Microstructural Instability

It has been reported that the permittivity of a material decreases with decreasing film thickness. As a result, the capacitance increase is not as large as expected based on the inverse relation between C and t. The reason is the existence of dielectric dead layers at the dielectric s surface and interfaces, or interfacial layers between the dielectric and its neighboring materials, which in both cases are characterized by a much lower permittivity. As the dead layer is in series with the dielectric, the effective permittivity is reduced as well. In the case of MOS devices, a notorious interfacial reaction is the formation of silicates between the dielectric and silicon, hence the importance of the thermodynamic stability of the dielectric on Si. In the case of MIM (metal-insulator-metal) devices, alloy formation or in-diffusion of the electrode is a possibility, for example, in the case of Pt. However, also in case no secondary phases are formed, permittivity can be thickness dependent. Possible reasons include breaking of the lattice periodicity or the presence of ion vacancies at the interface, which disturb or inhibit the soft phonon mode and other intrinsic effects [6]. It was shown that electrodes with a shorter electronic screening length, for example, Pt or Au, lead to a smaller dead layer effect compared with, for instance, SrRuOs electrodes [13]. 

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As an additional example of high practical significance, we refer here to copper depKJsits when used in microelectronics, mirrors, and other optical applications. Those deposits have been observed to soften in time even when stored at room temperature for only 4 to 6 weeks. Also, mirrors and other precision objects made of copper will undergo surface deformation after a few months. This type of degradation can be counterbalanced by a suitable metal overcoating. Another, not always practical way is heat treatment to about 300°C. These phenomena are the direct results of microstructural instabilities, often referred to as recrystallization in the copper. It is worth stressing that recrystallization is not limited to copper (5). 

The Si-C-0 fibers produced from oxygen cured PCS precursor fibers at 1200-1300°C are unstable at high temperatures. Si-C fibers, i.e., almost pure SiC or SiC + C mixtures, are thermodynamically stable to 2500°C. The only cause of microstructural instability is the extremely small grain size of the phases. When such fibers are maintained at high... 

In the opposite direction, the constant miniaturization of electronic components, driven by Moore s law, necessitated that both lateral sizes and different layer thicknesses in electronic components be decreased. This soon became a driving force for the investigation of the lower limit of film thicknesses, which could be deposited using sol-gel precursors by, for instance, spin coating, dip coating, spray coating, and so on. In early literature, several issues were identified such as microstructural instability and the so-called dead layer effects, which inhibited any functional properties to be observed below a critical film thickness. 

Figure 27.23 Some methods of obtaining nanostructures from sol-gel solutions, (a-c) Microstructural instability method showing how a thin film breaks into islands for a grain size to film thickness ratio. Images correspond to cross-sectional TEM photographs of films with different thickness. (Reprinted with permission from Ref. [62].) (d-f) Microemulsion-mediated synthesis method showing a scheme with the preparation method to obtain an array of nanostructures on polycrystalline Pt-coated silicon substrates. A micellar solution containing the building units...

Seifert, A., Vojta, A., Speck, J.S., and Lange, F.F. (1996) Microstructural instability in single-crystal thin films. /. 

Clearly, then, the chemical and physical properties of liquid interfaces represent a significant interdisciplinary research area for a broad range of investigators, such as those who have contributed to this book. The chapters are organized into three parts. The first deals with the chemical and physical structure of oil-water interfaces and membrane surfaces. Eighteen chapters present discussion of interfacial potentials, ion solvation, electrostatic instabilities in double layers, theory of adsorption, nonlinear optics, interfacial kinetics, microstructure effects, ultramicroelectrode techniques, catalysis, and extraction. 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

Although equation 33 gives a physical description of the mechanism of the instability that leads to microstructure formation during solidification, it is not rigorous because it does not consider the effects of the rates of heat and species transport on the evolution of the disturbance. Because of this deficiency, equation 33 cannot be used as a basis for further analysis of microstructure formation. This deficiency is shown clearly by the inability of equation 33 to predict the spatial wavelength of the microstructure formed along the interface. 

The Nicalon fibre (10-20 pm) available commercially consists of a mixture of P-SiC free carbon and Si02 [28], The properties of Nicalon start to degrade above about 600°C because of the thermodynamic instability of the composition and microstructure. Ceramic-grade Nicalon fibres, designated the NL series, having low oxygen content are also available. 

The performance of a cast nickel-based alloy is generally based on the microstructural quality, such as the amount of interdendritic segregation, secondary carbides, and intermetallic phases. With the same overall chemical composition, the corrosion rate of the same alloy can vary by several orders of magnitude, depending on its particular microstructure. The most important metallurgical factors that need to be considered are second-phase precipitation by thermal instability and the presence of cold work. The latter is especially important in cases where SCC may be expected. (Rebak)5... 

Wood has many excellent mechanical properties that result from its exceptional combination of microstructural, ultrastructural, and molecular features. The main drawback is its dimensional instability in the presence of moisture. One main reason for modifying wood chemically is to reduce this instability. Other chemical treatments of wood reach the crystalline region of cellulosic microfibrils, destroying the crystalline structure, thus eliminating most of the composite structure of wood. The resulting material does not have any of the characteristic properties of wood but it may be provided with thermoplasticity. A chemical treatment of wood as defined here excludes such radical modifications. We refer to a chemical treatment that may reduce some defects relative to wood utilization and enhance its properties while keeping the bulk of the superior mechanical properties of wood. 

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