1,720,955 research outputs found
Self-healing in B12P2 through Mediated Defect Recombination
Get PDFCitation: Self-healing in B12P2 through Mediated Defect Recombination. S. P. Huber, E. Gullikson, C. D. Frye, J. H. Edgar, R. W. E. van de Kruijs, F. Bijkerk, and D. Prendergast. Chemistry of Materials 28 8415--8428 (2016) 10.1021/acs.chemmater.6b04075The icosahedral boride B12P2 has been reported to exhibit “self-healing” properties, after transmission electron microscopy recordings of sample surfaces, which were exposed to highly energetic particle beams, revealed little to no damage. In this work, employing calculations from first-principles within the density functional theory (DFT) framework, the structural characteristics of boron interstitial and vacancy defects in B12P2 are investigated. Using nudged elastic band simulations, the diffusion properties of interstitial and vacancy defects and their combination, in the form of Frenkel defect pairs, are studied. We find that boron icosahedra maintain their structural integrity even when in a degraded state in the presence of a vacancy or interstitial defect and that the diffusion activation energy for the recombination of an interstitial vacany pair can be as low as 3 meV, in line with the previously reported observation of “self-healing”
Detection of defect populations in superhard semiconductor boron subphosphide B12P2 through X-ray absorption spectroscopy
Get PDFCitation: Detection of defect populations in superhard semiconductor boron subphosphide B12P2 through X-ray absorption spectroscopy. S. P. Huber, E. Gullikson, J. Meyer-Ilse, C. D. Frye, J. H. Edgar, R. W. E. van de Kruijs, F. Bijkerk, and D. Prendergast J. Mater. Chem. A 5 5737--5749 (2017) 10.1039/c6ta10935gRecent theoretical work has shown for the first time how the experimentally observed property of “self-healing” of the superhard semiconductor boron subphosphide (B12P2) arises through a process of mediated defect recombination. Experimental verification of the proposed mechanism would require a method that can detect and distinguish between the various defect populations that can exist in B12P2. X-ray absorption near-edge spectroscopy (XANES) is such a method and in this work we present experimentally collected spectra of B12P2samples with varying crystalline qualities. By simulating the X-ray spectroscopic signatures of potential crystallographic point defects from first-principles within the density functional theory framework, the presence of defect populations can be determined through spectroscopic fingerprinting. Our results find an increasing propensity for the presence of phosphorus vacancy defects in samples deposited at lower temperatures but no evidence for comparable populations of boron vacancies in all the samples that have been studied. The absence of large amounts of boron vacancies is in line with the “self-healing” property of B12P2
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
B₁₂P₂: improved epitaxial growth and evaluation of α irradiation on its electrical transport properties
Get PDFDoctor of PhilosophyDepartment of Chemical EngineeringJames H. EdgarThe wide bandgap (3.35 eV) semiconductor icosahedral boron phosphide (B₁₂P₂) has been reported to self-heal from radiation damage from β particles (electrons) with energies up to 400 keV by demonstrating no lattice damage using transmission electron microscopy. This property could be exploited to create radioisotope batteries–semiconductor devices that directly convert the decay energy from a radioisotope to electricity. Such devices potentially have enormous power densities and decades-long lifetimes. To date, the radiation hardness of B₁₂P₂ has not been characterized by electrical measurements nor have B₁₂P₂ radioisotope batteries been realized. Therefore, this study was undertaken to evaluate the radiation hardness of B₁₂P₂ after improving its epitaxial growth, developing ohmic electrical contacts, and reducing the residual impurities. Subsequently, the effects of radiation from a radioisotope on the electrical transport properties of B₁₂P₂ were tested.
B₁₂P₂ was grown epitaxially on 4H-SiC by chemical vapor deposition (CVD) over the temperature range of 1250-1450 °C using B₂H₆ and PH₃ precursor gases in a H₂ carrier gas. The epitaxial relationship between B₁₂P₂ and 4H-SiC was (0001)B₁₂P₂[1100]B₁₂P₂(0001)4H-SiC[1100]4H-SiC using hexagonal indices (or (111)B₁₂P₂[121]B₁₂P₂(0001)4H-SiC[1100]4H-SiC using rhombohedral indices for B₁₂P₂). X-ray diffraction (XRD) rocking curve measurements (assessing the crystal quality) about the B₁₂P₂ (0003) peak were minimized at 1300 °C indicating it was the optimum growth temperature studied. By miscutting the (0001) 4H-SiC substrate 4° to the (1100) plane, B₁₂P₂ rotational twinning, a type of crystal defect, was strongly suppressed to a twin density of <1% in comparison to a standard miscut to the (1120) plane which resulted in a twin density of 30%.
Cr/Pt (500/1000 Å) and Ni/Au (1000/1000 Å) ohmic contacts to B₁₂P₂ were developed. Ni/Au contacts annealed at 500 °C for 30 s in Ar proved to be the best contact studied with a specific contact resistance of 3x10⁻⁴ Ω-cm². Background impurities were measured by secondary ion mass spectrometry (SIMS), and Si (≈1x10¹⁹-5x10¹⁹ cm⁻³), C (≈5×10¹⁹-2x10²⁰ cm⁻³), and O (≈2×10²⁰ cm⁻³) were the primary residual impurities when B₁₂P₂ was grown on 4H-SiC at 1250 °C. The SiC substrate caused Si and C contamination, and the TaC-coated graphite heating element was a second C source. Since Si and C are p-type dopants in B₁₂P₂ (confirmed by Hall Effect measurements), the 4H-SiC substrate was exchanged with AlN/sapphire, and the TaC-coated graphite susceptor was replaced with Zr metal. With the new substrate and susceptor, the Si and C concentrations were decreased to 2-3x10¹⁸ cm⁻³ and 4x10¹⁸ cm⁻³, respectively. The radiation hardness of p-B₁₂P₂ grown on AlN/sapphire was compared to an epi-layer of p-4H-SiC (a conventional radiation hard material) by temperature-dependent Hall Effect measurements. A defect band was present in B₁₂P₂ prior to irradiation, and radiation further contributed to defect band conduction and lowered the hole mobility. Evidence for self-healing of B₁₂P₂ from electrical measurements remains inconclusive. However, the steps necessary to evaluate B₁₂P₂ for device applications has been clarified, and includes further reductions in crystalline defects and residual impurities, especially carbon and oxygen.
This work was performed under the auspices of the U.S. Department of Energy by
Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNLABS-
706086
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Titanium-indium oxy(nitride) with and without RuO2 loading as photocatalysts for hydrogen production under visible light from water
Get PDFTitanium-indium (oxy)nitride composite materials with and without RuO[subscript 2] loadings were produced by treating TiO[subscript 2]-In[subscript 2]O[subscript 3] mixed powders with ammonia at high temperature (700-850 °C) (Elemental analysis indicated an empirical formula of TiIn[subscript 0.029]O[subscript 0.63]N[subscript 1.4]). We have found that ammonolysis of TiO[subscript 2] to form Ti (oxy)nitride or In[subscript 2]O[subscript 3] to form In (oxy)nitride do not give composites active toward methanol-water under visible light, but TiO[subscript 2] and In[subscript 2]O[subscript 3] mixed together do give composites active toward methanol-water under visible light. Ti-In (oxy)nitride powders modified by surface loading with RuO[subscript 2] nanoparticles at 3 wt % achieves the highest H[subscript 2] evolution activity under visible light irradiation. The composite material did not react in the dark, but upon irradiation with visible light, the hydrogen production rate under illumination goes way up to 30 μmole∙h[superscript -1] and the turnover number shows that this is a photocatalytic reaction
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