B. M. Hasanien, I. E. Iyoub, O. N. Soliman, M. R. N. NazeerEffects of gamma rays on the capacitance...

Effects of gamma rays on the capacitance of Electric Double Layer Capacitors ( 27 )

ABSTRACT

KEYWORDS

Effects of gamma rays on the capacitance of Electric Double Layer Capacitors

B. M. Hasanien, I. E. Iyoub, O. N. Soliman, M. R. N. Nazeer

E.mail:[email protected]

Capacitor Compensation, Radiation Effect, EDLC, Capacitor System.

4th Int. Con. Rad. Res. Appl. Sci., Taba, Egypt (2014) PP. 27 : 39

1. Hot Labs Center, Atomic Energy Authority, Cairo, Egypt2. Faculty of engineering, South Valley University, Qena, Egypt3. Faculty of engineering, El-Azhar University, Qena, Egypt

Electric Double Layer Capacitors “EDLC” are widely used in many fields and wide applications, such as in power distribution systems depend-ing on Energy Capacitor Systems, and other electronic circuits. In nuclear installations, some of its infrastructures could be subjected to ionizing radia-tions; these infrastructures could contain control or power systems that is using EDLC. EDLCs could be subjected to ionizing radiations too in space, humans used to send scientific investigation trips to space, in space, the equipment and systems sent via the investigation trips will be subjected to ionizing radiation, many of these devices are using EDLCs. Our purpose here in this current research was to investigate the effect of gamma rays on the capacitance of EDLC. In some control systems and timing circuits it is very important to have stable values of electronic components in the control circuits. So that, it is very important to be aware of the effects of the operat-ing conditions on the components of the system. In this current research, we used 30 capacitors (EDLC type); these capacitors was divided into six groups, each group has different characteristics depending on the electrolyte type and the characteristics of the activated carbon layer exists on the two electrodes. We subjected these capacitors after preparing them to different gamma radiation doses. The irradiation process was accomplished in the “Gamma irradiation unit” in the National Center of Radiation Research and Technology – Egyptian Atomic Energy Authority “EAEA”. The capacitance of the capacitor samples was measured before and after irradiation process using RLC meter. In this research we did some theoretical and mathematical calculations in order to introduce a complete vision on the effect of gamma rays on each component of the EDLC. Some types of the EDLC showed an increase in capacitance with the higher radiation doses, while other types of these EDLC showed a sharp decrease in capacitance along with the radia-tion dose.

4th InternationalConference on Radiation

Sciences and Applications,13-17/10/2014, Taba, Egypt

B. M. Hasanien et al.( 28 ) 4th Int. Con. Rad. Res. Appl. Sci., Taba, Egypt (2014)

INTRODUCTION

Electronics and electrical devices ex-posed to ionized air and radiations on space or on the nuclear installations and facilities. One issue that should be taken in to account is the recovery

behavior of the capacitors; “P. Osmokrovic” studied the recovery behavior of capacitance of the irradi-ated capacitors after being left away from the radia-tion sources for given time. In 2004, “Ignacio Yasel-li” has studied the recovery behavior, giving results which are very close to “P. Osmokrovic” results. On the other hand, in the case of nuclear power installa-tions, the recovery behavior of the capacitors is not important, as the capacitors will be in a continuous use and a continuous exposure of the radiations ex-isted in these installations.

The important of this research is to take the ef-fects of the gamma radiation on the capacitance of EDLCs into consideration, as EDLCs nowadays are used widely in many applications due to its high ca-pacitance, its speed charging and discharging perfor-mance, it’s a clean energy device, also due to its high lifespan as the over-charging and over-discharging doesn’t affect its lifespan. EDLCs can be used within the following applications:

• Energy Capacitor Systems (ECS) used in isolated power distribution networks.

• In starting circuits for small-sized motors.

• In devices depending on solar cells as a power source.

• Backup power source.

• In timers and other equipment needs memory backup.

Most of the pre-mentioned applications can be found inside nuclear installations (such as nuclear power plants, nuclear laboratories, and radioactive waste treatment plants), or can be found within de-vices and equipment that will be sent to space (space vehicles, and space equipment). These devices

whether inside nuclear installations or space equip-ment will be subjected to different types of ionizing radiations, but the most practical case of ionizing radiation that is subjected to EDLCs is the case of gamma radiation.

Here we are trying to introduce a simple way to calculate the effect of gamma radiation on the capac-itance of EDLCs, in order to be taken into consid-eration in case of sensitive devices and equipments including an EDLC and subjected to this type of ion-izing radiation.

EXPERIMENTAL

Samples

Here in the current research we prepared 30 samples of EDLCs with a rated capacitance of 50 F. each 5 capacitors of the total 30 capacitors have different manufacturing characteristics. Noting that the actual capacitance of the sample capacitors was different from the named capacitance stated by the manufacturer, so we made combination of 6 groups of samples, each group contains 5 capacitors that have the same actual capacitance.

Fig. (1): A hierarchy indicating the samples used in the experimental part of this study.

To understand what is meant by the groups and zones mentioned in the hierarchy in Fig.1, please refer to paragraphs: «3-2-A» and «3-3-B». Every 5 capacitors in each zone within each group of the three groups that was subjected to different dose of gamma rays.


Number of

samples:Electrolyte type: Pore size (nm) of

activated carbon: Rated

voltage (V)

Group AZone 1 5 KOH–PC Composite

Polymer: (60 wt % PPC) > 2 2.7

Zone 2 5 KOH–PC CompositePolymer: (60 wt % PPC) < 1 2.7

Group BZone 1 5 KOH–PC Composite

Polymer: (40 wt % PKOH) > 2 2.7

Zone 2 5 KOH–PC CompositePolymer: (40 wt % PKOH) < 1 2.7

Group CZone 1 5 KOH–PC Composite

Polymer: (60 wt% PC PKPC) > 2 2.7

Zone 2 5 KOH–PC CompositePolymer: (60 wt% PC PKPC) < 1 2.7

Table.1 for indicating the details about the manufacturing characteristics of the capacitor samples:

Preparation of the samples:

The capacitors were fully charged before irradia-tion. The capacitance of the capacitors was measured before irradiation to find the actual value of the ca-pacitance using a RLC meter.

Results

We have 6 groups of capacitors, each group con-tains 5 capacitors, and each one of these 5 capacitors was subjected to different dose of gamma radiation (20 kGy – 50 kGy – 100 kGy – 150 kGy – 200 kGy). Then the capacitance of these 30 capacitors was measured using the RLC Meter after the irradiation process.

Irradiation process of the samples:

Irradiation of samples was carried out using the Indian Co-60-gamma cell type 4000 with a dose rate 3.82 - 4 kGy/h at the National Center of Radiation Research and Technology, Cairo, Egypt. The opera-tor used a Reference Alanine Dosimeter supplied by the National Physical Laboratory, UK, to determine the irradiator’s dose every 12 months. The dose ap-plied was determined each day by calculation.

It should be taken into account that for each sample, the radiation dose should be subjected to the four sides of the samples uniformly.

Table (1) : Samples characterization


Therefore, we had the following measures and data illustrated in Table.2:

Capacitance beforeirradiation (F):

20 kGy

Capacitance after irradiation (F):

50 kGy 100 kGy 150 kGy 200 kGy

Group A

Zone 1

49.8 50.6

49.8 50.5

49.8 50.3

49.8 50.11

49.8 49.9

Zone 2

49.77 49.7

49.77 49.56

49.77 49.1

49.77 48.5

49.77 48.11

Group B

Zone 1

49.7 50.23

49.7 50.15

49.7 50.22

49.7 50.3

49.7 49.91

Zone 2

49.7 49.65

49.7 48.14

49.7 47.58

49.7 47.13

49.7 46.88

Group C

Zone 1

49.7 51.1

49.7 50.88

49.7 51

49.7 51.32

49.7 51.35

Zone 2

49.68 49.55

49.68 49.5

49.68 49.04

49.68 48.89

49.68 49.88

Table (2) : Capacitance VS radiation dose


Group (A)

Group (B)

Group (C)

Fig. (2): curves showing the effect of radiation on different EDLCs types.


DATA ANALYSIS AND DISCUSSIONS

Electric Double Layer Capacitor (EDLC):

A simple capacitor (conventional capacitor) is consisted of 2-electrodes (conductive plates) and an insulator (dielectric) between them; see Fig.3 for an illustration drawing.

Here, after applying a voltage difference be-tween the two plates, positive charges will be accu-mulated on one plate, while negative charges will be accumulated on the other one. The capacitance «C» of the capacitor is equal to the magnitude of charge on anyone of the 2-plates which have an area of «A», with a distance «d» between them:

(1)C = Ԑ 𝐴𝐴𝑑𝑑 )1)

Where, “Ԑ” is the dielectric constant or the per-mittivity of the insulation between the 2-plates.

EDLC composed of 2-electrodes with an inside layer of activated carbon above them, an electrolyte between the two electrodes, and a separator in the center of the EDLC, fig.4 indicates the components of the EDLC.

Fig. (3): a simple sketch for conventional capacitor.

Fig. (4): a simple sketch of EDLC.

Fig. (5): ions entering activated carbon pores, forming EDL.

In EDLC the charges on the two electrodes leads to polarization of the electrolyte, making it electri-cally active. This helps in adding another dimen-sion of charges near to the 2-electrodes, see Fig.3 for more illustration. The new charges dimension created due to the ions entering the activated carbon pores as shown in Fig.5, this dimension «or layer» of charges called «Electric Double Layer». Each one of the EDL is working as a stand-alone capacitor [1].


Our main purpose here was to find the relation between each capacitor component and the capaci-tance, as here in the current research we are going to derive the relation between the capacitance and the radiation dose, through the analysis of the equa-tions and curves describes the relation between the radiation dose and each of the capacitor component. We tried to illustrate the experimental results we get, these calculations can be used through a computer program in order ti simplify an easy way to calculate the effects of a given radiation dose on the capaci-tance of the EDLC, as these calculations were veri-fied through the experimental results.

• First step: was to find the relation between EDLC components and its capacitance.

• Second step: was to find the effect of the gamma rays on each one of the EDLC components.

• Third step: was to use the previous two steps to calculate the relation between the radiation dose and the capacitance of the EDLC.

• Final step: was to illustrate and examine the results we get from the experimental part against the calculations and relation we get from step 1 & 2.

1-1- Relation between capacitance and the compo-nents of the EDLC:

EDLC consists of 2-Electrodes (current collec-tor with a layer of activated carbon particles), Elec-trolyte (could be any of those electrolytes: organic solution, sulfuric acid, or KOH), and a Separator (cellulose based material, or porous polypropylene) [2].

See Fig.6 for an equivalent circuit of EDLC in-dicating the effect of each component characteristic on the performance of the EDLC:

Where:

C1, C2: are the capacitances of the virtual capaci-tors formed due to the two EDLs occurs in the con-tact regions between activated carbon and the elec-trolyte.

Rs: resistance of the electrolyte and separator.

Re1, Re2: electrode resistance.

R1, R2: insulation resistance.

A- Relation between “Electrodes” and “Capaci-tance” of EDLC:

As shown in Eq.1, capacitance is directly pro-portional to the surface area on which the charges are collected. Charges are being collected over the elec-trode which is made mainly from activated carbon. We studied the relation of the capacitance of EDLC and the activated carbon characteristics. Activated carbon can be described by its surface area and pore size. In the past, it was thought that in order to get the largest capacitance you should use activated car-bon with the highest surface area and with the big-gest pores size [3], after that “shii” introduced another opinion, stating that for large pore sizes, the surface area of the activated carbon subjected to electrolyte ions will be decreased, decreasing the capacitance value [4], see Fig.7 for more illustration:

Fig. (6): equivalent circuit for EDLC.

Fig. (7): shii opinion about relation between pore size and surface area of activated carbon.

Through the upper sketch in Fig.7 we can find that there is an optimum pore size that gives the maximum surface area for the activated carbon ex-posed to ions from the electrolyte. In case #1 the pore size is very small to be accessed by the ions, which will lead to a small surface area. In case #2 the pore sizes are medium which allows the ions to


enter it, increasing the surface area of the activated carbon. In case #3 pore sizes are very large, leading to a smaller surface area of the activated carbon than in case #2.

Pore sizes could be more than 50 nm or lower than 2 nm, Fig.6 indicates the available pore sizes and the name of each size range:

Fig. (8): names and values of each size range.

Fig. (9): a graph indicating the relation between capaci-tance of EDLC and pore size of activated carbon electrode.

The most recent researches are working on ac-tivated carbons with sizes smaller than 1 nm. It is a strange trend of increasing capacitance with decreas-ing the size of pores lower than 1 nm. The ions of the electrolyte moving towards the electrodes in the form of ion shells which affects the distribution process of these ions on the activated carbon electrodes. In case of small activated carbon pores (smaller than 1 nm), these very small pores will distort these ion shells making the ions to move toward the electrodes freely, achieving a closer and high density distribution of these ions on the electrode, which will lead to higher capacitance [5], Fig.9 and Table.3 shows the relation between the capacitance of EDLC and the pore size of the activated carbon of its electrodes.

The activated carbon of the 2-electrodes of the EDLCs is designed within two zones, which are zone |||, and zone |. From Fig.9 we can clearly see that the chosen zones show two different relations between the pore size of the activated carbon and the normalized capacitance of the EDLC.

For zone |:

From Fig.9, it’s clear that the normalized capaci-tance is directly proportional to the pore size of the activated carbon (straight line – traditional view):

CN α PO Eq.2

This is the capacitance of one plate of the elec-trodes for 1 cm2 of its surface area. Referring to the equivalent circuit shown in Fig.4 it will be shown that the total capacitance of the EDLC “C” will be proportional to this normalized capacitance.

C α CN Eq.3

For zone |||:

Table.3 describes the points of the curve in zone |||:


The research paper didn’t derive an equation to describe the relation between pore size of activated carbon and the capacitance of EDLC. We used the data given in the paper to derive this equation:

From Table.1 we can find an interpolation formula between pore size and capacitance using Lagrange Method.

Pore size = X, Capacitance = f(X),

X0 = 1.1 f(X0) = Y0 = 6.115X1 = 0.8 f(X1) = Y1 = 7.615X2 = 0.75 f(X2) = Y2 = 9.688X3 = 0.7 f(X3) = Y3 = 10.125X4 = 0.68 f(X4) = Y4 = 13.4375

Pore size (nm): Normalized capacitance (µF/cm2):1.1 6.1150.8 7.6150.75 9.6880.7 10.1250.68 13.4375

Table (3) : Normalized capacitance VS pore size

l0(X) = 𝑋𝑋−𝑋𝑋1𝑋𝑋0−𝑋𝑋1 ×

𝑋𝑋−𝑋𝑋2𝑋𝑋0−𝑋𝑋2 ×


𝑋𝑋−𝑋𝑋4𝑋𝑋0−𝑋𝑋4

= 56.69 (X4 – 2.93 X3 + 3.215 X2 – 1.5658 X + 0.2856)





= -5555.56 (X4 – 3.23 X3 + 3.854 X2 – 2.0191 X + 0.3927)





= 16326.531 (X4 – 3.28 X3 + 3.978 X2 – 2.1188 X + 0.41888)





= -2500 (X4 – 3.33 X3 + 4.107 X2 – 2.2274 X + 0.4488)





= 14172.335601 (X4 – 3.35 X3 + 4.16 X2 – 2.2735 X + 0.462)

Thus the interpolating polynomial then is:

L(X) = f(X0) [56.69 (X4 – 2.93 X3 + 3.215 X2 – 1.5658 X + 0.2856)] – f(X1) [5555.56 (X4 – 3.23 X3 + 3.854 X2 – 2.0191 X + 0.3927)] + f(X2) [16326.531 (X4 – 3.28 X3 + 3.978 X2 – 2.1188 X + 0.41888)] – f(X3) [2500 (X4 – 3.33 X3 + 4.107 X2 – 2.2274 X +

0.4488)] + f(X4) [14172.335601 (X4 – 3.35 X3 + 4.16 X2 – 2.2735 X + 0.462)]

= 53528.26 X4 – 178241.253 X3 + 219923.915 X2 – 119413.67 X + 24121.582 = Y

Then we can say:

CN = 53528.26 (Po4 – 3.33 Po3 +4.1086 Po2 – 2.231 Po + 0.450633) Eq.4

Where: “CN” is the normalized capacitance for the EDLC; “Po” is the pore size of the activated car-bon powder on the electrode of EDLC (Zone |||).

A- Relation between the electrolyte and capaci-tance of EDLC:

Eq.1 in case of EDLC will be:

C = Ԑ

4𝜋𝜋𝜋𝜋 ∫𝑑𝑑𝑑𝑑 Eq.5

Where: is the dielectric constant of the electro-lyte; L is the distance between the center of the ion and the electrode interface; S is the surface area of the electrode interface [6].

For an electrolyte, the permittivity could be a complex amount, and can be written as [7],

Ԑ (ω) = Ԑ′ (ω) + j Ԑ" (ω) = Ԑr (ω) Ԑ0 + j 𝛿𝛿 (ω)

ω Eq.6 From Eq.5 & Eq.6, we can see that:

Capacitance of the electric double layer capacitor is directly proportional to the dielectric constantand the conductivity of the electrolyte [8].

C α Ԑ α 𝛿𝛿 Eq.7


A- Relation between the separator and the capaci-tance of the EDLC:

The separator of the EDLC is located in the cen-ter of the EDLC separating the 2-electrodes from each others, surrounded by the electrolyte. In some techniques the separator is used as a solid electro-lyte and a separator without using other electrolytes within the EDLC [9], [15].

For the separator, the main effective property that affecting the overall capacitance of the EDLC is the pore size of the separator. The ions from the electrolyte are passing through the separator pores to the both sides of the electrodes, which mean that the size of the separator pores should not be less than the ion size.

Then we can say that for the separator, if its pores size is larger than the ion size of the electro-lyte, it will not have an effect of the capacitance of the EDLC (especially if the EDLC is designed with an electrolyte with small ions size).

1-1- The effect of gamma rays on the EDLC com-ponents:

In order to get a complete understand of the ef-fect of the gamma rays on the EDLC, we are going to study the effect of gamma rays on each component of the EDLC. After all we should put in mind that our aim here is to describe the effect of the gamma rays on the capacitance of the EDLC, which led us to study the relation between the EDLC components and its capacitance. This helped us to have a com-

plete and comprehensive overview of what effects have the gamma rays on the EDLC and its compo-nents; this could lead to future researches that will study the solutions that could be implemented to avoid the bad effects of the gamma rays on the ca-pacitance of the EDLC or to utilize gamma rays ef-fects on EDLC to design more reliable and effective capacitors.

A) Effect of gamma rays on the activated carbon layer of the EDLC electrodes:

Fig. 8 and Table.2 shows the effect of gamma radiation on the total surface area and pores size of the activated carbon [10].

Fig. (10): Pore size distribution of activated carbon before (---) and after (ـــــ) irradiation.

Table (4) : Total surface area of the activated carbon before and after irradiation

Activated Carbon Samples: STOT (m2/g) SEXT (m2/g)

Before irradiation 1025 16 – 0.99 nm

After irradiation. 884 76 – 2.4 nm

Where, STOT is the total surface area of the acti-vated carbon, while SEXT is the area of pores in the activated carbon.

In this research paper, the activated carbon samples were irradiated with a 25 kGy using 60Co-

source. Here in this research, the total surface area and pore size of the activated carbon calculated us-ing Brunauer–Emmet–Teller (BET) method [11].Now we can derive an equation to describe the relation between radiation dose and the surface area and pore size of the activated carbon, as the research paper


stated that the relation is a straight line.

- Relation between radiation dose and total sur-face area of activated carbon:

Atot-γ = Atot – 5.64 D Eq.8

Where, Atot-γ is the total surface area of the acti-vated carbon after irradiation, Atot is the total surface area of the activated carbon before irradiation; D is the dose (gamma rays) subjected to the activated car-bon.

- Relation between radiation dose and the pore size of the activated carbon:

PO γ = 0.0564 D + PO Eq.9

Where, PO γ is the pore size of the activated car-bon after irradiation, PO is the pore size of the acti-vated carbon before irradiation; D is the dose (gam-ma rays) subjected to the activated carbon.

A) Effect of gamma rays on the electrolyte of EDLC:

EDLC electrolyte could be an organic solution, sulfuric acid, or KOH. Here we are going to derive the relation between radiation dose and the conduc-tivity of different types of “PVA–KOH–PC Compos-ite Polymer Electrolytes” [12]. The paper indicates the relation between the radiation dose and the conduc-tivity of three different groups of electrolytes:

Group A:

For low doses (from 0 to 40 kGy):

δγ = 1.175 e-10 D + 1 e-10 Eq.10

For high doses (more than 40 kGy):

δγ = -2.525 e-11 D + 5.81 e-9 Eq.11

Group B:

Fig. (11): shows the relation between the radiation dose and the conductivity of 60 wt % PPC (group A).

Fig. (13): shows the relation between the radiation dose and the conductivity of 60 wt% PC PKPC composites (group C).

Fig. (12): sshows the relation between the radiation dose and the conductivity of 40 wt % PKOH (group B).


δγ = 0.15 e-4 D + 3 e-4 Eq.12

For high doses (more than 20 kGy):

δγ = -8.3333 e-7 D + 6.1667 e-4 Eq.13

Group C:



δγ = 0.15 e-5 D + 0.055 e-3 Eq.14

For higher doses (from 20 to 100 kGy):

δγ = -1.875 e-7 D + 8.875 e-5 Eq.15

For very high doses (more than 100 kGy):

Here, as shown in Fig.11 for very high doses of radiation, conductivity of Group C electrolytes has not been affected and reached saturation.

CONCLUSION

The experimental results showed different trends of the capacitance of different EDLCs types, some types tends to show an increased capacitance with higher dose rates of radiation, while other EDLCs types shows a decreased capacitance with higher dose rates of radiation.

This could be easily illustrated through the equa-tions and curves mentioned in section “3”. The two main important factors that affecting the trend of ca-pacitance versus the dose rate of radiation are: the characteristics of pore size of the activated carbon layer of the electrodes of the EDLC – and the elec-trolyte type.

The characteristics of pore size of the activated carbon layer of the electrode could be classified into 2 main zones, as illustrated in Fig.9. While the type of electrolyte used in the EDLC could be classified into 3 groups, as illustrated in section 3-3-B.

We had 6 types of EDLCs with different types of electrolytes and different activated carbon character-istics (A-1, A-2, B-1, B-2, C-1, C-2).

For A-1 EDLC: As shown in Fig.2, this type of EDLC showed an increase in its capacitance as being subjected to a dose of gamma rays, as its electrolyte type is belonging to group A, which tends to show an increase in its conductivity as being subjected to radiation. Also the characteristics of the activated carbon of this EDLC belong to Zone 1, in which the normalized capacitance of the electrode is directly proportional to the radiation dose Eq.2 & Eq.3. Note

that in this type of EDLC the conductivity of the electrolyte tended to decrease after a dose rate higher than 20 kGy, which will affect its capacitance. So we can get the maximum efficiency from this type of ca-pacitors if we subjected its components to a radiation dose of 20 kGy. For B-1 EDLC it could be seen from its curve that it could be treated with gamma rays for best efficiency up to 140 kGy, but for the both types it is not preferred to be used in nuclear installations.

For A-2 EDLC: The electrolyte used in this EDLC type is the same as in (A-1 EDLC), while the activated carbon is differ. The activated carbon layer used here in this type showed a sharp decrease in normalized capacitance with higher radiation doses, which led to a decrease in the capacitance of the EDLC, see Eq.3 & Eq.4. This type of EDLCs should not be used with nuclear power applications, and should not be treated or subjected to ionized radia-tion. The same thing is considered for B-2 EDLC as it couldn’t be used in places where it could be sub-jected to ionized radiation.

From Fig.9, we can see that the most appropriate type of EDLC that can be used in nuclear installa-tions and within places that can be subjected to ion-ized radiation is (EDLC – group C – Zone 1). As above 200 kGy of radiation dose it reached a stable value of capacitance. The same recommendations are for C-2 EDLC, but it should be noted that for the doses range (20 > 180 kGy) the capacitance of this type of EDLC was decreased, so it is preferably to be irradiated with a dose of 200 kGy before being used in nuclear installations.

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[16] Pore-size ion-size correlations for carbon supercapacitors Doctor of Philosophy Thesis, February 2009 - John Chmiola

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B. M. Hasanien, I. E. Iyoub, O. N. Soliman, M. R. N. NazeerEffects of gamma rays on the capacitance...

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Transcript of B. M. Hasanien, I. E. Iyoub, O. N. Soliman, M. R. N. NazeerEffects of gamma rays on the capacitance...