Chem. Mater. 2002, 14, 3009-3015
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Hydrothermal Crystallization-Stabilization of Zirconia Xerogel in the Presence of Different Yttria (3 mol %) Precursors G. Dell’Agli and G. Mascolo* Dipartimento di Meccanica, Strutture, Ambiente e Territorio-Laboratorio Materiali, Via G. Di Biasio, 43-03043 Cassino (FR), Italy Received January 14, 2002. Revised Manuscript Received May 2, 2002
Yttria (3 mol %)-zirconia powders were crystallized under hydrothermal conditions at 110 °C for 7 days from mixtures of zirconia gel with different Y-based precursors. Zirconia xerogels mixed with crystalline Y2O3 or Y(OH)3 or coprecipitated with Y(OH)3 gel were hydrothermally treated in the presence of solutions of different concentrations of (KOH + K2CO3) mineralizer. The effects of both the Y-based precursor and the concentration of the mineralizer solution on the crystallization-stabilization, the reactivity, the average crystallite size, and the degree of agglomeration of the corresponding hydrothermally synthesized ZrO2 powders are discussed.
1. Introduction Among the wet-chemical methods used to obtain highpurity yttrium-doped zirconia (Y-ZrO2) with nanometer-sized primary particles, a narrow particle distribution, and weakly bonded primary particles, hydrothermal synthesis at low-temperature appears a simple and relatively inexpensive method.1-4 Unlike other wetchemical methods of synthesis, in fact, calcination and milling steps are not necessary for powders obtained under hydrothermal conditions. Many reports on the hydrothermal synthesis of yttrium-doped zirconia are reported in the literature. However, the different precursors and the various conditions adopted for the syntheses determine the formation of powders with significantly different characteristics, and consequently, it is practically impossible to make a correlation between the conditions of the synthesis and the characteristics of the resulting crystallized products. Cubic (c-) Y-ZrO2 powder was synthesized by high-temperature (190 °C) hydrolysis of coprecipitated hydroxides in the presence of an excess of NH4OH by Burkin et al.2 Cubic YSZ was also synthesized at pH 12 at 190 °C with NaOH.5 The resulting fine crystallite size of the powder made uncertain whether the phase was tetragonal- (t-) or c-ZrO2. A mixture of monoclinic (m-) and c-ZrO2 resulted at 160 °C in the ZrOCl2-YCl3-urea solution system, whereas at 220 °C, predominantly c-ZrO2 was detected.6 For a system containing 6 mol % yttrium, c-ZrO2 was (1) Dawson, W. Z. Am. Ceram. Soc. Bull. 1988, 67, 1673. (2) Burkin, A. R.; Saricimen, H.; Steele, B. C. H. Trans. J. Br. Ceram. Soc. 1980, 79, 105. (3) Pyda, W.; Haberko, K.; Bucko, M. M. J. Am. Ceram. Soc. 1991, 74, 2622. (4) Tsukada, T.; Venigalla, S.; Morrone, A. A.; Adair, J. H. J. Am. Ceram. Soc. 1999, 82, 1169. (5) Stambaugh, E. P.; Adair, J. H.; Sekercioglu, I.; Wills, R. R. U.S. Patent 4,619,817, Oct 28, 1986. (6) Hishinuma K.; Kumaki T.; Nakai Z.; Yoshimura M.; Somiya S. In Science and Technology of Zirconia III; Advances in Ceramics Series; Somiya, S., Yamamoto, N., Yanagidas, H., Eds.; American Ceramic Society: Westerville, OH, 1986; Vol. 24, p 201.
obtained at 250-300 °C in the presence of 0.01 M NaOH, whereas m-ZrO2 resulted in 1 M NaOH.7 For crystallization syntheses performed in open reaction vessels at 100 °C, Tsukada et al.4 obtained c- or t-phase from a coprecipitated (Y-Zr) hydroxide gel obtained at pH 13.9 with KOH and containing 6 mol % yttrium, whereas at pH 9.5, no significant crystallization was observed after 240 h. A mixture of m- and c-ZrO2 was obtained from a mechanical mixture of Y(OH)3 and Zr(OH)4 gels precipitated at pH 13.9. A diffusionless mechanism of yttrium-soluble species into zirconia gel for the transformation of the resulting (Y,Zr) hydroxide gel to Y-ZrO2 was proposed. Hydrothermal crystallizations at 250 °C for 8 h of Zr-Y coprecipitates containing 1.75, 2.0, and 7.0 mol % yttria in the presence of solutions of different mineralizers were performed by Pyda et al.3 In the presence of concentrated solutions of NaOH, the preferential formation of elongated crystallites of m-ZrO2 (some hundreds of nanometers long) were detected especially for coprecipitates with low Y2O3 contents. For a coprecipitate containing 7 mol % Y2O3, isometric and nanometer-sized crystallites (about 10 nm in size) of c-ZrO2 crystallized as the main phase in a mixture with secondary elongated crystallites of mphase. The content of the latter phase increased with the duration of the hydrothermal treatment. For crystallization in water or in water solutions of NaCl, the t- or c-form was attributed to the resulting small, isometric particles. From these results, some doubts arise about the effective and full stabilization of zirconia by hydrothermal treatment of the coprecipitated precursors, especially when powders with very fine crystallite sizes resulted. The questionable reasons are different. First, both the low Y2O3 content and its eventual amorphous (7) Kriechbaum, G. W.; Kleinschmit, P.; Peuckert, D. In Ceramic Powder Science IIA; Ceramic Transactions; Messing, G. L., Fuller, E. R., Jr., Hausner, H. J., Eds.; American Ceramic Society: Westerville, OH, 1988; Vol. 1, p 146.
10.1021/cm0211103 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/17/2002
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state8 make it difficult to ascertain the effective stabilization of zirconia. On the other hand, the homogeneous dispersion of the dopant in the coprecipitate and the high reactivity of the amorphous coprecipitate favor the formation of stabilized zirconia at very low calcination temperatures or at high hydrothermal treatment temperatures. The detected c- and t-phases of uncalcined zirconia might be related to the effect of the very small particle size of the powders,9 which favors the formation of metastable phases. Moreover, it is not easy to distinguish between the t- and c-phases, especially when the powders are nanometers in size.10 A further complication arises from the incorporation of some Na+ in the crystalline ZrO2 hydrothermally synthesized from ZrO2 xerogel in the presence of NaOH solution as the mineralizer, unlike the crystalline ZrO2 obtained in the presence of KOH.11 The incorporation of some impurity might also affect the polymorphism of ZrO2. Various attempts to stabilize ZrO2-7 mol % Y2O3 failed when mixtures of zirconia xerogel and crystalline Y2O3 were hydrothermally treated at 110 °C for 7 days and in the presence of a 0.5 M NaOH mineralizer solution. In fact, unreacted crystalline Y2O3 and crystallized metastable zirconia in the t-form were detected. When the same mixture was treated under the same conditions and in the presence of 0.5 M Na2CO3, crystalline Y2O3 disappeared from the corresponding XRD pattern, and a stabilized c-phase arose.12 Mechanical mixtures of zirconia xerogel and crystalline Y2O3 have recently been employed as precursors in the presence of mineralizer solutions of (KOH + K2CO3) for the hydrothermal synthesis of nanosized 3 mol % Y-ZrO2.13 The presence of the alkaline hydroxide and, in particular, the pH of the basic mineralizer favor the crystallization of zirconia gel, whereas the presence of the alkaline carbonate appears to favor the stabilization of zirconia with Y2O3. The aims of this work concern the effect of different Y-based precursors on the characteristics of 3 mol % Y-ZrO2 powders hydrothermally synthesized in the presence of mineralizer solutions of different concentrations. In particular, to verify the effective stabilization of ZrO2 with the dopant, crystalline Y2O3 in mixtures with ZrO2 xerogel, crystalline Y(OH)3 in mixtures with ZrO2 xerogel, and coprecipitated (Y,Zr) hydroxide xerogel have been employed as precursors.
Dell’Agli and Mascolo hydrothermal treatments. The crystalline Y(OH)3 source was prepared by treating the above-mentioned crystalline Y2O3 in 1 M KOH solution at 110 °C for 14 days. Zirconia xerogel and crystalline Y2O3 (3 mol %) were mechanically mixed and hydrothermally treated in the presence of solutions of (KOH + K2CO3) mineralizer. Taking the K2CO3/KOH molar ratio to be constant and equal to 3, the total concentrations of the mineralizer employed were 0.025, 0.05, 0.10, 0.15, 0.20, 0.3, and 2.0 M. A solid/solution weight ratio equal to 1:30 was adopted in all of the hydrothermal treatments. Setting all of the conditions constant, two mechanical mixtures of zirconia xerogel and crystalline Y(OH)3 (3 mol %) were hydrothermally treated in the presence of mineralizer solutions. One mixture was contacted with a solution having a total concentration equal to 0.15 M, and the other one was contacted with a 2.0 M concentration. A batch of a coprecipitated (Y-Zr) hydroxide xerogel containing 3 mol % yttria was prepared by adding to ammonia (∼4 M) a mixture of ZrOCl2 and Y(NO3)3 solution. The resulting precipitate was filtered and repeatedly washed with distilled water to remove the chloride and nitrate ions, until no reaction for Cl- ions (with 1 M AgNO3) was observed. Two fractions of the amorphous coprecipitate, previously dried at 60 °C, were also hydrothermally treated in the presence of 0.15 and 2.0 M concentrations of the mineralizer. The details of the hydrothermal synthesis are reported in a previous paper.12 X-ray diffraction analysis (XRD) was performed using a X′PERT diffractometer of Philips (Almelo, The Netherlands) and Cu KR radiation to detect the hydrothermally synthesized products. The powders were also characterized by simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) using a Netzsch model STA 409 thermoanalyzer (Selb/Bavaria, Germany), with R-Al2O3 as a reference and a 10 °C/min heating rate. Particle sizes of t-ZrO2 and m-ZrO2 were calculated by the Scherrer formula by measuring the half-width of the (111)t and (1 h 11)m diffraction peaks with Philips software for the correction of R1R2 overlap and using polycrystalline silicon for the correction of instrumental broadening. The crystalline content of t-ZrO2 was determined by adopting the intensity relationship
I(111)t I(111)t + I(1 h 11)m + I(111)m where m and t refer to m- and t-ZrO2, respectively. The specific surface areas of the powders were determined by the BET method using a Gemini instrument from Micromeritics (Norcross, GA) and utilizing nitrogen as the adsorbate after drying at 60 °C. The synthesized powders were calcined in air at a 10°C/min heating rate for 5 min at 600, 900, and 1050 °C to measure the corresponding coarsening.
3. Results and Discussion 2. Experimental Section The various 3 mol % Y-ZrO2 powders were hydrothermally prepared at 110 °C for 7 days at autogenous pressure. The zirconia xerogel source was obtained by adding GR-grade ZrCl4 (Merck, Germany) solution to concentrated ammonia (∼4 M) under continuous stirring. The precipitate was washed in water several times until no reaction for Cl- ions (with 1 M AgNO3) was observed and then dried at 60 °C. Crystalline Y2O3 (99.99 purity, Fluka Chemie AG, Switzerland) and Gr-grade K2CO3 and KOH (C. Erba, Italy) were also used in the (8) Bokhimi, X.; Morales, A.; Garcia-Ruiz, A.; Xiao, T. D.; Chen, H.; Strutt, P. R. J. Solid State Chem. 1999, 142, 409. (9) Garvie, R. C. J. Phys. Chem. 1965, 69, 4. (10) Srinivasan, R. R.; Simpson, S. F.; Harris, J. M.; Davis, B. H. J. Mater. Sci. Lett. 1991, 10, 352. (11) Dell’Agli, G.; Ferone, C.; Mascolo, G.; Pansini, M. Solid State Ionics 2000, 127, 223. (12) Dell’Agli, G.; Mascolo, G. J. Eur. Ceram. Soc. 2000, 20, 139. (13) Dell’Agli, G.; Mascolo, G. J. Eur. Ceram. Soc. 2001, 21, 29.
3.1. Characteristics of Powders. The powders hydrothermally synthesized from mechanical mixtures of ZrO2 xerogel and crystalline Y2O3 and in the presence of increasing concentrations of the mineralizer resulted mainly in t-ZrO2, a smaller amount of m-ZrO2, and a variable content of uncrystallized zirconia, as reported in Table 1. The t-phase content of the powders with respect to the crystallized phases increased from 78% in 0.05 M to 94% in 2.0 M (KOH + K2CO3). To compare the contents of uncrystallized zirconia of the powders, the heights of their exothermic peak in DTA at 420-440 °C, due to the crystallization of amorphous zirconia,14 were compared with the reference (14) Osendi, M. I.; Moya, J. S.; Serna, C. J.; Soria, J. J. Am. Ceram. Soc. 1985, 68, 135.
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Table 1. Characteristics of the Powders Hydrothermally Synthesized at 110 °C for 7 Days from Mechanical Mixtures of Zirconia Xerogel and Crystalline Y2O3 in the Presence of Increasing Concentrations of (KOH + K2CO3) Mineralizer (KOH + K2CO3) t-ZrO2 t-ZrO2 m-ZrO2 total molarity crystal contenta crystal uncrystallized (M) size (nm) size (nm) ZrO2 (%) 0.025 0.05 0.10 0.15 0.20 0.30 2.0
ndb 14.0 17.4 17.2 15.5 15.0 11.0
ndb 78 80 83 85 86 94
ndb 10.3 15.6 16.0 13.5 11.4 10.2
large amount small amount trace trace small amount small amount small amount
a Content with respect to the crystalline phases. b Not determined.
Figure 1. Profile fitting of XRD pattern of a hydrothermally synthesized powder in the 25-37° range (2θ) for unravelling overlapping peaks.
height of the corresponding exothermic peak of untreated zirconia gel. The uncrystallized zirconia content was also evaluated by the area of the XRD band obtained from profile fitting of the XRD pattern in the range 25-37° (2θ) for the powdered samples to unravel overlapping peaks, from which the amount of the crystalline phases and their crystallite sizes were also determined (Figure 1). The height of the DTA peak and the area of the XRD band show a good proportionality to the uncrystallized zirconia content. An important amount of uncrystallized phase is present in the powder obtained in 0.025 M mineralizer. A small amount was detected in the powders synthesized in 0.05, 0.20, 0.30, and 2.0 M mineralizer, whereas in 0.10 and 0.15 M, the resulting powders were quite fully crystallized. With the exception of the powder synthesized in 0.05 M, a slight decrease in the crystallite sizes of the main
t-phase can be observed at the larger concentrations of mineralizer. The crystal sizes change, in fact, from 17.4 to 11.0 nm in going from 0.10 to 2.0 M mineralizer. The corresponding crystal sizes of the secondary m-phase show an analogous behavior, but the corresponding crystallites are slightly smaller in size than those of the main t-ZrO2 phase. The crystallized zirconia powder obtained in dilute solution (0.15 M), is generally characterized by weakly bonded primary particles, unlike the powder obtained in concentrated solution (2.0 M).13 The characteristics of these two powders are compared with those of the powders obtained from the coprecipitated (Y-Zr) hydroxide and from the mechanical mixture of ZrO2 xerogel and crystalline Y(OH)3 after a same hydrothermal treatment in the presence of 0.15 and 2.0 M concentration, respectively (Table 2). The different Y-based precursors do not significantly influence the nature of the resulting products. Crystallized t-ZrO2 is, in fact, the main phase, whereas m-ZrO2 and uncrystallized zirconia are the secondary phases detected in the resulting powders. Unlike the powders synthesized from the mechanical mixtures, the powders crystallized from the coprecipitates do not contain m-ZrO2 (Figure 2). Such a result might be attributed to the higher reactivity of the Y(OH)3 gel than of the crystalline yttria precursors. The highest percentage content of m-ZrO2 results for the powders synthesized from crystalline Y(OH)3. Small amounts of uncrystallized zirconia are found in powders synthesized in dilute solutions, in contrast with the amounts detected in powders obtained in concentrated solutions of mineralizer and in the presence of crystalline yttria precursors. The average crystallite sizes of the t-phase of the powders synthesized in the concentrated solution of mineralizer (2.0 M) are smaller than those of the powders crystallized in the dilute solution (0.15 M). Among the Y-based precursors, crystalline Y2O3 favors the largest crystalline sizes, whereas the Y(OH)3 gel favors the smallest ones. Nevertheless, the powders differ in the content of both m-ZrO2 and uncrystallized zirconia, and their corresponding surface areas agree perfectly with the detected crystalline sizes of the main t-ZrO2 (Table 2). 3.2. Effect of the Calcination. In Table 3 are summarized the effects of the calcination step on both the coarsening and the fractioned t-ZrO2 content of the various powders. All of the thermal treatments were performed with a ramp at a heating rate of 10 °C/min with a hold of 5 min at the selected temperature of calcination. Unlike the Zr-Y coprecipitate, the hydrothermally synthesized powders calcined at 600 °C do
Table 2. Characteristics of the ZrO2-Y2O3 (3 mol %) Powders Hydrothermally Synthesized at 110 °C for 7 days from the ZrO2 Xerogel Mixed with Crystalline Y2O3, from ZrO2 Xerogel Mixed with Crystalline Y(OH)3 and from the Amorphous Coprecipitate of (Y,Zr) Hydroxide in the Presence of Mineralizer Solutions at 0.15 and 2.0 M (KOH + K2CO3) total molarity (M)
Y2O3 precursor
t-ZrO2 content (%)a
uncrystallized zirconia area (counts)
t-ZrO2 crystal size (nm)
surface areab (m2/g)
0.15 0.15 0.15 2.0 2.0 2.0
crystalline Y(OH)3 crystalline Y2O3 coprecipitated Y(OH)3 gel crystalline Y(OH)3 crystalline Y2O3 coprecipitated Y(OH)3 gel
81 83 100 83 94 100
587 459 640 1174 832 460
16.8 17.2 12.9 9.5 11.0 6.5
73 70 105 155 127 186
a
Content with respect to the crystalline phases. b Surface area of untreated ZrO2 gel ) 283 m2/g.
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Figure 3. XRD patterns of coprecipitated Zr-Y hydroxide (a) as-received and after calcination at (b) 600 and (c) 1050 °C.
calcination at low temperature between the coprecipitate and the hydrothermally synthesized powders confirms the positive effect of the hydrothermal treatment on the preparation of soft ceramic precursors. After calcination at 900 °C, no coarsening results for the sample obtained from crystalline Y(OH)3 in dilute solution of mineralizer, whereas a certain coarsening results for the remaining hydrothermally treated powders. However, a relatively small value of coarseness results for the other powders obtained from different precursors and synthesized in 0.15 M, whereas a higher coarsening results for the analogous precursors crystal-
Figure 2. XRD patterns of powders crystallized by hydrothermal treatment in 0.15 M from (a) a mechanical mixture of ZrO2 xerogel and crystalline Y(OH)3, (b) a mechanical mixture of ZrO2 xerogel and crystalline Y2O3, and (c) coprecipitated Zr-Y hydroxide.
not coarsen; rather, a slight decrease in the t-ZrO2 crystal size can be observed for all of the powders. In contrast, the effect of calcination on the Zr-Y coprecipitate determines a noticeable coarsening, with the subsequent formation of hard agglomerates, as can be seen in Figure 3. The very different behavior upon
Table 3. Effect of Calcination at Increasing Temperature on the Crystal Sizes and the Content of t-ZrO2 of the Powders Hydrothermally Synthesized from Different Y2O3 Precursors in the Presence of 0.15 and 2.0 M Solutions of Mineralizer t-ZrO2 crystal size (nm) (KOH + K2CO3) total molarity (M)
Y2O3 precursor
t-ZrO2 content (%)a
after hydrothermal treatment
after calcination at 600 °C
after calcination at 900 °C
0.15 0.15 0.15 2.0 2.0 2.0
crystalline Y(OH)3 crystalline Y2O3 coprecipitated Y(OH)3 gel crystalline Y(OH)3 crystalline Y2O3 coprecipitated Y(OH)3 gel
81 83 100 83 94 100
16.8 17.2 12.9 9.5 11.0 6.5
16.6 17.0 12.7 9.4 10.9 6.4
16.4 19.4 17.1 12.3 23.5 19.1
a
Content with respect to the crystalline phases.
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Table 4. Reaction Products of Y-Based Precursors in Contact with Different Solutions at 110 °C for 7 Days solution of contact 1 M KOH 1 M K2CO3 0.15 Mb 2.0 Mc
crystalline Y2O3
Y(OH)3 gela
reaction products
reaction products
crystalline mixture of Y2O3 and Y(OH)3 crystalline Y2O3
crystalline Y(OH)3
crystalline yttrium hydroxy carbonate crystalline Y(OH)3
crystalline yttrium hydroxy carbonate crystalline yttrium hydroxy carbonate crystalline yttrium hydroxy carbonate
a No transformations were detected by submitting the crystalline Y(OH)3 to the same tests. b 0.038 M KOH + 0.112 M K2CO3. c 0.5 M KOH + 1.5 M K CO . 2 3
lized in the presence of 2.0 M solution of mineralizer. This behavior confirms the effect of the mineralizer concentration on the degree of agglomeration of the hydrothermally crystallized powders. The different coarsening upon calcination can be attributed, in fact, either to a different reactivity of the powders, because of their different starting particle sizes, or to their different degrees of agglomeration, i.e., the presence of groups of firmly bound primary particles, or finally, to a possible reaction between the tetragonal crystallized zirconia as a metastable phase and the possible unreacted yttria precursor. The different coarsening detected for the samples obtained in dilute solution and in the presence of crystalline yttria precursors (Table 3) cannot be attributed to different reactivities of the powders. The comparable starting crystalline sizes of these powders (16.8 and 17.2 nm, respectively) exclude, in fact, different reactivities, and consequently, the different coarsening must be attributed to different degrees of agglomeration of the two powders. Regardless of the different crystal sizes of the starting powders, after the calcination at 1050 °C, all of the resulting powders show comparable crystal sizes (23-24 nm). After the calcination step at 1050 °C, a 100% t-ZrO2 content results for the powders obtained from the coprecipitates, in contrast with the 10-15% m-ZrO2 content in the mixture with the main product t-ZrO2 in the powders synthesized from the mechanical mixtures. It must also be pointed out that the calcination at 1050 °C of the powders obtained from the mechanical mixture determines an increase of the t-phase content. To explain the different characteristics of the powders synthesized by using different precursors, crystalline Y2O3, crystalline Y(OH)3, and Y(OH)3 gel were hydrothermally treated at 110 °C for 7 days in the presence of the following solutions: 1.0 M KOH, 1.0 M K2CO3, 0.15 M (KOH + K2CO3), and 2.0 M (KOH + K2CO3). The source of Y(OH)3 gel was obtained by adding GRgrade Y(NO3)3 solution to ammonia (∼4 M) under continuous stirring. The precipitate was then repeatedly washed with water to remove the byproducts. All of the hydrothermal treatments were carried out by adopting a solid/solution weight ratio equal to 1:30. The resulting reaction products were identified by XRD analysis and are summarized in Table 4. In 1.0 M KOH, a crystalline mixture of untransformed Y2O3 and Y(OH)315 results from the starting crystalline (15) Powder Diffraction File Inorganic Phases No. 24-1422; JCPDS International Center for Diffraction Data: Swarthmore, PA, 1993.
Figure 4. DTA and TGA curves of crystalline Y(OH)3.
Y2O3, whereas only crystalline Y(OH)3 forms from the Y(OH)3 gel. These products demonstrate the higher reactivity of Y(OH)3 gel compared to crystalline Y2O3. After the treatment in 1 M K2CO3, unmodified crystalline Y2O3 results, whereas a poor crystallized phase, presumably a yttrium hydroxy carbonate, forms from the Y(OH)3 gel. In the presence of 0.15 M (KOH + K2CO3) solution, analogous and poorly crystallized yttrium hydroxy carbonate results from both the crystalline Y2O3 and Y(OH)3 gel. In 2.0 M (KOH + K2CO3), only crystalline Y(OH)3 forms from crystalline Y2O3, whereas a yttrium hydroxy carbonate forms from Y(OH)3 gel. No transformations were detected for the crystalline Y(OH)3 submitted to the same treatments. From these results and from refs 16-18, the following scale of solubility can be proposed for the Y-based precursors in the alkaline carbonate media:
crystalline Y2O3 > Y(OH)3 gel > yttrium hydroxy carbonate > crystalline Y(OH)3 To verify the effective stabilization of zirconia with Y2O3, the presence of unreacted crystalline Y(OH)3 or a yttrium hydroxy carbonate-based phase was ascertained. From the XRD patterns, no such phase was detected. On the other hand, the low amount of Y2O3 as dopant, together with the poor crystallinity of such phases, justifies such a finding. Otherwise, the thermal behavior of both the pure crystalline Y(OH)3 and the various yttrium hydroxy carbonates was analyzed. The DTA and TGA curves of crystalline Y(OH)3 are shown in the Figure 4. The two significant endothermic peaks at 317 and 482 °C, along with the corresponding weight loss, perfectly agree with the following steps of decomposition
2Y(OH)3 f 2YO(OH) + 2H2O f Y2O3 + H2O (16) Baes, C. F. J.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976. (17) Adair, J. H.; Krarup, H. G.; Venigalla, S.; Tsukada, T. In Better Ceramics through Chemistry II. Proceedings of the Materials Research Society Symposium; Voight, J. A., Wood, T. E., Bunker, B. C., Casey, W. H., Crossey, L. J., Eds.; Materials Research Society: Pittsburgh, PA, 1997; Vol. 432, p 101. (18) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: Houston, TX, 1974.
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Figure 5. DTA and TGA curves of yttrium hydroxy carbonate obtained from crystalline Y2O3 in 0.15 M mineralizer.
The DTA and TGA curves of the yttrium hydroxy carbonate obtained from crystalline Y2O3 in 0.15 M mineralizer in Figure 5 show a significant and sharp endothermic peak at 352 °C, with a corresponding weight loss, whereas a broad endothermic peak appears at higher temperature, with a maximum at 820 °C joined by a corresponding weight loss. The first endothermic peaks of the remaining yttrium hydroxy carbonates listed in Table 2 were detected at 357, 369, and 373 °C, respectively, i.e., over a small range of temperature. The corresponding endothermic peaks at higher temperature were detected at 800, 654, and 600 °C, respectively, i.e., over a wide range of temperature. The DTA signal of the first endothermic peak was used to discriminate the possible presence of unreacted crystalline Y(OH)3 or yttrium hydroxy carbonate in the hydrothermally synthesized powders. For the powders synthesized in dilute solutions of mineralizer, no small endothermic peak was detected. In contrast, in the powders obtained in concentrated mineralizer solution and in the presence of both crystalline Y2O3 and crystalline Y(OH)3, a very small endothermic peak appears that resembles the first peak of a yttrium hydroxy carbonate or crystalline Y(OH)3. To this end, a mechanical mixture of ZrO2 xerogel and crystalline Y2O3 (8 mol %) was also tested in both dilute (0.15 M) and concentrated (2.0 M) mineralizer solution. The corresponding DTA curve of the powder obtained in 2.0 M and acquired at high sensitivity is reported in Figure 6. A small endothermic peak results at 347 °C followed by an exothermic peak at a temperature slightly above 400 °C. The first peak at 347 °C was assigned to the presence of a yttrium hydroxide or hydroxy carbonate phase. This signal, in fact, is between the first endothermic peak at 314 °C of Y(OH)3 and the hydroxy carbonate peak detected at 352 °C. For the powders obtained in the dilute solution of mineralizer (0.15 M), such a signal was not detected. The exothermic peak at the temperature slightly higher than 400 °C was attributed to the crystallizing effect of the small fraction of uncrystallized zirconia. From these results, it can be concluded that the two powders obtained from crystalline yttria precursors and in concentrated solution of mineralizer (Table 2) appear to be unstabilized phases. On the other hand, these samples are also characterized by a relatively large XRD band that was previously
Dell’Agli and Mascolo
Figure 6. DTA and TGA curves of powder obtained by hydrothermal treatment of mechanical mixture of ZrO2 xerogel and crystalline Y2O3.
attributed only to the amount of uncrystallized zirconia, but in these circumstances, the presence of amorphous and unreacted yttria precursor must be considered. 4. Conclusions Processing of 3Y-TZP powdered precursors obtained by hydrothermal treatment involves many parameters that can be changed to optimize the characteristics of the products. However, the advantage of hydrothermal synthesis in preparing soft powders that are nanometric in size and have different degrees of agglomeration is clear. Nevertheless, by using different precursors, i.e., crystalline Y2O3, crystalline Y(OH)3, and coprecipitated Y(OH)3 gel, less agglomerated and stabilized ZrO2 powders are obtained in the presence of a dilute solution (0.15 M) of the (KOH + K2CO3) mineralizer, which corresponds to the lowest nucleation rate for the crystallization-stabilization of zirconia. Some concomitant effects must be considered during hydrothermal treatment for the crystallization-stabilization of zirconia xerogel in the presence of yttria, namely, the crystallization rate of zirconia xerogel and the reaction rate of a soluble Y-containing species of the mineralizer solution with the crystallizing zirconia. This last reaction, in turn, depends on the formation rate of a soluble Y-containing species from the Y-based precursor. Hydrothermal treatments carried out at high pH and high temperature favor a rapid crystallization of the zirconia xerogel and the subsequent formation of the m-phase or metastable phases in the t- or c-form in relation to the crystallite sizes of the resulting powders. Under these circumstances, the stabilization of zirconia with the Y2O3 is unfavored. The predominant product, m-zirconia, detected at high hydrothermal treatment temperature and in the presence of a high concentration of NaOH by Pyda et al.3 and by Kriechbaum et al.7 suggests, in this case, favorable crystallization of zirconia gel without its stabilization. The effective stabilization of zirconia with Y2O3 requires a crystallization rate of zirconia xerogel comparable to the reaction rate of the yttria chemical species with the crystallizing zirconia. A moderate crystallization rate of ZrO2 xerogel must be expected in performing hydrothermal treatments at low temperature and at
Crystallization-Stabilization of Zirconia Xerogel
relatively low pH. In this case, a favorable stabilization of ZrO2 is to be expected, as found by Kriechbaum et al.7 with use of dilute 0.01 M NaOH and by Burkin et al.2 in the presence of ammonia at pH 10. During the crystallization-stabilization of zirconia, the mineralizer solution must constantly contain an available and soluble content of yttria-containing species. In this case, both the reactivity and the solubility of the different Y-based precursors must be considered. The high reactivity detected for the Y(OH)3 gel might explain both the stabilization of zirconia and the absence of the m-phase in the powders synthesized from the coprecipitates in contrast with the powders obtained from the mechanical mixtures, in which the less reactive and crystalline Y-based precursors are used. In addition, the lowest solubility of the crystalline Y(OH)3 precursor also explains the highest content of m-phase present in the corresponding powders, in comparison with the powders obtained with the relatively more soluble
Chem. Mater., Vol. 14, No. 7, 2002 3015
crystalline Y2O3 and, finally, with very reactive Y(OH)3 gel, for which the m-phase disappears. The stabilization of zirconia appears to be effective when dilute solution of mineralizer are used, whereas in the presence of concentrated solutions, the stabilization of zirconia needs a very reactive Y-based precursor such as the Y(OH)3 gel. However, the very fine powdered ceramic precursor appears to be the one obtained from crystalline Y(OH)3 in dilute solution. It is characterized, in fact, by the lowest degree of agglomeration, a necessary condition for obtaining defect-free ceramics upon sintering. Acknowledgment. Financial support provided by CNR “Materiali Speciali per Tecnologie avanzate II” is gratefully acknowledged. The authors thank Ing. Colantuono and Mr. Di Mambro for their technical assistance. CM0211103
