Metastable phase formation

Reactions that take place under hydrothermal conditions in CaO -SiOj - HjO system are complicative the number of silicates synthesized in this system approaches 20 [26]. The state of calcium hydroxide has a substantial effect on final products. If amorphous hydroxide is used, the reaction runs faster, but at the same time the probability of metastable phases formation increases. Detailed investigations that were carried out by the authors [26] allowed to state the preferable regions of the synthesis of various calcium compounds depending on Ca0/Si02 ratios and hydrothermal treatment temperature (450-850°C). 

The transformation of the crystalline into the glassy state by solid-state reactions is extensively reviewed in its theoretical and experimental aspects. First, we give some historical background and describe the thermodynamics of metastable phase formations, adding as well the kinetic requirements for the amorphization process. Then we discuss the different experimental routes into the amorphous state hydriding, thin diffusion couples, and other driven systems. In the discussion and the summary, we close the gap between the melting phenomena and the amorphization and provide a tentative outlook. 

FoUstaedt, D. 1985. Metastable phase formation in ion-implanted metals. Nucl. Instrum. Methods. Phys. 

Loe] Loeser, W., VoUemann, T., Metastable Phase Formation in Undercooled Stainless Steel Alloys , Mater. Sci. Forum, 225-221, 27-32 (1996) (Crys. Structure, Experimental, Phase Relations, 18)... 

The concentration profile c(x) in metastable phase formation must become so flat that the length exceeding la... 

Z. Chvoj, Z. Kozisek, J. Sestak Non-equilibrium Processes of Melt Solidification and Metastable Phases Formation a review , special issue of Thermochim. Acta,... 

Ion beam-induced epitaxial crystallization (IBIEC) has attractive features from the viewpoint of beam-solid interactions especially in semiconductors. Specific properties and phenomenological understandings of IBIEC in Si have been hitherto obtained on the basis of extensive investigations (41-45). The features of the process have provided not only a new field of investigation of beam-solid interactions but also a possibility of process application of Si because of its advantages, such as processing at low temperatures, local process capability, and metastable phase formation by impurity atom incorporation at a nonthermal equilibrium concentration. 

Under equiUbrium vapor pressure of water, the crystalline tfihydroxides, Al(OH)2 convert to oxide—hydroxides at above 100°C (9,10). Below 280°—300°C, boehmite is the prevailing phase, unless diaspore seed is present. Although spontaneous nucleation of diaspore requires temperatures in excess of 300 °C and 20 MPa (200 bar) pressure, growth on seed crystals occurs at temperatures as low as 180 °C. For this reason it has been suggested that boehmite is the metastable phase although its formation is kinetically favored at lower temperatures and pressures. The ultimate conversion of the hydroxides to comndum [1302-74-5] AI2O2, the final oxide form, occurs above 360°C and 20 MPa. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

The reason for the formation of anatase phase at such a high temperature might be explained as following. The as-prqiared ultrafine titania particles are liquefied at sufficimtly high temperature because melting point of nanoparticlra are lower than that of bulk titania (1850 C). The liquid titania particles are supercooled and became metastable states. The residence time in the flame is only in the order of miU-second so that the metastable phase has no time to become thermodynamically stable phase, rutile. 

Fig. 4.11. Unexpected formation of a metastable phase of acetaminophen as a result of exposure to the molten wax formulation during spray-congeal processing. Note The three curves have been manually offset on the F-axis from the normal zero milliwatt baselines in order to display the relative X-axis (temperature) differences between the three samples.

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

Fig. 8.6 Formation of metastable phases of higher Gibbs energy of formation during the discharge of Li FeSj. Upon recharging the electrode returns to the thermodynamically more favourable phases.

---