ORCID Profile
0000-0003-3402-0846
Current Organisation
Universita degli Studi di Milano-Bicocca
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: AIP Publishing
Date: 28-01-2005
DOI: 10.1063/1.1852072
Abstract: The properties of Te-doped Co(Sb1−yTey)3 and Te-Ni double-doped Co1−xNix(Sb1−yTey)3 nanostructured skutterudites were evaluated by means of x-ray powder diffraction, and transport properties measured on the synthesized s les have been compared with ab initio theoretical modeling. Theoretical optimal dopant contents have been evaluated according to the maximum value of the power factor, calculating the electronic transport properties from the ab initio material band structure using semiclassical Boltzmann transport theory. The s les have been synthesized by chemical alloying with Te substitution for Sb up to 2.5at.% and Ni substitution for Co up to 2.0at.%. X-ray powder diffraction has been performed on all s les to reveal information about phase purity and Rietveld refinement was performed for the phase composition and cell parameter. The thermoelectric properties of the resulting consolidates were investigated in a temperature range from 300to723K using various measurement facilities. A standardization and round robin program was started among the participating evaluation laboratories in order to ensure reliability of the data obtained. The significant reduction in thermal conductivity, when compared to highly annealed CoSb3, could be proved which is caused by the nanostructuring, resulting in a high concentration of grain boundaries. A combination of substitution levels for Ni and Te has been found resulting in the largest ZT value of 0.65 at 680K among unfilled skutterudite materials.
Publisher: Wiley
Date: 20-08-2004
Abstract: The experimental electron density of the high-performance thermoelectric material Zn4Sb3 has been determined by maximum entropy (MEM) analysis of short-wavelength synchrotron powder diffraction data. These data are found to be more accurate than conventional single-crystal data due to the reduction of common systematic errors, such as absorption, extinction and anomalous scattering. Analysis of the MEM electron density directly reveals interstitial Zn atoms and a partially occupied main Zn site. Two types of Sb atoms are observed: a free spherical ion (Sb3-) and Sb2(4-) dimers. Analysis of the MEM electron density also reveals possible Sb disorder along the c axis. The disorder, defects and vacancies are all features that contribute to the drastic reduction of the thermal conductivity of the material. Topological analysis of the thermally smeared MEM density has been carried out. Starting with the X-ray structure ab initio computational methods have been used to deconvolute structural information from the space-time data averaging inherent to the XRD experiment. The analysis reveals how interstitial Zn atoms and vacancies affect the electronic structure and transport properties of beta-Zn4Sb3. The structure consists of an ideal A12Sb10 framework in which point defects are distributed. We propose that the material is a 0.184:0.420:0.396 mixture of A12Sb10, A11BCSb10 and A10BCDSb10 cells, in which A, B, C and D are the four Zn sites in the X-ray structure. Given the similar density of states (DOS) of the A12Sb10, A11BCSb10 and A10BCDSb10 cells, one may electronically model the defective stoichiometry of the real system either by n-doping the 12-Zn atom cell or by p-doping the two 13-Zn atom cells. This leads to similar calculated Seebeck coefficients for the A12Sb10, A11BCSb10 and A10BCDSb10 cells (115.0, 123.0 and 110.3 microV K(-1) at T=670 K). The model system is therefore a p-doped semiconductor as found experimentally. The effect is dramatic if these cells are doped differently with respect to the experimental electron count. Thus, 0.33 extra electrons supplied to either kind of cell would increase the Seebeck coefficient to about 260 microV K(-1). Additional electrons would also lower sigma, so the resulting effect on the thermoelectric figure of merit of Zn4Sb3 challenges further experimental work.
Publisher: AIP Publishing
Date: 02-09-2004
DOI: 10.1063/1.1781762
Abstract: We present neutron and synchrotron powder-diffraction investigations as well as ab initio calculations to elucidate delicate structural features in doped skutterudites. S les with assumed Fe doping were investigated (FeyCo4Sb12, y=0.4, 0.8, 1.0, and 1.6), as well as s les with formal Ni substitution (Co4−xNixSb12, x=0, 0.4, 0.8, and 1.2). The present study serves as a case story for the determination of fine structural details of thermoelectric skutterudites by diffraction methods in combination with ab initio calculations. We illustrate the problem of fluorescence in the conventional x-ray powder diffraction on the Fe-doped s les by a comparison with the neutron powder-diffraction data. On the series of the Ni-substituted s les, the neutron powder-diffraction data were collected to investigate the exact sitting of the Ni. The s le with the highest Ni substitution (Co2.8Ni1.2Sb12) was also used for high resolution, high-energy synchrotron powder diffraction measurements. These revealed that the s le consists of two skutterudite phases. A complete description of the Ni-substituted s les was obtained in tandem with ab initio calculations, which show that the system contains a Ni-rich (Co0.38Ni3.62Sb12) and a Ni-poor (Co3.76Ni0.24Sb12)) skutterudite phases.
Publisher: Wiley
Date: 22-09-2003
Abstract: Periodic density functional calculations using pseudopotentials and a local basis set were performed on the type I clathrates A(8)Ga(16)Ge(30) (A=Sr, Ba). Both are known to show promising thermoelectric properties. Ab initio wave functions were analyzed within the framework of the quantum theory of atoms in molecules. This enabled us to analyze both the charge transfer and bonding properties of the clathrate from a rigorous quantum mechanical viewpoint. The Ba and Sr centers were found to be largely ionic (charge: ca. +1.7 e) both in the smaller 20-atom and in the larger 24-atom cages, consistent with a Zintl-phase view of these type I clathrates. The assertion that the Sr atoms in the different cages have similar oxidation states is shown to be consistent with multiwavelength diffraction experiments on Sr(8)Ga(16)Ge(30) while the assertion of ionicity of the Sr center is supported by the observation that the adsorption edge lies close to that previously found in the Sr K-edge XANES spectra of Sr(OH)(2).8 H(2)O. As such, this work contradicts previous experimental and theoretical studies that claim that the guest atoms are neutral. We show that the discrepancy is related to the definitions used for electron transfer. Definitions based on electron displacement (rearrangement) in space, as in previous works, do not account for the variation in shape and volume of the atomic catchment regions upon change in the number and average locations of the particles in the system. Eventually, such definitions lead to underestimation of charge transfer. The large binding energy found in earlier work for Ba and Sr in these materials is found to be consistent with a simple picture of charge transfer from the guest to the frame. Preliminary investigations on a clathrate of perfect stoichiometry appear to rule out any important relationship between the observed increase in the thermoelectric figure of merit with increasing external pressure and host-guest charge transfer.
Location: Italy
No related grants have been discovered for LUCA BERTINI.