Application of Carbon Nanotubes and Fullerenes for Thermal Management in Ceramics
- PDF / 1,795,732 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 41 Downloads / 161 Views
Application of Carbon Nanotubes and Fullerenes for Thermal Management in Ceramics Leonard L. Yowell, Brian M. Mayeaux1, Hsin Wang2, and Enrique V. Barrera Mechanical Engineering and Materials Science Department, Rice University, 6100 Main St., Houston, TX 77005 1 currently at NASA Johnson Space Center, Houston, TX 2 High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064 ABSTRACT New techniques for thermal management in ceramics at the nanoscale level have been investigated using low percentages of carbon nanotubes to reduce thermal conductivity of bulk ceramics. Samples of yttria-stabilized zirconia containing purified single-walled carbon nanotubes (SWNT) or vapor grown carbon fibers (VGCF) have been prepared by tape casting and analyzed using the laser flash method to evaluate reductions in thermal conductivity at high temperatures. New features in the samples due to the presence of carbon nanotubes have been characterized using Raman, SEM, TEM and, in the case of VGCFs, are related to significant reductions in thermal conductivity (>25%). The inclusion of a low percentage of nanoscale carbon fibers, the intimate relationship between the fibers and ceramic particles, and the indication that the fibers possess a crystalline overcoating, all contribute to the lowering of the thermal conductivity. INTRODUCTION Single walled carbon nanotubes (SWNTs) possess highly anisotropic thermal properties. Theoretical calculations as well as some initial experimental efforts [1] have demonstrated that the longitudinal thermal conductivity is very high and within an order of magnitude of graphite or diamond (∼2000 W/mK). In the transverse direction, the thermal conductivity is low and similar to that of C60 (0.4 W/mK) [2]. In addition to possessing low intrinsic thermal conductivity, C60 molecules act as point defects and scatter phonons effectively when dispersed among ceramic grains [3,4], significantly reducing thermal conductivity when low and intermediate temperatures were studied. The present effort involves the creation of nanoscale ceramic defects using nanotubes as structural members for the creation of positive ceramic templates for high temperature applications. Rao et al. [5] demonstrated that multi-walled carbon nanotubes could be coated with zirconia precursors and then burned out to leave behind nanoscale ceramic template structures. While at high temperatures nanotubes may not remain in the final ceramic product, it is essential that they remain intact long enough for the unique nanotube-based ceramic structures to be formed. The creation of new microstructural features or porosity is advantageous when attempting to reduce or control the thermal conductivity of ceramic insulating materials over a broad temperature range. Considering the high temperature stability of vapor grown carbon fibers (VGCFs), it is possible for some forms of carbon nanotubes to remain intact in the final system, but with a ceramic crystalline coating. Either situation should lead to reductions in the
Data Loading...
