Electroactive Polymers (EAP)

By boltzmann • May 3, 2007 • 2 Comments

Electroactive polymers (EAP) represent a relatively new (as compared with Nitinol) class of smart materials. EAPs can exhibit very high deformations coupled with low forces and are classified in two forms, dielectric and ionic based.

In the dielectric form polymers are layered with electrodes and a voltage is applied, the electrostatic forces then lead to deformation of the material. Dielectric EAPs therefore rely on the electrostatic forces between electrodes to induce actuation by expansion of the polymer layer. When a high voltage is applied electrostatic attraction between the electrodes leads to an expansion in the plane of the actuator.

Ionic EAPs function via the displacement of ions in the polymer. The required driving voltages are generally on the order of a few volts, less than that required for dielectric EAPs. However, a larger driving current is required. Common applications for EAPs include artificial muscles, body sensors, and peristaltic pump designs. Due to the viscoelastic nature of the base polymer material, EAP actuators generally exhibit low response times.

At the present time, the commercialization of EAP materials is not sufficiently advanced to provide a characterized material source to be considered for a smart materials characterization study. The long-term reliability of EAP actuators is also a concern, dielectric actuators need to be pre-stressed but degradation of the pre-stress stiffness can occur. The advent of new polymers will no doubt lead to improvement in EAP designs and reliability.

Categories: Electroactive Polymers (EAP), Smart Materials
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Shape Memory Alloys (SMA)

By boltzmann • May 3, 2007 • No Comments

The Shape Memory Alloy (SMA) category of smart materials is made up of metallic alloys with shape memory effects, which rely on a temperature driven phase transformation to realize shape change in the material. Essentially, a material can be deformed, and returned to its original shape by heating. Materials that heal themselves, actuators, nanotechnology, numerous medical devices, and many other applications exist for SMAs.

The most common SMA is Nitinol (Ti3Ni4), and research into it dates back to research by the United States Navy, hence it’s name, Nickel Titanium Navy Ordinance Labs (Nitinol). The morphing affect of SMAs relies on the temperature dependent phase transformation between austenite and martensite. The shape memory effect is possible through reversibility of the martensite transformation via self-accommodation of martensitic plates.

Shape memory alloys rely on a transformation sequence between martensite and austenite. A shape memory material may be mechanically deformed while exhibiting a martensitic phase, and return to its original shape when heated above its transformation temperature. When heated, the material goes through a reverse transformation, the phase changing from martensite to austenite. Upon cooling from the austenite phase, the material crystallography returns to martensite. The transformation is rather versatile, enabling the manufacture of three different types of shape memory effects: one-way, two-way, and mechanical shape memory.

The narrow composition tolerance of Nitinol, and cost of the raw materials makes it difficult to manufacture in large bulk. Advances are being made in the area of iron and copper based SMAs, which offer a lower material cost. Super elastic tendons and texture-changing surfaces are common applications of Nitinol-based smart material system designs.

However, the phase transformation dependence on temperature renders a poor actuation response time unless the material can be heated and cooled at high rates. This implies that the surface area to volume ratio of a Nitinol actuator should be very high, such as occurs with a Nitinol coating as opposed to a Nitinol beam actuator. Despite a great deal of research expenditure, Nitinol based smart material designs have seen limited success. One drawback of SMAs such as Nitinol is the accumulation of dislocations in the crystal lattice, which can limit the service life and long-term reliability of Nitinol actuators. Over millions of fatigue cycles degradation in shape recovery can be seen.

Categories: Shape Memory Alloys (SMA), Smart Materials
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Smart Materials

By boltzmann • May 3, 2007 • No Comments

Engineering fields have traditionally focused on using uni-functional materials with set mechanical properties to design and build structures and products which fulfill certain design intents. Materials such as wood, concrete, and metals have served vast uses throughout history; allowing the construction of cities and industries. The industrial introduction of metals such as titanium or aluminum alloys and fiber-based composite materials have allowed the production of lightweight structures such as airplane fuselages and numerous inventions. While great strides have been made in the field of materials, modern science and engineering does not tell how to design and manufacture multifunctional materials, which can be designed for multiple design intents such as actuation and sensing in addition to fulfilling load-bearing requirements.

Traditional materials such as wood, concrete and metals are generally highly engineered and optimized to certain design requirements. Systems in Nature however, use complex materials, which are genetically optimized through natural selection to accommodate the changing conditions of a certain operating environment. In many ways current material design concepts are being rethought with an eye towards systems, which integrate active components into traditional materials to create material systems which interact with their environment. One method of accomplishing this goal is the integration of sensors and actuators into laminate materials. This allows the engineering of multi-functional products, which can sense and respond to changes in their environment and thereby increase the functionality of the original design. The field of smart materials seeks to fill the gap between traditional uni-functional and controllable multifunction materials.

The goal of smart materials research is to enable the design of materials that can function beyond the traditional design interests such as strength, stiffness, thermal conductivity, etc. An active or smart material system can generally be thought of consisting of three essential elements, the active material, the passive material, and the control system.

The active material is defined as the actuator or sensor element. As a sensor, the device exists to receive information from the operating environment (such as vibration or mechanical deformation). As an actuator the device enables interaction with the environment via shape or vibration control. The term “passive material” or "host structure" denotes the material that the device is integrated into. Finally, the control system refers to the mechanism or program which collects information from the sensor elements and controls actuation of the material. Modeling of various control systems has been addressed in numerous works, but research into the coupling between device and structure is infrequently addressed. Ideally, the same device should be able to act as a sensor and actuator, thereby lending greater flexibility to fulfilling the specific design requirements of any given application.

Categories: Smart Materials
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