Dielectric Property Measurement Over a Wide Range of Temperatures Edward Ripley, Brian Warren, Kevin Williams Y-12 National Security Complex Technology Development Division presented by Edward Ripley Global World Congress Lake Biwa Japan August 5-8, 2008

Tokyo Zoo 1966 I used to live in Yokahama Japan from 1963 to This is my first trip to Japan in 41 years. Me and my brother at the Tokyo Zoo in 1966

Overview Definitions Key Concepts Measurement Methods Industry Standard Probe Y-12 High Temperature Probe Basic setup and equipment Typical Experimental Results Questions and answers

Electrical permittivity (ε), also known as the dielectric constant of a material, describes how a material interacts with an electric field The dielectric constant is not really constant over frequency or temperature Relative permittivity, εr, is the permittivity relative to the permittivity of free (empty) space Relative permittivity is a complex property that comprises both a real and imaginary part. Real part, ε’, is a measure of how much energy from an electric field can be stored in a material. Imaginary part, ε”, is called the loss factor, and is a measure of the electric loss of a material Definitions

Material Interactions When a material interacts with an electric field, one of the three following scenarios occurs: –Storage: A bidirectional exchange of energy between the material and the field that can be classified as either a reflection or transparency of the field by the material. This is represented by ε’, (the real portion of the electrical permittivity) –Absorption: Field energy permanently lost from the field and absorbed by the material in the form of heat. This is represented by ε”, (the imaginary portion of the electrical permittivity) –Loss: Combination of what can be stored into a material and what is absorbed and converted into heat.

Loss Tangent An additional term widely used in electromagnetic analysis is the loss tangent of the material tan δ. The loss tangent is the vector angle δ formed with the horizontal axis, εr’, when the complex permittivity is represented in vector format. This resulting vector, tan δ, is the measure of the lossiness of the material derived from the ratio of the energy lost to energy stored.

Dielectric Constant Vs Frequency Dielectric Constant versus Frequency for an Example Material

Dielectric Property Measurement Methods Capacitive Modeling Method Cavity Perturbation Method Printed Circuit Resonators Method Coaxial Probe Method Free Space Measurement Method Y-12 High Temperature Probe

Capacitive Modeling Method This simple method of dielectric measurement used only in low frequencies utilizes a parallel plate capacitor, with the material under test (MUT) sandwiched in between the plates. The value of the capacitance and quality factor can be acquired with the assistance of a LCR meter. Although the capacitive model is relatively simple to implement, it is not suitable for the high frequency (2.45 GHz) or high temperatures (~1000 °C) Agilent Capacitive Dielectric Measuring Device

Capacitive Modeling Method cont. Once the capacitance is measured, it is translated to dielectric constant via the following equation: Where: εr = Dielectric constant of material C = Capacitance in picoFarads D = Distance between plates in inches A = Area of plates in square inches

Cavity Perturbation Method The cavity perturbation method, also known as the resonant cavity method, involves loading a sample into a waveguide cavity, then perturbing the cavity structure high Q and resonant frequency with the sample under test. Dielectric properties can then be acquired from the transmission coefficient response of the cavity. The true highlight of this system is the extremely high accuracy in measurement of both the real and imaginary parts of the dielectric constant. While this method is applicable to both high and low loss materials, it is generally used for very high accurate measurements of low loss materials. Extremely difficult to use at high temperatures.

Cavity Perturbation Method cont. Illustration of a Cavity Resonator Measurement System

Printed Circuit Resonators Method The printed circuit resonator method utilizes a metallized substrate, and a ring resonator printed on the top surface of the substrate The bottom surface is blank (uncoated) Dielectric Ring Resonator

Printed Circuit Resonators Method cont. This fixture utilizes the measured unloaded Q to evaluate the surface resistance (conductivity) This test is fine to perform nondestructive screenings of the various thick-films, metallizations (inks) This test is considered destructive for larger samples of a ceramic dielectric. This test is not adequate for high temperature measurements.

Coaxial Probe Method This method, is optimized for liquids and semi-solids It can also be used for solids and powders if materials are sufficiently machined or are finely ground The open-ended coaxial probe consists of a truncated coaxial line connected to a vector network analyzer through a coaxial cable. An illustration of a commercial probe from Agilent Technologies

Coaxial Probe Method cont. The material under test (MUT) is placed in contact with the probe face intimate contact is maintained at all times during the measurement cycle Any air-gap will result in the averaging of εr’ = 1 into the results Air-gaps of a fraction of a millimeter can have strong influences on the measured input impedance A software package calculates the complex permittivity from the measured complex reflection coefficient Commercially available probes are not suited for high temperatures

Coaxial Probe Method applicability (HP) Frequency Range200 MHz to 20 GHz Temperature RangeRange: -40 °C to +200 °C Rate: < 10 °C per minute Sample Size Diameter: > 20 mm Thickness: > 20/√|εr*| mm Granule Size: < 0.3 mm Maximum Recommended εr’: < 100 Minimum Recommended tan δ: > 0.05 Sample Flatness100 micro-inches, typical, over-lapped surface

Free Space Measurement Method This method is comprised of a vector network analyzer and two antennae facing each other The dielectric sample in between Free space measurements work very well for sheet materials, powders, or liquids. This method also is a non-contacting test that lends itself well to high temperature measurements since all test equipment is clear of the hot materials The system requires a uniform wave front for the propagating wave and therefore is not well suited for lower frequency measurements The required transmit/receive horn antennae would be extremely large and the sample needing to be even larger to maintain a uniform wave front Therefore, this method will ultimately be rejected for this application

Free Space Measurement Method cont. Basic Free Space Measurement System

Y-12 Probe Design The y-12 high temperature coaxial probe was designed based on the fundamental equation The probe response is stable over a wide range of temperatures The measurements are nearly identical to the response of the HP up to 200C The additional temperature range allows us to observe and plot high temperature property data Comparing plots of all the material properties over the range of temperatures allows you to avoid undesired heating effects This probe allows us to characterize materials which are reflective at room temperature and absorb at elevated temperatures. Allows us to predict thermal runaway

Design and Simulation This probe design was simulated in the software package High Frequency Structure Simulator (HFSS) to determine what effect cross-section changes would have on the input return loss Results indicated insignificant return loss degradation at 2.45 HFSS will also account for all of the fringing fields using full 3D EM analysis. HFSS Dielectric Probe Model

Comparison of HP and Y-12 probe

Material Properties at Elevated Temperatures Agilent 85070A HP Probe Suitable for Isotropic Materials Low Loss Max. temp 200C Teflon loaded lines High Temp Probe Design Simulated using HFSS coaxial probes/no dielectric loading Larger Measuring Area to minimize Fringing High Temperature to 1000 C

Mixed Ceramic using Y-12 Probe

Hp Probe Vs Y-12 Probe (mixed Ceramic)

Y-12 and HP probes for Mixed Ceramic Y-12 probe above HP probe left

Y-12 and HP probes for Mixed Ceramic Y-12 probe above HP probe left

Explanation of Differences The Hp Probe is constructed of materials which have Coefficients of Thermal Expansion which are different The temperature (up to 200C) causes a shift in the a/b ratio and affects the probes accuracy at different temperatures in an non-linear fashion The Y-12 probe uses temperature invariant materials and the a/b ratio remains constant The material is a mixed transparent and absorbing ceramic, fringing effects are minimized in the Y-12 probe, however cannot be eliminated completely The Y-12 probe is able to show the inflection point where the dielectric heating would plateau and drop off This can be experimentally verified

Conclusions For high temperature measurement of dielectric properties, traditional probes cannot reach the ideal processing temperatures When dielectric properties vary as a function of temperature, the processes need to be adjusted accordingly The Y-12 probe is the most simple, direct and accurate way to measure the dielectric properties at the temperatures of interest The measurements made by the Y-12 probe can be confirmed by experiments The absorption (heating) curves line up and the inflection points line up with experimental data

Questions?