1 Hydrolysis Polycondensation Gelation Ageing Drying Densification Sol-gel process Crystallinity: Microcrystalline, Nanocrystalline, Amorphous, Glasses Dimensionality: Monoliths, Coatings, Films, Fibers, Nanoparticles Drying: Xerogels, Aerogels, Ionogels, Cryogels Heat Treatment: Powders, Glasses, Ceramics Composition: Oxides, Silicates, Phosphates, Nitrides, Carbon, InorganicOrganic Hybrid Materials Sol-Gel Methods PRECURSOR 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 = a nonfluid, porous, two-phase system of threedimensional, 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 3 Sol and Gel Gel point = point of incipient network formation Sol-to-Gel transition is difficult to define Rheological methods = viscosity increases 4 Sol-to-Gel transition 5 Sol-Gel Processes 6 Aqueous • Colloid Route – inorganic salts, water glass, change pH, hydrolysis, polycondensation • Metal-Organic Route – metal alkoxides, amides, addition of water, hydrolysis, polycondensation • Pechini and Citrate Gel Method – inorganic metal salts, complexing agent, chelate formation, polyesterification with polyfunctional alcohol Nonaqueous • Hydroxylation (= formation of MOH) • Heterofunctional Condensations (MX + YOM  XY + MOM) Sol-Gel Chemistry 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+ 8 Colloid Route OH2 AlH2O OH2 OH2 H2O OH2 3 OH2 AlH2O OH OH2 H2O OH2 2 pH < 3 pH = 4 - 5 B HB+ pH = 5 - 7 Al(OH)3 [Al(OH)4]- [Al(H2O)4(OH)2]the Keggin cation [Al13O4(OH)24(OH2)12]7+ Gibbsite Al(OH)3 Colloid Route 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 pH 10 Colloid Route Reduction Oxidation Acid addition Base addition Fe2+(aq) + CO3 2  ? Fe3+ (aq) + CO3 2  ? Oxidation state 11 Colloid Route The higher a charge on ion, the more acidic coordinated waters are  Partial charges on ions and H2O molecule Electronegativity Colloid Route 12 Olation = a hydroxo bridge (-OH- “ol” bridge) is formed between two metals centers Oxolation = an oxo bridge (–O–) is formed between two/three metals centers Only hydroxo groups can condense !! Colloid Route 13 Area I : monomeric and soluble cations Area II : condensation by olation (MOHM) Area III : condensation by olation or oxolation Area IV : condensation by oxolation (MOM) Area V : monomeric and soluble anions Electronegativity of a central atom M determines degree and mechanism of condensation for neutral hydroxo containing species Oxidation state Electronegativity Colloid Route 14 Oxygen Coordination Metal Coordination 15 Metal-Oxide Clusters 16 Metal-Oxide Clusters cuboctahedron 17 Pechini Sol-Gel Route Oxide Polyesterification Complexation Calcination 18 Pechini Sol-Gel Route Major components Dopants Gelling agents Doped YAG product Removal of organics Removal of solvents 19 Pechini Sol-Gel Route EG : CA : M Control of morphology 20 Metal / Metalloid Alkoxides [M(OR)x]n + H2O  ROH + MOH Si(OEt)4 = TEOS Metal Amides [M(NR2)x]n + H2O  R2NH + MOH 2 MOH  MOM + H2O Hydrolysis Polycondensation Sol – Gel – Oxide xerogel Metal-Organic (Alkoxide) Route 21 Metal-Organic Precursors Metal Alkoxides [M(OR)x]n formed by the replacement of the hydroxylic hydrogen of an alcohol (ROH) through a metal atom Heterometallic Alkoxides [MaM’b(OR)x]n 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 M = Metal or metalloid of valency x n = degree of molecular association 22 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 TEOS = Si(OEt)4 23 Modified Silicon Alkoxides as Precursors Terminal groups Bridging groups Polymerizable groups Functional groups Silsesquioxanes = RSiO1.5 (sesqui = 3/2) 24 Modified Silicon Alkoxides as Precursors Connectivity of Si 4 Connectivity of Si 3 R = Functional groups Hybrid Inorganic-Organic Materials 25 Self-Assembly of Bridged Silsesquioxanes 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 27 Polyhedral Oligomeric Silsesquioxanes (POSS) 28 Hydrolysis Condensation Polymers and Copolymers of POSS 29 1D linear polymers Polymers and Copolymers of POSS 30 a certain degree of crystallinity 3D crosslinked polymers 31 Mechanism of Sol-Gel Reactions in Silica Systems Si O Si OR H+ H2O -ROH Si OH SiHO SiRO -H2O -ROH Si O Si Hydrolysis PolycondensationSilicate (aq) Alkoxide (nonaq) Metal-Organic Route Metal alkoxide in alcoholic solution, water addition Silica network Silanol Colloid Route 32 H O H RO Si O RO RO R H OR Si O OR RO R H OR SiO OR OR H H O H + ROH + H Acid catalysed hydrolysis H O RO Si OR RO RO OR Si O OR RO R OR SiO OR OR HH O + RO Base catalysed hydrolysis Mechanism of Silicon Alkoxide Route Hydrolysis of silicon alkoxide in alcoholic solution by water addition is SLOW 33 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 Hydrolysis - Condensation Kinetics 34 400 310 220 130 040 301 211 121 031 202 112 022 103 013 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 35 Metal-Organic (Alkoxide) Route Si(OMe)4 + H2O GC of TMOS hydrolysis products 36 Metal-Organic (Alkoxide) Route Neg. ion ESI-MS and 29Si NMR of silicate aq with TMA ions Silicate anions in aqueous alkaline media detected by 29Si-NMR Q - Notation 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 M = OSiR3 D = O2SiR2 T = O3SiR Q = O4Si 38 Linear and Branched Silicate Anions 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 39 Cyclic and Polyhedral Silicate Anions 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 Oligomeric Silicate Anions 40 Si50O75(OH)50 three-dimensional clusters formed by (A) four-rings (B) six-rings Oligomers formed by hydrolysis-condensation 41 IR spectrum of silica 460800 1100 , cm-1 Amorphous Silica / Water Interface 42 H2O Cations Anions Protons SiO/H 43 The electrical double layer at the interface of silica and a diluted KCl solution  = local potential OHP = Outer Helmholtz plane u = local electroosmotic velocity y = distance from the surface 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 44 Isoelectric point = zero net charge at pH = 2 for silica Sol-Gel Methods SiOH SiOH2 + SiO deprotonated protonated neutral The slowest reaction at IEP 45 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 Precursor Substituent Effects Partial Charge Model (Livage and Henry) 46 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 (Electronegativity equalization) = the mean electronegativity Partial Charge Model (Livage and Henry) 47 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) 48 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  5ꞏ109 mol1s1 for Si(OEt)4 kh  103 mol1s1 for Ti(OEt)4 There is a problem to prepare mixed-metal oxides TiOSi Hydrolysis/condensation leads to phase-separated mixture of TiO2/SiO2 Partial Charge Model 49 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 50 51 H O H RO Si O RO RO R H OR Si O OR RO R H OR SiO OR OR H H O H + ROH + H Acid catalysed hydrolysis Hydrolysis Rate 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 52 Acid catalysed hydrolysis Hydrolysis Rate 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 Electron-donating Electron-withdrawing 53 Hydrolysis Rate H O RO Si OR RO RO OR Si O OR RO R OR SiO OR OR HH O + RO 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 54 Base catalysed hydrolysis Hydrolysis Rate Hydrolysis reaction rate increases as more alkoxy RO groups are hydrolyzed and replaced with OH groups Complete hydrolysis to Si(OH)4 Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi Electron-donating Electron-withdrawing 55 Si-OH becomes more acidic with increasing number of SiO-Si bonds Hydrolysis Rate Nucleophilic catalysis F- Si-F bonds HMPA N-methylimidazol N,N-dimethylaminopyridin acidity increases Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi 56 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 Hydrolysis Rate 57 Hydrophobic effect - Si(OR)4 are immiscible with water Cosolvent ROH is used to obtain a homogeneous reaction mixture and prevent phase separation Hydrolysis Rate Cosolvent properties affect rates: polarity, dipole moment, viscosity, protic behavior: Protic (EtOH) - bind to OSi Aprotic (THF) - bind to HOSi Alcohol produced during the reaction Alcohol/Alkoxide – transesterification Si(OR)4 + ROH  Si(OR)3(OR) + ROH Sonication – homogenization, emulsion Solvents affect drying 58 Acid catalysed condensation fast protonation, slow condensation Condensation Rate Positively charged transition state (TS), 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 Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi TS 59 Condensation Rate Base catalysed condensation fast deprotonation, slow condensation Negatively charged transition state (TS), 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 Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi Acid Catalysed Condensation 60 • 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 61 • 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 Acid/Base Catalysed Condensation 62 Reaction limited monomer-cluster growth (RLMC) or Eden growth Reaction limited cluster aggregation (RLCA) Acid catalysed Base catalysed 63 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 64 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 Gel point – modelling of a spannig cluster Kinetics of Sol-to-Gel Transition 65 29Si NMR Kinetics of Sol-to-Gel Transition 66 c = condensation degree, max 83 % viscosity Q0 = O4Si Q1 = O3SiOSi Q2 = O2Si(OSi)2 Q3 = OSi(OSi)3 Q4 = Si(OSi)4 67 The sol-gel reactions continue - unreacted species are retained in the porous structure - reactive groups on the surface Crosslinking condensation of the OH surface groups, stiffening and shrinkage Syneresis shrinkage causes expulsion of liquid from the pores Properties of the gel can be influenced by ageing time, ageing temperature, solvent Ageing of Gels 68 Coarsening Dissolution and re-precipitation process Materials dissolve from the convex surfaces and deposits at the concave surfaces: necks Ostwald Rippening Smaller particles have higher solubility than larger ones Phase Separation Fast gelation, different miscibility, isolated regions of unreacted precursor, inclusions of different structure, opaque, phase separation Strengthening Ageing at high temperature Soaking in a solvent with high solubilization power Ageing of Gels 69 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 r Pc  cos2  Drying of Gels 70 Water is wetting the surface: cos = 1 Water:  = 71.97 mN/m (25 C) Pore: r = 2.0 nm Pc = 72 MPa Coexistence of solid–vapor, liquid–vapor, and solid–liquid interfaces r P LV c  cos2  Solvent  (mN/m, at 20 C) Acetone 23.7 Isopropanol 21.7 n-Hexane 18.4 71 1. Supercritical drying – fluid state (= aerogel) 2. Freeze-drying - solvent substitution in the pores by another with high sublimation pressure and low expansion coefficient (= cryogel) 3. Ambient drying Surface modification by drying-control chemical additives - silylation by trimethylchlorosilane 4. Large pore gels 5. Ageing - stiffening Drying Methods r Pc  cos2 To avoid cracking: • No meniscus • Decrease surface tension • Increase wetting angle (isopropanol) • Increase pore size • Make a stiff gel cryogel xerogel aerogel 72 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 73 Aerogels - Supercritical Drying Silica aerogel • Wet gel is prepared • Byproducts, salts, water washing • Water replacement with acetone • Loading to autoclave • Supercritical drying Drain valve 74 Supercritical Drying Cold supercritical drying path in the Pressure (P) Temperature (T) phase diagram of CO2 - The gel containing excess amount of solvent (e.g., acetone) is placed in an autoclave - Liquid CO2 is pumped in at 4-10°C until the p = 100 bar (step 1) - The solvent extracted by the liquid CO2 is drained - The temperature is raised to 40 °C aboveTc of CO2 (step 2) - The fluid is slowly vented at constant T, resulting in a drop in p (step 3) - At ambient pressure, the system is cooled down to the room temperature (step 4) Liquid CO2 Gas CO2 75 Sintering Common ceramic and metallurgic manufacturing process A process of bonding, densification and/or recrystallization of powder compacts A treatment in which a green body is converted to a strong monolith Thermal (solid-state) sintering A powder pressed into a highly porous pellets 50-60 % of the maximum theoretical density = green pellets 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: • Liquid-phase sintering • Microwave sintering • Spark-plasma sintering • Oxidative sintering • Pressure-assisted sintering Control of sintering by sintering parameters: temperature, pressure, time, atmosphere 76 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 interior - 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 77 Sintering 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  As the contact area increases the dihedral angle widens Eventually it reaches a wide enough angle that the sintering stress is zero and sintering stops = the equilibrium dihedral angle e Sintering 78 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 of sintering is controlled by the diffusion rate and sintering stress The diffusion rate is affected by the defect concentration and temperature More defects = more atoms can diffuse simultaneously High temperatures = atoms to diffuse faster = sintering is done at high temperature 79 Sintering mechanisms - solid, liquid, gas phase 1. Evaporation-condensation and dissolution- precipitation 2. Volume diffusion – viscous flow (amorphous) 3. Surface diffusion 4. Grain boundary diffusion 5. Volume diffusion from grain boundaries 6. Volume diffusion of dislocations – plastic flow Sintering Mechanisms 80 Sintering Mechanisms Concave Convex Sintering 81 Stage 1 The particles increase their contact areas through the formation of necks, it ends once neck growth ceases to be the major mechanism Stage 2 The overall density increases as the pores decrease in size The contact areas grow into planes = grain boundaries The pores become columnar in shape as they shrink into tunnel systems on grain boundaries and triple junctions Stage 3 The 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 82 Agglomerates 3 m Agglomerates 0.5 m Larger agglomerates/grains = higher sintering temperature 83 Densification process Stage I. Below 200 C, weight loss, no shrinkage Desorption of liquid from pore surface 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 84 Densification - Sintering SPS/FAST – Spark Plasma Sintering 85 • 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, current 10 kA • Extreme heating rates: 1000 K/min • Near theoretical density at lower sintering temperature compared to conventional sintering techniques