Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel

X-ray amorphous precursor phases for the synthesis of spinel (MgAl₂O₄) have been prepared by grinding mixtures of gibbsite (Al(OH)₃) with brucite (Mg(OH)₂) or hydromagnesite (4MgCO₃·Mg(OH)₂·4H₂O). The mechanochemical treatment does not remove any water or carbonate, but converts some of the gibbsite octahedral Al sites into tetrahedral sites and other sites with a ²⁷Al MAS NMR resonance at about 38 ppm. The brucite-derived precursor forms spinel on heating at 850°C, by contrast with unground mixtures which show little spinel formation even at 1250°C. The hydromagnesite-derived precursor transforms at about 850°C into a mixture of spinel and hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O), the latter decomposing to spinel and MgO by 1050°C. Spinel derived from the hydromagnesite-containing precursor shows superior pressureless sintering properties at 1400–1600°C, producing a body of 97% theoretical bulk density at 1600°C. Under the same conditions, the brucite-derived spinel sintered to 72% theoretical density and showed a morphology consisting of widely disparate grain sizes.     

Gels from a double alkoxide: (BuO)2-Al-O-Si-(OEt)3

NMR study (²⁷Al,²⁹Si) of the double Si-Al precursor (BuO)₂Al-O-Si(OEt)₃ in alcoholic solution shows molecular associations resulting from acid-base interactions with O Al bondings. Gels have been prepared in the H₂O, HOⁱPr, (BuO)₂Al-O-Si(OEt)₃ ternary phase diagram for a large range of oxide concentrations. For all the compositions, the very fast hydrolysis of Al-OR groups leads to an amphoteric sol. Nevertheless, the gelation time is governed by the initial water concentration, showing that the hydrolysis of Si-OR groups is the rate-determining process for the gelation. The apparent activation energy is about 20 kcal mol^–1. The relative hardness of gels increases when increasing the precursor concentration. After drying, the oxide skeleton of the xerogel is relatively dense, but the open porosity remains high up to about 1000° C. At this temperature, samples turn into optically clear and dense glass-ceramics, formed of mullite crystallites in amorphous silica.     

Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate

The effect of the NaOH content and the presence of sodium silicate activators on the formation of crystalline phases from metakaolinite-based geopolymers were studied by X-ray powder diffraction (XRD), Rietveld quantitative XRD, solid-state MAS NMR and SEM in samples synthesized with varying NaOH contents and different curing times at 40 °C. Geopolymers activated with NaOH alone with Si/Na ratios of 4/4 or less formed the crystalline zeolite Na–A (Na₉₆Al₉₆Si₉₆O₃₈₄·216H₂O), but at ratios >4/4 nanosized crystals of another zeolite (Na₆[AlSiO₄]₆·4H₂O) were formed. The Si/Na ratio of 4/4 produces a product of greatest crystallinity. The addition of sodium silicate in addition to NaOH significantly reduces crystallite formation. The network units of all the materials containing NaOH and sodium silicate are essentially the same, namely, tetrahedral [SiO₄] units coordinated through four bridging oxygens to four aluminium atoms [denoted as Q⁴ Si(4Al) units]. A templating function of the various silicate units of the sodium silicate molecules is suggested to occur in geopolymerization, which differs from the reaction route operating when NaOH alone is used as the activator. This templating function is responsible for the suppression of crystallization and the increase in strength of the geopolymers activated with sodium silicate.     

Structural studies of geopolymers by <Superscript>29</Superscript>Si and <Superscript>27</Superscript>Al MAS-NMR

A systematic study of geopolymers by ²⁹Si and ²⁷Al MAS NMR has been carried out in an attempt to understand polymer structural details. ²⁷Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of ²⁹Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. ²⁹Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na²O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na²O content], a clear ²⁹Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na²O content (20 mol%) and varied SiO²/Al²O³ ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed ²⁹Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by ²⁹Si NMR as the signal from metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO²/Al²O³ molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.     

Micelle based synthesis of cobalt ferrite nanoparticles and its characterization using Fourier Transform Infrared Transmission Spectrometry and Thermogravimetry

Cobalt ferrite (CoFe₂O₄) nanoparticles of average size 4 nm with narrow size distribution are synthesized by reverse micelle approach. The nanoparticles are characterized using powder X-ray diffraction (XRD), Dynamic Light Scattering (DLS), Theromogravimetric analysis (TGA) and Fourier Transform Infrared Transmission Spectrometry (FTIR). Three successive transformations are observed in the thermogram that correspond to the loss of solvent and surfactant; onset of the amorphous to crystallize conversion; and isochemical transformation, i.e. migration of cations between octahedral and tetrahedral sites in the inverse spinel structure. The isochemical transformation is further confirmed by FTIR. The IR absorption bands observed at 460 and 615 cm⁻¹ in the as-prepared CoFe₂O₄ nanoparticles correspond to the ferrite skeleton of octahedral and tetrahedral sites, respectively. The peak intensity at 615 and 460 cm⁻¹ is shifted to 601 and 440 cm⁻¹, respectively upon annealing at 320 and 400 °C. These results confirm migration of cations from the octahedral to the tetrahedral sites.     

Effects of Co content and annealing temperature on the structure and optical properties of CoxMg1−xAl2O4 nanoparticles

Co_xMg_1−xAl₂O₄ (x = 0–0.8) nanoparticles were synthesized by sol–gel method, and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and ²⁷Al solid-state NMR spectroscopy were performed to study the chemical environments of cations in the nanoparticles as a function of cobalt content and annealing temperature. The results show that the crystallite size of the particles is about 20–40 nm. Besides the tetrahedral and octahedral coordinations, the second octahedrally coordinated Al³⁺ ions are observed in the samples. The inversion parameter (two times the fraction of Al³⁺ ions in tetrahedral sites) decreases with the increase of annealing temperature and cobalt content. The fraction of octahedral Mg²⁺ decreases with the increase of Co concentration. The absorption spectra indicate that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The intensity of the absorption peak corresponding to octahedral Co²⁺ ions (300–500 nm) decreases with increasing annealing temperature.     

29Si and 27Al n.m.r. study of steamed faujasites — evidence for non-framework tetrahedrally bound aluminium

The steaming of USY (ultrastable Y) zeolite at 500°C has been studied by both ²⁷Al and ²⁹Si n.m.r. The ²⁹Si n.m.r. data show a progressive removal of aluminium from the zeolite framework as a function of steaming time, and XRD confirms this observation in that the unit cell size also contracts with steaming. The ²⁷Al n.m.r. spectra of these samples contain peaks assigned to aluminium in tetrahedral (framework) and octahedral (non-framework) sites. In the first 0.5 h of steaming, the peak due to tetrahedrally bound aluminium decreased in intensity, and the peak due to octahedrally bound aluminium increased in intensity. Thereafter, the aluminium n.m.r. spectra showed no further change. This indicates that aluminium is removed from the framework but remains in a tetrahedrally bound state. Resonances due to tetrahedrally and octahedrally bound aluminium respond differently to sample treatment. Hydration is most effective in narrowing the tetrahedral resonance, while acetylacetone treatment is most effective in narrowing the octahedral resonance.     

Influence of calcination temperature of kaolin on the structure and properties of final geopolymer

In this paper, the structure of two types of metakaolins from kaolin calcined at 800 and 900 °C, respectively, and the obtained geopolymer were systematically characterized. It was found that calcination temperature had little effect on the environment of silicon atoms but had great effect on that of aluminum ones. ²⁷Al NMR analysis showed that tetrahedral aluminums in the metakaolin from kaolin calcined at 800 and 900 °C were in different environment, of the type AlQ₃(3Si) and AlQ₄(4Si), respectively, leading to different environment of aluminum atoms in the resulted geopolymer. Aluminum atoms in the geopolymer based on metakaolin from kaolin calcined at 800 °C were in the types of tetrahedral and octahedral, and silicon atoms were in the types of tetrahedral Q₄(3Al) together with a small amount of Q₄(0Al). However, geopolymer based on metakaolin from kaolin calcined at 900 °C consisted of Q₄(4Si) unit aluminum and Q₄(3Al) unit silicon. The results revealed that the calcination temperature had a great effect on environment of the aluminum atoms of the metakaolin, thus led to the different structure and properties including mechanical strength and thermal conductivity of the post obtained geopolymer.     

27Al and29Si magic angle spinning nuclear magnetic resonance spectroscopy of Al-substituted tobermorites

Solid-state²⁷Al and²⁹Si NMR spectroscopy with magic angle spinning (MAS) of samples was used to study several 1.13 nm tobermorites, most of which were deliberately substituted with aluminium.²⁷Al MASNMR clearly showed that aluminium is tetrahedrally co-ordinated in tobermorite structures. In addition two different aluminium environments resonating at 57 and 64 ppm from [Al(H₂O)₆]³⁺ were detected.²⁹Si MASNMR of pure, anomalous tobermorites showed resonances at –85.7 and –95.7 ppm from tetramethylsilane representing chain middle groups (Q²) and branching sites (Q₃), respectively. Anomalous Al-substituted tobermorites, on the other hand, showed two to four resonances representing different silicon environments. One Al-substituted tobermorite showed two resonances at –84.6 and –91.5 ppm which were assigned to Q²(0 Al) and Q³ (1 Al), respectively. In the above tobermorite aluminium appeared to have substituted into branching sites only. Two other Al-substituted tobermorites, however, showed four distinct resonances at –82.0, –85.2, –92.0 and –96.0 and these were assigned to Q² (1 Al), Q² (0 Al), Q³ (1 Al) and Q³ (0 Al), respectively. Thus these two tobermorites showed substitution of aluminium in the chain middle groups as well as branching sites. Another Al-substituted tobermorite which showed a normal thermal behaviour exhibited, as expected, only Q²(0 Al) and Q² (1 Al) sites resonating at –84.7 and –80.2 ppm, respectively. No Q³ sites were detected because few or no branching sites are present in this normal tobermorite. The results reported here clearly demonstrate the usefulness of solid-state²⁷Al and²⁹Si MASNMR spectroscopy for the investigation of short-range order in alumino-silicate materials.     

XPS study of basic aluminum sulphate and basic aluminium nitrate

Basic aluminium sulphate and nitrate crystals were prepared by forced hydrolysis of aluminium salt solution followed by precipitation with a sulphate solution or by evaporation for the basic aluminium nitrate. X-ray Photoelectron Spectroscopy (XPS) confirms the chemical composition determined by ICP-AES in earlier work. High resolution XPS scans of the individual elements allow the identification of both the central ^IVAlO₄ group and the 12 aluminium octahedra in the [^IVAlO₄Al^VI(OH)₂₄(H₂O)₁₂] building unit by two Al 2p transitions with binding energies of 73.7 and 74.2 eV in both the basic aluminium sulphate and nitrate. Four different types of oxygen atoms were identified in the basic aluminium sulphate associated with the central AlO₄, OH, H₂O and SO₄ groups in the crystal structure with transitions at 529.4, 530.1, 530.7 and 531.8 eV, respectively.     

Hydrolytic stability of sodium silicate gels in the presence of aluminum

Polycondensation in alkali silicate solutions comprises a fundamental process of the geopolymerization technology. Previous works had shown that the hydrolytic stability of sodium silicate gels depends on the SiO₂/Na₂O ratio. Sodium silicate gels totally insoluble in water can be produced at SiO₂/Na₂O molar ratios higher than 4.4. This article aims at elucidating the effect of tetra-coordinated aluminum addition on the hydrolytic stability of sodium silicate gels. According to the results, the aluminum addition stabilizes the sodium silicate gels in an aqueous environment. A sodium silicate gel with SiO₂/Na₂O molar ratio 3.48, which is totally soluble in deionized water at ambient temperature, can be transformed to insoluble sodium hydroaluminosilicates with the addition of tetrahedral aluminum at Al/Si molar ratios higher than 0.08. In addition, this article studies the structure of prepared sodium hydroaluminosilicates and draws very useful conclusions for the geopolymerization technology.     

Synthesis of sodium and potassium aluminogermanate inorganic polymers

Synthesis of aluminogermanate inorganic polymers containing sodium as the charge-balancing ion was attempted by reacting NaAlO₂ solution with GeO₂ in an adaption of a sol-gel synthesis for aluminosilicate inorganic polymers. XRD and ²⁷Al MAS NMR suggested that only a small degree of reaction had occurred, based on the presence of unreacted GeO₂ and a trace of Na₃HGe₇O₁₆·4H₂O, with only a small amount of the tetrahedral aluminium characteristic of a true inorganic polymer. The addition of KOH markedly enhanced the reaction, producing a (Na,K) product with properties characteristic of a true inorganic polymer (an amorphous X-ray powder pattern and predominantly tetrahedral aluminium). An attempt to synthesise the potassium end-member aluminogermanate compound by replacement of the NaAlO₂ in the above synthesis with KAlO₂ produced only crystalline K(AlGeO₄)H₂O and K₃HGe₇O₁₆·4H₂O containing solely tetrahedral aluminium. Attempts to extend these syntheses to the gallium analogues of these aluminogermanate compounds were unsuccessful, producing only the crystalline products K(GaGeO₄)₆·7H₂O and K₃HGe₇O₁₆·4H₂O. Thus, the most successful sol-gel synthesis of a germanate compound with the properties of an inorganic polymer was of an aluminogermanate containing Na⁺ and K⁺ as the exchangeable cations.     

The cause of surface tension increase with temperature in multicomponent aluminosilicates derived from coal-ash slags

An explanation is proposed for the increase of surface tension with temperature in multicomponent aluminosilicate systems such as those derived from coal-ash slags. Two major factors are considered: (1) depolymerization of aluminosilicates caused by rearrangements of intermediate structures in the surface layers, and (2) the increase in surface entropy caused by evaporation of some ash slag components. Electron spectroscopy for chemical analysis spectra were recorded for oxygen 1s photoelectrons on quenched bulk slags and on sessile drops to gain insight into the depolymerization of coal-ash slags with temperature. The tests performed on quenched bulk slags indicated replacement of bridging oxygen [Si-O] with non-bridging oxygen atoms [Si-O^–] as a function of increasing temperature. Mössbauer spectra showed an increase in ferrous iron from 4% to 12% of total iron as temperature rose from 1400 °C to 1500 °C. The increase in non-bridging oxygens resulted from the reduction of tetrahedrally coordinated Fe³⁺ to octahedrally coordinated Fe²⁺. Also, the intensity of the non-bridging oxygen 1s photoelectron peak was higher when detected on the surface of a sessile drop than when detected from the bulk of the drop.     

Effect of ultrastructure on crystallization of mullite

The effect of pre-mullite structure on crystallization of mullite from aluminosilicate gels was investigated. The aluminosilicate gels were prepared by hydrolytic poly condensation of silicon alkoxide aluminium with alkoxide, with colloidal alumina and with aluminium nitrate. The structural differences in these materials were characterized by²⁷Al and²⁹Si nuclear magnetic resonance and X-ray diffraction. Their thermal behaviour was monitored by differential thermal analysis. These investigations show that the crystallization process in this system is fundamentally affected by the nature of the ultrastructure. A spontaneous crystallization of mullite at 980°C is promoted by a high degree of homogeneity and the network connectivity between silicon and aluminium. A method that provides condensation of such an ultrastructure is also given.     

Role of bismuth and titanium in Na2O-Bi2O3-TiO2-P2O5 glasses and a model of structural units

0.60Na₂O-0.40P₂O₅ and (0.55−z)Na₂O-0.05Bi₂O₃-zTiO₂-0.40P₂O₅ glasses (0≤z≤0.15) were prepared by melting at 1000°C mixtures of Na₂CO₃, Bi₂O₃, TiO₂ and (NH₄)₂HPO₄. Differential Scanning Calorimetry (DSC) measurements give the variation of glass transition temperature (T_g) from 269°C (for 0.60Na₂O-0.40P₂O₅) to 440°C (for z=0.15). The density measurements increases from 2.25 to 3.01 g/cm³. FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO₃)_∞ chains for 0.60Na₂O-0.40P₂O₅ to P₂O₇⁴⁻ groups in the glasses containing Bi₂O₃ or both Bi₂O₃ and TiO₂. When bismuth oxide and titania are added to sodium phosphate glass, phosphate chains are depolymerized by the incorporation of distorted Bi(6) and Ti(6) units through POBi and POTi bonds. Bi₂O₃ and TiO₂ are assumed to be present as six co-ordinated octahedral [BiO_6/2]³⁻and [TiO_6/2]²⁻ units again with shared corners. This is accompanied by the simultaneous conversion of [POO_3/2] into [PO_4/2]⁺ units which achieves charge neutrality in the glasses.     

Aluminium oxide grafted on silica gel surface: Study of the thermal stability, structure and surface acidity

The synthesis of aluminium oxide grafted on silica gel surface was carried out by the reaction of a suitable aluminium precursor with the surface hydroxyl groups, SiOH, of the oxide support in non-aqueous solvent. The advantage of this preparation method, compared to the conventional ones (impregnation or precipitation and calcination), is that the oxide is highly dispersed on the surface (monolayer or submonolayer). The resulting material, SiO₂/Al₂O₃, was heat treated at temperature range of 423 to 1573 K. The Al/Si atomic ratios, determined by X-ray photoelectron spectroscopy (XPS), showed that aluminium is less mobile up to heat treatment of 1173 K and above this temperature part of it diffuses to the interior of the matrix. ²⁷Al solid state nuclear magnetic resonance spectroscopy (NMR) showed two different environments, tetrahedral and octahedral for sample calcined up to 1023 K and above this temperature, aluminium in a trigonal bipyramidal environment was also detected. Pyridine adsorbed on a Lewis acid sites were observed for samples calcined up to 1023 K, and above this temperature they were not detected.     

Synthesis and crystal structure of the inter-alkali metal iodide Cs2Li3I5

Cs₂Li₃I₅ was obtained from the reaction of CsI, Li and I₂ in a sealed tantalum tube and colorless single crystals grown from the melt by slow cooling. This first inter-alkali metal iodide crystallizes in the monoclinic crystal system (C2/m, Z= 2) with a= 1666.8(6), b= 472.1(1), c= 1098.7(4) pm, β= 115.73(3)°. As the result of a x-ray crystallographic structure determination (R= 0.069, R_W= 0.066), Li⁺ is surrounded tetrahedrally (two thirds) and octahedrally (one third) by iodide. Cornersharing double chains of [LiI₄] tetrahedra are connected with edge-sharing chains of [LiI₆] octahedra to a layer of the composition [Li₃I₅]. Bicapped trigonal prisms [CsI₃] share faces and edges so that a network of the composition [Cs₂I₅] is formed which contains the necessary tetrahedral and octahedral holes for Li⁺.     

About copper valence and superconductivity

We describe the copper valence in superconductors based on our arguments on La₂O₃ crystal structure. In order to explain the two oxygen sites in La₂O₃, it has been supposed that O _II ¹⁻ ions occupy the tetrahedral site while O _II ²⁻ occupy the octahedral site. Oxygen ions in tetrahedral site form a covalent bond with lanthanum, which can be written [LaO]¹⁺. Considering the chemical and crystallographic properties of Tl and Bi compounds, [TlO]¹⁺ and [BiO]¹⁺ groups appear as defined by strong covalent bonds between Bi or Tl and O. This leads to the supposition that the four oxygen ions which coordinate to copper in Tl and Bi copper oxide-based superconductors are O _II ¹⁻ ions. The bivalent character of copper is then obtained through covalent bonds. For La_2−x Sr _x O₄ compounds, copper is supposed to have valence three, but spectroscopic studies point out bivalent copper. We show that krypton shell of Sr is responsible for the lack of one unit of valence as expected from krypton compounds for example KrF₂.     

Multinuclear magnetic resonance characterization of various aluminosilicate gels in the presence of tetraalkylammonium ions

Four gels of initial compositions 8 TAABr:Al₂O₃:90 SiO₂: 1,000 H₂O (gel I), 8 TAAOH:Al₂O₃:90 SiO₂:1,000 H₂O (gel II), 2 TAAOH:Al₂O₃:200 H₂O (gel III), and 8 TAABr:4.5 Na₂O:Al₂O₃:90 SiO₂:1,000 H₂O (gel IV) were prepared and dried at 363 K (TAA = tetramethyl, tetraethyl, tetrapropyl, and tetrabutylammonium). The dried gels were characterized by ¹³C, ²⁷Al, and ²⁹Si n.m.r. and the calcined gels by ¹²⁹Xe n.m.r. of adsorbed xenon. The Al atoms are very well dispersed in a tetrahedral form of the gel structure in gel II, rather well dispersed in gel IV, and partially dispersed in gel I. In gels I and IV, large and medium size cavities contained aggregated (crystalline) TAABr and less aggregated species. Only gel II is able to disperse the TAA ions in monomeric forms where they are countercations to gel negative charges. The monomeric TAA species could play a direct role in zeolite synthesis.     

Chemical bonding of ion-exchange type sites in spinel-type manganese oxides Li1.33Mn1.67O4

The electronic structure and chemical bonding nature of spinel-type Li_1.33Mn_1.67O₄ system was calculated by a discrete-variational (DV)-Xα clusters method. In order to elucidate the reason for the selective exchange of Li⁺ ions, the bonding natures between tetrahedral and octahedral Li sites of the Li_1.33Mn_1.67O₄ were compared. Li in the manganese oxides is highly ionized in both sites, but the net charge of Li was greater for tetrahedral sites than octahedral. These calculations suggest that the tetrahedral sites have higher Li⁺/H⁺ exchangeability than the octahedral sites, and are preferable for the selective adsorption for Li⁺ ions.