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
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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
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 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
Mechanochemical Synthesis
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 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 m) + S (50 m) FeS in Argon
ZnCl2 + Na2S ZnS + 2 NaCl
CdCl2 + Na2S CdS + 2 NaCl
Mechanochemical Synthesis
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 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)
Mechanochemical Synthesis
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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 573-
673 K in oxygen gives ITO
FeCl2 + 2 CpNa 2 NaCl + Cp2Fe
Mechanochemical Synthesis
7
Example: SiC fibers
 polymer synthesis
Me2SiCl2 [Me2Si]6 [-SiMe2-]n
soluble preceramic polymer
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 -SiC fiber
Li 400 C, Ar
Na
Polymer Pyrolysis
Preparation of:
powders, monoliths, fibers, films, impregnation (PIP)
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Polymer Pyrolysis
Cl-CH2-SiCl3
(SiCH4)n
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Polymer Pyrolysis
Nature 440, 783-786 (6 April 2006) doi:10.1038/nature04613
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BN
B10H14 + en polymer BN powder
AlN
Al Al(NHR)3 Al2(NR)3 polymeric gel
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
1300 K, NH3
anodic dissolution
CH3CN, RNH2, R4N+
>1100 K, NH3
Polymer Pyrolysis
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
''
'
2 e
e
Dp



λ = wavelength of the microwave radiation.
Dp = several micrometers for metals and several tens of meters for lowloss
polymers
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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
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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
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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
Microwave-Assisted Synthesis
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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
Microwave-Assisted Synthesis
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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
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Microwave-Assisted Synthesis