Structure and Strain Properties of GaN Films Grown on Si（111） Substrates with AlxGa1-xN/AlyGa1-yN Superlattices

CaN films with an AlxGa1-xN/AlyGa1-xN superlattice （SL） buffer layer are grown on Si（111） substrates by metal-organic chemical vapor deposition （MOCVD）. The structure and strain properties of the samples are studied by optical microscopy, Raman spectroscopy, x-ray diffractometry and atomic force microscopy. The results show that the strain status and crystalline quality of the CaN layers are strongly dependent on the difference of the Al composition between AlxCa1-xN barriers and AlyCa1-yN wells in the SLs. With a large Al composition difference, the CaN film tends to generate cracks on the surface due to the severe relaxation of the SLs. Otherwise, when using a small Al composition difference, the crystalline quality of the CaN layer degrades due to the poor function of the SLs in filtering dislocations. Under an optimized condition that the Al composition difference equals 0.1, the crack-free and compressive strained CaN film with an improved crystalline quality is achieved. Therefore, the AlxGa1-xN/AlyGal-yN SL buffer layer is a promising buffer structure for growing thick CaN films on Si substrates without crack generation.

STRAIN INDUCED STRUCTURAL TRANSITION OF INTERFACES AND TWINS IN A HOT－DEFORMED DUAL－PHASE TIAL ALLOY

The structure characteristics of a2/γinterfaces and the features of deformation twins in a quasi-isothermal forged Ti-45Al-10Nb alloy were studied by highresolution transmission electron microscopy. Three types of strain induced a2/γinterfaces and two types of strain induced twin boundaries were identified The most,important features are high density of ledges and the existence of I/3[111] Frank partial dislocation. Mechanisms for the formation these interfaces were proposed Two types of deformation twins were observed These deformation twins always start from the ledges it seems that ledges at interfaces are important features of interfacial structure for the mechanical behavior of alloys.

Effects of Internal Relaxation under Inplane Strain on the Structural,Electronic and Optical Properties of Perovskite BaZrO3

We present the specific ab-initio calculations that detail the variations of perovskite BaZrO3 caused by in-plane strain. Specifically, the internal relaxation, which was not captured in the widely used biaxial strain model, was included in a complementary manner to lattice relaxation. Density functional theory as well as a hybrid functional method based on a plane wave basis set was employed to calculate the lattice structure, elastic constants, electronic properties and optical properties of perovskite BaZrO3. The lattice parameter c exhibited a clear linear dependence on the imposed in-plane strain, but the Poisson＇s ratio caused by internal relaxation was smaller than the elastic deformation, indicating an ＂inelastic＂ or ＂plastic＂ relaxation manner caused by the introduction of internal relaxation. As a result, the related electronic and optical properties of perovskite BaZrO3 were also strongly affected by the in-plane strain, which revealed an effective way to adjust the properties of perovskite BaZrO3 via internal relaxation.

Strain induced polymorphism and band structure modulation in low-temperature 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene single crystal

Organic semiconductors are inherently soft,making it possible to increase their mobilities by strains.Such a unique feature can be exploited directly in flexible electronics for improved device performance.The 2,7-dioctyl[1]benzothieno[3,2-b][1]-benzothiophene derivative,C8-BTBT is one of the best small-molecule hole transport materials.Here,we demonstrated its band structure modulation under strains by combining the non-equilibrium molecular dynamics simulations and first-principles calculations.We found that the C8-BTBT lattice undergoes a transition from monoclinic to triclinic crystal system at the temperature below 160 K.Both shear and uniaxial strains were applied to the low-temperature triclinic phase of C8-BTBT,and polymorphism was identified in the shear process.The band width enhancement is up to 8%under 2%of compressive strain along the x direction,and 14%under 4%of tensile strain along the y direction.The band structure modulation of C8-BTBT can be well related to its herringbone packing motifs,where the edge to face and edge to edge pairs constitute two-dimensional charge transport pathways and their electronic overlaps determine the band widths along the two directions respectively.These findings pave the way for utilizing strains towards improved performance of organic semiconductors on flexible substrates,for example,by bending the substrates.

Simultaneous measurement of axial strain and temperature based on twisted fiber structure

In this Letter, an alternative solution is proposed and demonstrated for simultaneous measurement of axial strain and temperature. This sensor consists of two twisted points on a commercial single mode fiber introduced by flame-heated and rotation treatment. The fabrication process modifies the geometrical configuration and refractive index of the fiber. Different cladding modes are excited at the first twisted point, and part of them are coupled back to the fiber core at the second twisted point. Experimental results show distinct sensitivities of 34.9 pm/με with 49.23 pm/℃ and -36.19 pm/με with 62.99 pm/℃ for the two selected destructive interference wavelengths.

Syntheses, Characterization and DFT Analysis of Two Novel Thiaheterohelicene Derivatives

Two novel thiaheterohelicene derivatives were synthesized from the corresponding 2,2'-(2,6-naphthalenediyl-di-2,1-ethenediyl) bis-thiophene and its dimethyl substituted analogue 2,2'-(2,6-naphthalenediyldi-2,1-ethenediyl) bis-2’’-methylthiophene using oxidative photo cyclization reaction. The compounds were characterized by 1H NMR, electron impact-mass spectrometry, elemental analyses, and the absolute molecular structures were determined by single crystal X-ray diffraction analysis. They crystallized under monoclinic system with space group P21/n for the unsubstituted compound and P21/c for the methyl substituted compound, respectively. The dihedral angle between the terminal thiophene ring and the molecular center was observed to be 20.82? for the unsubstituted compound and 14.27? for the methyl substituted compound, respectively. Furthermore, molecules oriented as herringbone structures by intermolecular π-π stacking in the crystals. The relative study of the actual arrangement of these molecules has been carried out using X-ray diffraction analysis. The two molecules have different crystal packing. The molecule 3b has herring bone like arrangement due to the substituent bulkiness and weak CH-π interaction. On the other hand, the molecular packing of molecule 3a is not herringbone probably due to the multiple weak intermolecular CH-S short contacts between columns consisting of stacked molecules.

Structural Strains during the Process of Social Change

The structural readjustments and innovations accompanying institutional change have brought about contradictions and conflicts among different interest groups. The many problems thus touched off can be attributed to the tensions resulting from ＂structural strain,＂ with ＂anomie＂ a typical structural problem. We have used survey data as a basis for analyzing and examining the relationship between structural strain and anomie. We find that differences and differentiation in social status, organization, role and power structures and in income and educational level structures have indeed led to the birth of a sense of anomie. To a large extent, such negative feelings as individual anomie, dissatisfaction, a sense of relative deprivation, status inconsistency, etc., are nothing other than the result of the imbalances and tensions brought about by an objective structural differentiation that is proceeding faster than institutional integration.

Separation and extraction of bridge dynamic strain data

Through comparing the measured data of dynamic strains due to loading and temperature by the stain gauge and temperature sensor at the same location,the information in the strain data was divided into three parts in the frequency domain by using the defined index named power spectral density(PSD)-ratio index.The three parts are dominated respectively by temperature varying,stresses,and noises and thus can be distinguished from the determined the separatrix frequencies.Also,a simple algorithm was developed to separate the three types of information and to extract the strain caused mainly by structural stresses.As an application of the proposed method,the effect of strain deformation and noises on the fatigue assessment was investigated based on the separated data.The results show that,the determined values of separatrix frequencies are valuable for the monitoring data from other bridges.The algorithm is a multiresolution and hierarchical method,which has been validated as a simple and effective method for data analyses,and is suitable for the compression and preprocessing of the great amount monitoring data and easy to be integrated into the structural health monitoring(SHM)soft system.The strain due to temperature varying attributes a little to the errors of fatigue assessment;however,the noises or random disturbance existed in the monitoring data have much responsibility for the errors,and the main reason is that the random disturbance shifts the real strain/stress amplitude picked up by real structural stress or strain.

Energy Absorption and Deformation Mechanism of Lotus-type Porous Coppers in Perpendicular Direction

As metallic foams used for energy absorption in the automotive and aerospace industries, recently invented lotus-type porous metals are viewed as potential energy absorbers. Yet, solid conclusion on their eligibility as energy absorbers is still in question, particularly when compression is in the direction perpendicular to the axial orientation of cylindrical pores. In this work, the energy absorption of lotus-type porous coppers in the perpendicular direction is investigated at strain rates from 0.001 s^（-1） to^2400 s^（-1）. The energy absorption capacity and the energy absorption efficiency are calculated to be4–16 k J/kg and 0.32–0.7, respectively, slightly inferior to metal foams and the same porous solid compressed in the parallel direction due to the shortened extent of the plateau stress region. The deformation mechanism is examined experimentally in conjunction with finite element modeling. Both suggest that gradual squeeze and collapse of pores are the mechanisms accommodating the energy absorption. The deformation is generally evenly distributed over pore ligaments and independent of strain rate.