Roderick Melnik

Computational Analysis of Temperature Rise Phenomena in Electric Induction Motors

Abstract In developing electric machines in general, and induction motors in particular, temperature limits is a key factor affecting the efficiency of the overall design. Since conventional loading of induction motors is often expensive, the estimation of temperature rise by tools of mathematical modelling and computational experiments becomes increasingly important. In the present paper we develop and validate experimentally a model accounting for losses and describing thermal phenomena in induction motors. The developed model has been implemented in FEMLAB, and has been applied to predict temperature rise in totally enclosed fan-cooled induction motors. Comparisons with experimental results obtained with a 1.5 kW standard squirrel-cage induction motor show the effectiveness of the developed model in predicting temperature rise for a range of operating conditions, in particular for different frequencies and voltages. Finally, a SIMULINK-based control loop has been developed by using the thermal model as an input.

Temperature dependent elastic constants and ultimate strength of graphene and graphyne

Based on the first principles calculation combined with quasi-harmonic approximation in this work, we focus on the analysis of temperature dependent lattice geometries, thermal expansion coefficients, elastic constants, and ultimate strength of graphene and graphyne. For the linear thermal expansion coefficient, both graphene and graphyne show a negative region in the low temperature regime. This coefficient increases up to be positive at high temperatures. Graphene has superior mechanical properties with Youngs modulus E = 350.01 N/m and ultimate tensile strength of 119.2 GPa at room temperature. Based on our analysis, it is found that graphenes mechanical properties have strong resistance against temperature increase up to 1000 K. Graphyne also shows good mechanical properties with Youngs modulus E = 250.9 N/m and ultimate tensile strength of 81.2 GPa at room temperature, but graphynes mechanical properties have a weaker resistance with respect to the increase of temperature than that of graphene.

Body-centered tetragonal B2N2: a novel sp3 bonding boron nitride polymorph

A novel polymorph of boron nitride (BN) with a body-centered tetragonal structure (bct-BN) has been predicted using first-principles calculations. The structural, vibrational, and mechanical calculations indicated that bct-BN is mechanically stable at zero pressure. When pressure is above 6 GPa, bct-BN becomes energetically more stable than h-BN. The bct-BN appears to be an intermediate phase between h-BN and w-BN due to a low energy barrier from h-BN to w-BN via bct-BN. Our results also indicated that the structure of unknown E-BN phase might be bct-BN.

Relative importance of grain boundaries and size effects in thermal conductivity of nanocrystalline materials

A theoretical model for describing effective thermal conductivity (ETC) of nanocrystalline materials has been proposed, so that the ETC can be easily obtained from its grain size, single crystal thermal conductivity, single crystal phonon mean free path (PMFP), and the Kaptiza thermal resistance. In addition, the relative importance between grain boundaries (GBs) and size effects on the ETC of nanocrystalline diamond at 300 K has been studied. It has been demonstrated that with increasing grain size, both GBs and size effects become weaker, while size effects become stronger on thermal conductivity than GBs effects.

Finite element analysis of coupled electronic states in quantum dot nanostructures

Nanostructures, created by confinement of the motion of an electron from all three dimensions and known as quantum dots (QDs), provide materials scientists with a wide range of potential applications. These structures are produced today with advances of QD growth technology, and computational tools are fundamental in providing a better understanding of such structures. In this paper QD nanostructures are analysed with due account for coupling effects between electronic states in the dot and the wetting layer regions. The analysis, performed on the basis of the finite element methodology and Arnoldi iterations, demonstrates that the effect of coupling may be essential. The numerical procedure applied here is more efficient compared to the QR algorithm typically used in the context of modelling low-dimensional nanostructures. We report results of computational experiments for cylindrical and truncated conical QDs and compare them with the earlier results obtained for fully conical QD nanostructures.

First principles molecular dynamics study of CdS nanostructure temperature-dependent phase stability

First principles molecular dynamics simulations are used to determine the relative stability of wurtzite, graphitic, and rocksalt phases of the CdS nanostructure at various temperatures. Our results indicate that in the temperature range from 300to450K, the phase stability sequence for the CdS nanostructure is rocksalt, wurtzite, and graphitic phases. The same situation holds for bulk CdS crystals under high pressure and 0K. Our work also demonstrates that although the temperature can affect the total energy of the CdS nanostructure, it cannot change its phase stability sequence in the temperature range studied in this letter.

Dynamic coupling of piezoelectric effects, spontaneous polarization, and strain in lattice-mismatched semiconductor quantum-well heterostructures

A static and dynamic analysis of the combined and self-consistent influence of spontaneous polarization, piezoelectric effects, lattice mismatch, and strain effects is presented for a three-layer one-dimensional AlN∕GaN wurtzite quantum-well structure (with GaN as the central quantum-well layer). It is shown that, contrary to the assumption of Fonoberov and Balandin [J. Appl. Phys. 94, 7178 (2003); J. Vac. Sci. Technol. B 22, 2190 (2004)], even in cases with no current transport through the structure, the strain distributions are not well captured by minimization of the strain energy only and not, as is in principle required, the total free energy including electric and piezoelectric coupling and spontaneous polarization contributions. Furthermore, we have found that, when an ac signal is imposed through the structure, resonance frequencies exist where strain distributions are even more strongly affected by piezoelectric-coupling contributions depending on the amount of mechanical and electrical losses in...

A finite difference method for studying thermal deformation in a thin film exposed to ultrashort-pulsed lasers

Ultrashort-pulsed lasers have been attracting worldwide interest in science and engineering. Studying the thermal deformation induced by ultrashort-pulsed lasers is important for preventing thermal damage. This article presents a finite difference method for studying thermal deformation in a thin film exposed to ultrashort-pulsed lasers. The method is obtained based on the parabolic two-step model. It accounts for the coupling effect between lattice temperature and strain rate, as well as for the hot-electron-blast effect in momentum transfer. The method allows us to avoid non-physical oscillations in the solution as demonstrated by numerical examples.

Thermoelectromechanical effects in quantum dots

Electromechanical effects are important in semiconductor nanostructures as most of the semiconductors are piezoelectric in nature. These nanostructures find applications in electronic and optoelectronic devices where they may face challenges for thermal management. Low dimensional semiconductor nanostructures, such as quantum dots (QD) and nanowires, are the nanostructures where such challenges must be particularly carefully addressed. In this contribution we report a study on thermoelectromechanical effects in QDs. For the first time a coupled model of thermoelectroelasticity has been applied to the analysis of quantum dots and the influence of thermoelectromechanical effects on bandstructures of low dimensional nanostructures has been quantified. Finite element solutions are obtained for different thermal loadings and their effects on the electromechanical properties and bandstructure of QDs are presented. Our model accounts for a practically important range of internal and external thermoelectromechanical loadings. Results are obtained for typical QD systems based on GaN/AlN and CdSe/CdS (as representatives of III-V and II-VI group semiconductors, respectively), with cylindrical and truncated conical geometries. The wetting layer effect on electromechanical quantities is also accounted for. The energy bandstructure calculations for various thermal loadings are performed. Electromechanical fields are observed to be more sensitive to thermal loadings in GaN/AlN QDs as compared to CdSe/CdS QDs. The results are discussed in the context of the effect of thermal loadings on the performance of QD-based nanosystems.

COUPLED THERMOMECHANICAL WAVES IN HYPERBOLIC THERMOELASTICITY

Using models incorporating a thermal relaxation time (hyperbolic models), we study the properties of spatially periodic thermoelastic waves propagating in an infinite rod. Analyzing the Lord-Schulman and Green-Lindsay linear models, we reveal dependencies of decay rates and frequency shifts of temperature and displacement upon the wave number for the case of weak thermoelastic coupling. We explore numerically a general nonlinear hyperbolic model, describing the time evolution of initially sinusoidal distributions of displacement and temperature. Mechanisms of nonlinear interaction between thermal and mechanical fields are qualitatively analyzed. It is demonstrated that larger relaxation times may provide smoother temperature profiles at an intermediate stage of the dynamics.Using models incorporating a thermal relaxation time (hyperbolic models), we study the properties of spatially periodic thermoelastic waves propagating in an infinite rod. Analyzing the Lord-Schulman and Green-Lindsay linear models, we reveal dependencies of decay rates and frequency shifts of temperature and displacement upon the wave number for the case of weak thermoelastic coupling. We explore numerically a general nonlinear hyperbolic model, describing the time evolution of initially sinusoidal distributions of displacement and temperature. Mechanisms of nonlinear interaction between thermal and mechanical fields are qualitatively analyzed. It is demonstrated that larger relaxation times may provide smoother temperature profiles at an intermediate stage of the dynamics.