Amorphous Precursors

The addition of sodium during the synthesis of TS-1 completely eliminates the activity for n-octane oxidation and also the IR band at 960 cm- (this IR band is present in the amorphous precursor, Ti02-Si02). It has been shown (12) that the presence of sodium in itie synthesis gel prevents the incorporation of titanium into the zeolite framework. However, the addition of sodium ter the zeolite crystallizes does not... 

Mechanism of Action Inhibits formation, growth, and dissolution of hydroxyapatite crystals and their amorphous precursors by chemisorption to calcium phosphate surfaces ... 

In industrial procedures catalyst preparation in organic solvents is preferred, producing a more amorphous precursor with high density of active sites.1022 In addition, phosphorous compounds are usually injected into the feed to replenish phosphorous lost through hydrolysis. In this way an appropriate surface phosphorous vanadium ratio is maintained ensuring prolonged catalytic activity and selectivity. 

The most plausible interpretation of the above results is as follows Al in aluminate solutions is present in the form of AKOH) anions irrespective of the Al concentration or the Na/Al ratio (150,181,182). When solution A, containing low molecular weight silicates and thus a high proportion of reactive SiOH groups, is mixed with the aluminate solution, a predominantly aluminous intermediate results. When, by contrast, solution B, richer in silicon and with fewer silanol groups, is used, a silca-rich intermediate is formed. But the composition of zeolite A (with Si/Al = 1.00) is achieved via structural rearrangements in the amorphous precursors and is dictated by the final structure itself. 

Many amorphous alloys were tested in the hydrogenation of ethylene. It was shown that the thermal pretreatment of a Cu7oZr3o amorphous precursor in hydrogen leads to structural modification and the resulting catalysts exhibit extremely high activities in ethylene hydrogenation197 (Table 10). 

Ceramics with fine SiC and Si3N4 grains can also directly produced from amorphous precursors with no sintering additives, however they are not dense after crystallisation and therefore have relatively low strength and fracture toughness [213, 609-611]. 

Brink R, Lange H (1994) Investigations on the Synthesis of fine-grained, high-purity j8-Si3N4 Powder by Crystallization of amorphous Precursors. In Hoffmann MJ, Becher PF, Petzow G (eds) Silicon Nitride 93. Key Eng Mat 89-91. Trans Tech Publications Ltd, Switzerland, p 73... 

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). 

Abstract The focus of this chapter is primarily directed towards nanocrystalline soft magnetic materials prepared by crystallization of amorphous precursors. The key elements involved in the development of this class of materials are three-fold (i) theoretical models for magnetic softness in nanostructures (ii) nanostructure-property relationships and (iii) nanostructural formation mechanisms. This chapter surveys recent research on these three areas with emphasis placed on the principles underlying alloy design in soft magnetic nanostructures. 

Figures 4(a) and 4(b) show the relationship between the average grain size and the coercivity in various Fe-based nanocrystalline soft magnetic alloys prepared by crystallization of amorphous precursors (For details, see Herzer [13], Yoshizawa [31], Muller and Mattem [32], Fujii et al. [33], and Suzuki et al. [34, 35]). As shown in Fig. 4(a), the coercivity Ha of the nanocrystalline Fe-Si-B-M-Cu (M = IVa to Via metal) alloys follows the predicted D6 dependence in a D range below LO ( 30 to 40 nm for this alloy system) although the plots deviate from the predicted D6 law in the range below H0 1 A/m where the effect of grain refinement on is overshadowed by magneto-elastic and annealing induced anisotropies. Hence, the experiments are better described by Hc [a2 + where a...

The role of Cu in FINEMET has been discussed almost solely from the viewpoint of microstructural refinement. However, it is equally important to discuss the effect of Cu in suppressing the formation of magnetically hard compounds upon crystallization of the amorphous precursors. Prevention of the magnetically harder compounds from forming upon crystallization is surely the primary role of Cu in NANOPERM. 

The full substitution of Zr by Nb is important since the oxygen reactivity of Nb is lower than that of Zr or Hf, and because the amorphous precursors of some Fe-Nb-B based alloys can be prepared under ambient atmosphere [70], making NANOPERM mass producible. 

Apart from its technical performance the material is based on the inexpensive raw materials iron and silicon. The amorphous precursor material for the Fe-Si-B-Nb-Cu alloys, furthermore, is easily accessible by rapid solidification from the melt - a well established technique for large scale production of amorphous metals. 

K. Suzuki, in Properties and Applications of Nanocrystalline Alloys from Amorphous Precursors, NATO Science Series II Mathematics, Physics and Chemistry, Vol. 184 , Eds. B. Idzikowski, P. Svec, M. Miglierini, Kluwer Academic, Dordrecht 2005, p. 1. 

Considering the case of crystalline complex salts and the amorphous precursor method, both chemical com-plexation methods have found important and innovative applications in recent years for the preparation of a wide variety of catalysts and of various perovskite-type catalysts and barium hexaaluminates, as required for high-temperature (>1500K) applications such as catalytic combustion. It is therefore worthwhile describing some applications, in the next two subsections. 

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. 

If one assumes that these different crystal species are merely metastable intermediates in the continuum HS-A-X-Y, the Si02 content of the precipitated amorphous precursor would specify the starting position in the continuum. The reaction environment would then dictate which species, with Si02/Al203 ratio to that of the precursor, will crystallize. Whenever the temperature is sufficient, the free energy relationship between these zeolites will permit any of them to form. 

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). 

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