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Emerald Group Publishing Limited
Copyright © 2010, Emerald Group Publishing Limited
Nanostructured smart materials to change engineering design
Article Type: Viewpoint From: Sensor Review, Volume 30, Issue 4
Ideally, engineers would like to design components that are almost unbreakable. However, we cannot create unbreakable materials. The next best situation would be to have a material that can report on its state of health, react to its environment, and self-repair small damage. Such a material would change engineering design. At present, we also do not have materials that report, react, and self-repair. In industry, we mostly use simple lifeless materials that do not do anything, except be there. However, things are changing. Recently, there is a new class of materials coming to town – nanostructured smart materials (NSM). NSM convert physical stimuli or energy from one form to another and respond to their environment in a beneficial way. NSM change strain, temperature, or photons into electricity for sensing or power harvesting, and respond to external loading by actuating and changing their shape or stiffness. Nanostructured means there are nanoscale or nanophase features or components within the material. These features and components include graded-composition, crystal structure, coatings, and various types of particles such as nano-tubes, flakes, shells, platelets, spheres, and wires. With the large number of possibilities for synthesizing nanoscale materials, designer NSM will provide enabling new sensing and actuation properties that will change the game in design engineering. This viewpoint describes possibilities that await in engineering design using NSM. To keep it simple describing applications, NSM are considered generally to encompass smart, intelligent, active, passive, reactive, and responsive materials.
A primary application of nanostructured smart materials is to enable the design of sensors and transducers that are smaller, require less power, and are more sensitive than macroscale sensors. NSM will make a major impact in industries ranging from mechatronics to medicine. The convergence of micro-nanotechnology and the life sciences is particularly important and will produce a revolutionary new generation of medical devices. NSM transducers will enable us to observe, interpret, and understand the physical world in ways not previously possible. Future NSM may integrate sensing, actuation, and computing functions into their architecture. This will enable them to become intelligent and to “think” and decide how to respond to their environment rather than always repeating the same response. As an example, flexible materials may be able to stiffen or change shape in response to complex dynamic loading or the material may become transparent on command.
Incorporating NSM into industrial applications starts with surveying different available materials including hybrid materials and selecting a candidate material that provides the critical properties needed for the application. Categories of NSM are tabulated (Table I) for reference in developing industrial applications. Every material will have some property limitations. After selecting the material that satisfies the most important requirement, examine the individual limitations of the material. Imagine the ideal way to overcome each limitation, and then work backwards toward what can be achieved considering innovative new approaches. After backing up on the properties as needed, the overall performance of the achievable material should be evaluated. An iterative process incorporating knowledge from each trial can be used to converge to the optimal material. An example of this approach would be to design a continuous sensor (a long sensor that measures along its entire length) to detect cracking, moisture, delamination, or a chemical change in a composite material. Candidate sensor materials could include a continuous electrical strain gage, a fiber optic cable, or a carbon nanotube (CNT) thread made by drawing and spinning CNT from a forest. Considering CNT thread as a new approach, on the positive side, it is very small in diameter, strong, tough, lightweight, and electrically conductive with supercapacitance properties in an electrolyte. However, on the down side, the electrical conductivity is a bit low when long sensors are needed. Ideally the electrical conduction should be as good as copper. A possible solution is to dope or sputter conductive particles onto the thread. Alternatively, transformers along the thread could boost the voltage just enough to maintain a satisfactory signal to noise level. Power loss through Joule heating of the thread is still a disadvantage. However, CNT can operate at 350 °C so thermally insulating the thread or using thread in a vacuum (space application) are options to mitigate power loss. If the thread still does not meet the application, other options include improving the properties of the nanotubes and improving the spinning process. Otherwise, improving the properties may require developing long-metallic nanotubes, which is an on-going research effort. The process of backing up from the ideal situation considering unconventional approaches is a way to learn innovation and may lead to the optimal solution.
Richard Feynman was the first to predict that developing tiny machines cannot be avoided because of their potential advantages in engineering and medicine. We might extend this prediction and suppose that tiny machines can be used to develop materials that have electromechanical properties. Components and products built up of electromechanical systems will then be capable of relatively complex behavior, especially with the incorporation of computers. Combining microscale motors, gears, levers, bearing, plates, sensors, power harvesting, and electrical fiber with powerful microscopic computers would provide the necessary ingredients for a new class of electronic and mechanical NSM that are programmable and perform just about any desired function. Electromechanical NSM could check their structural integrity, repair damage, and even be programmed to self-assemble into components and structures. Imagine smart 3-D auxetic materials that expand, interlock, and self-assemble into automobile components, or aerospace and civil infrastructure that can morph and change shape to optimize their performance. There are a few problems to work out in developing these kinds of electromechanical NSM including surviving the environment in which the materials might be formed, e.g. high temperature, and ensuring the safety of unleashing materials that are somehow intelligent yet still follow our commands.
The whole idea of this viewpoint is that developing materials and structures that are programmable, tunable, and smart is starting to follow a nanomaterials trend. A possible distinction is that nanomaterials can be classified as engineered nanomaterials or those based on nature. Bio-inspired, self-assembled nanomaterials are important for the “engineering design” process because nature builds materials using an efficient multi-functional, hierarchical approach. An example of copying nature using nanomaterials is using CNT forests to mimic the gecko’s feet acting as a dry adhesive. Summing up, the emergence of industrial nanotechnology and material change are providing nanostructured smart materials that can improve the performance and safety of all kinds of components and structures. Using nanotechnology, engineers from all fields can join in the excitement trying to develop programmable electromechanical smart materials and devices that will do what we want them to do.