The principle of memory alloy is not very clear. It is generally believed that shape recovery memory will occur when the memory alloy changes from a complex rhombic crystal structure to a simple cubic crystal structure. When the memory alloy returns to its original shape, it is accompanied by great force. Nickel-titanium alloy is as high as 60 kg square millimeter, which is far greater than the force applied during initial deformation. Generally speaking, it can reach ten times of the original deformation, which means that the output energy is much greater than the input energy. Scientists can't explain this. Physicist Ricardo Roberto Barreto da Rocha De said, "There is nothing wrong with the laws of thermodynamics, but these laws just don't apply to Nitinol ...".
Composition of memory alloy
The characteristics of memory alloys were discovered in the early 1950 s, and both gold cadmium and indium thallium alloys have this characteristic.
Application of memory alloy
Memory alloy has been used in pipeline assembly and automatic control. Bushing made of memory alloy can replace welding. The method is to make the inside of the pipe end fully expand by about 4% at low temperature, and put them together when assembling. Once heated, the sleeve shrinks back to its original shape and forms a tight bond. There are 6,543,800 such joints used in the hydraulic system of American Navy aircraft, and there has been no oil leakage or damage for many years. It is very convenient to repair damaged pipelines of ships and submarine oil fields with memory alloy fittings. In some inconvenient parts, the pin is made of memory alloy, put into the hole and heated, and the tail automatically curls separately to form a single-sided assembly.
Memory alloy is especially suitable for thermal machinery and constant temperature automatic control. The ventilation window can be opened in sunny days and closed automatically when the room temperature drops at night. There are also many design schemes of memory alloy heat engine, which can work between two kinds of media with small temperature difference, opening up new ways for the utilization of industrial cooling water, nuclear reactor waste heat, ocean temperature difference and solar energy. At present, the common problem is low efficiency, only 4% ~ 6%, which needs to be further improved.
The application of memory alloy in medical treatment is also very eye-catching. For example, the bone plate used for fracture can not only fix two broken bones, but also produce compressive force in the process of restoring to the original state, forcing the broken bones to join together. Dental orthodontic wires, long clips for ligating cerebral aneurysms and vas deferens, and stents for straightening the spine are all started by body temperature after being implanted in the human body. Thrombotic filter is also a new product of memory alloy. After the straightened filter is implanted in a vein, it will gradually return to a network, thus preventing 95% of blood clots from flowing to the heart and lungs.
Artificial heart is an organ with more complicated structure. Muscle fibers made of memory alloy can imitate the contraction movement of ventricle by cooperating with elastic membrane ventricle. Now the pumping has been successful.
Because memory alloy is a kind of "living alloy", various automatic control devices can be designed by using its shape change at a certain temperature, and its application field is expanding constantly.