Interface strain

Because of surface contributions as well as due to interface strains, the magnetostriction of thin films is expected to be different from that observed in bulk materials. Motivated by this observation and by possible technical applications of thin films of manganites and cobaltites, we decided to present in this paper mostly results of investigations of magnetostrictive effects in thin films. It has to be mentioned that no data exist in the literature on magnetostriction of cobaltite thin films. 

For the decompositions of the metal carboxylates, alternative mechanisms involving breaking of each of the different principal linkages in the R-CO-O-M group have been proposed as rate controlling (reference [40] p.210), in addition to mechanisms based on charge transfer, interface strain and catalysis by an active product. 

If the distortion of the interface by the higher harmonics generated can be neglected as a higher-order effect, the interface vibration is sinusoidal with the excitation frequency. Its displacement amphtude ao is determined by the stress and displacement continuity of the waves of fundamental frequency at the interface. If the transmission of the fundamental frequency shows no hysteresis relative to the interface vibration, we get the result ao=fiBi/k, fiBi = 2 /fi] — e. Here k=colVi is the wavenumber and Vl is the compressional sound velocity in the aluminum plates [10]. The strain amphtude in the interface is the ratio of Oq to the thickness of the interface, i.e., the interface strain amphtude is directly proportional to bi. 

Figs. 25.1-25.4 show the cahbrated measure of the interface strain vibration bi and the strain amphtudes of the transmitted waves of fundamental frequency and its second and third harmonic i, 2, and 3 as a function of the incident strain amphtude i. The sohd hues represent a hnear fit of Ebi and 1, a quadratic fit of 2, and a cubic fit of 3 of the first 12 measuring points up to a strain level 1.3x10. The strain amphtudes of the first six measured data points of the third harmonic were below the noise level. The experimentally observed power laws for 1, 2, and 3 render it possible to relate them to an expansion of the stress-strain curve which displays no hysteresis [9]. In this case... 

Fig. 25.1 Ultrasonic transmission data for a sample of two aluminum plates 4 mm thick bonded together by an adhesive epoxy layer of 30 p,m thickness showing the measure of the interface strain amplitude Bi = 2- /e — versus the input strain amplitude i and the linear fit of the first 12 measuring points.

The formation of tetragonal-Ge02 appears to be desirable for chemical stability and reduction of interface strain deformation. The tetragonal phase can grow epitaxially on the (110) plane of germanium and therefore is a promising candidate for MOS structures, but conditions for its formation are not well defined. 

Fig. 14.9. Interface internal energy eint plotted versus temp>erature T at Pc = Pm = 1 atm ( ), and its temperature derivative at constant interface strain and constant S3fstem volume, demt/dT) f ( ). Reprinted from [30] with wiitten permission from Elsevier...

Zhao Y and Ansari F (2002), Embedded fiber optic sensor for characterization of interface strains in FRP composite . Sensor Actuat A - Phys, 100(2-3), 247-251. 

All strains and stresses ate supposed to be uniform and homogeneous in the whole volume of each particle in all phases. Mutual interactions are realized at the phase interfaces. Strains and stresses ate distributed in a different manner for the different components as well as for the different volume distributions of the phases. Let us demonstrate the approximation for series and parallel connection of two phases (Fig. 7.25). In series connection, stress component along X3-ditection is the same in all phases, while the strain is different in the phases due to the different stiffness On the contraiy, strains ate the same in both phases in the direction perpendicular to the x3-axis, while the stresses are distributed differently in both phases. Assuming all interface conditions, following effective material properties could be calculated (similar method is applied also to the parallel connection of the elements)... 

Effects of Spin Coating and Substrate Interaction. Effects of spin coating and interactive interfaces on shear mechanical properties of ultrathin unannealed PEP elastomer films were discussed. It was found that a stressed boundary layer is formed, 7-10 Rg thick. This imexpected far-field effect for a polymer has recently also been observed for annealed PEP films (1), The degree of disentanglement depends strongly on the distance towards the interactive interface. Strained film surfaces were found to determine the film stability, and thus, can be responsible for spontaneous autophobicity. 

Mechanical stresses In Interfaces strain the adhesion layers between materials that are often unstable, so that frequently observed signs of aging processes are peeling coatings and eroding pigments and modifiers [36]. 

The adhesion theories of rubber friction fall into two main groups - molecular and macroscopic. The molecular theories are associated with the names of Schallamach , Bartenev >, and Rieger, and have much in common. The basic idea is that bonds are formed at the interface, strained and then broken, and one form or another of Eyring s rate-process theory is applied. In effect one finds that the theories end with two main factors multiplied together. 

Figure 30.2 shows the microhardness of TiN/ZrN, TiN/NbN, and TiN/CrN multi-layer films as a function of the layer number for films with a similar total thickness of nearly 2 nm. For TiN/NbN and TiN/ZrN films with 180 layers (with monolayer thickness of 10 nm), the hardness is about 70-80 GPa, i.e., approximately that of diamond. The different behavior of TiN/CrN is connected with (Ti, Cr)N solution formation for more than 100 layers, as XRD analysis revealed. This observation indicates the significance of interlayer mixing to the mechanical strength that is proportional to the local energy density and therefore the presence of interface strain and trapping. 

Fig. 30.2 The layer number dependence of the microhardness of nitride films [12] indicates the significance of interlayer mixing of the mechanical strength that is proportional to the local energy density that depends on the interface strain and quantum entrapment (reprinted with permission from [14])...

In this section, we describe the effect of hydrostatic pressure on the nanocrystallite Ge embedded in a rather soft matrix such as Si02. The evolution of the interface strain between Ge NCs and the matrix with pressure is discussed in terms of a simple elastic model. Such information provides an insight in understanding the formation mechanism of nanocrystallite Ge embedded in Si02. 

---