Preparation, amorphous

In 1824 Berzelius, generally credited with the discovery, prepared amorphous silicon by the same general method and purified the product by removing the fluosilicates by repeated washings. Deville in 1854 first prepared crystalline silicon, the second allotropic form of the element. 

Laser and electron beam processing are effective methods for preparing amorphous surface alloys covering conventional crystalline bulk metals... 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

Tab. 3.S. Performance of sonochemically prepared amorphous catalysts in the aerobic oxidation of cyclohexanes (Equation 3.33).

Figure 3.5 Various routes for preparing amorphous solids.

Preparation of Silicon Tetrachloride. To prepare amorphous silicon, mix 2 g of dry quartz sand comminuted in an iron mortar with 3 g of powdered magnesium. Spill the mixture into a refractory test tube, secure it at an angle in a stand, and carefully heat first the entire tube, and then its bottom end more intensively until the mixture ignites. When the reaction ends, break the test tube, extract the product, crush it, and wash it in a fume cupboard, wear eye protection. ) with dilute hydrochloric acid until the flashes of light vanish. Filter off the silicon, wash it on the filter with distilled water, and dry it in the air. 

Preparation of Amorphous Boron from Boric Acid Anhydride. (Each student performs only one of the experiments for preparing amorphous boron.) Put 2 g of boric acid anhydride into an iron crucible or onto an iron plate and roast it if the boric acid anhydride contains moisture, then when it is heated with magnesium an explosion may occur ). Triturate the boric acid anhydride as rapidly as possible (why ) in 4 mortar, weigh 1 g and mix it with 2 g of a magnesium powder. Place the mixture into a refractory test tube and fasten it in an inclined position in a clamp of a stand in a fume cupboard . First carefully heat the test tube with the mixture with the flame of a burner, and then strongly heat the bottom part of the tube. What do you observe ... 

The major application of Zn(OR)2 is connected with their catalytical activity in the polymerization of olefine oxides. It is rather interesting that the activity of file freshly prepared amorphous samples of the zinc methoxide, ethoxide, or Zn(OMe) (OEt)2.n as heterogeneous catalysts in these reactions is noticeably higher than for soluble R ZnOR >ZnR2 as homogeneous catalysts [808, 1604],... 

Several materials have been investigated as cathode activators. Among the most studied systems we find CuTi, CuZr, NiTi, NiZr, FeCo, NiCo. A variety of methods are available to prepare amorphous alloys [562] and, as expected, the resulting activity is largely dependent on them. Normally, amorphous phases are obtained by rapidly quenching a melt. The material can thus be obtained in the form of ribbons, but mechanical alloying by compaction is also possible [572]. The metallic components are usually alloyed with non-metallic components such as B, Si and P which stabilize the metastable non-crystalline structures. Electrodeposition is thus also a viable preparation route [573, 574],... 

This concise overview demonstrates the great promise of inexpensive and easy-to-prepare amorphous diamond-like carbon electrodes for electrosynthetic and elec-troanalytical applications. 

Rhodium metallocycles reacted with freshly prepared amorphous tellurium in refluxing absolute xylene to give tellurophene derivatives3 s. 

The metallocycles react only with freshly prepared amorphous tellurium. Metallocycles with aliphatic substituents in place of the phenyl or 4-methylphenyl groups did not yield... 

We started our investigation in 1957 and worked out initially reliable methods for preparing amorphous atactic polyacetaldehyde (16). Soon thereafter we discovered crystalline isotactic poly aldehydes (49, 51). Detailed studies of polymerization conditions, polymer structures, and stabilization of the polymers followed. 

Subsequently, and independently from us, Furukawa (17, 18, 19, 21) and Natta (34, 36) also succeeded in preparing amorphous and crystalline poly-aldehydes. 

Ball milling of native celluloses under dry conditions decreases their Cl together with severe depolymerization and is sometimes used for preparing amorphous cellulose samples [7]. Noncrystalline celluloses with virtually no depolymerization and with a stable noncrystalline structure under aqueous conditions can be prepared from various native celluloses by regeneration from their nonaqueous cellulose solutions in water [8]. 

FIGURE 27.5 Schematic diagram of the free energies of a CSD-derived amorphous film, a conventionally prepare amorphous material, and the crystaUine perovskite phase. AG, is the thermodynamic driving force for crystallization. Also shown is the influence of crystallization temperature on the magnitude of AG,. After Schwartz et al. and Roy. ... 

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