Atomic Structure of Ceramics. (different chemical bonding) ... Phase Diagrams of Ceramics. (binary SiO2 â Al2O3 system) ... e.g., asbestos. Ca2Mg5Si8O22(OH)2.
Ceramics, Glasses, Composite Materials Part 2. Atomic structure, Grain and Texture
Professor Igor L. Shabalin
1
Atomic Structure of Ceramics (different chemical bonding)
β-SiC,
covalent, Tdec=2830 0C, E=410 GPa, cubic ZnS, a=0.4349 nm
MgO,
ionic, Tm=2825 0C, E=320 GPa, cubic NaCl, a=0.4208 nm
β-SiC
TiC1-x (0.01≤x≤0.55),
quasi-metallic, Tm=30700C, E=485 GPa, cubic NaCl, a=0.430-0.433 nm 2
MgO, TiC1-x
Phase Diagrams of Ceramics (binary SiO2 – Al2O3 system)
Phase diagrams help to understand the evolution of ceramics microstructure, though diffusion is slower than in metals, so equilibrium is rarely reached
SiO2
ionic (51%), Tm = 1715 0C, E = 70 GPa, Hardness HV = 4 GPa, variety of forms
Al2O3
ionic (63%), Tm = 2050 0C, E = 380 GPa, Hardness HV = 18 GPa, variety of forms 3
Phase Diagrams of Ceramics (binary SiO2 – Na2O system)
4
Atomic Structure of Silica
17 crystalline forms are known for SiO2, as well as amorphous – silica glass; the most important polymorphs:
α (low) - cristobalite, tetragonal β - cristobalite, cubic β - tridymite, hexagonal β (high) - quartz, hexagonal made up of network of tetrahedral SiO4 units (bonding – about 50% covalent) shared corners 5
Atomic Structure of Silica In crystalline SiO2 the atoms are arranged in a regularly repeating pattern in 3D network, i.e. they have long-range order. In contrast, in amorphous form, the atoms exhibit only shortrange order.
6
Atomic Structure of Alumina
Alumina is also a family of oxides that can take many forms; the most important are: α-Al2O3 or corundum, hexagonal, a = 0.476, c = 1.299, c/a = 2.72 β-Al2O3, hexagonal, a = 0.564, c = 2.265, c/a = 4.02 γ-Al2O3, cubic, a = 0.791 δ-Al2O3, hexagonal, a = 0.570, c = 0.290, c/a = 1.18 θ-Al2O3, monoclinic, a = 0.592, b = 0.286, c = 0.1174, β = 103.3O χ-Al2O3, hexagonal, a = 0.557, c = 0.864 7
Lattice parameters in nm
Atomic Structure of Silicates
Mullite (nominally): Al6Si2O13 or 3Al2O3·2SiO2 has real chemical formula –
Al4+2xSi2-2xO10-x or AlVI2(AlIV2+2xSiIV2-2x)O10-x where x = 0.17–0.59 and posseses an orthorhombic structure (a = 0.7579, b = 0.7682, c = 0.2886), which is built up with chains of AlO6 octahedra running parallel to c-axis, which are interconnected by chains of AlO4 and SiO4 tetrahedra. Lattice parameters in nm
8
Atomic Structure of Silicates
Silicates, in contrast of SiO2, have two types of oxygen atoms: bridging and non-bridging; an oxygen atom that is bonded to two Si atoms is bridging, whereas one that is bonded to only one Si atom is non-bridging. Such non-bridging oxygens are formed by the addition of either alkali or alkali-earth metal oxides to SiO2 according to:
where O– denotes a non-bridging oxygen atom 9
Atomic Structure of Silicates
(1) Infinite sheets:
O/Si ratio = 2.5 number of O-atoms per Si: bridging – 3.0 non-bridging – 1.0 e.g., kaolinite clays Na2Si2O5
(2) Double chains:
O/Si ratio = 2.75 number of O-atoms per Si: bridging – 2.5 non-bridging – 1.5 e.g., asbestos Ca2Mg5Si8O22(OH)2 1
to be continued
Atomic Structure of Silicates
(3) Chains (SiO3)n2n–:
O/Si ratio = 3.0 number of O-atoms per Si: bridging – 2.0 non-bridging – 2.0 e.g., metasilicate Na2SiO3, enstatite MgSiO3
(4) Isolated tetrahedra SiO44–:
O/Si ratio = 4.0 number of O-atoms per Si: bridging – 0 non-bridging – 4.0 e.g., olivine Mg2SiO4, nesosilicate Li4SiO4
1
Phase Diagrams of Ceramics
(cross-sections of ternary SiO2 – Al2O3 – MgO system)
1
Phase Diagrams of Ceramics
(cross-sections of ternary SiO2 – Al2O3 – MgO system)
1
Phase Diagrams of Ceramics
(example of ternary system: Hf – B – C)
Components of the Hf – B – C system:
Metal hafnium Hf Elemental Carbon C Elemental Boron and compounds: Monocarbide HfC1-x Monoboride HfB Diboride HfB2 Carbide B4C
B4 C C
B HfB HfB 2
HfC1-x
1
Hf
Phase Diagrams of Ceramics
(cross-sections of ternary Hf – B – C system) HfB2 – C
HfB2 – HfC1-x
1800 0C
HfB2 – B4C
1
Atomic Structure of Titania
Titanium dioxide TiO2 (titania) appears in nature in 3 crystalline modifications: rutile, tetragonal, a = 0.4593, c = 2.959 anatase, b.c. tetragonal a = 0.3785, c = 0.9514 brookite, orthorhombic, a = 0.5456, b = 0.9182, c = 0.5143
Rutile structure Stacking of TiO6 octahedra and their relationship to the unit cell
1
Lattice parameters in nm
Atomic Structure of Zirconia
Zirconium dioxide (zirconia) exists in in 3 different crystal structures:
α-ZrO2, monoclinic, a = 0.5151, b = 0.5203, c = 0.5315, β =99.28O at low temperatures β-ZrO2, tetragonal, a = 0.364, c = 0.527, c/a = 1.445 at intermediate temperatures γ-ZrO2, cubic, a = 0.5086 at high temperatures
1
Lattice parameters in nm
Phase Diagrams of Ceramics (binary ZrO2 – MeOx systems)
1
Atomic Structure of Zirconia
Stabilized Zirconia:
Adding dopants, e.g. MgO, CaO, Y2O3, CeO2 and other RE oxides, stabilizes the cubic γ-ZrO2 to lower temperatures Transformation cubic γ-ZrO2 ↔ tetragonal β-ZrO2 is diffusional Transformation tetragonal β-ZrO2 → monoclinic α-ZrO2 is martensitic with a 6% increase in volume High MgO or CaO content can get metastable cubic form at room temperature About 2.5% Y2O3, can get metastable tetragonal form at room temperature
1
Atomic Structure of Zirconia
Partially Stabilized Zirconia (PSZ):
Sinter the material with a dopant in the cubic phase Lower temperature and heat treatment (aging) to nucleate small precipitates of tetragonal phase; these are grown to below the critical size for tetragonal → monoclinic transformation Cool to room temperature; remaining cubic phase does not get time to transform
2
Atomic Structure of Perovskite
Perovskite CaTiO3 represents a type of cubic structure with general formula ABX3:
The larger A cations, Ca2+ in this case, are surrounded by 12 oxygen atoms, and the smaller B (Ti4+) ions are coordinated by 6 oxygens This structural type or modified versions of it are able to accommodate a large number of cationic combinations as long as the overall crystal is neutral, e.g. BaTiO3, SrTiO3, SrSnO3, CaSnO3, CaZrO3, SrZrO3, Sr4Zr3O10, BaZrO3, CaHfO3, SrHfO3, NaWO3, YAlO3, ReO3, WO3, NbF3, TiOF2 and other 2
Atomic Structure of Silicon Carbide
Silicon carbide, carborundum occurs as 2 crystal groups of phases: α-SiC – hexagonal (more common) and β-SiC – cubic (stable below 1800 0C):
Covalent bonded (~90%) compound Both atoms of Si and C are in sp3-hybridisation α-SiC has a variety of faulted hexagonal structures based on wurtzite, like another super-hard substances such as lonsdaleite (hexagonal allotropy of diamond) and γ-BN 6H, 2H and 15R are the most common polytypes of α-SiC – number gives stacking repeat sequence, letter gives lattice type. 2
Atomic Structure of Silicon Nitride Silicon nitride Si3N4 exists in 2 crystal structures (α and β), both of them are hexagonal:
Atom of Si is surrounded by 4 N atoms to form SiN4 tetrahedra covalent bonded (~70%) in preference and similar in size to SiO4 tetrahedra in silicates; N corners are shared by 3 tetrahedra α and β forms are distinguished by the stacking sequence of Si-N layers in the structure; α form has β stacking plus a glide operator
2
Phase Diagrams of Ceramics (Si3N4 – SiO2 – Al2O3 – AlN system)
2
Atomic Structure of α′-Sialon
Based on the α-Si3N4 structure, α′-sialon has a general formula – MexSi12-m-nAlm+nOnN16-n (Me = Li, Mg, Ca, Sr, Y, RE; x = m, m/2, m/3; 0.1 ≤ x ≤ 0.9):
This kind of sialons (ceramic alloys) forms, if more Si is substituted by Al than N by O and so balancing cations (alkali, alkali-earth or rare-earth metals) are accommodated in interstices By adjusting cation dopants by Al and O levels it is possible to form 2-phase α – β sialon material 2
Atomic Structure of β′-Sialon
Based on the β-Si3N4 structure, β′-sialon has a general formula – Si6-xAlxOxN8-x (0 ≤ x ≤ 4.2):
This kind of sialons forms, if one Al (III) is substituted for each Si (IV) and each N (III) by an O (II), so charge neutrality is preserved Crystal structure is not too distorted, as SiN4 tetrahedra, composed Si3N4, have almost the same size with AlO4 tetrahedra in the β′-sialon structure With the substitution, the formula for the tetrahedral unit changes from SiN4 to (Si,Al)(O,N)4 and the dimensions of the unit cell increase; although it causes the composition to shift towards that of alumina, the structural coordination in solid solution is fourfold whereas in alumina it is sixfold (AlO 6) Negative deviation from Raoult’s Law so lower than expected N2 vapour pressure above it – atmospheric nitrogen strongly suppresses the decomposition reaction 2
Atomic Structure of O′-Sialon
O′-sialon, with general formula Si2xAlxO1+xN2-x (0 ≤ x ≤ 0.2), is a derivative of orthorhombic silicon oxynitride Si2ON2 with the substitutions Si-N pairs of atoms by Al-O pairs, which is similar to those in β′-sialon:
The lattice parameters of orthorhombic unit cell of O′-sialon and the cell volume increase with increasing x-value in phase formula of material Some amounts of metal (I-III) cations, e.g. yttrium (III), may enter the O′-sialon structure
2
Atomic Structure of Carbon
(hexagonal and rhombohedral graphite)
Carbon is the only chemical element to have the particular layered sp2-hybridised structure
Hexagonal α-phase of graphite with a -ABABAB- stacking order Rhombohedral graphite with the stacking order -ABCABC-; thermodynamically unstable, reverts to α-form above 13000C
2
Atomic Structure of Carbon
(cubic and hexagonal diamond) Being sp3-hybridised, carbon appears in nature in 2 crystalline forms:
Cubic β-phase of diamond; each diamond tetrahedron combines with 4 other to form 3D strongly-bonded, 100% covalent structure Hexagonal, wurtzite-like γ-phase of lonsdaleite, which is a special form of polymorph (polytypism) where the close-packed layers are identical but have a different stacking sequence The 2-layer hexagonal sequence (2H-polytype) is different from the 3-layer cubic sequence (3C-polytype) Revealed recently 6H-polytype has intermediate structure between hexagonal and cubic phases; the existence of 4H, 8H and 10H has yet to be confirmed 2
Atomic Structure of Carbon
(graphite and diamond: comparison)
The results of the inelastic scattering of optical photons by lattice vibration phonons, due to Raman spectroscopy, can determine with great accuracy the bonding states of the carbon atoms (sp2 for graphite and/or sp3 for diamond) 3
Atomic Structure of Boron Nitride Boron nitride BN, structurally isoelectronic to carbon, forms phases, crystallographically equivalent to graphite, diamond and lonsdaleite:
Hexagonal α-BN and its rhombohedral polytype Cubic β-BN, diamond-like modification Hexagonal γ-BN, lonsdaleite- or wurtzite-like phase
3
Conventional Ceramics Processing (fabrication of traditional ceramics)
Specific manufacture technique for ceramics processing was developed because of their high melting points, hardness and brittleness:
General manufacturing method is sintering of powders, hence porosity is inherent for the most part of ceramics Ceramic powders are typically in size range 0.1-100 μm Often a liquid suspension stage followed to give solid for compaction Dried powders are then used to form a “green body” 3
Ceramics Processing
(fabrication of advanced ceramics)
The variety of different methods are used to fabricate advanced ceramics e.g.: Hot pressing (HP) combines pressing and sintering procedures in one joint stage Hot isostatic pressing (HIP) uses high gas pressures for compaction of powders, contained in special metal or glass claddings 3
Advanced Ceramics Processing
(alternative fabrication methods) (1) Powder technology (superhigh temperature hot pressing and isostatic pressing, application 2 of plasma, super-high pressures, 1 nanoparticles and different inorganic / organic precursors) (2) Chemical vapour deposition (application of different gaseous mixtures and metal-organic compounds) 4 (3) Arc-melting and casting (high- 3 pressure protective gas media, special prepared electrodes) (4) Impregnation of porous media (ceramic and carbonaceous special porous structures) 3
Microstructures of Ceramics (sintering and porosity)
(1) Particles of quartz sand cemented by silicates (initial stage of sintering, formation of neck) (2) High-porous ceramic foam, silicon carbide (porosity >70%) (3) High-dense fine-grained sintered ceramics (porosity ~0%),
1
3
2
3
Microstructures of Ceramics (general view)
3
Microstructures of Ceramics (“healing” of pores)
Densification mechanisms of ceramics in dependence on different value of specific surface energy of material: a) low
b) high
c) intermediate 3
Microstructures of Ceramics (high-density materials)
(1) Single-phase titanium carbide, hot-pressed at 2700 0C (porosity ~5%), SEM (2) Pure alumina, sintered at 1700 0C (porosity ~3%) (3) Sialon ceramics (porosity ~10%) 3
3
1
2
Microstructures of Ceramics
(pottery: liquid-phase sintering of silicates)
Particularities of pottery microstructures:
Equilibrium never fully achieved Large quantities of cooled liquid forms “Solution rings” (SR) around partially dissolved quartz particles Regions of mullite (M) needle formation within glass
3
SR M
Microstructures of Ceramics (alumina)
Sintering mechanism depends on purity of feedstock: solid-state process – at high purity and liquidstate – in the case of contaminated alumina. High processing temperatures results to grain growth of material, which could be suppressed by pinning boundaries due to addition of some dopants
(1) Debased alumina (~95% Al2O3) with remnant glassy phase, sintered at 1500 0C (2) Practically pure alumina (~99% Al2O3) with MgO used to stabilize grain growth, sintered at 1650 0C 4
1
2
Microstructures of Ceramics (zirconia)
Tetragonal zirconia polycrystalline (TZP) ceramics, sintered at 1400-1550 0C Partially stabilized zirconia (PSZ) ceramics, sintered at 1800 0C: small tetragonal precipitates in a cubic matrix
4
TZP
PSZ
Microstructures of Ceramics (oxide ceramics)
1
(1) Beryllia BeO (2) Mixed oxide Ce0.8Gd0.2O1.9 (3) Mixed manganite La0.6Sr0.4MnO3 3
4
2
Microstructures of Ceramics (oxide electroceramics)
1
Mn-Zn Ferrite with (1) and without (2) Bi2O3 (3) Polarized Pb-La zirconate-titanate (PLZT) (4) Steatite ceramics 4
4
2
3
Microstructures of Ceramics (silicon carbide and nitride)
(1) Reaction bonded SiC (RBSC) (2) Metal – ceramic composite Al – SiC (3) SiC whiskers for reinforcement of metal and ceramic matrices (4) Hot-pressed Si3N4 (HPSN) 4
1
2
3
4
Microstructures of Ceramics (sialon ceramics)
(1) α′sialon (2) α′-β′sialon: (a) – ┴ (b) – II to HP direction (3) Y-α′sialon
4
1
3
2
Microstructures of Ceramics (cermets and hard alloys)
(1) Ceramic hard alloy WC – 10%Co, sintered at: (a)12800C, (b)13800C, (c)14300C (2) Ceramic hard alloy WC – 20%Co (3) Cermet ZrB2 – Ti 3
4
1
2
Microstructures of Ceramics
(carbon and carbon composite materials)
(1) SiC – carbon fibre (2) Carbon filaments (3) Carbon – carbon composite, fractography (4) Carbon foam
4
1
2
3
4
Microstructures of Ceramics (hetero-modulus ceramics)
(1) TiC – graphite, hot-pressed: (a) 10 vol% C, (c) 30 vol% C 2 (2) TiC – graphite, fused hypereutectic alloy, 30 vol% C (3) α′-sialon – 20 vol% α-BN, 2 hot-pressed 1
3 3
4
Microstructures of Ceramics (nanomaterials)
Carbon nanotubes fabricated using porous ceramic templates
4
To be continued
