Reoxidation Process Research Articles

Chemical engineering of Cu(In,Ga)Se2 surfaces: An absolute deoxidation studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy signatures

CIGS (Cu(In,Ga)Se2) absorbers are among the most efficient for photo-conversion. As part of their manufacturing cycle, absorber surface preparation is crucial for both the success of processes and the outcome of characterizations actions. This work tackles the question of an efficient deoxidation of CIGS surfaces. Chemical composition of surface is investigated thanks to XPS (X-ray Photoelectron Spectroscopy) characterizations. Present study focuses on the possible control of the CIGS absorber surface associated with simple HCl treatment and subsequent atmosphere ageing up to 20days. Using 1.0M HCl concentration solution, remarkable oxide elimination is achieved. Starting from an air aged as-grown surface (Cu and Se-poor), this treatment enables the generation of a deoxidized chalcopyrite reference sample. Furthermore, the reoxidation process of the treated surface at ambient atmosphere is studied considering the global evolution of the XPS signals as the best probe of the ageing process. The surface composition evolution represents a key parameter for the achievement of the following interfaces engineering. The main result is that the reoxidation process is not instantaneous (no significant oxide content before 2days) and involves In, Ga and Se elements while the Cu seems less affected, leading again to a Cu-poor surface different from the initial one.

CO2 decomposition using metal ferrites prepared by co-precipitation method

To catalytically decompose the greenhouse gas, CO2, spinel structure M-ferrites (M=Co, Ni, Cu, Zn) were synthesized by chemical co-precipitation using metal salts and sodium hydroxide as starting materials. The crystallite size of the newly-prepared M-ferrites increased and the BET surface area decreased with increasing calcination temperature. A thermal analysis of the reduction and reoxidation of M-ferrites indicated that substitution of divalent transition metals (i.e., Cu, Ni and Co) into Fe3O4 improved the reduction kinetics in the order of Cu>Ni>Co. ZnFe2O4 was the most difficult compound to completely reduce due to its stable structure. Commercial samples of the reduced Fe3O4, CoFe2O4 and ZnFe2O4 showed an increase in mass through the reoxidation process, but it was much more difficult for oxygen atoms to enter the structure of the reduced samples of NiFe2O4 and CuFe2O4. The M-ferrites in a batch type reactor showed better efficiency than the commercial Fe3O4. Also found was that CoFe2O4 showed a high regeneration potential, although it required a higher critical reaction temperature. NiFe2O4 and CuFe2O4 were excellent candidate materials for CO2 decomposition at lower temperatures.

The Effects of Low Oxygen Activity Conditions on the Phase Equilibria and Cation Occupancy of Strontium Barium Niobate

Strontium barium niobate (SBN) is a tungsten bronze family ferroelectric which shows promising thermoelectric properties under reducing conditions. It is found here that the enhanced electrical conductivity of oxygen‐deficient SBN correlates with the formation of a NbO2 secondary phase. The effects of the reducing environment and the NbO2 phase formation are studied via a detailed defect chemistry analysis. Increasing amounts of the NbO2 phase are accompanied by an interesting mechanism where the A‐site occupancy of the SBN matrix increases. The resulting donor defects source the large carrier concentrations which cause the enhanced electrical conductivity necessary for thermoelectric performance. In investigation of the phase equilibria, it is found that a solid solution between (Sr0.6,Ba0.4)Nb2O6 and (Sr0.6,Ba0.4)1.2Nb2O6 exists and that the A‐site filling is found to occur at more modest reduction conditions in Sr‐ and Ba‐rich compositions. Finally, thermogravimetric analysis of the reoxidation process is performed, and the results suggest that the A‐site filling is compensated ionically. Not only do the presented results explain the enhanced electrical conductivity of oxygen‐deficient strontium barium niobate but also modification of the site occupancies by reduction and reoxidation may widen the design space for property modification in tungsten bronze‐structured materials in general.

Regeneration of sintered Rh/ZrO2 catalysts via Rh re-dispersion and Rh–ZrO2 interaction

Rh/ZrO2 catalyst was prepared by the impregnation method and then aged in N2 atmosphere with 10% steam at 1050°C for 12 h. The regeneration of the aged catalyst was examined by slow cooling the catalyst in O2 following preheating at 950°C. The as-prepared, aged, and regenerated catalysts were characterized by X-ray diffraction, Brunauer–Emmett–Teller specific surface area analysis, and (CO+NO) reaction activity tests, CO chemisorption, and transmission electron microscopy. The results revealed that the Rh species on ZrO2 that underwent severe sintering during the high-temperature aging process could be regenerated by the slow re-oxidation process employed herein. Owing to the remarkable increase in the degree of Rh dispersion after the regeneration process, the activity could be markedly recovered. Further investigations revealed that such a regeneration phenomenon, involving the re-dispersion of the Rh species during the slow cooling process in O2, was strongly related to the phase transformation of the ZrO2 support and the interaction between the Rh and tetragonal ZrO2 species.

Surface-modified carbon nanotube coating on high-voltage LiNi0.5Mn1.5O4 cathodes for lithium ion batteries

Surface-modified carbon nanotubes were utilized as a coating for LiNi0.5Mn1.5O4 (LNMO) via a mechano-fusion method as a strategy to prevent unfavorable carbothermal reduction. Two types of carbon nanotubes were investigated as coating materials: carbon nanotubes (CNTs) and oxidized carbon nanotubes (OCNTs), which were prepared by a simple re-oxidation process. The samples coated with CNTs or OCNTs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy, Raman spectroscopy, and elemental analyses. The OCNT-coated LNMO presented a highly enhanced discharge capacity retention (95.5%) and a coulombic efficiency of 99.9% after 80 cycles between 3.5 and 4.9 V (versus Li/Li+), whereas the CNT-coated LNMO exhibited poor retention of 47.2% and a coulombic efficiency of 95.3%. In addition, post-mortem XPS and electrochemical impedance spectroscopy (EIS) analysis proved that the OCNT coating improved the surface electrochemical stability and rate capability, whereas the CNT coating formed a thick resistive solid electrolyte interphase (SEI) film by accelerating the surface side reactions.

Microstructural finite element modeling of redox behavior of Ni–YSZ based ceramic SOFC anodes

One of the main challenges in solid oxide fuel cell (SOFC) is the redox problem of Ni-based anodes. Therefore, the redox tolerance of the anode should be improved to increase the service life of the system. Since the real SOFC anodes have a heterogeneous structure with different grain sizes, a micro-level modeling is necessary. Especially, the re-oxidation step which creates large amount of stress is critical during the redox cycles. Therefore, in this study, the effects of re-oxidation temperatures and exposure times on the oxidation rate and stresses developed are investigated on a synthetically generated porous anode functional layer microstructure. A mathematical model is developed to characterize the re-oxidation process and solved numerically. The results indicate that the re-oxidation strongly depends on the re-oxidation temperature and exposure time. The time required for the full oxidation is found to be 7458.55, 313.14, 200.8 and 33.78s at 800, 850, 900 and 950°C, respectively for the generated anode microstructure. In addition, the created stresses in the anode structure are well above the critical material limits.

PTCR Behavior of CuO-Doped Ba0.96(Bi0.5K0.5)0.04TiO3 Ceramics

As a candidate of a lead-free positive temperature coefficient of resistivity (PTCR) material, Ba0.96(Bi0.5K0.5)0.04TiO3-xCuO (x = 0–0.80%) ceramics were prepared by the solid-state reaction technique. X-ray diffraction results indicated that the CuO doped Ba0.96(Bi0.5K0.5)0.04TiO3 ceramics sintered at 1360°C for 3h in N2 exhibited a tetragonal structure at room temperature. The PTCR characteristics of Ba0.96(Bi0.5K0.5)0.04TiO3-xCuO ceramics were investigated in terms of CuO content. The undoped sample sintered in N2 showed low resistivity and a small resistance jump. With the increasing CuO content, both the Curie temperature and the room-temperature resistivity increased. The Raman scattering spectra showed that the Cu2+ was partly reduced to Cu+ during sintered in N2 which increased the density of the grain surface acceptors and enhanced PTC effect without reoxidation process. In the range of 0.05 ≤ x ≤ 0.20%, the samples displayed a marked PTC effect. When x ≥ 0.40%, CuO and Cu2O in ceramic samples could act as sintering aids to promote grain growth and all the samples showed NTC effect.

Chemical Passivation of In0.53Ga0.47As(100) Using Ammonium Sulfide and Thiols

Dilute solutions of (NH4)2S in water and of 1-eicosanethiol (ET) in ethanol were used to deposit a protective layer to prevent the reoxidation of clean In0.53Ga0.47As(100) substrates. The surface composition of the InGaAs after oxide etching in 0.3 M HF, and after immersion in (NH4)2S or ET solutions was studied using XPS. Immersing clean InGaAs substrates in 4.0 mM ET for 20 h protected the surface for 4 min, after which oxide states were detected by XPS. A 20 min immersion in 3 mM (NH4)2S in a glove box slowed down the reoxidation process. After 1 h of atmosphere exposure, the amount of InGaAs oxides on the surface treated in (NH4)2S were lower than those of a control sample after only 4 min of air exposure. The sulfur layer deposited from aqueous ammonium sulfide and the self-assembled eicosanethiol layer blocked reoxidation and chemically passivated the surface only for brief ambient exposures.

Analysis of hematite re-oxidation in the chemical looping process

Very little attention has been dedicated to the carrier re-oxidation in chemical looping systems. The work presented in this paper is for the re-oxidation of partially reduced hematite from a cyclic chemical looping fixed bed process. The underlying purpose of this work is to develop engineering rates and mechanisms for the re-oxidation of partially reduced hematite that can be included in CFD models for a chemical looping process. To this end, experiments were run using nominally 1000g of hematite material in a fixed bed reactor cycling between reduction and re-oxidation. The cyclic processing began with the reduction step then proceeded to the oxidation step repeating this analysis for several cycles ranging from 5 to 10. The re-oxidation process was conducted at temperatures ranging from 745°C to 825°C and oxygen concentrations ranging between 9% and 11%. The reduction was carried out at the same temperature as the re-oxidation step at various CH4 concentrations from 5% to 9%. In this paper, cyclic induced variations in performance are presented as well as the kinetic parameters for the first cycle. The re-oxidation of the depleted hematite occurs through a 2 step parallel process in which oxygen reacts to fill the surface of each grain within the particles and then migrates through oxygen vacancy diffusion to the depleted cores of each grain.

Development of label-free impedimetric platform based on new conductive polyaniline polymer and three-dimensional interdigitated electrode array for biosensor applications

Novel label-free impedimetric platform based on a three-dimensional interdigitated electrode array (3D-IDEA) sensor and new conductive polymer as a transducer for oxidoreductases is introduced. This platform is cost-effective, simple to construct and miniaturize. Monomer of conductive polymer N-(N’,N’-diethyldithiocarbamoylethylamidoethyl) aniline (AnD) was deposited onto 3D-IDEA by chemical polymerisation. It was found that the polymer film resistance depends on the redox-potential of the solution. For the first time polyAnD was used as enzyme immobilisation matrix. Pyrroloquinolinequinone (PQQ) dependent alcohol and glucose dehydrogenases were immobilized on 3D-IDEA covered with polyAnD by two different methods. 3D-IDEA sensors with enzymes, which were immobilised by physisorption on polyAnD layer, showed specific response in the presence of 1μM of the corresponding substrates. Obtained results revealed that PQQ dependent dehydrogenases can re-oxidize on polyAnD via direct electron transfer (DET) from enzyme active site to the polymer surface. This process can be monitored by methods of electrochemical impedance spectroscopy (EIS) and chronoamperometry. Presented study shows that EIS method gives a useful tool for research of re-oxidation process and interaction of electroactive enzymes with conducting materials giving information required to construct and develop analytical devices.

Unravelling the fate of arsenic during re-oxidation of reduced wetland waters: Experimental constraints and environmental consequences

The presence of arsenic(As)-bearing Fe(III) oxyhydroxides in wetland zones may threat water quality due to the reduction processes that affect these zones. These processes have indeed the potential of releasing As into the soil solutions, and ultimately into the nearby river network, being given the hydrological connectivity that exists between wetlands and rivers. The effective transport of the released As into the river network is however dependent on the behaviour of As during the re-oxidation process that will occur at the wetland-river boundary, which could immobilize the released As into neo-formed Fe(III) oxyhydroxides. One of the key questions is, however, which is the impact of the organic-rich nature of wetland waters on this neoformation, which could instead favour the development of highly mobile As-bearing, organomineral colloids. In this study, we evaluated this possibility by carrying out oxidation experiments on humic acid (HA)- and As(III)/Fe(II)-rich waters. The ultrafiltration results showed that the presence of organic molecules during Fe oxidation events played a major role in the hydrolysis reaction of Fe oxyhydroxides. When Fe microparticles were formed in the absence of HA, the occurrence of HA during Fe oxidation events promoted the formation of amorphous nanosize Fe phase diffusely embedded within the organic matrix. These mixed Fe–HA colloids constrained the fate and distribution of As. These results were confirmed by nanoSIMS images that showed As sorption onto and within Fe microparticles (>0.2μm) in the absence of HA. By contrast, with HA, results showed preferential incorporation of As in the bulk of Fe–HA colloids (<0.2μm) during their formation rather than surface adsorption onto these colloids.

Characteristics of 600, 1200, and 3300 V Planar SiC-MOSFETs for Energy Conversion Applications

SiC-MOSFETs provide superior performance for next generation power electronics systems. High threshold voltage 600 V SiC-MOSFETs were realized utilizing a reoxidation process, which drastically improves a tradeoff between an ON-resistance and a threshold voltage. Low-loss SiC-MOSFETs with a 1200 V/100-A rating have been developed. Using the developed SiC-MOSFETs, 1200 V/800-A high-power full SiC module with an ON-resistance as low as 2.9 m $\Omega $ at 150 °C was successfully fabricated. The high-power module markedly reduces power loss especially at high carrier frequency. Large-area 3300 V SiC-MOSFETs with an ON-resistance of 52 m $\Omega $ at 175 °C exhibit an adequate reverse bias safe operating area and 3300 V SiC-MOSFETs screened by applying a body diode current stress show stable characteristics under a continuous current through their body diode for 1000 h.

Effect of Na2Ti6O13 addition on the microstructure and PTCR characteristics of Ba0.94(Bi0.5K0.5)0.06TiO3 ceramics

As a candidate of lead-free positive temperature coefficient of resistivity (PTCR) materials, Ba0.94(Bi0.5K0.5)0.06TiO3–xNa2Ti6O13 (x=0−0.20%) ceramics were prepared by the solid-state reaction technique. X-ray diffraction results indicated that there was no secondary phase detected in the Na2Ti6O13-doped Ba0.94(Bi0.5K0.5)0.06TiO3 ceramics. Scanning electron microscopy observations revealed that the grain size was improved with increasing Na2Ti6O13 content. The samples sintered under reducing atmosphere were reoxidized by annealing in air to induce the PTC effect. The PTCR characteristics of Ba0.94(Bi0.5K0.5)0.06TiO3 ceramics were investigated in terms of Na2Ti6O13 content, reoxidation temperature and time. For x≤0.15% samples, with the increasing Na2Ti6O13 content, both the Curie temperature (TC) and the room temperature resistivity increased. With x=0.15%, the obtained ceramic through a suitable reoxidation process possessed a relatively low room temperature resistivity about 2.27×103Ωcm, the TC about 145.4°C and a moderate PTC effect of ρmax/ρmin=2.39.

Self-assembled In2O3 truncated octahedron string and its sensing properties for formaldehyde

In this work, a continuous two-step route consisting of ammonolysis and re-oxidation process was developed for the growth of a novel In2O3 truncated octahedron string morphology. The as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), results of which indicated that the truncated octahedron strings were self-assembled by In2O3 crystals, and several tens of micrometers along their longitudinal axes. In addition, a gas sensor based on as-synthesized In2O3 truncated octahedron strings was fabricated using a ceramic tube as substrate in an in situ deposition manner. The sensor exhibited good selectivity toward formaldehyde, and showed a significant response for formaldehyde at a concentration lower to 5ppm. The present study may serve as a basis for designing other novel morphology from semiconductor oxides for gas-sensing applications.

Vapor-Phase-Mediated Phenomena Associated with High Temperature, High Water Content Oxidation of MCrAlX Bond Coats

Oxidation of MCrAlX (M = Ni and Co; X = Y or Re) bond coats was carried out at 1,125 °C in a range of N2–O2–H2O environments. A three-step process of (1) oxidation, (2) taper-polishing, and (3) re-oxidation was used to evaluate steady state development of thermally grown oxide (TGO). During initial oxidation, transient (Ni,Co)(Al,Cr)2O4 spinel formed above α-Al2O3. Following taper-polishing, no new spinel grew during 1–200 h of re-oxidation in any water vapor environment; spinel growth at the TGO surface by a steady state mechanism—owing to Al-depletion of the bond coat, as predicted elsewhere—was deemed unlikely. Observations of transient spinel volatilizing in wet environments were supported by measurements of nickel volatilizing from pre-fabricated NiAl2O4 spinel pellets as a function of humidity. In some cases, following volatilization, water promoted vapor phase-redeposition of spinel onto adjacent specimen surfaces. Spinel-related conclusions from past humid oxidation experiments for which volatilization phenomena were not considered—and especially for which Al-depletion of the bond coat is cited as the cause for spinel growth—should be reevaluated.

In-plane mesoporous graphene oxide nanosheet assembled membranes for molecular separation

In-plane mesoporous graphene oxide (GO) sheets were prepared by a re-oxidation process and subsequently assembled into lamellar membranes. The in-plane pores significantly shortened the mass transport paths, resulting in 2–3 times higher water permeance than that of the pristine GO membrane, but continued to reject 3 nm molecules.

Densification of Highly Defective Ceria by High Temperature Controlled Re-Oxidation

Highly enhanced densification and grain growth of Ce0.9Gd0.1O1.95-δ (CGO, gadolinium-doped ceria, with 10 mol% Gd) is achieved in low oxygen activity atmospheres. However, the material can suffer mechanical failures during cooling when the re-oxidation process is not controlled due to the large volume changes. In this work, the redox process of CGO is investigated using dilatometry, microscopy, electrochemical impedance spectroscopy and thermodynamic analysis. In addition, the conditions allowing controlled re-oxidation and cooling in order to preserve the mechanical integrity of the CGO component are defined: this can be achieved over a wide temperature range (800−1200°C) by gradually increasing the oxygen content of the atmosphere. It is found that the electrical conductivity of the CGO, particularly at low temperature (<450°C) is influenced by the sintering and controlled re-oxidation conditions. An increase in activation energy for conduction at low temperature is observed as the re-oxidation temperature decreases. Moreover it was observed that the ionic conductivity blocking effect, usually associated with grain boundary contributions, is not influenced by the grain size but rather by the chemical history of the material.

Reoxidation of direct reduced iron in ambient air

Fresh reduced direct reduced iron pellets produced in a commercial furnace were reoxidised in ambient air for 1–50 days at an average temperature of 27°C. The degree of reoxidation was determined by weight gain. The reoxidation process is controlled by an irreversible first order reaction with respect to oxygen. A model based on reaction between gas and porous solid was used to calculate the degree of reoxidation with time. The calculation results agree satisfactorily with the experimental data. The apparent rate constant of reaction varies between 1·65×10−3 and 4·55×10−3 g/cm2/day, and is inversely proportional to the initial radius of the pellet. The effect of reoxidation on degree of metallisation and relative metallisation loss was also investigated. In all cases, two distinct groups of data resulted, reflecting the behaviour of the two types of oxide pellets charged in the furnace.

Combined production and purification of hydrogen from methanol using steam iron process in fixed bed reactor

A research work is being conducted to study the combined production and purification of hydrogen by means of redox processes departing from biomass fast pyrolysis oils (bio-oils). To achieve that goal, methanol has been used as featured material because it is the most representative compound of the alcoholic fraction of bio-oils. The study has been carried out in a fixed bed reactor where methanol decomposes in H2 and CO when gets in contact with a reactive solid based in an iron oxide at temperatures above 600 °C. During the first stage of the “steam–iron” process, reactive gases reduce the iron oxide to metallic iron. Afterward, in a following step, the previously reduced iron is reoxidized by steam producing a high purity hydrogen stream. Although coke deposition does exist during the reducing stage, this behaves as inert during the reoxidation process. Coke inert role has been corroborated by GC, SEM and TEM techniques, showing that carbon deposits were constituted by ordered structures (carbon nanotubes). The determination of the hydrogen production along successive cycles allowed the evaluation of the effect of temperature and alternating reactive atmospheres on the stability of the solid, as well as the optimum conditions for such purpose.

Polaron relaxation and non-ohmic behavior in CaCu3Ti4O12 ceramics with different cooling methods

Structure, defects, non-ohmic behavior and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics with furnace-cooled and water-quenched methods have been systemically investigated. Cooling methods make an influence on reoxidation process and cause changes of electric properties in grains and at grain boundaries. SEM–EDS provides detailed imaging information about Cu-rich phase on the fracture of CCTO ceramics which is explained by the reoxidation reaction during cooling. Morphologies of the water-quenched sample show very small grains floating in bigger grains; whereas they reveal that many grains are surrounded by an exfoliated sheet of Cu-rich phases in the furnace-cooled sample. The polaron relaxation associated with Ti3+ ions and oxygen vacancies occurs at low temperatures, and more Ti3+ ions are remained in the water-quenched sample. The Schottky barrier height (I–V, non-ohmic behavior) associated with the reoxidation of grain boundaries can be modified by cooling method.