Multiferroic magnetoelectric materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities for their scientific interest and significant technological promise in the novel multifunctional devices. Natural multiferroic single-phase compounds are rare, and their magnetoelectric responses are either relatively weak or occurs at temperatures too low for practical applications. In contrast, multiferroic composites, which incorporate both ferroelectric and ferri-/ferromagnetic phases, typically yield giant magnetoelectric coupling response above room temperature, which makes them ready for technological applications. This review of mostly recent activities begins with a brief summary of the historical perspective of the multiferroic magnetoelectric composites since its appearance in 1972. In such composites the magnetoelectric effect is generated as a product property of a magnetostrictive and a piezoelectric substance. A...

Multiferroic magnetoelectric composites: Historical perspective, status, and future directions

After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

Lead zirconate titanate (PZT) based piezoelectric materials are well known for their excellent piezoelectric properties. However, considering the toxicity of lead and its compounds, there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union in this effect. 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 piezo materials emphasizing on their preparation, structure–property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail. From the above study, it is concluded that some lead-free compositions show stable piezoelectric responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large number of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of the others.

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Review: environmental friendly lead-free piezoelectric materials

A2B′B″O6 perovskites: A review

Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions.

Lead-Free Relaxor Ferroelectrics

Dielectric characteristics of Ba(Fe1∕2Nb1∕2)O3 ceramics were investigated over a broad temperature and a frequency range. Two dielectric relaxations following Arrhenius law were observed at 150–400 and 406–650K. An extremely high relaxorlike dielectric peak (202 270 at 5Hz, 91 930 at 1kHz, and 37 030 at 100kHz) with very strong frequency dispersion was observed at 406–650K, and it was not relaxor ferroelectric but an oxygen defect induced dielectric abnormity. The significant drop of dielectric constant at 146–400K was also frequency dependent, and it was nearly a Debye relaxation. Between these two dielectric relaxations, there was a frequency dependent dielectric constant plateau over a wide temperature interval.

Dielectric abnormities of complex perovskite Ba(Fe1∕2Nb1∕2)O3 ceramics over broad temperature and frequency range

Relaxor-like dielectric behavior in La2NiMnO6 double perovskite ceramics

Microstructure-dependent giant dielectric response in CaCu3Ti4O12 ceramics

Nonstoichiometric Mg2SiO4 forsterite dielectric ceramics with various Mg/Si ratios (R=2.025, 2.05, and 2.2) were prepared and characterized. The appearance of a MgSiO3 secondary phase could be effectively suppressed by adjusting the nonstoichiometry, and a single-phase forsterite structure was obtained in the present ceramics with Mg/Si ratios of 2.025 and 2.05. The microwave dielectric characteristics were significantly improved in the nonstoichiometric Mg2SiO4 ceramics on eliminating the MgSiO3 secondary phase, where an enhanced Q×f value and a suppressed temperature coefficient of resonant frequency λf were obtained. The best microwave characteristics were achieved in the nonstoichiometric forsterite ceramics with an Mg/Si ratio of 2.05: ɛr=7.5, Q×f=114 730 GHz, and λf=−59 ppm/°C.

Effects of Mg/Si Ratio on Microwave Dielectric Characteristics of Forsterite Ceramics

We have developed a novel route to reduce the dielectric loss of CaCu3Ti4O12 (CCTO) material based on the internal-barrier-layer-capacitance (IBLC) model. By the partial La-for-Ca substitution the dielectric loss was suppressed re-markably while the giant dielectric constant still remains. The sample of Ca0.8La0.2Cu3Ti4O12 exhibits a high dielectric con- stant over 3000 and a low dissipation factor below 0.015 at 10 kHz from 180 K to 300 K. The impedance spectroscopy analysis confirms that the decrease of dielectric loss is mainly due to the increase of conductivity in the CCTO grain/subgrain. Our result is expected to be able to promote practical applications. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X.M. Chen

Papers

Dielectric abnormities of complex perovskite Ba(Fe1∕2Nb1∕2)O3 ceramics over broad temperature and frequency range

Relaxor-like dielectric behavior in La2NiMnO6 double perovskite ceramics

Microstructure-dependent giant dielectric response in CaCu3Ti4O12 ceramics

Effects of Mg/Si Ratio on Microwave Dielectric Characteristics of Forsterite Ceramics

Decrease of dielectric loss in CaCu3Ti4O12 ceramics by La doping