Of amorphous phase

Figure 2. Bulk glass formation range in Pd-Ni-P system. Filled eireles denote the formation of amorphous phase, open eireles denote the formation of erystalline phases.

Heat capacity measurements at the glass transition temperature, Tg, are based on the same differential concept. The weight fraction of amorphous phase is calculated as the ratio of changes of heat capacity of the semi-crystalline sample ACp(S) over the change in heat capacity of the melt (ACp(m)) at the glass transition. For a two-phase system, the degree of crystallinity is given as ... 

Pickering and co-workers observed visible photoluminescence (PL) from PS at 4.2 K in 1984 [Pil], which they interpreted as due to a complex mixture of amorphous phases. The questions of why PS is transparent for visible light and why it is photoluminescent remained unanswered until 1990-91 when a quantum size effect was proposed as an explanation [Cal, Lei]. Two years later PL was also found for oxidized PS [Lel5, It2]. These astonishing optical properties of PS initiated vigorous research and resulted in more than a thousand publications, as well as several books and reviews [Cu2, Th7]. 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

Topological disorder that leads to formation of amorphous phases... 

There are a number of studies where the CALPHAD approach to GFA in amorphous systems has been used. Bormann and co-workers (1988, 1990, 1993) and Saunders and Miodownik (1986) have all used G/x diagrams to help explain the formation of amorphous phases in SSAR. This work has shown that as well as... 

Ciystallization of the as-anodized amorphous nanotubes takes place through nucleation and growth [154], Hence, a sufficiently high temperature and appropriate duration are needed for the complete transformation of the amorphous phase into crystalline. The removal of the amorphous phase is critical for effective device application, however even after annealing at 600°C Sample 20 showed the presence of amorphous phase. HRTEM studies indicate that rutile and hematite phases are formed in separate events depending upon the temperature and nature of the nucleation sites. In general, the nanotubes of moderate iron concentration consist mainly of rutile phase with hematite crystallites randomly distributed. 

These two inequalities can be proved for a more general model in which the film is assumed to consist of multiple parallel bands, each band containing an arbitrary fraction of amorphous phase which exhibits multiple relaxation processes. 

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

The catalytic activity of these oxometalates is well documented [360, 361]. An inactive surface of Ti02 becomes an efficient catalyst for H2 evolution as it is derivatized with silicotungstic acid [362, 363]. However, while real electrocatalytic effects seem likely for a pigmented Ni surface in view of the lower Tafel slope observed (which can also be due to some activation of the Ni itself), these are not completely established for the surface of pure oxometalates surface area effects could be entirely responsible for the apparent activation. The real surface state of these electrodes deserves to be further investigated since these materials might fall into the category of amorphous phases. 

Numerous physical techniques have been used to characterize the bulk and the surface of amorphous phases, including X-ray, AES, SEM, XPS, Mossbauer spectroscopy (MOS) and STM. MOS has been able to highlight the state of Fe in a elec-trodeposited FeP alloy [573]. Three different non-equivalent positions of Fe with different Fe-P distances have been identified. In Feg3P7, 16% of Fe is in the amorphous state. This indicates the extent by which the properties of these phases can be controlled. STM of FeCo alloys has shown that the surface may possess properties depending on the size of the homogeneous zones (cf. particle size effect) [588]. 

The fact that the thickness of the interphase estimated here stays unchanged at 34A in the molecular weight range of 30,000-100,000, while the mass fraction and thickness of amorphous phase change remarkably, is particularly meaningful. Flory et al. [6,7] anticipated in 1984 based on their lattice theory that the methylene chains that emerge from the basal plane of lamellar crys-... 

As can be seen from Table 4, two lower molecular weight samples actually comprise only the crystalline and noncrystalline interlamellar material, devoid of amorphous phase. On the other hand, the larger molecular weight samples comprise three phases the crystalline, amorphous, and crystalline-amorphous interphase in a similar fashion to the atmospheric-pressure crystallized samples. However, we note that the T2c s of the crystalline-amorphous interphase for two higher molecular weight samples is appreciably shorter than those of the atmospheric-pressure crystallized samples. This demonstrates that the molecular chain motion in the crystalline-amorphous interphase of these pressure-crystallized samples on a T2C time scale is more severely restricted. 

The thermal properties of the dimers 50a-d relative to the linear analogs were probed by DSC analysis. The pure phenylenevinylene cruciforms 50a and b still form crystalline phases with Tm = 223 and —258 °C, respectively. As the volume of the cruciforms is increased, the intermolecular interactions are reduced, which is documented by the occurrence of amorphous phases with glass transitions at 43 and 142 °C for 50c and d, respectively. A corresponding cruciform trimer 54 was also synthesized by Ma and coworkers [76] following a related protocol, as illustrated in Scheme 3.25. The DSC of 54 confirms the previously discussed trend and 54 is fully amorphous and shows only a Tg at 118 °C [76]. 

As a result one obtains that average size of compact supermolecules (H20)n in a cryogenic HDA-ice is equal to 18 nanometers that corresponds to n 105 molecules of water. The same as liquid water under normal conditions LDA-ice has an average size of compact supermolecules (H20)n about 1 nanometer that corresponds approximately n 30 molecules of water. These distinctions at a size of compact multiparticles can serve for explain observable density jump at polyamorphic transition of ice. According to scheme of this transition shown on Fig. 3 one comes to think of it. Indeed, knowing share distributions of amorphous phases it is possible to calculate percentage change of volume ... 

An interesting analogous in vitro experiment was performed by Xu el al. [92] in which the presence of a polyelectrolyte in solution resulted in a layer of amorphous calcium carbonate forming under a structured monolayer. This subsequently transformed into a thin layer of polycrystalline calcite. The crystal growth in this in vitro system occurs by phase transition of amorphous calcium carbonate into calcite and not by dissolution of amorphous phase and reprecipitation of calcite crystals. 

Thus, although substitution generally increases the proportion of amorphous phase and leads to more disordered crystalline phases, the reverse effect can be obtained using specific molecular interactions between side groups and conjugated chains. This suggests that interaction of a CP chain with another, nonconjugated polymer could be used to orient the former. In a sense, this is what is achieved in blends (see Section II.E). 

---