علم سرامیک

is well known to producers of tiles and bricks who modify the atmospheres – oxidizing or reducing – of the kilns; – by considering now the raw materials, we can find a ternary composition, because the three components of silicate ceramics are: i) clays, ii) sand and iii) fluxes – i.e. compounds contributing to the firing thanks to the development of phases with low melting points. As kaolinite clay can be written: Al2O3-2SiO2- 2H2O, quartz sand: SiO2, and potassic feldspar, which is frequently used as a flux: K2O-Al2O3-6SiO2, we again find the ternary SiO2-Al2O3-MxOy (if MxOy = K2O). Figure 1.1 shows the equilibrium diagram Al2O3-SiO2-MgO and locates some of the main compounds that come under it [KEI 52, KIN 76]. Magnesia MgO is useful for refractory materials in iron metallurgy; mullite 3Al2O3-2SiO2, a unique compound defined in the binary diagram Al2O3-SiO2, is a crystallized phase present in many ceramics; cordierite 2MgO-2Al2O3-5SiO2 is characterized by very poor thermal expansion: it is used for example as catalyst support in exhaust pipes, etc. Hydroxyls OH- are present in many hydrated raw materials and water H2O allows the plasticity of clays, but because the ions and the corresponding molecules are eliminated in the heat treatments (this is called ignition loss), they are not taken into account in the composition of ceramics after firing. It is important to distinguish between impregnated water (which occurs as a mixture with rock particles and whose reversible departure is caused by simple drying, with possibility of rehydration in a wet environment) and combined water (which corresponds to the hydroxyls of the hydrated phases, for example to the four OH- in the formula of kaolinite Al2(Si2O5)(OH)4). The departure of this “water” is accompanied by the disturbance of the crystallographic structure, hence the irreversible transformation at the end of firing beyond approximately 500°C. Silicate ceramics make the most of the versatility of silica (see section 1.5.2), which can exist in crystallized form (particularly quartz) or in amorphous form (silica glass) and, as a result, contain both crystallized phases and vitreous phases. The interatomic bonds brought into play in silicate ceramics are typically iono- covalent (SiO2 exhibiting a fine compromise, because its bonds are regarded as 50% ionic and 50% covalent), therefore these ceramics are almost always electrical insulators. The accentuation of the ionic nature yields hydrolysable compounds: halides can be regarded as ceramic compounds, but the salt-marsh workers are not classified among the producers of ceramic powders! 14 Ceramic Materials Alumina 1,925 2,050 Corundum 2,030 1,713 Silicia Two liquids Magnesia 2,800 Magnesia refractory materials 1,810 2,135 Figure 1.1. Main compounds in the diagram Al2O3-SiO2-MgO [KIN 76] 1.4. Non-silicate ceramics To classify any material, the user can consider two main categories: i) structural materials, whose operating performances are essentially mechanical, even thermal, in nature, and ii) functional materials, whose operating performances are primarily electrical, magnetic, optical, etc. We have said “primarily”, because we must underline, especially in a book of this nature, that no application can be exempt from mechanical properties. For example, the glasses in our spectacles are functional materials, designed in such a manner that their optical characteristics correct the defects in our vision, but their impact resistance or their scratch resistance are variables that are more difficult to improve than the optical properties. In addition to their functionalities, functional materials must in general exhibit a sufficient level of mechanical properties. Ceramic Compounds: Ceramic Materials 15 1.4.1. Structural ceramics The uses of these ceramics vary according to their characteristics: – for ceramics with high mechanical performances, the established markets include abrasives, cutting tools and tribological applications: wear resistance and friction resistance (see Chapters 6, 7, 8 and 9); – for ceramics used at high temperatures, the established markets include refractory materials, essential for equipments of iron and steel, glass, cement or incineration industries (see Chapter 10); – for ceramics that must combine high mechanical performances and high temperatures the markets are more recent, but are growing rapidly. Only these ceramics are sometimes referred to as structural ceramics, but it is better to call them thermomechanical ceramics [CHE 89] (see Chapter 7). Thermostructural composites form the vanguard of thermomechanical ceramics: we will not discuss these composites here. The first two subdivisions that we have classified here among structural ceramics (abrasives, cutting tools and wear parts on the one hand, industrial refractory materials on the other) are often classified outside the field of ceramics. This is logical for abrasives, because if abrasive grains are ceramic compounds (primarily alumina Al2O3 or silicon carbide SiC), abrasives are themselves multi-material systems, for example, grinding stones whose matrix can be a glass or a ceramic, but frequently also a polymeric resin or a metal, or fabrics and papers (sandpaper) whose base is organic. The most widely used cutting tools and wear parts are made of tungsten carbide (WC) grains bonded together by a metal matrix, typically of cobalt. These cemented carbides fall under the category of cermets (for “ceramic- metal”), which are materials prepared by powder metallurgy, and this explains why they are claimed by ceramists and metallurgists. Our choice has been to include cermets among structural ceramics and to cover them in Chapter 9. Finally, as regards industrial refractory materials, their importance sometimes justifies their being regarded as a distinct category when we speak of “ceramics and refractory materials”. Here again, our choice has been to include refractory materials among ceramics (see Chapter 10), which is currently the commonly accepted view, but this does not however imply that refractory materials are always classified among structural ceramics. These remarks are essential to decipher economic data: spread across the three categories that we are considering here, structural ceramics represent a larger market than that of functional ceramics about which we will speak later on, but if reduced to thermomechanical ceramics and thermostructural composites, they represent only a small market in comparison with functional ceramics. 16 Ceramic Materials 1.4.2. Functional ceramics Functional ceramics are characterized by their: – electrical properties: insulators (very often), semiconductors (often), conductors (less frequently) and superconductors (a scientifically exciting field, but whose industrial applications are yet to be explored at the time of writing); – magnetic properties: hard magnets (permanent magnets) or soft magnets (winding cores); the field of magnetic recording is among one of the most spectacular scientific and technical advances with enormous industrial stakes; – optical properties; – chemical properties: catalysis, sensors; – “nuclear” properties: fuels, moderators; – biological properties: biomaterials and prostheses; – monocrystals for varied uses, for example for ionizing radiation detectors. Unlike silicate ceramics, raw materials used for the preparation of non-silicate ceramics are generally synthetic powders and not mixtures of crushed rocks. But these synthetic powders can result from natural products, which the English terminology makes easy to understand by distinguishing between “starting materials” (for example, alumina powders) and “raw materials” (bauxite rocks, in this case, whose treatment by the Bayer process yields the alumina powders) [CAS 90]. Ceramic compositions offer in general a simple chemistry, but microstructural parameters are complex. 1.4.2.1. Oxide ceramics Alumina Al2O3 is by far the foremost basic compound for “technical ceramics”, because alumina exhibits exceptional versatility: abrasion, cut, friction and wear, refractory uses, electricity and electronics, optics, biomedical, jewelry and the list can go on and on [CAS 90]. Silica SiO2 is also a basic compound both for ceramists and glassmakers; the alumina-silica diagram has for ceramists the same importance as the iron-carbon diagram has for metallurgists. Ceramic Compounds: Ceramic Materials 17 Magnesia (MgO) and spinel (MgAl2O4) are primarily used as refractory materials in the iron and steel industry. Zirconia ZrO2 (not to be confused with zirconium silicate, called zircon, ZrSiO4) is used in the ceramic colors, but also for ionic conduction, mechanical purposes or in jewelry. Uranium oxide UO2 is the basic constituent of nuclear fuels, if necessary, as a mixture with a little plutonium oxide PuO2 (the mixture gives MOXs, or “mixed oxide nuclear fuels”). Barium titanate BaTiO3 is dielectric or a semiconductor, depending on its doping and its stoichiometry. It is the basic material in the industry of ceramic capacitors and it is also used for the manufacture of various types of probes and sensors. Soft ferrites and hard ferrites or hexaferrites are important materials for magnetic uses. “Soft” ferrites are crystals with a spinel structure whose reference is magnetite Fe3O4; “hard” hexaferrites are crystals with a hexagonal structure, whose type is BaFe12O19. Almost all metallic oxides have uses in ceramics, for example yttrium oxide Y2O3, beryllium oxide BeO, zinc oxide ZnO, tin oxide SnO2, superconductive cuprates like YBa2Cu3O7, and others. Most ceramic oxides are electrical insulators, whose electronic conduction is very weak (major exception: superconductors), but whose ionic conduction can be remarkable (for example, zirconia); those oxides that are semiconductors are frequently extrinsic semiconductors, whose performances vary considerably with the nature of the doping agents and their concentration. 1.4.2.2. Non-oxide ceramics Carbides form the main category of non-oxides [MCC 83], the most important of which are silicon carbide SiC, which is a semiconductor, but whose chemical is essentially covalent, and tungsten carbides, whose name comes from a typically metallic band structure, which therefore exhibits high electronic conductivity: tungsten carbide WC is the main industrial material in this class, which includes many other compounds, for instance, titanium carbide TiC. Nitrides primarily include silicon nitride Si3N4 and aluminum and silicon oxynitrides, also called sialons, aluminum nitride AlN, and various metallic nitrides, including TiN. 18 Ceramic Materials Some borides have industrial applications, for example titanium diboride TiB2 or lanthanum hexaboride LaB6, and some boron compounds are conventionally classified among borides, including boron carbide B4C and boron nitride BN, a material which has three polymorphs, including two isostructural carbon polymorphs: graphite and diamond. Silicides are numerous, but only one of them presents great industrial interest: molybdenum disilicide MoSi2 (not to be confused with molybdenum disulphide MoS2), which is used for the manufacture of the heating elements of very high temperature electric ovens (1,750°C), in air. Halides, finally, are more model materials in the chemistry of solids than usable ceramics, even if some of them are used for their optical properties; some chalcogenides could also enter the field of ceramics. This list omits a class of materials that has not yet been mentioned, in spite of its importance: carbonaceous materials – diamond, graphite, and more or less crystallized carbons that are obtained by heat treatments of tar and pitch, not to mention carbon fullerenes and nanotubes, which have not yet actually reached the stage of industrial products. Although some carbonaceous materials are prepared from organic raw materials, the trend is to classify the materials themselves under inorganic products: we endorse the term black ceramics [LEN 92]. Whereas oxides are mainly electrical insulators, non-oxides equally include insulators (for example, Si3N4 and AlN), semiconductors (for example, SiC) and conductors (for example, “metallic” carbides and borides and carbon products other than diamond, of which graphite is the most important). 1.5. Ceramic structures and microstructures 1.5.1. Ceramic structures This discussion on the crystalline structure of ceramics presupposes that the reader is familiar with the basics of crystallography [BUR 90, GIA 85, HAH 89]. Most oxides and silicates have crystalline structures obeying Pauling’s rules for ionic crystals, where the ions of small size (generally cations) enter the interstices of big ions (generally anions). The three main rules relate to the coordination number of cations, the coordination number of anions and the coordination number of polyhedra. Ceramic Compounds: Ceramic Materials 19 The cation coordination number (Nc): the geometry of the polyhedron of anions around a cation depends on the ratio R = rcation/ranion. The cation must be in contact with the anions. The ionic radius varies, for a given element, with the charge of the ion and its coordination number. The anion coordination number (Na): the geometry of the polyhedron of cations around an anion is such that the sum of the electrostatic attractions resulting in the anion is equal to the charge (p) of this anion. For Mq+ X p-: s = electrostatic attraction of the link = q/Na Σs = p The force of the link is obtained by dividing the charge of the cation by its coordination number: EXAMPLE 1.– NaCl: NaVIClVI: each Na+ at the center of an octahedron of 6 Cl- contributes + 1/VI = 1/6, therefore Na(Cl-) = 6. EXAMPLE 2.– SiO2: SiIVOII 2: each Si4+ at the center of a tetrahedron of 4 O2- contributes + 4/IV = 1, therefore Na (O2-) = 2. The linking of polyhedra: the stability of the crystal decreases if the cations are too close, it decreases; therefore a linking of the coordination polyhedra at the corners is more favorable than at their edges and, even more so than at their faces. Many ceramic compounds have structures that bring into play an appreciably compact stacking of anions with cations in tetrahedral (four neighbors) or octahedral (six neighbors) coordination (see Table 1.1). 20 Ceramic Materials Formula Cation: anion coordination Type and number of interstices occupied Compact cubic stacking Compact hexagonal stacking MX 6:6 4:4 All the octa. Half the tetra. NaCl, FeO, MnS, TiC ZnS blende, CuCl, AgI-γ NiAs, FeS, NiS ZnS würtzite, AgI-β MX2 8:4 6:3 All the tetra. Half the octa. Alternating layers with all the occupied sites CaF2 fluorine, ThO2, ZrO2, UO2 CdCl2 – CdI2, TiS2 MX3 6:2 1/3 of the octa. Alternating pairs of layers with 2/3 of the octa. occupied BiI3, FeCl3, TiCl3, VCl3 M2X3 6:4 2/3 of the octa. Al2O3 corundum, Fe2O3, V2O3, Ti2O3, Cr2O3 ABO3 2/3 of the octa. FeTiO3 ilmenite AB2O4 1/8 of the tetra. and 1/2 of the octa. MgAl2O4 spinel, MgFe2O4 inverse spinel Mg2SiO4 olivine Table 1.1. Coordination and stacking in a few typical structures The structures are varied and we will mention only five of the most important ones (MgO, ZrO2, BaTiO3, Al2O3 and diamond), before discussing the rudiments of the structure of silicates: – MgO is the example of oxides with NaCl structure (space group Fm 3m) with Mg in site 4a (0, 0, 0) and O in 4b (1/2, 1/2, 1/2); Ceramic Compounds: Ceramic Materials 21 – CaF2 (fluorine) and K2O (antifluorine) also crystallize in the space group Fm 3m, with Ca in 4a and F in 8c ± (1/4, 1/4, 1/4); zirconia ZrO2 and urania UO2 adopt this type of structure; – BaTiO3 adopts a perovskite structure, with the oxygen octahedra at the center of which are titaniums, linked at their corners and surrounding a perovskite cage occupied by the large barium. A “beads on rods” representation of this structure places titanium at the eight corners of the cube, oxygen at the twelve centers of the edges and barium at the center of the cube (or barium at the eight corners of the cube, oxygen at the six centers of the faces and titanium at the center of the cube). Cuprate superconductors frequently have structures based on the perovskite structure; – alumina defines the corundum structure where oxygens form a compact stacking with the hexagonal aluminum ions placed in two-thirds of the octahedral sites, which decreases the overall symmetry towards the rhombohedric space group R 3 c; – if it is true that most ceramics have iono-covalent bonds which lead to structures that reasonably obey Pauling’s rules, others are markedly covalent. This is the case with silicon carbide, whose structure is similar to that of diamond (or silicon). We can think of a giant covalent molecule, extended to the scale of a crystal: the network is cubic, face centered and the pattern is composed of two carbon atoms, one located at 0, 0, 0 and the other located at 1/4, 1/4, 1/4; – as regards the various silicates, the description of the structure depends on the manner in which the Si-O bond is modeled. The ionic model predicts a compact stacking of O2-, with Si4+ and the other cations that occur in the various interstices. However, most silicates do not have a compact stacking of O2- and the coordination numbers observed often violate the rules deduced from the rcation/ranion ratio: the ionic model is imperfect. The covalent model describes the Si-O bonds by bonding orbitals, which explains the tetrahedral coordination of silicon and the angles between the bonds are close to the theoretical value of 109.5°. But the covalent model stumbles on some hurdles and explains less well than the ionic model the chemical formulas of most silicates and the substitution of silicon by aluminum, which correspond to formal charges: Si4+, Al3+, O2-, etc. In fact, the Si-O bond is 50% ionic and 50% covalent, the structure of silicates having been described based on tetrahedra [SiO4]4- linked such that: i) the tetrahedra are linked at the corners, ii) a bridging oxygen is common only to two tetrahedra and iii) the formal charges of the ions are Si4+ and O2-. The sequencing of the tetrahedra makes it possible to classify the various silicates under six categories, based on an increasing degree of polymerization [PUT 92]: 1) tetrahedra isolated from one another, without bridging oxygens, Si/O ratio = 1/4, (for example, olivine Mg2SiO4); 22 Ceramic Materials 2) two tetrahedra forming a dimer, with oxygens bridging two tetrahedra, each tetrahedron having one bridging and three non-bridging oxygens: Si/O ratio = 1/3.5; charge of the dimer: [Si2O7]6-, (for example, rankinite Ca3Si2O7); 3) single chain silicates, each tetrahedron having two bridging and two non- bridging oxygens: Si/O ratio = 1/3; a chain with N links has a charge [SiO3]n 2n- (for example, enstatite MgSiO3); 4) double chain silicates, half of the tetrahedra with two bridging and two non- bridging oxygens (Si/O = 1/3) and other half three bridging and one non-bridging (Si/O = 1/(2.5): in total Si/O = 2/5.5 and the charge is [Si4O11]n 6n- (for example, anthophyllite Mg7Si8O22(OH)2, the OHs being independent of the tetrahedra); 5) silicates forming two dimensional layers, each tetrahedron with three bridging and one non-bridging oxygens: Si/O = 1/(2.5); charge of a layer [Si2O5]n 2n- (for example, minerals of clays and micas or talc: Mg6Si8O20 (OH)4, the OHs being here again independent of the tetrahedra); 6) lastly, silicates where the tetrahedra are linked at all their corners: four bridging oxygens per tetrahedron, Si/O = 1/2 (for example, quartz SiO2). Quartz is part, like diamond, of a covalent description where the molecule extends to the scale of the entire crystal, regularly in the three-dimensional space. In addition to this classification, we can observe that: – when Al substitutes Si in the tetrahedron, we must consider the (Al+Si)/O ratio: for example, plagioclase feldspars, which range from albite NaAlSi3O8 to anorthite CaAl2Si2O8, the (Al+Si)/O ratio always being 1/2; – Al is generally in a tetrahedral site, instead of Si, but can be in an octahedral