The large-scale setup is an induction furnace, containing a graphite crucible. The setup is sketched in Figure 85.
The furnace crucible is cylindrical. The graphite crucible is 400 mm high (outer height) and 150 mm wide (outer diameter). The internal walls of the crucible are 35 mm thick on the bottom, and 20 mm thick on the sides. The crucible is divided into a gas production zone and a condensation zone.
The gas production zone is located at the bottom of the crucible, at the highest temperatures. The SiO(g) producing pellets are placed into a smaller graphite crucible, called pellets chamber. The pellets chamber has a circular base of 100 mm inner diameter and is 70 mm high (outer height). Its walls are 4 mm thick. The carbon surface on top of the pellets chamber was drilled with 128 concentric holes of 3 mm diameter, to let the gas escape from the chamber. A larger hole (25 mm diameter) is drilled at the center of the pellets chamber roof.
This allows a shielding graphite tube to enter the pellets chamber. The chamber is placed at the bottom of the graphite crucible, with the drilled surface facing up. The crucible in the gas production zone was designed to separate the gas production materials from the condensation zone.
The space around the pellets chamber is filled with SiC particles, with size between 1-3 mm. Such particles fill the graphite crucible up to 70 mm from the crucible bottom, which is the same height as the height of the pellets chamber.
The condensation zone is the remaining space above the gas production zone. This area is 310 mm high. SiC particles at different size fill the crucible up to the top. The particles size distribution chosen were 5-8 mm, 3-10 mm, 12-20 mm and 8-14 mm, as described in the chapter about characterization of substrates. Condensation chambers were reused, when possible.
A shielding graphite tube protects a multi-point C-type thermocouple. C-type thermocouple wires were soldered at the desired measuring point before each experiment and inserted in the same alumina tube. The alumina tubes chosen are designed to provide up to five temperature measurement points. Temperature was measured every 70 mm, starting from Y = 1 cm (i.e. 1 cm from the bottom of the graphite crucible).
One of the experiments (IF6a) was used to check if there is a horizontal temperature gradient within the condensation zone. IF6a has a slightly different setup than the other experiments. The gas production chamber was moved to the edge of the crucible. Two graphite tubes were used in this experiment. The first was inserted in the gas production chamber, whereas the second was placed close to the opposite border of the chamber.
Each tube protected a multi-point C-type thermocouple. The first thermocouple is close to the center, and measures the temperature at all the 5 available positions (Y = 1, 8, 15, 22 and 29 cm), whereas the second one has 4 available measuring points located at the crucible edge, at Y = 8, 15, 22 and 29 cm.
101 Figure 85: Left: Large-scale setup; Right: Large-scale setup for lateral temperature gradient computation.
Running experiments in a larger setup allows to analyze better some parameters which are difficult to inquire in the small-scale setup. In the larger scale setup, the effects of the particle size distribution could be investigated easier, pictures of the crusts are easier to be taken, and the amount of condensates produced will be larger. In addition, this will simulate the industrial furnace better, as no inert gas is added.
Procedure
The gas production zone is first assembled. The pellets chamber is inserted in the graphite crucible. Then, the desired pellets mixture is inserted in the pellets chamber with a funnel. Afterwards, the same funnel is used to fill the surroundings of the pellets chamber, with SiC particles with size 1-3 mm. The SiC particles must not enter the pellets chamber, nor the pellets should come out from the bottom of the pellets chamber itself. Finally, the graphite tube is inserted in the central hole of the pellets chamber.
For the condensation zone, the mounting procedure consist simply in placing the SiC particles and the thermocouple in the crucible. First, the desired SiC particles are poured in the condensation chamber to fill the crucible to the top. Their amount is usually around 3 kg. Then, the thermocouple is inserted in the graphite tube and connected to the temperature measurement software.
The thermal history of each experiment is controlled at the second measurement point from the bottom of the crucible (Y = 80 mm). This point is located slightly above the gas production zone. It is assumed that the gas came out of the gas production zone at the temperature measured at the top of the pellets chamber, i.e. at 80 mm from the bottom of the graphite crucible. The temperature is recorded continuously during the experiment. The experiment is aborted if the thermocouple at Y= 80 mm brakes.
The induction furnace is turned on and the experiment is carried out with the chosen temperature and time conditions. The power supply history was kept constant for each experiment, giving a heating rate of 40°C/min.
The target temperature was fixed for all the experiments at 1890±10°C. The temperature at Y = 80 mm oscillated by 10 degrees, since the power was supplied manually to the furnace. When the target temperature is reached, the oscillation period lasted for ca. 20 minutes. An oscillation period is the time used for an increase from 1880°C to 1900°C, or vice versa.
Once the experiments were finished, the SiC charge is analyzed by excavating the crucible. The material is extracted with the help of a spoon. When the system is clogged, a steel pin is used to poke the charge. Partially reacted pellets are collected from the pellets chamber, then weighted and characterized.
The pellets chamber is cleaned from SiC and Si residues stuck on its surface. The extraction and cleaning procedure of the pellets chamber needs often the use of a hammer, and the pellets chamber could be damaged.
If that happens, a new chamber is used the next experiment. This is always the case for experiments containing Si-SiO2 pellets.
List of experiments
The parameters varied in the large-scale setup are the pellets composition, the holding time and the SiC substrate particle size distribution. The pellets are either the SiO2-SiC or the Si-SiO2 mixture. The holding time was fixed at 10, 30 or 120 minutes, since the instant when the target temperature was reached. The particle size distributions are those represented earlier in Figure 79. Table 9 gives an overview of the setup conditions. The letters “IF” indicate that the large-scale setup was used. The numbers identify a setup, whereas the letters label repetitions of each experimental setup.
As for the small-scale setup, a temperature gradient develops in the condensation chamber. The average temperatures are computed as described in the procedure section of the small-scale setup. An average value is computed and related to its position. After polynomial regression, the temperature gradient in the system is obtained. The thermal history and related temperature profile of experiment IF1b is shown in Figure 86. The relation between position and temperature follows a similar trend to a model previously computed by Kennedy [82], as shown in Figure 87. Appendix C contains all the polynomial expression for the temperature as a function of position in every experiment. The results of the horizontal temperature gradient computation (Experiment IF6a) are also collected in Appendix C.
Figure 86: Thermal history (a) and computed temperature gradient (b) of experiment IF1b.
103 Figure 87: Temperature gradient in large-scale setup computed by modelling [82].
Table 9: Overview of experimental conditions: time, pellets compositions and condensation substrate type.
Setup name Pellets used SiC substrate size Holding time (h)
IF 1 (a,b) SiO2-SiC 3-10 2 hours
IF 2 (a,b) SiO2-SiC 5-8 2 hours
IF 3 SiO2-SiC 12-20 2 hours
IF 4 SiO2-SiC 8-14 2 hours
IF 5 (a,b) Si-SiO2 3-10 2 hours
IF 6 (a,b) Si-SiO2 5-8 2 hours
IF 7 (a,b) Si-SiO2 12-20 2 hours
IF 8 Si-SiO2 8-14 2 hours
IF 9 SiO2-SiC 3-10 30 min
IF 10 Si-SiO2 3-10 30 min
IF 11 SiO2-SiC 3-10 10 min
IF 12 Si-SiO2 3-10 10 min