Metastable phase selection

Sha] Sha, G., O Reilly, K., Cantor, B., Worth, J., Hamerton, R., Growth Related Metastable Phase Selection in a 6xxx Series Wrought A1 Alloy , Mater. Sci. Eng. A, 304-306, 612-616 (2001) (Crys. Stracture, Phase Relations, Experimental, 9)... 

Ordering of vacancies also plays a key role in selective oxidation catalysis over perovskite-based catalysts such as CaMnOs oxides. CaMnOs has a CaTiOs (AMO3) perovskite structure which is made up of cations coordinated to 12 0 anions. They, in turn, are connected to corner-sharing MoOe octahedra. CaMnOs was used as a model catalyst on a laboratory scale by Thomas et al (1982) in propene oxidation to benzene and 2-methyl propene to paraxylene. In such reactions the compounds are found to undergo reduction to form anion-deficient metastable phases of the type CaMnOs-x where 0 < x < 0.5, forming several distinct phases. 

The development of the -modification is controlled by the relative crystallization thermodynamics and kinetics of the stable a-modification and of the smectic phase towards the metastable / -phase. For PP homopolymers, it is generally accepted that under isothermal conditions, the a-phase grows more rapidly at temperatures below 105 and above 140 °C than its counterpart, which in turn is more prone to develop in between these two temperatures in the presence of selective -promoters [52,70,122]. An elegant way to get fully nucleated /3-PP specimens would consist of pressing /3-PP pellets above their melting temperature (ideally more than 250 °C to erase any a-nuclei in the system), cool the melt quickly up to a crystallization temperature in between 100 and 130 °C, let the sample crystallize, and then quench it to room temperature [70]. However, such a processing method is too time-consuming to be of industrial relevance. 

Nanocrystallite S -ZrW MooiOg was prepared through a precursor route. The characterization by powder X-ray diffraction (XRD) and selected area electron diffraction (SAED) shows that the compound is orthorhombic crystal system with cell parameters a=0.8969(7) nm, b=0.7011(8) nm, c=0.596(l) nm. The possible space group is Pnnm (58) or Pnn2 (34). The compound is a metastable phase depending on temperature and time. 

By carrying out thermodynamic calculations, it is possible to predict the conditions for the appearance of different metastable phases during PVD coating processes and thereby assist in the selection of coating parameters required to produce coatings with optimum desired properties. 

Fig. 3.8 The published solubility data in the CaO-SiOj-HjO system (referenees in [13]). Bright spots correspond to the equilibrium with the metastable phases of C-S-H, dark spots relate to the equilibrium at early CjS hydration stage. For some selected spots the C/S ratio is given the values in brackets relate to the samples composed of C-S H and CH. (According to [13])...

Sprayed ceramic coatings can be made chemically active by selection of the spray parameters, which result in metastable phases within the coating. Ceramic bond coats are useful for difficult to bond materials such as ceramic components, including carbide-containing parts and refractory metals. These materials may be used in combination with a metallic bond coat on metallic substrates to mitigate stress differentials between the metallic substrate and the ceramic bond coat due to thermal or mechanical stress. 

In contrast to nucleation control, inhibition of phase transformation does not require selectivity, as interactions with both substrates (inhibition of dissolution of the metastable phase and growth of the stable phase) may be rate-controlling [22,50] (see also Sections n.D and ni.B and Fig. 6). It is therefore not surprising that both the anionic SDS and the cationic DDACl effectively inhibit the transformation of COD into the thermoynamically stable COM (Fig. 9) [68]. 

The oxovanadium phosphates constitute a subclass of the general family of metal oxides, which display important catalytic properties and a remarkable structural diversity. The syntheses selected for this volume do no reflect a comprehensive attempt to provide the most important representatives of oxovanadium phosphates that have been described since the late 1990s. Rather, the focus is on the critical role of the organic component as a structure-directing agent and on the efficacy of the hydrothermal technique for the isolation of crystalline metastable phases. 

The microwave chemistry can be applied to practically all inorganic families that do exist (see Introduction). Based on the literature, some interesting facts have been reported and can be summarized as follows stabilization of metastable and novel phases, increase of the phase purity and phase selectivity, short crystallization times and narrow particle-size distribution. 

Camblor et al. [4] suggested that the phase selectivity could also occur through a kinetic control in accordance with the thermodynamic calculations by Henson et al. [87]. Indeed, the latter authors found that pure silica zeoUtic materials with low FD are metastable with respect to denser phases. In reality, the phase selectivity may be in some systems quite... 

The kinetics and mechanisms of the C —> G transition in a concentrated solution of PS-fr-PI in the PS-selective solvent di-n-butyl phthalate was studied [137,149]. An epitaxially transformation of the shear-oriented C phase to G, as previously established in melts [13,50,150], was observed. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexag-onally perforated layer (PL) state. The C -> G transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. 

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