VTechWorks Repository :: Browsing by Author "Homa, Daniel S."

Browsing by Author "Homa, Daniel S."

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Abrasive Blasting with Post-Process and In-Situ Characterization
Mills, Robert Jeffrey (Virginia Tech, 2014-07-25)
Abrasive blasting is a common process for cleaning or roughening the surface of a material prior to the application of a coating. Although the process has been in practice for over 100 years, the lack of a comprehensive understanding of the complex interactions that exist with the process can still yield an inferior surface quality. Subsequently, parts can be rejected at one of many stages of the manufacturing process and/or fail unexpectedly upon deployment. The objective of this work is to evaluate the effect of selected input parameters on the characteristics of the blasted surface characteristics so that a more useful control strategy can be implemented. To characterize surface roughness, mechanical profilometry was used to collect average roughness parameter, Ra. Decreasing blast distance from 6” to 4” gave ΔRa = +0.22 µm and from 8” to 6” gave ΔRa = +0.22 µm. Increasing blast pressure from 42 psi to 60 psi decreased the Ra by 0.33 µm. Media pulsation reduced Ra by 0.56 µm and the use of new media reduced Ra by 0.47 µm. Although blasting under the same conditions and operator on different days led to ΔRa due to shorter blast times, there was no statistically significant variance in Ra attributed to blasting on different days. Conversely, a ΔRa = +0.46 µm was observed upon blasting samples with different cabinets. No significant ΔRa was found when switching between straight and Venturi nozzles or when using different operators. Furthermore, the feasibility of fiber optic sensing technologies was investigated as potential tools to provide real time feedback to the blast machine operator in terms of substrate temperature. Decreasing the blast distance from 6” to 4” led to ΔT = +9.2 °C, while decreasing the blast angle to 45° gave ΔT= -11.6 °C for 304 stainless steel substrates. Furthermore, increasing the blast pressure from 40 psi to 50 psi gave ΔT= +15.3 °C and changing from 50 psi to 60 psi gave ΔT= +9.9 °C. The blast distance change from 8” to 6” resulted in ΔT = +9.8 °C in thin stainless steel substrate temperature. The effects of substrate thickness or shape were evaluated, giving ΔT= +7.4 °C at 8” distance, ΔT= +20.2 °C at 60 psi pressure, and ΔT= -15.2 °C at 45° blasting when comparing thin stainless steel against 304 stainless steel (thick) temperatures. No significant ΔT in means was found when going from 6” to 8” distance on 304 stainless steel, 40 psi and 60 psi blasting of thin SS, as well as angled and perpendicular blasting of thin SS. Comparing thick 304 and thin stainless steel substrates at a 6” blast distance gave no significant ΔT.
The Adhesion Strength of a Plasma Sprayed Silicon Bond Coating on a Silicon Carbide Ceramic Matrix Composite
Scherbarth, Austin Daniel (Virginia Tech, 2020-10-19)
Silicon-based ceramics and ceramic matrix composites (CMCs), such as silicon carbide (SiC) fiber reinforced SiC, are promising candidates for hot section components in next generation turbine engines. Environmental barrier coatings (EBCs) are essential for implementing these components as they insulate and protect the substrate from reaction with water vapor in the engine environment. EBCs are typically deposited via atmospheric plasma spraying (APS) and preparing the component surfaces through cleaning and roughening prior to coating is a vital step to ensure sufficient coating adhesion. The adhesion of a plasma sprayed coating to the underlying component is one of the most important properties as the component will not be protected if the coating is not well adhered. Surface roughening of metallic components via grit blasting is well documented and understood, but much less is known about preparing ceramic and ceramic composite surfaces for thermal spray coating. Silicon coatings are often used as a bond coating between SiC-based components and EBC top layers, but the adhesion strength of plasma sprayed Si on these substrates, Si splat formation and the factors that affect coating formation and adhesion have not been well studied. The effects of automated grit blasting process parameters on surface roughness and material loss of a reaction bonded SiC (rb SiC) composite were evaluated. Surface roughness before and after grit blasting was evaluated with a confocal laser scanning microscope. The differences and advantages of automated grit blasting compared to manual grit blasting were observed. Most notably was the level of control at high nozzle traverse speeds resulting in reduction of material loss and consistency of roughening. At high nozzle traverse speeds, the amount of material loss decreased greatly with a small effect on induced surface roughness. The degree of grit blasting induced roughness and material loss was found to be largely dependent on the nature of the composite matrix and reinforcement, as well as blast nozzle traverse speed. A statistical model was developed to predict the substrate thickness loss and induced average roughness based on nozzle traverse speed and blast pressure for automated grit blasting. Additionally, laser ablation was used to create controlled, regularly patterned surface texture on rb SiC substrates to further investigate the role of texture parameters in Si coating adhesion. Si was plasma sprayed onto rb SiC substrates to deposit both thick coatings to evaluate adhesion strength and single splats to study splat formation. Surface roughness/texture, substrate preheat temperature and mean Si particle size were varied in plasma spray coating experiments to observe their role in coating adhesion strength. Si adhesion strength was found to be related to all three factors and a statistical model was developed to predict adhesion strength based on them. Substrate preheat temperature had a significant effect on both Si adhesion strength and Si splat formation on rb SiC. Single splat formation during plasma spraying of Si on SiC was simulated with software called SimDrop. Simulations of Si droplet impact, spreading and solidification during plasma spraying on smooth and textured SiC surfaces were used to investigate the effects of relevant process parameters on splat formation. Experimentally observed Si splats on smooth substrates at different temperatures during deposition were matched with simulated splats with the same spraying parameters. A change in thermal contact resistance with changing substrate preheat temperature was confirmed by the simulation results. The role of surface texture parameters for a regularly patterned surface texture in splat formation was demonstrated through simulation. This dissertation investigates methods of roughening and preparing a SiC composite substrate for plasma spray coating, as well as factors which affect the adhesion strength and splat formation of plasma sprayed Si through experiments and simulation. The observations made provide valuable insight for understanding and optimizing the manufacturing processes utilized to deposit strongly adhered coatings onto SiC-based composites. In addition, areas of interest in this field for future study and further investigation are introduced and suggested.
Application of Sapphire-Fiber-Bragg-Grating-Based Multi-Point Temperature Sensor in Boilers at a Commercial Power Plant
Yang, Shuo; Homa, Daniel S.; Heyl, Hanna; Theis, Logan; Beach, John; Dudding, Billy; Acord, Glen; Taylor, Dwyn; Pickrell, Gary R.; Wang, Anbo (MDPI, 2019-07-21)
Readily available temperature sensing in boilers is necessary to improve efficiencies, minimize downtime, and reduce toxic emissions for a power plant. The current techniques are typically deployed as a single-point measurement and are primarily used for detection and prevention of catastrophic events due to the harsh environment. In this work, a multi-point temperature sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (SFBGs) were fabricated via the point-by-point method with a femtosecond laser. The sensor was packaged and calibrated in the lab, including thermally equilibrating at 1200 °C, followed by a 110-h, 1000 °C stability test. After laboratory testing, the sensor system was deployed in both a commercial coal-fired and a gas-fired boiler for 42 days and 48 days, respectively. The performance of the sensor was consistent during the entire test duration, over the course of which it measured temperatures up to 950 °C (with some excursions over 1000 °C), showing the survivability of the sensor in a field environment. The sensor has a demonstrated measurement range from room temperature to 1200 °C, but the maximum temperature limit is expected to be up to 1900 °C, based on previous work with other sapphire based temperature sensors.
Coupled Mode Analysis for 3D Stress-Free Elastic Acoustic Waveguide
He, Jiaji; Homa, Daniel S.; Pickrell, Gary R.; Wang, Anbo (IEEE, 2019)
Acoustic sensors and acoustic measurements receive much attention in various applications. Because waveguides are commonly used in sensor design, theoretical means to study acoustic propagation and interaction in waveguides are necessary. However, current methods for elastic wave coupling, including the transfer matrix method and coupled mode theory in planar 2D waveguides, are not satisfactory. In this work, a coupled mode analysis for acoustic waves in 3D stress-free elastic waveguides is proposed. Similar to the coupled mode theory in optical waveguides, the analysis is presented by the evolution of modal amplitudes. It can solve various modal conversion and scattering problems in elastic waveguides with small changes of cross sections and stress-free boundaries. To demonstrate the practicability, the coupled mode analysis is used to calculate the reflection spectrum of the newly proposed structure, the acoustic fiber Bragg grating. In a notch-based grating fabricated on a thin cylindrical waveguide, the results from coupled mode analysis are in good agreement with those from the transfer matrix method, which has been already validated experimentally. The coupled mode analysis is a promising method to solve various scattering problems.
Cure Kinetics of Two Part Epoxy Resin and the Effect on Characterization of Thermal Barrier Coatings
Chang, Sunny (Virginia Tech, 2015-05-28)
The aerospace industry strives to develop new methods of refining gas turbine engines by increasing power and thermal efficiencies while simultaneously reducing cost. Turbine engines operate under high temperatures and therefore thermal barrier coatings (TBCs) composed of yttria-stabilized zirconia (YSZ) play an important role in improving the performance of the components that make up the engine. Failure of the TBC could lead to catastrophic events, thus requiring consistent and accurate characterization for supplier qualification and production quality assurance. However, due to porosity and the anisotropic behavior of the coating and variability in processing of TBCs, consistent characterization has proven to be extremely challenging. One of the reoccurring issues is the inconsistency in measuring percent porosity, which stems from the difficulty in distinguishing filled pores from damaged, unfilled voids. Sample preparation of TBCs involves sectioning, mounting, grinding, polishing, and characterization. Eliminating variability in characterization begins with mounting which is a critical step to protect the surface integrity and edge retention of the coating during grinding and polishing. The curing kinetics of a slow cure two part epoxy was investigated and the TBC samples were mounted and cured at heating rates of 2, 5, and 10°C/min to 55°C and 70°C. Grinding and polishing procedures simulated industry practices followed by characterization with optical microscopy. Results showed that heating rates of 2°C/min to 55°C and 70°C have the best impregnation properties while uncontrolled or high heating rates of 10°C/min had an increase in the amount of pullouts and lack of infiltration from the epoxy. The curing kinetics of the epoxy needs to be controlled to eliminate the ambiguity of filled and unfilled pores.
Devitrification Kinetics and Optical Stability of Optical Fibers at High Temperatures
Yakusheva, Anastasia A. (Virginia Tech, 2018-06-07)
Reliable sensing and monitoring systems based on optical fibers operating at high temperatures and in harsh environments are of high demand. One of the limitations of such systems is the devitrification of the fused silica based core and cladding glass at elevated temperatures. Crystallites can nucleate on the surface of the cladding and grow into the core. The formation of these crystalline flaws in the optical fiber causes stress concentration and extrinsic optical scattering and in addition leads to decreased mechanical properties and reduced optical stability. Commercial optical fibers of different compositions and core-cladding design were characterized in this study with respect to crystallization rate under various conditions. The optical stability was monitored with an optical spectrum analyzer. The crystallites were characterized with SEM and optical microscopy. The activation energies of crystallization for High OH and Low OH multimode fibers were estimated by measuring the crystal growth rate at different temperatures. The residual stress resulting from the formation of the crystals, which can lead to decreased mechanical performance of the fibers, was characterized with polarized light optical microscopy. The influence of water vapor in the atmosphere on the crystallization rate was determined. The features induced in the attenuation spectra were consistent with hydroxyl (OH) absorption peak. Spectral features such as thermal emission and hydroxyl absorption bands are discussed. The results obtained in this study can be used for selecting optical fibers for high temperature applications.
Dissolution and Diffusion-Based Reactions within YBa2Cu3O7-x Glass Fibers
Heyl, Hanna; Yang, Shuo; Homa, Daniel S.; Slebodnick, Carla; Wang, Anbo; Pickrell, Gary R. (MDPI, 2019-12-20)
This work presents a thorough identification and analysis of the dissolution and diffusion-based reaction processes that occur during the drawing of YBa₂Cu₃O_7-x (YBCO) glass-clad fibers, using the molten-core approach, on a fiber draw tower in vacuum and in oxygen atmospheres. The results identify the dissolution of the fused silica cladding and the subsequent diffusion of silicon and oxygen into the molten YBCO core. This leads to a phase separation due to a miscibility gap which occurs in the YBCO–SiO₂ system. Due to this phase separation, silica-rich precipitations form upon quenching. XRD analyses reveal that the core of the vacuum as-drawn YBCO fiber is amorphous. Heat-treatments of the vacuum as-drawn fibers in the 800–1200 °C range show that cuprite crystallizes out of the amorphous matrix by 800 °C, followed by cristobalite by 900 °C. Heat-treatments at 1100 °C and 1200 °C lead to the formation of barium copper and yttrium barium silicates. These results provide a fundamental understanding of phase relations in the YBCO–SiO₂ glass-clad system as well as indispensable insights covering general glass-clad fibers drawn using the molten-core approach.
Fabrication and Characterization of Superconducting Core Fibers with Fused Silica Cladding
Liang, Yongxuan (Virginia Tech, 2014-01-29)
Since the discovery of superconductivity, its fantastic properties have fascinated the scientific community. The discovery of high critical temperature (Tc) superconducting compositions further inspires the wide applications of superconductors with relatively inexpensive liquid nitrogen cooling. Recently, the integration of superconductivity and optical waveguides has put forward the potential for ultrasensitive, ultra-fast and ultralow noise light detectors. However, simple and cost effective superconductor designs and fabrication processes are still required to enable wide implementation. The objective of this research was to study the fabrication of the superconductor core fibers with a fused silica cladding via the melt-draw approach, as well as develop appropriate characterization techniques to describe the fibers produced. In addition, a further objective was to determine the cooling efficiency of ordered holes around a superconductor core and construction of a one dimensional (1-D) single-phase steady state model to predict the heat transfer during cryogenic liquid transfer inside glass tube. In this thesis, both Pb and YBCO superconductor core fibers with fused silica cladding have been demonstrated. The fibers were fabricated via the melt-draw technique and maintained overall diameters ranging from 200-900 μm and core diameters of 100-800 μm. Surface morphology, chemical composition, interface effect, and superconductivity were further investigated. Surface morphology analysis confirmed that the Pb and YBCO core fibers possessed good circularity and clean interfaces between the core and cladding. Both the Pb and YBCO cores were relatively dense after the melt-draw process. The melt-draw process avoided contamination during fabrication as indicated by the composition analysis. Limited PbO was examined on the Pb core surface but further action will be required to detect the source of oxygen. The YBCO core maintained a stoichiometric ratio comparable to the superconducting phase even after the melt forming process. The elemental mapping showed that limited cross-diffusion occurred between the Pb core and fused silica cladding. Conversely significantly more elemental cross interaction between the core and cladding was noted for the YBCO core fiber. Superconductivity of the Pb core was verified by a custom designed four-probe technique in liquid helium. The YBCO core was also confirmed to be superconductive after heat treatment with O₂ present. The feasibility of efficient cooling by the holey glass tubes was confirmed. A 1-D single-phase steady state model was constructed to evaluate the heat transfer mechanism. The experimental results are in reasonable agreement to the theoretical calculation.
Fiber Bragg Gratings Embedded in 3D Printed Prototypes
Pickrell, Gary R.; Homa, Daniel S. (2016)
In this study, we incorporated fiber optic sensors in 3D printed prototype parts. Fiber optic Bragg gratings embedded in polylactic acid were configured to measure strain and/or temperature. Residual non-uniform stresses in the 3D printed parts induced spectral distortions in the FBGs such as peak broadening and wavelength hopping. Local isolation of the FBG in a used quartz capillary tube minimized the spectral distortion and peak wavelengths were readily identifiable with commercial interrogation software. The seamless integration of robust optical fiber sensing techniques and additive manufacturing processes is readily feasible, via proper implementation and interrogation schemes, for a wide array applications to include structural health monitoring and real-time component diagnostics.
Fully Distributed Multi-Material Magnetic Sensing Structures for Multiparameter DAS Applications
Hileman, Zachary Daniel (Virginia Tech, 2022-06-29)
This dissertation demonstrates the first of its kind distributed magnetic field sensor based on a fiber optic distributed acoustic sensing (DAS) scheme. Ferromagnetic nickel and Metglas® were dispersed internally within a fiber optic preform and then drawn on an in-house fiber optic draw tower to lengths in the kilometers. Due to the close proximity of the ferromagnetic metals and fiber optic core, the magnetostrictive strain response of the ferromagnetic materials when exposed to a magnetic field would perturbate within the fiber cladding and transfer that strain, internally, to the fiber optic core. Strain resulting from the magnetostrictive effect allows the DAS based sensor to accurately translate strain into readable magnetic field data. Due to the high sensitivity seen in this sensor design, multiparameter sources, acoustic and magnetic fields, were tested and validated and a three dimensional magnetic-field vector sensor was proposed. Numerical analysis of the novel sensor design was first implemented using COMSOL Multiphysics, where inputs such as magnetostrictive element shape, size, distance, and number were first investigated. Upon optimizing system constraints, the sensor design was further modified such that single mode operation was consistent across multiple fiber draws while retaining high strain transfer from the ferromagnetic elements to the fiber optic core. Ferromagnetic material selection was evaluated as a function of the saturation magnetostriction constants and a total of 4 modules were used to fully characterize the complex physics involved in this sensor design. All fabrication and testing were performed in-house using a full scale 3-story fiber draw tower and custom environmental testing stations to imitate naturally occurring events such as magnetic or acoustic point sources. A unique stacking method was used to embed ferromagnetic nickel and Metglas® into a fiber optic preform which when combined with a custom fiber draw process resulted in consistent multi-material fibers drawn to lengths of 1-km. In-house testing facilities included different types of electromagnetic generators, in addition to a soil test bed, and an outdoor test bed which allowed 100 meters of fiber to be tested simultaneously. All tested sensors demonstrated high strain transfer capabilities on the order of 0.01-10 μϵ depending on the materials used, ferromagnetic rod number, and core to metal spacing. Due to the sensitivity of the system the difference between AC and DC was distinct, and directional magnetostriction was studied. Transverse and longitudinal magnetic wave propagation was controlled through a solenoid and rectangular Helmholtz coil, both built in-house. A three-dimensional magnetic field vector sensor was proposed due to the success of the magnetic field sensor, and a design was proposed and initially tested to validate direction as a function of field strength and distance. To summarize, this dissertation explores the first fully distributed magnetic field sensor using DAS based techniques and one of the first multi-material fiber draw processes which can produce consistent single mode fiber up to 1-km. Due to extensive FEA modeling, multiple iterations of the magnetic sensor were fully characterized and an equation describing the relationship between sensor design and strain transfer has been created and validated experimentally. Multi-parameter tests including acoustic and magnetic fields were implemented and an algorithm was developed to separate the mixed signals. Finally, a test was performed to demonstrate the feasibility of sensing magnetic fields directionally. Cumulative results demonstrate a high-quality sensor alternative to current designs which may surpass other magnetic sensors due to innate multi-parameter capabilities, in addition to the inexpensive production cost and extremely long operating lengths.
High-Temperature Superconducting Fiber
Homa, Daniel S.; Liang, Yongxuan; Pickrell, Gary R. (Springer, 2014-01-29)
In this study, we demonstrated superconductivity in a fiber with an yttrium barium copper oxide core and fused silica cladding. The fibers were fabricated via a modified melt-draw technique and post-process annealing treatment in excess oxygen. The fibers maintained overall diameters ranging from 100–900 microns and core diameters of 50–700 microns. Superconductivity of this fiber design was validated via the traditional four-point probe test method in a bath of liquid nitrogen at temperatures on the order of 93 K. The high-temperature superconducting fiber provides a glimpse of its cross cutting potential in fields of electromagnetism, healthcare, optics, and energy and lends credence to the promise for superconductivity.
Magnetic Field Sensing via Multi-Material Acoustic Sensing Optical Fibers with Magnetostrictive Cladding Inclusions
Dejneka, Zachary Bryce (Virginia Tech, 2024-03-28)
In this conducted research, optical fiber sensors are used to measure low strength alternating magnetic fields. Various fiber sensor configurations are tested and investigated to demonstrate sensing capabilities at different field magnitudes and frequencies. Distributed acoustic sensing fibers (DAS) have been largely studied and documented across a variety of applications and sensing systems. This research uses the DAS technology in tandem with magnetostrictive materials to create a distributed multi-material optical fiber magnetic sensor. Magnetic sensing has high demand across different fields and often runs into challenges of extreme environments including high temperature, corrosion, and areas with poor accessibility. The robust and distributed nature of optical fiber sensors which can be cheaply produced for long lengths is an attractive option over other single point magnetic sensors. In down hole applications specifically, having a distributed sensor able to be deployed easily and over long distances for magnetic sensing would be a large improvement to bulkier traditional magnetometers. In the conducted study, different magnetostrictive materials are implemented in distributed optical fiber sensors to analyze and compare the effective sensitivity and potential commercial viability. Nickel, galfenol alloy, and MetGlas alloy inclusions are drawn into fused silica optical fibers with Bragg gratings inscribed later on for DAS capability. Each was investigated for its response to varying AC magnetic fields to determine relative sensitivity and resolution for distributed magnetic field sensing.
Magnetic Sensing with Ferrofluid and Fiber Optic Connectors
Homa, Daniel S.; Pickrell, Gary R. (MDPI, 2014-02-25)
A simple, cost effective and sensitive fiber optic magnetic sensor fabricated with ferrofluid and commercially available fiber optic components is described in this paper. The system uses a ferrofluid infiltrated extrinsic Fabry-Perot interferometer (EFPI) interrogated with an infrared wavelength spectrometer to measure magnetic flux density. The entire sensing system was developed with commercially available components so it can be easily and economically reproduced in large quantities. The device was tested with two different ferrofluid types over a range of magnetic flux densities to verify performance. The sensors readily detected magnetic flux densities in the range of 0.5 mT to 12.0 mT with measurement sensitivities in the range of 0.3 to 2.3 nm/mT depending on ferrofluid type. Assuming a conservative wavelength resolution of 0.1 nm for state of the art EFPI detection abilities, the estimated achievable measurement resolution is on the order 0.04 mT. The inherent small size and basic structure complimented with the fabrication ease make it well-suited for a wide array of research, industrial, educational and military applications.
Non-Invasive Flow Measurement Via Distributed Acoustic Sensing Utilizing Frequency Spectra Analysis of Wall Pressure Fluctuations
Snider, Steven Michael (Virginia Tech, 2023-02-24)
This research describes a method of using distributed acoustic sensing to noninvasively measure volumetric flow rate via multiple unique sensor styles. This work modifies previously used methods of flow detection via fiber optic acoustic sensors affixed onto the exterior body of a flow apparatus. Flow rate measurement methods for two unique sensor styles are described. Weak trends are additionally observed as a function of flow temperature that may represent opportunity for future optimization. A discussion of current noninvasive flow rate measurement methods is given as well as their limitations. A background of distributed acoustic sensing is presented along with a summary of its fundamentals as well as its functionality in noninvasive flow rate measurement. A description of previous techniques that utilized distributed acoustic sensing in conjunction with fiber optic acoustic sensing is shown. The acoustic properties of the fluid-induced vibrations are measured as a function of flow rate and flow temperature utilizing a special type of fiber optic sensor. Numerically smoothed frequency domain acoustic peaks are evaluated by intensity, area, central frequency, and full width at half maximum as flow conditions vary. All tested sensors were found to yield a strong dependence between peak intensity and flow rate. A dependence between central frequency and flow temperature was observed in some cases. The sensor system developed was able to measure fluid-induced vibration intensity and vibrational central frequency and offers potential uses in a myriad of vibrational applications.
Phase Relations in the YBa2Cu3O7-x - SiO2 System and the Impact on Superconducting Fibers
Heyl, Hanna Verena (Virginia Tech, 2019-10-24)
This dissertation presents the first reported identification and analyses of the phase relations in the YBa2Cu3O7-x (YBCO)-SiO2 system at elevated temperatures. In this regard, a rigorous characterization study of the reaction phases within YBCO glass fibers, heat-treated YBCO+SiO2 pellets, rapid thermally annealed YBCO+SiO2 rods and rapid thermally annealed YBCO powder inside a fused silica tube is provided. These analyses are based on a vast set of generated novel results obtained using energy dispersive spectroscopy analyses on an environmental scanning electron microscope, X-Ray diffraction analyses, Raman spectroscopy, X-ray photoelectron spectroscopy analyses and a cross-polarized light study. First, original drawings of YBCO into glass fibers using the molten-core approach on a fiber draw tower in air and oxygen atmospheres are presented and analyzed. The performed analyses reveal the occurrence of reactions between the YBCO core and the silica cladding in as-drawn fibers as well as after additional heat-treatments. A detailed analysis and characterization of the occurring dissolution and diffusion based reaction processes is, then, provided along with the identification of the arising phase separation. Moreover, in order to analyze drawing YBCO glass fibers at lower temperatures, the use of borosilicate as the preform material is also investigated. This varied set of experiments and associated analyses reveal that the as-drawn YBCO fibers contain an amorphous core and that cuprite (Cu2O) is the first phase to crystallize out of the amorphous silicate matrix upon heat-treatment. Furthermore, the obtained results demonstrate the dissolution of the fused silica cladding into Si4+ and O2- ions and their subsequent diffusion into the molten YBCO core, leading to phase separation due to an occurring miscibility gap in the YBCO-SiO2 system as well as to silicate formation and amorphization of the YBCO core. This, as a result, prohibits the formation of the superconductive YBCO (Y-123) phase upon annealing. In addition, heat-treatment analyses show that higher temperatures or prolonged dwelling times at lower temperatures lead to the formation of barium copper and yttrium barium silicates. The analysis focusing on the use of borosilicate as the preform material reveals that drawing at lower temperatures reduces the dissolution and diffusion based reactions, but does not prevent them. Furthermore, the analysis on YBCO glass fibers with a fused silica cladding drawn in oxygen atmosphere shows that a higher oxygen content increases the dissolution of the fused silica cladding into its ions and their subsequent diffusion into the molten YBCO core. In addition, the performed heat-treatments on YBCO+SiO2 pellets in air and oxygen atmospheres demonstrate the gradual decomposition of the Y-123 phase with an increase in SiO2 content. Moreover, the rapid thermal annealing experiments with a subsequent quenching step on YBCO+SiO2 rods and on YBCO powder inserted inside a fused silica tube show the decomposition of the Y-123 phase and the formation of phases similar to the phases obtained in the YBCO glass fiber study, thus corroborating the results thereof. In summary, this dissertation enables the determination of the phase relations and reaction processes within the YBCO-SiO2 system, the identification of the direct effects of the silicon content on the Y-123 phase decomposition, as well as a rigorous characterization of the dissolution and diffusion based reactions within the YBCO-SiO2 glass-clad fiber system. The generated results and drawn conclusions build a fundamental understanding of phase relations in the YBCO-SiO2 system, which enables a definite assessment of the feasibility of manufacturing long-scale purely superconductive YBCO glass fibers using the molten-core approach and introduces advanced contributions to general glass-clad fiber systems manufactured using this method.
Powder-in-Tube Reactive Molten-Core Fabrication of Glass-Clad BaO-TiO2-SiO2 Glass–Ceramic Fibers
Yang, Shuo; Heyl, Hanna; Homa, Daniel S.; Pickrell, Gary R.; Wang, Anbo (MDPI, 2020-01-15)
In this paper we report the fabrication of glass-clad BaO-TiO₂-SiO₂ (BTS) glass–ceramic fibers by powder-in-tube reactive molten-core drawing and successive isothermal heat treatment. Upon drawing, the inserted raw powder materials in the fused silica tubing melt and react with the fused silica tubing (housing tubing) via dissolution and diffusion interactions. During the drawing process, the fused silica tubing not only serves as a reactive crucible, but also as a fiber cladding layer. The formation of the BTS glass–ceramic structure in the core was verified by micro-Raman spectroscopy after the successive isothermal heat treatment. Second-harmonic generation and blue-white photoluminescence were observed in the fiber using 1064 nm and 266 nm picosecond laser irradiation, respectively. Therefore, the BTS glass–ceramic fiber is a promising candidate for all fiber based second-order nonlinear and photoluminescence applications. Moreover, the powder-in-tube reactive molten core method offers a more efficient and intrinsic contamination-free approach to fabricate glass–ceramic fibers.
Single Mode Air-Clad Single Crystal Sapphire Optical Fiber
Hill, Cary; Homa, Daniel S.; Yu, Zhihao; Cheng, Yujie; Liu, Bo; Wang, Anbo; Pickrell, Gary R. (MDPI, 2017-05-03)
The observation of single mode propagation in an air-clad single crystal sapphire optical fiber at wavelengths at and above 783 nm is presented for the first time. A high-temperature wet acid etching method was used to reduce the diameter of a 10 cm length of commercially-sourced sapphire fiber from 125 micrometers to 6.5 micrometers, and far-field imaging provided modal information at intervals as the fiber diameter decreased. Modal volume was shown to decrease with decreasing diameter, and single mode behavior was observed at the minimum diameter achieved. While weakly-guiding approximations are generally inaccurate for low modal volume optical fiber with high core-cladding refractive index disparity, consistency between these approximations and experimental results was observed when the effective numerical aperture was measured and substituted for the theoretical numerical aperture in weakly-guiding approximation calculations. With the demonstration of very low modal volume in sapphire at fiber diameters much larger than anticipated by legacy calculations, the resolution of sapphire fiber distributed sensors may be increased and other sensing schemes requiring very low modal volume, such as fiber Bragg gratings, may be realized in extreme environment applications.
Superconducting fiber
Homa, Daniel S.; Liang, Y.; Pickrell, Gary R. (AIP Publishing, 2013-08-01)
In this study, we demonstrated superconductivity in a fiber with a lead core and fused silica cladding. The fibers were fabricated via a melt-draw technique and maintained overall diameters ranging from 200-900 mu m and core diameters of 100-800 mu m. Superconductivity of this fiber design was validated via the traditional four probe test method in a bath of liquid helium at temperatures on the order of 4 K. The superconducting fiber paves the way for applications in power transmission, magnetic sensing, and fundamental studies in the fields of electromagnetism. (C) 2013 AIP Publishing LLC.
Temperature Dependent Behavior of Optical Loss from Hydrogen Species in Optical Fibers at High Temperature
Bonnell, Elizabeth Ann (Virginia Tech, 2015-04-07)
This study reports on the behavior of silica based optical fibers in a hydrogen environment at high temperatures. The hydrogen response in the form of optical loss in the wavelength range of 1000-2500 nm of a germanium doped graded index 50/125 graded index fiber was examined in the temperature range of 20–800 °C. When the fiber was exposed to hydrogen at 800 °C two absorption bands appeared: ~1390 nm assigned to the first overtone of the hydroxyl stretch and ~2200 nm band with complex assignments including the combination mode of the fundamental hydroxyl stretch with SiO4 tetrahedral vibrations and the combination mode of SiOH bend and stretch. The growth rate of the 1390 nm band fits the solution to the diffusion equation in cylindrical coordinates while the 2200 nm band does not. Absorption for both bands persisted as the fiber is cooled to room temperature. Temperature dependent behavior was observed in that as temperature increases from room temperature, the absorption intensity decreases and band shifts slightly to longer wavelengths. Temperature dependence is repeatable and reversible. However, if no hydrogen is present in the environment at temperatures greater than 700 °C, the 1390 nm band will permanently decrease in intensity, while the 2200 nm band does not change. Changes in the structure of the glass appear to be causing this temperature dependent behavior. Other necessary conditions for structural changes to cause this temperature dependent behavior are examined.
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