Phase transformations, periodic

Figure 6.34 shows the dependence of the dimensionless period of phase transformations (i.e., the time between bubble venting), t, on boiling number Bo f =t/Ud, ... 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

The linear log-log plots of reaction rate (in terms of oxygen consumption) versus time show for many alloys a discontinuity, or increase of reactivity. It appears that this transition is associated with the phase transformation in the protective film of Zr dioxide. The initial film formed on Zr is the cubic polymorph of Zr dioxide. After a period of oxidation this transforms to the tetragonal, and finally to the monoclinic (stable) form of Zr dioxide. When certain alloying constituents... 

Figure 1 shows that the phase transformation, the crystallization rates as well as the length of the induction periods, depends drastically on the kind of template and the alkalinity. One has to take into account that all other synthesis parameters have been kept as constant as possible. 

Periodic reactions of this kind have been mentioned before, for example, the Liese-gang type phenomena during internal oxidation. They take place in a solvent crystal by the interplay between transport in combination with supersaturation and nuclea-tion. The transport of two components, A and B, from different surfaces into the crystal eventually leads to the nucleation of a stable compound in the bulk after sufficient supersaturation. The collapse of this supersaturation subsequent to nucleation and the repeated build-up of a new supersaturation at the advancing reaction front is the characteristic feature of the Liesegang phenomenon. Its formal treatment is quite complicated, even under rather simplifying assumptions [C. Wagner (1950)]. Other non-monotonous reactions occur in driven systems, and some were mentioned in Section 10.4.2, where we discussed interface motion during phase transformations. 

From a practical point of view, creep becomes an important natural phenomenon when the temperature at which a metal is loaded lies above about 0,4 to 0,5 of its melting point on an absolute scale. In some metals such as zirconium, which undergo a solid state phase change, creep becomes an important effect above about one-half of the temperature of the phase transformation. In many metals such as steel, creep is almost nonexistent at room temperatures if the metal is not loaded above its annealed yield strength. However, at 900"F (482"C> steel can creep readily al very small stresses, and equipment such as boilers and tubes for petroleum cracking stills, intended to operate at high temperatures for long periods... 

When the gel is kept in contact with the pore-filling liquid, its structure and properties keep changing as a function of time. This process is called aging. During the aging period, four processes affect the porous structure and surface area of the silica gel. These are polycondensation, syneresis, coarsening and phase transformation.6,12,13... 

The product of the initial reaction forms a protective layer on the C3S particles the induction period ends when this is destroyed or rendered more permeable by ageing or phase transformation (S53,B66,J15). 

However, even though this transformation is thermodynamically favored, the diamond allotrope still exists at high pressures and over long time periods. That is, if a particular phase transformation is predicted as spontaneous, the acmal rate of that process will depend on the kinetics of the transformation. Since the sp carbon bonds in diamond are extremely strong, the kinetics governing the migration of carbon atoms between diamond-graphite is extremely slow at normal temperatures and... 

From a view point of reaction time, the typical preparation of mesoporous material can be divided three main steps (1) interaction between surfactant and silica (or other inorganic) species in solution and the formation of ordered mesostmcture (2) the further reaction (polymerization or condensation for silica) at a certain temperature for a time period. A possible phase transformation may occur (3) recovery of solid product by filtration, washing, and drying. The phase transformation may also occur in this step (4) removal of template from the solid product by calcination or extraction with solvent. The phase transformation is also possible even in this step. 

In many systems, measurement of the solubility of a metastable form can be directly obtained if there is an energy barrier between the metastable polymorph and the stable polymorph that prevents interconversion during the lifetime of the measurement. If the free energy difference of the polymorphs, which is the driving force of the phase transformation, does not overcome the activation energy barrier, the metastable polymorph may stay unchanged for a sufficiently long period of time to permit a direct determination of solubility to be made. 

Hydroxyapatite/titania layers were spin-coated on the surface of TiZr alloy at a speed of 3000 r.p.m. for 15 s, followed by a heat treatment at 600 °C for 20 min in an argon atmosphere (Wen et al., 2007). The coating displayed excellent bioactivity when soaked in a SBF for an appropriate period. Differential scanning calorimetry, TGA, XRD and SEM in conjunction with energy dispersive spectroscopy were used to characterise the phase transformations and the surface structures and to assess the in vitro tests. The titania (anatase) layer exhibited a cracked surface and the HAp layer showed a uniform dense structure. Both layers were about 25 im thick. 

From Fig. 6 it is evident that the dopant elements have an influence on the transformation period of the metastable alumina phase to a-Al203. The longest transformation period is observed for p-NiAl + Ce with 150h, followed by (3-NiAl+Y with lOOh and undoped (3-NiAl with 50 h. The shortest transformation period is observed for (3-NiAl+Hf with 25 h. 

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