MAGNETODEFORMATION EFFECT AND VACUUM SEALING WITH A MAGNETOACTIVE ELASTOMER
The possibility of using magnetically active elastomers (MAE) in vacuum seals over an uneven surface is discussed. The material is characterised by high magnetodeformation and magnetostriction effects and is highly elastic. This material is able to be attracted to the sealed uneven surface under the action of the magnetic field and act as an effective sealant.
Magnetic elastomeric material of a novel type demonstrating the capability of changing its shape under the influence of various magnetic fields has been studied. As was indicated by the primary results, affected by homogeneous and non-homogeneous fields, this material can exhibit elongations by tens and hundreds of percent, respectively [1-3]. Besides, when the field is homogeneous, it demonstrates shape memory – the property of remaining as plastic as plasticine until the field is off. Removed from the field, the sample restores its geometrical parameters . One of the most complete survey of such behavior is given in ; another one covers elastomers with magnetically hard filler as well . Owing to the fact that magnetodeformaton continues to attract significant attention, ways of its application as well as the formulae of the corresponding elastomeric composites are subjects for patenting [6, 7] as is, for instance, in the case of using the material in the form of flow valve  or controlling the motion of an elastic worm .
One of the prospective areas of employment of the magnetodeformation of MAE may be based on gaskets creation for covering large surfaces or on using the phenomenon in vacuum equipment. The conventional way to provide sealing features pressing one surface against the other. The problem is that creation of the necessary pressure may require a significant effort if the area of sealing is large enough. The purpose of our prototype was to find out how high the efficiency of sealing rough surfaces with MAE may be. In the efficiency tests we considered pulling magnetoelastic rubber in the airspaces by the magnetic field and how this results in a sealed space between the two surfaces.
In the measurement of magnetodeformation, there was used an electromagnet, at a certain distance from which the sample was fixed. The pole creating a non-homogeneous field was to attract the elastomer. Investigation of magnetostriction was carried out using a cylindrical-shaped specimen placed between the poles of the electromagnet or in the center of the solenoid, the voltage signal created by the material in the field was recoded.
For the determination of the sealing properties demonstrated by the MAE, an innovative setup, the schematic of which is illustrated in Fig.1, was used.
The sequence of steps in the experiment is as follows. An evacuated vessel (1), on the side surface of which MAE (2) is affixed to its butt end, is set to the rough surface to be sealed (3). A magnet (4) is placed beneath it at the distance appropriate for the induction of the desired magnetic field. A certain vacuum level is created by means of a vacuum pump (7), after which the valve (6) is closed and the vacuum decay rate is determined on the basis of the indications of the manometer (5). The diameter of the part of the vessel being sealed is 1–15 mm and the width of the MAE ring (2) is 5 mm. The space being evacuated has a volume of 40 mL.
The MAE synthesized for these investigations features a composite material prepared on the basis of a silicone matrix filled with magnetic particles of carbonyl iron with a mean size of 5 µm and a concentration of 30 vol.%.
Measurements of the viscoelastic properties and magnetostriction were carried out in the stretching and compression mode on an И1158М-0,5-01-1 pull test machine, a product of TOCHPRIBOR Ltd., Russia, supplied with a strain-gauge sensor with a capacity of 10 N. In the range 0.2–10 N, the sensor has a sensitivity and fractional uncertainty of 0,0001 N and 1%, respectively.
Investigations of the elastic properties and magnetostriction exhibited by the material were conducted using a pull test machine and a cylinder-shaped specimen with a diameter and height of 14 and 35 mm, respectively. The results obtained are indicative of the fact that materials of this family practically obey Hooke’s law when subjected to deformation in zero field, and their elasticity modulus determined on the basis of stretching and compression is 100 kPa, which corresponds to line 1 in Fig.2. Under the influence of a homogeneous magnetic field, the material expands thus creating pressure on the end faces of the cylinder and the deformation (stress-strain) curve changes configuration to ellipsoidal, as may be seen by the example of curve 2. This phenomenon is based on the variation of the elastic properties with magnetic field, which simultaneously depend on the degree of deformation.
Influenced by a homogeneous magnetic field, MAE extending along direction of the induction vector exhibits a positive magnetostriction. Figure 3 presents the dependence of the pressure created by the sample on the end faces of the cylinder on the magnetic field affecting the specimen. At the same time, in the unconstrained case this material is capable of demonstrating an elongation of 10% on the average.
The way MAE demonstrates its capability to change shape when filling in airspaces may be considered using the example of a ribbed surface, a photo presented in Fig.4. Surface ribbing is sometimes deliberately made perpendicular to the motion direction of the medium leaking in. At the same time, on a regular basis, a sealed joint is provided by pressing an elastic material in the gaps and airspaces.
As a result of the study carried out on the apparatus presented in Fig.1, it was found out that that the vacuum decay rate in the system does depend on magnetic field strength. The curves shown in Fig.5 demonstrate how significantly magnetic field is capable of preventing air penetration.
The system was evacuated to a low vacuum of 5 kPa, the value taken as 100%. In the following steps measurements of the vacuum decay rate in the system were done. As may be seen from Fig.5, the variation of pressure inside the vessel exhibits a strong field-dependence. A typical tendency is brought in Fig.6. Apparently, there is a threshold field, beyond which no air leak-in occurs. In the present testing a field of 450 mT provided a stable vacuum for several hours, which corresponds to line 6 in Fig.5. Owing to the fact that the relief made on the surface in the direction of the air flow was quite deep, providing this degree of sealing required the application of a relatively strong magnetic field. This experiment demonstrates the possibility in principle to create sealants of a novel type on the basis of magnetoactive elastomers.
The results obtained may be explained by the unique properties possessed by MAE demonstrating high elasticity and the capability to change shape in magnetic fields. Schematically, the functioning principle of the sealant may be presented, as shown in Fig.7.
Attempts to provide sealing without the application of a magnetic field result in an insufficiently strong pressing of the MAE (1) against the uneven surface (2) leading to an incomplete elimination of airspaces conditioned by the relief. At the same time, application of a non-homogeneous magnetic field created by an electromagnet or a permanent magnet makes the MAE pull in the gaps filling in all the hollows. Depending on the specifics of the relief, the degree of sealing is a function of magnetic field. Pressing an evacuated vessel against the ribbed surface with a depth of grooves of 1 mm, 450 mT was sufficient for having the system completely sealed. It should be noticed, however, that the mentioned values were limiting. Creation of such a magnetic field required the application of a strong neodymium magnet with a size of 40 × 40 × 40 mm.
As is indicated by the results of our studies, magnetoactive elastomeric materials, which are characterized by a significant magnetostriction, may indeed be employed as sealant in equipment manipulating rough vacuum when the surface to be sealed demonstrates pronounced unevenness.
This work was supported by the RFBR grant 19-53-12039, Russian Science Foundation (Grant No. 19-13-00340) and the NATO Science for Peace program SfP 977998.
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.