Synthesis and Ultrasonic Characterization of Cu/PVP Nanoparticles-Polymer Suspensions

doi:10.4236/oja.2011.11002

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Open Journal of Acoustics

2011. Vol.1, No.1, 9-14

Synthesis and Ultrasonic Characterization of Cu/PVP

Nanoparticles-Polymer Suspensions

Giridhar Mishra1, Satyendra Kumar Verma1, Devraj Singh2*,

Pramod Kumar Yadawa2, Raja Ram Yadav1

1Department of Physics, University of Allahabad, Allahabad, India;

2Department of Applied Sciences, Amity School of Engineering and Technology, New Delhi, India.

*Email: [email protected]

Received June 2nd, 2011; revised June 19th, 2011; accepted June 24th, 2011.

A polymer colloidal solution having dispersed nanoparticles of Cu metal has been developed using a novel

chemical method. Colloidal solutions of representative concentrations of 0.2 to 2.0 wt% Cu-nanoparticles con-

tents in the primary solutions were prepared to study the modified ultrasonic attenuation and ultrasonic velocity

in polyvinyl pyrrolidone (PVP) polymer molecules on incorporating the Cu-nanoparticles. The synthesized cop-

per metal nanoparticles dispersed in the polymer solutions were characterized by UV-Visible absorption spec-

troscopy, X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The nanofluid sample showed

a symmetrical peak at 592 nm due to the surface plasmon resonance of the copper nanoparticles. XRD results

confirmed that copper nanoparticles were crystalline in the colloidal solution. The TEM micrograph revealed

spherical copper nanoparticles having diameter in the range 10 - 40 nm. A characteristic behaviour of the ultra-

sonic velocity and the attenuation are observed at the particular temperature/particle concentration. It reveals that

the colloidal suspension occurs in divided groups in the small micelles. The results are discussed in correlation

with the thermophysical properties predicting the enhanced thermal conductivity of the samples.

Keywords: Nanofluids, Colloids, Thermal Properties, Nanoparticles, Ultrasonic Properties

Introduction

Nanoparticles with sizes in the nm range are used extensively

(Kota, 2007; Gatos, 2007; Cheng, 2008; Giraldo, 2008; Arribas,

2009; & Singh, 2009). Most of work on nanohybrids deals with

materials in the solid state. However, nanofluids are no less

interesting. Such fluids containing small amounts of metal (Cu,

Ag etc.) or nonmetal (SiC, Al2O3, TiO2, CuO etc.) nanoparticles

exhibit substantially enhanced thermal conductivity compared

to the base fluids. Therefore, nanofluids can be used as heat

transfer fluids; the thermal conductivity of the latter determines

the efficiency of heat exchange systems. Heat transfer fluids

with higher efficiency allow reduction of the sizes of heat ex-

change systems, thus miniaturization of devices. For example, a

small amount ( < 2wt%) of Cu nanoparticles or carbon nano-

tubes dispersed in ethylene glycol or petroleum increases the

inherently poor thermal conductivity of the liquid by 40% and

150% respectively. The thermal conductivity of nanofluid plays

an important role in the development of energy-efficient heat

transfer devices. However, the thermal conductivities of the

working fluids such as ethylene glycol, water, and engine oil

are relatively lower than those of solid particles. Therefore, the

development of advanced heat transfer fluids with higher ther-

mal conductivity is thus in a strong need now a days. The cen-

tury-old technique used to increase cooling rates is to disperse

millimetre or micrometer-sized particles in heat transfer fluids.

The major problem with suspensions containing millimeter or

micrometer-sized particles is the rapid settling of these particles.

Furthermore, such particles are not applicable to microsystems

because they can clog micro channels. Nanomaterials have

unique mechanical, optical electrical, magnetic and thermal

properties. Nanofluids (nanoparticles-fluid suspensions) are

engineered by suspending nanoparticles in traditional heat

transfer fluids such as water, oil, polymers and ethylene glycol.

A very small amount of guest nanoparticles, when dispersed

uniformly and suspended stably in host fluids, can provide

dramatic improvements in the thermal properties of host fluids.

More recently there has been an increasing interest in the

acoustical properties of suspensions for acoustic telemetry

through drilling fluids as well as arising demand for ultrasonic

particle size instrumentation. Commercial instrument have been

developed to characterize the properties of suspensions using

ultrasound (Kytömaa, 1995). Several scientists have made the

study of ultrasonic propagation behaviour through the suspen-

sion of solid particles particularly in micrometer or millimeter

size in a liquid aiming to identify the mechanism that enable

useful information to be extracted from the behaviour of ultra-

sonic properties, such as particle size, concentration and me-

chanical properties of the constituents (Biwa, 2004; Mbhele,

2003; Ensminger, 2005; & Shin, 2004). Polymers have been

found effective stabilizers of colloidal metal nanoparticles.

Recently, polymer nanofluids are the subject of considerable

interest because of the unique properties that can be achieved

with these materials. At the same time, because of their high

surface to volume ratio, nanoparticles suspended polymer ma-

trix significantly are revealing some new properties which are

not present in either of the pure materials. Therefore, the inves-

tigation of the influence of nanoparticles on the properties of a

polymer matrix is necessary in order to be able to better predict

the final properties of the complex fluids. In the present work

we have made the study of the ultrasonic attenuation and veloc-

G. MISHRA ET AL.

ity in a polymer colloidal solution with dispersed Cu-metal

nanoparticles (nanofluid). We prepared stable nanofluids con-

taining copper metal nanoparticles suspended in the base fluid

polyvinyl pyrrolidone (PVP) and measured ultrasonic proper-

ties. Copper nanoparticles are of great interest because of their

use as coolant and its application in heat exchanger. The results

are analyzed and discussed in correlation with the microstruc-

ture and improved thermal properties of the complex nanofluid.

Experimental Details

Synthesis of Polymeric Nanofluids

CuCl2·2H2O and PVP were received from M/s Merck

Chemicals & Reagents. A freshly prepared homogeneous col-

ourless transparent PVP aqueous solution has been used. It was

obtained by dissolving 3.0 g PVP in 100 ml of double distilled

water. A continuous stirring over a magnetic stirrer at a con-

stant temperature of 25˚C - 30˚C promotes the PVP dissolution

in water in a clear solution. 1.0 M aqueous solution of

CuCl2·2H2O has been used to derive the nanofluids of Cu-PVP

having concentrations of Cu contents 0.2, 0.5, 1.0 and 2.0 wt%

in total solution using chemical route. The reactions were car-

ried out at room temperature (25˚C) with constant stirring over

a magnetic stirrer for 5 hours. These solutions were used as

stock solutions to perform the proposed ultrasonic velocity and

ultrasonic attenuation studies in Cu-PVP colloids.

Spectroscopy and Microscopy Me asurements

The absorption spectrum was recorded using a Lambda 35,

Perkin-Elmer double beam UV-visible absorption spectrometer.

A film of the nanofluid was dried on the glass plate for X-ray

diffraction analysis. XRD measurement was done by X’Pert-

Pro, PANalytical (with CuKα radiation λ = 1.5406 Å) operating

at room temperature. The particle size and its distribution were

analysed with E.M.-C.M.-12 (Philips) transmission electron

microscope operating at 200 KeV.

Ultrasonic Velocity and Ultrasonic Attenuation

Measurements

Ultrasonic velocity measurements have been made at 2 MHz of

frequency with help of a variable path interferometer. Water is

circulated around the sample using a specific thermostat. The

measured value of ultrasonic velocity is accurate to  0.1%

with an error of measurement of  0.5˚C in temperature. The

ultrasonic attenuation (α/f2) measurements have been made by a

pulse-echo technique. The measured value of (α/f2) is accurate

to  2%. The standard liquids have been used to check the cali-

bration and accuracy of the measurements. Pulses are sent by a

5 MHz quartz crystal and the decay is observed on the cathode

ray oscillograph. The decay is made exponential by adjusting

the levelling screws and adjusting the crystal and the reflector

parallel to each other. The measured value of the temperature is

accurate to  0.5˚C as in the ultrasonic velocity measurements.

Results and Discussion

Physical Properties of Polymeric Colloids

When adding a CuCl2·2H2O solution (1.0 M) to a PVP solu-

tion (3.0 g/100ml) in water, a polymer complex forms by a

redox reaction of CuCl2·2H2O with the PVP molecules of re-

freshed reactive nascent surfaces caused during the processing

using the mechanochemical stirring under heating conditions of

the solution. The reaction occurs in steps, depending on the

initial concentrations in the two solutions and other experimen-

tal conditions. Ultimately, a colloidal solution consisting of Cu

metal nanoparticles embedded in a moiety of modified PVP

molecules appears in a colloid complex in a characteristic equi-

librium colour.

Figures 1 and 2 show the UV-visible spectrum of the copper

nanoparticles-polymer suspension having concentrations 0.2

and 0.5 wt% Cu nanoparticles in PVP. The UV-visible spec-

trum shows strong absorption peaks at 588 nm and 590 nm due

to the plasmon oscillation modes of conduction electrons in the

colloidal nanoparticles-liquid suspensions. This indicates that

size of Cu nanoparticles increases with increasing the concen-

tration. This prediction is confirmed by the TEM results of the

synthesized samples (Figures 3 - 5). The crystal structure of the

400 500 600 700 800 9001000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Absorbance (a.u.)

Wavelength (nm)

Figure 1.

UV-Visible spectrum of 0.2 wt% Cu nanoparticles-PVP suspension.

400 500 600 700 800 9001000

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

Absorbance (a.u.)

Wavelength (nm)

Figure 2.

UV-Visible spectrum of 0.5 wt% Cu nanoparticles-PVP suspension.

G. MISHRA ET AL. 11

Figure 3.

TEM micrograph of 0.2 wt% Cu nanoparticles-PVP suspension.

Figure 4.

TEM micrograph of 0.5 wt% Cu nanoparticles-PVP suspension.

Figure 5.

TEM micrograph of 2.0 wt% Cu nanoparticles-PVP suspension.

Cu-nanoparticles dispersed in this polymer complex is exam-

ined with X-Ray diffraction. Figure 6 shows the XRD pattern

of the Cu naoparticles dispersed in PVP. All the peaks on the

XRD pattern can be indexed to that of pure Copper metal. The

peaks are corresponding to the 111, 200, 220 and 311 planes

respectively. The average crystalline size of the copper

nanoparticles was calculated to be 20 nm according to the half

width of the 111 diffraction peak using the Debye-Scherrer

formula. The Cu-nanoparticles occur in the usual cubic crystal

structure. TEM images of the Cu nanoparticles-polymer sus-

pensions with corresponding particles size distribution are

shown in Figures 4, 5 and 6 for three different concentrations.

The TEM images reveal that the Cu nanoparticles are spherical

in shape with sizes around 10 nm and clusters with size distri-

bution between 10 - 40 nm. The copper particles are well dis-

persed in colloidal solution as evidenced by TEM micrographs.

The corresponding selected area electron diffraction pattern is

displayed in the Figure 7 showing the crystalline structure of

Cu nanoparticles. It can be indexed to the reflection of

face-cantered cubic structure verifying the results obtained by

XRD pattern.

Figure 6.

XRD pattern of 0.5 wt% Cu nanoparticles-PVP suspension.

Figure 7.

SAED pattern of 0.5 wt% Cu nanoparticles-PVP suspension.

G. MISHRA ET AL.

Ultrasonic Velocity and Ultrasonic Attenuation

As the ultrasonic velocity/attenuation is highly sensitive to the

local structure, we applied it here to examine its value in

Cu-PVP polymer colloids at various temperatures. The results

of the temperature dependent ultrasonic velocity and ultrasonic

attenuation are presented in Figures 7 and 8 respectively.

The results are showing the effect of Cu nanoparticles on the

ultrasonic velocity and attenuation. Figure 8 shows that the

ultrasonic velocity in Cu-PVP increases with temperature and

for higher temperatures it becomes constant. From Figure 9, we

find that maximum value of attenuation (



/f2) is at 25˚C in 0.2

wt% Cu-PVP polymer colloid sample. Also, there are charac-

teristic minima for different concentrations respectively. A

perusal of ultrasonic velocity and attenuation plots reveals that

the temperature dependency of the ultrasonic velocity and the

ultrasonic attenuation does not follow a linear curve. This

seems to be in contrary to the results in a sample of pure Cu

metal. In general, as in other materials (Mougin, 2003), both the

Figure 8.

Temperature dependent ultrasonic velocity in Cu-PVP nanofluids.

Figure 9.

Temperature dependent ultrasonic attenuation in Cu-PVP nanofluids.

ultrasonic velocity and the ultrasonic attenuation are quite sen-

sitive to the particles size, morphology and dispersion of the

particles. A macroscopic interaction of Cu nanoparticles with

the PVP molecules appears to be a critical parameter to control

their final values in this specific example of an inorganic-or-

ganic polymer nanocolloidal solution. The effective attenuation

in this example can be expressed as:

α = αp +αm + αpm (1)

where αp is the contribution from the Cu-metal, αm is the coun-

terpart contribution from the polymer matrix, αpm describes the

change in the final α-value owing to a macroscopic interaction

between the two components in a Cu-PVP nanocolloid struc-

ture and associated modified thermophysical properties of the

nanofluid.

Biwa et al. (Biwa, 2004) analysed the ultrasonic attenuation

in millimeter sized particles-reinforced polymers by a differen-

tial scheme and found good agreement between the theory and

experiments. The wave attenuation in these composites is a

complex process where the viscoelastic loss and the scattering

loss coexist. It is also important to recognize that the relative

contributions of these loss mechanisms may change not only

depending on the acoustic properties of the constituent (matrix

and particles) but also according to the particle size, particle

concentration and the frequency of interest. In the differential

scheme, the changes of macroscopic properties of the complex

due to infinitesimal increase of particles concentration are given

in differential (incremental) forms. Thus the composite with the

particle volume fraction is regarded as a homogeneous effective

medium with the equivalent macroscopic properties given by

the Lame moduli λ and μ as well as the density (ρ). The com-

plex moduli λ and μ of an isotropic viscoelastic medium can be

given in terms of the ultrasonic velocities and ultrasonic at-

tenuation coefficients of longitudinal and shear waves. The

complete description of this theoretical model which we have

tried to apply is given in the literature (Biwa, 2004). The sig-

nificant attenuation due to scattering by the particles-reinforced

was incorporated in the total attenuation in their theoretical

model (Biwa, 2004). The observed attenuation in our case

could not be explained by the exact theoretical model following

the differential scheme. We found that the attenuation due to

scattering from the Cu-nanoparticles in the nanofluid is not

significant. It is also important to note that the characteristic

behaviour of the ultrasonic attenuation in the Cu + PVP

nano-colloids is not found in any of the individual components

of the composite (Abdul, 1982; & Awasthi, 2005). Calculated

value of the ultrasonic attenuation in the sample (0.2wt% of the

Cu nanoparticles) at 30˚C following the differential scheme

including the ultrasonic absorption due to nanoparticles and

thermo-elastic loss following the Mason scheme comes 85.54 ×

10−3 Np/cm. Here the thermal conductivity ‘K’ of the nanopar-

ticles for the calculation of thermo-elastic loss has been taken

following the molecular dynamics method. Here we incorpo-

rated the attenuation due to thermoelastic loss determined by

the formula αTh = ω2 <



> 2KT/(2ρ5

V). Here K is thermal

conductivity, ω(2πf) is frequency of the ultrasonic wave,



is Gruneisen number, T is the temperature in Kelvin scale, ρ is

the density, VL is the ultrasonic wave velocity. Here we have

not incorporated the thermal conductivity of complex thermoe-

lastic medium of our nanofluid as it is not known to us. As the

above formulations attenuation due to thermoelastic mechanism

G. MISHRA ET AL. 13

is directly proportional to the thermal conductivity of the sam-

ples. The total observed attenuation in our experiment for the

sample is 101.76 × 10−3 Np/cm. At this juncture it is interesting

to investigate the source of excess ultrasonic attenuation.

Most importantly scientists have been perplexed by the

thermal phenomena behind the recently discovered nanofluids

like the present samples. One fascinating feature of nanofluids

like copper-ethylene glycol is that they have anomalously high

thermal conductivities at very low nanoparticles concentrations

(Hong, 2006; & Eastman, 2001). To date, the exact mechanism

of thermal transport in nanofluids is not fully known, and sev-

eral possible mechanisms based on theoretical models, experi-

ments and previous heat transfer theory have been suggested to

describe experimental results on thermal conductivity of nan-

ofluids. Brownian motion of suspended nanoparticles is attrib-

uted as one of the key factors of the greatly enhanced thermal

conductivity performance and it was not considered in conven-

tional thermal transport theory. Recently Philip et al. have con-

firmed the anomalous enhancement in the thermal conductivity

of the nanofluids of Au nanoparticles in PVA suspensions by

photoacoustic measurements (The work was presented in the

National Symposium on Ultrasonics, India in 2007). This en-

hancement is seen characteristic in nature at particular tem-

perature and particle concentration of the Au nanoparti-

cles-suspensions. The Brownian motion of nanoparticles at the

molecular and nanoscale level is considered as a key mecha-

nism governing the thermal behaviour of nanoparticles-fluid

suspensions (nanofluids). Eastman et al. proposed the theoreti-

cal model that accounts for the fundamental role of dynamic

nanoparticles in the nanofluids (Eastman, 2001). They have

derived a general expression for the thermal conductivity of

nanofluids involving different modes of energy transport in the

nanofluids. The important mode is thermal interaction of dy-

namic or dancing nanoparticles with base fluid molecules. Even

though the random motion of nanoparticles is zero when time

averaged, the vigorous and relentless interactions between liq-

uid molecules and nanoparticles at the molecular and nanoscale

level translate into conductions at the macroscopic level, be-

cause there is no bulk flow.

In FTIR results of Ag-nanoparticles-PVA, disappearance of

several bands (837, 711, 650 and 570 cm−1) on increase in the

Ag-nano filler content in Ag-PVA suggests that the interaction

between Ag-nano particles and the matrix PVA molecule takes

place (Seok, 2004; Mbhele, 2003; Garcia-Serrano, 2004; &

Khanna, 2005). So on the basis of the FTIR results of Ag-PVA

we may say that there is interaction between Cu nanoparticles

and PVP molecules.

Thus we may postulate that the Brownian motion of the

Cu-nanoparticles in nanofluids produces convection like effects

at the nanoscale. Moreover, the thermal conductivity model not

only captures the concentration and temperature dependent

conductivity, but also predict strongly size-dependent conduc-

tivity. As we know, thermo-elastic ultrasonic attenuation is

directly proportional to the thermal conductivity of the compos-

ite and the attenuation due to scattering for the nanoparticles is

negligible. Therefore, we may predict that the effective in-

creased thermal conductivity of this nanofluid has such an im-

pressive effect as the excess attenuation on the total ultrasonic

attenuation behaviour. Thus, we have developed ultrasonic

mechanism to predict enhanced thermal conductivity due to

suspension of the metallic nanoparticles with very low concen-

tration into polymeric fluid. On the other hand the ultrasonic

velocity may be correlated to the viscoelastic property of the

complex fluid as follows:

If the polymer has sufficient molecular mobility due to

nanoparticles suspension, larger scale rearrangement of the

atoms may also be possible.

Since rate of conformational change α exp (−E*/RT) (Ar-

henius-type expression), where E* is an apparent activation

energy of the process and R is gas constant (Fridley, 1989).

More technically the leathery behavior can be understood as

viscoelastic. Its response is a combination of viscous fluidity

and elastic solidity. The value of Tg is an important descriptor

of fluid thermomechanical response, and is a fundamental

measure of the materials propensity for mobility. The viscoe-

lastic response can be a source of substantial energy dissipation

during the nanoparticles dispersion. On the basis of the above

description correlating the behavior of temperature dependency

of the ultrasonic velocity, we may have the idea of the modified

glass transition temperature of the complex showing viscoelas-

tic behavior of the complex fluid.

Conclusion

 We have successfully synthesized Cu-PVP nanofluids hav-

ing different concentrations of Cu metal nanoparticles in

ambient condition.

 The UV-Visible spectra, XRD, TEM image and SAED

pattern confirm the formation of crystalline Cu nanoparti-

cles dispersed in PVP and also UV-Visible results are con-

sistent with TEM results.

 The Brownian motion of the Cu-nanoparticles in nanofluids

produces convection like effects at the nanoscale.

 On the basis of the behaviour of ultrasonic wave propaga-

tion we have developed ultrasonic mechanism to predict

enhanced thermal conductivity due to suspension of the

metallic nanoparticles with very low concentration into

polymeric fluid.

Acknowledgements

The authors are thankful to the Department of Science and

Technology, Govt. of India (DST project no. SR/S2/CMP-

0069/2006) for the financial support to carryout the work. We

are very grateful to Emeritus Prof. O.N. Srivastava-F.N.A.,

Department of Physics, Banaras Hindu University for his kind

permission for the XRD and TEM measurements in his labora-

tory. We are thankful to Prof. Ram Gopal, Department of

Physics, University of Allahabad for UV-visible facility and Mr.

Dinesh Jaiswal for his technical assistance in BHU, Varanasi.

Authors are also grateful to Dr. A.K. Tiwari (BSNVPGC,

Lucknow), Dr. Priyanka Awasthi (DMO, Lucknow), Dr. D.K.

Pandey (PPNPGC, Kanpur), Dr. Akhilesh Mishra (DST, New

Delhi), Dr. A.K. Gupta (NIOS, Noida), Dr. D.K. Singh (GIC,

Jalaun), Dr. A.K. Yadav (Ambedkar University, Lucknow), Mr.

Meher Waan (AU, Allahabad) and Mrs. Neera Bhutani (ASET)

for many useful helps like reading the manuscript, discussion

and knowledgeable suggestions during the preparation of the

manuscript.

G. MISHRA ET AL.

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