Small toroidal antennas

The radiation fields of small air-cored toroidal antennas carrying a uniform in-phase current have been evaluated. It is shown that according to whether the number of turns is one or more, different radiation patterns result. For the single turn toroid the position of its null axis can be adjusted by altering the ratio of its mean radius to the radius of its cross-section.     

Dry etching and sputtering

Dry etching is an important process for micro- and nanofabrication. Sputtering effects can arise in two contexts within a dry-etch process. Incoming ions cause removal of volatile products that arise from the interaction between the dry-etch plasma and the surface to be etched. Also, the momentum transfer of an incoming ion can cause direct removal of the material to be etched, which is undesirable as it can cause electrical or optical damage to the underlying material. This is largely avoided in dry-etch processes by use of reactive chemistries, although in some processes this component of the etching can be significant. Etch processes, both machine type and possible etch chemistries, are reviewed. Methods of characterizing the electrical and optical damage related to ion impact at the substrate are described. The use of highly reactive chemistries and molecular constituents within the plasma is best for reducing the effects of damage.     

Measurement of radioactivity in the soil of Bahawalpur division, Pakistan

Bahawalpur is the largest division of the Punjab province in Pakistan. It is larger than many countries of the world. Gamma activity from the naturally occurring radionuclides namely 226Ra, 232Th, the primordial radionuclide 40K and the artificial radionuclide 137Cs was measured in the soil of the Bahawalpur division using gamma spectrometry technique. The mean activity of 226Ra, 232Th, 40K and 137Cs were found to be 32.9 +/- 0.9, 53.6 +/- 1.4, 647.4 +/- 14.1 and 1.5 +/- 0.2 Bq kg(-1), respectively. The mean radium equivalent activity Raeq, external hazard index, internal hazard index and terrestrial absorbed dose rate for the area under study are 158.5 +/- 4.1 Bq kg(-1), 0.4, 0.5 and 77.32 nGy h(-1), respectively. The annual effective dose equivalent to the public was found to be 0.5 mSv.     

Contents of Vol. 16 (2001)

Volume Contents

Contents of Vol. 16 (2001)     

Silicon nitride Micromesh Bolometer Array for Submillimeter Astrophysics

We present the design and performance of a feedhorn-coupled bolometer array intended for a sensitive 350-mum photometer camera. Silicon nitride micromesh absorbers minimize the suspended mass and heat capacity of the bolometers. The temperature transducers, neutron-transmutation-doped Ge thermistors, are attached to the absorber with In bump bonds. Vapor-deposited electrical leads address the thermistors and determine the thermal conductance of the bolometers. The bolometer array demonstrates a dark noise-equivalent power of 2.9 x 10(-17) W/ radicalHz and a mean heat capacity of 1.3 pJ/K at 390 mK. We measure the optical efficiency of the bolometer and feedhorn to be 0.45-0.65 by comparing the response to blackbody calibration sources. The bolometer array demonstrates theoretical noise performance arising from the photon and the phonon and Johnson noise, with photon noise dominant under the design background conditions. We measure the ratio of total noise to photon noise to be 1.21 under an absorbed optical power of 2.4 pW. Excess noise is negligible for audio frequencies as low as 30 mHz. We summarize the trade-offs between bare and feedhorn-coupled detectors and discuss the estimated performance limits of micromesh bolometers. The bolometer array demonstrates the sensitivity required for photon noise-limited performance from a spaceborne, passively cooled telescope.     

Spectroscopic investigations of Cu2+ in Li2O-Na2O-B2O3-Bi2O3 glasses

Pure and copper doped glasses with composition,x Li ₂ O-(40-x)Na ₂ O-50B ₂ O ₃-10Bi ₂ O ₃,have been prepared over the range 0 ≤ x ≤ 40. The electron paramagnetic resonance (EPR) spectra of Cu²⁺ ions of these glasses have been recorded in the X-band at room temperature. Spin Hamiltonian parameters have been calculated. The molecular bonding coefficients, α² and β², have been calculated by recording the optical absorption spectra in the wavelength range 200–1200 nm. It has been observed that the site symmetry around Cu²⁺ ions is tetragonally distorted octahedral. The density and glass transition temperature variation with alkali content shows non-linear behaviour. The IR studies show that the glassy system contains BO₃ and BO₄ units in the disordered manner.     

A continuum shape sensitivity method for fracture analysis of isotropic functionally graded materials

This paper presents a new continuum shape sensitivity method for calculating mixed-mode stress-intensity factors for a stationary crack in two-dimensional, linear- elastic, isotropic FGMs with arbitrary geometry. The method involves the material derivative concept taken from continuum mechanics, the mutual potential energy release rate, and direct differentiation. Since the governing variational equation is differentiated prior to discretization, resulting sensitivity equations are independent of approximate numerical techniques, such as the finite element method, boundary element method, mesh-free method, or others. The discrete form of the mutual potential energy release rate is simple and easy to calculate, as it only requires multiplication of displacement vectors and stiffness sensitivity matrices. By judiciously selecting the velocity field, the method only requires displacement response in a subdomain close to the crack tip, thus making the method computationally efficient. Seven finite-element based numerical examples, which comprise mode-I and mixed-mode deformations and/or single or multiple interacting cracks, are presented to evaluate the accuracy of the fracture parameters calculated by the proposed method. Comparisons have been made between stress-intensity factors predicted by the proposed method and available reference solutions in the literature, generated either analytically or numerically using various other fracture integrals or analyses. Excellent agreement is obtained between the results of the proposed method and previously obtained solutions. Therefore, shape sensitivity analysis provides an attractive alternative to fracture analysis of cracks in homogeneous and non-homogeneous materials.     

Study of Graphitic Crystallites in Some Carbonized Residues Prepared From Catalytic Cracking of Asphalt

This paper demonstrates the carbonization of asphalt collected from the Morgah Oil Refinery, Rawilpindi, Pakistan. The asphalt (80/90) was carbonized in a micro-autoclave under nitrogen environment at 300°C. To enhance cracking reactions of side chains, and condensation of polycyclic configurations, the sample was also loaded with catalysts such as Zeolite Socony Mobil No. 5 (ZSM-5), phosphotungstic acid, coal ash and Utmanzai clay (UTIMAC). Each carbonized residue was crushed with mortar and pestle and soxhlet extracted with n-pentane for removal of oil, dried and analyzed by X-ray diffractometery (XRD). The appearance of distinct bands correspond to crystallites in some samples particularly those loaded with UTIMAC has established graphitization.     

MAC‐assisted topology control for ad hoc wireless networks

We consider ad hoc wireless networks and the topology control problem defined as minimizing the amount of power needed to maintain connectivity. The issue boils down to selecting the optimum transmission power level at each node based on the position information of reachable nodes. Local decisions regarding the transmission power level induce a subgraph of the maximum powered graph G_max in which edges represent direct reachability at maximum power. We propose a new algorithm for constructing minimum‐energy path‐preserving subgraphs of G_max, i.e. ones minimizing the energy consumption between node pairs. Our algorithm involves a modification to the medium access control (MAC) layer. Its superiority over previous solutions, up to 60% improvement in sparse networks, demonstrates once again that strict protocol layering in wireless networks tends to be detrimental to performance. Copyright © 2005 John Wiley & Sons, Ltd.     

Evaluation of Minimal of Lubricant in End Milling

When considering machinability parameters, the use of coolant is indispensable in metal cutting operations. However, stricter environmental regulations are making the use of an ample amount of conventional coolant impossible because of its negative impact on the environment. Consequently, the use of minimal quantities of lubricant (MQL) can be regarded as an alternative solution in which the functionality of cooling and lubrication can be achieved by a tiny amount of cutting oil. In this study, flood coolant (42 l min _1) was used compared to the MQL amount of 8.5 ml h_1 and the comparative effectiveness was investigated in terms of cutting force, tool wear, surface roughness, and chip shape. Unlike the catastrophic tool failure in flood cooling, the use of MQL resulted in the cutting edge remaining intact, in spite of a higher flank width. The findings of this study show that MQL may be considered to be an economical and environmentally compatible lubrication technique.