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.     

High-throughput study of binary thin film tungsten alloys

Combinatorial magnetron co-sputtering from elemental sources was applied to produce W-alloy thin film composition spread materials libraries with well-defined, continuous composition gradients (film thicknesses between 1 and 2.5 μm). Three systems were studied: W-Fe (0–7 at.%), W-Ti (0–15 at.%) and W-Ir (0–12 at.%). High-throughput characterization of the materials libraries comprised of chemical, morphological and microstructural analyses. Scanning electron microscope investigations revealed that the films have a columnar structure of inverted cone-like units separated by voided boundaries, with a strong correlation to the alloying element content. Significant morphological changes occurred with an increase in the amount of the added element; W films with lower at.% of the alloying element had higher density and tighter grain boundaries, altering towards an increased amount of voids as the concentration of the alloying element increased. Electron backscatter diffraction scanning was used to determine microstructural components (grain size, grain shape, texture evolution), in dependence on the concentration of the alloying element.     

The dissolution behaviour of iron,chromium, molybdenum and copper from pure metals and from ferritic stainless steels

Dissolution rates of iron, chromium, molybdenum and copper as pure metals and as components of three ferritic stainless steels in neutral solutions (distilled H₂O and 0.5 M NaCl) and acid solutions (1 M and 9.2 M of HNO₃ and H₂SO₄) are presented. The relative position of dissolution rates of the elements is in general the same in pure metal and alloy dissolution. Selective dissolution of iron and simultaneous surface enrichment of chromium is observed under all the investigated conditions. A slight surface enrichment of molybdenum is found upon exposure in the neutral and sulfuric acid solutions but not in nitric acid solution. Large amounts of copper are found in the surface of the copper-containing steel exposed to 1 MH₂SO₄. Evidence is presented which shows that an increase in electrolyte volume decreases the thickness and changes the composition of the passive film. It is believed that this hardly ever mentioned effect is due to an increased amount of dissolution of the film components upon increased volume of electrolyte.     

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.     

Deposition and characteristics of chromium nitride thin film coatings on precision balls for tribological applications

Thin film hard coatings on rolling element surfaces can enhance the overall wear resistance of rolling element bearings, as demonstrated previously for coated tapered, cylindrical, and spherical roller bearings. Hard coatings in ball bearings are less common because of the difficulty in achieving uniform film thickness on a ball surface. This limitation is overcome by a new process for depositing chromium nitride coatings with uniform thickness on precision balls using ion beam assisted deposition (IBAD) e-beam evaporation. Scanning electron microscopy indicated that the deposited films were smooth and conformal on the ball surfaces with no areas of localized delamination. Auger electron spectroscopy confirmed that Cr₂N and CrN bulk film stoichiometry was achievable by modulating the argon to nitrogen process gas ratio during deposition. Transmission electron microscopy revealed dense, polycrystalline film structure. Film hardness and elastic modulus as measured using nanoindentation on the coated balls met expectations for chromium nitride, and tribological testing of the coated balls in angular contact ball bearings under moderate contact stress levels demonstrated adequate film adhesion for practical use of these coatings in bearing applications.     

多孔泡沫铜支撑锡薄膜阳极材料的制备及性能

以三维多孔泡沫铜为基底,采用化学镀的方法制备锂离子电池薄膜Sn负极材料.利用扫描电镜、X射线衍射分析以及恒电流充放电测试等手段研究不同厚度薄膜Sn电极的形态、结构和电化学行为.结果表明:化学镀工艺制备的Sn电极表面的大量微孔和岛状突起不仅增大电极的表面积,而且显著缓解电极在充放电过程中体积的变化;其中镀层较薄的样品C薄膜Sn电极的初始充电(脱锂)容量为660.6 mA·h/g,经100次循环后,容量保持在299.5 mA·h/g,具有较好的循环性能.     

Hydrogen reduction of tungsten oxides: Alkali additions,their effect on the metal nucleation process and potassium bronzes under equilibrium conditions

The addition of alkali compounds influences W particle growth during the reduction of tungsten oxide using hydrogen. Existing literature does not reveal the mechanism and clear mode of interaction. To improve understanding of the reduction process, lithium, sodium and potassium compounds were added to a highly pure tungsten oxide prior to reduction, and interrupted reduction experiments were carried out at 750 °C. The resulting powders were investigated by metallographic and chemical methods: X-ray diffraction, SEM and EDX analysis.The experiments confirmed that intermediately formed tungsten bronzes play an important role in the early stages of reduction. Further experiments showed that bronzes are formed from potassium containing compounds in equilibrium with mixtures of W and WO₂ or WO₂ and WO_2.72. Furthermore it is demonstrated that the kinetics of the reduction sequence differs significantly between doped and undoped tungsten oxide powders, as demonstrated by the on-line measurement of reaction water in the hydrogen process gas.The results help to understand how the presence of alkali compounds effects the reduction of tungsten oxides. Investigations in production scale with intentionally contaminated tungsten oxide showed that bronzes can play a role in industrialised production.     

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.     

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.     

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.     

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.