Solid-state amorphization reactions

In 1979, White [3.2] observed that, by milling elemental Nb and Sn powders, the distinct X-ray diffraction peaks of the elements disappeared and typical diffuse peaks of an amorphous pattern showed up. But these samples did not show the superconducting transition temperature of vapor-quenched amorphous Nb-Sn alloys. In 1983, Koch et al. reported on the Preparation of amorphous Ni60Nb40 by mechanical alloying [3.3]. After the detection of amorphization by solid-state reaction in evaporated multilayer films by Schwarz and Johnson [3.4] (see also Chap. 2), Schwarz et al. [3.5] proposed after investigating glass formation in Ni-Ti alloys, that amorphization by mechanical alloying is also based on the solid-state reaction process. Within the last couple... 

In the polymerization of DL-lactide, temperatures in the range of 135-155 C are typical. Since DL-Pl. is an amorphous polymer, solid-state polymerization does not occur as with PG and L-PL. The DL-PL polymerization reaction is also normally catalyzed by stannous octoate or stannic chloride dihydrate. 

Fig. 2. Time-Temperature-Transition diagram for a) liquid quenching, b) crystallization of the amorphous alloy at T / SSR) amorphization by solid state reactions at T. ...

In the solid state, small N0 species can diffuse through the amorphous zones, and clearly become associated with the oxidized domains in these amorphous zones. Thus the local N0 concentration adjacent to an -00H site (before photo-cleavage) is anticipated to be much greater than expected from the overallI NO1 concentration, and so increase the effectiveness of the scavenging process (reaction 7) as compared to the propagation step (reaction 2). 

X-ray powder diffractometry can be used to study solid state reactions, provided the powder pattern of the reactant is different from that of the reaction product. The anhydrous and hydrated states of many pharmaceutical compounds exhibit pronounced differences in their powder x-ray diffraction patterns. Such differences were demonstrated earlier in the case of fluprednisolone and carbamazepine. Based on such differences, the dehydration kinetics of theophylline monohydrate (CvHgN H20) and ampicillin trihydrate (Ci6H19N304S 3H2O) were studied [66]. On heating, theophylline monohydrate dehydrated to a crystalline anhydrous phase, while the ampicillin trihydrate formed an amorphous anhydrate. In case of theophylline, simultaneous quantification of both the monohydrate and the anhydrate was possible. It was concluded that the initial rate of this reaction was zero order. By carrying out the reaction at several... 

In industrial PET synthesis, two or three phases are involved in every reaction step and mass transport within and between the phases plays a dominant role. The solubility of TPA in the complex mixture within the esterification reactor is critical. Esterification and melt-phase polycondensation take place in the liquid phase and volatile by-products have to be transferred to the gas phase. The effective removal of the volatile by-products from the reaction zone is essential to ensure high reaction rates and low concentrations of undesirable side products. This process includes diffusion of molecules through the bulk phase, as well as mass transfer through the liquid/gas interface. In solid-state polycondensation (SSP), the volatile by-products diffuse through the solid and traverse the solid/gas interface. The situation is further complicated by the co-existence of amorphous and crystalline phases within the solid particles. 

The papers of Mallon and Ray [98, 123] can be regarded as the state of the art in understanding and modelling solid-state polycondensation. They assumed that chain ends, catalysts and by-products exist solely in the amorphous phase of the polymer. Because of the very low mobility of functional groups in the crystalline phase, the chemical reactions are modelled as occurring only in the amorphous phase. Additionally, the diffusion of by-products is hindered by the presence of crystallites. The diffusivity of small molecules was assumed to be proportional to the amorphous fraction. Figure 2.32 shows the diffusion coefficients for the diffusion of EG and water in solid PET. 

According to the principles of polycondensation, all of the above reactions will also take place during SSP. The conditions for the latter, however, are different as this process is carried out at lower temperatures in a non-homogeneous environment. In order to examine the kinetics of SSP, some assumptions have to be made to simplify the analysis. These are based on the idea that the reactive end groups and the catalyst are located in the amorphous regions. Polycondensations in the solid state are equilibrium reactions but are complicated by the two-phase character of the semicrystalline polymer. 

Either MA or MM processes drop under more general class of solid-state amorphization reactions, SSAR. Amorphization by irradiation of solids was observed yet in the era of study of materials for nuclear reactors. In 1962, Bloch [88] amorphized UgFe by exposing it to fluxes of nuclear fission fragments. Others obsawed amorphization... 

R.B. Schwarz, W.L. Johnson, Formation of amorphous alloy by solid-state reaction of the pure polycrystaUine metals, Phys. Rev. Lett. 51(5) (1983) 415 18. 

When thioamides 24f-h were irradiated in benzene with a high-pressure mercury lamp, iV-isopropyl-P-thiolactam 26 and 1,3,5-dithiazine 28 were obtained (Scheme 15 and Table 10, entries 1,3, and 5). Considerably different photochemical behavior was observed between that in solution and in the solid state. Powdered thioamide 24f was irradiated in the solid state at 0 °C until 19% conversion, because the solid changed to amorphous at around 20% conversion. In this case, only dithiazine 28f was obtained as the sole photoproduct (entry 2). On the contrary, photolysis of 24g gave a new type of p-lactam 27g as a main product in 88% yield in addition to dithiazine 28g (12%) the P-lactam 26g was not detected at all (entry 4). Photochemical ( ,Z) isomerization of (Z)-24g was also observed in the early stage of the reaction, where the ratio of the photostationary state was ZIE=1.9. In the case of 24h, P-thiolactam 27h was ob-... 

A further method of producing amorphous phases is by a strain-driven solid-state reaction (Blatter and von Allmen 1985, 1988, Blatter et al. 1987, Gfeller et al. 1988). It appears that solid solutions of some transition metal-(Ti,Nb) binary systems, which are only stable at high temperatures, can be made amorphous. This is done by first quenching an alloy to retain the high-temperature solid solution. The alloy is then annealed at low temperatures where the amorphous phase appears transiently during the decomposition of the metastable crystalline phase. The effect was explained by the stabilisation of the liquid phase due to the liquid—>glass... 

Many crystalline solids can undergo chemical transformations induced, for example, by incident radiation or by heat. An important aspect of such solid-state reactions is to understand the structural properties of the product phase obtained directly from the reaction, and in particular to rationalize the relationships between the structural properties of the product and reactant phases. In many cases, however, the product phase is amorphous, but for cases in which the product phase is crystalline, it is usually obtained as a microcrystalline powder that does not contain single crystals of suitable size and quality to allow structure determination by single-crystal XRD. In such cases, there is a clear opportunity to apply structure determination from powder XRD data in order to characterize the structural properties of product phases. 

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