P-phase nucleation

An alloy is cooled from a temperature at which it has a single-phase structure (a) to a temperature at which the equilibrium structure is two-phase (a -i- ji). During cooling, small precipitates of the P phase nucleate heterogeneously at a grain boundaries. The nuclei are lens-shaped as shown below. 

H-H interaction due to the lattice expansion becomes important and the hydride phase (P phase) nucleates and grows. The hydrogen concentration in the hydride phase is often found to be H M = 1. The volume expansion between the coexisting a- and P-phases corresponds in many cases to 10-20% ofthe metal lattice. Therefore, at the phase boundary high stress is built up and often leads to decrepitation of brittle host metals such as intermetaiiic compounds. The final hydride is a powder with a typical particle size of 10-100 pm (Figure 5.24). 

Outstanding properties a high purity, a very small particle size in the nanometric range with narrow crystal size distribution, increase the efficiency of the P-phase nucleation, acid scavenger, processing aid and impact modifier, enhances mechanical properties, improves process throughput, surface appearance, reduces cost, improves service hfe, improves material flow of neat resins... 

Fujiyama, M. (1996) Structure and properties of inflation films of P-phase nucleating agent-added polypropylene. Int. Polymer Process., 11, 159-66. 

Polymer supported xanthene derivatives have been used in the solid phase synthesis of 1-aminophosphinic acids, RCH(NH2)PH(0)0H, <%TL1647> and of C-terminal peptide amides <96JOC6326>. Xanthene units also feature in crown ethers <96JCS(P2)2091>, calixarenes <96JOC5670> and in a flexible template for a P-sheet nucleator <96JOC7408>. 

To determine the relationship between hydrate nucleation (requiring three phases) and the more usual type (two-phase nucleation) consider the theory of homogeneous and heterogeneous nucleation in crystallization, as reviewed by Mullin (1993, p. 172) and Kashchiev and Firoozabadi (2002b), from which much of the below discussion has been excerpted. 

A M. Gusak, K.P. Gurov. Peculiarities of intermediate phase nucleation in the process of chemical diffusion // Solid State Phenomena.- 1992 - V.23-24 - P. 117-122. 

R.W. Walser, R.W. Bene. First phase nucleation in silicon - transition-metal planar interfaces // Appl.Phys.Lett- 1976.- V.28, No. 10.- P.624-625. 

With CaC03, the spherulite size is significantly reduced (Ds = 10-15 pm) and the particle surface chemistry induces some specific microstructural characteristics of the PP matrix small size surface treated CaC03 particles promote formation of the p phase. Without surface treatment, CaC03 has a nucleating effect the degree of crystallisation is increased by about 20%(X(. = 65%). 

During charging and discharging, the overall composition of the Li-Al alloy electrode changes into a two-phase region limited on one side by a saturated solid solution of Li in A1 (a phase) and on the other side by the p phase of Li-Al alloy. These processes are accompanied by a first order phase transition, clearly indicated by the discontinuity of the AE relation (Fig. 3.56) and typical 3D nucleation and... 

Ardizzone, S. and Lazzari, P, Homogeneous nucleation of pure oxide phases. Eormation of rod-like particles. Mater. Lett., 12, 37, 1991. 

We assume that this is the case, since the a-phase nucleates on top of the P-phase (as concluded below) and should hence exhibit the same mosaic spread with respect to the surface normal. In particular we find that the relative fraction of the a-phase is larger for higher T. E.g., at the extremely high growth temperature 93 °C, only the a-phase is observed, whereas around room temperature (T = 23-28 °C) the phase ratio varies from 10% to 40%. 

Sheppard, J. C., H. Westberg, J. F. Hopper, K. Ganesan, and P. Zimmerman (1982). Inventory of global methane sources and their productions rates. J. Geophys. Res. 87, 1305-1312. Shneour, E. A. (1966). Oxidation of graphitic carbon in certain soils. Science 151, 991-992. Shugard, W. J., R. H. Heist, and J. J. Reiss (1974). Theory of water phase nucleation in binary mixtures of water and sulfuric acid. J. Chem. Phys. 61, 5298-5307. 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

X. Jiang, K. Schiffmann and C.-P. Klages, Nucleation and initial growth phase of diamond thin films on (100) silicon, Phys. Rev. B, 50, 8402-10 (1994). 

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