Geunsik Lim

Radiative properties of thermal barrier coatings at high temperatures

Surface radiation represents an important mechanism for heat loss at high temperatures. Thermal control may require improved heat dissipation of highly emitting surfaces in order to keep the maximum temperature below a certain critical value in high-temperature turbine systems. Emissivity allows determining the surface temperature based on thermal spectra measurement or thermal imaging of the turbine blades. In this study, the emissivities of different coating samples including the metal substrate have been measured over a wavelength range 0.4–1.08 µm in the temperature range 400–1150 °C and high values of emissivities are observed. The data are also compared with the theoretical values of emissivity. The comparison between the theory and experiment are, however, poor because the experimental data are obtained at high temperatures, while the theoretical values are calculated using the values of refraction and absorption indices at room temperature in the Fresnel reflection formula. The optical constants of the samples are computed by the Lorentz elastically bound electron theory of insulator and the Drude free-electron theory of metals.

Laser optical gas sensor by photoexcitation effect on refractive index

Laser optical gas sensors are fabricated by using the crystalline silicon carbide polytype 6H-SiC, which is a wide-bandgap semiconductor, and tested at high temperatures up to 650 degrees C. The sensor operates on the principle of semiconductor optics involving both the semiconductor and optical properties of the material. It is fabricated by doping 6H-SiC with an appropriate dopant such that the dopant energy level matches the quantum of energy of the characteristic radiation emitted by the combustion gas of interest. This radiation changes the electron density in the semiconductor by photoexcitation and, thereby, alters the refractive index of the sensor. The variation in the refractive index can be determined from an interference pattern. Such patterns are obtained for the reflected power of a He-Ne laser of wavelength 632.8 nm as a function of temperature. SiC sensors have been fabricated by doping two quadrants of a 6H-SiC chip with Ga and Al of dopant energy levels E(V)+0.29 eV and E(V)+0.23 eV, respectively. These doped regions exhibit distinct changes in the refractive index of SiC in the presence of carbon dioxide (CO(2)) and nitrogen monoxide (NO) gases respectively. Therefore Ga- and Al-doped 6H-SiC can be used for sensing CO(2) and NO gases at high temperatures, respectively.

Optical response of laser-doped silicon carbide for an uncooled midwave infrared detector.

An uncooled mid-wave infrared (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. 4H-SiC is one of the polytypes of crystalline silicon carbide and a wide bandgap semiconductor. The dopant creates an energy level of 0.30  eV, which was confirmed by optical spectroscopy of the doped sample. This energy level corresponds to the MWIR wavelength of 4.21  μm. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the semiconductor. This process modifies the electron density, which changes the refractive index, and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam, such as a He-Ne laser. This capability of measuring the detector response remotely makes it a wireless detector. The variation of refractive index was calculated as a function of absorbed irradiance based on the reflectance data for the as-received and doped samples. A distinct change was observed for the refractive index of the doped sample, indicating that the detector is suitable for applications at the 4.21  μm wavelength.

Noise sources and improved performance of a mid-wave infrared uncooled silicon carbide optical photodetector

An uncooled photon detector is fabricated for the mid-wave infrared (MWIR) wavelength of 4.21 μm by doping an n-type 4H-SiC substrate with gallium using a laser doping technique. The dopant creates a p-type energy level of 0.3 eV, which is the energy of a photon corresponding to the MWIR wavelength 4.21 μm. This energy level was confirmed by optical absorption spectroscopy. The detection mechanism involves photoexcitation of carriers by the photons of this wavelength absorbed in the semiconductor. The resulting changes in the carrier densities at different energy levels modify the refractive index and, therefore, the reflectance of the semiconductor. This change in the reflectance constitutes the optical response of the detector, which can be probed remotely with a laser beam such as a He-Ne laser and the power of the reflected probe beam can be measured with a conventional laser power meter. The noise mechanisms in the probe laser, silicon carbide MWIR detector, and laser power meter affect the performance of the detector in regards to aspects such as the responsivity, noise equivalent temperature difference (NETD), and detectivity. For the MWIR wavelengths of 4.21 and 4.63 μm, the experimental detectivity of the optical photodetector of this study was found to be 1.07×10(10)  cm·Hz(1/2)/W, while the theoretical value was 1.11×10(10)  cm·Hz(1/2)/W. The values of NETD are 404 and 15.5 mK based on experimental data for an MWIR radiation source with a temperature of 25°C and theoretical calculations, respectively.

MWIR room temperature photodetector based on laser-doped silicon carbide

MWIR photon detector in the mid-infrared wavelength (2-5 μm) range is developed using crystalline silicon carbide substrates. SiC, which is a wideband gap semiconductor, is laser-doped to create a dopant energy level corresponding to a quantum of energy for the required operating wavelength of the detector. The photons of the objects in the field of view excite the electrons of the detector, leading to changes in the refractive index. This change in the optical property of the detector can be measured remotely with a laser beam, such as a He-Ne laser beam of wavelength 632.8 nm, which makes it a wireless detector. While many IR detectors require cryogenic cooling (77 K) to suppress thermal generationrecombination processes in order to operate with good detectivity, the SiC-based detector can operate at room temperature with excellent performance. An n-type 4H-SiC substrate has been doped with Ga by a laser doping technique to create a detector element for the MWIR wavelength of 4.21 μm corresponding to the photon energy 0.30 eV. The dopant energy level is confirmed by optical absorption measurements. The change in the refractive index is studied as a function of absorbed irradiance on the detector. The experimental result shows that the Ga-doped 4H-SiC sample can be used for MWIR detectors.

Laser fabrication of silicon carbide detector for gas sensing and focal plane array imaging

A Mid-Wave Infra-Red (MWIR) detector is developed by doping an n-type 4H-SiC with an appropriate dopant to create a dopant energy level that matches with a quantum of energy for the wavelength of interest. The detector absorbs the photons and the absorbed photon energy modifies the electron density in the semiconductor by the photoexcitation, leading to changes in the refraction index. Ga is known to have an energy level of 0.30 eV in n-type 4H-SiC substrates, which corresponds to the wavelength 4.21 μm. A detector was fabricated for the MWIR wavelength of 4.21 μm by doping n-type 4H-SiC with Ga. The dopant energy level was confirmed by optical absorption measurements in the wavelength range of 4 to 5 μm. The optical response of the detector to the wavelength 4.21 μm was determined by measuring the reflectivity of the detector using a He-Ne laser of wavelength 632.8 nm as the probe beam. The reflectivity data were used to calculate the variation in the refraction index of the detector at the MWIR wavelength of interest and the selectivity of the detector was established by testing the sensor response to that of an as-received sample. The comparison yielded a distinct change in the refraction index curve for the detector, indicating that the detector is suitable for applications at the wavelength 4.21 μm.

Silicon carbide novel optical sensor for combustion systems and nuclear reactors

Crystalline silicon carbide is a wide bandgap semiconductor material with excellent optical properties, chemical inertness, radiation hardness and high mechanical strength at high temperatures. It is an excellent material platform for sensor applications in harsh environments such as combustion systems and nuclear reactors. A laser doping technique is used to fabricate SiC sensors for different combustion gases such as CO2, CO, NO and NO2. The sensor operates based on the principle of semiconductor optics, producing optical signal in contrast to conventional electrical sensors that produces electrical signal. The sensor response is measured with a low power He-Ne or diode laser.

Electron dynamics for uncooled MWIR SiC detector for digital imaging

An uncooled mid-wave infrared (MWIR) detector is developed by doping n-type 4H-SiC with Ga using a laser doping technique. Crystalline silicon carbide (SiC) is a wide bandgap covalent semiconductor material with excellent thermomechanical and optical properties. While the covalent bonding between the Si and C atoms allows n-type or p-type doping by incorporating dopant atoms into both the Si and C sites, the wide bandgap enables fabrication of optical detectors over a wide range of wavelengths. Doping SiC with Ga creates an acceptor energy level of 0.30 eV, corresponding to the MWIR wavelength of 4.21 μm. To fabricate the MWIR detector, an n-type 4H-SiC substrate is doped with Ga using a laser doping technique. Photons of wavelength ~4.21 μm excite electrons from the valence band to the acceptor level, altering the electron density, refractive index, and therefore the reflectance of the substrate. This change in reflectance constitutes the detector optical response. To understand the dynamic response of the detector, the photoexcited electron density and lifetime in the acceptor level is theoretically analyzed. This response is experimentally measured by projecting 633 nm radiation from a laser or high power light-emitting diode (LED) array off the detector at an angle towards a CMOS camera, and examining the digital output of the captured images pixel by pixel to determine the relative intensity of the reflected radiation across the detector. Through digital image processing, a distinct difference is observed in the measured intensity of light reflected off the as-received (undoped) detector sample over infrared temperatures ranging from 100°C to 600°C compared to that of the doped sample comprising quadrants characterized by different doping concentrations, evidencing a change in reflectance from MWIR exposure and thus detector response for the Ga doped SiC detector device.

Uncooled silicon carbide sensor producing optical signal

A novel approach will be discussed to design and fabricate sensors for a wide variety of wavelengths by selecting appropriate acceptor levels in a semiconductor material. An n-type 4H-SiC substrate has been doped with gallium using a laser doping method for sensing the MWIR wavelength of 4.21 mm. The incident MWIR photons change the electron densities in the valence band and the acceptor energy levels, modifying the reflectivity of the sensor. This change in the reflectivity is determined with a He-Ne laser as an optical signal and the sensor can be operated at room temperature. The effect of the photon collection optics on the sensor response has been studied. Also the dopant concentration has been found to affect the optical signal.

Laser-Doped SiC as Wireless Remote Gas Sensor Based on Semiconductor Optics

An uncooled SiC-based electro-optic device is developed for gas sensing applications. P-type dopants Ga, Sc, P and Al are incorporated into an n-type crystalline 6H-SiC substrate by a laser doping technique for sensing CO2, CO, NO2 and NO gases, respectively. Each dopant creates an acceptor energy level within the bandgap of the substrate so that the energy gap between this acceptor level and the valence band matches the quantum of energy emitted by the gas of interest. The photons of the gas excite electrons from the valence band to the acceptor level, which alters the electron density in these two states. Consequently, the refractive index of the substrate changes, which, in turn, modifies the reflectivity of the substrate. This change in reflectivity represents the optical signal of the sensor, which is probed remotely with a laser such as a helium-neon laser. Although the midwave infrared (3-5 mm) band is studied in this paper, the approach is applicable to other spectral bands.