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

University of Pavia, ItalyFilippo Maglia Grain size below 20 nm. Why? Small grain size produce modification of the defects concentrations, distribution and on transport properties 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. Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS X.Guo, R.Waser, Prog. Mater. Sci. 51 (2006) 151P. Knauth, Solid State Ionics 177 (2006) 2495

University of Pavia, Italy 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 Synthesis of bulk nanocrystalline ionic oxides with grain size below 20 nm Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Filippo Maglia

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

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

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

University of Pavia, Italy Characteristics of SPS/FAST Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Filippo Maglia 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

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

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

University of Pavia, Italy MaterialStarting powder grain size (nm) Heating rate (°C/min) Hold temp. (°C) Hold pressure (MPa) Hold time (min) Relative density (%) Grains size Direct measurement (nm) CeO > ± 2.1 Ce 0.7 Sm 0.3 O > ± 3.1 YFSZ (8%) > ± 3.2 Powder agglomeration has very little influence Example of materials obtained by HP-FAST Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Filippo Maglia 100 nm YSZ (8%) 100 nm CeO 2 (30%Sm) 100 nm CeO 2

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

University of Pavia, Italy Interaction of nanocrystalline YSZ with water Dehydration at 200°C for (b) 10 min, (c) 20 min, (d) 30 min, (e) 40 min, (f) 80 min Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Filippo Maglia 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

University of Pavia, Italy Nanocrystalline Microcrystalline Interaction of nanocrystalline YSZ with water Synthesis and electrical characterization of bulk fully dense nanocrystalline electrolytes prepared by high-pressure SPS Filippo Maglia 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

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