo (MC) simulation using the SRIM-2008 computer code. The observed increase in sputter yield versus incidence angle is closely correlated to Bi surface topography and crystalline structure changes under ion irradiation.     

Synthesis and characterization of lithium silicate powders

Lithium-based ceramics, such as Li₂O, LiAlO₂, Li₄SiO₄, Li₂SiO₃, Li₂TiO₃and Li₂ZrO₃, have long been recognized as promising tritium breeding-materials for D-T fusion reactor blankets. Among these candidate materials, lithium orthosilicate (Li₄SiO₄) and lithium metasilicate (Li₂SiO₃) are recommended by many ITER research teams as the first selection for the solid tritium breeder. Li₄SiO₄ has even been selected as the breeder material for the helium-cooled solid breeder test blanket module (HCSB TBM) in China and EU. In present study, the processes of solid-state reaction between amorphous silica and Li₂CO₃ powders was studied by thermogravimetry analysis-differential scanning calorimetry (TGA/DSC); the lithium silicate powders were synthesized at 700, 800 and 900 ° C with Li:Si molar ratios of 0.5, 1, 2 and 4, respectively, using solid-state reaction method. The as-prepared lithium silicates were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that the phase composition and morphology of the as-prepared samples change with the different synthesis conditions. At low temperature of 700 °C, all samples contain the amorphous silica, and the major crystalline phase is Li₂SiO₃ with different microstructure for Li/Si ratio of 0.5, 1 and 2. As for Li/Si=4, 98% purity of Li₄SiO₄ can be obtained at 700 °C. At high temperature of 900 °C, the significant sinterization effect will occur in all samples and Li₄SiO₄ will even decompose. The results also show that pure Li₄SiO₄ can be synthesized by calcining at 800 ° C for 4 h, and its’ solid-state reaction synthesis may be divided into two steps:

(1)

515-565 °C: Li₂CO₃+SiO₂→Li₂SiO₃+CO₂;

(2)

565-754 °C: Li₂CO₃+SiO₂→Li₂SiO₃+CO₂ and then Li₂SiO₃+Li₂CO₃→Li₄SiO₄+CO₂.

While Li/Si=2, 99% purity of and pure Li₂SiO₃ can be obtained at 800 and 900 °C, respectively.     

Effect of heating and cooling rate on the kinetics of allotropic phase changes in uranium: A differential scanning calorimetry study

The kinetic aspects of allotropic phase changes in uranium are studied as a function of heating/cooling rate in the range 10⁰-10² K min⁻¹ by isochronal differential scanning calorimetry. The transformation arrest temperatures revealed a remarkable degree of sensitivity to variations of heating and cooling rate, and this is especially more so for the transformation finish (T_f) temperatures. The results obtained for the α → β and β → γ transformations during heating confirm to the standard Kolmogorov-Johnson-Mehl-Avrami (KJMA) model for a nucleation and growth mediated process. The apparent activation energy Q_eff for the overall transformation showed a mild increase with increasing heating rate. In fact, the heating rate normalised Arrhenius rate constant, k/β reveals a smooth power law decay with increasing heating rate (β). For the α → β phase change, the observed DSC peak profile for slower heating rates contained a distinct shoulder like feature, which however is absent in the corresponding profiles found for higher heating rates. The kinetics of γ → β phase change on the other hand, is best described by the two-parameter Koistinen-Marburger empirical relation for the martensitic transformation.     

Phase relations in the U-Mo-Al ternary system

The phase relations in the U-Mo-Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo_2−xAl_20+x and U₆Mo_4+xAl_43−x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl₂ (cubic MgCu₂-type) giving a quite large extension (UAl_2−xMo_x, 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl_2−xMo_x with the hexagonal MgZn₂-type. There is no detectable solubility of Mo in UAl₄, and it is of the order of 1 at.% in UAl₃. The interaction layers between the γU-Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.     

Stability of nano-scaled Ta/Ti multilayers upon argon ion irradiation

The effects of argon ion irradiation on structural changes in Ta/Ti multilayers deposited on Si wafers were investigated. The starting structures consisted of sputter deposited 10 alternate Ta (∼23 nm) and Ti (∼17 nm) layers of a total thickness ∼200 nm. They were irradiated at room temperature with 200 keV Ar⁺, to the fluences from 5 × 10¹⁵ to 2 × 10¹⁶ ions/cm². The projected ion range was around mid-depth of the multilayered structure, and maximum displacements per atom ∼130. It was found that, despite of the relatively heavy ion irradiation, individual nanocrystalline Ta and Ti layers remain unmixed, keeping the same level of interface planarity. The changes observed in the mostly affected region are increase in lateral dimensions of crystal grains in individual layers, and incorporation of bubbles and defects that cause some stretching of the crystal lattice. Absence of interlayer mixing is assigned to Ta-Ti immiscibility (reaction enthalpy ΔH_f = +2 kJ/mol). It is estimated that up to ∼5 at.% interface mixing induced directly by collision cascades could be compensated by dynamic demixing due to chemical driving forces in the temperature relaxation regime. The results can be interesting towards developing radiation tolerant materials based on multilayered structures.     

Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation

Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 × 10¹⁶/cm² and 110 keV H ions at a fluence of 1 × 10¹⁶/cm², singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 μm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results.     

Effect of ion beam irradiation on magnetic and structural properties of Pt/Cr/Co multilayers

This paper discusses the effect of ion beam irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe Co-Cr-Pt ternary alloy phase formation in 1 MeV N⁺ ion irradiated [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]_×6/Si multilayers for a fluence of 1 × 10¹⁶ ions cm⁻² and beyond. The observed phase formation is accompanied by an enhancement in the average grain size, surface roughness and coercivity. Monte Carlo simulation has been performed to study ion-induced defect evolution and atomic displacements to correlate the above observed effects.     

The interdiffusion and solid-state reaction of low-energy copper ions implanted in silicon

At room temperature, single-crystal silicon was implanted with Cu⁺ ions at an energy of 80 keV using two doses of 5 × 10¹⁵ and 1 × 10¹⁷ Cu⁺ cm⁻². The samples were heat treated by conventional thermal annealing at different temperatures: 200 °C, 230 °C, 350 °C, 450 °C and 500 °C. The interdiffusion and solid-state reactions between the as-implanted samples and the as-annealed samples were investigated by means of Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD). After annealing at 230 °C, the XRD results of the samples (subject to two different doses) showed formation of Cu₃Si. According to RBS, the interdiffusion between Cu and Si atoms after annealing was very insignificant. The reason may be that the formation of Cu₃Si after annealing at 230 °C suppressed further interdiffusion between Si and Cu atoms.     

The ternary system: silicon-uranium-vanadium

Phase equilibria in the system Si-U-V were established at 1100 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed, U₂V₃Si₄ and (U_1−xV_x)₅Si₃, for which the crystal structures were elucidated by X-ray powder data refinement and found to be isotypic with the monoclinic U₂Mo₃Si₄-type (space group P2₁/c; a = 0.6821(3), b = 0.6820(4), c = 0.6735(3) nm, β = 109.77(1)°) and the tetragonal W₅Si₃-type (space group I4/mcm, a = 1.06825(2), c = 0.52764(2) nm), respectively. (U_1−xV_x)₅Si₃ appears at 1100 °C without any significant homogeneity region at x ∼ 0.2 resulting in a formula U₄VSi₃ which corresponds to a fully ordered atom arrangement. DTA experiments clearly show decomposition of this phase above 1206 °C revealing a two-phase region U₃Si₂ + V₃Si. At 1100 °C U₄VSi₃ is in equilibrium with V₃Si, V₅Si₃, U₃Si₂ and U(V). At 800 °C U₄VSi₃ forms one vertex of the tie-triangle to U₃Si and V₃Si. Due to the rather high thermodynamic stability of V₃Si and the corresponding tie-lines V₃Si + liquid at 1100 °C and V₃Si + U(V) below 925 °C, no compatibility exists between U₃Si or U₃Si₂ and vanadium metal.     

Interferences in electron emission from O2 by 30 MeV O impact

Interference structures in the ejected electron spectra for 30 MeV O^5,8+ + O₂ are investigated. The measured electron yields were studied for electron energies from 5 to 400 eV and observation angles of 30°, 60°, 90°, 120° and 150° with respect to the incident beam direction. Experimental molecular cross-sections were normalized to theoretical molecular one-center cross-sections revealing oscillatory structures suggestive of secondary interferences as evidenced by the independence on the observation angle. An oscillation interval for 30 MeV O^5,8+ + O₂ of Δk ∼ 4 a.u. is found, a value two times larger than that previously observed for 3 MeV H⁺ + N₂. No obvious evidence for primary Young-type interferences was seen.     

Resonant Rutherford backscattering spectrometry for carbon diffusion in silicon

300 keV C⁺ ion implantation onto Si(1 0 0) wafers was carried out at temperatures of 400, 500, 550, 600, 650 and 700 °C. Depth profile of C was determined by resonant Rutherford backscattering spectrometry (RRBS) measurements using ¹²C(α,α)¹²C resonant reaction with the α-particle energy of 4.27 MeV. The concentration of the implanted carbon at the surface as a function of inverse of implantation temperature shows an Arrhenius behaviour. The activation energy for diffusion of carbon in Si was measured and found to be 0.434 eV, which is smaller than the activation energy (0.88 eV) for the C diffusion in Si in equilibrium condition. The possible mechanism of C diffusion in Si during irradiation conditions existing in our experiments where large concentration of vacancies and interstitials are produced is discussed and we find that the C diffusion during irradiation conditions could be due to the drag the carbon towards the surface by the vacancy flux.     

Synthesis & fabrication of lithium-titanate pebbles for ITER breeding blanket by solid state reaction & spherodization

⁶Li produces tritium by (n, α) nuclear reaction, ⁶Li + ¹n → ⁴He + ³H. Lithium titanate (Li₂TiO₃) enriched with ⁶Li, is the most promising candidate for solid test blanket module (TBM) material for fusion reactors. Various processes are reported in literature for the fabrication of Li₂TiO₃ pebbles for its use as TBM material. A process has been developed based on the solid state reaction of lithium-carbonate and titanium-dioxide for the synthesis of lithium titanate and pebble fabrication by extrusion, spherodization and sintering. This paper discusses the sequence of steps followed in this process and the properties obtained. Analytical grade titanium-dioxide and lithium-carbonate were taken in stoichiometric ratio and were milled to ensure thorough intimate mixing and obtain fine particles less than 45 μm before its calcination at 900 °C. Following calcination, the agglomerated product was again milled to fine particles of size less than 45 μm. Aqueous solution of ploy-vinyl-alcohol was added as binder prior to its feeding in the extruder. The extruded strips were spherodized and spherical pebbles were dried and sintered at 900 °C for different duration. Pebbles of desired density and porosity were obtained by suitable combination of sintering temperature and duration of sintering. Properties of the prepared pebbles were also characterized for sphericity, pore size distribution, grain size, crushing load strength, etc. The values were found to be conforming to the desired properties for use as solid breeder. The attractive feature of this process is almost no waste generation.     

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.     

Fine-structure effects on the polarization of Kα1 radiation from heavy, highly-charged ions

Radiative decay of heavy, few-electron ions following electron capture is studied within the framework of the density matrix theory combined with the multiconfiguration Dirac-Fock approach. Special attention is paid to the linear polarization of the decay X-ray photons. Detailed calculations have been carried out, in particular, for the Kα₁ (1s2p_3/2^1,3P_1,2 →  1s²¹S₀) transition in helium-like uranium U⁹⁰⁺ ions. For this line, the characteristic radiation is almost unpolarized as a result of the superposition of its fine-structure ¹P₁ → ¹S₀ and ³P₂ → ¹S₀ components which are strongly linearly polarized in perpendicular directions.     

Structural and compositional characterization of LiNbO3 crystals implanted with high energy iron ions

Iron ions were implanted with a total fluence of 6 × 10¹⁷ ions/m² into lithium niobate crystals by way of a sequential implantation at different energies of 95, 100 and 105 MeV respectively through an energy retarder Fe foil to get a uniform Fe doping of about few microns from the surface. The implanted crystals were then annealed in air in the range 200-400 °C for different durations to promote the crystalline quality that was damaged by implantation. In order to understand the basic phenomena underlying the implantation process, compositional in-depth profiles obtained by the secondary ion mass spectrometry were correlated to the structural properties of the implanted region measured by the high resolution X-ray diffraction depending on the process parameters. The optimised preparation conditions are outlined in order to recover the crystalline quality, essential for integrated photorefractive applications.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.