Precursor phase

The phase relationship between the most commonly known tetragonal phase CaC2-I and phase(s) II (or III) is still in question. CaC2-I transforms reversibly into the cubic phase IV, as does the phase (II -r) III. It is interesting to note that the cubic phase IV seems to have a memory for its respective precursor phase (III or I) that is regained after cooling down phase IV. The synthesis of... 

Because both [PtClgl g) and Ru" are more equally adsorbed on alumina, the mobility of the Pt-precursor phase relative to the Ru-precursor phase is reduced. The data in Table III show that the increase in dispersion following dilution with pure alumina (Alon-C) is moderate with respect to that observed on silica. However, the redlsperslon of Pt is still substantially greater than that of Ru. 

Compounds of crucial importance for the RNA world or a precursor phase could have been formed in this or similar ways. Thus, a thioester world seems to support the RNA world hypothesis, although de Duve can be considered as a careful critic of the latter hypothesis. His opinions were expressed in a short article in Nature under the provoking title Did God make RNA (de Duve, 1988). 

The most complete model to date for describing Case II diffusion is that of Thomas and Windle (13-16). They envision the process as a coupled swelling-diffusion problem in which the swelling rate is treated as a linear viscoelastic deformation driven by osmotic pressure. This model leads to the idea of a precursor phase propagating ahead of a moving boundary, as we have depicted in Figure 4. While Thomas and Windle have used numerical methods to examine in detail the predictions of their model, this model is difficult to test with the data obtained here. 

The reptation model for polymer diffusion would predict that the thickness of the gel phase reflects the dynamics of disentanglement. The important factors here are chain length, solvent quality and temperature since they affect the dimensions of the polymer coils in the gel phase. The precursor phase, on the other hand, depends upon solvency and temperature only through the osmotic force it can generate in the system and the viscoelastic response of the system in the region of the front. These factors should be independent of the PMMA molecular weight. 

Schematic plot of free energy of formation of clusters from solution as a function of size (number of ions in the cluster). Curve A corresponds to the (thermodynamically) ultimately formed more stable phase, while curve B corresponds to the precursor phase.

As the precursor, e.g., an amorphous phase, precipitates and brings down the supersaturation of the solution, the more stable phase to be precipitated is using the precursor phase as a substrate for its own precipitation (Steefel and Van Cappellen, 1990). A classical example that documents this principle is the precipitation of calcium phosphates, where a metastable calcium phosphate precursor phase is nucleated initially and is then replaced, in some instances via an intermediate phase, by apatite. (Nancollas, 1990 Steefel and Van Cappellen, 1990). 

Recently, a new concept in the preparation of TPVs has been introduced, based on the reaction-induced phase separation (RIPS) of miscible blends of a semicrystalline thermoplastic in combination with an elastomer, with the potential for obtaining submicrometer rubber dispersions. This RIPS can be applied to a variety of miscible blends, in which the elastomer precursor phase was selectively crosslinked to induce phase separation. Plausible schematic representation of the morphological evolution of dynamic vulcanization of immiscible and miscible blends is shown in Fig. 9. For immiscible blends, dynamic vulcanization leads to a decrease in the size... 

The previous section describes different preparation methods, that produce materials with similar phase composition but markedly distinct morphological properties. To rationalize this behavior, the thermal evolution of the precursor phases obtained with the different preparation procedures has been thoroughly addressed in the literature. 

Hydrolysis of Alkoxides - A different thermal evolution of the precursor phase was observed for a BaAli20i9 sample prepared via hydrolysis of alkoxides.8,9 For this sample no XRD diffraction lines were detected after heating at 1000°C. At 1200°C Ba-hexaaluminate formed, and it was the only phase detected in the XRD spectra. Calcination at 1450 °C resulted in higher sharpness and intensity of the XRD reflection of Ba-hexaaluminate. No further changes were observed upon calcination at 1600°C. Also in this case, the appearance of Ba-(3-Al203 was accompanied by a marked drop of surface area. However, the... 

The nature of the functional groups of the HBP is a significant factor for the control of viscosity, miscibility with the thermoset precursors, phase separation during cure, and particle-matrix adhesion. 

Keller, W.D., Hanson, R.F., Huang, W.H. and Cervantes, A., 1971. Sequential active alteration of rhyolitic volcanic rock to endellite and a precursor phase of it at a spring in Michoacan, Mexico. Clays Clay Miner., 19 121-127. 

The reaction pathway can be complex with the formation of intermediate precursor phases (e.g., Katz and Mathews, 1977). An induction stage can be the slowest part of the reaction (Sibley et al., 1987). 

Raz, S., Weiner, S., and Addadi, L. "Formation of high-magnesian calcites via an amorphous precursor phase Possible biological implications". Advan. Mater. 12(1), 38-42 (2000). Riekel, C., Muller, M., and Vollrath, F. "In situ X-ray diffraction during forced silking of spider silk". Macromolecules 32(13), 4464 -4466 (1999). 

Case 4 Chemical interaction patches of monolayers or complete monolayer (Fig. 6). This category corresponds to situations where all the atoms or molecules of the supported precursor are exposed on the surface. The upper limit is encountered when the precursor phase combines with the carrier surface as a continuous monolayer. Figure 6 represents a partial coverage with one-molecule-thick patches (also called rafts). A typical example corresponds to the formation of a monolayer of M0O3 on the surface of y-Al203 as follows ... 

The deposition of unstable amorphous precursor phases requires a hydrated medium that has a very high ion concentration, and an inordinately high supersaturation relative to the corresponding crystalline phases. In order to stabilize, even transiently, such high supersaturations, specialized inhibitors of crystallization probably need to be present. Such high concentrations of ions cannot be regarded as a solution, but rather a structured colloidal phase. Concepts such as mechanisms of diffusion, levels of supersaturation and consequently kinetics of crystallization, must be reconsidered if crystallization does not occur from free solution. 

Control of fhe precursor phase is an imporfanf aspecf in fhe formafion of an active catalyst. The topotactic transformation to the pyrophosphate phase leaves many of fhe feafures of fhe precursor unchanged. Controlling the morphology and preferentially exposing the desired crystal faces have the potential to increase the activity of fhe catalyst or its selectivity to MA. 

Vanadyl pyrophosphate is widely considered to play an important catalytic role in the oxidation of -butane to MA, specifically the (100) face (Figure 18b), which is retained from the topotactic transformation (6,43,84—86) of the catalyst precursor phase (Figure 18a). Furthermore, this active phase has been reported to be an efficient catalyst for the oxyfimctionalization of light paraffins (a) for the oxidation of ethane to acetic acid (3,87), (b) for the oxidation and ammoxidation of propane to acrylic acid (88) and acrylonitrile (89,90), respectively, and (c) for the oxidation of n-pentane to maleic and phthalic anhydrides (90-102). 

Acid precursor phase Amine phase Thermally regenerable capacity, 20-80 °C (meq/ ... 

Measurements of the specific surface area, SSA, of the products grown at various times indicate that the initial formation of a microcrystalline or amorphous precursor leads to a rapid increase in SSA. The development of these phases is also observed by scanning electron microscopy, and dissolution kinetic studies of the grown material have indicated the formation of OCP as a precursor phase ( , 7). The overall precipitation reaction appears to involve, therefore, not only the formation of different calcium phosphate phases, but also the concomitant dissolution of the thermodynamically unstable OCP formed rapidly in the initial stages of the reaction. In the presence of magnesium ion the overall rate of crystallization is reduced and lower Ca P ratios are observed for the first formed phases (51). 

It appears that the magnesium ion stabilizes a precursor phase, the stoichiometry of which appears to correspond to a mixture of DCPD and OCP. The possibility of DCPD, the simplest calciiam phosphate phase, as a precursor to apatite precipitation was proposed by Francis (52). 

The effects of aqueous alteration are apparent in the least equilibrated ordinary chondrites such as Semarkona, Bishunpur, and Chainpur. The effects of alteration are most widely developed in the matrices of these chondrites (Hutchison et al., 1987 Alexander et al., 1989a,b), but alteration of chondrules has recently been recognized (Hutchison et al., 1998 Grossman et al., 2000, 2002). In these chondrites, alteration of the matrices is not complete and precursor phases are still present. The dominant alteration phase in the matrix is an amorphous phase and an iron-rich smectite phase replacing olivine. 

Van Cappellen and Berner (1991) smdied the growth kinetics of FAP in seeded precipitation experiments using carbonate-free solutions. The growth of FAP was inhibited by Mg " " at concentrations typical of marine pore waters and enhanced by H" " ions in the pH range of 7-8.5. At moderate degrees of supersaturation, a precursor phase similar in composition to octacalcium phosphate formed on the FAP... 

---