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

The 5d level positions of the trivalent lanthanides in inorganic compounds

In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO 2 , and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.

Substrates for gallium nitride epitaxy

The current state of efforts to understand the phenomenon of geometric magnetic frustration is described in the context of several key materials. All are transition metal oxides which crystallize with magnetic lattices which are geometrically or topologically prone to frustration such as those based on triangles or tetrahedra which share corners, edges or faces. These include the anhydrous alums, jarosites, pyrochlores, spinels, magnetoplumbites, garnets, ordered NaCl and other structure types. Special attention is paid to materials which do not undergo long range ordering at the lowest temperatures but instead form exotic ground states such as spin glasses, spin liquids and spin ices, and to S = 1/2 based materials.

Geometrically frustrated magnetic materials

Dielectric, ferroelectric and piezoelectric properties of perovskite ferroelectric and bismuth layer-structured ferroelectric (BLSF) ceramics are described as superior candidates for lead-free piezoelectric materials to reduce environmental damages. Perovskite type ceramics seem to be suitable for actuator and high power applications that are required a large piezoelectric constant, d 33 (>300 pC/N) and a high Curie temperature, T c (>200 °C). For BaTiO 3 (BT)-based solid solutions, that is, (1 −  x )BaTiO 3 − x (Bi 0.5 K 0.5 )TiO 3 [BTBK − 100 x ] ceramics, the T c increases with increasing the amount of x . BTBK-20 + MnCO 3 0.1 wt.% ceramic shows the high T c than 200 °C and the electromechanical coupling factor, k 33  = 0.35. In the case of a (Bi 1/2 Na 1/2 )TiO 3  −  b BaTiO 3  −  c (Bi 1/2 K 1/2 )TiO 3 [BNBK (100 a /100 b /100 c )] solid solution ceramics, the d 33 and T c are 191 pC/N and 301 °C for the BNBK (85.2/2.8/12), respectively. On the other hand, BLSF ceramics seem to be excellent candidates as piezoelectric sensors for high temperatures and ceramic resonators with high mechanical quality factor ( Q m ), and low temperature coefficient of resonance frequency (TC- f r ). Donor-doped Bi 4 Ti 3 O 12 ceramics such as Bi 4 Ti 3− x Nb x O 12 [BITN- x ] and Bi 4 Ti 3− x V x O 12 [BITV- x ] show high T c than 650 °C. The k 33 value of the grain-oriented (HF) BITN-0.08 ceramic is 0.39 and is able to keep the same value up to 350 °C. Bi 3 TiTaO 9 (BTT)-based solid solution system, Sr x −1 Bi 4− x Ti 2− x Ta x O 9 [SBTT2( x )] (1 ≦  x  ≦ 2), displays the high Q m value (=13500) in (p)-mode at the x  = 1.25 composition.

http://web1.see.asso.fr/electroceramics/html/cdrom/pdf/Papers/C07-01-Inv.pdf

Current status and prospects of lead-free piezoelectric ceramics

Growth and characterization of lanthanum gallium silicate La3Ga5SiO14 single crystals for piezoelectric applications

Mixed perovskites (AA‘)(BB‘)O3, where A = La or Nd, A‘ = Sr, B = Al or Ga, and B‘ = Ta or Nb, were investigated by X-ray photoelectron spectroscopy (XPS). The interpretation of XPS O(1s) signals in mixed oxides is discussed. The nonsingular O(1s) peak that occurs in the mixed oxides is due to the polarization of the oxygen valence electron density, as was already proposed by us for Y3Al5O12 and YAlO3. This interpretation seems to be true also for the case when four different metals are in the oxide. The O(1s) binding energies seem to depend on the direction in which they are ejected.

Interpretation of XPS O (1s) in Mixed Oxides Proved on Mixed Perovskite Crystals

Chirped-pulse amplification in the ultraviolet region is demonstrated by use of a broadband Ce3+:LiCaAlF6 laser medium. A modified bow-tie-style four-pass amplifier pumped by 100-mJ, 266-nm pulses from a Q-switched Nd:YAG laser has a gain factor of 370 and delivers 6-mJ, 290-nm pulses. After dispersion compensation, the output pulses can be compressed to 115  fs.

Kiyoshi Shimamura

Papers

Growth and characterization of lanthanum gallium silicate La3Ga5SiO14 single crystals for piezoelectric applications

Interpretation of XPS O (1s) in Mixed Oxides Proved on Mixed Perovskite Crystals

Chirped-pulse amplification of ultraviolet femtosecond pulses by use of Ce(3+):LiCaAlF(6) as a broadband, solid-state gain medium.

Microstructure of Al2O3/Y3Al5O12 eutectic fibers grown by μ-PD method

Growth and characterization of La3Nb0.5Ga5.5O14 single crystals