Why calcium is stored under vacuum
Answers
Answer:
Calcium, along with its alloys and compounds , is evaporated under vacuum to make optical storage media, superconductors, and
Answer:
Calcium is classified as an alkaline earth metal on the Periodic Table. It is silvery-white in appearance and soft enough that it can be cut with a sharp knife. It is known to easily oxidize when exposed to the atmosphere. Calcium has a density of 1.54 g/cc, a melting point of 839°C, and a vapor pressure of 10-4 Torr at 459°C. Calcium is an essential building block for human life as it is found in abundance in human skeletons and teeth. Calcium and its compounds are also used to produce commercial concrete and cement. Calcium, along with its alloys and compounds, is evaporated under vacuum to make optical storage media, superconductors, and anti-reflective layers.
Explanation:
One of the challenges of thermally evaporating calcium is controlling the deposition rate, which is dictated by the temperature. The temperatures required for calcium to evaporate makes it difficult to control the deposition rate. Calcium will evaporate at an appreciable rate until the temperature reaches ~500°C. At higher temperatures, there is risk of calcium oxide (CaO) contamination in films. However, this can be avoided as long as the temperature is controlled well below 1,000°C. Calcium oxide (CaO) will evaporate at an appreciable rate at ~1,700°C.
We have reported success evaporating calcium out of our EVCH12 heater with an EVC5AO crucible. Great care must be taken when installing the heater to prevent the outer shields from becoming warped which can cause a short in the heater, causing the welded joints to fail. The heater should be centered between the contacts and the outer shielding must be clear of the leads. Crucibles should be stored in a cool, dry place and always handled with gloves or forceps.
A more cost-effective alternative would be to use a dimple-style, tungsten.
A more cost-effective alternative would be to use a dimple-style, tungsten boat such as our EVS8B005W if using a KJLC system. However, it is more difficult to control the temperature with a tungsten boat than with the crucible heater/crucible set-up. It is important to note that temperature should be closely monitored while executing either method.
Since calcium has a relatively high vapor pressure at only 459°C, one must carefully consider executing calcium evaporation in any deposition system. Calcium is very difficult to remove from deposition chambers, and it could possibly contaminate subsequent films. For this reason, some users prefer to use only dedicated vacuum chambers for calcium deposition.
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