Jeffrey G. Nelson

Ferroelectric and structural properties of the tungsten bronze system K2Ln3+Nb5O15'Ln = La to Lu

The K2Ln3+Nb5O15 system has been investigated for a range of rare-earth ions (La3+ to Ho3+, Y3+). The temperature-dependent dielectric properties indicate that all but the La3+-containing materials have a tungsten bronze orthorhombic structure. Two phase transitions occur above room temperature, one paraelectric:ferroelectric and the other ferroelectric:ferroelastic. These transitions shift toward higher temperatures as the size of the rare-earth ion decreases. The lattice constant determination for this system indicates that the larger ions (La3+ to Gd3+) predominantly prefer the 12-fold coordinated site, Tb3+ to Ho3+ are distributed over both the 12- and 9-fold coordinated sites, and the smaller rare-earths (Er3+ to Lu3+) do not yield the tungsten bronze phase. Because of their low dielectric constants, these bronzes appear to be promising for some optical and pyroelectric detector applications.

Ferroelectric properties of the tungsten bronze M2+6M4+2Nb8O30 solid solution systems

Abstract Ferroelectric compositions based on the chemical formula M 2+ 6 M 4+ 2 Nb 8 O 30 , where M 2+ = Ba, Sr or Ca and M 4+ = Ti, Sn or Zr, have been synthesized and their characteristics have been compared with the results of earlier work. X-ray diffraction and dielectric constant measurements show that Ba 2+ analogs are tetragonal at room temperature as previously reported, with one phase transition, while the Sr 2+ analogs are orthorhombic with two phase transitions. The orthorhombicity becomes pronounced on the addition of Ca 2+ into the Sr 2+ site and two phase transitions, paraelectric:ferroelectric and ferroelectric:ferroelastic, have been established. The solid solution between Ba 6 Ti 2 Nb 8 O 30 and Sr 6 Ti 2 Nb 8 O 30 was studied and found to exhibit a morphotropic phase boundary near the Sr 2+ rich side. The structural and ferroelectric properties of these various bronzes are described with respect to the M 2+ and M 4+ cations.

Piezoelectric and ferroelectric properties of La-modified and unmodified tungsten bronze Pb0.6Ba0.4Nb2O6 dense ceramics

Abstract As part of a long-range project on piezoelectric materials for actuators and sensors, the ceramic densification of ferroelectric tungsten bronze Pb0.6Ba0.4Nb2O6 (PBN:60), with and without La3+-modification, has been investigated. Samples prepared using pressure during sintering exhibit high densification (≥ 95%) with improved piezoelectric and polarization properties. Our current best samples show a spontaneous polarization of up to 24 μcoul/cm2 and piezoelectric strain coefficients, d33, of up to 275 × 10−12 C/N. The addition of La3+ (6%) has been observed to significantly alter the polarization behavior and change the lattice symmetry from 4mm to mm2.

Grain oriented ferroelectric PZT thin films on lattice-matched substrates

Abstract The sol-gel technique has been used to produce highly grain-oriented perovskite PZT thin films on lattice-matched SBN:60, SrTiO 3 and MgO substrates. These films were deposited on both La 0.5 Sr 0.5 CoO 3 (LSC) laser ablated-coated and uncoated substrates and annealed in the range of 630–675°C in an air or oxygen atmosphere. In all cases the PZT thin films were highly crystalline and oriented, with dielectric constants higher than 1100. The measured remanent polarization in these films exceeded 18 μcoul/cm 2 indicating the potential for strong electro-optic and piezoelectric effects.

Properties of Dense PLZT Piezoelectrics

Perovskite PZT and PLZT ceramics have been hot- forged to densification > 98 % of their theoretical values. The piezoelectric d33 coefficient in the PLZT dense ceramics exceeds 650 pC/N compared to 550 pC/N for conventionally sintered ceramics. In addition, the microstructure in these samples is much improved, and a substantial amount of grain orientation is observed. This combination of properties results in a substantial increase in fatigue life, and we expect the hot-forging technique to extend the fatigue life to over 109 cycles.

Indexed experimental X-ray powder diffraction patterns of Tl 0.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O x (Tl,Pb-1223) and TlSr 2 Ca 2 Cu 3 O x (Tl-1223)

Indexed experimental powder diffraction patterns of Tl,Pb-1223 and Tl-1223 are presented with a method of determining good cell parameters for the former awkward case where c/a almost exactly equals 4. For Tl,Pb-1223, a = 3.814±0.001 A, c = 15.267±0.005 A, for Tl-1223, a = 3.814±0.001 A, c = 15.302±0.005 A, both tetragonal, space group P4/mmm.

Grain-oriented high-Tc superconductors and their applications

Ninety six percent grain-orientation was achieved in YBa2Cu3O(7-x) ceramics using the hot-forging technique. X-ray diffraction measurements show that the c-axis is oriented parallel to the direction of pressing while the a and b axes are in the plane perpendicular to the pressing direction. Optical and electron microscopy fully support the X-ray results. Highly grain-oriented samples show a sharp Tc at 92 K and in some samples, Jc was found to be close to 10,000 A/sq cm.