George, K C

Kunen, Siby

Mathew, Jose

Sebastian, Shajo

IACS Institutional Repository

dum J- Phys.  78 (9), 947-950 (2004)%< U P >Dielectric studies of nanocrystalline copper orthophosphateJose Mathew, Siby Kunen, Shajo Sebastian and K C George* Department of Physics, S t Berchmatis* College, ( ’hanganacherry-686 tOl, Kei.ila, IndiaB-mail kim kcgcorgcCoHancharnet inRet eived 29 Onohvt 2003, accepted 20 July 2004Abstract • Nanoerystallinc coppei orthophosphate {('u ,(P(),)J with different giain sizes was piepaicd by a simple and efticjenl method by (haiiemg the concentration of the reactants The grain si/es were deleimined through X-ta> diffiaction technique 1’he dielectiic piopeities of the Dupaied samples were analyzed as a function of fiequency and temperature It is found that the dielectric constant (r ') and dielectric loss (tan 3) of .ill the samples decrease sharply as the frequency is increased and attains a constant low value at higher frequencies The ac conductivity ) has a low \alue at smaller frequencies which increases, attains a maximum value and then decreases as the fiequency is increased. A stmilui natuie with lowet peaks IS observed m the very high frequency region also There is an upwaid shift in all these values as the lempeiature is laised An additional upwaid shift IS observed when the gram size of the samples is reducedke>words Nanoerystallinc materials, reactant concentration dielectric properties 1»\(’S Nos. 73 40 - c. 73 63 - b. 73 90 4 f.1. IntroductionThe enhanced dielectric properties o f nanoerystallinc materials exhibiting unusual properties play a vital role in the development of new materials [ 1-4J. N ot only the electronic, magnetic and optical properties but also catalytic and electrical properties o f  nanostrucrured materials are much different from those o f the hulk form and depend sensitively on size, shape, composition  n^d preparation conditions f5J. The large surface to volume latii) and the variations in geom etry and electronic structure have dramatic effects on transport and catalytic properties. The large portion o f  atoms located at surfaces or interfaces greatly uilluence the transport o f  ions and electrons through solids. In addition, short-range rearrangement or redistribution o f ions and electrons may cause electrical or chemical polarization. This ^^ e^ct is more pronounced or even  becom es dominant in tianophase dielectric materials. Each interface will polarize in its unique way when the system  is subjected to an applied electric h<^ ld [6], Studies on the effect o f  temperature and frequency on die dielectric behaviour and ac electrical conductivity offer valuable in form ation  about co n d u ctio n  phenom enon  in aanostructured materials \1 \ .  Dielectric behaviour can effectively used to study the electrical properties o f grain boundaries, C^ orresponding Authorsince majorily o f  atoms o f  nanom aterials reside m grain boundaries |8,9j. The origin o f resistance or capacitance, its dispersion with small signal frequencies and the role o f defects may also obtain from these studies [10). This paper reports the study of the effect o f temperature and frequency on the dielectric constant, dielectric loss and ac electrical conductivity o f  nanoerystallinc copper orthophosphate {Cu,(PO^)^} prepared in different grain sizes.2. ExperimentalNanocrystalline copper orthophosphate o f  different grain sizes were prepared by adding stoichiom etric amounts o f copper nitrate, 10%(w/v) aqueou.s solution o f PVA (the matrix) and the aqueous .solution ofsucro.se (the fuel) and then evaporated to a viscous liquid, as described in the literature 111]. This fluffy, voluminous, carbonaceous, pyrolysed mass is then thermolysed to get the nanosized powders. The carbonaceous precursor powders are then grounded and annealed at 600 for 5 hours. The crystal structure and particle size were determined by X-ray powder diffraction technique [ 121. The nanosized samples were consolidated into pellets o f  diameter 13 mm and thickness 1-2 mm at a pressure o f  -*'0.4 GPa. Capacitance (C^). dielectric constant (£ ') ,  dielectric loss tangent (tan S )  and ac conductivity©20041ACS948 Jose Mathew, Sihy Kurien, S/iajo Sebastian and K C Georgeobtained using a computer controlled Hewlett -  Packard 4192 A impedance analyzer [ I3j. The measurements were carried out al different frequencies from 100 Hz to 3 MHz over the temperature range from 3 0 0  K to 373 K.3. Results and discussionThe X-ray d iffraction  pattern o f  n anocrystalline copper  orthophosphate powder o f reactant concentrations 1.0 mL~‘,0.5 mL~* and 0.1 m lr ‘ (hereafter referred to as samples S 1, S2 and S3) are shown in P’lgure 1. The temperatures o f heat treatment significantly influence the reactivity and particle size. The sharp and sin gle X-ray diffraction peaks in the XRD pattern o f  Cu.^(PO^), powder sam ples suggest the formation o f single phase compounds. The particle sizes were estimated from the line broadening o f the diffraction peaks using Scherrer formula and were found to be 28 nm, 26 nm and 23 nm for samples S 1, S2 and S3 respectively. Even though the size o f the particles is not much different from each other and may be assumed within the range o f  experimental error, there is appreciable change in the dielectric properties o f  the present sample when the grain size is changed. This observation clearly indicates that even small changes in particle size may lead to measurable changes in the physical properties o f  nanostructered materials.Diffraction angle (20)Figure 1. The X- ray diffraction spectra of nanocrystallinc copper orthophosphate samples (a) sample SI, (b) sample S2 and (c) sample S3, annealed at 000 for 5 Hrs.The variation o f dielectric constant ( e ') with frequency o f  the applied field for temperatures from 300 K to 373 K o f sample S 1 is shown in Figure 2 (a). It is found that the dielectric constant, for all temperatures, has high value at low frequencies, which decreases as the frequency increases and attains a constant value at higher frequencies. For 300 K, the value o f  e '  is 607 at 100 Hz which decreased to 28 at 100 KHz. The corresponding values for 373 K are 1312 at 100 Hz and 28 at 100 KHz respectively. The nature is similar for the other samples also, but the values are shifted upwards when the grain size is decreased. Figure 2(b) shows the corresponding variations at 300 K. The value o f  a '  changed from 607 at 100H zto43  at 10 KHz for sample SI whileit is from 846 to 52 for sample S2 and from 1120 to 59 for sampleS3 respectively. The value o f  e '  at still higher frequenucs remains almost the sam e at about 28 for all samples.Figure 2. Variation o f d ic lcc ln c  constant (/ ') with rictjunu \ (a) sample Slat different temperatures jind (b) samples SI, S2 ami S^  .i 300 K.The variation ot dielectric loss (tan S )  o f  sample SI ;iv. function o f frequency and IcmperaUirc is shown in Figuic 3 i.n The nature o f  variation o f  tan S  is almost similar to thai nt / For 300 K, tan S  has a value o f 5.7 at 100 Hz, which decrease i<0.5 al 100 KHz and remains almo.st at the same constani l)v value at higher frequencies. At 373 K, the corrcspondmi: variation is from 13.26 to 1.33. The variation o f tan S voin frequency at 300 K for different gram sizes is shown m 1 i^ 'un 3 (b). At 100 Hz, the value is 5.23 for S I , which mcrca.scs to /"' for S2 and 8.93 for S3 respectively.aa-  3 0 0 K  — 3 2 3 K  — 3 4  3 K—  3 6 3 K  -SZ SK(a)242220181614121086H42OHLog fFiisure 3. Variation o f loss Uingent (tan with frequency of (a) sample SI al different temperatures and (b) samples SI, S2 and S3 at 300 KA c c o rd in g  to th eo ry , the d ie le c tr ic  behaviour ol nanostructured materials is prim arily due to different types ol polarizations present in the material [13]. Nanocrysiallint materials posses enormous number o f interfaces, and the larg‘d number o f defects present in these interfaces can cause a change o f positive or negative space charge distribution. When electric field is applied these space charges move and arc trapp*^ ^Dielectric studies o f nanocrystalline copper orthophosphate 949t>y these defects resulting in the formation o f  dipole moments. Phjs IS ca lled  sp ace  ch arge p o la r iza tio n . In terfaces in nanostruclured materials posses many oxygen or nitrogen ion vacancies^ which are equivalent to positive charges giving dipole moments. Exposed to an electric field, these dipoles will rotate, mving a resultant dipole m om ent in the direction t f^ the applied field. This is called rotation direction polarization [ 14], Thus, thehigh value o f e '  in the present study at low frequencies, is mainly due to space charge polarization and rotation direction polarization [13>15].Log fFigure 4, Variation of ac conductivity (rr J  with frequency of (a) sample SI i\{ diflercnt temperatures and (b) samples SI, S2 and S3 at 300 KThe dielectric constant ( s ' )  initially decreases rapidly with increase in frequency but remains fairly constant at higher frequencies for all the samples. This is due to the fact that beyond a certain frequency o f  the applied field, the electron exchange does not fo llow  the alternating field  [16J, and hence the polarization decreases resulting a decrease in . As temperature increases, more and more dipoles will be oriented [ 17], resulting an increase in the value o f  the dipole moment. This is why e /  increases with increase in temperature for fixed frequencies.There is a strong correlation  b etw een  the conduction  mechanism and the d ielectric  behaviour. The conduction  mechanism is due to hopping o f  electrons; as such when the hopping frequency is nearly equal to the frequency o f the externally applied electric field, a maximum o f  loss tangent may he observed [18j. In nanophase materials, the inhomogenities present in the interface layers produce an absorption current119) resulting in a d ielectric loss. This absorption current decreases with increase in frequency o f  the applied field, ^^ Jiulting the dielectric loss to be reduced o f as shown in Figure (^a). The hopping or jum ping probability per unit time increases '^ 'Jth increase in temperature [18j. Correspondingly, the loss ^^ngent also increases. The variation o f  tan S for different grain (shown in Figure 3(b)) is due to size effect [13]. From X-ray line broadening stud ies, it is found that the particle sizediminishes as the reactant concentration is reduced (Figure 1). Even though the change in particle size is very small, it leads to measurable changes in the values o f s '  and tan S .  The volume percentage o f interface boundaries and therefore the amount n f  detects that cause the various types o f polarizations, increase as the particle size is reduced This explains the upward shii’l in the values o f dielectric constant and dieicciric loss tangent with respect to reduction in particle sizeThe variation o f  ac electrical conductivity ) as a function o f frequency and temperature for sample SI is shown in Figure 4 (a). At low frequencies, has a small value, which increases as the fiequcncy increases, reaches a maximum and then decrea.ses with further incrca.se in frequency up to about I MHz. A similar variation o f ac electrical conductivity with reduced values is observed in the high frequency region (> 1 MHz) as shown in the same figure. The nature is similar for other tem peratures, but the values are shifted  upwards as the temperature is raised. The maximum value o f at 3(X) K is 2.6 X 10”  ^ while the corresponding value o f at 373 K is 16 x  10“\  The nature o f variation o f for samples S2 and S3 at 300 K is shown in Figure 4(b). For sample S 1, the maximum value is2.6 X 1(T^  while the corresponding values for samples S2 and S3, arc 7.45 X 10“'’ and 12.1 x 10“  ^ respectively.The ac conductivity is obtained from the data o f  dielectric constant (t'') and loss tangent (tan <S) using the relationO',,, = tan 5, (1)where isthepermittivily o f vacuum and a){~  I r t f )  the angular frequency [17]. Thus, depends strongly on the frequency o f the applied field. The conductivity initially increases with frequency, reaches a maximum and then decrea,ses with frequency for all temperatures.Electrical conductivity in copper orthophosphate is believed to arise through hopping o f  electrons between ions in different vacancy states |20j. The sample can carry a current due to an electron m oving from a Cu^ to a Cu^^ site. The dispersion in conductivity confirms the existence o f electron hopping around lattice imperfections.(7 ,^ is much more sensitive to temperature in the higher temperature regime than the lower temperature regime. The low  temperature ac conduction can be explained using bipolar hopping mechanism whereas the high temperature behaviour is due to thermally activated single polaron hopping |2 11. In copper orthophosphate, conductivity mcrca.ses with the ratio C = CuV Cu [22,23]. The effect is a consequence o f ionic conduction o f Cu+ ions. In other words, Cu3(PO ^ )2  exhibits a ’m ixed  conduction’ phenomenon in which ionic conduction as well as electronic conduction occur.950 Jose Mathew, Siby Kurien, Shajo Sebastian and K C GeorgeThe classical transport theory cannot be applied in the case of nanostructured materials, and striking new phenomena such as universal conductivity fluctuations were discovered. According to Elliot's barrier hopping model, the ac conductivity is given as£T„, =^n7i-NN,>e'(oR‘j 2 4 , (2)where n is the number of polarons involved in the hopping process, is proportional to the .square of the concentration of states, e' is the dielectric constant and R,„ is the hopping distance |21]. When the gram size of the sample is reduced, the hopping distance increa.ses which increases as is evident from the equation. The five-fold increase of when the grain size of the sample is reduced from 28 nm to 23 nm in the present study, is a direct confirmation of this theory.4. ConclusionNanocrystalline Cu3(P0 4 ), samples of different grain sizes were prepared and the particle sizes were determined from X-ray diffraction spectra. The dielectric properties of these samples were analyzed as a function of frequencies from 100 Hz to 3 MHz and temperature ranging from 300 K to 373 K. As the frequency is increased, e' and tan <5 decrease sharply and attain small constant values. The nature remains the same when the temperature is raised but the values are shifted upwards. cr„, strongly depends on frequency and temperature. The peculiar nature of a,,,, is thoroughly investigated as a function of frequency and temperature. There is an additional upward shift in the values of f ' ,  tan 6 and when the grain size of the sample is reduced.AcknowledgmentsThe authors express their gratitude to Mr. Joseph Mathai of the Applied Magnetics Lab., Department of Physics, Cochin University of Science And Technology (CUSAT) for carrying out the dielectric measurements in this work. Thanks are alsodue to the University Grants Commission, New Delhi tor th fellowship under FIP (IX plan .scheme).References111 J Lee, J H Hwang. J J Mashek, T O Mason. A E Millci ,nui r \\ Siegel iWaKT It) 2295 (1995)12] W Puin and P Haiysm  N (ina \in tri. Maivr. 6 88.513] M Abdul Khadai and Binny Tliomas /Viv.v. Siai Sol (a) isy (1995)[4] P Marquardt P/ivs./.<•« 12.3 .365 (1982)|5 | Joshy Jo.se and M Abdul Kludar Naninti M o u i 2(8) l()‘j| (|C)')i,[6] WO Kingery h iim durtion  to (Vw/wi-i (New York lohn Wun (1976)17] r  Mo, Z Yuan, L Zhang and C Xic Noiiour. M a ltr  218] W Sicgciy l>hy\ C lww Solut\ 559 1097 (1994)19] C Siiryanarana Bull Mater Set 17 307 (1994)110] L O Zhang, f  M Mo,T Wang.S Z (Yu and Y N Xie/'/iu S/.| Sol (a) 1.30 291 (1993)[II] P I^amamk ««// Mater. S ri 22 ( i )  .3.35 (1999)112] M Apama, T Bhima.sankarani. S V Suiyanaiayan.i, (i I’rjsjii ,iii,| (i S Kumar Hull Mater Sei 24 (5) 497 (2001)[13] Mo ('hi -  Mcl, Zhang Lide and Wang (luo/hong Nanosn M,u,' 6 823 (1995)114] J Maiei. S Prill and 13 Reichert Solid State lontrs 28 -  .V) 1 (n (1988)115] QTaicev Hhwics of U ieleetrir Mateinils (Moscow Mir I’ublishi: , (1979)] 16| AN Paul, M G Patil, K K Patankai. V I Malbe. K P Malw|.m .n.' S A Paul Bull. Mater. Sei 23 (5) 447 (2(KK))[17] Tanika Kar, R N ( ’houdhary, Scema Sharma and K S Singh In.lnn. J r i m  73 A(4) 453. (1999)[I8| I) Ravindcr and K Vijayakumar AfuOv .V(7 24(5) 505 <2hh0' 453 (1999)119] S Sankaranarayanan Polly and M AUlul Khadar Hull Mata 23 (5) 361 (2000)[20] S S Bcllad. S C Walawe and B K ('hougulc Mater Re^ Hull ,34> 109 (1999)121] Rajani .Shukla Indum J Pure AppI P Im  31 894 (1993)[22] L Murawski, C H Chung and J D Mackenzie J Noiu m  Sal .'2 91 (1979)123] J M Ar/eian and C A Hogarth J. Mater Sei. 26 5353 (1991'

Dielectric Studies of Nanocrystalline Copper Orthophosphate

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Dielectric Studies of Nanocrystalline Copper Orthophosphate

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