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Barium Ferrite Disc Magnet, Sintered at 1300 °C (Went, Philips 1956)
The dark grey ceramic magnet on fridges and in loudspeakers is barium ferrite. It is made from two cheap powders: one part barium carbonate to six of iron oxide, milled, fired, milled again, pressed and sintered. No cobalt or nickel is needed. This is Example 34 of the Philips patent. This iteration sinters at 1300 °C, where his rod held 1,850 gauss with a coercive force of 1,780 oersted.
Source: J. J. Went, G. W. van Oosterhout and E. W. Gorter (Philips), US 2,762,777 (1956). They pressed rods; the claim covers "a body of a desired shape", and the disc size is ours. The patent does not say how to magnetise it; our way is stated. Picture: ferrite magnets, Omegatron, Wikimedia Commons, CC BY-SA 3.0.
Advanced
2 days
Instructions
1
1
Weigh one part barium carbonate to six of iron oxide
Weigh one part barium carbonate to six of iron oxide
Weigh 39.5 g of barium carbonate (0.2 mol) and 191.7 g of ferric oxide, Fe2O3 (1.2 mol). Barium carbonate is poisonous if swallowed: ours, gloves and a dust respirator.
Materials for this step:
Barium Carbonate (Witherite)39.5 g
Ferric Oxide191.7 gTools needed:
Balance Scale
Rubber Gloves
P100 Respirator2
2
Mill with ethanol for 4 hours
Mill with ethanol for 4 hours
Grind the mixture with ethanol in an iron ball mill for 4 hours. Let the ethanol evaporate, then dry the powder in a drying oven.
Tools needed:
Iron Ball Mill
Ethanol - Lab Grade, 95%
Drying Oven3
3
Presinter at 1000 °C for 4 hours
Presinter at 1000 °C for 4 hours
Fire the dry powder in air at 1000 °C for 4 hours (ours: in a lidded porcelain crucible in a muffle furnace). Let it cool.
Tools needed:
Muffle Furnace
Crucible with Lid (Porcelain)
Crucible Tongs
Heat-Resistant Gloves4
4
Mill again for 4 hours
Mill again for 4 hours
Grind the presintered product with ethanol in the ball mill for another 4 hours. Let the ethanol evaporate.
Tools needed:
Iron Ball MillEthanol - Lab Grade, 95%Drying OvenP100 Respirator5
5
Press discs at 1.5 tons per cm²
Press discs at 1.5 tons per cm²
Press the powder at 1.5 tons/cm². The patent pressed rods; we press discs. Ours: a die 20 mm across, so about 4.7 tonnes on the punch (our arithmetic: 1.5 × 3.14 cm²).
Tools needed:
Hydraulic Press
Disc Pressing Die6
6
Sinter 4 hours at 1300 °C
Sinter 4 hours at 1300 °C
Sinter the discs for 4 hours at 1300 °C in oxygen or in air. The patent's rod at 1300 °C: 1,850 gauss and 1,780 oersted.
Tools needed:
Muffle FurnaceCrucible TongsHeat-Resistant Gloves7
7
Magnetise the discs
Magnetise the discs
The patent gives no magnetising method. Ours: lay a disc with its faces against the poles of a strong electromagnet, switch the current on, then off. Find which face is north with a compass.
Tools needed:
Electromagnet
CompassMaterials
2- Placeholder
- 191.7 gPlaceholder
Tools Required
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