R.J. De Angelis

Particle size distribution function of supported metal catalysts by X-ray diffraction

An X-ray diffraction method which is capable of determining average particle size, microstrain, and the particle distribution function existing in crystalline materials is presented. The method is based on the analysis of a single X-ray diffraction profile. Results obtained on coprecipitated nickel oxide on alumina- and silica-supported catalytic materials indicate that appreciable strains exist. It is suggested that the strains present in NiO could be due to the pressure developed in the small particles to balance the surface tension forces and the distortion produced by the deformation of face-centered cubic structure into a rhombohedral form. The changes in particle size distributions observed during sintering determined from three catalytic materials provide evidence that particle growth takes place by atomic migration mechanism. In one material the particle growth during sintering appears to be controlled predominantly by crystallite migration and coalescence. The sintering behavior appears to be controlled by the extent of the bimodal character of the initial distribution function and the average particle size in the as-received condition.

Alloy formation in Pt-Sn-alumina catalysts: in situ X-ray diffraction study

Changes in PtSnalumina catalysts, prepared from the (Pt3Sn8Cl30)2− complex, during reduction in flowing hydrogen were followed by in situ X-ray diffraction (XRD). For high metal loading (ca. 5% Pt) evidence was found for both PtSn and Pt phases: other PtSn phases, such as Pt3Sn, PtSn2, and PtSn4, were not observed. Metallic Pt was detected for a 0.6 wt% PtAl2O3 catalyst but only a PtSn alloy was observed for a 0.6 wt% Pt catalyst containing Sn. In situ XRD studies therefore support alloy formation with a stoichiometry of Pt: Sn = 1: 1; the Sn in excess of that needed to form this alloy is present in an X-ray “amorphous” form.

The sintering of a silica-supported nickel catalyst

Abstract Changes in the average particle size and particle size distribution (PSD) were monitored during sintering of a silica-supported nickel catalyst. Sintering was carried out in nitrogen and hydrogen atmospheres from 500 to 800 °C for times varying from 1 to 100 hr. The particle size distribution functions (PSDs) determined both by X-ray single profile analysis and transmission electron microscopy were in excellent agreement. The sintering process was found to occur very rapidly initially and then proceeded more slowly at longer times for temperatures higher than 600 °C. The effects of sintering temperature on the changes of PSD were found to be more pronounced than the effects of sintering time. As sintering progresses, the PSDs developed long tails to the larger diameter side. Fitting the data to the sintering power-law gave exponent values of 13 and 14 for sintering below 700 °C in nitrogen and hydrogen, respectively. Particle growth became much faster as sintering was carried out at 800 °C in both atmospheres. The values of the sintering exponent change from 13 to 6 in nitrogen atmosphere and from 14 to 4 in hydrogen atmosphere. The large values of the sintering exponent and the changes in the PSDs obtained during sintering at temperatures of 700 °C and below indicate that sintering occurs by a particle migration mechanism. However, the sintering results obtained at 800 °C tend to indicate that the atomic migration mechanism is predominating.

Dislocation dynamics during the growth of silicon ribbon

The thermal viscoplastic stresses and the dislocation densities in silicon ribbon are computed for an axially changing thermal profile by using an iterative finite difference method. A material constitutive equation (Haasen–Sumino model) which involves an internal variable (mobile dislocation density) is used. The results are interpreted as showing that there is a maximum width of silicon ribbon that can be grown when viscoplasticity and dislocations are considered. This maximum width limitation does not exist if the material behavior is elastic.

Structural characterization of cobalt catalysts on a silica support

Abstract In situ X-ray diffraction was employed to characterize the structure of several cobalt catalysts on a silica support. The catalysts were reduced in flowing hydrogen at 350 °C for about 16 h and X-ray diffraction patterns were collected. After reduction metallic cobalt was found to be present in the hcp and fcc forms in the ratio of 7:3 and 17% of the hexagonal close-packed planes were found to be faulted. If surface atoms adjacent to stacking faults are sources of active sites, the density of active sites was estimated to be 4.0 × 10 19 per gram of cobalt.

Characteristics of particle size distribution functions of a supported metal catalyst during sintering

It has been recognized for years that to arrive at an identification of the mechanism of thermal sintering of catalysts one requires detailed characterization of the metal particle size distribution (PSD) function during sintering. The purpose of this paper is to report the analysis performed on the existing PSDs and to show that the results provide further evidence that sintering at temperatures of 700/sup 0/C and below takes place by particle migrations and that during 800/sup 0/C sintering the atomic migration mechanism becomes operative. The analyses reported here are mainly concerned with fitting the existing PSDs to log-normal distribution functions (LNDF), and observing the behavior of the standard deviations of the PSD functions related to sintering time, sintering temperature, and the geometric mean average particle size. The results indicate that: (1) Nickel particles on a silica support follow a LNDF both in the as-reduced condition and after sintering at temperatures up to 800/sup 0/C; and (2) The values of the standard deviation of the particle size distribution functions increase with sintering temperature. The time-dependent increase of the standard deviation observed at the highest sintering temperature is a possible indicator of a change in sintering mechanism.

Starting materials and superconductor stoichiometry

Abstract Oxygen contents of as-received La 2 O 3 and Nd 2 O 3 were determined by fast neutron activation analysis. X-ray diffraction was used to determine the phases present in these materials. Results of these analyses show that significant amounts of hydroxides (in one case over 90 wt%) were present in all oxide samples. These results carry tremendous implications for the stoichiometry of the superconductor formation reaction and the composition of the final product. The most important conclusion to be drawn from this study is that many of the bulk superconducting ceramics produced to date may be of approximate composition at best.

Dynamic strain aging in Czochralski-grown silicon single crystals

Abstract Constant extension rate tensile tests were performed on dislocation-free Czochralski-grown ([O] = 24–38 ppma) silicon single crystals oriented for single slip in the temperature range 1173–1573 K. Evidence for dynamic strain aging, resulting from oxygen in solid solution, includes the observation of a plateau in the plot of the lower yield stress vs. strain rate at 1373, 1473, and 1573 K; an increase in the flow stress with temperature above 1373 K; a higher magnitude of the lower yield stress than that observed in float-zone (low-oxygen)-grown silicon for temperatures above 1373 K; and other observations resulting from the analysis of the later stages of deformation. Dynamic precipitation of oxygen caused by the supersaturated condition of oxygen in Czochralski-grown silicon complicates the application of solute hardening models for analyzing the experimental observations.

Twinned colloidal gold particles

Abstract The structural characteristics of colloidal gold particles prepared by reduction of gold chloride were studied. Images and microdiffraction patterns indicate that multiple twinning is commonly present in the gold particles which range in size from 15 to several hundred nm. Very good agreement was found between the twin-fault densities determined by TEM studies and X-ray diffraction results.

Omega-phase formation in Beta III titanium alloy

Abstract The kinetics of the formation of the omega phase in the Beta III alloy is followed during aging at 400°C after quenching from 825°C. Electron diffraction patterns indicate that the omega phase forms during the quench. The aging kinetics indicate that the omega particles that form on quenching grow to form ellipsoidal shaped particles during aging. After aging 49 hrs at 400°C the omega phase makes up greater than 90% of the volume of the material. During the first 20 min of aging a heavy grain boundary precipitate is formed which continues to grow as aging progresses.