Progress in ultrasonic spray pyrolysis for condensed matter sciences developed from ultrasonic nebulization theories since michael faraday PDF

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Progress in Ultrasonic Spray Pyrolysis for Condensed Matter Sciences Developed From Ultrasonic Nebulization Theories Since Michael Faraday Bonex W. Mwakikunga

a a

a

DST/CSIR National Centre for Nano-Structured Materials , P.O. Box 395, Pretoria , 0001 , South Africa b

Department of Physics , University of Malawi-The Polytechnic , Private Bag 303, Chichiri , Blantyre , 0003 , Malawi Published online: 23 Oct 2013.

To cite this article: Bonex W. Mwakikunga (2014) Progress in Ultrasonic Spray Pyrolysis for Condensed Matter Sciences Developed From Ultrasonic Nebulization Theories Since Michael Faraday, Critical Reviews in Solid State and Materials Sciences, 39:1, 46-80, DOI: 10.1080/10408436.2012.687359 To link to this article: http://dx.doi.org/10.1080/10408436.2012.687359

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Critical Reviews in Solid State and Materials Sciences, 39:46–80, 2014 c 2013 Council for Scientific & Industrial Research Copyright  ISSN: 1040-8436 print / 1547-6561 online DOI: 10.1080/10408436.2012.687359

Progress in Ultrasonic Spray Pyrolysis for Condensed Matter Sciences Developed From Ultrasonic Nebulization Theories Since Michael Faraday Bonex W. Mwakikunga

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DST/CSIR National Centre for Nano-Structured Materials, P.O. Box 395, Pretoria 0001, South Africa and Department of Physics, University of Malawi-The Polytechnic, Private Bag 303, Chichiri, Blantyre 0003, Malawi

This review outlines, in great detail, the history of the phenomenon of ultrasonic nebulization of liquids since the discovery of such an effect by Michael Faraday and the explanation of the phenomenon by capillary wave mechanism and “cavitation” hypothesis. Capillary waves described by the Kelvin equation are confirmed by Lang’s experiment and more theoretical models by Peskin & Raco and Jokanovic. Cavitation bubbles have been elaborated by the Rayleigh-Plesset equation as well as the correlation equations between predicted and experimentally measured droplet diameter. Correlation equations such as one by Rajan & Pandit and Avvaru et al. have been considered. Ultrasonic spray pyrolysis for materials processing and the theories that predict the final particle size distribution are introduced. The popularity of the technique is shown by the rising number of research groups in the world processing various materials by this method due to its cost-effectiveness, purity of its products, and controllability of particle size as well as final properties. Keywords pyrolysis, ultrasonic spray, surface tension, chemical vapor deposition, viscosity

Table of Contents 1.

INTRODUCTION ................................................................................................................................................47 1.1. Ultrasonic Nebulization Phenomenon ...............................................................................................................48 1.1.1. Capillary Wave Mechanism ..................................................................................................................48 1.2. Cavitation Mechanism and Formation of Nano-particles in the Spray ..................................................................49 1.3. Cavitation Mechanism and the Formation of Nano-particles in the Precursor Solution ..........................................50 1.4. Cavitation Mechanism and Bubble Dynamics ...................................................................................................50 1.5. Cavitation Mechanism and the Particle Nucleation Theories ...............................................................................51 1.6. A Combination of Capillary and Cavitation Hypotheses for Spray Pyrolysis ........................................................52

2.

EFFECTS OF PRESSURE AND TEMPERATURE ON SURFACE TENSION, DENSITY, AND VISCOSITY OF FLUIDS ..........................................................................................................................................................53 2.1. Surface Tension as a Function of Temperature and Pressure ...............................................................................53 2.2. Liquid Density as a Function of Temperature and Pressure .................................................................................55 2.3. Effect of Temperature and Pressure on Liquid Viscocity ....................................................................................55 2.4. Final Droplet Size Formula .............................................................................................................................56 2.4.1. Final Droplet Size Distribution after Droplet Coalescence .......................................................................57 2.4.2. Theory of Pyrolysis for Predicting Final Particle Size .............................................................................57 2.4.3. Popularity of Ultrasonic Spray Pyrolysis ................................................................................................58

∗

E-mail: [email protected]

46

PROGRESS IN ULTRASONIC SPRAY PYROLYSIS

47

3.

PARAMETER OPTIMIZATION IN USP: DROPLET RESIDENCE TIME .........................................................58

4.

VARIOUS FORMS OF USP .................................................................................................................................59 4.1. Asynchronous Pulse USP ................................................................................................................................59 4.2. Electrostatic Nebulizer USP ............................................................................................................................60 4.3. Infrared USP ..................................................................................................................................................60 4.4. Flame-Assisted USP .......................................................................................................................................62

5.

MORPHOLOGY, STRUCTURAL, AND OTHER PROPERTIES OF MATERIALS OBTAINED BY USP ...........62 5.1. Solid and Hollow Spheres ...............................................................................................................................62 5.2. One-Dimensional Nanostructures from USP: Nanowires, Nanoribbons, Nanorods ...............................................63

6.

CONCLUSION AND OUTLOOK ........................................................................................................................65

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ACKNOWLEDGMENTS ............................................................................................................................................65 REFERENCES ...........................................................................................................................................................65

1. INTRODUCTION Chemical vapor thermal deposition forms one of the largest groups of techniques for realizing a variety of materials in condensed matter science. The starting material is either a gas or liquid carefully chosen to end up into a stoichiometric material desired. The general process entails a source of chemical vapors/droplets which are carried into a heated zone for evaporation and decomposition and finally ending up either on a substrate (for thin films) or a filter (for powders) (see schematic in Figure 1). When dealing with vapors/gases as starting materials, the method is usually referred to as chemical vapor deposition (CVD); there are many forms of CVD. The term “spray pyrolysis” (SP) is used when dealing with liquid droplets or powders as precursor materials. The word pyrolysis is taken from a Greek word “pyre” which means “a pile of fuel or pile of wood” with specific reference to heating by flame.1 Since such heating raises the precursor material to a plasma state where radicals, electrons and ions prevail, this process can be used in in situ spectral analysis of elemental composition of the precursors in addition to the decomposition mechanisms, reaction kinetics, and formation of new condensed matter. The source of heat can be a furnace (thermal CVD), a hot wire/filament (HWCVD, HFCVD), an intense light source such an I.R. CO2 laser or a UV excimer laser (laser pyrolysis LP), plasma source (plasma enhanced PECVD), an I. R. lamp, or, simply, a heated substrate. A number of previous review articles have been presented on different forms of CVD: thermal CVD,2–11 plasmaenhanced PE-CVD.12–19 hot-wire, or hot filament (HWCVD or HFCVD)20–28 and not many of them have been as exhaustive in their respective areas. Pyrolysis, although classified under CVD in some text, has become a wide area of research and technology covering synthesis of new products, qualitative and quantitative spectroscopic analysis of fluids and, lately, alternative route to

production of debri-free x-ray sources; these aspects are elaborated further in the sections that follow. In spray pyrolysis the droplets or vapors can be generated either by pneumatic nozzles in whistle-type sprayers or ultrasonic nebulizer. In the former, the process is simply called spray pyrolysis (SP) and in the latter case, the process assumes the name “ultrasonic spray pyrolysis” (USP). An article on the versatility of spray pyrolysis by Pramod Patil28 among other aspects tabulated publications up to early 1999 listing materials and spray pyrolysis parameters. Other reviews have been on specific materials employing spray pyrolysis as one of the wide range of methods used in producing such materials: superconductors,29 carbon nanostructures,30 ceramic nano-composites,31 diamond,32 semicokes.33 and semiconductors.34 The present chapter restricts its discussion to ultrasonic spray (USP) technique on a wide range of materials especially from 1999 to the present and on laser spray (LP) pyrolysis. This is a period that has seen a lot of improvements to pyrolysis techniques to the extent that structures with new shapes and novel growth dimensionality have been produced in a controlled manner. The scarcity of specific review articles in a period like this one where numerous publications pertaining to materials synthesis by various versions of pyrolysis was the main motivation of the present compilation. First, a historical outline of the droplet generation phenomenon by ultrasonic nebulization is given. This has not been covered in most previous reviews except by Yule et al.,35 Barreras et al.,36 and Nevolin37. These reviews have not covered pyrolysis but restricted themselves to the nebulization phenomenon. The triumphs and challenges in ultrasonic spray pyrolysis are also presented. A tabulated literature survey and data-base from 1988– 2008 is given and some unsolved problems in pyrolysis for materials processing with regard to droplet and particle size under different pyrolysis parameters are discussed.

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B. W. MWAKIKUNGA

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FIG. 1. Generalized schematic of chemical vapor deposition systems on which ultrasonic spray pyrolysis is based. 1.1. Ultrasonic Nebulization Phenomenon Ultrasonic atomization is a very effective method for production of ultra-small droplets and, after the droplets are pyrolyzed, the realization of nano-sized materials. Quantum dots have been produced by spray pyrolysis.38 Three approaches are common in the droplet production: (1) passing the liquid across a standing ultrasonic wave; (2) depositing the liquid over an ultrasonic transducer; and (3) immersing a focussing ultrasonic transducer in the liquid in such a way that the liquid depth is equal to the focal length of the ultrasound lenses in the transducer. Generation of droplets by means of ultrasonic waves was first reported in 1927 by Wood and Lomis.39 A number of mechanisms have been proposed to explain this phenomenon. At low excitation frequencies (20–100 kHz), we can imagine that only surface molecules respond to form droplets; such waves are called capillary waves. At higher excitation frequencies (0.1–5 MHz) and intensities, a phenomenon which takes place in the bulk of the liquids come into play and this effect is called cavitation; this phenomenon is modeled by standard continuum hydrodynamics to be introduced in later sections. 1.1.1. Capillary Wave Mechanism The capillary wave proposal enjoyed intense research interest from the first known studies by Faraday40 in 1831 to the present. It was Lord Kelvin, as elaborated in Rayleigh’s book41 in 1871, who derived the well-known equation for the wavelength of capillary waves as:   2π σ 1/3 . [1] λ= ρf 2 Here, λ is the wavelength, σ is the surface tension, ρ is the liquid density. and f is the frequency of the surface waves. This equation was later modified by Rayleigh41,42 to give

origin of droplet formation relying on the simplified linear instability analysis. The 1962 experimental determination by Robert Lang49 of the relationship between the wavelength of the capillary waves and the size of the droplets so formed spurred the capillary wave mechanism to greater heights. Lang showed that the droplet size, DL , and the capillary wave length λ were related by the empirical equation: DL = 0.34λ.

[3]

The subscript L in Equation (3) signifies the Lang’s droplet diameter in distinction from other droplet diameter symbols to follow. Extra support from Sindayihebura and Bolle50 in 1998 brought more assurance that capillary waves were probably the main mechanism. How drop formation may occur by unstable surface capillary waves was illustrated schematically as reproduced in Figure 2 and this phenomenon is usually called the Taylor instability.52 In the Taylor instability the liquid capillary waves are composed of crests (peaks) and troughs. Atomization takes place when unstable oscillations tear off the crests of the capillary waves away from the bulk of the liquid. Thus, the droplets are produced at the crests whose size is proportional to the wavelength. A major revision to the Lang’s equation was done by Peskin and Raco52 in 1963 and later, 1996, by Jokanovic et al.53 who, rather than adopting an existing empirical equation, chose to derive a general equation from first principles. The analysis especially by Jokanovic et al. started from applying the Bernoulli’s equation to an incompressible fluid of density, ρ, surface tension, σ , under pressure, p, due to an ultrasonic excitation, f , from a depth, y, and thereby generating a disturbance of amplitude, ξ (x,t) given by:

[2]

∂ 2ϕ ∂ϕ + σ 2 = 0. [4] ∂t ∂t In this equation, ϕ is the rate potential. The boundary conditions employed were that when y = −h, v = 0 and ∂ 2ϕ/∂x2 = 0 then:

Note that F which is equal to 2f is not the frequency of the surface waves but rather the frequency of the forcing sound. The fact that the frequency of the surface waves is half the exciting frequency was empirically obtained from experimental measurements. A number of experimental workers in the 1950’s43–48 pointed to unstable surface capillary waves as the

1 dy cJ h [kJ (y + x)] eikh [5] h dt Here, cJ is a constant, kJ was taken to be the wave-number (2π /DJ ) where in turn DJ is the Jokanovic’s aerosol droplet diameter (again to distinguish it from that of Lang above). The Mathieu’s function was then adopted which was observed to explain the typical shape of the relationship between the amplitude

 λ=

8π σ ρF 2

1/3 .

ρgh + ρ

ϕ=

PROGRESS IN ULTRASONIC SPRAY PYROLYSIS

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of the liquid at the surface including one given by:54   ∂ϕ ∂ 2ξ σ ∂ 2ξ + 2 =0 + gξ − ∂t ρ ∂x 2 ∂z

FIG. 2. A sketch showing idealized droplet formation under (a) the capillary wave phenomenon at the surface of the liquid; droplets are formed from standing-wave crests showing one period of wall vibration (b) cavitation mechanism illustrating cavitation bubbles from the bulk which become hot spots upon their implosive collapse on which Rayleigh and Plesset based their theoretical models. of the oscillation of the meniscus surface and the wave-number. The Mathieu’s function was given as:  3  σk dy + hk ht − kght y = 0. [6] dt ρ The solution of Equation (6) for h  ξ (x,t), that is, for small disturbances, found by Jokanovic was seen to be similar to that previous found by Peskin and Raco using a different analysis route (not reproduced here):  DJ =

πσ ρf 2

1/3 =

1 DL . 0.68

[7]

Note that the relationship between droplet diameter and the Kelvin relation for capillary wave length can also be derived from dimensional analysis as shown in by Mwakikunga et al.55 given as:  D = kM

σ ρf 2

1/3 ,

[8]

where kM is a dimensionless constant which according to Lang is 0.68π 1/3 while, according to theoretical derivation by Peskin and Raco52 and Jokanovic53, the constant kM is equal to π 1/3. This means the droplet diameter as calculated by Lang’s equation is smaller by the factor of 0.68 in comparison with that determined by Jokanovic’s equation. Jokanovic et al. were able to show experimentally that their freshly derived equation yielded better agreement between calculated and experimentally determined droplet sizes. It must also be noted that Jokanovic et al. arrived at Equation (7) using various forms of the equation of motion

49

[9]

However, a number of more recent studies employing ultrasonic spray pyrolysis (an application to be discussed in the next section) and using either the Lang’s empirical formula (Equation (3 and/or the Jokanovic’s revision Equation (7)) have shown that both equations have limitations. A serious conflict between theory and experiment reported by Nedeljkovic et al.56 states: “Comparison of the theoretically [dJ = 195 nm, dL = 132 nm] obtained results with the experimentally determi...