Browsing by Subject "stability"

Now showing 1 - 3 of 3
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    High temperature phase behavior of 2D transition metal carbides
    (2024-08) Wyatt, Brian C.; Anasori, Babak; Trice, Rodney; Zhang, Jing; Hood, Zachary
    The technological drive of humanity to explore the cosmos, travel at hypersonic speeds, and pursue clean energy solutions requires ceramic scientists and engineers to constantly push materials to their functional, behavioral, and chemical extremes. Ultra-high temperature ceramics, and particularly transition metal carbides, are promising materials to meet the demands of extreme environment materials with their >4000 °C melting temperature and impressive thermomechanical behaviors in extreme conditions. The advent of the 2D version of these transition metal carbides, known as MXenes, added a new direction to design transition metal carbides for energy, catalysis, flexible electronics, and other applications. Toward extreme conditions, although MXenes remain yet unexplored, we believe that the ~1 nm flakes of MXenes gives ceramics scientists and engineers the ability to truly engineer transition metal carbides layer-by-layer at the nanoscale to endure the extreme conditions required by future harsh environment technology. Although MXenes have this inherent promise, fundamental study of their behavior in high-temperature environments is necessary to understand how their chemistry and 2D nature affects the high- temperature stability and phase behavior of MXenes toward application in extreme environments. In this dissertation, we investigate the high-temperature phase behavior of 2D MXenes in high temperature inert environments to understand the stability and phase transition behavior of MXenes. In this work, we demonstrate that 1) MXenes’ transition at high-temperatures is to highly textured transition metal carbides is due to the homoepitaxial growth of these phases onto ~1-nm- thick MXenes’ highly exposed basal plane, 2) the MXene to MXene interface plays a major role in the phase behavior of MXenes, particularly toward building layered transition metal carbides using MXenes as ~1-nm-thick building blocks, and 3) Defects are the primary site at which atomic migration begins during phase transition of MXenes into these highly textured transition metal carbides, and these defects can be engineered for different phase stability of MXenes. To do so, we investigate the phase behavior of Ti3C2Tx, Ta4C3Tx, Mo2TiC2Tx, and other MXenes using a combination of in situ x-ray diffraction and scanning transmission electron microscopy and other ex situ methods, such as secondary ion mass spectrometry and x-ray photoelectron spectroscopy, with other methods. By investigating the fundamentals of the high-temperature phase behavior of MXenes, we hope to establish the basic principles behind use of MXenes as the ideal material for application in future extreme environments.
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    Institutional review of the management of type II odontoid fractures: associations and outcomes with fibrous union
    (AANS, 2021-04) Wilson, Christopher; Hoyos, Mariana; Huh, Andrew; Priddy, Blake; Avila, Stephen; Mendenhall, Stephen; Anokwute, Miracle C.; Eckert, George J.; Stockwell, David W.; Neurological Surgery, School of Medicine
    OBJECTIVE Type II odontoid fractures may be managed operatively or nonoperatively. If managed with bracing, bony union may never occur despite stability. This phenomenon is termed fibrous union. The authors aimed to determine associations with stable fibrous union and compare the morbidity of patients managed operatively and nonoperatively. METHODS The authors performed a retrospective review of their spine trauma database for adults with type II odontoid fractures between 2015 and 2019. Two-sample t-tests and Fisher’s exact tests identified associations with follow-up stability and were used to compare operative and nonoperative outcomes. Sensitivity, specificity, and predictive values were calculated to validate initial stable upright cervical radiographs related to follow-up stability. RESULTS Among 88 patients, 10% received upfront surgical fixation, and 90% were managed nonoperatively, of whom 22% had fracture instability on follow-up. Associations with instability after nonoperative management include myelopathy (OR 0.04, 95% CI 0.0–0.92), cerebrovascular disease (OR 0.23, 95% CI 0.06–1.0), and dens displacement ≥ 2 mm (OR 0.29, 95% CI 0.07–1.0). Advanced age was not associated with follow-up instability. Initial stability on upright radiographs was associated with stability on follow-up (OR 4.29, 95% CI 1.0–18) with excellent sensitivity and positive predictive value (sensitivity 89%, specificity 35%, positive predictive value 83%, and negative predictive value 46%). The overall complication rate and respiratory failure requiring ventilation on individual complication analysis were more common in operatively managed patients (33% vs 3%, respectively; p = 0.007), even though they were generally younger and healthier than those managed nonoperatively. Operative or nonoperative management conferred no difference in length of hospital or ICU stay, discharge disposition, or mortality. CONCLUSIONS The authors delineate the validity of upright cervical radiographs on presentation in association with follow-up stability in type II odontoid fractures. In their experience, factors associated with instability included cervical myelopathy, cerebrovascular disease, and fracture displacement but not increased age. Operatively managed patients had higher complication rates than those managed without surgery. Fibrous union, which can occur with nonoperative management, provided adequate stability.
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    On the Significance and Predicted Functional Effects of the Crown-to-Implant Ratio: A Finite Element Study of Long-Term Implant Stability Using High-Resolution, Nonlinear Numerical Analysis
    (ASME, 2016-04) Sego, T. J.; Hsu, Yung-Ting; Chu, Tien-Min Gabriel; Tovar, Andres; Mechanical Engineering, School of Engineering and Technology
    With the rising popularity of short dental implants, the effects of the crown-to-implant (C/I) ratio on stress and strain distributions remain controversial. Previous research disagrees on results of interest and level of necessary technical detail. The present study aimed to evaluate the strain distribution and its functional implications in a single implant-supported crown with various C/I ratios placed in the maxillary molar region. A high-fidelity, nonlinear finite-element model was generated to simulate multiple clinical scenarios by laterally loading a set of single implants with various implant lengths (IL) and crown heights (CH). Strain distribution and maximum equivalent strain (MES) were analyzed to evaluate the effects and significance of the CH, IL and C/I. Predicted functional response to strain at the implant interface was analyzed by interface surface area. Results. Results were evaluated according to the mechanostat hypothesis to predict functional response. Overloading and effects of strain concentrations were more prevalent with increasing C/I. Overloading was predicted for all configurations to varying degrees, and increased with decreasing IL. Fracture in trabecular bone was predicted for at least one C/I and all IL of 10 mm or less. Higher C/I ratios and lower IL increase the risk of overloading and fracture. Increasing C/I augments the functional effects of other implant design factors. Greater C/I ratios may be achieved with implant designs that induce less significant strain concentrations.