Ostwald ripening

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Ostwald ripening

The Ostwald ripening is one of self-running colloid chemical ripening of dispersed matter, in 1900 the polymath and later Nobel laureate in chemistry, Wilhelm Ostwald was discovered and named after him.

Ostwald ripening is based on the curvature dependence of the vapor pressure or the solubility of a fine powder , in Ostwald's wording: "since, according to known principles, a fine powder must be more soluble than a coarse one, just as small droplets have a greater vapor pressure than large ones" ( Gibbs -Thomson Effect ). The difference in vapor pressure or concentration in a closed system is balanced out by a flow of matter flowing from the small to the large colloids . As a result, the small ones are shrinking, but the big ones continue to grow. As soon as the radius of a small colloid falls below a critical value, it becomes energetically unstable and dissolves completely (Kelvin instability). As a result, the number of colloids decreases with advancing evolution (coarsening) and there is a phase separation. During Ostwald ripening, the free energy ( surface tension ) of the system is minimized. This is of importance for practical use in the production of emulsions or ointments , in evaluating the stability of foams or in cloud point extraction . Ostwald ripening can be measured nephelometrically using the Tyndall effect .

A similar effect occurs in metallurgy during aging and during grain growth in polycrystalline solids , in particular alloys , especially during grain coarsening after grain growth has been completed. During the aging of a metallic alloy, precipitation particles form a second phase through nucleation in the matrix phase, which then grow and also coarser through Ostwald ripening (also called redissolution), which is described quantitatively by Carl Wagner's theory of Ostwald ripening. Carl Wagner developed his theory of Ostwald ripening independently of that of Lifshitz and Slyozov, so that the theory of Ostwald ripening is also called the LSW theory for short. The theory predicts the kinetics of Ostwald ripening in that the mean particle diameter increases proportionally to the third root of time, and the particle size distribution, which remains constant over time in relation to the mean particle size. The theory only applies to alloys corresponding to a dilute solid solution with diffusion as the slowest and thus rate-determining process of Ostwald ripening, so that the coarsening rate is proportional to the diffusion coefficient of the matrix and to the solubility and the interfacial energy of the precipitated particles. The correspondence between theory and experiment has often been established, for example in steels , whereby when several carbide phases precipitate side by side, not only the smaller particles dissolve in favor of the larger particles, but also the particles of the more soluble carbide phase in favor of the less soluble carbide phase. Also within the LSW theory, the effects of external forces on the course could Ostwald ripening are as described for the orientation readout precipitated Fe 16 N 2 particles in a Fe - N - mixed crystal by an external magnetic field and the orientation readout precipitated Au -rich particles in an Fe- Mo -Au mixed crystal was shown by an external mechanical stress .

Mats Hillert described the coarsening of the grain in polycrystalline metals and alloys in an analogous manner .

Web links

Individual evidence

  1. ^ Wilhelm Ostwald: About the supposed isomerism of the red and yellow mercury oxide and the surface tension of solid bodies. In: Journal of Physical Chemistry . Vol. 34, 1900, pp. 495-503.
  2. ↑ C. Wagner: Theory of the aging of precipitation by dissolving (Ostwald ripening). Zeitschrift für Elektrochemie Vol. 65, No. 7/8 (1961), pp. 581-591.
  3. ↑ М.Лифшиц, В.Слёзов // ЖЭТФ 35, 479 (1958); IM Lifshitz, VV Slyozov: The Kinetics of Precipitation from Supersaturated Solid Solutions . In: Journal of Physics and Chemistry of Solids . 19, No. 1-2, 1961, pp. 35-50. bibcode : 1961JPCS ... 19 ... 35L . doi : 10.1016 / 0022-3697 (61) 90054-3 .
  4. ↑ Wolfgang Pitsch and Gerhard Sauthoff : Kinetics and morphology of various structural reactions . In: Material Science Steel, Volume 1 , published by the Association of German Ironworkers, Springer-Verlag Berlin, Heidelberg, New York ( ISBN 3-540-12619-8 ) and Verlag Stahleisen, Düsseldorf ( ISBN 3-514-00251-7 ) 1984, Pp. 115-138.
  5. ↑ Gerhard Sauthoff: For the orientation readout of Fe 16 N 2 particles in the iron-nitrogen mixed crystal by means of an external magnetic field . In: Zeitschrift für Metallkunde . tape 68 , no. 1 , 1977, pp. 22-26 .
  6. ↑ Gerhard Sauthoff (winner of the Academy Prize for Chemistry 1977) (accessed on February 6, 2017): The effect of external forces on precipitation processes in metals. In: Yearbook of the Academy of Sciences in Göttingen . 1977. Presented at the public session on November 25, 1977.
  7. ^ M. Hillert: On the Theory of Normal and Abnormal Grain Growth . Acta Metallurgica, Vol. 13, No. 3 (1965), p. 227 ff.