Densification and properties of bulk nanocrystalline functional ceramics with grain size below 50 nm

The synthesis and functional characterization of dense bulk nanometric oxides are reviewed, with particular emphasis on the modifications that a grain size in the low nanometric range (10–50 nm) introduces in their physical properties. The preparation of ceramics with low porosity and extremely small grain size is particularly challenging and mostly relies on the sintering of extremely fine nanopowder. The most popular methods for the preparation of the starting nanopowders are introduced and briefly discussed as well as the most widely employed densification techniques. The role of nanostructure in controlling phase stability, electrical and thermal transport, optical and magnetic properties of nano-oxides is discussed in details. Several examples are given where bulk materials prepared with grain size equal or below 50 nm show characteristics that are either enhanced or, in some cases, completely different from those possessed by the same materials, but with larger grain sizes.     

Preparation of nano-size ZrB2 powder by self-propagating high-temperature synthesis

Preparation of nano-size ZrB₂ powder by SHS has been investigated. Zr and B elemental powders were mixed with 10–50 wt.% NaCl, and prepared pellets were reacted under argon. Adiabatic temperatures were calculated by HSC software. Increasing NaCl content led to a continuous decrease in adiabatic temperatures and reaction wave velocity. Products were subjected to XRD, SEM and FESEM analyses. Average crystallite size of ZrB₂, which was 303 nm without NaCl, decreased to 32 nm with 40% NaCl addition. Distinct decrease in ZrB₂ particle size was also observed from SEM analyses. 30% NaCl addition was found to be optimum for ensuring a stable SHS reaction and providing the formation of nano-size ZrB₂ particles. It was revealed from particle size distribution measurements that ZrB₂ powder obtained by 30 wt.% NaCl addition contained particles mostly finer than 200 nm. A mechanism, similar to solution-precipitation was proposed for the particle size refining effect of NaCl.     

Interactive anatomy online

The scientific community and general public can best take advantage of the benefits of 3D digital reconstructions if they are stored in a Web-accessible, easy-to-access database. We describe MorphologyNet/sup /spl copy// a Web-based digital library of realistic, 3D interactive and customizable images of anatomy currently in development at the University of Missouri-Rolla.     

High altitude retinal hemorrhages in the expeditions to 8,000 meter peaks. A study of 10 cases

BACKGROUND: Retinal haemorrhages are common at high altitude. Their pathogenesis is unknown. It has been suggested that they are less frequent in sherpas, and that possible predisposing factors might be the abscence of previous high-altitude experience, the extent of the high-altitude hypoxic exposure, polycythemia (because of hyperviscosity), history of cough and Valsalva manoeuvres during the expedition, existence of severe forms of mountain sickness (high-altitude pulmonary oedema and high-altitude cerebral oedema) and use of antiinflammatory drugs. The aim of this study is to know the incidence of retinal haemorrhages in the expeditions to mountains higher than 8.000 m and their relationship to the previously referred possible predisposing factors. SUBJECTS AND METHODS: Funduscopy was performed on 17 healthy subjects taking part in expeditions to Cho-Oyu (8.201 m) and to Shisha Pangma (8.046 m) and on six of their Nepali coworkers. RESULTS: Retinal haemorrhages were found in 10 of the European (59%) and in none of the Nepali mountaineers (p = 0.019). Other 2 Spanish climbers had tortuosity and engorgment of the retinal veins. No statistical association was found between retinal haemorrhages and maximal altitude attained prior to the expedition, maximal altitude reached during the present expedition, number of nights spent at extreme altitude, weight loss as an expression of chronic exposure to hypoxia, haemoglobin, history of cough or Valsalva manoeuvres during the expedition, existence of severe forms of mountain sickness or use of drugs. CONCLUSIONS: These results do not allow us to state that the mentioned factors predispose to high-altitude retinal haemorrhages.     

Self-Propagating Reactions in the Ti–Si System: A SHS-MASHS Comparative Study

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.     

Nanoscale Effects on the Ionic Conductivity of Highly Doped Bulk Nanometric Cerium Oxide

Nanometric ceria powders doped with 30 mol % samaria are consolidated by a high‐pressure spark plasma sintering (HP‐SPS) method to form > 99 % dense samples with a crystallite size as small as 16.5 nm. A conductivity dependence on grain size was noted: when the grain size was less than 20 nm, only one semicircle in the AC impedance spectra was observed and was attributed to bulk conductivity. In contrast to previous observations on pure ceria, the disappearance of the grain‐boundary blocking effect is not associated with mixed conductivity. With annealing and concomitant grain growth, the samples show the presence of a grain‐boundary effect.     

Controlled translocation of individual DNA molecules through protein nanopores with engineered molecular brakes

Protein nanopores may provide a cheap and fast technology to sequence individual DNA molecules. However, the electrophoretic translocation of ssDNA molecules through protein nanopores has been too rapid for base identification. Here, we show that the translocation of DNA molecules through the α-hemolysin protein nanopore can be slowed controllably by introducing positive charges into the lumen of the pore by site directed mutagenesis. Although the residual ionic current during DNA translocation is insufficient for direct base identification, we propose that the engineered pores might be used to slow down DNA in hybrid systems, for example, in combination with solid-state nanopores.     

The role of local structural distortions in the stabilisation of undoped nanocrystalline tetragonal zirconia

Nanopowders of pure or very lightly doped zirconia were studied by means of total scattering and pair distribution function analysis, with the aim of understanding how the size of the particles (if a size limit of about 20 nm is not exceeded) tends to stabilise the tetragonal polymorph at room temperature. Total scattering and PDF analyses, together with Rietveld refinements, showed that the tetragonal model is indeed applicable to the average structure of these nanocrystalline zirconia samples, and provided comparable results. However, all the samples, with no influence from the dopant content, showed a similar local distortion for r < 10 Å. In this region, the data were fitted with two slightly different tetragonal structures, with a tetragonal distortion that decreases with the R_max used, giving rise to a structure closer to the average tetragonal one (given by Rietveld as well as by average PDF refinements). In the first coordination shell, however, an orthorhombic distortion fits very well both in intensity and in position. This distortion may be responsible of the increased RMS strain local level, and therefore of the stabilisation of the high temperature structure at room temperature.     

Mechanism of Low‐Temperature Protonic Conductivity in Bulk,High‐Density,Nanometric Titanium Oxide

Uncovering the mechanism of low‐temperature protonic conduction in highly dense nanostructured metal oxides opens the possibility to exploit the application of simple ceramic electrolytes in proton exchange fuel cells, overcoming the drawbacks related to the use of polymeric membranes. High proton conducting, highly dense (relative density 94 vol%) TiO₂ samples are prepared by a fast pressure‐assisted sintering method, which allows leaving behind an interconnected network of open nanoporosity. Solid‐state ¹H NMR is used to characterize the presence of strongly associated water confined in the nanopores and hydroxyl moieties bonded to the pores walls, providing a model to explain the unusually high protonic conductivity. At the lowest temperatures (T < 55 °C) protons hop between confined water molecules, according to a Grotthuss mechanism. The resulting conductivity values are however much higher than those of liquid water, indicating a significant increase in the charge carriers concentration. At higher temperatures (up to 450 °C) unexpected proton conduction is still present, thanks to the persistence of hydroxyl groups, derived from water chemisorption, which still produce protons by ionization. The phenomenon is strongly dependent on grain size, and not explicable by simple geometric brick‐layer models, suggesting that the enhanced ionization could rely on space charge region effects.