Idalia Ramos

Pyrolysis temperature and time dependence of electrical conductivity evolution for electrostatically generated carbon nanofibers

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 carbon.

Synthesis and characterization of tin oxide microfibres electrospun from a simple precursor solution

Tin oxide (SnO2) microfibres in the rutile structure were synthesized using electrospinning and metallorganic decomposition techniques. Fibres were electrospun from a precursor solution containing 20 mg poly(ethylene oxide) (molecular weight 900 000), 2 ml chloroform and 1 ml dimethyldineodecanoate tin, and sintered in the air for 2 h at 400, 600 and 800 °C, respectively. Scanning electron microscopy, x-ray diffraction and Raman microspectrometry were used to characterize the sintered fibres. The results showed that the synthesized fibres are composed of SnO2.

Optical bandgap and photoconductance of electrospun tin oxide nanofibers

Optical and photoconductive properties of transparent SnO2 nanofibers, made from C22H44O4Sn via electrospinning and metallorganic decomposition, were investigated using Fourier transform infrared and ultraviolet (UV)/visible spectrometry and the two-probe method. Their optical bandgap was determined from their UV absorption edge to be 3.95–4.08eV. Their conductance responds strongly to UV light for a wavelength of 254nm: in air its steady-state on-to-off ratios are 1.31–1.56 (rise) and 1.25–1.33 (fall); its 90% rise and fall times are 76–96 and 71–111s, respectively. In a vacuum of about 10−4torr, its on-to-off ratios are higher than 35.6 (rise) and 3.4 (fall), respectively, and its 90% rise and fall times are longer than 3×104s.

Detection of Moisture and Methanol Gas Using a Single Electrospun Tin Oxide Nanofiber

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.

Characterization of an electrospinning process using different PAN/DMF concentrations

We performed an extensive characterization of an electrospinning process to evaluate how the process parameters and precursor solution characteristics affect the fibers morphology. The work was conducted using precursor solutions with different concentrations of polyacrylonitrile (PAN) diluted in a fixed amount of N,N/dimethylformamide (DMF). Fibers obtained with this process can find important applications in the field of nanosensors. The characteristics of the electrospun fibers were analyzed as a function of the solution viscosity, applied voltage and distance between the needle tip (positive electrode) and the collector plate (grounded electrode). The electrical current was monitored during the deposition process and its behavior was correlated with the characteristics of the fibers obtained. Our results demonstrate that the diameter of the fibers increases with increasing viscosity and applied voltage. The number of deposited fibers also increases with the applied voltage. Also, viscosity and applied voltage strongly affect the shape, length and morphology of the fibers. Of particular interest, we demonstrated that by monitoring the electrical current it is possible to control the fibers morphology and bead concentration. The distance between tip and collector plate determines the way the fibers arrive on the collector plate. A main contribution of this study was the definition of conditions to controllably obtain fibers that are smooth and that present diameters in the range between 140 and 300 nm.

Electrostatic deposition of nanofibers for sensor application

This work addresses the formation of nanofibers (with hundred of nanometers) by using electrospinning (electrostatic deposition) aiming at applications as sensors. Different quantities of a colloidal dispersion of graphite particles were blended with polyacrylonitrile (PAN) and N,N dimethylformamide (DMF), resulting in a series of solutions with carbon concentrations ranging from 0 to 25%. Precipitation was observed depending on the concentration of carbon added to the precursor blend. As a consequence, the relative viscosity decreases, due to PAN molecules removal from the solution by carbon particles adsorption, forming precipitates. The resulting fibers show an irregular shape, as observed by SEM and the diameters decrease with the increase of the carbon concentration in the precursor blend. The incorporation of carbon particles in the fibers was confirmed by FTIRS and Raman spectroscopy.

Electronic transport properties of incipient graphitic domains formation in PAN derived carbon nanofibers

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, /spl sigma/, was measured at temperatures between 1.9 and 300 K and transverse magnetic field between -9 and 9 T. Zero magnetic field conductivity /spl sigma/(0,T) was found to increase monotonically with the temperature with a convex /spl sigma/(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.

Electrical characterization of a single electrospun porous SnO2 nanoribbon in ambient air

The electrical conductance of a single porous SnO2 ribbon, synthesized from dimethyldineodecanoate tin using electrospinning and metallo-organic decomposition techniques, has been measured using the two-probe method in atmosphere following a cycle of heating from 300 to 660?K and subsequent cooling from 660 to 300?K. During the heating, the conductance (G) is relatively insensitive to the temperature below 380?K and, above that, follows an Arrhenius relation with a thermal activation energy of 0.918 ? 0.004?eV until 660?K; upon cooling, G follows the same Arrhenius relation until 570?K and, below that, observes another Arrhenius relation with a lower activation energy of 0.259 ? 0.006?eV. After a cycle of heating and cooling, G returns to a value higher than its initial one. The Arrhenius relations are attributed to the surface adsorption and desorption of moisture and oxygen, and the G hysteresis between 300 and 380?K is attributed to the partial replacement of adsorbed oxygen by moisture because of the porous nature of the ribbon surface.

Disordered grain growth in polycrystalline GaN obtained by the polymer-derived-ceramic route

Polycrystalline GaN fibers have been produced by the polymer-derived-ceramic (PDC) technique. The wurtzite-polymorphic fibers appear to emerge from complex nucleation and grain growth mechanisms, being mostly unconstrained during initial polymer to ceramic conversion. The importance of carrier polymer architecture and alignment is highlighted towards controlled microstructure.

Electrical Properties of Electrospun Sb-Doped Tin Oxide Nanofibers

Transparent and conducting tin oxide fibers are of considerable interest for solar energy conversion, sensors and in various electrode applications. Appropriate doping can further enhance the conductivity of the fibers without loosing optical transparency. Undoped and antimony-doped tin oxide fibers have been synthesized by our group in previous work using electrospinning and metallorganic decomposition techniques. The undoped tin oxide fibers were obtained using a mixture of pure tin oxide sol made from tin (IV) chloride : water : propanol : isopropanol at a molar ratio of 1:9:9:6, and a viscous solution made from poly(ethylene oxide) (PEO) and chloroform at a ratio of 200 mg PEO/10 mL chloroform. In this work, antimony doped fibers were obtained by adding a dopant solution of antimony trichloride and isopropanol at a ratio of 2.2812 g antimony trichloride/10 ml isopropanol to the original tin oxide precursor solution. The Sb concentration in the precursor solution is 1.5%. After deposition, the fibers were sintered 600°C in air for two hours. The electrical conductivity of single fibers measured at room temperature increases by up to three orders of magnitude when compared to undoped fibers prepared using the same method. The resistivity change as a function of the annealing temperature can be attributed to the thermally activated formation of a nearly stoichoimetric solid. The resistivity of the fibers changes monotonically with temperature from 714Ω-cm at 2 K to 0.1Ω-cm at 300 K. In the temperature range from 2 to 8 K the fibers have a positive magnetoresistance (MR) with the highest value of 155 % at 2 K and ±9 T. At temperatures of 10 and 12 K the sign of MR changes to negative values for low magnetic fields and positive for high magnetic fields. For higher temperatures (15 K and above) the MR becomes negative and its magnitude decreases with temperature.