Key Engineering Materials Vols. 280-283

Abstract: NbSi2 coatings were formed on niobium by halide-activated pack cementation process. The as-coated niobium samples were oxidized in air up to 1723 K by thermogravimetry method. The surface and cross-sectional morphology, phase composition and element distribution of the NbSi2 coatings before and after oxidation were characterized by SEM, XRD and EPMA. The results show that the as-formed coatings consist of single phase of hexagonal NbSi2 and the oxidation resistance of pure niobium can be greatly improved by pack siliconizing.

Abstract: Effects of plasma pretreatments to the TaSiN film surface on Cu nucleation were investigated. Scanning electron microscopy (SEM) was used to measure the Cu nucleation density and to observe the morphology of the Cu film. X-ray spectroscopy (XPS) and Auger depth profiling analyses were used to investigate the bonding state of atoms and the concentrations of oxygen and nitrogen at the TaSiN film surface, respectively. Cu nucleation in Cu MOCVD is effectively enhanced by treating the underlying Ta-Si-N film surface with hydrogen plasma prior to Cu MOCVD. The Cu nucleation density in Cu MOCVD increases as the rf-power and the plasma exposure time increase in the hydrogen plasma pretreatment, but it is saturated at the rf-power of 40W and the plasma exposure time of 2min. To increase the rf-power and the plasma exposure time further would increase the plasma radiation damage for the Si substrate. Therefore, 40W and 2min are the optimal process conditions for the hydrogen pretreatment. Copper nucleation is enhanced by hydrogen plasma pretreatment because the plasma treatment removes the nitrogen and oxygen atoms from the Ta-Si-N film surface. Since Ta-Si is a substrate more favorable for Cu nucleation than Ta-Si-N(O), Cu nucleation on the Ta-Si-N film is enhanced by hydrogen plasma pretreatment of the Ta-Si-N film surface.

Abstract: AC and DC conductivities have been measured by using the real (e¢) and imaginary (e¢¢) parts of the dielectric constant data of glass and glass-ceramics (GC) at different temperatures in the rage 297-642K and in the frequency range 100 Hz to 10 MHz. Using Anderson –Stuart model, we have calculated the activation energy, which is observed to be lower than that of the DC conductivity. The analysis for glass/glass-ceramics indicates that the conductivity variation with frequency exhibits an initial linear region followed by nonlinear region with a maximum in the high-frequency region. The observed frequency dependence of ionic conductivity has been analyzed within the extended Anderson–Stuart model considering both the electrostatic and elastic strain terms. In glass/glassceramic the calculations based on the Anderson-Stuart model agree with the experimental observations in the low frequency region but at higher frequencies there is departure from measured data.

Abstract: AC and DC conductivities have been measured by using the real (e¢) and imaginary (e¢¢) parts of the dielectric constant data of glass and glass-ceramics (GC) at different temperatures in the rage 297-642K and in the frequency range 100 Hz to 10 MHz. Using Anderson –Stuart model, we have calculated the activation energy, which is observed to be lower than that of the DC conductivity. The analysis for glass/glass-ceramics indicates that the conductivity variation with frequency exhibits an initial linear region followed by nonlinear region with a maximum in the high-frequency region. The observed frequency dependence of ionic conductivity has been analyzed within the extended Anderson–Stuart model considering both the electrostatic and elastic strain terms. In glass/glassceramic the calculations based on the Anderson-Stuart model agree with the experimental observations in the low frequency region but at higher frequencies there is departure from measured data.

Abstract: The effects of Bi3+ addition on the sintering performance and microstructural evolution of cordierite were analyzed in this paper. Cordierite powders were prepared by sol-gel processing. As nucleating agent, the Bi3+ additive was found to promote the phase transition from µ-cordierite to α-cordierite and decrease sintering temperatures. The effect of Bi3+ on the activation of cordierite can be attributed to the nucleation of cordierite. The doping of Bi3+ increased the expansion coefficient and dielectric constant. Too much Bi3+ will inhibit the cordierite crystallization and encourage anorthite crystallization.

Abstract: La2O3-Al2O3-B2O3 (LAB) glass with Al2O3 particulates is a lead-free, non-alkali, and low-sintering temperature glass-ceramic system, which has been developed for LTCC application. The glass melting and sintering process window of the LAB systems were investigated. A systematic study on the wetting behaviors of the LAB glass on alumina plate, microstructures and crystallization in the glass-ceramics had also been conducted. The densification and crystallization temperatures of the glass-ceramics took place as low as 800oC. The interface condition of layer glass-ceramics with Ag electrode was also characterized. No obvious interfacial diffusions were observed.

Abstract: Synthesis of La2O3-SiO2-B2O3 (LSB) based glass-ceramics using glass melting method has een investigated in this study. XRD result showed that some LSB glass systems in this study were ntirely amorphous phases. In addition, TMA results revealed that the LSB/mullite (LSBM) glassceramics ith a mass ratio of 60/40 could be densified at 850oC, which matches the requirements for theLTCC application. Moreover, dispersive behavior of the LSB glass powder with six kinds of commercial ispersants in MEK and toluene solvent had been studied. Furthermore, tape-casting process used for ow-temperature-cofired-ceramic (LTCC) chips was conducted in order to do crystal phase identification, microstructure analysis, and dielectric property measurement.

Abstract: Magnesium aluminum silicate glass-ceramics were prepared by controlled heat treatment of base glass. The Al2O3 content was varied from 7.58 to 14.71mol % keeping total Mg content almost constant in the form of (MgO+MgF2). The crystalline phases were identified by X-ray diffraction technique while scanning electron microscopy was employed for finding the microstructure in the sample. The dielectric behaviour of the material was studied over the frequency range from 10-107 Hz and in the temperature range from 603 to 793K using impedance analyzer. X- ray diffraction studies revealed the formation of pure and mixed phases of MgSiO3, fluorophlogopite and Mg2SiO4 depending upon the alumina content. Mixed phase material has higher dielectric constant (e′), loss factor (e²) and dissipation factor (tand) than the single-phase material at lower frequencies. Dielectric constant (e′) decreases with frequency rather sharply up to 104Hz, and then remains nearly constant at higher frequencies up to 107Hz. However, tand first increases and then decreases with frequency passing through a broad peak. In all the cases e¢ and tan d are seen to increase with temperature. The activation energies for dipolar relaxation were calculated for all samples and were found to vary from1.77Joule/mole to 13.82Joule/mole with variation of alumina from 7.58 to 14.71mol %.

Abstract: Lithium zinc silicate (LZS) glass-ceramics with compositions: (a) Li2O-ZnO-SiO2-Na2OB2O3- P2O5 and (b) Li2O-ZnO-SiO2-K2O-Al2O3-B2O3-P2O5 have been prepared by controlled nucleation and crystallization. The effect of ZnO/(ZnO+SiO2) ratio on various thermo-physical properties was investigated by changing the ratio from 0.1 to 0.31 in the case of (a). Different crystalline phases have been identified by X-ray diffraction studies in glass-ceramics including cristobalite, Li3Zn0.5SiO4 and Li2SiO3. Density ( r) was found to increase from 2.62 to 2.82 gm cm-3 while microhardness (VHN) decreased from 6.56 to 5.79 GPa with increase in ZnO/(ZnO+SiO2) ratio in the glass-ceramics. Average thermal expansion coefficient (TEC) in the temperature range 30 to 450oC increased from 125x10-7 to 185x10-7 /oC. The increase in TEC and decrease in microhardness are thought to be due to the formation of different phases, which in turn influence the rigidity/bonding in the glass-ceramics. A remarkable difference in the microstructure close to interface of the glass-ceramics to Cu seal was seen in both the cases [high ZnO content (a) of ZnO/(ZnO+SiO2) ratio 0.31 and low ZnO content (b) of ZnO/(ZnO+SiO2) ratio 0.024]. Both the microstructures showed globally two contrast phases of bright and dark dispersed in the glass matrix. An interesting dandritic phase observed towards core in the microstructure for the high zinc content is not seen in the microstructure for low zinc content glass-ceramic. The seal withstands a vacuum of ~ 10-6 torr at helium leak rate of 3x10-10 torr litre/sec.

Abstract: The fluorescence and up-conversion spectral properties of Er3+-doped TeO2-ZnO and TeO2-ZnO-PbCl2 glasses suitable for developing optical fiber amplifier and laser have been fabricate and characterized. Strong green (around 527-550 nm) and red (around 661 nm) up-conversion emissions under 977 nm laser diode excitation were investigated, corresponding to 2H11/2, 4S3/2, ® 4I15/2 and 4F9/2 ® 4I15/2 transitions of Er3+ ions respectively, have beenobserved and the involved mechanisms have been explained. The dependence of up-converted fluorescence intensity versus laser power confirm that two-photons contribute to up-conversion of the green-red emissions. The novelty of this kind of optical material has been its ability in resisting devitrification, and its promising optical properties strongly encourage for their further development as the rare-earth doped optical fiber amplifiers and upconversion fiber laser systems.