Influence of Spontaneous and Piezoelectric Polarizations on the Lattice Dynamics of III-Nitride Structures

2001 ◽  
Vol 680 ◽  
Author(s):  
Jérôme Gleize ◽  
Jean Frandon ◽  
Marie A. Renucci ◽  
Friedhelm Bechstedt
Keyword(s):  
Elasticity Theory ◽  
Lattice Dynamics ◽  
Effective Strain ◽  
Growth Direction ◽  
Stress And Strain ◽  
Free Standing ◽  
The Difference ◽  
Piezoelectric Fields ◽  
Nitride Layers ◽  
Theory Framework

ABSTRACTThe influence of pyroelectric and piezoelectric polarizations on the lattice dynamics of strained III-nitride based structures is investigated within a macroscopic framework. New relationships between stress and strain are derived, which take into account the piezoelectric fields, which appear in the strained III-nitride layers. Consequently, the strained phonon frequencies in such systems differ from those calculated within the elasticity theory framework. In the case of strained, free-standing GaN/AlN superlattices grown along the [0001] axis, the difference of the spontaneous polarizations of GaN and AlN also contributes to the change in the effective strain along the growth direction. The corresponding shift of the zone center phonon frequencies of GaN and AlN might be negligible or significant, depending on the value of the ratio of the GaN and AlN layer thickness.

Effect of Thickness on Mechanically Tunable Magnetic Anisotropy of FeGa Thin Films Deposited on Flexible Substrates

2015 ◽  
Vol 815 ◽  
pp. 227-232 ◽  
Author(s):  
Ying Yu ◽  
Shu Hong Xie ◽  
Qing Feng Zhan
Keyword(s):  
Thin Films ◽  
Magnetic Properties ◽  
Magnetic Anisotropy ◽  
Effective Strain ◽  
Anisotropy Field ◽  
Saturation Field ◽  
Effective Anisotropy ◽  
Magnetostriction Constant ◽  
The Difference ◽  
Different Strains

A practical way to manipulate the magnetic anisotropy of magnetostrictive FeGa thin films grown on flexible polyethylene terephthalate (PET) substrates is introduced in this study. The effect of film thickness on magnetic properties and magnetostriction constant of polycrystalline FeGa thin films was investigated. The anisotropy field Hk of flexible FeGa films, i.e., the saturation field determined by fitting the hysteresis curves measured along the hard axis, was enhanced with increasing the tensile strain applied along the easy axis of the thin films, but this enhancement via strain became unconspicuous with increasing the thickness of FeGa films. In order to study the magnetic sensitivity of thin films responding to the external stress, we applied different strains on these films and measure the corresponding anisotropy field. Moreover, the effective magnetostriction constant of FeGa films was calculated from the changes of both anisotropy field and external strain based on the Villari effect. A Neel’s phenomenological model was developed to illustrate that the effective anisotropy field of FeGa thin films was contributed from both the constant volume term and the inverse thickness dependent surface term. Therefore, the magnetic properties for the volume and surface of FeGa thin films were different, which has been verified in this work by using vibrating sample magnetometer (VSM) and magneto-optic Kerr effect (MOKE) system. The anisotropy field contributed by the surface of FeGa film and obtained by MOKE is smaller than that contributed by the film volume and measured by VSM. We ascribed the difference in Hk to the relaxation of the effective strain applied on the films with increasing the thickness of films.

45° REFLECTOMETRY OF SEMICONDUCTOR QUANTUM WELLS

2001 ◽  
Vol 15 (17n19) ◽  
pp. 683-687
Author(s):  
A. SILVA-CASTILLO ◽  
F. PEREZ-RODRIGUEZ
Keyword(s):  
Quantum Well ◽  
Quantum Wells ◽  
Growth Direction ◽  
Angle Of Incidence ◽  
Near Surface ◽  
New Information ◽  
Difference Formula ◽  
Exciton Polaritons ◽  
The Difference ◽  
First Time

We have applied the 45° reflectometry for the first time to study exciton-polaritons in quantum wells. The 45° reflectometry is a new polarization-modulation technique, which is based on the measurement of the difference [Formula: see text] between the p-polarization reflectivity (Rp) and the squared s-polarization reflectivity [Formula: see text] at an angle of incidence of 45°. We show that [Formula: see text] spectra may provide qualitatively new information on the exciton-polariton modes in a quantum well. These optical spectra turn out to be very sensitive to the zeros of the dielectric function along the quantum-well growth direction and, therefore, allow to identify the resonances associated with the Z exciton-polariton mode. We demonstrate that 45° reflectometry could be a powerful tool for studying Z exciton-polariton modes in near-surface quantum wells, which are difficult to observe in simple spectra of reflectivity Rp

CUSPS IN THE KAON DECAYS

2009 ◽  
Vol 24 (02n03) ◽  
pp. 214-221
Author(s):  
AKAKI RUSETSKY
Keyword(s):  
Effective Field Theory ◽  
Neutral Pion ◽  
Effective Field ◽  
S Wave ◽  
Kaon Decays ◽  
Cusp Region ◽  
The Difference ◽  
Scattering Lengths ◽  
Theory Framework

We consider two ways of the experimental determination of the S-wave ππ scattering lengths: from K → 3π decays and from Ke4 decays. In both cases the unitary cusps, which emerge due to the difference in the charged and neutral pion masses, play a crucial role. We construct a systematic effective field theory framework which can be used to describe decay amplitudes in the cusp region.

DETERMINATION OF GROWTH DIRECTION OF CARBON NANOTUBES FROM THE SHAPE OF THE CATALYST PARTICLE

2011 ◽  
Vol 10 (04n05) ◽  
pp. 1045-1050 ◽  
Author(s):  
K. P. S. S. HEMBRAM ◽  
K. REDDAPPA REDDY ◽  
G. MOHAN RAO
Keyword(s):  
Carbon Nanotubes ◽  
Tip Growth ◽  
Flow Direction ◽  
Catalyst Particle ◽  
Forward Direction ◽  
Growth Direction ◽  
Morphological Studies ◽  
Simple Geometry ◽  
The Difference

We present a mechanism that can explain the growth direction of the carbon nanotubes (CNT), based on the observation of the catalyst particle's shape, that is present along with the CNT. We claim the mechanism holds good for the tip growth as well as the base growth model for CNTs formation. The morphological studies performed using simple geometry show that for encapsulated catalyst, (i) the contact angle is less along the forward direction than that of backward direction, (ii) the growth direction of CNT is along the flow direction of the catalyst particle in the fluid state, (iii) the distance between the centre of two imaginary spheres (circles in 2D) drawn at the front and back of the catalyst particle, qualitatively predicts the difference in growth rate of the CNT and flow of the catalyst particle inside the CNT. In order to support the above claims, a generalized model is proposed, irrespective of the position of the catalyst.

scholarly journals Development of a novel high straining backward extrusion process

2018 ◽  
Vol 190 ◽  
pp. 06001 ◽  
Author(s):  
Qiang Wang ◽  
Zhimin Zhang ◽  
Xubin Li ◽  
Huifang Zhang
Keyword(s):  
Flow Direction ◽  
Effective Strain ◽  
Fibrous Tissue ◽  
Axial Direction ◽  
Extrusion Process ◽  
Backward Extrusion ◽  
New Process ◽  
Processed Sample ◽  
The Difference ◽  
Low Performance

In this study, a new method of backward extrusion is proposed. In this new process, a punch with a movable mandrel was designed. A hollow billet was firstly backward extruded and subsequently upset with the use of the punch after the mandrel returned. The extrusion and upsetting processes were successively executed in order for a higher effective strain to be imposed and a fibrous tissue flow direction to be controlled. In order for the capability of this process to be investigated, experimental and finite element (FE) methods were used. The effective strain of the final part prepared by both the conventional and the new process were compared along the bottom radial and wall axial direction respectively. In the results, it is shown that the plastic strain applied through the processed sample was approximately higher in twice the value of the sample processed via conventional backward extrusion. Consequently, this may improve the mechanical properties and anisotropy of the final products. The difference of the UTS and the TYS between radial and tangential at the bottom was less than 3%.This new process has proven to be promising for parts with a central hole at the bottom production in order for the parts low performance to be improved.

Creep Analysis of Orthotropic Rotating Cylinder

1980 ◽  
Vol 102 (4) ◽  
pp. 371-377 ◽  
Author(s):  
N. S. Bhatnagar ◽  
V. K. Arya ◽  
K. K. Debnath
Keyword(s):  
Strain Rate ◽  
Effective Stress ◽  
Anisotropic Material ◽  
Isotropic Material ◽  
Effective Strain ◽  
Angular Speed ◽  
Stress And Strain ◽  
Tangential Strain ◽  
Effective Strain Rate ◽  
Stress And Strain Rate

The stress and strain-rate distributions in the wall of a hollow thick-walled circular cylinder, rotating about its own axis with a constant angular speed, have been obtained using Norton’s law for the steady-state creep. The cylinder is assumed to be made of a homogeneous and orthotropic material. The numerical computations, for a number of steels and steel alloys commonly used to manufacture the cylinder, have been carried out for three cases of anisotropy. The effect of anistropy and of exponent n in creep law has been studied. It is observed that the stress and strain-rate distributions are significantly affected by the anisotropy of material and the value of exponent n. It is also noticed that the values of the effective stress for an anisotropic material for which the ratios of axial to tangential strain rate and of radial to tangential strain rate are equal to 1.2, are lower than the corresponding values for an isotropic material for which these ratios are 1.0. And, because of a power law between effective strain rate and effective stress, much lower values of the effective strain rate for the foregoing anisotropic material than those for the isotropic material will be obtained. Thus the use of the aforementioned anisotropic material may be beneficial for the manufacture of the cylinders because (i) it will result in a longer life for the cylinders (because of the lowest strain rate), or (ii) it will allow the cylinder to sustain larger forces without a risk of failure under creep.

The Extrusion Behavior of AZ31 Mg Alloys by Finite Element Simulation

2014 ◽  
Vol 906 ◽  
pp. 285-288
Author(s):  
Ping Li ◽  
Shou Ren Wang ◽  
Yong Wang ◽  
Guang Ji Xue
Keyword(s):  
Finite Element ◽  
Magnesium Alloy ◽  
Effective Stress ◽  
Effective Strain ◽  
Three Dimensional ◽  
Stress And Strain ◽  
Technical Parameters ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element ◽  
Extrusion Forming

The three-dimensional finite element (FE) numerical simulation of extrusion forming of AZ31 matrix magnesium alloy was analyzed in four extrusion velocities. The flow pattern and the influence of extrusion velocity for the temperature, the distribution of effective stress and strain of composites were analyzed. The results showed that, when the extrusion velocity increased from 1.5 mm/s to 4.5 mm/s, the heat flux under steady extrusion state would change from-2.77e+004 (Wm2) to 1.14e+005 (Wm2), meanwhile the effective stress and strain increased at first and then decreased, and the average effective strain and stress value were smallest when v = 4.5 mm/s. It showed that along with the increase of the extrusion velocity, the rise degree of the temperature increased and the distribution of the effective stress and strain tended to be more evenly. Finally, the best extrusion technical parameters of AZ31 magnesium alloy were determined, that was the extrusion velocity was equal to 4.5 mm/s when extrusion ratio was 25 and extrusion temperature was 350 °C.

Formation of Multiple Twinned Structural Units in Electrodeposited Nickel after Annealing

2014 ◽  
Vol 783-786 ◽  
pp. 2573-2578 ◽  
Author(s):  
Alexander Kahrimanidis ◽  
Uta Klement
Keyword(s):  
Grain Growth ◽  
Growth Direction ◽  
Zone Axis ◽  
Organic Additives ◽  
Free Standing ◽  
Structural Units ◽  
Ni Alloys ◽  
Orientation Change ◽  
Control Film ◽  
Electrodeposited Nickel

Electrodeposition is an advanced synthesis technique which involves the creation of a coating or free-standing material through an electrolytic process. Organic additives such as saccharin have been frequently used in electroplating operations to moderate deposit growth rates and to control film quality. In the present study, plating of Nickel without additives has resulted in a sub-microcrystalline microstructure and a <110>-fibre texture in growth direction. Structural units in form of groups of grains possessing a common <110>-zone axis in growth direction and low-Σ relationships between them have been found in the microstructure by use of EBSD. Upon annealing, grain growth sets in. However, the structural units and the texture are preserved up to 550°C. This means that the structural units stabilize the microstructure; there is no orientation change when grain growth occurs (e.g. by twinning). The low-Σ boundaries of the structural units are described in detail and texture development upon annealing is discussed in connection with results from previous studies on Ni and Ni-alloys of different initial texture.

Review of Residual Stress Determination and Exploitation Techniques Using X-Ray Diffraction Method

2006 ◽  
Vol 524-525 ◽  
pp. 229-234
Author(s):  
M. Belassel ◽  
J. Pineault ◽  
M.E. Brauss
Keyword(s):  
Residual Stress ◽  
Diffraction Method ◽  
Point Of View ◽  
Stress And Strain ◽  
Stress Determination ◽  
Calculation Methods ◽  
X Ray Diffraction ◽  
X Ray ◽  
True Meaning ◽  
The Difference

Although x-ray diffraction techniques have been applied to the measurement of residual stress in the industry for decades, some of the related details are still unclear to many production and mechanical testing engineers working in the field. This is because these details, specifically those associated with the transition between diffraction and mechanics, are not always emphasized in the literature. This paper will emphasize the appropriate calculation methods and the steps necessary to perform high quality residual stress measurements. Additionally, details are given regarding the difference between mechanical and x-ray elastic constants, as well as the true meaning of stress and strain from both diffraction and strain gage point of view. Cases where the material is subject to loading above the yield limit are also included.

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