Precursor Methods Goals - decrease difussion paths, shorten reaction times and temperatures Intimate mixing of components in solution, precipitation, filtration, washing, drying, calcination é* High degree of homogenization é* Large contact area é* Reduction of diffusion distances é* Faster reaction rates é* Lower reaction temperatures é* Metastable phases, smaller grain size, larger surface area i Coprecipitation Method Coprecipitation applicable to nitrates, acetates, oxalates, hydroxides, alkoxides, beta-diketonates Requires: similar salt solubilities similar precipitation rates no supersaturation Washing: water, organic solvents Drying: evaporation azeotropic distillation freeze-drying Disadvantage: difficult to prepare high purity, accurate stoichiometric phases if solubilities do not match 2 Coprecipitation Method Spinels oxalates: Zn(C02)2/Fe2[(C02)2]3/H20 1 : 1 mixing, H20 evaporation, salts coprecipitation Solid-solution mixing on atomic scale, filter, calcine in air Zn(C02)2 + Fe2[(C02)2]3 -> ZnFe204 + 4CO + 4C02 A1203 Bayer Process bauxite NaOH'p» Al(OH)4- ^% Al(OH)3 1^ra-Al203 I Fe(OH)3, Ti02, Si02 BaTi03 BaCl2 + TiOCl2 + 2 H2C204 + 4 H20 + Ln dopants -------------► BaTiO(C204)2.4H20 + 4 HCl filtration, washing, drying, calcination @ 730 °C 3 Coprecipitation Method Spinel A1(N03)3 + Mg(N03)2 + H20 freeze-drying gives amorphous mixture, calcination @ 800 °C !!! low T Mg(N03)2 + 2 A1(N03)3 -----►MgAl204 + 6 NOx + (10-3x)O2 random Ruby Ion exchange A1(N03)3 + Cr(N03)3 -----► Al(OH)3 + Cr(OH)3 sol freeze drying gives solid (Al/Cr)(OH)3 @ LN2 temperature, 5 Pa anealing @ 950 °C for 2.5 h gives solid solution Al2.xCrx03 Zirconia ZrSi04(zircon) + NaOH ____► Na2Zr03 + Na2Si03 ____► ZrOCl2 °^^ Zr(OH)4 / Y(OH)3 ^^ ► nano-Y/Zr02 Coprecipitation Method High-Tc Superconductors 1373 K La3+ + Ba2+ + Cu2+ + H2C204 —► ppt --------► LaL85Bao.15Cu04 Magnetic garnets, tunable magnetic materials Y(N03)3 + Gd(N03)3 + FeCl3 + NaOH -► YxGd3xFe5012 Firing @ 900 °C, 18-24 hrs, pellets, regrinding, repelletizing, repeated firings, removes REFe03 perovskite impurity Isomorphous replacement of Y3+ for Gd3+ on dodecahedral sites, solid solution, similar rare earth ionic radii complete family accessible, 0 < x < 3,2Fe3+ Oh sites, 3Fe3+ Td sites, 3RE3+ dodecahedral sites Pechini and Citrate Gel Method Aqueous solution of metal ions i—cooh HO—----COOH C O O TT Chelate formation with citric acid Polyesterification with polyfunctional alcohol on heating Further heating leads to resin, transparent glassy gel calcination provides oxide powder Control of stoichiometry by initial reagent ratio Complex compositions, mixture of metal ions Good homogeneity, mixing at the molecular level Low firing temperatures Pechini and Citrate Gel Method BaTi03 by conventional powder method at 1200 °C Ba2+ + Ti^Pr^ + citric acid at 650 °C Sc203 + 6 HCOOH -----------► 2Sc(HCOO)3 + 3 H20 MnC03 + 2 HCOOH -----------► Mn(HCOO)2 + C02 + H20 added to citric acid, water removal, calcination @ 690 °C gives ScMn03 without citric acid only mixture of Sc203 and Mn203 is formed Double Salt Precursors Double salts of known and controlled stoichiometry such as: Ni3Fe6(CH3COO)1703(OH). 12Py Burn off organics 200-300 °C, then 1000 °C in air for 2-3 days Product highly crystalline phase pure NiFe204 spinel Good way to make chromite spinels, important tunable magnetic materials Juggling the electronic-magnetic properties of the Oh and Td ions in the spinel lattice Chromite spinel Precursor Ignition T, °C MgCr204 (NH4)2Mg(Cr04)2.6H20 1100-1200 NiCr204 (NH4)2Ni(Cr04)2.6H20 1100 MnCr204 MnCr207.4C5H5N 1100 CoCr204 CoCr207.4C5H5N 1200 CuCr204 (NH4)2Cu(Cr04)2.2NH3 700-800 ZnCr204 (NH4)2Zn(Cr04)2. 2NH3 1400 FeCr204 (NH4)2Fe(Cr04)2 1150 Single Source Precursor Compounds containing desired elements in a proper stoichiometric ratio Easy chemical pathway for ligand removal M k___ H2° + (M-0-Si03)x 7v 9 Vegard's Law Vegard law behavior: Any property P of a solid-solution member is the atom fraction weighted average of the end-members The composition of the A1-XBX alloy can be calculated from Vegard's law The lattice parameter of a solid solution alloy will be given by a linear dependence of lattice parameter on composition: a(AlxBx) = x a(B) + (1-x) a(A) 10 Vegarcľs Law cíCdSe^SJ = x c(CdS) + (1-x) c(CdSe) 7.0C e,9c — í: í A] ■5.GC t—r—i—t—i—i—i—i—r É 70'____I------L_^J------1------1------1------1------J------"------1 0 10 20 3D 40 SO *0 70 50 Ů1 100 mole peRC&rr cast in Cds^t *i 11 Vegarcľs Law P (YxCe!xRhIn5) = x P (YxCe!xRhIn5) + (1-x) P (YxCe!xRhIn5) Any property P of a solid-solution member is the atom fraction weighted average of the end-members Tetragonal lattice constant a as a function of Y concentration x for the Ce1_xYxRhIn5 system 0 0.2 0.4 0.6 0.6 1 Y conoemiration (mol) 4,65 4.Ü4 -i------1------1------r-----r-----p------r-----1------1------1------1------r-----i-----r-----r-----■-----------1------■-----■ Lattice Paramater a vs. Y concentration in Ce v Rhln 1-í *. s Vrftafds Law b 4,62 4 R C A *íR —i__I__i__ĺ Vegarcľs Law A linear relationship exists between the concentration of the substitute element and the size of the lattice parameters The direction of the linear relationship, increasing or decreasing, depends upon the system being analyzed As the concentration of Y is increased, lattice constant a decreases, implying the cell is contracting along the a axis 13 Vegarcľs Law Vegard law behavior: P (YxGd3xFe5012) = x/3 P (Y3Fe5012) + (3-x)/3 P (Gd3Fe5012) Any property P of a solid-solution member is the atom fraction weighted average of the end-members 14 Tunable magnetic properties by tuning the x value in the binary garnet YGd3xFe5012 3 Td Fe3+ sites, 5 UPEs 2 Oh Fe3+ sites, 5UPEs Ferrimagnetically coupled material, oppositely aligned electron spins on the Td and Oh Fe3+ magnetic sublattices Counting spins Y3Fe5012 ferrimagnetic at low T: 3x5-2x5 = 5UPEs Counting spins Gd3Fe5012 ferrimagnetic at low T:3x7-3x5 + 2x5 = 16 UPEs YxGd3 xFe5012 creates a tunable magnetic garnet that is strongly temperature and composition dependent, applications in permanent magnets, magnetic recording media, magnetic bubble memories and so forth, similar concepts apply to magnetic spinels 15 Flux Method Molten salts (inert or reactive), oxides, metals MNO3, MOH, (M = alkali metal) FLINAK: LiF-NaF-KF M2Qx (M = alkali metal, Q = S, Se, Te) molten salts ionic, low mp, eutectics, completely ionized act as solvents or reactants, T = 250-550 °C enhanced diffusion, reduced reaction temperatures in comparison with powder method products finely divided solids, high surface area (SA) slow cooling to grow crystals separation of water insoluble product from a water soluble flux incorporation of the molten salt ions in product prevented by using salts with ions of much different sizes than the ones in the product (PbZr03 in a B203 flux) Flux Method Lux-Flood formalism oxide = strong base acid = oxide acceptor A + OB —► AO + B base = oxide donor 700 K Zr(S04)2 + eut. (Li/K)N03 --------► Zr02 540 K Zr(S04)2 + eut. (Li/K)N02 --------► Zr02 520 K ZrOCl2 + eut. (Na/K)N03------► Zr02 amorph. —► t- Zr02 720 K ZrOCl2 + YCI3 + eut. (Na/K)N03--------► Zr02 BaC03 + SrC03 + Ti02 + eut. (Na/K)OH _________^ cubic-Bao.75 Sr025TiO3 17 Flux Method fly ash (aluminosilicates) NaOH, NH4F, NaN03 ► zeolites (sodalite, cancrinite) NH4H2P04 + (Na/K)N03 + M(N03)2 ------------► (Na/K)MP04 4 SrC03 + A1203 + Ta205 ________► Sr2AlTa06 900 °C in SrCl2 flux 1400 °C required for a direct reaction K2Tex + Cu -------► K2Cu5Te5 K2Tex reactive flux, 350 °C 18 Flux Method Electrolysis in molten salts Reduction of Ti02 pellets to Ti sponge in a CaCl2 melt at 950 °C O2- dissolves in CaCl2, diffuses to the graphite anode insulating Ti02 —> Ti02 x conductive graphite anode anodic oxidation 2 O2- —> 02 + 4 e~ cathode Ti02 pellet cathodic reduction Ti4+ + 4 e- -> Ti 19 Ionic Liquids Organic cations (containing N, P) Inorganic anions: CI, A1C14, A12C17, A13C110~, PF6, SnCl3, BC13, BF4", NO3, OSO2CF3 (triflate), CH3C6H4S03, N(S02CF3)2, P043 20 Ionic Liquids Oldest known (1914): EtNH3+N03 mp 12 °C ■^Liquids at room temperature or low mp ■^Thermal operating range from -40 °C to 400 °C ■^Higly polar, noncoordinating, completely ionized ■^Nonvolatile - no detectable vapor pressure ^Nonflamable, nonexplosive, nonoxidizing, high thermal stability ■^Electrochemical window > 4V (not oxidized or reduced) ■^Immiscible with organic solvents ■^Hydrophobic IL immiscible with water 21 Ionic [NR„íW+ [SR^H^] [PIMW* lí+ 1 J 3 4 r \ 'S. 0 1 R s R, Ri R, R2 R5 Rí 7 s * A «, A R4 R2 it 11 liquids N .SCMľFi SO-,CFT #CH O BH ,CN CN 14 1í ]ft 22 Synthesis of Ionic Liquids NR3+RCI -> [NR4]+C1- Aluminates [NR4]+ Cl" + AICI3 -» fNR4]+ f AICIJ" Metal halide elimination [NR4]+ Cl" + MA -» MCI + [NR4]+A- Reaction with an acid [NR4]+ Cl" + HA -» HCl + [NR4]+A- Ion exchange [NR4]+ Cl" + Ion exchanger A -> [NR4]+A" 23 Halogenoaluminate(III) Ionic Liquids The most widely studied class of IL High sensitivity to moisture - handling under vacuum or inert atmosphere in glass/teflon RC1 + AICI3 £? R+ [A1C1J- 2 [A1C1J- *5 [A12C17]- + CI- autosolvolysis Keq = 1016 to 1017 at 40 °C 2[A12C17]-^ [A13C110]-+[A1C14]- Acidic: excess of A1C13 as [A12C17]- x(AlCl3) > 0.5 Basic: excess of CI" x(AlCl3) < 0.5 Neutral: [A1C1J- x(AlCl3) = 0.5 24 Equilibria in Halogenoaluminate(III) IL Equilibria in IL XI = ci- X4 = [AICI4]-X7 = [A12C17]-X10 = [A13C110]-X13 = [A14C113]-X6 = A12C16 * 1.0 *m (Xn) o.a » 0.6 - 04 U ^ 0.0 0-0 0-2 0,4 Q.G OS 1.0- y (AICIJ -+ 25 Halogenoaluminate(III) Ionic Liquids 2 [A1C14]- í; [A12C17]- + CI- autosolvolysis K^ = 1016 to 1017 at 40 °C Acidic IL with an excess of A1C13 HCl + [A12C17]- * H+ + 2 [A1C1J- Proton extremely poorly solvated = high reactivity Superacid [EMIM]C1/A1C13/HC1 H0 = -19 (HS03F: H0 = -15) Latent acidity MCI + [A12C17]- £? M+ + 2 [A1C14]- buffered IL B + JVT+ [AICIJ- t? MC1+ B-AICI3 26 Superacidity Liquid HF-£bF5 2S 24 ať JriLf in njf liquid HF-TüFj ľSO^I 100% H3St>4 HT 10 14 11 tf Solid 27 Superacidic [EMIM] C1/A1C13/HC1 i»ttii.»m#'hylti#r:*rt* ni 4-1.4 Blp-Fbnyl haphlhd+ne 3H-f lüOrfnt: 1 II III -!5 ^.Ď "í 1 CO n Cirrsepe 3-MnLhylomMľitel»nlhriunr X -6* I = not protonated II = slightly protonated III and IV = 10-20 % V = 75-90% VI-VIII = nearly completely IX and X = completely log Kb in HF 28 Ionic Liquids Completely inorganic ionic liquids Compound mp(K) Compound mp(K) Na13[La(TiW11039)2] 253.0 Na13[Tm(TiWn039)2] 260.2 Na13[Ce(TiW11039)2] 263.0 Na13[Yb(TiWn039)2] 267.2 Na13[Pr(TiWn039)2] 253.0 Na5[CrTiWn039] 261.5 Na13[Sm(TiWn039)2] 256.0 Na5[MnTiWn039] 253.0 Na13[Gd(TiWn039)2] 265.1 Na5[FeTiWn039] 257.6 Na13[Dy(TiWn039)2] 265.2 Na6[ZnTiWn039] 257.4 Na13[Er(TiWn039)2] 261.0 29 Melting Point of Ionic Liquids Melting point is influenced by: Cation - low symmetry, weak imtermolecular interactions, good distribution of charge Anion - increasing size leads to lower mp Composition - Phase diagram '00 Phase diagram of [EMIM]C1/A1C13 X {AlCIa) O.fl 30 Melting Point of Ionic Liquids ^N *- Me-" "^ "R R X mp/^C Me CI 125 f;l CI 87 n-Bu CI 65 e:l NO-, 38 Et AlClj 7 Et BFi * Et CF,303 -9 Et (CBS03)2N -3 Et CFjC02 -14 n-B u CFjSOj 16 31 Density of Ionic Liquids The density of IL decreases as the bulkiness of the organic cation increases: t P*i i lis I 1J [.*.] _l R-Ma R-lfc R-Ue Fl-Wf- R-Bu H-Mj H^' Rift *-Qu n>fcj 32 Viscosity of Ionic Liquids The viscosity of IL depends on: van der Waals interactions H-bonding Anion [A] n I* ľ] 'ÖV ^^ [A) C K,S Oj 90 fl-QF^SOj- 373 Ch.COO n n-CjFŤ<:ao 1S2 (CbySCVjnN 52 33 Solubility in/of Ionic Liquids Variation of the alkyl group Increasing nonpolar character of the cation increases solubility of nonpolar solutes. Water solubility depends on the anion water-soluble [BMIM] Br, CF3COO, CF3S03 Water-immiscilble [BMIM] PF6 (CF3S02)2N IL miscible with organic solvent IF their dielectric constant is above a certain limit given by the cation/anion combination Polarity by E(T)(30) scale [EtNH3] [N03] 0.95 between CF3CH2OH and water [BMIM] PF6 as methanol 34 Solubility in/of Ionic Liquids L-Iickcik / IL (wi - (CFjSOi^fr -CFjCOO 35 Applications of Ionic Liquids Electrodeposition of metals and alloys (also nanoscopic) Al, CoAlx, CuAlx, FeAlx, AlTix Semiconductors Si, Ge, GaAs, InSb, CdTe Electrodeposition of a Bi-Sr-Ca-Cu alloy (precursor to SC oxides) Melt of MeEtlmCl at 120 °C BiCl3, SrCl2, CaCl2, CuCl2 dissolve well Constituent BiCl3 SrCl2 CaCl2 CuCl2 Concentration 0.068 0.50 0.18 0.050 (mol kg1 MeEtlmCl) Substrate Al -1.72 V vs the Ag/Ag+ reference electrode 36 Applications of Ionic Liquids Biphasic solvent systems Preparation of aerogels 2HCOOH + Si(OMe)4 ----»ag-SiC^ + 2 MeOH + 2 HCOOMe Natural gas sweetening (H2S, C02 removal) Electrolytes in batteries or solar cells Dissolving spent nuclear fuel (U4+ oxidized to U6+) Extraction Enyzme activity 37 Applications of Ionic Liquids Olefin polymerization Ethene + TiCl4 + AlEtCl2 in acidic IL Ethene + Cp2TiCl2 + Al2Me3Cl3 in acidic IL Cp2TiCl2 + [cation]+[Al2Cl7]- *5 [Cp2TiCl] + + [cation]++ 2 [A1C1J- Olefin hydrogenation Cyclohexene + H2 + [RhCl(PPh3)3] (Wilkinson's catalyst) Sound Sound = pressure wave, periodic compression/expansion cycles traveling through a medium possessing elastic properties (gas, liqud, solid) Liquids and gases - longitudinal pressure waves - compression/rarefaction Solids - longitudinal and transverse waves The energy is propagated as deformations in the media The molecules oscillate about their original positions and are not propagated The propagation of a sound wave = the transfer of vibrations from one molecule to another 39 Sound In a typical liquid, the speed of sound decreases as the temperature increases, at all temperatures. The speed of sound in water is almost five times greater than that in (340 m s1) Longitudinal Pressure Waves C?-iprö5ii?- lara ,' r-Kbon Compression t ^r.ref'iiciion y j err F'= j? en J 41 Acoustic Pressure Pa = PAsin27ift Pa acoustic pressure PA pressure amplitude f sound frequency c = A,f (for 20 kHz, X = 7.5 cm) p = p _|_ p * total ra T rh Ph hydrostatic pressure compression compression displacement (x) graph 42 stojatá vina nízký tisk vysoký tlak [ÍLU rozpínáni bubliny kolaps bubliny TC . .P vyzářeni SvéUa 43 Acoustic Pressure PA = driving pressure amplitude [Pa] I = irradiation intensity [W nr2] p = liquid density [kg nr3] c = sound velocity in liquid [m s_1] (Water 1482 m s"1) 44 Speed of Sound Substance Speed of sound [m s Air 343 Helium 965 Water 1482 Lead 1960 Steel 5960 Granite 6000 The speed of sound The speed of sound (w) u2 = 1/Ksp = [ôP/ôp]s - l/(<(tV)2>) where ks is the adiabatic compressibility p is the density and P the pressure. 1600 3? 1300 —ŕ 1200/ 50 100 150 Temperature, °C 46 200 Sound Intensity Sound Intensity = Power / area = Watts/m2 Source of Sound Intensity (W/m. 2) Sou Jet Airplane 30 m away 102 140 Air-raid Siren, nearby 1 120 Threshold of Pain 101 120 Concert ~10i 115 Riveter 103 100 Busy Traffic 105 70 Normal Conversations 106 60 Whisper io-i° 20 Threshold of Hearing 10i2 0 0 dB (10 i2 W/m2) 10 dB = 10 as intense 20 dB = 102 as intense 30 dB = 103 as intense 120 dB = 10i2 as intense 47 Ultrasound Frequencies from 20 kHz to 50 MHz o 10 10 i 1 10 10 1 1 10 10 J» Human hearing ^ Conventional powsr ultrasound Extended range for sonochenistry Diagnostic ultrasound v, Hz 16Hz-18kHz 20kHz -100kHz 20kHz - 2MHz 5MHz- 10MHz 48 Sonochemistry Suslick, K. S.; Price, J. P. Ann. Rev. Mater. Sei. 1999,29,295-326. Mason, T. J.; Lorimer, J. P. Applied Sonochemistry, Wiley-VCH, Weinheim, 2002. Gedanken, A. Ultrason. Sonochem. 2004, //, 47-55. Mastai, Y.; Gedanken, A. In: Rao, C.N.R.; Mueller, A.; Cheetham, A. K. (Eds.), The Chemistry of Nanomaterials, Wiley-VCH, NY, 2004,113-169. 49 Sonochemical Reactions No direct interaction of US field with molecules Liquid phase reactions - chemical reactions driven by cavitation effects Solid state reactions - introduction of defects = speeding up diffusion 50 Hydrodynamic Cavitation the passage of liquid through (an orifice plate) the kinetic energy/velocity of the liquid increases at the expense of the pressure throttling causes the pressure to fall below the threshold pressure for cavitation (vapor pressure) cavities are generated the liquid jet expands, the pressure recovers the collapse of the cavities 51 Acoustic Cavitation Cavitation effects = creation, growth, and implosive collapse of bubbles in a liquid TRANSIENT CAVITATION: THE ORIGIN OF SONOCHEMISTRY Compression RAPID QUENCHING -I------- stable cavitation - bubbles oscillate for many cycles transient cavitation - transient cavities expand rapidly collapse violently 52 Time \ i*sac) Acoustic Cavitation Bubble formation = overcoming tensile strength of the liquid (pure water 1500 bar, only 50 bar available) Weak spots = dissolved gas molecules, solid particles, trapped gases slow bubble growth (300 jus), energy absorption, size oscillations critical size (170-300 urn) = most efficient energy absorption, rapid growth, inefficient energy absorption, collapse mnafaction rarďaction n&efaction tarďacticn raf&acth O o Q O O BUBBLE FORMS •GROWS IN SUCCESSIVE CYCLES REACHES UNSTABLE SIZE 53 Cavitation Bubble collapse = implosion ( 1 ns) • HOMOGENEOUS liquid: spherically symmetrical implosion, shear forces Hot spots = adiabatic compression, life time 2 jus temperature of the gas inside bubble 5 000 - 20 000 °C, surrounding liquid layer 2000 °C pressure 500 - 1000 bar cooling rate 1010 K s-1 -i red hot steel poured into water 2500 K s • HETEROGENEOUS liquid-solid interface: asymmetrical implosion, high speed microjets of liquid (400 km h_1) i Microjet - bubble implosion Homogeneous Sonochemistry Two-Site Mechanism Inside the cavity gases and vapors temperatures 5 000 - 20 000 °C pressure 500 - 1000 bar ■Surrounding liquid layer temperatures 2000 °C ^ Bulk liquid shear forces 55 o Two-site Mechanism -h $ IN THE CAVITY extreme conditions on collapse •"" <^ 5Q0Q°C and 2DD0 atmospheres A/N-t r^ ^^ ÍN THE BULK MEDIA Č h. ^ intense shear forces 56 Heterogeneous Sonochemistry Solid surfaces = implosion, microjets, Shockwaves 200 |iim minimum particle size at 20 kHz for microjets surface erosion removal of unreactive coatings (oxides, nitrides, carbonaceous) fragmentation of brittle materials, increased surface area 57 Heterogeneous Sonochemistry LARGE PARTICLES SMALL PARTICLES © surface cavitation due to defects leading to fragmentation © CD collision can lead to surface erosion or fusion 58 Heterogeneous Sonochemistry Solid particles in liquid = shock waves high speed interparticle collisions (500 km/s) surface smoothing, surface coating removal localized melting of metal particles at the impact point fragmentation, increased surface area intercalation rates enhanced SONOCHEMICAL METHOD ♦ Solid particles in liquid = shock waves, high speed interparticle collisions (500 km/s) surface smoothing, surface coating removal Ni catalytic activity in hydrogenation increased 105 fold by NiO removal localized melting of metal particles at the impact point fragmentation, increased surface area intercalation rates enhanced 200 fold in layered oxides and sulfides (V205, M0O3, MoS2, ZrS2, TaS2) 60 Heterogeneous Sonochemistry Metal powders Cr (mp 2130 K) and Mo (mp 2890 K) agglomerate W (mp 3683 K) does not temperature at the point of impact ~ 3000 °C ** * u* -i*\ ' i í f., . í-'■ * '„■' ... - j* i»• ; * ... . Before ultrasound m.p.: 2130 K W m.p.: 3Ě83K ll -m. - ä--."y- ;..■" * - r- • . - » 30 min. ultrasound Cavitational Corrosion of the Tip 62 Control of sonochemical reactions ■ frequency 20-40 kHz, the higher the frequency, the higher power needed to actuate cavitation, rarefaction phase shortens at high freq. ■volatile reactants, primary reaction site inside the bubbles, diameter 200 jum, 5000 °C, easier bubble formation, more vapors inside bubbles, but the cavitation is cushioned ■ nonvolatile reactants, reaction in the thin layer (200 nm) surrounding the bubble, 2000 °C, less cushioning, more energetic cavitation (collapse) ■use high boiling solvents, high vapor pressure inside the bubble cushions the implosion ■less cavitation in viscous liquids, viscosity resists shear forces ■reaction rates decrease with increasing temperature, more vapors in bubbles ■ low surface tension facilitates cavitation, in water add surfactants 63 Cavitation in Water 105 E 103 ^ 102 i^V * 1.0 0.1 — i J f 1 I í K 3 102 103 104 10s 106 10' Frequency (Hz) The frequency dependence of the intensity required to produce cavitation for degassed water at room temperature. The intensity required to produce vaporous cavitation above the frequency of 100 kHz rises rapidly. 64 The effect of temperature on cavitation c o 4 h- m "Í 3 > d 2 -* 10 20 30 40 50 60 70 Temperature (°C) The effect of temperature on cavitation and its associated hysteresis effect for tap water. The increase in intensity as the temperature is increased can be observed before it falls away at the boiling point. When the temperature is allowed to fall an increase in intensity is found in the region of 50-60 °C. This is quite a significant effect and appears to occur in all liquids. 65 Control of sonochemical reactions ■ ambient gas important energy developed on bubble collapse: monoatomic (Ar) > diatomic (N2) > triatomic (C02) Xe: low thermal conductivity, heat of the collapsing cavity retained He: high thermal conductivity, heat of the collapsing cavity dissipitated, no reaction ■external pressure, higher pressure suppresses bubble formation but makes cavitation more energetic, optimum pressure for given frequency ■temperature, higher temperature increases vapor pressure of a medium, lowers viscosity and surface tension, many bubbles formed at temps, close to solvent boiling point, a barrier to sound transmission ■ intensity, minimum intensity for cavitation threshold, depends on freq., optimum intensity for given reaction conditions, at high powers great number of bubbles hinder sound transmission, decoupling of a liquid from the source, breakdown of transducer material ■ sound attenuation is proportional to the frequency, more power needed at high freq. 66 Generation of Ultrasound a transducer - device converting one type of energy into other gas driven whistle (F. Galton), liquid atomizer siren liquid driven electromechanical liquid whistle homogeniser, a jet of liquid passed through an orifice on a thin metal blade, vibrations, cavitation, mixing of immiscible liquids, ketchup, mayonnaise magnetostrictive, Ni, Co/Fe, Al/Fe, Tb/Dy/Fe alloys shrink when placed in mg. field, solenoid, pulses, upper limit 100 kHz, cooling piezoelectric, oposite charges applied on crystal sides, contraction/expansion, quartz, Pb(Zr/Ti)03 ceramics (PZT), up to MHz Generation of Ultrasound casing containing transducer element upper fixed horn (booster) detachable horn replaceable tip generator screw fitting at null point 68 Sonochemical Reactor SONOCHEMICAL METHOD Ti alloy horn, minimum lenght is a half-wavelength of sound in a material, 26 cm for 20 kHz in Ti, multiples of 13 cm vibration amplitude 5-50 \xm Sandwich transducer operating at 1-200 kHz PZT wafers Back Ht , 7 H—r* i L Front Mass 10-2Újrm 0 Strass 71 Sonochemical Reactor Ultrasound Processor VCX 500 W Frequency 20 kHz 0 to 40 °C Argon (flow rate 62 cm3 min-1) TIME of ultrasound treatment PULSE irradiation and a dwell time 2:2 TEMP maximum temperature 50 °C AMPL amplitude 50 % SONOCHEMICAL METHOD ♦ Liquids = heating/cooling by cavity implosions H20 —► H + OH —► H2 + H202 precursor decomposition: metals Fe(CO)5 ___► Fe + 5 CO oxides Ga3+ + H20 -----► Ga(0)(OH), diaspóre nitrides, carbides, sulfides alkane cracking polymer degradation, lower MW, surface modification emulsification of immiscible liquids (oil-water, Hg-organics, polymer-inorganics) 73 SONOCHEMICAL METHOD ♦ Solid surfaces = implosion, microjets, Shockwaves 200 urn minimum particle size at 20 kHz for microjets surface erosion removal of unreactive coatings (oxides, nitrides, carbonaceous) fragmentation of brittle materials, increased surface area Li, Mg, Zn, Al, Cu react at room temperature MC15 + Na + CO —► M(CO)5 (M = V, Nb, Ta) Mo + 6 CO —► Mo(CO)6 r. t, 1 bar, normally needs 300 bar, 300 °C R2SiCl2 + Li -------► [-SiR2-SiR2-]n + LiCl monomodal MW distribution Protein microspheres diameter 2 um, hollow emulsification, crosslinking cysteine -S-S- by superoxide 74 SONOCHEMICAL METHOD LARGE PARTICLES SMALL PARTICLES surface cavitation due to defects leading to fragmentation collision can lead to surface erosion or fusion 75