VTechWorks
VTechWorks provides global access to Virginia Tech scholarship, including journal articles, books, theses, dissertations, conference papers, slide presentations, technical reports, working papers, administrative documents, videos, images, and more by faculty, students, and staff. Faculty can deposit items to VTechWorks from Elements, including journal articles covered by the University open access policy. Email vtechworks@vt.edu for help.
Open Access Policy
Virginia Tech's open access policy enables researchers to deposit the accepted version of scholarly articles with no embargo.
Theses and Dissertations
Virginia Tech was first in the world to require ETDs in 1997, and continues to add scans of older theses and dissertations.
Open Textbooks
More than 50 freely available and openly licensed textbooks are among our most downloaded items.
Communities in VTechWorks
Select a community to browse its collections.
Recent Submissions
Connecting the Opens: Open Education Week 2026
Lener, Edward F.; Walters, Tyler; Walz, Anita R.; Lord, James K.; Grohs, Jacob R.; Pullen, Brandie; Surprenant, Aimée (Virginia Tech, 2026-03-02)
The 2026 Connecting the Opens panel discussion features brief overviews of open educational resources, open access, and open data to provide some general information on the topics. Finally, presenters will discuss how aspects of open have impacted their career development as well as how they have incorporated open practices into their research and scholarship, and the audience will be invited to participate in the conversation.
High-Density, High-Efficiency Resonant Converters for Power Delivery to Modern Datacenters
Prakash, Pranav Raj (Virginia Tech, 2026-03-11)
Global demand for computational power—driven by large-scale artificial intelligence, machine learning, and high-performance cloud workloads—is accelerating at an unprecedented pace. As GPUs continue to grow in power, size, and current demand, datacenter power-delivery architectures have evolved from traditional 12 V distribution to 48 V systems and are now transitioning toward high-voltage 800 V and ±400 V rack buses. Although these architectural shifts reduce copper mass, mitigate busbar losses, and improve system scalability, they also impose new requirements on the isolated and non-isolated DC–DC converters embedded throughout the power-delivery chain. Despite differing roles—from tightly regulated front-end converters to unregulated high-ratio intermediate bus converters (IBCs)—all stages share the same overarching mandate: maximize efficiency and power density under tightening electrical, thermal, and spatial constraints.
To meet these demands, this dissertation investigates advanced methodologies for high-performance LLC resonant converters across multiple power-delivery stages. A unifying principle throughout the work is the deployment of PCB-integrated magnetics, which offer excellent manufacturability, repeatability, thermal performance, and ultra-low profile compared to conventional wound components. However, PCB transformers introduce stringent challenges in synchronous-rectifier (SR) termination, particularly when distributed output capacitors interact through parasitic inductances to create parallel resonances. These resonances can substantially increase transformer AC resistance at multi-hundred-kilohertz switching frequencies. Through analytical modeling, finite-element simulation, and hardware validation, this work characterizes these mechanisms and proposes optimized termination structures that suppress resonant peaking and lower conduction losses—recovering up to 3.5% full-load efficiency in 1 kW and 2 kW LLC prototypes.
Building on this foundation, the dissertation next addresses the emerging high-voltage DC-distribution architectures for AI datacenters. A 6 kW, 800 V/50 V stacked LLC converter is developed using 650 V GaN devices in a three-level topology, enabling operation from both 0–800 V and ±400 V buses. A custom four-leg PCB-integrated magnetic structure merges two EI-core transformers into a compact, symmetric, and thermally optimized assembly. A top-cooled SR termination network is introduced to accommodate high output currents within severe footprint constraints. Operating near 600 kHz, the prototype achieves 98.7% peak efficiency and a record power density of 2070 W/in3—demonstrating the feasibility of compact GaN-based conversion for next-generation megawatt-class server racks.
To support extreme-current GPU loads exceeding 1000 A, the dissertation also develops a high-density intermediate bus converter for emerging vertical power-delivery (VPD) architectures. A modular transformer unit-cell structure is introduced that maximizes magnetic utilization while enabling flexible series/parallel scalability. This concept is validated through an 840 W, 48 V/1.8 V LLC-DCX module achieving 2200 W/in3 and 95.5% peak efficiency, providing a viable pathway for shrinking regulator footprint, reducing PDN losses, and enabling back-side power delivery in future high-TDP GPUs.
Finally, to reduce bulk-capacitor requirements in multi-kilowatt front-end AC–DC PSUs, the effective gain range of the LLC converter must be increased. To this end, a selective synchronous-rectifier (SR) phase-shift control technique is developed that extends the achievable gain without compromising soft-switching behavior. By phase-shifting only a subset of SR bridges in matrix-transformer structures, the required frequency variation is significantly reduced compared to prior-art methods, thereby minimizing circulating energy, lowering SR switching losses, and reducing the likelihood of ZVS loss in the primary switches. Experimental validation on a 3 kW, 400 V/50 V LLC converter—achieving 98.7% peak efficiency and 1300 W/in3 power density—confirms that the technique enables up to a 36% reduction in hold-up capacitance while maintaining stable and efficient operation.
The approach is further extended to center-tapped rectifiers, where coordinated SR control eliminates reverse-conduction issues even when operating at the resonant frequency. Collectively, this dissertation advances the state of the art in high-density, high-efficiency LLC resonant conversion for modern datacenter power architectures. The contributions span device-level loss mechanisms, PCB-integrated magnetic design, high-voltage GaN converter topologies, extreme-current vertical power delivery, and advanced gain-extension control strategies. In addition, the work summarizes practical design guidelines for SR termination techniques for both center-tapped and full-bridge rectifiers across varying current levels, cooling configurations, and layout constraints. Together, these developments chart a clear path toward compact, thermally efficient, and scalable DC–DC converter platforms capable of meeting the escalating power demands of the AI era.
Distribution and Characterization of Herbicide-Resistant Italian ryegrass and Palmer amaranth in Virginia
Viric, Milos (Virginia Tech, 2026-03-11)
Weed infestation is the major reason for economic losses in agriculture. Italian ryegrass and Palmer amaranth are some of the most troublesome weed species in Virginia. These species are strong competitors with crops for growth resources which eventually leads to significant yield losses in absence of adequate control. One of the challenges for the control of these species is development of herbicide-resistant populations.
There is a limited knowledge about the distribution of resistant populations of Italian ryegrass and Palmer amaranth in Virginia. Palmer amaranth resistance to glyphosate was confirmed in 2011 and Italian ryegrass resistance to diclofop was confirmed in 1993. These are the only two confirmed cases of herbicide resistance in Virginia but based on control failure reports, resistance to these species is suspected to be more widespread in Virginia. To investigate the distribution and levels of resistance in populations from Virginia there is a necessity for more updated surveys.
A total of 32 populations of Italian ryegrass were collected. Plants were grown in the greenhouse to test for sensitivity to herbicides commonly used for burndown or in-crop control of Italian ryegrass: pinoxaden, diclofop, glyphosate, mesosulfuron, pyroxsulam, and pyroxasulfone. At 21 days after the herbicide treatments, visible injury ratings were recorded on a scale 0 to 100%, where 0 indicates no control and 100 represents complete plant necrosis. Populations exhibiting ≤49% control were suspected to be resistant. Based on this criteria, 10, 27, 0, 14, 0, and 7 populations were found to be resistant to pinoxaden, diclofop, glyphosate, mesosulfuron, pyroxasulfone, and pyroxsulam, respectively. Following the initial screening, dose-response assays with pinoxaden, diclofop, mesosulfuron and pyroxsulam were conducted. Resistance indices (R/S ratios), calculated based on GR50 (herbicide dose that reduced biomass by 50%) values for resistant and susceptible populations, were 20 for pinoxaden, 87 for mesosulfuron, and 161 for pyroxsulam. The R/S value for diclofop could not be determined because even the highest tested dose could not achieve 50% growth reduction in the resistant population. Cross and multiple resistance was observed in this study and 6% of populations were found resistant to pinoxaden, diclofop-methyl, mesosulfuron, and pyroxsulam.
A total of 68 Palmer amaranth populations were collected from corn, soybean and cotton fields across Virginia. Palmer amaranth seedlings grown in the greenhouse were treated with: trifloxysulfuron, 2,4-D, fomesafen, atrazine, mesotrione, glyphosate, glufosinate and dicamba. Visible control ratings were recorded on a 0 to 100% scale, where populations with up to 49% injury were considered resistant. Upon testing the populations, resistance was found in 46, 1, 3, 7, 3, 50, 0 and 0 populations to trifloxysulfuron, 2,4-D, fomesafen, atrazine, mesotrione, glyphosate, glufosinate and dicamba, respectively. Dose-response assay for glyphosate revealed that GR50 value for resistant population was 1,238 g ae ha-1, however R/S value could not be calculated as susceptible population was not available. The R/S values for trifloxysulfuron, fomesafen and atrazine were 47, 14 and 18, respectively. Approximately 69% of the populations showed multiple resistance to two or more herbicide sites of action. Overall, findings from these statewide surveys provide critical insights into the current herbicide resistance status for both Italian ryegrass and Palmer amaranth in Virginia. This information will help growers better understand the effectiveness of commonly used herbicides and make more informed management decisions.
Enhancing Web Privacy through Fine-Grained Program Analysis of Tracking JavaScript
Amjad, Abdul Haddi (Virginia Tech, 2026-03-11)
Advertisers and tracking services (ATS) are pervasive on the modern web. To counter these practices, millions of users rely on privacy-enhancing technologies (PETs), such as ad-blockers, which primarily depend on filter lists composed of regex-based rules to block network requests associated with data exfiltration. However, this approach has become increasingly ineffective in the face of an ongoing arms race. Currently, PETs struggle to handle mixed JavaScript (JS) scripts that combine tracking behavior with legitimate website functionality. These tools operate at the network level and can only block entire script requests rather than specific behaviors within a script. As a result, once a mixed script executes, any subsequent data exfiltration carried out through encrypted URL requests becomes invisible to network-based defenses, rendering existing PETs ineffective. Mixed JS scripts create a fundamental dilemma for PETs: aggressively blocking them risks breaking websites, while allowing them undermines user privacy. This thesis addresses this challenge by asking: How can code-aware program analysis be leveraged to design privacy-enhancing technologies that effectively mitigate tracking while preserving essential web functionality? This thesis makes three primary contributions. First, we focus on identification of mixed JS by introducing TrackerSift, a large-scale measurement framework that reveals mixed behavior in a substantial fraction of web JS scripts. Second, we validate that mixed JS scripts can be effectively handled through program analysis, demonstrating that functions inside mixed JS scripts can clearly separate tracking code from functional code. Finally, we address mitigation by presenting NoT.JS, a code-aware, ML–based system that accurately identifies tracking JS functions inside mixed JS scripts and refactors them to selectively remove tracking logic while maintaining legitimate functionality. Together, these contributions advance privacy-enhancing technologies by enabling principled mitigation of mixed JS scripts through code-aware program analysis.
Partial Courses of Fidaxomicin Followed by Oral Vancomycin and the Effect on Recurrence of Clostridioides difficile Infections
Papamanolis, Irene-Constantina; Stornelli, Nicholas; Everson, Nathan; Ahmad, Zayd; Kamrada, Meghan; Lockhart, Ellen Rachel; McDaniel, Lauren (Sage, 2025-12-01)
Background: Clostridioides difficile infection (CDI) causes a significant national health care burden. Literature has demonstrated lower rates of CDI recurrence with fidaxomicin compared with oral vancomycin. However, patients are sometimes switched to oral vancomycin before completing a fidaxomicin course. Objective: The objective of this study is to evaluate rates of CDI recurrence in full courses of fidaxomicin versus partial courses of fidaxomicin followed by a switch to oral vancomycin. Methods: In this single-center, retrospective, cohort study of adults with CDI, patients were screened for inclusion if they received either a full 10-day course of fidaxomicin or partial course of fidaxomicin followed by a switch to oral vancomycin. The primary outcome was the rate of CDI recurrence within 30 days after completion of initial therapy determined by a positive CDI test and initiation of treatment. Results: Ninety-nine patients received a full course of fidaxomicin, and 95 patients received a partial course of fidaxomicin followed by oral vancomycin. Mean age was lower in the full course group compared with the partial course (65.3 years vs 71.5 years, P < 0.002). Clostridioides difficile infection recurrence occurred in 5.1% of the full course group and 7.4% of the partial therapy group (P = 0.503) at 30 days and 13.1% versus 14.7% (P = 0.747) at 90 days. Clostridioides difficile infection–related readmissions at 30 days were similar in the full course and partial course groups (7.1% vs 4.2%, P = 0.389). Conclusion and Relevance: Partial courses of fidaxomicin followed by oral vancomycin had similar 30-day CDI recurrence compared with full course fidaxomicin.