Saturday, September 25, 2010

20 new things for today

  1. Agar cools quicker than I anticipated. Drop 3-4 drips of it and screw the ring quickly.
  2. Japan releases Chinese trawler captain, but refuses to apologize. It is said that China's banning on raw material (Rare earth) exportation to Japan, which is crucial for the high-tech industry in Japan, is the real stirring response for the Japan's release.
  3. One of the usages of rare earth elements is to produce phosphor. Got its name from element of phosphorus, but phosphor has a completely different mechanism on lightening—the former is because of chemiluminescence (some oxidization-reduction reaction), whereas the latter is because of phosphorescence (slow decay, >1ms) or fluorescence (quick decay, tens of nanoseconds).
  4. But why materials made of rare earth element emit light? Simply put, it's because of the electronic band structure in the crystals. An incoming particle (such as a UV photon) excites an electron from the valence band (trapped and cannot move under usual state) to either exciton band or conduction band. The excitons are loosely bound electron-hole pairs which wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). As for the elections excited to the conduction band, the holes associated with them in the conduction band are independent (compared to the loosely bound pairs in exciton band). Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanism, again results in light emission (slow component).
  5. Usage? Lighting, mainly on fluorescent lamps, and on some occasions used on metal halide lamps. Glow-in-the-dark toys. Electroluminescence (used commonly in LCD backlights). White LED (turning the blue light from LED into white). Cathode ray tube (television).

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