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The electron field emission of carbon nanotube ͑CNT͒/boron-doped diamond ͑BDD͒/carbon felt electrodes ͑CNT/BDD/felt͒ have been investigated. The composite electrode was initially prepared with the growth of BDD on carbon felt and the subsequent growth of CNT by chemical decomposition of methanol. The composite electrodes were characterised using scanning electron microscopy and transmission electron microscopy. For the CNT/BDD/felt samples, the electron field emission was observed at macroscopic fields as low as 1.1 V m −1 . The emission current versus time plot shows significant potential for future field emission applications. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2178247͔ Diamond thin films are a class of carbon materials that have been used for electron field emission devices since 1991. 1 Its electron emission properties depend on several factors, such as the work function, grain boundaries, 2 and small protuberances. [3][4][5][6][7] For example, it has been shown that the boundaries between grains, sites rich in sp 2 hybridization, emit electrons more easily than the diamond facets of boron-doped diamond ͑BDD͒ polycrystalline films. 7 Interestingly, despite boron being an acceptor state in diamond it appears to enhance the electron emission properties of the material. We believe this is based on the substrate playing an important role in the emission process of diamond films. In fact, it has been observed that better emission results from chemical vapor deposited diamond films grown on SnO 2 instead of silicon, suggesting that the electron supply layer or substrates plays an active role in the emission process. 8 However, despite the well-known attractive properties of diamond ͑negative electron affinity, thermal, mechanical, and chemical inertness͒, there are still several problems in the production of field emission devices based on this material associated with obtaining high current densities from emitters, obtaining uniform microstructures, etc., which affect long-term stability. 9 In addition, diamond films, as well as other carbon materials which have been used for emission, are subject to thermal effects at the interface between the substrate and the carbon film. The presence of heating at the interface of the film and substrate generates different mechanical stresses on the film. This will increase the resistance and/or the voltage barrier needed to maintain electron emission at low voltages for an extended periods of time.In order to circumvent the above problem, we present here an approach based on a combination of carbon nanotubes ͑CNTs͒ and BDD films mounted on a carbon felt to obtain reproducible electron emission at low fields. BDD mounts on a carbon felt have previously been explored for electrochemical applications. 10,11 CNTs grown on BDD films, as described in this letter, is a further interesting technique to increase its potential uses.The felt substrate was obtained from carbonization of polyacrinitrile at 2000°C. The obtained carbon fibers have a diameter of around 10...

Nanocomposites of poly (methyl methacrylate) (PMMA) and carbon nanotubes have a high potential for applications where conductivity and low specific weight are required. This piece of work concerns investigations of the level of dispersion and morphology on the electrical properties of in situ polymerized nanocomposites in different concentrations of multi-walled carbon nanotubes (MWCNT) in a PMMA matrix. The electrical conductivity was measured by the four point probe. The morphology and dispersion was analyzed by Transmission Electron Microscopy (TEM) and Small Angle X-ray Scattering (SAXS). The correlation between electrical conductivity and the MWCNT amount, presented a typical percolation behavior, whose electrical percolation threshold determined by power law relationship was 0.2 vol. (%) The exponent t from the percolation power law indicated the formation of a 3D network of randomly arranged MWCNT. SAXS detected that the structures are intermediate to disks or spheres indicating fractal geometry for the MWCNT aggregates instead of isolated rods. HR-TEM images allowed us to observe the MWCNT individually dispersed into the matrix, revealing their distribution without preferential space orientation and absence of significant damage to the walls. The combined results of SAXS and HR-TEM suggest that MWCNT into the polymeric matrix might present interconnected aggregates and some dispersed single structures.

Previous study with nanocomposites of acrylonitrile-butadiene-styrene copolymer (ABS) and organophylized montmorillonite clays (OMMT) investigated the effect of clay modifiers polarities on their performance. Nevertheless, the literature has shown that the use of compatibilizers plays a key role in the dispersion of OMMT in polymeric matrices. Therefore, in the present work, ABS/OMMT nanocomposites compatibilized with styrenic block copolymers prepared by melt intercalation in a twin-screw extruder was studied. Nanocomposites formulations followed a 22 factorial design having different kinds of Cloisite OMMT (30B and 20A + 30B) and of compatibilizers (SBS and SEBS- g-MA) as independent variables. Structural (X-ray diffraction) and morphological (transmission electron microscopy) analyses of the nanocomposites showed a better clay dispersion, partially exfoliated, for ABS/30B/SBS than the intercalated morphology for ABS/20A30B/SEBS- g-MA. Rheological dynamic shear tests indicated a tendency to the formation of a percolating network structure for the ABS/OMMT/SBS nanocomposites, independently of nanoclay kind. Besides, these materials presented non-dripping effect on flammability tests. ABS/OMMT/SBS nanocomposites show also the best performance in both uniaxial tensile and dynamic shear storage moduli. These results are probably due to improved affinity between components. In conclusion, among the styrene block copolymers, SBS compatibilizer is more effective than SEBS- g-MA to improve the properties of ABS/OMMT-based nanocomposites.

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RESUMO -Os Nanotubos de Carbono (NTCs) têm sido reconhecidos, desde seu descobrimento por Iijima em 1991, como materiais de grande interesse em nanotecnologia, devido às suas excelentes propriedades mecânicas, elétricas, térmicas, entre outras. Desde sua descoberta, os parâmetros de síntese têm sido estudados em busca de rotas que elevem a produtividade e a qualidade dos mesmos. Alguns trabalhos recentes reportam a melhoria na produtividade e na qualidade dos NTCs quando catalisadores contendo dois metais de transição são utilizados na síntese com reator tipo CVD (Chemical Vapour Deposition). Ainda que interessantes resultados tenham sido obtidos, estes trabalhos carecem de experimentos estatisticamente planejados para avaliar a existência ou não de interações significativas entre os parâmetros de síntese dos NTCs. Portanto, o objetivo desta pesquisa é explorar as rotas de síntese via CVD utilizando dois tipos de catalisadores binários, Ni/Co e Co/Mn, utilizando o planejamento fatorial de experimentos 2 3 (FED -Factorial Experimental Design), avaliando três parâmetros de processo: temperatura, razão de fluxo de CH 4 /N 2 e tempo de síntese. As repostas obtidas, como: diâmetro dos NTCs, quiralidade e pureza, serão tratados utilizando-se o software Statistica™, verificando as interações entre estes parâmetros e suas respectivas significâncias, buscando otimizar a síntese de nanotubos. INTRODUÇÃONanotubos de Carbono (NTCs) foram descritos, pela primeira vez, por Iijima (1991) e segundo Belin e Epron (2005), desde sua descoberta, eles têm contribuído para o desenvolvimento de estudos nas áreas da física, química e ciência dos materiais e têm sido um material extremamente promissor. Kumar e Ando (2010) citam que suas propriedades extraordinárias abriram uma ampla gama de aplicações e desencadearam uma corrida ao ouro na academia e laboratórios industriais por todo o mundo na busca por usos práticos dos NTCs. Odom et al. comentaram sobre as distintas características do NTCs que aparecem devidas á sua estrutura atômica e suas dimensões nanométricas. Por exemplo, eles podem ser metálicos ou Área temática: Engenharia de Materiais e Nanotecnologia 1

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