Low Oxygen Partial Pressure Atmosphere Research Articles

High-Temperature Behavior, Oxygen Transport Properties, and Electrochemical Performance of Cu-Substituted Nd1.6Ca0.4NiO4+δ Electrode Materials

In this study, Nd1.6Ca0.4Ni1−yCuyO4+δ-based electrode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) are investigated. Materials of the series (y = 0–0.4) are obtained by pyrolysis of glycerol-nitrate compositions. The study of crystal structure and high-temperature stability in air and under low oxygen partial pressure atmospheres are performed using high-resolution neutron and in situ X-ray powder diffraction. All the samples under the study assume a structure with Bmab sp.gr. below 350 °C and with I4/mmm sp.gr. above 500 °C. A transition in the volume thermal expansion coefficient values from 7.8–9.3 to 9.1–12.0 × 10−6, K−1 is observed at approximately 400 °C in air and 500 °C in helium.The oxygen self-diffusion coefficient values, obtained using isotope exchange, monotonically decrease with the Cu content increasing, while concentration dependence of the charge carriers goes through the maximum at x = 0.2. The Nd1.6Ca0.4Ni0.8Cu0.2O4+δ electrode materialdemonstrates chemical compatibility and superior electrochemical performance in the symmetrical cells with Ce0.8Sm0.2O1.9, BaCe0.8Sm0.2O3−δ, BaCe0.8Gd0.19Cu0.1O3−δ and BaCe0.5Zr0.3Y0.1Yb0.1O3−δ solid electrolytes, potentially for application in IT-SOFCs.

Improved ferroelectric and piezoelectric properties of (Na0.5K0.5)NbO3 ceramics via sintering in low oxygen partial pressure atmosphere and adding LiF

Dense ([Formula: see text][Formula: see text]NbO3 lead-free ceramics with the simple composition were prepared via sintering in low oxygen partial pressure (pO2, [Formula: see text][Formula: see text] atm) atmosphere and adding LiF. All the ceramics have pure orthorhombic structure. Compared to the LiF-added ([Formula: see text][Formula: see text]NbO3 ceramics sintered in air and the low pO2-sintered pure ([Formula: see text][Formula: see text]NbO3 ceramics without LiF addition, the present ceramics exhibit improved piezoelectric and ferroelectric properties. The piezoelectric constant [Formula: see text] is 125 pC/N, and the converse piezoelectric constant [Formula: see text]* is 186 pm/V. The dielectric constant and dielectric loss of the ceramics at room temperature and 1 kHz are 451 and 0.03, respectively. Under the measured electric field of 70 kV/cm, the remanent polarization is 25.9 [Formula: see text]C/cm2 and the coercive field is 13.9 kV/cm. Furthermore, if the base metals such as Cu and Ni powders were mixed into the green pellets and sintered in the low pO2 atmosphere, the base metals cannot be oxidized, suggesting possibility of using base metals as electrodes.

Growth Kinetics and Characterization of Chromia Scales Formed on Ni–30Cr Alloy in Impure Argon at 700 °C

The oxidation of a Ni–30Cr alloy at 700 °C in impure argon was studied in order to provide new elements of understanding on chromia scale growth in low oxygen partial pressure atmosphere (10−5 atm). Oxidation tests were carried out during 30 min to 50 h in a thermogravimetric analysis system using a symmetrical balance with in situ monitoring of the oxygen partial pressure. The oxidation kinetics were determined as parabolic with an estimated stationary parabolic constant value of 10−15 cm2 s−1, after a transient stage of about 3 h. The oxide scale was identified as a pure chromia layer by TEM and XPS characterisations. After 50 h at 700 °C, the scale thickness estimated by TEM cross section observation was about 100 nm. A slightly thicker and more porous oxide scale was observed above the alloy’s grain boundaries. The metal/oxide interface also exhibited a deeper recession towards the substrate above the alloy’s grain boundaries. The orientation of chromia grains was determined by TKD (transmission Kikuchi diffraction). A strong preference was noted for the orientation perpendicular to the surface, along the direction of the corundum structure. Finally, the type of semiconduction was determined for the scales formed after 7 h and 50 h of oxidation. For the shorter oxidation time, the chromia scale exhibited an n-type semiconduction, whereas for the longer exposure, both n-type and insulating semiconduction were identified.

Atmospheric controlled processing enabling highly textured NKN with enhanced piezoelectric performance

High-performance lead-free piezoelectrics are of interest as alternate materials to confront potential legislative actions on lifting the exemption on lead-based piezoelectric materials. In this work, we demonstrated a highly textured commercial-grade Li- and Ta- modified (Na, K)NbO3 (NKN) lead-free piezoceramic that was sintered under low oxygen partial pressure (pO2) atmospheres using the solid state template grain growth (TGG) method. It was found that a moderate low pO2 condition not only facilitated the reactive template grain growth but also lowered the temperature required for such process. Highly textured NKN with a converse piezoelectric coefficient d33*≈680 pm V-1 was obtained, this remarkable piezoelectric property can be explained by the position of the polymorphic phase boundary of NKN. With considerations on both performance and cost, this work not only presented the potential for NKN to meet the requirement of commercial applications, but also provided an effective processing strategy to utilize low cost base metal inner electrodes in the future.

Residual stress and electrical properties of Pb(Zr0.52,Ti0.48)O3 thin films RF sputtered directly on Cu foils

AbstractPolycrystalline Pb(Zr0.52Ti0.48)O3 (PZT) thin films between 250 and 1000 nm thick were deposited on Cu foils via RF magnetron sputtering. Samples were crystallized ex situ between 550°C and 750°C in a low oxygen partial pressure atmosphere, pO2, in order to avoid oxidation of the substrate. These were compared to films made on more common Pt/TiOx/SiO2/Si substrates also crystallized under low pO2 conditions. The mismatch of the coefficients of thermal expansion for Cu and PZT caused large compressive residual stresses to develop in the films, whereas films on Pt‐Si experienced more moderate tensile stresses. Stress was measured using the sin2ψ method. In addition to mechanical implications, i.e., film cracking and delamination, the effect of residual stress on electrical properties is discussed. Dielectric constants of PZT were lower on Cu than on Pt/TiOx/SiO2/Si. This could be due either to a dead layer effect or to the residual stress imposed by the substrate. The remanent polarizations for films on Cu were between 18 and 41 μC/cm2, while coercive fields were between 37 and 54kV/cm. Rayleigh analysis was used to describe the role of defects affecting domain wall mobility, as they act as pinning centers and decrease the extrinsic polarization response.

Anti-coking Effect of MnCr2O4 Spinel Coating during Light Naphtha Thermal Cracking

The research aimed at fabricating MnCr2O4 spinel coating on HP 40 alloy, and evaluating the influence of the spinel coating on coking behavior during thermal cracking of light naphtha. MnCr2O4 spinel coating formed on the alloy surface by selectively oxidized under low oxygen partial pressure atmosphere at 1050oC for 20h. The as-formed spinel coating was characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The results show that the spinel scale is uniform and compact, with blade-like structures on the top. The coking behavior was studied from the morphology and microstructure of coke, and characterized by scanning electron microscope (SEM) and Raman spectra, respectively. The results show that, only globular coke presents on MnCr2O4 spinel coating compared with blank alloy, and the coke exhibits more disordered graphite structures. Thus, the spinel scale has a favorable anti-coking property.

Effect of thermal treatment on TiO2 varistor properties in different atmospheres

TiO2 is a typical nonstoichiometric compound and defects such as oxygen vacancies in TiO2 grains and at TiO2 grain boundaries are very sensitive to the oxygen partial pressure in the environment. In this paper, commercial TiO2 doped with Nb2O5 and Bi2O3 was used as the raw material to prepare TiO2 varistors. The samples were thermally treated in three kinds of atmospheres: H2, vacuum, and O2 at 600°C. In the two low oxygen partial pressure atmospheres of vacuum and H2, due to the volatilization of lattice oxygen in TiO2, the oxygen vacancy concentration and the number of semiconducting grains increase, while the height and width of the potential barrier and the varistor voltage decrease. After the oxygen atmosphere thermal treatment, the acceptor state density and the height and width of the potential barriers increase, the conductivity decreases, and the varistor voltage improves significantly as a result of oxygen accumulation at grain boundaries.

Novel Glass-free LTCC Material co-fired with Cupper-Electrodes

Abstract Low Temperature Co-fired Ceramics (LTCC) have excellent high-frequency characteristics and have widely been used for microwave electronic components. By lowering the sintering temperature of the ceramics used as insulating layers, LTCC was co-fired with a high-conductivity wiring conductor, such as Cu or Ag. LTCC substrate has been expected as one of the most promising technologies to realize miniaturization of RF circuits in the field of wireless communications. There is no limitation to demand for further downsizing of RF circuits, suppression of electric loss and high mechanical strength of the substrate. However, conventional LTCC materials for substrates contain glass frit which causes defects, such as pores or cracks, and low mechanical strength. In this work, we have developed a novel LTCC material system BaO-Al2O3-SiO2-MnO-TiO2, without any glass frits. The material was co-fired with cupper electrodes, which have low resistivity and show less diffusion than silver in LTCC, under a low-oxygen partial pressure atmosphere (mixture of N2 and H2) at 980°C. Thin layers (8μm) of the material showed high insulating resistivity and reliability due to few defects, such as pores, in LTCC. Its dielectric and mechanical properties were measured as 6.8 (low-εr), 350 at 3GHz (high-Q-value) and 341MPa (high mechanical strength) respectively. This LTCC material will contribute to further miniaturizing of microwave applications and integration of passive elements.

Demonstration of Copper Co‐Fired (Na, K)NbO3 Multilayer Structures for Piezoelectric Applications

A Li and Ta modified (Na, K)NbO3 piezoelectric ceramic has been successfully co‐fired with inner copper electrodes in a reduced atmosphere. Highly dense NKN ceramics (95% relative density, 4.64 g/cm3) were obtained by sintering the samples in a low oxygen partial pressure (low pO2) atmosphere at 1050°C. The poly(propylene carbonate) binder system was used to permit a clean burnout at low temperature in N2 atmosphere, and also prevent the electrode copper particles from undergoing any oxidation. No interdiffusion of copper, chemical reactions, and/or carbon residues were observed in the grains, grain boundaries, or at the electrode–ceramic interface of the co‐fired samples from a detailed transmission electron microscopy (TEM) analysis. Dielectric and piezoelectric properties were characterized from those co‐fired prototyped samples. The samples displayed high relative dielectric permittivity above 800, with low dielectric loss about 3.6%. A normalized strain coefficient (max. strain/max. electric field) of = 220 pm/V was obtained under unipolar converse electromechanical measurement at 20 kV/cm. This paper presents the feasibility of co‐firing a Cu inner electrode with NKN ceramics toward multilayer lead‐free piezoelectric applications, providing an engineering route to narrow the performance differences between soft lead‐based piezoelectrics and lead‐free materials.

Multichannel mixed‐conducting hollow fiber membranes for oxygen separation

SignificanceA multichannel mixed‐conducting hollow fiber (MMCHF) membrane, 0.5 wt % Nb2O5‐doped SrCo0.8 Fe0.2O3‐δ (SCFNb), has been successfully prepared by phase inversion and sintering technique. The crystalline structure, morphology, sintering behavior, breaking load, and oxygen permeability of the MMCHF membrane were studied systematically. The MMCHF membrane with porous‐dense asymmetrical microstructure was obtained with the outer diameter of 2.46 mm and inner tetra‐bore diameter of 0.80 mm. The breaking load of the MMCHF membrane was 3–6 times that of conventional single‐channel mixed‐conducting hollow fiber membrane. The MMCHF membrane showed a high oxygen flux which was about two times that of symmetric capillary membrane at similar conditions as well as a good long‐term stability under low oxygen partial pressure atmosphere. This work proposed a new configuration for the mixed‐conducting membranes, combining advantages of multichannel tubular membrane technology and conventional hollow fibers. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1969–1976, 2014

Piezoelectric and dielectric properties of Sn-doped (Na0.5K0.5)NbO3 ceramics processed under low oxygen partial pressure atmosphere

Sn-doped (Na0.5K0.5)NbO3 (Sn-NKN) ceramics fired under various oxygen partial pressure (pO2) conditions have been investigated and discussed in terms of bulk piezoelectric and dielectric properties. X-ray diffraction measurements and Rayleigh analysis indicate that the substitution site of the Sn cations depend on the pO2 atmosphere in the firing process. For pO2 higher than 1.0 × 10−10 atm, Sn cations mainly substitute as Sn4+ at the B-site of perovskite NKN, whereas Sn2+ A-site substitution is favored under a low-pO2 atmosphere. Low-pO2 fired Sn-NKN ceramics exhibit higher relative permittivity, Curie temperature, and piezoelectric coefficient (d33). Sn2+ at A-site acts as a donor and reduces the p-type carrier concentrations that result from an electronic compensation of metal vacancies created through the high volatility of Na and K suboxides. The higher piezoelectricity and resistivity in low-pO2 fired Sn-NKN ceramics make this material suitable for base-metal cofired devices such as Ni-inner-electrode multilayer capacitors and actuators.

Crystallization Atmosphere and Substrate Effects on the Phase and Texture of Chemical Solution Deposited Strontium Niobate Thin Films

Strontium niobate (Sr:Nb = 1:1) thin films were prepared via chemical solution deposition on (001)‐oriented SrTiO3, (001)p‐oriented LaAlO3, (0001)‐oriented sapphire, and polycrystalline alumina substrates. Crystallization in oxygen at 1000°C yielded Sr2Nb2O7 films on all substrates with strong (010) orientation. Films on LaAlO3 and SrTiO3 single‐crystal substrates possessed a small amount of preferred in‐plane orientation, whereas films prepared on sapphire and polycrystalline alumina substrates were fiber textured. Films crystallized at 900°C in a low oxygen atmosphere (~10−21 atm pO2) formed a randomly oriented polycrystalline perovskite, SrNbO3−δ on all substrates. A similar set of films crystallized at 900°C at a slightly higher oxygen partial pressure (~10−15 atm pO2) was comprised of Sr2Nb2O7 and SrNbO3−δ phases, exposing the dependence of phase formation on oxygen partial pressure. When subjected to a high‐temperature anneal in oxygen, the SrNbO3−δ phase is shown to transform into Sr2Nb2O7, however, Sr2Nb2O7 did not significantly reverse transform into SrNbO3−δ after annealing in low oxygen partial pressure atmospheres.

Substrate-dependant chemical stability and conductivity of LaNiO3 − x thin films

This work studies the chemical stability and the changes of structure and electrical conductivity of LaNiO3−x films under the low oxygen partial pressure (pO2) atmosphere. Polycrystalline and epitaxial LaNiO3−x films were grown on a number of substrates including Si, LaAlO3 and SrTiO3 by sol–gel method. A range of structural and electrical characterizations were carried out for the grown films. The results showed that the chemical stability and oxygen stoichiometry of the LaNiO3−x films were substrate-dependent, as well as influenced by the preparation process. The compressive strain introduced by the heteroepitaxial growth helped the films to keep a higher oxygen stoichiometry and therefore showed a higher conductivity than the polycrystalline and stretched epitaxial films. The epitaxial films, in particular under the tensile strain, showed a higher stability with a slower oxygen loss rate in vacuum at high temperature.

Structure, electrical conductivity and oxygen permeability of Ba0.6Sr0.4Co1−xTixO3−δ ceramic membranes

Cubic perovskite oxides, Ba0.6Sr0.4Co1−xTixO3−δ (BSCT, x=0.12–0.30) are prepared by conventional solid state reaction process as oxygen permeation membranes. The effects of Ti-doping on the crystal structure, electrical conductivity and oxygen permeability are investigated. The chemical stability in low oxygen partial pressure atmospheres is enhanced by Ti-doping. The partial substitution of Ti for Co is charge-compensated by the reduction of oxygen vacancy concentration in BSCT. Both the electrical conductivity and the oxygen permeation flux of BSCT decrease with Ti doping level. The large bonding energy of Ti–O and the decreased lattice free volume with Ti substitution lead to increased activation energy of oxygen permeation, which, combined with the decreased oxygen vacancy concentration, is responsible for the decreased oxygen permeation flux. Nevertheless, sufficiently high permeation fluxes, ca. 2.12 and 1.46mLcm−2min−1 at 900°C, are obtained for the samples with x=0.12 and 0.30, respectively, which were comparable to that of the well known Ba0.6Sr0.4Co0.8Fe0.2O3−δ (BSCF5582) material. Furthermore the BSCT material shows much better phase stability compared to BSCF5582 material under oxygen permeation condition. The prepared BSCT materials are promising candidates for oxygen separation or POM applications.

Li4Ti5O12Heattreated under Nitrogen Ambient with Outstanding Rate Capabilities

The powders of spinel Li4Ti5O12were prepared by heat treating the mixture of rutile TiO2and Li acetate at800∘Cfor 3 h under a nitrogen atmosphere and, for comparison, under air as well. The powders heated underN2show a remarkably higher-rate capability and better cycle stability. The discharge capacity of Li4Ti5O12heated underN2at 19°C (corresponding to a 3.2-minute total discharge) reached 107 mA hg−1, 22 mA hg−1higher than that of Li4Ti5O12heated under air, which was 85 mA hg−1. The former material also shows a much better cycle stability, with no discharge capacity loss after 300 cycles at 6°C or 16.3°C. The results indicate that heat treatment under low-oxygen partial pressure atmosphere such asN2could significantly improve the high-rate performance of spinel Li4Ti5O12.

Selective Oxidation and Reactive Wetting of 1.0 Pct Si-0.5 Pct Al and 1.5 Pct Si TRIP-Assisted Steels

The effect of oxygen partial pressure on the selective oxidation and reactive wetting behavior of 1.0 pct Si-0.5 pct Al and 1.5 pct Si TRIP-assisted steels was studied. The annealing atmosphere affected the surface chemistry and the oxide morphology, which in turn affected reactive wetting. Similar wetting results were observed for the two TRIP steel compositions with good wetting being observed at the two lower oxygen partial pressure atmospheres (220 K (–53 °C) or 223 K (–50 °C) dew point (dp) and 243 K (–30 °C) dp) with poor reactive wetting at the higher oxygen partial pressure atmosphere (278 K (+5 °C) dp). The differences in wetting were attributed to the oxide morphology. The predominant oxide morphology at the two low oxygen partial pressure atmospheres was larger, widely spaced oxide nodules. At the 278 K (+5 °C) dp, the predominant oxide morphology was smaller, more closely spaced oxide nodules. TEM analysis revealed that at the 243 K (–30 °C) dp, Fe2Al5 was able to form between oxide nodules promoting good reactive wetting. At the 278 K (+5 °C) dp, the more closely spaced nodules may have impeded the formation of Fe2Al5, thereby producing numerous bare spot defects in the zinc coating.

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H2–H2O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO2 sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.

Conductive oxide thin films: Model systems for understanding and controlling surface plasmon resonance

Degeneratively doped conductive oxides represent a unique host for exploring the inter-relationship between the properties of charge carriers and their collective plasmonic response. These materials often lack interband transitions that obfuscate interpretation of spectral response in elemental metals, and unlike metals, the electronic transport properties of conductive oxides are easily tunable. This work explores the process-structure-property relationships that regulate surface plasmon resonance (SPR) in sputter deposited indium tin oxide (ITO) thin films. Film deposition conditions are used to regulate film microstructure and tune the electronic mobility to between 7 and 40 cm2 V−1 s−1. Postdeposition annealing in low oxygen partial pressure atmospheres is used to engineer the ITO defect equilibrium and modulate carrier concentrations to between 1020 and 1021 cm−3. These electronic transport properties are modulated with near independence enabling straightforward interpretation of their influence on the SPR response observed in the infrared reflectivity spectrum. Higher electronic mobilities favor narrower surface plasmon absorption bands, while higher carrier concentrations favor higher absorption band frequencies. A simple free electron model, having only electronic carrier density and electronic mobility as variables, can be used to describe ITO’s dielectric response. Calculations that combine this dielectric function and the Fresnel equations provide simulated reflectivity spectra that match experimental data with remarkable accuracy. Because these spectra use no fitting parameters and are calculated with well-studied material properties, it opens the opportunity for future design of plasmonic response in advanced material systems including degeneratively doped semiconductors, silicides, and nitrides.

Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatings

In the present study, the applicability of thermal barrier coatings (TBCs) on γ-TiAl alloys was investigated. Two alloys with the chemical compositions of Ti-45Al-8Nb-0.2B-0.15C and Ti-45Al-1Cr-6Nb-0.4W-0.2B-0.5C-0.2Si were used. Before TBC deposition, the specimens were pre-oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron-beam physical vapour deposition (EB-PVD). The oxidation behaviour of the γ-TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900 °C. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). No spallation of the TBC was observed for pre-oxidised specimens of both alloys when exposed to air at 850 °C for 1100 cycles of 1 h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO2/Al2O3 protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900 °C, the oxide scales on the alloy Ti-45Al-8Nb-0.2B-0.15C pre-oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900 °C.

The effect of sintering atmosphere on V2O5 substituted BiNbO4 microwave ceramics

In multiplayer passive devices, low sintering temperature dielectric materials were needed to co-fire with low melting point inner electrode such as copper or silver, a major problem of base metal electrode (BME) was that the devices must be fired under low oxygen partial pressure atmosphere to protect Cu from oxidation. In this paper, dielectric properties of Bi(VxNb1−x)O4 (x = 0.001, 0.004, 0.008, 0.016, 0.048) microwave ceramics sintered under air and N2 atmosphere have been investigated. The densification temperature sintered in different atmosphere decreased from 1010 to 830°C with the amount of V2O5 increasing from 0.001 to 0.048. Due to the increasing vacancy defects, the density of ceramics sintered in N2 was smaller than that sintered in air. The ceramics sintered under N2 have similar dielectric constant, and its Qf values are higher while x < 0.016.