Improving Production of Carbon Nanotube Composites

Transcription

1 Improving Production of Carbon Nanotube Composites Research Experience for Teachers (RET Program) Brigham Young University Provo, Utah Dan Broadbent Physics Teacher Timpanogos High School Orem, Utah 14 August

2 Abstract Carbon nanotube (CNT) composites offer great promise for making lighter, thinner and stronger structures. Producing CNT composites, however, can be tricky. The focus of this work is to improve production yields of CNT composites by doing research and development in two areas: Research the relationship between ethylene gas concentrations used during CNT growth and yields of usable composite films produced. Develop furnace for growing larger CNT samples, which will enable larger sizes and quantities of research product. 2

3 Table of Contents Abstract... 2 Table of Contents... 3 Motivation... 4 Background... 4 What is a CNT Composite... 5 Method... 6 CNT Growth... 6 The Problem... 7 What to Vary... 8 Measurement Inch Furnace Design Parameters Results CNT Growth Inch Furnace Completed Design Fabrication Conclusion CNT Growth Further Work Needed Inch Furnace Further Work Needed Acknowledgements References and Bibliography Appendix A Steps in CNT Composite Production Process to Grow CNTs on a Chip Appendix B and Data Furnace Calibration Data

4 Motivation To advance carbon nanotube (CNT) composites, many product samples must be made and tested. A significant amount of loss occurs between growing CNTs and converting them into usable composites. The goal of this work is to improve the amount of CNT samples grown and how many are able to be turned into usable composite films. Background Carbon-based composites offer excellent strength and weight properties. Carbon fiber composites have been used successfully in many applications including bicycles, tennis rackets and aircraft. Figure 1: Carbon fiber bicycle 1 Figure 2: Carbon fiber aircraft, B-2 Stealth Bomber 2 4

5 What is a CNT Composite A CNT composite is similar to a carbon fiber composite in that strong carbon-based filaments are placed next to each other and held in place with a binder. They differ from carbon fiber composites in that the carbon nanotubes are much smaller in diameter, may have different placement patterns and may use a different binding material. Carbon nanotubes can be much stronger than carbon fibers. A single carbon nanotube has been measured to withstand 150 GPa. This compares to a single carbon fiber, which has been measured to withstand 20 GPa. 3,4 Combining this strength per unit difference with the significant difference in diameters means CNT composites have the potential to be thousands of times stronger than carbon fiber composites for a given thickness of material. 5

6 Method CNT Growth CNTs are grown on silicon wafers that have iron traces deposited on them. CNTs grow up from the traces as shown in blue in the image below: Figure 3: Iron pattern to grow carbon nanotubes These wafers are snapped into smaller chips which are placed in a furnace where gasses are flowed over them. These gasses allow for CNTs to grow into a forest, shown in the image below: Figure 4: Carbon nanotube "forest" 5 The forests are then knocked over by rolling a Teflon cylinder over them as shown in the image below: 6

7 Figure 5: Direction of Teflon roller The CNTs are then infused with a polyimide binder and hot pressed. Hydrofluoric acid is then used to release the CNT composite from the chip resulting in individual composite films shown in the image below: Figure 6: CNT composites after being released from chips For specific details on producing CNT composites and growing CNTs on chips, please see Appendix A. The Problem As the forest is knocked over, sometimes the CNTs pull free from the chip they were grown on. Then, no composite films can be produced. Here is an example of how chips look after a successful knock over. 7

8 Figure 7: Example of good CNT chip Here is an example of how chips look when some of the CNTs have pulled free. Figure 8: Example of bad CNT chip What is needed is better adhesion between the CNTs and the chip. This can be affected by several things, described next. What to Vary Growing the CNT forest is a tricky process. There are several factors that can ruin growth on a set of chips. We experienced variability due to leaky connections in the furnace gas lines, contamination of the chips themselves, accumulation of chemicals in the quartz furnace tubes, and other factors that we could not clearly identify. 8

9 We chose to focus on the relationship between ethylene gas concentrations used during CNT growth and yields of usable composite films produced. Specifically, we compared ethylene gas concentrations of 35% and 50%. Measurement Sections of contiguous CNTs are needed to produce viable CNT composite films. The following yield categories were used to measure results: Categories Circular diameter of contiguous CNTs Unusable < 5 mm Possibly usable 5-9 mm Usable > 9 mm Figure 9: How to measure yield areas 9

10 6 Inch Furnace Current furnaces support a one inch diameter quartz tube, as shown in the images below: Figure 10: Existing furnace, closed Figure 11: Existing furnace, open, showing 1 inch quartz tube The new furnace, shown below, will support a six inch diameter tube. This will allow processing 4 inch silicon wafers. 10

11 Figure 12: New 6 inch furnace Figure 13: New furnace, larger heating tube diameter Design Parameters The new furnace needs to allow for the following: 1. Accommodate 4 inch wafer. 2. Minimize gas volume to reduce expense and increase safety. 3. Maintain uniform gas flow profile over wafer. 4. Maintain uniform temperature in region of wafer. 5. Allow for fast cooling. 11

12 Results CNT Growth We compared yield results for ethylene concentrations of 35% and 50% and obtained the following results: Table 1: results for different ethylene concentrations % Usable Possibly Unusable Total Samples 35% % Shown graphically, here are the results below: Figure 14: Graphical results for yields and ethylene concentrations The 50% ethylene concentration resulted in: significantly higher number of usable chips. slightly smaller number of possibly usable chips. significantly smaller number of unusable chips. 12

13 6 Inch Furnace Completed Design To meet the design parameters mentioned previously, the furnace was designed as follows. Shown below, the 6 inch furnace on the left will slide left and right, allowing the wafer holding assembly, shown on the right, to be inside the oven s tube for heating or outside for cooling. Figure 15: 6 Inch furnace and wafer holding assembly The wafer holding assembly will hold the silicon wafer and flow the gasses over it. It is made of Inconel so it will hold up under the temperature range of up to 900 C and possibly higher. The wafer holding assembly will be mounted on a fixed base, allowing a stationary structure to connect gas lines to. The furnace is connected to an electrical cable with enough slack to allow full range of motion, left and right. This cable will need a spring retainer to keep the cable clear as the furnace slides back and forth. 13

14 Figure 16: Detailed furnace design features Please note the following features shown in the figure above: Levelling base so furnace will roll back and forth in a predictable way. Rolling metal wheels and tracks for smooth motion. Stops at the ends of tracks with adjustable screws for precise positioning of the furnace in the open and closed positions. Adjustable side mount height so wafer holding assembly can be positioned vertically to be in the middle of the furnace tube. Spacer ring to allow extra room needed for attaching the quartz tubes and gas lines. Clamps to hold quartz tubes with enough width to minimize torque on the quartz tubes since the tubes only connect to the clamps but will extend to near the end of the Inconel box. Water-cooled ring to keep the heat away from heat sensitive O-rings and gas line connections. Here is a side view, below: 14

15 Figure 17: Side view Here are details on the inside of the wafer holding assembly from the side view: Figure 18: Detailed components of wafer holding assembly 15

16 Figure 19: Inside of furnace, showing where wheels will attach The above pictures show where the current rubber footings are that will be removed and replaced with the metal wheels. Fabrication The pictures below show the current stage of fabrication. Figure 20: Furnace base assembly with Jeremy Peterson, BYU Physics Machinist and Design Engineer 16

17 Figure 21: Furnace base assembly, other side view 17

18 Conclusion CNT Growth Our tests were conclusive that flowing ethylene at 50% did produce better yields of usable chips than at 35%. Further Work Needed We began growing samples with 65% ethylene. Initial yield results seemed worse than with 50%, however, the sample size was not sufficient to reach a conclusion. There are many other parameters that can be varied, including: Time of flowing ethylene gas. The main process we followed was for 8 minutes. We began to do 12 minutes for some growth sets but did not do sufficient for evaluating that factor by itself. Position in the furnace tube. Position in the tube (front or back on the quartz boat relative to the gas flow) was recorded in the data but we have not yet analyzed for that parameter. 6 Inch Furnace The design is complete and fabrication is underway. Further Work Needed Heat Shield When the furnace is slid to the left the Inconel box will be exposed and will be dangerously hot for several minutes while it is cooling. We need a way to protect operators and others in the lab from inadvertently touching or bumping into it. Here are some options to protect against injuries from the hot Inconel box: Hang a metal sheet or mesh down from the vent hood. It would need to cover the full distance, left-to-right, of the Inconel box and would need to stop high enough above the table top to allow access to the furnace programmable controls. Have an upside down "U" cage hinge-mounted on the back wall that lowers to cover the Inconel box once the furnace is moved. A cage would be lighter and allow better air flow for cooling than a solid sheet. Positioning on the wall is shown below: 18

19 A bigger cage that would be big enough to cover the entire furnace. It would rest on the table top with the furnace underneath. The furnace could be slid out from under it. This offers a simpler design that the wall-mounted cage, but would be significantly bigger. Furnace Handle Mounting a handle somewhere on the left end of the furnace will facilitate operators being able to slide the furnace back and forth. 19

20 Acknowledgements I would like to offer thanks to: National Science Foundation, Grant PHY , for funding this research. Dr. Robert Davis, for overseeing the project. Nathan Boyer, for day-today advice and direction on how to proceed. Jeremy Peterson, for expertise on designing and building the large furnace support structure. Scott Daniel, for electrical work on the large furnace. 20

21 References and Bibliography References 1. Image from Carbon Fiber Gear, Baltimore, MD, (accessed August 11, 2015). 2. Image from Northrop Grumman Corporation, 2980 Fairview Park Drive, Falls Church, VA 22042, (accessed August 11, 2015). 3. American Chemical Society National Historic Chemical Landmarks. High Performance Carbon Fibers. (accessed August 21, 2014). 4. Demczyk, Bg; Ym Wang; J. Cumings; M. Hetman; W. Han; A. Zettl; and Ro Ritchie. "Direct Mechanical Measurement of the Tensile Strength and Elastic Modulus of Multiwalled Carbon Nanotubes." Microscopy and Microanalysis 12.S02 (2006): 934. Web. 25 Aug Image from Oak Ridge National Laboratory, Oak Ridge, TN 37831, (2015). Bibliography Wang, X., et al. "Ultrastrong, Stiff and Multifunctional Carbon Nanotube Composites." (2012) Cheng, Q., et al. "High Mechanical Performance Composite Conductor: Multi-Walled Carbon Nanotube Sheet/Bismaleimide Nanocomposites." (2009) 21

22 Appendix A Steps in CNT Composite Production 1. Deposit alumina on 4inch silicon wafer. 2. Use photo lithography to establish pattern on wafer 3. Use thermal evaporator to deposit iron on pattern 4. Coat wafer with photoresist 5. Dice cut wafer 6. Remove photoresist with N-Methylpyrrolidone (NMP) solvent. 7. Grow CNT forest (see details in Process to Grow CNTs on a Chip below) 8. Measure height of CNT forest with optical microscope 9. Use light to determine orientation of CNTs 10. Flood with IPA 11. Knock over forest with Teflon tube 12. Measure yield areas 13. Dip chips in polyimide 14. Soft bake 15. Sandwich chip in wafer surround 16. Hot press chip to full cure 17. Remove CNT film using HF 18. Place film in rings 19. Do bulge testing Process to Grow CNTs on a Chip 1. Use diamond scribe to uniquely label the back of each chip. 2. Blow chips clean with nitrogen gas. 3. Place two chips on quartz boat. 4. Place quartz boat in the furnace s quartz tube, the first chip centered over the thermocouple, the second chip downstream relative to the gas flow. 5. Close the end of the quartz tube. 6. Program furnace to ramp up to 750 C. 7. Switch valve to growth gas. 8. Use mass flow controller to let 50% H 2 begin to flow. 9. Close furnace lid and allow furnace to ramp up to temperature. 10. When furnace has reached 750 C use mass flow controller to let gas (C 2 H 4 ) to flow. Start timer. a. We experimented with two different percentages, 35% and 50%, to see the effects. 11. After 8 minutes, stop the gas flow. 12. At 8:15 lift the lid to begin cooling the furnace. 13. At 8:30 turn off H 2 and turn on Argon gas. a. When we had trouble with the Argon lines we just left the H 2 flowing instead. 14. When the furnace has cooled to about 200 C switch the valve to air and begin flowing air at 20%. 15. After flowing air for about a minute the quartz tube is ventilated and ready to be opened. 22

23 16. Carefully remove the quartz boat and place the chips that now have CNTs on them, in a storage tray. 17. After running 3 to 5 growth cycles the tube gets very dirty from carbon buildup. Then we ran a clean cycle where we put the quartz boat in the tube with no chips on it. Then ramped up the furnace to 750 C with only air flowing through the tube to allow the carbon deposits to burn off. 23

24 Appendix B and Data Please note: height measurements are in microns and were obtained using an optical microscope Corners & Middle A % A % B % B % Corners & Middle A % A % B % B % C % C % Corners & Middle A % A % B % B % C % C % Corners Middle A % A % B % B % C % C % Corners Middle 24 A % Unusable A % Possibly B % Possibly Damage during removal from furnace

25 B % Unusable C % Possibly C2 N/A 35% Corners Middle 25 A % Unusable A % Usable B % Possibly B % Possibly C1 Bad N/A 50% Unusable C % Usable D % Usable D % Possibly E % Usable E % Usable F % Usable F % Possibly Corners Middle E1, E2, F1, F2 Were grown on also used 20% Ar on the dry argon MFC that is part of the water system. A1 Bad N/A 35% Unusable Really bad growth, Lawrence A2 Bad N/A 35% Unusable fixed a leak in the system B1 Bad N/A 35% Unusable afterwards. Only hydrogen in cool B2 Bad N/A 35% Unusable down, no argon. C % Unusable Not grown on 1.July, grown on 2.July much better growth that C % Possibly the previous 4. Damaged during removal. Only hyrdrogen in cool down. Corners Middle A % Possibly Wafer dirty and growth was not uniform. Only hydrogen in cool A % Unusable down, no argon Corners Middle A % Unusable A % Usable B % Unusable B % Unusable Corners Middle Weirdly shorter than other growths. Only hydrogen in cool down, no argon. A1 Bad N/A 50% Unusable Little to no growth

26 A2 Bad N/A 50% Unusable Little to no growth B % Unusable No Ar cooling, B2 Bad N/A 50% circle in middle where no growth Unusable occurred C % Usable No Ar cooling, C % Usable No Ar cooling, D % Clean cycle before. No Ar cooling, Unusable E % Unusable No Ar cooling, E % Unusable No Ar cooling, F % Unusable No Ar cooling, F % Possibly No Ar cooling, Corners Middle A % Clean cycle before. No Ar cooling, Unusable B % Unusable No Ar cooling, B % Unusable No Ar cooling, C % Unusable No Ar cooling, C % Unusable No Ar cooling, D % Unusable No Ar cooling, D % Unusable No Ar cooling, Corners Middle A % Possibly No Ar cooling, A % Usable No Ar cooling, B % Unusable No Ar cooling, B % Possibly No Ar cooling, C % Possibly No Ar cooling, C % Possibly No Ar cooling, Corners Middle A % Possibly No Ar cooling, A % Usable No Ar cooling, B % Unusable No Ar cooling, B % Possibly No Ar cooling, Corners Middle A1 Bad N/A 35% Unusable A2 Bad N/A 35% Unusable 26

27 B % Possibly B % Possibly C % Unusable C % Possibly Corners Middle A1 Bad N/A 36% Unusable 8 min growth, 89% - H2, 36% - B % Unusable C % Unusable 12 min growth, 89% - H2, 36% - C % Unusable Corners Middle A % Usable A % Possibly B % Possibly B % Usable C % Usable C % Usable Corners Middle A % Unusable 12 min growth. Accidentally left C2H4 on during cooling and H2 off. A % Possibly 12 min growth. Accidentally left C2H4 on during cooling and H2 off. B % Possibly B % Possibly C N/A % Possibly C2 Bad N/A 35% Unusable

28 Corners Middle A % Unusable A % Usable B % Unusable B % Usable C % Unusable C % Possibly Corners Middle A1 Bad N/A 65% Unusable A % Possibly B % Possibly B % Possibly C % Unusable C % Possibly Corners Middle A % A % B % B % C % C % 28

29 Furnace Calibration Data Furnace 2 Calibrations 2.May.2015 (20% regulator) Hydrogen (20% regulator) % MFC Sccm Slope b % MFC Sccm Slope b

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