Polycrystalline material, grain

Grains and their sizes are very important variables characterizing the microstructure of polycrystalline materials. Grain-boundary movement plays a significant role in the characteristic behavior of materials for creep application. 

The coexistence of crystals and deeply supercooled liquids was suspected already one century ago for bulk systems [45]. More recently, the ice-water coexistence was reported by experiments, especially in the temperature range 140-21 OK [46-49], and by simulations in NML [50,51], evidencing the presence of 15-20% of liquid water between nanometer-sized icecrystals [50]. Notably, recent simulations concluded that in polycrystalline materials grain boundaries exhibit the dynamics of glass-forming liquids [52]. 

In polycrystalline materials, ion transport within the grain boundary must also be considered. For oxides with close-packed oxygens, the O-ion almost always diffuses much faster in the boundary region than in the bulk. In general, second phases at grain boundaries are less close packed and provide a pathway for more rapid diffusion of ionic species. Thus the simplified picture of bulk ionic conduction is made more complex by these additional effects. 

Fig. 10.4. Ball bearings can be used to simulate how atoms are packed together in solids. Our photograph shows a ball-bearing model set up to show what the grain boundaries look like in a polycrystalline material. The model also shows up another type of defect - the vacancy - which is caused by a missing atom.

Other applications of REELM include monitoring variations like oxidation, segregation, and hydration in the surface chemistry of polycrystalline materials. Differences of 1 /10 of a monolayer in oxygen coverage due to variations in grain... 

The obvious application of microfocus Raman spectroscopy is the measurement of individual grains, inclusions, and grain boundary regions in polycrystalline materials. No special surface preparation is needed. Data can be obtained from fresh fracture surfeces, cut and polished surfaces, or natural surfeces. It is also possible to investigate growth zones and phase separated regions if these occur at a scale larger than the 1-2 pm optical focus limitation. 

The sputtering process is frequendy used in both the processing (e.g., ion etching) and characterization of materials. Many materials develop nonuniformities, such as cones and ridges, under ion bombardment. Polycrystalline materials, in particular, have grains and grain boundaries that can sputter at different rates. Impurities can also influence the formation of surface topography. ... 

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

Conventional electrodeposition from solutions at ambient conditions results typically in the formation of low-grade product with respect to crystallinity, that is, layers with small particle size, largely because it is a low-temperature technique thereby minimizing grain growth. In most cases, the fabricated films are polycrystalline with a grain size typically between 10 and 1,000 nm. The extensive grain boundary networks in such polycrystalline materials may be detrimental to applications for instance, in semiconductor materials they increase resistivity... 

The behavior of polycrystalline materials is often dominated by the boundaries between the crystallites, called grain boundaries. In metals, grain boundaries prevent dislocation motion and reduce the ductility, leading to hard and brittle mechanical properties. Grain boundaries are invariably weaker than the crystal matrix, and... 

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

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