The Mendeleev Periodic Table
This table shows the form of Mendeleev's Periodic Table of the chemical elements as published in 1872. The heading "Reihen" means "Row" and the heading "Gruppe" means "Group". The symbols R2O and RH4, etc., are written in the style of the time which uses superscripts to denote the number of atoms in molecules rather than the current style which uses subscripts. The gaps marked with hyphens ("-") represent chemical elements deduced by Mendeleev as existing but unknown in 1872. He was able to predict with considerable success the properties of some of the missing chemical elements such as germanium.
|8||Cs=133||Ba=137||?Di=138||?Ce=140||-||-||-||- - - -|
|12||-||-||-||Th=231||-||U=240||-||- - - -|
This is interesting. NASA scientists are examining a seemingly magical way to produce high-quality crystals.
Perhaps a NASA laboratory is an unlikely setting for a magic show. Nevertheless, this is where Frank Szofran and colleagues are growing high-quality crystals using a method as amazing as any conjuring trick. By carefully cooling a molten germanium-silicon mixture inside a cylindrical container, they coax it into forming a single large and extraordinarily well-ordered crystal. Such crystals have very few defects because, remarkably, they never touched the walls of the very container in which they grew.
Scientists at the Lawrence Berkeley National Laboratory in California, USA, have discovered that nanocrystals of germanium embedded in silica glass don't melt until the temperature rises almost 200 degrees Kelvin above the melting temperature of germanium in bulk. What's even more surprising, these melted nanocrystals have to be cooled more than 200 K below the bulk melting point before they resolidify. Such a large and nearly symmetrical "hysteresis" -- the divergence of melting and freezing temperatures above and below the bulk melting point -- has never before been observed for embedded nanoparticles.
"Melting and freezing points for materials in bulk have been well understood for a long time," says Eugene Haller (one of the authors) , "but whenever an embedded nanoparticle's melting point goes up instead of down, it requires an explanation. With our observations of germanium in amorphous silica and the application of a classical thermodynamic theory that successfully explains and predicts these observations, we've made a good start on a general explanation of what have until now been regarded as anomalous events."
The research was conducted because the properties of germanium nanoparticles embedded in amorphous silicon dioxide matrices have promising applications. "Germanium nanocrystals in silica have the ability to accept charge and hold it stably for long periods, a property which can be used in improved computer memory systems. Moreover, germanium dioxide (germania) mixed with silicon dioxide (silica) offers particular advantages for forming optical fibers for long-distance communication."1
- 1. Large Melting-Point Hysteresis of Ge Nanocrystals Embedded in SiO2,
, Physical Review Letters, Volume 97, Number 15, p.155701, (2006)
Abstract: The melting behavior of Ge nanocrystals embedded within SiO2 is evaluated using in situ transmission electron microscopy. The observed melting-point hysteresis is large (±17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.Large Melting-Point Hysteresis of Ge Nanocrystals Embedded in SiO2, , Physical Review Letters, Volume 97, Number 15, p.155701, (2006)
Workers at The University of Wisconsin-Madison in the USA have managed to release thin membranes of semiconductors from a substrate and transfer them to new surfaces. The freed membranes which are just tens of nanometers thick retain all the properties of silicon in wafer form but the nanomembranes are flexible. By varying the thicknesses of the silicon and silicon-germanium layers composing them, membrane shapes are possible ranging from flat to curved to tubular.
Potential applications include flexible electronic devices, faster transistors, nano-size photonic crystals that steer light, and lightweight sensors for detecting toxins in the environment or biological events in cells.
The scientists made a three-layer nanomembrane composed of a thin silicon-germanium layer sandwiched between two silicon layers of similar thinness. The membrane sat upon a silicon dioxide layer in a silicon-on-insulator substrate. The nanomembranes may be etched away from the oxide layer with hydrofluoric acid.
Although the Wisconsin team grew their nanomembranes on silicon-on-insulator substrates, the method should apply to many substances beyond semiconductors, such as ferroelectric and piezoelectric materials. The key requirement is a layer, like an oxide, that can be removed to free the nanomembranes.1
- 1. Elastically relaxed free-standing strained-silicon nanomembranes,
, Nature Materials, 5/2006, Volume 5, Issue 5, p.388 - 393, (2006)