Using Ultra Small Angle X-ray Scattering Research Articles

In Situ Observation of the Genesis of Mesoporous Silica SBA-15: Dynamics on Length Scales from 1 nm to 1 μm

We report on the mechanism of growth of mesoporous silica (SBA-15, plane group p6m). In situ studies of the formation using ultrasmall angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) covering length scales from 5 to 10,000 A, complemented with UV-vis and transmission electron microscopy (TEM), provide unique data on particle growth coupled with information regarding the progression of the mesostructure formation and the micellar evolution.

A Comparative Study of Characteristics of Ferrogels Prepared using Coated and Uncoated Fe3O4Nanoparticles

AbstractThis paper presents the results of our analysis of ferrogels prepared using uncoated and polyvinylpyrrolidone (PVP)-coated magnetite (Fe3O4) nanoparticles. The N-isopropylacrylamide (NIPAM) based hydrogel is used with 15-25 nm Fe3O4nanoparticles different concentrations in the range 1.25 -14%. These samples were analyzed using ultra small angle x-ray scattering (USAXS), Transmission Electron Microscopy (TEM), and Direct-Current Superconducting Quantum Interference Devices (DC-SQUID) magnetometry. Samples prepared with polyvinylpyrrolidone (PVP) coated nanoparticles showed a better single particle distribution compared to uncoated samples. USAXS data indicated that the gels prepared using uncoated nanoparticles have a large two particle agglomerations. In both cases, the volume fraction of the particles in the gel is linearly proportional to the initial particle concentrations. The DC-SQUID magnetometry analysis indicated that the magnetic moment of the gel samples prepared with uncoated particles is ∼ 2emu/g compared to ∼1.5emu/g of those having coated particles.

Protection of organic carbon in soil microaggregates via restructuring of aggregate porosity and filling of pores with accumulating organic matter

We examined relationships between the pore structure of microaggregates and the protection of organic matter (OM) within that structure. By using ultra-small angle X-ray scattering (USAXS) before and after combustion of microaggregates at 350 °C, we took advantage of differences in X-ray scattering contrast among soil minerals, OM, and air to evaluate the distribution of the total- and OM-filled porosity within microaggregates (53–250 μm in diameter). Systematic changes in microaggregate structure were observed for long-term field manipulations of land use (a chronosequence of tallgrass prairie restorations) and agricultural management (conventional tillage versus no-till at two levels of nitrogen fertilization). Our results imply that OM preservation arose from the evolution of the architectural system of microaggregates during their formation and stabilization. Soils and treatments with increasing OM in microaggregates were associated with encapsulation of colloidal OM by minerals, thereby creating protected OM-filled pores at the submicron scale within the microaggregate structure. For example, in the prairie chronosequence, microaggregates from the cultivated soil had the lowest concentration of OM, but 75% of the OM that had survived cultivation was in OM-filled pores. Following restoration, the concentration of OM in microaggregates increased rapidly, but the proportion of OM in OM-filled pores declined initially and then increased over time until 90% of the OM was in OM-filled pores. OM totally encapsulated within the pore structure can create spatial and kinetic constraints on microbial access to and degradation of OM. Encapsulation of OM increases the capacity for its protection relative to sorption on mineral surfaces, and comparison of its extent among treatments suggests important feedback loops. The use of USAXS, which has not previously been applied to the study of soil aggregate structures and the distribution of OM within those structures, provided new information on the mechanisms of OM protection in soil microaggregates, and insights relevant to strategies for enhancing carbon-sequestration in soil through changes in agricultural management practices and land use.

USAXS studies of monosaccharide gels. I. Dependence of the glucofuranose-based gel structure on the gelator concentration

Glucofuranose-derivative based gels with various gelator concentration were studied using ultra-small angle X-ray scattering (USAXS) in the scattering vector range 0.0028–0.25nm−1. A complex method of data processing was applied to extract the structural parameters of the gelator, such as the mass fractal and surface fractal dimensions, dm and ds, respectively, the radius of gyration, Rg, the distance distribution function, p(r), as well as the volume distribution function, D(R), of particles forming aggregates. The results of these analyses show that the aggregate structure changes with the gelator concentration: when it increases, the size of the aggregates decreases, rough aggregates of the fractal type evolve to better developed smooth ones, and their shape changes from disk-like to rod-like. These results suggest that only two different types of aggregates are formed depending on the gelator concentration, and a gradual change between them is observed.

An investigation of the properties of poly(dimethylsiloxane)-bioinspired silica hybrids

Elastomers typically require the incorporation of reinforcing fillers in order to improve their mechanical properties. For commercial silicone systems silica and titania are typically used as fillers. Fumed and precipitated silica are made on an industrial scale for many applications; however, we have shown recently that biological and synthetic macromolecules can generate new silica structures using a bioinspired route. Herein we have incorporated bioinspired silica fillers into poly(dimethylsiloxane) (PDMS) elastomers and investigated their mechanical, morphological and thermal properties as a function of filler loading. The equilibrium stress–strain characteristics of the PDMS-bioinspired silica hybrids were determined as a function of bioinspired filler loading and the Mooney–Rivlin constants (2 C 1 and 2 C 2) were calculated. The thermal characteristics, in particular glass transition temperatures ( T g) and melting points ( T m), of the PDMS-bioinspired silica hybrids were characterized using differential scanning calorimetry (DSC). The thermal stability of these hybrid materials were investigated using thermogravimetric analysis (TGA). The morphology of the samples was characterized using scanning electron microscopy (SEM), and the filler dispersion was characterized using ultra small angle X-ray scattering (USAXS) and scanning electron microscopy (SEM). Although spherical silica particles were used here, we have demonstrated elsewhere that this bioinspired synthetic route also enables highly asymmetric silica structures to be prepared such as fibres and sheets. This methodology therefore offers the interesting possibility of preparing new hybrid systems where the properties are highly anisotropic.

X-ray imaging with ultra-small-angle X-ray scattering as a contrast mechanism

A new transmission X-ray imaging technique using ultra-small-angle X-ray scattering (USAXS) as a contrast mechanism is described. USAXS imaging can sometimes provide contrast in cases where radiography and phase-contrast imaging are unsuccessful. Images produced at different scattering vectors highlight different microstructural features within the same sample volume. When used in conjunction with USAXS scans, USAXS imaging provides substantial quantitative and qualitative three-dimensional information on the sizes, shapes and spatial arrangements of the scattering objects. The imaging technique is demonstrated on metal and biological samples.

Agglomerate formation during drying

The evolution of agglomerate structure during drying of particles from suspension has been studied for a nanocrystalline Y2O3 (8% mol fraction)-stabilized ZrO2 powder. Agglomerates in drying and dried suspensions were examined at the smallest size scales (1 nm to 1 μm) using ultra-small angle x-ray scattering (USAXS) and at the largest size scales (100 nm to 10 μm) using scanning electron microscopy. The results were correlated with the degree of particle dissolution in each suspension (measured by flame absorption spectroscopy of the suspension filtrate) and the zeta potential of the particles in suspension prior to drying. Results show that large agglomerates readily form across a pH range from 2 to 9. The fact that Y+3 ion dissolution varies by over four orders of magnitude in this range leads to the conclusion that there is little direct correlation between the degree of Y dissolution and agglomeration in this system (Zr ion dissolution was below the detection limit at all pH values studied). The observation of large agglomerates well before the introduction of air-water interfaces into the drying mass likewise leads to the conclusion that capillary forces are not essential to agglomerate formation. Instead, agglomerates appear to form as a direct consequence of increasing suspension concentration. Zeta potential also plays a role. Specifically, there was a notable change in agglomerate morphology as the isoelectric point was approached, at approximately pH 8. Here USAXS shows the particles in suspension to have a layered interior structure, with small primary particles aggregated in sheets to form each blocky particle. This is in contrast to the more rounded agglomerates formed away from the isoelectric point, which appear to be composed of the same primary particles arranged in chainlike structures. USAXS of powders from the dried suspensions confirms that the structures seen after drying are the same as those present in suspension. The two structural morphologies are attributed to diffusion-limited (sheets) versus reaction-limited (chains) aggregation, respectively.