Raman spectroscopic investigations on the kinetics of gelation and densification of Cd- and Pb-doped silica glasses under basic conditions

In the present work, the evolution of the gelation of undoped and Cd²⁺-and Pb²⁺-doped silica sols has been studied. Tetramethyl orthosilicate and cadmium and lead nitrates were selected as suitable precursors for the synthesis of nanoporous undoped and doped silica systems. Raman spectroscopy was used both to follow the different species formed and the structural changes occurring at various stages of the sol-gel transformation. The combination of information obtained from Raman spectra and from textural measurements using nitrogen adsorption-desorption served to investigate the effects of doping metal ion on the structure of both the gels and on the resulting nanoporous silica glasses. Results showed that doping silica materials with different concentrations of Cd²⁺ or Pb²⁺ makes it possible both to decrease the densification time and to control the porosity of the resulting gels and glasses.     

Shear bands in metallic glasses

Shear-banding is a ubiquitous plastic-deformation mode in materials. In metallic glasses, shear bands are particularly important as they play the decisive role in controlling plasticity and failure at room temperature. While there have been several reviews on the general mechanical properties of metallic glasses, a pressing need remains for an overview focused exclusively on shear bands, which have received tremendous attention in the past several years. This article attempts to provide a comprehensive and up-to-date review on the rapid progress achieved very recently on this subject. We describe the shear bands from the inside out, and treat key materials-science issues of general interest, including the initiation of shear localization starting from shear transformations, the temperature and velocity reached in the propagating or sliding band, the structural evolution inside the shear-band material, and the parameters that strongly influence shear-banding. Several new discoveries and concepts, such as stick-slip cold shear-banding and strength/plasticity enhancement at sub-micrometer sample sizes, will also be highlighted. The understanding built-up from these accounts will be used to explain the successful control of shear bands achieved so far in the laboratory. The review also identifies a number of key remaining questions to be answered, and presents an outlook for the field.     

The aluminium effect on gel-derived iron silica glasses

Sol–gel aluminosilicate glasses containing 1 mol% of Fe₂O₃ and different amounts of Al₂O₃ (1; 2; 3; 4; 6 and 8 mol%) have been investigated. The ultraviolet–visible–near infrared spectrophotometry (UV–VIS–NIR), electron paramagnetic resonance (EPR), X-ray diffraction (XRD) and scanning electron microscopy (SEM), has been employed to obtain information about the structural evolution of the samples. The concentration of Al₂O₃, the treatment temperature and the furnace atmosphere play an important role in the structural incorporation of iron. The treatment of the samples, in air and under reducing conditions, results in remarkable changes in the UV–VIS–NIR and EPR spectra. In the samples were detected nanoparticles. The low temperature blocking of the nanoparticles magnetic moments has been clearly evidenced in the EPR derivative spectra at low temperatures.     

Annealing of shear bands in metallic glasses

Shear bands introduced into the as-quenched metallic glass alloy, Metglas^R 2826 (Fe₄₀Ni₄₀P₁₄B₆), were etched by immersion in a solution of CUS04 and HCI. Elimination of the etchability property through isothermal annealing had an apparent activation enthalpy of 250 kJ mol^–1, similar to that of stress relaxation. Direct microscopic observations indicated that the etching of shear bands may be a result of stress corrosion cracking. The electrochemical etchability of shear bands in the nickel-based alloy, BNi₂ (Ni_68.8Cr_6.6Fe_2.6B_14.1Si_7.9), polarized in a solution of perchloric acid and acetic acid at 12° C, could also be eliminated by thermal annealing. The loss of etchability had an apparent activation enthalpy of 580 kJ mol^–1, a value indicative of more complex atomic rearrangements taking place within the deformed material. Finally, shear bands introduced into the as-received BNi₂ material retained the ability to reverse shear after heat treatment at temperatures in the range of 192 to 375° C even if the electrolytic etchability was apparently diminished.     

The thickness of defect bands in high-pressure die castings

Defect bands following the casting contour are often observed in high-pressure die cast (HPDC) components. In this article, suitable methods for measuring the thickness (w) and grain size in the bands (d_sb) in HPDCs have been developed. The w/d_sb relationship of defect bands has been investigated in HPDC specimens from a range of alloys, casting geometries and band locations within castings. Samples include aluminum alloy, AlSi4MgMn and AlMg5Si2Mn, tensile test bars cast by cold-chamber (cc) HPDC and a magnesium alloy, AM50, hot-chamber (hc) HPDC steering-wheel component. The band thicknesses were measured to be in the range 7–18 mean grains wide. This is substantial evidence that defect bands form due to strain localization in partially solidified alloys during cc and hc HPDC. Additionally, within this range, the w/d_sb ratio decreases with increasing distance from the casting center in a cross-section containing multiple bands.     

Characterization of fluorine-doped silica glasses

Fluorine-doped silica glasses containing up to 2 wt% were prepared by the vapour-phaseaxial-deposition (VAD) sintering process. The characteristics of these glasses were investigated by Raman spectroscopy, vacuum-ultraviolet-ultraviolet (v.u.v-u.v.) spectroscopy, and viscosity measurements. From the Raman spectroscopic investigation, it has been shown that the Si-F bond structure in the fibres is the same as that in bulk glass and is not affected by codoped additives such as B₂O₃. From the u.v.-v.u.v, spectroscopic investigation, it has been shown that the absorption band at 7.6 eV in high-purity silica glass is removed with the addition of fluorine. This addition produces a silica glass with most excellent transparency in u.v. and v.u.v. regions. Also it was observed that the optimum addition is around 1 wt%. This fact proves that fluorine incorporation in glass prevents the generation of defects related to optical loss in fibres. From the viscosity study, it was found that the viscosity decreases with an increase of the fluorine content in glass and the activation energy also decreases with increasing fluorine content.     

Low-frequency Raman scattering in alkali tellurite glasses

Raman scattering has been employed to study the alkali-cation size dependence and the polarization characteristics of the low-frequency modes for the glass-forming tellurite mixtures, 0·1M₂O–0·9TeO₂ (M = Na, K, Rb and Cs). The analysis has shown that the Raman coupling coefficient alters by varying the type of the alkali cation. The addition of alkali modifier in the tellurite network leads to the conversion of the TeO₄ units to TeO₃ units with a varying number of non-bridging oxygen atoms. Emphasis has also been given to the low-frequency modes and particular points related to the low-frequency Raman phenomenology are discussed in view of the experimental findings.     

On the defect states in As-Se glasses

Magnetic susceptibility and ESR studies of As-Se glasses indicate that these glasses are diamagnetic containing only charged defect pairs and that they correspond to a chemically ordered network structure. Communication No. 106 from Solid State and Structural Chemistry Unit.     

Synthesis and densification of lutetium pyrosilicate from lutetia and silica

Cerium-doped lutetium pyrosilicate (Lu₂Si₂O₇:Ce) powder was synthesized by solid state reaction of Lu₂O₃ and SiO₂. Stoichiometric mixtures of the starting materials were heat treated at various different temperatures and their phase contents were measured by XRD technique. It was found that the first step in the formation of Lu₂Si₂O₇ (LPS) is the appearance of Lu₂SiO₅ (LSO). This takes place at 1100 °C, fully 300 °C below the first appearance of LPS. Between 1400 and 1500 °C both LSO and LPS coexist in the calcined batch, but by 1550 °C all LSO is completely converted to LPS. LPS formation temperature does not have appreciable effect on the density of the hot pressed samples. Hot pressed samples obtained from powder synthesized at 1650 °C are nearly transparent, although the particle size of the starting powder is higher than that of the powder formed at lower temperatures.     

Temperature dependence of silver-sodium interdiffusion in micro-optic glasses

The variation of the concentration-dependent diffusion process with ion exchange temperature from 280°C to 400°C has been investigated in two boro-aluminosilicate glasses suitable for micro-optic applications. Silver dopant concentration profiles are measured using energy dispersive X-ray spectroscopy and converted to concentration dependence using an analytical formulation of the Boltzmann-Matano analysis. Extensive error analysis of the Boltzmann-Matano method is presented. While the overall interdiffusion rates vary strongly with temperature, the temperature variation of the concentration dependence does not exceed experimental error. Temperature dependencies of Ag and Na self-diffusion coefficients are also presented. The activation energies for Ag and Na are both approximately 0.72 eV in these glasses.     

Effects of pores on shear bands in metallic glasses: A molecular dynamics study

The effects of nanoscale pores on the strength and ductility of porous Cu₄₆Zr₅₄ metallic glasses during nanoindentation and uniaxial compression tests are modelled and investigated using molecular dynamics (MD) simulations. In the MD simulations, atomistic amorphous samples were digitally prepared through fast quenching from the liquid states of copper and zirconium alloy. In both of the nanoindentation and uniaxial compression simulations, shear transformation zones and shear bands are observed through the local deviatoric shear strains in the samples. The results show that the existence of pores causes strain concentrations and greatly promotes the initialization and propagation of shear bands. Importantly, only pores reaching critical size can effectively facilitate the formation of multiple shear bands. It is also observed that hardening occurs through pore annihilation and the shear band stops in porous metallic glasses.     

Effect of Tb ions on X-ray-induced defect formation in phosphate containing glasses

The study of radiation-induced defect formation in glasses is of growing interest for applications in optics and photonics. The influence of Tb³⁺ ions on X-ray-induced defects has been examined in glasses with different phosphate contents. The defects have been characterized by optical absorption and ESR spectroscopy. (Tb³⁺)⁺ ions are formed by X-ray irradiation. They exhibit a broad charge transfer band at 370 nm. The formation of (Tb³⁺)⁺ hole centres suppresses the formation of intrinsic phosphate-related hole centres, absorbing in the visible region. PO₃ defects absorbing in the ultraviolet region contribute mainly to the electron centres corresponding to the (Tb³⁺)⁺ hole centres. Immediately after X-ray irradiation, 5–10% of the Tb³⁺ ions have been oxidized. The amount of (Tb³⁺)⁺ ions increases with increasing phosphate content. The stability of the (Tb³⁺)⁺ ions at room temperature depends on glass composition and melting conditions.     

Sol-gel synthesis of luminescent InP nanocrystals embedded in silica glasses

III-V semiconductor nanocrystals rarely exist as spherical inclusions inside glasses, due to difficulties during their preparation, such as high toxic reagents or fast oxidation under usual glass technology temperatures. In this letter a sol-gel method for synthesis of InP nanocrystals embedded in silica glasses was described. Gels were synthesized by hydrolysis of a complex solution of Si(OC2H5)4, InCl3.4H2O, and PO(OC2H5)3. Then, the gels were heated at 600 degrees C in the presence of H2 gas to form fine cubic InP crystallites. Raman spectrum showed an InP longitudinal-optic mode (342 cm(-1)) and a transverse-optic mode (303 cm(-1)). The size of InP nanocrystals was found to be from 2 to 8 nm in diameter by transmission electron microscopy. A strong photoluminescence with a peak at 856 nm was observed from InP nanocrystals embedded in silica glasses. The results suggest that it might be possible to synthesize other III-V semiconductor nanocrystals embedded in silica glasses through the sol-gel process.     

Strong overtones and combination bands in ultraviolet resonance Raman spectroscopy

Ultraviolet resonance Raman spectroscopy is carried out using a continuous wave frequency-doubled argon ion laser operated at 229, 244, and 257 nm in order to characterize the overtones and combination bands for several classes of organic compounds in liquid solutions. Contrary to what is generally anticipated, for molecules such as pyrene and anthracene, strong overtones and combination bands can show up; it is demonstrated that their intensity depends critically on the applied laser wavelength. If the excitation wavelength corresponds with a purely electronic transition--this applies to a good approximation for 244-nm excitation in the case of pyrene and for 257-nm excitation in the case of anthracene--mostly fundamental vibrations (up to 1700 cm(-1)) are observed. Overtones and combination bands are detected but are rather weak. However, if the laser overlaps with the vibronic region--as holds for 229- and 257-nm excitation for pyrene and 244-nm excitation for anthracene--very strong bands are found in the region 1700-3400 cm(-1). As illustrated for pyrene at 257 nm, all these bands can be assigned to first overtones or binary combinations of fundamental vibrations. Their intensity distribution can roughly be simulated by multiplying the relative intensities of the fundamental bands. Significant bands can also be found in the region 3400-5000 cm(-1), corresponding with second overtones and ternary combinations. It is shown that these findings are not restricted to planar and rigid molecules with high symmetry. Substituted pyrenes exhibit similar effects, and relatively strong overtones are also observed for adenosine monophosphate and for abietic acid. The reasons for these observations are discussed, as well as the potential applicability for analytical purposes.