CH5716

Processing of Materials

Ceramic Thick Film Processing

Lecture MC6 – Tape Casting

Tape Casting Background

• Doctor/Knife Blading long established in industry

•Blade scraped excess material from the moving substrate being coated •Paper – surface coatings

•Paints – coatings onto test substrates to assess opacity

•Polymers– coatings – rubber on to fabric

•First ceramic tape casting patent in 1952 for flat plates for electronic and

radio fields

•Introduction of moving carriers realised a truly mass manufacture and

continuous technique (mid 1950’s)

•Tape casting allows large area thinner layers to be produced •Too large/thin for pressing or extrusion

•Tapes can essentially be considered to be 2D objects

•Plasticisation also allowed for green post-processing •Shaping/Cutting

•Hole punching – electrical vias for multilayer ceramics

Applications for Thick film Components Telephones Cell Phones Pagers

Computers Modems Monitors

Keyboards Printers Photocopiers

Music Keyboards Guitars (electric) Microphones

Television VCR Laser Disc Players

Stereo CD Players Cassette Decks

Speakers Antennas (some) SEGA / Nintendo

Automobiles Video Cameras Disposable Cameras

Coffee makers Watches Washers/Dryers

Ovens/Ranges Digital Clocks Walkmans/Discmans

Irons Toasters Refrigerators

Airplanes Helicopters Missiles

Radar Sonar Satellites

Space Shuttle Christmas Ornaments pH meters

Ohmmeters Ammeters Intercom Systems

Oscilloscopes Scalpel Blades X-ray Machines

MRI machines Ultrasound Walkie Talkies

Metal detectors Golf Carts Motorcycles

Electric Toothbrushes Li-Ion Batteries Fuel Cells

Basically, if it contains electronics , very likely there will be a tape cast or

printed component somewhere. Probably both

The Basic Tape Casting Process

Casting

Head

Drying & Local

Ventilation

Casting Plate

Take up

spool

Carrier Film

Feed

Carrier motion

•Ceramic Slip (slurry) introduced to casting head

•Dragged under a preset gap between casting plate and doctor blade •This meters slip to a set thickness

•Solvent evaporates to form dry green tape

•Thickness can be 10’s to 100’s μm •Recent work has demonstrated tapes under 10μm

•Dry tape can be cut & shaped in green state

•Can have further layers applied •Lamination

•Casting

•Printing

•Technique widely used in mass manufacture of ceramics •Continuous and batch processes can be used

•Normally fixed casting head with moving carrier but moving casting head also seen at lab scale •Casting head normally sits on granite or tempered glass block for flatness •Casting plate either tempered glass or stainless steel

•Latter can be heated

Lab scale

The Tape Casting Machine

Industrial Scale

•Number of different carriers can be used •Polyester film (Mylar)

•Stainless steel belt

•Tempered glass

•Mylar and steel are moving carriers

•Glass is used in moving head designs

•Tape will generally stay attached to Mylar during rolling and storage •Protects tape, prevents sticking

•Steel belt common in continuous processes, tape to next process immediately on drying

•Glass for small scale lab batches, used soon after drying

The Casting Head – Doctor Blade

Slip

Reservoir

Doctor blade

Cast tape

Carrier film

Casting plate Casting direction

Micrometer

control

•Many variables on a theme •Mainly shape/geometry of blade

•Simplest flat bottomed <1cm thin

•Various materials used •Stainless or hardened steels

•Tungsten carbide, Al2O3

•Entire assembly rides on carrier •Minimise weight. normally Al construction

•Again surface contact must be smooth •Some coated with PTFE for low friction

•Easy dismantling for cleaning

•Gap between blade and carrier meters wet

thickness •Relative movement of carrier pulls material under blade

•Essentially a wiping process

•In theory a simple process but many

variables •Slip homogeneity, viscosity,

•blade height, reservoir height, blade geometry,

•carrier speed, wetting of slip on carrier

•Temperature, humidity

Factors Affecting Casting

Carrier motion

Hydraulic

pressure

•4 main factors affecting slip behaviour during casting •Rheology

•Hydraulic pressure

•Blade gap

•Carrier Speed

•Rheology defined during slip formulation

•Others are casting process variables

Blade

Gap

Hydraulic pressure

Carrier motion

Hydraulic

pressure

Friction

under

blade

•This pushes slip under the blade •Extent of this flow determined by slip viscosity

•This hydraulic pressure gives a slip velocity under the blade

•Occurs in both Newtonian and Non-Newtonian

•Welling often observed downstream in pseudoplastic slips

•Increase in apparent viscosity in lower shear conditions after the blade •Reduces flow rate

•Some polymer relaxation (lower influence)

•Increasing viscosity prevents sideways spreading during casting •Thixotropic behaviour will also influence this

Blade Gap

Carrier motion

Hydraulic

pressure

Friction

under

blade

•Increasing blade gap increases wet thickness •However not a linear correlation as viscosity effects also impact thickness

•Influences apparent viscosity through changing shear conditions

•Also affects influence of blade friction •Bigger gap – smaller influence of friction – higher slip velocity due to hydraulic pressure

•Higher slip relative to carrier velocity

•Increasing welling

•Therefore wet thickness may increase several percent more than the direct

increase in thickness

•Reverse is true for reducing blade height

Blade

Gap

Carrier Speed

V carrier > V Slip V carrier = V Slip V carrier < V Slip

•Also referred to as casting speed •Has a significant influence on tape thickness

•Only external applied force to slip

•In Pseudoplastic systems higher speed will result in lower slip viscosity

•Works in concert with hydraulic pressures •Wet tape thickness function of slip velocity and carrier velocity

•In industrial continuous processes will maintain constant reservoir level •Therefore hydraulic pressure remains constant

•In fixed slip volume systems, hydraulic pressure will reduces as slip is consumed

Dual Blade System

Slip

Carrier Film

Tempered Glass Plate

Doctor Blades

•In small lab systems variations in slip reservoir viscosity is manageable

•In bigger non-continuous systems it can become significant

•The effects of changing hydraulic pressures can be minimised by using a dual blade

system •Back (left hand) metering blade meters slip into casting pool

•Front (right hand casting blade controls final wet thickness

•Aim to minimise difference in height between casting pool and wet tape

•Reservoir depth now not so critical as back “metering” blade merely supplies slip to

casting pool

Casting pool

Metering blade

Casting blade

Multi Layer Tapes- Simultaneous or Multiple Casting

•Multiple casting heads can be used to layer up different tapes

•Complex casting heads and blade settings

•Care needed in control of slip properties •Maintain good interfacial bonding

•Avoid diffusion and mixing

•Useful for building up tape thicknesses of single materials

•Sequential multiple casts can also be used

Carrier Motion

Triple casting Head

•Commonly used for building up thicknesses •Thicker single material

•Multilayer devices

•Important in MLCCs

•Layers brought together in green tape once

fired result in monolithic ceramic part •Lamination function of pressure temperature and

time

•Plasticiser selection important •type I for bonding,

•type II for conforming to features

•Warm Isostatic pressing often used

lamination method •Layered parts placed in sealed bag then immersed

in water (or water glycerine mix)

•Pressure & heat applied, typically 5000-1000psi and

70-90°C for 3-10 minutes

•Even pressure distribution

•Good dimensional control

•Multiple parts processed in single operation

Multi Layer Tapes- Lamination

Drying •Complex field in its own right – larger scale casting about drying

• Downstream of casting head is essentially a large drying chamber

•Drying is a one sided process controlled by 2 significant parameters •Rate of solvent evaporation at surface

•Rate of solvent diffusion through the body of the tape to the surface

•Second is the rate limiting step

Surface Evaporation •Controlled by airflow and temperature

•Generally fast – skin formation

•Higher solvent partial pressure slows skinning

•Airflow opposite to casting direction

•Airflow calculations important •Avoid explosive mixtures

•Especially when heated

Solvent Diffusion •This is slower process

•Once skin forms slows further

•To improve diffusion underside heating

sometimes used

•In combination with unheated outer airflow

•Promotes diffusion while trying to avoid

too rapid evaporation •Best case when diffusion = evaporation

•Worst when evaporation much faster than

diffusion

Air Flow

Casting Direction

Drying Issues Drying results in tape shrinkage

This is mostly in z direction

This is can occur due to packing of ceramic particle and polymer reorientation

Shrinkage can occur in x and y directions, can cause issues –

most often in the cross cast direction

Curling – Edge, Full Body, Gulls Wing, Edge with release, Reverse •Often top layer shrinking more than underlying layers

•Good particle to particle adhesion

•Thickness, Particle size dist, carrier interface, will determine which type

•Addition of type II plasticiser, Heating drying air to soften tape, Slowing solvent evaporation

Cracking – Occurs when the particle/particle adhesion less than the carrier adhesion

“Centre” Cracking- can occur down length of tape •May be offset if tape inhomogeneous or defects present

•Minimise defects & improve homogeneity

•Increase binder content, additions of Type I & II plasticiser, slow drying rate

Crows feet cracking – often propagate from defects Can be more prevalent in thicker tapes – lower binder levels higher drying gradients

•Eliminate defects

•Type II plasticiser addition to help dissipate stress concentrations round defects

Further Drying Issues

Aqueous systems can also have casting & drying issues

High surface energy can dewet from polymer carrier films •Careful matching surface energies (high surface energy carrier)

•Use of wetting agents in tape slip

•Increase solids loading increasing SG, viscosity pseudoplasticity

Much slower drying than non-aqueous •Can be an issue in continuous processing

•However explosive mixtures less of an issue

•More flexibility in application of heating

Many other tape & drying effects exist but outside scope of this lecture series Refer to Mistler & Twiname or other relevant literature for full discussion of these

However many issues at the final stage of the process can be addressed by the first – slip formulation

Surface Defects

•Wrinkling, Mudflat Cracking, Orange Peel

•Surface effects •Do not penetrate full depth of tape

•Often a sign of two layer drying

•Some effects (such as wrinkling) may be transient

•Slow down surface drying rates •Reduce air temperatures or flow rates

•Skin retarding agents

•Addition of Type II plasticisers – Allow for plastic deformation & creep in outer layer

Still a developing Technology- Freeze Casting

J.D. McCrummen MSc Thesis Montana State Univ. 2008

•Based on standard tape casting process

•Aqueous based solvent system

•Caster modified to include freezing bed

•Ice crystals grow through tape acting as a

pore former

•Once fired creates a skeletal structure for

infiltration of catalytic species

Attraction of microstructure immediately

apparent Natural grading due to the nucleation and growth

of ice crystals

Current area of research

Interest in effects of process variables on

morphology of ice crystals Freezing temperature, casting speed, tape

thickness, solvent systems,