Three electrodes gas sensor based on ITO thin film

IntroductionThese days, the use of various industrial and personal equipment using hazardous gases is on the rising such as H2 for fuel cells. To utilize these gases, the low concentration sensor(~ppm) for leakage detection as well as the high concentration sensor (~%) for monitoring hazardous gasses in the industrial sites are essential. Semiconductor type and electrochemical type sensors are commonly used for gas detection but they are not suitable for high concentration because they are easily saturated in high concentration. In contrast, thermal conductivity type sensors are used as high concentration sensors because they measure resistance change of the heat loss of heated wire(heater) to a gas environment without sensor signal saturation in high concentration. However conventional thermal conductivity type sensors require large power (hundreds of milliwatts to watts) and relatively large size. In the previous study, a microscale bridge-type heater-based gas sensor was developed using MEMS technology for low power consumption and small size [1, 2]. This sensor used the 3ω-method to measure accurately heat loss to the gas with a high signal to noise ratio.Here, we developed a 3ω-method based high concentration gas sensor using a suspended nano-sized wire heater to minimize the required power and size. The suspended nano-sized wire heater consists of a suspended nanowire backbone, an eave structure and a thin gold heater on the suspended nanowire. The suspended nanowire backbone structure was fabricated by pyrolyzing suspended photoresist wire, and a thin gold layer as a heater material, which ensures high sensitivity due to its high-temperature coefficient of resistance, was selectively coated on the suspended carbon nanowire by virtue of the eave structure. The 3ω-method based gas sensor exhibited high sensitivity and wide linear range with ultra-small power consumption because of its suspended architecture, small size, high aspect ratio and high surface to volume ratio. In addition, all the processes of the 3D nanostructures were carried out at a wafer-level enabling cost-effective manufacturing owing to novel eave structures and carbon-MEMS processes (consisting of photolithography and pyrolysis) [3]. Method The suspended nanowire heaters were fabricated by a three-step process. First, eave structures for the selective metal coating on a suspended carbon nanowire was fabricated by oxide etching and isotropic silicon etching. Then, suspended microscale suspended polymer wires were patterned by two successive photolithography processes and the micro polymer wire was converted into a carbon nanowire by a dramatic volume reduction in pyrolysis. The pyrolysis temperature was set to 700℃ for low carbon electrical conductivity so that the electrical charge flows only through the gold. The last step was the deposition of gold as a heater line. A 50-nm-thick gold was deposited using evaporation. Owing to the eave structure and anisotropic evaporation, the gold layer is solely connected through the suspended wire as shown in Figure a. Results and Conclusions The gold-coated suspended carbon nanowire and the eave structure were well-defined as shown in Figure b. The gold-coated nanowire exhibited a very stable 3ω voltage output signal compared to bare carbon nanowire with high conductivity (Figure c). The thermal penetration depth reduces as the input frequency increases, the 3ω voltage decreases with increasing input frequency (Figure d). The 3ω voltage linearly changed with gas concentration depending on the relative thermal conductivity of target gas in comparison to that of N2 (Figure e). Therefore, selective gas detection is feasible. Figure f showed the response and recovery time of the sensor when it is exposed to 100% Ar and 5% H2. The sensor measures a gas concentration based on the thermal equilibrium between heater structure and gas environment only. Thus, very fast response and recovery time within 3s can be achieved even at high gas concentrations. In addition, the power consumption was only 0.107 mW due to suspended nanowire-type heater configuration.

Introduction These days, the use of various industrial and personal equipment using hazardous gases is on the rising such as H2 for fuel cells. To utilize these gases, the low concentration sensor(~ppm) for leakage detection as well as the high concentration sensor (~%) for monitoring hazardous gasses in the industrial sites are essential. Semiconductor type and electrochemical type sensors are commonly used for gas detection but they are not suitable for high concentration because they are easily saturated in high concentration. In contrast, thermal conductivity type sensors are used as high concentration sensors because they measure resistance change of the heat loss of heated wire(heater) to a gas environment without sensor signal saturation in high concentration. However conventional thermal conductivity type sensors require large power (hundreds of milliwatts to watts) and relatively large size. In the previous study, a microscale bridge-type heater-based gas sensor was developed using MEMS technology for low power consumption and small size [1, 2]. This sensor used the 3ω-method to measure accurately heat loss to the gas with a high signal to noise ratio.Here, we developed a 3ω-method based high concentration gas sensor using a suspended nano-sized wire heater to minimize the required power and size. The suspended nano-sized wire heater consists of a suspended nanowire backbone, an eave structure and a thin gold heater on the suspended nanowire. The suspended nanowire backbone structure was fabricated by pyrolyzing suspended photoresist wire, and a thin gold layer as a heater material, which ensures high sensitivity due to its high-temperature coefficient of resistance, was selectively coated on the suspended carbon nanowire by virtue of the eave structure. The 3ω-method based gas sensor exhibited high sensitivity and wide linear range with ultra-small power consumption because of its suspended architecture, small size, high aspect ratio and high surface to volume ratio. In addition, all the processes of the 3D nanostructures were carried out at a wafer-level enabling cost-effective manufacturing owing to novel eave structures and carbon-MEMS processes (consisting of photolithography and pyrolysis) [3]. Method The suspended nanowire heaters were fabricated by a three-step process. First, eave structures for the selective metal coating on a suspended carbon nanowire was fabricated by oxide etching and isotropic silicon etching. Then, suspended microscale suspended polymer wires were patterned by two successive photolithography processes and the micro polymer wire was converted into a carbon nanowire by a dramatic volume reduction in pyrolysis. The pyrolysis temperature was set to 700℃ for low carbon electrical conductivity so that the electrical charge flows only through the gold. The last step was the deposition of gold as a heater line. A 50-nm-thick gold was deposited using evaporation. Owing to the eave structure and anisotropic evaporation, the gold layer is solely connected through the suspended wire as shown in Figure a. Results and Conclusions The gold-coated suspended carbon nanowire and the eave structure were well-defined as shown in Figure b. The gold-coated nanowire exhibited a very stable 3ω voltage output signal compared to bare carbon nanowire with high conductivity (Figure c). The thermal penetration depth reduces as the input frequency increases, the 3ω voltage decreases with increasing input frequency (Figure d). The 3ω voltage linearly changed with gas concentration depending on the relative thermal conductivity of target gas in comparison to that of N2 (Figure e). Therefore, selective gas detection is feasible. Figure f showed the response and recovery time of the sensor when it is exposed to 100% Ar and 5% H2. The sensor measures a gas concentration based on the thermal equilibrium between heater structure and gas environment only. Thus, very fast response and recovery time within 3s can be achieved even at high gas concentrations. In addition, the power consumption was only 0.107 mW due to suspended nanowire-type heater configuration.

Influence of defects on rare-gas diffusion in solids

Surface acoustic wave (SAW) devices can be used as vapor phase chemical sensors. If we make use of SAW devices with high frequency properties and multi-channel structures, highly sensitive and multi gas sensors can be realized. However, it is generally difficult to deposit different gas sensitive films on the propagation paths of monolithic multi-channel SAW devices. This paper describes the new coating method by which several types of gas-sensitive molecular films can be coated on the paths. The method is called partial casting method on water surface. It makes use of the water surface segmented by a trapezoidal frame, and the molecular film spread in the frame is compressed by the vertical movement of the frame and deposited on the substrate placed on the frame top. Several bilayers of dialkyl polyion complex and dimyristoyl phosphatidyl ethanolamine were coated on 36°Y-X LiTaO 3 SAW delay line oscillators, and odor sensor characteristics were successfully measured. It was confirmed from the present experiments that the method can be applicable to fabrication of multi-channel SAW chemical sensors with various kinds of molecular films.

In this paper, effect of catalytic configuration on the sensing properties of nanoparticle gas sensitive thick film was investigated. Two types of catalytic configuration, mono and binary, were made on the nanoparticle. In case of mono catalytic system, Pd or Pt catalyst was doped onto the nanoparticle, respectively. In case of binary catalytic system, Pd and Pt was doped simultaneously with concentration ratio of 1:2 to 2:1 onto the nanoparticle. After doping, gas sensitive thick film was printed on alumina substrate and heat-treated at 450 to . Gas sensing properties was evaluated using 500 to 10,000 ppm gas. As a result, gas sensitive thick film with binary catalytic system showed unstable phenomena that the gas sensitivity was changed according to aging time. In contrary, the mono catalytic system showed relatively stable phenomena despite of aging time. Especially, gas sensitive thick film doped with Pt catalyst and heat-treated at showed good sensing properties such as 0.57 of and very small variation within after aging for 5 hours, and response time was very short less than 20 seconds.

The annealing effects on the properties of ITO and pure In2O3 thin films have been investigated. The thin films were deposited with various O2 flow ratios to total gas flow by pulsed dc magnetron sputtering. The post-deposition annealing of the thin films was carried out for 30 minutes at various temperatures ranging up to 500 degrees C in air. It was found through the comparison of the carrier density of ITO and In2O3 thin films that the carrier electrons of the ITO thin films came from both of the dopant Sn and oxygen vacancies under the annealing less than 400 degrees C. Therefore, the ITO thin films deposited with lower O2 flow ratio exhibited higher carrier density due to many oxygen vacancies; in consequence, they exhibited lower resistivity at the annealing up to 400 degrees C. On the other hand, the carrier density of ITO thin films was almost identical regardless of O2 flow ratio when they were annealed at 500 degrees C. This fact indicates that most carrier electrons of the ITO thin films were brought by the dopant Sn at the annealing temperature of 500 degrees C. However, the ITO thin films deposited with lower O2 flow ratio exhibited higher Hall mobility; as a result, they showed lower resistivity at the annealing of 500 degrees C. Atomic force microscope, X-ray diffraction and X-ray reflectivity measurements revealed that the ITO thin films deposited with lowe O2 flow ratio exhibited dense structure even after they were annealed at 500 degrees C. Hence, the carrier electrons of the dense ITO thin films deposited with low O2 flow ratio can conduct better, as a result, the ITO thin films exhibited high Hall mobility and low resistivity.

New gas sensitive films containing anthocyanin/curcumin for real-time monitoring of strawberries quality

The sensitivity of a nanocrystalline tin oxide gas sensitive thick film was successfully enhanced by controlling its resistance. For the precise control of the resistance, low temperature catalyst doping (LTCD) method was applied below 300/spl deg/C to dope a Pt catalyst onto nanocrystalline tin oxide particles. Also, a gas sensitive thick film was fabricated using the nanocrystalline tin oxide particles having a size smaller than 10 nm, and it led to an enhanced sensitivity with respect to methane gas. That is, a good and stable sensitivity (R/sub 3500//R/sub 1000/) of 0.64 was achieved after aging for 5 hours at 400/spl deg/C by doping with 5wt% of Pt catalyst onto the tin oxide particle.

Acetone detection is essential in industrial, pharmaceutical, and environmental applications due to its toxicity at elevated concentrations. This study reports the fabrication and optical analysis of a D-shaped fiber optic sensor coated with indium tin oxide (ITO, 10 nm), gold (Au, 40 nm), and bilayer Au: ITO (40:10 nm) thin films for room-temperature acetone sensing. The D-shaped geometry was produced via controlled mechanical polishing, while thin films were deposited using high-vacuum electron-beam evaporation. The sensing mechanism is governed by evanescent-field absorption and wavelength-resolved spectral modulation induced by acetone adsorption. Under static testing conditions across 0–870 ppm, the ITO coating achieved a sensitivity of 16 at 350 ppm (LOD = 66 ppm), Au exhibited 14 at 350 ppm (LOD = 75 ppm), and Au: ITO demonstrated 10.2 at 700 ppm (LOD = 205.9 ppm). The bilayer structure enhanced spectral stability at higher gas concentrations. The proposed low-cost, compact, and non-electrical configuration demonstrates strong potential for selective, real-time acetone vapor monitoring in industrial and environmental settings.

Surface plasmon resonance (SPR) technique is an easy and reliable method for detecting very low concentration of toxic gases at room temperature using a gas sensitive thin film layer. In the present work, a room temperature operated NH3 gas sensor has been developed using a laboratory assembled SPR measurement setup utilising a p-polarized He-Ne laser and prism coupling technique. A semiconducting gas sensitive tin oxide (SnO2) layer has been deposited under varying growth conditions (i.e., by varying deposition pressure) over the gold coated prism (BK-7) to excite the surface plasmon modes in Kretschmann configuration. The SPR reflectance curves for prism/Au/SnO2/air system for SnO2 thin films prepared at different sputtering pressure were measured, and the SnO2 film deposited at 10 mT pressure is found to exhibit a sharp SPR reflectance curve with minimum reflectance (0.32) at the resonance angle of 44.7° which is further used for sensing NH3 gas of different concentration at room temperature. The SPR reflectance curve shows a significant shift in resonance angle from 45.05° to 58.55° on interacting with NH3. The prepared sensor is found to give high sensing response (0.11) with high selectivity towards very low concentration of NH3 (0.5 ppm) and quick response time at room temperature.

Ammonia and greenhouse gas emissions from fattening pig house with two types of partial pit ventilation systems

Resistive losses in silicon heterojunction (SHJ) solar cells are partly linked to transport barriers at the amorphous silicon/crystalline silicon (a-Si:H/c-Si) and transparent conductive oxide (TCO)/a-Si:H interfaces. A key parameter is the position of the Fermi-level on either side of the junction which we modify by a systematic doping variation of the amorphous silicon and the transparent conductive oxide. We identify the charge carrier concentration to be the main driver for low contact resistance. For a-Si:H, this is achieved by using a sufficient but not too high doping gas concentration during deposition. For indium tin oxide (ITO) and aluminum zinc oxide (AZO), no or only a very low oxygen (O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gas concentration during deposition is needed. We show that a stack of low-oxygen ITO interlayer and an oxygen-rich ITO “bulk” layer is not only an effective means to combine efficient transport and low TCO absorption but also to improve the thermal stability of the a-Si:H/TCO/metal contact resistivity (ρ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sup> ). Such a layer stack helps to relax the constraints regarding the optoelectrical performance and improves the efficiency of SHJ solar cells.

This article concerns the modeling of the SO2 absorption and desorption by falling drops through air with low to high concentration of sulfur dioxide. In the liquid phase, a model based on local scales, interfacial liquid friction velocity and drop size diameter is used. In the continuous gas phase a more classical model is applied. Data obtained by the modeling of the SO2 absorption and desorption by a single water drop are compared to published experimental results and a fairly good correspondence was found. On the other hand, the model shows that at high gas concentration (&gt; 1 %) the internal resistance to diffusion dominates, while at low gas concentration (&lt; 1000 ppb) the external resistance to diffusion dominates.

Technetium‐99 is a mobile, long‐lived radionuclide and environmental risk driver at some nuclear waste sites. The feasibility of decreasing 99Tc mobility in vadose zone sediments using H2S and NH3 gases was evaluated in laboratory experiments. In untreated sediments, 75 to 95% of the 99Tc was leachable. Using combinations of H2S and NH3 gases, the 99Tc mobility was reduced to 14 to 48%. Individual H2S or NH3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H2S gas created reducing conditions at the pore water–mineral interface, which temporarily reduced and precipitated 99Tc, while the NH3 gas created alkaline pore water that caused mineral dissolution. As the pH neutralized, subsequent aluminosilicate precipitation probably coated 99Tc precipitates and rendered them less mobile. Surface phase analysis showed that 99Tc was associated with weathered basalt clasts and S, possibly from the precipitation of TcSx. Treatment performance was nearly the same at different 99Tc concentrations (1.3–240.5 Bq g−1), water contents (1–8%), and gas injection rates but was sensitive to gas concentrations. Low gas concentrations (&lt;3%) had insufficient reductant or slower mineral dissolution. High gas concentrations (&gt;30%) formed an NH4SH precipitate. The 14 to 48% mobile 99Tc remaining after gas treatment may have been caused by the limited time for aluminosilicates to precipitate in our experiments. Degradation of added NH3 was not observed during the 3‐mo experiment. Overall, this study showed that combined H2S and NH3 gas treatment of low‐water‐content sediments can be applied to significantly decrease 99Tc mobility.

Diffusion and trapping of rare gas xenon in calcium fluoride single crystals