We have used a modified Richardson-Dushman equation and have adopted a model where the collector temperature could be controlled through heat extraction in a calculated amount and a magnet can be attached on the back surface of the collector for future control of the space-charge effect. Such attractive electronic properties (e.g., good electrical conductivity and high dielectric constant) make engineered graphene a good candidate as an emitter and collector in a thermionic energy converter for harnessing solar energy efficiently. It is known that graphene can withstand temperatures as high as 4600 K in vacuum, and it has been shown that its work function can be engineered from a high value (for monolayer/ bilayer) of 4.6 eV to as low as 0.7 eV. Thermionic energy conversion (TEC) using nanomaterials is an emerging field of research. Theoretical studies of thermionic conversion of solar energy with graphene as emitter and collector," Abstract. © 2018 Society of Photo-Optical Instrumentation Engineers (SPIE). Phosphors like those are useful for both biological and engineering applications as biomarkers and for solar cell enhancers, respectively.
The UC luminescence CIE coordinates for the phosphors and the PMMA films are at the middle of the "green area" (0.3, 0.6). The PMMA composite films were synthesized by spin coating technique they have excellent transparency even after the incorporation of the phosphor content. The UC emission of these phosphors was the characteristic green UC emission from the Ho³⁺ ions, and the effect of Li⁺ and Yb³⁺ codoping was twofold (1) lithium's presence provokes a slight change into Gd2O3 structure, and (2) there is an energy transfer from the Yb³⁺ to Ho³⁺ ions. The phosphors were synthesized by the simple solvent evaporation technique and characterized by x-ray diffraction and scanning electron microscopy techniques (SEM) SEM observations reveal that these phosphors have a layered structure. Upconversion (UC) luminescence characteristics of Ho³⁺-, Yb³⁺-, and Li+-doped Gd2O3 phosphors and composite films of these phosphors into polymethyl methacrylate (PMMA) under continuous wave excitation with 980-nm laser light are reported. We also find that the model explains fairly well the J vs T data for carbon nanotubes, which is reported in a separate paper. The values of thermal expansion coefficient, α and surface density of charge, ns obtained with the use of the model are in excellent agreement with experiments.
The model gives us unique method of determination of the Fermi energy of graphene as a function of temperature. The corresponding values obtained for monolayer suspended graphene are: W0 = 4.592 ± 0.002 eV, EF0 = 0.203 ± 0.002 eV ns = 3.16x10¹² cm⁻². It provides a unique technique for accurate determination of work function, W0, Fermi energy, EF0 at 0 K and surface density of charge carriers, ns of graphene. For the first time we have derived an equation for the temperature (T) dependent work function (W(T)) containing terms up to fifth power of T which gives a modified Richardson-Dushman (MRDE) equation that fits excellently well the experimental data of thermionic current density, J vs temperature, T data for suspended monolayer graphene.
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