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Multi-Scale Computational Modeling of Metal/Ceramic Interfaces
Get PDFMulti-scale atomistic calculations were carried out to understand the interfacial features that dictate the mechanical integrity of the metal/ceramic nanolaminates. As such, first principles density functional theory (DFT) calculations were performed to understand the electronic and atomistic factors governing adhesion and resistance to shear for simple metal/ceramic interfaces, whereas molecular dynamics (MD) simulations were performed to observe the impact of interfacial structures, such as misfit dislocation network geometries and orientation relationships, on interfacial mechanical properties.
For the DFT investigation, we choose metals with different crystal structures, namely - Cu (fcc), Cr (bcc) and Ti (hcp) along with a variety of NaCl-type ceramics (TiN, VN and CrN) to identify common descriptors for metal/ceramic interfacial behavior. Electron density was found to be a good descriptor of shear barrier heights between metal layers. DFT calculations were also used to explore doping-assisted strengthening of metal/ceramic interfaces. As such, Ti/TiN metal/ceramic interface was doped with Al, V and Cr, whereas Cu/TiN was doped with Ni, Zn and Sn. Only Al dopants had negative enthalpies of mixing at the Ti/TiN interface. For Cu/TiN, enthalpy of mixing dictated that Sn was not energetically suitable for doping at the interface, whereas both Ni and Zn were. The addition of Al increased the barrier to shear displacement of pure Ti by ~18 % when they were in adjacent atomic layers. There was a general correlation between higher resistance to shear and the Al concentration at the Ti/TiN interface, which were attributed to two effects: destabilizing Al-Al interactions and Al drawing electron density from the ceramic into the Ti phase. In the case of Cu/TiN, Ni segregated at the interface forming sub-nanometer interlayers between Cu and TiN, whereas Zn formed a solid solution with Cu. Ni interlayers increased both the shear (by a factor of ~3.75) and tensile strength (~17%) to a significant degree coinciding with an increase in electron density between the layers. Using analysis form their partial density of states, Ni interlayers were found to accept more electrons from interfacial Ti into their more compact 3d-orbitals than Cu, which accepted more into available 4s-orbitals.
A new modified embedded atom method interatomic potential was developed to study semi-coherent metal/ceramic interfaces involving Cu, Ti and N, such as Cu/TiN and Ti/TiN. The MD simulations performed using our newly developed MEAM model suggested that interfacial energetics is not the dominant factor in selecting the orientation relationship of the Cu/TiN interface, and pointed to the role of kinetic pathways in selecting the actual orientation. In addition, the models were applied to study semi-coherent Ti(0001)/TiN(111) and Cu(111)/TiN(111) systems as well. Ti/TiN was stable with misfits accommodated away from the interface. Cu/TiN, in contrast, was more stable with misfits at the interface. A spiral pattern in the misfit dislocation networks was observed away from the Cu/TiN interface, similar to the metal/metal (111) semi-coherent interfaces. The theoretical shear strength calculated for Ti/TiN when the misfits were several layers away from the interface (ranging from 1200-1800 MPa) and for Cu/TiN with the misfit at the chemical interface (1.65 MPa), had reasonable agreement with experiment
Diamonds in the Rough
Get PDFBrennan Hilliard is a photographer from Shreveport, Louisiana. He currently attends Louisiana Tech University where he is pursuing a Bachelor’s degree in journalism. He formally studied photography at Bossier Parish Community College where he acquired two certifications
An Ethereal Afternoon
No full texthttps://digitalcommons.latech.edu/quatrain-gallery-volume-5/1024/thumbnail.jp
Keeny Hall
No full texthttps://digitalcommons.latech.edu/quatrain-gallery-volume-5-cropped/1021/thumbnail.jp
Melvin
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Epiphany
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An Evaluation of the Process Used to Develop and Administer an Employee Culture Survey in a Public Research University
Get PDFIn October of 2015, AROS, a faculty-supervised, student-led consulting group affiliated with the Louisiana Tech Industrial-Organizational Psychology doctoral program (arosconsulting.org), was contracted by the President and Vice President of a public research university to develop and administer a survey to gauge the climate, operations, and alignment of proceedings with the University’s strategic objectives (Valadez, Allen, Lovell, & Toaddy, 2015). AROS conducted interviews with key stakeholders and focus groups with University members (faculty, staff, and students), wrote and refined the list of survey items, programmed the survey into the online platform, and administered the survey to all faculty, staff, and students. Results were analyzed, reported, and fed back to members of the university. Feedback sessions were held with each unit leader to discuss unit-level results and action planning efforts. The current paper discusses the scientific literature that informed the process, evaluates the process, and provides suggestions for future improvements
The Role of Med12 in Adipogenesis of Human Adipose Stem Cells
Get PDFThe interest in adipogenesis stems from the high rate of obesity in the world and the motivation to better understand the process of cell fate in relation to human diseases. Mutations in MED12 lead to developmental disorders and certain malignancies. To date, specific roles of Mediator and MED12 in cell state regulation are unclear, nevertheless, they are critical for understanding stem cell regulation and enhancement of their clinical applications. This study investigates the role of MED12 in relation to the Mediator complex kinase domain to better understand transcriptional control of adipogenesis in hASCs. In this study, MED12 expression was diminished with MED12 specific siRNA and adipogenesis was induced/assessed at different time points (day 3,7 and 14) using phase imaging to detect lipid droplets with Oil red-O staining being used to further validate differentiation and visualize the cells. To validate and confirm the effects of the MED12 knockdown, qRT-PCR and western blot analysis was carried out. To determine the role of MED12 as it functions in the Mediator complex and its role in maintaining the structural integrity of the kinase module, Coimmunoprecipitation assays were carried out. Results from this study show that in the absence of MED12, there is a significant decrease in the number of lipid vesicles compared to the negative control as observed via oil red-O staining and a decrease in the expression of adipogenic transcription factors such as PPARγ and C/EBPα via qRT-PCR and western blot analysis. Results also indicated that MED12 interacts with the adipogenic transcription factors to direct proper gene expression and indicates a role for MED12 and the Mediator complex in directing the process of transcription during adipogenesis. Taken together, these data suggest a significant role for MED12 in regulating adipogenesis and will initiate new research into understanding the regulatory mechanisms of adipogenesis and offer insight into novel treatments for obesity and relevant human disease