Materials Science Forum Vol. 517

Abstract: A study was conducted on the effects of surface pre-treatment with 3- aminopropyltriethoxysilane (3-APS) on the water absorption and adhesion behaviour of two commercial alkyd and epoxy polyamide organic coatings on mild steel and glass substrates. The results indicated that using 3-APS as surface pre-treatment prior to coating resulted in a significant reduction of total water absorption for alkyd on both mild steel and glass substrate. Subsequently ile adhesion of alkyd is enhanced on both pre-treated mild steel and glass substrate. However 3-APS pre-treatment of mild steel increases water absorption of epoxy and lowers its adhesion strength. While 3-APS pre-treatment of glass reduces water absorption of epoxy and subsequently enhances the overall wet adhesion. These data are consistent with the known adsorption behaviour of 3-APS to steel, i.e. as an easily hydrolysed hydrogen-bond to the amine group, rather than as the more stable metal-oxide-silicon bond.

Abstract: This paper presents an analysis of the characteristics of two gallium nitride (GaN) films grown on (0001) plane sapphire substrates by low-pressure metalorganic chemical vapor deposition (MOCVD) is presented. The GaN films were characterized by a variety of methods, including scanning electron microscopy (SEM), x-ray diffraction (XRD), photoluminescence (PL), and Raman scattering. SEM micrographs revealed that different growth conditions will lead to different surface morphology of the films. XRD measurements indicated that both films were highly oriented and mono crystalline. PL spectra for both samples exhibited an intense and sharp band edge peak at 3.42 eV with full width at half maximum (FWHM) of 15 and 35 meV respectively. Raman scattering showed that the peaks of E2(high) phonon mode were observed at 568.1 and 570.1 cm-1 respectively. The different growth mode of these films were linked to the growth conditions, in which the growth mechanism could be correlated with the shift of E2(high) phonon mode in Raman scattering.

Abstract: A comparative study of the structural and electrical properties of GaN films grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) at 700°C, with and without AlN buffer layer is presented . Hydrogen plasma was used in addition to nitrogen plasma to produce GaN. The introduction of H-plasma is found to influence the properties of the films. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Hall Effect measurements show that the sample with AlN buffer layer possesses a smoother and more homogenous morphological characteristics as well as a lower background electron and higher Hall mobility as compared to the sample without buffer layer. X-ray diffraction (XRD) reveals that hydrogenation is capable of producing the epitaxial GaN films at reduced temperatures with the full width at half maximum (FWHM) of the x-ray rocking (XRC) of GaN (0002) reflection was found to be 54.8 and 256 arcmin for samples with buffer layer and without buffer layer, respectively.

Abstract: In this study, single-core and multi-core Tl1212 tapes were fabricated from Tl-1212 superconducting powder of Tl0.5Pb0.5Sr1.8Yb0.2CaCu2O7 starting composition derived by conventional solid state synthesis. Single core tapes showed maximum Jc and enhanced 1212 phase formation were achieved for tapes annealed at a higher temperature for a longer duration. Multi core tapes showed a general increase in Ic and Jc with the number of tapes core. The highest Jc of 521 A/cm2 was observed for the 4-core tapes annealed at 870 °C for 90 minutes. The initial drastic drop of the tapes Jc in low magnetic fields (≤1T) indicates limitation on tapes performance by weak links. The low Jc of the tapes may be due to the lack of texture in the tapes core as revealed by SEM.

Abstract: Longitudinal and shear ultrasonic attenuation measurements were performed at 10 MHz in HoBa2-xHoxCu3O7-δ (x=0.0 and 0.15) polycrystalline ceramics between 80 K and 285 K. Longitudinal attenuation profiles showed a large peak around 200 K for superconducting HoBa2Cu3O7-δ but not for oxygen-reduced non-superconducting HoBa2Cu3O7-δ. Two longitudinal attenuation peaks at 190 K and 250 K were observed for Ba-substituted HoBa1.85Ho0.15Cu3O7-δ. The high temperature (∼190 K-200 K) peaks are due to absorption of sound energy during interaction of ultrasound wave with mobile oxygen atoms involved in an oxygen rearrangement process involving Cu-O chains of the materials.

Abstract: We have synthesized the belt-like structures of tin oxide (SnO2) by carrying out the thermal evaporation of solid Sn powders. We have analyzed the samples with scanning electron microscopy, X-ray diffraction, transmission electron microscopy and photoluminescence (PL). The obtained nanobelts were single crystalline with a tetragonal rutile structure. PL spectrum exhibited the visible light emission. We have discussed the possible growth mechanisms.

Abstract: In the fabrication of nitride-based laser, the cavity plays a vital role in order to enhance the fundamental modes while suppressing the higher order modes. The critical problem faced between the designer and the grower is the idealistic approach in the design laboratory, which more often cannot be met in the fabrication lines. This is because the design does not take into account the limitation in the growth chamber in achieving the targeted mirror thickness and material composition. This paper discusses methods of designing DBR mirrors for vertical cavity surface emitting GaN based lasers. Our design considers variance in the thickness and material composition of the mirrors. The simulation results are given for the investigation of several properties of DBR mirror, as well as a comparison of same types of symmetric DBR mirror.

Abstract: In this paper we discussed the relation between depth errors that happened in films growth and incidence angle variation on DBR reflectivity. We assume that there is 10% depth error in high and low index materials, and there are four plus one situations to be considered. Four are combinations of Hi +/- 10% error and Lo +/- 10% error, and no error. Our simulation results show that the depth error makes the reflective band shift and it almost doesn’t reduce reflectivity. The thickness error of +/- 10% in (Al0.4Ga0.6N/GaN) DBR structure (15 pairs) at 420nm was 42nm. A theoretical analysis using Transfer Matrix Mode with MATLAB software on the influence of layer thickness and incidence angle variation in vertical-cavity surface-emitting lasers with distributed Bragg reflectors (DBRs) on lasing wavelength is presented. It is shown that changing the thickness of the layers in the DBR mirror by only 10% is sufficient to produce shifts in the peak reflectance wavelength up to ± 20 nm (for a blue laser at 420nm). This could limit the precision of a desired wavelength, which is its reproducibility.

Abstract: There is a particular interest in the development of wide band gap semiconductor gas sensor because of their potential for high temperature operation and the ability to integrate them with power or microwave electrodes or with UV solar-blind detector and emitters fabricated in the same materials. AlGaN based devices are attractive for gas sensing in automotive exhausts and flow-gas, because of strong spontaneous polarization of AlGaN (free carrier concentration profiles inside this material that is very sensitive to any manipulation of surface change). In this report, we characterized the Ni/AlGaN/Sapphire Schottky barriers as hydrogen gas sensor at temperature range of 25°C to 500°C. A change in forward current was obtained in response to a change in ambient from pure N2 to 2% H2/ 98% N2, higher than the change in forward current obtained in Ni/GaN or Ni/Si Schottky diodes measured under the same conditions. The sensor response time was independent on the rate of mass transport of gas into the test chamber, while at high temperature, dissociation of gas controlled by the diffusion of atomic hydrogen through the metal/AlGaN surface, increased the sensor response time.