# Thermal Diffusion

Also found in: Wikipedia.

## thermal diffusion

[′thər·məl di′fyü·zhən]
(physical chemistry)
A phenomenon in which a temperature gradient in a mixture of fluids gives rise to a flow of one constituent relative to the mixture as a whole. Also known as thermodiffusion.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Thermal Diffusion

the transport of the components of gaseous mixtures or solutions when subjected to a temperature gradient. If the temperature difference is held constant, thermal diffusion in a mixture will produce a concentration gradient. The production of such a gradient causes ordinary diffusion. Under steady-state conditions, when there is no mass flux, ordinary diffusion counterbalances thermal diffusion, and a concentration difference, which may be utilized for isotope separation, is established.

Thermal diffusion in solutions—the Soret effect—was discovered by the German scientist C. Ludwig in 1856 and studied by the Swiss scientist C. Soret between 1879 and 1881. Thermal diffusion in gases was predicted on the basis of the kinetic theory of gases by the English scientist S. Chapman and the Swedish scientist D. Enskog between 1911 and 1917 and was observed experimentally by Chapman and the British scientist F. Dootsen in 1917.

In the absence of external forces, the total diffusion mass flux in a binary mixture at constant pressure is equal to ji = – nD12 grad ci - DT/T) grad T, where D12 is the coefficient of ordinary diffusion, DT is the coefficient of thermal diffusion, n is the number of particles of the mixture per unit volume, and ci = ni/n is the particle concentration of the ith component (i = 1,2). The steady-state distribution of concentrations can be found from the condition ji = 0; in this case grad ci = – (kT/T) grad T, where kT = DT/D12 is the thermal diffusion ratio, which is proportional to the product of the component concentrations. Since the coefficient of thermal diffusion is highly dependent on molecular interactions, knowledge of this coefficient makes it possible to study intermolecular forces in gases.

### REFERENCE

Crew, K. E., and T. L. Ibbs. Termicheskaia diffuziia v gazakh. Moscow, 1956. (Translated from English.)

D. N. ZUBAREV

References in periodicals archive ?
Bedside monitors are used in thermal diffusion cerebral blood flow, infra-red spectroscopy, micro dialysis and others.
The thermal diffusivity is almost constant, with a mean value of D = 1.74 [+ or -] 0.03 [m.sup.2]/s, at least for this power range, showing that no other diffusive process is concomitant with the thermal diffusion process.
In the above condition, [D.sup.T.sub.eff] is the effective thermal diffusion coefficient of phosphorus, [t.sup.T] is the exposure time at a higher temperature, [D.sup.F.sub.eff] is the effective diffusion coefficient of phosphorus under neutron irradiation, and [t.sup.F] is the time of neutron irradiation.
This is mainly because the thermal diffusion coefficient of the liquid phase is substantially the same in the front of the gas slug, but the obstruction of the gas slug causes the heat flow in the local area close to the phase interface to be reduced.
Zinc thermal diffusion coating (TD) represents a newer, more technically advanced version of an older process called diffusion coating or Sherardizing, invented in the early 1900s by Sherard Cowper-Coles.
Differences in diffusion coefficients lead to formation of the so-called "tails" in the impurity distribution depthwise, which significantly reduces quality of the p-n junctions fabricated by means of thermal diffusion. Tails reduce the breakdown voltage, and traps associated with nitrogen (which are formed at high diffusion temperatures) increase the switching time of the diodes .
The basic link of the thermal diffusion length [mu] to the heating frequency / and to the mean material thermal diffusivity coefficient [alpha] is via the relationship:
Each line represents a pavement with a different thermal diffusion coefficient (a measure of how effectively the pavement is able to conduct heat).
 analyzed the effects of thermal diffusion and chemical reaction on MHD boundary layer flow of electrically conducting dusty fluid between two vertical heated plates.
Besides, these [n.sup.+] and [p.sup.+] regions are generally formed by thermal diffusion at a temperature higher than 800[degrees]C.

Site: Follow: Share:
Open / Close