Optimized Design

As it is found that the heating stage becomes too hot while the sample does not reach the goal temperature the conclusion is drawn that the energy loss from the sample is too large. To solve this issue, several solutions have been found and implemented. First of all, where previously the tantalum nest of springs made direct contact with the sample and SiC-disk, small Al2O3 blocks are inserted between the springs on one side and the sample and SiC-disk on the other, as shown in Figure 7a. These blocks form thermal resistors as Al2O3’s thermal conductivity is much smaller than that of tantalum.

The attachment points of the three base plate legs to the cooling shield, are thermally isolated by using zirconia (ZiO2) blocks. Zirconia has an extremely low thermal conductivity and therefore is useful as a thermal isolator. Note that the reason zirconia was not used in the hood or the nest of springs is that zirconia undergoes a phase transformation from monoclinic below 1170°C, to tetragonal between 1170 and 2370°C [33]. This transformation is accompanied by a 10% volume increase, which makes zirconia unsuited for the hood or the nest of springs. However, as the

Page 17 temperature remains below 1100°C at the end of the legs, zirconia is suited as a material for the endcaps of these legs.

Figure 7: (a) Nest of springs, with the specimen shown in turquoise and with the inserted alumina blocks shown in white, (b) base plate spider with the nest of springs and sapphire balls, shown in blue, and zirconia blocks, shown in white, (c) base

plate that is covered by the hood, shown in white, installed in the copper heat shield

Finally, to minimize the heat loss through conduction, the connection between the nest of springs and the base plate is altered. Instead of using recessed tantalum cylinders to centre the nest of springs, which form six line contacts, three sapphire balls (also recessed in the base plate) are used to form six point contacts. In addition, besides reducing the contact area, sapphire has a coefficient of thermal conduction that is twice as low as that of tantalum [9]. Additionally, the balls do not only thermally isolate the nest of springs but also increase the ease of manufacturing as no cylinders have to be milled out of the tantalum base plate, but three conical holes can be drilled to accommodate the sapphire balls, as shown in Figure 8.

Figure 8: Exploded view of the entire heating stage without the frame. The sample and SiC disk are placed inside the nest of springs which, in turn, is placed on top of the sapphire balls that are recessed in the base plate. The alumina hood is placed over the nest of springs and bolted to the base plate, which has zirconia isolators at the end of its legs. The base plate and isolators are bolted to the actively cooled heatshield, and the backside heatshield is bolted to the cooling to close of the

stage.

Page 18 In order to be able to use a standard optical fibre, it must be shielded from the baseplate. To this end, a heatshield is placed at the rear of the heating stage and is connected to the actively cooled copper tubing, as shown in Figure 8. With this heatshield and resulting increased distance of the fibre to the SiC-disk to keep the temperature of the fibre end connector low enough, a significant part of the light lands outside of the SiC-disk. This is solved by inserting an internally polished thin steel tube, which acts as an extension of the optical fibre to deliver all of the light to the SiC-disk. As this tube may reach temperatures well over 1000°C, it can be placed in the vicinity of the sample. This is illustrated in the exploded view of the entire heating stage shown in Figure 8. By aiming the laser beam in this tube, the light is delivered to the sample without the optical fibre reaching too high temperatures.

The applied changes have also been simulated, see Figure 9. The result is a sample temperature of 1650°C at a laser power of only 60W with a temperature distribution throughout the heating stage that is well within the limitations. The sapphire balls have been modelled as extremely small contact surface as the used software did not support point contact. Since a surface conducts heat better than a point, the sample temperature can only rise when using sapphire balls justifying this simplification.

Figure 9: (a) The temperature distribution of the heating assembly of the optimized design both with the alumina hood. (b) the temperature distribution inside the alumina hood. (c) The temperature distribution in the entire heating stage including

heat shield and cooling. The images above are for a laser input of 60W.

Page 19 In addition to the target temperature being met, the temperature in the cooling circuit rises to only 50°C which is amply sufficient to cool the heating stage. Additionally, the addition of the back heat shield lowered the fibre temperature to 65°C, enabling the use of a regular optical fibre. Finally, the exterior of the copper heatshield and cooling, and the frame remain at room temperature validating the assumption regarding the lack of thermal interaction between stage and SEM. This is shown in Figure 9c. Further results of the thermal simulations can be found in Appendix E. Thermal simulations 2.5.1 Manufacturing

Figure 10: Front and back of the optimized heating stage. Here, the heatshield and combined cooling circuit are brown, the frame is a blueish grey, the clamp that holds the optical fibre is depicted in black, and the zirconia isolators are depicted in

white.

As the coarse model has been constructed from a conceptual point of view, it is not yet fit for manufacturing. For this reason several adaptions have been made to (further) increase the ease of manufacturability. To be able to access all bolts that fix the base plate to the cooling circuit, the assembly is rotated 180° around its central axis, as can be seen by comparing Figure 7c to Figure 4bFigure 5. This is also necessary for the cooling water connections to provide the space they need.

Where first the cooling circuit has been modelled as two superimposed circular tubes, the design is adapted so that it forms one conduit and can be manufactured. Finally, the connection between frame and cooling is made by four bolts. The final design is found in Figure 10 and the detailed drawings for all the separate parts can be found in Appendix G. Detailed Drawings