High temperature oxidation behaviour of Ti3Al-Nb intermetallics

The oxidation behaviour of (alpha(2)-Ti3Al base alloys: Ti-25Al-11Nb, Ti-24Al-20Nb and Ti-22Al-20Nb was investigated in the present work. The alloys were used in as-received condition and after heat-treatment: 1060 degrees C, 4 hr., followed by air cooling and furnace cooling. The oxidation tests were done at 800 and 900 degrees C in air. At both the temperatures, kinetics were parabolic for the alloy, Ti-25Al-11Nb, while Ti-24Al-20Nb and Ti-22A1-20Nb showed nonparabolic kinetics with much higher oxidation rates. The massive spallation of the scale was observed at 900 degrees C for these two alloys. The analysis of the oxide scale, using SEM/EDAX and XRD, confirmed the scale to be a mixture of TiO2, Al2O3 With some Nb2O5. In situ oxidation analysis by AES confirmed that the initial growth was of titania based scale. The effect of variation of Al and Nb content on the oxidation behaviour is discussed

Synthesis and characterisation of lead magnesium niobate having exceptionally high dielectric constant

Exceptionally high dielectric constant, approximate to 28,000 (1 kHz) at T-c (-18 degrees C) has been achieved in stoichiometric lead magnesium niobate (PMN) ceramics prepared by partial oxalate route. Sintering at 1300 degrees C for 2 h in closed alumina crucible resulted in a highly dense PMN sample. Dielectric measurements with frequency showed a typical relaxer behaviour. Microstructure investigation along with electron probe microanalysis (EPMA) provide interesting clues to the observed material behaviour. Apart from grain size and density, lead oxide distribution controls the dielectric properties. (C) 1997 Elsevier Science Limited and Techna S.r.l

Combustion synthesis of Sr0.5Ba0.5Nb2O6 and effect of fuel on its microstructure and dielectric properties

Nano-sized Sr0.5Ba0.5 Nb2O6 (SBN50) powder has been synthesized, at very short reaction time, for the first time by a novel combustion method. Ba(NO3)(2) and Sr(NO3)(2) were used as source of Sr and Ba, respectively, while Nb-oxalate was used as the source of niobium. Urea, hexamethyltetramine (HMT) and glycine were used as fuel. The crystallite sizes in the powder ranged between 14-125 nm. X-ray diffraction analysis showed complete SBN50 phase formation at 700 degreesC, when urea/HMT was used as fuel, and at 900 degreesC when glycine was used as fuel. Ferroelectric-paraelectric phase transition temperature (T,) close to 40 degreesC was observed when urea and HMT were used and the T-c was -49 degreesC when glycine was used. When urea was used as fuel highest dielectric constant was observed for the pellets sintered at 1250 degreesC for 4 h. Low dielectric loss was observed when HMT was used as fuel. Larger grain sizes in the sintered pellets were observed when glycine was used as fuel. (C) 2002 .

Microstructure and dielectric properties of strontium barium niobate ceramics synthesized by partial coprecipitation

Strontium Barium Niobate, Sr0.5Ba0.5Nb2O6 (SBN50), has been synthesized, for the first time, by partial coprecipitation of SrCl2 and BaCl2 on Nb2O5. Powder X-ray diffraction study shows tetragonal tungsten bronze phase formation at 1200 degreesC. Particle morphology and size of calcined powder has been examined using Transmission Electron Microscopy. The particle size of calcined powder ranges between 250 and 300 nm. The green compacts have been sintered at 1250, 1300 and 1350 degreesC and at each temperature for 6, 12 and 24 h, respectively. Effect of sintering time and temperature on dielectric properties has been investigated. Scanning electron microscopy has been used for grain morphology studies. Grains have been found to be tetragonal in shape and show variation in size for different sintering conditions. Highest dielectric constant (epsilon) has been observed for the pellet sintered at 1350 degreesC for 6 h. A variation in T, from 93 to 119 degreesC has also been observed. (C) 2002

Dielectric and ferroelectric behavior of SBN50 synthesized by solid-state route using different precursors

In the present study, two different sets of precursors have been used in the synthesis of strontium barium niobate (SBN50) by conventional solid-state route: I. Sr(NO3)(2), Ba(NO3)(2), Nb2O5 and IL SrCO3, BaCO3 and Nb2O5. The formation of SBN50 phase was completed at 1150degreesC. The green compacts, of the calcined powder, were sintered at temperatures varying from 1250 to 1350degreesC. Microstructural characterization was done using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The aim was to study the dielectric and ferroelectric behavior depending on the precursors used and the sintering conditions and correlate with microstructure. The synthesis, involving the precursor set-I, showed a duplex microstructure with large number of small grains, of size 10-15 mum, sandwiched between very large grains, of size in the range of 50-200 mum. On the other hand the second set of precursor resulted in a microstructure devoid of duplex nature. The microstructure consisted of polyhedral grains having more or less uniform size. SBN from set-II showed abetter dielectric property compared to set-I. The discrepancies in this ferroelectric/dielectric behavior, have been explained in terms of density as well as microstructural differences. (C) 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Oxidation characteristics of Ti3Al-Nb alloys and improvement in the oxidation resistance by pack aluminizing

The oxidation behavior of three Ti-3-Al-Nb alloys: Ti-25Al-11 Nb, Ti-24Al-20 Nb, and Ti-22Al-20 Nb was investigated in the temperature range of 700-900 degrees C in air. The uncoated alloy Ti-25Al-11 Nb showed the lowest weight gain with nearly parabolic oxidation rate; while the other two alloys had much higher weight gain, accompanied by excessive oxide scale spalling. The scale analysis, using XRD, SEM/EDAX, and AES revealed that the scale was a mixture of TiO2, Al2O3, and Nb2O5 with the outer layer rich in TiO2. The effect of variation in Al and Nb content on the oxidation behavior is discussed A decrease in Al content of the alloy adversely affects the oxidation resistance, and it seems that a Nb content as high as 20 at.% is also not beneficial. Hence these alloys, especially Ti-24Al-20 Nb and Ti-22Al-20 Nb, should not be used in the as-received condition above 750 degrees C. An attempt was made to improve the oxidation resistance of these alloys by pack aluminizing which led to the formation of an Al rich TiAl3 surface layer doped with Nb. The coating process was gaseous-diffusion controlled with a parabolic Al deposition rate. The weight gains for the aluminized alloy specimens oxidized at 900 degrees C in air were much lower than that of the uncoated specimens. The weight gains were further decreased in the case of Si-modified aluminized specimens. The scale analysis revealed an alumina-rich scale with some amount of titania doped with Nb. The improvement in the oxidation resistance of the pack-aluminized alloys at 900 degrees C is attributable to the formation of the alumina-rich oxide scale. The addition of Si to the aluminizing pack seems to promote fur ther the growth of an alumina-rich scale by lowering the oxygen partial pressure in the system

Effect of sintering temperature on composition, microstructure and electrical properties of K0.5Na0.5NbO3 ceramics

Lead free potassium sodium niobate (K0.5Na0.5NbO3) ceramic powders were synthesized by colloidal coating method. The calcined powders (800 degrees C) were sintered conventionally at three different temperatures (1050 degrees C, 1100 degrees C and 1150 degrees C) and the effect of sintering temperature on density, microstructure, composition and electrical properties was investigated. All the samples showed a single phase perovskite structure with orthorhombic symmetry similar to KNbO3 ceramics. Microstructure examined under FEG-SEM revealed an optimum microstructure, in terms of grain size, porosity and uniformity, at the sintering temperature of 1100 degrees C, which also showed density of 92% rho(Th). As the sintering temperature increased the X-ray diffraction peaks shifted to lower 2 theta values indicating excess volatilization of Na at higher temperature as compared to K. This was further confirmed through elemental Probe X-ray microanalysis and ICP-AES studies. Dielectric constant (epsilon(r)), dielectric loss (tan delta), ferroelectric (P-E loop) and piezoelectric (d(33)) properties showed considerable improvement and leakage current decreased with increasing sintering temperature. The sample sintered at 1100 degrees C. showed marked improvement in maximum dielectric constant (573) at RT at 1 kHz, minimum tangent loss (0.04) at RT at 1 kHz, maximum remnant polarization (135 mu C/cm(2)), lower leakage current (7.6 x 10(-7) A/cm(2)) and maximum d(33) value (100 pC/N). (C) 2013 Elsevier B.V. All rights reserved