Santiago-Aviles, Jorge J

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Now showing 1 - 10 of 12
  • Publication
    Electronic Transport Properties of Incipient Graphitic Domains Formation in PAN Derived Carbon Nanofibers
    (2004-03-01) Wang, Yu; Ramos, Idalia; Santiago-Aviles, Jorge J; Furlan, Rogerio
    The carbon nanofibers used in this work were derived from a polyacrylonitrile (PAN)/N, N-dimethyl formamide (DMF) precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity, σ, was measured at temperatures between 1.9 and 300 K and transverse magnetic field between -9 and 9 T. Zero magnetic field conductivity σ(0,T) was found to increase monotonically with the temperature with a convex σ(0,T) versus T curve. Conductivity increases with the external transverse magnetic field, revealing a negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of - 75 % at 1.9 K and 9 T. The magnetic field dependence of the conductivity and the temperature dependence of the zero-field conductivity are best described using the two-dimensional weak localization effect.
  • Publication
    Low-Temperature Electronic Properties of Electrospun PAN-Derived Carbon Nanofiber
    (2004-06-01) Wang, Yu; Santiago-Aviles, Jorge J
    Although carbon nanofibers might have wide potentials in applications, most of their physical properties have yet to be investigated. This paper reports on the low-temperature electronic transport properties of an electrospun polyacrylonitrile-based carbon nanofiber, with its mean diameters around 120 nm. The resistance of the carbon fiber was measured using the four-point probe method from 295 down to 15 K. The semiconducting nature of the fiber is revealed by its positive temperature coefficient of conductance, i.e., an increase in conductivity with the temperature. The conductivity (σ) depends on temperature according to the relation, σ = 5768T 0.338exp(-2 x 10-6 eV/kT), suggesting an almost zero bandgap and a strong temperature dependence of carriers mobility. Such temperature dependence of conductivity is very similar to that found in carbon microfibers, and can be explained using a simple two-band model with temperature-dependent mobility.
  • Publication
    Synthesis of Palladium with Different Nanoscale Structures by Sputtering Deposition onto Fiber Templates
    (2008-11-13) Pantojas, Victor M.; Rodriguez, Diego; Morell, Gerardo; Rivera, Adamari; Ortiz, Carlos; Santiago-Aviles, Jorge J; Otaño, Wilfredo
    A flexible and versatile method combining sputtering and electrospinning techniques was used to shape different palladium morphological structures with nanoscale features. The samples were prepared by dc-magnetron sputtering onto thermally degradable polymer templates. The sputtering parameters were chosen to deposit the metal under low adatom-mobility conditions. After deposition, the template was removed by heat treatment, thereby forming different palladium morphologies with shapes resembling ribbons and half tubes, amongst others. X-ray diffraction studies demonstrated that they are composed of crystalline palladium or palladium oxide, depending on the heat treatment. The cylindrical walls are composed of 30 nm or smaller crystallites, as measured from transmission electron microscopy images. A mathematical simulation demonstrate that the morphological structures obtained are a consequence of the sputtering line-of-sight deposition process. This fabrication process can be varied to modify three types of structures at the nanoscale level: the external shape, the columnar shape of the walls, and the nano-crystallinity. The external shape can be modified by controlling the deposition time and the fiber template diameter. The columnar shape of the walls and the nano-crystallinity can be modified by changes in the sputtering process parameters. The nanoscale morphologies created have potential uses in sensing and photonic applications.
  • Publication
    Early Stages on the Graphitization of Electrostatically Generated PAN Nanofibers
    (2002-08-26) Wang, Yu; Santiago-Aviles, Jorge J
    Carbon nanofibers were produced from polyacrylonitrile/N, N-Dimethyl Formamide (PAN/DMF) precursor solution using electrospinning and vacuum pyrolysis at temperatures from 773K to 1273K for 0.5,2, and 5 hours, respectively. Their conductance was measured. It was found that the conductivity increases sharply with the pyrolysis temperature, and increases considerably with pyrolysis temperatures of 873, 973 and 1073K, but varies, less obviously, with pyrolysis time at the higher pyrolysis temperatures of 1173 and 1273K.
  • Publication
    Tin oxide micro/nano fibers from electrostatic deposition
    (2006-02-01) Wang, Yu; Aponte, M.; Leon, N.; Ramos, Idalia; Furlan, Rogerio; Pinto, N.; Santiago-Aviles, Jorge J
    SnO2 micro/nano fibers in the rutile structure were synthesized using electrospinning and metallorganic decomposition techniques. Fibers were electrospun using two different precursor solutions, one based on SnCl4 and the other on C22H44O4Sn. The fibers were sintered in air for two hours at 400, 500, 600, 700 and 800ºC. SEM, AFM, XRD, XPS and Raman microspectrometry were used to characterize the sintered fibers. The results showed that the fibers were composed of SnO2 and that the SnCl4 precursor led to better results in terms of uniformity/continuity of the fibers.
  • Publication
    Ceramic Microchips for Capillary Electrophoresis-electrochemistry
    (1999-07-01) Henry, Charles S.; Zhong, Min; Lunte, Susan M.; Bau, Haim; Kim, Moon; Santiago, Jorge J.
    A capillary electrophoresis–electrochemistry chip constructed from low-temperature co-fired ceramic (LTCC) tape is presented. This is the first report of such a chip constructed in this manner using these materials. Electroosmotic flow at pH 7 is demonstrated by the migration of a neutral marker, catechol. The separation and detection of two phenolic compounds are presented.
  • Publication
    Detection of Moisture and Methanol Gas Using a Single Electrospun Tin Oxide Nanofiber
    (2007-09-01) Wang, Yu; Ramos, Idalia; Santiago-Aviles, Jorge J
    This letter reports the fabrication of a gas sensor based on a single tin oxide nanofiber made from dimethyldineodecanoate tin using electrospinning and metallorganics decomposition techniques. The fabricated sensor has been used to detect moisture and methanol gas. It showed high sensitivity to both gases and the response times of the complete testing system are in the range of 108–150 s for moisture, and 10–38 s for methanol gas, respectively.
  • Publication
    Large Negative Magnetoresistance and Strong Localization in Highly Disordered Electrospun Pregraphitic Carbon Nanofiber
    (2006-09-21) Wang, Yu; Santiago-Avilés, Jorge J
    A highly disordered pregraphitic carbon nanofiber with the product of its quasi-Fermi wave vector and mean free path close to 1 was fabricated using electrospinning technique. Strong localization made the conductivity vary with temperature as ln σ ∝ T−1/2 from 300 to 5 K, suggesting variable range hopping as the conductivity mechanism, and resulted in a large negative magnetoresistance from 300 K down to 1.9 K that can still be quantitatively described using weak localization and electron interaction models.
  • Publication
    Pyrolysis Temperature and Time Dependence of Electrical Conductivity Evolution for Electrostatically Generated Carbon Nanofibers
    (2003-03-01) Wang, Yu; Santiago-Aviles, Jorge J; Furlan, Rogerio; Ramos, Idalia
    Carbon nanofibers were produced from polyacrylonitrile/N, N-Dimethyl Formamide (PAN/DMF) precursor solution using electrospinning and vacuum pyrolysis at temperatures from 773-1273 K for 0.5, 2, and 5 h, respectively. Their conductance was determined from I – V curves. The length and cross-section area of the nanofibers were evaluated using optical microscope and scanning probe microscopes, respectively, and were used for their electrical conductivity calculation. It was found that the conductivity increases sharply with the pyrolysis temperature, and increases considerably with pyrolysis time at the lower pyrolysis temperatures of 873, 973, and 1073 K, but varies, less obviously, with pyrolysis time at the higher pyrolysis temperatures of 1173 and 1273 K. This dependence was attributed to the thermally activated transformation of disordered to graphitic carbons.
  • Publication
    Large negative magnetoresistance and strong localization in highly disordered electrospun pregraphitic carbon nanofiber
    (2006-09-18) Wang, Yu; Santiago-Aviles, Jorge J
    A highly disordered pregraphitic carbon nanofiber with the product of its quasi-Fermi wave vector and mean free path close to 1 was fabricated using electrospinning technique. Strong localization made the conductivity vary with temperature as σ [proportional to]Τ-1/2 from 300 to 5 K, suggesting variable range hopping as the conductivity mechanism, and resulted in a large negative magnetoresistance from 300 K down to 1.9 K that can still be quantitatively described using weak localization and electron interaction models.