A Comparative Study of Dielectric Properties of PMN and 0.7PMN- 0.3PZN Ceramic

A Comparative Study of Dielectric Properties of PMN and 0.7PMN- 0.3PZN Ceramic

Band S. A. Takarkhede M .V.

Yeshwantrao Chavan College of Engineering , J D College of Engineering ,

Wanadongari – 4441110,Nagpur (M.S.), India Nagpur-441501(M.S.)India

Abstract

Lead magnesium niobate designated as PMN is a representative of ferroelectric relaxors, It exhibits unusually high relative permittivity 12000-20000 with Tc – 9 to 15oC. PMN is of special interest due to its wide technological applications such as MLCS , electrostrictive positioners , electro-optical valves , memory devices , pyroelectric bolometers etc. The dielectric properties (r and tan) of PMN can be tailored by forming solid solution with other ferroelectric materials such as normal (BaTiO3 , PbTiO3), antiferroelectric (PbZrO3), relaxor (PZN). PMN and 0.7PMN+ 0.3PZN ceramic is synthesized by precalcination route suggested by Swartz. XRD analysis confirmed major single phase pervoskite formation in both the ceramics. SEM analysis also revealed single phase formation. Dielectric measurements (r and tan) are taken in the temperature range from -50 to 100oC at four different frequencies 0.1,1 ,10 and 100kHz . Dielectric properties are compared and improvement in the dielectric properties are reported with a focus on microstructure , density and perovskite phase .

Key words: ferroelectric relaxors , perovskite , pyrochlore , microstructure ,dielectric properties .

Introduction

Lead magnesium niobate (PMN) and Lead Zinc niobate designated as PZN both shows ferroelectric relaxor behavior with transition temperature Tc –

15oC and 140oC respectively1-2. Relaxors are

ferroelectrics which show pronounced change in its permittivity with frequency at temperature near curie point and broad phase transition in the temperature dependence of permittivity. They are complex perovskites characterised by A(B B)O3 structure

where Amono or divalent , B is di or tri-valent , B is tetra or penta-valent .They exhibit diffuse phase transition and obey quadratic law in the certain temperature range above transition temperature. On the other hand normal ferroelectrics show sharp first order or second order transition and obeys Curie Weiss law above Tc . PMN and PMN based relaxors exhibit excellent dielectric properties

.However its commercial use is limited owing to its reproducibility problems due to inevitable formation of cubic type pyrochlore phase( Pb3 Nb4 O13 or Pb3 Nb2 O8) along with perovskite phase formation

which dilutes its dielectric properties 3-7. It is more

difficult to produce PZN in single phase than PMN by conventional solid state route although the intrinsic dielectric constant of PZN is also high8. In the present work we have synthesized PMN and 0.7PMN+ 0.3PZN ceramic and the dielectric properties are studied in the temperature range -50 to 100oC at four different frequencies. An attempt has been made to compare the dielectric properties with a focus on microstructure , perovskite phase and density .

Experimental Details

PMN and 0.7PMN+0.3PZN ceramic is synthesized by columbite two stage route suggested by Swartz etal9,10 . The Initial ingradients PbO, MgO , Nb2O5 , ZnO used have been of 99.9% purity (AG Fluka, Switzerland). All the constituents were well dried before weighing. Especiallly MgO being moisture sensitive is dried for prolonged period (24hrs) to ensure desired stoichiometry of the starting compositions.

In the first stage the columbite MgNb2O6 is synthesized by taking MgO and Nb2O5 in 1.02:1 molar ratio as per Eq. 1 and following flow chart Fig1. Due to high refractivity of MgO , 2 mole % of

excess MgO is taken . Similarly ZnNb2O6 is synthesised by taking ZnO and Nb2O5 in 1.:1 molar ratio as per Eq. (2) following flow chart Fig1

MgO + Nb2O5 MgNb2O6 (1)

ZnO + Nb2O5 ZnNb2O6 (2)

Weighing MgO and Nb2 O5 in 1.02 : 1 molar ratio

Or

ZnO and Nb2 O5 in 1. : 1 molar ratio

Weighing MgO and Nb2 O5 in 1.02 : 1 molar ratio

Or

ZnO and Nb2 O5 in 1. : 1 molar ratio

Crushing and thoroughly mixing in acetone

Crushing and thoroughly mixing in acetone

Calcination at 10000C for 2hrs

Calcination at 10000C for 2hrs

Crushing and thoroughly mixing in acetone

Crushing and thoroughly mixing in acetone

Recalcination at 10000C for 2hrs

Recalcination at 10000C for 2hrs

Columbite MgNb2O6 or ZnNb2O6

Columbite MgNb2O6 or ZnNb2O6

Fig1 : Flow chart of Material processing of MgNb2O6 or ZnNb2O6

3PbO +(0.7)MgNb2O6 +(0.3) ZnNb2O6 Pb3Mg0.7Zn0.3Nb2O9 = 3(0.7PMN-0.3PZN) (4)

During sintering the pellets are coverd with the powder of the same composition to overcome the PbO losses .

Weighing PbO and MgNb2O6 in 3: 1 molar ratio

Or

PbO , MgNb2O6 and ZnNb2O6 in 3: 0.7: 0.3 molar ratio.

Weighing PbO and MgNb2O6 in 3: 1 molar ratio

Or

PbO , MgNb2O6 and ZnNb2O6 in 3: 0.7: 0.3 molar ratio.

Crushing and thoroughly mixing in acetone

Crushing and thoroughly mixing in acetone

Calcinations at 800oC for 3 hrs

Calcinations at 800oC for 3 hrs

Crushing and thoroughly mixing in acetone

Crushing and thoroughly mixing in acetone

Pelletisation using uniaxial pressure 1 ton

Pelletisation using uniaxial pressure 1 ton

Heating at 4000C for 2hrs to remove organic

Heating at 4000C for 2hrs to remove organic

Sintering at 11800C for 2hrs

Sintering at 11800C for 2hrs

PMN or 0.7PMN- 0.3PZN ceramic

PMN or 0.7PMN- 0.3PZN ceramic

In the 2nd stage PMN ceramic is synthesised by taking PbO and MgNb2O6 in 3:1 molar ratio as per Eq 3 and following flow chart Fig2 .

3PbO + MgNb2O6 3( PbMg 1/3Nb 2/3 O3 ) =

3PMN (3)

Similarly 0.7PMN – 0.3PZN is synthesized by taking PbO , MgNb2O6 and ZnNb2O6 in

3: 0.7: 0.3 molar ratio as per Eq 4. and following

flow chart Fig 2 .

Fig2: Flow chart of Material processing of PMN

XRD scans of powder of the sintered pellet were recorded using PW 1700 X-ray diffractometer with 1710 controlling unit , using K /F radiation

,scanning rate 2.4o/min 2 range 10-120o. The

fractured surfaces of the sintered pallet has been examined by using 250 MK3 Cambridge UK , scanning electron microscope of high resolution

x20 to X800,000 at 10 mm at RSIC , Nagpur. The bulk density of the sintered pellet is measured by Archimedes principle. Dielectric measurements were taken ( r , tan) using HP4192A LF Impedence analyzer selected in parallel mode of operation in the temperature range -50 to 100o C at 0.1 ,1, 10 and

100 kHz .

Result and Discussion

X-ray diffraction patterns of PMN and 0.7PMN- 0.3PZN are shown in Fig 1 & 2. The appearance of 9-10 major characteristics peaks due to perovskite phase ensures its formation in maximum percentage in both cases. In addition to perovskite phase few lines of weak intensity corresponding to pyrochlore phase are also observed . A critical analysis of XRD data reveals cubic symmetry with lattice constant of 4.053Ao for PMN and that for solid solution is 4.056Ao at room temperature. Almost same value of lattice constant confirm the solid solution formation in 0.7PMN 0.3 PZN .

where I perv and I pyro are the intensities corresponding to major peaks of perovskite and pyrochlore phase respectively.

Perovskite phase

Pyrochlore phase

Perovskite phase

Pyrochlore phase

Fig.2 X-ray diffraction pattern of 0.7 PMN-0.3 PZN

The estimated volume percent of perovskie phase is 94 % in PMN and 91 % for 0.7PMN 0.3 PZN .

The high value of perovskite phase observed in

ht be due to high value of tolerance factor erance factor for PMN is (0.99) and that for

ht be due to high value of tolerance factor erance factor for PMN is (0.99) and that for

PMN mig

. The tol

ceramics

Perovskite phase

Pyrochlore phase

Perovskite phase

Pyrochlore phase

Fig 1. X-ray power diffraction patterns of PMN

PZN is 0.984 as reported by shrout etal13 . For stable perovskite phase t should be between 0.88 to

1. Hence it is difficult to produce PZN than PMN in perovskite phase . The sintered density determined in both the samples is same = 96 %.

   The microphotographs of both the samples are depicted in Fig 3 and 4 . A close look of microstructure reconfirms single major pervoskite phase formation. Few small angular grains of pyrochlore phase are also observed .

   The relative % of perovskite phase present in respective samples is quantitatively estimated using following equation similar to reported by Goo etal 11 and Chen etal 12

   vol % of perovskite phase = 100

   +

   (5)

   Fig.3 Scanning electron microphotographs of fractured surface of PMN ceramic

   Fig.4 Scanning electron miicrophotographs

   of fractured surface of 0.7PMN-0.3PZN ceramic

   The grain size calculated from microphotographs for PMN ceramic is 5.8±0.2 µm and is that of solid solution 6.9 ± 0.6µm.

   The graphs of r , tan versus temperature for four different frequencies are given in fig 5 and 6 respectively.The characteristic relaxor behavior

   ,broad phase transition,dispersion in maximum of r and tan peak with frequency is observed in both the samples. The Tc and r max values at four different frequencies for both samples are presented in table 1.

   Fig 5 (a)Relative permittivity (r)and (b)Dissipation factor(tan)) ,as a function of temperature and frequency for PMN ceramic

   For PMN ceramic at 100 Hz r max = 14200 at Tc

   =-90C and for 0.7PMN 0.3 PZN

   r max = 18200 at Tc =240C.

   The high relative permittivity is observed in case of 0.7PMN 0.3 PZN ceramic than PMN ceramic . This value obtained is also higher than reported value = 14400 by Landin etal 14 for the PMN-PZNceramic .

   Fig 6 (a)Relative permittivity (r)and (b)Dissipation factor(tan)) ,as a function of temperature and frequency for 0.7 PMN-0.3 PZN

   Table 1 : Relative permittivity (rmax) and Curie temperature ( Tc )

   PMN and 0.7PMN 0.3PZN ceramic at four different frequencies .

   Frequency	rmax and Tc
   	PMN	O.7PMN 0.3PZN
   100Hz	14200 , -9oC	18270 , 24oC
   1kHz	13500 , -6oC	17270 , 26.5oC
   10kHz	12730 , -2o C	16110 , 30oC
   100kHz	11842 , 3oC	15130 , 35oC

   Conclusion :

   A high value of relative permittivity along with favourable shift of Tc (= 24oc) near room temperature observed for 0.7PMN-.3 PZN solid solution than PMN ceramic is due to increased grain size and intrinsic high dielectric constant of PZN inspite of relatively low vol % of of perovskite phase.

   References

   1. Smolenskii G A and Agranaovskaya A I ,1958 Sov phy solid state 3 , 1380.

   2. Khuchua N P, Bova V F and Mylnikova I E ,1968 Sov phy solid state 10 [1] 194.

3 Lejeune M and Biolot J P , 1985 Mat Res Bull 20 , 493.

1. Swartz S L and Shrout T R ,1984 J Am Ceram Soc 67{5} 311.

2. Yan M F , Ling H C ,Rhodes W W , 1989 J Mat Res 4 [4] ,930 .

3. Chaput F, Boilot J P , 1990 J Am Cer Soc 72 [8]

   ,1355.

4. Himanshu A K , Choudhary B K ,Singh S , Gupta D C, Bandopadhaya S K and Sinha T P, 2010 Solid state sciences 12 ,1231 .

5. Agrawal D K , Halliyal A and Belsick J , 1988 Mat Res Bull 23, 159.

6. Swartz S L and Shrout T R ,1982 Mat Res Bull 17

   , 1245.

7. Swartz S Land Shrout T R , Schedule W A, Cross L E 1984 , J Am Ceram Soc (5) , 312.

8. Goo E, YamamotoT, Okazaki K ,1986 J Am Cer Soc 69 [8] C188.

9. Chen J , Harmer M P 1990 , 68 [73] C303.

13 Shrout T R , Halliyal A 1987 Am Ceram Soc Bull 66 , 704 ..

14.Landin SM and Schulze WA 1990 J Am Cer Soc 73 , 913