keV Xenon Ion Research Articles

Chemical and interface effects on glass forming ability under ion mixing in ZrTa system of positive heat of formation

Amorphous alloy films were formed by room temperature 200 keV xenon ion mixing of multilayers in a relatively broad composition range in the ZrTa system, which has a positive heat of formation of +4 kJ mol −1. A free energy diagram was constructed to include the free energy curves of the amorphous phase and the multilayers, to which the previously missing term of the excess interfacial free energy was calculated and added. It turned out that the interface plays a role in influencing the alloying behavior in such system and the calculated diagram provided a reasonable interpretation of the observed amorphization. Two new metastable crystalline phases of both fcc structures were obtained.

Amorphization induced by ion mixing in Fe–Mo multilayered films: Correlation with the number of metal layers

In the Fe–Mo system, amorphous alloys are thermodynamically unstable and are hard to form in the bulk state. In multilayered films, however, amorphous alloys were produced by room temperature 200 keV xenon ion mixing. It was found, when keeping the total thickness of the multilayered film constant at 40 nm, that the composition range favoring amorphization extended from the equiatomic stoichiometry towards the two metal sides with increasing number of metal layers. This alloying behavior is discussed in terms of a simple thermodynamic calculation.

Formation of amorphous alloys through the multilayer method in an immiscible Y - Nb system

Amorphization was achieved by room-temperature 190 keV xenon ion mixing of multilayered films in the Y - Nb system, which has a positive enthalpy of formation exceeding . To give further insight into the singularity of the amorphization behaviour of the system, a Gibbs free-energy diagram of the system was constructed by calculating the free energies of the amorphous phase and those of the as-deposited Y - Nb multilayers, in which the interfacial free energy was estimated and added. The established diagram can explain the irradiation-induced amorphization behaviour in the Y - Nb system well.

Hexagonal-close-packed face-centered-cubic transformation in irradiated hafnium thin films

Hafnium thin films, 45 nm thick were investigated by transmission electron microscopy and were shown to deposit with a hexagonal close packed structure, then transformed to a face centered cubic structure after being irradiated by 200 keV xenon ions at room temperature.

Size effect on glass-forming ability in the AuTa system upon ion mixing

Abstract Glass-forming ability was studied by room temperature 190 keV xenon ion mixing for the AuTa system, for which the atomic size difference is nearly zero. Several amorphous alloys were obtained and their compositions were located in a narrow range around Au23Ta77. The amorphization mechanism under ion irradiation process, as well as the size effect on the glass-forming range, is also discussed.

Ion-mixing-induced amorphization in an immiscible Au - Mo system with a small size difference

Amorphization in immiscible Au - Mo multilayered films with a very small atomic size difference was achieved by 200 keV xenon ion mixing at 77 K. The experimental results revealed that Au/Mo multilayered films can be amorphized in the Mo content range 75 - 85 at.% upon ion mixing. Thermodynamically, the excess interfacial free energy stored in multilayered films was responsible for the formation of amorphous alloys.

Metastable phases formed by ion mixing in the Hf-Nb system with a positive heat of formation

Amorphization was achieved in the Hf-Nb system with a positive heat of formation (+6 kJ mol-1) by room-temperature 200 keV xenon ion mixing of multilayered films. On the basis of thermodynamic calculation, in which the interfacial free energy of multilayered films was specially taken into account, a Gibbs free-energy diagram was constructed and a relevant interpretation to the observed amorphization behaviour was proposed together with the kinetic considerations for the ion irradiation process. Also, two metastable FCC phases of Nb-rich and Hf-rich concentrations with different sizes were obtained by ion mixing.

Role of interface in ion mixing induced amorphization in the Ag–Mo system with very positive heat of formation

A free energy diagram of the Ag–Mo system with a very positive heat of formation of +57 kJ/mol was constructed, with special consideration to the interfacial free energy in the multilayered films for ion mixing. It was found that amorphization was possible to achieve within two separate composition ranges toward Ag and Mo ends, respectively, if 12 layers, or about 13% of the interfacial atoms were included in the films. Accordingly, ion mixing of the Ag–Mo multilayered films was conducted at liquid nitrogen temperature by 190 keV xenon ions and the formation of amorphous phases was in good agreement with the calculation. Besides, a new metastable fcc phase was formed and, more interestingly, it transformed into an amorphous state upon aging, which can also be interpreted by a related free energy calculation.

Alloying behavior induced by ion mixing in a system with positive heat of formation

Amorphous alloys were formed by room-temperature 200 keV xenon ion mixing of multilayered films in the Zr-Nb system that has a positive heat of formation (+6 kJ mol−1) and the ion-induced amorphization was found to be discontinuous in the composition range. Thermodynamic calculation was conducted and a free-energy diagram, which concerns free-energy curves of the amorphous phase and the initial state of the multilayered films with excess interfacial energy, was constructed. An interpretation to the observed discontinuity of amorphization range was presented based on the calculated free-energy diagram. In addition, two new metastable crystalline phases both of fcc structure with different lattice parameters were formed in the composition range close to pure zirconium and niobium, respectively.

Microstructure evolution in Zr-Mo multilayers induced by ion irradiation

Amorphization was achieved by room-temperature 200 keV xenon ion mixing in Zr-Mo multilayered films. In addition, two new Mo-based metastable crystalline phases were obtained, which were of FCC and HCP structures, respectively. A Gibbs free-energy diagram that concerns the free-energy curves of the amorphous phase and the terminal solid solutions was constructed to give related thermodynamic explanations to the observations.

Microstructure of TiN films and interfaces formed by ion-beam-enhanced deposition and simple physical vapor deposition

The microstructure and composition of TiN films, formed by ion beam enhanced deposition (IBED) with different energy (40 keV and 90 keV) xenon ion bombardment and by simple physical vapor deposition (hereafter S-PVD) without any ion beam enhancement, and the interfaces between TiN films and Si substrates have been studied by cross-sectional view analytical electron microscopy in this work. Both the IBED TiN films prepared by Xe+ bombardment with either 40 keV or 90 keV energy ions and the S-PVD TiN film consist of nanocrystals. The TEM observations in the S-PVD case reveal an amorphous layer and a mixed layer of TiN grains and amorphous material at the TiN/Si interface. The thicknesses of the amorphous layer and the mixed layer are about 210 nm and at least 40 nm, respectively. Upon 40 keV Xe+ bombardment, an amorphous Si transition layer of about 50 nm thickness is found at the TiN/Si interface, and the TiN grains close to the TiN/Si interface are of weak preferred orientation. Upon 90 keV Xe+ bombardment, amorphous TiN and Si layers are found with a total thickness of 80 nm at the TiN/Si interface, and the TiN grains near the TiN/Si interface are of preferred orientation [111]TiN ‖ [001]Si. The energy of xenon ion bombardment has a strong effect on the microstructural characteristics of TiN films and the interfaces between the TiN films and the Si substrates, as well as the size and the preferred orientation of TiN grains.

In sito Ion Beam Analysis of Radiation Damage Kinetics in MgTiO3 Single Crystals at 170-470 K

AbstractRadiation damage kinetics in synthetic MgTiO3 (geikielite) single crystals have been studied using the in situ ion beam facility at Los Alamos National Laboratory. The geikielite samples were irradiated at temperatures of 170, 300, and 470 K with 400 keV xenon ions and the radiation damage was sequentially measured with Rutherford backscattering using a 2 MeV He ion beam along a channeling direction. Threshold doses of 1 and 5×1015 Xe/cm2 were determined for the crystalline-to-amorphous transformation induced by Xe ion irradiation at 170 and 300 K, respectively. However, geikielite retained its crystallinity up to a dose of 2.5x1016 Xe/cm2 at the irradiation temperature of 470 K. This study has shown that MgTiO3, which has a corundum derivative structure, is another radiation resistant material that has the potential for use in radiation environments.

Thermodynamics and crystallographic mechanism of metastable phase formation in Nb-Fe, Nb-Co, and Nb-Ni systems by ion mixing

Two metastable crystalline phases, i.e., hcp and fcc, were formed in Nb-rich Nb-Fe, Nb-Co, and Nb-Ni multilayered films by room temperature 200 keV xenon ion mixing. The experimental results revealed that the formation sequence was first hcp and then fcc through an abnormal two-step phase transition of bcc→hcp→fcc. To give insight into the phase formation, Gibbs free-energy diagrams of the systems were constructed by the calculation method proposed by Alonso, Gallego, and Somozar [Nuovo Cimento, 12, 587 (1990)]. Besides, steady-state thermal annealing experiments of the corresponding as-deposited multilayers were conducted and the results confirmed the existence as well as the energetic sequence of the hcp and fcc phases predicted by the calculated free-energy diagram. Concerning the growth kinetics of the two metastable crystalline phases, the two-step phase transition was determined to be a shearing plus sliding mechanism.

Solid state phase transition induced by ion irradiation in the Co-Nb system: experiments and thermodynamics

In the Co-Nb system, seven amorphous alloys were synthesized by room-temperature 200 keV xenon ion mixing of multilayered films with various compositions. The composition range favouring amorphization was determined to be from 23 to 80 at.% Co. In addition, two metastable crystalline (MX) phases were consecutively formed in the Co23Nb77 multilayered films, i.e. an HCP phase at a dose of 1*1015 Xe+ cm-2 and a FCC phase at a dose of 6*1015 Xe+ cm-2. The thermal stabilities of both ion-mixed amorphous alloys and the MX phases were examined by subsequent annealing. To obtain a deep insight concerning all the above observations, a free-energy diagram of the Co-Nb system was constructed by calculating the free-energy curves of the solid solutions, the amorphous phase, the equilibrium compound and the MX phases, which were considered as compound-like phases. To test the relevance of the constructed diagram in its outline, steady-state thermal annealing of the as-deposited Co23Nb77 multilayered films was performed and the results confirmed the calculated relative energy levels of the phases considered. The free-energy diagram can explain the amorphization range as well as the observed evolution of the ion-mixed phases.

Effect of Ion Irradiation on the Artificial Structure and the Interlayer Coupling of Copper-Cobalt Multilayers

ABSTRACTStructural defects like local ferromagnetic short-circuits are responsible for large reductions of the intrinsic interlayer coupling in the case of antiferromagnetically (AF) coupled multilayers (ML). We have investigated the role of such point defects in AF-coupled Cu-Co ML’s with tCu=8Å and tCo=12Å which have been irradiated with various doses of 200 keV xenon ions. A significant decrease of the giant magnetoresistance and of the saturation field is observed when the density of defects is increased. These results are compared with a micromagnetic model that takes into account local discontinuities of the AF coupling in an ideally AF-coupled ML. Evidence for the presence of such defects is investigated by TEM on cross-sectional specimens.

Irradiation induced texture of Ag and Fe grains in Ag/Fe multilayers

Ag/Fe multilayers were prepared by alternate deposition of pure silver and iron and then irradiated with 200 keV xenon ions at liquid nitrogen temperature to various doses to investigate the texture changes of Ag and Fe grains in the films. When the dose was low, the irradiation induced texture changes increased with increasing dose. When the dose exceeded a critical value, the irradiation induced texture changes decreased with increasing dose. The possible mechanism responsible for the changing texture structure is also discussed.

Radiation damage in ion-irradiated pyrolytic graphite

Several grades of graphite are used in nuclear applications; as moderators in fission reactors, primary fuel containment in high temperature gas-cooled reactors, and plasma facing materials in fusion reactors. Whereas high temperature (<800°C) radiation damage response is reasonably well understood, the low temperature response is less well understood, including amorphization under irradiation. This study examined this response for ion-irradiated, highly-oriented pyrolytic graphite (HOPG). Electron transparent flakes of HOPG were prepared by repeated cleavage. Folding grids were used to hold the flakes during ion irradiation and electron microscopy. Specimens were irradiated with either 35 or 165 keV carbon or 300 keV xenon ions to fluences up to 1 × 1016 and 3×1015 ions/cm2, respectively, at nominal temperatures from room temperature to 600°C. The low energy carbon and xenon ions were selected to give similar ion ranges (∼84 nm). The range of the 165 keV carbon ions was 330 nm.

Construction of free energy diagram and phase formation by ion mixing in the NiNb system

In the NiNb system, eight amorphous alloys were formed by 200 keV xenon ion mixing at room temperature of multilayered films with various compositions covering a range of 20–85 at.% of Ni. In addition, a new metastable crystalline (MX) phase was observed and identified to be of hexagonal structure with a stoichiometry around 75 at.% of Ni. The MX phase transformed into an amorphous state upon thermal annealing at 400°C for two hours. The free energy diagram of the system was constructed by calculating the free energy curves of the solid solution, the amorphous phase, the equilibrium compound and the MX phase. In calculation of the MX phase the identified structural characteristics and the ferromagnetic effect from Ni atoms were considered. The calculated diagram can explain the amorphization range upon ion mixing as well as the unexpected phase transition from the MX to the amorphous phase upon heating.

A Cubic Phase Formed in Immiscible Fe/Ag(Cu) System S by Ion Mixing

ABSTRACTFexAg1-x (x=0.20-0.65) and Fey Cu1-y (y=0.50-0.75) nano-multilayers were prepared by alternative deposition of pure constituent metals. Ion mixing of the multilayers by 200 keV xenon ions at 77K resulted in the formation of a similar metastable phase with a big lattice of cubic structure, when the ion dose exceeded 1×1016 and 1×1015 Xe+/cm2 for Fe/Ag and Fe/Cu systems, respectively. The fraction of the new phase emerged in the alloy films increased with increasing ion dose in both cases.

Metastable Crystalline Phases Synthesized by Ion Beam Mixing in Some Nb-Based Binary Metal Systems

ABSTRACTIn the Nb-rich Nb-Fe(Co,Ni) multilayered films, two metastable crystalline (MX) phases of hep and fee structures were formed by room temperature 200 keV xenon ion mixing. The phase formation was through a two-step transition, Le. a first shearing of bcc—>hcp and then a sliding of hep—>fcc. The free energy curves of the observed MX phases were calculated and included into the free energy diagram together with those of the terminal solid solutions, the amorphous phase and the equilibrium intermetaШc compounds. An independent comparative thermal annealing experiment was conducted with the corresponding as-deposited multilayers and the results certified the emerging sequence of the MX phases being exactly the same as the calculated diagram predicted. Moreover, a crystallographic model was figured out for the bec—>hcp—>fcc transition and the resultant lattice parameters agreed well with the experimental ones.