Sample mounting

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Metal crystals


crystals Polished metal single crystals are usually mounted on Ta heating wires (diameter 0.25 or 0.4 mm) annealed by passing current.

Round shape and 5-10 mm diameter is optimal, the thickness in the range 1-2 mm. The smaller the crystal the higher annealing temperatures can be easily reached. The annealing limit for small and thin crystals (5 mm diameter) is about 1100 °C/1400 K, larger crystals (10 mm diameter) will hardly reach 700 °C/1000 K.
crystals The crystal should have 2 slits (0.3-0.5 mm width). If not it can be spot-welded directly to the heating wires (only some materials). Spot-welding improves the heating contact and provides easier annealing and cooling.

The thermocouple can touch the crystal from the backside like a spring or be spot-welded to the crystal (only some materials). Spot-welding improves the thermal contact and provide more precise temperature reading. The error can be up to 100 K/°C.

For annealing usually the 70 V×24 A DC power supply is sufficient. Maximum annealing current is 20 A. The centre of the sample is theoretically at half of the annealing voltage so it is difficult to acquire spectra during annealing
crystals Example of Cu crystal (8 mm diameter × 2 mm thickness) mounted.
crystals Example of Ir crystal (5 mm diameter × 1 mm thickness) mounted.
crystals

Assembly:

item count name material notes
S 1 sample metal preferably 5-8 mm diameter (max. 10)
and 1-2 mm thickness (max. 3)
1 1 baseplate molybdenum 1 mm thickness
2A 1 thermocouple pins alumel
(magnetic)		 must stick out 3.8-4 mm below the baseplate bottom surface
2C 1 chromel
(non magnetic)
3a 6 ceramic spacers alumina thinner
3b 2 thicker
4 8 nut molybdenum M1.2 thread
5 1 thermocouple chromel
(non magnetic)
alumel
(magnetic)		 0.1-0.25 mm diameter wires, spot-welded together
and touching the sample or spot-welded to it
6 4 rod molybdenum M1.2 thread
7a 1 annealing contacts tungsten brushed on the left side, see surfaces highlighted in yellow;
must be 3.6-3.8 mm above the baseplate top surface
7b 1
8 1 heater tantalum 0.25-0.4 mm diameter wire; thinner one makes annealing more efficient (lower currents), thicker one makes position better fixed and cooling more efficient
9 2 clamp tantalum 0.25 mm thickness
crystals For the correct thermocouple pin contact the pins must stick out 3.8-4 mm below the baseplate bottom surface.
crystals For the correct connection the annealing contacts must be 3.6-3.8 mm above the baseplate top surface. This distance (if 3.5 mm or lower) can be increased by placing tiny spacers made of properly thick wire below the bottom ceramic spacers (3a) or swapping the thin ceramic spacers (3a) with the thicker ones (3b).

Check in the areas labelled by green circles that the (too big) sample (S) is not touching directly the annealing contacts (7ab). In such case the annealing current would not flow through the thin annealing wires (8) but directly through the thick crystal (S) which would make the annealing inefficient and high temperatures could not be reached.


Last Updated on Thursday, 09 March 2023 10:32