Order / Disorder thermal

Thermal stahility. Yor applications of LB films, temperature stability is an important parameter. Different teclmiques have been employed to study tliis property for mono- and multilayers of arachidate LB films. In general, an increase in temperature is connected witli a confonnational disorder in tire films and above 390 K tire order present in tire films seems to vanish completely [45, 46 and 45] However, a comprehensive picture for order-disorder transitions in mono- and multilayer systems cannot be given. Nevertlieless, some general properties are found in all systems [47]. Gauche confonnations mostly reside at tire ends of tire chains at room temperature, but are also present inside tire... 

Whereas only a few atomie jumps may be neeessary to enable ehanges in the equilibrium degree of SRO, without atomie movement over long distanees ehanges in LRO may not be suffieient to reaeh equilibrium. This ean lead to a eompetition between inerease of SRO in the matrix and formation of the new LR0-phase when lowering the temperature below an order/disorder phase boundary. In those cases, thermal and/or meehanieal pretreatment of the sample is of erueial importanee for what is observed in the sample. 

The region from the Helmholz plane into the solution, across which the potential varies exponentially attaining a value of zero at some distance in this region the ions are subjected to both ordering electrical forces and disordering thermal forces. 

Dilatometric methods. This can be a sensitive method and relies on the different phases taking part in the phase transformation having different coefficients of thermal expansion. The expansion/contraction of a sample is then measured by a dilatometer. Cahn et al. (1987) used dilatometry to examine the order-disorder transformation in a number of alloys in the Ni-Al-Fe system. Figure 4.9 shows an expansion vs temperature plot for a (Ni79.9Al2o.i)o.s7Feo.i3 alloy where a transition from an ordered LI2 compound (7 ) to a two-phase mixture of 7 and a Ni-rich f c.c. Al phase (7) occurs. The method was then used to determine the 7 /(7 + 7O phase boundary as a function of Fe content, at a constant Ni/Al ratio, and the results are shown in Fig. 4.10. The technique has been used on numerous other occasions,... 

Scientific awareness of a low-temperature transition in magnetite began in 1929 with the observation of a A-type anomaly in the specific heat at about 120 K. The anomaly was typical of an order-disorder transition, but it was well below the magnetic-ordering temperature Tc = 850 In 1931, Okamura observed an abrupt semiconductor-semiconductor transition near 120 K. The transition exhibits no thermal hysteresis, but the transition temperature is sensitive to the oxygen stoichiometry. More recent specific-heat measurements show the presence of two resolvable specific-heat peaks at the transition temperature the lower-temperature peak near 110 K appears to be due to a spin reorientation. 

CO/Rh(100) shows a very strong repulsion for CO molecules at nearest-neighbor positions and much weaker repulsion between molecules farther apart. The occurrence of different lateral interactions is typical for real systems and leads to complicated order-disorder phenomena, especially if there are different types of adsorbates. Thermal motion of the adsorbates may overcome some lateral interactions but not others. The phase diagram can be quite complex even if there is only one type of adsorbate, and the effect on the kinetics can be profound. 

We have used here that X is either CO or a vacant site and (CO) + ( ) = . The main problem with this mean-field approximation is that at temperatures below the order-disorder transition there is a strong correlation between the occupations of sites. As will be shown in Section 4.3 (pco,co for neighboring sites is about 24 kJ/mol, which corresponds to a thermal energy at 2880 K. This means that neighboring sites will not be occupied simultaneously at any realistic temperatures, and a more sophisticated approach is needed that describes this well. For weak interactions eqn. (11) may be acceptable, but one should be aware that there is some correlation in the occupation of sites even if there is no long-range order. 

Stern Combination of Parallel-Plate and Diffuse-Charge Models 9m = -9S = "[9h + J t 1 1 C Qi Qi maV=waVhaV Pole r R 1 of ,ial. linear variation V, ions are under the combined influence of the ordering electrical and the disordering thermal forces. Agrees with the experiment only for ions nonspecificaHy adsorbed on the electrode (e g. NaF). 

When the defect interaction energy is much larger than the thermal energy, it can lead to an ordering of defects into superlattice structures and to the appearance of phases having ordered arrays of defects. Other interactions may also become important as the spin-dependent interactions between the d electrons in the Fj gS system (24). We shall not consider these order-disorder phenomena, since they are discussed by Wadsley (30). Some of the structural-consequences of ordering are considered below. 

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