Materials Science and Engineering B 151 (2008) 140–145

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

osi

J. Dom

te foositios wi

zingnd tein theV/mme attaelopeower

Spray deposition of polymer nanocomp

Xin Zhao ∗, Claire Hinchliffe1, Colin Johnston, PeterDepartment of Materials, Oxford University, Parks Road, Oxford OX1 3PH, United Kingd

a r t i c l e i n f o

Article history:Received 3 March 2008Received in revised form 21 May 2008Accepted 27 May 2008

Keywords:Spray depositionNanocompositeDielectricBarium titanatePolymer films

a b s t r a c t

A scalable processing roudeveloped by spray depPolydiacrylate–BaTiO3 filmwere achieved by optimisubstrate traverse speed ananoparticles embeddeddown strengths of >100 kstability up to 150 ◦C werdeposition technique devnanocomposite films for p

1. Introduction

Polymer–ceramic nanocomposites have attracted interest fordielectric applications because they may combine the high dielec-tric constants of ceramic powders, such as ferroelectrics, with the

low temperature processability, low dielectric loss and high dielec-tric strength of polymer matrices. Potential applications includemicro and power capacitors, embedded dielectrics and energy stor-age [1–3].

In the last few years, the need for integrated, passive electri-cal components has spurred the development of polymer–ceramicnanocomposites for embedded capacitors. Most research has con-centrated on increasing the overall dielectric constant of thenanocomposites, and so increasing the energy density. By incor-porating ferroelectric ceramic nanoparticles at 80–90 vol% intopolymer matrices, dielectric constants of above 100 have beenreached [4–6]. However, at such high ceramic filler loadings, aninhomogeneous particle dispersion, poor adhesion and residualporosity tend to have a detrimental effect on other dielectricproperties such as loss and breakdown voltage, as well as themechanical and thermal stability of the nanocomposite films[7]. Processing and metallization of the nanocomposite filmswith areas (large as possible) and thicknesses (small as possible)for meaningful capacitor applications may also be problem-atic. Although relatively high filler fractions might be attractive

∗ Corresponding author. Tel.: +44 1865 283707; fax: +44 1865 848785.E-mail address: xin.zhao@materials.ox.ac.uk (X. Zhao).

1 Now at UK Engineering and Physical Sciences Research Council.

0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.mseb.2008.05.024

te films for dielectric applications

obson, Patrick S. Grant

r fabrication of polymer-based dielectric nanocomposite films has beenn and in situ polymerization of monomer–nanoparticle dispersions.

th filler loadings of 10 vol% to 30 vol% and controlled thickness of 5–30 �mspray conditions of spray liquid flow rate, atomizing gas pressure andmperature. Microstructural investigations showed well-dispersed BaTiO3

polydiacrylate matrix. Dielectric constants of ∼ 39 at 10 kHz, high break-and low leakage currents of 10−9 to 10−8 A/cm2 above 1000 V, and thermalined for PTPGDA–30 vol%BaTiO3 films. The low-cost, atmospheric sprayd in this work has potential for larger-scale manufacture of dielectriccapacitor and other applications.

© 2008 Elsevier B.V. All rights reserved.

for low voltage, low temperature embedded capacitor applica-tions in microcircuitry, they are unsuitable in power capacitorsoperating at higher voltage, frequencies and temperatures. Theoptimum filler volume fraction for power capacitor applicationsis generally accepted to be approximately 0.35, while the cor-responding dielectric constant at this loading rarely exceeds 30[7–9].

Spin coating and hot molding are the most convenient andextensively used technologies for processing of polymer–ceramic

nanocomposite films in the microelectronic industry. However,these techniques are restricted by the non-uniform thickness con-trol of the films and other practical limitations when hundreds ofm2 of continuous film are required. A high volume and low costprocessing technology, i.e. out of vacuum, is desirable for manu-facturing composite films with large area for power capacitor andother applications, while maintaining a homogeneous dispersionof ceramic nanoparticles.

In this work, an atmospheric spray deposition route has beendeveloped for the preparation of polymer–ceramic nanocompos-ite films with a thickness of 5–30 �m based on a diacrylatemonomer and barium titanate (BaTiO3) nanoparticle dispersionas a composite film precursor. While relatively small area filmsare reported here, few practical problems to process scale-upare envisaged. The spray deposited monomer films containingnanoparticles were polymerized by electron beam and thermaltreatments, and the resulting polydiacrylate films filled with well-dispersed BaTiO3 nanoparticles investigated in terms of micro/nanostructure, and dielectric performance (constant, loss and strength)as a function of frequency, temperature and volume fraction ofBaTiO3.

and En

voltage level, the current was allowed to settle for 2 min beforetaking the reading.

3. Results and discussion

3.1. Polymerization of TPGDA

The chemical structure of the TPGDA precursor monomer isshown in Fig. 2. The molecule contains two polymerizable acrylicgroups, is highly reactive and can be readily polymerized by theusual bulk, suspension and emulsion techniques. Moreover, a highoptical transparency along with a low viscosity and low vapourpressure make it an ideal cross-linking component and reactivethinner for radiation curing in the surface coating industry.

FTIR analysis was used to study the polymerization process of

X. Zhao et al. / Materials Science

Fig. 1. Schematic of spray deposition process.

2. Experimental

2.1. Thermal polymerized polydiacrylate–BaTiO3 composite films

As-received 120 nm BaTiO3 (K-Plus 8, Cabot Inc.) powderswere dispersed in 66:34 vol% 2-butanone:ethanol (MEK:EtOH)azeotropic under stirring, and then sonicated for 0.5–1 h. The sus-pensions were mixed with tripropylene glycol diacrylate (TPGDA,Surface Specialties Inc., ≥99%) monomer and sonicated for afurther 3–5 h to form stable dispersions. 1% mass fraction of 1,1′-azobis(cyclohexane-carbonitrile) (ACN, Aldrich, 98%) was chosenas radical initiator and added to the dispersions, and sonicated foranother 15 min. Dispersions were then spray deposited using theapparatus shown schematically in Fig. 1.

A high accuracy controlled flow rate syringe pump deliveredthe TPGDA/BaTiO3/MEK:EtOH suspension to a hypodermic needlewith an internal diameter of 0.6 mm at flow rates in the range2–5 ml/min. The point of the needle was placed at the end ofan atomizing nozzle operating with cleaned compressed air at∼200 kPa. The position of the needle in the gas flow was criticaland optimized using digital high speed video in order to obtain asymmetric and stable atomization regime at the needle tip. The dis-persion was atomized continuously into droplets with diameters of≤0.1 mm and velocities of ≥10 m/s. The TPGDA–BaTiO3 dispersionswere sprayed onto glass slides previously coated with 100 nm of Al

by thermal evaporation to act as an electrode. The glass slide sat ona pre-heated table (75 ◦C) that moved in the x–y plane according toa pre-programmed path at 50 mm/s. The atomizing gas pressure,the spray distance, the dispersion flow rate, and the traverse hotplate speed were optimized to ensure that the droplet spray wasstable and coated evenly onto the substrate. By controlling theseparameters, as well as the number of coated layers and the concen-tration of the MEK/EtOH solvent, the BaTiO3 fractions and thicknessof the deposited films could be adjusted independently, and filmswith thicknesses in the range 5–30 �m were produced.

The as-deposited films were heated from 30 to 90 ◦C at a heat-ing rate of 1 ◦C/min under nitrogen atmosphere and then held for10 h. Another layer of Al was then evaporated onto the top of thefilms through a mask arrangement to form a simple parallel platecapacitor.

2.2. Electron beam polymerized polydiacrylate–BaTiO3 compositefilms

Suspensions containing BaTiO3 nanopowders, TPGDA and sol-vent were prepared as described above but without adding initiator,

gineering B 151 (2008) 140–145 141

and the suspensions were spray deposited following the sameroute. The as-deposited films were dried at 90 ◦C for 0.5 h to removeany remaining solvent, and then polymerized by irradiation withan electron beam at 360 V under vacuum. Since the energy of thiselectron beam was relatively low, a thermal baking at 125 ◦C in airfor 5 h was used to ensure full polymerization of TPGDA. As before,another layer of Al was then deposited to act as the top electrode.

2.3. Characterization of polydiacrylate–BaTiO3 composite films

The polymerization behaviour of the nanocomposite films wasinvestigated using a Varian Excalibur Fourier-Transform Infrared(FTIR) spectrometer and UMA 600 FTIR microscope. Complemen-tary thermal measurements were performed using a PerkinElmerDiamond differential scanning calorimeter (DSC) and PerkinElmerPyris 1 thermogravimetric analyzer (TGA) at a heating rate of10 ◦C/min under nitrogen atmosphere and air atmosphere respec-tively. The surface morphologies and particle distributions of thenanocomposite films were observed by a confocal profilome-ter NanoFocus �Surf and field emission gun scanning electronmicroscopy (FEG-SEM, JEOL 6500 F). Thicknesses of the filmswere determined by a Veeco Dektak IIA surface profilometer. Theac dielectric constant and dissipation factor of the films weremeasured using a Solartron SI 1260 impedance analyzer in the fre-quency range 10 Hz to 1 MHz and the temperature range 25–150 ◦C.The leakage current measurements were performed using a Bran-denburg photomultiplier power supply model 472 R by sweepingthe applied voltage from 50 up to 1300 V, and the resulting leakagecurrent was monitored by a Keithley 414 A picoammeter. At each

the spray deposited TPGDA monomer. Fig. 3 shows the FTIR spectrafrom the TPGDA monomer and the as-deposited and polymerizedTPGDA–BaTiO3 films. The absorption band of the CH CH2 twistingvibration at 810 cm−1 is commonly used to follow the conversionof the TPGDA monomer, as shown in spectra (a) and (b). In spectra(c), this characteristic peak disappeared after the electron beam

Fig. 2. Chemical structure of tripropylene glycol diacrylate (TPGDA) monomer.

142 X. Zhao et al. / Materials Science and En

Fig. 3. FTIR spectra for (a) TPGDA monomer, (b) an unpolymerized TPGDA–BaTiO3

film and (c) a polymerized TPGDA–BaTiO3 film.

and thermal treatment, indicating that CH CH2 was transformedto CH CH2. The electron beam effectively cured the surface of thefilms, which prevented the re-introduction of oxygen into the filmswhen removed from the electron beam chamber. When the filmswere thermally baked, the mobility of residual free radicals insidethe films was increased, and their lifetime was sufficient to promote

fully polymerization and cross-linking of partially cured regions[10,11].

Fig. 4 shows the DSC and TGA traces as a function of tempera-ture from 25–400 ◦C for the polymerized TPGDA (PTPGDA)–BaTiO3films. The DSC trace showed no glass transition or meltingbehaviour within this temperature range. The TGA trace showedweight loss started only above 200 ◦C, suggesting that the PTPGDApossessed reasonable heat resistance up to 200 ◦C.

3.2. Microstructure of PTPGDA–BaTiO3 nanocomposite films

Confocal microscope images of the surface of PTPGDA–10vol%BaTiO3 and PTPGDA–20 vol%BaTiO3 nanocomposite films areshown in Fig. 5. For PTPGDA–BaTiO3 films with 10 vol% BaTiO3,there were small ridges on the surface attributed to the shrink-age of the matrix during polymerization. Despite the presence ofridges, relatively smooth films with a mean arithmetical rough-ness Ra ∼ 0.5 � m were prepared reproducibly. In all films preparedunder optimum conditions, there was no evidence of the remnantsof individual droplets. When the volume fraction of BaTiO3 wasgreater than 20%, the viscosity of the TPGDA–BaTiO3 feedstock

Fig. 4. DSC and TGA traces for a PTPGDA–BaTiO3 film.

gineering B 151 (2008) 140–145

suspension increased, and the corresponding film final surfaceroughness increased to Ra ∼ 1 �m.

One of the primary concerns in manufacturing large area andreliable nanocomposite films is the homogeneity of the nanopar-ticle dispersion in the matrix. Fig. 6 shows scanning electronmicrographs of the surface of PTPGDA–10 to 30 vol%BaTiO3nanocomposite films. The thermal polymerized PTPGDA–20vol%BaTiO3 nanocomposite films in Fig. 6(a) contained inhomo-geneous distribution of BaTiO3 nanoparticles, and agglomeratesof BaTiO3 nanoparticles are visible on the surface of the films.Compared with thermal polymerized films, in the electron beampolymerized films as shown in Fig. 6(b–d), the BaTiO3 nanoparticleswere separated and randomly dispersed in cross-linked PTPGDAnetworks. The comparatively well-dispersed structures here areattributed to the small physical size of the TPGDA monomer relativeto the BaTiO3 nanoparticles [12], and the Van der Waals inter-actions between the polar C–O bonds in TPGDA molecules andBaTiO3 nanoparticles. In comparison with direct mixing of poly-mer and nanoparticles, the liquid monomers can easily penetratebetween nanoparticles, and the wettability of the nanoparticles bythe matrix can be improved. The agglomeration of BaTiO3 nanopar-ticles in the thermal polymerized films indicates that the presenceof the initiator in the feedstock suspension may disrupt the criticalstability of the suspensions and lead to an inhomogeneous mixtureof the nanoparticles and monomers in the films.

Each of the PTPGDA–10 vol%BaTiO3 films in Fig. 6(b) andPTPGDA–20 vol%BaTiO3 films in Fig. 6(c) were prepared underidentical spray deposition conditions. The top layer of PTPGDA–20vol%BaTiO3 films contained some pores due to the increased vis-cosity of the droplets at the higher nanoparticle fraction, whichinhibited lateral flow to fill interstices between adjacent dropletimpacts, and also again because of the shrinkage of the matrixduring polymerization. However, by increasing the flow rate ofthe TPGDA-BaTiO3 suspension during manufacture, the size of thedroplets increased, they remained fluid for longer, the pores werereduced, and the continuity of the surface improved, as shown inFig. 6(d). All PTPGDA–10 to 30 vol%BaTiO3 nanocomposite filmsremained flexible when detached from substrate by immersion ina water bath for several hours.

3.3. Dielectric properties

The dielectric constant and dissipation factor (tan ı) of thePTPGDA–BaTiO3 nanocomposite films as a function of volume frac-

tion of BaTiO3 at 10 kHz and 25 ◦C are shown in Fig. 7. The dielectricconstant of pure PTPGDA polymer was ∼5, and the correspondingtan ı was ∼0.03 at 10 kHz. This relatively high dielectric constantand tan ı for PTPGDA compared with commercial epoxy materials(dielectric constant ∼4, tan ı ∼ 0.02 at 10 kHz) generally used asmatrices for embedded nanocomposite capacitors is attributed tothe relatively high polarity of the PTPGDA polymer [12]. The dielec-tric constant of the electron beam irradiated films was higher thanthat of the thermal polymerized films, because of the significantlyimproved dispersion of BaTiO3 nanoparticles and electron beaminduced polarization effects [13].

The frequency dependent dielectric constant and dissipationfactor (tan ı) of the electron beam polymerized PTPGDA–BaTiO3nanocomposite films with different volume fractions of BaTiO3at 25 ◦C are shown in Fig. 8. The dielectric constant of thePTPGDA–BaTiO3 nanocomposites increased with increasing BaTiO3fraction at all frequencies; while for a given fraction, the dielec-tric constant decreased with increasing frequency. For example,a dielectric constant of 39 at 10 kHz was achieved by adding 30vol%BaTiO3 nanoparticles into PTPGDA matrix, giving a capaci-tance density of ∼3 nF/cm2. Similarly to PTPGDA alone, the tan ı

X. Zhao et al. / Materials Science and Engineering B 151 (2008) 140–145 143

Fig. 5. Confocal micrographs of (a) PTPGDA–10 vol%

of PTPGDA–BaTiO3 nanocomposites increased dramatically at fre-quencies above 100 kHz, and in general tan ı proved sensitive tothe precise experimental conditions, such as the environmentalhumidity and the stability of the electron beam during curing. Weare uncertain about the reason for the high tan ı at frequenciesabove 100 kHz, but suggest it may be caused by small molecules ofresidual solvent embedded in the films or the polarization effectof the electron beam. However, it should be noted that in the fre-quency range of practical significance for power capacitors of 103–104 Hz, the spray deposited films offered a well balanced combi-nation of elevated dielectric constant, low tan ı losses and physicalflexibility for further processing into devices.

Fig. 6. Scanning secondary electron micrographs of PTPGDA–BaTiO3 films at an acceleratbeam polymerized PTPGDA–10 vol%BaTiO3, (c) electron beam polymerized PTPGDA–20 v

BaTiO3 and (b) PTPGDA–20 vol%BaTiO3 films.

The temperature dependency of the dielectric constant and tan ıof the electron beam polymerized PTPGDA–BaTiO3 nanocompositefilms with different volume fractions of BaTiO3 at 10 kHz are shownin Fig. 9. The dielectric constant was relatively stable in the range25–150 ◦C, with a gradual increase from 39 to 45 for PTPGDA–30vol%BaTiO3 films, because of the higher mobility of the polymerchains with increasing temperature. The dielectric constant of bulkBaTiO3 increases in the vicinity of the Curie temperature, whichis approximately 130 ◦C for bulk BaTiO3. However in this study,neither DSC investigations nor dielectric measurements showed asignificant change or phase transition as temperature increased,since it has been suggested that changes in the dielectric constant

ing voltage of 2.0 kV: (a) thermal polymerized PTPGDA–20 vol%BaTiO3, (b) electronol%BaTiO3 and (d) electron beam polymerized PTPGDA–30 vol%BaTiO3.

and En

Jayasundere–Smith model:

εpVp + εcVc((3εp/(εc + 2εp)))[1 + 3Vc((εc − εp)/(εc + 2εp))]

144 X. Zhao et al. / Materials Science

Fig. 7. Dielectric constant and dissipation factor (tan ı) of spray depositedPTPGDA–BaTiO3 nanocomposites at 10 kHz and 25 ◦C as a function of volume fractionof BaTiO3.

of BaTiO3 nanoparticle is less sensitive to temperature than bulkBaTiO3 [14]. Tan ı behaviour within this temperature range wasalso relatively constant, and there were again no indications of sig-nificant phase transitions taking place within the polymer matrixor BaTiO3 nanoparticles.

In order to understand the observed dielectric behaviour oftwo phase polymer–ceramic nanocomposites, a range of analyticalexpressions, experimental correlations and numerical simulationshave been proposed. Mixing laws and effective medium theory(EMT) approaches are used most commonly for modelling thedielectric constant of a continuous polymer matrix filled withuniformly distributed ceramic particles, including the following[15–18]:

Fig. 8. Dielectric constant and dissipation factor of electron beam polymerizedPTPGDA–BaTiO3 nanocomposites at 25 ◦C as a function of frequency for variousvolume fractions of BaTiO3.

gineering B 151 (2008) 140–145

Lichtenecker logarithmic mixing law:

log εt = Vp log εp + Vc log εc (1)

Maxwell–Garnett theory:

εeff = εp2εp + εc + 2Vc(εc − εp)2εp + εc − Vc(εc − εp)

(2)

Bruggeman theory:

Vcεc − εeff

εc + 2εeff+ (1 − Vc)

εp − εeff

εp + 2εeff= 0 (3)

εt =Vp + Vc((3εp/(εc + 2εp)))[1 + 3Vc((εc − εp)/(εc + 2εp))]

(4)

Yamada equation:

εeff = εp

[1 + nVc(εc − εp)

nεp + (εc − εp)(1 − Vc)

](5)

where Vc is the volume fraction of the ceramic particles, Vp

is the volume fraction of the polymer matrix, εt or εeff is thedielectric constant of the resulting composite, εc is the dielectricconstant of the ceramic particles, and εp is the dielectric constantof the polymer matrix. Note that these expressions are applicablefor non-percolating, well-dispersed particles. Percolating or non-percolating networks, e.g. those formed by high aspect ratios fillerssuch as carbon nanotubes or conductive metal clusters, e.g. Ag,require different approaches [19]. Partly because of its simplicityand partly because of its generally good fit to experimental data,

Fig. 9. Dielectric constant and dissipation factor of electron beam polymerizedPTPGDA–BaTiO3 nanocomposites at 10 kHz as a function of temperature for variousvolume fractions of BaTiO3.

X. Zhao et al. / Materials Science and En

References

Fig. 10. Comparision of the experimental data and theoretical predictions of dielec-tric constant for PTPGDA–BaTiO3 nanocomposites.

the Yamada Eq. (5) has become widely used. The constant n in Eq.(5) is a geometric parameter, which is dependent on the shape of theellipsoidal particles in the matrix, and their orientation in relationto the composite film surface [17].

Fig. 10 presents the measured dielectric constant of ourPTPGDA–BaTiO3 nanocomposite films with different volume frac-tions of BaTiO3 at 10 kHz and 25 ◦C, compared with the dielectricconstant calculated based on the various analytical models in Eqs.(1)–(5). Assumed values for εp and εc were 5 and 2000 at 10 kHzrespectively. The experimental data from the thermal polymer-ized only films fitted well to the Lichtenecker model at low BaTiO3fractions, while there remained some discrepancy between fit andexperiment at higher BaTiO3 fractions. A value for the geometricbest-fitting parameter n = 10 in Eq. (5) gave the overall best-fit tothe experimental data, suggesting an elliptical shape of the BaTiO3nanoparticles. However, since Fig. 6 showed that the shape ofthe BaTiO3 nanoparticles were approximately spherical, the phys-ical meaning of n = 10 in Eq. (5) is proposed to arise from 3Dheterogeneity in the micro-distribution of the BaTiO3 nanoparti-cles. The measured dielectric constant εeff for the electron beampolymerized films showed an approximately linear variation withBaTiO3 volume fraction between 10% and 30%, with absolute val-ues generally larger than those predicted by Eqs. (1)–(4), or Eq.(5) using the same value of n = 10 obtained from the thermal

polymerized films. This indicates a polarizing effect of the elec-tron beam that increased the dielectric constant of one or bothof the component phases, due to the well-known “poling” effect,and the better homogeneity of the electron beam polymerizedfilms.

While nanocomposite dielectric constants have been studiedextensively in the literature, tan ı losses, electrical (dielectric)breakdown strength and the leakage current are of equal prac-tical relevance, since the energy density of a capacitor varieswith the square of the breakdown strength and the leakagecurrent determines power losses. Fig. 11 shows the leakage cur-rent density of PTPGDA–30 vol%BaTiO3 nanocomposite films asa function of the applied electric field. The leakage currentincreased steadily as the electric field increased. Nonetheless,the leakage current density remained low in the range 10−9 to10−8 A/cm2 at fields of up to 100 kV/mm. Due to the limita-tion of the measurement instrument, for the thicknesses studiedhere of ∼15 �m, the maximum electric field that could beapplied was ∼100 kV/mm, up to which field no breakdown wasobserved.

[[[[

[

[[[[[

gineering B 151 (2008) 140–145 145

Fig. 11. Leakage current of PTPGDA–30 vol%BaTiO3 nanocomposite films.

4. Conclusions

A simple spray deposition processing technique has beendeveloped for the manufacture of polymer-based nanocompos-ite dielectric films in capacitor applications. The process hasbeen suggested to offer good potential for cost-effective scale-up. The critical process steps were the production of stable liquidnanoparticle/matrix suspensions of low viscosity, and control of theatomization parameters. Polydiacrylate–BaTiO3 films with moder-ate filler loadings of 10 vol% to 30 vol% and controlled thicknessof 5–30 �m were fabricated, and a dielectric constants of ∼ 39 at10 kHz, a high breakdown strength of >100 kV/mm and a low leak-age current of 10−9 to 10−8 A/cm2 above 1000 V were measuredfor PTPGDA–30 vol%BaTiO3 films, with good thermal stability up to150 ◦C.

Acknowledgements

The work is funded by Innovative Electronics ManufacturingResearch Center (IeMRC) and Defence Science and Technology Lab-oratory (DSTL), UK. The authors gratefully acknowledge Dr. HazelAssender (Department of Materials, University of Oxford) for heradvice and technical assistance, and Mr. Chris Salter (Oxford Mate-rials Characterization Services) for his help with SEM.

[1] Y. Cao, P.C. Irwin, K. Younsi, IEEE Trans. Dielect. Elect. Insul. 11 (2004) 797.[2] T. Tananka, G.C. Montanari, R. Mulhaupt, IEEE Trans. Dielect. Elect. Insul. 11

(2004) 763.[3] Q. Tan, P.C. Irwin, Y. Cao, IEEJ Trans. FM 126 (2006) 1153.[4] Y. Rao, S. Ogitani, P. Kohl, C.P. Wong, J. Appl. Polym. Sci. 83 (2001) 1084.[5] H. Windlass, P.M. Raj, D. Balaraman, S.K. Bhattacharya, R.R. Tummala, IEEE Trans.

Elect. Pack. Manuf. 26 (2003) 100.[6] Y. Rao, C.P. Wong, J. Appl. Polym. Sci. 92 (2004) 2228.[7] J. Xu, S. Bhattacharya, K. Moon, J. Lu, B. Englert, C.P. Wong, IEEE Proceedings of

the 56th Electronic Components and Technology Conference, 2006, p. 1520.[8] M. Kakimoto, A. Takahashi, T. Tsurumi, J. Hao, L. Li, R. Kikuchi, T. Miwa, T. Oono,

S. Yamada, Mater. Sci. Eng. B 132 (2006) 74.[9] Z. Dang, Y. Yu, H. Xu, J. Bai, Compo. Sci. Tech. 68 (2008) 171.10] J. DeGaspari, Mech. Eng. 121 (1999) 69.11] T. Scherzer, H. Langguth, Macromol. Chem. Phys. 206 (2005) 240.12] R. Popielarz, C.K. Chiang, R. Nozaki, J. Obrzut, Macromolecules 34 (2001) 5910.13] P. Bloss, A.S. DeReggi, H. Glas̈el, E. Hartmann, D. Hirsch, H. Schaf̈er, J. Appl. Phys.

86 (1999) 2712.14] Z. Zhao, V. Buscaglia, M. Viviani, M.T. Buscaglia, L. Mitoseriu, A. Testino, M.

Nygren, M. Johnsson, P. Nanni, Phys. Rev. B 70 (2004) 024107.15] R. Landauer, J. Appl. Phys. 23 (1952) 779.16] N. Jayasundere, B.V. Smith, J. Appl. Phys. 73 (1993) 2462.17] T. Yamada, T. Ueda, T. Kitayama, J. Appl. Phys. 53 (1982) 4328.18] D. Yoon, J. Zhang, B.I. Lee, Mater. Res. Bull. 38 (2003) 765.19] T.K.H. Starke, C. Johnston, P.S. Grant, Scripta Mater. 56 (2007) 425.