Characterization of carbon aerogels by transport measurements
- PDF / 1,202,019 Bytes
- 11 Pages / 576 x 792 pts Page_size
- 93 Downloads / 210 Views
Z. H. Wang Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 K. Lu Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
R.W. Pekala Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, California 94550 (Received 26 August 1992; accepted 8 April 1993)
Carbon aerogels are a special class of low-density microcellular foams. These materials are composed of interconnected carbon particles with diameters of approximately 10 nm. The temperature dependence of the dc electrical resistivity and magnetic susceptibility (x) fr°m 4 K to room temperature, magnetoresistance (MR) in a magnetic field up to 15 T, and Raman scattering were measured as a function of aerogel density. While Raman scattering measurements are not sensitive to variations in density, the x data show that there are more free carriers in samples of higher density. Aerogel samples with different densities all show a negative temperature coefficient of resistivity and a positive MR. The less dense samples exhibit a stronger temperature dependence of resistivity and a stronger field dependence of the MR, indicating that with decreasing density and increasing porosity, charge carriers are more localized. Data analysis precludes variable-range hopping in favor of nearest-neighbor hopping and fluctuation-induced tunneling as the most likely conduction mechanisms for carbon aerogels.
I. INTRODUCTION
Interest in the physics and chemistry of small clusters of atoms (1-100 nm in size) has grown considerably in recent years because these assemblies often have properties between the molecular and bulk solid-state limits. The different properties can be explained in terms of the large fraction of atoms that are at the surface of a cluster as compared to the interior. Although the synthesis and properties of metal and semiconductor clusters (e.g., quantum dots) are the subject of extensive investigations, little attention has been paid to clusterassembled porous materials.1'2 This oversight is of particular interest to us since we believe that aerogels are one of the few monolithic materials presently available where the benefits of cluster assembly can be demonstrated. In particular, the unique optical, thermal, acoustic, mechanical, and electrical properties of aerogels are directly related to their nanostructure, which J. Mater. Res., Vol. 8, No. 8, Aug 1993
http://journals.cambridge.org
Downloaded: 13 May 2014
is composed of interconnected clusters (3-25 nm) with small interstitial pores (0.4 g/cm 3 , where the resistivity is only weakly dependent on the mass density. At low temperatures, p (4.2 K) for carbon aerogels with different densities differs by several orders of magnitude, thus providing a more sensitive characterization par
Data Loading...
