43672-AC10
Thermal Transport across Organic/Inorganic Interfaces
While most nanostructures investigated to-date have been fabricated from purely inorganic materials, the study of organic/inorganic and bio/inorganic hybrid nanostructures is an important research frontier with enormous potential applications in areas such as bioMEMs and microelectronics. A key maintenance task in all nanostructures is to efficiently move this heat away from the active components into heat-management structures such that the functionality and structural integrity of the nanostructure are not compromised Indeed, there is a growing recognition that in many cases heat-management structures have to be designed into devices with the same care that is given to the design of the primary functionality. The issue of thermal management in organic/inorganic systems is potentially even more critical since the very structure of many organic materials can be irreversibly compromised by even rather modest temperatures increases.
We have addressed the question as to how high the interfacial conductance of an organic/inorganic interface could be under optimum conditions. To address this question, we used atomic-level simulation methods to explore the thermal transport properties across highly idealized polyethylene/diamond interfaces, which we take as a prototypical organic/inorganic system. We have found that, in principle organic/inorganic interfaces should be able to conduct heat as well as inorganic/inorganic interfaces if there is strong bonding at the interface itself.
Before addressing the interface itself, we determined the thermal conductivity of the polymer alone. We determined the axial thermal conductivity for a polyethylene crystal of infinite molecular weight to be 310 ± 190 W/m•K. This is considerably larger than either the extrapolated values of 40 and 70 W/m•K determined experimentally. One of the reasons for the discrepancy between our calculated values and the experimental values is probably the absence of imperfections in the simulated PE structure. We thus characterized the effects of adding defects in the form of unsaturated carbons. Because of the shorter, and thus stronger C-C bond lengths, these segments have a higher intrinsic thermal conductivity than the polyethylene itself. It turns out that the detrimental effect of the defects is almost exactly balanced by the increase in the thermal conductivity due to the superior intrinsic thermal transport properties of the segments added. By contrast, when polyethylene defects are added to polyacetylene, both the defects themselves and their lower intrinsic thermal conductivity lead to a strong decrease in the overall thermal conductivity.
The calculated values for the polyethylene/diamond interfacial conductance range from 690 to 930 MW/m2K, depending on the surface orientation of the diamond crystal. Although these values are extremely large, they are not without precedent. In particular, strongly bonded inorganic interfaces give interfacial conductances that reach values of hundreds of MW/ m2•K. Also, from the earlier molecular-dynamics simulations (partly funded by this project), the interfacial conductance in diamond was predicted to be 9-17 GW/m2•K depending on the type of grain boundary. Moreover, a simulation of a model diamond nanocrystal containing a number of high-angle (001) grain boundaries yielded an average interfacial conductance of ~4.5 GW/m2•K, a result that was reasonably consistent with the experimentally derived value ~3 GW/m2•K for ultrananocrystalline diamond thin films.
Our simulations demonstrate the possibility of extremely high thermal transport in polyethylene and across polyethylene/diamond interfaces. Adhesion of adsorbates to the diamond surface can be dramatically increased by surface dehydrogenation and creation of reactive sites such as C· radicals, polar C=O and π- bonded C=C groups. Thus polyethylene bonding to the diamond may be viable but will require additional chemical treatment. It may thus be anticipated that the increasing ability to functionalize surfaces may lead to methods for the systematic development of inorganic/organic interfaces with conductances similar to those of interfaces between inorganic materials.
