Plasma spray method

The other important factor to affect the operational conditions of the cell is the voltage increase between the carbon and copper lead. This problem has been solved individually in industry. For example, a 250 pm thick layer of nickel can be coated onto the upper part of the carbon anode using the atmospheric plasma spraying method.7 This electrode has been operated at 15 to 17 A dm-2 in a 1000 A scale industrial cell for 19 months. The cell voltage was 9.5 V and polarization did not occur with this electrode. Characteristic points of this new carbon electrode are low polarizability and no anode effect, and the concentration of carbon tetrafluoride contaminating the fluorine is below 2 ppm. 

The surface of upper disc is coated with a layer of TiC/Mo materials by a plasma spray method . The TiC surface has emissivity of approximately 0.9 and can be heated up to a temperature range of 1800 to 2000 [K] by solar concentration. Furthermore, such a FGM layer reduces the stress caused by difference of thermal expansion between TiC surface and Mo base. 

Oh, S. et. al., Multilayer ionic devices fabricated by the plasma-spray method. Solid State Ionics 53-56 (1992) 90-94. 

A plasma spray method is also a well-studied method for FGM fabrication. In this method, the source powder (metal and/or ceramic) is transported to the plasma jet by a torch nozzle. Then the source materials in a molten state by plasma are sprayed onto a substrate to form a coating. The composition gradient within the sprayed coating can be realized by changing the composition of the supply source. To date, various kinds of ceramic/metal, ceramic/ceramic and metal/metal FGMs (such as Zr02/NiCrAl, NiCr/Al and NiCrAlY/YSZ) have been fabricated by the plasma spray technique [40, 41]. 

Boron-reinfixced aluminum is a technologically mature continuous-fiber MMC (Fig. 2). Applications for this composite include tubular truss members in the mid-fiiselage structure of the space shuttle orbiter and cold plates in electronic microchip carrier multilayer boards. Fabrication processes for boron/aluminum composites are based on hot-press diffusion bonding of alternating layers of aluminum foil and boron fiber mats (foil-fiber-foil processing) or plasma-spraying methods. 

In Section 4.3 several recently developed and experimental surface preparation methods for steel such as conversion coatings, plasma spray and sol-gel that attempt to simultaneously improve durability and bond strength over grit blasting will be discussed. 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

Alternative lower cost fabrication methods to sintering and electrochemical vapor deposition (EVD) are receiving more attention. These methods include plasma spraying and chemical vapor deposition (CVD). Many development projects are being conducted in fabrication techniques. Examples of some of the work follow. 

In the field of metallic powder applications, a method of plasma spray coating suitable for biomedical materials has been developed using titanium and calcium phosphate composite powder. By means of the mechanical shock process, the appropriate composite powder was prepared, and plasma sprayed on Ti substrate under a low-pressure argon atmosphere. A porous Ti coating layer was obtained in which the surface and the inside of the pores were covered thinly with hydroxyapatite. This surface coating is expected to show excellent bone ingrowth and fixation with bone (21). 

The physical and metallurgical properties of plasma sprayed coatings are generally superior to all other flame spraying methods, with the exceptions of hypersonic combustion and detonation gun systems. 

There are two ways in which coatings can be applied thermomechanical processes (e.g. detonation gun, flame spraying and plasma spraying) and vapour phase deposition processes. The latter category can be subdivided into CVD (chemical vapour deposition) and PVD (physical vapour deposition). In the case of a CVD process, a chemical reaction takes place in an oven and as a result the coating material is formed and deposited on the object. Figures 11.7.9 and 11.7.10 are representations of two methods to apply coatings. 

The method of preparation is the most crucial and usually the least controllable variable, so much that it is usually taken as the last chance to explain any inconsistent behaviour. Unfortunately, this remains the least understood parameter, but also the most open one to originality and inventiveness. It is hard to list all methods, also because in many cases details have been left purposely obscure. However, to a first approximation, cathodes can be coated with an active layer by (i) electrodeposition [142], (ii) thermal decomposition [143], (iii) flame and plasma spraying [144-146], (iv) in situ activation [147, 148]. Cathodes may also be prepared from the powder... 

It has already been mentioned that one of most used forms of Ni is Raney Ni which is obtained from Ni-Al or Ni-Zn alloys by leaching A1 or Zn in alkaline solution. However, the properties of the resulting electrocatalyst appear to depend on the nature of the precursor [135], Methods of application of the alloys are various [135]. A particularly convenient one is the so-called LPPS (low pressure plasma spray) [146]. Raney Ni prepared in this way has shown that lower Ihfel slopes can be obtained, thus suggesting a real electrocatalytic effect (Fig. 11). On such highly porous Ni it is possible that the proportion of particularly active sites (at the edges and peaks of crystallites [262] increases considerably. However, the effect of temperature on the Tafel slope is more than anomalous [248] suggesting indeed some temperature-induced surface modifications. In fact, recrystallization phenomena are observed which can be minimized by means of small additions of Ti, Mo or Zr. The... 

In contrast to plasma spraying, low pressure plasma CVD does not require any remote handling technique. However, there is yet no experience with large scale applications of this method, particularly in metallic vessels. This is the first goal towards which future studies have to be directed. Very little is also known about the adherence of the coatings deposited by low pressure plasma CVD and their resistance against thermal shock. The choice of the best material for such coatings is presently open to discussion. 

The electrolyte (YSZ) layer ( 40 fim thick) is applied to the cathode by EYD (see Section 3.6.9). Because this is an expensive route efforts are made to use more cost-effective methods, for example plasma-spraying and electrophoretic deposition followed by sintering. 

The shielding blanket is composed of water-cooled steel modules, which are directly supported by the vacuum vessel and are effective in moderating the 14MeV neutrons, with a water-cooled copper mat bonded to the surface of the modules on the plasma side, and protected from interaction with the plasma by beryllium. Manufacturing considerations can be found elsewhere [48]. The first wall incorporates two start-up limiters located in two equatorial ports. With the aim to reduce cost and nuclear waste, the design includes a modular and separable first wall. This allows damaged or eroded blanket modules to be repaired inside the hot cell either by replacement of panels or by plasma spraying or other methods. 

The damaged surface of a ceramic after finishing is sometimes healed by coating. Coating methods include (1) sol—gel, (2) laser gazing, (3) chemical vapor deposition (CVD), (4) sputtering, (5) plasma spraying,... 

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