Recommended
More Related Content
Similar to Ceramics.pptx
Similar to Ceramics.pptx (20)
More from HafizMudaserAhmad
More from HafizMudaserAhmad (9)
Recently uploaded
Recently uploaded (20)
Ceramics.pptx
- 1. CERAMICS
- 2. Cerami cs An inorganic compound consisting of a metal (or semi-metal) and one or more nonmetals Examples: Silica or silicon dioxide (SiO2), the main ingredient in most glass products Alumina - aluminum oxide (Al2O3), used in various applications from abrasives to artificial bones More complex compounds such as hydrous aluminum silicate (Al2Si2O5(OH)4), the main ingredient in most clay products
- 4. Ceramic Crystal Structures- Oxide structures: 1. Anions are larger than cations. 2. Close packed oxygen in a lattice (usually FCC). 3. Cations fit into interstitial sites among anions.
- 6. ABX3 Crystal Structures - 6 • Perovskite (calcium titanium oxide) structure Ex: complex oxide Barium Titanate (BaTiO3)
- 7. Silicate Ceramics - • Most common elements on earth are Si & O 7 cristobalite • SiO2 (silica) polymorphic forms are quartz, cristobalite, & tridymite • The strong Si-O bonds lead to a high melting temperature (1710ºC) for this material Si4+ O2-
- 8. 8 Glass Structure - • Quartz is crystalline SiO2: • Basic Unit: • Glass is noncrystalline (amorphous) • Fused silica is SiO2 to which no impurities have been added • Other common glasses contain impurity ions such as Na+, Ca2+, Al3+, and B3+ (soda glass)- Adapted from Fig. 12.11, Callister & Rethwisch 8e. 4- Si0 4 tetrahedron Si 4+ O 2- Na Si 4+ O 2 - +
- 11. Thermal Conductivity (k) of Metals and Ceramics
- 12. Thermal Protection System of Ceramics
- 13. Properties of Ceramic Materials High hardness, electrical and thermal insulation, chemical stability, and high melting temperatures Brittle, virtually no ductility – can cause problems in both the processing and performance of ceramic products
- 14. Processing of Ceramics (Slip Casting) • A suspension of ceramic powders in water called a slip, is poured into a porous plaster of Paris mold, so that water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface. •The slip composition is 25% to 40% water.
- 15. Figure - Sequence of steps in drain casting, a form of slip casting: (1)slip is poured into the mold cavity, (2)Water is absorbed into the plaster mold to form a firm layer, (3)Excess slip is poured out, and
- 16. • Drying decreases shrinkage. : as water is removed interparticle spacings Drying and Firing wet body partially dry completely dry Drying too fast causes sample to warp or crack due to non-uniform shrinkage • Firing: -- heat treatment between 900-1400ºC -- vitrification: liquid glass forms from clay and flux – flows between SiO2 particles. (Flux lowers melting
- 17. Home Assignment Hydroplastic forming of Ceramics
- 18. Powder Pressing used for both clay and non-clay compositions. • Powder (plus binder) compacted by pressure in amold -- Uniaxial compression - compacted in single direction -- Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope -- Hot pressing - pressure + heat
- 19. Ceramic Products • Clay construction products - bricks, clay pipe, and building tile ,cement concrete • Refractory ceramics - ceramics capable of high temperature applications such as furnace walls, crucibles, and molds • White ware products - stoneware, fine china, porcelain, and other tableware, based on mixtures of clay and other minerals • Glass - bottles, glasses, lenses, window pane, and light bulbs • Glass fibers - thermal insulating wool, reinforced plastics (fiberglass), and fiber optics communications lines • Abrasives - aluminum oxide and silicon carbide • Cutting tool materials - tungsten carbide, aluminum oxide,
- 20. • Ceramic insulators - applications include electrical transmission components, spark plugs, and microelectronic chip substrates • Magnetic ceramics – example: computer memories • Nuclear fuels based on uranium oxide (UO2) • Bioceramics - artificial teeth and bones
- 21. Ceramic Materials- Applications Clay – Shaped, dried, and fired inorganic material Examples: Brick, tile, sewer pipe, chimney flue, china, porcelain, etc. Refractory – Designed to provide acceptable mechanical or chemical properties while at high temperatures Example: Space shuttle all-silica insulating tiles
- 22. Applications of Ceramic Materials Electrical Resistors – Create desired voltage drops and limit current Thermistors – Application of heat regulates current flow Rectifiers – Allow current to flow in one direction Heating elements for furnaces
- 23. • Alumina(Al2O3) is used as insulators in spark plug and electronic packaging, rocket nozzles etc. • Tungsten carbide and Titanium carbide along with metal binders like Ni, Co, Cr, Mo are known as cermets which are used as cutting tool materials. • Tungsten carbide is used as an abrasive material for grinding and polishing operations
- 25. Refractori es
- 26. Definition In general usage, products applied at temperatures > 600 °C are referred to as refractories. A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as Non-metallic materials having those chemical and physical properties that made them applicable for structures or as components of systems, that are exposed to environments above 1000 °F (800K, 500 °C)"
- 27. Refractory Materials Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion. The oxides of aluminum (alumina), silicon (silica), and magnesium (magnesia) are the most important materials used in the manufacturing of refractories
- 28. Types of Refractories Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications. There are four basic types of refractories: Acidic Refractories Basic Refractories Neutral Refractories Super Refractories
- 29. Acidic Refractory A refractory that is composed principally of silica and reacts at high temperatures with bases such as lime, alkalies, and basic oxides. These are used in areas where slag and the atmosphere are acidic. They are stable to acids but attacked by alkalis. The main components of these refractories are silica along with alumina. (Al2O3). The steel industries are the largest consumer of acidic refractories.
- 30. Basic Refractories These are used on areas where slags and atmosphere are basic, stable to alkaline materials but react with acids. The main raw material is magnesia (MgO) is a very common example. Other examples include dolomite (MgCO3 + CaCO3) and chrome-magnesia (Cr2O3 + MgO).
- 31. Neutral Refractories These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The common examples of these materials are alumina (Al2O3), chrome ( Cr₂O3), and carbon. Normally we have to use acidic and basic refractories combined but we use neutral bricks to avoid the reaction. The neutral bricks are made of graphite and chromite.
- 32. Super Refractories These refractories are manufactured for exceptional high-temperature application higher than 1800°C. For example, zirconia, its melting point is 2340 deg C - 2550 deg C , (Thoria) its melting point is 3200°C.
- 33. What is a Furnace • Equipment to melt metals – Casting – Change shape – Change properties • Type of fuel important – Mostly liquid/gaseous fuel or electricity • Low efficiencies due to – High operating temperature – Emission of hot exhaust gases
- 34. Introduction What are Refractories Materials that – Withstand high temperatures and sudden changes – Withstand the action of molten slag, glass, hot gases, etc – Withstand load at service conditions – Withstand abrasive forces – Conserve heat – Have a low coefficient of thermal expansion – Will not contaminate the load
- 35. Refractories Refractory lining of a furnace arc Refractory walls of a furnace interior with burner blocks (BEE India, 2005)
- 36. Properties of Refractories • Melting point – Temperature at which a ‘test pyramid’ (cone) fails to support its own weight • Size – Affects stability of furnace structure • Bulk density – Amount of refractory material within a volume (kg/m3) – High bulk density = high volume stability, heat capacity and resistance • Porosity – Volume of open pores as % of total refractory volume – Low porosity = less penetration of molten material • Cold crushing strength – Resistance of refractory to crushing • Creep at high temperature – Deformation of refractory material under stress at given time and temperature
- 37. Properties of Refractories • Pyrometric cones – Used in ceramic industries to test refractory bricks – Each cone is mix of oxides that melt at specific temperatures • Pyrometric Cone Equivalent (PCE) • Temperature at which the refractory brick and the cone bend • Refractory cannot be used above this temp • Volume stability, expansion & shrinkage – Permanent changes during the refractory service life – Occurs at high temperatures • Reversible thermal expansion – Phase transformations during heating and cooling
- 38. Type of Refractories Fireclay Refractories • Common in the industry: materials available and inexpensive • Consist of aluminum silicates • Fire Clay Brick is the most used refractory brick in the iron & steel industry, glass industry, non-ferrous industry, cement industry, pottery kilns, and many other industries with the advantages of its lower cost
- 39. Type of Refractories Silica Brick • >93% SiO2 made from quality rocks • Iron & steel, glass industry • Advantages: no softening until fusion point is reached; high refractoriness; high resistance to spalling, flux, and slag, volume stability Magnesite • Chemically basic: >85% magnesium oxide • Properties depend on silicate bond concentration • High slag resistance, especially lime and iron
- 40. Type of Refractories Chromite Refractories • Chrome- magnesite – 15-35% Cr2O3 and 42-50% MgO – Used for critical parts of high temp furnaces – Withstand corrosive slags – High refractories • Magnesite-chromite – >60% MgO and 8-18% Cr2O3 – High temp resistance – Basic slags in steel melting
- 41. Type of Refractories Zirconia Refractories • Zirconium dioxide ZrO2 • Stabilized with calcium, magnesium, etc. • High strength, low thermal conductivity, not reactive, low thermal loss • Used in glass furnaces, insulating refractory
- 42. Selecting the Right Refractory Selection criteria • Type of furnace • Type of metal charge • Presence of slag • Area of application • Working temperatures • Extent of abrasion and impact • Structural load of furnace • Stress due to temp gradient & fluctuations • Chemical compatibility • Heat transfer & fuel conservation • Costs
- 43. F U R N A C E
- 44. OLD FURNACE
- 45. To succeed in your mission, you must have single-minded devotion to your goal.
Editor's Notes
- A furnace is an equipment used to melt metals for casting or to heat materials to change their shape (e.g. rolling, forging) or properties (heat treatment). Since flue gases from the fuel come in direct contact with the materials, the type of fuel chosen is important. For example, some materials will not tolerate sulphur in the fuel, in which case you can use light diesel oil. Solid fuels generate particulate matter, which will interfere the materials placed inside the furnace, therefore coal is not often used as fuel. Furnace ideally should heat as much of material as possible to a uniform temperature with the least possible fuel and labor. The key to efficient furnace operation lies in complete combustion of fuel with minimum excess air. Furnaces operate with relatively low efficiencies (as low as 7 percent) compared to other combustion equipment such as the boiler (with efficiencies higher than 90 percent. This is caused by the high operating temperatures in the furnace. For example, a furnace heating materials to 1200 oC will emit exhaust gases at 1200 oC or more, which results in significant heat losses through the chimney.
- Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or corrosive solids, liquids or gases at high temperatures. The various combinations of operating conditions in which refractories are used, make it necessary to manufacture a range of refractory materials with different properties. Refractory materials are made in varying combinations and shapes depending on their applications. General requirements of a refractory material are: Withstand high temperatures Withstand sudden changes of temperatures Withstand action of molten metal slag, glass, hot gases, etc Withstand load at service conditions Withstand load and abrasive forces Conserve heat Have low coefficient of thermal expansion Should not contaminate the material with which it comes into contact
- Melting point: Pure substances melt instantly at a specific temperature. Most refractory materials consist of particles bonded together that have high melting temperatures. At high temperatures, these particles melt and form slag. The melting point of the refractory is the temperature at which a test pyramid (cone) fails to support its own weight. Size: The size and shape of the refractories is a part of the design of the furnace, since it affects the stability of the furnace structure. Accurate size is extremely important to properly fit the refractory shape inside the furnace and to minimize space between construction joints. Bulk density: The bulk density is useful property of refractories, which is the amount of refractory material within a volume (kg/m3). An increase in bulk density of a given refractory increases its volume stability, heat capacity and resistance to slag penetration.
- Pyrometric cones and Pyrometric cones equivalent (PCE): The ‘refractoriness’ of (refractory) bricks is the temperature at which the refractory bends because it can no longer support its own weight. Pyrometric cones are used in ceramic industries to test the refractoriness of the (refractory) bricks and thus determine what refractory bricks they should use. They consist of a mixture of oxides that are known to melt at a specific narrow temperature range. Cones with different oxide composition are placed in sequence of their melting temperature alongside a row of refractory bricks in a furnace. The furnace is fired and the temperature rises. One cone will bends together with the refractory brick as shown in the figure. This is the temperature range in oC above which the refractory cannot be used. This is known as Pyrometric Cone Equivalent temperatures.
- Fireclay Refractories Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others. Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO2) content of up to 78 percent and Al2O3 content of up to 44 percent. The table shows that the melting point (PCE) of fireclay brick decreases with increasing impurity and decreasing Al2O3. This material is often used in furnaces, kilns and stoves because the materials are widely available and relatively inexpensive. (Click once) High alumina refractories Alumina silicate refractories containing more than 45 percent alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals. Fireclay Refractories Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others. Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO2) content of up to 78 percent and Al2O3 content of up to 44 percent. The table shows that the melting point (PCE) of fireclay brick decreases with increasing impurity and decreasing Al2O3. This material is often used in furnaces, kilns and stoves because the materials are widely available and relatively inexpensive. (Click once) High alumina refractories Alumina silicate refractories containing more than 45 percent alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals. Fireclay Refractories Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others. Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO2) content of up to 78 percent and Al2O3 content of up to 44 percent. The table shows that the melting point (PCE) of fireclay brick decreases with increasing impurity and decreasing Al2O3. This material is often used in furnaces, kilns and stoves because the materials are widely available and relatively inexpensive. (Click once) High alumina refractories Alumina silicate refractories containing more than 45 percent alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.
- Silica brick Silica brick (or Dinas) is a refractory that contains at least 93 percent SiO2. The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel melting furnaces and the glass industry. Advantages are The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. This behavior contrasts with that of many other refractories, for example alumina silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points. High resistance to thermal shock (spalling) High refractoriness. Flux and slag resistance Volume stability (Click once) Magnesite Magnesite refractories are chemically basic materials, containing at least 85 percent magnesium oxide. They are made from naturally occurring magnesite (MgCO3). The properties of magnesite refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO2 ratio of less than two with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. The slag resistance is very high particularly to lime and iron rich slags.
- Chromite refractories Two types of chromite refractories are distinguished: Chrome-magnesite refractories, which usually contain 15-35 percent Cr2O3 and 42-50 percent MgO. They are made in a wide range of qualities and are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness. Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18 percent Cr2O3. They are suitable for service at the highest temperatures and for contact with the most basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance than chrome-magnesite.
- Zirconia refractories Zirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it before application as a refractory, which is achieved by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization, quantity of stabilizer and quality of the original raw material. Zirconia refractories have a very high strength at room temperature, which is maintained up to temperatures as high as 1500 oC. They are therefore useful as high temperature construction materials in furnaces and kilns. The thermal conductivity of zirconium dioxide is much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory. Zirconia exhibits very low thermal losses and does not react readily with liquid metals, and is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Glass furnaces use zirconia because it is not easily wetted by molten glasses and does not react easily with glass. (Click once) Oxide refractories (Alumina) Alumina refractory materials that consist of aluminium oxide with little traces of impurities are known as pure alumina. Alumina is one of the most chemically stable oxides known. It is mechanically very strong, insoluble in water, super heated steam, and most inorganic acids and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is used for lining furnaces operating up to 1850oC
- We discussed the different types of refractories earlier. But despite the advantages of some refractories over others, it is important to select the right refractory for the specific application. The selection of refractories aims to maximize the performance of the furnace, kiln or boiler. Furnace manufacturers or users should consider the following points in the selection of a refractory: Type of furnace Type of metal charge Presence of slag Area of application Working temperatures Extent of abrasion and impact Structural load of the furnace Stress due to temperature gradient in the structures and temperature fluctuations Chemical compatibility to the furnace environment Heat transfer and fuel conservation Cost considerations