Search

The CO 2 adsorption and CO 2 adsorption kinetics were evaluated by thermogravimetry on two Prussian blue analogues, K 2 x /3 Cu II [Fe II x Fe III 1- x (CN) 6 ] 2/3, with nominally K-free x  = 0.0 and K-rich x  = 1.0. Differential isosteric heats of adsorption were determined from adsorption isotherms using the Clausius-Clapeyron equation and integral values by differential scanning calorimetry. The average differential heats of CO 2 adsorption are 28 kJ/mol for x  = 0.0 and 33 kJ/mol for x  = 1.0. Both compositions show small maxima in differential heat at ∼1 mmol/g. The integral adsorption heats were determined to be 26 kJ/mol for both x  = 0.0 and x  = 1.0. The kinetic CO 2 adsorption/desorption curves can be modeled by a double exponential function describing two parallel processes with different rate constants. The activation energies for CO 2 adsorption on x  = 0.0 were 6 (1) kJ/mol for the faster component and 16 (1) kJ/mol for the slower one, while the corresponding values for x  = 1.0 were 9 (1) kJ/mol and 7 (1) kJ/mol, respectively. The maximum CO 2 uptake for both compositions was found to be ∼4.5 mmol/g, 19.8 wt %, at 1 bar and 273 K. The materials exhibited fast adsorption kinetics and stable cyclic performance at room temperature. The kinetics were slower for the samples with x  = 1.0 than for x  = 0.0 which may be attributed to interactions between CO 2 molecules and K + ions. [ABSTRACT FROM AUTHOR]

Abstract: We report physical properties of density, glass transition temperature, Vickers hardness and refractive index for 15 La–Al–Si–O glasses within the compositional region 10–28mol% La2O3, 11–30% Al2O3 and 45–78% SiO2. The glass transition temperature varies only slightly between 871 and 883°C. Both the density (3.25–4.33gcm−3) and refractive index (1.59–1.70) display similar compositional dependencies and are primarily dictated by the La2O3 content. The hardness (6.4–9.0 GPa) increases with decreasing SiO2 content and to a minor extent also with the amount of La2O3 of the glass. The physical properties, in particular the hardness, are discussed with reference to glass structure. The similarity in the observed properties between the present oxide-based glasses and those of analogous rare-earth (RE) element-containing RE–Si–(Al)–O–N oxynitride glasses is highlighted and discussed in relation to the apparent, but hitherto largely neglected, role of RE-ions to primarily control many enhanced physical properties of oxynitride glasses. [Copyright &y& Elsevier]

Abstract: Mixed alkali earth element containing high nitrogen content oxynitride glasses (Ca1− xAEx )1.2(1)SiO1.9(1)N0.86(6), with AE =Mg, Sr, Ba, x ≤0.30 for Mg and x ≤0.46 for Sr and Ba, and nominally constant (Ca/AE):Si:O:N ratios were prepared in order to investigate the compositional dependencies of physical properties on alkali earth element composition. The glasses were prepared by melting mixtures of AEH2, CaH2, SiO2 and Si3N4 powders in nitrogen atmosphere at 1600–1700°C and characterized by X-ray powder diffraction and scanning and transmission electron microscopy. Cation and anion glass compositions were determined by respectively energy dispersive X-ray analysis and combustion analysis. The determined physical properties were density, glass transition temperature, Vickers hardness, and refractive index. The physical properties were found to vary linearly with the degree of substitution of Ca by the AE elements. The density of the glasses increases substantially upon substitution by Sr and Ba, up to 3.99g/cm3. Glass transition temperatures are found to be higher for Mg and Sr substituted glasses, ca. 900°C, in comparison with Ba substituted glasses, ca. 850°C. The hardness increases upon substitution by Mg, up to 12.2 GPa at x =0.46, and decreases upon substitution by Sr and Ba. The refractive index increases upon substitution by Sr and Ba, up to 1.97 for Ba at x =0.46, and decreases upon substitution by Mg. The transparency of the glasses was found to increase upon increasing substitution by Mg and completely transparent glasses were obtained for x =0.24. [Copyright &y& Elsevier]

Abstract: The present work explores the glass-forming region in the Ca–Si–O–N system, particularly in the N-rich part of the system. Bulk oxynitride glasses in the Ca–Si–O–N system were prepared by melting mixtures of CaH2, SiO2 and Si3N4 powders in a nitrogen atmosphere at 1500–1650°C. The glasses were characterized by X-ray powder diffraction, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. Glass compositions were calculated from analyses obtained by energy dispersive X-ray (EDX) spectroscopy and combustion analysis, for cation and anion compositions, respectively. The glasses were found to be X-ray amorphous and gray to black in color. The glasses retain up to 58e/o of nitrogen and 42e/o of calcium. The glass formation depends on reaction kinetics and the precursor used. A strong exothermic reaction is observed at 850–1000°C, leading to formation of amorphous and crystalline oxynitride phases that melt at high temperatures upon further heating. Glass transition temperatures (T g) were observed to vary between 798°C and 1050°C, and crystallization occurs typically 130°C above the glass transition temperature. The glass densities vary between 2.79g/cm3 and 3.25g/cm3. [Copyright &y& Elsevier]

Abstract: Calcium aluminate is the main constituent in calcium aluminate cements, used in a wide range of applications in construction and mining industries and recently also as biomedical implant. In applications that demand very precise reaction features, such as the biomedical ones, the phase purity is of very high importance. In this paper the formation of CaAl2O4 from CaCO3–Al2O3 powder mixtures has been studied, varying holding times between 1 and 40h and temperatures between 1300 and 1500°C. Phase formation was studied in samples both quenched from the holding temperatures and in samples slowly cooled. Samples were characterized by X-ray powder diffraction (XRPD), using Guinier-Hägg film data and the Rietveld method, and scanning (SEM) and transmission (TEM) electron microscopy. Samples for TEM with very high site accuracy were produced using focused ion beam microscopy. In addition to CA (CaAl2O4) the samples contained major amounts of CA2 (CaAl4O7), C12A7 (Ca12Al14O33) and minor amounts of un-reacted A (Al2O3). Trace amounts of C3A (Ca3Al2O6) were observed only for samples heated to 1500°C. The amount of the Ca-rich phase C12A7 was found to decrease with time as it reacts with A and, to a less degree, CA2 to form CA. In agreement with previous studies the amount of CA2 formed decreases comparatively slowly with time. Its un-reactivity is due to that it is concentrated in isolated porous regions of sizes up to 100μm. The formation of the Ca aluminates is found to be in response to local equilibriums within small inhomogeneous regions, with no specific phase acting as an intermediate phase. Samples quenched from 1500°C were found to contain smaller amounts of poorly crystallized phases. A reaction between C and A takes place already at 900°C, forming a meta-stable orthorhombic modification of CA. The orthorhombic unit cell with a =8.732(2)Å, b =8.078(2)Å, c =√3·a =15.124(4)Å was verified by electron diffraction, revealing frequent twinning and disorder of the crystallites. [Copyright &y& Elsevier]

Abstract: In a series of Ba-based oxonitrido-silicate S-phases (Ba2Al x Si12−x N16−x O2+x ) spanning a compositional range up to x ≈3, we examine the incorporation of Al and O by 29Si and 27Al magic-angle spinning (MAS) solid state nuclear magnetic resonance (NMR) and 27Al triple-quantum MAS (3QMAS). The 3QMAS spectra reveal 27Al signals from two distinct structural environments, assigned to AlN4 or AlN3O tetrahedra, respectively, and with their relative amounts depending on the S-phase substitution parameter x. 29Si NMR show variable fractions of SiN4 and SiN3O environments. The NMR results accord overall with a structural substitution model for which O enters at one crystallographic position (occupied according to N4−x O x ), in conjunction with a random Al for Si substitution at two distinct crystallographic positions. This leads to S-phase frameworks built from SiN4, SiN3O, AlN4 and AlN3O tetrahedra. [Copyright &y& Elsevier]

Abstract: The nitrogen-rich part of the glass forming region in the La–Si–O–N system has been the subject of a comprehensive study. Glasses were prepared by heating powder mixtures of La metal, Si3N4 and SiO2 in a nitrogen atmosphere at 1650–1800°C. By this new synthesis route, glasses containing up to 68e/o of N and 62e/o of La were prepared, showing that the glass forming region is significantly larger than previously reported. The glasses were characterized by elemental analysis, differential thermal analysis, X-ray powder diffraction, and scanning electron microscopy. They were found to be X-ray amorphous and homogenous, with the majority of them containing small amounts of crystalline La silicides and elemental Si. Glass transition temperatures (T g) were found to vary between 900 and 1100°C and crystallization to occur typically 120°C above T g. The forming of the glasses was investigated by characterizing samples taken out at various steps of the heating cycle. The results indicate that the glass formation is strongly dependent on reaction kinetics. A strong exothermal reaction occurs at temperatures 900–1100°C, leading to the formation of assemblies of amorphous and crystalline (oxy)nitride phases that melt upon further heating at 1650–1800°C. [Copyright &y& Elsevier]

Abstract: Vickers hardness and refractive index have been determined for a series of La-Si-O-N oxy-nitride glasses containing 30–62e/o of La and 9–68e/o of N. The hardness varies between 7.7 and 11.5GPa at a load of 1kg and is dependent of the N content, while the La content does not influence it significantly. The increase of the hardness with N content is, contrary to reported findings for other oxy-nitride glasses, found not to be linear over the whole compositional range. The refractive index varies between 1.8 and 2.3 and increases non-linearly with increasing N content. The compositional variations of hardness and refractive index are compared with previously published results. [Copyright &y& Elsevier]

Studies of compound formation in the system Mn&z.sbnd;Ta&z.sbnd;O showed the existence of six new meta-stable phases. The compound MnTa2O6 was found to exhibit, in addition to the stable tantalite structure, modifications related to the tri-rutile, wolframite, and wodginite structures. These were characterized by powder X-ray diffraction and, for the wodginite modification, also single crystal X-ray diffraction. Three new phases, RI-Mn4Ta2O9, RII-Mn4Ta2O9 and R-Mn11Ta4O21 were found with rhombohedral symmetries and corundum related structures. The structures of RI-Mn4Ta2O9 and RII-Mn4Ta2O9 were determined and refined, respectively, from X-ray powder and single-crystal diffraction data. Electron diffraction studies indicate the existence of a chemical inter-growth between corundum and rock-salt type structures. [ABSTRACT FROM AUTHOR]

Abstract: The viscosity of three high-nitrogen content Ca–Si–O–N glasses, with 30–58 e/o N and 36–39 e/o Ca, was determined by micro-indentation. The measurements were made using an automated set-up, designed and built in-house, capable of measurements up to 1200°C with applied loads of 0.01–15N. The viscosity increases significantly with the nitrogen content and reaches viscosity values close to reported values for rare-earth silica oxynitride glasses. The glass transition temperatures range between 878 and 995°C and are in very good agreement with values measured by differential thermal analysis. The apparent viscosity activation energies are very high, ranging from 855 to 2170kJ/mol. The glasses can accordingly be classified as being both very refractory and very fragile. Implications of the viscosity values and mechanical properties of the glasses for their structures are discussed. [Copyright &y& Elsevier]

Abstract: Hierachically porous monoliths of ZSM-5 and Y zeolites have been prepared by pulsed current processing (PCP). The densification behaviour and structural characteristics during rapid thermal treatment of the zeolites have been determined and related to the influence of the Si:Al ratio on the thermal stability of the zeolites. Monoliths of macroscopic shape can be prepared with an insignificant loss of surface area and micropore volume when the PCP-treatment was performed at temperatures below a critical maximum PCP temperature (T PCP). Full-profile fittings of the powder X-ray diffraction patterns showed that the lattice strain of zeolite Y increases rapidly above the critical T PCP while the ZSM-5 zeolites undergo a phase transition from orthorhombic to monoclinic. The use of a novel ceramic processing route for the production of the zeolite monoliths that do not significantly influence the structural characteristics and surface area of the starting materials has a potential to be of importance in catalysis and gas separation applications. [ABSTRACT FROM AUTHOR]

Abstract: An aluminosilicogermanate with a DFT zeotype framework (denoted as SU-57) was synthesized for the first time by ethanol-assisted hydrothermal synthesis at 160 and 170°C. The compound was characterized by single crystal and powder X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetry (TG) and solid state nuclear magnetic resonance (NMR) spectroscopy. SU-57 crystallizes in space group P42/n with estimated a ≈10.38–10.46Å, c ≈8.88–8.91Å, V ≈957–975Å3. It has variable Al–Si–Ge composition with an approximate formula |C2H10N2| [Al(Si x Ge1− x )O4]2 (x ≈0.3–0.9), which results in a super-structure originated from different cation occupancies of the two unique tetrahedral (T) sites. Single crystal X-ray structure refinements, together with results from X-ray powder diffraction (XRPD) and EDS analysis, showed that (i) the AlO4 and (Si,Ge)O4 tetrahedra are only partially ordered over the DFT framework and do not follow a strict alternating manner. (ii) Al resides predominantly on one T site and Si and Ge predominantly on the other. (iii) The Al cation concentration (Al/(Al+Si+Ge)) is nearly constant and slightly less than 50at%, while the Si and Ge cation concentrations vary over a large range. (iv) Al and Ge occupy both T sites. The cation disorder was confirmed by 27Al and 29Si solid state NMR. TG analysis and in situ XRPD showed that SU-57 was stable up to 375°C in N2 atmosphere. [Copyright &y& Elsevier]

From a suite of advanced magic-angle spinning (MAS) NMR spectroscopy and powder X-ray diffraction (PXRD) experiments, we present a comprehensive structural analysis of pyrophosphate-bearing calcium phosphate cements that are investigated for bone-inductive biomedical implants. The cements consist mainly of poorly ordered monetite (CaHPO 4), along with minor Ca orthophosphate phases, and two distinct pyrophosphate constituents: crystalline β -Ca 2 P 2 O 7 and amorphous calcium pyrophosphate (ACPP), the latter involving one water-bearing portion and another anhydrous component. Independent 2D MAS NMR experiments evidenced close contacts between the amorphous pyrophosphates and the monetite phase, where ACPP is proposed to form a thin layer coating the monetite particles. Heteronuclear 1H–31P and homonuclear 31P–31P correlation NMR experimentation enabled us to detect, identify, and quantify even minor cement constituents (≲ 2 mol%) that could not be ascertained by the Rietveld method. Quantitative phase analyses of the cements, as determined independently by 31P NMR and PXRD, are contrasted and discussed. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen-rich Ca-α-Sialon (Ca x Si12−2x Al2x N16 with x =0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si3N4, AlN and CaH2 powders in a hot press at 1800°C using a pressure of 35MPa and a holding time of 4h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x =1.6 was prepared in the temperature range 600–1800°C using a pressure of 35MPa and a holding time of 2h. The α-Sialon phase was first observed at 1400°C but the α-Si3N4 and AlN phases were still present at 1700°C. Phase pure Ca-α-Sialon ceramics could not be obtained until the sintering temperature reached 1800°C. The phase pure nitrogen-rich Ca-α-Sialon exhibited no phase transformation in the temperature range 1400–1600°C. In general, mixed α/β-Sialon showed better oxidation resistance than pure α-Sialon in the low temperature range (1250–1325°C), while α-Sialons with compositions located at α/β-Sialon border-line showed significant weight gains over the entire temperature range tested (1250–1400°C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies. [Copyright &y& Elsevier]

Abstract: We investigate the local structures of oxynitride La–Si–(Al)–O–N glasses by 29Si and 27Al magic-angle spinning (MAS) solid state Nuclear Magnetic Resonance (NMR). The glasses studied span an unprecedented range of compositions, up to >50at.% lanthanum and nitrogen out of the cations and anions, respectively, and achievable through a recently introduced glass preparation route. Transmission as well as scanning electron microscopy verified homogeneous samples over length-scales down to 20nm. As the nitrogen content of the glasses increased, 29Si NMR evidenced a progressive formation of Si–N bonds, with SiO2N2 tetrahedra dominating in the nitrogen-rich glass networks. In the oxygen-rich end of the series, aluminum is predominantly present in tetrahedral coordination as AlO4, whereas the glasses with highest nitrogen contents have a major fraction of AlO3N structural units. Trends in isotropic 29Si and 27Al chemical shifts and 27Al quadrupolar couplings are compared with results of La–Si–Al–O glasses and are discussed in relation to the glass compositions and their proposed structures. [Copyright &y& Elsevier]