29Si 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.

Structure of amorphous materials in the NASICON system NaTi2Si x PO12

The structure of glasses in the sodium (Na) super-ionic conductor (NASICON) system NaTi2Si x PO12 with x = 0.8 and x = 1.0 was explored by combining neutron and high-energy x-ray diffraction with 29Si, 31P and 23Na solid-state nuclear magnetic resonance (NMR) spectroscopy. The 29Si magic angle spinning (MAS) NMR spectra reveal that the silica component remains fully polymerized in the form of Si4 units, i.e. the silicon atoms are bound to four bridging oxygen atoms. The 31P{23Na} rotational echo adiabatic passage double resonance (REAPDOR) NMR data suggest that the 31P MAS NMR line shape originates from four-coordinated P n units, where n = 1, 2 or 3 is the number of bridging oxygen atoms per phosphorus atom. These sites differ in their 31P-23Na dipolar coupling strengths. The results support an intermediate range order scenario of a phosphosilicate mixed network-former glass in which the phosphate groups selectively attract the Na+ modifier ions. Titanium takes a sub-octahedral coordination environment with a mean Ti–O coordination number of 5.17(4) for x = 0.8 and 4.86(4) for x = 1.0. A mismatch between the P–O and Si–O bond lengths of 8% is likely to inhibit the incorporation of silicon into the phosphorus sites of the NASICON crystal structure.

Probing oxide-based glass structures by solid-state NMR: Opportunities and limitations

This Primer critically reviews the possibilities and limitations of probing the short-range structure of an oxide-based glass by routine magic-angle spinning (MAS) or triple-quantum MAS (3QMAS) nuclear magnetic resonance (NMR) experiments. We briefly outline the structural features of oxide-based glasses and the basics of solid-state NMR. Besides suggesting guidelines for selecting favorable experimental conditions and important considerations for recording high-quality MAS NMR spectra amenable for subsequent analysis, we review options for extracting NMR observables and their distributions from spins-1/2 as well as half-integer spin quadrupolar nuclei. Considering that the isotropic chemical shift remains the primary probe of local structure by MAS NMR, we thoroughly review its dependence on the short-range oxide-based glass parameters from the viewpoint of a very simple and intuitive but qualitative model. The utility of deconvolutions of notably 29Si and 31P MAS NMR spectra are discussed critically. We also suggest alternative yet qualitative analysis options that are available whenever MAS NMR spectral deconvolutions are not warranted without additional information, which incidentally applies to a majority of modern multicomponent glasses.

Aluminosilicate-Supported Catalysts for the Synthesis of Cyclic Carbonates by Reaction of CO2 with the Corresponding Epoxides.

Functionalized aluminosilicate materials were studied as catalysts for the conversion of different cyclic carbonates to the corresponding epoxides by the addition of CO2. Aluminum was incorporated in the mesostructured SBA-15 silica network. Thereafter, functionalization with imidazolium chloride or magnesium oxide was performed on the Al_SBA-15 supports. The isomorphic substitution of Si with Al and the resulting acidity of the supports were investigated via 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and NH3 adsorption microcalorimetry. The Al content and the amount of MgO were quantified via inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The anchoring of the imidazolium salt was assessed by 29Si and 13C MAS NMR spectroscopy and quantified by combustion chemical analysis. Textural and structural properties of supports and catalysts were studied by N2 physisorption and X-ray diffraction (XRD). The functionalized systems were then tested as catalysts for the conversion of CO2 and epoxides to cyclic carbonates in a batch reactor at 100 or 125 °C, with an initial CO2 pressure (at room temperature) of 25 bar. Whereas the activity of the MgO/xAl_SBA-15 systems was moderate for the conversion of glycidol to the corresponding cyclic carbonate, the Al_SBA-15-supported imidazolium chloride catalysts gave excellent results over different epoxides (conversion of glycidol, epichlorohydrin, and styrene oxide up to 89%, 78%, and 18%, respectively). Reusability tests were also performed. Even when some deactivation from one run to the other was observed, a comparison with the literature showed the Al-containing imidazolium systems to be promising catalysts. The fully heterogeneous nature of the present catalysts, where the inorganic support on which the imidazolium species are immobilized also contains the Lewis acid sites, gives them a further advantage with respect to most of the catalytic systems reported in the literature so far.

Direct Identification of Zeolite Beta Polymorphs by Solid-State 29Si Nuclear Magnetic Resonance Spectroscopy.

Stacking disorder and polymorphism in zeolite and zeolite-like materials hinder their structural characterization. In this work, we propose an advanced approach based on applying "pure shift" solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy for the structural investigation of zeolitic materials containing intergrown polymorphs. The approach developed in the case study of zeolite beta allows for the resolution of 21 29Si signals, attributing them to non-equivalent T sites in polymorphs A, B, and C, reconstruction of individual 29Si magic angle spinning NMR spectra for each polymorph, and determination of the polymorph composition with higher accuracy than X-ray diffraction. The results reveal that two widely used synthetic routes for zeolite beta, alkaline and fluoride synthesis, lead to different polymorph compositions. These findings indicate that "pure shift" solid-state 29Si NMR can serve as a superior tool for the elucidation of polymorphism in zeolites.

Elaboration of geopolymers based on clays by-products from phosphate mines for construction applications

Yellow clays from Moroccan phosphate mines are extracted with other waste rocks lithologies and stored in large dumps within the mine site. The present paper investigates the use of this by-product as a new aluminosilicate source for geopolymers elaboration at the laboratory scale. Yellow clays studied are mainly composed of montmorillonite, dolomite and quartz as major phases. The Toxicity Characteristic Leaching Procedure (TCLP) showed no existence of contaminant. In order to reach thermal activation, the raw clays were first calcined at different temperatures between 500 and 900 °C. Raw and calcined clays were characterized using thermogravimetric analysis (TGA), X-Ray Diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy. For the synthesis of geopolymers, calcined powder was mixed with an alkaline solution presenting different NaOH/Na2SiO3 ratios. The compressive strength was measured at 7, 14 and 28 days and material porosity was determined using Mercury Intrusion Porosimetry method in order to explain the mechanical behavior. XRD, 27Al and 29Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy, Fourier transformed infrared and TGA have been used to investigate the gel structure.The thermal treatment at 900 °C allowed the destruction of crystalline structure of montmorillonite clays, as well as the formation of new phases such as periclase (MgO) and gehlenite (Ca2Al2SiO7). Furthermore, the 27Al - 29Si MAS-NMR and the XRD analysis of the elaborated materials confirmed the formation of two cementitious gels C-A-S-H (Calcium aluminum silicate hydrate) and N-A-S-H (Sodium aluminum silicate hydrate). The compressive strength was around 25 MPa at 28 days. The obtained results are promising; they have shown that elaborated materials can be adapted for construction applications. In this case, it is recommended that economic and commercial study be developed.

Evolution mechanism of surface hydroxyl groups of silica during heat treatment

The surface hydroxyl groups of nano-silica have a great influence on its application properties. In this paper, the changes of the number of various hydroxyl groups after dehydroxylation and rehydration progress of silica were quantitatively analyzed by thermogravimetric (TG) analysis and 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. According to the different stability of Si-O-Si bond formed after dehydroxylation, we propose two types of hydroxyl groups depending on the relative positions of them: chain adjacent hydroxyl groups (C-OHs) and spatially adjacent hydroxyl groups (S-OHs). DFT calculations indicate that two C-OHs dehydroxylation form a four-atoms ring structure, and the resulting OSiO bond angle deviation from optimal value is up to 19.56°, and the average bond length of the SiO bond increase more than 4.66%. Further more, its rehydration reaction activation energy (Ear) is very low, even only 0.21 kJ/mol, so it is easy to re-open and return to hydroxyl groups under environmental conditions. Conversely, the S-OHs dehydroxylation form a stable multi-atoms ring. The DFT calculations are a good illustration of the phenomenon in which some unstable siloxane bonds in a dehydroxylated sample are restored to hydroxyl groups in TG analysis and 29Si MAS NMR measurements.

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.

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle.

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO4 and SiO4 structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al2O3-SiO2 (CMAS) glass composition was modified by substituting MgO for CaO and SiO2 for Al2O3 to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), 29Si and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO4 and AlO4 tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of 29Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO4 and AlO4 tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al[4]-O-Al[4] bonds; this region coexists with a predominantly SiO4-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO4 and AlO4 structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO4-rich region at higher temperatures, leading to the formation of additional crystalline phases.

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.

Application of Ca-doped mesoporous silica to well-grouting cement for enhancement of self-healing capacity

Carbonation has been considered as an alternative to prevent CO2 leakage. In this study, calcium-doped mesoporous silica (CDMS) was introduced into Portland cement as a promoter of carbonation. An admixture of CDMS and American Petroleum Institute (API) Class G Portland cement exposed to CO2-saturated water was analyzed under geologic sequestration conditions (40°C and 80MPa) to assess the carbonation properties and self-healing effect of CDMS for CO2 storage. The capacity of CDMS to synthesize CaCO3 from CO2via carbonation was identified in an in vitro crystallization test. Analysis of the cut surface of a cement core showed the rapid synthesis of CaCO3 including calcite and aragonite. Rietveld analysis was employed for quantitative phase analysis. The quantitative analysis of the cement carbonation showed that tricalcium silicate (C3S) and dicalcium silicate (C2S) play important roles in cement carbonation. Several crystal phases of CaCO3 were identified in this study including calcite, aragonite and amorphous CaCO3. X-ray diffractometry (XRD), a field emission-scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray spectrometer (EDS), 29Si MAS-NMR (magic angle spinning–nuclear magnetic resonance) spectrometry, thermogravimetry-differential thermal analysis (TG-DTA) and FT-IR (Fourier transform infrared spectroscopy) were applied to characterize the admixture.

Novel activator for synthesis of fly ash based geopolymer

Lithium hydroxide as a novel activator for synthesis of fly ash based geopolymer was reported. The effects of LiOH concentration on the mechanical properties, morphology and mineral structure were investigated by X-ray diffraction (XRD), scanning electron microscopy, thermogravimetric analysis/differential thermogravimetric analysis and 29Si magic angle spinning–nuclear magnetic resonance (MAS-NMR) spectra. The results showed that the optimum concentration of LiOH was 4 mol L−1, corresponding to the compressive strength of 8·29 MPa at curing age of 28 days after 85°C heating for 9 h. The mineral phase of spodumene (LiAlSi2O6) was formed from the results of XRD. The 29Si MAS-NMR spectra indicated that the network structure of fly ash based geopolymer was built by the replacement of one SiO4 with one AlO4 at a given site, causing a low field shift of ∼10 ppm.

Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O–CaO–Al2O3–SiO2 glasses: A multinuclear NMR study

The incorporation mechanisms of fluorine (F) into peralkaline and peraluminous Na2O–CaO aluminosilicate glasses with ∼65mol% SiO2 (model system for phonolites) were investigated by magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In 19F MAS NMR spectra of the fluorine-bearing peralkaline glasses at least five F sites could be distinguished, while only three of these sites could be found in the corresponding peraluminous glasses, which shows that there are more F incorporation mechanisms in peralkaline than in peraluminous glasses. In the peralkaline glasses containing up to 6.2mol% F the following F environments were identified: F–Ca(n) at ∼−113ppm, Si–F–Na(n) or Al–F–Ca(n) at ∼−146ppm, Al–F–Al at ∼−168ppm, Al–F–Na(n) at ∼−188ppm and F–Na(n) at ∼−225ppm (“n” indicates that the number of atoms is variable or uncertain). F–Ca(n) is the most abundant site which is surprising as Ca is the least common cation in the glasses. The fraction of F–Ca(n) sites increases from 42% to 53% as the F content increases from 1.2 to 6.2mol%. The addition of up to 16.5mol% (5.3wt%) water strongly affects F speciation in peralkaline glasses and results in a decrease in the fraction of F–Al sites compared to F–Ca(n) sites. It seems that hydroxyl groups (OH) and F occupy similar Al environments and that F cannot compete with OH.In the peraluminous glasses containing up to 18.3mol% F only three F environments Si–F–Na(n) or Al–F–Ca(n) at ∼−149ppm, Al–F–Al at ∼−170ppm and Al–F–Na(n) at ∼−190ppm are observed. Al–F–Na(n) is the most abundant site with a fraction of 54–61%. The F speciation also changes with the F concentration, with a minimum in Al–F–Na(n) sites between 3.5 and 9.7mol% F.Fluorine has only a small effect on the 23Na and 29Si MAS NMR spectra. 27Al MAS NMR spectra of the peralkaline glasses show only four-coordinated Al while in the peraluminous glasses ∼5% of the Al was found to be five-coordinated. The amount of five-coordinate Al does not change with increasing F content, but the environment of the five-coordinate Al becomes more symmetric with increasing F.

Sol–gel chemistry synthesis and DTA–TGA, XRPD, SIC and 7Li, 31P, 29Si MAS–NMR studies on the Li-NASICON Li3Zr2−ySi2−4yP1+4yO12 (0 ⩽ y ⩽ 0.5) system

Five selected compounds of Li-NASICON, Li3Zr2−ySi2−4yP1+4yO12 (0⩽y⩽0.5), were synthesized by sol–gel chemistry in order to obtain pure polycrystalline powder and then analyzed by different physicochemical characterizations such as coupled DTA (differential thermal analysis)–TGA (thermogravimetric analysis), XRPD (X-ray powder diffraction), CIS (complex impedance spectroscopy) and MAS (magic angle spinning)–NMR (nuclear magnetic resonance). So the calcined temperature of each sample has been deduced from its corresponding DTA–TGA thermogram. However, the recorded X-ray powder diffractograms were indexed in the rhombohedral system with R3¯c space group which corresponds to the ideal structure of NASICON. Whereas, the complex impedance spectroscopy study showed that these Li-NASICON materials are excellent lithium fast cation conductors with total electric conductivity maximal value 1.97×10−3Scm−1 at 293K in the case of Li3Zr1.5P3O12. Furthermore, 7Li, 31P and 29Si MAS–NMR spectroscopy study and DFT/B3LYP theoretical calculations of chemical shifts were performed to discuss the ambiguousness that exists between the resonance peak number in the experimental spectrum and the crystallographic site number relative to Li3Zr2Si2PO12.

Characterisation of a remineralising Glass Carbomer® ionomer cement by MAS-NMR Spectroscopy

ObjectivesThe purpose of this study was to characterize commercial glass polyalkenoate cement (GPC) or glass ionomer cement (GIC), Glass Carbomer®, which is designed to promote remineralization to fluorapatite (FAp) in the mouth. The setting reaction of the cement was followed using magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. MethodsGlass Carbomer® initial glass powder and cements were subjected to 27Al, 31P, 19F and 29Si MAS-NMR analysis. X-ray powder diffraction (XRD) was employed to determine the presence of crystalline phases. Results27Al MAS-NMR showed the Al to be predominantly four coordinate, Al(IV), and the presence of Al–O–P species in the glass. The proportion of Al(IV) was reduced with setting reaction of the cement and significant amount of six coordinate Al, Al(VI), was found in the cement. The 31P MAS-NMR spectra showed clearly a decrease of the orthophosphate peak of apatite on initial setting. 19F MAS-NMR showed only a small fraction of FAp. 29Si MAS-NMR demonstrated the presence of largely Q4(2Al) in the glass which changed only little in the aged cement. SignificanceThis study also demonstrated how the setting reaction in Glass Carbomer® cement and other GICs can be followed by 27Al MAS-NMR examining the conversion of Al(IV) to Al(VI). Our data revealed that the apatite in this cement was not FAp but largely hydroxyapatite, which was partially consumed during the cement formation.

Degree of reactivity of two kaolinitic minerals in alkali solution using zeolitic tuff or silica sand filler

Four geopolymers were synthesized by NaOH-activation of a mixture of kaolinite (Jordanian kaolinite or Ukrainian kaolinite) and a filler (zeolitic tuff or silica sand). X-ray powder diffraction (XRD), Fourier-transform infrared spectrometry (FTIR), and solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) were employed to monitor the extent of reaction and to characterize the phases in the geopolymer. Remaining kaolinite in all produced geopolymer specimens unambiguously indicated an incomplete reaction. The 29Si MAS-NMR spectra of the geopolymers revealed the presence of tetrahedral-SiO4 whereas the 27Al MAS-NMR spectra revealed the presence of both tetrahedral-AlO4 and octahedral-AlO6. The XRD patterns of geopolymers showed the formation of a new feldspar mineral. Replacing silica sand filler by zeolitic tuff enhanced markedly the specific surface area of the corresponding geopolymers.

Synthesis of monodispersed lithium silicate particles using the sol–gel method

Lithium silicate particles were prepared by the sol–gel process based on the Stober method using tetraethoxysilane and lithium ethoxide as starting materials; lithium dodecyl sulfate (LDS) was used as a surfactant. Lithium ion concentration of the obtained particles increased with an increase of Li/Si ratios from 1 to 4. Scanning electron microscope images showed that the obtained particles were rather monodispersed with diameter of 100–300 nm, and the particle size was not influenced by the amount of added LDS but the Li/Si ratios. Fourier-transform infrared spectra of the particles showed that the intensity of the peaks due to CO32− increased with an increase of the Li/Si ratios. X-ray diffraction patterns and 29Si magic-angle spinning-nuclear magnetic resonance spectra of the particles indicated that Q3 and Q2 units were present as amorphous state in the particles prepared with Li/Si ratios of 1 and 2, respectively. In the case of Li/Si ratios of more than 3, lithium metasilicate crystals formed, and Q1 and Q2 units were dominant.

Local structure around phosphorus and silicon in the CaO–SiO2–PO2.5 system

The local structure of phosphorus and silicon in the molten CaO–SiO2–PO2.5 slag system was investigated by magic angle spinning nuclear magnetic resonance (MAS-NMR). The 31P MAS-NMR spectra revealed that phosphorus was present primarily as the monophosphate complex ion PO43−, with a small amount of diphosphate ion also present. Their relative ratio to total phosphorus was independent of the phosphate concentration of the sample. In the case of the 29Si MAS-NMR, the mean number of the non-bridging oxygen atoms associated with tetrahedrally coordinated silicon decreased as the phosphate concentration increased at a fixed CaO/SiO2 ratio. This indicates that the nonbridging oxygen atoms around the silicon were replaced by bridging oxygen atoms around the phosphorus as the phosphate concentration in the samples increased.To elucidate the basicity dependence of the structure of slag, the relationship between the structure and optical basicity was also investigated. The relative ratio of Qn (Qn means the silicon atoms tetrahedrally bonded with “n” number of bridging oxygen atoms) strongly depends on the optical basicity. These optical basicity dependencies of the structures of phosphorus and silicon can be explained clearly by the basicity equalization concept (Duffy and Ingram, 1976) [12].

Preparation of a liquid boron‐modified polycarbosilane and its ceramic conversion to dense SiC ceramics

AbstractA boron‐modified ethynylhydridopolycarbosilane (B‐EHPCS) was successfully prepared via the hydroboration reaction of ethynylhydridopolycarbosilane (EHPCS) with 9‐borabicyclo‐[3.3.1]nonane (9‐BBN). The as‐synthesized B‐EHPCS with a branched structure was characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The structural evolution of ceramic conversion of B‐EHPCS was investigated by solid‐state NMR. The 13C magic angle spinning (MAS) NMR results indicated that the CC and CC groups of B‐EHPCS take part in the hydrosilation cross‐linking at a relatively low temperature (170°C). According to the 29Si MAS NMR analysis, the CSiH3 end groups are most reactive hydride functionality involved in the hydrosilation cross‐linking. With increasing curing temperature, the C2SiH2 and CSiH3 units are completely consumed, while C3SiH units remain even after curing at 600°C. The TGA results show the 1200°C ceramic yield of B‐EHPCS reaches 86%, which is 10% higher than that of the parent EHPCS (76%). At high temperatures, the introduction of <1 wt% boron significantly inhibits silicon carbide (SiC) crystallization. The 1800°C ceramics derived from B‐EHPCS are found to be significantly denser than that from EHPCS. Copyright © 2010 John Wiley & Sons, Ltd.