Chapter II. Starting materials and methods
II.2 High temperature treatments
Silicon is a highly reactive material at high temperature and is commonly used in metallurgy as a melting point depressant. Phase diagrams including silicon are then numerous and very useful when designing a high temperature process including silicon. The choice of the material and atmosphere at the sample surrounding is thus crucial and will be discussed. Then experimental methods (thermogravimetry and dilatometry) will be presented. Finally, the water vapor partial pressure controller that has been designed specifically for our experiments is described.
II.2.1 Furnace geometry and materials at the sample surroundings
Sintering experiments are carried out in a Setaram dilatometric and thermogravimetric apparatus that are similarly designed. The furnace tube of these equipments is schematically represented in Figure II.7.
Measurement probe and electronics, TGA (Thermogravimetric Analysis) or Dilatometry
Figure II.7: Picture of a Setaram equipment (Setsys) and schematic representation of the furnace tube.
The furnace tube (280 mm height and 18 mm diameter) is made of alumina and is located in a graphite resistor. Platinum is commonly used as a thermocouple material because of its high reliability. However, Pt is not suitable as a surrounding material for silicon because of the existence of low temperature eutectics. This is the reason for the use of a tungsten-rhenium thermocouple (5%/26%) located at the sample underneath. The temperature is homogeneous over a height ± zh (± 15 mm) from the center of the tube and tube ends are cooled by water flowing.
Graphite resistor under Ar
zh
...
Alumina furnace tube
W-Re 5%/26%
thermocouple
280 mm
18 mm
zf z
r Sample, z = 0
Water cooling
Water cooling zf: Diffusion length zh: Heated zone He-4mol.% H2-2 l h-1
Gas inlet
Gas outlet
Vacuum is performed in the furnace tube at room temperature before each experiment. The tube is first filled with Ar and vacuum is applied a second time before filling with the carrier gas at 150 °C. A 2 l h-1 gas flow is commonly used as a carrier gas. We shall later on show that for such small flow conditions, the gas mixture can be considered as stagnant (Appendix B.3.3). In most of the experiments, He-4 mol.% H2 (supplied by Air Liquide, mélange crystal) gas mixture was used. Oxygen impurities are water molecules and their amount is less than 5 ppm (0,5 Pa) from supplier specifications. In some experiments a 2 l h-1 Ar (Air Liquide, Ar1) gas flow is used. In this case, oxygen impurities are typically O2
molecules and are also less than 5 ppm.
II.2.2 Thermogravimetry
Silicon powder compacts undergo mass loss during sintering. As will be seen in Chapter III, Thermogravimetric Analysis (TGA) is a highly suitable technique in order to follow mass variation during the whole thermal process. The thermogravimetric equipment used is a Setaram Setsys apparatus that is schematically represented in Figure II.8.
Figure II.8: TGA equipment and sample configurations.
The sample is hung up to suspensions that are connected to a beam equilibrated on a tight ribbon. Any sample mass deviation induces a slight rotation of the beam that is measured by a photo-detector associated with a laser diode. The deviation is instantaneously corrected by inductors that apply a counter electromagnetic force. The beam being always in its
....
Sample
Tungsten suspension Quartz suspension Steel suspension Counterweight Tight ribon Equilibrated beam Photo-detector Inductor
Silicon compacts hung up with a tunsgsten wire Vertical or horizontal
position
Furnace tube
....
W-Re 5%/26%
thermocouple Uncompacted powder
in an alumina crucible covered with papyex®
equilibrium position, the electromagnetic force applied to the beam is directly related to the sample mass.
Suspensions used are successively made of steel, quartz and tungsten from the measurement to the heated area of the TGA. In this case, the use of a hydrogenated atmosphere is a requirement as tungsten would oxidize because of the presence of O2 traces under neutral atmosphere. The behavior of un-compacted powders is first studied in Chapter III.2. In this case, powders are placed in an alumina crucible covered with papyex®. Powder compact oxidation kinetics is also studied. Compacts are then hold in a tungsten wire and are either in a horizontal (Chapter III.2) or vertical (Chapter V.2) position.
Suspension and crucible mass variations (~ 0.1 mg) are measured after each experiment and are much less than the sample mass variation (~ 10 mg). TGA experimental curves are corrected with a blank that corresponds to a TGA measurement along the same thermal cycle without sample. The W-Re (5%/26%) thermocouple is regularly calibrated by measuring the melting temperature of pure metals using the Differential Thermal Analysis (DTA) equipment which can be introduced in the same tube.
II.2.3 Dilatometry
Shrinkage kinetics during sintering is studied in a Setaram TMA92 vertical dilatometer that is schematically represented in Figure II.9. The sample is placed on an alumina support and sandwiched by appropriate spacers to avoid any chemical interaction. An alumina pushrod is in contact with the top spacer and follows the course of the sample during the thermal cycle.
The load applied by the pushrod on the sample (typically 5 g) is controlled by inductors at the top of the equipment. The applied load takes into account the pushrod weight that is previously measured.
The measured signal, z, depends on the compact dilation and shrinkage,
compact
z , as well as the dilation of the pushrod,
pushrod
z , and of the alumina tube,
ztube, in Equation (II.6).
tube pushrod
compact z z
z
z (II.6)
During blank experiments, the pushrod is in contact with the bottom of the alumina tube. The measured signal,
blank
z , then becomes Equation (II.7).
tube pushrod
blank z z
z (II.7)
To obtain the compact dilation and shrinkage,
compact
z , the measured signal, z, is thus corrected with the blank using Equation (II.8), where the term CTE .h. T
3 2O
Al accounts for the dilation of the pushrod on the sample height, h,
3 2O
CTEAl being the coefficient of thermal expansion of alumina and T the difference between the actual and initial temperature.
T h CTE z
z
z . .
3 2O blank Al
compact
compact (II.8)
Figure II.9: Vertical dilatometer and sample configurations.
The W-Re (5%/26%) thermocouple is regularly calibrated by measuring the transition temperatures and associated size changes of a pure iron sample.
Shrinkage is first studied under standard reducing atmosphere (Chapter III) as explained in section II.2.1. The sample is then sandwiched by silicon and alumina spacers to avoid contact between the silicon powder compact and alumina (Figure II.9). Spacers are replaced for each new experiment performed. Sintering kinetics is also studied under controlled silicon monoxide atmosphere (Chapter IV). The compact with a silica spacer is then placed in a silica crucible filled with a mix of silicon and silica powders (Figure II.9). In the following, these conditions will be respectively referred to as “sintering under reducing atmosphere” and
“sintering under silicon-silica powder bed”.
...
Silicon spacer Alumina spacer
Silica crucible Silica spacer
Alumina tube
Alumina pushrod Inductor
Sample Furnace tube
W-Re 5%/26%
thermocouple Pushrod
Sample
Silicon and silica powder bed Sample
II.2.4 Water vapor partial pressure controller
For some experiments, variable water vapor pressure at the sample surroundings are used by monitoring the water vapor in the gas flux upstream, at the tube inlet. The water vapor pressure is monitored with a humidity controller system made of two gas lines and schematically represented in Figure II.10. One line is composed of dry gas while the other is composed of humidified gas bubbled in a water container. Both gases are mixed and a humidity probe (Vaisala HUMIDICAP® HMT333) associated with a controller (West N8800) allows to regulate thermal mass flow (Brooks SLA5850S) to give a water vapor pressure, HProbeO
P 2 , comprised between 100 and 2000 Pa. We shall see from modeling arguments in Chapter V.2 that these water vapor pressures actually correspond to approximately 10 and 200 Pa at the sample surrounding.
Figure II.10: Water vapor pressure controller designed during this study.