Sill/ace and Coatings Technology, 54/55 (1992) 219-223

Chemical vapor deposition in fluidized bed reactors

219

A. Sanjurjo, K. Lau and B. WoodInorganic Materials and SUI/aces Program, Materials and Chemical Engineering Lahoratory, SRI International, Menlo Park, CA 94025·3493 (USA)

Abstract

The intrinsic high rates of heat and mass transfer of fluidized bed reactors can be uscd to significantly expand applications forchemical vapor deposition. This paper illustrates some of the uses of fluidized bed reactors for depositing homogeneous coatings bychemical vapor deposition. Conventional approaches to coating particles and new developments for coating large substrates arepresented. The usc of su bhalide chemistry for coating at low temperatures is also described.

1. Introduction

Chemical vapor deposition (CVD) has become a stan­dard coating technique with applications in almost everyfield of the technology of materials. Some of the advan­tages of CVD include the capability to coat conformallycomplicated geometries, its relatively low cost, particu­larly for atmospheric CVD, and its flexibility. Manymaterials can be deposited on different substrates usinga variety of chemistries.

The limitations of CVD include the occasional needto use relatively high temperatures of deposition, thepartial depletion of the reactants as deposition proceeds,which may lead to coating inhomogeneities and varia­tions in thickness, and (in some cases) difficulty inmaintaining the substrate at uniform temperature.

The use of CVD could be further extended if some ofthese limitations were minimized. Indeed, a great efforthas been made to extend the use of CVD in every area.For example, the use of labile organometallic moleculesor plasmas allows deposition at the relatively low tem­peratures that most inorganic substrates can withstand.Specially designed gas injectors, reactors and heatingsystems have also helped to minimize some of theselimitations. Nevertheless, the future potential is clear for(1) lower and more uniform temperatures and (2) betterheat and mass transfer which avoids the effects ofreactant depletion and coating non-uniformity as depos­ition proceeds. The use of fluidized bed reactors canovercome some of these limitations and provide anotherimportant choice to the CVD coater when selecting acoating technique.

The theory and practice of fluidized bed reactors(FBRs) are described in great detail in many textbooksand journals (see e.g, ref. I). Briefly, particles varying insize from a few micrometers to a few millimeters are

0257-8972/92/$5,00

loaded in a conical or cylindrical reactor with a bottomfrit or distribution plate through which a fluidizing gasis injected (Fig. I). At low gas linear flow velocities theparticles in the bed remain in repose. As the linear flowvelocity increases, it reaches a value above which theparticles become suspended in the gas stream and thebed of particles behaves as a liquid. At even highervalues the particles are entrained and eleutriated out ofthe bed. The main characteristic of a fluid bed is that ithas very large heat and mass transfer rates. Conse­quently, the temperature and the gas composition in thebed are uniform; therefore the deposition rates are alsouniform, resulting in uniform and homogeneous coat­ings. Because of its attributes, the FBR has been preferredby chemical engineers for many petrochemical, energy

Thermocouple

Furnace

Substrates -+-l-~"

I-----J.-_+_ FkJldlzed BedReactor

'-----Distribution Plate

,..-H,FkJldlzlng Gas

Fig.!. Fluid bed reactor with particles of an element M*.

© 1992 - Elsevier Sequoia. All rights reserved

220 A. Sanjurjo et al. I CVD in fluidized bed reactors

generation (coal combustion), metallurgical, catalytic,thermal (drying) and polymer-coating applications.

This paper is divided into three sections. First wedescribe some typical inorganic coating applications inwhich the particles in the bed are the substrate. Secondlywe describe some applications reported in the literaturein which beds of inert particles are used to coat largeobjects in an FBR. Thirdly we review a relatively newdevelopment in which the particles in the bed are reactiveand serve as the source of the coating material as wellas a medium with high heat and mass transfer rates.

2. Coating of particles

Many papers have been published on this topic. Wegive here only a few examples to illustrate the capabilitiesof FBRs. In the 1960s and 1970s most of the workfocused on coating the small spherical (0.1-1 mm diame­ter) nuclear fuel pellets (U02, Th02 UC, ThC) that areused in gas-cooled reactors [2-8]. The main objectivewas to provide a coating that could contain the fissionproducts generated during reactor operation at temper­atures up to 1500°C and protect the particles fromerosion. Pyrolytic carbon was deposited in the temper­ature range 1200-2000°C from hydrocarbons such asCH4 , C2H2, or C3H6 injected into an argon fluidizingstream. Coatings of 30-40)lm were deposited [5]. Intotal, hundreds of tons have been coated with thistechnique.

Lower pyrocarbon deposition temperatures (477 0C)were used successfully by Neuman and Koffer [8J byadding a nickel catalyst in the form of nickel carbonylvapors to a C2H2-H2 mixture. Deposition rates variedfrom 0.1 to 0.7 urn min - 1, depending on the concen­trations of reactants and catalyst precursor. The coatingscontained some nickel and were smooth, uniform andin some cases optically reflective.

Silicon carbide coatings were also deposited on fuelpellets from methyltrichlorosilane or from mixtures of asilicon precursor (such as trichlorosilane) and carbonprecursors such as methane or hydrocarbons (see e.g,refs. 2, 3, 6, 7 and 9).

McCreary [lOJ deposited thin metal shells (0.5-5 11m)on hollow microspheres (40-200 urn) in a FBR to encap­sulate LiD for use as laser targets in nuclear fusionexperiments. Hollow microspheres of glass or metalswere coated with nickel from Ni(CO)4 at 112-122°C,molybdenum using MO(CO)6 at 352°C and rheniumusing Re2 (COho at 352°e. The pressure was 200 Torrin all cases. The deposition rate for nickel was 0.1 urnmin -1. The coatings were a few micrometers thick, veryuniform (some surfaces were smooth with a few nodules),ductile, compact and pinhole free and they showed afine-grained microstructure.

Arthur et al. [llJ coated B4C particles (100 urn) withlayers of TiC lfl um thick by adding a 7: 1 mixture ofTiCI4-C7Hs (toluene) to the fluidizing gas and deposit­ing at 1200"C, These coated particles were incorporatedinto a WC-Co matrix to produce improved rock-drillingbits.

Wood et al. [12J coated particles with large surface­to-volume ratio such as mica microflakes with aluminumin an FBR by thermal decomposition of triisobutyl­aluminum.

Kaae [9J, among others, reported on codeposition ofcompound coatings in FBRs. For example, SiC-C, ZrC­C and Co-C composite coatings were deposited byadding CH 3, SiCI3, ZrCl4 or CoI 2 and a hydrocarbonto the fluidizing gas. The coatings consisted of a finedistribution of small crystallites of the carbide in acarbon matrix. Codeposition of HfC and SiC wasachieved when HfC14 , CH3SiCl3, and CH4 were addedto the carrier gas, resulting in a coarse coating containing20 vol.% of acicular SiC grains distributed in Hfe.Mixtures of ZrCI4 , propylene and CH3SiCl3 resulted incoatings containing 40 vol.% SiC in ZrC. HfC-TiCcoatings were deposited from gaseous mixtures contain­ing vapors of HfCI4, TiCI3 , and propylene. The effectsof gaseous composition and deposition temperature onthe microstructure of these types of composites werestudied by Lackey et al. [13].

A particularly interesting application is the productionof high purity materials. For example, silicon can bedeposited on silicon seeds in a state of high purity.Padovani et al. [14] deposited silicon by reduction ofHSiC13 in H 2 on silicon seeds in an FBR with an SiCliner. Noda [15J described a similar approach usingSi02-1ined steel reactors. Hsu et al. [16J deposited siliconon seeds by pyrolysis of SiH4 in an FBR.

3. Coating of large objects using beds of inert particles

There are two main modes of coating objects that aresignificantly larger than the particles in the bed. Thesubstrate can be immersed in the bed and allowed tomove freely during fluidization, or it can be fixed orconstrained in a region of the reactor by means of wirebaskets, supporting rods, shafts or other mechanicalmeans.

In the simple mode of operation, large objects can beimmersed in a bed of inert particles (typically alumina)which act as a heat transfer medium. Diffusion coatingsor surface modification can then be obtained by addinga reacting vapor species to the carrier gas. For example,steel can be nitrided or carburized or carbonitrided byaddition of NH3 or CH4 or mixture to the carrier gas[17].

A. Sanjurjo et al. / CVD in fiuidized bed reactors 221

A second mode of operation consists of performingconventional CVD chemistry (e.g. TiCl4 and NH 3 ) in abed of inert alumina particles [18]. The uniformity andhomogeneity of the TiN coating produced are reportedto be high, as expected because of the high heat andmass transfer and good mixing. Note that the particlesin the bed also get coated; thus only a small part of thereactants is used to coat the sample.

4. Coatings of large objects using beds ofreactiveparticJes

In a new and emerging technique, the particles of thebed act as a source of the material to be deposited aswell as an efficient heat and mass transfer medium. Wedescribed this coating technique in a previous article[19]. Briefly, the fluidized bed is made out of activeparticles of the same element (M*) to be coated (insteadof inert particles). The sample to be coated is immersedin the bed. A reactive gas stream is then mixed with thefluidizing gas so that reactive (coating) species are gener­ated in situ in the bed. For example, when HCI reactswith silicon, some of the equilibria that take placeinclude

Si+4HCI~SiCI4+2H2

Si+2HCI~SiCh + H 2

Si + SiCl4~ 2SiCl2

Si+ 3HCI3~SiHCI3+ H2Si+2HCI~SiH2CI2

SiCI2 , SiH2Cl2 and SiHC13 are more reactive thanSiCI4 and help in depositing silicon at low temperatures.Other even more reactive species such as SiH3Cl, SiCI3and SiCI may also be produced, but their lifetimes arevery short so their participation in the coating may beless important.

When the powder is a transition metal such as tita­nium or zirconium, then subhalides such as TiCI3, TiCI2 ,

and ZrCl2 are produced in addition to TiCl4 and ZrCl 4 •

As the coating species reach the surface of the samplewhere the activity of M* is lower than in the bed, theydisproportionate and/or reduce at the sample surface,depositing a coating of M* on it. For example, amolecule of SiCl2 disproportionates, depositing siliconaccording to

2SiCI2~Si + SiCl4

whereas SiHCl3 is reduced according to the overallequilibrium

SiHCl 3 +%H 2-Si+3HCI

Typically, the gaseous reaction byproducts generatedduring deposition react again with particles in the bedand, in turn, regenerate more reacting coating species

and so on. In this manner the bed acts as an unlimitedsource of reactants and as a sink for gaseous byproducts.Because the bed is in intimate contact with the sampleto be coated (practically there is no boundary layer), therate of mass transfer from the gas phase in the bed tothe surface of the substrate is very fast. As a consequencethe overall deposition rate is not limited by gas diffusionas is the case for high temperature in conventional CVD.The deposition may be driven by-a difference in activitybetween the bed and the surface, where the depositedelement may form a solid solution or a compound orboth. It may also be driven by a temperature gradient,e.g. when the substrate is heated directly.

To determine the best conditions for coating, we firstestimate the equilibrium composition for a given system,then we check the composition experimentally by massspectrometry [19]. Thermochemical estimates of thevapor pressure of the gaseous species in equilibriumwith the reactive metal and the substrate have beenmade for a variety of temperatures, pressures and reac­tant ratios using a Gibbs free-energy minimization pro­gram. These results are then checked first by hightemperature mass spectrometry using a Knudsen celland quenching the chemistry in the molecular regime[19]. The kinetics are further studied by determining thegas composition of the bed at atmospheric pressure.From this information we select the best conditions forcoating by choosing a temperature at which the partialpressures of in situ generated species are highest.

We normally use HCI because of its low cost andavailability and because it reacts at lower temperaturesthan the halide of the coating elements (e.g. SiCI4, BCI3 ,

TiCI4). For example, if SiCl4 is used as the reactivespecies in atmospheres of H 2, it is expected that itsreaction by reduction will not be appreciable below500°C. However, if HCI is injected, it can react attemperatures as low as 350°C [19] to form Si-Cl-Hspecies such as SiCI4, SiHCI3, SiClz, and SiH2C1 2• whichcan be used for coating.

The production of coating species can be catalyzedby impurities such as copper, which may also promotethe preferential formation of less stable species such asSiHCI3 , SiH2Cl2 and SiCI2 • which can serve as goodcoating precursors. The advantage gained by using thecatalyst is that these hydrogenated and less stable speciesproduced can be reduced or disproportionated to depositsilicon at much lower temperatures than SiCI4. Thesecatalytic effects have been described in detail in thescientific and patent literature (see e.g. ref. 20)for siliconand we have observed similar effects for titanium andzirconium.

Some of the results obtained using this technique aresummarized in Table 1. Samples with relatively compli­cated shapes, including threaded conjectors and helicoi­dally wound heat exchangers, can be coated. The column

222 A. Sanjurjo et al. / CVD injluidized bed reactors

TABLE I. Examples of coating used beds of reactive particles

Substrate Coating Chemistry Temperature (cC)

Cu Si Si-Br-H 600Cu Si Si-Cl-H 500

600Cu Ti Ti-Br-H 750Cu Ti Ti-CI-H 600

650Cu Ti02 _ .< Ti-C1-H 600

O2

Cu TiN Ti-CI-H 650NH 3

Cu AI AI-CI-H 480Cu-Ni (10%) Ti Ti-CI-H 600Cu~Ni (10%) TiN Ti-CI-H 650

NH 3

Low C steel Ti Ti-Br-H 650Low C steel TiN Ti-Br-H 850

NH 3

Low C steel Zr Zr-Br-H 900Steel rebar Si Si-CI-H 500Steel rebar Si+Ti Si-Ti-Cl-H 550Graphite felt Si3N4 Si-Br-H 600-800

NH 3

Deposition rate (11m min - 1)

4-6>10

4-6

<0.1

Ti>\TiN<O.1

<I<\

<I2-5I

Comments

Deep diffusionDeep diffusion

Shallow diffusionShallow diffusion

Continuous amorphous

headed Chemistry in Table 1 lists the main elementspresent in the gas phase. Ti-Br-H indicates that weused titanium powder and HBr to generate the coatingspecies. When O 2 or NH 3 is added, it indicates that,after coating with the metal, the coating was oxidizedor nitrided in situ. Note that the temperatures of depos­ition are much lower than those commonly used inconventional CVD or pack cementation.

5. Discussion and conclusions

The intrinsic attributes of FBRs are particularly wellmatched with some of the requirements for obtaininghomogeneous, uniform coatings at high deposition ratesusing CVD-based processes. Particles ranging from I 11mto 2 mm can be easily coated at atmospheric pressure.Smaller particles (0.1-1 11m) can be coated at reducedpressures. Particles larger than 2 mm can also be coatedusing the same ideas, but using a moving bed reactorsuch as a rotary kiln.

Coated particles allow the engineer new degrees offreedom to design new metal-metal, metal-ceramic andceramic-ceramic composites. Fabrication can also bene­fit from the availability of coated particles.

The use of subhalide chemistry in FBRs also holdsgreat promise. Because the temperature-time require­ments are significantly lower than for conventional CVD,new applications for CVD should open. The coatingsobtained by this technique are very homogeneous and

uniform. In overlay coatings the microstructure is finelygrained, presumably because of the constant generationof nucleation sites due to the mechanical interactionbetween substrate surface and particles.

The diffusion coatings we produced were always veryuniform even when complicated geometries were used.The diffused coating provides some in-depth protectionin case the coating is scratched. Depending on therelative diffusion rates of coating and substrate elements,some precautions must be taken. For example, wheniron was coated with silicon, the formation of a thick(greater than 30 J..Lm) coating led to formation of silicidesthat spalled off during cooling. When the concentrationof silicon was kept below 14% so that it remained as asolid solute in iron, the coatings were protective.

An interesting property of FBRs is that in many casesthe surface of the sample does not need to be speciallycleaned. The mechanical action of the bed scratches thesurfaces and can be used to remove residual oxide scalesor other surface contamination. A final advantage ofusing FBRs for CVD is the capability to directly oxidizeor nitride in situ. For many applications this providesan added level of protection or passivation at no extracost.

In conclusion, the FBR reactor can be used to depositvery uniform and homogeneous simple and compositecoatings. By in situ generation of the coating species, thetemperature-time requirements for coating can be sub­stantially lower than for conventional CVD. The combi­nation of low temperatures, short times, atmospheric

A. Sa njurjo et al. I C VD in fiuid ized heel reactors 223

pressure and /or fast deposition rates should result inlower costs and expanded potential for CVD in manyapplications.

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