Structural and dielectric characterization of barium strontium titanate synthesized by hydrothermally assisted modified Pechini method

U okviru ove doktorske disertacije proučavan je uticaj novog načina sinteze tj. hidrotermalno potpomognutog modifikovanog Pećinijevog postupka na formiranje strukture i karakteristike barijum-stroncijum-titanatnog (BST) praha, kao i na svojstva dobijene keramike. Prekursorski rastvor barijum stroncijum titanata (Ba0.8Sr0.2TiO3) dobijen je modifikovanim Pećinijevim postupkom, tj. metodom kompleksne polimerizacije polazeći od rastvora titan-citrata, barijum- i stroncijum-acetata. Razblaženi prekursorski rastvor BST tretiran je hidrotermalnim postupkom u reakcionom sudu autoklava na temperaturi od 225 ºC, pritisku od 6.8 MPa, i vremenu trajanja reakcije od 5 h. Prekursorski prah BST je dalje termički tretiran na različitim temperaturama (500-800 °C) u cilju formiranja kristaliničnog BST praha. Fazna evolucija tokom kristalizacije BST prahova praćena je metodom difrakcije rendgenskog zračenja i Ramanske spektroskopije. Rezultati rendgenske difrakcije i Ramanske spektroskopije BST prahova dobijenih hidrotermalnim postupkom, pokazali su da kalcinacijom na 700 ºC dolazi do formiranja kristaliničnog tetragonalnog BST praha, koji ne sadrži organske i karbonatne faze. Morfologija prahova analizirana je skanirajućom elektronskom mikroskopijom. Prahovi dobijeni hidrotermalnim postupkom su aglomerisani i sastoje se od sfernih čestica veličine do 2 μm u prečniku. Keramički uzorci dobijeni su postupkom jednoosnog presovanja BST praha kalcinisanog na 700 ºC u tablete prečnika ~ 8 mm i njihovim sinterovanjem na temperaturi od 1280 ºC u trajanju od 1 do 32 h. Ispitivan je uticaj vremena sinterovanja na fazni sastav, strukturne i mikrostrukturne parametre, dielektrična i feroelektrična svojstva dobijene keramike...In this doctoral dissertation the influence of new synthesis method, i.e hydrothermally assisted modified Pechini method, on the structure formation and properties of barium strontium titanate powder, as well as properties of obtained ceramics. The BST (Ba0.8Sr0.2TiO3) precursor solution was prepared using modified Pechini method, i.e complex polymerization method starting from titanium citrate, Ba- and Sr- acetate. Precursor solution was hydrothermally treated in high pressure autoclave for 5 h, at the temperature of 225 °C and pressure of 6.8 MPa in order to obtain BST precursor powder. Such obtaned precursor powder was further thermally treated at different temperatures (500-800 ºC) in order to obtained crystalline BST powder. XRD and Raman studies were carried out in order to estimate the phase evolution and crystalline structures of BST powders calcined at various temperatures. XRD and Raman measurements confirmed complete crystallization of tetragonal BST at 700 ºC, with no presence of organic and carbonate phases. The morphology of BST powders was analysed using scanning electron microscopy. Thermal treatment of precursor powder at 700 °C, led to formation of low agglomerated BST powder consists of spherical particles up to 2 μm in size. After detailed examination of the BST powder characteristics, using uniaxial press and sintering at 1280 ºC for a various sintering time (1-32 h), BST ceramic samples were obtained. The sintering effect on phase composition, structural and microstructural parameters, dielectric and ferroelectric properties was investigated..

Eu doped barium cerium oxide as a promising electrolyte for intermediate temperature SOFCs

BaCe0.9Eu0.1O2.95 (BCE) powder was synthesized by citric-nitric autocombustion method. According to Rietveld analysis, BCE also possesses slightly larger unit cell volume than mostly investigated BaCe0.9Y0.1O2.95 (BCY), which allows higher proton mobility through the perovskite lattice. Sinterability of BaCeO3 is enhanced by doping with Eu since dense electrolyte microstructure with 1–2 μm grains can be obtained at temperatures below 1500 ºC. Conductivity measurements revealed separate bulk and grain boundary contributions to the total electrolyte conductivity below 200 ºC. The grain boundary conductivity was one order of magnitude higher than the bulk conductivity, indicating the blocking effect of the grain boundaries to the mobility of charge carriers. As this effect ceased with temperature, it was possible to determine only total conductivities above 500 ºC. Conductivity of BCE in a wet hydrogen atmosphere at 600 ºC reached 1.2 × 10–2 S/cm, which can be considered as one of the highest conductivities among BaCeO3 based proton conductors. Thus, doping of BaCeO3 with europium offers multiple improvements that can eventually lead to decrease in operating temperature of SOFCs based on this type of proton conducting electrolyt

Influence of anodization voltage on photocatalztic activity of TiO2 nanotubes

TiO2 under UV light generates charge carriers (electrons and holes) that take part in the process of decomposition of pollutants, which is a well-known fact for the past few decades. For this reason, TiO2 is used for photocatalytic purification water and air. Photoactivity of TiO2 depends on the amount of generated charge carriers, and one part of those carriers are lost through the process of recombination. In order to improve photoactivity, it is necessary to speed up transport of electrons and holes. For better and faster charge carrier transport a unidirectional path is desired, which can be obtained by synthesizing 1D morphology. Anodization of titanium foil is a good way to obtain perpendicular nanotubular morphology onto a substrate. Nanotubular morphology can be optimized via operative conditions of anodization: applied voltage, anodization time and type of electrolyte. In this work, TiO2 nanotube arrays were synthesized by anodization of titanium foil at different voltages: 10 V, 15 V, 20 V and 25 V. Based on FESEM (Field Emission Scanning Electron Microscopy) micrographs, the microstructure of nanotubes was analysed and data concerning wall thickness, outer diameter of nanotubes and active surface were considered. Also, the influence of nanotube morphology on optical properties was determined. Results of our research show that the increasement in anodization voltage influenced on the appearance of redshift in the absorption spectrum. The sample synthesized at 20 V showed the highest photocatalytic activity due to the optimal nanotube length and nanotube diameter

Influence of anodization voltage on photocatalztic activity of TiO2 nanotubes

TiO2 under UV light generates charge carriers (electrons and holes) that take part in the process of decomposition of pollutants, which is a well-known fact for the past few decades. For this reason, TiO2 is used for photocatalytic purification water and air. Photoactivity of TiO2 depends on the amount of generated charge carriers, and one part of those carriers are lost through the process of recombination. In order to improve photoactivity, it is necessary to speed up transport of electrons and holes. For better and faster charge carrier transport a unidirectional path is desired, which can be obtained by synthesizing 1D morphology. Anodization of titanium foil is a good way to obtain perpendicular nanotubular morphology onto a substrate. Nanotubular morphology can be optimized via operative conditions of anodization: applied voltage, anodization time and type of electrolyte. In this work, TiO2 nanotube arrays were synthesized by anodization of titanium foil at different voltages: 10 V, 15 V, 20 V and 25 V. Based on FESEM (Field Emission Scanning Electron Microscopy) micrographs, the microstructure of nanotubes was analysed and data concerning wall thickness, outer diameter of nanotubes and active surface were considered. Also, the influence of nanotube morphology on optical properties was determined. Results of our research show that the increasement in anodization voltage influenced on the appearance of redshift in the absorption spectrum. The sample synthesized at 20 V showed the highest photocatalytic activity due to the optimal nanotube length and nanotube diameter

Encapsulated Clove Bud Essential Oil: A New Perspective as an Eco-Friendly Biopesticide

In this work by encapsulation technique we have synthetized three new clove bud essential oil (CEO) Emulsifiable Concentrate (EC) formulations depending on the carrier (synthetic zeolite-F-CSZ, nature zeolite-F-CNZ and gelatin-F-CG). The main idea was to develop an eco-friendly biopesticide that can find use in plant protection as an alternative to the use of conventional pesticides. By encapsulation we wanted to enable water solubility and ensure prolonged efficacy of the essential oil. Biological activity of designed CEO formulations was tested on potato tuber moth Phthorimaea operculella (fumigant mode of action), gray mold fungal pathogen Botrytis cinerea (preserver coatings), and soft rotting bacterial pathogens Pectobacterium carotovorum (subsp. carotovorum and brasiliensis) and Dickeya dianthicola (direct competition). CEO formulations evinced a prolonged action on mortality of P. operculella during the insects’ exposure to the concentration of the emulsions of 40 µL L−1 air. The mortality gradually decreased from a probability of 100% after the first 24 h to 50% after 5 days for F-CSZ or after 4 days for F-CNZ and F-CG. The most promising formulation is F-CSZ enabling activity during 14 days of exposure, while the effect of the other two formulations lasted 10 days. All three formulations produced a strong fungicidal effect against B. cinerea by preventing infection and disease development. The best efficacy was evidenced with F-CSZ (synthetic zeolite as a carrier) showing 100% efficacy when it was used even at the lowest tested concentration of active CEO (1%). The results of in vitro testing against soft rot pathogens determined the MIC value of CEO formulations to be 1% of active CEO. By this research, we present a novel perspective on the use of essential oils as an alternative, environmental biopesticide. CEO formulations can be commercially exploited as a fumigant or preserver coatings to extend the shelf life of stored products or the fresh-fruit market

Ferroelectric properties of BiFeO3 ceramics with cation substitutions at Bi-site (La3+, Eu3+) and Fe-site (Nb5+, Zr4+)

BiFeO3 is one of the few multiferroic perovskites that exhibits magnetic and ferroelectric properties at room temperature. However, it is also distinguished by high leakage current, low remnant electric and magnetic polarization, and high electric coercive field. These features keep it away from any practical use in electronics. Therefore, many attempts have been made to improve the properties of BiFeO3 by Bi- or Fe-site doping or by both. Previous investigations suggest that doping with Nbat Fe-site can positively affect the magnetic behavior of BiFeO3 and decrease the leakage current. In this study, various cation substitutions at Bi-site (La3+, Eu3+) and Fe-site (Nb5+, Zr4+) were examined to investigate their possible synergism and benefit for the ferroelectric properties. The role of the cations with higher valence is to suppress the formation of structural defects during synthesis, such as oxygen and bismuth vacancies. These defects are responsible for high leakage currents and, consequently, low breakdown voltages characteristic of the pure BiFeO3. On the other hand, rare earth cations at the Bisite usually enable densification of the ceramics in a wider range of temperatures, preventing bismuth loss and forming defects and secondary phases during sintering. However, do pant concentrations above 10–15mol% may give rise to transition from polar, rhombohedral (R3c) to non-polar, orthorhombic (Pnma) symmetry. The carefully selected compositions of doped BiFeO3 were synthesized by a simple hydro-evaporation method. The ceramics samples were characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and polarization techniques, including leakage current measurements. Although the introduction of Nb5+or Zr4+decreased the leakage current, they surprisingly deteriorated the ferroelectric properties even at concentrations as low as 1 mol%. This effect was more pronounced for the samples containing Nb. On the contrary, both La3+ and Eu3+ (incorporated at the Bi-site) improved the ferroelectric properties as their concentrations increased. The La-doped samples exhibited higher remnant electric polarizations at observed electric fields. The highest remnant electric polarization of31.9 μC/cm2at 150 kV/cm was measured for Bi0.85La0.15Fe0.998Zr0.002O3, indicating the synergetic effect of La3+ and Zr4+, which is limited to very low Zr4+concentrations

Photodegradation of organic dye using BiFeO3 particles synthesized by ultrasound route

BiFeO3 precursor powder was synthesized by ultrasound asissted sol–gel route at relatively low temperature, starting from Bi-nitrate, Fe-nitrate, and ethylene glycol. Structural, optical, and photocatalytic properties of the obtained powder were investigated. X-ray diffraction analysis confirmed that thermal treatment of precursor powder at 500 °C led to formation of pure phase BiFeO3. The determined band gap was 2.20 eV, indicating its potential application as visible-light-response photocatalyst. The powder is used for photocatalytic degradation of typical organic azo dye Mordant Blue 9 in concentration of 50 mg/l. Measurements were performed for different times of irradiation and pH of the dye solution. Changes in UV-Vis absorption spectra revealed the decolorization and decomposition of organic dye during the photodegradation process. Photodegradation products were analyzied by HPLC technique, and mechanism of photocatalytic degradation of organic dye was proposed

Influence of MN doping on the evolution of microstructure and optical properties of mechanically activated SrTiO3 powders

Manganese doped SrTiO3 powders with various manganese dioxide weight percentages in the range of 1.5, 3 and 6 wp% were prepared by a solid-state method in the presence of mechanical activation (10, 30 and 120 minutes). A systematic investigation by X-ray diffraction (XRD), scanning electron microscopy (SEM), particle size analisys (PSA), Brunauer-Emmett-Teller (BET) methods and Raman spectroscopy has been undertaken to evaluate the role of dopants on the microstructural and morphological study of the perovskite oxide obtained. The optical properties of the different manganese doped and activated SrTiO3 powders have been also evaluated. Mn insertion in SrTiO3 is discussed considering the possibility for Mn ions to occupy both Ti4+ and Sr2+ sites as well as manganese segregation and Mn incorporation-related non-homogeneities. The results demonstrated that Mn has substituted into the lattice and surface layers of the particles of SrTiO3 powders and the absorption onset shifted to higher values of wavelengths with increasing time of activation and dopant concentration. The lowest value of the band gap (Eg=3.10 eV) was registered with the longest activation for 120 minutes and the highest concentration of dopant (6 wp%). Combining doping with mechanical activation lower values of band gap can be achieved and that fact could be used in subsequent studies to make Mn-SrTiO3 more suitable visible-light photocatalysts

TUNING OF FERROELECTRIC PROPERTIES OF BiFeO3 CERAMICS BY CATION SUBSTITUTIONS AT Bi-SITE AND Fe-SITE

In this study, we tried various cation substitutions at Bi-site (La3+, Eu3+) and Fesite (Nb5+ , Zr4+ ) to explore their possible synergism and improvement of the ferroelectric properties of bismuth ferrite. The cations with higher valence ought to suppress the formation of structural defects during syntheses, such as oxygen and bismuth vacancies. These defects are responsible for high leakage currents and low breakdown voltages characteristic of pure BiFeO3. On the other hand, rare earth cations at the Bi-site usually enable densification of the ceramics at a broader range of temperatures, preventing bismuth loss and formation of defects and secondary phases during sintering. However, dopant concentrations above 10–15 mol% may give rise to a transition from polar, rhombohedral (R3c) to non-polar, orthorhombic (Pnma) symmetry. Thus, we synthesized pure and selected compositions doped BiFeO3 by a hydroevaporation method and determined the optimal calcination temperature by thermal analyses of the precursor powders. Then we characterized ceramics samples using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and polarization techniques. Although only 1 mol% Nb5+ decreased the leakage current, it surprisingly deteriorated the ferroelectric properties of BiFeO3. Similar effect exhibited the samples containing Zr4+ that showed no improvement compared with undoped bismuth ferrite. On the contrary, La3+ and Eu3+ (incorporated at the Bi-site) improved the ferroelectric properties as their concentrations increased, whereby the samples doped with 15 mol% La exhibited higher remnant electric polarizations at observed electric fields. The highest remnant electric polarization of 31.9 µC/cm2 at 150 kV/cm, was measured for Bi0.85La0.15Fe0.998Zr0.002O3, indicating the synergetic effect of La3+ and Zr4+, which is limited to low Zr4+ concentrations

Tuning of BiFeO3 multiferroic properties by light doping with Nb

BiFeO3, as one of few multiferroic perovskites, is distinguished by some drawbacks such as high leakage current, low remnant magnetic polarization, high electric coercive field, and difficulties of pure phase synthesis that still keep it away from any practical use in electronics. There have been many attempts to improve the overall properties of BiFeO3 by A- or B-site doping or by both. It was found that B-site doping with Nb can potentially improve both the ferroelectric and magnetic properties of BiFeO3. In this study, BiFe1–xNbxO3 (x=0.002; 0.005 and 0.01) bulk ceramics were investigated in the quest for a more integral understanding of changes in multiferroic properties caused by light Nb doping (≤ 1 mole%). BiFe1–xNbxO3 powders were synthesized by hydro evaporation method and only traces of the secondary phases were observed. Multiferroic properties of bulk ceramics were investigated using X-ray diffraction (XRD) analysis, scanning electron spectroscopy (SEM), polarization (PMTS), and magnetization (SQUID) techniques. It was shown that even small percentages of Nb could notably change the electric and magnetic behavior of BiFeO3. The electric conductivity differed by two orders of magnitude between samples doped with 0.2 and 1% Nb. The ferroelectric behavior strongly depended on the conduction mechanism, and the transition from space-charge-limited current (SCLC) conduction to trap-filled Limited (TFL) conduction regime reflected on a change in hysteresis patterns, particularly for the samples with 0.2 and 0.5% Nb. ZFC-FC magnetization curves were separated for all Nb concentrations and the degree of separation increased with Nb doping. Weak ferromagnetic behavior was observed from the hysteresis measurements and the increase of remnant magnetization with Nb concentration. The coercive magnetic field changed drastically compared to the pure BiFeO3, namely, the sample with 1% Nb exhibited a very high coercive magnetic field of ~10 kOe