Thermocouple guide

At ID06LVP, thermocouple insertion is performed from one of the ends of a sample column of the octahedron. An example below is a rough idea of preparing an assembly for experiments with a thermocouple. You are free to use your technics for a TC installation.

Experiments with a thermocouple (TC) require the usual octahedron to be modified by cutting a groove (Fig. 1) Use a diamond saw or any other instrument you find useful. Ideally, the groove should be as narrow as a TC wire, except for spots where it crosses isolation/furnace and goes out (around 0.8-0.9 mm based on the size of TC isolation, see below).

Figure 1.  A 3-D illustration and a cross-section of an octahedron + inner assembly parts prepared for a thermocouple insertion.

The Top (TC side) inner part of the assembly has a 4-hole alumina (Al₂O₃) rod (diameter varies with assembly size) and MgO/ZrO₂ drilled (alumina rod diameter) (Fig. 2). TC junction is a cross of wires and is isolated from the capsule by an alumina disk. TC wires should be 10-12 cm long (depending on the assembly dimensions). ID06LVP lab operates with two types of TC: W₉₇Re₃-W₇₅Re₂₅ (Type D) and W₉₅Re₅-W₇₄Re₂₆ (Type C). Consult with your LC about availability.

Figure 2. Instruction for a TC installation into the octahedron.

Prepare short alumina one-hole rods (x2) for the TC isolation from the furnace/cubes. Its length depends on the assembly; however, it should be long enough to protect TC from the furnace. A copper coil (x2) is necessary for the prevention of the TC wire being broken during compression. To make one, ask your LC for a proper copper wire. Use a pin vise with a 0.6-0.8 drill bit fixed. The length is around 12-15 mm (even longer for bigger assemblies), so it should be long enough to stick in the octahedron and be at least twice longer than the width of the pyrophyllite gasket. Once done, [1] pass the end of a TC wire through one of the one-hole rods. [2] Bend the wire and fix the rod so that the TC wire is isolated from the furnace (Fig. 3). I recommend keeping the alumina rod as close to the bending place as possible (use a tiny drop of superglue if needed). [3] Pass the end of a TC wire through a copper coil and stick the coil in the octahedron. I suggest that the coil be quite deep inside, perhaps even bent along with the TC wire. [4] Bend the TC wire along the cut in the octahedron (use a razor blade or any other thin instrument you may find useful). Mind that the copper should go out of the octahedron exactly (or very close) in the middle of its edge. [5] In the end, fix/fill this cut in the octahedron using a special cement (ask your LC) with a consistency of toothpaste. Use a razor blade to stick the cement in the cut of the octahedron, and try to make it dense. Parts of cement protruding from the octahedron must be removed, so the shape of the assembly is untouched. Repeat for another side. You may need to keep the assembly at >100 ^oC for around 30 - 60 min.

Figure 3. Instructions for TC bending in the octahedron.

During assembling the cubes, some alternations from a no-TC experiment are needed (Fig. 4). [1] Mark those gaskets through which TC wires pass and [2] mark where exactly. TC path depends on your octahedron assembly, but if everything was assembled according to this tutorial, it should be similar to the one shown in Fig. 4. Make sure the orientation of the octahedron is so that x-ray windows are along the beam path! [3] Drill holes through the gaskets so TC wires and a copper coil could stick through (a pin vise with a 0.9-1 mm drill bit fixed may be useful).

Figure 4. Orientation of the octahedron with a TC.