Tony Withers: Lab: Casting Octahedra

The starting materials

Our 12-TEL octahedra are cast from Aremco Ceramacast 584-OS (specify OS when ordering), a two part ceramic potting compound that consists primarily of MgO, with additional SiO2 and Al2O3 components. We add 2.5 wt. % Cr2O3 to the powder, which may reduce radiative heat transfer. The fired Cr2O3-bearing octahedra have a mottled green colour. To prepare a new batch of Cr2O3-doped powder, weigh enough 584 powder to half fill a large reagent bottle. Weigh the appropriate amount of Cr2O3 into a separate container, and sequentially add powder from the large bottle, shaking the mixture between additions to homogenize the mix. Be careful not to release the fine powder into the atmosphere. When the Cr2O3 mix is diluted to at least 1:10, add it to the large bottle and secure the lid. To homogenize the mixture, secure the bottle by its lid in the lathe chuck and support at the base using a live centre. Spin the lathe at its slowest speed for an hour; provided the bottle is half full this will result in a well homogenized mixture. New batches of activator should be divided between reagent containers of varying size, being sure to tighten the lids and wrap with plastic tape to prevent evaporation. Store the activator in the refrigerator. Activator from the smallest containers is used for casting, and the small containers are occasionally replenished from the larger containers. Decanting into small containers helps to minimize evaporation of the activator. If the activator is stored in a poorly sealed container, it will thicken and precipitate crystals.

Instructions for casting three octahedra for 12mm TEL multi-anvil experiments

You will need:

	Casting jig
	Three metal binding rings
	24 plastic cubes (8 per mould)
	One set of PTFE inserts for each mould
	Plastic spacers
	Plastic beaker
	Small glass beaker
	Stirring rod

Method

1. Arrange the cubes into groups of four, carefully identifying the 12 mm truncations and placing them in the centre of the group. Look carefully for mismatches in truncation length – it’s very easy to make a mistake and end up with a useless octahedron. Three of the cube sets should contain one of the cubes with a slotted face.
   
2. Place assembled PTFE inserts between each set of four cubes.
   
3. Drop a metal binding ring over each of the cube groups that don’t have a slotted cube.
   
4. Insert packing to secure cubes within the binding ring.
   
5. Place remaining PTFE pieces over the cubes in the binding rings.
   
6. Insert remaining cubes
   
7. Remove the three slotted cubes, and place these nearby balanced on the 12 mm truncation, with the slotted face to the rear (so that they can be quickly slid into position in the moulds at a later stage)
   
8. Adjust the position of the binding rings and place each mould in the casting jig. Tighten the wing nuts to hold all of the cubes securely in place.
   
9. Weigh 32 g of 584 powder into a plastic beaker
   
10. Weigh 14g of activator into the small beaker, and quickly add to the powder, stirring for a few seconds to remove any lumps, then pour immediately into the moulds. This stage must be done quickly because the mixture starts to harden very rapidly. Be careful not to overfill the moulds – the ceramic mixture shouldn’t extend above the opening formed by the 12 mm truncations. Throw the plastic beaker into water to prevent the residue from hardening any further. Spend the next 30 seconds poking around with the stirring rod, trying to remove the bubbles that accumulate under the edges of the PTFE inserts. Just as the mixture is hardening, squeeze the slotted cubes into place.
    
11. Clean up. Add the excess Cr2O3-bearing ceramic to the appropriate waste container.
    
12. After a few hours, loosen the wing nuts and gently pry out one cube from each mould. Allow the opened moulds to sit for a few hours, preferably overnight, before disassembling. The octahedra will have the consistency of clay after a few hours, and in this state it’s very difficult to extract them without breaking a fin.
    
13. Clean ceramic residue from cubes and inserts.

After casting:

	Let the octahedra dry for at least 24 hours. Drying for longer in high humidity may help to prevent cracks.
	Inspect octahedra for enlarged fins (too much mixture in mould) and excess bubble cavities at the surface. Add rejected octahedra to the Cr2O3-bearing ceramic waste.
	Fire octahedra at 1000 °C for 12 hours.
	Drill the furnace holes. Use the drilling jig with a 5/32” carbide drill, and drill from each end of the octahedron.
	Inspect internal bore for bubble cavities, and discard any octahedra that look bad.
	When selecting an octahedron for an experiment, weigh and record the weight of the octahedron. Weight should be ?9 ± 0.3 g. Widen the bore of the furnace hole using a 4 mm carbide reamer so that the furnace has a tight fit.

 

 

ACW 2006

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