High-temperature phase transitions in a quaternary lead based perovskite structured materials with negative temperature coefficient of resistance (NTCR) behavior

Impedance spectroscopy measurements were carried out on lead based, 0.25 (PbZr_0.52Ti_0.48O₃) + 0.25 (PbFe_0.50Ta_0.50O₃) + 0.25 (PbFe_0.67W_0.33O₃) + 0.25 (PbFe_0.50Nb_0.50O₃) (PZT–PFT–PFW–PFN) solid solution over a wide range of temperatures (400–650 K) and frequencies (100 Hz–1 MHz). Impedance data showed the presence of both grains and grain boundaries effects in the electrical transport properties of quaternary. The role of the grains and grain boundaries to the impedance become more prominent around the phase transition (~420 K). Two thermally activated processes were found from the temperature dependences of the relaxation time (τ). Activation energies calculated from relaxation times obtained from imaginary part of impedance were estimated ~1.21 and ~1.84 eV over 400–490 K and 490–650 K respectively. The sum of the activation energies for the grain and grain boundary resistances is basically of the same order of magnitude that is from the impedance at high temperatures. A constant phase element is used in the equivalent electrical circuits for fitting of experimental impedance data. The nature of variation of the grain and grain boundary resistance with temperature suggested negative temperature coefficient of resistance behavior.     

Calculation of dry out and post-dry out heat transfer in rod bundles using a three field model

The critical heat flux (CHF) condition is an important parameter in the design and operation of nuclear reactor fuel rod assemblies. It has traditionally been evaluated using look-up tables or empirical correlations. In the present paper, a set of relations, in the form of a one-dimensional three-field model, is presented which enables a mechanistic prediction of high-quality CHF known as dry out. This also allows a more fundamental treatment of post-dry out heat transfer. The ensemble of relations is validated by comparing the predictions with data obtained from the TPTF test series of JAERI and the THTF test series of ORNL. The rod bundles have been modelled on a sub-channel basis. The effect of spacer grids on the flow and heat transfer has been studied by including their geometrical characteristics only. Good predictions of the dry out location and the post-dry out variation of the rod temperature are obtained over the range of parameters investigated in these tests, namely, in the pressure range of 30-130 bar, mass flux range of 50-800 kg/m² s and inlet condition ranging from sub-cooled to an inlet quality of 0.89.     

Augmented infinitesimal perturbation analysis: An alternate explanation

Augmented infinitesimal perturbation analysis (APA) was introduced by Gaivoronski [1991] to increase the purview of the theory of Infinitesimal Perturbation Analysis (IPA). In reference [Gaivoronski 1991] it is shown that an unbiased estimate for the gradient of a class of performance measures of DEDS represented bygeneralized semi-Markov processes (GSMPs) (cf. [Glynn 1989] can be expressed as a sum of an IPA-estimate and a term that takes into account the event order changes. In this paper we present an alternate approach to establishing the result of Gaivoronski, and from this we derive a necessary and sufficient condition for the validity of the IPA algorithm for this class of performance measures. Finally we validate our results by simulation examples.This research was supported by the National Science Foundation under grant number ECS-85-15449, Office of Naval Research Grants Nos. N00014-90-K-1093 and N00014-89-J-1023 and by Army Grant No. DAAL-03-86-K-0171.     

The relevance of short communication in scholarly journals: An empirical study

This study aims to examine whether rapid communications exert more influence/impact on subsequent research. Citation analysis of Short Communications (SCs) and Main Articles (MAs) from 1983 and 1990 for 5 high impact biomedical journals was carried out for a five year period following publication. The mean citations cumulated for the five year period showed no consistent trend. Some journals showed more citations for SCs while some showed more for MAs. The mean citations (range) for SCs and MAs for the 1983 and 1990 papers respectively were as follows:Gene: 14.13 (0-61) and 38.79 (0-677), 9.73 (0-93) and 13.17 (0-44);Journal of Clinical Investigation (JCI): 79.77 (3-202) and 27.52 (0-86), 50.52 (0-254) and 33.53 (0-151);Journal of Experimental Medicine (JEM): 39.80 (0-200) and 49.20 (0-403), 47.26 (0-258) and 50.27 (0-173); andJournal of Biological Chemistry (JBC); 36.21 (0-380) and 19.67 (0-53), 37.19 (0-273) and 26.84 (0-185). SCs ofJournal of Cell Biology (JCB) had a mean citation of 25.84 per article with a range of 0-98, while the MAs had a mean citation of 33.13 with the range 4-122 during 1983-87. The citation peak was seen about three years after publication for all the journals during both the periods. The mean cumulative citations showed a progressive increase over the five years for both types of papers, in all journals and for both the 5 year periods. The initial differences observed persisted even four years after the year of publication. No significant differences were observed in the distribution of the cumulative 5 year citations between the SCs and MAs. An index of speed of citation per article showed no substantial differences between SCs and MAs with MAs showing an edge over SCs. Both MAs and SCs of all the journals showed nearly same average time per citation per article further confirming that the SCs do not enjoy the advantage of speedier citation. The results show that the generally perceived feeling of SCs getting cited more frequently and faster does not appear to be valid. Hence, the practice of publishing SCs on a priority basis is perhaps not warranted.     

Ordinal optimization of DEDS

In this paper we argue thatordinal rather thancardinal optimization, i.e., concentrating on finding good, better, or best designs rather than on estimating accurately the performance value of these designs, offers a new, efficient, and complementary approach to the performance optimization of systems. Some experimental and analytical evidence is offered to substantiate this claim. The main purpose of the paper is to call attention to a novel and promising approach to system optimization.This work is supported by NSF grants CDR-88-03012, DDM-89-14277, ONR contracts N00014-90-J-1093, N00014-89-J-1023, and army contracts DAAL-03-83-K-0171, DAAL-91-G-0194.     

A general method for resonance power supply analysis

The resonance power supply is widely used and proved to be an efficient method to supply accelerator magnets. The literature describes several power supply circuits, but no comprehensive theory of operation is presented. The authors present a mathematical method which describes the operation of the resonance power supply and its use for the accurate design of components     

Zero-crossing based spectral analysis and SVD spectral analysis forformant frequency estimation in noise

The authors discuss a method for spectral analysis of noise corrupted signals using statistical properties of the zero-crossing intervals. It is shown that an initial stage of filter-bank analysis is effective for achieving noise robustness. The technique is compared with currently popular spectral analysis techniques based on singular value decomposition and is found to provide generally better resolution and lower variance at low signal to noise ratios (SNRs). These techniques, along with three established methods and three variations of these method, are further evaluated for their effectiveness for formant frequency estimation of noise corrupted speech. The theoretical results predict and experimental results confirm that the zero-crossing method performs well for estimating low frequencies and hence for first formant frequency estimation in speech at high noise levels (~0 dB SNR). Otherwise, J.A. Cadzow's high performance method (1983) is found to be a close alternative for reliable spectral estimation. As expected the overall performance of all techniques is found to degrade for speech data. The standard autocorrelation-LPC method is found best for clean speech and all methods deteriorate roughly equally in noise     

Micro-kinetic modeling of NH3 decomposition on Ni and its application to solid oxide fuel cells

This paper presents a detailed surface reaction mechanism for the decomposition of NH₃ to H₂ and N₂ on a Ni surface. The mechanism is validated for temperatures ranging from 700 to 1500 K and pressures from 5.3 Pa to 100 kPa. The activation energies for various elementary steps are calculated using the unity bond index-quadratic exponential potential (UBI-QEP) method. Sensitivity analysis is carried out to study the influence of various kinetic parameters on reaction rates. The NH₃ decomposition mechanism is used to simulate SOFC button cell operating on NH₃ fuel.