Papers by Author: László Péter Biró

Abstract: Photonic crystals are periodic dielectric nanocomposites, which have photonic band gaps that forbid the propagation of light within certain frequency ranges. This property enables one to manipulate light with amazing facility. Such nanoarchitectures frequently occur in living organism like butterflies and beetles. Butterfly scales are particularly well suited to be used as optical gas sensors as their nanoarchitecture is an open sponge-like type, composed of chitin and air. The open nanoarchitecure allows fast gas exchange. The spectral change of the reflected light depends on the composition of the ambient atmosphere and also on the wing nanostructure. In this work we show the results of recent measurements on nine Polyommatine species with dorsal blue coloration. Their color is generated by similar pepper-pot type nanoarchitectures which exhibit species specific characteristics, associated with species specific color. Experiments were carried out changing the concentration and nature of test vapors while monitoring the spectral variations in time. Proper data processing results gas-selective and concentration dependent signals. Our work shows a way to a prospective integrated biological - optical sensor combining light-weight and low power consuming with environmental friendly production.

Abstract: Photonic crystal type nanoarchitectures have an important advantage over conventional displays: they do not fade under solar illumination; on the contrary, more intense illumination generates more intense color. We present a simple method based on cooling in ambient air - to observe the color change of several butterfly wings colored by various photonic nanoarchitectures. The color change can be attributed to the condensation of atmospheric humidity in the nanocavities of the photonic nanoarchitecture. The effects were investigated by controlled cooling combined with the in-situ measurement of the changes in the reflectivity spectra. For certain species the reflectivity maximum (color) has almost completely disappeared. A correlation was also found between the openness of the nanostructure and the time of the color change. Cooling experiments, using thin copper wires showed that color alteration could be limited to millimeters; this may offer a possible alternative for display technology.

Abstract: Silicon nitride based composites with 3wt% of single wall carbon nanotubes and exfoliated graphite have been prepared. Optimisation of the manufacturing processes has been conducted to preserve the carbon nanotubes and exfoliated graphite in composites and to avoid damaging during high temperature processing. The first results show that carbon nanotubes and exfoliated graphite have a good contact to the surface of silicon nitride grains. In the case of increase of sintering pressure an increase of bending strength was achieved. It was found that microstructure features achieved by properly designed sintering parameters are the main responsible factors for the strength improvements.

Abstract: Multiwall carbon nanotube reinforced silicon nitride composites have been prepared by hot isostatic pressing. A manufacturing process has been worked out to avoid the damage of nanotubes during sintering. This method provides their preservation even in severe circumstances at temperature 1700°C and gas pressure 20 MPa. As shown by scanning and transmission electron microscopy after low and high pressure processing, carbon nanotubes have good adherence to the silicon nitride grains. Moreover, carbon nanotubes have been found to be located not only at grain surfaces, but in several cases they are well integrated with the silicon nitride grains. Composites with higher strengths can be obtained by increasing the nitrogen gas pressure.

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Abstract: This work is focusing on exploring preparing processes to tailor the microstructure of carbon nanotube (CNT) reinforced silicon nitride-based ceramic composites. Samples with different porosity’s and different amount (1, 3 or 5 wt%) of carbon nanotubes have been prepared by using gas pressure sintering or hot isostatic pressing. In comparison, composites with 1wt%, 5wt% or 10wt% carbon black and graphite have been manufactured. We measured the room temperature mechanical and electrical properties, examined the micro and nano structure by X-ray diffraction and electron microscopy. It was found that it is possible to develop CNT-silicon nitride composite for applications where a decent electric conductivity and good mechanical properties are required.

Abstract: The effect of the dispersion state of carbon nanotubes in polycarbonate on the rheological, mechanical and electrical properties of melt-extruded compounds is presented. The dispersion state was monitored by means of grey values distribution histograms of optical micrographs of the composites. Increasing the processing residence time, and hence the deformation induced by the surrounding polymer, increased the level of dispersion of CNTs in the matrix. This, in turn, resulted in a large improvement of the CNTs reinforcement effect and decreased composite electrical resistivity. Rheological behaviour is in agreement with these observations.