Thermal stability, conditions

Dielectric Strength. Dielectric failure may be thermal or dismptive. In thermal breakdown, appHed voltage heats the sample and thus lowers its electrical resistance. The lower resistance causes still greater heating and a vicious circle, leading to dielectric failure, occurs. However, if appHed voltage is below a critical value, a stabilized condition may exist where heat iaput rate equals heat loss rate. In dismptive dielectric failure, the sample temperature does not iacrease. This type of failure is usually associated with voids and defects ia the materials. 

The major reasons for the beluu ior of vertical temperature in water bodies are the low thermal condnctii ity and the absorption of heat in the first few meters. As tlie surface waters begin to heat, transfer to low er layers is reduced and a stability condition develops. The prediction of thermal behavior in lakes and reser oirs is an important power plant siting consideration and also is a major factor in preienting e.xcessive thermal effects on sensitive ecosystems. Furthermore, the extent of thermal stratification influences the vertical dissolved ox)gen (DO) profiles where reduced DO often results from minimal exchiuige with aerated water. ... 

In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

Fukutomi, M., Kitajima, M., Okada, M., and Wanatabe, R., Silicon Carbide Coating on Molybdenum by Chemical Vapor Deposition and its Stability under Thermal Cycle Conditions, J Nucl Mater., 87(1) 107-116 (Nov. 1979)... 

Equation (8.29) provides no guarantee of stability. It is a necessary condition for stability that is imposed by the discretization scheme. Practical experience indicates that it is usually a sufficient condition as well, but exceptions exist when reaction rates (or heat-generation rates) become very high, as in regions near thermal runaway. There is a second, physical stability criterion that prevents excessively large changes in concentration or temperature. For example. An, the calculated change in the concentration of a component that is consumed by the reaction, must be smaller than a itself Thus, there are two stability conditions imposed on Az numerical stability and physical stability. Violations of either stability criterion are usually easy to detect. The calculation blows up. Example 8.8 shows what happens when the numerical stability limit is violated. 

It has been proposed (17) that the portion of coal which is mobile under liquefaction conditions, contributes to the stabilization of thermally-generated radicals. Thus, coals which are highly fluid or contain large contents of extractable material might be expected to provide hydrogen and thus promote conversion. Collins has reported that vitrinite is a better donor of hydrogen than is Tetralin (20). Our own measurements of the aromatic content and elemental analyses of the coals (16,21) (or coal products) before and after conversion at short time are insufficient to confirm or deny the supposition that coal acts as its own H-donor even at short times. 

Dreier et al. [44] determined sterols in lacustrine sediments. Samples of wet lacustrine sediments were heated under anoxic conditions at 150, 175, 200 and 250°C for five days at 175°C for five days with influx of potassium hydroxide and methanol to remove sterols and at 175°C for 12, 18, 24 and 48h, after which extraction was performed. Heating the sediment increased the amounts of extractable sterols provided that the temperature did not exceed 200°C, because degradation became rapid above that temperature. The behaviour of sterol ketones was similar, but the temperature limit was slightly higher. The various levels of the sterols extracted are tabulated 4-methylsterols had a high stability towards thermal degradation under the conditions used. 

The 1.5-nm nanoparticles readily react with thiol or amine-terminated ligands under mild conditions to yield thiol- or amine-stabilized nanoparticles. Triphenylphosphine-stabilized particles thermally decompose with the production of (PPh3)AuCl and metallic gold. 

Second-derivative conditions such as (5.17) are known as stability conditions, expressing the self-restorative property of thermal equilibrium. 

McDaniel and Maher (159-161) were the first to report that upon thermal treatment of NH4-Y, under a particular set of conditions, thermal stability of the zeolite is considerably increased. The product retains crystallinity at temperatures in excess of 1000°C, while the decomposition of the sodium form of the zeolite takes place at ca. 800°C. This process is known as ultrastabilization. Ultrastable zeolite Y is very well suited as a catalyst for hydrocracking reactions—much more so than the as-prepared zeolite, which is too acidic and has insufficient thermal stability. 

Since TG and DTA complement each other, it is an obvious move to attempt both investigations simultaneously [173]. TG-DTA measures mass and energy changes as a function of temperature or time. Depending on the atmospheric conditions (vacuum, inert or air conditions) thermal or oxidative stability is measured. Typical TG-DTA... 

The inequalities in Eq. (7.8) can now be written in terms of the transport coefficients of the thermal conductivity and mass diffusivity by using the thermodynamic stability condition (dpfilwfjp > 0... 

Although the ring system of monocyclic 1,2,3-triazoles shows a remarkable stability towards oxidation, reduction and hydrolysis, under forced conditions thermal extrusion of nitrogen can be achieved at elevated temperatures. Some such reactions are reviewed in (B-76MI41103). Similarly, photochemical extrusion of nitrogen can be effected, and this is consistent with their behavior upon electron impact (cf. Section 4.11.3.2.8). 

When we intend to apply organic molecular materials, especially photochromic dyes, to optical memory media, the indispensable condition is stability, both thermal and photochemical. The photogenerated isomers are required never to return to the initial isomers in the dark, even at elevated temperatures, e.g., 80 °C. In addition, the coloration/decoloration can be cycled many times while the photochromic performance is maintained, and the memory media are provided with nondestructive readout capability. Although several molecules which fulfill the former condition have been developed, some problems still remain to gain access to molecules and systems which fully satisfy the latter condition. 

Depending on the polymer sample (and presence of various stabilizers), as well as the heating conditions, thermal decomposition of PVC may show some variations. The curve showing the variation of weight loss % as a function of temperature (TG curve) for a 3.5 mg PVC sample with M = 85,000 is shown in Figure 6.3.1. The heating was done between 30° C and 830° C at a rate of 10° C/min. in air. 

Equations (2.302) and (2.303) enable us to explicitly calculate the temperatures t U at time = t0 + At from the initial temperature distribution .. These can then be used to calculate the temperature of the next time level etc. This difference method is relatively simple to program. It is also suitable for a temperature dependent thermal power density W(d), because WU is calculated for the already known temperature The disadvantage of this explicit difference method lies in its limited stability, cf. 2.4.1.2. The stability condition, from which none of the coefficients on the right hand side of (2.302) are allowed to be negative, limits the time step... 

For these models, thermal stability conditions were first discussed as the absolute prerequisite condition. Consequently, the nucleation field of the each dot arrays... 

---