Assessment of the final metrological characteristics of a MOEMS-based NDIR spectrometer through system modeling and data processing

A model of a miniaturized non-dispersive infrared (NDIR) gas analysis system, aiming to predict the final system specifications, is presented. It comprises the different elements of the NDIR detector, including a surface micromachined Fabry-Perot tunable filter. These models have been used to estimate the response of the NDIR system to different gas mixtures. Multivariate regression methods like partial least squares allow recovering the true sample composition from the IR absorption spectra measured with the NDIR system, despite the limited selectivity of the filter. Combining model and data processing permits to predict the effect on the final system specification of design parameters. Here, we compare the effect of the technology used for the filter on the system errors.     

Thermal AFM: a thermopile case study

In this work, an atomic force microscope (AFM) with an integrated thermal sensor has been used to obtain the local spatial distribution of temperatures in a micromachined thermopile with submicron resolution. In this communication, we will show how the dimensional, structural and functional characteristics of a thermopile suits well with the requirements for AFM thermal imaging, and how a deeper insight of the thermopile operation can be gained with the aid of these advanced scanning probe-based tools.     

Planar Thermoelectric Microgenerators Based on Silicon Nanowires

Silicon nanowires have been implemented in microfabricated structures to develop planar thermoelectric microgenerators (??TEGs) monolithically integrated in silicon to convert heat flow from thermal gradients naturally present in the environment into electrical energy. The compatibility of typical microfabrication technologies and the vapor?Cliquid?Csolid (VLS) mechanism employed for silicon nanowire growth has been evaluated. Low-thermal-mass suspended structures have been designed, simulated, and microfabricated on silicon-on-insulator substrates to passively generate thermal gradients and operate as microgenerators using silicon nanowires as thermoelectric material. Both electrical measurements to evaluate the connectivity of the nanowires and thermoreflectance imaging to determine the heat transfer along the device have been employed.     

Improved Thermal Behavior of Multiple Linked Arrays of Silicon Nanowires Integrated into Planar Thermoelectric Microgenerators

Low-dimensional structures have been shown to be promising candidates for enhancing the thermoelectric properties of semiconductors, paving the way for integration of thermoelectric generators into silicon microtechnology. With this aim, dense arrays of well-oriented and size-controlled silicon nanowires (Si NWs) obtained by the chemical vapor deposition (CVD)-vapor–liquid–solid (VLS) mechanism have been implemented into microfabricated structures to develop planar unileg thermoelectric microgenerators (μTEGs). Different low-thermal-mass suspended structures have been designed and microfabricated on silicon-on-insulator (SOI) substrates to operate as microthermoelements using p-type Si NW arrays as the thermoelectric material. To obtain nanowire arrays with effective lengths larger than normally attained by the VLS technique, structures composed of multiple ordered arrays consecutively bridged by transversal microspacers have been fabricated. The successive linkage of multiple Si NW arrays enabled the development of larger temperature differences while preserving good electrical contact. This gives rise to small internal thermoelement resistances, enhancing the performance of the devices as energy harvesters.     

Adsorption and Decomposition of Glycerol on Pristine and Oxygen Modified Au(111) Surfaces

Topics in Catalysis - Research on biomass derived raw materials for conventional catalytic processes, especially those directed to replace human dependence on fossil-based energy, is a high...     

Variations in Reactivity on Different Crystallographic Orientations of Cerium Oxide

Cerium oxide is a principal component in many heterogeneous catalytic processes. One of its key characteristics is the ability to provide or remove oxygen in chemical reactions. The different crystallographic faces of ceria present significantly different surface structures and compositions that may alter the catalytic reactivity. The structure and composition determine the number of coordination vacancies surrounding surface atoms, the availability of adsorption sites, the spacing between adsorption sites and the ability to remove O from the surface. To investigate the role of surface orientation on reactivity, CeO₂ films were grown with two different orientations. CeO₂(100) films were grown ex situ by pulsed laser deposition on Nb-doped SrTiO₃(100). CeO₂(111) films were grown in situ by thermal deposition of Ce metal onto Ru(0001) in an oxygen atmosphere. The chemical reactivity was characterized by the adsorption and decomposition of various molecules such as alcohols, aldehydes and organic acids. In general the CeO₂(100) surface was found to be more active, i.e. molecules adsorbed more readily and reacted to form new products, especially on a fully oxidized substrate. However the CeO₂(100) surface was less selective with a greater propensity to produce CO, CO₂ and water as products. The differences in chemical reactivity are discussed in light of possible structural terminations of the two surfaces. Recently nanocubes and nano-octahedra have been synthesized that display CeO₂(100) and CeO₂(111) faces, respectively. These nanoparticles enable us to correlate reactions on high surface area model catalysts at atmospheric pressure with model single crystal films in a UHV environment.     

Reaction Between Ethylene and Acetate Species on Clean and Oxygen-Covered Pd(100): Implications for the Vinyl Acetate Monomer Formation Pathway

Abstract  

The reaction between gas-phase ethylene and adsorbed acetate species on Pd(100)-p(2 × 2)-O and Pd(100)-c(2 × 2)-O surfaces is studied using infrared spectroscopy. It is found that acetate species are removed more rapidly by gas-phase ethylene on oxygen-covered Pd(100) than on Pd(111). However, in contrast to reaction on Pd(111), where vinyl acetate monomer (VAM) formation is detected by infrared spectroscopy, only CO is found on oxygen-covered Pd(100) surfaces. In the case of Pd(111), it has been shown that VAM is stabilized on the crowded, ethylidyne-covered surface. Since ethylidyne species do not form on Pd(100), any VAM that is formed can thermally decompose. The reaction shows an isotope effect when C₂D₄ is substituted for C₂H₄, indicating the hydrogen is involved in the rate-limiting step. Based on the surface chemistry found for VAM on a Au/Pd(111) alloy, where 30 to 40% ML of gold inhibits VAM decomposition, it is suggested that the VAM formation rate will increase on (100) alloy surfaces, while it will decrease at higher gold coverages since acetate formation is inhibited.     

A MEMS-based thermal infrared emitter for an integrated NDIR spectrometer

A novel micromachined thermal infrared emitter using a heavily boron doped silicon slab as radiating element is presented. The fabrication process has been designed to allow the integration of such infrared emitters with an array of thermopile infrared detectors, with the aim of achieving an integrated non-dispersive infrared microspectrometer. A first set of infrared emitters with a common size for the doped silicon radiating slab (1,100?×?300?×?8?μm³) has been successfully fabricated and characterized. The working temperature of the Joule-heated radiating slabs has been controlled by means of DC and pulsed electrical signals, achieving temperatures well beyond 700°C. The thermal time constant measured in pulsed operation, around 50?ms, is adequate to enable the direct electrical modulation of the emitted radiation up to a frequency of 5?Hz while maintaining the full modulation depth. The temperature distribution in the radiating elements has been analyzed using two different thermal imaging methods.     

Feasibility of a flip-chip approach to integrate an IR filter and an IR detector in a future gas detection cell

In this work we have studied the feasibility of integrating an infrared filter and an infrared detector by means of a flip-chip technique. This filter and detector combination should be the heart of a future gas detection cell based on infrared absorption. In our case the filter is a surface micromachined Fabry-Perot interferometer, and the infrared detector is a bulk micromachined thermopile. The flip-chip technique is an elegant solution to assure the optical micro-alignment of both devices and allows the electrical contact needed to actuate active optical filters.Work originally presented at DTIP 2003. Spanish CICYT projects no TIC-98-0987-C03-03 and DPI-2001-3213-C02-01 have financed this work.