Characterization of MgAl₂O₄ Sintered Ceramics

Single phase MgAl2O4 was made from a one-to-one molar ratio of MgO and Al2O3 powders mixed using ball-milling. Mixtures of MgO and Al2O3 were subsequently treated in planetary ball mill for 30, 60, 90 and 120 minutes in air. The aim of this study was to examine phase composition, microstructure, and densification behavior of sintered specimens. After sintering in dilatometer at 1500 °C, the powder was converted to single phase MgAl2O4. The results show that mechanical activation improved the densification behavior of MgAl2O4 sintered specimens, and it reduced the onset temperature for sintering by approx. 100°C. Based on dilatometer data, powders were subsequently densified at 1450°C by hot pressing. Almost all specimens exhibited full density, while sample activated for 30 minutes showed the fastest densification rate

Effects of citric ion on hexagonal NaYF4: Yb/Er phase formation during solvothermal synthesis

Up-converting nanoparticles (UCNPs) which have ability to emit a visible light upon excitation with NIR photons are with wide applications in optoelectronic devices, forensic, biomedicine and security. The synthesis of the most efficient hexagonal β- NaYF4:Yb/Er phase in shape of uniformly sized spherical nanoparticles is usually performed through thermal decomposition of complex organic precursor in high-boiling organic solvents, which rise deep concerns regarding their potential citotoxicity. In this work, citric acid and Na-citrate are used for the stabilization of hexagonal polymorph during solvothermal processing of NaYF4:Yb/Er phase from common inorganic precursor (rare earth nitrate salts). Additionally, effect of precipitation agent, i.e. fluorine source (NaF, NH4F and NH4HF2) on the final particle morphology is deduced based on comprehensive scanning and transmission electron microscopy analyses. Detailed refinement of the X-ray powder diffraction data (XRPD) revealed simultaneous nucleation of a cubic and hexagonal phase during solvothermal reaction. The use of citric acid resulted in formation of micro- and nano- spherical particles of a hexagonal and cubic NaYF4:Yb/Er phase, respectively, while Na-citrate provoked nucleation of well crystallized hexagonal crystals of β-NaYF4:Yb/Er phase. Size of the crystallites is determined by the choice of the precipitant, and is smallest in the case when NH4F is used. All synthesized powders emit bright and intense green light due to the intensification of the 2H11/2,4S3/2 →4I15/2 electronic transitions upon the excitation by the infrared light (λ=978 nm)

Clogging of water supply wells in alluvial aquifers by mineral incrustations, central Serbia

The formation of incrustations on public water supply well screens reduces their performance considerably. The incrustations increase hydraulic losses, reduce the capacity of the well and screen, affect the quality of the pumped water and increase maintenance costs. In alluvial environments, the most common deposits are iron and manganese hydroxides. However, the rates of formation, compositions and levels of crystallization vary, depending on the geochemical characteristics of the alluvial environment, the microbiological characteristics of the groundwater and the abstraction method. Samples of 15 incrustations were collected from wells that tap shallow alluvial aquifers and were found to be dominated by iron. XRD analyses detected low-crystalline ferrihydrite and manganese hydroxide in the samples collected from the water supply source at Trnovče (Velika Morava alluvial). The incrustations from the Belgrade Groundwater Source revealed the presence of ferrihydrite and a substantial amount of goethite α-FeOOH. Apart from goethite, greigite (Fe3S4) was detected in three samples, while one sample additionally contained bernalite Fe(OH)3 and monoclinic sulfur S8. Among carbonates, only siderite was detected. Iron oxidizing bacteria generally catalyze deposition processes in wells, while sulfate reducing bacteria (SRB) play a role in the biogenic formation of greigite. Determining the nature of the deposited material allows better selection of rehabilitation chemicals and procedure. [Projekat Ministarstva nauke Republike Srbije, br. TR37014

ZnFe2O4 antiferromagnetic structure redetermination

Magnetic structure of ZnFe2O4 normal spinel is re- examined. Antiferromagnetic structure non-collinear model is established within C(a)2 space group having four different crystallographic/magnetic sites for 32 Fe3+ spins within magnetic unit cell

Dielectric and ferroelectric properties of BST ceramics obtained by a hydrothermally assisted complex polymerization method

Ba0.8Sr0.2TiO3 (BST) powder, obtained by a hydrothermal treatment of precursor solution previously prepared by a complex polymerization method (HT-CPM), was uniaxially pressed and sintered at 1280 degrees C for 1-32 h. The sintering effect on structural parameters, phase composition, dielectric and ferroelectric properties was investigated. Rietveld refinement analysis of BST ceramics showed that predominant phase was tetragonal BST with monoclinic Ba6Ti17O40 (B6T17) as a secondary phase. Phase contents determined by Rietveld refinement match well with the results of energy dispersive X-ray analysis. Dielectric and ferroelectric properties of BST ceramics have been correlated with density of sintered compacts, phase composition, structural parameters and microstructure. The sample sintered for 16 h showed the highest density (93% rho(t)), as well as the highest value of dielectric constant (similar to 2300) and remanent polarization - Pr (2 mu C/cm(2)) among investigated samples

Synthesis, Structure, Morphology and Properties of Biphasic ZnO–ZnMn2O4

Transition metal oxides are important materials that have found many applications, as capacitors, sensors or in energy storage [1]. Synthesis of these compounds has been realized by various methods, such as hydro(solvo)thermal synthesis, precipitation, microwave synthesis or sol-gel synthesis [2]. Recently, the thermolysis of coordination compounds as precursors has been considered as a new approach in obtaining functional nanosized materials. [3]. In this way, by selecting the proper precursor, it is possible to control the phase composition, morphology and particle size of a resulting material [3]. The biphasic powder composed of ZnO (zincite) and ZnMn2O4 (hetaerolite), (I) has been obtained by thermolysis of bimetallic complex [MnZn2(dipya)3(tpht)3(H2O)4]·2H2O (dipya = 2,2’-dipyridylamine, tpht = dianion of 1,4-benzenedicarboxylic acid) at 450 °C during 1 h in air atmosphere. Scanning Electron Microscopy (SEM) was used to investigate the morphology of I (Figure 1). It can be observed that the morphology consists of deformed spherical grains of ZnO with an average diameter of 67 nm and elliptical grains of hetaerolite whose average diameter and length were 156 and 290 nm, respectively. The X-ray powder diffraction (XRPD) was applied to investigate the structure of I. In Figure 2 two-phase Rietveld refinement Electron Microscopy of Nanostructures ELMINA 2018, August 27-29, 2018 172 pattern of I is presented (ZnO to ZnMn2O4 phase-ratio of 62:38 wt. %). The main crystallographic data and Rietveld refinement parameters for ZnO phase are: hexagonal, space group P63mc, a = 3.2574(1), c = 5.2175(2) Å, V = 47.945(3) Å3 ; for ZnMn2O4 phase are: tetragonal, space group I41/amd, a = 5.7299(3), c = 9.3000(8) Å, V = 305.34(3) Å3 ; Rwp = 4.80 %, Rp = 3.82 %, Rexp = 3.80 % and χ2 = 1.5960. UV- Vis-NIR absorption spectrum was measured in order to investigate the direct band gap (Eg) of I. Due to the existence of two phases in I, two different Eg values of 2.4 and 3.3 eV for ZnMn2O4 and ZnO phase, respectively, were determined using Kubelka-Munk function. The mean size, polydispersity index (PDI) and zeta potential of spherical grains were measured using Zetasizer Nano Series, Nano ZS. The mean size was 418.6±53.1 nm while PDI value was found to be 0.354±0.099. Relatively high values of PDI and low apostate value of zeta potential (–6.55 mV) are indications of incipient instability of colloidal dispersion of I, probably due to the formation of agglomerates [4, 5]. Photoluminescence measurements were carried out at room temperature on Fluorolog-3 Model FL3-221 spectrofluorimeter system upon excitation at 350 nm, in order to study the optical properties of I. This analysis revealed one band centred at 422 nm in the blue region of the visible part of the spectrum, which can possibly be associated with defects in the crystal structure of the ZnO phase [6]. References: [1] C Yuan et al, Angewandte Chemie International Edition 53 (2014), 1488. [2] CNR Rao and B Raveau in “Transition Metal Oxides: Structure, Properties, and Synthesis of Ceramic Oxides” 2nd edition (WILEY-VCH, New York). [3] MY Masoomi and A Morsali, Coordination Chemistry Reviews 256 (2012), 2921. [4] R Greenwood and K Kendall, Journal of the European Ceramic Society 19 (1999), 479. [5] M Staiger et al, Journal of Dispersion Science and Technology 23 (2002), 619. [6] H Zeng et al, Advanced Functional Materials 20 (2010), 561. [7] The authors acknowledge funding from the Ministry of Education, Science and Technological Development of the Republic of Serbia, Grant Number III45007

An integrated study of thermal treatment effects on the microstructure and magnetic properties of Zn-ferrite nanoparticles

The evolution of the magnetic state, crystal structure and microstructure parameters of nanocrystalline zinc-ferrite, tuned by thermal annealing of similar to 4 nm nanoparticles, was systematically studied by complementary characterization methods. Structural analysis of neutron and synchrotron x-ray radiation data revealed a mixed cation distribution in the nanoparticle samples, with the degree of inversion systematically decreasing from 0.25 in an as-prepared nanocrystalline sample to a non-inverted spinel structure with a normal cation distribution in the bulk counterpart. The results of DC magnetization and Mossbauer spectroscopy experiments indicated a superparamagnetic relaxation in similar to 4 nm nanoparticles, albeit with different freezing temperatures T-f of 27.5 K and 46 K, respectively. The quadrupole splitting parameter decreases with the annealing temperature due to cation redistribution between the tetrahedral and octahedral sites of the spinel structure and the associated defects. DC magnetization measurements indicated the existence of significant interparticle interactions among nanoparticles (superspins). Additional confirmation for the presence of interparticle interactions was found from the fit of the Tf(H) dependence to the AT line, from which a value of the anisotropy constant of K-eff = 5.6 x 10(5) erg cm(-3) was deduced. Further evidence for strong interparticle interactions was found from AC susceptibility measurements, where the frequency dependence of the freezing temperature T-f(f) was satisfactory described by both Vogel-Fulcher and dynamic scaling theory, both applicable for interacting systems. The parameters obtained from these fits suggest collective freezing of magnetic moments at T-f

Luminescence of Mn4+ ions in CaTiO3 and MgTiO3 perovskites: Relationship of experimental spectroscopic data and crystal field calculations

Herein, the synthesis, structural and crystal field analysis and optical spectroscopy of Mn4+ doped metal titanates ATiO(3) (A = Ca, Mg) are presented. Materials of desired phase were prepared by molten salt assisted sol-gel method in the powder form. Crystallographic data of samples were obtained by refinement of X-ray diffraction measurements. From experimental excitation and emission spectra and structural data, crystal field parameters and energy levels of Mn4+ in CaTiO3 and MgTiO3 were calculated by the exchange charge model of crystal-field theory. It is found that crystalline field strength is lower (Dq = 1831 cm(-1)) in the rhombohedral Ilmenite MgTiO3 structure due to the relatively longer average Mn4+-O2- bond distance (2.059 angstrom), and higher (Dq = 2017 cm(-1)) in orthorhombic CaTiO3 which possess shorter average Mn4+-O2- bond distance (1.956 angstrom). Spectral positions of the Mn(4+2)Eg - GT (4)A(2g) transition maxima is 709 nm in MgTiO3 and 717 nm in CaTiO3 respectively in good agreement with calculated values. (C) 2017 Elsevier B.V. All rights reserved.6th International Conference of Excited States of Transition Elements (ESTE) / Workshop on Luminescence, Aug 21-26, 2016, Polanica Zdroj, Polan

Fe Doped TiO₂ Prepared by Microwave-Assisted Hydrothermal Process for Removal of As(III) and As(V) from Water

Elevated concentrations of arsenic in groundwater, which is used as a source for drinking water, is a worldwide problem. Use of TiO₂ and iron doped TiO₂ synthesized by a microwave-assisted hydrothermal method for As­(III) and As­(V) removal were examined. Synthesized sorbents were characterized with XRD and nitrogen physisorption. Synthesized sorbents have predominantly anatase structure, and no peaks for iron could be observed. Doping of iron increases the surface area of synthesized sorbents. Sorption experiments show that increase of iron in sorbents increases the sorption capacity for As­(III) and As­(V). Increase of pH from 3 to 11 has no influence on As­(III) sorption but decreases the sorption of As­(V). Batch isotherm studies were performed to determine the binding capacities of As­(III) and As­(V). As­(III) followed the Freundlich isotherm model, while for As­(V) a better fit was with the Langmuir isotherm. The results of competition of SO₄^2– and PO₄^3– anions on adsorption of As­(III) indicated that both anions reduced substantially the efficiency of adsorption on both adsorbents while for As­(V) only the presence of PO₄^3– anion interfered with adsorption. Testing 10Fe/TiO₂ sorbent with arsenic contaminated natural water showed that this material could be used for removal of arsenic to the level recommended by WHO without pretreatment

On the tungsten carbide synthesis for PEM fuel cell application - Problems, challenges and advantages

Fuel cell application of tungsten carbide is revisited starting with four different tungsten carbide precursors used for high temperature synthesis. It was shown that the final products greatly depend on the nature of the precursor. Using tungsten peroxide/2-propanol derived precursor almost pure WC was obtained which was subjected to further electrochemical investigation. It was shown that it is necessary to decorate WC with Pt nanoparticles in order to obtain satisfactory fuel cell performance, but catalytic activity of Pt/WC anode catalyst is not expected to overcome the activity of Pt/C. It is argued that new synthetic routes for the preparation of WC should be directed towards obtaining highly dispersed WC, that is, WC with high external surface area available for Pt deposition, rather than high specific surface area WC with large contribution of micropores having no importance when it comes to the use of WC as a catalyst support. The true benefit of the use of WC as catalyst support is found in increased CO tolerance/CO oxidation activity of WC-supported Pt catalysts. Qualitative mechanistic view on increased CO oxidation activity of Pt/WC is offered. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved