CERAMIC MATERIALS I. Asst. Prof. Dr. Ayşe KALEMTAŞ

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1 CERAMIC MATERIALS I akalemtas@mu.edu.tr, akalemtas@gmail.com, Phone: Metallurgical and Materials Engineering Department

2 Traditional Ceramics Clay products Main Components Clay Feldspar Silica

3 Clay products Main Components Clay When mixed with water the crystals can easily slide over each other (like a pack of cards), and this phenomenon gives rise to the plasticity of clays. Provides plasticity, when mixed with water Hardens upon drying and firing (without losing the shape) Adding water to clay -- allows material to shear easily along weak van der Waals bonds -- enables extrusion -- enables slip casting Silica Silica, SiO 2, is mixed with clay to reduce shrinkage of the ware while it is being fired, and thus prevent cracking, and to increase the rigidity of the ware so that it will not collapse at the high temperatures required for firing. Silica is useful for this purpose becasue it is hard, chemically stable, has a high melting point and can readily be obtained in a pure state in the form of quartz. Feldspar Feldspars are used as a flux in the firing of ceramic ware. When a body is fired, the feldspar melts at a lower temperature than clay or silica, due to the presence of Na +, K + or Ca 2+ ions, and forms a molten glass which causes solid particles of clay to cling together: when the glass solidifies it gives strength and hardness to the body.

4 SILICA (SiO 2 ) Silica (SiO 2 ) is an important raw material for the production of glass, glazes, enamels, refractories, abrasives and whiteware. Major SiO 2 sources are in the polymorphic form quartz, which is the primary constituent of sand, sand-stone, and quartzite. Quartz is the second most abundant mineral in Earth s crust. The major use (accounting for about 38% of U.S. production) is in glass manufacture. For example, incandescent lamp bulbs are made of a sodalime silicate glass containing about 70 wt% SiO 2. The SiO 2 content of high-quality optical glasses can be as high as 99.8 wt%. The United States is the largest producer of industrial sand in the world. Annual production of silica in the United States is approximately 30 Mt, valued at around $700 million.

5 Quartz Minerals Quartz Silica tetrahedra alone can form a neutral three-dimensional framework structure with no need for other cations. This arrangement forms a very stable structure. Popular as ornamental stone and as gemstones Amethyst is the purple gemstone variety. Citrine is a yellow to orange gemstone variety that is rare in nature but is often created by heating Amethyst. Milky Quartz is the cloudy white variety. Rock crystal is the clear variety that is also used as a gemstone. Rose quartz is a pink to reddish pink variety. Smoky quartz is the brown to gray variety.

6 SILICA (SiO 2 ) Polymorphs are materials that have the same chemical composition but different crystal structures. Many ceramic materials show this behavior, including SiO 2, BN, BaTiO 3, ZrO 2 and BeO. Transitions between the different polymorphs may occur as a result of changes in temperature or pressure.

7 Phase Transitions Displacive transition is a transition in which a displacement of one or more kinds of atoms or ions in a crystal structure changes the lengths and/or directions of bonds, without severing the primary bonds. Reconstructive transition is a transition which involves a major reorganization of the crystal structure and a change of local topology, during which primary bonds are broken and reformed so that there is no immediate relationship between the crystal structures of the parent and product phases.

8 Phase Transitions The relationships between the polymorphic forms of silica High Quartz Reconstructive 867 C High Tridymite Reconstructive 1470 C High Cristobalite Displacive 160 C Displacive 573 C Middle Tridymite Displacive C Displacive 105 C Low Quartz Low Tridymite Low Cristobalite

9 Phase Transitions The transformations between the basic structures (quartz, tridymite, and cristobalite) are necessarily reconstructive. Therefore, they are relatively slow and thermally activated. Some conversion of quartz to cristobalite may occur during anneals at high temperature, but the reverse transformation will not occur simply because the kinetics of transformation are extremely slow in the temperature regime in which, say, quartz is thermodynamically stable. For whitewares the duration of firing is usually too short for reconstructive transformations to be significant, although this may not the case for silica refractories which are typically fired above 1400 C. Significant conversion to tridymite and cristobalite may also occur during service at high temperature, and the presence of a lime flux will enhance the rate of conversion. In sharp contrast, the displacive transitions within a polymorph cannot be suppressed and are extremely rapid.

10 Phase Transitions The key consequence of interest in ceramic processing is the dimensional changes which occur during the - to -quartz, low- to high-tridymite, and low- to high-cristobalite transformations. The molar volume of quartz is a smooth function of temperature except for the phase transformation which occurs at 573 C which causes a sudden change in volume on the order of one percent. Interestingly, high quartz exhibits a negative thermal expansion coefficient. Tridymite exhibits a smaller displacive volume change at 105 C, but also exhibits negative expansion above 575 C. In the case of cristobalite the transformation occurs at a low temperature, about 215 C, and the volume change is roughly three percent, that is, significantly larger than in the case of the other two polymorphs.

11 SILICA (SiO 2 ) Polymorph Density (g/cm 3 ) Crystal Structure Tridymite 2.28 Hexagonal Cristobalite 2.33 Cubic Quartz (Beta) 2.53 Hexagonal Quartz (Alpha) 2.65 Rhombohedral Cristobalite and tridymite also have high and low forms. Low tridymite is orthorhombic and pseudohexagonal, low cristobalite is tetragonal and pseudo-cubic. At pressures around 2 GPa, quartz transforms into coesite. At even higher pressures, around 7.5 GPa, coesite transforms to stishovite. The high pressure forms have been prepared experimentally and are also found at the famous Canon Diablo Meteor site in Arizona.

12 SILICA (SiO 2 ) Influence of temperature on the expansion of the various forms of silica

13 SILICA (SiO 2 ) When a ceramic is fired, the sand can react, particularly with the fluxes. This reaction is seldom complete. The transformation of residual quartz into cristobalite can then start from 1200 C onwards. It is favored by the rise in temperature, the use of fine grained sand, the presence of certain impurities and a reducing atmosphere. The form in which silica is found determines the thermal properties of silicate ceramics. Thus, quartz and cristobalite do not have the same influence on the expansion of the shard. Quartz can also cause a deterioration of the mechanical properties of the finished product owing to the abrupt variation in dimensions (ΔL/L 0.35%) associated, at 573 C, with the reversible transformation quartz β quartz α. As the crystal of cristobalite formed from the flux are usually small, the transition cristobalite β cristobalite α, which occurs at about 220 C, often causes less damage to the finished product. It can even contribute to the shard/enamel fit by compressing it after cooling at room temperature.

14 SILICA (SiO 2 ) For each form, at low temperatures (the α phase) we find a structure that is a distortion of the high-temperature form (the β phase). In each case, changing from the α to β structure involves a displacive phase transformation; the atoms need to move only slightly relative to one another. However, to change from one form to another requires breaking bonds. This process is much more difficult and is known as a reconstructive phase transformation. The Si O Si arrangement of ions does not always lie exactly on a straight line, especially for the low temperature forms. If the bonding were purely ionic, the line would be straight and the O 2 should lie exactly in the middle: the reason in each case is that we want to maximize the electrostatic attractive forces and minimize the electrostatic repulsion. However, the Si O bond is 60% covalent, so there is a strong tendency toward directional bonding.

15 CARBON Elemental carbon exists in nature mainly as two allotropes, diamond and graphite. Graphite has a variety of uses: writing medium in pencils; electrodes; high-temperature devices (crucibles, rocket nozzles); and strong graphite fibers. High-quality diamonds are used in jewelry. Most diamonds are used as abrasives, in industrial drill bits, saw blades, etc. Diamond is the hardest substance known and has a high thermal conductivity (dissipates heat quickly). Carbon also exists in amorphous forms, such as coke, charcoal, and carbon black.

16 CARBON FORMS Elemental carbon exists in nature mainly as two allotropes, diamond and graphite. An allotrope is a variant of a substance consisting of only one type of atom. It is a new molecular configuration, with new physical properties. Substances that have allotropes includecarbon, oxygen, sulfur, and phosphorous. Allotropes of a given substance will often have substantial differences between each other. For example, one allotrope of carbon, fullerene, is many times stronger and lighter than steel. An allotrope should not be confused with phase, which is a change in the way molecules relate to each other, not in the way that individual atoms bond together.

17 Carbon Forms - Diamond Carbon black amorphous surface area ca m 2 /g Diamond: Carbon with a cubic crystalline structure with covalent bonding between atoms *hard no good slip planes *brittle can cleave (cut) it *large diamonds jewelry *small diamonds often man made - used for cutting tools and polishing diamond films *hard surface coat cutting tools, medical devices, etc.

18 Carbon Forms - Diamond Carbon with a cubic crystalline structure with covalent bonding between atoms This accounts for high hardness Industrial applications: cutting tools and grinding wheels for machining hard, brittle materials, or materials that are very abrasive; also used in dressing tools to sharpen grinding wheels that consist of other abrasives Industrial or synthetic diamonds date back to 1950s and are fabricated by heating graphite to around 3000 C (5400 F) under very high pressures

19 Carbon Forms - Graphite Form of carbon with a high content of crystalline C in the form of layers Bonding between atoms in the layers is covalent and therefore strong, but the parallel layers are bonded to each other by weak van der Waals forces This structure makes graphite anisotropic; strength and other properties vary significantly with direction As a powder it is a lubricant, but in traditional solid form it is a refractory When formed into graphite fibers, it is a high strength structural material

20 Carbon Forms - Graphite layer structure weak van der Waal s forces between layers planes slide easily, good lubricant

21 Carbon Forms - Fullerenes and Nanotubes Fullerenes or carbon nanotubes wrap the graphite sheet by curving into ball or tube Buckminister fullerenes Like a soccer ball C60 - also C70 + others Adapted from Figs & 12.19, Callister 7e.

22 THE END Thanks for your kind attention

23 Any Questions