Papers by Author: Norbert Hegman

Abstract: In this paper CuZrAl amorphous alloys with different Al content were prepared by centrifugal casting. The master alloy ingots were prepared by arc melting the mixture of the constituents on a water cooled copper boat in argon atmosphere. Wedge-shaped samples were prepared from the ingots by centrifugal casting into copper mold. The microstructure of the samples was examined by Scanning Electron Microscope (SEM) and the phases were analyzed by X-ray diffractometry (XRD) to define the amorphous fraction.

Abstract: Master alloys with different compositions of the Cu-Hf-Ti alloy system were prepared by arc-melting. The pieces were then cast into a wedge- and rod – shaped Cu mould by centrifugal casting. The different mould shapes generated various cooling conditions and produced various microstructures in the samples. Moreover due to the wedge shape, the change of the microstructure can be observed within the samples. The wedge samples became amorphous in 1-1.5 mm thickness, and dendritic structure formed in the thicker parts. The change in the microstructure in the rod samples along the radius can be studied as well. The solidified phases were also investigated by X-ray diffraction and SEM-EDAX.

Abstract: Bulk Metallic Glasses (BMGs) have been widely investigated due to their excellent physical and chemical properties [1]. The copper based BMG occupies a special place in the family of BMGs since they are relatively low priced. The Cu-Zr-Ag ternary system has been examined on the basis of the ternary phase diagram [2]. We have changed the concentration of the alloys from the Cu58Zr42 to the concentration of the deep eutectic point. Wedge-shaped samples have been cast from the master alloys by centrifugal casting into a copper mould, consequently analyse the influence of the cooling rate on the crystallization. The cooling rate has been estimated from the secondary dendrite arm distances by using a Cu-Sn crystalline alloy. Near the tip of the wedge the samples were amorphous and near the base of the wedge the samples were fully or partially crystallized. The structures of the samples have been characterized by scanning electron microscope and X-ray diffraction.

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: 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.

Abstract: Our research focused on the adhesion properties of mirror coating on silica float glass due to applying different quantity of tin-chloride sensitizing agent. Investigation was carried out to detect the distribution and quantity of tin containing clusters deposited on the glass surface working as crystallization centre. The adhesion properties of coatings were tested by scratch test where critical peeling forces were evaluated. The morphology of the formed scratch track was measured with scanning electron microscope. The surface topography was scanned by AFM technique.