arXiv subjects

Hong Fang

Publications and source records attributed to Hong Fang.

8 recordsLinked to original sources

arXiv2020

U-carbon: metallic and magnetic

We report the discovery of a pristine crystalline 3D carbon that is magnetic, electrically conductive and stable under ambient conditions. This carbon material, which has remained elusive for decades, is synthesized by using the chemical vapor deposition (CVD) technique with a particular organic molecular precursor 3,3-dimethyl-1-butene (C6H12). An exhaustive computational search of the potential energy surface reveals its unique sp2-sp3 hybrid bonding topology. Synergistic studies involving a large number of experimental techniques and multi-scale first-principles calculations reveal the origin of its novel properties due to the special arrangement of sp2 carbon atoms in lattice. The discovery of this U-carbon, named such because of its unusual structure and properties, can open a new chapter in carbon science.

cond-mat.mtrl-sci↗

arXiv2020

Efficient mesh deformation using radial basis functions with a grouping-circular-based greedy algorithm

A grouping-circular-based (GCB) greedy algorithm is proposed to promote the efficiency of mesh deformation. By incorporating the multigrid concept that the computational errors on the fine mesh can be approximated with those on the coarse mesh, this algorithm stochastically divides all boundary nodes into $m$ groups and uses the locally maximum radial basis functions (RBF) interpolation error of each group as an approximation to the globally maximum one of all boundary nodes in each iterative procedure for reducing the RBF support nodes. For this reason, it avoids the interpolation conducted at all boundary nodes and thus reduces the corresponding computational complexity from $O\left({N_c^2{N_b}} \right)$ to $O\left( {N_c^3} \right)$. Besides, after $m$ iterations, the interpolation errors of all boundary nodes are computed once, thus allowing all boundary nodes can contribute to error control. Two canonical deformation problems of the ONERA M6 wing and the DLR-F6 Wing-Body-Nacelle-Pylon configuration are computed to validate the GCB greedy algorithm. The computational results show that the GCB greedy algorithm is able to remarkably promote the efficiency of computing the interpolation errors in the data reducing procedure by dozens of times. Because an increase of $m$ results in an increase of $N_c$, an appropriate range of $\left[ {{N_b}/{N_c},{\rm{ }}2{N_b}/{N_c}}\right]$ for $m$ is suggested to prevent too much additional computations for solving the linear algebraic system and computing the displacements of volume nodes induced by the increase of $N_c $. The results also show that the GCB greedy algorithm tends to generate a more significant efficiency improvement for mesh deformation when a larger-scale mesh is applied.

math.NA↗

arXiv2019

An accurate moving wall boundary algorithm for Direct Simulation of Monte Carlo in unsteady rarefied flow

An accurate algorithm is proposed to improve the prediction of a particle in collision with a moving wall within the direct simulation Monte Carlo (DSMC) framework for the simulation of unsteady rarefied flows. This algorithm is able to predict the particle-wall collision in a coupled manner by removing the assumption employed by the approximate algorithm, in that the wall is frozen during the collision. The trajectory equation of the particle is theoretically constructed in a moving object coordinate system. It can accurately describe the geometries of the collision between a particle and an arbitrarily shaped object of which the motion incorporates both translation and rotation, thus allowing to deal with complex problems. In contrast, the approximate algorithm ignores the effect of the moving wall on the particle movement during the collision, and therefore induces error that is an increasing function of the wall velocity. Four rarefied flow problems are applied to validate the accurate algorithm. It is shown that the algorithm can produce results perfectly consistent with the Maxwellian theoretical solutions and ensure particle conservation to avoid gas leakage. It is also shown in a three-dimensional case of a re-entry module that the steady simulation fails to reproduce the hysteresis effect while the unsteady simulation using the accurate algorithm can do that, indicating that the unsteady simulation with an appropriate algorithm as proposed in the present work is essentially required in such applications.

physics.comp-ph↗

arXiv2014

The Ag...Ag dispersive interaction and exotic physical properties of Ag3Co(CN)6

We report a density functional theory (DFT) study of Ag3Co(CN)6, a material noted for its colossal positive and negative thermal expansion, and its giant negative linear compressibility. Here we explicitly include the dispersive interaction within the DFT calculation, and find that it is essential to reproduce the ground state, the high-pressure phase, and the phonons of this material; and hence essential to understand this material's remarkable physical properties. New exotic properties are predicted. These include heat enhancement of the negative linear compressibility, a large reduction in the coefficient of thermal expansion on compression with change of sign of the mode Grueneisen parameters under pressure, and large softening of the material on heating. Our results suggest that these are associated with the weak Ag...Ag dispersive interactions acting with an efficient hinging mechanism in the framework structure.

cond-mat.mtrl-sci↗

arXiv2014

Common origin of exotic properties in ceramic and hybrid negative thermal expansion materials

Many ceramic and hybrid metal-organic framework materials show negative thermal expansion (NTE): they \textit{contract} instead of expanding on heating \cite{Barrera_Miller_Lind_Romao 2005}. Their structures are invariably characterised as a network of polyhedral groups of atoms that are connected through sharing of corner atoms or by shared ligands. Empirically, NTE materials tend to show pressure-induced softening, pressure enhancement of NTE, and the reduction of NTE on heating. But such effects have only been investigated in a small number of materials \cite{Pantea 2006,Chapman 2005,Chapman 2007,Fangexp 2013}, and as yet there is no general framework for understanding the whole suite of properties together. By studying models with Hamiltonians chosen to reflect the physical picture generally accepted as responsible for NTE in framework materials, we demonstrate that NTE, pressure-enhanced NTE, and pressure-induced softening naturally emerge together. We then show how anharmonic interactions lead to structural warm hardening---something that has only previously been seen in laser-excited warm-dense matter \cite{Ernstorfer 2009}---as well as to the transition from NTE to positive thermal expansion and the disappearing of the pressure-induced softening at high temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Temperature-dependent pressure-induced softening in Zn(CN)$_{2}$

We investigate the temperature dependence of the pressure-induced softening in the negative thermal expansion material Zn(CN)$_2$ using neutron powder diffraction and molecular dynamics simulations. Both the simulation and experiment show that the pressure-induced softening only occurs above a minimum temperature and also weakens at high temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Simulation study of pressure and temperature dependence of the negative thermal expansion in Zn(CN)$_2$

Pressure and temperature dependence of the negative thermal expansion in Zn(CN)$_2$ is fully investigated using molecular dynamics simulations with a built potential model. The advantage of this study allows us to reproduce all the exotic behaviours of the material, including the negative thermal expansion (NTE), the reduction of NTE with elevated temperature, the pressure enhancement of NTE and the pressure-induced softening. Results of the study provide us detailed data to link the properties in the energy space and the real space, giving us insights to understand the properties and the connections between them.

cond-mat.mtrl-sci↗

arXiv2013

Pressure-induced softening as a common feature of framework structures that have negative thermal expansion

Results of a series of molecular dynamics simulations of cubic siliceous zeolites suggest that pressure-induced softening -- the phenomenon in which a material becomes progressively more compressible under pressure -- is likely to be a common feature of framework materials that show negative thermal expansion. The correlation between the negative thermal expansion and the pressure-induced softening is investigated on the basis of thermodynamics.

cond-mat.mtrl-sci↗