• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 7
  • 1
  • Tagged with
  • 10
  • 10
  • 4
  • 4
  • 4
  • 4
  • 4
  • 4
  • 4
  • 3
  • 2
  • 2
  • 2
  • 2
  • 2
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Heat capacity measurements of Sr₂RuO₄ under uniaxial stress

Li, You-Sheng January 2018 (has links)
The most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, pₓ ± p[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 Hc₂ 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.

Uniaxial stress technique and investigations into correlated electron systems

Barber, Mark E. January 2017 (has links)
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.

Uniaxial Pressure Studies of the Unconventional Superconductor Sr₂RuO₄

Jerzembeck, Fabian 09 January 2024 (has links)
This thesis concentrates on the effect of Lifshitz transitions and associated Van Hove singularities on the superconducting state of Sr₂RuO₄. I will start by giving a short summary of the unconventional superconducting state of Sr₂RuO₄, and discuss how a Lifshitz transition can be accessed using uniaxial pressure. I will then discuss recent results of measurements under uniaxial stress, which have changed our view of the nature of the superconductivity of this material considerably.

Punching Shear Failure Analysis of Reinforced Concrete Flat Plates Using Simplified Ust Failure Criterion

Zhang, Xuesong, n/a January 2003 (has links)
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.

Electrical Properties of n-MOSFETs under Uniaxial Mechanical Strain

Tsai, Mei-Na 18 January 2012 (has links)
Metal-oxide-semiconductor field-effect transistors (MOSFETs) are major devices inintegrated circuit, extensively used in various electronic products. In order to improve the electrical characteristics, scaling channel width and length, using high-£e gate dielectric insulator, and strained silicon may be utilized to increase the driving current and circuit speed. Nevertheless, the scaling of the channel width and length must overcome the limitation of the photolithographytechnology and cost. Once the method is employed, the MOSFETs will face a serious short-channel effect and gate leakage current. In the aspect of high-£e gate dielectric insulator, there still have problems, containing the trap states, phonon scattering, dipole-induced threshold voltage variation, needed to be solved. This dissertation focuses on the properties of MOSFETs experienced an external-mechanical strain, where the channel will be strained. Hence, the mobility, driving current, and circuit speed will increase. Our research can be divided into three topics: fabricating process-induced strained Si, external mechanical stress-induced strained Si, and the properties of strained Si MOSFETs at different temperatures. Except the electrical measurement, we also used the ISE-TCAD to simulate the electrical characteristic of MOSFETs under stress. Firstly, we apply the stress on n-MOSFETs by utilizing the nitride-capping layer. Once the lattice is strained, the mobility will increase, hence resulting in the operating speed. Secondly, the electrical characteristics under external stress is explored by introduced the external mechanical stress along the channel length of nMOSFETs. In addition to the fabricating process-induced strain, the fabricating process condition will also influence the device characteristics. As a result, we propose a new strain technology for our following research. Thirdly, the device performance of strained Si under different temperatures is investigated. Finally, we discuss the gate leakage current in strained Si depending on the ultra-thin gate oxide layer.

Manipulation of time reversal symmetry breaking superconductivity in Sr₂RuO₄ by uniaxial pressure

Ghosh, Shreenanda 30 September 2021 (has links)
Unconventional superconductivity continues to be one of the most striking chapters in condensed matter physics, by posing challenges to our theoretical understanding of its origin. During the last three decades a large number of unconventional superconductors with exotic properties have been found arising great interest, such as the heavy fermion systems, high Tc cuprates as well as the Iron based superconductors etc. Sr2RuO4, the material I have studied, can be considered as an exemplary case in this regard. In spite of more than two decades of comprehensive research, Sr2RuO4 remains one of the most compelling superconductors till date. Various experimental results give evidence that the superconductivity of Sr2RuO4 is chiral: including measurements of the Kerr effect, sound velocities, critical currents across junctions, and muon spin relaxation(μSR), the experimental technique at the heart of this dissertation. Recent NMR Knight shift measurements suggests that the pairing is most likely spin-singlet, and in the tetragonal lattice of Sr2RuO4, the combination of singlet pairing and chirality compels consideration of an seemingly unlikely order parameter: dxz ± idyz. It is unlikely because it comes along with a horizontal line node at kz = 0, whereas Sr2RuO4 has a very low c-axis conductivity. And that makes the question whether or not the superconductivity of Sr2RuO4 is chiral, of great importance. This calls for an unique scenario in regard to our understanding of unconventional superconductivity, as the presence of chirality in Sr2RuO4 might imply a new form of pairing, which is yet to be firmly determined. Chiral superconductors break time reversal symmetry by definition, and in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two component order parameters. Probing Sr2RuO4 under uniaxial pressure offers the possibility to lift the degeneracy between such components. However, despite strenuous efforts, a splitting of the superconducting and TRSB transitions under uniaxial pressure has not been observed so far. In this thesis, I report muon spin relaxation measurements on Sr2RuO4 samples, placed under uniaxial stress. The relatively large sample size suitable for μSR demanded for a customized uniaxial pressure cell in order to perform our experiments. It has been a technically challenging task to have a fully fledged uniaxial pressure cell with stringent requirements, that is suitable for time restricted facility experiments like μSR. The technical advancement has been documented thoroughly in this thesis. Using the dedicated uniaxial pressure cell, we observed the much awaited stress induced splitting between the onset temperatures of superconductivity and time reversal symmetry breaking, consistent with the qualitative expectations for a chiral order parameter in Sr2RuO4. In addition to that, we report the appearance of a bulk magnetic order in Sr2RuO4 under higher uniaxial stress for the first time, above the critical pressure at which a Lifshitz transition is known to occur. The signal in the state appearing at high stress qualitatively differs from that in the TRSB state in unstressed Sr2RuO4, which provides evidence that the enhanced muon spin relaxation at lower stresses is not a consequence of conventional magnetism. As a whole, our results strongly support the idea of two-component superconducting order parameter in Sr2RuO4, that breaks time-reversal symmetry.

Probing Hund’s-Metal Physics through the Hall Effect in Microstructured Sr₂RuO₄ under Uniaxial Stress

Yang, Po-Ya 01 April 2022 (has links)
Uniaxial stress is a powerful technique to tune the electronic structure of very pure materials. The novel piezoelectric-based techniques developed by our group, which allow application of large and homogeneous uniaxial pressure in a continuously-tunable manner, make uniaxial pressure an independent axis in the parameter space for the study of quantum materials. Many exciting experiments have been performed that combine different measurement methods with this uniaxial stress technique in the past few years. In this thesis, I demonstrate the first electrical transport measurement under uniaxial pressure of a free-standing microstructure single-crystalline sample patterned by focused ion beam (FIB) milling. With the microstructuring technique that I developed, the transport properties transverse to the force direction can be more accurately probed. The ability to resolve the anisotropy introduced by the uniaxial pressure lets us have a better understanding of how the electronic structure of Sr₂RuO₄ changes under uniaxial stress. Moreover, the microstructure technique opens new roads for smaller crystals (∼ 100 µm) to be studied under uniaxial pressure. In addition, higher stresses and better sample homogeneity could be achieved by working with smaller samples. For Sr₂RuO₄, one of the three Fermi-surface sheets can be driven through a Lifshitz transition by applying uniaxial stress along the [100] direction. Superconductivity and resistivity have been observed to be strongly enhanced at the singularity. In addition, a spin-density wave (SDW) has been observed at stresses beyond the Lifshitz transition. Measurement of the Hall effect under uniaxial stress allows us to probe Hund’s metal physics in Sr₂RuO₄. The Hall coefficient of unstressed Sr₂RuO₄ goes through two sign reversals, at 30 K and 120 K. Under the Hund’s metal scenario, this temperature dependence has been proposed to result from orbital differentiation of the inelastic scattering rate, which is a key property expected of Hund’s metals. In the present study, it is shown that at a temperature where electron-electron scattering dominates (≳ 5 K), the Hall coefficient becomes less electron-like while approaching the VHS, which is consistent with increased scattering in the d_xy band. Beyond the transition, the Hall coefficient becomes much more electron-like, which is opposite to expectations from the change in Fermi surface topology, but can be explained by a combination of Hund’s metal physics and strong suppression in the d_xy scattering rate. At very low temperature (0.5 K), the Hall coefficient is essentially unchanged across the Lifshitz transition, despite the change in the Fermi-surface topology. In contrast to the longitudinal resistivity that has a strong peak at the VHS but does not respond to the SDW, the resistance transverse to the force direction shows a strong response to the SDW, but only a small response at the VHS. In addition, I obtain ρ(T) at the Lifshitz transition below Tc by subtracting off the magnetoresistance and find that T² ln(1/T) fits better than T^3/2, which suggests a saddle point rather than an extended saddle point at the VHS.:1. Introduction to Sr2RuO4 1.1. Normal-State Properties Van Hove Singularity and Lifshitz Transition in Sr2RuO4 1.2. Hall Effect in Sr2RuO4 Weak-field Hall Coefficient Experimental Hall Coefficient in Sr2RuO4 and Related Systems 1.3. Hund’s Metal Scenario Dynamical Mean-Field Theory Experimental Evidence for Orbital Differentiation in Sr2RuO4 Hall Coefficient of Sr2RuO4 within Hund’s Metal Scenario 1.4 Uniaxial-Pressure Projects on Sr2RuO4 2. Experimental Setup 2.1. Stress and Strain 2.2. Uniaxial Stress Technique Uniaxial-Stress Cell Sample Carrier 2.3. Imperfections of the Stress Cells 2.4. Sample Preparation Needle Sample Preparation Microstructure Sample Preparation Comparison of the Two Samples 2.5. Measurement Setup 3He Cryostat Transport Measurement Setup 3. Hall Coefficient and Resistivity Measurements 3.1. Basics of Resistivity Measurement Stress Ramps 3.2. Basics of Hall Measurement Setup Field Dependence of Hall Resistivity Temperature Dependence of Hall Coefficient 3.3. Stress Ramps under Constant Magnetic Field 3.4. Stress Dependence of Hall Coefficient and Resistivity 3.5. Resistivity Measurements below Tc 3.6. Field Sweeps within the Magnetic Phase 3.7. Summary 4. Measurements Transverse to the Stress Axis 4.1. Setup for Transport Measurements Transverse to the Uniaxial Stress 4.2. Simulations Based on Finite Element Method 4.3. Resistance Measurements Transverse to Applied Stress 4.4. Summary 5. Data Analysis and Discussion 5.1. A Tight-Binding Model under Uniaxial Pressure 5.2. Analysis of Hall Coefficient across the Lifshitz Transition Hall Coefficient Analysis under the Isotropic-l or Isotropic-τ Approximations Hall Coefficient Analysis under Hund’s Metal Scenario 5.3. Magnetoresistance Subtraction in Temperature Ramps 5.4. Transport Properties at 5 K 5.5. Summary 6. Conclusions and Outlook Appendices A. Si-Gap-Platform Microstructure Project A.1. Si-Gap Platform A.2. Sample Preparation with PFIB-Microstructuring A.3. Microstructure Stress Cells B. Other results B.1. Hall Effect from the Hall Pair 2 B.2. Magnetoresistance in Longitudinal and Transverse Configurations B.3. Toward -1.5 GPa B.4. Comparison of RH(T) in Sr2RuO4 Compressed along [100] Direction and YBa2Cu3O6.67 Compressed along the b-axis Bibliography

The mechanochemistry in heterogeneous reactive powder mixtures under high-strain-rate loading and shock compression

Gonzales, Manny 07 January 2016 (has links)
This work presents a systematic study of the mechanochemical processes leading to chemical reactions occurring due to effects of high-strain-rate deformation associated with uniaxial strain and uniaxial stress impact loading in highly heterogeneous metal powder-based reactive materials, specifically compacted mixtures of Ti/Al/B powders. This system was selected because of the large exothermic heat of reaction in the Ti+2B reaction, which can support the subsequent Al-combustion reaction. The unique deformation state achievable by such high-pressure loading methods can drive chemical reactions, mediated by microstructure-dependent meso-scale phenomena. Design of the next generation of multifunctional energetic structural materials (MESMs) consisting of metal-metal mixtures requires an understanding of the mechanochemical processes leading to chemical reactions under dynamic loading to properly engineer the materials. The highly heterogeneous and hierarchical microstructures inherent in compacted powder mixtures further complicate understanding of the mechanochemical origins of shock-induced reaction events due to the disparate length and time scales involved. A two-pronged approach is taken where impact experiments in both the uniaxial stress (rod-on-anvil Taylor impact experiments) and uniaxial strain (instrumented parallel-plate gas-gun experiments) load configurations are performed in conjunction with highly-resolved microstructure-based simulations replicating the experimental setup. The simulations capture the bulk response of the powder to the loading, and provide a look at the meso-scale deformation features observed under conditions of uniaxial stress or strain. Experiments under uniaxial stress loading reveal an optimal stoichiometry for Ti+2B mixtures containing up to 50% Al by volume, based on a reduced impact velocity threshold required for impact-induced reaction initiation as evidenced by observation of light emission. Uniaxial strain experiments on the Ti+2B binary mixture show possible expanded states in the powder at pressures greater than 6 GPa, consistent with the Ballotechnic hypothesis for shock-induced chemical reactions. Rise-time dispersive signatures are consistently observed under uniaxial strain loading, indicating complex compaction phenomena, which are reproducible by the meso-scale simulations. The simulations show the prevalence of shear banding and particle agglomeration in the uniaxial stress case, providing a possible rationale for the lower observed reaction threshold. Bulk shock response is captured by the uniaxial strain meso-scale simulations and is compared with PVDF stress gauge and VISAR traces to validate the simulation scheme. The simulations also reveal the meso-mechanical origins of the wave dispersion experimentally recorded by PVDF stress gauges.

Uniaxial-stress response, electron-phonon interaction, and magnetic interactions in topological semimetals and narrow-gap semiconductors

Schindler, Clemens 24 November 2021 (has links)
Materialien mit einer geringen, aber endlichen Zahl an beweglichen Ladungsträgern bieten eine interessante Plattform für die experimentelle Erforschung von niederenergetischen elektronischen Anregungen. Derartige Halbmetalle und Halbleiter mit geringer Bandlücke zeigen starke Effekte in Magnetfeldern, wie z. B. Quantenoszillationen und Magnetwiderstandseffekte, welche ein hilfreiches Werkzeug zur Untersuchung der elektronischen Eigenschaften darstellen. In Kombination mit verschiedenen experimentellen Techniken wie elektrischen und thermischen Transportmessungen, der Anwendung uniaxialer Spannung, und Ultraschallmessungen, kann man umfassende Informationen über die Wechselwirkungen und Symmetriebeziehungen in solch einem Material gewinnen. In letzter Zeit sind vor allem die topologischen Eigenschaften der elektronischen Bänder in den Fokus der Festkörperphysik gerückt, deren Beitrag zu den Transporteigenschaften insbesondere in Halbmetallen und Halbleitern mit geringer Bandlücke zu klären ist. In der vorliegenden Dissertation wurden drei solcher Materialien hinsichtlich ihrer außergewöhnlichen elektronischen Eigenschaften untersucht. In NbP, einem Halbmetall mit komplex geformter, anisotroper Fermi-Fläche, welche aus mehreren räumlich entarteten Taschen besteht, wurden die Effekte der Gitterdeformation untersucht. Die Anwendung uniaxialer Spannung führt zur Brechung der Kristallsymmetrie und damit zur Aufhebung der räumlichen Entartung der Fermi-Taschen, was mittels Analyse der Shubnikov-de Haas-Oszillationen im Magnetwiderstand nachgewiesen werden konnte. Weiterhin konnte durch Messung der im Ultraschall auftretenden Quantenoszillationen eine genaue Untersuchung der Anisotropie der Elektron-Phonon-Wechselwirkung durchgeführt werden. ZrTe5 ist ein aus zweidimensionalen Schichten bestehender Halbleiter mit geringer Bandlücke, welcher kürzlich aufgrund seiner besonderen Niedrigtemperatur-Magnetotransporteigenschaften größere Aufmerksamkeit erfahren hat. So weist ZrTe5 plateau-ähnliche Features im Hall-Widerstand, sowie einen ungewöhnlichen Magnet- und Hall-Widerstand im Quanten-Limit auf. Im Rahmen dieser Arbeit wurde der Effekt uniaxialer Spannung auf diese Transportphänomene untersucht, was dazu beitragen kann, deren bislang umstrittene Ursache aufzuklären. Schließlich wurden die elektrischen und thermischen Magnetotransporteigenschaften von GdPtBi untersucht, einem Halbleiter mit geschlossener Bandlücke, welcher sich durch das Vorliegen starker, lokalisierter magnetischer Momente ausgehend von den 4f-Elektronen des Gd auszeichnet. Es konnte gezeigt werden, dass das Auftreten von Anomalien im elektrischen Magnetotransport, welche ursprünglich den topologischen Eigenschaften der im Magnetfeld gekreuzten elektronischen Bänder zugeschrieben wurden, auch durch magnetische Wechselwirkungen zu erklären ist. Desweiteren konnte durch die Messung magnetfeldabhängiger thermischer Transporteigenschaften das Auftreten von Wechselwirkungen zwischen Phononen und magnetischen Momenten, sowie möglicherweise auch magnetischen Spinwellen, nachgewiesen werden. / Materials with a low, but finite density of charge carriers offer an interesting experimental platform for the investigation of electronic low-energy excitations. Such semimetals and narrow-gap semiconductors exhibit large magnetic-field responses, e.g., quantum oscillations (QOs) and magnetoresistance (MR) effects, that can be used as a powerful tool to study the electronic properties. In combination with experimental techniques such as electrical- and thermal-transport measurements, uniaxial-stress application, and measurement of the ultrasound velocity, a lot can be learned about the interactions and symmetry dependences in the materials. Recently, the topological properties of electronic bands became an important research field in condensed matter physics. Especially in semimetals and narrow-gap semiconductors, it is to be clarified to what extent exotic transport phenomena are related to topological effects. In this thesis, three such materials with intriguing electronic properties have been investigated. In NbP, a semimetal with a complex, anisotropic Fermi surface, consisting of spatially degenerate pockets whose degeneracy is tied to the symmetry of the crystal lattice, the effects of lattice deformation have been studied. Application of uniaxial stress breaks the crystalline symmetries and, thereby, lifts the degeneracy of the Fermi-surface pockets, which could be traced via analyzing Shubnikov-de Haas oscillations in the MR. Furthermore, the measurement of QOs in the ultrasound allowed for a detailed analysis of the anisotropy of the electron-phonon interaction in NbP. ZrTe5 is a layered narrow-gap semiconductor that recently attracted a lot of attention due to its remarkable low-temperature magnetotransport, namely plateau-like features in the Hall resistance as well as unconventionalMRand Hall resistance in the quantum limit. Here, the uniaxial-stress response of those features was investigated as a contribution to clarify their origin, which, to date, remains under discussion. Lastly, the electrical and thermal magnetotransport properties of GdPtBi were studied. GdPtBi is a zero-gap semiconductor that features the presence of large localized magnetic moments stemming from Gd’s 4 𝑓 -electron shell. The occurrence of anomalous features in the electrical MR was previously attributed to the topological properties of magnetic-field induced crossings of the electronic bands. However, in the course of this thesis it could be shown that those features can also be explained by magnetic interactions. Further, the presence of interactions between phonons and magnetic moments, and potentially also between phonons and magnetic spin waves, was demonstrated via measurement of a magnetic-field-dependent thermal resistance.

Modelling and Evaluation of the Methods for Compression Testing of Thermoplastics / Modellering och utvärdering av metoder för kompressionsprovning av termoplaster

Itani, Abdul Rahman January 2021 (has links)
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.

Page generated in 0.0719 seconds