• Complex Permittivity Measurements
• Microwave Processing Furnace and Temperature Measurement System
• Extensometer

Complex Permittivity Measurements

Open-Ended Coaxial Probe

The absorbed microwave in a material is determined by the imaginary part of the complex permittivity and the electric field. The local electric field depends on the magnitude and spatial distribution of the complex permittivity within the material. Since there is a lack of knowledge in the complex permittivity of ceramics with respect to temperature and density, it is difficult to model the sample power absorption, temperature distribution and densification.

In order to measure the complex permittivity of materials within conventional and microwave furnaces, a high-temperature open-ended coaxial probe was designed, built, and modeled. The coaxial probe is made out of stainless steel with air as the dielectric material between the conductors. "The spring loading of the inner conductor insured that the probe maintained contact with the sample up to 1000^oC and eliminated errors due to differential thermal expansion of the probe. Comparison with an industry standard probe demonstrated that the spring loaded probe accurately and reproducibly measured the complex permittivity of several samples over a broad frequency range of 0.3 to 6 GHz at room temperature. At temperatures up to 1000^oC, dielectric measurements of a glass ceramic and of a porous alumina composite performed with both a spring loaded probe and a resonant cavity agreed to within 8% for the real part and 15% for the imaginary part of the complex permittivity. The probe's insensitivity in measuring low loss materials constrained accurate dielectric measurements to materials with loss tangent > 0.05 [7]," where loss tangent is the ratio of the imaginary part to the real part of the complex permittivity.

Complex permittivity measurements are performed by placing a flat side of a sufficiently thick sample onto the probe. Microwaves are transmitted through the coaxial probe and reflect off the interface between the sample and probe. The magnitude and phase of these reflected waves is a function of the complex permittivity of the sample. Numerical codes calculate the complex permittivity from the measured reflection coefficient.

The figure illustrates the experimental setup for conducting measurements up to 1000^oC. A conventional furnace heats the ceramic sample and the probe, which is cooled by a water jacket. The probe is housed inside a ceramic tube with a reducing atmosphere to prevent oxidization. A computer triggers the vector network analyzer (VNA) at specified temperatures and then calculates the complex permittivity. Since the probe thermally expands, it was necessary to adjust for the change in phase with respect to temperature.

Nondestructive Resonant Cavity

The complex permittivity of low loss tangent alumina composites was measured with a resonant cavity. When a material is placed inside a resonant cavity, the resonant modes of the cavity change depending upon the material's dimensions and complex permittivity.

Resonant cavity method is usually considered destructive because the sample has to be machined to a specific shape and size. However, our project goals require dielectric measurements of materials with variable dimensions. Our solution was to design, machine, and model a resonant cavity with a moveable wall. This cavity can adjust its height to match the sample height. An exact analytic solution can solve this configuration with a variable sample radius.

As shown, our resonant cavity wave was made out of OFHC copper with an inner diameter of 16.4 cm. The TM modes of the cavity are exited by two loop couplers, which are placed 90^o apart. These loop couplers then connect to VNA, which measures the transmitted signals. The moveable wall and sample plug, which enables loading the sample, have gold contact strips on their sides for making electrical contact with the cavity walls [22]

Microwave Processing Furnace and Temperature Measurement

Microwave processing is performed in the highly overmoded applicator (Model 101 from Microwave Materials Technology, Inc.) as shown in the figures below [13,22]. This modified microwave system operates at 2.45 GHz and/or 28 GHz.

Microwave processing system with computer control cart

2.45 GHz magnetron with a maximum power of 3 kW

28 GHz gyrotron from CPI with a maximum power of 10 KW A mode stirrer mixes the electromagnetic modes and attempts to produce a uniform field distribution within the applicator.

A sample, which is surrounded by alumina insulation, is loaded into the microwave applicator.

Pyrometer array attached to microwave furnace door.

Temperature measurements of the sample is performed by an array of single- and two-color pyrometers as shown above. The light from the sample surface shines down which then reflects off a gold mirror into a pyrometer. This noncontact technique can accurately measure the surface temperature from room temperature to 2500⁰C.

A new control system was designed and built for this microwave system [13]. The computer control cart houses an IBM compatible PC, three temperature controllers, and two thermocouple controllers. The PC interface cards control the magnetron power, gyrotron power, linear stage with pyrometer array, and output displays. Our microwave system can be controlled by a power, temperature and/or (eventually) densification profile [13].

Extensometer

Extensometer measuring the shrinkage of ceramic
sample in a conventional tube furnace.