Ferroelectric Phase Stability Studies in Spray Deposited KNO3:PVAComposite Films

Navneet Dabra and Jasbir S. Hundalw

Applied Physics and Materials Science Laboratory, Department of Applied Sciences, Baba Farid College of Engineeringand Technology, Bathinda, Punjab, India

Kopple C. Sekhar, Arvind Nautiyal, and Rabinder Nath

Ferroelectric Materials and Devices Research Laboratory, Department of Physics, Indian Institute of Technology,Roorkee, Uttarakhand, India

The composite films of potassium nitrate (KNO3):poly(vinyl al-cohol) (PVA) have been prepared at different temperatures byspray-deposition technique. Ferroelectric hysteresis loops weretraced at room temperature for the composite films using mod-ified Sawyer and Tower circuit. The X-ray diffraction studiesconfirm the existence of ferroelectric phase III of KNO3 in thecomposite films at the room temperature, where this phase inpure KNO3 films is known to exist in the temperature range1101–1241C. The composite films deposited at 2001C shows theoptimum remanent polarization, PrB17 lC/cm2

. The Pr wasstudied in the frequency range 10 Hz–1 kHz. The stability of thePr in the composite films exhibits improved fatigue comparedwith that of quenched KNO3 films. The capacitance–voltage (C–V) characteristics exhibits butterfly features which supports thepresence of ferroelectric phase in the composite films. Theatomic force microscopy images show that the composite filmshave uniform dispersion of KNO3 particles in the PVA matrix.

I. Introduction

POTASSIUM NITRATE (KNO3) is an interesting material becauseof its ferroelectric properties in one of its phases.1–6 The pure

KNO3 crystals exists in phase II with orthorhombic (aragonite)crystal structure at room temperature and atmospheric pressure.It changes to phase I (rhombohedral crystal structure) above1301C.4,5 On cooling, phase I of KNO3 does not directly revertback to phase II but through an intermediate metastable phaseIII, which is ferroelectric in region B1241–1101C, on furthercooling it changes to phase II.4,5 The ferroelectric phase IIIwhich appears only in the process of cooling results from theshift by 0.55 A of the nitrate groups from the centre of the unitcell, which creates dipoles and hence spontaneous polarizationalong the c-axis.4,7 The phase III has attracted much attentionbecause of its extremely useful ferroelectric properties.5 It hasbeen reported that the exact temperature region over whichphase III exists at atmospheric pressure, not only depends on theprevious treatment of the specimen, but also strongly on thecooling rate.

Many studies such as far infrared,4,7 neutron diffraction,8 in-elastic neutron scattering,9 Raman spectroscopy,1,10,11 X-ray,11–13

differential scanning calorimetric,14–16 dielectric,5,17 electrical17

and thermal conductivity,18 and ferroelectric,19–22 have beencarried out in past by many researchers to examine the natureof the phase transitions in KNO3.

The stabilized phase III of KNO3 in thin film form have ad-vantages such as, low operating voltage,23 fast switching re-sponse,23,24 small size and cost effectiveness.25,26 These are beingused as ferroelectric capacitors, memory element, nonvolatile mem-ory devices (NV-RAMs), and dynamic random access memories.23

Several attempts have been made to stabilize phase IIIof KNO3 at room temperature in thin fused films,19 with theapplication of high pressure,5 mixing it with starch,13 poly-vinyl fluoride,25 poly(vinyldiene fluoride) (PVDF),26 carbon,27

and silicon carbide.28,29 The KNO3–polymer composite filmsshowed a considerable improvement in the ferroelectric proper-ties and phase stabilization.25,26 The surface electric field effect,1

strain,30 departures from the stoichiometry,31 enhancement offerroelectric phase with decreasing thickness20 have been studiedin past that explain the phase stabilization in the material even inthe thermodynamically unstable regions.

Usually, the ceramic–polymer composites have been preparedby embedding ceramic particles in a suitable polymer matrix bymelt press,25,26 sol–gel,32 solvent cast,33,34 and intercalation tech-niques.35 However, it was found that under suitable conditionsthe solvent cast technique yields uniform composition of ce-ramic and polymer due to proper interactions that occur at mo-lecular level.36 Recently, the spray-pyrolysis37 and ultrasonicnebulizer38 techniques have been used to prepare the thin filmsof various pure ceramics.

In the present article, KNO3:poly(vinyl alcohol) (PVA) com-posite films have been prepared using spray-deposition tech-nique where PVA can provide an appropriate environment toKNO3 to retain its ferroelectric phase III at room temperature.Moreover, PVA has high viscosity, common aqueous solubilityas KNO3 and it can also give the mechanical strength39 to thecomposite films in comparison with the pure KNO3. The ferro-electric behavior was studied by tracing the hysteresis loop andcapacitance–voltage (C–V) characteristics. The surface morpho-logical and structural studies have been carried out using atomicforce microscopy (AFM), and X-ray diffraction (XRD).

II. Experimental Procedure

The crystals of purified KNO3 supplied by E.Merck (India) Ltd.(Mumbai, India) were dissolved in the double distilled water tillsaturation level and kept in a closed environment for a few days.The crystals grown in the solution were dried and powdered andthen used to prepare the composite films. Different weight per-cent of KNO3 and PVA weighing a total of 200 mg were dis-solved in 50 mL of double-distilled water at 401C while stirring.PVA is known to have maximum aqueous solubility withoutprecipitation at 401C.36,39 The deposition of the composite filmsof KNO3:PVA have been done in ambient atmosphere with thespray-deposition set up assembled in the laboratory. The aver-age thickness of each film was found to be 14 mm. The smooth

D. C. Lupascu—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: jshundal@yahoo.comManuscript No. 24773. Received May 26, 2008; approved January 8 2009.

Journal

J. Am. Ceram. Soc., 92 [4] 834–838 (2009)

DOI: 10.1111/j.1551-2916.2009.02976.x

r 2009 The American Ceramic Society

834

well-polished circular brass disks of diameter 1 cm and thickness2 mm were used as substrates for the deposition of the compos-ite films at different temperatures. It has been reported that thePZT and derived PZT glass–ceramics thin films deposited on thedifferent metal substrates shows excellent ferroelectric, electrical,and dielectric properties.40–42 The composite films were pre-pared at temperatures 1501, 2001, and 2501C. Each film was thenannealed at their respective deposition temperatures for 2 daysand then slowly cooled down to room temperature.

Circular electrodes of Indium24 having cross-sectional areaB1.867� 10�2, 3.253� 10�2, and 5.726� 10�2 cm2 were depos-ited by thermal evaporation under the vacuumB2.0� 10�5 mbaronto the respective composite films. Modified Sawyer–Tower cir-cuit43 connected with computer has been used to display the hys-teresis loop characteristics. The values of the Pr were calculatedfor different electrode areas. The dependence of Pr on frequencyof applied signal and reversal cycles has also been investigated.X’Pert-Pro X-ray diffractometer (PANalytical, B.V Lelyweng,the Netherlands) with Ni-filtered CuKa radiation of wavelength0.154 nm was used for XRD studies of the composite films. AFMhas been used to investigate the morphology and microstructureof the composite films. The capacitance–voltage (C–V) charac-teristics were recorded at room temperature using Keithley (590model) C–V analyzer (Keithley Instruments, Cleveland, OH).

III. Results and Discussions

(1) Structural Properties

(A) XRD: The XRD measurements were performed toobtain the information about the existence of the ferroelectricphase III in the composite films deposited at different temper-atures. Figure 1 shows the X-ray scans of pure KNO3 powder,composite films of KNO3:PVA (containing 50 wt% of KNO3)deposited at different temperatures and pure PVA film. It wasreported by Schaffer and Mikkola22 that KNO3 exhibits thediffraction peak at 2y5 29.801 for (003) reflection for the phaseIII and 2y5 29.451 for (012) reflection for the phase II. The mostcommon diffraction peaks of phase II at 2y5 23.571, 23.831,29.401, 32.331, and 32.821 were observed in the KNO3 powderfor the planes of (h k l) values (111), (021), (012), (102), and

(112), respectively. The XRD peak corresponding to phase IIIwas not seen in the XRD pattern of pure KNO3 powder. Thecomposite films exhibit the ferroelectric phase III peak at2y5 29.711 corresponding to (003) reflection and another peakat 2y5 29.521 for (012) reflection for the phase II in addition tothe peak around 2y5 19.61 that corresponds to PVA.44 Theearlier X-ray studies by other workers also suggest the existenceof ferroelectric peak around 2y5 29.81 in addition to peak cor-responding to phase II around 2y5 29.51 in mixed KNO3 sys-tems.22,26,43,45 The composites films of the KNO3:PVDF are alsoknown to exhibit enhanced phase III peak at 2y5 29.81.26

Therefore, the present X-ray studies suggest that the new peakscould be due to the ferroelectric phase III.

Figure 2 shows the expanded XRD scans of pure KNO3

powder and the composite films prepared at different tempera-tures. The peak of ferroelectric phase III in the composite filmswere found to occur in the range of 2y5 (29.7170.01)1 and thatof phase II within 2y5 (29.5270.01)1. Such small shift in thediffraction peaks have also been reported earlier in KNO3:starchand KNO3:PVDF composite samples.13,26 The relative intensityof phase III to phase II was estimated and was found to be 1.68,2.63, and 1.53 in the composite films prepared at 1501, 2001, and2501C. It was maximum for the composite films prepared at2001C. Thus, it is inferred that the composite film deposited at2001C have more stable ferroelectric phase. The average crys-tallite size was determined using Scherrer relation46

t ¼ 0:91lbðyÞcos y (1)

where b(y) is the full-width at half-maximum of the peak, l isthe wavelength of X-rays used in X-ray scan and t is the averagecrystallite size. The crystallite size was found to be 76.8, 79.2,and 84.5 nm for the composite films prepared at temperatures1501, 2001, and 2501C, respectively. The crystallite size wasfound to increase with the deposition temperature. It has beenreported in literature that large crystallite size of the phase III inthe composite film of KNO3:PVDF is responsible for producingstable ferroelectric phase at room temperature.26

(B) AFM: The surface morphology of the compositefilms (containing equal proportions of KNO3 and PVA) hasbeen analyzed by AFM. Figure 3 shows the two- and three-dimensional images depicting 5 mm� 5 mm area of these com-posite films prepared at the temperatures 1501, 2001, and 2501C,respectively. The AFM images of the composite films preparedat 1501 and 2001C clearly show well-grown spherical-shapedKNO3 particles, which are homogenously dispersed in PVA ma-trix and average grain size was estimated to be 186 and 230 nm.

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Fig. 1. X-ray scans of the (a) pure potassium nitrate (KNO3) powderand KNO3:poly(vinyl alcohol) (PVA) composite films (containing50 wt% of KNO3) prepared at temperature, (b) 1501C, (c) 2001C,(d) 2501C, and (e) pure PVA Film.

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Fig. 2. Expanded X-ray scans of the (a) pure potassium nitrate (KNO3)powder and KNO3:poly(vinyl alcohol) (PVA) composite films (contain-ing equal proportions of KNO3 and PVA) prepared at temperatures,(b) 1501C, (c) 2001C, and (d) 2501C.

April 2009 KNO3:PVA Composite Films 835

The root mean square (rms) roughness of the compositefilms deposited at 1501 and 2001C was found to be 74 and61 nm, respectively. However, the interconnected clusters ofKNO3 grains were seen in the AFM image of the compositefilm deposited at 2501C. The average grain size was found to be290 nm with very large rms roughness of 170 nm. This might bedue to melted PVA, which creates the clustering of KNO3 par-ticles in the composite films.

(2) Electrical Properties

(A) Hysteresis Loop Measurements: The ferroelectrichysteresis loops (P–E) of the composite films were traced usingmodified Sawyer and Tower circuit.43 Figure 4 shows the de-pendence of normalized Pr in composite films deposited at2001C, containing different weight percent of KNO3. The valueof Pr was found to be maximum in the composite films withequal proportion of KNO3 and PVA.

Figure 5 shows the P–E loops of the composite films (com-prising of equal weight percentages of KNO3 and PVA) preparedat different temperatures. The value of Pr was estimated in eachcomposite film and the optimum value was found to beB17 mC/cm2 for the films prepared at substrate temperature 2001C. Thismight be due to the distance dependent repulsive forces, which

exist between the neighboring domains. These forces are knownto weaken as the particle size increases.40 This in turn facilitatemore domains to orient in the field direction at elevated tem-perature causing large Pr. The composite films deposited at

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Fig. 3. The two- and three-dimensional atomic force microscopic images of the composite films (containing equal weight percentage potassium nitrateand poly(vinyl alcohol)) prepared at temperatures (a) 1501C, (b) 2001C, and (c) 2501C.

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Fig. 4. Composition dependence of the normalized polarization inpotassium nitrate:poly(vinyl alcohol) composite films.

836 Journal of the American Ceramic Society—Dabra and Hundal Vol. 92, No. 4

2501C gave lower Pr though the average crystallite and grainsizes are slightly larger than those of the composite films depos-ited at 2001C. The decrease in Pr might be due to melted PVA,which create the porosity between the KNO3 clusters. The valueof Pr was calculated for different electrode areas and was foundto be 17.070.5 mC/cm2. Thus, Pr was observed to be almostindependent of electrode area. The deposition temperature doesnot appear to affect the coercive field (Ec) in these compositefilms. The ferroelectric composite films deposited at 2001C con-taining equal proportions of KNO3 and PVA show compara-tively better results, hence, the composite films prepared underthese conditions were used for further studies.

The frequency dependence of Pr in the optimized compositefilm was studied in the frequency range of 10 Hz–1 kHz, asshown in Fig. 6. It was found that the value Pr at 300 Hz de-creases to 0.84 times its peak value at 10 Hz (inset of Fig. 6).Further increase in the frequency of the applied field does notaffect Pr much, it remains B17 mC/cm2. The decrease in Pr withfrequency may be due to steady decrease in the contribution ofspace charge effect36 to the total polarization. However, thespace charge contribution become negligible at higher frequen-cies and Pr may really be due to ferroelectric domains switch-ing.36

The value of the PrB17 mC/cm2 in the optimized KNO3:PVAcomposite films prepared by spray-deposition technique is morethan three times as reported in the poled KNO3:PVA filmsprepared by solvent cast technique47 and about twice that ob-served in pure fused KNO3 films.22 This value of Pr is slightlylower than that reported for KNO3:PVDF composite films.26

The KNO3:PVDF composite films could not be prepared byspray-deposition technique for want of common solvent.

(B) Fatigue Study: Figure 7 shows the normalized po-larization versus number of reversal cycles in the KNO3:PVAcomposite film prepared under the optimum conditions. Thevalue of normalized polarization at 2� 106 reversal cycles re-duces to 0.65 times of its initial value. Thus, these compositefilms show better fatigue life span than the fused KNO3 films inwhich Pr drops to 50% after 105 cycles,20 but this value of lifespan is still less than that of other competitive materials in thiscategory.

(C) Capacitance–Voltage (C–V) Characteristics: Figure 8shows the typical C–V characteristics (frequencyB100 kHz) stud-ied at room temperature for the composite films (containing50 wt% of KNO3) prepared at 1501, 2001, and 2501C, respec-tively. The voltage was swept from �15 to 115 V in the forwardbias and from 115 to �15 V in the reverse bias with a step biasof 1V. The capacitance shows a strong bias dependence and ex-hibit nonlinear behavior. The two sharp peaks at �4 and 15 Vwere observed in all the composite films and the values of Cmax

were measured to be 0.16, 0.21, and 0.12 nf, respectively, for thecomposite films prepared at 1501, 2001, and 2501C. The compositefilm prepared at 2001C exhibited better dielectric properties. Thesesharp peaks may be attributed to the polarization switching phe-nomena.36,43 The initial increase in the capacitance with bias volt-age may be due to orientation of domains in the field direction.Moreover, the maxima in C–V curves occur in the vicinity of co-ercive voltage (Vc). In the neighborhood of coercive voltage thedomain walls are known to be more active due to nucleation ofnew domains or the maximization of the domain wall mobility.Thus, the contribution from the domain wall motion to the di-

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Fig. 6. Hysteresis loops of the optimized potassium nitrate (KNO3):poly(vinyl alcohol) (PVA) composite film at frequency. (a) 10 Hz,(b) 50 Hz, (c) 100 Hz, (d) 300 Hz, and (e) 1 kHz. (Inset) Frequencydependences of Pr in the optimized KNO3:PVA composite film.

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Fig. 8. C–V characteristics of the potassium nitrate (KNO3):poly(vinylalcohol) composite films (containing 50 wt% of KNO3) prepared attemperatures (a) 1501C, (b) 2001C, and (c) 2501C.

April 2009 KNO3:PVA Composite Films 837

electric properties is the greatest48 resulting in the sharp peaksaround Vc in C–V characteristics. The decrease in the capacitancein the forward and reverse cycles could be due to the reductionin domain moment.48,49 The strong bias dependence of capaci-tance with applied voltage can be useful in tunable microwaveapplications and voltage controlled oscillators in electronics.36

The butterfly features of C–V curves in the composite films likeother ferroelectric films strongly corroborate the presence of theferroelectric nature of the composite films at room temperature.

IV. Conclusions

The composite films of KNO3:PVA containing equal propor-tions of KNO3 and PVA prepared by spray-deposition tech-nique show improved Pr and stability of ferroelectric phase IIIat room temperature. The X-ray diffraction studies on thesefilms reveal more intense phase III peak relative to phase IIpeak. The optimum value of Pr is obtained for the films depos-ited at 2001C. The hysteresis loop and the C–V characteristicsconfirm the existence of ferroelectric phase III in KNO3:PVAcomposite films at room temperature.

Acknowledgments

The authors acknowledge the Chairman, Baba Farid College of Engineeringand Technology, Bathinda for providing the research facilities in the college andDirector, Sophisticated Analytical Instrumentation Facility (SAIF), Panjab Uni-versity, Chandigarh for providing the facility for the X-ray scans.

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