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Material Measurement Techniques and Characterization

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Overview of material measurement techniques and characterization research.

Overview of Electromagnetic Material Characterization:

The faculty and students of the Electromagnetics Laboratory have many years of experience in the electromagnetic characterization of materials. It has become common to synthesize the electromagnetic properties of materials to control the behavior of circuits and devices at RF, VHF, UHF, microwave and mm-wave frequencies. Examples include enhancing the performance of microwave integrated circuits, modification of scattering from material bodies, shielding for EMC purposes and improvement of antenna bandwidth. The synthesis of such materials is largely empirical, which consequently leads to the requirement for experimental measurement of their electromagnetic properties.

The materials are typically assumed to be linear and isotropic and the frequency dependence of their complex permittivities and permeabilities are measured. Two different measurement schemes, either bounded or free-field, are commonly used depending upon the nature of the materials being characterized. If the materials are solid and can be shaped appropriately, samples are placed in a transmission system (coaxial line, stripline or waveguide) and the terminal scattering parameters of the field applicator are measured. The scattering parameters of the material sample are subsequently de-embedded from the measured terminal parameters. Non-rigid materials such as foams are usually available only in large sheets, and their characteristics would be altered by placing them in a field applicator. Such materials are characterized by free-field measurements in which the back and forward scattering of an incident wave by material sheets are measured using an appropriate antenna system; this technique has the advantage that oblique wave incidence can be accommodated. Again the scattering parameters of the material sample are de-embedded from antenna terminal measurements.

In either measurement scheme the scattering parameters are measured over various frequency bands using an automatic network analyzer. Analytical expressions relate the sample-region scattering parameters to the complex permittivity and permeability of the material under test. By equating the analytical results to the experimental measurements at each frequency the desired material parameters can be extracted numerically. 

The Electromagnetics Laboratory has been active in the design of appropriate field applicators. It has also developed elaborate schemes for de-embedding the material sample parameters from measurements of terminal scattering parameters as well as for the characterization of multi-layered materials. Calibration of the field-applicator or free-field measurement systems is an important consideration, and advances have been made in the development of calibration procedures.

History of Material Measurement Technique Development:

  • 1963 - 1980's -- K.M. Chen
    • Plasma diagnostics
    • Biological tissues: TEM cell
    • Superconductors: free space
    • Anisotropic composites: coaxial cavity, coaxial probe
  • 1990 - 2000's -- D.P. Nyquist
    • Magneto-electric materials: stripline, parallel plate, integrated optics guide
  • 1990 - Present -- Rothwell, Kempel, Balasubramaniam, Chahal
    • Magnetic (MagRAM) and dielectric materials: free space, waveguide, contact probe, THz
    • Metamaterials: free space, waveguide, antenna
    • Soils: coaxial waveguide
    • Biaxial and gyromagnetic materials: waveguide

Material Characterization Frequency Regimes:

  • 1 MHz - 1GHz
    • TEM Cell
    • Stripline
    • Material analyzer
  • 1 - 20 GHz
    • Stripline
    • Coplanar waveguide
    • Microwave Cavity
    • Free Space
    • Waveguide
    • Waveguide Probes
    • Coaxial Probe
    • Coaxial Waveguide
  • 20 - 40 GHz
    • Free Space
    • Wafer Probes
  • 50 - 110 GHz
    • Guided Wave
    • Free Space
    • Wafer Probes
  • 0.1 - 4 THz
    • Guided Wave
    • Free Space

Material Characterization Research: Material Types

  • Isotropic
    • Low-loss dielectrics
    • Circuit boards
    • Absorbers (Shielding)
    • MagRAM
    • R-cards
    • Superconductors
    • Biological Materials
    • Soils
  • Anisotropic
    • Ferrites
    • Plasmas
  • Artificial Materials
    • Honey Comb
    • FSSs
    • Layered dielectrics
    • RF Polymers
    • Metamaterials