Nebraska Center for Materials and Nanoscience

Nebraska City, NE, United States

Nebraska Center for Materials and Nanoscience

Nebraska City, NE, United States
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News Article | February 14, 2017
Site: www.cemag.us

In 2011, chemists and engineers met the MXenes: a large family of two-dimensional nanomaterials whose members have already shown real talent for storing energy, purifying water and protecting against electromagnetic interference. Knowing the family could find employment in those fields for years to come, researchers have since launched the equivalent of a background check into job performance, versatility, stability, and quirks. Scientists from the University of Nebraska-Lincoln and Drexel University recently published findings on an especially promising candidate that includes three titanium atoms and two carbon atoms. Their paper demonstrated that modifying the traditional method of synthesizing the MXene can substantially affect the structure and related properties of its individual, nanoscopic flakes. The researchers then measured the electrical conductivity of their synthesized flakes, which substantially outperformed those previously reported. By doing so, they also established a threshold for conductivity that engineers could eventually target when incorporating the MXene in lithium-ion batteries, transistors, capacitors and other devices. “MXenes are synthesized as thin sheets that are then processed for different applications,” says co-author Alexander Sinitskii, associate professor of chemistry at the University of Nebraska-Lincoln. “An important research direction in this field is to develop synthetic approaches to produce MXene sheets with high structural quality and electrical conductivity.” MXenes begin their lives in the so-called MAX phase, whose name describes its signature components: the “M,” a transition metal such as titanium or chromium; an element such as aluminum from the “A” group of the periodic table; and the “X,” representing carbon or nitrogen atoms. To synthesize MXenes, chemists have used acidic solutions to etch away the “A” group while leaving the other layers intact — a relatively simple, high-yield technique. But previous solutions have produced relatively small flakes peppered with nanoscopic pinholes that limit the movement of conductivity-driving electrons while offering plenty of opportunities for oxidation to degrade the material. By tweaking the ratio of solution to MAX phase, the Nebraska-Drexel team managed to synthesize defect-free flakes that were about 25 times larger, significantly more conductive and far less susceptible to degradation than those prepared via other approaches. “We found that these slight variations in the chemical procedures result in pretty dramatic differences in the qualities of the products we obtain,” says Sinitskii, a member of the Nebraska Center for Materials and Nanoscience. “Our measurements revealed that electrical conductivity of MXene sheets is actually close to that of graphene, the two-dimensional material that holds the present record for conductivity. Information about the intrinsic properties of individual MXene sheets is important for optimizing the performance of energy-storage devices that consist of multiple sheets.” The team’s paper appeared in the December issue of Advanced Energy Materials, which featured the study on its back cover. Sinitskii authored the paper with Alexey Lipatov, research assistant professor of chemistry; Alex Boson, graduate student in chemistry; along with Drexel University’s Yury Gogotsi, Mohamed Alhabeb, and Maria Lukatskaya. The team received support from the National Science Foundation, the NSF-funded Nebraska Materials Research Science and Engineering Center, and the U.S. Department of Energy’s Office of Science.


Hua Y.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience | Trogdon M.,University of Nebraska - Lincoln
International Journal of Adhesion and Adhesives | Year: 2012

The objective of this paper was to investigate the performance of recessed single-lap joints with dissimilar adherends through the finite element method. The influence of material and geometric nonlinearity of the adhesive as well as the impact of the recess length was examined in terms of maximum principal stresses. The strength of the joint was obtained as the load to initiate the crack propagation. Results suggested that either adding a spew fillet or considering the adhesive plasticity led to reduced peak stresses at the edge of the adhesive layer. The presence of a spew fillet in the single-lap joint with a recess length of 50% of the overlap length reduced the peak stress concentrations in the adhesive layer by 45.2% and subsequently improved the strength of the joint by 36.3%. Mitigation of stress concentration was observed in cases of an adhesive layer with a smaller recess length. The strength of recessed joints with a gap less than 50% of the overlap length decreased slightly. For the recess length as 70% and 90% of the total overlap length, the strength of the joints reduced 36.4% and 66.3%, respectively. This study suggested a recess of less than 50% of the overlap length may be beneficial for the performance of the joints. © 2012 Elsevier Ltd.


Hua Y.,University of Nebraska - Lincoln | Akula P.K.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience
Composites Part B: Engineering | Year: 2014

The objective of this paper is to investigate the structural response of carbon fiber sandwich panels subjected to blast loading through an integrated experimental and numerical approach. A total of nine experiments, corresponding to three different blast intensity levels were conducted in the 28-inch square shock tube apparatus. Computational models were developed to capture the experimental details and further study the mechanism of blast wave-sandwich panel interactions. The peak reflected overpressure was monitored, which amplified to approximately 2.5 times of the incident overpressure due to fluid-structure interactions. The measured strain histories demonstrated opposite phases at the center of the front and back facesheets. Both strains showed damped oscillation with a reduced oscillation frequency as well as amplified facesheet deformations at the higher blast intensity. As the blast wave traversed across the panel, the observed flow separation and reattachment led to pressure increase at the back side of the panel. Further parametric studies suggested that the maximum deflection of the back facesheet increased dramatically with higher blast intensity and decreased with larger facesheet and core thickness. Our computational models, calibrated by experimental measurements, could be used as a virtual tool for assessing the mechanism of blast-panel interactions, and predicting the structural response of composite panels subjected to blast loading. © 2013 Elsevier Ltd. All rights reserved.


Chafi M.S.,University of Nebraska - Lincoln | Ganpule S.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience | Chandra N.,University of Nebraska - Lincoln
International Journal of Applied Mechanics | Year: 2011

Blast wave induced a frequency spectrum and large deformation of the brain tissue. In this study, new material parameters for the brain material are determined from the experimental data pertaining to these large strain amplitudes and wide frequencies ranging (from 0.01 Hz to 10 MHz) using genetic algorithms. Both hyperelastic and viscoelastic behavior of the brain are implemented into 2D finite element models and the dynamic responses of brain are evaluated. The head, composed of triple layers of the skull, including two cortical layers and a middle dipole sponge-like layer, the dura, cerebrospinal fluid (CSF), the pia mater and the brain, is utilized to assess the effects of material model. The results elucidated that frequency ranges of the material play an important role in the dynamic response of the brain under blast loading conditions. An appropriate material model of the brain is essential to predict the blast-induced brain injury. © 2011 Imperial College Press.


Lin S.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience
Materials | Year: 2015

The mechanical properties of type I collagen gel vary due to different polymerization parameters. In this work, the role of crosslinks in terms of density and stiffness on the macroscopic behavior of collagen gel were investigated through computational modeling. The collagen fiber network was developed in a representative volume element, which used the inter-fiber spacing to regulate the crosslink density. The obtained tensile behavior of collagen gel was validated against published experimental data. Results suggest that the cross-linked fiber alignment dominated the strain stiffening effect of the collagen gel. In addition, the gel stiffness was enhanced approximately 40 times as the crosslink density doubled. The non-affine deformation was reduced with the increased crosslink density. A positive bilinear correlation between the crosslink density and gel stiffness was obtained. On the other hand, the crosslink stiffness had much less impact on the gel stiffness. This work could enhance our understanding of collagen gel mechanics and shed lights on designing future clinical relevant biomaterials with better control of polymerization parameters. © 2015 by the authors.


Hua Y.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience
Composites Part B: Engineering | Year: 2013

The objective of this paper was to predict the thermomechanical behavior of 2080 aluminum alloy reinforced with SiC particles using the Mori-Tanaka theory combined with the finite element method. The influences of particle volume fraction, stiffness, aspect ratio and orientation were examined in terms of effective Young's modulus, Poisson's ratio and coefficient of thermal expansion (CTE) of the composite. The microstructure induced local stress and strain field was obtained through the numerical models of the representative volume element. Results suggested that particle volume fraction had significant impact on the effective Young's modulus, Poisson's ratio and CTE of the composite. Stiffer particles could improve the effective Young's modulus of the composite, while the overall sensitivity of the effective Poisson's ratio and CTE with respect to the particle stiffness was minimal. Particles with larger aspect ratio generally led to a composite with increased effective Young's modulus, as well as reduced Poisson's ratio and CTE. The overall material properties of the composite were insensitive to the particle aspect ratio beyond 10. The particle orientations significantly impacted the effective material properties of the composite, especially along the longitudinal direction. Random 3D dispersed particles exhibited the effective isotropic behavior, whereas anisotropy has been observed for random 2D and unidirectional aligned particles. Our results could help create tailorable bulk composite. © 2012 Elsevier Ltd. All rights reserved.


Hua Y.,University of Nebraska - Lincoln | Kasavajhala A.R.M.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience
Composites Part B: Engineering | Year: 2013

The objective of this paper is to investigate the performance of adhesive joints of carbon/epoxy wind turbine blade subjected to combined bending and tension loadings through finite element method. The influence of adhesive material properties and geometrical details including fillet and imperfections was examined in terms of interlaminar stresses in the adhesive layer. The variation of stress intensity with change in adhesive shear modulus has also been investigated, while contour integral method was used for evaluating the stress intensity factors (SIF) at the imperfection tip. Furthermore, the strength of the joint was assessed through the crack initiation and propagation analysis. Results suggested that either adding a fillet or considering the plasticity led to the reduced peak stresses at the edge of the adhesive layer and redistributed the load to low stress regions. Inclusion of imperfections has resulted in high stress concentrations in the adhesive layer and reduction in the strength of the joint. Compared to the filleted adhesive, the strength of the joint reduced 2.4% and 4.8% considering a flat adhesive and filleted adhesive with through-thickness imperfection, respectively. Large shear modulus of the adhesive diminishes the fracture strength with the increased SIF. © 2012 Published by Elsevier Ltd.


Yuan Y.,University of Nebraska - Lincoln | Bi Y.,University of Nebraska - Lincoln | Bi Y.,CAS Institute of Semiconductors | Huang J.,University of Nebraska - Lincoln | Huang J.,Nebraska Center for Materials and Nanoscience
Applied Physics Letters | Year: 2011

We report efficient laminated organic photovoltaic device with efficiency approach the optimized device by regular method based on Poly(3-hexylthiophene- 2,5-diyl) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The high efficiency is mainly attributed to the formation of a concrete polymer/metal interface mechanically and electrically by the use of electronic-glue, and using the highly conductive and flexible silver film as anode to reduce photovoltage loss and modifying its work function for efficiency hole extraction by ultraviolet/ozone treatment, and the pressure induced crystallization of PCBM. © 2011 American Institute of Physics.


Hua Y.,University of Nebraska - Lincoln | Gu L.,University of Nebraska - Lincoln | Gu L.,Nebraska Center for Materials and Nanoscience | Watanabe H.,University of Nebraska Medical Center
International Journal of Engineering Science | Year: 2013

The mechanical behavior of TiO2 nanoparticle-reinforced resin-based dental composites was characterized in this work using a three-dimensional nanoscale representative volume element. The impacts of nanoparticle volume fraction, aspect ratio, stiffness and interphase zone between the resin matrix and nanoparticle on the bulk properties of the composite were characterized. Results clearly demonstrated the mechanical advantage of nanocomposites in comparison to microfiber reinforced composites. The bulk response of the nanocomposite could be further enhanced with the increased nanoparticle volume fraction, or aspect ratio, while the influence of nanoparticle stiffness was minimal. The effective Young's modulus and yield strength of the composite was also significantly affected by the interphase stiffness. Results obtained in this work could provide insights for the optimization of nanoparticle-reinforced dental composites. © 2013 Elsevier B.V.All rights reserved.


Yang B.,University of Nebraska - Lincoln | Yang B.,Nebraska Center for Materials and Nanoscience | Cox J.,University of Nebraska - Lincoln | Yuan Y.,University of Nebraska - Lincoln | And 5 more authors.
Applied Physics Letters | Year: 2011

Photovoltaic characteristics of a low bandgap polymer, poly[(4,4′- bis(2-lethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1, 3-benzothiadiazole)-4,7-diyl], based bulk hetero-junction organic photovoltaic were investigated from room temperature (RT) to 145 °C to evaluate its applications in harsh environments. The power conversion efficiency was found to increase from 4.1% at RT to 4.5% at 105 °C with increased short circuit current density (Jsc) and fill factor (FF) despite the decreased open circuit voltage (Voc). Detailed investigation revealed that J sc and FF improvements were caused by the increased and balanced carrier mobilities at higher temperatures. The Voc of the low bandgap polymer solar cell is determined not only by the energy levels and dark currents, but also by the binding energy of charge transfer excitons (CTEs). A slower reduction of Voc is observed at high temperatures due to the decreased binding energy of CTEs. © 2011 American Institute of Physics.

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