Effect of heating rate

Mechanisms during FAST 4: 

Oxide ceramics 

Olivier Guillon 

March 24, 2011 | FAST School | O. Guillon | 1

Outline 

Lower temperatures, higher densities, smaller grain sizes 

…compared to free sintering! 

Question: What really happens during FAST? 

Methodology to answer this question: 

� FAST vs. Hot Pressing 

• Identification of main densification mechanism 

• Evaluation of microstructure 

� Effect of heating rate 

� Effect of electric field/current 


Goal 

� Transition from the „black box“ era to the intelligent „tool box“ approach 

? 

� When you know how a tool works, you can get more out of it! 

� Based on experimental evidence 

March 24, 2011 | FAST School | O. Guillon | 3 

!

FAST vs. Hot Pressing 

� FAST (HP 25/1, FCT Systeme) 

Measuring system: 

• Pyrometer (P) from 450°C (standard) 

• Thermocouple (T) from 

room temperature (optional) 

� HP (HPW 150, FCT Systeme) 

Measuring system: 

• Thermocouple (T) 

Temperature calibration 

By melting copper powder 


(T) 

(P) 

FAST 

Sample 

Graphite 

felt 

HP 

(T)

a - alumina (Al 2O 3) 

� Insulator (dielectric) 

� Model material for sintering studies 

Purity: 99.99 % 

Average particle size: 150 nm 

(TMDAR, Taimei Chem., JP) 

Purity: 99.80 % 

Average particle size: 700 nm 

(CT 3000 SG, ALMATIS, USA) 

Theoretical density: 3.986 gcm -3 

J. Langer, M. Hoffmann, O. Guillon, Acta Materialia (2009) 


Powder filling 

Ø 20mm pressing tool 

Initial compaction 

(50 MPa, 3 min) 

Load adjustment 

15-50 MPa 

Start of the sintering experiments 

Heating rate: 10 Kmin -1 

Max. temperature: 1100-1250 °C 

Atmosphere: Vacuum 

Dwell time: up to 2 h

Relative density 

Sintering behavior of alumina 

Time 

� Curves for FAST and HP show similar trends, for both particle sizes 

� Densification starts earlier for FAST but same final densities 

� Differences in relative density at the beginning of dwell time: 

150 nm: Dr rel = 0.076 DT ≈ 25 K 

700 nm: Dr rel = 0.026 DT ≈ 05 K 



Dwell time Dwell time 

Time

Analysis of the sintering mechanism 

True strain: 

� 

d� 

dt 

z 

� 

� z 

r 

� h � 

� ln � � 

� � 

� h0 

� 

dr 

dt 

r = relative density p a = applied pressure 

H = numerical constant T = temperature 

f = stress intensification factor * G = grain size 

n = stress exponent m = grain size exponent 

G 

HD 


1 

� 

m 

kT 

* after Montes et al., Comp. Mat. Sci. (2006) 

ρ 0 green density 

or Helle et al., Acta Metall. (1985) 

� � n 

fp 

a 

� � 1 � r �� 

f � �1 

� � � 

� � 

� 

1 

� 

� � r 0 �� 

f � 

r ² 

1 � r 

0 

�r � r � 

0 

�1 

Mechanism Stress 

exp. n 

Grain size 

exp. m 

Lattice diffusion 1 2 

Grain boundary 

diffusion 

1 3 

Viscous flow 1 0 

Grain boundary 

sliding 

Plastic 

deformation 

1 or 2 1 

≥3 0 

M.N. Rahaman, Ceramic Processing and Sintering (2003)

Grain size measurement 

h 

Sample cross-section 

Grain size analysis 

(linear intercept method on 

SEM micrographs) 


Homogeneous grain size 

in the whole sample for all 

investigated densities

Grain size measurement 


R. Zuo, E. Aulbach, J. Rödel 

Acta Mater. (2003) 

� Marked grain growth from ρ rel > 0.95 

� Identical sintering trajectories for FAST and HP 

� Reduced grain growth in comparison to free sintering



1 dr 

� � 

r dt 

Time 

HD 

kT 

� 

� � 

� 





G 


1 

m 

Dwell time 

� 

� 

� 

� 

� � n 

fp 

a 

T & G 

const. 

T & (fp a) 

const.



1 dr 

� � 

r dt 

Time 

HD 

kT 

� 

� � 

� 





G 


1 

m 

Dwell time 

� 

� 

� 

� 

� � n 

fp 

a 

T & G 

const. 

T & (fp a) 

const. 

n ≈ 1 

for FAST and HP 

Densification is controlled 

by grain boundary 

diffusion 

m ≈ 3 

for FAST and HP


Calculation of the activation energy 

Time 

D � D � e 0 

� Q 

� � 

� RT 

D 0 = diffusion coefficient T = absolute temperature 

D 0 = pre-exponential factor R = gas constant 

Q = activation energy 


Dwell time 

� 

� 

� 

Similar value for the activation energy: 

Grain boundary diffusion 

HP Q = 430 ± 50 kJ/mol 

FAST Q = 420 ± 35 kJ/mol 

Wang & Raj, 

J. Am. Ceram. Soc. (1990) 

Q = 440 ± 45 kJ/mol

Results for other materials 

Comparative studies FAST / HP: 

� Cubic zirconia (8YSZ), ionic conductor: same densification mechanism and 

sintering trajectory 

� Zinc oxide (ZnO), semi-conductor: same densification mechanism (grain size very 

sensitive to temperature variations) 

� Tetragonal zirconia (TZ3Y): same densification mechanism and sintering 

trajectory 

� AlCuFeB quasi-crystals: same densification mechanism 


J. Langer, M. Hoffmann, O. Guillon, J. Am. Ceram. Soc. (2011) 

J. Langer, M. Hoffmann, O. Guillon, J. Am. Ceram. Soc. (2011) 

G. Bernard-Granger, A. Addad, G. Fantozzi, G. Bonnefont, C. Guizard, D. Vernat, Acta Mater. (2010) 

L. Ramond, G. Bernard-Granger, A. Addad, C. Guizard, Acta Mater (2010)

Results for other materials 


with grain size 

MgAl 2O 4 spinel 

SPS 

Similar mechanisms as for hot pressing 

� Low stress regime: diffusion 

� High stress regime: climb-controlled dislocation creep 

K. Morita, B.N. Kim, H. Yoshida, K. Hiraga, 

Scripta Mater. (2010) 

Densification maps for MgO 

R. Chaim, M. Margulis 

Mat.Sci. Eng. A (2005)

Investigation of the initial sintering stage 

� Thermal conductivity proportional to the contact area between particles 

� Same values for HP und FAST samples above 65% density 

� Effect of the temperature overshoot at the beginning of the FAST process? 


Thermal conductivity 

Alumina 


Laser Flash 

measurements 

J. Langer, M. Hoffmann, O. Guillon, 

Acta Materialia (2009)

Temperature profile during FAST process 


� Overheating up to 130 °C at the 

beginning of the heating process 

(pyrometer control only above ~400°C; 

input power fixed) 

� Effects on neck formation and neck 

growth (especially for nano-powders)

Effect of the pressing tool and 

temperature control 

FAST Thermocouple: 

without overshoot 

Density difference depends on: 

• the pressing tool used 

• the temperature control 


Relative Density, r rel 

1.00 

0.95 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

0.60 

0.55 

0.50 

TMDAR 

50 MPa 

Alumina 

1) FAST Pyrometer 

2) FAST Thermocouple 

3) HP Thermocouple 

4) HP FAST-Tool 

0 1 2 3 4 5 6 7 8 

Time, 10 3 *t [sec] 

Begin of 

Dwell Time 

T max = 1200 °C

Effect of temperature control 

Relative Density, r rel 

1.00 

0.95 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

0.60 

0.55 

0.50 

0.45 

(a) 

Starting point 

Dwell time 

NG20 

750 °C 

50 MPa 

FAST 

HP 

0 1 2 3 4 


Dramatic effect of transient overheating on densification kinetics 

for sensitive materials like ZnO (20 nm particle size) 

(b) 

NG20 

550 °C 

50 MPa 


Dwell time 

FAST Pyro 

FAST Thermo 

HP 

0.0 0.5 1.0 1.5 

J. Langer, M. Hoffmann, O. Guillon, J. Am. Ceram. Soc. (2011)

Resistivity, r el [�cm] 

Consequences on properties 

2x10 3 

10 3 

FAST r rel = 0.74 

HP r rel = 0.75 

NA90 

50 MPa / 750 °C 

ZnO 90 nm 

400 450 500 550 600 650 700 750 

Temperature, T [°C] 

� Slightly lower resistivity for FAST samples, but same order of magnitude 

� Correlates with the larger interparticle contact area shown by 

Young‘s modulus measurements (due to initial temperature overshoot)

Outline 











Effect of heating rate: alumina 

Relative Density 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 


35 K/min 

50 K/min 

100 K/min 

150 K/min 

-800 -600 -400 -200 0 200 400 600 

Time (s) 

1200°C 

Isothermal step 

� Same final density is reached 

� Is there a change in the densification mechanism? 

O. Guillon & J. Langer, 

J Mater Sci, 2010

Effect of heating rate: alumina 

Heating rate 

[K/min] 

Slope 

[10 -3 K -1 ] 

10 1.7 ± 0.2 

35 1.6 ± 0.2 

50 1.7 ± 0.2 

100 1.5 ± 0.2 

150 1.4 ± 0.3 

� Slope independent of the heating rate 

� Same results for 8YSZ 


Master Sintering Curve approach 

Free sintering: 

FAST/HP: 

dr 3�� 

� � D � D 

V V b b 

� � � 

3 

4 

rdt 

kT � G G 

� ( t, 

T ( t)) 

1 dr 

� 

r dt 

HD 


k 

p 

n 

� 

G 

r 

� 

0 a r 0 

r 

t 1 Q k G 

� exp � �dt 

� 

0 

� 

T RT �� 

D 3r 

� 

HD 

m 

G 

rf 

kT 

m 

n 

� � 

� 

� � n 

fp 

a 

1 

dr 

� � T 

t 

0 

� 

� 

exp 

� 

� 

� 

� 

� � 

� 

with 

0 

Q 

RT 

r 

0 

rHD 

D � D exp 

kG 

� 

�dt 

� 

( r ) 

0 

( r ) 

m 

p 

n 

a 

m 

f 

n 

0 

dr 

� � ( t, 

T ) 

� � Q 

� 

� RT 

T 

� 

� 

� 

1 

� exp 

A unique MSC ρ=f(Θ) can be obtained if: 

• only one diffusion mechanism is dominant during sintering 

• the microstructure is function only of density 

Su &Johnson, J Am Ceram Soc (1996) 

� 

� � 

� 

Q 

RT 

� 

�dt 

�


MSC: alumina 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

-16 -15 -14 -13 -12 -11 -10 

Log(�) 


Residual sum of squares 

0.50 

0.45 

0.40 

0.35 

0.30 

0.25 

0.20 

0.15 

0.10 

200 250 300 350 400 450 500 

Activation energy (kJ/mol) 

� Master Sintering Curve obtained for the whole sintering cycle 

� Apparent activation energy of 290 kJ/mol 

� No change in the densification behavior

Activation energy values 

Dry pressing 

Pressure filtration 


Free sintering of alumina (same powder) 

Literature: Q = 400-1100 kJ/mol (!) 

Dry pressing: 700 � 20 kJ/mol 

Pressure filtration: 605 � 15 kJ/mol 

� Interplay between different diffusion mechanisms 

when whole sintering curve taken into account 

� In the density range 70-85%: 

450 kJ/mol for both sample types 

M. Aminzare, F. Golestani-fard, O. Guillon, M. Mazaheri, H.R. Rezaie, 

Materials Science & Engineering A, 2010

High(er) heating rates 

� Exothermic reaction to produce heat (SHS) 

� Applied pressure: 60-120 MPa 

� Heating rate of 1600°C/min 

F. Meng, Z. Fu, J. Zhang, H. Wang, W. Wang, Y. Wang, Q. Zhang, 

J Am Ceram Soc (2007) 


Dense alumina (99%) 

in a few minutes


J. Zhang, F. Meng, R. Todd, Z. Fu, Scripta Mater. (2010) 

Same grain size and density, but different resistance to mechanical abrasion 


(1) 

(3)


(1) 

(4) 

As-sintered 

Annealed at 1500°C 


Grain boundaries in non-equilibrium: 

� Diffuse, open structure as opposed to 

relaxed boundaries? 

� Higher diffusion coefficient and thicker 

GB? 

Also to be seen in other materials?

High heating rates: ZnO 


20 nm particles 

50 MPa 

Higher heating rates improve sinterability 

S. Schwarz, O. Guillon

High heating rates: ZnO 

50°C/min (ρ= 67%) 

100°C/min (ρ= 76%) 

Curved GB 

Faceted GB 


S. Schwarz, A. Thron, K. van Benthem, O. Guillon

Outline 











Carbon contamination? 

Secondary Ion Mass Spectroscopy (SIMS) 

8YSZ 




h 

y 

TM 

Cross section x 

� Carbon hardly diffuses into the specimen (constant C-signal at depth of ~2 µm) 

� Oxygen vacancies are responsible for specimen darkening 

� No influence of carbon on electrical conductivity / sintering behavior 

TR 

S

XPS measurements on pure ZnO 

• Identical emission spectra for FAST und HP 

• Only ZnO could be detected 

• No trace of carbon

Resistivity changes in pure ZnO 

Sample 

Sample resistance 

Al 2O 3-felt 

T = 750 °C 2.5x10 2 Ω 

Graphite tool resistance 

T = 750 °C 2.0x10 -3 Ω 

Vanmeensel et al. J. Mater Sci. (2008) 

Relative density, r rel 

1.0 

0.9 

0.8 

0.7 

0.6 


Resistance 

NA90 

SPS / 750 °C / 

50 MPa / 5 min 

100 200 300 400 500 600 700 

Temperature, T [°C] 

Semi-conductor behavior 

a) 

10 6 

10 5 

10 4 

10 3 

Resistance, R [ �]

Electrical boundary conditions 

direction of current 

Al 2O 3-discs 

p a 

sample 



Relative density, r rel 

1.00 

0.95 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

0.60 

0.55 

(a) Standard 

(b) Electrically insulated 

FAST 

NA90 

750 °C 

50 MPa 

ZnO 

0 1000 2000 3000 


dwell time 

750 °C 

0 1 2 3 


No effect of electrical boundary conditions 

1.00 

0.95 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

0.60

Sintering trajectory of pure ZnO 

(electrically insulated) 

� No significant effect of the electric field / current 

� Effect of the temperature overshoot 

Standard FAST 

z 

(a) r rel = 0.97 

z 

(b) 

x 

Insulated FAST 

x 

2 µm 

r rel = 0.97 2 µm

Behavior of doped ZnO 

Sample resistance: 

T = 750 °C ~ 5 Ω 

Insulated SPS 

Standard SPS 

T. Misawa, N. Shikatani, Y. Kawakami, T. Enjoji, Y. Ohtsu, H. Fujita 

J Mater Sci (2009) 


Magnetic 

current 

probe 

Error due to positioning

Current flow through the sample 

I sample/I total 

Graphite tool 


Resistivity of sample material [Ωm] 

M. Herrmann, B. Weise, K. Sempf, A. Bales, J. Raethel, I. Schulz 

Workshop IFAM Dresden (2006)

Electrically conductive composite materials 

Electrical conductivity of a composite material depends on: 

- volume fraction of conductive and insulating phases (incl. porosity) 

- temperature 

Polder-Van Santen mixture rule: 

Similar equation for thermal conductivity 


* 

* 

with 

m matrix 

p particles 

V* m volume fraction of 

matrix phase in a 

partially sintered compact 

K. Vanmeensel, A. Laptev, O. Van der Biest, J. Vleugels 

Acta Mat (2007)

Electrical conductivity 

90 vol.% TiN (grey) - ZrO 2 (white) 

pores (black) 


J Eur Ceram Soc (2007) 


Dense ZrO 2-TiN composites

Percolation threshold 


Percolation: a continuous path for current is created 


Acta Mater (2007)

Effect on densification 


ZrO 2-TiN (60/40) 

Transition from insulator to conductor-like behavior during sintering

Effect of electric field on surface diffusion 

� Before densification takes place 

� Neck growth kinetics estimated from I-V curves 

(without additional Joule heating) 

� No effect of electric field 

(limited to 10 V/cm) 

� Specific Surface Area measurements confirm this result 

(identical with and without electric field) 


TZ-3Y @ 1050°C 

Sample thickness: 

2 mm 

M. Cologna, R. Raj 

J. Am .Ceram. Soc. (2010)

Flash sintering 

TZ-3Y 

DC-Field 

Proposed explanation: 

Joule heating at grain boundaries (several hundreds of °C) 


But why then: 

M. Cologna, B. Rahkova, R. Raj 

J. Am .Ceram. Soc. (2010)


Flash sintering: an effect of current 

8YSZ 

AC Field 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

0.60 

0.55 

0.50 

400 600 800 1000 

Temperature 


0V 

40V/cm; max. 0.02 A/cm² 

from room temperature 

40V/cm; max. ~1.5 A/cm² 

40V/cm at 1150°C; max ~6 A/cm² 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

0.60 

0.55 

1200 20 40 60 80 100 120 0.50 

Isothermal time [min] 

Absolute Density [g/cm³ ] 

4.75 

4.70 

4.65 

4.60 

4.55 

4.50 

4.45 

4.40 

4.35 

current switch off 

induces a 

temperature drop 

of ~ 450 °C 

8YSZ flashed at 1150°C with 40V/cm; max. 5A 

0 2 4 6 8 10 12 14 16 18 

Isothermal time [min] 

R. Baraki, S. Schwarz, O. Guillon

Comparison Flash Sintering / FAST 

FAST: 

� limited voltage, applied from the beginning 

� electric equipotential affected by the conducting pressing tool 

DV sample 

=10 V 

ZrO2 (sinter-forging) 

Flash sintering conditions are not expected in standard FAST 


electrode 

DV sample 

=2.68 V 

ZrO2 (in graphite tool) 

S. Schwarz, 

O. Guillon

Conclusions 

� Identification of sintering mechanism requires „clean“ experimental 

conditions to avoid misunterpretations 

� For „insulating“ oxide ceramics: 

• Same densification mechanism for HP and FAST 

(not only based on activation energy considerations) 

• Identical grain growth behavior 

• No effect of electrical field during sintering in FAST 

� Transient heating affects subsequent densification 

� Densification behavior may depend on the heating rate (or not: MSC) 

� Large electric current (not field) may lead to additional phenomena 

(Flash sintering)

Effect of heating rate

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