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Eingeladener Vortrag
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Calcium cobaltite is a promising p-type oxide thermoelectric material for high temperature applications due to its high figure of merit between 600 °C and 900 °C in air. The solid-state-reaction is well known for large scale powder synthesis of functional materials. As a high temperature process, the powder synthesis consumes a lot of energy. In different studies, different synthesis conditions were used for the preparation of calcium cobaltite powder. This study showed that a higher energy input (elevated temperatures, longer dwell times, or repeated calcinations) during powder synthesis does not increase but decrease the Seebeck coefficient and the electrical conductivity. The same correlation was determined for the densification. As a higher energy input leads to a larger grain size and therefore to a reduced sinter activity, it can be concluded that the thermoelectric properties are correlated with the sinter activity of the powder. These results can be used to minimize the energy demand for the powder synthesis of Ca₃Co₄ O₉.
Entwicklung oxidkeramischer Werkstoffe und Folien für thermoelektrische Multilayergeneratoren
(2016)
Calcium cobaltite is a promising oxide thermoelectric materials for applications between 600 °C and 900 °C in air to convert waste heat directly into electrical power. The solid-state reaction, well known for large scale powder synthesis of functional materials, is used for the production of thermoelectric oxides. As a high temperature process, the powder synthesis consumes a lot of energy. In different studies, different synthesis conditions were used for the preparation of calcium cobaltite. To the author’s knowledge, a systematic study of the synthesis conditions of calcium cobaltite and calcium manganate has not yet been published.
Therefore, the synthesis conditions for calcium cobaltite (temperature, dwell time, and particle size of raw materials) were studied with a statistical design of experiments (2³) and investigated regarding phase composition (XRD), densification, and thermoelectric properties.
This study showed that a higher energy input (elevated temperatures, longer dwell times, or repeated calcinations) during powder synthesis does not improve but deteriorate the thermoelectric properties of calcium cobaltite. The same correlation was determined for the shrinkage. As a higher energy input during powder synthesis leads to a larger grain size and therefore to a reduced sinter activity the shrinkage at a given sinter profile is minimize as well as the thermoelectric properties. These results can be used to minimize the energy demand for the powder synthesis of oxide thermoelectric materials.
In addition an increase of power factor by factor 10 can be achieved by applying pressure assisted sintering.
Pressure-assisted sintering of tape cast calcium cobaltite Ca3Co4O9 with varied powder compositions
(2017)
Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and the morphology of Ca3Co4O9 are strongly anisotropic because of its layered crystal structure. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. Hot-pressing of tablets is a well-known technology for grain alignment of Ca3Co4O9 and increases the thermoelectric properties in a/b-direction remarkably. However, hot-pressing of tablets is limited by the tablet size. An interesting alternative for larger components is the pressure assisted sintering of panels from tape casted layers. Tape casting already leads to grain orientation during green body forming. By combining tape casting and pressure assisted sintering (50 kN maximum force) of Ca3Co4O9, high densities and high thermoelectric properties can be reached for large components up to 200 mm edge length.
The morphology of Ca3Co4O9-grains can be designed by doping as well as by varying the powder synthesis conditions. For example Bi-doping increases the anisotropy of the grains, and reaction sintering of uncalcined powder leads to a fine grained microstructure and increases the electrical conductivity for pressure-less sintered specimens.
Doped and undoped Ca3Co4O9 powders were successfully tape cast with the doctor blade technique. Several layers of tape were stacked and laminated to 7 cm x 7 cm panels. These panels were sintered in a LTCC sintering press with combined in-situ shrinkage measurement. Pressure-less sintered panels from undoped powder have a 2.5 times higher electrical conductivity at room temperature than dry-pressed test bars with randomly orientated particles. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity (σ25°C = 15000 S/m) increases by the factor of 6 compared to the pressure-less sintered panels, which is in good accordance to the values reported in literature for conventional hot pressing [1, 3]. It is not possible to assign the increased anisotropy of Ca2.7Bi0.3Co4O9 to the pressure-assisted sintered panels, as Bi leads to an abnormal grain growth (up to 500 µm) with randomly oriented grains. This decreases the electrical conductivity (σ25°C = 5000 S/m). Such an abnormal grain-growth is reported for Bi over-doped Ca3Co4O9 but not because of hot-pressing.
Lowering the sintering temperature of calcium manganate CaMnO3 for thermoelectric applications
(2018)
Thermoelectric materials can convert waste heat directly into electrical power by utilizing the Seebeck effect. Calcium cobaltite (p-type) and calcium manganate (n-type) are two of the most promising oxide thermoelectric materials. The development of cost-effective multilayer thermoelectric generators requires the co-firing of these materials and therefore the adjustment of sintering temperatures. Calcium manganate is conventionally sintered between 1200 °C and 1350 °C. Calcium cobaltite exhibits an undesired phase transition at 926 °C but can be sintered to high relative density of 95 % at 900 °C under axial pressure of 7.5 MPa. Hence, co-firing at 900 °C would be favourable. Therefore, strategies for lowering the sintering temperature of calcium manganate have been investigated. Basically, two approaches are common: i) addition of low melting additives like Bi2O3-ZnO-B2O3-SiO2 (BBSZ) glass or Bi2O3, and ii) addition of additives that form low-melting eutectics with the base material, for example CuO. In this study, several low melting additives including BBSZ glass and Bi2O3, as well as CuO were tested regarding their effect on calcium manganate densification. Bi2O3 did not improve the densification, whereas BBSZ glass led to 10 % higher relative density at 1200 °C. An addition of 4 wt% CuO decreases the temperature of maximum sinter rate from above 1200 °C to 1040 °C. By reducing the particle size of the raw materials from 2 μm to 0.7 μm the maximum sinter rate could be further shifted 20 K towards lower temperatures and the sinter begin decreased from 920 °C to 740 °C. It is shown that eutectic phase formation is more effective in lowering sintering temperature and accelerating densification than low-melting additives.
Thermoelektrische Materialien können durch die Nutzung des Seebeckeffektes einen Temperaturunterschied direkt in eine Spannung umwandeln. Calciumcobaltit (p-typ) und Calciummanagant (n-typ) sind 2 der vielversprechendsten oxidischen thermoelektrischen Materialien. Für die Entwicklung von kostengünstigen Multilayergeneratoren ist das Co-sintern dieser beiden Materialien notwendig und deshalb eine Anpassung der Sintertemperatur nötig. Calciummangant wird herkömmlicherweise zwischen 1200°C und 1350°C gesintert. Calciumcobaltit erfährt einen ungewünschte Phasenumwandlung bei 926°C, es kann allerding bei 900°C unter 7.5MPa zu 95% dicht gesintert werden. Demzufolge, ist eine Co-sintertemperatur von 900°C anzustreben. Aus diesem Grund wurden mehrere Strategien zur Absenkung der Sintertemperatur von Calciummanaganat untersucht. Zum einen die Zugabe niedrigschmelzender Additive, zum anderen die Zugabe von Additiven, die eine eutektische Schmelze bilden. Es konnte gezeigt werden, dass für Calciummanganat die Verwendung von eutektischen Schmelzen besser geeignet ist als die Verwendung von niedrigschmelzenden Additiven um die Sintertemperatur zu senken.“
Calciumcobaltit und Calciummanganat gehören zu den vielversprechendsten thermoelektrischen Oxiden im Temperaturbereich zwischen 600 °C und 800 °C an Luft. Mittels thermoelektrischer Generatoren kann ein Temperaturgradient direkt in elektrische Leistung umgewandelt werden. Für die kostengünstige Pulverherstellung von Funktionsmaterialien wird im industriellen Maßstab meist die Festphasenreaktion (bzw. Kalzinierung) verwendet. Da es sich dabei um einen Hochtemperaturprozess handelt, ist diese Kalzinierung sehr energieintensiv. In der Literatur werden sehr unterschiedliche Prozessbedingungen zur Pulversynthese thermoelektrischer Oxide genutzt. Soweit dem Autor bekannt, ist keine systematische Untersuchung des Einflusses der Pulversynthesebedingungen auf die thermoelektrischen Eigenschaften publiziert.
Deshalb wurde eine systematische Untersuchung des Einflusses der Pulversynthesebedingungen (Temperatur, Haltezeit, Partikelgröße, Wiederholungen) auf die thermoelektrischen Eigenschaften von Calciumcobaltit und Calciummanganat durchgeführt.
Es konnte gezeigt werden, dass sich ein höherer Energieeintrag während der Kalzinierung negativ auf die thermoelektrischen Eigenschaften auswirkt.
A wide range of solid-state synthesis routes for calcium manganate is reported in the literature, but there is no systematic study about the influence of the solid-state synthesis conditions on thermoelectric properties. Therefore, this study examined the influence of calcination temperature and calcination cycles on the Seebeck coefficient, electrical conductivity, and thermal conductivity. Higher calcination temperatures and repeated calcination cycles minimized the driving force for sintering of the synthesized powder, leading to smaller shrinkage and lower densities of the sintered specimens. As the electrical conductivity increased monotonously with increasing density, a higher energy input during calcination caused deterioration of electrical conductivity. Phase composition and Seebeck coefficient of sintered calcium manganate were not influenced by the calcination procedure. The highest thermoelectric properties with the highest power factors and figures of merit were obtained by means of reaction-sintering of uncalcined powder.
Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca3Co4O9 powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.
Calciumcobaltit ist eines der vielversprechendsten thermoelektrischen Oxide, welche zur direkten Wärmerückgewinnung genutzt werden können. Calciumcobaltit weist sowohl eine anisotrope Partikelform als auch anisotrope thermoelektrische Eigenschaften auf. Durch gezielte Ausrichtung der Partikel mittels Foliengießens und druckunterstützter Sinterung kann diese Anisotropie gezielt auf ein Bauteil übertragen werden. Die Partikelmorphologie kann durch Dotierung und Kalzinierung beeinflusst werden. Verschiedene Calciumcobaltitpulver wurden druckgesintert und das Gefüge sowie die thermoelektrischen Eigenschaften untersucht. Die Dotierung mit Bismut führte zu einer Verschlechterung der elektrischen Leitfähigkeit bei druckgesinterten Proben im Gegensatz zu undotierten Pulvern.
Influence of solid-state-synthesis conditions on properties of oxide thermoelectric materials
(2017)
Calcium cobaltite and calcium manganate are promising oxide thermoelectric materials for applications between 600 °C and 900 °C in air to convert waste heat directly into electrical power. The solid state reaction, well known for large scale powder synthesis of functional materials, is used for the production of thermoelectric oxides. As a high temperature process, the powder synthesis consumes a lot of energy. In different studies, different synthesis conditions were used for the preparation of calcium cobaltite and calcium manganate powder. To the author’s knowledge, a systematic study of the synthesis conditions of calcium cobaltite and calcium manganate has not yet been published.
Therefore, the synthesis conditions for calcium cobaltite (temperature, dwell time, and particle size of raw materials) were studied with a statistical design of experiments (2³) and investigated regarding phase composition (XRD), densification and thermoelectric properties. The gained knowledge was used to optimize the solid state reaction of calcium manganate.
This study showed that a higher energy input (elevated temperatures, longer dwell times, or repeated calcinations) during powder synthesis does not improve but deteriorate the thermoelectric properties of calcium cobaltite. The same correlation was determined for the densification. As a higher energy input during powder synthesis leads to a larger grain size and therefore to a reduced sinter activity the densification at a given sinter profile is minimize as well as the thermoelectric properties. These results can be used to minimize the energy demand for the powder synthesis of oxide thermoelectric materials.