J Pak Mater Soc 2008; 2 (1)

A Review on Perovskites and investigation into Phase and Microstructure of PbTiO3 Samples Prepared by Mixed Oxide Route Abdul Manan, Yaseen Iqbal, Mujeeb ur Rahman and Matiullah Khan Department of Physics, University of Peshawar, NWFP, Pakistan Corresponding author E-mail: [email protected] ABSTRACT Technological development has been a key to meeting new demands of the evolving society. Therefore there is a constant need for new and improved materials. The perovskite structure has and continues to attract interest, because of its suitability for applications in telecommunication. Lead titanate (PbTiO3) has strong ferroelectric behaviour because of the Pb-O and Ti-O covalent bonds as compared to BaTiO3 where covalent bonding exists only between Ti and oxygen atoms. In this study PT ceramics were prepared through solid state route and were characterised through XRD for its phase purity and SEM for its Morphology. The main finding of the present work includes loss of PbO affecting the phase purity and product disintegration. Keywords: Perovskites, Dielectric resonator, Lead titanate, Lead oxide, Microwaves applications materials INTRODUCTION Gustav Rose discovered CaTiO3 in Ural mountains in 1839 and named it Perovskite after an eminent Russian mineralogist, Count Lev Alexevich Von Perovski1. Perovskite structure is described by the general formula ABX3 with A and B representing cations and X as anions. The anions are generally Oxygen in naturally occurring perovskites but they can also halogens in synthetic materials e.g. in LiBaF3 and CsCdBr32. The ideal perovskite structure is cubic with BX6 octahedra sharing their corners and the cations “A” occupy the space created by 8 BX6 octahedra. The A site cation has 12-fold coordination with X anions as shown in Figure 1. Under ambient conditions, the BX6 octahedra tilt or rotate about a certain crystallographic axis resulting in a phase transition from a higher symmetry to a lower one, reducing the volume of the unit cell as shown in Figure 2. The tilting may be due to change in temperature or pressure5.

Figure 2: The lattice structure perovskite-type oxides [6].

of

tetragonal

ABO3

The structure of perovskite can be maintained by controlling the tolerance factor (ϒ), a quantity first suggested in 1926 by Goldschmidt7 and this is mathematically given by equation (1)

τ=

R A + RO 2 ( R B + RO )

------------ (1)

Where RA and RB stand for the radii of A and Bsite cations respectively and RO stands for the X-site anions. For ideal perovskite structure (cubic) τ =1, however; the structures other than the cubic remain stable in the range 0.75 < τ <1 i.e. Materials with a lower tolerance factor exhibit a lower symmetry and vice versa8.

Figure 1: Ideal cubic perovskite structure of SrTiO3 [3-4]

The first ceramic ferroelectric material with a high (~1100) dielectric constant ‘BaTiO3’ was discovered in the early 1940s. During the period of World War II, BaTiO3-based dielectric

Abdul Manan, Yaseen Iqbal, Mujeeb ur Rahman, Matiullah Khan : A Review on Perovski……

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J Pak Mater Soc 2008; 2 (1) materials attracted considerable interest because of their use in radar systems during the Second World War 9. In the beginning of mobile telecommunication technology, air-filled metallic cavity was utilized as a microwave resonator making the microwave circuitry too large for practical applications. In 1939 the concept of using a solid dielectric material as a microwave resonator instead of the traditional air-filled metallic cavity resonator was suggested by Richtmyre10. Dielectric Resonator (DR) which is a small ceramic component made from a dielectric material is fundamental to the operation of filters and oscillators in several microwave systems11. In its simplest form, a dielectric resonator is cylindrical in shape and it must have sufficiently high dielectric constant so that it can sustain standing electromagnetic waves within its body because of the reflection at the dielectric-air interface as shown in the Figure 3.

temperature changes. In order to meet the specifications of current and future systems, improved or new microwave components based on engineered dielectric materials with new shape designs are required. Recent developments in microwave telecommunication, satellite broadcasting and intelligent transport systems (ITS) have resulted in an increased demand for DRs. With the recent revolution in cellular phones and satellite communication systems using microwaves as the propagation media, research and development in the field of device miniaturization has been a major challenge in contemporary materials science. In a cellular phone communication, the message is sent from a phone to the nearest base station, and then via a series of base stations to the other phone. There is a combiner/filter unit at the heart of each base station which receives the messages, resolves them from one another, amplifies the signals and finally sends them onto the next base station. For such a microwave circuit to work, part of it needs to resonate at a specific working frequency. The frequency determining component (resonator) used in such a high frequency device must satisfy three criteria including: i. its dielectric constant should be large enough to reduce the size of the resonator because of the inverse relationship of the size of resonator with the square root of the dielectric constant. In practice, the range of dielectric constant for a material is restricted to approximately 20<εr<100 because of the inverse proportionality of its square root with the resonant frequency as given in the equation (2).

f0 =

Figure 3: Interaction of electric and magnetic fields with a resonating cylinder for the simplest resonant mode [12].

In microwave communications, dielectric resonator filters are used to discriminate between wanted and unwanted signal frequencies so as to select the receiving signals from those in the transmitted. When the wanted frequency is extracted and detected, it is necessary to maintain a strong signal. For clarity, it is also critical that the wanted signal frequencies are not affected by seasonal

c

λd ε r

=

c D εr

----------------(2)

ii. The dielectric loss (tanδ) should be small enough to ensure maximum signal discrimination. Thus the quality factor (Q=1/tanδ) should be maximum (Q>30,000 at 1GHz) this is also a frequency (fo) dependent parameter and it decreases with increase in frequency. iii. The temperature coefficient of resonant frequency (TCf) should be ~±2ppm/ºC so that the signal does not drift during the device operation13. Equation (2) shows that if the size of the resonator is fixed, an increase in the dielectric constant causes a decease in the resonant frequency. Similarly at a constant resonant

Abdul Manan, Yaseen Iqbal, Mujeeb ur Rahman, Matiullah Khan : A Review on Perovski……

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J Pak Mater Soc 2008; 2 (1) frequency, the diameter of the resonator decreases with increase in the dielectric constant. Thus a high dielectric constant (up to a certain) limit is required for the miniaturization of the component size which is a requirement of a microwave circuitry. Change in temperature also affects the resonant frequency, dielectric constant and the diameter of resonator. The variation in these parameters can be obtained by differentiating equation (2) w.r.t T

1 ∂f 0 1 ∂D 1 1 ∂ε r =− − ---------- (3) f 0 ∂T D ∂T 2 ε r ∂T

Where

1 ∂f 0 = TCf is the temperature f 0 ∂T

coefficient of resonant frequency,

1 ∂D = αL is the temperature coefficient of D ∂T

linear expansion of the material and

1 ∂ε r = TCε is the temperature coefficient of ε r ∂T

relative permittivity, and hence equation (3) can be written as TCf = - TCє - 2αL ---------- (4) To compensate for the coefficient of linear expansion, the coefficient of resonant frequency must be non-zero. During the past few decades, tremendous amount of work has been carried out regarding the development of new materials or improvement in properties of the existing materials. In 1960, Okaya investigated the dielectric properties of TiO2 (rutile) and showed that it possessed a high dielectric constant (єr~100) and Q-factor (~10000 at 4GHz). However, it was not suitable to be used as resonator because of its high temperature coefficient of resonant frequency (TCf~400 ppm/ºC)14. Dielectric materials were not utilized as dielectric resonators until 1970, when Ba2Ti9O20 with improved dielectric properties (εr=39.8, Q>8000 at 4 GHz, and TCf =2ppm/оC) was developed11. Consequently, different series of compounds with layered perovskite structure such as AnBnO3n+215, AnBn-1O3n16, An+1BnX3n+117 and An+1BnO3n+3 have been developed. A summary of the currently available commercial dielectric materials (with perovskite related structures) for base station applications as dielectric resonators with TCf~0 ppm/ºC is given in Table 1.

Table 1: .Commercially materials used as dielectric station applications Material єr 24 BaMg1/3Ta2/3O3 29 BaZn1/3Ta2/3O3 Ba(Co,Zn)1/3Nb2/3O3 34 SrTiO3–LaAlO3 39 CaTiO3–NdAlO3 45 47 ZrTiO4–ZnNb2O6

available dielectric resonators for base Q×fo(GHz) 250000 150000 90000 60000 48000 34000

Ref. 13 18 18 13 19 20

Thus microwave dielectric materials with perovskite or perovskite related structure have played a key role in the development of telecommunication industry with a wide range of applications, from terrestrial and satellite communication including software radio, GPS and DBS TV to environmental monitoring via satellite. PbTiO3 is an important ferroelectric material with high Curie temperature (TC=765K) and a dielectric constant ~20021. The polarization value for PbTiO3 is 97 µC/cm2 which is due to the presence of Pb-O covalent bond which does not exist in Ba-O pair of atoms in BaTiO36. Pure phase of lead titanate is very difficult to synthesize because of the large c/a value (~1.06) giving rise to stresses in these ceramics22. Therefore, chemical methods including sol-gel, hydrothermal reactions, coprecipitation, molten salt method and emulsion technique have been employed to fabricate PbTiO3. These chemical routes are normally complicated and expensive and are therefore, not considered suitable for mass production. The simplest and economical method is the solid state reaction between lead oxide and titanium dioxide22, however, at high temperatures, loss of PbO occurs leading to cracking of the sample, increased porosity and formation of second phases22. the aim of the present study is to perform investigation of phase and micro-structural analysis of PbTiO3 prepared via mixed oxide route. Experimental work AR grade Lead oxide (litharge) and GPR grade TiO2 and PbO powders were weighed and mixed with a molar ratio of 1:1 to prepare stoichiometric composition of lead titanate. The mixture was subjected to wet ball milling in 2peopanol in polyethylene jars for 24 hours using ½" diameter Yt-toughened ZrO2 balls as grinding media at Materials Research Laboratory (MRL), department of physics University of Peshawar,

Abdul Manan, Yaseen Iqbal, Mujeeb ur Rahman, Matiullah Khan : A Review on Perovski……

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J Pak Mater Soc 2008; 2 (1) Pakistan. The slurry thus obtained was dried overnight in an electric oven. Differential thermal analysis (TGA/DTA) of as mix-milled powders was carried out using TGA/DTA unit (Perkin Elmer Diamond TG-DTA) at the Centralized Resource Laboratory, Department of Physics University of Peshawar. The resulting powders were calcined for two hours at 750ºC with 3ºC/min heating/cooling rate. The calcined powders were pressed into pellets at 100 MPa in a die with 13 mm diameter in a Carver Manual Press. The pellets were then sintered at 1060ºC for 2 hours with heating/cooling rate of 5ºC/min. Phase analysis of as-milled, calcined and sintered samples was carried out using a JEOL X-ray diffractometer (model JDX 3532) with copper (Cu) Kα radiation (λ=1.5418Å) operated at 40 kV voltage and 30mA current. For scanning electron microscopy, samples (5x5x5 mm3) were cut with fine cutting saw and smoothened using a polishing wheel with sand paper and finally polished using a polishing wheel with Nylon cloth and 1µ diamond paste. After fine polishing, the samples were thermally etched at 1000ºC for 40 minutes at 5ºC/min. Finely polished samples were mounted on aluminum stubs using silver past and goldcoated for 240 seconds using a gold sputter coater to avoid the charging problem in SEM. Microstructure of the sample was studied using SEM (JSM5910, JEOL, Japan) operated at voltage 25kV. Results and Discussion A small weight loss (1.85%) was recorded in the temperature range 230-300ºC and the curve remained unchanged onwards up to 1050ºC. Above 1050ºC, ~6% weight loss was recorded. An overall average weight loss of 8.5% was recorded in the temperature range of 30 to1200ºC. The small weight loss at the beginning (below 300ºC) may be due to the escape of moisture or release of air trapped in the mixture. Lead Oxide is volatile above 750ºC, however the critical temperature is still debatable. The weight loss at temperatures ≥1050ºC may be due to the volatilization of PbO content as reported in a number of previous studies23-25. The DTA curve is not very straight forward to be interpreted. However, after a careful analysis, two variations, appearing as exotherms could be identified. The DTA curve appeared to begin rising at 540ºC and continued the same behavior

up to 750ºC. Another rise in the curve appeared to begin at ~900ºC and continued onwards. The presence of two exotherms is not consistent with a previous study45 which reported only one exotherm in the temperature range 500-720ºC. Phase Analysis X-Ray diffraction patterns of as-milled, calcined at 750ºC and sintered at 1060ºC samples are shown in Figure 4. The d-spacings for as milled powder (Figure 4a) matched with the ICDD card#381477 (for Massicot, PbO), ICDD card#40477 (for Anatase, syn TiO2). Most of the d-spacings and intensities corresponding to the XRD peaks from the calcined sample shown in Figure 4b, matched with ICDD card#60452 (macedonite, syn PbTiO3), however, there were also a small number of low intensity peaks whose d-values matched with ICDD card#30573 for PbO. Similarly most of the d-spacings corresponding to XRD peaks from the sintered sample shown in Figure 4c matched with ICDD card#60452 for macedonite, syn (PbTiO3), however, in this sample as well, there were a few low intensity peaks whose d-values matched with ICDD card#30573 for PbO. This indicated that the formation of lead titanate (PT) phase initiated at temperatures ≤750ºC and most of the material had been crystallized after 2 hours at temperature. The observation of PbO as a secondary phase is consistent with the previous results.22-23. Most of the samples sintered at temperatures ≥1060ºC cracked spontaneously or sometimes within a couple of days after sintering. This behavior of lead titanate ceramics has been reported earlier and according to Forrester et al.23, the grain size has a critical role in microcracking and porosity in PT bulk ceramics. The grain size increases with increase in temperature and is therefore, considered responsible for micro-cracking and spontaneous fracturing. Figure 5 shows secondary electron SEM images of lead titanate sintered at 1060ºC for 2h. The bulk of the sample comprised grains with diameters ranging from 1-2.5 µm. The microstructure of the sintered sample revealed the presence of two distinct grain morphologies a) pentagon-shaped ~1 to ~2.5 µm grains in the bulk, which is typical of lead titanate (PT) ceramics and b) elongated grains only on the surface. The sample was highly cracked because of the internal stresses developed in

Abdul Manan, Yaseen Iqbal, Mujeeb ur Rahman, Matiullah Khan : A Review on Perovski……

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J Pak Mater Soc 2008; 2 (1) the ceramics as can be confirmed from the loose packing of the grains. These stresses develop due to high lattice anisotropy in the c/a ratio22-23. 300

M… (PbTiO3), ICDD Card# 60452, P1…(PbO), ICDD Card# 381477, t....(TiO2, Anatase) ICDD Card# 40477, P…(PbO) ICDD card# 30573 M P

Intensity

200 M M P

P P

M M

M

M P

M P M M M

(c)

M

MM

100 P1

P1

t

0 10

25

t

(b) P1 P1

P1 P1 t

2θ→

40

P1

P1

(a)

55

70

Figure 4: X-ray diffractograms of a) as mix-milled powders, b) calcined at 750ºC and c) sintered at 1060°C.

A

B

º

Figure 5: SEM images of the sample sintered at 1060 C and º etched at 1000 C showing PbTiO3 pentagonal grains (top) in the bulk and elongated TiO2-rich lead titanate grains on the surface on the right hand side in image B as well as loosely arranged PbTiO3 gains on the left side of the image.

CONCLUSION 1. The loss of PbO at high temperatures caused the formation of a small volume percent of secondary phase(s) which affected the phase purity of the final product. 2. The grain morphology of PbTiO3 was pentagonal and the secondary phase appeared elongated. 3. Fabrication of pure PbTiO3 ceramics requires low temperature methods to control lead loss and suppress formation of secondary phase(s). REFERENCES 1. Wolfram T, Ellialtioglu S. Electronic and optical properties of D-Band Perovskites. 2006, Cambridge University Press, New York, USA, pp-1 2. Wall F, Jones A P. Rare Earth Mineral: Chemistry, Origin and Ore Deposits, 1996 the mineralogical Society, Chapman & Hall, London, 1996; pp- 42. 3. http://wikis.lib.ncsu.edu/index.php/Perov skite, Accessed on: 05/06/2008 4. http://www.princeton.edu/~cavalab/tutori als/public/structures/perovskites.html accessed on: 05/06/2008 5. Angel R J, Zhao J, Ross, N L. General rules for predicting phase transitions in perovskites due to octahedral tilting. Phys Rev Let 2005;95; 1-4. 6. Miyazawa H, Ishii F, Natori E, Shimoda T, Oguchi T. Contribution of Pb to ferroelectricity in perovskite-type oxides. Ferroelectrics, 2004; 301; 49–53. 7. Woodward D I, Reaney I M, Electron diffraction of tilted perovskites. Acta. Cryst. 2005;B (61); 387-399. 8. Keer H V. Principles of the Solid State. 1st edition 1993, New Age International (P) Ltd Publishers, pp-37. 9. Wise P L. Structure-microwave dielectric property relations in Sr and Ca titanates. PhD Thesis, Department of Engineering Materials University of Sheffield, 2001, pp- 16. 10. Richtmyre R D. Dielectric resonators. J Appl phys. 1939; 10; 391-398. 11. Plourde J K, Linn D F, O′Bryan JR, H M, Thomson J JR. Ba2Ti9O20 as microwave dielectric resonator. J Am Ceram Soc. 1975; 58 (9-10); 418-420. 12. Ubic F J. Processing and analysis of microwave dielectric resonators in the system BaO.Nd2O3.TiO2. PhD Thesis,

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J Pak Mater Soc 2008; 2 (1) Department of Engineering Materials, 13. Freer R, Azough F. Microstructural engineering of microwave dielectric ceramics. J Eur Ceram Soc. (2008); 28; 1433–1441. 14. Ratheeshi R, Sreemoolanadhan H, Suma S, Sebastian M T, Jose, K A, Mohanan P. New high permittivity and low loss ceramics in the BaO-TiO2Nb2O5 composition., Mater. Sci: Matter electron. 1998; 9; 291-294. 15. Lichtenberg F, Herrnberger A, Wiedenmann K, Mannhart J. Synthesis of perovskite-related layered AnBnO3n+2 = ABO3 type niobates and titanates and study of their structural, electric and magnetic properties. Prog Solid State Chem. 2001;29; 1–70. 16. Rawal R, Feteira A, Flores A A, Hyatt N C, West A R, Sinclair D C, Sarma K, Alford N M, Dielectric properties of the “Twinned” 8H-hexagonal perovskite Ba8Nb4Ti3O24. J Am Ceram Soc. 2006; 89 (1); 336-339. 17. Liu Y f, Lu Y N, Xu M, Zhoun L F, Formation mechanisms of platelet Sr3Ti2O7 crystals synthesized by the Molten Salt Synthesis Method. J Am Ceram Soc. 2007; 90(6); 1774-1779. 18. Reaney I M, Idles D. Microwave dielectric ceramics for resonators and filters in mobile phone networks. J Am Ceramic Soc. 2006; 89(7); 2068-2072. 19. Jancar B, Suvorov D, and Valant M, Microwave dielectric properties of CaTiO3-NdAlO3 ceramics. J Mater Sci Lett. (2001); 2071-2072.

University of Sheffield, 1997. 20. Riazikhoei P, Azough F, Freer R. Influence of ZnNb2O6 on the microwave dielectric properties of ZrTi2O6 ceramics. J Am Ceram Soc. 2006; 89(1); 216-223. 21. Dai S B, Juang Y D, Hwang J S, Chen D E, Chu S Y. Effects of calcium on the phase transition and the dielectric properties of Sm-modified PbTiO3 ceramics. J Cryst Growth. 2003; 255; 298-302. 22. Udomporn A, Pengpat K, Ananta S. Highly dense lead titanate ceramics from refined processing. J Eur Ceram Soc. 2004; 24; 185-188. 23. Forrester J S, Zobec J S, Phelan D, Kisi E H, Synthesis of PbTiO3 Ceramics using mechanical alloying and solid state sintering. J Solid Stat Chem. 2004; 177; 3553-3559. 24. Kim S, Cheol J M, Cheol H S. Preparation of undoped lead titanate ceramics via sol-gel processing. J Am Ceram Soc. 1999; 82(2); 289-296. 25. Korotkov L N, Konstantinov S A, Barmin Y V, Bondarev A V, Posmet’ev V V, Kozhukhar S N. Variation of the structure of amorphous lead titanate in the course of crystallization, Tech Phys Lett. 2002; 28 (10); 848-850. 26. Narayanan T R, Padmini P. A method of preparation of high purity lead titanate zirconate solid solutions by carbonategel composite powder precipitation, J. Mater Chem. 1997; 7 (3); 521-526.

Abdul Manan, Yaseen Iqbal, Mujeeb ur Rahman, Matiullah Khan : A Review on Perovski……

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A Review on Perovskites and investigation into Phase ...

J Pak Mater Soc 2008; 2 (1). 16 materials attracted considerable interest because of their use in radar systems during the. Second World War 9. In the beginning of mobile telecommunication technology, air-filled metallic cavity was utilized as a microwave resonator making the microwave circuitry too large for practical.

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