27Al Magic Angle Spinning Nuclear Magnetic Resonance Research Articles

Monitoring the Geopolymerization Reaction of Geopolymer Foams Using 29Si and 27Al MAS NMR

This study aims to investigate the geopolymerization reaction of geopolymer foams produced with three different foaming agents: aluminum powder, zinc powder, and hydrogen peroxide. The geopolymerization reaction of geopolymer foam was monitored using the 27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy technique. 27Al MAS-NMR was used to monitor the reaction at an early stage, while 29Si and 27Al MAS-NMR analyses were employed at specific time intervals of 3, 6, 10, 15, and 28 days to examine the changes that occurred in the formed gel over time. We discussed in detail how the type of foaming agent used and the duration of the reaction both influence the quantity of gel formed and the amount of remnant fly ash. Our findings indicate that the type of foaming agent used affects the formation and structure of the gel, with aluminum powder leading to the highest gel formation. Additionally, the duration of the reaction plays a significant role in determining the quantity of remnant fly ash, with longer reaction times resulting in decreased fly ash content. This study sheds light on the relevance of understanding the role of foaming agents in the geopolymerization reactions of geopolymer foams and the influence of reaction time on the formed gel properties.

Structural Insight into Protective Alumina Coatings for Layered Li-Ion Cathode Materials by Solid-State NMR Spectroscopy.

Layered transition metal oxide cathode materials can exhibit high energy densities in Li-ion batteries, in particular, those with high Ni contents such as LiNiO2. However, the stability of these Ni-rich materials often decreases with increased nickel content, leading to capacity fade and a decrease in the resulting electrochemical performance. Thin alumina coatings have the potential to improve the longevity of LiNiO2 cathodes by providing a protective interface to stabilize the cathode surface. The structures of alumina coatings and the chemistry of the coating-cathode interface are not fully understood and remain the subject of investigation. Greater structural understanding could help to minimize excess coating, maximize conductive pathways, and maintain high capacity and rate capability while improving capacity retention. Here, solid-state nuclear magnetic resonance (NMR) spectroscopy, paired with powder X-ray diffraction and electron microscopy, is used to provide insight into the structures of the Al2O3 coatings on LiNiO2. To do this, we performed a systematic study as a function of coating thickness and used LiCoO2, a diamagnetic model, and the material of interest, LiNiO2. 27Al magic-angle spinning (MAS) NMR spectra acquired for thick 10 wt % coatings on LiCoO2 and LiNiO2 suggest that in both cases, the coatings consist of disordered four- and six-coordinate Al-O environments. However, 27Al MAS NMR spectra acquired for thinner 0.2 wt % coatings on LiCoO2 identify additional phases believed to be LiCo1-xAlxO2 and LiAlO2 at the coating-cathode interface. 6,7Li MAS NMR and T1 measurements suggest that similar mixing takes place near the interface for Al2O3 on LiNiO2. Furthermore, reproducibility studies have been undertaken to investigate the effect of the coating method on the local structure, as well as the role of the substrate.

Local Structure and Mixed Proton-Hole Conduction in Y and Al-Doped BaZrO3

A mixed proton-hole conductor is expected to be an excellent cathode material for proton-conducting solid oxide fuel cells (PCFCs). Accepter-doped BaZrO3 has oxygen vacancies that can be either proton defects in wet or holes in oxidizing conditions. When both proton and hole concentrations are high enough, mixed proton-hole conduction is achieved. Our group has investigated the relationship between local structural distortion and hydration energy of accepter-doped BaZrO3. It was suggested that large Y3+ prefers the compensation by protonic defects; on the other hand, small Al3+ does holes or oxygen vacancies. Thus, by doping Y and Al into BaZrO3 simultaneously, high mixed proton-hole conduction is expected without transition metals. This study focuses on the local structure around dopants, especially around Al, which can be observed by nuclear magnetic resonance (NMR) spectroscopy. The coordination number of oxygen around Al can be converted to the number of oxygen vacancies. Based on the local structural analysis, the defect equilibrium and mixed conductivity of Y and Al co-doped BaZrO3 can be discussed. BaZr0.8Y0.2-xAlxO3-δ (BZYA) was prepared by solid-state reaction method. Proton concentration was evaluated by thermogravimetric analysis at 350 ºC. AC impedance measurement was performed to evaluate the electrical conductivity of BAYA in dry or wet Ar. The proton transport number was measured at 600 ºC using a water-vapor concentration cell. 27Al Magic-Angle Spinning (MAS) NMR was performed to investigate the configuration around Al. The proton concentration of BZY10A10 and BZY15A5 was 0.6 mol%, 1.8 mol%, respectively. These proton concentrations were much smaller than that of BZY20, 18.6 mol%. This indicates that Al substitution led to a decrease in proton concentration. Meanwhile, those BZYAs showed a relatively high proton transport number, 0.84. This was found to be due to the unique oxygen configuration around Al. The local structure around Al and its effect on proton concentration and mixed conduction will be discussed in the presentation.

Structure, thermal properties and crystallization behavior of binary Y2O3–Al2O3 glasses with high alumina content

Five compositions in the system Al2O3–Y2O3 with high level of homogeneity were prepared in the form of glass microspheres by flame synthesis. The amorphous nature of prepared glasses with highly disordered structure was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman and nuclear magnetic resonance (NMR) spectroscopy. In the NMR spectra, typical signals with chemical shifts of 75, 42 and 12 ppm were observed, which were attributed to the presence of AlO4, AlO5 and AlO6 motifs in the glass structure. The ratio of individual motifs in glass samples did not change significantly with the composition. The crystallization of yttrium-aluminium garnet (YAG) phase was observed as a major process in the glasses thermally treated up to 1450 °C, with slow crystallization of θ- and α-Al2O3 phases detected in the temperature interval 980–1450 °C. IR and Raman spectra of the microspheres crystallized at 998, 1300 and 1500 °C for 4 h contained typical bands, that were assigned to the vibrations of AlO4 and AlO6 groups in YAG and Al2O3 structures. The comparison of 27Al and 89Y magic angle spinning (MAS) NMR spectra showed the presence of only YAG and α-Al2O3 phase in the samples crystallized at 1500 °C and the presence of a trace amount of θ-Al2O3 in the sample crystallized at 998 and 1300 °C. The yttrium aluminium perovskite (YAP) and yttrium aluminium monoclinic (YAM) phases, expected in this system, were no detected.

Effect of coordination nature of aluminum on its activation from minerals and Oxisols during their acidification

The types and amounts of phyllosilicate minerals in soils play an important role in soil acidification, as the acid buffering capacity of the soil varies with the composition of the phyllosilicate minerals. In addition to aluminum-oxygen (Al-O) octahedrons, a certain number of Al-O tetrahedrons exist in phyllosilicate minerals due to the isomorphic substitution of silicon ion (Si4+) by Al3+ in Si-O tetrahedrons of minerals. However, the effect of the two coordination structures of Al on the release of Al during mineral acidification has not yet been investigated. Therefore, the differences in Al activation in phyllosilicate minerals and soils containing different Al coordination structures were investigated through constant pH experiments and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) measurements. The results of 27Al MAS-NMR spectra showed that kaolinite contained Al-O octahedron, phlogopite and illite contained Al-O tetrahedron, and vermiculite-composite contained both octahedral and tetrahedral Al. At pH < 5.1, the amount of Al released from the minerals during simulated acidification followed the order: illite > vermiculite-composite > phlogopite > kaolinite, which was consistent with the cation exchange capacity and tetrahedral Al of the minerals. According to the rate constants, Al release rates were in the order of phlogopite > illite > vermiculite-composite > kaolinite at pH 4.8. Except for phlogopite, Al release rates in these minerals increased with decreasing suspension pH. Therefore, the amount and rate of Al release in phlogopite, illite, and vermiculite-composite containing Al-O tetrahedron were greater than those in kaolinite containing only Al-O octahedron. Two Oxisols derived from basalt with different ages were selected for similar study. 27Al MAS-NMR spectra of the Oxisols showed that the 0.01 million years (Ma) Oxisol contained both octahedral and tetrahedral Al, while the 1.33 Ma Oxisol contained only Al-O octahedrons. The results showed that the amounts of both exchangeable and soluble Al released from 0.01 Ma Oxisol were greater than those from 1.33 Ma Oxisol when the soils were acidified to the same pH. The results from minerals and soils confirmed that Al in Al-O tetrahedrons was more readily released into solution and exchangeable sites as soluble Al and exchangeable Al than that in Al-O octahedrons during the acidification of soils and minerals. The results of this study will provide useful references for investigating the mechanisms of solid phase Al release in different soil types and for mitigating soil acidification and inhibiting Al activation.

Quantitative analysis of C-(K)-A-S-H-amount and hydrotalcite phase content in finely ground highly alkali-activated slag/silica fume blended cementitious material

For the development of CO2-reduced binders for structural engineering and for predicting their performance, knowledge of the amount formed in weight fractions of the reaction products is essential. This paper presents a method to specify the hydration products of an alkali-activated slag/silica fume blended cementitious material by weight. The 29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to determine the C-A-S-H phases was coupled with energy dispersive analysis (EDX) technology to determine the bound K/Si ratio. Furthermore, the 27Al MAS NMR was used to define the amount of hydrotalcite. With the help of the developed model, the hydration phases of alkali-activated slag/silica fume blended cementitious material in dependency of the activator can be calculated. The results of the mass ratios deduced by NMR with coupled EDX was in agreement with the loss of ignition measured by thermogravimetric analysis (TGA). Consequently, the hydration products can be self-checked.

Rechargeable Aluminum-chalcogen Batteries: Molecular-Level Mechanistic Insights & Scientific Perspectives

Rechargeable aluminum metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric and gravimetric capacities. Commercialization of rechargeable aluminum battery systems has been limited due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal and (ii) positive electrode materials that are (electro)chemically within those electrolytes while providing high capacity and cycle life. Metal-chalcogen (e.g., sulfur, selenium) batteries are among the most promising candidates for low-cost, high-energy-density electrochemical energy storage systems; however, to date, rechargeable aluminum-chalcogen batteries have seldom been explored scientifically or technologically.Here, rechargeable aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were prepared with a chloroaluminate ionic liquid electrolyte (AlCl3:[EMIm][Cl], molar ratio of 1.5:1) and their electrochemical reaction mechanisms and products were studied at a molecular-level with solid-state nuclear magnetic resonance (NMR) spectroscopy. Galvanostatic cycling of both Al-S and Al-Se cells showed similar initial capacities and overpotentials to previously reported systems. For Al-S systems, the higher specific capacity upon charge, compared to discharge, points towards either an unwanted side reaction or deleterious polysulfide-shuttle-like behavior. This phenomenon is heavily prevalent at low current densities and affects reversibility. For Al-Se systems, two different reaction mechanisms that have been previously reported were found to be dependent on the applied current density and the crystallinity of the selenium. We were able to consolidate both reaction mechanisms during a single charge/discharge step, significantly increasing specific capacity and energy density. For both Al-S and Al-Se systems, significant capacity fade was also observed over multiple galvanostatic cycles. Solid-state 27Al single-pulse magic-angle-spinning (MAS) NMR spectra acquired on sulfur electrodes cycled to different states-of-charge reveal the presence of both solid and liquid discharge products. Solid-state 2D 27Al{27Al} multiple-quantum (MQ-)MAS measurements reveal the amorphous nature of the solid discharge product, while 2D 27Al{27Al} exchange spectroscopy (EXSY) NMR spectra reveal chemical exchange between electrolyte-soluble sulfur species coordinating with AlCl4 - and Al2Cl7 - after discharge. For Al-Se systems, solid-state 27Al and 77Se single-pulse MAS NMR measurements also reveal liquid-state reaction products in the electrolyte, leading to loss of active material from the cathode, while revealing compositional changes in the selenium electrode. Scientific perspectives on these aluminum-chalcogen battery chemistries will be discussed. This work also highlights the importance that molecular-level analytical tools, such as solid-state NMR spectroscopy, can have in clarifying electrochemical mechanisms in emerging electrochemical systems.

Nuclear magnetic resonance (NMR) studies of sintering effects on the lithium ion dynamics in Li1.5Al0.5Ti1.5(PO4)3

Abstract Various nuclear magnetic resonance (NMR) methods are combined to study the structure and dynamics of Li1.5Al0.5Ti1.5(PO4)3 (LATP) samples, which were obtained from sintering at various temperatures between 650 and 900 °C. 6Li, 27Al, and 31P magic angle spinning (MAS) NMR spectra show that LATP crystallites are better defined for higher calcination temperatures. Analysis of 7Li spin-lattice relaxation and line-shape changes indicates the existence of two species of lithium ions with clearly distinguishable jump dynamics, which can be attributed to crystalline and amorphous sample regions, respectively. An increase of the sintering temperature leads to higher fractions of the fast lithium species with respect to the slow one, but hardly affects the jump dynamics in either of the phases. Specifically, the fast and slow lithium ions show jumps in the nanoseconds regime near 300 and 700 K, respectively. The activation energy of the hopping motion in the LATP crystallites amounts to ca. 0.26 eV. 7Li field-gradient diffusometry reveals that the long-range ion migration is limited by the sample regions featuring slow transport. The high spatial resolution available from the high static field gradients of our setup allows the observation of the lithium ion diffusion inside the small (&lt;100 nm) LATP crystallites, yielding a high self-diffusion coefficient of D = 2 × 10−12 m2/s at room temperature.

Structure of crystalline and amorphous materials in the NASICON system Na1+xAlxGe2-x(PO4)3.

The structure of crystalline and amorphous materials in the sodium (Na) super-ionic conductor system Na1+xAlxGe2-x(PO4)3 with x = 0, 0.4, and 0.8 was investigated by combining (i) neutron and x-ray powder diffraction and pair-distribution function analysis with (ii) 27Al and 31P magic angle spinning (MAS) and 31P/23Na double-resonance nuclear magnetic resonance (NMR) spectroscopy. A Rietveld analysis of the powder diffraction patterns shows that the x = 0 and x = 0.4 compositions crystallize into space group-type R3̄, whereas the x = 0.8 composition crystallizes into space group-type R3̄c. For the as-prepared glass, the pair-distribution functions and 27Al MAS NMR spectra show the formation of sub-octahedral Ge and Al centered units, which leads to the creation of non-bridging oxygen (NBO) atoms. The influence of these atoms on the ion mobility is discussed. When the as-prepared glass is relaxed by thermal annealing, there is an increase in the Ge and Al coordination numbers that leads to a decrease in the fraction of NBO atoms. A model is proposed for the x = 0 glass in which super-structural units containing octahedral Ge(6) and tetrahedral P(3) motifs are embedded in a matrix of tetrahedral Ge(4) units, where superscripts denote the number of bridging oxygen atoms. The super-structural units can grow in size by a reaction in which NBO atoms on the P(3) motifs are used to convert Ge(4) to Ge(6) units. The resultant P(4) motifs thereby provide the nucleation sites for crystal growth via a homogeneous nucleation mechanism.

Rice hush ash as supplementary cementitious material for calcium aluminate cement – Effects on strength and hydration

Replacing part of calcium aluminate cement (CAC) with reactive rice husk ash (RHA) can directly reduce the impact of rice husk disposal on environment and lower the CO2 footprint of cement. RHA is comparable in chemical composition to silica fume, and thus has the potential to similarly improve the hydration of CAC. In the present study the development of strength and hydration of CAC with various amount of RHA was investigated. By means of calorimetry, X-ray diffraction analysis, 27Al magic-angle spinning-nuclear magnetic resonance spectroscopy, mercury intrusion porosimeter (MIP) and high-resolution scanning electron microscopy (SEM), a detailed insight on the changes of CAC hydration by addition of RHA is gained. Results indicate that RHA does not only accelerate hydration but also improve the early strength of CAC. A high dosage (60%) of RHA densifies the microstructure locally, but the dilution effect induces the decrease of strength. The formed hydrates are C-A-S-H phases with smaller size than CAH and thus a refined pore size develops. Considering the effect of RHA on strength, the amount of substitution should be less than 40%.

Structure of type A CAI-like melts: A view from multi-nuclear NMR study of melilite (Ca2Al2SiO7-Ca2MgSi2O7) glasses

Exploring the polymerization and structural disorder of Ca-Al-rich inclusion (CAI)-like melts is a key question in cosmochemistry due to strong implications for macroscopic properties of melts (i.e., viscosity and diffusivity). Here, we report experimental results showing the effect of composition on the structure of melilite glasses and melts [åkermanite (Åk, Ca2MgSi2O7) - gehlenite (Gh, Ca2Al2SiO7)] in type A CAIs with varying composition using high-resolution solid-state nuclear magnetic resonance (NMR). The 27Al magic angle spinning (MAS) and 3Q (triple quantum) MAS NMR spectra of melilite glasses show predominant [4]Al. A non-negligible fraction of [5]Al is observed in Åk50Gh50 and Åk72Gh28 glasses and it slightly increases with increasing åkermanite content. The 17O 3QMAS NMR spectra of melilite glasses show that bridging oxygens (BOs, Si–O–Si, Al–O–Al, and Si–O–Al) and non-bridging oxygens (NBOs, Ca–O-Si, Ca-O-Al, and mixed {Ca, Mg}–NBO) are partially resolved. Despite the strong preference of Si over Al for NBOs, for the first time, Ca-O-Al is observed in natural melts (i.e., gehlenite and Åk25Gh75 glasses and melts). The results show that Al-NBO (~150 ppm in MAS dimension) can be distinguished from Si-NBO (~110 ppm) in melilite glasses and melts. The fraction of NBO increases with increasing åkermanite content. The experimental results suggest that composition-induced structural changes should be considered to interpret the melt viscosity, diffusivity, and oxygen isotopic composition of CAI-like melts in the early Solar System.

Li-Ion Diffusion in Nanoconfined LiBH4-LiI/Al2O3: From 2D Bulk Transport to 3D Long-Range Interfacial Dynamics.

Solid electrolytes based on LiBH4 receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH4 can be stabilized via the incorporation of LiI. If the resulting LiBH4-LiI is confined to the nanopores of an oxide, such as Al2O3, interface-engineered LiBH4-LiI/Al2O3 is obtained that revealed promising properties as a solid electrolyte. The underlying principles of Li+ conduction in such a nanocomposite are, however, far from being understood completely. Here, we used broadband conductivity spectroscopy and 1H, 6Li, 7Li, 11B, and 27Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH4-LiI/Al2O3. In particular, diffusion-induced 1H, 7Li, and 11B NMR spin–lattice relaxation measurements and 7Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. 27Al magic angle spinning NMR revealed surface interactions of LiBH4-LiI with pentacoordinated Al sites, and two-component 1H NMR line shapes clearly revealed heterogeneous dynamic processes. These results show that interfacial regions have a determining influence on overall ionic transport (0.1 mS cm–1 at 293 K). Importantly, electrical relaxation in the LiBH4-LiI regions turned out to be fully homogenous. This view is supported by 7Li NMR results, which can be interpreted with an overall (averaged) spin ensemble subjected to uniform dipolar magnetic and quadrupolar electric interactions. Finally, broadband conductivity spectroscopy gives strong evidence for 2D ionic transport in the LiBH4-LiI bulk regions which we observed over a dynamic range of 8 orders of magnitude. Macroscopic diffusion coefficients from PFG NMR agree with those estimated from measurements of ionic conductivity and nuclear spin relaxation. The resulting 3D ionic transport in nanoconfined LiBH4-LiI/Al2O3 is characterized by an activation energy of 0.43 eV.

Composition and pressure effects on the structure, elastic properties and hardness of aluminoborosilicate glass

A series of calcium-aluminoborosilicate glasses with different ratios of B2O3/SiO2 have been isostatically compressed at 1 and 2 GPa at temperatures above their glass transition temperatures. Boron and aluminum coordination numbers were quantified for recovered samples by 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) and 27Al MAS NMR spectroscopy at ambient pressure and temperature. The average coordination numbers increase as the glass was compressed to higher pressure and the glass with higher concentration of boron shows higher recoverable densification. The atomic structure changes can be represented by oxygen packing density, Young's modulus and Vickers hardness number show good correlations with oxygen packing density as well. In general Poisson's ratio decreases as the average coordination number of network formers increases, however the interpretation of the changes in Poisson's ratio with pressure for each glass composition presents a more complicated case and no straightforward trend with compaction was found.

29Si, 27Al, 31P and 11B magic angle spinning nuclear magnetic resonance study of the structural evolutions induced by the use of phosphor- and boron–based additives in geopolymer mixtures

This study aims to develop a geopolymer binder, which requires a control of the reactive mixture setting time, viscosity and pH value. A previous study underlines the possibility of controlling the setting time, viscosity and pH value of geopolymer binders by the addition of phosphate- or boron–based compounds. To understand the structural evolutions responsible for these operating property changes, magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis were performed on the different binders. A batch of ten specific samples was studied by 11B or 31P (depending on the additive nature), 27Al and 29Si MAS NMR. The NMR spectral decomposition results were correlated with the operating properties of the different binders. The setting time increase was associated with the formation of a secondary phase based on boron or phosphorous. The pH variation is connected with both the modified Si/Al ratio and the asymmetric BIII species associated with one non-bridging oxygen (NBO) atom. Thus, determination of the various NMR environments is a useful tool to predict the setting time and pH value evolutions.

SiAlOC glasses derived from sol-gel synthesized ladder-like silsesquioxanes

The main aim of this study was to describe both the formation process of SiOC glasses doped with Al3+ cations and the resulting differences in their structures in comparison with undoped silicon oxycarbide glasses. A novel material was formed by means of the sol-gel method with the application of alkyl-substituted alkoxysilanes and a butoxy derivative of aluminum (Al(OC4H9)3) as a source of Al3+ ions. After the synthesis of a sol, the material was treated with a two-step thermal processing including aging (70 °C) and annealing (800 °C). As a result, yellowish xerogel (aging) and black glasses (annealing) were obtained and subjected to structural investigation, including X-ray diffraction (XRD), middle infrared spectroscopy (MIR), Raman, electron dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR) methods. The obtained results were compared to corresponding findings for SiOC glasses, obtained with the same experimental procedure. An additional thermogravimetric analysis (TGA) study was carried out for the xerogel in order to determine its thermal behavior in a protective atmosphere up to 1000 °C. The resultant mass loss curve was much smoother for the Al-doped material, indicating the beneficial impact of Al3+ addition on the formation process. XRD and spectroscopic (MIR, Raman, magic angle spinning (MAS) NMR) research revealed the formation of an amorphous silsesquioxane xerogel with a slightly modified ladder-like structure, and silicon oxycarbide glasses successfully doped with Al3+ ions, after aging and annealing, respectively. Moreover, NMR studies confirmed the introduction of aluminum into the glass network, based on the presence of Q4(1Al) and [AlO4] species in the 29Si and 27Al MAS NMR spectra, respectively. Additionally, the results from EDS and Raman spectroscopic studies suggested the partial release of free carbon phase from the glass network because of cationic substitution and ordering influence of Al3+ cations on the glass structure.

Elementary Steps of Faujasite Formation Followed by in Situ Spectroscopy

Ex situ and in situ spectroscopy was used to identify the kinetics of processes during the formation of the faujasite (FAU) zeolite lattice from a hydrous gel. Using solid-state 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), the autocatalytic transformation from the amorphous gel into the crystalline material was monitored. Al X-ray absorption near-edge structure shows that most Al already adopts a tetrahedral coordination in the X-ray-amorphous aluminosilicate at the beginning of the induction period, which hardly changes throughout the rest of the synthesis. Using 23Na NMR spectroscopy, environments in the growing zeolite crystal were identified and used to define the processes in the stepwise formation of the zeolite lattice. The end of the induction period was accompanied by a narrowing of the 27Al and 23Na MAS NMR peak widths, indicating the increased level of long-range order. The experiments show conclusively that the formation of faujasite occurs via the continuous formation and...

Understanding the Formation of CaAl2Si2O8 in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies.

An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO–9.7MgO–12.9Al2O3–38.7SiO2 mol %), used as a sealing material in solid oxide fuel cells, in view of the detrimental effect of anorthite on the sealing properties. Several advanced characterization techniques are employed to assess the material after prolonged heat treatment, including neutron powder diffraction (ND), X-ray powder diffraction (XRD), 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy. ND, 29Si MAS-NMR, and 27Al MAS-NMR results revealed that both Si and Al adopt tetrahedral coordination and participate in the formation of the network structure. In situ XRD measurements for the CMAS glass demonstrate the thermal stability of the glass structure up to 850 °C. Further heat treatment up to 900 °C initiates the precipitation of melilite, a solid solution of akermanite/gehlenite crystalline phase. Qualitative XRD data for glass-ceramics (GCs) produced after heat treatment at 850 °C for 500 h revealed the presence of anorthite along with the melilite crystalline phase. Rietveld refinement of XRD data indicated a high fraction of glassy phase (∼67%) after the formation of crystalline phases. The 29Si MAS-NMR spectra for the CMAS-GC suggest the presence of structural units in the remaining glassy phase with a polymerization degree higher than dimer units, whereas the 27Al MAS-NMR spectra revealed that most Al3+ cations exhibit a 4-fold coordination. In situ Raman spectroscopy data indicate that the formation of anorthite crystalline phase initiated after 240 h of heat treatment at 850 °C owing to the interaction between the gehlenite crystals and the remaining glassy phase.

Short-range structure and thermal properties of alumino-tellurite glasses

Alumino-tellurite glasses with wide range of Al2O3 concentration (1 to 20-mol%) are prepared and their short-range structural and thermal properties are studied by Raman spectroscopy, 27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and Differential Scanning Calorimetry (DSC). Glass sample with 1-mol% of Al2O3 has TeO co-ordination of 3.66, which is close to the value of 3.68 reported in pure TeO2 glass. TeO co-ordination decreases steadily to 3.33 on increasing alumina content to 20-mol%. 27Al MAS-NMR studies found that Al ions exist in hexa, penta and tetrahedral co-ordination in the alumino-tellurite network. On increasing alumina content from 3 to 20-mol%, the concentration of hexa-co-ordinated AlO units decreases from 54% to 35% with a simultaneous increase in the concentration of penta and tetra co-ordinated AlO units. Glass transition temperature increases with increase in Al2O3 concentration and correlates directly with the average single bond enthalpy of the glass network.

Combining 27Al Solid-State NMR and First-Principles Simulations To Explore Crystal Structure in Disordered Aluminum Oxynitride.

The nuclear magnetic resonance (NMR) technique gives insight into the local information in a crystal structure, while Rietveld refinement of powder X-ray diffraction (PXRD) sketches out the framework of a crystal lattice. In this work, first-principles calculations were combined with the solid-state NMR technique and Rietveld refinement to explore the crystal structure of a disordered aluminum oxynitride (γ-alon). The theoretical NMR parameters (chemical shift, δiso, quadrupolar coupling constants, CQ, and asymmetry parameter, η) of Al22.5O28.5N3.5, predicted by the gauge-including projector augmented wave (GIPAW) algorithm, were used to facilitate the analytical investigation of the 27Al magic-angle spinning (MAS) NMR spectra of the as-prepared sample, whose formula was confirmed to be Al2.811O3.565N0.435 by quantitative analysis. The experimental δiso, CQ, and η of 27Al showed a small discrepancy compared with theoretical models. The ratio of aluminum located at the 8a to 16d sites was calculated to be 0.531 from the relative integration of peaks in the 27Al NMR spectra. The occupancies of aluminum at the 8a and 16d positions were determined through NMR investigations to be 0.9755 and 0.9178, respectively, and were used in the Rietveld refinement to obtain the lattice parameter and anion parameter of Al2.811O3.565N0.435. The results from 27Al NMR investigations and PXRD structural refinement complemented each other. This work provides a powerful and accessible strategy to precisely understand the crystal structure of novel oxynitride materials with multiple disorder.

Direct Observation of Lattice Aluminum Environments in Li-Ion Cathodes NCA and Al-Doped NMC Via 27al MAS NMR Spectroscopy

Direct observations of local lattice aluminum environments have been a major challenge for aluminum bearing Li-ion battery materials such as LiNi1-y-zCoyAlzO2 (NCA) and aluminum doped LiNixMnyCozO2 (NMC). 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the only structural probe currently available that can qualitatively and quantitatively characterize lattice and non-lattice (i.e. surface, coatings, segregation, secondary phase etc.) aluminum coordination and provide information that helps discern its effect in the lattice. In the present study, we use NMR to gain new insights into transition metal (TM)-O-Al coordination and evolution of lattice aluminum sites upon cycling. With the aid of first principles DFT calculations, we show direct evidence of lattice Al sites, non-preferential Ni/Co-O-Al ordering in NCA, and the lack of bulk lattice aluminum in aluminum doped NMC. Aluminum coordination of paramagnetic (lattice) and diamagnetic (non-lattice) nature is investigated for Al-doped NMC and NCA. For the later, the evolution of the lattice site upon cycling is also studied. A clear re-ordering of lattice aluminum environments due to nickel migration is observed in NCA upon extended cycling.