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.

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.


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


References in periodicals archive ?
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.
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:
Firstly, the ta-C film is prepared by the FCVA technique; then the prepared film is boronated by the thermal diffusion technique.
There might be a thermal diffusion plate, located away from the processor to dissipate the heat.
The lower-Tg composition enables subsequent thermal diffusion of the dopants into the host layer uniformly at a very low temperature (below 80[degrees] C) without the assistance of solvent.
The investigation concluded that the vapor pressure gradient is the critical driving potential for moisture transfer, and thermal diffusion is not significant.
Abstract The thermal diffusion coefficient of the intumescent insulating layer, which forms when a fire-retardant coating is subjected to heat in a fire, is a key factor used in assessing the fire protection performance of the coating.
Greenkote is a patented thermal diffusion process that applies proprietary zinc-based coatings to ferrous metals and alloys.
The energy caused by a composition gradient is called the Dufour or the diffusion-thermo effect, also the mass fluxes can also be caused by the temperature gradient and this is called the Soret or thermal diffusion effect; that is, if two regions in a mixture are maintained at different temperatures so that there is a flux of heat, it has been found that a concentration gradient is set up and in a binary mixture, one kind of a molecule tends to travel toward the hot region and the other kind toward the cold region.
They describe evoked potentials and electroencephalography; radiological techniques; techniques for cerebral blood flow, such as neurosonology, laser Doppler and thermal diffusion flowmetry, jugular bulb oximetry, and near infrared spectroscopy; monitoring techniques for intracranial pressure, brain oxygen, cerebral microdialysis, and brain temperature; and device development, engineering, simulation, telemedicine, robotics, information processing, data acquisition and storage, medical informatics and multimodality monitoring, noninvasive brain monitoring, and future developments.
When applied to tissue, the plasma stream creates a thin, flexible coagulation layer that prevents bleeding and lymphatic oozing by removing liquid from the wound surface without thermal diffusion to surrounding tissue or fluids.