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Journal of Advanced PhysicsVol. 3, pp. 1–6, 2014(www.aspbs.com/jap)

Dielectric and Electrical Properties of SrxBa1-xNb2O6

(0.40≤ X≤0.75) CeramicsShridhar. N. Mathad1�∗, Pragathi Jadhav2, and Vijaya Puri3

1Department Physics, K.L.E.I.T, Hubli 560030, India2Department Physics, Shivaji Univesrty, Kolhapur, India3Department Physics, Shivaji Univesrty, Kolhapur, India

The purpose of this paper is to describe dielectric and electrical properties of strontium barium noibates(SrxBa1-xNb2O6� ceramics were prepared by the low-cost solid state reaction technique. Composition depen-dent variations of complex permittivities were studied at room temperature exhibit normal dielectric behavior,attributed to Maxwell-Wagner type interfacial polarization. Strontium barium noibates (SBN) ceramics showspositive temperature (PTC) behavior. The themistor constant, activation energy, sensitivity index (�) and stabilityfactor (SF) of SBN are also reported. The themistor constant lies in the range of 204 K to 707 K.

KEYWORDS: Strontium Barium Noibates (SBN), Electrical Properties, PTC, Activation Energy, Thermistor Constant.

1. INTRODUCTIONMonitoring and control of temperature is of overridingimportance in of our everyday life. Temperature sen-sors are present not solely in domestic and industrialactivities however additionally in laboratory and medi-cal procedures. NTC thermistors find intensive use incircuit compensation, aerospace, cryogenic, automotivetemperature measure and management applications.1–3 Theelements based on the materials with the positive tem-perature coefficient of resistivity (PTCR) found the wideapplication practically in all the branches of engineering.4

PTC thermistor applications build use of the characteris-tics inherent in their composition. They’re used as pro-tection devices, heating components, temperature sensors.typically for fabricating such elements the ceramic materi-als, supported the semiconductor barium titanate(BaTiO3�,are used.5 One of the perspective directions in using PTCRmaterials is their application in surface heaters with regula-tion of temperature and temperature sensors in varied elec-tronic devices and integrated circuits. The interest groupduring this association raises an issue of creating thin filmstructures on the basis of materials with positive tempera-ture coefficient of electric resistance.6

In regard of environmental issues, lead free materialsare being considered for several applications and stron-tium barium noibates (SrxBa1−xNb2O6� is among the fewmaterials that are within the fore-front thanks to glorious

∗Author to whom correspondence should be addressed.Email:Received: 9 November 2014Accepted: 28 November 2014

novel technological application properties. The presentstudy focuses on the study of strontium barium nio-bate(SBN) ceramics (0.40 ≤x ≥0.75) DC electrical resis-tivity, thermistor constant, activation energy and dielectricstudies. For the first time the thermal resistor propertiesare reported for this material. Tungsten bronze structuredstrontium barium noibates were synthesized by easy solid-state reaction.7�8 This powder was pressed into pellets inan exceedingly press at ten ton/cm2 for five minutes. Thesesamples were named Sr40, Sr50, Sr61 and Sr75 respec-tively. The DC electrical resistance for all compositionswas measured by two-probe methodology applying sil-ver conductor paste on each side. At room temperature,commercially available impedance analyser (SOLATRON1260A) in the frequency region twenty hz to 1 megacyclewas used for complex permittivity studies.

2. EXPERIMENTAL RESULTS ANDDISCUSSIONS

2.1. Dielectric Studies of SBNThe frequency dependent permittivity for all the bulk sam-ples was studied at room temperature. Figure 1 showsthe variation of dielectric constant with frequency from20 Hz to 1 MHz (using Impedance Analyzer (SOLATRON1260A)). The variation of dielectric loss tangent (tan�)with frequency is shown in Figure 3. The loss tangentdecreases with increase in frequency and attains a con-stant value at higher frequencies. The decrease in permit-tivity with frequency can be explained on the basis ofKoop’s theory. The features of the plot corresponding toare compared with a curve obtained for condenser model

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Fig. 1. Variation of dielectric constant with frequency of bulk SBNceramic.

of the parallel plates designed for the interfacial polariza-tion where a rapid fall with frequencies in the mid fre-quency region, a long tail in the high frequency region. Ingeneral, the decreases in permittivities are rapid at lowerfrequencies and slower at higher frequencies. This is anormal behavior of dielectrics because at lower frequen-cies different types of polarizations (i.e., dipole, atomic,ionic, electronic, interfacial) are present in the compound.A monotonic decrease in permittivity with increase infrequency is known as relaxation dispersion typical fordipole and migrational or interfacial polarization. Ionicand electronic polarization experiences resonance disper-sion, where the permittivity first grows, then decreases,traverses the minimum and reaches a constant value atvery high frequencies,9 as observed in other ferroelec-tric systems.10–13 The low frequency dispersion in SBNis observed due to space charge effect and slowly allpolarization vanishes at higher frequency except electronicpolarization. When the frequency increases, the num-ber of mechanisms involved in the dynamic polarizationdecreases. At very high frequencies, only the electroniccontribution of the polarization remains.14�15 The dielectricstructure is assumed to be consisting of well-conducting

Fig. 2. Variation of dielectric loss (tan�) with frequency of bulk SBNceramic.

grains which are separated by poorly conducting grainboundaries. The grain boundaries are more effective forconductivity and permittivity than grains at lower frequen-cies. i.e., relaxation mechanism was explained in termsof a hopping process of space charge migration boundinside the ferroelectric grains, while the other was associ-ated with interface charges at the grain boundary layers.Therefore permittivity (�’) is high at lower frequenciesand decreases as frequency increases. Since it is observedthat in dielectrics the permittivity (�’) is directly propor-tional to the square root of conductivity (��.16 Hence thepermittivity (�’) high at lower frequencies and decreasesas frequency increases, decrease in permittivity is due tojumping frequency of electrics charge carriers can not fol-low the alternation of the applied ac field beyond a certaincritical frequency.High frequency studies in SBN ceramics the effect of

the hopping of off-centred ferroelectric ions has also beenrecommended by several authors.18 The tetragonal tung-sten bronze phase (SBN) ceramics exhibits high frequencyrelaxation in the paraelectric phase and its origin wasascribed to the crystalline network owing to the motionof active ferroelectric niobate (Nb+5� ions. At lower fre-quency, the dielectric constant of Sr40, Sr61, and Sr75samples shows more prominent values than that of Sr50.It is observed that dielectric constant of these ceramicsdecreases from 4�31×103 to 1�63×103 for Sr40 and Sr50at 20 Hz respectively. Disorder usually may takes placein in A1 and A2 sites. i.e., its origin is probably relatedto the relaxor behavior of this material resulting from thecationic disorder of Sr+2 and Ba+2 in A1 and A2 sites. Intungsten bronze type relaxor ferroelectrics, the complexityof the structure makes interpretations more difficult. Therelaxation phenomenon depends on the strontium content.The dispersion action is much pronounced in strontium-rich compositions.19–21

Thus the grain boundaries are more effective at lowfrequencies while the grains are more effective at higherfrequencies. It is found that dielectric constant (�’)

Fig. 3. Variation AC conductivity with frequency of bulk SBN ceramic.

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LEdecreases on increase in frequency which indicates a nor-mal behavior of the dielectric materials. However theseSBN ceramics have moderate dielectric constants, whichmight be applicable in dynamic random access mem-ory (DRAM), microwave telecommunications, multilayercapacitors applications and MIC’s.22

2.2. AC Conductivity Studies of SBNTo understand the conduction mechanism and the type ofpolarons responsible for conduction, the variation of ACconductivity carried out at room temperature as a func-tion of frequency is represented Figure 3. The ac conduc-tivity (�ac� was calculated from dielectric data using therelation,23

�a�c = ��0�′′ (1)

The conductivity is calculated from dielectric constantand dielectric loss data. The electrical conductivity isdue to migration of ions and this ionic transport stronglydepends on the angular frequency (��. Thus it has beenshown that the ac conductivity is directly proportional toangular frequency. The treatment of conduction by polaronis discussed by Austin and Mott.21 In large polaron hop-ping the ac conductivity decreases with frequency and insmall polaron hopping conductivity increases with angularfrequency. From the graph it is seen that the ac conductiv-ity increases with increase in frequency for all the samples.It has been observed that, conductivity of Sr40 is quitelarge at lower frequencies and Sr61 sample shows lowestconductivity compared to other samples for all frequencyrange.

The electron hopping mechanisms between two adjacentsites are promoted by active conductive grains attributableto increase within the applied frequency. due to hoppingof charge carriers between localized states the linear vari-ation of ac conductivity is ascertained. On the premiseof theoretical models it will understood as indicating thatthe ac conductivity originates from a migration of ionsby hopping between neighboring potential wells In ionic

Fig. 4. Variation of log and reciprocal of absolute temperature(1000/T) for the bulk SBN.

solids, the electrical conduction is attributable to migra-tion of ions and this ionic transport depends on the angu-lar frequency.22�24 At higher frequencies the frequencyof the hopping ions couldn’t follow the applied fieldfrequency and it lags behind. so conductivity decreasesat higher frequencies. Thus, ac conductivity is propor-tional to angular frequency confirming linear nature, thateventually provides rise to dc conductivity at the low-est frequencies.25 All the plots are linear confirming thatthe conductivity altogether SBN samples is attributable tosmall polaron hopping. In few circumstances the conduc-tivity slightly decreases, attributing to the conductivity bymixed polarons.26

2.3. Electrical Properties of SBNThe electrical properties of ceramics are quantitativelyconsidered as sum properties of their individual electri-cal and ionic behavior is obtained. The graph of log and reciprocal of the absolute temperature (1000/T) for thebulk ceramic is shown in Figure 4. The resistivity increaseswith increase in temperature, i.e., PTC (positive temper-ature coefficient) behavior is obtained. The resistivity at300 �C and 900 �C, thermistor constant (B30/90�, sensitivityindex, activation energy and stability factor are tabulatedin Table II. The thermistor constant B30/90 was calculatedby the relation,27

B30/30 =lnR30/R90�

1/T30�− 1/T90�(2)

Where R30 and R90 are resistivity measured at 30 �Cand 90 �C respectively.It is also observed that as strontium concentration

increases resistivity of bulk. It is observed that, the roomtemperature resistivity of Sr 40 is ∼ 3�7×107 � cm, Sr50is ∼ 5�9×107 � cm, Sr61 is ∼ 4�4×107 � cm and Sr75is ∼ 4�4×107 � cm for bulk, the room temperature resis-tivity, and it increases as strontium concentration increasesabove 350 K for bulk. Resistivity is actually the result ofthe combined factors such as grain size, crystal structure,imperfections and microstructure homogeneity.The properties of the PTC result that shows up increase

in resistance of a ferroelectric material with temperature,once it passes from the ferroelectric phase into paraelectricphase. The PTC result underlines the operational principleof thermally sensitive resistors. This is often in contrast tosemiconductor primarily based thermistors during whichresistance diminishes with increase in temperature. ThePTC region lies among the temperature interval of crystallattice transcription throughout phase transition and thusthe ferroelectric with a diffused phase transition showsa smooth increase in resistivity over the whole tempera-ture interval wherever the diffused phase transition takesplace.10

The conduction in ferroelectric can be explained interms of polaron hopping process.28 There are experimen-tal evidences for the existence of polarons and its hopping

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process.29�30 The most popular barrier model for the PTCeffect in ceramics has been proposed by Heywang,31 whoproposed that this phenomenon is caused by the Schottkybarrier at the grain boundaries in polycrystalline structures.To explain the PTCR effect, he proposed a model basedon the presence of two-dimensional resistive grain bound-ary layers consisting of discrete electron traps located inenergy between the conduction and valence bands. It isbased on a description of grain boundaries as insulatingbarrier layers separating two semi-conducting grains. Thewhole dc resistivity jump at the phase transition tempera-ture of the grains is then fully explained by a change inthe barrier height because of the structural transition inthe grains. The theory for PTCR has not been establishedcompletely.In PTCR ceramics, the grain boundaries are the natu-

ral lattice breaking places where charge depletion occurs.The resistance of a PTC thermistor increases with tem-perature, but only over a limited temperature range near aphase transition. The resistance change is very large at thistemperature because of grain boundary effects. Explana-tion of the PTC effect rests upon understanding the defectstructure. The grain boundary region changes during cool-ing. Oxygen is adsorbed on the surface of the ceramic anddiffuses to grain boundary sites, altering the defect struc-ture along the grain boundaries. The added oxygen ionsattract electrons from nearby Nb5+ ions, thereby creat-ing an insulating barrier between grains. Large currentscause the temperature of the thermistor to rise into thePTC range, thereby raising the resistance and lowering thecurrent.32

In PTC effect it is necessary to consider the ferroelec-tric phase transition in SBN and its effect on the insulat-ing barriers between grains. Below Curie temperature theperovskite structure distorts, due to which a large sponta-neous polarization (Ps) develops at the grain boundaries.Near room temperature the resistance of PTC thermis-tors is low because the electron charge trapped in grainboundary regions is partially neutralized by spontaneouspolarization. Wherever the domain structure is advanta-geously positioned, positive polarization charge will can-cel the negatively charged barriers between conductivegrains, thereby establishing low resistance paths across theceramic. When sintered at high temperature, SBN may beexpected to become an n-type semiconductor through thefollowing reaction

SrxBa1−xNb2O6 → Sr2+x Ba3+1−xNb

4+1−xNb

3+x O2−

6

Furthermore, when the cation to anion ratio in theseoxides departs from the ideal value, the oxygen vacanciesin sintered ceramics, on thermal excitation, can providethe trapped electrons to give rise to n-type conductivity ina oxygen deficient ferroelectric. Similarly, P -type conduc-tivity has also been observed.33 To reduce the P -type con-duction, we can use donor dopants, such as Nb 5+ or La3+,

(a)

(b)

(c)

Fig. 5. Composition dependent thermistor constant (B), activationenergy (Ea), sensitivity index (�) and stability factor (SF) of bulk SBNceramics.

replacing divalent (Ba2+, Pb2+ etc.) in order to provideelectrons required to convert O− ions to O2−, thus reducingthe conductivity.33 It is important to reduce the conductiv-ity because high conductivity makes it difficult to applya high field for poling for a long period of time. In thiscase, these additives will enhance domain reorientation andhence increase spontaneous polarization, dielectric con-stant. The oxygen in the porous ceramics is adsorbed atthe grain boundaries during sintering in air, thus increasing

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LETable I. Composition dependent thermistor constant (B), activationenergy (Ea), sensitivity index (�) and stability factor (SF) of bulk SBN

Resistivity Resistivity Thermistor Activation Sensitivity StabilitySample at 30 �C at 90 �C constant energy index factor

(M�cm) (M�cm) B(K) (eV) (�) (SF)

Sr40 37�40 46�75 409�44 0.068 0.005 0.097Sr50 59�06 66�08 204�09 0.059 0.002 0.048Sr61 44�88 53�43 319�92 0.037 0.004 0.076Sr75 44�88 66�00 707�64 0.065 0.008 0.167

the electrical potential barrier height, which results froman increase of surface states density. An increased resis-tivity is attributed to an increase in the electrical barrierheight of grain boundaries, porosity and the grain bound-ary area.34

The activation energies calculated by using Arrheniusrelation, E = 2�303∗K∗ slope of log versus 1000/T (K−1�curve are tabulated in Table II. The sensitivity index (��,activation energy (E) and stability factor (SF) were calcu-lated using the following formula.34

�= −�

T 2� SF = log

Rmax

Rmin

(3)

It is well known that the electron and hole hoppingbetween Sr2+/Sr3+, Ba2+/Ba3+and Nb3+ /Nb4+ ions, withactivation energy value is responsible for electrical con-duction. The material behaves like a semiconductor, butelectron mobility is also changed at the Curie temperature.As a result resistivity increases with increase in tempera-ture. The resistivity (30 �C and 90 �C), thermistor constant,sensitivity index, stability factor, and activation energy forSBN are summarized in Table I and shown in Figure 5.

It occurs only in polycrystalline ceramics. The thermis-tor constant lies in the range of ∼204K to ∼707K. Theconduction electrons are scattered by the surface of thematerial to alter the path of the electrons. The most sig-nificant scattering mechanisms are scattering from grainboundaries, scattering from rough surfaces, and scatter-ing due to impurities. However, what is clear about thesescattering mechanisms is that processing techniques andimpurity concentration will have a larger effect on thebulk resistivity and that grain boundary size and roughsurface scattering are more prominent for smaller filmthicknesses.35 This SBN thermistors PTC properties maybe used as a temperature sensor, constant temperatureheater (PTC heater), honeycomb air heater (hair dryer),auto fuel evaporator, current limiter, circuit timer, sensorfor motor protection motor start assist and high tempera-ture material due to its PTC behavior.36

3. CONCULSION AND FUTUREWORKIn SBN ferroelectrics, the substitution of strontium pro-duces appreciable changes within the dielectric and elec-trical properties. The PTCR-ceramics was prepared by the

traditional ceramic technology. The variation of complexpermittivity properties with frequency show dielectric dis-persion. For the first time the thermistor properties arereported for this material. The thermistor constant lieswithin the range of 204 K to 707 K. These tunable varyof sensitive lead free ferroelectrics (SBN) could also bepromising material in industrial application level for hightemperature PTC kind thermistor applications.

Acknowledgment: The author Vijaya. Puri gratefullyacknowledges UGC, India for the Award of Research Sci-entist “C”. Shridhar. N. Mathad is very much thankful tothe management and Principal of K.L.E. Institute of Tech-nology, Hubli.

References and Notes1. J. G. Fagan, V. R. W. Amarakoon, Am. Ceram. Soc. Bull. 72, 70

(1993).2. G. Lavenuta, Sensors 14, 46 (1997).3. R. N. Jadhav, S. N. Mathad, Vijaya Puri, Ceramics International

38, 5181 (2012).4. Energoizdat, Semiconducting Barium Titanates, Moscow (1982),

p. 328.5. B. Huybrechts, K. Ishizaki, and M. Takata, Review. The positive

temperature coefficient of resistivity in barium titanate. J. Mater. Sci.30, 2463 (1995).

6. S. R. Syrtsov, V. N. Shut, I. F. Kashevich, A. S. Sedlovsky, andB. E. Watts, Materials Science in Semiconductor Processing 5, 223(2003).

7. S. N. Mathad, R. N. Jadhav, R. P. Pawar, and Vijaya Puri, AdvancedScience Engineering and Medicine 5, 1 (2013).

8. S. N. Mathad and Vijaya Puri, Archives of Physics Research 3, 106(2012).

9. B. M. Tareev, N. V. Korotkova, V. M. Petrov, and A. A. Preobrazhen-sky Electrical and Radio Engineering Materials, MIR Publishers,Moscow (1980).

10. J. Topfer and J. B. Goodenough, J. Solid State Chem. 130, 117(1997).

11. D. W. Kim, K. S. Hong, C. H. Kim, and K. Char, Appl. Phys. A79, 677 (2004).

12. D. Pal, J. L. Pandey, Bull. Mater. Sci. 33, 691 (2010).13. K. Sambasiva Rao, D. Madhava Prasad P. Murali Krishna, and

T. Swarna latha, Ceramics - Silikáty 52, 190 (2008).14. M. George, S. S. Nair, K. A. Malini, P. A. Joy, and M. R. Anthara-

man J. Phys. D: Appl. Phys. 40, 1593 (2007).15. A. Buixaderas, M. Savinov, M. Kempa, S. Veljko, S. Kamba, J. Pet-

zelt, R. Pankrath, and S. Kapphan, J. Phys.: Condens. Matter. 17, 653(2005).

16. A. M. Shaikh, S. S. Bellad, and B. K. Chougule, J. Magn. Magn.Mater. 195, 384 (1999).

17. R. Guo, A. S. Bhalla, C. A. Randall, and L. E. Cross, J. Appl. Phys.67, 6405 (1990).

18. C. Elissalde and J. Ravez, J. Mater. Chem. 11, 1957 (2001).19. C. Elissalde and J. Ravez, J. Mater. Chem. 10, 681 (2000).20. P. A. Jadhav, M. B. Shelar, and B. K. Chougule, Arch. Phys. Res.

1, 92 (2010).21. I. G. Austin and N. F. Mott, Adv. Phys. 18, 41 (1969).22. A. Lakshmanan, Sintering of Ceramics—New Emerging Techniques,

Published by InTech, ISBN 978-953-51-0017-1 (2012).23. S. N. Mathad, R. N. Jadhav, R. P. Pawar, and Vijaya Puri, Electronic

Materials Letters 9, 87 (2013).24. R. C. Kambale, P. A. Shaikh, C. H. Bhosale, K. Y. Rajpure, and

Y. D. Kolekar, Smart Mater. Struct. 18, 085014 (2009).

J. Adv. Phys., 3, 1–6, 2014 5

Dielectric and Electrical Properties of SrxBa1−xNb2O6 (0.40≤ X≤0.75) Ceramics Mathad et al.

ARTIC

LE

25. P.Dasaria, K. SambasivaRao, P. Murali Krishna, and G. GopalaKrishna, Acta Physica Polonica A 119, 387 (2011).

26. S. L. Kadam, C. M. Kanamadi, K. K. Patankar, and B. K. ChouguleMater. Lett. 59, 215 (2005).

27. R. N. Jadhav, S. N. Mathad, and Vijaya Puri, Physica Scripta87, 065801 (2013).

28. Om Prakash, K. D. Mandal, C. C. Christopher, andM. S. Sastry Devendra Kumar, Bull. Mater. Sci. 17, 253(1994).

29. N. F. Mott and R. W. Gurney Electronic Processes in Ionic CrystalsOxford University Press, London (1978).

30. M. I. Kinger, J. Phys. C (GB) 1, 3395 (1975).31. H. Heywang, J. Mater Sci. 6, 1214 (1971).32. R. E. Newnham, Properties of Materials Anisotropy, Symmetry,

Structure, Oxford University Press (2005).33. K. C. Kao, Dielectric Phenomena in Solids, Elsevier Academic Press

(2004).34. S. N. Mathad, R. N. Jadhav, R. P. Pawar, and Vijaya Puri Science of

Advanced Materials 12, 1276 (2012).35. Fred Lacy, Nanoscale. Res. Lett. 6, 636 (2011).36. T. Panmatarith, N. Hayeemayang, N. Baka-lee, and Songklanakarin

J. Sci. Technol. 27, 1113 (2005)

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