Functionally Graded Ceramics Fabricated with Side‐by‐Side Tape Casting for Use in Magnetic Refrigeration

Functionally graded ceramic tapes have been fabricated by a side‐by‐side tape casting technique. This study shows the possibility and describes the main principles of adjacent coflow of slurries resulting in formation of thin plates of graded ceramic material. Results showed that the small variations of solvent and binder system concentrations have a substantial effect on slurry viscosity. Varying these parameters showed that side‐by‐side tape casting with a well‐defined interface area is possible for slurries with viscosities above 3500 mPa s at a casting shear rate of 3.3 s^?1. As it was expected, the choice of de‐bindering and sintering regimes significantly influences crack formation, and a three‐step heating programme was found to result in tapes of the highest quality. The interface regions of green graded tapes were investigated structurally by scanning electron microscopy; for a distinct identification of the interface region and analysing the degree of cross‐interface diffusion, the isothermal entropy change was measured by a vibrating sample magnetometer as the magnetic transition temperature (Curie temperature) is very sensitive to the dopant level in ceramics. Also the purpose of developing this graded ceramic tape casting was applications of these specific magnetocaloric properties within the magnetic refrigeration technology.     

L- and S-Band Compact Octave Bandwidth 4-bit MMIC Phase Shifters

This paper presents the design approach and test results of L- and S-band compact monolithic microwave integrated circuit (MMIC) phase shifters (PSs) developed for octave band applications. The PS designs were based on an optimum topology selected for each bit for compact size and low insertion loss, and have on-chip integrated digital control. The MMIC PSs were fabricated employing GaAs-based high-performance 0.4-mum MSAG MESFET technology with multilevel-plating process. We achieved a chip size of 2.6 mm², the smallest size reported to date.     

Post spinning modification of PAN fibres — a review

Post spinning modifications play an important role in improving the mechanical properties, density and structure of PAN fibres. Post spinning modifications affect the thermal and kinetic parameters during stabilization and alter the overall stabilization mechanism. The present paper presents a review of the different types of post spinning modifications previously attempted on PAN fibres and the effects of these modifications on the resulting carbon fibres.     

A cone-based distributed topology-control algorithm for wireless multi-hop networks

The topology of a wireless multi-hop network can be controlled by varying the transmission power at each node. In this paper, we give a detailed analysis of a cone-based distributed topology-control (CBTC) algorithm. This algorithm does not assume that nodes have GPS information available; rather it depends only on directional information. Roughly speaking, the basic idea of the algorithm is that a node u transmits with the minimum power p/sub u,/spl alpha// required to ensure that in every cone of degree /spl alpha/ around u, there is some node that u can reach with power p/sub u,/spl alpha//. We show that taking /spl alpha/=5/spl pi//6 is a necessary and sufficient condition to guarantee that network connectivity is preserved. More precisely, if there is a path from s to t when every node communicates at maximum power then, if /spl alpha//spl les/5/spl pi//6, there is still a path in the smallest symmetric graph G/sub /spl alpha// containing all edges (u,v) such that u can communicate with v using power p/sub u,/spl alpha//. On the other hand, if /spl alpha/>5/spl pi//6, connectivity is not necessarily preserved. We also propose a set of optimizations that further reduce power consumption and prove that they retain network connectivity. Dynamic reconfiguration in the presence of failures and mobility is also discussed. Simulation results are presented to demonstrate the effectiveness of the algorithm and the optimizations.     

Distributed fair scheduling in a wireless LAN

Fairness is an important issue when accessing a shared wireless channel. With fair scheduling, it is possible to allocate bandwidth in proportion to weights of the packet flows sharing the channel. This paper presents a fully distributed algorithm for fair scheduling in a wireless LAN. The algorithm can be implemented without using a centralized coordinator to arbitrate medium access. The proposed protocol is derived from the Distributed Coordination Function in the IEEE 802.11 standard. Simulation results show that the proposed algorithm is able to schedule transmissions such that the bandwidth allocated to different flows is proportional to their weights. An attractive feature of the proposed approach is that it can be implemented with simple modifications to the IEEE 802.11 standard.     

Effect of carbon fiber surface functional groups on the mechanical properties of carbon-carbon composites with HTT

The present investigation describes the quantitative measurement of surface functional groups present on commercially available different PAN based carbon fibers, their effect on the development of interface with resol-type phenol formaldehyde resin matrix and its effect on the physico-mechanical properties of carbon-carbon composites at various stages of heat treatment. An ESCA study of the carbon fibers has revealed that high strength (ST-3) carbon fibers possess almost 10% reactive functional groups as compared to 5.5 and 4.5% in case of intermediate modulus (IM-500) and high modulus (HM-45) carbon fibers, respectively. As a result, ST-3 carbon fibers are in a position to make strong interactions with phenolic resin matrix and HM-45 carbon fibers make weak interactions, while IM-500 carbon fibers make intermediate interactions. This observation is also confirmed from the pyrolysis data (volume shrinkage) of the composites. Bulk density and kerosene density more or less increase in all the composites with heat treatment up to 2600 °C. It is further observed that bulk density is minimum and kerosene density is maximum upon heat treatment at 2600 °C in case of ST-3 based composites compared to HM-45 and IM-500 composites. It has been found for the first time that the deflection temperature (temperature at which the properties of the material start to decrease or increase) of flexural strength as well as interlaminar shear strength is different for the three composites (A, B and C) and is determined by the severity of interactions established at the polymer stage. Above this temperature, flexural strength and interlaminar shear strength increase in all the composites up to 2600 °C. The maximum value of flexural strength at 2600 °C is obtained for HM-45 composites and that of ILSS for ST-3 composites.     

Fluorine-intercalated carbon fibers-I. structural and transport properties

The intercalation of fluorine in various types of carbon fibers (PAN-based or pitch-based, asreceived or high-temperature treated) has been investigated at room temperature in the presence of gaseous HF. Stage-1 compounds with C_2.5F to C₄F compositions are obtained for 10 bar F₂ pressures, whereas lower pressures (1 bar F₂) lead to stage-2 compounds. Although in higher stages (≥2) the electrical conductivity is generally larger than in the pristine fiber, in stage-1 compounds a drastic increase of resistivity is observed, ρ being more than one order of magnitude larger than that of the starting material. Finally, fluorine-intercalated GICs have been found appropriate to investigate the effects of disorder and reduced dimensionality.     

Studies on bitumen production from North Rumaila crude oil

In the present investigation, the effect of three process variables viz., air rate, temperature and time of oxidation has been studied to determine the possibility of making paving bitumens from North Rumaila Crude Oil. In order to have flexibility in the choice of proper feedstock for bitumen production, the effect of these process parameters was studied on three different vacuum residues. Air rate and temperature of oxidation were optimised to produce a bitumen product having less temperature susceptibility. The composition studies of typical feeds and oxidised products indicate that mainly “saturates” components get converted to “asphaltics” on air blowing.     

Sample modulated Raman spectroscopy and frequency modulated visible light--resonance Raman spectrum of a polydiacetylene fiber

Methods for obtaining sample modulated Raman (SMR) spectra and frequency modulated visible light are described. The sample modulated Raman method is one in which the frequency of the Raman scattered light is modulated by the sample which is responding to an externally applied periodic stress. The Raman active modes of the sample which are due to the vibrational motion of those molecular-scale units that change in response to the external stress are modulated and appear in the frequency-locked phase sensitively detected Raman spectrum. Those modes due to units that do not respond significantly to the stress do not appear. In principle the SMR method permits determination of those molecular units of a complex substance that are responsible for the substance's response to an external stress. The Raman scattered light is frequency modulated, has the wave form nu=nu(0)-nu(i)-(deltanu) sin(2pi ft), where nu(0) is the incident monochromatic light, nu(i) is the vibrational frequency of the statically stressed material, and (deltanu) is the magnitude of the frequency shift caused by the maximum stress applied at the frequency f.