Kinetic studies of Cr and Al deposition using CVD-FBR on different metallic substrates
Introduction
The industrial use of the fluid bed phenomenon has become widespread since its first use in the early 1920s. Its role in catalytic processing as well as in ore roasting is by now well known. Today, fluid bed technology covers a diverse range of applications in the chemical processing industries, including a broad range of gas/solid reactions and chemical vapour deposition (CVD) coatings [1]. The coatings obtained by CVD offer better environmental control [2], and thus industrial operations can be performed without the use of salt baths or toxic and hazardous solvents.
Briefly, the principles of the chemical vapour deposition in fluidized bed reactor (CVD-FBR) are as follows: particles of a few micrometres to a few millimetres are loaded in a conical or cylindrical reactor with a distribution plate through which the fluidizing gas is introduced (see Fig. 1) [3]. The existence of a coarse-grained refractory material placed at the bottom of the reactor over the distribution plate makes the preheating of the fluidizing gases unnecessary [4]. The gas flow in the entrance orifice must be greater than the terminal velocity of the particles, so that particles do not fall down into the orifice, and the terminal velocity of the gas in the parallel section above the orifice must be less than the terminal velocity of the particles, so they are not blown out of the coater. In order to fluidize the particles, the gas velocity in the parallel section must be greater than the minimum fluidization velocity, Umf, which is given by:where d is the particle diameter, ρp is the particle density, ρg is the gas density, G is the acceleration due to gravity, Uo is the superficial gas velocity, and μ is the gas viscosity [5].
The design of a proper reactor geometry which ensures uniform movement of solids and efficient gas–solid contact is important [6]. One has to bear in mind that the gas leaving the top of the bed carries with it entrained solid particles. It has been shown that as the gas travels upwards from the bed surface, the flux of entrained solids decreases until a certain height above the bed surface, called the transport disengaging height (TDH), is reached. Thus, industrial columns are designed so that the gas exits above the TDH [7].
For the coating of objects, two methods are available: in the first, when objects are not too large, they can be placed loosely in the particle bed; in the second, when the objects are larger, they can be suspended on a fixture that holds them in a fixed position in the particle bed.
The conditions imposed by a FB reactor, such as high and uniform heat transfer rate, temperature uniformity, high degree of mixing of the gases with fluidized particles, lead to a nearly complete reaction of all the active species in the fluidized bed [4], [8]. On the other hand, the main disadvantages of the fluidized bed CVD reactor are the necessity of the special design of the complicated gas distributor which is associated with the possibility of its plugging with deposit solids, and attrition of the agglomerates due to frequent relative movements [9].
This paper presents the fluidizing conditions for the deposition of aluminium and chromium onto substrates of AISI 304 stainless steel and IN-100 Ni-base alloy using subhalide chemistry [10]. Some of the coatings obtained under different conditions are also discussed.
Section snippets
Thermodynamic calculations
Before performing any fluidization experiments, thermochemical estimates were calculated by means of the HSC chemistry software [11] to ascertain the subhalide chemistry that would take place during the deposition experiments. For such purpose, fixed amounts of 3 and 4 mol of aluminium and chromium, respectively, were chosen. Then, the reactive gas input amounts were ranged in steps of 0.5 vol% from 0.5 to 3 vol% HCl(g) and from 5 to 30 vol% H2(g) in steps of 5 vol% (the rest being inert Ar gas).
Fluidization regimes
To
Thermochemical calculations
Fig. 2 shows the thermochemical estimations of equilibrium compositions of the different species of interest when 10 vol% H2(g) and 1 vol% HCl(g) is added to the aluminium system from room temperature up to 1000°C. It has to be noted that pure aluminium melts at 657°C, but when alloyed the melting temperature is higher. Therefore, this graphic is also valid when an aluminium master alloy is employed. It can be seen that Al2Cl6(g) is the main species formed up to about 410°C. Thereupon, AlCl3(g)
Conclusions
AlCl3(g) and CrCl2(g) have been shown to be the precursors for the deposition of aluminium and chromium, respectively, onto either the AISI 304 stainless steel or the IN-100 Ni-base substrates at the temperatures of interest. The reducing agent [H2(g)] is produced in the system even if this gas is not added to the gas mixture.
Very low flows have been found to be the most suitable at the deposition temperatures which allow the process to be cost effective. The total gas flows need to be
Acknowledgements
The present authors are very much obliged to the Comisión Interministerial de Ciencia y Tecnologı́a (C.I.C.Y.T.) for financial support of the project MAT96-0917 and to SRI-International for technical support.
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