of 12

Aaa

Published on November 2016 | Categories: Documents | Downloads: 6 | Comments: 0
90 views

Comments

Content

1. Traditional raw materials nelerdir ve seramıklere ne gibi özellikler katarlar ? Many of the raw materials used by the ancient civilizations are still utilized today and form the basis of a sizable segment of the ceramic industry . These ceramic products are often referred to as traditional ceramics. Clays The clay minerals are hydrated aluminosilicates that have layer structures. The importance of clays in the evolution of traditional ceramic processing cannot be over emphasized. The plasticity developed when water is added provides the bond and workability so important in the fabrication of pottery, dinnerware, brick, tile, and pipe. Clay Minerals Kaolen, Montmorillonite, Mika, İllite Kaolin This group has other members (kaolinite, dickite, nacrite and halloysite) and a formula of Al2O3.2SiO2.2H2O. It also known as China clay. It contains coarse grains (10-0.1μm). Fired color is white. In ceramic industry, kaolinite mostly used for sanitary ware, porcelain and insulator materials. For tiles processing it uses max.20 % of kaolinite. Quartz Quartz is the most common mineral found on the surface of the Earth. Silica (Si02) is a major ingredient in glass, glazes, enamels, refractories, abrasives, and white ware. Its major sources are in the polymorphic form quartz, which is the primary constituent of sand, sandstone, and quartzite. Quartz; •reduces drying shrinkage of ceramic structure •helps to regulate plasticity •allow gas outlet during firing without deformation. Feldspar In the manufacture of ceramics, feldspar is the second most important ingredient after clay. Feldspars are used as fluxing agents to form a glassy phase at low temperatures and as a source of alkalies and alumina in glazes. They improve the strength, toughness, and durability of the ceramic body, and cement the crystalline phase of other ingredients, softening, melting and wetting other batch constituents. Wollastonite Wollastonite forms from the interaction of limestones, that contain calcite, CaCO3, with the silica, SiO2, in hot magmas( high temperature and pressure). It forms by the following formula: CaCO3 + SiO2 ----> CaSiO3 + CO2 Some of the properties that make wollastonite so useful are its high brightness and whiteness, low moisture and oil absorption, and low volatile content. Wollastonite is used primarily in ceramics, friction products (brakes and clutches), metal making, paint filler, and plastics. It improves strength of ceramic products and reduces firing time of tiles.

The increase of clay makes easy molding processes. The increase of feldspar lowering the temperature of sintering and introduces in a liquid phase sintering. The increase in silica limit shrinkages and deformations in sintering. 2. Advanced ceramic genel özellikleri ve Chemical composition a göre kaça ayrılır? During the past 50 years scientists and engineers have acquired a much better understanding of ceramic materials and their processing and have found that naturally occurring minerals could be refined or new compositions synthesized to achieve unique properties. These refined or new ceramics are often referred to as modern ceramics or advanced ceramics. The wide variety of application possibilities for advanced ceramic materials arise from their specific properties which in many respects cannot be achieved by other materials. To highlight some positive properties: •low density, •high hardness, •high mechanical strength, •dimensional stability (specific stiffness), •resistance to wear, •resistance to corrosion (resistance to chemical attack), •weathering resistance, •high working temperature, •low or high thermal conductivity, •good electrical insulation and •dielectric and ferroelectric properties In accordance with their chemical composition the advanced ceramic materials can be divided into main groups: •SILICATE-CERAMICS technical porcelain-The natural starting materials of technical porcelain are quartz, feldspar and kaolin. steatite-major component: soapstone, additives: clay and flux, Mg 3 Si4 O10 (OH)2 cordierite-These magnesium silicates occur during the sintering of soapstone with added clay, kaolin, corundium and mullite. Mg2Al4Si5O18 mullite-ceramic -3 Al2O3.2 SiO2 •OXIDE CERAMICS aluminium oxide-Al2O3 magnesium oxide-MgO zirconium oxide-ZrO2 aluminium titanate-Al2TiO5 or Al2O3.TiO2 piezo ceramic-lead zirconate to lead titanate

•NONOXIDE CERAMICS CARBIDES silicon carbide-SiC tungsten carbide-WC boron carbide-B4C titanium carbide-TiC NITRIDES silicon nitride-Si3N4 aluminium nitride-AlN boron nitride-BN SIALON Titanium nitride-TiN 3. Bayer process aşamaları? Processing of Alumina •Alumina occurs abundantly in nature, most often as impure hydoxides which are the essential constituents of bauxite ores. • Bauxite is an impure mixture of gibbsite, boehmite and diaspore. • The most common process for the extraction and purification of alumina is the ‘Bayer’ process. Bayer process involves the following operations: (1) Digestion (2) Clarification (3) Precipitation (4) Calcination

Digestion •The exact procedure required for digestion, depends on the nature of the ore deposits. In order to remove the iron oxides and most of the silicon oxides present, the ore is first treated with sodium hydroxide. The digestion process takes advantage of the solubility of amphoteric aluminum oxides to form a solution of aluminate ions, whilst the basic iron oxides which form do not dissolve and are separated by filtration. •Gibbsite Al2O3.3H2O + 2NaOH → 2 NaAlO2 + 4 H2O (135-150 °C) •Boehmiite Al2O3.H2O + 2NaOH → 2 NaAlO2 + 2 H2O (205-245 °C) •Diaspore Al2O3.H2O + 2NaOH → 2 NaAlO2 + 2 H2O (high T and P)

Clarification of the liquor stream •The alumina-bearing solution is separated from the insoluble impurities that were part of the original bauxite.

•With all solids removed, the pregnant liquor leaving the filter area, contains alumina in clear supersaturated solution. It is cooled by flash evaporation, the steam given off being used to heat spent liquor returning to digestion.

Precipitation •The alumina is precipitated or crystallised from the solution as crystals of alumina trihydrate. The solution is mixed in tall vessels with recycled seed crystals. When completed the solid alumina hydrate is passed on to the next stage and the remaining liquor, which contains caustic soda and some alumina, goes back to the digesters. •Dissolved alumina is recovered from the liquor by precipitation of crystals. Alumina precipitates as the trihydrate Al2O3 .3H2O in a reaction which is the reverse of the digestion of trihydrate •2NaAlO2 + 4H2O → Al2O3.3H2O + 2 NaOH

Calcination •In the final stage the alumina trihydrate is washed to remove any remaining caustic . It is heated to about 1050°C in special calciners or kilns to drive off the water of crystallisation, leaving the alumina, which is a dry, white, sandy material. •In the calcination process water is driven off to form alumina: •2Al(OH)3 ---> Al2O3 + 3H2O 4. ZrO2 genel özellikleri? Zirconium oxide (ZrO2) has gained importance in the last few years due to its •high fracture toughness, •low thermal expansion similar to cast iron, •extremely high bending strength, •high resistance to wear and to corrosion, •low thermal conductivity, •oxygen ion conductivity and •very good tribological properties (it is very well suited for slide rings).

4. What are the key properties of SiC, and production methods, explain acheson process briefyl? General View of SiC Properties Thermal Properties

•Well known in its refractory applications •Excellent thermal conductivity and a low thermal expansion coefficient , thermal shock resistance. Mechanical Properties •Sintered silicon carbide is one of the strongest ceramic materials. •Its strength is limited by the presence of a variety of flaws , crystallite agglomerates, oversize, elongated grains, and porosity. Electrical Properties •Silicon carbide is a semiconductor. Its energy gap is related to the structure (polytype). It ranges from 2.2 eV for the cubic material to 3.3 eV for the simple hexagonal polytype.(2H, Wurtzite form) . •Resistivity can vary by as much as seven orders of magnitude. If pure, s ilicon carbide is an insulator. Optical Properties •It can be prepared in a wide range of colors, •colorless (pure α / hexagonal ), •yellow (β / cubic), •green ( nitrogen / or phosphorous doped), •blue (aluminium doped)

PRODUCTION of SiC Acheson Process • A mixture of silica, carbon, sawdust and common salt (e.g. 50% silica, 40% coke, 7% sawdust and 3% common salt) is heated in an electric furnace. The heating is accomplished by a core of graphite and coke placed centrally in the furnace. The mixture of reactants is placed around this core. •The mixture is then heated to reach a maximum temperature of approximately 2700o C, after which the temperature is gradually lowered. The reaction within the furnace is complex but, in general, following the formula SiO2 + 3 C → SiC + 2 CO

5. Si3N4 üretimi nasıl yapılır? Özellikleri nelerdir? Key Properties •Low density •High temperature strength •Superior thermal shock resistance •Excellent wear resistance •Good fracture toughness •Mechanical fatigue and creep resistance •Good oxidation resistance •Electrical resistivity •High hardness •Not wetted by molten metals •Does not melt like other ceramics, but it decomposes above 1800°C and sublimates into Si and N. Mainly three techniques are used for the production of silicon nitride powders : 1.Carbothermal Reduction and Nitridation of Silica 2.Nitridation of Silicon Powder 3.Vapor Phase Reactions

Carbothermal Reduction of Silica Raw material powder of the silica, Si3N4 and carbon is blended and mixed at a spesific ratio, and this raw material is heated in flowing N2 gas. SiO2 + C = SiO + CO SiO + C = Si + CO 3Si + 2N2 = Si3N4 3 SiO2 + 6 C + 2 N2 = Si3N4 + 6 CO ΔH = 1268 kJ/ mol (1700K; 1423°C) Direct Nitridation of Silicon Primary and Secondary Reactions : 3 Si + 2 N2 (g) = Si3N4 It is an exothermic reaction because of the control of the reaction temperature during the formation of α Si3N4 is so hard and an original method is used to produce stable α -type silicon nitride. ΔH = -732 kJ/mol (1600K, 1323°C) Vapor Phase Reactions Some of the gaseous silicon compounds is used, i.e. silicon tetrachloride and silane with ammonia. Vapor Phase Reactions are : •3 SiCl4 + 2 N2 + 6 H2 = Si3N4 +12 HCl •3 SiCl4 + 4 NH3 = Si3N4 +12 HCl •3H2SiCl2 + 4 NH3 = Si3N4 + 6 HCl + 6 H2 •3SiH4 + 4 NH3 = Si3N4 + 12 H2 •3SiH4 + 2 N2 = Si3N4 + 6 H2 6. Raw selection criteria başlıkları?

Raw Materials Selection Criteria Purity Particle Size and Reactivity Purity Purity strongly influences high-temperature properties such as strength, stress rupture life, and oxidation resistance of ceramic materials. Impurities present as inclusions do not appreciably affect properties such as creep or oxidation, but do act as flaws that can concentrate stress and decrease component tensile strength. The effects of impurities are important for mechanical properties, but may be even more important for electrical, magnetic, and optical properties. Particle Size and Reactivity Particle size distribution is important, depending on which consolidation or shaping technique is to be used.Large amounts of porosity are difficult to eliminate during densification. Low porosity and fine grain size are beneficial to achieve a ceramic with high strength. 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. 7. Size reduction methods nelerdir. roll milling i acıkla ve wet milling in ne gibi avantajları vardır?

Particle Size Reduction Ball Milling Attrition Milling Vibratory Milling Fluid Energy Milling Hammer Milling Roll Crushing Ball Milling Ball milling is one of the most widely used. Ball milling consists of placing the particles to be ground (the "charge") in a closed cylindrical container with grinding media (balls, short cylinders, or rods) and rotating the cylinder horizontally on its axis so that the media cascade. The ceramic particles move between the much larger media and between the media and the wall of the mill and are effectively broken into successively smaller particles.

Milling can be conducted either dry or wet. Advantages of wet milling •Low power required •No dust problems •Higher rotational speeds •Can wet screen through fine screen •Good homogenization •Smaller particle size than dry •Narrower particle size distribution than dry •Compatible with spray drying and casting processes Advantages of dry milling •Avoids drying of the powder •Avoids reaction of the powder/liquid •Less media and lining wear than wet •Can be started/stopped any time •Easier to optimize 8. What are the shape formıng methods, what are dry pressings and explain die presssing briefly? Forming Processes DRY FORMING PROC. Die-pressing Isostatic pressing WET FORMING PROC. Slip casting Tape Casting PLASTIC FORMING PROC. Extrusion Injection Moulding 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-Wet pressing 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.Wet pressing: Involves a feed powder containing 10-15 % moisture and often used with clay-containig 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 %.

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 Slip Casting It involves ceramic particles suspended in water and cast into porous plaster mold. 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.

Tape Casting Some applications such as substrates and packages for electronics and dielectrics for capacitors require thin sheets of ceramics. 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 9. what is sintering main purpose, tell sintering types and explain hot pressing? Sintering commonly refers to processes involved in the heat treatment of powder compacts at elevated temperatures, usually at T > 0.5Tm [K], in the temperature range where diffusional mass transport is appreciable. Successful sintering usually results in a dense polycrystalline solid. •The following criteria must be met before sintering can occur: 1. A mechanism for material transport must be presentdiffusion and viscous flow 2. A source of energy to activate and sustain this material transport must be presentheat is source of energy, in conjuction with energy gradient due to particle-particle contact and surface tension.

Why do we need Sintering? •The principal goals of sintering are; 1.Bond individual grains into a solid mass 2.Increase density 3.Reduce or eliminate porosity The sintering process is usually accompanied by other changes within the material, some desirable and some undesirable. The largest changes occur in: - strength, elastic modulus - hardness, fracture toughness - electrical and thermal conductivity - permeability to gases and liquids - average grain number, size and shape - distribution of grain size and shape - average pore size and shape - distribution of pore size and shape - chemical composition and crystal structure Sintering types 1.Solid State Sintering 2.Liquid Phase Sintering 3. Pressure Assisted Sintering

SOLID STATE SINTERING Solid state sintering involves material transport by volume diffusion. Diffusion can consist of movement of atoms or vacancies along; •a surface •grain boundary or •through the volume of the material. •Surface diffusion like vapour-phase transport, does not result in shrinkage. •Volume diffusion, whether along grain boundaries or through lattice dislocations, does result in shrinkage. •The driving force for solid-state sintering is the difference in free energy or chemical potential between the free surface of particles and points contact between the adjacent particles. Solid State Sintering Stages 1st Stage (initial) Rearrangement Neck Formation 2 nd Stage (intermediate) Neck Growth Grain growth High shrinkage Pore phase continuous 3 rd Stage (final) Much grain growth

Discontinuous pore phase Grain boundary pores eliminated HOT PRESSING In a heated state, pressure transmition is carried out from the upper punch; pressure produced is received by the graphite case Restrictions: •Simple shapes •Poor pressure transmission for thick objects

HOT ISOSTATIC PRESSING Here, GAS is the pressing medium, with pressure capabilities ~ 200 to 300 MPa. The pressure can only be transmitted to the interior of the component if the surface is SEALED using metal or glass cans, or pre-densification until closed (>92%TD) porosity is produced. In HIP-ping driving force is the same as in hot pressing , however additional diffusivity is provided from increase in stress at contact area. Sample must be jacketed , otherwise open pores will not be removed. The jacket materials include metal can (Pt. Ni, Fe, Ti, W ) or glass , must be viscous and non-penetrating the component.

Sponsor Documents


Recommended


View All
Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close