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41.
This paper deals with the availability and reliability analysis of two different 1-server n-unit systems with preventive maintenance and repair. Initially, one unit operates and the remaining n - 1 units are kept as cold standbys. In the first system the time to failure and the time to preventive maintenance of a unit are arbitrarily distributed. In the second system, each unit consists of 2 components connected in series. When a unit fails, the failed component is taken up for repair while the other waits for preventive maintenance. Explicit expressions for the Laplace transform of the mean down-time of the system in [0, t] and for the mean time to system failure are obtained. Steady-state availability of the system is also discussed. A few special cases have been studied. 相似文献
42.
A commonly made assumption for stochastic analysis of maintainable systems is that continuous monitoring at a modular level is incorporated. A provision for continuous monitoring will add to the complexity of the design and/or to the cost of operation. However, it may not necessarily improve system performance. An effective monitoring policy will depend on the properties of modules and can be selected by carrying out prototype studies. The analysis will depend on the policy adopted, because some state transitions become unobservable. Such an analysis of a standby redundant system is discussed in this paper. 相似文献
43.
This paper deals with the s?-expected busy period of a n-unit system with a single repair facility. The service facility is called for whenever r (1 ≤ r ≤ n) units are in failed condition and is retained until no unit is waiting for repair. The analysis is carried out under the assumption that the time to failure of a unit is arbitrarily distributed while the service rate is constant. The case when n = 2 with the failure time distribution is 2-stage Erlangian is discussed. 相似文献
44.
David A. Scrymgeour Venkatraman Gopalan 《Metallurgical and Materials Transactions A》2004,35(8):2287-2290
We report the presence of an unexpected asymmetry in the local electromechanical response across a single ferroelectric domain
wall in congruent lithium niobate at room temperature. We provide evidence that this is related to the presence of nonstoichiometric
defects in the crystal, which interact with the domain wall.
This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures
and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003. 相似文献
45.
N.P. Gopalan 《International Journal of Engineering Science》1984,22(5):617-626
Consummating our earlier work [1], the unsteady flow of a fairly concentrated suspension due to a single contraction or expansion of the walls of a tube is studied. A comparison of the results obtained by using two different formulae for the additional drag terms in the governing equations has been made. A region of circulation in the flow field is observed when the volume fraction Z ? 0.3, the Schmidt number Sc < 1 andthedensityratio (densityoftheparticulatephase/densityofthefluidphase) > 1. 相似文献
46.
Lauren M. Garten Shyam Dwaraknath Julian Walker John S. Mangum Paul F. Ndione Yoonsang Park Daniel A. Beaton Venkatraman Gopalan Brian P. Gorman Laura T. Schelhas Michael F. Toney Susan Trolier‐McKinstry Kristin A. Persson David S. Ginley 《Advanced materials (Deerfield Beach, Fla.)》2018,30(25)
Many technologically critical materials are metastable under ambient conditions, yet the understanding of how to rationally design and guide the synthesis of these materials is limited. This work presents an integrated approach that targets a metastable lead‐free piezoelectric polymorph of SrHfO3. First‐principles calculations predict that the previous experimentally unrealized, metastable P4mm phase of SrHfO3 should exhibit a direct piezoelectric response (d33) of 36.9 pC N?1 (compared to d33 = 0 for the ground state). Combining computationally optimized substrate selection and synthesis conditions lead to the epitaxial stabilization of the polar P4mm phase of SrHfO3 on SrTiO3. The films are structurally consistent with the theory predictions. A ferroelectric‐induced large signal effective converse piezoelectric response of 5.2 pm V?1 for a 35 nm film is observed, indicating the ability to predict and target multifunctionality. This illustrates a coupled theory‐experimental approach to the discovery and realization of new multifunctional polymorphs. 相似文献
47.
MCM-41 materials were synthesized using alkyl(decosane, dodecyl)trimethyl ammonium bromide as structure directing surfactants. X-ray diffraction (XRD) analysis and nitrogen adsorption measurements reveal that the pores are hexagonal with tunable textural properties through the choice of surfactant and experimental condition. Poly(diphenylamine), PDPA was entrapped into the pores of MCM-41 by initial sorption of diphenylamine (DPA, monomer) in a medium (napthalein sulfonic acid) that provides self-assembling of DPA inside the pores and subsequent oxidative of polymerization with peroxydisulphate. Clear presence of an additional peak (around 9-10°) in XRD pattern for the DPA loaded MCM-41 provides evidence for self-assembled structure. Upon polymerization the self-assembly of DPA molecules resulted tubular PDPA inside the pores of MCM-41. PDPA thus formed shows different electronic property than the PDPA prepared by conventional method. XRD and FTIR spectroscopic analysis of PDPA loaded MCM-41 clearly informs that PDPA are entrapped in channels of MCM-41. 相似文献
48.
Yu‐Ping Zhang Se‐Hee Lee Kakarla Raghava Reddy Anantha Iyengar Gopalan Kwang‐Pill Lee 《应用聚合物科学杂志》2007,104(4):2743-2750
SiO2/Poly(3‐aminophenylboronic acid) (PAPBA) composites were synthesized under different experimental conditions, using ultrasonic irradiation method. Polymerization was carried out in the presence of sodium fluoride and D ‐fructose to anchor 3‐aminophenylboronic acid groups on to SiO2 surface. The SiO2/PAPBA nanocomposite prepared by NaF and D ‐fructose in the polymerization medium was found to show different morphology, electrical properties, thermal behavior and structural characterization in comparison to the nanocomposites prepared under other conditions. Ultrasonic irradiation minimizes the aggregation of nanosilica and promotes anchoring of PAPBA units over SiO2 surface. The morphology of PAPBA/ SiO2 nanocomposite was investigated by using transmission electron microscopy, UV‐visible spectroscopy; thermogravimetric analysis, Fourier transform infrared spectroscopy, and X‐ray diffraction analysis were used for characterization. Transmission electron microscope of the nanocomposites observation shows that SiO2/PAPBA composite, prepared with D ‐fructose and NaF under ultrasonication has a core–shell morphology. The thermal and crystalline properties of core‐shell SiO2/PAPBA nanocomposite was prepared via ultrasonication method is different from the SiO2/PAPBA nanocomposite prepared via conventional stirring method, in which SiO2 nanoparticles are submerged in PAPBA. Conductivity of the composite prepared via ultrasonication shows around 0.2 S/cm. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2743–2750, 2007 相似文献
49.
Vibrational Energy Transport in Hybrid Ordered/Disordered Nanocomposites: Hybridization and Avoided Crossings of Localized and Delocalized Modes 下载免费PDF全文
Taishan Zhu Krishnan Swaminathan‐Gopalan Kevin J. Cruse Kelly Stephani Elif Ertekin 《Advanced functional materials》2018,28(17)
Vibrational energy transport in disordered media is of fundamental importance to several fields spanning from sustainable energy to biomedicine to thermal management. This work investigates hybrid ordered/disordered nanocomposites that consist of crystalline membranes decorated by regularly patterned disordered regions formed by ion beam irradiation. The presence of the disordered regions results in reduced thermal conductivity, rendering these systems of interest for use as nanostructured thermoelectrics and thermal device components, yet their vibrational properties are not well understood. Here, the mechanism of vibrational transport and the reason underlying the observed reduction is established in detail. The hybrid systems are found to exhibit glass‐crystal duality in vibrational transport. Lattice dynamics reveals substantial hybridization between the localized and delocalized modes, which induces avoided crossings and harmonic broadening in the dispersion. Allen/Feldman theory shows that the hybridization and avoided crossings are the dominant drivers of the reduction. Anharmonic scattering is also enhanced in the patterned nanocomposites, further contributing to the reduction. The systems exhibit features reminiscent of both nanophononic materials and locally resonant nanophononic metamaterials, but operate in a manner distinct to both. These findings indicate that such “patterned disorder” can be a promising strategy to tailor vibrational transport through hybrid nanostructures. 相似文献
50.
The Impact of Sequential Fluorination of π‐Conjugated Polymers on Charge Generation in All‐Polymer Solar Cells 下载免费PDF全文
Kakaraparthi Kranthiraja Seonha Kim Changyeon Lee Kumarasamy Gunasekar Vijaya Gopalan Sree Bhoj Gautam Kenan Gundogdu Sung‐Ho Jin Bumjoon J. Kim 《Advanced functional materials》2017,27(29)
The performance of all‐polymer solar cells (all‐PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors ( P1 – P3 ) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all‐PSCs is investigated. Sequential fluorination of the polymer backbones increases the dipole moment difference between the ground and excited states (Δµge) from P1 (18.40 D) to P2 (25.11 D) and to P3 (28.47 D). The large Δµge of P3 leads to efficient exciton dissociation with greatly suppressed charge recombination in P3 ‐based all‐PSCs. Additionally, the fluorination lowers the highest occupied molecular orbital energy level of P3 and P2 , leading to higher open‐circuit voltage (VOC). The power conversion efficiency of the P3 ‐based all‐PSCs (6.42%) outperforms those of the P2 and P1 (5.00% and 2.65%)‐based devices. The reduced charge recombination and the enhanced polymer exciton lifetime in P3 ‐based all‐PSCs are confirmed by the measurements of light‐intensity dependent short‐circuit current density (JSC) and VOC, and time‐resolved photoluminescence. The results provide reciprocal understanding of the charge generation process associated with Δµge in all‐PSCs and suggest an effective strategy for designing π‐conjugated polymers for high performance all‐PSCs. 相似文献