Synthesis of HfO2 from hafnium hydroxide hydrate

Accepted Manuscript Synthesis of HfO2 from hafnium hydroxide hydrate Ilya B. Polovov, Yakov S. Bataev, Yurii D. Afonin, Vladimir A. Volkovich, Andrey V. Chukin, Aydar Rakhmatullin, Miroslav Boča PII:

S0925-8388(19)30910-7

DOI:

https://doi.org/10.1016/j.jallcom.2019.03.103

Reference:

JALCOM 49867

To appear in:

Journal of Alloys and Compounds

Received Date: 17 October 2018 Revised Date:

2 February 2019

Accepted Date: 5 March 2019

Please cite this article as: I.B. Polovov, Y.S. Bataev, Y.D. Afonin, V.A. Volkovich, A.V. Chukin, A. Rakhmatullin, M. Boča, Synthesis of HfO2 from hafnium hydroxide hydrate, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.103. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of HfO2 from hafnium hydroxide hydrate

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Ilya B. Polovov,1 Yakov S. Bataev,1 Yurii D. Afonin,1 Vladimir A. Volkovich,1 Andrey V. Chukin,2 Aydar Rakhmatullin,1,3 and Miroslav Boča4 1

Department of Rare Metals and Nanomaterials, Institute of Physics and Technology, Ural

Federal University, Ekaterinburg, Russian Federation Department of Theoretical Physics and Applied Mathematics, Institute of Physics and

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2

Technology, Ural Federal University, Ekaterinburg, Russian Federation 3

Conditions Extrêmes et Materiaux: Haute Température et Irradiation, CEMHTI, Orléans,

France

Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Slovakia

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4

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Corresponding author Ilya B. Polovov, e-mail: [email protected]

Abstract

A comprehensive study of processes taking place during synthesis and subsequent thermal treatment of hafnium oxide was performed. Synthesis of HfO2 involves following stages: stepwise dehydration of hydrated hafnium hydroxide, crystallization of metastable tetragonal

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phase of HfO2 and subsequent irreversible transition to the stable monoclinic modification. Preliminary heat treatment lowered temperature of crystallization of HfO2 and resulted in

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changing process mechanism.

Keywords: ceramics, oxide materials, transition metal compounds, chemical synthesis,

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hafnium

1. Introduction

Hafnium oxide has a number of applications in technology and currently there is sharp growing demand for this material in the world. Leading companies manufacturing microprocessors use HfO2-based ceramics as dielectric gates in field-effect transistors resulting in more compact, fast and power effective nano-CMOS integrated circuits [1, 2]. Hafnium oxide has high refractive index and, at the same time, low light absorption coefficient. As a result, HfO2 is used in manufacturing wear resistant coatings of optical elements that can be used in a wide wavelength range [3]. In addition, HfO2 is used for producing ceramics and coatings with high hardness, catalysts, gas sensors, and fuel cell electrolytes [4–8].

HfO2 forms three crystalline modifications, monoclinic, tetragonal and cubic. Polymorphous

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transformation in hafnium dioxide were studied employing high temperature X-ray diffraction, differential scan calorimetry, thermal analysis, vibrational spectroscopy, dilatometry, etc. [9–21]. Metastable modifications of hafnium oxide were investigated [22–35] but the results are often contradictory. Several groups reported that only monoclinic HfO2 can be formed from the amorphous phase [23–25, 30, 31, 33–35]. Stefanic et al. [27] and Lu et al. [32] noted

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formation of the metastable cubic form of hafnium oxide. At the same time Tang et al. [26] combining X-ray diffraction and Raman spectroscopy studies concluded that metastable HfO2 modification is not cubic but tetragonal. Robinson et al. [29] indicated that the mixture of tetragonal and monoclinic phases was formed after heating of amorphous nanomaterial at 600

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°C in air for 1 h.

Information concerning temperatures of formation of monoclinic or metastable modifications

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of HfO2 is also in a poor agreement. For example, Dautel and Duval [22] reported crystallization of HfO2 at 350 °C, Blanc et al. [24] observed the formation of monoclinic HfO2 at 450–500 °C, whereas Komissarova et al [23] stated higher temperature of 610–630 °C. Preliminary thermal treatment can affect crystallization temperature [27]. Combining hydrothermal and ultrasound treatment produced monoclinic hafnium oxide at 250 °C [30]. One of the reasons for poor agreement of the data reported in the literature is that the majority

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of researchers used rather limited number of experimental techniques. The aim of the present work was therefore comprehensive studying processes taking place during formation of HfO2. One of the methods of producing HfO2 is based on precipitation of hydrated hafnium hydroxide from aqueous solutions followed by thermal treatment. Very little attention was

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paid to the mechanism of transforming hydrate of hafnium hydroxide to dioxide, and the data available on phase composition and dehydration temperatures are inconsistent and

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contradictive [22, 23, 27, 30, 34, 35]. Komissarova et al [23] observed 2 endothermic effects attributed to loss of moisture at 95–120 and 190–235 °C, whereas Stefanic et al. [27] and Nikishina et al. [34, 35] noticed only one broad endothermic maximum on the DSC and DTG curves at 90–150 °C. Meskin et al. [30] reported that the exothermic peak at 224 °C was caused by the formation of amorphous HfO2. Other research groups noticed the exothermic effect at higher temperatures [23, 26–28, 32, 34, 35] but the observed temperatures of DSCor DTA-maximum differ from each other. Therefore another aim of the present work was studying mechanism of formation of hafnium oxide from hydroxide hydrate. 2. Experimental To avoid introducing impurities into the final product, synthesis of hydrate of hafnium hydroxide was performed using the reagents that would have no effect on the phase formation

process [36]. Samples were obtained by precipitating hydrate of hafnium hydroxide from

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hydrochloric solutions by an aqueous ammonia solution. Composition of the precipitate thus obtained corresponded to the formula of HfO2-0.5n(OH)n·xH2O (where n can vary from 2 to 4 and x on average equals to 30–40). The precipitate was filtered off on a suction filter and thoroughly washed by distilled water to remove mother liquor and excess precipitant. Table 1 shows the impurities content in the hydroxide prepared.

Mg

Al

50

60

30

K not found

Ti

Cr

Mn

1320

60

10

Fe

Ni

Cu

770

30

1

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Na

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Table 1. Impurities content in samples of synthesized hydrate of hafnium hydroxide (in ppm)

In the first series of experiments the processes taking place during thermal treatment of

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hydrate of hafnium hydroxide were investigated. The second series of experiments was devoted to studying samples obtained after heating hydrated hafnium hydroxide in air at different temperatures for various length of time. Thermal and X-ray diffraction analysis, vibrational spectroscopy and specific surface area determination were employed to study the processes taking part in preparation of hafnium dioxide.

Thermogravimetric analysis (TGA) with simultaneous mass-spectrometry of the gas phase

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and single-pan differential thermal analysis (SDTA) of the samples of hydrated hafnium hydroxide were performed at the heating rate of 10 oC/min and the air flow through the furnace at the rate of 100 ml/sec. A METTLER TOLEDO STARe TGA/SDTA851e LF/1600°С instrument with a quadrupole PFEIFER Thermostar GSP 301T mass-spectrometer

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was employed. Sample weight was determined using built-in METTLER TOLEDO MT5 microbalances (weight limit of 5 g, discreteness of 1 microgram).

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Differential scanning calorimetry (DSC) was used for determining heat effects of chemical reactions and phase transformations taking place during heating hydrated hafnium hydroxide. The measurements were performed employing a METTLER TOLEDO STAR DSC823 calorimeter with FRS 5 sensor equipped with 56 Au-Au/Pd thermocouples (working range of 10–700 oC, resolution <0.04 µW). Experimental conditions were the same as used for TGA measurements. Vibrational spectra were recorded using an IR-Raman spectrometer Vertex 70 (Bruker) with a RAM II attachment in the range of 400–4000 cm–1 (IR) and 70–1300 cm–1 (Raman). IR spectra were measured in a diffuse reflection mode at temperatures up to 700 oC; potassium bromide acted as a reference.

Changes in the phase composition occurring during the thermal treatment were assessed using

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high temperature X-ray diffraction analysis (Ni-filtered Сu Kα, 1.5406 Å radiation) on an X’PERT PRO MPD diffractometer equipped with a PIXCEL high speed solid state detector and an Anton Paar HTK 1200N high temperature camera. X-ray diffraction patterns were analyzed using a full profile Rietveld analysis and PDF2 database. All X-ray diffraction measurements were conducted in air.

method (BET) on a Nova 1200e instrument. 3. Results and discussion 3.1. Dehydration of hydrated hafnium hydroxide

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Specific surface area of the samples was determined by low temperature nitrogen sorption

Studying dehydration process showed that the total weight decrease on average was 85 % and

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the major part of water (99 %) was removed below 250 oC, Fig. 1. SDTA curve (Fig. 1)

contained two endothermic effects (one starting at 35 oC with the peak at 131 oC, and another

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starting at 189 oC with the peak at 235 oC) and one very intense exothermic effect (starting at 531 with the peak at 555 oC). Mass-spectrometry analysis (Fig. 2) showed that the endothermic peaks were accompanied by evolution of water. At the same time carbon dioxide absorbed from the air was removed (25–350 oC) and this was followed by decomposition of carbonates formed in the surface layer due to chemosorption of CO2 (350–550 oC), Fig. 2. DSC analysis (Fig. 3) showed that the heat effects of the endothermic processes were 146 and

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22 J/g and that of the exothermic process 96 J/g. The data obtained showed that the dehydration process occurred in two stages. At the heating rate of 10 oC/sec removing non-structural water, i.e. water molecules that were not part of the crystal lattice and were bound with OH-groups via hydrogen bonding [37], was completed at

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160 oC. The second stage of dehydration took part at 160–250 oC. According to the weight change data at this stage hafnyl hydroxide, or γ-hafnium hydroxide, HfO(OH)2, decomposed

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forming amorphous HfO2. Taking into account the tendency of zirconium and hafnium to form tetramers [38–41], the composition of γ-hafnium hydroxide should be more correctly expressed as Hf4O4(OH)8. After the transformation accompanied by the second endothermic effect (Fig. 1 and 3) sample weight essentially did not change and at 250–1200 oC the weight decrease did not exceed 1 %. To obtain additional information on the mechanism of decomposition of hydrated hafnium hydroxide, IR spectra were recorded in the course of heating the samples from the room temperature to 700 oC. At the room temperature (Fig. 4) the samples contained bidentate carbonate groups (as indicated by the corresponding bands at 1320 and 1580 cm–1 [42, 43]) formed due to reaction of hafnium hydroxide with absorbed CO2, and non-structural water (band in the spectra around 1650 cm–1 corresponding to the bending vibration of water [37]).

Increasing temperature led to decomposition of carbonates (intensities of the corresponding

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bands in the spectra decreased). Above 200 oC formation of γ-hydroxide of hafnium, HfO(OH)2, began, as witnessed by the appearance and growth of bands at 1410 and 1560 cm– 1

(Fig. 4) that, by the analogy with hydroxides of other transition metals [37], can be related to

bending vibrations of Hf–OH and O–Hf–O (or Hf–O–Hf). Further heating resulted in decomposition of γ-HfO(OH)2 and formation of HfO2. Removing non-structural water can stretching vibrations of water molecules [37]. 3.2. Phase transformations during synthesis of HfO2

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also be assessed by changing intensity of a wide band around 3450 cm–1 (Fig. 5) arising from

To determine the temperature of beginning of crystallization and temperatures of phase

transitions, a series of X-ray diffraction patters was recorded at heating samples from the

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room temperature to 950 oC with the heating rate of 10 oC/min. This heating rate was the same as used in TGA, SDTA and DSC measurements. Starting material was X-ray

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amorphous as shown by the absence of peaks in the diffraction patterns up to 540 oC, Fig. 6. Fig. 6 shows that crystallization of HfO2 started at 540–560 oC, i.e. at the same conditions where the exothermic effect was observed on the thermograms, Figs. 1 and 3. X-ray diffraction analysis of the powders obtained showed the presence of nanocrystalline phases with the crystallites ranging from 7 to 40 nm (depending on the regime of thermal treatment).

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Crystalline hafnium oxide forms three modifications [36], i.e. monoclinic, stable at the room temperature, and high temperature tetragonal and cubic phases. Transformation of the monoclinic HfO2 into tetragonal takes place by the martensitic mechanism around 1750 oC; and tetragonal phase changes into cubic around 2600 oC [19]. In case of crystallization of

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small particles of HfO2 formation of metastable tetragonal [26, 29] and cubic [27, 32] phases is possible at considerably lower temperatures. Presence of the monoclinic HfO2 phase is

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manifested by the presence in the diffraction patterns of maxima labeled as m( 1 11 ) and m(111) in Fig. 6. At the heating rate of 10 oC/min this phase started to form at 730–750 oC. In the samples heated to 950 oC and above m-HfO2 phase became predominant. Monoclinic phase is formed from the metastable cubic or tetragonal modification. An indirect analysis of the diffraction patterns was used to elucidate the source phase of the monoclinic modification. The necessity of using indirect analysis is determined by the fact that the tetragonal crystal lattice of hafnium oxide only slightly differs from the cubic one of fluorite type. Their diffraction patterns can be distinguished by splitting lines at large diffraction angles or by the presence of a weak line at small angles. Formation of nanocrystalline powders, as in our case, resulted in broadening diffraction lines and impossibility to detect their splitting. Only asymmetry of some maxima could be observed. In the present case the

fact that the metastable phase was tetragonal rather than cubic can be deduced from the shape

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of line at diffraction angles of 58–61o, Fig. 7, where reflections t(103) and t(211) of the tetragonal phase are situated. Intensities of these reflections has the ratio around 1 : 2. The diffraction pattern showed that the resulting line was stretched towards smaller angles, Fig. 7. Therefore, upon heating γ-hydroxide of hafnium the tetragonal phase (to which the peak t in Fig. 6 is related) crystallized first. Further temperature increase led to a gradual growth of the amount of the tetragonal phase from the amorphous component and the size of its grains up to

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the temperature of the phase transformation, which resulted in the appearance of monoclinic phase crystallites. Subsequent heating was accompanied by increasing amount of the monoclinic phase due to decreasing fraction of the tetragonal one, Fig. 6.

Previous works [27, 32] reported that homogeneous formation of particles of the cubic phase

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was possible upon heating amorphous hafnium oxide, and these cubic particles were

transformed into monoclinic with increasing temperature. However, in the present study no

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convincing evidence of the formation of cubic HfO2 phase was obtained. Therefore we conclude that the metastable HfO2 phase has tetragonal crystal lattice. This conclusion agrees with the results of high temperature Raman spectroscopy measurements [26]. Mechanisms of the processes taking part upon crystallization and subsequent phase transformations are different. Crystallization from the amorphous phase is homogeneous, i.e. for its beginning it is necessary to form a critical size nucleus of the new crystalline phase by

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a fluctuation mean. Phase transformations from the tetragonal to the monoclinic and, possibly, from the cubic to the tetragonal phase take place without diffusion within each crystallite. However, for crystallization in the bulk of the sample, the number of crystals reached critical size must be large. Gradual increasing temperature leads to growing number of such crystals

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and creating conditions for crystallization. Cooling monoclinic HfO2 did not lead to changes in the phase composition, i.e. the phase

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transitions taking place were irreversible. Thermal treatment of hafnium hydroxide resulted in formation of stable (up to 1700 oC) monoclinic phase through the intermediate metastable tetragonal modification of HfO2. At the heating rate of 10 oC/min crystallization of tetragonal hafnium oxide occurred at 555–580 oC. The heat effect of this process was 96 J/g HfO2. Further heating resulted in growing grains of the tetragonal phase. Starting from 730 oC an irreversible t' → m transition was observed. The latter process was spread in time and was not accompanied by a significant heat effect. 3.3. Effect of preliminary heat treatment on HfO2 crystallization In a number of works [27, 30, 44] it was shown that preliminary treatment (hydrothermal, ultrasound, mechanical) could have an effect on dynamics of phase transformations during heat treatment of zirconium and hafnium hydroxides. We made an assumption that similar

effect might have preliminary heating of samples to the temperatures below crystallization

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temperature. To estimate the influence of the preliminary heat treatment, a series of experiments was performed employing DSC and high temperature X-ray diffraction methods. According to the calorimetry data, Fig. 8, increasing time of holding at 400 oC resulted in shifting the exothermic peak associated with crystallization of HfO2 towards lower temperatures. X-ray diffraction analysis of samples of hydrated hafnium hydroxide heated to 400 oC with subsequent holding for 2 h, did not reveal any crystalline phases, Fig. 9(a and b).

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Further heating to 460 oC led to the beginning of HfO2 crystallization, Fig. 9(c). If such sample was held at 460 oC for 30 min, then the major phase would be monoclinic modification of HfO2, Fig. 9(d).

Thus, preliminary thermal treatment leads to lowering crystallization temperature. Such

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observation explains the discrepancy in phase transformation temperatures during synthesis of HfO2 given in different works. Obtaining reliable data on processes taking part during

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preparation of hafnium oxide from hydrate of its hydroxide requires performing the measurements using different methods but at the same heating conditions. It is obvious that the method of preparing hydroxide, ultrasound and hydrothermal treatment also influence the changes of crystallization temperature. As a result the data available on crystallization temperature are in a poor agreement [22–35].

The data obtained here for the crystallization temperature from the thermograms (Fig. 8) and

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diffraction patterns (Fig. 9) were somewhat different. Since the crystallization is homogeneous, it can be assumed that the beginning of this process strongly depended on the sample volume and method of its preparation, particularly the conditions of preliminary heat treatment.

temperatures

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3.4. Analysis of samples obtained after holding hydrated hafnium hydroxide at different

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Analysis of samples produced by calcining hydrate at various temperatures (400, 600 and 1000 oC) for 4 hours did not show the presence of metastable tetragonal hafnium dioxide phase, Fig. 10. The sample calcined at 400 oC was amorphous and those heated at 600 and 1000 oC consisted of the monoclinic phase. This points out that at a prolonged heating transformation of the tetragonal phase into monoclinic was already completed at 600 oC. The results also showed that the four hours heating of hydrated hafnium hydroxide at 400 oC did not lead to the formation of the crystalline phase. IR spectra of the samples obtained by holding samples of hydrated hafnium hydroxide at various temperatures are presented in Fig. 11. IR active vibrations of monoclinic hafnium oxide are observed at 400–900 cm–1 [31]. Monoclinic hafnium oxide (Aldrich, 98 %) was used as a standard. Spectra of the samples produced at 140 and 400 oC did not contain

absorption lines. Spectra of the samples synthesized at higher temperatures (600 and 1000 oC)

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contained bands at 750, 650, 512 and 406 cm–1 corresponding to the expected frequencies of vibrations in monoclinic HfO2 [45]. Spectral profiles obtained in the present study agreed with reported in the literature [25] and corresponded to the IR spectrum of standard monoclinic hafnium oxide. Analysis of Raman spectra, Fig. 12, also showed that the four-hours holding of hydrated hafnium hydroxide did not result in the formation of a crystalline phase, and that the samples

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calcined at 600 oC and above contained only monoclinic hafnium oxide. Raman spectra were interpreted by comparing with the spectrum of standard HfO2 and with the literature data [18, 46].

3.5. Estimation of the effect of crystallite size on phase transformation processes

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Particle size of hafnium hydroxide was estimated from the results of specific surface area determination assuming spherical shape of the particles. Specific surface area and calculated

particles of hafnium oxide.

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particle sizes are given in Table 2. Increasing calcination temperature led to growth of

Table 2. Particle size and specific surface area of samples obtained after heating hydrated hafnium hydroxide at different temperatures for 4 h T heating / oC

400

600

1000

Specific surface area / m2/g

210

125

60

30

Average particle size r, 10–10 m

15

25

55

105

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170

Carvie [47] on the example of thermal treatment of zirconium hydroxide noted that the

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particles having the size below critical had stabilizing effect on stability of the metastable phase. Transition from one modification to another took place after crystallites reached certain

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critical size [47]. The results obtained in the present study and concerning changes particle size with temperature correspond to that suggested mechanism of phase transformations. The following equation was used to estimate the energy change in the system associated with decreasing surface area [48]: ∆φ = 3εV(1/r1 - 1/r2),

(1)

where ∆φ is the change of surface energy of the system, J/mol; ε is the specific surface energy of HfO2 (cub. 111) crystal, (1.078 J/m2); V is the molar volume of HfO2, 2.2·10–5 m3; r1 and r2 are the average values of particle radii for the states 1 and 2. Change of the surface energy of the samples at increasing temperature of the thermal treatment from 400 oC (below crystallization temperature) to 600 oC (above crystallization

temperature) was 70 J/g. This value is close to the heat effect observed upon crystallization of

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tetragonal hafnium oxide, Fig. 3. 4. Conclusions Processes taking place during formation of hafnium dioxide from hydrate of its hydroxide were studied. The following stages were determined: two-stage dehydration; crystallization of the metastable tetragonal phase; subsequent irreversible transition of the metastable phase to monoclinic modification of HfO2. Dehydration process was accompanied by desorption of

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carbon dioxide from the surface of the samples. No evidence of formation of cubic HfO2 phase was obtained.

To obtain reliable information about the processes taking place during synthesis of hafnium oxide from hydrated hydroxide it is necessary (when various experimental technique are

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employed) to use identical conditions for performing measurements. Preliminary thermal treatment resulted in lowering crystallization temperature of HfO2 and changing mechanism

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of the process.

Radii of the oxide phase nuclei had an influence on the crystallization processes, and size of the crystalline phase grains influenced stability of the metastable phase.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public,

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commercial, or not-for-profit sectors.

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Figure 1. TGA and SDTA curves recorded during dehydration of HfO2-0.5n(OH)n·xH2O.

Figure 2. Mass spectra (at 18 and 44 atomic mass units) of the gas phase recorded during heating dehydration of hydrated hafnium hydroxide.

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Figure 3. DCS curves recorded during heat treatment of hafnium hydroxide hydrate.

Figure 4. IR spectra recorded during heating HfO2-0.5n(OH)n·xH2O (region of frequencies of bending vibrations of molecular water, Hf–OH and surface carbonates).

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Figure 5. IR spectra recorded in the course of heating HfO2-0.5n(OH)n·xH2O (region of

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stretching vibrations of structural water).

Figure 6. XRD patterns recorded during heating HfO2-0.5n(OH)n·xH2O at 10 °C/min rate. Temperature, °C: 20–520 (a); 520–540 (b); 540–560 (c), 560–580 (d), 650–670 (e), 730–750 (f); 770–790 (g), 790–810 (h), 830–850 (i), 870–890 (j), 910-930 (k), 930–950 (l), 950–970 (m).

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Figure 7. Part of an XRD pattern of sample of hafnium hydroxide hydrate at 560–580 °C

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(heating rate 10 °C/min).

Figure 8. Changes of the heal flux during thermal treatment of hydrated hafnium hydroxide (heating rate 10 °C/min). Insert shows thermograms of samples that were preliminary held at 400 °C (time is given for each curve).

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Figure 9. XRD patterns measured during heating hydrated hafnium hydroxide at 50 °C/min rate to 400 °C (a); after subsequent holding at 400 °C for 2 h (b); after heating to 460 °C (c);

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and after holding at 460 °C for 30 min (d).

Figure 10. XRD patterns of samples of HfO2 measured after various heat treatments (conditions are given for each pattern).

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Figure 11. IR spectra of samples obtained during heating HfO2-0.5n(OH)n·xH2O for 4 h at different temperatures (HfO2 marked as “standard” was commercially available monoclinic

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phase (Aldrich, 98 %)).

Figure 12. Raman spectra of calcined samples of hydrated hafnium hydroxide (HfO2 marked as “standard” was commercially available monoclinic phase (Aldrich, 98 %)).

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1. Synthesis of HfO2 from hydrated hafnium hydroxide. 2. In situ combined DTA, DSC, XRD and IR investigation of HfO2 formation. 3. Effect of preliminary heat treatment on crystallization of HfO2 and its structure.