Relaxation Of Internal Strain Research Articles

In-situ synchrotron high energy X-ray diffraction study on the internal strain evolution of D019-α2 phase during high-temperature compression and subsequent annealing in a TiAl alloy

The residual stress in the D019-α2 phase is known to be significantly higher than that in the L10-γ phase in TiAl alloys after deformation due to the poor plasticity and strong mechanical anisotropy of the α2 phase. However, the internal stress accumulation and relaxation in the α2 phase during high-temperature deformation and annealing are scarcely investigated. In this study, for the first time, the internal strain evolution and load partitioning between the α2 and γ phases at high temperatures are characterized by in-situ synchrotron high energy X-ray diffraction (HEXRD) technique. The plastic deformation is at least initiated at a stress of roughly 200 MPa in the γ phase and 775 MPa in the α2 phase. The intergranular strains in the α2 phase are generated by the onset of dislocation glide in the γ phase, and accentuated with the accumulated dislocations and the ensuing twinning activity. After unloading, great intergranular strains are preserved in the α2 phase constrained by the heavily plastically deformed γ phase. During subsequent heating from 400 to 1000 °C, the internal strains in the α2 phase are almost fully relaxed by substantial dislocation annihilation and rearrangement in the γ phase. During annealing at 800 °C, the internal strain relaxation is rapid in the initial 10 min, whereas considerably retarded subsequently. The extent of relaxation after holding at 800 °C for 1 h is equivalent to that of heating in an effective temperature range of 680–880 °C for 10 min. The in-situ lattice strain measurements with various thermal relaxation schemes provide guidance for the stress relief annealing of TiAl components.

Electronically phase separated nano-network in antiferromagnetic insulating LaMnO3/PrMnO3/CaMnO3 tricolor superlattice

Strongly correlated materials often exhibit an electronic phase separation (EPS) phenomena whose domain pattern is random in nature. The ability to control the spatial arrangement of the electronic phases at microscopic scales is highly desirable for tailoring their macroscopic properties and/or designing novel electronic devices. Here we report the formation of EPS nanoscale network in a mono-atomically stacked LaMnO3/CaMnO3/PrMnO3 superlattice grown on SrTiO3 (STO) (001) substrate, which is known to have an antiferromagnetic (AFM) insulating ground state. The EPS nano-network is a consequence of an internal strain relaxation triggered by the structural domain formation of the underlying STO substrate at low temperatures. The same nanoscale network pattern can be reproduced upon temperature cycling allowing us to employ different local imaging techniques to directly compare the magnetic and transport state of a single EPS domain. Our results confirm the one-to-one correspondence between ferromagnetic (AFM) to metallic (insulating) state in manganite. It also represents a significant step in a paradigm shift from passively characterizing EPS in strongly correlated systems to actively engaging in its manipulation.

Unveiling Growth Pathways of Multiply Twinned Gold Nanoparticles by In Situ Liquid Cell Transmission Electron Microscopy.

A mechanistic understanding of the growth of multiply twinned nanoparticles (MTPs), such as decahedra (Dh) and icosahedra (Ih), is crucial for precisely controlled syntheses and applications. Despite previous successes, no consensus has been reached regarding the multiple competing growth pathways for MTPs proposed thus far, in part due to the lack of information about their nucleation and growth dynamics. Here, we used decahedral and icosahedral gold nanoparticles as a model system in conjunction with in situ liquid cell transmission electron microscopy (LCTEM) to investigate the nucleation and growth dynamics of MTPs in aqueous solution; two growth pathways were successfully identified: (A) nucleation-based layer-by-layer growth from a rounded multiply twinned seed and (B) the successive twinning and growth of tetrahedra. The LCTEM results enabled us to directly and conclusively identify the growth behaviors of intermediate products. The internal strain relaxation mechanisms and growth kinetics differ for the two pathways: in pathway A, a MTP grew by the opening and closing of re-entrant grooves at the twin boundaries, which was not found in pathway B. We also analyzed different MTP growth pathways from an energetic perspective and discussed how the preferred pathway (A or B) is related to factors, such as the initial seed yield and the size- and morphology-dependent formation of MTPs. Our results contextualize the current understanding of MTP formation mechanisms and provide insightful guidance for the precisely controlled synthesis of MTPs for practical applications.

Direct lift-off and the piezo-phototronic study of InGaN/GaN heterostructure membrane

Internal polarizations, caused by the intrinsic material properties and lattice mismatch during material epitaxial growth, exist inherently in III-nitrides and limit significantly their practical applications. Here, InGaN/GaN single quantum well (SQW) structure grown on sapphire substrate was directly exfoliated by electrochemical (EC) etching. The separated InGaN/GaN SQW heterostructure membranes (SQW-HMs) exhibit substantial relaxation of internal strain and polarization compared to the as-grown samples. To further modulate/tune the internal polarizations and corresponding opto-electronic properties of the SQW-HMs, the piezo-phototronic effect was introduced by applying external stress on the freestanding membrane under ultraviolet (UV) light illumination. The obtained photoluminescence (PL) intensity can be effectively modulated in different manners under tensile and compressive straining conditions. Energy diagrams of the SQW-HMs under free and straining conditions were carefully analyzed to illustrate the working mechanisms of the piezo-phototronic modulations. This research provides a new approach to optimize the performances of III-nitride devices and expand their further applications in optical communications and optical integration systems.

Gallium Nitride Nanowires and Heterostructures: Toward Color-Tunable and White-Light Sources.

Gallium-nitride-based light-emitting diodes have enabled the commercialization of efficient solid-state lighting devices. Nonplanar nanomaterial architectures, such as nanowires and nanowire-based heterostructures, have the potential to significantly improve the performance of light-emitting devices through defect reduction, strain relaxation, and increased junction area. In addition, relaxation of internal strain caused by indium incorporation will facilitate pushing the emission wavelength into the red. This could eliminate inefficient phosphor conversion and enable color-tunable emission or white-light emission by combining blue, green, and red sources. Utilizing the waveguiding modes of the individual nanowires will further enhance light emission, and the properties of photonic structures formed by nanowire arrays can be implemented to improve light extraction. Recent advances in synthetic methods leading to better control over GaN and InGaN nanowire synthesis are described along with new concept devices leading to efficient white-light emission.

Origin of nonlinear piezoelectricity in III-V semiconductors: Internal strain and bond ionicity from hybrid-functional density functional theory

We derive first- and second-order piezoelectric coefficients for the zinc-blende III-V semiconductors, {Al,Ga,In}-{N,P,As,Sb}. The results are obtained within the Heyd-Scuseria-Ernzerhof hybrid-functional approach in the framework of density functional theory and the Berry-phase theory of electric polarization. To achieve a meaningful interpretation of the results, we build an intuitive phenomenological model based on the description of internal strain and the dynamics of the electronic charge centers. We discuss in detail first- and second-order internal strain effects, together with strain-induced changes in ionicity. This analysis reveals that the relatively large importance in the III-Vs of non-linear piezoelectric effects compared to the linear ones arises because of a delicate balance between the ionic polarization contribution due to internal strain relaxation effects, and the contribution due to the electronic charge redistribution induced by macroscopic and internal strain.

Effect of strain relaxation in InGaN/GaN multi-quantum wells with self-assembled Pt nanoclusters.

We report effect of the strain relaxation in InGaN/GaN multi-quantum well (MQW) structures grown on platinum nanocluster-coated sapphire substrate (PNSS) by metal organic chemical vapor deposition. The photoluminescence (PL) intensity of InGaN/GaN MQWs on PNSS was significantly enhanced compared to that of the InGaN/GaN MQWs on flat sapphire substrate due to the reduction of defect density and residual strain by self-assembled Pt nanoclusters. We confirmed the reduction of strain-induced piezoelectric field by the power dependence of the PL in InGaN/GaN MQWs on PNSS. Cathodoluminescence shows that a large bright area with overall strong peak intensity is attributed to the suppression of In inhomogeneity and strain relaxation in InGaN/GaN MQWs on PNSS. Based on these results, we suggest that the self-assembled Pt nanocluster can be applied to increase the quantum efficiency through improved crystal quality and internal strain relaxation in MQWs.

Heat-induced Internal Strain Relaxation and its Effect on the Microstructure of Polyacrylonitrile-based Carbon Fiber

The transformation of the internal strain and its effect on the microstructure of polyacrylonitrile-based carbon fiber during the high-temperature graphitization were investigated. The internal compressive strain within the carbon turbostratic structure was confirmed through a careful analysis by wide-angle X-ray diffraction and Raman spectroscopy. Heat-induced strain/stress relaxation along the fiber axis was observed and was found to have a profound effect on the structure of both the crystallites and microvoids. The results indicated that, the relaxation of residual strain changed the graphite layers from a wrinkled and distorted morphology to a straight and smooth one, and consequently led the crystallites to stack closely and orderly with increasing stack height. The strain relaxation also changed the morphology of crystallites and microvoids, resulting in an anisotropic growth for the latters.

Domain configuration and phase transition for BaTiO 3 thin films on tensile Si substrates

Ferroelectric BaTiO 3 films with different thicknesses were deposited on “tensile” Si substrates buffered with 100 nm LaNiO 3 by radio frequency sputtering. X-ray diffraction measurements demonstrate a high (0 0 1)-orientation and in-plane tensile strain in all BaTiO 3 films. With increasing BaTiO 3 thickness from 200 to 400 nm, the in-plane lattice constant is decreased and out-of-plane lattice constant increased. High-resolution transmission electron microscopy observations reveal two different in-plane lattice constants in the adjacent twinning domains, thus confirming that the BaTiO 3 film is under an in-plane polarization state. The temperature-dependent dielectric measurement results show that the ferroelectric to paraelectric phase transition temperature is enhanced for both BaTiO 3 films. However, the enhanced phase transition temperature shows a decrease tendency from 200 to 400 nm BaTiO 3, which coincides with the change of lattice constants and can also be interpreted as the result of the internal strain relaxation.

Mechanism of polarization enhancement in La-doped Bi4Ti3O12 films

Bi 4 − x La x Ti 3 O 12 (La-doped BIT, x=0–0.6) thin films prepared by metalorganic decomposition method, were studied using x-ray diffraction (XRD), polarization hysteresis and Raman scattering techniques. The XRD and polarization studies showed that La substitution reduces structural distortion, relaxes internal strain and enhances polarization, while the Raman data indicate that the added La occupies mainly at A sites in the BIT films. Raman modes also indicate an increase in TiO6 symmetry, internal strain relaxation, lower Ti–O hybridization, and decrease in polarization along the a axis. Further, the polarization in the highly c-oriented BIT films significantly increases with the added La, hence a polarization tilt towards the c axis is implied. The higher polarization observed can be attributed, therefore, to microscopic changes involving decreased distortion in the TiO6 octahedra and a polarization tilt towards the c axis, caused by doping with the electronically less active ion (e.g., La, Y) onto the A sites in the BIT structure. This understanding is important to the commercialization of high-density ferroelectric random access memories.

Grain refinement versus voids accumulation during severe plastic deformations of polycrystals: mathematical simulation

We view the emergence of disorientation boundaries in crystals and the formation of micropores as complementary mechanisms of relaxation of internal strain during severe plastic deformations of polycrystals. Using an assumption of self-similarity of the grain refinement process, we obtain kinetic equations for two scalar parameters characterizing the structure of the material: the total area of fragment boundaries and the total volume of micropores per unit volume of material. We show that the model captures the main regularities of large plastic deformations of metals with high stacking-fault energy. The analysis of the model yields a number of recommendations for increasing the effectiveness of severe plastic deformation processes.

Thermal relaxation of internal strain in two-phase Cu–Nb wire

Thermal relaxation of internal strain in high-strength two-phase Cu–18wt.%Nb wire was studied using neutron diffraction at temperatures from 100 to 450 °C. The average axial tensile strain in the Nb phase reduced significantly at temperatures above 300 °C. This was shown to correspond to the onset of spheroidization of the Nb phase.

Yttrium oxide thin films: chemistry- stoichiometry-strain and microstructure

Yttrium oxide thin films were in-situ deposited by ion beam sputtering on Si, MgO and SrTiO 3 substrates. These Y 2O 3 thin films were investigated mainly by means of x-ray diffraction. The strained state of the oxide layers was studied by the sin 2ψ method as a function of the deposition parameters as well as the post annealing treatments. An in situ study of the kinetics of the internal strain relaxation process was performed as a function of temperature. The Arhenius plot of relaxation rate gives the activation energy of this strain relaxation process, which is 1.3 eV. The results obtained in this work were interpreted in terms of crystal chemistry and the stoichiometry-microstructure relationship.

Generation and Relaxation of Microstrains in GaN Nanocrystals under Extreme Pressures

Nanocrystalline powders of GaN with grain sizes ranging from 2 to 30 nm were examined under high external pressures by in situ diffraction techniques in a diamond anvil cell at DESY (HASYLAB, Station F3). The experiments on densification of pure powders under high pressure were performed without a pressure medium. The mechanism of generation and relaxation of internal strains and their distribution in nanoparticles was deduced from the Bragg reflections recorded in situ under high pressures at room temperature. The microstrain was calculated from the full-width at half-maximum (FWHM) values of the Bragg lines. It was found that microstrains in GaN crystallites are generated and subsequently relaxed by two mechanisms: generation of stacking faults and change of the size and shape of the grains occurring under external stress.

Microstructure and magnetic properties of nanosized Fe–Co alloy powders synthesized by mechanochemical and mechanical alloying process

An optimum route to synthesize nanosized Fe–Co alloy powder with enhanced magnetic properties was investigated. Two methods of mechanical alloying (MA) and mechanochemical alloying (MCA) for developing a nanosized alloy powder were compared on the basis of the resulting microstructural characteristics and magnetic properties. The alloy powder, synthesized by MCA process with ball milling and hydrogen reduction using Fe 2O 3 and Co 3O 4 powders, showed ordered BCC structure with the grain size of 40 nm. Also, this powder exhibited low coercivity of 43 Oe and good permeability compared with MA powder. Enhanced magnetic properties of the MCA powder were explained by the formation of ordered structure and relaxation of internal strain.

Microwave-stimulated relaxation of internal strains in GaAs-based device heterostructures

We have studied the effect of microwave radiation (frequency, 2.45 GHz; specific power density, 1.5 W/cm2) on the relaxation of internal mechanical strains in the (i)n-n+-GaAs structures, (ii) Au-Ti-n-n+-GaAs diode structures with Schottky barriers, and (iii) GaAs-based Schottky-barrier field-effect transistors (SFETs). It is shown that exposure of the samples to the microwave radiation for a few seconds leads to relaxation of the internal mechanical strains and improves the quality of the semiconductor surface layer structure. This results in improved parameters of the GaAs-based device structures of both (diode and SFET) types, as manifested by increased Schottky barrier height, reduced ideality factor and back current in the diode structures, and increased gain slope and initial drain current in the SFETs.

高分子材料 ビスフェノールＡポリカーボネートの副分散 動的複屈折による検討

Dynamic birefringence and dynamic viscoelasticity of bisphenol A polycarbonate were simultaneously measured over a wide range of temperature from -150 to 180°C. Large γ dispersion observed for the complex Young's modulus was found to be inactive for the complex strain-optical coefficients, which is the ratio of birefringence to the strain. The present result indicates that γ dispersion cannot be attributed to the reorientation of any atomic groups, but to the relaxation of internal strain by translational motions. Dynamic birefringence was anticipated to be a new experimental method for the subrelaxation of glassy polymers.

Vapor-phase growth mechanism of the Bi-based 2212 superconducting whiskers

Bi2Sr2CaCu2O8 (2212) superconducting whiskers can be grown out from an annealed melt-quenched precursor plate in an atmosphere of flowing oxygen. It was found that other than the appropriate annealing temperature and time, glass precursor, flowing oxygen atmosphere and temperature gradient in the furnace are vital factors. The relaxation of internal strain in the melt-quenched precursor provides the driving force for the whisker growth. Flowing oxygen atmosphere and temperature gradient both play the role of mass transport and accelerate the whisker growth rate.

N + ion implantation effects on microhardness and adhesion in TiO 2 films

The epitaxial films of TiO 2 rutile (100) could be grown on sapphire (0001) substrates at 500°C by means of reactive ionized cluster beam (RICB) deposition. To achieve epitaxial growth at low substrate temperature, ionization of Ti vapor and O 2 gas was necessary. The difference of mechanical properties in epitaxial rutile films on sapphire and polycrystal rutile films on Si wafer has been studied by microhardness and scratch adhesion measurements. Epitaxial films are more hard about 1.9 times and adherent about 1.3 times than polycrystal films. High-energy (3 MeV) implantation has a stronger hardening and adhesion effects than low-energy (90 keV) implantation. In epitaxial films, the spread of interplanar spacing of rutile (200) planes occurred due to implantation-induced internal stress and strain. In polycrystal films, a phase transformation of crystalline rutile to anatase of TiO 2 occurred due to the relaxation of internal strain. The adhesion of TiO 2 films to the substrates could be increased about 27% in both epitaxial and polycrystal films by high energy (3 MeV and 400 keV respectively) implantation due to interface mixing and reaction between film and substrate.

Strain-induced birefringence in GaAs.

We present the results of density-functional calculations of the dielectric function and the photoelastic tensor---the susceptibility for strain-induced birefringence---in GaAs for photon frequencies below the direct band gap. These calculations were performed in the Kohn-Sham local-density approximation in a pseudopotential/plane-wave scheme which includes local-field effects and self-energy corrections. We find that traditional special-points integration methods are inadequate for performing the Brillouin-zone integrals involved in computing the photoelastic tensor. Very high resolution of the critical point at {ital k}=0 is needed to obtain even the correct {ital qualitative} behavior of the photoelastic tensor. Accurate expansions of the integrand in spherical harmonics for small {ital k} and plane waves elsewhere in the Brillouin zone were obtained and an integration approach which correctly integrates the expansions was used. Dramatic improvement in the qualitative frequency dispersion of the photoelastic tensor, in comparison with experiments, is obtained despite a large (50%) shift of the static value away from the measurement. We also present the result of a calculation of the internal strain relaxation associated with strains along the bonding [111] direction. These results are in excellent agreement with two previous {ital ab} {ital initio} calculations and with recent measurements.