1 Mechanochemical Synthesis • Precursor powder mixing • High-energy ball-milling for several hours • Ball-to-powder ratio (20:1) • Vial (250 ml) and balls (d = 10-20 mm) • WC, stainless steel, zirconia • 250 rotations per minute • Controlled atmosphere Reaction Setup 2 Mechanochemical Synthesis • Particles repeatedly subjected to deformation, cold welding, and fracture, homogenization on an atomic scale • On impact, high energy concentrated in a small spot, stress 200 MPa, duration of microseconds • Fragmentation, atomically clean surface exposed • Balance between fragmentation and coalescence • Grain size 10 nm • Amorphization, product nucleation and crystallization 3 Mechanochemical Synthesis Phase Transitions (to denser structures) Oxide Before V, Å3 After V, Å3 GeO2 quartz 40.3 rutile 27.6 TiO2 anatase 34.1 rutile 31.2 ZrO2 baddaleyite 35.2 fluorite 32.8 V = volume per formula unit Mechanical Alloying Ni + Nb  Nb40Ni60 amorphous 4 Mechanochemical Synthesis Preparation of mixed oxides Al2O3(corundum) + SiO2 (xerogel)  mullite Al6Si2O13 Al2O3 + La2O3  LaAlO3 120 min La2O3 + Mn2O3  LaMnO3 room temp., 180 min SnO + B2O3 + P2O5 + Li2O  (Li2O)2(Sn2BPO6)4 in dry N2, anodic material for lithium batteries Preparation of chalcogenides Fe (powder 4 m) + S (50 m)  FeS in Argon ZnCl2 + Na2S  ZnS + 2 NaCl CdCl2 + Na2S  CdS + 2 NaCl 5 Mechanochemical Synthesis Preparation of carbides, borides, nitrides, silicides Nb + C (graphite)  NbC (Fe impurities from abrasion) Nb + C + Cu + Fe  NbC/Cu/Fe cermet Ti + N2  TiN 60 h Ti + C  TiC 35 h Ti + 2 B  TiB2 15 h TiO2 + 2 Mg + C  TiC + 2 MgO (MgO removed by HCl) WO3 + 3 Mg + C  -W + 3 MgO + C explosive -W + 3 MgO + C  WC 50 h, 4-20 nm (MgO removed by HCl) 6 Polymer Pyrolysis Preparation of: powders, monoliths, fibers, films, impregnation (PIP) SiC fibers • polymer synthesis Me2SiCl2 + Li  [Me2Si]6 + LiCl  [-SiMe2-]n soluble preceramic polymer Me2SiCl2 + MePhSiCl2 + Na  [-SiMe2-SiMePh-]n • melt spinning or drawing from solution gives continuous polymer fiber • curing in O2, heat to 400 - 500 C, thermoset, crosslinking to prevent melting • pyrolysis at 1000 - 1500 C to polyxtalline -SiC fiber 400 C 7 Polymer Pyrolysis Cl-CH2-SiCl3 Mg in Et2O LiAlH4 SiC fiber 8 Polymer Pyrolysis Nature 440, 783-786 (6 April 2006) doi:10.1038/nature04613 spin-coating ink-jet printing silicon filmssilicon thin-film transistors 9 Polymer Pyrolysis BN B10H14 + en  polymer  BN powder 1300 K, NH3 AlN Al  Al(NHR)3 anodic dissolution, CH3CN, RNH2, R4N+ Al(NHR)3  Al2(NR)3 polymeric gel Al2(NR)3 polymeric gel  AlN powder 1100 K, NH3 10 Thermolysis of Organometallic Coordination Polymers Prussian blue (Me3Sn)nM(CN)6 n = 3,4; M = Fe, Co, Ru thermolysis in Ar or H2 gives intermetallics FeSn2, CoSn2, Ru3Sn7 thermolysis in air gives oxides Fe2O3/SnO2, Co2SnO4, RuO2 Resorcinol-Formaldehyde Polymers 11 TEM of carbon xerogel carbonized at 1200 C 12 Microwave radiation = electromagnetic radiation Microwaves:  = 1 mm to 1m,  = 0.3 to 300 GHz Microwave ovens 2.45 GHz,  = 12.24 cm All kitchen microwave ovens and all microwave reactors for chemical synthesis operate at a frequency of 2.45 GHz to avoid interference with telecommunication and cellular phone frequencies Power up to 1 kW, pulses, magnetron, microwaveguide, microwave cavity Microwave-Assisted Synthesis 13 14 The energy of the microwave photon in this frequency region is too low (10─5 eV) to break chemical bonds lower than the energy of Brownian motion at 298 K Microwaves cannot induce chemical reactions Microwave-enhanced chemistry The heating of materials by “microwave dielectric heating” effects = the ability of a material (solvent or reagent) to absorb microwave energy and convert it into heat Microwave-Assisted Synthesis 15 Microwave-Assisted Synthesis Dielectric heating Electric dipole reorientation in the applied alternating field The dipoles or ions aligning in the applied electric field applied field oscillates, the dipole or ion field attempts to realign itself with the alternating electric field Energy is lost in the form of heat through molecular friction and dielectric loss If the dipole does not have enough time to realign, or reorients too quickly with the applied field, no heating occurs 16 Microwave-Assisted Synthesis Resistive heating polarization current, a reorientation phase lag Joule heating ionic current, ionic conduction, ions drift in the applied field Electronic transport metal powders, semimetallic and semiconducting materials Rotational excitation: weak bonds (interlayer bonds in graphite and other layer materials) Eddy currents: metal powders, alternating magnetic fields Microwave absorption = f (frequency, temperature) Thermal runaway = increased dielectric loss at higher T 17 Dielectric Properties Dipolar polarization, P Dipole moment per volume P = ε0(εr − 1)E E = external electric field (V) ε0 = permittivity of free space εr = relative permittivity of a material ε* permittivity is a complex quantity: ε* = ε0εr ε* = ε′ + iε″ ε′ = time-independent polarizability of a material in the presence of an external electric field ε″ = time-dependent component of the permittivity, quantifies the efficiency with which electromagnetic energy is converted to heat  18 Dielectric Properties The ability of a substance to convert electromagnetic energy into heat at a given frequency and temperature Loss factor tan tan = ’’/’ ’’ is the dielectric loss, the efficiency of radiation-to-heat conversion ’ is the dielectric constant, the ability of molecules to be polarized by the electric field A high tan value is required for efficient absorption and for rapid heating 19 Loss Factors (tan) of different solvents (2.45 GHz, 20 ºC) Solvent tan Solvent tan ethylene glycol 1.350 DMF 0.161 ethanol 0.941 1,2-dichloroethane 0.127 DMSO 0.825 water 0.123 2-propanol 0.799 chlorobenzene 0.101 formic acid 0.722 chloroform 0.091 methanol 0.659 acetonitrile 0.062 nitrobenzene 0.589 ethyl acetate 0.059 1-butanol 0.571 acetone 0.054 2-butanol 0.447 tetrahydrofuran 0.047 1,2-dichlorobenzene 0.280 dichloromethane 0.042 NMP 0.275 toluene 0.040 acetic acid 0.174 hexane 0.020 Microwave absorbing properties high tan > 0.5 medium tan 0.1–0.5 low tan < 0.1 20 Dielectric Heating The applied field potential E of electromagnetic radiation E = Emax.cos(t) If the polarization lags behind the field by the phase (, radians, phase lag) then the polarization (P, coulombs) varies as P = Pmax.cos(  ) Pmax is the maximum value of the polarization Emax = the amplitude of the potential (V)  = the angular frequency (rad s-1)  = time (s) 21 Dielectric Heating The current (I, A) varies as I = (dP/dt) =   Pmax sin(t - ) The power (P, watts) given out as heat is the average value of (current x potential) P is zero if there is no lag (i.e., if  = 0), otherwise P = 0.5 PmaxEmax.sin() The penetration depth, Dp, is the distance into the sample at which the electric field is attenuated to 1/e of its surface value '' ' 2 e e Dp    λ = wavelength of the microwave radiation Dp = several micrometers for metals and several tens of meters for low-loss polymers 22 Interaction of materials with microwaves: Reflectors: metals, alloys (Dp  skin depth, large E gradients, discharges) Transmitters: quartz, zircon, glasses, ceramics (TM free), Teflon Absorbers: amorphous carbon, graphite, powdered metals, metal oxides, sulfides, halides, water Microwave-Assisted Synthesis 23 Temperature Gradients MW Oil bath Microwave heating profiles for pure water () 0.03 M sodium chloride solution () at constant 150 W power Solvent T, °C ' '' Skin, cm tan Ethylene glycol 25 37 49.95 0.55 1.35 Water 25 78 10.33 3.33 0.13 24 Microwave-Assisted Synthesis Examples of Microwave-assisted syntheses Si + C  -SiC G298 = 64 kJ/mol silica crucible, 1 kW, 4-10 min, 900 C, inert ambient (I2), conventional process requires 1400 C metal + chalcogenide  ME evacuated quartz ampoules, 5-10 min, 900 W, melting, light emission PbSe, PbTe, ZnS, ZnSe, ZnTe, Ag2S Mo + Si + graphite  MoSi2 high mp, oxidation and carbidation resistance, metallic conductivity, heating elements and high-T engine parts 25 Microwave-Assisted Synthesis Mixed oxides Y2O3 + BaO + CuO  YBa2Cu3O7-x 200 W, 25 min BaO + WO3  BaWO3 500 W, 30 min Amorphous carbon is a secondary susceptor, does not react with reagents or products (carbothermal reduction) C burns and initiates decomposition of carbonates or nitrates BaCO3 + TiO2 + C  BaTiO3 + CO2 Pb(NO3)2 + TiO2 + C  PbTiO3 + CO2 NaH2PO4.2H2O = good MW susceptor, rotational excitation of water, dehydrates to NaPO3, melts, 700 C in 5 min Na2HPO4.2H2O, KH2PO4 no MW heating NaH2PO4.2H2O + ZrO2  NaZr2(PO4)3 NASICON superionic conductor, 8 min Microwave-Assisted Synthesis 26 Microvawe-Active Elements, Natural Minerals, and Compounds (2.45 GHz, 1 kW) element/ mineral/compound time (min) of microvawe exposure T, K element/ mineral/compound time (min) of microvawe exposure T, K Al 6 850 MnO2 6 1560 C (amorphous, < 1 m) 1 1556 NiO 6.25 1578 C (graphite, 200 mesh) 6 1053 V2O5 11 987 C (graphite, < 1 m) 1.75 1346 WO3 6 1543 Co 3 970 Ag2S 5.5 925 Fe 7 1041 Cu2S 7 1019 Mo 4 933 CuFeS2 (chalcopyrite) 1 1193 V 1 830 FeS2 (pyrite) 6.75 1292 W 6.25 963 MoS2 7 1379 Zn 3 854 PbS 1.25 1297 TiB2 7 1116 CuBr 11 995 Co2O3 3 1563 CuCl 13 892 CuO 6.25 1285 ZnBr2 7 847 Fe3O4 (magnetite) 2.75 1531 ZnCl2 7 882 Wire Explosion 27 Nano tungsten oxide WO3 particles W wire in low pressure of oxygen TEM