Otis Singleton
Shape-memory alloys (SMAs) are smart materials that can remember their original shape. They are often used in aerospace and medical applications due to their lightweight, flexible, and tough nature. SMAs can be deformed when cold but will return to their pre-deformed shape when heated. This unique property has led to their use in various applications, including dental braces, root canal files, and automotive valves. SMAs can be actuated electrically, with a current passing through and heating the material, causing it to contract. This actuation method has some limitations, such as high power consumption and slow system response times. However, recent studies have proposed improved heating methods, such as using carbon nanotubes and silver paste, to enhance the efficiency and stability of SMA actuation.
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What You'll Learn
Electric heating
To address these issues, researchers have proposed alternative heating methods for SMAs used in textiles, such as using silver paste and coating the basic fabric with carbon nanotubes (CNTs). Silver paste requires lower power consumption, while CNTs support heating stability and heat uniformity. The efficiency of these methods has been evaluated by comparing direct heating and silver layer heating, as well as examining the uniformity of heat distribution using infrared images.
In the field of smart wearables, there is interest in replacing rigid, heavy, and large electric actuators with SMAs. This is due to the ability of SMAs to "remember" their initial shape and return to it when heated. The "memorized geometry" of an SMA can be modified by fixating the desired geometry and subjecting it to a thermal treatment. For example, a wire can be taught to remember the shape of a coil spring.
SMA-based devices have applications in various fields, including the biomedical and robotics fields. In the biomedical field, SMA is used in orthodontic wires and endoscopes for intestinal exploration. In robotics, SMA is used in four-fingered robot hands and portable tactile displays for the blind. To prevent overheating and thermal fatigue in SMA-based devices, controllers are designed to consider the maximum heating current at which an SMA element can be safely heated.
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Joule heating
Shape-memory alloys (SMAs) are alloys that can be deformed when cold but return to their pre-deformed ("remembered") shape when heated. The two most prevalent shape-memory alloys are copper-aluminium-nickel and nickel-titanium (NiTi).
SMAs are actuated electrically, with an electric current resulting in Joule heating. This is a fast and easy method of actuation, but it can lead to overshooting, instability, and high power consumption. As the length or thickness of the alloy increases, the power consumption increases, which has resulted in limited use of SMAs.
To overcome these challenges, researchers have proposed using a heating method that combines silver paste and carbon nanotubes (CNTs). This method requires lower power consumption and improves heating stability and uniformity. The CNTs are coated onto the fabric, supporting the silver paste and enabling more uniform heat distribution.
SMA actuation is typically asymmetric, with a fast actuation time and a slow deactivation time. Deactivation occurs through free convective heat transfer to the ambient environment.
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Energy harvesting
Shape-memory alloys (SMAs) are materials with the ability to change shape. They can be deformed when cold but return to their pre-deformed ("remembered") shape when heated. The two most prevalent SMAs are copper-aluminium-nickel and nickel-titanium (NiTi).
SMAs are actuated electrically, with an electric current resulting in Joule heating. However, this method of actuation has been shown to have high power consumption, especially with increases in the length or thickness of the alloy. To combat this, researchers have suggested a heating method using silver paste and carbon nanotubes (CNTs) to support heating stability and heat uniformity.
SMAs have been identified as a potential technology for energy harvesting, particularly for low-power electrical equipment. They can be used to harvest waste energy in the form of vibrational, thermal, and acoustic energy. This is because SMAs have a unique combination of large displacement, rapid mechanical responses, and great actuation capability.
Recent studies have focused on the use of magnetic shape memory alloys (MSMAs) for energy harvesting. MSMAs are supplied with actuation energy through a magnetic field and have been shown to provide higher output voltage compared to piezoelectric materials. The use of MSMAs for energy harvesting has been explored in industries such as aerospace, automotive, and marine, where there is a demand for mobile electricity.
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Superelasticity
Shape-memory alloys (SMAs) are materials that can be deformed at one temperature but when heated or cooled, return to their original shape. This is known as one-way shape memory. Some materials also show a memory effect on subsequent cooling, which is known as two-way shape memory. SMAs are unique because they can be taught a formal shape to return to in their memory state.
SMAs that show superelasticity can also exhibit the shape memory effect (SME) due to the heating and cooling process. However, the available SMAs on the market with superelasticity are supposed to be used at low phase transformation temperatures, typically less than room temperature.
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Aerospace applications
Shape-memory alloys (SMAs) have a wide range of applications, including in the aerospace industry. SMAs are metals that can be deformed when cold but return to their original, "remembered" shape when heated. This unique property makes them useful in various aerospace applications, where they can be used to create lightweight, solid-state alternatives to traditional actuators.
One of the key advantages of SMAs in aerospace is their ability to replace heavier, motorized actuators. For example, in the 2014 Chevrolet Corvette, SMA actuators were used to open and close the hatch vent, making it easier to close. SMAs also have benefits over traditional actuators, such as lower noise, weight, form factor, and power consumption.
SMAs are electrically actuated using Joule heating, where an electric current generates heat. This makes them particularly useful in applications where precise control and rapid response are required. However, one challenge with SMAs is that their power consumption increases with their length or thickness, which can limit their use in certain applications. Researchers are working on improving the heating methods for SMAs, such as using carbon nanotubes and silver paste, to overcome this issue.
In aerospace, SMAs have been used in both atmospheric and space flight applications. For example, SMAs can be used in wing-morphing technologies, where their ability to change shape with temperature can be utilized to optimize aerodynamics. SMAs are also being explored for energy harvesting in aerospace applications, where they can convert waste energy from sources such as structural vibration into electricity.
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Frequently asked questions
A shape-memory alloy (SMA) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated.
Shape-memory alloys are activated by direct current (DC) passed through them, heating the material, contracting the SMA, and hence reducing its height.
The two most prevalent shape-memory alloys are copper-aluminium-nickel and nickel-titanium (NiTi).
Shape-memory alloys are strong, flexible, easy to sterilize, corrosion-resistant, and lightweight. They are also capable of operating at high temperatures.
Shape-memory alloys suffer from functional fatigue, where the SMA loses its shape-memory characteristics over time. They also have slow system response times due to issues with temperature control and high power consumption.
