MTI

Notes

1. The conductivity of Borosilicate (Pyrex) glass is much less than Soda-lime glass. Typical Borosilicate glasses contain 4% Na₂O versus 15% for Soda-lime glasses. Try a piece of Borosilicate glass in place of Soda-lime glass and see what happens.

 

2. As suggested by the authors, the temperature dependence of electrical conductivity in other ceramics can be demonstrated. Zirconium oxide and silicon carbide are suggested. It is also suggested to use a steel nail to illustrate that the nail will conduct over the entire temperature range, and the bulb will not dim perceptibly when the nail is heated. (See page 445-446 of article)

 

3. Use a DC power supple capable of 110 V, 2 A instead of a AC source. After the current flows for a few minutes, the glowing bulb will dim and will not conduct well. This is due to electrolysis of the glass. (See page 447 of article)

 

4. One could also connect a ammeter and measure the amperage versus temperature (use a thermocouple to determine approximate temperature).

    

   One method to melt glass for production is Electric Melting. Initially, gas-fired combustion is used to heat the glass to a temperature where it will conduct. Electrodes (normally molybdenum) are used to conduct electricity into the melt which then heats by Joule heating. You may notice Joule heating if you remove the torch and see the light bulb continue to glow for a few minutes. (See page 445 of article)

    

   Carbon electrodes were inserted in the soil at Hanford to demonstrate the vitrification of contaminated soil. This project was called "In-Situ Vitrification" and was developed by Battelle Pacific Northwest Laboratories for the Dept. of Energy. A mixture of powdered graphite and glass was first laid between the electrodes on the surface of the soil. This mixture provided the initial conductivity.

    

   You can easily make glass from borax by using a Bunsen burner or propane torch. This makes an interesting demonstration as borax can be obtained from your local grocery store (20 Mule Team). Prospectors used a borax bead test to help determine the mineral content of their samples. This was called the borax bead test.

Here are a few tricks to make this experiment work. The main problem is to get a good contact between the copper wire holder and glass specimen.

1. Clean inner contact wires with file or steel wool, or heat up in reducing flame (but this never worked for me!)

 

2. Bend the copper wires into proper position to hold glass before you place glass into "clips" ... problem is when there is some "spring" to the wires... when you heat up the wires will spring away from the glass.

 

3. Use a small piece (1") of glass to cut down on heat loss.

 

4. Try it out with propane torch... moving torch back and forth (thermal shock is not a big problem with the small pieces of glass)... need to keep whole piece of glass hot. Sometimes I come in from the side or use two torches.

 

5. If the contact is not good... you can still tell if the glass is conducting because you will get arcing at the contacts. If you get arcing.... use this trick..

• Heat up the glass WITHOUT ELECTRICITY PLUGGED IN and squeeze the wire contacts into the soft glass.

 

• Now plug it in and heat up.

• As the glass softens and becomes conductive you may witness joule heating: the glass starts to sag and gets thin in the center, current density increases and glass begins to self heat, and you will see a "white" hot center area. Asthe temperature goes up , viscosity decreases and the glass sags until it necks down to breaking.

I used both a regular light bulb and a small-base light bulb. For the standard light bulb it was easy to find a ceramic base. But for the small-base light bulb I could not find a ceramic base and used a phenolic base. A phenolic base is preferred over a plastic base if you can not find a ceramic base for the light bulb.

When you heat up the light bulb stem containing the lead wires, there is another interesting effect you can discuss. When the class cools you should notice a metallic or "chrome" surface on the glass where it was heated. The sealing glass used for the stem probably contains lead oxide. The lead oxide is reduced at the surface to give the metallic coating. I used to use this method to put a metallic or smoked surface on the openings of hand-blown lead glass vases!