Drilling of hybrid Al-5%SiCp-5%B4Cp metal matrix composites

Hybrid metal matrix composites consist of at least three constituents—a metal or an alloy matrix and two reinforcements in various forms, bonded together at the atomic level in the composite. Despite their higher specific properties (properties/unit weight) of strength and stiffness, the nonhomogeneous and anisotropic nature combined with the abrasive reinforcements render their machining difficult. The work piece may get damaged and the cutting tools experience high wear rates, which may lead to an uneconomical production process or even make the process impossible. This work reports on the drilling of Al-5%SiC_p-5%B₄C_p hybrid composite with high-speed steel (HSS), not expensive PCD, or carbide drills in an attempt to explore the viability of the process. Drilling of Al-5%SiC-5%B₄C composites with HSS drills is possible with lower speed and feed combination. The cutting conditions for minimized tool wear and improved surface finish are identified. Characterization of tool wear and surface integrity are also carried out.     

Transhybrid loss with RC balance circuits for primary-rateISDN transmission systems

The authors report a method for the reduction of the near-end echo in the subscriber loop environment for primary-rate ISDN full-duplex data transmission. The technique uses an RC network as the matching input line impedance at the transmitter to reduce the near-end echo. Data rates ranging from the ISDN basic rate with (2B+D) channels to primary rate with (23B+D) channels are covered. The results indicate that a single basic configuration for the balancing line impedance may be adapted to keep the transhybrid loss in the range of about 22 dB in the worst case over the entire frequency range of interest. The balancing procedure for the hybrid takes place in a training sequence during which the component values of the RC matching circuit are adjusted from their initial values by an iterative adaptation procedure. Two techniques for optimizing the component values have been verified. For all cases examined, including worst-case line configurations, the components of the RC matching circuit have converged to final values, for which the minimum transhybrid loss had a maximum above 20 dB     

Heat transfer analysis of plain and dimpled tubes with different spacings

Heat transfer enhancement by modifying the surface of tubes is commonly practiced throughout the world. Grooves, dimples, flutes or corrugations are placed inside and outside the surface of tubes and channels for enhancement. In this article, a novel method for heat transfer enhancement by varying the spacing between the tubes is reported. A comparison is made between the heat transfer performance of plain tubes and dimpled tubes at different spacings. For analysis, an experimental setup is fabricated and assembled. The flow is externally forced laminar flow of air over a hot tube maintained at constant temperature. Four different velocities of air 0.4, 0.6, 0.8, and 1.0 m/s are considered in this study. Tube surface temperature, heat transfer coefficient, heat transfer rate and Nusselt number are the parameters studied to analyze the thermal behavior of tubes at different spacings of 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 cm. From the experimental investigation it is found that, apart from heat transfer enhancement by providing dimples on the tube surfaces, there is an optimal spacing between the tubes after which no further improvement is obtained. In this study, 3.0 cm is found to be the optimal spacing for both plain and dimpled tubes. However, the percentage value of heat transfer enhancement is greater with optimal spacing and for all velocities of air in dimpled tubes.     

A Tutorial on Two-Wire Digital Transmission in the Loop Plant

This paper explores the constraints on the design of twowire repeaterless digital subscriber loop (DSL) systems. Broadly categorized, the design depends on the technical feasibility of the approach used to achieve two-wire transmission, constraints related to compatibility with other systems sharing the same cable, and immunity to central office noise. Each of these varies With the choice of system parameters including the transmission rate, transmit power, choice of line codes, etc. Technical feasibility is evaluated by computer simulation studies. Compatibility with other systems is explored by crosstalk calculations. Noise immunity considerations, as they translate into digital line power levels, are also explored.     

A Piezoactuated Droplet-Dispensing Microfluidic Chip

A microfluidic dispensing device that is capable of generating droplets with volumes varying between 1 nL and 50 pL at an ejection frequency of up to 6 kHz is presented. In this device, a piezoactuator pushes onto an elastic membrane via piston tips; the mechanical bending of the membrane generates a pressure pulse pushing droplets out. An analytical model was developed solving bending characteristics of a plate-actuated fluidic dispensing system and used to calculate the displaced volume. The model was extended to perform stress analysis to find the optimum piston tip radius by minimizing design stresses. The optimum piston tip radius was found to be 67% of the chamber radius. The actuation force estimated using the analytical model was then used as input to a finite element model of the dispenser. A detailed numerical analysis was then performed to model the fluid flow and droplet ejection process and to find critical geometric and operating parameters. Results from both models were used together to find the best design parameters. The device contains three layers, a silicon layer sandwiched between two polydimethylsiloxane (PDMS) polymer layers. Silicon dry etching, together with PDMS soft lithography, was used to fabricate the chip. PDMS oxygen plasma bonding is used to bond the layers. Prototypes developed were successfully tested to dispense same-sized droplets repeatedly without unwanted droplets. The design allows easy expansion and simultaneous dispensing of fluids.$hfill$[2009-0099]      

Effect of Nano-ZrO2 on the Properties of Al-Al2O3 Nanocomposites Prepared by Mechanical Alloying

In the present work, mechanical alloying was used to prepare Al-20wt.% Al₂O₃ metal-matrix nanocomposites having up to 4wt.% ZrO₂ at the expense of Al₂O₃. The powders were milled for different time intervals. To characterize the powders after milling, x-ray diffraction and transmission electron microscopy were used to identify the phase composition, crystallite size and morphology. In order to study the sinterability, the milled powders were cold pressed and sintered in argon atmosphere at different firing temperatures up to 470 °C for 1 h. The relative density and apparent porosity of the sintered composites were determined according to Archimedes principle. Moreover, the microstructure was examined by a scanning electron microscope attached with an energy dispersive spectrometer (EDS). Microhardness and AC conductivity of sintered composites were also measured. The results pointed out that the increasing of milling time is responsible for uniform distribution of Al₂O₃-ZrO₂ particles in the Al matrix as well as remarkable increases in relative density, microhardness and AC conductivity of the sintered specimens. Also, the relative density was affected considerably by the increasing of sintering temperature. Moreover, increasing of ZrO₂ content led to a significant decrease in the crystal size of the milled powders and increase in the microhardness of the sintered compacts. No changes were observed on the conductivity after addition of ZrO₂.

     

Enhanced dielectric properties of PVA/PEDOT:PSS/MnO2 based composites for electronic applications

Journal of Materials Science: Materials in Electronics - Polymer composite (PC) films comprising polyvinyl alcohol (PVA) and poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)...     

A formal context specific trust model (FTM) for multimedia and ubiquitous computing environment

In order to ensure secure sharing of resources in an ad-hoc network of handheld devices in a multimedia and ubiquitous computing environment, mutual collaboration is essential. However, the limitations, such as poor storage and computational capability of these multimedia and ubiquitous devices stand as the bottleneck for effective sharing of resources. As a result of this drawback, the adversaries are obtaining access to the new doors for security breaches. Mutual Trust is the weapon used to combat security violations by restricting malicious devices from participating in any interaction in such an open and dynamic environment. In this paper, we present a context-specific and reputation-based trust model along with a brief survey and a comparative study of the existing trust models found in the literature. To the best of our knowledge, our model is the first formal trust model for multimedia and ubiquitous computing, which incorporates multi-hop recommendation capability and flexible behavioral model to handle interactions among devices. The other major contribution of this universal trust model is a simple protocol for circumventing malicious recommendations and handling them for achieving accuracy. This paper also illustrates the implementation and evaluation detailed of our proposed omnipresent formal trust model.     

Enhancing Privacy and Security of RFID System with Serverless Authentication and Search Protocols in Pervasive Environments

One of the recent realms that gathered attention of researchers is the security issues of Radio Frequency Identification (RFID) systems that have tradeoff between controlled costs and improved efficiency. Evolvement and benefits of RFID technology signifies that it can be low-cost, efficient and secured solution to many pervasive applications. But RFID technology will not intermingle into human lives until prevailing and flexible privacy mechanisms are conceived. However, ensuring strong privacy has been an enormous challenge due to extremely inadequate computational storage of typical RFID tags. So in order to relieve tags from responsibility, privacy protection and security assurance was guaranteed by central server. In this paper, we suggest serverless, forward secure and untraceable authentication protocol for RFID tags. This authentication protocol safeguards both tag and reader against almost all major attacks without the intervention of server. Though it is very critical to guarantee untraceability and scalability simultaneously, here we are proposing a scheme to make our protocol more scalable via ownership transfer. To the best of our knowledge this feature is incorporated in the serverless system for the first time in pervasive environments. One extension of RFID authentication is RFID tag searching, which has not been given much attention so far. But we firmly believe that in near future tag searching will be a significant issue RFID based pervasive systems. So in this paper we propose a serverless RFID tag searching protocol in pervasive environments. This protocol can search a particular tag efficiently without server’s intervention. Furthermore they are secured against major security threats.