MSE 3143 Ceramic Materials

Transcription

1 MSE 3143 Ceramic Materials Production Methods of Ceramic Materials Assoc.Prof. Dr. Emre YALAMAÇ Res.Asst. B.Şölen AKDEMİR Fall 1 OUTLINE Definitions of basic terms Powder preparation Powder preparation Preconsolidation Powder characterization Shaping methods Shaping agents Shaping techniques By microscopy Sieving By light scattering By X-ray diffraction BET surface area analysis Particle composition & purity analysis 2

2 DEFINITIONS OF BASIC TERMS Primary particles: the smallest clearly identifiable unit in the powder. Primary particles may be crystal-line or amorphous and cannot easily be broken down into smaller units. Agglomerates: clusters of bonded primary particles. Soft agglomerates are easily broken up; hard agglomerates, because of the stronger interparticle bonds, are more difficult to break up. Particles: general term applied to both primary particles and agglomerates. Granules: large agglomerates, usually mm in diameter, that are formed by the addition of a granulating agent (e.g., a polymer binder). 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. Aggregates: coarse constituents, >1 mm, in a mixture. 3 DEFINITIONS OF BASIC TERMS 4

3 DEFINITIONS OF BASIC TERMS 5 POWDER PREPARATION Powder Preparation Mechanical Methods Chemical Methods Vapor-Phase Processing use coarse-grained materials that have generally been derived from naturally occurring minerals. They are subjected to a series of processes, collectively referred to as comminution, in which the particle size is gradually reduced. The final step is known as milling, which produces particles of the desired size. Mechanical methods of powder production are used widely in the production of traditional ceramic products where high purity powders are not required and cost is one of the most important requirements. 6

4 POWDER PREPARATION Powder Preparation Mechanical Methods Chemical Methods Vapor-Phase Processing such as sol-gel processing, offer several advantages over mechanical methods because they allow exceptional control over particle morphology and purity. Chemical processes are used widely in the production of advanced ceramic materials. Alumina production by Bayer Process is given as an example of precipitation process. 7 POWDER PREPARATION Chemical Methods SOL-GEL 8

5 POWDER PREPARATION Powder Preparation Mechanical Methods Chemical Methods Vapor-Phase Processing can be used to produce ceramic powders. They tend to be expensive, but offer many advantages, such as the ability to produce particles of nonoxides. Vapor phase techniques are also used to produce nanoparticles (particles with diameters of a few to 10s of nanometers). Nanoparticles with narrow size distributions 9 POWDER PREPARATION Mechanical Methods Communition causes reducing particle size of raw materials by Crushing Grinding Milling Particle size Surface area Population of particles Particle Size Primary Crushing Up to 0.3 m down to 1 cm in diameter Crushing Secondary Crushing Down to ~ 1 mm in diameter Fine Grinding or Milling Down to as low as 1.0 µm in diameter 10

6 POWDER PREPARATION Mechanical Methods Communition techniques are most effective on brittle materials such as cement & metallic ores Purpose of communition Liberate impurities Break up aggregates Modify particle morphology & size distribution Facilitate mixing & forming Produce more reactive material for firing Aim of size reduction create appropriate particle sizes improve material blending and prevent segregation increase the material s surface area control a material s bulk density liberate impurities reduce porosity of the particles modify shape of the particles 11 POWDER PREPARATION Mechanical Methods Milling For traditional raw materials like clay and the oxides produced from ores, it is often necessary to eliminate aggregates and to reduce the particle size. Compound formation during firing and densification during sintering require diffusion between neighboring particles. Diffusional processes are proportional to the square of the particle size. The most common method for reducing particle size is ball milling. A ball mill is a barrel (usually made of a ceramic) that rotates on its axis and is partially filled with a grinding medium (called media) in the form of spheres, cylinders, or rods. 12

7 POWDER PREPARATION Mechanical Methods Milling where A is numerical constant that is specific to the mill being used and the powder being milled, a is the radius of the mill, r is the density of the balls, d is the particle size of the powder, and r is the radius of the balls 13 POWDER PREPARATION Mechanical Methods Milling There are many mechanical techniques to achieve different particle size as shown in table Carter, C.B.; Norton, M.G.; Ceramic Materials: Science and Engineering, Springer,

8 POWDER PREPARATION Preconsolidation 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. Some of these treatments are shown in table Richerson, D.W.; Modern Ceramic Engineering: Properties, Processing and Use in Design, 3rd edition, Taylor&Francis, POWDER PREPARATION Preconsolidation The preconsolidation steps are essential to minimize severe fabrication flaws that can occur in later processing steps such as; 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 Improper viscosity control of a casting slurry can result in incomplete fill of the mold or a variety of other defects during slip casting 16

9 POWDER PREPARATION Preconsolidation - Additives Binders is a component that is added to hold the powder together while we shape the body. Shaping Agents Lubricants Deflocculants Surfactants or wetting agents Decrease particle-particle and particle-tool friction during compaction is an additive which break up floccules present in a liquid into fine particles producing a dispersion. In other words, deflocculation is the opposite of coagulation. Controls the surface charge of particle and provides dispersion in liquid as deflocculants Plasticizers is the component of a binder that keeps it soft or pliable; it improves the rheological properties. lar 17 POWDER PREPARATION Preconsolidation - Additives The most important additives are binders. Binders keep the particles together and provide easy shaping, green strength of ceramic, remaining shape during drying and sintering. There are two types of binders: Organic & Inorganic Beeswax & resin They are not used in wet processes because they are insoluable in water. They are used in melting and injection processes. While organic binders leave the system by burning during shaping process, inorganic binders remain in structure and becoming one of the component of ceramic. 18

10 POWDER PREPARATION Preconsolidation Additives Richerson, D.W.; Modern Ceramic Engineering: Properties, Processing and Use in Design, 3rd edition, Taylor&Francis, POWDER PREPARATION Preconsolidation - Granulation For the production of technical ceramics, the poor flowability of the micron or submicron powders makes it necessary to form press granulates by the controlled agglomeration of the primary particles. Granulation methods can be divided into agitation, pressure, or spray techniques. Main method of granulation is spray drying: produce spherical particles (~20 μm) high productivity (e.g. ~ kg/h) suitable for subsequent pressing process. 20

11 POWDER PREPARATION Preconsolidation Spray Drying The variables in spray drying are Droplet size Solution concentration and composition Temperature and flow pattern of the air in the drying chamber Chamber design It is used widely for preparing ferrites, titanates, and other electrical ceramics. Fine droplets produced by an atomizer are sprayed into a drying chamber and the powder is collected. 21 POWDER CHARACTERIZATION There are various techniques to measure particle size & distribution Carter, C.B.; Norton, M.G.; Ceramic Materials: Science and Engineering, Springer,

12 POWDER CHARACTERIZATION By Microscopy Visible light microscopy (VLM) If the size of the particle is >1μm Particle size measurements are made either directly at the microscope or from micrographs (photographs taken using the microscope). The main challenge is in determining the size of three-dimensional grains on the basis of planar images. 23 POWDER CHARACTERIZATION By Microscopy Electron Microscopy For submicron particles it is necessary to use an electron microscope. For scanning electron microscopy (SEM), and in particular transmission electron microscopy (TEM), the total amount of material that can be examined is quite small, and so it is essential to make sure that the sample examined is representative of the entire powder batch. 24

13 POWDER CHARACTERIZATION Sieving Actually, sieving is used for sorting particles according to size rather than measuring their size. Typically, sieves with decreasing mesh size are stacked with the largest mesh at the top. The term mesh size denotes the number of openings per linear inch in the sieve screen. It is particularly suited for powders with particle size >56 μm. Carter, C.B.; Norton, M.G.; Ceramic Materials: Science and Engineering, Springer, POWDER CHARACTERIZATION By Light Scattering Smaller particles scatter a small amount of light through a large angle. Large particles scatter a greater amount of light but through a smaller angle. The reliable particle size range is μm. Light scattering methods have the following advantages: Accuracy Speed Small sample size Can be automated 26

14 SHAPING METHODS Many shaping methods are used for ceamic products and these can be grouped into three basic categories, which are not necessarily independent. Powder Compaction Dry pressing CIP (Cold Isostatic Pressing) HP (Hot Pressing) HIP (Hot Isostatic Pressing) Casting Slip casting Pressure casting Tape Casting Plastic Forming Extrusion Injection Molding 27 SHAPING METHODS Powder Compaction Dry pressing This method; In a double-action press both the top and bottom punches are movable. Ceramic raw materials mixed ingredients (powder element or master alloyed powders and binders are facilitate shaping of powders) place mould cavity and with hydraulic or mechanic pressing by top or bottom pistons, raw material is shapeable. In dry pressing, priory pressing, powders are with 1-8% water and binder mixture moisten and pressing. This is simple and easily applicable method. This method is usually used in duplicate production of small and simple parts. Floor and walls coating, electroporcelain and several ornament can be product with this method. In this method we need consider mould life because of high hardness of ceramic powders, size percision of shape of mould and friction. 28

15 SHAPING METHODS Powder Compaction COLD ISOSTATIC PRESSING (CIP) CIP is used for shaping of complex shape and quality products. Dry pressing HIP CIP Powder is weighed into a rubber bag and a metal mandrel is inserted that makes a seal with the mouth of the rubber bag. The sealed bag is placed inside a high-pressure chamber that is filled with a fluid (normally a soluble oil/water mixture) and is hydrostatically. For production units the pressure is usually 400 MPa. Once pressing is complete, the pressure is released slowly, the mold is removed from the pressure chamber, and the pressed component is removed from the mold. 29 SHAPING METHODS Powder Compaction The advantages of the wet-bag process are CIP Wide range of shapes and sizes can be produced Uniform density of the pressed product Low tooling costs The disadvantages are Poor shape and dimensional control (particularly for complex shapes) Products often require green machining after pressing Long cycle times (typically between 5 and 60 minutes) give low production rates 30

16 SHAPING METHODS Powder Compaction thigh bone Dry pressing HIP CIP Sintered zirconia ceramic 31 SHAPING METHODS Powder Compaction HOT PRESSING (HP) Pressing can also be performed at high CIP Dry pressing HP temperatures; this process is known as hot pressing. That die assembly used for hot pressing is very similar dry pressing. The main difference is that in hot pressing the die assembly is contained within a high temperature furnace. During hot pressing the ceramic powders may sinter together to form a high density component. 32

17 SHAPING METHODS Powder Compaction HP We can summarize the advantages of this process. The powder does not have to be of the highest quality. Large pores that are caused by nonuniform mixing are easily removed. We can densify at temperatures lower than those needed for conventional pressureless sintering. Extensive grain growth or secondary recrystallization does not occur when we keep the temperature low during densification. We can densify covalently bonded materials such as SiC without additives. The principal disadvantage is also important Dies for use at high temperatures are expensive and do not generally last long. 33 SHAPING METHODS Powder Compaction HOT ISOSTATIC PRESSING (HIP) The hot isostatic press uses the simultaneous application of heat and pressure. CIP Dry pressing HIP A furnace is constructed within high pressure vessel and the objects to be pressed are placed inside. Temperatures can be up to 2000 o C and pressures are typically in the range of MPa. A gas is used as the pressure medium, unlike the CIP in which a liquid is often used. Argon is the most common gas that is used for HIPing, but oxidizing and reactive gases can also be used. Now HIPing is used for a wide variety of ceramic components, such as alumina based tool bits and the silicon nitride nozzles used in flue gas desulfurization plants by the utility industry. The advantages of the HIPing process are becoming more important as interest in structural ceramics. Nonoxide ceramics can be HIPed to full density while keeping the grain size small and not using additives. Very high densities combined with small grain sizes lead to products with special mechanical properties Disadvantage: COST 34

18 SHAPING METHODS Powder Compaction CIP Dry pressing HIP silicon nitride nozzles 35 SHAPING METHODS Slip Casting Slip casting is a low cost way to produce complex shapes and in the traditional pottery industry it is the accepted method for the production of teapots, jugs, and figurines although handmade items will likely be hand-thrown. In the process, a slurry is poured into a microporous plaster of mold. The porous nature of the mold provides a capillary suction pressure, which draws the liquid from the slurry into the mold. After reach the thickness and remained slurry is poured back. Drying rate is decreasing over time cause of porous are saturated liquid, depending on shell s thickness. Gypsum (CaSO 4.2H 2 O), is the most common mold material, obtained reaction of plaster of paris and water. Water is the principal liquid using in slip casting. 36

19 SHAPING METHODS Slip Casting Fill the mold with slip the mold extracts liquid, forming a compact along the mold walls; after excess slip is drained and after partially dryed green ceramic is removed. 37 SHAPING METHODS Slip Casting 38

20 SHAPING METHODS Pressure Slip Casting This method resembling a pressure filtering. In pressure filtering, liquid separated by slurry pressure with mold and punch's to flat filter. In pressure casting slurry compressing in a mold. Special polymer molds using this method. 39 SHAPING METHODS EXTRUSION Extrusion involves forcing a deformable mass through a die orifice (like toothpaste from a tube). The process is widely used to produce ceramic components having a uniform cross section and a large length-to-diameter ratio such as ceramic tubes and rods. Clay with a suitable rheology for the extrusion process (essentially a paste) can be made by controlling the amount of water. Carter, C.B.; Norton, M.G.; Ceramic Materials: Science and Engineering, Springer,

21 SHAPING METHODS EXTRUSION Clay-free starting materials, such as Al 2 O 3, are mixed with a viscous liquid such as polyvinyl alcohol or methylcellulose and water to produce a plastically deformable mass. Carter, C.B.; Norton, M.G.; Ceramic Materials: Science and Engineering, Springer, SHAPING METHODS EXTRUSION Extrusion is also used to produce the alumina shells for sodium vapor lamps and the honeycomb-shaped catalyst supports for automotive emission-control devices. The catalyst supports are designed to give a high surface area and can consist of hundreds of open cells per square centimeter with wall thicknesses <100 μm. To produce these shapes, cordierite ceramic powder is mixed with a hydraulic-setting polyurethane resin. The mix is extruded into a water bath at a rate that matches the rate of cure of the polyurethane (about 2 mm/s). It is then fired to produce the final ceramic. two ceramic extruded cordierite honeycomb substrates for catalytic converters. 42

22 SHAPING METHODS INJECTION MOLDING Injection molding can be applied to shaping and forming ceramic components if the ceramic powder is added to a thermoplastic polymer. When forming ceramics by injection molding, the polymer is usually referred to as the binder (but we could instead have called the material a ceramic-loaded polymer). The ceramic powder is added to the binder and is usually mixed with several other organic materials to provide a mass that has the desired rheological properties. 43 SHAPING METHODS INJECTION MOLDING Table shows the additives that have been used to form SiC shapes by injection molding. The organic part of the mix accounts for about 40 vol%. Carter, C.B.; Norton, M.G.; Ceramic Materials: 44 Science and Engineering, Springer, 2007

23 SHAPING METHODS INJECTION MOLDING The plastic mass is first heated, at which point the thermoplastic polymer becomes soft and is then forced into a mold cavity. The heated mixture is very fluid and is not self-supporting (this is different from the situation encountered in extrusion). The mixture is allowed to cool in the mold during which time the thermoplastic polymer hardens. Because of the large volume fraction of organic material used in the mixture, there is a high degree of shrinkage of injection-molded components during sintering. Shrinkage of 15 20% is typical, so precise control of component dimensions is difficult. However, complex shapes are retained with very little distortion during sintering since the densities, although low, are uniform. Carter, C.B.; Norton, M.G.; Ceramic Materials: Science and Engineering, Springer, SHAPING METHODS TAPE CASTING Tape casting is used to make flat ceramic sheets having a thickness about 1mm. Microelectronics, piezoelectrics, heat changers, and sensors (for sub-layers) produced in this way. In tape casting, a slurry containing a powdered ceramic together with a complex mixture of solvents and binders is spread onto a moving polymer sheet. The thickness of the deposited layer is determined by the height of the doctor blade above the polymer sheet. After these processes solvents are voparize and the remaining tape can be firing. 46

24 SHAPING METHODS TAPE CASTING 47 SHAPING METHODS TAPE CASTING 48