The 0-by-micro actuator is an important component of microelectromechanical systems (MEMS).
The smart materials used as actuators in the past mainly include shape memory alloy (SMA), piezoelectric ceramic (PZT), electric (magnetic) rheological fluid and giant magnetostrictive materials. 1121. These materials are used as actuators. There are advantages and disadvantages. Ion exchange polymer metal composite (IPMC) is a new type of smart material. It consists of an ion exchange membrane and an electrode consisting of a precious metal (usually platinum, gold or silver) chemically coated on both sides of the membrane. According to the properties of membrane exchange ions, it can be divided into cation exchange type and anion exchange type. 134. IPMC is an electroactive material, because it constitutes an actuator structure that does not require traditional mechanical moving parts, at low voltage (less than 1). Can produce a large amount of deformation and a high force density, in some cases use more than 40 times its own weight 151, this actuator is soft and tough like plastic, can be within a certain size range It is easy to divide into the desired shape, works well in a humid environment, and works in a dry environment by a capsule encapsulation technique. 6. This study selected a cation exchange type in an ion exchange polymerization membrane and a metal composite structure. The fluorine-based polymer film-platinum structure was studied as a research object. After a large number of experiments, actuator samples were successfully fabricated.
Fund Project: The National Natural Science Foundation of China (50377022) obtained some basic characteristics of the actuator for the tested samples.
1IPMC actuation mechanism and characteristics The actuation mechanism of IPMC is complex, involving the mutual conversion of electrical energy, chemical energy and mechanical energy. Although the current mechanism of IPMC activation has not been fully quantified, it is generally considered to be the result of interaction between intrinsic ions inside the IPMC and chemically displaced ions and solution molecules attached to these ions under electric field induction. Taking cation exchange type IPMC as an example, the base film Nafion 117 is a perfluorinated ion exchange membrane with high ionic conductivity and can absorb a large amount of polar solution (such as water). Its internal structure has a fixed anion belt grid, and the movable cations can migrate or diffuse through the network chain.
When an electric field is applied across the diaphragm, under the action of Coulomb force, the movable hydrated cation (entrained water molecules) accumulates toward the cathode, causing the membrane to expand on one side of the cathode and contracting on one side of the anode to produce an anode direction. Bending deformation (the opposite is true for the anion exchange type IPMC), as shown, where a and a are initial states, and b and b are effects after an electric field is applied.
The electromechanical effect can be briefly described by two forms of migration, namely ion migration (with the bending effect of IPMC under electric field with current density perpendicular to the diaphragm) and solvent migration (with flow Q). The common point force including electric field E and pressure gradient-Vp gives the standard Onsagei formula according to the linear irreversible thermodynamics theory: Equations (1) and (2) provide a basic theoretical basis for understanding the actuation principle of IPMC, and also explain IPMC. Has a sensing effect.
Into the research, and several models proposed by other researchers have been revised. They assume that the internal current of the IPMC actuator is composed of three parts when the electric field is applied, that is, the ion current caused by the migration of the cation to the cathode (this is the main factor of IPMC actuation), a similar plate capacitor composed of the IPMC metal electrode and the diaphragm. The displacement current generated by the structure and the electron current generated by the resistive path between the two electrodes. This is a model that considers the energy band. When the voltage exceeds a certain limit value, the electrons are generated when the electrons jump from the valence band to the conduction band. Therefore, when the voltage is lower than this limit, the electron current can be ignored. The IPMC actuator equivalent circuit model thus composed is as shown.
Ion migration phenomenon; the left side through the branch of the resistor Ri simulates the electron current caused by electron leakage. If the applied voltage is greater than the limit value, the diode Di-D2 is allowed to conduct; the intermediate path through the capacitor Ci is the analog IPMC actuator The displacement current generated by the capacitor structure is the resistance R2 corresponding to the surface resistance of the platinum electrode. The electrical characteristics derived from this model are consistent with the experimental results. Applying a 2 5V sinusoidal voltage input exhibits nonlinear characteristics at low frequencies (less than 2 Hz).
What we are most concerned about is the relationship between the input voltage and the internal current and the characteristics of the actuator. However, this model does not reflect this relationship, and the external deformation is not linearly added with the large internal current, so it needs further study.
IPMC actuator preparation IPMC preparation process can be mainly divided into the following steps: 1 surface roughening of ion polymerization membrane; 2 ion exchange process; 3 metal reduction process; 4 surface electrodeization.
In this study, a cation exchange type perfluorinated ionized membrane (Nafion 117) electroless platinum plating process was used. It contains two main processes that are significantly different, namely the initial reduction process of deep molecular metallization and the coating process of surface electrodeization. 7. Before the initial reduction, the membrane must be pretreated. Firstly, the two sides of the membrane are ground, washed, and then boiled in dilute hydrochloric acid (or nitric acid) to remove organic impurities, then boiled with deionized water to fully expand, and then put a certain concentration of platinum salt solution (such as Cl2-H2) In the middle, the ion exchange was carried out by soaking for 2 hours or more at room temperature.
The ion exchanged membrane was first subjected to initial reduction. The reducing agent (NaBH4) at the initial reduction is gradually added in stages, and the temperature rise of a certain gradient is maintained for more than 2 hours. After the initial reduction, the depth of penetration into the two surfaces of the membrane is 1~2 (the range of Fm forms a nanometer-sized particulate silver-gray metal platinum layer, and its structural structure, such as dendrites, extends from the surface to the middle, which is the next step of surface polarization. Lay the foundation.
The initially reduced membrane was then placed in a platinum salt solution (Cl2-H2O) and reduced with a reducing agent (NH2OH-HCl and H2NH2-H2O). Similarly, the reducing agent is added successively and maintained at a certain gradient temperature rise for more than 3 h to raise the temperature to 60 until the reducing solution and a certain amount of the same amount of NaBH4 solution are boiled and remain black.
Tests have shown that IPMC is a highly process-dependent material. The quality of the initial restoration is the key to the success of the IPMC actuator. Normally, during the initial reduction, the film gradually turns black from the initial light brown from the edge to the center, and then a silver-gray metallic platinum layer is formed. If the desired platinum layer (such as a dark gray exhibiting black or no metallic luster) is formed on the surface of the film during the initial reduction stage, the next surface electrode formation process will not be achieved, or the post-action formation will be achieved. The activity of the device is very poor (the deformation is little or almost no deformation under the action of voltage). The performance of the actuator can be optimized by varying various process parameters such as the concentration of the platinum salt and the soaking time, the temperature rise gradient of the different reduction stages, the type of reducing agent and the concentration (dose). In addition, the surface electrodeization process can be repeated to obtain an IPMC actuator of the desired electrode thickness.
IPMC Actuator Performance Test and Results Analysis 3.1 Test Conditions and Test Items Striped IPMC samples were first tested for maximum deformation at a given voltage. The test as shown was performed with a No. 7 (AAA) dry battery (1.5 V) with significant deformation.
For formal testing, clamp the end of the diaphragm with two electrodes and place it vertically into the water. AC voltage commutation is used to generate alternating square wave signals, from 0 to 4V, each time boosting a5V, repeat the test multiple times, see IPMC deformation test photos. Take the maximum distance at the lowermost left and right extreme positions (shown as a and c respectively) of the suspended diaphragm in the same cycle as the maximum deformation amount, and the commutation frequency is to make the diaphragm reach the maximum deformation amount. The relationship between the measured voltage E and the maximum deformation amount / is as shown.
In addition, the relationship between frequency and deformation and performance variability (deformation attenuation) at a given input voltage amplitude was also tested.
3.2 Analysis and discussion of test results Preliminary tests on the sample show that the general trend is that the higher the input voltage, the larger the deformation, the higher the frequency, and the smaller the deformation. This phenomenon can be explained by the inertia of hydrated ions. When the electric field changes too fast, since the inertia of the hydrated ions is much larger than the inertia of the electrons, the movement of the ions is not the same as the transformation speed of the electric field at the maximum deformation amount, and the test curve is not enough to move sufficiently from one end of the electrode to the other end. The expansion and contraction are incomplete, resulting in a reduction in deformation. When the voltage is lower than 1.5V, the repeatability of the measurement is higher. When the voltage is higher than 3V, the temperature at the electrode is significantly increased (the hand touches the electrode is hot), and the bubble on the surface of the diaphragm is obviously visible due to the electrolysis. It is slower and slower and smaller as the number of repetitions increases. When a long-term DC stimulus is applied to the diaphragm, the diaphragm will produce a return in the opposite direction when the bend reaches a maximum. This phenomenon is thought to be caused by the migration of water from a cathode having a large cation concentration to an anode having a small cation concentration under the action of a pressure gradient, resulting in expansion of the originally contracted anode surface and contraction of the originally expanded cathode surface. In addition, there is no such phenomenon as "there is an over-displacement of the IPMC in the opposite direction, causing permanent deformation" once the electric field is cancelled.
Under the 5V square wave voltage, the deformation attenuation increases significantly with the increase of the frequency. When the frequency is a75Hz, the deformation of the actuator is hardly felt by the naked eye after the operation is less than 1 hour, and when the frequency is lowered to a5 Hz, the deformation of the actuator is not felt until the operation is continued for about 9 hours. It was noted in the test that the left and right deformations were asymmetrical and unstable when the test was repeated at a given voltage, and the deformation zone was biased toward one side. This may be caused by poor or uneven thickness of the two-sided metal plating, as is the case for multiple samples. Whether this phenomenon is caused by the characteristics of the diaphragm itself or due to improper preparation of IPMC is still to be further studied. When the diaphragm reaches the maximum deformation in a certain direction and maintains the current direction, so that the diaphragm returns to the opposite direction, the deformation region will shift in the opposite direction in the next cycle. If the commutation frequency is low enough (to ensure that the movable hydrated ions have enough time to move back to the other electrode end), the total amount of deformation is large, but the response speed is slow. After several cycles, it will return to its original state.
IPMC application prospects and future research directions IPMC as an actuator application, foreign research is still in the laboratory stage. Shahinpooi et al 131 used four robots with a weight of 0i1g as the finger to lift 1013g of stone at 5V, and the required power was 25mW. This mechanism would be very complicated if it was realized by mechanical means. In addition, the application as an underwater micro-bionic robot propulsion device is also under study 110111. Because of its simple mechanism (no traditional mechanical moving parts), light weight, high efficiency relative to propeller propulsion, low noise, this has unique advantages for military applications. . In addition, due to its low driving voltage and safe operation, it also has great potential in biomedical applications.
For example, Wang et al. 1121 studied the use of IPMC as an actuator for the in-vivo probe fiber scanning drive mechanism used in optical coherence tomography (0CT), and obtained satisfactory test results.
Although the relative power and deformation of the IPMC (compared to its own weight) is large, the absolute output force is still small for some applications.
As mentioned above, the robot is only a short-term test. As a practical device, there are still many problems to be solved, such as performance degradation at high voltage, displacement recovery, and the like. At present, foreign scholars are studying ways to improve performance from the manufacturing process, such as adding a dispersant to reduce the surface metal particles as an electrode (nanoscale) in the process of reducing the metal electrode, and then plating gold or silver on the outer layer of the platinum electrode. In order to reduce the outward penetration of moisture and reduce the surface resistance of the electrode; the three-dimensional IPMC151 is made by using the recasting process and the thickness of the diaphragm. Although certain effects have been achieved, further research and improvement are needed.
Since there is no standard IPMC commercial product, it is generally produced by the researchers themselves. The performance varies greatly due to unstable process conditions. Some parameters cannot be directly measured. It is difficult to obtain a general model suitable for all actuator samples. . Therefore, in terms of drive control, it is necessary to study the method of optimizing the amplitude, waveform and frequency of the input voltage for different samples to achieve the best response characteristics, so that the output force, the deformation amount and the response speed meet the requirements of use. In addition, according to the characteristics of IPMC, it is also one of the next research directions to develop strengths and avoid weaknesses, construct appropriate drive mechanisms, find suitable uses for them, and expand their application range.
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