الفهرس 
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Abstract
Energy supply to develop the power source is increasingly demanded. Conventional fossil fuel is a limited resource and nuclear power. Renewable energy such as solar, wind, hydroelectric and biomass energy is expected to become increasingly important as future energy sources [1-2]. Another source for energy is the waste heat where it is heat produced as a byproduct in power generation, industrial processes and electrical machines. Also it is available from natural sources such as geothermal reservoirs and solar energy. Thermoelectric materials which convert the waste heat into a usable electricity would save a significant amount of money through increasing efficiency and lowering fuel costs as well as being beneficial to the environment [3].
This fraction of heat can be converted depends on the intake and exhaust temperature which less efficient. However, since such a large amount of energy is freely available, the engineering problem then becomes one of economics, and choosing the technology and configuration which produces the best utilization of heat.
The thermoelectric devices used to generate electricity, thermoelectric generators, depend on temperature gradients to cause an electromotive-force that propels charge carriers (electron or hole) to one end of a material, and produce potential difference. The reverse is also true, thermoelectric refrigerators use a power source to drive a current of charge carriers that result in a temperature gradient [4].
Improving the efficiency of thermoelectric devices could make it possible to utilize waste heat from currently neglected sources. The sources of automotive waste heat, e.g. exhaust, brakes, and engines, can be used to increase the efficiency of future hybrid automobiles. Industrial plants, electrical plants, and nuclear power plants, could provide a vital new source of energy by harnessing the heat they produce [4].
Thermoelectric devices have the potential to provide an alternate energy source and be part of the solution to break our dependence on fossil fuel energy. The present thermoelectric research is split into two ways:
(a) Searching for new bulk-materials.
(b) Focusing on low-dimensional systems.
New bulk materials of particular interest act like glass with regards to thermal conductivity (low), while simultaneously acting like a crystal when it comes to electrical conductivity (high). These bulk materials are referred to as phonon- glass electronic-crystals and contain weakly bound atoms that provide scattering centers for phonons [5]. The other field of primary focus is the study of low-dimensional thermoelectricity. Reducing dimensions from 3D to 0D (bulk - slab - wire - dot) has shown to introduce quantum confinement effects that alter the electronic structure and enhance the thermoelectric properties over bulk[6-8]. It is believed that the surfaces introduced by lowering the dimensions play a major role in reducing the thermal conductivity [4].
1.2. Thermoelectricity:
1.2.1. Brief History of Thermoelectricity:
In 1822 Seebeck began to study thermoelectrics [9], where he noticed that two dissimilar metals in a closed loop caused a compass needle to deflect when the two metals were held at different temperatures. This means that an electric field was created between the two metals, thus inducing a magnetic field to deflect the needle. Seebeck later discovered that some metals were able to create stronger fields with the same temperature difference, and that the amount of deflection in the needle was proportional to the temperature difference between the two conducting metals. These principles make up the foundations of thermoelectrics, and for his discoveries the Seebeck coefficient (the voltage produced between dissimilar conductors where a uniform temperature difference of 1K exists between those conductors) was named after the founding father of thermoelectrics.
In 1834 Peltier [10] discovered an applied voltage could create a temperature difference between the two dissimilar metals. Although Peltier is generally credited with the discovery of thermoelectric cooling, after four years, Emil Lenz, who showed that a DROP of water on a bismuth-antimony junction would freeze when electrical current was applied one way, and melt again when the current was reversed.
1.2.2. Thermoelectric energy conversion:
Current is generated from a temperature gradient when the heated charge carriers of a thermoelectric device are driven from the hot end to the cold end of the temperature gradient, Fig.1.1. Conversely, when a potential is applied, the charge carriers create a temperature gradient by flowing in the opposite direction.
The thermoelectric effect is a solid state effect in which thermal energy is directly converted into electrical energy and electrical energy into thermal energy. Fig.1.2. is a schematic of a thermoelectric module consisting of an n-type and p-type that is electrically in series and thermally in parallel. The charge carriers can be diffuse from the hot side to the cold side. The result referred to as power generation. However, the charge carriers can be forced to move to one side of the module through an electromotive force, which causes one side to get hot, and the other side to get cold. This configuration would be considered as a solid state cooling device in which the working fluid is made of electrons.