Small-Angle X-Ray Scattering Analysis of Nucleation in Glass: II, Selenium Ruby Glasses

Nucleation studies were conducted on several selenium ruby glasses, of different colorant concentrations, using the technique of small-angle X-ray scattering. The glass rnicrostructure increased from homogeneous to 6-nm (60-Å) precipitating particles as the colors changed from colorless through yellow and orange to red. The glasses showed little difference in particle morphology despite differences in color, supporting the explanation of ruby color formation as due to energy band-gap phenomena. Matrix intensities, particle volume concentrations, and interfacial surface areas were also determined.     

Small-Angle X-Ray Scattering Analysis of Nucleation in Glass: III, Gold Ruby Glasses

Small-angle X-ray scattering (SAXS) studies of the striking gold ruby glasses show that both tin oxide and gold are necessary for a satisfactory ruby. Increasing tin oxide decreases the striking speed, whereas increasing gold concentration increases it. Once the ruby color has been struck, further heat treatment causes little change. The presence of tin oxide aids the nucleation and solubility of gold in the glass. Particles in the glass which struck to purple are fairly large (22 nm (220 Å)). Particle sizes for the livery ruby are >100 nm (1000 A).     

Specific Surface of Hardened Portland Cement Paste as Determined by Small-Angle X-Ray Scattering

Small-angle X-ray scattering was used to determine the specific surfaces of hardened portland cement pastes subjected to various treatments. Reasonably mature pastes yielded values of the order of 700 m²/g of ignited paste when examined in the original saturated condition. Drying to an intermediate relative humidity, P-drying, and D-drying reduce the surface areas to values of the order of 200 to 300 m²/g, but vacuum resaturation results in recovery of all the lost surface. Oven drying produces an additional loss in surface area, and in this case recovery on resaturation is not quite complete. The effect of w/c ratio appears to be minor, and pastes of hydrated C₃S yield essentially the same surface areas as corresponding portland cement pastes.     

Characterization of Ceramics by X-Ray and Neutron Small-Angle Scattering

This paper reviews some recent advances in small-angle X-ray and neutron scattering methods and their application to address complex issues in ceramic systems of technological importance. It is shown how small-angle scattering (SAS) can be applied to ceramic systems in order to extract statistically representative microstructure information (e.g., void volume fraction size distributions, internal surface areas, pore morphologies) that complements the information obtained from diffraction methods, X-ray microtomography, or electron microscopy. It is demonstrated how SAS studies provide insights, not obtainable by other means, on the processing–microstructure–property relationships that frequently govern technological performance.     

碳纤维中微孔缺陷的小角X射线散射研究方法

碳纤维的小角X射线散射表明存在沿纤维轴方向取向的针形微孔。应用小角X散射可以得到微孔的尺寸和分布等的信息。文章主要阐述了微孔长度L、取向角Beq,孔隙率P,回转半径Rt,横截面积S等参数的计算方法。     

Anomalous Small-Angle X-Ray Scattering on a New,Nonpyrophoric Raney-Type Ni Catalyst

The nickel part of a new, nonpyrophoric Raney-type catalyst was characterized by using anomalous small-angle X-ray scattering. The mean particle size distribution determined by the method as meant by C. G. Shull and L. C. Roess (J. Appl. Phys.18, 295 (1947)) was in good agreement with the domain size obtained from the broadening of the (111) Bragg reflection peak of the metallic nickel using the Scherrer equation and with the size observed by TEM. Moreover, it was found that the nickel particles have a cylindrical shape and do not exhibit surface fractal behavior.     

Stereological Macropore Analysis of a Controlled Pore Glass by use of Small-Angle Scattering

A first measurement (1) of the isotropic Small-Angle Scattering intensity I(h) of a defined controlled mesoporous glass before and a second measurement (2) after alkaline removing of the mesoporous colloidal silica in the macropores of the main silica framework were performed. Consequently, (1) considered a mesoporous glass (MePG) and (2) considered the immediately-afterwards-outcoming macroporous glass (MaPG).The SAS correlation functions and the chord-length distribution densities of the macropores were determined in both cases (so-called filled (1) empty (2) macropores). There exist no differences between outer size and shape and geometrical arrangement of filled/empty macropores at all. Certainly, as a consequence of the missing filling of the macropores in the MaPG-case, the scattering intensities are changed by a constant factor for small h-values and absolutely different for large h-values. The geometric arrangement of the macropores, distances 20 nm < r < 150 nm, involves a high symmetry in the corresponding MePG and MaPG type glass. There exists a relatively restricted placing of the pore axis and the pore positions in space.     

Detection and Measurement of Inhomogeneities in Glass by Small-Angle Scattering of X Rays

Slope analyses of small-angle scattering curves were used to determine the particle size of submicroscopic, immiscible regions in a lead silicate glass heat-treated for various times at constant temperature. Particle-size estimates made from electron micrographs indicated relatively good agreement between the two methods.     

Surface Nucleation and Crystal Orientation in Lithium Silicate Glass Fibers

The effect of inorganic surface treatments on the orientation of lithium disilicate crystals formed in Li₂O·2.75SiO₂ glass fibers was studied. Glass fibers 0.5 mm and 9μ in diameter were subjected to various surface treatments at room temperature and then heated between 550O and 800°C. It was observed that metal salt solutions applied to the fiber surfaces at room temperature decreased the degree of orientation during heating only if the metal entered the glass and formed nucleation sites within the fibers. Orientation could also be decreased by removing lithium from the glass surfaces. Comparison of the crystallization characteristics of untreated and AgNO₃-treated fibers indicated that the crystallization behavior was controlled by either a growth process or a nucleation process, depending on whether the temperature was below or above 625°, respectively.     

Real-Time Nucleation and Crystallization Studies of a Fluorapatite Glass–Ceramics Using Small-Angle Neutron Scattering and Neutron Diffraction

Real-time small-angle neutron scattering (SANS) and neutron diffraction (ND) studies have been performed on a calcium fluorapatite (Ca₅(PO₄)₃F) (FAP) glass–ceramic composition. The cast glass exhibited scattering at low q and a peak in I ( q ) at higher q . The scattering at low q is thought to arise from a larger-scale nucleated structure, while the peak in I ( q ) is thought to arise from a finer spinodally decomposed structure. High temperature viscoelastic measurements show two reductions in the storage modulus ( E ') and two peaks in the damping factor (tan δ) consistent with a glass that has undergone amorphous phase separation (APS) during the casting process. On heating to 780°C, the scattering at low q increased in intensity, while the peak in I ( q ) increased in intensity and moved to lower q , consistent with the coarsening of the finer scale phase separated structure. During isothermal experiments, the scattering at low q increased in intensity and the peak in I ( q ) moved to lower q , corresponding to a final spacing of about 35 nm. After about 30 min at 740°C, and 12 min at 750°C the coarsening process effectively stopped and is inhibited by the glass transition temperature ( T _g) of the second glass phase. ND showed the glass to crystallize on heating to FAP and then mullite (2SiO₂·3Al₂O₃). At high temperatures, both the FAP and mullite crystal phases partially re-dissolved, but were found to re-crystallize rapidly on subsequent cooling. The results indicate that the proposed crystal growth hold is actually a crystal dissolution hold, with re-crystallization occurring rapidly on cooling. The results indicate that it is important to control not only the heat-treatment cycle but also the cooling cycle.     

Advances,Applications, and Perspectives in Small-Angle X-ray Scattering of RNA

RNAs exhibit a plethora of functions far beyond transmitting genetic information. Often, RNA functions are entailed in their structure, be it as a regulatory switch, protein binding site, or providing catalytic activity. Structural information is a prerequisite for a full understanding of RNA-regulatory mechanisms. Owing to the inherent dynamics, size, and instability of RNA, its structure determination remains challenging. Methods such as NMR spectroscopy, X-ray crystallography, and cryo-electron microscopy can provide high-resolution structures; however, their limitations make structure determination, even for small RNAs, cumbersome, if at all possible. Although at a low resolution, small-angle X-ray scattering (SAXS) has proven valuable in advancing structure determination of RNAs as a complementary method, which is also applicable to large-sized RNAs. Here, we review the technological and methodological advancements of RNA SAXS. We provide examples of the powerful inclusion of SAXS in structural biology and discuss possible future applications to large RNAs.     

Fractal Analysis of DNA Sequences Using Frequency Chaos Game Representation and Small-Angle Scattering

The fractal characteristics of DNA sequences are studied using the frequency chaos game representation (FCGR) and small-angle scattering (SAS) technique. The FCGR allows representation of the frequencies of occurrence of k-mers (oligonucleotides of length k) in the form of images. The numerically encoded data are then used in a SAS analysis to enhance hidden features in DNA sequences. It is shown that the simulated SAS intensity allows us to obtain the fractal dimensions and scaling factors at various scales. These structural parameters can be used to distinguish unambiguously between the scaling properties of complex hierarchical DNA sequences. The validity of this approach is illustrated on several sequences from: Escherichia coli, Mouse mitochondrion, Homo sapiens mitochondrion and Human cosmid.     

Small-Angle Neutron Scattering and Rheological Characterization of Aluminosilicate Hydrogels

Structure and formation of aluminosilicate hydrogels were investigated as a function of chemical composition. Oscillatory rheometry was used to measure the dynamic moduli, G ' and G ", of the gels during setting. The formation of aluminosilicate hydrogels was monitored over time as a function of the storage modulus for gels of low solids volumes, φ < 0.030. Overall gel strength correlated well to the fractal dimension, D , observed by neutron scattering. Scattering data were interpreted using the power law, and the Guinier relationship was used to determine the size of the constructing colloidal/gel particles. Aluminosilicate hydrogels were observed to consist of large fractal aggregates of indeterminable size constructed from smaller subunits of ∼45 Å in dimension. Structure and formation of the gels were highly dependent on [Al₂O₃] and [Na₂O] content.     

X-Ray Stress Analysis of WC-Co Cermets: I,Procedures

The factors that influence the measurement of stress by the “two-exposure” X-ray diffractometer technique in the WC phase of WC-Co cermets were studied. Chromium Karadiation was used on the (1012) line of WC (2θ= 135.8°). The X-ray values of Young's modulus and Poisson's ratio are 105×10⁶ psi and 0.19, respectively, and the X-ray stress factor is 1.51 × 10⁸ psi/Å. The measured stresses are biaxial. Because penetration is limited to less than 5 μm for CrKa radiation, extreme care in surface preparation and heat treatment is required. A suitable preparation is metallographic polishing with 6 μm diamond followed by annealing in hydrogen at 750°C. This treatment allows measurement of about 40, 000 psi compressive temperature stress inherent in the WC particles resulting from differences in coefficients of thermal expansion between WC and Co. Mechanical stresses can also be measured; they arise from external forces such as grinding, polishing, and sand blasting. Mechanical and temperature stresses cannot always be clearly distinguished, however.     

Bubble-Induced Nucleation and Crystal Growth in Glass

Evidence that bubbles can cause annular nucleation and subsequent crystal growth on the surface of Bao-Al₂O₃-TiO₂-SiO₂, glasses is presented. The crystalline rings were identified as hexacelsian, BaAl₂Si₂O₈ (the hexagonal or high-temperature polymorph of celsian). Size, character, and suggestions as to the genesis of these rings are given.     

Practical Particle-Size Analysis of Clays: I, Sample Preparation

Wide variations in particle-size data from different laboratories for the same clay were thought to be due to dissimilar preparatory techniques. It was proposed to attempt to establish universally acceptable conditioning procedures based on determination of deflocculant requirement, most effective electrolyte, and method of dispersion. Three commonly employed dispersing agents were tested over a range of percentages on four selected whiteware clays. Both high-intensity agitation and low-intensity stirring were tried. It was found that all the sodium polyphosphate salts tested were equally effective and that with them complete dispersions could be attained by a minimum of agitation. Electrolyte requirements for complete dispersion were found to relate directly to points of minimum viscosity of deflocculated 50% clay-water slips.     

Characterization of Creep Cavitation in Sintered Alumina by Small-Angle Neutron Scattering

Small-angle neutron scattering experiments were performed on compression-crept alumina. The results suggest that grain-boundary cavities, on the order of 100 nm in diameter, probably are present initially. For compression strains up to 8%, the average cavity size changes very little. However, the cavity density approximately triples, and it appears that the cavities become elongated in the direction of the compression axis .     

Nucleation and Crystallization of a Lithium Aluminosilicate Glass

An aluminosilicate glass of composition 61SiO₂6Al₂O₃10MgO6ZnO·12Li₂O·5TiO₂ (mol%) has been prepared by a melting process and investigated as far as crystallization is concerned. Glass-ceramic is easily obtained because glass shows a high tendency to crystallize starting from 700°C. The crystalline phases evolve with temperature, showing the aluminosilicates to be the main phase up to 1050°C, followed by metasilicates and silicates, some of which have lower melting points. The titanates of Mg and Zn develop from the phase-separated glass, soon after T _g, and grow to form nucleation centers for the other crystalline phases. The evolution from phase-separated glass to glass-ceramic has been followed by many thermal, diffractometric, spectroscopic, and microscopic techniques.     

High-Temperature Failure of Polycrystalline Alumina: I, Crack Nucleation

A combination of techniques has been used to compare the compositional and microstructural characteristics of two commercially hot-pressed aluminas. Second-phase material containing Ni was observed in one material, mainly as small intergranular particles. Thermal treatment in air caused Ni to concentrate at the surfaces, in small precipitates. Large-grained heterogeneities, with Ti at the core, were identified in the same material. Hot-pressing flaws were observed in the second material. The large-scale heterogeneities act as crack nucleation sites during creep. Stress concentrations associated with these heterogeneities are considered to contribute to the premature crack nucleation, in conjunction with the presence of localized regions of amorphous second phase.