Shalini Bahel

Low fire M-phase lithium based dielectric ceramics for microwave applications: A review (I)

ABSTRACT Microwave dielectric ceramics have been used for various applications such as wireless communications including mobile communications, local-area networks, ultra high speed local area networks, intelligent transport systems, satellite communications and consumer electronic products. The continuous trend towards portable, light weight and multifunctional electronic components has motivated scientists to enhance the properties of existing materials and develop new dielectric materials. During the past 3 decades, a lot of research has been taken place in this field. Various low fire systems having sintering temperature ≤1300°C like nickel-niobates, bismuth-niobates, bismuth-zinc-niobates, lithium-niobates and lithium-tantalates have been reported. However, the information on these useful materials is sprinkled. The main purpose of this review is to bring together the data on the low fire M-phase lithium based dielectric ceramics with remarkable dielectric properties. Sintering temperature of all the M-phase Li based solid solution has been reduced from 1100°C approximately to about (9000-850°C) with the help of sintering aids. It should be of great help to researchers and technologists across the globe.

Structural and Dielectric Properties of Ca Doped Barium Iron Niobate

Ceramic materials with high dielectric constant are being used in many electric and electro-optics applications. Ca substituted barium iron niobate with general formula (Ba1-xCax) (Fe0.5Nb0.5)O3 with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by conventional solid state reaction method. Structural properties were investigated using X-ray diffraction and scanning electron microscope. Two types of phases were observed for 0.4 ≤ x ≤ 08. Dielectric constant and loss tangent were measured using impedance analyzer in the frequency range of 20 Hz–1 MHz at room temperature. Dielectric constant and loss tangent were found to decrease with increasing the frequency. A relatively good combination of dielectric constant and loss tangent was obtained for x = 0.2 (ϵr = 1441 and tan δ = 0.19) at 10 KHz.

Complex permittivity, permeability and microwave absorbing properties of Co–Ti substituted strontium hexaferrite

Abstract M-type strontium ferrite with compositions SrFe(12-2x)CoxTixO19 (x = 0.0, 0.3, 0.5, 0.7, 1.0), were prepared by two route ceramic method. The effects of Co–Ti substitution on their microstructure, electromagnetic properties, and microwave absorptive behavior were analyzed. The complex permittivity (∊′-j∊″) and complex permeability (μ′-jμ″) have been measured from 8.2 to 12.4 GHz using a network analyzer. Scanning electron microscope was used to analyze the grain size distribution and porosity of the ferrite. X-ray diffraction confirmed the M-type structure of the doped strontium ferrite. Vibrating sample magnetometer was used to study hysteresis loop of the ferrite. This study suggests that the control of grain size, decrease in coercivity and enhanced values of dielectric constant and loss are effective means to improve microwave absorption. The dielectric constant and loss were enhanced in comparison to the permeability constant and loss over the entire frequency range.

TEMPERATURE DEPENDENT DIELECTRIC PROPERTIES OF Ba-La-Sm-Bi-Ti-O CERAMIC SYSTEM

ABSTRACT The temperature dependent dielectric properties of some of the compounds in Ba-La-Sm-Bi-Ti-O ceramic system have been studied for developing temperature stable high dielectric constant ceramics. Two series, Ba4(La0.5-zSm0.5Biz)9.33Ti18O54, and Ba4(La0.3-zSm0.7Biz)9.33Ti18O54, where z = 0.0, 0.1, 0.2, have been synthesized by conventional mixed oxide route and characterized. The study revealed that the substitution of rare earths (Sm, La) and Bi has been found effective in achieving high dielectric constant along with stabilizing the variation in dielectric constant with temperature. Ba4(La0.5-zSm0.5Biz)9.33Ti18O54 with z = 0.2 and Ba4(La0.3-zSm0.7Biz)9.33Ti18O54 with z = 0.1 posses high dielectric constant with small temperature variation of the resonant frequency (± 5 ppm/°C). These could be dielectric materials for practical applications.

Effects of Temperature Variation on Dielectric Properties of Ba4(La(1 - z)Biz)9.33Ti18O54 Solid Solutions

Bi modified Ba4La9.33Ti18O54 solid solutions were synthesized by conventional mixed oxide route. The three composition with molar formula Ba4(La(1 - z)Biz)9.33Ti18O54 where z = 0.0, 0.1 and 0.2 were characterized for structural and dielectric properties. A detailed study related to variation in dielectric properties namely dielectric constant and dielectric loss with change in temperature from 298 K to 423 K was carried out in the frequency range 10 kHz–10 MHz. It was observed that dielectric constant decreases as the temperature increase from 298 K to 423 K in the entire frequency range for all the three samples. There was very small change in dielectric loss. The temperature coefficient of resonant frequency was estimated to improve from 273.8 ppm/K to 54.5 ppm/K as Bi contents increases from z = 0.0 to 0.2. Such materials would be a suitable candidate for applications in mobile communications.

Relative analysis of AODV & DSDV routing protocols for MANET based on NS2

To design any wireless network, routing and protocol selection are of important consideration. In Mobile Adhoc Networks (MANET), routing protocols should have best throughput with minimum delay. So the performance in critical situations is the main concern of mobile networks. In this paper, wireless network of mobile nodes has been designed with defined parameters and the relative analysis is carried out on Adhoc On-demand Distance Vector (AODV) and Destination Sequenced distance Vector (DSDV) protocols using NS2 simulator. The Throughput of generating and receiving packets, Jitter on sending and receiving side and End to end delay are the common parameters used for the comparison.

Synthesis and Dielectric Properties of a Temperature Stable Ceramic: Ba4La3.73Sm4.66Bi0.93Ti18O54

A temperature stable high dielectric constant Ba4La3.73Sm4.66Bi0.93Ti18O54 ceramic was synthesized by conventional solid state route. The structural and micro-structural properties were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The compound crystallizes in the orthorhombic system with unit cell parameters as a = 12.56Å, b = 22.25Å, c = 3.83Å and v = 1070.33Å3. The microwave dielectric properties were measured in the frequency range 0.3 to 3.0 GHz using a network analyzer and it shows a high dielectric constant of 86.1 and low loss tangent of 4.3 × 10–3 at 3.0 GHz along with a very low temperature coefficient of resonant frequency of 4.3 ppm/°C. It could be a promising candidate to be used as dielectric resonator in microwave communication.

Absorption Properties of Lanthanum and Samarium Substituted Barium Titanates

This paper deals with the investigation of structural and absorption properties of microwave dielectric ceramics with the general formula Ba6–3xR8+2xTi18O54 where R = La, Sm and x = 2/3. The X-ray diffraction analysis revealed tungsten bronze type crystal structure with orthorhombic symmetry for these ceramics. The scanning electron micrograph shows uniform needle like elongated grains. Absorption properties i.e. absorption loss, return loss and insertion loss have been measured for the synthesised samples with respect to the frequency in the range 8.0 GHz–12.0 GHz at room temperature.