Thermal phenomena

This chapter elucidates the origin of the thermal phenomena observed in the amorphous materials at temperatures Tu/3 and below, down to the so far reached millikelvins. The nature of these phenomena can be boiled down to the existence of excitations other than elastic strains of a stable lattice. The... 

The fact that WCirtz reactions take a long time to start with halogenous derivatives and sodium is due to a concentration effect which can lead to an acceleration of the reaction once the concentration of reagents is critical. It is aggravated by thermal phenomena. 

As already indicated, Tian s equation supposes (1) that the temperature of the external boundary of the thermoelectric element 8e, and consequently of the heat sink, remains constant and (2) that the temperature Oi of the inner cell is uniform at all times. The first condition is reasonably well satisfied when the heat capacity of the heat sink is large and when the rate of the heat flux is small enough to avoid the accumulation of heat at the external boundary. The second condition, however, is physically impossible to satisfy since any heat evolution necessarily produces heat flows and temperature gradients. It is only in the case of slow thermal phenomena that the second condition underlying Tian s equation is approximately valid, i.e., that temperature gradients within the inner cell are low enough to be neglected. The evolution of many thermal phenomena is indeed slow with respect to the time constant of heat-flow calorimeters (Table II) and, in numerous cases, it has been shown that the Tian equation is valid (16). 

This simplified equation is equivalent to Tian s equation [Eq. (16)], and it appears that n is indeed the time constant r of the calorimeter. Thence, the successive coefficients n in Eq. (29) may be called the calorimeter time constants of 1st, 2nd,. .., ith order. When the Tian equation applies correctly, all time constants r, except the first r may be neglected. Since the value of the coefficients n of successive order decreases sharply [the following values, for instance, have been reported (40) n = 144 sec, r2 = 38.5 sec, r3 = 8.6 sec, ri 1 sec], this approximation is often valid, and the linear transformation of many thermal phenomena produced by the thermal lag in the calorimeter may actually be represented correctly by Eqs. (16) or (30). It has already been shown (Section IV.A) that the total heat produced in the calorimeter cell is then proportional to the area limited by the thermogram. 

It has been shown recently, however, that these equations may be solved 62), by means of the state functions theory (64) and/or the time-domain matrix methods 63). Figure 14 shows, for instance, that the computer calculations allow us to determine, with a good approximation, the time-dependence of thermal phenomena taking place in the calorimeter, although all significant details of their kinetics are completely blurred on the thermogram 62). This method has been recently used to correct... 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

This energy dissipation in the core of materials results in a much more uniform temperature than classical heating. Classical thermal phenomena (conduction, convection, radiation, etc.) only play a secondary role in the a posteriori equilibration of temperature. 

Differential scanning calorimetry (DSC) is a technique which aims to study the same thermal phenomena as DTA, but does so on a rather different principle. Hence, although the data obtained are very similar, they may differ in detail. Typical DSC equipment will operate over the temperature range from ambient to ca. 700°C. However, as with DTA, specially modified equipment can extend this substantially in both directions. 

DSC essentially studies the same thermal phenomena as DTA, albeit using a different principle. Thus DTA and DSC provide very much the same information and their applications are similar. Reference back to the section on the applications of DTA will suffice to indicate the scope of DSC. Some differences in the quality of the information obtained sometimes exist however, leading to a preference for one technique over the other for particular purposes. 

In the DTA measurement, an exothermic reaction is plotted as a positive thermal event, while an endothermic reaction is usually displayed as a negative event. Unfortunately, the use of power-compensation DSC results in endothermic reactions being displayed as positive events, a situation which is counter to IUPAC recommendations [38]. When the heat-flux method is used to detect the thermal phenomena, the signs of the DSC events concur with those obtained using DTA, and also agree with the IUPAC recommendations. 

The thermal phenomena - heat flux, heat quantity, and temperature gradient -occurring at the zinc electrode during the electrolysis from aqueous solutions containing zinc salt were investigated [215]. 

Observe the thermal phenomena occurring when 0.1 mol of the following salts is dissolved in 50 ml of water ammonium nitrate, anhydrous sodium sulphate, and sodium sulphate decahydrate (Glauber salt). For this purpose, pour 50 ml of water into a 100-ml beaker, measure its temperature, and, after pouring in the relevant amount of a salt, see how the temperature changes. Explain what occurs. 

The first law of thermodynamics seems to allow many thermal phenomena that are not observed experimentally. Indeed, the high symmetry of first-law-type relationships such as (3.99) would seem to suggest that the time sequence between initial and final states of a thermal process could be arbitrarily reversed with no special consequence (other than the expected sign change of AH). Yet experience says that this is not so. 

In this section we review the results from positron annihilation experiments, predominantly those performed using the lifetime and positron trap techniques described in section 6.2. Comparisons are made with theory where possible. The discussion includes positron thermalization phenomena and equilibrium annihilation rates, and the associated values of (Zeff), over a wide range of gas densities and temperatures. Some studies of positron behaviour in gases under the influence of applied electric fields are also summarized, though the extraction of drift parameters (e.g. mobilities) is treated separately in section 6.4. Positronium formation fractions in dense media were described in section 4.8. 

The correct assessment of thermal phenomena requires a special knowledge. 

Couple LBM to be developed in step 4 with LBM schemes descriptive for air bearing and thermal phenomena (Figures 24e and 25e). 

The governing equations (1) - (10) are completed by an appropriate set of state and thermodynamic equations as well as boundary conditions and constitutive relationships. The latter ones express some inherent couplings between chemical and hygro-thermal phenomena and medium deformations. 

Dimensional calculations are simplified if the unit for each kind of measure is expressed in terms of special reference units. The reference dimensions for mechanics are length, mass, and time. Other measurements performed are expressed in terms of these reference dimensions units associated with speed contain references to length and time—mi/hr or m/s. Some units are simple multiples of the reference unit—area is expressed in terms of length squared (m2) and volume is length cubed (in3). Other reference dimensions, such as those used to express electrical and thermal phenomena, will be introduced later. 

We recently observed anomalous thermalization phenomena in Er +-doped Y2O2S nanocrystals of 20-nm diameter at low temperature (Liu et al., 2002). It is well known that the population of the lanthanide ions doped in the host among the energy levels generally obeys the Boltzmann distribution characterized by the Boltzmann factor. For the / th level,... 

The unusual thermalization phenomena take place not only in the 4Ii5/2 4F7/2 excitation... 

Additional evidence of anomalous thermalization phenomena was found in the excitation spectmm 2Hn/2 4Ii5/2, 4S3/2 -> 4Ib/2 emission spectrum and upconversion excitation... 

Recently, we also observed an anomalous thermalization phenomenon in Er Gd203 (1 at%) nanocrystals with diameters of 40-50 nm. In the excitation spectra at 2.9 K, hot bands originating from the upper stark level of 4Ii5/2 (38 cm-1) were observed. These hot bands disappear when temperature goes up to 5 K. Our preliminary results show that the anomalous thermalization phenomena in this system are more complicated, because they depend on the laser power and temperature. The effect of laser heating or temperature fluctuation in nanocrystals must be ruled out before a definite conclusion can be reached. 

Figures 12 and 14 show first and second DSC runs of Nylon 6 and Nylon 6-6 using seventy to eighty microgram quantities of fiber. In the initial run on this particular sample of Nylon 6, no thermal phenomena are discernable up to the melt, which is a...

Two distinct families of models for the CR origin of non-thermal phenomena in galaxy clusters have been proposed so far i) the electronic and the ii) hadronic models. 

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