Lead magnesium niobate-lead titanate fibres by a modified sol-gel method

Article abstract of DOI:10.1016/j.materresbull.2005.05.024

Lead magnesium niobate-lead titanate (PMN-PT) ceramic fibres with the nominal composition of 0.65Pb(Mg_1/3Nb_2/3)O ₃-0.35PbTiO₃ have been fabricated by a modified sol-gel method. Due to the difficulty of dissolving the magnesium component, the mixed oxide method was used together with the traditional sol-gel method. To obtain crack-free fibres, pyrolysis was carried out at a very slow heating rate under specific atmosphere to control the organic burnout. The thermal and microstructural properties were investigated using thermogravimetric analysis, scanning electron microscopy and X-ray diffraction. The optimum sintering temperature is 1200°C and yields a fibre with a final diameter of around 100 μm. A single PMN-PT fibre has been poled and its electrical properties were measured. The properties of the fibre are found to be better than that of a ceramic disc.

Full text of DOI:10.1016/j.materresbull.2005.05.024

Materials Research Bulletin 40 (2005) 1955–1967
www.elsevier.com/locate/matresbu
Lead magnesium niobate–lead titanate fibres by
a modified sol–gel method
*
Kwok-Ho Lam , Kun Li, Helen Lai-Wa Chan
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University,
Hunghom, Kowloon, Hong Kong, China
Received 5 August 2004; received in revised form 10 May 2005; accepted 27 May 2005
Available online 22 June 2005
Abstract
Lead magnesium niobate–lead titanate (PMN–PT) ceramic fibres with the nominal composition of 0.65Pb(Mg1/
3Nb2/3)O3–0.35PbTiO3 have been fabricated by a modified sol–gel method. Due to the difficulty of dissolving the
magnesium component, the mixed oxide method was used together with the traditional sol–gel method. To obtain
crack-free fibres, pyrolysis was carried out at a very slow heating rate under specific atmosphere to control the
organic burnout. The thermal and microstructural properties were investigated using thermogravimetric analysis,
scanning electron microscopy and X-ray diffraction. The optimum sintering temperature is 1200 8C and yields a
fibre with a final diameter of around 100 mm. A single PMN–PT fibre has been poled and its electrical properties
were measured. The properties of the fibre are found to be better than that of a ceramic disc.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; B. Sol–gel chemistry; D. Microstructure
1. Introduction
There are many piezoelectric materials used in ultrasonic transducer applications. The most common
material is the lead zirconate titanate (Pb(Zr,Ti)O3, PZT) family which is well-known to have high
piezoelectric strain constant, dielectric constant and electromechanical coupling [1]. In recent years, lead
magnesium niobate–lead titanate (PMN–PT) single crystals have attracted extensive interest due to their
* Corresponding author. Tel.: +852 2766 4162; fax: +852 2333 7629.
E-mail address: Kwok-HoLam*02900857r@polyu.edu.hk (K.-H. Lam).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.05.024

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ultrahigh piezoelectric property compared with conventional piezoelectric ceramics [2]. It is a solid
solution of relaxor ferroelectrics (PMN) with a normal ferroelectric (PT). Properties of PMN–PT are
critically dependent on its composition and PMN–PTwith 35 mol% PbTiO3 is at its morphotrophic phase
boundary (MPB) region [3,4]. At this composition, PMN–PT has anomalously high dielectric and
piezoelectric properties as a result of the enhanced polarizability arising from the coupling between two
equivalent energy states. It has found wide applications as transducers and actuators [5].
Piezoelectric ceramic/polymer 1–3 composites have found wide applications as transducers for
medical ultrasound and underwater acoustics [6]. The dice and fill technique has been widely used to
fabricate 1–3 composites in various laboratories and in commercial productions [6]. Due to the
limitations of pillar geometry (ꢀ120 mm), the operating frequency cannot be further increased.
Incorporation of ceramic fine fibres into 1–3 composites is a promising way of scaling down the
piezoelectric phase and reducing the interference of lateral resonance frequencies [7]. There are three
major methods used to fabricate ceramic fibres including the sol–gel processing [8], the relic process [9]
and the viscous suspension spinning process (VSSP) [10]. Among these methods, the sol–gel technique is
useful in fabricating a large quantity of fibres and resulting in high purity and homogeneity. In the past,
various types of fibres of Al2O3 [11], SrZrO3 [12], PbTiO3 [13] and Pb(ZrxTi1ꢁx)O3 [14–16] have been
fabricated successfully by the sol–gel method. Recently, fine PZT fibres (<80 mm diameter) have been
fabricated by various methods [17,18]. However, there are few reports on fabrication of PMN–PT fibres.
In this work, the fabrication of 65PMN–35PT fibres was based on the sol–gel technique. The
technology is based on solution chemistry in which the metal–oxygen–metal bonds are produced. It relies
on the hydrolysis and condensation reactions of a metal alkoxide precursor solution. During this process,
the gel network grows and so it is important to control the gel homogeneity. Addition of solvent
facilitated the reactions. After the viscosity of the gel became higher, it was spun into fibres. The extruded
fibres were dried at room temperature and then fired to induce a gel to ceramic conversion. Due to the
difficulty of dissolving the magnesium solution, the traditional sol–gel method cannot be used to
fabricate a crack-free PMN–PT fibre. A modified sol–gel method has been developed such that the
magnesium solution is replaced by the oxide and the experimental details are given in the following
sections. Besides studying the fabrication procedures, the properties of the fibres were also studied.
2. Experimental procedures
2.1. Synthesis of the precursors
Two precursor systems were prepared to fabricate the PMN–PT fibre and the procedures for
fabricating the fibres are illustrated in Fig. 1. In the traditional sol–gel method, the precursor system
includes lead(II) acetate trihydrate [Pb(O2C2H3)2ꢂ3H2O; Acros, reagent ACS], magnesium nitrate salt
[Mg(NO3)2], niobium(V) ethoxide [Nb(OC2H5)5; Strem, >99.9% Nb] and titanium(IV) n-butoxide
[Ti(OC4H9)4; Acros, 99%]. Two solvents were used for this precursor system: N,N-dimethylformamide
(DMF; Aldrich, 99%) as a common solvent and 2,4-pentanedione (Acros, >99%) as a solvent of
magnesium nitrate. Before preparing the precursor solution, lead(II) acetate trihydrate was dried at
130 8C and magnesium nitrate was produced by drying the mixing solution of magnesium oxide and
nitric acid. After mixing lead(II) acetate trihydrate and magnesium nitrate, niobium(V) ethoxide and
titanium(IV) n-butoxide were added. The mixture was heated and stirred until it became a solution. The

K.-H. Lam et al. / Materials Research Bulletin 40 (2005) 1955–1967
1957
Fig. 1. Schematic diagram of PMN–PT fibre fabrication.
solution was filtered and then refluxed with a rotary vacuum evaporator for at least 2 h. The sol to gel
conversion occurred and a high viscosity gel was formed. The color of the precursors changed from pale
yellow to dark orange during the concentrating process.
Another precursor system of modified sol–gel method is similar to the previous one except that the
magnesium precursor and the solvent have been changed. This method is a sol–gel method with partially
mixed oxide and is a new way for fabricating PMN–PT fibres. Magnesium oxide (Acros, reagent ACS)
was used instead of magnesium nitrate. Since the magnesium precursor was changed, 2-methoxyethanol
(MOE; Acros, >99%) can be used as a common solvent. During refluxing in a rotary vacuum evaporator,
the oxide powders can be evenly distributed in the solution. The oxide powders in the solution will react
with the precursors during subsequent heat treatments.
2.2. Fibre drawing and heat treatments
Near the gel point of the solution, PMN–PT fibres were extruded through a mold containing 200 mm
diameter holes. The fibres were collected on a spindle. The gel fibres were then kept at room temperature
in air for 2 weeks for drying. Ceramic fibres were obtained after a series of proper heat treatments.
Since the fibres contained a high amount of organic solvent, pyrolysis was carried out under a
controlled atmosphere to regulate the organic burnout. A very slow heating rate of 0.5 8C/min was used
and the pyrolysis temperature was up to 600 8C. The heating profile of pyrolysis was determined by a

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thermogravimetric analysis (TGA). Various atmospheres were tried to reduce the organic burnout rate
including air and carbon monoxide (CO). The optimum pyrolysis condition can be determined by the
quality of fibres produced.
After pyrolysis, the fibres were then calcined and sintered. Since the calcination and sintering
temperatures of fibres relate to that of the bulk ceramic samples, several temperatures were tried and the
optimum temperature and dwell time were determined by the quality of the resulting fibres. For
determining the burnout process and the fibre quality, thermogravimetric analysis, scanning electron
microscopy (SEM) and X-ray diffraction (XRD) were used. TGA (Netzsch, STA 449C) with the heating
rate of 10 8C/min was used to show the effect of heat on the changing weight of the fibres in order to
optimize the profile of pyrolysis. The microstructure of the fibres was monitored by SEM (Leica, Stereo
440). The crystallinity of ceramic fibres was investigated using XRD (Philips, PW3710). With the help of
SEM and XRD, the optimal heating temperature was determined. For evaluating the quality of the fibres,
the density was measured. Since the fibres are assumed having cracks and pores, density was measured
based on the Archimedes’ principle using an electronic balance. The density of sample can be determined
using the following equation:
mdry
r ¼
ꢃ rwater
(1)
mdry ꢁ mwater
where r is the density of the sample, mdry the dry mass of the sample, mwater the mass of the sample
suspended in water and rwater is the density of water which is assumed to be 1000 kg/m3.
2.3. Electrical characterization
Because of the weak mechanical characteristics of a single ceramic fibre, the single fibre of around
0.25 mm long was embedded in an epoxy matrix with a diameter of around 2 mm to carry out electrical
measurements. The chromium–gold electrode was only coated on the cross-sectional area of ceramic
fibre. After poling under an electric field of 3 kV/mm at 60 8C for 0.5 h, the impedance and phase of the
fibre was measured using the impedance gain phase analyzer (Agilent 4294A) with a test fixture (Agilent
16034E). With the resonant and anti-resonant frequencies in the impedance spectrum, the electro-
mechanical coupling coefficient in the thickness direction (kt) and frequency constant in the thickness
direction (NtD) can be calculated based on the IEEE standard on piezoelectricity [19]. Besides, the
capacitance of the fibre can also be measured and so the relative permittivity of the poled ceramic fibre
can be obtained.
3. Results
3.1. Thermal analysis of gel fibres
Long, continuous gel fibres with around 180 mm diameter were drawn by controlling the drawing
speed. The flexible gel fibres became brittle after drying.
The gel fibres contained high organic content and these organic substances were burned out during
pyrolysis and the weight of fibres was reduced. From the TGA curves, the information on the stages of
pyrolysis and on the temperature needed for each stage can also be extracted. It is useful for tailoring the

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Fig. 2. Thermogravimetric analysis of PMN–PT gel fibres fabricated by both the sol–gel and modified sol–gel methods.
Samples were heated at 10 8C/min.
heating profile to prevent fibre cracking during pyrolysis. Fig. 2 shows the TGA curves of fibres
fabricated from both precursor systems. Comparing the two curves, it is seen that the fibre prepared by the
modified sol–gel method is 5 wt.% higher in organic content. In the figure, many points with large
changes in slopes can be observed. Those points indicate that there are rapid mass changes of the samples
at the corresponding temperatures. Those rapid mass changes are the processes of chemical reactions,
which are very important for the pyrolysis of fibres. According to the turning points of the TGA curves,
the burnout process of fibres can be tailored and 1-h dwell time was kept at each stage.
3.2. Optimization of pyrolysis atmosphere
With information from the TGA curve, the organic burnout rate can be controlled and so the degree of
fibre cracking can be reduced. In Fig. 3, the surface of a sol–gel fibre after pyrolysis is shown. The
pyrolysis temperature is 600 8C for 1 h with a very slow heating rate of 0.5 8C/min. In Fig. 3(bottom), the
pyrolysis profile was tailored using a TGA result, 1-h dwell time was kept at each turning point of a TGA
curve. The cracking is greatly reduced after using the TGA result for pyrolysis. Nevertheless, cracks still
existed because the oxygen pressure was high under an air ambient in the crucible, the organic
components in the fibres were burnt out rapidly and produced lots of carbon monoxide and carbon
dioxide (CO2). These gases produced many pores in the fibres or even blew off the surface layer of the
fibre. In order to prevent the fibre from cracking, CO atmosphere was tried to reduce the oxygen content
within the crucible. During pyrolysis, the black carbon powder was put inside the crucible to create CO
atmosphere so that the organic burnout rate can be reduced. Fig. 4 shows the effect of CO atmosphere on
the cross-section and surface of a sol–gel PMN–PT fibre that was heated using the TGA information. It

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Fig. 3. SEM micrographs of a fibre using an air atmosphere (top) without the help of TGA and (bottom) with the help of TGA
during pyrolysis.
Fig. 4. SEM micrograph of the sol–gel fibre using a CO atmosphere during pyrolysis.

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1961
Fig. 5. SEM micrograph of a fibre prepared by the modified sol–gel method using CO atmosphere during pyrolysis.
was found that the fibre is nearly crack-free using a CO atmosphere during pyrolysis. Subsequently, the
modified sol–gel fibres also used a CO atmosphere during pyrolysis and the fibres were entirely crack-
free as shown in Fig. 5.
3.3. Gel to ceramic conversion
After pyrolysis, all the organic content inside the fibres was assumed to be burnt out. However, the
fibres are still in an amorphous state. The gel fibres can be converted to ceramic fibres through calcination
and sintering in lead atmosphere. In order to create the lead atmosphere, the fibres were covered with the
PMN–PT powder during the high temperature heat treatment. The heating rate was increased from
0.5 8C/min to 3.0 8C/min. In order to determine the optimal sintering schedule, XRD analysis was used to
determine the constituent phases in the calcined and sintered samples. From the XRD spectra, the amount
of pyrochlore phase existed in the sample can be calculated by:
Ipyro
Pyrochlore phase ð%Þ ¼
ꢃ 100
(2)
Iperov þ Ipyro
where Ipyro refers to the (2 2 2) pyrochlore peak and Iperov refers to (1 1 0) perovskite peak [20]. As shown
in Fig. 6, the pyrochlore phase decreases from 11.5% to 4.9% with an increase in the calcination
temperature from 800 8C to 850 8C.
In order to find the optimal sintering temperature, XRD analysis and SEM are used to determine the
phase change and monitor the microstructure of fibres, respectively. XRD spectra of sol–gel powder with
different sintering temperatures are shown in Fig. 7. From the spectra, the perovskite phases are similar at
different sintering temperatures and the difference in the resulting pyrochlore phase is only 1%.
To determine the optimal sintering temperature, the SEM technique is used. With different sintering
temperatures, the microstructure of the fibres prepared by the modified sol–gel method changes as shown
in Figs. 8–10. At 1150 8C, the grain is not well grown and the grain boundaries are not sharp. There are
many pores within the fibre. As the temperature increases, the grain size increases. However, the grain
growth became inhomogeneous at high sintering temperature (1250 8C). In comparison, the grains of

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Fig. 6. Comparison of XRD spectra of modified sol–gel powders calcined at 800 8C and 850 8C for 2 h. The solid circle
represents the perovskite phase and the open circle represents the pyrochlore phase.
Fig. 7. Comparison of XRD spectra of modified sol–gel powders sintered at 1150 8C, 1200 8C and 1250 8C for 2 h. The solid
circle represents the perovskite phase and the open circle represents the pyrochlore phase.

K.-H. Lam et al. / Materials Research Bulletin 40 (2005) 1955–1967
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Fig. 8. SEM micrograph of a fibre prepared by the modified sol–gel method sintered in a lead atmosphere at 1150 8C for 2 h.
Fig. 9. SEM micrograph of a fibre prepared by the modified sol–gel method sintered in a lead atmosphere at 1200 8C for 2 h.
Fig. 10. SEM micrograph of a fibre prepared by the modified sol–gel method sintered in a lead atmosphere at 1250 8C for 2 h.

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Fig. 11. SEM micrograph of a sol–gel fibre sintered in a lead atmosphere at 1200 8C for 2 h.
fibres have grown homogeneously at 1200 8C and so the optimal sintering temperature should be
1200 8C. With a series of specific heat treatment, crack-free PMN–PT fibres can be fabricated
successfully using a modified sol–gel method. From Fig. 9, the grain size of PMN–PT ceramic fibre
sintered at 1200 8C for 2 h is around 1 mm in diameter. Since the organic content was burnt out during
pyrolysis, the dimension of fibres was reduced. The average final diameter of the fibres is around 100 mm.
However, the fibre prepared by the sol–gel method still has many cracks after sintering at any temperature
as shown in Fig. 11.
3.4. Performance of a single ceramic fibre
The measured impedance and phase versus frequency spectra of a single PMN–PT ceramic fibre is
shown in Fig. 12. The calculated parameters are summarized in Table 1. The parameters of the ceramic
fibre are compared with the parameters measured from the ceramic disc prepared by the sol–gel powder.
The ceramic disc of 10 mm diameter and 1 mm thick was fabricated by the dry pressing method, which
was sintered at 1200 8C and then poled under conditions similar to that of the fibre. The SEM micrograph
of the cross-sectional of the ceramic disc in Fig. 13 shows that the microstructure and grain size of the
ceramic disc are similar to that of the ceramic fibre (Fig. 9). In Table 1, it is found that the ceramics in both
disc and fibre forms approach 95% theoretical density. Both the relative permittivity K3T3 and NtD of the
fibre and disc are comparable. Because of the geometry, the k33 coefficient of the single fibre was obtained
instead of the kt coefficient. The result is reasonable that the calculated k33 coefficient of the single fibre is
0.50, which approaches the k33 value of the bulk ceramics.
4. Discussion
With the modified sol–gel method, crack-free PMN–PT fibres can be prepared after pyrolysis and
sintering. With the sol–gel method, the fibres had small cracks after pyrolysis and they were cracked
severely after sintering. Comparing the two methods, the main difference is the solvent. Due to the
difficulty of dissolving the magnesium, during the preparation of sol–gel PMN–PT fibre, high boiling

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Fig. 12. The thickness mode of a PMN–PT single ceramic fibre prepared by the modified sol–gel technique.
Table 1
Comparison between the piezoelectric coefficients of the PMN–PT ceramic fibre prepared by the modified sol–gel technique and
that of the ceramic disc prepared by the modified sol–gel technique
Parameters
Ceramic disc
Ceramic fibre
Density (kg/m3)
kt
k33
K3T3 (poled)
NtD (Hz m)
7563
0.39
–
7583
–
0.50
2991
1886
2772
1878
solvents (DMF; 2,4-pentanedione) were used. Since higher temperature will boil away the niobium(V)
ethoxide (boiling point ꢀ 140 8C), those high boiling solvents cannot be totally evaporated out during
distillation and refluxing. Those organics will be liberated during further heat treatments and so cracks
are formed easily. In the modified sol–gel method, low boiling solvent (MOE) was used. It was almost
completely evaporated during refluxing, and so the resulted ceramic fibres can be crack-free.
5. Conclusion
Crack-free ceramic PMN–PT fibres have been successfully fabricated by a modified sol–gel method.
To ensure that the organic content can be completely removed during pyrolysis, magnesium oxide and the
low boiling solvent (MOE) were used. With the help of TGA information, a very slow heating rate and

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Fig. 13. SEM micrograph of a disc prepared by the modified sol–gel method sintered in a lead atmosphere at 1200 8C for 2 h.
specific atmosphere were used to control the organic burnout rate. After calcination and sintering under
optimized conditions, the fibres are well crystallized with nearly complete perovskite structure. The final
diameter and grain size of ceramic fibres are around 100 mm and 1 mm, respectively. The electrical
characterization of the single ceramic fibre reveals that the modified sol–gel method can be used to
fabricate well-crystallized PMN–PT ceramic fibres with good properties especially the electromecha-
nical coupling coefficient. In future work, the PMN–PT fibre will be used to fabricate PMN–PT fibre/
epoxy 1–3 composites for high-frequency ultrasonic transducer applications.
Acknowledgements
This work was supported by the Hong Kong Research Grants Council (RGC, project No. PolyU 5190/
99P) and the Centre for Smart Materials of the Hong Kong Polytechnic University.
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