Nanostructured Nb-substituted CaMnO3 n-type thermoelectric material prepared in a continuous process by ultrasonic spray combustion

One way to further optimize the thermoelectric properties toward a higher ZT is a temperature stable nanoengineering of materials, where the thermal conductivity is reduced by increasing the phonon scattering at the grain boundaries. To study this, Nb-substituted CaMnO3 perovskite-type material was synthesized by ultrasonic spray combustion (USC). The grain growth has been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Finally, the thermoelectric properties of compacted and sintered bulk samples from powder prepared by a continuous scalable USC process were measured up to 1050 K. The thermoelectric legs were prepared by an adapted sintering process. Here, a compromise between enhanced porosity to reduce the thermal conductivity and securing of mechanical stability and low resistivity should be obtained. Based on the grain growth mechanisms, an advanced sintering process for additional interconnection of the particles without particle growth is needed to further increase the thermoelectric performanc

Thermoelectric properties of CaMnO3 films obtained by soft chemistry synthesis

Polycrystalline randomly oriented CaMnO3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K < T < 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO3 films at 900 °C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900 °C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavio

Half-Heusler (TiZrHf)NiSn Unileg Module with High Powder Density

(TiZrHf)NiSn half-Heusler compounds were prepared by arc melting and their thermoelectric properties characterized in the temperature range between 325 K and 857 K, resulting in a Figure of Merit ZT ≈ 0.45. Furthermore, the prepared samples were used to construct a unileg module. This module was characterized in a homemade thermoelectric module measurement stand and yielded 275 mW/cm2 and a maximum volumetric power density of 700 mW/cm3. This was reached using normal silver paint as a contacting material; from an improved contacting, much higher power yields are to be expected

Étude de la transition de Mott dans V2O3 dopé avec chrome par mesures de la vitesse du son et du pouvoir thermoélectrique

This work presents an experimental study of the high temperature Mott transition in chromium doped V2O3 by means of two different experimental techniques at pressures between ambient pressure and 6 kbar and temperatures from room T to 500 K. Ultrasound measurements as a direct probe of the compressibility were used to estimate the effects of the lattice on the Mott transition. We were able to present the first systematic study of the influence of the lattice degrees of freedom on the high temperature Mott transition in V2O3 and could verify the existence of a critical electronic temperature Tel c that differs in the order of 3% or 4% from the temperature of the transition Tc of the bulk sample. Furthermore, observation of the behaviour of the transversal mode of the speed of sound allowed to conclude that this transition occurs without symmetry breaking and the order parameter is a scalar. In the second part the evolution of the Seebeck coefficient was investigated under the same experimental conditions. The studies around the critical point allowed us to establish an experimental phase diagram. Furthermore, we were able to show that the change of the Seebeck coefficient at the Mott transition is driven by the change in the resistivity rather than by the change of the particle-hole symmetry. As a consequence of this we could relate the scaling laws for the critical parameters related to conductivity to our data. In the vicinity of the transition our data were quite well reproduced by the mean field exponents found in the conductivity experiments.Cette thèse présente une étude expérimentale de la transition de Mott dans V2O3 dopé au chrome par deux différentes techniques expérimentales entre la température ambiante à 500 K et sous pressions jusqu'à 6 kbar. La technique ultrasonore fournit une mesure directe de la compressibilité et a été utilisé pour estimer les effets de la réseau sur la transition de Mott. Nous avons été en mesure de mener la première étude systématique de l'influence des degrés de liberté du réseau sur la transition de Mott dans V2O3. On a vérifiée l'existence d'une température électronique critique Tel c qui diffère de l'ordre de 3% ou 4% par rapport à la température de transition effectivement observée Tc. L'observation du comportement du mode transverse de la vitesse du son a permis de conclure que la transition de Mott à haute température a lieu sans brisure de symétrie et le paramètre d'ordre est un scalaire. Dans la deuxième partie de la thèse l'évolution de coefficient du Seebeck a été étudiée dans les mêmes conditions expérimentales. Les études autour du point critique nous ont permis d'établir un diagramme de phase expérimentale. Nous avons montré que le changement de S à la transition est dominé par le changement de la résistivité plutôt que par le changement de la symétrie particule-trou. En conséquence, nous avons pu relier les lois d'échelle pour les paramètres critiques relatifs à la conductivité à nos données. Dans le voisinage de la transition nos données sont assez bien reproduites par les exposants de champ moyen observés dans les mesures de conductivité

Investigation de la transition de Mott dans V2 O3 dope avec chrome par mesure de la vitesse du son et du pouvoir thermoélectrique

Cette thèse présente une étude expérimentale de la transition de Mott dans V2O3 dopé au chrome par deux différentes techniques expérimentales entre la température ambiante à 500 K et sous pressions jusqu à 6 kbar. La technique ultrasonore fournit une mesure directe de la compressibilité et a été utilisé pour estimer les effets de la réseau sur la transition de Mott. Nous avons été en mesure de mener la première étude systématique de l'influence des degrés de liberté du réseau sur la transition de Mott dans V2O3. On a vérifiée l'existence d'une température électronique critique Telc qui diffère de l'ordre de 3% ou 4% par rapport à la température de transition effectivement observée Tc. L'observation du comportement du mode transverse de la vitesse du son a permis de conclure que la transition de Mott à haute température a lieu sans brisure de symétrie et le paramètre d'ordre est un scalaire. Dans la deuxième partie de la thèse l'évolution de coefficient du Seebeck a été étudiée dans les mêmes conditions expérimentales. Les études autour du point critique nous ont permis d'établir un diagramme de phase expérimentale. Nous avons montré que le changement de S à la transition est dominé par le changement de la résistivité plutôt que par le changement de la symétrie particule-trou. En conséquence, nous avons pu relier les lois d échelle pour les paramètres critiques relatifs à la conductivité à nos données. Dans le voisinage de la transition nos données sont assez bien reproduites par les exposants de champ moyen observés dans les mesures de conductivité.This work presents an experimental study of the high temperature Mott transition in chromium doped V2O3 by means of two different experimental techniques at pressures between ambient pressure and 6 kbar and temperatures from room T to 500 K. Ultrasound measurements as a direct probe of the compressibility were used to estimate the effects of the lattice on the Mott transition. We were able to present the first systematic study of the influence of the lattice degrees of freedom on the high temperature Mott transition in V2O3 and could verify the existence of a critical electronic temperature Telc that differs in the order of 3% or 4% from the temperature of the transition Tc of the bulk sample. Furthermore, observation of the behaviour of the transversal mode of the speed of sound allowed to conclude that this transition occurs without symmetry breaking and the order parameter is a scalar. In the second part the evolution of the Seebeck coefficient was investigated under the same experimental conditions. The studies around the critical point allowed us to establish an experimental phase diagram. Furthermore, we were able to show that the change of the Seebeck coefficient at the Mott transition is driven by the change in the resistivity rather than by the change of the particle-hole symmetry. As a consequence of this we could relate the scaling laws for the critical parameters related to conductivity to our data. In the vicinity of the transition our data were quite well reproduced by the mean field exponents found in the conductivity experiments.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Half-Heusler (TiZrHf)NiSn Unileg Module with High Powder Density

(TiZrHf)NiSn half-Heusler compounds were prepared by arc melting and their thermoelectric properties characterized in the temperature range between 325 K and 857 K, resulting in a Figure of Merit ZT ≈ 0.45. Furthermore, the prepared samples were used to construct a unileg module. This module was characterized in a homemade thermoelectric module measurement stand and yielded 275 mW/cm2 and a maximum volumetric power density of 700 mW/cm3. This was reached using normal silver paint as a contacting material; from an improved contacting, much higher power yields are to be expected.ISSN:1996-194

Charge-Carrier Hopping in Highly Conductive CaMn1-xMxO3-δ Thermoelectrics

Highly dense CaMn1-xMxO3-δ (with M = Nb, Mo, Ta, W and 0 ≤ x ≤ 0.08) n-type thermoelectric materials with low electrical resistivities are prepared from nano-crystalline powders. Their room temperature power factors outperform the best reported results by 30% or more. In combination with the thermal conductivities promising figure-of-merits of ZTM=Ta,x=0.04 = 0.21 and ZTM=W,x=0.04 = 0.20 were achieved at 1160 K. The relative changes and temperature dependencies of the Seebeck coefficient, the electrical resistivity, and the power factor are described with a small-polaronhopping-based mechanism. In the limits of high-temperatures and low substitution levels the Seebeck coefficients are in good agreement with Heikes formula. At high substitutions the efficiency of the doping presumably decreases due to trapping states caused by the formation of bands from Jahn-Teller lowered eg orbitals of Mn3+. Jahn-Teller distortion of Mn3+ also leaves its footprints in the orthorhombic distortion of the crystal structure along the b-axis

Optical and transport properties of LaTi1-xMx(O,N)3±δ (x=0; 0.1, M=Nb5+, W6+) thin films prepared by plasma ammonolysis

Oxynitride thin films of composition LaTi1−xMx(O,N)3±δ with x=0; 0.1 and M=Nb5+, W6+ were prepared by nitridation via microwave-induced plasma (MIP) ammonolysis, which allowed keeping the nitridation time short (16 min). The higher possible oxidation states of the B-site substituents (Nb5+, W6+) with respect to Ti4+ caused higher N content for LaTi0.9Nb0.1(O,N)3±δ and LaTi0.9W0.1(O,N)3±δ compared to LaTiO2N due to charge-compensation. XPS O 1s and N 1s core level and valence band spectra evidenced for increasing N content in the order LaTiO2N<LaTi0.9Nb0.1(O,N)3±δ<LaTi0.9W0.1(O,N)3±δ. All films were N deficient comparing with their stoichiometric formulas and assuming cations in highest oxidation states. Along with increasing N content, the bandgaps decreased from 2.4 to 2.2 and 2.1 eV for LaTiO2N, LaTi0.9Nb0.1(O,N)3±δ, and LaTi0.9W0.1(O,N)3±δ and the electronic conductivities increased along with the decreased bandgaps. The Seebeck coefficients at 860 K indicated the highest charge-carrier density for LaTi0.9W0.1(O,N)3±δ.Fil: Maegli, Alexandra E.. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Sagarna, Leyre. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Populoh, Sascha. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Penkala, Bartosz. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Otal, Eugenio Hernan. Eidgenössische Materialprüfungs- und Forschungsanstalt; Suiza. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; ArgentinaFil: Weidenkaff, Anke. Eidgenössische Materialprüfungs- und Forschungsanstalt; Suiz