<p>We have developed fiber-based optical thermocouples (OTCs) for fast temperature sensing in extreme environments. Our OTCs consist of a thin film of dysprosium-doped yttrium aluminum garnet (Dy:YAG)—a well-known two-color thermometry phosphor—deposited on the end of a sapphire fiber using pulsed laser deposition. Temperature sensing is achieved by comparing the relative intensities of photoluminescence arising from two closely spaced Dy<sup>3 + </sup> excited states. Using a combination of time-gated detection and blackbody background subtraction, we are able to measure Dy:YAG’s photoluminescence up to 2033 K, which is one of the highest temperatures obtained in literature. However, we are only able to use the photoluminescence spectra for temperature sensing up to 1773 K due to poor signal-to-noise ratio for higher temperatures. These results suggest the possibility of measuring higher temperatures with time-gated detectors designed for low-light levels. After characterizing the fiber-based OTCs’ temperature response, we next demonstrate their functionality using subsecond pulsed CO<sub>2</sub> laser heating using both intensified charge-coupled device detection and a photodiode-based software time-gating technique. In the lab, we have utilized this technique to measure temperatures at rates up to 80 kHz. In addition, we comment on the applicability of OTCs to fast temperature sensing in turbulent flows and estimate rise times on the order of several hundred microseconds for a 1-μm OTC film.</p>
Fossil fuel combustion processes generate CO<sub>2</sub>, a greenhouse gas of considerable concern. To reduce this emission, the National Energy Technology Laboratory (NETL) is evaluating alternative combustion approaches, including chemical looping combustion (CLC). This technique generates relatively pure CO<sub>2</sub>, suitable for subsequent capture and storage. CLC uses oxygen-carrier particles (OCPs) such as iron oxides, copper oxides, calcium sulfates, etc. to provide oxygen for the combustion process. Optimization of the overall combustion process requires knowledge of the oxidation state (e.g., content of Fe<sub>2</sub>O<sub>3</sub> vs. Fe<sub>3</sub>O<sub>4</sub>) of the OCPs during the different stages of the CLC process. Unfortunately, the ability to make on-line measurements of the oxidation state of OCPs in harsh environments is lacking and new sensors need to be developed.<p> </p> We are evaluating non-contact, stand-off Raman spectroscopy to determine the relative concentrations of the oxidized and reduced forms of OCPs at temperatures between 800 °C and 1000 °C, and pressures of about 10 atm. Using cw and pulsed Raman spectroscopy, in combination with a pressurized high-temperature sample chamber, we have optimized the operating parameters such as laser wavelength, laser intensity, collection optic design, focal spot size, etc. and measured Raman spectra of various OCP materials at high temperatures. To extract from the Raman spectra relevant information such as the concentration ratio of a material in different oxidation states, the measured data needs to be processed, and statistical modeling and multivariate calibration need to be performed.
Authentication/tamper-indication is required in a wide range of applications, including nuclear materials management and product counterfeit detection. State-of-the-art techniques include reflective particle tags, laser speckle authentication, and birefringent seals. Each of these passive techniques has its own advantages and disadvantages, including the need for complex image comparisons, limited flexibility, sensitivity to environmental conditions, limited functionality, etc. We have developed a new active approach to address some of these short-comings. The use of an active characterization technique adds more flexibility and additional layers of security over current techniques. Our approach uses randomly-distributed nanoparticles embedded in a polymer matrix (tag/seal) which is attached to the item to be secured. A spatial light modulator is used to adjust the wavefront of a laser which interacts with the tag/seal, and a detector is used to monitor this interaction. The interaction can occur in various ways, including transmittance, reflectance, fluorescence, random lasing, etc. For example, at the time of origination, the wavefront-shaped reflectance from a tag/seal can be adjusted to result in a specific pattern (symbol, words, etc.) Any tampering with the tag/seal would results in a disturbance of the random orientation of the nanoparticles and thus distort the reflectance pattern. A holographic waveplate could be inserted into the laser beam for verification. The absence/distortion of the original pattern would then indicate that tampering has occurred. We have tested the tag/seal’s and authentication method’s tamper-indicating ability using various attack methods, including mechanical, thermal, and chemical attacks, and have verified our material/method’s robust tamper-indicating ability.
We perform laser-induced photodissociation fluorescence spectroscopy on mononitrotoluenes (MNTs) and dinitrotoluenes (DNTs) in the vapor phase and observe the spectrally overlapping fluorescence from nitric oxide (NO) and carbon (C). Energy-dispersive x-ray spectroscopy (EDS) and Raman spectroscopy of deposits found in the sample chamber confirm the presence of carbon. By comparing the observed fluorescence intensities with the Franck-Condon factors for NO, we are able to identify the presence or absence of fluorescence from carbon. 2-nitrotoluene and 4- nitrotoluene show carbon fluorescence for gate delays of up to 500 ns, while 2,4-dinitrotolune, 3,4-dinitrotolune, and 2,6-dinitrotolune show carbon fluorescence for gate delays of at least up to 1500 ns. The spectroscopic signal from atomic carbon in the vapor phase is observed at concentrations as low as 10 ppt. Based upon the observed S/N, detection at even lower concentrations appears feasible. Several non-nitrotoluene molecules including nitrobenzene, benzene, toluene, and CO<sub>2</sub>, are tested under identical conditions, but do not show any carbon emission. The presence of extra NO (simulation of NO pollutants) in the samples improves the S/N ratio for the detection of carbon. Energy transfer from laser-excited molecular nitrogen to NO, multiple decomposition channels in the electronic excited state of the nitrotoluene molecules, and interaction of NO with the excited-state decomposition process of the nitrotoluene molecules may all play a role.
We have synthesized nanophase yttria via gas-phase condensation with CO<sub>2</sub>-laser heating. Transmission Electron Microscopy (TEM) shows particles of about 3 - 8 nm in diameter. X-ray diffraction (XRD) indicates a diffracting domain size L440 of 2 - 5 nm, and a lattice constant a0440 ranging from 10.60 Å to 10.70 Å. Vacuum sintering, two-stage sintering, and combined vacuum/two-stage sintering are used to consolidate the Y<sub>2</sub>O<sub>3</sub> nanoparticles into ceramics.
Nanophase yttria co-doped with erbium and ytterbium (Er,Yb:Y<sub>2</sub>O<sub>3</sub>) was successfully synthesized using gas-phase condensation. As-prepared nanoparticles were about 2 nm in diameter, while annealed nanoparticles were about 20 nm in diameter. Two-stage sintering of the nanoparticles led to ceramics with grain sizes of less than 160 nm in diameter, while vacuum sintering led to ceramics with grain sizes of about 5 μm. The annealed nanopowder, as well as the two-stage and vacuum sintered ceramics showed fluorescence from erbium and ytterbium ions.
Avionic engineers are increasingly replacing CRTs with LCDs in both head-up displays and head down displays. Indeed, LCDs have made considerable progress with regards to adequate brightness, dimmability and reliability. Image quality issues in terms of resolution, viewing angle, gray scale and color gamut have also been improved. However, much more progress is required and manufacturing cost cannot be ignored. Quantum Vision is actively developing an alternate approach, the resonant microcavity anode. This emissive component is based upon rugged thin film phosphors capable of generating high brightness and high resolution images. Current theoretical predictions indicate that resonant microcavities can lead to an order of magnitude increase in brightness while having a cost profile consistent with high volume products.
In this paper we will outline the theoretical and practical advantages of projection displays based on resonant microcavities. We will present results recently obtained for Eu:Y<SUB>2</SUB>O<SUB>3</SUB> activated microcavities, compare them with theoretical models and discuss the impact of such devices. The extension to other optical systems will also be discussed.