High-Temperature Routes

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. 

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 ... 

A range of porphyrins and phthalocyanines exist syntheses often involve the high-temperature routes typical of the transition metals but recently a high-yield low-temperature route has been utilized ... 

A range of porphyrins and phthalocyanines exist syntheses often involve the high-temperature routes typical of the transition metals thus when ScCh is refluxed with H2TTP (H2TTP = meso-tetratolylporphyrin) in 1-chloronaphthalene, Sc(TTP)Cl is formed. This has the expected square pyramidal structure with Sc 0.68 A above the N4 basal plane (Sc—Cl = 2.32 A Sc— N = 2.17-2.18 A). 

The use of organic molecules in the synthesis of zeotype solids is an especially interesting preparative method for extended inorganic solid materials. Organic molecules cannot survive the harsh conditions of the classical high-temperature route involving reaction of the components in the solid state. Structure-directed synthesis thus belongs to the "soft chemistry" routes for the preparation of solid-state compounds [12, 13]. 

As mentioned, the sinterability of pure silicon carbide strongly depends on the properties of the powder used. Powders made by high-temperature routes are predominantly composed of different hexagonal SiC polytypes (e.g., 2H, 4H, and 6H). In contrast, SiC produced by polymeric routes almost exclusively consists of p-SiC. In principle, SiC can be formed by the pyrolysis of either polysilanes or polycarbosilanes. 

Aluminum nitride powder synthesized by high-temperature routes can be sintered to a density of more than 97% of the theoretical density by adding calcium or yttrium compounds as sintering aids [52-54]. Therefore, new processing routes to aluminum nitride predominantly aim at powders with lower quantities of cationic impurities. In addition, the formation of aluminum nitride coatings [55-58] or fibers [59] has become subject of extensive research. 

Schnick and co-workers, for example, have prepared nitridophosphates with zeolite-like frameworks by high-temperature routes, including analogues of... 

The ASi used in this route can be prepared by either conventional high temperature routes or via low temperature reaction of the element. Si, with the metal hydrides [44]. The reaction to prepare clathrates with hydrogen encapsulation was performed either in solution (dioctyl ether) at 250 °C or as a solid state reaction at 200 °C under vacuum. The solid state reaction is prepared by mechanical mixing (or ball-milling) of the two solids, ASi and NH4X, pressing them into a pellet which is then placed into a furnace and heated under vacuum to 200 °C. The solution reaction uses the same precursors, but they are dispersed in a solvent and... 

Non-oxide ceramics are typically synthesized via high-temperature routes, which convert molecular precursors into the desired structures. For instance, SiC (carborundum) may be produced from the direct reaction of siUca sand with carbon in an electric furnace (Eq. 53). Industrially, a mixture of 50 wt% Si02,40 wt% coke, 7 wt % sawdust, and 3 wt% NaQ is heated together at ca. 2,700°C - known as the Acheson process. The purpose of the salt is to remove metalUc impurities via formation of volatile metal chlorides e.g., FeQg, MgCl2, etc.). To yield highly crystalline SiC, the Lely process uses the subhmation of SiC powder or lumps at 2,500°C under argon at atmospheric pressure. 

Recently Revaparasadu and co-workers who synthesized CdTe by a hybrid solution based high temperature route.Briefly, the method involves the addition of an aqueous suspension or solution of a cadmium salt (chloride, acetate, nitrate or carbonate) was to a freshly prepared NaHTe solution. The isolated bulk CdTe was then dispersed in tri-octyl-phosphine (TOP) and injected into pre-heated HDA at temperatures of 190, 230 and 270 °C for 2h. The as prepared CdTe nanoparticles were then isolated by the addition of methanol, followed by centrifugation and finally... 

The hot-soup method has also been applied to the production of semiconductor core/shell nanostructures, where a semiconductor nanocrystal is coated with a second semiconductor of wider bandgap. Shown in Figure 23 is a scheme for the synthesis of CdSe/CdS core/shell nanocrystals. These nanostructures were synthesized via a high-temperature route in a mixture of TOP and TOPO (223,224) and via a low-temperature route in pyridine (225). Among the semiconductor core/shell nanomaterials produced were CdSe/ZnS, CdSe/CdS, InAs/InP, InAs/CdSe, InAs/ZnSe, and InAs/ZnS (223,225-228). In these structures, the shell type and thickness allow further control of the optical, electronic, and other properties of semiconductor nanocrystals. For example, the shell may be used to passivate the imperfect surface of the core semiconductor, resulting in significantly improved luminescence efficiency. 

The results obtained by [2001War] showed that complete nitridation of ferroboron precmsor in a mixture of NH3 + H2 under ambient pressure with BN formation can be achieved at 400°C with 20 vol.% NH3. The nitrided product consists of amorphous BN, Fe and metastable Fe nitrides. An alternative route using 100 vol.% N2 is also possible, but a higher reaction temperature (>1430°C) is needed. The product from the high temperature route contains partially crystallized ABN, Fe and virtually no metastable Fe nitrides [2001War]. 

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