SYNTHESIS AND ELECTRICAL CHARACTERIZATION OF BULK FULLY DENSE NANOCRYSTALLINE ELECTROLYTES PREPARED...

SYNTHESIS AND ELECTRICAL CHARACTERIZATION OF SYNTHESIS AND ELECTRICAL CHARACTERIZATION OF BULK FULLY DENSE NANOCRYSTALLINE ELECTROLYTES BULK FULLY DENSE NANOCRYSTALLINE ELECTROLYTES

PREPARED BY HIGH-PRESSURE SPARK PLASMA PREPARED BY HIGH-PRESSURE SPARK PLASMA SINTERINGSINTERING

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

University of Pavia, ItalyFilippo Maglia

Grain size below 20 nm. Why?Grain size below 20 nm. Why?

•Small grain size produce modification of the defects concentrations, distribution and on transport properties

NANOIONICSNANOIONICS

•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 SPSSynthesis 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 Synthesis of bulk nanocrystalline ionic oxides with grain size below 20 nmwith grain size below 20 nm


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200 m 20 nm

Powders obtained by solution chemistry are usually characterized by a high level of agglomeration

Synthesis of nanopowders (d<10 nm)Synthesis of nanopowders (d<10 nm)

This makes the densification much more difficult


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SPARK PLASMA SINTERING (SPS)FIELD ASSISTED SINTERING (FAST)

Densification of nanopowdersDensification of nanopowders

Dept. Chemical Engineering and Materials Science

University of California, Davis

Prof.Z.A.Munir


Filippo Maglia


SPARK PLASMA SINTERING (SPS)FIELD ASSISTED SINTERING (FAST)

Voltage = 0 – 10 V

Current = 1000 – 10000 A

Pulse size = 3 ms


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Characteristics of SPS/FAST


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•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


HIGH-PRESSURE FIELD ASSISTED SINTERING (HP-FAST)HIGH-PRESSURE FIELD ASSISTED SINTERING (HP-FAST)


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HIGH-PRESSURE FIELD ASSISTED SINTERING (HP-FAST)HIGH-PRESSURE FIELD ASSISTED SINTERING (HP-FAST)

Sintering time 5 min


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>98% relative density


Material Starting powder grain size (nm)

Heating rate(°C/min)

Hold temp.(°C)

Hold pressure(MPa)

Hold time(min)

Relative density(%)

Grains sizeDirect measurement(nm)

CeO2 7 200 625 600 5 > 98 11.6 ± 2.1

Ce0.7Sm0.3O2 8 200 750 610 5 > 98 16.9 ± 3.1

YFSZ (8%) 6.6 200 850 530 5 > 98 15.5 ± 3.2

Powder agglomeration has very little influence

Example of materials obtained by HP-FAST


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100 nm

YSZ (8%)

100 nm

CeO2 (30%Sm)

100 nm

CeO2


YSZ (8%)YSZ (8%)

Grain size 15.5 nm T=200°C

Very large apparent grain boundary resistance

dr = 98%


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The difference between gb and bulk reduces with grain size

Guo X, Zhang Z. Acta Mater 2003;51:2539


Interaction of nanocrystalline YSZ with waterInteraction 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


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•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


Nanocrystalline

Microcrystalline

Interaction of nanocrystalline YSZ with waterInteraction of nanocrystalline YSZ with water


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•Results suggests protonic conductivity

•Protonic conductivity in YSZ has been proposed before but never observed

OXOO OHOVOH )(22

•Nanostructure can enhance the localization of protons at the grain boundary


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 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


Filippo Maglia

SYNTHESIS AND ELECTRICAL CHARACTERIZATION OF BULK FULLY DENSE NANOCRYSTALLINE ELECTROLYTES PREPARED...

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