Calcium cobaltate (Ca3Co4O9) remains one of the most promising p-type oxide materials for high-te... more Calcium cobaltate (Ca3Co4O9) remains one of the most promising p-type oxide materials for high-temperature thermoelectric energy conversion. While much progress has been made in refining our understanding of the unique structure of the material, as well as optimization of the transport properties for thermoelectric efficiency, there remains a gap in the knowledge, both experimental and theoretical, of the thermodynamics of the system. Presented herein is an analysis of the heat of formation of the Ca3Co4O9 phase from sol-gel precursors using a highly sensitive differential scanning calorimeter, as well as some observations of its decomposition into the Ca3Co2O6 phase. The reaction enthalpy of forming Ca3Co4O9 from CaCO3 and Co3O4 sol-gel precursors has been determined to be +284 (±2%) kJ/mol, leading to a standard enthalpy of Ca3Co4O9 of –3307 (±3.5%) kJ/mol.
Composites of Mo3(Sb,Te)7 materials with C60 were synthesized and characterized.The carbon additi... more Composites of Mo3(Sb,Te)7 materials with C60 were synthesized and characterized.The carbon additions form amorphous nanoparticles with sizes of the order of 50 nm.The Seebeck effect remains basically unaffected from the carbon additions.Electrical and thermal conductivity decrease differently with increasing carbon content.The figure-of-merit was slightly improved, most noteworthy at higher temperatures.About 18 g of Ni0.05Mo3Sb5.4Te1.6 were prepared by heating the elements in the stoichiometric ratio at 1000 K. The product was divided into four parts, and then C60 was added to three of these four parts at 1, 2, and 3 mass%, respectively. Each part was hot-pressed at 150 MPa and 923 K. The sample with 1% C60 was characterized via a Rietveld refinement and TEM analyses. Measurements of the three thermoelectric key properties revealed that the Seebeck coefficient barely depends on the carbon amount added, while both the electrical and the thermal conductivity decrease with increasing amount of carbon. Depending on the amount of C60 used and on the temperature, the thermoelectric performance was either enhanced or decreased, depending on whether the electrical conductivity decreased less or more than the thermal conductivity. At the highest temperature measured, all carbon-containing samples performed better than the unmodified bulk sample, namely up to 14%. These improvements are within the error margin, however.Figure-of-merit of various Ni0.05Mo3Sb5.4Te1.6/C composites.
Journal of the European Ceramic Society, Apr 30, 2014
A rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modificat... more A rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gel method where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the advantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes coming from the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used to fabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||) and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 °C, power factors of 506 μW/m K2 (⊥) and 147 μW/m K2 (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.
ABSTRACT The past few decades have witnessed an increasing demand for renewable energy technology... more ABSTRACT The past few decades have witnessed an increasing demand for renewable energy technology, including that of direct thermal to electrical energy conversion via thermoelectricity. Central to high efficient thermoelectric (TE) energy conversion is a high figure of merit, ZT, of the TE material, which requires high electrical conductivity, high thermopower and low thermal conductivity. For simple bulk materials it is hard to simultaneously satisfy these criteria because these physical quantities are inter-dependent: optimizing one quantity often adversely affects the others. Over the past decade, the pursuit of higher ZT materials has culminated into a new paradigm, namely, nanocomposite thermoelectric materials (NcTMs). A NcTM is typically multi-phased, and the characteristic length scale of at least one constituent is on the order of nanometers. The resulting classical and quantum size effects arising from the nanophase(s) and at the interfaces help decouple the inter-dependence of those TE properties, leading to outstanding TE performance. In this paper, we present a brief survey of relevant patent disclosures for NcTMs, focusing on the preparation methods and the peculiar micromorphologies.
ABSTRACT Ca3Co4O9+δ samples were synthesized by solid-state (SS) and sol–gel (SG) reactions, foll... more ABSTRACT Ca3Co4O9+δ samples were synthesized by solid-state (SS) and sol–gel (SG) reactions, followed by spark plasma sintering under different processing conditions. The synthesis process was optimized and the resulting materials characterized with respect to their microstructure, bulk density, and thermoelectric transport properties. High power factors of about 400 μW/m·K2 and 465 μW/m·K2 (at 800°C) were measured for SS and SG samples, respectively. The improved thermoelectric performance of the SG sample is believed to originate from the smaller particle sizes and better grain alignment. The SG method is suggested to be a beneficial means of obtaining high-performance thermoelectric materials of Ca3Co4O9+δ type.
Freeze-dried wheat gluten foams were evaluated with respect to their thermal and fire-retardant p... more Freeze-dried wheat gluten foams were evaluated with respect to their thermal and fire-retardant properties, which are important for insulation applications. The thermal properties were assessed by differential scanning calorimetry, the laser flash method and a hot plate method. The unplasticised foam showed a similar specific heat capacity, a lower thermal diffusivity and a slightly higher thermal conductivity than conventional rigid polystyrene and polyurethane insulation foams. Interestingly, the thermal conductivity was similar to that of closed cell polyethylene and glass-wool insulation materials. Cone calorimetry showed that, compared to a polyurethane foam, both unplasticised and glycerol-plasticised foams had a significantly longer time to ignition, a lower effective heat of combustion and a higher char content. Overall, the unplasticised foam showed better fire-proof properties than the plasticized foam. The UL 94 test revealed that the unplasticised foam did not drip (form droplets of low viscous material) and, although the burning times varied, self-extinguished after flame removal. To conclude both the insulation and fire-retardant properties were very promising for the wheat gluten foam.
Zirconium diselenide (ZrSe2) is one of many members of the layer-structured transition-metal dich... more Zirconium diselenide (ZrSe2) is one of many members of the layer-structured transition-metal dichalcogenide family. The structure of these materials features a weakly bonded van der Waals gap between covalently bonded CdI2-type atomic layers that may host a wide range of intercalants. Intercalation can profoundly affect the structural, thermal, and electronic properties of such materials. While the thermoelectric potential of layer-structured transition metal dichalcogenides has been formerly studied by several groups, to our best knowledge, neither the thermoelectric properties of ZrSe2 nor the impact of intercalation on its thermoelectric properties have been reported (specifically, the full evaluation of the dimensionless figure of merit, ZT, which includes the thermal conductivity). In this proof-of-principle study, ZrSe2 single crystals have been synthesized using an iodine-assisted vapor transport method, followed by a wet-chemistry lithium intercalation process. The results of resistivity, thermopower, and thermal conductivity measurements between 10 K and 300 K show that Li intercalation induced additional charge carriers and structural disorder that favorably affected the thermoelectric properties of the material. As a result, a dimensionless figure of merit ZT 0.26 has been attained at room temperature in a Li-intercalated sample, representing nearly a factor of three improvement compared with the pristine sample. These improvements, along with the abundance, relatively low toxicity, and low cost of such materials, merit further thermoelectric investigations of intercalated zirconium diselenide, especially in conjunction with a substitutional doping approach.
Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates s... more Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates study of the interfaces between the oxide TE elements and the interconnect materials used to transfer current between them. In this study, interfaces between pure nickel and undoped calcium cobaltate (Ca3Co4O9) have been formed directly by spark plasma sintering (SPS). An intermediate NiO phase is formed during the SPS processes, which grows during post-heating with Co entering from the cobaltate side to form a graded Ni1xCoxO interfacial layer. The electrical and thermal transport across these interfaces, as well as the long-term chemical stability of the intermediate layers, have been studied and are discussed.
Polycrystalline samples of nickel intercalated (0 to 5%) TiSe2 were attempted via solid-state rea... more Polycrystalline samples of nickel intercalated (0 to 5%) TiSe2 were attempted via solid-state reaction in evacuated quartz tubes followed by densification using a spark plasma sintering process. X-ray diffraction data indicated that mixed NiSe2 and TiSe2 phases were present after initial synthesis by solid-state reaction, but a pure TiSe2 phase was present after the spark plasma sintering. While EPMA data reveals the stoichiometry to be near 1:1.8 (Ti:Se) for all samples, comparisons of the measured bulk densities to the theoretical densities suggest that the off stoichiometry is a result of the co-intercalation of both Ni and Ti rather than Se vacancies. Due to the presence of excess Ti (0.085-0.130 per formula) in the van der Waals gap of all the samples, the sensitive electron-hole balance is offset by the additional Ti-3d electrons, leading to an increase in the thermopower (n-type) over pristine, stoichiometric TiSe2. The effects of the co-intercalation of both Ni and Ti in TiSe2 on the structural, thermal, and electrical properties are discussed herein.
Mo3Sb7− xTex was earlier reported to be a promising p-type thermoelectric material for high tempe... more Mo3Sb7− xTex was earlier reported to be a promising p-type thermoelectric material for high temperature applications, with Ni0. 06Mo3Sb5. 4Te1. 6 achieving a ZT of 0.93 at 1023K. In order to investigate the effect of using different transition metal atoms and to further ...
Indium-filled CoSb3 skutterudites have been shown previously to have promising thermoelectric pro... more Indium-filled CoSb3 skutterudites have been shown previously to have promising thermoelectric properties but the thermal conductivity still remains somewhat high. In order to further decrease the thermal conductivity, the double-filling approach has been adopted using ytterbium, in conjunction with indium, due to its heavy mass and small size. The In0.1YbyCo4Sb12 (y=0.00, 0.05, 0.10, and 0.20) samples have been prepared by a melting method and subsequently characterized by means of electron microscopy, electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall coefficient measurements. The results show that the ytterbium filling effectively decreases the thermal conductivity without degrading the power factor, resulting in an enhancement of the dimensionless figure of merit ZT. A state-of-the-art ZT value of 0.97 is attained in In0.1Yb0.1Co4Sb12 at 750 K.
Calcium cobaltate (Ca3Co4O9) remains one of the most promising p-type oxide materials for high-te... more Calcium cobaltate (Ca3Co4O9) remains one of the most promising p-type oxide materials for high-temperature thermoelectric energy conversion. While much progress has been made in refining our understanding of the unique structure of the material, as well as optimization of the transport properties for thermoelectric efficiency, there remains a gap in the knowledge, both experimental and theoretical, of the thermodynamics of the system. Presented herein is an analysis of the heat of formation of the Ca3Co4O9 phase from sol-gel precursors using a highly sensitive differential scanning calorimeter, as well as some observations of its decomposition into the Ca3Co2O6 phase. The reaction enthalpy of forming Ca3Co4O9 from CaCO3 and Co3O4 sol-gel precursors has been determined to be +284 (±2%) kJ/mol, leading to a standard enthalpy of Ca3Co4O9 of –3307 (±3.5%) kJ/mol.
Composites of Mo3(Sb,Te)7 materials with C60 were synthesized and characterized.The carbon additi... more Composites of Mo3(Sb,Te)7 materials with C60 were synthesized and characterized.The carbon additions form amorphous nanoparticles with sizes of the order of 50 nm.The Seebeck effect remains basically unaffected from the carbon additions.Electrical and thermal conductivity decrease differently with increasing carbon content.The figure-of-merit was slightly improved, most noteworthy at higher temperatures.About 18 g of Ni0.05Mo3Sb5.4Te1.6 were prepared by heating the elements in the stoichiometric ratio at 1000 K. The product was divided into four parts, and then C60 was added to three of these four parts at 1, 2, and 3 mass%, respectively. Each part was hot-pressed at 150 MPa and 923 K. The sample with 1% C60 was characterized via a Rietveld refinement and TEM analyses. Measurements of the three thermoelectric key properties revealed that the Seebeck coefficient barely depends on the carbon amount added, while both the electrical and the thermal conductivity decrease with increasing amount of carbon. Depending on the amount of C60 used and on the temperature, the thermoelectric performance was either enhanced or decreased, depending on whether the electrical conductivity decreased less or more than the thermal conductivity. At the highest temperature measured, all carbon-containing samples performed better than the unmodified bulk sample, namely up to 14%. These improvements are within the error margin, however.Figure-of-merit of various Ni0.05Mo3Sb5.4Te1.6/C composites.
Journal of the European Ceramic Society, Apr 30, 2014
A rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modificat... more A rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gel method where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the advantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes coming from the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used to fabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||) and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 °C, power factors of 506 μW/m K2 (⊥) and 147 μW/m K2 (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.
ABSTRACT The past few decades have witnessed an increasing demand for renewable energy technology... more ABSTRACT The past few decades have witnessed an increasing demand for renewable energy technology, including that of direct thermal to electrical energy conversion via thermoelectricity. Central to high efficient thermoelectric (TE) energy conversion is a high figure of merit, ZT, of the TE material, which requires high electrical conductivity, high thermopower and low thermal conductivity. For simple bulk materials it is hard to simultaneously satisfy these criteria because these physical quantities are inter-dependent: optimizing one quantity often adversely affects the others. Over the past decade, the pursuit of higher ZT materials has culminated into a new paradigm, namely, nanocomposite thermoelectric materials (NcTMs). A NcTM is typically multi-phased, and the characteristic length scale of at least one constituent is on the order of nanometers. The resulting classical and quantum size effects arising from the nanophase(s) and at the interfaces help decouple the inter-dependence of those TE properties, leading to outstanding TE performance. In this paper, we present a brief survey of relevant patent disclosures for NcTMs, focusing on the preparation methods and the peculiar micromorphologies.
ABSTRACT Ca3Co4O9+δ samples were synthesized by solid-state (SS) and sol–gel (SG) reactions, foll... more ABSTRACT Ca3Co4O9+δ samples were synthesized by solid-state (SS) and sol–gel (SG) reactions, followed by spark plasma sintering under different processing conditions. The synthesis process was optimized and the resulting materials characterized with respect to their microstructure, bulk density, and thermoelectric transport properties. High power factors of about 400 μW/m·K2 and 465 μW/m·K2 (at 800°C) were measured for SS and SG samples, respectively. The improved thermoelectric performance of the SG sample is believed to originate from the smaller particle sizes and better grain alignment. The SG method is suggested to be a beneficial means of obtaining high-performance thermoelectric materials of Ca3Co4O9+δ type.
Freeze-dried wheat gluten foams were evaluated with respect to their thermal and fire-retardant p... more Freeze-dried wheat gluten foams were evaluated with respect to their thermal and fire-retardant properties, which are important for insulation applications. The thermal properties were assessed by differential scanning calorimetry, the laser flash method and a hot plate method. The unplasticised foam showed a similar specific heat capacity, a lower thermal diffusivity and a slightly higher thermal conductivity than conventional rigid polystyrene and polyurethane insulation foams. Interestingly, the thermal conductivity was similar to that of closed cell polyethylene and glass-wool insulation materials. Cone calorimetry showed that, compared to a polyurethane foam, both unplasticised and glycerol-plasticised foams had a significantly longer time to ignition, a lower effective heat of combustion and a higher char content. Overall, the unplasticised foam showed better fire-proof properties than the plasticized foam. The UL 94 test revealed that the unplasticised foam did not drip (form droplets of low viscous material) and, although the burning times varied, self-extinguished after flame removal. To conclude both the insulation and fire-retardant properties were very promising for the wheat gluten foam.
Zirconium diselenide (ZrSe2) is one of many members of the layer-structured transition-metal dich... more Zirconium diselenide (ZrSe2) is one of many members of the layer-structured transition-metal dichalcogenide family. The structure of these materials features a weakly bonded van der Waals gap between covalently bonded CdI2-type atomic layers that may host a wide range of intercalants. Intercalation can profoundly affect the structural, thermal, and electronic properties of such materials. While the thermoelectric potential of layer-structured transition metal dichalcogenides has been formerly studied by several groups, to our best knowledge, neither the thermoelectric properties of ZrSe2 nor the impact of intercalation on its thermoelectric properties have been reported (specifically, the full evaluation of the dimensionless figure of merit, ZT, which includes the thermal conductivity). In this proof-of-principle study, ZrSe2 single crystals have been synthesized using an iodine-assisted vapor transport method, followed by a wet-chemistry lithium intercalation process. The results of resistivity, thermopower, and thermal conductivity measurements between 10 K and 300 K show that Li intercalation induced additional charge carriers and structural disorder that favorably affected the thermoelectric properties of the material. As a result, a dimensionless figure of merit ZT 0.26 has been attained at room temperature in a Li-intercalated sample, representing nearly a factor of three improvement compared with the pristine sample. These improvements, along with the abundance, relatively low toxicity, and low cost of such materials, merit further thermoelectric investigations of intercalated zirconium diselenide, especially in conjunction with a substitutional doping approach.
Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates s... more Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates study of the interfaces between the oxide TE elements and the interconnect materials used to transfer current between them. In this study, interfaces between pure nickel and undoped calcium cobaltate (Ca3Co4O9) have been formed directly by spark plasma sintering (SPS). An intermediate NiO phase is formed during the SPS processes, which grows during post-heating with Co entering from the cobaltate side to form a graded Ni1xCoxO interfacial layer. The electrical and thermal transport across these interfaces, as well as the long-term chemical stability of the intermediate layers, have been studied and are discussed.
Polycrystalline samples of nickel intercalated (0 to 5%) TiSe2 were attempted via solid-state rea... more Polycrystalline samples of nickel intercalated (0 to 5%) TiSe2 were attempted via solid-state reaction in evacuated quartz tubes followed by densification using a spark plasma sintering process. X-ray diffraction data indicated that mixed NiSe2 and TiSe2 phases were present after initial synthesis by solid-state reaction, but a pure TiSe2 phase was present after the spark plasma sintering. While EPMA data reveals the stoichiometry to be near 1:1.8 (Ti:Se) for all samples, comparisons of the measured bulk densities to the theoretical densities suggest that the off stoichiometry is a result of the co-intercalation of both Ni and Ti rather than Se vacancies. Due to the presence of excess Ti (0.085-0.130 per formula) in the van der Waals gap of all the samples, the sensitive electron-hole balance is offset by the additional Ti-3d electrons, leading to an increase in the thermopower (n-type) over pristine, stoichiometric TiSe2. The effects of the co-intercalation of both Ni and Ti in TiSe2 on the structural, thermal, and electrical properties are discussed herein.
Mo3Sb7− xTex was earlier reported to be a promising p-type thermoelectric material for high tempe... more Mo3Sb7− xTex was earlier reported to be a promising p-type thermoelectric material for high temperature applications, with Ni0. 06Mo3Sb5. 4Te1. 6 achieving a ZT of 0.93 at 1023K. In order to investigate the effect of using different transition metal atoms and to further ...
Indium-filled CoSb3 skutterudites have been shown previously to have promising thermoelectric pro... more Indium-filled CoSb3 skutterudites have been shown previously to have promising thermoelectric properties but the thermal conductivity still remains somewhat high. In order to further decrease the thermal conductivity, the double-filling approach has been adopted using ytterbium, in conjunction with indium, due to its heavy mass and small size. The In0.1YbyCo4Sb12 (y=0.00, 0.05, 0.10, and 0.20) samples have been prepared by a melting method and subsequently characterized by means of electron microscopy, electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall coefficient measurements. The results show that the ytterbium filling effectively decreases the thermal conductivity without degrading the power factor, resulting in an enhancement of the dimensionless figure of merit ZT. A state-of-the-art ZT value of 0.97 is attained in In0.1Yb0.1Co4Sb12 at 750 K.
Uploads