Solid-state amorphization reactions interface

It has been well known for a couple of decades that noncrystalline metallic alloys can be made by vapour- and melt-quenching. Recent results show that an amorphous phase can also be formed directly when a crystalline metallic alloy is subjected to various types of disordering processes. Solid-state amorphization can be induced through a variety of methods including absorption of atomic hydrogen, thermal interdiffusion reaction along the interface separating... 

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. 

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. 

Stoichiometry has long been used by solid-state chemists to control the final products of a reaction. However, traditional synthetic techniques do not have the ability to control reaction intermediates and all stable phases will form as illustrated in Figure 2. For example, in the iron-silicon system, thin film diffusion couples have been used to determine the sequence of phase formation (79). FeSi was always found to nucleate first, followed by the crystallization of FeSi2 at the FeSi-Si interface and FeSi3 at the FeSi-Fe interface. The following paragraphs provide evidence that stoichiometry of the amorphous intermediates can be used to control nucleation to obtain the desired crystalline compounds directly. Thus, we use stoichiometry to control the mechanism of the reaction. 

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