Effects of sintering temperature on the physical and Contents Rainer Schmidt Effects of sintering temperature on the physical and chemical properties of CaCu3Ti4O12 ceramics 1. Introduction 2. Experimental 3. Dielectric properties 4. Thermal gravimetry and X-ray diffraction 5. Scanning Electron Microscopy Analysis 6. XANES and Raman spectroscopy 7. Conclusions

Crystal structure of 1:2 A-site ordered perovskite CaCu3Ti4O12 1. Introduction: Crystal structure Rainer Schmidt Crystal structure of 1:2 A-site ordered perovskite CaCu3Ti4O12 Ti within the green octahedra Ca Square-planar Cu O

Giant dielectric permittivity in CaCu3Ti4O12 1. Introduction: Giant dielectric permittivity Rainer Schmidt Giant dielectric permittivity in CaCu3Ti4O12 Dielectric spectra represented in the dielectric permittivity notation: Real part e1 vs frequency w Solid Lines are fits to a Debye – model: C2 C2 R C1 C1 For one resistor only one relaxation with one relaxation peak is expected Homes et al., Science 293 (2001) p.673

Polycrystalline CCTO Dielectric spectra represented in the 1. Introduction: Impedance spectroscopy of CCTO Rainer Schmidt Polycrystalline CCTO Dielectric spectra represented in the complex impedance notation: Imaginary part –Z’’ vs real part Z’ The presence of two semicircles indicates the presence of 2 relaxations with 2 resistors Sinclair et al., Appl.Phys.Lett. 80 (2002) p.2153

Brickwork Layer Model (BLM) 1. Introduction: Brick-work layer model Rainer Schmidt Brickwork Layer Model (BLM) Semiconducting bulk: EA ≈ 0.01 – 0.1 eV Insulating grain boundaries (GB) EA ≈ 0.6 eV high dielectric permittivity has extrinsic origin Adams et al., Phys.Rev.B 73 (2006) p.094124 Rgb Rb Cgb Cb Cgb ~ 1 n F Cb ~ 1 pF where Rgb and Cgb are >> Rb and Cb

Internal Barrier Layer Capacitor 1. Introduction: Brick-work layer model Rainer Schmidt Potential sources for chemical differences in bulk and GB areas: Variations in oxygen content Ti valence state changes: Ti4+/Ti3+ Cu loss/non-stoichiometry/valency Exact mechanisms are not known Internal Barrier Layer Capacitor In this work we investigate the effect of sintering temperature on the chemical and physical properties of CCTO ceramics Moulson & Herbert, Electroceramics, Chapman & Hall London (1990) p.261 In this way we hope to find indications for oxygen/Cu loss/non-stoichiometry or valence state changes

Chemical and Physical Properties 2. Experimental Rainer Schmidt Powder Processing Mixed oxide route using dried CaCO3 99.99% , CuO 99.99% , TiO2 99.98% Furnace reaction temperature of 975°C Pellet Production Pellet pressing using a 1 ton uni-axial hydraulic press Pellet sintering at 975°C, 1000°C, 1025°C, 1050°C, 1075°C, 1100°C Chemical and Physical Properties Dielectric properties by impedance spectroscopy Thermal Gravimetry, Lattice parameter determination by XRD, Composition & Microstructure by SEM, Quantitative analysis by EDAX Oxidation states by XANES and phonon modes by Raman spectroscopy

3. Results: Dielectric properties Rainer Schmidt Impedance spectroscopy data at 100 Kelvin from sintered pellets (975°C – 1100°C) 10-7 10-8 10-9 10-10 10-11 10-12 1,5x105 105 0,5x105 100 K 975C Grain boundary 1100C 1075C 1050C 1000C -Z'' (Wcm) 1025C Real part capacitance C' (F/cm) 1000C 975C 1025C 1050C Bulk 100 K 1075C 101 102 103 104 105 106 107 0,5x105 105 1,5x105 Frequency (Hz) 1100C Real part impedance Z' (Wcm) The grain boundary capacitance significantly increases with sintering temperature The bulk resistance significantly decreases with sintering temperature

Cole-Cole and conductivity plots of impedance spectroscopy data 3. Results: Dielectric properties Rainer Schmidt Cole-Cole and conductivity plots of impedance spectroscopy data 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 103 100 K 1100C 1100C 1075C 1050C 1075C 1025C 1000C 975C C' ' (F) Real part conductivity s' (1/Wcm) 1050C 1025C 1000C 100 K 0 2,5x10-10 5x10-10 7,5x10-10 10-9 101 102 103 104 105 106 107 Real part capacitance C' (F) Frequency (Hz) 975C The grain boundary capacitance significantly increases with sintering temperature The bulk conductivity significantly increases with sintering temperature

Reversible and unreversible weight loss was detected 4. Results: Thermal gravimetry Rainer Schmidt Reversible and unreversible weight loss was detected Reversible: oxygen content changes? 25C hold 10 min 25 - 700C heat 10C/min 700 - 1100C heat 3C/min 1100C hold 60 min 1100 - 900C cool 3C/min 900C hold 30 min 900 - 25C cool 10C/min 25- 1100C – 25 C heat/ cool 3C/min Unreversible: Cu content changes?

CCTO lattice parameter vs sintering temperature 4. Results: Lattice parameter Rainer Schmidt CCTO lattice parameter vs sintering temperature from X ray diffraction using an internal Si standard

SEM/EDAX analysis on sintered CCTO pellets demonstrate Cu segregation 5. Results: Composition & microstructure by SEM Rainer Schmidt SEM/EDAX analysis on sintered CCTO pellets demonstrate Cu segregation 1025ºC 1075ºC Secondary electrons Ti Ti EDAX line scans using backscattered electrons Cu Cu Ca Ca O O C C

5. Results: Grain average by quantitative EDAX Rainer Schmidt Quantitative EDAX analysis by averaging over large areas (large area average) and localised EDAX measurements on single grains averaging over 25 grains expected Cu 0,375 The measurements were calibrated using a CuTi alloy and ceramic CaTiO3

X-ray Absorption Near Edge Spectroscopy suggests mainly Cu2+ 6. Results: XANES Rainer Schmidt X-ray Absorption Near Edge Spectroscopy suggests mainly Cu2+ Cu1+ standard Hints of Cu3+ ?? Cu2+ standard 975C 1100C 1100C:quenched

X-ray Absorption Near Edge Spectroscopy suggests Ti4+ exclusively 6. Results: XANES Rainer Schmidt X-ray Absorption Near Edge Spectroscopy suggests Ti4+ exclusively 975C 1100C Ti3+ standard Ti4+ standard Ti2+ standard Ti0 standard

Raman spectroscopy suggests no changes in lattice structure 6. Results: Raman spectroscopy Rainer Schmidt Raman spectroscopy suggests no changes in lattice structure

Conclusions Rainer Schmidt 1.) Changes in sintering temperature of CCTO ceramics between 975 -1100C leads to dramatic changes in bulk resistivity (factor ≈103) 2.) A Cu rich phase segregates out of the ceramic by heating above 1025C 3.) Irreversible weight loss can occur suggesting Cu volatilization 4.) EDAX, XANES and Raman spectrocsopy analysis suggests that the resulting Cu deficiency 5.) Only weak indications can be found that Cu segregation/volatilization leads to increased bulk conductivity

Questions Rainer Schmidt