Increased Si NMR sensitivity in glasses with a Carr-Purcell-Meiboom-Gill echotrain

The use of a Carr-Purcell-Meiboom-Gill (CPMG) echotrain to increase the ²⁹Si NMR sensitivity in glasses was investigated. The echo intensity decay follows a stretched exponential behavior M(t) = M₀ exp[−(t/t2)^β] with values for the exponent β in the range of 0.41-0.65. The signal to noise in the spectra can be increased by a factor of up to 4 by taking the weighted sum of the spectra obtained from the individual echoes. However, differential T2 relaxation for the different Qⁿ species is observed, with a shorter relaxation time for Q⁴ than Q³. Thus, summing of the echoes leads to distorted spectral intensities and quantitative information can no longer be obtained from the spectra. To circumvent the problem with differential T2 relaxation, an alternative approach is developed in which the spectral intensity for each chemical shift value is determined from the stretched exponential fit to its echo decay. With this approach, the sensitivity can be increased by a factor of up to 2.4, while quantitative information can still be obtained from the spectra. The increased sensitivity permitted the detection of five-coordinated silicon in a potassium silicate glass with natural ²⁹Si abundance at ambient pressure.     

Crystallization and dielectric properties of low temperature dielectrics containing Li2O filler

Crystallization and dielectric properties of typical low temperature co-fired ceramics (LTCC) consisting of calcium zinc aluminoborosilicate glass and Al₂O₃ filler were investigated by substituting the Al₂O₃ filler partially with Li₂O at the levels of 2-10 wt%. Depending on the content of Li₂O, densification was found significantly affected by early crystallization that resulted from the formation of unexpected crystalline phases including LiAlSiO₄, Ca₂SiO₄, LiAlO₂, and LiAlSi₃O₈. The effect of hindering sintering via earlier crystallization became enormous regardless of firing temperature when >5 wt% Li₂O substitution occurred. It was observed that the substitution of 2 wt% Li₂O for Al₂O₃ was beneficial in producing promising performance at the low temperature of 750 °C, which can be highlighted with k ∼ 8.7 and tan δ ∼ 0.009 at 1 MHz.     

Electrical conductivity studies in single and mixed alkali doped cobalt-borate glasses

The glasses of the type (Li₂O)_x-(CoO)_0.2-(B₂O₃)_0.8−x and (Li₂O)_0.2-(K₂O)_x-(CoO)_0.2-(B₂O₃)_0.6−x were prepared by melt quench technique and their non-crystallinity has been established by XRD studies. The glasses were investigated for room temperature density and dc electrical conductivity in the temperature range 300-550 K. Molar volumes were estimated from density data. Composition dependence of density and molar volume in both the sets of glasses has been discussed. Conductivity data has been analyzed in the light of Mott’s Small Polaron Hopping (SPH) Model and activation energies were determined. Variation of conductivity and activation energy with Li₂O content in single alkali glasses indicated change over conduction mechanism from predominantly electronic to ionic, at 0.4 mole fraction of Li₂O. In mixed alkali glasses, the conductivity has passed through minimum and activation energy has passed through maximum at x = 0.2. This has been attributed to the mixed alkali effect. It is for the first time that a change over of predominant conduction mechanism in lithium-cobalt-borate glasses and mixed alkali effect in lithium-potassium-cobalt-borate glasses has been observed. Various physical and polaron hopping parameters such as polaron hopping distance, polaron radius, polaron binding energy, polaron band width, polaron coupling constant, effective dielectric constant, density of states at Fermi level have been determined and discussed.     

Probing alkali coordination environments in alkali borate glasses by multinuclear magnetic resonance

Five series of binary alkali borate glasses were prepared to compare the alkali dependence of network and modifier short-range order. ¹¹B magic-angle spinning (MAS) NMR reveals that the fraction of four-coordinate boron depends strongly upon alkali type at high-alkali concentrations: heavier alkalis favour the formation of non-bridging oxygens, whereas lithium borates contain a much higher concentration of tetrahedral boron units. The alkali modifiers were observed directly by MAS NMR to measure the change in chemical shift with composition. All alkali peaks shift to higher frequency with increasing loading, indicative of decreasing average coordination numbers. Relative to their known chemical shift ranges, the heavier alkalis exhibit the greatest shifts, whereas the lithium shifts are subtle. This is interpreted in terms of the availability of charged and partially charged coordinating oxygens in the network. Moreover, the ¹³³Cs chemical shifts plateau at 40 mol%, implying that the Cs⁺ coordination number reaches a lower limit at this composition. This work demonstrates that NMR instrumentation and methodology have reached a level where even challenging nuclei like ³⁹K and ⁸⁷Rb can be probed to yield structural information in glasses.     

Ab initio calculation of vibrational frequencies of GexPxS1−2x glass

The vibrational frequencies of GeS₄, GeP₄, Ge₂S₆, GeP₃, Ge₃P, Ge₂P₂, P₂S₂, P₃S, P₄S₃, α-P₄S₄, β-P₄S₄, α-P₄S₅, β-P₄S₅, P₄S₇, P₄S₉ and P₄S₁₀ are theoretically computed from the first principles. The Raman frequencies of Ge_xP_xS_1−2x glass are obtained for x varying from 0.05 to 0.019. The computed fundamental frequencies of clusters are compared with those experimentally found. In this way, we are able to identify the vibrating clusters in the real glass. The clusters identified in the real glass are found to be Ge₂P₂, P₄S₃, α-P₄S₄, β-P₄S₄, β-P₄S₅, P₄S₇, P₄S₉, β-P₄S₅, Ge₂S₆, Ge₃P.     

Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption

Glasses of the systems Bi₂O₃-SiO₂, Bi₂O₃-PbO-Ga₂O₃, Bi₂O₃-PbO-Ga₂O₃-GeO₂ and Bi₂O₃-GeO₂-Li₂O have been prepared and the interaction of their melts with crucibles of different materials has been analytically determined. Silica and porcelain crucibles were very strongly corroded and the glass composition was noticeably altered. Instead platinum crucibles are not affected if the Bi₂O₃ content is not too high. The color of the glasses changes in all cases from pale yellow to deep brown when the melting temperature reaches approximately 1000 °C. The higher the temperature and the Bi₂O₃ content the darker the brown color, independently of the nature of the employed crucible. The addition of oxidizing ions (Sb⁵⁺, As⁵⁺ or Ce⁴⁺) to the glass batch prevents darkening. Nanoparticles of elementary bismuth Bi⁰ are observed by transmission electron microscopy in the glasses melted above 1000 °C. The partial thermoreduction of the Bi₂O₃ during the heating of the glass melt is proposed as the mechanism responsible for the observed darkening.     

Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality

We report on Raman scattering measurements of all Raman-active phonons in wurtzite and zinc blende structure GaN epilayers grown on GaAs (001), GaAs (111)A, and GaAs (111)B oriented substrates by means of molecular beam epitaxy (MBE). Raman spectra are taken from these epilayers at room temperature and 77 K in backscattering geometry. The measured values of the phonon frequencies are in agreement with other studies and with lattice dynamic calculations of phonon modes in GaN zinc blende and wurtzite structures. We show that crystal quality is much better in samples grown on GaAs (111) substrates than in samples grown on GaAs (001) substrates. The observation of disorder-activated modes gives information about sample quality. Comparison of the spectra from different thickness epilayers shows that the GaN is more highly disordered close to the substrate, particularly for the (001) substrates. Received 16 July 1999     

Studies of Er doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

An erbium doped germanate-oxyfluoride glass 60GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (GPOF) and a tellurium-germanate-oxyfluoride glass 30TeO₂ · 30GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (TGPOF) were prepared in the bulk form. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained with the formation of β-PbF₂ nanocrystals in the glass matrix confirmed by X-ray diffraction. Optical absorption and photoluminescence measurements were performed on as-prepared glass and glass-ceramics. The luminescence of Er³⁺ ions in transparent glass-ceramics revealed sub-band splitting generally seen in a crystal host. The intensity of red and near infrared luminescence significantly increased in transparent glass-ceramic compared to that in as-prepared glass. Two luminescence bands at 758 nm from ⁴F_7/2 → ⁴I_13/2 and at 817 nm from ²H_11/2 → ⁴I_13/2 transitions were observed from transparent glass-ceramic but cannot be seen from the corresponding as-prepared glass. These results are attributed to the change of ligand field of Er³⁺ ions and the decrease of effective phonon energy when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Analysis of serum from type II diabetes mellitus and diabetic complication using surface-enhanced Raman spectra (SERS)

In this paper, we show surface-enhanced Raman spectra (SERS) of serums from type II diabetes mellitus and diabetic complication (coronary disease, glaucoma and cerebral infarction), and analyze the SERS through the multivariate statistical methods of principal component analysis (PCA). In particular, we find that there exist many adenines in these serums, which maybe come from DNA (RNA) damage. The relative intensity of the band at 725±2 cm⁻¹ assigned to adenine is higher for patients than for the healthy volunteers; therefore, it can be used as an important ‘fingerprint’ in order to diagnose these diseases. It is also shown that serums from type II diabetes mellitus group, diabetic complication group and healthy volunteers group can be discriminated by PCA.     

Numerical study of optical properties of InGaN multi-quantum-well laser diodes with polarization-matched AlInGaN barrier layers

The optical properties of InGaN multi-quantum-well laser diodes with different polarization-matched AlInGaN barrier layers have been investigated numerically by employing an advanced device simulation program. The use of quaternary polarization-matched AlInGaN barrier layers enhances the electron–hole wave function overlap due to the compensation of polarization charges between InGaN quantum well and AlInGaN barrier layer. According to the simulation results, it is found that, among the polarization-matched quantum-well structures under study, lower threshold current and higher slope efficiency can be achieved simultaneously when the aluminum composition in AlInGaN barrier layers is about 10–15%. The optimal polarization-matched InGaN/AlInGaN laser diode shows lower threshold current and higher slope efficiency compared to conventional InGaN/InGaN laser diodes.