Magnesium Vapor Pressure

[Juliane Bari]

Thermodynamicproperties of liquid Al-Mg alloys have been determined by vapor pressure measurements over pure liquid magnesium and liquid Al-Mg alloys, in the composition range of 5 to 94 at. pct magnesium between 900 and 1245 K, employing transpiration technique. The results obtained have been compared with those reported by earlier workers and possible reasons for the observed inconsistencies have been discussed. The present investigations indicate that the liquid Al-Mg system nearly conforms to ideal solution behavior, with slight negative deviation. The minimum activity coefficient value for magnesium and the maximum integral molar heat of solution for the alloy have been found to be equal to 0.6 at 10 at. pct and -2.49 kJ/g atom at 50 at. pct magnesium respectively at 1000 K.

Magnesium is a grey-white, alkaline earth metal with a melting point of 649C, a density of 1.74 g/cc, and a vapor pressure of 10-4 Torr at 327C. It is extremely flammable, especially in powder form, and fires are difficult to extinguish. Magnesium is also an essential element to life. Magnesium is responsible for regulating nerve function, blood pressure, and the levels of nutrients in the human body. It is extensively used in the production of aircraft, engine casings, laptops, cell phones, and cameras. Magnesium and its alloys are evaporated under vacuum for the development of magnetic storage media, optical storage media, and semiconductors.

Another alternative is to change crystals frequently and ignore the error. The graph below shows the % Error in Rate/Thickness from using the wrong Z Factor. For a crystal with 90% life, the error is negligible for even large errors in the programmed versus actual Z Factor.

Cleaning material overspray is a concern for many users depositing magnesium films. Proper cleaning procedures are essential to preventing film contamination. Magnesium has a relatively high vapor pressure versus temperature curve. If a process chamber contains a substrate heater, subsequent films run the risk of contamination from residual magnesium when working at temperatures above 250C. The vapor pressure of magnesium at this temperature is around 2 X 10-6 Torr. Generally, systems that are used to deposit magnesium are dedicated for magnesium film work alone because of this high vapor pressure versus temperature curve.

Another concern is the pyrophoric nature of magnesium overspray. The overspray is invariably highly granular and, depending on the surface area to volume ratio, can spontaneously combust in air. Water presents another problem as its presence can help ignite the magnesium flakes. Users should consult their lab safety manager for more information about avoiding and extinguishing magnesium fires.

A key process note is to consider the fill volume in the e-beam application because we find that the melt level of a material in the crucible directly affects the success of the crucible liner. Overfilling the crucible will cause the material to spill over and create an electrical short between the liner and the hearth. The outcome is cracking in the crucible. This is the most common cause of crucible liner failure. Placing too little material in the crucible or evaporating too much material before refilling can be detrimental to the process as well. When the melt level is below 30%, the e-beam is likely to strike the bottom or walls of the crucible which immediately results in breakage. Our recommendation is to fill the crucible between 2/3 and 3/4 full to prevent these difficulties. Crucible liners should be stored in a cool, dry place and always handled with gloves or forceps.

KJLC recommends a fill rate between 67-75%. Overfilling the crucible will cause the material to spill over and create an electrical short between the liner and the hearth causing the crucible to crack. Placing too little material in the crucible or allowing the melt level to get too low can be detrimental to the process as well. When the melt level is below 30%, the e-beam is likely to strike the bottom or walls of the crucible which immediately results in breakage.

The fill rate above assumes that the material is fully melted and does not take into account packing density. It should be noted that the crucible liner may need to be loaded multiple times, pre-melted, and topped off in order to achieve the final desired melt level/fill rate. When loading the crucible, do not load more than 80% of the height of the crucible liner.

This calculator is for estimation purposes only. The estimated mass is that needed to produce a desired film thickness (1 micron in this case) on a flat substrate at a point directly above the source, accounting for the approximate plume distribution of the selected source type. It does not account for any expected nonuniformity in thickness over a larger substrate area. More complex geometries, e.g. those with offset and/or tilted sources, may have higher material requirements. The estimated mass is for the film deposition only; additional margin should be included to account for loss during ramp-up, burn-in, stabilisation and ramp-down.

Magnesium (Mg) is another metal that, when heated in a vacuum, can also volatilize quite easily, and should therefore (like Zn and Cd) never be used in any vacuum furnace used for high-temp aerospace brazing of stainless or super-alloy base metals.

Fig. 1 Magnesium (Mg) plays a very important role in the vacuum-brazing of aluminum components because of its strong oxygen-gettering capability. Illustration Kelly Brogan MD, and is reproduced courtesy of Kelly Brogan MD from her newsletter at kellybroganmd.com.

As mentioned in my article last month, it is critically important to be aware of the vapor pressures of any materials that are processed at elevated temperatures in a vacuum-furnace, because a vacuum can effectively lower the temperature at which a particular material will volatilize (outgas). We learned that you should never try to vacuum-braze brass, a copper-alloy which contains zinc (Zn), because Zn is a metallic element which can easily volatize when heated. The same is true for cadmium (Cd), a metallic element that is added to a number of silver-based brazing filler metals (BFMs) to lower its melting temp and improve wetting (such as in AWS A5.8, Class BAg-1).

Magnesium (Mg) is another metal (see Fig. 1) that, when heated in a vacuum, can also volatilize quite easily, and should therefore (like Zn and Cd) never be used in any vacuum furnace used for high-temp aerospace brazing of stainless or super-alloy base metals, since Mg contamination in such furnaces could ruin the furnace, rendering it non-useable ever again for any critical high-temp aerospace applications.

IMPORTANT REMINDER: Aluminum should ONLY be brazed in vacuum furnaces specifically built for aluminum brazing. Aluminum should NEVER be brazed in standard high-temp aerospace-brazing furnaces!

Mg can be added as a constituent of the aluminum base-metal chemistry, or perhaps added to the brazing filler metal (BFM) chemistry, or it may be added as a separate entity (as chips or powder), such as that shown in Fig. 3, and placed in a small crucible placed near the parts that are being brazed. As the temps increase in the vacuum furnace, the Mg (in powder or chip form) may be expected to volatilize and become most active at temperatures above approximately 1000F (525C). If the Mg is alloyed into either the aluminum base metal or the BFM, then the temperature needed for volatilization will be a bit higher, in the range of about 1050-1060F (570C), since the Mg is alloyed with the aluminum and silicon in the base materials.

Then, as mentioned earlier, when Mg volatilizes, it forms a gaseous Mg-cloud that will grab onto any available oxygen (to form MgO) thus preventing that oxygen from reacting with the clean aluminum base metal surfaces revealed at the bottom of the cracked Al-oxide layer.

The amount of magnesium needed for efficient gettering will vary according to the size of the vacuum-furnace hot-zone and the size of the part being brazed, and will probably need to be determined experimentally.

According to Duke Singleton of Singleton Technologies, Richmond, VA (Duke is an expert in vacuum-brazing of aluminum) the use of about 3-to-10 grams of magnesium (Mg) per cubic meter of furnace volume is a good place to start when trying to determine how much total Mg to use for effective gettering.

The effectiveness of Mg-gettering is also dependent on the leak-tightness of the vacuum chamber, and the temperature at which the cooling water is operated in the furnace walls. If there is a significant leak-rate of outside atmosphere into the furnace, then the oxygen and moisture in the air leaking into the furnace can quickly destroy the effectiveness of the Mg-gettering.

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