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Otakaari 1 grandhall. Photo: Esa Kapila
 

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Recent Submissions

Growth and properties of diamond and diamond-like materials
(1999) Kaukonen, Markus
Helsinki University of Technology | Doctoral thesis (monograph)
Engineering of lignin and lignin-carbohydrate complexes for high-value applications
(2026) Diment, Daryna
School of Chemical Engineering | Doctoral thesis (article-based) | Defence date: 2026-01-23
Industrial development has led to a dramatic increase in fossil fuel consumption. These nonrenewable energy sources, while currently accessible and cost-effective, contribute heavily to greenhouse gas emissions, therefore accelerating anthropogenic climate change. Addressing this global environmental challenge requires a transition toward sustainable, low-carbon energy alternatives. Within this paradigm, lignocellulosic biomass has emerged as a promising solution, offering a renewable, carbon-neutral feedstock that can be converted into biofuels and bioproducts. Its valorization not only supports the reduction of fossil fuel dependence but also contributes to the mitigation of climate change by harnessing a circular bio-based economy. However, despite its high availability and low cost, lignocellulosic biomass exhibits significant resistance to degradation, presenting substantial challenges for its effective valorization. While considerable progress has been made in the efficient conversion and application of cellulose, lignin remains a challenging component of lignocellulosic biomass, often assigned to combustion or disposal, thus extremely underestimating its unique chemical potential. To address the challenges in lignin utilization, a novel strategy was developed based on structure–property–performance correlation, representing a first step toward efficient lignin engineering. The approach involves selective modification of targeted functional groups while keeping others unchanged, followed by comprehensive property and performance evaluation. It was particularly useful for investigating how specific functional groups affect the physicochemical behavior of lignin and its performance in methylene blue adsorption. For instance, benzylic −OH groups were found to contribute approximately 3 and 2.3 times more than phenolic and aliphatic −OH groups, respectively. Overall, this work established a robust framework for tailoring lignin properties to meet the demands of high-value applications. Following that, a recently developed green biorefinery concept (AqSO Omni) was advanced using the power of artificial intelligence (AI) to provide simultaneous maximization of both lignincarbohydrate complexes (LCCs) yield and content in acetone-extracted lignin as LCCs hold great potential for high-value applications, yet achieving high yields remains challenging. Using Bayesian Optimization, optimal processing conditions were identified, achieving LCC yields of 8–15 wt% and carbohydrate contents up to 60/100 Ar. Importantly, LCCs with higher carbohydrate content showed lower glass transition (Tg) and surface tension, highlighting a significant step toward scalable production of tailor-made LCCs with tuned properties. Based on the previous outcomes, lignin demonstrated strong radical scavenging potential, that marked the urgent need for the development of a reliable and fast screening method for quantitative evaluation of the antioxidant properties of lignin. It was achieved by assessing the impact of the solvent, time, and the type of substrate on the antioxidant activity using a well-established 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. This work unveiled the importance of appropriate solvent choice as it predetermines the DPPH scavenging mechanism, steady state establishment, and the DPPH stability with 90 vol% acetone (aq.) exhibiting the greatest suitability for the antioxidant evaluation. While a small-scale and rapid performance evaluation method for lignin engineering is concerned, the concluding stage of the study involved the development of the UV-shielding lignocellulosic film, where a small addition of lignin into the formulation allowed to block over 90% of the UV rays.
Enabling cryogenic technologies for superconducting quantum devices
(2026) Hätinen, Joel
School of Science | Doctoral thesis (article-based) | Defence date: 2026-01-23
Low-temperature refrigerators cool systems down to cryogenic temperatures near absolute zero, where thermal noise and decoherence are suppressed. This allows quantum phases, such as superconductivity, to emerge in certain materials and enables the harnessing of individual quantum states for scientific and high-performance applications. However, the refrigerators used for these purposes are large and rely on cryoliquids, such as scarce and expensive 3He, which can be a limiting factor depending on the technological application. To enable more scalable, costeffective cryogenic platforms, new refrigeration technologies must be developed. To this end, chip-scale coolers based on superconducting tunnel junctions have been envisioned to provide a fully solid-state alternative. Proof-of-principle operation of these coolers has been demonstrated at temperatures below 1.5 K, but to link them with commercially available 4He pulse tube cryocoolers, stage operating above 2.0 K is required. Additionally, thermally isolating and electrically conducting methods are needed to cascade coolers operating at different temperature ranges. In this thesis, the fundamental components of a multi-stage chip-scale cooler operating at temperatures compatible with 4He pulse tube cryocoolers are developed. A superconducting flipchip assembly fabricated with In-bumps was characterized in the sub-kelvin temperature range, and the inter-chip thermal resistance was found to be suitable for chip-scale cooling applications. A through-chip signal routing method utilizing ALD TiN-based TSVs was developed, and the demonstrated critical temperature of 2.0 K enables dissipationless DC transport for multi-chip assemblies, such as cascaded coolers. Additionally, ALD MoCx was shown to exhibit a superconducting transition temperature up to 4.4 K and high conformality, showing promise as a TSV-compatible material. The key achievement of electronic cooling of Al thin film from a bath temperature of 2.4 K down to 1.6 K was demonstrated using Nb-based superconducting tunnel junctions, probed by an onchip junction thermometer. Thermal model calculations highlighted the emergence of superconductivity in the Al beneath the cooler junctions, persisting up to a bath temperature of 2.4 K: one kelvin higher than the nominal critical temperature of the Al thin film. The single-stage cooler operating above 2.0 K enables solid-state on-chip cooling from 4He pulse-tube compatible temperature without the use of magnetic fields. Additionally, Al- and V-based tunnel junctions were fabricated at the wafer scale using degenerately doped Si as the normal electrode. The junctions exhibited suitable low-temperature electrical characteristics for cooling applications. From superconducting interconnects to tunnel-junction components supporting high cooling power density above 1 K, the achievements presented in this thesis enable modular design of chip-scale cascade coolers. This technology is envisioned to support the scaling of several superconducting quantum devices from proof-of-principle to multi-component systems beyond experimental lab environments.
Measuring spectral signatures of forest floor and trees
(2026) Palviainen, Petri
School of Engineering | Doctoral thesis (article-based) | Defence date: 2026-01-23
Forests contain much of the Earth’s biodiversity and carbon storage and are being increasingly studied using methods based on remote sensing. Optical remote sensing provides means to study forest ecosystems based on measuring complex interactions between sunlight and plants. Ground reference measurements of vegetation spectral signatures, measurements of composition, size, shape, and structure of plants, as well as observations of growing conditions and seasonal changes are essential in understanding the links between these diverse characteristics and for accurate interpretations of satellite images within and between ecosystems. While ground reference measurements are widely recognized as an important part of Earth observation, the study of the optical properties of individual forest components is largely an unexplored topic. To emphasize the matter, spectral-directional properties of the largest contributors to overall spectral signatures in most forests, i.e., trees, have not been measured prior to this thesis. This thesis presents, improves, and evaluates several laboratory and field measurement methods. These methods were used to measure the spectral signatures of forest floors, plant components, and single trees in monoangular, multiangular, and hemispherical measurement geometries. The spectral sampling of different vegetation types used different types of spectral probes in tandem with a spectrometer capable of recording the shortwave spectrum between 350 and 2500 nm with 1 nm spectral resolution output. Laboratory measurements were made (1) of the seasonal spectral dynamics of lingonberry and blueberry dwarf shrubs over a growing season, (2) of the spectral-directional characteristics of pine, spruce, and oak trees, and (3) of birch and maple leaves to compare different leaf-level spectral measurement methods. Data were analyzed from field measurements of forest floors from 67 study stands in four different types of forests in boreal, hemiboreal, and temperate vegetation zones in Europe. Ancillary data collection included using photographic techniques to obtain estimates of the light availability at the forest floors and fractional cover classification. While the measurements were challenging to implement, and required both practical and theoretical considerations, each of the applied measurement methods produced high-quality spectral data previously missing from spectral libraries. Spectra of lingonberry and blueberry shrubs showed seasonal trends influenced by different phenological stages. The effects of lingonberry flowers and berries on shrub spectra were observed. The mean spectral signatures of forest floors were different between the four ecosystems, with ecosystem-specific variation linked to growing conditions, forest floor fractional cover, and tree canopy structure. Trees exhibited hemispherical spectral patterns that were different between the species, most notably in view directions towards and away from the illumination source. The data measured may be used as empirical evidence when validating the interpretation of remote sensing data and in reflectance modeling of forests by including new information on factors influencing overall forest reflectance. The results of this thesis assist the research community in selecting appropriate methods for measuring forest spectra. Additionally, they underline the importance of incorporating ecosystem-specific spectra of forest components in analyzing Earth observation data to enhance the accuracy of measurements relating to the compositional and structural properties of forests. The spectral libraries of this thesis have been published as open data.
Robust Tracking of Heterogeneous Objects in Sparse Point Clouds
(2025-12-27) Semin, Nikolai
School of Engineering | Master's thesis
Object detection, tracking, and classification are fundamental tasks for smart urban environments and autonomous driving systems. Covering large outdoor spaces with LiDAR sensors typically requires either many sensors with overlapping fields of view or fewer sensors with non-overlapping fields of view. The latter approach is more cost-effective but introduces a handover problem: when a tracked object exits one sensor's field of view and then enters another field of view, maintaining its identity is crucial. This thesis proposes a method for tackling such scenarios by using a constant velocity motion model with a Kalman filter operating in prediction-only mode: updating the state estimate forward in time without measurement corrections until the object re-enters a field of view of one of the sensors. The method is implemented as a ROS2 node and evaluated on real-world data collected specifically for the research. The results are discussed and improvements for robustness are proposed.