Materials Chemistry and Physics 86 (2004) 83–87

Fabrication of Sm- and Mn-doped lead titanate ceramic powderand ceramics by sol–gel methods

Kun Li a,∗, Jin-hua Lib, Helen Lai Wah Chanca Department of Materials Science and Engineering, Jiangsu Polytechnic University, Changzhou, Jiangsu 213016, PR China

b Department of Information Science, Jiangsu Polytechnic University, Changzhou, Jiangsu 213016, PR Chinac Department of Applied Physics, Hong Kong Polytechnic University, Kowloon, Hong Kong, PR China

Received 25 October 2003; received in revised form 25 January 2004; accepted 17 February 2004

Abstract

Samarium- and manganese-doped lead titanate (Pb0.85Sm0.1Ti0.98Mn0.02O3, PSmT) ceramic powder was prepared by sol–gel method.The particle size of the sol–gel powder was about 250 nm. PSmT ceramics were also fabricated from the fine powder. The sintering temper-ature of these ceramics was lower than that of the ceramics made from mixed oxides. The X-ray diffraction patterns show that the ceramicshave a pure tetragonal crystal structure. Thec anda of the unit cell were 4.068 and 3.897 Å, respectively. The piezoelectric and dielectricproperties were measured by using an impedance analyzer and ad33 meter. The dielectric permittivity, piezoelectric constant and thick-ness electromechanical coupling coefficientkt of the ceramics were 180, 51 and 0.44 pC N−1, respectively. The planar electromechanicalcoupling coefficientkp value was lower than 0.05.© 2004 Elsevier B.V. All rights reserved.

Keywords: Sol–gel; Anisotropic ceramics; Doped lead titanate

1. Introduction

Samarium- and manganese-doped lead titanate ceramics(Pb0.85Sm0.1Ti0.98Mn0.02O3, PSmT) have a large piezoelec-tric anisotropy and low dielectric permittivity[1–4]. Theyare good candidates for fabricating piezoelectric devices[5,6]. Large piezoelectric anisotropic property is good for in-creasing the power utility of transducers and decreasing thetemperature coefficient of delay of the devices. In small sizehigh frequency device applications, when low dielectric per-mittivity ceramics were used as active elements, the electricimpedance of elements can be easily adjusted to match the50 conventional standard. So this ceramic has a high po-tential for high frequency piezoelectric device applications.But because the fabrication process strongly affects the ce-ramic properties, it is difficult to control the quality and uni-formity of the products made from mix-oxide method. Inthis work, the fine PSmT ceramic powder was prepared bysol–gel method. Then they were pressed into disks and sin-tered into ceramics. The fabrication procedures were inves-tigated.

∗ Corresponding author.E-mail address: lizunchang@yahoo.com (K. Li).

2. Experiments

2.1. Preparation of PSmT ceramic powder

According to the formula Pb0.85Sm0.1Ti0.98Mn0.02O3,lead acetate tri-hydrate, manganese acetate hydrate andsamarium acetate hydrate were weighed out and placed ina beaker. They were heated to 135C for more than 8 hand dissolved in methoxy-ethonal (MOE). Titanium bu-toxide was added into the solution and the solution wasrefluxing for 1 h. After cooling down, it was filtered intoa beaker and hydrolyzed in moisture for 2 days to forma stiff gel. The brown glass-like gel was dried at about120C for 12 h and milled into powder. This powder waspyrolyzed at 450C for 1 h and annealed at 550C for 1 haccording to the DSC and TGA thermogram. The crys-talline structure was analyzed by X-ray diffraction method.Seen inFig. 2. Finally, the powder was ball-milled in anagate jar with well-polished zirconium dioxide balls. Usingcetyltrimethyl ammonium bromide or n-dodecanethiol assurfactant, the dispersibility of the powder can be improvedand a semitransparent solution could be obtained. But thesechemicals contain bromine and sulfur, which were diffi-cult to be removed in the sintering process. They were not

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2004.02.010

84 K. Li et al. / Materials Chemistry and Physics 86 (2004) 83–87

added into the powder that was used for fabricating ceramicdisks.

2.2. Fabrication of the ceramic disks

The fine PSmT powder was pressed into disks with diam-eter of 12.5 or 25.0 mm. The fine sol–gel powders seriouslyagglomerate together because of the large specific surfacearea, high potential and surface charge. Binder was not usedin disk fabrication. The disks were put into an alumina cru-cible, buried with PSmT powder and covered with a pieceof alumina sheet. They were heated to 850C at the rate of3.5C min−1 and calcinated at this temperature for 2 h. Thenthey were sintered at different temperatures, ranging from1100 to 1285C, to find an optimal sintering temperature.

The ceramic disks were grounded, polished and slicedinto proper shapes according to a standard of the AmericanCeramic Society[7]. The samples were painted with silverpaste on both sides, fired at 650C for 30 min and then poledunder a schemed electric field. After being aged for 2 days ina piece of aluminum foil, the poled samples were measuredby an HP4294A impedance analyzer and ad33 meter.

After an optimal fabrication condition has been deter-mined, the samples fabricated by using this condition weremeasured and the piezoelectric parameters of the ceramicswere calculated. For each shape and fabrication condition,more than five samples were fabricated and measured. Thedata shown in the tables or graphs normally are the average.

Finally the bulk ceramics used in measuring the piezoelec-tric properties were pressed under a pressure of 400 MPa,sintered at 1150C for 1.5 h and poled under an electric fieldof 4.5 kV mm−1 at 120C for 15 min.

3. Results and discussion

Fig. 1shows the DSC and TGA thermogram of the PSmTgel and four peaks are seen. The first peak in the temperatureregion of 80–150C is an endothermic process resultingfrom the evaporation of the organic materials (solvent and

100 200 300 400 500 600 700 80040

60

80

100

DTA

TGA+

T

Wei

ght

(%

)

Temperature (oC)

Fig. 1. DSC and TGA thermogram of PSmT gel.

20 30 40 50 600

1000

2000

3000

Ind

ensi

ty

(110

)

(112

)(1

21)(

211)

(201

)(2

10)(

120)

(102

)(002

)(2

00)(1

11)

(101

)

(001

)(1

00)

Bulk ceramic1150oC 1.5hr

5500C 1hr

2q

Fig. 2. X-ray diffraction patterns of PSmT ceramics sintered at 1150Cfor 1.5 h.

by products) and some water absorbed from the wet air.The second peak is an exothermic peak in the region of280–330C, which may be caused by the organic groupdecomposition. The third one is a large peak in the region of390–450C. It may result from the carbon burning. Thesetemperatures are very important in the pyrolysis process.The fourth one is in the region of 450–500C, which isrelated to the PSmT crystallization process. So the annealingtemperature was set at 550C.

Fig. 2 shows the X-ray diffraction patterns of PSmTpowder and bulk ceramics. The PSmT sol–gel powder wasannealed at 550C for 1 h and the bulk ceramics were sin-tered at 1150C for 1.5 h. It can be seen that both of themhave a pure perovskite structure. After fitting the curve,the accurate 2θ angles of all the main peaks were found.Using these data, the calculateda and c values are 3.897and 4.068 Å, respectively. Comparing these two curves, itis seen that the peaks of the powder were much wider thanthose of the bulk ceramics. This results from the differenceof the crystallite size in the samples. Because the peaks(0 0 1) and (1 0 0), (1 0 1) and (1 1 0), (0 0 2) and (2 0 0)were not well separated, the peak (1 1 1) was selected forcalculating the crystallite size of powder and ceramics. Thefull width at half maximum (FWHM) was calibrated by us-ing a standard sample of polycrystalline silicon. AccordingScherrer’s formulaD = Kλ/B cosθ, the average diameterof the crystalliteD can be calculated, whereλ is the wave-length of the X-ray,θ the diffraction angle,B FWHM andKis a constant, 0.89. The calculated grain sizes of the powderand bulk ceramics are 73 and 2100 nm, respectively. Theparticle size distribution of the PSmT powder was measuredby using a Horiba Capa-700 particle analyzer.Fig. 3 showsthe particle size distribution of the PSmT powder. The aver-age size of the particles was about 250 nm according to theweight average. This means that each particle was normallycomposed of several grains. The calculated grain size ofthe ceramic was smaller than that observed by a scanningelectron microscope (SEM). Because the FWHM decreasesalong with the grain size increasing, the FWHM of largegains will be very small. Even if a slight deviation in FWHM

K. Li et al. / Materials Chemistry and Physics 86 (2004) 83–87 85

0.0 0.1 0.2 0.3 0.40

2

4

6

8

10

12

14

16vo

lum

e (

%)

partickle size ( m)

Fig. 3. Particle size distribution of sol–gel PSmT powder.

calibration and measurement could cause a large deviationin the results. According to the SEM micrograph, the grainsize of the PSmT ceramic is in the region of 2000–3500 nm.

It was found that the disk-shaped ceramics pressed undera lower pressure have a lowerd33 value. So the effect ofapplied pressure during pelleting was investigated.Fig. 4shows theρ, kt and d33 of the ceramic disks plotted asa function of the fabrication pressure. The samples weresintered at 1150C for 1.5 h and polarized under an electricfield of 4.5 kV mm−1 at 120C for 15 min. We can see thatthe disks formed under a low pressure have a lower density.It indicates that there are a lot of pores in these samples.So the mechanical quality factor is very low. The leakagecurrent during the poling process is also very high. Whenthe pelleting pressure was increased to 400 MPa, the ceramicdensity was increased to 7490 kg m−3; andkt andd33 werealso increased to 0.44 and 51 pC N−1, respectively. If thepressure was further increased, theρ, kt and d33 can onlybe increased slightly; but this pressure was too high forconventional mold.

The fine scale of the sol–gel powder may cause this pres-sure effect. The particle size distribution is from 0.1 to0.5m. They have a very large specific surface area andit is difficult to pack them tightly under lower pressure.In the sintering process, the tightly packed particles canbe well degassed and form dense ceramics. If the particles

100 200 300 400 500 600

25

30

35

40

45

50

55

60

d33

(p

C/N

)

Pressure (MPa)

0.0

0.1

0.2

0.3

0.4

6500

7000

7500

8000

Den

sity

(kg

/m3)

kt

d33

kt

Fig. 4. The effect of fabrication pressure on the piezoelectric propertiesof PSmT ceramics.

1100 1150 1200 1250 1300

35

40

45

50

55

d33

(p

C/N

)

Sintering temperature 0C

0.0

0.1

0.2

0.3

0.4

6500

7000

7500

8000

d33

kt

kt

Den

sity

(kg

/m3 )

Fig. 5. Effect of sintering temperature on the PSmT ceramics.

do not have good contacts, pinholes may form during thesintering.

Fig. 5 shows the effect of sintering temperature on theceramic density,d33 coefficient and electromechanical cou-pling coefficientkt. The disk-shaped ceramic samples wereused in the measurements. They were fabricated under apressure of 400 MPa, and sintered for 1.5 h. The poling con-dition was the same as that mentioned above. It was foundthat PSmT materials could not form a dense ceramics whenthey were sintered below 1100C. The densityρ, electrome-chanical coupling coefficientkt and piezoelectric coefficientd33 of the ceramic disks sintered at 1100C are 6790 kg m−3,0.26 and 25–31 pC N−1 respectively. When the sinteringtemperature was increased to 1150–1250C, theρ, kt andd33 of the ceramic disks are 7410–7520 kg m−3, 0.42–0.46and 51 pC N−1, respectively. But when the sintering temper-ature was further increased to 1285C, the ceramic disksbent and stuck together; and the resultedρ andkt were de-creased while thed33 value did not change significantly.

Fig. 6 shows the effect of the poling field on thekt andd33. We can see that when the electric field is higher than4.5 kV mm−1 the piezoelectric properties only increasedslightly with increasing in the poling field. This electricfield is higher than that for poling soft PZT, but lower thanthat for poling manganese doped lead titanate PT. It can beexplained from the difference of domain structure andc/aratio of these ceramics.

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

35

40

45

50

55

60

d33

d33

(p

C/N

)

Poling field (kV/mm)

0.2

0.3

0.4

0.5

0.6

kt

kt

Fig. 6. The effect of poling field on the ceramic properties.

86 K. Li et al. / Materials Chemistry and Physics 86 (2004) 83–87

In ceramics, the grains are randomly oriented. Sponta-neous polarizations of the domains are also oriented alongdifferent directions. In the poling process, spontaneous po-larization of the domains will be switched under the highelectric field. PSmT ceramic has a tetragonal crystal struc-ture. The structure may be regarded as consisting of TiO6octahedra surrounded by Pb (or Sm) cations. Consideringthe lattice site of the ions, dipole direction of the domainsnormally can be switched by 180 or 90. According to theX-ray diffraction patterns of the ceramic, the parameters ofthe PSmT crystal cell wasa = 3.899 Å,c = 4.054 Å. Whenthe polarization of the domain switches by 180, shape (aandc axis) of the unit cell will only change slightly. If thepolarization of the domain switches by 90, the originalaaxis of unit cell will change into thec axis. The size andshape of the cell will also change. The shape of the rela-tive domain will change too. But the clamping effect of thelayer structure 90 domains makes it difficult to change theshape of the domains. So PSmT ceramic has a high coerciveelectric field.

Comparing with pure PT, samarium ions in PSmT take theplace of some lead ions. Manganese ions substitute Ti ionsat the B site. Because Sm ions have a smaller size and moreelectric charges than lead, the parameters of PSmT unit cellchanged froma = 3.894 Å, andc = 4.140 Å (of pure PT) toa = 3.899 Å andc = 4.054 Å. Thec/a ratio changed to 1.04.On the other hand, some A site vacancy broke the balanceof the electric field of lead ions. So, domain switching inPSmT is little easier than that in PT.

Comparing with PZT ceramics, PSmT ceramic has a lowerrelative permittivity. According to the polarization mech-anism, dielectric properties of the ionic crystal under lowelectric field are usually regarded as ionic displacementand electron displacement. In doped PT and PZT systems,the sizes of the lead and oxygen ions were big. So theelectronic cloud distortion of these ions was larger thanthat of the other ions. Ti4+ and Zr4+ ions having a smallsize and high charge are placed in the center of the oc-tahedron. Ionic displacement in polarization mainly resultsfrom these ions. Comparing the composition and the crys-

2.0x106 4.0x106 6.0x106 8.0x106 1.0x107100

101

102

103

104

105

-100

-80

-60

-40

-20

0

20

40

60

80

Impe

danc

e ( Ω

)

Frequency (Hz)

diameter:11.15 mmthickness: 0.563 mm

Pha

se (

deg.

)

Fig. 7. The impedance and phase spectra of a PSmT ceramic disk.

2.0x106 4.0x106 6.0x106 8.0x106 1.0x107104

105

106

107

108

109

-150

-100

-50

0

50

1004.89 x 0.62 X 0.62 mm3

Impe

danc

e (Ω

)

Frequency (Hz)

Pha

se (

deg.

)

Fig. 8. The impedance and phase spectra of a PSmT ceramic rod.

talline structure of doped PT with PZT, the main reasonfor their different relative permittivity may be on the dif-ference in the crystalline structure. Doped PT has a tetrag-onal structure. Thec/a ratio is in the region of 1.04–1.06.Doped PZT has a rhombohedron or tetragonal structure, de-pending on the Zr/Ti ratio. Thec/a ratio is only 1.00–1.02[8]. Ionic displacement under low electric field is relatedto their occupied space. Under the same electric field, thelarger the space is, the more their displacement will be.In the tetragonal structure of doped PT ceramic, there isa small spacing along thea axis, so the displacement inthis direction should be small. Thec axis of the dopedPT is longer than thea axis; but spontaneous polariza-tion made the Ti4+ ions deviate to one side of the octahe-dron. The extent of further displacement under a low elec-tric field is small. In PZT ceramic with a pseuocubic orrhombohedral structure,a andc values are almost the same.There is a large freedom for Ti4+ (or Zr4+) displacementin 3-dimension. So doped PT ceramics have a lower rela-tive permittivity than that of PZT. Furthermore, the samereason can explain that PZT ceramics with a composition

Table 1Measured structural parameters and properties of PSmT bulk ceramics

Parameter Value Parameter Value

c (Å) 4.0684 sE11 (pm2 N−1) 7.49

a (Å) 3.8973 sD33 (pm2 N−1) 7.11

c/a 1.0428 sE33 (pm2 N−1) 8.92

α = β = γ 90 sD44 (pm2 N−1) 15.2

V (Å3) 61.796 cD33 (GPa) 151.2

Formula weight 287.2 σ 0.21Theoretical density

(kg m−3)7720 tanδ 0.003–0.01

Density (kg m−3) 7490 Np (m Hz) 2665kt 0.44 Nt (m Hz) 2063kp >−0.05 Qm (thickness) 561k31 >−0.07 Tc (C) 300–354k33 0.45 εT

33 180d33 (pC N−1) 51 εS

33 141d31 (pC N−1) −1.5 εT

11 202g33 (mV m N−1) 32.0 εS

11 175

K. Li et al. / Materials Chemistry and Physics 86 (2004) 83–87 87

near the morphtropic phase boundary have a larger relativepermittivity.

Figs. 7 and 8show the impedance and phase spectra ofa PSmT ceramic disk (11.15 mm diameter and 0.563 mmthick) and a ceramic rod (4.89 mm× 0.62 mm× 0.62 mm),respectively. It is seen that this ceramics have a stronganisotropy. The planar resonance (at below 1 MHz inFig. 7) was almost too weak to be seen. The parameterscalculated on the basis of the IEEE standards are listed inTable 1.

4. Conclusion

The fine PSmT powder was prepared by using a sol–gelmethod. The average particle size was about 250 nm. Us-ing this sol–gel powder as precursor, a higher pressurewas needed in disk formation. The sintering tempera-ture of PSmT ceramics made from sol–gel powder waslower than that made from mixed oxides. The fabricationprocess can be repeated easily. The ceramics have a ho-mogenous composition and a nearly uniform piezoelectricproperty.

Acknowledgements

The work was supported by Jiangsu Polytechnic Univer-sity and the centre for Smart Materials of the Hong KongPolytechnic University.

References

[1] D. Damjanovic, T.R. Gururaja, S.J. Jang, L.E. Cross, Am. Ceram.Soc. Bull. 66 (1987) 699–703.

[2] B. Jimenez, J.M. Vicente, R. Jimenez, J. Phys. Chem. Sol. 57 (4)(1996) 389–396.

[3] H. Takeuchi, S. Juomura, E. Yamamoto, Y. Ito, J. Acoust. Soc. Am.72 (4) (1982) 1114–1120.

[4] S.Y. Chu, C.H. Chen, Sens. Actuators A 89 (2001) 210–214.[5] Y. Ito, K. Nagatsuma, H. Takeuchi, S. Jyomura, J. Appl. Phys. 52 (7)

(1981) 4479–4486.[6] J.Z. Zhao, C.H.F. Alves, K.A. Snook, J.M. Cannata, W.H. Chen, R.J.

Meyer Jr., S. Ayyappan, T.A. Ritter, K.K. Shung, in: Proceedings ofthe 1999 IEEE Ultrasonic Symposium, vol. 2, 1999, pp. 1185–1189.

[7] An American National Standard IEEE Standard on Piezoelectricity,ANSI/IEEE Standard 176, 1987, pp. 227–273.

[8] E. Nakamura, M. Adachi, J. Harada, T. Ikeda, Landolt-BornsteinGroup III Condensed Matter (perovskite-type oxides), Springer-VerlagHeidelberg, Vol. 16, Subvolume A, (1981), 45–147.