Thermophoretic deposition

The deposition of particles on macroscopic surface is the primary goal in CVD processes, bnt rednces the efficiency of vapor phase particle synthesis. Particles can deposit by Brownian motion, bnt in high-temperature reactors, thermophoretic deposition often dominates. Thermophoresis is the migration of small aerosol particles as a resnlt of a temperatnre gradient. It causes particles carried in a hot gas to deposit on a cool surface. Eor small particles, Kn 1, a dimensionless group can be created to describe thermophoresis, Th ... 

Turning now from equilibrium considerations, let us investigate the deposition of gas-phase particles, called soot, on the tube walls. The Si02 soot particles are 0.02-0.1 p.m in diameter and are entrained in the gas flow. Without the proper temperature gradient, they would remain in the gas stream and be exhausted. However, the traveling hot zone produces a temperature gradient so that the particles drift toward and deposit on the wall by thermophoresis (cf. Section 7.2.4.1). The thermophoretic deposition efficiency for this process is about 60%. 

Park, H.M. and Rosner, D.E., 1989a, Boundary layer coagulation effects on the size distribution of thermophoretically deposited particles. Chem. Eng. Sci. 44, 2225 - 2231. 

A mathematical model for thermophoretic deposition [24], experimentally verified, concluded that deposition efficiency (ratio of Si02 equivalent entering tube to that contained in the exhaust) may be expressed as e=0.8(1-Te/Trxn) where T is the gas reaction temperature and Te is the temperature downstream of the torch at which the gas and the tube wall equilibrate and deposition ceases. Typically, Te is about 400°C and T,xr about 2000°C, giving an efficiency on the order of 60%. 

K. L. Walker, F. T. Geyling and S. R. Nagel, Thermophoretic deposition of small particles In the modified chemical vapor deposition process. J. Am. Ceram. Soc., 63,96-102 (1980). 

The validity of Eq. (IX.50) has been demonstrated by studies of the thermophoretic deposition of particles with particle concentrations of 5-70 mg/cm, on the walls of horizontal and vertical tubes with diameters of 4,7,14, and 24 mm, when the stream is cooled by water from 500 to 100° C, over a range of Reynolds numbers from 100 to 300 (air-flow rate 33.3-133.2 cm /sec). 

The phoretic processes, thermophoresis and diffusiophoresis, are the deposition processes likely to be susceptible to manipulation by accident management measures. They are of particular interest because unlike many aerosol processes, the phoretic processes are relatively insensitive to aerosol particle size, which will not be known well for the design of accident management strategies. Thermophoresis is the tendency for aerosol particles to move from the hot gas toward a cool surface. The rate of particle deposition is proportional to the gradient in the temperature from the gas to the surface. Accident management efforts that increase this gradient will increase thermophoretic deposition of radionuclide particles in the reactor coolant system. 

Consequently, all codes predict that the main location for retention is the steam generator tube and more wedfically the upward part of this tube. Retention is (uiven by thermophoretic deposition of aerosol particles and wall condensation (the latter predicted... 

The basic operations in dust collection by any device are (1) separation of the gas-borne particles from the gas stream by deposition on a collecting surface (2) retention of the deposit on the surface and (3) removal of the deposit from the surface for recovery or disposal. The separation step requires (1) application of a force that produces a differential motion of a particle relative to the gas and (2) a gas retention time sufficient for the particle to migrate to the coUecting surface. The principal mechanisms of aerosol deposition that are apphed in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. Thermal deposition is only a minor factor in practical dust-collectiou equipment because the thermophoretic force is small. Table 17-2 lists these six mechanisms and presents the characteristic... 

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. 

Air movement indoors is much slower than outdoors, but it is usually enough to ensure that concentrations are fairly uniform in a room. Convection from heating appliances gives air speeds typically in the range 0.05-0.5 m s-1 (Daws, 1967). However, to undergo deposition, vapour molecules or particles must be transported across the boundary layer, typically a few millimetres thick, of almost stagnant air over surfaces. This may be achieved by sedimentation, molecular or Brownian diffusion, or under the action of electrostatic or thermophoretic forces. 

Deposition other than in rain is termed dry deposition, and this includes sedimentation of particles, molecular and Brownian diffusion to surfaces, impaction on roughness elements and deposition under electrical or thermophoretic forces. The velocity of deposition is defined... 

Thermophoretic forces produce very obvious effects near areas of significant temperature gradients. For instance, one can often observe a black deposit on the wall just above a hot-water radiator or pipe. Convection currents conduct the warm gas and particles over the radiator, but since the cooler surfaces nearer the radiator are not protected by a dust-free space, deposition takes place. On a ceiling or on walls of rooms heated by convection, one can often see a replica of the construction behind the plaster formed by deposited particles. Again, the dust is deposited on the cooler portions of the surface on spaces between the laths if the laths are poor heat conductors and directly opposite the laths if they are good conductors. In a room that is heated by direct radiation, such as by an open fire, the walls and furniture of the room are warmer than the air, so that particles suspended in the air are not deposited by thermal forces (Lodge, 1883 Gibbs, 1924). 

Values of the dimensionles.s thermophoretic velocity are. shown in Fig. 2.9 as a function of the Knudsen number with kg/kp as a parameter. For Knudsen numbers larger than unity, the dependence of the dimensionless thermophoretic velocity on particle size and chemical nature Is small. Particle sampling by theimophoresis in this range offers the advantage that particles are not selectively deposited according to size. 

An added benefit of a sharp thermal gradient is that since the deposition surface is the hottest surface in the reactor (other than possibly the heater assembly) thermopho-retic effects act to buoyantley drive particles from the surface [9]. This, fortuitously, naturally results in low particle incorporation and helps with resulting device performance. Interestingly enough, in systems for the deposition of organic semiconductors where the surface is cold, thermophoresis actually drives particles to the surface, which can further complicate the process. This thermophoretic process is actually used to collect nanoparticles in some application areas [10, 11]. 

Rosener, D.E. and Kim, S., 1984, Optical experiments on thermophoretically augmented submicron particle deposition from "dusty" high temperature gas flows. J. Chem. Eng. 29, 147 - 157. 

Deposition of nanoparticles was investigated in the free molecular regime approximation for thermophoretic force and the Brownian motion. The analytical solution was obtained by the Galerkin method for the heat transfer between gas flow and substrates and convective diffusion. Relative roles of two channels of nanoparticle deposition are discussed. 

Emphasize that for the higher modes, obtained by the Galerkin method, characteristic lengths are much shorter. In turn, if there is no significant temperature gradient, we can neglect the influence of thermophoretic force on deposition of nanoparticles. 

Figure 6.26. The morphology of a titanium nitride deposit made by thermophoretic CVD under different conditions (a) a dense polycrystalline layer (b) a fine-grained dense layer (c) a porous deposit of fairly large particles sintered together (d) a porous sintered deposit concentrated on isolated particles of the porous substrate. ...

Deposition of Particles from a Heated Stream. The reason for deposition of aerosol particles in a hot stream onto cold surfaces is the movement of the particles in a nonuniformly heated medium in a direction opposite to the temperature gradient, i. e., from a high-temperature zone to a lower-temperature zone [265]. Under the influence of thermophoretic force, we find a radial component of velocity from the center of the heated stream to the cooler wall, and an additional possibility of contact between the particles and the surface. 

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