Rock Porous Structure Characterization: A Critical Assessment of Various State-of-the-Art Techniques

Transport in Porous Media - The porous structure of geomaterials is of utmost importance for various industrial and natural processes. In this study, various conventional porous structure...     

Studies of Hindered Rotation and Magnetic Anisotropy by 1H, 13C and 19F NMR. The Diels-Alder Adduct of N- Pentafluorophenylmaleimide and Phencyclone: A Model for Drugs.

The potent Diels-Alder diene, phencyclone, 1, reacts with N-pentafluorophenylmaleimide, 2, to form an adduct, 3, characterized by ¹H, ¹³C, and ¹⁹F NMR at 300, 75 and 282 MHz, respectively. The one-dimensional (1D) and two-dimensional (2D) ¹H and ¹³C NMR spectra of 3 at ambient temperatures imply a slow exchange limit (SEL) regime with respect to rotation of the unsubstituted bridgehead phenyl groups about severely hindered C(sp²)-C(sp³) bonds. Major non-bonded interactions are expected between the ortho protons of the C₆H₅ groups and H-1, 8 of the phenanthrenoid moiety of 3. ¹⁹F 1D and 2D (COSY) NMR spectra show that the SEL regime also obtains for rotation about the N-C₆F₅ bond of 3, with five separate fluorine signals seen, consistent with a preferred conformation in which the C₆F₅ may lie roughly perpendicular to the plane of the pyrrolidinedione moiety, and may be in the mirror symmetry plane of 3. The results are considered relevant to hindered aryl rotations in numerous Pharmaceuticals. Selected spectral data for 2 and precursors are also presented.     

High temperature /spl pi//2-SQUID

A new DC-SQUID is proposed that exploits the properties of the grain boundary junctions in high T/sub c/ superconductors. The orientations of the grain boundaries are chosen in such a way to establish a /spl pi//2 (rather than 0 or /spl pi/) phase difference between the equilibrium phases of the two Josephson junctions in the SQUID loop. This property is exploited to increase the sensitivity and direction dependence of the SQUID for measuring magnetic flux without additional flux generating coils.     

Starting energy and current for a large diameter finite lengthbackward wave oscillator operated at the fundamental mode

We study the starting conditions for a large diameter (diameter/wavelength=4.8) finite length backward wave oscillator designed for 24-GHz operation at the fundamental TM₀₁ mode. This geometry is very promising for high power handling capability. We analyze two separate threshold conditions. First, finite length effects give rise to a threshold in electron beam energy below which oscillations cannot be sustained at any beam current. The second is the more familiar current threshold known as a start current. It is also found that the growth rate for the fundamental mode can be much larger than those of other higher order modes thus leading to coherent operation of large diameter sources free from mode competition     

High spectral resolution time-frequency distribution kernels

A new class of time-frequency distribution (TFD) kernels is introduced. Members in this class satisfy the desirable TFD properties and simultaneously provide local autocorrelation functions (LAF) that are amenable to high-frequency resolution modeling techniques. It is shown that members of the proposed class are product kernels, fast implementation multiplication-free kernels, recursive kernels, and optimum kernels with respect to autoterm localization     

Time-frequency distribution kernels using FIR filter designtechniques

In this correspondence, time-frequency distribution (TFD) kernels are obtained using finite impulse response (FIR) filter design methods, namely, the windowing method and the equiripple approximation method based on Chebyshev criterion. It is shown that the class of the window-designed kernels are simple to obtain and can handle most time-varying environments     

Power series solutions for the KPP equation

In this paper we solve the KPP equation by a non numerical method. To this end we find power series solutions where the coefficients are computed recursively. We also prove convergence of the series and illustrate the method by few examples.      

Time-frequency kernel design by the two-dimensional frequencytransformation method

A subclass of Cohen's class of time-frequency (TF) distributions is introduced in which the TF distribution kernels are provided via the frequency transformation method (FTM), used in two dimensional (2-D) filter design. The FTM kernels have finite extent in time and frequency and allow the TF distribution to be efficiently implemented in all four domains     

Order-recursive spectrum estimation

The use of a multiple-pole filter in the time-average estimation of the autocorrelation allows the power spectrum estimates to be recursive in the order of multiplicity of the filter pole. The recursive generation of the estimates from various filter orders provides the flexibility to select the estimator of interest in terms of the variance and spectral and temporal resolution