Amir Pakdel

Boron Nitride Nanosheet Coatings with Controllable Water Repellency

The growth, structure, and properties of two-dimensional boron nitride (BN) nanostructures synthesized by a thermal chemical vapor deposition method have been systematically investigated. Most of the BN nanosheets (BNNSs) were less than 5 nm in thickness, and their purity was confirmed by X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. The effects of the process variables on the morphology and roughness of the coatings were studied using atomic force microscopy and scanning electron microscopy. A smooth BN coating was obtained at 900 °C, while compact BNNS coatings composed of partially vertically aligned nanosheets could be achieved at 1000 °C and higher temperatures. These nanosheets were mostly separated and exhibited high surface area especially at higher synthesis temperatures. The nonwetting properties of the BNNS coatings were independent of the water pH and were examined by contact angle goniometry. The present results enable a convenient growth of pure BNNS coatings with controllable levels of water repellency, ranging from partial hydrophilicity to superhydrophobicity with contact angles exceeding 150°.

Large-surface-area BN nanosheets and their utilization in polymeric composites with improved thermal and dielectric properties.

High-throughput few-layered BN nanosheets have been synthesized through a facile chemical blowing route. They possess large lateral dimensions and high surface area, which are beneficial to fabricate effectively reinforced polymeric composites. The demonstrated composites made of polymethyl methacrylate and BN nanosheets revealed excellent thermal stability, 2.5-fold improved dielectric constant, and 17-fold enhanced thermal conductivity. The results indicate multifunctional practical applications of such polymeric composites in many specific fields, such as thermoconductive insulating long-lifetime packaging for electrical circuits.

A comprehensive analysis of the CVD growth of boron nitride nanotubes

Boron nitride nanotube (BNNT) films were grown on silicon/silicon dioxide (Si/SiO(2)) substrates by a catalytic chemical vapor deposition (CVD) method in a horizontal electric furnace. The effects of growth temperature and catalyst concentration on the morphology of the films and the structure of individual BNNTs were systematically investigated. The BNNT films grown at 1200 and 1300 °C consisted of a homogeneous dispersion of separate tubes in random directions with average outer diameters of ~30 and ~60 nm, respectively. Meanwhile, the films grown at 1400 °C comprised of BNNT bundles in a flower-like morphology, which included thick tubes with average diameters of ~100 nm surrounded by very thin ones with diameters down to ~10 nm. In addition, low catalyst concentration led to the formation of BNNT films composed of entangled curly tubes, while high catalyst content resulted in very thick tubes with diameters up to ~350 nm in a semierect flower-like morphology. Extensive transmission electron microscopy (TEM) investigations revealed the diameter-dependent growth mechanisms for BNNTs; namely, thin and thick tubes with closed ends grew by base-growth and tip-growth mechanisms, respectively. However, high catalyst concentration motivated the formation of filled-with-catalyst BNNTs, which grew open-ended with a base-growth mechanism.

Facile synthesis of vertically aligned hexagonal boron nitride nanosheets hybridized with graphitic domains

Motivated by the recent quest for producing novel two dimensional nanomaterials, we developed a facile synthetic method for growing boron nitride–carbon (BN–C) phase-separated composite nanosheet coatings on silicon/silicon dioxide (Si/SiO2) substrates. The coatings were composed of compact partially vertically aligned nanosheets with a nanoscale roughness. The majority of the obtained BN–C nanosheets were less than 5 nm in thickness, mostly consisting of 2–15 atomic layers. Electron energy loss spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed the natural sp2 hybridization of the product, and cathodoluminescence spectroscopy measurements showed strong luminescence emission in the ultraviolet region at room temperature. Ultraviolet-visible spectroscopy demonstrated that the composite structure of alternating BN and C domains has different optical band gap features compared to pure h-BN nanosheets and graphenes, making it a promising material for further fundamental physical studies and potential applications in optoelectronics. Moreover, due to the rough morphology and nanoscale features of the BN–C coatings, they exhibited excellent water repellency (superhydrophobicity).

Plasma-Assisted Interface Engineering of Boron Nitride Nanostructure Films

Today many aspects of science and technology are progressing into the nanoscale realm where surfaces and interfaces are intrinsically important in determining properties and performances of materials and devices. One familiar phenomenon in which interfacial interactions play a major role is the wetting of solids. In this work we use a facile one-step plasma method to control the wettability of boron nitride (BN) nanostructure films via covalent chemical functionalization, while their surface morphology remains intact. By tailoring the concentration of grafted hydroxyl groups, superhydrophilic, hydrophilic, and hydrophobic patterns are created on the initially superhydrophobic BN nanosheet and nanotube films. Moreover, by introducing a gradient of the functional groups, directional liquid spreading toward increasing [OH] content is achieved on the films. The resulting insights are meant to illustrate great potentials of this method to tailor wettability of ceramic films, control liquid flow patterns for engineering applications such as microfluidics and biosensing, and improve the interfacial contact and adhesion in nanocomposite materials.

Morphology-Driven Nonwettability of Nanostructured BN Surfaces

Designing geometrical structures is an effective route to tailoring the wettability of a surface. BN-based hierarchical nano- and microstructures, in particular, vertically aligned and randomly distributed tubes and cones, were synthesized and employed as a platform for studying the influence of surface morphology on their static and dynamic interactions with water droplets. The variation of the contact angle in different hierarchical BN films is attributed to the combined effects of surface roughness and partial liquid-solid contact at the interface. Moreover, the impact response of water droplets impinging on BN arrays with different wetting properties is distinct. In the case of superhydrophobic films, the water droplet bounces off the surface several times whereas in less hydrophobic films it does not rebound and remains pinned to the surface. These results provide a facile route for the selective preparation of hierarchical BN nanostructure array films and a better understanding of their tunable water-repelling behavior, for which a number of promising applications in microelectronics and optics can be envisaged.

Enhanced Thermoelectric Performance of Bi-Sb-Te/Sb2O3 Nanocomposites by Energy Filtering Effect

Engineering of thermoelectric materials through hybridization with nanoparticles has been proved effective to boost their thermoelectric efficiency by providing the means to decouple thermal and electrical transport phenomena. Here, we report the synthesis of p-type Bi0.5Sb1.5Te3/X wt% Sb2O3 (X = 0, 1, 2, 4, 6) nanocomposites, in which the Sb2O3 nanoparticles are dispersed mainly at the grain boundaries of the Bi0.5Sb1.5Te3 matrix. It is shown that incorporation of up to 4 wt% Sb2O3 into the matrix results in simultaneous enhancement of the Seebeck coefficient (by filtering of low energy charge carriers) and decline of thermal conductivity (mainly by charge carrier scattering at the interfaces), both of which contribute to improving the thermoelectric figure of merit to a maximum of 1.51 at 350 K. Moreover, the nanocomposites with 2, 4, and 6 wt% Sb2O3 demonstrate ZT > 1.0 up to 450 K, making them commercially appealing for thermoelectric applications in a wide temperature range. Furthermore, it is shown that Bi0.5Sb1.5Te3/4 wt% Sb2O3 samples exhibit excellent thermal and chemical stability in ambient atmosphere and 300–475 K temperature range over a 24 month period.

Nonwetting behavior of “white” graphene coatings

Nanometer-thick coatings consisting of partially vertically aligned boron nitride nanosheets (BNNSs) were grown on silicon/silicon dioxide (Si/SiO2) substrates by a thermal chemical vapor deposition (CVD) technique. Surface morphology and roughness of the coatings were controlled by growth temperature. As a result, their wetting behavior could be adjusted from hydrophilic to superhydrophobic.

Influence of Hot Extrusion Process on the Mechanical Behavior of AA6061/SIC Composites

The effect of hot extrusion process on the microstructure, mechanical properties and fracture behavior of metal-matrix composites (MMCs) of AA6061 alloy reinforced with 10 volume percent particulate SiC with the average size of 46 µm was studied. The MMC ingots were fabricated by the stir casting method and were extruded at 450°C at a ram speed of 1mm/s and at the extrusion ratios of 6:1, 12:1 and 18:1. Various techniques including metallography, density measurement, tensile testing, and SEM fractography were utilized to characterize the mechanical behavior of the MMCs. Results demonstrated that extruded composites possessed considerably lower porosity contents, higher strength, and enhanced ductility in comparison with the as-cast samples. In addition, further improvement in the mechanical properties of the extruded composites was noticed by increasing the extrusion ratio. Fractographic observations revealed that the brittle fracture behavior of the as-cast specimens was promoted by cracking of the large SiC particle clusters. Whereas, the fracture surfaces of extruded composites showed extensive tear ridge formation by initiation and growth of shallow dimples, around the cracked particles, which is characteristic of a ductile fracture process. This change in the fracture behavior and improvement in mechanical properties is attributed to the break up of particle clusters and diminishment of pores during the extrusion process.