Congliang Huang

Experimental Study on Thermal Conductivity and Hardness of Cu and Ni Nanoparticle Packed Bed for Thermoelectric Application

The hot-wire method is applied in this paper to probe the thermal conductivity (TC) of Cu and Ni nanoparticle packed beds (NPBs). A different decrease tendency of TC versus porosity than that currently known is discovered. The relationship between the porosity and nanostructure is investigated to explain this unusual phenomenon. It is found that the porosity dominates the TC of the NPB in large porosities, while the TC depends on the contact area between nanoparticles in small porosities. Meanwhile, the Vickers hardness (HV) of NPBs is also measured. It turns out that the enlarged contact area between nanoparticles is responsible for the rapid increase of HV in large porosity, and the saturated nanoparticle deformation is responsible for the small increase of HV in low porosity. With both TC and HV considered, it can be pointed out that a structure of NPB with a porosity of 0.25 is preferable as a thermoelectric material because of the low TC and the higher hardness. Although Cu and Ni are not good thermoelectric materials, this study is supposed to provide an effective way to optimize thermoelectric figure of merit (ZT) and HV of nanoporous materials prepared by the cold-pressing method.

THERMAL CONDUCTIVITY OF NANOPOROUS GLASS ALUMINA FILM AND COMPOSITES

In this paper, cross-plane thermal conductivities of the nanoporous glass alumina film (NGAF) and its composites (Ag/NGAF, with Ag nanowires embedded in) were measured. And a model was setup to predict the thermal conductivity of the nanoporous material. Results show that the thermal conductivity of the NGAF is about 50 times smaller than that of the ceramic alumina. It is about 0.5 W ⋅ m-1⋅ K-1 and depends on both the pore radius and the porosity. The thermal conductivity of the Ag/NGAF is not larger than that of the NGAF. The contact resistance and the unfilled space between Ag nanowires and the matrix are responsible for that.

Specific heat capacity of nanoporous Al2O3

Based on Lindemanns criterion, a specific heat capacity model for nanoporous material was proposed by defining the surface-atom layer, to take the surface atoms and the volume atoms separately into account. The height of the surface-atom layer was determined from the experiment, and results show that only the first layer atoms on the surface should be separately considered for nanoporous Al2O3. The shape factor of the pore was also introduced in the model with values between 2 (for cylindrical pore) and 3 (for spherical pore) to characterize the morphology of the pore. It turns out experimentally that the specific heat capacity of the analyzed nanoporous Al2O3 is much larger than that of the bulk, which can be interpreted as due to the fact that the surface atom plays a more important role than the volume one. And the smaller the radius and/or the larger the porosity, which lead to a larger surface-volume ratio, the larger the specific heat capacity becomes. The nanoporous material could be a better heat storage medium than the corresponding bulk with a much lighter weight, smaller volume but higher heat storage capacity.

Influence of Nanopore Shapes on Thermal Conductivity of Two-Dimensional Nanoporous Material

The influence of nanopore shapes on the electronic thermal conductivity (ETC) was studied in this paper. It turns out that with same porosity, the ETC will be quite different for different nanopore shapes, caused by the different channel width for different nanopore shapes. With same channel width, the influence of different nanopore shapes can be approximately omitted if the nanopore is small enough (smaller than 0.5 times EMFP in this paper). The ETC anisotropy was discovered for triangle nanopores at a large porosity with a large nanopore size, while there is a similar ETC for small pore size. It confirmed that the structure difference for small pore size may not be seen by electrons in their moving.

Decreased Thermal Conductivity of Polyethylene Chain Influenced by Short Chain Branching

In this paper, we have studied the effect of short branches on the thermal conductivity of a polyethylene (PE) chain. With a reverse non-equilibrium molecular dynamics method applied, thermal conductivities of the pristine PE chain and the PE-ethyl chain are simulated and compared. It shows that the branch has a positive effect to decrease the thermal conductivity of a PE chain. The thermal conductivity of the PE-ethyl chain decreases with the number density increase of the ethyl branches, until the density becomes larger than about 8 ethyl per 200 segments, where the thermal conductivity saturates to be only about 40% that of a pristine PE chain. Because of different weights, different types of branching chains will cause a different decrease of thermal conductivities, and a heavy branch will leads to a lower thermal conductivity than a light one. This study is expected to provide some fundamental guidance to obtain a polymer with a quite low thermal conductivity.

Self-diffusion of lignite/water under different temperatures and pressure: A molecular dynamics study

Temperature and pressure have direct and remarkable implications for drying and dewatering effect of low rank coals such as lignite. To understand the microenergy change mechanism of lignite, the molecular dynamics simulation method was performed to study the self-diffusion of lignite/water under different temperatures and pressure. The results showed that high temperature and high pressure can promote the diffusion of lignite/water system, which facilitates the drying and dewatering of lignite. The volume and density of lignite/water system will increase and decrease with temperature increasing, respectively. Though the pressure within simulation range can make lignite density increase, the increasing pressure showed a weak impact on variation of density.

Near-field radiative heat transfer in mesoporous alumina*

The thermal conductivity of mesoporous material has aroused the great interest of scholars due to its wide applications such as insulation, catalyst, etc. Mesoporous alumina substrate consists of uniformly distributed, unconnected cylindrical pores. Near-field radiative heat transfer cannot be ignored, when the diameters of the pores are less than the characteristic wavelength of thermal radiation. In this paper, near-field radiation across a cylindrical pore is simulated by employing the fluctuation dissipation theorem and Green function. Such factors as the diameter of the pore, and the temperature of the material are further analyzed. The research results show that the radiative heat transfer on a mesoscale is 2~4 orders higher than on a macroscale. The heat flux and equivalent thermal conductivity of radiation across a cylindrical pore decrease exponentially with pore diameter increasing, while increase with temperature increasing. The calculated equivalent thermal conductivity of radiation is further developed to modify the thermal conductivity of the mesoporous alumina. The combined thermal conductivity of the mesoporous alumina is obtained by using porosity weighted dilute medium and compared with the measurement. The combined thermal conductivity of mesoporous silica decreases gradually with pore diameter increasing, while increases smoothly with temperature increasing, which is in good agreement with the experimental data. The larger the porosity, the more significant the near-field effect is, which cannot be ignored.

Thermal Conductivity of Mesoporous MCM-41 Studied by Molecular Dynamics Methods and Kinetic Theory

MCM-41 consists of a hexagonal array of long, unconnected cylindrical pores with diameters that can be tailored within the range 1.6–10nm. As a porous silica nano-material, MCM-41 is thought to have a special thermal conductivity and is a promising porous substrate for mesoporous composites with high or low thermal conductivity. The Equilibrium Molecular Dynamics numerical simulations of thermal conductivity of MCM-41 are performed in this paper. FB potential equation and procedure of annealing are employed to get the structure of MCM-41. The Green-Kubo method is used to calculate the thermal conductivity of MCM-41. At the same time, the kinetic method is used to predict the thermal conductivity of MCM-41 for comparison. It turns out that the shell thermal conductivities of MCM-41 distribute within a reasonable range and increases linearly as porosity decreases, approaching the thermal conductivity of aerogels.© 2010 ASME