what is a smart memory alloy | NITINOL WIRE PROVIDER
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Nitinol performance characteristics
A. Phase transformation and properties of Nitinol
As the name implies, nickel-titanium alloy is a binary alloy composed of nickel and titanium. Due to changes in temperature and mechanical pressure, there are two different crystal structure phases, namely austenite phase and martensite phase. The phase transformation sequence of nickel-titanium alloy during cooling is the parent phase (austenite phase) -R phase-martensite phase. The R phase is rhombohedral, and austenite is a state when the temperature is higher (greater than the same: the temperature at which austenite starts), or when the load is removed (external force is removed from Deactivation), cube, and hard. The shape is relatively stable. The martensite phase is the state when the temperature is relatively low (less than Mf: the temperature at which the martensite ends) or loaded (activated by external force), hexagonal, malleable, repeatable, less stable, and easier Deformed.
B. Special properties of Nitinol
a. shape memory (shape memory) shape memory is when the mother phase of a certain shape is cooled from above the Af temperature to below the Mf temperature to form martensite, the martensite is deformed at a temperature below the Mf and heated to below the Af temperature, With the reverse phase change, the material will automatically restore its shape in the parent phase. In fact, the shape memory effect is a heat-induced phase transformation process of nickel-titanium alloy.
B. Superelastic The so-called superelasticity refers to the phenomenon that the sample produces a strain that is much larger than the elastic limit strain under the action of external force, and the strain can automatically recover when unloaded. That is, in the state of the parent phase, due to the effect of applied stress, the stress-induced martensite transformation occurs, so that the alloy exhibits a mechanical behavior different from that of ordinary materials, and its elastic limit is much greater than that of ordinary materials, and no longer observes the tiger. Law. Compared with shape memory properties, superelasticity does not involve heat. In a word, superelasticity means that the stress does not increase with the increase of strain within a certain deformation range. Superelasticity can be divided into linear superelasticity and nonlinear superelasticity. The stress-strain curve in the former is close to a linear relationship between stress and strain. Non-linear superelasticity refers to the results of stress-induced martensitic transformation and its inverse phase transformation during loading and unloading in a certain temperature range above Af. Therefore, nonlinear hyperelasticity is also called phase transition pseudoelasticity. The pseudo-elasticity of the nickel-titanium alloy can reach about 8%. The superelasticity of the nickel-titanium alloy can be changed with the change of heat treatment conditions. When the archwire is heated above 400ºC, the superelasticity begins to decrease.
C. Sensitivity to temperature changes in the oral cavity: The curative power of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in the oral cavity. The orthodontic power of superelastic nickel-titanium alloy orthodontic wire changes with the change of oral temperature. When the amount of deformation is constant. As the temperature rises, the correction power increases. On the one hand, it can accelerate the movement of teeth, because the temperature changes in the oral cavity will stimulate the blood flow in the stagnation of blood flow caused by the correction device, so that the repair cells are fully nourished during the movement of the teeth. Maintain its vitality and normal function. On the other hand, orthodontists cannot accurately control or measure the correction power in the oral environment.
D. Corrosion resistance: Studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire
Anti-toxicity: The special chemical composition of nickel-titanium shape memory alloy, that is, this is an atomic alloy of nickel-titanium, containing about 50% nickel, and nickel is known to have carcinogenic and pro-cancer effects. In general, the titanium oxide on the surface layer acts as a barrier to make Ni-Ti alloys have good biocompatibility. TiXOy and TixNiOy of the surface layer can suppress the release of Ni.
F. Soft orthodontic power: currently commercially available orthodontic wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and ß-titanium alloy wire. The load-displacement curve of these orthodontic correction wires under tensile test and three-point bending test conditions. Nitinol’s unloading curve platform is the lowest and flattest, indicating that it can provide long-lasting and gentle healing power.
Good shock absorption characteristics: the greater the vibration caused by the chewing and nocturnal teeth to the arch wire, the greater the damage to the tooth root and periodontal tissue. Through the results of different arch wire attenuation experiments, it is found that the vibration amplitude of the stainless steel wire is larger than that of the superelastic nickel-titanium wire. The initial vibration amplitude of the superelastic nickel-titanium arch wire is only half that of the stainless steel wire. The good vibration and shock absorption characteristics of the arch wire The health of teeth is very important, and traditional arch wire, such as stainless steel wire, has a tendency to increase the absorption of tooth roots.
Clinical application of nickel-titanium alloy wire
a. For the early alignment of the patient’s dentition. Due to the superelasticity and shape memory properties of the nickel-titanium alloy arch wire and the low stress-strain curve, the nickel-titanium alloy arch wire is currently routinely included in the correction system in clinical practice. The arch wire, so that the patient’s discomfort will be greatly reduced. Because there are several different straight wire bow correction techniques, MBT technology recommends the use of 0.016-inch heat activated nickel-titanium alloy wire (HANT wire), DEMON self-locking bracket technology recommends the use of copper-containing heat activated nickel titanium produced by Omcro Alloy arch wire (the phase transition temperature is about 40 degrees), O-PAK correction technology recommends the use of 0.016 inch superelastic nickel titanium alloy arch wire for early alignment.
b. Nitinol springs: Nitinol push springs and tension springs are a kind of spring used for orthodontics. They are particularly elastic with nickel-titanium. They are suitable for orthodontic treatment to develop gaps between teeth and pull teeth in different directions. Nickel-titanium coil springs extend 1mm to produce a force of approximately 50g. Nickel-titanium coil springs have high elastic properties and can produce a soft and stable continuous force under tension. The attenuation of the force is very small, which can produce the ideal orthodontic force required for clinically moving teeth. Meet the physiological requirements. The high elasticity and permanent deformation rate of the nickel-titanium wire tension spring is very low. Compared with the stainless steel wire of the same diameter, the release force is 3.5-4 times different. Therefore, in the application of orthodontic treatment, the patient not only has light pain, but also feels soft and long-lasting strength, and the time for returning to the clinic is reduced, which shortens the course of treatment and improves the curative effect. It is a new and excellent mechanical device in orthodontic treatment.
c. L-H arch wire is researched and developed by Dr. Soma in Japan and produced by Tomy. “LH” is named after “LowHysteresis”, that is, when the arch wire is ligated to the bracket, that is, the stress generated when the arch wire is activated and the tooth is slowly restored when the tooth is moved. The difference in the stress generated at the time is small. That is, the lag is very small. SOMA et al. Compared the stress-strain curves of LH arch wire and other nickel-titanium alloy wires. The hysteresis range of LH arch wire is the smallest. This characteristic gives the arch wire the advantage of low load and continuous light force. At the same time, the initial slope of the curve is low. This shows that the stiffness of the archwire is low, and the hysteresis curve of the other types of nitinol archwires shows that the rigidity is relatively large. Obviously, the LH archwire has obvious mechanical advantages. Because the content of titanium in the nickel-titanium component of the LH wire is higher than that of the general nickel-titanium arch wire, it is called titanium-nickel wire, and experiments have shown that its shock absorption effect is stronger. Another feature of LH nickel-titanium wire is that it can be bent, and can be heated and shaped with heat treatment equipment, so LH nickel-titanium wire can also be aligned from the line, open to close to the gap, and the final completion stage, one arch wire up and one down That is, the treatment can be completed, as long as the arch wire is taken out and bent to the desired shape at each stage, and then set with a heat treatment instrument to strengthen the hardness. At present, L-H arch wire is used in clinical expansion treatment to correct open jaws, partial jaws and anti-jaws. Because of its continuous stability and softness, the effect is better. At the same time, J hook is often used to improve the weakness of the arch wire. Although MEAW technology also has an ideal effect for the correction of the above wrong jaw, the complicated arch wire bending often makes many doctors discouraged. Therefore, some doctors use a rocking chair-type nickel-titanium arch wire similar to the mechanical system plus vertical traction of the front teeth. Although this has a similar effect, I always feel that when compared with MEAW, the movement of each individual tooth is not as good as MEAW technology. The rocking chair-type nickel-titanium wire is a continuous arch wire, and there is no way to bend it. Therefore, the angle of bracket bonding and the curvature of the rocking chair of the arch wire determine the angle of each tooth. Unlike the MEAW technology, the angle of each tooth has more room for individual adjustment. Use LH nickel titanium to bend the rocking chair, and then use the bow wire shaper to bend in the mouth and then bend backward or forward, the effect is quite ideal.