Boron nitride nanotubes (BNNTs) have exceptional thermal stability, thermal conductivity, mechanical properties, neutron radiation shielding, and piezoelectricity. Due to their multifunctional properties, BNNTs are potential candidates for sensory materials in harsh environments. Brittleness and non-conformity of conventional piezoelectric ceramics have limited their broad applications. Flexible and ultra-light piezoelectric sensors based on BNNTs could be an alternative solution in high temperature, high radiation, high shock, and severe vibration environments. In this study, BNNTPolyurethane (PU) composites were fabricated and their converse piezoelectric constant of d33 was assessed using a laser Doppler vibrometer (LDV). This study demonstrated that BNNT could be an excellent piezoelectric nanofiller for flexible sensor applications.
Scientists have predicted that carbon’s immediate neighbors on the periodic chart, boron and nitrogen, may also form perfect nanotubes, since the advent of carbon nanotubes (CNTs) in 1991. First proposed then synthesized by researchers at UC Berkeley in the mid 1990’s, the boron nitride nanotube (BNNT) has proven very difficult to make until now. Herein we provide an update on a catalyst-free method for synthesizing highly crystalline, small diameter BNNTs with a high aspect ratio using a high power laser under a high pressure and high temperature environment first discovered jointly by NASA/NIA/JSA. Progress in purification methods, dispersion studies, BNNT mat and composite formation, and modeling and diagnostics will also be presented. The white BNNTs offer extraordinary properties including neutron radiation shielding, piezoelectricity, thermal oxidative stability (> 800°C in air), mechanical strength, and toughness. The characteristics of the novel BNNTs and BNNT polymer composites and their potential applications are discussed.
Tailoring the solar absorptivity (αs) and thermal emissivity (ƐT) of materials constitutes an innovative approach to solar energy control and energy conversion. Numerous ceramic and metallic materials are currently available for solar absorbance/thermal emittance control. However, conventional metal oxides and dielectric/metal/dielectric multi-coatings have limited utility due to residual shear stresses resulting from the different coefficient of thermal expansion of the layered materials. This research presents an alternate approach based on nanoparticle-filled polymers to afford mechanically durable solar-absorptive and thermally-emissive polymer nanocomposites. The αs and ƐT were measured with various nano inclusions, such as carbon nanophase particles (CNPs), at different concentrations. Research has shown that adding only 5 wt% CNPs increased the αs and T by a factor of about 47 and 2, respectively, compared to the pristine polymer. The effect of solar irradiation control of the nanocomposite on solar energy conversion was studied. The solar irradiation control coatings increased the power generation of solar thermoelectric cells by more than 380% compared to that of a control power cell without solar irradiation control coatings.
A carbon nanocomposite-based contact mode interdigitated center of pressure sensor (CMIPS) has been developed.
The experimental study demonstrated that the CMIPS has the capability to measure the overall pressure as well as
the center of pressure in one dimension, simultaneously. A theoretical model for the CMIPS is established here
based on the equivalent circuit of the CMIPS configuration as well as the material properties of the sensor. The
experimental results match well with the theoretical modeling predictions. This theoretical model will provide
guidelines for future advanced sensor development based on the CMIPS. A system mapped with two or more pieces
of the CMIPS can be used to obtain information from the pressure distribution in multi-dimensions. As an
intelligent system component, the inexpensive CMIPS can be used broadly for improving sensing and control
capabilities of aircraft and measurement capabilities of biomedical research as well as chemical industries.
The focus of this paper is on our experimental observation that addition of single wall carbon nanotubes (SWNTs) to a
non-polar polyimide induces an electromechanical behavior, where bending strains are observed in response to applied
electric fields. Percolation studies are carried on the nanocomposites. The samples used for studying actuation are above
the percolation threshold. The electromechanical mechanism is identified as electrostriction, and the coefficients of
electrostriction are calculated under different conditions of SWNT loading and applied electric fields. The
electrostriction is believed to be a result of induced polarization in these materials, which shows an increase with the
SWNT content. Furthermore, the polarizability of SWNTs modifies the local electric field in the surrounding polymer
matrix, resulting in a low actuation voltage.
Electrospinning of a SWNT-polyimide composite is accomplished under DC electric field. The resulting composite fibers are characterized to assess the alignment of the SWNTs in the polyimide. Polarized Raman spectroscopy is performed using a Nicolet dispersive Raman spectrometer with a polarizer. The Raman spectrum of SWNT-polyimide fibers is recorded at several angles between the SWNT axis and the incident polarization, in the range of 0° to 180°. The Raman peak in each spectrum corresponds to the tangential mode (1590 cm-1) of the SWNT in the composite. Inspection of the spectra reveals that the maximum intensity is obtained when the polarization of incident radiation is parallel to the SWNT axis, while the smallest intensity is obtained when the polarization of incident radiation is perpendicular to the SWNT axis. Difference in the intensities when the radiation is parallel and perpendicular to the SWNT axis indicates preferential alignment of SWNTs in the polyimide fibers.
Single wall carbon nanotube (SWNT)-polymer composites aligned by an AC electric field were characterized using Raman spectroscopy and electrical conductivity measurements to assess the resulting alignment. The Polarized Raman spectra was recorded at several angles between the SWNT axis and the incident polarization ranging from 0° to 180°. Inspection of the spectra revealed that maximum intensity is obtained when the polarization of incident radiation is parallel to the SWNT axis (0° and 180°), while the smallest intensity is obtained when the polarization of incident radiation is perpendicular to the SWNT axis (90°). The electrical measurements were made in three directions; parallel to the aligned SWNTs and perpendicular to the aligned SWNTs. Based on the electrical conductivity and polarized Raman spectroscopy measurements, it can be concluded that the SWNTs in the polymer matrix were preferentially aligned by applying an AC electric field of 43.5 V/mm at a frequency of 1 Hz, 10 Hz, 10 KHz and 100 KHz.
High performance piezoelectric polymers are of interest to NASA as they may be useful for a variety of sensor applications. Over the past few years research on piezoelectric polymers has led to the development of promising high temperature piezoelectric responses in some novel polyimides. In this study, a series of polyimides have been studied with systematic variations in the diamine monomers which comprise the polyimide while holding the dianhydride constant. The effect of structural changes, including variations in the nature and concentration of dipolar groups, on the remanent polarization and piezoelectric coefficient is examined. Fundamental structure-piezoelectric property insight will enable the molecular design of polymers possessing distinct improvements over state-of-the-art piezoelectric polymers including enhanced polarization, polarization stability at elevated temperatures, and improved processability.