Carla Veríssimo

Ionic conductivity and structural characterization of Na1.5Nb0.3Zr1.5(PO4)3 with NASICON-type structure

Abstract The NASICON-type Na1.5Nb0.3Zr1.5(PO4)3 was prepared by solid state reaction of Nb2O5 and the precursor γ-NaHZr(PO4)2 at 700 °C. The EPR spectra showed a signal with a g factor of 1.984 assigned to Nb (IV) species in octahedral oxygen environments. The X-ray powder diffraction pattern obtained with monochromatic radiation was indexed on the basis of a rhombohedral cell, the hexagonal parameters being aH = 8.8061(2) and c H = 22.7638(7) A . The Na+ ion conduction was measured by the complex impedance method (frequency range: 0.1–105 Hz; temperature range: 20–500 °C) on four pellets previously sintered at 450, 750, 900 and 1000 °C. The conductivity data are discussed in relation to the sintering temperature. An activation energy of 0.60 eV for the movement of Na+ ions in the NASICON framework has been found.

Nucleation and growth of carbon nanotubes in catalytic chemical vapor deposition

The process of nucleation of multiwall carbon nanotubes in chemical vapor deposition process with nickel as catalyst and methane as a carbon precursor is analyzed. The nucleation is considered as a specific instability developed on the surface of a metal catalyst particle supersaturated with carbon. The energy released in graphitization of carbon from the metal-carbon solution is shown to be crucial for the nanotube nucleation. The energy released may be high enough for substantial metal heating resulting in partial liquefaction of the catalyst particle. The proposed mechanism can be called vapor-solid-liquid-solid (VSLS) as the catalyst particle may be in a mixed solid-liquid (or liquidlike) state during nucleation and unstable phases of nanotube growth.

Different carbon nanostructured materials obtained in catalytic chemical vapor deposition

Different carbon nanostructured materials, such as nanotubes, nanofibers, nanosprings and nanooctopus, were grown by changing the metal catalyst and experimental parameters of the thermal chemical vapor deposition process. These experiments were performed using a tubular furnace and methane or acetylene as carbon feedstock gases. Thin films of Ni or Cu were deposited onto a SiO2/Si substrate and employed as catalysts. The effect of the growth temperature, metal catalyst and carbon gas precursor (methane or acetylene) on the final carbon nanoestructured material was studied by scanning electron microscopy, Raman spectroscopy and grazing incidence X-ray diffraction. Growth of multiwall carbon nanotubes (MWCNTs) was observed using both metal films and carbon precursor gases, whereas partially oxidized Ni films promoted formation of nanosprings. Experiments with reduced supply of methane resulted in octopus-like carbon nanostructures when a Cu film was used as a catalyst.

Effect of TiO2 nanoparticles on the thermal properties of decorated multiwall carbon nanotubes: A Raman investigation

We have investigated multiwalled carbon nanotubes decorated with TiO2 nanoparticles. Scanning electron microscopy and transmission electron microscopy measurements revealed that the TiO2 incorporates on the nanotubes forming large cauliflowerlike aggregates and/or small crystalline particles attached to the nanotube wall, depending on the growth conditions. A detailed Raman study was performed in pristine and a series of decorated nanotubes, where we analyzed both the Raman signal from the nanotubes and from the TiO2 nanoparticles. We demonstrate that the attached TiO2 nanoparticles affect significantly the thermal properties of the resulting hybrid nanostructure, as revealed by differentiated reactions to laser heating. This is a crucial point for modeling and optimization of devices based on nanotubes, which properties are strongly temperature dependent.

Controlled Deposition and Electrical Characterization of Multi-Wall Carbon Nanotubes

A method of ac dielectrophoresis was applied to align and deposit metallic multi-wall carbon nanotubes between pre-fabricated metal (Au, Pd) electrodes with a micron scale separation. For improvement of nanotube contacts with electrodes, Ni and Pd electroless processes were developed, and significant reduction of 2 terminals resistances was demonstrated. Further, using electron and ion beam deposited Pt contacts in two different configurations (“Pt-on-CNT” and “CNT-on-Pt”), 4 terminals measurements have been performed to evaluate intrinsic nanotube resistances. The values between 90 and 130 kΩ/μm were obtained, while systematically lower values (30-70 kΩ/μm) were estimated using 2 terminals method. The 4 terminals method was applied to study the effect of ion irradiation on the electrical parameters of supported nanotubes.

Preparation of the conducting nanocomposites using molded inorganic matrix: fibrous cerium(IV) hydrogenphosphate as a self-supported pyrrole polymerization agent

New supported polypyrrole nanocomposites have been prepared and studied in this work. Self-supported sheets of fibrous cerium(IV) hydrogenphosphate (CePf) were used as the inorganic matrix, since the fibrous morphology presented by this material makes molding possible, giving a desired shape to the final polypyrrole/CePf conducting nanocomposites. The influence of pyrrole concentration on the nanocomposite preparation was studied, as well as the influence of CePf on the polypyrrole characteristics. XRD, Raman, SEM and conductivity measurement data show the conducting polypyrrole coating CePf fibers, yielding self-supported sheets of organic–inorganic conducting nanocomposites.

Laser irradiation of carbon nanotube films: Effects and heat dissipation probed by Raman spectroscopy

We investigate the thermal properties of thin films formed by single- and multi-walled carbon nanotubes submitted to laser irradiation using Raman scattering as a probe of both the tube morphology and the local temperature. The nanotubes were submitted to heating/cooling cycles attaining high laser intensities (∼1.4 MW/cm2) under vacuum and in the presence of an atmosphere, with and without oxygen. We investigate the heat diffusion of the irradiated nanotubes to their surroundings and the effect of laser annealing on their properties. The presence of oxygen during laser irradiation gives rise to an irreversible increase of the Raman efficiency of the carbon nanotubes and to a remarkable increase of the thermal conductivity of multi-walled films. The second effect can be applied to design thermal conductive channels in devices based on carbon nanotube films using laser beams.

Fabrication of Multi-point Test Structures Using Focused Ion Beam and Selective Carbon Nanotubes Deposition by Dielectrophoresis

In this work, we present experimental procedures developed for fabrication of multi-point test structures using focused ion beam. The test structure is fabricated in two steps: (i) metal (Au, Pd) electrodes are fabricated by lift-off technique. (ii) nanocontacts are fabricated by ion beam induced deposition of platinum and milling with Ga ion beam. The multi-wall nanotubes (MWNT) are deposited by dielectrophoresis method between metal electrodes. Our test structure allows measurements using 2 and 4 terminals methods.

The Influence of Substrates on the Carbon Nanotube Growth Process

Carbon nanotubes (CNTs) have attracted much attention due to their extraordinary properties. This nanostructured material has been synthesized by various methods and the chemical vapor deposition (CVD) has been shown to be an efficient and versatile technique. In this work, catalytic thermal CVD method, using a mixture of methane and hydrogen at atmospheric pressure on a horizontal tubular quartz furnace, was used to grow carbon nanotubes. Silicon wafers with SiO2 or Al2O3 layers were used as substrates whereas thin nickel film was deposited over the substrates and used as catalyst. The interaction between catalyst nickel film and two different oxide layers supported on silicon wafers was studied as well as the influence of both substrates (Si/SiO2 and Si/Al2O3) on the carbon nanotube growth. It was observed a completely different interaction between Ni film and both oxide layers, affecting strongly the growth of CNTs.