H.C. Shih

WELL-ALIGNED CARBON NITRIDE NANOTUBES SYNTHESIZED IN ANODIC ALUMINA BY ELECTRON CYCLOTRON RESONANCE CHEMICAL VAPOR DEPOSITION

Vertically aligned carbon nitride nanotubes with a uniform diameter of about 250 nm have been synthesized on a porous alumina membrane template (50–80 μm thick) in a microwave excited plasma of C2H2 and N2 using an electron cyclotron resonance chemical vapor deposition system. A negative dc bias voltage was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nanochannels of the alumina template. This allowed the physical, and subsequent chemical, absorption of species on the walls of the nanochannels that resulted in the formation of the carbon nitride nanotubes. The hollow structure and vertically aligned properties of the nanotubes have been clearly verified by field-emission scanning electron microscope images. The absorption band between 1250 and 1750 cm−1 in the Fourier transform infrared spectroscopy spectrum proves that nitrogen atoms have been incorporated into an amorphous network of carbon.

Bias-enhanced nucleation and growth of the aligned carbon nanotubes with open ends under microwave plasma synthesis

Aligned carbon nanotubes with open ends have been fabricated on silicon wafer in one step using a microwave plasma enhanced chemical vapor deposition system with a mixture of methane and hydrogen as precursors. High concentration hydrogen plasma and high negative bias voltage to the substrate induce anisotropic etching of carbon nanotubes and can effectively reduce the randomly oriented carbon nanotubes. The mechanism of aligned carbon nanotubes with open ends is proposed in this letter.

The effect of boron on the corrosion resistance of the high entropy alloys Al0.5CoCrCuFeNiBx

High entropy alloys are a newly developed family of multicomponent alloys that consist of various major alloying elements, including copper, nickel, aluminum, cobalt, chromium, iron, and others. Each element in the alloy system is present at between 5 and 35 atom %. A high entropy alloy has numerous beneficial mechanical, magnetic, and electrochemical characteristics. This investigation discusses the corrosion resistance of the Al 0.5 CoCrCuFeNiB x alloys with various amounts of added boron. Surface morphological and chemical analyses verified that the addition of boron produced Cr, Fe, and Co borides. Therefore, the fraction of Cr outside borides precipitates was scant. The anodic polarization curves and electrochemical impedance spectra of the Al 0.5 CoCrCUFeNiB x alloys, obtained in 1 N H 2 SO 4 aqueous solution, clearly reveal that the general corrosion resistance decreases as the concentration of boron increases.

Role of extrinsic atoms on the morphology and field emission properties of carbon nanotubes

Extrinsic atoms were doped into multiwalled carbon nanotubes (MWCNTs) using microwave plasma-enhanced chemical vapor deposition. Doped nitrogen atoms alter the original parallel graphenes into highly curved ones including some fullerene-like structures. Doped nitrogen atoms could replace carbon atoms in MWCNTs and therefore increase the electronic density that enhances the electron field emission properties. On the other hand, the incorporation of boron into the carbon network apparently increases the concentration of electron holes that become electron traps and eventually impedes the electron field emission properties. Fowler–Nordheim plots show two different slopes in the curve, indicating that the mechanism of field emission is changed from low to high bias voltages. β values could be increased by an amount of 42% under low bias voltages and 60% under high bias voltages in the N-doped MWCNTs, but decreased by an amount of 8% under low bias region and 68% under high bias voltage in the B-doped MWCNTs.

Synthesis and characterization of the aligned hydrogenated amorphous carbon nanotubes by electron cyclotron resonance excitation

Abstract Aligned hydrogenated amorphous carbon nanotubes on porous anodic alumina have been synthesized by electron cyclotron resonance chemical vapor deposition (ECR-CVD) using the precursor gases, acetylene and argon. The composite film, with the aligned hydrogenated amorphous carbon nanotubes embedded in the porous anodic alumina, was found to be robust and is expected to have potential application in optic, electronic and optoelectronic devices. It is possible to prepare a large area of such a film by taking advantages of the ECR-CVD process, e.g. high plasma density at low temperature, less ionic damage, contamination-free and high deposition rate. By adjusting the pore size of anodic alumina, hydrogenated amorphous carbon nanotubes of various diameters can be produced in a range from 230 down to 30 nm. Characterization of the nanotubes in anodic alumina was carried out by field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), transmission electronic microscopy (TEM) and electron energy loss spectroscopy (EELS). The results indicate that the nanotubes consist of amorphous hydrogenated carbon, which are grown at a temperature of ∼100°C for 4 min.

The lifetime assessment of hot-dip 5% Al–Zn coatings in chloride environments

Abstract Batch-type hot-dip zinc and 5% Al–Zn coatings were investigated for comparison of their corrosion resistance, electrochemical behavior and microstructures. The 5% Al–Zn coatings possess prominent electrochemical passivation behavior. Intermetallics formed mainly between iron, aluminum and zinc adhering to the substrate were identified with energy-dispersive X-ray analysis and are believed to be responsible for the passivation phenomena observed in the electrochemical polarization. The 5% Al–Zn coatings exhibit much better corrosion resistance than the conventional hot-dip galvanizing steels under salt spray tests. Although the corrosion potential of both coatings increases toward the noble potential as the immersion time increases, 5% Al–Zn coatings are always nobler than hot-dip zinc coatings.

Mechanical properties of a bulk Cu0.5NiAlCoCrFeSi glassy alloy in 288°C high-purity water

The tensile properties and fatigue lives of a bulk Cu0.5NiAlCoCrFeSi glassy alloy were measured in 288°C water and compared with those obtained in air at ∼25°C. The bulk glassy alloy retained good tensile properties in 288°C water, i.e., tensile fracture strength and tensile fracture elongation were, respectively, 2660MPa and 2.01% in air and a little lower, 2000MPa and 1.49%, in 288°C water. The results of fatigue tests confirmed the significant decrease in fatigue life in high-temperature water. A decrease in strain rate from 0.5to0.001%s−1 decreased fatigue life by a factor of ∼5.

A novel approach to the formation of amorphous carbon nitride film on silicon by ECR-CVD

Amorphous carbon nitride films have been synthesized on silicon by using an ECR-CVD system equipped with a DC bias and a mixture of C2H2, N2 and Ar. Excess argon together with the application of DC bias can increase the ratio of nitrogen to carbon in the film up to 41% as determined by XPS. FTIR spectrum shows an absorption band between 1000 and 1700 cm−1 which proves the incorporation of nitrogen atoms into the amorphous network of carbon. The plasma chemistry of the system was also analyzed by OES to investigate the active chemical species that were involved in the formation of carbon nitride. The result indicated that the addition of excess argon (four times more than nitrogen) can effectively enrich the excited-state CN radicals which subsequently promotes the concentration of nitrogen in the amorphous carbon nitride film. This observation is likely due to the lower ionization energy of argon (15.8 eV), argons larger cross-section area for collision and its massive weight in comparison with the indispensable hydrogen gas as employed in the synthesis of other related materials.

The cavitation-erosion phenomenon of chromium nitride coatings deposited using cathodic arc plasma deposition on steel

Abstract The cavitation-erosion behavior of chromium nitride (CrN) coatings on AISI 4140 steel in fresh water and 3.5% NaCl solution has been investigated using an ultrasonic vibration system. The CrN coatings were deposited by cathodic arc plasma deposition (CAPD) with and without an intermediate layer of electroplated hard chrome, i.e. CrN/AISI 4140 and CrN/Cr/AISI 4140. The composition and structure of the CrN coatings have been studied using X-ray diffraction (XRD), using θ/2θ diffraction mode and Schulz reflection methods, which revealed a preferred orientation of CrN(220) for CrN/Cr/AISI 4140. The microstructure and crystalinity of the CrN coatings have been examined using cross-sectional transmission electron microscopy (XTEM). Both CrN/AISI 4140 and CrN/Cr/AISI 4140 coatings exhibited strong microcolumnar structures, however, the shape and grain size of the coatings were approximately the same. In addition to the microstructural investigations, cavitation-erosion tests were conducted under free corrosion conditions in fresh water and in chloride containing water. Subsequent electrochemical evaluation was made to elucidate the mechanism of cavitation-erosion. The resulting mechanical damage has been assessed using weight loss measurements and scanning electron microscopy (SEM) observations. Notwithstanding a significant improvement in the cavitation resistance of the coated specimens in fresh water and NaCl solution, the damage resulting from the cavitation-erosion in the 3.5% NaCl solution exhibited a higher weight loss than those in freshwater.

Electron field emission from various morphologies of fluorinated amorphous carbon nanostructures

Unlike general fluorination, amorphous fluorinated carbon (a‐C:F) nanostructures have been synthesized directly and efficiently by an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system using a mixture of C2H2, CF4, and Ar as precursors. The electron field-emission properties of the a‐C:F nanostructures were investigated. The a‐C:F nanoporous films with a low turnon field (1.8V∕μm) are apparently lower than other types of a‐C:F nanostructures. The a‐C:F nanostructures have a greater field-enhancement factor (2500–4000) than other nonaligned multiwall nanotubes. However, the a‐C:F nanostructures follow the Fowler–Nordheim characteristics only in the medium emission current region and they deviate from the characteristics in the low and high emission current regions.