Kavitha Arunachalam

The feasibility of non-contact and non-destructive evaluation (NDE) of planar aerospace dielectric composites using microwave is examined in this paper. Free space microwave measurement set up includes spot focusing horns to gather the scattering parameters of the composites with the help of (Gated-thru reflect line) G-TRL calibration. The dielectric properties are studied from the measured scattering parameters to characterize the material's defective and non-defective region. Particulate reinforced composite with an air gap of 0.1 mm thickness and 10 mm width is used for validation of the technique.

A hybrid microwave NDE technique employing near field sensor for electric field measurement in plane wave regime is proposed in this paper. A custom made spot focusing horn antenna has been used to illuminate the sample with defect. A graphene based electric field sensor is employed to measure the electric field with less field perturbation. Thin dielectric composites of thickness less than quarter wavelength has been inspected using this method. Samples of size 300 mm × 300 mm with machined defect radius of 10 mm and 5 mm with 0.5 mm depth were measured using the proposed hybrid method. The hybrid technique has a higher spatial resolution and insignificant perturbation to the electric field compared to the conventional methods.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

Micro-scale removal of Cu from a dielectric substrate has applications in microelectronics, patch antenna fabrication and frequency selective surface (FSS) manufacturing. Pulsed laser-based micro-scribing of Copper (Cu) from a dielectric is a preferred technique to avoid the adverse effects of chemical etching, such as toxicity and corrosive nature of the etchant, difficulty in fabrication of mask etc. However, pulsed laser-assisted removal of Cu from a dielectric in the air will produce recast layer/ redeposit, oxide layer near the ablation zone and thermal damage to the dielectric is another challenge. In this study, a hybrid technique with nanosecond laser-activated electrochemical micro-scribing of Cu is demonstrated. The technique was extended to remove 35 μm Cu from Rogers-RO4003 dielectric with a thickness ≈0.75 mm to fabricate FSS samples in X-band. The Cu-deposited dielectric substrate was immersed in Sodium Chloride (NaCl) solution, the laser beam was directed through a negatively biased tool electrode and the sample was biased positively. In this hybrid technique, along with laser-assisted material removal, laser-activated electrochemical etching also removed Cu selectively. The laser irradiation coupled with the NaCl solution induced preferential micro-etching, resulting in improved surface morphology without re-deposition and recast layer and thermal protection to the dielectric substrate. The FSS sample produced with the laser-hybrid micro-scribing was working at 10.3 GHz.

A non-contact microwave sensor is presented for in situ process monitoring of nuclear waste glass melts inside a cold crucible induction melting (CCIM) furnace. The level and thermal steady state of the molten glass inside a 1400 °C CCIM furnace were measured during 6-hour long vitrification process using a corrugated horn antenna operating over 20–24 GHz designed for high temperature measurement. The non-contact in situ microwave measurements indicate the ability to measure absolute level, identify thermal steady state of the glass melt, and remotely monitor the vitrification process for safe immobilization of the liquid radioactive waste.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

This article presents a compact precision free-space microwave measurement setup with a choice of three dielectric lenses to tailor the antenna focal plane characteristics for extracting complex dielectric permittivity of small samples. Custom designed spot-focusing horn antenna pairs were used to achieve a compact setup with antenna separation distance, 2f l : 4λ c-8λ c and focal spot size, fs : 1λ c-1.5λ c , where λ c is the wavelength at center frequency. Using the compact free-space setup, relative complex permittivity (j) was extracted over 8-12 GHz for low-and high-loss dielectrics with lateral dimensions, 3.3 λ c× 3.3λ c and 10 λ c × 10 λ c. For large materials under test (MUTs), i.e., 10 λ c × 10 λ c , measurement accuracy in dielectric constant, Δ% was <0.65% and ≤1.14% for low-and high-loss dielectrics, respectively. For smaller MUTs ( 3.3 λ c × 3.3 λ c ), Δ % was <0.89% and ≤2.29% for low-and high-loss MUTs, respectively. The error in loss tangent ( Δ) varied over 0.002-0.016 and 0.015-0.056 for large ( 10 λ c × 10 λ c ) and small MUTs ( 3.3 λ c × 3.3 λ c ), respectively. For large MUTs, biconvex lens pair with the smallest f-s ( 1 λ c ) and f-l ( 4 λ c ) among the three lenses yielded the best accuracy in dielectric constant due to tight field focusing at the focal plane. The plano-convex lens pair yielded the best accuracy in loss tangent for large MUTs due to slow variation in the phase of the local plane wave. By tailoring antenna focal plane characteristics, a compact free-space setup that is 6×-10× smaller than the classical setup for handling MUTs that are 1/5th of the size used in classical setup is demonstrated without compromising the measurement accuracy.

A specially designed air-cooled compact antenna is proposed for non-contact real-time radio detection and ranging (RADAR)-level measurement in furnace. 3-D modeling is used to optimize the surface profile of the RADAR antenna for beam symmetry and low sidelobe level over 20-24 GHz. The fabricated RADAR sensor deployed in mono-static mode has 0.19-mm measurement error for a static target. Measurements in a closed furnace at 1100 °C demonstrate the feasibility of non-contact in-situ monitoring of the level of molten metal for long duration (11 hours) without degradation in the sensor performance.

Non-contact real time microwave measurement and signal analysis techniques to extract high temperature material parameters from the mono-static reflections gathered by a compact air cooled corrugated horn are presented in this work. Non-contact in situ microwave measurements gathered over 20-24 GHz inside a closed furnace were processed to identify the thermodynamic phase change temperature of metal and glass melts. The melting point of aluminum alloy and glass transition of a borosilicate glass matrix extracted from the time gated and processed microwave measurements were in good agreement with differential scanning calorimetry measurements. Thus, the ability to measure high temperature material process parameters using non-contact microwave measurements is demonstrated.

Step frequency continuous wave (SFCW) RAdio Detection And Ranging (RADAR) sensor is proposed for non-contact measurement of the absolute level of molten solids in industrial furnaces. A conical horn is optimized for operation over 10-16 GHz in mono-static configuration for level measurement. The absolute distance of a stationary target was measured with 3 mm accuracy using the SFCW RADAR sensor in mono-static mode. Stable measurements were recorded for 4.35 h in a furnace at 450 °C. Level measurements of molten glass and aluminum in 1100 °C furnace indicate the feasibility of industrial-level gauging.