1. Relationship between microstructure and experimental conditions
23-27 june 2013, Limoges, France
Relationship between microstructure and experimental conditions
in the flash sintering of YSZ ceramics
Marlu César STEIL, Daniel MARINHA
Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces – UMR 5279 Université de Grenoble BP Saint Martin d’Hères Cedex - FRANCE
Purpose/Objectives
We concentrated our attention on the microstructures
obtained by varying the flash-sintering parameters:
Flash sintering
Two-electrode-experiments using AC electrical fields 1,2
furnace temperature (initial temperature T0); initial power spike
the steady state current density, the flash time
V(volt)
i(A)
Porous
Sample
dense
t(seconds!!)
flash-sintering
First set of results : initial temperature of 900°C was selected
and typical flash conditions were applied, varying the current density
and flash time over the 7-50 s range.
Second part of this work is focused on the initial period,
limited to 1s, that we will call the hyper-flash.
Flash-sintering on the conventional 8 mol% Y2O3 doped ZrO2
Experimental
-----Sample processing and characterization-----
Samples: pellets (8 mm in diameter, 3 mm thick)
uniaxial pressing and IP (250 MPa).
Green relative densities: 50,O ± 0,5% TD (Theoretical Density).
Powder : 8 mol.% yttria stabilised zirconia (Tosoh TZ-8Y)
Post-sintering characterizations :
Densities: determined geometrically and by the Archimede method; Microstructures : FEG-SEM on fracture surfaces
Electrical characterization by Impedance Spectroscopy
-----Flash-sintering equipment and procedures-----
Sample is heated in the experimental furnace (temperature ramp: 10°/mn) to the the initial sample temperature (900°C) and then electric signals are applied (AC signals, 1000Hz, to avoid electrolysis)
To avoid any electrical breakdown, we used a current limitation
(Pacific Smart Source 115ASX AC power generator)
flashes were performed under constant current densities
and not under constant electric fields
The current increases very rapidly
and is limited to the selected value
(from 5 to 20 A.cm-2)
The flash was triggered by applying
an appropriate electric field
Input Parameters: Furnace Temperature T0; Current max i; Initial voltage V0; Flash time Dt
Results
Grain size ~ ; Density
time
1 Conventional flash sintering
furnace temperature : 900°C;Initial elect. field E= 170Vcm-1
10A/cm2 – 7s
10A/cm2 – 30s
10A/cm2 – 45s - t
14A/cm2
Dt = 30s
Initial power :
2100 W.cm-3
i (A/cm2)
t (s)
Relative density (%TD)
Grain size
(µm)
Geom.
Arch. 10 7 81 --
0.5 30 89 87
0.3 45 96 98 14 92 94 93
1 - 2 97
1-2 20 3
2-3
2 -3
Current
14A/cm2 – 7s
14A/cm2 – 30s
Conventional sintering 1350°C/1h
Grain size
Density
20A/cm2 – 7s
20A/cm2 – 14s
Impedance measurements
Conductivity of flashed samples is equivalent to conventional YSZ
2Hyper flash sintering (t1s)
furnace temperature : 900°C
E=150Vcm A/cm2
Initial power : 1500W.cm-3
E=300Vcm A/cm2 Initial power : 3000W.cm-3
Electrical Field
80%TD
Grain size and density ~
78%TD
Conclusions
AC flash sintering: versatile investigation tool for the sintering studies.
Appropriate selection of experimental parameters lead to a wide variety of microstructures and different states of densification
The microstructures do not reveal any specific feature associated with the flash-sintering process.
The quality of the sintered samples was confirmed by impedance spectroscopy
Major part of densification (up to 80%) occurs within a short time interval of about 1s with a minor grain growth
Acknowledgements
This work was partially funded by the French National Research Agency
The authors thank Dr. F Roussel Dherbey and Dr. F. Charlot (CMTC, INP Grenoble)
for SEM observations.
A. Cordier , M. Kleitz, M. C. Steil, Journal of the European Ceramic Society, 32 (2012) 473–1479
M. C. Steil, D. Marinha, Y. Aman, J. R.C. Gomes, M. Kleitz, Journal of the European Ceramic Society
33 (2013) 2093–2101