Rastko Vasilić

Investigation of plasma electrolytic oxidation on valve metals by means of molecular spectroscopy - a review

A review of results of molecular spectroscopic investigations during plasma electrolytic oxidation of valve metals is presented. Particular attention is paid to three spectral systems, B1Σ+ → X1Σ+ of MgO, and B2Σ+ → X2Σ+, and C2Π–X2Σ+ of AlO. It was shown that a reliable assignment of the observed spectral features can only be carried out by critical comparison with the data obtained from high-resolution spectroscopy, and by using the results of quantum mechanical structure calculations. Assuming the existence of partial local thermal equilibrium, we used our spectroscopic results to determine the plasma temperature. Although limited in quality, the obtained spectra are very rich, they cover large wavelength regions, and are used to obtain information about physical and chemical processes that take place in the course of plasma electrolytic oxidation of light metals and their alloys.

The galvanoluminescence spectra of porous oxide layers formed by aluminum anodization in oxalic acid

This paper presents the result of the most recent investigations of the galvanoluminescence (GL) spectra of porous oxide layers formed by aluminum anodization in oxalic acid. A new experimental procedure for GL spectra recording was established. This method can be easily applied to GL spectral measurements on porous anodic films. The spectra were recorded for different values of temperature and anodization current density. Two spectral peaks at 446 and 485 nm, respectively, were observed, indicating the probable existence of two kinds of luminescence centers incorporated in the oxide. In addition, it was discovered that the intensity ratio for the two peaks changes with the current density. Those GL measurements, with further photoluminescent (PL) measurements should lead to a definitive explanation of the GL phenomena, as well as of the nature and distribution of luminescent centers in both porous and barrier oxide films on aluminum.

Spectroscopic Investigation of Direct Current (DC) Plasma Electrolytic Oxidation of Zirconium in Citric Acid

Plasma electrolytic oxidation of zirconium in citric acid was investigated using optical spectroscopy. A rich emission spectrum consisting of about 360 zirconium and 170 oxygen atomic and ionic lines was identified in the spectral regions 313–320, 340–516, and 626–640 nm. It was shown that the remaining features observed in the spectrum could be ascribed to various molecular species, which involve zirconium, oxygen, hydrogen, and carbon. The temperature of the plasma core (T = 7500 ± 1000 K) was determined using measured Zr line intensities, and the temperature of peripheral plasma zone (T = 2800 ± 500 K) was estimated from the intensity distribution within a part of an OH spectrum. The composition of the plasma containing zirconium, oxygen, and hydrogen, under assumption of local thermal equilibrium, was calculated in the temperature range up to 12 000 K and for pressure of 105 and 107 Pa, in order to explain the appearance of the observed spectral features.

Real-time imaging, spectroscopy, and structural investigation of cathodic plasma electrolytic oxidation of molybdenum

In this paper, the results of the investigation of cathodic plasma electrolytic oxidation (CPEO) of molybdenum at 160 V in a mixed solution of borax, water, and ethylene glycol are presented. Real-time imaging and optical emission spectroscopy were used for the characterization of the CPEO. During the process, vapor envelope is formed around the cathode and strong electric field within the envelope caused the generation of plasma discharges. The spectral line shape analysis of hydrogen Balmer line Hβ (486.13 nm) shows that plasma discharges are characterized by the electron number density of about 1.4 × 1021 m−3. The electron temperature of 15 000 K was estimated by measuring molybdenum atomic lines intensity. Surface morphology, chemical, and phase composition of coatings formed by CPEO were characterized by scanning electron microscopy with energy dispersive x-ray spectroscopy and x-ray diffraction. The elemental components of CPEO coatings are Mo and O and the predominant crystalline form is MoO3.

The characterization of cathodic plasma electrolysis of tungsten by means of optical emission spectroscopy techniques

Cathodic plasma electrolysis (CPE) of tungsten is investigated by means of optical emission spectroscopy in order to measure the electron temperature T e and the electron number density N e from CPE micro-arc discharges. The relative intensity of W I lines recorded from seven spectral ranges are used in conjunction with the Boltzmann plot (BP) technique to measure . For N e diagnostics, the complex line shape of the and the lines are analyzed in the frame of the spectral line broadening theory. The results showed two N e values, and , corresponding to different regions of CPE micro-arc discharges. The self-absorption effect on both studied H I lines was revealed and discussed as well. The SEM and XRD analysis showed a weak formation of rough oxide coating featured by the number of grains, channels, and holes.

Luminescence During the Electrochemical Oxidation of Aluminum

Electrochemical oxidation of aluminum has attracted considerable attention because of the widespread use of aluminum oxide layers in different areas of technology and industry. Traditionally, the anodization of aluminum is used to protect metals from corrosion and to increase the abrasive and adsorption properties, hardness, etc. The oxide layers on aluminum are widely used in electronics due to its excellent dielectric properties, perfect planarity, and good reproducibility during production. In recent years, scientists have focused on the formation of self-ordered oxide structures on aluminum in various electrolytes. This is a result of the application of porous oxide layers arranged pores with dimensions ranging from micrometers to nanometers, as the mold in nanotechnologies for the synthesis of nanotubes, nanowires, solar cells, micro-optical elements, photonic crystals, etc.

The determination of micro-arc plasma composition and properties of nanoparticles formed during cathodic plasma electrolysis of 304 stainless steel

This paper presents the research focused on the determination of micro-arc plasma composition during cathodic plasma electrolysis of AISI304 stainless steel in water solution of sodium hydroxide. The complex line shape of several Fe I spectral lines was observed and, by means of a dedicated fitting procedure based on the spectral line broadening theory and H2 O thermal decomposition data, the mole fraction of micro-arc plasma constituents (H2 , Fe, O, H, H2 O, and OH) was determined. Subsequent characterization of the cathodic plasma electrolysis product formed during the process revealed that it consists of Fe-nanoparticles with median diameter of approximately 60 nm.

The study of micro-arc discharges during cathodic plasma electrolysis of refractory metals using the spectral line shape of Na I lines

The micro-arc discharges during cathodic plasma electrolysis of refractory metals (Zr, Ti, Ta) are studied by means of optical emission spectroscopy. The fitting procedure based on three mutually shifted profiles is developed to analyze the complex line shape of Na I 568.64 nm and 615.86 nm doublets. Each profile includes effects of instrumental, Doppler, Stark, van der Waals and resonance broadening. The results show the existence of three discharge zones with electron number density values , and while those of sodium ground-state atoms are , and .