Abstract:There are many different techniques for the synthesis of carbon nanotubes (CNTs), and plasma technologies experience a significant competitor in thermal chemical vapor deposition (CVD) processes. A particular process is, therefore, selected according to the specific requirements of an application, which clearly differ for the development of composites as compared to nanoelectronics, field emission, displays, sensors, and the like. This paper discusses the method for the synthesis of CNTs using an atmospheric-pressure microwave (MW) torch. It was successfully applied in the fast deposition of multiwalled nanotubes (MWNTs) on a substrate without the necessity of any vacuum or heating equipment. Dense straight-standing nanotubes were prepared on Si substrates with and also without barrier SiO x layer. Therefore, it was possible to produce CNTs directly on conductive Si and to use them as an electron-emitting electrode of the gas pressure sensor. The CNTs grown in MW torch were also used to create a gas sensor based on the changes of electrical resistance measured between two planar electrodes connected by the CNTs.
Cobalt-cemented hard metals present an example of samples with a complicated matrix consisting of components differing in chemical and physical properties and with extremely low volatility of all components. The purpose of this study was to compare particle formation of a set of real samples with similar matrices but different content of major components. The laser ablation process was studied using a Q-switched quintupled (213 nm) nanosecond Nd:YAG laser. Five samples of Co-cemented tungsten carbides, actually WC-TiC-(Ta,Nb)C-Co with a varied content of main constituents, were selected as representatives of a family of 15 miscellaneous tungsten carbide hard metal products. Physical and chemical properties vary over this specimen selection and therefore the effect on particle size formation and distribution was expected. The size distributions by number of ablated particles in different size ranges were measured using an optical aerosol spectrometer. The results proved the relationship between particle formation and sample composition. The structure of laser generated particles and the properties of ablation-craters were additionally studied by scanning electron microscopy (SEM). Spherical particles in the diameter range of 0.25-2 mm and mm-sized agglomerates composed of primary nano-particles were observed. The W and Co content in the aerosol particles was determined by energy dispersive X-ray spectroscopy (EDS). The volumes of ablation craters were measured by an optical profilometer. The laser ablation study of selected Co-cemented tungsten carbide hard metals indicates a similar total volume of formed particles with composition-dependent particle-size distributions.
Institute of Scientific Instruments of the ASCR, v.v.i., Královopolská 147, 612 64 Brno, Czech Republic Instrument and methodology is presented for very low energy scanning transmission electron microscopy. The detector system provides simultaneous acquisitions of total reflected and transmitted electron fluxes. Introductory experiments incorporated examination of ultrathin foils of gold, carbon and graphene flakes. Extremely sensitive thickness contrast obtained at units of eV is demonstrated. The phenomenon of electron transmissivity apparently exceeding 100% owing to the contribution of secondary electrons released near to the exit surface is described and discussed.
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