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Eingeladener Vortrag
- nein (20)
Thermoelektrische Effekte beschreiben die direkte Verknüpfung von thermischer Energie und elektrischer Energie in Festkörpern. Durch Thermodiffusionsströme entsteht direkt, ohne beweg¬liche Teile, ein elektrisches Feld als Folge einer Temperaturdifferenz. Diese Material-eigenschaft wird durch den Seebeckkoeffizienten beschrieben. Je nach Art der Ladungs¬träger sind die indu¬zierte Spannung und der Seebeckkoeffizient positiv (p-Typ) oder negativ (n-Typ). Thermo¬elek¬trische Effekte lassen sich beispielsweise in Thermo¬elementen zur Temperatur-messung, in Pel¬tierelementen zum Kühlen oder Heizen und in thermo¬elektrischen Generatoren zur Umwandlung von thermischer Energie in elektrische Energie nutzen.
In thermoelektrischen Generatoren werden Schenkel aus p-Typ- und n-Typ-Materialien elek-trisch in Reihe und thermisch parallel verschaltet. Konventionell werden einzelne Schenkel aus Bismut¬tellurid auf ein metallisiertes Substrat gelötet. Man spricht vom π-Typ-Design. Aufgrund auf¬wendiger Fertigung und nicht optimaler Flächennutzung stellt dieses Design nicht die best-mög¬liche Lösung dar. Neben Telluriden gibt es noch andere vielversprechende thermoelektrische Material¬systeme wie die oxidischen Thermoelektrika. Im Temperatur¬bereich oberhalb von 700 °C können oxidische thermoelektrische Materialien mit nichtoxidischen konkurrieren. Zudem sind sie oxidationsbeständig und können aus weniger toxischen und besser verfügbaren Rohstoffen her¬ge¬stellt werden. Da es sich um keramische Materialien handelt, können unter Nutzung der Multi¬layer¬technologie (auch Vielschicht- oder Mehrlagentechnik) Generatoren im Multilayer¬design hergestellt werden. Keramische Multi¬layer¬generatoren sind aufgrund der höheren Leis¬tungs¬dichte, der Möglichkeit der gezielten Texturierung und des hohen möglichen Auto¬mati¬sierungs¬grades des Herstellungs¬prozesses eine viel¬versprechende Alternative zu konven¬tionellen π-Typ-Generatoren. Alle Lagen werden in einem Schritt co-gesintert. Die beiden zum jetzigen Zeit¬punkt wohl viel¬versprechendsten oxi¬dischen Thermoelektrika sind Calcium-cobaltit Ca3Co4O9 als p-Typ und Calciummanganat CaMnO3 als n-Typ. Die Sinter¬tem¬peratur von Ca3Co4O9 ist durch eine Phasenumwandlung bei 926 °C beschränkt. Texturiertes, dichtes Ca3Co4O9 mit einer hohen Festigkeit kann nur über Hei߬pressen hergestellt werden. Das Co-Sintern von Ca3Co4O9 und CaMnO3 war wegen der Tem¬pera¬tur¬differenz von 350 K zwischen den jeweiligen Sinterintervallen bisher nicht möglich. Ziel dieser Arbeit war deshalb die Ent¬wick¬lung von kompatiblen oxidkeramischen Werkstoffen und Folien für thermoelektrische Multilayer-generatoren auf der Basis von Ca3Co4O9 und CaMnO3.
Daraus resultieren vier wesentliche Arbeitspakete. Zunächst die Materialentwicklungen von Ca3Co4O9 (p-Typ) und CaMnO3 (n-Typ) für ein Co-Sintern bei 900 °C mit akzeptablen thermoelek¬trischen Eigenschaften, dann die Entwicklung der weiteren im Generator benötigten Kompo¬nenten wie der Isolationsschicht und abschließend die Fertigung und Bewertung von Demonstra¬toren im Multi¬layer¬design.
Foliengießen und druckunterstütztes Sintern ermöglichen die Herstellung von dichtem, tex-turier¬tem Ca3Co4O9 mit hoher Festigkeit und hohem Leistungsfaktor. Letzterer ist das Produkt der elek¬trischen Leitfähigkeit und dem Quadrat des Seebeckkoeffizienten. Für die elektrische Leit¬fähigkeit zeigte sich in dieser Arbeit ein kombinierter Einfluss von Sinterdichte und Textur. Die thermo¬elektrischen Eigen¬schaften lassen sich somit über die Einstellung der Mikrostruktur gezielt steuern.
Durch die Optimierung der Pulversynthese, die Einführung des Sinteradditives CuO und die Kombi¬nation mit dem druckunterstützten Sintern (7,5 MPa) konnte die Sintertemperatur des CaMnO3 bei gleichbleibendem Leistungsfaktor von 1250 °C auf 950 °C gesenkt werden. Druck-unter¬stütztes Sintern von CaMnO3 ist bei 900 °C möglich, führt aber zu einem Werkstoff mit geringerem Leistungsfaktor, geringerer Dichte und ungenügender Festigkeit.
Zur elektrischen Isolation der beiden thermoelektrischen Materialien wurde ein Glas-Keramik-Kompo¬sit mit hohem Volumenwiderstand und angepasstem Wärmeausdehnungs¬koef¬fizienten ent¬wickelt.
Aus den zu Folien vergossenen thermoelektrischen Materialien, der siebgedruckten Iso¬lations-schicht und der siebgedruckten Metallisierung wurden mittels Multilayertech¬nologie De¬mons-tratoren hergestellt. Neben dem pn-Generator aus Ca3Co4O9 und CaMnO3 wurden auch Unileg-generatoren aus Ca3Co4O9 gefertigt. Bei Unileggeneratoren wird die Komplexität des Aufbaus durch die Verwendung von nur einem thermoelektrischen Material verringert. Die Simulation der Demonstratoren zeigte, dass der pn-Generator aus Ca3Co4O9 und CaMnO3 keine höheren Leis-tungsdichten erbringt als der aus nur Ca3Co4O9 bestehende Unileg¬generator. Auf¬grund des ge-ringen Leistungsfaktors und der geringen Festigkeit des bei 900 °C gesinterten CaMnO3 er¬scheint die Fertigung von pn-Multilayer¬generatoren aus Ca3Co4O9 und CaMnO3 derzeit nicht sinn¬voll. Die Unileg¬generatoren aus Ca3Co4O9 erreichen mit sehr hoher Reproduzier¬barkeit 2 mW/cm² bei einer Temperaturdifferenz von 230 K, dies entspricht 80 % der simulierten elektrischen Leistung. Es handelt sich hierbei um den ersten Machbarkeits¬nachweis zur Herstellung von Multilayer-generatoren auf Basis von texturiertem Ca3Co4O9 mit hohem thermoelektrischem Leistungsfaktor, hoher Dichte und hoher Festigkeit.
Solch thermoelektrische Multilayergeneratoren könnten zukünftig Systeme mit geringen elek-trischen Leistungsanforderungen wie Sensoren autark und nachhaltig mit elektrischer Energie ver¬sorgen.
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.
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.
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.
Thermoelektrische Generatoren können zum „Energy harvesting“ für den autarken Betrieb von bspw. Sensoren eingesetzt werden. Eine interessante Alternative zu den herkömmlichen π-Typ Generatoren sind auf Grund der höheren Leistungsdichte und der guten Automatisierbarkeit thermoelektrische Multilayergeneratoren. Calciumcobaltit ist ein vielsprechendes oxidisches Thermoelektrika (p-Typ) mit stark anisotropen Eigenschaften. Die hier vorgestellte Studie zeigt die Entwicklung von texturierten Unileg-Multilayer-Generatoren mittels keramischer Multilayertechnologie.
Calciumcobaltit wird durch Foliengießen und druckunterstützte Sinterung texturiert. Im Vergleich zur konventionellen Sinterung verbessert sich die Festigkeit um den Faktor 10. Die thermoelektrischen Eigenschaften können je nach verwendetem Druckniveau hinsichtlich maximalem Power Factor oder hinsichtlich maximalem Gütefaktor optimiert werden.
Ein Glaskeramikkomposit wird als Isolationsmaterial mit hohem Volumenwiderstand und angepasstem Wärmeausdehnungskoeffizienten entwickelt. Der Unileg-Multilayer-Generator wird in einem Schritt co-gesintert. Die hergestellten Demonstratoren erreichen 80% der simulierten Output-Leistung. Diese Ergebnisse stellen den ersten Machbarkeitsnachweis für die Herstellung von co-gesinterten Multilayer-Generatoren aus texturiertem Calciumcobaltit mit hohem Power Factor und hoher Festigkeit dar.
Calcium cobaltite Ca3Co4O9, abbreviated Co349, is a promising thermoelectric material for high‐temperature applications in air. Its anisotropic properties can be assigned to polycrystalline parts by texturing. Tape casting and pressure‐assisted sintering (PAS) are a possible future way for a cost‐effective mass‐production of thermoelectric generators. This study examines the influence of pressure and dwell time during PAS at 900°C of tape‐cast Co349 on texture and thermoelectric properties. Tape casting aligns lentoid Co349. PAS results in a textured Co349 microstructure with the thermoelectrically favorable ab‐direction perpendicular to the pressing direction. By pressure variation during sintering, the microstructure of Co349 can be tailored either toward a maximum figure of merit as required for energy harvesting or toward a maximum power factor as required for energy harvesting. Moderate pressure of 2.5 MPa results in 25% porosity and a textured microstructure with a figure of merit of 0.13 at 700°C, two times higher than the dry‐pressed, pressureless‐sintered reference. A pressure of 7.5 MPa leads to 94% density and a high power factor of 326 µW/mK2 at 800°C, which is 11 times higher than the dry‐pressed reference (30 MPa) from the same powder.
Glass-ceramic composites as insulation material for thermoelectric oxide multilayer generators
(2021)
Thermoelectric generators can be used as energy harvesters for sensor applications.
Adapting the ceramic multilayer technology, their production can be
highly automated. In such multilayer thermoelectric generators, the electrical insulation material, which separates the thermoelectric legs, is crucial for the performance of the device. The insulationmaterial should be adapted to the thermoelectric regarding its averaged coefficient of thermal expansion α and its sintering temperature while maintaining a high resistivity.
In this study, starting from theoretical calculations, a glass-ceramic Composite material adapted for multilayer generators fromcalciummanganate and Calcium cobaltite is developed. The material is optimized towards an α of 11 × 10−6 K−1 (20–500◦C), a sintering temperature of 900◦C, and a high resistivity up to 800◦C.
Calculated and measured α are in good agreement. The chosen glass-ceramic composite with 45 vol.% quartz has a resistivity of 1 × 107 Ωcm and an open porosity of <3%. Sintered multilayer samples from tape-cast thermoelectric oxides and screen-printed insulation show only small reaction layers. It can be concluded that glass-ceramic composites are a well-suited material class for insulation layers as their physical properties can be tuned by varying glass composition or dispersion phases.
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.
Thermoelectric generators can be used as energy harvesters for sensor applications. Multilayer thermoelectric generators (ML-TEGs) are a promising alternative to conventional π-type generators due to their high filling factor, high capability of automated production and the texturing potential during the production process. Calcium cobaltite is a promising thermoelectric oxide (p-type) with highly anisotropic properties. The following study shows the development of a textured unileg ML-TEG using ceramic multilayer technology.
Tape-casting and pressure assisted sintering are applied to fabricate textured calcium cobaltite. Compared to conventional sintering, pressure assisted sintering increases the strength by the factor 10. Thermoelectric properties can be tuned either towards maximum power factor or towards maximum figure of merit depending on the pressure level.
As electrical insulation material, a screen-printable glass-ceramic with high resistivity and adapted coefficient of thermal expansion is developed. From various commercial pastes a metallization with low contact resistance is chosen. The unileg ML-TEG is co-fired in one single step. The demonstrators reach 80% of the simulated output power and the power output is highly reproducible between the different demonstrators (99%). These results provide the first proof-of-concept for fabricating co-fired multilayer generators based on textured calcium cobaltite with high power factor, high density, and high strength.
Thermoelectric materials can convert waste heat directly into electrical power by utilizing the Seebeck effect. Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and morphology of Ca3Co4O9 are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. The electrical conductivity is for example 13.5 times higher in a/b-direction than in c-direction. 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. It also increases the relative density. 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 a grain alignment 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.
Ca3Co4O9 was successfully tape casted with the doctor blade technique (binder: polyvinyl butyral, organic solvent). Several layers of tape were stacked and laminated to 5 cm x 5 cm panels. These panels were sintered with different applied pressures in a LTCC sintering press with combined in-situ shrinkage measurement. Pressure-less sintered panels have a 2.5 times higher electrical conductivity at room temperature than test bars with randomly orientated particles. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity increases by the factor of 6 compared to the pressure-less sintered panels. About 40 % linear shrinkage are reached in pressing direction. The Seebeck coefficient (S25 °C=146 μV/K) and the electrical conductivity (σ25 °C=15100 S/m) are in good agreement with the values published in literature for hot-pressed tablets.
Thermoelectric generators can be used for energy harvesting by directly transforming a temperature gradient into a voltage. Multilayer generators based on low-temperature co-fired ceramics technology (LTCC) are an interesting alternative to conventional π-type generators. They exhibit several advantages like high filling factor, possibility of texturing, co-firing of all materials in one single-step, and reduction of production costs due to the high possible degree of automation. Pressure-assisted sintering enables the theoretical possibility of co-firing two promising oxide thermoelectric materials: Ca3Co4O9 (p-type) and CaMnO3 (n-type). Due to the large difference in sintering temperature (300 K) the process is very challenging.
In this work we show the material development of Ca3Co4O9, CaMnO3, insulation and metallization for multilayer generators co-fired under pressure at 900 °C. The materials are tailored regarding their sintering behavior, electrical performance and coefficients of thermal expansion. Different generator designs (unileg and pn-type) were fabricated and analyzed regarding crack formation, interaction layers and thermoelectric performance. Simulated stresses during cooling in the multilayers are compared with actual crack formation for different sintering conditions. This study shows that a lower pressure level and a lower level of complexity are beneficial for co-firing and performance.
Texturing of calcium cobaltite for thermoelectric applications by pressure assisted sintering
(2021)
Thermoelectric materials can convert waste heat directly into electrical power by using the Seebeck effect. Calcium cobaltite (CCO) is considered as a promising thermoelectric p-type oxide for energy harvesting applications at temperatures above 500 °C. The properties and morphology of single-crystal CCO are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. By aligning the plate-like grains, the anisotropic properties of the grains can be assigned to the poly-crystalline parts.
In this study, the combination of tape casting and pressure-assisted sintering is used to texture and densify large scale components (50 cm²). Thereby, the influence of powder preparation and applied pressure during sintering on texturing and thermoelectric properties is investigated.
The analysis of XRD pole figures revealed that tape casting already leads to highly textured CCO. By pressure variation during sintering, the microstructure of CCO can be tailored either toward maximum power factor as required for energy harvesting or toward maximum figure of merit as required for energy recovery. Low pressure lead to a porous microstructure and maximum figure of merit and higher pressure to full densification and maximum power factor. The electrical and thermal conductivity of CCO seem depending on both texture and sinter density.
Reaction sintering and sintering additives for cost-effective production of thermoelectric oxides
(2020)
Thermoelectric oxides attract much interest recently. Although their thermoelectric properties are inferior to non-oxides, they exhibit distinct advantages. Thermoelectric oxides are stable in air at higher temperatures, their raw materials are less toxic, and more abundant. To enhance attractivity of these materials for industrial applications, production costs need to be reduced.
Conventionally, the legs of thermoelectric generators are sintered from green bodies of previously synthesized powder. Reaction-sintering is a fabrication method without a powder synthesis step, as the final phase is formed during the sintering from a raw material mixture. Moreover, the reduction of chemical potential during reaction-sintering is effective as an additional driving force for sintering. We show that reaction-sintering increases the densification of CaMnO3 (n-type, Sm doped). Consequently, the electrical conductivities improved by about 100 % leading to superior power factors (PF = 230 µW/mK² for CaMnO3).
Another approach to reduce the production costs is to lower the sintering temperature by adding sinter additives. The addition of 4 wt% CuO to CaMnO3 lowers the sinter temperature from 1250 °C to 1050 °C. The achieved power factor PF = 264 µW/mK is more than two times higher as reported in literature for the same dopant.
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.
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.
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.
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.
Thermoelectric generators can be used for energy harvesting by directly transforming a temperature gradient into a voltage. Multilayer generators based on ceramic multilayer technology are an interesting alternative to conventional π-type generators. They exhibit several advantages like high filling factor, possibility of texturing, co-firing of all materials in one single-step, and reduction of production costs due to the high possible degree of automation. But, co-firing of promising oxide thermoelectric materials, Ca3Co4O9 (p-type) and CaMnO3 (n-type), is very challenging due to the large difference in sintering temperature (300 K).
In this work we show the material development of Ca3Co4O9, CaMnO3, and insulation for multilayer generators co-fired under uniaxial pressure at 900 °C. The materials are tailored regarding their sintering behavior, electrical performance and coefficients of thermal expansion. Tape-casting and pressure assisted sintering are applied to fabricate textured Ca3Co4O9. Compared to conventional sintering, pressure assisted sintering increases the strength by the factor 10 and the power factor by the factor of 20. The combination of sintering additives and uniaxial pressure is used to decrease the sintering temperature of CaMnO3 to 900 °C while maintaining acceptable thermoelectric properties.
Different generator designs (unileg and pn-type) were fabricated and analyzed regarding microstructure and thermoelectric performance. A lower level of complexity is beneficial for co-firing and performance. The unileg demonstrators reach 80% of the simulated output power and the power output is highly reproducible between the different demonstrators (99%).
Thermoelectric materials can convert waste heat directly into electrical power by using the Seebeck effect. Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and morphology of Ca3Co4O9 are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. Pressure-assisted sintering (PAS), as known from large-scale production of low temperature co-fired ceramics, was used to sinter multilayers of Ca3Co4O9 green tape at 900 °C with different pressures and dwell times. In-situ shrinkage measurements, microstructural investigations and electric measurements were performed. Pressure-less sintered multilayers have a 2.5 times higher electrical conductivity at room temperature than dry pressed test bars with randomly oriented particles. The combination of tape casting and PAS induces a pronounced alignment of the anisotropic grains. Relative density increases from 57 % after free sintering for 24 h to 94 % after 2 h of PAS with 10 MPa axial load. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity (at 25°C) improves by a factor of 15 compared to test bars with randomly oriented particles. The high temperature thermoelectric properties show the same dependencies. The smaller the applied axial load, the lower the relative densities, and the lower the electrical conductivity. Longer dwell times may increase the density and the electrical conductivity significantly if the microstructure is less densified as in the case of a small axial load like 2 MPa. At higher applied pressures the dwell time has no significant influence on the thermoelectric properties. This study shows that PAS is a proper technique to produce dense Ca3Co4O9 panels with good thermoelectric properties similar to hot-pressed tablets, even in large-scale production.