1,721,006 research outputs found
Synthesis and Characterization of a Novel Fluorinated Bismaleimide via a Nucleophilic Addition-Elimination Reaction and its Polymeric Networks
No full textIncludes supplementary material.The commercially available octafluorocyclopentene (OFCP) as both a fluorinated building block and a linker has been successfully utilized to prepare a new fluorinated bismaleimide monomer OFCP-BMIvia a nucleophilic addition-elimination reaction. The resulting OFCP-BMI is characterized by 1H, 13C, 19F nuclear magnetic resonance (NMR) spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and high resolution mass spectrometry. The monomer OFCP-BMI reacts with a free radical initiator or self-cures to prepare its resins. No obvious glass transition temperature (Tg) below 260°C was observed for the self-curing resin suggesting that a highly cross-linked polymeric network was formed. The self-curing resin exhibits excellent thermal stability with the on-set weight loss temperature (Ton-setd) of 445°C and a 50% char yield at 800°C under a nitrogen atmosphere. The resin obtained by a free radical polymerization process exhibits less thermal stability and a lower char yield than the self-curing resin due to its lower cross-linking density. The self-curing resin exhibits a low dielectric constant (Dk = 2.5)
Synthesis and Characterization of a Holmium 2,2′-bipyridine-5,5′-dicarboxylate MOF: Towards the Construction of a Suitable Holmium Carrier
No full textDue to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is limited to current UTD affiliates (use the provided Link to Article).Supplementary material is available on publisher's website. Use the DOI link below.Based on the 2,2′-bipyridine-5,5′-dicarboxylic acid ligand (H₂bpdc), a new metal-organic framework using holmium as the coordinating metal site was synthesized through a solvothermal reaction. The 3D compound {[Ho₂(bPdc)₃(DMF)₂] · 2(DMF)}ₙ (1) was characterized by single crystal X-ray diffraction, FTIR, TGA and PXRD. The coordination between H₂bpdc, the solvent, and holmium (CN = 7) allows for the construction of a MOF containing potentially free metal sites and uncoordinated N-donor sites. These features make this MOF an attractive holmium carrier for radiotherapeutic applications.Robert A. Welch foundation (Grant No. AT-1153)School of Natural Sciences and Mathematic
Bulk Fermi Surface of The Weyl Type-II Semimetallic Candidate NbIrTe₄
No full textIncludes supplementary material.Recently, a new group of layered transition-metal tetra-chalcogenides was proposed via first-principles calculations to correspond to a new family of Weyl type-II semimetals with promising topological properties in the bulk as well as in the monolayer limit. In this paper, we present measurements of the Shubnikov-de Haas (SdH) and de Haas-van Alphen effects under high magnetic fields for the type-II Weyl semimetallic candidate NbIrTe₄. We find that the angular dependence of the observed Fermi surface extremal cross-sectional areas agree well with our density functional theory (DFT) calculations supporting the existence of Weyl type-II points in this material. Although we observe a large and nonsaturating magnetoresistivity in NbIrTe4 under fields all the way up to 35T, Hall-effect measurements indicate that NbIrTe₄ is not a compensated semimetal. The transverse magnetoresistivity displays a fourfold angular dependence akin to the so-called butterfly magnetoresistivity observed in nodal line semimetals. We conclude that the magnetoresistivity and its unconventional angular dependence are governed by the topography of the Fermi surface and the resulting anisotropy in effective masses and in carrier mobilities. © 2019 American Physical Society.DOE-BES through award DE-SC0002613; NSF DMR-1700030, NSF-DMR-1644779, NSF-DMR-1555163School of Natural Sciences and Mathematic
Spin density wave instability in a ferromagnet
Full text linkAbstract Due to its cooperative nature, magnetic ordering involves a complex interplay between spin, charge, and lattice degrees of freedom, which can lead to strong competition between magnetic states. Binary Fe3Ga4 is one such material that exhibits competing orders having a ferromagnetic (FM) ground state, an antiferromagnetic (AFM) behavior at intermediate temperatures, and a conspicuous re-entrance of the FM state at high temperature. Through a combination of neutron diffraction experiments and simulations, we have discovered that the AFM state is an incommensurate spin-density wave (ISDW) ordering generated by nesting in the spin polarized Fermi surface. These two magnetic states, FM and ISDW, are seldom observed in the same material without application of a polarizing magnetic field. To date, this unusual mechanism has never been observed and its elemental origins could have far reaching implications in many other magnetic systems that contain strong competition between these types of magnetic order. Furthermore, the competition between magnetic states results in a susceptibility to external perturbations allowing the magnetic transitions in Fe3Ga4 to be controlled via temperature, magnetic field, disorder, and pressure. Thus, Fe3Ga4 has potential for application in novel magnetic memory devices, such as the magnetic components of tunneling magnetoresistance spintronics devices
One-Dimensional Tellurium Chains: Crystal Structure and Thermodynamic Properties of PrCuₓTe₂ (x ~ 0.45)
No full textDue to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is limited to current UTD affiliates (use the provided Link to Article).X-ray diffraction, crystal structure, magnetization, and heat capacity results are presented for the rare earth chalcogenide ternary system PrCu₀.₄₅Te₂ and its non-4⨍ analogue LaCu₀.₄₀Te₂. The crystal structure of PrCu₀.₄₅Te₂ is characterized by chains of edge and corner-sharing CuTe₄ tetrahedra and Pr centered in polyhedra comprised of Cu and Te. The Cu site is partially occupied and exhibits signatures of local disorder. Magnetic susceptibility measurements show a Curie-Weiss temperature dependence consistent with a Pr³⁺ state. No magnetic ordering is observed down to 1.8 K, but the negative Curie-Weiss temperature suggests an antiferromagnetic exchange interaction. Importantly, the low temperature heat capacity of PrCu₀.₄₅Te₂ is strongly enhanced by comparison to LaCu₀.₄₀Te₂, suggesting that there is a build-up of entropy that is associated with the 4f-electrons from the Pr³⁺ ions. These features reveal possible spin frustration behavior and introduce this family of materials as a template for studying new phenomenon.National Science Foundation CAREER Award 1541230; National Science Foundation Division of Materials Research 1700030; Department of Energy SC0002613; Agreements No. Division of Materials Research 1157490 and No. Division of Materials Research 1644779School of Natural Sciences and Mathematic
Utilization of a Meldrum's Acid Towards Functionalized Fluoropolymers Possessing Dual Reactivity for Thermal Crosslinking and Post-Polymerization Modification
No full textIncludes supplementary informationNew thermally cross-linkable and/or post-functionalizable perfluorocyclobutyl (PFCB) polymers containing Meldrum's acid moieties have been successfully prepared via the thermal cyclopolymerization of a new Meldrum's acid functionalized aromatic trifluorovinyl ether (TFVE) monomer.Funded in part by the Welch Foundation (grant AT-0041); National Science Foundation (CEHMS: NSF-1/UCRC grant 1035024) and (NMR grant CHE-1126177
Transition from a 1D Coordination Polymer to a Mixed-Linker Layered MOF
No full textDue to copyright restrictions full text access from Treasures at UT Dallas is restricted to current UTD affiliates (use the provided Link to Article).A novel copper(II) metal-organic framework (MOF) has been synthesized by modifying the reaction conditions of a 1D coordination polymer. The 1D polymer is built by the coordination between copper and 2,2′-(1H-imidazole-4,5-diyl)di-1,4,5,6-tetrahydropyrimidine (H-L1). The geometry of H-L1 precludes its ability to form extended 3D framework structures. By adding 1,4-benzenedicarboxylic acid (H 2 BDC), a well-studied linker in MOF synthesis, we achieved the transition from a 1D polymer chain into porous 2D layered structures. Hydrogen bonding between L1 and BDC directs the parallel stacking of these layers, resulting in a 3D structure with one-dimensional channels accessible by two different pore windows. The preferred growth orientation of the crystal produces prolonged channels and a disparity in pore size distribution. This in turn results in slow diffusion processes in the material. Furthermore, an isoreticular MOF was prepared by substituting the BDC linker by 2,6-naphthalenedicarboxylic acid (H₂NDC). © 2019 American Chemical Society.Robert A. Welch foundation (Grant No. AT-1153).School of Natural Sciences and Mathematic
Detailed study of the Fermi surfaces of the type-II Dirac semimetallic candidates XTe2 (X =Pd, Pt)
Full text linkWe present a detailed quantum oscillatory study on the Dirac type-II semimetallic candidates PdTe2 and PtTe2 via the temperature and the angular dependence of the de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects. In high quality single crystals of both compounds, i.e. displaying carrier mobilities between 10 3 and 10 4 cm 2 /Vs, we observed a large non-saturating magnetoresistivity (MR) which in PtTe 2 at a temperature T = 1. 3 K, leads to an increase in the resistivity up to 5 × 10 4 % under a magnetic field μ 0 H = 62 T. These high mobilities correlate with their light effective masses in the range of 0.04 to 1 bare electron mass according to our measurements. For PdTe2 the experimentally determined Fermi surface cross-sectional areas show an excellent agreement with those resulting from band-structure calculations. Surprisingly, this is not the case for PtTe2 whose agreement between calculations and experiments is relatively poor even when electronic correlations are included in the calculations. Therefore, our study provides a strong support for the existence of a Dirac type-II node in PdTe 2 and probably also for PtTe2. Band structure calculations indicate that the topologically non-trivial bands of PtTe2 do not cross the Fermi-level (εF). In contrast, for PdTe2 the Dirac type-II cone does intersect εF, although our calculations also indicate that the associated cyclotron orbit on the Fermi surface is located in a distinct kz plane with respect to the one of the Dirac type-II node. Therefore it should yield a trivial Berry-phase
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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