ORCID Profile
0000-0003-0700-0858
Current Organisations
Lawrence Livermore National Laboratory
,
University of Oxford
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Publisher: IOP Publishing
Date: 10-10-2022
Abstract: Gold-bismuth alloys are of interest as catalysts and catalytic sensing systems, electrochemical sensors, superconductors, and hohlraums for magnetically assisted inertial confinement fusion implosions. Radiation-hydrodynamics simulations with the Lasnex code of laser-driven hohlraums predict higher x-ray drive from Au-Bi alloys compared with cases of Au-Ta or pure Au and Bi hohlraums. Here, we use direct current magnetron sputtering in Ar gas, with co-sputtering from two elemental targets, to deposit Au-Bi alloys with Bi content of 9–77 at.% and thicknesses up to ∼20 µ m. Films are characterized by a combination of x-ray diffraction, Rutherford backscattering, scanning electron microscopy, substrate-curvature-based residual stress, and electronic transport measurements. Experiments are complemented by Monte Carlo simulations of ballistic sputtering and gas phase transport of depositing species and Ar gas atoms. Results show that all films are polycrystalline, with three distinct compositional regimes dominated by Au, Au 2 Bi, and Bi crystallographic phases. A metallic behavior of the temperature dependence of electrical resistivity is observed for all the films. Films with Bi content above ∼30 at.% exhibit porosity, which is tolerable to hohlraum x-ray drive based on Lasnex simulations.
Publisher: Elsevier BV
Date: 2017
Publisher: Elsevier BV
Date: 05-2021
Publisher: IOP Publishing
Date: 03-12-2020
Abstract: Gold–tantalum alloy films are attractive for hohlraums used in indirect drive magnetized inertial confinement fusion. A high electrical resistivity of over ∼100 µ Ω cm at cryogenic temperatures is an essential requirement for allowing an externally imposed pulsed magnetic field to soak through a hohlraum and magnetize the fusion fuel. Here, we systematically study properties of Au–Ta alloy films in the entire compositional range from pure Au to pure Ta with thicknesses up to 30 µ m. These films are made by direct current magnetron co-sputtering on planar substrates. Films are characterized by a combination of high-energy ion scattering, x-ray diffraction, electron microscopy, nanoindentation, and electrical transport measurements. Results show that an alloy with ∼80 at.% of Ta forms a metallic glass exhibiting a maximum electrical resistivity of ∼300 µ Ω cm with a weak temperature dependence in the range of 5–400 K. The deposition of a film with ∼80 at.% of Ta onto a sphero-cylindrical substrate for hohlraum fabrication is also demonstrated.
Publisher: Elsevier BV
Date: 08-2018
Publisher: AIP Publishing
Date: 03-10-2022
DOI: 10.1063/5.0106774
Abstract: Heavy-metal-based films with high electrical resistivity are needed for hohlraums for magnetically assisted inertial confinement fusion. Here, we study ternary Au–Ta–O films deposited by reactive direct-current magnetron co-sputtering from elemental Au and Ta targets in an oxygen containing atmosphere. By varying the O content, the electrical resistivity of films can be tuned in a wide range of ∼100–40 000 μΩ cm. With increasing O content, a drastic increase in resistivity occurs at ∼45 at. % of O, separating regimes with two different dominant conduction mechanisms attributed to metallic conduction through the Au–Ta alloy matrix (for ≲45 at. % of O) and tunneling across insulating Ta2O5 layers separating conducting islands (for ≳45 at. % of O). Post-deposition annealing at 300 °C leads to the segregation of Au into ∼50-nm islands, sharply decreasing the resistivity for films with ≳45 at. % of O but not for the metal-like films with lower O content.
Publisher: IOP Publishing
Date: 29-03-2018
Publisher: AIP Publishing
Date: 25-10-2021
DOI: 10.1063/5.0050901
Abstract: Gold-tantalum alloy films are of interest for biomedical and magnetically-assisted inertial confinement fusion applications. Here, we systematically study the effects of substrate tilt (0°–80°) and negative substrate bias (0–100 V) on properties of ≲3-μm-thick films deposited by high-power impulse magnetron sputtering (HiPIMS) from a Au–Ta alloy target (with 80 at. % of Ta). Results reveal that, for all the substrate bias values studied, an increase in substrate tilt leads to a monotonic decrease in film thickness, density, residual compressive stress, and electrical conductivity. Larger substrate bias favors the formation of a body-centered cubic phase, with films exhibiting lower column tilt and higher density, electrical conductivity, and residual compressive stress. These changes are attributed to metal atom ionization effects, based on the lack of correlation with distributions of landing energies and incident angles of depositing species as calculated by Monte Carlo simulations of ballistic collisions and gas phase atomic transport. By varying substrate tilt and bias in HiPIMS deposition, properties of Au–Ta alloy films can be controlled in a very wide range, including residual stress from −2 to +0.5 GPa, density from 12 to 17 g/cm3, and the electrical resistivity from 50 to 4500 μΩ cm, enabling optimum deposition conditions to be selected for specific applications.
Publisher: American Chemical Society (ACS)
Date: 14-08-2018
Publisher: MDPI AG
Date: 29-06-2022
Abstract: Tantalum-based films with tailored composition, density, and electrical resistivity are of interest for next generation hohlraums for magnetized indirect-drive inertial confinement fusion. Here, we use reactive direct-current magnetron sputtering to deposit tantalum suboxide films with O content in the range of 46–71 at.%. In contrast to a common approach involving varying reactive gas contents, compositional control is achieved kinetically by changing the total chamber pressure and the deposition rate, while keeping the working gas mix of Ar-5%O2 constant. The resultant films are X-ray amorphous with electrical resistivity varying by over seven orders of magnitude. The dominant conduction mechanism changes from metallic to activated tunneling above ∼55 at.% of O, which is characterized by a sharp increase in resistivity and a decrease in the carrier density at low temperatures.
Publisher: Elsevier BV
Date: 09-2020
Publisher: Elsevier BV
Date: 09-2022
Publisher: IOP Publishing
Date: 20-05-2019
Publisher: AIP Publishing
Date: 20-02-2019
DOI: 10.1063/1.5082563
Abstract: A better understanding of the reaction of Mg and B in the solid-phase regime is needed for the low-temperature synthesis of MgB2 films. Here, we study the kinetics of reactive inter-diffusion of Mg and B multilayers on glassy carbon substrates in the temperature range of 400−650°C. Results show that, at these temperatures, inter-diffusion is characterized by a single activation energy of ∼0.45eV. The formation of the superconducting MgB2 phase with critical temperatures of 25–31 K occurs at reaction temperatures of 450°C and above, with the rate of inter-diffusion obeying a power law with a kinetic exponent of ∼0.3. This suggests that rate-limiting processes are the nucleation and growth of MgB2 grains rather than diffusion and interfacial reactions. Implications of these results to the low-temperature synthesis of MgB2 films are discussed.
Location: United States of America
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Alexander Baker.