Copper Hyper-Stoichiometry: the Key for the Optimization Of

Copper Hyper-Stoichiometry: The Key for the Optimization of Thermoelectric Properties in Stannoidite Cu8+xFe3–xSn2S12 Ventrapati Pavan Kumar, Tristan Barbier, Vincent Caignaert, Bernard Raveau, Ramzy Daou, Bernard Malaman, Gérard Le Caër, Pierric Lemoine, Emmanuel Guilmeau To cite this version: Ventrapati Pavan Kumar, Tristan Barbier, Vincent Caignaert, Bernard Raveau, Ramzy Daou, et al.. Copper Hyper-Stoichiometry: The Key for the Optimization of Thermoelectric Properties in Stannoidite Cu8+xFe3–xSn2S12. Journal of Physical Chemistry C, American Chemical Society, 2017, 121 (30), pp.16454-16461. 10.1021/acs.jpcc.7b02068. hal-01580457 HAL Id: hal-01580457 https://hal-univ-rennes1.archives-ouvertes.fr/hal-01580457 Submitted on 14 Sep 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Page 1 of 36 The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 Copper Hyper-stoichiometry, the Key for the 9 10 11 12 Optimization of Thermoelectric Properties in 13 14 15 16 Stannoidite Cu 8+ xFe 3-xSn 2S12 17 18 19 20 21 † † † † 22 Ventrapati Pavan Kumar, Tristan Barbier, Vincent Caignaert, Bernard Raveau, Ramzy 23 24 † ‡ # § ,† 25 Daou, Bernard Malaman, Gérard Le Caër, Pierric Lemoine, and Emmanuel Guilmeau * 26 27 28 † 29 Laboratoire CRISMAT, UMR 6508 CNRS/ENSICAEN, 6 bd du Marechal Juin, 14050 Caen 30 31 32 Cedex 4, France 33 34 35 ‡ 36 Institut Jean Lamour, UMR 7198, CNRS, Université de Lorraine, Faculté des Sciences et 37 38 Technologie, B.P. 70239, 54506 Vandœuvre-lès-Nancy Cedex, France 39 40 41 # 42 Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes I, Campus de 43 44 Beaulieu, Bâtiment 11A, F-35042 Rennes Cedex, France 45 46 47 § 48 Institut des Sciences Chimiques de Rennes (ISCR) - UMR CNRS 6226, Université de Rennes 49 50 I, Campus de Beaulieu, Bâtiment 10A, 35042 Rennes Cedex, France 51 52 53 54 KEYWORDS. thermoelectric, sulfide, stannoidite, mechanical alloying, SPS 55 56 57 58 59 60 ACS Paragon Plus Environment 1 The Journal of Physical Chemistry Page 2 of 36 1 2 3 ABSTRACT 4 5 6 A univalent copper hyper-stoichiometric stannoidite Cu 8+ xFe 3-xSn 2S12 with 0 ≤ x ≤ 0.5 has been 7 8 synthesized using mechanical alloying followed by spark plasma sintering. The X-ray diffraction 9 10 57 119 11 analysis combined with Fe and Sn Mössbauer investigations has allowed the charge 12 13 distribution of the cationic species on the various sites to be established and suggests the 14 15 possibility of a small tin deficiency. The transport properties show a remarkable crossover from a 16 17 18 semiconducting to a metal-like behaviour as the copper content increases from x = 0 to x = 0.5, 19 20 whereas correlatively the Seebeck coefficient decreases moderately, with S values ranging from 21 22 310 to 100 µV/K. The thermal conductivity decreases as the temperature increases showing low 23 24 25 values at high temperature, far below those reported in related stannite materials. The 26 27 investigation of the thermoelectric properties shows that the ZT figure of merit is dramatically 28 29 enhanced by the copper hyper-stoichiometry by a factor of 5 going from 0.07 for x = 0 to 0.35 30 31 32 for x = 0.5 at 630 K. This thermoelectric behaviour is interpreted on the basis of a model 33 34 involving the Cu-S framework as the conducting electronic network where the Fe 2+ /Fe 3+ species 35 36 37 play the role of hole reservoir. 38 39 40 41 1. INTRODUCTION 42 43 44 The demand in energy consumption has been escalating this last decade, leading to renewed 45 46 interest on thermoelectric (TE) technology.1 Beyond the promising results obtained for telluride 47 48 based materials which exhibit high performances, 2–4 the need to conciliate efficiency with 49 50 51 environmental and cost constraints has triggered research recently toward copper-based sulfides 52 53 which have, for most of them, the advantage to contain eco-friendly and abundant elements. The 54 55 2 56 figure of merit ZT values ( ZT = S T/ ρκ, where S is the Seebeck coefficient, T is the absolute 57 58 59 60 ACS Paragon Plus Environment 2 Page 3 of 36 The Journal of Physical Chemistry 1 2 3 4 temperature, ρ is the electrical resistivity and κ is the thermal conductivity) observed for the 5 5–7 6 simple phases Cu 2-xS, ranging from 0.5 to 1.7 are very attractive. However, the electro- 7 8 migration of copper that appears in the Cu-S system weakens the stability and durability of these 9 10 11 TE materials. The introduction of other metallic (M) elements besides copper has allowed this 12 13 problem to be mostly overcome, opening the route to the exploration of complex copper based 14 15 sulfides. Actually, from the viewpoint of thermoelectricity, two classes of copper based sulfides 16 17 18 can be distinguished exhibiting an n-type and p-type conductivity, respectively. The n-type class 19 20 is obtained for lower copper content materials, i.e. Cu/M ratio ≤ 1, as shown for chalcopyrite 21 22 8 9 23 CuFeS 2-type sulfides such as CuFe 1-xIn xS2 and Cu 1-xZn xFeS 2 and for the cubic isocubanite 24 10 25 CuFe 2S3 , suggesting that the Fe 3 d states play a major role in the formation of the electronic 26 27 band structure of these n-type thermoelectrics. The p-type class of copper based sulfides covers a 28 29 30 broad range of compositions, from Cu/M ratio equal or close to 1 as exemplified by the stannites 31 11,12 32 Cu 2ZnXS4 (X=Sn, Ge) and copper-rich sulfides, with Cu/M ratios ranging from 2 to 5 as 33 34 shown by Cu SnS ,13 the derivatives of the tetrahedrite Cu Sb S ,14–16 of the colusite 35 2 3 12 4 13 36 17–19 20–22 37 Cu 26 V2Sn 6S32 , and of the bornite Cu 5FeS 4. 38 39 From these studies, the p-type copper rich sulfides appear as most promising for thermoelectric 40 41 applications. However, the improvement of the thermoelectric performances of these materials 42 43 44 remains a challenge, due to the interdependent and contrary effects of their electrical and thermal 45 46 transport parameters S, ρ and κ. The nature of the chemical bonds, especially Cu-S bonds and of 47 48 49 the distribution of charges in these complex structures is still a matter of debate, which is of 50 51 capital importance for the optimization of their TE properties. In this respect, the stannoidite 52 53 Cu (Fe,Zn) Sn S mineral appears to be an attractive material in relation with (i) its 54 8 3 2 12 55 23 56 orthorhombic structure (Figure 1) closely related to that of stannite, (ii) the possibility to 57 58 59 60 ACS Paragon Plus Environment 3 The Journal of Physical Chemistry Page 4 of 36 1 2 3 prepare the synthetic isostructural Cu Fe Sn S sulfide,24 and (iii) the distribution of the cationic 4 8 3 2 12 5 + 2+ 3+ 4+ 2- 6 charges according to the formula (Cu )8(Fe )(Fe )2(Sn )2(S )12 determined by Mössbauer 7 8 spectroscopy.25 Curiously, the thermoelectric properties of this sulfide have not been investigated 9 10 11 to date, in spite of its potential to be used as a well characterized model material for 12 13 understanding the TE phenomena in these materials. We report herein on the synthesis by 14 15 mechanical alloying followed by spark plasma sintering (SPS) and on the TE properties of the 16 17 18 copper hyper-stoichiometric stannoidite Cu 8+ xFe 3-xSn 2S12 (x ≤ 0.5) series. A drastic change in the 19 20 TE properties is observed with the copper content, characterized by a crossover from a 21 22 semiconducting poor TE ( ZT = 0.07) state for the stoichiometric phase ( x = 0) to a metallic- 23 630 K 24 25 like attractive TE ( ZT 630 K = 0.35) state for the Cu-hyper-stoichiometric sulfide (x = 0.5). The 26 27 modification of these performances is explained in the frame of the iono-covalent model of the 28 29 chemical bonds. 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Figure 1. View of the crystal structure of stannoidite Cu 8Fe 3Sn 2S12 . 51 52 53 54 55 56 2. EXPERIMENTAL SECTION 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 36 The Journal of Physical Chemistry 1 2 3 Polycrystalline samples of Cu Fe Sn S (0 ≤ x ≤ 0.5) were synthesized by mechanical 4 8+ x 3-x 2 12 5 26 6 alloying followed by Spark Plasma Sintering. All sample preparation and handling of powders 7 8 were performed in Argon filled glove box with oxygen content of < 1 ppm. Stoichiometric 9 10 11 amounts of Cu (99.9 %, Alfa Aesar), Fe (99.9 %, Alfa Aesar), Sn (99.9 %, Alfa Aesar) and S 12 13 (99.99 %, Alfa Aesar) were loaded in a 25 ml tungsten carbide jar under argon atmosphere.

Load more