- Nano Express
- Open Access
Experimental Study on Thermal Conductivity and Hardness of Cu and Ni Nanoparticle Packed Bed for Thermoelectric Application
© The Author(s). 2017
- Received: 3 January 2017
- Accepted: 28 February 2017
- Published: 11 March 2017
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
- Thermoelectric materials
- Nanoparticle packed bed
- Nanoporous material
- Vickers hardness
For thermoelectric materials, the efficiency of thermoelectric energy conversion can be described by the materials’ thermoelectric figure of merit (ZT), which can be defined as ZT = S 2 σT/(k p + k e + k bipolar) , where S, σ, T, k p , k e , and k bipolar are the Seebeck coefficient, electrical conductivity (EC), temperature, and the lattice, electronic, and bipolar parts of the thermal conductivity (TC), respectively. ZT is mainly determined by the materials’ effective TC, EC, temperature, and Seebeck coefficient. A good thermoelectric material should possess high S to achieve high voltage output, high σ to reduce Joule heat loss, and low k (effective TC) to maintain a large temperature difference. A normal way to optimize ZT of a material is to increase the power factor S 2 σ by optimizing the carrier concentration and to reduce k p by introducing scatter centers to the phonons. In order to improve the thermoelectric materials’ ZT, the method of increasing σ and/or S and/or decreasing k by tuning the structure of thermoelectric materials is widely applied. The reduction of TC sometimes also leads to a decrease of EC, contrary to our goal to decrease TC and increase EC at the same time. To minimize the contradiction, nanoporous materials have drawn wide attention . Nanoporous materials with high density of interfaces offer an alternative to superlattices as potential thermoelectric materials , because the presence of closed or connected pores in porous materials provides an effective scattering mechanism for mid- and long-wavelength phonons that contribute heavily to TC . So, the nanoporous materials are not only potential thermoelectric materials, but also high-performance insulation materials [5, 6]. In spite of the potential applications, the study of the thermal physical properties of nanoporous materials is also valuable for understanding the underlying mechanism. In this paper, we focus on the TC and hardness of a kind of nanoporous material to illustrate the potential advantage of nanoporous materials for thermoelectric application.
There are already some methods to prepare nanoporous materials, for example, the dealloying method [7–10], organics pyrolytic method [11, 12], sol-gel method [13–15], etc. While these methods are usually applied to prepare isotropic nanoporous materials, there is no interface existing in these nanoporous materials which is preferable for getting a low TC. For introducing interfaces into nanoporous materials, the nanoparticle tableting method [16–21] is applied to prepare the nanoporous materials in this paper. A force is usually applied to press the nanoparticles into large-scale nanoporous materials in the tableting method. There are three different ways to prepare nanoparticle tablets [22–28]: the hot-pressing method, the cold-pressing method, and the self-forming method. Different methods will lead to different porosities. The porosity of a nanoparticle powder can be larger than 85% in the absence of the pressing method applied [29, 30]. For the hot-pressing method, the porosity of the nanoparticle tablet can be lower than 10% , while the porosity could span a large scale for the cold-pressing method. Nanoporous tablet which is composed of air and nanoparticles is also called a nanoparticle packed bed (NPB). The cold-pressing method is applied in this paper to obtain a large span of porosity, which ranges from 15 to 40%.
In this paper, with the Cu and Ni nanoparticles pressed into NPBs, respectively, the relationship between the tableting pressure and the porosity is investigated to indicate the connection between the porosity and the nanostructure. Then, the TC of NPBs at low porosities (15–40%) are experimentally studied, resulting in the discovery of a different decreasing tendency of TC versus porosity than that currently known, which is explained herein. Meanwhile, the Vickers hardness (HV) of the NPBs at different porosities is also measured to assess the mechanical properties of NPBs additionally. Finally, the TC and the HV are both discussed for figuring out a proper porosity which is suitable for thermoelectric application.
A Tukon™ 1102/1202 Vickers hardness tester is applied to measure the HV hardness. In the experiment, a force of 9.8 N is applied to press the diamond probe into the samples, and the force is maintained for 15 s. Finally, the HV of the materials can be reflected by the size of the surface indentation. To obtain the average hardness, five different locations on each NPB are selected to measure the HV. The relative error of the HV is less than 8.3%. The randomness of the testing locations should be responsible for this deviation.
To reveal the relationship between TC and nanostructures of NPBs, we firstly probe the influence of pressing pressure on the porosity of NPBs. Then, the TC and the HV at different porosities are discussed. Finally, with the thermal and mechanical properties considered, the most suitable porosity in NPBs is indicated for thermoelectric application.
For convenience, the tendency is divided into three parts denoted as parts I, II, and III, respectively. In part I (φ ≥ 0.3), the TC increases rapidly with decreasing porosity. This is caused by the increase of heat-transfer routes due to the increases of particle contacts. In part II (0.26 < φ < 0.30), the increase of thermal conductivity with decreasing porosity is much lower than that in part I. This is thought to be caused by the inhomogeneous porosity in NPBs. With a larger pressure loaded in part II, extrusion deformation emerges among some nanoparticles. Although a larger pressure will lead to more extrusion deformation among nanoparticles, there are still some nanoparticles that remain unchanged in the inner part of NPBs. The large thermal resistance in these unchanged separated nanoparticles will dominate the resistance. Therefore, if there are still some nanoparticles that have not been deformed, the effective TC will not be obviously increased. At the end of part II, extrusion deformation has happened for most of the nanoparticles. With a larger pressure loaded, severe deformation will occur in part III (this can be seen from the SEM image in Fig. 5a). A severe deformation will lead to a larger contact interface. With an increase of the area of the contact interface, the TC of Cu NPBs increases rapidly. At the end of the part III, the TC tends to be a constant. Similar tendencies of TC-versus-porosity are also observed for Ni NPBs in Fig. 5b, which confirms the above discussed result about Cu NPBs. In conclusion, the porosity dominates the TC of the NPBs in part I while the inhomogeneous porosity dominates in part II, and the TC depends on the contact area between nanoparticles in part III.
A kind of nanoporous material which is a potential thermoelectric material is discussed in this paper. We try to probe a way to decrease the TC of NPBs and to enhance the hardness of NPBs at the same time. With nanostructures of NPB illustrated by SEM, the relationship between the pressing pressure and the porosity is discussed. The TC of NPBs is measured with a hot-wire method, and the TPS method is applied as a testing group. The porosity dominates the TC of the NPB in large porosities, while the TC depends on the contact area between nanoparticles in small porosities. The HV of NPBs at different porosities is also measured with a Vickers hardness tester. Results turn out that when the porosity of NPBs becomes smaller than 0.25, the HV tends to be a constant like what happens for TC. 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 in this paper, because of the low TC and high hardness. Although Cu and Ni are not good thermoelectric materials, the discussion in this paper should be also true for other materials, for which the proper porosity may be different.
NPB Nanoparticle packed bed
TC Thermal conductivity
HV Vickers hardness
ZT Thermoelectric figure of merit
k Effective thermal conductivity
kp Phonon thermal conductivity
k e Electronic thermal conductivity
k bipolar Bipolar thermal conductivity
S Seebeck coefficient
ρ 1 Density of the NPB
ρ 2 Density of the bulk
σ Electric conductivity
* Dimensionless quantity
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51406224 and 51436001).
All sources of funding for the research reported should be declared. We admit that no other funding should be declared except grants from the National Natural Science Foundation of China (Grant Nos. 51406224 and 51436001), which were added in the Acknowledgements section, for funding, interpretation of the data, and writing the manuscript.
ZL contributed to the experimental method setup, experiment, and manuscript preparation. CH contributed to the experimental method setup. WZ contributed to the experiment. YF contributed to the manuscript preparation. XZ contributed to the experimental method setup. GW contributed to the experimental method setup. All authors read and approved the final manuscript.
All financial and non-financial competing interests must be declared in this section. The authors declare that they have no competing interests.
Ethics Approval and Consent to Participate
We admit that ethical identity is not involved.
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