Network Formers Research Articles

Urea and deuterium mixtures at high pressures.

Urea, like many network forming compounds, has long been known to form inclusion (guest-host) compounds. Unlike other network formers like water, urea is not known to form such inclusion compounds with simple molecules like hydrogen. Such compounds if they existed would be of interest both for the fundamental insight they provide into molecular bonding and as potential gas storage systems. Urea has been proposed as a potential hydrogen storage material [T. A. Strobel et al., Chem. Phys. Lett. 478, 97 (2009)]. Here, we report the results of high-pressure neutron diffraction studies of urea and D2 mixtures that indicate no inclusion compound forms up to 3.7 GPa.

The fate of silicon during glass corrosion under alkaline conditions: A mechanistic and kinetic study with the International Simple Glass

International Simple Glass – a six oxide borosilicate glass selected by the international nuclear glass community to improve the understanding of glass corrosion mechanisms and kinetics – was altered at 90°C in a solution initially saturated with respect to amorphous 29SiO2. The pH90°C, was fixed at 9 at the start of the experiment and raised to 11.5 after 209d by the addition of KOH. Isotope sensitive analytical techniques were used to analyze the solution and altered glass samples, helping to understand the driving forces and rate limiting processes controlling long-term glass alteration. At pH 9, the corrosion rate continuously drops and the glass slowly transforms into a uniform, homogeneous amorphous alteration layer. The mechanisms responsible for this transformation are water penetration through the growing alteration layer and ion exchange. We demonstrate that this amorphous alteration layer is not a precipitate resulting from the hydrolysis of the silicate network; it is mostly inherited from the glass structure from which the most weakly bonded cations (Na, Ca and B) have been released. At pH 11.5, the alteration process is very different: the high solubility of glass network formers (Si, Al, Zr) triggers the rapid and complete dissolution of the glass (dissolution becomes congruent) and precipitation of amorphous and crystalline phases. Unlike at pH 9 where glass corrosion rate decreased by 3 orders of magnitude likely due to the retroaction of the alteration layer on water dynamics/reactivity at the reaction front, the rate at pH 11.5 is maintained at a value close to the forward rate due to both the hydrolysis of the silicate network promoted by OH− and the precipitation of CSH and zeolites. This study provides key information for a unified model for glass dissolution.

Short and medium range order in two-component silica glasses by positron annihilation spectroscopy

The dependence of chemical composition on the average sizes of subnanometer-scale intrinsic structural open spaces surrounded by glass random networks in two-component silica-based glasses was investigated systematically using positronium (Ps) confined in the open spaces. The average sizes of the open spaces for SiO2-B2O3 and SiO2-GeO2 glasses are only slightly dependent on the chemical compositions because the B2O3 and GeO2 are glass network formers that are incorporated into the glass network of the base SiO2. However, the open space sizes for all SiO2-R2O (R = Li, Na, K) glasses, where R2O is a glass network modifier that occupies the open spaces, decrease rapidly with an increase in the R2O concentration. Despite the large difference in the ionic radii of the alkali metal (R) atoms, the open space sizes decrease similarly for all the alkali metal atoms studied. This dependence of the chemical composition on the open space sizes in SiO2-R2O observed by Ps shows that the alkali metal atoms do not randomly occupy the structural open spaces, but filling of the open spaces by R2O proceeds selectively from the larger to the smaller open spaces as the R2O concentrations are increased.

Unique sodium phosphosilicate glasses designed through extended topological constraint theory.

Sodium phosphosilicate glasses exhibit unique properties with mixed network formers, and have various potential applications. However, proper understanding on the network structures and property-oriented methodology based on compositional changes are lacking. In this study, we have developed an extended topological constraint theory and applied it successfully to analyze the composition dependence of glass transition temperature (Tg) and hardness of sodium phosphosilicate glasses. It was found that the hardness and Tg of glasses do not always increase with the content of SiO2, and there exist maximum hardness and Tg at a certain content of SiO2. In particular, a unique glass (20Na2O-17SiO2-63P2O5) exhibits a low glass transition temperature (589 K) but still has relatively high hardness (4.42 GPa) mainly due to the high fraction of highly coordinated network former Si((6)). Because of its convenient forming and manufacturing, such kind of phosphosilicate glasses has a lot of valuable applications in optical fibers, optical amplifiers, biomaterials, and fuel cells. Also, such methodology can be applied to other types of phosphosilicate glasses with similar structures.

Mixed Network Former Effects in Oxide Glasses: Spectroscopic Studies in the System (M2O)1/3[(Ge2O4)x(P2O5)1–x]2/3

The structure of mixed-network former glasses in the system (M2O)0.33[(Ge2O4)x(P2O5)1–x]0.67.(M = Na, K) has been studied by 31P and 23Na high-resolution and dipolar solid state nuclear magnetic resonance (NMR) techniques, O-1s X-ray photoelectron spectroscopy, and Raman spectroscopy. Using an iterative fitting procedure, a quantitative structural model has been developed that is consistent with all of the experimental data and which provides a detailed description of network connectivities, network modification processes, and spatial cation distributions. Formation of heteroatomic P–O–Ge linkages is generally preferred over homoatomic P–O–P and Ge–O–Ge linkages, as shown by a detailed comparison with a random linkage model. An exception occurs in glasses with low germanium contents (x = 0.2) where a pronounced nonlinear dependence of the glass transition temperature on x can be related to a cross-linking of the sodium ultraphosphate network by fully polymerized germanium species, possibly including also 5- and 6-fold coordination states. At higher x values, the Ge component is modified as well, however, the fraction of anionic nonbridging oxygen atoms bound to germanium is always lower than expected for proportional modifier sharing between both network formers. Rather, the phosphate component is preferentially modified by the cations, leading to the formation of P(1) units at high x values. Consequently, the local coordination of the cations is dominated by phosphorus, as is clearly evident from the 23Na{31P} rotational echo double resonance (REDOR) results. This preferred association, combined with the formation of P(1) units, results in partially clustered cation distributions, which can be detected by 23Na spin echo decay spectroscopy. Finally, the joint interpretation of all of the data from NMR, Raman, XPS, and thermal analysis measurements offers indirect evidence for the formation of higher germanium coordination states in this glass system.

Gels of DNA Nanostars Never Crystallize

Using state-of-the-art numerical techniques, we show that, upon lowering the temperature, tetravalent DNA nanostars form a thermodynamically stable, fully bonded equilibrium gel. In contrast to atomic and molecular network formers, in which the disordered liquid is always metastable with respect to some crystalline phase, we find that the DNA nanostar gel has a lower free energy than the diamond crystal structure in a wide range of concentrations. This unconventional behavior, here verified for the first time in a realistic model, arises from the large arm flexibility of the DNA nanostars, a property that can be tuned by design. Our results confirm the thermodynamic stability of the recently experimentally realized DNA hydrogels.

Investigation of the Viscosity and Structural Properties of CaO-SiO2-TiO2 Slags

The viscosity of CaO-SiO2-TiO2 slags was measured via the rotating cylinder method to reveal the effect of TiO2 on viscous flow of the slags. Furthermore, the structure of the ternary slags and the role of Ti4+ were investigated by Fourier transform infrared and Raman spectroscopy techniques. The results are beneficial for a better understanding of the behaviors of Ti-bearing silicate slags. The TiO2 additions lowered the viscosity and apparent activation energy of the slags. However, the degree of polymerization (DOP) of silicate network was found to be enhanced with increasing the TiO2 content, which is suggested by the increase in mole fraction of Q3 ([SiO4]-tetrahedra with three bridging oxygens) and the decrease in Q0. The Eq. [2] Q2 ↔ Q1 + Q3 was appropriate to express the relationship of different Qn species. The introduction of Ti4+ into the silicate network as network formers increased the DOP but weakened the strength of slag structure at the same time. Besides, a large proportion of Ti4+ exists in the slag in the form of \( TiO_{4}^{4 - } \) monomers, resulting in a decrease of viscosity with increasing TiO2 content.

Common Physical Framework Explains Phase Behavior and Dynamics of Atomic, Molecular, and Polymeric Network Formers

Atoms, organic molecules, and polymerized DNA can all form polygon networks, despite enormous differences in their sizes and interactions. Scientists find the geometry, and strength of interactions, of building blocks to be the unifying factors for network assembly and codify them in the concept of an effective, material-dependent ``patchy particle.''

Speciation and amphoteric behaviour of water in aluminosilicate melts and glasses: high-temperature Raman spectroscopy and reaction equilibria

Combination of Raman spectroscopy and theoretical considerations give fundamental insights about water incorporation within the structure of aluminosilicate melts and glasses. Raman spectroscopy allows quantifying both water concentration and speciation. Particularly, it highlights the existence of OH groups differently bonded to (Si,Al) network formers and network- modifying cations, thus confirming previous results obtained by NMR spectroscopy, for instance. In situ experiments using Raman spectroscopy on basalt, rhyolite and albite melts show that the water speciation is profoundly affected by temperature, and that the amphoteric behaviour of water drives its diffusion and exsolution processes.

Investigation of Self-Diffusion and Structure in Calcium Aluminosilicate Slags by Molecular Dynamics Simulation

Molecular dynamics simulation is applied to investigate the mechanism and variation of self-diffusion in calcium aluminosilicate slags. The self-diffusion coefficients are calculated for eleven slag compositions with varying Al2O3/SiO2 ratios at a fixed CaO content. In practice, the results of the study are relevant to the significant changes in transport phenomenon caused by the changes in chemical composition during continuous casting of steels containing high amounts of dissolved aluminum. The cooperative movement between O atoms and network formers is discussed since [AlO4] and [SiO4] tetrahedra are the elementary structural units in the CaO-Al2O3-SiO2 (CAS) slag system. The diffusivities for four atomic types are affected by the degree of polymerization (DOP) of slag network characterized by the proportions of non-bridging oxygen (NBO) and Qn species in the system. On the other hand, a sudden increase in 5-coordinated Al as network modifiers in high alumina regions slightly increases the self-diffusion coefficient for Al. As another structural defect, oxygen tricluster plays an important role in the behavior of self-diffusion for O atoms, while the diffusivity for Ca is deeply influenced by its bonding and coordinating conditions.

Alpha Decays Impact on Nuclear Glass Structure

This paper reviews the main results on the long-term behavior of SON68 nuclear glass towards alpha decay accumulation. The effects of the radiation damage induced by alpha decay were investigated by doping a glass with a short-lived actinide 244Cm, by using ionic irradiations and by molecular dynamic simulations.The analysis of the behavior of the glass structure subjected to ballistic effects with various spectroscopic studies has identified some local order changes around born and silicon atoms. Moreover a modification of the medium-range order has also been demonstrated through changes in the bond angles between network formers and broadening of the ring size distributions, indicating increasing disorder of the glass structure. This structural evolution induced by alpha decays would be driven by the reconstruction of the glass disorganized by displacement cascades of the recoil nuclei, freezing a glass structure with a higher fictive temperature. This “ballistic disordering (BD) fast quenching” event induce a new glassy state characterized by a higher enthalpy state.

Electrical conductivity and local structure of lithium iron tungsten vanadate glass

A relationship between physical properties and local structure of 20Li2O·10Fe2O3·xWO3·(70–x)V2O5 glass, abbreviated as xLFWV glass (x = 0 − 25 in mol%), was investigated by 57Fe-Mossbauer spectroscopy, Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA), leaching test using 20 vol% HCl and DC two- or four-probe method. 57Fe-Mossbauer spectra of xLFWV glass showed an increase of quadrupole splitting (Δ) from 0.67 to 0.73±0.02 mm s − 1 and a constant isomer shift (δ) of 0.39±0.01 mm s − 1 with an increase of ‘x’ from 0 to 25. This suggests that FeIIIO4 tetrahedra gradually increase their local distortion along with a substitution of WO3 for V2O5. DTA of xLFWV glass showed an increase in glass transition temperature (Tg) from 252 to 298\(_{\pm 5}^{\circ}\)C with an increase of ‘x’. Composition dependency of Tg and Δ indicates that FeIII atoms occupy substitutional sites of WO6 octahedra as network former (NWF), since a large slope of 680 K (mm s − 1) − 1 was obtained in Tg − Δ plot. Comparable electrical conductivities (σ) of 2.5 × 10 − 6, 1.9 × 10 − 6, 8.4 × 10 − 7 and 2.9 × 10 − 6 S cm − 1 obtained for xLFWV glasses with ‘x’ of 0, 10, 20 and 25, respectively increased to 2.4 × 10 − 2, 2.4 × 10 − 3, 3.5 × 10 − 4 and 8.8 × 10 − 5 S cm − 1 after annealing at 400 °C for 100 min. Smaller Δ values of 0.58 and 0.67±0.02 mm s − 1 obtained in annealed xLFWV glasses with ‘x’ of 0 and 10, respectively indicate that structural relaxation occurs in VO4 units of vanadate glass units, as had been observed in other vanadate glasses.

Specific outcomes of the research on the radiation stability of the French nuclear glass towards alpha decay accumulation

This paper presents an overview of the main results of the French research on the long-term behavior of SON68 nuclear glass towards alpha decay accumulation. The effect of the radiation damage induced by alpha decay and also helium build-up were investigated by examining glass specimens, doped with a short-lived actinide 244Cm, irradiated by light and heavy ions. Additionally, atomistic simulations by molecular dynamics have provided further information on the atomic-scale effects of the macroscopic phenomena observed.These studies have shown that some macroscopic properties vary with the accumulation of alpha decay, but then stabilize after integrated doses of the order of 4×1018αg−1. For example, the glass density diminishes by about 0.6%, its Young’s modulus by about 15%, and its hardness by about 30%, while its fracture toughness increases by around 50%. The SEM and TEM characterization showed that the glass is still homogeneous. No phase separation, crystallization or bubbles formation was noticed up to an alpha decay dose corresponding to several thousand years of disposal of nuclear glass canister. Moreover the initial alteration rate of the glass is not significantly affected by the glass damage induced by alpha decays or heavy ions irradiations.The comparison of the macroscopic evolutions of the Cm doped glass with those obtained for glasses irradiated with light or heavy ions (from either experimental and molecular dynamic studies) suggests that the macroscopic evolutions are induced by the nuclear interactions induced by the recoil nuclei of alpha decay.The analysis of the behavior of the glass structure subjected to ballistic effects with various spectroscopic studies, together with the results of atomistic modeling by molecular dynamics, have identified some slight changes in the local order around some cations. Moreover a modification of the medium-range order has also been demonstrated through changes in the bond angles between network formers and broadening of the ring size distributions, indicating increasing disorder of the glass structure. This structural evolution induced by alpha decays would be driven by the reconstruction of the glass disorganized by displacement cascades of the recoil nuclei, freezing a glass structure with a higher fictive temperature. This “ballistic disordering (BD) fast quenching” event induces a new glassy state characterized by a higher enthalpy state. Accumulation of α decays induce similar phenomena of “BD-fast quenching”, increasing the fraction of the sample volume characterized by a “high enthalpy state”. At dose around 4×1018αg−1 the entire sample volume has been affected by “BD-fast quenching” events at least once, which explain the stabilization of the evolutions of glass structure and properties.Helium behavior was also studied by measuring the helium solubility constants and diffusion coefficients. Helium atoms are incorporated into the glass free volume with a solubility constant that varies less than 10% around a value of about 1011atcm−3Pa−1 and a density of solubility sites accessible for helium around 2×1021 sitescm−3 which is larger than helium production in a glass package. Helium diffusion experiments performed on infused and Cm doped SON68 glasses indicate that helium migration is controlled by a classical thermally activated diffusion process, whose activation energy (e.g. 0.6±0.03eV) is not affected by an alpha decay dose of around 1019αg−1.Helium implantation studies suggest that helium trapping could exist in nanometer size bubbles. SEM and TEM analysis performed on a Cm doped glass damaged by an alpha decay dose of around 1019αg−1, showed a homogeneous glass without crystallization, phase separation or bubbles with a spatial resolution limit of 10nm. Bubbles of significant size seem very unlikely to form at room temperatures. But, the ability to form helium bubbles of nanometer size, at temperature below the glass vitreous transition temperature cannot be excluded. However, all these studies agree on one point, the absence of macroscopic consequence on the glass integrity of accumulation of high helium concentration in the glass.

Linear and non-linear optics and FTIR characteristics of borosilicate glasses doped with gadolinium ions

Borosilicate glasses have been prepared using the high-temperature melt components of ingredients Gd2O3 doped borosilicate glasses. FTIR spectra were measured in the wavenumber range (4000–400cm−1) to explore the state and influence of Gd3+ ions in the structure of the glasses. Data indicated that B2O3 is acting as dual network formers (BO3) and (BO4) structural units whereas the gadolinium ions playing the role of network modifier in these glasses. Optical transmission spectra were recorded in the range 190–2500nm and different optical parameters such as the direct and the indirect optical band gap, Urbach energy, refractive index and optical dielectric constant, have been determined. The molar refraction, electronic polarizability and the optical basicity results have been determined using the measured glass refractive indices. Gadolinium-doped borosilicate glasses are found to be characterized by different optical parameters.

Raman spectroscopic study of the structural properties of CaO–MgO–SiO2–TiO2 slags

Raman spectra of CaO–MgO–SiO2–TiO2 slags were carefully analyzed to obtain the effect of TiO2 on the slag structures and the results are important for a better understanding of the behaviors of Ti-bearing slags. Gaussian deconvolutions of the Raman curves provide us with a deep insight into various structural units of the slags. The silicate network was more polymerized because of the increase of TiO2. Q2 (silica tetrahedra with two bridging oxygens) and Q3 species increase at the cost of Q0 and Q1 species. A significant fraction of Ti4+ plays the role of network formers and enters the silicate network, while some other Ti4+ exist in the slags in the form of TiO44− monomers suggested by ~790cm−1 bands. A small content of 6-coordinated Ti4+ occurs when the fraction of TiO2 is relatively high. 5-Coordinated Ti4+ indicated by 870cm−1 may also appear and help the initial addition of Ti4+ into silicate network.

Toward the effective design of steam-stable silica-based membranes

Pure silica and 5mol% Nb2O5-, TiO2- and Zr2O-doped silica nanoparticles were synthetized via sol-gel method and used for preparing porous and dense unsupported membranes. After annealing at 723K, samples of the porous membranes were hydrothermally treated for 2days at 393K. The materials were characterized by low temperature nitrogen sorption and calorimetric analysis before and after steam-exposure. The specific surface area loss after hydrothermal treatment for the porous pure SiO2 sample and the TiO2-, the ZrO2-, and the Nb2O5-doped SiO2 samples was respectively 51%, 40%, 29% and 28%, confirming a higher steam-stability for the doped silica membranes. The glass transition temperature (Tg) for membrane materials with different pore structure and thermal history was determined from their heat capacity curves, after densification at a temperature ⩾1373K. The Tg increases in the sequence: pure SiO2<TiO2-doped SiO2<ZrO2-doped and Nb2O5-doped SiO2. Thus, the higher the Tg is, the lower is the specific surface area loss after hydrothermal treatment. These results support the hypothesis that Nb(V), Ti(IV), and Zr(IV) ions act as network formers in silica-based membranes and stabilize their porous structure by enhancing their network connectivity. Moreover, these results indicate that calorimetric analysis is a useful tool for comparing the structure and the stability of silica-based membranes with different composition but identical thermal history, and thus for developing basic knowledge for the effective design of steam-stable silica membranes.

ToF-SIMS analysis for leaching studies of potash–lime–silica glass

In this work the durability to acidic solutions of two kinds of potash–lime–silica glasses with compositions typical for mediaeval stained glass was investigated. The low amount of network formers such as silica and alumina, and the high amount of network modifiers such as potassium and calcium, give to the glass a lower chemical stability compared to modern glass. Studies on its durability are of interest to understand degradation mechanisms. In particular the leaching procedure was focused on determining any correlation between the type of acid and the corrosion of glass independently from the pH value, which was kept constant during the different acidic treatments.Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a valuable tool to study compositional variations of glass, giving useful information concerning provenance, effects of the conservation environment, of weathering or leaching processes and about the compositional differences between the corroded layer and the bulk as a function of depth. In spite of that the insulating properties of glass, the surface roughness and the parameters used for the measurements can lead to possible misinterpretations of the results; in this paper these difficulties are discussed, in order to better interpret the analyses performed on leached glass. ToF-SIMS data are influenced by strong matrix effects making quantification difficult; for this reason the quantitative composition and surface morphology of the leached layer were additionally investigated with scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM–EDX).

Effect of the replacement of aluminum by magnesium on the compressibility and degree of polymerization of silicate glasses

The refractive index of Na2MgSi4O10 and Na2Al2Si4O12 glasses in the pressure range to 6.0 GPa was measured using a polarization-interference microscope and apparatus with diamond anvils. The changes in relative density, which characterize the compressibility of glasses, were calculated in the studied range of pressures from the measured refractive indices within the framework of the theory of photoelasticity. For the studied glasses, the degree of depolimerization was found to be NBO/T = 0.28 and 0.25 for Na2MgSi4O10 and Na2Al2Si4O12 glasses, respectively. It is shown that the replacement of aluminum by magnesium in silicate glasses does not lead to a considerable decrease in compressibility, which suggests the predominant role of magnesium ions as network formers. A minor increase in compressibility when passing from Na2MgSi4O10 to Na2Al2Si4O12 glass conforms with the corresponding decrease in the degree of depolymerization.

Organic sol–gel synthesis of microporous molecular networks containing spirobifluorene and tetraphenylmethane nodes

AbstractThe microporous molecular networks based on rigid tetrafunctional units are synthesized via organic sol–gel polymerization of 2,2′,7,7′‐tetraamino‐9,9′‐spirobifluorene (TASBF) and/or tetrakis(4‐aminophenyl)methane (TAPM) with a diisocyanate, hexamethylene diisocyanate (HDI), or p‐phenylene diisocyanate. This study is performed as an extension of our previous report on the first organic sol–gel method, which enabled the synthesis of microporous molecular networks via a two‐stage mechanism involving the formation of colloidal dispersions of the nanoparticulate molecular networks and their subsequent growth to monolithic networks on solvent evaporation. The sol–gel‐synthesized molecular networks obtained by incorporating TASBF as a network former show improved porosity, processability, and thermal stability than the TAPM‐based system. The improved porosity of TASBF‐based networks is attributed to higher rigidity of the spirobifluorene compared with the tetraphenylmethane units. We also demonstrate the synthesis of mixed organic molecular networks by sol–gel copolymerization of the two network formers, TASBF and TAPM, and a diisocyanate monomer. The sol–gel transformation of TASBF/TAPM/HDI occurred at longer reaction times with increasing the amount of TASBF in the TASBF/TAPM/HDI mixture. The results indicate that the organic sol–gel method can be further optimized by adjusting various synthesis parameters to create new functional organic molecular network materials. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Statistics of modifier distributions in mixed network glasses

The constituents of any network glass can be broadly classified as either network formers or network modifiers. Network formers, such as SiO2, Al2O3, B2O3, P2O5, etc., provide the backbone of the glass network and are the primary source of its rigid constraints. Network modifiers play a supporting role, such as charge stabilization of the network formers or alteration of the network topology through rupture of bridging bonds and introduction of floppy modes. The specific role of the modifiers depends on which network formers are present in the glass and the relative free energies of modifier interactions with each type of network former site. This variation of free energy with modifier speciation is responsible for the so-called mixed network former effect, i.e., the nonlinear scaling of property values in glasses having fixed modifier concentration but a varying ratio of network formers. In this paper, a general theoretical framework is presented describing the statistical mechanics of modifier speciation in mixed network glasses. The model provides a natural explanation for the mixed network former effect and also accounts for the impact of thermal history and relaxation on glass network topology.