D.K. Bhattacharya

Ferroelectric relaxor behaviour and impedance spectroscopy of Bi2O3-doped barium zirconium titanate ceramics

Bi2O3-doped barium zirconate titanate ceramics, Ba1−xBix(Zr0.05Ti0.95)O3, have been prepared by the conventional solid-state reaction method. The ferroelectric relaxor behaviour and dielectric properties have been investigated in detail. By XRD analysis, it is suggested that up to x = 0.04, Bi3+ substitutes A-site ion, and thereafter with higher Bi3+ content, it enters the B-site sub lattice. Substitution of Bi3+ ions induces ferroelectric relaxor behaviour and the degree of relaxation behaviour increases with bismuth concentration. The remanent polarization and strain behaviour show a slight increase with the substitution level. The degree of hysteresis (strain versus electric field) also reduces from 21.4% to 4.6% with bismuth substitution. Impedance measurements were made on the prepared sample over a wide range of temperatures (300–723 K) and frequencies (40 Hz–1 MHz), which show the presence of both bulk and grain boundary effects in the material. The bulk and grain boundary conductivities determined from impedance study indicate the Arrhenius-type thermally activated process. Impedance spectroscopy is shown to be an efficient method capable of detecting the contributions of the resistances of grains and grain boundaries to the complex impedance of a ceramic system, accurately estimating its electrical conductivity as well as its corresponding activation energies and drawing conclusions on its structural properties.

Effect of Double Doping in Lead Zirconate Titanate (PZT) Lattices by Sol-Gel Technique for MEMS Applications

We report on the effect of double doping (La2+ and Sm3+) in PZT lattices in thin film form, keeping La (mol%) fixed at 8 mol% [Pb0.92-x La 0.08 Smx(Zr0.65Ti0.35)O3: x = 0, 0.02, 0.04, 0.06, 0.08]. Both the cation substitutions are being substituted at A site only. The thin films were grown on Pt/Si<111> substrates by sol gel technique. XRD diffractogram shows single phase for the films for x = 0.02. Thereafter, an unidentified phase starts appearing which increases with further substitution of Sm3+. Surface morphology was studied using atomic force microscope (AFM). The films show well-defined ferroelectric behaviour. The dielectric constant decreases with Sm substitution initially and then increases slowly as the Sm substitution is increased but never attains the pure PLZT (8/65/35) dielectric value. The loss tangent (tanδ) initially increases and then decreases. The phase transition temperature (Tc) increases drastically with 2 mol% Sm substitution and decreases slightly as the Sm substitution is increased further. The I-V behaviour shows a consistent shift towards the –ve bias voltage as the Sm substitution is increased from 2 mol% to 8 mol%. The logI-logV curve show ohmic behaviour with the electrode for all the samples. The P-E loop shows more hysteresis for the 2 mol% Sm substitution. The appearance of P-E loop qualitatively shows the piezoelectric nature of the films. Optical band gap was calculated from transmittance studies using a UV-Vis-NIR spectrophotometer. The optical band gap was estimated from (αhυ)2 vs hυ curve. The band gap decreases first with Sm substitution and then increases as the Sm substitution is increased. The results are discussed.

Improvement in Electrical Properties of Laser Ablated Barium Strontium Titanate Thin Films by Controlled Oxygen Atmosphere

Thin films of Ba0.6Sr0.4Ti0.99Fe0.01O3 (BST) were deposited on Pt/Si substrates at two different substrate temperatures (650 and 700 °C) by pulsed laser deposition. The perovskite structure of the films was confirmed by X-ray diffraction (XRD) analysis and atomic force microscopy (AFM) was carried out to observe the surface morphology. The detailed electrical properties such as dielectric constant, loss tangent, tunability and leakage current of the films were investigated in order to determine their suitability for application in microwave tunable devices.

Effect of Ball Milling Time on the Electrical and Piezoelectric Properties of Barium Titanate Ceramics

Barium Titanate (BaTiO3) is the first piezoelectric ceramic developed and most widely used though for few decades it has been replaced by better performed lead based materials like PZT, PLZT, etc. for piezoelectric applications. Recently, however, due to the environmental concern caused by lead based materials, researchers have been giving importance to this material and trying to improve its piezoelectric and electromechanical properties by performing various treatments. The present work is also an effort in this direction. Barium titanate (BaTiO3) powder was synthesized by solid state reaction of BaCO3 and TiO2 at 1050°C. X-ray diffraction analysis of the powder showed the single phase perovskite structure of BaTiO3. The powder was milled to produce nanocrystalline powders using a planetary ball mill. Different samples were prepared by varying the milling time from 1 to 30 hours, keeping the milling speed fixed at 250 rpm. All the milled powders were examined with TEM. With increasing milling time, the particle size first decreases and attains a minimum of 28.59 nm at 20 hours of milling time. However, further increase in milling time results in increase of the particle size. The electrical and piezoelectric properties of the ceramic samples formed by these nanocrystalline powders were studied.

Low Temperature Deposited BST Thin Films for RF MEMS Switch

BST thin films of optimized target composition of Ba0.6Sr0.4TiO3 have been deposited on Cr-Au coated Si substrate by Pulsed Laser Deposition technique with low substrate temperature of 100°C using different laser energies (250 mJ, 300 mJ and 350 mJ). The structural, morphological and electrical studies have been carried out for all the films. The value of dielectric constant and dielectric loss for the film deposited with different laser energies were in the range of 18–20 and 0.04–0.07 respectively. The thickness of the film was 0.4 μm. All the films show higher resistivity (∼107Ω-cm) and high dielectric breakdown strength (∼500 kV/cm). The observed results suggest that the low temperature deposited BST thin films are suitable for RF MEMS Switch application.

Trench formation in silicon for millimeter-wave switching device

Anisotropic etching using alkaline solution has been adopted to form trenches in silicon while fabricating surface oriented bulk window SPST switches. An array pattern has been etched on silicon with good control on depth of trenches. KOH-water solution is seen to yield a poor surface finish. Use of too much of additive like isopropyl alcohol improves the surface condition but distorts the array pattern due to loss of anisotropy. However, controlled use of this additive during the last phase of trench etching is found to produce trenched arrays with desired depth, improved surface finish and minimum distortion of lateral dimensions.