Effects of electrolyte on the structure of pyrolytic graphite surfaces in anodic oxidation

Anodic oxidation effects on the structure of the basal and edge surfaces of pyrolytic graphite in alkaline electrolytes have been studied. Laser Raman spectroscopy, a gas-phase chemical modification method, coupled with X-ray photoelectron spectroscopy and secondary ion-mass spectroscopy techniques, were used. Anodic oxidation of the surfaces of pyrolytic graphite in alkaline electrolytes does not cause destruction of their surface structure, even at a higher level of treatment, unlike oxidation of acid electrolytes. In alkaline electrolytes, the number of hydroxyl groups added on the edge surface gradually increases with the increase in treatment level, whereas the number of carboxyl groups does not increase. It was found that anodic oxidation in alkaline electrolytes has a wider permitted range of treatment, in which hydroxyl groups can be added without destroying the edge surface structure, than that found in acid electrolytes. On the other hand, the number of hydroxyl groups added by treating with alkaline electrolytes is smaller than that with acid electrolytes. At a higher treatment level with acid electrolytes, oxidation occurs, even to a depth of 40 nm from the edge surface, whereas with alkaline electrolytes, oxidation occurs only at the surface. On the basis of these results, the effects of electrolytes on the adhesion between carbon fibres and epoxy resin matrix are discussed.     

High temperature oxidation behavior of Ti-Al-Nb ternary alloys

The oxidation behavior of four Ti-Al-Nb ternary alloys with different microstructures were investigated at 1000°C using interrupted oxidation test in air. Alloys with single-phase -TiAl, two-phase -TiAl + ₂-Ti₃Al, multi-phase -TiAl + ₂-Ti₃Al + Nb₂Al and two-phase ₂-Ti₃Al + Nb₂Al were prepared. The oxidation resistance of the Ti-Al-Nb ternary alloy at high temperature was found to be better than that of the binary Ti-Al alloy. Among the four Ti-Al-Nb ternary alloys, the + ₂ two-phase alloy has the best oxidation resistance. The presence of Nb-enriched phase such as Nb₂Al and Nb₃Al decrease the oxidation resistance at elevated temperature presumably due to the formation of Nb₂O₅, which would accelerate the exfoliation of oxide.     

Formation of nanocrystalline phases in the Cu-Zn system during mechanical alloying

The evolution of various nanocrystalline phases in the Cu-Zn system during the course of mechanical alloying of elemental powder blends, manifests a similar sequence of phase formation in all the compositions. Zinc-rich phases were always first to form, which can be attributed to the diversities of diffusivities and diffusion distances in the constituents. The extent of zinc in -phase becomes significant only after turns nanocrystalline. The crystallite size of reached a minimum (18 nm) near the - phase boundary, while the and phases showed quite coarse crystallite size due to their low melting points. Alloying was sluggish during dry milling compared to wet milling, or at lower milling speed, which demonstrates the effects of oxidation during milling and lower milling energy.     

Interfacial debonding strength between the edge surfaces of pyrolytic graphite and epoxy resins

The edge surfaces of pyrolytic graphite have been treated by anodic oxidation and the interfacial debonding strength (IFDS) between the oxidized edge surfaces and epoxy resin measured. The unoxidized and oxidized edge surfaces were examined by X-ray photoelectron spectroscopy. The edge and epoxy resin surfaces after debonding test were examined by a field-emission scanning electron microscope. Carboxyl groups were expected to be added in great quantities by breaking the carbon-carbon bonds at the edge surface. On the other hand, hydroxyl groups could be added to edge carbon atoms at the edge surface without breaking the carbon-carbon bonds. The rise in IFDS with anodic oxidation does not correspond directly to the O/C ratios or the amount of carboxyl groups present on the edge surface. Hydroxyl groups added to the edge surface are considered to play an important role in improving the adhesion between the edge surface and epoxy resin through covalent bonds between hydroxyl and epoxy groups. In the case of epoxy resin used in this study, IFDS initially increased to 9 MPa with increasing -OH/C ratio up to about 0.02; thereafter it remains constant at this value. This strength is lower than those of PG and epoxy resin.     

Properties of evaporated Mo-Re thin-film superconductors

Mo-Re films have been deposited by electron-beam coevaporation in ultrahigh vacuum in the composition range between 25 and 40 at % Re. The films had either a single-crystal A15 structure, single-crystal -Mo (bcc) structure, or polycrystalline -Mo structure, depending on the substrate temperature during deposition. The superconducting transition temperature was approximately 12 K for films with both the A15 and -Mo structures, the same as literature values for bulk samples of the -Mo structure, even for films as thin as 20 nm. XPS measurements showed that the surface oxide thickness of samples exposed to ambient air was approximately 0.5 nm. Artificial tunnel barriers of oxidized Al were used with Pb or Mo-Re counterelectrodes to form low-leakage tunnel junctions for measurements of the superconducting energy gap.     

γ ? α2 Phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%)

The ₂ phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%) alloy has been studied by analytical electron microscopy. ₂ and phases were found at grain boundaries. ₂ layers that suspended in layers and interfacial ledge higher than 2d ₍₁₁₁₎ at /₂ interfaces were observed in the lamellar grains. These facts indicated that ₂ phase transformation and dynamic recrystallization have occurred during high temperature stress rupture deformation. It can be concluded that deformation induced ₂ phase transformation and dynamic recrystallization resulted in the presence of particles at grain boundaries. A structural and compositional transition area between deformation-induced ₂(or ) and its adjacently original (or ₂) phases was found by HREM and EDS and is suggested as a way to transform between and ₂ phase during high temperature stress rupture deformation. The transition area was formed by slide of partial dislocations on close-packed planes and diffusion of atoms.     

The mechanism of simultaneous sintering and phase transformation in alumina

The sintering behaviour of boehmitic alumina gels during the transformation to the stable phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis. The specimens for transmission electron microscopy were prepared from gel specimens, sintered to various predetermined temperatures, using an ion-beam thinning technique. The transmission electron microscope study and X-ray analysis have revealed a characteristic sintering behaviour which is associated with the to phase transition. The transformation to the phase occurs by a nucleation and growth process. During the growth process considerable redistribution of the fine porosity existing within the transition alumina matrix occurs, in the form of large elongated interconnected pores trapped within the nucleating grains. These pores grow rapidly to a size approximately one hundred times that of the grains. This process results in a rapid fall-off in sintering rate at the end of the transformation. A study of the/ interphase interface by transmission electron microscopy has led to the development of a model that accounts satisfactorily for the redistribution of the porosity.     

Microhardness and microbrittleness of several crystalline glass ceramic phases

The micromechanical properties of crystals of -cordierite, spinel, rutile, mullite, -cristobalite, and magnesium aluminosilicate have been studied. It was shown that the maximum Vickers hardness and maximum Dertev microbrittleness are found for crystals of magnesium aluminosilicate, cordierite, and spinel. The minimum values of these parameters are found for crystals of -cristobalite.Translated from Problemy Prochnosti, No. 10, pp. 29–31, October, 1992.     

Influence of the martensitic transformation on the deformation behaviour of an AISI 316 stainless steel at low temperatures

The deformation behaviour of an AISI 316 stainless steel under uniaxial tension was examined at 25, – 70 and – 196° C. The flow curves exhibited peculiar shapes and the work hardening rates were found to increase with strain beyond certain values of plastic strain. X-ray diffraction analyses showed that the transformation to-martensite commenced at these values of plastic strain and thereafter the volume fraction of increased steadily with strain. On the other hand, the amount of the-martensite was found to increase with plastic strain initially, reach a maximum and then decrease gradually. The contribution of the-phase to the flow stress of the alloy was found to be directly proportional to the volume fraction of. It is shown that the analysis of the flow curves provides a simple method of detecting the onset of the strain-induced martensitic transformation as well as estimating the amount of this martensite during further deformation.     

Analytical electron microscopy of Al2O3 implanted with iron ions

Single crystals of -Al₂O₃ were implanted with iron ions at room temperature to fluences ranging from 4×10¹⁶ Fe cm^–2 to 1×10¹⁷ Fe cm^–2. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques. Special emphasis was placed on monitoring the microstructural changes which take place during post-implantation annealing. Clusters of metallic -Fe were identified in the specimen after implantation to a dose of 1×10¹⁷ Fe cm^–2. Analytical electron microscopy of implanted specimens annealed in oxygen revealed the redistribution of the implanted iron and the formation of surface precipitates of -Fe₂O₃, subsurface precipitates of various forms of spinel, and, in some cases, subsurface precipitates of iron, depending on the annealing temperature. Examination of implanted specimens annealed under reducing conditions revealed the presence of precipitates of -Fe.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.     

Effect of phase transitions in Fe-Cr-Al alloys on their corrosion resistance in gases and liquid metals

We investigate the process of oxidation of Fe-Cr-Al alloys in argon and liquid sodium at a temperature of 650°C for 1000 h, analyze distinctive features of the process of formation of surface oxide and sub-oxide layers, and demonstrate the effect of phase transitions in alloys on the corrosion losses in these media. The process of oxidation of ferrochrome alloys in the region of homogeneity of -solid solutions in both media results in the formation of oxide layers based on Al₂O₃ on the surface of aluminum-containing alloys. In an atmosphere of argon, the intense growth of the oxide layer promotes the formation of residual stresses followed by its destruction and exfoliation, which leads to an increase in corrosion losses. In liquid sodium, aluminum improves the corrosion resistance of ferrochrome alloys, which is explained both by the suppression of formation of unstable compounds (sodium chromites and ferrates) and the appearance of an interlayer of -phase inclusions between the Al₂O₃ film and the matrix. This interlayer inhibits the growth of the protective oxide layer and enhances its adhesion to the matrix. The -phase is formed in homogeneous ferrochrome -alloys as a result of saturation of their surface layers with carbon present in sodium. If the composition of Fe-Cr alloys is close to equiatomic, their corrosion resistance catastrophically decreases as a result of the bulk transformation which is typical of both media.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 6, pp. 59–66, November – December, 1995.     

Structural studies of nylon 13,13

A structural study of nylon 13,13 has been carried out by means of X-rays and electron microscopy. Both X-ray and electron diffraction data, obtained from oriented films as well as from lamellar crystals prepared in solution, evidenced that nylon 13,13 adopts a structure in a monoclinic lattice of parameters a _o=0.488 nm, b _o=0.473 nm, c _o=3.40 nm, ==90°, =121°. As expected from the constitution of the nylon, no trace of structure with chains in fully extended conformation (-form) was observed throughout this study.     

Microstructure and crack sensitivity of laser-fusion zones of Ti-46 mol % Al-2 mol % Mo alloy

Laser surface melting and laser welding were performed on Ti-46 mol % Al-2 mol % Mo using a 2.5 kW CO₂ laser. Microstructures of the fusion zones were changed in the following way as the cooling rate increased: massive ₂+massive +lamellar(₂+) massive ₂+massive ₂. In laser surface melting, a single-phase structure of ₂ was seen when the calculated average cooling rates between 1773 and 1273 K were above approximately 4000 Ks^–1. In laser welding, the microstructure of the fusion zones was mainly composed of massive ₂+massive +lamellar. The hardness of the fusion zones increased with increasing cooling rate and the single-phase structure of ₂ showed hardness above 500 Hv. While all of the laser surface-melted zones included cracking, in laser welding, crack-free welds could be obtained at traverse speeds below 50.0 mm s^–1 and pre-heating temperatures above 573 K. As a result, cracking was prevented by selecting optimum welding parameters which result in calculated cooling rates between 1073 and 873 K below approximately 30 Ks^–1 and the hardness of the fusion zones below approximately 400 Hv. In tensile tests, the laser-welded specimens without weld cracking fractured in the base metal. Thus, laser welding can be applied to the joining of Ti-46 mol % Al-2 mol % Mo.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

Incorporation of amino acids within the surface reactive layers of bioactive glass in vitro: an XPS study

Surface reaction layers grown on bioactive glass (Bioglass^®), by immersion in either simulated body fluid (SBF) or minimal essential medium (-MEM) for 2, 5, 32 and 72 h, were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Layers grown in -MEM exhibited delamination when observed in SEM. Low resolution XPS analysis detected nitrogen at the surface of the Bioglass^® exposed to -MEM for 72 h (8 relative at%), whereas insignificant nitrogen was found at the surface of any sample immersed in SBF. XPS depth profiling (argon) showed the presence of nitrogen throughout the depth of the surface layer of the sample incubated in -MEM for 72 h. Deconvolution of the nitrogen envelope in a high resolution XPS spectrum demonstrated nitrogen characteristic of the amine bonds present in amino acids. Carbon concentration also considerably increased over time with exposure to -MEM (24–55%), whereas it remained in the 20–25% range in SBF. These results demonstrate that the amino acids contained in the culture medium were incorporated within the growing calcium phosphate rich surface reaction layer of Bioglass^®     

Examination of the suitability of α-tocopherol as a stabilizer for ultra-high molecular weight polyethylene used for articulating surfaces in joint endoprostheses

The lifetime of articulating surfaces in joint endoprostheses made of ultra-high molecular weight polyethylene (UHMW-PE), especially of UHMW-PE-cups of hip-endoprostheses, is usually limited to 10–15 years due to material failure as a result of oxidation of the UHMW-PE in vivo. In this study the suitability of the natural antioxidant -tocopherol (vitamin E) as a stabilizer for UHMW-PE in these applications was investigated. Specimens with 0.1%, 0.2%, 0.4% and 0.8% w/w -tocopherol as well as unstabilized samples were sintered and sterilized with -rays at 25 kGy in accordance with standard processing methods of cups for total hip-endoprostheses. These specimens were aged in pure oxygen at 70 °C and 5 bar as well as in aqueous H₂O₂ at 50 °C. The degree of oxidation was observed by means of FTIR-spectroscopy, DSC analysis and mechanical testing. The FTIR-measurements showed that -tocopherol can prolong the lifetime of UHMW-PE in an oxidative environment by a factor of more than 2.5. In the mechanical tests no embrittlement could be observed with the stabilized samples. A comparison with the standard antioxidant system Irganox® 1010/Irgafos® 168 (Ciba-Geigy, Switzerland) was carried out and revealed that -tocopherol can even exceed the stabilization effect of this widely-used antioxidant system.     

Eutectoid decomposition in Fe2Si5 based alloys with a small amount of 10(Pd, Pt) and 11(Cu, Ag, Au) group elements

The addition of a small amount of Cu is effective in accelerating the + Si eutectoid decomposition. Some elements (Pd, Pt, Ag and Au) that are expected to have similar chemical properties were added to an Fe₂Si₅ based alloys up to 1.0 at.% to examine whether a similar effect could be revealed. The microstructures, X-ray diffraction and differential thermal analysis (DTA) of slowly solidified or heat treated specimens were investigated in detail. The solubility of all containing elements into the phase was less than 0.2 at.% for the slowly solidified specimen. The excess of the additive solidified as a phase of a eutectic with the Si phase. On the other hand, the solubility of these additives in the phase was classified into two groups. The first group had higher solubility in the phase than that in the phase. The solubility of the other group was as low as that in the phase. Pd and Au belonged to the former and Pt and Ag belonged to the later. The acceleration of the eutectoid decomposition was only observed in the former group but it was negligible in the later group. The existence of the eutectic melt at the annealing temperature was effective on the coarsening of the eutectoid structure but not essential for the acceleration. The eutectoid decomposition strongly depended on the solubility of these elements into the phase.     

The evaporation signal from α particles stopped in superfluid helium

Alpha particles stopped in a 3 liter volume of liquid helium at 30 mK are observed by the calorimetric detection of helium atoms evaporated from the free surface of the liquid. Quantum evaporation of the helium is produced by the rotons that are created by the particle. While the energy spectrum of the 5.5 MeV 's from the²⁴¹Am source has a width of less than 0.5%, the energy distribution of the observed evaporation signals extends from the low energy threshold of several keV up to a maximum of several 100 keV, depending on geometrical factors and the collection area of the calorimeter. The origin of the observed distribution may result in part from the presence of the substrate and a dependence on the direction of the track of the particle. A simple model of the generation of rotons by the particle will be discussed.