Lalitha Sirdeshmukh

Dielectric properties and electrical conduction in yttrium iron garnet (YIG)

The dielectric properties (dielectric constant and loss) of a single crystal of yttrium iron garnet (Y3Fe5O12) were measured in the temperature range 77–725 K and in the frequency range 100 Hz-1 MHz. AC conductivity was derived from dielectric constant and loss. DC conductivity was measured in the temperature range 30–725 K. Thermoelectric power (TEP) was measured from 77–800 K. On the basis of the results, conduction in this garnet is interpreted as due to small polarons. The nature of conduction at different temperature ranges is discussed in the light of existing reports on defect formation.

Systematic hardness measurements on some rare earth garnet crystal

Microhardness measurements were undertaken on twelve rare earth garnet crystals. In yttrium aluminium garnet and gadolinium gallium garnet, there was no measurable difference in the hardness values of pure and nominally Nd-doped crystals. The hardness values were correlated with the lattice and elastic constants. An analysis of hardness data in terms of the interatomic binding indicated a high degree of covalency.

Atomistic properties of solids

Introduction.- Crystal growth.- Crystallography.- Diffraction of radiation by crystals.- Crystal structure determination.- Cohesion.- Tensor nature of crystal properties.- Mechanical properties.- Thermal properties.- Lattice vibrations.- Dielectric properties.- Pyro-, Piezo- and ferroelectricity.- Optical properties of insulators.- Defects in crystals: I Point defects.- Defects in crystals: II Dislocations.- The other crystalline states (quasi crystals, nano crystals, polycrystals, thin films, liquid crystals).

Reactivity ratios and thermal and dielectric studies of copolymers of acrylamide with butyl methacrylate

Abstract Free radical copolymerization of acrylamide with butyl methacrylate was carried out in the presence of 2,2′-azobisisobutyronitrile in dimethyl formamide at 60°. The compositions of copolymers were determined from their nitrogen contents. Reactivity ratios were calculated by the Fineman-Ross and Kelen-Tudos methods. The copolymers were characterized by i.r., 1H-NMR and 13C-NMR, and by thermal and dielectric studies. Glass transition temperatures were determined by DSC. Molecular weights of the copolymers were of the order of 105.

Dielectric behavior of NaClO3 and NaBrO3

A systematic measurement of the dielectric constant (e) and dielectric loss (tan δ) has been carried out on NaClO3 and NaBrO3 in the frequency range 100 Hz to 10 MHz at temperatures ranging from — 180°C to 240°C. The dielectric constants become frequency independent above 50 KHz. The data on temperature variation of e at 106 Hz have been fitted to Curie—Weiss type equations and also to polynomials in the temperature.The controversy regarding ferroelectricity in these crystals is briefly discussed and it is suggested that it is preferable to represent data on temperature variation of dielectric constant by a polynomial.The data on tan δ are utilised to evaluate the conductivity and thence, the activation energy for conduction.

Temperature dependence of dielectric constant of crystals with fluorite structure

The experimental data on the temperature variation of dielectric constant of six crystals with fluorite structure are analysed using an approach proposed by Havinga and Bosman for ionic crystals. The temperature variation of dielectric constant is resolved in three components related to the thermal expansion, the pressure dependence of dielectric constant and the temperature variation of polarizability (theA, B, C terms). In the present work, theB term is calculated semiempirically, such that the analysis can be extended to crystals like EuF2 for which high pressure dielectric constant data are not available. For the first time, such calculations have been made for EuF2 and PbF2 at elevated temperatures. TheC term, which is related to the temperature variation ofir polarizability is seen to play a dominant role in determining the temperature variation of dielectric constant.

Dielectric properties of Bi4(GeO4)3 and Bi4(SiO4)3

Dielectric constant, dielectric loss and conductivity of Bi4(GeO4)3 and Bi4(SiO4)3 single crystals have been measured as a function of frequency and in the temperature range from liquid nitrogen temperature to 400° C. The values of the static dielectric constant at room temperature are 16·4 and 13·7 for Bi4(GeO4)3 and Bi4(SiO4)3 respectively. The plots of log (σ) against reciprocal temperature at different frequencies of these crystals merge into a straight line beyond 250°C and the activation energies calculated in this region are found to be 0·95 eV and 1·2 eV for Bi4(GeO4)3 and Bi4(SiO4)3 respectively.

Mechanical Properties of Solids

As we proceed along,we will see that the elastic properties of solids have twofold importance. Firstly,they indicate the mechanical strength of the solid. Secondly,they are very important in understanding the nature of the interatomic forces and in the analysis of lattice vibrations.

Dielectric properties of RbCl-RbBr mixed crystals

A systematic measurement of dielectric constant and loss on RbCl-RbBr mixed crystals in various compositions has been carried out in the frequency range 100 Hz to 100 kHz and in the temperature range from room temperature to 320°C. From these measurements the static dielectric constant, the Szigeti charge, the conductivity and the activation energy for conduction are evaluated. All these properties show a nonlinear composition dependence. Semiempirical equations proposed earlier are employed to evaluate the dielectric constant as a function of composition. The validity of these relations is also discussed.

Characterization and dielectric properties of almandine-pyrope garnet

Some garnets collected from the Kothagudem area of Khammam district in Andhra Pradesh were characterized by chemical analysis. The results show the garnets to be of almandine (Fe+23 Al2Si3O12) pyrope (Mg3Al2Si3O12) group. Dielectric constant (ɛ) and dielectric loss (tanδ) were measured as a function of frequency and temperature in the frequency range of 100 Hz to 100 KHz and from room temperature to 400°C. The room temperature measurement was extended to 10 MHz, AC conductivity was calculated from the data on ε and tan δ. DC conductivity was also measured.