1,720,969 research outputs found
Thermal study on decomposition of LiBH4 at non-isothermal and non-equilibrium conditions
No full textThermal decomposition measurements for lithium borohydride (LiBH4) are performed at non-isothermal and non-equilibrium conditions by means of differential thermal analysis (DTA). A simplified alternative procedure is introduced for evaluating thermodynamic and kinetic parameters simultaneously using a single set of measurements. Rate constant (k) and enthalpy (??H = ???102.1 ± 0.7 kJ mol???1 LiBH4) are archived. Temperature dependence for activation energy (E a) is found taking advantage of Guggenheim???Arrhenius method; the mean activation energy is E?????????a 93.9 ± 0.9 kJ mol???1 LiBH4 in the range of heating rate ?? 1???50 K min???1
"Boron Effect" on the Thermal Decomposition of Light Metal Borohydrides MBH4(M = Li, Na, Ca)
No full textThe hydrogen release rate of thermal decomposition, after the melting, for the borohydride of lithium (Li), sodium (Na), and calcium (Ca) and their boron mixtures, at selected molar ratios, is investigated under 1 bar hydrogen pressure and nonisothermal conditions. The reaction is studied by means of manometric measurements. The maximum hydrogen release rate for all pure borohydrides is 8 × 10 -3 bar/min. By adding boron to the borohydride systems, the hydrogen release rate is affected and, generally, is lowered. For the decomposition process of LiBH 4+B, maximum rate is 2 × 10 -3 bar/min. On the opposite, hydrogen rate is suppressed in boron mixtures of NaBH 4. The addition of boron changed slightly the maximum rate of hydrogen release for Ca(BH 4) 2+B for first and second decompositio
Enhanced hydrogen release of metal borohydrides M(BH4)n (M = Li, Na, Mg, Ca) mixed with reduced graphene oxide
No full textMixtures of light metal borohydrides M(BH4)n, M = Li, Na, Mg, Ca and reduced graphene oxide (rGO) or graphite with molar ratio 70:30 were prepared by high energy ball milling. Samples were investigated with thermal programmed desorption (TPD) volumetric analysis to test their gas release profiles. The presence of rGO led to a lower decomposition temperature for every mixture and to a different gas-release profile compared to pristine borohydrides and mixtures with graphite. The onset of decomposition temperature was reduced by 200 °C for LiBH4, 360 °C for NaBH4, 130 °C for Mg(BH4)2 and 50 °C for Ca(BH4)2. The comparison of XRD profiles of decomposed samples showed a different pattern for each metal cation, confirming an interaction of the decomposition products with rGO sheets and suggesting a different pathway for the decomposition reaction. Reduced graphene oxide was prepared by clean thermal treatment in H2 atmosphere from graphene oxide obtained by modified wet chemical synthesis, ..
The structural change of graphene oxide in a methanol dispersion
No full textGraphene oxide is a derivative of graphene which contains oxygen domains. Its debatable structure depends on the specific functional groups bonded to the graphene basal plane which have an impact on its reactivity. Here, we report the influence of methanol which affects the functionalization of pristine graphene differently to water
Time-Dependent Structure and Solubilization Kinetics of Graphene Oxide in Methanol and Water Dispersions
No full textCarbonaceous materials have long been attracting attention due to their physical and chemical properties as well as their low production costs. Among them, graphene oxide is considered to be an extremely promising material in the context of a wide spectrum of emerging technologies. However, since graphene oxide is charaterized by a variable and therefore elusive chemical composition, complete control on the structure and the reactivity of this complex material has yet to be achieved, which clearly represents a knowledge gap that needs to be closed in order to confidently enable as many future applications as possible. Here, we present a spectroscopic study of graphene oxide obtained via the Sun method showing structural alteration of the material upon utilization of solvents of different polarity. In particular, our data provide clear experimental evidence that the use of methanol results in a graphene oxide material essentially enriched in carbonyl and hydroxyl groups. Moreover, the conjugated character of the material is also influenced by the nature of the solvent. The observed structural changes, which are due to the functionalization of the graphene basal plane, are time-dependent and correspond to different solubility properties of the resulting material. Finally, the modification of the graphene oxide structure alters the rate and the activation energy of solubilization
A case of chemical scalp burns after hair highlights: experimental evidence of oxidative injuries.
No full text<p>Hair highlights are quite common procedures carried out in hair salons by using a mixture of a lightening powder containing persulfates with a suspension containing hydrogen peroxide: a representative case of chemical scalp burns is described as a consequence of this treatment. The aim of the paper is to demonstrate the strict relationship between the scalp damage and the commercial products used in a case of hair highlighting. The results of some chemical analyses have been reported, showing, in particular, that the chemical reactivity of the mixture changes in the time, thus strongly suggesting that the procedure for the application of the mixture is critical for the occurrence of possible accidents. The presence in the powder of chemical compounds bearing aliphatic chains as surfactants explains the appearance of dramatic symptoms after days due to a slow dissolution of the oxidant compounds in the stratum corneum of skin with no effect in reducing injury of palliative treatments. Safety suggestions and recommendations for producers and workers are also included.</p
Self-assmebly of molecules on nanostructured graphene
Full text linkTesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica de la Materia Condensada. Fecha de lectura: 27-07-2014In this thesis the growth and morphological characterisation for the self-assembly
of various electron acceptor molecules were investigated by mean of Scanning Tunnelling
Microscope (STM) in Ultra High Vacuum. Molecules, from the quinone
and xanthine classes, such as 11,11, 12,12 - tetracyano-9,10-anthraquino dimethane
(TCAQ), 13,13,14,14 - tetracyano-5,12 -tetracenequino dimethane (TCTQ), 15,15,
16,16 -tetracyano -6,13-pentacenequino dimethane (TCPQ) and 1,3,7-Trimethyl xanthine
(caffeine), were chosen. Two different substrate, graphene/Ru(0001) and Cu
(111), were used to study the influence of the substrate molecule interaction in
the self-assembly processes. The metallic substrate allows the hybridisation of the
molecular states and the metallic DOS. On the other hand, the graphene decouples
the molecules from the metal providing a good playground to study the moleculemolecule
interactions. These molecules aggregate and eventually self-assembly on
the surface and the aggregation process is influenced by the number of the rings
in the backbone of the molecule, the interaction with the substrate and the coverage
of the surface. Basically, the TCxQ molecules show on Cu(111) a very limited
aggregation character with the formation of small clusters, such as chains or
dimers, and eventually, disordered structures. A qualitative explanation for the
cluster formation can be given using the molecular potential maps, where the negative
potential, associated with the cyano groups, interacts with the positive potential,
located on the hydrogens in the molecular backbone, forming a hydrogen
bond network. Analysing the apparent size in the STM images on the energetics in
theoretical calculations it can be concluded that all TCxQ molecules adopt the socalled
X-configuration upon adsorption on Cu(111). Unlike, the deposition of those
molecules on graphene/Ru(0001) brings to a very different structure which is characterized
by a formation of an extensive hydrogen bond network and a very weak interaction
with the substrate. Each of the quinone derived molecules self-assemblies
in a different way depending on the number of the aromatic rings in the backbone
of the molecule and the coverage of the surface. At low coverage, only symmetric
molecules stay on the surface and occupy the low area of the graphene moiré with
unordered clusters. By increasing the coverage TCAQ molecules self-assembly into
representative “railroad tracks” structure covering low and top area of the moiré .
By contrast, TCTQ and TCPQ molecules formed a porous network filling the low
area only. When the monolayer conditions are reached, a compact self-assembly
structure is obtained only for the symmetric molecules. To investigate the influence
of the rigid molecular structure on the surface, caffeine molecules were deposited
on Cu(111). Caffeine is basically adsorbed flat-lying on Cu(111) and forms dimers
arranged into parallel rows. The molecules are easy to move indicating the absence
of a preferential adsorption position and the important of the intermolecular interactions
in the formation of the self-assembled structure.En esta tesis fueron investigadas el crecimiento y la caracterización morfológica para
el self-assembly de diferentes moléculas aceptoras de electrones por medio de Scanning
Tunnelling Microscope (STM) en Ultra Alto Vacío. Fueron elegidas moléculas
que van desde las clases quinona y xantina, tales como 11,11, 12,12 - tetraciano -
9,10 - Dimethane anthraquino ( TCAQ ), 13,13,14,14 - tetraciano - 5, Dimethane 12 -
tetracenequino ( TCTQ ), 15,15, 16,16 tetraciano Dimethane -6,13 - pentacenequino
( TCPQ ) a la 1,3,7- trimetil xantina (cafeína). Para estudiar la influencia de la interacción
molécula-sustrato en los procesos de self-assembly han sido utilizados dos
diferentes sustratos, grafeno / Ru(0001) y Cu(111). El sustrato metálico permite la
hibridación de los estados moleculares y el metálico DOS. Por otro lado, el grafeno
desacopla las moléculas del metal proporcionando una buena zona para estudiar las
interacciones molécula-molécula. Estas moléculas se agregan y, al final, se autoensamblan
en la superficie. En concreto el proceso de agregación está influenciado por
el número de anillos en la estructura de la molécula, la interacción con el sustrato
y el recubrimiento de la superficie. Básicamente, las moléculas TCxQ muestran en
Cu (111) un carácter de agregación muy limitado con la formación de pequeños
grupos, tales como cadenas o dímeros, y, al final, estructuras desordenadas. Una
explicación cualitativa para la formación del clúster se puede hallar utilizando los
mapas de potencial molecular, donde el potencial negativo, asociado con los grupos
ciano, interactúa con el potencial positivo, que se encuentra en los átomos de
hidrógeno en el esqueleto molecular, formando una red de enlace de hidrógeno.
Analizando el tamaño que aparece en las imágenes de STM en la energética en cálculos
teóricos, se puede concluir que todas las moléculas TCxQ adoptan la denominada
configuración-X de la adsorción en Cu(111). Por otro lado, la deposición de las
moléculas en el grafeno / Ru(0001) lleva a una estructura muy diferente, caracterizada
da la formación de una extensa red de enlaces de hidrógeno y una muy débil
interacción con el sustrato. Cada una de las moléculas quinonas derivadas se autoensabla
de manera diferente en función del número de los anillos aromáticos en la
cadena principal de la molécula y del recubrimienti de la superficie. En condiciones
de bajo recubrimiento, sólo las moléculas simétricas permanecen en la superficie y
ocupan la zona baja del moiré de grafeno con clusters desordenados. A medida que
aumenta el recubrimiento las moléculas de TCAQ se autoensemblan en estructuras
representativas “railroad tracks”cubriendo la parte del área superior y inferior del
moiré. Por el contrario, las moléculas de TCTQ y TCPQ forman una red porosa
que llena sólo la zona inferior. Se comprobó tambien que en el caso de tener una
monocapa, la estructura self-assembly compacta se obtiene sólo para las moléculas
simétricas. Por último, fueron depositadas moléculas de cafeína en superficies de
Cu(111) para poder investigar la influencia de la estructura molecular rígida en la
superficie. La cafeína es básicamente adsorbida en manera planar con respecto al
Cu(111) y forma dímeros dispuestos en filas paralelas. En concreto, las moléculas
presentan facilidad de movimientos, que indica la ausencia de una posición de
adsorción preferencial y la importancia de las interacciones intermoleculares en la
formación de la estructura self-assembl
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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