Tuesday, July 24, 2012

Copper Wire - Cooler than it sounds (at 5,000x)

An undesirable phenomenon called charging can occur when using a scanning electron microscope.  The electrons from the electron gun need a conductive pathway.  Nonconductive specimens will accumulate a charge - they become very bright on the screen.  The worst part is that at some point they will discharge which can distort the image.

There are lots of ways to deal with this, like reducing the gun voltage, but I wanted to try my hand at using some high voltages.  My solution - use copper wire. 

I found an old telephone line, opened it up and stripped the insulation off one of the small wires inside.  (I used a pocket knife - not such a good idea as you will see.)

Here are my lovely copper wires at 37x  (Gun voltage = 15kv)

A closer look at 1,000x

At 5,000x the smooth wires are looking a little rough.
Notice the striations that show where the wire was drawn through the die.

All of the images above were made using the secondary electron detector, thus the "SE" on the bottom of the picture.  SE images are made in a high vacuum.  The problem is, I purposely left some the insulation on the wire.  Talk about charging!!  Another solution to the charging problem is to use the back scattered electron detector.  This detector can work in a partial vacuum.  That little bit of air can solve the charging problem.  Notice on the images below the "BSE" as well as the atmosphere in the scope.  In the next image shows an atmosphere of 6 Pascals.  (Standard atmospheric pressure is 101,325 Pa.)

 Why you shouldn't strip wires with a pocket knife (42x)


Copper is soft.  Looks like I have effectively reduced the diameter of the wire by half. (400x)

 Back to SE mode at a lower voltage.  The surface of the wire shows what appears to be crystallization. (1,600x) I am going to have to go back to the lab and take another look. 

Have something you would like to see up close and personal?  Give me a call.

Murry Gans
Scanning Electron Microscope Lab Coordinator
Eastfield College


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