asst. prof. dr. asst. prof. dr. sirirat siriratsirirat tubsungnoen...

Tape Casting Processing

Asst. Prof. Dr. Asst. Prof. Dr. Asst. Prof. Dr. Asst. Prof. Dr. SiriratSiriratSiriratSirirat TubsungnoenTubsungnoenTubsungnoenTubsungnoen RattanachanRattanachanRattanachanRattanachanSchool of ceramic engineering School of ceramic engineering School of ceramic engineering School of ceramic engineering

426206 Ceramic Fabrication Processes426206 Ceramic Fabrication Processes426206 Ceramic Fabrication Processes426206 Ceramic Fabrication Processes

R.Sirirat Cera. Fab. Proc. : Tape casting 2

Outline: for 2-3 h.

• The principle of tape casting• Tape casting application• Tape forming methods• Doctor Blade Processing• Preparation of tape casting slurry• Drying of tape • Problems and defects


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The principle of tape casting

Tape casting ��ก��ก�� ก�� ก!"�#�$�� %�ก�� &'�ก��$��'ก'��()��$* กก �'�� “doctor blade” ��*��+ก��'� +'��,��ก��#�(�)��)�� *ก� �*�,'$��$� (green tape) ��ก��ก��ก�-ก�-��-�� ()- ' ��$��.ก/ �"��$��#��('��0�� *ก� �*��ก��,'$�� two-dimensional.

Tape casting #�$�ก � slip casting � ��#()��$� +'�� () slurry ��ก �-��ก��ก��+��$��$�-��ก � #()• Tape casting � evaporation• Slip casting � absorption in porous mold


Tape applications:

• Thin layers of dielectric materials that could be electroded, stacked and sintered into monolithic multilayered capacitors

• Multilayered capacitors• Multilayered ceramic packages• Mullite substrate for photovoltic solar energy cells

• Substrates of Al2O3 and AlN � thin or thick film circuit

• BaTiO3 for capacitors• Solid electrolytes for sensors or energy conversion


Tape applications

Functionally graded ferroelectric ceramics (FGM) have been fabricated for pyroelectric applications by an aqueous tape casting technology. The FGM consisted of a porous Lead Zirconate Titanate (PZT) tape-cast ceramic, which was made by starch inclusions; sandwiched between two dense PZT layers by stacking and lamination.


Tape applications:

Courtesy to www.synkera.com/technology/electrochem.htm

Multi Chip Module

Courtesy to www.micro-circuit.com/.../index.asp

Courtesy to www.ami.ac.uk/courses/topics/0135_cc/index.htmlCourtesy to www.sanwacomponents.com/Ceramics/ceramic.htm

Printed ceramic substrates for electronics


Tape casting processes: waterfall technique

Recirculatingpump

Curtain of slip

Substrate carrier

Conveyor belt

Drying stage

Collection

This technique has been used to form thin tape for capacitor dielectrics and thicker tape for porous electric for fuel cells


Use of the paperpaper--casting processcasting process to fabricate a honeycomb structure for a heat exchanger

The thickness of adherence depends on the viscosity of the slurry and the nature of the paper.


Principle of one-side tape coating unit, the slurry is stripped off from one side of the support by a knife-edge shaped blade. The tape thickness decreases with increasing angle of inclination.

Continuously working type-coating machine

Slurry reservoir Knife blade


• The adherence of a certain amount of slurry to a carrier tape while it passes through the slurry reservoir.• In order to avoid an excessive reflow of material and drop formation, a fast increase of velocity of deposited slurry is necessary.• Tape thickness increase with: increasing slurry viscosity, increasing velocity of the supporter, decreasing specific weight of slurry, decreasing wetting angle between slurry and support. • For tapes with thickness ranging from about 30 to 150 µm


Factors for tape casting

• Slurry viscosity

• The velocity of substrate movement

• The angle of declination

• Specific weight of slurry


Roll compaction schematics (Source: Ragan)

For roll compaction: moisture 7-13%


Storage A Storage B Storage C

Formulation weighting -bags

Ball milling waterDispersant

Q.C particle size

Blunger DefoamerBinder and plasticizer

Wet sieve and magnetic separator

Q.C specific gravity, viscosity, % solid

Spray dryer and screen (40 mesh)

Storage bag

Feeder

Roll compaction , speed 5-10 cm Slit, punch Sintering


Advantages:• Water-based technology• easily incorporated into a factory with a spray drying facility• more akin to conventional ceramic processing• Low, uniform shrinkage

Disadvantages: • need a spray dryer• can not produce very thin tape ( <0.25 mm)• surface quality not adequate for thin film substrates• Tapes can not be laminated for multilayers

Roll compaction


Doctor bladeDoctor blade

Casting methods:• Non-continuous working: moving the casting head over a fixed substrate• Continuous working : pulling the substrate under the fixed reservoir


ก��+��)� Doctor blade


Continuously working industrial casting unit showing the long drying tract and a closed drying tunnel (courtesy of C. v. Stein, Siemens AG)

Non-continuously working laboratory casting units. A casting head with a gap that can be adjusted or chosen is drawn over a glass plate.

Movable infrared lump for drying

R.Sirirat Cera. Fab. Proc. : Tape casting 20เมนู ถัดไปยอนกลับ

Courtesy to Courtesy to Courtesy to Courtesy to www.keramverband.de/brevier_engl/4/1/4_1_3.htmwww.keramverband.de/brevier_engl/4/1/4_1_3.htmwww.keramverband.de/brevier_engl/4/1/4_1_3.htmwww.keramverband.de/brevier_engl/4/1/4_1_3.htm


Double-blade construction for the production of tapes with narrow thickness tolerances. A first blade reduces the hydrostatic pressure in the slurry reservoir to a lower value in the casting chamber between the two blades. During casting, this pressure in front of the second blade, which determines the tape thickness, remains approximately constant ( after Runk and Andrejco, 1975)


��ก)��)� Sheet casting

• Basic components: ��#��" 25 30- ��ก$��" 1 ��-�• Carried film: Teflon (polytetrafluorethylene)

Mylar (glycol terephthalic acid polyester)Cellulose acetate#��" 0.015 �� )*��)��" 1 �� )�'$�-$)�ก�� )' � +,�� ,�� slurry ��$�,��#�-�'�� carried film �-� slurry #��ก��'�ก�� carried film 7)��#��"�� 7(�)#�#0�ก��'- �)��


Solvent Ceramic powder Dispersant

Milling

Mixing PlasticizerBinder

Degassing

Tape casting

Drying

Shaping

Binder burning

Sintering Quality control

Doctor BladeDoctor BladeDoctor BladeDoctor Blade


Courtesy to www.britannica.com


Typical multilayered ceramic process

Tape cast sheets

Punch holes

Metallize holes

Print patterns

laminate

Punch final shape

Sinter

The stack is laminated at high P. and slightly elevated temperature � promotes interlayer adhesion

www.ami.ac.uk/courses/topics/0135_cc/index.htmlwww.ami.ac.uk/courses/topics/0135_cc/index.htmlwww.ami.ac.uk/courses/topics/0135_cc/index.htmlwww.ami.ac.uk/courses/topics/0135_cc/index.html

Courtesy to Courtesy to Courtesy to Courtesy to global.kyocera.comglobal.kyocera.comglobal.kyocera.comglobal.kyocera.com/.../capacitor//.../capacitor//.../capacitor//.../capacitor/ceramic.htmlceramic.htmlceramic.htmlceramic.html


Schematic cross section showing the complexity of the metallized circuit patterns in a ceramic multilayer integrated circuit package. (From E. Ryshkewitch and D. Richerson, Oxide Ceramics, 2nd ed., General Ceramics, Inc., N.J., 1985, p. 466

Process steps for fabrication of multilayer capacitors by tape casting (Courtesy J. Adair, University of Florida)


Tape casting slurry

• ก��()ก- �+��- ��(�)�� (Binder-solvent selection)• ก�ก��*��)�� (Dispersion of ceramic power)• ก��-�� (Preparation of slurries)• �+��$��)�� (Drying of the tape)• �8��-+�� (Problems and defects)

Tape casting slurry ��ก)�'$�• Ceramic powders• Solvents• Dispersants• Binders• Plasticizers• Other additives �� homogenizers

�7(�)��- Substrates �� uniform dielectric and mechanical properties *��-$)�#+��%�� ก��-��ก�#�#0� slip ��#��#ก�� (casting)


Materials and milling procedure (from Mister et.al., Ceramic Processing before firing)


Inorganic additives• To lower sintering temperature• To control grain growth• To modify the properties of the ceramic for special applications Ex. MgO, Y2O3

Inorganic components: Powder characteristics• Small particle size: requires a smooth surface, minimum defect size and high density• Soft agglomerates• Sol-gel technique � homogeneity, low temperature, densification, small grain size, greater surface smoothness, purity• Disadvantage: Higher costs, strong shrinkage, form crack


Solvents:

• it should dissolve the binder, plasticizer and dispersant and should not react with the inorganic powders.• Homogenization of the components of the slurry • Low viscosity• Low boiling point• Low evaporation heat• High vapor pressure (��,'$��ก�� +) �+��$��$ก�)��$�� )0"�;��$��ก�)�ก.-�+�� -��)**��+��$�ก�'��7�!,'$• it can be recycle

Boiling point and heat of vaporization

Solvent B.P. (°C) Heat of vaporization (J/g)

Water 100 2257Ethanol 70.0 856Trichlorethylene 85.7 240Trichlorethylene/Ethanol 70.9 -


ก�"� Aqueous system *��+��$�ก�'ก��,'$�$ ��(�)��ก��-�� binder �ก*��+��$��ก��ก�'7 �<� hydrogen bond ��*��+��$�ก�' flocculation- )�� SiC: Trichlorethylene / ethyl alcohol

AlN: Toluene/ethanol + PVB + dibutyl-phathalateAzeotropic mixtures: constant solvent composition- )�� 66/34 vol% of MEK (methyl ethyl ketone)/ethanol

72/28 vol% of trichlorethylene / ethanol

Some solvent systems: MEK/ 95% ethanol TCE/ anhydrous ethanolMEK/ anhydrous ethanol TCE/95% ethanolxylenes/ 95% ethanol TCE/MEK/ethanolxylenes/ anhydrous ethanol toluene/ethanol/cyclohexanoneMEK/ tolueneMEK/ acetoneToluene1,1,1 trichloroethane (TCE)


Dispersants/Deflocculant/Wetting agent

• To stabilize the dispersion with a high ceramic powder concentration• effect on slurry viscosity, rheology, tape packed density• to separate or hold separate the primary particles so the binder can coat them individually• to decrease the amount of solvent in the powder suspension in order to dry the slip faster and with less shrinkage• To burn out cleanly prior to sintering in order not to contaminate the final fired part.


Molecular structure of glyceryltrioleate and triglycerides in fish oil deflucculant

The absorption of menhaden fish oil and glycerol trioleate from tolulene onto an alumina powder. (E.S. Tormey et. al., 1984)


Some reported dispersants/deflocculants used for Tape Casting

Polyisobutylene Polyvinyl butyralLinoleic acid EthoxylateOleic acid Phosphate esterCitric acidStearic acidLanolin fatty acidsSalts of polyacrylic acidsSalts of methacrylic acidsBlown menhaden fish oilCorn oil Glycerol trioleateDibutyl amineSubstituted imidazolinesPolyethlene glycol


มี 2 models ใชอธิบาย action of dispersants

1. Electrostatic repulsion

2. Steric-hindrance (Ceramic powder in organic sol.)


Si----OH

Si----OH

Si----OH

H+

Acidic s

olutio

n

OH -Basic solution

Si---OH2+

Si---OH2+

Si---OH2+

Si---O-

Si---O-

Si---O-

SiO2 particle+ H2O

The production of surface charges on SiO2 particles by adsorption of ions

++

++++

+

++

++

--

-

--

-

-

-

-

The distribution of positive and negative charges in the electrical double layer


Si---OH

Silica particle

+ X-Si-[CH2-CH2]n - R

Si—O-Si- [CH2-CH2]n-R + XH

Chemical attachment of a polymer chain to the surface of a silica particleThus, good solubility in the solvent (�=)�ก �ก��ก�)

Steric-hindrance (Ceramic powder in organic sol.)


Binder

Ceramic powders

Polymer:7( ��Binder: ��-�) • #��.��)��• #�� #��('��0��)��• #��)��• ก�7��7>��

Binder: Binder: ��-�)��-�) •• #��.��)��#��.��)��•• #��#�� #��('��0��)��#��('��0��)��•• #��)��#��)��•• ก�7��7>��ก�7��7>��

�8** ��-�)ก��()ก binder: • -$)��%��,'$��( )��'��ก �• ��#��('�� lower molecular weight• �#��• �+��$�ก�'#��.��)��7)�7��• ��ก�,��))ก�� : )0"�;�� ก@ก�� กA��ก�'��

�8** ��-�)ก��()ก�8** ��-�)ก��()ก binder: binder: •• -$)��%��,'$��( )-$)��%��,'$��( )��'��ก ��'��ก �•• ��#��('��#��('�� lower molecular weightlower molecular weight•• �#��#��•• �+��$�ก�'#��.��)��7)�7��+��$�ก�'#��.��)��7)�7��•• ��ก�,��))ก��ก�,��))ก�� :: )0"�;��)0"�;�� ก@ก��ก@ก�� กA��ก�'�� กA��ก�'��


การเลือก Binder

• เพิ่ม Strength ให green tape (มีการยึดเกาะของอนุภาคตางๆใน green tape) โดยเติมในปริมาณที่นอยที่สดุ• กําจัดออกงาย ไมมีกากเหลืออยู•ไมมีผลตอ decomposition ของ oxidation• ไมแพง• มี molecular weight สูงจะทาํใหเทปที่ไดมีความเหนียวและความแข็งแรงสูง

• Low glass transition temperature (Tg) •The anchoring of binder on the ceramic particles depends on the characteristics of particle surface and on the chemistry of binder.• ขึ้นกับชนิดของ casting support และควรจะทําให slurry มีคุณสมบัติ pseudoplastic behavior• Setting must be avoided to ensure a homogeneous distribution and to avoid the adhesion of tape on the support.


• Vinyl : (poly) vinyl alcohol (PVA) for water –based binder, (poly) vinyl butyral (PVB). The vinyl family is typically used for powder systems to be fired in an oxidizing atmosphere. After firing, the vinyl polymer is remain to be carbon�CO or CO2

• Acrylics: or acrylates. Consideration: cost, cleaner removal in neutral or reducing atmospheres, strength, solubility. Acrylic binders are also plasticized or made flexibility by a wide variety of additives. • Cellulose: in aqueous or non-aqueous systems. The biggest drawback is their thickening properties. Methyl cellulose and ethyl cellulose form very viscous aqueous solutions, resulting requires excess solvent (water), which lowers that slurry solidsloading. Low solids loading increase drying shrinkage, cracking tendency, and tape porosity.


Effect of binder content on particle separation in green tape. [modified from R.A. Gardner and R.W. Nufer, Solid State Technol., 5,40 (1974)]

An excess of binder may increase the particle separation and reduce the packing faction of the particles in the tape.


Plasticizer

• เพื่อใหเทปโคงงอไดโดยไมเกิดการแตกหัก และสามารถทําการเจาะรูหรือตัดขอบเพื่อทําเปนแผนวงจรได • Plasticizers เปนสารพอลิเมอรที่จะมีการสรางพันธะกับ binder เพื่อใหเกิดความยืดหยุนของ matrix และปองกันการแตกของเทปเมื่อเทปแหง Type I: a chemical that softens the polymer chains (below Tg) between particles, allowing them to stretch more easily. Slope of stress/strain curve would be expected to be less than that of unmodified binder. Type II: it will display a lower strain to failure because the binder remains stiff without a lubricant to soften the polymer.


Binder and plasticizers used in tape casting systemsBinder and plasticizers used in tape casting systems

Binder PlasticizerNonaqueous Polyvinyl butyral Dioctyl phthalate

Polymethylmethacrylate Dibutyl phthalatePolyvinyl alcohol Benzyl butyl phthalatePolyethylene Polyethylene glycol

Aqueous Acrylics GlycerineMethyl cellulose Polyethylene glycolPolyvinyl alcohol Dibutyl phthalate


Materials Processing: slip preparation

Dispersion milling:

• To break apart any agglomerated particles• To coat the broken-apart particles with a dispersant

Solid loading in the dispersion mill: % solid = powder weight / (powder weight + solvent weight)During dispersion milling � solid loading 20 -90% (depend on the powder density, size and dispersant effectiveness.)

Mill rotation speed should be at a fraction of the critical speed (35-115% of critical speed):

DNc 6.76=Nc = Critical rotation speed (rpm)D= mill inside diameter in feet

Slip volume (+ grinding media) = 1/2 or 2/3 of total volume


The effect of different times for each of the stages of slip preparation on the final fired density of the aluminum oxide substrate material. Fired density is plotted as a function of both total time in the mill and the split of the time between the dispersion mill and the binder/plasticizer mixing.

��ก��


Slip de-airing

ฟองอากาศที่มีอยูในน้ําสลิปนี้จะเปนสาเหตุใหเกิดตําหนิ รูเข็มหรือ “crows foot” (cracks that radiate from the pinhole) De-airing เปนการไลฟองอากาศที่อยูในสลิปที่มาจากการบดหรือผสมน้ําสลิป • Vacuum range 635-700 mm of Hg• Depending on the volume of slip to be de-airing, the time required will range from eight minutes to more than one hour.


Slip Characterization

• viscosity : most tape casting slips fall in the

range of 500-6000 m.Pa.s (cP) when tested

using an RV4 spindle at 20 rpm.

• specific gravity

• particle size distribution


•Most tape casting slips are pseudoplastic and exhibit a decrease in viscosity with an increase in shear rate. • Viscosity tends to decrease as a function of increasing shear rate.•This behavior is beneficial in the tape casting process, since the slip will display a lower viscosity under the shear of doctor blade, yet a higher viscosity downstream from the blade, thereby resisting motion within the metered film.

Pseudoplastic slips (shear-thinning)


Tape thickness H on the carrier velocity V

Where A = a constant (depend on the amount of side flowDr = the ratio of the density of the slurry/density of as-dried tape,ho = the cast thickness at the blade,∆P = the pressure-motivating flow,ηs= the viscosity of the slurry on casting,L = length of the cast

)]6/(1[ 200 LPhhADH sr νη∆+=


Influence of the various casting parameters Influence of the various casting parameters on the thickness of the deposited layeron the thickness of the deposited layer

Vo

Model casting unit for tape thickness calculation. D, thickness of the dried tape, h, gap height; L, gap length; Vo, casting velocity

For a slurry with Newtonian rheological behavior and a laminar flow in the gap of a simple casting unit.

)6

1.(2

.0

2

LV

PhhD

ηα

∆+=

D = thickness of dried tapeα= a factor which reflects the thickness shrinkage of the wet layer during dryingh = height of blade gapL = length of blade gapη= viscosity of the slurry ∆P = pressure difference between entry and exit of gap (�#��)��' � slurry)V0 = relative velocity of casting unit and substrate


Values of the calculated normalized tape thickness D [2/(αh)] for different sets of casting parameters


Large gaps can give rise to appreciable deviations from this sample linear thickness dependence on the gap height. This effect is more pronounced with small casting velocity (V0), low slurry viscosities and small gap lengths L.

For gap heights below 200µm and casting velocity higher than about 5 cm/s, D.2/αh�1, so it will have only a small effect on the tape thickness D.The height of slurry level (∆P) will exert a strong influence on the tape thickness. For pseudoplastic fluids, the tape thickness depends strongly on the gap width and the casting velocity.

Large gaps can give rise to appreciable deviations from this sample linear thickness dependence on the gap height. This effect is more pronounced with small casting velocity (V0), low slurry viscosities and small gap lengths L.

For gap heights below 200µm and casting velocity higher than about 5 cm/s, D.2/αh�1, so it will have only a small effect on the tape thickness D.The height of slurry level (∆P) will exert a strong influence on the tape thickness. For pseudoplastic fluids, the tape thickness depends strongly on the gap width and the casting velocity.

ก��


Frictional forces and flow under a doctor blade during tape castFrictional forces and flow under a doctor blade during tape castinging

�The viscosity of a slip greatly affects the flow characteristics under the doctor blade.�The doctor blade wipes off the excess slip deposited in the carrier surface. �This flow, referred to as welling, depends not only on the slip viscosity but also on the head pressure formed by the reservoir height, blade gap, carrier speed and blade shape.

Pool or head

Hydraulic force stems from the weight of the slip in the reservoir (head pressure) and affects the volume flow under the blade during thecasting process.


Surface Tension and wetting at the slip/carrier interface

ก�� ก�� ก!"#� $��% ��!�� !��!��&'��(�!��&��!��&��)&��*))�+��ก�� (, ก��(��!� �� -+��ก�&��!��, ��&�ก!ก��.+ก��!��)&�� (, ��!��+� �+��,�ก/�� +�� *))�+�0(�%��ก , �ก� �� -��)��'�&��!� �� 0��&��ก12&ก��,��!�+��ก�� +


�Energy balance will induce the edges of the wet film to taper at an angle θ defined by the interfacial energy balance. �Slip viscosity, specific gravity and rheology can modify the actual contact angle and therefore the size of the tapered area. �To overcome several dewettingproblems in aqueous slips simply by increasing the solids loading in the slip, thereby increasing specific gravity, viscosity, and pseudoplasticity and avoiding the dewetting.

�*3��ก��"#�$ ��


The effect of carrier velocity on wet tape thickness

The volume of slip flows through a fixed area (orifice) defined by the carrier surface on the bottom, the doctor blade on top and the side walls of the reservoir. Net slip velocity = slip volume flow/ the fixed orifice area

The wet thickness of the cast layer (wet tape thickness) downstream from the doctor blade is defined by the ratio of slip velocity to carrier velocity.

Good!


Fitting it all together

There are 4 measurable and controlled variables that work together to determine the wet thickness of the cast film.• Slip homogeneity, rheology, surface tension and wetting are all factors that affect the viscosity (slip friction) under the casting blade•Casting speed determines the externally applied force that compels the fluid to flow under the blade• Blade gap and the effective thickness of the blade determine the flow orifice and thus the force of resistance applied by the doctor blade on the moving slip against the direction of flow (blade friction) • Pressure applied to the slip reservoir compels the slip to flow out of the reservoir under the doctor blade.

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asst. prof. dr. asst. prof. dr. sirirat siriratsirirat tubsungnoen...

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