U N D E R C O N S T R U C T I O N
Text by Arne Lüker
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This article emerged from one of my unpublished scientific papers
The Quest for Pb-free piezoelectrics and ferroelectrics - a review
Lead zirconate titanate (PZT) based piezoelectric materials are well known for their excellent piezoelectric properties. However, considering the toxicity
of Pb and its compounds, there is a general awareness for the development of environmental friendly lead-free materials as regulated from the European Union.
Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been
made to review the recent developments on lead-free piezoelectric and ferroelectric materials. In this context, perovskite systems such as barium strontium titanate,
bismuth sodium titanate, alkali niobates (ANbO3), etc., and non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail.
From this study, it is concluded that some Pb-free compositions show stable piezoelectric and/or ferroelectric responses even though they do not match the overall
performance of PZT. The aim of this paper is to stimulate the growing research on this subject. This topic is of current interest to the researchers worldwide as
evidenced from the large number of recent research publications. This has motivated us to come out with a review article hoping it would motivate the researchers
already working in this area and to trigger the attention of researcher working on other topics of material sciences. In more or less short detours to linked
subjects we have made the effort to make this review more interesting to a larger field of researchers.
Toxic effects of lead
Lead-free ferroelectrics with perovskite structure
The (Bi0.5Na0.5)TiO3 system
The Bi0.5(Na0.5,K0.5)TiO3 system
The BNT-Ba(Ti,Zr)O3 system
The Bi(Na,K,Li)TiO3 system
The BNT-BKT-BT system
The KNbO3-NaNbO3 system
Non-perovskite structured lead-free ferroelectrics
Bismuth layer-structured ferroelectrics
Non-perovskites with lead-free tungsten-bronze structure
Remarks and limitations
Figure 1: Rochelle Salt crystal
Ferroelectrics are a subclass of piezoelectrics, implying that all ferroelectrics are piezoelectric but not the other way round. Ferroelectricity was discovered by Joseph Valasek in Rochelle salt in 1920 [1, 2]. The term ›ferro‹ describes the apparent analogy of ferroelectrics to ferromagnetism, viz. a permanent polarisation which can be changed with an applied electrical or magnetic field in case of ferromagnetic materials (hysteresis curve). PZT and BaTiO3 were the first commercial ferroelectric materials used mainly for sonar systems to detect submarines and other military applications in the Second World War. Everyone who watched the 1981 German epic war movie "Das Boot" (engl. The Boat) remembers the sharp ›chirp‹-sound when the allied Destroyers detected the submarine - that was the sound of the sonar system. Later on ferroelectrics conquered the civil markets, in particular high capacitance, small volume capacitors in early televisions and radio circuits, as well as active elements for phonograph pick-ups, accelerometers, and ultrasonic generators. Pb-based materials became the workhorses in these applications since they are easy to fabricate and show superior properties in a variety of attributes like a very high dielectric constant, low loss, high tunability, large d33- and d11-values and so on .
However Pb-oxide, which is a component of PZT, is highly toxic and its toxicity is further enhanced due to its volatilization at high temperature particularly during calcination and sintering causing environmental pollution during manufacturing.
Nowadays international efforts in removing toxic substances from everyday applications are increasing. The EU passed the »Waste Electrical and Electronic Equipment« (WEEE) and »Restriction of the use of certain Hazardous Substances in electrical and electronic equipment« (RoHS) in 2003 . While the WEEE regulates the disposal, reuse and recycling of electronic equipment, the RoHS is a necessary requirement to ensure this can be accomplished safely without endangering the environment or people´s health. Mercury, cadmium, hexavalent chromium, the flame-retardants Polybrominated biphenyl (PBB) and Polybrominated diphenyl ethers (PBDE) and, the focus of this work, Pb, have been identified as a primary risk during recycling, disposal or just improper use.
Figure 2: The PZT crystal structure above (left) and below (right) the phase transition temperature (TC); (left) cubic
paraelectric and (right) tetragonal ferroelectric.
The perovskite type ferroelectrics are promising candidates for Pb-free piezoelectric ceramics because its anisotropy in piezoelectric properties are large compared to other ferroelectrics. A list of lead-free piezoelectric materials and their properties is presented in Table 1.
(Na0.5Bi0.5)0.92-Ba0.08TiO3+x mol% MnCO3
AE = Mg, Ca, Sr, Ba
The large piezoelectric response of PZT results from two factors. Primarily, the stereo-chemical activity of the 6s2 lone pair on the Pb-ion causes large structural distortions from the cubic perovskite phase that results in strong coupling between the electronic and structural degrees of freedom . Bi-based compounds have similar or larger levels of ion off centering than Pb-based compounds, driven by the stereo chemically active 6s2 lone pairs on the Bi3+ ion. This leads to large ferroelectric polarizations. In most common ferroelectrics, ion off centering is mainly contributed by the B-site cations to the ferroelectricity so as to increase the chemical bonding between their valence d-orbital and the surrounding oxygen 2p-orbital, the so-called second-order Jahn Teller-effect . In contrast to Pb, bismuth is non-toxic in its oxide forms; indeed, the active ingredient of the popular antacid (a mild drug which neutralizes stomach acidity) is bismuth salicylate. Some of the toxic effects of Pb and its hazardous consequences on health are discussed in the next section
Gingival lead line
*) Encephalopathy means disorder or disease of the brain. In modern usage,
encephalopathy does not refer to a single disease, but rather to a syndrome of global brain dysfunction; this syndrome can be caused by many different illnesses.
**) Arthralgia literally means joint pain; it is a symptom of injury, infection, illnesses (in particular arthritis) or an allergic reaction to medication.
The main route of absorption in adults is the respiratory tract where 30-70% of inhaled Pb (mostly the inorganic form like oxides and salts) goes into the circulatory system. High blood lead levels in adults are also associated with decreases in cognitive performance and with psychiatric symptoms such as depression and anxiety . It was found in a large group of current and former inorganic Pb-workers in Korea that blood Pb-levels in the range of 20-50 mg/dL were correlated with neuro-cognitive defects . Increases in blood Pb-levels from about 50 to about 100 mg/dL in adults have been found to be associated with persistent, and possibly permanent, impairment of central nervous system functions . Pb has three important biochemical properties that contribute to its toxic effects on humans. First, Pb being an electropositive metal has high affinity for enzymes, which are essential for the synthesis of hemoglobin. Second, divalent Pb acts in a manner similar to calcium inhibiting mitochondrial oxidative phosphorylation thus reducing the intelligence quotient. Pb can also affect the genetic transcription of DNA by interacting with nucleic acid binding proteins . The most important initial aspect of management of Pb poisoning is the removal of the patient from the source of exposure  and second by using chelating agents (EDTA) that form complexes with Pb and hence are excreted out [15-18].
An interesting historical remark: Since 1982, signs of a high exposure to Pb have been identified in the human remains of members of John Franklin's so-called lost expedition to the Arctic, 1845-8. Franklins crew just turned mad at the end as a result of Pb poisoning. They left their ships behind and moved on with nothing else than empty dinghies and silver spoons towards the north pole. Tinned food has been suggested as the source of this Pb. But there is a strong evidence that the primary source of this Pb was not tinned food, which was in widespread use in the Royal Navy at the time, but the unique water system fitted to the expedition's ships .
In this review, an overview of current developments in various Pb-free piezoelectric and ferroelectric ceramics, and the effects of various dopants to enhance the piezoelectric and ferroelectric properties is presented.
Figure 3: Mica in its natural form (a) and capacitors made of mica (b)
However, the Germans forced the Americans to develop a new material for their capacitors and in 1941 Thurnmaurer and Deaderick at the American Lava Corporation filed the U.S, Patent No. 2,429,588 for mixed BaO-TiO2 ceramics .
The high permittivities were found by measurements made at the Erie Resistor Company, with dielectric constants exceeding 1000, ten times greater than any other ceramic known at that time, such as TiO2 (er=110).
Figure 4: The effect of Sr-doping on the transition temperatures of BT, based on 
BaTiO3 (BT) has a relatively high electromechanical coupling factor (k33) and has been partially used for piezoelectric applications such as sonar although its main use is for capacitor applications.
It has the advantage of easy manufacture by various ceramic techniques. BT has a low Curie temperature (TC = 120 °C) causing the working temperature range of this ceramic narrow for actual piezoelectric applications.
For ferroelectric devices it is an ideal candidate because its TC can be decreased with various dopants below room temperature (Fig. 4). The most prominent dopant is strontium (Sr). Since that discovery, (Ba,Sr)TiO3 began to rock the military market
For that reason it is worthwhile to make a little detour to the subjects of phase shifters.
The most impressive change in the size of devices can be seen in phase shifters for radar technology (Fig. 5). Phase shifters are used to change the transmission phase angle (phase of S21) of a network. Ideally phase shifters provide low insertion loss, high power handling, instantaneous phase change response, and approximately equal loss in all phase states. While the loss of a phase shifter is often overcome using an amplifier stage, the less loss, the less power that is needed to overcome it. Most phase shifters are reciprocal networks, meaning that they work effectively on signals passing in either direction (which comes in handy when you are designing a transmit/receive system like in a mobile phone).
Figure 5: Phase shifters for radar applications. (a) 1960 - an intermediate-frequency six-bit digital phase shifter. Each bit
consists of a length of coaxial cable that can switched into the signal path to produce the desired phase shift. (b) 1966 - a Westinghouse production model of a four-bit C-band ferrite phase
shifter, with the waveguide cover removed. (c) 1961 - a four bit low-loss hybrid L-band diode phase shifter. The stripline ground planes have been removed for clarity.
(d) 1995 - A MEMtronics phase shifter based on capacitive MEMS switches. (e) 2002 - A distributed phase shifter with voltage-tunable BST-varactors between the signal- and ground lines.
Phase shifters can be controlled electrically, magnetically or mechanically. Phase shifters can be analog or digital. Analog phase shifters provide a continuously variable phase, perhaps controlled by a voltage. Electrically controlled analog phase shifters can be realized with varactor diodes that change capacitance with voltage, or nonlinear dielectrics such as barium strontium titanate (BST), or ferroelectric materials such as yttrium iron garnet. A mechanically-controlled analog phase shifter is really just a mechanically lengthened transmission line - as perfectly seen in Figure 5a .
Currently, most phased array antenna systems rely on ferrite - Figure 5b  - or MEMS phase shifters - Figure 5d . Ferrite phase shifters are slow to respond to control signals and cannot be used in applications where rapid beam scanning is required. MEMS (micro-electro-mechanical systems) phase shifters have much faster response speeds (measure in milliseconds), however their major drawback is that they have high losses at microwave and millimeter-wave frequencies. Other disadvantages with MEMS phase shifters is that they have limited power-handling capability (perhaps 100 mW) and they may need expensive packaging to protect the movable MEMS bridges against the environment. MEMtronics developed phase shifters based on their own proprietary low-loss capacitive MEMS switches to enable passive high performance phased array antennas from X-band through Ka-band and beyond. The most important features of these phase shifters are their very low insertion loss, negligible power consumption (10s of nanojoules per switch cycle), and high linearity (third order intercept points exceeding +66 dBm). The chip size is 1.7mm x 5.4mm. Figure 5e shows a newly proposed device topology by Robert A. York et al. . Their approach is to periodically load a coplanar waveguide transmission line with tunable BST parallel plate capacitors. This new process provided 240° phase shift with an insertion loss of only 3 dB at 10 GHz at room temperature with only 17.5 Volts. The circuit has demonstrated a record figure of merit 93°/dB at 6.3 GHz and 87°/dB at 8.5 GHz at room temperature.
Actually this approach is a mixture of the ferroelectric phase shifter and MEMS phase shifter technology since it uses both the advantages of these technologies. It combines the low-loss properties of BST at microwave frequency with the distributed transmission line philosophy of the MEMS phase shifter which provides wide bandwidth and ease of design.
PIN diodes can also be used to make very low-loss phase shifters, as seen in figure 5c , but who wants to deal with thousands of devices that are controlled by current, not voltage?
The main drawback of this material is its high conductivity, consequently giving problems in the poling process. In addition, BNT ceramics need high sintering temperature (>1200 °C) to obtain densely packed ceramic. However BNT is considered to be a promising candidate for lead-free piezoelectric ceramics with balanced ferroelectric properties. In the following sections we discuss the effect of various dopants on BNT ceramics.
Effect of dopants on BNT ceramics
For the pure BNT system, d33 lies in the range of 57- 64 pC/N as discovered by researchers pointed out in Table 3. The large piezoelectricity is expected in the BNT-based solid solutions with a morphotropic phase boundary (MPB). It has been reported that BNT-based compositions modified with BaTiO3, BiKTiO3, NaNbO3, BiFeO3, MnO2, Sc2O3, La2O3, CeO2, etc. [29-42] showed improved properties and easier treatment in the poling process when compared with pure BNT ceramics.
(1-x) (Na0.5Bi0.5)TiO3 - x BaTiO3
Figure 6: Phase relation between (Bi0.5Na0.5)TiO3, KNbO3 and (Bi2O3⋅Sc2O3)0.5 
Meanwhile, Ce4+ with a radius of 0.94 Å goes into the Ti4+ site (r = 0.74 Å) and changes the space charge. La2O3 - a typical soft additive for PZT ceramics - is occupying the Bi3+ site or the Ba2+ site .
x = 0
x = 0.56
x = 2.6
x = 2.97
Figure 7: Hysteresis curves of (white) BNBT+0.56 mol% Mn and (turquoise) PZT
At this point it is worthwhile to take a closer look at a ferroelectric hysteresis loop. The important characteristic of ferroelectric materials is polarisation reversal (or switching) by an applied electric field. One consequence of the domain-wall switching in ferroelectric materials is the occurrence of the ferroelectric hysteresis loop (figure 8). The hysteresis loop can be observed experimentally by using a Sawyer-Tower circuit . At small values of the AC electric field, the polarization increases linearly with the field amplitude, according to relation
cij ⋅ Ej
where cij [F/m] is known as the dielectric susceptibility of the material. This corresponds to segment AB in figure 8. In this region, the field is not strong enough to switch domains with the unfavourable direction of polarisation. As the field is increased the polarisation of domains with an unfavourable direction of polarisation will start to switch in the direction of the field, rapidly increasing the measured charge density (segment BC). The polarisation response in this region is strongly nonlinear and the former equation is no longer valid.
Figure 8: Ferroelectric (P –E) hysteresis loop. The hexagons with the gray and black regions represent schematically repartition of two polarisation states in
the material at different fields. The symbols are explained in the text. The actual loop is
measured on a (111)-oriented 1.3 mm thick sol-gel Pb.Zr0:45Ti0:55/O3 film .
The coercive field, spontaneous and remanent polarization and shape of the loop may be affected by many factors including the thickness of the film, the presence of charged defects, mechanical stresses, preparation conditions, and thermal treatment. The tilt of the white loop for BNBT doped 0.56 mol.% Mn in figure 7 can be explained by the presence of a constant-valued capacitance layer at the interfaces of the electrodes and the ferroelectric/piezoelectric film [60, 61]. This layer separates the bound charges that are due to the ferroelectric polarisation from the compensating charges on the electrode. The depolarising field will thus be incompletely compensated even if the top and bottom electrodes are shorted.
Figure 9: A schematic view of a ferroelectric device and its circuit diagram.
where d and ei are the thickness and permittivity of the constant value dielectric layer, and P and d are the polarisation and thickness of the ferroelectric layer. A simplified schematic of this arrangement is shown in figure 9. Since the interface layer forms itself predominantly under the top electrode because it arises from surface contaminations and roughness of the ferroelectric/piezoelectric film, nucleation or reaction layers at the film/electrode interfaces, or changes in the defect chemistry at the dielectric-electrode interfaces. The dependence of the inverse of the zero bias capacitance density of the film to its thickness is often attributed to the presence of a constant-valued capacitance density, Ci/A, represented by the nonzero intercept, in series with the thickness-dependent capacitance density of the bulk of the film. The apparent capacitance density at zero bias may then be expressed as
A/Ci + A/CB
d/eie0 + (d-d)/eBe0
where A is area of the top electrode, Capp the apparent capacitance, Ci the interfacial capacitance, CB the bulk film capacitance, eB the film bulk permittivity, ei the interfacial layer permittivity, e0 the permittivity of free space, d the total film thickness and d the interfacial layer thickness . The interface layer is often described as a dead layer because it gives no contribution to the ferroelectric/piezoelectric performance of the working device.
In other words, a ferroelectric device with a tilted P-E-loop has more losses than a material with a precipitous loop (see figure 7).
However, as the temperature was increased to 1170 °C, the piezoelectric properties degraded seriously. An MPB-like phase transition from tetragonal to rhombohedral symmetry was found in BNKT20 and BNKT22 specimens sintered between 1150 and 1170 °C. Enhanced electrical properties were obtained for the BNKT22 samples sintered at an optimal temperature of 1150 °C, in which d33, Pr, and K were 192 pC/N, 19.5 µC/cm2, and 1007, respectively.
However, when the addition of BZT exceeds 9 mol% there is a small increase in the TC. The reason for this small shift is not perfectly understood at present, but the BZT addition may also influence the evolution of the sintered microstructure of the crystal, and thus the Curie temperature in addition to its chemical effect.
The [Bi1-z(Na1-x-y-z)KxLiy)0.5]BazTiO3 (BN-x/y/z) multicomponent lead-free piezoelectric system proposed by Lin et al. were prepared by conventional ceramic techniques and their electrical properties were studied  (Table 13). The piezoelectric constant d33 has a maximum value of 178 pC/N at x = 0.15. The maximum values of d33 (198 pC/N) of the BN-0.15/0.10/z ceramics occur at z = 0.02. The ceramics with x = 0.15-0.20 and z = 0.01-0.04 provide better piezoelectric properties, which may be attributed to the compositions near the MPB where the number of spontaneous polarization directions increases . However, the addition of Li significantly improves the sintering performance, decreases the sintering temperature of BNT ceramics, and greatly assists in densification of BNT-based ceramics.
Figure 10: Phase relation between (Bi0.5Na0.5)TiO3-BaTiO3-(Bi0.5K0.5)TiO3 (BNBK) system.
A conventional ceramic fabrication technique was used to prepare these materials. The piezoelectric and dielectric properties were also reported . They concluded that the dielectric constant and d33 increases with increasing x, reaches a maximum value of 150 pC/N at x = 0.035 and then decreases. These observations demonstrate that the compositions near the MPB have relatively high piezoelectric and electromechanical activities due to the increase in the number of possible spontaneous polarization and to the coexistence of rhombohedral and tetragonal phases.
Li et al. prepared (1-5x)BNT-4xBNT-xBT ceramics by conventional ceramic fabrication technique . The piezoelectric and ferroelectric properties of these ceramics were studied. They came to the result that the piezoelectric constant d33 attains a maximum value of 149 pC/N at x = 0.03. This property demonstrates again that compositions near the MPB have relatively high piezoelectric activities.
Lead-free piezoelectric ceramic 0.90(Bi0.5Na0.5)TiO3-0.05(Bi0.5K0.5)TiO3-0.05BaTiO3 (abbreviated as BNT-BKT-BT5) has been used recently as the driving element in a cymbal actuator with titanium end caps. It was found that the lead-free ceramic cymbal actuator has reasonable piezoelectric coefficients and low density. Hence its performance was comparable to those fabricated using hard PZT ceramic .
(x = 0.01, 0.02, 0,025, 0.03, 0.035, 0.04) at x = 0.035
(x = 0, 0.01, 0.02, 0,024, 0.028, 0.03, 0.032) at x = 0.03
Figure 11: Phase transitions sequences in KNbO3 (white) and BaTiO3 (turquoise)  for comparison
The piezoelectric data for the air-fired samples are in the range of d33 = 80 pC/N and density of the sample is around 4.25 g/cc . One of the main obstacles for the development of potassium sodium niobates (KNN) as a commercial piezoceramic material by conventional method is the difficulty in processing and densification. Furthermore; the volatility of potassium oxide makes it difficult to maintain stoichiometry. To optimize the processing conditions and to obtain reproducible properties, KNN ceramics were doped with suitable materials. Matsubara et al.  and Seo et al.  found that the addition of CuO greatly enhanced the sinterability of KNN-based ceramics. CuO is often used because of its low melting point and the formation of a liquid phase.
Figure 12: Phase diagramm for the system KNbO3-NaNbO3
The Cu2+ ion replaced the Nb5+ ion and produced an oxygen vacancy to maintain the charge neutrality. A defect dipole consisting of a Cu2+ ion and oxygen vacancy provided the pinning effect, which eventually transformed the KNN ceramics into hard materials. TC and EC were slightly increased with CuO addition.
Bernard et al.  found that the densification of KNN ceramics can be improved by the addition of a small amount (from 0.5 to 4 mass%) germanate, which melts at around 700 °C. Germanate-modified KNN ceramics can be sintered to high density (95.6% TD) at 1000 °C without degrading the piezoelectric properties.
Egerton and co-workers reported the electrical properties of KNN in which they indicated relatively low dielectric constants over a wide compositional range. Hence to achieve sufficient densification, hot-pressed KNN ceramics (~ 99% of the theoretical density) have been reported to possess a high Curie temperature (TC = 420 °C), a large piezoelectric longitudinal response (d33 = 160 pC/N), and a high planar coupling coefficient (kp = 45). KNN samples have been prepared by conventional air sintering in order to reach high densities over 95% which yielded superior piezoelectric properties (d33 = 100 pC/N) than those obtained by the same method as reported previously . It is important to note that KNN material prepared by spark plasma sintering showed significantly higher dielectric and piezoelectric properties than those prepared by conventional method (K ~ 700 and d33 ~ 148 pC/N) [87, 88]
Recently, Saito et al. fabricated textured-based KNN ceramics by the reactive grain-growth method which produced d33 value as high as ~416 pC/N . Comparisons of properties of alkali niobates (ANbO3) obtained by different processing methods are given in Table 15.
Air-sintered (Jaeger-Egerton) K0.5Na0.5NbO3
Air-sintered (Kosec) K0.5Na0.5NbO3
Air-sintered (Birol) K0.5Na0.5NbO3
Hot-pressed (Jaeger-Egerton) K0.5Na0.5NbO3
Hot-forged (Schultze) K0.5Na0.5NbO3+4mol% Ba
Reactive grain growth (Saito)
Effect of dopants on KNN-based ceramics: Potassium sodium niobate is a promising lead-free material with low dielectric constant and high electromechanical coupling coefficient. However, it is difficult to prepare KNN ceramics with high density by conventional sintering technique. Sintering may be activated by the addition of Nb5+ or Mg2+ ions in the crystal lattice thereby producing high-density samples [90 - 93].
Addition of LiSbO3 to the above system increased the d33 to 373 pC/N . Many authors have been studied the effect of LiSbO3 (LS) on pure KNN ceramics [102-104]. Yang et al.  studied the effect of LS on pure KNN ceramics and reported that increasing the LS content, the d33 and kp values of the ceramics initially increased, and then began to decrease at higher LS concentration. In addition, dielectric study revealed that TC shifted toward the lower temperature regions and a normal ferroelectric KNN-based ceramics changed to relaxor ferroelectric by increasing LS content. The KNN-LS ceramics are promising lead-free piezoelectric materials for electromechanical transducer applications. Effect of other ions such as Ag, Nb, Ta, Bi, Cu, Ca, etc. and combinations of two or three these dopants on KNN-LS ceramics were also studied by many authors [105-117] and promising results were reported.
Wang et al. reported that an addition of Ag in KNN-LiTaO3 ceramics increases the Curie temperature (TC = 438 °C) and piezoelectric properties (d33 = 252 pC/N) of the ceramic by normal sintering technique .
However, adding Mg2+ to KNN can decrease the cell parameters and density, increase TC, and seriously deteriorate the electrical properties. Adding Ba2+ to KNN can increase the cell parameters, decrease the density, decrease the phase transition temperatures, and also significantly deteriorate the electrical properties.
Figure 13: Aurivillius type structure oxides with general formula Bi2An-1BnO3n+3
Text by Arne Lüker
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