Chemical changes/reactions induced by ultrasound No direct interaction of ultrasound field with molecules (in contrast to photochemistry,...) •Liquid phase reactions - chemical reactions driven by cavitation effects • Solid state reactions - introduction of defects = speeding up diffusion Sound Sound = pressure waves = 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 (tensile/compressive stress) 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 Longitudinal Pressure Waves en 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 air (340 m s1) Substance Speed of sound, u [m s_1] Air Helium Water Lead Steel Granite 343 965 1482 1960 5960 6000 Sound Intensity Sound Intensity = Power / area = Watts/m2 Source of Sound Intensity (W/m2) Sound level (dB) Jet Airplane 30 m away 102 140 Air-raid Siren, nearby 1 120 Threshold of Pain 101 120 Concert -101 115 Riveter 103 100 Busy Traffic 105 70 Normal Conversations 106 60 Whisper 1010 20 Threshold of Hearing 10~12 0 0 dB (10 12 W/m2) 10 dB = 10 as intense 20 dB = 102 as intense 30 dB = 103 as intense 120 dB = 1012 as intense Pa = PA sin 271 ft Pa acoustic pressure PA pressure amplitude f sound frequency c = A,f (for 20 kHz, X = 7.5 cm) p = p + p 'total ra ^ rh Ph hydrostatic pressure compression compression displacement (x) graph AAA raref action M J \J \J Pressure (P) graph r\ r\ r \ f \J 1 ▲ Pa p = liquid density fkg m~3] c = sound velocity in liquid fm s_1] (Water 1482 m s"1) PA = 620 700 Pa = 6.2 bar Utrasound frequencies from 20 kHz to 50 MHz 0 10 10 10 10 10 7 10 Human hearing ► Conventional povrer ultrasound frequency, Hz 16Hz- 18kHz 20kHz-100kHz Extended range for sonochenistry B | 20kHz - 2MHz Diagnostic ultrasound 5MHz-10MHz Generation of Ultrasound Transducer - a device converting one type of energy into another gas driven liquid driven electromechanical whistle (F. Galton), liquid atomizer siren 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)Os ceramics (PZT), up to MHz Generation of Ultrasound casing containing transducer element generator upper fixed horn (booster) detachable horn screw fitting at null point Sonochemical Reactor Piezoelectric Ultrasound Generator Ultrasound Processor VCX 500 W Reaction vessel Sonochemical Reactor Ultrasound Processor VCX 500 W Frequency 20 kHz 0 to 40 °C Argon (flow rate 62 cm3 mhr1) TIME of ultrasound treatment PULSE irradiation and a dwell time 2:2 TEMP maximum temperature 50 °C AMPL amplitude 50 % Sonochemical Reactor 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 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 (Bernoulli) below the threshold pressure for cavitation (vapor pressure) cavities are generated the liquid jet expands, the pressure recovers energetic collapse of the cavities © im S.A. Kinn» FACE SHEET CAVITATION M. Versluis, B. Schmitz, A. von der Heydt, D. Lohse, How Snapping Shrimp Snap: Through Cavitating Bubbles. Science 289, 2114-2117 (2000) Snapping Shrimp -LOO -0.75 -0.25 -0.00 TINfE tm&) D. Lohse, B. Schmitz, M.Versluis, Nature 413, 477-478 (2001) EL m . . ■ i L^L /^l Snapping Shrimp cavitation bubbles D. Lohse, B. Schmitz, M.Versluis, Nature 413, 477-478 (2001) Relaiiv* acousiic pressure IT r.: a. 3 R/fiO (bubble radius / initial radius) Relalna acoustic pressure 7-r t. I a> a> R/BO (bubble radius /initial radius) Acoustic Cavitation Cavitation effects = creation, growth, and implosive collapse of bubbles (1-2 jus) in a liquid = implosion HOT SPOT (1 ns) TRANSIENT CAVITATION: THE ORIGIN OF SONOCHEMISTRY Compression 111/WWWWW Expansion E z ■ IMPLOSION SHOCKWAVE FORMATION 200 300 400 Time ( u sec) RAPID QUENCHING I 500 transient cavitation - transient cavities expand rapidly collapse violently Acoustic Cavitation Cavitation effects = creation, growth, and implosive collapse of bubbles in a liquid Bubble formation = breakage of liquid during expansion, overcoming tensile strength (pure water 1500 bar, only 6.2 bar available) Weak spots needed = dissolved gas molecules, solid particles, trapped gases Bubble growth (300 jus), energy absorption, size oscillations critical size (170-300 jum) = most efficient energy absorption, rapid growth, inefficient energy absorption, collapse compression compression compressor) compnsssion rarefaction rarefaction rarefaction rarefacticn rarefaction. O BUBBLE FORMS BUBBLE GROWS IN SUCCESSIVE CYCLES UNSTABLE SIZE Acoustic Cavitation Standing wave I Low pressure _ ' >_ Bubble expansion Bubble collapse Light emisssion High pressure Acoustic Cavitation Bubbles collapse = spherically symmetrical implosion, shear forces, adiabatic compression, life time 1-2 jus Hot spot = end of the collapse temperature of the gas inside bubble 5 000 - 20 000 °C (for 1 ns) surrounding liquid layer 2000 °C pressure 500 - 1500 bar Extreme cooling rates 1010 K s-1 red hot steel poured into water 2500 K s-1 Homogeneous Sonochemistry Mechanism •••• SurrdVinding interface layer ■ A-B diffusion of volatile reagents Bulk liquid CD** nonvolatile reagents Shock waves, shear forces • ■ ■ How to Measure the Temperature inside a Bubble ? Sonoluminescence - Light generated during the implosive collapse of bubbles in liquids irradiated with ultrasound 95% H2S04(aq.) under Ar 20 kHz (14 W/cm2) Ti horn directly immersed T = 298 K Kenneth S. Suslick University of Illinois 22 WJcm2 ) & 7 mm B Apparent blackbody temperature Ar emission + SO and 02 emission 8 000 - 15 000 K G30 14Wflttsfcm2 22 Wattsfcm2 30 Watts/cm' 30 w/cm2 Wavelength (nm) Temperature/Pressure inside a Bubble Neppiras Equation t = t max 0 pa (r -1) q p =q max V p. (r -1) G J Pa = acoustic pressure T0 = solution temperature Y = Cp/Cv Q = gas pressure inside a bubble upon initiation of the collapse, at its maximum size Gas Y = Cp/Cv Kr 1.66 Ar 1.66 He 1.63 o2 1.41 Fate of Bubbles under Ultrasonic Irradiation Ultrasound «Bubble nuclei Dissolution \ Coalescence Rectified diffusion Fragmentation Buoyancy Resonance Collap&e VV* SL Rectified diffusion - during expansion phase the bubble has larger surface area - more gas diffuses inside than during compression gets out Single Bubble Sonoluminescence SBSL D. F. Gaitan, L. A. Crum, 1990 a method to trap a single sonoluminescing bubble within an acoustic standing wave field Standing acoustic wave field One bubble trapped The bubble oscillates for many cycles Bubble sonoluminescence Single Bubble Sonoluminescence SBSL D. F. Gaitan, L. A. Crum, 1990 Standing acoustic wave field 1 bar One bubble levitates in the acustic field The bubble oscillates for many cycles Bubble sonoluminescence Bjerknes force sound pressure p(x,t) I 1=0 location bubble force at the time t=0: volume V(t) force at the time t=T/2: t hue - aver age d f or ce: C. A. and V. Bjerknes The force on an object in a liquid depends on its volume and the pressure gradient, the time averaged force drives the bubble towards the antinode of sound pressure and keeps it there. Single Bubble Sonoluminescence SBSL Proper conditions for a single sonoluminescing bubble within an acoustic standing wave field 'S, 30 V 40' s SBSL intensity Single Bubble Sonoluminescence SBSL Red - MBSL in dodecane Blue - MBSL in water, 16 kHz Green - SBSL in water, 43 kHz Black - blackbody curve for 16200 K Single Bubble Sonoluminescence i - SBSL Red - bubble radius Green - bubble temperature Blue - acoustic pressure 1.3 bar/25 kHz 2560 2570 2580 TIME (microseconds) 2590 8120 6960 c 5800 ^ 31 HI 4640 § CO 3480 £ 2320 1160 290 LU 2581.5 ■6860 * -4640 2581.6 2581,7 TIME (microseconds) 2320 LU i LU CL S LU 290 Multi Bubble Sonoluminescence MBSL Multi-bubble sonoluminescence Spatial and temporal average 250 bar Sonoluminescence 95% H2S04(aq.) blackbody temperature 2QQ 300 400 500 600 .JLHJ 800 Ar emission an optically opaque plasma core Sonoluminescence 95% H2S04(aq.) SO and 02+ emission with vibronic progression 160H | i J= 12CH 1 1 80 40 2-v' JTU*- iron -n-n1=v' 1112 1314=ir" J V'-0\ I I I I I I I I I I V'=4 5 6 7 8 9 1011 121314 SO 1,& 1.2 0.8- 04 250 300 350 0.& 1580 ± 110 K at 3.3 bar 2470 ± 170 K at 4.2 bar 3480 ± 240 K at 5.1 bar Wavelength (rm) 200 250 Wavelength (nm) 300 Sonofusion Fraud D + D^ ^{0.82MeV) + n{2A5MeV) Power Measurement in Sonochemistry Calorimetry P = power, W P el = input power to generator P hf = high-freq. power output P th = power input into liquid © 1 • 1 • H Power Measurement in Sonochemistry Calorimetry P = power, W T = temperature, K t = time, s cp = heat capacity, J g1 K1 m = mass, g Volume 50 cm3 Argon atmosphere Error 5% heat capacity, J g1 K1 Water Tetraglyme 4.2 2.08 54 49 O' o 19 Calorimetric measurement for water 75% amplitude y = 0,2284x + 20,76 20 40 60 80 Time (s) 100 120 140 Weissler Reaction cci4 + h2o ci2 + co + 2 HCl 2 ki + ci2 i2 + 2 s2o3 i2 + 2 kc1 21+ S4Q6 020! is s '0.10 0.00 ki t-'---r 0 20 40 60 Calorimetrically determined ultrasonic power (W) 80 Power Measurement in Sonochemistry Chemical dosimetry The Fricke reaction Volume 50 cm3 (NH4) Fe(S04)2.6H20 0.001 M IHiO-+OH Fe2+ + OH-► FeJ" + OH 3+ H2S04 0.4 M NaCl 0.001 M Time 30 min Fe3+ ^max = 304 nm s = 2197 dm3 mol1 cm 1 Power Measurement in Sonochemistry Chemical dosimetry Porphyrin decomposition ratio TPPS 3.3 10 6 M Volume 50 cm3 TPPS U = 412 nm 6 = 500000 dm3 mol1 cm 1 Power Measurement in Sonochemistry Temperature [*C] Reactor Optimization cavitating bubbles in the optimised cell (water, 20 kHz, Pus = 10 W) and simulated intensity distribution for the same geometry Heterogeneous Sonochemistry Solid surfaces = implosion, microjets, shock waves 200 jum minimum particle size at 20 kHz for microjets surface erosion removal of unreactive coatings (oxides, nitrides, carbonaceous) fragmentation of brittle materials, increased surface area LARGE PARTICLES SMALL PARTICLES Heterogeneous Sonochemistry 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 (V2Os, Mo03, MoS2, ZrS2, TaS2) Cr (mp 2130 K) and Mo (mp 2890 K) agglomerate W (mp 3683 K) does not temperature at the point of impact ~ 3000 °C Before ultrasound 30 min. ultrasound Cavitational Corrosion of the Tip Control of Sonochemical Reactions sound intensity - minimum for cavitation threshold, depends on frequency, 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, 10 - 100 W cm2 sound frequency - 20 - 100 kHz, the higher the frequency, the higher power needed to actuate cavitation, stronger cavitation effects, rarefaction phase shortens at high frequency sound attenuation - proportional to the frequency, more power needed at high frequencies 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. Control of Sonochemical Reactions volatile reactants - primary reaction site inside the bubbles, diameter 200 jum, 5000 °C, easy bubble formation, more reactant vapors inside bubbles, but the cavitation is cushioned Fe(CO)5 Fe(acac)3 FeS04 nonvolatile reactants - reaction in the thin layer (200 nm) surrounding the bubble, 2000 °C, less cushioning, more energetic cavitation (collapse) high boiling solvents - high vapor pressure inside the bubble cushions the implosion, nonvolatile solvents give less cushioning, more energetic cavitation less cavitation in viscous liquids, viscosity resists shear forces low surface tension facilitates cavitation, in water add surfactants Control of Sonochemical Reactions temperature - higher temperature increases vapor pressure of a medium, lowers viscosity and surface tension, many bubbles formed at temperatures close to solvent boiling point, a barrier to sound transmission, reaction rates decrease with increasing temperature, more vapors in bubbles ambient gas 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 a given frequency Effect of Temperature on Cavitation in Water 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. Sonochemical Reactions Solid surfaces = implosion, microjets, shock waves 200 jum 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 Homogeneous Sonochemical Reactions 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), diaspore nitrides, carbides, sulfides alkane cracking polymer degradation, lower MW, surface modification emulsification of immiscible liquids (oil-water, Hg-organics, polymer-inorganics) M(acac)n as Precursors 1 Well studied class of compounds 'Many elements form acac complexes » Metal complexes - precursors in CVD, Me sol-gel, thermolysis routes to oxides M(acac) Easily chemically modified Volatile, organics soluble Nontoxic Ismail, H. M. J. Anal. Appi Pyrolysis 1991, 21, 315-326. Pinkas, J.; Huffman, J. C; Baxter, D. V.; Chisholm, M. H.; Caulton, K. G. Chem. Mater. 1995, 7, 1589-1596. Cao, X.; Prozorov, R.; Koltypin, Y; Kataby, G.; Feiner, I.; Gedanken, A. J. Mater. Res. 1997, 12, 402-406. Cao, X.; Koltypin, Yu.; Prozorov, R.; Katabya, G.; Gedanken, A. J. Mater. Chem. 1997, 7, 2447-2451. ))))) / r Fe203 \ Fe(acac)3 -► ( amorphous 1 hexadecane Sonochemical Synthesis of Iron Oxide Nanoparticles Corundum Amorphous sono-Fe203 Fe(acac) J. Pinkas, V. Reichlova, R. Zboril, Z. Moravec, P. Bezdicka, J. Matejkova: Sonochemical synthesis of amorphous nanoscopic iron(lll) oxide from Fe(acac)3 IR Spectrum of Sono-Fe203 1.0 0.9 0.8 : 0.7 ; 0.6 E CD O § _ -S 0.5 = o co < _ 0.4 0.3 0.2 0.1 0.0 = as-synthesized Fe203 (red) after calcination to 500 °C (blue) 4000 30 00 2000 Wave number (cm-1) 1000 Acetate stretching