Dejan Poleti

Mechanochemical treatment of α-Fe2O3 powder in air atmosphere

Abstract Powder of α -Fe 2 O 3 was mechanochemically treated in a planetary ball mill in an air atmosphere. Structural changes were followed by X-ray diffraction analysis, magnetization measurements and differential scanning calorimetry after various milling times. It was found that complete transformation of α -Fe 2 O 3 to Fe 3 O 4 is possible during milling in an air atmosphere under appropriate milling conditions. Presumably, the decrease in the oxygen partial pressure during milling has a critical influence on promoting the dissociation of α -Fe 2 O 3 . Before nucleation of the Fe 3 O 4 phase, the crystallites of the α -Fe 2 O 3 phase are reduced to a minimal size accompanied by the introduction of atomic-level strain. Local modeling of a collision event, coupled with a classical thermodynamic assessment of the Fe 2 O 3 -Fe 3 O 4 system, were used to rationalize the experimental results. It is proposed that the mechanochemical reactions proceed at the moment of impact by a process of energization and freezing of highly localized sites of a short lifetime. Excitation on a time scale of ∼10 −5 s corresponds to a temperature rise of the order of (1–2)×10 3 K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 10 6 K s −1 .

Synthesis and characterization of biodegradable poly(butylene succinate-co-butylene fumarate)s

Abstract A series of aliphatic biodegradable polyesters modified with fumaric residues was synthesized by transesterification in the melt of dimethyl succinate, dimethyl fumarate and 1,4-butanediol. The amount of unsaturation, originating from the fumaric acid residues in the polyesters chains was varied from 5 to 20 mol%. The molecular structure and composition of the polyesters were determined by 1H NMR spectroscopy. The effects of the content of fumaric residues on the thermal and thermo-oxidative properties of the synthesized polyesters were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis. The degree of crystallinity was determined by DSC and wide angle X-ray scattering. The degrees of crystallinity of the unsaturated copolyesters were reduced, while the melting temperatures were higher in comparison to poly(butylene succinate). Biodegradation of the synthesized copolyesters was estimated in enzymatic degradation tests using a buffer solution with Rhizopus arrhizus lipase at 37 °C. Although the degree of crystallinity of the copolyesters decreases slightly with increasing unsaturation, the biodegradation is not enhanced suggesting that not only the chemical structure and molecular stiffness but also the morphology of the spherulites has an influence on the biodegradation properties. The highest biodegradability was observed for the copolyesters containing 5 and 10 mol% of fumarate units.

Structural and electrical properties of ZnO varistors containing different spinel phases

Abstract Structural and electrical properties of ZnO varistors were investigated as a function of spinel composition. Six varistor mixtures differing only in chemical composition of spinel, were prepared by mixing separately synthesized constituent phases (DSCP method). Compositions of constituent phases in sintered samples were investigated by changes of lattice parameters of the phases, as well as by EDS analysis of the constituent phases. It was found that compositions of ZnO, intergranular and spinel phases were partially changed during sintering due to redistribution of additives, that was controlled by starting spinel composition and its stability. Electrical characterization showed significant difference in electrical properties of investigated varistors: nonlinearity coefficients ranging from 22 to 55 and leakage currents differing by the order of magnitude. Activation energies of conduction were obtained from ac impedance spectroscopy measurements. Calculated values of activation energies were in the range 0.61–1.0 eV confirming difference in defect structure of ZnO grain boundaries in varistors containing different spinel phases.

Mixed ligand Co(II), Ni(II) and Cu(II) complexes containing terephthalato ligands. Crystal structures of diaqua(2,2′-dipyridylamine)(terephthalato)metal(II) trihydrates (metal=cobalt or nickel)

Abstract A series of nine ternary Co(II), Ni(II) and Cu(II) complexes with some aromatic diamines and dianion of terephthalic acid (tpht) have been synthesized and characterized by elemental analysis, magnetic susceptibility measurements, IR and diffuse-reflectance spectroscopy. All compounds were obtained as microcrystalline solids, stable in air and insoluble in common solvents. Two of them, [M(tpht)(dipya)(H 2 O) 2 ]·3H 2 O [M=Co(II), Ni(II); dipya=2,2′-dipyridylamine], were obtained as single crystals and their crystal structures have been determined by X-ray structure analysis. Both structures consist of discrete [M(tpht)(dipya)(H 2 O) 2 ] 2+ complex units with two H 2 O molecules in the trans position. Three additional H 2 O molecules together with coordinated H 2 O and O atoms from COO groups make a three-dimensional network of hydrogen bonds. Terephthalato ligands are coordinated by only one COO group, which acts as a chelating ligand. Central metal ions are in a distorted octahedral environment. Based on the above-mentioned analysis and known crystal structures, the possible structures and geometry of the complexes are discussed. All Co(II) and Ni(II) complexes are pseudooctahedral and mononuclear, with only one of the tpht COO groups coordinated in a chelate mode. Cu(II) complexes have probably distorted octahedral geometry with bridging role of tpht ions and monodentate or strongly asymmetric coordination of COO groups.

Synthesis and structure of aquamethanol-2′,2‴-(2,6-pyridin-diyldiethylidene)-dioxamohydrazidocobalt(II) perchlorate

Abstract The complex [Co(H 2 L)(H 2 O)(MeOH)](ClO 4 ) 2 [H 2 L] = 2′,2‴-(2,6-pyridindiyl-diethylidene)-dioxamohydrazide] has been prepared by direct and template syntheses and characterized by elemental analysis, IR spectroscopy, and magnetic susceptibility measurements. The crystal structure of the complex has also been determined. The cobalt atom is seven coordinated with approximately pentagonal-bipyramidal geometry. The H 2 O and methanol are axial with the planar pentadentate H 2 L ligand forming the equatorial plane. The possible reaction mechanism has been proposed.

Disodium hexaaquacobalt(II) bis[dihydrogen 1,2,4,5-benzenetetracarboxylate(2 –)] tetrahydrate

The title structure, Na 2 [Co(H 2 O) 6 ](C 10 H 4 O 8 ) 2 .4H 2 O, consists of [Co(H 2 O) 6 ] 2+ and Na + cations, dihydrogen 1,2,4,5-benzenetetracarboxylate(2-) (H 2 btc) anions and water of crystallization. The [Co(H 2 O) 6 ] 2+ and H 2 btc ions, as well as the water molecules, are linked together by a three-dimensional network of hydrogen bonds. A short symmetrical intramolecular hydrogen bond between the COO and COOH groups is found to exist in the H 2 btc ions.

Cation distribution and magnetic properties of ternary spinels

The series of spinels was prepared by ceramic technology. The crystal structures were refined from the x-ray diffraction data using the Rietveld profile method. The high-temperature magnetic susceptibility was measured by the Faraday method in the temperature range 290 - 600 K and the data were fitted to the Curie - Weiss law. The cation distribution in these diluted magnetics is obtained from x-ray diffraction and high-temperature magnetic susceptibility measurements. It was found that, for , ions occupy exclusively the octahedral position in the spinel type of structure. In the low-concentration region a strong clustering of magnetic ions is observed. In the range , ions are distributed in both tetrahedral and octahedral cationic sites. The Curie - Weiss paramagnetic temperature is negative in the whole range of concentrations, as the consequence of a predominant antiferromagnetic interaction. The appearance of the ions in tetrahedral sites (around x = 1.33) leads to a drop in the absolute -value. The effective magnetic moments of ions are about for and depend on the concentration for x>1.33.

Preparation, characterization and redox properties of o,o′‐geometrical isomers of μ‐α and μ‐β‐aminoisobuty‐rato co(ii) complexes with n,n′,n′,n‴‐tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane

Abstract Two new dinuclear cobalt(II) complexes which, besides macrocyclic ligand N′,N′,N″,N‴-tetrakis (2-pyridylmethyl)-l,4,8,11-tetraazacyclotetradecane (tpmc) contain α- or β-aminoisobutyrato ions (α-aibu − and β-aibu −, respectively) were synthesized. The complexes having general formula [Co2(aibu)tpmc](CIO4)3 were characterized by elemental analysis, electronic and 1R spectroscopy, magnetic measurements and cyclic voltammetry. It was assumed that in both complexes an exo coordination of the macrocyclic ligand is achieved. Cobalt(II) ions are bridged with O,O′-aminoisobutyrato ligand. Both complexes are stable against air oxidation, whereas the oxidation of the complex containing the α-aibu− ion by hydrogen-peroxide gave the coresponding mixed-valence Co11/Co111 complex. The complexes are also electrochemically stable in the potential range from-0,6V to 0,4V vs SCE.

Solid state synthesis of extra phase-pure Li4Ti5O12 spinel

Extra phase-pure Li4Ti5O12 spinel with particle sizes less than 500 nm was synthesized by solid state reaction of mechanochemicaly activated mixture of nano anatase and Li2CO3 for a very short annealing time, 4 h at 800 °C. Structural and microstructural properties, the mechanism of solid state reaction between anatase and Li2CO3 as well as thermal stability of prepared spinel were investigated using XRPD, SEM and TG/DSC analysis. The mechanism of reaction implies decomposition of Li2CO3 below 250 oC, formation of monoclinic Li2TiO3 as intermediate product between 400 and 600 °C and its transformation to Li4Ti5O12 between 600–800 oC. The spinel structure is stable up to 1000 oC when it is decomposed due to Li2O evaporation.

MIXED DINUCLEAR CO(II) COMPLEXES WITH N, N', N , N '-TETRAKIS-(2-PYRIDYLMETHYL)-1,4,8,11-TETRAAZACYCLOTETRADECANE AND ALPHA - AND BETA -AMINOCARBOXYLA TO LIGANDS

Abstract Four new mixed complexes of Co(II) with N,N′ N″,N′″-tetrakis(2-pyridylmethyl)-1, 4, 8, 11-tetraazacyclotetradecane (tpmc) and bridged α- or β-aminocarboxylato ligands of general formula [Co2(Y)tpmc](C1O4)3 zH2O where Y = glycinato, S-alaninato, S-aminobutyrato, β-aminobutyrato ion, and z = 0, 0.5 or 1 were isolated. The complexes were characterised by elemental analysis, electronic and IR spectroscopy, magnetic measurements and cyclic voltam-metry. A structure with μ-O, O′-coordination of the aminocarboxylato ligand, and exo coordination of Co(II) ions and tpmc is proposed. The complexes exhibit different electrochemical activities; glycinato and S-alaninato complexes are electrochemically active, whereas S-aminobutyrato and β-aminobutyrato complexes are electrochemically inactive under the given conditions.