Gabriel Constantinescu | University of Aveiro (original) (raw)
Papers by Gabriel Constantinescu
![Research paper thumbnail of Improvement of the thermoelectric properties of [Bi1.68Ca2O4−δ]RS[CoO2]1.69 cobaltite by chimie douce methods](https://attachments.academia-assets.com/53893343/thumbnails/1.jpg)
](Journal of Solid State Chemistry, 2010
Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to ... more Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Bi 2 Sr 2 Co 1.8 O x ceramics with improved thermoelectric properties, compared to the classical solid state reaction. The products obtained by these different methods have been compared using DTA-TGA, powder X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show higher homogeneity and lower content of secondary phases. The main differences in thermoelectrical properties are due to the decrease of electrical resistivity in samples obtained by solution methods, compared with the solid state obtained samples. Between them, the decrease is especially high for those samples prepared by the polymer solution method. Therefore, the polymeric solution synthesis route is shown to yield a power factor four times higher than the obtained for the solid state and sol-gel methods at room temperature.
Journal of Electroceramics, 2014
Materials Today: Proceedings, 2015
Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit h... more Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit high thermopower and relatively low electrical resistivity, as well as high chemical stability at high temperatures. They show anisotropic thermoelectric properties linked to their layered structures. As a consequence, different texturing methods developing oriented grains, such as sinterforging, and template grain growth processes have been successfully used to improve the performances of these materials. There are some other less used methods, like the directional growth from the melt, which have already shown their applicability to this kind of compounds. One of these methods is the laser floating zone (LFZ) melting technique. This process has demonstrated its suitability in the Co-oxide based thermoelectric materials processing in the last years in our laboratories. In this work, two examples of the versatility and usefulness of the LFZ technique will be shown. The first one will be based on the processing of the well known Ca 3 Co 4 O 9 ceramic material. In this case, a very dense and homogeneous material is obtained in two steps (growth and annealing), reaching PF values as high as 0.42 mW/K 2 m at 800 ºC. In the second case, a new and well textured thermoelectric composite (Bi 2 Ca 2 Co 1.7 O x + Ca 3 Co 4 O 9 ) is obtained, reaching PF values of 0.31 mW/K 2 m at 650 ºC.
Boletín de la Sociedad Española de Cerámica y Vidrio, 2015
ABSTRACT Ca3Co4-xNixO9 (x=0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been... more ABSTRACT Ca3Co4-xNixO9 (x=0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. As a result of the Ni addition an increase in porosity has been detected. Moreover, the presence of Ni has been related with the increase of Ca2Co3O6 secondary phase and the appearance of a new NiO-CoO solid solution. However, for the 0.01-Ni doped samples an improvement in the thermoelectric performances has been measured. This effect has been related with a decrease in the resistivity values and an increase in the Seebeck coefficient. The raise in the power factor for the 0.01-Ni doped samples, compared with the undoped ones, is between 10 and 25% at 50 and 800 degrees C respectively. Moreover, the maximum power at 800 degrees C, around 0.25 mW/K-2.m, is significantly higher than the best results obtained in Ni doped samples reported previously in the literature. (C) 2015 Sociedad Espanola de Ceramica y Vidrio. Published by Elsevier Espana, S.L.U. This is an open access article under the CC BY-NC-ND license
Ceramics International, 2015
ABSTRACT Ca3−xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.0... more ABSTRACT Ca3−xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.01, 0.03, 0.05, 0.07, and 0.10) have been prepared using the classical solid state method. Microstructural characterization has shown that Na has been incorporated into the Ca3Co4O9 phase and that no Na-based secondary phases have been produced. It has also been found that Na addition promotes grain growth and favours sintering due to the formation of a small amount of liquid phase. Electrical resistivity decreases when Na content increases until 0.07Na addition while Seebeck coefficient is maintained practically unchanged. The improvement in electrical resistivity leads to higher power factor values than the usually obtained in samples prepared by more complex and expensive techniques.
Journal of Materials Science: Materials in Electronics, 2015
ABSTRACT Ca3−xBaxCo4O9 bulk polycrystalline thermoelectric ceramics with x = 0.00, 0.01, 0.03 and... more ABSTRACT Ca3−xBaxCo4O9 bulk polycrystalline thermoelectric ceramics with x = 0.00, 0.01, 0.03 and 0.05 have been prepared through a classical solid state method. Microstructural characterizations of sintered samples have shown that Ba has been incorporated into the Ca3Co4O9 and/or Ca3Co2O6 structures and that no Ba-based secondary phases have been produced. The apparent density values increase when the amount of barium is raised with respect to the undoped samples, reaching a maximum value of around 77 % of the Ca3Co4O9 theoretical density, for samples with the highest amount of barium. Electrical resistivity of doped samples decreases in the whole measured temperature range with respect to the undoped ones, while Seebeck coefficient remains practically unchanged. In conclusion, the improvement in resistivity leads to power factor values which are higher than the ones measured in undoped samples, fact which makes this bulk polycrystalline compound a promising candidate for high-temperature power generation applications in oxidative environments.
ABSTRACT Ca3Co4−xTixO9 polycrystalline thermoelectric ceramics with small amounts of Ti have been... more ABSTRACT Ca3Co4−xTixO9 polycrystalline thermoelectric ceramics with small amounts of Ti have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, they show that Ti has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, until 0.03-Ti doped ones. Further Ti addition produces an increase of resistivity with respect to the 0.03 Ti doped samples. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Ti content. The improvement in electrical resistivity leads to about 55 % higher power factor values for the 0.03 Ti-doped samples than that obtained in the undoped ones. The maximum power factor at 800 °C, around 0.33 mW/K2 m, is slightly higher than the obtained in higher density samples, clearly indicating the good thermoelectric performances of these doped samples.
Bi 2 Ca 2 Co 1.7 O x bulk polycrystalline ceramics were prepared by the solid state method and by... more Bi 2 Ca 2 Co 1.7 O x bulk polycrystalline ceramics were prepared by the solid state method and by directional growth. Moreover, the effect of annealing on the textured materials has been studied. Microstructure has shown randomly oriented grains in the classical sintered materials while in the textured samples they were well oriented with their c-axis nearly perpendicular to the growth direction. Furthermore, the annealed samples showed much lower amount of secondary phases than the as-grown ones. These microstructural changes are reflected on the thermoelectric properties which increase with the grain orientation and with the decrease on the secondary phases content mainly due to the electrical resistivity reduction. As a consequence, a raise on the power factor of about 6 and 9 times, compared with the classically sintered samples, was obtained for the as-grown and annealed samples, respectively. The maximum power factor obtained at 650 ºC in the annealed samples (~ 0.31 mW/K 2 m) is about 50 % higher than the obtained in sinter-forged textured materials at the same temperature.
Boletín de la Sociedad Española de Cerámica y Vidrio, 2013
Boletín de la Sociedad Española de Cerámica y Vidrio, 2014
Ca 3-x Mg x Co 4 O 9 polycrystalline thermoelectric ceramics with small amounts of Mg have been s... more Ca 3-x Mg x Co 4 O 9 polycrystalline thermoelectric ceramics with small amounts of Mg have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Mg has been incorporated into the Ca 3 Co 4 O 9 structure and no Mg-based secondary phases have been identified. Apparent density measurements have shown that samples do not modify their density until 0.05 Mg content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Mg content increases until 0.05 Mg addition. The improvement in both parameters leads to higher power factor values than the usually obtained in samples prepared by the conventional solid state routes and close to those obtained in textured materials.
JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 2014
ABSTRACT Ca3Co4-xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have be... more ABSTRACT Ca3Co4-xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. XRD data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, it has been found that the Zn has been incorporated into these two phases. Slight Zn doping decreases electrical resistivity compared with the values obtained in undoped samples. The minimum values have been obtained for the 0.01-Ni doped samples, increasing for further Zn substitution. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Zn content. The improvement in electrical resistivity leads to higher power factor values for the 0.01 Zn-doped samples (about 30 %) than for the undoped ones. The maximum power factor at 800 A degrees C, around 0.27 mW/K-2 m is significantly higher than the best results obtained in Zn doped samples reported in the literature.
Scripta Materialia, 2013
ABSTRACT Bi2Ba2Co2Ox thermoelectric ceramics were textured from the melt using the laser floating... more ABSTRACT Bi2Ba2Co2Ox thermoelectric ceramics were textured from the melt using the laser floating zone method, at a growth rate of 5 mm h−1. The microstructure showed good grain alignment with the growth axis. These microstructural features are reflected in the thermoelectric performance, with a significant increase in the power factor values, reaching ∼0.4 mW K−2 m−1 at 650 °C, much higher than the values typically obtained in these materials.
Scripta Materialia, 2014
Bi 2 Ca 2 Co 1.7 O x nominal composition ceramic rods were textured by the Electrically Assisted ... more Bi 2 Ca 2 Co 1.7 O x nominal composition ceramic rods were textured by the Electrically Assisted Laser Floating Zone technique. A new composite formed by three phases, two of them with well known thermoelectric properties, Bi 2 Ca 2 Co 1.7 O x and Ca 3 Co 4 O 9 , has been obtained for the first time when the growth process is performed with 300 mA applied current. Under these conditions the thermoelectric performances of this new composite reach values above the highest ones reported in the literature for polycrystalline Bi 2 Ca 2 Co 1.7 O x materials.
Materials Today, 2013
Thermoelectric materials can directly convert a temperature gradient into an electric voltage tha... more Thermoelectric materials can directly convert a temperature gradient into an electric voltage thanks to the Seebeck effect. Since its discovery in 1823, this characteristic has allowed the use of metallic junctions to control temperatures. These thermocouples possess a low electrical resistivity and Seebeck coefficient with high thermal conductivity. The thermoelectric performances of these materials are evaluated by the dimensionless figure-of-merit (ZT = S 2 sT/k, where S is the Seebeck coefficient, T is the absolute temperature, s is the electrical conductivity, and k is the thermal conductivity) which is very small, indicating that metallic materials possess poor thermoelectric behavior. The introduction of intermetallic semiconductor materials, with higher thermoelectric performances than their metallic counterparts, has resulted in their practical application in thermoelectric modules. These modules can be found as Peltier devices, when used for refrigeration purposes, or Seebeck devices when used for electrical energy generation. Current applications of this kind of material include the environmentally friendly recovery of industrial and automobile waste heat, and in the production of electrical energy in radioisotope thermoelectric generators installed on spacecrafts, or in lighthouses located in isolated regions.
Journal of the European Ceramic Society, 2012
Two solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obta... more Two solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Ca 3 Co 4 O 9 misfit compounds with improved thermoelectric properties, compared to the classical solid state reaction. A comparison among the final products obtained by these different methods has been performed using DTA, TGA, FTIR X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show a very significative reduction on the secondary phases content. As a consequence, an important decrease on the electrical resistivity values is produced, compared to the solid state prepared samples, leading to a relatively important power factor raise.
Journal of the European Ceramic Society, 2012
Bi 2 Sr 2 Co 1.8 O x /Ag composites with small amounts of Ag have been synthesized by a sol-gel v... more Bi 2 Sr 2 Co 1.8 O x /Ag composites with small amounts of Ag have been synthesized by a sol-gel via nitrates reaction and directionally grown from the melt. Some of the obtained samples were annealed in order to obtain the thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases distribution and alignment. Moreover, thermoelectric and mechanical characteristics of annealed samples were determined by the four-probe technique and by three point flexural strength tests, respectively. Scanning electron microscopy revealed that Ag particles appear dispersed among well oriented ceramic grains with large size, providing a plastic flow region which increases the flexural strength for the optimally Ag added samples (1 wt.%). The composites electrical resistivity is lower than that of pure Bi 2 Sr 2 Co 1.8 O x while Ag addition does not significantly affect thermopower values. The resistivity reduction leads to power factor improvements of ∼50%, compared with pure samples, for Ag additions of 1 wt.% Ag.
Journal of the European Ceramic Society, 2014
ABSTRACT Bi2−xPbxSr2CaCu2Oy textured materials (x = 0.0, 0.2, 0.4, and 0.6) have been successfull... more ABSTRACT Bi2−xPbxSr2CaCu2Oy textured materials (x = 0.0, 0.2, 0.4, and 0.6) have been successfully prepared by the laser floating zone technique. Microstructure and electrical properties (JC and TC) have been clearly affected by Pb addition. From the E–I curves, slope of the transition between the superconducting and the normal state (n) at 77 K reaches a maximum of about 16 for the 0.4 Pb doped samples. This value is much higher than the typical ones for the Bi-2212 materials. Moreover, when the electrical properties of the 0.4 Pb doped samples are measured at lower temperatures (between 65 and 77 K), n values increase when the temperature is decreased. A maximum n value of 32 has been reached at 65 K which makes this material very attractive for its use as resistive fault current limiters.
Journal of the European Ceramic Society, 2013
High-performance Ca 3 Co 4 O 9 thermoelectric ceramic has been prepared from a Ca 1-x Co x O/Ca y... more High-performance Ca 3 Co 4 O 9 thermoelectric ceramic has been prepared from a Ca 1-x Co x O/Ca y Co 1-y O divorced eutectic structure produced by a directional meltgrown using the laser floating zone technique. This material has been grown at very high solidification rate in order to produce a very fine microstructure to reduce the necessary annealing time to recover the Ca 3 Co 4 O 9 thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases and estimate the extent of Ca 3 Co 4 O 9 formation with time and related with their thermoelectric performances. The optimum annealing time, 72 h, has been determined by the maximum power factor value (about 0.42 mW K -2 m -1 ), which is around the best values reported in textured materials (~ 0.40 mW K -2 m -1 ). This high power factor outcome from the high Ca 3 Co 4 O 9 phase content, apparent density and Co 3+ /Co 4+ relationship determinations performed in the present work.
Journal of Superconductivity and Novel Magnetism, 2013
Bi-2212 samples prepared by the classical solid-state method have been grown from the melt using ... more Bi-2212 samples prepared by the classical solid-state method have been grown from the melt using the Laser Floating Zone (LFZ) method. They have shown good grain alignment and transport critical current densities (J c ). After postannealing processes designed to produce the Bi-2212 phase controlled decomposition, J c values have been increased in an important manner. Maximum values have been achieved when samples were thermally treated at 680 ºC for 168 h with improvements around 80 %, compared with the original textured samples. The results clearly indicate that postannealing processes, when adequately controlled, produce the formation of effective pinning centers which are responsible for the raise in the measured J c values.
Journal of Solid State Chemistry, 2010
Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to ... more Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Bi 2 Sr 2 Co 1.8 O x ceramics with improved thermoelectric properties, compared to the classical solid state reaction. The products obtained by these different methods have been compared using DTA-TGA, powder X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show higher homogeneity and lower content of secondary phases. The main differences in thermoelectrical properties are due to the decrease of electrical resistivity in samples obtained by solution methods, compared with the solid state obtained samples. Between them, the decrease is especially high for those samples prepared by the polymer solution method. Therefore, the polymeric solution synthesis route is shown to yield a power factor four times higher than the obtained for the solid state and sol-gel methods at room temperature.
Journal of Electroceramics, 2014
Materials Today: Proceedings, 2015
Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit h... more Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit high thermopower and relatively low electrical resistivity, as well as high chemical stability at high temperatures. They show anisotropic thermoelectric properties linked to their layered structures. As a consequence, different texturing methods developing oriented grains, such as sinterforging, and template grain growth processes have been successfully used to improve the performances of these materials. There are some other less used methods, like the directional growth from the melt, which have already shown their applicability to this kind of compounds. One of these methods is the laser floating zone (LFZ) melting technique. This process has demonstrated its suitability in the Co-oxide based thermoelectric materials processing in the last years in our laboratories. In this work, two examples of the versatility and usefulness of the LFZ technique will be shown. The first one will be based on the processing of the well known Ca 3 Co 4 O 9 ceramic material. In this case, a very dense and homogeneous material is obtained in two steps (growth and annealing), reaching PF values as high as 0.42 mW/K 2 m at 800 ºC. In the second case, a new and well textured thermoelectric composite (Bi 2 Ca 2 Co 1.7 O x + Ca 3 Co 4 O 9 ) is obtained, reaching PF values of 0.31 mW/K 2 m at 650 ºC.
Boletín de la Sociedad Española de Cerámica y Vidrio, 2015
ABSTRACT Ca3Co4-xNixO9 (x=0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been... more ABSTRACT Ca3Co4-xNixO9 (x=0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. As a result of the Ni addition an increase in porosity has been detected. Moreover, the presence of Ni has been related with the increase of Ca2Co3O6 secondary phase and the appearance of a new NiO-CoO solid solution. However, for the 0.01-Ni doped samples an improvement in the thermoelectric performances has been measured. This effect has been related with a decrease in the resistivity values and an increase in the Seebeck coefficient. The raise in the power factor for the 0.01-Ni doped samples, compared with the undoped ones, is between 10 and 25% at 50 and 800 degrees C respectively. Moreover, the maximum power at 800 degrees C, around 0.25 mW/K-2.m, is significantly higher than the best results obtained in Ni doped samples reported previously in the literature. (C) 2015 Sociedad Espanola de Ceramica y Vidrio. Published by Elsevier Espana, S.L.U. This is an open access article under the CC BY-NC-ND license
Ceramics International, 2015
ABSTRACT Ca3−xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.0... more ABSTRACT Ca3−xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.01, 0.03, 0.05, 0.07, and 0.10) have been prepared using the classical solid state method. Microstructural characterization has shown that Na has been incorporated into the Ca3Co4O9 phase and that no Na-based secondary phases have been produced. It has also been found that Na addition promotes grain growth and favours sintering due to the formation of a small amount of liquid phase. Electrical resistivity decreases when Na content increases until 0.07Na addition while Seebeck coefficient is maintained practically unchanged. The improvement in electrical resistivity leads to higher power factor values than the usually obtained in samples prepared by more complex and expensive techniques.
Journal of Materials Science: Materials in Electronics, 2015
ABSTRACT Ca3−xBaxCo4O9 bulk polycrystalline thermoelectric ceramics with x = 0.00, 0.01, 0.03 and... more ABSTRACT Ca3−xBaxCo4O9 bulk polycrystalline thermoelectric ceramics with x = 0.00, 0.01, 0.03 and 0.05 have been prepared through a classical solid state method. Microstructural characterizations of sintered samples have shown that Ba has been incorporated into the Ca3Co4O9 and/or Ca3Co2O6 structures and that no Ba-based secondary phases have been produced. The apparent density values increase when the amount of barium is raised with respect to the undoped samples, reaching a maximum value of around 77 % of the Ca3Co4O9 theoretical density, for samples with the highest amount of barium. Electrical resistivity of doped samples decreases in the whole measured temperature range with respect to the undoped ones, while Seebeck coefficient remains practically unchanged. In conclusion, the improvement in resistivity leads to power factor values which are higher than the ones measured in undoped samples, fact which makes this bulk polycrystalline compound a promising candidate for high-temperature power generation applications in oxidative environments.
ABSTRACT Ca3Co4−xTixO9 polycrystalline thermoelectric ceramics with small amounts of Ti have been... more ABSTRACT Ca3Co4−xTixO9 polycrystalline thermoelectric ceramics with small amounts of Ti have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, they show that Ti has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, until 0.03-Ti doped ones. Further Ti addition produces an increase of resistivity with respect to the 0.03 Ti doped samples. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Ti content. The improvement in electrical resistivity leads to about 55 % higher power factor values for the 0.03 Ti-doped samples than that obtained in the undoped ones. The maximum power factor at 800 °C, around 0.33 mW/K2 m, is slightly higher than the obtained in higher density samples, clearly indicating the good thermoelectric performances of these doped samples.
Bi 2 Ca 2 Co 1.7 O x bulk polycrystalline ceramics were prepared by the solid state method and by... more Bi 2 Ca 2 Co 1.7 O x bulk polycrystalline ceramics were prepared by the solid state method and by directional growth. Moreover, the effect of annealing on the textured materials has been studied. Microstructure has shown randomly oriented grains in the classical sintered materials while in the textured samples they were well oriented with their c-axis nearly perpendicular to the growth direction. Furthermore, the annealed samples showed much lower amount of secondary phases than the as-grown ones. These microstructural changes are reflected on the thermoelectric properties which increase with the grain orientation and with the decrease on the secondary phases content mainly due to the electrical resistivity reduction. As a consequence, a raise on the power factor of about 6 and 9 times, compared with the classically sintered samples, was obtained for the as-grown and annealed samples, respectively. The maximum power factor obtained at 650 ºC in the annealed samples (~ 0.31 mW/K 2 m) is about 50 % higher than the obtained in sinter-forged textured materials at the same temperature.
Boletín de la Sociedad Española de Cerámica y Vidrio, 2013
Boletín de la Sociedad Española de Cerámica y Vidrio, 2014
Ca 3-x Mg x Co 4 O 9 polycrystalline thermoelectric ceramics with small amounts of Mg have been s... more Ca 3-x Mg x Co 4 O 9 polycrystalline thermoelectric ceramics with small amounts of Mg have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Mg has been incorporated into the Ca 3 Co 4 O 9 structure and no Mg-based secondary phases have been identified. Apparent density measurements have shown that samples do not modify their density until 0.05 Mg content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Mg content increases until 0.05 Mg addition. The improvement in both parameters leads to higher power factor values than the usually obtained in samples prepared by the conventional solid state routes and close to those obtained in textured materials.
JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 2014
ABSTRACT Ca3Co4-xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have be... more ABSTRACT Ca3Co4-xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. XRD data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, it has been found that the Zn has been incorporated into these two phases. Slight Zn doping decreases electrical resistivity compared with the values obtained in undoped samples. The minimum values have been obtained for the 0.01-Ni doped samples, increasing for further Zn substitution. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Zn content. The improvement in electrical resistivity leads to higher power factor values for the 0.01 Zn-doped samples (about 30 %) than for the undoped ones. The maximum power factor at 800 A degrees C, around 0.27 mW/K-2 m is significantly higher than the best results obtained in Zn doped samples reported in the literature.
Scripta Materialia, 2013
ABSTRACT Bi2Ba2Co2Ox thermoelectric ceramics were textured from the melt using the laser floating... more ABSTRACT Bi2Ba2Co2Ox thermoelectric ceramics were textured from the melt using the laser floating zone method, at a growth rate of 5 mm h−1. The microstructure showed good grain alignment with the growth axis. These microstructural features are reflected in the thermoelectric performance, with a significant increase in the power factor values, reaching ∼0.4 mW K−2 m−1 at 650 °C, much higher than the values typically obtained in these materials.
Scripta Materialia, 2014
Bi 2 Ca 2 Co 1.7 O x nominal composition ceramic rods were textured by the Electrically Assisted ... more Bi 2 Ca 2 Co 1.7 O x nominal composition ceramic rods were textured by the Electrically Assisted Laser Floating Zone technique. A new composite formed by three phases, two of them with well known thermoelectric properties, Bi 2 Ca 2 Co 1.7 O x and Ca 3 Co 4 O 9 , has been obtained for the first time when the growth process is performed with 300 mA applied current. Under these conditions the thermoelectric performances of this new composite reach values above the highest ones reported in the literature for polycrystalline Bi 2 Ca 2 Co 1.7 O x materials.
Materials Today, 2013
Thermoelectric materials can directly convert a temperature gradient into an electric voltage tha... more Thermoelectric materials can directly convert a temperature gradient into an electric voltage thanks to the Seebeck effect. Since its discovery in 1823, this characteristic has allowed the use of metallic junctions to control temperatures. These thermocouples possess a low electrical resistivity and Seebeck coefficient with high thermal conductivity. The thermoelectric performances of these materials are evaluated by the dimensionless figure-of-merit (ZT = S 2 sT/k, where S is the Seebeck coefficient, T is the absolute temperature, s is the electrical conductivity, and k is the thermal conductivity) which is very small, indicating that metallic materials possess poor thermoelectric behavior. The introduction of intermetallic semiconductor materials, with higher thermoelectric performances than their metallic counterparts, has resulted in their practical application in thermoelectric modules. These modules can be found as Peltier devices, when used for refrigeration purposes, or Seebeck devices when used for electrical energy generation. Current applications of this kind of material include the environmentally friendly recovery of industrial and automobile waste heat, and in the production of electrical energy in radioisotope thermoelectric generators installed on spacecrafts, or in lighthouses located in isolated regions.
Journal of the European Ceramic Society, 2012
Two solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obta... more Two solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Ca 3 Co 4 O 9 misfit compounds with improved thermoelectric properties, compared to the classical solid state reaction. A comparison among the final products obtained by these different methods has been performed using DTA, TGA, FTIR X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show a very significative reduction on the secondary phases content. As a consequence, an important decrease on the electrical resistivity values is produced, compared to the solid state prepared samples, leading to a relatively important power factor raise.
Journal of the European Ceramic Society, 2012
Bi 2 Sr 2 Co 1.8 O x /Ag composites with small amounts of Ag have been synthesized by a sol-gel v... more Bi 2 Sr 2 Co 1.8 O x /Ag composites with small amounts of Ag have been synthesized by a sol-gel via nitrates reaction and directionally grown from the melt. Some of the obtained samples were annealed in order to obtain the thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases distribution and alignment. Moreover, thermoelectric and mechanical characteristics of annealed samples were determined by the four-probe technique and by three point flexural strength tests, respectively. Scanning electron microscopy revealed that Ag particles appear dispersed among well oriented ceramic grains with large size, providing a plastic flow region which increases the flexural strength for the optimally Ag added samples (1 wt.%). The composites electrical resistivity is lower than that of pure Bi 2 Sr 2 Co 1.8 O x while Ag addition does not significantly affect thermopower values. The resistivity reduction leads to power factor improvements of ∼50%, compared with pure samples, for Ag additions of 1 wt.% Ag.
Journal of the European Ceramic Society, 2014
ABSTRACT Bi2−xPbxSr2CaCu2Oy textured materials (x = 0.0, 0.2, 0.4, and 0.6) have been successfull... more ABSTRACT Bi2−xPbxSr2CaCu2Oy textured materials (x = 0.0, 0.2, 0.4, and 0.6) have been successfully prepared by the laser floating zone technique. Microstructure and electrical properties (JC and TC) have been clearly affected by Pb addition. From the E–I curves, slope of the transition between the superconducting and the normal state (n) at 77 K reaches a maximum of about 16 for the 0.4 Pb doped samples. This value is much higher than the typical ones for the Bi-2212 materials. Moreover, when the electrical properties of the 0.4 Pb doped samples are measured at lower temperatures (between 65 and 77 K), n values increase when the temperature is decreased. A maximum n value of 32 has been reached at 65 K which makes this material very attractive for its use as resistive fault current limiters.
Journal of the European Ceramic Society, 2013
High-performance Ca 3 Co 4 O 9 thermoelectric ceramic has been prepared from a Ca 1-x Co x O/Ca y... more High-performance Ca 3 Co 4 O 9 thermoelectric ceramic has been prepared from a Ca 1-x Co x O/Ca y Co 1-y O divorced eutectic structure produced by a directional meltgrown using the laser floating zone technique. This material has been grown at very high solidification rate in order to produce a very fine microstructure to reduce the necessary annealing time to recover the Ca 3 Co 4 O 9 thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases and estimate the extent of Ca 3 Co 4 O 9 formation with time and related with their thermoelectric performances. The optimum annealing time, 72 h, has been determined by the maximum power factor value (about 0.42 mW K -2 m -1 ), which is around the best values reported in textured materials (~ 0.40 mW K -2 m -1 ). This high power factor outcome from the high Ca 3 Co 4 O 9 phase content, apparent density and Co 3+ /Co 4+ relationship determinations performed in the present work.
Journal of Superconductivity and Novel Magnetism, 2013
Bi-2212 samples prepared by the classical solid-state method have been grown from the melt using ... more Bi-2212 samples prepared by the classical solid-state method have been grown from the melt using the Laser Floating Zone (LFZ) method. They have shown good grain alignment and transport critical current densities (J c ). After postannealing processes designed to produce the Bi-2212 phase controlled decomposition, J c values have been increased in an important manner. Maximum values have been achieved when samples were thermally treated at 680 ºC for 168 h with improvements around 80 %, compared with the original textured samples. The results clearly indicate that postannealing processes, when adequately controlled, produce the formation of effective pinning centers which are responsible for the raise in the measured J c values.