Direct Energy Conversion System
Direct energy conversion system
Direct energy conversion systems that could operate in the 100-700°C temperature range with high efficiencies (>30%) provide an attractive compact alternative to internal combustion engines for many military applications. They will also expand the possibilities for waste heat recovery applications. The core of the solution we are proposing is a metal/semiconductor nanocomposite that will allow us to modify four intrinsic material properties in order to fabricate more efficient thermoelectric systems. The concept of a metal/semiconductor nanocomposite as a solid-state thermionic material represents a radical alternative to conventional homogeneous thermoelectric materials. Instead of focusing on semiconducting materials with a highly asymmetric density of states about the Fermi level yielding an optimal doping level of about 10^19/cm^3 at room temperature, the metal/semiconductor nanocomposite concept utilizes the Schottky barrier to filter the electron energy distribution, creating a large difference between the average energy of the conduction electron and the Fermi energy. The high electron density in the metal (>10^22/cm^3) compensates for the negligible contribution to the conductivity from the majority of the electrons in the metal that have energies below the top of the barrier. The high interface density and/or nanoscale embedded nanoparticles in a metal/semiconductor nanocomposite are expected to suppress the transport of mid-long wavelength phonons. This is very important to reduce the lattice contribution to thermal conductivity below the alloy limit. The ability to tune the properties by controlling layer thicknesses and nanoparticles sizes as well as manipulating lattice mismatch and barrier height by alloying offers additional degrees of freedom for materials design. Modeling of the transport properties of metal/semiconductor superlattices suggested that ZT values in excess of 4-5 should be possible if the barrier
Direct energy conversion systems that could operate in the 100-700°C temperature range with high efficiencies (>30%) provide an attractive compact alternative to internal combustion engines for many military applications. They will also expand the possibilities for waste heat recovery applications. The core of the solution we are proposing is a metal/semiconductor nanocomposite that will allow us to modify four intrinsic material properties in order to fabricate more efficient thermoelectric systems. The concept of a metal/semiconductor nanocomposite as a solid-state thermionic material represents a radical alternative to conventional homogeneous thermoelectric materials. Instead of focusing on semiconducting materials with a highly asymmetric density of states about the Fermi level yielding an optimal doping level of about 10^19/cm^3 at room temperature, the metal/semiconductor nanocomposite concept utilizes the Schottky barrier to filter the electron energy distribution, creating a large difference between the average energy of the conduction electron and the Fermi energy. The high electron density in the metal (>10^22/cm^3) compensates for the negligible contribution to the conductivity from the majority of the electrons in the metal that have energies below the top of the barrier. The high interface density and/or nanoscale embedded nanoparticles in a metal/semiconductor nanocomposite are expected to suppress the transport of mid-long wavelength phonons. This is very important to reduce the lattice contribution to thermal conductivity below the alloy limit. The ability to tune the properties by controlling layer thicknesses and nanoparticles sizes as well as manipulating lattice mismatch and barrier height by alloying offers additional degrees of freedom for materials design. Modeling of the transport properties of metal/semiconductor superlattices suggested that ZT values in excess of 4-5 should be possible if the barrier