Mechanisms during FAST 4: Oxide ceramics · f *= stress intensification factor G = grain size n =...

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

Mechanisms during FAST 4:

Oxide ceramics

Olivier Guillon


Outline

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

Lower temperatures, higher densities, smaller grain sizes

…compared to free sintering!

Question: What really happens during FAST?


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

?

!



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)

Sample

HP FAST

(P)

(T)

Graphite

felt


a - alumina (Al2O3)

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

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


Sintering behavior of alumina

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: Drrel = 0.076 DT ≈ 25 K

700 nm: Drrel = 0.026 DT ≈ 05 K

Rela

tive d

ensity

Time

Dwell time Dwell time

Time

Re

lative

den

sity


Analysis of the sintering mechanism

r = relative density pa = applied pressure

H = numerical constant T = temperature

f = stress intensification factor* G = grain size

n = stress exponent m = grain size exponent

0

lnh

hz

n

am

zp

kTG

HD

dt

d

dt

df

r

r

1

True strain:

1

01

11

r

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

ρ0 green density

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

0

0

²

1

rrr

rf

Mechanism Stress

exp. n

Grain size

exp. m

Lattice diffusion 1 2

Grain boundary

diffusion

1 3

Viscous flow 1 0

Grain boundary

sliding

1 or 2 1

Plastic

deformation

≥3 0

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


Grain size measurement

Sample cross-section

Homogeneous grain size

in the whole sample for all

investigated densities

Grain size analysis

(linear intercept method on

SEM micrographs)

h


Grain size measurement

Marked grain growth from ρrel > 0.95

Identical sintering trajectories for FAST and HP

Reduced grain growth in comparison to free sintering

R. Zuo, E. Aulbach, J. Rödel

Acta Mater. (2003)







n

amp

GkT

HD

dt

df

r

r

11

T & G

const.

T & (fpa)

const.

Rela

tive d

ensity

Time

Dwell time







n

amp

GkT

HD

dt

df

r

r

11

T & G

const.

T & (fpa)

const.

n ≈ 1

for FAST and HP

m ≈ 3

for FAST and HP

Densification is controlled

by grain boundary

diffusion

Rela

tive d

ensity

Time

Dwell time


Calculation of the activation energy

RT

Q

eDD0

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

D0 = diffusion coefficient T = absolute temperature

D0 = pre-exponential factor R = gas constant

Q = activation energy

Rela

tive d

ensity

Time

Dwell time


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

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

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

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



Results for other materials

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

Scripta Mater. (2010)

MgAl2O4 spinel

SPS

with grain size

Similar mechanisms as for hot pressing

Low stress regime: diffusion

High stress regime: climb-controlled dislocation creep

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?

Th

erm

al conductivity

Relative density

Laser Flash

measurements

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

Acta Materialia (2009)

Alumina


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

0 1 2 3 4 5 6 7 80.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1) FASTPyrometer

2) FASTThermocouple

3) HPThermocouple

4) HPFAST-Tool

TMDAR

50 MPaR

ela

tive

De

nsity,

rre

l

Time, 103*t [sec]

Begin of

Dwell Time

Tmax

= 1200 °C

Density difference depends on:

• the pressing tool used

• the temperature control

Alumina

FASTThermocouple:

without overshoot

Effect of temperature control

Dramatic effect of transient overheating on densification kinetics

for sensitive materials like ZnO (20 nm particle size)


0 1 2 3 40.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

FAST

HP

NG20

750 °C

50 MPa

Re

lative

De

nsity,

rre

l

Time, 103*t [sec]

Starting point

Dwell time

(a)

0.0 0.5 1.0 1.5

FASTPyro

FASTThermo

HP

Starting point

Dwell time

(b)

NG20

550 °C

50 MPa

Consequences on properties

400 450 500 550 600 650 700 750

103

2x103

Re

sis

tivity,

re

l [

cm

]

FAST rrel

= 0.74

HP rrel

= 0.75

NA90

50 MPa / 750 °C

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)

ZnO 90 nm


Outline











Effect of heating rate: alumina

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

0.5

0.6

0.7

0.8

0.9

1.0

Rel

ativ

e D

ensi

ty

Time (s)

35 K/min

50 K/min

100 K/min

150 K/min

Isothermal step

Same final density is reached

Is there a change in the densification mechanism?

O. Guillon & J. Langer,

J Mater Sci, 2010

1200°C


Effect of heating rate: alumina

Slope independent of the heating rate

Same results for 8YSZ

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


Master Sintering Curve approach

n

amp

kTG

HD

dt

df

r

r

1

RT

QDD exp

0

dtRT

Q

Td

pHD

kG

nn

a

m

exp

1

0

rfr

),(exp1

00 0

TtdtRT

Q

Td

G

pHD

kt

n

m

n

a

r

rf

r

r

43

3

G

D

G

D

kTdt

d bbVV

r

r

r

rrr

r

0

)(3

)(exp

1))(,(

00

mt G

D

kdt

RT

Q

TtTt

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

Free sintering:

FAST/HP:

with

Su &Johnson, J Am Ceram Soc (1996)


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

0.5

0.6

0.7

0.8

0.9

1.0

Rel

ativ

e D

ensi

ty

Log()

200 250 300 350 400 450 500

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Res

idua

l sum

of s

quar

es

Activation energy (kJ/mol)

MSC: alumina

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

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

Materials Science & Engineering A, 2010

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

Dry pressing

Pressure filtration


High(er) heating rates

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

J Am Ceram Soc (2007)

Exothermic reaction to produce heat (SHS)

Applied pressure: 60-120 MPa

Heating rate of 1600°C/min

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)



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?

(1)

(4)

As-sintered

Annealed at 1500°C


High heating rates: ZnO

S. Schwarz, O. Guillon

20 nm particles

50 MPa

Higher heating rates improve sinterability


High heating rates: ZnO

50°C/min 100°C/min (ρ= 67%)

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

Curved GB Faceted GB

(ρ= 76%)


Outline











Carbon contamination?

Secondary Ion Mass Spectroscopy (SIMS)

S

Cross section x

y

h

TM TR

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


J. Am. Ceram. Soc. (2011) 8YSZ

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

Al2O3-felt

Sample resistance

T = 750 °C 2.5x102 Ω

Graphite tool resistance

T = 750 °C 2.0x10-3 Ω

100 200 300 400 500 600 700

0.6

0.7

0.8

0.9

1.0

Relative density

Re

lative

de

nsity,

rre

l

Temperature, T [°C]

a)

103

104

105

106

Resistance

NA90

SPS / 750 °C /

50 MPa / 5 min

Re

sis

tan

ce

, R

[

]

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

Semi-conductor behavior

Electrical boundary conditions

sample

Al2O3-discs

pa direction of current

0 1 2 30.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

(a) Standard

(b) Electrically insulated

FAST

NA90

750 °C

50 MPa

Re

lative

de

nsity,

rre

l

Time, 103*t [sec]

0 1000 2000 3000

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Starting point

dwell time

750 °C

ZnO


J. Am. Ceram. Soc. (2011)

No effect of electrical boundary conditions

Sintering trajectory of pure ZnO

No significant effect of the electric field / current

Effect of the temperature overshoot

(electrically insulated)

(b)

z

x

rrel = 0.97 2 µm

(a) rrel = 0.97

z

x

2 µm

Standard FAST

Insulated FAST


Behavior of doped ZnO

Sample resistance:

T = 750 °C ~ 5 Ω

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

J Mater Sci (2009)

Standard SPS

Insulated SPS

Magnetic

current

probe

Error due to positioning


Current flow through the sample

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

Workshop IFAM Dresden (2006)

Resistivity of sample material [Ωm]

Graphite tool

I sam

ple/I

tota

l


Electrically conductive composite materials

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

Acta Mat (2007)

Electrical conductivity of a composite material depends on:

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

- temperature

Polder-Van Santen mixture rule:

m matrix

p particles

Similar equation for thermal conductivity

with

V*m volume fraction of

matrix phase in a

partially sintered compact

*

*


Electrical conductivity

90 vol.% TiN (grey) - ZrO2 (white)

pores (black)

Dense ZrO2-TiN composites


J Eur Ceram Soc (2007)


Percolation threshold


Acta Mater (2007)

Percolation: a continuous path for current is created


Effect on densification

ZrO2-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

M. Cologna, B. Rahkova, R. Raj

J. Am .Ceram. Soc. (2010)

TZ-3Y

DC-Field But why then:

Proposed explanation:

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


400 600 800 10000.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

Rela

tive D

ensity

Temperature

0V

40V/cm; max. 0.02 A/cm²

40V/cm; max. ~1.5 A/cm²

40V/cm at 1150°C; max ~6 A/cm²

1200 20 40 60 80 100 1200.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

Isothermal time [min]

Flash sintering: an effect of current

0 2 4 6 8 10 12 14 16 18

4.35

4.40

4.45

4.50

4.55

4.60

4.65

4.70

4.75

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

current switch off

induces a

temperature drop

of ~ 450 °C

Ab

so

lute

De

ns

ity

[g

/cm

³ ]

Isothermal time [min]

R. Baraki, S. Schwarz, O. Guillon

8YSZ

AC Field from room temperature


Comparison Flash Sintering / FAST

FAST:

limited voltage, applied from the beginning

electric equipotential affected by the conducting pressing tool

Flash sintering conditions are not expected in standard FAST

S. Schwarz,

O. Guillon

ZrO2 (sinter-forging)

DVsample =10 V

electrode

ZrO2 (in graphite tool)

DVsample =2.68 V


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)

Mechanisms during FAST 4: Oxide ceramics · f *= stress intensification factor G = grain size n =...

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