Oxygen Gettering Research Articles

Reduction chemistry of platinum group metal perovskites

The reduction chemistry of the perovskite and perovskite-related compounds, CaRuO 3, SrRuO 3, BaRuO 3, SrIrO 3, and BaIrO 3, was examined. Samples were reduced at low temperatures (≤ 600°C) by direct reduction with hydrogen gas and by a method that utilized zirconium metal as an oxygen getter. No evidence for oxygen deficient perovskites was observed. In almost all cases, the mixed metal oxide was reduced to form the zero valent platinum group metal and the binary oxide or hydroxide (AO or A(OH) 2). One exception was seen in the reduction of SrIrO 3 via the getter method where the intermediate Sr 4IrO 6 was found.

Model for dislocation locking by oxygen gettering in silicon crystals

Oxygen gettering to dislocations slows and stops dislocation motion caused by applied stress in silicon crystals. A model is presented that quantitatively describes the inhibition of dislocation motion by accounting for the drag caused by the oxygen atmosphere in the crystal around the dislocation and for oxygen aggregates inside the dislocation core. The oxygen distribution is computed by analysis of diffusion and stress-assisted migration in the crystalline lattice. The predictions of the model agree quantitatively with the experimental data of Imai and Sumino. Hysteresis is predicted in the dependence of the dislocation velocity on applied stress and explains the difference in the unlocking and locking stresses for dislocation motion.

Proton-induced reduction ofR s, Jc, andT c in YBa2Cu3O7?? thin films

The authors explored the effect of 2-MeV H{sup +} irradiation on the superconducting transport properties of thin films of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({Tc}, J{sub c} (B = 0; 77 K, 4.2 K), and R{sub s} (36 GHz; T)). The inductively measured critical temperature {Tc} changed slowly and uniformly ({approximately}2 K per 10{sup 16}/cm{sup 2}) for fluences less than {approximately}3 {times} 10{sup 16}/cm{sup 2}. Beginning at {approximately}3-4 {times} 10{sup 16}/cm{sup 2}, the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwave {Tc} (as defined by the sharp transition in R{sub s} as a function of temperature) resembled the low-frequency inductive {Tc} measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance ({approximately}6-10 m{Omega}) was not affected for fluences up to 5 {times} 10{sup 16}cm{sup 2}. The sudden and reproducible reduction in the microwave {Tc} transition was interpreted as a sensitive indicator of disruption in the copper-oxygen chain sublattice, and the proton-induced change was compared to that observed in oxygen gettering studies of bulk materials.

An experimental study of the influence of oxygen on silicide formation with tungsten deposited from tungsten hexafluoride

Tungsten disilicide (WSi2) was formed by annealing tungsten films deposited by low-pressure chemical vapor deposition on 〈100〉-silicon substrates. The influence of oxygen on the silicidation rate was studied. Si wafers with different oxygen content in the form of Czochralski, float-zone, and epitaxial wafers were used. Oxygen was also ion implanted into either the silicon substrate or the as-deposited tungsten film. The Rutherford backscattering technique was used to follow the progress of the silicidation. The silicidation rate was found to be dependent on the oxygen content of the Si substrates. The rate was lowest for Czochralski substrates and highest for float-zone substrates. Secondary ion mass spectroscopy was used to study the oxygen and fluorine profiles in the films prior to and after silicidation. Growth of WSi2 was found to be retarded concurrently with a pile-up of fluorine at the tungsten side of the W/WSi2 interface and a gettering of oxygen from the annealing atmosphere at the interface. Growth of WSi2 was then transferred to the tungsten surface. Oxygen implantation into silicon and tungsten, respectively, reduced the rate of silicide formation. Oxygen implantation into tungsten altered the distribution of fluorine and suppressed WSi2 growth at the tungsten surface. The observations led to a conceptual model, which ascribes the retardation in the growth of the inner WSi2 to a‘‘poisoning’’ effect caused by the increase of oxygen and fluorine levels at the interface.

Oxidation behaviour of boron carbide

The oxidation behaviour of various boron-carbide samples have been studied by photoelectron spectroscopy. The exposure of sputter-cleaned surfaces to molecular oxygen at room temperature did not cause a measurable surface oxidation. An increased reactivity has been found by irradiating boron carbide with energetic oxygen ions. The highest reactivity against oxidation was obtained during the growth of a boron-carbide surface phase which revealed a marked oxygen gettering effect.

Results of JET operation with beryllium

The 1989 JET experimental campaign assessed the properties of beryllium as a plasma-facing material. This campaign had three phases: (i) an all graphite phase to recommission the tokamak, (ii) a phase with beryllium evaporation on graphite to isolate the effect of oxygen gettering and (iii) a beryllium limiter phase. Beryllium evaporation removed oxygen from the plasma and reduced carbon concentrations by a factor of two. Strong deuteron pumping was obtained which resulted in a significant, long-term improvement in density control and eliminated the deconditioning effect of disruptions. The use of beryllium limiters reduced the plasma radiation, increased the density limit, reduced the incidence of disruptions and enhanced the deuteron pumping. In the beryllium phases impurity influxes were lower during ICRH heating which permitted the first ICRH-only H-modes. The deuteron pumping allowed heavy gas fuelling to be used to reduce impurity levels of limiter and X-point plasmas.

Preparation of hypostoichiometric UO2-x at low temperatures and study of some properties

Preparation of UO2-x samples by means of thermal treatment of zirconium or quartz ampoules containing powdered UO2 and thorium metal having properties of oxygen getter is described. UO2-x samples were identified by X-ray technique. Oxide prepared was a black powder with crystal lattice parameter exceeding that of stoichiometric UO2.000. It is shown that under such conditions UO2-x may be prepared within the temperature range of 500–1200°C and fixed by quenching at room temperature. Oxygen coefficient values of oxides were estimated by method of solid electrolyte coulometry (SEC). Lattice parameter versus O/U ratio was determined. Maximum value of UO2-x lattice parameter was 0.54760 nm which agreed with UO1.98.

Modification of zirconia film properties by low-energy ion bombardment during reactive ion-beam deposition

The effect of low-energy ion bombardment on the structure and properties of zirconia films deposited by reactive ion-beam process is investigated. Bombardment of 100-eV Ar+ during film growth is shown to induce a substantial relaxation of the residual stress as a result of structural modification by the increased mobility of adatoms. Concurrently, the oxygen gettering capability, or film stoichiometry, is improved by the enhanced diffusion of oxygen on substrate surface. The structural change by thermal annealing, up to 600 °C, demonstrates that fine-grained (200–400 Å) cubic zirconia can be stabilized down to room temperature without alloying stabilizers. The impingement of low-energy Ar+ during film growth results in an as-deposited film microstructure which promotes the growth of the cubic-phase crystallites upon post-deposition annealing as well as the texturing of film plane into (111) orientation. The refractive index and optical transmission of the zirconia films are shown to be dependent predominantly upon the oxygen content in the film and less sensitive to the improved atomic packing due to the low-energy Ar+ bombardment.

Concept for a beryllium divertor with in-situ plasma spray surface regeneration

Two serious problems with the use of graphite tiles on the ITER divertor are the limited lifetime due to erosion and the difficulty of replacing broken tiles inside the machine. Beryllium is proposed as an alternative low-Z armor material because the plasma spray process can be used to make in-situ repairs of eroded or damaged surfaces. Recent advances in plasma spray technology have produced beryllium coatings of 98% density with a 95% deposition efficiency and strong adhesion to the substrate. With existing technology, the entire active region of the ITER divertor surface could be coated with 2 mm of beryllium in less than 15 h using four small plasma spray guns. Beryllium also has other potential advantages over graphite, e.g., efficient gettering of oxygen, ten times less tritium inventory, reduced problems of transient fueling from D/T exchange and release, no runaway erosion cascades from self-sputtering, better adhesion of redeposited material, as well as higher strength, ductility, and fracture toughness than graphite. A 2-D finite element stress analysis was performed on a 3 mm thick Be tile brazed to an OFHC soft-copper saddle block, which was brazed to a high-strength copper tube. Peak stresses remained 50% below the ultimate strength for both brazing and in-service thermal stresses.

Analysis of Oxygen Gettering and Dislocation Locking in Silicon

ABSTRACTThe motion under an applied stress of dislocations in silicon crystals is slowed and stopped by the presence of oxygen inthe material. A model is presented that quantitatively describes the inhibition of dislocation motion by accounting for the oxygen gettering to dislocations caused by diffusion and stress-enhanced migration. Drag on the dislocation motion ismodelled using the elastic interactions between the interstitial oxygen and the dislocation and the energy needed to breakbonds formed between silicon and aggregated oxygen atoms within the dislocation core. The predictions of the model agree quantitatively with the experimental data of Imai and Sumino.

Effect of beryllium evaporation on the performance of ICRH on JET

Beryllium films (150 to 300 Å) have been evaporated on all JET in-vessel components (carbon limiters, nickel screen, inconel walls). Immediate and lasting improvements of the plasma have been observed in all plasma regimes. We concentrate on the results obtained with ICRH and NBI on the density limit and on H-mode plasmas which are particularly sensitive to impurity release. Specific effects produced by RF waves are reduced to a negligible level when the following 3 aspects are combined - beryllium evaporation, dipole phasing, correct screen angle. The density limit obtained with ICRF is now identical to NBI heated cases and substantially higher than the values obtained in the pre-beryllium phase. Long H-modes (1.3 s) have been obtained with RF alone. The H mode confinement is similar to that of NBI cases. In limiter plasmas Z eff decreased to 1.2–1.4 at high density and for 10 MW input power. The results are related to oxygen gettering, suppression of nickel sputtering from the antenna screens and to the use of dipole phasing.

High tempeature oxidation of Fe-31 Mn-9Al-xCr-0.87C alloys (x = 0, 3 and 6)

Three austenitic Fe-31 Mn-9Al-0.87C based alloys with various contents of chromium are oxidized in dry air from 800 to 1100° C. Decarburization takes place in the first stage of oxidation, and results in a porous initial alumina layer followed by a uniform and bulky oxide layer. In the second stage of oxidation or at higher temperature, the oxidation is dominated by the metallic elements. The chromium addition in this study acts as the oxygen getter to retard the oxidation of iron and especially decarburization in the first stage of oxidation. Qualitative and quantitative microscopic techniques are employed to investigate the morphological development and the elemental redistribution in the alloy system.

水冷銅ハースによるプラセオジムおよびネオジムの電子ビーム溶解挙動

Commercial-grade purity Pr and Nd metal rods were repeatedly melted in a 10kW EB-furnace to grasp the features of EB-melting of rare earth metals and the properties of the EB-melted rare earth metals. Initial amount of the melting samples was about 40g, and the beam current added was 0.22A and net melting time was 25X2min every double-melting for both metals.(1) The average weight-loss of Pr and Nd metals were respectively 7% and 25% every double-melting, suggesting about 350°C and 250°C higher temperatures of the melting pools than each melting point of Pr and Nd, respectively.(2) The significant decrease of the pressure inside the furnace after the melting of Nd rods reveals that the vapour of the metal can act as a strong getter of oxygen and nitrogen.(3) Among the gaseous impurity elements (C, H, O, N) with a total amount of 5000 wt.ppm and the metallic ones with each amount of several wt. ppm, only H in Pr was effectively removed from 76 to 4 wt. ppm. EPMA images showed the presence of a large amount of inclusion-like deposits for oxygen, but not for nigrogen.(4) Vickers hardness of Pr and Nd metals lowered from 58 to 32 and 50 to 36, respectively, while the grain size increased from 60 to 120μm in both metals.(5) Initiation temperature of oxidation detected by weight increase was 450°C for Pr metal and above 900°C for Nd metal. But the oxidation of the former before EB melting proceeded rapidly owing to the formation of cracks and the collapse of specimen.

Al 3Ti formation by diffusion of aluminum through titanium

We study the effect of oxygen contamination on the reaction of Al/Ti bilayers on thermal annealing. The samples are annealed in a vacuum better than 5 × 10 -7 Torr at temperatures varied between 400 and 550°C. The samples are, moreover, wrapped in a titanium foil used as a getter for oxygen. The presence of a thin oxide film at the Al-Ti interface reduces the number of nucleation sites of the aluminide phase. The diffusion of aluminum is not affected significantly by this interfacial oxide layer. As the aluminum diffuses through the titanium layer to the free surface, an aluminide layer forms there. Similarly to the interfacial aluminide layer, this top layer is very sensitive to oxygen contamination.

Titanate Ceramics for the Immobilization of Sodium‐Bearing High‐Level Nuclear Waste

The phase chemistries and microstructures of titanate‐based ceramics containing simulated high‐level nuclear waste with varying sodium contents were compared. Incorporation of relatively low sodium levels (∼0.9 wt% Na2O) resulted in more complex phase assemblages. The principal hosts for sodium were hibonite and freudenbergite, and, when iron and sodium were present in combination, loveringite was also stabilized. During fabrication, oxygen potential was controlled by Ti‐TiO2 or TiH2‐Ti3O5 solid‐state buffers. These metal and hydride oxygen getters behaved similarly, neither disturbing the phase assemblages nor significantly altering the partitioning of waste elements between radiophases. It is believed that the hydrothermal stability of the sodium‐bearing ceramics (containing up to 2.7 wt% Na2O) will be comparable to sodium‐free material, since less durable sodium‐rich phases are encapsulated in a resistant matrix. Extensive formation of glassy phases may cause embrittlement at higher sodium loadings.

Properties of high-purity AlxGa1−xAs grown by the metalorganic vapor-phase-epitaxy technique using methyl precursors

The metalorganic vapor-phase epitaxy (MOVPE) of AlxGa1−xAs most commonly employs the methyl precursors Al(CH3)3 and Ga(CH3)3. These precursors were used in the growth of AlxGa1−xAs over the entire range of alloy composition in a low-pressure horizontal MOVPE reactor. A complete chemical, electrical, and optical characterization of high-purity MOVPE AlxGa1−xAs grown over the entire range of growth temperatures (600–800 °C) was carried out in order to determine the relationship of the materials properties to the growth conditions. Carbon, the primary impurity in the layers, dominates the electrical properties of the epitaxial layers. A superlinear dependence of carbon incorporation on AlAs mole fraction is observed, along with a two-slope dependence on growth temperature. Photoluminescence spectra (2 K) were obtained from materials with AlAs mole fraction over the range 0≤x≤0.80. The photoluminescence intensity of the layers also exhibits a systematic dependence on alloy composition and growth temperature. The in situ gettering of oxygen by the growth reactants is necessary to achieve high luminescence intensity and low electrical compensation. The influence of the reactor design on this gettering process is modeled.

Carbon monoxide formation in UO 2 kerneled HTR fuel particles containing oxygen getters

Mass spectrometric measurements of CO in irradiated UO 2 kerneled HTR fuel particles containing various oxygen getters are summarized and evaluated. Uranium carbide addition in the 3 to 15% range reduces the CO release by factors between 25 and 80, up to burn-up levels as high as 70% FIMA. Unintentional gettering by SiC in TRISO coated particles with failed inner pyrocarbon layers results in CO reduction factors between 15 and 110. For ZrC, only somewhat ambiguous results have been obtained; most likely, ZrC results in CO reduction by a factor of about 40. Ce 2O 3 and La 2O 3 seem to be somewhat less effective than the three carbides; for Ce 2O 3, reduction factors between 3 and 15 have been found. However, these results are possibly incorrect due to premature oxidation of the getter already during fabrication. Addition of SiO 2 + Al 2O 3 has no influence on CO release at all.

Examination of the titanium environment in a René 41 nickel base superalloy by X-ray absorption spectroscopy

Results of an X-ray absorption study show that titanium has acted as an internal getter of oxygen, confirming the conjecture that the diminished performance of Rene 41 honeycomb panels was related to internal oxidation during the prebrazing oxidation heat treatment. Two Rene 41 0.0015-in. foils were examined after mill-annealing at 1975 F with and without being air oxidized in an electric furnace at 1600 F for 14 minutes.

チタン焼結体の機械的性質に及ぼす含有酸素の影響

The mechanical properties of sintered titanium were examined in relation to the oxygen content, and the sintering process using calcium as the getter of oxygen was investigated.The results obtained are summarized as follows:(1) The elongation of titanium sintered in vacuum by the conventional method was much lower than that of vacuum-melted titanium containing about the same amount of oxygen.(2) The ductility of sintered titanium was considerably improved by using calcium as the getter of oxygen.(3) The mechanical properties of sintered titanium were affected not only by the total amount of oxygen content, but also by the oxygen gathering around the pores during sintering.

Gettering of Carbon and Oxygen in Silicon Processing

The control of carbon and oxygen is of great importance in today's silicon materials and device processing, and it will become a critical parameter in the immediate future for VLSI device processing. Although considerable technology and some understanding are available today in the control of oxygen in silicon materials through processing, there is little technology and even less understanding of carbon control in silicon wafer processing. We have determined, for high carbon content silicon, that carbon can be gettered to damage sites such as dislocations, stacking faults, and/or wafer surfaces, thus creating a carbon‐denuded zone in epitaxial silicon, as well as in bulk Czochralski silicon substrates. The gettering effects of carbon have been investigated using controlled time, temperature, and ambient gas thermal anneals. The characterization of these materials was carried out using chemical etches, Fourier transform infrared spectroscopy, and scanning Auger microprobe analysis.