Papers by Keyword: Ostwald Ripening

Abstract: A discrete multi-particle model of Ostwald ripening based on direct pairwise interactions between precipitates with incoherent interfaces is presented. Although based on the mean field concept, it is a valid alternative to the classical LSW theory. The main differences with respect to the classical approach can be summarized as follows: i) Particles interact the one another; ii) The first Fick’s law is considered to evaluate the fluxes of matter instead of the quasi stationary solution of the concentration field around particles; iii) The rate of matter exchange depends on the average surface-to-surface interparticle distance, a characteristic feature of the system which naturally incorporates the effect of volume fraction of second phase; iv) The multi-particle diffusion is described through the definition of an interaction volume containing all the particles involved in the exchange of solute. The model is in excellent agreement with the experimental data available in the literature. The shape of the quasi-stationary 3D particle size distribution of solid-solid and solid-liquid systems is well predicted from volume fractions of 0.07, 0.30, 0.52 and 0.74. Similarly, a very good prediction of the dependence of the kinetic constant of the coarsening process on the volume fraction of precipitates is obtained with reference to literature data on solid-liquid mixtures in the volume fraction range from 0.20 to about 0.75. For volume fractions below about 0.1 the model predicts broad and right-skewed stationary size distributions resembling a lognormal function. Above this value, a transition to sharper, more symmetrical but still right-skewed shapes occurs.

Abstract: Liquid phase sintering (LPS) is widely used as a materials processing technique for hightemperature applications. In LPS, particle-particle contact size and distribution, 3-D coordination number, connectivity, and contiguity are important microstructure parameters which, to a large extent, determine the mechanical properties of the sintered materials. These features all depend on the grain size, solid volume fraction and dihedral angle during sintering. The dihedral angle is an important parameter in LPS. It is the angle formed between the 2 solid-liquid interfaces at the intersection of a grain boundary with the liquid. A higher solid volume fraction, on the other hand, favors a larger 3-D coordination number, connectivity, and contiguity. In practice, studying the correlation between these parameters and direct measurement of them is not a trivial task. Among them, 3-D measurement of dihedral angle is believed to be the most challenging one. In the current study, phase-field modeling is employed to simulate LPS in two phase systems (solid and liquid). Simulations are performed for the different ratios of grain boundary to solid-liquid energies and the different solid volume fractions. To create initial structures with high solid volume fraction, an advanced particle packing algorithm is employed. An extended sparse bounding-box algorithm is used to speed-up the computations and makes it computationally efficient for 3-D simulations. Contiguity, connectivity, and three dimensional coordination number were measured in the self similar regime. The results were compared with empirical rules and experimental data and are used to estimate the mean 3-D dihedral angle.

Abstract: The effect of peak temperature on austenite grain size was investigated by using thermal simulation and OM, TEM analysis. Grains grow slowly below 1250°C, and dramatically grow up as the temperature exceeds 1350°C. The primary precipitates in X80 pipeline steel are TiN and NbC, TiN particles cannot absolutely dissolve in steel during holding for 2s at 1300°C.Although TiN particles exist in steel, they cannot hinder grain growth of CGHAZ during welding process. The phenomenon can be explained by Ostwald Ripening mechanism.

Abstract: This paper continues our previous work on preparation of triangular silver nanoparticles. The method proceeds with reaction of silver nitrate with hydrazine hydrate in the presence of polyvinyl pyrrolidone in aqueous solution. Effects of the concentration of PVP on the morphologies of Ag NPs were systematically investigated. The obtained Ag NPs were characterized by transmission electron microscopy and UV-visible spectrophotometer. The results showed that, triangular Ag NPs with edge lengths in the range of 50-200 nm were obtained using PVP as protective agent with lower concentration. As the concentration of PVP increased, spherical Ag NPs with their sizes about 6.2 nm were prepared and triangular Ag NPs were not obtained. The formation mechanism of triangular Ag NPs has been studied. Ostwald ripening is the driving force on the conversion of spherical Ag NPs to triangular Ag NPs in the presence of PVP.

Abstract: The effect of excessive intermetallic growth to the reliability of solder joints become major problem in electronic devices industry. In this study, we used Sn-Cu-Si₃N₄ composite solder to observe the intermetallic compound (IMC) growth during low and high temperature aging. 50°C and 150°C represent low and high aging temperature respectively. Various isothermal of aging times were carried out by using 24hrs, 240hrs and 720hrs. The IMC thickness increases with increasing of aging temperature and time. Cu6Sn5 phase appear at low aging temperature whilst Cu₆Sn₅ together with Cu₃Sn phases has been observed at high aging temperature. The growth kinetics for low and high aging temperature is 1.63x10^-18μm2/s and 2.75 x10^-18μm2/s.

Abstract: Hardness heterogeneous on weld metal and base metal in complicate structure of water storage tank was found after examination. Replica metallographic examination in hardness heterogeneous zone shows abnormal microstructures. Ostwald Ripening structure was caused by uneven temperature control during heat treatment. Lower cooling speed is the main cause of forming tempered sorbite in weld joints.

Abstract: The mechanism of the intercalation/de-intercalation processes during the formation of α-Co (OH)₂ and its conversion to β-Co (OH)₂ is carefuly examined using a new non-invasive technique in a gel matrix. The incorporation of rhodamine molecules inside the interlayer gallery of α samples allows us to monitor the variation of its fluorescence intensity throughout different stages of the reactions. We show after calculating the corresponding activation energies that the formation/transformation reactions exhibit different dynamics depending on the nature of intercalated anions. Moreover, the morphological changes that are revealed from scanning electron micrographs suggest the occurence of an Ostawald ripening mechanism in which perfect and stable crystals are produced at the expense of smaller ones.

Abstract: We have developed a new, simple, green and very reproducible aqueous synthesis method for the preparation of different sizes CdTe QDs without the use of any pyrophoric organometallic precursors. Transmission electron microscope image demonstrates the shape, monodispersity, average size and size distribution. Two different sizes CdTe QDs are mixed and standing for three months in aqueous solution. Through UV-Vis absorption spectroscopy and fluorescence spectrum detection, we confirm Ostwald ripening process can also occur in CdTe QDs mixed solution for a long time at room temperature.

Abstract: The paper researched the isothermal time and temperature affects the solid particles evolution of semisolid AZ91D alloy, and the alloy be produced by the method of strain induced melt activation (SIMA) ,and the method of compression ratio is 40%. The research demonstrated that the solid fraction decreased and solid particle size increased when increasing temperature and time. The isothermal coarsening of solid particles obeys Ostwald ripening which depending on the curvature of the interface, thus creating concentration gradients and the diffusion transport of material. As a result, the large particles coarsening and little particles minish even dissolved and creating change of distribution frequency of solid particle size. The fitting curves reveal that the experimental data are well fitted to the coarsening equation. The quantitative analysis of solid fraction and solid particle size demonstrated that the appropriate isothermal holding temperature is 550°C -590°C and corresponding isothermal holding time is (45-60min)-(6-20min) in 40% compression ratio.

Abstract: We compare three models of 2D precipitation kinetics that give access to different time-space scales. Kinetic Monte Carlo simulations (KMC), cluster dynamics (CD) and nucleation-growth-coalescence model (NGCM), based on a same atomic model, lead to an excellent agreement as long as the interfacial free energy is evaluated accurately and the interaction between diffusion fields is taken into account in the CD. The NGCM model noticeably improves the previous approaches of the same kind by using a constrained-equilibrium hypothesis to describe the solid solution. Moreover, in the coalescence regime, we show that CD leads to cluster distributions that are wider and more symmetric than the LSW distribution due to the probabilistic feature of the growth law of a cluster, that makes it differ from the purely deterministic NGCM approach.