Silicon vapor

Gao, T. Gao, J. Sailor, M. J., Tuning the response and stability of thin film mesoporous silicon vapor sensors by surface modification, Langmuir. 2002, 18, 9953 9957... 

Our experiments have shown that hydrogen is critical for growth. The required presence of both hydrogen and metal catalysts, and the virtual absence of silicon vapor suggest that totally new reaction paths assist in the growth of these nanowires under the conditions studied. If silicon does not come directly from the wafer substrate, then it is required to become airborne in some form, as in the CVD production of SiNW, thus enabling tip growth. 

Silicon vapor reacts with alkanes, alkenes, alkynes, arenes, ketones, alkyl halides, and ethers to give mainly polymers (104). 

Consequently such a microvaporizer uses a glass-silicon-glass stack with a microstructured silicon evaporator. This evaporator is heated up by a pulsed NIR-laser diode. Thus, the liquids which are in contact with the evaporator within a very short period of time are heated beyond their evaporation temperature. This arrangement not only requires a low energy to evaporate in comparison to the commonly used resistive heating, but the very rapid heating of the silicon vaporizer also allows for simultaneous evaporation of sample mixtures with components of different evaporation temperatures and partial pressures, respectively. Furthermore,... 

A variety of metallic silicides are obtained by melting silicon with a transition metal. Condensation of a transition-metal vapor with a silicon compound, or silicon vapor with a transition-metal complex has not so far yielded a species with a discrete silicon-transition-metal bond. 

The contact failure appears within a limited current and voltage range [14]. It is well known that there is a relationship between silicone vapor concentration and the relay life. To get the long life of switching cycles, a reduction of the low molecular weight siloxane included in products is needed. 

When silicon is deposited from the vapor phase at ambient temperature, it solidifies as amorphous silicon. Vapor deposited bilayers and multilayers of silicon with metals thus consist of polycrystallinc metal and amorphous silicon. The earliest observations of amorphous silicide formation by SSAR were made on such diffusion couples [2.51, 54], Similar results were also obtained earlier by Hauser when Au was diffused into amorphous Tc [2.56], Figure 2.15 shows an example of an amorphous silicide formed by reaction of amorphous silicon with polycrystallinc Ni-metal at a temperature of 350"C for reaction times of 2 and 10 s [2.55,57], The reaction experiments were carried out by a flash-healing method (see [2.55] for details). In this example, the amorphous phase grows concurrently with a crystalline silicide. The amorphous phase is in contact with amorphous Si and the crystalline silicide in contact with the Ni layer. As in the case of typical mctal/metal systems, the amorphous interlayer is planar and uniform. It is also interesting that the interface between amorphous silicon and the amorphous silicide appears to be atomically sharp despite the fact that both phases are amorphous. This suggests that amorphous silicon (a covalently bonded non metallic amorphous phase with fourfold coordinated silicon atoms) is distinctly different from an amorphous silicide (a metallically bonded system with higher atomic coordination number). These two phases are apparently connected by a discontinuous phase transformation. 

The infiltration of carbonaceous structures by liquid silicon or silicon vapor is also an interesting technique to produce SiC components. Especially the Si-inhltration of C-structures coining from natural materials like wood offers the possibility for low cost fabrication of SiC, and has already been described in Sect. 4. Infiltration of polymer-matrix derived carbon fiber reinforced carbon is schematically drawn in Fig. 14 [255, 256]. The production of complex SiC-composite parts and components as well as big parts is possible by this route. 

Liu, R., Schmedake,T. A., Li, Y.Y., Sailor, M. X and Fainman,Y. Novel porous silicon vapor sensor based on polarization interferometry , (2002) Sensor Actual. B. 87, 58-62. 

The morphological pattern of the products of silicon vapor combustion in gaseous nitrogen at condensation synthesis with skeleton crystal formation as well as denchite growth of silicon nitride crystals in melted metal salts proves the existence of the nonequilibrium mechanism of structure formation in the case of SHS. The mechanism appears to be the basis of the conception of nanodispersed particle formation under the combustion mode [28]. 

Nanoporous SiC-based materials are difficult to obtain due to the high formation temperature. Disordered nanoporous silicon carbide ceramics can be fabricated by a solid-gas reaction of ordered mesoporous carbon replica with silicon vapor [82], and a chemical vapor infiltration of dimethyldichlorosilane inside mesoporous silica following by the removal of silica [83]. Recently, highly ordered mesoporous SiC materials with uniform pore sizes and ultralarge surface areas have been synthesized by fully impregnating polycarbosilane... 

Skell and Owen " have studied the reactions of Si atoms with methylsilanes by heating solid silicon, resistively, to 1400°C in vacuo followed by co-condensation with the substrates on the liquid-nitrogen-cooled walls. The silicon vapor thus obtained may contain some Si2 and Si3 species, but the monoatomic Si atoms should be predominant at low pressures." ... 

As mentioned in Section II.2, Skell and Owen " have produced silylenes through Si atom insertion reactions by the co-condensation of thermally produced silicon vapor with methylsilanes. The molecules they employed are... 

For example, in the chloride method of silicon vapor-phase homoepataxy, the temperature at which a perfect epitaxial layer grows is 1250°C (the so-called diffusion region where the limiting stage of the process is the transfer of reagents to the solid-phase surface), which is... 

Figure 2.9 shows a CVD system consisting of a gas metering system, a heated reaction chamber, and a system for the treatment and disposal of exhaust gases [13]. The aluminum or silicon vapor precursor (e.g., AICI3) in a carrier gas is introduced into the reaction chamber that is heated to the desired temperature in advance. This vapor precursor will be chemsorbed on the pore surface to form intermediate species (e.g., -O-AICI2). Subsequently, the pores are evacuated by vacuum to remove all the precursors in the gas phase, and then exposed to water... 

The values of A, and B, for different systems are presented in Table 2. This equation describes vapor pressure with good accuracy for the system SiC-Si. When approaching the temperature of peritectic transformation Tp the curves deviate from linear ones in the coordinates Ig p = f(llT) (Fig. 3f). The silicon vapor pressure data presented are approximately two times higher than the most reliable experimental data. Vapor pressures over other polytypes can be calculated using the expression... 

The values of fli are shown in Table 3. Vapor pressures of Si2C and SiC2 according to the constants of dissociative evaporation reactions depend on silicon vapor pressure. In order to control the vapor composition over silicon carbide, it is extremely important to know the equations... 

One method of this type is direct synthesis. In this case silicon is loaded in the low-temperature part (2000-2200°C) of the crucible. Silicon vapors react with the crucible walls and form Si2C, SiC2, and SiC molecules. These molecules form nuclei of silicon carbide crystals and then crystal growth takes place. This method also has disadvantages mentioned above. 

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