The effect of mineralizers on the crystallization of zirconia gel under hydrothermal conditions

Solid State Ionics 123 (1999) 87–94

The effect of mineralizers on the crystallization of zirconia gel under hydrothermal conditions G. Dell’Agli, A. Colantuono, G. Mascolo* Dipartimento di Meccanica, Strutture, Ambiente e Territorio della Facolta` d’ Ingegneria, Universita` di Cassino, Via G. Di Biasio, 43 -03043 Cassino ( FR) Italy Received 2 February 1999; accepted 12 April 1999

Abstract Solutions of different MOH mineralizers (M 5 Li 1 , Na 1 , K 1 and (CH 3 ) 4 N 1 ), with concentration level changing between 0.01 to 3.0 M, were utilized for the crystallization of zirconia gel under hydrothermal conditions at 1108C for 7 days. An increasing crystallization rate of zirconia resulted at increasing concentration of each basic mineralizer. The corresponding and progressive diminution in the crystallite size of products determined the formation of monoclinic, tetragonal and cubic sequence of zirconia. The crystallization rate of gel was found to depend also on the nature of mineralizer solution; higher cationic radius of MOH mineralizer, lower crystallization rate was observed. A nucleation and growth mechanism is proposed for the crystallization of zirconia gel during the hydrothermal treatment in the presence of different basic mineralizers.  1999 Elsevier Science B.V. All rights reserved. Keywords: Zirconia gel; Hydrothermal treatment; Mineralizer solutions; Crystallization of zirconia gel

1. Introduction Very fine particle sizes of zirconia, prepared by wet chemistry [1–8], do not always ensure an appropriate sinterability due to the formation of hard agglomerates on drying and calcination of such fine powders [9–15]. To avoid the interaction between agglomerates and / or primary particles of powder, some preliminary treatments have been proposed before the precalcination treatment. Of these treatments, freeze drying [15], hydrothermal treatment [16–18], washing with alcohols [19–21], represent *Corresponding author. Tel.: 139-0776-299-710; fax: 1390776-310-812. E-mail address: [email protected] (G. Mascolo)

ways of treating hydrous and amorphous zirconia powders. In all cases these methods require the removal or substitution of non-bridging hydroxo groups present in the precursor [12–14]. The condensation reactions involving the non-bridging hydroxo groups of the precursor are, in fact, responsible for the formation of hard agglomerates during drying and calcination. Among such preliminary treatments, the hydrothermal treatment at low temperature appears to be a simple and advantageous method. On the other hand this method also allows preparation of fine powders of zirconia solid solutions with different dopants [22,23]. In such a way two aims can be pursued, soft and doped zirconia powders are simultaneously prepared in a single step under hydrothermal conditions. For the crystalliza-

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tion–stabilization of zirconia by hydrothermal treatment at low temperature two simultaneous conditions must occur. In the first place the crystallization of zirconia gel is easily favoured by the presence of a basic solution of mineralizer in the starting mixture reaction. A high pH value of mineralizer promotes, in fact, the condensation reactions of non-bridging hydroxo groups of gel precursor. Such reactions allow a structural rearrangement and consequent crystallization of the gel. The other condition, concerning the stabilization of zirconia, requires the presence of a certain (small) concentration of the ionic form of the dopant in the mineralizer solution during the crystallization. This last condition needs a small solubility of dopant in the basic solution of precursor. In some cases such a condition can only be pursued by coupling the basic mineralizer with a substance able to dissolve partly the dopant to be mixed with the zirconia gel. As the crystallization mechanism of zirconia gel in the formation of metastable forms at low temperature is not yet completely understood [17,24], we report a systematic study on the crystallization of zirconia gel under hydrothermal treatment at low temperature and in the presence of various basic mineralizers. Such knowledge allows to synthesize powders with programmed crystallite sizes of both undoped and doped zirconia, respectively.

conia gel and the mineralizer solution was transferred to half-filled and sealed Teflon vessel (250 mm 3 ) held in an outer pressure vessel made of stainless steel. The vessels were rotated in an air thermostated oven at 25 rpm to mix the gel precursor with mineralizer solution during the treatment. After 7 days of hydrothermal treatment, the products were filtered and repeatedly washed with distilled water to remove Li 1 , Na 1 , K 1 or (CH 3 ) 4 N 1 cations, and dried on silica gel. The reaction products were characterized by X-ray powder diffractometry (XRD) using a X’Pert of Philips diffractometer and CuKa radiation. Crystallite sizes of polymorphs were calculated from XRD peaks using the Scherrer formula, the Warren correction [25] and calibration with polycrystalline silicon. The powders were also characterized by simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) using a Netzsch thermoanalyzer model STA 409, a-Al 2 O 3 as reference and a 108C min 21 heating rate. The specific surface area of powders was determined by the BET method using a Gemini of Micromeritics and utilizing nitrogen as adsorbate after drying at 608C. The morphology of powders was analysed by scanning electron microscopy (SEM) using a model XL30 of Philips.

3. Results and discussion 2. Experimental procedure

3.1. Reaction products of hydrothermal treatment Suspensions of gel mixed with different mineralizer solutions were employed for the crystallization of zirconia under hydrothermal conditions. A batch of zirconia gel precursor was precipitated from GR grade ZrCl 4 (Merck, Germany) solution with ammonia, filtered and repeatedly washed with distilled water until removal of the chloride ions. The ZrO 2 content of this precursor was determined by thermogravimetric analysis (TGA). GR grade LiOH ? H 2 O, NaOH, KOH and (CH 3 ) 4 NOH (TMAH) of C. Erba (Italy) were the bases utilized for the preparation of mineralizer solutions. The concentration level of such solutions was ranged from 0.01 to 3.0 M. The temperature of hydrothermal treatment was 1108C and a solid / mineralizer solution ratio equal to 1 / 40 was adopted. The suspension containing zir-

The prepared batch of ZrO 2 -based precursor resulted amorphous to XRD, showed in DTA a very sharp exothermic peak of crystallization into tetragonal zirconia at 4308C and gave in TGA a weight loss of 23%. Phases present of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of nature and concentration of mineralizer solution are reported in Table 1. The treatments in all the basic mineralizers with 0.01 M concentration resulted in amorphous products. The tests carried out in 0.05 M solutions resulted instead in products with a crystallization degree depending on the nature of basic mineralizer. In particular the crystallization degree of zirconia decreased with the increasing

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Table 1 Phases present of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of nature and concentration of mineralizer Concentration

0.01 M 0.05 M 0.10 M 0.50 M 1.0 M 3.0 M

Mineralizer LiOH

NaOH

KOH

(CH 3 ) 4 NOH

A*** (A), M ** , T ** M ** , T ** M ** , T ** M*, C ** (M), C ***

A*** A** , M*, (T) M *** , T* M*, T ** (A), C *** C ***

A*** A*** , (M), (T) M *** , T* M ** , T ** M*, T ** M*, C **

A*** A*** (A), M *** M ** , T* M*, T ** [C ** ] M ** , T*[C*]

A, M, T and C are amorphous, monoclinic, tetragonal and cubic zirconia, respectively. Phase content: *** high, ** medium, * small. Parentheses indicate the presence of traces of a phase.

cationic radius of mineralizer. A quite full crystallization, in fact, was detected in LiOH, while no crystallization resulted in TMAH. Intermediate crystallization degrees resulted in NaOH and KOH, respectively. The treatments in 0.10 M resulted in fully crystallized products consisting, with the exception of TMAH, of mixtures of monoclinic and tetragonal zirconia. In this case the amount of tetragonal form is favoured by the presence of basic mineralizer with lower cationic radius as can be

observed in Fig. 1. The measured crystallite sizes of the monoclinic form is 30 nm in TMA, 21 nm in KOH, 13 nm in NaOH and 12 nm in LiOH. The absence of the tetragonal form in TMA (Fig. 1) shows that its formation, as a metastable phase and in term of surface-energy theory [26], requires a crystallite size in the range of 31 nm to 21 nm according to the value # 30 nm determined by Garvie [26]. In the presence of higher concentrations of miner-

Fig. 1. Phase present of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of nature of mineralizer and taking constant the mineralizer concentration (0.10 M).

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alizers, the content of monoclinic form decreased instead of tetragonal form which, in turn, transformed in the cubic zirconia especially in highly concentrated solutions, i.e. in 1.0 and 3.0 M base. The crystallite sizes of monoclinic zirconia measured on products obtained in the presence of each mineralizer and as a function of concentration are reported in Fig. 2. It is evident the progressive diminution in the crystallite size of monoclinic ZrO 2 with the increasing concentration of each mineralizer. The crystallite sizes of monoclinic, tetragonal and cubic ZrO 2 as a function of concentration and for each mineralizer are plotted in Figs. 3–6. All the polymorphic forms of ZrO 2 show a diminution in the crystallite sizes at increasing concentrations. Such a diminution is more evident for monoclinic zirconia. The presence in the products of monoclinic crystals with sizes , 20 nm is not easy to explain. The feeling is that when the product contains a single phase as hydrated monoclinic crystals, crystallite sizes must be higher than 20 nm, while monoclinic crystal sizes , 20 nm are compatible only in the presence of other polymorphs. The contemporaneous presence of monoclinic and cubic ZrO 2 in the products, it is possible to evaluate, in terms of surface-energy theory [26], a critical value of the crystallite size of phase precursor for the formation of metastable cubic zirconia. In LiOH (Fig. 3), NaOH (Fig. 4) and KOH (Fig. 5) mineralizers such

Fig. 3. Crystallite size of monoclinic, tetragonal and cubic ZrO 2 of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function decreasing of LiOH concentration.

Fig. 4. Crystallite size of monoclinic, tetragonal and cubic ZrO 2 of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of NaOH concentration.

Fig. 2. Crystallite size of monoclinic ZrO 2 of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of nature and concentration of mineralizer.

value appears to be lower than 5 nm. In the presence of higher concentrations of TMAH (Fig. 6), the excessive diminution of crystallite size of both monoclinic form and of coupling phase makes uncertain the designation of the polymorphic form of the coupling phase as reported in Table 1. From

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Fig. 5. Crystallite size of monoclinic, tetragonal and cubic ZrO 2 of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of KOH concentration.

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observed upon ageing of zirconia gel in 1.0 M NaOH [23]. However the different temperatures adopted in the two conditions must be taken into account. Nishizawa et al. [27] observed that in NaOH solutions higher than 1 M, cubic zirconia crystallized initially at 130–1408C and transformed to monoclinic zirconia at 3008C. Such transformation can be related to the increasing crystallite sizes of products with the increasing treatment temperature. In addition the crystallite size of cubic zirconia decreased as the concentration of the NaOH mineralizer was increased. The results show that, taking the temperature and duration of hydrothermal treatment constant, both concentration and nature of mineralizer solution influence the structural rearrangement of particles of zirconia gel during crystallization. In particular both the increasing concentration and the lower cationic radius of mineralizer solution favour the diminution of the crystallite size of products determining the following sequence of phase crystallization of zirconia: amorphous → monoclinic → tetragonal → cubic.

3.2. Surface area of products

Fig. 6. Crystallite size of monoclinic, tetragonal and cubic ZrO 2 of products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of TMAOH concentration.

these results it is possible to predict, in terms of surface-energy theory, the crystallite size of phase precursor for the formation of metastable cubic ZrO 2 . A single and fully crystallized cubic form was obtained only in 3.0 M NaOH. Such findings partly agree with the semicrystalline tetragonal zirconia

The surface area of hydrothermally synthesized products are reported in Table 2. Each mineralizer determines a similar behaviour of the surface area of products as a function of increasing concentrations. The minima in the curves represent the concentration of each mineralizer that separates the full crystallization of zirconia gel from that of uncompleted crystallization. Starting from 0.01 M solutions, the crystallization degree of ZrO 2 gel increases, in fact, with increasing concentration of mineralizer so reducing the corresponding surface area of products. It can be seen that the value of minimum in the curves depends on the nature of mineralizer. Higher cationic radii of the mineralizer, in fact, required higher concentrations for the minimum and consequently for full crystallization. Fig. 7 shows the increasing surface area of products obtained for concentrations higher than the minimum for full crystallization. A higher driving force for structural rearrangement of the gel is to be expected for concentrations higher than that of the minimum. In this circumstance the crystallization rate of zirconia increases so justifying

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Table 2 Surface area (m 2 / g) of products listed in Table 1 Concentration

0.01 M 0.05 M 0.10 M 0.50 M 1.0 M 3.0 M

Mineralizer LiOH

NaOH

KOH

(CH 4 ) 3 NOH

229 107 50 100 119 224

274 202 90 73 95 122

329 241 47 50 77 92

317 313 134 58 68 69

crystallization of gel in the presence of basic mineralizers.

3.3. Weight loss of products

Fig. 7. Surface area of fully crystallized products obtained by hydrothermal treatment of zirconia gel at 1108C for 7 days as a function of nature and concentration of mineralizer.

the smaller crystallite sizes of products observed in more concentrated solutions. Such behaviour suggests a nucleation and growth mechanism during the

The weight loss of hydrothermal products are reported in Table 3. The results show a trend similar to that found for the surface area where the minimum corresponds to the fully crystallized product. As amorphous zirconia is characterized by the maximum weight loss, the decreasing value for products obtained at low and increasing mineralizer concentration can be related to the diminution of amorphous zirconia and consequently to the increasing crystallization degree of zirconia. The successive weight loss increase, observed for products treated in more concentrated solutions, can be related to their lower crystallinity. The observed weight losses of products are due, in addition to both crystallinity. As the weight loss of products is due, in addition to both chemically coordinated and physisorbed water, to the oxo-bridging and non-bridging structural hydroxyl groups, the minimum value corresponds to products

Table 3 Weight loss (%) of products listed in Table 1 Concentration

0.01 M 0.05 M 0.10 M 0.50 M 1.0 M 3.0 M

Mineralizer LiOH

NaOH

KOH

(CH 4 ) 3 NOH

24.0 13.5 9.3 11.9 12.4 15.1

24.1 19.0 7.5 11.5 11.8 13.8

24.2 20.0 7.0 8.9 10.3 13.6

27.0 22.7 12.6 9.1 10.3 13.3

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obtained in mineralizer solutions characterized by the minimum driving force for structural rearrangement. Nishizawa et al. [27] suggested that during the crystallization of metastable cubic zirconia in NaOH mineralizer, Na 1 (and OH 2), absorbed on the gel, ruptured the Zr–O–Zr bridges collapse and dehydration of structure with consequent formation of cubic zirconia occurs. To this end, some additional experiments have been performed on zirconia gel in the presence of mineralizers characterized by a lower pH value. LiCl, NaCl and KCl were employed, in fact, as mineralizer solutions. A concentration 0.5 M was chosen; i.e. a value higher than that of the minima in Table 2. After 7 days of hydrothermal treatment at 1108C, uncrystallized products resulted. Such results show that the pH of basic mineralizers represents the main feature for zirconia crystallization, while the nature of M 1 , and in particular the cationic radius influences the crystallization rate. From Tables 2 and 3 it is confirmed that the crystallization rate decreases according to the following sequence of decreasing radius of basic mineralizer: Li 1 .Na 1 .K 1 .(CH 3 ) 4 N 1 . Another hypothesis might be formulated in this case; the strong base OH 2 might favour the formation of Zr–O–Zr bridges between the non-bridging structural hydroxyl groups of the gel. Such a rearrangement is also affected by the presence of M 1 in term of steric hindrance. The higher the cationic radius of MOH, the higher the difficulty for structural rearrangement. In such cases it is easy to explain the faster rate of crystallization in the presence of Li 1 with respect to other mineralizers and the relatively slow rate in the presence of (CH 3 ) 4 N 1 . The faster rate of crystallization might involve an incomplete formation of Zr–O–Zr bridges so determining a higher weight loss as observed for products obtained in the presence of LiOH mineralizer.

4. Conclusions A nucleation and growth mechanism may be proposed for the crystallization of zirconia gel hydrothermally treated at 1108C in the presence of basic mineralizers. The strong base OH 2 favours the formation of Zr–O–Zr bridges between the nonbridging structural hydroxyl groups present in the gel

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so favouring its rearrangement, its nucleation and its consequent crystallization. The rate of such rearrangement increases with the increasing concentration of MOH mineralizer and decreases with the increasing cationic radius of MOH. This last effect has been related to the steric hindrance of higher M 1 radius during the structural rearrangement. The two effects determine a wide change in the crystallite sizes of products so favouring, in terms of surfaceenergy theory, the formation of metastable tetragonal and cubic zirconia.

Acknowledgements The financial support given by CNR ‘Materiali speciali per tecnologie avanzate II’ is gratefully acknowledged. The authors thanking Dal Vecchio for technical assistance.

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