Che5700 陶瓷粉末處理 Casting Techniques Slip casting (e.g. drain casting, solid casting), tape casting, gel casting, centrifugal casting, etc Prepare proper slurry (proper viscosity), fill into mold or direct forming, dehydration (or solvent), de-molding, drying, sintering, post-fabrication, product Can be used to get complex shape products Uniformity of slurry: very important Dispersion of slurry: can be judged by sediment height (an index) anisometric particles, easy to show preferred direction, e.g. plate particle, tend to be parallel to casting surface

Schematics of Slip Casting Drain casting: dilute solution (5%), porous mole, particle adsorb on mold wall, form thin layer, pour extra solution, de-molding and other processing Rate of dehydration affect particle packing

Mechanism of Slip Casting Che5700 陶瓷粉末處理 Mechanism of Slip Casting Flow resistance: mold and cake Darcy law applied to get relationship between pressure drop and velocity; usually mold resistance greater than cake resistance PT = applied pressure + suction pressure from mold Filtration theory: sample thickness and time relation

Viscosity of Slurry viscosity too high or low: not ideal Che5700 陶瓷粉末處理 Viscosity of Slurry viscosity too high or low: not ideal Also slurry may have yield strength Good stability for storage life

Schematics of Solid Casting Solid casting: concentrated slurry pour into mold, dehydration, de-molding and other processing;

Strength of Product Product strength increase gradually Che5700 陶瓷粉末處理 Strength of Product Product strength increase gradually Due to increasing density, strength comes from interaction between particles, with help from binder

Che5700 陶瓷粉末處理 Filter Pressing In general: slip casting rely on capillary action to remove water, filter pressing: from applied pressure, can reach higher density Cake may be compressible: S = compressibility index another possibility: vacuum filtering

Sedimentation Casting/Centrifugal Casting Che5700 陶瓷粉末處理 Sedimentation Casting/Centrifugal Casting Different casting process Sedimentation: slower rate; add centrifuge to increase rate of sendimentation Well dispersed suspension, very slow sendimentation, can reach higher density; different particle size, different rate  different effect, i.e. may be size distribution inside the cake (sediment)  to product, poor effect  always differential shrinkage Centrifugal casting: suitable for tube making; r2 for g as the acting force

取自TA Ring, 1996

取自TA Ring, 1996;

Che5700 陶瓷粉末處理 Porous Mold Materials Gypsum: frequently used; plaster of Paris CaSO4. .5H2O + H2O  gypsum CaSO4. 2H2O Today’s technology: good smooth surface, high ultimate porosity, micron-sized pores, short setting time, small dimensional expansion on setting (for easy release), low cost Often use hot water to get uniform mold Porosity: depending on mixing, electrolyte in water, T etc. parameters Limitations : low compressive strength when partially saturated with water  erosion; when in contact with acid or alcohol, gypsum life becomes shorter New generations of polymer mold, with similar pore structure, high strength, corrosion resistant

when making mold, too much water, low strength, greater adsorption Taken from JS Reed, 1995; when mold drying, avoid too high temperature, will cause dehydration

Dip Coating Pull at an angle and speed to get coating; Che5700 陶瓷粉末處理 Dip Coating Pull at an angle and speed to get coating; film thickness depend on slurry rheology; for Newtonian fluid

Particle Orientation Taken from TA Ring, 1996; Che5700 陶瓷粉末處理 Particle Orientation Taken from TA Ring, 1996; Dip coating: shear force may cause special arrangement of particles to minimize resistance

Problems of Slip Casting Che5700 陶瓷粉末處理 Problems of Slip Casting Implies optimized viscosity and gelation  by proper additives; slurry rheology, casting rate, cast property, drying and burn-out rate = balance of these parameters

Taken from JS Reed, 1995; case A, range of “good” cast is wider and favorable; S = soft; H = hard;

Che5700 陶瓷粉末處理 Gel Casting Refer to cases where binder (monomer) can polymerize, make system a gel; fill in a mold (of complex shape)  stimulate reaction to gelation  de-mold  thermal treatment to product Can be used for dense product or porous product, former case: concentrated slurry to get high density packing Linear shrinkage is about 0.5 – 1.5% Volume of binding phase/volume of particles ~ 16% Mold not necessary porous material, can be made of a variety of materials

Control of Defects in Cast Che5700 陶瓷粉末處理 Control of Defects in Cast

Discussions on Defects Che5700 陶瓷粉末處理 Discussions on Defects Non-uniformity in wall thickness: due to non-uniformity from mold, non-uniformity in water adsorption or casting rate; or erosion non-uniformity in mold etc. distortion: come from stress in green body, or differential shrinkage during drying, e.g. warping crack: difference in adhesion between object and mold wall, differential volumetric shrinkage, especially at joint; or very large agglomerates void: could be due to trapped air Bubbles and pin-holes: comes from during pouring; pin-hole on the surface may come from removing excess slurry Surface irregularity: come from irregularity on mold surface, water adsorption non-uniformity, during pouring of high viscosity slip etc. Microscopic defects: incomplete dispersion, contamination, unadsorbed additive; etc.

Che5700 陶瓷粉末處理 Tape Casting Or called doctor blade method; often used to fabricate thin, plate-like products, such as substrate High productivity, if continuous process, green tape first, then cut into appropriate sizes; can be multilayer “laminated” products Appropriate viscosity is of vital importance In general, particle size around 1 μm, or specific surface area of 2 – 5 m2/g An-isometric particles may align preferentially during casting, to form special structure; Tapes of Tyflon, Mylar are often used as carrier, may be coated with de-molding compound (脫模劑) for easy removal

Common tape casting equipment; from JS Reed, 1995

Che5700 陶瓷粉末處理 Doctor Blade Belt moving velocity + pressure gradient  effect of motion

MLCC= multilayer ceramic capacitor LTCC = low temperature co-fired ceramics Taken from TA Ring, 1996

Che5700 陶瓷粉末處理 Slurry Compositions Change to water system: a common trend for environmental reasons; need to overcome drying-shrinkage problem

Che5700 陶瓷粉末處理 Additives

Che5700 陶瓷粉末處理 Quantity Effects Too much lubricant, lower tensile strength; increase strain-to-failure

Binder Quantity Tape casting: Vb/Vp ~ 15 – 25% Che5700 陶瓷粉末處理 Binder Quantity Tape casting: Vb/Vp ~ 15 – 25% high MW binder, can offer higher strength and toughness green body, not moving during drying (migration with solvent) Yet binder has to be removed during calcination

Che5700 陶瓷粉末處理 Tape Thickness Pressure flow under the blade + planar laminar flow by the carrier Thickness = f(height under blade, speed of carrier, drying shrinkage, viscosity of slurry) Dr = (density of slurry)/(density of as-dried tape);ho cast thickness at the blade; L = length of tape Drying stage: shrinkage mostly in the thickness; not much in the lateral direction  thus use Dr

Che5700 陶瓷粉末處理 Uniformity Issue High rate and high viscosity, beneficial to product uniformity Industrial scale: 25 m long, several meters wide, 1500 mm/min speed, tape thickness: 25-1250 m are common;

Che5700 陶瓷粉末處理 Drying Drying rate ~ temperature and solvent content of the drying air; initial: solvent evaporation, drying rate about constant Capillary force to transport liquid inside green body: mechanism during rate decreasing region Shrinkage occurs: particle closer to each other Binder often not moving, lubricant can move Last item: vapor transport may be rate determining step, tape becomes more elastic Final density ~ 55-60% of TD; 35% organics, 15% porosity

Shaping and Laminating Che5700 陶瓷粉末處理 Shaping and Laminating Individual steps: scale become smaller and smaller; 0402; 0201, etc.

Che5700 陶瓷粉末處理 Binder Burnout Need certain gas permeability to allow organics to burn out and vented; Bond strength between layers: important issue during burnout stage; Strength related to pressure during forming, temperature and time

Che5700 陶瓷粉末處理 Defects Agglomerate inhibit sintering in that region; differential shrinkage lead to crack; bubbles & poor surface cause poor contact between layers Difference between metal line and neighboring ceramics; gas pressure from binder burnout may cause problem

Effects of Some Parameters Che5700 陶瓷粉末處理 Effects of Some Parameters Effects bubble skinning crack More solvent decrease NA may increase Higher temperature decrease increase NA Increase solvent evaporation rate NA increase increase Faster pouring rate NA NA NA Air rate up NA NA increase Inorganic NA lower NA Ideal slip: high solid content, low viscosity, solvent not causing skinning and trap air bubble, drying system can remove gas and rapid drying

取自JS Reed, 1995;