Factors influencing the chemical vapor deposition of ZrC

Journal of Nuclear Materials 62 (1976) 221-228 0 North-Holland Publishing Company

FACTORS INFLUENCING THE CHEMICAL VAPOR DEPOSITION OF ZrC *

P. WAGNER, L.A. WAHMAN, R.W. WHITE, C.M. HOLLARAUGH and R.D. REISWIG Los Alamos Scientific hboratory, Universityof California,Los Alamos, NM87545, USA Received 3 March 1976

Experiments have been performed to determine the effects on the ZrC coating process of varying the composition of the gas mixture used for chemical vapor deposition of ZrC. The ZrC was deposited in a fluidized bed of ThO2 particles using a gas mixture of CH4, Ha, ZrC4 and At. The ZrC4 flow was controlled using a powder feeder. Effects of varying CH4 and Ha concentrations and particle bed area on the rate of deposition of the ZrC, the appearance of the ZrC, the composition of the ZrC, and the microstructure of the ZrC were studied. Increases in CH4 and Ha concentration were effective in increasing the linear coating rate of ZrC. Increases in the ratio of CH4 to ZrC4 in the coating gas resulted in a decreased metallic appearance of the coat and an increase in the C/Zr in the deposit. Increases in Hz inhibit these effects. Des experiences ont 6te effect&s en vue de determiner les effects sur le processus de revdtement en ZrC des variations de la composition du melange de gaz utilids pour realiser la deposition en phase vapeur de ZrC. Le carbure ZrC est depose dam un lit fluidise de particules de Th02 en utilisant un milange de gaz (CH4, Hz, ZrC4 et Ar). Le courant gazeux itait control& en utilisant un m&ngeur de poudres. Les effets de variations de concentration en CH4 et Hs et de la surface du lit de particules sur la vitesse de deposition du carbure ZrC, l’apparition de ZrC, la composition et la microstructure de ZrC sont ktudides. L’augmentation de la concentration en CH4 et Ha joue un r6le effectif sur l’augmentation de la vitesse lineaire de deposition de ZrC. L’augmentation du rapport CH4/ZrC4 darts le melange de gaz a pour resultat une diminution de l’kclat mktallique du dip& et une augmentation du rapport C/Zr darts le dep6t. L’augmentation de la teneur en Ha inhibe ces effets. Es wurden Versuche durchgefiihrt, mit denen der Einfluss einer Anderung der Gaszusammensetzung auf den ZrC-Beschichtungsprozess durch chemische Abscheidung von ZrC aus der Gasphase bestimmt wurde. Das ZrC wurde in einem Fliessbett auf ThOz-Teilchen mit einem CH4-Hs-ZrC4-Ar-Gasgemisch abgeschieden. Der ZrC4-Fluss wurde mit Hilfe einer Pulver-Zuleitung kontrolliert. Es wurden der Einfluss wechselnder CH4- und Ha-Konzentrationen und Fliessbettquerschnitte auf die ZrC-Abscheidegeschwindigkeit sowie das Aussehen, die Zusammensetzung und das Gefiige des ZrC untersucht. Eine Zunahme der CH4- und Ha-Konzentration wirkt sich auf eine ErhShung der linearen ZrC-Beschichtungsgeschwindigkeit aus. Ein Anstieg des CHe/ZrC4-Verhaltnisses im Beschichtungsgas hat eine Abnahme des metallischen Aussehens und eine Zunahme des C/Zr-Verhiiltnisses der Beschichtung zur Folge. Eine Hz-Zunahme verhindert diese Einfliisse.

1. Introduction

will yield a higher coolant temperature for a given coated particle operating temperature and a given fuel element design. The second is the potential to increase the operating temperature of the coated particles under irradiation, to increase attainable coolant temperatures further [3]. ZrC, with its high temperature capability and good neutronic properties, appears to have all the necessary qualifications to replace Sic. Several studies have described the chemical vapor deposition of zirconium carbide and carbon-zirconium carbide alloys [4-141. All utilized the reaction of a zirconium halide with a hydrocarbon gas to deposit

Replacement by ZrC of the currently used SIC coat in high temperature, fission-product-retaining, coated nuclear fuel particles has two, potentially major, advantages. The first is to increase the temperature to which the graphite fuel body, containing the particles, can be heated during the fabrication procedure, in order to produce a fuel body matrix with a higher thermal conductivity [ 1,2]. The improved heat transfer * Work performed under the auspices of the United States Energy Research and Development Administration. 221

P. Wagner et al. / Chemical vapour deposition of ZrC

222

zirconium carbide. The zirconium halide vapor was obtained either by a reaction of a halide vapor with zirconium sponge [4-IO] or by sublimation of ZrCb [ 1l-141. In both methods, control of the flow rate of zirconium halide to the CVD chamber was neither reliable nor flexible. Consequently, a detailed study of deposition parameters for a wide range of conditions was not practicable. This limitation has been overcome with the recent development of a ZrCb powder feeder [ 151 in which the mass flow rate of the salt can be varied and controlled accurately. This report describes the experimental conditions and results of a detailed parametric study of the deposition of ZrC on ThO2 microspheres in a fluidized bed. The purposes of the experiments were to define optimum coating conditions with respect to rate, stoichiometry, and quality in the ZrC coat.

2. Apparatus The fluidized bed particle coater apparatus used in these experiments is shown schematically in fig. 1. Microspheres are fluidized in the coating chamber which has a 25.4 mm diameter with 60” apex angle cone and 3.2 mm diameter inlet. The graphite coater wall is heated inductively. ZrC14 powder is supplied from the powder feeder, its rate controlled by the auger speed and metered by the output of the load cell on which the powder feeder is hung. The powder is swept by Ar to the coater base where it is mixed

COATER : . : : :

: : INDUCTION: HEATER_: 0

Fig. 1. Schematic representation particle coater.

of

ZrC4powder feeder and

0

M

40 6D 80 100 DISTANCEFROM ORIFICE,mm

120

Fig. 2. Typical axial temperature profile in fluid bed.

with the other coating gases supplied from a gas manifold. The gases are metered by calibrated flowmeters. The ZrC14 powder in the gas stream is vaporized in the coater base before entering the coating chamber. Coater temperatures are measured with Pt-Pt 10% Rh thermocouples, one is inserted in the coater base to a position 34.8 mm below the apex of the coater cone and held to monitor the temperature at that point through the coating run. Another thermocouple is lowered through the top of the coater to measure the coating chamber temperature at several axial positions. These measurements are made in the bed of microspheres fluidized in Ar, and the thermocouple is removed before introduction of the active coating gases. Fig. 2 is a plot of a typical temperature profile in the coater used in this work.

3. Experimental description The experiments were designed to establish the effects of changing single variables in the ZrC coating process. By the nature of the coating process, particles of some size distribution change in size and density as the coating proceeds, thus changing the fluidization behavior of the bed, and probably changing the depletion mechanism of the coating gases as well. Therefore, the goal of observing an effect of a single variable will only be approached, not truly attained. The independent variables change8 in these experiments were: (1) H2 concentration, (2) CH4 concentration, (3) bed area (surface area of the particles).

223

P. Wagneret al. / Chemical vapour deposition of ZrC Table 1 Effects of coating gas compositions Run ID no.

on ZrC coating rates, coat appearances and compositions

Mol fraction --____.___Ar

Ha

CH4

ZrC4

Total flow (cm3/s)

Thickness rate bm/min)

Volume rate (cm3/min)

Surface

Coating gas ClZr

Chemical analysis of ZrC* C,/Zr CT/Zr

1 2 3 4 5

0.82 0.81 0.80 0.78 0.77

0.14 0.14 0.14 0.13 0.13

0.007 0.016 0.034 0.049 0.060

0.034 0.034 0.033 0.032 0.032

88.0 88.8 90.5 91.9 93.0

0.09 0.30 0.38 0.57 0.63

0.005 0.017 0.023 0.037 0.042

Smooth Smooth Matte Tumulose Tumulose

0.21 0.47 1.0 1.5 1.9

0.87 0.84 0.89 0.63 0.45

0.87 0.89 1.18 1.88 2.40

6 7 8 9 10

0.69 0.69 0.68 0.67 0.66

0.27 0.27 0.27 0.26 0.26

0.006 0.013 0.029 0.042 0.05 1

0.029 0.029 0.028 0.028 0.028

104.0 104.8 106.5 107.9 109.0

0.14 0.18 0.40 0.46 0.61

0.005 0.0098 0.024 0.028 0.040

Smooth Smooth Smooth Matte Tumulose

0.21 0.45 1.0 1.5 i.8

0.76 0.84 0.90 0.94 0.96

0.76 0.88 0.93 1.12 1.19

11 12 13 14 15

0.49 0.43 0.48 0.47 0.47

0.48 0.48 0.47 0.47 0.46

0.005 0.012 0.027 0.038 0.047

0.026 0.026 0.026 0.025 0.025

114.1 114.9 116.6 118.0 119.1

0.14 0.26 0.38 0.49 0.66

0.006 0.015 0.023 0.031 0.045

Smooth Smooth Smooth Smooth Tumulose

0.19 0.46 1.0 1.5 1.9

0.74 0.87 0.83 0.99 0.58

0.74 0.91 0.97 1.09 1.08

__-__ *CT= total carbon content in ZrC coat; Cc = combined carbon in ZrCx.

The particles tribution,

to be coated

had a very narrow

time and temperature

size dis-

were held constant

throughout, the ZrC14 flow was kept constant to about +5%, the same furnace and associated equipment were used throughout, and dependent variables such as argon content and total gas flow were changed only to the extent required to keep the bed properly fluidized. Enough ThO2 microspheres for these experiments were carefully sieved to a 230-250 pm diameter size range. A representative sample, obtained by multichute riffling, was radiographed and measured. The particles had a mean diameter of 244.7 /.nn with a standard deviation of 6.2 pm. The furnace charge for each experiment was weighed to 0.01 g; e.g., for experiments with a constant bed area of 0.05 m2, the charge was 20 * 0.01 g. Each experiment lasted one hour, and the initial temperature profile in the coating furnace was that shown in fig. 2. All experiments involved coating ThO2 with ZrC from a gas mixture of CH4, Hz, ZrCl4 and Ar. The overall chemical reaction for the formation of substoichiometric zirconium carbide is:

xCH4 + ZrCl4 + 2(1- x)Hz = ZrC, t 4HCl.

(1)

ZrC, is monophase and stable in the region 0.59
* The total flow rate in the coating furnace was effectively constant and was changed only to the extent required to keep the bed fluidized. However, the flow rate of CH4 was varied by a factor about nine. In this situation, the relative concentration of the components in the gas stream is most conveniently described by their mol (or volume) fraction. The mol fraction is used in this paper in the discussion of the effects of Ha and CH4 on the ZrC coating rates.

P. Wagneret al. / Chemical vapour deposition of ZrC

224

clear that increasing the amount of CI-I4(i.e. increasing C/Zr) in the coating gas results in ZrC coats with increased amounts of carbon. The chemical analysis indicates that this carbon exists as free carbon and chemically bound carbon in ZrC,. The combined carbon is listed as C,/Zr (this is x in ZrC,). There is a direct relationship between the value of C/Zr where it is 1, the relationship becomes less certain, and it is under these conditions that the coat loses its metallic appearance. The dramatic changes in the appearance of the coats at high values of C/Zr in the gas stream suggest changes in the deposition and character of the ZrC; however, this is speculative. The higher Hz extends the range of CH4 concentrations which produce ZrC coats with a metallic lustre. After coating, representative samples were radiographed and the ZrC thicknesses obtained from diameter differences before and after coating. Because of the inherently large errors associated with such a method, these measurements were later repeated by making direct measurements of the coat thickness using metallographic mounts. The agreement between the two methods was quite good and the average standard deviation for the measured ZrC coat thickness is calculated to be 9%. When coat volumes are calculated from coat thicknesses, this introduces an uncertainty of about 12% in the calculation of the volume. Metallographic sections of representative samples were taken after each run. These were prepared to

evaluate the appearance, and structure of the ZrC coats. Samples were also submitted for quantitative carbon and zirconium chemical analysis to determine the C/Zr in the coats. Coating rates were computed on the basis of the thickness (linear rate) or volume (volumetric rate) deposited after one hour and thus are actually mean values for a one hour coating time. As shown in table 1, increases in either H2 or CH4 increase the coating rate. The three sets of coating experiments compiled in table 1 were done using a constant initial bed area of 0.05 m2 (20 g). To determine the effect of bed area on the deposition parameters, additional experiments were done with bed areas of 0.075 m2 and 0.1 m2. The results of these experiments are summarized in table 2. Once again, an attempt was made to hold everything constant except for one independent variable, the bed area. This was done for three different Hz concentrations. In each case, the linear coating rate decreased with increased bed area. The appearance of the ZrC coats made with XHa = 0.47, the highest H2 concentration, is more metallic (smooth, silvery) than those made with XH2= 0.14, the lowest Hz concentration, (xi is the mol fraction of the ith species). 4. Lkcus3ion According to Wallace [ 141, chemical vapor deposition of stoichiometric ZrC may be expressed by the overall equation, ZrCh + CI& + ZrC + 4HC1,

(2)

Table 2 Effects of bed area on Zr coating rates and appearance Run ID no.

Total flow

Mol fraction

Bed area Cm21

(cm3/s1

Thickness rate

Surface

Olmlmin)

Volume rate (cm3 /min)

Ar

Hz

CHq

zIc4

3 16 17

0.80 0.80 0.80

0.14 0.14 0.14

0.034 0.034 0.034

0.033 0.033 0.033

90.5 90.5 90.5

0.05 0.075 0.10

0.38 0.29 0.24

0.023 0.025 0.026

Matte Matte Smooth

8 18 13

0.68 0.68 0.68

0.27 0.27 0.27

0.029 0.029 0.029

0.028 0.028 0.028

106.5 106.5 106.5

0.05 0.075 0.10

0.43 0.33 0.28

0.026 0.029 0.032

Smooth Smooth Smooth

13 20 21

0.48 0.48 0.48

0.47 0.47 0.47

0.027 0.027 0.027

0.026 0.026 0.026

116.6 116.6 116.6

0.05 0.075 0.10

0.38 0.35 0.29

0.023 0.031 0.033

Smooth Smooth Smooth

P. Wagneret aL / Chemical vapour depositoin of ZrC

which establishes the lower thermodynamic boundary for the coating gas composition. One must, however, also consider the H2 reduction reactions,

22s I

I

I

I A 0 l

ZrCl4 t 3H2 + ZrClj t HCl ,

(3) 0

ZrC13t $Hz + ZrC12 t HCl

A Il

for the influence of H2 can be explained only through these relationships. In his analysis Wallace found that the deposition rate may be expressed by R=

:A

PCH4P(Hn-2)‘2%lt 1+ aP&,

A exp(-BIT) ,

l

(4)

A

where

l MOLFRACTION H2=0.14

l

n MOLFRACTlON H2-p27

A

n = (4PZrCLj + 3PZrC13 + 2PZrC12YPZrCl~ 9

AMOL FRACTION H2'0.47

and

Psalt =pzlC4

0

+pzrc13

(5)

+pzrc1*

A similar analysis in this work showed that the coating rate depends on ZrC14, CH4, and H2, but not on HCl. This is illustrated by fig. 3 where the volumetric coating rate is plotted against xZrC4 xc& x g,f. The data confirm that for this experiment the volumetric rate can be expressed by l

R, = KXZrC4

’ XCH4 ’ Xy;

.

$rN [(q) t ty -

6

a

10

l

(6)

is similar in form to (4) for the case where cr < 1. Eq. (6) is useful to describe the coating process, but for simplicity in application, the linear (thickness) coating rate plotted against only CH4 is more useful. Results of the experiment plotted thus are shown in fig. 4. The volumetric and thickness of the coats are related by, =

4

. 104 . 'CH4 . ,112' H2 Cl4

Fig. 3. ZrC volumetric coating rates showing the effects of composition of the coating gas stream.

0.6L

This

VZrC

2 'Zr

ri] ,

5

$

0.5

4

Y

z 0.4 az Y =

$ B 2

0.2 0.31

0

MOL FRACTION H2=0.14

l MOt FRACT10NH2' 0.27

where N is the number of microspheres, r. the radius of microspheres, and t the thickness of ZrC. This demonstrate that the relationship depends on the number of particles and particle size. In order to investigate, quantitatively, the effect of number of particles, the bed area, N in eq. (7) was varied holding all other coat-

A

MOLFRACT10NH2=0.47 0.03 0.04 MOL FRACTION CH4

0.05

0.06

Fig. 4. ZrC linear coating rates showing the effects of CH4 andHz.

P. Wagner et al. / Chemical vapour deposition of ZrC

226

O.B-

MCX FRACTIONHp = 0.47 - 0.4

0.25-

0.20'

- 0.3 I 0.05

I 0.06

I 0.07

I 0.08

I 0.09

I 0.10

0.2

BEDAREAdi

ing parameters constant. The effects on the coating rates are shown in fig. 5. Figs. 6,7, and 8 show how the appearance of the ZrC coated particles and the microstructure of the ZrC change with coating gas composition. These figures have been prepared to allow direct comparisons to be made with the data summary in table 1. Fig. 6 is comprised of the coating results from runs l--5, fig. 7 from runs 6-10 and fig. 8 from runs 1l-l 5 inclusive. The low magnification photographs showing the surface appearance are paired with the corresponding photomicrographs of the ZrC coat. The overall observation to be made is that as the C/Zr in the coating gas is increased, the ZrC deposit progresses by stages from a silvery, smooth, metallic appearing coat to one which is rough and black. Whether the non-metallic appearance of these high carbon coats is due to light scattering at the particle surface, or whether the carbon reflection characteristics dominate those of ZrC is not clear. At sufficiently high C/Zr values in the coating gas, the particle surfaces actually become rough and bumpy (tumulose). This progression in surface appearance and microstructure with increases in

Fig. 5. Comparisons of volumetric and linear coating rates vs bed area. Solid line, volumetric rate; dashed line, hear rate.

Fig. 6. Surface and microstructure

of ZrC coats; run nos. l-5,

table 1.

P. Wagneret aL / Chemical vapour deposition of ZrC

Fig. 7. Surface and microstructure

of ZrC coats; run nos. 6-10, table 1.

Fig. 8. Surface and microstructure

of ZrC coats; run nos. 11-15, table 1.

221

228

P. Wagner et al. / Chemical vapour deposition of ZrC

CH4 concentration is seen in each of the figures. The three figures also show the effect of H2 on surface appearance and microstructure of the ZrC coats. The Hz concentration is lowest in fig. 6 and highest in fig. 8. The overall effect of the H2 is to inhibit the effect of increasing the CI-$ concentration. That is, with higher H2 concentration, the increased C/Zr in the coating gas still results in degradation of the metallic lustre of the ZrC coat, but a higher value of C/Zr is required to achieve a given result. This effect is also clearly demonstrated in figs. 6,7 and 8. The microstructure observed in coats from runs 3, 4, 5,9, 10 and 15 also show a layered or “tree ring” structure. Superficially these bear a resemblance to the ring structure seen in low temperature isotropic carbon coats that have been plasma oxidized [ 161. In the case of the carbon coats, it has been shown that these rings have different,anisotropies. It has been speculated that this structure is a natural consequence of coating particles in a spouting bed in a vertical coating furnace with an axial temperature gradient, as the particles circulate the coating takes place at different rates in different temperature regimes. We think that a similar phenomenon is responsible for the “tree ring” structure seen in some of the ZrC coats. As the particles move up and down in the spouting bed of the coating furnace, they are exposed to regions of slightly different gas compositions and temperatures. It is possible that an analysis on a fine enough scale would reveal small compositional differences in the different layers, thus the ringlike appearance on some of the coats. 3. summary A set of ZrC coating experiments has been described in which effects of CI-14,H2 and particle bed area on

coating rate, coat appearance and coat composition have been evaluated. Increases in CH4 and H2 concentration are effective in increasing the coating rate of ZrC on a bed of fluidized ThO2 particles. Increases in the ratio of C!H4to ZrCh in the coating gas (C/Zr) results in ZrC coats of decreasing metallic appearance. Increases in H2 concentration inhibits, but does not eliminate, this effect.

References 111P. Wagner, C.M. HolIabaugh and R.J. Bard, Proc. IAEA Sfmp. on Gas Cooled Reactors with Emphasis on Advanced Systems, JueIich, FRG (1975). 121P. Wagner, Carbon (in press). I31 J.D. Balcomb and P. Wagner, Proc. BNES Intern. Conf., London (1974). 141 K. Ikawa and K. Iwamoto, J. Nucl. Mater. 45 (1972/73) 67. [51 K. Ikawa, J. Less Common Metals 27 (1972) 325. WI K. Ikawa, J. Less Common Metals 29 (1972) 233. 171 K. Ikawa and K. Iwamoto, J. Ceram. Sot. of Japan (Yogyo-Kyakai-Shi) 81 (1973) 403. PI K. Ikawa and K. Iwamoto, J. Nucl. Mat. 52 (1974) 128. PI K. Ikawa and K. Iwamoto, J. Nucl. Sci. and Tech. 11 (1974) 263. [lOI G.H. Reynolds, J. Nucl. Mat. 50 (1974) 215. (111 R. Harris, E. Kelley, D.H. Leeds and W.V. Kotlensky, Chemical Vapor Deposition (Electrochemical Society) 3 (1972) 183. [121 A.R. Driesner, E.K. Storms, P. Wagner and T.C. Wallace, Chemical Vapor Deposition (Electrochemical Society) 4 (1973) 473. [13] P. Wagner, LASL report LA-5224 (1973). [ 141 T.C. Wallace, Chemical Vapor Deposition (Electrochemical Society) 4 (1973) 91. [15] C.M. Hollabaugh, R.D. Reiswig, P. Wagner, L.A. Wahman and R.W. White, J. Nucl. Mat. 57 (1975) 325. 1161 E. Balthesen, K. Ehlers, K.G. Hackstein and H. Nickel, Proc. ANS meeting on Gas-Cooled Reactors, Gatlinburg, TN (1974) p. 201.