Structure of Glasses in the Na2SO4–P2O5–H2O System

The structure of sulfate–phosphate glasses is investigated by high-resolution ³¹P solid-state nuclear magnetic resonance with magic-angle sample spinning (³¹P MAS NMR spectroscopy). The structure parameters that account for the number of different types of phosphorus–oxygen structural units involved in the glass network are determined, and the main structural units of the glasses under investigation are revealed. The role of H₂O and Na₂SO₄ as modifiers of the structural network of sulfate–phosphate glasses is clarified.     

Structural densification of lithium phosphoaluminoborate glasses

Lithium aluminoborate glasses have recently been found to undergo dramatic changes in their short-range structures upon compression at moderate pressure (~1 GPa), most notably manifested in an increase in network forming cation coordination number (CN). This has important consequences for their mechanical behavior, and to further understand the structural densification mechanisms of this glass family, we here study the effect of P₂O₅ incorporation in a lithium aluminoborate glass (with fixed Li/Al/B ratio) on the pressure-induced changes in structure, density, and hardness. We find that P₂O₅ addition results in a more open and soft network, with P-O-Al and P-O-B bonding, a slightly smaller fraction of tetrahedral-to-trigonal boron, and an unchanged aluminum speciation. Upon compression, the cation-oxygen CNs of both boron and aluminum increase systemically, whereas the number of bridging oxygens around phosphorous (Qⁿ) decreases. The glasses with higher P₂O₅ content feature a larger decrease in Qⁿ (P) upon compression, which leads to more non-bridging oxygen that in turn fuel the larger increase in CN of B and Al for higher P₂O₅ content. We find that the CN changes of Al and B can account for a large fraction (around 50% at 2 GPa) of the total volume densification and that the extent of structural changes (so-called atomic self-adaptivity) scales well with the extent of volume densification and pressure-induced increase in hardness.     

Characterization of the Free-Carbon Phase in Precursor-Derived Si-C-N Ceramics: I, Spectroscopic Methods

Polyvinylsilazane, as a precursor for Si-C-N ceramics, was prepared by ammonolysis of functionalized chlorosilanes. Pyrolysis under inert atmospheres at T _p= 1000°C led to an amorphous Si-C-N-(H) ceramic. Further heat treatment caused the transformation to the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterizing the solid intermediates via solid-state magic angle spinning nuclear magnetic resonance spectroscopy. Moreover, Raman spectroscopy, electron spin resonance spectroscopy, and chemical analysis were used. Based on these methods, a comprehensive picture of the formation and behavior of the free-carbon phase present in polymer-derived ceramics was obtained.     

Pressure-induced transparency of MgO,Al2O3, AlN,and cBN ceramics at room temperature

The in situ axial X-ray diffraction patterns of four ceramic powder samples (MgO, Al₂O₃, AlN, and cBN) that were compressed in a diamond anvil cell under uniaxial non-hydrostatic conditions were recorded. The microscopic deviatoric stress as a function of the pressure was determined from the X-ray diffraction peak broadening analysis: the curves increased approximately linearly with the pressure at the initial compression stage and then levelled off under further compression. Pressure-induced transparency was observed in all of the samples under compression, and the pressure at the turning point on the curves of the microscopic deviatoric stress versus pressure corresponded to the pressure at which the samples became transparent. Analysis of the microstructural features of the pressure-induced transparent samples indicated that the compression caused the grains to fracture, and the broken grains bonded with each other. We demonstrated that the ceramics’ pressure-induced transparency was a process during which the grains were squeezed and broken, the pores were close between the grains, and the broken grains were re-bonded under compression.     

Structure of Phosphorus Oxynitride Glasses

The structures of phosphourus oxynitride glasses have been determined using a combination of solid-state ³¹P, ¹⁵N, and ²³Na nuclear magnetic resonance and Raman spectroscopies. Raman spectra of model phosphazene compounds with different types of P–N bonding have been used to confirm spectral assignments. Results indicate that nitrogen replaces oxygen in the phosphrus atoms (via one double bond and one single bond) and as nitrogen bonded to three phosphorus atoms via three single bonds. The observed structural features are consistent with data which show that nitrogen influences the chemical durability, thermal expansion, and other properties of phosphate glasses by cross-linking ploymeric phosphate chain to gether in the glass network.     

High-Pressure Effects on Oxide Glasses: I, Densification in Rigid State

Densification of SiO₂, GeO₂, and B₂O₃ glasses in the rigid state was studied at pressures up to 80 kb and at temperatures up to 600°C in different pressure-transmitting media. The infrared, ultraviolet, and nuclear magnetic resonance absorptions of the compressed glasses were examined. Densification increased with time, temperature, pressure, and more important, with applied shear. This process of volumetric shrinkage in the rigid state at low temperatures is fundamentally different from that at temperatures at or above the glass transition. A qualitative model involving mutual entanglement of parts of the random glassy network is postulated to explain the observed results.     

Structural and Thermophysical Properties of (PbO)0.5(SiO2)0.5 Glasses: Effect of SnO2 Incorporation

¹¹⁹Sn and ²⁹Si solid-state nuclear magnetic resonance studies on lead silicate glasses containing different amounts of SnO₂ confirmed that tin exists in the glass as distorted SnO₆ polyhedra and there is no direct interaction between tin and silicon structural units. Transmission electron microscopic studies have established that tin structural units are uniformly distributed in the glass. Significant changes in the values of glass transition temperature, microhardness, and thermal expansion coefficient with SnO₂ incorporation into the glass have been attributed to the increased rigidity of the glass network brought about by the replacement of weaker Pb–O linkages with stronger Sn–O linkages.     

Structure,crystallization, and performances of alkaline-earth boroaluminosilicate sealing glasses for SOFCs

We study the structure, crystallization, and performances of the sealing glasses with the composition (mol.%) of 12Al₂O₃·8B₂O₃·40SiO₂·40RO (R = Mg, Ca, Sr) for solid oxide fuel cells (SOFCs) before and after isothermal treatment at 700°C, which is within the operation temperature range (600-800°C) of SOFCs. The crystallization behavior has been investigated by differential scanning calorimetry and X-ray diffraction under both dynamic and isothermal conditions. The structural evolution is probed using the Raman and nuclear magnetic resonance spectroscopies. The performances of the sealing glasses are characterized in terms of the coefficient of thermal expansion, the crystallization-induced stress at glass–steel interface. We find that strong crystallization occurs at the operation temperature (700°C) far below the crystallization onset temperature measured by DSC. The structure origin of this anomalous crystallization is discussed in terms of structural heterogeneity of the three studied glasses. We determine the residual stress at the interface between the Ca-containing glass and the steel after isothermal treatment at 700°C for 48 h, but this stress does not lead to falling off the glass layer from the steel. This indicates that this glass is a good candidate to be applied in SOFCs.     

Field strength effect on structure,hardness, and crack resistance in single modifier aluminoborosilicate glasses

Although the interactions among glass formers and modifiers, for example, connectivity and charge distribution, have been studied extensively in oxide glasses, the impact of a particular modifier species on the mechanical performance of aluminoborosilicate (ABS) glasses is not well understood. This work compares the indentation properties of six ABS glasses, each of which contains a different network modifier (NWM) with varying field strength (FS). Three alkali and three alkaline earth ABS glasses were designed with low NWM content and [NWM] ≈ [Al₂O₃], to test the modifier FS effect at low concentrations and to maximize three-coordinated boron. It has been found that both hardness and crack resistance increase with increasing FS in these ABS systems, which is surprising in the context of historical reports. Using ¹¹B, ²⁷Al, and ²⁹Si solid-state nuclear magnetic resonance, this work provides evidence of how charge distributions differ as a function of NWM species, and how this relates to the observed indentation behaviors.     

Proton Spin–Spin Relaxation Study of the Effect of Temperature on White Cement Hydration

The chemical and microstructural changes within a white cement paste were characterized in situ using proton nuclear magnetic resonance spin–spin relaxation at 30 MHz, and X-ray diffraction. Paste samples with a water-to-cement ratio of 0.42 were cured at constant temperatures of 2°, 20°, 60°, and 100°C. Proton nuclear magnetic resonance spin–spin relaxation allows tracking the evolution of the mixing water into the solid fractions of calcium silicate hydrate, calcium hydroxide, and monosulfate, and the liquid phases: the calcium silicate hydrate interlayer water, gel pore water, and capillary pore water. It is shown that the hydration process is markedly accelerated with increasing hydration temperature, and that proton nuclear magnetic resonance relaxation measurements can quantitatively determine the proportions of water phases, their magnetic resonance characteristics, as well as the setting times of the cement during the hydration process.     

A solid-state and suspended-state magic angle spinning nuclear magnetic resonance spectroscopic investigation of a 9-ethyladenine molecularly imprinted polymer

Suspended-state high resolution/magic angle spinning nuclear magnetic resonance spectroscopy and solid-state cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy were employed to study the interactions between 9-ethyladenine and a 9-ethyladenine molecularly imprinted polymer, and a non-imprinted polymer, respectively, both are copolymers of methacrylic acid and ethyleneglycol dimethacrylate. Template-related structural differences between the materials were revealed by contact time measurements and solid-state nuclear magnetic resonance. Rebinding of the template to the imprinted polymer resulted in shorter contact times for nuclei believed to be involved in the binding site interactions whereas the non-imprinted polymer did not exhibit such effects. This indicates that binding site reoccupation has a stiffening effect lowering the mobility of nearby nuclei. More detailed information was obtained from suspended-state saturation transfer difference high resolution/magic angle spinning nuclear magnetic resonance experiments. These revealed molecular level details concerning the interactions of the adenine guests with the polymer binding sites. Thus, a relatively larger transfer of magnetization was observed in the solute when bound to the molecularly imprinted polymer at a position where multiple hydrogen bonds between the analyte and the template can be expected to take place in the molecularly imprinted polymer only.     

Structural impact of nitrogen incorporation on properties of alkali germanophosphate glasses

The structure, atomic packing density, calorimetric glass transition, and hardness of mixed sodium–lithium germanophosphate oxynitride glasses with varying Ge/P and N/P ratios were investigated. The combined influences of nitridation and mixed network former effect (MNFE) on the glass structure were analyzed using Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and ³¹P nuclear magnetic resonance (NMR) spectroscopy. Evidence for the existence of germanium in a higher coordination state, i.e., five‐ or sixfold coordination, was obtained by performing XPS analysis of the oxide glasses, with indication of conversion to tetrahedral coordination upon nitridation. Raman spectroscopy measurements implied that the germanate network was modified upon nitridation, including the removal of ring‐like germanate structures and P–O–Ge mixed linkages. The partial anionic N‐for‐O substitution gave rise to the linear dependence of the glass transition temperature (T_g) and hardness (H_V) on nitrogen content (expressed as N/P ratio), especially for lower Ge/P ratio. However, nitridation also caused an unexpected increase in liquid fragility and decrease in density. This suggests that the governing structural parameter for property evolution in such LiNaGePON glasses is not only the increased degree of cross‐linking of the phosphate chains, but rather the short‐ and intermediate‐range structural modifications within the germanate component of the oxynitride glasses.     

Free volume,mobility, and structural relaxations in poly(ethylene oxide)/poly(methyl methacrylate) blends

Time-dependent structural relaxations in a melt-mixed 38/62 vol% poly(ethylene oxide)/atactic poly(methyl methacrylate) blend were studied using several techniques: differential scanning calorimetry, pressure-volume-temperature analysis, positron annihilation lifetime spectroscopy, dynamic mechanical analysis, and solid-state nuclear magnetic resonance. The internal volume (free volume hole size) and the external volume (specific volume) of the blend are found to decrease with aging time. The time scale of the volume changes is the same, suggesting that internal and external volumes can be calculated from each other. Increasing mobility of poly(ethylene oxide), composition fluctuations, and shifting glass transition temperatures are observed upon aging. Phase separation in terms of spinodal decomposition below an upper critical solution temperature occurs within minutes and results in two amorphous phases of different composition. Subsequent crystallization then causes further structural changes.     

Heteronuclear Spin Decoupling in Solid-State Nuclear Magnetic Resonance: Overview and Outlook

Heteronuclear spin decoupling is a vital element in most of the solid-state nuclear magnetic resonance experiments for resolution and sensitivity reasons. The field has seen remarkable advancement in ideas, pulse schemes, and theoretical understanding. This review will take a look at a few such developments in the last two decades and the current perspectives. We expect and hope that the reader will get an overall idea of the decoupling field without getting into the rigour of the design and understanding of the pulse schemes.     

Large Photoinduced Densification in Organically Modified Germanosilicate Glasses

Organically modified germanosilicate (ORMOGSIL) glasses prepared by a sol-gel method showed a large refractive index change on ultraviolet exposure. The large photoinduced refractive index change in the ORMOGSIL glasses is mainly due to the structural densification caused by ultraviolet irradiation. The shifts in frequency of the Raman bands measured at room temperature reveal structural densification by reduction of the average intertetrahedral bonding angle θ in the ORMOGSIL glasses. Surface relief patterns by photoinduced densification were directly inscribed on the ORMOGSIL glasses.     

Structural origin of thermally induced refractive index changes in Yb3+/Al3+/P5+/F−-co-doped silica glass

In this work, the relationship among the thermal history, refractive index (RI), and structures of Yb³⁺/Al³⁺/P⁵⁺/F⁻-co-doped silica glass is studied. We reveal the structural origin of the thermally induced RI change in the Heraeus F300 and Yb³⁺/Al³⁺/P⁵⁺/F⁻-co-doped silica glasses with different states of thermal history by advanced solid-state nuclear magnetic resonance and Raman scattering techniques, and the effect of the thermal history on the beam quality of Yb³⁺-doped large-mode-area photonic crystal fiber (LMA PCF). The structural analysis demonstrates that the RI change in the glass is inextricably linked to the change in the AlPO₄ domains, coordination number of the Al and Si–O–Si rings. An intimate and direct connection of the thermally induced physical behaviors is developed with the structural pictures of glasses, and the beam quality of the LMA PCF is greatly improved by proper thermal annealing. The results provide a significant reference for the precise manipulation of the numerical aperture of Yb³⁺-doped active fiber.     

Investigation of Griffiths phase,spin reorientation and magnetism in double perovskite Gd2FeMnO6

In this study, double perovskites of Gd₂FeMnO₆ (GFMO) are successfully prepared using the solid-state reaction method. X-ray diffraction results indicate that GFMO has an orthogonal structure (space group Pnma). Moreover, X-ray photoelectron spectroscopy results show mixed-valence states of 3d transition ions. A further analysis of thermomagnetic data suggests that, in addition to the canted antiferromagnetism and spin reorientation, there is also a Griffiths phase with an antiferromagnetic ground state caused by ferromagnetic short-range interactions and confirmed by electron spin resonance analysis. Antisite-disordered B-site ions give rise to different short-range magnetic orders, which disrupt the long-range ferromagnetic order of Fe–O–Mn, leading to the formation of a short-range ferromagnetic order. Furthermore, with magnetocaloric magnitudes of 17.0 J kg⁻¹ K⁻¹ for 0–50 kOe, GFMO polycrystalline is a promising candidate for magnetic refrigerants in the ultra-low temperature range. Finally, the change in magnetic entropy due to spin reorientation at room temperature (285K) has potential applications in thermostatic water bath switches.     

Spin Polarization in Single Spin Experiments on Defects in Diamond

Generation of pure spin states is an important step towards coherent control of single spin systems. Especially for nitrogen-vacancy defects in diamond, where readout of single spins using optical detection is available, fast initialization of spin is significant in the context of quantum computing applications. In this system the spin polarization is caused by an intersystem crossing process via the meta-stable singlet state ¹ . The slowest relaxation rate is the intersystem to the triplet ground-state ³ on a timescale of 400 ns. The intersystem crossing process mainly populates the ms = 0 spin sublevels of the paramagnetic ground state. Although no direct nuclear spin polarization is seen for low magnetic fields, frequency selective microwave pulses can be used to transfer electron spin polarization to ¹³C nuclear spin states.     

Unfolding under Pressure: An NMR Perspective

This review aims to analyse the role of solution nuclear magnetic resonance spectroscopy in pressure-induced in vitro studies of protein unfolding. Although this transition has been neglected for many years because of technical difficulties, it provides important information about the forces that keep protein structure together. We first analyse what pressure unfolding is, then provide a critical overview of how NMR spectroscopy has contributed to the field and evaluate the observables used in these studies. Finally, we discuss the commonalities and differences between pressure-, cold- and heat-induced unfolding. We conclude that, despite specific peculiarities, in both cold and pressure denaturation the important contribution of the state of hydration of nonpolar side chains is a major factor that determines the pressure dependence of the conformational stability of proteins.     

Heats of Solution, Density, and Refractive Index Changes in Sodium Borate Glasses Subjected to High Pressure

The densification produced in sodium borate glasses by pressures to 40 kbars at 25° and 250°C was measured. At a constant temperature and pressure, the densification decreased with increasing alkali content. For specimens pressed at 40 kbars and 250°C, the densification ranged from 14.2% for B₂O₃ to 6.3% for the 33.3 mole % Na₂O composition. Under the same conditions, the refractive index increase ranged from 4.8 to 1.1%, and the decrease of the molar refraction of the oxygen ions ranged from 2.5 to 1.5%. The heats of solution of the pressed glasses were more negative than those of the corresponding unpressed glasses. The heats of solution of both the pressed and unpressed glasses pass through a minimum near 20 mole % Na₂O. A decrease in density was observed at room temperature for all pressed specimens. Electron micrographs were made of two of the pressed specimens. The results could be explained on the basis of a repacking of structural units.