Niobium Oxide Layers Research Articles

Complementary Resistive Switching in Niobium Oxide-Based Resistive Memory Devices

For the applications of resistive random access memory (RRAM), we study the complementary resistive switch (CRS) behavior of a bilayer Nb2O5-∞/NbOy RRAM. The CRS effect is explained by the redistribution of oxygen vacancies inside the two niobium oxide layers. Improved CRS effects were observed using W top electrode (TE) instead of Pt, which can be attributed to the oxygen barrier layer derived from a self-formed WO∞ layer between the W TE and the Nb2O5-∞ oxide film. The niobium oxide-based CRS devices within a single memory cell can be directly integrated into a crossbar memory array without the need of extra diodes; this can significantly reduce the fabrication complexity.

NaF 전해질 양극산화에 의한 마이크로콘 구조 니오븀 산화물 제조

In this study, we show that by anodization of Nb in NaF electrolytes microcone niobium oxide layers can be formed under a range of experimental conditions. It is found that a single NaF electrolyte leads to the formation of microcones. At 1 M NaF, 40 V, 1 h, well-ordered microcones were generated on Nb discs. XRD results show that the initially formed anodic oxide is amorphous, but an amorphous to crystalline transition occurs during anodization. For the formation of favorable microcones, it is considered that proper parameters such as electrolyte concentration, voltage, anodizing time are necessary according to the kind of electrolytes.

Preparation and characterization of Nb2O5–Al2O3 composite oxide formed by cathodic electroplating and anodizing

► We fabricate Nb 2 O 5 –Al 2 O 3 /Al film for high performance thin film capacitor. ► The optimum condition of electrolyte composition will coat NbO x on Al without corrosion of Al during the cathodic electroplating. ► Increasing annealing temperature will form Nb 2 O 5 crystalline. ► The Al 2 O 3 layer will form between Nb 2 O 5 layer and metal Al after anodizing and the thin film capacitor with Nb 2 O 5 –Al 2 O 3 /Al improve dielectric properties. Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb 2 O 5 –Al 2 O 3 composite oxide was approximately 27% higher than that of Al 2 O 3 without a Nb 2 O 5 layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb 2 O 5 –Al 2 O 3 composite oxide film is a suitable material for thin film capacitors.

Characterization and performance of UNS S63019 (21-4N) as bipolar plate material in a simulated polymer electrolyte membrane fuel cell environment

The austenitic stainless steel UNS S63019 was evaluated regarding its potential as bipolar plate material in a polymer electrolyte membrane fuel cell (PEMFC) environment. Segregated grains of niobium carbide (NbC x ) were identified in polished cross-sections of the alloy, offering a possible pathway for enhanced electrical conductivity through the passive surface oxide. Additionally, the alloy was tested for corrosion resistance in a simulated PEMFC environment. It was considered that perhaps the elevated nitrogen concentration in the alloy would provide some benefit for corrosion resistance. Results for interfacial contact resistance (ICR) testing of the air-formed surface film on UNS S63019 showed decreased electrical conductivity as compared to UNS S30400. Niobium carbide particles did not improve film conductivity due to a non-conductive niobium oxide layer that formed on the surface. Corrosion resistance of the alloy was also poor as compared with UNS S30400, demonstrating that elevated nitrogen concentration in the alloy was not adequate in itself to enhance corrosion resistance. Poor corrosion resistance was attributed primarily to high carbon content in the alloy which combined with a significant amount of chromium to form carbides.

Electrochemical synthesis, characterization and thermal properties of niobium ethoxide

Niobium(V) ethoxide (Nb(OEt)5) was synthesized by electrochemical reaction of ethanol with niobium plate as the sacrificial anode, stainless steel as the cathode and tetraethylammonium chloride (TEAC) as the conductive additive. The condensates were isolated by vacuum distillation under 5 kPa. The product was characterized by Fourier transform infrared (FT-IR) spectra, Raman spectra and nuclear magnetic resonance (NMR) spectra. The results indicate that the product is niobium ethoxide. Thermal properties of niobium ethoxide were analysed by TG/DTG. Vapour pressure was calculated from the Langmuir equation and the enthalpy of vaporization was calculated from the vapour pressure-temperature data using the Clausius-Clapeyron equation. The concentrations of impurity metallic elements in the sample were detected by ICP-MS. It is shown that the purity can reach 99.997%. The volatility and purity of the niobium ethoxide ensure that it could be a good precursor for chemical vapor deposition and atomic layer deposition of niobium oxide layers.

Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells

Nanostructured porous zinc oxide electrodes for use in dye-sensitized solar cells (DSSCs) were coated with thin niobium oxide layers by using sol–gel transformation of niobium pentaethoxide in air. Coating solutions were prepared by mixing niobium pentaethoxide and ethanol. A dip-coating technique was adopted at a low withdrawal speed of 100μms−1. The coated electrodes were then heat-treated at temperatures between 400 and 600°C. The presence of niobium in the coated electrodes was confirmed by X-ray photoelectron spectroscopy. As expected, the niobium oxide layers worked as an energy barrier between the ZnO electrode and electrolyte. Open-circuit voltage (VOC) of the cells using the coated electrodes was then enhanced up to 0.768V, which was attributable to the suppression of the recombination of photogenerated electrons with oxidized species in electrolytes. An additional benefit of the coating was that grain growth of ZnO particles in the electrodes was hindered and short-circuit photocurrent density (JSC) was kept relatively high due to large amounts of adsorbed dye. An overall light-to-electricity conversion efficiency was increased to a maximum of 5.19%, indicating that the proper coating technique was the key for improving the performance of ZnO-based DSSCs.

A series of new phases in the alkali metal–NbV/TaV–SeIV/TeIV–O systems

Six new phases in the alkali metal-Nb(V)/Ta(V)-Se(IV)/Te(IV)-O systems have been prepared by solid-state reactions at high-temperatures. Their structures were determined by single-crystal X-ray diffraction studies. AM(3)O(6)(QO(3))(2) (A = K, Rb, M = Nb, Ta, Q = Te; A = K, M = Nb, Q = Se) are isomorphous and their structures feature a 3D network with 1D 4- and 6-MRs tunnels along the a-axis which is composed of 2D layers of corner-sharing MO(6) octahedra bridged by QO(3) groups. The alkali metal ions are located at the above 1D tunnels of 6-MRs. The structure of Cs(3)Nb(9)O(18)(TeO(3))(2)(TeO(4))(2) features a thick Nb-Te-O layer built of corner-sharing NbO(6) octahedra, TeO(3) and TeO(4) groups. The 2D layer of the NbO(6) octahedra with 1D tunnels of 6-MRs along the c-axis are formed by 1D chains of NbO(6) chains along the c-axis and linear Nb(4)O(21) tetramers by corner-sharing. The TeO(3) and TeO(4) groups are grafted on both sides of the niobium-oxide layer via Nb-O-Te or/and Te-O-Te bridges. The caesium(i) ions are located at the above 1D tunnels of 6-MRs. TGA, UV-vis and infrared spectral measurements as well as electronic structure calculations have also been performed.

Synthesis and Characterization of Open-Framework Niobium Silicates: Rb2(Nb2O4)(Si2O6)·H2O and the Dehydrated Phase Rb2(Nb2O4)(Si2O6)

A new niobium(V) silicate, Rb(2)(Nb(2)O(4))(Si(2)O(6)).H(2)O, has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and solid-state NMR spectroscopy. It crystallizes in the tetragonal space group P4(3)22 (No. 95) with a = 7.3431(2) A, c = 38.911(3) A, and Z = 8. Its structure contains tetrameric units of the composition Nb(4)O(18), which share corners to form a layer of niobium oxide. The Nb-O layer is a slice of the pyrochlore structure. Neighboring Nb-O layers are linked by vierer single-ring silicates generating eight-ring and six-ring channels running parallel to <100> directions, in which the Rb(+) cations and water molecules reside. The tantalum analogue was prepared and characterized by powder X-ray diffraction. Upon heating to 500 degrees C, Rb(2)(Nb(2)O(4))(Si(2)O(6)).H(2)O loses lattice water molecules, while the framework structure is retained to give the anhydrous compound Rb(2)(Nb(2)O(4))(Si(2)O(6)), whose structure was also characterized by single-crystal X-ray diffraction. The dehydrated sample absorbs water reversibly, as indicated by powder X-ray diffraction. Rb(2)(Nb(2)O(4))(Si(2)O(6)) crystallizes in the tetragonal space group I4(1) (No. 80) with a = 10.2395(6) A, c = 38.832(3) A, and Z = 16.

Preparation of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films Via a Polyvinylpyrrolidone‐Assisted Aqueous Sol–Gel Process and Dielectric Properties

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) thin films were prepared by spin coating using aqueous solutions of metal salts containing polyvinylpyrrolidone, where niobium oxide layers and lead—magnesium–titanium oxide layers were laminated on Pt(111)/TiOx/SiO2/Si(100) substrates and fired at 750° or 800°C. 250 ± 20 nm thick 0.7PMN–0.3PT thin films of a single‐phase perovskite could be prepared, and the film fired at 750°C had dielectric constants and dielectric loss of 1900 ± 350 and 0.13 ± 0.03, respectively, exhibiting polarization‐electric field hysteresis with a remanent polarization of 5.1 μC/cm2 and a coercive field of 21 kV/cm.

EELS Investigations of Reference Niobium Oxides and Anodically Grown Niobium Oxide Layers

Extract Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Microstructure and dielectric properties of nanoscale oxide layers on sintered capacitor-grade niobium and V-doped niobium powder compacts

Abstract Electrolytic anodization was used to form amorphous niobium oxide layers on niobium and V-doped niobium powder compacts which act as dielectric layers, e.g. in niobium-based solid electrolyte capacitors. The microstructure development within the layered structure niobium-niobium oxide was studied by scanning- and transmission electron microscopy in order to investigate the influence of processing parameters. It could be shown that the thickness as well as the quality of the oxide layers on niobium vary in a considerable range, depending on processing parameters. Examination of the influence of heat treatments on the structural properties of the oxide layers revealed remarkable changes at the niobium – niobium oxide interface upon heat impact exceeding 300°C. In contrast, the interface remains stable with respect to the as-anodized sample when annealing was performed at lower temperature.

Grey, brown and blue coloring sol–gel electrochromic devices

Pure and doped niobium oxide (Nb 2O 5) layers are electrochromic (EC) materials which change their color by insertion of Li + ions from transparent to brown, grey or blue depending on the crystallinity of the layer. EC-devices with the configuration K-glass/EC-layer/composite electrolyte/ion-storage (IS) layer/K-glass, were produced using different Nb 2O 5 EC-layers, a (CeO 2) x (TiO 2) 1− x ( x = 0.45) IS-layer and an inorganic–organic composite electrolyte to which a small amount of water (up to 3 wt.%) was added. The grey coloring all-solid-state sol–gel devices fabricated with Nb 2O 5 : Mo coatings show a high reversible coloration (ΔOD = 0.3) and a long-term stability of more than 55 000 switching cycles. Large area EC-devices (30 × 40 cm) show a transmittance change between 60% and 25% at 550 nm after galvanostatic coloration and bleaching for 3 min and a coloration efficiency of 27 cm 2/C. The results obtained with blue and brown coloring Nb 2O 5 EC-layers and a comparison with blue coloring WO 3 layers are also presented.

Synthesis and superconducting properties of Li-intercalated niobium oxide Li xAB 2Nb 3O 10 (A: alkaline metal, B: alkaline earth metal) by electrochemical technique

We prepared Li-intercalated layered niobium oxide Li x AB 2Nb 3O 10 (A=K, Rb, Cs, B=Ca, Sr) by an electrochemical technique. The host compounds AB 2Nb 3O 10 were synthesized by a solid state reaction method at 850–1050 °C for 10–12 h. Electrochemical intercalation of Li was carried out at room temperature under a constant potential using a potentio-galvanostat. A three-electrode cell was set up as AB 2Nb 3O 10|1.0 M LiClO 4/PC|Pt, where the working electrode was a pellet of AB 2Nb 3O 10 and counter electrode was a Pt wire. LiClO 4 (1.0 M) dissolved in propylene carbonate was used as an electrolyte. An Ag/AgCl electrode was used as a reference electrode connected to the LiClO 4/PC solution via the salt bridge. The cyclic voltammograms of KCa 2Nb 3O 10 had one cathodic peak at ∼−2.8 V, indicating that the Li intercalation occurred at this potential. The c-axis length decreases with increasing x, though a-axis length does not change with increasing x, in Li x KCa 2Nb 3O 10. All of the samples of Li 0.34AB 2Nb 3O 10, which is obtained for only 1.5∼2.5 h of the intercalation treatment, showed superconductivity at 4–6 K.

Electrochemical intercalation of lithium in layered niobates AB 2Nb 3O 10 (A=K, Rb, Cs; B=Ca, Sr) and their superconductivity

We have succeeded in preparing Li-intercalated layered niobium oxide Li x AB 2Nb 3O 10 (A=K, Rb, Cs; B=Ca, Sr) by an electrochemical technique. The host compounds AB 2Nb 3O 10 were synthesized by a solid state reaction method at 850–1150°C for 10–12 h. All of the intercalated compounds showed superconductivity at 4–6 K. In the cases of A=Rb, B=Ca, and A=Cs, B=Ca, no superconductivity appeared up to x=0.17; for x=0.34 and 0.46 T c was constant with maximum value, and for x=0.62 T c decreased slightly. On the other hand, the correlation between T c (onset) and a-axis length demonstrates that T c tends to increase with a-axis length increasing, which was found in Li 0.34 AB 2Nb 3O 10 with optimum Li content.

Cytochrome c immobilization into mesoporous molecular sieves

The immobilization of cytochrome c into the mesoporous moleclar sieves MCM-48 (Mobil Composition of Matter) and SBA-15 (Santa Barbara Amorphous), as well as the layered niobium oxide Nb-TMS4 (Niobium Transition Metal Oxide Molecular Sieve) is described. The cytochrome c loading was influenced by the molecular sieve structure, i.e., one-dimensional vs. three-dimensional. Cytochrome c immobilization was accomplished by absorption into the molecular sieves followed by silylation of the pore openings. The molecular sieves containing cytochrome c were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV/Vis, and FT-IR spectroscopy. The protein was found to be stable in the molecular sieve under conditions that would denature the protein in solution. The immobilized cytochrome c retained its redox activity following immobilization for several months as demonstrated by cyclic voltammetry (CV).

Synthesis and characterization of niobium oxide layers on silica and the interaction with nickel

A surface phase oxide containing niobiuma oxide, deposited on non-porous silica was prepared by two different methods. The photoelectron spectroscopy (XPS) and infrared results show that the method using five-stage attaching reactions between Nb(OC 2O 5) 5 and silanol led to a very close one-atomic-layer niobium oxide structure on SiO 2. This sample presented two oxygen species comparable to model systems and it was possible to distinguish them in the XPS spectral even when using a silica powder. Cyclohexane dehydrogenation was used to characterize the reduced metallic surface. The nickel compounds interact preferentially with niobia rather than with silica. These nickel species favored lower dehydrogenation activity, suggesting the choice of niobia as a nickel trap.

酸化物超伝導体の合成と評価 電気化学的手法による層状ニオブ酸化物ＡＣａ２Ｎｂ３Ｏ１０（Ａ＝Ｋ，Ｒｂ，Ｃｓ）へのＬｉインターカレーション

We prepared Li-intercalated layered niobium oxide LixACa2Nb3O10(A=K, Rb, Cs) by an electrochemical technique. The host compounds ACa2Nb3O10 were synthesized by a solid state reaction method at 850-1150°C for 10-12h. Each of the cyclic voltammograms of these compounds had one cathodic peak at--3V, indicating that the Li-intercaration occurred at this voltage. All of the intercalated compounds LixACa2Nb3O10 (A=K, Rb, Cs) showed superconductivity at 4-5K, and their x-ray diffraction pattern remained almost unchanged in spite of the Li-intercalation. For LixRbCa2Nb3O10, no superconductivity appeared up to x = 0.17, for x=0.34 and 0.46 Tc (determined as the onset of the Missner effect) was constant at 4 K, and for x = 0.62 Tc decreased slightly to 3K.

Preparation and crystal structure of layered niobium oxides: Rb(Mg 0.34Nb 1.66)O 5 and K(Fe 0.43Nb 1.57)O 5

Single crystals of layered niobium oxides, Rb(Mg 0.34Nb 1.66)O 5 and K(Fe 0.43Nb 1.57)O 5, were prepared at 1200°C. The niobium oxides crystallized in the KTiNbO 5-type structure and have the orthorhombic space group Pnma with Z = 4 and the lattice parameters are a = 6.571(1), b = 3.858(1), and c = 19.077(1) Å for Rb(Mg 0.34Nb 1.66)O 5, and a = 6.533(1), b = 3.820(2), and c = 18.682(2) Å for K(Fe 0.43Nb 1.57)O 5. The single-crystal structure refinements led to R = 0.032 and R w = 0.031 for Rb(Mg 0.34Nb 1.66)O 5 and to R = 0.049 and R w = 0.064 for K(Fe 0.43Nb 1.57)O 5. The niobium and iron/magnesium atoms are statistically distributed in a unit of doubled octahedra sharing edges. Alkaline ions in the interlayer were ion exchanged with protons to yield hydrated protonated compounds, H(M,Nb) 2O 5·nH 2O (M = Mg,Fe).

Layered niobium oxides pillaring and exfoliation

Several ion-exchangeable layered niobates were applied to prepare modified layered photocatalysts. K 1- x La x Ca 2- x Nb 3O 10 ( x = 0, 0.25, 0.50, 0.75), a layered perovskite series, with variable interlayer cation density, were used for the modification of the interlayer space by pillaring of silica and titania. The silica-pillaring process was monitored by NMR and the thermal stability was also examined. Silica-pillar was successfully introduced to all the series of perovskites which showed good photocatalytic activity. The preparation of titania-pillared layered niobates was more difficult. In the case of KCa 2Nb 3O 10 ( x=0), we could not introduce a titania pillar, but in the case of K 0.5La 0.5Ca 5Nb 3O 10 ( x = 0.5) carrying less interlayer cation density, titania-pillar was introduced although the thermal stability was very poor. K 4Nb 6O 17, another type of layered niobate, was used for the exfoliation and restacking of the niobate sheets. The structure and photocatalytic activity of the product was examined.

The Influence of Oxide on the Electrochemical Processes in K 2NbF7 ‐ NaCl ‐ KCl Melts

Transient electrochemical techniques showed that in melts the reduction of occurs through a two‐step reaction Nb(V) → Nb(IV) → Nb. When oxide ions were introduced, cyclic voltammetry indicated that the waves corresponding to reduction of the complex progressively disappeared. Oxo complexes, such as , are reduced at a potential close to that of the Nb(IV) fluoro complexes. Niobium metal deposition was perturbed by the formation of niobium suboxides like or . For molar ratios of oxide to Nb greater than one a black layer of niobium oxide , appeared at the electrode surface. The present study confirms the high oxygen affinity of niobium and shows that a careful purification of the electrolyte and feeding materials is required for producing niobium with a low oxygen content.