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Metal Oxide Varistor: Pure Zinc Oxide at 1300 °C, Bismuth Oxide Paste Fired at 800 °C (Masuyama 1972, Ex. 4)
A varistor made the other way round: a disc of pure zinc oxide, sintered, with bismuth oxide fired into its faces from a paste instead of mixed in. Sintered at 1300 °C and fired with the paste at 800 °C, the disc measured a C-value of 100 at 1 mA and a nonlinearity exponent n of 7.5. Masuyama found that firing on the bismuth oxide paste raises n a great deal over mixing it in.
Source: T. Masuyama, M. Matsuoka and T. Nishi (Matsushita), US 3,663,458 (1972), Example 4. Unknown: what the silver paint is, and how strong the epoxy solution of the paste is. The spraying is ours, after US Army Corps of Engineers EM 1110-2-3401 (1999). The 10 g batch is our arithmetic. Picture: the patent's drawing.
Advanced
2 days
Instructions
1
1
Press pure zinc oxide at 340 kg/cm²
Press pure zinc oxide at 340 kg/cm²
Press zinc oxide powder at 340 kg/cm². Example 4 gives no disc size; ours: discs 13 mm across and 2.5 mm thick in the mould of Example 1.
Materials for this step:
Zinc Oxide Powder1 as requiredTools needed:
Balance Scale
Hydraulic Press
Disc Pressing Die
Vernier Caliper2
2
Sinter 1 hour at 1300 °C and cool
Sinter 1 hour at 1300 °C and cool
Heat the pressed disc for 1 hour at 1300 °C, then let it cool to room temperature.
Tools needed:
Muffle Furnace3
3
Lap both faces
Lap both faces
Lap both faces with silicon carbide abrasive. Example 4 gives no thickness.
Tools needed:
Silicon Carbide GritVernier Caliper4
4
Coat with bismuth oxide paste and fire at 800 °C
Coat with bismuth oxide paste and fire at 800 °C
Coat both lapped faces with a paste of 50 % by weight bismuth oxide in a solution of epoxy resin in butyl alcohol, and fire it in air at 800 °C for 30 minutes. The bismuth oxide diffuses into the zinc oxide. Not given: how much paste, or the strength of the epoxy solution.
Materials for this step:
Bismuth Oxide1 as requiredTools needed:
Epoxy Resin
n-ButanolMuffle Furnace5
5
Spray aluminium electrodes and attach leads
Spray aluminium electrodes and attach leads
Spray a metallised film of aluminium on both faces. Attach a lead wire to each with conductive silver paint, as in Example 1. Ours, after US Army Corps of Engineers EM 1110-2-3401 (1999): wire-arc spray the aluminium from two 3.2 mm wires melted by an 18-40 V DC arc and atomised by compressed air, 15-20 cm from the disc and never at less than 45°, in at least two coats at right angles of 50-75 µm each. Unknown: the paint.
Materials for this step:
Aluminium Ingot1 as 3.2 mm spray wire
Silver Conductive Paint1 as required
Copper Wire2 piecesTools needed:
Vernier Caliper
Wire-Arc Spray Gun
Industrial Air CompressorMaterials
5- 1 as requiredPlaceholder
- 1 as requiredPlaceholder
- 1 as 3.2 mm spray wirePlaceholder
- 1 as requiredPlaceholder
- 2 piecesPlaceholder
Tools Required
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