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The broadband (10 kHz–18 GHz) complex dielectric permittivity of electrically lossy aluminum silicate–pyrolytic carbon composites is studied as a function of carbon content. The composites are made from a commercial machinable ceramic that is treated by oxalic acid refluxing, followed by sucrose pyrolysis within the open porosity. The resulting composites have a slightly percolating conductor configuration that exhibits non-Debye dielectric properties. Such composites are useful as microwave absorbers in vacuum electronic amplifiers, for suppressing spurious oscillations, and for bandwidth control. Four contributions to the dielectric response were observed and analyzed. These consist of a direct current conductivity contribution that is significant only at frequencies below 1 MHz, an irregular low-frequency relaxation, a strong non-Debye, high-frequency relaxation with its upper portions extending into the microwave regime, and an underlying, broadband loss consistent with a universal dielectric response. The microwave behavior is dominated by changes in the position, intensity, and Cole–Cole exponent of the high-frequency relaxation as the carbon content increases.

Reliable and accurate temperature measurement during microwave processing of ceramic bodies is controversial. Although thermocouples are routinely used in conventional thermal furnaces, their presence in microwave furnaces can locally distort the electromagnetic field, conduct heat away from the sample, induce thermal instabilities and microwave breakdown, and lead to serious measurement errors. These thermocouple effects have been studied and found to be more pronounced in low- and medium-loss ceramic materials. To decrease the thermocouple effects during the processing of advanced ceramic materials, an optical, noncontact temperature sensing system has been developed, calibrated, and incorporated into a computer-controlled microwave furnace.

The frequency-dependent microwave complex permittivity of composites based on an Al2O3–CuO system is investigated at room temperature. The composites are formed by solution infusion of copper precursors into a porous Al2O3 matrix, followed by thermal decomposition to copper oxides and localized formation of copper aluminate compounds, and finally, H2 firing. The material exhibits a strong amplitude, with relatively narrowband dielectric resonance in the microwave frequency regime at intermediate levels of mass gain, which is reminiscent of a plasmon resonance. Large mass gains cause negative permittivity behavior below 6 GHz, consistent with a collisional Drude model of a semiconducting oxide phase.