Dhaneshwar Mishra

Polarization characteristics of semipolar (112̄2) InGaN/GaN quantum well structures grown on relaxed InGaN buffer layers and comparison with experiment

Partial strain relaxation effects on polarization ratio of semipolar (112̄2) InxGa1−xN/GaN quantum well (QW) structures grown on relaxed InGaN buffers were investigated using the multiband effective-mass theory. The absolute value of the polarization ratio gradually decreases with increasing In composition in InGaN buffer layer when the strain relaxation ratio (ε0y′y′−εy′y′)/ε0y′y′ along y′-axis is assumed to be linearly proportional to the difference of lattice constants between the well and the buffer layer. Also, it changes its sign for the QW structure grown on InGaN buffer layer with a relatively larger In composition (x > 0.07). These results are in good agreement with the experiment. This can be explained by the fact that, with increasing In composition in the InGaN subsrate, the spontaneous emission rate for the y′-polarization gradually increases while that for x′-polarization decreases due to the decrease in a matrix element at the band-edge (k‖ = 0).

Partial strain relaxation effects on polarization anisotropy of semipolar (112¯2) InGaN/GaN quantum well structures

Partial strain relaxation effects on polarization anisotropy of semipolar (112¯2) InGaN/GaN quantum well (QW) structures were investigated using the multiband effective-mass theory. In the case of strain relaxation of ϵx′x′ along x′-axis, the polarization ratio gradually decreases with increasing strain relaxation. Also, with the strain relaxation by the same amount, the variation of the polarization ratio along x′-axis is shown to be much larger than that along y′-axis. However, the polarization switching is not observed even at a high In composition of 0.4 due to a small strain component (ϵx′x′0) with no strain relaxation. On the other hand, in the case of strain relaxation of ϵy′y′ along y′-axis, the polarization switching is observed, and the optical anisotropy is found to change from positive to negative with increasing strain relaxation. Also, the absolute value of the polarization ratio gradually decreases with increasing carrier density. However, the polarization switching due to the carrier densi...

Elliptical Inclusion Problem in Antiplane Piezoelectricity: Stress Concentrations and Energy Release Rates

An elliptical piezoelectric inclusion embedded in an infinite piezoelectric matrix is analyzed in the framework of antiplane piezoelectricity. The stress distribution patterns for different elliptical inclusion and cavity shapes with different material constants are discussed based on the closed-form solution obtained by the authors. It is found that the stress concentration at the interface of the inclusion and the matrix becomes significantly high when piezoelectric constants of the inclusion and matrix have opposite sign. The variation in the energy release rates of selfsimilarly expanding and rotating defects, expressed by the M-and L-integrals, respectively, are discussed as a function of the applied electric field. Information on the stress concentration and the energetics of such a system can be quite useful in predicting failures, hence, aid in properly designing piezoelectric electromechanical systems.

Quaternary AlInGaN/InGaN quantum well on vicinal c-plane substrate for high emission intensity of green wavelengths

Electronic and optical properties of non-trivial semipolar AlInGaN/InGaN quantum well (QW) structures are investigated by using the multiband effective-mass theory and non-Markovian optical model. On vicinal c-plane GaN substrate miscut by a small angle (θ < 40°) from c-plane, the AlInGaN/InGaN system is shown to have ∼3 times larger spontaneous emission peak intensity than the conventional InGaN/GaN system at green wavelength. It is attributed to much larger optical matrix element of the quaternary AlInGaN/InGaN system, derived from the reduction of internal electric field induced by polarizations. This effect exceeds the performance-degrading factor of smaller quasi-Fermi-level separation for the quaternary AlInGaN/InGaN system than that for the conventional InGaN/GaN system. Results indicate that the use of quaternary III-nitride QWs on vicinal substrates may be beneficial in improving the performance of optical devices emitting green light.

Future of CAE and Implication on Engineering Education

In this paper, the trend of FEM which is one of core tools of CAE and the future of CAE are examined in terms of TRIZ concept. This observation leads to the necessity of engineering creativity. This paper reports our successful experiences of implementing systematic innovation tools into engineering design classes. Also potential revolutionary change of engineering education is hinted by extending the accomplishments of TRIZ. It can innovate the current engineering curricula and an on-going study of classifying the engineering activities is introduced.

Modeling of stresses and electric fields in piezoelectric multilayer: Application to multi quantum wells

Exact closed-form expressions have been derived for the stresses and the electric fields induced in piezoelectric multilayers deposited on a substrate with lattice misfit and thermal expansion coefficient mismatch. The derived formulations can model any number of layers using recursive relations that minimize the computation time. A proper rotation matrix has been utilized to generalize the expressions so that they can be used for various growth orientations with each layer having hexagonal crystal symmetry. As an example, the influence of lattice misfit and thermal expansion coefficient mismatch on the state of electroelastic fields in different layers of GaN multi quantum wells has been examined. A comparison with the finite element analysis results showed very close agreement. The analytical expressions developed herein will be useful in designing optoelectronic devices as well as in predicting defect density in multi quantum wells.Exact closed-form expressions have been derived for the stresses and the electric fields induced in piezoelectric multilayers deposited on a substrate with lattice misfit and thermal expansion coefficient mismatch. The derived formulations can model any number of layers using recursive relations that minimize the computation time. A proper rotation matrix has been utilized to generalize the expressions so that they can be used for various growth orientations with each layer having hexagonal crystal symmetry. As an example, the influence of lattice misfit and thermal expansion coefficient mismatch on the state of electroelastic fields in different layers of GaN multi quantum wells has been examined. A comparison with the finite element analysis results showed very close agreement. The analytical expressions developed herein will be useful in designing optoelectronic devices as well as in predicting defect density in multi quantum wells.

Evaluation of Fracture Toughness of Human Dentine-Enamel Junction (DEJ) by Finite Element Analysis

Human teeth system is one of the most important organs, plays vital role in mastication process. It consist 3 layers, outer hard and brittle cover enamel and porous dentine separated by very thin interfacial region called Dentine Enamel Junction (DEJ) or crown dentine. Dentine and enamel has almost isotropic material property while DEJ has different microstructure near enamel and dentine represented by functionally graded microstructure. Experimental evidences show that the crack initiated in the hard enamel region grows towards the crown dentine, it starts healing after 10–15 micron. Finite element analysis has been carried out and fracture toughness has been evaluated for layered model of human teeth with crack initiated at enamel region grows towards DEJ with its healing capabilities and graded structure. Comparison of the result with experimental value reported in literature shows acceptable matching suggests; FE results can be taken as basis for estimating Fracture toughness of human teeth system especially at interface of Enamel and Dentine, the DEJ.© 2009 ASME

A STUDY ON PARTIAL SIDE WEARS FOR THE BRAKE SYSTEM OF RAILWAY VEHICLES

The HSR-350x (high-speed rail 350x), also known as Korean G-7, is the experimental high speed train which was developed independently by G-7 R&D Project. The G-7 is now being investigated for its performance and tested at the high speed line. Partial side wear occurred in proposed braking system more than in KTX (Korean Train Express) according to experiment. In this study, comparison between KTX, KTX-II and G7 models of caliper unit of brake system has been done using FEM. The factor of generated partial side wear was investigated by comparing with results. Suitability of the caliper unit with KTX-II would be verified.

DETERMINATION OF THE MATERIAL PROPERTY FOR THE REAR FIXATION DEVICE OF THE VERTEBRAE BY FEM

Since Boucher technique used pedicle screw for the fixation of the spine in 1959, various kinds of pedicle screws have been developed continuously. The treatments of degenerative spine disease include spinal fusion and fixation using spinal devices. Spinal fusion surgery is classified into fusion and non fusion. The spine device market in which pedicle screws are used is getting bigger. However, it is one of the most challenging fields because of lack of the experimental and analytic data. In this study, a finite element model which includes non fusion device of human lumbar is developed and studied to understand the stability of the technique. The material property of the non fusion device is decided through comparison of intact model with fixed model. This result can be used as a basic source to new apparatus. This dynamic device offers a system that may alter the motion preservation and load sharing of a spinal segment without sacrificing stability.