Sol-Gel Methods 1 Hydrolysis Polycondensation Gelation Ageing Drying Densification Sol-gelprocess Powders: microcrystalline, nanocrystalline, amorphous Monoliths, Coatings, Films, Fibers Xerogels, Aerogels, Ionogels, Cryogels Glasses, Ceramics, Hybrid materials Sol-Gel Methods PRECURSOR Sol-Gel Methods 2 Sol = a fluid system of stable suspension of colloidal (1 nm – 1 m) solid particles or polymeric molecules in a liquid (Below 1 m – Brownian motion, larger than 1 m – sedimentation) Gel = nonfluid, porous, three-dimensional, continuous solid network (elastic or rigid) surrounded by a continuous liquid phase Colloidal (particulate) gels = agglomeration of dense colloidal particles Polymeric gels = agglomeration of polymeric particles made from subcolloidal units Agglomerate = assemblage of particles rigidly joined together, as by partial fusion (sintering) or by growing together, covalent bonds, hydrogen bonds, polymeric chain entanglement Aggregate = assemblage of particles which are loosely coherent, van der Walls forces Sol-Gel Methods Sol-Gel Methods 3 Sol and Gel Gel point = point of incipient network formation Sol-to-Gel transition is difficult to define Rheological methods = viscosity increases Sol-Gel Methods 4 Sol-to-Gel transition Sol-Gel Methods 5 Sol-Gel Methods Sol-Gel Methods 6 Aqueous • Colloid Route – inorganic salts, water glass, pH, hydrolysis, polycondensation • Metal-Oragnic Route – metal alkoxides, amides, hydrolysis, polycondensation • Pechini and Citrate Gel Method – inorganic metal salts, complexing agent, chelate formation, polyesterification with polyfunctional alcohol Nonaqueous • Hydroxylation • Heterofunctional Condensations Sol-Gel Chemistry Sol-Gel Methods 7 Colloid Route Metal salts in aqueous solution, pH and temperature control Solvation – water molecule becomes more acidic Mz+ + :OH2  [M  OH2]z+ For transition metal cations, charge transfer occurs from the filled bonding orbital of the water molecule to the empty d orbitals of the transition metal. Therefore, the partial positive charge on the H of water molecule increases, making the water molecule more acidic. Hydrolysis [M(H2O)b]Z+ ⇄ [M(H2O)b-1OH](Z-1)+ + H+ Condensation-polymerization [M(H2O)b]Z+ ⇄ [(H2O)b-1M(OH)2M(H2O)b-1](2Z-2)+ + 2H+ Sol-Gel Methods 8 Colloid Route the Keggin cation [Al13O4(OH)24(OH2)12]7+ Gibbsite Al(OH)3 Colloid Route Sol-Gel Methods 9 [M(OH2)]z+ ⇄ [M–OH](z-1)+ + H+ ⇄ [MO](z-2)+ + 2 H+ Depending on the water acidity and the charge transfer, the following equilibria are established: Aqua Hydroxo Oxo Only hydroxo can condense Oxidation stateComposition of complexes depends on: - nature of transition metal - oxidation state - charge - ionic radius - electronegativity - nature of ligands - coordination abilities - pH of solution Colloid Route Sol-Gel Methods 10 Olation = a hydroxo bridge (-OH- “ol” bridge) is formed between two metals centers Oxolation = an oxo bridge (–O–) is formed between two metals centers Only hydroxo groups can condense Sol-Gel Methods 11 Colloid Route Reduction Oxidation Acid addition Base addition Fe2+(aq) + CO3 2  ? Fe3+ (aq) + CO3 2  ? 12 Colloid Route The higher a charge on ion, the more acidic coordinated waters are Partial charges on ions and H2O moleculeElectronegativity Colloid Route 13 Area I : monomeric and soluble cations Area II : condensation by olation Area III : condensation by olation or oxolation Area IV : condensation by oxolation Area V : monomeric and soluble anions Electronegativity of a central atom M determines degree and mechanism of condensation for neutral hydroxo containing species Sol-Gel Methods 14 Pechini Sol-Gel Route The transesterification reaction between citric acid and ethylene glycol Calcination Oxide Sol-Gel Methods 15 Pechini Sol-Gel Route The transesterification reaction between citric acid and ethylene glycol Sol-Gel Methods 16 Pechini Sol-Gel Route Major components Dopants Gelling agent Doped YAG product Removal of organics Removal of solvents Sol-Gel Methods 17 Pechini Sol-Gel Route EG : CA : M Sol-Gel Methods 18 Metal Alkoxides [M(OR)x]n + H2O  ROH + M-O-H Metal Amides [M(NR2)x]n + H2O  R2NH + M-O-H 2 M-O-H  M-O-M + H2O Hydrolysis Polycondensation OXIDE Metal-Organic (Alkoxide) Route Sol-Gel Methods 19 Metal-Organic (Alkoxide) Route Oligomers formed by hydrolysis-condensation process -linear -branched -cyclic -polyhedral Never goes to pure SiO2 n Si(OR)4 + 2n+(ab)/2 H2O  SinO2n(a+b)/2(OH)a(OR)b + (4nb) ROH Sol-Gel Methods 20 Metal Alkoxides and Amides as Precursors Metal Alkoxides [M(OR)x]n formed by the replacement of the hydroxylic hydrogen of an alcohol (ROH) through a metal atom Most frequently used precursor for sol-gel: TEOS = Si(OEt)4 Metal Amides [M(NR2)x]n formed by the replacement of one of the hydrogen atoms of an amine (R2NH) through a metal atom Sol-Gel Methods 21 Metal Alkoxides and Amides as Precursors Homometallic Alkoxides General Formula: [M(OR)x]n Heterometallic Alkoxides General Formula: MaM’b(OR)x]n Metal Amides General Formula: [M(NR2)x]n M = Metal or metalloid of valency x O = Oxygen Atom N = Nitrogen atom R = simple alkyl, substituted alkyl or aryl group n = degree of molecular association Sol-Gel Methods 22 Modified Silicon Alkoxides as Precursors Functional groups Sol-Gel Methods 23 Modified Silicon Alkoxides as Precursors Terminal groups Bridging groups Polymerizable groups Functional groups Silsesquioxanes = RSiO1.5 ( = 3/2) Hybrid Inorganic-Organic Materials Sol-Gel Methods 24 Organization in Xerogels of Bridged Silicon Alkoxide Precursors Sol-Gel Methods 25 Self-Assembly of Bridged Silsesquioxanes Sol-Gel Methods 26 Nanostructuring of hybrid silicas through a Self-Recognition Process - the crystallization of the hydrolyzed species by H-bonding followed by their polycondensation in solid state 1,4-Bis(triethoxysilyl)propylureidobenzene Templating Porosity in Bridged Polysilsesquioxanes Sol-Gel Methods 27 Polyhedral Oligomeric Silsesquioxanes (POSS) Sol-Gel Methods 28 Polymers and Copolymers of POSS Sol-Gel Methods 29 Polymers and Copolymers of POSS Sol-Gel Methods 30 a certain degree of crystallinity Sol-Gel Methods 31 Sol-Gel in Silica Systems Hydrolysis PolycondensationSilicate (aq) Alkoxide (nonaq) Sol-Gel Methods 32 Acid catalysed hydrolysis Base catalysed hydrolysis Metal-Oragnic (Alkoxide) Route Metal-Oragnic (Alkoxide) Route Sol-Gel Methods 33 Isotope labelling experiments Sol-Gel Methods 34 Metal-Oragnic (Alkoxide) Route Oligomers formed by hydrolysis-condensation process -linear -branched -cyclic -polyhedral Never goes to pure SiO2 n Si(OR)4 + 2n+(ab)/2 H2O  SinO2n(a+b)/2(OH)a(OR)b + (4nb) ROH Sol-Gel Methods 35 GC of TMOS hydrolysis products Si(OMe)4 + H2O Sol-Gel Methods 36 Neg. ion ESI-MS and 29Si NMR of silicate aq with TMA ions Q - notation Sol-Gel Methods 37 Q0 = O4Si Q1 = O3SiOSi Q2 = O2Si(OSi)2 Q3 = OSi(OSi)3 Q4 = Si(OSi)4 The notation of Qa b ‘‘Q’’ stands for the maximum 4 siloxane bonds for each Si ‘‘a’’ is the actual number of siloxane bonds on each Si ‘‘b’’ is the number of Si in the unit Sol-Gel Methods 38 Silicate anions in aqueous alkaline media (detected by 29Si-NMR) M = OSiR3 D = O2SiR2 T = O3SiR Q = O4Si Q0 = O4Si Q1 = O3SiOSi Q2 = O2Si(OSi)2 Q3 = OSi(OSi)3 Q4 = Si(OSi)4 Sol-Gel Methods 39 Silicate anions Sol-Gel Methods 40 Silicate anions Sol-Gel Methods 41 Si50O75(OH)50 three-dimensional clusters formed by (A) four-rings (B) six-rings Oligomers formed by hydrolysis-condensation Sol-Gel Methods 42 IR spectrum of silica , cm-1 460800 1100 Amorphous silica/water interface Sol-Gel Methods 43 Sol-Gel Methods 44 The electrical double layer at the interface of silica and a diluted KCl solution  = local potential OHP = outer Helmholtz plane u = local electroosmotic velocity Negative surface charge stems from deprotonated silanols Shielding of this surface charge occurs due to adsorbed ions inside the OHP and by mobile ions in a diffuse layer The shear plane = where hydrodynamic motion becomes possible Zeta = potential at the shear plane The Electrical Double Layer Silica surface Solution Zeta Sol-Gel Methods 45 Isoelectronic point: zero net charge pH = 2.2 for silica Sol-Gel Methods SiOH SiOH2 + SiO Sol-Gel Methods 46 Rate of H+ catalyzed TEOS hydrolysis (gel time) as a function of pH Effects on hydrolysis rate: pH substituents solvent water Sol-Gel Methods Longest TEOS gel time = the slowest reaction Sol-Gel Methods 47 Precursor substituent effects: Steric effects: branching and increasing of the chain length LOWERS the hydrolysis rate Si(OMe)4 > Si(OEt)4 > Si(OnPr )4 > Si(OiPr)4 > Si(OnBu)4 Inductive effects: electronic stabilization/destabilization of the transition state (TS) Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi Sol-Gel Methods Partial Charge Model (Livage and Henry) Sol-Gel Methods 48 Electron transfer occurs when atoms combine to give a molecule Charge transfer causes each atom to acquire a partial positive or negative charge, i This transfer mainly depends on the electronegativity difference between atoms The electronegativity i of an atom varies linearly with its partial charge i i = o i + k i Electron transfer must stop when all electronegativities have the same value = the mean electronegativity Partial Charge Model (Livage and Henry) Sol-Gel Methods 49 The mean electronegativity of a molecule z = the electric charge for ions k = a constant that depends on the electronegativity scale (k = 1.36 in Pauling's units) The partial charge i on an atom in the molecule Partial Charge Model (Livage and Henry) Sol-Gel Methods 50 Alkoxide Zr(OEt)4 Ti(OEt)4 Nb(OEt)5 Ta(OEt)5 VO(OEt)3 W(OEt)6 Si(OEt)4  (M) +0.65 +0.63 +0.53 +0.49 +0.46 +0.43 +0.32 The hydrolysis rate depends on the  (M): The more positive partial charge i the faster hydrolysis reaction kh  5·10-9 mol-1s-1 for Si(OEt)4 kh  10-3 mol-1s-1 for Ti(OEt)4 Partial Charge Model Sol-Gel Methods 51 the number of valence electrons n* on the central atom of a radical AB N = the number of valence electrons on the free atom A p = the number of valence electrons supplied by B when forming the A–B bond. m = the number of bonds between A and B s = the number of resonance contributions from A B+ Group electronegativity g rA = the covalent radius of atom A in the radical AB. Partial Charge Model Sol-Gel Methods 52 Sol-Gel Methods 53 Acid catalysed hydrolysis Hydrolysis Transition State Acidic conditions: Hydrolysis reaction rate decreases as more alkoxy groups are hydrolyzed Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi TS (+) is destabilized by increasing number of electron withdrawing OH groups (wrt OR) The reaction at terminal Si favored, as there is only one electron withdrawing SiO group Linear polymer products are favored, leading to fibers RSi(OR)3 is more reactive than Si(OR)4 Sol-Gel Methods 54 Acid catalysed hydrolysis Hydrolysis Hydrolysis reaction rate decreases as more alkoxy RO groups are hydrolyzed and replaced with OH groups Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi Sol-Gel Methods 55 Hydrolysis Base catalysed hydrolysis Transition State Basic conditions: Hydrolysis reaction rate increases as more alkoxy groups are hydrolyzed Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi TS () is stabilized by increasing number of electron withdrawing OH groups (wrt OR) The reaction at central Si favored, as there is more electron withdrawing SiO groups Branched polymer products are favored, spherical particles, powders RSi(OR)3 less reactive than Si(OR)4 Sol-Gel Methods 56 Base catalysed hydrolysis Hydrolysis Hydrolysis reaction rate increases as more alkoxy RO groups are hydrolyzed and replaced with OH groups Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi Sol-Gel Methods 57 Si-OH becomes more acidic with increasing number of Si-O-Si bonds Sol-Gel Methods Nucleophilic catalysis: F- Si-F bonds HMPA N-methylimidazol N,N-dimethylaminopyridin Sol-Gel Methods 58 Water-to-Si ratio (k) stoichiometric ratio for complete hydrolysis k = 4 Si(OR)4 + 4 H2O  Si(OH)4 + 4 ROH additional water comes from condensation Si-OH + HO-Si  Si-O-Si + H2O Small amount of water (k  4) = slow hydrolysis due to the reduced reactant concentration Condensation of incompletely hydrolyzed species Large amount of water (k  4) = slow hydrolysis due to the reactant dilution Condensation of completely hydrolyzed species Reverse reaction promoted - depolymerization of Si-O-Si Sol-Gel Methods Sol-Gel Methods 59 Hydrophobic effect Si(OR)4 are immiscible with water cosolvent ROH is used to obtain a homogeneous reaction mixture and prevent phase separation Sol-Gel Methods Solvent properties: polarity, dipole moment, viscosity, protic behavior: Protic (EtOH) - bind to OSi Aprotic (THF) - bind to HOSi Affect rates alcohol produced during the reaction alcohols - transesterification Sonication - homogenization Solvents affect drying Sol-Gel Methods 60 Acid catalysed condensation fast protonation, slow condensation Condensation Positively charged transition state, fastest condensation for (RO)3SiOH > (RO)2Si(OH)2 > ROSi(OH)3 > Si(OH)4 TS (+) is destabilized by increasing number of electron withdrawing OH groups Hydrolysis fastest in the first step, i.e. the formation of (RO)3SiOH Condensation for this species also fastest, the formation of linear chains TS Sol-Gel Methods 61 Condensation Base catalysed condensation fast deprotonation, slow condensation Negatively charged transition state, fastest condensation for (RO)3SiOH < (RO)2Si(OH)2 < ROSi(OH)3 < Si(OH)4 TS () is stabilized by increasing number of electron withdrawing OH groups Hydrolysis speeds up with more OH, i.e. the formation of Si(OH)4 Condensation for the fully hydrolysed species fastest, the formation of highly crosslinked particles TS Acid Catalysed Condensation Sol-Gel Methods 62 • For k  4 : complete hydrolysis at early stage • Reaction limited cluster aggregation (RLCA) • Q0 or terminal groups Q1 on chains • Irreversible reactions in acidic pH • Condensation to linear chains or weakly branched • For k  4 : incomplete hydrolysis at early stage • Unhydrolysed chains, highly concentrated solution without gelling • Spinnable to fibers • Small primary particles • Microporosity, Type I isotherms pH  2 Base Catalysed Condensation Sol-Gel Methods 63 • For k  4 : complete hydrolysis at early stage • Reversible reactions in basic pH • Chains cleaved at Q1, source of Q0 • Condensation to highly crosslinked particles • Reaction limited monomer-cluster growth (RLMC) • Compact nonfractal structure • For k  4 : incompletely hydrolysed species incorporated • Fractal uniformly porous structure • Large primary particles • Mesoporosity, Type IV isotherms pH  7 Sol-Gel Methods 64 Reaction limited monomer-cluster growth (RLMC) or Eden growth Reaction limited cluster aggregation (RLCA) Acid catalysed Base catalysed Hydrolysis - Condensation Kinetics Sol-Gel Methods 65 400 310 220 130 040 301 211 121 031 202 112 022 103 013 x y z 004 Si(OR)x(OH)y(OSi)z x + y + z = 4 Si(OR)4 Si(OH)4 Si(OSi)4 Hydrolysis Condensation Sol-Gel Methods 66 Gel point - a spannig cluster reaches across the container, sol particles, oligomers and monomer still present a sudden viscosity increase at the gel point further crosslinking - increase in elasticity Gelation = Sol-to-Gel Transition Bond Percolation Sol-Gel Methods 67 p = the fraction of created links sav(p) = average cluster size lav(p) = average spanning length P(p) = percolation probability = a bond is added to a spanning cluster Sol-to-Gel Transition Sol-Gel Methods 68 Sol-to-Gel Transition Sol-Gel Methods 69 c = condensation degree, max 83 % Sol-Gel Methods 70 Ageing Crosslinking condensation of the OH surface groups, stiffening and shrinkage Syneresis shrinkage causes expulsion of liquid from the pores Coarsening materials dissolve from the convex surfaces and deposits at the concave surfaces: necks Rippening Smaller particles have higher solubility thean larger ones Phase separation Fast gelation, different miscibility, isolated regions of unreacted precursor, inclusions of different structure, opaque, phase separation Ageing of Gels Sol-Gel Methods 71 1. The constant rate period the gel is still flexible and shrinks as liquid evaporates 2. The critical point the gel becomes stiff and resists further shrinkage and deformation by surface tension, the liquid begins to recede (meniscus with a contact angle ) into the pores (radius r), surface tension  creates large capillary pressures Pc, stress, cracking 3. The first falling-rate period a thin liquid film remains on the pore walls, flows to the surface and evaporates, the menisci first recede into the largest pores only, as these empty, the vapor pressure drops and smaller pores begin to empty 4. The second falling-rate period liquid film on the walls is broken, further liquid transport by evaporation Drying of Gels Drying of Gels Sol-Gel Methods 72 Pc = 2/r Wetting surface: cos = 1 Water:  = 72.75 mN/m n-Pentan:  = 16.0 mN/m Pore: r = 2 nm Pc = 73 MPa Drying of Gels Sol-Gel Methods 73 Sol-Gel Methods 74 1. Supercritical drying 2. Freeze-drying 3. Drying control chemical additives 4. Ageing 5. Large pore gels Drying Methods To avoid cracking: •No meniscus •Decrease surface tension •Increase wetting angle (isopropanol) •Increase pore size •Make a stiff gel Sol-Gel Methods 75 25 mmol (5.2 g) of tetraethoxysilane (TEOS) TEOS:H2O:HNO3:ammonium acetate molar ratio of the solution is 1 : 10 : 0.002 : 0.02 Sol-Gel Methods 76 Aerogels 1931 Steven S. Kistler J. Phys. Chem. 34, 52, 1932 Aerogels = materials in which the typical structure of the pores and the network is largely maintained while the pore liquid of a gel is replaced by air The record low density solid material - 10 mg/cm3 density of air 1.2 mg/cm3 Sol-Gel Methods 77 Aerogels - Supercritical Drying Silica aerogel • Byproduct, salt, water washing • Water replacement with acetone • Replacement of acetone with CO2 (l) • Supercritical drying Sol-Gel Methods 78 Aerogels - Supercritical Drying Silica aerogel • Byproduct, salt, water washing • Water replacement with acetone • Replacement of acetone with CO2 (l) • Supercritical drying Sol-Gel Methods 79 Supercritical Drying Cold supercritical drying path in the Pressure (P) Temperature (T) phase diagram of CO2 Sol-Gel Methods 80 Supercritical Drying Sol-Gel Methods 81 Sintering Common ceramic and metallurgic manufacturing process Thermal sintering A powder is first pressed into a highly porous pellets 50-60% of the maximum theoretical density = green pellet heating, the pellet densifies, reducing surface area and surface energy of individual particles without reaching melting point, sintering time - several hours to several days Other methods of sintering: • two-phase sintering • microwave sintering • spark-plasma sintering • oxidative sintering Control of sintering sintering parameters: temperature, pressure, time, atmosphere Sol-Gel Methods 82 Sintering Sintering - self-diffusion of atoms in the crystal lattice Atoms diffuse randomly through the lattice by moving into adjacent vacant lattice sites = vacancies A vacant lattice site increases the energy of the lattice Atoms on the surface of particles have higher energies than the atoms in the particle interiors Energy is lower if the particle is in contact with another particle of the same material than if it is in contact with the atmosphere or a different material The lattice sites that increase the contact area between particles are preferred = around the edges of the contact area Sol-Gel Methods 83 Sintering When atoms move out of the bulk and to the contact area - vacancies are created within the bulk The overall energy change - the difference of the energy reduced by increasing the surface area and the energy increased by creating a vacancy = the sintering stress The magnitude of the sintering stress depends on the contact angle between the particles = the dihedral angle Sharper contact angles reduce the overall energy, as the contact area increases the dihedral angle widens Eventually it reaches a wide enough angle that the sintering stress is zero and sintering ceases = the equilibrium dihedral angle Sintering Sol-Gel Methods 84 Sintering stress R [nm] 300 10 3 1000 Sintering Sol-Gel Methods 85 Sintering Sol-Gel Methods 86 Agglomerates  3m Agglomerates 0.5m Larger agglomerates = higher sintering temperature Sintering Sol-Gel Methods 87 Because of the limit on the dihedral angle, it is possible for sintering to reach equilibrium with pores still present in the material The rate at which sintering occurs is controlled by the diffusion rate and sintering stress The diffusion rate is affected by the defect concentration and temperature More defects mean more atoms can diffuse simultaneously, while higher temperatures allow individual atoms to diffuse faster = sintering is done at high temperature Sintering Sol-Gel Methods 88 Stage I. the powder particles increase their contact areas through the formation of necks, it ends once neck growth ceases to be the major mechanism Stage II. the overall density increases as the pores decrease in size, the contact areas grow into planes called grain boundaries, the pores become more columnar in shape as they shrink into tunnel systems on grain boundaries and triple junctions Stage III. begins when pores become closed off to the surface, grain boundary motion begins as the lattice continues to decrease its overall energy by decreasing the surface area between grains, large grains grow at the expense of smaller grains Sintering Sol-Gel Methods 89 Grain boundaries move - pick up vacancies, impurity atoms, and even small pores. These small pores can come in contact with one another as grains are eliminated = grain boundary sweeping Reduction in the number of pores, defect concentration in regions near moving grain boundaries In the final stage of sintering, atmospheric pressure becomes important. The pores are closed off from the surface - gas is trapped in the pores. As the pores decrease in volume the pressure inside the pore increases, pushing back against further pore shrinkage If the temperature is raised suddenly, reverse sintering = the pores increase in volume The gas will diffuse into the solid lattice, relieving pressure and allowing sintering to continue - the rate of sintering depends on the gas solubility Sol-Gel Methods 90 Densification Stage I. Below 200 C, weight loss, no shrinkage pore surface liquid desorption Stage II. 150 - 700 C, both weight loss and shrinkage loss of organics - weight loss further condensation - weight loss and shrinkage structural relaxation - shrinkage Stage III. Above 500 C, no more weight loss, shrinkage only close to glass transition temperature, viscous flow, rapid densification, large reduction of surface area, reduction of interfacial energy, termodynamically favored Densification Mechanical Properties Sol-Gel Methods 91 • τ – yield stress • H – microhardness • d – average grain size Smaller grains – higher yield stress and hardness! H H0 Mechanical Properties Sol-Gel Methods 92 Sol-Gel Methods 93 Densification - Sintering Sol-Gel Methods 94 Sol-Gel Methods 95 Densification Densification - Sintering Sol-Gel Methods 96 Sintering mechanisms - solid, liquid, gas phase 1. Evaporation-condensation and dissolution-precipitation 2. Volume diffusion 3. Surface diffusion 4. Grain boundary diffusion 5. Volume diffusion from grain boundaries 6. Volume diffusion from dislocations – plastic flow Volume diffusion from dislocations vacancies Viscous flow Sintering mechanisms Sol-Gel Methods 97 Sintering Mechanisms Sol-Gel Methods 98 Sintering Mechanisms Sol-Gel Methods 99 Sintering Mechanisms Sol-Gel Methods 100 Sintering Mechanisms SPS/FAST – Spark Plasma Sintering Sol-Gel Methods 101 • Field assisted sintering technique • No spark, No plasma • Pulsed electric current sintering • High pressure – limitation by high temperature fracture strength • graphite 100-150 MPa, WC or SiC – 1GPa • High temperature – up to 2400 °C • Joule heating – resistence - at contact points • Up to 10 V, 10 kA • Extreme heating rates: 1000 K/min Sol-Gel Methods 102 Dehydration sequence of hydrated alumina in air Path (b) is favored by moisture, alkalinity, and coarse particle size (100µm) path (a) by fine crystal size (<10µm) Sol-Gel Methods 103 HT-XRD of the phase transitions g = Gibbsite -Al(OH)3 b = Boehmite -Al(O)OH  = -Al2O3 alumina  = -Al2O3 Corundum Sol-Gel Methods 104 Gibbsite to Boehmite to Gamma Gibbsite -Al(OH)3 to Boehmite -Al(O)OH to -Al2O3 alumina (defect spinel) CCP Sol-Gel Methods 105 27Al Solid-State NMR spectra Sol-Gel Methods 106 Bayerite to Diaspore to Corundum Bayerite -Al(OH)3 to Diaspore -Al(O)OH to -Al2O3 Corundum HCP Sol-Gel Methods 107 Oxygen Coordination Metal Coordination Sol-Gel Methods 108 Metal-Oxide Clusters Sol-Gel Methods 109 Metal-Oxide Clusters