Formation of Metastable Phase

Metastable phases such as amorphous silica (SiOa) and wurtzite (ZnS) are common in mid-oceanic ridge deposits. These phases form fi om highly supersaturated mixed fluid of hydrothermal solution and seawater. Solubility of metastable phases is higher than that of stable phases. Therefore, metastable phases dissolve and stable phases form after the formation of ore deposits. Wurtzite (metastable phase of ZnS) is observed in active chimney, but sphalerite (stable phase of ZnS) is in nmiactive chimney. It is inferred that sphalerite formed by the dissolution of wurtzite. 

Anhydrite is abundant in active chimney but not inactive one. Anhydrite dissolves by the interaction of cold seawater because solubility of anhydrite at lower temperature is higher than that at higher temperature. 

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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.

One way to rationalize and remember the rule is to think that "nature is lazy" and hence would like to accomplish a process with the least effort. Because ions and molecules in a liquid are more or less randomly distributed, one may guess that the phases with simple structure and with low degree of order (meaning ions and molecules do not have to be arranged in a specific way) tend to form more easily than the phases with complicated structure and high degree of order. This often means the formation of metastable phases such as opal. 

As introduced above, different forms of the same molecule can be observed in the solid state. The phenomenon is known as polymorphism, i.e., the concurrent presence of more crystal forms, only one of which is thermodynamically stable at a given pressure and temperature. However, more polymorphs can be observed simultaneously when kinetic conditions allow formation of metastable phases together with (or even in the absence of) the thermodynamically stable one. It might even occur that metastable phases are not recognized as such, simply because the most stable polymorph is (as yet) unknown. This might produce the extraordinary phenomenon of disappearing polymorphs [97]. 

The tendency of B-C disorder, i.e. mutual exchange of B and C atoms in the RNi2B2C compounds in not well heat-treated samples (see Section 3.1) is also an example for the formation of metastable phases in the R-T-B-C system. 

In 1897, W. Ostwald formulated a general rule that remains important to this day for synthesizing materials If a chemical reaction produces several modifi cations of a compound, it is not the most stable but the most labile modifica tion that forms first, while more stable modifications are produced via further transformations of the labile one [1,2,3]. Hence, the Ostwald step rule indi cates the possibility of the formation of metastable (labile) intermediate phases on phase transitions due to the kineticaUy preferential formation of metastable phases rather than a thermodynamically stable phase immediately. 

Reversibility of the phase transitions formation of metastable phases... 

Because of long relaxation times, especially in the transition vicinity, lipid phase transitions often are not reversible and end up with the formation of metastable phases, which replace the equilibrium phases in cooling scans. The metastable phases can be very long-lived and display no spontaneous conversion to the ground state in sensible time scales. 

Penn RL, Banfield JF (1998b) Oriented attachment and growth, twinning, polytypism, and formation of metastable phases Insights from nanociystalline Ti02. Am Mineral 83 1077-1082 Post JE, Bish DL (1989) Rietveld refinement of crystal stmctnres nsing powder X-ray diffraction data. Rev Mineral 20 277-308... 

B.Y. Tsaur Ion-Beam-Induced Modifications of Thin Film Structures and Formation of Metastable Phases. Ph.D. Thesis, Calif. Inst, of Tech. (1980)... 

Magneli-type TiH02H 1 phases should have been formed (Berger et al., 2001 Naumann et al., 2003). However, brookite formation may be the result of a thermodynamic process akin to Oswald s rule of steps, whereby the steep temperature gradient attained during plasma spraying will cause the formation of metastable phases. 

Formation of metastable phases (H-In203, y-Fe203, SnO, H-M0O3) Developed specific surface area Elevated activity in a course of adsorption processes Increased additive solubility as compared to the equilibrium values... 

Diamond thin films can be produced from microwave-discharge-heated methane or from carbon that is sputtered by ion bombardment [63-65]. Discuss the methods of preparation and the possible reasons for the formation of metastable phases by these methods. 

It is well established that most metallic and ionic materials undercool to approximately 80% of the equilibrium melting point before crystallisation commences. The cooling rate is also large ranging form 10 — 10 K/s. These effects may result in the formation of metastable phases as discussed in detail by McPherson for alumina. The crystalline structure formed may be influenced by a second component. For example Al Oj favours the formation of rutile form of TiO rather than anastase... 

In alloys of the other category, the preferentially sputtered component is segregating towards the surface. Here PtsSn serves as an example, in which Sn is segregating and preferentially sputtered (wsn < mpt and Tsn < 7pt). The PtsSn exhibits a strict chemical order of the LI2 type, that is, fee structure with Pt at the comer sites and Sn at the face sites of the unit cell. The depletion in Sn in the surface region leads to a smaller lattice constant (apt < JptsSn)- All three low-indexed surfaces of PtsSn respond to this depletion by formation of metastable phases with characteristic stress compensation features (Table III). A mesoscopic dislo-... 

Only very rarely are the traces of the stl2 phase observed in Ge nanoindentations [38]. In view of the Si nanoindentation results, where only the elbow in the unloading curve and never a pop-out (associated with the Si-II -> Si-XII transformation [91, 123]) is observed below some load threshold ( 10 mN for Si) [12, 121], we conjecture that a minimum volume of material may be necessary for the abrupt transformation from the metallic into a metastable phase to occur. The maximum loads reported for Ge nanoindentation experiments did not exceed 120 mN [8], and perhaps the lattice constraints in this case inhibit the abrupt Ge-II Ge-III transformation. On the other hand, at higher loads (> 1 N), the formation of metastable phases in Ge indentations has been experimentally confirmed (see Fig. 28). 

It is general experience in materials science that alloy can exhibit qualities that are unobtainable with parent metals. This is particularly true for electrodeposited alloys, mainly due to formation of metastable phases and intermediate layers. Some important properties of materials, such as hardness, ductility, tensile strength. Young s modulus, corrosion resistance, solderability, wear resistance, antifriction service, etc., may be enhanced. At the same time, some properties that are not characteristic for parent metals, such as high magnetic permeability, other magnetic and electrical properties, amorphous structure, etc., can also be obtained. In some cases, alloy coatings may be more suitable for subsequent electroplate overlayers and conversion chemical treatments [1],... 

One recent development in the static immersion testing from Newman et al. [116] originated with Bengough and May [JJ5]. Recent experimentation [116] involved exposing samples of alpha-brass in NaCl solutions with additions to simulate the local environment in crevices or otherwise inhibited transport. Under these severe conditions, dealloying can be shown to occur without an applied potential and may be reduced or eliminated by addition of arsenic to the alloy. The authors suggest that these results support the percolation mechanism with surface diffusion for dealloying rather than mechanisms that rely on formation of metastable phases or disproportionation of the less noble element. 

Mechanical alloying has been shown to cause significant microstructural changes, including the formation of a nanocrystalline grain structure, extended solid solubility, disordering, formation of metastable phases, amorphization and disproportionation of pre-... 

The high rate of the recasting process gives opportunities for formation of metastable phases and considerable decreasing of grain size. The electrolyte type is of great importance for the chemical composition, microstructure and properties of the modified layer. By these experiments the electrolyte is on water basis and contains boron or silicon compounds. At short times of treatment it is not observed diffusion of elements from the electrolyte in the modified surface, but it is available diffusion process inside the workpiece between the white layer and the matrix - Table 3. The strong carbide-formed elements such as Mo, W, and V diffuse from the white layer to the matrix and Cr, Co in the op>posite side. 

Bag59] Bagariatskh, Yu.A, Nosova, G.I., and Tagunoya, TV, Factors in the Formation of Metastable Phases in Titanium-Base Alloys, Sov. Phys. Dokl., Vol 3, 1959, p. 1014-1018 [transl. of Dokl. Akad. Nauk SSSR, Vol 122, 1958, p. 593-596]... 

The high-temperature environment in the flame reactor, as well as short residence times and rapid cooling directly after production of the particles, often result in the formation of metastable phases or polymorphs which are not easily accessible by conventional techniques. Depending on the composition, the rapid quenching can preserve the amorphous state of the material [20, 21]. As a result of the process characteristics and parameters, the primary particles produced are spherically shaped with different degrees of a omeration [22, 23]. Based on models and experimental data, the primary particles have a log normal particle size distribution [24, 25]. 

For PAs and PGs there is a clear dependence of the binding behavior on the nature (i.e. probably the size) of the divalent cation. behaves differently compared to Mg or Sr ", the phases formed in an excess of divalent cations are probably not the same and the thermotropic behavior is complicated by the formation of metastable phases [48], As an example, DSC-curves of DMPA at various PA M ratios is shown in Figure 18 for the case of Ca and Mg binding. The DSC scans at pH 3.5 always correspond to the singly charged form of DMPA, i.e. 2 1 DMPAiM " complexes, whereas at pH 1 1 1 complexes are formed. 

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