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There is a growing need for a continuous monitoring system that can detect explosives such as triacetone triperoxide (TATP) in the vapor phase at trace levels. TATP is an energetic material commonly used by terrorists in improvised explosive devices both as the initiator and the energetic material itself. TATP is still going largely undetected in many densely populated venues such as train stations and airports. No electronic trace detection system currently exists that is capable of continuously monitoring TATP, or its precursors, in the vapor phase. Recently, we developed a thermodynamic sensor that can detect TATP and other nitrogen-based explosives in the vapor phase at the ppb level. These ultrathin sensors showed unparalleled sensitivity and could even be operated at low enough powers to enable them to be deployed on drones. The sensor relies on specific oxidation-reduction reactions between the energetic molecules and metal oxide catalysts. Here, catalyst coated microheaters measure the heat effects associated with redox reactions occurring on the surface of the metal oxide catalyst. Enhanced sensitivity and selectivity were achieved through implementation of ultrathin ( $8~\mu \text{m}$ thick) yttria-stabilized-zirconia (YSZ) as the substrate for the sensor platform. This reduction in the thermal mass combined with the catalytic amplification associated with Pd microheaters and optimized catalyst porosity enabled us to detect TATP in the vapor phase at unprecedented levels.

Surface temperature measurements of ceramic matrix composite (CMC) based engine components within the hot section of a gas turbine engines are needed to verify engine designs and cooling requirements. Traditional temperature sensors cannot be applied to silicon carbide (SiC) based CMC's for this purpose, since they cannot be welded or embedded in the material. For these reasons, a novel thermocouple was developed for surface temperature measurement of p -type SiC-SiC CMC's. This novel thermocouple uses the p -type SiC-SiC CMC itself as one of the thermoelements and indium-tin-oxide (ITO) as the other thermoelement. Since both thermoelements are wide bandgap semiconductors, this thermocouple has exceptional thermoelectric output. A thermoelectric output of 272 μV/°K was observed for this ITO:SiC CMC thermocouple, which is an order of magnitude larger than that of type- K thermocouples. A drift rate of 3.28 °C/h was measured for this ITO:SiC CMC thermocouple. In addition, indium tin oxynitride (ITON) protective coatings deposited via reactive sputtering enabled these thermocouples to operate at 1000 °C. The ITON coating served as an oxygen diffusion barrier to prevent oxidation at the ITO:SiC CMC junction. Nitrogen and air anneals at 550 and 1000 °C, respectively, were used to improve the high temperature stability of the ITO and ITON coatings. Performance, fabrication details, and challenges using these ITO:SiC CMC thermocouple are discussed within.

There is a growing need for an electronic trace detection system that can continuously monitor explosives at trace levels in the vapor phase. Triacetone triperoxide (TATP) is a common explosive used by terrorists as the initiator or the energetic material in improvised explosive devices. TATP still goes largely undetected in many densely populated public venues. Currently, no detection system exists that is capable of continuously monitoring TATP, as well as nitrogen-based explosives such as 2,4-dinitrotoluene. We have demonstrated a thermodynamic sensor platform employing thin film microheaters and metal oxide catalysts that can detect TATP and 2,4-DNT at the parts per billion (ppb) level. The microheaters and catalysts were deposited onto ultrathin alumina ceramic substrates, which are responsible for extraordinary sensitivity. Recently, we have added a conductometric sensor platform to this thermodynamic platform to form an orthogonal sensor capable of interrogating the same catalyst using two different modalities. The orthogonal sensor probes the same catalyst using a thermodynamic protocol, as well as a conductometric protocol where electrical resistivity changes in the catalyst are measured as a function of analyte exposure. The advantages of this added orthogonality in terms of mitigating false-positives and negatives are described within.

There is a growing need for a continuous monitoring system that can detect explosives at trace levels in the vapor phase. Triacetone triperoxide (TATP) is a common explosive used by terrorists in improvised explosive devices (IED’s) that still goes largely undetected in many densely-populated venues. No electronic trace detection system currently exists, which is capable of continuously monitoring TATP or its precursors. However, we have demonstrated that a thermodynamic sensor employing low mass microheaters and metal oxide catalysts can detect TATP and 2, 4-DNT at the ppb level. The improved selectivity and sensitivity of these low-mass sensors enabled us to investigate the mechanism and pathway responsible for the detection of explosives. We now believe that the sensor response relies on specific oxidation-reduction reactions taking place on the surface of the metal oxide catalyst. These surface reactions are specific to the explosive molecule of interest, catalyst (metal oxide), and the oxidation states of the catalyst. Experiments employing SnO, Cu2O, and ZnO as catalysts were performed to establish a mechanism responsible for the detection of various explosives at trace levels using our thermodynamic sensor. The responses of our trace detection system to TATP and 2, 4-DNT at trace levels will be presented as well as the proposed detection mechanism.

As increasingly more SiCSiC ceramic matrix composites (CMCs) are being used in the hot sections of gas-turbine engines, there is a greater need for in situ strain measurement in these harsh conditions. Thin-film strain sensors are ideally suited for this task since they have minimal mass and are nonintrusive. However, if the bulk piezoresistive properties of SiCSiC CMCs could be used instead of the thin-film piezoresistive properties, superior resolution and stability could be realized. Toward this end, we have fabricated strain gages in which the active strain element is the SiCSiC CMC itself. As a result of this approach, a Pt:SiC CMC strain gage that is capable of operating at 500C for prolonged periods was demonstrated. The large piezoresistive responses associated with the SiC fibers and SiC matrix resulted in gage factors larger than 100. The performance of the Pt:SiC CMC strain gages under both tension and compression are reported, and the IV characteristics of the strain gages, as well as the piezoresistive response as a function of temperature, are presented. In addition, the potential for embedded strain gages in advanced CMC engine components will be discussed.

A platinum:silicon carbide thermocouple has been developed to measure the surface temperature of ceramic matrix composites (CMC) with high resolution. Platinum was deposited by rf sputtering onto a SiC-SiC CMC substrates coated with a dielectric, such that the SiC-SiC CMC was one thermoelement and the platinum film was another thermoelement comprising the Pt:SiC(CMC) thermocouple. The purpose of the dielectric was to electrically isolate the platinum leads from the SiC-SiC CMC. The thermoelectric output, hysteresis and drift of the Pt:SiC(CMC) thermocouples were measured at temperatures ranging from 600 °C to 1000 °C. The thermoelectric powers generated by the Pt:SiC thermocouples were an order in magnitude greater than conventional Pt:Pd or Type K thermocouples. Thermoelectric powers as large as 250 μV/K were reported for these thermocouples, as compared to thermoelectric powers of 10 μV/K reported for Pt:Pd and Type K thermocouples. The results presented within show that the Pt:SiC(CMC) thermocouples exhibit excellent stability at high temperatures, relatively low drift rates, and little hysteresis during thermal cycling. However, the Pt:SiC junctions were prone to oxidation effects as well as the formation of platinum silicides at high temperature, which can compromise the junction and lead to excessive drift. Therefore, a number of diffusion barrier coatings were applied to the Pt:SiC junctions in an attempt to improve stability and lower drift in this promising new class of thermocouples.

As more and more SiC–SiC ceramic matrix composites or CMC’s are being used in the hot sections of gas turbine engines, there is a greater need for surface temperature measurement in these harsh conditions. Thin film sensors are ideally suited for this task since they have very small thermal masses and are nonintrusive due to their thickness. However, if the bulk properties of SiC contributed to the sensor performance (thermoelectric response) rather than those of the thin films, superior resolution, and stability could be realized. Therefore, thermocouples utilizing the SiC–SiC CMC itself as one thermoelement and thin film platinum as the other thermoelement were developed. Large and stable thermoelectric powers (as large as 250 μV/°K) were realized with these Pt:SiC (CMC) thermocouples. The advantages in using this approach for surface temperature measurement are presented as well as the effects of fiber orientation on thermoelectric response and drift.

With the recent terrorist attacks in Paris and the continued use of IED’s employing TATP for delivering these threats, there is a real need for explosives detection at trace levels. This work describes the fabrication and characterization of metal oxide nanowires used as catalysts for the detection of energetic materials at trace levels. Recently, several oxide nanowires, based on zinc oxide and copper oxide, have been incorporated into our solid-state gas sensors as catalysts. These nanowire catalysts produced a dramatic increase in sensor response with improved selectivity for threat molecules of interest. The improved responses were attributed to a large increase in surface area available for catalyst/analyte interaction. Zinc oxide and copper oxide nanowires were grown by hydrothermal and controlled oxidation reactions, and were characterized using XRD, XPS and SEM to determine extent of crystallinity, oxidation state and morphology. Results indicated that energetic materials such as TATP and 2-6 DNT could be detected at the part per billion level using these nanowire catalysts. Other oxide nanowires are being considered as catalysts for the detection of explosives and are discussed as well.

INTRODUCTION In recent years, the use of improvised explosive devices (IEDs) have become the preferred vehicle for terrorist attacks since they can be easily fabricated from readily available materials. This has raised concerns for the travelling public and prompted the need for a cost-effective, continuous monitoring system for explosives detection [1]. Recent works have been published regarding sensors and their ability to detect trace level explosives. Although these sensors address some of the needs for DHS, there is a limit to the modifications that can be made to these sensors in order to decrease the limit of detection. In this work, a pre-concentrator was developed as a means to increase the sensitivity of sensors used to detect vapor-phase explosives. Pre-concentrators employ sorbents that possess critical adsorption/desorption properties, thus allowing the delivery of a highly concentrated pulse of explosive vapor to the sensor. An order of magnitude improvement in detection limit was demonstrated for 2, 6-DNT and our pre-concentration approach shows considerable promise for the detection of other explosives at trace levels. EXPERIMENTAL Two microheaters were employed in our dynamically controlled sensor platform; one microheater was coated with a metal-oxide catalyst and a second microheater was left uncoated. The pre-concentrator, located upstream employed a spin-coated polystyrene film, which was deposited onto a third microheater, as shown in Figure 1. A carrier gas delivered the target molecules to the pre-concentrator for 180 seconds allowing time for collection of the explosive vapor. The pre-concentrator was then heated to desorb the explosive molecules, delivering a high concentration burst of 2, 6-DNT to both microheaters downstream. RESULTS Figure 2 illustrates the difference in signals after the thermal desorption of explosive molecules from the polymer. The response (signal) due to the pre-concentrator was nearly double that of the sensor alone. The results suggest an increase in power, signifying that an endothermic reaction occurs between the analyte and the catalyst. Figure 3 shows the detection limit obtained using the dual sensor (dynamically controlled) platform for TATP, which provides the baseline for comparison purposes; i.e. enhanced response due to pre-concentration. Further experiments were conducted to determine a gas flow that optimizes signal response, collection time and polymer film thickness and their role in the adsorption/desorption kinetics. CONCLUSION AND FUTURE WORK We have demonstrated that the addition of a polystyrene pre-concentrator to our sensor platform lowered the detection limit of 2, 6-DNT by an order of magnitude. Upon thermal desorption, there was an approximate 47% increase in sensor response. Future work will focus on developing other polymer pre-concentrators that will decrease the detection limit of other vapor phase explosives of interest. REFERENCES [1] Ras, M. R., F. Borrull, and R. M. Marcé. "Sampling and Preconcentration Techniques for Determination of Volatile Organic Compounds in Air Samples." TrAC Trends in Analytical Chemistry, 28.3 (2009): 347-61. Figure 1

Sensors used for trace explosives detection frequently exhibit poor sensitivity and selectivity for non-nitrogen based explosives such as triacetone triperoxide (TATP). Selectivity between hydrogen peroxide (H2O2) and TATP molecules for example, is marginal in many types of gas sensors. In this study, we focused on improving sensor response and selectivity by employing combinatorial chemistry techniques. Specifically, Pd:SnO2 nanocomposite catalysts were prepared and the most promising candidates screened using combinatorial chemistry techniques. The effect of palladium loadings on the response of Pd:SnO2 nanocomposites to TATP were investigated, and select compositions greatly improved the sensitivity to TATP and the selectivity of TATP over H2O2. Both SEM and TEM were employed to characterize the morphology of the nanocomposite catalysts and XRD and XPS were used to identify the crystal structure and the oxidation state of the catalyst. Based on the combinatorial chemistry experiments, Pd:SnO2 nancomposite catalysts with an 8 wt.% palladium loading yielded the greatest selectivity of TATP over H2O2, while a 12 wt.% palladium loading yielded the largest response signal to TATP. The relative merits of using this approach to identify catalysts for the detection of non-nitrogen based explosives are discussed.

Thin film nanocomposites based on semiconducting oxides and palladium have been systematically studied using combinatorial chemistry techniques to optimize their thermoelectric properties. These nanocomposites have considerable potential for energy harvesting applications in harsh environments and were prepared by embedding palladium nanoparticles into an indium oxide matrix via co-sputtering from metal and ceramic targets. The as-deposited films were largely amorphous and thus were subsequently heat treated in nitrogen ambients to produce crystalline nanocomposites. These nanocomposite films were screened for their thermoelectric properties at room temperature as a function of composition, and the resulting films exhibited thermoelectric power factors that were significantly greater than the semiconducting oxides deposited directly from ceramic targets. Based on these rapid screening protocols employing hundreds of micro-thermocouples, the most promising nanocomposite films in terms of thermoelectric power factor were down-selected. Transmission electron microscopy, selected area electron diffraction, and scanning electron microscopy were then used to study the microstructural changes in these films as a function of temperature. When thermally cycled to 800 °C, an abrupt transition from normal, equiaxed grain growth to abnormal (spherulitic growth) was observed at a threshold palladium loading which was also optimal for thermoelectric energy harvesting.

Thin film platinum:palladium thermocouples were fabricated on alumina and mullite surfaces using radio frequency sputtering and characterized after high temperature exposure to oxidizing environments. The thermoelectric output, hysteresis, and drift of these sensors were measured at temperatures up to 1100 °C. Auger electron spectroscopy was used to follow the extent of oxidation in each thermocouple leg and interdiffusion at the metallurgical junction. Minimal oxidation of the platinum and palladium thermoelements was observed after high temperature exposure, but considerable dewetting and faceting of the films were observed in scanning electron microscopy. An Arrhenius temperature dependence on the drift rate was observed and later attributed to microstructural changes during thermal cycling. The thin film thermocouples, however, did exhibit excellent stability at 1000 °C with drift rates comparable to commercial type-K wire thermocouples. Based on these results, platinum:palladium thin film thermocouples have considerable potential for use in the hot sections of gas turbine engines.

Based on results obtained utilizing combinatorial chemistry techniques to screen the thermoelectric power factor of materials in the system ZnxIny Ox+1.5y, several multiphase candidates were down-selected and investigated in terms of their thermoelectric response from room temperature to 1050°C. While the screening experiments suggested that peaks in the power factor occur at relatively high indium oxide content, only the thermoelectric properties of zinc-oxide-rich homologous layered phases in the system (In2O3)(ZnO)k have been well documented, since the phases where k < 3 cannot be easily formed. In the present study, indium-oxide-rich materials in the system In2O3–(In2O3)(ZnO)3 were fabricated and their figures of merit were determined. The results suggest that the indium-oxide-rich phases have improved figures of merit, especially at elevated temperatures, relative to the best performing k phases by combining the high power factor of In2O3 and the low thermal conductivity of (In2O3)(ZnO)k.

This paper focuses on the microstructure and deformation properties of structural steel at elevated temperatures. The amounts and morphology of carbides present were monitored as a function of thermal exposure parameters. It was observed that as the temperature approaches the ferrite-austenite phase transformation (727 °C), considerable diffusion of cementite plates and abnormal grain growth occurs. A viscoplastic constitutive model has been employed to simulate the flow behavior of the steel. Monotonic and cyclic tests were carried out to determine the kinematic and isotropic hardening parameters required for full identification of the model variables. Results of the model were compared with those obtained experimentally. The material parameters were shown to be sensitive to the microstructure and temperature. Variation in carbide amounts and morphology in the post thermal exposed specimens result in differences in the kinematic hardening, as compared to the as received material. Furthermore, the temperature sensitivity of the isotropic hardening is indicated by the presence a cyclic hardening/softening transition in the temperatures 600–700 °C. Validity of the model in capturing time dependent behavior of the structural steel is examined using a sequentially coupled thermal-stress finite element analysis of single steel beam. Results of this simulation are discussed in terms of effects of temperature and strain-rate sensitivity on the material's viscoplastic deformation response.

Triacetone triperoxide (TATP) is commonly used in improvised explosive devices (IEDs) due to its relatively simple preparation and readily available precursors. In this study, a small footprint gas sensor utilizing metal oxide catalysts was fabricated and tested, which is capable of detecting the heat of reaction during the catalytic decomposition of TATP. Due to its relatively high vapor pressure compared to other explosives, TATP is an ideal target molecule for this type of gas sensor. TATP and its decomposition products, hydrogen peroxide and acetone were successfully detected using this thermodynamic based sensor platform. Each tested catalyst exhibited a specific response at a given temperature and the detection limit of TATP was determined to be in the range of parts per million. X-ray photoelectron spectroscopy (XPS) was employed to study the oxidation state of selected catalysts prior to and after exposure to the target gases. Morphology of the catalysts in the as-deposited and tested condition was followed using scanning electron microscopy (SEM).

Dissipative particle dynamics (DPD) with energy conservation was applied to simulate forced convection in parallel-plate channels with boundary conditions of constant wall temperature (CWT) and constant wall heat flux (CHF). DPD is a coarse-grained version of molecular dynamics. An additional equation for energy conservation was solved along with conventional DPD equations, where inter-particle heat flux accounts for changes in mechanical and internal energies when particles interact with surrounding particles. The solution domain was considered to be two–dimensional with periodic boundary condition in the flow direction and additional layers of particles on the top and bottom of the channel to apply no-slip and wall temperature boundary conditions. The governing equation for energy conservation was modified based on periodic fully developed velocity and temperature conditions. The results were shown via velocity and temperature profiles across the channel cross-section. The Nusselt numbers for CWT and CH...

The effects of impact loading on changes in microstructure have been studied in low carbon steel. Low to moderate shock loading tests have been carried out on steel specimens using a single stage gas gun with projectile velocities ranging from 200 to 500 m/s. Stress history at the back face of the target specimen and projectile velocity prior to impact were recorded via manganin stress gauges and velocity lasers, respectively. A Johnson–Cook constitutive material model was employed to numerically simulate the material behavior of low carbon steel during impact and obtain the particle velocity at the impact surface as well the pressure distribution across the specimens as a function of impact duration. An analytical approach was used to determine the twin volume fraction as a function of blast loading. The amount of twinning within the α-ferrite phase was measured in post-impact specimens. A comparison between experimental and numerical stress histories, and analytical and experimental twin volume fraction were used to optimize the material and deformation models and establish a correlation between impact pressure and deformation response of the steel under examination. Strain rate controlled tensile tests were carried out on post-impact specimens. Results of these tests are discussed in relation to the effects of impact loading on the yield and ultimate tensile strength as well as the hardening and strain energy characteristics.

Indium oxide (In2O3) and indium tin oxide (ITO) thin films have been investigated for high temperature thermocouple and strain guage applications. Reactive sputtering in nitrogen-rich plasmas was used to improve the high temperature stability of indium oxide-based films in air and scanning electron microscopy was used to follow the microstructural changes in the nitrogen-processed films. When thermally cycled at temperatures above 800°C, a partially sintered microstructure comprised of nanometer-sized crystallites was revealed. A densified layer was also formed on the surface, which acted as an oxygen-diffusion barrier in the bulk film. This combined with a network of partially sintered oxynitride crystallites lead to considerable open porosity and a stabilizing effect on the ensuing electrical properties. In this article, the thermoelectric properties of nitrogen-processed films were evaluated at temperatures up to 1400°C. To study the effect of nitrogen plasma processing on the sintering kinetics and associated densification, the constrained sintering of the resulting films was followed as a function of time and temperature. Based on the measured thermoelectric properties of the nitrogen processed films, drift rates on the same order of magnitude as commercial type K wire thermocouples were realized for these all-ceramic thermocouples.

This work investigates experimentally the near-infrared optical properties of SiO(2) thin film embedded with tungsten (W) nanoparticles at varying volume fractions. The samples are prepared by using the technique of magnetron sputtering. The formation and distribution of W nanoparticles are characterized using transmission electron microscopy, and the volume fraction of W nanoparticles is validated by Auger electron spectroscopy. Near- and mid-infrared diffuse reflectance measurements are conducted using Fourier transform infrared spectroscopy. The samples exhibit wavelength selective optical response in the near-infrared region and are suitable for applications involving selective thermal emitters/absorbers. Measured reflectance data is utilized to estimate the effective dielectric function of the nano-composites. Calculated reflectance spectra in different samples are compared to the measured spectra using the experimentally measured dielectric function of these samples in the near-infrared region. Reflectance spectra after thermal annealing at different temperature are compared to show how the thermal treatment affects the optical properties of samples. Optimized structures are proposed for thermal emitters and absorbers with different volume fractions of W nanoparticles.

The effects of rib-patterned surfaces and surface wettability on liquid flow in microchannels were experimentally investigated in this study. Microchannels were fabricated on single-crystal silicon wafers by photolithographic and wet-etching techniques. Rib structures were patterned in the silicon microchannel, and the surface was chemically treated by trichlorosilane to create hydrophobic condition. Experiments with water as the working fluid were performed with these microchannels over a wide range of Reynolds numbers between 110 and 1914. The results for the rib-patterned microchannels showed that the friction factor with the hydraulic diameter based on the rib-to-upper-wall height was lower than that predicted from incompressible theory with the same height. The friction factor-Reynolds number products for the hydrophobic condition increased as Reynolds number increased in the laminar flow regime. The experimental results were also compared with the predictive expressions from the literature, and it was found that the experimental data for the small rib/cavity geometry was in good agreement with those in the literature.

Ceramic thermocouples are being developed to replace noble-metal thermocouples that are unable to withstand the harsh environments inside the hot sections of turbine engines used for power generation and propulsion. A number of alloys in the system indium oxide (In2O3):tin oxide (SnO2) were systematically investigated as thermocouples. Specifically, solid solutions containing up to 10 wt% SnO2 were initially tested relative to a platinum reference electrode and the resulting thermoelectric properties were measured. The results indicated that the thermoelectric response was dependent on the SnO2 content in the alloy. Seebeck coefficients ranged from 53 to 224 μV/°C at temperatures up to 1300°C, which are considerably larger than those generated from metal thermocouples. Bi-ceramic thermocouples based on selected solid solutions of indium tin oxide (ITO) exhibited high temperature stability and Seebeck coefficient on the order of 160 μV/°C. Postdeposition treatments had a significant effect on the stability of the ceramic thermocouples. High-temperature annealing improved the film uniformity, stability, and reproducibility of the ITO thin-film thermocouples. A bi-ceramic thermocouple consisting of In2O3 and In2O3:SnO2 (95:5 wt%) was the best-performing thermocouple of all compositions studied.

Indium tin oxide (ITO), alumina doped zinc oxide (ZnO) and NiCrCoAlY/alumina nanocomposites were systematically investigated as thermoelements. These ceramic thermoelements were initially tested relative to a platinum reference electrode and the resulting thermoelectric properties were evaluated. Bi-ceramic junctions comprised of the most stable and responsive ceramic thermoelements, i.e. those thermoelements with the largest and most stable Seebeck coefficients relative to platinum, were fabricated and tested. A bi-ceramic junction based on nitrogen-doped ITO:oxygen-doped ITO exhibited excellent high temperature stability and reproducibility, however, this thermocouple pair had a relatively low Seebeck coefficient (6 μV/°C). Alumina doped ZnO:ITO thermocouples generated a very large electromotive force at low temperatures but lacked high temperature stability. When nitrogen-doped ITO was combined with a NiCoCrAlY/alumina nanocomposite, a very large and stable Seebeck coefficient (375 μV/°C) was realized. Ceramic thermocouples based on several candidate materials were demonstrated at temperatures up to 1200 °C and the potential of using these materials in other thermoelectric devices including those for energy harvesting is discussed.

We report the optical properties of SU-8 in the mid-infrared (mid-IR) region before and after UV treatment. Samples consisted of SU-8 films of thickness ranging from 10 um to 157 um deposited on gold coated silicon substrates and were prepared using spin coating. Mid-IR diffuse reflectance measurements were conducted using Fourier transform infrared spectroscopy. Spectra measurements imply a change in optical properties of SU-8 upon exposure to UV and heat treatment. A gradual change in optical properties is seen after each step of UV treatment and the baking process. Reflectance spectra of thin-films were also observed to be thickness dependent. We calculate the dielectric function of SU-8 in the range 2 um to 15 um using the reflectance spectra of the samples.

Indium–tin-oxide (ITO) thin films strain gages were prepared by reactive sputtering onto both high purity alumina and lanthanum stabilized zirconia substrates. We report the piezoresistive response of these ITO gages in the temperature range 350–1500 °C up to 1000 microstrain (μe). Strain response and gage factors are reported for ITO in both tension and compression. The data suggest that not only is there an annealing component but also a strain component of the piezoresistive change with time at temperature. The effects in both tension and compression are reported.

In this theoretical study, we present a near-field thermal modulator that exhibits change in radiative heat transfer when subjected to mechanical stress/strain. The device has two terminals at different temperatures separated by vacuum: one fixed and one stretchable. The stretchable side contains one-dimensional grating. When subjected to mechanical strain, the effective optical properties of the stretchable side are affected upon deformation of the grating. This results in modulation of surface waves across the interfaces influencing near-field radiative heat transfer. We show that for a separation of 100 nm, it is possible to achieve 25% change in radiative heat transfer for a strain of 10%.

Radiative thermal transport of metamaterials has begun to play a significant role in thermal science and has great engineering applications. When the key features of structures become comparable to the thermal wavelength at a particular temperature, a narrowband or wideband of wavelengths can be created or shifted in both the emission and reflection spectrum of nanoscale metamaterials. Due to the near-field effect, the phenomena of radiative wavelength selectivity become significant. These effects show strong promise for applications in thermophotovoltaic energy harvesting, nanoscale biosensing, and increased energy efficiency through radiative cooling in the near future. This review paper summarizes the recent progress and outlook of both near-field and far-field radiative heat transfer, different design structures of metamaterials, applications of unique thermal and optical properties, and focuses especially on exploration of the tunable radiative wavelength selectivity of nano-metamaterials.

Recovered pipe bomb fragments, exploded under controlled conditions, have been characterized using scanning electron microscopy, optical microscopy and microhardness. Specifically, this paper examines the microstructural changes in plain carbon-steel fragments collected after the controlled explosion of galvanized, schedule 40, continuously welded, steel pipes filled with various smokeless powders. A number of microstructural changes were observed in the recovered pipe fragments: deformation of the soft alpha-ferrite grains, deformation of pearlite colonies, twin formation, bands of distorted pearlite colonies, slip bands, and cross-slip bands. These microstructural changes were correlated with the relative energy of the smokeless powder fillers. The energy of the smokeless powder was reflected in a reduction in thickness of the pipe fragments (due to plastic strain prior to fracture) and an increase in microhardness. Moreover, within fragments from a single pipe, there was a radial variation in microhardness, with the microhardness at the outer wall being greater than that at the inner wall. These findings were consistent with the premise that, with the high energy fillers, extensive plastic deformation and wall thinning occurred prior to pipe fracture. Ultimately, the information collected from this investigation will be used to develop a database, where the fragment microstructure and microhardness will be correlated with type of explosive filler and bomb design. Some analyses, specifically wall thinning and microhardness, may aid in field characterization of explosive devices.

Scanning electron microscopy combined with microchemical analysis has evolved into one of the most widely used instruments in forensic science today. In particular, the environmental scanning electron microscope (SEM) in conjunction with energy dispersive spectroscopy (EDS), has created unique opportunities in forensic science in regard to the examination of trace evidence; i.e. the examination of evidence without altering the evidence with conductive coatings, thereby enabling criminalists to solve cases that were previously considered unsolvable. Two cold cases were solved at URI using a JEOL 5900 LV SEM in conjunction with EDS. A cold case murder and a cold missing person case will be presented from the viewpoint of the microscopist and will include sample preparation, as well as image and chemical analysis of the trace evidence using electron microscopy and optical microscopy.

An optical sensor is described which can be attached to a structure and used as a gage for measuring bending strain. This device can be adjusted to maximize the gage factor for predetermined strain ranges. The sensor consists of glass capillaries coated on the outer surfaces with an optical absorbing layer followed by a reflecting layer. A mechanical strengthening layer can be included to extend the range of strain response. A source laser beam from an optical fiber is injected into one end of the gage. The light remaining in the beam after traveling through the gage is collected via another optical fiber. The optically active layer is adjusted during manufacture to provide a predetermined gage factor. For a given thickness of the absorber layer, the detected light is proportional to the amount of bending. Thus, by rigidly affixing the sensor to a structural member, the strain experienced by the member can be monitored.

A ceramic thermocouple based on indium-tin-oxide (ITO) thin films is being developed to measure the surface temperature of gas turbine engine components employed in power and propulsion systems that operate at temperatures in excess of 1500/spl deg/C. By fabricating ITO elements with substantially different charge carrier concentrations, it was possible to construct a robust ceramic thermocouple. A thermoelectric power of 6.0 /spl mu/V//spl deg/C, over the temperature range 25-1250/spl deg/C, was realized for an unoptimized ITO ceramic thermocouple. The charge carrier concentration difference in the legs of the ITO thermocouple was established by r.f. sputtering in oxygen-rich and nitrogen-rich plasmas. SEM micrographs revealed that after high-temperature exposure, the surfaces of the nitrogen prepared ITO films exhibited a partially sintered microstructure with a contiguous network of ITO nanoparticles. Thermal cycling of ITO films in various oxygen partial pressures showed that the temperature coefficient of resistance was nearly independent of oxygen partial pressure at temperatures above 800/spl deg/C and eventually became independent of oxygen partial pressure after repeated thermal cycling below 800/spl deg/C. Based on these results, a versatile ceramic sensor system has been envisioned where a ceramic thermocouple and strain sensor can be combined to yield a multifunctional ceramic sensor array.

Ceramic strain sensors based on reactively sputtered indium tin oxide (ITO) thin films doped with aluminum are being considered to improve the high-temperature stability and response. Ceramic strain sensors were developed to monitor the structural integrity of components employed in aerospace propulsion systems operating at temperatures in excess of 1500 °C. Earlier studies using electron spectroscopy for chemical analysis (ESCA) studies indicated that interfacial reactions between ITO and aluminum oxide increase the stability of ITO at elevated temperature. The resulting ESCA depth files showed the presence of two new indium–indium peaks at 448.85 and 456.40 eV, corresponding to the indium 3d5 and 3d3 binding energies. These binding energies are significantly higher than those associated with stoichiometric indium oxide. Based on these studies, a combinatorial chemistry approach was used to screen large numbers of possible concentrations to optimize the stability and performance of Al-doped ceramic strain sensors. Scanning electron microscopy was used to analyze the combinatorial libraries in which varying amounts of aluminum were incorporated into ITO films formed by cosputtering from multiple targets. Electrical stability and piezoresistive response of these films were compared to undoped ITO films over the same temperature range.

This paper presents an experimental investigation of laminar gas flow through microchannels. The independent variables: relative surface roughness, Knudsen number and Mach number were systematically varied to determine their influence on the friction factor. The microchannels were etched into silicon wafers, capped with glass, and have hydraulic diameters between 5 and 96 μm. The pressure was measured at seven locations along the channel length to determine local values of Knudsen number, Mach number and friction factor. All measurements were made in the laminar flow regime with Reynolds numbers ranging from 0.1 to 1000. The results show close agreement for the friction factor in the limiting case of low Ma and low Kn with the incompressible continuum flow theory. The effect of compressibility is observed to have a mild (8 percent) increase in the friction factor as the Mach number approaches 0.35. A 50 percent decrease in the friction factor was seen as the Knudsen number was increased to 0.15. Finally, the influence of surface roughness on the friction factor was shown to be insignificant for both continuum and slip flow regimes.

We theoretically analyze two near-field thermal rectification devices: a radiative thermal diode and a thermal transistor that utilize a phase change material to achieve dynamic control over heat flow by exploiting metal-insulator transition of VO2 near 341 K. The thermal analogue of electronic diode allows high heat flow in one direction while it restricts the heat flow when the polarity of temperature gradient is reversed. We show that with the introduction of 1-D rectangular grating, thermal rectification is dramatically enhanced in the near-field due to reduced tunneling of surface waves across the interfaces for negative polarity. The radiative thermal transistor also works around phase transition temperature of VO2 and controls heat flow. We demonstrate a transistor-like behavior wherein heat flow across the source and the drain can be greatly varied by making a small change in gate temperature.

A robust high temperature strain gage based on indium-tin-oxide (ITO) has been used to measure static and dynamic strains at temperatures up to 1400 °C. These thin film, ceramic strain gages have several advantages over metal strain gages including a large gage factor and increased chemical and electrical stability at very high temperatures. Electron spectroscopy for chemical analysis (ESCA) of ITO films deposited onto high purity alumina substrates and subjected to temperatures up to 1400 °C indicated that the ITO films had undergone an interfacial reaction with the substrate. In addition, the interfacial reaction appears to have been responsible for high temperature stabilization through the formation of an ITO/Al2O3 solid solution. When similar ITO films were deposited onto alumina with a platinum diffusion barrier, there was no evidence of an interfacial reaction. Thermodynamic calculations indicated that bulk ITO may not be stable in air ambients at temperatures above 1300 °C but the alumina substrates stabilized the ITO to temperatures well beyond this value.

Thin films of PVDF were deposited from a variety of solution conditions and examined by IR spectroscopy and scanning electron microscopy. Methods to rapidly assess the film thickness and the phase composition of PVDF films have been developed. In particular, the formation of the ferroelectric β phase can be controlled by the composition of the solvent, notably the water content. Using a hydrated salt in the casting solvent reproducibly formed films with high β phase content. However, the water also induces increased surface roughness in the deposited films. The nature of the PVDF films also is influenced by the kinetics of the evaporation process, and this was followed by IR spectroscopy. The rate constants depend on both the film thickness and the gas-phase humidity, indicating that solvent diffusion is an important contributor to both the bulk phase composition and the surface structure of the polymer films.

A robust strain gage based on alloys of indium–tin–oxide (ITO) has been developed, which is capable of measuring strain at temperatures up to 1450 °C. These thin film sensors are ideally suited for in-situ strain measurement in harsh environments since they are non-intrusive, have minimal impact on vibration patterns due to their negligible mass and are robust enough to withstand the high ‘g’ loading associated with rotating components. Thus, this ITO strain gage is well suited to meet instrumentation requirements in advanced propulsion systems. Static strain tests performed at temperatures as high as 1400 °C have resulted in a relatively large and repeatable piezoresistive response. However, in the vicinity of 950 °C, a change in sign of the piezoresistive response from −G to +G was observed, suggesting that the active ITO strain element had been converted from a net ‘n-carrier’ to a net ‘p-carrier’ semiconductor. The ‘n’ to ‘p’ transition has been shown to be reversible over many temperature cycles from room temperature to 1400 °C. This repeatability implies that the carrier species is the predominate factor controlling the observed changes in the resistance and gage factor. Consistent with this change in sign of the gage factor was a change in the sign of the slope of emf vs. temperature (dV/dT) for hot probe measurements made on the same ITO films that were thermally cycled over the temperature range (600 °C to 1300 °C). This finding supports the premise that change in sign of the gage factor from −G to +G occurred within the same temperature range (∼950 °C) and that a change in the charge carrier type was responsible for observed transition in both cases.

A ceramic thin film strain gage has been developed for static strain measurements at temperatures up to 1400°C. These thin film sensors are ideally suited for in situ strain measurement in harsh environments, such as those encountered in the hot sections of gas turbine engines. However, the wide bandgap semiconductor used as the active strain elements in these devices also exhibited a large temperature coefficient of resistance (TCR). Thus, to reduce the apparent strain or thermally induced strain in these static strain sensors to an acceptable level, a novel self-compensation scheme was demonstrated using thin film platinum resistors placed in series with the active strain element. A mathematical model was developed and design rules were established for the self-compensated circuitry using this approach. Close agreement between the model and actual static strain results have been achieved using this approach, and reliable static strain measurements have been made over an extended temperature range.

The α phase (or form II) of poly(vinylidene fluoride), PVF2, produced from an acetone/N,N-dimethylformamide (DMF) solution gave different surface morphologies, depending on the solvation temperature of the PVF2 solution, the DMF concentration, and the relative humidity when deposited onto a smooth silicon substrate. Solutions prepared at less than 30 °C always gave rise to transparent films. However, solutions prepared at temperatures greater than 50 °C resulted in a rough and opaque, white surface when depositing at high humidity and in a transparent film when depositing at low humidity. The aging behavior of the polymer solutions as measured by the viscosity revealed an enormous dependence on the DMF concentration. Optical light and atomic force microscopies and infrared spectroscopy characterized the differences of these film surfaces.

A new type of biosensor for pathogens has been developed. The sensor produces spectral fingerprints of biological systems by using surface-enhanced infrared absorption (SEIRA) spectroscopy. Antibodies were immobilized onto a 10-nm-thick film of gold which had been previously deposited on a Si wafer. SEIRA spectra of the antibodies measured in the external reflection mode exhibited two new bands at 1085 and 990 cm-1. These new bands were observed with p-polarized radiation but were absent with s-polarized radiation. The spectrum of water on the surface of the sensor was observed under both directions of polarization. The sensor was first tested with a model system consisting of glucose oxidase (GOX) and the antibodies for glucose oxidase (anti-GOX). In addition to the bands due to the anti-GOX at 1085 and 990 cm-1, new bands were observed at 1397, 1275, and 930 cm-1 when the GOX antigens were present. The same type of sensor was prepared for Salmonella (SAL) by immobilizing antibodies for Salmonella (anti-SAL) on a gold-surfaced Si water. The SEIRA spectra for anti-SAL antibodies were very similar to those for anti-GOX, with bands at 1085 and 990 cm-1; however, a sharp new band was observed at 1045 cm-1 after the sensor was exposed to the SAL antigens. In addition to specific new bands due to antigens, both GOX and SAL sensors exhibited changes in the regions of water absorptions at approximately 3500 and 850 cm-1 when the antigens were present.

Overcoat protection schemes for thin-film sensors have typically focused on inhibiting the growth of native oxides formed on the sensor surface, rather than on improving the passivating nature of these native oxides. In this work, 730Asputtered Cr overcoats and heat treatments in varying partial pressures of oxygen enhance the passivating nature of native Cr 2 O 3 formed on PdCr thin-film strain gages. Results of dynamic strain tests are presented and the implications in using this approach are discussed. PdCr gages with sputtered Cr overcoats withstood 12,000 strain cycles of 1100 μe during 100 h of testing at a temperature of 1000°C in air. Gage factors of 1.3 with drift rates as low as 0.1 ω h 1 were achieved for devices having a nominal resistance of approximately 100 ω. TCRs ranging from +550 ppm °C 1 to + 798 ppm°C 1 were realized depending on the overcoat scheme used and the thermal history of the film. Possible mechanisms for an anomaly in the electrical characteristics of these films at 800°C and improvements in stability due to the use of overcoats are presented.

Hot section temperatures in gas turbine engines can reach 1500°C, an environment in which implementing sensors for in situ measurement is rather difficult. Therefore, there is a need to develop instrumentation for these turbine engines that can survive the harsh conditions. Thin film instrumentation is targeted for this purpose because it is non-invasive to the physical and thermodynamic properties of the engine components. Type-S and type-K thin film and wire thermocouples are currently being utilized for surface temperature measurement during the testing phase of gas turbine engines. These metallic thin film thermocouples suffer from drift and degradation due to microstructural changes and oxidation. Therefore, thin film thermocouples based alternatively on platinum:palladium and indium oxynitride:indium tin oxynitride, as well as their oxide counterparts, have been developed and proved to be more stable than conventional type-S and type-K thin film thermocouples operating in these high temperature (> 1000°C) oxidizing environments. Furthermore, the metallic and ceramic thin film thermocouples described within this paper exhibited remarkable stability and have drift rates similar to bulk (wire) thermocouples operating in the same temperature range.

An orthogonal detection system comprised of two independent sensing platforms integrated into a single sensor was recently developed to detect trace levels of explosives including TATP, 2,6-DNT and ammonium nitrate in the vapor phase. The orthogonal sensor consists of a thermodynamic based sensing platform, which measures the heat effect associated with the interaction of specific target molecules with various metal oxide catalysts (ZnO and SnO2), and a conductometric sensing platform, which monitors the electrical conductivity changes of the same catalyst when exposed to the explosive molecules. The experimental results to date indicated that the orthogonal sensor platform was very effective in identifying explosives at the ppb level. At the same time, the additional sensor signature provides redundancy in the sensor response, which helps mitigate false positives. The orthogonal sensor response is linear when plotted as a function of target molecule concentration. XPS was employed to confirm the oxidation state of metal oxides used as catalysts, which were a function of deposition and post deposition heat treatments.

Combinatorial chemistry techniques were used to study the thermoelectric properties of sputtered thin films in the system copper oxide (CuO) and indium oxide (In2O3). Seven hundred seventy thin film thermocouples or combinatorial library elements were simultaneously deposited, each with a unique spatially dependent chemistry, based on the relative position of the thermocouples to each sputtering target. The resulting thermoelectric properties of each element were determined along with electrical resistivity as a function of composition. Energy dispersive spectroscopy was used to identify the composition of each thermo-element, and electron and X-ray diffraction were used to determine the degree of crystallinity and phases present. Transmission electron microscopy was used to characterize the microstructure of selected thermo-elements. A change in sign of the thermoelectric voltage was observed in the thermo-element containing 40.0 atomic percent indium, which suggests a change in the dominant carrier type occurred, from p-type to n-type. Based on this finding, the fabrication of thermoelectric p-n junctions using the same base Cu-In-O semiconductor appears feasible.