K. Singh

A compendium of Tc-Us and Ps-Δz data for displacive ferroelectrics

Abstract An attempt has been made to relate transition temperature (Tc ) with spontaneous polarization (Ps ) for ‘Twenty Two’ displacive ferroelectrics belonging to different families. A possible Ps -Δz relation is predicted on the basis of our previously reported Tc -Δz relationship. It exhibits a fairly good agreement between experimental and theoretical results. A new classification of ferroelectrics has been proposed wherein the concept of ‘strong’ and ‘weak’ ferroelectrics is introduced.

Antiferroelectric lead zirconate, a material for energy storage

Abstract Lead zirconate (PbZrO3) is an antiferroelectric material melting at about 12507deg;C and having the Curie temperature of 230°C.1 The free energy of the antiferroelectric orthorhombic phase is very close to that of a ferroelectric rhombohedra1 especially near the Curie point.1 An evidence of the existence of a stable ferro-electric phase over a temperature range of a few degree has been reported.2 Some impurities e.g. Ba, Ti help to stabilize the ferroelectric phase,3 others, e.g. Ca, Sr, tend to supress it.2 Also it is reported that on the application of strong electric field to the material in the antiferroelectric state and when this field is increased beyond some threshold value the ferroelectric phase becomes energetically favourable and gives rise to antiferroelectric (AFE) to ferroelectric (FE) transition. Jaffe4 has reported that this leads to high energy storage for DC field, because of the large increase in polarization that accompanies the transition. Field of 60 kV/cm has induced AFE-F...

Effect of Sintering Temperature on Dielectric Properties of Pb(Fe1/2Nb1/2)O3 Perovskite Material

Lead iron niobate [Pb(Fel/2Nb1/2)]O3 (PFN) are synthesized by using precursor method. Three different sintering temperatures (1000, 1040 and 1100°C) are selected to optimize the PFN processing technique. X-ray powder diffraction studies confirm the formation of single-phase perovskite PFN. The scanning electron microscopy (SEM) study revealed increase in grain size with increasing sintering temperature. Quadrupole splitting observed in room temperature Mossbauer spectra for sample sintered at 1000°C disappears on sintering it at 1040 and 1100°C. The decrease in isomer shift with increasing temperature is attributed to change in oxidation states of Fe. The dielectric constant of these samples, measured in the frequency range (100 Hz to 1 MHz) and temperature range from 25–175°C, show giant values and diffuse phase transition behavior (DPT around 104.5°C). Although, the dielectric constant increases with increase in sintering temperature, however, the transition temperature (104.5°C) is seen to be independent of frequency. Both these properties predict non-relaxor nature of PFN. High value of dielectric constant is interpreted on the basis of the formation of barrier at the interface of grain and grain-boundaries. High dielectric loss is understood to be due to hopping of electrons between Fe+2 to Fe+3.

Qualitative Analysis of Tolerance Factor, Electronegativity and Chemical Bonding of Some Ferroelectric Perovskites Through MOT

Qualitative analysis of the factors governing variation in T c and P s of PbTiO3, KNbO3 and BaTiO3 members of ABO3 perovskite family considering the parameters like tolerance factor, t, electronic configuration, electronegativity, polarizability and character of bonding using molecular orbital theory (MOT) reveals that (i) T c and P s vary inversely with t, (ii) increased interaction between ions decreases t as packing in unit cell increases, (iii) the size of A and B cations and their interaction with oxygen governs the ferroelectric behaviour, (iv) the covalent interaction between A─O and B─O is responsible for the other transitions in the ferroelectric phase and (v) the occurrence of P s is due to the presence of mixed ionic-covalent B─O bonds.

Relation between transition temperature, atomic displacement and spontaneous strain with special reference to structural symmetry

An analysis on the structrually classified displacive type ferroelectrics when further divided on the basis of space groups and the order of transition (n), gave fairly good agreement between the observed and calculated values of transition temperature (Tc) from the knowledge of Δz, the atomic displacement. The relation between transition temperature and spontaneous strain is least affected by structural symmetry.

PC based ferroelectric analyzer using modified sawyer and tower circuit

Abstract Though several modifications and improvements have been made in past in the hysteresis loop tracer suggested by Sawyer and Tower, a more versatile PC based tracer with the facility to analyse the ferroelectric parameters is designed by looking at the present trend and requirement of computer automation. The use of a PC based automated ferroelectric analyzer, in addition to performing the tasks of measurement and plotting, is also utilised for extracting the ferroelectric parameters with more accuracy and speed. It also facilitates storage and retrieval of a large amount of data. Furthermore, the loop tracer designed using solid state integrated components reduces the complexity and bulkness. In this system, the output of the modified loop tracer is given to the 2 channels of a 12 bit successive approximation ADC card interfaced to PC instead of CRO in conventional circuits. A program written in BASIC and assembly languages implement the tasks of fastest data acquisition, file creation, plotting a...

Effect of X-ray irradiation on dielectric properties of lead zirconate ceramics

The dielectric constant e and loss tangent, tan δ, of unirradiated and X-ray irradiated lead zirconate ceramic, prepared from reagent grade PbO and ZrO2 in equimolar proportion, have been studied as a function of temperature and frequency, in the temperature and frequency range from 300°K to 673°K and 50 kHz to 30 MHz respectively. The dielectric study shows that PbZrO3 undergoes a first order phase transition at about 565°K. The Curie-Weiss temperature is estimated to be about 521°K. The ionizing radiation decreases dielectric constant e, increases loss tangent tan δ and shifts the transition temperature by 16°K. These effects are attributed to increased density of free electrons and free holes after bombarding the material with X-rays.

Electrical Conductivity of Non-Stoichiometric LiNbO3 Single Crystals

Single crystals of LiNbO3 with different melt composition, (Li/Nb)m = 0.945, 1.0, 1.1 and 1.2, are grown by slow cooling of the melt. The crystals subjected to chemical analyses reveal a marked difference in starting/melt and crystal composition. Practically, evaporation losses of initial ingredients during the crystal growth are found to be responsible for nonstoichiometry in LiNbO3 single crystals. Partial replacement of monovalent Li+ by excess pentavalent Nb+5 creates additional cation vacancies to attain electro-neutrality in crystal. Complex impedance spectroscopy technique is applied to resolve the displacement current and ionic current through the crystals with P O2 > 1 atm. Oxygen partial pressure (> 1 atm.) dependent conductivity, along the c-axis, obeys the Arrhenius law in the temperature range from 750 to 1000°C. The strong dependence of O−2 conductivity on crystal composition is understood to be because of defects. Although, the dc conductivity of LiNbO3 single crystals is very low compared to ionic, but it follows the Arrhenius behavior. The results are discussed in the light of defect chemistry.

An investigation of lithium conducting borate glasses in the light of preparative mode

Abstract Lithium borate glasses of four different compositions have been prepared by using Al-mould, Al-blocks, Cu-drum and Twin roller quenching techniques and characterised by XRD and DTA. It is found that the quenching technique has a profound effect on the glass formation and its electrical conductivity. With Cu-drum quenching technique, it has been possible to extend the glass formation up to 70 mol% Li2O. This sample exhibited maximum conductivity. A close scrutiny of glasses for 40 ≤ Li2O ≤ 50 mol% prepared by Al-blocks quenching technique reveals that, the composition with 42.5 mol% Li2O gave maximum conductivity. This can be clearly understood on the basis of ‘N4′, the fraction of four co-ordinated boron.

Enhanced Electrode Kinetics in Electrochemical CO2 and SO2 Gas Sensors with Ferroelectric Dispersed Composite

The demand for reliable and economical SO 2 and CO 2 gas sensors with a large detection range, high sensitivity, good selectivity, rapid response and high accuracy has become evident. In addition, these gases are very difficult to detect by conventional gas sensors for the reason of its high stability at ordinary temperatures. Many kinds of compact sensors using various materials, such as solid electrolytes, mixed oxides, zeolites, and so on have been investigated so far. None of them seems to have achieved sufficient performance characteristics for practical use. Among these, solid electrolytes based sensors are of particular interest again from the viewpoint of low-cost, simple structure, high sensitivity and simplicity of above mentioned gas detection mechanism. The electrochemical SO 2 and CO 2 gas sensors are made using ferroelectric materials dispersed sulphate and carbonate electrolytes, respectively. The performance of sensing electrode sensors is improved due to use of ferroelectric materials with proper proportion. Also, both the response time and the recovery time are found decreased considerably, i.e. sensors quickly respond to change in gas concentration. The ferroelectric property of second phase not only enhances the defect concentration at the grain boundary interface, leading to increase in ionic conductivity, but also promotes oxy-ion activity with improved reaction kinetics. These pronounced effects are discussed in the light of presence of ferroelectric phase in the electrolyte as well as electrodes.