Quantitative Characterization of Structural and Mechanical Properties of Boron Nitride Nanotubes in High Temperature Environments

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.

Figure 1

(a) A schematic illustration of the experiment setup for in situ temperature-dependent Raman spectroscopy and optical microscopy characterization of BNNT microfibrils. (b) Selected in situ optical images showing the morphology evolutions of one tested BNNT microfibril being heated at different temperatures (the scale bar represents 20 μm). (c) Selected in situ Raman spectra of the BNNT microfibril shown in (b) that were recorded, in situ, at different temperatures ranging from 25 °C to 900 °C. (d) Temperature dependence of the active G (E_2g mode) peak frequency for the tested BNNT sample. The solid line represents a quadratic polynomial function fitting curve. The inset plot shows the dependence of the corresponding FWHM of the E_2g mode on temperature.

Figure 2

Selected AFM images of one individual BNNT that stayed on a silicon substrate at room temperature (a), and after being heated subsequently at 850 °C (b), 900 °C (c), 925 °C (d) and 950 °C (e), each for 10 minutes in air. All of the AFM images shown in (b–e) were taken after the samples were cooled down to room temperature. The scale bar represents 200 nm.

Figure 3

HRTEM images of individual BNNTs deposited on Ni grid after being heated at 850 °C (a) and 900 °C (b,c) in air for 10 mins (the displayed image in (c) is an assembled image from a set of HRTEM images that were taken at different portions of the same tube); (d) The EDS spectrum for the selected area (blue box) of the BNNT displayed in (c). The insert EDS spectrum is for the selected area (red box) of the BNNTs displayed in (a). All scale bars represent 10 nm.

Figure 4

The XPS spectra of a deposited BNNT film after being heated at 900, 925, and 950 °C, respectively, each for 10 minutes in air.

Figure 5

(a) A schematic illustration of the AFM-based compression testing scheme on the radial elasticity of an individual BNNT on a flat substrate. (b) The recorded compressive load versus nanotube height profiles for five selected BNNTs that were thermally annealed at different temperature in air. The dots represent experimental measurements, while the solid curves represent the respective fitting curves based on the Hertzian contact mechanics model. (c) The dependence of the measured effective radial modulus of BNNTs on the tube outer diameter at room temperature and after being annealed at 300, 600, 800 and 850 °C, respectively. The solid curves represent the power-function fitting curves of the effective radial elastic moduli data of single- to quadruple-walled BNNTs at room temperature, which are reproduced from ref..