Silicon, crystal structure melting point

Exists in two adotropic modifications. Crystalline sihcon is made up of grayish-black lustrous needle-hke crystals or octahedral platelets cubic structure Amorphous sdicon is a brown powder. Other physical properties are density 2.33g/cm3 at 25°C melts at 1,414°C high purity liquid silicon has density 2.533 g/cm at its melting point vaporizes at 3,265°C vapor pressure 0.76 torr at 2,067°C Mohs hardness 6.5. Brinell hardness 250 poor conductor of electricity dielectiric constant 13 critical temperature 4°C calculated critical pressure 530 atm magnetic susceptibility (containing 0.085%Fe) 0.13x10 insoluble in water dissolves in hydrofluoric acid or a mixture of hydrofluoric and nitric acids soluble in molten alkalies. 

Silicon and germanium crystallize in the diamond structure. However, they have somewhat weaker covalent bonds than carbon as a consequence of less efficient orbital overlap. These weaker bonds result in lower melting points for silicon (1420°C for Si and 945°C for Ge, compared with 4100°C for diamond) and greater chemical reactivity. Both sihcon and germanium are semiconductors, described in Chapter 7. 

The randomizaton takes place by a a diffusion of atoms that is implicit in our earlier description of the initial randomization process as being akin to melting [1]. Later it was shown that the root-mean-square displacement of each atom must be of the order of the nearest-neighbor distance in order that the network lose all memory of the original crystal structure as measured by the structure factor S q) [21]. In this context, the melting point can be defined as that temperature for which the mean square displacement increases linearly with time. It appears, though, that a sequence of bond switches as illustrated in Fig. 1 is not the primary mechanism for self-diffusion in silicon [31,32]... 

Each constituent atom of a covalent crystal is linked to its neighbours through directed covalent bonds. The crystal structure is determined by the spatial dispositions of these bonds. Because primary valence forces are involved, such solids are hard and have high melting points, e.g. diamond, silicon carbide, etc. Relatively few entirely covalent solids have been studied at elevated temperatures and it is, therefore, premature to comment on their decomposition characteristics. 

The densities of liquids under normal pressures are not too dissimilar to those of their solids between the melting and boiling points. Generally a liquid is less dense than its solid at the melting point, but there are a few exceptions of which water is one ice floats on water. These less dense solids have rather open crystal structures. Silicon, germanium and tin are other examples. Another similarity between normal liquids and solids is that they have a low compressibility due to there not being a great deal of space between the molecules in a liquid. 

Polycrystalline-alumina-based fibres can at present not compete with silicon-carbide-ba.sed fibres when low creep rates are required. Fibres with higher resistance to creep by dislocation motion could be provided by oxides with high melting point and complex crystal structure, a tendency to order over long distances and the maintenance of this order to high fractions of the melting temperatures (Kelly, 1996). Experimental development of monocrystalline fibres by Czochralski-derived techniques from chrysoberyl... 

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