Materials Science Forum Vols. 654-656

Abstract: The structures of Au particles on CeO2 surfaces were observed by an analytical transmission electron microscopy (TEM) equipped with annular dark field scanning transmission electron microscopy (HAADF-STEM) systems. The Au/CeO2 model catalysts were prepared by using the poly-crystalline CeO2 substrates. The Au particles of 2-5 nm in diameter were deposited on the substrates. The preferential orientation relationship of (111)[1-10]Au//(111)[1-10]CeO2 was frequently observed in profile-view HRTEM images on CeO2 (111) surface. High resolution HAADF-STEM images were also obtained for Au-CeO2 interfaces. The position of atomic columns of Au and Ce at Au-CeO2 interface is directly investigated from HAADF-STEM images. The structure of the interfaces between Au particles and CeO2 (111), (100), (110) surfaces were discussed.

Abstract: Atom probe tomography provides compositional information in three dimensions at the atomic scale, and is therefore extremely suited to the study of nanocrystalline materials. In this paper we present atom probe results from the investigation of nanocomposite TiSi¬Nx coatings and nanocrystalline Al. We address some of the major challenges associated with the study of nanocrystalline materials, including specimen preparation, visualisation, common artefacts in the data and approaches to quantitative analysis. We also discuss the potential for the technique to relate crystallographic information to the compositional maps.

Abstract: In the present study, the relationship between the fluctuation of indium (In) composition and wavelength change involved in internal quantum efficiency of InGaN based LEDs has been investigated. The In composition of commercial InGaN based LEDs with and without annealing at 900 oC has been analyzed by using atom probe tomography (APT). The results revealed that the decrease in In composition caused by annealing at 900 oC for 30 min. exhibited the wavelength change (approximately 6.13 nm blueshift) in the photoluminescence (PL). Wavelength change in the PL of InGaN based LEDs will be discussed in terms of the composition and distribution of In in InGaN/GaN.

Abstract: Free-standing samples of plasma sprayed cordierite with a high level of porosity were heat treated at different temperatures and for different holding times in order to study the densification process. Optimisation of a new characterisation methodology for the interconnected defects network in the specimens of plasma sprayed cordierite based on fluorescence confocal scanning laser microscopy (CSLM) information was the main objective of the present work. Optical, electron and confocal microscopes were use to determine percentage, distribution and morphology of the defects in the specimens. Fractal geometry combined to the CSLM images was useful for evaluating structural characteristics, complexity and level of connectivity of the defects network of these samples. All this information allowed studying the level of densification accomplished by the samples under the different thermal treatments conditions. After the evaluation of the results produced, this methodology can be validated for other applications or processes such as conventional sintering, etc. Results showed low percentage of porosity with dispersion the latter related to the degree of anisotropy of the samples. However an increase of porosity with the crystallisation was also observed.

Abstract: Screen printing is the most common method of transferring silver thick film paste to a low temperature co-fired ceramic tape. Silver paste with varying rheology will have a varying print quality. The effect of silver paste rheology on the microstructure is investigated. Although there are many parameters to be considered, this parameter is a critical parameter which will control the line width and roughness on the substrate. To study the effect of paste rheology, a test pattern design was printed on the Heraeus CT2000 LTCC tape using silver paste. Then the LTCC tape was stacked and laminated at pressure of 28 MPa and temperature of 65 oC for 10 minutes. The laminated sample then was fired up to 850 oC. The fired silver conductor printed on the LTCC tape then was analyzed. Prints were evaluated by scanning electron microscopy. It was found that silver paste with high viscosity produce a good microstructure.

Abstract: Alumina powder is used to improve the heat conduction and physical properties of heat radiation sheets, these properties can be improved by increasing the filling rate of alumina. Spherical alumina powder is being actively developed to improve the filling rate of alumina in sheets. In this study, a high-temperature flame was created by the flame-fusion method using an oxygen burner to produce spherical alumina. We investigated how the chemical composition and particle size of starting materials affect their fusion behavior under flaming conditions during spherical alumina production using the flame-fusion method. Boehmite and aluminum hydroxide, which accompany the endothermic reaction due to the dehydration of the starting materials, showed lower spheroidization ratios than sintered alumina. The spheroidization ratio improved as the particle size of the starting materials decreased.

Abstract: Tantalum nitride (Ta3N5) is a nontoxic red pigment that is being developed as a substitute for Cd-related pigments Ta3N5 is produced by the nitridation and heat treatment of amorphous Tantalum precursors. Doping elements were added in the precursor manufacturing stage to improve the red color tone of tantalum nitride. Grain growth was observed in nitrides that formed second phases, such growth led to an increase in the average grain size comprared to undoped nitrides, and the colors declined as the oxygen content increased. Nitrides that did not form second phases in response to doping elements remained a single-phase Ta3N5 and exhibited an excellent red color with a high nitrogen content. We determined that a change in the oxygen/nitrogen contents affected the color manifestation, which depended on the amount by which doping was increased.

Abstract: The cBN-cBN direct bonding structure of some cBN compacts made by the method of the direct conversion of the pyrolytic-BN (pBN) or wurtzite-BN (wBN) to cubic-BN at the pressure of 7GPa and temperature 2000 °C and the cBN compact made by the method of sintering the cBN powder along with polyvinylidene chloride (PVDC) at the condition of 5.5GPa and 1600 °C has been studied. The XRD, SEM, TEM analysis and the wear resistance on the ductile cast iron machining has been investigated. While the XRD analysis of the above cBN samples showed that all samples consist only of cBN, and the SEM observation showed that they all have the cBN-cBN direct bonding structure, A STEM analysis showed the existence of the impurities at the cBN crystal boundary. The starting materials and the sintering method affect the degree of these impurities.

Abstract: This paper reports observations of defects in laminated eight layers of a glass-ceramic composite system fabricated by a standard low temperature co-fired ceramic (LTCC) technology. The layers were laminated at 3000 psi and 70 °C for 10 minutes and were fired at 850 °C for 15 minutes. Material characterizations of the green compact and fired substrate were carried out on density, surface roughness and microstructure. The crack and warpage of the substrate were related to the microstructure and densification process of the system. It was found that the presence of these defects could be due to a mismatch of the sintering kinetics of the glass-ceramic composite system and silver conductor materials which lead to the development of stresses which act on both materials. The detailed microscopic observation of the internal and surface defects is explained.

Abstract: The thermal decomposition of pyrite was investigated under reducing atmosphere, e.g. p(O2)~10-6 bar. Thermogravimetric measurements were coupled with differential scanning calorimetry and mass spectrometry. Multi-scan method was used to calculate the values of the activation energy of the reaction of thermal decomposition of the pyrite. It was shown that the value of the activation energy of the decomposition of pyrite into pyrrhotite varies with the reaction progress. The values of activation energy vary from 250 to 350 kJ.mol-1.