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

Transcription

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.

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5 Feldspars A group of alumino-silicates, tetrahedra form three-dimensional frameworks with Ca, Na and K as the balancing cations. The very abundant feldspar are subdivided in K-Na bearing alkali feldspars and the Ca-Na solid-solution series called the plagioclase feldspars.

6 Feldspar Group light silicates (K-Na-Ca, Al) K-feldspar Most common mineral group Orthoclase Plagioclase 2-directions of cleavage (at 90 degrees) Ca/Na-feldspar

7 orthoclase KAlSi 3 O 8... K-feldspar Is very hard Has multiply colors Has cleavage Is a mineral... weathers to kaolinite: Al 2 Si 2 O 5 (OH) 4... which can weather further to bauxite: AlO(OH)

8 Four feldspathic minerals are likely to enter the composition of silicate ceramic pastes. They are: Orthoclase, a mineral rich in potassium with the composition K 2 O.Al 2 O 3.6SiO 2 Albite, a mineral rich in sodium with the composition Na 2 O.Al 2 O 3.6SiO 2 Anorthite, a mineral rich in calcium with the composition CaO.Al 2 O 3.2SiO 2 Petalite, a mineral rich in lithium with the composition Li 2 O.Al 2 O 3.8SiO 2 The plagioclase feldspars are solid solutions of albite and anorthite. Na + Si 4+ is replaced by Ca 2+ Al 3+ The alkali feldspars are solid solutions of albite and orthoclase. Na is replaced by K There is virtually no solid solution between anorhite and orthoclase.

9 Feldspars constitute an abundant mineral group and make up an estimated 60% of the earth s crust. They are present in many sedimentary deposits and are found in almost all igneous and metamorphic rocks. The glass industry uses most of the feldspar produced. Feldspar is a source of Al 2 O 3, which improves the mechanical properties of glass such as its scratch resistance and its ability to withstand thermal shock. Feldspar is also used in whiteware bodies as a flux, which produces a glassy phase during firing increasing the strength and translucency of the body. The Republic of Korea is the largest producer of feldspar in the world.

10 Orthoclase and albite, which form eutectics with silica, respectively, at 990 and 1050 C, are widely used as flux. Anorthite is rather regarded as a substitute to chalk. The use of petalite, especially owing to its negative expansion coefficient, is marginal. Potassic feldspar is particularly appreciated by ceramists because its reaction with silica leads to the formation of a liquid whose relatively high viscosity decreases slightly when the temperature increases. This behavior is considered as a guarantee against the excessive deformation of the pieces during the heat treatment. Natural feldspars used for the preparation of ceramics are mineral mixtures. Thus, the commercial potassium products can contain between 2.5 and 3.5% of albite mass, whereas anorthite and a small quantity of orthoclase, between 0.5 and 3.2%, are often present in the available sodium feldspars.

11 Feldspars rarely occur in nature as pure minerals. Albite and anorthite form a complete solid solution series. Even though orthoclase and albite form only limited solid solutions, deposits of orthoclase always contain some albite. The rock nepheline syenite is a mixture of orthoclase, albite and nepheline with minor impurities. These materials are typically ground to a relatively coarse powder, on the order of 70 to 100 m, for use in ceramic (or glass) production.

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13 The tendency to form a glass is strongly correlated to the viscosity of a melt. In general, molten feldspars are rather viscous which is ascribed to the existence of polymerized silicon-aluminum-oxygen tetrahedra in the liquid. Despite lower melting points, the alkali feldspars produce much more viscous liquids than anorthite. In the case of albite this is interpreted as evidence for a higher degree of polymerization in the melt. In the case of orthoclase it is due to the formation of leucite, KAlSi 2 O 6, crystals. In all cases glasses are produced under the cooling rates normally encountered in ceramic processing. Albite melts at the lower temperature than orthoclase, but the addition of anorthite increases the melting temperature of soda feldspar while decreasing that of the potash feldspar (down to a minimum at about 22% anorthite). Similarly a 50%: 50% mixture of albite and orthoclase melts at a lower temperature than either end member. Often mixtures of fluxes are employed in order to take advantage of eutectic melting. Lithium bearing minerals are often very effective fluxes when used in conjunction with feldspar since such combinations form deep eutectics.

14 Traditional Ceramics The materials treated at higher temperatures or in the presence of a large quantity of flux are generally the least porous. Whiteness is primarily the result of the use of raw materials free from iron and titanium or containing only small contents of transition metals. Increasing the relative amount of ball clay generally improves the plasticity as well as the green strength, but often leads to discoloration as a result of contamination by iron-bearing accessory minerals. Therefore applications where green strength is at a premium, and color is of less importance, employ larger amounts of ball clay. Fine china represents the opposite end of the spectrum where aesthetics take priority. China clays are used in these formulations, because they are nearly phase pure and do not occur as iron bearing solid solutions (as do the smectites and illites). The extent of glass formation affects properties such as dimensional stability and degree of densification. In systems which require high dimensional stability such as structural clay products (i.e., large ceramic pipes and tiles) the extent of glass formation is kept to a minimum. In contrast, dental porcelains must fuse at low temperatures to be compatible with metal substructures and therefore may contain in excess of 80% feldspars.

15 Traditional Ceramics The substitution of alumina for quartz The substitution of alumina for quartz increases the strength of the fired ceramic However, it increases density, decreases translucency, and reduces the effective thermal expansion coefficient. The density of alumina, 3.96 g/cm 3, is roughly 50% larger than quartz, 2.65 g/cm 3, therefore in formulations containing fifty weight percent filler the density difference is substantial. There is also a decrease in translucency due to the alumina's higher index of refraction, 1.76, which leads to a greater degree of internal light scattering. The reduction in thermal expansion coefficient is the result of a modestly lower thermal expansion coefficient and the absence of a phase transformation.

16 THE END Thanks for your kind attention

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