23 research outputs found
Optical pulse induced ultrafast antiferrodistortive transition in SrTiO3
The ultrafast dynamics of the antiferrodistortive (AFD) phase transition in
perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using
femtosecond optical pulses, we induce a thermally driven tetragonal-to-cubic
structural transformation and detect notable changes in the frequency of
Brillouin oscillations (BO) induced by propagating acoustic phonons. First, we
establish a fingerprint frequency of different regions across the temperature
phase diagram of the AFD transition characterized by tetragonal and cubic
phases in the low and high temperature sides, respectively. Then, we
demonstrate that in a sample nominally kept in tetragonal phase, deposition of
sufficient thermal energy induces an instantaneous transformation of the
heat-affected region to the cubic phase. Coupling the measured depth-resolved
BO frequency with a time and depth-resolved heat diffusion model, we detect a
reverse cubic-to-tetragonal phase transformation occurring on a time scale of
hundreds of picoseconds. We attribute this ultrafast phase transformation in
the perovskite to a structural resemblance between atomic displacements of the
R-point soft optic mode of the cubic phase and the tetragonal phase, both
characterized by anti-phase rotation of oxygen octahedra. Evidence of such a
fast structural transition in perovskites can open up new avenues in the field
of information processing and energy storage.Comment: 15 Pages, 4 Figure
Spatially localized measurement of thermal conductivity using a hybrid photothermal technique
A photothermal technique capable of measuring thermal conductivity with micrometer lateral resolution is presented. This technique involves measuring separately the thermal diffusivity, D, and thermal effusivity, e, to extract the thermal conductivity, k = (e2/D)1/2. To generalize this approach, sensitivity analysis is conducted for materials having a range of thermal conductivities. Application to nuclear fuel is consider by performing experimental validation using two materials (CaF2 and SiO2) having thermal properties representative of fresh and high burnup nuclear fuel. The measured conductivities compare favorably with literature values
Kapitza Resistance of Si/SiO₂ Interface
A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO 2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance
Comprehensive characterization of irradiation induced defects in ceria: Impact of point defects on vibrational and optical properties
Validation of multiscale microstructure evolution models can be improved when standard microstructure characterization tools are coupled with methods sensitive to individual point defects. We demonstrate how electronic and vibrational properties of defects revealed by optical absorption and Raman spectroscopies can be used to compliment transmission electron microscopy (TEM) and x-ray diffraction (XRD) in the characterization of microstructure evolution in ceria under non-equilibrium conditions. Experimental manifestation of non-equilibrium conditions was realized by exposing cerium dioxide (CeO2) to energetic protons at elevated temperature. Two sintered polycrystalline CeO2 samples were bombarded with protons accelerated to a few MeVs. These irradiation conditions produced a microstructure with resolvable extended defects and a significant concentration of point defects. A rate theory (RT) model was parametrized using the results of TEM, XRD, and thermal conductivity measurements to infer point defect concentrations. An abundance of cerium sublattice defects suggested by the RT model is supported by Raman spectroscopy measurements, which show peak shift and broadening of the intrinsic T2g peak and emergence of new defect peaks. Additionally, spectroscopic ellipsometry measurements performed in lieu of optical absorption reveals the presence of Ce3+ ions associated with oxygen vacancies. This work lays the foundation for a coupled approach that considers a multimodal characterization of microstructures to guide and validate complex defect evolution models
Recommended from our members
How are the energy waves blocked on the way from hot to cold?
Representing the Center for Materials Science of Nuclear Fuel (CMSNF), this document is one of the entries in the Ten Hundred and One Word Challenge. As part of the challenge, the 46 Energy Frontier Research Centers were invited to represent their science in images, cartoons, photos, words and original paintings, but any descriptions or words could only use the 1000 most commonly used words in the English language, with the addition of one word important to each of the EFRCs and the mission of DOE energy. The mission of CMSNF to develop an experimentally validated multi-scale computational capability for the predictive understanding of the impact of microstructure on thermal transport in nuclear fuel under irradiation, with ultimate application to UO2 as a model syste
First-principles determination of the phonon-point defect scattering and thermal transport due to fission products in ThO2
This work presents the first principles calculations of the lattice thermal
conductivity degradation due to point defects in thorium dioxide using an
alternative solution of the Pierels-Boltzmann transport equation. We have used
the non-perturbative Green's function methodology to compute the phonon point
defect scattering rates that consider the local distortion around the point
defect, including the mass difference changes, interatomic force constants and
structural relaxation near the point defects. The point defects considered in
the work include the vacancy of thorium and oxygen, substitution of helium,
krypton, zirconium, iodine, xenon, in the thorium site, and the three different
configuration of the Schottky defects. The results of the phonon-defect
scattering rate reveals that among the considered intrinsic defects, the
thorium vacancy and helium substitution in the thorium site scatter the phonon
most due to substantial changes in the force constant and structural
distortions. The scattering of phonons due to the substitutional defects
unveils that the zirconium atom scatters phonons the least, followed by xenon,
iodine, krypton, and helium. This is contrary to the intuition that the
scattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the mass
difference. This striking difference in the zirconium phonon scattering is due
to the local chemical environment changes. Zirconium is an electropositive
element with valency similar to thorium and, therefore, can bond with the
oxygen atoms, thus creating less force constant variance compared to iodine, an
electronegative element, noble gas helium, xenon, and krypton. These results
can serve as the benchmark for the analytical models and help the
engineering-scale modeling effort for nuclear design.Comment: 10 page
Photoresponse mechanism of superconducting MgB<sub>2</sub>
Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2008.The recent discovery of superconductivity in MgB2, with its BCS-like Cooper pairing mechanism and the 40-K critical temperature, and the demonstration of efficient single-optical-photon detection in superconducting NbN nanowire meanders inspired an interest in the development of superconducting radiation detectors based on MgB2. We report the results of our experimental and theoretical studies of a photoresponse mechanism in superconducting MgB2 thin films and microbridges. We demonstrate that despite the two-gap nature of this material, the nonequilibrium superconducting recovery dynamics in MgB2 is similar to conventional one-gap, both low- and high-temperature superconductors and is governed by quasiparticle recombination, limited by the phonon bottleneck mechanism. Our measured 100-ps-wide responses in MgB2 superconducting microbridges, operated at temperatures above 20 K, make this material promising for superconducting photodetector applications
Recommended from our members
Mechanical Properties of Nuclear Fuel Surrogates using Picosecond Laser Ultrasonics
Detailed understanding between microstructure evolution and mechanical properties is important for designing new high burnup nuclear fuels. In this presentation we discuss the use of picosecond ultrasonics to measure localize changes in mechanical properties of fuel surrogates. We develop measurement techniques that can be applied to investigate heterogeneous elastic properties caused by localize changes in chemistry, grain microstructure caused by recrystallization, and mechanical properties of small samples prepared using focused ion beam sample preparation. Emphasis is placed on understanding the relationship between microstructure and mechanical propertie
Research of dynamics of turning of machine-tractor aggregate with tractor on wheeled-crawler mover
In the article theoretical preconditions of a description of dynamics of manoeuvrability of machine-tractor, aggregates with a wheeled-tracked mover are considered. For a machine-tractor aggregate with half-tracked progress theoretical formulas of determination of an actual turning radius, the moment of resistance of turn and torque for rotation are obtained. The theoretical preconditions are confirmed by experimental research of the manoeuvrability of the machine-tractor aggregate with the tractor on a halftracked progress, made as the experimental sample. The dependences of the turn coefficient and the resistance coefficient of the turn are obtained, and the correlation coefficients and their significance have confirmed the existence of a stable connection between the changing parameter and the response function. Proceeding from theoretical and experimental research, it is possible to draw a conclusion that the manoeuvrability of the tractor with a wheeled-crawler mover does not concede to the tractor in the basic execution
Machine learning potential assisted exploration of complex defect potential energy surfaces
Abstract Atomic-scale defects generated in materials under both equilibrium and irradiation conditions can significantly impact their physical and mechanical properties. Unraveling the energetically most favorable ground-state configurations of these defects is an important step towards the fundamental understanding of their influence on the performance of materials ranging from photovoltaics to advanced nuclear fuels. Here, using fluorite-structured thorium dioxide (ThO2) as an exemplar, we demonstrate how density functional theory and machine learning interatomic potential can be synergistically combined into a powerful tool that enables exhaustive exploration of the large configuration spaces of small point defect clusters. Our study leads to several unexpected discoveries, including defect polymorphism and ground-state structures that defy our physical intuitions. Possible physical origins of these unexpected findings are elucidated using a local cluster expansion model developed in this work