Isotropic Matrices Research Articles

Triplet spin alignment of “stretched porphycenes”

The photoexcited triplet states of 22π and 26π acetylene—cumulene porphycenes (“stretched porphycenes”) were investigated by time-resolved continuous wave electron paramagnetic resonance spectroscopy. Experiments were carried out in isotropic (toluene) and anisotropic (a nematic liquid crystal) matrices. These porphyrinoid compounds exhibit a red-shift in the absorption spectra as a function of molecular length and compared to the 18π porphycene. On the other hand, the zero-field splitting parameter, D, does not exhibit a dependence on the molecular length. This anomaly is interpreted in terms of the sign and magnitude of D, resulting from triplet spin alignment in the molecular plane.

Effective Thermal Conductivity of Uniaxial Composite with Cylindrically Orthotropic Carbon Fibers and Interfacial Thermal Barrier

The theories of Hasselman et al. and Benveniste et al. were extended to derive expressions for the longitudinal and transverse effective thermal conductivity of an isotropic matrix uniaxially reinforced with dilute concentrations of cylindrically orthotro pic fibers for finite interfacial thermal conductance values. The transverse thermal conduc tivity of the composite was found to be a function of both the radial and tangential compo nents of the transverse thermal conductivity of the fibers. Comparison with the effective thermal conductivity of an isotropic matrix reinforced with isotropic fibers indicates the effective thermal conductivity of cylindrically orthotropic fibers equals the geometric mean of the radial and tangential transverse thermal conductivity. As for isotropic matrices reinforced with isotropic fibers, a finite value for the interfacial thermal conductance in composites reinforced with cylindrically orthotropic fibers leads to a lowering of the com posite transverse thermal conductivity and also introduces a dependence on fiber diameter. Based on experimental data for the transverse thermal conductivity of a uniaxial carbon fiber-reinforced aluminoborosilicate glass matrix composite in nitrogen at atmospheric pressure and in vacuum, estimates of the interfacial thermal conductances indicate that in nitrogen at atmospheric pressure interfacial gaseous heat transfer contributes about one- third of the total interfacial thermal conductance. The theory should be applicable to other isotropic matrices reinforced with cylindrically orthotropic fibers.

Dynamics of triplet fullerene-60 probed by electron paramagnetic resonance. Motional analysis in isotropic and liquid crystalline matrixes

Detailed analysis of time-resolved electron paramagnetic resonance (EPR) line shapes of photocited triplet states of C[sub 60] in isotropic and anisotropic matrices has been carried out. Two main motional models have been tested over a very wide temperature range. For most temperatures, the best fit to the experimental line shapes was obtained with a model of discrete jumps between two configurations, which can be formally defined as an exchange between the longitudinal (Z) dipolar axis and one of the two transverse axes (X,Y). This exchange process is probably related to two distorted Jahn-Teller (JT) configurations of the molecule. For very high temperatures, the best fit was obtained with a recently developed model of Brownian rotational diffusion. In the latter cases, relating to the liquid phase of the solvent, the triplets appear to be in thermal spin equilibrium, and their kinetics can be described by a special form of transient nutations. In the former cases, related to the glass and amorphous phases (toluene) or nematic phase (E-7), the triplets are clearly spin polarized. For very low temperatures the motional (jump) rate is practically temperature independent. This seems to be due to the nature of the motion, i.e., exchange between JT configurations,more » rather than actual molecular motion. 45 refs., 9 figs., 2 tabs.« less

Numerical prediction of the mechanical properties of anisotropic composite materials

A numerical method is presented to predict the mechanical properties of composite materials with anisotropic constituents and used to predict the properties of a Fiber-Reinforced Composite (FRC). The FRC is treated as an anistotropic but homogeneous continuum and the elastic constants are determined using an Energy Equivalence method. The strain energy (or complementary energy) of the composite, calculated analytically is equated with the summation of the strain energies (or complementary energies) of the components (i.e. fiber and matrix) calculated by finite element analysis. Comparison is made with previous techniques to determine the longitudinal, transverse moduli and Poisson's ratios for isotropic and transversely isotropic fibers in isotropic matrices. The method is also applicable to any nonlinear elastic stress-strain relationship.

Theory of averaging of elastic fields in media with a monoclinic structure

The proposed relations of averaging theory, together with complex Kolosov-Muskhelishvili potentials for isotropic matrices and Lekhnitskii potentials for rectilinearly anisotropic matrices with prismatic fillers, constitute a closed system of equations in the problem of determining the internal fields and the complete set of effective elastic constants of composite media with uniform external static stresses.

On the Interface Determined Growth of Precipitates in Finite Isotropic Matrices

On the Interface Determined Growth of Precipitates in Finite Isotropic Matrices

On the Interface Determined Growth of Precipitates in Finite Isotropic Matrices

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Young's Moduli of Compact Matrices for HTGR Fuel

Young's moduli of matrices of compacted graphite samples were determined from measured values of ultrasonic-wave propagation through the sample at room temperature. The results were analyzed with particular interest directed to the effects of differences in matrix density and graphite orientation on the propagation velocity, from which the modulus was calculated. The characteristics of the matrices were observed by X-ray diffractometry on the crystallite sizes, and the shapes of the graphite powders constituting the main substance of the compact matrices were scrutinized by scanning electron microscope. The value of Young's modulus was found to increase with binder content in the matrices constituted uniquely of one kind of graphite material, while in the case of composite matrices embodying a mixture of petroleum coke graphite and natural graphite, the modulus decreased with increasing natural graphite content. The velocity of ultrasonic-wave propagation—from which the value of Young's modulus is derived—depends on the density alone in isotropic matrices prepared from milled isotropic coke graphite, whereas in anisotropic matrices embodying needle coke graphite it depends on graphite orientation as well as on density. The contributions of density (p) and of reflective intensity ratio (IR) to the propagation velocity (V) are expressed ΔV=a (Δo) n and ΔV=b (ΔIR), where the symbol Δ indicates increment, while a, b and n are constants depending upon the characteristics of the compact matrix.

Random Walk and Random Roughness Models of Drainage Networks

A random walk model of a drainage network is generated on an underlying matrix by selecting at random the drainage direction out of the elementary areas. In a random roughness model, roughness heights of the elementary areas rather than drainage directions are assigned at random. The resulting topography of that model then uniquely determines the drainage pattern. An equivalence of the two models is suggested by the similarities, found in the mode of construction and by the equality of the probabilities of occurrence of some simple configurations. The random roughness model can yield values of primary probabilities, i.e., probabilities which can be used in the construction of a random walk model. The various types of drainage patterns are classified by the sets of primary probabilities which would generate them. Thus outcomes of homogeneous and isotropic matrices are classified as pure dendritic patterns. Outcomes of homogeneous matrices with probability sets derived from a sloping plane roughness model are classified as general dendritic (including parallel) patterns. Trellis, annular, and other patterns are considered as subgroups of the class of patterns generated on all other possible matrices.