Themes of PhD Thesis

5.2.48 "Physical Engineering"

  • Theme: Composite materials with controlled physical properties

    Supervisor: doc. Ing. F. Gömöry, DrSc. ( Department of Superconductor Physics )
    Abstract: Reasearch will focus on development of preparation technologies for the composites containing powder material in a suitable binder. The use is foreseen for two different applications: the ferromagentic material for a magnetic invisibility cloak, and the cover layer with suitable thermal properties for tapes considered in the superconducting faule current limiter.Investigation of procedures for manufacturing of composites with reproducible properties will be combined with the study of relation between the structural characteristic of the starting powder (distribution of grain sizea and shapes) and the physical properties of the composite.
    The goal is to establish the procedures allowing to prepare the material with requested essential property. For the first application mentioned above this is the magentic permeability, in case of the second one these are the thermal conductivity and specific heat.
     
  • Theme: Influence of mechanical deformation on the electrical properties of high-temperature superconductor tapes

    Supervisor: doc. Ing. F. Gömöry, DrSc. ( Department of Superconductor Physics )
    Abstract: Research will focus of the influence of mechanical deformation experienced by the tapes from high temerature supercondcutors during a cabling process on the electric properties.Experimental part of activities will consist in the developement of apparatus for in situ testing of critical current, i.e. at the temperature of liquid nitrogen boilig, during the deformation process. Results of experiments will be confronted with theoretical predictions obtained with help of numerical methods.
    The goal is to establish the recommendations for the cable architecture as well as the parameters of the cabling process necessary to avoid an excessive degradation of properties.
     
  • Theme: Thermal stability of composite MgB2 superconductors at high current densities

    Supervisor: Ing. P. Kováč, DrSc. ( Department of Superconductor Physics )
    Abstract: Utilization of MgB2 superconductors is the most actual for the range of medium and low magnetic fields (1-5 T) and temperature around 20 K (MRI magnets, generators and engines), where the transport currents are very high and requirement of thermal stability especially needed. Therefore, thermal stability of composite MgB2 superconductors with well-conductive elements (Cu, Al) will be modelled and studied experimentally at low temperatures (4.2-20K). Current-voltage characteristics will be measured far above the level of critical current and also generated heath and critical warming of differently stabilized MgB2 wires and cables will be examined by pulse currents. Obtained results allows to optimize the design of MgB2 superconductor for well stable working conditions. 
  • Theme: Technology of advanced and homogeneous superconductors with ceramic fibres

    Supervisor: Ing. P. Kováč, DrSc. ( Department of Superconductor Physics )
    Abstract: High temperature superconducting materials discovered in the last decades have dominantly ceramic character (BSCCO, magnesium boride  and iron pniktides). For practical applications of these materials they have to be integrated into composite wires consisting of several metallic sheaths, which requires a special demands on the deformation process of composite components having different mechanical properties and also on the heat treatment conditions with used barriers to avoid interactions between aby filaments and sheaths.  In the frame of present topic a composite wires with superconducting and highly uniform filaments will be prepared (especially with Mg core surrounded by boron powder). The uniformity of filaments will be tested from the transversal and longitudinal sections of wire samples as well as by RTG tomography. Chemical interactions on the interfaces will be analysed by scanning electron microscopy with EDS. The measurements of current-voltage characteristics will be used for the estimation of critical currents and also for heat generation and thermal stability of composite wires at high DC currents.  Obtained results allow to optimise the architecture of composite wire having not only high current densities but also sufficient thermal stability at working conditions.

  • Theme: Electro-thermal computer modelling of superconducting large-scale applications

    Supervisor: Mgr. E. Pardo, PhD. ( Department of Superconductor Physics )
    Abstract: Superconductors may reduce the green-house effect emissions by applying them to renewable energies, such as wind turbine generators, and highly efficient electric power applications, such as motors for air-planes transformers and power-transmission cables. This is thanks to the fact that hard type II superconductors present virtually no resistance below a certain temperature, current density (critical-current density) and magnetic field. If the current density overcomes the critical-current density in a portion of the superconductor, local heat is generated, which may turn the superconductor into normal and generating even more heat. This is known as thermal quench, which may cause dramatical damage. However, thermal quench could be beneficial, if well managed: self-limitting power-transmission cables use thermal quench to “switch off” under current peaks and protect the electric grid. Therefore, electro-thermal modelling is a must in the design of superconducting power applications. The PhD is intended to develop a computer program in order to model the electro-thermal behaviour in superconductors, based on the minimum entropy production principle. The program will be developed in C++ and use parallel computing concepts to apply it to computer clusters or GPUs, which might evolve into super-computer computations. However, no previous programming experience or knowledge of superconductivity is required to enrol into PhD but a motivation to learn. The work will be done in close collaboration with the supervisor, ensuring the success of the PhD dissertation. The student will also benefit from international collaborations and attendance to conferences abroad. This PhD represents a balance of fundamental and applied research that may open a professional future both in academy and industry with an international projection.
    Requirements: Be in possession of a degree in physics, engineering, mathematics, informatics or similar at the beginning of the PhD and citizenship of a country in EU. Knowledge of a multi-purpose programming language (such as C++, python or Fortran) is welcome, although not necessary at the beginning.
    Benefits: In addition to the standard benefits for a PhD candidate (stipendium and important discount on accommodation), the candidate may attend to international conferences in Europe, United States or Asia. There are also several possibilities to make short research stays abroad, thanks to the international contacts of the supervisor.
    Please, do not hesitate to write an email for more information (enric.pardo@savba.sk)

  • Theme: Superconducting properties and microstructure of thin films for superconducting radio frequency cavities

    Supervisor: Mgr. E. Seiler, PhD. ( Department of Superconductor Physics )
    Abstract: Superconducting radio frequency resonant cavities (RF cavities) used to accelerate and shape the particle beams in particle accelerators require high quality superconducting layer deposited at the inner walls of the cavity. Thin niobium (Nb) layer is used in most cases. The proposed topic is focused at complex characterization of superconducting thin films for RF cavities prepared at the leading European laboratories with the help of various deposition and substrate treatment methods. The work will cover the “classic” Nb films as well as films and multilayers of other alternative materials as NbN, Nb3Sn and MgB2. Superconducting properties and parameters of the films will be studied using electric and magnetic characterization methods, like transport measurements of electric resistance, magnetic moment measurements in the vibrating sample magnetometer, AC magnetic susceptibility measurements etc. Characterization of the electric and magnetic properties will be interconnected with analysis of the microstructure of the films, employing scanning and transmission electron microscopy and other suitable techniques. The results will help to select the most suitable materials and deposition methods for preparation of superconducting films in RF cavities in future accelerators. 
  • Theme: Advanced superconducting conductors for fault current limiters

    Supervisor: Ing. M. Vojenčiak, PhD. ( Department of Superconductor Physics )
    Abstract: Superconducting fault current limiters (SCFCLs) allow further development of the power distribution grid without increase of fault currents level. Principle of SCFCLs is based on steep increase of superconductor resistance when electric current exceeds critical current of the conductor. However, temperature of the conductor rises during limitation of fault current; this can lead to damage of the supercoducting material.
    The thesis will include detail study of thermal and electrical processes in the SCFCL. The most important of them are dissipation (AC loss) during normal opration, changes of electrical resistance, temperature and heat transfer during limiting and recooling period.
    The goal of the thesis is identification of superconducting conductor properties influencing operation of the SCFCL, design of improved architecture of the conductor and/or architecture of the SCFCL and experimental verification. It is expected to be manufactured small experimental model using superconducting conductor of length between  0.1 and 1 m. Preferably there will be studied concept of advanced conductor with increased heat capacity.
  • Theme: Crystal optics for high resolution X-ray imaging

    Supervisor: Mgr. Bohumír Zaťko, PhD. ( Oddelenie mikroelektroniky a senzoriky )
    Supervisor specialist: Ing. Zdenko Zápražný, PhD. ( Oddelenie mikroelektroniky a senzoriky - Piešťany )
    Abstract: High resolution X-ray imaging in real or reciprocal space requires high resolution X-ray optics, which effectively increases the resolution (spatial or spectral) and also transmits sufficiently X-ray beam intensity. The work will be focused mainly on X-ray optics performing expansion or focussing of the X-ray beam in one (1D) to three dimensions (2D, 3D) without an image distortion. A perfect surface planarity and reduced roughness is of paramount importance in advanced X-ray metrology or X-ray imaging, where the angle of incident or diffracted X-ray beams is very low, the examples being grazing incidence monochromators and X-ray magnifiers. The shape of monochromator has to be designed with functional surfaces open for modern finishing methods as nano-machining and post-polishing. Prior to the production of X-ray optical element, ray tracing simulations and proposing the optimal design for a specified application will be needed. High resolution diffractometer and AFM microscope will be used for characterization of the quality of active optical surfaces (roughness, planarity, subsurface damage). Testing of imaging or focusing ability will be done mostly using high intensity laboratory sources in combination with other optical elements and detectors based on direct converting Medipix camera with silicon or GaAs chip, which are in our department also developed and tested.
    The work will be realized in detached place of IEE SAS in Piešťany.
     

    5.2.13 "Electronics and photonics"

  • Theme: Development and characterization of MOS gate structures for GaN vertical switching transistors

    Supervisor: Ing. Milan Ťapajna, PhD. ( Department of III-V Semiconductors )
    Abstract: Despite an immature technology, lateral GaN heterostructure field-effect transistors (HFETs) can nowadays over-perform state-of-the-art Si devices, so that converters with GaN devices reach conversion effectivity on the level of 99%. However, further power boost of the lateral GaN HFETs faces limitations related to maximum operating blocking voltage (~ 1 kV), thermal effects and associated reliability issues, frequency dispersion, and packaging. To fully exploit the excellent properties of the GaN material, it is therefore necessary to develop new concepts for vertical GaN switching devices. The goal of the PhD study will aim the technology development and detail characterization of metal-oxide-semiconductor (MOS) gate structure with 3D architecture for vertical switching GaN MOSFETs. The research will focuse on understanding of the formation and charge distribution in GaN MOS gate structures, their technological control enabling adjustment of the transistor’s threshold voltage, and optimization of the oxide/semiconductor interface quality to suppress undesired device instabilities. The project is based on an original concept of inversion-type MOSFET, the state-of-the-art technologies available at IEE SAS, and combination of detail electrical and structural characterisation of MOSFET structures and simulations. The work will be performed at IEE SAS in cooperation with Slovak University of technology and international partners (Hokkaido University, Japan).
  • Theme: Study of preparation of electronic and sensoric structures for applications at extreme conditions

    Supervisor: Ing. Gabriel Vanko, PhD. (Department of Microelectronics and Sensors )
    Popis: The proposed work is motivated by need for implementation of new materials for design and fabrication of Micro(Nano)ElectroMechanical Systems – M(N)EMS operating at extreme conditions of high temperatures and chemically agressive environments. It is aimed on design, processing technology and characterization of sensoric and electronic devices on the base of III-nitride semiconductor materials. All above, there are AlGaN/GaN heterostructures in hybrid integration with nanocrystalline diamond layers primarily used as: a)porous absorbing layers (for chemical gas sensors); b)supporting layers for membrane structures (for pressure and force sensors, accelerometers), biocompatible layers (for biosensors) and/or heat sinking layers (for electronic devices using 2D electron gas).
  • Theme: Pixel detectors based on GaAs for X-ray imaging

    Supervisor: Mgr. Bohumír Zaťko, PhD. ( Department of Microelectronics and Sensors )
    Abstract:  The aim of the thesis is technology preparation of ionizing detectors, study of electrical and detection properties and interpretation of obtained results. Used detection materials are semi-insulating GaAs. At first the work will be concentrated on design and preparation of pixel detection structures. Following the electrical characterization will be realized. Selected suitable pixel detection structures can be conneted to spectrometric set-up. After evaluation of results the final pixel structures will be realized and conneted to Timepix3 readout chip. The connection of pixel detector with readout chip produces a pixel detection system and its spectrometric and imaging performance will be tested.
  • 4.1.3 Physics of condensed matter and acoustics 

  • Theme: III-N quantum structures for new generation of ultra-fast tranzistors

    Supervisor: RNDr. Dagmar Gregušová, DrSc. ( Department of III-V Semiconductors )
    Abstract: GaN-based high-electron mobility transistors are intensively studied because of their outstanding properties as the next-generation power electronic devices and for special high-frequency applications. Conventional HFET devices typically have negative threshold voltage, working in the normally-on mode. This work is aimed at the investigation of the new technological concepts (electro-chemical etching, O2 plasma oxidation) for normally-off HFET preparation. Their use is monitored by the measurement and analysis of transistor data.

  • Theme: Technology and properties of GaN-based vertical switching tranzistors

    Supervisor: Ing. Ján Kuzmík, DrSc. ( Department of III-V Semiconductors )
    Abstract: Massive deployment of GaN-based switching transistors promises huge world-wide energy savings. This anticipation stems from material parameters of III-N semiconductors. Consequently, GaN-based transistors show extremely efficient switching performance, high robustness and temperature stability. On the other hand, state-of-the-art GaN device technology is almost completely based on planar concepts, i.e. contacts are located only on side of the substrate. This approach simplifies processing issues; however electrons drift in the vicinity of the surface. Particularly, devices may suffer from parasitic surface charges, surface breakdown, and insufficient heat sinking. Thus planar GaN transistors may not reach theoretical switching speed, electrical break-down field is low and also pre-mature thermal break-down before the avalanche appears.  Consequently, despite excellent material parameters of GaN, nowadays applications above 1 kV are exclusively dominated by SiC. PhD thesis will be focused on proposal, technology and analysis of vertical GaN-based switching transistors. Emphasis will be given to 3-dimensional patterning of the device, to the preparation of non-planar contacts and also to electrical and thermal characterization of prepared vertical structures. Work will be performed in collaboration with foreign laboratories.  

  • Theme: Computer modeling of superconducting motors for aviation

    Supervisor: Mgr. E. Pardo, PhD. ( Department of Superconductor Physics )
    Abstract: Hard type II superconductors present virtually no resistance below a certain temperature, current density and magnetic field. However, time-varying magnetic fields generate local heat in the superconductor, which may overcome the critical temperature and become normal, thus  generating even more heat. Thus, the interplay between the electromagnetic and thermal systems create thermal avalanches of fractal dentritic structures. This topic is not only of fundamental importance but also highly relevant for superconducting power and magnet applications. Electro-thermal effects create thermal quench, which may cause dramatical damage in large-scale magnets like the Large Hadron Collider in CERN. Electro-thermal studies are also a must for the design of wind-turbine generators, electrical motors for air-planes or fault-current limiters. The PhD is intended to develop a computer program in order to model the electro-thermal behaviour in superconductors, based on the minimum entropy production principle. The program will use parallel computing concepts to apply it to computer clusters or GPUs, which might evolve into super-computer computations. However, no previous programming experience or knowledge of superconductivity is required to enrol into PhD but a motivation to learn. The work will be done in close collaboration with the supervisor, ensuring the success of the PhD dissertation. The student will also benefit from international collaborations and attendance to conferences abroad. This PhD represents a balance of fundamental and applied physics research that may open a professional future both in academy and industry with an international projection.
    Requirements: Be in possession of a degree in physics (any specialization), engineering, mathematics or similar at the beginning of the PhD. Knowledge of a multi-purpose programming language (such as C++, Python or Fortran) is welcome, although not necessary at the beginning.
    Benefits: In addition to the standard benefits for a PhD candidate (stipendium and important discount on accommodation), the candidate may attend to international conferences in Europe, United States or Asia. There are also several possibilities to make short research stays abroad, thanks to the international contacts of the supervisor.
  • Theme: Technology and properties of GaN-based vertical switching tranzistors

    Supervisor: Ing. Milan Ťapajna, PhD. ( Department of III-V Semiconductors )
    Supervisor specialist: Ing. Stanislav Hasenöhrl ( Department of III-V Semiconductors )
    Abstract: Topic of the work will be growth and characterization of GaN vertical structures prepared using homo-epitaxial growth on GaN substrate. Developing of this technique is necessary for the preparation of new generation of GaN-based high-power switches. Structures will be prepared using MOCVD technique; special emphasis will be given to various methods of GaN nucleation and its influence on subsequent epitaxial layers growth. Carbon-based compensation will be studied together with p and n-type doping, as well as formation of the pn junction. Structural quality of epi-structures will be accessed using XRD and PL techniques. Electrical properties and mechanisms of electron transport in the vertical structures will be studied using I-V techniques in the dc as well in the pulsed mode and at different ambient temperature. By measuring at high electrical field we will analyze mechanism of the electrical break-down and possibilities of enhancing the electrical strength. Work will be performed in collaboration with labs abroad.
  • Theme: Numerické simulácie magnetických skyrmiónov

    Supervisor: Ing. Jaroslav Tóbik, PhD. ( Department of Physics and Technology at Nanoscale )
    Supervisor specialist: Dr. Michal Mruczkiewicz ( Department of Physics and Technology at Nanoscale )
    Abstract: A skyrmion is a particle-like, topologically protected magnetic configuration [1]. Since the experimental observation, skyrmions are intensively investigated and are expected to play an important role for next generation memory or logic devices.  It is due to their high stability and high mobility under the influence of weak currents. However, a generation and observation of the skyrmion in nanostructures, such as nanodisk, is still a challenge [2,3]. The aim of the work will be to perform analytical calculations and numerical simulations using open source (e.g., mumax3, OOMMF) to study the influence of geometry and magnetic parameters on stabilization and nucleation of skyrmions. Next, the focus will be put on investigation of dynamical response of magnetization under influence of external magnetic field and control of skyrmions [4, 5].  The results will support for the experiments performed at IEE SAS (active participation of candidate in experiment is possible and can be part of the thesis). The analysed structures will be considered for potential applications in memory and logic devices. The work will be performed within international collaboration (Adam Mickiewicz University in Poznań, Poland/University of the Basque Country, Spain).
    [1] https://en.wikipedia.org/wiki/Magnetic_skyrmion
    [2] Beg, Marijan, et al. "Ground state search, hysteretic behaviour, and reversal mechanism of skyrmionic textures in confined helimagnetic nanostructures." Scientific reports 5 (2015).
    [3] Heo, Changhoon, et al. "Switching of chiral magnetic skyrmions by picosecond magnetic field pulses via transient topological states." arXiv preprint arXiv:1601.08212 (2016).
    [4] Kim, Joo-Von, et al. "Breathing modes of confined skyrmions in ultrathin magnetic dots." Physical Review B 90.6 (2014): 064410.
    [5] Ma, Fusheng, et al. "Skyrmion-based dynamic magnonic crystal." Nano letters 15.6 (2015): 4029-4036.
Last build: 03.03.2017