Silicon structural phases

Needs R J and Mujica 1995. First-principles Pseudopotential Study of the Structural Phases of Silicon. Physical Review B51 9652-9660. 

Silicon microstructures can be categorized according to the dimensionality of the confinement. Most PL studies deal with silicon structures confined in three dimensions such dot-like structures are designated zero-dimensional (OD). An overview of size-dependent properties of silicon spheres is given in Table 6.1. Standard methods of generating such microstructures are gas-phase synthesis [Di3, Li7, Scl2], plasma CVD [Ru2, Col, Ta8] or conventional chemical synthesis [Mal5]. 

M. Khayyat, G. Banini, D. Hasko, and M. Chaudhri, Raman microscopy investigations of structural phase transformations in crystalline and amorphous silicon due to indentation with a Vickers diamond at room temperature and at 77 K, J. Phys. D—Appl. Phys. 36, 1300-1307 (2003). 

Many studies on the direct reaction of methyl chloride with silicon-copper contact mass and other metal promoters added to the silicon-copper contact mass have focused on the reaction mechanisms.7,8 The reaction rate and the selectivity for dimethyldichlorosilane in this direct synthesis are influenced by metal additives, known as promoters, in low concentration. Aluminum, antimony, arsenic, bismuth, mercury, phosphorus, phosphine compounds34 and their metal complexes,35,36 Zinc,37 39 tin38-40 etc. are known to have beneficial effects as promoters for dimethyldichlorosilane formation.7,8 Promoters are not themselves good catalysts for the direct reaction at temperatures < 350 °C,6,8 but require the presence of copper to be effective. When zinc metal or zinc compounds (0.03-0.75 wt%) were added to silicon-copper contact mass, the reaction rate was potentiated and the selectivity of dimethyldichlorosilane was enhanced further.34 These materials are described as structural promoters because they alter the surface enrichment of silicon, increase the electron density of the surface of the catalyst modify the crystal structure of the copper-silicon solid phase, and affect the absorption of methyl chloride on the catalyst surface and the activation energy for the formation of dimethyldichlorosilane.38,39 Cadmium is also a structural promoter for this reaction, but cadmium presents serious toxicity problems in industrial use on a large scale.41,42 Other metals such as arsenic, mercury, etc. are also restricted because of such toxicity problems. In the direct reaction of methyl chloride, tin in... 

Pressure volume relation of three main group elements. Aluminium and sodium exhibit continuous compression behaviour in the selected pres sure range. The discontinuous volume changes of silicon are caused by structural phase transitions. 

Aluminium chlorohydrate is a widely used active ingredient for antiperspirant compositions. This salt requires a hydrophilic solvent at low pH values to remain stable. Structured microemulsions can be used to formulate clear and viscous products with this active. These products can be used in solid applicators such as firm stick applicators [35]. The o/w microemulsions can be based on fatty alcoholethoxylates with an organic or silicon oil phase [36]. An improved temperature stability in the range of 0-70°C for such formulations can be achieved by a combination of oxyalkylene-modified siloxanes, pentacyclosiloxane and glycol [37]. The addition of alpha-hydroxylic acids results in an improved skin feel with decreased tackiness [38]. 

Fig. 3 23. (a) Grofh of energy vs volume (scale normalised to the diamond structure) for eleven phases of silicon (b) Enthalpy-pressure plot for the same eleven phases relative to the body-centred cubic phase (Figures redrawn from Needs R f and A Mujica 1995. First-principles pseudopotential study of die structural phases of silicon. Physical Review B51-9652-9660.)... 

High-pressure silicon dioxide phase stishovite (rutile structure) W. Sinclair, A. E. Ringwood, Nature 1978, 272, 714. 

Figure 2. Proposed structure of the amorphous silicon carbide phase obtained from polycarbo-...

Using the PDFM, Yin and Cohen looked at various real and hypothetical crystalline forms of silicon including close-packed structures such as the face-centered-cubic form and open structures such as the diamond and hexagonal diamond structures [7]. They evaluated the total energy for each structural phase and plotted the energies as a function of crystal volume. In doing so, they created a phase diagram for silicon at zero temperature as shown in Fig. 1. 

The decrease in intensity of the 520-cm Raman band as the scratch is approached in the RSNOM data is probably indicative of the silicon having undergone a structural phase... 

Bois L., Maquet J., Babonneau F., Bahloul D. Structure characterization of sol-gel derived oxycarbide glasses. 2. Study of the thermal stability of the silicon oxycarbide phase. Chem. Mater. 1995 7 975-981... 

Figure 13 shows three IPMC actuator strips embedded in a soft silicone structure. The twisting motion is produced by phase shifting the same input signal by 180° between the outer two IPMCs. The center IPMC is not actuated in order to not obstruct the movement of either adjacent IPMCs. A square-wave signal of 4 V at 50 mHz was applied to drive the boot-IPMC. As shown in Fig. 13, the maximum twist angle measured at this input is approximately 12°. Photographs indicate that... 

Structural phase transitions that occur during lithiation are typically undesirable since they often lead to slow kinetics and poor cycle life. In that respect, the electrochemically driven amorphization of silicon is advantageous since it allows lithiation to occur while b3q)assing multiple crystallographic transitions. However, the electrochemical properties of silicon and germanium could likety... 

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