Study of structural and magnetic properties of different polymorphs of manganese dioxides

Predmet rada ove doktorske disertacije usmeren je na detaljno ispitivanje strukturnih, morfoloških, i magnetnih svojstava različitih polimorfa mangan-dioksida sintetisanih hidrotermalnom metodom. Proučavani su polimorfi β-MnO2 tetragonalne rutilne strukture i α-KxMnO2 tetragonalne holanditne strukture. Ispitivani materijali dobijeni su različitim postupcima sinteze i oni su ispitivani rendgenostrukturnom analizom, skenirajućom elektronskom mikroskopijom, metodom energetski disperzivne spektrometrije, ramanskom i infracrvenom spektroskopijom, metodom elektronske paramagnetne rezonancije i primenom merenja magnetizacije pomoću SQUID-magnetometra u konstantnom (DC) i promenljivom (AC) spoljašnjem magnetnom polju. Na osnovu merenja skenirajuće elektronske mikroskopije utvrĎeno je da svi ispitivani materijali poseduju morfologiju nanoštapića, što je potvrĎeno i mikrostrukturnom karakterizacijom na osnovu rendgenostrukturnih merenja. Na osnovu merenja vibracione spektroskopije potvrĎene su strukturne karakteristike ispitivanih materijala. Kod β-MnO2 materijala koji poseduje jednostavniju rutilnu strukturu, infracrvenom spektroskopijom detektovana je dodatna traka koja potiče od čestica drugačije veličine i/ili oblika, dok su ramanskom spektroskopijom uspešno detektovane dve dodatne trake koje potiču od strukturno neureĎenog i nestehiometrijskog regiona MnO2-δ na površini nanoštapića. PotvrĎeno je da β-MnO2 materijal ima antiferomagnetni prelaz na 93 K, a za α-KxMnO2 materijale ustanovljeno je kompleksnije niskotemperatursko ponašanje. α-KxMnO2 materijali holanditne strukture ispoljavaju reentrant ponašanje spinskog stakla: stanje spinskog stakla se javlja na temperaturama niţim od temperature na kojoj se javlja slabo feromagnetno ureĎenje. Stanje superspinskog stakla na niskim temperaturama kod oba α-KxMnO2 materijala nedvosmisleno je potvrĎeno kroz ispitivanje magnetnih relaksacija, starenja i memorijskih efekata. Merenja elektronske paramagnetne rezonancije kod β-MnO2 i α-K0,15MnO2 materijala pokazala su prisustvo dva tipa Mn4+ jona koja se nalaze u različitom lokalnom katjonskom okruţenju: (i) Mn4+ joni u čijem su lokalnom katjonskom okruţenju uglavnom samo Mn4+ joni i (ii) Mn4+ joni oko kojih je mešovito Mn4+/Mn3+ okruţenje. Region sa mešovitom valencom mangana kod β-MnO2 materijala potiče od nestehiometrijskog sloja MnO2-δ na površini nanoštapića, a kod α-K0,15MnO2 materijala, lokalno katjonsko okruţenje sa mešovitom valencom mangana javlja se u neposrednoj blizini tunelnih kalijumovih katjona. Rezultati dobijeni u ovoj disertaciji značajni su za bolje razumevanje strukturnih i magnetnih karakteristika nanostrukturnih mangan-dioksida, naročito nedovoljno istraţenih magnetnih karakteristika α-KxMnO2 holanditne strukture.The research topic of this doctoral dissertation is a detailed study of structural, morphological and magnetic properties of different polymorphs of manganese dioxides that are hydrothermally synthesized. Polymorphs β-MnO2 with tetragonal rutile-type structure and α-KxMnO2 with tetragonal hollandite-type structure were studied. Investigated materials have been obtained by different synthesis pathways and they were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman and infrared spectroscopy, electron paramagnetic resonance (EPR) and DC and AC magnetization measurements. SEM images showed nanorod shape of all specimens, while microstructure characterization done by XRD measurements revealed that crystallites have an elongated rod-like shape and hence, it confirmed the anisotropic morphology. Based on vibrational spectroscopic measurements, structural characteristics of investigated materials have been confirmed. In the infrared spectrum of β-MnO2 with a simpler rutile-type structure, the additional band that originates from particles of different shape and/or size has been obtained, while in the Raman spectrum, additional bands that originate from the presence of nonstoichiometric and disordered MnO2-δ on the surface of nanorods, have been obtained. Magnetic ordering of β-MnO2 is antiferromagnetic below 93 K, while α-KxMnO2 materials possess more complicated low-temperature behaviour of the reentrant spin-glass type: the onset of the spin-glass state occurs at temperatures below the weak ferromagnetic ordering. Low-temperature superspin-glass state has been confirmed through the investigations of magnetic relaxations, aging and memory effects. EPR measurements of β-MnO2 and α-K0.15MnO2 show the appearance of two types of Mn4+ ions having different local cationic environment: (i) Mn4+ ions in Mn4+-rich environment and (ii) Mn4+ ions in mixed-valence Mn4+/Mn3+ environment. For β-MnO2 material mixed-valence Mn4+/Mn3+ environment match up with nonstoichiometric MnO2-δ at the surface of the nanorods, while for α-K0.15MnO2 material local mixed-valence Mn4+/Mn3+ environment appears close to the tunnel potassium cations. The results obtained in this dissertation are important for a better understanding of the structural and magnetic properties of nanostructured manganese dioxides, in particular the insufficiently investigated magnetic properties of the α-KxMnO2 with holandite-type structure

Aluminium Underpotential Deposition from AlCl3+NaCl Melts and Alloy Formation with Vanadium Substrate

Aim of this work was to study underpotential deposition of aluminium onto polycrystalline vanadium electrode from equimolar AlCl3+NaCl melt at 473, 523 and 573 K. It was found that aluminium was deposited and incorporated into polycrystalline vanadium electrode at potentials more positive than the Al reverse potential. Applied electrochemical techniques: linear sweep voltammetry and potentiostatic deposition/ galvanostatic striping, showed clear evidence of formation of three intermetallic compounds whose presence depends on temperature and applied deposition time. Deposits were studied via scanning electron microscopy (SEM), energy dispersive spectrometry and X-ray spectroscopy (EDS and EDX), atomic force microscopy (AFM) and X-ray diffraction (XRD)

Copper Electrodeposition onto Palladium from a Deep Eutectic System Based on Choline Chloride

Recently, there has been an increasing interest in developing nonaqueous electrolytes which have been widely employed as an alternative media for a range of metals and metal alloys electrodepositions. A promising and new class of electrolytes among ionic liquids (Ils) are deep eutectic solvents (DESs)1 . The purpose of the copper deposition study from DESs is the application of copper coating and copper alloys in both, industry and fundamental research. In this work, the electrochemical deposition of copper onto palladium working substrate from ChCl/EG (1:2 ratio) DES electrolyte at 50°C was investigated. Additionally, the Cu(II) electroreduction process was studied by potentiodynamic measurements, cyclic voltammetry, chronoamperometry, in the electrolytes with different concentrations of Cu(II) ions ranging from 0.1 M to 0.5 M. The cyclic voltammetry results indicated that the bulk deposition of Cu(II) begins to occur at around –0.080 V vs. Cu. It was found that copper deposition onto the Pd cathode from ChCl:EG electrolyte under potentiostatic conditions is achievable. Data collected from X-ray diffraction (XRD) analysis proved that the cathodic deposits are composed of Cu and CuPd intermetallic. CuPd alloys with different Pd-Cu ratios were prepared by constant potential of –0.100 V vs. Cu from ChCl/EG containing 0.1 M and 0.5 M Cu(II). It is worth noting that the Xray data indicated that the composition of the produced Pd-Cu films could be varied by changing the concentration of Cu(II) ions in the electrolyte or changing the deposition mode

Synthesis, structural and magnetic properties of Y1-xYbxF3 solid solution

Many works devoted to obtaining nanodispersed BaTiO3 powder modified with different dopants for suitable properties providing. In particular, recently considerable attention has been given to obtaining modified nanopowders BaTiO3 possessing relaxor behavior order to ensure reliable work of dielectrics. Generally, Ca,Zr,Mn, ,Pb and rare earth elements such as Nb,Y adds order to provide stress, inhibit grain growth and provide Pinching effect, and hence to increase dielectrics relaxor behavior. However, there is still an issue associated with obtaining satisfactory stoichiometry of the obtained powder. From this viewpoint Ca,Zr-doped BaTiO3 were prepared with co-precipitation method via multiligand complexes formation and influence of the precursor type on Ca,Zr-doped BaTiO3 stoichiometry were investigated. Their stoichiometry, crystal structure was examined in order to determine preferential solubility site of Ca,Zr ions in perovskite structure. Stoichiometry Ca,Zr-modified BaTiO3 will be evaluated considering different precursor type. X-ray, IR spectroscopy and X-ray fluorescence analysis were carried out to obtain the knowledge on the occupation site in the Ba1-xСaxTi1-yZryO3 perovskite structure. These results proved influence complex formation on Ca,Zr-modified BaTiO3 stoichiometry

Investigation on the Electrochemical Behaviour and Deposition Mechanism of Neodymium in NdF3–LiF–Nd2O3 Melt on Mo Electrode

Neodymium was electrochemically deposited from NdF3–LiF–Nd2O3 molten salt electrolyte onto the Mo electrode at temperatures close to 1273 K. Cyclic voltammetry and chronoamperometry measurements were the applied electrochemical methods. Metallic neodymium is obtained by potentiostatic deposition. The optical microscopy and XRD were used to analyze the electrolyte, the working electrode surface, and the deposit on the electrode. It was established that Nd(III) ions were reduced to Nd metals in two steps: Nd(III) + e− → Nd(II) at potential ≈−0.55 V vs. W and Nd(II) + 2e− → Nd(0) at ≈−0.83 V vs. W. Both of these processes are reversible and under mass transfer control. Upon deposition under the regime of relatively small deposition overpotential of −0.10 V to −0.20 V, and after the electrolyte was cooled off, Nd metal was observed at the surface of the Mo electrode. CO and CF4 were gases registered as being evolved at the anode. CO and CF4 evolution were observed in quantities below 600 ppm and 10 ppm, respectivel

Formation of the honeycomb-like MgO/Mg(OH)(2) structures with controlled shape and size of holes by molten salt electrolysis

Synthesis of the honeycomb-like MgO/Mg(OH)(2) structures, with controlled shape and size of holes, by the electrolysis from magnesium nitrate hexahydrate melt onto glassy carbon is presented. The honeycomb-like structures were made up of holes, formed from detached hydrogen bubbles, surrounded by walls, built up of thin intertwined needles. For the first time, it was shown that the honeycomb-like structures can be obtained by molten salt electrolysis and not exclusively by electrolysis from aqueous electrolytes. Analogies with the processes of the honeycomb-like metal structures formation from aqueous electrolytes are presented and discussed. Rules established for the formation of these structures from aqueous electrolytes, such as the increase of number of holes, the decrease of holes size and coalescence of neighbouring hydrogen bubbles observed with increasing cathodic potential, appeared to be valid for the electrolysis of the molten salt used

Electrochemical study of Nd and Pr co-deposition onto Mo and W from molten oxyfluorides

Electrodeposition processes of neodymium and praseodymium in molten NdF3 + PrF3 + LiF + 1 wt.%Pr6O11 + 1 wt.%Nd2O3 and NdF3 + PrF3 + LiF + 2 wt.%Pr6O11 + 2 wt.%Nd2O3 electrolytes at 1323 K were investigated. Cyclic voltammetry, square wave voltammetry, and open circuit potentiometry were applied to study the electrochemical reduction of Nd(III) and Pr(III) ions on Mo and W cathodes. It was established that a critical condition for Nd and Pr co-deposition in oxyfluoride electrolytes was a constant praseodymium deposition overpotential of ≈−0.100 V, which was shown to result in co-deposition current densities approaching 6 mAcm−2 . Analysis of the results obtained by applied electrochemical techniques showed that praseodymium deposition proceeds as a one-step process involving exchange of three electrons (Pr(III)→Pr(0)) and that neodymium deposition is a two-step process: the first involves one electron exchange (Nd(III)→Nd(II)), and the second involves an exchange of two electrons (Nd(II)→Nd(0)). X-ray diffraction analyses confirmed the formation of metallic Nd and Pr on the working substrate. Keeping the anodic potential to the glassy carbon working anode low results in very low levels of carbon oxides, fluorine and fluorocarbon gas emissions, which should qualify the studied system as an environmentally friendly option for rare earth metal deposition. The newly reported data for Nd and Pr metals co-deposition provide valuable information for the recycling of neodymium-iron-boron magnets

Electrochemical deposition of neodymium and praseodymium on molybdenum from molten fluoride

Neodymium and praseodymium were electrochemically co-deposited onto Mo cathode applying constant potential, from fluoride-based molten salts containing the corresponding rare earth oxides. According to the recorded voltammograms, it appears that in the investigated system, the electrodeposition of neodymium proceeds as a two-step reduction process: Nd(III)→Nd(II) and Nd(II)→Nd(0), whilst the praseodymium deposition proceeds as an one-step reduction process: Pr(III)→Pr(0). However, it was also recognized that at the same time a substantial amount of NdF2 was formed as a result of the disproportionation reaction between the electrodeposited Nd metal and Nd(III) present in the electrolyte. The deposit on the working electrode surface was recorded by optical microscopy and analyzed by X-ray diffraction (XRD). The analysis made upon the applying the potentiostatic deposition regimehas shown Nd/Pr metals present on the molybdenum cathode

Copper Electrodeposition onto Palladium from a Deep Eutectic System Based on Choline Chloride

In this work, the electrochemical deposition of copper onto palladium working substrate from ChCl/EG (1:2 ratio) DES electrolyte at 50°C was investigated. Additionally, the Cu(II) electroreduction process was studied by potentiodynamic measurements, cyclic voltammetry, chronoamperometry, in the electrolytes with different concentrations of Cu(II) ions ranging from 0.1 M to 0.5 M

Zn/Au alloys formation by Zn electrodeposition from a deep eutectic system

The electrochemical deposition of Zn onto Au from a choline chloride/ethylene glycol deep eutectic system containing different ZnCl2 concentrations has been investigated. The voltammetric results demonstrated that Zn electrodeposition commences in the zinc underpotential deposition (UPD) and proceeds through to the zinc overpotential deposition (OPD) region. The results of X-ray diffraction analysis revealed that the deposit prepared at a relatively low Zn overpotential of – 0.050 V vs. Zn was composed of cubic AuZn alloy. The morphology of the deposit has been characterized by means of a scanning electron microscope and displays a relatively compact and dendrite-free Zn/Au alloy deposit formed