Aem Lect16

Advanced Electronic Ceramics I (2004)

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

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

PEG additionto PVA

In order to reduce Tg

1. Use of less rigid side groups2. Reduction of the # of polar group3. Lowered molecular weight4. Lowered intermolecular contact


Plasticizer

plasticizer1. Low molecular weight

- decrease Tg

2. Significant decrease in green strength

Strength of alumina green sheetas a function of plasticizer, DBP



Plasticizer


Plasticizer concentration

low plasticizer concentrationporosity decrease due to the increaseinflexibility

too high plasticizer concentrationincreasing interparticle distancedecreasing the green density

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


σ

ε

σ

ε

σ

ε

σ

ε

No plasticizer Type II plasticizer

Type I plasticizerType I & Type II

- lower Tg

- binder solvent

-Lubricant-Easy to release of green tapefrom carrier film

Advanced Electronic Ceramics I (2004)D. J. Shanefield, Organic Additives & Ceramic Processing

H

C

OH

H

H

C

OH

H

C

OH

H

Glycerin

P

HOHCH

HCH

O Hn

Polyethylene Glycol (PEG)

D P B

Various Plasticizers

Plasticizer m.p.(oC) b.p .(oC)Water 0 100Ethylene glycol -16 197Diethylene glycol - 8 245Tetraethylene glycol - 7 288Poly(ethylene glycol) -10 >330Glycerine 18 290Dimethyl phthalate 1 284Dibutyl phthalate 1 340Octyl phthalateBenzyl butyl phthalateDiethyloxalate



Common binder(binder + plasticizer) choice

Application

Tape Casting

Screen Printing

Binders

Polyvinyl ButyralMethacrylate Solution(in MEK)Methacrylate EmulsionAmmonium Polyacrylate

AlginatesGumsEthyl CellulosePolyvinyl Butyral

System

NonaqueousNonaqueous

AqueousAqueous

AqueousAqueousNonaqueousNonaqueous

Advantages

StrongEasy burnout

Easy burnoutHigh Solids

InexpensiveInexpensivePseudoplasticEasy Burnout

D. J. Shanefield, Organic Additives & Ceramic Processing


Binder Burnout 1


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 plasticizerPlasticizer(liquid) can evaporate at lower Tmostly plasticizer, residual solvent, small amount of binder

When large amount of plasticizer is added evaporation of plasticizer leaves many pores for the air, CO2, and solvent vapor.


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 burnout1. 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

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

(TMA)ThermoMechanicalAnalysis

Heightmonitoringusing(LVDT)LinearVoltageDifferentialTransformer


Binder Burnout: TMA study

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

Various stage of Binder Removalcan be resolved using TMAanalysis


Binder Burnout: Design of Belt Furnace

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

Design factor1. Belt speed2. Flow rate of purging gas


Dispersent

Surfactant (SURFace ACTive AgeNT)

The role1. 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 romaintain 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

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


Dispersent


Measures1. 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 ofoptimum 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


Polymethacrylic Acid(PMA)

HCH

CH3

C

C O

OHn

D B

C=C

C=O

OH

C=C

C=O

OH

+

- C - C - C - C -

C=O

OH

C=O

OH

n

acrylic Acid Polyacrylic Acid

D

D

OC

O ONa

Sodium Alginate(water soluble)

B


Dispersent


C3H4(OH) 3 + 3C17H33COOH → C3H5(C17H33COO) 3 + 3H2OGlycerin Oleic Acid Glyceryl Triolate(Olein)

D

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

Cf. saturated fat (glyceryl tri stearate found in red meat)- Stimulate the production of cholesterol - 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.)

→ sodium stearate soap→ solid soap


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


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

Menhaden Fish OilPrimarily(not entirely)“Steric hindrance”deflocculant

Phosphate ester- function both asionic repulsion andSteric hindrancedeflocculant


Dispersent


1. Unsaturation(double bond)

2. m.w.~1000- high m.w.- long moleculecause bridging &tangling

3. Ester linkage

Aqueous : high ε, high ionic concentration - electrostatic stabilizationNonaqueous: low ε, low ionic concentration -Steric stabilization

1000e2Na 1/2

κ = ΣZi2MiεkT


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 sticking1. 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 on1. Slurry viscosity2. Casting carrier speed3. Doctor-blade gap setting4. 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


ProblemTape sticks excessively to

carrier film

Tape releases from carrier film too soon and curls up

Tape is too week to handle once it is released from carrier film

Tape is hard but brittle

Cracks during drying

Solution1. More release agent (dispersant of liquid, and.or

plasticizer), or use2.Silicon surface-treated carrier film1. Less of above, or2. Add some more-powerful solvent (such as

methylene chloride), to attack carrier film slightly1. More plasticizer, up to the point of almost filling the

pores between ceramic powder particles2. Shorter milling time after adding binderMore 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

of dispersant per 100 gm of ceramic powder), or2. More binder and plasticizer, up to the point of

preventing high fired density3. 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


ProblemCracks during firing

Warpage during firing

Fired density too low

Solution1. High green density(a. better dispersant, or more dispersant, or more

milling before adding binder, orb. less total organics)

2. Slower firing heat-up (if cracked pieces do not match up)3. Slower firing cool down1. Try the same 3 things above2. Use optimized weight of porous pre-fired

ceramic plates1, More milling before adding binder, or2. More milling after adding binder, or3. Less total binder and plasticizer


- 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.

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”.

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

Automatic stacking andPrinting machine


Heating Press for Lamination of green tape

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


Shrinkage matching in co-firing of two different layers1. Shrinkage matching during the sintering

T TT

shrinkage

Cracking during sintering Cracking after sintering Good adhesion

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


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



Matching thermal expansionbetween alumina and YSZ

Matching sintering temperatureby controlling particle size andby 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

Technology

Aem Lect16