ABSTRACT
Boron nitride nanotubes (BNNTs) are a unique class of light and strong tubular nanostructure and are highly promising as reinforcing additives in ceramic materials. However, the mechanical strength of BNNT-ceramic interfaces remains largely unexplored. Here we report the first direct measurement of the interfacial strength by pulling out individual BNNTs from silica (silicon dioxide) matrices using in situ electron microscopy techniques. Our nanomechanical measurements show that the average interfacial shear stress reaches about 34.7 MPa, while density functional theory calculations reveal strong bonded interactions between BN and silica lattices with a binding energy of -6.98 eV nm-2. Despite this strong BNNT-silica binding, nanotube pull-out remains the dominant failure mode without noticeable silica matrix residues on the pulled-out tube surface. The fracture toughness of BNNT-silica ceramic matrix nanocomposite is evaluated based on the measured interfacial strength property, and substantial fracture toughness enhancements are demonstrated at small filler concentrations.
ABSTRACT
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ABSTRACT
Due to recent advances in high-throughput synthesis, research on boron nitride nanotubes (BNNTs) is moving toward applications. One future goal is the assembly of macroscopic articles of high-aspect-ratio, pristine BNNTs. However, these articles are presently unattainable because of insufficient purification and fabrication methods. We introduce a solution process for extracting BNNTs from synthesis impurities without sonication or the use of surfactants and proceed to convert the extracted BNNTs into thin films. The solution process can also be used to convert as-synthesized material-which contains significant amounts of hexagonal boron nitride ( h-BN)-into mats and aerogels with controllable structure and dimension. The solution extraction method, combined with further advances in synthesis and purification, contributes to the development of all-BNNT macroscopic articles, such as fibers and 3-D structures.
ABSTRACT
The interaction of boron nitride nanotubes (BNNTs) with Al has been investigated by means of quantum chemical calculations. Two model structures were used: a BNNT adsorbing a four atom Al4 cluster, and a BNNT adsorbed on Al surfaces of different crystallographic orientations. The BNNTs were modeled as: (i) pristine, and (ii) having a boron (B-) or a nitrogen (N-) vacancy defect. The results indicated that the trends in binding energy for Al4 clusters were, similar to those of the adsorption on Al surfaces, while the Al surface orientation has a limited effect. In all cases, the calculations reveal that Al binding to a BNNT was strongly enhanced at a defect site on the BNNT surface. This higher binding was accompanied by a significant distortion of the Al cluster or the Al lattice near the respective vacancy. In case of a B-vacancy, insertion of an Al atom into the defect of the BNNT lattice, was observed. The calculations suggest that in the Al/BNNT metal matrix composites, a defect-free BNNT experiences a weak binding interaction with the Al matrix and tthe commonly observed formation of AlN and AlB2 was due to N- or B-vacancy defects within the BNNTs.
ABSTRACT
The structural stability and mechanical integrity of boron nitride nanotubes (BNNTs) in high temperature environments are of importance in pursuit of their applications that are involved with extreme thermal processing and/or working conditions, but remain not well understood. In this paper, we perform an extensive study of the impacts of high temperature exposure on the structural and mechanical properties of BNNTs with a full structural size spectrum from nano- to micro- to macro-scale by using a variety of in situ and ex situ material characterization techniques. Atomic force microscopy (AFM) and high resolution transmission electron microscopy measurements reveal that the structures of individual BNNTs can survive at up to 850 °C in air and capture the signs of their structural degradation at 900 °C or above. In situ Raman spectroscopy measurements reveal that the BN bonds in BNNT micro-fibrils undergo substantial softening at elevated temperatures of up to 900 °C. The AFM-based nanomechanical compression measurements demonstrate that the mechanical integrity of individual BNNTs remain intact after being thermally baked at up to 850 °C in air. The studies reveal that BNNTs are structurally and mechanically stable materials in high temperature environments, which enables their usages in high temperature applications.
ABSTRACT
Space exploration missions require sensors and devices capable of stable operation in harsh environments such as those that include high thermal fluctuation, atomic oxygen, and high-energy ionizing radiation. However, conventional or state-of-the-art electroactive materials like lead zirconate titanate, poly(vinylidene fluoride), and carbon nanotube (CNT)-doped polyimides have limitations on use in those extreme applications. Theoretical studies have shown that boron nitride nanotubes (BNNTs) have strength-to-weight ratios comparable to those of CNTs, excellent high-temperature stability (to 800 °C in air), large electroactive characteristics, and excellent neutron radiation shielding capability. In this study, we demonstrated the experimental electroactive characteristics of BNNTs in novel multifunctional electroactive nanocomposites. Upon application of an external electric field, the 2 wt % BNNT/polyimide composite was found to exhibit electroactive strain composed of a superposition of linear piezoelectric and nonlinear electrostrictive components. When the BNNTs were aligned by stretching the 2 wt % BNNT/polyimide composite, electroactive characteristics increased by about 460% compared to the nonstretched sample. An all-nanotube actuator consisting of a BNNT buckypaper layer between two single-walled carbon nanotube buckypaper electrode layers was found to have much larger electroactive properties. The additional neutron radiation shielding properties and ultraviolet/visible/near-infrared optical properties of the BNNT composites make them excellent candidates for use in the extreme environments of space missions.
ABSTRACT
The length of nanotubes is a critical structural parameter for the design and manufacture of nanotube-based material systems and devices. High-precision length control of nanotubes by means of mechanical cutting using a scriber has not materialized due to the lack of the knowledge of the appropriate cutting conditions and the tube failure mechanism. In this paper, we present a quantitative nanomechanical study of the cutting of individual boron nitride nanotubes (BNNTs) using atomic force microscopy (AFM) probes. In our nanotube cutting measurements, a nanotube standing still on a flat substrate was laterally scribed by an AFM tip. The tip-tube collision force deformed the tube, and eventually fractured the tube at the collision site by increasing the cutting load. The mechanical response of nanotubes during the tip-tube collision process and the roles of the scribing velocity and the frictional interaction on the tip-tube collision contact in cutting nanotubes were quantitatively investigated by cutting double-walled BNNTs of 2.26-4.28 nm in outer diameter. The fracture strength of BNNTs was also quantified based on the measured collision forces and their structural configurations using contact mechanics theories. Our analysis reports fracture strengths of 9.1-15.5 GPa for the tested BNNTs. The nanomechanical study presented in this paper demonstrates that the AFM-based nanomechanical cutting technique not only enables effective control of the length of nanotubes with high precision, but is also promising as a new nanomechanical testing technique for characterizing the mechanical properties of tubular nanostructures.
