Isabel Sobrados

Influence of metakaolin purities on potassium geopolymer formulation: The existence of several networks

Geopolymer materials are obtained by the alkaline activation of aluminosilicate sources, the best of which is metakaolin. However, every raw material is different, and very few comparative studies have been done on different metakaolin sources. The aim of this work is to develop methods for the prediction of the working properties of geopolymer materials based on the reactivity of the metakaolin employed. Infrared spectroscopy showed direct relationships between the wettability, the Si/Al ratio and the kinetics of conversion of Si-O-Si bonds to Si-O-Al bonds. Moreover, it was demonstrated that the presence of impurities and the reactivity of the metakaolin can generate the formation of one or several networks. Finally, a descriptive model of the mechanism of geopolymer formation was proposed that takes into account the quality of metakaolin used.

Thermal Evolution of Alumina Prepared by the Sol-Gel Technique

The thermal evolution of an alumina gel synthesized by hydrolysis of aluminium alkoxide (sol-gel technique) was studied by thermal analysis (DTA and TGA), X-ray diffraction, FTIR and NMR spectroscopies, and specific surface area measurements. Between 400 and 900°C, γ- and δ-aluminas were formed showing aluminium vacancies preferentially located in tetrahedral sites. The atomic rearrangements produced during α-alumina formation are oriented to the progressive elimination of tetrahedral aluminium in the ultimate phase. The evolution of the specific surface area during heating is explained by changes in structure and microstructure.

Analysis by nuclear magnetic resonance and raman spectroscopies of the structure of bioactive alkaline-earth silicophosphate glasses

Abstract Structural knowledge of bioactive glass systems is necessary to understand their biological properties and improve their mechanical and clinical performance. In this work, the local structure of P and Si in bioactive glasses of the system CaO-SiO 2 -P 2 O 5 -CaF 2 with different Mg or Sr contents was determined, by means of 31 P and 29 Si magic-angle sample-spinning nuclear magnetic resonance (MASS NMR) and Raman spectroscopies. The results show that phosphorus is located in calcium-orthophosphate domains, surrounded by silicate species containing 1, 2 and 3 bridging-oxygens per SiO 4 tetrahedra. The type and relative distribution of the silicate units do not depend on the nature of the alkaline earth cations present in the system. However, the increase of RO content decreases the degree of polymerization of silicon tetrahedra, while PO 4 monomeric units remain unchanged. An interpretation of the medium-range structure of the glass is suggested.

Solutions of hybrid silica microgels as precursors of sol-gel coatings

A two-step process to synthesize a new type of organic–inorganic hybrid sol, that may be used for a variety of processes related to sol–gel chemistry, is analyzed. The first step consisted of a typical inorganic polycondensation of tetraethoxysilane (TEOS) and 3-methacryloxypropyl-trimethoxysilane (MPMS) leading to a conventional sol. In the second step a convenient amount of a solvent (isopropanol), and a vinyl co-monomer (2-hydroxyethylmethacrylate, HEMA) were added. The free-radical co-polymerization of the CC groups of MPMS and those of the co-monomer was carried out by using a suitable initiator (2,2′-azobis(isobutyronitrile), AIBN). The advance in the organic and inorganic polymerizations was followed employing different experimental techniques: FTIR, 13C NMR, 29Si NMR and viscosity measurements. The key to the new process was the amount of solvent added before the second stage such that gelation did not occur after complete consumption of CC groups. This led to a solution of hybrid microgels (a hybrid sol) that was characterized by scanning electron microscopy (SEM) and exhibited the following characteristics: a) it could be stored for prolonged periods at room temperature without gelation, b) it led to extremely fast gelation by solvent evaporation and c) it had a high wettability that allowed the production of coatings over different substrates. Besides, in this new process it is possible to add co- and ter-monomers in the second step to provide a desired functionality to the resulting coating. These concepts may be used to synthesize a variety of sols for the extended field of sol–gel processes.

The effect of an activation solution with siliceous species on the chemical reactivity and mechanical properties of geopolymers

Precursors are critical parameters in geopolymerization mechanisms because they govern the reaction kinetics as well as the working properties of the final materials. This study focuses on the effect of alkaline solutions on geopolymer formation. Toward this end, several geopolymer samples were synthesized from the same metakaolin and various alkaline solutions. First, the solutions were characterized by thermogravimetric analysis as well as DTA–TGA, infrared spectroscopy, and MAS-NMR spectrometry. The structural evolution of the formed geopolymers was investigated using infrared spectroscopy. The measurement of mechanical strength was tested by compression. The results provide evidence of relationships between the chemical composition, the extent of depolymerization of the alkaline solutions, the kinetics of Si–O–Si bond substitution by Si–O–Al and the compressive strength. For a given aluminosilicate source, the nature and the quantity of siliceous species in the activation solution appear to lead to variation in the reactivity and, consequently, to the formation of various networks that control the kinetics of formation of geopolymers and their mechanical properties.

NMR study of Li distribution in Li7−xHxLa3Zr2O12 garnets

Despite the large number of NMR studies performed on lithium conductors with a garnet-type structure, the distribution of the lithium ions in Li7La3Zr2O12 (LLZO), and their contribution to ionic conductivity are still a matter of controversy. In this work we present a magic-angle spinning (MAS) NMR study of enriched 6Li7−xHxLa3Zr2O12 (0 ≤ x ≤ 5) garnets with the aim of identifying the bands arising from the different lithium sites occupied in the garnet lattice. Taking advantage of the known sensitivity of this material to moisture and facile proton-for-lithium exchange, we have been able to alter the relative population of tetrahedral and octahedral sites (the exchange is favoured in the latter) by submitting the samples to different post-treatments to obtain samples with varying lithium content. This has allowed the identification of three different bands that we ascribe to Li in different environments within the garnet structure. In addition, variable temperature measurements have indicated the presence of dynamic exchange processes between the octahedral and tetrahedral Li sites. Protons inserted in the garnet structure were analyzed using 1H-MAS-NMR and Raman spectroscopies. 6Li-1H-CP-MAS experiments have allowed the investigation of the relative distribution of protons and lithium ions in partially exchanged samples.

Geomaterial foams: role assignment of raw materials in the network formation

The geomaterial foams studied is based on geopolymerization reactions, which is a type of geosynthesis that involves silico-aluminates. Its study during formation has however revealed a different behavior than geopolymer, suggesting the formation of various networks. This work investigates the interaction between initial compounds (metakaolin, silica fume, potassium-based solution) by a kind of mixture decomposition to ultimately understand the formation mechanism of foam. The structural evolution was determined using thermal analysis, FTIR spectroscopy and 27Al and 29Si MAS-NMR measurements. The use of different raw materials in combination with various solutions demonstrates the formation of various species in solution. The reactivity of the solution will then evolve in different ways. The Si/K ratio controlled the type of species created and, particularly, the reactivity in the mixture. From the various reactions of dissolution and polycondensation that were deduced, we could identify the composition of the four networks (K0.5SiAl0.75O6.8H8.6; K2Si2O5; KAlSi2O4,1.5H2O; and amorphous silica) constituting the foam.

Insight into Ramsdellite Li2Ti3O7 and Its Proton-Exchange Derivative

Despite being proven to be a good lithium-ion conductor 30 years ago, the crystal structure of the ramsdellite-like Li(2)Ti(3)O(7) has remained uncertain, with two potential models for locating the lithium ions in the structure. Although the model presently accepted states that both lithium and titanium occupy the octahedral sites in the framework, evidence against this model are provided by (6)Li and (7)Li MAS NMR spectroscopy. Thus, about 14% of these octahedral positions are empty since no lithium in octahedral coordination is present in the material. When Li(2)Ti(3)O(7)-ramsdellite is treated with nitric acid a complete exchange of lithium by protons is produced to yield H(2)Ti(3)O(7). The crystal structure of this proton-exchanged ramsdellite has been re-examined combining X-ray diffraction (XRD), neutron powder diffraction (NPD), and spectroscopic ((1)H and (7)Li MAS NMR) techniques. Two kinds of protons are present in this material with different acidity because of the local environments of oxygen atoms to which protons are bonded, namely, low acidic protons strongly bonded to highly charged oxygen atoms (coordinated to two Ti(4+) and a vacancy); and protons linked to low charged oxygen atoms (bonded to three Ti(4+) ions) which will display a more acidic behavior. H(2)Ti(3)O(7) absorbs water; proton mobility is enhanced by the presence of absorbed water, giving rise to a large improvement of its electrical conductivity in wet atmospheres. Thus, it seems that water molecules enter the tunnels in the structure providing a vehicle mechanism for proton diffusion.

Tautomerism of 1-(2′,4′-dinitrophenyl)-3-methyl-2-pyrazolin-5-one: theoretical calculations, solid and solution NMR studies and X-ray crystallography

To gain a better understanding of the tautomerism of 1-(2′,4′-dinitrophenyl)-3-methyl-2-pyrazolin-5-one, 2, different studies were performed. In order to simulate the gas phase, several MO calculations at the semiempirical (AM1 and PM3) and abinitio (HF/6-31G* and B3LYP/6-31G*) levels were carried out on the different tautomers of this compound and on those of the corresponding 1-phenyl derivative 4. The 1H and 13C NMR spectra were recorded in solution for compound 2. Finally, to investigate the solid state, 13C CPMAS NMR studies and the crystal structure analysis of this pyrazolinone and that of its isomer [1-(2′,4′-dinitrophenyl)-3-hydroxy-5-methylpyrazole, 3, whose chemical structure was incorrectly reported in the literature] were performed. In solution, the most abundant tautomer for both pyrazolinones, 2 and 4, depends on the solvent used. For compound 2 it was found that the CH tautomer was the most stable in the gas and solid states as opposed to its 1-phenyl analogue, which appears as the CH form in the gas phase and as NH and OH tautomers in the crystal.

An X-ray and 13C CP/MAS NMR study of C,C-linked bipyrazoles and bispyrazolylmethanes

Abstract 4,4′-Methylene-bispyrazole (1) crystallizes in the P21/n space group with Z = 4, a = 5.4199(8) A, b = 16.194(6) A, c = 8.381(5) A, β = 98.18(5)°. 4,4′-Methylene-bis (3,5-dimethylpyrazole) (2) crystallizes in the Pbca space group with Z = 8, a = 8.350(3) A, b = 16.078(3) A and c = 17.154(5) A. Finally, 3,3′-bipyrazole (4) crystallizes in the P21/n space group with Z = 2, a = 5.465(2) A, b = 5.491(3) A, c = 10.058(4) A and sol; = 92.88(2)°. The packing adopted by molecules 1 and 2 is related to the disposition of the pyrazole rings. In compound 4, all the molecules are joined to their neighbors by two double hydrogen bond systems forming zigzag chains. Using solid state 13C NMR spectroscopy, no dynamic proton transfer was observed in these crystals, and not on those of 3,5,3′,5′-tetramethyl-4,4′-bipyrazole (3) either.