Royal Kessick

Microscale polymeric helical structures produced by electrospinning

Microscale helical coils consisting of a composite of one conducting and one nonconducting polymer were produced using electrospinning. The nonconducting polymer was poly(ethylene oxide) and the conducting polymer was poly(aniline sulfonic acid). The coil structures were studied over a range of processing conditions and fiber composition. The data suggest that the helical structures are formed due to viscoelastic contraction upon partial neutralization of the charged fibers. Polymeric microcoils may find applications in microelectromechanical systems, advanced optical components, and drug delivery systems.

An electrospray-based, ozone-free air purification technology

A zero-pressure-drop, ozone-free air purification technology is reported. Contaminated air was directed into a chamber containing an array of electrospray wick sources. The electrospray sources produce an aerosol of tiny, electrically charged aqueous droplets. Charge was transferred from the droplets onto polar and polarizable species in the contaminated air stream and the charged contaminants were extracted using an electric field and deposited onto a metal surface. Purified air emerged from the other end of the chamber. The very small aqueous electrospray droplets completely evaporate so that the process is essentially dry and no liquid solvent is collected or recirculated. The air purification efficiency was measured as a function of particle size, air flow rate, and specific system design parameters. The results indicate that the electrospray-based air purification system provides high air purification efficiency over a wide range of particle size and, due to the very low power and liquid consumption ...

Microscale electrospinning of polymer nanofiber interconnections

Polymer fiber interconnects were produced between microscale features on a substrate using only electrostatic forces. Electric-field-driven directed growth of nanoscale carboxymethylcellulose fibers was achieved between microscale droplets of a concentrated polymer solution. The fibers were studied using atomic force and scanning electron microscopy and were observed to emerge from the tip of conical protrusions formed at the surface of the droplets. The conical structures appear to be analogous to the characteristic Taylor cones formed in an electrospinning process and the process is interpreted as a microscale version of electrospinning requiring significantly lower driving potentials.

Investigation of the electronic properties of cadmium zinc telluride surfaces using pulsed laser microwave cavity perturbation

The spectroscopic performance of cadmium zinc telluride (CZT) room temperature radiation detectors is currently limited by both bulk and surface imperfections introduced during the growth, harvesting and fabrication of these devices. Bulk imperfections including impurities, vacancies, interstitials, grain boundaries and dislocations have been relatively well studied and are known to trap charge and reduce detector performance. However, the effect of specific traps on the electronic decay process has been difficult to quantify. Surface imperfections including mechanical damage or adsorbed chemical species are also known to trap charge or increase leakage current, but it has proven difficult to characterize the electronic properties of CZT surfaces prior to electrode deposition. Here it is shown that the pulsed laser microwave cavity perturbation method can provide important information on the electronic decay process both in the bulk and at the surface of high pressure Bridgman CZT crystals. Electronic decay process both in the bulk and at the surface of high pressure Bridgman CZT crystals. Electronic decay time was measured as a function of temperature and surface conditions. It is shown that the electronic decay process in bulk CZT is consistent witha single deep hole trap at an energy between 600meV and 700meV. The effect of surface quality was resolved by analyzing distinct features in the photoconductivity decay curves. Atomic force microscopy was used to characterize the surface roughness. Rough or damaged surfaces exhibited persistent photoconductivity, which could be eliminated by etching with a bromine methanol solution.

Nanoelectrospray aerosols from microporous polymer wick sources

Nanoelectrospray aerosols were formed from microporous polymer wick sources. Current-voltage characteristics were measured as a function of solution electrical conductivity and surface tension and two distinct electrospray modes were observed. In the first mode, when the maximum capillary flow rate through the wick exceeds the electrospray flow rate, a single electrospray forms from a droplet at the end of the wick. In the second mode, when the maximum capillary flow rate is less than the electrospray flow rate, a multitude of microscopic nanoelectrospray sources are formed from within the surface of the wick tip.

Photosensitive 2,5-distyrylpyrazine particles produced from rapid expansion of supercritical solutions

Solvent-free, photoreactive particles of 2,5-distyrylpyrazine (DSP) monomer were developed by rapid precipitation from an expanding supercritical chlorodifluoromethane solution. DSP polymer particles were produced by solid-state photopolymerization. DSP particles below a critical diameter of about 0.5 μm were found to be mechanically stable and did not fragment upon photopolymerization. The rate of DSP photopolymerization was shown to be size-sensitive. Nano-scale particles demonstrated superior photoreactivity in the solid state in comparison to micro-scale crystals. UV spectra of DSP at different degrees of conversion were investigated and the extinction coefficients were calculated for the DSP monomer and polymer in sulfuric acid.

Contactless thermally stimulated lifetime measurements in detector-grade cadmium zinc telluride

Contactless thermally stimulated lifetime measurements were performed on detector-grade Cd1−xZnxTe (x∼0.1) crystals using a pulsed laser microwave cavity perturbation method. The carrier lifetime decreased from approximately 30 μs at 110 K to 4 μs at 160 K, and then remained relatively constant from 160 to 300 K. The sudden drop in carrier lifetime within a particular temperature range is consistent with the thermal activation of a charge trap with a detrapping time longer than the carrier lifetime. The maximum trap activation temperature and the minimum detrapping time are estimated from the lifetime versus temperature curve to be approximately 160 K and 10−6 s, respectively.

Contactless measurements of charge traps and carrier lifetimes in detector-grade cadmium zinc telluride and mercuric iodide

An understanding of compensation and trapping in Cd1-xZnxTe and HgI2 is necessary in order to improve the size and spectroscopic performance of radiation detectors fabricated from these materials. Although several electron and hole traps have been identified, very little is currently understood about the effect of specific carrier traps on the mean free path of the charge carriers. Characterization techniques such as Thermally Stimulated Current (TSC) or Thermoelectric Emission Spectroscopy (TEES) have been used for trap identification, while time-of-flight techniques have been employed to determine carrier mobility and lifetime but it has proven difficult to correlate the results of these independent measurements. Furthermore, these characterization methods are complicated by the need to make electrical contacts to the material. Here we report on contactless, thermally stimulated lifetime measurements performed on detector-grade Cd1- xZnxTe (x approximately 0.1) and HgI2 crystals using a microwave cavity perturbation method. The microwave technique is complimentary to contact-based methods and provides both trap identification and lifetime determination in a single measurement. The results provide evidence of lifetime-limiting deep traps in these materials. The trap activation energies and the minimum detrapping times are estimated and the results are compared to previous TSC and TEES investigations.

Chemical microsensors based on polymer fiber composites

There is an urgent need for new chemical sensors for defense and security applications. In particular, sensors are required that can provide higher sensitivity and faster response in the field than existing baseline technologies. We have been developing a new solid-state chemical sensor technology based on microscale polymer composite fiber arrays. The fibers consist of an insulating polymer doped with conducting particles and are electrospun directly onto the surface of an interdigitated microelectrode. The concentration of the conducting particles within the fiber is controlled and is near the percolation threshold. Thus, the electrical resistance of the polymer fiber composite is very sensitive to volumetric changes produced in the polymer by vapor absorption. Preliminary results are presented on the fabrication and testing of the new microsensor. The objective is to take advantage of the very high surface to volume ratio, low thermal mass and linear geometry of the composite fibers to produce sensors exhibiting an extremely high vapor sensitivity and rapid response. The simplicity and low cost of a resistance-based chemical microsensor makes this sensing approach an attractive alternative to devices requiring RF electronics or time-of-flight analysis. Potential applications of this technology include battlespace awareness, homeland security, environmental surveillance, medical diagnostics and food process monitoring.

Temperature-dependent electronic decay profiles in CZT: probe of bulk and surface properties

The electronic performance of CZT-based gamma radiation spectrometers is governed by a synergism of bulk and surface properties. Compensation is used to increase the bulk resistivity of Cd1-xZnxTe (x~0.1), but the same electronic states that are introduced to increase the material resistivity can also trap charge and reduce the carrier lifetime. Electrical and mechanical surface defects introduced during or subsequent to crystal harvesting are also known to interfere with device performance. Using a contactless, pulsed laser microwave cavity perturbation technique, electronic decay profiles were studied in high pressure Bridgman CZT as a function of temperature. The electronic decay profile was found to depend very strongly on temperature and was modeled using a function consisting of two exponential terms with temperature-dependent amplitudes and time constants. The model was used to relate the observed temperature dependent decay kinetics in CZT to specific trap energies. It was found that, at low temperatures, the electronic decay process is dominated by a deep trap with an energy of approximately 0.69 ± 0.1 eV from the band edge. As the temperature is increased, the charge trapping becomes dominated by a second trap with an energy of approximately 0.60 ± 0.1 eV from the band edge. Surface damage introduces additional charge traps that significantly alter the decay kinetics particularly at low temperatures.