Thermal stability of organo-chromium or chromium organic complexes and vapor pressure measurements on tris(2,4-pentanedionato)chromium(III) and hexacarbonyl chromium(0) by TG-based transpiration method

Chemical Engineering Science 57 (2002) 3603 – 3610

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Thermal stability of organo-chromium or chromium organic complexes and vapor pressure measurements on tris(2,4-pentanedionato)chromium(III) and hexacarbonyl chromium(0) by TG-based transpiration method R. Pankajavallia , C. Mallikaa , O. M. Sreedharana; ∗ , V. S. Raghunathana , P. Antony Premkumarb , K. S. Nagarajab a Thermodynamics

and Kinetics Division, Materials Characterisation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, TN 603 102, India b Department of Chemistry, Loyola College, Chennai, TN 600 034, India Received 10 October 2001; accepted 17 May 2002

Abstract Thirteen di7erent organo-chromium and chromium organic complexes were synthesized and thermogravimetric studies had been carried out on these complexes in order to search for materials exhibiting high vapor pressure for chemical vapor deposition applications. These studies revealed only two complexes namely tris(2,4-pentanedionato)chromium(III), Cr(acac)3 and hexacarbonyl chromium(0), Cr(CO)6 to be vaporizing congruently even at temperatures as low as 500 K, without any decomposition and identifying themselves to be promising candidate materials for coating purposes. A horizontal thermo analyzer was adopted as a transpiration apparatus to measure the vapor pressure of Cr(acac)3 and Cr(CO)6 after ascertaining the equilibrium nature of these measurements and by exploiting Dalton’s law of partial pressure for ideal gas mixtures. The vapor pressures of Cr(acac)3 and Cr(CO)6 were computed to be log(p=Pa) = 14:16(±0:07) − 5830(±157) K=T and log(p=Pa) = 11:54(±0:04) − 3308(±110) K=T , valid over the ranges 374 –418 K and 309 –347 K and yielding values of 111:6 ± 3:0 and 63:3 ± 2:1 kJ=mol for their standard enthalpies of sublimation, respectively. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Materials processing; Phase equilibria; Thermodynamic process; Vaporisation; Chromium complexes; TG based transpiration

1. Introduction Organo-chromium and chromium organic complexes were often reported to be employed for chemical vapor deposition (CVD) on substrates such as nuclear fuels, stainless steels and borosilicate glasses either in the elemental form as Cr (Blasko, 1965; Hampden Smith & Kodas, 1995), Cr 2 O3 (Maruyama & Akagi, 1996; Hertz, Audisin, Defort, & Idrissi, 1992), carbide (Schuster, Maury, & Nowak, 1990; Maury et al., 1990) or nitride (Kuppusami et al., 2001; Schuster, Maury, Nowak, & Bernard, 1991). The Cr organic complex, tris(2,4-pentanedionato)chromium(III) commonly known as chromium acetylacetonate, Cr(acac)3 was employed by Hertz and co-workers in 1992 for ∗ Corresponding author. Tel.: +91-4114-480116; fax: +91-4114480081. E-mail address: [email protected] (O. M. Sreedharan).

coating the nuclear fuel pellets as well as the inner walls of the zircalloy cladding with Cr 2 O3 by thermal CVD at temperatures in the range of 573–873 K for minimizing fuel cladding chemical interaction and to enhance the retention of Gssion products by the fuel. The same complex was also used for hard facing of stainless-steel components with CrN by a plasma-assisted CVD process developed by Kuppusami and co-workers in 2001. A Huidized bed CVD of Cr on nuclear fuel particles to prevent their attrition and contamination due to Gssion product release was reported by Oxley, Browning, Veigel, and Blocher (1962) who employed an organo-chromium complex namely, bis(6 -isopropyl benzene)chromium(0) (commonly known as DiCumene Chromium, DCC). Maruyama and Akagi (1996) had reported the use of not only Cr(acac)3 but also Cr(CO)6 for coating of borosilicate glass with thin Glms of Cr 2 O3 by photo-CVD for enhancing the eIciency of solar heaters. In view of the wide ranging technological

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Table 1 Thermal decomposition behavior of chromium complexes by TGa studies Sl. Complex no.

Initial Mass % of Cr in the weight complex 10−3 ×W (g)

Residue

Final Mass of Cr, T (K) (10−3 ×W )(g) in

Gravimetric Theoretical 10−3 ×W (g) Nature analysis 1 Tris(2,4-pentanedionato)Cr(III); Cr(acac)3 2 3 4 5 6 7 8 9 10 11 12 13

Hexacarbonyl Cr(0); Cr(CO)6 Bis(isopropyl benzene)Cr(0); DCC Reinecke salt; (NH4 )[Cr(NH3 )2 (NCS)4 ] (NH4 )[Cr(NH3 )2 (NCS)4 ] + biphenyl; (1:1) (NH4 )[Cr(NH3 )2 (NCS)4 ] + biphenyl; (1:2) (NH4 )[Cr(NH3 )2 (NCS)4 ] + biphenyl; (1:3) [Cr(NH2 CH2 CO2 )3 ] · 0:5H2 O [Cr(C7 H6 NO2 )3 ] · 2H2 O [Cr(C9 H6 NO)2 ] · 2H2 O [Cr(NO)2 (C9 H6 NO)2 ] · 0:5H2 O (NH2 NH3 )[Cr(NH3 )2 (NCS)4 ] (NH4 )[Cr(C9 H6 ON)(NCS)4 ]

a In

11.02

14:9 ± 0:01 14.9

0.00

23.17 32.26 13.46 9.35 16.19 17.93 10.47 8.80 16.72 8.55 10.29 9.33

na 16:1 ± 0:01 15:5 ± 0:03 na na na 18:4 ± 0:01 10:5 ± 0:03 13:8 ± 0:02 12:7 ± 0:03 14:8 ± 0:04 11:7 ± 0:01

0.00 1.19 5.04 3.13 2.53 2.03 3.46 6.69 4.54 1.76 3.48 6.75

na 17.8 15.5 na na na 19.0 10.9 13.8 12.7 14.8 11.7

Initial Final sample residue

Volatile; no end 523 1.64 residue No end residue 403 5.48 Cr 2 O3 + C 773 3.00 CrNCS 792 2.08 Cr(NCS)2 1003 0.99 CrNCS 874 1.31 CrNCS 894 1.17 CrC0:43 + 3C 794 1.92 794 0.92 Cr( − NH2 C6 H4 O)3 Cr(C9 H6 NO)0:45 750 2.31 Cr(NO) 923 1.09 CrNCS 792 1.52 [Cr(C9 H6 ON)(NCS)2 ] 792 1.09

0.00 0.00 0.50 2.38 0.97 1.20 0.96 2.03 0.93 2.02 1.12 1.64 1.12

dynamic helium; How rate: 6 dm3 =h; linear heating rate: 0:17 K=s; na: not analyzed.

applications of CVD of Cr as metal or its binary compounds namely oxide, nitride or carbide, several Cr organic and organo Cr complexes were synthesized and the thermal stabilities were analyzed using thermogravimetry (TG) in the present investigation for an assessment of their volatilization=thermal decomposition behavior. Such an assessment of 13 complexes revealed only Cr(acac)3 and Cr(CO)6 as the materials which vaporize congruently and most suited for CVD applications. Hence, accurate measurements of vapor pressures of these two compounds were undertaken in the present study. 2. Experimental 2.1. Materials The complexes of chromium subjected to TG analysis in a dynamic helium environment are listed in Table 1. These complexes were either synthesized in the laboratory or procured commercially. Cr(acac)3 , synthesized in the laboratory from reagent grade CrCl3 · 6H2 O, urea and acetyl acetone (Loba Chemie, 98%) was maroon in color, compared to the purple color of the commercial sample (Merck, Germany, purity better than 98%). Cr(CO)6 was commercially procured (Lancaster, UK, purity better than 99%). The starting materials for all the other complexes were either from Loba Chemie or from Merck and of purity better than 98%. DCC was synthesized by modifying the procedure reported by Hess and Mailey (1964). The fourth complex in Table 1 (known as Reinecke salt) was prepared by a procedure published elsewhere (Braver, 1965). To examine whether the tendency of the complex to enter the vapor phase by sublimation (which pro-

cess is deGned by its vapor pressure, VP) could be enhanced by mixing it with a complexant of Cr, Reinecke salt was mixed with biphenyl in the molar mass ratios 1:1, 1:2 and 1:3 (referred to as complexes 5, 6 and 7 in Table 1) by physical blending. Complexes 8 and 9 in Table 1 were prepared as per the standard procedures (Braver, 1965; Rochow, 1960). Complexes 10 and 11 were derived from tris oxine Cr(III), whereas Reinecke salt was the starting material for complexes 12 and 13. The purity of all the complexes was established by powder X-ray di7raction (XRD) and estimating the chromium content by gravimetric analysis. The results of the quantitative analysis for Cr along with the theoretical values are given in Table 1. Since Cr(acac)3 is a chiral molecule, the optical activity of the two sources (synthesized as well as commercially procured) were individually checked. Solutions of these samples in chloroform under irradiation with the sodium D spectral line of wavelength 589 nm were found ◦ to rotate the plane of polarization by an angle of −27 and ◦ −39 , respectively, indicating them to be mixtures of Q(D) and (L) forms in di7erent ratios. However, with respect to other physico-chemical properties including VP, the two sources of Cr(acac)3 were indistinguishable. A fast atom bombardment mass spectrometric (FABMS) study was conducted on this complex to identify the nature of the major vaporizing species. 2.2. Non-isothermal TG characterization The complexes listed in Table 1 were subjected to non-isothermal TG runs in a TG/di7erential thermal analysis (DTA) thermal analyzer (Model Seiko 320) at a linear heating rate of 0:17 K=s under He atmosphere purged at a

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Fig. 1. Block diagram of the TG-based transpiration apparatus.

rate of 5:7 dm3 =h. The purpose of these runs was to identify the mass loss steps and the Gnal temperature to attain a somewhat constant mass of the residue. 2.3. Vapor pressure measurements The thermal analyzer referred to in Section 2.2 was adopted as a transpiration apparatus for VP measurements by suitable modiGcations and precautions. This dual arm horizontal balance with both the sample and the reference pans located in the same uniform temperature zone of a horizontal tubular furnace had enabled the use of a common environment provided by a suitable purge gas. Such a design minimizes=eliminates errors arising from convection, buoyancy, thermo molecular drag and electrostatic charge e7ects. The block diagram of the TG-based transpiration setup is given in Fig. 1. The temperature of the sample as well as DTA signals were sensed by Type R thermocouples, which serve as the arms of the balance. The How rate of the carrier gas was measured with a capillary glass How meter, which was calibrated by a soap bubble displacement method using a horizontal burette as described by Pankajavalli, Mallika, Sreedharan, Premila, and Padma Gopalan (1998). A wet test meter (Model Toshniwal, India) used for total volume measurement at the outlet was in turn calibrated with the help of the glass capillary How meter. Though the precision in the How rate measurements by the glass capillary How meter was ±0:5%, the overall precision in the integral volume How was of the order of ±1% of the total volume of the carrier gas. To facilitate the saturation of the carrier gas with the vaporizing species, the sample was spread over a larger area on a Hat Pt foil shaped as a shallow crucible and placed on the sample pan. Further, the narrow chamber tube was found to be conducive for vapor saturation without resorting to the use of baRes. Even though high-purity He (99.995%) was used as the carrier gas, due to unavoidable ingress of oxygen, the oxygen partial pressure in the carrier gas as monitored

by a solid electrolyte sensor was as high as 10 –100 Pa. However, the analysis of the carrier gas (saturated with the vaporizing species) at the outlet by FABMS did not show any peak due to the oxidation of the sample by the residual oxygen in He. The transpiration experiments were carried out at temperatures in the range 300 –420 K, which range is very much lower than what is required to oxidize the Cr complexes by 100 –1000 ppm of oxygen. Prior to transpiration experiments, blank runs were conducted with How rates of 6 –18 dm3 =h of He gas at isothermal temperatures within the range 300 –900 K and keeping both sample and reference pans empty. The apparent mass gain was of the order of 9:0×10−7 g=100 K change in temperature encompassing the full range of How rate. However, at any given isothermal temperature in this range, the drift in mass with time was of the same magnitude namely 9:0×10−7 g=h. This apparent mass loss was subtracted from all the recorded isothermal mass losses. From Table 1, it is seen that only Cr(acac)3 and Cr(CO)6 sublime at relatively lower temperatures leaving no residue and hence these two complexes were chosen for VP measurements. The samples were taken in quantities of 0.05 – 0:1 g and spread over on a shallow Pt crucible located on the sample pan with an equivalent mass of Al2 O3 on the reference pan. Before starting each experiment, the apparatus was purged with He at least for 7:2×103 s at the ambient temperature. Rapid heating was resorted to until the approach of temperature, which was within 20 K of the set point. Further heating to reach the desired temperature was done at 0:08 K=s to avoid overshooting of the temperature limit. Mass losses were recorded manually for equal lengths of time at a given How rate of the carrier gas (He) until constancy of mass loss was observed in three consecutive readings. In the Grst set of experiments, the How rate of the carrier gas was varied from run to run over the range 6 –18 dm3 =h at an isothermal temperature which was chosen to be either the mean or a value slightly higher than the temperature

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Fig. 2. TG and DTA curves of tris(2,4-pentanedionato)chromium(III).

Fig. 3. TG and DTA curves of hexacarbonyl chromium(0).

range of VP measurements. The How rate could be calculated within ±0:5% precision as checked by capillary How meter and an accuracy of the same order in the integral volume of the gas assessed by wet test meter. Further experiments for VP determination were carried out by using a How rate in the middle of the plateau obtained in the previous set of experiments. 3. Results and discussion 3.1. Non-isothermal TG The purpose of the non-isothermal TG was merely to identify the Cr complexes that could function as precursors

for CVD of Cr or its oxide, etc., besides identifying mass loss steps and their inception temperatures. For meeting the Grst mentioned objective, one would require to know the initial Cr content of the complex and that of the Gnal residue for an assessment of overall loss of Cr bearing vapor species through He cover gas acting as the carrier. These two details are also furnished in Table 1. It could be seen that there was 100% mass loss in the case of Grst two and 82% loss of Cr in the third complex. Owing to the high volatilities of the Grst two complexes, TG=DTA results had to be made use of for the choice of the temperature range over which the vapor pressure could be experimentally determined in the present investigation. For this purpose, typical TG=DTA traces of these two complexes are reproduced in Figs. 2 and 3. The

R. Pankajavalli et al. / Chemical Engineering Science 57 (2002) 3603–3610

3.2. Vapor pressure measurements The rate of mass loss of a sample over a Gxed surface area under isothermal conditions owing to vaporization should be constant since sublimation is a zero order process. The inert He gas sweeping over the sample carries the saturated vapor and the mass loss is recorded by TG analyzer as a function of time. If W is the mass loss of the sample at the isothermal temperature T , which was caused by the How of Vc dm3 of the carrier gas, the apparent vapor pressure, papp could be calculated using Dalton’s law of partial pressure for ideal gas mixtures as given by papp = WRT=MVc ;

(1)

where M is the molar mass of the sample. For VP measurements using transpiration, ‘a priori’ knowledge of the vapor species (monomer, dimer or fragmented) is required for ascribing a molar mass M . For this purpose, the results of FABMS studies on Cr(acac)3 complex was made use of which showed the 100% peak to correspond to mass numbers of 349 along with 80% and 20% peaks corresponding to 250 and 150, respectively, with unipositive charge on the species. These mass numbers indicated the major vaporizing species as the monomer later fragmenting into Cr(acac)+ 2 and Cr(acac)+ , respectively. In order to establish the measured values of papp to be the equilibrium VP data at a given isothermal temperature, it is necessary to demonstrate the existence of a chair-shaped curve in the plot of papp against How rate. Such a plot for Cr(acac)3 vaporizing at an isothermal temperature of 404 K is shown in Fig. 4. The existence of a plateau over the How rates of 8–12:4 dm3 =h shows that the vaporization rate was adequate to saturate the carrier gas for any How rate within this range. Of course, it is well known that at higher How rates the carrier gas is under saturated with slow vaporiza-

Apparent p/Pa

0.7

0.5

0.3

0.1

5

20

10 15 3 Flow rate (dm /h)

Fig. 4. Variation of apparent VP of Cr(acac)3 as a function of How rate of the carrier gas at 404 K. 2

1

log (p/Pa)

range of temperature was so chosen to be signiGcantly below the melting point as to be amenable for VP determination using this novel TG transpiration technique. The TG=DTA trace shown in Fig. 3 indicated rapid volatilization commencing from 345 K and the broad endotherm coincides with the rapid single stepped loss of total mass ending by 403 K. Thus, the upper limit of temperature range for VP measurements was set to be below 350 K. All other complexes listed in Table 1 showed insigniGcant loss of Cr. Even if one of the intermediate products of thermal decomposition were volatile, it would have resulted in a detectable loss of Cr content due to its entrainment in a carrier gas whose volume could be signiGcant even over a span of 100 –200 s of resident time prior to substantial change in the temperature. The absence of signiGcant volatility of the complexes referred to in the remaining part of Table 1 did not warrant further discussion on their TG results. Hence, identiGcation of the intermediate decomposition steps of these complexes was not considered as relevant and are omitted here.

3607

Series1 Series2

0

-1

-2 23.5

24.5

25.5

26.5

27.5

10 4 K/T

Fig. 5. Temperature dependence of the equilibrium VP of Cr(acac)3 .

tion for this conGguration of the sample, etc., while at How rates lower than that of the plateau, papp is greater than pequilibrium owing to back di7usion of the heavier species to the colder end (Cater, 1970). The VP measurements at all other temperatures covering the range 374 –418 K were carried out at How rates in the middle of the plateau region to avoid unsaturation as well as back di7usion. The VP results derived from the mass losses of Cr(acac)3 are plotted as log p against reciprocal temperature in Fig. 5. The least-squares expression for log p corresponding to the VP data plotted in Fig. 5 is given as log(p=Pa) = 14:16(±0:07) − 5830(±157) K=T:

(2)

From the slope of Eq. (2), a value of 111:6 ± 3:0 kJ=mol could be derived for the standard enthalpy of sublimation, ◦ QHsub of Cr(acac)3 . At this juncture, it should be pointed ◦ out that this QHsub (Cr(acac)3 ) is a weighted average of the VP results of two sources of the complex (as mentioned in Section 2.1), one of which is indicated by di7erent legends of the points in Fig. 5. It might be possible to Gt the two sets of points to two di7erent least-squares expres◦ sions corresponding to slightly di7erent values of QHsub . However, such a computation was not resorted to since the di7erence in the values of papp are within the limits of ◦ experimental scatter. The values of QHsub derived for one

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Table 2 Sublimation enthalpy of Cr(acac)3 ; derived from TG transpiration and literature values ◦

Sl. no.

Technique

QHsub = (kJ=mol)

Temperature range, T (K)

Reference

1 2 3

E7usion measurements Isoteniscopy Sublimation bulb= spectrophotometry TG method TG-DSC TG transpiration

91:6 ± 1:3 27:6 ± 2:9 110:9 ± 0:8

— 389 –397 390 – 408

Ref. 8 in Melia and MerriGeld (1970) Wood and Jones (1964) Melia and MerriGeld (1970)

85.9 141:5 ± 5:6 111:6 ± 3:0

335 –356 513– 633 373– 418

Ashcroft (1971) Murray and Hill (1984) This work

mole of Cr(acac)3 were compared with the literature data in Table 2. It is seen that the value of 111:6 ± 3:0 kJ is in good agreement with 110:9 ± 0:8 kJ reported by Melia and MerriGeld (1970) who employed sublimation bulb=spectrophotometric technique. The other values namely 91:6 ± 1:3 kJ (cited by Melia & MerriGeld as Ref. 8 (Frankhauser, 1965) in their work) using e7usion measurements, 27:6 ± 2:9 kJ from isoteniscopic studies (Wood & Jones, 1964), 85:9 kJ by TG method (Ashcroft, 1971) and 141:5 ± 5:6 kJ by TG-DSC technique (Murray & Hill, 1984) are scattered much beyond the limits of quoted precision and should not be considered for taking a mean value. This omission was called for due to the fact that the measurements were reported either over a very narrow range (Ashcroft, 1971) or at a much di7erent temperature span (Murray & Hill, 1984) and hence could not be reliably extrapolated. Therefore, a value of 111:3 ± 2:0 kJ=mol averaged between the present TG transpiration results and those from spectrophotometric sublimation bulb technique should be considered as quite reasonable. In addition, the magnitude of the equilibrium VP measured in the present studies agree with the results reported by Melia and MerriGeld (1970) as seen from the values of 1:7 ± 0:02 and 0:4 ± 0:01 Pa, respectively, at 400 K. 3.3. Vapor pressure of Cr(CO)6 A plot of apparent VP, papp of Cr(CO)6 against How rate at an uniform temperature of 314 K of the carrier gas, He (shown in Fig. 6) facilitated the identiGcation of the plateau region to be between 8.0 and 12:4 dm3 =h of How rate as in the case of the earlier samples. By employing a How rate of He in the middle of this plateau region, the TG transpiration runs were carried out on Cr(CO)6 as described earlier and the results so obtained are listed in Table 3. A plot of the values of log p from Table 3 against inverse temperature dependence of Cr(CO)6 shown in Fig. 7 was found to conform to the least-squares expression log(p=Pa) = 11:54(±0:04) − 3308(±110) K=T

(3)

valid over a rather narrow range of temperature namely 309 –347 K. From Eq. (3) a value of 63:3±2:1 kJ=mol could ◦ be assessed for QHsub (Cr(CO)6 ).

16

Apparent p/Pa

4 5 6

13

10

7

4

9

14

19

Flow rate/(dm3 /h)

Fig. 6. Plot of apparent VP of Cr(CO)6 against How rate of the carrier gas at 314 K. Table 3 Mass loss, How rate of the carrier gas and VP of Cr(CO)6 determined by TG transpiration method Run no.

T (K)

Mass loss, (10−3 ×W )=(g)

Vc =(dm3 =h)

p(Pa)

1 2 3 4 5 6 7 8 9 10 11 12

332 315 313 311 323 310 309 319 347 315 314 314

30.88 10.11 8.56 7.05 17.48 7.72 5.39 12.01 44.29 10.11 9.33 10.78

10.909 10.909 10.909 10.909 10.909 10.909 10.909 10.909 5.456 10.909 9.001 12.414

35.50 11.02 9.28 7.59 19.57 8.30 5.76 13.27 106.50 11.02 12.30 10.30

Hampden Smith and Kodas (1995) had reported its VP by a direct pressure technique which when plotted as log p against inverse temperature yielded a straight line over the range 309 –424 K. Their temperature range could be seen to encompass not only that of the present investigation quantifying log p by Eq. (3) but also the regime of rapid weight loss observed in the TG=DTA curve presented in Fig. 3 (which recorded zero weight by 403 K). The exact process occurring in this massive weight loss step commencing from

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the Gnal residue, it was concluded that the other complexes were either non volatile or exhibiting insigniGcant volatility. Hence, the vapor pressures of Cr(acac)3 and Cr(CO)6 were determined by adopting a recently developed TG-based transpiration technique from which values, the enthalpies of sublimation were found to be 111:6 ± 3:0 and 63:3 ± 2:1 kJ=mol valid over the ranges 374 – 418 and 310 –332 K, respectively.

log (p/Pa)

2

1.5

1

0.5 28.5

3609

30

31.5

33

Acknowledgements

4

10 K / T

Fig. 7. Temperature dependence of the equilibrium VP of Cr(CO)6 .

345 K and ending up by 403 K (cf. Fig. 3) appears to be obscure. This carbonyl was reported to melt over a range of temperatures from 323 to 422 K in a sealed tube (Braver, 1965), to sinter at 363 K and to explode at 483 K (Windholz, 1983) and to decompose above 373–393 K (Braver, 1965; Audrieth, 1950). In short, the broad endotherm attended to by a single stepped mass loss commencing from 336 K and ending up by 403 K (with nil residue) should correspond to the combination of two or more of the processes namely melting, vaporization and decomposition taking place simultaneously in such a way not to permit any deciphering of the same. The VP data reported by Hampden Smith and Kodas (1995) over the afore mentioned wide range of temperature was Gtted by the present authors into the least-squares expression log(p=Pa) = 12:72(±0:01) − 3276(±10) K=T:

(4)

From Eq. (4) a value of 62:7 ± 0:2 kJ=mol could be derived ◦ for the enthalpy change, QH(4) which cannot be merely the ◦ QHvap (of liquid Cr(CO)6 ) but a sum of enthalpy changes for vaporization and decomposition processes. If it were a ◦ simple vaporization of the liquid, the magnitude of QH(4) should be much smaller than 63:3 ± 2:1 kJ=mol which was determined to be the standard sublimation enthalpy of ◦ ◦ Cr(CO)6 (which is a sum of QHfus + QHvap ). 4. Conclusion Only two complexes of Cr namely tris(2,4pentanedionato)chromium(III), Cr(acac)3 , and hexacarbonyl chromium(0), Cr(CO)6 were identiGed to be completely volatile at temperatures around 500 K leaving no residue of Cr, for functioning as precursors for CVD applications from a series of 13 complexes screened by non-isothermal TG analysis. A third complex namely bis(6 -isopropyl benzene)chromium(0), DCC also had undergone substantial vaporization leaving only 18% of Cr as residue. Owing to its high reactivity with ambient atmosphere, DCC was not considered as a useful precursor for CVD. From an estimate of the chromium content in

The authors are indebted to Dr. Baldev Raj, Director, MCRG, IGCAR and Rev. Fr. John Pragasam, Director, LIFE, Loyola College, Chennai, for their keen interest and constant encouragement throughout the course of this work. References Ashcroft, S. J. (1971). The measurement of enthalpies of sublimation by thermogravimetry. Thermochimica Acta, 2, 512–514. Audrieth, L. F. (1950). Inorganic synthesis, Vol. III (p. 159). New York: Mc-Graw Hill. Blasko, J. (1965). Chromium plating on metal surface coated with non metal layer. Netherlands Patent Application, 6, 413, 438. Braver, G. (1965). Handbook of preparative inorganic chemistry (2nd ed.), Vol. II (pp. 1376, 1382, 1743). New York: Academic Press. Cater, D. E. (1970). Vapor pressure measurements. In Rapp, R. A., & Shores, D. A. (Eds.), Physico-chemical measurements in metals research, Vol. IV, Part 1 (p. 21). California: Interscience. Hampden Smith, M. J., & Kodas, T. T. (1995). Chemical vapor deposition of metals: Part 1. An overview of CVD processes. Chemical Vapor Deposition, 1, 8–23. Hertz, T. D., Audisin, C. S., Defort, F., & Idrissi, H. (1992). Process for forming a chromium oxide insulating layer between the pellets and the cladding of a nuclear fuel element and fuel element having such an insulating layer. US Patent 5,137,683. Hess, L. G., & Mailey, E. A. (1964). Bishydrocarbon compounds of Cr. US Patent 3,129,237. Kuppusami, P., Dasgupta, A., Sreedharan, O. M., Lawrence, F., Raghunathan, V. S., Baldev, R., Nagaraja, K. S., & Premkumar, A. (2001). An improved process for hard chromium nitride coating on substrates and a coated substrate so obtained. Indian Patent Application 673. Maruyama, T., & Akagi, H. (1996). Chromium oxide thin Glms prepared by chemical vapor deposition from chromium acetylacetonate and chromium hexacarbonyl. Journal of the Electrochemical Society, 143, 1955–1958. Maury, F., Oquab, D., Manse, J. C., Morancho, R., Nowak, J. F., & Gauthier, J. P. (1990). Structural characterization of chromium carbide coatings deposited at low temperature by low pressure chemical vapor decomposition using dicumene chromium. Surface and Coating Technology, 41, 51–61. Melia, T. D., & MerriGeld, R. (1970). Vapor pressures of the tris(acetylacetanato) complexes of scandium(III), vanadium(III) and chromium(III). Journal of Inorganic and Nuclear Chemistry, 32, 1489–1493. Murray, J. P., & Hill, J. O. (1984). DSC determination of the fusion and sublimation enthalpy of tris(2,4-pentanedionato)chromium(III) and iron(III). Thermochimica Acta, 72, 341–347. Oxley, J. H., Browning, M. F., Veigel, N. D., & Blocher, J. H. (1962). Micro miniaturized fuel elements by vapor deposition techniques. Industrial and Engineering Chemistry Product Research and Development, 1, 102–107.

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