POLITECNICO DI TORINO Collegio di Ingegneria Chimica e dei Materiali Corso di Laurea Magistrale in Ingegneria dei Materiali Tesi di Laurea Magistrale Magnesium Silicide: a novel, silicon-based material for printable thermoelectric devices Relatori prof. Fabrizio Giorgis prof. Isabel Ferreira Candidato Davide Miglietta Luglio 2018 Alla mia famiglia, il mio porto sicuro Table of contents Riassunto .................................................................................................................................. IV 1 Introduction ......................................................................................................................... 1 1.1 Thesis Structure ....................................................................................................................... 3 2 Thermoelectricity ................................................................................................................ 5 2.1 Thermoelectric effects [6, 7] ................................................................................................... 5 2.2 P-type and n-type semiconductors [8] ..................................................................................... 6 2.3 ZT Figure of merit and TE efficiency [8, 9] ............................................................................ 6 2.4 Thermoelectric generators (TEGs) [10] .................................................................................. 8 2.5 TEGs applications [10] .......................................................................................................... 10 2.6 Traditional Thermoelectric Materials [12, 13] ...................................................................... 12 3 Magnesium Silicide: A Novel Thermoelectric Material ................................................... 15 3.1 Magnesium Silicide: structure and physico-chemical properties .......................................... 15 3.2 Chemical reactivity of Magnesium Silicide .......................................................................... 16 3.3 Mg2Si Fabrication Routes and State of the Art ..................................................................... 17 3.3.1 Fabrication Routes: Melting Techniques ....................................................................... 18 3.3.2 Fabrication Routes: Solid-state synthesis ...................................................................... 19 3.4 Doped Magnesium Silicide and its Solid Solutions .............................................................. 22 3.4.1 N-type dopants .............................................................................................................. 22 3.4.2 P-type dopants ............................................................................................................... 25 4 Characterization Techniques ............................................................................................. 27 4.1 Scanning Electron Microscopy (SEM) .................................................................................. 27 4.2 X-ray Diffraction (XRD) ....................................................................................................... 28 4.3 Raman Spectroscopy ............................................................................................................. 29 4.4 Infrared Spectroscopy (FTIR) ............................................................................................... 30 4.5 DSC/TGA .............................................................................................................................. 30 5 Magnesium Silicide Synthesis .......................................................................................... 33 5.1 Ball milling and mechanical alloying [49-54] ....................................................................... 33 5.2 Ball mill ................................................................................................................................. 33 5.2.1 Planetary ball mill ......................................................................................................... 34 5.2.2 Selection of tools materials ........................................................................................... 34 5.2.3 Ball dimension and number, mass ratio balls to powder ............................................... 35 5.2.4 Dry and Wet Milling ..................................................................................................... 36 5.2.5 Overheating ................................................................................................................... 37 5.2.6 Inert Atmosphere ........................................................................................................... 37 5.3 Increase of internal energy and alloying mechanism ............................................................ 37 I 5.3.1 Alloying Mechanism ..................................................................................................... 37 5.4 Experimental setting .............................................................................................................. 37 5.4.1 Materials and milling parameters .................................................................................. 39 5.4.2 Calculations ................................................................................................................... 39 5.4.3 Procedure ....................................................................................................................... 40 5.5 Annealing temperature choice by DSC/TGA analysis .......................................................... 41 5.5.1 Results: .......................................................................................................................... 41 5.5.2 DSC results comparation and discussion: ..................................................................... 43 5.5.3 TGA results comparation and discussion: ..................................................................... 44 5.6 Mg2Si formation reaction by thermal annealing and inert atmosphere comparation ............ 45 5.6.1 Experimental setting ...................................................................................................... 45 5.6.2 Five hours milled powder .............................................................................................. 46 5.6.3 Synthesis from commercial powder (unmilled) ............................................................ 50 5.7 Characterization of annealed powder .................................................................................... 51 5.7.1 XRD results ................................................................................................................... 51 5.7.2 Raman results ................................................................................................................ 59 5.7.3 IR results........................................................................................................................ 60 5.7.4 SEM results ................................................................................................................... 62 6 3d Printing of silicon and magnesium silicide powder ..................................................... 69 6.1 3d printing technologies ........................................................................................................ 69 6.2 Experimental Setting: 3D Printer .......................................................................................... 69 6.3 Extrudable Paste and After-Printing Treatment .................................................................... 71 6.4 Paste formulation ................................................................................................................... 71 6.4.1 Materials ........................................................................................................................ 72 6.5 Paste formulation: Preliminary study .................................................................................... 75 6.5.1 Procedure ....................................................................................................................... 76 6.5.2 Tested materials and formulations ................................................................................. 78 6.5.3 Selected formulations .................................................................................................... 84 6.5.4 Influence of powder grain size ...................................................................................... 84 6.6 DSC/TGA Analysis for de-binding temperature determination ............................................ 86 6.6.1 Polystyrene formulations: DSC/TGA results ................................................................ 86 6.6.2 Ethyl cellulose formulations .......................................................................................... 89 6.6.3 Resume: Burnout Temperatures .................................................................................... 90 6.7 3D printing of Mg2Si powder ................................................................................................ 90 6.7.1 Post processing: de-binding and sintering ..................................................................... 91 6.7.2 Experimental setting ...................................................................................................... 92 7 Conclusions ......................................................................................................................