Nucleation and formation mechanisms

Nucleation and Formation Mechanisms of Hydrothermally Derived Barium Titanate... 

Barium titanate powders were produced using either an amorphous hydrous Ti gel or anatase precursor in a barium hydroxide (Ba(OH)2) solution via a hydrothermal technique in order to discern the nucleation and formation mechanisms of BaTi03 as a function of Ti precursor characteristics. Isothermal reaction of the amorphous Ti hydrous gel and Ba(OH)2 suspension is believed to be limited by a phase boundary chemicd interaction. In contrast, the proposed BaTi03 formation mechanism from the anatase and Ba(OH>2 mixture entails a dissolution and recrystallization process. BaTi03 crystallite nucleation, studied using high resolution transmission electron microscopy, was observed at relatively low temperatures (38°C) in the amorphous hydrous Ti gel and Ba(OH)2 mixture. Additional solution conditions required to form phase pure crystallites include a C02-free environment, temperature >70°C and solution pH >13.4. Analysis of reaction kinetics at 75°C was performed using Hancock and Sharp s modification of the Johnson-Mehl-Avrami approach to compare observed microstructural evolution by transmission electron microscopy (7). 

The previous discussion has shown that the CIPS technique allows one to produce macroporous epoxy networks with either a narrow or bimodal size distribution. However, no indication has been given on the type of phase separation mechanism to yield these morphologies. As discussed earlier, the formation of a closed cell morphology can result either from a nucleation and growth mechanism or from spinodal decomposition. 

Since the start of modern interpenetrating polymer network (IPN) research in the late sixties, the features of their two-phased morphologies, such as the size, shape, and dual phase continuity have been a central subject. Research in the 1970 s focused on the effect of chemical and physical properties on the morphology, as well as the development of new synthetic techniques. More recently, studies on the detailed processes of domain formation with the aid of new neutron scattering techniques and phase diagram concepts has attracted much attention. The best evidence points to the development first of domains via a nucleation and growth mechanism, followed by a modified spinodal decomposition mechanism. This paper will review recent morphological studies on IPN s and related materials. 

The nucleation and growth mechanism of CdTe thin films at polycrystalline gold surface [203] and on the rough face side of a Si(lOO) were studied as a function of different potential steps. Schematic representation of CdTe formation is shown in [204]. 

MPa, and 81.5% only at 4.8 MPa [16], In absorption process, the reaction of magnesium with hydrogen is a nucleation and growth mechanism where the nucle-ation rate is pressure dependent. They estimated the enthalpy and corresponding entropy of MgH2 formation as -70.0 kJ/mol and -126 J/mol K, respectively. 

The formation of highly dispersed particles or crystallites in the synthesis process of, for example, a supported metal catalyst, is governed by nucleation and growth mechanisms (vide supra) that have been described in the literature [15, 16, 21-23]. For sintering or redispersion (spreading and film formation) to occur, particles or atoms, molecules or clusters of the active... 

The entire region enclosed by the outer dome represents immiscibility. The inner dome is known as the spinodal. The outer dome is known as the binodal. In the region of composition between the binodal and spinodal lines, phase separation occurs by the nucleation and growth mechanism and leads to the formation of dispersed micro-spherical glass particles in the matrix (see also Shelby, 1997). Spinodal decomposition which takes place inside the dotted region is a special type of phase separation. In order to understand this, consider two materials A and B, melts of which... 

As a consequence of the above mentionned scheme, alpha alumina formation would proceed by a nucleation and growth mechanism. 

The formation of bulk phases is most common in the case of oxygen. Transformation of the chemisorbed phase into oxide proceeds generally through a nucleation and growth mechanism. As an example. Fig. 2.26a shows an STM image from a Ru(0 001) surface that had been exposed to O2 at elevated temperature [41 ]. The right part is still the Ru(0 001) surface covered by a 1 x 1 O adlayer, while the left part had been transformed into a thin... 

At elevated temperatures (>530K), the chemisorbed O atoms are more mobile and form larger 1x1 Oad islands, but nucleation and formation of the oxide proceed along with similar mechanisms [43]. 

S.P. Nunes, T. Inoue, Evidence for spi-nodal decomposition and nucleation and growth mechanisms during membrane formation. Journal of Membrane Science 111 (1996) 93. 

---