U. Balachandran

Negative thermal expansion behavior in single crystal and ceramic of Nb2O5-based compositions

The thermal expansion coefficients of a single crystal and ceramic of Nb2O5 are measured in the temperature range of −200–500u200a°C by the dilatometer technique. Both single crystals and ceramics of Nb2O5 and Nb2O5(1−x):xTiO2 show negative thermal expansion in this temperature range. Some contribution to the result could be due to the presence of the Magneli phases. The main phase transition temperature, which also matches with the dielectric anomaly, occurs at ≈150u200a°C.

Intermediate phases produced in growth of Bi2Sr2Ca2Cu3O10+δ

High‐temperature in situ x‐ray measurements have been undertaken during the growth of Bi2Sr2Ca2Cu3O10+δ in order to determine the sequence of phases that form and the kinetics of growth. With a Ag‐coated thin pellet of the starting mixture of Pb‐doped Bi2Sr2Ca1Cu2O8+δ, Ca‐cuprates, and CuO in an air atmosphere, major changes occur in the x‐ray spectrum at temperatures as low as 790u2009°C. A triplet of lines associated with Ca2PbO4 forms as soon as the pellet reaches 790u2009°C and then disappears after 20 or 30 minutes. The Bi2Sr2Ca2Cu3O10+δ phase begins to grow at about 810u2009°C. For these rather porous pellets, about 20% of the sample transforms to Bi2Sr2Ca2Cu3O10+δ, as indicated by both x‐ray intensities and superconducting magnetization studies.

Low-temperature dielectric behavior of Nb2O5–SiO2 solid solutions

Dielectric properties of Nb2O5(0.92):SiO2(0.08) ceramic were measured in the temperature range of 10–300 K by the cryostat system. Frequency-dependent dielectric loss suggests the relaxation behavior of this material. The relaxation mechanism was analyzed by the Arrhenius relationship and the Cole–Cole plot. Calculated distribution of relaxation time reveals deviation from the pure Debye relaxation.

Growth studies of (Nb2O5)(1-X):XTiO2 & (Nb2O5)(1-X):XSiO2 single crystals and their dielectric behaviors

Abstract Nb2O5:TiO2 and Nb2O5:SiO2 systems show interesting dielectric properties. This presentation describes recent progress in growing (Nb2O5)1-x:xTiO2, (Nb2O5)1-x:xSiO2, ×=0.05, single crystals by the laser heated pedestal growth (LHPG) technique and their dielectric behavior. Ceramic rods were used as seed and feed prepared by the mixed oxide method. Growth conditions such as temperature of melting zone and pulling rate were adjusted for each composition. Crystal symmetry and Growth direction were investigated by X-ray diffraction and Laue back-reflection analysis. Temperature dependences of the dielectric constant and loss tangent were measured over a broad frequency range (1 KHz-1 MHz) on sample cut parallel and the perpendicular to the growth direction.

Growth of nucleation sites on Pb‐doped Bi2Sr2Ca1Cu2O8+δ

In the growth of Bi2Sr2Ca2Cu3O10+δ from mixed powders of Pb‐doped Bi2Sr2Ca1Cu2O8+δ and other oxides, it has been discovered that a dense array of hillocks or mesas grow at the interface between a Ag overlay and Pb‐doped Bi2Sr2Ca1Cu2O8+δ grains during the ramp up to the reaction temperature. As viewed in an environmental scanning electron microscope, the Ag coated grains develop a texture that looks like ‘‘chicken pox’’ growing on the grains at about 700u2009°C. These hillocks are about 100 nm across and are spaced at about 500 to 1000 nm. If there is no Ag, this texture does not develop. Preliminary measurements indicate that the hillocks are a recrystallization of (Bi,Pb)2Sr2Ca1Cu2O8+δ, and are definitely not a Pb rich phase.

Low temperature dielectric properties of magnetoplumbite family of materials

Abstract The magnetoplumbite family of materials exhibit properties that make them suitable to be used as substrates materials for microwave application. Four members of the family studied in this work are LaMgAI 11 O 19 , NdGaMgAl 10 O 19 , CaGa 6 Al 6 O 19 and CaGa 12 O 19 . Dielectric studies have been carried out over a temperature range of 4–300 K and a wide frequency range. All four exhibit a low dielectric constant with good temperature stability, low dielectric loss and favorable frequency dependence characteristics.

Growth and Dielectric Behavior of Nb 2 O 5 (1-X):XTiO 2 Single Crystals

This paper reports recent progress in growing Nb 2 O 5 (1 m x):xTiO 2 , x = 0.05, 0.08, 0.11 single crystal using the laser heated pedestal growth (LHPG) technique. The growth conditions such as molten zone temperature, the seed and feed pulling rate were adjusted for each composition. XRD was performed to study their crystal structures. Temperature dependence of dielectric constant and loss tangent were measured over broad frequency range (1 KHz-1 MHz). The pure Nb 2 O 5 single crystal was also grown and was measured the dielectric properties to be the reference data. All three compositions exhibit the dielectric relaxation type. The relaxation mechanism of dielectric loss analyzed by Arrhenius law and Vogel-Fucher relation has been demonstrated in the single crystal at composition x = 0.05.

Quantum paraelectric-like behavior of the paratitanate family of materials

Abstract Quantum paraelectric-like behavior has been observed in the paratitanate family of materials. Experiments performed on the dielectric properties of (La 1/2 Na 1/2 )TiO 3 and (Nd 1/2 Na 1/2 )TiO 3 over the temperature range 4 K–300 K suggest quantum paraelectric-like behavior and the dielectric data fits Barrett’s expression. Other characteristics known to possess quantum paraelectrics, like a strong E -field dependence of dielectric constant at low temperature, were not observed and are discussed in this paper.