Brian Geist

Dielectric constant and breakdown field studies of electrostatic self-assembled materials

Electrical properties of electrostatic self-assembled (ESA) films were investigated using a conductive polymer and metal bar. These nanostructured polymeric films were fabricated on gold-coated glass slides using the ESA method. The thicknesses of the films were in the range 120?630?nm, and the films were obtained by depositing numbers of bilayers of negatively charged Poly s-119 (PS-119) or heparin and positively charged Poly(diallyldimethylammonium chloride) (PDDA). Measurement electrodes were fabricated on the ESA films using conductive silver grease and a brass bar. Capacitance measurements were conducted to determine the dielectric constant of the ESA films over various temperature and frequency ranges at 1?mV and 15% relative humidity, while electric field breakdown tests were performed at 1000?Hz, 15% relative humidity and varying temperatures and voltages. The test results showed that dielectric constant values were between 1.8 and 2.4 and breakdown field values were approximately 9?kV?mm?1. Based on the test results, it is concluded that this is a technique that might prove useful in estimating the capacitance, dielectric constant and breakdown field values of nanostructured ESA films.

A Fiber Bragg Grating Temperature Sensor for 2–400 K

We demonstrate fiber optic, multiplexible temperature sensing using a fiber Bragg grating (FBG) with an operational range of 2-400 K, and a temperature resolution better than 10 mK for temperatures <;12 K. This represents a significant reduction in the lowest usable temperature as well as a significant increase in sensitivity at cryogenic temperatures compared with previously reported multiplexible solutions. This is accomplished by mounting the section of the fiber with a FBG on a polytetrafluoroethylene coupon, which has a non-negligible coefficient of thermal expansion down to <;4 K. The sensors exhibit a good stability over multiple temperature cycles and acceptable sensor-to-sensor repeatability. Possible applications for this sensor include distributed temperature sensing across superconducting elements and cryogenic temperature measurements in environments where electrical measurements are impractical or unsafe.

Radiation stability of visible and near-infrared optical and magneto-optical properties of terbium gallium garnet crystals

Perspectives of terbium gallium garnet, Tb₃Ga₅O₁₂ (TGG), for the use of radiation-resistant high magnetic field sensing are studied. Long-term radiation stability of the TGG crystals was analyzed by comparing the optical and magneto-optical properties of a radiation-exposed TGG crystal (equivalent neutron dose 6.3×10¹³ n/cm²) to the properties of TGG control samples. Simulations were also performed to predict radiation damage mechanisms in the TGG crystal. Radiation-induced increase in the absorbance at shorter wavelengths was observed as well as a reduction in the Faraday effect while no degradation of magneto-optical effect was observed when at wavelengths above 600 nm. This suggests that TGG crystal would be a good candidate for use in magneto-optical radiation-resistant magnetic field sensors.

Thermal cycling and the optical and electrical characterization of self-assembled multilayer Nile Blue A–gold thin films

Some laser applications produce high power densities that can be dangerous to equipment and operators. We have fabricated thin-film coatings by using molecular electrostatic self-assembly to create a spectrally selective absorbing coating that is able to withstand thermal fluctuations from -20 degrees C to 120 degrees C. We made the thin-film coatings by alternating deposition of an organic dye and gold colloidal nanoparticles onto glass substrates. Nile Blue A perchlorate, with a maximum absorbance slightly above 632 nm, was chosen as the organic dye. Strong coupling between the dye molecules and the gold nanoparticles provides a redshift that increases as the films thickness is increased. The incorporation of the gold colloidal nanoparticles also decreases the resistivity of the film. The resistivity of the film was measured with a four-point probe and found to be approximately 10 omega/cm for the two samples measured. Atomic-force microscopy was used to show that film thickness increased 2.4 nm per bilayer. The optical properties of the film were measured at the end of every 5 thermal cycles from -20 degrees C to 120 degrees C, and negligible degradation was observed after 30 cycles.