Mechanochemical Synthesis Reaction Setup 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 1 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 2 Mechanochemical Synthesis Phase Transitions (to denser structures) Oxide Before V, Á3 After V, Á3 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 3 Mechanochemical Synthesis ^ Preparation of mixed oxides Al2O3(corundum) + SiO2 (xerogel) _^ mullite Al2O3 + La2O3 -^ LaAlO3 120 min Al2O3 + 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 jim) + S (50 \im) -► FeS in Argon ZnCl2 + Na2S -► ZnS + 2 NaCl CdCl2 + Na2S -► CdS + 2 NaCl 4 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 —► a-W + 3 MgO + C explosive a-W + 3 MgO + C -► WC 50 h (4-20 nm, MgO removed by HCl) Mechanochemical Synthesis ^Reactive milling Na2CO3 + SeO2 -► Na2SeO3 + CO2 2In + 3 urea.H2O2 + SnO2 —► In2O3 + SnO2 + 3 H2O + 3 urea heating to 473 K for 4h to remove organics and calcination at 573673 K in oxygen gives ITO FeCl2 + 2 CpNa —► 2 NaCl + Cp2Fe 6 Polymer Pyrolysis Preparation of: powders, monoliths, fibers, films, impregnation (PIP) Example: SiC fibers © polymer synthesis Li 400 °C, Ar Me2SiCl2 -► [Me2Si]6 -► [-SiMe2-]n soluble preceramic polymer Na Me2SiCl2 + MePhSiCl2 -► [-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 f$-SiC fiber 7 Polymer Pyrolysis Cl-CH2-SiCl3 8 Polymer Pyrolysis BN 1300 K, NH3 B10H14 + en -► polymer -► BN powder AlN anodic dissolution Al-► Al(NHR)3 CH3CN, RNH2, R4N+ >1100 K, NH3 -► AlN powder Thermolysis of Organometallic Coordination Polymers (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 9 Al2(NR)3 polymeric gel Thermolysis of Organometallic Coordination Polymers (Me3Sn)nM(CN)6 n = 3,4; M = Fe, Co, Ru thermolysis in Ar or H2 gives intermetallics 10 ll Microwave-Assisted Synthesis Microwave radiation = electromagnetic radiation Microwaves: X = 1 mm to lm, v = 0.3 to 300 GHz Microwave ovens 2.45 GHz, X = 12.24 cm power up to 1 kW, pulses, magnetron, microwaveguide, microwave cavity 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. 12 Microwave-Assisted Synthesis The energy of the microwave photon in this frequency region too low to break chemical bonds (0.0016 eV) lower than the energy of Brownian motion Microwaves cannot induce chemical reactions Microwave-enhanced chemistry is based on 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 13 Microwave-Assisted Synthesis Interaction of materials with microwaves: Xreflectors: metals, alloys (8 skin depth, large E gradients, discharges) X transmitters: quartz, zircon, glasses, ceramics (no TM), Teflon X absorbers: amorphous carbon, graphite, powdered metals, metal oxides, sulfides, halides, water 14 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 15 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 16 Dielectric Heating The applied field potential E of electromagnetic radiation E = Emax.cos(ox) Emax = the amplitude of the potential (V) co = the angular frequency (rad s-1) t = the time (s) If the polarization lags behind the field by the phase (8, radians) then the polarization (P, coulombs) varies as P = Pmax.cos(ct)T - 8) Pmax is the maximum value of the polarization 17 Dielectric Heating The current (I, A) varies as I = (dP/dt) = - co Pmax sin((QT — 8) 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 8 = 0), otherwise P = 0.5 PmaxEmaxG).sin(8) 18 Dielectric Properties The ability of a substance to convert electromagnetic energy into heat at a given frequency and temperature Loss factor tan8 tan8=s"/s' s" is the dielectric loss, indicative of the efficiency of radiation-to-heat conversion e' is the dielectric constant, the ability of molecules to be polarized by theelectricfield a high tan8 value required for efficient absorption and for rapid heating solvents can be classified as microwave absorbing high (tan8 >0.5) medium (tan8 = 0.1 - 0.5) low (tan8 <0.1) 19 Loss factors (tan8) of different solvents (2.45 GHz, 20 °C) Solvent tan 8 Solvent tan8 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 20 Temperature Gradients Microwave-Assisted Synthesis Examples of Microwave-assisted syntheses Si + C ^ p-SiC AG°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 22 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 PbNO3 + 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 23 Microvawe-Active Elements, Natural Minerals, and Compounds (2.45 GHz, 1 kW) element/ mineral/compound time (min) of T, K element/ time (min) of T, K microvawe exposure mineral/compound microvawe exposure Al 6 850 MnO2 6 1560 C (amorphous, < 1 |J,m) 1 1556 NiO 6.25 1578 C (graphite, 200 mesh) 6 1053 V2O5 11 987 C (graphite, < 1 |J,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 24 Microwave-Assisted Synthesis 25