A comparative study of the electrochemical deposition of molybdenum oxides thin films on copper and platinum

Molybdenum oxides thin films electrochemical deposition was performed using solutions of peroxo-polymolybdate at pH 2.3 and ammonium molybdate at pH 5.5 as precursors and smooth copper and platinum as supports. The deposition has been carried out at constant potentials in the range of −600 to −800 mV vs. Ag/AgCl (sat. KCl). The thin films deposited on copper were then heated at 350 and 450 °C in argon. In the case of thin films deposited from ammonium molybdate and heated at 450 °C, the XRD spectra reveal, along with MoO₂, the presence of Cu₆Mo₅O₁₈ phase. For the thin films prepared from peroxo-polymolybdate and subjected to the same heat treatment, the only XRD phase present was MoO₂. Thermogravimetric (TG) analysis was performed on samples prepared by scraping away the thin films (molybdate precursors) from the copper support. Before heat treatment, the AFM images of the as-deposited thin film reveal a granular morphology, with diameters in the 20–80 nm range.     

Differential reflectometry of corrosion products of copper

The new technique of differential reflectometry was applied to study some fundamental corrosion processes for copper. Free corrosion in air for one day yielded differential reflectograms characteristic of mixed oxides of Cu₂O and CuO. With increasing time of exposure to air, the reflectograms evolved into typical CuO patterns. Similarly, free corrosion of copper in distilled water yielded mixed oxides of Cu₂O and CuO. Holding copper potentiostatically near the boundary between these two corrosion products as shown on the Pourbaix diagram always yielded at first a mixed oxide which turned into the stable species after several days of polarization. It was further found that copper held potentiostatically first in the CuO region and then in the Cu₂O region of the Pourbaix diagram does not lose the characteristic patterns of CuO. This suggests that the sudden shift in potential induces growth of Cu₂O under the existing CuO layer without dissolving the CuO.     

Differential positron annihilation spectroscopy in nondestructive testing of thin chromium coatings on copper

The differential positron annihilation spectroscopy (DPAS) is used for the nondestructive investigation of thin metallic coatings on metallic substrates when the defectiveness of the coating applied is judged from the difference between the spectra of the individual substrate and the substrate covered with the coating. Using the DPAS angular correlation curves of positron annihilation radiation (ACAR), the presence of nanoscale void clusters in the X-ray amorphous galvanic chromium coatings with a weight thickness of 9.15 mg/cm² (1.3 μm) is determined on the copper substrate. The ACAR curves of the coatings have a complex structure compared to those of the specimens of Cr₃C₂ and Cr₂₃C₆ carbide powders and involve a narrow peak with a half width FWHF = 3 mrad and an intensity about 2% assigned to void clusters with a size of 0.86 nm, which act as positron traps, in the coating.     

Characteristics of ZnO thin film deposited on various metal bottom electrodes

ZnO films for electronic applications were deposited by radio-frequency (rf) sputtering onto various metal bottom electrodes (Pt/Ti, W, Ni) to investigate such structural properties as crystallinity and surface morphology. The crystallinity, surface morphology and composition of the as-deposited films were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Rutherford back-scattering spectrometry (RBS), respectively. The preferred orientation and surface morphologies were strongly influenced by the type of bottom electrodes. The ZnO films with (200) texturing deposited on Pt/Ti/SiO₂/Si showed a smoother and smaller grain size than those deposited on W and Ni. The ZnO films on Pt and W electrodes exhibited compressive residual stress. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials”, organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Technology of metal oxide thin film deposition with interruptions

The idea to obtain metal-oxide films with small grain size is to use a special regime of thin film deposition by r.f. sputtering of pure metal or metal oxide targets. This regime includes the deposition of thin films with one or several interruptions during the deposition process. WO₃ films were r.f. sputtered onto pure and oxidized silicon wafers. Four types of films were prepared, i.e. using continual deposition, one, two and three interrupted depositions with an actual deposition time of 40 min. The interruption time changed from 0.5 min to 5.0 min for the different samples. It was found that the total thickness of WO₃ films decreased with the increase of the number of interruptions and the increase in interruption time. Phase composition and features of surface morphology of the films deposited and annealed in the temperature range from room temperature to 900 °C have been investigated by XRD and AFM, respectively. It is shown that grain size in the metal oxide films decreased essentially with the increase of the number of interruption during the deposition process.     

An XPS study of anodic behavior of amorphous nickel-phosphorus alloys containing chromium,molybdenum or tungsten in 1 m CHl

The surfaces of amorphous Ni-18P, Ni-IOCr-20P, Ni-9Mo-19P and Ni-5W-18P alloys immersed or anodically polarized in 1 M HCl solution were analyzed in connection with their corrosion and anodic behavior. All alloys were more corrosion-resistant than crystalline nickel metal because of formation of phosphate-containing surface films on the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys and because of spontaneous passivation due to formation of passive hydrated chromium oxyhydroxide film on the Ni-10Cr-20P alloy. The latter alloy was stable up to the transpassive region of chromium although intrusion of phosphate in the film was responsible for the higher passive current density in comparison to the amorphous Fe-Cr-13P-7C alloy of the same chromium content. The formation of thick porous phosphate films containing nickel, and molybdenum or tungsten by anodic polarization was not effective in passivating the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys, and they suffered pitting corrosion by anodic polarization.     

Induction plasma spheroidization of tungsten and molybdenum powders

The melting, evaporation and oxidation behaviors as well as the solidification phenomena of tungsten and molybdenum in induction plasma were studied. Scanning electron microscopy was used to examine the morphology and the cross section of plasma-processed powders. X-ray diffraction was used to analyze the oxides formed on the particle surface of these two metals. The influence of spray chamber pressure on the spheroidization and oxidation phenomena was discussed. The results show that fewer Mo particles than W particles are spheroidized at the same powder feed rate under the same plasma spray condition although molybdenum has a lower melting point. A small fraction of tungsten is evaporized and condensed either on the surface of tungsten particles nearby or on the wall of spray chamber. Tungsten oxides were found in tungsten powder processed under soft vacuum condition. Extremely large grains form inside some spheroidized particles of tungsten powder.     

Syntheses of metal nitrides, metal carbides and rare-earth metal dioxymonocarbodiimides from metal oxides and dicyandiamide

We design a facile and efficient solid-state reaction method by selecting an organic reagent dicyandiamide and metal oxides as precursors to prepare metal nitrides, carbides and rare-earth metal dioxymonocarbodiimides in sealed ampoules. Some fine divided nitride and carbide nanoparticles with small and uniform size can be easily obtained at the relatively low temperatures. It is interesting to find that dicyandiamide is not only a highly efficient nitridation reagent but also a highly efficient carburization reagent, and can be used as a precursor to directly synthesize rare-earth metal dioxymonocarbodiimides. A possible mechanism is proposed to explain the results of the reactions between the organic reagent and metal oxides.     

Effect of certain oxides on the recrystallization of molybdenum

Metal Science and Heat Treatment -     

Electro-optical response of tungsten oxide thin film nanostructures processed by a template-assisted electrodeposition route

Tungsten oxide (WO₃) thin film mesophases encompassing a well-organized network of grains with a triclinic phase and pores with diameters in the range of 2–5 nm were generated at room temperature by template-assisted electrodeposition involving a novel combination of ionic and polymeric surfactants. Electron microscopy studies revealed the surface texture of the films annealed at 250 and 500 °C to be primarily composed of randomly oriented rod and sheet-like shapes and the bulk structure of these films to ensconce fibril-like shapes and distinct spherical nanoparticles, along with elongated pores. Coloration efficiency, transmission modulation and charge density for lithium insertion are greater, switching rates are faster (colors in 8.8 s and bleaches in 1.5 s for a 50% transmittance change) and cycling stability is superior for the as-deposited film as compared with the annealed WO₃ films. The superior electrochromic performance characteristics of the as-deposited WO₃ coatings are a direct manifestation of the mesoporosity over multiple length scales, a good lateral organization of grains and pores which facilitate ion insertion and extraction, and a large number of accessible surface sites which ensure a high proportion of W⁵⁺ color centers in the film upon lithium intercalation. In spite of the retention of a porous structure by the film annealed at 500 °C, the poor electrochromic response of the annealed films is attributed to the coarse grain rod-, sheet- and fiber-like framework, which efficiently obstructs lithium ion movement and uptake.     

Effects of working on strength and ductility of molybdenum and tungsten alloys

The variations in strength and ductility characteristics during industrial working of dispersion-strengthened molybdenum and tungsten alloys are described. With progressive working the ductile-brittle transition is decreased. Both strength and fracture elongation increase, in contrast to the usually observed inverse relation between these two properties. This is explained by the beneficial effects of dynamic strain aging which operates while the ingot is deformed at certain critical temperatures. However, such a twofold improvement is not observed in all alloys. It is not possible when the dispersates cannot dissociate; then, the alloying elements are not dissolved in the matrix and are not available for initiating dynamic strain aging, nor may the fracture elongation improve when extensive static age hardening is superimposed.     

Oxidation of copper and electronic transport in copper oxides

Oxidation of copper and electronic transport in thermally-grown large-grain polycrystals of nonstoichiometric copper oxides were studied at elevated temperatures. Thermogravimetric copper oxidation was studied in air and oxygen at temperatures between 350 and 1000°C. From the temperature dependence of the oxidation rates, three different processes can be identified for the oxidation of copper: bulk diffusion, grain-boundary diffusion, and surface control with whisker growth; these occur at high, intermediate, and low temperatures, respectively. Electrical-conductivity measurements as a function of temperature (350–1134°C) and oxygen partial pressure (10^–8–1.0 atm) indicate intrinsic electronic conduction in CuO over the entire range of conditions. Electronic behavior of nonstoichiometric Cu₂O indicates that the charge defects are doubly-ionized oxygen interstitials and holes. The calculated enthalpy of formation of oxygen ( ) and hole-conduction energy (E_H) at constant composition for nonstoichiometric Cu₂O are 2.0±0.2 eV and 0.82±0.02 eV, respectively.This work was supported by the U.S. Department of Energy, under Contract W-31-109-Eng-38.     

Effect of molybdenum and chromium on hardenability of low-carbon boron-added steels

The hardenability of low-carbon boron-added steels containing molybdenum or chromium was studied using dilatometry, thermodynamic calculations, and secondary ion mass spectroscopy (SIMS). The combined addition of boron and molybdenum was found to be more effective than that of boron and chromium in enhancing the hardenability of boron-added steels. In particular, the addition of 0.5 wt.% molybdenum to the boron-added steel almost completely suppressed the formation of polygonal ferrite even at a slow cooling rate of 0.5°C/s. The synergistic effect of the combined addition of molybdenum and boron is thought to be due to both the suppression of M₂₃(C,B)₆ precipitation resulting from the deterioration of phase stability and the reduction of carbon diffusivity by the molybdenum addition.     

Tensile and compressive behavior of tungsten,molybdenum, tantalum and niobium at the nanoscale

In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of 〈0 0 1〉-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log–log slope of strength vs. nanopillar diameter, is ?0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (?0.44). In tension, however, Ta and Nb show higher size effect slopes (?0.80 and ?0.77) as compared with W and Mo (?0.58 and ?0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.