Search

This study reports on the behaviour of the thermoelectric properties of n- and p-type hydrogenated microcrystalline silicon thin films (µc-Si: H) as a function of applied uniaxial stress up to ±1.7%. µc-Si: H thin films were deposited via plasma enhanced chemical vapour deposition and thermoelectric properties were obtained through annealing at 200 °C (350 °C) for n-(p-) type samples, before the bending experiments. Tensile (compressive) stress was effective to increase the electrical conductivity of n-(p-) type samples. Likewise, stress induced changes in the Seebeck coefficient, however, showing an improvement only in electron-doped films under compressive stress. Overall, the addition of elevated temperature to the bending experiments resulted in a decrease in the mechanical stability of the films. These trends did not produce a significant enhancement of the overall thermoelectric power factor, rather it was largely preserved in all cases.

The existing researches do not consider that coal of different particle sizes in the goaf is affected by the axial stress generated by the collapsed coal rock and the residual coal pillars, which will cause major changes in the coal permeability, fracture structure and other parameters, thereby affecting the temperature rise process of coal spontaneous combustion and oxidation. In order to study the temperature rise characteristics of spontaneous combustion and oxidation of coal with different particle sizes under the effect of thermal-stress coupling, the change law of the temperature rise rate, oxygen consumption rate and heat production rate of coal with different particle sizes is measured by using load-and-pressure coal spontaneous combustion characteristic parameter measuring device and a gas chromatograph. The experiment show the results as follows. ① The temperature rise rate of coal samples of different particle sizes under different uniaxial stresses has a consistent trend with the increase of temperature, and the overall trend is to increase first and then decrease. When the uniaxial stress is 2, 4, 6, 8 MPa, as the particle size of the coal sample increases, the maximum temperature rise rate first increases and then decreases. When the uniaxial stress is 10 MPa, the maximum temperature rise rate increases with the increase of particle size. ② The oxygen consumption rate of coal samples with different particle sizes increases rapidly and then increases slowly during the temperature rise process. The oxygen consumption rate of coal samples with different particle sizes generally tends to increase with the increase of uniaxial stress. ③ The overall heat production rate of coal samples with different particle sizes tends to remain stable first and then increase and then decrease during the temperature rise process. As the particle size of coal samples increases, the maximum heat production rate of coal samples under appropriate uniaxial stress is greater than that without uniaxial stress.

Oxides, materials composed of oxygen atoms, have several remarkable functional properties : ferroelectricity, piezoelectricity, magnetism, superconductivity ... and a large number of technological advantages : stability, integrability on silicon and flexibility of their properties according to the desired functionality. These materials have numerous applications in spintronics, in quantum information technologies or in energy production and harvesting.Among this family of materials, I have studied the properties and excitations of several compounds : SrTiO3 and the multiferroics BiFeO3, TbMnO3 and CuO by implementing Raman spectroscopy, synchrotron Thz measurements and time-resolved acoustic measurements.On the perovskite SrTiO3, I have carried out Raman studies on the calcium and carrier doping. This study highlights the appearance of ferroelectricity in the 2D gas at the interface of SrTiO3by calcium doping. Under electric field, I participated to show the creation of a polarization at the interface of the material induced by a migration of charges.Multiferroics are materials in which at least two ferroic orders coexist in the same phase.By combining picosecond acoustic experiments with numerical simulations, the elastic constants of BiFeO3 and TbMnO3 have been determined. The knowledge of the elastic properties of BiFeO3 allows to consider its use in piezotransducers for example. On this same compound, measurements under uniaxial tension in Raman spectroscopy show the enhancement of its polarization and the tuning of the magnetic state and in particular the spin waves.Finally in the compound CuO, multiferroic between 208 and 230 K, we have high-lighted by Thz spectroscopy an additional electromagnon (polar spin wave) at high energy. Under hydrostatic pressure, the multiferroic phase expands in temperature towards room temperature and the low energy electromagnon sees its polar activity strongly enhanced., Les oxydes, matériaux composés d’atomes d’oxygène, présentent plusieurs propriétés fonctionnelles remarquables : ferroélectricité, piézoélectricité, magnétisme, supraconductivité. . . et un grand nombre d’avantages technologiques : stabilité, intégrabilité sur silicium et flexibilité de leurs propriétés selon la fonctionnalité désirée. Les perspectives d’applications de ces matériaux sont nombreuses en spintronique, dans les technologiques quantiques de l’information ou encore dans la production et la préservation d’énergie.Parmi cette famille de matériaux, j’ai étudié les propriétés et les excitations de plusieurs composés : SrTiO3 et les multiferroïques BiFeO3, TbMnO3 et CuO en mettant en œuvre la spectroscopie Raman, des mesures Thz en synchrotron et des mesures acoustiques résolues en temps.Sur la perovskite SrTiO3, j’ai mené des études Raman sur le dopage en calcium et en porteurs. Cette étude met en évidence l’apparition de la ferroélectricité dans le gaz 2D à l’interface de SrTiO3 par dopage en calcium. Sous l’effet d’un champ électrique, j’ai participé à montrer la création d’une polarisation à l’interface du matériau induite par une migration des charges.Les multiferroïques sont des matériaux dans lesquels au moins deux ordres ferroïques coexistent dans une même phase.En combinant des expériences d’acoustique picoseconde à des simulations numériques, les constantes élastiques de BiFeO3 et TbMnO3 ont été déterminées. La connaissance des propriétés élastiques de BiFeO3 permet d’envisager par exemple son utilisation dans des piézotransducteurs. Sur ce même composé, des mesures en tension uniaxiale en spectroscopie Raman ont permis d’exalter sa polarisation et de modifier l’état magnétique et en particulier les ondes de spins.Enfin pour le composé CuO, multiferroïque entre 208 et 230 K, nous avons mis en évidence par spectroscopie Thz un potentiel électromagnon (onde de spin polaire) additionnel à haute énergie. Sous pression hydrostatique, la phase multiferroïque s’élargie en température en direction de la température ambiante et l’électromagnon de basse énergie voit son activité polaire décuplée.

Evolution of spectroscopic properties of a soda–lime silicate glass with different thermal history and under applied uniaxial stress was investigated using Raman and Brillouin spectroscopies as well as Nd3+ photoluminescence techniques. Samples of soda–lime silicate with a cooling rate from 6 × 10−4 to 650 K/min were prepared either by controlled cooling from the melt using a differential scanning calorimeter or by a conventional annealing procedure. Uniaxial stress effects in a range from 0 to −1.3 GPa were investigated in situ by compression of the glass cylinders. The spectroscopic observations of rearrangements in the network structure were related to the set cooling rates or the applied uniaxial stress to calculate an interrelated set of calibrations. Comparing the results from Raman and Brillouin spectroscopy with Nd3+ photoluminescence analysis, we find a linear dependence that can be used to identify uniaxial stress and cooling rate in any given combination concurrently. The interrelated calibrations and linear dependence models are established and evaluated, and equations relating the change of glass network due to effects of cooling rate or uniaxial stress are given. Deutsche Forschungsgemeinschaft

The perovskite rare-earth titanates are model Mott insulators with magnetic ground states that are very sensitive to structural distortions. These distortions couple strongly to the orbital degrees of freedom and, in principle, it should be possible to tune the superexchange and the magnetic transition with strain. We investigate the representative system (Y,La,Ca)TiO$_3$, which exhibits low crystallographic symmetry and no structural instabilities. From magnetic susceptibility measurements of the Curie temperature, we demonstrate direct, reversible and continuous control of ferromagnetism by influencing the TiO$_6$ octahedral tilts and rotations with uniaxial strain. The relative change in $T_C$ as a function of strain is well described by ab initio calculations, which provides detailed understanding of the complex interactions among structural, orbital and magnetic properties in rare-earth titanates. The demonstrated manipulation of octahedral distortions opens up far-reaching possibilities for investigations of electron-lattice coupling, competing ground states, and magnetic quantum phase transitions in a wide range of quantum materials., 6 pages, 4 figures, nearly published version

Evolution of spectroscopic properties of a soda–lime silicate glass with different thermal history and under applied uniaxial stress was investigated using Raman and Brillouin spectroscopies as well as Nd3+ photoluminescence techniques. Samples of soda–lime silicate with a cooling rate from 6 × 10−4 to 650 K/min were prepared either by controlled cooling from the melt using a differential scanning calorimeter or by a conventional annealing procedure. Uniaxial stress effects in a range from 0 to −1.3 GPa were investigated in situ by compression of the glass cylinders. The spectroscopic observations of rearrangements in the network structure were related to the set cooling rates or the applied uniaxial stress to calculate an interrelated set of calibrations. Comparing the results from Raman and Brillouin spectroscopy with Nd3+ photoluminescence analysis, we find a linear dependence that can be used to identify uniaxial stress and cooling rate in any given combination concurrently. The interrelated calibrations and linear dependence models are established and evaluated, and equations relating the change of glass network due to effects of cooling rate or uniaxial stress are given.

Tallennetaan OA-artikkeli, kun julkaistu /KS The effect of uniaxial stress on iron losses of M400-50A grade non-oriented electrical steel sheets cut by punching process is experimentally studied. Samples cut along the rolling and transverse directions and having different number of cutting edges are used for this purpose. Measurements are carried out in the range of 10-100 Hz frequency of sinusoidal excitations at different magnetization levels under varying uniaxial stress by using a single sheet tester. The iron losses are obtained from the measurements and comparative analyses are made for different cases. The study shows that the effect of stress on the iron losses of the punched samples varies depending on the degradation level that the samples have after the cutting process. By considering this varying effect, when the combined effect of stress and punching is analysed, it is observed that the iron losses increased up to 55.2% under compressive stress. It is also observed that the increase in the losses due to the effect of cutting can be recovered by applying tensile stress.

The application of thermoplastics as load-carrying components in furniture applications is an attractive prospect at IKEA. Thermoplastics can be used instead of metal where advanced geometries and aesthetics are considered. In this thesis work in solid mechanics, it is of interest to investigate different test setups proposed in the literature for compression testing of thermoplastics while taking into account the complex stress fields induced in the test samples during the compression test. The simulations have showed that the main cause of complex stress fields in compression test specimens is the presence of friction between the specimen and the machine heads. Friction prevents perfect Poisson contractions during the test which shifts the orientation of the local stress tensors from the direction of loading. Out of the shapes investigated in this thesis, it has been concluded that cube-shaped specimens were the most suitable to utilize in compression tests. The results were based on exploiting measures that pertain to accuracy and robustness parameters within stress and strain. Furthermore, manufacturing impact and ease of test measurements were considered. Användningen av termoplaster som lastbärande komponenter i möbelapplikationer har ökat under de senaste åren inom IKEA. Då det förutom att vara estetiskt tilltalande för hela möbler är främsta användningen i komponenter som har mer komplicerade geometrier. I detta examensarbete i hållfasthetslära är det av intresse att undersöka olika provkroppar vilket föreslås i litteratur för kompressionsprovning av termoplaster med hänsyn till de komplexa spänningstillstånden som orsakas under provning. Simuleringarna har påvisat de komplexa spänningstillstånd i kompressionsprov som uppkommer på grund av friktion mellan provstaven och grepen. Friktion förhindrar perfekt tvärkontraktion, vilket förändrar orienteringen av de lokala spänningstensorerna från belastningsriktningen, vilket är enaxlig. Utifrån de former som undersöktes i detta examensarbete blev slutsatsen att kubformade provkroppar var de mest lämpliga att använda i kompressionsprovning. Resultaten baserades på att utnyttja åtgärder som avser noggrannhetsparametrar inom spänning och töjning men även tillverknings och provmätningens lätthet.

When Heisenberg spins interact antiferromagnetically on a triangular lattice and nearest-neighbor interactions dominate, the ground state is 120$^{\circ}$ antiferromagnetism. In this work, we probe the response of this state to lifting the triangular symmetry, through investigation of the triangular antiferromagnet PdCrO$_2$ under uniaxial stress by neutron diffraction and resistivity measurements. The periodicity of the magnetic order is found to change rapidly with applied stress; the rate of change indicates that the magnetic anisotropy is roughly forty times the stress-induced bond length anisotropy. At low stress, the incommensuration period becomes extremely long, on the order of 1000 lattice spacings; no locking of the magnetism to commensurate periodicity is detected. Separately, the magnetic structure is found to undergo a first-order transition at a compressive stress of $\sim$0.4 GPa, at which the interlayer ordering switches from a double- to a single-q structure., Comment: 9 pages, 5 figures, accepted by New Journal of Physics

In the modelling and simulation of reinforced concrete structures, accurate definition of material properties is important in terms of obtaining precise results. Among the parameters defining the behaviour of concrete in structural analyses, stress–strain behaviour has great importance. Since the stress–strain behaviour of concrete is difficult to determine experimentally, many relations have been proposed in which this behaviour is predicted numerically. However, it is seen that the most commonly used of these stress–strain relations are produced with a limited number of samples, and that most of the relations are based on test results obtained from a single laboratory. Accordingly, the stress–strain behaviour relations constituted in different studies for the same concrete are quite distant from each other. Therefore, there is a need for an inclusive and valid relation for low- and normal-strength concrete. In this study, a simple and realistic stress–strain relation for low- and normal-strength ordinary Portland cement concretes in compression was proposed by using stress–strain data obtained from different laboratories by different researchers. The proposed relation, which predicts stress–strain behaviour with relatively low error rates when compared with similar relations, is presented, with comparative graphs, to the benefit of researchers and designers.

Understanding of defect kinetics under stress fields is important for multiscale modeling of nuclear materials degradation. By means of molecular dynamics (MD) simulation, the formation and migration energies were evaluated for self-interstitial atom (SIA) and SIA clusters (1~3 interstitials) in alpha Fe. Effects of 0~3% uniaxial tensile [100] strains were tested for SIAs of and dumbbell configurations. Regarding the stability, the dumbbell configurations becomes more stabilized at larger strains and larger clusters. For the mobility, the diffusion of single SIA defects under tensile stresses were traced. Under the free-strain condition, the diffusivity of the SIA clusters has a gradual transition from three dimensional (3D) to one dimensional (1D) at saturated strain. The 1D transition was observed for large clusters and large strain while the 3D transition was for small clusters and lower strains and presented mainly for the SIA alignment configuration. Under the tensile stress and for small clusters, diffusivity enhancement is bigger at a higher temperature. However, the temperature effect was small for larger clusters. These effects of strain fields can be explained by elastic interaction between defects and applied stress fields.

Experimental investigations of THz emission from silicon (Si) doped by shallow donors under uniaxial stress along the [001] crystal axis upon selective intracenter excitation by an infrared free electron laser are presented. spectral dependences of Si output integral emission on the pump wavelength and Si laser output terahertz emission spectra have been analyzed. Two mechanisms of stimulated emission have been recognized as inversion-population-based lasing and stimulated Raman scattering. It is shown that uniaxial stress allows to control the Stokes shift and by this the output Raman emission frequency in the active medium. The obtained results demonstrate the feasibility of broad range THz tuning in doped silicon by means of uniaxial stress.

The most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, 𝑝ₓ ± 𝑖𝑝[sub]𝑦, whose degeneracy is protected by the lattice symmetry. When the lattice symmetry is lowered by the application of a symmetry-breaking field, the degeneracy can be lifted, potentially leading to a splitting of the superconducting transition. To lift the degeneracy, the symmetry breaking field used in this study is uniaxial stress. Uniaxial stress generated by a piezo-electric actuator can continuously tune the electronic structure and in situ lower the tetragonal symmetry in Sr₂RuO₄. Previous studies of magnetic susceptibility and resistivity under uniaxial stress have revealed that there is a strong peak in T[sub]c when the stress is applied along the a-axis of Sr₂RuO₄. In addition, it has been proposed that the peak in T[sub]c coincides with a van Hove singularity in the band structure, and measurements of 𝐻[sub]𝑐₂ at the maximum T[sub]c indicate the possibility of an even parity condensate for Sr₂RuO₄ at the peak in Tc. In this thesis, the heat capacity approach is used to study the thermodynamic behavior of Sr₂RuO₄ under uniaxial stress applied along the crystallographic a-axis of Sr₂RuO₄. The first thermodynamic evidence for the peak in T[sub]c is obtained, proving that is a bulk property. However, the experimental data show no clear evidence for splitting of the superconducting transition; only one phase transition can be identified within the experimental resolution. The results impose strong constraints on the existence of a second phase transition, i.e. the size of the second heat capacity jump would be small or the second T[sub]c would have to be very close to the first transition. In addition to these results, I will present heat capacity data from the normal state of Sr₂RuO₄. The experimental results indicate that there is an enhancement of specific heat at the peak in T[sub]c, consistent with the existence of the van Hove singularity. The possibility of even parity superconductivity at the maximum T[sub]c has also been investigated. However, the heat capacity measurements are shown to be relatively insensitive to such a change, so it has not been possible to obtain strong and unambiguous evidence for whether it takes place or not.

In view of possible practical applications of transparent crystals for mechanical stress sensors, theoretical investigation of piezo(elasto)optic effect in crystals might be useful for searching for proper materials, possessing large coefficients of elasto-optic effect, and for determination of uniaxial stress directions of maximum piezo-optical sensitivity. The influence of the uniaxial stresses on the electronic band structure, density of states, and optical properties of potassium sulphate crystal β-K2SO4 have been studied in the framework of the density functional theory using the ab initio CASTEP code. Increase in the band gap, Eg of the crystal takes place for three crystallographic directions and for moderate uniaxial stresses, σ < 1.5 GPa. Dependencies of principal refractive indices and polarizabilities upon principal uniaxial stresses are discussed together with other features of electronic structure of the crystal.

In the repertoire of an experimental condensed matter physicist, the ability to tune continuously through features in the electronic structure and to selectively break point-group symmetries are both valuable techniques. The experimental technique at the heart of this dissertation, uniaxial stress, can do both such things. The thesis will start with a thorough discussion of our new technique, which was continually developed over the course of this work, presenting both its unique capabilities and also some guidance on the best working practices, before moving on to describe results obtained on two different strongly correlated electron materials. The first, Sr₂RuO₄, is an unconventional superconductor, whose order parameter has long been speculated to be odd-parity. Of interest to us is the close proximity of one of its three Fermi surfaces to a Van Hove singularity (VHs). Our results strongly suggest that we have been able to traverse the VHs, inducing a topological Lifshitz transition. T[sub]c is enhanced by a factor ~2.3 and measurements of H[sub](c2) open the possibility that optimally strained Sr₂RuO₄ has an even-parity, rather than odd-parity, order parameter. Measurements of the normal state properties show that quasiparticle scattering is increased across all the bands and in all directions, and effects of quantum criticality are observed around the suspected Lifshitz transition. Sr₃Ru₂O₇ has a metamagnetic quantum critical endpoint, which in highly pure samples is masked by a novel phase. Weak in-plane magnetic fields are well-known to induce strong resistive anisotropy in the novel phase, leading to speculation that a spontaneous, electronically driven lowering of symmetry occurs. Using magnetic susceptibility and resistivity measurements we can show that in-plane anisotropic strain also reveals the strong susceptibility to electronic anisotropy. However, the phase diagram that these pressure measurements reveal is consistent only with large but finite susceptibility, and not with spontaneous symmetry reduction.

Nonequilibrium process of shock-wave loading of aluminum nanocrystals were investigated numerically under uniaxial stress conditions using molecular dynamics simulations (MD). Deformation curves, time dependence of strain rate and the change in potential energy were constructed. Calculated properties of aluminium using embedded atom interatomic potential are compared with each other. Relationship between the calculated characteristics are discussed. The behaviour of deformation curves for two different aluminium nanosrystals is explained. The problems of modeling the deformation hardening of aluminum nanocrystals are considered.

In this study, effects of uniaxial stress on the dielectric properties of ceramics in PMN-PZT system were investigated. The ceramics with a formula (1-x)Pb(Mg1/3Nb2/3)O3-(x)Pb(Zr0.52Ti0.48)O3 or (1-x)PMN-(x) PZT when x = 0.0 0.3 0.5 0.7 and 1.0 were prepared by a conventional mixed-oxide method. Phase formation behavior of these ceramics was studied by an X-ray diffraction (XRD) method. In addition, other physical properties, e.g. firing shrinkage and density, were measured. The dielectric properties under the uniaxial stress of the poled PMN-PZT ceramics were observed at low and high-stress levels using a uniaxial compressometer. It was found that at low stress level (0-800 kPa) there was no significant change in the dielectric constant and the dielectric loss tangent with the applied stress. As the stress level was raised higher (0-5 MPa), changes of the dielectric constant with the applied stress were dependent on the ceramic compositions. On the other hand, changes of the dielectric loss tangent with the applied stress were independent of the ceramic compositions.

This paper is to commemorate the work of Leszek Dobaczewski 1 who devoted much of his life to the development and application of high resolution DLTS. Under good experimental conditions Laplace DLTS provides an order of magnitude higher energy resolution than conventional DLTS techniques. This has had a profound effect on electrical defect spectroscopy enabling the effect of external probes, such as uniaxial stress, and internal perturbations, such as the proximity of atoms isovalent with the host, to be quantified in terms of electronic behaviour. Laplace DLTS provides a synergy with other techniques that was difficult or impossible to achieve previously. In this paper we present an overview of the development of LDLTS and illustrate some of its uses by describing its application in a number of key areas of defect research. Leszek Dobaczewski was born on 25th December 1954. He received his education in Warsaw taking his PhD in 1986 with Jerzy Langer at the Institute of Physics on Recombination Processes at defects with the large lattice relaxation. He held a research position at the institute in Warsaw until he came to Manchester in 1990 and thereafter alternated between Manchester and Warsaw. He worked primarily on the development and application of high resolution DLTS. He was awarded the degree of DSc in 1994 for his work on DX centres and held an appointment as full professor in Warsaw with Visiting Professor posts at Manchester and Aarhus. © 2011 Elsevier B.V. All rights reserved.

This paper is to commemorate the work of Leszek Dobaczewski1 who devoted much of his life to the development and application of high resolution DLTS.Under good experimental conditions Laplace DLTS provides an order of magnitude higher energy resolution than conventional DLTS techniques. This has had a profound effect on electrical defect spectroscopy enabling the effect of external probes, such as uniaxial stress, and internal perturbations, such as the proximity of atoms isovalent with the host, to be quantified in terms of electronic behaviour. Laplace DLTS provides a synergy with other techniques that was difficult or impossible to achieve previously. In this paper we present an overview of the development of LDLTS and illustrate some of its uses by describing its application in a number of key areas of defect research.Leszek Dobaczewski was born on 25th December 1954. He received his education in Warsaw taking his PhD in 1986 with Jerzy Langer at the Institute of Physics on “Recombination Processes at defects with the large lattice relaxation”. He held a research position at the institute in Warsaw until he came to Manchester in 1990 and thereafter alternated between Manchester and Warsaw. He worked primarily on the development and application of high resolution DLTS. He was awarded the degree of DSc in 1994 for his work on DX centres and held an appointment as full professor in Warsaw with Visiting Professor posts at Manchester and Aarhus.

Обчислено радіус екранування для та - мінімумів зони провідності кристалівз різною концентрацією домішки. Отримано залежності екрануючого множника від концентрації домішки для відповідних мінімумів. Показано, що інверсія типу абсолютного мінімуму зони провідності, яка обумовлена одновісним деформуванням ( 2,8 ГПа) вздовж кристалографічного напряму [100], змінює як радіус екранування, так і екрануючий множник. Чутливість даних параметрів при переході від недеформованих до сильно деформованих кристалів пояснюється різним значенням ефективної маси густини станів для та - мінімумів., Определён радиус экранирования для и - минимумов зоны проводимости кристаллов с различной концентрацией примеси. Получены зависимости экранирующего множителя от концентрации примеси для соответствующих минимумов. Показано, что инверсия типа абсолютного минимума зоны проводимости , обусловленная одноосным деформированием ( 2,8 ГПа) вдолькристаллографического направления [100], меняет как радиус экранирования, так и экранирующий множитель. Чувствительность данных параметров при переходе от недеформированных до сильно деформированных кристаллов объясняется различным значением эффективной массы плотности состояний для и - минимумов., The screening radius was calculated for the - and - minimums of the conduction band of crystals with different impurity concentration. The dependences of screening factor on the impurity concentration were obtained for the corresponding minimums. It was shown that the absolute minimum type inversion of the conduction band, caused by uniaxial stress ( 2.8 GPa) along the crystallographic direction [100], changes the radius of screening as well as the screening factor. Sensitivity of the parameters to pressure under the transition from undeformed to strongly deformed crystal is explained as being caused by different effective mass values of states density for - and - minimums.

En esta tesis se presenta una ecuación que modela el comportamiento tenso-deformacional de cualquier material sometido a ensayos de tensión uniaxial bajo cualquier tasa de incremento de la tensión o la deformación y a cualquier temperatura, por lo que puede considerarse como modelo universal de comportamiento uniaxial, en la que los modelos clásicos -lineal, perfectamente plástico, elástico-perfectamente plástico o los que obedecen una ley potencial- se contemplan como simples casos límite de dicha ecuación. Consecuentemente, la ecuación representa una única línea continua y derivable en todos sus puntos que no necesita ser dibujada a trozos para caracterizar las distintas fases o puntos singulares que aparecen, por ejemplo, en las curvas de los aceros de construcción a temperatura ambiente, incluyendo los distintos picos de fluencia (superior e inferior), la meseta plástica o incluso el efecto Portevin-Le Chatelier que presenta a menudo dicha meseta plástica., This thesis presents an equation that govems the stress-strain behavior of any material subjected to uniaxial stress tests under any stress or strain increase rates and under any temperature, thus it can be considered as a universl:l.l model of uniaxial behavior, in which the classical models - lineal, perfectly plastic, elastic-perfectly plastic or those which obey a power law- are regarded as simple limiting cases of such equation. Consequently, the equation represents one line only, which is continuous and differentiable at every point and it is not necessary to draw it separately in sections to characterize the different phases or singular points that appear, for example, in the curves of building steels at room temperature, including the different yield peaks (upper and lower), plastic plateau or even the Portevin-Le Chatelier effect that such plateau often presents.

In the lithium-caesium sulphate crystal in the temperature range of ferroelastic phase transition, the uniaxial stress σ X induced changes in velocity and attenuation of the longitudinal ultrasonic wave propagating in the direction [010] are studied. The phase transition is close to the three-critical point and the critical exponent is κ = 0.27 ± 0.02. The stress applied drastically decreases the stepwise change in the wave velocity at T C up to its disappearance at 2 MPa. In the temperature range between T C and T C − 6 K, the stress leads to an increase in the wave velocity and a decrease in its attenuation. This range was interpreted as that of co-existence of ferroelastic and incommensurate ordering, in which the stress influences the density of solitons leading to stiffening of the crystal lattice.

Effect of uniaxial stress application to Fe-Si-B amorphous metal has been investigated below and above glass transition temperature. Uniaxial stress effect was characterized, as a magnetic permeability measurement, by using Magnetic Permeability Resonance (MPR) method. The values of horizontal and vertical permeabilities from stress applied direction were measured. Amorphous metal, which is heat treated at near glass transition temperature, showed significant anisotropy of permeabilities with stress application. This anisotropy is the result from atomic structural change, which is caused by viscosity flow under amorphous state.

In order to design and optimize high-sensitivity silicon nanowire-field-effect transistor (SiNW FET) pressure sensors, this paper investigates the effects of channel orientations and the uniaxial stress on the ballistic hole transport properties of a strongly quantized SiNW FET placed near the high stress regions of the pressure sensors. A discrete stress-dependent six-band k.p method is used for subband structure calculation, coupled to a two-dimensional Poisson solver for electrostatics. A semi-classical ballistic FET model is then used to evaluate the ballistic current-voltage characteristics of SiNW FETs with and without strain. Our results presented here indicate that [110] is the optimum orientation for the p-type SiNW FETs and sensors. For the ultra-scaled 2.2 nm square SiNW, due to the limit of strong quantum confinement, the effect of the uniaxial stress on the magnitude of ballistic drive current is too small to be considered, except for the [100] orientation. However, for larger 5 nm square SiNW transistors with various transport orientations, the uniaxial tensile stress obviously alters the ballistic performance, while the uniaxial compressive stress slightly changes the ballistic hole current. Furthermore, the competition of injection velocity and carrier density related to the effective hole masses is found to play a critical role in determining the performance of the nanotransistors.

A compact model of the low-field effective carrier mobility is developed for n- and p-type MOSFETs with either polySi or TiN gate, ultrathin SiO2/HfO2 gate stacks, and silicon under biaxial or uniaxial stress conditions. Physical insights, theoretical analyses and experimental investigations are used to develop and accurately calibrate the model.

The spontaneous electrification of marble samples was studied while they were subjected to uniaxial stress. The Pressure Stimulated Current (PSC) technique was applied to measure the charge released from compressed Dionysos marble samples, while they were subjected to cyclic loading. The experimental results demonstrate that, in the linear elastic region of the sample, no PSC is recorded, while beyond the stress limit (s>0.60), observable variations appear, which increase considerably in the vicinity of sample failure, reaching a maximum value just before the failure. The emitted current is reduced on each loading cycle and it has a reciprocal dependence to the normalized Young modulus. The MCD model, applied out of the vicinity of sample failure explains successfully the above findings. The existence of a "memory-like" behavior of the sample, could justify the weakness or absence of electrical earthquake precursors, during an aftershock sequence.

Failure criteria play a vital role in the numerical analysis of reinforced concrete structures. The current failure criteria can be classified into two types, namely the empirical and theoretical failure criteria. Empirical failure criteria normally lack reasonable theoretical backgrounds, while theoretical ones either involve too many parameters or ignore the effects of intermediate principal stress on the concrete strength. Based on the octahedral shear stress model and the concrete tensile strength under the state of triaxial and uniaxial stress, a new failure criterion, that is, the simplified unified strength theory (UST), is developed by simplifiing the five-parameter UST for the analysis of reinforced concrete structures. According to the simplified UST failure criterion, the concrete strength is influenced by the maximum and intermediate principal shear stresses together with the corresponding normal stresses. Moreover, the effect of hydrostatic pressure on the concrete strength is also taken into account. The failure criterion involves three concrete strengths, namely the uniaxial tensile and compressive strengths and the equal biaxial compressive strength. In the numerical analysis, a degenerated shell element with the layered approach is adopted for the simulation of concrete structures. In the layered approach, concrete is divided into several layers over the thickness of the elements and reinforcing steel is smeared into the corresponding number of layers of equivalent thickness. In each concrete layer, three-dimensional stresses are calculated at the integration points. For the material modelling, concrete is treated as isotropic material until cracking occurs. Cracked concrete is treated as an orthotropic material incorporating tension stiffening and the reduction of cracked shear stiffness. Meanwhile, the smeared craclc model is employed. The bending reinforcements and the stirrups are simulated using a trilinear material model. To verify the correctness of the simplified UST failure criterion, comparisons are made with concrete triaxial empirical results as well as with the Kupfer and the Ottosen failure criteria. Finally, the proposed failure criterion is used for the flexural analysis of simply supported reinforced concrete beams. Also conducted are the punching shear analyses of single- and multi-column-slab connections and of half-scale flat plate models. In view of its accuracy and capabilities, the simplified UST failure criterion may be used to analyse beam- and slab-type reinforced concrete structures.

Brick masonry assemblage composed of brick mass, head joint, and bed joint is subjected to unlade, compressive stress regime. The compressive stress is increased until failure occurs as the bed joint is rotated through O x/2. The results of this test compare favorably with previous finite element simulation. The orthogonal plane of weakness theory, a modified version of a similar phenomenon found in rock mechanics is now applied to masonry assemblages.

The emission spectra of microring lasers fabricated from π -conjugated polymer films casted on nylon microfibers with diameters in the range of 35-90 μm were studied upon application of uniaxial stress with strain up to ∼12%. The laser emission spectra substantially change with the applied stress, showing enhanced sensitivity to stress over changes induced in the fiber diameter alone. This is explained as due to the induced change in the polymer refractive index spectrum upon stress, causing an unexpected increase in the refractive index dispersion and, consequently, also in the effective refraction index for lasing at emission wavelengths. © 2007 American Institute of Physics.

The internal structure of free and bound exciton luminescence and reflectance in n and p-type CdTe is investigated through the Zeeman effect, application of uniaxial stress, optical spin orientation and lifetime studies. The doublet stress splitting observed in each component of the bound excitons is contrasted with that of the free exciton. The different stress splitting and polarization of various components of the bound excitons is compared with that of the Zeeman pattern in order to interpret the origin of the luminescence lines. A comparison of the spin orientation and lifetimes between all exciton lines is made. For the free exciton, its energy dependence is discussed on the polariton model.

Nous avons mesure la variation avec les contraintes uniaxiales de la temperature de Neel du terbium et de la temperature de transition ferromagnetique-helimagnetique du dysprosium. Dans une etude preliminaire nous avons determine par photoelasticite la repartition des contraintes a l'interieur d'echantillons en resine photoelastique de forme cubique ou spherique.