Preface
Pages 4-6
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Crystallization and crystal structure of lysozyme in the presence of nanosized Titanium dioxide
Original Research Article
Pages 7 – 14
H. I. Sbirkova-Dimitrova, S. Georgieva, V. Ganev, B. L. Shivachev
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Abstract
We present a single crystal XRD, LA-ICP-MS and cyclic voltammetry studies on the observed interaction of the protein lysozyme and TiO2 nanosized particles (JRC NM-101, anatase). The diffraction quality of lysozyme single crystals grown in presence and absence of TiO2 was comparable. The X-ray structure solution reveled that lysozyme crystallizes in the P43212 space group and disclosed the presence of electron density that was assigned to Na+ and Ti4+ ions. LA-ICP-MS analyses were conducted on both lysozyme-TiO2 single crystals and SDS-PAGE featuring lysozyme-TiO2 interaction. LA-ICP-MS data confirmed the presence of Ti4+ ions and the increased concentration of Na+. Cyclic voltammetry and differential pulls polarography results suggest that titanium binds successfully with the enzyme and that the complex formation is irreversible, at least in slightly acidic conditions (pH 6.5).

Spectroscopic analysis of sewage sludge and wood ash from biomass used for land remediation
Original Research Article
Pages 15 – 22
E. S. Serafimova, V. V. Stefanova
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Abstract
The present study aims to determine the possibility of using soil improvers (mixtures of active sludge and wood ash in different proportions) for possible remediation of metal contaminated soils from the mining area near Chelopech village, Bulgaria. Analyzes of the soil improvers show that the prepared mixtures do not meet the requirements of the Fertilizers Regulation 2003/2003, so they are classified as soil improvers. The chemical analysis shows high calcium content (13.56 wt. %) in sample M1, 8.52% in M2 and 8.43% in M3, which make them suitable for treatment of acidic soils and immobilization of heavy metals in contaminated soils. Of the trace elements found (B, Cu, Fe, Mo, Mn and Zn) in the enhancers, only Fe has favorable concentration. The infrared spectroscopy proved rich contents of acids, proteins, carbohydrates, lipids and their byproducts, which are very important for soils and plants. The thermal investigations proved this prediction and the available functional groups that are defined by IR. The obtained results from FT-IR measurements show that samples M1 and M2 are more suitable for soil improvers because of the established process of mineralization.

Conformational polymorphism in (3-acetamidophenyl)boronic acid
Original Research Article
Pages 23 – 30
V. M. Dyulgerov, L. T. Dimowa, R. Rusev, R. P. Nikolova, B. L. Shivachev
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Abstract
This study focuses on the structural peculiarities of two conformational polymorphs of (3-acetamidophenyl)boronic acid, C8H10BNO3. The two polymorphs were generated by crystallization from different solvents: chloroform and ethanol. The crystal structures of both polymorphs have been characterized by single-crystal X-ray diffraction analyses, DTA/TG and FTIR. Single crystal analyses showed that the title compound crystallizes in the triclinic system space group P–1 (No 2) and in the monoclinic crystal system, space group P21/c (No 14) in function of the employed crystallization solvent. The differences between the two crystal structures are centered on the different hydrogen bonding pattern, producing a different three-dimensional arrangement of the molecules. The DTA/TG and FTIR spectra of the two polymorphs are nearly identical and therefore they are not very suitable for differentiation. The DFT calculations showed that the energy minima of the two polymorphs differ by 0.9 kcal.mol–1 while the generated potential energy surface revealed a low value of 5.8 kCal mol–1 for the barrier of rotation of the acetamide group.

Synthesis and characterization of willemite ceramic pigments in the system xCoO. (2–x)ZnO. SiO2
Original Research Article
Pages 31 – 37
Ts. Ibreva, Ts. Dimitrov, R. Titorenkova, I. Markovska, E. Tacheva, O. Petrov
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Abstract
Zinc silicate α-willemite (Zn2SiO4) is an orthosilicate with rhombohedral symmetry. Cobalt doped willemite is a pigment with application in high-temperature ceramics and glaze production. A full series of cobalt containing willemite ceramic pigments with composition xCoO. (2–x)ZnO. SiO2, where x = 0.125, 0.250, 0.375, 0.50, 0.625, 0,75, 0.875 and 1, were syntesized via solid-state high temperature sintering. The resulting ceramic pigments were examined by powder X-ray diffraction analysis, electron microscopy, infrared spectroscopy and the color was determined spectrophotometrically. It was found that the pigmentwith composition 0,375CoO.1,625ZnO.SiO2 sintered at 1000 °C has the brightest blue color as defined after spectrophotometric measurments of the coloring efficience. The results confirmed that the synthesized pigments are suitable for application in sanitary ceramics and glaze tiles.

Investigation on crystallization and transformation processes in amorphous alloy Fe81B13.5Si3.5C2
Original Research Article
Pages 38 – 41
D. Paneva, Z. Cherkezova-Zheleva, V. Petkova, B. Kostova, H. Kolev, G. Avdeev, G. Stefanov
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Abstract
Amorphous Fe81B13.5Si3.5C2 ribbons produced by the melt spinning technique were used for the study of crystallization as a model system. Thermal treatment up to 1000 °C was performed in three media – vacuum, air and argon. Crystallization process was registered by thermal analysis, X-ray diffraction and Mössbauer spectroscopy. The thermal treatment above the crystallization temperature of the amorphous Fe81B13.5Si3.5C2 alloy results in formation of multiphase crystalline structure composed by α-Fe and iron borides and silicides. Mössbauer data show rearrangement of iron neighbors as a result of thermal treatment.

Sol-gel synthesis and properties of Sm modified TiO2 nanopowders
Original Research Article
Pages 42 – 48
S. I. Yordanov, A. D. Bachvarova-Nedelcheva, R. S. Iordanova, I. D. Stambolova
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Abstract
The present investigation deals with the sol-gel synthesis and properties of samarium doped TiO2 nanopowders and the impact of Sm3+ on the structural and thermal properties of the obtained samples was established. By XRD was found that the heat treated up to 300 °C gels exhibit a predominantly amorphous phase and its amount gradually decreases with increasing the temperature (above 400 °C). The first TiO2 (anatase) crystals were detected at about 400 °C and the average crystallite size of the samples heat treated at 400 °C is about 25–30 nm. By DTA was established that the organics decomposition is accompanied by strong weight loss occurring in the temperature range 240–350 °C. The completeness of the hydrolysis – condensation reactions was verified by IR and UV-Vis analyses.

TEM analysis of gold nanoparticles attached on the surface of organoclay and obtained by two different methods
Original Research Article
Pages 49 – 53
V. A. Angelov, E. H. Ivanov, R. K. Kotsilkova
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Abstract
This article studies two different methods for synthesis of gold nanoparticles over organoclay. The preparation of the organoclay suspension is done with two different solvents – i-propanol and distilled water. The study is focused on the determination of the size of gold nanoparticles synthesized with the two different methods, using TEM and corresponding computer software. The results from the TEM analysis show that the sample prepared with the solvent distilled water provides better control over the size of the Au nanoparticles.

Characterization of a novel geopolymer based on acid-activated fayalite slag from local copper industry
Original Research Article
Pages 54 – 61
A. Nikolov, R. Titorenkova, N. Velinov, Z. Delcheva
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Abstract
In the present study the copper smelter slag from the Aurubis enterprise (Pirdop, Bulgaria) was used as geopolymer precursor. The X-ray fluorescence and powder diffraction analysis showed that the slag contains significant amount of iron, presented by the minerals fayalite and magnetite. The slag was activated with phosphoric acid solution to prepare fresh geopolymer paste. Rapid exothermic reaction took place and the material hardened in minutes. The obtained inorganic polymer material was characterized by XRD, FTIR, DSC and Mössbauer spectroscopy. In conclusion, our study shows that the waste slag from local copper industry could be used as a potential geopolymer precursor to produce building materials. This novel method could reduce the huge amounts of fayalite slag generated during the last decades.

XRD study on the structural evolution of Zn-exchanged titanosilicate ETS-4 during thermal treatment
Original Research Article
Pages 62 – 72
V. Kostov-Kytin, R. Nikolova, G. Avdeev
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Abstract
In situ time-resolved powder X-ray diffraction technique has been applied to investigate the structural evolution of Zn-exchanged polycrystalline titanosilicate ETS-4 upon heating within the temperature interval from 25 to 375 °C. The facilities of the Rietveld method as implemented in the software package GSAS have been used to control the plausibility of the obtained crystal-chemical characteristics at each stage. Previous single crystal structure determination of the title compound has served as initial model for the refinement procedure held on a sample at room temperature. Subsequently, the structure model of each increasing temperature step has been taken from the previous refinement. The structural evolution has been evaluated in terms of the unit cell parameters changes, water molecules site occupancies (during the dehydration period), the titano-silicate framework flexibility (pore sizes), and possible atomic motion during the thermal treatment. The obtained results and previously accumulated knowledge on the crystalchemical peculiarities of ion-exchanged ETS-4 and its behavior upon heating have been interpreted in terms of the elastic properties of this titanosilicate structure and its thermal stability.

Physicochemical characterization on clinically retrieved TriTanium orthodontic archwires
Original Research Article
Pages 73 – 79
A. Stoyanova-Ivanova, I. Ilievska, V. Petrova, M. Gueorgieva, V. Petrov, L. Andreeva, A. Zaleski, V. Mikli
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Abstract
The orthodontic archwire TriTanium® has three distinct thermally activated force regions which release the correct force in the anterior, premolar, and molar arch-regions in order to efficiently level, align and torque. During patient treatment, in the leveling phase of teeth alignment, fixed appliances in the frontal area require weak forces while for the lateral section greater forces are needed. This work aims to identify the chemical composition, structure and thermal behavior of clinically retrieved (at least six weeks wearing) TriTanium (0.41×0.56 mm2) archwires. The studies were conducted in the three regions of elasticity: anterior – including the four incisions teeth, middle teeth – including the canine teeth and the premolars and the posterior e.g. the molars. To achieve the aim the following methods are used: XRD, EDX, SEM and DSC. The EDX analysis shows that Ni and Ti are the main elements in the composition of the examined archwires and the 1:1 ratio of elements is kept during treatment. The room temperature XRD patterns show typical peaks for a Ni-Ti alloy with austenite type structure. SEM micrographs show different morphology in the three (3) zones of the investigated archwires. The DSC measurements were conducted in the
–50 °C to +50 °C temperature range and revealed three phase transitions (austenite, martensite and R-phase) in the 3 zones. Тhe wearing of the archwires in the patients’ mouth alters the thermal phase transitions in the three investigated regions of TriTanium archwires.

Elastic behavior of the titanosilicate framework in Mn-ETS-4
Original Research Article
Pages 80 – 85
L. Tsvetanova, V. Kostov-Kytin, S. Ferdov, R. Nikolova
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Abstract
Microporous titanosilicate Na-K-ETS-4 has been synthesized and subsequently exchanged on Mn2+. The crystal structures of the as-synthesized and the exchanged forms have been analyzed by single crystal X-ray diffraction at 290 K and 150 K, correspondingly. The elasticity and stability of the titanosilicate framework have been investigated as important characteristics of the studied material with impact on its ion-exchange and sorption properties. The obtained results reveal that the crystal structure of ETS-4 is stable at temperatures of 150 K and well adapts to the new conditions through non-destructive, mutually compensating each other deformations of the pores and the channel systems within the titanosilicate framework. The exchange on Mn2+ ions affects the degree of structural deformation at low temperature conditions.

Synthesis, structure and properties of hybrids doped with copper ions in the silica-hydroxypropyl cellulose system
Original Research Article
Pages 86 – 92
N. Rangelova, L. Aleksandrov, Ts. Angelova, N. Georgieva
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Abstract
In the present study, the formation of copper-doped hybrids and their structure were examined. The XRD analysis showed that all samples are in amorphous state. The IR spectra are characterized with typical bands for silica network (~460, 650, 795, 960, 1085, 1200 and 3480 cm–1), and vibration of Cu-O bonds (460–480 cm–1). The thermal stability decreased with increasing of Cu content from 15 to 25 mass%. By optical microscopy it was found that in the sample containing 5 wt.% Cu, CuSO4*nH2O separated from the amorphous matrix and it was no able to incorporate into the hybrid material. After heat treatment of as prepared copper-doped hybrids the XRD, IR and UV-VIS analysis were done. The amorphous state of the hybrids remains except the sample containing 5 wt.% Cu. There are three refraction peaks on the amorphous halo observed assigned to the CuO. The calculated band gap energy (Eg) values sharply decreased after heat treatment process: 3.66–3.97 eV for raw materials up to 2.54–1.30 eV for heated samples. The obtained materials were tested as antibacterial agents against Escherichia coli K12 used as a model microorganism.

Challenges at characterization of particulate matter – a case study
Original Research Article
Pages 93 – 98
Z. Cherkezova-Zheleva, D. Paneva, B. Kunev, H. Kolev, M. Shopska, I. Nedkov
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Abstract
Nowadays, the particulate matter (PM) has been recognized as one of the major air pollutants with established scientific evidences on risks to human health [1–4]. Major interdisciplinary and multilayer research efforts have to be done to improve scientific understanding of airborne PM and its effects on human health. This new information will help to reduce uncertainties in the framework for assessing the public health risks from emissions of airborne particles and their gaseous precursors. The aim of our project is to investigate PM samples collected in Sofia, to identify global and regional particle sources in order to develop tools needed to formulate effective control strategies. This study reports an example of the challenges ahead for Particulate Matter physicochemical characterization. In this regard several different approaches have been utilized including techniques of powder XRD, elemental analysis, FTIR, SEM microscopy, XPS and Mössbauer spectroscopy. The main focus of the study were the smallest magnetic grains (<10 µm) as the most dangerous ones for the human health [1–5]. On the other hand, the mixture of multiple phases presented in PM and also their small size makes difficulties in their identification [3–5]. Therefore the preparation of standard procedures and methodology for investigation and control of PM is both challenge and priority [6].

Mg- and Zn-modified calcium phosphate fine powders examined by Rietveld refinement
Original Research Article
Pages 99 – 106
K. Sezanova, D. Kovacheva, D. Rabadjieva, R. Gergulova
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Abstract
Fine powders, consisting of single phase Mg- or Zn-modified β-tricalcium phosphate (Mg/Zn-β-TCP) or biphase mixture of Mg/Zn-β-TCP and hydroxyapatite (HA) with a (Ca+Mg+Zn)/P ratio of 1.3–1.4 and Me/(Me+Ca) (Me = Mg, Zn) molar ratios from 0 to 0.10 were prepared by a two-step method: (i) continuous precipitation in biomimetic electrolyte systems of simulated body fluids, keeping pH=8; and (ii) step-wise calcination to 1000 °C. Rietveld refinement of the XRD data was performed. The refinement confirms that both Mg2+ and Zn2+ ions substitute the Ca2+ ions in the octahedral Ca(5) sites of Mg/Zn-β-TCP, which results in a decreasing trend of Ca(5)-O average distances and a and c cell parameters. The contraction of the crystal lattice is more pronounced for the Zn-substituted samples due to differences in the preferred coordination polyhedrons оf Zn and Mg. The results also show that in the case of Mg substitution, as well as for the low levels of Zn substitutions, Ca(1), Ca(2) and Ca(3) positions remain fully occupied by Ca2+ ions. For high levels of Zn substitution Ca(1) and Ca(2) are partially occupied by Ca2+ ions, thus calcium vacancies appear. The latter affects the values of Ca(2)-O and Ca(4)-O mean distances by increasing them.

Polydentate ligands combining pirlinidole and piperazine fragments
Original Research Article
Pages 107 – 114
V. B. Kurteva, B. L. Shivachev, R. P. Nikolova
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Abstract
A series of polydentate ligands possessing pirlindole and piperazine fragments are synthesized. Their structures are assigned by 1D and 2D NMR spectra and confirmed by single crystal XRD of selected samples.

Antibacterial activity of novel quaternary ammonium salts of quinoline and 4-pyrolidino pyridine
Original Research Article
Pages 115 – 125
R. I. Rusew, V. B. Kurteva, B. L. Shivachev
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Abstract
Three new quaternary ammonium compounds were synthesized by Menshutkin reaction adapted for aromatic tertiary amines. The purity of the novel compounds was confirmed by 1H-NMR and 13C NMR spectroscopic techniques. Single crystal X-ray diffraction studies showed that all three compounds crystalize in the monoclinic crystal system (two in P21/c and one in P21/n space group) as bromide or iodide salts. The crystal structures are stabilized by a network of strong intermolecular halogen bonding interaction of C-H…X type (X = I, Br). DTA/TGA analyzes confirmed the stabilizing role of the counter ion by showing that the compounds decompose immediately after the elimination of the halogen atom. The quaternary ammonium compounds were tested for antibacterial activity against Gram-positive and Gram-negative bacterial strains using Kirby-Bauer disk diffusion test. Antibacterial effect was observed only on Gram-positive bacteria namely Bacillus subtilis and Staphylococcus aureus. The minimum inhibitory (MIC) and non-inhibitory (NIC) concentrations for all three compounds were obtained using the Gompertz function. Compound 2 showed the best results with MIC values of 0.321, 0.504 μM and NIC values of 0.053, 0.030 μM for S. aureus and B. subtilis respectively. .

Bi-axial dielectric anisotropy of crystalline materials and its characterization by resonance microwave methods
Original Research Article
Pages 126 – 134
P. I. Dankov, M. T. Iliev, V. P. Levcheva
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Abstract
In this paper, the dielectric parameters (permittivity and dielectric loss tangent) of different single and poly-crystalline or crystalline-like samples have been characterized in the microwave range (especially in the C and X bands; 4–12 GHz). A proven specific property of many crystalline materials is their dielectric anisotropy – different dielectric parameters along different directions and/or crystallographic axes. We consider the anisotropy as a very informative parameter for the sample structure, composition, inclusions, used technology and conditions for the crystal growth or layer deposition. The authorship two-resonator method or some its variants have been used for characterization of the crystalline samples. A lot of results has been presented for the anisotropy of different materials selected as typical examples (crystals, ceramics, glasses, semi-conductor wafers, unknown composites, carbon-content samples: graphite, Graphene, etc.), with different shapes (disks, prisms, cylinders, multilayer composites, thin films, etc.) and different orientation (sample axes orientated along crystallographic axes or in arbitrary directions). Depending on the samples’ kind, the observed anisotropy falls in the intervals from 0.35 up to 3.50. The presented data set for the dielectric properties show good correlation with the sample structure and used technology and allow a reliable microwave characterization of the samples, which can add new useful information to the results, obtained by other standard experimental methods applicable for crystals.

Åkermanite based bioactive ceramics: structural and in-vitro bioactivity characterization
Original Research Article
Pages 135 – 142
I. K. Mihailova, L. N. Radev
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Abstract
Ceramics with chemical composition corresponding to åkermanite (2CaO.MgO.2SiO2) was synthesized by using sol gel technique. The obtained dried gel was subjected to a two-step thermal treatment firstly at 700 °C for 2 hours and then at 1000, 1100, 1300 °C respectively for 2 hours. X-ray diffraction showed the temperature dependent structure evolution. The peculiarities of the crystallization of gels proceeding in the system CaO-MgO-SiO2 provide the synthesis of materials of an identical chemical but with various phase composition, microstructure аnd relevant properties. All obtained samples from 700 to 1300 °C were multiphase. The quantity of åkermanite was increased with the increasing of the temperature. The structural behavior of the synthesized after two-step thermal treatment at 700 and 1300 °C ceramics was examined by means of X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Åkermanite, as the main crystalline phase, merwinite and diopside, as the minor phases, were identified. The XRD results were in good agreement with FTIR analysis. The main purpose of the paper was the evaluation of the in vitro bioactivity of the åkermanite ceramics in static conditions for different periods of time – 7, 14 and 28 days in Simulated Body Fluid (SBF). The formation of carbonated apatite layer on the surface of the immersed samples was verified by FTIR, SEM and Energy Dispersive Spectroscopy (EDS) techniques. The change of ions concentrations in SBF was also carried out by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES).

The influence of the modification of zirconium tungstate with Eu(III) on the α → β phase transition temperature and optical band gap
Original Research Article
Pages 143 – 149
E. D. Encheva, M. K. Nedyalkov, M. P. Tsvetkov, M. M. Milanova
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Abstract
The hydrothermal method was used to obtain pure and Eu(III)-modified ZrW2O8. Tungstates modified with 1, 2, and 5 mol% Eu(III) were synthesized in order to investigate the influence of Eu(III) doping on the properties of zirconium tungstate. The samples obtained were phase homogeneous as shown by XRD. High temperature XRD was used to follow the temperature of the alpha-beta phase transition. It was observed that the higher the content of Eu(III), the higher the temperature of the transition. The unit cell parameters decrease both with the Eu(III) content and the temperature increasing. Тhe values of the thermal expansion coefficients obtained decreased with increasing Eu(III) for both the alpha and beta phases of ZrW2O8. Doping with increasing amounts of Eu(III) increased the energy of the optical band gap as well..

Structural characterization of Au/Cu1–хMnхFe2O4 catalysts suitable for WGSR
Original Research Article
Pages 150 – 157
T. M. Petrova, N. I. Velinov, I. B. Ivanov, T. T. Tabakova, V. D. Idakiev, I. G. Mitov
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Abstract
The spinel ferrites are known to have many important properties as magnetic, optical, catalytic etc., which provokes the scientific interest. Copper-manganese-iron samples with nominal compositions Cu1–хMnхFe2O4 (x=0; 0.2; 0.4; 0.6; 0.8 and 1) were prepared by auto-combustion sol-gel method and modification by gold was carried out by depositionprecipitation procedure. Their performance in the water-gas shift reaction (WGSR) was investigated. Structural characteristics of samples were determined by X-Ray diffraction and Mössbauer spectroscopy. Spinel ferrite phase and gold phase were proved in all synthesized samples. In copper-rich composition additional phase of CuO was present. The cation distribution in octahedral and tetrahedral positions in spinel lattice and the presence of superparamagnetic particles were evaluated by Mössbauer spectroscopy. Catalytic activities in the WGSR of studied samples followed the order: Au/Cu0.2Mn0.8Fe2O4 ≥ Au/Cu0.5Mn0.5Fe2O4 > Au/Cu0.8Mn0.2Fe2O4 > Au/CuFe2O4 ≥ Au/MnFe2O4. Some changes in the phase composition and structure of both gold and spinel ferrite phases were found after WGSR. Full or partial alloying of gold with copper was evidenced. The main samples transformation in reactive atmosphere was partial reduction of the ferrite phase. The result of this reduction was the formation of metallic copper and copper and/or manganese substituted magnetite in mixed copper-manganese containing samples. Simultaneous presence of both phases could be considered as a reason for better catalytic activity of mixed ferrites.

TEM characterization of silver and gold nanoparticles synthesized by а ‘green’ method using water extract of Rosa Damascena petals waste and beer yeast
Original Research Article
Pages 158 – 162
B. C. Georgieva, D. B. Karashanova, R. R. Angelov, A. M. Slavov, I. N. Vasileva, T. M. Dodevska
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Abstract
In this study we present “green” synthesis of Ag and Au nanoparticles by reduction of AgNO3 and HAuCl4 using water extracts of solid waste from Rosa Damascena essential oil industry, as well as pasteurized and live beer yeasts and combinations thereof. Мorphology, microstructure and phase composition of the Ag and Au NPs obtained by ten different recipes are investigated by High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED). Histograms of nanoparticles distributed by their diameters are constructed using the data acquired with Image J computer program. The histograms demonstrate that the predominant population of
Ag NPs has diameter between 0 and 5 nm, regardless some of Ag NPs reach larger sizes, up to 50 nm. In the same time, the main part of Au NPs possesses diameters in the range 0–15 nm. The indexing of SAED patterns revealed the presence of cubic and hexagonal metal Ag and cubic metal Au phases. The relation between synthesis conditions, phase composition and size distribution of Ag and Au nanoparticles is followed and discussed.

A new phase obtained by oxidation of nanosized spinel MnFe2O4
Original Research Article
Pages 163 – 166
T. I. Lazarova, D. G. Kovacheva
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Abstract
In a bulk form MnFe2O4 is a partially inverse spinel where about 80% of Mn2+ ions are located at tetrahedral (A) coordination sites. MnFe2O4 is used as catalyst, as adsorbent for removing heavy metals in water, as ferrofluids, in biomedicine, in energy storage devices and others. Solution combustion synthesis was used for the preparation of nanosized MnFe2O4 with a mixture of two fuels – glycine and glycerol (0.75:0.25 reducing power ratio). The resulting material was characterized by X-ray diffraction (XRD). As-prepared sample is single phase spinel with unit cell parameter 8.470 Å. After a thermal treatment in argon flow at 40 °C for 2 hours, the unit cell parameter increases to 8.488 Å. Additional thermal treatment at 400 °C in air resulted in a new diffraction pattern, indicative for structural transformation. Indexing the entire set of diffraction lines gave a solution with a good reliability factor within a rhombohedral space group (unit cell parameters a = 6 Å and c = 28.6 Å). Possible space groups were R3, R3m and R–3m. Preliminary structural data for the new phase are presented. The new structure is closely related to that of the spinel. It was found that the oxygen layer packing sequence remained unchanged, but the displacement of the oxygen positions resulted in lowering of the cubic symmetry to hexagonal. The arrangement of the cation positions also remains close to that of the spinel, alternating the Oh3 layers and T2O layers. The cation to anion ratio of the new phase is considered to be close to that of γ-Fe2O3 and is cation deficient.

The effect of different acid stabilizers on the morphology and optical properties of ZrO2 sol-gel films
Original Research Article
Pages 167 – 172
O. Dimitrov, I. Stambolova, K. Lazarova, T. Babeva, S. Vassilev, M. Shipochka
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Abstract
Transparent and homogeneous ZrO2 films were obtained by sol-gel technique using different acids as stabilizing agents. Tetragonal ZrO2 phase with small nanosized crystallites is characteristic for the studied films. The surface morphology of the samples is smooth and continuous without visible cracks. An average transmittance of about 80% was observed for the samples in the visible range. The refractive indexes of the films prepared using CH3COOH and HNO3 are 1.672 and 1.602, respectively. The chemical composition of the films as revealed by XPS analysis show the presence of oxygen defects. Photoluminescence (PL) properties were also studied in the range of 300–550 nm. The PL spectra of the samples obtained from solutions with CH3COOH and HNO3 exhibit broad emission band with maximum at 401 and 415 nm, respectively. The later PL peak has stronger intensity, due to the larger size of crystallites of the HNO3 films, as opposed to the CH3COOH samples.

Rare Earth doped silicate glass-ceramics for LED application
Original Research Article
Pages 173 – 178
I. Koseva, P. Tzvetkov, P. Ivanov, A. Yordanova, V. Nikolov
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Abstract
Rare earth doped glasses from the system Li2O-Al2O3-SiO2-LiBO2 (19.6:19.6:39.1:21.7 mol%) are prepared with concentration of the dopants 0.5 at.% for Tb3+ and 0.1 at.% for Eu3+. Glass-ceramics are obtained after thermal treatment of the parent glass at two different temperatures (580 and 630 °C) and for three different durations (2, 5 and 24 hours). The crystallizing phases, the crystallization degree and the particle size are determined. The main crystallizing phase after thermal treatment with different duration time is LiAlSiO4 in two different structural modifications – β-Eucryptite and γ-Eucryptite. Powder XRD analyses show the presence of additional phases with quantity less than 5%. The crystallinity of all samples is about 90%. The particle size depends slightly on the Rare Earth ion and varies between 80 and 120 nm according to the thermal treatment regime. Emission and excitation spectra of the glassceramics show the characteristic peaks of Tb3+ and Eu3+. The main emission peak of Tb3+ is 5D4 → 7F5 transition at 545 nm, corresponding to green color. The main emission peak of Eu3+ is 5D0 → 7F2 transition at 613 nm, corresponding to orange-red color. CIE coordinates of the samples show different emission colors, which depend on the active ion and the thermal treatment regime. The obtained results show that as-prepared terbium doped glass-ceramics could be used as a blue-green phosphor. Europium doped glass-ceramics could be used as an orange-red phosphor. Different emission colors could be obtained by using different thermal treatment regimes.

Catalytic oxidation of ethyl acetate by copper modified Ce-Mn and Ce-Ti mesoporous nanostructured oxides
Original Research Article
Pages 179 – 185
R. N. Ivanova, M. D. Dimitrov, G. S. Issa, D. G. Kovacheva, T. S. Tsoncheva
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Abstract
Ceria-based materials continue to be investigated for their structural and chemical reduction behaviour and nonstoichiometry, oxygen storage capacity and metal-ceria interactions. These materials show promising application as catalysts for environmental protection. In the current investigation mesoporous ceria-titania and ceria-manganese binary oxides were used as a host matrix of nanosized copper oxide species. The textural, structural and redox properties of the obtained composites were investigated using Nitrogen physisorption, X-ray diffraction, UV-Vis, Raman spectroscopy as well as temperature-programmed reduction with hydrogen. Their catalytic activity for total oxidation of volatile organic pollutants was studied using ethyl acetate as a probe molecule. The state of the loaded copper species and the related catalytic behavior depends on the degree of deffectness of the metal oxide support, which could be controlled by using binary oxide systems..

Spectroscopic and XRD analysis of sulphuric acid treated biodegradable waste
Original Research Article
Pages 186 – 191
E. Serafimova, V. Petkova, B. Kostova
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Abstract
The number of realized projects in Bulgaria for recovery of poultry wastes as secondary raw material energy resource is limited. However, the problem with recovery of poultry wastes has also no sustainable solution worldwide. In this paper we investigate the possibility of recovery two types of solid wastes – chicken litter from poultry farm and wood ash from cellulose factory. For the purpose we prepared mixtures of mentioned above wastes and sulfuric acid. The obtained samples were investigated by elemental analysis, powder X-Ray diffraction and Fourier transformed infrared spectroscopy. It was found that the raw materials have a structure and composition, classifying them as suitable components to obtain soil improvers, due to their content of essential micro-nutrients without excessive of heavy and toxic elements.

Sol-gel silica hybrid materials – characterisation and long-term stability
Original Research Article
Pages 192 – 197
E. Todorova, G. Chernev
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Abstract
Main goal of the present work was the synthesis of reactive silica hybrid materials and the investigation of their water stability for a long period of time. For the improvement of silica reactivity, which is the base structural component of these hybrids different types of natural and synthetic organic materials were used – chitosan (CS), 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA) and methacrylic acid (MA). The structural characteristics of the obtained silica hybrids are investigated via XRD and SEM analysis. The XRD data reveals successfully formation of amorphous silica hybrid material. The SEM micrographs show the specific chemical properties of the chosen organic components, play important role on the final hybrid morphology .EDX analysis is used for establishment of the hybrid materials chemical/elemental composition.. In order to investigate their long-term stability, the synthesised sol-gel silica hybrids are immersed in a water solution for 48 and 168h. Their weight changes were evaluated. The experimental results of the immersed samples presented high stability for a long period for the specimens from the following compositions-silica/MMA and silica/CS/MMA. The opposite behaviour (low stability) was shown from the silica hybrids with participation of HEMA and combination of CS and HEMA.

Heterogeneous photocatalytic degradation of Reactive Black 5 in aqueous suspension by La-modified ZnO powders
Original Research Article
Pages 198 – 204
N. Kaneva, A. Bojinova, K. Papazova, D. Dimitrov
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Abstract
La-modified ZnO powders with different dopants concentration (0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol%) are prepared by simple thermal method. The obtained homogeneous charge is annealed at 100 °C. The structural and photocatalytic properties of the samples are characterized by X-ray diffraction, Scanning electron microscopy, BET surface area and UV−vis spectroscopy. The result of XRD shows that La3+ is successfully introduced into ZnO lattice. It is found out that the crystallite size of La-modified ZnO is greater as compared to pure ZnO and increases with the increasing La content. Surface area (BET) measurements show higher surface areas and pores volume for La–ZnO catalysts in comparison to pure ZnO. The photocatalytic investigations revealed that all the La-modified ZnO powders exhibited excellent photocatalytic degradation of Reactive Black 5, compared with the ZnO, under UV and visible light irradiation. The optimal dopant concentration is experimentally found to be 2 mol% La in terms of the photocatalytic efficiency. The result shows that La3+ doping concentration has a remarkable effect on the efficiency of photocatalytic activity.

Characterization of in-situ-sampled particulate matter in air pollution localized by lidar monitoring
Original Research Article
Pages 205 – 210
I. V. Grigorov, G. V. Kolarov, L. L. Gurdev, Z. P. Cherkezova-Zheleva, L. S. Slavov, Ch. G. Ghelev, R. V. Ilieva, M. V. Iliev, V. I. Groudeva, D. V. Stoyanov, I. I. Nedkov
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Abstract
The work presents part of comprehensive studies performed during 2018 that dealt with particulate matter (PM), more specifically such with size ≤ 10 µm (PM10) and ≤ 2.5 µm (PM2.5), in the atmospheric aerosol. The experiments combined lidar monitoring over a densely populated urban area (city of Sofia, capital of Bulgaria) with on-the-spot sampling at sites with high concentration of dust products. Once the place of high PM concentration was localized by the lidar, samples were taken using a Hygitest 106 (Maimex) – a high-efficiency portable device for sampling and concentration determination of PM in atmospheric aerosol. X-ray diffraction and Mössbauer studies at room temperature were used to study the PM inorganic part. The material collected was also subjected to microbiological investigation by first applying a standard procedure for isolation of pure microbial cultures. The bacterial and fungal obtained isolates were identified on the basis of their morphological, physiological and biochemical characteristics. Thus, it was found that the surface of the micron-sized particles can adsorb both mechanical and microbial contaminants, while the liquid envelope, when the PM is dispersed as aerosols, may preserve this nano-world and, in some cases, create conditions favoring the occurrence of chemical and bio-processes. The lidar maps constructed can be further used for tracing the full air mass transport carrying contamination from a number of pollution sources (chemical, biological, dust, etc.), distributed over the scanned region.

Preparation and antimicrobial properties of silver nanoparticles supported by natural zeolite clinoptilolite
Original Research Article
Pages 211 – 218
M. I. Panayotova, N. N. Mintcheva, O. T. Gemishev, G. T. Tyuliev, G. D. Gicheva, L. P. Djerahov
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Abstract
Nanocomposites (AgNPs-zeolite) comprising of silver nanoparticles supported by the natural Bulgarian zeolite clinoptilolite have been prepared by two-step synthesis: silver ions (Ag+) immobilization on the zeolite followed by their thermal reduction and formation of silver nanoparticles (AgNPs). The structure and surface chemistry of as-prepared nanocomposites, morphology and size distribution of the obtained AgNPs were characterized by various methods (XRD, BET, XPS, SEM, and TEM). SEM and EDS analyses have shown around 11 wt% Ag which is uniformly dispersed in the zeolite host. On TEM images it can be seen that the composites contain small (3–5 nm) AgNPs situated in the microstructural defects of the zeolite crystals, and bigger AgNPs (20–25 nm) that are located on the surface of zeolite crystallites. The AgNPs-zeolite composites, at low amount added to Peptone water, cease the Escherichia coli cell growth. The synthesized AgNPs-clinoptilolite composites could find an application in the water disinfection.