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A methodology is developed to determine the microwave backscatter from inhomogeneous materials and structures using a free‐space focused‐beam apparatus. This quantitative method determines the scattering coefficient for inhomogeneous surfaces and echo width for linear discontinuities. Measured data from a random rough surface are compared to compact radar cross‐section (RCS) range measurements, scatter data from a periodic array are compared to numerical simulations, and edge‐diffraction measurements are compared to model calculations. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 201–205, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20252

Impedance spectroscopy and modeling were used to investigate the partitioning of 0-40 V dc bias in composites containing alumina and different volume fractions of silicon carbide whiskers (SiCw), which formed low-connectivity percolated clusters. Differences in response between long (∼25 cm) composite rods and thin (∼1.7 mm) slices thereof were interpreted in terms of the relative contributions to the impedance from the electrodes and SiCw-percolated clusters of the composite samples. Bias had minimal effect on the impedance of rods, because its distribution across the long percolated clusters within translated to low electric fields at the SiCw-SiCw interfaces. The impedance of thin slices was more sensitive to bias and was mainly due to such interfaces. The associated dc resistance and effective capacitance decreased significantly with increasing dc bias. A model for symmetrical Schottky energy barriers at interfaces fit the capacitance trend and outputted a parameter Φi/κ¯i. Different models for the non-linear current-voltage behavior were related to each other and indicated weak varistor-like behavior, i.e., 1.15 ≤ αV ≤ 2.57. With increasing SiCw content and composite dc conductivity, Φi/κ¯i increased and the varistor non-linearity strength αV decreased. Also, the exponent t describing conductivity divergence at percolation was reduced at large dc bias. A new model of percolated clusters was proposed and correctly predicted the qualitative character and some quantitative aspects of these experimental results. The model is based on tendencies of the current distribution, which are expected from the topological structure and contrast between interface/whisker electrical behavior. Accordingly, it outputs the voltage-distribution tendencies.

In the second part of this general study, the carbon fiber–PEEK interfacial shear strength is measured by means of a fragmentation test on single‐fiber composites. Different thermal treatments (continuous cooling from the melt, isothermal treatments and long melting temperature time) are applied to these model composites prior to testing. The results are systematically compared with the previously determined reversible work of adhesion between carbon fiber and PEEK. It is shown that physical interactions at the interface determine, to a large extent, the magnitude of the interfacial shear strength between both materials. However, it appears that the magnitude of the stress transfer from the matrix to the fiber is affected either by the existence of an interfacial layer or by a preferential orientation of the polymer chains near the fiber surface. The results obtained on systems that have been subjected to isothermal treatments (isothermal crystallization of PEEK) seem to confirm the existence of a transcrystalline interphase, the properties of which are dependent upon the crystallization rate of the matrix and the interfacial adhesion energy.

Ceramic composite rods of alumina filled with varying volume fractions of silicon carbide whiskers (SiC w ) were made by extrusion and pressureless-sintering. The electrical response was measured over a wide range of frequency (1 mHz-1.8 GHz) and was compared to that of disks made by dry-pressing and pressureless-sintering the same starting powder blends. Extrusion resulted in increased whisker alignment and electrical conductivity along the processing direction compared to drypressing. Pressureless sintering resulted in anisotropic shrinkage and the rods having lower density (D), dc conductivity (r dc ), and SiC w -SiC w connection quality compared to fully dense hot-pressed disks. Porosity (P = 1 À D) increased with SiC w loading and was linearly related to the rod elastic modulus, which was determined via sonic resonance. SiC w percolation in the extruded rods required greater SiC w loading, took place over a wider compositional range, and resulted in significantly decreased r dc for the rods compared to hot-pressed disks. The r dc of percolated rods having composition near the percolation threshold was sensitive to small changes in porosity and was correlated to the elastic modulus. The weak percolation transition was correlated to a gradual evolution of the complex permittivity spectra. As SiC w content was increased, such spectra exhibited dc-conductivity tails in the loss which grew in magnitude, and dielectric relaxations which grew in strength, shifted to lower frequency, spread out over wider frequency ranges, and seemed to evolve toward power-law behavior for the dielectric constant.R. Cutler-contributing editor Manuscript No. 29560.