Tuning up bi-ae-co-o (ae: alkaline earth) thermoelectric performances via processing and doping
- Autores: Shahed Vazeh Rasekh Modabberi
- Directores de la Tesis: Andrés E. Sotelo Mieg (dir. tes.), Juan Carlos Díez Moñux (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: M. Pilar Villar Castro (presid.), María Antonieta Madre Sediles (secret.), Nuno Miguel Freitas Ferreira (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad de Zaragoza
- Materias:
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- Resumen
- Introducción o motivación de la tesis The epic challenge of the 21st century is trying to fill the gap between energy supply and demand using clean, reliable and cheap energy. While sources of green energy are gradually changing the landscape, products derived from fossil fuels still continue to heat our homes, fuel our cars, and producing electricity. Despite the extraordinary advances in technology, rapid economic growth in developing countries like China and India will require a huge amount of energy. These requirements will be, for sure, reached by using fossil fuels as energy sources instead of green ones. As a consequence, the use of fossil fuel resources is still exponentially growing, following the trends started at the Industrial Revolution. As it is well known, the fossil fuels are the accumulated remains in the Earth’s crust of prehistoric organisms which have been anaerobically decomposed over millions of years. Nowadays, it is commonly believed that the fossil fuel energy is non-renewable and that its uncontrolled usage over the past centuries has led to serious environmental problems, such as the global warming. It is considered that the production rate over time of these resources follows the Hubbert curve [1], signifying that the production of each type of fossil fuel will peak at some point in time, followed by a gradual decrease. At the present time, the main battle in reducing the emissions of greenhouse gasses (mainly CO2) is being carried out on two fronts: the increase of today’s machines and applications performances and the search for alternative energy sources. Considering the enormous efforts and costs required for providing energy in the first place, it is perhaps shocking to realize that ~60% of the total energy is lost as waste heat in the different energy transforming processes [2]. For example, ordinary fossil fuel power plants can transform between 36% and 48% of the fuel’s energy into electricity, the rest being lost in the form of wasted heat. These low yields are generally due to the operating principle involving the heat release, i.e. in condensation towers. A vehicle powered by an internal-combustion engine uses only about 25% of the fuel energy for mobility and accessory power, while the remaining is lost in the form of waste heat in the exhaust and coolant [3]. For a conventional incandescent light bulb (today considered as obsolete), only around 10% of the input energy is used for creating the visible light, the rest (90%) is mainly lost as non-visible electromagnetic radiation.
Some solutions for these challenges are being implemented today, but many will come from the next generation of entrepreneurs, engineers and scientists. In order to answer to this great challenge, the following issues should be considered: A large portion of the globally produced oil is directly processed into transportation fuels like gasoline and diesel, as they combine reliability, affordability and performance. As their consumption is exponentially increased, alternatives to these transportation technologies are vital in order to meet global demand over the next 50 years.
Oil and gas producers are trying to make as little environmental impact as possible taking initiatives such as drilling multiple wells from a single location, employing environmentally friendly chemicals, and ensuring a seamless transition from the wellhead to the consumer.
Still, the most important goal is energy sustainability to insure enough supply for several more decades. Alternative energy sources are among the pioneering technologies that will aim to be sustainable and economically competitive with today’s fossil fuels. While wind, solar and biofuels appear to be among the most promising, significant breakthroughs are still required to make them viable sources of future energy supply. The use of renewable (“green”) energy leads to advantages like non-pollution, constant replenishment, economic viability, good geodistribution, which are of great interest in the actual global context. However, meeting energy demands will require not only producing more, but also a more efficient use of what it is produced, as well as supplying consumers with cheap energy to make it accessible to the whole population. For these objectives, not only new technologies will be necessary, but also new consumer habits.
As a consequence of above mentioned challenges, it is evident that new technologies, and/or improving the existing ones, must be necessary to reduce wasted heat production and increase the energy efficiency. As a consequence, lower fossil fuels consumption should be achieved to keep the air cleaner and fight against global warming. Current technological advancements produced new paths to various novel techniques for recovering lost energy in industry, traffic and households. In any case, in order to compete at a high level with the fossil fuels, the efficiency of devices using this type of energy must be greatly increased, and their costs lowered.
Among the new techniques and technologies adequate to fulfil the above mentioned challenges, thermoelectricity emerges as a very promising solution that can radically change the situation. It is defined as the science and technology associated with thermoelectric generation and refrigeration. Thermoelectric (TE) effect refers to the direct conversion of temperature differences to electric voltage and vice-versa. The system consists of two different connected thermoelectric materials, known as thermoelectric module, which are solid state devices, without moving parts and, consequently very reliable. These modules can be classified in several groups, based on their applications: Thermoelectric generators can be used for converting heat generated by many sources, such as solar radiation, automotive exhausts, and industrial processes, to electricity.
Thermoelectric coolers can be used to make refrigerators and other cooling systems.
Other applications are based on their high sensibility, as infrared sensors.
All these materials can be designed in various shapes and sizes, due to their simplicity, which can be adapted to meet the demanded requirements. In other words TE materials can be used as standalone devices in new installations or as an addition into the already existing equipments, increasing their overall performance and efficiency. These assets, together with the absence of pollutant emissions and practically no maintenance, are the main reasons why these devices are in the centre of today’s “green technology” debate, especially to recover waste heat and convert it directly into electrical power. Today’s scientific as well as industrial progresses have multiplied the heat sources, especially at high temperatures (incinerators, thermal power plants, factories, vehicle exhaust/radiator systems, solar concentrators, aircrafts, etc.). In these conditions, thermoelectricity can be used not only to harvest all the wasted heat [4-6], but also exploiting natural heat sources. However, the best performing TE materials with potential industrial applications, from the scientific point of view, are alloys mostly containing toxic and scarce elements such as tellurium or antimony and, more importantly, can be oxidized and/or degraded releasing hazardous heavy elements when they are exposed to high temperatures, in air, drastically limiting their working temperatures. On the basis of this background, metal oxides have attracted much attention as thermoelectric power generation materials at high temperatures on the basis of their potential advantages over heavy metallic alloys from the point of view of their chemical and thermal robustness. Additionally, TE oxide materials with acceptable performances are mostly composed of abundant and environmental-friendly compounds with reasonable costs, and can work for long periods of time at relatively high temperatures without degradation, even in oxidative environments due to their oxidic nature.
In 1997 TE oxides started to attract attention by the discovery of large thermoelectric properties in Nax CoO2 [7]. This discovery boosted up research on these materials, leading to the identification of other members of the CoO based family, [Mm A2 O(m+2)]q [CoO2] (M = Co, Bi, Pb, Tl; A = Ca, Sr, Ba; m = 0, 1, 2; and q≥0.5) [8]. Among all these compounds, Bi-AE-Co-O (AE= Ca, Sr, Ba) has got attention due to their properties, such as relatively low electrical resistivity, low thermal conductivity, and large Seebeck coefficient. However, their complex structure and chemical composition are challenging, especially from the point of view of possible improvements on sample preparation and fabrication. As a consequence Bi-based cobalt oxides are less studied than the well-known Ca3 Co4 O9 phase. Therefore, in order to be used in the most suitable practical applications (waste-heat recovery systems, for example), their TE performances should be further improved.
- Desarrollo teórico Among the CoO thermoelectric materials, Bi-AE-Co layered oxides attracted lot of attention due to their properties and their structural similarity to the widely studied Bi based high temperature superconductors. Powder X-ray diffraction patterns with the help of electron diffraction studies of this family revealed the superposition of two orthorhombic systems of reflections, typical of the CoO based materials. These compound are composed of the two layers, the rock salt type and the CdI2 type ones.
studies by T. Fujii, et al. and T. Yamamoto [9,10] have shown large anisotropy in these materials. A similar large anisotropy has also been observed in Bi based high temperature superconductors which possess the CuO conducting planes. Moreover, it was observed that both systems similarly tend to grow preferentially along to their respective conducting planes. Therefore, it has been proposed that bulk Bi based cobaltite thermoelectric [Bi2 AE2 O4 ]y [CoO2] (where AE= Ca, Sr, and Ba) materials can be produced using directional growth techniques to enhance their electrical properties. Furthermore, their thermoelectric properties can be also tuned by RS type layer modification (without significant changes in the CoO2 layers structure), controlling crystal symmetry, using chemical pressure, and modifying carrier concentration, using well established sintering processes, doping, etc. Additionally, the presence of Bi in the system drastically reduces thermal conductivity, compared with the one obtained in Ca349, especially the lattice/phonon component by shortening the phonon mean free path due to larger RS-layer unit cell.
Further research established that doping and/or substitution of cations in the rock salt layer of this system would simultaneously modify c and b1 parameters, affecting the misfit ratio. It is evident that increasing the AE cation ionic radii in 6 fold coordination, from 1Å for 〖Ca〗(2+) to 1.18Å for 〖Sr〗(2+) and 1.35Å for 〖Ba〗(2+) will result in the increase of b1/b2 ratio.
Many reports have been published in order to explain the good TE properties of this family, such as an increased effective mass of the charge carriers due to the strongly correlated nature of this system [11], degeneration of d orbitals in of Co(3+) and Co(4+) low-spin-state [12], electronic structure [13], and the effect of in-plane stress [14].
Therefore, the introduction of bigger atoms in the unit cell produces the system expansion, allowing the accommodation of more oxygen, and inducing an increase of oxidation state of cobalt in the [CoO2] layer. The raise of Co valence decrease the Seebeck coefficient, as it was pointed out by Koshibae et al. [15], assuming that the total electrical charge of the RS type layer does not change.
On the other hand, the decrease of Seebeck coefficient is lower when 〖Bi〗(3+), with ionic radii of 1.03Å, is partially substituted with elements with bigger ionic radii but different oxidation state, e.g. 〖Pb〗(2+) (1.13Å in 6 fold coordination). This minor decrease is due to Seebeck coefficient compensation resulted from changes on the total RS layer electrical charge which, in turn, provoke the formation of oxygen vacancies (increasing the Co^(3+) proportion in the [CoO] layer). As a result, it can be deduced that the changes on the RS layer parameters and/or their total electrical charge, will affect the carrier concentration on the [CoO] one. However, these modifications have different influence on thermoelectric performances due to the interconnection between all the thermoelectric parameters. For example, increasing the charge carriers concentration will result in decrease of the Seebeck coefficient while reducing the electrical resistivity of the system and increasing thermal conductivity. Therefore, enhancing the overall thermoelectric performance should involve the optimization, at the same time, of these three parameters.
From all the considerations presented so far, it becomes obvious that the TE properties of Bi-AE-Co-O could be improved, especially in polycrystalline form, by employing a variety of methods and techniques. This statement is the core of the present work, which will explore some of these routes, with the objective to bring this TE material closer to its large-scale use in high-temperature practical applications.
The goal of this work is studies on Bi2 Ca2 Co1.7 Oδ, Bi2 Sr2 Co1.8 O_δ, and Bi2 Ba2 Co2 Oδ materials in order to enhance their thermoelectric properties and producing bulk polycrystalline materials with thermoelectric properties as close as possible to the reported values for single crystals.
As it is well known, the ceramic materials properties are basically controlled by their composition. However, the grain size and porosity of specimens play an important role in their characteristics, being strongly influenced by the fabrication methods. As a consequence, a close control of both starting materials and preparation conditions is essential for obtaining ceramic materials with the desired characteristics. Therefore, the effect of different wet chemistry routes, i.e. sol-gel (SG) and polymer solution (PEI), on the microstructure and thermoelectric properties of the final bulk samples, using the classical solid state (SS) method as a reference will be studied.
Other approach which will be used in this work is related to the grain orientation taking advantage of the preferential growth habit in this family. As it was previously mentioned, the grains alignment may improve the electrical conduction, leading to higher thermoelectric performances. Therefore, these materials will be grown by this method at different growth rates (5,15,30, and 90 mm/h) in order to determine the influence of these parameters on their final thermoelectric performances. These studies will allow determining the best growth rate/thermoelectric performances ratio.
Moreover, the effect of cation substitution in the RS layer, which can modify the cell parameters and the Co^(4+)/(Co(4+)+Co(3+)) relationship thus, allowing thermoelectric performances modification. For this purpose, Bi will be substituted with Pb in different proportions, using the preparation methods described in the first part of these chapters, and textured by the LFZ technique, using the previously determined growth rates.
Finally determine the role and effect of metallic Ag additions (in different weight % amount) in the final performances of these LFZ textured materials has been determined.
Furthermore, some non-conventional alternative sample preparation and fabrication were studied, such as coprecipitation synthesis rout, or electrically assisted LFZ, etc.
- Conclusión Through these work, the structural, microstructural and thermoelectric properties of Bi-AE-Co-O (AE= Ca, Sr, Ba) ceramics have been studied. It has been found that thermoelectric properties can be enhanced through several strategies: synthesis methods, texturing, doping, metallic additions, and thermal treatments.
It has been found that solution synthesis methods are very effective to enhance the thermoelectric performances of sintered materials, compared with the results obtained through the classical solid state method.
Texturing has clearly enhanced the thermoelectric performances due to a good grain orientation and a lower charge carrier concentration produced in their processing. Nevertheless, the performances have been found to be strongly dependent on the growth rate, leading to the highest ones at the lowest growth speed.
Some differences have been found in the final thermoelectric performances of as-grown materials depending on the synthesis route.
The use of EALFZ process has allowed an impressive improvement of thermoelectric performances when compared with the classical LFZ processing system.
Bi substitution by Pb in textured materials has been observed to decrease electrical resistivity due to the Pb lower oxidation state, which promotes the formation of a higher amount of Co(4+) in the conducting layer and, as a consecuence, increasing the charge carrier concentration. Furthermore, the optimal Bi substitution has led to enhanced grain orientation, which also results in lower electrical resistivity.
Metallic Ag addition in textured materials also produces a decrease of electrical resistivity, leading to enhanced thermoelectric performances. Moreover, Ag is maintained as metallic after all processes.
Combined Bi substitution by Pb with metallic Ag additions in textured materials further improved the thermoelectric performances of these materials, mainly producing a drastic decrease of electrical resistivity.
Finally, annealing some of the previously textured materials has allowed reaching thermoelectric performances higher than the measured in single crystals and, consequently, achieving the main goal proposed in this work.
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