Boriana Tzaneva

Effect of phosphoric acid concentration on corrosion of Cr–Mn–N and Cr–Ni stainless steels

Abstract Phosphoric acid concentration (5–85%) effects on the corrosion behaviour of austenitic Fe–18Cr–12Mn–N steel have been studied by potentiodynamic polarisation measurements. After the anodic polarisation, both the film composition and the electronic structure have been investigated by X-ray photoelectron spectroscopy. The specimen surface examinations have been carried out by scanning electron microscopy. The results of the corrosion behaviour of the steel at issue have been compared to those relevant to two trademark materials [austenitic stainless steels AISI 304 (Fe–18Cr–9Ni) and X14AΓ15 (Fe–14Cr–15Mn–N)] and developed under the same test conditions.

Effect of anodization conditions on the breakdown voltage of nanoporous aluminium oxide

Within the work presented the effect of voltage, electrolyte nature and time of anodization on breakdown voltage of nanoporous aluminium oxide layers are investigated. The dielectric layers formed under various conditions feature a pore diameter from 20 to 90nm, and the thickness lies within 1÷14μm. The results reveal that reanodization up to 300V significantly enhances the breakdown voltage. For Al/Al2O3 structure, it is found out that breakdown voltage depends mainly on the layer thickness and significantly less on the pore diameter.

Engineering of III-Nitride Semiconductors on Low Temperature Co-fired Ceramics

This work presents results in the field of advanced substrate solutions in order to achieve high crystalline quality group-III nitrides based heterostructures for high frequency and power devices or for sensor applications. With that objective, Low Temperature Co-fired Ceramics has been used, as a non-crystalline substrate. Structures like these have never been developed before, and for economic reasons will represent a groundbreaking material in these fields of Electronic. In this sense, the report presents the characterization through various techniques of three series of specimens where GaN was deposited on this ceramic composite, using different buffer layers, and a singular metal-organic chemical vapor deposition related technique for low temperature deposition. Other single crystalline ceramic-based templates were also utilized as substrate materials, for comparison purposes.

Investigation of the efficiency of the heat dissipation for the heat-conducting circuit boards made of aluminum with the nanoporous alumina layer

The results of the investigation of the efficiency of the heat dissipation from the circuit board made of aluminum with the nanoporous alumina layer and conducting copper layer. In the construction of these circuit boards nanoporous alumina was used as insulator, which provided heat transfer from topology elements to the aluminum substrate. The conducting copper layer was formed by electroplating followed by chemical deposition of thin nickel layer. The diode MBRB16H60 was used as an element possessing high level of heat release. The temperature on the surface of the circuit board was measured by the thermocouple and the infrared images of the samples were studied by the thermal imaging camera. It was shown that proposed circuit boards allow significantly increasing the efficiency of the heat dissipation from the circuit board elements and reducing their operating temperature. The thermal conductivity of the nanoporous anodic alumina layer was determined to be 1.56 W K−1 m−1.

Study of thermal conductivity of nanoporous anodic alumina layer formed in sulphuric acid using steady-state heat flow technique

The paper presents the results of studies of thermal characteristics of nanoporous anodic alumina layer formed in sulphuric acid. One-sided heating of aluminum boards by heating element from carbon filament was used. The temperature of the back side of the board was kept constant by using an aluminum heatsink. Thermograms of aluminum board surface were taken by thermal imaging camera MobIR M4.

Kinetics of reanodization of porous anodic oxide films on aluminium formed in pore-forming solutions of various acids

Investigations are carried out towards reanodization of porous anodic oxide films on aluminium formed in aqueous solutions of sulphuric, oxalic and chromic acid. These films are prepared in a way to have one and the same porosity. It is found out that in case of equal porosity the slopes of the kinetic voltage-time curves recorded during reanodization are different. The effect of the anionic species incorporated from electrolyte solution into the porous template could be a probable explanation for the dissimilar rates of barrier layer growth.

Fabrication of microcuvettes using anodic aluminum oxide membrane

A variant for preparing an anodic aluminium oxide (AAO) membrane in copper ring with deposited tin last layer is proposed in the work. It allows to create closed microholders (microcuvettes) with developed nanostructured surface by soldering. Classical techniques as photolithography, electrochemical oxidation and metal deposition, etching and soldering was used. Is it approved by FTIR-analysis, that the prepared by proposed technological scheme cuvettes can be used as closed volumes containing gas and volatile liquids with designation of optical sensors.