Blends of polyacrylonitrile (PAN) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) as precursors may offer an opportunity to reduce the cost of carbon fiber (CF) production. The all-carbon ring ...formations and volatile gas production during the heating and carbonization for 9 PAN/PBO blend precursors with varying mixing ratios are studied via the ReaxFF reactive molecular dynamics simulations. Evolutions of oxygen-containing (O-containing) and nitrogen-containing (N-containing) groups are detailed, in order to reveal the reaction mechanisms of the O and N-based CF precursors. Particularly, the O-containing groups are identified to be more efficient for initiating the carbonization, whereas N-containing groups are far longer retained in the graphitic materials and play a key role in capturing and converting carbon radical species into the graphitic networks. Additionally, the PAN/PBO blend precursor with a mole ratio of 1:1 is compared with the pre-oxidized PAN, PAN, and PBO. It is found that the PAN/PBO blends could be a promising alternative for the cost-effective PAN-based CF precursors, since they can decrease the cost of the CF production by means of: (a) eliminating the pre-oxidation process, (b) having considerable all-carbon ring formations within a short period, and (c) having a relatively fast conversion rate, reaching 95% 6-membered carbon ring formation.
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Combining two- and three-dimensional (2D/3D) materials provides a unique route to enabling next-generation hot electron transistors (HETs)—a vertical ballistic device, promising for high-frequency ...applications since they are not limited by electron velocity saturation, fabrication limitations, or short channel effects. The early demonstrations of HETs suffered from poor material and interface qualities and thick device components. The revival of the HET, with a cut-off predicted frequency above 1 THz, can be correlated with the arrival of 2D materials. Here, we discuss HET operating principles, examine HET material architectures with and without tunneling barriers, and review heterostructure considerations. We discuss material and interface properties that control barrier and base performance and critically review recent 2D/3D HETs for tunneling efficiency, output current density, current gain, and output conductance. Finally, we provide an overview of 2D and 3D semiconductors that form Schottky barriers with graphene that may be utilized as a collector while considering the device physics and growth issues.
Graphene can substantially enhance the mechanical properties of PAN-derived carbon fibers by minimizing porosity and defects.
The superlative strength-to-weight ratio of carbon fibers (CFs) can ...substantially reduce vehicle weight and improve energy efficiency. However, most CFs are derived from costly polyacrylonitrile (PAN), which limits their widespread adoption in the automotive industry. Extensive efforts to produce CFs from low cost, alternative precursor materials have failed to yield a commercially viable product. Here, we revisit PAN to study its conversion chemistry and microstructure evolution, which might provide clues for the design of low-cost CFs. We demonstrate that a small amount of graphene can minimize porosity/defects and reinforce PAN-based CFs. Our experimental results show that 0.075 weight % graphene-reinforced PAN/graphene composite CFs exhibits 225% increase in strength and 184% enhancement in Young’s modulus compared to PAN CFs. Atomistic ReaxFF and large-scale molecular dynamics simulations jointly elucidate the ability of graphene to modify the microstructure by promoting favorable edge chemistry and polymer chain alignment.
During the carbonization process of raw polymer precursors, graphitic structures can evolve. The presence of these graphitic structures affects mechanical properties of the carbonized carbon fibers. ...To gain a better understanding of the chemistry behind the evolution of these structures, we performed atomistic-scale simulations using the ReaxFF reactive force field. Three different polymers were considered as a precursor: idealized ladder PAN (polyacrylonitrile), a proposed oxidized PAN, and poly(p-phenylene-2,6-benzobisoxazole). We determined the underlying molecular details of polymer conversion into a carbon fiber structure. Because these are C/H/O/N-based polymers, we first developed an improved force field for C/H/O/N chemistry based on the density functional theory data with a particular focus on N2 formation kinetics and its interactions with polymer-associated radicals formed during the carbonization process. Then, using this improved force field, we performed atomistic-scale simulations of the initial stage of the carbonization process for the considered polymers. On the basis of our simulation data, the molecular pathways for the formation of low-molecular-weight gas species and all-carbon ring formation were determined. We also examined the possible alignment of the developed all-carbon 6-membered ring clusters, which is crucial for the further graphitic structure evolution.
Graphene inclusion in a polymer matrix is a promising route to significantly enhance the mechanical properties of low-grade carbon fibers (CFs). Using ReaxFF molecular dynamics simulation, the ...atomistic mechanism leading to this enhancement is investigated. We demonstrate that the graphene edges along with the nitrogen and oxygen functional groups play a catalytic role and act as seeds to expedite alignment of the all-carbon rings, which are starting sites for the growth of graphitic structures. To examine the role of this proposed mechanism that enhances the graphitic structure of PAN/graphene CFs, we discuss the experimental results wherein the PAN/graphene CFs carbonized at 1250 °C demonstrate 91% (from 632 to 1207 MPa) increase in strength and 101% (from 88 to 177 GPa) enhancement in Young’s modulus compared to PAN-based CFs carbonized at 1500 °C. These enhanced mechanical properties of low-grade carbon fibers achieved via graphene inclusion at decreased carbonization temperature provide a means to realize both energy savings and cost reduction.
Display omitted The graphene edges play a catalytic role and act as seeds in expedite graphitic structure growth, while the graphene surface carbon atoms are relatively inactive due to their conjugated sp2 electronic configuration.
Blending polyacrylonitrile (PAN) with plastic wastes and bio-based polymers provides a convenient and inexpensive method to realize cost-effective carbon fiber (CF) precursors. In this work, ...PAN-based blend precursors are investigated using ReaxFF reactive molecular dynamics simulations with respect to the formation of all-carbon rings, the evolutions of oxygen-containing and nitrogen-containing species, and the migration of carbon atoms to form turbostratic graphene. From these simulations, we identify that PAN/cellulose (CL) blend manifests the highest carbon yield and the most substantial all-carbon ring formation. This ReaxFF-based finding is confirmed by Raman and TEM experiments indicating high crystallinity for PAN/CL-derived blend CFs. We trace the pathway of gasification and carbonization of PAN/CL to elaborate the mechanism of the formation of all-carbon ring networks. We discover that the acetals of CL can catalyze the cyclization of the blend precursor, allowing for the search for CL derivatives or the other kinds of bio-based polymers with similar functionalities as alternative blends. In addition, we examine the structural characteristics using the carbon–carbon (C–C) radial distribution functions, C–C bond length distributions, and sp2 C atom ratios for the four representative precursors, i.e., PAN, oxidized PAN, PAN/nylon 6,6, and PAN/CL. Our simulation results show the most extensive all-carbon ring cluster and graphitic structure growths for PAN/CL. Here, we propose PAN/CL as a cost-effective alternative CF precursor, since (a) CL is naturally abundant and eco-friendly for production, (b) the blend precursor PAN/CL does not require oxidation treatment, (c) PAN/CL has a high carbon yield with substantial all-carbon ring formation, and (d) PAN/CL based CFs potentially provide a mechanical property enhancement.
This dissertation has two distinct research foci: Carbon fibers and 2-Dimensional materials. As such, this abstract describes these separately. Carbon fibers (CFs), composed of 92 to 100 weight % (wt ...%) anisotropic carbon, are typically manufactured through a series of controlled thermal treatments of precursor polymer fibers. As connected and automated vehicles (CAVs) have been reshaping the transportation sector, there is a growing desire to capitalize on this opportunity to enhance fuel efficiency and reduce emissions by replacing metal components with light-weight, low-cost CF-reinforced composites. However, over 50% of their cost is attributed to the PAN precursor, a drawback that has triggered the search for low-cost alternatives. The work presented in this dissertation explores the strategy of improving the low-grade CFs properties by the graphene inclusion into the polymer matrix. Specifically, this dissertation presents studies regarding implementing ReaxFF molecular dynamic simulations to investigate the effects of graphene inclusion on CFs mechanical properties.The other research focus of this dissertation is atomically-thin, or 2-dimensional (2D) materials beyond van der Waals materials. 2D materials have gained tremendous interest in a wide range of scientific communities due to fundamental research afforded by single or few-atom-thick materials, as well as the potential for impact and application in electronic, optical, sensing, and quantum technologies. This dissertation presents investigation on stabilizing 2D group-III alloys with tunable properties via an intercalation process called Confinement Heteroepitaxy (CHet). These alloys exhibit tunable electronic and optical properties.This thesis is broken up into two main sections. First, after a short introduction on carbon fibers and 2-Dimensional materials (Chapter 1), Chapter 2 investigates how graphene inclusion into polymer matrix affect the chemistry and mechanical properties of low-grade carbon fibers. Chapter 3 introduces Confinement Heteroepitaxy (CHet) as a novel synthesis platform to stabilize group-III alloys with tunable electronic and photonic properties. We demonstrate that the electronic, superconducting and optical properties of air-stable two-dimensional metals can be controllably tuned by the formation of alloys. Chapter 4 introduces two ReaxFF force fields (GaCH-2020 and InCH-2020) which can be used to simulate the interaction of group-III metals with graphene. These force fields enable the investigation of intercalation mechanism in CHet and this knowledge can be used to extend the library of materials which can be stabilized by CHet. Chapter 5 will present ongoing and future work with a summary of findings in this thesis.
Blending polyacrylonitrile (PAN) with plastic wastes and bio-based polymers provides a convenient and inexpensive method to realize cost-effective carbon fiber (CF) precursors. In this work, ...PAN-based blend precursors are investigated using ReaxFF reactive molecular dynamics simulations with respect to the formation of all-carbon rings, the evolutions of oxygen-containing and nitrogen-containing species, and the migration of carbon atoms to form turbostratic graphene. From these simulations, we identify that PAN/cellulose (CL) blend manifests the highest carbon yield and the most substantial all-carbon ring formation. This ReaxFF-based finding is confirmed by Raman and TEM experiments indicating high crystallinity for PAN/CL-derived blend CFs. We trace the pathway of gasification and carbonization of PAN/CL to elaborate the mechanism of the formation of all-carbon ring networks. We discover that the acetals of CL can catalyze the cyclization of the blend precursor, allowing for the search for CL derivatives or the other kinds of bio-based polymers with similar functionalities as alternative blends. In addition, we examine the structural characteristics using the carbon–carbon (C–C) radial distribution functions, C–C bond length distributions, and sp2 C atom ratios for the four representative precursors, i.e., PAN, oxidized PAN, PAN/nylon 6,6, and PAN/CL. Furthermore, our simulation results show the most extensive all-carbon ring cluster and graphitic structure growths for PAN/CL. Therefore, we propose PAN/CL as a cost-effective alternative CF precursor, since (a) CL is naturally abundant and eco-friendly for production, (b) the blend precursor PAN/CL does not require oxidation treatment, (c) PAN/CL has a high carbon yield with substantial all-carbon ring formation, and (d) PAN/CL based CFs potentially provide a mechanical property enhancement.
Two-dimensional (2D) materials exhibit a wide range of optical, electronic, and quantum properties divergent from their bulk counterparts. To realize scalable 2D materials, metal–organic chemical ...vapor deposition (MOCVD) is often used. Here, we report two ReaxFF reactive force fields, GaCH-2020 and InCH-2020, which were developed to investigate the MOCVD gas-phase reactions of Ga and In film growth from trimethylgallium (TMGa) and trimethylindium (TMIn) precursors, respectively, and the surface interactions of TMGa and TMIn with graphene. The newly developed force fields were applied to determine the optimal conditions for the thermal decomposition of TMGa/TMIn to achieve Ga/In nanoclusters with low impurities. Additionally, the cluster formation of Ga/In on a graphene substrate with different vacancies and edges was studied. It was found that a graphene with Ga-functionalized monovacancies could help conduct directional Ga cluster growth via covalent bonds. Moreover, under specific growth conditions, we found that Ga atoms growing on armchair-edged graphene not only exhibited a superior growth ratio to In atoms but also produced a widely spread 2D thin layer between graphene edges.
Blending polyacrylonitrile (PAN) with plastic wastes and bio-based polymers provides a convenient and inexpensive method to realize cost-effective carbon fiber (CF) precursors. In this work, ...PAN-based blend precursors are investigated using ReaxFF reactive molecular dynamics simulations with respect to the formation of all-carbon rings, the evolutions of oxygen-containing and nitrogen-containing species, and the migration of carbon atoms to form turbostratic graphene. From these simulations, we identify that PAN/cellulose (CL) blend manifests the highest carbon yield and the most substantial all-carbon ring formation. This ReaxFF-based finding is confirmed by Raman and TEM experiments indicating high crystallinity for PAN/CL-derived blend CFs. We trace the pathway of gasification and carbonization of PAN/CL to elaborate the mechanism of the formation of all-carbon ring networks. We discover that the acetals of CL can catalyze the cyclization of the blend precursor, allowing for the search for CL derivatives or the other kinds of bio-based polymers with similar functionalities as alternative blends. In addition, we examine the structural characteristics using the carbon-carbon (C-C) radial distribution functions, C-C bond length distributions, and
sp
2
C atom ratios for the four representative precursors,
i.e.
, PAN, oxidized PAN, PAN/nylon 6,6, and PAN/CL. Our simulation results show the most extensive all-carbon ring cluster and graphitic structure growths for PAN/CL. Therefore, we propose PAN/CL as a cost-effective alternative CF precursor, since (a) CL is naturally abundant and eco-friendly for production, (b) the blend precursor PAN/CL does not require oxidation treatment, (c) PAN/CL has a high carbon yield with substantial all-carbon ring formation, and (d) PAN/CL based CFs potentially provide a mechanical property enhancement.
PAN/CL-derived blend carbon fiber precursor eliminates oxidation and shows high carbon yield and potentially enhanced mechanical properties.
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