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    Nd:YAG Laser Welding of NiTi-SS Wires by Creating In-Situ High Entropy Joints
    (University of Waterloo, 2026-09-28) Creaser, Aidan
    Welding of NiTi and stainless steel (SS) wires is of great importance for device fabrication within the medical industry. Devices such as implants, surgical equipment, and even orthodontic arc-wires would benefit from cost effective and efficient welding between NiTi and SS. The production of Ni-Ti and Fe-Ti based intermetallic compounds (IMCs) in the fusion zone (FZ) makes joining extremely challenging. This study aims to eliminate IMCs in the FZ by generating an in-situ high entropy alloy (HEA) FZ of NiTi-SS wires. A custom Co-Cr-Ni powder was created to target a face-center-cubic (FCC) CoCrNiFe HEA FZ. Temporal pulse shaping was investigated by using different laser power/duration during the welding process. It was discovered that long; low-power pulses were effective at obtaining a fully FCC HEA joint. However, IMC precipitation occurred at the NiTi-FZ interface. The results observed a 60% increased tensile strength and 47 % increased strain. Ultrasonic assistance was then applied under the same conditions to mechanically alter the interfacial region. Using the acoustic streaming and cavitation effects, the thickness of the interface was drastically reduced. Instead, a Ti-rich area of the FZ, with lower hardness, served as the point of failure. As a result, the mechanical properties of the joint increased and the welds came remarkably close to the target, the super-elastic plateau of the base NiTi wire. Achieving a weld capable of the functional properties of the base NiTi has rarely been reported in the literature, especially for wire joining. A chemical route was then further developed, using a CoVNi powder. It was observed that the V was successful at migrating into previously Ti-rich regions and forming more ductile IMCs which lowered hardness. However, the V could not diffuse far enough into the interface, and the results matched those observed with the CoCrNi powder.
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    Breaking Information Silos: Advancing Search System for Unified Information Seeking
    (University of Waterloo, 2026-09-28) Ma, Xueguang
    Information seeking is central to how people learn, make decisions, and advance knowledge. At the center of any search system is the retriever, the component that provides access to world knowledge: a system can surface only what its retriever can find. Retrievers, however, have traditionally been specialized, each trained for a particular task, fitted to a single language, restricted to one modality, and limited to a single retrieval pass. Each such specialization makes the retriever effective for what it was built for but walls off everything else, fragmenting the world's information into information silos: bodies of relevant information that no single search system can cohesively reach. This thesis breaks these silos by replacing traditional retrieval's specialized components, one for each task, language, and modality, with general-purpose models that learn relevance, perceive documents, and reason over information needs, lifting each underlying constraint rather than building another pipeline. The thesis then studies how to make this unified access general, efficient, verifiable, and reproducible, across three dimensions: domain and language, modality, and representation space. For domain and language silos, I show that fine-tuning a large language model (LLM) as a dense retriever yields strong generalization across retrieval tasks, even without the task-specific pretraining or data augmentation that BERT-era retrievers relied upon. I then show that this relevance-modeling capability need not remain locked inside a large model: it can be distilled into small, efficient retrievers that generalize across both tasks and languages, and it can be exploited with no relevance supervision at all by letting an LLM generate a hypothetical document that an off-the-shelf encoder grounds to the corpus. A recurring consequence is that such retrievers improve in step with the underlying language models. For modality silos, I introduce a paradigm shift from extracting text out of documents to encoding documents as they appear. Document Screenshot Embedding encodes a document's screenshot directly with a vision-language model, preserving text, images, tables, and layout in a single dense representation and outperforming parsing- and OCR-based pipelines on mixed-modality retrieval. I further argue that unifying access is only half of the goal, since a unified system must also be trustworthy: VISA extends the visual paradigm to the output side, attributing each generated answer to the specific region of the source document that supports it, and yielding a fully visual pipeline from retrieval to verifiable generation. For representation-space silos, I address the limitation that a single query embedding reaches only a local neighborhood, while the evidence for a complex, reasoning-intensive need is dispersed across the space. Crossing this silo requires an agent that can reason and search iteratively. I therefore cultivate general reasoning by scaling reinforcement learning with verifiable rewards beyond mathematics and code, bring that reasoning into the retrieval pipeline through a reranker trained to reason before ranking, and construct a benchmark over a fixed, curated corpus that, for the first time, evaluates deep-search agents fairly by disentangling the contribution of the retriever from that of the reasoning agent. This evaluation shows that, even with strong agents, retrieval quality remains pivotal. Underpinning these contributions, I develop and maintain open-source toolkits that make the unified paradigm efficient and reproducible: one for reproducible first-stage retrieval and evaluation, and one for training neural retrievers at scale and across languages and modalities. Together, these works move document retrieval from a collection of specialized pipelines toward unified, efficient, verifiable, and agentic access to world knowledge. I conclude by outlining how representations, search agents, and interaction must evolve to make such access seamless across both digital and physical worlds.
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    Microbiology of pre-saturated gap fill bentonite compacted in pressure vessels
    (University of Waterloo, 2026-09-28) Schofield, Luke
    Deep geologic repositories (DGRs) are the planned long-term management and disposal approach for used nuclear fuel in many countries. Designs include the use of multiple engineered barriers, including used fuel containers, bentonite buffer material, and the surrounding host rock. In Canada’s design, used fuel containers (UFCs) will be encased in highly compacted bentonite blocks and placed at depths of 650-800 m below ground surface in emplacement rooms excavated from crystalline rock, with spaces between bentonite blocks and the host rock filled with a granular bentonite (i.e., “gap fill material”). Past research has shown that microorganisms exist in as-received bentonites, some of which could negatively influence the longevity of a DGR, for example through microbially influenced corrosion of copper-coated UFCs particularly by sulfate-reducing bacteria (SRB). Previous research has focused on the use of pump-saturated pressure vessels to simulate early stages of groundwater infiltration in a DGR. Results showed suppression of microbial abundances in compacted bentonite after a relatively slow saturation process is completed and high swelling pressures have been established. To examine post-saturation microbial suppression, while minimizing changes that occur during slow saturation, pressure vessels were used to rapidly compress pre-wetted gap fill material to varying dry densities under oxic or anoxic conditions. This approach targeted nominal 100% saturation prior to incubation of the bentonite “plugs” for periods of up to three months. Following incubation, bentonite plugs were analyzed using cultivation and 16S rRNA gene sequencing. Results revealed decreasing abundances of culturable aerobic heterotrophs with increasing bentonite dry density. Relative to the as-received bentonite starting material, no significant increases in the abundance of culturable microorganisms were detected for the highest dry density tested (1.45 g cm-3). In contrast, all lower dry densities tested showed significantly greater abundances of culturable aerobic heterotrophs compared to as-received bentonite. These samples were associated with high relative abundances of three Bacillus-associated ASVs that were mostly undetected in 1.45 g cm-3 samples. Both cultivation and sequencing results demonstrated that a bentonite dry density of 1.45 g cm-3 limited detectable increases in culturable abundances under the tested conditions, which is a target average dry density under consideration for repository gap fill material. No significant increases in SRB counts were detected by cultivation methods for any conditions tested, and 16S rRNA gene profiles also did not reveal increases in taxa affiliated with putative SRB. This research also presents a rapid saturation and compaction method for studying microbial suppression in bentonite, while minimizing the influence of slow saturation effects, allowing for rapid testing of post-saturation microbial processes.
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    Crystal Structures and Nonlinear Optical Properties of Semiconducting Main Group Chalcogenides
    (University of Waterloo, 2026-09-25) Licskai, Tristan
    I have investigated ten compounds where the experimental crystal structures have not been reported. Of these compounds, five are noncentrosymmetric, two are oxychalcogenides, and four are new structure types. The noncentrosymmetric compounds discovered are part of two distinct families: A2BQ4 and A6M4T4Q16 (A = Sr–Ba; M = Cu–Ag; B = Si–Sn; Q = O–Te). The A2BQ4 family contains Sr2SiSe4 (P21, a = 6.8866(19) Å, b = 7.0062(19) Å, c = 8.520(2) Å, β = 108.200(6)°) and Sr2SnSe4 (Ama2, a = 10.4802(10) Å, b = 10.6965(10) Å, c = 7.5411(7) Å). DFT calculations revealed Sr2SiSe4 and Sr2SnSe4 to have direct band gaps of 2.31 eV and 2.05 eV, respectively. Sr2SiSe4 was determined to have a second harmonic generation intensity of 0.5 × AGS, a laser-induced damage threshold of 3.1 × AGS, and a thermal conductivity of 0.46 W m-1 K-1 at 523 K. The A6M4T4Q16 family contains Sr6Ag3.91Si4Se16 (I4¯3d, a = 14.621(2) Å), Sr6Ag3.63Sn4Se16 (I4¯3d, a = 14.9295(5) Å), and Sr6Cu2.33Ag1.55Si4Se16 (I4¯3d, a = 14.4904(5) Å). DFT calculations revealed Sr6Ag3.91Si4Se16, Sr6Ag3.63Sn4Se16, and Sr6Cu2.33Ag1.55Si4Se16 to have direct band gaps of 1.14 eV, 1.09 eV, and 1.28 eV, respectively. Sr6Ag3.63Sn4Se16 was determined to have a second harmonic generation intensity of 0.5 × AGS, and a laser-induced damage threshold of 3.3 × AGS. The centrosymmetric compounds discovered are part of three distinct families: ABQ3, A3B2Q7, and oxychalcogenides. The ABQ3 family contains SrSiSe3 (P21/c, a = 6.6948(2) Å, b = 17.4606(4) Å, c = 15.8647(3) Å, β = 117.0100(10)°) and BaGeSe3 (P21/c, a = 15.6603(7) Å, b = 6.0379(3) Å, c = 14.0072(6) Å, β = 116.3830(10)°). DFT calculations revealed SrSiSe3 and BaGeSe3 to have band gaps of 2.15 eV and 1.65 eV, respectively. The A3B2Q7 family contains Sr3Sn2S7 (C2/m (no. 12), a = 23.207(2) Å, b = 7.9768(8) Å, c = 17.216(3) Å, β = 132.1738(17)°), which possesses rare SnS5 polyhedra in addition to SnS4 tetrahedra. DFT calculations revealed Sr3Sn2S7 to have a direct band gap of 1.10 eV. The oxychalcogenide family contains Sr3Ge3OS8 (Pnma, a = 15.6628(6) Å, b = 13.9919(5) Å, c = 6.6687(3) Å) and Ba4Ge2O3Se5 (Pnma, a = 11.536(14) Å, b = 10.539(11) Å, c = 11.836(13) Å). DFT calculations revealed Sr3Ge3OS8 and Ba4Ge2O3Se5 to have band gaps of 2.55 eV and 2.25 eV, respectively.
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    Quantum Information Probes in Holography: Shell Geometries and Black Hole Singularities
    (University of Waterloo, 2026-09-25) Joerstad, Eivind Hilmen
    A remarkable aspect of the AdS/CFT correspondence and related holographic dualities is the way quantities related to quantum information in the non-gravitational boundary theory are geometrized by the bulk gravitational theory. The most well known example is the relationship between minimal surfaces in the bulk and the boundary entanglement entropy, as described by the Ryu-Takayanagi formula. Another example is measures of quantum complexity, whose bulk duals are believed to be a class of geometric bulk observables called ``complexity=anything'' that we explain in the main body of the thesis. In this thesis, we study these holographic quantum information probes in two families of geometries that arise either in AdS/CFT or in related string-theory derived dualities. We explore the two families of geometries for essentially different purposes. First, we consider so-called shell geometries, which are sourced by a spherical distribution of Dp-branes. We are interested in these geometries as they contain within the shell a region of flat spacetime. Hence, this setup gives us a clean way to study a region of flat spacetime geometry in a setting with a controlled holographic dual theory. Our motivation is then to use this setup to extract lessons relevant for the quest to understand holography in asymptotically flat spacetimes. To this end, we study the holographic entanglement entropy and associated entropic c-functions in these backgrounds. We also explore the properties of so-called internal RT surfaces in these geometries, and draw a connection to RT surfaces in asymptotically flat spacetimes. We also briefly evaluate a holographic complexity observable in this background, and confirm that the result is compatible with the conclusion of the entanglement entropy calculations. We conclude that the creation of the flat space region entails a drastic reduction in effective IR degrees of freedom in the boundary theory. Then, we investigate how so-called complexity=anything observables encode the interior geometry of (d+1)-dimensional asymptotically AdS black holes. The flexibility of this class of observables allows the corresponding extremal surfaces to be pushed arbitrarily close to the spacelike singularity, and we show that their late-time growth rate carries information about the near-singularity geometry. After resolving a puzzle concerning codimension-one observables near the singularity, we evaluate observables defined on constant-mean-curvature slices and find that the late-time growth rate depends on the value of the mean curvature, which controls how deeply the slices probe the interior. This family of observables then encodes properties of the interior region. Comparing the uncharged and charged cases, we find that for AdS-Reissner–Nordström black holes these observables only probe the interior up to the inner horizon, so in this case the observables do not reveal the presence of a singularity.