External Tensile Strain Research Articles

Microtwin Structure and Its Influence on the Mechanical Properties of REBCO Coated Conductors

The elastic properties of REBCO tape (i.e., REBa2Cu3 O7-d, RE = Y, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strain Ahkll, which consists of thermal strain AhklT and lattice strain Ahkl, was analyzed using a microscopic structure model, with two types of microtwin configuration along the tensile axis, i.e., with the [100] or [110] crystal axis oriented parallel to the longitudinal direction of the tape. Experimental results were: (a) slope dAh00l/dA is larger than dA0k0l/dA (where A is the external tensile strain), which is attributed to the elastic modulus Ea along the a-axis being smaller than Eb along the b-axis; (b) both dAh00l/dA and dA0k0l/dA are smaller than unity, but dA110l/dA is almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different as Ah00T ≠ A0k0T due to the elastic behavior of the superconducting layer being modified because it is constrained by the substrate and the outer metallic layer; and (d) a large Poisson ratio is attributed to this constraint state. It is suggested that the microtwin structure is critical to understand and model this unusual stress/strain behavior.

Strain‐Induced Orientation‐Selective Cutting of Graphene into Graphene Nanoribbons on Oxidation

Cut to ribbons: Making atomically well-controlled graphene nanoribbons (GNRs) is prerequisite for many graphene applications. Ab initio calculations reveal that, on applying a uniaxial external tensile strain, O atoms adsorbed on graphene form parallel epoxy chains, and subsequent cutting by oxygen attack gives GNRs instead of the quantum dots that are obtained from unstrained graphene (see picture).

Strain effect on diffusion properties of oxygen vacancies in bulk and subsurface of rutile TiO2

The influences of external strain on diffusion properties of the bulk and subsurface oxygen vacancy (OV) in rutile TiO2 are systematically studied using first-principle calculations. For OVs in bulk, we find that tensile (compressive) strain applied in the [001] direction or isotropically applied in the equivalent [110] and [11¯0] directions reduces (increases) the energy barriers of diffusion. Anisotropic strain applied in [110] and [11¯0] increases the energy barriers of diffusion in the two directions. Meanwhile it results in anisotropic diffusion behaviors. Between [110] and [11¯0], the bulk OV prefers to diffuse along the one in which more compressive or less tensile strain is applied. From subsurface to surface, the most energetically favorable OV pathway is along the [110] rows terminated with the surface bridging oxygen atoms. The diffusion barrier of the OV in the first trilayer is much lower than that of a bulk OV. External in-plane tensile strain can further reduce the energy barrier of the subsurface OV diffusion, and thus help to improve the diffusion of OVs from bulk to surface.

First-principles study on the tension-induced magnetic phase transition in Fe3C-single walled carbon nanotube compounds

We report a first-principles investigation on the mechanical and magnetic properties of the compound of a single Fe3C cluster and single walled carbon nanotubes (SWNT) under uniaxial tensile stress. We find that the external tensile strain can induce a remarkably magnetic phase transition in the Fe3C-SWNT compounds. The results on geometrical and electronic structure suggest that the strong Fe-SWNT interaction plays an important role in determining the magnetic properties of the compounds.

An atomic resolution scanning tunneling microscope that applies external tensile stress andstrain in an ultrahigh vacuum

We have developed an ultrahigh vacuum scanning tunneling microscope with an in situexternal stress application capability in order to determine the effects of stress and strainon surface atomistic structures. It is necessary to understand these effects becausecontrolling them will be a key technology that will very likely be used in futurenanometer-scale fabrication processes. We used our microscope to demonstrate atomicresolution imaging under external tensile stress and strain on the surfaces of wafers ofSi(111) and Si(001). We also successfully observed domain redistribution induced byapplying uniaxial stress at an elevated temperature on the surface of a wafer of vicinalSi(100). We confirmed that domains for which an applied tensile stress is directed along thedimer bond become less stable and shrink. This suggests that it may be feasible tofabricate single domain surfaces in a process that controls surface stress and strain.

The intermixing and strain effects on electroluminescence of SiGe dots

Secondary-ion mass spectroscopy, energy dispersion spectrometry, and Raman spectroscopy reveal that SiGe dots grown by ultrahigh-vacuum chemical vapor deposition at 600°C exhibit significant intermixing with an average Ge composition of ∼50%. Raman spectroscopy shows the top SiGe quantum dots of the 20-layer sample to be more relaxed than those of the 5-layer samples. As a result, the electroluminescence from the top SiGe quantum dots of the 20-layer sample has the higher peak energy at ∼0.84eV as compared to ∼0.82eV for the 5-layer sample. The external tensile mechanical strain can compensate the built-in compressive strain of SiGe quantum dots and increase electroluminescence energy.

Investigation of effective mass of carriers in Bi2Te3̸Sb2Te3 superlattices via electronic structure studies on its component crystals

The electronic structures of Bi2Te3 and Sb2Te3 were computed and related to the thermoelectric properties of Bi2Te3̸Sb2Te3 superlattices. The authors found that the similarity of the electronic structure of the two materials permits the Bi2Te3̸Sb2Te3 superlattices inherit high band edge degeneracy, and thus have high electrical conductivity. From the calculated effective mass along the superlattice growth direction, we infer that presence of more Sb2Te3 than Bi2Te3 in the superlattice leads to a smaller effective mass and enhanced carrier mobility. Furthermore, our results suggest that external tensile strain parallel to the interface may further improve the thermoelectric performance of the Bi2Te3̸Sb2Te3 superlattices.

Strained Pt Schottky diodes on n-type Si and Ge

The variation of electron barrier height and built-in voltage of Pt Schottky diodes on the mechanically strained n-type Si and Ge is investigated experimentally and theoretically. The mechanical strain is measured by Raman spectroscopy and analyzed by the finite element method. The built-in voltage and barrier height measured by capacitance-voltage and current-voltage methods, respectively, decrease with increasing external tensile strain. The reduction of the built-in voltage and barrier height originates mainly from the conduction band lowering with strain. The extracted value of conduction band lowering is consistent with the theoretical calculations using the “stress-free” boundary condition.

Fracture behavior of steam-grown oxide scales formed on 2–12%Cr steels

A room temperature three point bent-beam test was conducted for preoxidized coupon specimens of ferritic 2%Cr (T22), 9%Cr (T91), and 12%Cr (T122) steels in order to examine the fracture/spalling behavior of steam-grown oxide scale. Test specimens were reacted with atmospheric 100% steam at 550–750°C for 1000-4800 h. Oxide scale thickness of the tested steels was 15–1150 μm for the 2Cr steel, 30–450 μm for the 9Cr steel, and 25–60 μm for the 12Cr steel. External tensile strain of up to 1.86% was loaded to each preoxidized specimen surface and the fracture/spalling behavior of steam-grown oxide scale was investigated visually and by microscopic observation. For the 2Cr steel, scale greater than 380 μm exfoliated without applying any external strain. For thin-scaled specimens of 15–20 μm thick, a tensile strain of 0.25% and more caused through-scale cracking perpendicular to the scale/metal interface. For these specimens, cracks along the scale/metal interface also resulted, and the oxide scale became separated from the base metal. For the 9Cr steel, scale exfoliation due to cooling was not prominent even for specimens with 450 μm thick scales. External tensile strain of 0.91% and 1.86% caused through-scale cracking to the oxide scale, but the scale/metal interface remained intact and scale exfoliation did not take place. This was the same for the 12Cr steel. Clearly, spalling resistance of the steam-grown oxide scale was significantly higher for the 9Cr and 12Cr steels than for the 2Cr steel.

Biaxial stress ring applications to magneto-optical studies of semiconductor films

We present a magneto-optical system to study semiconductor heterostructures in the presence of an external biaxial tensile strain. The pressure cell is based on the deflection of a plate (the sample) placed between a sphere and a ring. This externally applied stress is easily controlled and can achieve a deformation of up to ∼0.25% for GaAs films. This device is very useful for band structure study and optical resonance experiments in heterostructures. We also present the application of the device to study the behavior of the magneto-excitons in InP epitaxial layer as a function of the biaxial strain. We observed that the diamagnetic and Zeeman effects in InP films are affected by the biaxial tensile strain.

Mechanically Strained Strained-Si NMOSFETs

The drain-current enhancement of the mechanically strained strained-Si NMOSFET device is investigated for the first time. The improvements of the drain current are found to be /spl sim/3.4% and /spl sim/6.5% for the strained-Si and control Si devices, respectively, with the channel length of 25 μm at the external biaxial tensile strain of 0.037%, while the drain-current enhancements are /spl sim/2.0% and /spl sim/4.5% for strained-Si and control Si devices, respectively, with the channel length of 0.6 μm. Beside the strain caused by lattice mismatch, the mechanical strain can further enhance the current drive of the strained-Si NMOSFET. The strain distribution due to the mechanical stress has different effect on the current enhancement depending on the strain magnitude and channel direction. The smaller current enhancement for strained-Si device as compared to the control device can be explained by the saturation of mobility enhancement at large strain.

V–I characteristics of MgB2 PIT composite tapes: n-values under strain, in high fields, or at high temperatures

Abstract V–I characteristics of monocore MgB 2 PIT composite tapes are examined in n -values under strain, in high fields, or at high temperatures. n -values are determined by using V–I curves that are obtained for short samples by dc four-probe resistive method. The n -value increases with increasing external tensile strain up to 0.5% in the same way that critical current, I c , increases with increasing the strain. When the strain exceeds 0.5%, I c and n -value show a large degradation. In high field tests, in situ processed MgB 2 (+10 at.% SiC) tapes show n -values of 25.8–29.0 ( J c (core)=12,000–15,000 A/cm 2 ), 18.3–20.1 (5300–6200 A/cm 2 ), and 4.8–6.5 (970–1100 A/cm 2 ) at 10, 12, and 16 T, respectively. In high temperature tests, the tapes show n -values around 40 at 18 K in 4 T, 21 K in 3 T, 25.5 K in 2 T, and 29 K in 1 T. I c and n -value show a strong correlation. MgB 2 conductors with the larger I c can lead to the higher n -value.

The influence of thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)2223 tapes with different matrices

The behaviour of the critical current in longitudinally strained Bi,Pb(2223) tapes shows a strain-insensitive plateau up to an irreversible strain limit ε irr. For higher strains, the formation of cracks induces an abrupt decrease of the critical current. We investigate the relationship between precompression and irreversible strain limit with a set of tapes made with different filling factors as well as an in situ Inconel 600-reinforced tape. I c( ε) curves were measured in a longitudinal strain apparatus at 77 K. The precompression at the measurement temperature is numerically estimated for each sample as well as the evolution of precompression during the cool-down. These calculated values are compared to the fracture susceptibility of extracted filaments, which gives an empirical estimate of the precompression. The main hypothesis of the “Irreversible I c Reduction Model” is confirmed, i.e. the irreversible strain limit essentially depends on precompression. However, we also found that the regime where I c remains constant contains a tensile component: the plateau extends beyond the external tensile strain needed to relieve the thermal precompression and includes a regime where the ceramic is further elongated non-destructively. This non-destructive deformation can be understood as a “connected-grains” behaviour, and extends the strain-insensitive plateau ∼0.1% beyond the precompression strain. This value is confirmed with a three points bending experiment performed on single filaments which gives a similar value for the bending failure strain. A comparable regime was found to exist also under compressive strain. These non-destructive regimes are of great importance for practical applications since up to a certain level the precompression can develop without any I c degradation.

Evaluation of magnetostriction constant in amorphous ribbons utilizing the velocity change of a magnetoelastic wave

The magnetoelastic coupling constant B, or the saturation magnetostrictive constant λ s in highly magnetostrictive amorphous ribbons was evaluated indirectly from the change of the sound velocity under application of external field or uniaxial tensile strain by comparing with the theory based upon the one-dimensional elastic beam model.