Natural biological systems make use of capillary-type hierarchical structures in order to enhance surface functionality within limited size. This paper discusses fabrication of similar synthetic structures by grafting carbon nanotubes (CNTs) on microcellular substrates such as graphitic foam. A major hurdle so far had been deposition of dense CNT layers inside uneven pores. This has been overcome in this study by pre-coating the porous surface with plasma-derived silica molecules. It is seen that the pre-coating not only increases the density of nanocatalyst attachment on the surface but also makes each nanocatalyst more effective in nucleation and growth of nanotubes. The CNT layers formed are strongly attached to the substrate, which makes them particularly suitable for use in robust hierarchical devices in the future.
Polycrystalline diamond thin films are grown on a p-type Si (100) single crystal substrate at a low surface deposition temperature of 455°C using a microwave plasma enhanced chemical vapor deposition process in an Ar-rich Ar∕H2∕CH4 plasma containing different oxygen levels from 0% to 0.75%. The surface deposition temperatures are measured and monitored by an IR thermometer capable of working in a plasma environment without any interference from the plasma emissions. The lower surface deposition temperature at high microwave power of 1300W and higher gas pressure of 95torr is achieved by active cooling of the substrate from the backside using a specially designed cooling stage. An enhanced growth rate from 0.19to0.63μm∕h is observed with varying oxygen from 0% to 0.75% in the plasma. Diamond grain size also increased from 0.69μm for the sample with no oxygen to 1.74μm for the sample with 0.75% oxygen. The diamond films are characterized using x-ray diffraction, environmental scanning electron microscopy field emission gun, Raman spectroscopy, and x-ray photoelectron spectroscopy. The enhanced growth rate is correlated with the enhanced atomic hydrogen to C2 ratio with increasing oxygen concentration in the plasma, which is measured by an in situ optical emission spectroscopy.
This paper demonstrates greatly improved specific power (W/g) for encapsulated phase change materials (EPCM) as a result of modified interface morphology. Carbon nanotubes are strongly attached to the interior walls of the graphitic foam encapsulation. Microstructure analysis using scanning electron microscopy (SEM) indicates that the wax infiltrates into the carbon nanotubes (CNT) forest and creates an intimate contact with increased interfacial area between the two phases. Specific power has been calculated by measuring thermal response times of the phase change materials using a custom system. The carbon nanotubes increase the specific power of the encapsulated phase change materials by about 27% during heating and over 146% during the more important stage of latent heat storage. Moreover, SEM images of the interface after repeated thermal cycling indicate that the presence of CNT may also improve durability of the EPCM by preventing interfacial gaps and maintaining improved contact between the graphite and PCM phases.
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