Najaf Tomaraei, Golnaz
(2024)
Dynamic Processing to Manufacture Tailored Carbon Nanotubes and Nanofibers.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Vertically aligned carbon nanotube (VACNT) forests grown via chemical vapor deposition (CVD) hold promise for applications requiring high thermal and electrical conductivity. However, challenges related to process complexity, consistency, and sensitivity to uncontrolled parameters have impeded scalable manufacturing. This dissertation aims to enhance the manufacturability of VACNT forests through innovative reactor design and dynamic recipe engineering. A custom-designed rapid thermal CVD reactor with independent control of temperatures for gas-phase reactions, catalyst treatment, and CNT growth, enabled unprecedented decoupling of growth. Moreover, the rapid nature of the infrared heating system uniquely enables dynamic recipes with spatiotemporal control of temperature distributions. In particular, elevated catalyst pretreatment temperatures suppressed subsurface diffusion into the alumina support, delaying deactivation and boosting yield by 3 folds. Moreover, tailoring the spatial temperature profiles across the catalyst-coated silicon chip, greatly increased geometric uniformity of macroscopic forests.
Combining statistical data analytics with incorporating a helium purifier and redesigning the sample holder significantly improved process reproducibility with respect to forest height and density: a 17-fold decrease of coefficient of variation (CV) for heights and a 2-fold decrease of CV for densities. Leveraging and modeling of the thermochemical history of reactor walls enabled order-of-magnitude enhancement of both mass yield and catalytic lifetime, based on slow release of adsorbed water vapor, which acts as a growth promoter. Importantly, deliberately adding even small amount of reducing agent like H2S (<0.02% of total flow rate) greatly influenced the as-grown CNT morphology and atomic structure, enabling growth of single-walled CNTs with controlled kinks. In addition to gaseous growth additives, we show that employing solid promoters like molybdenum can be used as a method to tune alignment and density of CNT based on proximity and localized diffusion effects.
In sum, this dissertation combines experimental and modeling efforts towards both understanding and improving different aspects of CNT growth based on reactor design and dynamic recipe engineering. Importantly, findings in this work provide the scientific basis for practical strategies to improve scalability, consistency, uniformity, as well as to tailor morphology and properties of CVD-grown VACNTs to propel manufacturability for targeted applications.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
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| ETD Committee: |
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| Date: |
11 January 2024 |
| Date Type: |
Publication |
| Defense Date: |
1 September 2023 |
| Approval Date: |
11 January 2024 |
| Submission Date: |
28 September 2023 |
| Access Restriction: |
1 year -- Restrict access to University of Pittsburgh for a period of 1 year. |
| Number of Pages: |
186 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Industrial Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Carbon nanotube, Vertically aligned carbon nanotubes, Carbon nanotube forest, Catalytic chemical vapor deposition, Rapid thermal processing, |
| Related URLs: |
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| Date Deposited: |
11 Jan 2024 19:29 |
| Last Modified: |
11 Jan 2024 19:29 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/45402 |
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