The Metastable Amorphous State

The extensively studied (especially during the recent years) transitions of solids from the metastable amorphous state to the polycrystalline state (see ref. 58 and the references therein) are of autowave character and resemble very much the above regimes of solid-state cryochemical reactions. The action of autodispersion, which facilitates phase transition by allowing it to proceed on the surface of a fracture instead of in the glass volume, cannot be excluded in the case of those processes either. Actually, the two classes of processes are similar in their physical nature both are connected with rearrangement of the solid matrix and are of exothermic character, differing only in the extent of the thermal effect. It should be added that fracturing and autodispersion of the sample are very typical of the autowave destruction of amorphous states and can be seen even by the unaided eye. 

A summary in the form of a diagram of metastable amorphous states is depicted in Fig. 13, in which the upstroke-downstroke hysteresis has been averaged out. The vertical arrow represents the pressure annealing from... 

This was the first proven example of the operation of the principle that free energy stored in the metastable amorphous alloy can be used to create a catalytically active species which is still metastable against phase separation and recrystallization, but which is low enough in residual free energy to maintain the catalytically active state for useful lifetimes. 

Many foods contain biopolymers and low molecular weight carbohydrates. These can be present in a metastable amorphous state that is sensitive to temperature and the state... 

Starting from this initial state G0, the free enthalpy of the system can be reduced either by the formation of the metastable amorphous phase or by the formation... 

An important characteristic that must be optimized in the development of low-molecular-weight orgaiuc materials is the ability to form morphologically stable amorphous films. Thermal stress during the operation of a device can lead to phase transitions of the metastable amorphous film into the thermodynanucaUy stable polycrystaUine state. It has been found that this effect leads to a fast degradation of the device. Moreover, grain boundaries... 

In summary it was the aim of this lecture to discuss a new mechanism without rapid quenching which produces amorphous metals by solid state reactions. All parameter known so far summarize in the critical condition to be fast enough for the competing crystalline phases. The main subject was on the gas-crystal reaction were an interface limited process is expected for the reaction kinetic. This remains one on the vice versa case of the polymorphic crystallization of some metallic glasses. Pure metallic diffusion couples seem to exhibit a /t-law for the growth of the planar amorphous layers at least for longer times. This case comes close to the eutectic crystallization in the reverse subject. All amorphization processes lead into the same metastable amorphous state, which is far from being only a "frozen in" liquid. Solid state reactions are just a new way into the same minimum. 

Since polymer chains are largely immobilized below Tg, if they are cooled rapidly through T to below Tg, it is sometimes possible to obtain a metastable amorphous state in polymers which at equilibrium would be crystalline. As long as the material is held below Tg, this metastable amorphous state will persist indefinitely. When annealed above Tg and below T ,), the polymer will crystallize, as the chains gain the mobility necessary to pack into a lattice. 

During precipitate ageing, a gradual transformation of an initially precipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amorphous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated substance (Mullin, 2001). In 1896, Ostwald promulgated his rule of stages which states that an unstable... 

When determining the solubility and dissolution rate of amorphous or partially crystalline solids, the metastability of these phases with respect to the highly crystalline solid must be considered. While the low diffusivity of the molecules in the solid state can kinetically stabilize these metastable forms, contact with the solution, for example during measurements of solubility and dissolution rate, or with the vapor, if the solid has an appreciable vapor pressure, may provide a mechanism for mass transfer and crystallization. Less crystalline material dissolves or sublimes whereas more crystalline material crystallizes out. The equilibrium solubility measured will therefore approach that of the highly crystalline solid. The initial dissolution rate of the metastable form tends to reflect its higher... 

Three different forms of beryllium hydroxide in the solid state have been described (10, 52, 53, 100). The amorphous form of Be(OH)2 is obtained as a gelatinous precipitate when alkali is added to a beryllium-containing solution at ambient temperatures. The gelatinous precipitate is slowly transformed into the metastable a form when the mixture is allowed to stand. The stable fi form is obtained after the mixture has aged for some months or by precipitation at 70°C. The value of log K for the equilibrium between a and (3 forms,... 

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