Atm Oxygen Research Articles

Roughness and oxidation: application to NiO growth on Ni at 800°C

The changes with oxidation time of the oxide outer surface roughness, the oxide grain size and the oxidation rate constant k c for NiO growth on Ni were studied at 800°C in 1 atm oxygen. The roughness was determined by optical interferometry establishing 2D-profiles and 3D-images. Correlations were found between the variations of the oxide outer surface roughness or the k c values and the oxide grain size. These are attributed to the decrease of the oxide grain boundary density with time, which induces a decrease of the effective diffusion coefficient.

Lateral grain size and electron mobility in ZnO epitaxial films grown on sapphire substrates

We have fabricated ZnO thin films on sapphire substrates at temperatures ranging from 350 to 1000°C by laser molecular-beam epitaxy. With increasing growth temperature, lateral grain size evaluated by X-ray reciprocal space mapping increased resulting in improved electron mobility. When the film was grown at 1000°C, an electron mobility as large as 90 cm 2/V s was achieved. By annealing in 1 atm of oxygen at 1000°C, thin films having much larger grain size (>5 μm 2) and higher mobility (∼120 cm 2/V s) comparable with those for bulk single crystals could be obtained.

Subsolidus phase relations and dielectric properties in the SrO–Al2O3–Nb2O5 system

Subsolidus phase equilibria in the SrO–Al2O3–Nb2O5 system were determined by synthesis of 75 compositions in air in the temperature range 1200–1600°C. Phase assemblages were determined by X-ray powder diffraction at room temperature. Two new ternary compounds, Sr4AlNbO8 and Sr5.7Al0.7Nb9.3O30, were observed to form in addition to the known double perovskite, Sr2AlNbO6 (Fm3̄m, a=7.7791(1) Å). Sr4AlNbO8 crystallizes with a monoclinic unit cell (P21/c; a=7.1728(2), b=5.8024(2), c=19.733(1) Å; β=97.332(3)°) determined by electron diffraction studies; the lattice parameters were refined using X-ray powder diffraction data, which are given. This compound decomposes above 1525°C; attempts to grow single crystals from neat partial melts, or using a strontium borate flux, were unsuccessful. The phase Sr5.7Al0.7Nb9.3O30 (Sr6−xAl1−xNb9+xO30, x=0.3) forms with the tetragonal tungsten bronze structure (P4bm; a=12.374(1), c=3.8785(1) Å), melts incongruently near 1425°C, and occurs essentially as a point compound, with little or no range of x-values; indexed X-ray powder diffraction data are given. The tungsten bronze structure exhibits a narrow region of stability in the SrO–Al2O3–Nb2O5 system, which is probably related to the small size of Al3+. The existence of an extensive cryolite-type solid solution, Sr3(Sr1+xNb2−x)O9−3/2x, occurring between Sr4Nb2O9 (x=0) and Sr6Nb2O11 (x=0.5), was confirmed, with cubic lattice parameters ranging from 8.268(2) to 8.303(1) Å, respectively. The dielectric properties of the three ternary compounds occurring in the system were measured using the specimen as a TE011 or TE0γδ dielectric resonator: Sr2AlNbO6: εr=25, τf=−3 ppm/°C, tan δ=1.9×10−3 (7.7 GHz); Sr4AlNbO8: εr=27, tan δ=2.8×10−3 (10.5 GHz); Sr5.7Al0.7Nb9.3O30: εr=168, tan δ=3.8×10−2 (3.1 GHz). Sr2AlNbO6, when sintered in 1 atm oxygen, exhibited a reduced permittivity (εr=21) and a significantly improved dielectric loss tangent (tan δ=5.2×10−4, 8.3 GHz), resulting in a four-fold increase in Q×f as compared to the specimen sintered in air.

An investigation of the growth mechanism of scale formed on chromium containing low amounts of sulfur, chlorine, and phosphorus

The growth mechanism of the flat and adherent scale formed at 900°C in 0.1 atm oxygen on chromium, which contained 1 ppm (wt) sulfur, 0.08 ppm (wt) chlorine, and 0.2 ppm (wt) phosphorus, has been found to be very different in adjacent parts of the scale, consistent with the results of previous tracer studies of the growth mechanism of chromia formed on chromium. In some places, it grew primarily by chromium transport, while in others it grew primarily by oxygen transport and in still others it grew by a mixture of both. New oxide formed within the outer part of the scale and, in some cases, throughout the scale. A tentative hypothesis is proposed to explain why growth mechanisms were different in different parts of the scale on the same specimen. The scale formed at 950°C on Fe–20%Cr–0.11%Si, which contained 15 ppm sulfur, grew predominantly by cation transport, with only a small amount of oxygen transport. In this case also, new oxide formed within the outer part of the scale.

High Oxidation State Alkali-Metal Late-Transition-Metal Oxides

ABSTRACTLi2NaCu2O4 has been prepared by solid state reaction under high-pressure (250 Atm) oxygen. A structural study, using time-of-flight powder neutron diffraction on a sample made with 7Li, shows a material isostructural with Li3Cu2O4, with sodium occupying the octahedral and lithium the tetrahedral A-cation sites. A 7Li MAS-NMR study of Li3Cu2O4, Li2NaCu2O4 and Li2CuO2 has been used to confirm the Li/Na site ordering.

Influence of calcium substitution on the formation and thermal stability of the YBa 2Cu 4O 8 superconductor

In order to study the influence of calcium substitution on the thermal stability and phase composition of the YBa 2Cu 4O 8 (Y-124) superconductor, precursor powders with composition Y : Ca : Ba : Cu = 1 − x : x : 2 : 4 ( x = 0.0–0.1) are prepared using an acetate–tartrate based sol–gel method. The phase composition of the products formed after different stages of heat treatment at 800°C and under 1 atm oxygen pressure is examined by X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). We found that in the first stage of the heat treatment, among stable mixed oxides and carbonate, mainly Y 2Ba 4Cu 7O y (Y-247) and CuO are formed. Upon further heating Y-247 can react with CuO to form Y-124, but can also decompose into Y-124 and YBa 2Cu 3O 7 − y (Y-123). The formation of Y-123 is favoured by an increasing amount of calcium substitution. We also demonstrate that the TGA technique makes it possible to detect and distinguish between the presence of the Y-123, Y-124 and Y-247 when they are simultaneously present in a sample, while this is less obvious using only XRD.

Thermal Oxidation Kinetics of MoSi2‐Based Powders

The oxidation process of MoSi2 is very complex, and controversial results have been reported, especially for the early‐stage oxidation before the formation of passive SiO2 film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi2 powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO2 film. The oxidation kinetics of commercial MoSi2‐SiC and MoSi2‐Si3N4 powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X‐ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO3 phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo5Si3 phase formed first and was subsequently oxidized to solid SiO2 and volatile MoO3, resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi2 and Mo5Si3 are proportional to their fractions in the system describes the experimental data well.

A simple and reproducible way to synthesize PrBa 2Cu 4O 8 under 1 atm of oxygen by amorphous citrate method

A simple aqueous solution technique utilizing citric acid (CA) as a complexant was applied to prepare PrBa 2Cu 4O 8 (Pr124) at ambient oxygen pressure. Heating of a mixed-solution of CA, water (H 2O) and ions of Pr, Ba and Cu with a molar ratio of [CA]:[H 2O]:[Pr+Ba+Cu]=3.6:40:1 on a hot-plate set at 200°C produced a transparent amorphous citrate “gel” without any precipitation, which after heat-treatment at 450°C was converted to a precursor for Pr124. The Pr124 compound with only minor traces of BaCuO 2, PrBaO 3 and CuO formed after heat-treating the precursor twice at 850°C for 60 h under 1 atm oxygen pressure. The amount of impurities in the citrate-derived Pr124 was approximately half of that in the best sample previously prepared by the more elaborate high-pressure technique.

Two-beam coupling of a series of reduced BaTiO 3 :Rh at wavelengths of 514 nm and 633 nm

Dedicated to Prof. Dr. Eckard Kratzig on the occasion of his 60th birthday.Five annealed BaTiO3:Rh crystals, including as-grown and those reduced at 10-5 atm, 10-10 atm, 10-12 atm, and 10-14 atm oxygen partial pressures, were produced and investigated by two-beam coupling at wavelengths of 514 nm and 633 nm. The carrier type is a function of reduction degree and incident wavelength. The compensation points are around 10-3 atm and 10-11 atm oxygen partial pressures for 514 nm and 633 nm, respectively. The electrooptic gain is controlled by the photoconductivity–dark conductivity ratio, effects from the deep and shallow traps, and hole–electron competition, which are dependent on the reduction degree and incident wavelength.

Synthesis of PrBa2Cu4O8by Solution Method under Ambient Oxygen Pressure

A highly homogeneous and nearly monophasic PrBa2Cu4O8 compound has been synthesized at 1 atm of oxygen via a route employing polymeric ethylenediamine/citric acid precursors. The PrBa2Cu4O8 compound with a trace amount of BaCuO2 was synthesized for the first time by sintering the pellets at the temperature of 875 °C under ambient oxygen pressure. The temperature coefficient of the resistivity is positive below 190 K while above 190 K is negative. The superconducting transition was not detected down to 4.2 K. Samples were characterized by X-ray diffraction, scanning electron microscopy, thermal analysis, and transport and magnetic measurements.

Yttrium-promoted selective oxidation of aluminium in the oxidation at 1100°C of an eutectic Ni–Al–Cr 3C 2 alloy

The effects of an yttrium addition of 0.2 wt% on the oxidation behaviour of an eutectic Ni–Al–Cr 3C 2 alloy of average composition Ni–12.3 Cr–6.9 Al–1.8 C (wt%) have been investigated at 1100°C in 1 atm oxygen. Experiments have been carried out under isothermal and thermal cycling conditions on the as-cast alloy, in which Cr 3C 2 was present as a finely dispersed eutectic phase, and on directionally-solidified specimens having an aligned structure. In all cases, Al 2O 3 formed almost entirely as an external scale. Although Cr 2O 3 formed in some regions of the surface in the early stages, compact subsurface Al 2O 3 layers generally developed beneath such locations, protecting the alloy from severe internal attack. This behaviour is quite different from that observed for similar yttrium-free alloys, where aluminium was oxidized internally beneath continuous external Cr 2O 3 scales. In the present case, while it was almost certainly formed during the transient stage, the growth rate of Cr 2O 3 was probably reduced due to the presence of the reactive element and the extent of chromium depletion in the alloy was thereby decreased. This yielded an increase in the concentration of chromium in the alloy at its interface with the transient Cr 2O 3 scale compared to the yttrium-free alloy, with a corresponding decrease in the oxygen activity at the same location. As a consequence, the inward flux of oxygen in the alloy was reduced, allowing a transition from internal to external oxidation of aluminium to become possible on longer periods of oxidation. The presence of carbides at the alloy/scale interface was highly detrimental to the adhesion of the oxide scale to the metal substrate, and the scales tended to crack and/or lift on cooling. However, no massive spalling occurred, even during thermal cycling for periods up to 124 h, while the Al 2O 3 scales tended to re-form rapidly after mechanical damage.

Oxidation behavior of nanocrystalline Al alloys containing 5 and 10 at.% Ti

The oxidation behavior of mechanically alloyed(MA) Al-Ti alloys containing 5 and 10 at.% Ti wereinvestigated at 500-600°C under 1 atm of oxygen. Ateach temperature, alloys oxidized linearly during the initial stage and later followed the parabolicrate law. During the initial stage, the oxidation ratesof nanocrystalline (≤50 nm) Al-Ti alloys were fasterthan those of conventional (≤200 nm) alloys. It is suggested that more grain boundaries innanocrystalline alloys provide more nucleation sites foroxides, so that the oxide scales grew faster as denseprotective layers. During the parabolic stage, the nanocrystalline alloys had greater oxidationresistance than conventional alloys because of the denseprotective layer. Oxide scales on both alloys consistedof a mixture of γ-Al2O3 andTiO2 in the outer layer andγ-Al2O3 near the alloy as aprotective layer.

Effects of In-Situ DC-Bias on the Composition, Microstructures and Dielectric Properties of RF Magnetron Reactive Sputtered (Ba,Sr)TiO3 Films

AbstractThin films of (Ba,Sr)TiO3 (BST) have been prepared by rf magnetron reactive sputtering using single alloy target, and in-situ negative DC-bias. Post annealing was applied to the asdeposited films at 620°C for 2 hours in 1 atm oxygen atmosphere. Thin film microstructures and grain sizes were studied and calculated by means of X-ray diffraction, while the film composition was analyzed by Electron Probe X-ray Microanalyzer (EPMA). The dielectric constant of the films was also studied. As the applied DC-bias increased, the dielectric constant increased from 176 to 912, the x-ray diffraction patterns became more intense and narrower, and the (Ba+Sr)/Ti molar ratio increased from 0.70 to 0.83. In-situ DC-bias was observed to be a feasible fabrication process to increase the grain size and relative permittivity of sputtered BST films.

Influence of oxygen partial pressure on growth morphologies in unidirectionally solidified YBCO-Ag superconducting composites

YBCO/Ag superconducting composites were fabricated by the unidirectional solidification method under various oxygen partial pressures. The effect of oxygen partial pressure on the growth interface stability and the morphology of (Y123)-Ag composite was investigated. The growth interface changed from planar to not planar with decreasing oxygen partial pressure. This phenomenon indicated that the interface stability was dominantly affected by the slope of liquidus, which is a significant function of the oxygen partial pressure. The morphology of the Y123-Ag composite was also strongly influenced by the oxygen partial pressure. In the case of 0.21 atm oxygen partial pressure , rod-like silver particles elongated along the growth direction were formed around the bottom of the facet crystal. On the other hand, spherical silver droplets were entrapped at the grain boundaries in the case of atm.

Compact laser molecular beam epitaxy system using laser heating of substrate for oxide film growth

A high-temperature, oxygen compatible, and compact laser molecular beam epitaxy (laser MBE) system has been developed. The 1.06 μm infrared light from a continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to achieve a wide range and rapid control of substrate temperature in ultrahigh vacuum and at up to 1 atm oxygen pressure. The maximum usable temperature was limited to 1453 °C by the melting point of the nickel sample holder. To our knowledge, this is the highest temperature reported for pulsed laser deposition of oxide films. The efficient laser heating combined with temperature monitoring by a pyrometer and feedback control of the Nd:YAG laser power by a personal computer made it possible to regulate the substrate temperature accurately and to achieve high sample heating and cooling rates. The oxygen pressure and ablation laser triggering were also controlled by the computer. The accurate growth parameter control was combined with real-time in situ surface structure monitoring by reflection high energy electron diffraction to investigate oxide thin film growth in detail over a wide range of temperatures, oxygen partial pressures, and deposition rates. We have demonstrated the performance of this system by the fabrication of homoepitaxial SrTiO3 films as well as heteroepitaxial Sr2RuO4, and SrRuO3 films on SrTiO3 substrates at temperatures of up to 1300 °C. This temperature was high enough to change the film growth mode from layer by layer to step flow.

Synthesis and transport properties of substituted Tl2Mn2O7 pyrochlore

Pyrochlore-type Tl 2 Mn 2 O 7 with various levels of substitution by Ru, Bi and Sb has been prepared by high-pressure synthesis at 6 GPa and 1150 °C. The ruthenium system shows a complete solid solution from Tl 2 Mn 2 O 7 to Tl 2 Ru 2 O 7 . All three substituents induce an increase in cell parameter; in the case of Sb, this indicates that the substitution involves rather Sb 5+ on Mn sites than Sb 3+ on Tl sites. These pyrochlores start releasing thallium oxide when annealed at temperatures as low as 150 °C under 1 atm oxygen pressure; the pyrochlore phase, however, is still present at 750 °C. The partial replacement of Mn by Ru or of Tl by Bi in Tl 2 Mn 2 O 7 induces a metal-semiconductor transition, in both cases at low levels of substitution (x<0.2). In the ruthenium case, transport measurements in magnetic fields up to 8 T yielded a magnetoresistance (R 0 –R H )/1v\R 0 =96% in the vicinity of the metal-semiconductor transition.

Optical and Photorefractive Properties of Strongly Reduced BaTiO3

Strong reduction of BaTiO3 induced the appearance of an absorption peak around 600–610 nm and a high dark conductivity, both of which increase with increasing reduction level. The carriers change from holes to electrons after reduction. Because of the influence of the high dark conductivity, beam coupling gain becomes highly dependent on incident light intensity, and response time changes little with intensity. For the sample reduced at 10-15 atm oxygen partial pressure and a temperature of 900°C, the dark conductivity is estimated to be 1.6×10-10 ( Ω·cm)-1, which increases by about four orders of magnitude after reduction.

Floating zone partial melting and solidification of SmBCO superconductor under low oxygen partial pressure

Microstructure control of SmBCO superconductor was carried out using the floating zone partial melting and solidification method under 0.01 atm oxygen partial pressure which is a preferable atmosphere to obtain a crystal with stoichiometric SmBCO. The growth rate, initial composition, and addition of small amount of platinum dependences on the microstructure formations of the (Sm211 + L) mixture during melting and the Sm123 or Sm123/211 during solidification were investigated. Furthermore, superconductive properties of the solidified Sm123/211 were measured by SQUID after appropriate oxygen annealing. Estimated critical current density of the single crystalline Sm123/211 was 3.6 × 104 A/cm at 77 K, 1 T.

Wavelength dependence of carrier-type in reduced BaTiO3:Rh

We report that in BaTiO3:Rh the carrier-type depends not only on the postprocessing condition but also on the incident wavelength in two beam coupling. Carriers are the holes for the as-grown sample, and change to electrons for the sample reduced in the atmosphere of 10−14 atm oxygen partial pressure. However, for the sample reduced in the atmosphere of 10−10 atm oxygen partial pressure, the carrier is an electron for the incident wavelength of 514 nm, and a hole for 633 nm. Its absorption, photoinduced absorption, and two beam coupling are investigated and discussed.

Process for high growth rate and high superconducting properties of REBCO single crystals

Process developments for both higher growth rate and superconducting properties are two main objectives of RE 1Ba 2Cu 3O 7- δ (REBCO or RE123) single crystal growth. In this paper, we will review several processes for increasing growth rate and achieving higher T c REBCO superconductors. There are two effective methods to enhance the growth rate. One is to grow single crystals under high oxygen partial pressure, the other is to partially or completely replace Y by RE elements with higher solubilities in the Ba-Cu-O solution, for instance RE = Nd, Sm. Using these methods the growth rate was increased. The unprecendently large REBCO single crystals with the size 25 × 25 mm 2 on the a-b plane and up to 20 mm in thickness were obtained. Regarding superconductivity, the critical temperature T c of about 93 K was achieved from the Y123 single crystals both grown under air and 1 atm oxygen pressure, indicating that T c is insensitive to the growth atmosphere of the oxygen pressure in the YBCO system. Furthermore, the high T c of about 95 K NdBCO superconductors with a sharp transition was obtained by using Ba-rich liquid (Ba:Cu ratio = 0.75–0.80 of solvent), suggesting that the liquid composition is an important thermodynamic parameter for controlling T c. Moreover, the partial substitution RE (RE = Nd, Sm) at the Y sites up to 30% shows no obvious reduction of T c remaining at about 91±1 K.