Waste Recycling in Thermoelectric Materials

Abstract

power plants, factories, motor vehicles, computers, or even human bodies into electricity. TEGs are suitable for small applications because of their compatibility, simplicity, and scalability. For example, electricity can be generated from small heat sources and by taking advantage of small temperature differences for powering wristwatches or wearable devices such as miniaturized accelerometers or electroencephalography (EEG) devices. [3,4] However, low efficiency and high fabrication cost are the main disadvantages of TEGs which hinder their vast application in the market. Although many studies have explored the ways to increase the efficiency of TEGs in converting the thermal to electric energy, cost of TE materials synthesis and miniaturized TEGs fabrication has not been addressed widely.TEGs require materials with high electrical conductivities and low thermal conductivities, as well as high thermopower (Seebeck coefficient, S) which in turn constrains the types of materials that can be used as a TE material. The direct conversion of thermal to electrical energy in response to a temperature gradient across a material can be calculated by using the following equationwhere ΔV is the generated voltage, S is the Seebeck coefficient or thermopower, and ΔT is the temperature difference. The performance of a TE material is defined by the dimensionless figure of merit (zT), which relates the material properties to its conversion efficiency, in the following way 2 l e ZT S T σ κ κ ) ( = +where σ is the electrical conductivity, κ l and κ e are the lattice and electronic contributions to the thermal conductivity of TE material, respectively, and T is the absolute temperature. High S 2 σ (power factor, PF) and low thermal conductivity, κ = κ l + κ e , lead to better performance. In practice, however, the mutual dependence between these material properties makes an optimization of zT difficult. For many decades since the 1950s, when research and development of bulk homogenous materials for TE applications started to increase, research and development efforts focused on elements derived from raw materials for synthesizing TE materials. The compounds synthesized from raw elements such as Thermoelectric (TE) technology enables the efficient conversion of waste heat generated in homes, transport, and industry into promptly accessible electrical energy. Such technology is thus finding increasing applications given the focus on alternative sources of energy. However, the synthesis of TE materials relies on costly and scarce elements, which are also environmentally damaging to extract. Moreover, spent TE modules lead to a waste of resources and cause severe pollution. To address these issues, many laboratory studies have explored the synthesis of TE materials using wastes and the recovery of scarce elements from spent modules, e.g., utilization of Si slurry as starting materials, development of biodegradable TE papers, and bacterial recovery and recycling of tellurium from spent TE modules. Yet, the outcomes of such work have no...

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