Columnar deposits

Effect of Temperature. Generally, isotropic deposits are obtained at higher temperatures (>1400°C) and laminar and columnar deposits at lower temperatures. 

In summary, isotropic deposits are obtained at high temperature, low pressures, and low C/H ratio. The opposite conditions favor the deposition of laminar and columnar deposits. 

Figure 7.16. Schematic cross section (perpendicular to the substrate) of the columnar deposit.

The products are collected from different deposition areas inside the apparatus. A mixture of the collected soot is used for the experiments. Soft cathode soot is deposited around the copper rod behind the cathode. On the inner wall of the cooled copper tube cloth-like soot is produced. Occasionally, outer tube soot is found on the outside of the large copper pipe around the viewing hole. Similar to the arc-discharge of pure graphite rods in MWCNT production, a columnar deposit is formed on the surface of the cathode. A small amount of puffy powders is found on the inner wall of the reactor, but it is not collected. 

Electric Fuel Ltd. which was modified by the inclusion of an anionic polymeric membrane. The polymeric membrane was composed of interpenetrated network of two polymers. A polycationic cross-linked polyepichlorohydrin was used as the ionic network and poly(hydroxyl ethyl methacrylate) was used as the structural polymer to provide mechanical stability and reduced swelling. The cyclic performance of the cell using a saturated aqueous solution of LiOH and untreated ambient air is shown in Fig. 13b. Relatively high capacities were obtained however, on cycling the lithium metal formed a porous or columnar deposit that increased in volume and caused a loss of contact between the hthium metal and O-LATP [50]. The hfetime of this composite air electrode when used with untreated air in 5 M or saturated LiOH aqueous solution was increased firom 10 h without the anion exchange membrane to 1000 h. 

Columnar Structure and Grain Size. Most of the deposited films reported in the Hterature have a so-called columnar stmcture (see Fig. 

The stmcture of the polysihcon depends on the dopants, impurities, deposition temperature, and post-deposition heat annealing. Deposition at less than 575°C produces an amorphous stmcture deposition higher than 625°C results in a polycrystalline, columnar stmcture. Heating after deposition induces crystallization and grain growth. Deposition between 600 and 650°C yields a columnar stmcture having reasonable grain size and (llO)-preferred orientation. 

Fibrous stmctures represent a grain refinement of columnar stmcture. Stress-reHeving additives, eg, saccharin or coumarin, promote such refinement, as do high deposition rates. These may be considered intermediate in properties between columnar and fine-grained stmctures. 

During the discharge one electrode is moved in such a way that the discharge remains stable (this can be monitored by the current value). After this period, a deposit (10-15 mm long) forms on the cathode, which is composed of a hard grey shell formed, and a black inner core. An eye observation of the black core easily reveals a columnar texture in the direction of the deposit growth. The columns are actually formed by bundles of CNTs. 

At higher temperatures, deposits tend to be columnar (Fig. 2.13 a and b) as a result of uninterrupted grain growth toward the reactant source. The structure is also often dependent on the thickness of the deposit. For instance, grain size will increase as the thickness increases. A columnar-grain structure develops, which becomes more pronounced as the film becomes thicker. 

After the silicon is completely reduced. Reaction (3) stops and Reaction (1) is resumed. The temporary deposition of sacrificial silicon interrupts the tungsten grain growth as new nucleation sites are created. Larger grain and columnar growth are essentially eliminated... 

CVD graphite can have several structural forms columnar, laminar, or isotropic. The columnar form is shown in Fig. 7.2. The crystallites are deposited with the basal planes (ab directions) essentially parallel to the deposition surface. Their structure tends to be columnar (cone-like) as a result of uninterrupted grain growth toward the reactant source. 

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