Materials Science Forum Vols. 610-613

Abstract: TiN films have been deposited on K211 super alloy substrates by chemical vapor deposition (CVD) using a gaseous mixture of TiCl4, NH3, and Ar as carrier gas between 400 and 700°C. The films were characterized by means of scanning electron microscopy and energy dispersive spectrometer. In this paper, the wear and corrosion resistance of TiN coatings have been investigated by abrasive experiments and corrosion tests. It was shown that coated samples demonstrated significantly higher wear resistance than those uncoated by weight loss measurements in the test procedure. Under the conditions of the corrosion experiments, substrates with TiN coatings showed better corrosion resistance than uncoated samples and stainless steel.

Abstract: Aluminum nitride ceramics doped carbon nanotubes were fabricated by dispersed multi-walled carbon nanotubes (MWNTs) into AlN powders homogenously. The densification, electric resistance, thermal conductivity and relative dielectric constant of AlN ceramics were investigated. It was found that MWNTs additives could purify the grain boundaries. Furthermore, it induced the increase of the thermal conductivity and the decrease of the relative dielectric constant. However, the linear shrinkage and electric resistance decreased with increasing MWNTs amount. AlN ceramics with high thermal conductivity and low relative dielectric constant were obtained without apparent decrease of electric resistivity.

Abstract: A newly developed nano/microcrystalline diamond composite film for thermal applications was prepared in this investigation. A microcrystalline diamond (MCD) film was deposited onto silicon substrate by hot filament chemical vapor deposition (HFCVD) method, and then a nanocrystalline diamond (NCD) film was grown onto this MCD film to obtain a NCD/MCD composite film. The root-mean-square (RMS) value of surface roughness for the composite film estimated from the atomic force microscope image was 42.7nm. Compared with 85.9nm for the MCD film. And it was also found that the thermal diffusivity increased from 32.61mm2/s to 37.63mm2/s by further growing a NCD film. Results indicated that the deposition of NCD film reduced the rough surface of the MCD film with grain sizes of the order of microns, and thus increased the efficiency of diamond films as thermal spreading device. It was found that the NCD/MCD composite film had a smoother surface and a higher thermal diffusivity compared with MCD film.

Abstract: A new method for the MEMS die package process was reported. Using the method of bionic package, a novel MEMS vector hydrophone was fabricated from a typical mass-beam accelerometer structure. According to the auditory principle of fish’s lateral line and its nature’s fundamental acoustic sensors-stereocilia, a rigidity cylinder was fixed on the center of the mass as a stereocilia, and the sensitivity structure of the hydrophone was obtained. By means of a specific package process, the sensor was fabricated properly for the underwater sound detection application. From the test results of the sensor, we can see that, this vector hydrophone has a receiving sensitivity of -165dB (0dB=1V/µPa, the amplification factor of the preamplifier is 100.) and a good directional pattern in form of “8”-shape. The hydrophone can work well under the maximum pressure of 2MPa.

Abstract: The nano-Al2O3 reinforced copper matrix composite was prepared by powder metallurgy technique and the fretting test against 440C stainless steel was performed at room temperature by using the ball-on–flat configuration with 300μm amplitude at various normal loads in the range of 0.1N to 1N. The influences of load and proportion of Al2O3 on the friction coefficient and volume wear -loss were investigated. Results showed that the volume wear-loss decreased firstly and then increased as the proportion of Al2O3 increased from 0.5% to 4%, and the minimum proportion was about 2%, in which the electrical conductivity can reach up to 81% according to International Annealed Copper Standard (IACS). The friction coefficient of the copper will slightly increases as the load increased. However, in the case of composites, it increased firstly then decreased. The volume wear-loss of the composite is always lower than that of copper, giving the highest relative wear-ability of 1.55, which indicated a higher wear resistance. The wear mechanism of copper is adhesive wear, whereas it becomes oxidation wear in composites.

Abstract: Two types of helical and straight carbon nanofibers, have been synthesized by the decomposition of acetylene using cupric carbonate basic as catalyst precursor at low temperature. The obtained carbon nanomaterials were characterized by transmission electron microscope, scanning electron microscope, and X-ray power diffraction. The size of the catalyst nanoparticles remaining inside the resultant nanofibers was determined. The carbon nanofiber diameters, ranging from 30 to 400 nm, closely correlated with the size of the catalytic nanoparticle. The growth mechanism of carbon nanomaterials was also studied. The nanocopper particle size had a considerable effect on the morphology of carbon nanofibers. The helical carbon nanofibers with a symmetric growth mode were synthesized with the nanocopper catalyst particles having a grain size less than 50 nm. When the average catalyst particle size determined was around 50–300 nm, the straight carbon nanofibers were obtained dominantly.

Abstract: A film with a single-layer of size controlled silver nanocrystals embedded in silicon dioxide (SiO2) dielectric film by magnetic sputtering has been fabricated for nonvolatile memory applications. The effects of sputtering power, deposition time and substrate temperature on Ag nanocrystals formation were investigated. Transmission electron microscopy (TEM) images showed the as-prepared Ag nanocrystals had high uniformity in their size and distribution. The relationship between Ag nanocrystal size, density and electron storage capability as well as date retention time has been discussed.

Abstract: Cu99.8Y0.2, Cu99.2Y0.8 and Cu98Y2 alloy ribbons were prepared by single roller melt spinning. The microstructure was studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), high voltage electron microscope (HVEM) and high resolution electron microscopy (HREM). The results showed that α-Cu was the dominative phase in the rapid solidification ribbons of three alloys. A secondary phase Cu4Y was detected in the Cu98Y2 ribbon by XRD. The grain size was in a range of 50-200 nm in the Cu99.2Y0.8 and Cu98Y2 ribbons. Many nano-scale twins and some dislocations existed inside the larger grains. However, the grains in Cu99.8Y0.2 ribbon were in the size of microns and the sub-grains with small misorentations were in 100-200 nm. To understand the deformation mechanism, in situ tensile test were carried out at a High Voltage Electron Microscope (HVEM). The results showed that the deformation is predominated by the dislocation slip in larger grains. To accommodate the deformation, elastic deformation occured in the small grains in the initial stage of the deformation. Meanwhile, some small grains maybe deform by grain rotations. With strain increasing, some fractures generated and propagated along the grain boundaries or across the grains.

Abstract: The correlation between the energy band-gap of AlxGa1-xN epitaxial thin films and lattice strain was investigated using both High Resolution X-ray Diffraction (HRXRD) and Spectroscopic Ellipsometry (SE). The Al fraction, lattice relaxation, and elastic lattice strain were determined for all AlxGa1-xN epilayers, and the energy gap as well. Given the type of intermediate layer, a correlation trend was found between energy band-gap bowing parameter and lattice mismatch, the higher the lattice mismatch is, the smaller the bowing parameter (b) will be.

Abstract: Wet chemical etching with dry metal deposition method has been developed to fabricate large-area aligned silicon nanowire (SiNW) arrays. The combination of nanoclusters Ag film with proper interconnection and interspaces (of about 20 nm thick), and proper temperature during etching (from 20 to 80 °C) is vital to successfully fabricating large-area uniform SiNW arrays. Raman and photoluminescence spectra of the SiNW arrays indicated their potential applications in chemical detection and optical devices ,respectively.