gh LITHIUM IN ALLOYS OR INSTANT OF TRUTH FOR MiG AIRCRAFT AND OTHERS

Chemists from the Lomonosov Moscow State University have developed a method that enables to quickly, precisely and practically without detriment to the subject of inquiry determine the lithium content in aluminum alloys and other materials containing this metal. The method is based on sample evaporation from the surface by a focused laser beam with subsequent spectrum analysis of laser plasma. However, in this case the basic concept is not the main thing. The truth is in details. So, to discover them, i.e. to find optimal conditions for the experiment, the researchers had to accomplish an enormous volume of research. However, the work was worthy of it.

It should be noted that it is absolutely necessary to know the lithium content in aluminum alloys, first of all – particularly as applied to aircraft construction and rocket production. The point is that the lithium- magnesium-aluminium type alloys possess a number of unique properties, the most important of which is a surprising combination of lightness and the highest durability. Lithium is in many respects responsible for these, although its content in the alloy is quite low – no more than several percents. But how much in particular – that is what should be determined very accurately. As it is often required to analyze not only the metal ingot but also a complete product, for example – a rocket, the analysis should be made separately – the principal part and welding seam independently, so the method should be non-destructive. For example, like the one suggested by Professor Nikita Zorov and his colleagues.

So, the essence is as follows. A powerful laser beam is directed at the surface of the subject of inquiry. Should the beam be “thicker” and should it shine for a longer time, there would be a hole in a piece of metal. However, a lens helped to focus a beam of radiation on a minor “patch” of surface, its square making only 0.34 mm, at that each impulse duration made about 15 nanoseconds (15 billionth part of a second). So, billionth parts of a gram of metal had time to evaporate thus turning into plasma. The image of obtained laser torch was projected by another lens onto the spectrograph slot. And then, the device recorded emanation at different wave-lengths with the help of a charge-transfer device of a ruler. This enabled to identify the plasma content, and consequently – subject of inquiry per se. First of all, this enabled to determine the lithium content.

Generally speaking, identifying lithium by its ability to color flame in the red color is the classics. Excited atoms of this alkali element emit radiation in the red (670nm) and orange (610nm) spectral regions, which is perceived in total as a carmine-red tint. But what wave length should be chosen and when should the spectrum be recorded? If plasma radiation spectrum is recorded simultaneously with the laser impulse or immediately after that, spectral lines of different elements’ atoms (the alloy and air components) will overlap and it will be difficult to single out the lines related to lithium. If there is a delay, the laser torch will simply go out. These are the questions the authors answered. They have been supported in their effort by the Russian Foundation for Basic Research.

Interesting details were discovered in the course of investigation. For example, it has turned out that the first laser impulses should not be taken into account – at that time all “dirt” from the surface flies into the plasma. But after approximately two hundred impulses have been made, the material of interest to the researchers begins to evaporate. It is best to record the spectrum approximately in 500 to 600 nanoseconds after completion of a laser impulse – at that time the lines are sufficiently narrow and are still sufficiently intense. However, it is better to “accumulate” the data – to “shoot” about fifty times. Nevertheless, the crater from the entire series of laser hits remains practically unnoticeable on the surface.

Besides, it has turned out that the most informative is the 610 nm lithium line. The researchers have discovered that sensitivity of a definition is in this case more than three times higher than if the 670 nm line is used. However, the method developed by the university chemists enables to identify the lithium quantity in very different aviation alloys. This is done very quickly – altogether, within several milliseconds. The analysis will do no harm to the airplane or a rocket – because it takes no more than the ten thousandths of a milligram. This is very little, but the information about the lithium content is complete and accurate.

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