A new fast method for ceramic foam impregnation: Application to the CCVD synthesis of carbon nanotubes

A new process that allows preparing, in a single step, good washcoats of catalytic materials for the catalytic chemical vapour deposition (CCVD) synthesis of carbon nanotubes (CNTs) in reticulated ceramic foams is reported. It is shown that the washcoats, obtained by impregnation using viscous slurries made of finely divided powders dispersed in different media, cover the total surface of foams with good adhesions. The catalytic activity with regards to the CNT synthesis is finally verified, showing that our new fast impregnation process makes possible to get materials with final architectures suitable for heterogeneous catalysis applications

In situ CCVD synthesis of carbon nanotubes within a commercial ceramic foam

Consolidated nanocomposite foams containing a large quantity of carbon nanotubes (CNTs) within millimetre-sized pores are prepared for the first time. A commercial ceramic foam is impregnated by a 60 g L21 slurry of a (Mg(12x)(Co0.75Mo0.25)xO solid solution (x = 0.01, 0.05, 0.1 and 0.2) powder in ethanol. Three successive impregnations led to deposits several tens of mm thick, with a good coverage of the commercial-ceramic pore walls but without closing the pores. The materials were submitted to a CCVD treatment in H2–CH4 atmosphere in order to synthesise the CNTs. When using attrition-milled powders, the carbon is mostly in the form of nanofibres or disordered carbon rather than CNTs. Using non-milled powders produces a less-compact deposit of catalytic material with a higher adherence to the walls of the ceramic foam. After CCVD, the carbon is mostly in the form of high-quality CNTs, as when using powder beds, their quantity being 2.5 times higher. The so-obtained consolidated nanocomposite materials show a multi-scale pore structuration

Catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes from α-(Al1−xFex)2O3 powders and self-supported foams

An investigation of the potential interest of α-alumina–hematite foams, as opposed to powders, as starting materials for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapor deposition method was performed. The oxide powders and foams as well as the corresponding CNT–Fe–Al2O3 composite powders and foams are studied by X-ray diffraction, specific surface area measurements, electron microscopy, Raman spectroscopy and Mössbauer spectroscopy. The latter technique revealed that four components (corresponding to α-Fe, Fe3C, γ-Fe-C and Fe3+) were present in the Mössbauer spectra of the composite powders, and that an additional sextet, possibly due to an Fe1−yCy alloy, is also present in the Mössbauer spectra of the composite foams. Contrary to some expectations, using foams do not lead to an easier reduction and thus to the formation of more α-Fe, Fe3C and/or γ-Fe–C potentially active particles for the formation of CNTs, and hence to no gain in the quantity of CNTs. However, using foams as starting materials strongly favors the selectivity of the method towards SWCNTs (60% SWCNTs and 40% DWCNTs) compared to what is obtained using powders (5% SWCNTs, 65% DWCNTs and 30% MWCNTs)

Numerical Simulation of Low-Frequency Noise in Polysilicon Thin-Film Transistors

3 pagesInternational audienceNumerical simulations of low-frequency noise are carried out in two technologies of N-channel polysilicon thin-film transistors (TFTs) biased from weak to strong inversion and operating in the linear mode. Noise is simulated by generation/recombination processes. The contribution of grain boundaries on the noise level is higher in the strong inversion region. The microscopic noise parameter that is deduced from numerical simulations is lower than the macroscopic one defined according to the Hooge empirical relationship and deduced from noise measurements. The higher macroscopic value is attributed to the drain-current crowding induced by nonconducting spots in the devices due to structural defects. The ratio of these two noise parameters can be considered as an indicator to qualify TFT technology

Numerical simulation of conduction and low-frequency noise in polysilicon thin film transistors

4 pagesInternational audienceNumerical simulations of static conduction and low-frequency noise are carried out in N-channel polysilicon thin film transistors. The Meyer-Neldel effect associated with the drain current is related to trapping/detrapping processes of carriers from dangling bonds located at the interface. Low-frequency noise is simulated by generation-recombination processes. The sources responsible of noise in the thin film transistors are mainly located close to the interface. The microscopic parameter deduced from numerical simulation is lower than the macroscopic one deduced from noise measurements. The ratio of these two parameters is considered as a factor of merit to qualify thin film transistor technology

Application of homogeneously precipitated nanosized Fe-doped alumina powders to carbon nanotube growth.

Homogeneous precipitation of hydroxides was investigated as an alternative method to synthesize Fe-doped aluminum oxide (α-Al2−2xFe2xO3) particles over which carbon nanotubes (CNTs) were grown via a catalytic chemical vapor deposition (CCVD) method. Performance of the homogeneously precipitated particles for CNT growth was quantitatively compared with that of the combustion-synthesized particles. The main advantage of the homogeneous precipitation of hydroxides and subsequent calcination process against to the combustion synthesis and other commonly practiced chemical routes is the ability to tailor the Fe-doped Al2O3 precursor powder characteristics such as size and specific surface area (SSA) without requiring any milling step and also to control the phase composition of the oxide powder with high Fe content, and subsequently the quality and quantity of CNTs during CCVD process. The particle size of the precipitated and calcined α-Al2−2xFe2xO3 powders varies between ∼50 and 400 nm for 5–10 cat.% Fe-containing systems. The monodispersed particle size distribution and optimum phase composition of the homogeneously precipitated powders, particularly for a 10 cat.% Fe content in the starting oxide, and their much higher SSA than similar materials prepared by other chemical routes lead to production of high amounts of good quality CNTs