Synthesis of Multifunctional One-dimensional and Two-dimensional Highly Conductive Nanomaterials for Charge Storage Applications

In this thesis, highly conductive 1D and 2D nanomaterials including silver nanowire (AgNW), graphene, and MXene were synthesized and used for charge storage. AgNW with high purity and aspect ratio was incorporated in poly(methyl methacrylate) (PMMA) to fabricate a high performance dielectric material. To improve the dielectric performance of the nanocomposites, a hybrid structure, containing AgNW and MnO2NW, was utilized to not only enhance the dielectric constant but also minimize the dielectric loss. The fabricated AgNW:MnO2NW (2.0:1.0 vol %) hybrid nanocomposite showed a high dielectric constant (64 at 8.2 GHz) and low dielectric loss (0.31 at 8.2 GHz), which were among the best reported values in the literature in the X-band frequency range (8.2–12.4 GHz). This superior dielectric performance of the hybrid nanocomposites was attributed to (i) dimensionality match between the nanofillers, (ii) better dispersion state of AgNW in the presence of MnO2NW, (iii) positioning of ferroelectric MnO2NW in between AgNWs, (iv) barrier role of MnO2NW, and (v) the aligned structure of AgNWs, as nanoelectrodes. In the second phase of this thesis, graphene was synthesized via electrochemical exfoliation of graphite in different inorganic salt electrolytes. Electrochemically exfoliated graphene (EEG) sheets synthesized in this thesis had a good dispersibility in water as well as a high electrical conductivity. This eliminated extra steps to increase conductivity such as reduction of graphene oxide. By utilizing these advantages, polyvinyl alcohol (PVA)/EEG nanocomposites were prepared for dielectric application. The prepared nanocomposites had an aligned structure of co-doped EEG (doped with nitrogen and sulfur) and demonstrated excellent dielectric performance. The aligned 4.0 wt% co-doped EEG in the PVA matrix led to a high dielectric constant (201) and low dielectric loss (0.2) in X-band frequency. Different chemical and structural characteristics of the nanocomposite account for the enhanced dielectric properties: (i) good dispersion of co-doped EEG in the polymer matrix, (ii) enhanced polarization centers in the graphene due to nitrogen/sulfur doping, (iii) aligned co-doped EEG sheets, (iv) functional groups at the surface of EEG as barriers between graphene sheets. In the third phase of this thesis, a modified minimally intensive layer delamination (MILD) synthesis approach was introduced to synthesize a highly conductive Ti3C2Tx MXene. By utilizing the modified approach, the electrical conductivity was significantly improved to 2.4 ×〖10〗^4 S/cm, five times more than that obtained from traditional MILD approach which is about 5.8 ×〖10〗^3 S/cm. This significant improvement in electrical conductivity was attributed to better quality of the synthesized MXene with the modified approach as well as higher flake sizes. Furthermore, the modified approach enhanced the synthesis aspects such as synthesis yield (up to ⁓ 60%) and MXene colloidal concentration (up to 31 mg/ml). The prepared MXene demonstrated a promising application as a supercapacitor, with a high capacitance of ⁓ 490 F/g at 1 A/g. Our synthesis approach has great potential to be used as the next MXene synthesis (etching) approach since it demonstrates improved synthesis yield, high colloidal concentration, and enhanced electrical conductivity

Silver Nanowire/MnO₂ Nanowire Hybrid Polymer Nanocomposites: Materials with High Dielectric Permittivity and Low Dielectric Loss

This study reports the fabrication of hybrid nanocomposites based on silver nanowire/manganese dioxide nanowire/poly­(methyl methacrylate) (AgNW/MnO₂NW/PMMA), using a solution casting technique, with outstanding dielectric permittivity and low dielectric loss. AgNW was synthesized using the hard-template technique, and MnO₂NW was synthesized employing a hydrothermal method. The prepared AgNW:MnO₂NW (2.0:1.0 vol %) hybrid nanocomposite showed a high dielectric permittivity (64 at 8.2 GHz) and low dielectric loss (0.31 at 8.2 GHz), which are among the best reported values in the literature in the X-band frequency range (8.2–12.4 GHz). The superior dielectric properties of the hybrid nanocomposites were attributed to (i) dimensionality match between the nanofillers, which increased their synergy, (ii) better dispersion state of AgNW in the presence of MnO₂NW, (iii) positioning of ferroelectric MnO₂NW in between AgNWs, which increased the dielectric permittivity of nanodielectrics, thereby increasing dielectric permittivity of the hybrid nanocomposites, (iv) barrier role of MnO₂NW, i.e., cutting off the contact spots of AgNWs and leading to lower dielectric loss, and (v) AgNW aligned structure, which increased the effective surface area of AgNWs, as nanoelectrodes. Comparison of the dielectric properties of the developed hybrid nanocomposites with the literature highlights their great potential for flexible capacitors

Enhanced Dielectric Performance of Polymer Nanocomposites Based on CNT/MnO₂ Nanowire Hybrid Nanostructure

We report a new highly efficient polymer nanocomposite for charge storage applications based on carbon nanotube (CNT) and MnO₂ nanowire (MnO₂NW). Our study suggested that combination of conductive filler (CNT) and ferroelectric filler (MnO₂NW) is an effective method to fabricate nanocomposite with outstanding dielectric permittivity and low dielectric loss if two fillers share similar length and geometry. This strategy leads us to fabricate a hybrid nanocomposite (CNT:MnO₂NW (3.0:4.5 wt %)) with a high dielectric permittivity (50.6) and low dielectric loss (0.7), which are among the best-reported values in the literature in the X-band frequency range (8.2–12.4 GHz). We postulated that superior dielectric properties of the new hybrid nanocomposites were attributed to (i) better dispersion state of CNT in the presence of MnO₂NW, which increases the effective surface area of CNTs, as nanoelectrodes, (ii) dimensionality match between the nanofillers, which increases their synergy, and (iii) barrier role of MnO₂NWs, cutting off the contact spots of CNTs and leading to lower dielectric loss. Comparison of the dielectric properties of the developed hybrid nanocomposites with the literature highlights their great potential for flexible capacitors

Numerical modeling of coupled heat transfer and phase transformation for solidification of the gray cast iron

In the present study the numerical model in 2D is used to study the solidification bahavior of the gray cast iron. The conventional heat transfer is coupled with the proposed micro-model to predict the amount of different phases, i.e. total austenite (c) phase, graphite (G) and cementite (C), in gray cast iron based on the cooling rate (R). The results of phase amount are evaluated to find the proper correlation in respect to cooling rate. The semi-empirical formulas are proposed to find a good correlation between mechanical property (hardness Brinell) and different phase amounts and the cooling rate. Results show that the hardness of the gray cast iron decreases as the amount of graphite phase increases. It alsoincreases by increased amount of the cementite and the cooling rate. These formulas were developed to correlate the phase fractions to hardness. Results are compared whit experimental data and show good agreement