Silicon phase transformation

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). 

In bulk material, the resistivity is independent of crystal orientation because silicon is cubic. However, if the carriers are constrained to travel in a very thin sheet, eg, in an inversion layer, the mobility, and thus the resistivity, become anisotropic (18). Mobility is also sensitive to both hydrostatic pressure and uniaxial tension and compression, which gives rise to a substantial piezoresistive effect. Because of crystal symmetry, however, there is no piezoelectric effect. The resistivity gradually decreases as hydrostatic pressure is increased, and then abrupdy drops several orders of magnitude at ca 11 GPa (160,000 psi), where a phase transformation occurs and silicon becomes a metal (35). The longitudinal piezoresistive coefficient varies with the direction of stress, the impurity concentration, and the temperature. At about 25°C, given stress in a (100) direction and resistivities of a few hundredths of an O-cm, the coefficient values are 500—600 m2/N (50—60 cm2/dyn). 

In our investigation, sodium sulfate was selected as the electrolyte. Rare earth sulfates Lj CSO, (LnsY and Gd) were added in order to increase the electrical conductivity. Silicon dioxide was ad ed so as to obtain the network structure which is effective for Na cation conduction and to prevent the electrolyte from becoming too soft. A thinner electrolyte was possible to prepare by mixing in SiC. The suppression of the phase transformation(15, 16) from Na2S0,-I(a high temperature phase) to Na2S0 -IH(a low temperature phase was also achieved by mixing rare earth sulfates(Ln=Y and Gd) and silicon dioxide into sodium sulfate. 

Turan, S. and Knowles, K.M., (1996a), a — (3 reverse phase transformation in silicon carbide in silicon nitride-particulate-reinforced-silicon carbide composites ,, /. Am. Ceram. Soc., 79 (11), 2892-2896. 

In this section, a hydrothermal treatment that produces phase transformations in clinoptilolite and allows us to synthesize zeolites X and Y is described. Zeolites X and Y have the FAU-type framework however, the zeolite X has a silicon/aluminum ratio in the range 1 < Si/Al < 1.5 and zeolite Y has a silicon/aluminum ratio in the range 1.5 < Si/Al < =7.0. 

The electron-hole annihilation serves as decay process.) Figure 73 refers to Au-doped silicon. If the rate constant is tuned by a bias one can switch from an insulating to a conductive behavior (cf. nonequilibrium phase transformation). 

The hyperbolic description implies that to a reasonable approximation, tetrahedral water, silicate, silicon and germanium frameworks are characterised by a preferred area per vertex group and a preferred Gaussian curvature. Thus, identical tessellations of isometric surfaces, with equal areas and curvatures at corresponding points on the surface, should offer alternative possibilities for stable frameworks. Indeed this is the case for the zeolite frameworks, faujasite and analcime, which are related to each other through the Bonnet transformation. Within an intrinsic two-dimensional description, these two frameworks are indistinguishable. We have seen in section 2.6 that the Bonnet transformation describes well a number of characteristics of the fee -> bcc martensitic phase transformation in metals and alloys. The success of this model suggests that the hyperbolic picture, intuitive and obvious for zeolites, is also valid for other atomic structures. 

There were two major objectives of this study. Firstly, the effects of the addition of potassium and/or silicon on catalyst deactivation rates and changes in catalyst properties with TOS were investigated. Secondly, the possible causes of catalyst deactivation were examined by following aged catalyst properties and reactor conditions as a function of TOS for each catalyst. The FTS was carried out in a continuous-flow stirred slurr) reactor to ensure uniformity in catalyst aging and reactor conditions throughout the reactor. The aged catalyst properties examined as a ftinction of TOS were total surface areas, carbon deposits and phase transformations. 

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. 

P. F. Becher, G. S. Painter, N. Shibata, S. B. Waters, H-T. Lin, Effects of rare-earth (RE) intergranular adsorption on the phase transformation, microstructure evolution, and mechanical properties in silicon nitride with RE203 4- MgO additives RE=La, Gd, and Lu, 7. Amer. Ceram. Soc., 91 [7], 2328-2336, (2008). 

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