Mechanochemical synthesis and electrical conductivity of nanocrystalline δ-Bi2O3 stabilized by HfO2 and ZrO2

A powder mixture of α-Bi2O3 and HfO2, in the molar ratio 2:3, was mechanochemically treated in a planetary ball mill under air, using zirconium oxide vials and balls as the milling medium. After 50 h of milling, the mechanochemical reaction led to the formation of a nanocrystalline δ-Bi2O3 phase (fluorite-type solid solution Bi0.78Hf0.59Zr0.63O3.61), with a crystallite size of 20 nm. The mechanochemical reaction started at a very beginning of milling accompanied by an accumulation of ZrO2 arising from the milling tools. The samples prepared after various milling times were characterized by X-ray powder diffraction and DSC analysis. The electrical properties of the as-milled and pressed Bi0.78Hf0.59Zr0.63O3.61 powder were studied using impedance spectroscopy in the temperature range from 100 to 700°C under air. The electrical conductivity was determined to be 9.43×10-6 and 0.080 S cm-1 for the temperatures of 300 and 700°C, respectively.Smeša prahova α-Bi2O3 i HfO2 u molarnom odnosu 2:3 mehanohemijski je tretirana u planetarnom mlinu u atmosferi vazduha, koristeći cirkonijumske posude i kuglice kao medijum za mlevenje. Posle 50 h mlevenja, mehanohemijska reakcija dovodi do stvaranja nanokristalne δ-Bi2O3 faze (čvrsti rastvor fluoritske strukture Bi0,78Hf0,59Zr0,63O3,61), veličine kristalita 20 nm. Mehanohemijska reakcija otpočinje u samom početku mlevenja i praćena je akumulacijum ZrO2 koji potiče od medijuma za mlevenje. Uzorci dobijeni posle različitih vremena mlevenja karakterisani su rendgenskom strukturnom i termijskom analizom. Električna svojstva mlevenih i presovanih Bi0,78Hf0,59Zr0,63O3,61 prahova ispitivana su impedansnom spektroskopijom u temperaturnom opsegu od 100 do 700°C. Dobijena električna provodnost je 9,43·10-6 i 0,080 S cm-1 za temperaturu 300 i 700°C, redom

Phase composition of Bi2O3 specimens doped with Ti, Zr and Hf

Powder mixtures of α-Bi2O3 containing 2, 5 and 10 mole % of TiO2, ZrO2 or HfO2 were homogenized, heated at 820ºC for 24 h and quenched in air. X-ray powder diffraction technique was used to characterize the prepared samples. In all cases metastable Bi2O3 polymorphs, γ-Bi2O3 or β-Bi2O3, are found as single or major phases. Addition of Ti4+ ions stabilizes γ-Bi2O3 polymorph, while either Zr4+ or Hf4+ ions stabilize β-Bi2O3 polymorph. In the samples with 2 and 5 mole % of TiO2 the presence of even two γ-Bi2O3 phases (Bi12TiO20 compound and a very low Ti-doped γ-Bi2O3) was established. Similarly, in the sample with 2 mole % of HfO2 two β-Bi2O3 phases were found. Phase composition of prepared samples, values of unit cell parameters and the appearance of two polymorphs with identical crystal structure but different unit cell parameters are discussed and compared with known data

Uticaj parametara mlevenja na odnos amorfne i kristalne faze u jedinjenju Bi4Ti3O12

Poster presented at the XLVI Savetovanje srpskog hemijskog društva, Beograd, 29. mart (21. februar) 200

Crystal structure of picotpaulite, TlFe2S3, from Allchar, FYR Macedonia

The crystal structure of the mineral picotpaulite, TlFe2S 3, was solved and refined using single-crystal X-ray diffraction data collected at room temperature. The symmetry is orthorhombic, space group Cmcm, with unit-cell parameters: a = 9.083(6), b = 10.754(6), c = 5.412(4) Å, V = 528.6(6) Å3, Z = 4. The structure was refined to the conventional R factor 0.0532 for 226 independent reflections with I > 2σ(I) and 21 variables. Picotpaulite is isostructural with minerals rasvumite (KFe2S3) and pautovite (CsFe2S 3), as well as with a number of synthetic compounds belonging to the CsCu2Cl3 structure type. The structure consists of double chains of FeS4-tetrahedra running along [001] interconnected by TlS10 coordination polyhedra, which form zig-zag chains along the same direction. The Fe-Fe distances between neighbours along the chain direction and perpendicular to it are 2.706(2) and 2.693(6) Å, respectively, indicating strong Fe-Fe interactions. The average oxidation state of Fe is +2.5, achieved by electron transfer over the close Fe sites. Thallium coordination polyhedron can be described as a combination of a square antiprism adjacent to a trigonal prism. The 6s2 electrons of Tl behave as an inert pair and the atom is situated in the centroid of its coordination

Tm(III)-doped d-Bi2O3 for solid - oxide fuel cells

The oxide ion conductors have widely been investigated because of their application in many devices with high economical and ecological interests, such as solid oxide fuel cells (SOFC). -Bi2O3 polymorph possesses the highest known O2– ion conductivity, which is one to two orders of magnitude higher than that of stabilized zirconia at corresponding temperatures [1]. At the moment, the application of this high-temperature polymorph as an electrolyte in SOFC requires temperatures above 730 °C. However, the doping allows -Bi2O3 stabilization to room temperature and opens the possibility for construction of SOFC that will operate at intermediate temperatures (ca. 350 °C) [2]. As it is found that some lanthanides are suitable dopants [3], the possibility to stabilize O2– ion conductors related to the -Bi2O3 polymorph in the Bi2O3–Tm2O3 system was investigated. Two starting mixtures with compositions (Bi1–xTmx )2O3 (x = 0.04 and 0.20) were homogenized in an agate mortar, heat treated at 750 °C for 3 h and then slowly furnace cooled. The samples were characterized by XRPD, DTA and SEI techniques. Based on XRPD, the single-phase tetragonal β-Bi2O3 was identified in the sample with x = 0.04. Its unit cell parameters, a = 7.742(2) and c = 5.650(2) Å, well-correspond to those of undoped β-Bi2O3 [4]. On the other hand, the cubic -Bi2O3 phase was obtained in the sample with x = 0.20. Its unit cell parameter was greater than the value reported for Tm-doped -Bi2O3 sample with x = 0.25 [3] (5.5033(9) vs. 5.478 Å). Both values are smaller than reported for undoped -Bi2O3 [4]. This means that the unit cell parameter of cubic -Bi2O3 decreases as Tm-content increases and it is in accordance with Tm3+ and Bi3+ ionic radii [5]. For the sample with x = 0.04, cyclic DTA curves showed one reversible β-Bi2O3 ↔ -Bi2O3 transition with corresponding temperatures: on heating, 660 °C, and, on cooling, 600 °C. Surprisingly, no phase transitions were observed in the sample with x = 0.20 which indicates that the obtained -Bi2O3 is stable within the whole investigated interval, i.e., from room temperature to 1000 °C. Electrochemical impedance of -Bi2O3 phase was measured in the following temperature range: 300 – 800 °C. At higher temperatures (600 – 800 °C) the conductivities are similar (0.11 – 0.32 S cm –1 ), but with lowering temperature they rapidly decrease, and amount, for example, 2.1·10–5 S cm–1 at 300 °C. As a consequence, two activation energies are found: 0.45(4) eV (600 – 800 °C), and 1.33(2) eV (300 – 600 °C). References: [1] P. Shuk, H.-D. Wiemhöfer, U. Guth, W. Göpel, M. Greenblatt, Solid State Ionics 89 (1996) 179 [2] E. D. Wachsman, K.T. Lee, Science 334 (2011) 935 [3] H. T. Cahen, T. G. M. Van Den Belt, J. H. W. De Wit, G. H. J. Broers, Solid State Ionics 1 (1980) 411 [4] H. A. Harwig, Z. anorg. allg. Chem. 444 (1978) 151 [5] R. D. Shannon, Acta Cryst. A 32 (1976) 75

Hydrothermal treatment of nanoanatase with alkali and alkaline earth hydroxides

Nanoanatase powder was hydrothermally treated with LiOH, NaOH, KOH or Ca(OH)2 at T = 120 ºC for 8 h. In all cases, the molar TiO2 : hydroxide ratio was identical. The obtained samples were washed with distilled water, centrifuged and dried at room temperature. Characterization of the samples was done by X-ray powder diffraction, TG/DTA/DSC and SEM analysis. Nanoanatase showed very diverse reactivity toward different alkali and alkaline earth cations. The final products showed different phase composition, crystallinity and microstructure. With Li+ and Ca2+ anatase forms different titanate structures while with Na+ and K+ it mainly retain original anatase structure

HIGHLY CONDUCTIVE LANTHANOIDE STABILIZED δ-Bi2O3 PHASES

Due to the increasing demands for new highly efficient and environmentally friendly energy conversion technologies, the oxide ion conductors applicable in solid oxide fuel cells (SOFCs) have widely been investigated. The aim is to find a suitable electrolyte with the ionic conductivity high enough at intermediate temperatures in order to reduce the operating temperature of SOFCs. The high temperature δ-Bi2O3 phase has been proposed as a good candidate for electrolyte in SOFCs because it is the fastest known ionic conductor. In this study, the possibility to stabilize O 2– ion conductors related to the -Bi2O3 polymorph in Bi2O3–Tm2O3 and Bi2O3–Lu2O3 systems was investigated. Six starting mixtures with the following compositions (Bi1–xTmx)2O3, x = 0.11, 0.14 and 0.20, and (Bi1–yLuy)2O3, y = 0.15, 0.20 and 0.25, were dry homogenized in an agate mortar, heat treated at 750 °C for 3 h and then slowly furnace cooled. The samples were characterized by XRD, TEM/SAED, SEM, DTA and SEI techniques. Based on XRD and TEM/SAED, the targeted cubic δ-Bi2O3 single-phase samples (space group Fm3m ) were successfully obtained within all six systems. The unit cell parameter of both Tm- and Lu-doped -Bi2O3 decreases as dopant content increases. By comparing Tm- and Lu-doped δ-Bi2O3 phases mutually, an expected increase of the unit cell parameters with larger ionic radii of dopant was found [ri(Tm3+) = 0.88 Å, and ri(Lu3+) = 0.86 Å in the octahedral environment1 ]. Electrochemical impedance of -Bi2O3 phases was measured between 300 and 800 °C. At temperatures 550 – 800 °C the conductivities are of the same order of magnitude (0.1 – 0.4 S cm –1 ), but with lowering temperature they rapidly decrease resulting in two activation energies. This is due to the changes in conductivity mechanism which will be discussed.According to the cyclic DTA curves, no phase transitions were observed in the following samples: (Bi0.8Tm0.2)2O3, (Bi0.8Lu0.2)2O3 and (Bi0.75Lu0.25)2O3, indicating that these -Bi2O3 phases are stable within the whole investigated interval, i.e., from room temperature to 985 °C. This means that the application of these electrolyte materials could result not only in the significant enhancement of IT-SOFC electrochemical performance, but also in their good structural stability over long time service in a wide temperature range. 1. R. D. Shannon, Acta Cryst. A, 32 (1976) 75

ZnO varistors prepared by direct mixing of constituent phases

ZnO varistors with non-linearity coefficients reaching 50 and low values of the leakage current (approximately 0.01 A/m2), were obtained by a method referred to as `direct mixing of constituent phases'. The method is based on sintering of a mixture of the constituent phases with the following compositions: ZnO phase uniformly doped with Co2+ and Mn2+; intergranular phase: γ-Bi2O3 stabilized with Mn2+ or Zn2+; spinel phase: Zn1.971Ni0.090Co0.030Cr0.247Mn0.090Sb0.545O4. As proved, the obtained varistors are composed of the same phases as those introduced into the system. The influence of composition as well as milling and sintering parameters on the varistor characteristics were investigated. Some varistor powder mixtures were intensively milled before sintering. As a result, high density ceramics with highly homogeneous microstructure were obtained after sintering

Towards understanding intermolecular interactions in hydantoin derivatives: the case of cycloalkane-5-spirohydantoins tethered with a halogenated benzyl moiety

A series of cycloalkane-5-spirohydantoins bearing a halogeno substituted benzyl group (X = Cl and Br) in position 3 has been synthesized and their structures (1-6) have been determined by a single crystal X-ray diffraction method. These compounds have multiple functional groups, which allow greater competition and/ or cooperation among the different intermolecular interactions in the formation of their crystal structures. The molecules are linked together by paired N-H... O hydrogen bonds in R22(8) rings, while the CH. O interactions lead to their further association into double chains. The contribution of the cycloalkyl ring depends on its conformational flexibility and the multiple C-H donor implications. In the case of compounds 1-4 bearing the cyclopentyl or the cyclohexyl ring, halogen bonding (X...O) interactions give rise to a supramolecular pseudo-hexagonal network. In addition, the C-H... X interactions with a higher degree of multifurcation at the halogen acceptor have an important role in the formation of the crystal structure. Regarding compounds 5 and 6 with the cycloheptane ring, the X. O interaction is absent, and along with the C-H. X interactions, these compounds realize an alternative crystal structure with an emphasis on the X. p interactions. The lattice energies of all these crystal structures, as well as the intermolecular pair energies, have been calculated using PIXEL and further partitioned into coulombic, dispersive, polarization and repulsive factors. The crystal structures have also been subjected to Hirshfeld surface analysis which reveals that approximately 75% of the close contacts correspond to relatively weak interactions. The application of both concepts has provided a new insight into the relationship between the molecular interactions and crystal structures of the hydantoin derivatives.This is the peer-reviewed version of the following article: Crystengcomm, 2017, 19, 3, 469-483 [https://dx.doi.org/10.1039/C6CE02210C][http://cer.ihtm.bg.ac.rs/handle/123456789/2204

Mikrotalasna sinteza i karakterizacija Pt i Pt-Rh-Sn katalizatora za oksidaciju etanola

Carbon-supported Pt and Pt-Rh-Sn catalysts were synthesized by the microwave-polyol method in ethylene glycol solution and were investigated in the ethanol electro-oxidation reaction. The catalysts were characterized in terms of structure, morphology and composition employing the X-ray diffraction (XRD), scanning tunneling microscopy and energy-dispersive X-ray spectroscopy techniques. The STM analysis indicated rather uniform particles and particle sizes below 2 nm for both catalysts. The XRD analysis of the Pt/C catalyst revealed two phases, one with the main characteristic peaks of the face-centered cubic crystal structure (fcc) of platinum and the other related to the graphite-like structure of the carbon support, Vulcan XC-72R. However, in the XRD pattern of the Pt-Rh-Sn/C catalyst, diffraction peaks for Pt, Rh or Sn could not be resolved, indicating extremely low crystallinity. The small particle sizes and homogeneous size distributions of both catalysts could be attributed to the advantages of the microwave-assisted modified polyol process in ethylene glycol solution. The Pt-Rh-Sn/C catalyst was highly active for ethanol oxidation with the onset potential shifted by more than 150 mV to more negative values and with currents nearly 5 times higher in comparison to the Pt/C catalyst. The stability tests of the catalysts, as studied by chronoamperometric experiments, revealed that the Pt-Rh-Sn/C catalyst was evidently less poisoned than the Pt/C catalyst. The increased activity of Pt-Rh-Sn/C in comparison to Pt/C catalyst was most probably promoted by the bi-functional mechanism and the electronic effect of the alloyed metals.Pt i Pt-Rh-Sn katalizatori na ugljeniku razvijene površine su sintetizovani poliol-mikrotalasnim postupkom u rastvoru etilenglikola i ispitivani za reakciju eletrohemijske oksidacije etanola u kiseloj sredini. Katalizatori su okarakterisani strukturno, morfološki i po sastavu korišćenjem XRD, STM i EDX tehnika. STM analiza je potvrdila da su Pt i Pt-Rh-Sn čestice uniformne veličine i prečnika manjeg od 2 nm. XRD analiza Pt/C katalizatora pokazala je prisustvo dve faze, jedne sa glavnim karakterističnim pikovima za pljosno-centriranu kubnu kristalnu strukturu platine (111, 200, 220 i 311) i druge sa difrakcionim pikom na 2q oko 25° karakterističnim za heksagonalnu strukturu vulkana XC-72R (ugljeničnog nosača). XRD analiza Pt-Rh-Sn/C katalizatora nije pokazala karakteristične pikove, što je indikacija veoma male kristaličnosti katalizatora. Aktivnost katalizatora ispitivana je potenciodinamičkim i hronoamperometrijskim merenjima. Pt-Rh-Sn/C katalizator je veoma aktivan za oksidaciju etanola sa početkom reakcije na potencijalima za oko 150 mV pomerenim ka negativnijim vrednostima i strujama koje su oko pet puta veće u poređenju sa Pt/C katalizatorom. Stabilnost katalizatora ispitivana hronoamperometrijski pokazala je da se Pt-Rh-Sn/C katalizator manje truje od Pt/C katalizatora. Mala veličina i homogena distribucija čestica mogu se pripisati prednostima mikrotalasne sinteze i modifikovanog poliol postupka u rastvoru etilenglikola. Veća aktivnost Pt-Rh-Sn/C katalizatora u poređenju sa Pt/C katalizatorom posledica je bi-funkcionalnog mehanizma i elektronskog (ligand) efekta metala u sintetizovanoj leguri