Abstract
The grain size of CVD (Chemical Vapor Deposition) graphene was controlled by changing the precursor gas flow rates, operation temperature, and chamber pressure. Graphene of average grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm was synthesized in high quality and full coverage. The possibility to tailor the thermoelectric conversion characteristics of graphene has been exhibited by examining the grain size effect on the three elementary thermal and electrical properties of σ, S, and k. Electrical conductivity (σ) and Seebeck coefficients (S) were measured in a vacuum for supported graphene on SiO2/Si FET (Field Effect Transistor) substrates so that the charge carrier density could be changed by applying a gate voltage (). Mobility (µ) values of 529, 459, and 314 cm2/V·s for holes and 1042, 745, and 490 cm2/V·s for electrons for the three grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm, respectively, were obtained from the slopes of the measured σ vs. graphs. The power factor (PF), the electrical portion of the thermoelectric figure of merit (ZT), decreased by about one half as the grain size was decreased, while the thermal conductivity (k) decreased by one quarter for the same grain decrease. Finally, the resulting ZT increased more than two times when the grain size was reduced from 4.1 µm to 0.5 µm.
Highlights
The thermoelectric effect enables direct energy conversions between temperature and electric voltage differences
When a temperature gradient is applied, the momentum difference between charge carriers causes them to shift to one side, yielding voltage potential inside the materials
Since it allows for the conversion of wasted heat into electrical energy, having control over the thermoelectric effect would give rise to one of the most promising sources of renewable energy, because the eco-friendly generation of electrical energy only requires a temperature difference to reuse the wasted heat energy
Summary
The thermoelectric effect enables direct energy conversions between temperature and electric voltage differences. Graphene has high potential for becoming a thermoelectric material due to its high electrical conductivity and Seebeck coefficient. In order to enhance graphene’s thermoelectric properties, many ideas have been proposed aimed at lowering its thermal conductivity, including defecNtancoomnatterioallsli2n01g8,[81, ]x FaOnRdPmEEaRnRaEgVeIEmWent of grain size [2,3].
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