University of Pavia, Italy

1. SYNTHESIS AND ELECTRICAL CHARACTERIZATION OF BULK FULLY DENSE NANOCRYSTALLINE ELECTROLYTES PREPARED BY HIGH-PRESSURE SPARK PLASMA SINTERING U. Anselmi Tamburini1, F. Maglia1, G. Chiodelli2, G. Spinolo1, Z.A. Munir3 1Dept. of Chemical Engineering and Materials Science, UC Davis, USA 2IENI - CNR, Dipartimento di Pavia, Italy 3Dipartimento di Chimica fisica, Universita’ di Pavia, Italy

2. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Grain size below 20 nm. Why? • Small grain size produce modification of the defects concentrations, distribution and on transport properties P. Knauth, Solid State Ionics 177 (2006) 2495 X.Guo, R.Waser, Prog. Mater. Sci. 51 (2006) 151 NANOIONICS • These effects become particularly evident when the grain size becomes comparable with the Debye lenght. • In most ionic materials this requires grain size close to 10 nm. Filippo Maglia University of Pavia, Italy

3. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Synthesis of bulk nanocrystalline ionic oxides with grain size below 20 nm • In principle there are two possible routes: • Densification of nanopowders • General applicability • Grain growth difficult to control • Very sensitive to the characteristics of the nanopowders • Controlled crystallization of amorphous precursors • Applicable only to few systems • Single phase samples are difficult to obtain Filippo Maglia University of Pavia, Italy

4. 20 nm 200 m Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Synthesis of nanopowders (d<10 nm) Powders obtained by solution chemistry are usually characterized by a high level of agglomeration This makes the densification much more difficult Filippo Maglia University of Pavia, Italy

5. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Densification of nanopowders SPARK PLASMA SINTERING (SPS) FIELD ASSISTED SINTERING (FAST) Dept. Chemical Engineering and Materials Science University of California, Davis Prof.Z.A.Munir Filippo Maglia University of Pavia, Italy

6. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS SPARK PLASMA SINTERING (SPS) FIELD ASSISTED SINTERING (FAST) Voltage = 0 – 10 V Current = 1000 – 10000 A Pulse size = 3 ms Filippo Maglia University of Pavia, Italy

7. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Characteristics of SPS/FAST • Fast heating rates (up to 1000°C/min) • Main difference with Hot-pressing (10-20°C/min) • Enhances sintering reducing grain growth (less surface diffusion) • Pressure (up to 150 MPa) • Enhances particle rearrangement • Increase driving force • Plastic or superplastic deformation • Very short sintering times (minutes instead of hours) • Lower sintering temperatures Particularly suited for the densification of nanopowders Filippo Maglia University of Pavia, Italy

8. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS HIGH-PRESSURE FIELD ASSISTED SINTERING (HP-FAST) Filippo Maglia University of Pavia, Italy

9. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS HIGH-PRESSURE FIELD ASSISTED SINTERING (HP-FAST) >98% relative density Sintering time 5 min Filippo Maglia University of Pavia, Italy

10. 100 nm 100 nm 100 nm Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Example of materials obtained by HP-FAST Powder agglomeration has very little influence YSZ (8%) CeO2 (30%Sm) CeO2 Filippo Maglia University of Pavia, Italy

11. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS YSZ (8%) Very large apparent grain boundary resistance The difference between gb and bulk reduces with grain size Grain size 15.5 nm T=200°C dr = 98% Guo X, Zhang Z. Acta Mater 2003;51:2539 Filippo Maglia University of Pavia, Italy

12. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Interaction of nanocrystalline YSZ with water • If exposed for 400h to air saturated with water the conductivity increases almost 2 orders of magnitude at 200°C • The change involves both bulk and grain boundaries • It is totaly reversible • It is observed only in samples with grain size below 20 nm Dehydration at 200°C for (b) 10 min, (c) 20 min, (d) 30 min, (e) 40 min , (f) 80 min Filippo Maglia University of Pavia, Italy

13. Microcrystalline Nanocrystalline Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Interaction of nanocrystalline YSZ with water • Results suggests protonic conductivity • Protonic conductivity in YSZ has been proposed before but never observed • Nanostructure can enhance the localization of protons at the grain boundary Filippo Maglia University of Pavia, Italy

14. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Conclusions • HP-SPS represent the first reliable method for the synthesis of large amounts of bulk ceramic materials with grain size < 20 nm and relative density >98% • This offers for the first time the possibility to investigate systematically the physical properties of these materials • Nanostructure represent a powerful new parameter (beside doping) to control the electrical properties of ionic materials • Approaching grain size of 10 nm traditional ionic compounds show unexpected new properties as interface and confinement effects become dominant • This suggest the possibility of a new class of ionic materials whose properties are totally controlled by the nanostructure Filippo Maglia University of Pavia, Italy