Renáta Hanzlíková

Synthesis of carbon nanotubes by plasma-enhanced chemical vapor deposition in an atmospheric-pressure microwave torch

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 SiOx 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.

Study of aerosols generated by 213 nm laser ablation of cobalt-cemented hard metals

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 μm and μm-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.

Tungsten carbide precursors as an example for influence of a binder on the particle formation in the nanosecond laser ablation of powdered materials

A study of LA-ICP-MS analysis of pressed powdered tungsten carbide precursors was performed to show the advantages and problems of nanosecond laser ablation of matrix-unified samples. Five samples with different compositions were pressed into pellets both with silver powder as a binder serving to keep the matrix unified, and without any binder. The laser ablation was performed by nanosecond Nd:YAG laser working at 213 nm. The particle formation during ablation of both sets of pellets was studied using an optical aerosol spectrometer allowing the measurement of particle concentration in two size ranges (10-250 nm and 0.25-17 microm) and particle size distribution in the range of 0.25-17 microm. Additionally, the structure of the laser-generated particles was studied after their collection on a filter using a scanning electron microscope (SEM) and the particle chemical composition was determined by an energy dispersive X-ray spectroscope (EDS). The matrix effect was proved to be reduced using the same silver powdered binder for pellet preparation in the case of the laser ablation of powdered materials. The LA-ICP-MS signal dependence on the element content present in the material showed an improved correlation for Co, Ti, Ta and Nb of the matrix-unified samples compared to the non-matrix-unified pellets. In the case of W, the ICP-MS signal of matrix-unified pellets was influenced by the changes in the particle formation.