Fundamentals of Ceramic Materials PDF 2024-2025 Fall

FUNDAMENTALS OF CERAMIC MATERIALS Prof.Dr. Filiz Şahin 2024-2025 Fall Glass Raw Materials – Base materials (glass makers, melts, stabilizers) and – Auxiliary materials (fining agents, dyes, discoloration agents, opal glass and melting accelerators)....

FUNDAMENTALS OF CERAMIC MATERIALS Prof.Dr. Filiz Şahin 2024-2025 Fall Glass Raw Materials – Base materials (glass makers, melts, stabilizers) and – Auxiliary materials (fining agents, dyes, discoloration agents, opal glass and melting accelerators). The raw materials for making glass are all oxides. The composition of any sample of glass can be given in terms of the percent of each oxide used to make it. Glass Raw Materials The basic materials for glass production are: – Quartz (Silicon dioxide (𝑺𝒊𝑶𝟐 )) – Soda (Sodium Carbonate (𝑵𝒂𝟐 𝑪𝑶𝟑 ) (using in soft glass)) – Limestone (Calcium Carbonate (𝑪𝒂𝑪𝑶𝟑 )) – Potash (Potassium Carbonate (𝑲𝟐 𝑪𝑶𝟑 ) (using in hard glass)) – Dolomite (Magnasium Calcium Carbonate (𝑴𝒈𝑪𝒂(𝑪𝑶𝟑 )𝟐 )) – Crushed glass forms 25-30% of the whole mixture and has to be of the right size; the crushed glass pieces must not be too large and also too small, since the latter make the clarification process more difficult Auxiliary Materials Auxiliary materials are added to base materials: – Materials for glass discoloring and clarification of glass mixture (Manganese dioxide), – Materials for coloring are metal oxides, – Materials for opaque glass texture (Titanium and Zirconium oxides) Composition of Glass When sand is mixed with metal oxides, melted at high temperatures, and then cooled to a rigid condition without crystallization, the product is called glass. By adding soda (𝑵𝒂𝟐 𝑪𝑶𝟑 ) to the sand, its melting point and viscosity are both lowered, making it much easier to work it. Lime (CaO) is added to the sand and soda mixture so that the " soda-lime " glass will not dissolve in water. Components of Glass Because glass is used in so many different ways, there is no chemical composition for each glass sample (composition range). There are so many different glass compositions but only 3 categories of substances in all glass. These are: – Network Formers – Network Formation – Network Modifiers (Fluxes, Softeners) – Intermediate Oxides (Stabilizers – Provide Chemical Resistance) Components of Glass The network former makes up the bulk of the glass. Silica (𝑆𝑖𝑂# ) in the form of sand is the most common type Other possible formers include 𝐵# 𝑂$ and 𝑃# 𝑂%. The network modifiers (fluxes) change the temperature at which the formers melt during the manufacturing of glass. 𝑁𝑎# 𝐶𝑂$ and 𝐾# 𝐶𝑂$ The stabilizers (or intermediate oxides) strengthen the glass and make it resistant to water. – 𝐶𝑎𝐶𝑂$ is the most frequently used  Network Formers, Modifiers and Intermediates Glass network formers – Form the interconnected backbone glass network Glass network modifiers – Present as ions to alter the glass network – Compensated by non-bridging oxygen (NBO) in oxide glasses – Usually reduce glass network connectivity Intermediates – Can behave as network formers or modifiers depending on glass composition – Improve chemical resistance (especially Al2O3) – Behave as stabilizing agent (e.g. TiO2, ZrO2, CeO2) Network Formers, Modifiers and Intermediates § 3D glass network predominantly consisting of corner-sharing 𝑆𝑖𝑂& tetrahedral interconnected by bridging oxygen (BO) § High network connectivity: high softening point, low diffusion coefficient, small coefficient of thermal expansion (CTE) ü Each alkali ion creates one non bridging oxygen ü Reduced network connectivity viscosity decreases (compared to silica at the same T), diffusion coefficient Si: Glass Former and CTE increases Na: Network Modifier ü Increased ionic conductivity, reduced chemical resistance Network Formers, Modifiers and Intermediates Glass former: high valence state, covalent bonding with oxygen Modifier: low valence state, ionic bonding with oxygen Alkali - Alkaline Earth Silicate Glass Each alkaline earth ion creates two NBOs Increased network connectivity compared to alkali silicates stabilized glass network (Na2O), improved chemical resistance Approximate composition of commercial soda lime glass (window glass) 16𝑁𝑎2𝑂·10𝐶𝑎𝑂·74𝑆𝑖𝑂2 11 Borate Glass 𝐵2𝑂3:the glass former consisting of corner sharing. 𝐵𝑂3 triangles connected by bridge oxygen 12 Network Formers, Modifiers and Intermediates Example: Window glass composition Example: Borosilicate glass composition  Glass Manufacturing Melting Forming and Shaping Annealing Finishing Melting Raw materials in proper proportions are mixed It is finely powdered and intimate mixture called ʹ batch ʹ is fused in furnace at high temperature of 1800 C this charge melts and fuses into a viscous fluid 𝐶𝑎𝐶𝑂3+𝑆𝑖𝑂2→𝐶𝑎𝑆𝑖𝑂3+𝐶𝑂2↑ 𝑁𝑎2𝐶𝑂3+𝑆𝑖𝑂2→𝑁𝑎2𝑆𝑖𝑂3+𝐶𝑂2↑ After removal of 𝐶𝑂2 decolorizes like 𝑀𝑛𝑂2 are added to remove traces of ferrous compounds and carbon Heating is continued till clear molten mass is free from bubbles is obtained and it is then cooled to about 800 C 15 Forming and Shaping The Viscous mass obtained from melting is poured into molds to get rid of different types of articles of desired shape by either blowing or pressing between the roller 16 Annealing Glass articles are the allowed to cool gradually at room temperature by passing through different chambers with descending temperatures This reduces the internal strain in the glass. 17 Finishing Finishing is the last step in glass manufacturing It involves following steps ü Cleaning ü Grinding ü Polishing ü Cutting ü Sand Blasting 18 19 Characterization GLASS-CERAMICS: PROCESSING, PROPERTIES and APPLICATIONS  Glass-Ceramic § Enhancing mechanical properties of glasses with crystallization § Controlled microstructure, crystalline phases (+ residual glassy phase) Controlled Heat Treatment nuclei crystal glass formation growth ceramic  Nucleating Agents v Nucleating agents promotes heterogeneous nucleation, which could be controlled much more easily comparing with homogenous crystallization surface crystallization mechanism. v Nucleating agents increase the number of heterogeneous nucleation sites. v Nucleating agents are composed of v either metal elements dispersed as colloids (Au, Ag, Pt,Pd, etc.) v or simple oxides (TiO2, ZrO2, P2O5, Ta2O5, etc.) v Agents are added to glass composition as 1-8 mol% for oxides and less than 1 mol% for colloids for the efficient volume crystallization with material that has enhanced properties  v Processing and thermal, optical, chemical, biological, mechanical, electrical and magnetic properties could be controlled by considering composition, morphology, amount and orientation of crystals in the glass-ceramics. Electrical and Processing Thermal Optical Chemical Biological Mechanical Magnetic Method Controllable Translucency or Resorbability Biocompatibility Machinability Isolation Limited and expansion opacity High chemical Bioactivity High strength and Ion conductivity controllable depending on Pigmentation durability toughness Superconductivity shrinkage temperature Photo-induction Ferromagnetism No porosity in High temperature Opalescence, monolithic G/C stability fluorescence  Military High thermal stability High radar wave transparence Low CTE Optical Electrical High ion High transparence conductivity Chemical Zero porosity stability Cost-effective Good formability Application areas Construction Medical High strentgh High strength Aesthetic Aesthetic Low cost Bioactive q Kitchenware Aesthetic Low CTE  Glass-Ceramic Production Designing of Glass Ceramics 1. Choice of the parent glass composition for desired crystalline phase. 2. Synthesis of the glass via a melting process (followed by quenching) and shaping. 3. Controlled crystallization of the glass. (microstructure) https://wellcomecollection.org/works/w935sf9w v The composition determines; § The nature of the future crystalline phase(s), § Nucleation and growth § Thermodynamics and the kinetics of the system § Properties  Glass-Ceramic Production Fabrication from glass batch; §Melting of glass §Casting and cooling of molten glass §Nucleation and grain growth to convert G/C Fabrication from glass powder; §Sintering of the glass powder by holding during heating §Cooling of the bulk Glass-Ceramics in Dentistry: A Review - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/The-two- manufacturing-processes-of-glass-ceramics-a-The-classic_fig4_339500415 [accessed 11 Sep, 2022] Glass–Ceramic Glazes for Ceramic Tiles: A Review - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Evolution-of-the-microstructure-from-glass-to-glass-ceramic-through-volume_fig1_225719990 [accessed 11 Sep, 2022] https://www.ivoclar.com/en/p/all/all-ceramics/ips-emax-system-technicians/ips-emax-cad/  Glass-Ceramic – LAS (Lithium Aluminosilicate) v Most common glass ceramic LAS (Li2O-Al2O3-SiO2) v Transparent v Heat – resistant v Low thermal expansion of coefficient (CTE) v It could be colored with the addition of nucleating agents like TiO2, ZrO2 LAS glass-ceramics are used in cooktop panels for kitchen stove and in fireplace glass doors  Glass-Ceramic – MAS (Magnesium Aluminosilicate) v Most important crystal phase in MAS is cordierite (Mg2Al4Si5O18) v High mechanical strength v Excellent dielectric properties v Good thermal stability v Thermal shock resistance § The development of MAS glass–ceramic was motivated by the need arose for a ceramic missile nosecone for a missile to be guided by an internal antenna.  Glass-Ceramic - MACOR® § Machinable glass ceramics with nucleated fluoromica crystals in glass vAerospace industry: More than 200 special parts of the U.S. space shuttle orbiter are made of this glass-ceramic. These parts contain rings at all hinge points, windows, and doors. v Medical equipment: Machinability of the material and its inert character are particularly significant in the fabrication of specialized medical equipment. vUltrahigh vacuum applications: Excellent insulators and pore-free material which is used to manufacture equipment for vacuum technology. vWelding: It is used in welding equipment which has excellent nonwetting properties with regard to oxyacetylene. vNuclear-related experiments: Irradiation resistant material. https://m.turkish.industrial-ceramicparts.com/photo/pt33610095- macor_machinable_glass_ceramic.jpg  Glass-Ceramic - DICOR® § Machinable glass ceramics which are used for dental applications § Translucency and chemical durability are improved comparing to MACOR® § Includes tetrasilisic mica: KMg2,5AlSi4O10F2 § Good strength (~150 MPa) is associated with the development of anisotropic at relatively high temperatures (> 1000°C). § Ceria addition is made to catch fluorescency of natural teeth. HÖLAND, W., & RHEINBERGER, V. (2008). Dental glass-ceramics. Bioceramics and Their Clinical Applications, 548–568. doi:10.1533/9781845694227.2.548 Two usage areas in dentistry; § Dental crowns or inlays are produced with lost-wax technique. § Machining of dental restorations by DICOR® MGC  Glass-Ceramic - DICOR® A schematic (a) and an example (b) demonstrating interlocking effect in glass–ceramics. The interlocking microstructure of the mica glass-ceramic allows the material to be machined. Cracks propagate in a zig path, instead of propagating in a direct path, which effectively consumes the energy of cracks and slows crack propogation. Fu, L., Engqvist, H., & Xia, W. (2020). Glass–Ceramics in Dentistry: A Review. Materials, 13(5), 1049. doi:10.3390/ma13051049  Glass-Ceramic – BIOVERIT I and BIOVERIT II § Biocompatible and machinable depending on the high mica content § Machinability (BIOVERIT III) § in orthopedic surgery, § especially different types of head and neck surgery, § middle ear implants Middle-ear implants of BIOVERIT II  What are Refractory Materials? Refractories are non-metallic inorganic materials capable of withstanding high temperatures and not degrading in a furnace environment when in contact with corrosive liquids, gases and solids. Refractory insulators are used in high-temperature applications – to provide proper processing environment – to reduce heat losses and – to save fuel 44 45 Properties of Refractory Materials Withstand high temperatures under high load (high creep resistance) High volume stability. Withstand sudden changes in temperature. Low coefficient of thermal expansion. High thermal shock resistance Withstand the action of molten metal slag, glass, hot gases, etc. (high corrosion resistance) Withstand the abrasive/wear/erosive forces. Able to conserve heat. Thermal conductivity (low or high depending on properties of thermal shock and insulation) Density and porosity (depending on dense or insulating character requirements) 46 FIGURE 1.1 Industry wise global refractory consumption (in percent) 47  Manufacturing and Properties Crushing of Refractories about 25 mm Shaped Refractories Grinding 200 mesh Refractory raw materials are normally available as hard lumpy materials, which are crushed first in primary crusher like jaw Screening crusher (if the size of the lumps is above 50 mm) and then in the secondary crusher like hammer mill. The crushed materials are screened to several size fractions like 3–5 Batch Weighting mm, 1–3 mm, 0–1 mm, and fines. The different size fractions are mixed together along with a binder Mixing and pressed in a mold of desired size in high capacity press to form the brick. Shaping The brick is then dried to remove the moisture inside and fired at high temperature (1150–1750 °C) to impart the strength and desired properties to the refractory. Drying Firing 48 (1150–1750 °C)  Physical Properties of Refractories; 1) Refractoriness 2) Strength of refractories under load 3) Dimensional stability 4) Porosity 5) Thermal spalling 6) Thermal expansion 7) Thermal conductivity 49  Physical Properties of Refractories 1) Refractoriness Ability of a material to withstand the heat, without appreciable deformation or softening under particular servise conditions. In general, measured as the softening or melting temperature of the material. Most of the refractory materials are mixture of metallic oxides, they do not have a sharp melting point. Pyrometric Cones Test (Seger Cones Test) The softening temperature of the refractory materials are determined by using Pyrometric Cones Test. Expressed in terms Pyrometric Cone Equivalents (PCE) Softening Temp. of Mat. to be used as Refractory ≫ Operating Temp. 50  Pyrometric Cones Test (Seger Cone Test) Refractoriness is determined by comparing the behaviour of the heat on cone of material to be tested with that of a series of Seger Cones of standart dimensions. Pyrometric cone is a refractory specimen of standart dimension (38 mm hight abd 19 mm triangular base) and composition Seger cones melt or fuse at definite temperature when heated under standart conditions of 10 C/min. ISO 528:1983--; EN 993-12: The temp. at which the fusion or softening of the test cones occures is indicated by its apex touching the base. 51  Pyrometric Cone Equivalent (PCE) Number PCE Number: Value representing the number of standard cones which also fuses with the test cone for a given refractory. 52 2) Strength of Refractories Under Load (RUL) It is used to determine the temperature at which a standard dimensioned specimen of a refractory undergoes 10 % deformation with constant load of 3.5 or 1.75 kg/cm2 ( standart heating rate 10 C/min.) The RUL test gives only an index of refractory quality rather than a figure which can be used in a refractory design. TS EN ISO 1893 53  TS EN ISO 1893 Ta; temperature at which 0.6 % def. occurs (3 mm deflection) Tc ; temperature at which 40 % def. occurs (20 mm deflection) Usage temp. of refractory＜ Ta Safe Ref. having narrow range between Ta-Tc 54 3)Dimensional Stability Resistance of a material to any volume change which may be occur on its exposure to high temperature, over a prolonged time. Dimensional change can be permanent or reversible Irriversible change may result either in the contraction or expansion of a refractory 55 56 4) Porosity All refractories contain pores, either due to manufacturing methods or deliberately made (by incorporating saw-dust or cork during manufacture). Porosity is the ratio of its pore’s volume to bulk volume. TS EN 993-1 57 58 5) Thermal Spalling Property breaking, cracking or peeling of refractory material under high temperature. Thermal spalling mainly due to 1.Rapid change in temperature This causes uneven expansion and contraction with the mass of a refractory, leads to development of uneven stresses and strain. 2.Slag penetration This causes variation in the coefficient of expansion, leads to spalling. Thermal spalling can be decreased by a)Using high porosity, low CTE and good thermal conductivety b)Avoiding sudden temperature change c)By modifiying furnace design. 59 6)Thermal expansion Expansion of a refractory material when exposed to heat for a longer duration. Refractory expansion has an impact on the capacity of the furnace lifetime. Reapeted expansion and contraction of refractory materials due to thermal impact causes wear, tear, breakdown… Refractory materials should have least possible thermal expansion. TS EN 993-19 60 7) Thermal conductivity A good heat conductivity of the refractory material is desirable for effective heat transmission in furnace construction. The densest and least porous brick have the highest thermal conductivity, owing to the absence of air-voids. In porous bricks, entrapped air in pores acts as a non heat conducting materials. Thermal conductivity depends on Temperature Chemical and mineralogical composition of the brick Porosity, pore size Th.cond. at 1000C (W/m.K) Brick firing temperature TS EN 993-15 61  Classification of Refractories Refractories are generally classified based on different parameters, e.g.: (a) Basicity of oxides (b) Form (c) Manufacturing process (d) Method of application (e) Special chemistry (f) Insulating property 62  Refractories By Basicity 1. Acidic refractories are those refractories which readily react with bases at high temperatures. The most common examples of acidic refractories are fire clay and silica. 2. Basic refractories Complimentary to the idea of acidic refractories are basic refractories. These often react with acids at high temperatures. Magnesite and dolomite. 3. Neutral refractories Neutral oxides are prized for their lack of reactivity with acids and bases, and are often considered superior refractories for their wide utility and performance over a broad number of applications. Neutral refractories are used across many applications, as they are tolerant to both acidic and basic atmospheres and slags Table: Classification of refractories based on basicity. 63  By Insulating Property of Refractory Most refractory linings are composite in nature and made of the combination of - dense refractories, in front, to contain the high temperature and to withstand the harsh operating condition and - insulating refractories, at back, to contain the heat and protect the energy loss from the system. The main features of insulating refractories are the high porosity and low bulk density which leads to low thermal conductivity and low mechanical strength of the insulating refractory. Insulating refractories are made out of aluminosilicate range of materials because of their inherent lower thermal conductivity compared to many other materials. The insulation product can be designed for continuous use at a very high temperature (1600 °C). 64  Besides normal insulating bricks and castables, there are other types of insulating refractories available made of ceramic fiber. The advantages of ceramic fiber lining are: 1. Ceramic fibers have very low bulk density. 2. Ceramic fibers are virtually completely resistant to thermal shock. Ceramic fiber lined furnaces can be heated up very fast and can be cooled also very fast. 3. Ceramic fiber can withstand thermo-mechanical stresses better than insulation bricks or castables. 4. Ceramic fiber lining is easier and cheaper to install. 65  Silica brick made from naturally occurring sources of silica and bonded by adding 3.0–3.5% CaO to promote liquid phase sintering. Semisilica brick. A silica brick containing 18–25% Al2O3. Fireclay brick. Made from kaolinite (Al2O3.2SiO2.2H2O) with 25–45% Al2O3. High-alumina brick. Alumina content in the range 45–100 wt.%. Dolomite brick. Made from dolomite (CaCO3.MgCO3). Magnesia brick. Contains mainly MgO (typically >90% MgO). Chrome brick. Made from naturally occurring chrome ore. Contains 34% Al2O3 and 30% Cr2O3. Often MgO is added to produce chrome-magnesia brick. Zircon refractory brick. Zircon is ZrO2.SiO2. Bricks may contain 4% CaO. 66 Diagram of a blast furnace indicating the type of refractories used in each region. 67  Silica Refractories Silica refractories contain a minimum of 95% SiO2. The common impurities are CaO, Fe2O3, and Al2O3. The main raw material used, is quartzite. The combined Al2O3 and TiO2 should be as low as possible, ＜2% Alkali ＜0.3% and for high-quality products it should be below 0.1%. Various grades of silica brick have found extensive use in the iron and steel melting furnaces and the glass industry. 68 Properties of Silicate Refractories; High resistance to thermal shock( above 600 C) and high thermomechanical resistance up to 1600 C. It does not begin to soften under high loads until its fusion point is approached. (alumina silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points. ) High flux and slag resistance, volume stability and high spalling resistance 69  Quartz shows polymorphic transformation to different crystalline forms when heat treated at different temperatures with some being reversible and some irreversible in nature. Volume changes occur in silica associate with polymorphic transformation. The horizontal arrows in figure indicate the transformations which occur slowly, and the vertical arrows show the transformations that occur fast. The suitable quartzite for manufacturing of silica refractories must not degrade fast on heating!!! 70  The degeneration can be measured by testing the sp. gravity of the quartzite after firing at 1460 °C from ambient temperature and holding for 1 h at peak temperature. Degeneration behavior of quartzit. The milk of lime (slurry of Ca(OH)2 in water) is used as binder which provides 1–2.5% CaO, in the composition, that acts as a mineralizer and speeds up the polymorphic transformations in the horizontal direction. The addition of Fe2O3 also promotes the conversion of quartz to desirable phase. 71  CaO reacts with SiO2 above 600 °C and forms β-2CaO.SiO2(C2S) and 3CaO.SiO2 (C3S), which play a significant role in the development of mechanical strength in silica refractory. Tridymite is the most desirable phase in silica refractories although all the crystalline forms of silica exist in silica refractory. Residual or free quartz is the most undesirable phase in a silica refractory. The main problem associated with the existence of free quartz is its conversion to tridymite and cristobalite in service which is accompanied by 14% volume expansion and causes crack to the refractory. The determination of specific gravity also gives a good indication about the presence of free quartz in the refractory body. A silica brick without free quartz should have a sp. gravity below 2.34. Silica refractories are prepared from grounded quartzit mixing with CaO for mineralizing and sulphite solution as binder. After shaping with hydraulic press, bricks are fired 1450-1500ºC for 200-330 hours. 72  Aluminosilicate Refractories The main chemical constituents of this category of refractories are alumina (Al2O3) and silica (SiO2). If the Al2O3% is less than 50%, it is normally categorized as fireclay refractory, and refractory with more than 50% Al2O3 is called high alumina refractory, Fireclay The main raw material used for fireclay refractories is calcined fireclay or chamotte, along with plastic fireclay. Fire-clay brick comprise about 75% of the production of refractories on a volume basis. Fireclay refractories have alumina and silica as main constituents. The other oxides which remain present as impurities are Fe2O3, TiO2, CaO, MgO, K2O, and Na2O. All these oxides are acting as flux and bring down the softening temperature of the refractory Typical composition consists of SiO22) Densities ~3.50 g/cc or more Large periclase crystal sizes (1000 microns minimum) 83  Due to its relatively high chemical stability, strength, resistance to abrasion, and excellent corrosion resistance, refractory-grade fused magnesia is used in high wear areas in steelmaking, like the molten metal and slag contact areas. The important properties to characterize the magnesia raw material are as follows: 1. Bulk density and grain porosity 2. The impurities and their distribution 3. CaO/SiO2 ratio 4. Boron content 5. Periclase crystal size 84 85 Phase diagram of magnesia with lime 86 (CaO) and iron oxide (Fe2O3) 87  MAIN APPLICATION AREAS OF MAGNESIA Highly pure magnesia refractories (MgO ∼95%–97%, CaO ∼2%–1.5%, and SiO2 ∼1%– 0.5%) are useful for the upper part of the glass tank furnace regenerator checker work. Resistance to alkali and sulfur attack is of prime importance in that region. The next-level purity class magnesia refractories (MgO ~95%, CaO ~2%, and SiO2 ~2%) are used in a lime kiln, where lime stone is calcined, and high basic environments prevail. Magnesia refractory with 92% MgO and impurities about 2% CaO and ∼4% SiO2 are used for the hot metal mixer in the iron and steel industries. Backup of the lining of converters, ladles, and electric arc furnaces of steel industries are done by using relatively less pure magnesia refractories (MgO ∼87%, CaO ∼2%, and SiO2 ∼6%). Relatively inferior magnesia refractories containing about 50% MgO are used for the reheating furnace hearth applications. 88  ZIRCON AND ZIRCONIA REFRACTORIES Zircon (ZrSiO4) is the silicate of zirconium, containing 67% ZrO2 and 33% SiO2. It is used as a refractory material directly and also used as a raw material for zirconia (ZrO2). The main reason for these materials to become a refractory is the presence of zirconia in them, which is high melting, chemically inert, and imparts special mechanical properties in the refractories. Zircon has a wide range of applications as a ceramic and refractory material, for example, As refractory, it is mainly used for the construction of glass tank furnaces and nozzles for iron and steel industries. As molds and cores in precision, investment casting. As protective coatings on steel molding tools. As an opacifier in the glaze for ceramic and whiteware industries due to its high refractive index. As the principal precursor for the preparation of metallic zirconium and its compounds, like zirconia. 89 The wide range of applications of zircon is due to its excellent thermophysical properties such as low thermal expansion, good thermal stability, high corrosion resistance against glass melts, slag, and liquid metals and alloys. Zircon is one of the most chemically stable compounds, and mineral acids other than HF cannot attack zircon. Very aggressive reaction conditions are required to break down the strong bonding between the zirconium and silicon parts of the compound. The main feature of the zircon refractories is their acidic slag resistance, excellent thermal shock resistance, good mechanical strength with wear and erosion resistances. 90  Dissociation of Zircon Zircon (ZrSiO4) decomposes by a solid-state reaction, and chemically pure zircon decomposes in solid state at 1676 °C to form a mixture of tetragonal zirconia and cristobalite. ZrSiO4 t-ZrO2 + SiO2 (Cristobalite) But the presence of impurities reduces the decomposition temperature, which may even start at 1285°C. The more the impurities present in zircon, the lower the onset temperature of dissociation. To reduce the degree of zircon degradation, impurities, like iron, titanium, aluminum, and alkali must be minimum. 91 Zircon refractories are used as ladle nozzles for steel pouring, tundish metering nozzles, in furnaces for melting aluminum, as compounds and coatings. Zircon refractories with at least 63% zirconia and maximum 20% apparent porosity are used in glass tank furnace. It is also an ideal mold and chill sand due to its low thermal expansion rate, high thermal conductivity, and non-wettability with molten metal. Zircon is also used in core and mould washes to improve surface finish. 92  Preparation of Zircon Based Refractories Zircon-based refractories are prepared by using beneficiated zircon aggregate or sand, fine- milled zircon, and a temporary binder. It is then pressed into desired shape and size, and finally the shapes are fired. The firing temperature must be lower than the decomposition temperature of zircon. 93  ZIRCONIA REFRACTORIES Zirconia is mainly obtained by chemical processing of zircon or by carbothermal reduction (fusion) of zircon in an electric arc furnace. Zirconia has a natural source, called baddeleyite, commonly found in igneous rocks containing felspar, zircon, etc. Baddeleyite is chemically homogeneous, Main impurities ; hafnium (Hf), titanium (Ti), and iron (Fe). Zirconia is an important refractory material due to its excellent high-temperature properties and chemical inertness. But high cost and cracking tendency (due to polymorphic transformation) have restricted its wide application!!! At ambient condition, zirconia has a monoclinic structure, which instantaneously changes to tetragonal form during heating around 1180°C and from the tetragonal to the cubic form around 2370°C. This polymorphic transformation causes a huge volume expansion during cooling and cracking and failure of the material. Hence for any high-temperature application, zirconia has to be stabilized, with CaO/MgO/CeO2/Y2O3. 94  Zirconia refractories have refractoriness above 2000°C and also have excellent chemical resistance to the action of melts, alkalis, and most of the acids. But due to the high cost, they are only used for very specific critical applications. The main application areas are the metering nozzle in tundish (for continuous casting of steel) inserts in the bore area in the slide gate plates (in steel ladles). In combination with carbon, it is used as a band in the subentry nozzle (SEN) to resist the corrosive attack of mold powders during continuous casting, etc. Zirconia is mostly used as a component in a refractory composition mainly to improve mechanical, corrosion, and thermal shock properties. It is also used as crucibles for melting platinum, palladium, and other metals and quartz glass, and also in the construction of nuclear reactors. Lightweight zirconia products, fibers, and granular powders are suitable for high-temperature thermal insulation. It is also used as heating elements at temperatures up to 2200°C in furnaces with resistive and induction heating. 95  Fused Cast Al2O3–ZrO2–SiO2 (AZS) Refractories Fused cast Al2O3–ZrO2–SiO2 refractories, abbreviated as AZS, are important for their corrosion resistance and wear properties mainly due to its compositions and considerable ZrO2 content in them. They are the most widely used materials both in glass contact areas and superstructure of glass melting furnaces due to their very high resistances against glass and alkali vapor. These refractories are further divided as per ZrO2 content. According to ASTM C 1547-02, these refractories are classified by the amount of monoclinic zirconia (ZrO2) present, as determined by chemical analysis or quantitative image analysis. 96 Heindrich R., Gupta A.,”Fused Cast AZS Glassy Phase Exudation: Intrinsic or Pathologic Property?” RHI Bulletin , 2, 2011, pp.24-28  The refractory consists of two major interlocked crystalline phases (i.e., corundum and zirconia) and one glassy/vitreous phase that fills the interspace of the crystalline structure. Therefore, fused cast AZS has no open porosity, which is one reason for the high corrosion resistance to molten glass. The glassy phase can, above its transition temperature, relax mechanical stress present in fused cast refractories that could result in block cracking. The composition of this viscous alumina silicate melt phase is approximately: 70% SiO2, 20% Al2O3, 5% Na2O, and 2% ZrO2 with some wt.% of other oxides. Whilst the weight percent of glassy phase in an AZS block amounts to about 22%, the volume equivalent is approximately 33%. 97