Principles and applications of CVD powder technology Constantin Vahlas, Brigitte Caussat, Philippe Serp and George N. Angelopoulos Centre Interuniversitaire de Recherche et d’Ingénierie des Matériaux, CIRIMAT-UMR CNRS 5085, ENSIACET-INPT, 118 Route de Narbonne, 31077 Toulouse cedex 4, France Laboratoire de Génie Chimique, LGC-UMR CNRS 5503, ENSIACET-INPT, 5 rue Paulin Talabot, BP1301, 31106 Toulouse cedex 1, France Laboratoire de Catalyse, Chimie Fine et Polymères, LCCFP-INPT, ENSIACET, 118 Route de Narbonne, 31077 Toulouse cedex 4, France Department of Chemical Engineering, University of Patras, University Campus, 26500 Patras, Greece Abstract Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro- fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique. Keywords: Powders; Chemical vapor deposition; Fluidization; Coatings; Films 1. Introduction 2. Gas-powder contactors and CVD processes: chemical engineering aspects 2.1. Ways to ensure gas–solid contact in CVD on particles 2.2. Basic principles of fluidization and of powder classification 2.3. Behavior of powders in FBCVD reactors, advanced classifications 2.4. Constraints on fluidized bed due to the CVD process 2.5. Types of reactor for performing CVD on powders 2.5.1. Classical FBCVD reactors 2.5.2. Spouted-bed reactors 2.5.3. Specific technologies 2.5.4. PECVD and other activated technologies 2.6. Modeling of FBCVD reactors 3. Materials produced by fluidized bed CVD 3.1. Supported catalysts 3.1.1. Catalytic performance of CVD-FB prepared materials 3.1.2. Some important statements and phenomena linked to CVD preparation of supported catalysts 3.1.2.1. Selection of the precursor and of the support 3.1.2.2. Design of the FBCVD reactor 3.1.2.3. Role of the substrate surface and of the gas phase on the dispersion and the chemical nature of the active phase 3.2. Wear, oxidation and temperature-resistant materials 3.3. Microelectronic and photovoltaic applications 3.4. Nuclear materials 3.5. New materials 4. Other powders involving CVD processes 4.1. Coating parts in FBCVD reactors 4.1.1. Principles of the process 4.1.2. Types of coatings 4.1.2.1. Ti-coatings 4.1.2.2. Cr-coatings 4.1.2.3. V-coatings 4.1.2.4. Boriding 4.1.2.5. Si-coatings 4.1.2.6. Al-coatings 4.2. Fabrication of powders by CVD 5. Conclusions Acknowledgements References 1. Introduction Modification of surface properties of powders that are used as solid process materials in industrial applications, through either grafting or deposition of films or coatings, is an area of science and technology which interests numerous fields: fabrication of monolithic or composite parts, mechanics, transport (vehicle structure and motors), catalysis, energy production, microelectronics, optoelectronics, leisure industry, etc. The properties required for engineering structures are protection against corrosion, oxidation or wear, biocompatibility, wetting, adhesion, durability, catalytic activity, toughness, etc. Surface treatments of particles are applied through various techniques, either wet, such as sol–gel and impregnation, or dry, such as pyrolysis and chemical vapor deposition (CVD). Choosing the most appropriate depends on the requirements of the surface treatment and on the characteristics of the particles to be coated, such as shape, density, porosity, size and size distribution. Dry techniques involving surface modification of powders imply the use of a reactive gas phase. In this case, a complex process prevails, composed of two elemental processes taking place simultaneously: (i) mixing of powders in such a way that their entire surface is presented to the reactive gas phase and (ii) either homogeneous or heterogeneous gas–solid chemical reactions. Fluidization is a well-known particle treatment process among all those used to meet the requirement of gas–solid contact [1]. It has been traditionally used for several decades in very large units of petroleum and mineral industries. On the other hand, gas–solid reactions are often used in the context of various CVD processes: those involving either halogen or organometallic chemistry ensuring robust or flexible, low temperature operation, respectively, or even those involving plasma activation, taking thus advantage of non- equilibrium plasmas providing active species at low temperatures compatible with a wide range of substrate materials. Therefore, the association of such mature techniques, namely fluidized bed chemical vapor deposition (FBCVD) is expected to lead to innovative, flexible and cost-effective particle treatment processes [2]. Fig. 1 presents the evolution of published papers dealing with CVD on—mainly fluidized powders as a function of the year of publication. The histogram and the curve illustrate the percentage (left) and the cumulated percentage (right), respectively, for a sample of 150 papers. Although this amount is not exhaustive, it can be considered as representative of the scientific and technical production in this field in the open literature and in patents. The period covered in this diagram concerns the years 1981–2004. Only a limited number of publications are available for period 1960–1980, and they essentially deal with particle coatings for nuclear applications as discussed later. This indicates that CVD on powders is a relatively new technique. During its childhood (1980–1990), its extension was relatively slow since yearly published documents up to 1994 were merely the double of those published in 1981, with an exception in 1991 when, interestingly, numerous reports were published which aimed at applications other than nuclear. Fig. 1. Numbers of papers dealing with CVD on powders as a function of the year of publication. The histogram illustrates the percentage (left) and the curve the cumulated percentage (right) for a sample of 150 papers. Since 1994 and especially since 2000, the slope has become steeper. Trends in the first months of 2005 (not reported) are compatible with those observed in this figure. Indeed, process and material investigations on the previously mentioned application domains, also including the use of CVD for the production of powders, for example, ultrapure silicon, and for the surface treatment of bulk pieces immersed in a FB, are far from being exhaustive. They concern rather thick and continuous deposits on dense (non-porous) powders. The numerous experimental and theoretical studies performed on this topic improved the understanding of various undesirable phenomena occurring during deposition, such as bed clogging and formation of parasitic fines. However, the recent strong rise in the volume of the scientific and technical literature in this field is mainly attributed to new classes of materials for which large-scale production through a reliable, reproducible and robust process is a necessary condition of development. In general, such new trends in powder processing by CVD concern deposition of nanometric films on powders consisting of particles which present one or more of the following characteristics: meso- or micro-porosity, submicron size, wide size distribution or shape factor much less than one. Production of supported catalysts on particles is a domain where CVD on powders can potentially replace more traditional, wet methods like liquid impregnation, which prevails nowadays for the production of such materials. This field of application is of great interest to industry and can hardly be overestimated: in the USA, the “catalysis industry” is the largest single economic activity, with an annual turnover in solid catalysts as such of around US$ 7 billion [3]. The importance of nanomaterials for society and economy is foreseen to become extremely strong in the future. Among them, carbon nanotubes (CNTs), is a unique material, whose wide application development is critically dependent on the availability of methods for low-cost, large-scale production. It will be shown below that FBCVD is a promising response to this limitation, which also restricts fundamental research on CNTs and nano-objects in general. However, the extension of the application of CVD on powders to these and other new domains of nanotechnology requires additional hard points of the process to be overcome. These can be summed up as individual particle treatment, efficient gas–solid mixing and confinement of submicron powders, while for instance, operating at sub-atmospheric pressure, in continuous mode, with a bed whose characteristics are continuously modified during the operation. Such constraints require appropriate reactor design and finely tuned operating conditions. The present review aims to provide information to assist the chemical engineer and the materials scientist to meet these constraints. The manuscript is organized as follows: first, the different ways to induce an efficient contact between a gas and particles will be presented and discussed, followed by the basic principles of fluidization and the classification of powders as a function of their behavior in a FBCVD reactor. Then, the constraints on FBs brought about by the simultaneous operation of CVD will be developed. This will lead to the presentation of the different types of CVD reactors for deposition on powders, and for their surface treatment. This first part, essentially dealing with chemical engineering aspects, will be completed with a detailed presentation of modeling of FBCVD reactors and processes. The second and third parts will develop the state of the art on materials produced by FBCVD. The organization of the second part will be based on the different application domains, namely surface treatment of powders for the preparation of supported catalysts, of wear, oxidation and heat resistant materials, of materials for microelectronics and photovoltaics, of nuclear fuel and finally for new materials. The fourth part mainly focuses on the use of FBCVD reactors for the coating of bulk parts, mainly for the formation of wear and corrosion resistant protective coatings on metals. Finally, the fabrication of powders (including nano-objects; e.g. nanotubes) by CVD is discussed, before providing concluding remarks. 2. Gas-powder contactors and CVD processes: chemical engineering aspects 2.1. Ways to ensure gas–solid contact in CVD on particles Ensuring efficient contact between the gas phase and the growing surface in CVD processes is rarely a problem for deposition on flat substrates. A different situation prevails when deposition is attempted on the entire available surface of a piece lying on a support. CVD on powders corresponds to this latter case. The technical solutions developed to face this situation are shown schematically in Fig. 2, adapted from ref. [4]. The simplest ones are fixed bed and flat hearth, where gas flows, respectively, through and over powders whose particles do not move relative to each other. In this case, if the gas flow rate is sufficiently low, the gas merely percolates through the void species between the stationary particles. This mode of gas solid contact has often been used for CVD processes devoted mainly to catalyst preparation and CNT synthesis (see refs. [5] and [6] and references therein) and recently to grafting metalorganic species in mesoporous silica [7]. However, there are numerous drawbacks related to these simple solutions: (i) as in the case of chemical vapor infiltration, diffusion limitations may affect the efficient contact between the gas reactants and the inner particles; (ii) it is impossible to perform deposition on the entire surface of each particle due to contact points among them and with the reactor walls; (iii) when high growth rates and/or high thickness is aimed, clogging phenomena can occur; (iv) finally, due to the previous drawbacks, the scale-up of these processes is problematic. An improvement of this situation is to impose a movement to the particles of the powder relative to each other. Fig. 2 depicts technical solutions that have been used to induce relative particles movement. These are rotary cylinders, pneumatic conveying, vertical moving beds and fluidized beds. In such cases, it is expected that each particle will present its entire surface to the gaseous reactants during the process. The use of rotary cylindrical reactors has been reported for the deposition of tungsten carbide (WC) on fine titanium nitride (TiN) powders [8]. However, strong thermal gradients detrimental to the control of the process and to deposit quality often occur in such reactors. Fig. 2. Contacting modes for CVD on powders in gas–solid reactors. Adapted from ref. [4]. The most frequently employed method to treat powders by CVD is by maintaining them in a FB regime. Through this technique, a bed of solid particles over a gas-distributing plate (often called the grid), is made to behave like a liquid by passing gas through it at a flow rate above a certain critical value [1]. Compared with deposition on flat surfaces (wafers), FBCVD presents considerable differences. Indeed, in order to ensure satisfactory uniformity of deposits on each wafer and from wafer to wafer, most classical industrial CVD processes, especially those running under low pressure, are organized in terms of temperature, pressure and gas flow rates so as to be limited by surface kinetics. In fluidized bed deposition, the situation is drastically different. Substrates are powders and often (micro-)porous, resulting in extremely high available growth surface to heated volume ratios (S/V) in the deposition area. Fig. 3 illustrates this trend [9]. The histogram reports typical S/V values for atmospheric pressure single wafer and low pressure multi-wafer industrial reactors, for laboratory scale fluidised-bed reactors operating with non-porous and with porous powders (specific surface area of 1 and 160 m2/g, respectively), and for industrial reactors for the chemical vapor infiltration of pyrolytic carbon. It can be noticed that the S/V ratios for these processes extend over 6 orders of magnitude. CVD on flat surfaces does not exceed 102 m−1, to be compared with FBCVD for which this ratio is between 104 and 3 × 107 m−1. As expected, S/V for chemical vapor infiltration (CVI) is very high, but it is still more than 100 times lower than FBCVD on porous particles. Fig. 3. Typical values of growth area per heated volume in the deposition region for different CVD processes [9]. Although Fig. 3 provides trends rather than precise values, it does illustrate why surface reactions are so extended that gaseous precursors are very often totally consumed a few centimeters after their entrance into the fluidised-bed reactor in a laboratory scale contactor. Fortunately, the vigorous mixing of the particles always compensates for this potential source of error, and ensures uniform deposition. Due to the high degree of gas–solid mixing, diffusive phenomena only occur near the boundaries of bubbles, which may be formed by the fluidizing gas, and mass transfer rates are mainly convective, ensuring homogeneous deposition. A further consequence of the high degree of contact between gases, powders and reactor walls in FBCVD, in contrast with classical CVD processes, is that fluidized contactors intrinsically lead to high heat transfer rates, thus ensuring isothermal conditions both radially and axially in the bed [10]. Due to these intrinsic advantages, FBCVD has been extensively used for deposition on particles [9]. However, the evolution of technological requirements in terms of: (i) types of powders to be treated, (ii) properties of use and (iii) environmental constraints means new techniques have been proposed in the open literature or in patents to ensure deposition on powders. Based on this situation, the basic principles of classical fluidised-bed CVD reactors will be first presented in this part. Then, the constraints imposed either by the CVD process or by the powders to be treated will be developed and technical solutions to face these new situations will be reviewed. Finally, modeling of CVD fluidised-bed reactors as a tool to understand and optimize processes and materials will be briefly discussed. 2.2. Basic principles of fluidization and of powder classification The behavior of a fluidised-bed reactor is so complex that, according to Geldart … it is more difficult to predict it than to calculate the arrival time of a space probe traveling to Saturn … ([10], p. 11)! This difficulty is systematically pointed out in the numerous books dedicated to fluidization [1], [4], [10], [11] and [12]. In essence, the behavior of the bed depends on the particle characteristics and on gas velocity. Low flow rates correspond to a fixed bed. At higher velocity, fluidization takes place and generally gives rise to heterogeneous systems, characterized by gas bubbles which form at the bottom of the bed, just above the distributor, travel through the bed, grow, coalesce and then burst at the upper surface. These two regimes are separated by a third one, characterized by a homogeneous situation for which practically no influence of bubbles has to be taken into account. Bed expansion is initiated in this latter regime, which corresponds to the minimum fluidization velocity, U . For gas velocities mf higher than U , the drag force on an individual particle exceeds the force exerted by gravity, mf thus leading to fluidization. The morphology of the particle bed, which is representative of the different fluidization regimes, is schematically illustrated in the upper part of Fig. 4(a). Crossing of the bed by the gas flow results in a pressure drop (∆P) through the bed. ∆P variations with gas velocity provide essential information on the characteristics of the bed. For this reason, the hydrodynamic behavior of the latter is investigated by ∆P measurements between the bottom and the top of the bed. Such variations are schematically reported in the lower part of Fig. 4(b). In zone (a), corresponding to low gas velocity, the bed is fixed and ∆P is proportional to gas velocity. U is the gas velocity at the intersection point between this mf straight line and the plateau observed during fluidization (zone c), for which ∆P equals the weight of particles per unit area. Measurements performed at increasing and decreasing flow rates lead to the hysteresis observed in zone (b), due to lower interparticle forces for defluidization conditions. For this reason, measurements of U performed in practice at mf decreasing flow rates. For velocities higher than the terminal velocity of particles, ∆P decreases due to powder entrainment. Optimum operation conditions for laboratory scale contactors generally correspond to three or four times the U and to a quantity of powders mf yielding a ratio between the height of the static bed and the diameter of the reactor of between 2 and 4. Fig. 4. Pressure drop–gas velocity relationship, characteristic of fluidized-bed reactors (lower part) and schematics of the contacting form of the powder by the gas, representative of the different domains in this relationship (upper part).