Pilling-Bedworth Ratio Research Articles

Efficient growth of anodic films on magnesium in organic electrolytes containing fluoride and water

The present study reports, for the first time, the highly efficient formation of barrier-type anodic films, with flat and parallel metal/film and film/electrolyte interfaces, on magnesium in ethylene glycol electrolytes containing ammonium fluoride and water. The anodizing voltage increases linearly with time during galvanostatic anodizing at 10Am−2 up to 350V. The anodic film formed to 200V is 247nm thick, containing a crystalline MgF2 phase. Analysis by Rutherford backscattering spectroscopy discloses the film composition of MgF1.8O0.1 and Pilling–Bedworth ratio (PBR) of 1.67. The PBR value greater than unity and the formation of chemically stable fluoride-based films may contribute to the film growth at high current efficiency.

Influence of the density of oxide on oxidation kinetics

A new kinetic model is proposed to describe the isothermal oxidation of metals and alloys in the form of sphere, flat plate and fiber shape with considering oxidation induced volume change or Pilling–Bedworth Ratio. The effects of temperature, oxygen partial pressure, particle size, sample shape on the oxidation reaction fraction are analyzed in the model through a physical meaning explicit function. The oxidation experimental data of zinc powders and plates were used to check the model. The results show that the calculated curves agree well with the experimental data and the model can give a good theoretical prediction. All of these will be useful for studying the oxidation of intermetallics either.

Corrosion and characterisation of dual phase Mg–Li–Ca alloy in Hank’s solution: The influence of microstructural features

The influence of the microstructure and the oxide film of the Mg–9.29Li–0.88Ca alloy on its corrosion behaviour was investigated using SEM, EPMA, XPS and corrosion measurements. The results demonstrated that the fine-grained microstructure improved the mechanical and corrosion resistance of the alloy and shifted pitting corrosion to overall corrosion. The oxide film contained a multi-layered structure, with the outer layer being enriched in lithium-bearing compounds; the interior layer predominantly consisting of oxides, hydroxides and carbonates of lithium and magnesium; and the bottom layer containing oxides. The Pilling–Bedworth ratio for chemical compounds was proposed, and the corrosion rates were characterised.

Selective oxidation behavior of an ignition-proof Mg-Y-Ca-Ce alloy

A Mg-Y-Ca-Ce magnesium alloy was optimized for high ignition-proof property, which did not burn in air at 1233 K up to 30 min. Oxidation behavior of the alloy was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermodynamics calculation at 673, 773 and 873 K. XRD and SEM analysis indicated that dense and compact oxide films composed of MgO, Y2O3 and CaO formed. The oxidation behavior was characterized by the selective oxidation. Based on Pilling-Bedworth ratio (PBR) and energy dispersive spectrometer (EDS) analysis, Y2O3 contributed more to form the compact surface oxide film, which led to the excellent ignition-proof performance. The thermodynamics analysis and EDS results implied that the Y-rich areas were preferred paths for the selective oxidation.

Nanostructuring of Refractory Metal Surfaces by Electrochemical Oxidation: Nb and the Binary Systems Ti-Ta and Nb-Ta

Metal oxide nanotubes were produced on Ti, Nb, and TiTa and TiNb alloy surfaces by anodic oxidation in fluoride containing electrolytes. Tube diameters from 7 to 110 nm and tube lengths from 50 nm to 1.6 μm were achieved, depending on the substrate material, the anodization parameters, and the composition of the electrolyte. With the application of a voltage of 20 V for 1 h to Nb substrates tubes with 124.3±8.3 nm length were formed, while the same anodization conditions resulted in a tube length of only 66.7±14.2 nm, when applied in a glycol based electrolyte. While there was a linear connection between tube diameter and voltage for Ti anodization, Nb showed limits in the achievable tube dimensions. Furthermore, significant influences on the ion transport mechanisms were found for variations of electrolyte viscosity and stirring frequency, which highly affected the morphology and the size distribution of the obtained nanotubes. Very different anodization behaviour was found for alloys of Ti with Nb resp. Ta, leading to the conclusion that the Pilling- Bedworth ratio must be kept at an average value below 2.44, if fabrication of isolated nanotubes on an alloy surfaces should be successful. Keywords: Nanotubes, anodization, self-organization, metal oxide, Pilling-Bedworth ratio

Oxidation behaviour of zirconium alloys and their precipitates – A mechanistic study

The precipitate oxidation behaviour of binary zirconium alloys containing 1wt.% Fe, Ni, Cr or 0.6wt.% Nb was characterised in TEM on FIB prepared transverse sections of the oxide and reported in previous studies [1,2]. In the present study the following alloys: Zr1%Cu, Zr0.5%Cu0.5%Mo and pure Zr are analysed to add to the available information. In all cases, the observed precipitate oxidation behaviour in the oxide close to the metal-oxide interface could be described either with delayed oxidation with respect to the matrix or simultaneous oxidation as the surrounding zirconium matrix. Attempt was made to explain these observations, with different parameters such as precipitate size and structure, composition and thermodynamic properties. It was concluded that the thermodynamics with the new approach presented could explain most precisely their behaviour, considering the precipitate stoichiometry and the free energy of oxidation of the constituting elements.The surface topography of the oxidised materials, as well as the microstructure of the oxide presenting microcracks have been examined. A systematic presence of microcracks above the precipitates exhibiting delayed oxidation has been found; the height of these crack calculated using the Pilling–Bedworth ratios of different phases present, can explain their origin. The protrusions at the surface in the case of materials containing large precipitates can be unambiguously correlated to the presence of these latter, and the height can be correlated to the Pilling–Bedworth ratios of the phases present as well as the diffusion of the alloying elements to the surface and their subsequent oxidation. This latter behaviour was much more considerable in the case of Fe and Cu with Fe showing systematically diffusion to the outer surface.

Anodic film growth on Al–Li–Cu alloy AA2099-T8

AA2099-T8 aluminium alloy was anodized at a current density of 5mA/cm2 to selected voltages in 0.1M ammonium pentaborate electrolyte at 293K. It was found that the growth of barrier-type anodic film on the alloy was accompanied by oxidation of intermetallics, formation and rupture of oxygen gas-filled voids in the anodic film and healing of the anodic film at the sites of rupture. It was revealed that the formation of oxygen gas-filled voids was related to the oxidation of copper-rich nanoparticles in the copper-enriched layer at the film/alloy interface, and that the oxidation process of copper-rich nanoparticles depended on grain orientation. Further, the significantly reduced Pilling–Bedworth ratio for formation of anodic lithium oxide compared with that for formation of anodic alumina resulted in the formation of fine voids at the alloy/film interface and sequentially, detachment of the anodic film from the alloy surface.

Characterization of the Surface Film Formed on Molten AZ91D Magnesium Alloy in Atmospheres Containing SO2

The surface film formed on molten AZ91D magnesium alloy in an atmosphere containing SO2 was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The surface film primarily contained MgO and MgS and had a network structure. MgS increased the Pilling–Bedworth ratio of the film and enhanced its protective capability. The films with a few pores at the surface consisted of two layers with an outer MgO layer and an inner layer of MgO and MgS. The film without pores at the surface also contained MgS and small amounts of MgSO4 in the outer layer. Increasing the SO2 content in the atmosphere promoted film growth and the formation of the protective film was prevented with the increased temperature.

Advanced Slurry Formulations for New Generation Chemical Mechanical Planarization (CMP) Applications

ABSTRACTChemical Mechanical Planarization (CMP) is widely used to ensure planarity of metal and dielectric surfaces to enable photolithography and hence multilevel metallization in microelectronics manufacturing. The aim of this study is to establish a fundamental understanding on the dynamic growth of nano-scale protective oxide thin films during CMP to enable the selection of proper oxidizer concentrations for slurry formulations. Tungsten was selected as the model metal film to study the formation of these metal oxide films in various oxidizers and Atomic Force Microscope (AFM) was used to measure the surface roughness of the samples conditioned in the oxidizer environment before and after the CMP was conducted. The affect of surface roughness on wettability of the surfaces were also studied through contact angle measurements on the treated tungsten films. Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance FTIR/ATR technique in combination with the X-Ray Reflectivity (XRR) were utilized to determine the thicknesses of the oxidized nano films on the tungsten surface. The results were evaluated through the material removal responses reported in the literature for the W-CMP in addition to the comparison of the Pilling-Bedworth ratios of the oxidized nano films to determine the ability of the created oxide film as a self-protective oxide.

TEM characterization of Au-based alloys to join YSZ to steel for SOFC applications

The microstructures of two gold-based alloys with compositions (in wt.%) of 96.4Au-3Ni-0.6Ti and 97.5Au-0.75Ni-1.75V following oxidation at 850 Degree-Sign C for 200 min were characterized by analytical transmission electron microscopy with energy dispersive spectroscopy and by scanning electron microscopy. In the oxidized 96.4Au-3Ni-0.6Ti interlayer, a dense scale composed of nickel oxide (NiO) and nickel titanate (NiTiO{sub 3}) formed at the alloy surface. No evidence of titanium oxide was found because there was not enough Ti present to form titanium oxide. In the oxidized 97.5Au-0.75Ni-1.75V interlayer, loose vanadium oxide (V{sub 2}O{sub 5}) and nickel vanadate (Ni{sub 2}V{sub 2}O{sub 7}) formed and were distributed within the oxidized 97.5Au-0.75Ni-1.75V interlayer. Similarly, because of the low Ni content in the alloys, no NiO formed. The oxide products in the 96.4Au-3Ni-0.6Ti and 97.5Au-0.75Ni-1.75V interlayers after oxidation are consistent with the Pilling-Bedworth (PB) ratio considerations. - Highlights: Black-Right-Pointing-Pointer Two commercial Au-based reactive metallic interlayers were oxidized at 850 Degree-Sign C for 200 min. Black-Right-Pointing-Pointer The oxidized products at the surface were characterized by TEM/EDS and SEM. Black-Right-Pointing-Pointer NiO and NiTiO{sub 3} formed at the oxidized 96.4Au-3Ni-0.6Ti interlayer. Black-Right-Pointing-Pointer V{sub 2}O{sub 5} and Ni{sub 2}V{sub 2}O{sub 7} were found in the oxidized 97.5Au-0.75Ni-1.75V interlayer. Black-Right-Pointing-Pointer These oxidemore » products are consistent with the Pilling-Bedworth (PB) ratio considerations.« less

A theoretical model and phase field simulation on the evolution of interface roughness in the oxidation process

An oxidation kinetics model is developed to account for the effects of the oxidation interface curvature and the oxidation-induced volume change or Pilling–Bedworth ratio. For the oxidation of Fe–Cr–Al–Y alloy fiber, the predictions agree well with experimental results. By considering the influence of the oxidation interface curvature on oxidation rates, the evolution of fluctuant oxidation interface is predicted. We also developed the phase field method (PFM) to simulate the evolution of the interface roughness. Both the theoretical model and the PFM results show that the interface will become smooth during high temperature oxidation. Stress distribution and evolution are calculated by PFM, which indicates that the stress level decreases as the interface morphology evolves.

Key factor influencing the ignition resistance of magnesium alloys at elevated temperatures

Magnesium alloys exhibit poor ignition resistance after the addition of elements whose solubility limit in the matrix is extremely low, although these elements can thermodynamically bring about the formation of an additional protective oxide layer by alloying. The solute concentration of the alloying elements dissolved in a matrix, in conjunction with conventionally used factors such as the Pilling–Bedworth ratio and the Gibbs free energy change for oxidation, should be considered as an important factor influencing the ignition resistance of magnesium.

Product/metal ratio (PMR): A novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys

Product/metal ratio (PMR) was introduced as a novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. The criterion initially sprang from Pilling-Bedworth ratio (PBR), focused on the roles of electrolytes for the compactness of the fabricated coatings, and took attention on properties of reactants/products during MAO. Meanwhile, based on our experiments as well as the results from literatures, the effects of electrolyte additives on morphologies and compositions of the fabricated MAO coatings of Mg alloys were exploited for verification and supplement of the initial criterion. In combination of the initial PMR criterion and experimental verification, PMR could be represented by special mode (PMRs=Voxide products/Valloy substrates) and general mode (PMRg= PMRs+ PMRd). The ideal PMRs should be between 1 and 2, while PMRd is related to the coating deposition during MAO. PMRd is a supplement to PMRs when the effect of the overlaying property (O) of the coatings and the effective deposition (D) of electrolyte composites are considered (PMRd=f(O, D). O is related to the melting point (MP) and boiling point (BP) of the MAO products. D is related to the effective reactions between alloy substrates and electrolytes during MAO. micro-arc oxidation, Mg alloys, Pilling-Bedworth ratio, product/metal ratio

Direct electroreduction of oxides in molten fluoride salts

A new kind of electrolyte composed of molten fluorides has been evaluated in order to perform a feasibility study of the direct electroreduction reaction. The direct reduction of SnO 2 and Fe 3O 4 was realised in LiF–NaF at 750 °C and in LiF–CaF 2 at 850 °C for TiO 2 and TiO. The electrochemical behaviour of these oxides was studied by linear sweep voltammetry: a current corresponding to the oxide reduction was evidenced for TiO 2, SnO 2 and Fe 3O 4. After galvanostatic electrolyses, a complete conversion was obtained for all oxides, except TiO, and the structure of reduced Ti and Fe samples had a typical coral-like structure while dense drops of Sn were recovered (Sn is liquid at operating temperature). After TiO electrolysis, a thin external metallic titanium layer was detected, acting as a barrier for the oxide ion diffusion and no complete reduction can be achieved. This could be explained by a Pilling–Bedworth ratio around 1 for Ti/TiO.

Growth of ZnO nanostructure on Cu 0.62Zn 0.38 brass foils by thermal oxidation

Cu 0.62Zn 0.38 brass foil was heated at temperatures of 400–700 °C in flowing N 2–5%O 2 at a pressure of 1 atm. for 1–24 h. The oxidized specimens were characterized with a scanning electron microscope, an X-ray diffractometer and a transmission electron microscope. The results show that hexagon ZnO nanowires and nanowalls grew on convoluted oxide scales when brass foils were oxidized at 500 and 600 °C. Thermodynamics of forming ZnO is analyzed based on oxidation theory of an alloy. Pilling–Bedworth ratio of Cu–Zn alloy is calculated based on the volume differences between the formed oxide and the consumed metal. The growth stresses caused by Pilling–Bedworth ratio of the alloy and stress relief of oxide scale at different oxidation temperature are analyzed. The mechanism of forming nanostructural ZnO on the convoluted oxide scales is explained by the change in the growth stresses and stress relief of oxide scale during thermal oxidation.

Improved electrical properties of silicon-incorporated anodic niobium oxide formed on porous Nb–Si substrate

In the present study, porous Nb–Si alloy films with isolated nano-column morphology have been successfully developed by oblique angle magnetron sputtering on to aluminum substrate with concave cell structure. The deposited films are amorphous with the 15 at% silicon supersaturated into niobium. The porous Nb-15 at% Si films, as well as niobium films with similar morphology, are anodized at several voltages up to 50 V in 0.1 mol dm −3 ammonium pentaborate electrolyte. Due to the presence of sufficient gaps between neighboring columns, the gaps are not filled with anodic oxide, despite the large Pilling–Bedworth ratio (for instance, 2.6 for Nb/Nb 2O 5) and hence, a linear correlation between the reciprocal of capacitance and formation voltage is obtained for the Nb-15 at% Si. From the comparison with the anodic films formed on porous niobium films, it has been found that silicon addition improves the thermal stability of anodic niobium oxide; the change in capacitance and increase in leakage current become small for the Nb–Si. The findings indicate the potential of oblique angle deposition to tailor porous non-equilibrium niobium alloy films for high performance niobium-base capacitor.

Formation of porous niobium films by oblique angle deposition: Influence of substrate morphology

The present work demonstrates the formation of porous niobium films with separated columnar structures by oblique angle magnetron sputtering for capacitor application. The niobium films deposited on textured aluminium substrates, which had concave cell structures with the cell sizes ranging from 125 nm to 550 nm, consist of isolated columns of niobium with wider gaps between columns developing on the substrates with larger cell sizes. The surface areas of the deposited films, evaluated by the capacitance of the anodic films formed at several voltages, increased with an increase in the cell size of substrate. The surface area decreases with an increase in the formation potential of anodic films from 2 V to 10 V vs Ag/AgCl, because the gaps are filled with anodic oxide as a consequence of the large Pilling–Bedworth ratio of 2.6 for the Nb/Nb 2O 5 system. The reduction of the surface area is suppressed when the substrate with larger cell size is used, due to the formation of niobium columns with wider gaps, which are not filled with anodic oxide. The high surface area even at higher formation voltages of the anodic films is a requisite for capacitor application.

Formation of Porous Aluminum Films with Isolated Columnar Structure Using Physical Vapor Deposition for Medium-Voltage and High-voltage Capacitors

An attempt was made to tailor porous aluminum films with an isolated columnar structure by magnetron sputtering for potential application to medium-voltage and high-voltage electrolytic capacitors. The aluminum film was deposited at a relatively high Ar pressure of 3.3 Pa on rough aluminum substrate with a cellular texture. Shadowing effects, which were enhanced by the random incident angle of aluminum atoms at the high Ar pressure and the use of a rough substrate, produced the isolated columnar structure of the deposited film. However, gaps separating neighboring columns were not high enough to maintain high surface roughness after anodic oxide formation, due to the large Pilling-Bedworth ratio for the Al/Al2O3 system. The gaps became filled with anodic oxide during anodic film formation. Slight alkaline etching of the deposited film the increased the gaps such that high surface area was maintained to high formation voltages.

Corrosion resistance of dilute CuMg alloys at elevated temperature

In comparison with CuAl (Al: 0.2 and 0.5 wt.%) alloys, corrosion resistance (CR) of CuMg (Mg: 0.12 and 0.34 wt.%) alloys was studied at 673–1173 K in atmospheric O 2. All the samples were pre-annealed at 873 K in atmospheric H 2. The CR of CuMg alloys at 673–973 K is improved in contrast to a pure Cu but much poorer than that of CuAl alloys, while the improvement can hardly be observed for CuMg alloys at and above 1073 K, which is similar to CuAl alloys. The poorer CR of CuMg alloys compared with that of CuAl alloys at 673–973 K is largely attributed to the incorporation of Cu in the MgO surface layer and the low Pilling-Bedworth ratio of CuMg–O system smaller than unity, and the vanishing of CR for CuMg alloys at and above 1073 K is ascribed to the instability of the MgO layer at the Cu 2O/CuMg interface.

Anodic oxide growth on Zr in neutral aqueous solution

Anodization and subsequent cathodic reactions on a thin-film sample of Zr were studied with in-situ neutron reflectometry (NR) and electrochemical impedance spectroscopy (EIS). The NR results during anodization showed the originally 485 A thick Zr film generally behaved similar to a bulk electrode in neutral solution. The anodization ratio measured at applied potentials increased in steps of 0.5 V was somewhat higher than the value determined by coulometry, while the Pilling Bedworth ratio is in good agreement with published data. Thickening of the oxide layer, accelerated immediately after each potential increase, gradually decreased over several hours, but remained nonzero even after ∼12 h. The thickened oxide eventually cracked when its thickness reached ∼120 A, causing loss of passivation. Surprisingly, neither the anodization ratio nor the Pilling Bedworth ratio showed any discontinuity at the time of oxide cracking, and the EIS behaviour remained qualitatively as before. This observation is taken as the evidence that the cracked and intact regions of the electrode behave more or less independently as parallel electrodes. When the potential was eventually switched to cathodic polarity, NR shows, as expected, that the effects of oxide cracking were irreversible. However, the electrode resistance recovered partially suggesting the cracks were rapidly plugged with newly formed oxide.