Highly sensitive hydrogen sensor based on graphite-InP or graphite-GaN Schottky barrier with electrophoretically deposited Pd nanoparticles

Depositions on surfaces of semiconductor wafers of InP and GaN were performed from isooctane colloid solutions of palladium (Pd) nanoparticles (NPs) in AOT reverse micelles. Pd NPs in evaporated colloid and in layers deposited electrophoretically were monitored by SEM. Diodes were prepared by making Schottky contacts with colloidal graphite on semiconductor surfaces previously deposited with Pd NPs and ohmic contacts on blank surfaces. Forward and reverse current-voltage characteristics of the diodes showed high rectification ratio and high Schottky barrier heights, giving evidence of very small Fermi level pinning. A large increase of current was observed after exposing diodes to flow of gas blend hydrogen in nitrogen. Current change ratio about 700,000 with 0.1% hydrogen blend was achieved, which is more than two orders-of-magnitude improvement over the best result reported previously. Hydrogen detection limit of the diodes was estimated at 1 ppm H2/N2. The diodes, besides this extremely high sensitivity, have been temporally stable and of inexpensive production. Relatively more expensive GaN diodes have potential for functionality at high temperatures

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

One of the most important environment monitoring problems is the detection of oxidizing gases in the ambient air. Negative impact of noxious oxidizing gases (ozone and nitrogen oxides) on human health, sensitive vegetation, and ecosystems is very serious. For this reason, palladium (II) oxide nanostructures have been employed for oxidizing gas detection. Thin and ultrathin films of palladium (II) oxide were prepared by thermal oxidation at dry oxygen of previously formed pure palladium layers on polished poly-Al2O3, SiO2/Si (100), optical quality quartz, and amorphous carbon/KCl substrates. At ozone and nitrogen dioxide detection, PdO films prepared by oxidation at T = 870 K have demonstrated good values of sensitivity, signal stability, operation speed, and reproducibility of sensor response. In comparison with other materials, palladium (II) oxide thin and ultrathin films have some advantages at gas sensor fabrication. Firstly, for oxidizing gas detection, PdO films with p-type conductivity are more perspective than the material with n-type conductivity. Secondly, at ambient conditions, palladium (II) oxide is insoluble in water and does not react with it. These facts are favorable for the fabrication of gas detectors because they make possible to minimize the air humidity influence on PdO sensor response values. Thirdly, the synthesis procedure of PdO films is rather simple and is compatible with planar processes of microelectronic industry

Gas sensing properties of Ceo2 nanostructures

>Magister Scientiae - MScThe industrial safety requirements and environmental pollution have created a high demand to develop gas sensors to monitor combustible and toxic gases. As per specifications of World Health Organization (WHO) and Occupational Safety and Health Administration (OSHA), lengthy exposure to these gases lead to death which can be avoided with early detection. Semiconductor metal oxide (SMO) has been utilized as sensor for several decades. In recent years, there have been extensive investigations of nanoscale semiconductor gas sensor

HEAT TRANSFER AND CHEMICAL PROCESSES IN CHEMICAL VAPOR DEPOSITION REACTOR FOR SYNTHESIS OF CARBON NANOTUBES

A small-scale model of a CVD reactor was built. Axial and radial of major species concentrations and temperature profiles were obtained with a micro gas chromatograph and a fine thermocouple. Those temperature and species concentrations revealed detailed thermal and chemical structures of the CVD reactor. The concentrations of argon plus hydrogen, methane, and C2Hx (C2H2 + C2H4 + C2H6) resulting from xylene decomposition were measured along the CVD at different temperatures. Ferrocene was added to xylene to investigate the effect of a catalyst on composition profiles. The results with ferrocene indicated an increase in CH4 and C2Hx concentrations. At 1000 C and above, the increase of C2Hx concentration is higher than that for CH4. The effect of ferrocene was very minor on the concentration of the gases. Finally composition and temperature profiles were measured and plotted for the radial direction at X=75 cm and T=1200 C. The overall rate constant for the gas-phase reaction was calculated based on the measured species concentration data using the Benson and Shaw reaction mechanism. Our study showed that the Benson and Shaw mechanism could be used in the temperature range lower than 800 C. Also the effect of hydrogen in the syntheses of CNTs, in the CVD reactor using xylene and ferrocene, was studied. Both single-step and two-step methods were applied. In the single-step method, the ferrocene was dissolved in the xylene. In the two step-method the catalyst preparation step was performed first; ferrocene powder was placed in the preheater for a certain period of time and carried by a mixture of argon and hydrogen at fixed concentration to get catalyst nanoparticles deposited on the reactor wall. Xylene then was injected to the reactor. To study the effect of hydrogen, the synthesized materials were observed by SEM and TEM. The results showed that the presence of hydrogen is essential for CNTs to be synthesized by the CVD method, and also the concentration of hydrogen in the reactor has a great effect on the quality of CNTs. The yield of CNTs in the two-step method was slightly higher than that in the one-step method

Nanomaterials characterization and bio-chemical sensing using microfabricated devices

textA variety of nanostructured materials have been synthesized in recent years. These nanomaterials have potential applications in areas spanning computing, energy conversion, sensing, and biomedicine. Because of size confinement effects, furthermore, these nanomaterials are expected to show very different physical properties from those of their bulk counterparts. The measurement of their properties, however, has been very challenging due to their small dimensions. Similarly, it remains a challenge to detect chemical and biomolecular species due to their small dimensions. This dissertation presents the development of microelectromechanical systems (MEMS) devices for the characterization of thermophysical properties of nanomaterials and for the detection of chemical species and biological cells. The thermophysical property of one-dimensional (1D) nanomaterials was measured using a batch-fabricated microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams. Three methods were developed to assemble nanomaterials with the measurement devices. Those three methods include a wet deposition process, an in-situ chemical vapor deposition technique, and an electric-field-assisted assembly method. During the measurement, one membrane is Joule-heated to cause heat conduction through the nanomaterials to the other membrane, allowing for the measurement of thermal conductance and Seebeck coefficient. The electrical conductance can also be measured using the microdevice. The temperaturedependent properties of an individual single-wall carbon nanotubes (SWCNs) and SWCN bundles were measured. Measurement sensitivity, errors, and uncertainty were examined. The obtained thermal conductivity of an individual SWCN is found to be much higher than bundles of SWCNs in the range of 2000-11000 W/m-K at room temperature, in agreement with theoretical predictions. Furthermore, the thermal conductivity of bundles of SWCNs are found to be suppressed by contact resistance between interconnected SWCNs in the bundle. The microdevice has also been integrated with metal oxide nanobelts for chemical sensing. The sensing mechanism is based on surface oxidation-reduction (redox) processes that change the electrical conductance of the nanobelt. The sensor was found to be highly sensitive to inflammable and toxic gas species including nitrogen dioxide (NO2), ethanol, and dimethyl methylphosphonate (DMMP). Furthermore, it eliminated the sensor poisoning effects that have limited the wide use of polycrystalline metal-oxide based sensors. The experiment is a step towards the large scale integration of nanomaterials with microsystems, and such integration via an electric-field-directed assembly approach can potentially enable the fabrication of low-power, ultra-sensitive, and selective integrated nanosensor systems. The electric field manipulation technique has not only been used to assemble nanomaterials with MEMS, but also been used to focus biological cells in a microfluidic channel for cytometry applications. Flow cytometry is a powerful and versatile method of rapidly analyzing large populations of cells and other particulate or molecular analytes that have been captured on the surface of carrier particles. However, the key components of the system, hydrodynamic focusing and optical systems, make conventional cytometers complex, large, and expensive. To eliminate these drawbacks, a dielectrophoretic particle focusing technique combined with MEMS is explored to replace the hydrodynamic focusing mechanism. To focus particles, microelectrodes are patterned on the circumference of the channel to generate AC fringing fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the channel. An ellipticlike microfluidic channel has been fabricated by isotropic etching of soda lime glass wafers and a subsequent wafer-bonding process. Experiments with microbeads and human leukemia HL60 cells and an analysis using a thin shell model indicate that biological cells can be focused using an AC voltage of an amplitude up to 15 Vp-p and a frequency below 100 kHz, respectively. This design eliminates the sheath flow and the fluid control system that makes conventional cytometers bulky, complicated, and difficult to operate, and offers the advantages of a portable standalone instrument as well as a module that could potentially be integrated with on-chip impedance or optical sensors into a micro total analysis system.Mechanical Engineerin

Prevention, Detection, and Suppression of Hydrogen Explosions in Aerospace Vehicles

Prevention, detection, and suppression of hydrogen explosions in aerospace vehicle

Modeling and simulation of ultrahigh sensitive AlGaN/AlN/GaN HEMT based hydrogen gas detector with low detection limit

Presented through this work is a steady state analytical model of the GaN HEMT based gas detector. GaN with high chemical and thermal stability provides promises for detectors in hazardous environments. However, HEMT sensor resolution must be improved to develop high precision gas sensors for automotive and space applications. The proposed model aids in systematical study of the sensor performance and prediction of sensitivities. The linear relation of threshold voltage shift at thermal equilibrium is used in predicting the sensor response. Numerical model for the reaction rates and the electrical dipole at the adsorption sites at the surface and metal/semiconductor interface have been developed and the sensor performance is analyzed for various gas concentrations. The validation of the model has been achieved through surface and interfacial charge adsorption-based gate electrode work function, Schottky barrier, 2DEG and threshold voltage deduction using MATLAB and SILVACO ATLAS TCAD. Further the applicability of gd (channel conductance) as gas sensing metric is also presented. With high ID and gd percentile sensitivities of 118.5% and 92 % for 10 ppm hydrogen concentration. The sensor shows capability for detection in sub-ppm levels by exhibiting a response of 0.043% for 0.01ppm (10 ppb) hydrogen concentration. The detection limit of the sensor (1% sensitivity) presented here is 169 ppb and the device current increases by 34.2 μA for 1ppb hydrogen concentration