Doctoral School

Reading date: 22/11/2024

• BALLESTAR DE LAS HERAS, RICARDO LUIS: Manufacturing and recycling of polyolefins and biopolymers in a circular economy model. Mechanical, thermal and morphological characterization of polymers and high-performance films.
  
  Author: BALLESTAR DE LAS HERAS, RICARDO LUIS
  Thesis file: (contact the Doctoral School to confirm you have a valid doctoral degree and to get the link to the thesis)
  Programme: DOCTORAL DEGREE IN POLYMERS AND BIOPOLYMERS
  Department: Department of Chemical Engineering (EQ)
  Mode: Article-based thesis
  Deposit date: 26/07/2024
  Reading date: 22/11/2024
  Reading time: 16:00
  Reading place: Defensa pública a ESEIAAT (Escola Superior d'Enginyeries Industrial, Aeroespacial i Audiovisual de Terrassa) Edifici TR1. C. Colom, 1 08222 Terrassa
  Thesis director: COLOM FAJULA, XAVIER | CAÑAVATE AVILA, FRANCISCO JAVIER
  Committee:
       PRESIDENT: SAURINA CANALS, JOAN
       SECRETARI: GARRIGA SOLE, PERE
       VOCAL: MARÍN GENESCÀ, MARC
  Thesis abstract: On April 29th, 2015, the EU enacted Directive 2015/720, aimed at reducing plastic consumption, promoting the use of reusable bags and encouraging the replacement of single-use plastic bags with biodegradable alternatives. This European legislation was transposed into Spanish law by Royal Decree 293/2018, dated May 18th.This legislation and its accompanying regulations have significantly impacted the transformation sector, forcing it to reinvent itself to continue its activities. This work aims to provide answers to the challenges faced by manufacturers due to this new context, as well as address the needs resulting from it. Distinctive features of this project are its practical nature and industrial scale.A primary objective is to establish the processability and quality windows for recycled post-industrial and post-consumer materials, ensuring the production of a final product with predefined quality standards. The study examines three types of polyethylene: virgin, post-industrial, and post-consumer. Each type undergoes a combined cycle simulating an industrial recycling process, with post-industrial material being processed up to twelve times and post-consumer material up to four times. This methodology of cyclic processing supports the principle of circularity in sample characterization.For each cycle, involving extrusion and recycling, the study examines micro and macroscopic changes, analyzing morphological and structural changes (using DSC, TGA, GPC, and FTIR techniques) and the evolution of mechanical properties (tensile strength, elongation at break, tear resistance, impact resistance (dart test), and colorimetric changes).At the microstructural level, two types of crystalline structures are observed, and they evolve differently through the processing cycles due to the thermo-oxidative degradation phenomena that occur along the process. The changes include a reduction in crystallinity due to the breakdown of more linear and less branched chains, and an increase in cross-linking among more branched structures. These microstructural changes affect post-industrial and post-consumer recycled polyethylene differently.Macroscopically, tear resistance is the most critical property affected during the recycling cycles, decreasing by up to 40% from the initial value and potentially becoming the main limitation for the successive reuse of these polymers.In line with the previously mentioned regulations, biodegradable polymers are another focus of interest. The study examines biodegradable compostable polymers used to manufacture single-use bags. A secondary objective is to determine how long these biodegradable polymers retain their usability properties under standard storage conditions, analyzing the samples over twelve consecutive months.Macroscopically, a progressive loss in tensile, elongation, and impact properties is the most significant phenomenon. Microscopically, two degradation mechanisms—hydrolysis and thermo-oxidation—are observed, both occurring in the amorphous phase of the material.The study concludes that the tested bags have a shelf life of over twelve months under standard storage conditions.
  

• BARRIOS AVALOS, JUAN JOSE: Reestructuración urbana de la Región Hidrológica 36 Nazas - Aguanaval, México: Modelos agroproductivos y sus sistemas hidrográficos
  
  Author: BARRIOS AVALOS, JUAN JOSE
  Thesis file: (contact the Doctoral School to confirm you have a valid doctoral degree and to get the link to the thesis)
  Programme: DOCTORAL DEGREE IN URBANISM
  Department: Department of Urbanism, Territory and Landscape (DUTP)
  Mode: Normal
  Deposit date: 30/09/2024
  Reading date: 22/11/2024
  Reading time: 11:30
  Reading place: Face-to-face (Room C-B1. ETSAB. Main floor)
  Thesis director: FRANQUESA SANCHEZ, JORDI
  Committee:
       PRESIDENT: LLOP TORNE, CARLOS JUAN
       SECRETARI: MENDOZA ARROYO, MARIA DEL CARMEN
       VOCAL: SAINZ GUTIERREZ, VICTORIANO
  Thesis abstract: Hydrological Region 36 (RH36) is one of the 37 hydrological regions identified in Mexico by the National Water Commission. Within this region lie the basins of the Nazas and Aguanaval rivers, two endorheic rivers in northwestern Mexico. The waters of these rivers have enabled the Metropolitan Area of La Laguna to become a prosperous region with a significant agro-industrial sector. This success was achieved through the regulation of the rivers, primarily the Nazas River, via the construction of dams and canals between 1930 and 1990. However, the control established over the Nazas River through the Lázaro Cárdenas and Francisco Zarco dams, as well as the network of canals in the lower basin, exacerbated the loss of the Mayrán Lagoon at its mouth and led to the disappearance of flow in the lower basin in the area of Torreón, Gómez Palacio, and Lerdo.In the lower basins of RH36, there is significant water stress resulting from the overexploitation of major aquifers and the reduced availability of water resources due to the impacts of climate change. This research is based on the hypothesis that the severe water stress is partly caused by the disconnection between the regional infrastructure for water storage and distribution, agricultural irrigation systems, rural and urban populations, and their productive agricultural areas. Additionally, it is argued that hydraulic infrastructures have been planned without considering the regional urban context of the lower basin and its logical dimensions, leading to disruptions in the ecological matrix and the organization of rural populations.The research employs a basin approach to define the scope of the study. It sustained that for the restructuring of populations and the potential fluvial restoration of the Nazas and Aguanaval rivers, it is essential to understand the organization of all communities within the geographical boundaries. Each of the 33 sub-basins that make up RH36 are subject to an in-depth analysis of its urban and rural fabric. In addition to populations, the study of agricultural areas, hydraulic infrastructure, and road networks is a fundamental part of this basin approach.The thesis is organized into four chapters. The first chapter outlines the methodology, hypothesis, objectives, and the general structure of RH36. It also includes references to water storage structures in various contexts with geographical conditions similar to RH36. The second chapter is an atlas aimed at constructing morphological information about the 33 sub-basins of the region, including their urban, rural, and agricultural areas. This atlas is one of the main contributions of the research and serves as a prelude to Chapters III and IV. Chapter III explores global references on river restoration and dam removal, which serve as a foundation for Chapter IV, where the strategic approach to fluvial restoration in RH36 is presented.Finally, the research presents conclusions based on a thorough analysis of the urban and rural fabric, the hydrographic network, and hydraulic infrastructure. It argues that hydraulic infrastructures located in the urban-rural transition zones of intermediate cities within RH36 can provide ecosystem services and promote ecological and productive balance in the region. Furthermore, it maintains that the hydrological restoration in the lower courses of the Nazas and Aguanaval rivers is directly dependent on the restructuring of hydraulic infrastructure in both urban and rural populations throughout Hydrological Region 36.
  

• BUOB, SANDRA: Quantum-gas microscopy of strontium Bose- and Fermi-Hubbard systems
  
  Author: BUOB, SANDRA
  Thesis file: (contact the Doctoral School to confirm you have a valid doctoral degree and to get the link to the thesis)
  Programme: DOCTORAL DEGREE IN PHOTONICS
  Department: Institute of Photonic Sciences (ICFO)
  Mode: Normal
  Deposit date: 16/10/2024
  Reading date: 22/11/2024
  Reading time: 10:00
  Reading place: ICFO, Mediterranean Technology Park, Avinguda Carl Friedrich Gauss, 3, 08860 Castelldefels, Barcelona
  Thesis director: TARRUELL PELLEGRIN, LETICIA
  Committee:
       PRESIDENT: HOLGER ESSLINGER, TILMAN
       SECRETARI: LOZA ALVAREZ, PABLO
       VOCAL: GROSS, CHRISTIAN
  Thesis abstract: Ultracold atoms have proven to be a valuable asset to study and understand complex quantum many-body systems in a well-controlled setting. In particular, quantum-gas microscopes provide unprecedented access to local observables and allow one to investigate those systems at the level of each individual particle, giving new insights on their behaviour. While so far most of these microscopes used alkali atoms, the distinct properties of alkaline-earth atoms, in particular strontium, combined with quantum-gas microscopy are expected to shed new light on a broad variety of many-body problems. This thesis describes the realization of single-site-resolved imaging for both bosonic and fermionic strontium atoms in a Hubbard-regime optical lattice, which unlocks the possibility to study novel types of Bose- and SU(N) Fermi-Hubbard systems.An essential step in ultracold-atom experiments is the preparation of a quantum degenerate cloud. In the first part of this thesis, we discuss the methods we have implemented in our apparatus to achieve this goal. We developed a new resonant-shielding method to double the atom number during the first cooling stage in a broad-linewidth blue magneto-optical trap. During the second cooling stage in a narrow-linewidth red magneto-optical trap, the hyperfine structure of the fermionic atoms adds additional challenges which are addressed by mixing the hyperfine states with an additional laser for efficient trapping and cooling. After the laser-cooling stages, we perform evaporative cooling in a far red-detuned optical potential before loading the atoms into a two-dimensional optical lattice. The lattice laser operates at the clock-magic wavelength of strontium (813.4nm) which will enable high-precision measurements in future experiments.To image the individual atoms in the optical lattice, we place a high-NA objective in close vicinity to the atoms. We demonstrate single-atom resolution for bosonic and fermionic strontium and successfully reconstruct the lattice occupation for both of them, reaching fidelities as high as 96%. For the bosonic Sr-84 atoms, we induce fluorescence on the blue broad-linewidth transition and simultaneously perform attractive Sisyphus cooling on the red narrow-linewidth transition. Moreover, combining this imaging method with momentum-space detection, we observe the matter-wave interference arising from the phase coherence of the Bose-Hubbard superfluid. For the fermionic Sr-87 atoms, we image with the red transition only, which allows us to obtain for the first time for a fermionic alkaline-earth atom both single-atom resolution and spin-resolved detection.This thesis has combined, for the first time, quantum-gas microscopy with ultracold strontium and its distinct spectral properties. This platform should enable a broad range of future studies. For the bosons, it unlocks investigation of the single-atom-resolved dissipative Bose-Hubbard systems and the exploration of collective atom-photon scattering in ordered atomic arrays. For the fermions, the spin-dependent single-atom detection provides the ideal setting for investigations of antiferromagnetic correlations in SU(N≤10) Fermi-Hubbard systems and the realization of exotic magnetic phases.
  

• DADJO TAVAKOLI, SAMAN: Control Design for Converters in Modern Power System
  
  Author: DADJO TAVAKOLI, SAMAN
  Thesis file: (contact the Doctoral School to confirm you have a valid doctoral degree and to get the link to the thesis)
  Programme: DOCTORAL DEGREE IN ELECTRICAL ENGINEERING
  Department: Department of Electrical Engineering (DEE)
  Mode: Normal
  Deposit date: 25/09/2024
  Reading date: 22/11/2024
  Reading time: 11:00
  Reading place: Aula 28.8 - ETSEIB- UPC - Avda. Diagonal, 647, Barcelona
  Thesis director: GOMIS BELLMUNT, ORIOL | PRIETO ARAUJO, EDUARDO
  Committee:
       PRESIDENT: WANG, XIONGFEI
       SECRETARI: RENEDO ANGLADA, FRANCISCO JAVIER
       VOCAL: LINDNER, MARCO
  Thesis abstract: Dynamic behavior of modern power system is increasingly influenced by the dynamics of power converters. At the generation level, a high volume of wind and solar energy with their large-scale power converters are replacing conventional synchronous generators. At the transmission level, many High Voltage Direct Current (HVDC) projects based on Modular Multilevel Converter (MMC) technology are commissioned to transfer offshore wind to the load centers, interconnect two non-synchronous AC grids, or two different nodes of the same AC grid (as an embedded link) to enhance the security of supply. The consumption level also faces such high integration of power converters since the modern flexible loads, such as electric vehicles, industrial motor drives, and recent hydrogen electrolyzers, are connected to the power system via power converters. Such transformation in generation, transmission, and consumption levels leads to the power dynamics that have never been experienced before in the conventional power systems.The high penetration of power converters may cause several challenges: reduction in the system total inertia and short-circuit current, excitation of undesirable interactions among control loops, reduction in stability margin, and loss of devices in the context of fault-ride through (FRT). Such challenges can be addressed by appropriate adjustments in the control systems of power converters.At the transmission level, this thesis focuses on improving the dynamic behavior of HVDC systems. A methodology is suggested to study the small-signal stability and evaluate potential interactions that may happen among control loops of two HVDC converter stations. Next, a multivariable controller based on H-inf design practice is developed to optimize the dynamic behavior of the system. As a follow-up study, mu-synthesis control approach is adopted to make HVDC control resilient against AC grid impedance variation. In the context of FRT performance, the fault operation of the HVDC system is improved by appropriate modification of the MMC control loops. Finally, a brief study on the DC voltage droop control design is conducted on a multi-terminal HVDC system. At the generation level, this thesis investigates the dynamic behavior of grid-forming control during fault events in a low-strength power system. It investigates the complication related to the existing grid codes and suggests two FRT controls for AC voltage support during fault recovery. As the last step, this thesis evaluates the feasibility of modern flexible loads to provide grid-forming services to the low-strength power grid. A hydrogen electrolyzer is used as an example and the operational constraints related to the hydrogen production are linked to the grid-forming functionalities.
  

• DE LA CRUZ JESUS, LUCIA GUADALUPE: Multifunctional eco-friendly aerogel composites for thermal insulation and fire resistant applications.
  
  Author: DE LA CRUZ JESUS, LUCIA GUADALUPE
  Thesis file: (contact the Doctoral School to confirm you have a valid doctoral degree and to get the link to the thesis)
  Programme: DOCTORAL DEGREE IN MATERIALS SCIENCE AND ENGINEERING
  Department: Department of Materials Science and Engineering (CEM)
  Mode: Article-based thesis
  Deposit date: 01/10/2024
  Reading date: 22/11/2024
  Reading time: 12:00
  Reading place: Defensa pública EEBE, Sala Polivalente del Edificio A, planta baja, Campus Diagonal-Besòs
  Thesis director: SANCHEZ SOTO, MIGUEL ANGEL | ABT, TOBIAS MARTIN
  Committee:
       PRESIDENT: MASPOCH RULDUA, MARIA LLUÏSA
       SECRETARI: CASTILLO LÓPEZ, GERMAN
       VOCAL: ECEIZA MENDIGUREN, MARÍA ARÁNZAZU
  Thesis abstract: Replacing petroleum-based polymers with natural alternatives requires overcoming the limitations of their thermal and mechanical properties, as well as the inherently low fire resistance of the biomass from which they are derived. A common method for improving thermal insulation is through the use of porous systems such as technical foams. Aerogels, as highly porous materials, are lightweight and have low thermal conductivity, making them effective barriers for this purpose. However, the challenge remains in finding a solution where all the aforementioned properties are fully optimized. This thesis aims to demonstrate that by using environmentally friendly materials and processes, it is possible to create multifunctional aerogels with optimized properties, positioning them as viable alternatives, among others, to conventional foams.Polyvinyl alcohol (PVA), a biodegradable synthetic polymer, was initially studied with the introduction of alginate and tannic acid (TA) as natural modifiers and clay playing the role of structural scaffold. The hydrogen bonding between the polar groups of the components resulted in a nearly tenfold improvement in the mechanical properties as compared to pure PVA aerogels. The addition of NaOH was necessary to overcome the high hydrogen bonding attraction between TA and PVA that causes a fast precipitation of a solid which hindered gel formation. The PVA aerogel matrix was further cross-linked using calcium and borate ions, leading to an additional enhancement in mechanical performance, along with increased thermal stability and fire resistance, with the peak of heat release decreasing five-fold with respect to PVA.Gelatin—an abundant and cost-effective protein—was utilized as a novel system for developing optimized aerogel composites. Tannic acid played a crucial role in establishing strong bonds with the active sites of gelatin, a process that was carried out under alkaline conditions to stabilize the hydrogel complex. This approach resulted in exceptionally strong aerogels, achieving up to a 15-fold increase in yield stress. The protective action of well-dispersed nanoclay particles enhanced the pyrolytic decomposition and charring effects of tannic acid, imparting excellent flame-retardant properties to the aerogels. Although gelatins’ highly hydrophilic nature posed an initial challenge, a simple dip-coating treatment with polydimethylsiloxane and SiO2 nanoparticles effectively conferred superhydrophobic surfaces.Finally, the performance of modified ammonium alginate/clay/tannic acid composites was studied based on the orientation induced during the freeze-drying process. Alginate, a naturally abundant polysaccharide, was chosen as the base polymer for the aerogels due to its high fire resistance and the release of nitrogen upon heating, which reduces the amount of combustible gases. Orientation and crosslinking were key steps in enhancing the mechanical properties, achieving values as high as 305 MPa for Young’s modulus and 7 MPa cm³/g for yield strength, placing these systems among the highest reported in the literature. Thermal conductivity fit well with existing model predictions, increasing in axially oriented samples due to heat convection contribution through large vertical pores. Thermal stability and fire resistance were significantly improved by the addition of tannic acid, resulting in a minimal peak heat release of just 21 kW/m² and no detectable flame.New types of eco-friendly composite aerogels with an ultralight, porous structure have been developed. They were successfully fabricated using freeze-drying and post-cross-linking techniques, achieving an excellent balance of mechanical strength, thermal stability, and fire retardancy. In summary, this work demonstrates that bio-based and biodegradable aerogels can serve as a viable approach when seeking greener alternatives with properties comparable to current traditional foam-like materials.