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Morphologies and growth mechanisms of zirconium carbide films by chemical vapor deposition.

Abstract Zirconium carbide films were grown on graphite slices by chemical vapor deposition using methane, zirconium tetrachloride, and hydrogen as precursors. The growth rate of zirconium carbide films as a function of temperature was investigated. The morphologies of these films at different temperatures were also observed by scanning electron microscopy. The results indicated that the deposition of zirconium carbide was dominated by gas nucleation at temperatures below 1523 K, and by surface process at temperatures higher than 1523 K. By comparison of the deposition activation energies for zirconium carbide and deposited carbon, it was determined that the carbon deposition was the controlled process during the growing of zirconium carbide films. The effect of temperatures on the morphologies of zirconium carbide films was also discussed, based on the carbon deposition process.

Keywords Chemical vapor deposition, Surface morphology, Growth mechanism, Zirconium carbide

Introduction

Zirconium carbide (ZrC) is an important refractory material that has attracted much attention within the industry. It has been widely used for coatings on atomicfuel particles (thoria and urania) for nuclear-fusion power plants due to its low neutron cross section. (1), (2) ZrC has also been used as a cutting tool because of its great hardness, (3) and as an ultrahigh temperature composite because of its extremely high melting point ([T.sub.m] = 3813 K) and excellent thermal stability (does not decompose until the melting point). (4) Moreover, ZrC could be applied in electronic devices because of its metallic, thermal, and electrical conductivities. (5)

ZrC is applied in the industry primarily as a coating. ZrC coatings have been growth by electron-beam bombardment, (6) laser cladding, (7) pulsed laser ablation-deposition, (8) magnetron sputter deposition, (9) and chemical vapor deposition (CVD) (10-16) and Among these methods, CVD has advantages over the others in that it could be used to uniformly coat not only flat surfaces, but also complex-shaped components like particles and cutting tools. It therefore is the most investigated deposition method for the preparation of ZrC coatings. The CVD processes--including, chloride, (10-13) bromide, (14), (15) and iodide (16) processes--have previously been examined. The various parameters such and flux of gas precursors, have also been heavily investigated. (12), (15), (17) Thermodynamic studies of the ZrC deposition process have also been carried out previous studies. (13), (18), (19)

To better control the ZrC deposition process, it is necessary to understand the deposition mechanism. However, the study has seldom been done to elucidate such a mechanism. In this paper, ZrC films were fabricated via CVD using methane ([CH.sub.4]), zirconium tetrachloride ([ZrCl.sub.4]), and hydrogen ([H.sub.2]) as precursors. The deposition morphologies and the deposition rates were studied at different temperatures to understand the deposition mechanisms. From these results, a model based on the carbon deposition was introduced to account for the ZrC deposition process. The ZrC morphologies at various temperatures were also discussed based on this model.

Experimental procedure

The deposition process was performed in a horizon hot-wall deposition apparatus. Schematic representation of the deposition furnace is shown in Fig. I. The horizon tube is an alumina tube with an inner diameter of 20 mm. The ZrC films were deposited on graphite slices via CVD using Zr[Cl.sub.4], [H.sub.2], [CH.sub.4], and argon (Ar). The graphite slices were polished to 1 [mu]m and cleaned in ethanol by ultrasonic before they were put into the deposition furnace. The furnace was then heated to the desired temperature at a rate of 5 K/min. After the setting temperature was reached, the deposition furnace was backfilled with Ar and [H.sub.2] to stabilize the temperature for a while. Simultaneously, the heat belts heated the Zr[Cl.sub.4] to 463 K in 5 min and stabilized the temperature for 5 min. All the temperatures were controlled with a margin of error of [+ or -]5 K by programs. Subsequently, the flowing argon gas carried the sublimated Zr[Cl.sub.4] into the coating chamber. [CH.sub.4] mixed with [H.sub.2] was supplied directly into the deposition chamber. The purities of the precursors and deposition parameters are listed in Table 1. When the deposition process finished, the heat belts were turned off immediately. The Ar and [CH.sub.4] gas were also shut down. [H.sub.2] gas was kept until the temperature in the chamber dropped to 573 K.
Table 1: The purities of precursors and deposition parameters

Deposition precursors  Zr[Cl.sub.4]  [CH.sub.4]  [H.sub.2]    Ar

Purities               99.5+%        99.99%      99.999%    99.999%

Flow rates             1 g/h         10 ml/min   40 ml/min  20 ml/min

Deposition             1423-1673 K
temperatures

Deposition pressure    5 kPa

Duration time          2 h


After deposition, the morphologies of ZrC films on the graphite slices were observed by scanning electron microscopy (SEM, JEOL6700F, Tokyo, Japan). The thickness of the films also was directly measured by SEM. The phases of the thin films were characterized by x-ray diffraction (XRD), which was carried out by using a Rigaku D/max-2400 diffractometer (Tokyo, Japan) with copper K[alpha] radiation.

Results and discussion

The deposition rate for ZrC films is expressed as the thickness growth per unit of time. For each temperature, at least five samples were used for measurement to get an average value. The deposition rate for ZrC films as a function of reciprocal temperature is shown in Fig. 2. The apparent activation energy of the deposition process can be determined from the deposition rates at different temperatures using the Arrhenius law

K = A exp (-[E.sub.a]/RT)

where k is the deposition rate, A is a constant. [E.sub.a] is the apparent activation energy, R is the gas constant, and T is the deposition temperature. As shown in Fig. 2, the activation energy for deposition is 305 kJ/mol at temperatures higher than 1523 K. When the temperatures are below 1523 K, the deposition rates become less dependent on temperature, and the activation energy needed for deposition is calculated to be about 85 kJ/mol. It is believed that the deposition mechanism changes at around 1523 K. During the deposition process, the surface kinetics have a strong dependence on the deposition temperature, which should have high activation energy. (20) Therefore, ZrC deposition at temperatures higher than 1523 K should be a surface kinetic process. The gas nucleation process, which has low activation energy, (20) will become dominant at temperatures lower than 1523 K.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The surface kinetics process is preferred during the deposition practice. A previous study indicates that the zirconium and carbon is deposited separately in ZrC deposition from a Zr [CI.sub.4]-[CH.sub.4]-[H.sub.2] system. (13) Hence, the surface kinetics for ZrC growth should be dominated by one of the following three processes: carbon deposition, zirconium deposition, or the reaction of carbon and zirconium to form carbide. In this study, the deposition activation energy for ZrC was calculated as 305 kJ/mol at temperatures higher than 1523 K. In comparison with the activation energy of carbon deposition from [CH.sub.4] (280-380 kJ/[mol.sup.21]), it is reasonable to assume that carbon deposition is the controlled process for ZrC deposition.

This assumption was confirmed by the similarity of morphologies for ZrC films and carbon growth in the same temperature ranges. As shown in Fig.3a, the ZrC films deposited at 1573 K show column growth. Each column has a tip on the top and grows larger downwards. The tips are composed of a number of small ZrC grains. At 1673 K, the morphology (Fig.3b) of the ZrC films resembles a cauliflower shape--an isotropic form. The grain size of ZrC grows larger compared with that at or over 1573 K. As for the morphologies of deposited carbon, they showed column growth at 1573 K and isotropic forms at temperatures higher than 1673 K, (4), (22) which coincide with the morphologies of ZrC films at 1573 K and 1673 K. Our assumption is further supported by the CVD depesition practice in a previous study, in which the growth rates of ZrC films were determined by the carbon deposition rate. (14)

[FIGURE 3 OMITTED]

Consequently, a deposition model for Zr[CI.sub.4]-[CH.sub.4]-[H.sub.2] system is being proposed on the basis of the carbon deposition process. During the deposition of carbon on a substrate, the gaseous hydrocarbon is generally decomposed to form a liquid or plastic droplet of complex organic materials, and then the droplets impinge on the substrate surface. (23) Our model for ZrC deposition process is based on this process: The gaseous hydrocarbon deposits form liquid droplets on the substrate, then the reduced Zr[CI.sub.4] or zirconium reacts with the organic droplets to form ZrC.

It would be easy to understand the morphologies of the deposited ZrC coatings based on this model. Regarding the ZrC coatings deposited at or above 1573 K, the [CH.sub.4] is first decomposed to form droplets on the substrate. Since the temperature is not high enough for the droplets to fuse together. The materials prefer column growth, as shown in a previous study. (23) The zirconium or reduced Zr[CI.sub.4] will react with these droplets to form ZrC grains. It is believed that the needle tip is the preferred place for liquid droplet deposition. The ZrC will nucleate in the liquid and grow in the diameter of the needle. When the temperature increases to 1673 K, [CH.sub.4] will be decomposed more easily and form more droplets per unit area of the substrate. The temperature is also high enough for these droplets to fuse together. Simultaneously, zirconium or reduced Zr[CI.sub.4] is dissolved in the droplets to form ZrC. The particle size of ZrC becomes larger than those at 1573 K due to the faster growth rate at the higher temperatures. These fused liquid droplets result in a cauliflower shape in the final morphology of ZrC films.

As for the ZrC films deposited at 1473 K, its morphology is different from those deposited at temperatures of 1573 K and 1673 K. As shown in Fig.4, the deposited ZrC presents a star shape. It is composed of six arms and each arm consists of many parallel layers. The adjacent arms are crossed at about a 60[degress] angle. The XRD results (Fig.5) indicate that the film has an orientation of (220). From the above discussion, it is known that gas nucleation is the dominant process at this temperature. At lower temperatures, it is difficult for [CH.sub.4] to decompose. Mean-while, the deposition rate of liquid droplets on the substrate is also very slow. The reaction of Zr [CI.sub.4], [CH.sub.4], and [H.sub.2] in the gas phases to form ZrC will become dominant at low temperatures. These resultant ZrC will directly impinge on the substrate. The supersaturation of ZrC in the gas should be low temperatures. Under such conditions, ZrC film will prefer to grow epitaxially.

As mentioned previously, the surface kinetics process is preferred in CVD deposition. In the present study, it was found that such a process happens at temperatures higher than 1523 K. Therefore, the ZrC deposition should be carried out at temperatures higher than 1523 K for Zr[Cl.sub.4]-[CH.sub.4]-[H.sub.2] systems. Although the temperature range could be varied in different facilities or conditions, most successful CVD deposition practices for ZrC using Zr[Cl.sub.4]-[CH.sub.4]-[H.sub.2] system were utilized in this temperature range. (11), (15) The fact that the carbon deposition is the controlled process in ZrC deposition could also guide our deposition practice by using the fruitful knowledge of the carbon deposition process.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Conclusion

In this paper, zirconium carbide films were synthesized by chemical vapor deposition using Zr[Cl.sub.4], [CH.sub.4], and [H.sub.2] as precursors. The deposition activation energies for ZrC growth were calculated. The morphologies of deposited films were also observed. From these results, the following conclusions can be drawn:

* At temperatures below 1523 K, the ZrC deposition process was dominated by gas nucleation. The films preferred epitaxial growth.

* At temperatures higher than 1523 K, surface kinetics was the dominant process during ZrC deposition. The controlled step in this process was the carbon deposition, which determined not only the ZrC deposition rates but also the morphologies of deposited films.

* A model based on carbon deposition was introduced to account for the surface kinetics process for ZrC deposition. The carbon would form liquid droplets on the susbstrate, and zirconium or reduced Zr[Cl.sub.4] would dissolve in the droplets and react with them to form ZrC.

Acknowledgment This work is financially supported by the Chinese Natural Science Foundation (Grant # 90176023).

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Q. Liu, L. Zhang, L. Cheng, Y. Wang

National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi' an, China 710072

e-mail: wangyiguang@nwpu.edu.cn

[C] FSCT and OCCA 2008
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Author:Liu, Qiaomu; Zhang, Litong; Cheng, Laifei; Wang, Yiguang
Publication:JCT Research
Date:Jun 1, 2009
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