Fundamentals of Ceramic Materials PDF 2024-2025 Fall

FUNDAMENTALS of CERAMIC MATERIALS Prof.Dr. Filiz Şahin 2024-2025 Fall  Mining Powder precipitation Ore Vapour phase synthesis Preparation Powder synthesis Sol-gel...

FUNDAMENTALS of CERAMIC MATERIALS Prof.Dr. Filiz Şahin 2024-2025 Fall  Mining Powder precipitation Ore Vapour phase synthesis Preparation Powder synthesis Sol-gel processing Carbothermal reduction Milling Powder Processing Mixing with additives Granulation Die pressing Isostatic pressing Forming Extrusion Injection Moulding Slip Casting Tape Casting Solid state sintering Sintering Liquid phase sintering Hot Pressing Pressure assisted sintering Hot Isostatic pressing Spark plasma sintering Surface finihsing Quality controll CERAMIC PRODUCTS2 Raw Materials Selection Criteria 1.Purity 2.Particle Size 3.Reactivity 3  1.Purity Purity strongly influences high-temperature properties such as strength, stress rupture life, and oxidation resistance of ceramic materials. For instance; Si3N4 + MgO; Ca is concentrated at gb (grain boundries) and decreases the softening point- decrease creep resistance Si3N4 + Y2O3 ; Ca is absorbed into solid solution, do not reduce the refractoriness of the system Impurities present as inclusions do not affect properties such as creep or oxidation, but do act as flaws that can concentrate stress and decrease component strength. The effect on strength is dependent on the size of the inclusion compared to the grain size of the ceramic and on the relative thermal expansion and elastic properties of the matrix and inclusion. – WC inclusions in Si3N4 have little effect on the strength; – Fe and Si have a large effect. The effects of impurities are important for mechanical properties, but may be even more important for electrical, magnetic, and optical properties. Electrical, magnetic, and optical properties are usually carefully tailored for a specific application, often by closely controlled addition of a dopant. Slight variations in the concentration or distribution of the dopant severely alter the properties. 4  2.Particle Size Particle size distribution is important, depending on which consolidation or shaping technique is to be used. In most cases the objective of the consolidation step is to achieve maximum particle packing and uniformity, so that minimum shrinkage and retained porosity will result during densification. A single particle size does not produce good packing. Optimum packing for particles all the same size results in over 30% void space. Adding particles of a size equivalent to the largest voids reduces the void content to 26%. Adding a third, still smaller particle size can reduce the pore volume to 23% Therefore, to achieve maximum particle packing, a range of particle sizes is required. 5  Real ceramic particles are generally irregular in shape and do not fit into ideal packing. Porosity after compaction of these powders is generally greater than 35% and sometimes greater than 50%. Large amounts of porosity are difficult to eliminate during densification. For example, 40% , initial porosity resulted in only 0.25% porosity and 2 μm grain size after sintering, 50% initial porosity resulted in 2% porosity and over 10 μm grain size after sintering 60% initial porosity resulted in over 10% porosity and 55μm grain size after sintering. Low porosity and fine grain size are beneficial to achieve a ceramic with high strength. However, there are many applications where strength is not the primary criterion. Refractories are a good example. Most refractories contain either large particles or high porosity as an important constituent in achieving the desired properties such as low thermal conductivity and high thermal shock resistance. 6  3.Reactivity Another important aspect of the starting powder is reactivity. The primary driving force for densification of a compacted powder at high temperature is the change in surface free energy. Very small particles with high surface area have high surface free energy and thus have a strong thermodynamic drive to decrease their surface area by bonding together. Very small particles, approximately 1 μm or less, can be compacted into a porous shape and sintered at a high temperature to near-theoretical density. – Transparent polycrystalline alumina for sodium vapor lamp envelopes is a good example. – To achieve transparency, virtually all the pores larger than about 0.4 μm must be removed during sintering. Pores larger than 0.4 μm scatter the visible wavelengths of light (~0.4 to 0.8 μm) and prevent transmission. Use of a highly reactive starting Al203 powder with an average particle size of about 0.150 μm aids in elimination of pores over 0.4 μm. 7  Particle size distribution and reactivity are also important in determining the temperature and the time at temperature necessary to achieve sintering. Typically, the finer the powder and the greater its surface area, the lower are the temperature and time at temperature for densification. This can have an important effect on strength. Long times at high sintering temperature result in increased grain growth and lower strength. To optimize strength, a powder that can be densified quickly with minimal grain growth is desired. 8 Primary particles: Types of Powders The smallest clearly identifiable unit in the powder. Primary particles may be crystalline or amorphous and cannot easily be broken down into smaller units. Agglomerates: Clusters of bonded primary particles. Ø Soft (easily broken up) and Ø Hard (difficult to break up bcs of stronger interparticle bonds, should be avoided) agglomerates Particles: General term applied to both primary particles and agglomerates. Granules: Large agglomerates (0.1–1 mm in diameter) that are formed by the addition of a granulating agent (e.g., a polymer binder). The mixture is tumbled, producing large, nearly spherical granules that flow freely and can be used to fill complex molds and in automated processes. 9 10 Flocs: Clusters of particles in a liquid suspension held together electrostatically. Colloids: Very fine particles (they can be as small as 1 nm in diameter) held in fluid suspension by Brownian motion. Colloidal particles settle very slowly. Aggregates: Coarse constituents (>1 mm) in a mixture (e.g., addition of gravel to cement to make concrete) 11 12  5.3 POWDER PREPARATION AND SIZING Control of particle size and particle size distribution is required to achieve the optimum properties for the intended applications. Each application has specific requirements. – High-strength ceramics require very fine particles (typically M(OH)n + nH+ or M n+ + OH- ® M(OH)n -M-O-H- + -H-O-M- ® -M-O-M- + H2O 45 Example of Sol-Gel 46  The sol-gel process can be used to make single or multicomponent oxides. In the case of a one-component system—silica. Multicomponent sols can be prepared by mixing Of the many available silicon alkoxides, TEOS is different precursors, which are selected to give commonly used. It is insoluble in water, but eventually an oxide of the desired composition. water is necessary for the hydrolysis reaction; There may be problems if the hydrolysis rates of the hence we need to select a solvent for both the precursors are different,and this can create alkoxide and water. Ethanol is a suitable solvent, inhomogeneities in the subsequent gel. and a typical formulation contains three main We can allow for this possibility by partially components: 43 vol% Si(OC2H5) 4, 43 vol% hydrolyzing the less reactive component [e.g., C2H5OH, and 14 vol% H2O. Si(OC2H5) 4 before adding the more reactive one [e.g., Ti(OiC3H7)4]. 47 48 Ceramic powders for structural and electronic applications have been synthesized by sol-gel processing of alkoxides. As an example, Pb(Fe2/3 W1/3)03 powders are synthesized from mixed solutions of; -lead propoxide {Pb(OC3H7)2} and iron propoxide {(Fe(OC3H7)3} in methoxyethanol {(CH30CH2CH20H)} containing acetyl acetone by fluxing at 125°C, - adding a W(OC2H5)6 solution in methoxyethanol, refluxing the solution mixture at 80°C, - hydrolysis with a H20/ethylene glycol mixture in methoxyethanol, - gelation, drying, and comminution, and - calcination at 870°C. These powders have applications in multilayer ceramic capacitors due to their low sintering temperatures and high dielectric constants. 49 50  An overview of the sol-gel process is presented in a simple graphic work below. 51 Advantages of the sol-gel process Simplicity; easy to process Homogenty; By sol-gel processing which using chemical precursors, ceramics and glasses with better purity and homogeneity obtain comparing high temperature conventional processes. Nano scale powders can be produced by sol-gel process. Flexibility; Sol gel has produced a wide range of compositions (mostly oxides) in various forms, including powders, fibers, coatings, thin films, monoliths, composites, and porous membranes. Low temperatures; No need to high temperature to produce ceramic powders by sol-gel technique. Small capital investment 52 5-Spray Roasting Spray roasting involves spraying fine atomized droplets of a solution of precursors in water or other fluid into a heated chamber. The temperature in the chamber is selected such that evaporation and chemical reaction occur to yield a high-purity powder containing fine crystallite size. 1. Atomization of a salt solution, 2. Mixture of droplets with a heated gas, 3. Evaporation of the solvent (often water), 4. Thermal decomposition of the dried salt particles in the heated part of the equipment. 53 Spray Roasting One variant of spray roasting is the pyrohydrolysis process for synthesis of high-purity metal oxides such as lanthanum strontium manganite(La1-xSrxMnO3, LSM), barium yttrium zirconate (BaZr1-xYxO3, BYZ), lanthanum nickelate (La2NiO4, LNO) and gadolinium-doped ceria (Ce1-xGdxO2-δ, CGO). A solution of a metal chloride in water is sprayed into a heated ceramic-lined chamber. Depending on the specific metal chloride, a temperature of 300 to 950°C results in reaction of the metal chloride with the water to form the metal oxide plus hydrochloric acid. The resulting oxide powder consists of crystallites approximately 0.2 to 0.4 μm in diameter agglomerated into hollow spheres 100 to 200 μm in diameter. 6-Decomposition Decomposition reactions are commonly used in ceramic carbonates, nitrates, sulfates, oxalates, and other compounds containing oxygen ions. These then decompose at elevated temperatures during calcining or sintering to yield the oxide. For example, MgCO3 decomposes to yield MgO. 55  7-Hydrothermal Process Hydrothermal synthesis involves crystallization of a composition in hot, pressurized water. Typical temperatures range from 100 to 350°C at pressures up to 15 MPa. Under these conditions, a wide variety of pure, fine-particle ceramic compositions can be synthesized. The feedstock can be oxides, hydroxides, salts, gels, organics, acids, and bases. The conditions can be oxidizing or reducing. The particle size can be controlled by residence time, temperature, and pressure. The resulting powder consists of single crystals of the final composition. No heat treatments or milling operations are required. Hydrothermal synthesis has been demonstrated on a laboratory scale, but has not yet been scaled up to commercial production. BaTiO3, Pb(ZrxTi1-x)O3, HA, quartz, zeolites, vanadates, phosphates, etc. 56  8-Plasma A variety of ceramic powders of high purity and very small particle size (10 to 20 nm) have been synthesized in high- temperature plasma* environments. The particles essentially condense in a flowing gas, which accounts for the high purity. Two types of plasma reactors have been successfully used. One is 1) The dc arc jet system; In this system the plasma is in direct contact with the metal electrode that supplies current. This reactor has very high efficiency, but can result in trace impurities from the electrode. 2)The rf (radio frequency) induction system; the current is transferred to the plasma through the electromagnetic field of the induction coil. No direct contact occurs, so no contamination results in the powder being synthesized. The efficiency of the rf induction system is lower than the dc arc jet system. However, both have produced SiC particles with greater than 70% efficiency using SiCI4, CH4, and H2 as the gaseous precursors. Si3N4 has also been synthesized by plasma techniques. Dc-arc jet *Plasma is a high-temperature, ionized gas. Because it is electrically conductive, a high degree of electrical heating can be achieved, that is, temperatures in the range 4000 to 10.000°c. 57 9-Laser It has been used successfully to produce controlled particle sizes of silicon and SiC. The SiC powder was prepared using a CO2 laser and a mixture of silane (SiH4) and methane (CH4). The CO2 laser energy is absorbed by this gas mixture. The resulting localized high temperature decomposes the gases and allows reaction to form SiC particles directly in the gas stream. Purity is very high. Particle sizes in the range of from 5 to 200 nm have been achieved. Organosilicon compounds such as 1,1,1,3,3,3- hexamethyldisilazane have also been successfully used. Efforts to scale up the laser synthesis approach have not been successful. The laser technique of powder synthesis is not likely to become commercial. 58  5.5 PRECONSOLIDATION The sized powders are compacted into the desired shapes by techniques such as pressing, slip casting, and injection molding and then strongly bonded or densified. To achieve a final component having uniform properties and no distortion requires a uniform particle compact. To achieve the required uniformity, the powder usually requires special treatments or processing prior to compaction. The preconsolidation steps are essential to minimize severe fabrication flaws that can occur in later processings steps. – For instance, a powder that is not free-flowing can result in poor powder distribution in the pressing die and distortion or density variation in the final part. – Similarly, improper viscosity control of a casting slurry can result in incomplete fill of the mold or a variety of other defects during slip casting. – Inadequate de-airing of either a slurry or an injection molding mix can result in a strength-limiting pores in the final slip-cast or injection-molded part. Such fabrication flaws can reduce the strength of a material to a fraction of its normal value. 59 Table 9.11 summarizes some of these special preconsolidation considerations for several compaction or consolidation approaches. 60  Additives for preconsolidation Additives are required for different reasons, depending on the specific forming process. However, several general comments are relevant to most forming approaches: 1. Binders are added to provide enough strength in the "green" body (unfired compact) to permit handling, "green" machining, or other operations prior to densification. 2. Lubricants are added to decrease particle-particle and particle-tool friction during compaction. 3. Sintering aids are added to activate densification. 4. Deflocculants, plasticizers, wetting agents, and thermoplastics are added to yield the rheological (flow) properties necessary for the specific shape-forming process. Table 9.12 further summarizes the function of additives. 61 62  A wide variety of binders are available, as shown by the partial listing in Table 9.13. Selection of the binders depends on a number of variables, including; – green strength needed, – ease of machining, – compatibility with the ceramic powder, – nature of the consolidation process. Gums, waxes, thermoplastic resins, and thermosetting resins are not soluble in water* and do not provide a benefit for slip casting, but are excellent for the warm mixing used to prepare a powder for injection molding. Organic binders can be burned off at low temperature and result in minimal contamination, where as inorganic binders become a part of the composition. 63 64  Binders, plasticiezers, PVA;binder, PEG; plasticizer Organic and inorganic materials used as a binder Most binders and plasticiezers are organic Inorganic binders also exist. Kaolinite is an example. It is not burn off. It is becomming part of ceramic. Lubricants and compaction aids; Stearic acid, synthetic wax, zinc stearate are lubricants 65  Spray Drying Spray drying is used in ceramic processing to achieve a uniform, free- flowing powder. The powder to be spray dried is suspended in a slurry with the appropriate additives. Slurry preparation are most frequently done by ball milling. The slurry is fed into the spray dryer through an atomizer and is swirled around by hot air circulating in the conical spray dryer chamber. The fluid evaporates and the powder forms into roughly spherical, soft agglomerates. Spray dried powder ranging from 30 to 250µm can be achieved. 66  GRANULATION It is another approach to achieving better flow properties of a powder. Instead of slurry, only a damp or plastic mix is prepared with equipment such as a mix muller, a sigma mixer or other mixer. The damp or plastic mixture is then forced through orifices of the desired size or screened. The resulting particle agglomerates are usually harder and more dense than spray- dried agglomerates and irregular shape. They do not flow as readily, but do tend to pack to a lower volume. 67  Shape Forming Processes Die-pressing DRY FORMING PROC. Isostatic pressing Slip casting WET FORMING PROC. Tape Casting Freeze Casting Gel Casting Extrusion PLASTIC FORMING PROC. Injection Moulding 68  DRY FORMING 1- Die Pressing; involves the compaction of powder into a rigid die by applying pressure along a single axial direction through a rigid punch, plunger or piston. a-Dry pressing b-Semi-dry pressing Uniaxial powder compaction showing the die- punch assembly during different stages. 69 a. Dry pressing: conducted with granulated powder or spray-dried powder containing 0-4 % moisture. Compaction occurs by crushing of the granules and mechanical redistribution of the particles in to a close-packed array. The lubricant and binder usually aid in this redistribution and the binder provides cohesion. Dimensional tolerances to ±1 % are achived. b. Semi-dry pressing: Involves a feed powder containing 10-15 % moisture and often used with clay-containing composition. These feed powder deforms plastically during pressing and conforms to the contour of the die cavity. Thin sheets of material at edges where the material extruded between die parts. So it is not well suited to automation. Dimensional tolerances are usually only held to ±2 %. 70 71 2- Isostatic Pressing: Uniaxial pressing has some limitation which can be overcome by applying pressure from all directions instead of only one or two directions. This is referred to as isostatic pressing or cold isostatic pressing(CIP). – Greather uniformity of compaction – Increased shape capability Two types of isostatic pressing are commonly used: a. Wet bag isostatic pressing b. Dry-bag isostatic pressing Isostatic Pressing 72  a. Wet-bag isostatic pressing: Powder is sealed in a water-tight die. The walls of the die are flexible. The sealed die is immersed in a liquid contained in a high pressure chamber.The chamber is sealed using a threaded or breach lock cover. The pressure of the liquid is increased by hydrolic pumping. The walls of the die deform and transmite the pressure uniformly to the powder, resulting in a compaction. Water or hydrolic oil and glycerin can be use as compaction fluids. Flexible Dies are made of an elastomer such as rubber or polyuretane. 35-1380 MPa is pressure capacity of laboratory isostatic press, however production units operate at 400 MPa or less. Advantages of Wet-bag isostatic pressing; 1. Density uniformity 2. Versatility 3. Low cost of tooling Disadvantages; 1. Long cycle time 2. High labor requirement 3. Difficult to automate 73  b.Dry-bag Isostatic Pressing: The tooling is built with internal channels into which the high pressure fuild is pumped. This minimizes the amount of pressurized fluid required and allows the use of stationary tooling. The major challange is constructing the tooling so that pressure is uniformly transmitted to the powder to achive the desired shape. Advantages of dry-bag isostatic pressing; 1. Increased production rate- 1000 to 1500 cycles per hour 2. Close dimensional tolerance It has been used for many years to press spark plug insulators. Schematic of a die for dry-bag isostatic pressing of a spark plug insulator. 74 75  WET FORMING Forming ceramic green articles with a more homogeneous microstructure and a lower amount of microstructural flaws when compared with dry or semi-dry pressing. A wide variety of techniques are available for forming simple or complex shapes. Homogeneous particle packing and high a green density. The slurry used to produce these characteristics should have; – as high a particle concentration as possible, – the requisite colloidal and rheological properties, and – a high enough casting rate for economical production. 76 WET FORMING 77  WET FORMING 1. Slip Casting Production of thin or thin-walled articles with simple or complex shapes. It is widely used in the traditional ceramics industry and its use in the production of advanced ceramics has been steadily increasing. In slip casting, a slurry is poured into a permeable mold, commonly made from gypsum (CaSO4.2H2O). The microporous nature of the mold provides a capillary suction pressure, that draws the liquid (filtrate) from the slurry into the mold. 78  WET FORMING The cast (consolidated layer of solids) forms on the walls of the mold. After a sufficient thickness of the cast has formed, the excess slurry is poured out and the mold and cast are allowed to dry. The cast shrinks away from the mold during drying and can be easily removed. Once fully dried, the cast is heated to burn out processing additives and sintered to produce the final article. In general, slip casting formulations typically consist of 40–50 vol.% particles, 50–60 vol.% water, plus small concentrations of a dispersant and a binder. The volume fraction of particles in the dried cast is often in the range 60%–70%. 79  Slip Casting WET FORMING It involves ceramic particles suspended in water and cast into porous plaster mold. SLIP PREPARATION: Slip can be done by a variety of techniques. The most common is wet ball milling or mixing. The ingredients, including the powder, binders, wetting agents, sintering aids, and dispersing agents are added to the mill with the proper proportion of the selected casting liquid and milled to achieve thorough mixing, wetting, and particle size reduction. The slip is then allowed to age until its characteristics are relatively constant. Then, it is ready for final viscosity checking, de-airing, and casting. Plaster molds are prepared by mixing water with plaster of Paris powder, pouring the mix into a pattern mold and allowing the plaster to set. Once the mold has been fabricated and properly dried and an optimum slip has been prepared, casting can be conducted. 80. Slip preparation in ball mill Slip casting in plaster moulds and demoulding 81  Parameters of Slip Casting; Slip Preparation and Rheology; Rheology is the study of the flow characteristics of matter. Viscosity is controlled by the volume fraction of solids. Particle size, particle shape, particle surface charge, and degree of agglomeration versus dispersion effect viscosity. The viscosity is determined by how close particles approach each other and by the degree of attraction and repulsion between the particles. Particle shape and size distributions also important to achieve a high green density and to minimize shrinkage during the densification. This objective of close packing can be achieved best by a distribution of particle size. A distribution of particle sizes also helps to achieve increased solid loading in the slip. When the powder A and B mixed together 50 vol %-50 vol.% (A; 2-18µm and B; 0.35- 7 µm), slip were successfully prepared containing 50 vol.%solid. Mostly finer powder have highest viscosity (lowest flow rate), coarser powder intermediate viscosity, and mixed powder the lowest viscosity. 82  Particle Surface Effects; For high solid content, particle-particle attraction results in the formation of agglomerates. These agglomerates act like roughly spherical particles and result in a decrease in viscosity. In other cases, especially very high solid content, the agglomerates can interact with each other and increase the viscosity. The degree of agglomeration can be controlled with additives. Dispersion and flocculation of ceramic particles in a fluid are strongly effected by; the electrical potential at the particle surface, adsorbed ions, and the distribution of the ions in the fluid adjacent to the particle. Thus, the chemical and the electronical structure of the solid, the pH of the fluid, and the presence of impurities are all critical consideration in the preparation of a slip for casting. Two approaches are commonly used to control and manipulate the surface characteristics of ceramic particles in a suspension: 1. Electrostatic repulsion 2. Steric stabilization 83 1. Electrostatic repulsion involves build up charges same polarity on all the particles. Like charge repel, so the particles are held apart in the solution by electrostatic forces. The higher the electrical charge at the surface of particles, the better the degree of dispersion and the less agglomeration. The charge at the surface of the particles is controlled by pH of the liquid and by addition of Na + -Clay interaction chemicals that supply monovalent cations (Na , + NH4+, Li+). To obtain optimum dispersion ; pH, zeta potential and viscosity of the slip should be measured. On the zeta potential-pH curve; the higher absolute value of zeta potential, the greater electrostatic repulsion between particles and the greater degree of powder dispersion in the slip. 2. Steric Stabilization; involves chainlike organic molecules that are adsorbed on onto the ceramic particles and a buffer zone around each particle. These are referred to as nonaqueous (non-water based). Nonaqueous slip utilizing steric hindrance are commonly used for tape casting. 84 85  WET FORMING 2. Tape Casting Tape casting is widely used to form thin sheets (~10 μm to ~1mm) for applications as substrates, dielectrics and multi layer components in the electronic packaging industry. Tape casting has been developed to fabricate these thin sheets in large quantity and at low cost. It is similar to slip casting, except that the slip is spread onto a flat surface rather than being poured into a shaped mold. 1. Doctor Blade Process 2. Waterfall technique 3. Paper-casting process 86  WET FORMING Doctor Blade Process Solvent removal is achieved by evaporation. The most common approach for As with slip casting, the fluid must be removed tape casting is the doctor blade slowly to avoid cracking, bubbles, or process. distortion. The technique consists of casting a This is the purpose of the long portion of the slurry onto a moving carrier surface tape-casting apparatus between the doctor blade and the take-up reel. (usually a thin film of cellulose acetate, Teflon™, Mylar™, or The evaporation is achieved either by controlled heating or airflow. The dry flexible cellophane) and spreading the tape is rolled onto a reel to be stored for use. slurry to a controlled thickness with the edge of a long, smooth blade. The slurry contains a binder system dissolved in a solvent. Enough binder is present so that a flexible tape will result when the solvent is removed. 87 WET FORMING 88  WET FORMING Preparation of Tape-Casting Slurries Tape-casting slurries are rheologically similar to slip-casting slurries, but contain a larger quantity of binder. the binder/plasticizer system is generally selected to be thermoplastic, that is, it can be softened by heating to moderate temperatures. This allows layers to be bonded together by lamination. Solvents; methyl ethyl ketone (MEK), alcohols, toluene, hexane, trichloroethylene (TCE), and water. Binders; polyvinyl butyral, polyvinyl acetate, polyvinyl chloride, polyvinyl alcohol, polyacrylic emulsion, polystyrene, polymethacrylates, and cellulose nitrates. The criteria for the binder include: (1) forms tough, flexible film when dry; (2) volatilizes to a gas when heated and leaves no residual carbon or ash; (3) remains stable during storage, especially with no change in molecular weight; and (4) is soluble in an inexpensive, volatile, nonflammable solvent. 89 WET FORMING 90 91 WET FORMING 92  WET FORMING 3. Freeze Casting Freeze casting consists of – freezing a stable (well-dispersed) slurry in a mold, – removing the frozen object from the mold, and – subliming the frozen liquid. Then, the green article is heated to burn out processing additives and sintered to densify the ceramic phase. The use of low temperatures in the freezing step (–20˚C when water is the liquid ) is a disadvantage. However, this can be alleviated by the use of nonaqueous sublimable materials (such as camphene) that allow freeze casting to be performed near room temperature. 93  WET FORMING A freeze tape casting process can be used to form ceramic sheets with oriented or graded porosity. The equipment is similar to that for conventional tape casting, except that it is augmented with a freezing bed. By casting on the cold surface, the liquid in the slurry is frozen from the bottom up through the thickness. Crystals nucleate and grow in the direction of the thickness. Sublimation of the frozen liquid, followed by sintering, leads to porous sheets in which the pores are oriented in the direction of the thickness. Potential applications of freeze tape casting include the production of catalysts, gas sensors, and filters. 94  WET FORMING 4. Gel-Casting Slurry of ceramic particles (dispersed in a monomer solution) is poured into a mold and the monomer is polymerized to form a gel-like bonding phase, immobilizing the particles. The system is removed from the mold while still wet, dried by evaporation of the liquid, heated to burn out organic additives, and sintered. Gel casting commonly employs aqueous solvents (although organic solvents can also be used), dispersants, and processing methods similar to those used in traditional slip casting to prepare the slurry. The slurry properties normally required are also similar to those for slip casting: stability against flocculation, high particle concentration (~50vol.% for gel casting), and a low enough viscosity for casting. A key difference in gel casting is the addition of an organic monomer to the solvent that is polymerized in situ to form a strong, interconnected, cross- linked gel. The gel prevents segregation or settling of the particles, and gives strength to the article to withstand capillary stresses during drying. 95  Thin parts as well as thick parts( thicker than those in slip casting) can be formed by gel casting. As in slip casting, the amount of processing additives in the dried article is low (2–4 wt%), so binder burn out is not normally a limiting step in the gel casting process. The commonly used mold materials include Al, glass, PVC, polystyrene, and polyethylene. Al and anodized Al are widely used for permanent production molds, while glass and polymeric materials are useful for laboratory experiments. 96 Journal of the American Ceramic Society, Volume: 100, Issue: 2, Pages: 458-490, First published: 28 January 2017, DOI: (10.1111/jace.14705)  PLASTIC FORMING Plastic forming involves producing shapes from a mixture of powder and additives that are deformable under pressure. Such a mixture can be obtained in systems containing clay minerals by addition of water and small amounts of a flocculant, a wetting agent, and a lubricant, In systems not containing clay ; such as pure oxides and carbides and nitrides, an organic material is added in place of water or mixed with water or other fluid to provide the plasticity. About 25 to 50 vol %. organic additive is required to achieve adequate plasticity for forming. A major difficulty in plastic-forming processes is removing the organic material prior to firing. – In the case of a water-clay system, substantial shrinkage occurs during drying, increasing the risk of shrinkage cracks. – In the case of organic additives, the major problems are forming a flaw-free green part and extraction of the organic. Too rapid extraction causes cracking, bloating, or distortion. Inadequate removal results in cracking, bloating, or contamination during the later high-temperature densification process. 98  PLASTIC FORMING Plastic processes are used extensively in the fabrication of traditional ceramics such as pottery and tableware. The compositions contain clay and have been made workable by addition of water. Modern ceramics generally do not contain clay and require organic additions to achieve plasticity. 1.Extrusion 2.Injection Molding 99  PLASTIC FORMING 1. Extrusion Extrusion is a plastic-forming method that has been used extensively for many years for fabrication of ceramics for furnace tubes, bricks, insulators, pipe, tile, tubular capacitors, catalyst supports, magnets, heat-exchanger tubes, and other parts with a constant cross section. The extrusion process consists of forcing a highly viscous, doughlike plastic mixture of ceramic powder plus additives through a shaped die. 100  PLASTIC FORMING Type of Extruder 1. An auger-type extruder in which 2. Piston type extruder; A piston is the plasticized mix is forced through used in place of an auger. The a shaped die by the rotation of an piston-type extruder generally auger. results in less contamination by wear. 101  PLASTIC FORMING Binders and Additives for Extrusion Additives to a ceramic powder are required to achieve a mixture that has characteristics suitable for extrusion. The nature of the additives depends on whether the extrusion is conducted at room temperature with a combination of a binder and fluid or at elevated temperature with a thermosetting polymer. The following are some of the key characteristics that must be considered: 1. The mixture must be plastic enough to flow under pressure into the desired cross- section, yet rigid enough (high enough wet strength) to resist deformation due to slumping or handling. 2. The mixture must not stick to the die or other tooling and must yield smooth surfaces after extrusion. 3. The fluid and ceramic must not separate under an applied pressure. 4. The mixture must have reproducible porosity such that shrinkage during drying and firing are predictable. 5. Organics must be low-ash content to leave minimal residue during firing. 102  PLASTIC FORMING The nature of the additives selected depends on the ceramic powder and the liquid. Compositions containing no clay require a substantial percentage of organic additives and either water or a solvent. Compositions containing a clay mineral such as kaolinite can be plasticized with water additions and do not require organics. Clay has a structure that absorbs water between the sheet layers, resulting in natural plasticity. Modern ceramic compositions such as Al2O3, ZrO2, and BaTiO3 do not contain clay and thus must have organic additives. The major additive is the binder. It provides a coating over each ceramic particle to allow flow during extrusion and green strength after extrusion. The plastizer modifies the rheology of the binder to achieve plastic behavior at the temperature of extrusion. The lubricant reduces particle-particle and die-wall friction, prevents sticking and aids in achieving an acceptable surface finish. Surfactants enhance the wetting ability of the binder onto the ceramic particles. Dispersants and flocculants control the degree of dispersion or agglomeration of the particles. For some particle size distributions, dispersion is preferred. For others, a high degree of flocculation works better so coagulants are added. The extrusion characteristics can be broadly varied by changing the degree of dispersion. As with slips, the dispersion characteristics can be altered by control of pH and by additives that provide electrostatic surface charges or steric hindrance. 103 PLASTIC FORMING 104  PLASTIC FORMING Extrusion Steps They include powder sizing, batch formulation, mixing, extrusion, drying, densification, and quality control. As with other forming processes, particle size and shape and degree of agglomeration are extremely important. Fine particles (under 1 µm) are generally easiest to extrude. 105  PLASTIC FORMING Mixing is a critical step in extrusion. All particles must be uniformly coated with the binder-liquid solution. Obtaining uniformity is difficult because the final extrudable mix is so stiff. Imagine stirring flour into honey. Without high-intensity mixing, pockets of binder and powder can remain that can cause nonuniform extrusion and also end up as strength-limiting defects in the final part. The first mixing techniques is the "brute force" technique in which the ingredients are weighed out to their final formulations and mixed directly to the final, stiff consistency in a high-shear mixer such as a pug mill or muller. A second technique is to add excess liquid to provide improved wetting and then partially dry to the required consistency after thorough mixing. Another technique is to mix the water and powder first to form a low viscosity mixture and then to add the binder incrementally to bring the mix uniformly up to the extrudable viscosity. 106  PLASTIC FORMING Extruded material contains a substantial percentage of liquid. This is removed by evaporation using the same type of careful control. In addition to liquid, the extruded compact contains a much higher binder content than a cast compact. This binder must be removed before the compact can be densified. Most binders used for extrusion can be burned off in an oxidizing atmosphere in the temperature range of 300 to 1000°C. This burn off must be done slowly and carefully to allow the carbonaceous gases generated by the decomposition of the organics to escape through the pores of the compact. The allowable rate of burn off is largely determined by – the particle size distribution, – the degree of particle packing, and – the thickness of the compact. Most organic binders will not burn off properly in a nonoxidizing atmosphere. Instead, they will decompose and leave a carbon residue. The acrylic binders are an exception. They burn out cleanly in inert and reducing atmospheres as well as oxidizing atmospheres. 107 108 Extruded ceramic parts 109  PLASTIC FORMING 2.Injection Molding Injection molding utilizes equipment that has across section similar to an extruder. However, the process is very different from extrusion. The feed material for injection molding generally consists of a mixture of the ceramic powder with a thermoplastic polymer plus a plasticizer, wetting agent, and antifoam agent. The mixture is preheated in the "barrel" of the injection-molding machine to a temperature at which the polymer has a low-enough viscosity to allow flow if pressure is applied. A ram or plunger is pressed against the heated material in the barrel by either a hydraulic, pneumatic, or screw mechanism. The viscous material is forced through an orifice into a narrow passageway that leads to the shaped tool cavity. This helps compact the feed material and remove porosity. At the end of the passageway the strand of viscous material passes through another orifice into the tool cavity. The mixture is much more fluid at this point than an extrusion mix and could not form a self- supporting shape. The strand piles on itself until the cavity is full and the material has ‘‘knit’’ or fused together under the pressure and temperature to produce a homogeneous part. The shaped tool is cooler than the injection-molding mix such that the mix becomes rigid in the tool cavity. The part can be removed from the tool as soon as it is rigid enough to handIe without deformation. Cycle times can be rapid providing the potential for injection molding to be a high-volume, low-cost process for fabrication of ceramics into complex shapes. 110  Injection molding is used extensively in the plastics industry to make everything from garbage cans to ice cube trays to surprisingly complex constructible toys such as model boats and airplanes. Ceramic parts are made with the same injection-molding equipment, but with dies made of harder, more wear-resistant metal alloy. The ceramic powder is essentially added to the plastic as a filler. After injection molding, the plastic is then removed by careful thermal treatments. 111  Particle Sizing Particle size distribution is important in injection molding, to achieve the densest packing and to minimize the amount of organic material. Both particle packing and sizing affect viscosity. it has been reported that viscosity starts increasing rapidly at about 55 vol % solids for a unimodal suspension of spheres, but that the solids loading can be increased to over 70% before the viscosity starts increasing rapidly for a bimodal distribution containing about 25% fine spheres. By using a graduated particle size distribution and ⍺/𝛄 Al2O3 , complex shapes of alumina parts having 50-60 vol % solids were successfully injection-molded by using High density Polyethylene/paraffin wax binder. Injection were conducted at a pressure of 1100 bar and a temperature 150°C above the melting temperature of the organic binder. Preconsolidation for Injection Molding. Preconsolidation consists primarily of mixing the ceramic powder homogeneously with the organic additives. The mixing is conducted in a high-shear mixer at a temperature above the softening point of the binder/plasticizer mixture. The objective is to coat each particle with a thin layer of the polymers. Once mixing is complete, the mixture is granulated or pelletized and cooled. The cooled material is hard like plastic or wax. Consolidation. The objective of the consolidation step is to inject the ceramic powder/binder mixture such that it completely fills the die or mold without leaving porosity, cracks, or other defects. Many factors affect this and must be considered. Major factors include die design, material rheology, and injection parameters, all of which are interactive. 112 113 114 Injection molded parts 115