Preparation mechanical solid-state reaction

The methods of synthesis of fluorapatite have been widely dis cussed (J ). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in a-queous solution at room temperature, or through a solid state reaction at 800°C. It can also be prepared by the action of 6-tricalcium phosphate on calcium fluoride at about 800°C. Its solubility and thermal stability have already been established. While much is known about fluorapatite, many questions still exist concerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapatite by a solid state reaction between calcium fluoride and apa-titic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 

A novel and simple one-step solid-state reaction in the presence of a suitable surfactant (PEG-400) has been developed to synthesize uniform polyoxometalate nanorods with an average diameter of ca. 20 nm and a length of up to 400 nm. Polyoxometalate nanoparticles were also prepared by one-step solid-state reaction at room temperature. The polyoxometalate nanorods and nanoparticles were characterized by IR, elemental analyses, XRD and TEM. The uniform nanoparticles have an average size of 8 10 nm. The possible formation mechanism of these polyoxometalate nanomaterials was speculated. 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

The current status of the models of fluctuational and deformational preparation of the chemical reaction barrier is discussed in the Section 3. Section 4 is dedicated to the quantitative description of H-atom transfer reactions. Section 5 describes heavy-particle transfer models for solids, conceptually linked with developing notions about the mechanism of low-temperature solid-state chemical reactions. Section 6 is dedicated to the macrokinetic peculiarities of solid-state reactions in the region of the rate constant low-temperature plateau, in particular to the emergence of non-thermal critical effects determined by the development of energetic chains. 

The main difference between the solid-state reaction synthesis route and hydro(sol-vo)thermal synthesis route lies in reactivity , which is reflected in reaction mechanisms. Reactions in solid-state synthesis depend on the diffusion of the reactants at the interface, whereas individual reactant molecules existing in the liquid phase can react with each other in hydro(solvo)thermal synthesis. Variation in the reaction mechanism leads to the formation of different structures from the same or similar starting materials. In addition, even the same material that can be obtained by both preparation routes can have totally different morphology and properties due to different formation mechanisms. For instance, perfect single crystals can usually be formed from liquid-phase synthesis, while being very difficult to obtain in solid-state synthesis. 

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... 

Quasicrystalline phases have received much attention since first reported by Shechtman et al. [3.92] in 1984 for Al-Mn. Within the last few years, various preparation techniques have been applied for the preparation of alloys in the quasicrystalline state. Liquid-phase quenching [3.92, 93] and relatively slow cooling from the melt [3.94], sputter or vapor deposition [3.95,96], ion-beam techniques [3.97-100], heat treatment of the amorphous phase [3.93,101, 102], and solid-state reaction during interdiffusion [3.103-106]. Recently it was demonstrated that the quasicrystalline phase can be produced by mechanical alloying [3.107-112],... 

Ceramic powders usually consist of particles with different sizes that are distributed over a certain range. Some powders have a very narrow size distribution, such as those prepared by chemical precipitation under well-controlled conditions, whereas others may exhibit a broad size distribution, such as those made through mechanical milling or solid-state reaction. Some particles have spherical, near spherical, or equiaxial shapes, while there are many cases where the powders are irregular in shape, including rod, wire, fiber, disk, plate, and so on. The importance of characterizing size and size distribution of ceramic particles is due to their effects in the consohdation and sintering behaviors of the powders. 

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