Temperature homogeneity

The stretching properties of polymers are investigated by examining the effect of polymer orientation, polymer chain length, stretching rate, and temperature. Homogeneity of polymer films and consistency between lots of polymer films also are investigated. Statistical analysis of data includes Q-tests and f-tests. 

This is the so-called water-gas shift reaction (—AG29gl9.9kJmoP ) and it can also be effected by low-temperature homogeneous catalysts in aqueous acid solutions. The extent of subsequent purification of the hydrogen depends on the use to which it will be put. 

In order to determine reaction rate constants and reaction orders, it is necessary to determine reactant or product concentrations at known times and to control the environmental conditions (temperature, homogeneity, pH, etc.) during the course of the reaction. The experimental techniques that have been used in kinetics studies to accomplish these measurements are many and varied, and an extensive treatment of these techniques is far beyond the intended scope of this textbook. It is nonetheless instructive to consider some experimental techniques that are in general use. More detailed treatments of the subject are found in the following books. 

Some apparently specific effects can, however, arise from the supports. Mineral supports are usually poor heat conductors, i. e. significant temperature gradients can develop inside the vessels under the action of conventional heating, whereas they behave as efficient absorbers of microwave energy with consequently more temperature homogeneity (Scheme 3.2). 

The most common crucible form in the laboratory is the cylindrical form (see Fig. 8 b). The size with respect to volume depends mainly on the expected weight change and on the homogeneity of the studied sample. For these types of crucibles lids are often used which do not close the liner hermetically, but rather influence the temperature homogeneity and the equilibrium of reaction by the self-generated atmosphere. 

Very good temperature homogenity over a wide temperature range. Convection in the furnace has only a very small influence on the balance without necessity for special shielding. The disadvantages of this construction are the position of the sample holder and its lever arm, its sensitivity to vibrations. 

The disadvantages are Strong shielding for protection of the balance is required, poor temperature homogenity, change of samples is complicated and time consuming. 

The second microhotplate design is derived from this circular microhotplate, in contrast to the first device, it does not feature a silicon island underneath the heated area, but exhibits a network of temperature sensors in order to assess the temperature distribution and homogeneity (Sect. 4.2). The measured temperature distribution was compared to simulations, and the model described in Chap. 3 was validated. Furthermore, the influence of the tin-oxide droplet on the temperature distribution was studied. A microhotplate without silicon island is much easier to fabricate, though the issue of sufficient temperature homogeneity has to be evaluated. 

The membrane layout should be as symmetric as possible to achieve good temperature homogeneity over the membrane area and, as a consequence, low stress gradients. This includes also thermal stress owing to the mismatch of the thermal expansion coefficients of the layer materials. 

Although this device has no feature for improving the temperature homogeneity (silicon island etc.), the temperature gradient at T3 at 300 °C hotplate temperature is only... 

C/ tm. With a Si-island underneath, the temperature homogeneity is further improved. For a comparable device with a Si heat spreader a relative deviation of less than 2% equivalent to a temperature gradient of 0.07 °C/pm at 300 °C in the active area was achieved (see Sect 4.4.4 and [81]). 

Another possibihty to improve the temperature homogeneity is to introduce an additional polysiHcon plate in the membrane center. The thermal conductivity of polysilicon is lower than that of crystalline siHcon but much higher than the thermal conductivity of the dielectric layers, so that the heat conduction across the heated area is increased. Such an additional plate constitutes a heat spreader that can be realized without the use of an electrochemical etch stop technique. Although this device was not fabricated, simulations were performed in order to quantify the possible improvement of the temperature homogeneity. The simulation results of such a microhotplate are plotted in Fig. 4.9. The abbreviations Si to S4 denote the simulated temperatures at the characteristic locations of the temperature sensors. At the location T2, the simulated relative temperature difference is 5%, which corresponds to a temperature gradient of 0.15 °C/pm at 300 °C. 

The conclusion from the results of this chapter is, that a sihcon island fabricated by ECE is not absolutely necessary, if a relative temperature difference of 5% within the active area between the electrodes is acceptable. A microhotplate with a dielectric membrane and a polysilicon heat spreader in the center features sufficient temperature homogeneity. Moreover, the tin-oxide droplet serves as additional heat spreader and smoothes out temperature gradients. 

In conclusion, a simple KOH-etching process without ECE is applicable for future microhotplate designs, although the best temperature homogeneity is achieved with the silicon island heat spreader. The island remains an important design feature, especially for the use of thin-film sensitive layers, where the additional heat spreading effect of the sensor materials is small. 

The microhotplate with the transistor heater was electrothermally characterized similarly to the procedures presented in Sect. 4.1.3. Special care was taken to exclude wiring series resistances by integration of on-chip pads that allow for accurate determination of Fsg and sd- With the two on-chip temperature sensors in the center (Tm) and close to the transistor (Tt) the temperature homogeneity across the heated area was assessed as well. Both sensors were calibrated prior to thermal characterization. The relative temperature difference (Tj - Tm)/Tm was taken as a measure for the temperature homogeneity of the membrane. The measured thermal characteristics of a coated and an uncoated membrane are summarized in Table 4.6. The experimental values have been used for simulations according to Eq. (4.10). 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

The actual configuration of the reactor may take various forms depending on the precise requirements of the process. For example, for a high-temperature homogeneous gas-phase reaction such as naphtha cracking, the reactor may be simply a long tube in a furnace [Fig. 6(a)]. In other cases, the single tube is replaced by a number of tubes in parallel as shown in Fig. 6(b). 

In geothermometry, geologists use information in a rock to infer the temperature history of a rock. For example, if a rock contains sillimanite, it can be inferred that it must have experienced temperatures above the triple point of sillimanite, kyanite, and andalusite. If a rock contains sanidine, it must have experienced high temperatures and have cooled down rapidly. The appearance of specific minerals only gives a limit on the temperature. To infer the specific temperature, homogeneous or heterogeneous reactions with continuous variation of K on temperature are required. 

Only two high-temperature homogeneous reactions have been investigated in detail for their kinetics by geochemists. One is the Fe-Mg order-disorder reaction in orthopyroxene, and the other is the hydrous species interconversion reaction in rhyolitic melt. The two reactions have been applied as geospeedometers in various geochemical and meteoritic problems. Because they are often encountered in geochemical kinetics literature, the two reactions are discussed in depth below. 

Remove supernatant, wash pellet in 30 ml 5 mmol/1 Tris (pH 10) at room temperature Homogenize the solution in homogenizer (3 strokes only)... 

Sulfur chemistry is important both in combustion and in the petrochemical industry. Most fossil fuels contain sulfur, and also biofuels and household waste have a sulfur content. As a consequence sulfur species are often present in combustion processes. Knowledge of gas-phase sulfur chemistry occurring in combustion has bearing on pollutant emissions and on system corrosion. Air pollution by SO2 still constitutes a major environmental concern and search for control techniques has motivated research also on high-temperature homogeneous sulfur chemistry. However, more recent work on sulfur chemistry has been concerned mainly with the effect of sulfur on other pollutant emissions, such as NO and CO, and with the SO3/SO2 ratio, which is important for the corrosive potential of the flue gas and for formation of sulfur containing aerosols. 

The main aspects of the reactor design are temperature homogeneity and homogeneous gas flow for all 96 reactor positions. In Figs. 2.8 and 2.9, homogeneities of temperature and gas distribution are indirectly verified by an IR-thermo-gram and a product distribution of a partial oxidation reaction, measured over all 96 catalyst samples. 

Fig. 2.8 (a) and (b) shows the temperature homogeneity of the whole reactor system with a symmetrical distributionof equal materials compositions in an 8x 12 MTP array. 

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