Coarsening in which small particles dissolve and disappear while large particles grow. The driving force is the curvature-dependent difference in solubility (the Gibbs–Thomson effect).
Japanese name: オストワルド成長
Ostwald ripening appears in 2 practice questions across 2 topics, including Thin films, surfaces and nanomaterials, Nanomaterials.
Atoms at a particle's surface are less stable than those inside by the amount of the interfacial energy. The equilibrium solubility around a particle of radius of curvature r is given by the Gibbs–Thomson equation c(r) ≈ c∞{1 + 2γVm/(rRT)} (c∞ is the solubility at a flat interface and Vm the molar volume), so the smaller the particle, the higher the solubility. As a result, a concentration difference forms in the parent phase from around small particles to around large ones, and solute moves by diffusion. It is coarsening driven by interfacial energy, proceeding in the direction that reduces the total interfacial area.
For diffusion-controlled Ostwald ripening, the Lifshitz–Slyozov–Wagner (LSW) theory gives the mean radius as growing according to r̄³ − r̄₀³ = kt. The rate constant k is proportional to the interfacial energy, the diffusion coefficient and the solubility, and rises steeply with temperature. If the initial radius is small enough, doubling the mean radius takes 8 times as long, and over that time the number of particles falls roughly in inverse proportion to time. The size distribution settles into a shape that stays the same over time, and in LSW theory the largest particle is 1.5 times the mean.
Ostwald ripening is a different mechanism from coalescence, in which particles collide and merge; it proceeds even if the particles do not move, simply by atoms transferring through the parent phase. It also differs from ordinary growth, in which all particles grow by taking solute from a supersaturated parent phase: here the total volume of precipitates stays nearly constant while their number falls and their mean size increases. The telltale sign is that small particles actually shrink and vanish. Grain growth after sintering or recrystallization is a separate phenomenon driven by grain-boundary migration, but it shares curvature as its driving force.
When precipitation-strengthened alloys are used at high temperature, Ostwald ripening coarsens the precipitates and lowers the strength (overaging). To suppress it, k must be made small, for example by choosing coherent precipitates with low interfacial energy, or by forming precipitates from elements with low solubility in the matrix and slow diffusion. The γ′ phase in Ni-based superalloys, highly coherent with the matrix and with low interfacial energy, is a classic example of slow coarsening at high temperature. In nanoparticle synthesis, by contrast, Ostwald ripening broadens the size distribution as it proceeds, so the time at which the reaction is stopped is controlled.
Answer: A. Smaller particles, around which solubility is higher, dissolve and disappear, and their atoms diffuse to larger particles, which grow
A particle with a small radius of curvature has a chemical potential raised by the interfacial energy, so the equilibrium solubility in the surrounding matrix is higher (the Gibbs–Thomson effect). The solute concentration is therefore high around small particles and low around large ones, and solute diffuses from small particles to large ones. Small particles thus disappear and large ones grow, reducing the total interfacial area (interfacial energy). This is the main cause of softening by overaging in precipitation-strengthened alloys and of the coarsening of nanoparticles. The choice with the solubility relation reversed is wrong, and collision and coalescence of particles is a mechanism distinct from Ostwald ripening.
Yes, they refer to the same phenomenon. In English it is usually called Ostwald ripening; in metallurgy the process is often described as particle coarsening, and in colloid science and crystallization as ripening.
When ice cream is stored for a long time, small ice crystals disappear and large crystals grow, giving a grainy texture. Along with temperature fluctuations during storage, Ostwald ripening is one of the main causes.
Because the rate constant is proportional to the diffusion coefficient and the solubility, both of which increase with temperature in Arrhenius fashion. Small temperature differences change the coarsening rate greatly, so controlling service temperature is important when estimating the life of high-temperature parts.
The choice with the solubility relation reversed is wrong, and collision and coalescence of particles is a mechanism distinct from Ostwald ripening.
When an alloy containing dispersed precipitate particles of…Ostwald ripening is the dissolution of small particles while large ones grow, not small particles catching up; when it proceeds in the monomer-depleted stage, the distribution generally broadens.
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