Aem Lect16

Plasticizer
- structurally expands the binder
- improves the distribution of the binder in the slurry (binder solvent)
- causes flexibility (by lowering glass transition point, Tg)
of green tape

In order to reduce Tg
1. Use of less rigid side groups
PEG addition
2. Reduction of the # of polar group
to PVA
3. Lowered molecular weight
4. Lowered intermolecular contact

R. Moreno, Am.Ceram.Soc.Bull., 71(11), 1647 (1992)

Advanced Electronic Ceramics I (2004)

Plasticizer

plasticizer
1. Low molecular weight
- decrease Tg
2. Significant decrease
in green strength

Strength of alumina green sheet
as a function of plasticizer, DBP

Plasticizer

Plasticizer concentration

low plasticizer concentration
porosity decrease due to the increase
inflexibility

too high plasticizer concentration
increasing interparticle distance
decreasing the green density


R. E. Mistler and E. R. Twiname, Tape Casting, Theory and Practice

No plasticizer Type II plasticizer
-Lubricant
-Easy to release of
σ σ green tape
from carrier film

ε ε
Type I & Type II Type I plasticizer

- lower Tg
σ σ - binder solvent

ε ε


Various Plasticizers
DPB P
H H H
HH
HO C C O H C C C
H H
HH n
OH OH OH
Polyethylene Glycol (PEG) Glycerin
Plasticizer m.p.(oC) b.p .(oC)
Water 0 100
Ethylene glycol -16 197
Diethylene glycol -8 245
Tetraethylene glycol -7 288
Poly(ethylene glycol) -10 >330
Glycerine 18 290
Dimethyl phthalate 1 284
Dibutyl phthalate 1 340
Octyl phthalate
Benzyl butyl phthalate
Diethyloxalate
D. J. Shanefield, Organic Additives & Ceramic Processing R. Moreno, Am.Ceram.Soc.Bull., 71(11), 1647 (1992)

Common binder(binder + plasticizer) choice

Application Binders System Advantages

Tape Casting Polyvinyl Butyral Nonaqueous Strong
Methacrylate Solution Nonaqueous Easy burnout
(in MEK)
Methacrylate Emulsion Aqueous Easy burnout
Ammonium Polyacrylate Aqueous High Solids

Screen Printing Alginates Aqueous Inexpensive
Gums Aqueous Inexpensive
Ethyl Cellulose Nonaqueous Pseudoplastic
Polyvinyl Butyral Nonaqueous Easy Burnout

D. J. Shanefield, Organic Additives & Ceramic Processing

When large amount of
plasticizer is added
evaporation of plasticizer
Binder Burnout 1
leaves many pores for
the air, CO2, and solvent
vapor.
Without the powder, the dotted
line shows the same starting
weight of binder alone dropping
more rapidly
TGA of the binder plus the ceramic powder.

Non-evaporating binder being fired in N2

Mostly binder + small amount of plasticizer
Plasticizer(liquid) can evaporate at lower T
mostly plasticizer, residual solvent, small amount of binder

Binder Burnout 2

1. Typical temperature schedule for binder burnout
- halted at 500oV for ~ 1h and then rise to the sintering temperature

2. Single binder with narrow range of m.w.
- binder burnout occurs at the small range of temperature
- abrupt burnout might cause crack

3. For the gradual binder burnout (desirable)
- employs wide range of molecular weight
- employs the mixture of different molecular species


Binder Burnout 3

Problems in binder burnout
1. Green ceramic body is so large that both the oxygen diffusion inward
to the binder and the outdiffusion of combustion products are slow
- burnout in pressurized air for the thick body(diffusion is slow)
- addition of ~100ppm of transition metal (Mn, Pd) to catalyze oxidation
- use of self-oxidizing binder (cellulose nitrate) that is soluble in ethyl
acetate or methanol (restriction in the choice of solvent)

2. Binder can not burn without oxygen when the non-oxidizing
atmosphere is required for protecting the ceramic or its metallization.
- use of wet hydrogen(water vapor oxidize the binder)
- electrical conductor (Mo or W ) on the insulating alumina


Binder Burnout: TMA study

(TMA)
Thermo
Mechanical
Analysis

Height
monitoring
using
(LVDT)
Linear
Voltage
Differential
Transformer
H.T.Kim et al., Am. Ceram. Soc. Bull., 80(10), 34 (2001)

Binder Burnout: TMA study

Various stage of Binder Removal
can be resolved using TMA
analysis

H.T.Kim et al., Am. Ceram. Soc. Bull., 80(10), 34 (2001)

Binder Burnout: Design of Belt Furnace
Design factor
1. Belt speed
2. Flow rate of
purging gas

T.C.K.Yang et al., Am. Ceram. Soc. Bull., 80(10), 43 (2001)

Dispersent
Surfactant (SURFace ACTive AgeNT)

The role
1. To separate the primary particles so the binder coat them
individually (Trapped air in the interstitial space trouble in
deairing and sintering)
2. To increase solids loading in the powder suspension in order ro
maintain moderate viscosities after binder addition (Many
2ndary particles result in the loose packing in the green tape.)
3. To decrease the amount of solvent in the powder suspension in
order to save money on solvent or in order to dry the slip faster
and with less shrinkage
4. To burn out clearly prior to sintering in order not to contaminate
final part


Dispersent
Measures
1. Sedimentation height (the lower, the more effective dispersion)
- difficult to analyze the very fine powder
2. Minimum viscosity
- used in scientific studies of dispersent (precise determination of
optimum dispersent amount)
- powerful in tape casting technique
- can not ordinarily be used with the high solids loadings because the
slips are nearly solids except right near the point of minimum viscosity.
3. Maximum solid loading
at maximum usable viscosity
- at the constant dispersent amount,
measure the viscosity increment
as increasing solid loading
- helpful to know the best dispersent conc.
- used in engineering field.


Dispersent
H CH3 DB
CC
DB
H C On O ONa
C
OH
O
Polymethacrylic Acid(PMA)

D
C=C C=C - C - C - C - C -n
C=O + C=O C=O C=O Sodium Alginate
(water soluble)
OH OH OH OH
acrylic Acid Polyacrylic Acid


Dispersent
D
C3H4(OH) 3 + 3C17H33COOH → C3H5(C17H33COO) 3 + 3H2O
Glycerin Oleic Acid Glyceryl Triolate(Olein)

Naturally occurring liquid (vegetable, fishes)
- Polyunsaturated (contains several double bond)

Cf. saturated fat (glyceryl tri stearate found in red meat)
→ sodium stearate soap
- Stimulate the production of cholesterol
→ solid soap
- heart disease

1. Olein heating with NaOH → Reverse reaction (glycerin + oleic acid)
2. NaOH + oleic acid → sodium oleate (soap) [saponification reaction]
Manufactured by heating vegetable oil with alkali (liquid soap.)


Most of dispersents for organic solvent contain various fatty acid and esters

Advanced Electronic Ceramics I (2004)Twiname, Tape Casting, Theory and Practice
R. E. Mistler and E. R.

Menhaden Fish Oil
Primarily
(not entirely)
“Steric hindrance”
deflocculant

Phosphate ester
- function both as
ionic repulsion and
Steric hindrance
deflocculant

R. E. Mistler and E. R.
Twiname, Tape Casting,
Theory and Practice


1000e2Na 1/2
κ= ΣZi2Mi
Dispersent εkT
Aqueous : high ε, high ionic concentration - electrostatic stabilization
Nonaqueous: low ε, low ionic concentration -Steric stabilization
1. Unsaturation
(double bond)
2. m.w.~1000
- high m.w.
- long molecule
cause bridging &
tangling

3. Ester linkage


Steric Hindrance
♦ Steric: comes from Greek word
that means ‘solid’, and refers to size
and shape being the important factors.
♦ definition : mechanical prevention of
two particles approaching each other

♦ Uniform coating of non-polar organic
compound
♦ Two reasons for not sticking
1. Non-polar materials have much less
van der Waals attraction than the
higher polar oxide surface
2. Coatings are weak and will break
easily even if they do adhere


Dispersent

W.R.Cannon et.al, Advances in Ceramics, Vol.26, p525 (1989)

Two-stage milling
1st stage
(ceramic powder + part of solvent + dispersant)
- low-viscosity-slurry preparation
- breaking down agglomerates
- uniformly distribute a dispersant on the surface
of ceramic powder

2nd stage
( + binder + plasticizer + part of solvent)
- homogeneous mixing of binder and plasticizer


Vacuum deairing
Purpose
- remove bubbles inside the slurry
- agitation + vacuum deairing (635 - 710 mmHg)
- till the moderate viscosity(1000 - 5000 mPa·sec)


Tape casting

Thickness of film depends on
1. Slurry viscosity
2. Casting carrier speed
3. Doctor-blade gap setting
4. Reservoir depth behind the doctor blade

In the most cases,
(The thickness of dried green tape) = 1/2 x (The blade gap setting)
Casting speed : 5 -100 cm / min

Polymeric carrier film
: Mylar, Teflon, cellulose triacetate, Aclar, silicon-coated Mylar,
polyethylene


Tape casting: Problem shooting 1
Problem Solution
Tape sticks excessively to 1. More release agent (dispersant of liquid, and.or
carrier film plasticizer), or use
2.Silicon surface-treated carrier film
Tape releases from carrier film 1. Less of above, or
too soon and curls up 2. Add some more-powerful solvent (such as
methylene chloride), to attack carrier film slightly
Tape is too week to handle 1. More plasticizer, up to the point of almost filling the
once it is released from pores between ceramic powder particles
carrier film 2. Shorter milling time after adding binder
Tape is hard but brittle More plasticizer, up to the point of 2.5 times the binder
weight (which would make the tape too sticky)
1. Higher solid loading in slip (might require up to 4 gm
Cracks during drying of dispersant per 100 gm of ceramic powder), or
2. More binder and plasticizer, up to the point of
preventing high fired density
3. More release agent (dispersant or plasticizer)
4. Slower drying (lower temp. and/or air flow, or more
highly saturated solvent vapor in the air)


Tape casting: Problem shooting 2

Problem Solution
Cracks during firing 1. High green density
(a. better dispersant, or more dispersant, or more
milling before adding binder, or
b. less total organics)
2. Slower firing heat-up
(if cracked pieces do not match up)
3. Slower firing cool down
Warpage during firing 1. Try the same 3 things above
2. Use optimized weight of porous pre-fired
ceramic plates
Fired density too low 1, More milling before adding binder, or
2. More milling after adding binder, or
3. Less total binder and plasticizer


Automatic stacking and
Printing machine

The machine is designed for
stacking ( soft pressing ) the
ceramic
green sheets on the carrier
palettes and for screen printing of
electrodes in MLC production,
based on highly refined “print on
stack technology”.

- Uses both carrier and freestanding tapes
- Print on stack yields superior stacking at high layer counts
- Perfect carrier film removal (optional) ensures high quality products
- Sheets of tape are cut automatically from the roll and pressed onto the stack.
- Screen printer is specially designed for precise printing of electrodes.
- It enables very accurate alignment of electrodes and uniform printing
conditions for printing all the layers of stacks.

http://www.keko-equipment.com/

Heating Press for Lamination of green tape

http://www.tester.co.jp/sa03.html

Shrinkage matching in co-firing of two different layers
1. Shrinkage matching during the sintering
2. Shrinkage matching after sintering
~ the matching of ∆L/L during and after the sintering in order to avoid the
cracking
3. The matching of thermal expansion coefficient to improve the
resistance against cyclic thermal shock
- particle size and distribution of powder and organic content
should be controlled
Cracking during sintering Cracking after sintering Good adhesion

shrinkage

T T T

Shrinkage matching: Example,
wide range air-to-fuel ratio sensor



Matching sintering temperature
Matching thermal expansion
by controlling particle size and
between alumina and YSZ
by adding flux
S.Iwanaga et al., from Hitach Co. Ltd


Matching shrinkage by
controlling the content of
organic binder

S.Iwanaga et al., from Hitach Co. Ltd

Aem Lect16

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