[Show abstract][Hide abstract] ABSTRACT: The pore–solid structure of selected high‐compressive‐strength metakaolin geopolymers has been characterized to facilitate quantitative prediction of their physical properties. Geopolymers are multiphase materials with pore widths ranging from subnanometre to several tenths of a millimetre. Ultramicrotoming of resin‐embedded grains was found to be an effective method for producing electron‐transparent sections. Scanning and transmission electron microscopy showed the existence of a bi‐level pore system and heterogeneity of the pore morphology. Ultra‐small‐angle neutron scattering, of sufficiently thin specimens, was found to be useful in detecting the length scales on which statistically significant structural changes occur as the geopolymer chemical composition is varied. Contrast variation experiments confirmed that the small‐angle neutron scattering from an Si:Al:Na = 2.5:1:1.2 geopolymer before and after dehydration was dominated by scattering from pores. These experiments suggested the presence of closed (under current experimental conditions) pores in the dehydrated geopolymer. A three‐phase analysis was developed for this system, and the scattering of the solid, open pore and closed pore phases was determined as a function of scattering length density ρ. The scattering from all three phases had the same q dependence over the range of likely ρ within the uncertainties. A lower limit of 4.21 (6) × 1010 cm−2 was determined for the scattering length density ρw of the nondehydrated geopolymer by assuming the pore fluid to be water. This scattering length density is significantly higher than the expected value of approximately 3.4 × 1010 cm−2. Small‐angle neutron scattering from the dehydrated and nondehydrated Si:Al:Na = 2.5:1:1.2 geopolymer showed that dehydration does not cause a severe change in morphology of the nanoporosity on the length scale probed.
Full-text · Article · Aug 2011 · Journal of Applied Crystallography
[Show abstract][Hide abstract] ABSTRACT: The influence of concentration and added chloride salts on the solution speciation of zirconyl chloride solutions, and the precipitate formed upon addition of aqueous ammonia, has been investigated. Crystalline zirconium oxychloride octahydrate samples available on an industrial scale were investigated using ICP-OES, XRD and SEM. The samples had a remarkably consistent level of the trace elements and LOI and contained approximately 2 wt.% hafnium. Zirconyl chloride solutions at industrially relevant concentrations of 0.81 and 1.62 M were studied by small angle X-ray scattering, and the particle radii were found to be unchanged within experimental error. Yttrium-zirconium mixed solutions relevant to the Solid Oxide Fuel Cell market (containing 3, 5, 8 and 10 mol% yttrium) were also investigated, and it was found that the added yttrium did not significantly change the particle radii or particle-particle distances. Solutions at the same concentrations were then precipitated using a continuous double jet precipitation apparatus with aqueous ammonia as the base. Using DLS it was found that the zirconyl chloride solution at higher concentration yielded a larger precipitated particle size (1.0 ± 0.1 μm, 4.2 ± 0.1 μm). The yttrium-zirconium mixed solutions were found to give a consistent increase in particle size with increasing yttrium levels (2.0 ± 0.1 μm, 3.7 ± 0.2 μm, 4.5 ± 0.1 μm and 4.9 ± 0.2 μm). To investigate if the growth effect was most influenced by the cations or the increasing chloride concentrations, sample solutions containing mixtures of caesium chloride/zirconyl chloride and calcium chloride/zirconyl chloride with the same concentration of added chloride anions as that of the 8 mol% yttrium-zirconium sample were precipitated. The increased particle size was found to be most dependent on the type of cation and did not appear to be as significantly dependent on the concentration of chloride ions.
No preview · Article · Jan 2009 · Powder Technology
[Show abstract][Hide abstract] ABSTRACT: A sample of MCM-41 at 77 K was shown to reversibly absorb 1.6 excess wt% of hydrogen at ∼ 3.5 MPa. Though the excess hydrogen adsorption peaked, the total gravimetric wt% of hydrogen adsorption continued to increase and reached a value of 2.7 wt% by ∼ 4.5 MPa. The correlation function has been used here for the first time in an attempt to determine the average pore size, distance between the pores and an estimate of the wall thickness by modelling the entire small angle X-ray scattering (SAXS) curve of the MCM-41 sample. At this stage of the model development, only a lower limit on the average pore size can be determined.
No preview · Article · Dec 2005 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: In this study we report on the stability of microemulsion templated mesoporous silica materials as a function of exposure time to 0.01 M phosphate buffered saline (PBS) at pH 7.4, determined via nitrogen sorption. The sorption isotherms were seen to have retained their characteristic shape even size distributions, BET surface areas and total pore volumes, indicates that the structural integrity of the materials has largely been maintained. Preliminary analysis of the small angle x-ray scattering (SAXS) data of unexposed cell size was seen to increase as the template size increased, a trend that is seen to reflect the polydispersity present in the template as previously determined via small angle neutron scattering (SANS).
No preview · Article · Jan 2005 · Studies in surface science and catalysis
[Show abstract][Hide abstract] ABSTRACT: This paper presents a preliminary structural and interfacial study of the iron chalcogenide glass [i.e., Fe(x)(Ge(28)Sb(12)Se(60))(100-x)] ion-selective electrode (ISE) using small angle neutron scattering (SANS) and electrochemical impedance spectroscopy (EIS). SANS detected variations in the neutron scattering as a function of iron content in the chalcogenide glass. Furthermore, a change in the chalcogenide glass structure was observed at elevated iron dopant levels. Conversely, EIS was used to show that the iron chalcogenide membrane comprises various time constants, and the interfacial charge transfer reaction depends on the membrane iron content. Equivalent circuit modeling revealed that the charge transfer resistance decreases at elevated iron levels, and this may be related to the presence of iron defects in the glass. It is proposed that the iron chalcogenide membrane comprises an iron nanostructural network embedded in the amorphous matrix, and this directly influences the electrical conductivity and concomitant electrochemical reactivity of the glass.
[Show abstract][Hide abstract] ABSTRACT: A boehmite-derived gamma-alumina (γ-Al2O3) system was studied using various complementary techniques to examine surface area and pore size, the amount of hydrogen-containing species, the nature of hydrogen bonding environments, and the location of these species. Using small-angle X-ray scattering, the material examined was shown to have a significantly higher surface area than that typically expected for highly crystalline boehmite-derived γ-Al2O3. This higher surface area was associated with the presence of closed nanopores, the size of which was found to complement observations from transmission electron microscopy. More hydrogen was determined to be in the structure when measured using prompt-gamma activation analysis than indicated by loss on ignition experiments, suggesting that hydrogen-containing species other than water were also present. Neutron vibrational spectroscopy and infrared spectroscopy showed a reduction in signals associated with water and hydroxide species as the calcination temperature increased. Measurements from small-angle X-ray scattering and prompt-gamma activation analysis show that the surface area and the amount of hydrogen present reduce with increasing temperature treatment. This is associated with a reduction in the amount of amorphous material in the structure and an increase in pore and crystallite size. The evidence obtained suggests that the bulk crystalline structure is relatively well-ordered and contains no interstitial hydrogen. Hydrogen is therefore located at the pore surfaces and within amorphous regions, which themselves are located near pore surfaces.
Full-text · Article · Apr 2004 · Chemistry of Materials