Preatomization behavior of palladium in electrothermal atomic-absorption spectrometry

Under certain exptl. conditions-short pyrolysis times, fast heating rates and presence of reducing agent-increasing vaporization temps. led to decreasing losses of Pd as the initial mass was increased between 0.3 and 5.0 ng. When Pd is introduced into the tube at mg levels, as is typical for modifiers, no similar tendency was established and max. loss-free temps. approach 1300 Deg. A procedure for deriving the enthalpy of vaporization was developed which characterizes low-temp. Pd release during pyrolysis. The evaluated enthalpy of vaporization increases with increasing Pd mass at ng levels. This could be explained by the enhancement of the dimensions of the vaporized particles/droplets. Because of the small mass of the injected sample, the particles obtained are sufficiently small and the behavior of the droplets at higher pyrolysis temps. could be explained by the Kelvin equation. In such a case the variation of the particle size leads to variation in their vapor pressure and, as a result, the enthalpy of vaporization is changed. At mg levels the Pd particles obtained are sufficiently large and the vapor pressure and enthalpy of vaporization are not a function of the particle size. Variation of the initial mass of Pd does not influence the shape of pyrolysis curves when longer pretreatment times were used. The prolonged pyrolysis times possibly influence the rates of nucleation and crystal growth and favor the diffusion of Pd into and on to the graphite surface, thus the initial sample distribution is changed. [on SciFinder (R)

electronic reprint Journal of Applied Crystallography Structure development in aerogel-processed nanocrystalline alkaline earth oxides as revealed by SANS Structure development in aerogel-processed nano- crystalline alkaline earth oxides as revealed by SA

Nanocrystalline MgO, CaO and SrO were prepared according to a modified aerogel process (AP). Small-angle neutron scattering (SANS) was used to probe the nanoscale structural features of these materials after each stage of the synthetic process, including hydrolysis, supercritical drying and calcining. SANS data were interpreted using a classical analysis involving power-law and Guinier regimes, and by application of the maximum entropy method. Results are compared with previously published structural data based on X-ray diffraction, electron microscopy and gas adsorption. It is found that the gel hydrolysis product suspended in methanol and toluene exhibits rod-like scattering at small length scales. This is consistent with a filiform morphology previously reported for air-dried Mg(OH) 2 alcogel, yet SANS data for air-dried alcogels tested in this study indicate no evidence for low-dimensional structure on any length scale. A previous assertion of mass fractal structure in the AP aerogels and oxides was not confirmed by the present data. Instead, surface fractal scattering was found to be the most dominant characteristic feature associated with the SANS data for all AP powders examined. Additionally, MgO and CaO exhibited a correlation peak that corresponds to liquid-like ordering at Bragg length scales of 5.9 nm and 20.3 nm, respectively. These values are roughly consistent with previous independent estimates of primary particle size, suggesting that local packing of primary crystallites is facilitated by the calcination/dehydration process. An alternative interpretation treats these features as Guinier scattering regions. Fitting of results using the unified Guinier/power-law equation yields sphere-equivalent radii for the primary particles that are nearly identical to the Bragg lengths calculated from the positions of the maxima. Air-dried alcogels produced very weak maxima that could be interpreted either as correlation peaks or as Guinier regions. No maxima were observed for aerogel samples. Maximum entropy analysis using a spherical shape factor produced interesting but complex results for the calculated volume size distributions of these materials. Overall, the observed trend shows an increase in structural feature size with increasing metal cation size

Nanocrystal superlattice imaging by atomic force microscopy

Applicability of Atomic Force Microscopy (AFM) for structural characterization of nanocrystal superlattices is demonstrated on high-resolution imaging of superlattices formed by thiol stabilized gold nanoparticles on carbon coated and hydrophobic supports. Thin (<1 nm) uniform coating of the samples with metal film before imaging was found to eliminate the undesirable effects of tip-sample interaction. Size and interparticle spacing are in excellent agreement with transmission electron microscopy results. AFM can be used as a complementary technique for nanocrystal superlattice structural characterization providing possibilities for crystal growth investigation on a variety of supports of practical interest and high resolution of the surface structure of superlattice structures

Reversible transformations of gold nanoparticle morphology

Herein is reported a metamorphosis taking place in a gold nanosized system. The observed phenomenon of shape and size transformations was found to be completely reversible. Unlike most procedures in the literature where shape and size control occur in the synthetic step by adding growth- and shape-controlling agents such as surfactants or polymers, in this system postsynthetic changes in shape and size can be carried out simply by changing the ratio of reactive, competing reagents, more specifically, alkylthiols versus tetralkylammonium salts. Interestingly, the transfer of gold metal occurs (large prismatic particles to small particles and vice versa) under the influence of reagents that do not cause such interactions with bulk gold. All intermediate steps of the morphology change were observed using HRTEM and electron diffraction. The processes of breaking down and “welding back” solid metal nanoparticles occur under mild conditions and are remarkable examples of the unique chemical properties of nanomaterials. The described process is expected to be relevant to other nanoscale systems where similar structural circumstances could occur

Face-centered cubic and hexagonal closed-packed nanocrystal superlattices of gold nanoparticles prepared by different methods

Dodecanethiol-stabilized gold nanoparticles with similar average size organize into different superlattice structures depending upon the method of preparation of the nanocrystals. Particles synthesized by the inverse micelle technique preferentially assemble into face-centered cubic (fcc) structures with long-range translational and orientational ordering. Gold nanoparticles obtained by the Solvated Metal Atom Dispersion (SMAD) method behave like “hard” spheres and predominantly organize into hexagonal close-packed (hcp) nanocrystal superlattices with long-range translational ordering. Different packing behavior results from differences in nanoparticle core morphologies induced by the synthetic method; fcc ordering is preferred by single crystalline nanoparticles, while hcp is preferred by polycrystalline nanoparticles. A combination of optical microscopy, Transmission Electron Microscopy (TEM and HRTEM), Selected Area Electron Diffraction (SAED), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) were used to characterize both the dispersed nanoparticles and the nanocrystal superlattices

DOL-BIP-Critical: a tool chain for rigorous design and implementation of mixed-criticality multi-core systems

International audienceMixed-criticality systems are promoted in industry due to their potential to reduce size, weight, power, and cost. Nonetheless, deploying mixed-criticality applications on commercial multi-core platforms remains a highly challenging problem. To name a few reasons: (i) Industrial mixed-criticality applications are usually complex reactive applications, which cannot be specified by traditional, e.g., dataflow-based, models of computation. Appropriate mixed-criticality models of computation built upon Vestal's assumptions are missing; (ii) Scheduling such applications on multicores with shared resources, such as memory buses, requires that any timing interference among applications of different criticality is bounded in order to guarantee-the necessary for certification-temporal isolation and to enable incre-mental design; (iii) The implementation of isolation-preserving mixed-criticality schedulers is itself subject to certification. Hence, it needs to be not only efficient, but also provably correct. This paper proposes, for the first time, a complete design flow covering all aspects from specification, using a novel mixed-criticality aware model of computation (DOL-Critical), to correct-by-construction implementation, using the principle 'what you verify is what you generate' which is based on a novel variant of task automata (BIP). We demonstrate the applicability of our design flow with an industrial avionic test case on the state-of-the-art Kalray MPPA R-256