Centrifuge

Technical specifications of the single stage non-endloaded piston cylinder

• Pressure range: 3 - 18 kbar (0.3 – 1.8 GPa)
  Temperatures up to 1250°C with standard straight graphite furnace (currently we limit the slip-ring stack to 300 amps of current), higher temperatures are achievable using stepped furnaces
• Piston bore: 14 mm
• Piston ram: 35 tons
• Assembly: NaCl (or talc) -pyrex - MgO, with straight or stepped furnace
• Sample capacity: max. 5 mm capsule O.D., max. 8 mm length
• Construction: 2002-2003, in service since Febr. 2004

Technical details

Heating and thermal gradient

Although the piston cylinder is minimized and reduced to 13 pieces, it hosts a standard salt/talc-pyrex-graphite-crushable MgO 14 mm O.D. assemblage with a furnace of 36 mm length, 8 mm O.D. and 6 mm I.D, thus resulting in standard piston cylinder temperature gradients of 5-10°C (at 1100-1250°C) over the capsule length (typically 4 mm O.D., 6-7 mm length). Temperature is measured by a B-type PtRh-thermocouple and controlled by a standard Eurotherm control unit to nominally within ±1°C. Up to 300 A current are transmitted through a water-colled 40-ring slip-ring stack located at the lower end of the hollow rotational axis. Cooling water to the piston cylinder is provided through the center of the lower slip-ring stack and through tubing positioned exactly in the axis. Water flows radially through the piston cylinder pressure vessel and is finally centrifuged outwards, where the water spray is collected by a conical ring running around the rotation table.

Pressure

Oil pressure in the hydraulic ram is measured and monitored on-board by a standard pressure transducer mounted vertically in the axis and connected to the hydraulic ram (which in turn is integrated into the piston cylinders base plate) by a NovaSuisse 4000 bar capillary. Pressure cannot be changed during centrifuging but only statically through a standard hand pump. The pressure and temperature mV readings are converted on-board to a 4-20 mA signal, resulting in a perfectly smooth and rotation speed independent control signal. The latter signals is transmitted through a 16 channel slip ring stack at the upper end of the rotation axis.

Static equilibration

Many experiments consist of two steps: first, chemical equilibration of e.g. melts and solids at the temperature and pressure of interest at static conditions for 12-120 hours, and secondly, physical segregation of the equilibrated melts/crystals by centrifuging for 3-36 hours at typically 400-1000 g. When chemically equilibrating the experiments at static conditions, the heating current is directly provided to the piston cylinder without using the lower slip ring stack.

Safety

The essential safety parameter is centrifuge vibration which is kept to less than 0.8 mm/s at 1800 rpm (equivalent to ca. 1000 g) by balancing the table to better than 10 gram. Nevertheless, the centrifuge does not have a breaking system and it takes about 25 minutes to cast down from 1800 rpm. As an additional safety, oil pressure, temperature, cooling water flow, furnace resistance, and heating power are computer monitored and any significant deviation after the startup phase of the experiment would result in immediate shutdown of the centrifuge and heating.