arXiv subjects

Qing Xi

Publications and source records attributed to Qing Xi.

12 recordsLinked to original sources

arXiv2022

Nanoporous Structure of Sintered Metal Powder Heat Exchanger in Dilution Refrigeration: A Numerical Study

We use LAMMPS to randomly pack hard spheres to simulate the heat exchanger, where the hard spheres represent sintered metal particles in the heat exchanger. We simulated the heat exchanger under different sphere radii and different packing fractions of the metal particle and researched pore space. To improve the performance of the heat exchanger, we adopted this simulation method to explore when the packing fraction is 65%, the optimal sintering particle radius in the heat exchanger is 30~35nm.

physics.comp-ph↗

arXiv2021

Role of Magnon-Magnon Scattering in Magnon Polaron Spin Seebeck Effect

The spin Seebeck effect (SSE) signal of magnon polarons in bulk-Y3Fe5O12 (YIG)/Pt heterostructures is found to drastically change as a function of temperature. It appears as a dip in the total SSE signal at low temperatures, but as the temperature increases, the dip gradually decreases before turning to a peak. We attribute the observed dip-to-peak transition to the rapid rise of the four-magnon scattering rate. Our analysis provides important insights into the microscopic origin of the hybridized excitations and the overall temperature dependence of the SSE anomalies.

cond-mat.mtrl-sci↗

arXiv2021

Dimension reduction induced anisotropic magnetic thermal conductivity in hematite nanowire

The thermophysical properties near the magnetic phase transition point is of great importance in the study of critical phenomenon. Low-dimensional materials are suggested to hold different thermophysical properties comparing to their bulk counterpart due to the dimension induced quantum confinement and anisotropy. In this work, we measured the thermal conductivity of $\alpha$-Fe$_2$O$_3$ nanowires along [110] direction (growing direction) with temperature from 100K to 150K and found a dip of thermal conductivity near the Morin temperature. We found the thermal conductivity near Morin temperature varies with the angle between magnetic field and [110] direction of nanowire. More specifically, an angular-dependent thermal conductivity is observed, due to the magnetic field induced movement of magnetic domain wall. The angle corresponding to the maximum of thermal conductivity varies near the Morin transition temperature, due to the different magnetic easy axis as suggested by our calculation based on magnetic anisotropy energy. This angular dependence of thermal conductivity indicates that the easy axis of $\alpha$-Fe$_2$O$_3$ nanowires is different from bulk $\alpha$-Fe$_2$O$_3$ due to the geometric anisotropy.

cond-mat.mes-hall↗

arXiv2021

Thermal percolation and electrical insulation in composite materials with partially metallic coated fillers

We propose a type of thermal interface materials incorporating insulating nanowires with partially metallic coating in insulating polymer matrix.Large thermal conductivity can be obtained due to thermal percolation while the electrical insulation is maintained by controlling $C_{\rm M}\varphi < \varphi_{\rm c}^{\rm e}$ and $\varphi > \varphi_{\rm c}^{\rm th}$, where $\varphi$ is the volume fraction of filler, $C_{\rm M}$ is the metallic coating fraction, $\varphi_{\rm c}^{\rm e}$ and $\varphi_{\rm c}^{\rm th}$ are the electrical and thermal percolation thresholds, respectively. The electrical conductivity of such composite materials can further be regulated by coating configuration. In this regard, we propose the concept of "thermal-percolation electrical-insulation", providing a guide to design efficient hybrid thermal interface materials.

physics.app-ph↗

arXiv2020

A Ubiquitous Thermal Conductivity Formula for Liquids, Polymer Glass, and Amorphous Solids

The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time. There have been several different approaches to explain the thermal conductivities for these systems, for example, the Bridgman's formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers. Here, we present a ubiquitous formula to explain the thermal conductivities of liquids and amorphous solids in a unified way. The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data for these systems. Our formula is not only providing detailed implications on microscopic mechanisms of heat transfer in these systems, but also solves the discrepancies between existing formulae and experimental data.

cond-mat.soft↗

arXiv2020

Thermal relaxation of magnons and phonons near resonance points in magnetic insulators

We theoretically investigate the energy relaxation rate of magnons and phonons near the resonance points to clarify the underlying mechanism of heat transport in ferromagnetic materials. We find that the simple two-temperature model is valid for the one-phonon/one-magnon process, as the rate of energy exchange between magnons and phonons is proportional to the temperature difference between them, and it is independent of temperature in the high temperature limit. We found that the magnon-phonon relaxation time due to the one-phonon/one-magnon interaction could be reduced to 1.48 $\mu s$ at the resonance point by applying an external magnetic field. It means that the resonance effect plays a significant role in enhancing the total magnon-phonon energy exchange rate, apart from the higher order interaction processes.

cond-mat.mes-hall↗

arXiv2019

Phonon renormalization induced by electric field in ferroelectric P(VDF-TrFE) nanofibers

We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the phonon spectrum when the atoms are pulled away from their equilibrium positions by the electric field. Our finding provides an efficient way to manipulate the thermal conductivity by tuning external fields in ferroelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

A Thermal Resistance Network Model for Heat Conduction of Amorphous Polymers

Thermal conductivities (TCs) of the vast majority of amorphous polymers are in a very narrow range, 0.1 $\sim$ 0.5 Wm$^{-1}$K$^{-1}$, although single polymer chains possess TC of orders-of-magnitude higher. Entanglement of polymer chains plays an important role in determining the TC of bulk polymers. We propose a thermal resistance network (TRN) model for TC in amorphous polymers taking into account the entanglement of molecular chains. Our model explains well the physical origin of universally low TC observed in amorphous polymers. The empirical formulae of pressure and temperature dependence of TC can be successfully reproduced from our model not only in solid polymers but also in polymer melts. We further quantitatively explain the anisotropic TC in oriented polymers.

cond-mat.soft↗

arXiv2018

Off-center Rattling Triggers High-Temperature Thermal Transport in Thermoelectric lathrates: Non-Perturbative Approach

Lattice thermal conductivities $\kappa_{\rm L}$ of type-I clathrates encapsulating "off-center" guest atoms behave in a way that is sharply different from those of conventional crystals such as on-center type-I clathrates. The latter decreases with increasing temperature according to 1/$T$ above a few tens\,K. While, $\kappa_{\rm L}$ of off-center clathrates increase T-linearly above a few tens K. Further increasing temperature above $T\gtrsim$100 K, $\kappa_{\rm L}$ saturates without exhibiting appreciable T-dependence. In this temperature regime, the standard formulation of $\kappa_{\rm L}$ based on the perturbation theory is unfeasible. We herein present a theory of $\kappa_{\rm L}$ based on "non-perturbative" approach by taking into account the interaction between non-vibrational off-center rattling states of guest atoms and cage shells. It is remarkable that our theory shows excellent agreement with observed $\kappa_{\rm L}(T)$ for off-center clathrates in magnitudes, the cage volume $\Omega$-dependence and T-dependence.

cond-mat.mtrl-sci↗

arXiv2018

Thermal Conductivity of Suspended Few-Layer MoS2

Modifying phonon thermal conductivity in nanomaterials is important not only for fundamental research but also for practical applications. However, the experiments on tailoring the thermal conductivity in nanoscale, especially in two-dimensional materials, are rare due to technical challenges. In this work, we demonstrate in-situ thermal conduction measurement of MoS2 and find that its thermal conductivity can be continuously tuned to a required value from crystalline to amorphous limits. The reduction of thermal conductivity is understood from phonon-defects scatterings that decrease the phonon transmission coefficient. Beyond a threshold, a sharp drop in thermal conductivity is observed, which is believed to be a crystalline-amorphous transition. Our method and results provide guidance for potential applications in thermoelectrics, photoelectronics, and energy harvesting where thermal management is critical with further integration and miniaturization.

cond-mat.mes-hall↗

arXiv2018

Dimensional crossover of heat conduction in amorphous Polyimide nanofibers

The mechanism of thermal conductivity in amorphous polymers, especially polymer fibers, is unclear in comparison with that in inorganic materials. Here, we report the observation of across over of heat conduction behavior from three dimensions (3D) to quasi-one dimension (1D) in Polyimide(PI) nanofibers at a given temperature. A theoretical model based on the random walk theory has been proposed to quantitatively describe the interplay between the inter-chain hopping and the intra-chain hopping in nanofibers. This model explains well the diameter dependence of thermal conductivity and also speculates the upper limit of thermal conductivity of amorphous polymers in the quasi-1D limit.

physics.app-ph↗

arXiv2017

Hopping Processes Explain T-linear Rise of Thermal Conductivity in Thermoelectric Clathrates above the Plateau

Type-I clathrate compounds with off-center guest ions realize the phonon-glass electron-crystal concept by exhibiting almost identical lattice thermal conductivities $\kappa_{\rm L}$ to those observed in network-forming glasses. This is in contrast with type-I clathrates with on-center guest ions showing $\kappa_{\rm L}$ of conventional crystallines. Glasslike $\kappa_{\rm L}$ stems from the peculiar THz frequency dynamics in off-center type-I clathrates where there exist three kinds of modes classified into extended(EX), weakly(WL) and strongly localized(SL) modes as demonstrated by Liu $et.\,al.$, Phys. Rev. B $\textbf{93}$, 214305(2016). Our calculated results based on the hopping mechanism of SL modes via anharmonic interactions show fairly good agreement with observed ${\it T}$-linear rise of $\kappa_{\rm L}$ above the plateau. We emphasize that both the magnitude and the temperature dependence are in accord with the experimental data of off-center type-I clathrates.

cond-mat.other↗