Energy development is a significant application area for shape memory alloys. Utilizing the property of shape memory alloys to change shape during heating and cooling, low-grade thermal energy such as waste heat, geothermal energy and solar energy can be directly converted into mechanical energy for use.
Based on this principle, a variety of heat engines have been successfully developed, accelerating the miniaturization and portability of heat engines.
In recent years, researchers around the world have developed materials with memory functions, mainly shape memory, temperature memory and color memory. Among them, the development of shape memory alloy materials is the fastest.
Due to the superior performance and broad application prospects of shape memory alloy materials in areas such as automobiles, robotics, energy development, medical equipment and household appliances, they have become one of the key new materials to be developed in the 21st century.
Shape memory alloys were the first materials with memory function discovered. In the early 1950s, experts from the University of Illinois in the United States accidentally discovered during an experiment that gold-cadmium alloys have a shape memory function.
Later, it was discovered that indium-thallium alloys also have a similar memory effect, but due to their high cost they did not attract attention. It was not until 1963, when the US Naval Weapons Research Institute accidentally discovered during its research on nickel-titanium alloys that these alloys had a significant shape memory effect.
They also have many unique advantages that other metals do not have, such as excellent super-elasticity, corrosion resistance, and vibration resistance. This discovery attracted the attention of researchers and research into it began.
In recent years, shape memory alloys have emerged with the rapid development of science and technology, and several dozen alloys with shape memory functions have been developed.
Currently, the most prominent shape memory alloys in practical use include the nickel-titanium, copper, and iron (or stainless steel) series.
I. Development of Shape Memory Alloys
1. Nickel-titanium shape memory alloys
Nickel-titanium alloys are among the most superior and widely used materials in shape memory alloys. They are characterized by good ductility, resistance to shape memory, deformation, corrosion resistance, electrical resistance and stability, but they also have high cost.
These alloys exhibit unidirectional and bidirectional shape memory behaviors, and the temperature range in which they exhibit these behaviors can be expanded or reduced through alloy improvements.
In recent years, many countries have been working on a series of improved nickel-titanium alloys, adding other elements to further improve their properties and reduce their costs. For example, adding copper or vanadium, aluminum, chromium, zirconium and traces of calcium can significantly improve its toughness, machinability and cutting ability.
Furthermore, the addition of rare earth elements and boron, silicon, phosphorus, sulfur, etc., to nickel-titanium-copper alloys can result in shape memory alloys with greatly improved recovery characteristics.
Sumitomo Electric Industries in Japan produces aesthetically pleasing shape memory alloy wires by adding copper (or aluminum, zirconium, vanadium, cobalt, iron) to nickel-titanium alloys and drawing the wire after surface treatment, meeting the decorative demands.
Cantoku, in Japan, developed a nickel-titanium alloy wire with half the diameter of a strand of hair, which has good compounding and weaving ability, elasticity and thermal sensitivity. These properties make it widely applicable in areas such as automobile valves, water heaters, water regulators, miniature electrical thermal regulators and bras.
2. Copper-based shape memory alloys
Copper-based shape memory alloys are cheaper and easier to shape than nickel-titanium memory alloys, so they have significant development potential.
However, the strength of copper-based shape memory alloys is lower than that of nickel-titanium memory alloys, and their memory capacity decreases rapidly with repeated heating.
To improve the mechanical properties of copper-based memory alloys, small amounts of titanium, manganese and zirconium can be added. The best performing and most widely used copper-based shape memory alloy is copper-zinc-aluminum alloy.
This alloy has high thermal conductivity and is sensitive to temperature changes, making it suitable for manufacturing thermosensitive components. Countries like the US and Japan use copper-zinc-aluminum alloys in automatic window openers for greenhouses and nurseries to regulate room temperature.
However, the resistivity of copper-zinc-aluminum alloys is lower than that of nickel-titanium alloys, making them unsuitable for electrical heating situations. Adding iron or silicon can improve its corrosion resistance.
A company in Tokyo, Japan, has successfully developed a copper-nickel-aluminum alloy with color memory effects. This alloy changes from red to gold at different temperatures and can be widely used in the production of crafts, decoration, toys and household appliances.
3. Iron-based shape memory alloys
Iron-based shape memory alloys are cheap and abundant, making them more competitive. Iron-based shape memory alloys developed include ferro-manganese alloys, iron-platinum alloys, and stainless steel shape memory alloys.
By adding silicon to manganese alloys, iron-manganese-silicon alloys with good shape memory effects can be obtained. These have high strength but low corrosion resistance.
Adding chromium can significantly improve its corrosion resistance. Iron-based shape memory alloys are now widely used in the manufacture of pipe joints, rivets and other connectors, as well as fittings. They are not only easy to install and operate, but also safe and reliable, which makes them promising functional materials.
The newly developed stainless steel shape memory alloys in Japan not only exhibit excellent shape recovery and corrosion resistance characteristics, but also exhibit good machinability and high-temperature oxidation resistance.
In addition to the main components of chromium, manganese, silicon and iron, these alloys contain a certain amount of nickel or cobalt, copper and nitrogen.
These stainless steel shape memory alloys can be melted in traditional steel furnaces and manufactured into finished products using common methods, making them extremely versatile.
II. Applications of shape memory alloys
Shape memory alloys, due to their superior properties, are widely used in automatic control devices, robotics, automotive industry, energy development, healthcare and daily consumer goods.
1. Use in Automotive, Robotics and Power Development for Automatic Control
Shape memory alloys function as temperature sensors and actuator elements, making them extremely sensitive for automatic control. One of the most common uses of these alloys is in brakes.
More than a hundred different types of shape memory brakes are currently used in the automotive industry, primarily to control engines, transmissions and suspensions to increase safety, reliability and comfort.
Shape memory alloys are also used in noise prevention devices in manual transmission systems and in engine fuel gas control devices. The memory effect and superelasticity of these alloys can be applied to displacement sensors and brakes on industrial robots.
The application of shape memory alloy brakes to robots has achieved encouraging results. Using shape memory alloy springs and their alloy wires, small robots can be assembled. By controlling the contraction of the alloy, the robot's fingers can open, close and flex.
The position, actions and speed of the robot are controlled by direct insertion of pulsed current of variable frequency into the alloy element. The force of form recovery is controlled by the size of the chain, making the movements mimic biological muscle movements.
Furthermore, energy development is another important field of application for shape memory alloys. Utilizing the shape-changing properties of shape memory alloys during heating and cooling, low-grade thermal energy such as waste heat, geothermal energy and solar energy can be directly converted into mechanical energy for use.
Based on this principle, a variety of heat engines have been successfully developed, accelerating the miniaturization and downsizing of heat engines.
2. Use in medical devices for dental and vascular diseases
The superelasticity, biocompatibility and non-toxicity of shape memory alloys have led to their successful application in the medical field.
In dentistry, shape memory alloys can be used to create artificial tooth roots, dental aligners and dental crowns. The artificial tooth is fixed on a nickel-titanium alloy gum and below it are double-layer alloy root sheets.
Through surgery, these root sheets are joined together at low temperatures and implanted into the alveolar bone. Under the influence of body temperature, the root layers invert and regain their original figure-eight shape, anchoring themselves firmly in the alveolar bone.
These artificial tooth roots are corrosion-resistant and biocompatible and are widely used in dentistry.
In addition, shape memory alloys are also used as tooth straightening wires, which are superior to those made of stainless steel. The super elasticity of the memory alloy clip can maintain its elasticity for a long time, eliminating the need for replacement due to elastic relaxation.
This long-term elasticity can shorten the treatment period. Shape memory alloys are also used to make dental crowns, which can automatically adjust as children's teeth develop, reducing the discomfort and pain of wearing dental crowns.
Medical devices for vascular diseases made from nickel-titanium shape memory alloys have many advantages.
For example, a vascular embolism removal knife made of a shape memory alloy can be processed at low temperature to form a cylindrical shape.
When inserted into the patient's blood vessels and heated to body temperature, the knife blade automatically flattens due to the shape memory effect, and by rotating it, embolisms or vascular deposits can be removed.
Medical experts have also successfully performed brain aneurysm removal surgeries using forceps made from a combination of shape memory alloy wires and silver wires.
3. Use in household appliances
Shape memory alloys are also widely used in household appliances such as food warmers, air conditioners and rice cookers.
Sharp Corporation in Japan uses shape memory alloy components to control the conversion between microwave heating and convection heating in food warmers, significantly reducing the volume and weight of the warmers and reducing costs by two-thirds .
Panasonic Corporation uses shape memory alloys to manufacture dual-use air conditioners with wind direction conversion devices.
The wind direction changes significantly: when the wind temperature is between 32-37°C, it blows downwards; when the wind temperature drops to 25-32°C, it automatically switches to blowing horizontally.
This device is stable, light weight, low cost and no noise when the wind port is started, making it popular among users. Shape memory alloys are also widely used in the manufacture of thermostats for rice cookers.
When the temperature rises, it will automatically open and disconnect the circuit. When the temperature drops to a certain level, it will reconnect the power supply and automatically maintain the temperature.
III. Shape memory alloys are progressing toward commercialization
In recent years, “memory” products made from various shape memory alloys have begun to sweep the market, with an increasing number of these “memory” items entering everyday life. The application of shape memory alloys is moving further towards commercialization.
A “memory” bra produced by a Japanese daily products company was immediately preferred by women after it hit the market. This bra is made of thin and soft nickel-titanium shaped memory alloy wire, which is lighter, softer, more elastic and more comfortable than traditional bras.
Its biggest advantage is that it can return to its original shape by detecting body temperature, even if it deforms after washing.
Furukawa Electric Co., Ltd. in Japan produces shape memory alloy eyeglass frames that can change shape as the lens expands and contracts, maintaining a close bond with the lens. The company also makes dolls that can change their hairstyle using the unique properties of shape memory alloys.
The doll's hair is made of copper-based shape memory alloy wire, and the hairstyle can change with changing temperature, which makes it extremely popular among children.
A Japanese shoe manufacturer has launched the world's first leather shoes made with shape memory alloys, which are not only soft and comfortable, but also effectively prevent shoes from deformation.
Energy development is a significant application area for shape memory alloys. Utilizing the property of shape memory alloys to change shape when heated and cooled, low-grade thermal energy such as waste heat, geothermal energy and solar energy can be directly converted into mechanical energy for use.
Based on this principle, numerous heat engines have been successfully developed, accelerating the miniaturization and portability of heat engines.
