Search Results (11)

Spinel-structured hausmannite (Mn(II)Mn(III)₂O₄) is a vital intermediate in Mn mineralogy and a key player in redox chemistry in the environment. Its transformation into other Mn oxides is a critical factor in controlling its environmental occurrence and reactivity. Yet structural impurities and solution pH, as well as the fate of impurities during transformation, which influence hausmannite transformation processes and products, remain largely unknown. In the present work, we address this knowledge gap by investigating pristine and metal-substituted hausmannite, specifically nickel (Ni) or cobalt (Co), equilibrated at two time periods (8 h and 30 days) and three different pH levels (4, 5, and 7). Solution chemistry data revealed that both the equilibration period and pH had a significant impact on hausmannite dissolution rates and the concomitant repartitioning of Ni or Co. Hausmannite with Ni or Co substitution exhibited lower dissolution rates than pristine mineral under acidic conditions. Mineralogy and crystal chemistry data indicated that hausmannite was the major host phase after 30-day equilibration, followed by minor transformed products, including birnessite and manganite. Although minor, birnessite became more abundant than manganite at low pHs. Analytical high-resolution transmission electron microscopy (HRTEM) analyses revealed a poorly crystalline, nano-scaled MnO₂ formed from hausmannite and the majority of metal impurities remaining in the host hausmannite. Yet Co was associated with both hausmannite and the newly formed birnessite, whereas Ni was only found with hausmannite, indicating the strong sequestration of Co by Mn(II/III) and Mn(IV) mineral phases. This study highlights the significant impacts of metal impurities and pH on the stability of hausmannite and its transformation into birnessite, as well as the control of Mn-oxide minerals on the solubility and sequestration of transition metals in the environment. Full article

For the first time, neodymium nickel manganites NdNa₂NiMnO₅ and NdK₂NiMnO₅ were synthesized via the solid-state interaction method, and they crystallize in a cubic system. Using experimental dynamic calorimetry in the temperature range of 298.15–673 K, the temperature dependences of the heat capacity of NdNa₂NiMnO₅ and NdK₂NiMnO₅ were studied. At 423 K, both compounds exhibited anomalous heat capacity jumps on the C⁰_p~f(T) dependency, likely corresponding to second-order phase transitions. Considering the phase transition temperatures, equations for the temperature dependence of heat capacity were derived, accurately describing the experimental data. Based on the experimental C⁰_p(T) data and calculated S⁰ (298.15) values, temperature dependences of C⁰_p(T) and the thermodynamic functions S⁰(T), H°(T)–H⁰(298.15), and Φ^xx(T) were determined for the studied compounds within the 298.15–673 K range. The analysis of electrophysical data confirmed the semiconducting and metallic nature of the conductivity, as well as identified the band gap and activation energy of conductivity. These results are valuable for the application of these materials in electronics and for controlling conductivity. Full article

A wide class of materials with different crystal and electronic structures including quasi-2D unconventional superconductors, such as cuprates, nickelates, ferropnictides/chalcogenides, ruthenate Sr${}_2$RuO${}_4$, and 3D systems, such as manganites RMnO${}_3$, ferrates (CaSr)FeO${}_3$, nickelates RNiO${}_3$, silver oxide AgO, are based on Jahn–Teller $3d$ and $4d$ ions. These unusual materials, called Jahn–Teller (JT) magnets, are characterized by an extremely rich variety of phase states, spanning from non-magnetic and magnetic insulators to unusual metallic and superconducting states. The unconventional properties of JT magnets can be attributed to the instability of their highly symmetric Jahn–Teller “progenitors” with the ground orbital E-state with repect to charge transfer, anti-Jahn–Teller d-d disproportionation, and the formation of a system of effective local composite spin–singlet or spin–triplet, electronic, or hole S-type bosons moving in a non-magnetic or magnetic lattice. We consider specific features of the anti-JT-disproportionation reaction, properties of the electron–hole dimers, possible phase states and effective Hamiltonians for single- and two-band JT magnets, concluding with a short overview of physical properties for actual JT magnets. Full article

Copper-substituted nickel manganites Ni_(1−x)Cu_xMn₂O₄ (Ni-TCE-NPs) were produced by co-precipitation route (sol–gel) at room temperature. Ni_(1−x)CuxMn₂O₄-Bio (NCB) NPs were studied by powder X-ray diffraction technique, scanning electron microscopy and Raman spectroscopy. XRD spectra authenticated the copper-doped nickel manganites’ formation with particle size 23–28 nm. A significant decrease in the lattice parameter confirmed the doping of copper ions into the nickel manganites. Microscopy (SEM) was used to estimate the grain size, shape and uniformity, revealing the non-uniform agglomerated polygon and plate-like microstructure. The NCB-NPs showed anticoagulant activity by enhancing the coagulation time of citrated plasma of human beings. NCB-NPs with x = 0.35 and 0.45 have increased clotting time from control 133 ± 4 s to 401 ± 7 s and 3554 ± 80 s, respectively, and others around 134 s. Additionally NCB-NPs with x = 0.35, 0.45 inhibited the platelet aggregation by 80% and 92%, while remaining inhibited with only 30%. NCB-NPs did not show hemolytic activity in RBC cells intimate its non-toxic nature. Finally, NCB-NPs were non-toxic and known to exhibit anti-blood-clotting and antiplatelet activities, which can be used in the field of biomedical applications, especially as antithrombotic agents. Full article

Nanocrystalline nickel manganite (NiMn₂O₄) powder with a pure cubic spinel phase structure was synthesized via sol-gel combustion and characterized with XRD, FT-IR, XPS and SEM. The powder was mixed with sodium alginate gel to form a nano-biocomposite gel, dried at room temperature to form a thick film and characterized with FT-IR and SEM. DC resistance and AC impedance of sensor test structures obtained by drop casting the nano-biocomposite gel onto test interdigitated PdAg electrodes on an alumina substrate were measured in the temperature range of 20–50 °C at a constant relative humidity (RH) of 50% and at room temperature (25 °C) in the RH range of 40–90%. The material constant obtained from the measured decrease in resistance with temperature was determined to be 4523 K, while the temperature sensitivity at room temperature (25 °C) was −5.09%/K. Analysis of the complex impedance plots showed a dominant influence of grains. The decrease in complex impedance with increase in temperature confirmed the negative temperature coefficient effect. The grain resistance and grain relaxation frequency were determined using an equivalent circuit. The activation energy for conduction was determined as 0.45 eV from the temperature dependence of the grain resistance according to the small polaron hopping model, while the activation energy for relaxation was 0.43 eV determined from the Arrhenius dependence of the grain relaxation frequency on temperature. Full article

Nickel manganite nanocrystalline fibers were obtained by electrospinning and subsequent calcination at 400 °C. As-spun fibers were characterized by TG/DTA, Scanning Electron Microscopy and FT-IR spectroscopy analysis. X-ray diffraction and FT-IR spectroscopy analysis confirmed the formation of nickel manganite with a cubic spinel structure, while N₂ physisorption at 77 K enabled determination of the BET specific surface area as 25.3 m²/g and (BJH) mesopore volume as 21.5 m²/g. The material constant (B) of the nanocrystalline nickel manganite fibers applied by drop-casting on test interdigitated electrodes on alumina substrate, dried at room temperature, was determined as 4379 K in the 20–50 °C temperature range and a temperature sensitivity of −4.95%/K at room temperature (25 °C). The change of impedance with relative humidity was monitored at 25 and 50 °C for a relative humidity (RH) change of 40 to 90% in the 42 Hzπ1 MHz frequency range. At 100 Hz and 25 °C, the sensitivity of 327.36 ± 80.12 kΩ/%RH was determined, showing that nickel manganite obtained by electrospinning has potential as a multifunctional material for combined humidity and temperature sensing. Full article

The objective of this paper is to manufacture free-standing solid oxide cells (SOCs) through the atmospheric plasma spray process (APS), without the aid of a metallic support nor the need for a post-process heating treatment. A five-layered cell was fabricated. Fused and crushed yttria-stabilized zirconia (YSZ) powder in the 5–22 μm particle size range was used in order to achieve a dense electrolyte layer, yet still permitting satisfactory ionic diffusivity. Nickel oxide (NiO) powder that was obtained by in-house flame spray (FS) oxidation of pure nickel (Ni) powder was mixed and sprayed with the original Ni-YSZ feedstock, so as to increase the porosity content in the supporting electrode. Two transition layers were sprayed, the first between the support electrode and the electrolyte (25% (Ni/NiO)–75% YSZ) and the second at the electrolyte and the end electrode interface (50% YSZ–50% lanthanum strontium manganite (LSM)). The purpose of intercalation of these transition layers was to facilitate the ionic motion and also to eliminate thermal expansion mismatches. All the as-sprayed layers were separately tested by an in-house developed acetone permeability comparative test (APCT). Electrodes with adequate porosity (25–30%) were obtained. Concerning electrolytes, relatively thick (150–200 µm) layers derived from fused and crushed YSZ were found to be impermeable to acetone, while thinner YSZ counterparts of less than 100 µm showed a low degree of permeability, which was attributed mostly to existent microcracks and insufficient interparticle cohesion, rather than to interconnected porosity. Full article

Toward the development of NTCR thermistors, nanocrystalline Mn–Ni–Cu–O powder was synthesized from a mixed chloride aqueous solution by a simple co-precipitation method.The introduction of an oxidizing agent (H₂O₂) into the solution led to the partial oxidation of Mn²⁺ ions into Mn³⁺ ions, which enabled the collected powder to be well crystallized at 650 °C. Such a low calcining temperature resulted in fine particles with a mean size of 60 nm, which significantly promoted densification of the resulting ceramics. As a result, a dense and homogenous microstructure with a relative density up to 97.2% was achieved for pellets sintered at 1100 °C. Furthermore, these sintered ceramics exhibited a room temperature resistivity (ρ₂₅) of 67 Ω·cmand a thermistor constant (B_25/85) of 2843 K, which make them suitable for use in industrial thermistors. In addition, electrical stability was greatly improved when the ceramics were prepared by a new two-step sintering method. The results suggest that the co-precipitation route with the introduction of H₂O₂ is suitable for the fabrication of cubic spinel thermistor nanopowders. Full article

This study focuses on two different techniques for preparation of nickel manganite (NiMn₂O₄) system. One approach is conventional ceramic method, and the second is based on replacing the ceramic route with green synthesis mediated by egg white. The goal of this strategy is produce a single nanomagnetic phase of NiMn₂O₄ using a mostly simple one-step method with specific characteristics as seen in the second route compared to the ceramic method. The as synthesized system was characterized by using various techniques such as X-ray diffraction (XRD), scanning electron micrographs (SEM), and Energy dispersive X-ray spectrometry (EDS). XRD, EDS and SEM analyses confirm a successful synthesis of NiMn₂O₄ single phase with cubic spinel and sponge crystal structures. The particles are polycrystalline in their nature and average crystallite size ranged between 76 and 90 nm. Egg white assisted combustion method imparted amelioration in the system crystallization, size of grain, distribution of cation and magnetic properties of the as prepared materials. The magnetic mensuration suggested that the obtained nickel manganite shows ferromagnetism at room temperature with an optimum value (3.56 emu/g) of saturation magnetization. Full article

A nickel-ceria-yttria stabilized zirconia (Ni-CYSZ) cermet material was synthesized and tested as the anode for the direct oxidation of methane in a solid oxide fuel cell (SOFC) with YSZ as the electrolyte and strontium-doped lanthanum manganite (LSM) as the cathode. Initially, the electrochemical behavior was investigated under several load demands in wet (3% H₂O) CH₄ at 850 °C during 144 h using I-V curves, impedance spectra, and potentiostatic measurements. Long-term tests were subsequently conducted under 180 mA·cm^–2 in wet CH₄ for 236 h and dry CH₄ for 526 h at 850 °C in order to assess the cell stability. Material analysis was carried out by SEM-EDS after operation was complete. Similar cell performance was observed with wet (3% H₂O) and dry CH₄, and this indicates that the presence of water is not relevant under the applied load demand. Impedance spectra of the cell showed that at least three processes govern the direct electrochemical oxidation of methane on the Ni-CYSZ anode and these are related to charge transfer at high frequency, the adsorption/desorption of charged species at medium frequency and the non-charge transfer processes at low frequency. The cell was operated for more than 900 h in CH₄ and 806 h under load demand, with a low degradation rate of ~0.2 mV·h^–1 observed during this period. The low degradation in performance was mainly caused by the increase in charge transfer resistance, which can be attributed to carbon deposition on the anode causing a reduction in the number of active centers. Carbon deposits were detected mostly on the surface of Ni particles but not near the anode/electrolyte interface or the cerium surface. Therefore, the incorporation of cerium in the anode structure could improve the cell lifetime by reducing carbon formation. Full article

Mechanical damage is a major factor limiting the long-term stability of solid oxide fuel cells (SOFCs). Here, the mechanical stability of planar SOFCs consisting of Ni-YSZ anode/YSZ electrolyte/LSM-YSZ cathode (Ni=Nickel, YSZ=yttria-stabilized zirconia, LSM=lanthanum strontium manganite) is analyzed by a structural mechanics model with composition dependent mechanical properties. Influencing factors considered include: the Ni and LSM volume fractions, the thicknesses of anode, cathode and electrolyte layers, and the cell types of anode-, cathode-, and electrolyte-supported designs. It is found that (i) the anode failure probability increases with the Ni content. However, SOFCs remain mechanically safe if the Ni volume fraction is below 65%. (ii) An LSM volume fraction of over 40% is required to maintain the mechanical integrity of cathode. (iii) For an anode-supported cell with a 20 μm thick electrolyte, the anode thickness should be more than 0.5 mm to be mechanically stable. (iv) The anode-supported cell is found to be mechanically safer than that of the electrolyte-supported cell. Full article