Browsing by Author "Lilova D."

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    A study on the kinetics of the electrodeposition of Ni, Co and Ni-Co alloy in citrate electrolyte part 1. The kinetic study of the independent electrodeposition of Ni and Co
    (2015-01-01) Ignatova K.; Lilova D.
    The elecrodeposition of Ni and Co was studied by electrochemical techniques in varying compositions of slightly acidic citrate electrolyte. Cyclic voltammetry and current transient measurements were used to characterize the initial stages of nucleation, growth mechanism and electrodeposition of Ni and Co. The voltammetry analysis showed that the electrodeposition of both Ni and Co occurs under diffusion-kinetic control associated with a typical nucleation process. The deposition of Ni is in one act of charge transfer with exchange of two electrons, the deposition of Co is a two-stage process of reduction of free cobalt ions to cobalt, and reduction of complex ions type CoCit- to cobalt. The application of the method of chronoammetry proved that for cobalt the mechanism of nucleation is spontaneous, with three-dimensional nucleation and nuclei growth, while for nickel the slower progressive mechanism of nucleation is typical. The differences in the deposition mechanisms of the two metals is a possible explanation for their anomalous co-deposition in the electrolyte examined.
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    Comparative study on electrodeposition of Co-Cu alloys from ammonia-sulphate and citrate electrolyte
    (2016-01-01) Ignatova K.; Lilova D.
    The kinetics of the electrodeposition of Cu and Co in Co-Cu alloy, chemical composition, cathodic current efficiency and morphology of Co-Cu alloys electrodeposited from an ammonia-sulphate electrolyte (AMSE) and from a weakly acidic citrate electrolyte (CE) without and with addition of saccharin (CE+SHR) at constant potential mode are compared. The results indicate that the deposition of Co-Cu alloys from AMSE occurs without initial polarization, while the deposition from CE is characterized by long initial polarization and the overall polarization is higher compared to that in AMSE as a result of formation of more stable citrate complexes of the metals. The individual deposition of Co as well as Cu from CE is a two-stage process of reduction of their complex ions. It is found that the Co-Cu alloys with Co up to 50 - 60 mass % deposited from AMSE have coarse-grained crystalline structure with comparable amount of elongated and spheroid crystals with average size in the range from 2 to 10 μm, as compared to those deposited from CE with average size of about and less than1μm. The increase of the Co content in the alloys up to 80 mass % results in decrease of the average size of the crystallites in AMSE to less than 5 μm, and in CE- of about 500 nm. The addition of saccharin in the citrate electrolyte yields glossy and smooth coatings with close to nano-sized structure with average size of the crystallites in the range from 100 to 300 nm.
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    Effect of organic additives in citrate electrolyte on the properties of Ni-Co alloy
    (2015-01-01) Ignatova K.; Lilova D.; Piroeva I.
    The kinetics of independent and joined deposition of Ni and Co in citrate electrolyte in the presence of three types of additives: saccharine (SCH), urotropine (URT) and gelatine (GLT), is examined. Comparison is made between the chemical composition, the current cathodic efficiency (CCE) and the morphology of Ni-Co alloys depending on the type of the additive, both in constant potential mode (CPM) and in pulse potental mode (PPM). The study specifies the effect of GLT and SCH for formation of smooth, shiny, nanostructured Ni-Co coatings with average crystallite size less than 100 nm with a high CCE (about 80 % for SCH, and over 90 % for GLT), in both potential modes. The presence of SCH in the electrolyte with simultaneous application of PPM increases the content of Ni in the alloy to 25-30 mass % Ni. The optimum additive of SCH containing 0.5 - 1.0 g dm-3 is specified, wherein the electrolyte remains stable during continuous operation.
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    ELECTRODEPOSITION OF Sn-Co POWDERS BY STATIONARY AND PULSE MODE
    (2021-01-01) Ignatova K.; Lilova D.; Vladimirova L.; Alakushev M.
    Sn-Co powders with homogeneous distribution of components in powder particles were obtained by constant and pulse potential modes from a fluoride-chloride electrolyte. Morphology, elemental and phase compositions by SEM, EDX, SEM-Mapping andXDR analysis were investigated. Under stationary conditions, the potential increment leads to an increase in cobalt content from 3.6 wt.% to 56.4 wt.%, and to a structural alteration of the powders from network-shaped to a dendritic form with an average particle size between 15 and 30 µm, depending on the total metal concentration. The powders, obtained by pulse mode at a lower pulse frequency (100 Hz), contain particles with an irregular dendritic structure, characterized by the greatest thickness of the main axes of the dendrites. Increasing the pulse frequency to 1000 Hz results in a finer dispersed structure (r < 10 µm) and the cobalt content achieves 70.5 wt.% under these conditions. The obtained Sn-Co powders are mixed structures of amorphous phase and inclusions of microcrystalline phases of tetragonal β-Sn, hexagonal α-Co, CoO and SnO2 and only in stationary mode an additional phase of SnF2 exists.
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    Kinetic study on electrodeposited Ni-Co alloy in citrate electrolyte part 2. the kinetics of joint electrodeposition of Ni and Co in Ni-Co alloy system
    (2015-01-01) Ignatova K.; Lilova D.
    In the paper the results from the study of the kinetic of joint electrodeposition of Ni and Co in Ni-Co alloy system are reported. It is found that the co-deposition of Ni and Co in Ni-Co alloy system in slightly acidic citrate electrolyte occurs under combined diffusion-kinetic control with a typical mechanism of nucleation similar to those in the case of Co. The application of the method of chronoammetry proved that the mechanism of nucleation for Ni-Co alloy is spontaneous with three-dimensional nucleation and nuclei growth. The difference in the independent deposition mechanisms of the two metals is a possible explanation for their anomalous co-deposition in the electrolyte examined. The anomalous codeposition is more clearly noticeable in electrolytes with higher content of cobalt than nickel. The SEM-analysis proves that the alloys enriched in cobalt have crystallites with more even and rounded shape, which is expected in the case of spontaneous nucleation.
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    Membrane electrochemical preparation of three-polyphosphate non-cyanide electrolyte for coppering
    (2015-01-01) Gadzhov I.; Lilova D.; Ignatova K.
    The membrane electrochemical technology for obtainment of three-polyphosphate non-cyanide electrolyte for coppering is developed and optimized. For the purpose of characterizing the selectivity of the ionic membranes used, the processes of mass transfer are examined through determining the transmission numbers of the ions in the three-chamber electrolyzer. Detailed balance of the voltage between the electrodes and quantitative evaluation of the anodic output on current during the membrane preparation of the electrolyte are made. It is found that Cu is actively solved without passivation up to current density ia = 8 A dm-2. The anodic polarization during the operation of the electrolyzer is insignificant and the current efficiency is close to 100 %. It is found that the optimal environment for the membrane preparation of the electrolyte is anolyte of 140 g dm-3 Na5P3O10 and citric acid C6H8O7 up to pH = 5.3, and catholyte solution of 380 g dm-3 Na2CO3.