Temperature Routes

Lowering the temperature at which cluster-forming solid-state reactions can be carried out is of some interest, particularly as a means of gaining a measure of kinetic control over the process. Several decades ago, it was discovered that the yields for forming tungsten halide cluster phases could be improved by 

Parent Phase Compound0 Dimensionality Compound Dimensionality References 

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Allcock HR, Crane CA, Morrissey CT, Nelson JM, Reeves SD, Honeyman CH, and Manners I. Living cationic polymerization of phosphoranimines as an ambient temperature route to polyphosphazenes with controlled molecular weights. Macromolecules, 1996, 29, 7740-7747. 

The ability of nanocrystals to fuse at relatively modest temperatures provides a low-temperature route to thin-film growth. This approach has been successfully used for formation of metallic (Au,53 Ag54) and semiconducting (CdS,52 CdSe,55 HgTe56) thin films with good electrical conductivity. However, a serious limitation of this approach is associated with the presence of organic... 

Synthetic fluor-containing apatites are prepared and investigated for biomedical applications and serve also as models to understand the formation of biological fluorapatites and some of their properties. The synthesis of fluoridated apatites has been accomplished in various ways from simple ion exchange in solution to more elaborate techniques involving sol-gel routes or thermal processes. Two main classes of synthesis routes are presented in this chapter high-temperature routes and low-temperature solution routes. 

A number of low temperature routes to the preparation of oxides have begun to receive attention. In many cases these involve the decomposition of organometallic precursors such as oxalates. These alternative ceramic approaches will be discussed elsewhere in this volume. 

Carbon dioxide is the drying agent of choice if the goal is to stabilize kinetically constrained structure, and materials prepared by this low-temperature route are referred to by some people as carbogeh In general, carbogels are also different from aerogels in surface functionality, in particular hydrophilicily. 

Process streams and utilities Heat-capacity flow rate FCp (kW/K) Input temperature Tm (K) Output temperature rout (K)... 

Cluster formation under such conditions is still poorly understood. Thus far, perhaps the most complete glimpse of a cluster nucleation process was granted in the investigation of a lower temperature route to tungsten iodide clusters (55). 

Fig. 17. Two schematic routes for the H NMR spectral changes associated with TV-methyl exchange in 46 upon increase of temperature route A exchange within each diastereomer route B exchange between diastereomers.47 Reproduced with permission from the American...

These metal compounds can be made by high temperature (>100°C) reactions such as that of NH3 with the metal or metal oxide, or by lower temperature routes where metal halides are heated with Li3N or Na3Nn the latter method has been used for Ti, Cr, Mn, Hf, Y, and Sm nitrides. Thin films of nitrides can be made by chemical vapor deposition12 using amido compounds such as Ti(NMe2)4 and NH3 at 150-450°C at 1 atm. 

These superacidic solutions of d- and /-transition element cations can provide a controllable low-temperature route to compounds containing transition metals in unusually low oxidation states. Also they are perceived as the precursors to transition metal carbonyl cations, as indicated in Sec. 11.3.4.3 immediately below. 

Relatively few complexes of N-donor ligands are known. One of the best characterized is with the tridentate ligand terpyridyl, which reacts with scandium nitrate to form Sc(terpy )(N03 )s here scandium is connected to 9 atoms, but one Sc-O bond is considerably longer than the others, so a coordination number of 8.5 has been assigned. A range of porphyrin and phthalocyanine complexes exist syntheses often involve the high-temperature routes typical of the transition metals but recently a high-yield low-temperature route has been utilized to make octaethylporphyrin (OEP) complexes ... 

Because water is a product and the recycle ethanol will be wet, the ethanol feedstock need not be the highest grade, but instead the easier to produce hydrous ethanol (95% ethanol). If the reaction temperature is low, there should be no contamination from acetylene, which is a problem with higher temperature routes. 

Non-oxide ceramic nanomaterials, such as carbides, nitrides, borides, phosphides and silicides, have received considerable attention due to their potential applications in electronics, optics, catalysis, and magnetic storage. In contrast with the traditional processes, such as solid state reactions, CVD, MOCVD and PVD, which involve using high temperatures, toxic organometallic precursors, or complicated reactions and posttreatments, solvothermal method is a low temperature route to these materials with controlled shapes and sizes. 

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