Superplasticity in a high strength powder aluminum

The superplastic properties of a rapidly solidified, high strength P/M Al alloy and the same alloy reinforced with SiC particulates (SiC _p ) have been studied. To prepare superplastic test materials, a matrix alloy powder of composition 7.2Zn-2.4Mg-2Cu-0.2Zr-0.12Cr-0.2Co (Kaiser PM-64) and the powder mixed with 10 to 20 vol pct SiC _p (~5 μm diameter) were thermomechanically processed to very fine equiaxed grain structures of ~6 μm and ~8 μm, respectively. Superplasticity in these materials was evaluated by characterizing (1) high temperature stability, (2) dynamic grain growth, (3) strain rate sensitivity, (4) flow stress behavior, (5) cavitation and cavitation control, and (6) total superplastic strain. It was observed that the PM-64 alloy could achieve a total elongation of over 800 pct, while the SiC_p reinforced alloy could attain an elongation greater than 500 pct before failure. Also, it was shown that with the use of hydrostatic pressure during superplastic flow, cavitation could be controlled. Observations were made of the effect SiC _p reinforcement particles had on the superplastic flow stress behavior. Interpretations are proposed to explain the role of particulates during superplastic straining.     

On the improvement of detection characteristics of trans-stilbene crystals by improving crystal perfection

Organic molecular scintillating crystals are noted for their good timing and particle discrimination process. Trans-stilbene is one such candidate noted for its good particle detection characteristics for the past five decades. Progressive strengthening of detection characteristics of trans-stilbene has been attempted by improving crystal perfection. A series of timing resolution studies have been carried out for the Bridgman grown trans-stilbene crystals under different experimental conditions. The results were compared with the previously reported values. Pulse shape discrimination process has been carried out for ²⁴¹Am and ²⁵²Cf sources and good discrimination has been obtained for gamma-alpha and gamma-neutron sources from the grown organic phosphor crystal. Electronic Publication     

Development of nano indium tin oxide (ITO) grains by alkaline hydrolysis of In(III) and Sn(IV) salts

Indium tin oxide (ITO) nano powders of different compositions (In: Sn = 90: 10, 70: 30 and 50: 50) were prepared by heat treatment (300-450°C) of mixed hydroxides of In(III) and Sn(IV). The hydroxides were obtained by the reaction of aq. NH₃ with mixed aq. solutions of In(NO₃)₃ and SnCl₄. FTIR and TG/DTA studies revealed that powders existed as In(OH)₃H₂O—SnO₃H₂H₂O in the solid state and then they transformed to In₂O₃—SnO₂ via some metastable intermediates after 300°C. Cubic phase of In₂O₃ was identified by XRD for the oxides up to 30% of Sn. Particle size measurements of the solid dispersed in acetone and SEM study for microstructure showed that the oxides were in the nano range (55-75 nm) whereas the size range determined from Debye-Scherrer equation were 11–24 nm.     

Stability and throughput analysis of unslotted CDMA‐ALOHA with finite number of users and code sharing

We consider a system comprising a finite number of nodes, with infinite packet buffers, that use unslotted ALOHA with Code Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition for the stability of the packet buffers; we interpret this as the capacity of the access method. We calculate and compare the capacities of CDMA‐ALOHA (with and without code sharing) and TDMA‐ALOHA; we also consider carrier sensing and collision detection versions of these protocols. In each case, saturation throughput can be obtained via analysis of a continuous time Markov chain. Our results show how saturation throughput degrades with code‐sharing. Finally, we also present some simulation results for mean packet delay. Our work is motivated by optical CDMA in which “chips” can be optically generated, and hence the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful in the optical CDMA context as it reduces the number of optical correlators at the receivers. Our throughput results help to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA‐ALOHA yields better capacity than TDMA‐ALOHA. This revised version was published online in June 2006 with corrections to the Cover Date.     

A novel scheme for protection of power semiconductor devicesagainst short circuit faults

Power semiconductor devices find wide application in modern power electronic converters. Protection of these devices against overload/short circuit conditions is of paramount importance. Present day protection topologies employing different circuits have invariably one main drawback in that the fault current reaches the set value before action is initiated to trip the system. This poses a severe stress on the device. Hence an adequate safety margin has to be necessarily provided to prevent excessive device stresses and care has to be taken to see that the device is operated well within its safe operating areas. The present paper proposes a method wherein the slope or rate of rise of the fault current is detected and once the slope exceeds the set reference, action is initiated to trip the system much before the fault current reaches dangerous levels. The method provides a fast means of detection of overload and short circuit currents and can be conveniently adopted for the protection of devices in power transistor/IGBT based inverters against short circuited load conditions or shoot through faults. The possible reduction of stresses in the power devices are also highlighted     

Finite element crack growth algorithm for dynamic fracture

A new finite element crack growth algorithm has been developed to simulate dynamic fracture. In this algorithm, pseudo elements with very high initial density are placed below the crack plane and the density is reduced to zero in a gradual manner as the crack passes the element. A number of linear elastic and elasto-viscoplastic problems have been carried out to test the new algorithm. The results are compared with some of the existing crack growth models.