arXiv subjects

Chris Bell

Publications and source records attributed to Chris Bell.

5 recordsLinked to original sources

arXiv2026

PCB-Integrated CoPt Micromagnets for Magnetophoresis

Integration of magnetic material with scalable microfluidic platforms can significantly improve the throughput and precision in biotechnology processes. In this work we have developed a new magnetic platform based on printed circuit board (PCB) technology. Cobalt-Platinum (CoPt) micromagnets are electroplated on copper pads with an arbitrary footprint on a Kapton substrate to enable generation of different magnetic field gradients. The magnets are characterized by XRD and VSM, before and after thermal annealing. The ordered L10 phase appear in XRD results after annealing at 600 {\deg}C, and VSM results show around six times increase for in-plane magnetic remanence from 0.24T to 1.4T. The near equiatomic ratios of Co:Pt is confirmed by EDX observations. The performance of these magnets is experimentally validated by trapping magnetic nanoparticles in microfluidic channels. These results are in excellent agreement with FEA models presented in COMSOL Multiphysics.

cond-mat.other↗

arXiv2026

Local Structure of Epitaxial Single Crystal UO$_{2+x}$ Thin Films

The influence of oxygen stoichiometry on the uranium local environment is explored in epitaxial single crystal uranium oxide thin films grown by DC magnetron sputtering. Through post-growth annealing, the stoichiometry of as-grown UO$_{2}$ films are tuned over an approximate stoichiometry range of $0.07 \leq x \leq 0.20$, estimated with X-ray photoelectron spectroscopy measurements of the U$-4f$ and O$-1s$ peaks. The local structure of the thin films are then probed using extended X-ray absorption fine structure measurements at the U $L_{3}$ absorption edge. We observe both the evolution of the U local environment of as a function of oxidation in UO$_{2+x}$, and that the near stoichiometric UO$_{2}$ film replicates the local structure of bulk UO$_{2}$ material standards well. The series of stoichiometrically varied samples highlights the non-trivial transitional behaviour of the UO$_{2+x}$ oxygen sublattice with increasing oxygen content in this stoichiometric regime, while also demonstrating the efficacy of this thin film synthesis route for actinide studies beyond their established use as idealised surfaces, which could be readily adapted for further stoichiometrically tailored material studies and UO$_{2+x}$ device fabrication.

cond-mat.mtrl-sci↗

arXiv2024

Polyepitaxial grain matching to study the oxidation of uranium dioxide

Although the principal physical behaviour of a material is inherently connected to its fundamental crystal structure, the behaviours observed in the real-world are often driven by the microstructure, which for many polycrystalline materials, equates to the size and shape of the constituent crystal grains. Here we highlight a cutting edge synthesis route to the controlled engineering of grain structures in thin films and the simplification of associated 3-dimensional problems to less complex 2D ones. This has been applied to the actinide ceramic, uranium dioxide, to replicate structures typical in nuclear fission fuel pellets, in order to investigate the oxidation and subsequent transformation of cubic UO$_{2}$ to orthorhombic U$_{3}$O$_{8}$. This article shows how this synthesis approach could be utilised to investigate a range of phenomena, affected by grain morphology, and highlights some unusual results in the oxidation behaviour of UO$_{2}$, regarding the phase transition to U$_{3}$O$_{8}$.

cond-mat.mtrl-sci↗

arXiv2022

Structure and Phase Transitions of Metastable Hexagonal Uranium Thin Films

We report a simple technique for the synthesis of uniaxially textured, metastable hexagonal close-packed-like uranium thin films with thicknesses between 175-2800 \r{A}. The initial structure and texture of the layers have been studied via X-ray diffraction and reflectivity and the time-dependent transitions of the samples into various orientations of orthorhombic ${\alpha}$-U have been mapped by similar techniques. The final crystallographic orientations of each system and the timescales on which the transitions occur are found to depend on the lattice parameters of the original layer. The absence of the ${\alpha}$-U (001) orientation in the transition products suggests that the transitions in these layers are mediated by mechanisms other than the [110] transverse acoustic phonon mode previously suggested for the cubic ${\gamma}$-U(110) to hcp-U(00.1) to ${\alpha}$-U(001) displacive phase transition. Alternative transition pathways are discussed.

cond-mat.mtrl-sci↗

arXiv2012

Measurements of the gate tuned superfluid density in superconducting LaAlO3/SrTiO3

The interface between the insulating oxides LaAlO3 and SrTiO3 exhibits a superconducting two-dimensional electron system that can be modulated by a gate voltage. While gating of the conductivity has been probed extensively and gating of the superconducting critical temperature has been demonstrated, the question whether, and if so how, the gate tunes the superfluid density and superconducting order parameter is unanswered. We present local magnetic susceptibility, related to the superfluid density, as a function of temperature, gate voltage and location. We show that the temperature dependence of the superfluid density at different gate voltages collapse to a single curve characteristic of a full superconducting gap. Further, we show that the dipole moments observed in this system are not modulated by the gate voltage.

cond-mat.supr-con↗