Amorphous Oxide Phase Research Articles

Understanding the Anomalous Thermoelectric Behavior of Fe-V-W-Al-Based Thin Films.

We investigated the thermoelectric and thermal behavior of Fe-V-W-Al-based thin films prepared using the radio frequency magnetron sputtering technique at different oxygen pressures (0.1-1.0 × 10-2 Pa) and on different substrates (n, p, and undoped Si). Interestingly, at lower oxygen pressure, formation of a bcc-type Heusler structure was observed in deposited samples, whereas at higher oxygen pressure, we have noted the development of an amorphous structure in these samples. Our findings indicate that the moderately oxidized Fe-V-W-Al amorphous thin film deposited on the n-Si substrate possesses a large magnitude of S ∼ -1098 ± 100 μV K-1 near room temperature, which is almost double the previously reported value for thin films. Additionally, the power factor (PF) indicated an enormously large value of ∼33.9 mW m-1 K-2 near 320 K. The thermal conductivity of the amorphous thin film is also found to be 2.75 Wm-1 K-1, which is quite lower compared to bulk alloys. As a result, the maximum figure of merit is estimated to be extremely high, i.e., ∼3.9 near 320 K, which is among one of the highest reported values so far. The anomalously large value of Seebeck coefficient and PF has been ascribed to the unusual composite effect of the metallic amorphous oxide phase and insulating substrate possessing a large Seebeck coefficient.

Effect of mechanical milling on microstructure evolution and mechanical properties of Al–Zn–Mg–Cu–Si alloy fabricated via spark plasma sintering

We investigated the effect of high-energy ball milling (HEBM) on the microstructure and mechanical properties of powders and spark plasma sintered samples of an Al–Zn–Mg–Cu–Si alloy. HEBM produced a nanocrystalline powder with an average grain size of 0.16 μm while increasing the amount of solid solution and the formation of fine amorphous aluminium oxide. The sintered alloy without HEBM consisted of η-Mg(Zn,Cu,Al)2, T-Mg32(Al,Zn)49, β-Mg2Si, and Q-Al5Cu2Mg8Si6 phases. The grain size of the sintered alloy decreased from 2.66 to 0.40 μm due to the application of HEBM. The amorphous aluminium oxide phase in the milled powder was transformed into MgO particles during sintering. The formation of MgO particles caused the depletion of Mg solid solutions, which resulted in the formation of Mg-free phases during sintering. High-energy ball milling (HEBM) improved the microhardness of the sintered alloy from 94 to 134 HV owing to grain refinement and the formation of fine secondary phases and MgO particles.

Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species

AbstractThe degradation of Pt‐containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO4 by combining operando high‐energy surface X‐ray diffraction, online mass spectrometry, and density functional theory. Our atomic‐scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe‐like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO2 and starts to grow when the coverage of the stripe‐like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential‐independent after the stripe‐like oxide has reached its saturation coverage.

Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species.

The degradation of Pt-containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO4 by combining operando high-energy surface X-ray diffraction, online mass spectrometry, and density functional theory. Our atomic-scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe-like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO2 and starts to grow when the coverage of the stripe-like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential-independent after the stripe-like oxide has reached its saturation coverage.

Effects of Copper Content on the Microstructural, Mechanical and Tribological Properties of TiAlSiN–Cu Superhard Nanocomposite Coatings

The effects of the Cu content on the microstructural, mechanical and tribological properties of the TiAlSiN–Cu coatings were investigated in an effort to improve the wear resistance with a good fracture toughness for cutting tool applications. A functionally graded TiAlSiN–Cu coating with various copper (Cu) contents was fabricated by a filtered cathodic arc ion plating technique using four different (Ti, TiAl2, Ti4Si, and Ti4Cu) targets in an argon-nitrogen atmosphere. The results showed that the TiAlSiN–Cu coatings are a nanocomposite consisting of (Ti,Al)N nano-crystallites (~5 to 7 nm) embedded in an amorphous matrix, which is a mixture of TiOx, AlOx, SiOx, SiNx, and CuOx phase. The addition of Cu atoms into the TiAlSiN coatings led to the formation of an amorphous copper oxide (CuOx) phase in the coatings. The maximum nanohardness (H) of ~46 GPa, H/E ratio of ~0.102, and adhesion bonding strength between coating and substrate of ~60 N (LC2) were obtained at a Cu content ranging from 1.02 to 2.92 at.% in the TiAlSiN–Cu coatings. The coating with the lowest friction coefficient and best wear resistance was also obtained at a Cu content of 2.92 at.%. The formation of the amorphous CuOx phase during coating growth or sliding test played a key role as a smooth solid-lubricant layer, and reduced the average friction coefficient (~0.46) and wear rate (~10 × 10−6 mm3/N·m).

Role of glycerine on formation & corrosion characteristic of PEO layer formed over Mg alloy in a high-concentrated mixed silicate-phosphate-based electrolyte

PEO coatings on AM50 magnesium alloy were synthesized under constant current mode exploiting high-concentrated mixed silicate-phosphate-based alkaline electrolyte with and without glycerine as an additive, i.e., gmix-PEO and bmix-PEO respectively. The development of PEO coatings was studied as a function of PEO treatment time for their microstructures, element compositions, phase compositions, and corrosion behaviour. For all the treatment times, gmix-PEO exhibited relatively smaller pores and lower coating thickness than bmix-PEO. The elemental composition of PEO coatings was independent of PEO processing time for both bmix-PEO and gmix-PEO. Throughout PEO processing the incorporation of Si was considerably higher than P into the synthesized coatings. MgO and Mg2SiO4 crystalline phase content was higher for gmix-PEO compared to bmix-PEO for all the treatment times studied. A complex amorphous oxide phase was observed from 4 min PEO treatment time for bmix-PEO while glycerine addition delayed the formation of the same. The study of electrochemical behaviour revealed improved corrosion performance of gmix-PEO particularly during initial immersion (up to 25 h) time for up to 4 min PEO treatment time than for bmix-PEO as glycerine addition leads to higher MgO and Mg2SiO4 phase content and thicker/compact inner barrier layer. For the final 8 min PEO processing time, bmix-PEO offered enhanced corrosion performance particularly due to its thicker coating compared to gmix-PEO.

Transformation of amorphous passive oxide film on Zr65Cu17.5Ni10Al7.5 metallic glass ribbons

Zr-based metallic glass with favorable properties and a stable surface oxide layer is foreseen as an alternative material for water/nuclear reactor pressure systems. The high stability of Zr65Cu17.5Ni10Al7.5 metallic glass (MG) in anodic and cathodic regions within 0.5 M H2SO4 solution is evidenced by the negligible shift in the corrosion potential upon linear sweep voltammetry cycles. The recorded passivation current density of 15 mA cm–2 is very similar to 430 stainless steel. The formation of an amorphous Zr-rich oxide phase and slight dissolution of the native ZrO2 layer are confirmed by scanning transmission electron microscopy – energy dispersive X-ray analysis, which result from the interaction of hydronium ions (H3O+) with the oxide layer during linear polarization. Furthermore, promising hydrogen absorption of the produced Zr-based MG ribbons (1.33 %wt, non-reversible) by Sieverts-type apparatus were recorded at 400 °C and under 6 MPa, which evidences that this alloy can minimize the hydrogen evolution within the chamber due to the coverage with an oxide film and thereby prevent accidents.

Size effects on the formation of LaAlO3 phase in La-doped γ-Al2O3 after hydrothermal treatment

A series of La-doped alumina ceramics were prepared by a strong electrostatic adsorption technique in comparison with a conventional incipient wetness impregnation method. The commercial La-doped alumina support was used as a reference sample. The samples were calcined in a temperature range of 800–1200 °C. The characterization of the ceramics was performed using low-temperature nitrogen adsorption, transmission electron microscopy, X-ray diffraction analysis, and photoluminescence. A strong electrostatic interaction between alumina and lanthanum was found to affect the stabilization of alumina and the resistance of La2O3 towards agglomeration during the wet procedures. The size effects related to the formation of LaAlO3 of ∼10 nm in size from the amorphous oxide phase take place on the surface of alumina doped with lanthanum. In cases of the commercial La-modified alumina sample and the sample prepared by wetness impregnation with lanthanum nitrate, the hydrothermal treatment under mild conditions has two effects. Firstly, the La2O3 particles weakly bonded with the support grow in size. Secondly, the high-temperature calcination of the samples leads to the formation of the LaAlO3 phase. Oppositely, the samples prepared by the adsorption of La(EDTA)- complexes showed no formation of the LaAlO3 phase neither before nor after the hydrothermal treatment procedure. The key factor accelerating the dissolution of the La2O3 particles was found to be the regime of the suspension stirring.

Electrodeposition of Complex High Entropy Oxides via Water Droplet Formation and Conversion to Crystalline Alloy Nanoparticles.

A combination of electrodeposition and thermal reduction methods have been utilized for the synthesis of ligand-free FeNiCo alloy nanoparticles through a high-entropy oxide intermediate. These phases are of great interest to the electrocatalysis community, especially when formed by a sustainable chemistry method. This is successfully achieved by first forming a complex five element amorphous FeNiCoCrMn high-entropy oxide (HEO) phase via electrodeposition from a nanodroplet emulsion solution of the metal salt reactants. The amorphous oxide phase is then thermally treated and reduced at 570-600 °C to form the crystalline FeNiCo alloy with a separate CrMnOx cophase. The FeNiCo alloy is fully characterized by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy elemental analysis and is identified as a face-centered cubic crystal with the lattice constant a = 3.52 Å. The unoptimized, ligand-free FeNiCo NPs activity toward the oxygen evolution reaction is evaluated in alkaline solution and found to have an ∼185 mV more cathodic onset potential than the Pt metal. Beyond being able to synthesize highly crystalline, ligand-free FeNiCo nanoparticles, the demonstrated and relatively simple two-step process is ideal for the synthesis of tailor-made nanoparticles where the desired composition is not easily achieved with classical solution-based chemistries.

Precursor Effect on Mn-Fe-Ce/TiO2 Catalysts for Selective Catalytic Reduction of NO with NH3 at Low Temperatures

Preparation of Mn/TiO2, Mn-Fe/TiO2, and Mn-Fe-Ce/TiO2 by the deposition-precipitation (DP) method can afford very active catalysts for low-temperature NH3-SCR (selective catalytic reduction of NO with NH3). The effect of precursor choice (nitrate vs. acetate) of Mn, Fe, and Ce on the physiochemical properties including thermal stability and the resulting SCR activity were investigated. The resulting materials were characterized by N2-Physisorption, XRD (Powder X-ray diffraction), XPS (X-ray photoelectron spectroscopy), H2-TPR (temperature-programmed reduction with hydrogen), and the oxidation of NO to NO2 measured at 300 °C. Among all the prepared catalysts 5MnAce/Ti, 25Mn0.75AceFe0.25Nit/Ti, and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti showed superior SCR activity at low temperature. The superior activity of the latter two materials is likely attributable to the presence of amorphous active metal oxide phases (manganese-, iron- and cerium-oxide) and the ease of the reduction of metal oxides on TiO2. Enhanced ability to convert NO to NO2, which can promote fast-SCR like pathways, could be another reason. Cerium was found to stabilize amorphous manganese oxide phases when exposed to high temperatures.

Mechanical properties and fatigue crack growth in tungsten deposited by RF-plasma

Fracture toughness and fatigue crack growth rates of free-standing thick W deposits produced in a protective atmosphere using radio frequency inductively-coupled plasma process were investigated. Stress-strain behavior of the deposits was studied using four-point bending and their fracture toughness was estimated from three-point bending following the ASTM E1820 standard. The fatigue crack growth rates were measured in symmetrical bending using a resonance approach. The results were compared with commercially available bulk tungsten prepared by powder metallurgy that served as a reference. The results showed that the deposits had significantly lower stress-strain properties: the elastic modulus and tensile strength ranged from 30% to 50% of the reference bulk values. The crack growth rate tests together with fractographic analysis suggested that a static failure by inter-granular decohesion predetermines the fatigue properties of the deposits and causes high slopes of the corresponding fatigue crack growth rate curves. Contrary to this, the reference sheet failed trans-granularly. The inter-granular failure mode of the deposits was caused by grain boundary embrittlement, a phenomenon probably related to oxides. The lowest obtained oxygen concentration in deposit was nearly by two orders of magnitude higher, than in reference sheet. The distribution of oxygen was studied by TEM and the amorphous and crystalline oxide phases were identified at splat interfaces. The presence of oxygen in the deposits produced by the inherently non-oxidizing RF technology calls for control of oxygen content in the feedstock powder. The performed characterization provided baseline data of mechanical and fatigue properties of RF plasma sprayed tungsten.

Truly Electroforming‐Free Memristor Based on TiO2‐CoO Phase‐Separated Oxides with Extremely High Uniformity and Low Power Consumption

AbstractOxide‐based memristor devices are being extensively studied as one of the most promising technologies for next generation nonvolatile memory and neuromorphic computing. However, the switching process of such devices relying on the formation and rupture of conductive filaments has not been easily controlled, and thus induces large cycle‐to‐cycle and device‐to‐device variations in resistive switching, which hinders the development of high‐performance memristors. High‐performance memristors that meet the requirements for truly electroforming‐free, highly uniform, and low‐power switching are yet to be developed. Here, a phase‐separated oxide memristor is demonstrated based on a spontaneous phase separation process to form amorphous TiO2 switching medium distributed among the crystalline CoO grains. The confinement of conductive filaments into the intergrain amorphous oxide phase effectively minimizes the stochasticity of filament formation and rupture, resulting in drastically enhanced switching uniformity. The designed microstructure also facilitates filament formation and dissolution during switching processes and leads to truly electroforming‐free switching and low switching power (simultaneous low switching voltage 0.4 V and low current 2.5 µA).

Atomic composition, structure, and electrical properties of In1-xGaxSb films deposited by magnetron sputtering

This article reports the formation of polycrystalline In1-xGaxSb films (x = 0, 0.2, 0.4, 0.5, and 1) deposited by magnetron sputtering on SiO2/Si substrates and a comprehensive characterization of their atomic composition, structure, and electrical properties. The structural and electrical characteristics of the films can be easily tuned by varying the stoichiometry of the compounds. The atomic composition of the films was investigated with particle induced x-ray emission and Rutherford backscattering spectrometry (RBS) techniques (the latter as a function of depth), while grazing incidence x-ray diffraction analysis yielded valuable information about the structure of the films. In1-xGaxSb films were polycrystalline, with crystallite sizes in the range 18–42 nm. Combining RBS and scanning electron microscopy results, it was possible to infer the overall density and also the density of the crystalline portion of the films, which turned out to be significantly distinct. A non-negligible fraction of oxygen atoms has been detected in all films, most prominently in the ternary ones, contributing to the formation of high density amorphous In2O3 and Ga2O3 in films with 40% and 50% Ga, while amorphous antimony oxide phases are predominant in the binaries as well as the ternary compound with 20% Ga. The electrical characterization of the films was performed using four point and Hall effect methods and the results show a consistent increase in resistivity with Ga concentration.

Formation and Properties of Amorphous Multi-Component (CrFeMoNbZr)Ox Thin Films

In this work, a new multi-component (CrFeMoNbZr)Ox system was developed. The thin films presented dual-phase amorphous structures, comprising a dominant amorphous alloy phase and a small amount of an amorphous oxide phase. The thin films showed higher hardness and better corrosion resistance than a commercial Zr-based alloy. The combined properties of high hardness and superior corrosion-resistance make the amorphous thin film a candidate for coating materials on commercial Zr-based alloys for engineering applications.

Oxidation Behavior and Structural Transformation of (CrTaTiVZr)N Coatings

(CrTaTiVZr)N coatings were prepared on Si substrates through the reactive magnetron sputtering system to investigate the oxidation behaviors and structural evolution of the coatings at different annealing temperatures in air. The (CrTaTiVZr)N coating had a face-centered cubic structure with an oxidation temperature of up to 300 °C, but its surface changed into the amorphous oxide phase and then into the rutile TiO2 phase when the annealing temperature was increased to 500 °C. The rutile TiO2 phase continued to grow, and an additional solid solution phase of body-centered tetragonal I41/amd was formed at annealing temperatures beyond 600 °C. The high annealing temperature promoted the oxidation to progress along the thickness direction and synergistically developed the porosity. As a result, the hardness and the electrical performance of the coating deteriorated. The hardness decreased from 34.30 GPa to 1.52 GPa, and the electrical resistivity increased from 142 µΩ·cm to 17.5 Ω·cm.

Investigation of the reduction roasting of saprolite ores in the Caron Process: microstructure and thermodynamic analysis

ABSTRACT Studies have been undertaken to investigate the thermodynamic aspects of the reduction roasting of saprolite ores in the Caron Process. Reduction experiments have been conducted in controlled oxygen partial pressure conditions at selected temperatures. Detailed microstructure characterisation and thermodynamic analysis of reduced ore samples, focusing on the phase transformations, nickel recoveries and Ni-Fe alloy compositions produced, were carried out. Comparison has been made between the experimental results and FactSage thermodynamic predictions for these conditions. The differences between between the experimental and predicted results are discussed. It has been shown that the system below 700°C is approximated thermodynamically by considering the nickel–iron alloy product to be in chemical equilibrium with a metastable amorphous oxide phase.

Nanostructured IrOx Coatings for Efficient Oxygen Evolution Reactions in PV-EC Setup

Abstract New heteroleptic iridium compounds exhibiting high volatility and defined thermal decomposition behavior were developed and tested in plasma-enhanced chemical vapor deposition (PECVD). The iridium precursor [(COD)Ir(TFB-TFEA)] (COD = 1,5-cyclooctadiene; TFB-TFEA = N-(4,4,4-Trifluorobut-1-en-3-on)-6,6,6-trifluoroethylamin) unifies both reactivity and sufficient stability through its heteroleptic constitution to offer a step-by-step elimination of ligands to provide high compositional purity in CVD deposits. The substitution of neutral COD ligands against CO groups further increased the volatility of the precursor. PECVD experiments with unambiguously characterized Ir compounds (single crystal X-ray diffraction analysis) demonstrated their suitability for an atom-efficient (high molecule-to-precursor yield) gas phase deposition of amorphous iridium oxide (IrOx) phases. Thin films of IrOx were well suited as electrocatalyst in oxygen evolution reaction so that an efficient coupled system in combination with solar cells is viable to perform water-splitting reaction without external bias.

In Situ Studies of 30% Li-Doped Bi25FeO40 Conversion Type Lithium Battery Electrodes.

One of the important discharge mechanisms for lithium batteries is the conversion reaction mechanism, where a metal oxide (fluoride) can decompose into metallic nanoparticles embedded in a Li2O (LiF) matrix. Here, 30% Li-doped Bi25FeO40 is successfully synthesized and displays an electrochemical discharge capacity of ∼300 mAh/g above 1.5 V (vs Li/Li+). During the electrochemical cycling process, 30% Li-doped Bi25FeO40 is decomposed into metallic Bi. During the subsequent charging process, the metallic bismuth can be first converted into an amorphous bismuth oxide phase, which contributed to the electrochemical discharge activities observed between 2 and 2.5 V. At a higher charging voltage between 3.5 and 5 V, metallic Bi can be oxidized to BiOx2–O3–2x–, which contributes to the discharge activities observed above 2.5 V. Using graphite as current collectors can prevent the corrosion from O– species and the discharge capacity is greatly enhanced at the voltage region between 1.5 and 2.5 V. This work provides a deeper understanding over the role of oxygen ions during the conversion reaction process and is beneficial for the future design of battery systems based on the conversion reaction.

Fate and dynamics of metal precipitates arising from acid drainage discharges to a river system

Neutralisation of acid drainage creates metal-rich precipitates that may impact receiving water bodies. This study assessed the fate of over seven years of acid drainage discharges on the sediments of the lower River Murray (Australia), including the potential for periodic water anoxia to enhance risk via reductive dissolution of amorphous (Fe, Mn and co-precipitated and bound metal) oxide phases. With the exception of one site with restricted water exchange, elevated reducible/reactive metal(oid) (Fe, Ni, As, Co, Zn) concentrations were only observed in the localised wetland-riparian area within approximately 100 m of the discharges. Only a minor exceedance of national sediment quality guideline values occurred for Ni. In the main river channel, elevated reactive metal (Fe, Mn, Ni, Zn) concentrations were also only observed less than approximately 100 m from the drainage discharge point. This appears due to (a) rapid neutralisation of pH leading to metal precipitation and deposition in the localised discharge area, and/or (b) dilution of any metal precipitates entering the main channel with natural river sediments, and/or (c) flushing of precipitates downstream during higher flow conditions. The influence of deoxygenation on metal release was profound with large increases in the concentration of dissolved Fe, Mn, Zn, Ni, and As in the overlying water during laboratory experimental simulations. However, given in situ sediment metal contamination is very localised, it appears on a river reach scale that the acid drainage precipitates will not significantly contribute, over and above, the background release of these metals during these conditions.

Catalyst for Selective Hydrogenolysis of Glycerol, Prepared from Hydrotalcite-Like Structures

A process of transforming hydrotalcite, containing Cu–Zn–Al (2:1:1) to a catalysts for selective hydrogenolysis of glycerol to 1,2-propanediol has been studied. A two steps process, consisted of the calcination at increased temperature and subsequent reduction with hydrogen, as well as a one step process, in which the hydrotalcite was heated at hydrogen atmosphere so the calcination and reduction were joined to one operation, were verified experimentally. The influence of the temperature in the calcination step in the two steps process on composition, texture and catalytic properties of the active form of catalyst has been studied in detail. X Ray diffraction, thermogravimetric analyses, nitrogen physisorption, mercury porosimetry, temperature-programmed reduction and high-resolution transmission electron microscopy were used for the catalysts characterization. A study of calcination of a Cu–Zn–Al containing hydrotalcite at different temperatures showed, that at temperature ending with 350 °C, predominantly amorphous, mixed oxides containing phase was formed. The most active and most selective catalyst was obtained by the reduction of this amorphous oxide phase. In contrary, highly crystalline material was obtained at higher calcination temperatures, i.e at 450 or 700 °C respectively, but the activity of the reduced form of these crystalline oxides was lower. It was also proven, that by simultaneous calcination and reduction, a catalyst of the same activity and even better selectivity towards 1,2-propanediol could be prepared, comparing with the catalyst prepared by the two steps method.