In situ 1H NMR Study of Nanoreactor Interaction NH3–H2O in Zeolite Nanopores

The interaction between ammonium NH₃ and H₂O molecules in zeolitic nanopores is studied by in situ ¹H nuclear magnetic resonance (NMR) method. The powder and single crystal samples of natural zeolites, heulandites Ca₄[Al₈Si₂₈O₇₂]·24H₂O and clinoptilolite (Na, K,Ca_1/2)₆[Al₆Si₃₀O₇₂], were used as the model system. It is shown that penetration of NH₃ into the zeolitic nanopores is accompanied by disordering of the hydrogen sublattice of zeolitic water and by the fast proton exchange NH₃ + H₂O ? [NH₄]⁺ + [OH]^? characterized by correlation frequency v _c = ~40 kHz. Another nanoreactor interactions are represented by interaction of [NH₄]⁺ ions with exchangeable Na⁺ and Ca²⁺ ions of the zeolitic structure. The slow ionic exchange [NH₄]⁺ → [Na,Ca_1/2]⁺ and binding of [NH₄]⁺ in cationic sites of the framework were visualized by NMR spectroscopy along with stepwise release of (Na,Ca_1/2)OH from zeolitic pores to the external surface of zeolite grains.     

Local relaxation in lanthanum silicate oxyapatites by Raman scattering and MAS‐NMR

To characterize the local relaxation in the structure of lanthanum silicate oxyapatite materials, six compositions with different cation and oxygen stoichiometries (La₈Ba₂Si₆O₂₆, La₉BaSi₆O_26.5, La₁₀Si_5.5Mg_0.5O_26.5, La_9.33SiO₂₆, La_9.67SiO_26.5 and La_9.83Si_5.5Al_0.5O_26.5) were investigated by combining Raman scattering and ²⁹Si and ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS‐NMR) spectroscopies. Only [SiO₄]⁴⁻ species were evidenced and the hypotheses of [Si₂O₇]⁶⁻ and [Si₂O₉]⁸⁻ entities were ruled out. Both oxygen excess and cation vacancies induce local distortions in the structure, which leads to nonequivalent [SiO₄]⁴⁻ species, characterized by different ²⁹Si MAS‐NMR signals and by splitting of Raman signals. Copyright © 2011 John Wiley & Sons, Ltd.     

Novel clays: Solid-state synthesis,characterization and cation exchange selectivity

Several novel swelling mica-type clays have been synthesized by solid-state processes. Synthetic clays of ideal compositions such as Na₂Si₆Al₂Mg₆O₂₀F₄ · xH₂O (Na-2-mica), Na₃Si₅Al₃Mg₆O₂₀F₄ · xH₂O (Na-3-mica) and Na₄Si₄Al₄Mg₆O₂₀F₄ · xH₂O (Na-4-mica) have been prepared and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and ²⁷Al and ²⁹Si solid-state magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Powder XRD showed that all syntheses yielded water swollen micas with c-axis spacing of ∼1.2 nm except in the case of Na-2-mica which also showed a small peak of anhydrous mica phase with a c-axis spacing of 0.96 nm. Solid-state ²⁷Al MASNMR spectroscopy revealed that almost all the Al is present in the tetrahedral environment of the different micas. Solid-state ²⁹Si MASNMR spectroscopy revealed different Si (Al) nearest neighbor environments depending upon the composition of the various micas. Selective cation exchange studies were performed on the various micas using 0.5 N NaCl solution containing 12.9 ppm Sr²⁺ or 8.12 ppm of La³⁺. The results showed, for the first time, that Na-3-mica has a high selectivity for the trivalent cation tested. The previously reported high selectivity of these synthetic micas for the divalent cations has been confirmed. These selective cation exchange studies are of relevance in cation separations from drinking and waste waters.     

Interfacial reactions between Al7075 alloy and BaAl2Si2O8 + CaAl2Si2O8 mixture

The present work reports the SEM, EPMA and TEM examination of reactions at the interface of Al7075 alloy and a 50/50 wt% mixture of BaAl₂Si₂O₈ + CaAl₂Si₂O₈ feldspars at 850 °C, 1150 °C and 1250 °C. Sintering of the feldspar mixture at 1450 °C caused dissolution of ~1.0 wt% Ca in BaAl₂Si₂O₈ and 0.5 wt% Ba in CaAl₂Si₂O₈. The interaction of the Al alloy with the sintered feldspars shifted the alloy composition to the Al–Si–αBaAl₂Si₂ and Al–Si–βaAl₂Si₂ compatibility triangles. The feldspars underwent a series of phase transformations, leading ultimately to the formation of Al₂O₃.     

Phase transition in hexacelsian at about 580 K

Temperature-induced structure and microstructure changes in hexacelsians (BaAl₂Si₂O₈) that have been synthesised from the Ba-exchanged LTA and FAU zeolites (hexacelsian_LTA and hexacelsian_FAU) show that the phase transition near 580?K exists only in hexacelsian_LTA. The X-ray powder diffraction method has been used to follow the evolution of the structure during the phase transition, as described here. The excess thermodynamic quantities Gibbs free energy (G), entropy (S) and enthalpy (H) are obtained through the Landau theory of phase transition. The constants of proportionality between the G and ordering parameter (Q) are: h?=??170345?J?mol^?1, a?=??66.6?J?mol^?1?K^?1 and b?=??410534?J?mol^?1. The abrupt change in the trigonal distortion of the single six-member tetrahedral [SiO₄]^4? and [AlO₄]^5? ring near 580?K is responsible for the phase transition. The phase transition is non-convergent, ferroelastic, pure and proper.     

Structural and spectroscopic characterization of the quenched hexacelsian

The hexacelsian with disorder distribution of the Si⁴⁺ and Al³⁺ is synthesized by thermally induced transformation of a Ba-LTA zeolite. The initial Ba-LTA zeolite is annealed and quenched to room temperature. The crystal structure and microstructure i.e. long-range ordering is investigated from the polycrystalline material by the Rietveld refinement procedure while short-range ordering is investigated by the ²⁹Si and ²⁷Al MAS NMR, Raman and luminescence (Eu³⁺ doped hexacelsian) spectroscopy's. The crystal structure (space group P6₃/mcm) and microstructure (size-strain analysis) results indicate disorder distribution of the Si⁴⁺ and Al³⁺. Analysis of the spectra indicates disorder distribution of the Si⁴⁺ and Al³⁺ (²⁹Si and ²⁷Al MAS NMR spectroscopy), dynamical disorder in the structure and site symmetry lowering for the Ba²⁺ site at a low temperature (Raman and luminescence spectroscopy's).     

Mössbauer spectroscopy study of the disordering process of Fe-rich FeAlSi alloys

In this work we study systematically the influence of different Al/Si ratios on the magnetic and structural properties of mechanically disordered powder Fe₇₅Al_25?x Si _x , Fe₇₀Al_30?x Si _x and Fe₆₀Al_40?x Si _x alloys by means of Mössbauer spectroscopy and X-ray diffraction measurements. In order to obtain different stages of disorder the alloys were deformed by ball milling annealed (ordered) alloys during different number of hours. X-ray and Mössbauer data show that mechanical deformation induces the disordered A2 structure in these alloys. The results indicate that addition of Si to binary Fe–Al alloys makes the disordering more difficult. The study of the hyperfine fields indicates that depending on the Fe content the magnetic behaviour of these ternary alloys varies. For Fe₇₅Al_25?x Si _x series, the alloys have different magnetic behaviours with deformation depending on the Si content. The magnetization of the alloys with high Si content decreases with deformation, as it happens to binary Fe₇₅Si₂₅ and the magnetization of the alloys with low Si content increases with deformation, as it happens to binary Fe₇₅Al₂₅. For Fe₇₀Al_30?x Si _x series the mean hyperfine fields show that there are two different stages with the disordering, in a first stage the mean hyperfine fields decrease and in the second stage they increase. Finally, for Fe₆₀Al_40?x Si _x alloys there is a magnetic transition, from a paramagnetic ordered state to a magneticdisordered state.     

Pyroxene inclusions in paleo-Christian mosaic tesserae: a new tool for constraining the glass manufacturing temperature

The mineral inclusions of two orange glass tesserae from paleo-Christian mosaics were investigated in order to derive the melting temperature reached during their production (sourced from Padua and Vicenza, Veneto region, Italy). In particular, clinopyroxene crystals were studied by single-crystal X-ray diffraction and electron microprobe WDS analysis. The crystals show C2/c symmetry, typical of disordered Ca/Na and Mg/Al distributions indicating high-temperature of formation (>700°C). The cation site populations were obtained by combining results from the two experimental techniques enabled us to derive the following stoichiometric formula:

Effect of ultrasound on the synthesis of low-modulus zeolites from a metakaolin

It was studied the effect of ultrasonic processing (22 kHz) of the aqueous suspension of metakaolin, sodium hydroxide and alumina with a molar ratio 2Al₂Si₂O₇:12NaOH:2Al₂O₃ on the low-modulus zeolite synthesis processes. To investigate the XRD, SEM, IR, EDS had been used. It was shown that after ultrasonic processing, sodium aluminates are formed, what leads to a change in process of further synthesis. It was found that without ultrasonic processing on the stage of thermal treatment at 650 °C, SOD zeolite (|Na₆|[Al₆Si₆O₂₄]) and sodium aluminosilicate (Na₆Al₄Si₄O₁₇) are synthesized. In the sample after ultrasound during thermal treatment, only sodium aluminosilicates of cubic syngony (Na₆Al₄Si₄O₁₇ and Na₈Al₄Si₄O₁₈) are formed. It was demonstrated that sodium aluminosilicates are precursors for the formation of LTA zeolite (|Na₁₂|[Al₁₂Si₁₂O₄₈]). As a result zeolitization of sodium aluminosilicates after the hydrothermal crystallization in alkaline solution, the sonicated sample contained 97 wt% LTA. Without ultrasonic processing, the product of synthesis contained 50 wt% SOD and 40 wt% LTA.     

Thin-film formation of Si clathrates on Si wafers

In this study, we prepared Si clathrate films (Na₈Si₄₆ and Na_xSi₁₃₆) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapour to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10⁻² Pa) resulted in a film of Si clathrates having a thickness of several micrometres. Furthermore, this technique enabled the selective growth of Na₈Si₄₆ and Na_xSi₁₃₆ using the appropriate crystalline orientation of Si substrates.     

1H NMR Study of Zeolite Water Structure and Broensted Acid Centers in Chabazite

The disordered structure of absorbed water in a highly porous zeolite chabazite Ca₂[Al₄Si₈O₂₄]·nH₂O (2 ≤ n ≤ 12.8) is studied from motionally narrowed ¹H nuclear magnetic resonance spectra at room temperature. Concentration phase transitions were observed at n ≈ 8.3 and n ≈ 10.2. The transitions are accompanied by displacement of Ca²⁺ ions and variation of the Broensted acid centers at inner surface of zeolite pores.     

Electronic structures of Al–Si clusters and the magic number structure Al8Si4

The low-energy structures of Al₈Si_m (m = 1–6) have been determined by using the genetic algorithm combined with density functional theory and the Second-order Moller-Plesset perturbation theory (MP2) models. The results show that the close-packed structures are preferable in energy for Al–Si clusters and in most cases there exist a few isomers with close energies. The valence molecular orbitals, the orbital level structures and the electron localisation function (ELF) consistently demonstrate that the electronic structures of Al–Si clusters can be described by the jellium model. Al₈Si₄ corresponds to a magic number structure with pronounced stability and large energy gap; the 40 valence electrons form closed 1S²1P⁶1D¹⁰2S²1F¹⁴2P⁶ shells. The ELF attractors also suggest weak covalent Si–Si, Si–Al and Al–Al bonding, and doping Si in aluminium clusters promotes the covalent interaction between Al atoms.     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Raman investigation of fibrous zeolites of the natrolite group at high pressures of an aqueous medium

The intermediate phases preceding overhydration are observed by Raman spectroscopy both in scolecite Ca[Al₂Si₃O₁₀] · 3H₂O and in thomsonite NaCa₂[Al₅Si₅O₂₀] · 6H₂O upon compression in an aqueous medium. The first intermediate phase of scolecite is attributed to a phase precursor revealed earlier using XRD at pressure of ～1 GPa. The widening of the Raman bands of O-H vibrations caused by the disordering of H₂O, which appears after additional water molecules are embedded in the zeolite channels, is typical of this intermediate phase. It is assumed on the basis of the Raman spectroscopy data that scolecite contains second overhydrated and second intermediate phases.     

On the Coexistence of Chemically Similar Stable and Metastable Phases in the BeO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> System

The joint crystallization of the stable phase (a number of solid solutions of chromium-containing beryllian indialite) and metastable phases (crystalline modifications) of the compound with β-quartz structure (Si_0.64Al_0.28Mg_0.21Be_0.09)^IVO₂ ~ Mg_1.89Be_0.81Al_2.52Si_5.76O₁₈ admixed with Cr₂O₃ phases of khmaralite Mg_1.21Cr_0.01Be_0.46Al_1.78Si_3.38O₂₀ and spinel {(Mg_0.95Be_0.045Si_0.005)^IV(Al_1.31Cr_0.67Mg_0.02)^VI}O₄ is performed using melted chromium–beryllian indialite preliminarily obtained by solid-phase synthesis as a precursor. Simultaneously, the residual X-ray amorphous melt of composition Mg_1.83Cr_0.01Be_1.04Al_2.64Si_5.57O₁₈ is hardened. Thus, a reconstructive transition from the beryllian indialite melt to a phase with the β-quartz structure is implemented, and the chemical similarity of these compounds is demonstrated. The rate of change in the crystallization isotherm of 2°С/h and increased heat outflow through the highly heat-conducting walls of the Pt–Rh crucible (taper) contribute to this process.     

Structure,ionic conduction,and phase transformations in lithium titanate Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>

The spinel structure of lithium titanate Li₄Ti₅O₁₂ is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li₄Ti₅O₁₂ compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li₄Ti₅O₁₂ compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)_8a[Li_0.33Ti_1.67]_16dO₄ → [Li□]_16c[Li_1.33Ti_1.67]_16dO₄ → [Li_1.33□_0.67]_16c[Ti_1.67□_0.33]_16dO₄. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → ....     

Al versus Si competition in FeSiAl alloys

In FeSiAl alloys, when Si substitutes for Al, important changes take place in the magnetism as well as in the structural properties. Alloys in the two composition series Fe₇₅Al_25−xSi_x (x=0, 7.5, 12.5, 17.5, 25) and Fe₇₀Al_30−xSi_x (x=0, 9, 15, 21, 30) were prepared by induction melting; afterwards they were crushed and then annealed in order to recover the DO₃ stable phase. The deformed FeAl samples show larger lattice parameters than the ordered ones; however, this difference (Δa) decreases when Si substitutes for Al until it becomes zero (i.e. until the ordered samples and the deformed ones have the same lattice parameters). This trend is the same for both sample series and does not depend on the Fe content of the alloy. However, the magnetization has a different behaviour depending on the Fe content. For deformed Fe₇₅Al_25−xSi_x alloys the saturation magnetization decreases with increasing Si content while for Fe₇₀Al_30−xSi_x deformed alloys the saturation magnetization has a plateau in which the saturation magnetization values do not vary.     

Structure of Al2O3 thin layers synthesized on the silicon surface by atomic layer deposition

The distribution of the phase and chemical composition at an Al₂O₃/Si interface is studied by depth-resolved ultrasoft x-ray emission spectroscopy. The interface is formed by atomic layer deposition of Al₂O₃ films of various thicknesses (from several to several nanometers to several hundreds of nanometers) on the Si(100) surface (c-Si) or on a 50-nm-thick SiO₂ buffer layer on Si. L _2,3 bands of Al and Si are used for analysis. It is found that the properties of coatings and Al₂O₃/Si interfaces substantially depend on the thickness of the Al₂O₃ layer, which is explained by the complicated character of the process kinetics. At a small thickness of coatings (up to 10–30 nm), the Al₂O₃ layer contains inclusions of oxidized Si atoms, whose concentration increases as the interface is approached. As the thickness increases, a layer containing inclusions of metallic Al clusters forms. A thin interlayer of Si atoms occurring in an unconventional chemical state is found. When the SiO₂ buffer layer is used (Al₂O₃/SiO₂/Si), the structure of the interface and the coating becomes more perfect. The Al₂O₃ layer does not contain inclusions of metallic aluminum, does not vary with the sample thickness, and has a distinguished boundary with silicon.     

Active metal brazing of Al2O3 to Kovar® (Fe–29Ni–17Co wt.%) using Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%)

The application of an active braze alloy (ABA) known as Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%) to join Al₂O₃ to Kovar® (Fe–29Ni–17Co wt.%) has been investigated. This ABA was selected to increase the operating temperature of the joint beyond the capabilities of typically used ABAs such as Ag–Cu–Ti-based alloys. Silica present as a secondary phase in the Al₂O₃ at a level of ~5 wt.% enabled the ceramic component to bond to the ABA chemically by forming a layer of Si₃Ti₅ at the ABA/Al₂O₃ interface. Appropriate brazing conditions to preserve a near-continuous Si₃Ti₅ layer on the Al₂O₃ and a continuous Fe₃Si layer on the Kovar® were found to be a brazing time of ≤15 min at 1025 °C or ≤2 min at 1050 °C. These conditions produced joints that did not break on handling and could be prepared easily for microscopy. Brazing for longer periods of time, up to 45 min, at these temperatures broke down the Si₃Ti₅ layer on the Al₂O₃, while brazing at ≥1075 °C for 2–45 min broke down the Fe₃Si layer on the Kovar® significantly. Further complications of brazing at ≥1075 °C included leakage of the ABA out of the joint and the formation of a new brittle silicide, Ni₁₆Si₇Ti₆, at the ABA/Al₂O₃ interface. This investigation demonstrates that it is not straightforward to join Al₂O₃ to Kovar® using Copper ABA®, partly because the ranges of suitable values for the brazing temperature and time are quite limited. Other approaches to increase the operating temperature of the joint are discussed.     

Thermoluminescence experiments to study lattice defects in aluminosilicates

Summary The thermoluminescent emissions of β-eucryptite and β-spodumene have been recordedvs. temperature and wavelength. Arguments are advanced which allow the observed emission to be ascribed to point defects originated by the exchange of Si⁺⁴ and Al⁺³ ions in (Si, Al)O₄ tetrahedra.