Vacuum plasma etching of 1 wt% La2O3 dispersed tungsten

Vacuum plasma etching of 1 wt% La₂O₃ doped tungsten alloy surfaces were carried out to refine the surface morphology for enhancing its bonding characteristics with copper for fusion reactor components. Three different gas compositions containing argon with zero, 14.3 and 25 vol% hydrogen were used to carry out the plasma etching from 30 to 120 s on the given samples. Mitutoyo surface roughness (R_a) measurement, FORM TALYSURF and scanning electron microscopy (SEM) were employed to measure the changes in the surface roughness. Plasma etching with 14.3 vol% hydrogen mixture was found to be the best in micro-roughening the alloy surface. The maximum increase of 44% in R_a value was obtained with this gas mixture, when etched for 90 s.     

Depth profiling of Al2O3 + TiO2 nanolaminates by means of a time-of-flight energy spectrometer

Atomic layer deposition (ALD) is currently a widespread method to grow conformal thin films with a sub-nm thickness control. By using ALD for nanolaminate oxides, it is possible to fine tune the electrical, optical and mechanical properties of thin films. In this study the elemental depth profiles and surface roughnesses were determined for Al₂O₃ + TiO₂ nanolaminates with nominal single-layer thicknesses of 1, 2, 5, 10 and 20 nm and total thickness between 40 nm and 60 nm. The depth profiles were measured by means of a time-of-flight elastic recoil detection analysis (ToF-ERDA) spectrometer recently installed at the University of Jyväskylä. In TOF-E measurements ⁶³Cu, ³⁵Cl, ¹²C and ⁴He ions with energies ranging from 0.5 to 10 MeV, were used and depth profiles of the whole nanolaminate film could be analyzed down to 5 nm individual layer thickness.     

Defect volumes of BO2 doped Y2O3 (B = Ti, Zr, Hf and Ce)

Atomistic simulations have been employed to study the effect of BO₂ (fluorite) incorporation into the bixbyite oxide Y₂O₃. The energetically preferred defect mechanism and the associated lattice parameter changes that occur from BO₂ doping have been predicted. The addition of Group IV elements into Y₂O₃ can follow three different mechanisms. The energetically favourable method is through a mediated reaction for ZrO₂ and HfO₂ while for TiO₂ and CeO₂, reducing B⁴⁺ to B³⁺ provides the lowest energy reaction. ZrO₂ and HfO₂ doping results in the lowest volume changes.     

Thermodynamic properties of ThxU1−xO2 (0 < x < 1) based on quantum-mechanical calculations and Monte-Carlo simulations

Th_xU_1−xO_2+y binary compositions occur in nature, uranothorianite, and as a mixed oxide nuclear fuel. As a nuclear fuel, important properties, such as the melting point, thermal conductivity, and the thermal expansion coefficient change as a function of composition. Additionally, for direct disposal of Th_xU_1−xO₂, the chemical durability changes as a function of composition, with the dissolution rate decreasing with increasing thoria content. UO₂ and ThO₂ have the same isometric structure, and the ionic radii of 8-fold coordinated U⁴⁺ and Th⁴⁺ are similar (1.14 nm and 1.19 nm, respectively). Thus, this binary is expected to form a complete solid solution. However, atomic-scale measurements or simulations of cation ordering and the associated thermodynamic properties of the Th_xU_1−xO₂ system have yet to be determined. A combination of density-functional theory, Monte-Carlo methods, and thermodynamic integration are used to calculate thermodynamic properties of the Th_xU_1−xO₂ binary (ΔH_mix, ΔG_mix, ΔS_mix, phase diagram). The Gibbs free energy of mixing (ΔG_mix) shows a miscibility gap at equilibration temperatures below 1000 K (e.g., E_exsoln = 0.13 kJ/(mol cations) at 750 K). Such a miscibility gap may indicate possible exsolution (i.e., phase separation upon cooling). A unique approach to evaluate the likelihood and kinetics of forming interfaces between U-rich and Th-rich has been chosen that compares the energy gain of forming separate phases with estimated energy losses of forming necessary interfaces. The result of such an approach is that the thermodynamic gain of phase separation does not overcome the increase in interface energy between exsolution lamellae for thin exsolution lamellae (10 Å). Lamella formation becomes energetically favorable with a reduction of the interface area and, thus, an increase in lamella thickness to >45 Å. However, this increase in lamellae thickness may be diffusion limited. Monte-Carlo simulations converge to an exsolved structure [lamellae || ] only for very low equilibration temperatures (below room temperature). In addition to the weak tendency to exsolve, there is an ordered arrangement of Th and U in the solid solution [alternating U and Th layers || {1 0 0}] that is energetically favored for the homogeneously mixed 50% Th configurations. Still, this tendency to order is so weak that ordering is seldom reached due to kinetic hindrances. The configurational entropy of mixing (ΔS_mix) is approximately equal to the point entropy at all temperatures, indicating that the system is not ordered.     

Effects of MeV Si ions bombardment on the thermoelectric generator from SiO2/SiO2 + Cu and SiO2/SiO2 + Au nanolayered multilayer films

The defects and disorder in the thin films caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We prepared the thermoelectric generator devices from 100 alternating layers of SiO₂/SiO₂ + Cu multi-nano layered superlattice films at the total thickness of 382 nm and 50 alternating layers of SiO₂/SiO₂ + Au multi-nano layered superlattice films at the total thickness of 147 nm using the physical vapor deposition (PVD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used to determine the stoichiometry of the elements of SiO₂, Cu and Au in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardments have been performed using the AAMU-Center for Irradiation of Materials (CIM) Pelletron ion beam accelerator to make quantum (nano) dots and/or quantum (quantum) clusters in the multilayered superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured Seebeck coefficient, cross-plane electrical conductivity, and thermal conductivity in the cross-plane geometry for different fluences.     

Effect of Nd and Pr addition on the thermal and mechanical properties of (U, Ce)O2

The thermal conductivity, Young’s modulus, and hardness of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ (x = 0.01, 0.08, 0.12) were evaluated and the effect of Pr and Nd addition on the properties of (U, Ce)O₂ were studied. The polycrystalline high-density pellets were prepared with solid state reactions of UO₂, CeO₂, Pr₂O₃, and Nd₂O₃. We confirmed that all Ce, Pr, and Nd dissolved in UO₂ and formed solid solutions of (U, Ce, Pr, Nd)O₂. We revealed that the thermal conductivity of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ (x = 0.12) was up to 25% lower than that of x = 0.01 at room temperature. The Young’s modulus of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ decreased with x, whereas the hardness values were constant in the investigated x range.     

Modelling of deposition processes on the TiO2 rutile (1 1 0) surface

Deposition of Ti_xO_y clusters onto the rutile TiO₂ (1 1 0) surface has been modelled using empirical potential based molecular dynamics. Deposition energies in the range 10-40 eV have been considered so as to model typical deposition energies of magnetron sputtering. Defects formed as a function of both the deposition energy and deposition species have been studied.The results show that in the majority of cases Ti interstitial atoms are formed, irrespective of whether Ti was contained within the deposited cluster. Furthermore that the majority of these interstitials are formed by displacing a surface Ti atom into the interstitial site. O surface atoms are also relatively common, with Ti and TiO₂ surface units often occurring when the deposited cluster contains Ti but becoming less frequent as the deposition energy is increased. Structures that would give rise to the growth of further layers of rutile are not observed and in the majority of the simulations the energy barriers for diffusion of the end-products is high.     

Film growth by polyatomic C2H5 bombarding a diamond (1 0 0) surfaces: Molecular dynamics study

The deposition of polyatomic C₂H₅⁺ ions is studied using classical molecular dynamics simulations with a new improved Brenner potentials developed by Brenner. The simulation results show that when the incident energy is less than 65 eV, the deposition coefficient of H is larger than that of C atoms. When the incident energy is larger than 65 eV, the deposition of H is less than that of C atoms. With increasing incident energy, a transition from Csp3-rich to Csp2-rich in the grown films is found.     

Molecular dynamics simulations of C2, C2H, C2H2, C2H3, C2H4, C2H5, and C2H6 bombardment of diamond (1 1 1) surfaces

The sticking and erosion of C₂H_x molecules (where x=0-6), at 300 and 2100 K onto hydrogenated diamond (1 1 1) surfaces was investigated by means of molecular dynamics simulations. We employed both quantum-mechanical and empirical force models. Generally, the sticking probability is observed to somewhat increase when the radical temperature increases and strongly decrease with increasing number of H atoms in the molecule.     

Nanostructuring of BaF2 (1 1 1) surfaces by single slow highly charged ions

The creation of surface nanostructures in BaF₂ (1 1 1) surfaces was studied after irradiation with slow highly charged Xe ions from the Dresden-EBIT (electron beam ion trap). After irradiation, the crystals were investigated by scanning force microscopy (SFM). Using specific ion parameters, the topographic images show nanohillocks emerging from the surface. Additionally, we used the technique of selective chemical etching to reveal the lattice damage created by ion energy deposition below and above threshold needed for surface hillocks formation. The role of both potential and kinetic energy as well as a comparison with results for swift heavy ion irradiations of BaF₂ single crystals are presented.     

Enhanced susceptibility of CaF2(1 1 1) to adsorption due to ion irradiation

We have investigated morphological changes of freshly cleaved CaF₂(1 1 1) single crystal surfaces before and after ion irradiation. We show that with or without irradiation the surface undergoes serious changes within minutes after the cleavage if the samples are exposed to ambient conditions. This is most likely due to the adsorption of water and could be avoided only if working under clean ultra-high-vacuum conditions. Ion-induced modifications on this surface seem to act as centers for an increased rate of adsorption so that any quantitative numbers obtained by atomic force microscopy in such experiments have to be treated with caution.     

Combustion of Na2B4O7 + Mg + C to synthesis B4C powders

Boron carbide powder was fabricated by combustion synthesis (CS) method directly from mixed powders of borax (Na₂B₄O₇), magnesium (Mg) and carbon. The adiabatic temperature of the combustion reaction of Na₂B₄O₇ + 6 Mg + C was calculated. The control of the reactions was achieved by selecting reactant composition, relative density of powder compact and gas pressure in CS reactor. The effects of these different influential factors on the composition and morphologies of combustion products were investigated. The results show that, it is advantageous for more Mg/Na₂B₄O₇ than stoichiometric ratio in Na₂B₄O₇ + Mg + C system and high atmosphere pressure in the CS reactor to increase the conversion degree of reactants to end product. The final product with the minimal impurities’ content could be fabricated at appropriate relative density of powder compact. At last, boron carbide without impurities could be obtained after the acid enrichment and distilled water washing.     

Kinetics of initial stage of sintering of UO2 and UO2 with Nd2O3 addition

The kinetics of initial stage sintering of UO₂ powder were reinvestigated, using Ar-10% H₂ atmosphere. The effect of the addition of neodynium oxide was studied. The results revealed that surface and grain boundary diffusion mechanisms act simultaneously. The values of activation energies were found to be $\text{48.48 ± 3.51 kcal/mole}$ in the temperature range 870–942°C and $\text{89.88 ± 9.87 kcal/mole}$ in the temperature range of 942–1030°C for UO₂, and $\text{115.61 ± 7.77 kcal/mole}$ in the temperature range 1030–1150°C for UO₂ + Nd₂O₃. An important decrease in the calculated diffusion coefficient occurs by the addition of Nd₂O₃.     

Effect of 200 MeV Ag ion irradiation on structural and electrical transport properties of Fe3O4 thin films

Thin films of Fe₃O₄ have been deposited on single crystal MgO(1 0 0) and Si(1 0 0) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly 〈1 1 1〉 oriented. Film thicknesses are 150 nm. These films have been irradiated with 200 MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5 × 10¹⁰ ions/cm² to 1 × 10¹² ions/cm². We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but 〈1 1 1〉 oriented film. The pristine film on Si substrate shows Verwey transition (T_V) close to 125 K, which is higher than generally observed in single crystals (121 K). After the irradiation with the 5 × 10¹⁰ ions/cm² fluence value, T_V shifts to 122 K, closer to the single crystal value. However, with the higher fluence (1 × 10¹² ions/cm²) irradiation, T_V again shifts to 125 K.     

Transformation of Al2O3 to LiAlO2 in Pb–17Li at 800 °C

A FeCrAl substrate was pre-oxidized for 2 h at 1000 °C to thermally grow an external Al₂O₃ scale and then isothermally exposed to Pb–17 at.% Li for 1000 h at 800 °C to determine if this layer would protect the underlying alloy from dissolution. After exposure, a small mass gain was measured, indicating that the layer did inhibit dissolution. However, characterization of the external layer determined that it had transformed to LiAlO₂ with an increased thickness and a much larger grain size than the original layer. This observation has implications for the use of Al₂O₃ as a permeation barrier in Pb–Li cooled fusion blanket systems.     

Computer simulation of sputtering at the low index (1 0 0), (1 1 0) and (1 1 1) surfaces of Ni3Al in a STEM

The present study is relevant to the preferential Al sputtering and/or enhancement of the Ni/Al ratio in Ni₃Al observed by the scanning transmission electron microscopy fitted with a field emission gun (FEG STEM). Atomic recoil events at the low index (1 0 0), (1 1 0) and (1 1 1) surfaces of Ni₃Al through elastic collisions between electrons and atoms are simulated using molecular dynamics (MD) methods. The threshold energy for sputtering, E_sp, and adatom creation, E_ad, are determined as a function of recoil direction. Based on the MD determined E_sp, the sputtering cross-sections for Ni and Al atoms in these surfaces are calculated with the previous proposed model. It is found that the sputtering cross-section for Al atoms is about 7-8 times higher than that for Ni, indicating the preferential sputtering of Al in Ni₃Al, in good agreement with experiments. It is also found that the sputtering cross-sections for Ni atoms are almost the same in these three surfaces, suggesting that they are independent of surface orientation. Thus, the sputtering process is almost independent of the surface orientation in Ni₃Al, as it is controlled by the sputtering of Ni atoms with a lower sputtering rate.     

High-temperature specific heat of UO2 ThO2, and A12O3

The high-temperature specific heat of solid UO₂, ThO₂, and Al₂O₃ can be represented by an equation of the form $\text{C}{}_{\mathrm{p(s)}}\text{= 3}{}_{\mathrm{n}}\text{RF(?}{}_{\mathrm{D}}\text{/T) + dT}{}^3$, (1) where ?_D is the Debye temperature, F(?_D/T) is the Debye function, $\text{d}$ represents contributions of the anharmonic vibrations within the lattice, and $\text{n}$ denotes the number of atoms per molecule. In the liquid the corresponding equation is $\text{C}{}_{\mathrm{p(1)}}\text{= 3}{}_{\mathrm{n}}\text{RF(?}{}_{\mathrm{D}}\text{/T) +}\text{h}\text{T}{}^2$, (2) where h is the anharmonic term. It is shown that for Al₂O₃ and UO₂, where experimental data for the liquid phase are also available, $\text{d}\text{h}$ has the same value, Indicating that both materials behave identically. If we compare the thermodynamic relationship $\text{C}{}_{\mathrm{p}}\text{? C}{}_{\mathrm{v}}\text{= Vα}{}^2\text{KT}$, (3) where V is the volume, $\text{α}$ the volume expansion coefficient, and $\text{K}$ the bulk modulus, with equation (1), It follows that d must be equal to $\text{Vα}{}^2\text{K}\text{T}{}^2$; the value of $\text{Vα}{}^2\text{K}\text{T}{}^2$ is calculated in the temperature region where d was obtained; within experimental error they are equal.     

Microstructural evolution of Y2O3 and MgAl2O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing

Different ODS EUROFER steels reinforced with Y₂O₃ and MgAl₂O₄ were elaborated by mechanical milling and hot isostatic pressing. Good compromise between strength and ductility could be obtained but the impact properties remain low (especially for the Y₂O₃ ODS steel). The materials were structurally characterized at each step of the elaboration. During milling, the martensite laths of the steel are transformed into nano-metric ferritic grains and the Y₂O₃ oxides dissolve (but not the MgAl₂O₄ spinels). After the HIP, all the ODS steels remain ferritic with micrometric grains, surrounded by nano-metric grains for the Y₂O₃ ODS steels. The mechanisms in the Y₂O₃ ODS steels are complex: the Y₂O₃ oxides re-precipitate as nano-Y₂O₃ particles that impede a complete austenitization during the HIP. The quenchability of the ODS steels is modified by the milling process, the oxide nature and the oxide content. Eventually, the advantages and drawbacks of each oxide type are discussed.     

Effect of thermal spike energy created in CuFe2O4 by 150 MeV Ni swift heavy ion irradiation

Effects of 150 MeV Ni¹¹⁺ swift heavy ion (SHI) irradiation on copper ferrite nanoparticles have been studied at the fluences of 1 × 10¹¹, 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴ and 5 × 10¹⁴ ions/cm². The XRD pattern shows the irradiation fluence dependant preferential orientation. Scanning electron microscope analysis displays fine blocks of material for pristine while partial agglomeration on irradiation. Notably, a large number of holes are present at the fluence of 5 × 10¹⁴ ions/cm². The magnetization measurements performed in these samples exposes that the coercivity and remanence magnetization value increases due to the magnetocrystalline anisotropy up to the fluence of 1 × 10¹³ ions/cm². At 1 × 10¹⁴ ions/cm² fluence, the induced thermal energy overcomes the magnetocrystalline anisotropy constant and causes a decrease in coercivity and remanence values. The saturation magnetization decreases up to the fluence of 1 × 10¹³ ions/cm² and then it increases for further irradiation. The change of crystalline orientation observed from XRD, the creation of holes from SEM and the change in magnetic properties are discussed on the basis of electro-phonon coupling and it invokes the thermal spike theory.