Präsentation
Filtern
Erscheinungsjahr
- 2017 (3) (entfernen)
Dokumenttyp
- Vortrag (3)
Referierte Publikation
- nein (3)
Schlagworte
- Calcium cobaltite (3) (entfernen)
Eingeladener Vortrag
- nein (3)
The development of competitive thermoelectric generators using oxide ceramics and multilayer technology requires the investigation of suitable sintering procedures. The sintering of Ca3Co4O9, which is the most promising p-type oxide regarding its figure of merit, is a special challenge, as the material starts to decompose at 926 °C. 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. Relative density increases from 57.4 % after free sintering for 24 h to 93.7 % after 2 h of PAS with 10 MPa axial load. The combination of tape casting and PAS induces a pronounced alignment of the anisotropic grains. In comparison to freely sintered multilayers, the electrical conductivity in the casting direction is thereby strongly increased by a factor of 5. With respect to dry-pressed bars, the improvement amounts to a factor of 15. 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.
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.
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.