Ž .Sensors and Actuators 74 1999 198–202

Microstructures of the monomorph piezoelectric ceramic actuators withfunctional gradients

Xinhua Zhu ), Jianmin Zhu, Shunhua Zhou, Qi Li, Zhiguo LiuNational Laboratory of Solid State of Microstructures, Department of Physics, Nanjing UniÕersity, Nanjing 210093, China

Abstract

In this paper, a monolithic ceramic bar with a gradient of piezoelectric activity across its thickness is introduced, which is fabricatedby interdiffusion between a high piezoelectric and dielectric compositions. The microstructures of the interdiffusion layer have greateffects on the electric field-induced displacement characteristics and interfacial strength of the monomorph piezoelectric actuator. Thecompositional profiles, phase distributions, and short-range phenomena in the interdiffusion layer formed in the PZTrPNNŽ .PbNi Nb O : PNN monomorph ceramic actuators with functional gradients are investigated by electron probe microbeam analyses1r3 2r3 3Ž . Ž . Ž .EPMA and transmission electron microscopy TEM , respectively. The results show that the thickness of the interdiffusion layer d canbe ordered as d2q)d5q)d4q)d4q. An interface between the rhombohedral and pseudocubic phases is found to exist in theNi Nb Ti Zr

Ž .interdiffusion layer by TEM observations and selected area electron diffraction SAED patterns. The SAED studies also reveal thew x w xpresence of the 1r2 111 superlattice spots along the 111 direction of perovskite cubic unit cell, and the origin of the superlattice is

discussed. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Microstructures; Actuators; Monomorph; Piezoelectric ceramics; Functional gradients

1. Introduction

In recent years micro-electromechanical systemsŽ . w xMEMS have been attracted in various fields 1,2 . Theimportance of the microactuators utilizing piezoelectricity

w xas the driving elements in MEMS have been increasing 3 .Piezoelectric actuators are usually constructed as eitherstacked or bimorph forms. The stacked piezoelectric actua-tors are used where the great force is needed. However, thegenerated small displacement is not sufficient for someapplications where the larger displacement are required. Incontrast, the bimorph ones should be selected. For thoseactuators, they usually have an uneven distribution ofstresses when voltage is applied, which could result inlifetime limitations of devices. It is very desirable todevelop a monomorph with few or no internal stress peakswhen voltage is applied and no structural joint that cancause failure of devices with repeated strain reversals. Inthis paper, a monolithic ceramic bar with a gradient ofpiezoelectric activity across its thickness is introduced,

) Corresponding author

which is fabricated by interdiffusion between a high piezo-electric and dielectric compositions. The application of avoltage can cause the monomorph to bend due to thedifferential stresses induced by the driving field. The mi-crostructures of the interdiffusion layer have great effectson the electric field-induced displacement characteristicsand interfacial strength of the monomorph piezoelectric

w xactuator 4 . The compositional profiles, phase distribu-tions, and short range phenomena in the interdiffusion

Ž .layer formed in the PZTrPNN PbNi Nb O : PNN1r3 2r3 3

monomorph ceramic actuators with functional gradientsare investigated by electron probe microbeam analysesŽ . Ž .EPMA and transmission electron microscopy TEM ,respectively.

2. Experimental procedure

The interdiffusion couple A–B was constructed by firstŽ Ž . Žpressing A powder composition mol% : 0.8Pb Ni1r3

. Ž . .Nb O –0.2Pb Zr Ti O in a mould and then adding2r3 3 0.6 0.4 3Ž Ž . Ž .B powder composition mol% : 0.2Pb Ni Nb O –1r3 2r3 3

Ž . .0.8Pb Zr Ti O and pressing again, both at 30 MPa,0.6 0.4 3Ž .to give pellets Fs20 mm in diameter of about 4 mm

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 98 00315-X

( )X. Zhu et al.rSensors and Actuators 74 1999 198–202 199

w xeach in thickness, details in Ref. 5 . In this way, optimalcontact between the reactant pellets was assured. Theinterdiffusion couple was sintered at 12508C in air for 2 h.The compositional profiles were obtained by means of

Ž .electron probe microbeam analyses EPMA on transversecuts of the annealed interdiffusion couple. The EPMA

Žexperiment was performed in the EPMA-8705QH Daojin,.Japan , operated at 20 kV with 10 mm step width.

Specimens for electron microscopy were thinned me-chanically and Arq-ion beam milled after they had beenmounted onto 3 mm Mo grids for transmission electron

Ž .microscopy TEM observation. Samples were coated withcarbon before examination in the electron microscope.Structural investigations were carried out with JEM-200CXTEM operating at 200 kV, using a double-tilt stage.

3. Results and discussion

3.1. Compositional profiles

The compositional profiles of the diffusion obtainedafter diffusion for 2 h at temperature of 12508C are shownin Fig. 1. The left side, corresponding to the compositionA pellet is taken as zero length. It is revealed that in Fig. 1the development of the profile along the length perpendic-ular to the interface shows first a zone of high constantniobate concentration, whereas that of the zirconate is low.At about 550 mm the niobate concentration starts to de-crease and the zirconate one to increase, as expected. Agraded region means the interdiffusion layer, which islocated between about 550 mm and 1106 mm. On theright, a constant concentration of the niobate and zirconateis reached, which corresponds to the initial concentrationof the Nb and Zr in composition B. Similar phenomena arealso observed in the compositional profiles of the Ni andTi ions. The normalized concentration profiles for thediffusion ions are simulated numerically by the method

w xdescribed in Ref. 5 , the results are shown in Fig. 2. It canŽ .be seen that the width of the interdiffusion layer d

Fig. 1. Compositional profiles of the diffusion ions in the A–B interdiffu-Ž .sion couple diffusion conditions: 12508Cr2 h .

Fig. 2. The normalized concentration profiles simulated numerically forŽthe diffusion ions and that obtained from the EPMA examination diffu-

.sion conditions: 12508Cr2 h .

controlled by ionic diffusion can be ordered as d2q)d5qNi Nb

)d4q)d4q. That can be interpreted as the effects of theTi Zrw xionic radii and valence 5 .

3.2. Distribution of the phases and short-range ordering

The bright-field TEM micrograph of the interdiffusionlayer formed in the A–B diffusion couple is shown in Fig.Ž .3 a . It is observed that an interface exists in the interdiffu-

sion layer. To identify the phase structures of the interdif-² :fusion layer, the room temperature 110 zone-axis se-

lected area electron diffraction patterns were taken fromŽ .the regions marked by I and II in Fig. 3 a , which were

Ž . Ž . Ž .shown in Fig. 3 b and Fig. 3 c , respectively. In Fig. 3 b ,splitting from 718 or 1098 domains is skewed from the² :110 , as marked by an arrow, which provides a directevidence for the existence of the rhombohedral phase on

Ž .the left side of the interface. It is clear that in Fig. 3 cŽ .besides the strong allowed reflections originating from

Ž .the cubic perovskite structure, extra weak superlatticeŽ . Žreflections F-spots appear at positions of hq1r2, kq.1r2, lq1r2 from the fundamental reflections for a cubic

perovskite unit cell with a lattice constant a. The intensityw xof the superstructure spot along the 111 axes varies from

one crystal to another one. The existence of the F-spotsclearly confirms that the ordered regions have a doubledperovskite unit cell, as shown in Fig. 4. Similar results are

Ž . w xreported for Pb Mg Nb O 6,7 . Since the average1r3 2r3 3

NirNb ratio in the PNN–PZT system is 1:2, according to

( )X. Zhu et al.rSensors and Actuators 74 1999 198–202200

Ž . Ž . Ž . ² :Fig. 3. a Bright-field TEM micrograph of the interdiffusion layer formed in the A–B diffusion couple, b and c for the room temperature 110Ž .zone-axis selected area electron diffraction patterns taken from the regions marked by I and II in a , respectively.

w xthe model proposed by Harmer et al. 7,8 , the orderedregions in the present pseudo-ternary system have a Ni:Nbratio that is 1:-2, they carry an overall negative charge

Ž .with respect to the global disordered matrix Nb-rich , andcan be directly charge compensated by the surroundingdisordered matrix, which is effectively donor doped sinceits Ni:Nb ratio is 1:)2.

In the present PNN–PZT system, the Ni and Nb ionsoccupy on the B-site of the perovskite structure, so as dothe Zr and Ti ions from considerations of their ionic radii.Therefore, several ionic substitutions can be hypothesized.One possible substitution of Nb ions by Zr ions would leadto a nonstoichiometric 1:1 Ni–Zr ordering between twoB-site cation sublattices. However such local 1:1 Ni–Zrshort-range ordered ions in the PNN–PZT solid solutioncarries the local B-site valence of q3, which is electrostat-

Žically less favorable than the local 1:1 Zr–Nb ions local.valence q4.5 formed by the substitution of Ni ions with

Zr ions on the B-site of perovskite structure. Because of

the high valence Zr 4q ions replacing the low valence Ni2q

ions, such donor doping effect is expected to compensatethe charge imbalance resulted from the local 1:1 Ni–Nbions. That can in turn stabilize the ordered microdomainsand promote to increase the degree of the short-rangeorder. Therefore, the substitution of Ni ions with Zr ions ismuch more probable than the substitution of Nb ions withZr ions.

Since the 1:1 order leads to compositional partitioning,it is reasonable to expect that the most ordered system will

w xtend to phase separate the earliest 8 . Due to an elasticenergy effect, the stability of the 1:1 ordering in the

Ž X Y .Pb B B O partly depends upon the size difference1r3 2r3 3

of the B-site ions. If a large size difference exists betweenthe BX and BY ions, it is believed that the lattice strain isless for the 1:1 ordering than for disordering or 1:2 stoi-

w xchiometric ordering 7 . The ionic size difference betweenthe Zr 4q and Nb5q ions on the B-site of the perovskite

Ž . w xstructure coordination number 6 is 0.008 nm 9 , and this

( )X. Zhu et al.rSensors and Actuators 74 1999 198–202 201

� 4Fig. 4. Proposed model for the atomic configuration of the 1r2 111 -ordered structure in the PNN–PZT crystalline solution, illustrating theformation of two distinct B-site cation sublattice. b and b for nickelI IIŽ .or zirconate and niobium, respecrively.

value is greater than the difference between the Ni2q and5q Ž . w xNb ions 0.005 nm 9 . Therefore, the stability of the

1:1 Nb–Zr short-range ordering is much higher than thatof the 1:1 Ni–Nb short-range ordering. Consideration ofthe possible 1:1 ordering of Ni and Ti ions yields anaverage valence of q3 on B-site and is therefore electro-statically less favorable than the Ni:Nb order. The Nb:Tiorder is also unlikely since this type of 1:1 orderinginvolves the substitution of small-sized Ti ion with rela-tively large-sized Ni ion which requires less favorable

Ž 2qstrain energy than the Ni:Nb ordering r s0.069 nm,Ni4q . w xr s0.0605 nm 9 . Ti ions are believed to dilute theTi

w xforces responsible for the ordering process 10 . The de-gree of the B-site 1:1 order is decreased with increasingthe PT content, which is identified by the PT contentdependence of the domain morphologies for the

Ž . Ž .0.2Pb Ni Nb O –0.8Pb Zr Ti O samples. Fig.1r3 2r3 3 x 1yx 3Ž .5 a–c illustrate the effect of PT content on the bright field

images of domain morphologies at room temperature inŽ . Ž .the 0.2Pb Ni Nb O –0.8Pb Zr Ti O specimens1r3 2r3 3 1yx x 3

for compositions xs0.30, 0.50 and 0.60, respectively. ItŽ .is revealed that in the rhombohedral-rich side xs0.30 ,

only local random contrast representing short-range orderpolar clusters or nanodomains is observed, and no evi-dence of micro-sized domains is found. For compositions

Žnear the PNN–PZ–PT morphotropic phase boundary xs.0.50 , normal micron-sized domains appear, and become

Ž .stable in the tetragonal-rich field xs0.60 , in which awell-defined 908 ferroelectric macro-domains can readilybe seen.

Based on the facts discussed above, it can be concludedthat the phase structures of the interdiffusion layer formedin the PNN–PZT system are composed of the rhombohe-

w xdral and pseudocubic phases. The 1r2 111 superlatticeŽ .reflections F-spots are originated from the enhanced

short-range, non-stoichiometric 1:1 ordering of the Zr andNb cations.

Fig. 5. Bright-field TEM micrographs at room temperature showing theŽ . Ž .domain morphologies of the 0.2Pb Ni Nb O –0.8Pb Zr Ti O1r3 2r3 3 1yx x 3

Ž . Ž . Ž .samples for compositions a xs0.30, b xs0.50, and c xs0.60,respectively.

( )X. Zhu et al.rSensors and Actuators 74 1999 198–202202

4. Conclusions

Using the EPMA and TEM techniques, the microstruc-Žtures compositional profiles, distribution of the phase and

.short-range ordering phenomena of the interdiffusion layerof the PZTrPNN monomorph piezoelectric ceramic actua-tor with functional gradients have been investigated. It is

Ž .found that the thickness of the interdiffusion layer d ofthe Ni2q, Nb5q, Ti4q and Zr 4q ions decreases in the orderd2q)d5q)d4q)d4q. An interface between the rhombo-Ni Nb Ti Zr

hedral and pseudocubic phases exists in the interdiffusionw x Ž .layer. The 1r2 111 superlattice spots F-spots are origi-

nated from the enhanced short-range, non-stoichiometric1:1 ordering of the Zr and Nb cations on an F-centeredsuperlattices, whose intensities vary from one crystal to

w xanother one along the 111 axes.

Acknowledgements

The authors would like to acknowledge the use of JEOLJEM-200CX TEM at the Analytical Center of NanjingUniversity. This work is supported by the China Postdoc-toral Science Foundation and the Opening Project of Na-

Ž .tional Laboratory of Solid State Microstructures NLSSMS ,Nanjing University.

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Xinhua Zhu was born in 1967. He received the B.S., M.S. and Ph.D.degrees in electronic material and devices all from Xi’an Jiaotong Univer-sity, Xi’an, China in 1989, 1992 and 1995, respectively. From Jan. 1996to Dec. 1997, he worked as a postdoctor research fellow at the National

Ž .Laboratory of Solid State Microstructures NLSSMS , Nanjing Univer-sity. He joined the Department of Physics, Nanjing University in Jan.1998 as an associate professor. His current research interests involve thedevelopment and processing of ferroelectric thin films for memory appli-cations, piezoelectric actuators, sensors and transducers applications,effects of the defect structure on the dielectric and piezoelectric propertiesof ferroelectric materials, and structural studies on the interfaces anddefects in ferroelectrics by TEM and HREM.

Jianmin Zhu was born in 1964. He received the B.S. and M.S. degrees incondensed physics from Nanjing University in 1984 and 1992, respec-tively. He is presently an associate professor of Department of Physics,Nanjing University. His research areas involve the characterization of thedefects and microstructures in nanometer materials, semiconductors, met-als, and oxide thin films by HREM.

Qi Li was born in 1939. He received the B.S. and M.S. degrees fromDepartment of Physics, Nanjing University in 1961 and 1964, respec-tively. He is presently a professor of Department of Physics, NanjingUniversity and the director of Research Laboratory of High ResolutionElectron Microscopy. His current research interest involves studies on thedefects, microstructures and phase transformation of crystals, defects andmicrostructures in nanometer materials.

Zhiguo Liu was born in 1943. He received the B.S., M.S. degrees fromDepartment of Physics, Nanjing University in 1966 and 1980 respec-tively, and Ph.D. degree from Institute of Metals Physics, GoettingenUniversity in 1984. He is presently a professor of Department of Physics,Nanjing University. His current research activities involve several aspectsin physics of condensed matter and materials science and engineering,especially in physical fabrication and application of ferroelectric anddielectric thin films, integrated ferroelectrics, structure and properties ofintermetallics, dynamics of diffusive phase transformations and atom-probe field ion microscopy.