Cl effects on structure of Zr and Ti alkali aluminosilicate glasses

Na-aluminosilicate glasses with Na/Al > 1 and added Ti or Zr have been analyzed by Raman spectroscopy to determine the effect of Cl on glass structure. Spectra of Ti-bearing glasses show a significant difference between the Cl-free and the Cl-bearing composition. The Cl-free glass spectrum contains a strong, asymmetric peak at 900 cm⁻¹ which is associated with Ti in fivefold coordination. This peak is shifted to higher frequency and becomes more symmetric with the addition of 0.3 wt% Cl. Deconvolution of the high-frequency waveband indicates differences between spectra are the result of a contribution from a peak at 945 cm⁻¹ resulting from Ti–O vibrations in fully polymerized titanate tetrahedra. It is proposed that addition of Cl destabilizes ^[5]Ti in favor of tetrahedral coordination as a result of competition between Cl and titanate groups for alkalis. The spectra of Zr-bearing glasses show a strong peak at 980 cm⁻¹ which is not seen in spectra of the base glass, indicating a vibrational mode associated with a zirconosilicate structure. There is no discernible difference between the Raman spectra of Zr-bearing glasses with and without added Cl, indicating that the local structure around Zr is insensitive to the presence of Cl.     

Structural aspects of volume nucleation in silicate glasses

An interrelationship between parameters of short and intermediate range order in silicate glasses and the tendency to nucleate homogeneously in the volume is tested. Changes in the average coordination number and metal-oxygen distance of network modifying cations as well as changes in the concentration of constitutive silica tetrahedra accompanied with the crystallization of 18 stoichiometric glass compositions into their crystalline analogs are determined. The intermediate range structure of the glasses is investigated by configurational entropy and flow birefringence. The changes in structural parameters are analyzed in terms of the reduced glass transition temperature T_rg, which is negatively correlated with the maximal rate of volume nucleation. The results indicate that the short-range structure in stoichiometric glasses is, in general, very similar to the corresponding crystal structure but independent of the T_rg-scale and for this reason independent of nucleation properties. In contrast to the short range of the glass structure, birefringence induced by a forced flow above the glass transition temperature and configurational entropy are positively correlated with increasing T_rg. The results indicate increased structural order in the intermediate range for melts with a high supercool limit (T_rg < 0.58). It is concluded that this order phenomena may promote nucleation events and may help to explain the tendency to volume nucleation of silicate glasses with T_rg < 0.58.     

Synthesis and characterization of SnO-containing phosphorous oxynitride glasses

SnO-containing oxynitride phosphate glasses have been obtained by ammonolysis and their structure studied by nuclear magnetic resonance. The nitrided glasses are characterized by tetrahedral units P(O,N)₄ in which nitrogen atoms have substituted both bridging and non-bridging oxygen atoms. The N/O substitution in the anionic network induces important changes in the glass properties such as an increase in the glass transition temperature and a decrease in the coefficient of thermal expansion. ³¹P MAS NMR shows that PO₄, PO₃N and PO₂N₂ units coexist within the vitreous network and their relative proportions are a function of the nitrogen content as well as of the base glass composition. The influence of the different modifiers on the nitridation process is explained through a comparative study of the LiNaSnPON and LiNaPbPON systems. Unlike lead in oxynitride glasses, tin affects the nitridation mechanism by limiting the nitrogen/oxygen substitution in the anionic network, so that the substitution model is assumed to be closer to the one taking place in alkali phosphate glasses LiNaPON.     

Thermophysical properties of Zr-Cu-Al metallic glasses during crystallization

The crystallization behavior of Zr-Cu-Al metallic glasses was studied using thermophysical property measurements. When the Zr content of Zr-Cu-Al metallic glass decreased from 65 at.% to 45 at.%, the thermal conductivity gradually increased and the maximum value obtained was the composition of Zr:Cu:Al = 50:39.3:10.7(at.%). These metallic glasses were not crystallized upon heat treatment below the glass transition temperature T_g, and the thermophysical properties of these metallic glasses were almost constant. In contrast, these metallic glasses started to crystallize upon heat treatment above T_g after a certain derived time, and their thermal conductivity increased with the crystallinity of the metallic glass.     

Temperature-assisted electrical poling of TeO2-Bi2O3-ZnO glasses for non-linear optical applications

The suitability for effective thermal poling of the ternary tellurite glasses with the compositions (100 − 2x)TeO₂-xBi₂O₃-xZnO (x = 5, 10 and 15, in molar percentage) for the second harmonic generation (SHG) was analyzed. The glass transitions and crystallization temperatures were studied via differential thermal analysis. The structural properties of the annealed glasses and furtherly heat-treated samples were probed by extended X-ray absorption fine structure (EXAFS) spectroscopy. Thermal poling of the glasses was undertaken conventionally at various temperatures close to the glass transition temperature under high vacuum and the second harmonic generated signals were compared. A new technique of two stage poling was tested for comparison. The non-linear second harmonic signal of the poled glasses was analyzed using the Maker-fringe technique and it was found that the two stage poling enhanced the non-linear efficiency when compared to the conventionally poled samples.     

An X-ray diffraction study of the structure of barium fluorozirconate and fluorohafnate glasses

Two binary glasses have been prepared in the ZrF₄BaF₂ and HfF₄BaF₂ systems, containing 64 mol.% ZrF₄ and 66 mol.% HfF₄, respectively. The two plate-like glass samples were studied by X-ray diffraction, and the resulting data were subjected to a Fourier analysis. The pair correlation functions thus obtained yield a ZrF (or HfF) nearest neighbor coordination number of ～ 7.6 for both glasses. The second peak in the correlation functions has been assigned to a combination of FF and BaF interactions, while the third peak is probably due in part to ZrZr (Hf-Hf) interactions. The diffraction data have been compared with the predictions of structural models for barium fluorozirconate and fluorohafnate glasses.     

Silver valence and local environments in borosilicate and calcium aluminoborate waste glasses as determined from X-ray absorption spectroscopy

Silver K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected and analyzed to characterize silver (Ag) environments in borosilicate and Ca-aluminoborate glass formulations developed as potential candidates for the immobilization of certain nuclear wastes. Silver is found in some nuclear waste streams and must be encapsulated in glass during waste vitrification processes. A related concern deals with phase separation within these glasses and whether colloidal silver would be present in the glass melt, which could present processing issues, or in the waste glass product. Characterization of the silver environments provides useful information for optimizing the silver incorporation ability of such glasses. Data were also gathered on four crystalline standards: Ag-foil, Ag₂O, argentojarosite (AgFe₃(SO₄)₂(OH)₆), and AgO. XANES data indicate Ag⁺ as the dominant species in the glasses. XANES and EXAFS data show that the average Ag environment in the Ca-aluminoborate glass is different compared with those in the two borosilicate glasses investigated. EXAFS analyses show that Ag in the borosilicate glasses is coordinated by two oxygens in a similar environment to that in crystalline Ag₂O, except that the associated Ag–O distances are approximately 0.10 Å longer in the glass. Silver in the Ca-aluminoborate glass may be within one highly disordered site, or possibly, several different sites, where the average Ag–O distance, coordination number, and Debye–Waller factor are larger than those determined for the borosilicate glasses. Despite their relatively high silver contents, there is no evidence from XANES or EXAFS of colloidal silver in the glasses investigated.     

Rate of homogeneous nucleation in alkali disilicate glasses

Rates of crystal nucleation in alkali disilicate glasses were measured by counting the number of crystals under an optical microscope. The viscosities of these glasses were measured by the method of beam-bending and penetration. Using the data of rate of nucleation and viscosity obtained in the present study and the data of free energy measured by Takahashi and Yoshio, crystal-glass interfacial energies for alkali disilicate systems were estimated on the basis of homogeneous nucleation theory as follows: 196 erg/cm² for Li₂O·2SiO₂, 126–144 erg/cm² for Na₂O·2SiO₂ and 88–104 erg/cm² for K₂O·2SiO₂. The effects of the viscosity of glass, the free energy difference between crystal and glass and crystal-glass interfacial energy on the rate of nucleation were discussed, and the remarkably higher rate of crystal nucleation in the Li₂O·2SiO₂ glass was attributed to the larger free energy difference.     

Amorphous nanostructuring in potassium niobium silicate glasses by SANS and SHG: a new mechanism for second-order optical non-linearity of glasses

Potassium niobium silicate (KNS) glasses xK₂OxNb₂O₅(1−2x)SiO₂ with x=0.167; 0.182; 0.200; 0.220 and 0.250 have been subjected to prolonged heat treatments in a wide temperature range above T_g. As a result, glasses exhibiting liquid-type phase separation phenomena have been isolated. Moreover for each glass composition, the temperature zones have been determined to produce transparent, opalescent or opaque materials which have been studied by X-ray diffraction (XRD), small-angle neutron scattering (SANS) and second harmonic generation (SHG) techniques. SANS data unambiguously point at nanostructuring of KNS glasses in the scale of 5-20 nm under appropriate heat treatments near T_g. In contrast to initial KNS glasses, nanostructured glasses exhibit SHG activity. For earliest stages of phase separation SHG-active glasses are characterized by fully amorphous XRD patterns. Further development of phase separation in glasses with increasing of their opalescence leads to diminishing SHG, and subsequently partial crystallization takes place giving opaque materials. Since relative maximum of SHG efficiency corresponds to non-crystalline nanostructured glasses, such new transparent second-order non-linear media may be of both scientific and practical interest. With regard to non-crystalline structure of nano-inhomogeneities, SHG mechanism in the glasses is supposed to be due to a combination of third-order non-linearity with a spatial modulation of linear polarizability.     

Leaching of some lithium silicate glasses and glass-ceramics by HCl

The leaching of some binary and ternary lithium silicate glasses and their respective glass-ceramics by HCl is reported.The leaching rate of lithium silicate glasses gradually decreases with the decrease of the percentage of Li₂O or by the introduction of small amounts of a third component, e.g. Al₂O₃, MgO, ZnO or B₂O₃. With a further increase in the proportions of B₂O₃ or ZnO the rate of leaching increases. The rate of leaching is also substantially modified by the conversion of glasses into glasses-ceramics.The results obtained are discussed in terms of the effects of the different ions on the rate of the interdifussion of the lithium and hydrogen ions in the glass and the leached layer, the phase separation developed in the glass, the type and concentration of crystalline phases developed in glass-ceramics and the composition of the residual glass phase.     

Phase separation and controlled crystallization of non-oxide glasses

This paper reviews the experimental results associated with the phase separation and controlled crystallization of non-oxide glasses, mainly chalcogenide glasses and ZrF₄-based fluoride glasses. Experiments demonstrate that the phase separation is not always the precursor of the glass-ceramic process. It has been found that some non-oxide glasses could be converted into glass ceramics without phase separation. Different mechanisms of controlled crystallization of non-oxide glasses are discussed.     

Characterization of the disordered phosphate network in CaO-P2O5 glasses by P solid-state NMR and Raman spectroscopies

In this study, the disordered network of calcium phosphate glasses is investigated by Raman scattering and ³¹P magic angle spinning (MAS) solid-state NMR spectroscopies. The use of both spectroscopies in a combined approach allows drawing a detailed understanding of the structure of these glasses. The P―O―P connectivity between successive PO₄ tetrahedra is probed using through-bond double quantum-single quantum (DQ-SQ) and triple quantum-single quantum (TQ-SQ) MAS NMR correlation experiments. Over the broad range of glass compositions studied here, two very different phosphate network topologies are encountered. The results obtained for the polyphosphate compositional range (above 50 mol% Ca) allow determining the phosphate chain-length distribution in the glass as a function of the modifier cation content. For the ultraphosphate region (below 50 mol% Ca), the network topology undergoes a sudden change close to 39 mol% Ca which can be interpreted in terms of a rigidity transition.     

Electrical conductivity of silicate glasses with tetravalent cations substituting Si

The effects of substituting Si by M^4 + cations in soda-lime silica glasses were analyzed by impedance spectroscopy in the frequency range of 1 Hz–1 MHz. The glass composition was (mol%) 22Na₂O·8CaO·65SiO₂·5MO₂, M = Si, Ti, Ge, Zr, Sn, and Ce. Although the Na⁺ concentration in the glasses is constant, the Zr-containing glass exhibits the highest dc conductivity and the lowest activation energy, while the Ce-containing glass exhibits the lowest conductivity. The activation energies obtained experimentally agree with those obtained by a theoretical equation proposed by Anderson and Stuart. The differences in electrical conductivity presented by the several M-containing glasses are attributed to the effect that the M^4 + ion has on the mobility of the diffusing Na⁺ ion.     

The structure of borosilicate glasses studied by Raman scattering

Raman spectra of alkali and alkaline earth borosilicate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of Al₂O₃ additions on the structure of borosilicate glass is also discussed. It is shown that the same type of groups are present in borosilicate glasses as in borate and silicate glasses. The presence of large borate groups such as tetraborate and metaborate groups is strongly suggested by the Raman spectra. It appears that boron ions are hardly taken up in the silicon-oxygen network. Our results suggest that the region of phase separation is larger than the region presently acknowledged.     

The structural role of Zr within alkali borosilicate glasses for nuclear waste immobilisation

Zirconium is a key constituent element of High Level nuclear Waste (HLW) glasses, occurring both as a fission product and a fuel cladding component. As part of a wider research program aimed at optimising the solubility of zirconium in HLW glasses, we have investigated the structural chemistry of zirconium in such materials using X-ray Absorption Spectroscopy (XAS). Zirconium K-edge XAS data were acquired from several inactive simulant and simplified waste glass compositions, including a specimen of blended Magnox/UO₂ fuel waste glass. These data demonstrate that zirconium is immobilised as (octahedral) six-fold coordinate ZrO₆ species in these glasses, with a Zr-O contact distance of 2.09 Å. The next nearest neighbours of the Zr species are Si at 3.42 Å and possibly Na at 3.44 Å, no next nearest neighbour Zr could be resolved.     

Formation of quasicrystals and metallic glasses in relation to icosahedral clusters

It has been widely accepted that quasicrystals and at least some metallic glasses are built up with icosahedral clusters. Information about the cluster structures can be obtained from crystalline counterparts. In this paper, we will describe the formation rules of bulk metallic glasses, originally developed for quasicrystals, by combining cluster structures with phase diagram features. We will introduce the e/a-constant and e/a-variant criteria for ternary systems, and e/a-constant and atomic size constant criteria for quaternary systems. We will show how the glass forming composition optimization is realized by applying these rules in the Zr–Al–Ni and Zr–Al–Ni–Cu systems. In both systems the optimized glass-forming composition is related to a common binary icosahedral cluster Zr₉Ni₄ derived from the fcc Zr₂Ni phase. A novel route to reach amorphous forming composition is also attempted by mixing Al-based quasicrystal-forming compositions with Zr.     

Structural investigation of amorphous FeZr,CoZr and NiZr alloys with low zirconium concentration

A structural investigation has been made of alloy glasses a low concentration of zirconium: compositions M_100?xZr_x with M = Fe, Co and Ni, and x = 9 at.% using X-ray diffraction. The characteristic second peak splitting in the radial distribution function is found for all samples presently investigated. The partial radial distribution functions of amorphous FeZr and CoZr alloys were derived from the measured total distribution function data by applying the concentration method with the anomalous scattering technique. The amplitude in the oscillation of the radial distribution function for M-M and M-Zr pairs is more enhanced in comparison with that of glasses with high Zr content, x = 40–45 at.%. The estimated coordination number of nearest neighbor Fe atoms (11.6 ± 0.5) for iron in the Fe₈₄Zr₁₆ glass is larger than that (10.7 ± 0.5) found in the Fe₈₃B₁₇ glass. This is consistent with the measurements of magnetic properties of these glasses with low zirconium concentration. The possible structural features of intertransition metal alloy glasses with low zirconium concentration is also discussed based on the present experimental data.     

Effect of silver doping on the structure and phase separation of sulfur-rich As–S glasses: Raman and SEM studies

We report on the structural details and microphase separation of the bulk glasses Ag_x·(As₃₃S₆₇)_100-x for 0x25. Glass–glass phase separation occurs over a wide range of Ag content, i.e. 4x20. An off-resonant polarized Raman spectroscopic study has been carried out to elucidate structural aspects at the short- and medium-range structural order of the glasses. Analysis of Raman spectra revealed quantitative changes of the sulfur-rich microenvironments that reduce upon adding Ag. Scanning electron microscopy combined with X-rays microanalysis have been utilized to examine the type and extent of phase separation, and to provide quantitative details on the atomic concentrations in the Ag-poor and Ag-rich phases. It has been shown that at 7 at.% Ag the Ag-rich phase percolates through the structure; this effect can be associated with an ionic-to-superionic behavior of these glasses in accordance with similar studies on the stoichiometric arsenic sulfide glass; although the phase separation observed in the present glasses is qualitatively different.     

Evaluation of phase separation in glasses with the use of atomic force microscopy

Glass forming melts frequently exhibit liquid–liquid immiscibility resulting in phase separation. The chemical and spatial variation of phase separated morphologies in glasses can range from a few angstroms to microns, often requiring very high magnification for detection. Historically, phase separated glasses have been characterized by transmission electron microscopy (TEM). This technique is very time consuming and costly, requiring specialized equipment and training. Atomic force microscopy (AFM) provides an inexpensive alternative to TEM and has proven to be a powerful tool in the characterization of phase separation in glasses. AFM provides rapid and accurate evaluation of the type, degree and scale of phase separation in glasses down to the nanometer level. Using a combination of topographical and phase imaging AFM we were able to quantitatively determine the microstructures of phase separated glasses with a resolution down to 50 nm. Additionally we were able to quantitatively confirm the time dependence of the chemical segregation and growth processes for phase separation in glass by spinodal decomposition. This paper will present sample preparation techniques and results for evaluation of phase separation in alkali borosilicate and sodium silicate glass systems.     

Influence of zirconium on the structure of pristine and leached soda-lime borosilicate glasses: Towards a quantitative approach by O MQMAS NMR

¹⁷O MQMAS NMR was used to characterize the influence of zirconium on the structural organization of soda-lime borosilicate glasses. A new method of quantitative analysis of the ¹⁷O MQMAS spectra is presented, by a direct fit of the two-dimensional MQMAS spectrum which provides the resolution of all the structural groups in glasses containing up to five oxides. Additional data were also obtained from the quantitative deconvolution of the ¹¹B MAS NMR spectra, with the help of the direct fit of MQMAS data as well. Excess of non-bridging oxygen is clearly identified in these glasses. Sixfolded zirconium is preferentially compensated rather than the tetrahedral boron and calcium only partially compensate the tetrahedral boron. Alteration gels arising from glass leaching were probed by oxygen-17 supplied by the alteration solution. Most of the zirconium is inserted in the silicate network forming Si-O-Zr bonds with the same configuration in the glass and in the gel. During leaching, calcium clearly remains in the alteration gel, either near non-bridging oxygen or as a zirconium charge compensator. This quantitative approach applied to ¹⁷O MQMAS spectra demonstrates its potential for investigating the structure of increasingly complex glass and gel compositions.