Structural memory effect

The conversion of the mixed metal oxides into LDHs has been variously referred to as regeneration, reconstruction, restoration, rehydration or the calcination-rehydration process , structural memory effect or simply memory effect . This method is usually employed when large guests are intercalated. It also avoids the competitive intercalation of inorganic anions arising from the metal salts. The procedure is more complicated than coprecipitation or ion-exchange methods, however, and amorphous phases are often produced simultaneously. 

More recently, a closer inspection of the transient stress rheology for thickening systems has revealed more complicated patterns, such as structural memory effects. Berret et al. [78] and Oeschlager et al. [93,107] have observed that the transient mechanical response also depended on the thermal and shear histories. Samples having been treated thermally, e.g., heated up to 90 °C for 2h, behaved very differently from samples freshly prepared or already sheared. The induction time couid last several hours, and was not proportional to the inverse shear rate, as mentioned previously. It was concluded that the lack of reproducibility under certain thermal and shear conditions might indicate that these surfactant solutions were characterized by long-lived metastable states. 

There are a number of techniques that have been successfully applied to synthesize modified hydrotalcites (48). The most commonly method used is the co-precipitation of two metal salts in alkaline solution at a constant pH value of about 10. Another method uses the classical ion exchange process in which the guest anions are exchanged with the anions in the interlayer spaces of preformed layered double hydroxides to produce specific anion intercalated modified hydrotalcites. StiU another method is a lattice reconstruction after heating, i.e., calcination, which is based on the structural memory effect of these materials, due to which the original structure is reproduced after rehydration. 

Thermal history as one of key factors in developing the catalytic active structure of metastable nature, minimizing in the same time the tendency of its reconstruction towards the initial structure (memory effect) ... 

Abstract This review deals with spin crossover effects in small polynuclear clusters, particularly dinuclear species, and in extended network molecular materials, some of which have interpenetrated network structures. Fe(II)Fe(II) species are the main focus but Co(II)Co(II) compounds are included. The sections on dinuclear compounds include short background reviews on (i) synergism of SCO and spin-spin magnetic exchange (ii) cooperativity (memory effects) in polynuclear compounds, and (iii) the design of dinu-... 

A similar effect was observed in our work and in the work of others (5), where voltammetry curves changed after extended cycling, particularly if the cathodic sweep was reversed before the full Pb deposition coverage. The observed "cathodic memory effect" may be due to the proposed structural transformation phenomenon and subsequent step density growth, initially facilitated by a high step density on a UHV-prepared or chemically polished (6) Ag(lll) substrate. Post electrochemical LEED analysis on Ag(lll)-Pb(UPD) surfaces provided additional evidence of a step density increase during Pb underpotential deposition, which will be discussed later in this text. (See Figure 3.)... 

Koh, C.A. (2005). Search for memory effects in methane hydrate Structure of water before hydrate formation and after hydrate decomposition. J. Chem. Phys. 123 (16), Art. No. 164507. 

Regeneration Many LDH materials show a unique phenomenon called memory effect, which involves the regeneration of the layered crystalline structure from their calcinated form when the latter is dispersed in an aqueous solution containing suitable anions [96]. This property is often used to synthesize and modify LDH with different types of intercalating anions. The regeneration property shown by LDH is extensively reported in the literature [97, 98]. 

In this contribution, we would like to show how such an approach allowed us to synthesize both soft and very hard molecule-based magnets. This contribution is organized as follows first, we define briefly the field of molecular magnetism, then we indicate the successive steps which led us to three-dimensional molecule-based magnets with fully interlocked structures. We describe these original structures in detail. Finally, we focus on the physical properties of these objects, with special emphasis on the huge coercivity of one of the compounds, which confers a memory effect on this compound. 

There has been a general consensus among hydrate researchers that hydrates retain a memory of their structure when melted at moderate temperatures. Consequently, hydrate forms more easily from gas and water obtained by melting hydrate, than from fresh water with no previous hydrate history. Conversely, if the hydrate system is heated sufficiently above the hydrate formation temperature at a given pressure, the memory effect will be destroyed. Some experimental observations of the memory effect phenomenon are summarized in Table 3.3. 

The memory effect has important implications for the gas industry. For example, after hydrates initially form in a pipeline, hydrate dissociation should be accompanied by the removal of the water phase. If the water phase is not removed, the residual entity (i.e., residual structure, persistent crystallites, or dissolved gas) will enable rapid reformation of the hydrate plug. Conversely, if hydrate formation is desired, the memory effect suggests that hydrate formation can be promoted by multiple dissociation and reformation experiments (provided the melting temperature is not too high, or melting time is not too long). 

If the temperature for melting hydrate is close to the dissociation temperature, or insufficient time is given to melt hydrate, a memory effect is observed (attributed to residual structure, persistent hydrate crystallites remaining in solution, or dissolved gas) to promote future more rapid hydrate formation. This memory effect is destroyed at temperatures greater than 28°C, or after several hours of heating. 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

The atomic mechanism, based on the previously proposed inhomogeneous shear, leading to the formation of twinning and antiphase boundaries in TiNi with the CsCl-type structure is described. The twinning mechanism described herein explains the electrical resistivity anomaly due to incomplete thermal cyclings observed previously in TiNi. This explanation is in keeping, in a qualitative manner, with the "memory effects observed in relation to the electrical resistivity anomaly. 

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