Low-temperature heat conduction characteristics of diamond/Cu composite by pressure infiltration method

In this paper,diamond/CuCr and diamond/CuB composites were prepared using the pressure infiltration method.The physical property measurement system(PPMS)was adopted to evaluate the thermal conductivity of diamond/Cu and MoCu composites within the range of100–350 K,and a scanning electron microscope(SEM)was utilized to analyze the microstructure and fracture appearance of the materials.The research indicates that the thermal conductivity of diamond/Cu composite within the range of100–350 K is 2.5–3.0 times that of the existing MoCu material,and the low-temperature thermal conductivity of diamond/Cu composite presents an exponential relationship with the temperature.If B element was added to a Cu matrix and a low-temperature binder was used for prefabricated elements,favorable interfacial adhesion,relatively high interfacial thermal conductivity,and favorable low-temperature heat conduction characteristics would be apparent.     

Comparison of the Isothermal Oxidation Behavior of As-Cast Cu–17%Cr and Cu–17%Cr–5%Al. Part II: Scale Microstructures

The isothermal oxidation kinetics of as-cast Cu–17%Cr and Cu–17%Cr–5%Al in air were studied between 773 and 1,173 K under atmospheric pressure. Details of the oxidation kinetics of these alloys were discussed in Part I. This paper analyzes the microstructures of the scale and its composition in an attempt to elucidate the oxidation mechanisms in these alloys. The scales formed on Cu–17%Cr specimens oxidized between 773 and 973 K consisted of external CuO and subsurface Cu₂O layers. The total thickness of these scales varied from about 10 μm at 773 K to about 450 μm at 973 K. In contrast, thin scales formed on Cu–17%Cr–5%Al alloys oxidized between 773 and 1,173 K. The exact nature of these scales could not be determined by X-ray diffraction but energy dispersive spectroscopy analyses were used to construct a scale composition map. Phenomenological oxidation mechanisms are proposed for the two alloys.     

Comparison of the Isothermal Oxidation Behavior of As-Cast Cu–17%Cr and Cu–17%Cr–5%Al Part I: Oxidation Kinetics

The isothermal oxidation kinetics of as-cast Cu–17%Cr and Cu–17%Cr–5%Al in air were studied between 773 and 1,173 K under atmospheric pressure. These observations reveal that Cu–17%Cr–5%Al oxidizes at significantly slower rates than Cu–17%Cr. The rate constants for the alloys were determined from generalized analyses of the data without an a priori assumption of the nature of the oxidation kinetics. Detailed analyses of the isothermal thermogravimetric weight change data revealed that Cu–17%Cr exhibited parabolic oxidation kinetics with an activation energy of 165.9 ± 9.5 kJ/mol. In contrast, the oxidation kinetics for the Cu–17%Cr–5%Al alloy exhibited a parabolic oxidation kinetics during the initial stages followed by a quartic relationship in the later stages of oxidation. Alternatively, the oxidation behavior of Cu–17%Cr–5%Al could be better represented by a logarithmic relationship. The parabolic rate constants and activation energy data for the two alloys are compared with literature data to gain insights on the nature of the oxidation mechanisms dominant in these alloys.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Tantalum Oxidation in Steam Atmosphere

The oxidation behavior of tantalum in steam was investigated in the temperature range of 600–1100 °C. Thermogravimetric measurements were used to determine the oxidation behavior of the Ta metal at different steam partial pressures (10, 50 and 100 kPa). As a result of the oxidation tests, the samples show parabolic behavior for the initial period of oxidation, which is almost not visible at temperature higher than 700 °C. After the short parabolic kinetics, breakaway occurs and turns the kinetics from parabolic to linear. In this work kinetic parameters of the linear oxidation were determined. The influence of the samples´ shape was investigated using both plate and cylinder specimens. Experiments aimed at quantifying the hydrogen uptake of tantalum were conducted at temperatures between 600 and 1000 °C, and results differ from the expected ones applying the Sieverts’ law. Post-test optical microscopy was performed in order to analyze the post-oxidation appearance of the materials. The oxide scale appeared non-protective, especially at low temperature.     

Short-circuit diffusion in the growth of nickel oxide scales on nickel crystal faces

A study has been made of the growth of nickel oxide layers on the (100), (110), and (111) crystal faces of nickel exposed to oxygen for periods up to 100 hr at temperatures in the range 500–800°C, These layers grow at a rate dependent upon the crystallographic orientation of the nickel face even at thicknesses within the scaling range. The (100) face oxidized more rapidly than the (110) and (111) nickel faces while the latter faces oxidized at different relative rates dependent upon temperature. A diffusional model is employed to interpret the oxidation kinetics wherein nickel transport proceeds in nickel oxide both by short-circuit diffusion at grain boundaries and by lattice diffusion.This work forms part of a research program sponsored by the National Research Council of Canada.     

A Critical Compressive Stress for Increasing the Oxidation Kinetics of Fe–20Cr Alloy Oxidized at 900 °C

The effects of compressive stresses on the oxide-scale morphologies formed on an Fe–20Cr alloy were investigated by comparison of the oxidation behavior in air under classical conditions, i.e., without any applied mechanical stresses and under static compressive stresses, at 900 °C. The study was carried out mainly by comparisons of oxidation kinetics gained by thermogravimetric analysis (TGA), surface morphologies of oxidized specimens observed by scanning electron microscopy (SEM), oxidized products examined by X-ray diffraction (XRD). It was found that the application of compressive stresses induced an increase in oxidation rate, but a decrease of oxide grain size. When the stresses are in the range of 5–8 MPa, both chromium- and iron-oxides formed but, at other stresses, only chromia was present. In particular, there was a maximum in oxidation rate when the applied stress was 5 MPa. The paper places emphasis on analyzing the cause of this phenomenon.     

Effect of Substrate Grain Size on the Growth, Texture and Internal Stresses of Iron Oxide Scales Forming at 450 °C

The oxidation behavior of iron polycrystals and single crystals with (110) surface orientation was studied at 450 °C. Energy-dispersive diffraction with synchrotron radiation provided in situ information regarding the evolution of stress gradients and fiber texture in the oxide scales. Within this low-temperature regime, grain boundaries caused the oxidation kinetics of polycrystalline iron to be more rapid than iron single crystals only during the first minutes of oxidation. Epitaxial growth of iron oxides occurred only on single crystal substrates during the initial oxidation. In situ stress analyses suggested that stress relief occurred invariably in the magnetite layer due to the formation of a fine-grained seam near the iron substrates. Above the magnetite and in the hematite layer, the growth stresses depend initially on volumetric strains and later on inner oxide formation and creep of the hematite.     

Oxidation of Advanced Zirconium Cladding Alloys in Steam at Temperatures in the Range of 600�C1200?��C

The oxidation kinetics of the classical pressurized water reactors (PWR) cladding alloy Zircaloy-4 have been extensively investigated over a wide temperature range from operational conditions to beyond design basis accident (BDBA) temperatures. In recent years, new cladding alloys optimized for longer operation and higher burn-up are used in Western light water reactors (LWR). This paper presents the results of thermo-gravimetric tests with Zircaloy-4 as the reference material, Duplex DX-D4, M5^® (both AREVA), ZIRLO? (Westinghouse), and the Russian E110 alloy. All materials were investigated in isothermal and transient tests in a thermal balance with steam furnace. Post-test analyses were performed by light-microscopy and neutron radiography for investigation of the hydrogen absorbed by the metal. Strong and varying differences (up to 800%) in oxidation kinetics between the alloys were found at up to 1000 °C, where the breakaway effect plays a role. Less but significant differences (ca. 30%) were observed at 1100 and 1200 °C. Generally, the M5^® alloy revealed the lowest oxidation rate over the temperature range investigated whereas the behavior of the other alloys was considerably dependent on temperature. A strong correlation was found between oxide scale structure and amount of absorbed hydrogen.     

Oxide Growth Characterization During Short-Time Oxidation of a Commercially Available Chromia-Forming Alloy (HR-120) in Air at 1,050 °C

This study focuses on the oxidation behavior of commercially available HR120 in air at 1,050 °C from 30 min to 100 h. The oxidation kinetics were first studied by thermogravimetry and isothermal exposure. The oxidation products were fully characterized using ex and in situ X-ray diffraction (XRD) and FEG-SEM observations. HR120 experienced at 1,050 °C a non protective transient stage and formed a multilayered oxide scale (SiO₂–Cr₂O₃–XCr₂O₄ with X = Mn and/or Fe, Ni). A series of complementary characterization methods (gold and isotopic marker experiments, photoelectrochemistry (PEC)) were implemented to elucidate the oxidation mechanism. The study identified a n-type semi-conductivity accompanied by an inward growth of the scale. Thus, assuming that diffusion in the oxide scale controlled chromia-scale growth, the oxygen vacancy was the major point defect governing the solid state transport. This result was attributed to the presence of a MnCr₂O₄ spinel layer at the top of chromia that strongly decreased the oxygen pressure at the interface spinel/chromia.     

Internal Oxidation of Ternary Alloys Forming a High Oxygen Conductive Oxide

Five ternary alloys consisting of a noble base metal (Ni, Co, Fe, Cu) and two reactive metals (Zr + Y, Ce + Gd) being able to form a high oxygen ion conductive oxide were internally oxidized under low oxygen partial pressures. All alloys developed either a continuous yttria-stabilized zirconia phase or a continuous gadolinia-doped ceria phase behind the front of internal oxidation. A Ni–Ce–Gd alloy showed extraordinarily high internal oxidation rates of up to 120 µm²/s at 900 °C. High internal oxidation rates in these ternary alloys were not limited to low concentrations of the reactive metals. The type of the internal oxide phase was found to be more important for the internal oxidation kinetics than the noble base metal.     

Isothermal and Cyclic Oxidation Behavior of Fe–B Based Alloys at 927 K

The high temperature oxidation behavior of an Fe–16Cr binary alloy, oxidized under different compressive stresses in air at 900 °C, was investigated. Surface and cross-sectional micrographs, observed by scanning electron microscopy, indicated that the resulting morphology of the thermally grown oxide scale depended on the compressive stress. Results showed that oxide scales were infact below 5 MPa stress after 10 h of oxidation. Delamination developed at the outer/inner oxide scale interface in the case of compressive stress above 5 MPa. Growth kinetics measurements revealed that the rate of oxide-scale growth increased by the compressive stress.     

Oxidation of Zirconium Alloys in Oxygen at High Temperatures up to 1600 °C

The oxidation kinetics of the classic pressurized water reactors (PWR) cladding alloy Zircaloy-4 has been extensively investigated over a wide temperature range. In recent years, new cladding alloys optimized for longer operation and higher burn-up are being increasingly used in Western light water reactors (LWR). These alloys were naturally optimized regarding their corrosion behavior for operational conditions. The publicly available data on high temperature oxidation of the various cladding materials are very scarce. This paper presents the results of a first test series with Zircaloy-4 as reference material, Framatome Duplex cladding, Framatome M5^® and the Russian E110 alloy. The first two are Zr–Sn, the latter two Zr–Nb alloys. All materials were investigated in isothermal and transient tests in a thermal balance under argon–oxygen atmosphere. Strong and varying differences (up to 500%) of oxidation kinetics between the alloys were found till 1000 °C, where the breakaway effect plays a role. Smaller but still significant differences (20–30%) were observed at higher temperatures. Generally, the advanced cladding alloys here studied show also a favorable behavior at high temperatures during accident scenarios.     

Transition Between Different Oxidation Modes of Binary Fe–Si Alloys at 600–800 °C in Pure O2

The oxidation behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si has been studied at 600–800 °C under 1 atm O₂. Fe–5Si and Fe–9Si followed multi-stage parabolic kinetics, while Fe–13Si showed more complex kinetics at all temperatures. The increase in Si content resulted in a transition from the internal oxidation of Si beneath an external scale of iron oxides to the exclusive external formation of silica. The critical contents required for the transitions between the various possible oxidation modes of the binary Fe–Si alloys were calculated and compared with the experimental results. A thermodynamic mechanism for the formation of Fe-rich oxide nodules observed in the oxidation of Fe–5Si and Fe–9Si has been proposed.     

The Effects of Nickel/Tin Ratio on Cu Induced Surface Hot Shortness in Fe

Surface hot shortness is of concern in scrap-based electric arc furnace (EAF) steelmaking. The excessive amount of residual copper (Cu) in the steel scrap enriches during the oxidation of the solid steel, and as a result a Cu-rich layer could form which causes inter-granular grain boundary cracking. Other residuals can also influence hot-shortness. Ni is known to improve the resistance to hot shortness whereas Sn is known to worsen it. In this paper, the mechanism through which nickel (Ni) counters the detrimental effects of tin (Sn) on surface hot shortness are investigated in detail. A series of Fe?C0.3 wt%Cu-x wt%Ni?C0.03 wt%Sn alloys with x content ranging from 0.03 to 0.45 wt% was oxidized in air at 1,423 K (1,150 °C) for 60, 300 and 600 seconds using thermogravimety (TG). The microstructure investigation under scanning electron microscopy (SEM) showed that, significant grain boundary cracking was suppressed by eliminating the highly embrittling Sn containing Cu-rich liquid, through increasing Cu solubility and/or solidifying the liquid phase by the presence of sufficient amount of Ni. Occlusion and decreased oxidation kinetics caused by Ni were shown not to play a significant role. A previous proposed diffusion numerical model was applied to the Fe?CCu?CNi?CSn system to quantitatively support experimental results and provide insight to the enrichment kinetics at the oxide/metal interface.     

Oxygen Atom Adsorption on and Diffusion into Nb(110) and Nb(100) from First Principles

To understand the dynamics of oxidation of Nb, we examine the adsorption, absorption, and diffusion of an oxygen atom on, in, and into Nb(110) and Nb(100) surfaces, respectively, using density functional theory. Our calculations predict that the oxygen atom adsorbs on the threefold site on Nb(110) and the fourfold hollow site on Nb(100), and the adsorption energy is ?5.08 eV and ?5.18 eV, respectively. We find the long and short bridge sites to be transition states for O diffusion on Nb(110), while the on-top site is a rank-2 saddle point. In the subsurface region, the oxygen atom prefers the octahedral site, as in bulk niobium. Our results show also that the O atom is more stable on Nb(110) subsurface than on Nb(100) subsurface. The diffusion of oxygen atoms into niobium surfaces passes through transition states where the oxygen atom is coordinated to four niobium atoms. The diffusion barriers of the oxygen atom into Nb(110) and Nb(100) are 1.81 eV and 2.05 eV, respectively. An analysis of the electronic density of states reveals the emergence of well-localized electronic states below the lowest states of clean Nb surfaces due to d–p orbital hybridization.     

High-Temperature Oxidation of Zircaloy-4 in Oxygen–Nitrogen Mixtures

Isothermal oxidation experiments with cladding tube segments of Zircaloy-4 (Zr-1.3%Sn) in oxygen–nitrogen model mixtures were performed at 800, 1000, and 1200 °C for 6, 1 h, and 15 min, respectively. The gas compositions varied between 0 and 100 vol% nitrogen including 1 and 99 vol%. A strong accelerating effect of nitrogen on the oxidation kinetics was seen for a wide range of boundary conditions. At 800 °C, oxidation in all mixtures with 1–99 % nitrogen resulted in higher reaction rates compared to the pure gases, especially after transition from protective to non-protective oxide scales. At 1000 and 1200 °C, only starvation of oxygen in mixtures with low oxygen contents resulted in lower rates compared to pure oxygen. The oxide scales formed in the mixtures were very porous due to the formation of zirconium nitride at the metal-oxide interface and its oxidation during continuing reaction. The extension of the oxide-nitride zone increased with temperature and with nitrogen content in the gas mixture. Nitrogen seems also to affect the pre-transition reaction kinetics. The mechanism of the faster oxidation kinetics of zirconium alloys in atmospheres containing nitrogen will be discussed in this paper.     

Formation of oxidation-resistant Cu-Mg coatings on (001) Cu for oxide superconducting tapes

The formation of oxidation-resistant buffer layers on (001) oriented Cu for coated high-temperature superconducting tape applications was investigated. The approach employed Cu/Mg multilayer precursor films that were subsequently annealed to form either Mg-doped fcc Cu or intermetallic Cu₂ Mg. The precursor consisted of an Mg/Cu multilayer stack with 5 each of 25 nm thick Mg and 25 nm thick Cu layers which were grown at room temperature by sputter deposition. At annealing temperature of 400 °C, formation of the intermetallic Cu₂ Mg was observed. X-ray diffraction showed that the Cu₂ Mg (100) oriented grains were epitaxial with respect to the underlying Cu film, possessing a cube-on-cube orientation. In order to test oxidation resistance, CeO₂ films were deposited at elevated temperature on Ni/(Cu,Mg)/Cu/MgO structures. In case of the CeO₂ film on Ni/Cu/MgO, significant surface roughness due to the metal oxidation is observed. In contrast, no surface roughness is observed in the SEM images for the CeO₂/Ni/(Cu,Mg)/Cu/MgO structure.     

Formation of metal (Iron)-oxide nanostructures and nanocomposites by reactive sputtering and low-temperature reoxidation

Morphology and composition of nanostructured metal-oxide coating prepared by iron reactive sputtering and subsequent low-temperature oxidation in air at 50°C were studied by atomic-force microscopy (with digital processing of images), X-ray diffraction, and resistance measurements. The growth kinetics of metal-oxide nanoparticles constituting the sputtered metal-oxide composite was evaluated by the direct processing of surface AMF-images. According to the results of morphological studies after the low-temperature oxidation, the surface layer has complicated structure: the metal nanoparticles surrounded by the α-Fe₂O₃ oxide phase merge to extended rod-like structures (globules) sized, on the average, 100–200 nm lengthwise, 20–30 nm in diameter. This structure allows explaining the coatings’ functional properties important for their applications. Original Russian Text ? V.A. Kotenev, D.N. Tyurin, A.Yu. Tsivadze, M.A. Petrunin, L.B. Maksaeva, T.P. Puryaeva, , 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 627–630.     

Early-Stage Oxidation Behavior of Pt-Modified γ′-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified γ′-Ni₃Al-based alloys of composition (in at.%) Ni–22Al–30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al₂O₃ phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 °C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 °C; however, it was found that Hf delayed the metastable-to-α-Al₂O₃ phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al₂O₃ layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to α-Al₂O₃, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.