Physical and chemical evolutions occurring in glass formation from alkoxides of silicon,aluminum and sodium

A multicomponent glass was obtained on using the new method of glass preparation from gel. The reactions occurring in the ethanolic solution of the parent alkoxides give condensation of the monomers so that hydrolysis of the remaining alkoxides groups followed by polycondensation appears to proceed on preformed oligomers. DTA, TGA, density and chemical durability measurements indicate that the transition from gel to the final glass is a continuous process. During the gel to glass conversion, scanning electron microscopy results show considerable compacting of the material.     

Electrical conductivity of low-temperature carbons as a function of frequency

The electrical conductivity of anthracene carbon samples, heat treated to temperatures of 600, 650, 730 and 800°C is measured at variable frequency up to 100 MHz. The carbons heat-treated to 600 and 650°C have a specific behaviour of amorphous solids, whereas the carbons HTT 730° and 800°C have an electrical conductivity independent of the frequency. The experimental results are correctly explained by means of the equations developed by Pollak and Geballe. Analysis of the results points to the conclusion that a hopping conduction between localized states could be a possible process of conduction in such solids.     

Characterization studies of AgIAg2OMoO3 glasses

Glass formation has been investigated in the system AgIAg₂OMoO₃. X-ray absorption edge chemical shifts, infrared spectra and heat capacities of the various glass compositions have been measured. The probable anion structures in the oxygen excess and deficient regions have been discussed.     

Magnetic properties of amorphous metals

Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe_xNi_1?x)₇₅P₁₆B₅Al₃ and Fe_xB_1?x. The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mössbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.     

Electrical conductivity of 2,4-dinitrosoresorcinol and its complexes

Electrical conductivity measurements of 2,4-dinitrosoresorcinol (DNR) and its complexes were measured in the temperature range 20–115°C to give faint semiconducting behaviour for these systems. The conductivity values of the bis Co, Ni, Cu and Zn systems indicated that the metal ion forms a bridge between the two ligands facilitating the transfer of current carriers with some degree of delocalization in the excited state. As the iron content in the complexes increased, a gradual increase of electrical conductivity occurred, that might be due to lattice imperfections.     

Structure of lithium borate and strontium borate glasses with high modifier contents

The structure of lithium borate and strontium borate glasses with high Li₂O and SrO contents was examined using the experimental results of ESR and the optical absorption of Cu²⁺ ions, and the Mössbauer spectra of Fe³⁺ ions and Raman spectra of these glasses. The results indicate the existence of non-bridging oxygens belonging to orthoborate and pyroborate groups, structural groups of diborate and high pressure forms of metaborate above 30 mol% of the modifier contents in these glasses. Among these four structural groups, the diborate group rapidly decreased with an increase of the modifier contents, while the other three groups increased with an increase of the modifier contents.     

High-field behaviour of amorphous AsSe films

Thin films of composition AsSe_χ where 0.8 ≤ χ ≤ 1.0 obtained by the evaporation of both crystalline and glassy As₂Se₃ have been examined for their high-field behaviour. The films exhibit two essentially linear I–V regions with a smooth nonlinear change over. While the initial linear region may perhaps correspond to a Poole-Frenkel emission, the very high-field linear region is likely to arise from a distortion of the band shapes and lowering of the mobility gap.     

The mixed alkali effect: Theoretical considerations

A cooperative theory for the composition dependence of activated processes in binary ionic oxide glasses (the mixed alkali effect) is proposed. It employs the regular solution model and introduces a molecular model, both of which are used in conjunction with appropriate spectroscopic data for obtaining an estimate of the magnitude of the theory's parameters. The nonlinear composition dependences of certain physical properties of mixed alkali systems, as well as their separation into exponential and pre-exponential factors, are shown to follow from these theoretical considerations.     

Stable long-period composition variations in demixing systems: I. Theory

In a binary mixture with components of different space requirements it is adequate to consider not only the concentration as an order parameter, but also the strain. If this is done in the framework of a Landau theory the leading coefficients of the local free energy expansion in powers of the concentration and its gradient are renormalized. This has two consequences: first, the miscibility gap is shifted to higher temperatures. Second, the coefficient of the gradient term is reduced and eventually will become negative. If this is the case, a modulated structure will become stable near to the top of the miscibility gap, and the phase diagram will be modified. The modulated structure may be characterized by its wavelength, which is independent of temperature, and by its amplitude. The latter may be used as an order parameter. Its growing from a zero value implies that the transition from the homogeneous state to the modulated structure is second order. The line of second order phase transitions merges into a line of first order phase transitions, where, for low temperatures, “normal” demixing behaviour is recovered.     

Crystallization in fluoride glasses: I. Devitrification on reheating

The devitrification of glass from the BaF₂LaF₃ZrF₃AlF₃ quaternary system is studied as a function of sample size and heating rate by differential scanning calorimetry. Results are analyzed in terms of a theory of non-isothermal crystallization kinetics similar to that of Matusita and Sakka but modified to allow for diffusion controlled growth of the crystallites.     

On the character of local electric fields in amorphous semiconductors

On the basis of the agreement of calculated and measured parameters of some amorphous semiconductors, an argument is stated in favour of the central-symmetric type of random local electric fields. A possible mechanism of the transport of electrons is discussed.     

Optical properties of Tm3+ ions in alkali germanate glass

Tm-doped alkali germanate glass is investigated for use as a laser material. Spectroscopic investigations of bulk Tm-doped germanate glass are reported for the absorption, emission and luminescence decay. Tm:germanate shows promise as a fiber laser when pumped with 0.792 μm diodes because of low phonon energies. Spectroscopic analysis indicates low non-radiative quenching and pulsed laser performance studies confirm this prediction by showing a quantum efficiency of 1.69.     

Effect of composition and forming parameters on the conductance of amorphous chalcogenide threshold switches

The effect on the conductance of formed STAG threshold switches of changing both the composition of the virgin films and the ON time of the switching pulse during the forming process has been studied. It was found that the silicon content of the glass has a large effect on the activation energy and conductivity parameters of virgin devices. After forming, the conductance of the device is much higher than in the unformed state; however, silicon no longer controls the conductivity parameters. When switching STAG threshold switches it was found that increasing the ON time of each switching cycle causes a decrease in the conductance of the formed device from its value when formed with very short ON times. For low pulse repetition frequencies the conductance saturates for ON times typically greater than 100 μs. The rate at which the conductance decreases is dependent upon the silicon content of the virgin glass. For high pulse repetition frequencies the conductance initially falls, following the low pulse repetition frequency case, and then rises, finally locking on to a memory state. The observations have been explained in terms of an initial large structural change of the glass, caused through adiabatic heating during switching. Relaxation of this structure back to more stable glass phases is possible during the ON time if the ON state temperature is sufficiently high. For high pulse repetition frequencies the temperature of the device is much higher, allowing for the possibility of crystalline growth. This leads to an increase in conductance and eventual lock on.     

Optical properties and valence state of Sm ions in aluminoborosilicate glass under β-irradiation

Sm-doped borosilicate glasses exposed to β-irradiation with doses from 8 × 10⁵ up to 4 × 10⁹ Gy have been studied by luminescence, Raman and electron paramagnetic resonance (EPR) spectroscopies. The luminescence spectra for pristine and irradiated glasses reveal that the β-irradiation process affects valence state of samarium ions. Intense emission at 684 and 727 nm excited by Ar+ laser (514.5 nm) due to the transition of Sm²⁺ ion was observed after irradiation. Relative proportion of Sm²⁺ ions estimated as a function of both Sm₂O₃ content and irradiation dose has the tendency to increase with increasing irradiation dose. In contrast, the EPR spectra of the studied samples reveal a decrease of the defect content, which are mostly hole defects, produced during irradiation, as a function of Sm₂O₃ content. Finally, the addition of Sm₂O₃ leads to a decrease of the Si–O–Si bending vibration modes shift and polymerisation changes under irradiation.     

Optical properties of pure and metal ions doped ammonium sulfate single crystals

A study of the optical properties of pure‐and some metal ions doped ammonium sulfate crystals (AS) were made. Optical constants of AS crystals were calculated at room temperature. The optical absorption coefficient (α ) was analyzed and interpreted to be in the allowed direct transition. The introduction of Rb⁺ or Cs⁺ ions gives rise to an intense charge transfer band with a maximum at λ= 310 nm in the optical spectrum. In case of Cr³⁺ ‐doping, the absorption shows a shoulder just before the onset band to band transition. The values of the allowed direct energy gap E_g for undoped and doped crystals were calculated. It was found that E_g values were decreased with metal ions doping. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were calculated as a function of photon energy. The validity of Cauchy‐Sellimeier equation was checked in the wavelength range 4.9 ‐ 5.6 eV and its parameters were calculated. Applying the Single‐Effective‐Oscillator model, the moments of ε (E ) could be estimated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mössbauer spectra in x PbO · (1 − x) B2O3 doped with Fe2O3

A Mössbauer spectroscopic study has been made on x PbO · (1 ? x) B₂O₃ containing 10 mol% Fe₂O₃ in melt before quenching. X has been varied between 0 and 0.85 in steps of 0.05. In samples with χ ≥ 0.25, a quadrupole doublet was observed. The corresponding isomershift and quadrupole spliting have shown a dependence on χ. A second type of spectrum has been observed in samples with χ < 0.25. It consisted of a six finger hyperfine pattern convulating over a central doublet spectrum. The corresponding internal field and the relative peak intensities are lower than observed in polycrystalline Fe₂O₃. The possible explanations for both types of spectrum are given.     

Synthesis,characterization and local changes induced by a cw argon laser in silica glass xerogel doped with transition metal ions

We report on the synthesis, characterization and local changes induced by a cw argon laser in SiO₂ xerogel doped with low metal ion concentrations, and obtained by the sol–gel process. The V, Mn and Cr metal ions were introduced into the SiO₂ matrix as inorganic salts and as an oxide in the case of Mo. The characterization of these materials showed that an amorphous system with high porosity was obtained; the metals were incorporated as ions with several different oxidation states, and the thermal diffusivity was around 10⁻⁹ m²/s. In these xerogel monoliths it is possible to induce local changes in the refractive index with the incidence of a focused laser beam at 488 nm, about 80 μm in diameter, with power intensities from 1 to 48 mW and an incidence time from 0.5 to 5 min. After this process, a high contrast in the transmitted light between the processed area and the homogenous matrix is obtained, resulting in an optical memory effect.