The crystallization process of HfO2 and ZrO2 under hydrothermal conditions

Journal of Alloys and Compounds 387 (2005) 300–307

The crystallization process of HfO2 and ZrO2 underhydrothermal conditions

G. Stefanic∗, S. Music, K. Molcanov

Ruer Boskovic Institute, P.O. Box 180, HR-10002 Zagreb, Croatia

Received 16 April 2004; received in revised form 22 June 2004; accepted 22 June 2004

Abstract

Hydrothermal crystallization of hydrous hafnia at 90 and 120◦C, was monitored at pH 3, 7, 9.5 and 13, using X-ray powder diffractionanalyses. The obtained results showed that hydrothermal crystallization proceeded much more slowly in a neutral pH medium than in an acidicor alkaline medium. Phase analysis of the products of hydrothermal treatment showed that, regardless of pH, hydrous hafnia crystallized intom-HfO2. Stabilization of a metastable cubic or tetragonal HfO2 occurred upon adding a sufficient amount of stabilizing Na+ ions. Similaritiesa Itw range.©

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nd differences between the hydrothermal crystallization of HfO2 and the hydrothermal crystallization reported for ZrO2 were discussed.as concluded that in both cases hydrothermal crystallization proceeds via a dissolution/precipitation mechanism in the whole pH2004 Elsevier B.V. All rights reserved.

eywords:Hydrothermal crystallization; HfO2; ZrO2; XRD; Metastable phase

. Introduction

The mechanism of hydrothermal crystallization of zirco-ia was investigated in several papers[1–9]. It was found

hat hydrothermal treatment of highly acidic solutions oruspensions of hydrous zirconia produced monoclinic zirco-ia,m-ZrO2 [1–3], whereas hydrothermal treatment at highH values yielded, besidem-ZrO2, a metastablet-ZrO2 andlso a metastable cubic zirconia,c-ZrO2, in the presence ofaCl2 [4] or NaOH[5,6] as a stabilizing agent. Tani et al.

7] concluded that the formation ofm-ZrO2 by hydrothermalreatment proceeded via a dissolution/precipitation mecha-ism [3,7], whereas the formation oft-ZrO2 occurred as aesult of structural rearrangement of amorphous hydrous zir-onia (topotactic crystallization)[7]. Denkewicz et al.[8]roposed a model of hydrothermal crystallization of ZrO2ased on three control regimes. At low pH solubility isigh, and hydrothermal crystallization occurs via a dissolu-

ion/precipitation mechanism, producingm-ZrO2. In a neu-

∗ Corresponding author. Tel.: +385-1-456-1111.

tral or a mild acidic medium solubility is very low, so thcrystallization occurs in situ by structural (topotactic) rerangement of hydrous zirconia. The product of hydrothecrystallization in this region will be predominantlyt-ZrO2,and the presence ofm-ZrO2 can be attributed to the tranformation t-ZrO2 → m-ZrO2 with prolonged hydrothermtreatment. At high pH the solubility of hydrous zirconiavery high and similar to the solubility at low pH. Howevin situ topotactic crystallization prevails because of a hienergy state of the obtained hydrous zirconia gel[8].

In an earlier investigation we examined the kineticshydrothermal crystallization of hydrous zirconia suspenat pH 2, 7, 9.5 and 13[6]. The obtained results showedanalogy between the rate of the hydrothermal crystallizaof ZrO2 and the solubility of hydrous zirconia. Although oobservations of crystallization kinetics can be accommodwithin the model of Denkewicz et al.[8], no evidence of thexistence of a gel structure-controlled regime could be foby a phase analysis of the products of hydrothermal tment. The results of structural characterization of hydzirconia[10,11], as well as its thermal behaviour[12,13], in-

E-mail address:[email protected] (G.Stefanic). dicate that the in situ crystallization of amorphous hydrous

925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2004.06.064

G. Stefanic et al. / Journal of Alloys and Compounds 387 (2005) 300–307 301

zirconia is not a simple topotactic process. It was concludedtherefore that the hydrothermal crystallization of hydrous zir-conia proceeds via the dissolution/precipitation mechanismin the whole pH range[6].

Hydrothermal synthesis of hafnia was examined in sev-eral papers. Blanc et al.[14] used hydrothermal synthesis atlow pH to produce a stable sol of monoclinic HfO2 used inthe production of ultrafiltration membranes. Yoshimura andSomiya[15] described the application of hydrothermal pro-cessing in preparing fine powders of rare earth doped zirconiaand hafnia. However, to the best of our knowledge there wasno attempt to investigate the influence of pH on the rate andmechanism of hydrothermal crystallization of HfO2, whichcould also throw more light on the mechanism of hydrother-

Table 1The notation of the samples and the results of XRD analysis

Hydrothermal treatment conditions XRD analysis

pH T (◦C) Time (h) Base Phase composition D (nm) %

3 – – NaOH Amorphous – 03 90 6 NaOH Amorphous – 03 90 24 NaOH Amorphous +m-HfO2 – 13 90 72 NaOH Amorphous +m-HfO2 9.7 (6) 53 90 165 NaOH m-HfO2 + amorphous 8.3 (4) 843 120 4 NaOH Amorphous – 03 120 12 NaOH Amorphous +m-HfO2 9.9 (6) 93 m3 m7 A7 A7 A7 A7 A7 m7 A7 A7 A7 m7 m9 A9 A9 A9 2

9999999111111111111

mal crystallization of ZrO2. Physical and chemical proper-ties of hafnia are very similar to those of zirconia. Dependingon temperature, HfO2 and ZrO2 appear in monoclinic (m-),tetragonal (t-) and cubic (c-) polymorphs, of which only themonoclinic polymorph is thermodynamically stable at roomtemperature (RT) and standard pressure. High temperaturepolymorphs of ZrO2 become stable at above 1170◦C (t-ZrO2)and 2300◦C (c-ZrO2), whereas the stabilization oft- andc-HfO2 requires considerably higher temperatures, 1720 and2600◦C, respectively. For that reason, metastable tetragonalor cubic polymorphs of hafnia are less often present in crys-tallization products, compared with the products of zirconia[16,17], which enables a better insight into the mechanism ofhydrothermal crystallization.

120 24 NaOH120 48 NaOH– – NaOH90 70 NaOH90 144 NaOH90 336 NaOH90 672 NaOH120 1344 NaOH120 48 NaOH120 168 NaOH120 336 NaOH120 672 NaOH120 792 NaOH

.5 – – NaOH

.5 90 24 NaOH

.5 90 72 NaOH

.5 90 168 NaOH

.5 90 208 NaOH

.5 90 240 NaOH m

.5 90 336 NaOH

.5 120 24 NaOH

.5 120 76 NaOH m

.5 120 120 NaOH m

.5 120 168 NaOH3 – – NaOH A3 90 12 NaOH m3 90 24 NaOH m3 120 4 NaOH c-3 120 12 NaOH m3 120 24 NaOH m3 – – TMAH A3 90 12 TMAH A3 90 24 TMAH A3 90 48 TMAH m3 90 72 TMAH m3 120 24 TMAH m

-HfO2 + amorphous 8.5 (5) 64-HfO2 + amorphous 9.3 (7) 96morphous – 0morphous – 0morphous – 0morphous +m-HfO2 – 1morphous +m-HfO2 28 (4) 26

-HfO2 + amorphous 37 (7) 77morphous – 0morphous +m-HfO2 29 (5) 11morphous +m-HfO2 29 (5) 24

-HfO2 + amorphous 33 (5) 97-HfO2 34 (5) 100morphous – 0morphous – 0morphous – 0

Amorphous +m-HfO 27 (4) 2
Amorphous +m-HfO2 40 (8) 14-HfO2 + amorphous 34 (6) 48m-HfO2 39 (8) 78Amorphous – 0-HfO2 + amorphous 38 (8) 50-HfO2 + amorphous 34 (6) 88m-HfO2 + amorphous 40 (8) 92morphous – 0
-HfO2 + c- or t-HfO2 + amorphous 4.7 (2) –-HfO2 + amorphous 4.7 (1) –or t-HfO2 +m-HfO2 + amorphous – –

-HfO2 + c- or t-HfO2 + amorphous 5.5 (2) –-HfO2 5.6 (2) 100morphous 0morphous 0morphous +m-HfO2 3.4 (8) 5-HfO2 12.6 (9) 99-HfO2 13 (1) 98-HfO2 – 97

302 G. Stefanic et al. / Journal of Alloys and Compounds 387 (2005) 300–307

In the present investigation we have examined themechanism of hydrothermal crystallization of hydrous haf-nia at different pH values of starting aqueous suspen-sions.

2. Experimental

All chemicals were of analytical purity. The aqueous sus-pensions of hydrous hafnia at pH 3, 7, 9.5 and 13 were pre-pared by adding NaOH solution to the aqueous solution of

Fa

HfOCl2·8H2O, obtained by dissolving 1 g of salt in twicedistilled water. In order to determine the influence of Na+ions, the suspension at pH = 13 was prepared by adding(CH3)4NOH (TMAH, 25%, w/w, aqueous solution). In allcases the final volume of suspensions was adjusted to 40 cm3.While adding a base the suspension was subjected to me-chanical stirring. Thus obtained suspensions were put intoan autoclave (volume 50 cm3) and subjected to hydrothermaltreatment at 90◦C or 120◦C for different times, then washedwith twice distilled water using a Sorvall RC2-B ultra-speedcentrifuge (max. 20,000 rpm) and dried at 60◦C for 24 h. The

ig. 1. Portions of observed (circles) and calculated XRD patterns of samplet 90◦C (left) or 120◦C (right) for different times. The difference between the o

s precipitated at pH = 3 by theaddition of NaOH (aq.) and hydrothermally treatedbserved and calculated patterns is shown in the same scale below each pattern.


notation for samples and corresponding synthesis conditionsare given inTable 1.

The kinetics of hydrothermal crystallization of HfO2 wasmonitored using X-ray powder diffraction (XRD). The phasecomposition of samples was determined at RT using X-raypowder diffraction (Philips Counter Diffractometer, modelMPD 1880). In order to make possible a comparison betweenvarious diffraction patterns, the same amount (15 wt.%) of�-Al2O3 standard was added to the sample before XRD mea-surements. The integral intensities of the diffraction lines(1 1 1) and (1 1 1) ofm-HfO2 were normalized using the in-

Fa

tegral intensities of the diffraction lines (0 1 2), (1 1 0) and(1 1 3) of standard�-Al2O3. After that, the fraction ofm-HfO2 in a products obtained at a given temperature and pHwas defined as a ratio of the value obtained for the sum of thenormalized integral intensities of the diffraction lines (1 1 1)and (1 1 1) and the value obtained for the sum of the nor-malized integral intensities of these lines for the product hy-drothermally treated at pH = 7 and 120◦C for 792 h. for thisproduct we assumed that the crystallization process is com-pleted. For samples which showed a pronounced broadeningof diffraction lines, the crystallite size was estimated using

ig. 2. Portions of observed (circles) and calculated XRD patterns of samplet 90◦C (left) or 120◦C (right) for different times. The difference between the o

s precipitated at pH = 7 by theaddition of NaOH (aq.) and hydrothermally treatedbserved and calculated patterns is shown in the same scale below each pattern.


the Scherrer equation (d = 0.9λ/βcosθ), λ being the X-raywavelength,θ the Bragg angle andβ the pure full width atone-half of the maximum intensity. The values ofβ werefound from the observed full width at half the maximum in-tensity (FWHM) of the diffraction lines, after a correction forinstrumental broadening, for which the corresponding widthof the diffraction lines of�-Al2O3 was used, following theprocedure given in literature[18]. The integrated intensitiesand FWHM of the diffraction lines were determined usingan individual profile-fitting method (SHADOW profile fittingprogram)[19]. The results of phase analysis and estimateddvalues are given inTable 1.

Fa

3. Results and discussion

3.1. Phase analysis

Before hydrothermal treatment all samples were amor-phous. The results of XRD analysis indicate the existence ofa certain induction time during which no sign of the crys-tallization process can be observed[20]. The first sign of acrystal phase in samples obtained from the suspensions at pH3, 7, 9.5 and 13 accommodated using NaOH appeared after24, 336, 168 and 12 h of hydrothermal treatment at 90◦C,and after 12, 168, 76 and 4 h of hydrothermal treatment at

ig. 3. Portions of observed (circles) and calculated XRD patterns of samplest 90◦C (left) or 120◦C (right) for different times. The difference between the o

precipitated at pH = 9.5 by the addition of NaOH (aq.) and hydrothermallytreatedbserved and calculated patterns is shown in the same scale below each pattern.


Fig. 4. Characteristic parts of XRD patterns of samples precipitated at pH = 13 by the addition of NaOH (aq.) and hydrothermally treated at 90◦C (a) or 120◦C(b) for different times.

120◦C, respectively (Table 1). Apart from the diffractionlines of standard (�-Al2O3), only the diffraction lines ofm-HfO2 can be observed in an XRD pattern of samples obtainedfrom suspensions at pH 3, 7 and 9.5 (Figs. 1–3). In the caseof samples obtained from suspension accommodated to pH= 13 using NaOH, in addition to the diffraction lines ofm-HfO2, the crystallization products contained the diffractionlines of a metastablec- or t-HfO2 (Fig. 4). The diffractionlines of these samples are very broad, which makes it diffi-cult to distinguish between the cubic and tetragonal phases.For the same reason we could not precisely determine de-gree of crystallization in those samples (significant overlapof the diffraction line (1 1 1) ofc-HfO2 or (1 0 1) oft-HfO2and the diffraction lines (1 1 1) and (1 1 1) ofm-HfO2). How-ever, the time needed for the appearance and the increase ofthe diffraction lines indicated the highest rate of HfO2 crys-tallization at pH = 13. Differently from the results obtainedfrom suspension accommodated to pH = 13 using NaOH, nosign of the diffraction lines of a metastable tetragonal or cubicphase could be observed in the XRD patterns of the samplesobtained from suspension accommodated to pH = 13 using

(CH3)4NOH (Fig. 5). The first sign of a crystal phase ap-peared after 24 h as very broad diffraction lines (D = 3.4 nm)typical ofm-HfO2. The intensity of these lines increased withan increase in the time of hydrothermal treatment and the linesbecame sharper. The obtained results clearly showed that re-gardless of the pH value hydrous hafnia crystallized into them-HfO2. The appearance of the metastable phase in the sam-ples processed with NaOH at pH = 13 can be attributed tothe stabilizing influence of Na+ ions, similarly as in the caseobserved forc-ZrO2 [5,6].

3.2. Mechanism of the hydrothermal crystallization ofhafnia

The relationship between the time of hydrothermal treat-ment at 90 and 120◦C and the fraction of crystal phase isshown inFig. 6. The curves indicate the increase in the frac-tion of the crystal phase with the increase in the hydrothermaltreatment time. The kinetics of this process showed to be pHand temperature dependent.Fig. 7 shows the influence ofpH on the time needed to crystallize 75% of sample, i.e.,


Fig. 5. Portions of observed (circles) and calculated XRD patterns of sam-ples precipitated at pH = 13 by the addition of (CH3)4NOH and hydrother-mally treated at 90◦C for 12, 24 or 48 h. The difference between the observedand calculated patterns is shown in the same scale below each pattern.

Fig. 6. The fraction of crystal phase as a function of hydrothermal treatment time at 90◦C (left) and 120◦C (right).

Fig. 7. The pH dependence of the time needed to crystallize 75% of the sam-ples, hydrothermally treated at 90◦C (left) or 120◦C (right), as determinedby DSC/TG (filled symbols) or XRD analysis (empty symbols).

the time after which the diffraction lines ofm-HfO2 reached75% of the integral intensities obtained for the completelycrystallized material. Although the kinetic of hydrothermalcrystallization was significantly faster at 120◦C, at both tem-peratures the process of hydrothermal crystallization of HfO2showed to be pH-dependent in a way similar to the hydrother-mal crystallization of ZrO2 [6]. Hydrothermal crystallizationproceeds much more slowly in a neutral pH medium than inacidic or alkaline media. However, the important difference


from the hydrothermal crystallization of ZrO2 is in the phasecompositions of the obtained crystallization product. In thecase of ZrO2 crystallization in a neutral or high pH mediumproduced, besidem-ZrO2, a metastablet- or c-ZrO2 phase,whereas the hydrothermal crystallization of HfO2 producedonly a monoclinic polymorph, regardless of the process-ing pH value (Table 1). The appearance of these metastablephases in the ZrO2 crystallization products, obtained after hy-drothermal treatment of neutral or alkaline suspensions, wasa basis for the proposed model of topotactic crystallization[7,8]. However, due to a great physical and chemical similar-ity between ZrO2 and HfO2 materials, as well as similaritiesin the pH dependence of the rate of their crystallization, it isreasonable to assume that the process of hydrothermal crys-tallization of ZrO2 proceeds by the same mechanism as theprocess of hydrothermal crystallization of HfO2. In the caseof HfO2, this process yields only a thermodynamically sta-ble monoclinic polymorph, so the topotactic model of crys-tallization proposed for ZrO2 could not be used. An analogybetween the pH dependence of the rate of hydrothermal crys-tallization of ZrO2 and HfO2 and their solubility[21], indi-cates that in both cases hydrothermal crystallization proceedsvia a dissolution/precipitation mechanism.

4

rys-t tedf pHm dif-fZ liza-t lo et fOc roush ofc .

Due to the analogy between the pH dependence of the rateof hydrothermal crystallization of ZrO2 and HfO2 and theirsolubility, it was concluded that in both cases hydrothermalcrystallization proceeds via a dissolution/precipitation mech-anism.

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. Conclusion

The results of XRD analysis show that hydrothermal callization of HfO2, in a way similar to the one earlier reporor ZrO2 [6], proceeded much more slowly in a neutraledium than in acidic or alkaline media. The important

erence between hydrothermal crystallization of HfO2 andrO2 is in the phase compositions of the obtained crystal

ion product. In the case of ZrO2 crystallization in a neutrar high pH medium produced, besidem-ZrO2, a metastabl

- or c-ZrO2 phase. The results of phase analysis of H2rystallization products show that, regardless of pH, hydafnia crystallized intom-HfO2, so the topotactic modelrystallization proposed for ZrO2 [7,8] could not be used

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The crystallization process of HfO2 and ZrO2 under hydrothermal conditions