Import substitution of W cathodes for scanning electron microscopes

The history of self-manufacturing of tungsten cathodes began with the MREM 200 scanning microscope, about which I posted an article here in 2020. The reason was the high cost of the original cathodes. The supplier requested 18,000 rubles apiece. And in a microscope, this is a consumable, like a light bulb on a landing. As a result, I had to master the whole technology, including equipment for forming the filament, equipment for precise alignment of the filament with holders during welding. We also had to make a welding machine that can even weld tungsten filaments together. And the obligatory annealing of the finished cathode in vacuum to clean the surface of the filament and relieve mechanical stress. Annealing is carried out in a thermal vacuum deposition unit. Cathodes in a special holder are installed instead of the evaporator. Otherwise, when the microscope is turned on for the first time, the high-voltage cathode assembly begins to close with evaporating impurities and the tip of the filament is displaced from the adjustment position. The finished cathode for MREM 200 looks like this

For the sake of interest, a photograph of the burnt cathode MREM 200 was taken

The development of technology resulted in the manufacture of cunning pointed cathodes for an experimental microscope being developed by my friends in Chernogolovka. Filament blanks with a welded tip look like this

After all the dances with a tambourine during manufacture, the finished cathode turned out like this (at the request of the Customer, a cathode holder from MREM was chosen as a holder)

I took photos of the cathode annealing process in vacuum. Cathodes are annealed in pairs

The finished tip was also sharpened before annealing. I took a photo of the tip

Recently, another friend asked me to make a sample cathode for the FEI microscope. They sent a box of burnt cathodes, on the holders of which a new filament needs to be welded. The microscope is used 24/7 and there are almost no cathodes left. It will be difficult to get new ones from the USA, to put it mildly. So far, using a bypass technology – a maximum of manual labor was made by one cathode per sample. One of these days I will send it for testing. Then you will have to manufacture all the equipment for the technical process for this type of cathode.

This is what the finished FEI cathode looks like. I made a couple at once, but as it usually happens – one dropped it with a filament down to the floor. I still use the USSR filament from old stocks. In my microscope, it showed excellent performance and long life. Modern tungsten filament is often of poor quality and even brand new cathodes quickly burn out. I hope there will be a sequel….

PS Thanks to my last article about the electron microscope, I met interesting people – enthusiasts of electron microscopy. Thanks XABR.

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