Aerosol processing thermal

Figure 7.37 Schematic illustration of gas-to-particle synthesis via thermal aerosol processing. From Carbide, Nitride, and Boride Materials Synthesis and Processing, A. W. Weimer, ed., p. 308. Copyright 1997 by Chapman Hall, London, with kind permission of Kluwer Academic Publishers.

Sample Introduction Samples can be introduced into the ICP by argon flowing at about 1 L/min through the central quartz tube. The sample can be an aerosol, a thermally generated vapor, or a fine powder. The most common means of sample introduction is the concentric glass nebulizer shown in Figure 28-9. The sample is transported to the tip by the Bernoulli effect. This transport process is called aspiration. The high-velocity gas breaks up the liquid into fine droplets of various sizes, which are then carried into the plasma. 

Various aerosol processes have been developed for the generation of ultrafine powders at laboratory s e, such as flame (2), tube furnace (5), gas-condensation (4), thermal plasma (5), laser, sputtering and a variety of other aerosol processes named after the energy sources which are applied to provide the high temperatures during gas-to-particle conversion. However, until now, only flame processes have been scaled up to produce commercial quantities of ceramic particulates, such as silica, titania, etc., at low cost (about 1/lb). 

Sample introduction into the plasma is a critical part of the analytical process in atomic emission spectroscopy (AES). Since the ICP is the most commonly used source, the sample introduction schemes described below will focus more on it than the other sources mentioned previously. Sample is carried into the plasma at the head of a torch by an inert gas, typically argon, flowing in the centre tube at 0.3-1.5 L min". The sample may be an aerosol, a thermally or spark generated vapour, or a fine powder. Other approaches may also be taken to facilitate the way the analyte reaches the plasma. These procedures include hydride generation and electrothermal vaporization. 

Recently, nanosize particles (diameters below 100 nm) have demonstrated enhanced properties in a number of applications. For example, ceramic layers formed from nano-particles demonstrate improved adhesion, ductility, and mechanical strength (7). Changes in chemical, physical and mechanical properties compared to bulk materials, such as a lower melting point, are attributed to the relative number of atoms or molecules on the surface of the particle becoming comparable to that inside the particle (7). The predominant methods of preparing these particles are based on aerosol processes, such as flames (2), tube furnaces (3), gas-condensations (4), thermal plasmas (5), etc., designed to provide sufficient temperature to promote gas-to-particle conversion. However, until now, only flame... 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

Other CVD Processes. CVD also finds extensive use in the production of protective coatings (44,45) and in the manufacture of optical fibers (46-48). Whereas the important question in the deposition of protective coatings is analogous to that in microelectronics (i.e., the deposition of a coherent, uniform film), the fabrication of optical fibers by CVD is fundamentally different. This process involves gas-phase nucleation and transport of the aerosol particles to the fiber surface by thermophoresis (49, 50). Heating the deposited particle layer consolidates it into the fiber structure. Often, a thermal plasma is used to enhance the thermophoretic transport of the particles to the fiber walls (48, 51). The gas-phase nucleation is detrimental to other CVD processes in which thin, uniform solid films are desired. 

However, the thermal fluctuations are not the only process that could perturb the propagation of ultrashort laser pulses through the atmosphere. Aerosol particles, like water droplets or dust, can have dimensions of several tens of microns, comparable with the filament diameter they could seriously harm the delicate dynamical balance required to propagate filaments. 

As noted above, in situ production of H2O2 within illuminated clouds and aquated aerosols is most likely a very important process governing the fate of oxidizable inorganic and organic compounds. Hydrogen peroxide is a potent thermal oxidant in its own right (i.e., = + 1.76 V H2O2 + 2 e +... 

It is easily observed that at high aerosol concentrations, individual particles coalesce to form larger chains or floes made up of many par-tides. The process of coagulation may be brought about solely by the random motion and subsequent collision of partides (often called thermal coagulation) or the collisions could be caused by such external forces as turbulence or electricity. In general, these external forces will act to increase the rate of coagulation. 

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