3. THE APPROXIMATE LINEAR ACTUATOR
3.3 The Simulation of Bending Motion
The simulation of the bending motion of IPMC strip with different thickness is according to the equation shown below [30]. And the result is shown in Figure 3.6.
Surface electrodes
t = 0.5 (sec) time
Y = 80 × 106 (Pa) Young modulus d = 2.3 × 10–5 (m) thickness of IPMC R = 8.314 (Pa·m3/mol·K) universal gas constant T = 303.15 (K) absolute temperature C = 0.001 (F) capacitance of IPMC
E = 30000 (V/m) electric field
N = 1 valence of the cations
E = 1.602192×10–19(C) charge of an electron
Av = 6.022×1023 Avogadro’s number
r = 100 (Ω) cross-resistance V = 2×10-8 (m3) strip volume
tg1 = 0.3 (m) thickness of IPMC
tg2 = 1 (m) thickness of IPMC
tg3 = 3 (m) thickness of IPMC
Figure 3.7 The simulation result of different thickness IPMC strips.
From the simulation result, the curvature of IPMC strips is inversely proportional to the thickness. And the result implies the linear type actuation of thick IPMC.
IV. PREPARATION OF ION EXCHANGE MEMBRANE
4.1 The Choice of Ion Exchange Resin
The main component of ionic polymer metal composite ( IPMC ) strip is ion exchange resin. There are several kinds of ion exchange resin could be chosen to make IPMC [17].
As shown in table 4.1.
Table 4.1 several kinds of ion exchange resin provided by different company
Company Product(s)
DuPont® Nafion®
Tokuyama® Neosepta®
Asahi Chemical® Aciplex®
Asahi Glass® Flemion® ; elemion®
The Nafion® , product of DuPont®, is a kind of common commercial ion exchange resin.
In this project, Nafion® is chosen to compose IPMC strips. Because of the convenience of acquiring the material and most of the papers reported the choice of same material. The Nafion® dispersion and membrane that directly bought from commercial agent are both applied in the experiment of this project.
4.2 The Process of Making Nafion® Membrane
To make a Nafion® membrane from Nafion® dispersion, the first step is to make a mold. The mold is used to contain the Nafion® dispersion, and the shape of the mold is arbitrary. The height of the mold should be calculated during the process of mold design. The second step is pouring the Nafion® dispersion into the mold. The process of pouring the dispersion can be accomplished easily by using a common plastic 5mL dropper. And the quantity of Nafion® dispersion could be calculated at first. According to the Chapter 2.4, the computing needs to
know the mass density of the Nafion® dispersion and Nafion® membrane, mass percentage temperature, and the airflow in the room. In fact, the drying process is separated into several steps. The Nafion® membrane would split easily due to the shrinkage during the drying process. Because, the shrinkage would induce internal stress that cause the breaking of the membrane. To fix the breaking on the membrane, the second time pouring should be executed after the membrane dried first time. The required quantity of Nafion® dispersion is just to submerge the dried membrane in the mold. After the second time drying, the membrane should totally separate from the wall of the mold. If the membrane is still broken, the third time pouring should be executed. The third step is finished until the membrane is formed into a unity. Then the forth step is the heating process. The main purpose of heating process is to change the characteristic of the membrane. If a Nafion® membrane is produced by evaporation of Nafion® dispersion at room temperature, the fluorocarbon molecule chains are dispersed. This kind of Nafion® membrane would be dissolved again by alcohol and water.
And the mechanical strength of the membrane is very weak. In order to prevent the situation occur, the Nafion® membrane should be heated to about 160 ℃ [31]. The fluorocarbon molecules in the membrane will extend and fuse together. After the heating process, the mechanical strength of the membrane will increase, and the membrane will become insoluble.
The final step is putting the Nafion® film into ionic solution, such as sodium hydroxide (NaOH) solution, lithium hydroxide (LiOH). The purpose of final step is to exchange the original H+ ( proton ) in the Nafion® film with the ions in the ionic solution. And the ion
exchange process is optional. The flow chart is shown in figure 5.1.
Figure 4.1 the flow chart of heating process 4.3 The Different Kinds of Molds
In the experiment, many kinds of ideas are tried. And there are several varieties of mold.
The first type mold is acrylic mold. It was made of acryl and glass. Acryl was made into the wall of the mold, and the glass was used to be the bottom plate. The acryl was glued to glass plate with polydimethylsiloxane ( PDMS ). The design is briefly shown in figure 4.2 and figure 4.3. And the photo is shown in figure 4.4.
Figure 4.2 the oblique drawing of first type acrylic mold
acryl
glass
Figure 4.3 the dimensions of the acrylic mold
Figure 4.4 the photo of the acrylic mold
But the mold had a problem. During the milling work of acrylic component, the milling cutter would produce heat at the cutting position. And the heat would cause the deformation of acrylic material. So the bottom of the acrylic mold wall is a little curvy.
The gap between the acryl and glass plate caused some pattern on the bonding area. But that had no influence on the air tightness of the mold. The picture is shown in figure 4.5.
Figure 4.5 the photo of the pattern
The second type mold was totally made of glass. Because of the manufacturing process is more convenient than first type. The curve phenomenon of acryl was also improved. The difference between first type and second type mold is the material of the wall of mold.
The wall of the second type mold is made of glass slide( 25.4 mm ×76.2 mm ×1 mm), and the bottom of the mold is also plane glass just like first type mold. The pictures are shown in figure 4.6 and figure 4.7.
Figure 4.6 the oblique drawing of second type mold
Figure 4.7 the photo of the second type mold
The third type mold was also made of glass, but the volume and thickness of mold wall are different to the second type mold. Because of the dispersion of Nafion® is expensive, the third type mold was designed as a small volume mold to decrease the quantity of usage of Nafion® dispersion. The wall of the mold is composed of cover glasses, and the bottom of the mold is composed of glass slide. The pictures are shown in figure 4.8 and figure 4.9.
Figure 4.8 the oblique drawing of third type mold
Figure 4.9 the photo of the third type mold
The next is forth type mold. The forth type mold is a variation of third type mold. So, the material used to make forth type mold is the same of third type. The main difference between this two type molds are the width of the mold. The width of the forth type mold is about 0.3 mm so the manufacturing procedure is more complicated than third type mold.
The key points are how to fix the distance between the two cover glasses and how to keep the parallel arrangement of the two cover glasses during manufacturing. The main purpose of forth type mold is to make a very thick and very narrow IPMC strip by vertical casting technique. The vertical casting will also be illustrated later. Pictures of forth type mold are shown in figure 4.10 and figure 4.11.
Figure 4.10 the oblique drawing of forth type mold
Figure 4.11 the photo of the forth type mold
In fact, the forth type mold doesn’t need a bottom. Because the gap between the two cover glasses are really small. So the capillary phenomenon is strong. The solution that pouring into the mold wouldn’t leakage out. But the bottoms of the molds still need to avoid coming into contact with the table. So, a shelf was be designed and applied. The pictures are shown in figure 4.12, 4.13, 4.14 and 4.15.
Figure 4.12 the oblique drawing of the shelf
Figure 4.13 the oblique drawing of the assembly of forth type mold and shelf cover glass
Figure 4.14 the photo of the shelf
Figure 4.15 the photo of the assembly of forth type mold and shelf
But the forth type mold has a great problem. Due to the small distance between the two cover glasses and the flexibility of the thin cover glasses, the capillary phenomenon would be very significant. Indeed, the cover glass has obvious flexibility. And the solution that poured into the mold has cohesion force. So the distance between the cover glasses would be changed. The changing of the distance could cause the not uniform capillary
force distribution, and the manufacturing would be failed because of the not uniform distribution of the dispersion in the mold. The figure of illustration is shown in figure 5.16 and 4.17.
Figure 4.16 the concept of deformation of forth type mold
Figure 4.17 the photo of the not uniform distribution of dispersion
Due to the failure of forth type mold, the fifth type mold was designed to solve the problem. The main reason of the failure of forth type mold is the flexibility of the thin cover glasses is really great in this scale ( the distance between the two cover glasses is only about 0.3 mm). To avoid the problem, the glass slides were be used. Because of the glass slides are thicker than cover glasses; the deformation due to the flexibility is smaller than cover glasses. In fact, if the force applied on glass slides was big enough, the deformation would still be very obvious. Fortunately, the cohesion force of Nafion® dispersion is not big enough to induce the phenomenon on glass slides. The figures are
Nafion® dispersion
shown in figure 4.18 and 4.19.
Figure 4.18 the photo of the fifth type mold and the uniform distribution of dispersion
Figure 4.19 the illustration of the assembly of the fifth type mold
side view composition diagram
0.3 mm
25.4 mm
76.2 mm 22 mm
22 mm
glass slide cover glass
the fifth type mold
the support bracket
4.4 The Pouring of Dispersion
To pour Nafion® dispersion into a mold, a 3 mL cheap plastic dropper was be used. The photo is shown in figure 4.20.
Figure 4.20 the 3 mL plastic dropper
The pouring process of first type, second type and third type are easy. But the pouring process of forth type and fifth type has a problem. Because of the narrow gap of the mold, the injection of Nafion® dispersion seems to be a precise motion. And a 1 mL plastic syringe was also be used. The photo is shown in figure 4.21.
Figure 4.21 the 1 mL plastic syringe
In fact, after the test of the injection process, the 3 mL plastic dropper is precise enough.
Figure 4.22 the droplets injected in a mold Droplets of Nafion dispersion
4.5 The Drying Process
After the pouring process, it needs time to let the dispersion dry. The problems that would occur in the process are listed below.
1. the break of membrane 2. the estimation of time
3. the control of the thickness of membrane
The main problem is the break of membrane. The internal stress occurs during the drying process of Nafion® dispersion due to the shrinkage of the polymer. So the fixing process of the membrane is important. The break of membrane usually occurs at the first time cast of dispersion. The following are four examples of second type mold and fifth type mold.
The first two examples are the fabrication of 2 mm and 1 mm thickness chunks of Nafion®. And the last two examples are the fabrication of vertical casting membrane. The heights of the membranes are about 3 mm. First, two second type mold were made, and the dimensions of the two molds are about 4 cm (L) × 2 cm (W) × 2 cm (H) and 7 cm phenomenon in the bottom-less fifth type mold. The membrane manufactured in this kind of fifth type mold almost has no fissures. It is a coincidence to design out this kind of fissure-less mold.
Figure 4.23 the top view of fissures of the 2 mm thick chunk
Figure 4.24 the side view of the fissures of the 2 mm thick chunk fissures
fissures
Figure 4.25 the oblique view of the fissures of the 1 mm thick chunk
Figure 4.26 the bottom view of the fissures of the 1 mm thick chunk
Figure 4.27 the oblique view of fissures of the vertical casting membrane in the 0.4 mm wide bottom-added fifth type mold
Figure 4.28 the side view of fissures of the vertical casting membrane in the 0.4 mm wide bottom-added fifth type mold
fissures
fissures
Then, the bottom-added fifth type mold was abandoned. Because the bottom-less fifth type mold is obviously better than that mold.
Then, the fixing process should be applied to the chunks. To fix the fissures, pouring a little quantity of Nafion® dispersion in the mold, until the chunks submerged in dispersion.
Then, wait for the dispersion drying. Finally, the whole chunks were finished, as shown in figure 4.29, 4.30, 4.31, 4.32 and 4.33.
Figure 4.29 the top view of the 2 mm thick whole chunk
Figure 4.30 the bottom view of the 2 mm thick whole chunk The whole
Nafion 2 mm thick chunk
The second type mold
Figure 4.31 the top view of the 1 mm thick whole chunk
Figure 4.32 the side view of the 1 mm thick whole chunk
Figure 4.33 the oblique view of the membrane in the bottom-less fifth type mold
After several days, the chunk would keep shrinkage, and the chunk looks like to be light yellow, as shown in figure 4.34, 4.35, 4.36 and 4.37.
Figure 4.34 the top view of the 2 mm thick light yellow chunk The bottom-less fifth type mold
The light yellow 2 mm thick Nafion chunk
Figure 4.35 the side view of the 2 mm thick light yellow chunk
Figure 4.36 the oblique view of the 1 mm thick light yellow chunk The light yellow 1 mm thick Nafion chunk
Figure 4.37 the side view of the 1 mm thick light yellow chunk
If the requirement of precision of the product is high, the estimation of time and the control of thickness are not easy. Because the Nafion® will shrink during the drying process, and the residual fragment on the mold wall causes the tolerance of the estimation of quantity of Nafion® dispersion. The breaking of the membrane also causes problem in the estimation of time. Because re-pouring process may not need only one time. And the thickness would also be influenced.
4.6 The Heating Treatment
The actual heating process is heating the membrane up to 100 ℃ for 40 minutes to fully evaporate the volatile components that still remain in the membrane. Then increase the temperature up to 120 ℃ also for 20 minutes. After that, rises the temperature up to 160
℃ for 10 minutes. Then turn off the oven to let the film decrease temperature naturally.
That is an annealing process to let the molecules in the film arrange slowly to avoid the internal strain occurs. The concept of actual heating progress is shown in figure 4.38.
Figure 4.38 the concept of heating progress
The apparatus used in this heating treatment is the simple vacuum oven, composed of a rotary vane vacuum pump and a vacuum chamber, as shown in figure 4.39, 4.40.
Figure 4.39 the rotary vane vacuum pump
Figure 4.40 the vacuum chamber
The vacuum apparatus is very simple, so the heating treatment also did not be progressed precisely. Although the temperature and the vacuum circumstances did not be controlled well, the sample treated was still usable. The following are the photos of the samples, said above, after the heating treatment.
Figure 4.41 the top view of the 2 mm thick chunk after heating treatment
Figure 4.42 the side view of the 2 mm thick chunk after heating treatment
Figure 4.43 the bottom view of the 2 mm thick chunk after heating treatment
Figure 4.44 the side view of the 1 mm thick chunk after heating treatment
Figure 4.45 the oblique view of the 1 mm thick chunk after heating treatment
Figure 4.46 the oblique view in another angle of the 1 mm thick chunk
Figure 4.47 the side view of the vertical casting membrane after heating treatment
To get out the membrane from the narrow mold, a burin blade was inserted into the mold to separate the two glass slides, as shown in figure 4.48.
Figure 4.48 the side view of the vertical casting membrane after heating treatment
After the separation, DI-water was injected to let the membrane swell, so the membrane would leave the glass slide by itself, as shown in figure 4.49.
Figure 4.49 The membrane swelled and left the glass slide.
Figure 4.50 The membrane was picked up by a stainless steel tweezers.
An obvious phenomenon was found in the vertical casting membrane. The color of the membrane has a gradient. From the bottom to the top of the membrane, the color become more and more light, from deep brown to the very light yellow. The samples showed in this chapter would be treated with the coating process in the next chapter.
4.7 The Fabrication of the Very Narrow and Very Thick IPMC Sample by Vertical Casting Method
To realize the idea brought up in chapter 4, the vertical casting method was designed. And the sandwich structure is introduced below.
Figure 4.51 the sandwich structure of vertical casting IPMC
The bottom of the structure is a thin layer of Nafion®, the purpose of the layer is to avoid the lower electrode break to pieces. Because the bottom of this IPMC is more dry than any other upper layers, as shown in figure 4.50. If let the electrode layer be the bottom layer, the electrode would be very dry, and would break before heating treatment, as shown in figure 4.52, 4.53.
Figure 4.52 a multi-layer IPMC under producing
Figure 4.53 The example of broken electrode
The dispersion used to make electrode layer was made by mixing Nafion® dispersion with silver powder. By trial and error method, the proper mixture density is about 0.060g / 1c.c.
(Nafion dispersion). The improved electrode is shown in figure 4.53.
Figure 4.54 the improved electrode And the IPMC sample is shown in figure 4.55.
Figure 4.55 a complete IPMC sample made by vertical casting method electrodes
Nafion medium
electrode
V. COATING OF ELECTRODES ON NAFION
®MEMBRANE SURFACE
One of the main composite layer of IPMC is the electrode on surface. There are many kinds of method to coat an electrode on Nafion® membrane. In this project, the electro-less coating technique was used.
5.1 Fundamental Concept
Electro-less coating is a kind of chemical method to coating electrodes on various materials. For instance, glass, PCB ( printed circuit board ), and, the main material in this project, ion exchange membrane. The electro-less coating in this project is according to the following chemical reaction.
First, mix the silver nitrate ( AgNO3 ) solution with sodium hydroxide ( NaOH ) solution.
Then, the brown silver oxide ( Ag2O ) precipitation would precipitate. The second step is mixing the ammonia ( NH3 ) with the reactant solution. The brown silver oxide precipitation will be dissolved, and the solution will be achromatic. The final step is mixing the glucose ( C6H12O6 ) solution with the reactant solution. The silver ( Ag ) precipitation will precipitate.
5.2 Apparatus and Chemicals
The apparatus, consumables and chemical materials used in coating process is listed below.
Apparatus :
1. Electronic balance: Precisa®, XS 365M
3 2 2 3
2. Heater: Elifemall®, Electronic warmer 3. Ultrasonic cleaner: DELTA®, D80
4. Stainless steel precision tweezers: XYTRONIC®, SS-sa Chemical materials:
1. Silver nitrate ( AgNO3 ), Fisher® Chemical, SS73-100 ( 0.1 N, 100 mL )
2. Sodium hydroxide ( NaOH ), unknown, degree of purity: 99.3%, solid state granule
2. Sodium hydroxide ( NaOH ), unknown, degree of purity: 99.3%, solid state granule