Daniela Kovacheva | Bulgarian Academy of Sciences (original) (raw)

Papers by Daniela Kovacheva

Proceedings of the Bulgarian Academy of Sciences

This work aims to characterize serpentine soils in Bulgaria and examine the biogeochemistry of pe... more This work aims to characterize serpentine soils in Bulgaria and examine the biogeochemistry of pedogenic Ni and its relation to Ni available forms. Soils were collected from serpentine soils of the Eastern and Southern Rhodopes (Bulgaria). The chemical composition of serpentine soils is similar to serpentine soils from the Balkan Peninsula and is characterized by high contents of Ni (919.9–2397.2 mg.kg–1). The soils had a neutral to slightly alkaline reaction (pH 6.4 to 7.6) and a medium to high organic matter content. Of the primary minerals, those of the serpentine group (antigorite and lizardite), the talc group (willemseite), the amphibole group (riebeckite), the feldspar group (albite) and quartz are mainly present in the soil samples. The secondary minerals present in the samples include calcite and dolomite, and also representatives of the clay minerals, clinochlore and montmorillonite. The contents of free iron oxides (Fed), amorphous iron oxides (Feo), well-crystallized iro...

Polymers

Here, we report results on the magnetic and microwave properties of polycrystalline Y-type hexafe... more Here, we report results on the magnetic and microwave properties of polycrystalline Y-type hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave-absorbing material. The reflection losses in the 1–20 GHz range of the Y-type hexaferrite powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in the absence and in the presence of a magnetic field. A permanent magnet was used with a strength of 1.4 T, with the magnetic force lines oriented perpendicularly to the direction of the electromagnetic wave propagation. In the case of using an external magnetic field, an extraordinary result was observed. The microwave reflection losses reached a maximum value of 35.4 dB at 5.6 GHz in the Ku-band without a magnetic field and a maximum value of 21.4 dB at 8.2 GHz with the external magnetic field applied. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by...

Nanomaterials

In the present study, we report on the facile prepared nanocomposites of reduced graphene oxide R... more In the present study, we report on the facile prepared nanocomposites of reduced graphene oxide RGO with Cu and Ag. The synthesis was performed through an environmentally friendly and easy method by simultaneous reduction in solutions containing Cu2+ or Ag+ and graphene oxide (GO) using zinc powder as a reducing agent in aqueous acidic media. The composites are characterized by powder X-ray diffraction, low-temperature nitrogen adsorption, X-ray photoelectron and FTIR and Raman spectroscopies, as well as Scanning and Transmission electron microscopies. The antibacterial activity of the composites was tested for Staphylococcus aureus, Escherichia coli and antifungal activity for Candida albicans. The cytotoxicity of the materials was studied towards two types of eukaryotic cells—MDCK II and A549 cell lines. The composites obtained consist of homogeneously distributed Cu and Ag nanoparticles on the surface of graphene sheets and manifest good antimicrobial activity and high cytotoxici...

Minerals, 2021

BiFeO3 is an interesting material due to its multiferroic properties. It attracts attention due t... more BiFeO3 is an interesting material due to its multiferroic properties. It attracts attention due to its potential applications in spintronics and in microelectronics for data storage, among others. Single-phase bulk material from BiFeO3 is difficult to synthesize. The kinetics of perovskite phase formation most often leads to the presence of impurity phases. It has been shown that low levels of replacement of Bi with rare earth ions lead to stabilization of the perovskite phase. In the present work, Rietveld refinement of the crystal structure based on powder X-ray diffraction patterns was applied to study the influence of partial substitution of Bi by rare-earth (RE) elements with different ionic radii on structural and morphological properties of the ferrite phase. Substitution by large RE ions was found to preserve the rhombohedral symmetry of BiFeO3, whereas substitution by smaller RE ions led to the coexistence of two polymorphic perovskite phases with rhombohedral R3c and ortho...

Data in Brief, 2020

Data supporting the results of the characterization of the phases and structures appearing during... more Data supporting the results of the characterization of the phases and structures appearing during the synthesis process of Ba 0.5 Sr 1.5 Zn 2-x Ni x Fe 12 O 22 by auto-combustion, Data in Brief (2020), doi:

ECS Meeting Abstracts, 2010

not Available.

Biotechnology & Biotechnological Equipment, 2019

In this study, spherical hollow mesoporous silica (HMS) particles with average size of 550 nm wer... more In this study, spherical hollow mesoporous silica (HMS) particles with average size of 550 nm were loaded with dopamine-receptor agonist pramipexole, as a model drug. High drug loading (55%) was achieved by its incorporation in the reservoir structure of HMS. The newly developed drug-delivery systems were further double-coated with bioadhesive polymers chitosan and sodium alginate to achieve sustained release. The double chitosan/sodium alginate coating diminished the initial burst effect and lowered pramipexole release at two pH values (1.2 and 6.8) as shown by in vitro release studies. The haemolytic assay and MTT-test in human neuroblastoma SH-SY5Y cells showed good safety profile of the novel pramipexole-loaded HMS particles. Moreover, a more pronounced protection of human neuroblastoma SH-SY5Y cells against H 2 O 2-induced oxidative damage was observed upon treatment with pramipexole-loaded (noncoated or chitosan-coated) HMS particles compared to the free drug. In conclusion, we found that HMS particles might be of great interest as a promising drug-delivery system for pramipexole. The coating with biopolymer chitosan modified both the drug-delivery process and the in vitro protection against oxidative stress in neuroblastoma SH-SY5Y cells.

Journal of Environmental Chemical Engineering, 2018

Highlights  TiO2-ZrO2 oxides with 2:8; 5:5 and 8:2 Zr/Ti mol ratio obtained by various methods. ... more Highlights  TiO2-ZrO2 oxides with 2:8; 5:5 and 8:2 Zr/Ti mol ratio obtained by various methods.  Hydrothermal procedure provides 2-7 times higher porosity than the urea one.  The catalytic activity of binary oxides increases due to the improved texture.  Simple regulation of catalytic behaviour by Zr/Ti ratio and preparation method.

Journal of Magnetism and Magnetic Materials, 2017

The possibilities for preparation of bilayers containing magnetic Fe-oxide (Fe-O) and high temper... more The possibilities for preparation of bilayers containing magnetic Fe-oxide (Fe-O) and high temperature superconducting (HTS) YBa 2 Cu 3 O 7-x (YBCO) thin films were investigated. For this purpose, Fe-O films were deposited using reactive magnetron sputtering at comparatively low temperatures T ≤ 250 o C onto dielectric (for example, LaAlO 3 (LAO)) substrates, covered with a HTS YBCO film. The sputtering of the Fe-O layer at such conditions did not lead to a crucial damage of the critical temperature T C of the YBCO film, but could affect the width of the superconducting transition. A decrease of the critical temperature of the (Fe-O)/YBCO/LAO bilayer kept at ambient conditions was observed, possibly due to the negative effects of the water vapour on the sample characteristics. The double peak structure of the imaginary component of the response signal to the AC harmonic magnetic field, observed in such a (Fe-O)/YBCO/LAO sample, was ascribed from two possible views: as a consequence of morphology determined inter-and intra-granular contributions and/or as transitions from dominant irreversible processes as Bean-Livingston barrier to vortex state chains formation.

Journal of Alloys and Compounds, 2017

Influence of the type of fuel used for the solution combustion synthesis on the structure, morpho... more Influence of the type of fuel used for the solution combustion synthesis on the structure, morphology and magnetic properties of nanosized NiFe 2 O 4 ,

Acta Crystallographica Section A Foundations of Crystallography, 2009

Applied Catalysis A: General, 2015

Abstract Two series of ceria–zirconia mixed oxide samples with wide variation in the composition ... more Abstract Two series of ceria–zirconia mixed oxide samples with wide variation in the composition were synthesized using homogeneous precipitation with urea and template-assisted hydrothermal procedures. The samples were characterized by nitrogen physisorption, XRD, SEM, Raman, XPS, UV–vis, FTIR, pyridine-FTIR and TPR techniques. The catalytic activity in ethyl acetate total oxidation as a representative of VOCs was tested. The effect of preparation procedure on the texture, structure, morphology, acidic and redox properties was discussed in details in close relation with mixed oxides composition and possibilities for fine tuning of their catalytic activity. As compared to urea procedure, the hydrothermal treatment provides more homogeneous insertion of Zr ions within cubic ceria lattice even at high Zr content. As a result higher degree of distortion of the cubic lattice, creation of Ce3+ and oxygen vacancies and an increase in both Lewis acidity and redox ability is observed and these properties are not affected by the presence of template during the synthesis. The differences in the redox and surface acidic properties of both series of materials probably promote the realization of the catalytic process by various reaction pathways and thus influence in a complex way the catalytic activity and selectivity.

ChemInform, 2003

ABSTRACT For Abstract see ChemInform Abstract in Full Text.

Journal of The Electrochemical Society, 2005

... Extensive investigations of the structure of electroless deposited films and their evolution ... more ... Extensive investigations of the structure of electroless deposited films and their evolution withthermal annealing have been conducted. ... On the basis of the data obtained from the structural and magnetic investigations one can state that W and P are located predominantly ...

Journal of Applied Electrochemistry, 2008

The effect of polypropylene glycol with molecular mass 620 (PPG 620) and glycine as additives in ... more The effect of polypropylene glycol with molecular mass 620 (PPG 620) and glycine as additives in electrodeposition of Zn–Cr alloy coatings from acidic sulfate electrolyte containing (NH4)2SO4 and H3BO3, at ambient temperature, without agitation is investigated. PPG 620 inhibits the Zn reaction causing significant proximity of the deposition potentials of Zn and Cr, and a co-deposition at a less negative

2017 40th International Spring Seminar on Electronics Technology (ISSE)

Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22<... more Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf> is a Y-type hexagonal ferrite known for a relatively high temperature (∼200 K) of the magnetic transition to a spiral spin arrangement and an easy magnetization axis lying in a plane perpendicular to the c crystal axis. The multiferroicity exists in the absence of an external magnetic field H; a longitudinal-conical spin arrangement sets below 50 K; and the direction of the electric polarization P can be governed by relatively low magnetic fields (<0.02 T). The structural, magnetic and ferroelectric parameters have been mostly studied on monocrystals and, in spite of the attractive possibilities of designing promising composite multiferroic structures, the information on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders is scarce. We report on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders studied by X-ray and neutron powder diffraction, and magnetometry. The samples were prepared by two different techniques: sonochemical co-precipitation and sol-gel auto-combustion. The structures retained the rhombohedral crystal symmetry, but mixed occupancies of the 14 (Mg, Fe) cation sites was a new feature. The magnetizations at a magnetic field of 60 kOe at 300 K were 22.78 emu/g for the sample prepared by sol-gel auto-combustion, and 24.95 and 25.06 emu/g for the samples obtained by sonochemical co-precipitation and annealed at 1170 °C and 1200 °C, respectively.