Heating System

C.1 Possible choices

As reaching an extremely high temperature is the main goal of this study, the heating of the sample is of great importance. Therefore, a choice must be made between various heating solutions. As there are multiple options to achieve high temperatures it is worth looking into the one that best suits the purpose of this research, especially when combining the heating with tensile testing. Some of the criteria are that the thermal load it exerts on the SEM should be as small as possible, it should minimally influence data acquisition, it should be as compact and simple as possible and it should be applicable in an already existing tensile stage.

When analysing previous research done at high temperatures, the primary modes of heating are found to be heating by conduction or radiation [12-26]. In the case of conduction, a heated block of electrically non-conductive material is placed under the sample. This block is heated by a coil either underneath or inside it, by applying an electric current to it. For radiation, a coil is placed in the vicinity of the sample and radiates heat towards it.

Both aforementioned techniques rely on the Joule-heating effect, also known as Ohmic-heating, to heat the sample. This heating effect is the process by which a current passed through a material produces heat. A well-known application of this effect can be found in incandescent light bulbs where a filament heats up due to current passing through it. Taking such a filament as an example shows the effect’s potential for heating, as the temperature reached in a light bulb lies at around 2800-3000°C, well above the goal temperature of 1500°C.

With these techniques temperatures of around 1000 °C have been regularly reached for EBSD, some studies even reached 1450°C for SE imaging. However, at these temperature, when using a coil, shielding of this coil becomes necessary as thermionic emission starts to occur from the coil and not only the sample. Additionally, introducing such a coil or furnace block increases the hot mass inside the vacuum chamber and therefore also the thermal load on the components inside it.

Where conduction and radiation by a furnace block or coil heat the sample indirectly, it is also possible to heat the sample directly with the Joule-effect by passing current through the sample itself. Comparing it to a light bulb filament, the sample would then be the filament. However, as current is directly applied to the sample, a potential downside to this technique is the possibility that the current applied to the sample interferes with the electron beam. Therefore it would be advisable to test this mode of heating both while the sample is being heated and in between heating to determine if and what the influence of the applied current might be. An advantage to this technique is that the only hot mass is the sample itself and therefore the hot mass is kept to a minimum. A major downside to this technique is the requirement it sets for the conductivity of the sample, i.e.

non-conductive samples are not suited to be heated by this technique.

The drawback of being limited to using conductive samples can, however, be overcome by using a principle that has frequently been used in SEM cathodes; the Mini-Vogel-Mount [46]. This principle clamps two pyrolytic graphite (PG) blocks together with the cathode, made of electrically conductive material, together in a V-shaped mount as can be seen in Figure 31. This mount is then electrified and the two PG blocks act as resistive heaters by the Joule-heating. In this case, of course, the current still passes through the cathode in between the two PG blocks keeping the sample’s

Page 49 conductivity condition. However, it is also possible to electrify a single PG block and place the sample on top of it and using conduction to heat the sample. As no current passes through the sample, it does not need to be electrically conductive. However, the amount of hot mass does increase as both the pyrolytic graphite block and the sample have to reach the high target temperature.

Figure 31: Schematic image of a Mini Vogel Mount as used for SEM cathodes [47].

Finally, a technique has been proposed in the past where the sample is indirectly heated by an infrared laser [48, 49]. Here, a laser heats a small silicon-carbide block that also functions as sample holder, this block then heats the sample through conduction. The largest benefit to this technique is the fact that no electrical current is introduced into the SEM chamber. This means that the electron beam does not risk being influenced by such a current. It also eliminates the need to apply electricity to the sample and therefore will definitely not influence the interaction between sample and electron beam. However, this technique also has drawbacks: both the lasers, which should have enough power, and the optical components, needed to feed the laser into the SEM, are quite expensive. Also, due to the necessity of a light absorbing block between laser and sample, the hot mass is also larger than for the direct Joule-heating technique (but not for the indirect Joule-heating technique). This block is necessary since metals don’t generally absorb light very well due to their reflectivity.

C.2 Validation design

Based on the various techniques proposed in the previous section, a choice was made to test several techniques in order to validate their potential use in a final design. First of all, the use of a heating block or coil in previous research makes it redundant to test its working as this has already been done by others [13, 15, 26]. As there has also been some research done by laser heating [48, 49], and the optical components for laser heating are quite expensive, the use of laser heating will not be validated separately . Finally, an induction coil and further necessary appliances to make induction possible are also very costly. Since no relevant prior research has been done with this technique, the combination with its cost results in the exclusion of this technique.

The choice not to test the three aforementioned techniques leaves direct-Joule heating and the MVM-principle to be tested. As there are no heating stages available that use these techniques, a new design has been made to validates them. Since the choice has not been made with respect to

Page 50 which heating technique will be chosen, it is important that the design remains simple and cheap as it will be discarded after its use.

Since the direct Joule-heating and Mini Vogel Mount(MVM) derived heating techniques both use a current passing through a material to heat the sample, the Joule-heating device combines these two techniques. Below, in Figure 32, the proposed design is shown both with and without pyrolytic graphite block.

Figure 32: Joule-heating base, with (left) and without pyrolytic graphite block (right) which is depicted in black.

The sample holder consists of a base and two spring rods that hold the sample in place, but also function as electric connection to the sample or PG block. The sample is placed on top of the sample holder and is pressed against the V-shape by the opposite spring rod. This ensures the placement and hold of the sample. Where the right spring in Figure 32 holds the sample in place and is a connector to the sample or PG block, the left spring only is present to provide an electrical connection.

Therefore the stiffness of the right spring is higher than its left side counterpart.

When using the MVM principle, a PG block (black object in Figure 32), is introduced. If the spring were to come into contact with the sample, the principle would not work anymore as the current would prefer to flow through the sample and not the PG block since the sample’s resistivity is lower.

Therefore the block is wider than the sample. Additionally since the spring does not come into contact with the sample anymore, the spring also doesn’t push the sample into the V-groove. Thus, the PG block is T-shaped to fulfil the role of the spring. This is shown in Figure 33 below where the springs are depicted as grey lines and the sample is depicted as the red block.

When a current is applied to the springs, an electric circuit is created and the sample or PG block is heated. One condition for this to happen is that the base should be non-conductive. Therefore the base is made of PEEK, a high temperature resistant polymer that can be used continuously at temperatures of 250°C. Evidently, this temperature is quite low in comparison with the target temperature. This doesn’t pose a problem as any influence of the heating technique will also be perceived at this temperature. Naturally, ceramics were also considered to take the place of PEEK, however due to the price this option was ignored.

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Figure 33: Cross-section of Joule-heating base with(left) and without(right) PG block. The sample is depicted in red, the spring rods are depicted as grey lines.

C.3 Experimental Validation C.3.1 Initial testing outside the SEM

Having obtained the necessary elements in order to perform the initial heating experiments, a first trial has been done. By applying a current on the graphite block, and monitoring the temperature with a fluke thermometer, the block should have heated up. Unfortunately no significant increase in temperature was found to occur at the surface of the block. However, on the bottom of the block, a major temperature increase occurred as the PEEK support started to melt and deform. Moreover, the solder with which the electrical wires had been connected with the stage had melted.

This is most likely due to the fact the contact area between the electrodes and the PG block was insufficient, resulting in a very high electrical resistance and therefore a high temperature. As copper, the electrode material, conducts heat much better than graphite, the wires heated up faster than the graphite.

Subsequently, the set-up was taken apart in order to analyse the parts themselves instead of the entire assembly at once. It was found that a current of 15A, the graphite block started glowing within seconds. Since at this range of temperature oxidation of the sample occurs, this current hasn’t been repeated. Also, as the temperature rose too fast for the thermometer to measure, the temperature is estimated to have lain around 1000°C based on the colour of the glow.

From calculations for the aluminium sample, it was found that neglecting convection, 4.5A of current should suffice to heat the sample up to 220°C. Applying this current outside of the vacuum, where the previous assumption does not hold, resulted in a temperature increase to 76°C before levelling out. As convection was neglected, it makes sense that the achieved temperature outside the SEM was lower than predicted. However, a difference of 144°C cannot be explained by convection loss.

This merits further investigation.

As the contact between electrodes and sample was made manually in the previous trials, another solution had to be found for the heating inside the SEM. Since the influence of the heating system is the only interest at this point of the research, sample interchangeability and structural change inside the sample do not matter as much as it will in the final design. Therefore, the electrodes are soldered to the sample.

Page 52 C.3.2 Initial testing inside the SEM

Testing of the sample inside the SEM while the electrodes were soldered to it, resulted in the conclusion that the application of current has an influence on the SEM image. In order not to exceed temperatures previously attained in the experiments outside the SEM, the current range was limited to 2 Ampère.

First, with the secondary electron image, a recognisable point on the sample surface was chosen.

Then, a current was applied and the point was located again. The applied current was set at 0.2A, and was increased with steps of 0.2A and resulted in the following displacement graph.

Figure 34: Displacement of the secondary electron image as a function of the current applied to the sample.

This displacement has been found to be caused by the magnetic field originating from the electrification of the sample and is linearly dependent on the current magnitude and directly dependent on the orientation of the sample. The linearity can be seen in Figure 34 and the direction dependency can is sown by the fact that rotating the sample a certain angle results is the same rotation for the displacement direction.

Furthermore, a vibration was noticed when the electrical wires were connected and the power source turned on. Since any vibration is detrimental to the quality of any SEM image, including EBSD, a solution must be found.

The cause of the vibration is suspected to be the formation of a ground loop. A ground loop is where two different systems are separately grounded and connected to each other. The solution would be to ground both systems with to the same ground. However, the connection and the two interfering systems have not been determined.

C.3.4 Validation conclusion

Since the application of a current to a component of a heating assembly results in the formation of magnetic fields that disrupt the image acquisition process, the choice is made to use a laser heated system. This choice is made since previous research has shown this technique to be effective in heating and, due to no current inside the SEM, does not influence the data acquisition.

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