3 process
3.1 preparation of shape memory alloy
The preparation of shape memory alloy is usually to prepare alloy ingot first, then hot rolling, die forging, extrusion, and finally cold working. Shape memory treatment (heat treatment) is an important step to realize the shape memory function of alloys.
One way memory effect of TiNi shape memory alloy can be treated by three processes: medium temperature, low temperature and aging. Low temperature treatment: the alloy is annealed above 800 ℃, processed at room temperature according to the application requirements, and then heated at 250 ~ 350 ℃ for 0.5 h to form. The alloy has good toughness and is easy to be processed into products with complex shape and small size, but its stability is poor and it is not wear-resistant; Medium temperature treatment: the alloy is processed and formed in low temperature environment (MS below), and then put into high temperature environment (MS above) for a period of time to form; Aging treatment: the alloy was solution treated at 1000 ℃, then quenched, and then aged at 400 ℃ for 1 h. When the alloy is rich in Ni, it has the conditions to precipitate the second phase particles and strengthen the alloy, which is suitable for aging treatment. This method can not only effectively avoid the appearance of Ti2Ni type particles in the alloy melting process, but also improve the SME and se of the alloy.
The two-way memory effect of TiNi shape memory alloy is due to the existence of directional stress field or crystal defects in the alloy. Martensite is easy to nucleate at the defects during phase transformation, and preferential growth occurs at the same time. Through memory training (forced deformation), the two-way memory ability was obtained: (1) first, the shape of high temperature phase was memorized through one-way memory effect（ 2) Then, when the temperature is lower than MS, a certain amount of recoverable deformation is carried out according to the required shape（ 3) When heated to the temperature above as, the sample returns to the high temperature shape, and then decreases to below MS, and then deforms the sample to the required shape at low temperature（ 4) After repeated processing, bidirectional memory effect can be obtained.
3.2 preparation of shape memory alloy films
The preparation methods of Ti Ni SMA thin films include magnetron sputtering, melt quenching, flash evaporation, ion beam sputtering, pulsed laser deposition, electron beam deposition, rapid solidification, pulsed laser deposition, evaporation deposition, plasma rapid evaporation and ion beam assisted deposition.
Magnetron sputtering deposition is a method in which the positive ions produced by glow discharge bombard the target surface under the action of electric field in a vacuum chamber, and the bombarded particles are deposited on the substrate to form a film. This method has the advantages of simple operation, fast deposition speed, good compatibility with the preparation process of silicon-based micro devices, high coating density, and the composition of the film is basically the same as that of the target material. It is the main method to prepare Ti Ni SMA films at present.
Melt rapid quenching is a method of giving a certain pressure to the molten metal or alloy in vacuum state and injecting it into the water-cooled copper roller with high-speed rotation, so that it can be solidified under great supercooling degree, so as to obtain the non-equilibrium structure film with ultra-fine structure. This method has the advantages of low cost and uniform chemical composition of alloy film, and can obtain nano or amorphous film grains. Process flow: master alloy smelting → ingot casting → ingot remelting in film nozzle tube → melt spraying → high speed rotating cooling roller → solidification → separation of film and roller → collection of film → crystallization annealing → crushing and pulverizing → SPS sintering. At present, although the research on the preparation of Ti Ni based SMA films by melt quenching has made great progress, there are still many problems, such as the influence of melt temperature, pressure, roll speed, alloy composition and heat treatment process on the microstructure and properties of the films, which need to be further studied in order to optimize the composition, process, microstructure and properties of the films.
4 Application
SMA can obtain 4% ~ 8% reversible recovery strain in a narrow temperature range. If the strain recovery is prevented during heating, SMA can produce larger resistance stress. That is to say, SMA can output force or displacement by changing temperature under certain conditions. SMA has been widely used in automotive, aerospace, robotics, biomedicine and other fields due to its dual functions of sensing and driving, as well as its ability to produce large reversible shape response to stress and strain.
Application of SMA
4.1 automobile
In modern vehicles, the demand for more safe and comfortable vehicles makes the market demand for sensors and drivers soar. In automotive applications, SMA is mostly used as line brake (such as mirror folding, temperature control aileron, lock control) and thermal brake (such as engine temperature control, carbon compound engine lubrication, power clutch).
The miniature SMA actuator is of great significance to reduce the size, weight and cost of automobile components. For example, the electrically driven zoom mirror, the SMA flexible rolling aileron (wheel) instead of the traditional electromagnetic pneumatic effector, an automatic pedestrian protection system (pop-up valve cover) to reduce the impact injury of pedestrians, a cost-effective side mirror brake, and a micro scanning system using FSMA brake for light sensing of target distance and target angle.
One of the challenges of automotive applications is the compatibility of SMA with automotive batteries.
Application of SMA in automobile
4.2 Aerospace
Shape memory alloys have been used in aviation and space devices. SMA has been successfully applied in aerospace field, such as actuator, structural connector, vibration damper, sealing material, release or deployment mechanism, inflatable structure, manipulator and Pathfinder. For example, the shape memory alloy (SMA) used in the hydraulic system of military aircraft is being developed in Europe and the United States. Due to the limitation of helicopter's high vibration and high noise, the main sources of noise and vibration are the eddy current interference of blade and the small deviation of blade profile. This requires a device to balance the blade pitch, so that each blade can accurately rotate in the same plane. At present, a kind of blade trajectory controller has been developed, which uses a small dual tube shape memory alloy driver to control the position of the small vane on the blade edge trajectory, so as to minimize the vibration. It can also be used to make lunar antennas for exploring the mysteries of the universe. People use shape memory alloy to make the antenna at high temperature, and then compress it into a small iron ball at low temperature, reducing its volume to one thousandth of its original size. In this way, it can be easily transported to the moon, and the strong radiation of the sun makes it return to its original shape, Send back valuable cosmic information to the earth according to the demand. In addition, a shape memory release device with an open container is used in the satellite to protect the sensitive germanium detector from contamination during assembly and launch.
Application of shape memory alloy in aerospace
4.3 robots
SMA has many successful applications in robotics, such as micro actuator, artificial muscle and so on. But it also faces many challenges: the performance and miniaturization of hardware platform, and the intellectualization of integrated system (small, fast, reliable and automatic). The technical problems to be solved include fixing difficulties, low resistance, micro electronic links (for micro robots), small strain output, control problems and ultra-low efficiency.
The response speed of SMA actuator is obviously affected by its shape and size. Resistance heating is generally used for small SMA actuators (the maximum diameter is 400 microns), and indirect heating is used for thick actuators. In order to increase the driving frequency, the capacitor and thick actuator can be combined together to obtain fast heating reaction. Cooling measures can be used to promote the cooling process, but it will cause heavy equipment; In addition, to increase the degree of freedom of the robot, the number of actuators must be increased, which will lead to complex control problems.
The existing and potential applications of shape memory alloys in Robotics
4.4 biomedicine
Although NiTi alloy is much more expensive than stainless steel, SMA shows excellent properties in biomedical applications, such as high corrosion resistance, good biocompatibility, non magnetism, and some other unique physical properties. Therefore, it has been applied in many fields of medical equipment and devices, including orthopedics, neurology, cardiology, media Radiology, etc Dentistry, etc. Products include endovascular stents, catheters, medical tweezers, bone anchors, variable stiffness implants, aneurysm treatment, artificial myocardium, frames and leads, etc.
Shape memory alloys have been widely used in biomedical field
4.5 potential applications
The development of new SMA or improved SMA will greatly promote the properties and performance of SMA.
In the future, the development of SMA can be expected at three different levels: (1) the development of new SMA or improved SMA（ 2) The functional properties of SMA are combined with the structure of other materials（ 3) Looking for new markets.
This entry is reviewed by the project of "science popularization China" science encyclopedia entry compilation and application. Shape memory polymer (SMP) is a new kind of functional polymer materials. It is a new branch of research, development and application of polymer materials. It has the characteristics of both plastics and rubber. With the deepening of the understanding of polymer structure and properties, and the development of polymer synthesis technology, it has become a reality for polymer materials to obtain the expected structure and properties through molecular design. Shape memory polymer is a kind of polymer materials obtained by molecular combination and modification of general polymer materials by using modern polymer physics theory and polymer synthesis and modification technology, such as polyethylene, polyisoprene, polyester, copolyester, polyamide, copolyamide, polyurethane and other polymer materials for molecular design and molecular structure adjustment, Under certain conditions, they are given a certain shape (initial state). When the external conditions change, they can change the shape and fix it accordingly. If the external environment changes again in a specific way and law, they will reversibly return to the initial state. So far, the cycle of "memory initial state - fixed variable form - recovery initial state" has been completed. Chinese name shape memory polymer foreign name shape memory polymer has the characteristics of memory, can be restored to the original category of polymer materials, application of industrial, military, medical theoretical basis of polymer chemistry catalogue 1. Classification of shape memory polymers Shape memory properties and basic principles of 2-Polymer 3 thermally induced shape memory polymer and its memory effect Application of shape memory polymer Shape memory polymer classification editing speech 1. Thermally induced SMP (thermally induced SMP) is a kind of polymer which can be deformed above room temperature, and can be stored at room temperature for a long time. When the temperature is raised to a specific response temperature, the product can quickly return to its original shape. It is widely used in medical and health, sports, architecture, packaging, automotive and scientific experiments, such as medical equipment, foam plastics, cushion, optical information recording medium and alarm. 2. Electrospinning (SMP) is a kind of composite material of thermally induced shape memory functional polymer and conductive materials (such as conductive carbon black, metal powder and conductive polymer). The heat generated by the current increases the temperature of the system, resulting in shape recovery. Therefore, the composite not only has conductivity, but also has good shape memory function. It is mainly used in the fields of electronic communication and instrumentation, such as electron beam tubes, electromagnetic shielding materials, etc. 3. The Photochromic Group (PCG) is introduced into the main chain and side chain of polymer. When exposed to ultraviolet light, PCG will undergo photoisomerization reaction, which makes the state of molecular chain change significantly. On the macro level, the material shows photodeformation; When the light stopped, the reversible photoisomerization of PCG occurred, the state of molecular chain was restored, and the material was also restored to its original state. The material can be used as printing material, optical recording material, "light driven molecular valve" and drug release agent. 4. Chemically induced SMP can stimulate material deformation and shape recovery by changing the properties of surrounding media. The common chemical induction methods include pH value change, equilibrium ion replacement, chelation reaction, phase transition reaction and redox reaction, etc. these substances include partially saponified polyacrylamide, polyvinyl alcohol and polyacrylic acid mixed film, etc. The material is used in protein or enzyme separation membrane, chemical engine and other special fields[ 1] Shape memory properties and basic principles of polymers All shape memory polymers have two-phase structure, that is, they are composed of a stationary phase which can remember the initial shape and a reversible phase which can solidify and soften reversibly with temperature. The stationary phase can be the cross-linked structure of polymer, the partial crystalline structure of polymer, the glassy state of polymer, or the winding of super polymer chain. The reversible phase can be a partial crystalline phase with reversible change of crystallization and melting, or a phase structure with reversible transition between glass state and rubber state (glass transition temperature Tg). SMP can be a single component polymer, or a copolymer or mixture of two components with different softening temperatures but good compatibility. The process of shape memory is as follows. Shape memory process Shape memory process The real reason of polymer memory effect needs to be analyzed from the structure. Due to the long chain structure of flexible polymer materials, the length and diameter of the molecular chain are very different, the chains are soft and easy to entangle with each other, and the length of each molecular chain is different, so it is very difficult to form a regular complete crystal structure. These structural characteristics of polymers determine that the macrostructure of most polymers is a coexistence system of crystalline and amorphous states, such as PE, PVC, etc. When the polymer is not cross-linked, once the heating temperature exceeds its crystallization melting point, it shows a temporary flow property, and no memory property can be observed; After crosslinking, the original linear structure of the polymer becomes a three-dimensional network structure. When heated above its melting point, the polymer no longer melts, but exhibits the properties of elastomer in a wide temperature range. Therefore, shape memory polymer materials must have the following conditions: 1) The polymer material itself should have crystalline and amorphous two-phase structure, and the proportion of two-phase structure should be appropriate; 2) In a wide temperature range above the glass transition temperature or melting point, it presents high elastic state and has a certain strength, which is conducive to the implementation of deformation; 3) In a wide range of ambient temperature conditions, it has glass state, which ensures that the frozen stress will not release in the storage state. Many thermoplastic elastomers with glassy state at room temperature, such as thermoplastic polyester elastomer, thermoplastic polystyrene butadiene elastomer, thermoplastic polyurethane elastomer, etc., and thermoplastic plastics with cross-linked structure, such as cross-linked PE, cross-linked EVA, cross-linked PVC, etc., can be prepared into shape memory materials by proper process. Natural rubber and other elastomers can not be used as shape memory materials, but only as elastomers, because they are in a high elastic state at the service temperature and can not freeze and maintain their tensile stress. Based on the elastic theory of rubber, the shape memory properties of polymer materials and the factors affecting the shape memory properties of materials can be analyzed[ 1] Thermally induced shape memory polymer and its memory effect Shape memory polymer (SMP) is generally composed of the same phase which prevents the resin from flowing and remembers the initial state, and the reversible phase which can be reversibly solidified and softened with the change of temperature. The reversible phase is physical cross-linking structure, such as crystalline state, glass state, etc., while the stationary phase can be divided into physical cross-linking structure or chemical cross-linking structure. SMP with chemical cross-linking structure as the stationary phase is called thermal SMP. SMP with physical cross-linking structure as the stationary phase is called thermoplastic SMP. There are many varieties of heat induced SMP. Japan has four kinds of industrial production technologies of SMP, namely, polynorbornene, polyurethane, high trans polyisoprene, and styrene, 7-butadiene copolymer. Other varieties include fluororesin, polycaprolactone, polyamide, etc. As far as the shape memory effect of this kind of polymer is concerned, the reversible phase has a greater influence on the deformation characteristics of SMP, and the fixed phase has a greater influence on the shape recovery characteristics. With the increase of the flexibility of the reversible molecular chain, the deformation of SMP increases and the deformation stress decreases. Compared with thermoplastic SMP, thermosetting SMP has the advantages of fast shape recovery, high precision and high stress, but it can not be recycled. Thermal induced SMP has been applied and is being developed in the fields of electronic communication, medical and health care, machinery manufacturing, commodity identification, entertainment and sports, daily necessities, modern agriculture, science and energy, etc. Compared with shape memory alloy, SMP has the following characteristics 1. The shape recovery temperature of SMP can be adjusted by chemical methods. For example, the recovery temperature of shape memory polyurethane ranges from - 30 ℃ to 70 ℃. The shape memory alloys of specific varieties are generally the same and fixed. 2. The shape memory alloy (SMA) has a low deformation, generally less than 10%, while the SMP is higher. The shape memory polyurethane and TPI are both higher than 400% 3. The number of repeated deformation of SMA can reach 10 * order of magnitude, while that of SMP is only slightly higher than 5000 times, so the fatigue resistance of SMP is not ideal. 4. The shape recovery stress of SMP is generally low, ranging from 9.81MPa to 29.4mpa, while that of SMA is higher than 1471mpa. 5. SMP has only one-way memory function, while shape memory alloys have found two-way memory and omni-directional memory. Unidirectional memory means that when the material is heated to its original state, it will not change its shape when the temperature is lowered; Bi directional memory materials can not only remember the shape at higher temperature, but also the shape at lower temperature. When the temperature changes repeatedly between high and low temperature, the shape changes constantly; Omnidirectional memory is a special case of bidirectional memory, that is, the shape of the beginning at lower temperature is opposite to that at higher temperature, that is, the shape of the beginning at lower temperature is opposite to that at higher temperature[ 2] Application of shape memory polymer in speech editing 1. Application in textile industry After deformation and fixation, the shape memory polymer will automatically return to its original shape under specific external conditions, such as thermal, chemical, mechanical, optical, magnetic and electrical effects. 1) There are three main forms of application of shape memory materials in textile. The first is shape memory yarn: the shape memory material is made into filaments and then spun into yarn; The two is shape memory chemicals: the shape memory polymer is made into emulsion, the fabric is finished, laminated or coated, and the shape memory function of the Fuzi fabric is made. The shape memory polymer is made into resin or adhesive and made of nonwoven fabric together with short fibers. Third, shape memory fabric: shape memory yarn woven into a variety of woven fabrics and knitted fabrics. The shape memory polymer and natural fiber / synthetic fiber are used to form composite materials. 2) Humidity sensitive polymer applications. Humidity activated shape memory materials are suitable for disposable sanitary products, such as diapers, training pants, sanitary napkins and incontinence products. These products can be folded or retracted. When the material is subjected to one or more external forces, it can deform at least in one direction; When the external force is released, a certain degree of deformation can be maintained in at least one direction. The material has the ability to deform in at least one direction and partially recover when exposed to humid or watery environment. Disposable products may deform or become uncomfortable when immersed in liquid or used under high temperature and human body temperature conditions, and the change of shape may cause leakage problems. The developed product and the adopted method can keep the deformation as much as possible, so as to prevent leakage. 3) Temperature sensitive polymer applications. It can be used as functional coating and finishing of fabrics to obtain waterproof and breathable fabrics, such as military uniforms, sportswear, mountaineering suits, tents, etc. Using the shape memory function of polyurethane, adjust the appropriate memory trigger temperature for clothing Interlinings (cuffs, necklines, etc.) to make them have good wrinkle resistance and wear resistance. By increasing the temperature, the creases produced in the process of use can be restored to the original shape. 2. Application in Engineering The shape memory polymer (SMA) JJ port is inserted into a bar with a larger diameter than that of the polymer to enlarge the diameter. After cooling, the bar is removed to make a heat shrinkable tube. When using. Insert the pipe to be bonded. Shrink and fasten the polymer pipe by heating. It can be used for insulation protection of line terminal. Communication cable joint is waterproof. And the joint protection of steel pipe line. 3. Application in medicine The application of shape memory polyurethane fiber in medical homomorphic material, sports sheath, fabric and artificial hair. In particular, it has a broad application prospect in the field of biodegradable medical suture. 1) Blood vessel suture, hemostatic forceps, medical tissue suture. Stretch the suture 200%, and then set it. After the operation, with the increase of body temperature, the shape memory of the operation line recovered. The wound was gradually tightened and closed. 2) Implant material. Thermosensitive shape memory polyurethane can be implanted in the body, placed in the place where medical treatment is needed, and the shape needed can be obtained through body temperature. When its physiological function is completed, the implanted material is slowly degraded, absorbed or discharged in the body. This kind of material does not need a second operation to take out the implanted material, which greatly reduces the pain of patients. The molecular design of biodegradable implant materials includes the selection of appropriate relay nodes to fix the permanent deformation of polymer, the selection of appropriate molecular segments to act as switch links, and the selection of appropriate raw materials and synthesis methods to minimize toxicity. Biocompatible components must also be considered. 3) Homomorphic materials. Shape memory polyurethane has the advantages of convenient acid shape, solid shape, shape recovery, easy shape memory temperature adjustment, light weight, good biocompatibility, air permeability and antibacterial. It has been widely used in medical orthopedics. It is an ideal substitute for gypsum[ 2] |