Piezoelectric properties of (Li, Ag, Sb) modified (K0.50Na0.50)NbO3 lead-free ceramics
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A Raman scattering study was performed in a variety of single crystals of both tetragonal and orthorhombic Ba 2 YCu 3 O y structures at room temperature. It is found that Raman spectra observed in the tetragonal crystal are much different from those in the orthorhombic one. Three modes with 500 cm -1 580 cm -1 and 637 cm -1 in energy in the orthorhombic crystal are tentatively assigned to be the copper-oxygen vibrational modes. The 500 cm -1 mode is assigned as the oxygen-stretching mode in Cu-O-Cu bond along the c -axis and its frequency is found to shift to 470 cm -1 in the tetragonal crystal.
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There is a general observation that the Curie temperature and piezoelectric property of the ferroelectric ceramics can be enhanced only at the expense of each other, i.e., higher piezoelectricity, lower Curie temperature, thus, limits their applications over broad temperature range. In this research, Sm-modified 0.15 Pb(Mg1/3Nb2/3)O3-(0.85-x)PbZrO3-xPbTiO3 ceramics have been studied, where excellent piezoelectric coefficients d33 = 720 pC/N, d33∗ = 950 pm/V and high Curie temperature TC = 293 °C were simultaneously achieved for x = 0.42 composition by designing the morphotropic phase boundary (MPB) with local structural heterogeneity. Of particular significance is that a high thermal stability was observed with piezoelectric variation below 20% with temperature up to 280 °C, demonstrating that the x = 0.42 composition is a good candidate for piezoelectric application over broad temperature range where high temperature stability is required. This work provides a good paradigm for designing high-performance piezoelectric ceramics with high thermal stability via a combination of MPB and local structural heterogeneity.
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KNbO3 (KN) nanowires were synthesized using various process conditions and their structures and morphologies were investigated. Homogeneous KN nanowires were formed in specimens synthesized at 130 °C for 24.0–48.0 h. These KN nanowires have a tetragonal structure that is known to be stable at high temperatures in the range of 225–435 °C. Tetragonal KN nanowires changed to orthorhombic KN nanoplates when the process time increased and homogeneous orthorhombic KN nanoplates existed for specimens synthesized for 144.0 h. In addition, tetragonal and orthorhombic structures coexisted in KN nanoplates synthesized at 130 °C for 72.0 h. For specimens synthesized at 100 °C, a long process time of 144.0 h was required to develop homogeneous KN nanowires that were also considered to have both tetragonal and orthorhombic structures. On the other hand, for specimens synthesized at 150 °C for 8.0 h, KN nanowires and a cube-shaped KN phase coexisted. Furthermore, a K4Nb6O17 second phase was formed in specimens synthesized for short periods of time (<8.0 h), indicating that the formation of homogeneous KN nanowires is difficult at 150 °C. Therefore, homogeneous KN nanowires with a tetragonal structure can be obtained at a low temperature of 130 °C with a short process time in the range of 24.0–48.0 h.
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Ba3.75Nd9.5Ti18O54 is known to be an orthorhombic phase built of chains of octahedra parallel to the z axis with pentagonal, tetragonal and triangular tunnels between them. It is shown here that this phase is a representative of a family of orthorhombic phases related to the tetragonal tungsten bronzes (TTB). It is assumed that some of these new phases can be ferroelectric. Crystal chemical aspects of these phases are discussed.
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The orthorhombic to tetragonal phase transition in (Ba 0.92 Ca 0.08 )(Zr 0.05 Ti 0.95 )O 3 was investigated using high-temperature X-ray diffraction between 260 and 333 K. The results established the presence of tetragonal ( P 4 mm ) and orthorhombic ( Amm 2) phase co-existence in the temperature range of 293 ≤ T ≤ 313 K. The tetragonal phase was found to increase from 27% at 293 K to 76% at 313 K. The structural refinement and line-profile analysis ruled out the presence of an intermediate monoclinic structure during P 4 mm → Amm 2 crossover. The analysis shows a pure orthorhombic ( Amm 2) structure for T < 293 K and tetragonal for T > 313 K.
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The relative stability of the tetragonal and monoclinic phases in the (1-x)BiFeO3-xPbTiO3 solid solution system has been investigated under externally applied stress. It is shown that external stress can transform monoclinic compositions partially to the tetragonal phase in the vicinity of the morphotropic phase boundary (MPB) leading to an extension of the MPB region from Δx ≈ 0.03 for annealed samples to Δx ≈ 0.17 for the stressed samples toward the BiFeO3 richer end. The tetragonality of the stress-induced tetragonal phase for x = 0.20 is shown to be higher than the highest value reported so far for x = 0.31 composition in annealed samples.
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CBN crystals show a one- and a two-dimensionally modulated modification. The former is isotypic with orthorhombic Ba 4 Na 2 Nb 10 O 30 and the latter with the tetragonal tungsten bronze type of crystal structure. The orthorhombic form irreversibly transforms to the tetragonal polymorph at the ferroelectric phase transition near 603 K. Orthorhombic and tetragonal CBN24 slightly differ in the distribution of the Ba and Ca atoms over the incompletely filled Me1 and Me2 sites. The tetragonal symmetry is further broken in orthorhombic CBN24 by different amplitudes of the positional modulations of O atoms which are symmetrically equivalent in the TTB structure. A similar orthorhombic phase of CBN31 could be obtained by quenching from 1473 K.
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