- Email: [email protected]
Addition of carbon nanotubes during the preparation of zirconia nanoparticles: influence on structure and phase composition
Powder Technology 139 (2004) 118 – 122 www.elsevier.com/locate/powtec
Addition of carbon nanotubes during the preparation of zirconia nanoparticles: influence on structure and phase composition T.Y. Luo, T.X. Liang *, C.S. Li Institute of Nuclear Energy Technology, Tsinghua University, Beijing 100084, PR China Received 8 July 2003; received in revised form 19 November 2003; accepted 1 December 2003
Abstract Zirconia powder with an average particle size of 10 nm was prepared by adding carbon nanotubes (CNTs) in the hydrolytic process of ZrO(NO3)2. The formation of zirconia nanoparticles was attributed to the addition of CNTs, which affected the zirconia precursor structure. The relation between the stability of cubic (c) and tetragonal (t) phase zirconia at room temperature, and the zirconia particle size and addition of carbon was studied. It was revealed that zirconia particles with nanometer size had took the form of t-phase and that the addition of carbon tended to favor m-phase ZrO2 to c-phase ZrO2 transformation and c-phase ZrO2 stabilization at low temperature. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Carbon and graphite; Phase transformations; Carbon nanotubes (CNTs)
1. Introduction Among advanced ceramics, zirconia (ZrO2) has an important role due to its excellent chemical resistance, refractory character and ionic conductivity. In the recent years, there is an increasing interest in nanostructured ceramics due to their lower sintering temperature and improved mechanical properties. ZrO2 exists in three polymorphic crystalline structures, namely, monoclinic (m) tetragonal (t) and cubic (c). At room temperature, t- and c-phases are metastable, while m-phase is stable. t- and c-phases are high temperature phases. They can be partially or fully stabilized at room temperature by adding a small amount of oxides (e.g. CaO, MgO and types of rare earth oxides) [1– 3]. Wang et al. [4] reported that tZrO2 could be obtained by carburizing pure m-ZrO2 powder at 1500jC. t-ZrO2 was also found in ZrO2-C films deposited by RF magnetron sputtering, while pure ZrO2 film only formed m-phase [5]. Stabilization of metastable phases also was found to depend on crystallite size and the process of preparing the powders [6– 10]. Thermal decomposition of amorphous hydrous ZrO2, zirconium alkoxides and zirconium salt, ball milling of m-ZrO2, vapor phase reactions and * Corresponding author. Tel.: +86-10-8979-6090; fax: +86-10-69771464. E-mail address: [email protected] (T.X. Liang). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2003.12.001
hydrothermal treatments of amorphous hydrated ZrO2 are known to produce c-phase ZrO2. Jagadish et al. [7] prepared pure ZrO2 nanoparticles by chemical synthesis route using sucrose, polyvinyl alcohol (PVA) and metal nitrates. At 200 jC, pure ZrO2 formed c-phase, which was stable up to 600 jC, and then slowly transformed into m-phase. Srinivasan et al. [8] reported that a pure c-phase was originated at 500 jC starting from a precursor precipitated at pH = 13.5 using a solution of NaOH. Nishizawa et al. [9] stabilized c-ZrO2 by inserting Na+ or Ca2 + ions into the initial ZrO2 gel through some hydrothermal reactions. Berry et al. [10] synthesized ZrO2 using aqueous solution of zirconium acetate. He found that high pH value and the addition of alkali to the solution tended to stabilize t-ZrO2. In the process of preparing ceramic powders, it is most desirable that the particles are completely dispersed without any aggregates. Aggregates usually can form in submicron powders due to ubiquitous attractive van der Waals force. There are two general ways to prevent the aggregates. One is to introduce charge to the particles so that they repel one another. This is so-called electrostatic stabilisation and is achieved by altering the pH. The other is to introduce a certain type of polymeric molecule, which, when adsorbed onto the powder surface, prevents the particles physically coming close [11]. In this paper, carbon nanotubes (CNTs) were added in the process of preparing ZrO2 precursor in an attempt to obtain
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
119
nanometer size ZrO2 powders. The phase structure of the obtained powders and the effect of carbon on the phase stability of ZrO2 were studied.
2. Experimental CNTs with a BET-specific surface area of 203 m2/g were provided by the department of Mechanical Engineering of Tsinghua University. Solid zirconium nitrate (ZrO(NO3)22H2O) was dissolved in water to produce a 0.5 mol/l solution; 0.3 g CNTs were added into the solution and dispersed by ultrasonic vibrations for 10 min. The solutions were boiled for 3 h. Then, the solutions were let cool and evaporate to dry. The products then underwent calcination in argon at 600 jC for 2 h, after which a gray powder was obtained (sample A). Then, sample A was annealed at 600 jC for 2 h in air to remove CNTs and it transformed into a white powder (sample B). In another experiment, the above process was repeated except that CNTs were replaced by natural graphite powder with a diameter of 200 nm. The BET-specific surface area of the graphite powders is 184 m2/g. Sample C powder was obtained after products were burned in argon at 600 jC for 2 h. Sample D was obtained after sample C was annealed at 600 jC for 2 h in air to remove carbon. H-800 transmission electron microscopy (TEM) and AMRAY-1910 scanning electron microscopy (SEM) were used to observe the microstructure of the powders. X-ray powder diffraction patterns (XRD) were recorded with a D/ MAX-III B diffractometer using CuKa radiation. The values of lattice constant determined from XRD data have ˚ . The Delsa440sx apparatus was an accuracy of F 0.003 A used to measure the zeta potential of powders or sols in the solution and to measure the size of particles.
3. Results and discussion 3.1. The effect of adding CNTs on the formation of ZrO2 nanoparticles Table 1 shows the average size of the particles in samples A, B, C and D. ZrO2 nanoparticles about 10 nm in diameter were obtained in sample A, while in samples C and D, where graphite powders were used, the size of ZrO2 particles increased to 37.5 and 47.5 nm, respectively.
Table 1 The average size of ZrO2 particles Sample
Average size (nm)
A B C D
10.2 15.5 37.5 47.5
Fig. 1. ZrO2 particles adhered to the surface of CNT.
The following decomposition reaction of aqueous solution of zirconium nitrate happens when the solution is heated: ZrOðNO3 Þ2 þ 3H2 O X ZrðOHÞ4 þ 2HNO3
ð1Þ
Nitric acid is a strong oxidizer and volatile in nature. When the zirconium nitrate solution with CNTs additive was heated, oxidation –reduction reaction occurred on some nitric acid with CNTs, and other nitric acid volatilized and escaped from the solution. As a result, the consumption of the nitric acid in the solution accelerated the hydrolytic process of zirconium nitrate. At the same time, ZrO2 precursor [ZrOx(OH)4 2xyH2O]n was formed. The definite structure of the precursor was unclear. Some researchers reported that the increase in ZrO2 particle size had relation with the collapse of the structure in the drying process [12,13]. According to Ref. [14], after the reflux digestion of nitric acid at boiling point (140 jC), the hydroxyls or other organic-functional groups were introduced on the surface of CNTs. With these groups, CNTs became hydrophilic and they were easily dispersed into the aquatic phase with high stability. These groups could also strengthen the cohesion between CNTs and the ZrO 2 precursor [ZrO x (OH) 4 2xyH2O]n. As shown in Fig. 1, after burned at 600 jC in argon, some ZrO2 particles retained adhering to the surface of CNTs after ultrasonic vibrations. Since ZrO2 particles generated from the precursor dewatering, the attachment of ZrO2 nanoparticles on the surface of the CNTs suggested that there had been strong adhering force between CNTs and the ZrO2 precursor. The fast cohesion was due to hydrophilic groups on the surface of CNTs. The groups also tended to help CNTs dispersing into aqueous solution while absorbing the ZrO2 precursor. When the particles in the solution had a higher value of zeta potential, they dispersed more stably. The zeta potential for different CNTs in ZrO2 hydroxide solution is shown in Fig. 2. Fig. 2a shows two separate distributingintensity peaks at very low zeta potential. This meant the initial CNTs without hydrophilic groups and ZrO2 particles, separated mutually, flocculated easily in the solution. Fig. 2b shows most particles distribute at high zeta potential and has
120
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
Fig. 2. The zeta potential for different CNTs in zirconia hydroxide solution: (a) initial CNTs and zirconia; (b) surface-hydroxyl CNTs with zirconia hydroxide. Fig. 4. The X-ray diffraction patterns of sample A.
no separate peaks. This indicated that hydroxyls helped to increase the stability of CNTs in the aqueous solution. No separate peaks also meant the fast cohesion between CNTs and ZrO2 precursor. Just because of the fast cohesion with the precursor and the better dispersion in the solution, CNTs could ultimately scatter in the sol of ZrO2 precursor and function as skeleton of the sol. The dehydration process of the precursor and crystallization of the ZrO2 nanoparticles happened in the limited space defined by CNTs networks. In this way, CNTs prevented the structure-collapse of the precursor, which would result in the increase in the ZrO2 particle size. In the subsequent burning process in argon, CNTs also played an important role in preventing particles from aggregating and growing up, and all above resulted in the formation of 10-nm particles. The surface microstructure of the sample A observed by SEM is shown in Fig. 3. Separate ZrO2 nanoparticles scattered among CNTs. If graphite powders were used to replace CNTs, they cannot form 3-D framework in the solution; ZrO2 particles will aggregate and grow up; as a result, larger ZrO2 particles will be formed and the shape of ZrO2 particle was not as good as that of sample A.
3.2. The relation between ZrO2 particle size and its phase structure Fig. 4 indicates sample A consisted of only c-ZrO2. The XRD patterns of all samples from 27j to 32.5j are shown in Fig. 5. Sample B consisted of mostly t-ZrO2 and some mZrO2 particles, sample C of mostly c-ZrO2 and sample D mainly of m-ZrO2. The occurrence of t-ZrO2 or c-ZrO2 at room temperature in these samples should be due to the influence by the particles size and the introduction of carbon. The stability of t-ZrO2 or c-ZrO2 at room temperature with a certain crystallite size was reviewed by Garvie [6]. He reported that the t-ZrO2 was more stable than m-phase at room temperature when the crystal size was below 30 nm [15]. But the calculation of crystallite size from XRD patterns and the observation of particle size by SEM and TEM suggest that the occurrence of tetragonal in ZrO2 depends not on crystallite size but on particle size. The average particle sizes of different samples are shown in Table 1. The particles’ microfigures of samples B and D are shown in Fig. 6. The crystallite size of ZrO2 was estimated using Debye– Scherrer equation: D¼
Fig. 3. The surface microstructure of sample A.
0:89k bcosh
ð2Þ
where D is the crystallite size, k is the wavelength of CuKa radiation, h is the Bragg diffraction angle and b is theffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi corrected peak width in radians calculated from b ¼ p B2 b2 . B is the observed peak width and b is the instrumental broadening. The instrumental broadening was assumed to be constant in the same XRD pattern. Widths of characteristic diffraction peaks can be used to get qualitative crystallite size information for different phases. The D(101) of t(101) of sample B was 18.74 nm calculated by Eq. (2) according to some parameters in the XRD data, and the D(1¯11) of m(1¯11) of sample D was 18.85 nm. Both of them were less than 30 nm in crystallite size.
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
Sample B was in tetragonal form, while sample D in monoclinic form. As shown in Table 1, the average diameter of particles in sample B was 15.5 nm, much smaller than 30 nm. The particle diameter of sample D was 47.5 nm and its structure was m-phase. By calculation with Garvie’s theory [6], the critical size for m –t transformation was about 28 nm. Since the particle size of sample D was far larger than the critical size, it was understandable that ZrO2 took the form of m-phase. It can be concluded that the occurrence of tetragonal in ZrO2 depends not on crystallite size but on particle size, and t-ZrO2 was more stable than m-phase when particle size was below 28 nm.
121
Fig. 6. The TEM images of ZrO2 particles: the left is sample B and the right is sample D.
Both samples A and C took the form of c-phase although they have very different average particle sizes, 10.2 and 37.5 nm, respectively. This fact could be attributed to the existence of carbon. After carbon was driven out, samples A and C transformed to samples B and D. c-phase transformed to t-phase in sample B and to m-phase in sample D, depending on the size of ZrO2 particles. These transformations were clearly caused by the loss of carbon. In Fig. 5, the only high peak of the curve (a) indicates the c-phase (111). The interplanar distance corresponding to the ˚ when CNTs were added, a little under the peak was 2.919 A ˚ . When graphite standard distance of c-phase (111), 2.930 A powders were used, the interplanar distance of c(111) ˚ . After annealing in air, c-phase disappeared, became 2.90 A and the interplanar distances of m(1¯11) and m(111) were consistent with the standard value. The addition of CNTs or carbon powders to prepare the ZrO2 particles tended to favor the transition from m-ZrO2 (or t-ZrO2) to c-ZrO2 and the stabilization of c-ZrO2 at low temperature. According to Refs. [5,16], the formation of c-phase and the reduction of the interplanar distance of c(111) were related to the entering of carbon into the lattice of ZrO2 or the high internal compressive stress induced by carbon dispersing among ZrO2 microcrystals. This is a subject that needs to be further studied.
4. Conclusion
Fig. 5. The comparison of different samples on X-ray diffraction while the double Bragg diffraction angle ranged from 27j to 32.5j (c: cubic, m: monoclinic, t: tetragonal).
ZrO2 powder with an average particle size of 10 nm was prepared by adding CNTs in the hydrolytic process of ZrO(NO3)2. CNTs supported the gel structure of the precursors and prevented the initial zirconium hydrate particles from aggregating in the drying process. The stabilization of t-phase at room temperature in ZrO2 powder depended on the particle size, not on the crystallite size. When the particle size was reduced to 28 nm, t-ZrO2 was more stable than m-ZrO2. The addition of CNTs or carbon powders when preparing ZrO2 particles tended to favor the transformation from mZrO2 to c-ZrO2 and to the stabilization of c-ZrO2 at room temperature. The experiment results showed that, after the carbon in the powder was removed by annealing in air, cphase structure of the ZrO2 particles transformed to t-phase or m-phase, depending on whether the ZrO2 particle size was smaller or larger than the critical size for t-phase to mphase transformation, 28 nm.
122
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
References [1] J.C. Ray, C.R. Saha, P. Pramanik, Journal of the European Ceramic Society 22 (2002) 851 – 862. [2] P. Peshev, I. Stambolova, S. Vassilev, P. Stefanov, V. Blaskov, K. Starbov, N. Starbov, Materials Science and Engineering B97 (2003) 106 – 110. [3] A. Sekulic, K. Furic, M. Stubicar, Journal of Molecular Structure 410 – 411 (1997) 275 – 279. [4] D. Wang, K. Liang, J. Wan, Journal of the Chinese Ceramic Society (in Chinese) 26 (1998) 11 – 17. [5] N. Aidani, V. Micheli, M. Anderle, Thin Solid Films 382 (2001) 23 – 29. [6] R.C. Garvie, The Journal of Physics and Chemistry 82 (1985) 218 – 224. [7] C.R. Jagadish, K.P. Ranjan, P. Pramanik, Journal of the European Ceramic Society 20 (2000) 1289 – 1295.
[8] R. Srinivasan, R. DeAngeles, B.H. Davis, Journal of Materials Research 1 (4) (1986) 583 – 588. [9] H. Nishizawa, T. Tani, K.J. Matsuoka, Materials Science 19 (1984) 1921 – 1926. [10] F.J. Berry, S.J. Skinner, I.M. Bell, R.J.H. Clark, C.B. Ponton, Journal of Solid State Chemistry 145 (1999) 394 – 400. [11] R.J. Hunter, Foundations of Colloid Science, vol. 1, Oxford: Clarendon Press, 1989. [12] G.K. Chuah, S. Jaenick, S.A. Cheong, Applied Catalysis. A, General 145 (1996) 267 – 269. [13] G.K. Chuah, S. Jaenick, B.K. Pong, Journal of Catalysis 175 (1998) 80 – 92. [14] Y.H. Li, C. Xu, B. Wei, X. Zhang, M. Zheng, Chemistry of Materials 14 (2002) 483 – 485. [15] R.C. Garvie, R.H.J. Hannink, R.T. Pascoe, Nature 258 (1975) 703. [16] D. Wang, K. Liang, Journal of Materials Science Letters 17 (1998) 343 – 344.
Addition of carbon nanotubes during the preparation of zirconia nanoparticles: influence on structure and phase composition T.Y. Luo, T.X. Liang *, C.S. Li Institute of Nuclear Energy Technology, Tsinghua University, Beijing 100084, PR China Received 8 July 2003; received in revised form 19 November 2003; accepted 1 December 2003
Abstract Zirconia powder with an average particle size of 10 nm was prepared by adding carbon nanotubes (CNTs) in the hydrolytic process of ZrO(NO3)2. The formation of zirconia nanoparticles was attributed to the addition of CNTs, which affected the zirconia precursor structure. The relation between the stability of cubic (c) and tetragonal (t) phase zirconia at room temperature, and the zirconia particle size and addition of carbon was studied. It was revealed that zirconia particles with nanometer size had took the form of t-phase and that the addition of carbon tended to favor m-phase ZrO2 to c-phase ZrO2 transformation and c-phase ZrO2 stabilization at low temperature. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Carbon and graphite; Phase transformations; Carbon nanotubes (CNTs)
1. Introduction Among advanced ceramics, zirconia (ZrO2) has an important role due to its excellent chemical resistance, refractory character and ionic conductivity. In the recent years, there is an increasing interest in nanostructured ceramics due to their lower sintering temperature and improved mechanical properties. ZrO2 exists in three polymorphic crystalline structures, namely, monoclinic (m) tetragonal (t) and cubic (c). At room temperature, t- and c-phases are metastable, while m-phase is stable. t- and c-phases are high temperature phases. They can be partially or fully stabilized at room temperature by adding a small amount of oxides (e.g. CaO, MgO and types of rare earth oxides) [1– 3]. Wang et al. [4] reported that tZrO2 could be obtained by carburizing pure m-ZrO2 powder at 1500jC. t-ZrO2 was also found in ZrO2-C films deposited by RF magnetron sputtering, while pure ZrO2 film only formed m-phase [5]. Stabilization of metastable phases also was found to depend on crystallite size and the process of preparing the powders [6– 10]. Thermal decomposition of amorphous hydrous ZrO2, zirconium alkoxides and zirconium salt, ball milling of m-ZrO2, vapor phase reactions and * Corresponding author. Tel.: +86-10-8979-6090; fax: +86-10-69771464. E-mail address: [email protected] (T.X. Liang). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2003.12.001
hydrothermal treatments of amorphous hydrated ZrO2 are known to produce c-phase ZrO2. Jagadish et al. [7] prepared pure ZrO2 nanoparticles by chemical synthesis route using sucrose, polyvinyl alcohol (PVA) and metal nitrates. At 200 jC, pure ZrO2 formed c-phase, which was stable up to 600 jC, and then slowly transformed into m-phase. Srinivasan et al. [8] reported that a pure c-phase was originated at 500 jC starting from a precursor precipitated at pH = 13.5 using a solution of NaOH. Nishizawa et al. [9] stabilized c-ZrO2 by inserting Na+ or Ca2 + ions into the initial ZrO2 gel through some hydrothermal reactions. Berry et al. [10] synthesized ZrO2 using aqueous solution of zirconium acetate. He found that high pH value and the addition of alkali to the solution tended to stabilize t-ZrO2. In the process of preparing ceramic powders, it is most desirable that the particles are completely dispersed without any aggregates. Aggregates usually can form in submicron powders due to ubiquitous attractive van der Waals force. There are two general ways to prevent the aggregates. One is to introduce charge to the particles so that they repel one another. This is so-called electrostatic stabilisation and is achieved by altering the pH. The other is to introduce a certain type of polymeric molecule, which, when adsorbed onto the powder surface, prevents the particles physically coming close [11]. In this paper, carbon nanotubes (CNTs) were added in the process of preparing ZrO2 precursor in an attempt to obtain
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
119
nanometer size ZrO2 powders. The phase structure of the obtained powders and the effect of carbon on the phase stability of ZrO2 were studied.
2. Experimental CNTs with a BET-specific surface area of 203 m2/g were provided by the department of Mechanical Engineering of Tsinghua University. Solid zirconium nitrate (ZrO(NO3)22H2O) was dissolved in water to produce a 0.5 mol/l solution; 0.3 g CNTs were added into the solution and dispersed by ultrasonic vibrations for 10 min. The solutions were boiled for 3 h. Then, the solutions were let cool and evaporate to dry. The products then underwent calcination in argon at 600 jC for 2 h, after which a gray powder was obtained (sample A). Then, sample A was annealed at 600 jC for 2 h in air to remove CNTs and it transformed into a white powder (sample B). In another experiment, the above process was repeated except that CNTs were replaced by natural graphite powder with a diameter of 200 nm. The BET-specific surface area of the graphite powders is 184 m2/g. Sample C powder was obtained after products were burned in argon at 600 jC for 2 h. Sample D was obtained after sample C was annealed at 600 jC for 2 h in air to remove carbon. H-800 transmission electron microscopy (TEM) and AMRAY-1910 scanning electron microscopy (SEM) were used to observe the microstructure of the powders. X-ray powder diffraction patterns (XRD) were recorded with a D/ MAX-III B diffractometer using CuKa radiation. The values of lattice constant determined from XRD data have ˚ . The Delsa440sx apparatus was an accuracy of F 0.003 A used to measure the zeta potential of powders or sols in the solution and to measure the size of particles.
3. Results and discussion 3.1. The effect of adding CNTs on the formation of ZrO2 nanoparticles Table 1 shows the average size of the particles in samples A, B, C and D. ZrO2 nanoparticles about 10 nm in diameter were obtained in sample A, while in samples C and D, where graphite powders were used, the size of ZrO2 particles increased to 37.5 and 47.5 nm, respectively.
Table 1 The average size of ZrO2 particles Sample
Average size (nm)
A B C D
10.2 15.5 37.5 47.5
Fig. 1. ZrO2 particles adhered to the surface of CNT.
The following decomposition reaction of aqueous solution of zirconium nitrate happens when the solution is heated: ZrOðNO3 Þ2 þ 3H2 O X ZrðOHÞ4 þ 2HNO3
ð1Þ
Nitric acid is a strong oxidizer and volatile in nature. When the zirconium nitrate solution with CNTs additive was heated, oxidation –reduction reaction occurred on some nitric acid with CNTs, and other nitric acid volatilized and escaped from the solution. As a result, the consumption of the nitric acid in the solution accelerated the hydrolytic process of zirconium nitrate. At the same time, ZrO2 precursor [ZrOx(OH)4 2xyH2O]n was formed. The definite structure of the precursor was unclear. Some researchers reported that the increase in ZrO2 particle size had relation with the collapse of the structure in the drying process [12,13]. According to Ref. [14], after the reflux digestion of nitric acid at boiling point (140 jC), the hydroxyls or other organic-functional groups were introduced on the surface of CNTs. With these groups, CNTs became hydrophilic and they were easily dispersed into the aquatic phase with high stability. These groups could also strengthen the cohesion between CNTs and the ZrO 2 precursor [ZrO x (OH) 4 2xyH2O]n. As shown in Fig. 1, after burned at 600 jC in argon, some ZrO2 particles retained adhering to the surface of CNTs after ultrasonic vibrations. Since ZrO2 particles generated from the precursor dewatering, the attachment of ZrO2 nanoparticles on the surface of the CNTs suggested that there had been strong adhering force between CNTs and the ZrO2 precursor. The fast cohesion was due to hydrophilic groups on the surface of CNTs. The groups also tended to help CNTs dispersing into aqueous solution while absorbing the ZrO2 precursor. When the particles in the solution had a higher value of zeta potential, they dispersed more stably. The zeta potential for different CNTs in ZrO2 hydroxide solution is shown in Fig. 2. Fig. 2a shows two separate distributingintensity peaks at very low zeta potential. This meant the initial CNTs without hydrophilic groups and ZrO2 particles, separated mutually, flocculated easily in the solution. Fig. 2b shows most particles distribute at high zeta potential and has
120
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
Fig. 2. The zeta potential for different CNTs in zirconia hydroxide solution: (a) initial CNTs and zirconia; (b) surface-hydroxyl CNTs with zirconia hydroxide. Fig. 4. The X-ray diffraction patterns of sample A.
no separate peaks. This indicated that hydroxyls helped to increase the stability of CNTs in the aqueous solution. No separate peaks also meant the fast cohesion between CNTs and ZrO2 precursor. Just because of the fast cohesion with the precursor and the better dispersion in the solution, CNTs could ultimately scatter in the sol of ZrO2 precursor and function as skeleton of the sol. The dehydration process of the precursor and crystallization of the ZrO2 nanoparticles happened in the limited space defined by CNTs networks. In this way, CNTs prevented the structure-collapse of the precursor, which would result in the increase in the ZrO2 particle size. In the subsequent burning process in argon, CNTs also played an important role in preventing particles from aggregating and growing up, and all above resulted in the formation of 10-nm particles. The surface microstructure of the sample A observed by SEM is shown in Fig. 3. Separate ZrO2 nanoparticles scattered among CNTs. If graphite powders were used to replace CNTs, they cannot form 3-D framework in the solution; ZrO2 particles will aggregate and grow up; as a result, larger ZrO2 particles will be formed and the shape of ZrO2 particle was not as good as that of sample A.
3.2. The relation between ZrO2 particle size and its phase structure Fig. 4 indicates sample A consisted of only c-ZrO2. The XRD patterns of all samples from 27j to 32.5j are shown in Fig. 5. Sample B consisted of mostly t-ZrO2 and some mZrO2 particles, sample C of mostly c-ZrO2 and sample D mainly of m-ZrO2. The occurrence of t-ZrO2 or c-ZrO2 at room temperature in these samples should be due to the influence by the particles size and the introduction of carbon. The stability of t-ZrO2 or c-ZrO2 at room temperature with a certain crystallite size was reviewed by Garvie [6]. He reported that the t-ZrO2 was more stable than m-phase at room temperature when the crystal size was below 30 nm [15]. But the calculation of crystallite size from XRD patterns and the observation of particle size by SEM and TEM suggest that the occurrence of tetragonal in ZrO2 depends not on crystallite size but on particle size. The average particle sizes of different samples are shown in Table 1. The particles’ microfigures of samples B and D are shown in Fig. 6. The crystallite size of ZrO2 was estimated using Debye– Scherrer equation: D¼
Fig. 3. The surface microstructure of sample A.
0:89k bcosh
ð2Þ
where D is the crystallite size, k is the wavelength of CuKa radiation, h is the Bragg diffraction angle and b is theffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi corrected peak width in radians calculated from b ¼ p B2 b2 . B is the observed peak width and b is the instrumental broadening. The instrumental broadening was assumed to be constant in the same XRD pattern. Widths of characteristic diffraction peaks can be used to get qualitative crystallite size information for different phases. The D(101) of t(101) of sample B was 18.74 nm calculated by Eq. (2) according to some parameters in the XRD data, and the D(1¯11) of m(1¯11) of sample D was 18.85 nm. Both of them were less than 30 nm in crystallite size.
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
Sample B was in tetragonal form, while sample D in monoclinic form. As shown in Table 1, the average diameter of particles in sample B was 15.5 nm, much smaller than 30 nm. The particle diameter of sample D was 47.5 nm and its structure was m-phase. By calculation with Garvie’s theory [6], the critical size for m –t transformation was about 28 nm. Since the particle size of sample D was far larger than the critical size, it was understandable that ZrO2 took the form of m-phase. It can be concluded that the occurrence of tetragonal in ZrO2 depends not on crystallite size but on particle size, and t-ZrO2 was more stable than m-phase when particle size was below 28 nm.
121
Fig. 6. The TEM images of ZrO2 particles: the left is sample B and the right is sample D.
Both samples A and C took the form of c-phase although they have very different average particle sizes, 10.2 and 37.5 nm, respectively. This fact could be attributed to the existence of carbon. After carbon was driven out, samples A and C transformed to samples B and D. c-phase transformed to t-phase in sample B and to m-phase in sample D, depending on the size of ZrO2 particles. These transformations were clearly caused by the loss of carbon. In Fig. 5, the only high peak of the curve (a) indicates the c-phase (111). The interplanar distance corresponding to the ˚ when CNTs were added, a little under the peak was 2.919 A ˚ . When graphite standard distance of c-phase (111), 2.930 A powders were used, the interplanar distance of c(111) ˚ . After annealing in air, c-phase disappeared, became 2.90 A and the interplanar distances of m(1¯11) and m(111) were consistent with the standard value. The addition of CNTs or carbon powders to prepare the ZrO2 particles tended to favor the transition from m-ZrO2 (or t-ZrO2) to c-ZrO2 and the stabilization of c-ZrO2 at low temperature. According to Refs. [5,16], the formation of c-phase and the reduction of the interplanar distance of c(111) were related to the entering of carbon into the lattice of ZrO2 or the high internal compressive stress induced by carbon dispersing among ZrO2 microcrystals. This is a subject that needs to be further studied.
4. Conclusion
Fig. 5. The comparison of different samples on X-ray diffraction while the double Bragg diffraction angle ranged from 27j to 32.5j (c: cubic, m: monoclinic, t: tetragonal).
ZrO2 powder with an average particle size of 10 nm was prepared by adding CNTs in the hydrolytic process of ZrO(NO3)2. CNTs supported the gel structure of the precursors and prevented the initial zirconium hydrate particles from aggregating in the drying process. The stabilization of t-phase at room temperature in ZrO2 powder depended on the particle size, not on the crystallite size. When the particle size was reduced to 28 nm, t-ZrO2 was more stable than m-ZrO2. The addition of CNTs or carbon powders when preparing ZrO2 particles tended to favor the transformation from mZrO2 to c-ZrO2 and to the stabilization of c-ZrO2 at room temperature. The experiment results showed that, after the carbon in the powder was removed by annealing in air, cphase structure of the ZrO2 particles transformed to t-phase or m-phase, depending on whether the ZrO2 particle size was smaller or larger than the critical size for t-phase to mphase transformation, 28 nm.
122
T.Y. Luo et al. / Powder Technology 139 (2004) 118–122
References [1] J.C. Ray, C.R. Saha, P. Pramanik, Journal of the European Ceramic Society 22 (2002) 851 – 862. [2] P. Peshev, I. Stambolova, S. Vassilev, P. Stefanov, V. Blaskov, K. Starbov, N. Starbov, Materials Science and Engineering B97 (2003) 106 – 110. [3] A. Sekulic, K. Furic, M. Stubicar, Journal of Molecular Structure 410 – 411 (1997) 275 – 279. [4] D. Wang, K. Liang, J. Wan, Journal of the Chinese Ceramic Society (in Chinese) 26 (1998) 11 – 17. [5] N. Aidani, V. Micheli, M. Anderle, Thin Solid Films 382 (2001) 23 – 29. [6] R.C. Garvie, The Journal of Physics and Chemistry 82 (1985) 218 – 224. [7] C.R. Jagadish, K.P. Ranjan, P. Pramanik, Journal of the European Ceramic Society 20 (2000) 1289 – 1295.
[8] R. Srinivasan, R. DeAngeles, B.H. Davis, Journal of Materials Research 1 (4) (1986) 583 – 588. [9] H. Nishizawa, T. Tani, K.J. Matsuoka, Materials Science 19 (1984) 1921 – 1926. [10] F.J. Berry, S.J. Skinner, I.M. Bell, R.J.H. Clark, C.B. Ponton, Journal of Solid State Chemistry 145 (1999) 394 – 400. [11] R.J. Hunter, Foundations of Colloid Science, vol. 1, Oxford: Clarendon Press, 1989. [12] G.K. Chuah, S. Jaenick, S.A. Cheong, Applied Catalysis. A, General 145 (1996) 267 – 269. [13] G.K. Chuah, S. Jaenick, B.K. Pong, Journal of Catalysis 175 (1998) 80 – 92. [14] Y.H. Li, C. Xu, B. Wei, X. Zhang, M. Zheng, Chemistry of Materials 14 (2002) 483 – 485. [15] R.C. Garvie, R.H.J. Hannink, R.T. Pascoe, Nature 258 (1975) 703. [16] D. Wang, K. Liang, Journal of Materials Science Letters 17 (1998) 343 – 344.