1 Mechanochemical Synthesis 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 4 Mechanochemical Synthesis 5 Mechanochemical Synthesis 6 Mechanochemical Synthesis 7 Polymer Pyrolysis Preparation of: powders, monoliths, fibers, films, impregnation (PIP) 8 Polymer Pyrolysis Cl-CH2-SiCl3 (SiCH4)n 9 Polymer Pyrolysis Nature 440, 783-786 (6 April 2006) doi:10.1038/nature04613 10 Polymer Pyrolysis 11 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 12 Resorcinol-Formaldehyde Polymers 13 TEM of carbon xerogel carbonized at 1200 C 14 Microwave radiation = electromagnetic radiation Microwaves:  1 mm to 1m,  = 0.3 to 300 GHz Microwave ovens 2.45 GHz,  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. Microwave-Assisted Synthesis 15 16 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 17 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 18 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 19 Dielectric Properties Dipolar polarization, P P = ε0(εr − 1)E E = external electric field of strength E, potential (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 20 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 required for efficient absorption and for rapid heating 21 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 22 Dielectric Heating The applied field potential E of electromagnetic radiation E = Emax.cos( 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) = the time (s) 23 Dielectric Heating The current (I, A) varies as I = (dP/dt) =   Pmax sin  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 λ = wavelength of the microwave radiation. Dp = several micrometers for metals and several tens of meters for lowloss polymers 24 Interaction of materials with microwaves: reflectors: metals, alloys ( 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 Microwave-Assisted Synthesis 25 26 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 27 Microwave-Assisted Synthesis 28 Microwave-Assisted Synthesis 29 30 Microwave-Assisted Synthesis