Sol-Gel Transparent PLZT Ferroelectric Ceramics

M. Płońska1,a, W. A. Pisarski2,b, Z. Pędzich3c, Z. Surowiak1,d 1Department of Materials Science, University of Silesia,

2 Śnieżna St., 41-200 Sosnowiec, Poland

2Institute of Materials Science, University of Silesia,

12 Bankowa St., 41-200 Katowice, Poland

3Department of Advanced Ceramics, AGH University of Science and Technology,

al.Mickiewicza 30, 30-059 Kraków, Poland

amplonska@ultra.cto.us.edu.pl; bwpisarsk@us.edu.pl; cpedzich@uci.agh.edu.pl;

dsurowiak@us.edu.pl

Keywords: sol-gel method, PLZT, ferroelectric properties, transparent ceramics

Abstract:

Lead lanthanum zirconate titanate (known as PLZT) ceramic powders have been prepared by

the modified sol – gel method, and underwent consolidation by the hot uniaxial pressing

method. Application of such technique of preparation permitted to receive fine-grained

transparent PLZT x/65/35 ceramics, with x = 8 -10 La at.%. The present publication gives a

detailed account of the relationships between technology and physical properties of obtained

materials. To analyze all ceramics SEM, EDS and mercury porosimetry were performed, and

dielectric properties were studied too. Quite wide light transparency from the visible to near-

infrared range for PLZT ceramics was detected using optical absorption and infrared

spectroscopy.

Introduction

For many years ferroelectric ceramics, based on the lead zirconate – titanate, are

important group of advanced materials. Inserting lanthanum ions into simple Pb(Zr1-xTix)O3

solid-state systems caused, that such ceramics are very important materials for dielectric,

piezoelectric, pyroelectric, ferroelectric and electro-optic applications [1].

The nature and majority properties of lead lanthanum zirconate titanate, shortly names

as PLZT ceramics, are function of the La concentration, and also the Zr/Ti ratio. As described

Y. Xu [2], the 65/35 – Zr/Ti ratio composition yields the most transparency ceramics for

La = 8-16 at.% whereas the 10/90 – Zr/Ti composition gives similar transparency in the 22-28

at.% La range. But receiving transparent ceramics is a very complexes process, because in

PLZT a lot of light diffusions and absorption centers are present, and those phenomena decide

about transparency. Using only the proper method, of powders synthesis and consolidation,

can influence on improvement of quality of such materials.

Conventionally, the PLZT powders are prepared using oxides, as the starting

materials, during a solid – state reaction. Unfortunately, such process usually has a multiple

Advances in Science and Technology Vol. 45 (2006) pp 2489-2494Online available since 2006/Oct/10 at www.scientific.net© (2006) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AST.45.2489

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steps, which have to take places at higher temperature, above 950°C. Such procedure,

involving repeated of milling and firing processes often lead to volatility of lead ions, and to

inhomogeneities of Pb-based materials. As were shown in many works [i.e.3], only chemical

homogeneity and sinterability of powders can give a result in fabrications of high-quality

electrooptic ceramics. The alternatives to conventional method are the wet chemical route of

preparation of ceramic powders, i.e. synthesis by the sol – gel method. It is a low temperature

process, which utilizes chemical precursors as raw materials, and makes it possible to

obtained high density homogeneous powders, with a control on their stoichiometry [4].

The aim of this work was to (i) get knowledge about technology – the conditions of

synthesis and consolidation of PLZT transparent ceramics, and (ii) to study their basic

physical properties.

Sample preparation

For the measurements were chosen the (Pb1-xLax)(ZryTi1-y)1-0,25xO3, compositions

corresponding to the x/65/35 ratio (La/Zr/Ti), with lanthanum at contents x = 8-10 at.% .

The fabrication of PLZT ceramics included two steps: (i) obtaining amorphous PLZT

nanopowders, by the modifying low temperature sol – gel method, and (ii) consolidation of

prepared powders by hot uniaxial pressing method. Figure1. illustrates a flow chart for the

samples preparation, described in detail in our previous works [i.e.5].

In the sol-gel synthesis lead(II) acetate trihydrate (Pb(Oac)2×3H2O), lanthanum(III)

acetate hydrate, (La(Oac)3×1,5 H2O), zirconium n – propoxide (Zr(Oprn)4), and

titanium n – propoxide (Ti(Oprn)4) were used as the starting materials. All components in the

proper ratio were dissolved in n – propyl alcohol, heated and still mixed for 2h, below the

boiling point of the solution. The 5% excess amount of Pb precursor was inserted to all of

prepared compositions, and with amount of 80% acetic acid the pH of this solution was

adjusted to 10. The solution were refluxed and distilled at 125 °C (398 K) and cooled to the

room temperature. It hydrolyzed with adding of distilled water, and next stabilized with

amount of acaetyloacetone (acac). The solution was then aged to yield a gel. Dried gel was

removal of organic parts at 600 °C/6h (873 K). After calcinations obtained powders were

crushed and mixed. An essential part of this step in the sol-gel preparation of powders is

aggregation of small particles. Powder packing is important in the compaction process, and it

is also well known, that aggregation can strongly influence the uniformity of powder packing

[6]. Therefore, to break up the agglomerates, powders were ball – milled in attritor, in ethyl

alcohol for 2h. Such prepared powders were formed into pellets at 200 MPa, by cold pressing

method, and then were sintered by the hot uniaxial pressing method (TS = 1200 °C

(1473 K)/ 2h in air).

Fig.1. Flow chart of the fabrication process of PLZT x/65/35 ceramics.

2490 11th International Ceramics Congress

Experimental procedure

The pore and particle size distributions of green bodies pellets were determined by

mercury porosimetry in a PoreMaster 60 (Quantachrome Instruments), with a wide range of

measuring for 3 nm - 250 µm.

The microstructure and grain size of PLZT sintered ceramics were investigated using a

Scanning Electron Microscopy (SEM) (HITACHI S – 4700) with system of microanalysis -

NORAN Vantage. It was used to qualification chemical composition of samples

Dielectric and ferroelectric measurements were carried out with Quad Tech 1920

impedance meter at an excitation frequency of 1 kHz, and with virtual Sawyer – Tower bridge

with digital registration of the date at frequency 1 Hz, respectively.

Optical absorption spectra were recorded using a VARIAN-CARY UV-VIS-NIR

spectrophotometer. The IR transmission spectra in the frequency region 400 – 4000 cm-1

were

stored on FT-IR BRUKER spectrometer using the KBr pellet disc technique. The spectra were

carried out with a resolution of 2 cm-1

.

Results and discussion

During mixing process the primary particles were raised to soft agglomerates, which

are not crushed when they were pressed. The green pellets of all PLZT samples existed with

powder aggregates. Therefore, porosity and particle sizes were determined in the aggregate.

The results obtained by mercury porosimetry are presented in Fig.2.

The pore size distribution, as the function of the La concentration in PLZT x/65/35 is

shown in Fig.2.(A1, B1, C1). The distributions obtained for the compositions with x = 8 - 9

have relatively narrow character, in the range of 15 -100 [nm], while for PLZT 10/65/35 it is

wider, in the range of 8 - 160 [nm]. It testifies that, in this composition, such value is a major

diversification of the grains size. Since for measurements, all samples were pressed with

P = 200 MPa, the increase of lanthanum content, in investigated compositions, caused change

in pore size distribution. In those samples, with an increasing of dopant amount increased

volume of the smallest pore fractions, which are probably resulted in larger amount of small

grains. Such effect is especially clearly evidenced for PLZT 10/65/35 sample, and an

existence of large content of smallest pore (< 20 [nm]), surely is a result of very fine grains

appearing. Of course, such phenomenon worsens ability to consolidation, during pressing

process, and it hence, increasing of samples porosity (pore volume) with increase of La

content: PLZT 8/65/35 - 90,46 [mm3/g]; PLZT 9/65/35 - 99,07 [mm

3/g]; PLZT 10/65/35 -

112,12 [mm3/g]. The measured pore diameter median value for PLZT 8/65/35 is 65,25 [nm],

however in the case of samples with 9 and 10 La 3+

at.% decreased insignificantly to 61,3

[nm]. The particle size distribution, calculated on the basis of pore size distribution, for three

investigated compositions is presented in Fig.2.(A2, B2, C2). The value of grains median

decrease with an increases of lanthanum content: PLZT 8/65/35 – 148,54 [nm]; PLZT 9/65/35

– 140,7 [nm]; PLZT 10/65/35 – 134,3 [nm]. The increase of introduced dopant quantity

effectively inhibits of crystallites growth. It causes that is that PLZT 10/65/35 composition is

characterized by the smallest particle diameters.

The EDS measurements confirmed the qualitative and quantitative chemical

compositions of sintered PLZT samples. Obtained materials were unporous and crystallized in

form well shaped grains, which size decreased with an increase of lanthanum content in PLZT

x/65/35.

Advances in Science and Technology Vol. 45 2491

Sample Pore size distribution Sample Particle size distribution

A1

PLZT

8/65/35

→

A2

PLZT

8/65/35

→

B1

PLZT

9/65/35

→

B2

PLZT

9/65/35

→

C1

PLZT

10/65/35

→

C2

PLZT

10/65/35

→

Fig.2. Pore size distribution of investigated samples measured by mercury porosimetry

(left column) and calculated particle size distribution (right column).

The EDS patterns with SEM micrographs on the fracture surface of sintered ceramics

are shown in Fig.3.(A, B, C). In the table 1, are presented values of the theoretical and

measured chemical compositions in wt %, for each of the investigated samples, in the count

on oxides. Results of EDS analysis for all samples are very close to the stoichiometric ratio in

each of prepared composition, and confirm a high purity and homogeneity of obtained

materials.

The low - frequency (1 kHz) temperature dependence on the dielectric constant for

PLZT x/65/35 samples, with x = 8, 9, 10 La at.%. is presented in Fig. 4. In those

compositions, as the La content increases, the maximum values of Curie temperature

decreases (TC = Tm), and displace it to the low temperature range. Obtained results are in a

good agreement with those reported in literature [i.e.7], and confirmed well dielectric

properties of prepared ceramics.

Ferroelectric properties of all PLZT materials were proved by measurements of the

ferroelectric hysteresis loops. The results of these investigations are shown in Fig.5. Also in

this case, as can be seen at the P(E) dependence graph, the increase of dopant amount in

ceramics composition change the character of the hysteresis loop. From 8 to10 La at.% for

x/65/35 decrease of values of residual polarization PR and coercive field E were observed.

From the three prepared compositions of PLZT ceramics, only two of them are

transparent, whereas third sample is translucent (Fig.6A). For that reason, optical absorption

and infrared spectra were recorded only for transparent ceramics. Fig.6B. presents optical

absorption spectra of the PLZT ceramic materials. The edge for the both investigated samples

is located in the visible region near 0.4 µm (25000 cm-1

) and does not depend on La content in

ceramic composition

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A.

Table 1.

Theoretical and determined chemical

composition of PLZT x/65/35

B.

C.

Fig.3. EDS patterns and SEM

micrographs on the fracture surface

of sintered samples: PLZT 8/65/35 (A),

PLZT 9/65/35 (B), PLZT 8/65/35 (C).

Fig.4. Temperature variation of dielectric

constant of PLZT x/65/35 (for x = 8 -10 La at%).

Fig.5. P-E hysteresis loops of

PLZT x/65/35 ceramics.

Theoretical PLZT

compositions,

in the count on oxides

[wt%]

Sample x/65/35

PbO

%

La2O3

%

ZrO2

%

TiO2

%

8/65/35 63,32 4,02 24,23 8,45

9/65/35 62,81 4,53 24,21 8,45

10/65/35 62,28 5,05 24,21 8,45

Determined PLZT

compositions ,

in the count on oxides

[wt%]

Sample

x/65/35

PbO

%

La2O3

%

ZrO2

%

TiO2

%

8/65/35 62,92 4,10 24,21 8,77

9/65/35 63,48 4,32 23,12 9,08

10/65/35 62,46 4,97 24,47 8,10

Advances in Science and Technology Vol. 45 2493

A. B. C.

Fig.6. Obtained transparent PLZT ceramics (A), absorption (B) and IR transmission (C)

spectra.

Quite different situation is observed for IR transmission spectra of sol-gel ceramic

materials, which depend on La content in PLZT composition. In this case, the light

transmission is higher about 15 % for PLZT 8/65/35 sample than PLZT 9/65/35 one, as

clearly seen in Fig 6C. However the same location of the IR multiphonon edge for the both

samples takes place. The IR edge is not shifted with changing of La content. Thus, the IR cut-

off defined as the intersection between the zero base line and the extrapolation of the IR edge

is found to be nearly 8 µm (1250 cm-1

). It indicates that our PLZT samples are more

transparent materials than other traditional glasses or ceramics. The similar results were

obtained for the optical glasses based on lead oxide. However, the IR cut-off for PLZT

ceramics is slightly higher than that observed for Pb-based glass materials [8]. It proves our

earlier observations for glasses and ceramics based on heavy metal oxides, that IR

transparency is shifted in direction of longer wavelengths.

Conclusions

Using the modify sol-gel method the nanopowders of PLZT x/65/35 ceramics were

prepared. By employing of the hot uniaxial pressing method, transparent and translucent

ceramics were sintered, with a x = 8-9 and 10 of content La at.% respectively. The high

qualities of all samples were proved by measurement of pore and particles size distribution,

chemical compositions, dielectric and ferroelectric properties. The both PLZT ceramic

materials are an optically transparent from the visible (0.4 µm) to the near-infrared (8 µm)

range.

Acknowledgement

The Committee for Scientific Research partially supported this work under grant

No. 3 T08D 009 29.

References

[1] G. H. Haertling, J. Am. Ceram. Soc. Vol. 82 (1999), p.797

[2] Y. Xu, Ferroelectric Materials and Their Application (Elsevier Science Pub., USA 1991)

[3] Rahaman, M.N., Ceramic Processing and Sinterring (Marcel Dekker Inc., USA 1995)

[4] B. I. Lee, E. J. A. Pope: Chemical Processing of Ceramics (Marcel Dekker, USA 1994)

[5] M. Płońska, D. Czekaj, Z. Surowiak Z., Mat. Science- Poland Vol.21, 4 (2003), p 431

[6] J. Zheng, P.F. Johnson, J.S. Reed: J. Amer. Ceram. Soc. Vol. 73 (1990), p.1392

[7] R. C. Buchanan: Ceramic Materials for Electronics, 2nd

edn. (Marcel Dekker,USA 1991)

[8] W.A. Pisarski, T. Goryczka, B. Wodecka – Duś, M. Płońska, J. Pisarska, Mater. Sci. Eng.

B Vol.122 (2005) 94

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Sol-Gel Transparent PLZT Ferroelectric Ceramics

10.4028/www.scientific.net/AST.45.2489 DOI References[8] W.A. Pisarski, T. Goryczka, B. Wodecka – Du, M. Poska, J. Pisarska, Mater. Sci. Eng. BVol.122 (2005) 94doi:10.1016/j.jallcom.2005.02.020