Nanotubular Boron-Carbon Heterojunctions

Linear nanotubular boron-carbon heterojunctions are systematically constructed and studied with the help of ab initio total energy calculations. The structural compatibility of the two classes of materials is shown, and a simple recipe that determines all types of stable linear junctions is illustrated in some detail. Our results also suggest the compatibility of various technologically interesting types of nanotubular materials, leading to novel types of nanotubular compound materials, and pointing out the possibility of wiring nanotubular devices within heterogeneous nanotubular networks.Comment: 7 pages, 5 figures, accepted by J. Chem Phy

Numerical range for random matrices

We analyze the numerical range of high-dimensional random matrices, obtaining limit results and corresponding quantitative estimates in the non-limit case. For a large class of random matrices their numerical range is shown to converge to a disc. In particular, numerical range of complex Ginibre matrix almost surely converges to the disk of radius $\sqrt{2}$. Since the spectrum of non-hermitian random matrices from the Ginibre ensemble lives asymptotically in a neighborhood of the unit disk, it follows that the outer belt of width $\sqrt{2}-1$ containing no eigenvalues can be seen as a quantification the non-normality of the complex Ginibre random matrix. We also show that the numerical range of upper triangular Gaussian matrices converges to the same disk of radius $\sqrt{2}$, while all eigenvalues are equal to zero and we prove that the operator norm of such matrices converges to $\sqrt{2e}$.Comment: 23 pages, 4 figure

Contact Dewatering of Cellulose Nanofibers for Biopolymer Composite Applications

Cellulose Nanofibrils (CNFs) are promising materials for reinforcement of polymer matrices attributable to their impressive physical and mechanical properties, as well as their biodegradability. However, the utilization of these materials in composites is made challenging by the water content of CNF slurries, the tendency of CNFs to agglomerate as they dry, and incompatibility between hydrophilic CNFs and hydrophobic polymer matrices. The most commercially viable drying methods to produce small-scale dry CNFs, such as spray drying, are very energy intensive, can only dry the materials down to micron-scale agglomerates, and do not preserve fibrillar aspect ratios. “Contact dewatering,” or the removal of bound water from CNF suspensions using wood flour (WF) and mechanical pressing, may provide a solution to the challenge of excess water and preservation of nanoscale CNF dimensions after drying. This project explored the use of contact dewatering with maple wood flour to remove water from CNF suspensions and incorporate dry CNF into poly(lactic) acid (PLA) composites as part of hybrid WF-CNF furnishes. This concept was further explored by using cryocrushed PLA powder instead of wood flour to dewater and process CNF into PLA-CNF composites. The use of WF showed preservation of nanoscale CNFs on the surface of wood particles after drying and high energy efficiency in water removal from CNF suspensions, with mixed results in terms of mechanical properties when incorporated into wood-plastic composites. These results included a slight increase in tensile modulus compared to PLA with WF at the lowest wt.%CNF/LCNF hybrid furnish, a drop in tensile strength with higher wt.%CNF/LCNF loading levels, and a reduction in flexural strength at lower wt.%CNF/LCNF loading levels. When successfully scaled-up to produce 50lbs of a PLA/WF/CNF composite for additive manufacturing, the process showed an 84% decrease in specific drying energy per gram of dried CNF compared to empirical values for spray-dried CNF (SDCNF). The use of small polymer particles as opposed to wood in the contact dewatering process for drying and direct compounding of CNF into composites has not been extensively studied, so the viability of contact dewatering with poly(lactic) acid (PLA) powder as a dewatering method for compounded PLA/CNF composites in terms of energy efficiency, preservation of nanoscale CNF morphology, and mechanical properties was also evaluated. Initial results that utilized high or equivalent amounts of CNF to PLA in the dewatering process showed some preservation of nanoscale morphology of dry CNFs, but with overall structures that favored spherical agglomerates with low aspect ratios that reduced the mechanical properties of the final composites compared to PLA and PLA/SDCNF. Using lower amounts of CNF in the dewatering process reduced the formation of larger-scale agglomerates, which was optimized to produce micron-to-nano-scale fibrillar CNF structures observed under polarized light microscopy (PLM). Attempting to model CNF agglomeration around PLA particles by assuming the formation of spherical agglomerates based on CNF weight percent and PLA particle size was not accurate to observed results for agglomeration based on the weight percent of CNF. Using PLA for contact dewatering showed more efficient water removal from the CNF suspension than previous wood flour-CNF furnishes, and specimens laser cut from compression molded films of dried materials showed an increase in tensile strength of up to 31% compared to pure PLA. Shear-mixed specimens showed an increase in tensile strength and modulus of 1.7% and 4.2% compared to PLA at 1 and 2wt.%CNF loading levels, respectively, and had equivalent or better properties than PLA-Spray Dried CNF at the same loading levels. None of these results were statistically significant save for significantly higher tensile modulus in dewatered CNF composites compared to SDCNF composites at 1 and 2wt.% total CNF loading levels post-shear-mixing. Theoretical energy for drying dewatered CNF to produce micron-to-nanoscale dry fibrils after one press was 67-205x lower than empirical values for drying SDCNF depending on initial CNF loading level

Development of Cislunar Space Logistics Networks for Satellite Constellation Support Using Event-Driven Generalized Multi-Commodity Network Flows

As space becomes an increasingly congested domain, the risk of damage to satellite constellations is increasing. In response, there is an increasing need for capabilities for unmanned repair, refueling, and reconstitution (R3) of those constellations. Cislunar orbits offer a promising storage and low-cost transfer solution for on-orbit service vehicles and replacement satellites to leverage those capabilities. This research makes use of mixed-integer linear programming-based logistics models to determine the situations in which a cislunar mission architecture would offer a cost-effective alternative to Earth-based R3. The network models presented in this research make use of the latest developments in Event-Driven Generalized Multi-Commodity Network Flows (ED-GMCNF), a new method of optimization that enables variable time steps between events. This research combines a new version of an ED-GMCNF with cislunar trajectory optimization to evaluate both the feasibility of cislunar orbits as well as the potential effects of lunar fuel production on R3 costs. This investigation finds, through an exhaustive numerical simulation campaign, that cislunar logistics networks provide cost-effective means of R3 regiments for Earth-orbiting and cislunar satellites when a lunar fuel supply is taken into consideration. The ED-GMCNF methodology also offers a promising foundation for future work in the mission planning field

When Party Does Not Matter: An Examination of Conditions that Influence a Senator to Vote Against Party Leadership

The United States Senate as the upper chamber of the United States Congress possesses great power and responsibility. The way that the 100 men and women who make up the chamber vote has long been of importance to those of us in the field of political science. This paper will look at influences that affect a senator’s decision to vote against leadership. The research will test these influences simultaneously in order to find the degrees of influence each has. The research will focus on the Senate from 1994 – 2008, when the Gingrich senators led to an increasingly polarized body but Citizens United had not yet changed the rules of campaigning

Chemical underpinning of the tea bag index: An examination of the decomposition of tea leaves

Decomposition is a key flux of terrestrial carbon to the atmosphere. Therefore, gaining a better understanding of how plant litter decomposes in soil, and what governs this process, is vital for global climate models. The Tea Bag Index (TBI) was introduced by Keuskamp et al. (2013) as a novel method for measuring litter decomposition rate and stabilisation. The TBI uses two types of tea bags representing fast (green tea) and slow (rooibos tea) decomposition substrates as standardised litter bags. To date, the TBI method has been used in over 2000 locations across the globe. However, before now, there has been no information on how the composition of the tea leaves change during incubation. These data are crucial in determining the validity of the use of the TBI method globally, to ensure the tea leaves decompose in a way that is representative of so-called “native” litters. To provide chemical underpinning of the TBI method, a laboratory incubation of the tea bags was conducted with destructive sampling at 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, and 91 d. The incubated tea was analysed for total C and N. In addition, C was characterised as alkyl, O-alkyl, aromatic, or carbonyl C using solid-state 13C nuclear magnetic resonance spectroscopy with cross-polarization and magic angle spinning (CPMAS NMR). The results suggest that changes in carbon in both tea types are comparable to other litter studies, with a net decrease in total C and relative proportion of O-alkyl C fraction, which contains carbohydrates and cellulose. We conclude that the decomposition of tea leaves in the bags used in the TBI is representative of other litters