Papers by Author: Balazs Fényi

Abstract: Electrical properties of the insulator silicon nitride ceramics may be improved by addition of electrical conductive parts. The conductive carbon parts were mixed with the base ceramic matrix to form a percolation network. Electrical current can flow through the ceramic by using the connected carbon channels. In air atmosphere however, the carbon can oxidize and burn out. Heat treatments were performed to observe the carbon degradation in composites in atmosphere. As resulted, the carbon exhaust started at 400°C from surface and finished above 750°C. Electrical measurements showed the conductor-insulator transformation. Thermo-gravimetric measurements suggested that some carbon inclusion still remained in isolated closed porosities.

Abstract: There is a continuous need to develop structural and functional components to sustain fusion plasma under the very severe environment such as intense radiation or high temperature in fusion reactors. The reference Si3N4 and novel Si3N4 based nanocomposites with carbon nanotube, graphene or carbon black additions were irradiated in a controlled-temperature irradiation rig inside the Budapest Research Reaktor (BRR) at a temperature of 270 °C. In the irradiation channel of BRR the 1,5×1013 n/cm2 fluence can be reached in 700 hours. The effect of the neutron radiation on the mechanical properties of Si3N4 based nanocomposites with different carbon additives was investigated. In the case of 3-point bending test graphene and carbon nanotube added samples showed an increase in strength ~100-300 MPa after irradiation. 4-point bending strength measurements resulted in a significant increase of strength in the case of reference samples ~200-400 MPa and carbon nanotube added composites ~200-300 MPa after irradiation. Other samples showed no change in strength after irradiation.

Abstract: The Si3N4 is one of the leader high-tech ceramic nowadays. As a result of numerous developments excellent mechanical properties of the material are increased by carbon additives. In our previous investigation the electrical effect of the additives was examined. In our work thermo radiation measurements were used by infrared camera technique on conductor Si3N4 ceramic composites in infrared wavelength range. The thermal properties of the developed ceramic composites were determined. This is not easy with the ordinary methods because of the fashioning and geometrical parameters of the material. During the experiments the emissivity of our composites was determined by infrared thermography measurements. Based on a theoretical thermal conductivity model an evaluation method that allows a derivation of thermal conductivities of special geometry conductive materials from results of infrared measurements was established.

Abstract: The Si3N4 ceramics are usually known as strongly refractory and enduring materials and they have typical insulating material properties on room temperature. If reinforcing phase of the Si3N4 matrix composite is a good electrical conductor, in that case it is worth to investigate the composite in electrical aspect. In our case carbon nanotubes, black-carbon and graphite with good electrical conductivity were mixed in the base ceramic. During our electrical investigations DC resistivity measurements were used to determine the percolation threshold and the conductivity of the composites. In case of high resistance samples AC impedance spectroscopy was applied. As result of the impedance spectroscopy capacitive properties were found. In some cases of conductor samples combined mechanical – electrical measurements were done to study the integrity of the additions in the matrix.

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.