By means of ab initio calculations, we have estimated stability of 2D Me[Formula: see text] ([Formula: see text], Ca, Sr, Ba and [Formula: see text], Ge, Sn) in the T and Td phases, which are similar to the ones of 2D transition metal chalcogenides, in addition to their phonon spectra. The T phase is found to be more stable for 2D Ca[Formula: see text], Sr[Formula: see text] and Ba[Formula: see text], whereas the Td phase is predicted to be the ground state for 2D Mg[Formula: see text]. We have also discussed that imaginary frequencies in the calculated phonon spectra of 2D Me[Formula: see text], which appeared in the vicinity of the [Formula: see text] point, were not necessarily associated with the dynamic instability.
We present the results of an extended theoretical study of the structure, phonon, electronic and optical properties of 2D alkaline-earth metal silicides, germanides and stannides (2D Me2X, where Me=Mg, Ca, Sr, Ba and X=Si, Ge, Sn). The performed analysis has shown the occurrence of the pseudo passivation effect and ionic chemical bonding in these 2D Me2X. In addition, the preformed investigation of their phonon spectra has shown the absence of imaginary frequencies indicating the stability of these 2D structures. The band structure calculations performed using the hybrid functional have revealed that all 2D Me2X are semiconductors with the gap varying from 0.12 to 1.01 eV. Among them Mg- and Ca-based 2D materials are direct band-gap semiconductors with the first direct transition having appreciable oscillator strength. We also propose to consider ternary 2D silicides, germanides and stannides with different Me atoms as a feasible way to modify properties of parent 2D Me2X.
The majority of heat-exchange apparatus operate in a regime where the parameters of one flow differ considerably from those of another (pressure, temperature, etc.); A marked difference in the coefficients of convective heat exchange from both sides of the separating wall of the heat exchanger is observed in that case. Relatively lower coefficients of convective heat exchange can be partially compensated by development of the heat-exchange surface due to finning (it may be increased 10-25 times as compared with the surface of smooth tubes [1]).One promising method of improving the effectiveness of finning is the method of gas-dynamic spraying [2], which consists in the deposition of particles of a powder formed from a high-speed cold or negligibly heated slightly dust-laden flow on the surface of the fin. The method makes it possible to use not only metals, but also alloys and cermets (ceramic-metal mixtures) for spraying, and, therefore, to form surfaces with different properties. An element of a heat exchanger formed from finned tubes using the method of gas-dynamic spraying is shown in Fig. 1.Let us examine the possibility of improving the effectiveness of finning. The ratio of the average difference in the temperatures of the fin Tr and surrounding medium 1",.., to the difference in the temperatures of the surface supporting the finning (the surface of the base) Tb.= and the surrounding medium is understood as finning effectiveness (f'm efficiency OF): owhere Ft is the surface area of the fro.Computed values [3] of the efficiency of circular fins of trapezoidal shape are presented in Fig, 2a. These relationships are obtained on the assumptions that the coefficient of convective heat exchange is constant, the vector of the thermal flux in the fm is normal to the surface on which it is mounted, and the contact resistance at the base of the I'm is negligibly small.In practice, the contact resistance at the base of the fm is one of the factors restricting use of tube finning. Aluminum fins with an interference fit to the tube have high contact resistances, which increase rapidly with rising temperature. They can therefore be used to temperatures of 100~ since attachment of the f'ms to the tubes is weakened at higher temperatures as a result of the high thermal expansion of the aluminum. Fins set in grooves are suitable for tubes, the wall thickness of which is increased by the depth of the groove [I]. With wire firming, the overall length of the unwelded segments may be 30% [4]; this makes itself felt on the characteristics of the heat exchangers.Computed and experimental data on the efficiency of the wire f-tuning of tubes are presented in Fig. 2b. The pronounced difference in the values is explained by the effect of contact resistance.To assess the effect of contact resistance near the base of the fins on T/F, we performed an experimental investigation of the elements of heat exchangers formed from tubes finned using gas-dynamic spraying.A schematic diagram of the experimental stand is shown in Fig. 3. The stan...
Single-phase films of semiconductor and semimetallic calcium silicides (Ca2Si, CaSi, and CaSi2), as well as films with a significant contribution of Ca5Si3 and Ca14Si19 silicides, were grown on single-crystal silicon and sapphire substrates. The analysis of the crystal structure of the grown films was carried out and the criterion of their matching with silicon and sapphire substrates was determined. Some lattice matching models were proposed, and the subsequent deformations of the silicide lattices were estimated. Film’s optical functions, including the optical transparency, were calculated from the optical spectroscopy data and an extended comparison was performed with the results of ab initio calculations. The real limits of the optical transparency for the films on sapphire substrates were established. The maximum transparency limit (3.9 eV) was observed for the CaSi film. Based on an analysis of the photoelectric properties of Ca2Si/Si diodes on n- and p-type silicon substrates, a perspective of their applications in silicon optoelectronics was discussed.
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