NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

Studies On The Dielectric Behavior In (tOO-x) wt, % MethylCellulose (MC) +x wt. % NH4N03 Polymer Electrolytes

N.E.A. Shuhaimi, S.R. Majid and A.K. Arof*

Center for lonics University Malaya, Department of Physics, Faculty of Science, Universiof Malaya, 50603 Kuala Lumpur

*Corresponding Author: akarof@um.edu.my

Abstract

This paper reports the dielectric behavior of (IOO-x)MC + x NH4N03 (x=5, 10, 15, 20,)and 30 wt. %) electrolyte system calculated using data from impedance measurement at rootemperature over the frequency range 50 Hz to 1 M Hz. The results obtained show tbdielectric constant, dielectric loss, imaginary part of electrical modulus and loss tangeshows changes with frequency. The dielectric properties of the samples at low frequene'have been explained on the basis of space charge polarization. The relaxation time for thesamples is determined from the variation of loss tangent at different frequencies at rootemperature. The relaxation time decreases with conductivity of the complexes.

Keywords: methyl cellulose, ammonium nitrate, dielectric behavior

1. INTRODUCTION

There has been considerable interest on thestudy of the dielectric behavior in polymerelectrolytes because it gives importantinformation even though these materialshave adequately high ionic conductivity [1].By studying the dielectric behavior, some ofthe physical and chemical properties of thepolymer can evaluate and the structure ofmaterial can be understood [2]. Ramesh eta1. [3] stated that, studying the dielectricbehavior help in understanding theconductive behavior of polymer electrolytes.Jang et a1. [4] have studied the effect ofchitosan concentration on the electricalproperty of chitosan-blended cellulo eelectroactive paper (B APap) and foundthat th relaxation time d creased while theion mobility and th conductivity incre edwith incr a ing chit an-bl nding rati inB APap. Th aim f in e tig ting

diele tri b ha i r f m thyl llul (M)

doped with ammonium nitrate (NH4N03).this work is to understand the conductifbehavior of the electrolyte films. MeWcellulose is used as polymer host ~ammonium nitrate as the doping salt Vieused in this study because only li~attention has been paid to proton conductllpolymer electrolytes using M host. OWammonium salts used as proton donors'ammonium triflate (NH4S03CF3) (5,1ammonium sulfate [(NH4)2S04] [7], te~methyl ammonium bromide [(CH3)4] [[8] ammonium thiocyanate (N&SCN)10] and ammonium iodide (NH4I) [11-12]

2. EXPERIMENTAL

2.1 Material'(1j(M ) from It

t, amm nium nit~a alt d di till

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

2.2 Sample Preparation

Polymer electrolytes were prepared usingsolution casting technique. Samples wereprepared using a general formula (100-x) wt.% MC + x wt. % NH4N03 (x=5, 10, 15,20,25 and 30). MC was dissolved in distilledwater and stirred until clear viscoussolution. NH4N03 was added accordinglyand stirred again until 24 hours for solutioncompletely dissolve. After completedissolution, the solutions were cast in plasticpetri dishes and left to dry at roomtemperature to form films.

2.3 Impedance Measurements

The solid (100-x) MC + X NH4N03electrolyte film were sandwiched betweenstainless steel and impedance measurementswere performed with electrochemicalimpedance spectroscopy (HIO KI 3531-01-LCR bridge) that has been interfaced with acomputer in the frequency range 50 Hz to 1M Hz at room temperature. The dielectricconstant, dielectric loss, imaginary part ofmodulus and loss tangent was calculatedfrom the impedance data.

3. RESULTS AND DISCUSSION

3.1 Impedance Analysis

The impedance plot of 70 wt. % MC + 30wt. % NH4N03 at room temperature isshown in Fig. 1. The -z, versus r; plotsshows a straight line at lower frequencyregion and semicircular arc at the higherfrequency region. The bulk resistance of theelectrolyte film was taken from the intercepton the real X-axis at the higher frequencyside.

The ionic conductivity of the samples wascalculated using the equation as follows inRef. 13-16.The value of the bulk resistance andconductivity are tabulated in Table 1.

6.E+04 -,-------------,

g 3.E+04~-

O.E+OO ~-----r--------1

O.E+OO 6.E+043.E+04Zr(!!)

Fig. 1 Impedance plot of 70 wt. % Me + 30 wt. %NH4N03.

The value of Rb for the samples is decreasewith addition of NH4N03. This implies thatthe conductivity has increase with additionof NH4N03. The conductivity of the systemwas optimized for sample containing 25 wt.% salt.

3.2 Dielectric Behavior

The effect of N~N03 concentration on thefrequency-dependent dielectric behavior of(100-x) MC + X NH4N03 system wasanalyzed using the equation as follows inRef. 17-19.

3.2.1 Dielectric loss and dielectric constant

Fig. 2 and 3 shows the variation of dielectricconstant, e; and dielectric loss, Ej as afunction of frequency for MC-N~N03complexes at room temperature.

(tOO-x) wt. % MC + x wt. % NH4N03

TABLE 1. The values of bulk resistance, Rb and conductivity, (J of samples with respective composition

Bulk resistance, a,(!!)

Conductivity, (J

(8 em")

100 wt. % Me6.74 x 10-

4.31 x 107 3.08 x lO-lJ

2.52 x 10

90 wt. % Me + 10 wt. % NH4N03 5.41 x 105 2.49 x 10-9

3.10xlO-785wt. % Me + 15wt. %NH4N03 3.74 X 104 5.75 x 10-8

80 wt. % Me + 20 wt. % NH4N03

2.10 X 10-66.43 X 103

70 wI. % MC + 30 wt. % NH~N03 3.35 X 104 6.40 x 10-875 wt. % Me + 25 wt. % NH4N03 7.81 X 102

86

4500 ,-----------,

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

oS-;3000-'"c8 :.::CJ

~1500Q,IQ:j

Q

o rrc San- m::10an+ m::1SanI:J. m::20an:.::m::2Sano m::30anx pure rrc

123 4 567Log frequency (Hz)

Fig.2 Dielectric constant versus log frequency plotfor (lOO-x) MC + x NH4N03 system (x= 5, 10, 15,20, 25 and 30 wt. %)

4500 -r-----------,

os.,; 3000'"..eCJ'i:-CJ~ 1500Q

o m::San-m::10an+ 1SanI:J. m::20an:.::m::2San<> m::30anx pure rrc

o~~ .. ~~ .... ._~1234567

Log frequency (Hz)

Fig. 3 Dielectric loss versus log frequency plot for(100-x)MC+xN~N03 system(x=5, 10, 15,20,25and 30 wt. %).

It can be seen that the value of e- and e,decrease with increasing the frequency up to106 Hz. The decrease in the value of e. and e,may be attributed to the electrical relaxationprocess [20] and indicating that electrodepolarization due to charge accumulation hastaken place in space [21-22]. Baskaran et al.[23] reported that non-Debye behavior washappen because the formation of spacecharge regions at the electrode-electrolytint rfaces at low frequencie. At highfrequencies, the periodic rever al of theelectric field occur at the interfac , thcontribution of m bile ion toward e,decrea e with increa ing frcqu n y.

In the b th f th figur ,n ppr ciablrela cati n p ak are r d in th tudi dfre uen y r nge. hein c ndu ti it j. due lnumb r d nsity

variation in e, and e, are observed follow jsame trends as in the conductiv'composition relationship. The sample ~the highest conductivity value has jhighest dielectric constant and dielecvloss.

3.2.2 Real And Imaginary Part Of Electri'Modulus.

Depicted in Fig. 4 is the vananonimaginary (M;) part of the electri'modulus at room t mperature for the varioconcentration ofNHtN03 salt.

0.3 ,------------..,;

mc5an::l 0:; 0.25 - mc10an"00 ..... + mc15ane.... 0.2 -,0 1:0"5 :E 0.15Q, oy)+~.... 0.1I-C':C'6iJ 0.05C': 0_ +-fe.... - .+#+0

1 2 3 4 5 6 1Log frequency (Hz)

Fig. 4 Imaginary part of modulus, M, log frequency Ifor (IOO-x) Me + x NH4N03 system (x= 5, 10, 15,20.and 30 wt. %).

The existence of a long tail at the lo~frequency end is attributed to the 1811capacitance associated with the electroO[18]. In M/ versus log frequency plots, ~appearance of peaks which correspondthe conductivity relaxation [20] ,observed to shift from low to bi#frequency region and it follows the tren~the conductivity variation [24].presence of uch peaks in M/ plots indjC~.Ithat the electrolyte y tem an 10

conductor [21].

3.2.3 Lo Tangent

ig. 5 d pict th fr qu n y d p nd nC1I tang nt at r m t mp ratur .angular fr qu n f the appli d field, (~" hi h th tan ')m L1r • th r laxLltJ'time ~ r the i ni h g n

I ulate r th r lati n:r(o 1 (1

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

where to is the angular velocity, (j) = Znf, f isthe frequency value corresponding tomaximum tan b.

Fig. 5 Loss tangent versus log frequency plot for(lOO-x) Me + x NH4N03 system (x= 5, 10, 15,20,25and 30 wt. %)

The occurrence of relaxation time is theresult of the efforts carried out by ioniccharge carriers within the polymer materialto obey the change in the direction ofapplied field. The shift of (tan b)max towardshigher frequency with increasingconductivity indicates that relaxation timedecrease with conductivity as shown in Fig.6.

1.E-05 1.E-02-..- Conductivity_ Relaxation time

-7 /~E 1.E-06 I \ 1.E-03 ::0(j

I '~

00 I> \ ;-'-' ~>. ~.... .....:E 1.E-07 1.E-04 o·.... ==(j .....= s·-ec

1.E-08 1.E-05 ~0 ,....._u '"'-'

1.E-09 1.E-06o 5 10 15 20 25 30 35Salt concentration (wt. %)

Fig. 6 The dependence of conductivity andrelaxation time on NH4N03 salt concentration atroom temperature

4. CONCLUSIONS

The value of dielectric constant, s, anddielectric loss, G; of (1OO-x) MC + X

NH4N03 (x = 5, 10, 15,20,25 and 30 wt. %)electrolyte films are decreased with

frequency is attributed to the polarizationdue to the charge accumulation. Thepresence of such peaks in M; plots shows theelectrolyte system is ionic conductor. The(tan b)max are shifted from lower to higherfrequency with increasing conductivityindicates that relaxation time decreases withconductivity.

ACKNOWLEDGEMENT

Authors are thankful to University ofMalaya for providing financial supportunder the project scheme PPP(PS223/2008C).

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NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

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