Phase transformations unstable

However, at some specific pressure the high-density polymorph becomes mechanically unstable. This low-pressure limit is seldom observed, since it often corresponds to negative pressures. When the mechanical stability limit is reached the phase becomes unstable with regard to density fluctuations, and it will either crystallize to the low-pressure polymorph or transform to an amorphous phase with lower density. 

Mitrovic and Knezic (1979) also prepared ultrafiltration and reverse osmosis membranes by this technique. Their membranes were etched in 5% oxalic acid. The membranes had pores of the order of 100 nm, but only about 1.5 nm in the residual barrier layer (layer AB in Figure 2.15). The pores in the barrier layer were unstable in water and the permeability decreased during the experiments. Complete dehydration of alumina or phase transformation to a-alumina was necessary to stabilize the pore structure. The resulting membranes were found unsuitable for reverse osmosis but suitable for ultrafiltration after removing the barrier layer. Beside reverse osmosis and ultrafiltration measurements, some gas permeability data have also been reported on this type of membranes (Itaya et al. 1984). The water flux through a 50/im thick membrane is about 0.2mL/cm -h with a N2 flow about 6cmVcm -min-bar. The gas transport through the membrane was due to Knudsen diffusion mechanism, which is inversely proportional to the square root of molecular mass. 

Our XPS and AES results indicate that the 300 L (saturated) NiO(lll) layer is thermally unstable and it undergoes a phase transformation at temperatures above 525 K [24,25]. Such a phase separation is also supported by the HREELS spectroscopic changes from Figure Id to Figure le. After heating the NiO/Ni(100) layer to 800 K,... 

The X-ray penetration power enables the use of cells under a controlled atmosphere. Currently, much development work concerns in situ XRD, a valuable aide in the study of fragile or unstable solids in ambient conditions but also offering possibilities of monitoring reaction kinetics in phase transformations. 

If a phase is unstable with respect to phases infinitesimally different from it, then it will disappear and give rise to one or more neighbouring phases. This process will be repeated until we arrive at a phase which is stable with respect to adjacent phases. In fact, since all matter undergoes molecular fluctuations, small amounts of phases infinitesimally different from the initial phase will be formed continuously, and so by means of such fluctuations, the system will be transformed spontaneously into a perturbed state. If a phase is not stable with respect to the perturbed state, then the phase will disappear. 

The latter instrument is of particular value in work of this kind because it allows continuous observation of a diffraction line. For example, the temperature below which a high-temperature phase is unstable, such as a eutectoid temperature, can be determined by setting the diffractometer counter to receive a prominent diffracted beam of the high-temperature phase, and then measuring the intensity of this beam as a function of temperature as the specimen is slowly cooled. The temperature at which the intensity falls to that of the general background is the temperature required, and any hysteresis in the transformation can be detected by a similar measurement on heating. 

During calcination, the precursor undergoes a continuous crystallization and recrystallization process during which thermodynamically unstable or metastable phases are converted to more stable ones. X-ray diffraction patterns in Fig. 7 show phase transformation of hydrous titanium oxide from 100°C to 550°C.P l... 

Upon release of supersaUiration, the initially dissolved compound will be separated from the solution and form a secondary phase, which could be either oil, amorphous solid, or crystalline solid. Crystalline materials are solids in which molecules are arranged in a periodical three-dimensional pattern. Amorphous materials are solids in which molecules do not have a periodical three-dimensional pattern. Under some circumstances with very high supersaturation, the initial secondary phase could be a liquid phase, i.e., oil, in which molecules could be randomly arranged in three-dimensional patterns and have much higher mobility than solids. Generally, the oil phase is unstable and will convert to amorphous material and/or a crystalline solid over time. At a lower degree of supersaturation, an amorphous solid can be generated. Like the oil, the amorphous solid is unstable and can transform into a crystalline solid over time. Even as a crystalline solid, there could be different solid states with different crystal structures and stability. The formation of different crystalline solid states is the key subject of polymorphism, which will be mentioned below and... 

In the metastable region between the binodal and spinodal curves, phase separation has to occur by the mechanism of nucleation and growth. In this region, the one-phase-state is Indeed stable against small concentration fluctuations but unstable against separation into two phases of more different concentrations. Phase transformations in one-component systems like condensation, evaporation or solidification as well as the crystallization of solutes from solvents occur by the nucleation and growth mechanism. The well known phenomena of oversaturation and hindered-phase transformation can be explained by discussing the nucleation as an equilibrium reaction with the creation of the "critical nucleus" (6, 7). 

The Al-Sc quasicrystalline phase was unstable, and after 300 days holding at room temperature ribbon phase composition transformed to a-Al+Al3Sc. Ribbon hardness increased with growing the content of the intermetallic Al3Sc phase to x=9 and practically stabilized at the level of about 2.3 GPa (Fig. 3). The appearance of the quasicry stal component did not increase this level. 

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