Showing papers by "V. R. K. Murthy published in 2005"

Raman spectroscopic study of (Pb1−xBax)(Yb1∕2Ta1∕2)O3 ceramics

Abstract: The ordered complex perovskite lead ytterbium tantalate undergoes a change in structural and dielectric properties as the concentration of Ba increases. The x-ray diffraction patterns show a decrease in the intensity of superlattice reflections due to Pb-antiparallel displacement as well as to B-site ordering. The structure approaches cubic symmetry with increase in Ba content. The dielectric response shows a gradual change from antiferroelectric to relaxor ferroelectric and then the relaxor behavior starts decreasing as Ba content increases. Raman-scattering study of the samples shows that some of the modes seen for Pb(Yb0.5Ta0.5)O3 disappear and an additional mode appears at 420cm−1 with increase in Ba concentration. A qualitative analysis of various modes suggests that the substitution of Ba for Pb leads to a decrease in the degree of ordering and occurrence of local distortions in the lattice as evidenced by the broadening of certain peaks in the Raman spectra.

Dielectric and Raman studies of (BaxPb1−x)(Yb0.5Nb0.5)O3

Abstract: Dielectric dispersion and Raman studies are carried out on the solid solution series (BaxPb1−x)(Yb0.5Nb0.5)O3 from x = 0.0 to 0.3. The x-ray diffraction analysis indicates that with the substitution of Ba2+, the intensity of the peaks corresponding to antiparallel displacement of Pb2+ decreases and the structure becomes cubic. The low frequency dielectric dispersion studies indicate that the series undergoes a transition from an antiferroelectric to a relaxor ferroelectric with variation of x. Raman spectra of the compositions indicate the presence of a greater number of modes for the compounds with cubic structure (). A new mode at 420 cm−1 is observed for the compounds with Ba2+ substitution. The intensity of this mode increases monotonically with increase in Ba2+ substitution and this has been attributed to the commencement of local distortion. This could be responsible for the appearance and increase in the diffuseness around Tmax, the temperature corresponding to .

Two-dimensional microwave band-gap structures of different dielectric materials

Abstract: We report the use of low dielectric constant materials to form two-dimensional microwave band-gap structures for achieving high gap-to-midgap ratio. The variable parameters chosen are the lattice spacing and the geometric structure. The selected geometries are square and triangular and the materials chosen are PTFE (e = 2.1), PVC (e = 2.38) and glass (e = 5.5). Using the plane-wave expansion method, proper lattice spacing is selected for each structure and material. The observed experimental results are analyzed with the help of the theoretical prediction

Study of one dimensional photonic band gaps at microwave frequencies for microwave filters

Abstract: There has been a great deal of interest in periodic dielectric structures that exhibit photonic band gaps. Periodicity in dielectric constant results in a range of frequencies within which transmission is forbidden, known as Photonic Band Gaps (PBG). PBG structures can be used as tunable filters whose bandwidth can be varied depending on the periodicity, dielectric constant etc. It is possible to construct PBG structures with one-dimensional periodicity. In this paper, we report the experimental observation of the band gaps between 10 and 20 GHz for a structure made of lossy glass slabs (ϵ r ′ = 6.1, ϵ r ” = 0.06). This paper also presents the results on the numerical computation using transfer matrix method for the above structure for comparison. The nature of the band gaps is also investigated with respect to the spacing between the dielectric slabs and dielectric constant.

Microwave Propagation in a Square Lattice Using Different Dielectric Materials for Device Applications

Abstract: This paper presents the investigation on the propagation of microwaves through the square lattice structure constructed by two different materials, glass (ϵ r = 5.5) and poly vinyl chloride (PVC) (ϵ r = 2.38). We report the transmission spectra of these structures that were recorded between 7 and 20 GHz. It is observed that the glass structure of lattice constant 1.4 cm gave a band gap centered around 17 GHz with a gap width of 3.5 GHz and attenuation of 30 dB. With PVC rods, the spectra were recorded for two different lattice constants 1.4 cm and 1.1 cm. The structure of 1.4 cm shows a band gap centered at 16.8 GHz with a gap width of 1.48 GHz where as the structure of 1.1 cm lattice constant shows a band gap centered at 11.17 GHz with a gap width of 2.67 GHz. The attenuation in both cases is around 25 dB. The analysis shows that the gap width is low for structures made of PVC rods when compared to structure made of glass as expected because of the low dielectric constant of PVC. It is also observed that...