The Cvd Gas Systems For Carbon Nanotube Growth Engineering Essay

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The oven used for CVD is a tube furnace from carbolite [1]. The oven chassis provides the power supply for the cylindrical oven which contains a 300 mm long ceramic work tube with an inner diameter of 38 mm. The oven is heated with a resistance wire wound around the work tube. The heated length is 250 mm long. The maximal reachable temperature is 1200 0C. A quartz tube with a length of 1 m and a diameter of 30 mm is placed within the boring of the work tube and closed on both sides with stainless steel plates (figure 1). Ring clips are mounted on the tube on both sides and three screws with wing nuts are used to fix the plates. This mounting is very delicate, since the tube breaks when the ring clips are tightened too much, and the system leaks when they are not tightened enough. A heat shield inside of the quartz tube reduces the heat flow out of the oven.

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Figure 1: Schematics of the CVD oven. The samples are mounted from the right side.

The uniformity of temperature within the tube is tested. When the tube is closed on both sides then the temperature fluctuates by ±10 0C expect for the last 10 cm at both ends. The fluctuation within the middle zone of 95 mm is only ±5 0C. The samples are placed in the middle of the oven on a quartz sample holder with a length of 70 mm. This means that it might be assumed that the temperature is approximately equal for all samples grown in one run.

The inlet of the oven is connected to the gas system with a flexible metal tubing. The outlet of the oven and a water cooler are joined with a short piece of teflon tube, since a plastic tube would be destroyed by the hot and aggressive exhaust gas. The cooler is connected with a piece of plastic tube to a bubbler and this again to the outside. The cooler is used to reduce the temperature of the exhaust gas to prevent the damage of the plastic tube. It is only necessary when methane with a flow of 5 lit/min is used, and when the growth temperature is higher than 1050 0C. The cooler is not used for lower growth temperatures and flows. The presence of bubbles within the bubbler indicates that there is no major leak in the system. However there is a more important effect of the bubbler, since it serves as a diffusion barrier. There is always some unwanted gas diffusion into the oven since the system is open to the outside. This flow is reduced since it has to pass the water inside the bubbler. This means there is surely some water vapour inside the reaction tube however the amount is limited to the vapour pressure of water. Other gases can pass the bubbler but they have to overcome two water gas boundary layers with areas of only 1 cm2 and 5 cm2, the surface of the bubble and the water air boundary of the bubbler, respectively. The reaction tube is flushed with argon during heating (around 30 minutes) which reduces the content of environmental gases which are present due to the fact that the tube has to be opened to mount the sample. However there is always a small quantity of water vapour, nitrogen, oxygen and the other trace gases of the environment present in the oven during growth in addition to the used reaction gases. The reaction tube is flushed with argon (and in the actually used process with hydrogen too) during cooling-down to prevent the oxidation of the grown nanotubes [2].

2 THE GAS SYSTEMS

The gas system provides the oven with the desired reaction gases. Each of the presented gas systems contains at least one flowmeter which allows the correct setting of the desired gas flow. All gas systems consist of several gas lines for different gases which join before the oven. The origin of each gas line is a gas bottle equipped with a reduction valve which allows the setting of the desired gas pressure. The next part is a one-way valve which protects the connected gas bottle in the case of an explosion within the gas line. It opens only when the pressure from the side of the gas bottle is higher then the pressure from the other side. The pressure stroke of an explosion would close the one-way valve and prevent the hot explosion gases from penetrating the gas bottle. A following two way valve permits the fast opening and closing of the gas line without changing the settings of the dedicated reduction valve and flowmeter. Manometers before and after the flowmeter allow the setting of a desired pressure drop over the flowmeter. The different parts are connected with stiff stainless steel tubes with an outer diameter of 6 mm. With one exception: more flexible tubes with an outer diameter of 3 mm are used to connect the reduction valves with the one way valves to guarantee the necessary flexibility which is needed when the gas bottles have to be changed [2].

2.1 GAS SYSTEM I, WITH ONE VARIABLE AREA FLOWMETER

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This is the most simple setup which is used (Figure 2.1). The gas lines for all used gases (argon, methane and ethylene) are connected to the same variable area flowmeter. The gas flow is controlled by closing and opening the valves. The gas lines have an overall length from the gas bottles to the oven of approximately 3 m [3,4].

Used gases: argon, methane and ethylene.

Figure 2.1: Schematics of gas system I, with one variable area flowmeter.

2.2 GAS SYSTEM II, WITH THREE THERMAL PROFILE FLOWMETERS AND A VARIABLE AREA FLOWMETER

This setup (Figure 2.2) belongs to the CVD oven mentioned above. A Brooks instrument controller controls three Brooks instrument thermal profile flowmeters gauged to Ar, H2 and N2. An additional variable area flowmeter can be used for high flows of methane. The gas system is connected to the oven with a 12 m long tube of a diameter of 3 mm. The gas lines have an overall length from the gas bottles to the oven of approximately 15 m [4,8].

Used gases: argon, hydrogen, methane, ethylene and acetylene.

Figure 2.2: Schematics of gas system II, with three thermal profile flowmeters and a variable area flowmeter.

2.3 GAS SYSTEM III, WITH TWO VARIABLE AREA FLOWMETERS, A NEEDLE VALVE AND A SWITCH

The distinctiveness of this setup (Figure 2.3) is a switch which is used to toggle between argon and the carbon feedstock, which is usually methane. For some experiments a small amount of ethylene is added to the methane flow with a needle valve. The argon and the methane gas lines were equipped with variable area flowmeters. The gas flow is controlled by using the switch. It has two inlets (for argon and methane) and two outlets (to the oven and outward). The switch has two positions. In position 1 the methane line is connected to the oven and the argon line is connected outward. In position 2 the argon line is connected to the oven and the methane line is connected outward. The pressure after the switch is ≈0.1bar ±10% relative (due to the fluctuations of the air pressure from day to day) and the pressure before the flowmeters is set to 0.2 bar relative using the manometers (this corresponds to a pressure drop of ≈0.1 bar over the flowmeters). A special feature of this system is a needle valve which is used to add a small quota of ethylene to the methane flow. The gas lines have an overall length from the gas bottles to the oven of approximately 3 m [7,8].

Used gases: argon, methane and ethylene.

Figure 2.3: Schematics of gas system III, with two variable area flowmeters, a needle valve and a switch.

2.4 GAS SYSTEM IV, WITH THREE VARIABLE AREA FLOWMETERS

This is the currently used gas system (Figure 2.4). It allows the simultaneous use of argon, methane and hydrogen. All gas lines are equipped with variable area flowmeters. Manometers before and after the flowmeters allow the exact setting of the pressure drop over the flowmeters (the pressure after the flowmeter depends from the actual air pressure and fluctuates by maximally 10% from day to day). Since the flowmeters are gauged to 0.2 bar the pressure drop over the flowmeters is set to 0.2 bar, using the reduction valves of the gas bottles considering the readout of the manometers in front of each flowmeter. This makes a pressure correction unnecessary. A correction for the used gas is necessary since the flowmeters are gauged to air (A correction table is usually used). The gas lines have an overall length from the gas bottles to the oven of approximately 3 m [4].

Used gases: argon, hydrogen and methane.

Figure 2.4: Schematics of gas system IV, with three variable area flowmeters.

3. CONCLUSION

The Gas System provides the oven with the desired reaction gases. Gas System IV has three variable flowmeters for argon, hydrogen and methane. The first experiment was performed by Gas System I which proved the possibility to grow Carbon Nanotubes with the CVD oven. However, it does not allow the simultaneous use of two or more gases. The Gas System II belongs to the old CVD system and was connected to the oven with a 12m long tube of a diameter of 3 mm. Gas System III is less sophisticated than Gas System II and allows flow of only two gases methane and argon. The Gas System IV is currently in use now-a-days.

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