Thermo-Electric Properties of Cu and Ni Nanoparticles Packed Beds.

The hot-wire method and the four-probe resistivity method are applied to probe the thermal conductivity (k) and the electric conductivity (σ) of Cu and Ni nanoparticle packed beds (NPBs). A fitting method based on classical physical theory is devised to separate ke (electronic thermal conductivity) and kp (phonon thermal conductivity) from k at room temperature. Results turn out that kp only accounts for a small proportion of k (4-20%); the proportion decreases with increasing porosity or temperature. Most importantly, this fitting method provides a simple way to separate ke and kp from k at room temperature. The Wiedemann-Franz law is checked and is found to be unsuitable for NPBs. The Lorenz number (L) is calculated from measurements of ke, k, and σ. Results turn out that L is found to be 50-60 times that of the bulk. With a Seebeck coefficient (S) measured, the thermoelectric property of NPBs is also calculated. We find that the NPB possess an advantage in thermoelectric property than bulk, the thermoelectric figure of merit (ZT) of Ni (Cu) NPBs can be 20.17 (1.87) times that of bulk Ni (Cu). The effect of porosity on ZT is also discussed, and results show that a NPB with a small porosity is more preferable as a thermoelectric material. With a small porosity, ZT can be even 1.73 times that of a large porosity. Although metals are not good thermoelectric material, the method in this paper supplies a way to improve the thermoelectric property of other thermoelectric materials.

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.

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.