Temperature monitoring passive

The active Cu containing catalyst is also very air sensitive (like the Ni reforming catalyst) and will spontaneously oxidize generating uncontrolled reaction heats. Thus it must be passivated before discharged and exposed to air. A small amount of air is added to the reactor and the temperature monitored. This process is continued until the exotherm is small enough that the catalyst can be safely removed from the reactor. 

From this discussion one might conclude that layer thickness measurements are the better choice for monitoring passive corrosion. While this is true from the point of resolution (ellipsometry allows the determination of scales down to the nm scale) it is difficult for high-temperature in situ recording and it implies dense transparent layers, which may not be the case. 

Liquid Metal Sealing, and Passive Temperature Monitoring. 

Passive temperature monitors can be used to determine the maximum temperature a substrate has reached in processing. Passive temperature monitors involve color changes, phase changes (e.g. melting of indium), or crystallization of amorphous materials. 

Diffusive samplers, also called diffusive monitors, passive samplers or passive monitors, are utilized for sampling without the need for an air mover, that is, without a pump. Manmade diffusive sampling operates by allowing gas or vapor molecules to diffuse through a defined volume of still air or through a polymer membrane, until they reach a sorbent bed. The principles of uptake are to consider that the passive sampling medium is uniform and porous and that it traps PAHs from the atmosphere by gaseous diffusion, and sorption. The mass collected is a function of the external concentration and the diffusion coefficient of the molecules. The diffusion coefficient varies in a known manner with temperature and pressure, and so the result can be corrected for these parameters [107]. 

Type of Interior Sensor Passive infrared (PIR) Presently the most popular and cost-effective interior sensors. PIR detectors monitor infrared radiation (energy in the form of heat) and detect rapid changes in temperature within a protected area. Because infrared radiation is emitted by all living things, these types of sensors can be very effective. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

The surface chemical composition of InP as a function of thermal cleaning temperature was studied by Cheng, et al. (19), also using AES. They used an arsenic molecular beam and temperature of about 500 C to clean a freshly oxide passivated InP. The surface oxides are replaced by arsenic oxides which then vaporize at these temperatures. An atomically flat and carbon contamination free surface was obtained, as monitored in situ with AES and RHEED OJ). 

These two examples illustrate how Mossbauer spectroscopy can reveal the identity of iron phases in a catalyst after different treatments. The examples are typical of many applications of the technique in catalysis - a catalyst is reduced, carburized, sulfided, or passivated and, after cooling down, its Mossbauer spectrum is monitored at room temperature. However, a complete characterization of phases in a catalyst sometimes requires that spectra are measured at cryogenic temperatures, in particular when catalysts are highly dispersed. 

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