Electrical Properties of Silver Nanoparticles

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been incorporated into a polyvinylidene fluoride-trifluoroethylene ... amount of lithium tantalite [(LiTaO3), LT] powder and nano silver particles are then added to ...

Electrical Properties of Silver Nanoparticles Reinforced LiTaO3:P(VDF-TrFE) Composite Films A. K. Batra, John Corda, Padmaja Guggilla, M. D. Aggarwal and M. E. Edwards Department of Physics, PO Box 1268, Alabama A&M University, Normal, AL 35762 ABSTRACT Pyroelectric infrared Lithium tantalite [(LiTaO3), LT] ceramic particles and silver nanoparticles have been incorporated into a polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE) 70/30 mol%] copolymer matrix to form composite films. The films were prepared using solvent casting method. Electrical properties such as the dielectric constant, dielectric loss, and pyroelectric coefficient have been measured as a function of temperature. In addition, materials’ figures-of-merit have also been calculated to assess their use in infrared detectors. The results show that the fabricated silver nanoparticles incorporated lithium tantalite: polyvinylidene fluoride-trifluoroethylene composite films may have a good potential for uncooled infrared sensor applications. Key words: Pyroelectric composite, Infrared detectors, Lithium tantalite, Polyvinylidene fluoridetrifluoroethylene, Ag nanoparticles

1. INTRODUCTION A pyroelectric detector exposed to infrared radiation absorbs the radiation and its temperature rises. The rise in the temperature changes the spontaneous polarization, and thus photocurrent is obtained. Infrared imaging devices have numerous applications, including military night vision, security surveillance, fire detection, medical diagnostics, and automotive vision enhancement, imaging systems for cars, ships, aircraft, and others. Pyroelectric infrared sensing devices have several advantages over photon infrared sensors: i.) Sensitivity over a larger spectral bandwidth; ii.) Sensitivity over a wide temperature range without the need of cooling; iii.) Low power requirements; iv.) Relatively fast response; v.) Generally low cost of materials; vi.) Detection is limited, being a passive device; vii.) Temperature range of operation can be changed in certain materials by the variation of the amount of its constituents (such as Lead zirconate titanate, Potassium titanate niobate and others); and viii) Suitable for space applications because of light weight; consuming less power having no bulky cooling equipment. Ferroelectric ceramic and polymer composites are a well-established alternative to conventional ferroelectrics for sensors and actuator applications because they combine the mechanical compliance and flexibility of polymer with the high piezoelectric and pyroelectric activities of electro-ceramic [1-3]. In the present work, Lithium tantalite [(LiTaO3), LT] ceramic particles have been incorporated into a polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE) 70/30 mol%] copolymer matrix to form 0-3 composite films. Both of these materials have excellent ferroelectric and pyroelectric properties. LT having low relative permittivity has been chosen for increasing the pyro-activity and performance of infrared detectors as per findings of triglycine sulfate (TGS) composites [4]. However, pyroelectric particles in the 0-3 composites can not be fully poled due to the screening effect of the polymer matrix i.e. important properties of the composite are reduced. Sakamoto et al [5] indicated that PZT/PU composite doped with graphite particles improved the poling behavior of PZT phase and in turn improved pyroelectric and piezoelectric properties. Oltean et al [6] studied some electrical properties of metallic iron reinforced polymeric composite materials subjected to stress state: electric field, temperature variation and mechanical load.

Infrared Systems and Photoelectronic Technology IV, edited by Eustace L. Dereniak, John P. Hartke, Paul D. LeVan, Randolph E. Longshore, Ashok K. Sood, Proc. of SPIE Vol. 7419, 741904 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.824384 Proc. of SPIE Vol. 7419 741904-1

In the present work, silver nano-particles were embedded in LT:P(VDF-TrFE) composite. The dielectric and pyroelectric properties of these resultant composites were measured. To assess their use as pyroelectric infrared detectors, various figures-of-merit of composite films have been calculated and compared with pure P(VDF-TrFE) film fabricated at authors’ laboratory.

2. EXPERIMENTAL The 0-3 connectivity composite films were fabricated using solution-cast technique. The first step in the preparation of P(VDF-TrFE): LiTaO3 (LT) composite is, dissolving a suitable amount of polymer, P(VDF-TrFE) [named PVTF] in methyl-ethyl-ketone (MEK) to form a solution (PMix). A requisite amount of lithium tantalite [(LiTaO3), LT] powder and nano silver particles are then added to form nPMix and this mixture is ultrasonically agitated for several hours to break-up the agglomerates and disperse the ceramic powder uniformly in the copolymer solution. The obtained nPMix composite solution is kept in a suitable container for solvent to evaporate. The film is then annealed for 2-3 hours in air, at 130 ºC for the present case. Full face electrodes are deposited of the films for testing. The electroded samples were poled at 100 0C with 50 kV/cm voltages for 2 hours. After the poling process the samples were short circuited and annealed at 50 0C for 2 hours. The volume fraction (φ) of LT powder in co-polymer matrix chosen for the present work was 2% (sample named PLT1). The amount of silver nano particles added remains unknown because the particles were in the organic solution (sample named PLT1Ag). The real (ε') and imaginary (ε'') parts of dielectric constant were calculated by measuring capacitance and dielectric loss. The DC conductivity was measured by usual method using 610 Keithley electrometer. To measure the dynamic pyroelectric current, direct method of Byer and Roundy [7] was used. The detailed measurement procedures of these parameters are described in our earlier publication [8]. The pyroelectric current Ip was measured, and the pyroelectric coefficient (p) was calculated using a relationship: p=(

Ip A

)/(

dT ), dt

where A is the electrode area (identical areas for the opposite electrodes used in each sample), and dT is dt

the rate of change of temperature which was kept constant through out the measurement. To provide the measure of the efficiency of a given material for pyroelectric applications, the figures-of-merit are defined. Using the foregoing electrical parameters, the material figure-of-merits for assessing the characteristics of single element pyroelectric detector, operating in optimum manner, were calculated as follows: [9-10] FI = p for high current responsivity, Fv = p / ε′ for high voltage responsivity, and FD = p / √ε′′ for high detectivity, where p is the pyroelectric coefficient.

3. RESULTS AND DISCUSSIONS Figure 1 and Figure 2 show the temperature dependence of the dielectric constants: ε' and ε'' of copolymer and composite films respectively. As expected in ferroelectrics, these parameters increase with increase in temperature. The variation of pyroelectric coefficient of co-polymer and composite films is depicted in Figure 3. The pyroelectric performance figures-of-merit (FI, FV and FD) increase with the volume fraction of LT (PLT1) and with addition of silver nanoparticles (PLT1Ag) in the polymer matrix

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(Figure 4 and Figure 5). The properties of co-polymer and composite films are summarized in table 1. Preliminary data on Figures-of-Merit of composite films (PLT1 and PLT1Ag) are higher than P (VDFTrFE) film poled under similar conditions. A theoretical model has been developed which successfully explained the effect of the matrix’s electrical conductivities on the electrical properties and poling behaviors of the 0-3 ferroelectric composites [1112]. According to Wong’s model [11], the increment of conductivity of matrix enhances the poling degree of the ceramic and hence the pyroelectric and piezoelectric properties are enhanced. The increment of the matrix’s electrical conductivity will increase the leakage current and the dielectric loss (ε''), which will be harmful to the poling process. Our data are in agreement with the expected results [11].

4. CONCLUSIONS The experimental results obtained of composite films (PVTF, PLT1, PLT1Ag) can be summarized as follows: (i)

Films of LT:P(VDF-TrFE) composites with 2% volume fraction of LT powder and with Ag nanoparticles particles have been fabricated using ‘solution cast’ method. This technique is a very useful and inexpensive technique for manufacturing composite pyroelectric sensors because composite films can be fabricated with less energy, time and effort as compared to ceramic and single crystal fabrication.

(ii)

The composites were characterized for their dielectric and pyroelectric properties, in order to determine their usefulness in uncooled infrared detector applications.

(iii)

The calculated pyroelectric figures-of-merit (FI, FV, FD) of the composite films were found to be higher than pure P(VDF-TrFE) films.

(iv)

Based on the preliminary results obtained, LT:P(VDF-TrFE) and LT:P(VDF-TrFE)+Ag films are attractive for use in infrared sensing elements especially where low level applications and curved surface detectors are needed. Further work is in progress to ascertain the mechanisms for enhancement of dielectric and pyroelectric properties of PLT1Ag films.

ACKNOWLEDGEMENTS The financial support for this work through NSF-RISE grant # HRD-0531183 is gratefully acknowledged. Also the work was partially supported by NSF subcontract # 083052T6181283.

REFERENCES [1] [2] 4448. [3] [4] [5]

K. H. Lam and H.L.W. Chan, Composite Science and Technology, 65 (2005) 1107. J. Kulek, I. Szafraniak, B. Hilczer and M. Polomska, J. Non-Crystalline Solids, 353 (2007) Y. Feng-Xia, Z. Duan-Ming, D. Zhong-Wei, C. Zhi-Yuan and J. Sheng-Lin, J. Phys D: Appl. Phys. 41 ( 2008) 055408. Y. Yang, H. L. W. Chan and C. L. Choy, J. Mat. Sci.41 ( 2006) 251. W. K. Sakamoto, P. Marin-Franch and D. K. Das. Gupta, Sensors and Actuators, A, 100 (2002) 165.

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[6] [7] [8] [9] [10] [11] [12]

D. I. Oltean, D. Luca motoc and D. Rosu, J. opt. and advn. Mat. 10 ( 2008) 3328. R. L. Byer and C. B. Roundy,. Ferroelectrics, 3 (1972)333. A. K. Batra, M. Simmons, P. Guggilla, M. D. Aggarwal and R. B. Lal , Integrated Ferroelectrics, 63 (2004) 161. R. B. Lal, and A. K. Batra, Ferroelectrics, 142 (1993) 51. Sidney B. Lang and Dilip K. Das-Gupta, Ferroelectrics Review, 2 (2000) 217. C. K. Wong and F. G. Shin, J. Appl. Phys., 97 (2005) 064111. D. M. Zhang, N. Wei, F. X. Yang, X. Y. Han, Z. C. Zhong and K. Y. Zhen, J. Phys. D: Appl. Phys., 39 (2006) 1963.

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60

PVTF 50

PLT1 P LT1Ag

ε'

40

30

20

10

0 0

10

20

30

40

50

60

70

80

T emp er at ur e( 0 C )

Figure 1: Temperature Dependence of ε' for PVTF, PLT1 and PLT1Ag films at 1kHz measured upon heating 50 PVTF PLT1

40

P LT1Ag

ε''

30

20

10

0 0

10

20

30

40

50

60

70

80

Tem perature( 0C)

Figure 2: Temperature Dependence of ε'' for PVTF, PLT1 and PLT1Ag films at 1kHz measured upon heating

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Pyroelectric Coefficient(nC cm -² 0C-1)

290 PVTF 240

PLT1 P LT1Ag

190 140 90 40 -10 0

10

20

30

40

50

60

70

80

Tem perature( C) 0

Figure 3: Temperature Dependence of the Pyroelectric Coefficient (p) of PVTF, PLT1 and PLT1Ag films 6 PVTF

5

PLT1 PLT1Ag

FV (nC cm -2 0C-1)

4 3 2 1 0 -1 0

10

20

30

40

50

60

70

80

Tem perature( C) 0

Figure 4 Temperature Dependence of Figure-of-Merit, FV , for PVTF, PLT1 and PLT1Ag films

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50

PVTF 40

PLT1 PLT1Ag

MD (nC cm

-2 0

-1

C )

30

20

10

0

-10 0

10

20

30

40

50

60

70

80

0

Temperature( C)

Figure 5 Temperature Dependence of Figure-of-Merit, MD , for PVTF, PLT1 and PLT1Ag films

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Table 1 Physical Properties and Figures-of-Merit of Films Investigated

At 60(0C)

Parameter

Units

p

At 70(0C)

PVTF

PLT1

PLT1Ag

PVTF

PLT1

PLT1 Ag

nCcm-2 0C-1

0.28

0.89

44.9

0.637

5.52

132

ε'

--

17.58

26.98

40.99

18.97

29.25

48.28

ε"

--

2.21

2.29

29

3.35

3.47

40.73

FI

nCcm-2 0C-1

0.28

0.89

44.9

0.64

5.52

132

FV

nCcm-2 0C-1

0.02

0.03

1.09

0.03

0.19

2.73

FD

nCcm-2 0C-1

0.19

0.59

8.34

0.35

2.96

20.68

PLT1 composite: 2 % volume fraction of lithium tantalite particles in P (VDF-TrFE) matrix PLT1Ag composite: 2 % volume fraction of lithium tantalite particles in P (VDF-TrFE) matrix + silver nanoparticles

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