Determination of phase diagrams and investigation of thermoelectrical properties in CaO-XO-CoO (X= Na, Ba, Sr, La) systems

The scientes have made many efforts in the field of renewable energy with the aim of meeting the energy needs of the rapidly growing world population. The negative effects of fossil fuels and nuclear energy on the ecosystem have made alternative energy sources more important. The fact that CO2 emissions from fossil fuel use causes global warming and environmental pollution has now made more popular recyclable energy sources such as waste heat recovery, hydroelectric, geothermal, wind and solar. While electricity remains the most convenient form of energy, heat has been an abundant but low quality source of energy as more then half of the energy generated is lost as waste heat. Waste heat recovery is the process of recycling of the waste heat obtained as a by-pruduct from the operating machines and moving parts. Thermoelectrisity is the simplest technology applicable for direct electricity-heat conversion. Especially in the automotive industry, thermoelectrical modules used in radiator and exhaust systems have an important application area in the recovery of waste heat. Due to global demand regarding to energy, new designs are being made using new technologies in order to increase the performance of renewable energy systems. Thermoelectric materials have drawn vast attentions for decades, due to the fact that thermoelectric effect enables direct conversion between thermal and electrical energy, thus providing an alternative for power generation and refrigeration. By the discovery of thermoelectric effects by Seebeck, Peltier and Thomson in the 19th century, it was understood that a temperature gradient could be created between the two ends of a meterial by applying an external voltage to the material by Peltier effect. On the contrary, electrical energy could be obtained from the temperature gradient between the two ends of a material by Seebeck effect. As a result, the thermoelectric technology can be divided in two categories according to the thermoelectric effect, which are thermoelectric generators and thermoelectric coolers. Thermoelectric generators are used for power generation when there is a temperature gradient between the two ends of the p-type and n-type materials. In contrast with this phenomenon, thermoelectric coolers ares used for cooling when the voltage applied onto the two different semi conductor materials. Compared with the conventional energy conversion methods, thermoelectric generators and thermoelectric coolers offer many advantages such as reliability, no moving parts and environmentally friendly. Thermoelectrical modules are obtained by the junction of two different semiconductor materials electrically in series and thermally in parallel. The thermoelectric generators can be used in many areas such as buildings, space vehicles and automobiles. As more then two-thirds of the fuel is dissipating to the surroundings as waste heat, the thermoelectric generators can be used to convert heat energy to electricity to improve total efficiency of the engine. They can be mounted to exhaust system of an internal combustion diesel engine to recover the waste heat. Low efficiency of the thermoelectric devices is the drawback of these modules which limits their application areas. The performance of energy conversion is characterized by a coefficient named ZT. This value is related to the thermal conductivity coefficient, electrical resistivity and Seebeck coefficient of the material. Energy conversion performance increases with increasing Seebeck coefficient, decreases with increasing thermal conductivity coefficient and electrical resistivity. Thermoelectric materials are typically classified by their structure and composition. Some of the main classification are chalcogenides, clathrates, skutterudites, silicides and oxides. The Ca3Co4O9 system has attracted great attention in thermoelectric application due to high thermal stability and lack of toxicity. Many oxide systems have been investigated, and some of them have good thermoelectric properties. In the literature, thermoelectric performance improvement by adding metal oxides to Ca3Co4O9 is recently reported. However there is limited studies on the phase equilibria. In order to determine the dopant amount, solubility limits and phase stubility regions have to be known. Therefore this work aimed to investigate thermeoelectrical properties of doped Ca3Co4O9 and ternary phase equilibria in the CaO-XO-CoO oxide system (X=Na, La, Sr, Ba) in order to investigate thermodynamic stability of doped Ca3Co4O9 ternary compound. The Ca3Co4O9 compound is a misfit-layered oxide consisting of two monoclinic subsystems, namely the Ca2CoO3 layer and the CoO2 layer. It does not only exhibit surprising thermoelectric properties due to a large thermopower at room temperature, but also has low electrical resistivity and good thermal stability up to 923°C which makes the compound an ideal material for high temperature thermoelectric application. Doped calcium cobaltites have also been considered as a potential material for high temperature thermoelectric applications. Recent experimental works showed that the thermoelectrical performance of calcium cobaltite (Ca3Co4O9) can be improved by metal oxide doping such as Na2O, La2O3, SrO, BaO by changing the misfit ratio and by increasing the carrier concentration. However, there is no detailed literature information on phase equilibria of the CaO-XO-CoO (X=Na, La, Sr, Ba) ternary oxide system at 800°C. This work aims to investigate the phase equilibria in the doped Ca3Co4O9 system. In order to obtain thermodynamic equilibrium, the samples were prepared by wet chemistry and calcined at 800°C. The samples obtained were uniaxially pressed and sintered at 800°C for thermoelectrical characterizations. Since Ca-Co-based oxides easily decompose at high temperature above 926°C, phase equilibria investigated below 926°C and phase diagrams constructed at 800°C. Structural characterizations were carried out at room temperature using a Phillips X-ray powder diffractometer with Cu Kα radiation, with a 2θ scanning range from 10° to 70°. Additionally, thermoelectrical characterizations were also performed. Electrical resistivity and thermal conductivity coefficient of the samples were measured (by four prob and laser flash methods respectively) at room temperature. As a result of this study, for the first time in the literaure, phase diagrams of CaO-Na2O-CoO and CaO-BaO-CoO ternary systems are determined. Solubilty limits and phase stability regions are identified by X-ray diffraction analysis. Solubility limits of Sr and La in Ca3Co4O9 structure are also investigated. The results were in good agreement with those of the litterature. According to the thermoelectrical characterizations, the lowest electrical resistivity is obtained by the Sr addition with the amount of x=0.0.25 (molar) into the Ca3Co4O9 by substituting Ca atoms by Sr atoms. Thermal conductivity coefficient measurement with laser flash method showed that the minimum thermal conductivity can be also obtained by Sr addition with the amount of x=0.01 (molar).