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The thickness and stretch dependence of the electrical breakdown strength of an acrylic dielectric elastomer
The performance of dielectric elastomer actuators is limited by electrical breakdown. Attempts to measure this are confounded by the voltage-induced thinning of the elastomer. A test configuration is introduced that avoids this problem: A thin sheet of elastomer is stretched, crossed-wire electrodes are attached, and then embedded in a stiff polymer. The applied electric field at breakdown, EB, is found to depend on both the deformed thickness, h, and the stretch applied, λ. For the acrylic elastomer investigated, the breakdown field scales as EB = 51 h ^(− 0.25) λ ^0.63. The test configuration allows multiple individual tests to be made on the same sheet of elastomer.
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Mechanism of electrical breakdown
Hi Jiangshui,
I notice this paper reports the break down field of VHB decreases with thickness and increases with prestretch, but I did not quite follow the mechanism you proposed. Can you briefly explain it? Thank you.
Best,
Qiming
Hi Qiming, Thanks for
Hi Qiming,
Thanks for your interest.
The thickness dependence of dielectric strength on thickness has been
known for many years. The empirical relation between breakdown electrical field
Eb and thickness is “Eb proportional to h^-n”. According to experimental
observations, 'n' depends on the thickness range: the thinner range, the larger
the 'n'. The thickness dependence can be simply understood as following: There
exist a possibility for electric breakdown and under same measuring conditions (Same
material, same tension, same temperature, same size of electrodes), at the same
electrical field the possibility is lower for thinner membrane. For more
details and deep understanding the mechanism, you can take a look of the theoretical
models (ref# 1-9).
To the best of my
knowledge, the stretch dependence of dielectric strength was discovered
recently in the study of dielectric elastomers, and was first reported by Kofod
(ref # 25). The mechanism is still not clear, although there are several
explanations. One is that the defects in the material which would induce
electrical breakdown are eliminated to a certain extent by the mechanical
stretch; one is that the stretch increases the stiffness of the material (According
to other reports, dielectric strength depends on Young's modulus). In the
paper, we mentioned: “Another possibility is that in stretching the elastomer,
the vibrational amplitudes of the individual molecules are reduced and
consequently the activation volume is decreased through which mobile charges
can migrate under field.” As I said previously, “There exist a possibility for
electric breakdown”, and the large volume measured, the higher the possibility
of electric breakdown and the lower the dielectric strength measured. Therefore,
reducing the vibrational amplitudes of the individual molecules decreases the possibility
of electric breakdown.
Sincerely,
Jiangshui
Thank you for your reply
Hi Jiangshui,
Thank you for your nice reply.
As for high prestretch leading to higher breakdown field, we proposed an possbile explanation in our Soft Matter Paper 2011. (thank you very much for citing our paper.) Our understanding is the following: when rigid electrodes contact the polymer film, air bubbles are inevitably trapped on polymer-electrode interface. These air bubbles provide free surfaces where creasing-cratering instability sets in. We expect the breakdown fields measured in this experiment propabaly corresponde to the critical field of creasing instability of polymer under voltage. We demonstrated that higher prestretch of the polymer film gives higher critical field of electro-creasing instability. So higher prestretch gives higher breakdown field. We further reasonalized our hypothesis with matching between theory and experimental data(Fig 6&7 in our paper ).
Now as for your creative experiment, I expect what you test is the intrinsic breakdown field of the polymer. I am pretty interested in your creative experimental setup. In my previous test, I used metal rod with spherical tip which contacts with the polymer film. The polymer film is bonded on substrate and The rod is fixed from the top. So the setup can eliminate pull-in instability and electro-creasing instability. Now I expect your paper provides another method.
THank you.
Best,
Qiming
Hi Qiming, Thanks a lot
Hi Qiming,
Thanks a lot for giving an excellent summary of your nice work. In fact, I have a technique question to ask you: How did you control the displacement of metal rod to well touch the dielectric elastomers but without indentation? By precisely monitoring the force? Thanks.
Sincerely,
Jiangshui
Hi
Hi Jiangshui,
Thanks for sharing this paper. Two years ago, I had proposed a topic asking if the electrical breakdown strength of the soft dielectric was dependent of stretch, see http://imechanica.org/node/8038, and we later reported this behavior in one of our papers http://imechanica.org/node/10012 (figure 9). In our group, we would to attribute this improvment of dieletric strength to the stiffening of the material.
For dielectric material, ceramics have a higher breakdown field than the elastomers, so the stiffer the material is, the higher of the breakdown strenght. Besides the stiffening due to limited chain length, elastomer may also show a local crystalization at large stretch state. So what do you think of this mechanism?
Thank you
Best.
Bo
Ph.D.student
School of Mechanical Engineering
28 West Xianing Road
Xian Jiaotong University
Xian, Shaanxi, 710049
Email: blancolee@gmail.com boli.xjtu@stu.xjtu.edu.cn
Hi Bo, Thanks for
Hi Bo,
Thanks for reminding me that stretch would result in local crystalization in the elastomers. Sure the occurance of crystalization would increase the breakdown electric field. I forgot to mention this point previously. However, I don't think it is reasonable to get a conclusion that "the stiffer the material is, the higher of the breakdown strenght" , through comparing the dielectric strength of ceramics and elastomers, since they are very different material and may have differnt breakdown mechanism. Instead, scientists tested this argument by using same polymer with different amount of crossing link. Maybe I misunderstand your point.
Thanks a lot for sharing the paper. In fact, I read the paper previously, but sorry for I missed that part. Again, thanks for reminding me.
Sincerely,
Jiangshui
Bo Li Ph.D.student School of
Bo Li
Ph.D.student
School of Mechanical Engineering
28 West Xianing Road
Xian Jiaotong University
Xian, Shaanxi, 710049
Email: blancolee@gmail.com boli.xjtu@stu.xjtu.edu.cn