29-Si and 27-Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Thermal Transformations of Pyrophyllite

The thermal transformation of pyrophyllite was analyzed using 27-Al and 29-Si MAS-NMR spectroscopies, thermal analysis (DTA-TG), dilatometry, and X-ray diffraction techniques. Above 800°C, dehydroxylation of pyrophyllite is the origin of a broad component in the 27-Al NMR spectra which is associated with a distorted pentahedral coordination of Al. At 1000°C, tetrahedral sheet breakdown takes place and a partial segregation of amorphous SiO₂ is detected by NMR spectroscopy. This process permits rearrangements of aluminum ions and favors the formation of small disordered nuclei of mullite. The 27-Al NMR signals observed at 59.5, 40.7, and 1 ppm have been assigned to 4- and 6-coordinated Al in mullite, and the 29-Si NMR signals observed at −110 and −88 ppm are assigned to well-crystallized mullite and cristobalite, respectively, formed during thermal decomposition of pyrophyllite.     

Silicon-29 Magic Angle Spinning Nuclear Magnetic Resonance Study of Calcium Silicate Hydrates

The reaction products formed in a series of fully "equilibrated," roomtemperature-hydrated, fumed colloidal silica plus lime water mixtures were examined using ²⁹Si magic angle spinning nuclear magnetic resonance. The data suggest that two structurally distinct calcium silicate hydrate (C-S-H) phases exist in the system CaO–SiO₂–H₂O. The more silica-rich C-S-H (Ca/Si = 0.65 to 1.0) consists predominantly of long chains of silica tetrahedra (Q² middle units) similar to those found in 1.4-nm tobermorite. The studied more lime-rich C-S-H (Ca / Si = 1.1 to 1.3) consists of a mixture of dimer (Q¹) and shorter chains (Q¹ end units and Q² middle units) similar to that reported for synthetic jennite. No monomer units (Q⁰) were detected.     

Investigations of Calcium Silicate Hydrate Structure Using Silicon-29 Nuclear Magnetic Resonance Spectroscopy

Silicon-29 nuclear magnetic resonance has been used to probe the effects of temperature and the addition of reactive silica on the silicate structure of amorphous calcium silicate hydrate (C-S-H), produced by the hydration of alite (Ca₃SiO₅). Both an increase in the temperature of formation and the addition of reactive silica increase the degree of silicate polymerization in the C-S-H. However, in all cases the data are consistent with the formation of linear chain silicates and the absence of any cross-linking. The initial formation of a hydrated orthosilicate is observed, and it is suggested that this species may form during the adsorption of water onto the surface of alite during storage.     

Analysis of Local Cation Arrangement in Mullite Using 29Si Magic-Angle Spinning Nuclear Magnetic Resonance Spectra

Local cation arrangements for mullites with various chemical compositions were examined using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra and the X-ray Rietveld method. The structure of mullite was refined using the Rietveld method, and the relations between chemical composition and lattice constants were revised. ²⁹Si NMR spectra of mullites showed a main resonance at −86 ppm accompanied by weak resonances at −94, −90, and −81 to −82 ppm. These resonances were assigned to each local cation arrangement of TO₄ tetrahedra in mullite by calculating the chemical shift according to Sherriff and Grundy's method. The resonances at −81 to −82 ppm were assigned to the arrangement of the T(Si),T(Al)–O^*_c–T^* (Al) cluster, which has been considered to be rather unusual, and their population was found to increase with increase in the Al₂O₃ content in mullite.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: I, Metakaolinite

Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid-state high resolution ²⁹Si and ²⁷AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI-0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1-0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well-defined X-ray pattern and the persistence of ∼10% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new ²⁹Si and ²⁷AI NMR data.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: 11, High-Temperature Transformations of Metakaolinite

Solid-state ²⁹Si and ²⁷Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al₂O₃· 2SiO₂ composition. (4) The spinel phase contains insufficient Si to be detected by ²⁹Si NMR but has a ²⁷Al NMR spectrum consistent with γ-Al₂O₃. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.     

Solid-State27Al and 29Si Magic-Angle Spinning NMR of Aluminosilicate Gels

The environment of the Al and Si atoms of diphasic and single-phase aluminosilicate gels was determined by solid-state ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy, The tetrahedral coordination of Al increased with decreasing Al/Si ratio in single-phase gels but not in diphasic gels. The results showed that ultrahomogeneous mixing on a next-nearest-neighbor level is achieved in single-phase aluminosilicate gels, while in diphasic gels this has successfully been avoided .     

Errors in the Calculation of 27Al Nuclear Magnetic Resonance Chemical Shifts

Computational chemistry is an important tool for signal assignment of ²⁷Al nuclear magnetic resonance spectra in order to elucidate the species of aluminum(III) in aqueous solutions. The accuracy of the popular theoretical models for computing the ²⁷Al chemical shifts was evaluated by comparing the calculated and experimental chemical shifts in more than one hundred aluminum(III) complexes. In order to differentiate the error due to the chemical shielding tensor calculation from that due to the inadequacy of the molecular geometry prediction, single-crystal X-ray diffraction determined structures were used to build the isolated molecule models for calculating the chemical shifts. The results were compared with those obtained using the calculated geometries at the B3LYP/6-31G(d) level. The isotropic chemical shielding constants computed at different levels have strong linear correlations even though the absolute values differ in tens of ppm. The root-mean-square difference between the experimental chemical shifts and the calculated values is approximately 5 ppm for the calculations based on the X-ray structures, but more than 10 ppm for the calculations based on the computed geometries. The result indicates that the popular theoretical models are adequate in calculating the chemical shifts while an accurate molecular geometry is more critical.     

核磁共振(NMR)测试技术进展及应用

介绍了核磁共振（NMR）测试技术进展，着重介绍固体高分辨核磁共振和二维核磁共振技术的进展及它们在高分子材料领域的主要应用举例。     

利用核磁共振技术研究岩心的润湿性

润湿性是存在于固体和液体表面的一种特性,核磁共振T2谱能够反映孔隙壁与孔隙流体的相互作用,它就可以成为一种评价岩石润湿性的方法。本文利用数值模拟的方法模拟出了在不同的润湿性、不同的含水饱和度条件下岩心的核磁共振T2谱的特征,并对水湿岩心和经过老化处理恢复润湿性的油湿岩心分别进行了核磁共振实验,对比不同含水饱和度下的T2谱特征来判断岩石的润湿性。     

29Si Magic-Angle-Spinning Nuclear Magnetic Resonance Study of Hydrated Cement Paste and Mortar

This paper presents ²⁹Si magic-angle-spinning nuclear magnetic resonance measurements that trace the cement hydration process in cement paste and mortar specimens made from ordinary portland cement, type I. These specimens were moist-cured for 3, 7, 14, and 28/31 d at temperatures ranging from 21° to 80°C. Compressive strength for all tested specimens was also determined. The results show that the degree of hydration ( Q ¹+ Q ²) and the compressive strength increase with curing times and temperatures. However, at 80°C, the compressive strength decreases while the degree of hydration increases.     

Characterization of Synthetic and Naturally Occurring Clays by 27Al and 29Si Magic-Angle Spinning NMR Spectroscopy

Magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy (²⁷Al) detected the coordination of Al in clays containing as little as 0.26% Al₂O₃. The ²⁹Si MASNMR of fluorphlogopite showed three distinct Si chemical environments which suggested short-range ordering. Synthetic laponite and mica-montmorillonite showed broad ²⁹Si resonances indicative of short-range disorder. A saponite showed four ²⁹Si resonances. Considerable insight into the short-range ordering of clays can be gained by ²⁷Al and ²⁹Si MASNMR.     

核磁共振氢谱法在食用油研究中的应用

主要综述了核磁共振氢谱技术在油脂指标的测定、油脂产地区分、油脂掺伪以及油脂氧化过程产物的鉴定等方面的应用,为进一步开发核磁共振技术在油脂上的应用提供一些参考。     

核磁共振技术及其应用进展

核磁共振技术是有机物结构测定的有力手段,不破坏样品,是一种无损检测技术.从连续波核磁共振波谱发展为脉冲傅立叶变换波谱,从传统一维谱到多维谱,技术不断发展,应用领域也越广泛.核磁共振技术在有机分子结构测定中扮演了非常重要的角色,核磁共振谱与紫外光谱、红外光谱和质谱一起被有机化学家们称为"四大名谱".     

Thermal Reactions of Pyrophyllite Studied by High-Resolution Solid-state 27Al and 29Si Nuclear Magnetic Resonance Spectroscopy

Water is lost in two overlapping steps from well-crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the ²⁹Si and ²⁷Al solid-state NMR spectra. Detailed analysis of the ²⁹Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the ²⁷A1 spectra is not possible because of electric field gradient effects which result in the loss of ∼90% of the A1 spectral intensity due to the formation of five-coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the ²⁹Si and ²⁷Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.     

柴油结构特点的核磁共振波谱法研究

采用硅胶色谱和尿素络合法分离柴油，得到正构烷烃，异构烷烃，芳烃，胶质和沥青质。并利用１＾ＨＮＭＲ谱对其中正构烷烃和异构烷烃组分进行定性定量分析，计算出二者的支化度。结果表明，异构烷烃具有单个支链的结构特征。     

Nuclear Magnetic Resonance Studies of Silicon Carbide Polytypes

The ²⁹Si MAS NMR spectra of the 2H, 4H, 6H, and 3C polytypes of silicon carbide are presented. An attempt is made to correlate differences in the chemical shifts with local atomic environment. The results of the analysis of the spectra of pure polytypes are used as a basic for the interpretation of the spectra of mixed polytypes and a discussion of the crystallinity and impurity levels of different samples. Carbon-13 chemical shifts obtained from spectra of the same polytypes are also tabulated.     

Preliminary Characterization of Gel Precursors and Their High-Temperature Products by 27Al Magic-Angle Spinning NMR

The environment of the Al atom in alumina gels, single-phase and diphasic mullite gels, and their transformation products was studied by ²⁷Al magic-angle spinning nuclear magnetic resonance (MASNMR). Alumina gels made from inorganic and organic precursors showed exactly the same ²⁷Al MASNMR spectra. Heat treatment of gels at 500°C caused artial conversion of octahedral Al to tetrahedral Al. The Al in single-phase mullite gel appears to be highly ordered compared to that in diphasic mullite gel.     

固体核磁共振技术及其在氟聚合物研究中的应用

固体核磁共振作为一种重要的现代分析研究手段，在许多研究领域都有广泛的用途。通过对常用的固体核磁共振技术的介绍结合其在聚四氟乙烯、聚偏氟乙烯、四氟乙烯-六氟丙烯共聚物分析研究中的应用，说明固体核磁共振技术是研究常见氟聚合物结构和分子运动的重要方法。