- Email: [email protected]
ZrO2 by pyrolysis of modified zirconium alkoxide precursors
SOLID STATE IONICS
Solid States lonics 63-65 (1993) 45-51 North-Holland
Synthesis of ZrC/ZrO2 by pyrolysis of modified zirconium alkoxide precursors E.L. Sham 1, E.M. Farf~ln-Torres ~, S. Bruque-G~imez Departamento de Qulmica Inorgdnica, Cristalografiay Mineralogla, Universidadde Mdlaga, Apartado 59, 290 71-Mdlaga, Spain
and J.J. Rodriguez-Jimdnez Departamento de Ingenieria Qulmica, Universidadde Mdlaga, Apartado 59, 29071-Malaga, Spain
A new route to prepare zirconium carbide preceramicpowders has been developedusingalkoxides polymerizedwith polyalcohols. Preceramicswere pyrolizedunder vacuum to form metal carbides/metal oxide materials. Best carbide yieldswere obtained using very low heating rates (0.5°C/min) and reaction time at final temperature higher than 30 min. An explanation of the possible parameters that governzirconiumcarbide formation is presented.
1. Introduction
Due to the very high hardness (8-9 Mobs) and refractivity of ZrC, it can be employed in several mechanical and chemical processes [ l ]. The use of ZrC in interaction with an adequate support or additives could improve its properties by sinergic effects. In our case, we have chosen ZrO2 as additive, because it was shown to be a good stabilizer when mixed with high technical ceramics [2]. The simplest way to produce conventional powder metal carbides is to react the corresponding metal or metal oxide in the solid state with carbon, under reducing conditions. In the case of ZrC, this implies a very high temperature of reaction ( T > 2 0 0 0 ° C ) , producing also iron-contaminated products which compromise its performance characteristics. On the other hand, it is not possible to obtain supported systems when carbides are prepared by this procedure. The synthesis of non-oxide ceramics from pyrolysis of chemically designed polymeric precursors has attracted wide interest during the recent years, because On leavefrom:INIQUI,UNSa-CONICET,BuenosAires 177, 4400-Salta, Argentina.
it offers several advantages as compared to the traditional methods [3-6]. For example, lower processing temperatures improve impurity control, formation of powders with smaller crystallite size and uniform mixing of the reacting elements on a molecular or even atomic level, could be achieved by this method. The possibility of controlling the structure and composition of preceramic polymers allows to obtain the necessary chemical bonding required in the final material, and the use of controlled pyrolysis conditions could lead to the preparation of mixed materials such as: M - C - O , M - C - N , M - N - O , MaMb-C-O, etc., opening a wide field of applications [4]. Alkoxides polymerized with polyalcohols, esters and carboxilic acids have been used for the production of carbides. This was first reported in a Japanese patent for the preparation of TiC [ 7 ]. Then several articles and patents were published for other metal carbides [ 8-11 ]. This paper provides a study of the preparation of mixed systems ZrC/ZIO2 by pyrolysis of powder precursors prepared by reacting a zirconium alkoxide with different crosslinked agents.
0167-2738/93/$ 06.00 © 1993Elsevier SciencePublishers B.V. All fights reserved.
E.L. Sham et aL /Synthesis of ZrC/Zr02 by pyrolysis
46
2. Experimental Preceramic precursors were prepared by reacting zirconium n-propoxide Zr(OCH2CH2CH3)4 (ZrnP) with ethyleneglycol C2H602 (EG) or 1,4-benzenediol C6H602 (BD), using n-propanol as solvent. In the first step of the reaction a solution of EG was dropwise added to a diluted solution 40% of Zr-nP at six different molar ratios of C/Zr: 1, 2, 3, 5, 8, 33. Upon addition of the EG solution a homogeneous gel was formed. The reaction was conducted by heating the gel under constant magnetic stirring, and allowing it to reflux at boiling temperature. After a 24 h reflux, the solvent was eliminated by centrifugation and the solid product washed several times with acetone. Then the material was dried at 60°C and a very fine white powder was obtained. Using this general procedure, another three solids were prepared using BD instead of EG with molar ratios C/Zr of 3, 4 and 6. In table 1 the different samples and preparation parameters are summarized. These materials were characterized by elemental analysis, XRD (Siemens D-501 diffractometer using monochromatic Cu Kct radiation), D T A - T G (Rigaku Thermoflex apparatus at a heating rate of 10 K min -~) and IR (Perkin Elmer 883). The preceramic polymers were pyrolyzed at various temperatures in vacuum ( ~ 10 - 3 mbar), in a quartz tube heated at different rates, using a tubular furnace (Carbolite MTF 12/38A). The structural transformations that occurred upon pyrolysis were characterized by XRD and X-ray photoelectron spectroscopy. XPS analyses were carried out with a
Leybold Heraus LHS 10 spectrometer interfaced to a data system which allowed accumulation of spectra. The spectrometer was equipped with a magnesium anode, Mg I ~ = 1253.6 eV, operated at 12 kV and 10 mA. The ZrC percentage obtained in each case was estimated from the ratio of the intensity of the diffraction line 111 of the ZrC (2.70 A) to the - 1 1 1 line of monoclinic ZrO2 (3.163 •) lz~:/ lzro2 [ 12 ]. Free carbon was determined by elemental chemical analysis.
3. Results and discussion 3. I. Preceramics preparation
Preceramic products are white powders when EG is used as crosslinking agent, and dark brown very fine powders when BD is used. No occurrence of any organic liquid phase was observed for the chosen C/Zr ratios for ZrOEG solids. This favours powder processing because the morphology and average grain size are not drastically affected by the pyrolysis procedure as in the case of other carbides reported in previous studies [ 10,11 ]. In the case of ZrOBD samples formation of two oily phases is observed when C / Z r < 3 ratios are used, then higher C/Zr ratios were chosen to work. Carbon content of the ZrOEG samples increases from 19 w/o for ZrOEGI to reach a maximum of 30 w/o for ZrOEG3 and then remains constant for the samples prepared with higher C/Zr initial ratios (fig. 1 ). In the case of ZrOBD the carbon content in the solids reaches a constant value near to 30 w/o in all cases.
Table 1 Preceramics precursors and their preparation parameters. Sample
Crosslinked agent
Initial C/Zr molar ratio
ZrOEG1 ZrOEG2 ZrOEG3 ZrOEG5 ZrOEG8 ZrOEG33 ZrOBD3 ZrOBD4 ZrOBD6
Ethyleneglycol Ethyleneglycol Ethyleneglycol Ethyleneglycol Ethyleneglycol Ethyleneglycol 1,4-benzenediol 1,4-benzenediol 1,4-benzenediol
1 2 3 5 8 33 3 4 6
35 25 15 •
0
,
•
,
.
,
.
10 20 30 40 C / Zr ( initial molar ratio)
Fig. 1. Carbon content of the ZrOEG preceramic materials related to the initial C/Zr molar ratio.
E.L. Sham et al. / Synthesis of ZrC/ZrO2 by pyrolysis
From XRD and 1R data it can be inferred that the preceramic precursors are amorphous solids with the corresponding alkoxy and phenoxy radicals present in their structure. When the solids are heated in air, the formation of tetragonal crystalline ZrO2 is detected at 600°C in all the samples. ZrOBD solids exhibit a stabilization of the ZrO2 tetragonal phase in a wider temperature range than ZrOEG samples. According to the thermogravimetric data presented in fig. 2 there is a first weight loss of 20 to 30% during the exothermic peak at 300°C and a 3 to 4% weight loss during the exothermic peak at 610°C, for the ZrOEG samples. No further weight losses are detected for temperatures higher than 650°C, when the samples begin to crystallize in the ZrO2 tetragonal phase. The transformation of the tetragonal to monoclinic phase in ZrO2 single crystals and polycrystals is a martensitic one, without atomic diffusion and athermal [ 13 ]. Therefore there are no more changes detected in D T A - T G curves for tern-
47
peratures above 650°C, however this change is detected in the XRD patterns. For ZrOBD3 and ZrOBD6 a multiple step transformation takes place, leading to a total weight loss of 50%. Here again, no further weight losses are observed for T > 7 0 0 ° C (fig. 3).
3.2. Pyrolysis of preceramic powders XRD spectra show that both kinds of preceramic powders, ZrOEG and ZrOBD, form ZrC/ZrO2 powder materials with variable free carbon content (58 w / o ) , upon vacuum pyrolysis at 1200°C. However, the pyrolysis parameters are different in each case. Since, there are a great variety of parameters that could influence the formation of ZrC during pyrolysis, we have chosen to study only three of them: (i) the rate of heating (r in ° C / m i n ) , (ii) the reaction time at the final temperature of reaction (tf) and (iii)
0
Ot""'-"""""
30 60
o~
0
....
15
''''--4.
0
"...
A
2s ~.5o
== V
v
0
0
....... ..,.,
25
J
............. .....................
~o2~o
~o
~
looo
TEMPERATURE "C
Fig. 2. DTA-TG curves of: (a) ZrOEGI, (b) ZrOEG2 and (c) ZrOEG5.
50
I
I
TEMPERATURE "C Fig. 3. DTA-TG curves of: (a) ZrOBD3, (b) ZrOBD4 and (c) ZrOBD6.
48
E.L. Sham et al. / Synthesis of ZrC/ZrOz by pyrolysis
the nature of the preceramic powder. The different parameters of pyrolysis and the resulting ZrC w / o formed are listed in table 2. In order to study the influence of the heating rate r, ZrOEG3 was pyrolized at 1200°C at different r: 0.5, 1 and 2°C/rain, and a tr of 3 min. In all cases ZrC is formed, but at different percentages (see table 2). Even the total concentration of ZrC formed when r--0.5 is the lowest, XPS studies show that, the concentration of ZrC phase in surface is the higher one. This could be related with the fact that at 800 °C the absorption of carbon dioxide or monoxide (produced during decomposition of the organic material) leads to the formation of an activated intermediary phase, that after favours the production of ZrC at higher reaction temperatures [ 14]. If the heating rate is too fast, the concentration of this intermediate phase will be lower, and therefore the total amount of ZrC obtained will be lower too. For this reason we have chosen the rate of 0.5 ° C/ min for all the other preparations. However this heating rate was used only between 200°C and 800 ° C, since in A T D - T G curves, this zone of temperature corresponds to the decomposition of the organic matter present in the preceramic powders, and therefore the formation of the activated phases could then be affected.
The most difficult challenge in the production of carbides from polymeric precursors is to obtain a preceramic powder with the correct amount and type of carbon. Previous work reported for TiC shows that phenyl derivatives produced higher yields of carbides, however, in the alkyl derivatives a more important oxide formation is favoured [ 11 ]. We have chosen ethyleneglycol and 1,4-benzenediol as alkoxide modifiers in order to compare this difference in reactivity. XRD patterns of the pyrolysis products show that in both cases the ZrC formation is observed, however in the case of ZrOBD solids, longer reaction times tf are necessary. Whilst in the case of ZrOEG material carbides begin to be formed at 1100°C; for ZrOBD solids more than 30 min at 1200 °C are needed to observe the formation of ZrC phases. Moreover, in the case of ZrOEG materials, the principal oxide phase corresponds to a monoclinic crystalline structure, and for ZrOBD there is only a tetragonal ZrO2 structure. In order to explain the different behaviour of the two kinds of preceramics solids, we have studied the structural transformations that take place during vacuum pyrolysis. For this. the structural modifications of the samples at different stages of the pyrolysis reaction were characterized using XRD (figs. 4 and 5). At 600°C the ZrO2 tetragonal phase begins
Table 2 Pyrolysis parameters and ZrC content (%) of the pyrolized materials. Preceramic polymer
Heating rate r (°C/rain)
Final temp. of pyrolysis (°C)
Reaction time tf (rain)
ZrC a) (% )
ZrOEGI ZrOEG2 ZrOEG3
0.5 b) 0.5 b) 0.5 1.0 2.0 0.5 b) 0.5 b~ 0.5 b)
1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
30 30 3 3 3 30 90 180 360 30 90 180 360
7 7 9 10 10 9 12 25 26 14 7 c) 30 c) 100
ZrOEG5
0.5 b)
ZrOEG8 ZrOBD4
0.5 b) 0.5 b) 0.5 b~ 0.5 b)
a) Estimated from X R D patterns. b) r for interval temperature between 200 ° C and 800 ° C, besides these temperatures a r of 10.0 ° C / m i n was used. c) ZrC+ZrO2t phases are only present.
49
E.L. Sham et aL / Synthesis of ZrC/Zr02 by pyrolysis
r~ D A
o
d
E.--
z
20
I
30
40
I
50
60
2 0 DEGREES Fig. 4. X-ray powder diffraction patterns of the ZrOEG5 sample heated in vacuum at: (a) 600°C, (b) 800°C, (c) 1000°C, and (d) 1200 ° C. The reaction time at 1200°C was 30 min: ( [] ) ZrO2 monoclinic phase, (/x ) ZrO2 tetragonal phase, ( © ) ZrC.
Z E-
z
"---'---" " ~
20
El
I
I
30
I
40
50
60
2 0 DEGREES Fig. 5. X-ray powder diffraction patterns of the ZrOBD4 sample heated in vacuum at: (a) 600°C, (b) 800°C, (c) 1000°C, and (d) 1200 ° C. The reaction time at 1200 ° C was 90 min: (/x ) ZrO2 tetragonal phase, ((3) ZrC.
to be detected in Z r O E G materials, while for Z r O B D it begins only at 800 ° C. At 1000 °C the ZrO2 monoclinic phase becomes very i m p o r t a n t for the Z r O E G samples, but for Z r O B D only a poorly crystalline tetragonal phase is formed. This b e h a v i o u r could explain the fact that Z r O B D solids need longer reac-
tion time at 1200°C in order to detect the ZrC phase, assuming that Z r C formation takes place growing epitaxially from the ZrO2 tetragonal phase. In order to verify this hypothesis, we have pyrolyzed two sampies: Z r O E G 5 and Z r O B D 4 (which present the same carbon content), with a final reaction time at 1200°C
50
E.L. Sham et al. /Synthesis of ZrC/ZrOe by pyrolysis
o
20
I 30
I 40
I 50
60
2 0 DEGREES Fig. 6. X-ray powder diffraction patterns of the ZrOBD4 sample heated in vacuum at 1200°C during: (a) 90, (b) 180 and (c) 360 minutes: ( A ) ZrO2 tetragonal phase, (O) ZrC.
of 360 min. As expected, Z r O B D 4 is completely t r a n s f o r m e d in Z r C (fig. 6). F o r Z r O E G 5 there are no changes in the a m o u n t o f the ZrC phase compared with the solid pyrolyzed with a tf o f 180 rain (table 2), because the ZrO2 monoclinic phase can not be t r a n s f o r m e d in ZrC. Finally, the reaction time tf determines the a m o u n t o f the ZrC phase formed, in the case o f the best preceramic precursors: Z r O B D solids. On the other hand, Z r O E G samples reach a m a x i m u m a m o u n t o f ZrC f o r m a t i o n after 180 min and r e m a i n constant even after 360 min o f reaction at 1200°C.
taining a good concentration o f carbide on the surfaces o f the materials.
Acknowledgement
The authors wish to express their grateful acknowledgement to C I C Y T ( S p a i n ) M A T 9 0 / 2 9 8 for financial support. E.M. Farfan Torres and E.L. Sham also thank the Ministerio de Educaci6n y Ciencia ( S p a i n ) for the provision o f Research Fellowships.
References
4. Conclusions
Reacting z i r c o n i u m alkoxides with a p r o p e r crosslinked agent (like polyalcohols), p r e c e r a m i c p o w d e r precursors can be designed to provide the f o r m a t i o n o f Z r C / Z r O 2 materials. According to the k i n d o f the polyalcohol used, differences in the crystalline structure o f the final products are observed. W h e n 1,4benzenediol is used for crosslinking, longer times o f reaction are necessary to arrive at the f o r m a t i o n o f ZrC phases. Slow heating rates are the best for ob-
[ l ] P. Pascal, Nouveau Trait6 de Chimie Min6rale, Tome IX. Titane, Zirconium, Thorium (Mason et Cie., Paris, 1963). [2] W.A. Sanders and D.M. Mieskowski, Adv. Ceram. Mater. 1 (1986) 166. [3]R.W. Chorley and P.W. Ledner, Adv. Ceram. Mater. 3 ( 1991 ) 474. [4] J.E. Sheats, C.E. Carraher Jr., C.V. Pittman Jr., M. Zeldin and B. Currell, eds., Inorganic and Metal-Containing Polymeric Materials (Plenum, New York, 1990). [ 5 ] M. Peuckert, T. Vaahs and M. Briick, Adv. Mater. 9 (1990) 398. [ 6 ] D.L. Segai,Chemical Routes for the Preparation of Powders, Nato ASI Series E (Kluwer, Dordecht, 1990).
E.L. Sham et aL / Synthesis of ZrC/Zr02 by pyrolysis [7]Ube Industries Japanese Patent, Pub. 56 155 013 (1 December 1987). [8] J.D. Birchal, M.J. Mockford and D.R. Stanley, Eur. Patent, Appl. 239 301 ( 13 March 1987). [9] S.J. Ting, C-J. Chu, E. Limatta, J.D. Mackenzie, T. Getman and M,F. Hauthorne, Better Ceramics Through Chemistry IV, Mater. Res. Soc. Symp. Proc. 180 (1990) 457. [ 10] Z. Jiang and W.E. Rhine, Chem. Mater. 3 ( 1991 ) 1132.
51
[l l] T. Gallo, C. Greco, B. Simms and F. Cambria, Covalent Ceramics, Mater. Res. Soc. Ext. Abs. 23 (1990) 29. [ 12] H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedure (Wiley, New York, 1974) p. 531. [ 13 ] G.K. Bausal and A.H. Heuer, Acta Metall. 20 (1972) 1281. [ 14] W.G. Guldner and L.A. Wooten, J. Electrochem. Soc. 93 ( 1948 ) 223.
Solid States lonics 63-65 (1993) 45-51 North-Holland
Synthesis of ZrC/ZrO2 by pyrolysis of modified zirconium alkoxide precursors E.L. Sham 1, E.M. Farf~ln-Torres ~, S. Bruque-G~imez Departamento de Qulmica Inorgdnica, Cristalografiay Mineralogla, Universidadde Mdlaga, Apartado 59, 290 71-Mdlaga, Spain
and J.J. Rodriguez-Jimdnez Departamento de Ingenieria Qulmica, Universidadde Mdlaga, Apartado 59, 29071-Malaga, Spain
A new route to prepare zirconium carbide preceramicpowders has been developedusingalkoxides polymerizedwith polyalcohols. Preceramicswere pyrolizedunder vacuum to form metal carbides/metal oxide materials. Best carbide yieldswere obtained using very low heating rates (0.5°C/min) and reaction time at final temperature higher than 30 min. An explanation of the possible parameters that governzirconiumcarbide formation is presented.
1. Introduction
Due to the very high hardness (8-9 Mobs) and refractivity of ZrC, it can be employed in several mechanical and chemical processes [ l ]. The use of ZrC in interaction with an adequate support or additives could improve its properties by sinergic effects. In our case, we have chosen ZrO2 as additive, because it was shown to be a good stabilizer when mixed with high technical ceramics [2]. The simplest way to produce conventional powder metal carbides is to react the corresponding metal or metal oxide in the solid state with carbon, under reducing conditions. In the case of ZrC, this implies a very high temperature of reaction ( T > 2 0 0 0 ° C ) , producing also iron-contaminated products which compromise its performance characteristics. On the other hand, it is not possible to obtain supported systems when carbides are prepared by this procedure. The synthesis of non-oxide ceramics from pyrolysis of chemically designed polymeric precursors has attracted wide interest during the recent years, because On leavefrom:INIQUI,UNSa-CONICET,BuenosAires 177, 4400-Salta, Argentina.
it offers several advantages as compared to the traditional methods [3-6]. For example, lower processing temperatures improve impurity control, formation of powders with smaller crystallite size and uniform mixing of the reacting elements on a molecular or even atomic level, could be achieved by this method. The possibility of controlling the structure and composition of preceramic polymers allows to obtain the necessary chemical bonding required in the final material, and the use of controlled pyrolysis conditions could lead to the preparation of mixed materials such as: M - C - O , M - C - N , M - N - O , MaMb-C-O, etc., opening a wide field of applications [4]. Alkoxides polymerized with polyalcohols, esters and carboxilic acids have been used for the production of carbides. This was first reported in a Japanese patent for the preparation of TiC [ 7 ]. Then several articles and patents were published for other metal carbides [ 8-11 ]. This paper provides a study of the preparation of mixed systems ZrC/ZIO2 by pyrolysis of powder precursors prepared by reacting a zirconium alkoxide with different crosslinked agents.
0167-2738/93/$ 06.00 © 1993Elsevier SciencePublishers B.V. All fights reserved.
E.L. Sham et aL /Synthesis of ZrC/Zr02 by pyrolysis
46
2. Experimental Preceramic precursors were prepared by reacting zirconium n-propoxide Zr(OCH2CH2CH3)4 (ZrnP) with ethyleneglycol C2H602 (EG) or 1,4-benzenediol C6H602 (BD), using n-propanol as solvent. In the first step of the reaction a solution of EG was dropwise added to a diluted solution 40% of Zr-nP at six different molar ratios of C/Zr: 1, 2, 3, 5, 8, 33. Upon addition of the EG solution a homogeneous gel was formed. The reaction was conducted by heating the gel under constant magnetic stirring, and allowing it to reflux at boiling temperature. After a 24 h reflux, the solvent was eliminated by centrifugation and the solid product washed several times with acetone. Then the material was dried at 60°C and a very fine white powder was obtained. Using this general procedure, another three solids were prepared using BD instead of EG with molar ratios C/Zr of 3, 4 and 6. In table 1 the different samples and preparation parameters are summarized. These materials were characterized by elemental analysis, XRD (Siemens D-501 diffractometer using monochromatic Cu Kct radiation), D T A - T G (Rigaku Thermoflex apparatus at a heating rate of 10 K min -~) and IR (Perkin Elmer 883). The preceramic polymers were pyrolyzed at various temperatures in vacuum ( ~ 10 - 3 mbar), in a quartz tube heated at different rates, using a tubular furnace (Carbolite MTF 12/38A). The structural transformations that occurred upon pyrolysis were characterized by XRD and X-ray photoelectron spectroscopy. XPS analyses were carried out with a
Leybold Heraus LHS 10 spectrometer interfaced to a data system which allowed accumulation of spectra. The spectrometer was equipped with a magnesium anode, Mg I ~ = 1253.6 eV, operated at 12 kV and 10 mA. The ZrC percentage obtained in each case was estimated from the ratio of the intensity of the diffraction line 111 of the ZrC (2.70 A) to the - 1 1 1 line of monoclinic ZrO2 (3.163 •) lz~:/ lzro2 [ 12 ]. Free carbon was determined by elemental chemical analysis.
3. Results and discussion 3. I. Preceramics preparation
Preceramic products are white powders when EG is used as crosslinking agent, and dark brown very fine powders when BD is used. No occurrence of any organic liquid phase was observed for the chosen C/Zr ratios for ZrOEG solids. This favours powder processing because the morphology and average grain size are not drastically affected by the pyrolysis procedure as in the case of other carbides reported in previous studies [ 10,11 ]. In the case of ZrOBD samples formation of two oily phases is observed when C / Z r < 3 ratios are used, then higher C/Zr ratios were chosen to work. Carbon content of the ZrOEG samples increases from 19 w/o for ZrOEGI to reach a maximum of 30 w/o for ZrOEG3 and then remains constant for the samples prepared with higher C/Zr initial ratios (fig. 1 ). In the case of ZrOBD the carbon content in the solids reaches a constant value near to 30 w/o in all cases.
Table 1 Preceramics precursors and their preparation parameters. Sample
Crosslinked agent
Initial C/Zr molar ratio
ZrOEG1 ZrOEG2 ZrOEG3 ZrOEG5 ZrOEG8 ZrOEG33 ZrOBD3 ZrOBD4 ZrOBD6
Ethyleneglycol Ethyleneglycol Ethyleneglycol Ethyleneglycol Ethyleneglycol Ethyleneglycol 1,4-benzenediol 1,4-benzenediol 1,4-benzenediol
1 2 3 5 8 33 3 4 6
35 25 15 •
0
,
•
,
.
,
.
10 20 30 40 C / Zr ( initial molar ratio)
Fig. 1. Carbon content of the ZrOEG preceramic materials related to the initial C/Zr molar ratio.
E.L. Sham et al. / Synthesis of ZrC/ZrO2 by pyrolysis
From XRD and 1R data it can be inferred that the preceramic precursors are amorphous solids with the corresponding alkoxy and phenoxy radicals present in their structure. When the solids are heated in air, the formation of tetragonal crystalline ZrO2 is detected at 600°C in all the samples. ZrOBD solids exhibit a stabilization of the ZrO2 tetragonal phase in a wider temperature range than ZrOEG samples. According to the thermogravimetric data presented in fig. 2 there is a first weight loss of 20 to 30% during the exothermic peak at 300°C and a 3 to 4% weight loss during the exothermic peak at 610°C, for the ZrOEG samples. No further weight losses are detected for temperatures higher than 650°C, when the samples begin to crystallize in the ZrO2 tetragonal phase. The transformation of the tetragonal to monoclinic phase in ZrO2 single crystals and polycrystals is a martensitic one, without atomic diffusion and athermal [ 13 ]. Therefore there are no more changes detected in D T A - T G curves for tern-
47
peratures above 650°C, however this change is detected in the XRD patterns. For ZrOBD3 and ZrOBD6 a multiple step transformation takes place, leading to a total weight loss of 50%. Here again, no further weight losses are observed for T > 7 0 0 ° C (fig. 3).
3.2. Pyrolysis of preceramic powders XRD spectra show that both kinds of preceramic powders, ZrOEG and ZrOBD, form ZrC/ZrO2 powder materials with variable free carbon content (58 w / o ) , upon vacuum pyrolysis at 1200°C. However, the pyrolysis parameters are different in each case. Since, there are a great variety of parameters that could influence the formation of ZrC during pyrolysis, we have chosen to study only three of them: (i) the rate of heating (r in ° C / m i n ) , (ii) the reaction time at the final temperature of reaction (tf) and (iii)
0
Ot""'-"""""
30 60
o~
0
....
15
''''--4.
0
"...
A
2s ~.5o
== V
v
0
0
....... ..,.,
25
J
............. .....................
~o2~o
~o
~
looo
TEMPERATURE "C
Fig. 2. DTA-TG curves of: (a) ZrOEGI, (b) ZrOEG2 and (c) ZrOEG5.
50
I
I
TEMPERATURE "C Fig. 3. DTA-TG curves of: (a) ZrOBD3, (b) ZrOBD4 and (c) ZrOBD6.
48
E.L. Sham et al. / Synthesis of ZrC/ZrOz by pyrolysis
the nature of the preceramic powder. The different parameters of pyrolysis and the resulting ZrC w / o formed are listed in table 2. In order to study the influence of the heating rate r, ZrOEG3 was pyrolized at 1200°C at different r: 0.5, 1 and 2°C/rain, and a tr of 3 min. In all cases ZrC is formed, but at different percentages (see table 2). Even the total concentration of ZrC formed when r--0.5 is the lowest, XPS studies show that, the concentration of ZrC phase in surface is the higher one. This could be related with the fact that at 800 °C the absorption of carbon dioxide or monoxide (produced during decomposition of the organic material) leads to the formation of an activated intermediary phase, that after favours the production of ZrC at higher reaction temperatures [ 14]. If the heating rate is too fast, the concentration of this intermediate phase will be lower, and therefore the total amount of ZrC obtained will be lower too. For this reason we have chosen the rate of 0.5 ° C/ min for all the other preparations. However this heating rate was used only between 200°C and 800 ° C, since in A T D - T G curves, this zone of temperature corresponds to the decomposition of the organic matter present in the preceramic powders, and therefore the formation of the activated phases could then be affected.
The most difficult challenge in the production of carbides from polymeric precursors is to obtain a preceramic powder with the correct amount and type of carbon. Previous work reported for TiC shows that phenyl derivatives produced higher yields of carbides, however, in the alkyl derivatives a more important oxide formation is favoured [ 11 ]. We have chosen ethyleneglycol and 1,4-benzenediol as alkoxide modifiers in order to compare this difference in reactivity. XRD patterns of the pyrolysis products show that in both cases the ZrC formation is observed, however in the case of ZrOBD solids, longer reaction times tf are necessary. Whilst in the case of ZrOEG material carbides begin to be formed at 1100°C; for ZrOBD solids more than 30 min at 1200 °C are needed to observe the formation of ZrC phases. Moreover, in the case of ZrOEG materials, the principal oxide phase corresponds to a monoclinic crystalline structure, and for ZrOBD there is only a tetragonal ZrO2 structure. In order to explain the different behaviour of the two kinds of preceramics solids, we have studied the structural transformations that take place during vacuum pyrolysis. For this. the structural modifications of the samples at different stages of the pyrolysis reaction were characterized using XRD (figs. 4 and 5). At 600°C the ZrO2 tetragonal phase begins
Table 2 Pyrolysis parameters and ZrC content (%) of the pyrolized materials. Preceramic polymer
Heating rate r (°C/rain)
Final temp. of pyrolysis (°C)
Reaction time tf (rain)
ZrC a) (% )
ZrOEGI ZrOEG2 ZrOEG3
0.5 b) 0.5 b) 0.5 1.0 2.0 0.5 b) 0.5 b~ 0.5 b)
1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
30 30 3 3 3 30 90 180 360 30 90 180 360
7 7 9 10 10 9 12 25 26 14 7 c) 30 c) 100
ZrOEG5
0.5 b)
ZrOEG8 ZrOBD4
0.5 b) 0.5 b) 0.5 b~ 0.5 b)
a) Estimated from X R D patterns. b) r for interval temperature between 200 ° C and 800 ° C, besides these temperatures a r of 10.0 ° C / m i n was used. c) ZrC+ZrO2t phases are only present.
49
E.L. Sham et aL / Synthesis of ZrC/Zr02 by pyrolysis
r~ D A
o
d
E.--
z
20
I
30
40
I
50
60
2 0 DEGREES Fig. 4. X-ray powder diffraction patterns of the ZrOEG5 sample heated in vacuum at: (a) 600°C, (b) 800°C, (c) 1000°C, and (d) 1200 ° C. The reaction time at 1200°C was 30 min: ( [] ) ZrO2 monoclinic phase, (/x ) ZrO2 tetragonal phase, ( © ) ZrC.
Z E-
z
"---'---" " ~
20
El
I
I
30
I
40
50
60
2 0 DEGREES Fig. 5. X-ray powder diffraction patterns of the ZrOBD4 sample heated in vacuum at: (a) 600°C, (b) 800°C, (c) 1000°C, and (d) 1200 ° C. The reaction time at 1200 ° C was 90 min: (/x ) ZrO2 tetragonal phase, ((3) ZrC.
to be detected in Z r O E G materials, while for Z r O B D it begins only at 800 ° C. At 1000 °C the ZrO2 monoclinic phase becomes very i m p o r t a n t for the Z r O E G samples, but for Z r O B D only a poorly crystalline tetragonal phase is formed. This b e h a v i o u r could explain the fact that Z r O B D solids need longer reac-
tion time at 1200°C in order to detect the ZrC phase, assuming that Z r C formation takes place growing epitaxially from the ZrO2 tetragonal phase. In order to verify this hypothesis, we have pyrolyzed two sampies: Z r O E G 5 and Z r O B D 4 (which present the same carbon content), with a final reaction time at 1200°C
50
E.L. Sham et al. /Synthesis of ZrC/ZrOe by pyrolysis
o
20
I 30
I 40
I 50
60
2 0 DEGREES Fig. 6. X-ray powder diffraction patterns of the ZrOBD4 sample heated in vacuum at 1200°C during: (a) 90, (b) 180 and (c) 360 minutes: ( A ) ZrO2 tetragonal phase, (O) ZrC.
of 360 min. As expected, Z r O B D 4 is completely t r a n s f o r m e d in Z r C (fig. 6). F o r Z r O E G 5 there are no changes in the a m o u n t o f the ZrC phase compared with the solid pyrolyzed with a tf o f 180 rain (table 2), because the ZrO2 monoclinic phase can not be t r a n s f o r m e d in ZrC. Finally, the reaction time tf determines the a m o u n t o f the ZrC phase formed, in the case o f the best preceramic precursors: Z r O B D solids. On the other hand, Z r O E G samples reach a m a x i m u m a m o u n t o f ZrC f o r m a t i o n after 180 min and r e m a i n constant even after 360 min o f reaction at 1200°C.
taining a good concentration o f carbide on the surfaces o f the materials.
Acknowledgement
The authors wish to express their grateful acknowledgement to C I C Y T ( S p a i n ) M A T 9 0 / 2 9 8 for financial support. E.M. Farfan Torres and E.L. Sham also thank the Ministerio de Educaci6n y Ciencia ( S p a i n ) for the provision o f Research Fellowships.
References
4. Conclusions
Reacting z i r c o n i u m alkoxides with a p r o p e r crosslinked agent (like polyalcohols), p r e c e r a m i c p o w d e r precursors can be designed to provide the f o r m a t i o n o f Z r C / Z r O 2 materials. According to the k i n d o f the polyalcohol used, differences in the crystalline structure o f the final products are observed. W h e n 1,4benzenediol is used for crosslinking, longer times o f reaction are necessary to arrive at the f o r m a t i o n o f ZrC phases. Slow heating rates are the best for ob-
[ l ] P. Pascal, Nouveau Trait6 de Chimie Min6rale, Tome IX. Titane, Zirconium, Thorium (Mason et Cie., Paris, 1963). [2] W.A. Sanders and D.M. Mieskowski, Adv. Ceram. Mater. 1 (1986) 166. [3]R.W. Chorley and P.W. Ledner, Adv. Ceram. Mater. 3 ( 1991 ) 474. [4] J.E. Sheats, C.E. Carraher Jr., C.V. Pittman Jr., M. Zeldin and B. Currell, eds., Inorganic and Metal-Containing Polymeric Materials (Plenum, New York, 1990). [ 5 ] M. Peuckert, T. Vaahs and M. Briick, Adv. Mater. 9 (1990) 398. [ 6 ] D.L. Segai,Chemical Routes for the Preparation of Powders, Nato ASI Series E (Kluwer, Dordecht, 1990).
E.L. Sham et aL / Synthesis of ZrC/Zr02 by pyrolysis [7]Ube Industries Japanese Patent, Pub. 56 155 013 (1 December 1987). [8] J.D. Birchal, M.J. Mockford and D.R. Stanley, Eur. Patent, Appl. 239 301 ( 13 March 1987). [9] S.J. Ting, C-J. Chu, E. Limatta, J.D. Mackenzie, T. Getman and M,F. Hauthorne, Better Ceramics Through Chemistry IV, Mater. Res. Soc. Symp. Proc. 180 (1990) 457. [ 10] Z. Jiang and W.E. Rhine, Chem. Mater. 3 ( 1991 ) 1132.
51
[l l] T. Gallo, C. Greco, B. Simms and F. Cambria, Covalent Ceramics, Mater. Res. Soc. Ext. Abs. 23 (1990) 29. [ 12] H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedure (Wiley, New York, 1974) p. 531. [ 13 ] G.K. Bausal and A.H. Heuer, Acta Metall. 20 (1972) 1281. [ 14] W.G. Guldner and L.A. Wooten, J. Electrochem. Soc. 93 ( 1948 ) 223.