Harmonics and the fundamental-frequency negative conductance of Gunn diodes

Parametric interactions of X-band Gunn diodes between the frequencies 10 and 20 GHz are studies with the help of a coaxial resonator. A diode is placed at various positions along the central conductor so that different load impedances are applied. Very strong variations in microwave amplitude behaviour of the two frequencies were observed, which indicate pronounced interactive effects. These are studied by measuring the negative-conductance as a function of r.f. diode voltage. So far, only a decrease in fundamental-frequency conductance function has been observed due to the presence of harmonic power.     

SPM levels of an urban area (Patiala City) and its relation to automobile exhaust

Concentration levels of suspended particulate matter (SPM), organic tarry matter (BSPM), ignitable matter (LOI) and lead in the ambient air of an urban area are measured by using a high volume sampler. SPM levels in different zones of the city are reported. Organic tarry matter measured as benzene soluble particulate matter (BSPM) constitutes about 4–9% of the SPM. BSPM in the ambient air varies in a range of 10–40 g m^–3. Ignitable matter measured as loss on ignition (LOI) forms about 24–40% of the SPM. Absolute values of LOI vary between 50 and 150 g m^–3. Concentrations of BSPM, LOI and Pb are correlated to automobile exhaust as the probable source.The studies indicate that BSPM and LOI are integral parts of the SPM but their concentrations are independent of the SPM levels.     

Construction and preliminary evaluation of an Aspergillus flavus reporter gene construct as a potential tool for screening aflatoxin resistance

Effective preharvest strategies to eliminate aflatoxin accumulation in crops are not presently available. The molecular biology of aflatoxin biosynthesis has been extensively studied, and genetic and molecular tools such as reporter gene systems for the measurement of fungal growth have been developed. A reporter construct containing the Aspergillus flavus beta-tubulin gene promoter fused to Escherichia coli beta-glucuronidase (GUS) has been shown to be a reliable tool for the indirect measurement of fungal growth in maize kernels. Since cost-saving alternative methods for the direct measurement of aflatoxin levels are needed to facilitate more widespread field and laboratory screening of maize lines, a new reporter gene construct involving the promoter region of the omtA gene of the aflatoxin biosynthetic pathway was constructed and tested. Expression of GUS activity by this construct (omtA::GUS) was correlated with aflatoxin accumulation in culture. In the fungal transformant GAP26-1, which harbors this construct, aflatoxin production and GUS expression on sucrose-containing medium showed the same temporal pattern of toxin induction. Furthermore, GUS expression by GAP26-1 was shown to be associated with aflatoxin accumulation in maize kernels inoculated with this strain. Our results suggest that this and other reporter gene pathway promoter constructs may provide superior alternatives to direct aflatoxin quantification with respect to time, labor, and materials for the screening of maize lines for resistance to aflatoxin accumulation.     

Effect of silver dissolution on electrical properties of As2S3

The paper reports the effect of dissolution of silver into the chalcogenide As₂S₃. The conductivity of As₂S₃ decreases with doping at lower temperatures. The a.c. conductivity is proportional to ^s for undoped as well as doped samples. The value of the frequency exponent s increases with doping. On the other hand, the dielectric constant of As₂S₃ also increases with doping and the effect is quite prominent at higher temperatures. A new structural model together with the correlated barrier hopping model has been used to explain these results. © 2001 Kluwer Academic Publishers     

Conductivity studies of lithium zinc silicate glasses with varying lithium contents

The electrical conductivity of lithium zinc silicate (LZS) glasses with composition, (SiO₂)_0.527 (Na₂O)_0.054(B₂O₃)_0.05(P₂O₅)_0.029(ZnO)_0.34−x (Li₂O) _x (x = 0.05, 0.08, 0.11, 0.18, 0.21, 0.24 and 0.27), was studied as a function of frequency in the range 100 Hz–15 MHz, over a temperature range from 546–637 K. The a.c. conductivity is found to obey Jonscher’s relation. The d.c. conductivity (σ _d.c.) and the hopping frequency (ω _h), inferred from the a.c. conductivity data, exhibit Arrhenius-type behaviour with temperature. The electrical modulus spectra show a single peak, indicating a single electrical relaxation time, τ, which also exhibits Arrhenius-type behaviour. Values of activation energy derived from σ _d.c., ω _h and τ are almost equal within the experimental error. It is seen that σ _d.c. and ω _h increase systematically with Li₂O content up to 21 mol% and then decrease for higher Li₂O content, indicating a mixed alkali effect caused by mobile Li⁺ and Na⁺ ions. The scaling behaviour of the modulus suggests that the relaxation process is independent of temperature but depends upon Li⁺ concentration.     

Epitaxial Growth of High-J c NdBa2Cu3O7−δ Films on RABiTS by Pulsed Laser Deposition

NdBa₂Cu₃O_7−δ (NdBCO) films were grown on rolling-assisted biaxially textured substrates (RABiTS) via pulsed laser deposition. c-Axis-oriented epitaxial NdBCO films with high performance were obtained under optimal deposition conditions. Transmission electron microscopy analysis shows that the NdBCO film grown on RABiTS has a clear interface with a CeO₂ cap layer and a nearly perfect lattice structure. The NdBCO film exhibits higher T _c of 93.7 K and better in-field J _c in magnetic fields and at all field orientations, compared to pure YBCO films.     

Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors

The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene molecule [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), have been investigated and compared. The two copolymers have the same aromatic backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irrespective of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphology of blend layers. The effect of bipolar transport in PCBM will also be discussed.     

A novel methodology to measure the responsiveness of RMTs in reconfigurable manufacturing system

Reconfigurable manufacturing systems are designed to deliver exact functionality and capacity that is needed, when it is needed. The reconfigurable machine tool (RMT) plays a pivotal role in the accomplishment of this objective through their built in modular structure consisting of basic and auxiliary modules along with the open architecture software.     

A vendor managed inventory model under contractual storage agreement

Vendor managed inventory (VMI) is a supply chain partnership strategy that allows a supplier to place orders on behalf of its customers. This paper considers a supply chain composed of a single vendor and multiple retailers operating under a VMI contract that specifies limits on retailers' stock levels. We address the problem of synchronizing the vendor's cycle time with the buyers' unequal ordering cycles by developing a mixed integer non-linear program that minimizes the joint relevant inventory costs under storage restrictions. We also propose a cost efficient heuristic to solve the developed optimization problem. We conducted computational experiments to assess the reduction in the total supply chain costs resulting from relaxing the restriction of equal ordering cycles. It is found that the heuristic generates greater cost savings in cases of increased variability in retailers' demand and cost parameters.