Microstructures multilayers

It is now well established that a fiber coating must be deposited on the fiber prior to infiltration of the matrix, in order to control the fiber-matrix bonding and the mechanical behavior of the composite. Pyrocarbon (PyC), boron nitride or (PyC-SiC)- and (BN-PyC)-multilayers, with an overall thickness ranging from about 0.1 pm to about 1 pm, and displaying a layered crystal structure (PyC, BN) or a layered microstructure (multilayers), are the most common interphase materials in nonoxide CMCs. The main role of the interphase is to deflect the microcracks which form in the matrix under loading, and hence to protect the fiber from notch effect. 

Laminographical approaches can be used for layer-by-layer visualization of the internal microstructure for the flat objects (multilayers, PCBs etc.), that caimot be reconstructed by computerized tomography because of the limited possibilities in rotation. Depth and lateral spatial resolutions are limited by the tube, camera and rotation accuracy. Microfocus X-ray tubes and digital registration techniques with static cameras allow improving resolution. Precision object manipulations and more effective distortion corrections can do further improvement. 

An intriguing recent review of size effects in materials due to microstructural and dimensional constraints with a focus on mechanical properties, including those of multilayers, is by Arzt (1998). 

Apart from molecular, or supramolecular design, modified electrodes can be improved by microstructuring. Two different strategies have been developed multilayered electrodes 272) of microelectrodes 6,273)... 

Delplancke-Ogletree, M. R, Monteiro, O. R., and Brown, I. G., Preparation of TiC and TiC/DLC Multilayers by Metal Plasma Immersion Ion Implantation and Deposition Relationship Between Composition, Microstructure and Wear Properties," Materials Research Society Symposia Proceedings, Voi.438,1997,p.639. 

Wang, Z. J., "Microstructure and Magnetic Properties of Fe-N/TiN Nano-Multilayer, Ph.D. Thesis, University of Science and Technology Beijing, 1996. 

Molecular calculations provide approaches to supramolecular structure and to the dynamics of self-assembly by extending atomic-molecular physics. Alternatively, the tools of finite element analysis can be used to approach the simulation of self-assembled film properties. The voxel4 size in finite element analysis needs be small compared to significant variation in structure-property relationships for self-assembled structures, this implies use of voxels of nanometer dimensions. However, the continuum constitutive relationships utilized for macroscopic-system calculations will be difficult to extend at this scale because nanostructure properties are expected to differ from microstructural properties. In addition, in structures with a high density of boundaries (such as thin multilayer films), poorly understood boundary conditions may contribute to inaccuracies. 

In addition to bilayered electrodes with a functional layer and a support layer, electrodes have also been produced with multilayered or graded structures in which the composition, microstructure, or both are varied either continuously or in a series of steps across the electrode thickness to improve the cell performance compared to that of a single- or bilayered electrode. For example, triple-layer electrodes commonly utilize a functional layer with high surface area and small particle size, a second functional layer (e.g., reference [26]) or diffusion layer with high porosity and coarse structure, and a current collector layer with coarse porosity and only the electronically conductive phase (e.g., reference [27]) to improve the contact with the interconnect. 

The combination of various SOFC component performance, microstructural, and property requirements has led to a variety of structures, such as the composite, graded, and multilayered electrodes and electrolytes described above. The need... 

Song HS, Hyun SH, Moon J, and Song RH. Electrochemical and microstructural characterization of polymeric resin-derived multilayered composite cathode for SOFC. J. Power Sources 2005 145 272-277. 

In summary, it is clear that there are substantial effects that vary systematically with the wavelength of the multilayer due both to internal stresses and the microstructure of the coatings. It has also been seen that deformation can occur not just by dislocation flow, as the initial analyses have assumed, but by mechanisms such as lattice rotations and shear along column boundaries. In addition, the use of indentation complicates the deformation field, so that the assumption that equal strains in both layers are required need not be correct. These effects all influence the hardness but have not so far been included in analyses. 

It can be seen that ceramic multilayer structures have been produced with increments of the hardness of up to 60 GPa, increasing the hardness by up to a factor of almost 3. Initial work in this area has developed a number of ideas, such as the effect of modulus mismatch, which in some cases give good agreement with the models suggested but in many others do not. It is suggested that at least some of this discrepancy can be accounted for by differences in the microstructure and residual stress-state of the film, both of which are often poorly characterized. Furthermore there is very little direct evidence about how these structures deform and in particular about how different layers must be strained in order to accommodate the indenter when it is pressed into the sample. Further advances in this area will require the greater use of numerical techniques to analyse the complex stress and strain behaviour under the indentation, coupled with the use of recently developed techniques that allow the localized deformation behaviour to be observed in detail. 

Ljungcrantz, H., Engstrom, C., Hultman, L., Olsson, M., Chu, X., Wong, M.S. and Sproul, W.D. (1998), Nanoindentation hardness, abrasive wear, and microstructure of TiN/ NbN polycrystalline nanostructured multilayer films grown by reactive magnetron sputtering , Journal of Vacuum Science and Technology A, 16, 3104-3113. 

Figure 1.3. The microstructure of clay dispersed in the polymer matrix (a) conventional composite (b) extended polymer chains intercalated between the silicate layers, resulting in a well-ordered multilayer with alternation polymer/inorganic layers, and (c) silicate layers (1 nm thickness) exfoliated and dispersed in a continuous polymer matrix.

Michaels et al (5) further studied the barrier effects of the skin in terms of the composition of the pathways, as well as the microstructure, permeability and permselectivity of the particular pathways. In particular, the barrier to permeation in the stratum corneum was attributed not only to the interstitial lipids, but also to their structure as ordered multilayers for nonpolar alkanols. 

Specimens that contain materials with very different ion-milling rates, such as metallic multilayers grown on silicon substrates, often tend to form bridges of material across the perforated area. Ion-milling at very low angles of incidence ( 1-2°) in a direction parallel to the interface can sometimes be used to overcome or at least alleviate these bridging problems. Finally, it should be noted that the use of a crystalline substrate such as silicon provides a convenient reference material for specimen orientation purposes in the TEM. Examination of the substrate EDP can be used to ensure that the substrate normal is aligned exactly perpendicular to the electron beam direction. The thin-film microstructure can then be easily determined. 

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