Arab J Sci Eng (2012) 37:1491–1498DOI 10.1007/s13369-012-0239-9

RESEARCH ARTICLE - PHYSICS

Kh. S. Karimov · M. Abid · M. Saleem · M. Farooq ·Z. M. Karieva · Adam Khan

Cu2O–PEPC Composite Based Pressure Transducer

Received: 31 January 2011 / Accepted: 11 June 2011 / Published online: 21 April 2012© King Fahd University of Petroleum and Minerals 2012

Abstract In this study, Al/Cu2O–PEPC/Al pressure transducer was designed and fabricated by drop-castingthe blend of Cu2O–PEPC microcomposite thin films of copper oxide (Cu2O) micropowder, (3 wt.%) and poly-N -epoxypropylcarbazole, PEPC, (2 wt.%) in benzol on Al substrates. The thicknesses of the Cu2O–PEPCfilms were in the range of 30–100 µm. The AC resistance and capacitance at 120 Hz of the transducer wasdecreased by 1.1–1.4 and increased by 1.2–1.8 times, respectively, as the pressure was increased up to 100 kPa.The resistance-pressure and capacitance-pressure relationships were simulated.

Keywords Copper oxide · Poly-N -epoxypropylcarbazole · Composite · Micropowder · Pressure transducer ·Resistance · Capacitance

Kh. S. Karimov · M. Abid · M. Farooq · A. KhanGIK Institute of Engineering Sciences and Technology,Topi, Swabi 23640, Pakistan

Kh. S. KarimovPhysical Technical Institute of Academy of Sciences, Rudaki Ave. 33,Dushanbe 734025, Tajikistan

M. Saleem (B)Department of Physics, Government College of Science,Wahdat Road, Lahore 54570, PakistanE-mail: msaleem108@hotmail.com; drsaleem@ymail.comURL: http://www.gcslahore.edu.pk

Z. M. KarievaTajik Technical University,Rajabov St. 10, Dushanbe 734000, Tajikistan

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1 Introduction

Pressure transducers are mostly used with resistance, capacitance, inductance and piezoelectric sensors andwith devices such as diaphragm and bellows [1,2]. The network of pressure sensors with organic transistorsbased on pentacene was fabricated [3]. The drain-source current was increased from 15 nA to 6.7 µA underthe applied pressure of 30 kPa. Unusual electromechanical effects (piezoelectricity and electrostriction) wereobserved in organic semiconductor Schottky junctions due to a presence of non-uniform spatial electric fielddistribution in the junction and softness of organic semiconductors. These effects can be potentially used forthe fabrication of electromechanical transducers [4].

Complexes of poly-N -epoxypropylcarbazole (PEPC) are known as photosensitive organic semiconductors,having good adhesive properties and are used for the fabrication of solar cells and photocapacitors [5]. Copperoxide (Cu2O) is a photosensitive p-type semiconductor with a band gap of 2.0 eV [6,7]. Cu2O is nontoxicand its fabrication process is simple, due to an abundance of copper in nature. Fabrication of nanodots andnanostructure thin films of Cu2O are reported in [8,9]. A number of resistance strain gauges have been fabri-cated and investigated [10] on the basis of low molecular organic semiconductors tetracyanoquinodimethane(TCNQ) ion-radical salts crystals. High sensitive resistance strain gauges based on PEPC were fabricated andinvestigated as well [11].

Fabrication of the pressure transducers and investigations of squeezing effect to the copper oxide and PEPCcomposites would be useful from practical point of view and for deepening of the knowledge about the physicalproperties of the composites. It would be reasonable to investigate the resistance-pressure and capacitance-pressure relationships. In this paper we have designed, fabricated and investigated the sandwich-type pressuretransducers based on resistance and capacitance Cu2O–PEPC composite sensors.

2 Experimental Procedure

Molecular structure of the PEPC is presented in Fig. 1. The PEPC was synthesized in the laboratory [11] andCu2O micropowder was purchased from WINLAB, UK, and used without further purification. The aluminumsubstrates of thickness 2 mm were initially cleaned ultrasonically by acetone for 10 min, followed by a plasmacleaning for 5 min in vacuum chamber. The blend of copper oxide micropowder, (3 wt.%) and poly-N -epoxy-propylcarbazole, (2 wt.%) in benzol were drop-casted on the aluminum substrates to fabricate Cu2O–PEPCmicrocomposite thin films. The sizes of Cu2O microparticles were in the range 3–4 µm. Thicknesses of theCu2O–PEPC films were in the range 30–100 µm.

Figure 2 shows optical images of Cu2O–PEPC film obtained by optical microscope Leica DM 6000 M.It is seen that the structure of the film is not uniform and contains clusters of particles. Figure 3 shows SEMmicrographs of the Cu2O–PEPC film obtained by JEOL JSM-6460 at different magnifications. On the top ofCu2O–PEPC film, a conductive film of aluminum was deposited by thermal evaporation. As a top electrode,thin aluminum foil of thickness of 40 µm and size of 5 mm × 5 mm was used to make the sandwich-typeAl/Cu2O–PEPC/Al resistance and capacitance pressure transducer as shown in Fig. 4. Experimental setupused for the investigation of the pressure transducer’s properties is shown in Fig. 5. The setup consists ofsupport (1), weight holder (2) with weights (3). The pressure transducer consists of a metallic squeezing disk(4) of diameter 8 mm and elastic rubber film (5) of 0.5 mm thickness, aluminum foil (6), aluminum film (7),Cu2O–PEPC composite film (8), aluminum substrate (9) and terminals (10, 11). The value of the pressure

N

CH=CH-CH2O-

n

Fig. 1 Molecular structure of poly-N -epoxypropylcarbazole (PEPC)

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Fig. 2 Optical microscope image of the Cu2O–PEPC composite film

Fig. 3 SEM images of the Cu2O–PEPC composite film at different magnifications

Al substrate

Al foil

Pressure

Terminals

Metallic squeezing disk

Elastic rubber film

Film

Aluminum film

Fig. 4 Schematic design of the Al/Cu2O–PEPC/Al pressure transducer

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1

2

3

4

5

6

7

8

9 10

11

Fig. 5 Experimental setup for the investigation of pressure transducer’s properties with installed pressure transducer: support (1),weight holder (2) with weights (3), pressure transducer: metallic squeezing disk (4), elastic rubber film (5), aluminum foil (6),aluminum film (7), Cu2O–PEPC composite film (8), aluminum substrate (9) and terminals (10, 11)

was changed by changing weights, in the conventional laboratory setup “Flexor: Cantilever Flexure Frame”.The AC dissipation (D) and capacitance of the sensor at 120 Hz was measured by FLUKE 87 true root meansquare (rms) multimeter and ESCORT ELC-132 Ammeter. The resistance (R) of the samples was determinedfrom the values of the dissipation using the following expression [12]:

R = 1

2π f C D(1)

where f is the frequency and C is capacitance.

3 Results and Discussion

Figure 6 shows the relative resistance-pressure and capacitance-pressure relationships for the Al/Cu2O–PEPC/Al transducers. The AC resistance and capacitance at 120 Hz of the transducer were decreased by1.1–1.4 and increased by 1.2–1.8 times respectively as the pressure was increased upto 100 kPa. The effectsof pressure to the resistances and capacitances were more significant at thinner Cu2O–PEPC films (∼30 µm)and less at thicker films (∼100 µm). The transducer’s resistance (R) is calculated by [13]:

R = dρ

A= d

σ A(2)

where d is the length or inter-electrode distance and A is the cross-section of the aluminum foil electrode, andρ is the resistivity (ρ = 1

σ, where σ is the conductivity).

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0 20 40 60 80 100 120

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Rel

ativ

e ca

paci

tanc

e/re

sist

ance

Pressure (kPa)

C/Co

R/Ro

Fig. 6 Relative resistance-pressure and relative capacitance-pressure relationships of the Al/Cu2O–PEPC/Al transducer

As Cu2O–PEPC is a microcomposite (Figs. 2, 3), the resistance-pressure relationship may be due to thedecrease in thickness (d) of the film by squeezing the disk or by increase of the conductivity (σ ) of the com-posite due to the decrease of the distances between the Cu2O particles in the PEPC matrix (Eq. 2) (Figs. 5, 6).Actually, resistance can be changed (Eq. 2), firstly, due to change of geometrical parameters as the length ofinterelectrode distance and cross-section area of the sample. Secondly, resistance can be changed by changingresistivity, i.e. intrinsic properties of material. In this case, the resistivity can be changed mostly due to thechange of concentration of charges. As change of resistance due to change of geometrical parameters is muchless (around of few %) than due to change of resistivity, we can assume that in the case of the investigatedsamples, the effect of resistivity change is larger, as total change of the resistance is large (up to 30 %). Usingan exponential function given in Eq. 3, Eq. 2 can be represented as given in Eq. 4 [14]:

f (x) = e−x (3)

R

Ro= e−pK1 (4)

where p is pressure, K1 is the resistance pressure factor. From the experimental data shown in Fig. 6, thepressure factor K1 = 0.00357 1/kPa for p = 100 kPa was determined. Experimental and simulated (usingEq. 4) results are plotted in Fig. 7 and are observed to be in good agreement.

The composite consists of Cu2O and PEPC. The value of the conductivity of the PEPC (4×10−9 cm−1 �−1

[11]) is less than the conductivity of Cu2O (4×10−4 cm−1 �−1 [6]). Therefore, the conductivity of the compos-ite (0.3×10−10 cm−1 �−1) is controlled by PEPC. Taking into account that the conductivity of the compositeis less than the conductivity of the PEPC and Cu2O, it can be assumed that in the interface of the PEPC–Cu2Oparticles junctions, there are depleted regions. Therefore, the PEPC–Cu2O system can be considered as abulk heterojunction system. This may be one of the reasons that why the composite shows high sensitivity tocompression, as observed from the resistance-pressure relationships (Fig. 6).

The mechanism of conductivity in PEPC can be considered as thermally assisted hopping transitionsbetween spatially separated sites, molecules or particles that can be attributed to the Percolation Theory[15,16]. The average conductivity (σ ) of one component (in this case PEPC), according to Percolation Theory,can be calculated as:

σ = 1

L Z(5)

where L is a characteristic length, depending on the concentration of the sites, Z is the resistance of the pathwith the lowest average resistance. With an increase in the pressure, the composite film will be squeezed

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0 20 40 60 80 100

0.72

0.80

0.88

0.96

1.04

R/R

o

Pressure (kPa)

Fig. 7 Experimental (solid line) and simulated (dashed line) relative resistance-pressure relationships of the Al/Cu2O–PEPC/Altransducer

between aluminum substrate and aluminum foil (Figs. 4, 5) that may cause, firstly, the decrease of L and, sec-ondly reduce Z . As a result, the conductivity increases and the resistance of the sample decreases accordinglyas is observed experimentally (Fig. 6).

Figure 6 shows that the relative capacitance of the Al/Cu2O–PEPC/Al transducer is changed more than thecorresponding relative resistance with pressure. At room temperature and at zero pressure, AC capacitance at120 Hz of the Al/Cu2O–PEPC/Al transducer was in the range of 0.015–0.030 nF. The sensor’s capacitance(C) is calculated as [17]:

C = εεoA

d(6)

where A is the area of the plates of the capacitor, ε is the relative permittivity, ε◦ is the permittivity of the freespace and d is the distance between plates of the capacitor. The capacitance-pressure relationship shown inFig. 6, may be due to the decrease of the thickness (d) of the film under pressure or/and the increase of therelative permittivity of the composite due to the decrease of the distances between the Cu2O particles in thePEPC matrix (Eq. 5) and by the change in the polarizability of the composite. We consider that the contributionof the second process is dominating in the change of capacitance of the sensor due to the change in pressure.

For the simulation of the relative capacitance-pressure relationships (Fig. 6), the following phenomeno-logical expression [18] can be used:

C

Co=

(εs

ε

)n(7)

where ε and εs are the permittivity coefficients of the composite at normal and squeezed states, respectively,Co and C are the corresponding capacitances. The factor n is related to the morphology of the dielectric. TheEq. 5 can be represented as:

C

Co=

(εs

ε

)n = (1 + K2 p)n (8)

where

εs = ε(1 + K2 p) (9)

K2 is the capacitance pressure factor: it is equal to 0.0224 1/kPa at p = 100 kPa for the curve in Fig. 6 showingthe largest capacitance-pressure response.

Figure 8 shows experimental and simulated (using Eq. 8) relationships of the relative capacitance versespressure for the Al/Cu2O–PEPC/Al transducer at n = 0.5. Simulated and experimental results are observed in

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0 20 40 60 80 100

1.05

1.20

1.35

1.50

1.65

1.80

C/C

o

Pressure (kPa)

Fig. 8 Experimental (solid line) and simulated (dashed line) relative capacitance-pressure relationships for the Al/Cu2O–PEPC/Altransducer

good agreement. As the experimental relative resistance-pressure and the relative capacitance-pressure (Fig. 6)relationships for the Al/Cu2O–PEPC/Al transducer are slightly non-linear and can be linearized by nonlinearop-amps [19].

4 Conclusion

The sandwich-type Al/Cu2O–PEPC/Al pressure transducer was designed, fabricated and investigated. Resis-tance of the transducer decreases and capacitance increases as the pressure is increased. The resistance-pressureand capacitance-pressure relationships were simulated. For the explanation of the conduction mechanism, thePercolation Theory is used. The PEPC–Cu2O system is assumed as a bulk heterojunction system that resultsto high sensitivity of the composite due to squeezing effect.

Acknowledgments The authors acknowledge the enabling role of Higher Education Commission (HEC) Pakistan and appreciatethe financial support through indigenous PhD Fellowship Program. The authors are also thankful to GIK Institute of EngineeringSciences and Technology for its support.

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