Carbon Nanotube Sensors Developed for Stress and Strain Detection in Airplanes, Automobiles, Buildings, Bridges And Other Structures

Rice University (Houston, TX) earned U.S. Patent 7,730,547 for sensor devices comprising carbon nanotubes that are capable of detecting displacement, impact, stress, and/or strain in materials as well as methods of manufacturing the sensors. Exemplary uses for such sensors include stress and strain detection in airplanes, spacecraft, automobiles, buildings, bridges, dams, and other structures.

The Rice University inventors include Chair of the Department of Mechanical Engineering and Material Science  Enrique V. Barrera,   Professor of Civil Enginering and Professor of Mechanical Engineering Smart Structures Satish Nagarajaiah and researchers Prasad Dharap, Li Zhiling and Jong Dae Kim.  

The sensors are very small and can be considered micro-electromechanical systems (MEMS) or even nano-electromechanical systems (NEMS). Despite the small size of some of these sensors, they can still be made to sense mechanical conditions in large objects (e.g., buildings or airplanes) by strategically positioning them, making them mobile, and/or utilizing large numbers of them. The sensors can also be made with macroscopic dimensions. Furthermore, the sensors can be made to sense mechanical conditions in multiple directions--either sequentially or all at once. Such multi-directional sensing can be generated with multiple sensors or sensing elements, or with a single sensor

Carbon nanotube sensing elements can be incorporated into airplane wings (or other parts of the airplane) to serve as indicators of potential structural failure. Carbon nanotube sensors can be potentially useful in detecting damage after lightning strikes an aircraft. 

FIG. 1 depicts SEM images of the films, wherein (a) is a carbon nanotube film made up of entangled bundles of SWNT, and (b) shows the thickness of the carbon nanotube film which is used to make the sensors. 

Carbon nanotubes can be used in an application such as gaskets.  When a gasket begins to fail, pressure inconsistencies will register as changes in electronic properties. For industrial facilities using such gaskets in pipes, a potential rupture may be averted by such gasket-sensors alerting as to the loss of gasket integrity. 

Changes in photoluminescence of the carbon nanotube sensing element as a result of mechanical conditions can be used to sense displacement, stress, strain, etc. The carbon nanotubes naturally absorb light at certain wavelengths, then re-emit them at the same or different wavelengths. A spectral analyzer determines the wavelengths at which the emission is occurring and the detector  records the intensity. By understanding how the emission spectrum changes as a direct result of certain mechanical conditions, the mechanical conditions can be evaluated in a quantifiable manner. Such methods and devices relying on photoluminescence offer some advantages and may be an attractive alternative to sensing changes in electrical properties for some applications. Perhaps most advantages is the ability to do this type of sensing remotely and through glass barriers

According to the inventors the devices and methods all rely on mechanically-induced electronic perturbations within the carbon nanotubes to detect and quantify such stress/strain. Such detection and quantification can rely on techniques which include electrical conductivity/conductance and/or resistivity/resistance detection/ measurements, thermal conductivity detection/measurements, electroluminescence detection/measurements, photoluminescence detection/measurements, and combinations thereof. All such techniques rely on an understanding of how such properties change in response to mechanical stress and/or strain.

FIG. 2 depicts a sensor that shows a carbon nanotube film with insulating PVC film attached to a brass specimen.   

  

To analyze mechanical conditions in a quantifiable manner, data must be compiled which correlates detectable thermal, electronic, and photoluminescence properties with fully understood mechanical conditions. Such understanding requires knowledge of the mechanical conditions' value, as well as a complete understanding of the environment (temperature, pressure, atmosphere, etc.) in which the correlatable data is obtained. 

Such data compilations or data bases may contain up to the tens of thousands (or more) of data points and computational hardware and software may be used to access and retrieve such information. Such information provides for a calibration measure by which observed thermal, electronic, and photoluminescence properties in unknown circumstances can be compared to and quantified. Such a database of calibration data is key to understanding the responses generated by the sensing element in response to various mechanical conditions.