Reactions in polyethoxysilane coatings using solution and solid-state29Si NMR spectroscopy

Solution and solid-state²⁹Si nuclear magnetic resonance (NMR) spectroscopy was used to follow the prehydrolysis and subsequent film formation of a polyethoxysilane mixture. Significant differences in intensity of the Q₂ resonances in the cross-polarization and dipolar decoupled spectra suggest a mobile phase is present in the cured film. Incorporation of solvent in the film is observed directly by¹³C cross polarization magic-angle-spin (CP MAS) NMR. The NMR results support the conclusions reached from a parallel chemical study.[/p]     

Characterization of alinite cements through X-ray diffraction and mas 29Si NMR studies

Alinite cements have been synthesized using mining and steel plant wastes and pulverized fuel ash (fly ash) as raw materials and a clinkering temperature of 1150°C. The cements possess hydration characteristics comparable to those of portland cements. X-ray diffraction studies on these samples confirm the presence of alinite as the predominant phase. MAS ²⁹Si NMR spectra have been used to distinguish alinite and alite cements. While both show resonances characteristic of Q° type silicate species, the portland cements exhibit three distinct peaks corresponding to three inequivalent SiO₄ units present, while alinite shows a single sharp peak corresponding to the unique Si position.     

29Si MAS NMR study of the structure of calcium silicate hydrate

This paper presents the results of a comprehensive investigation of single-phase calcium silicate hydrate (CSH) with known compositions using the combined capabilities of ²⁹Si magic angle spinning (MAS) NMR, powder X-ray diffraction (XRD), and chemical analysis of the solution and solid. CSH gels with C/S ratios ranging from 0.4 to 1.85 have been synthesized by hydration of highly reactive β-C₂S and aqueous reaction of fumed silica with CaO and separately with highly reactive β-C₂S. The main findings include the following. (1) CSH shows continuity and diversity in both composition and structure and forms a continuous structural series. (2) Phase-pure CSH has C/S ratios between 0.6–1.54. (3) SiOH and CaOH bonds both occur in CSH, with the abundance of the former decreasing and that of the latter increasing with increasing C/S ratio. Model calculation indicates that there are between 0.13–0.43 SiOH bonds per tetrahedron and the CaOH/Ca ratio varies between nearly 0 and 0.64. An ideal formula for CSH with a C/S ratio of 1.5 is: Ca_4.5[Si₃O₈(OH)](OH)₄ · nH₂O. A defect-tobermorite structural model is proposed for CSH in which individual layers have the basic structure of 1.4-nm tobermorite but contain a significant concentration of defects and are more disordered. Stacking disorder between adjacent layer and disorder within individual layers may both contribute to the local and long-range disorder and thus to the diversity of CSH.     

27Al and 29Si NMR study of sol-gel derived aluminosilicates and sodium aluminosilicates

Solid state ²⁷Al and ²⁹Si NMR was used to examine the structures of aluminosilicates and sodium aluminosilicates prepared by the sol-gel method from metal alkoxides. In contrast to the borosilicate system, where B-O-Si bonds are not formed until heat treatment above 150° C, Al-O-Si formation appears complete upon gelation. Aluminium occupies tetrahedral [AlO₄]^– sites in the polymer network and octahedral [Al(H₂O)₆]³⁺ (or similar) sites in the intersticies for charge balance. When sodium is added as a counter ion the octahedral aluminium is converted to tetrahedral aluminium in the oxide network. In gels of high aluminium content prepared from (Bu^sO)₂Al-O-Si(OEt)₃, some aluminium in five coordinate environments is also observed. All gels remain amorphous on heating to 800° C.     

29Si固体核磁共振技术在水泥化学研究中的应用

^29Si核磁共振技术常用于研究Si原子核周围的中短程相互作用,与X射线衍射、中子衍射、电子衍射等研究长程整体结构的方法互为补充,在水泥化学的研究当中具有重要的作用。本文从水泥混凝土中硅酸盐物质结构的分析、各种水泥的水化、矿渣的碱激发以及混凝土碱集料反应等方面,通过分析Si原子结构特征在^29Si固体核磁共振测试结果中所反映出来的变化,阐述了^29Si NMR技术在水泥化学研究中的应用。     

XRD, TEM, IR and 29Si MAS NMR characterization of NiO-SiO2 nanocomposites

The present study focuses on the structural properties of a NiO-SiO₂nanocomposite with 14 mol % of nickel oxide obtained by a sol-gel method and a gradual heating the gel in the 350–900°C range. NiO nanoparticles and their dispersion in the amorphous silica matrix were studied through TEM and XRD. The behaviour at the nanoparticle/matrix interface was investigated through IR and ²⁹Si MAS NMR spectroscopy comparing the spectra of the nanocomposite with that of a silica sample obtained with the same preparation method and submitted to the same thermal treatments. The results indicate that nanoparticles, formed in cavities of the silica matrix, act as an obstacle towards the spontaneous silica polymerization process with heat treatments.     

The thermal reactions of montmorillonite studied by high-resolution solid-state29Si and27Al NMR

Only very slight changes are observed in the²⁹Si and²⁷Al solid-sate magic-angle spinning NMR spectra of a montmorillonite containing almost equal numbers of octahedral aluminium and magnesium ions when its interlayer water is driven off by heating. The²⁹Si NMR spectra are unaffected by dehydroxylation which begins at 450° C, but the²⁷Al spectra show a decrease in total intensity, possibly due to the formation of 5-coordinated aluminium, with a slight increase in the intensity of the tetrahedral aluminium resonance. On the basis of these results, a structural model is proposed for the dehydroxylate phase and its formation mechanism is discussed. The destruction of the dehydroxylate X-ray pattern at 850° C and the subsequent recrystallization of the high-temperature products (-quartz, enstatite and high-cordierite at 1100° C;-cristobalite, enstatite and sapphirine at 1200° C) is accompanied by changes in the silicon and aluminium NMR spectra and in the⁵⁷Fe Mbssbauer spectra which are fully consistent with the known structural features of these phases.     

Measurement of the partial molar volume and solubility of hydrogen in its dilute solutions in palladium-silver alloys

Linear expansion of thin-wall tubes of palladium-silver alloys containing 10, 20, 27, and 40 wt% silver was measured on absorption of small amounts of hydrogen (hydrogen-to-metal atom ratio, 0.005 to 0.25) at temperatures between 150°C (423 K) and 350°C (623 K) for equilibration with gaseous hydrogen at pressures up to around 1 bar. Hydrogen uptake by the test specimens was determined concurrently with the expansion measurements. Thermal expansion coefficients for the hydrogen-free materials were also obtained. Analysis of the data indicates that fractional length change is linearly dependent on hydrogen-to-metal atom ratio within the experimental range of hydrogen concentrations, independent of temperature, and varies only slightly with alloy composition. Partial molar volumes of hydrogen (with 95% confidence limits) vary from 1.77±0.08cm³·(mol H)⁻¹ for the 10% Ag alloy to 1.92±0.03cm³·(mol H)⁻¹ for the 40% Ag alloy. Hydrogen solubility results are correlated by a relation that facilitates linear extrapolation for determination of limiting values for equilibrium constants. Heats of solution of hydrogen at infinite dilution and standard entropy changes are essentially independent of temperature within the range of experimental conditions. Paper dedicated to Professor Edward A. Mason.     

29Si and 17O NMR investigation of the structure of some crystalline calcium silicate hydrates

This paper presents the results of a systematic investigation of the structure of ¹⁷O-enriched, hydrothermally synthesized 1.1-nm tobermorite, 1.4-nm tobermorite, jennite, calciochondrodite, xonotlite, and hillebrandite, using ²⁹Si magic angle spinning (MAS) NMR, ¹H-²⁹Si cross-polarization magic angle spinning (CPMAS) NMR, and ¹⁷O MAS NMR. The ¹⁷O and most of the ¹H-²⁹Si CPMAS results are the first reported for these phases. Six types of oxygen sites were observed in tobermorite and jennite, including both SiOH CaOH linkages. The structure of 1.4-nm tobermorite is similar to that of 1.1-nm tobermorite with about 26% of the Ca²⁺′s in the interlayers. The results support the proposed jennite structure in which silicate chains and rows of OH⁻ groups alternately occur along the CaO layers [1]. Jennite contains long, single silicate chains similar to those in 1.4-nm tobermorite, with SiOH sites primarily occurring on bridging tetrahedra, and there seems to be no interlayer Ca²⁺′s. Although the Si sites in xonotlite and calciochondrodite cross-polarize well, neither contains SiOH linkages.     

In situ solid-state NMR studies of Ca3SiO5: hydration at room temperature and at elevated temperatures using 29Si enrichment

²⁹Si isotopic enrichment was used for acquisition of multiple ²⁹Si magic-angle spinning (MAS) and cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra, in situ in an NMR probe, from a single sample of hydrating Ca₃SiO₅ (C₃S). Data with excellent signal-to-noise ratios were obtained at 20, 50 and 75 °C, with minimal use of spectrometer time, and without the need for the quenching of multiple samples. Spectral line widths and polymer-chain lengths derived from the spectra had no detectable differences from experiments in which the quenching was carried out with propan-2-ol. Furthermore, the effects of the MAS technique on the hydration reaction appeared to be minimal. At 20 °C, the bulk hydrate initially produced was dimeric; at later stages of the reaction, polymerization occurred. Arrhenius energies of 35 and 100 kJ mol^–1, respectively, were calculated for these two reactions. The cross-polarization (CP) spectra acquired throughout the hydration showed that at 20 °C, 2% of the hydrated monomeric Q _o ^(H) species persisted from after the induction period through to the late stages of the hydration reaction; this indicates that this species is unlikely to result from surface hydroxylation of C₃S; an upfield shift of this species occurred with increasing hydration, indicating a possible change of environment for the silicate species. The amount of Q _o ^(H) produced was found to increase at higher temperatures. Potential mechanisms for polymerization were assessed and a model in which dimeric-silicate units are linked together by insertion of monomers (dimer pentamer octomer) was found to give the best fit to the observed data; these results support a dreierketten model for the structure of the hydrate.     

Measurement of the magnetic susceptibility of a rod-shapedsuperconductor using a solenoid coil (SRPM method)

An effective method for the measurement of the magnetic susceptibility (χ) of a circular rod-shaped superconductor, in which χ is estimated by substituting the magnetic penetration depth (λ) into a formula after obtaining λ from the difference in the impedance between a solenoid coil with a sample and an identical one without a sample, is proposed. The temperature dependence of χ obtained from this method has correlated well with the values measured with a SQUID magnetometer     

A29Si magic angle spinning NMR and DTA study of thermal crystallization of sphene and zircon gels

²⁹Si MAS NMR spectra of gels of sphene and zircon stoichiometry show wide variations in local silicon environments at intermediate stages of crystallization, depending on the pH of the starting mixtures. The broad NMR peaks of the calcined gels give way on heating to spectra of varying complexity, including a cristobalite peak which then diminishes in intensity again as the sphene or zircon peak becomes predominant on further heating. Differential thermal analysis as a function of heating rate for selected zircon and sphene gels yields activation energies of crystallization of about 300 kJ mol^–1.     

A study of silver species on silver-exchanged ETS-10 and mordenite by XRD, SEM and solid-state 109Ag, 29Si and 27AI NMR spectroscopy

Silver zeolites, especially Ag-ETS-10 and Ag-mordenite, actively bind xenon and iodine, two prime contaminants common to nuclear accidents. The evolution of silver species on silver exchanged ETS-10 (Ag/ETS-10) and mordenite (Ag/Mor) has been investigated by exposing the materials to a series of activation conditions in Ar, air and H2. The samples were characterized by XRD, SEM and solid-state 109Ag, 29Si and 27AI MAS NMR. The silver reduction and structural evolution have been illustrated by those techniques. The effectiveness of one sample of each type of sieve was tested for its ability to trap mercury from a gas stream. However, the results from this study demonstrate that the adsorption characteristics of silver-loaded sieves cannot necessarily be predicted using a full complement of structural characterization techniques, which highlights the importance of understanding the formation and nature of silver species on molecular sieves.     

Oscillatory convection in a dilute3He-superfluid4He solution

Convective instabilities in a rectangular, unity-aspect-ratio Rayleigh-Bénard cell with a solution of 1.46%³He in superfluid⁴He have been studied in the temperature range 0.70–1.05 K, with a corresponding Prandtl number range of 0.045<<0.15. The onset of stationary convection is much like that in a classical, one-component fluid. The oscillatory instability is studied by using an extremely sensitive local temperature probe. It is found that the total heat transport efficiency is suppressed by the oscillations in the entire range of Prandtl number we have studied. The local temperature probe indicates a striking difference in the oscillatory amplitude when the sense of rotation of the convective rolls is reversed. The magnitude of the convective velocity is deduced from both the initial slope of the Nusselt number near the onset of the stationary convection and the frequency of the oscillations. Determinations of the temperature dependence of the convective velocity using these two methods agree very well. The observed behavior of the oscillatory frequency and onset condition supports the theory of oscillatory convection for a classical, low-Prandtl-number, one-component fluid.     

Measurement of profiles of temperature and local vapor content near a heat-generating surface in water boiling in a large volume

Results are presented of simultaneous measurement of temperature profiles and local vapor content in a boundary layer in the boiling of water.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 2, pp. 197–203, August, 1979.     

29Si MAS NMR study of the hydration of tricalcium silicate in the presence of finely divided silica

²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy has been used to investigate the effect of finely divided silica on the hydration of tricalcium silicate (C₃S). In order to reduce the time needed to obtain quantitative results, low levels of paramagnetic iron oxide were added to materials to increase the nuclear relaxation rate. The reactions of the C₃S and the finely divided silica could then be monitored separately and without chemical modification. The results were correlated with data from microcalorimetry, thermogravimetric analysis and electron microscopy. Under the conditions used here the presence of the silica accelerates greatly the hydration process, and results in a markedly increased degree of polymerization in the resulting gel, without significantly affecting the induction period of the reaction. The significance of these results for understanding the hydration process of C₃S in the presence of silica is outlined.